Optical switch, optical amplifier and optical power controller as well as optical add-drop multiplexer

The first present invention provides an optical switch including the following elements. At least a plurality of optical transmission lines are provided for transmissions of optical signals. Each of the at least plurality of optical transmission lines have at least an impurity doped fiber. At least an excitation light source is provided for emitting an excitation light. At least an excitation light switch is provided which is connected to the excitation light source and also connected to the at least plurality of optical transmission lines for individual switching operations to supply the excitation light to the at least plurality of optical transmission lines to feed the excitation light to the impurity doped fiber on the at least plurality of optical transmission lines, thereby causing an excitation of the impurity doped fiber on selected one of the at least plurality of optical transmission lines so as to permit a transmission of the optical signal through the excited impurity doped fiber, whilst unselected one of the impurity doped fibers is unexcited whereby the optical signals are absorbed into the unselected one of the impurity doped fibers thereby to discontinue transmission of the optical signal by the unselected one of the impurity doped fibers.

BACKGROUND OF THE INVENTION

The present invention relates to an optical switch, an optical amplifier and an optical power controller as well as an optical add-drop multiplexer.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a novel optical switch free from the above problems.

It is a further object of the present invention to provide a novel optical amplifier.

It is a still further object of the present invention to provide a novel optical power controller.

It is yet a further object of the present invention to provide a novel optical add-drop multiplexer.

The first present invention provides an optical switch including the following elements. At least a plurality of optical transmission lines are provided for transmissions of optical signals. Each of the at least plurality of optical transmission lines have at least an impurity doped fiber. At least an excitation light source is provided for emitting an excitation light. At least an excitation light switch is provided which is connected to the excitation light source and also connected to the at least plurality of optical transmission lines for individual switching operations to supply the excitation light to the at least plurality of optical transmission lines to feed the excitation light to the impurity doped fiber on the at least plurality of optical transmission lines, thereby causing an excitation of the impurity doped fiber on selected one of the at least plurality of optical transmission lines so as to permit a transmission of the optical signal through the excited impurity doped fiber, whilst unselected one of the impurity doped fibers is unexcited whereby the optical signals are absorbed into the unselected one of the impurity doped fibers thereby to discontinue transmission of the optical signal by the unselected one of the impurity doped fibers.

The above and other objects, features and advantages of the present invention will be apparent from the following descriptions.

DISCLOSURE OF THE INVENTION

The first present invention provides an optical switch including the following elements. At least a plurality of optical transmission lines are provided for transmissions of optical signals. Each of the at least plurality of optical transmission lines have at least an impurity doped fiber. At least an excitation light source is provided for emitting an excitation light. At least an excitation light switch is provided which is connected to the excitation light source and also connected to the at least plurality of optical transmission lines for individual switching operations to supply the excitation light to the at least plurality of optical transmission lines to feed the excitation light to the impurity doped fiber on the at least plurality of optical transmission lines, thereby causing an excitation of the impurity doped fiber on selected one of the at least plurality of optical transmission lines so as to permit a transmission of the optical signal through the excited impurity doped fiber, whilst unselected one of the impurity doped fibers is unexcited whereby the optical signals are absorbed into the unselected one of the impurity doped fibers thereby to discontinue transmission of the optical signal by the unselected one of the impurity doped fibers.

It is preferable that the optical switch further includes: a single input side optical transmission line; and a single input side optical coupler connected to the single input side optical transmission line, and wherein the at least plurality of optical transmission lines comprise first and second optical transmission lines which are connected through the single input side optical coupler to the single input side optical transmission line, and the first and second optical transmission lines have first and second impurity doped fibers, and wherein the at least excitation light source comprises a single excitation light source, and the at least excitation light switch comprises a single excitation light switch which has first and second output terminals for selecting any one of the first and second output terminals, and the first output terminal is connected through a first optical coupler to the first impurity doped first to feed the excitation light to the first impurity doped fiber only when the first output terminal is selected by the single excitation light switch, and the second output terminals is connected through a second optical coupler to the second impurity doped fiber to feed the excitation light to the second impurity doped fiber only when the second output terminal is selected by the single excitation light switch.

It is preferable further comprise first and second optical filers. The first optical filter is provided on the first optical transmission line and positioned between the first optical coupler and an output terminal of the first optical transmission line so as to remove a noise from the first optical signal when the first impurity doped fiber is excited. The second optical filter is provided on the second optical transmission line and positioned between the second optical coupler and an output terminal of the second optical transmission line so as to remove a noise from the second optical signal when the second impurity doped fiber is excited.

It is preferable further comprise the following elements. A first optical reflective mirror is provided on one end of the first optical transmission line for reflecting the first optical signal passed through the first impurity doped fiber excited so that the reflected first optical signal is again transmitted through the first impurity doped fiber excited to an opposite end as an output terminal of the first optical transmission line. A first optical isolator is provided between the input side optical coupler and the first optical transmission line for permitting a unidirectional transmission of an optical signal from the input side optical coupler to the first optical transmission line. A second optical reflective mirror is provided on one end of the second optical transmission line for reflecting the second optical signal passed through the second impurity doped fiber excited so that the reflected second optical signal is again transmitted through the second impurity doped fiber excited to an opposite end as an output terminal of the second optical transmission line. A second optical isolator is provided between the input side optical coupler and the second optical transmission line for permitting a unidirectional transmission of an optical signal from the input side optical coupler to the second optical transmission line.

It is preferable further comprise the following elements. A first optical reflective mirror is provided on one end of the first optical transmission line for reflecting the first optical signal passed through the first impurity doped fiber excited so that the reflected first optical signal is again transmitted through the first impurity doped fiber excited to an opposite end as an output terminal of the first optical transmission line. A second optical reflective mirror is provided on one end of the second optical transmission line for reflecting the second optical signal passed through the second impurity doped fiber excited so that the reflected second optical signal is again transmitted through the second impurity doped fiber excited to an opposite end as an output terminal of the second optical transmission line. A circulator is provided as the input side optical coupler and an optical isolator provided between the input side optical transmission line and the first and second optical transmission lines.

It is preferable that the first optical coupler is inserted between the first impurity doped fiber and an output terminal of the first optical transmission line so as to feed the excitation light to the first impurity doped fiber in an opposite direction to a transmission of the first optical signal through the first impurity doped fiber excited, and also the second optical coupler is inserted between the second impurity doped fiber and an output terminal of the second optical transmission line so as to feed the excitation light to the second impurity doped fiber in an opposite direction to a transmission of the second optical signal through the second impurity doped fiber excited.

It is preferable that the first optical coupler is inserted between the first impurity doped fiber and the input side optical coupler so as to feed the excitation light to the first impurity doped fiber in the same direction as a transmission of the first optical signal through the first impurity doped fiber excited, and also the second optical coupler is inserted between the second impurity doped fiber and the input side optical coupler so as to feed the excitation light to the second impurity doped fiber in the same direction as a transmission of the second optical signal through the second impurity doped fiber excited.

It is preferable that the optical switch has two inputs and two outputs and comprises a pair of first and second optical switches connected to each other through at least an interconnecting optical transmission line, and wherein each of the first and second optical switches further comprises the following elements. A single input side optical coupler is provided which is connected to the single input side optical transmission line. First and second optical transmission lines are connected through the single input side optical coupler to the single input side optical transmission line. The first and second optical transmission lines have first and second impurity doped fibers. A single excitation light source is provided. A single excitation light switch is provided which has first and second output terminals for selecting any one of the first and second output terminals. The first output terminal is connected through a first optical coupler to the first impurity doped fiber to feed the excitation light to the first impurity doped fiber only when the first output terminal is selected by the single excitation light switch. The second output terminal is connected through a second optical coupler to the second impurity doped fiber to feed the excitation light to the second impurity doped fiber only when the second output terminal is selected by the single excitation light switch.

It is preferable that each of the first and second optical switches further comprises first and second optical filters. The first optical filter is provided on the first optical transmission line and positioned between the first optical coupler and an output terminal of the first optical transmission line so as to remove a noise from the first optical signal when the first impurity doped fiber is excited. The second optical filter is provided on the second optical transmission line and positioned between the second optical coupler and an output terminal of the second optical transmission line so as to remove a noise from the second optical signal when the second impurity doped fiber is excited.

It is preferable that each of the first and second optical switches further comprises the following elements. A first optical reflective mirror is provided on one end of the first optical transmission line for reflecting the first optical signal passed through the first impurity doped fiber excited so that the reflected first optical signal is again transmitted through the first impurity doped fiber excited to an opposite end as an output terminal of the first optical transmission line. A first optical isolator is provided between the input side optical coupler and the first optical transmission line for permitting a unidirectional transmission of an optical signal from the input side optical coupler to the first optical transmission line. A second optical reflective mirror is provided on one end of the second optical transmission line for reflecting the second optical signal passed through the second impurity doped fiber excited so that the reflected second optical signal is again transmitted through the second impurity doped fiber excited to an opposite end as an output terminal of the second optical transmission line. A second optical isolator is provided between the input side optical coupler and the second optical transmission line for permitting a unidirectional transmission of an optical signal from the input side optical coupler to the second optical transmission line.

It is preferable that each of the first and second optical switches further comprises the following elements. A first optical reflective mirror is provided on one end of the first optical transmission line for reflecting the first optical signal passed through the first impurity doped fiber excited so that the reflected first optical signal is again transmitted through the first impurity doped fiber excited to an opposite end as an output terminal of the first optical transmission line. A second optical reflective mirror is provided on one end of the second optical transmission line for reflecting the second optical signal passed through the second impurity doped fiber excited so that the reflected second optical signal is again transmitted through the second impurity doped fiber excited to an opposite end as an output terminal of the second optical transmission line. A circulator is provided as the input side optical coupler and an optical isolator provided between the input side optical transmission line and the first and second optical transmission lines.

It is preferable that, for each of the first and second optical switches, the first optical coupler is inserted between the first impurity doped fiber and an output terminal of the first optical transmission line so as to feed the excitation light to the first impurity doped fiber in an opposite direction to a transmission of the first optical signal through the first impurity doped fiber excited, and also the second optical coupler is inserted between the second impurity doped fiber and an output terminal of the second optical transmission line so as to feed the excitation light to the second impurity doped fiber in an opposite direction to a transmission of the second optical signal through the second impurity doped fiber excited.

It is preferable that, for each of the first and second optical switches, the first optical coupler is inserted between the first impurity doped fiber and the input side optical coupler so as to feed the excitation light to the first impurity doped fiber in the same direction as a transmission of the first optical signal through the first impurity doped fiber excited, and also the second optical coupler is inserted between the second impurity doped fiber and the input side optical coupler so as to feed the excitation light to the second impurity doped fiber in the same direction as a transmission of the second optical signal through the second impurity doped fiber excited.

It is preferable that the optical switch has two inputs and two outputs and comprises a pair of first and second optical switches connected to each other through at least an interconnecting optical transmission line, and a common excitation light source connected to the first and second optical switches, and wherein each of the first and second optical switches further comprises the following elements. A single input side optical coupler is provided which is connected to the single input side optical transmission line. First and second optical transmission lines are provided which are connected through the single input side optical coupler to the single input side optical transmission line. The first and second optical transmission lines have first and second impurity doped fibers. A single excitation light switch is provided which is connected to the common excitation light source, the single excitation light switch having first and second output terminals for selecting any one of the first and second output terminals, and the first output terminal being connected through a first optical coupler to the first impurity doped fiber to feed the excitation light to the first impurity doped fiber only when the first output terminal is selected by the single excitation light switch, and the second output terminal being connected through a second optical coupler to the second impurity doped fiber to feed the excitation light to the second impurity doped fiber only when the second output terminal is selected by the single excitation light switch.

It is preferable that the at least plurality of optical transmission lines are separated from each other for separate transmission of different optical signals on the plurality of separated optical transmission lines. Each of the separated optical transmission lines has a single impurity doped fiber. The at least excitation light source comprises a single excitation light source. The at least excitation light switch comprises a single excitation light switch for separate switching operations to the at least plurality of optical transmission lines to separately control individual excitations of the impurity doped fibers on the least plurality of optical transmission lines.

It is preferable that the at least plurality of optical transmission lines are separated from each other for separate transmission of different optical signals on the plurality of separated optical transmission lines, and each of the separated optical transmission lines has a single impurity doped fiber, and the at least excitation light source comprises two excitation light source, and further at least excitation light switch comprises a single optical cross connector for separate switching operations to the at least plurality of optical transmission lines to separately control individual excitations of the impurity doped fibers on the at least plurality of optical transmission lines.

The second present invention provides an optical switch comprising the following elements. A first optical transmission line is provided for transmitting a first optical signal. An optical reflectivity variable mirror is provided which is capable of varying a reflectivity in a range of 0% to 100% for reflecting the first optical signal. The optical reflectivity variable mirror is connected with the first optical transmission line. A second optical transmission line is provided which is connected through the optical reflectivity variable mirror to the first optical transmission line. An optical transmitter is provided which is connected through the second optical transmission line to the optical reflectivity variable mirror for transmitting a second optical signal. If the optical reflectivity variable mirror sets the reflectivity at less than 100%, then the first optical signal is reflected by the optical reflectivity variable mirror so that the first optical signal is outputted from the first optical transmission line, if the optical reflectivity variable mirror sets the reflectivity at 100%, then the first optical signal is transmitted through the optical reflectivity variable mirror, whilst the second optical signal transmitted from the optical transmitter is also transmitted through the optical reflectivity variable mirror to be outputted from the first optical transmission line.

The third present invention provides an optical add-drop multiplexer comprising at least a single set of the following elements. A first optical transmission line is provided for transmitting a first optical signal. An optical coupler is provided on the first optical transmission line for dividing the first optical signal into first and second divided optical signals. A fourth optical transmission line is provided which is connected with the optical coupler for transmitting the first divided optical signal. An optical receiver is provided which is connected through the fourth optical transmission line to the optical coupler for receiving the first divided optical signal. An optical reflectivity variable mirror is provided which is capable of varying a reflectivity in a range of 0% to 100%. The optical reflectivity variable mirror is connected with first optical transmission line for reflecting the second divided optical signal. A second optical transmission line is provided which is connected through the optical reflectivity variable mirror to the first optical transmission line. An optical transmitter is provided which is connected through the second optical transmission line to the optical reflectivity variable mirror for transmitting a second optical signal. If the optical reflectivity variable mirror sets the reflectivity at less than 100%, then the first optical signal is reflected by the optical reflectivity variable mirror so that the first optical signal is outputted from the first optical transmission line. If the optical reflectivity variable mirror sets the reflectivity at 100%, then the first optical signal is transmitted through the optical reflectivity variable mirror, whilst the second optical signal transmitted from the optical transmitter is also transmitted through the optical reflectivity variable mirror to be outputted from the first optical transmission line.

It is preferable that the optical add-drop multiplexer comprises a plurality of the optical add-drop multiplexers, and further comprising an optical device having at least any one of multiplexing function and demultiplexing function so that the optical add-drop multiplexers are operable to different wavelength optical signals.

The fourth present invention provides an optical add-drop multiplexer comprising at least a single set of the following elements. An input side optical transmission line is provided for transmitting a first optical signal. An input side optical coupler is provided on the first optical transmission line for dividing the first optical signal into first and second divided optical signals. First and second optical transmission lines are provided which are connected with the input side optical coupler for transmissions of the first and second divided optical signals respectively. The first and second optical transmission lines have first and second impurity doped fibers. An optical receiver is provided which is connected through the first optical transmission line to the first impurity doped fiber for receiving the first divided optical signal only when the first impurity doped fiber is excited. An optical transmitter is provided which is connected through the second optical transmission line to the second impurity doped fiber for transmitting a second optical signal through the second impurity doped fiber to the input side optical transmission line for output of the second optical signal only when the second impurity doped fiber is excited. At least an excitation light source is provided for emitting an excitation light. An excitation light switch is provided which is connected to the excitation light source and also connected to the first and second optical transmission lines for selective switching operations to supply the excitation light to any one of the first and second optical transmission lines to feed the excitation light to selected one of the first and second impurity doped fibers, thereby causing an excitation of the selected one of the first and second impurity doped fibers, whilst unselected one of the first and second impurity doped fibers is unexcited.

It is preferable that the optical add-drop multiplexer comprises a plurality of the optical add-drop multiplexers, and further comprising an optical device having at least any one of multiplexing function and demultiplexing function so that the optical add-drop multiplexers are operable to different wavelength optical signals.

The fifth present invention provides an optical gate switch comprising the following elements. A first optical transmission line is provided for transmitting an input optical input signal. A second optical transmission line is provided for transmitting an optical output signal. A fourth optional transmission line is connected through an optical coupler to both the first and second optional transmission lines. The fourth optional transmission line has at least a impurity doped fiber and a wavelength band selective optical reflecting mirror capable of selecting a wavelength band of a light to be reflected. The impurity doped fiber is positioned between the wavelength band selective optical reflecting mirror. An excitation light source is provided which is connected to the wavelength band selective optical reflecting mirror for controlling an emission of an excitation light so that if the excitation light source emits the excitation light to feed the excitation light to the impurity doped fiber so as to excite the impurity doped fiber, whereby the optical input signal is transmitted through the excited impurity doped fiber and amplified by the excited impurity doped fiber and subsequently the amplified optical signal is reflected by the wavelength band selective optical reflecting mirror before the reflected optical signal is then transmitted through the excited impurity doped fiber and further amplified by the excited impurity doped fiber for subsequent output of the further amplified optical signal through the output signal optical transmission line.

The sixth present invention provides an optical add-drop multiplexer comprising at least a single set of the following elements. A first optional transmission line is provided for transmitting an input optical input signal. A second optical transmission line is provided for transmitting an optical output signal. A fourth optional transmission line is provided which is connected through an optical coupler to both the first and second optional transmission lines. The fourth optional transmission line has at least a impurity doped fiber and a wavelength band selective optical reflecting mirror capable of selecting a wavelength band of a light to be reflected. The impurity doped fiber is positioned between the wavelength band selective optical reflecting mirror. An optical receiver is provided which is connected through a second optical coupler to the fourth optical transmission line so that the second optical coupler is positioned between the first optical coupler and the impurity doped fiber for allowing the optical receiver receives a part of the optical input signal. An optical transmitter is provided which is connected through a fourth optical coupler to the output signal transmission line for transmitting a second optical signal as a substitute output signal only when no output signal is supplied from the impurity doped fiber. An excitation light source is provided which is connected to the wavelength band selective optical reflecting mirror for controlling an emission of an excitation light so that if the excitation light source emits the excitation light to feed the excitation light to the impurity doped fiber so as to excite the impurity doped fiber, whereby the optical input signal is transmitted through the excited impurity doped fiber and amplified by the excited impurity doped fiber and subsequently the amplified optical signal is reflected by the wavelength band selective optical reflecting mirror before the reflected optical signal is then transmitted through the excited impurity doped fiber and further amplified by the excited impurity doped fiber for subsequent output of the further amplified optical signal through the output signal optical transmission line.

It is preferable that the optical add-drop multiplexer comprises a plurality of the optical add-drop multiplexers, and further comprising an optical device having at least any one of multiplexing function and demultiplexing function so that the optical add-drop multiplexers are operable to different wavelength optical signals.

The seventh present invention provides an optical transmission line junction structure comprising at least three optical transmission lines for transmuting optical signals and an optical device having at least any one of wavelength multiplexing and demultiplexing functions connected to the at least three optical transmission lines, so that the optical device having at least any one of multiplexing and demultiplexing functions serves as a same roll as an optical coupler so as to reduce an optical power loss when the optical signal is transmitted through the optical transmission line junction structure.

It is preferable that the optical device comprises an optical multiplexer/demultiplexer.

It is preferable that the optical device comprises an optical multiplexer.

It is preferable that the optical device comprises an optical demultiplexer.

The eighth present invention provides an optical transmission line junction structure comprising at least three optical transmission lines for transmuting optical signals and an optical circulator connected to the at least three optical transmission lines, so that the optical circulator serves as a same roll as an optical coupler so as to reduce an optical power loss when the optical signal is transmitted through the optical transmission line junction structure.

The ninth present invention provides an optical loop-structured circuit having at least a plurality of looped optical transmission lines having at least a plurality of optical transmission line junctions from which at least three optical transmission lines extend, wherein at least one of the plurality of optical transmission line junctions has an optical device having at least any one of wavelength multiplexing and demultiplexing functions, which is connected to the at least three optical transmission lines, so that the optical device having at least any one of multiplexing and demultiplexing functions serves as a same roll as an optical coupler so as to reduce an optical power loss when the optical signal is transmitted through the optical transmission line junction structure.

It is preferable that all of the plurality of optical transmission line junctions have the optical devices.

It is preferable that at least one of the plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that the optical loop-structured circuit has a function of an optical amplifier.

It is preferable that the at least one of the plurality of looped optical transmission lines is further connected to at least two set of an optical receiver and an optical transmitter so that the optical loop-structured circuit has a function of an optical add-drop multiplexer.

It is preferable that at least one of the plurality of looped optical transmission lines has at least single set of an optical attenuator and an optical isolator so that the optical loop-structured circuit has a function of an optical equalizer.

It is preferable that at least two of the plurality of looped optical transmission lines are connected to an optical multiplexer/demultiplexer, whilst a single looped optical transmission line is separated by the at least two of the plurality of looped optical transmission lines from the optical multiplexer/demultiplexer, so that optical signals are individually transmitted along the plurality of looped optical transmission lines, and wherein all of the plurality of optical transmission line junctions have the optical devices.

It is preferable that each of the plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that the optical loop-structured circuit has a function of an optical amplifier.

It is preferable that each of the plurality of looped optical transmission lines is further connected to at least two set of an optical receiver an optical transmitter so that the optical loop-structured circuit has a function of an optical add-drop multiplexer.

It is preferable that each of the plurality of looped optical transmission lines has at least a single set of an optical attenuator and an optical isolator so that the optical loop-structured circuit has a function of an optical equalizer.

It is preferable that the optical device comprises an optical multiplexer/demultiplexer.

It is preferable that the optical device comprises an optical multiplexer.

It is preferable that the optical device comprises an optical demultiplexer.

The tenth present invention provides an optical loop-structured circuit having at least a plurality of looped optical transmission lines having at least a plurality of optical transmission line junctions from which at least three optical transmission lines extend, wherein at least one of the plurality of optical transmission line junctions has an optical circulator, which is connected to the at least three optical transmission lines, so that the optical circulator serves as a same roll as an optical coupler so as to reduce an optical power loss when the optical signal is transmitted through the optical transmission line junction structure.

It is preferable that all of the plurality of optical transmission line junctions have the optical circulators.

It is preferable that at least one of the plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that the optical loop-structured circuit has a function of an optical amplifier.

It is preferable that at least one of the plurality of looped optical transmission lines is further connected to at least two set of an optical receiver and an optical transmitter so that the optical loop-structured circuit has a function of an optical add-drop multiplexer.

It is preferable that at least one of the plurality of looped optical transmission lines has at least a single set of an optical attenuator and an optical isolator so that the optical loop-structured circuit has a function of an optical equalizer.

It is preferable that at least two of the plurality of looped optical transmission lines are connected to an optical multiplexer/demultiplexer, whilst a single looped optical transmission line is separated by the at least two of the plurality of looped optical transmission lines from the optical multiplexer/demultiplexer, so that optical signals are individually transmitted along the plurality of looped optical transmission lines, and wherein all of the plurality of optical transmission line junctions have the optical circulators.

It is preferable that each of the plurality of looped optical transmission lines has at least a single set of an optical amplifier and an optical isolator so that the optical loop-structured circuit has a function of an optical amplifier.

It is preferable that each of the plurality of looped optical transmission lines is further connected to at least two set of an optical receiver and an optical transmitter so that the optical loop-structured circuit has a function of an optical add-drop multiplexer.

It is preferable that each of the plurality of looped optical transmission lines has at least a single set of an optical attenuator and an optical isolator so that the optical loop-structured circuit has a function of an optical equalizer.

The eleventh present invention provides an optical gate switch comprising the following elements. A main optical transmission line is provided. First and second optical multiplexer/demultiplexers are also provided on the main optical transmission line so that the first and second optical multiplexer/demultiplexers are separated from each other. The first and second optical multiplexer/demultiplexers are connected with first and second subordinate optical transmission lines respectively. An impurity doped fiber is provided on the main optical transmission line and positioned between the first and second optical multiplexer/demultiplexers. An excitation light source is provided which is connected through the first subordinate optical transmission line to the first optical multiplexer/demultiplexer so that the excitation light source emits an excitation light which is transmitted through the first subordinate optical transmission line and the first optical multiplexer/demultiplexer to the impurity doped fiber. The second optical multiplexer/demultiplexer transmits the optical signal onto the main optical transmission line and also transmits a leaked part of the excitation light onto the second subordinate optical transmission line.

It is preferable to further comprise an optical reflecting mirror provided on the second subordinate optical transmission line for reflecting the leaked part of the excitation light to the impurity doped fiber.

It is preferable to further comprise a secondary excitation light source on the second subordinate optical transmission line.

Preferred Embodiments

First Embodiment

A first embodiment according to the present invention will be described in detail with reference toFIG. 1which is a diagram illustrative of a first novel optical switch having a single input and two outputs. The optical switch has an input side coupler21which is connected to a first optical transmission line110on which an optical input signal is transmitted and then inputted into the optical switch. The optical input signal has a wavelength of 1550 nanometers and an intensity of 0 dBm. The optical input signal is divided by the input side coupler21into two parts. The optical switch has second and third optical transmission lines120and121which are connected to the input side coupler21. The two divided optical signals are then transmitted through the second and third optical transmission lines120and121for output thereof. The second optical transmission line120is connected to a first output side coupler22. The third optical transmission line121is connected to a second output side optical coupler23. A first erbium doped fiber EDF11is provided on the second optical transmission line120between the input side coupler21and the first output side coupler22. A second erbium doped fiber EDF12is provided on the third optical transmission line120between the input side coupler21and the second output side coupler23. The first and second erbium doped fibers EDF11and EDF12have a length of 50 meters. The first and second erbium doped fibers EDF11and EDF12may be replaced by rare earth doped fibers. The two divided optical signals are transmitted through the first and second erbium doped fibers EDF11and EDF12respectively. The optical switch further has an excitation light switch41which is connected through a first excitation light transmission line111to the first output side coupler22as well as which is connected through a second excitation light transmission line112to the second output side coupler23. The optical switch further has an excitation light source31which is connected to the excitation light switch41. The excitation light source31emits an excitation light with a wavelength of 1480 nanometers The excitation light switch41is operated to switch the excitation light to any one of the first and second excitation light transmission lines111and112to supply any one of the first and second erbium doped fibers EDF11and EDF12.

If the excitation light switch41is operated to switch to supply the excitation light to the first erbium doped fiber EDF11, then the first erbium doped first EDF11is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF11without any optical absorption and then the optical signal with an intensity of 0 dBm is outputted from the second optical transmission line120. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the excitation light switch41to be fed through the first excitation light transmission line111and the first output side optical coupler22to the first erbium doped fiber EDF11. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the excitation light switch41whereby a leaked excitation light is then fed through the second output side optical coupler23to the second erbium doped fiber EDF12. However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF11, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF11. As a result, an optical output signal from the third optical transmission line121has an intensity of −60 dBm or less. The excitation light switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

As a modification to the above first embodiment, the above excitation light switch41may be replaced by a polymer optical switch.

If the polymer optical switch41is operated to switch to supply the excitation light to the first erbium doped fiber EDF11, then the first erbium doped fiber EDF11is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF11without any optical absorption and then the optical signal with an intensity of 0 dBm is outputted from the second optical transmission line120. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the polymer optical switch41to be fed through the first excitation light transmission line111and the first output side optical coupler22to the first erbium doped fiber EDF11. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the polymer optical switch41whereby a leaked excitation light is then fed through the second output side optical coupler23to the second erbium doped fiber EDF12. However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF12, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF12. As a result, an optical output signal from the third optical transmission line121has intensity of −60 dBm or less. The polymer optical switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

If the polymer optical switch41is operated to switch to supply the excitation light to the second erbium doped fiber EDF12, then the second erbium doped fiber EDF12is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF12without any optical absorption and then the optical signal with an intensity of 0 dBm is outputted from the third optical transmission line121. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the polymer optical switch41to be fed through the second excitation light transmission line112and the second output side optical coupler23to the second erbium doped fiber EDF12. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the polymer optical switch41whereby a leaked excitation light is then fed through the first output side optical coupler22to the first erbium doped fiber EDF11. However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF11, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF11. As a result, an optical output signal from the third optical transmission line121has an intensity of −60 dBm or less. The polymer optical switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

As a further modification to the above first embodiment, the excitation light has a wavelength of 980 nanometers in order to shorten the wavelength for a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

In the above embodiment, the number of the wavelength multiplexing on each optical transmission line is one. Notwithstanding, 8, 16, 32, 64-wavelength multiplexing are available, wherein the batch-switching operation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal at not only 1550 nanometers but also other wavelengths, for example, 1330 nanometers.

It is also possible to set the wavelength of the excitation light at not only 1480 nanometers or 980 nanometers but also other wavelengths provided that such wavelength is capable of exciting the impurity doped fiber. It is preferable to set the wavelength of the excitation light in consideration of both the wavelength of the optical input signal and the kind of the impurity doped fiber.

The above excitation light switch may also be replaced by an acousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical output signal by controlling an optical power of the excitation light to be fed to the impurity doped fiber. It is possible to control the optical power of the excitation light to be fed to the impurity doped fiber by controlling an injection current to the excitation light source or by use of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rare earth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing to different excitation lights emitted separately from plural different excitation light sources in order to input the polarization-multiplexed excitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at the optical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source and the single excitation light switch permit ON-OFF switching operations of the plural gate switches by a simple structure. The above switch exhibits such a gain property as a sharp rising, for which reason there is substantially no influence due to a leaked light from the excitation light switch. This makes the switch available to switches having relatively large crosstalk levels such as a polymer type switch or LiNbO.sub.3 switch, thereby realizing a low crosstalk and low insertion loss optical switch. In addition, the use of the impurity doped fiber serving as an optical power amplifier can obtain a gain as the optical switch.

Second Embodiment

A second embodiment according to the present invention will be described in detail with reference toFIG. 2which is a diagram illustrative of a second novel optical switch having a single input and two outputs. A structural difference of the second novel optical switch from the first novel optical switch is only in further providing first and second optical filters on two output sides in order to eliminate or remove amplified noises from the optical output signals.

The optical switch has an input side coupler21which is connected to a first optical transmission line110on which an optical input signal is transmitted and then inputted into the optical switch. The optical input signal has a wavelength of 1550 nanometers and an intensity of 0 dBm. The optical input signal is divided by the input side coupler21into two parts. The optical switch has second and third optical transmission lines120and121which are connected to the input side coupler21. The two divided optical signals are then transmitted through the second and third optical transmission lines120and121for output thereof. The second optical transmission line120is connected to a first output side coupler22. The third optical transmission line121is connected to a second output side optical coupler23. A first erbium doped fiber EDF11is provided on the second optical transmission line120between the input side coupler21and the fist output side coupler22. A second erbium doped fiber EDF12is provided on the third optical transmission line120between the input side coupler21and the second output side coupler23. The first and second erbium doped fibers EDF11and EDF12have a length of 50 meters. The first and second erbium doped fibers EDF11and EDF12may be replaced by rare earth doped fibers. The two divided optical signals are transmitted through the first and second erbium doped fibers EDF11and EDF12respectively. The optical switch further has an excitation light switch41which is connected through a first excitation light transmission line111to the first output side coupler22as well as which is connected through a second excitation light transmission line112to the second output side coupler23. The optical switch further has an excitation light source31which is connected to the excitation light switch41. The excitation light source31emits an excitation light with a wavelength of 1480 nanometers. The excitation light switch41is operated to switch the excitation light to any one of the first and second excitation light transmission lines111and112to supply any one of the first and second erbium doped fibers EDF11and EDF12.

Further, in this second embodiment, a first optical filter51is provided on the second optical transmission line120and positioned closer to the output side than the first output side optical coupler22. If the first erbium doped fiber EDF11is excited, then this first erbium doped fiber EDF11also serves as an optical power amplifier which, however, amplifies not only the divided optical signal from the first optical transmission line110but also noises induced in the optical signals, for which reason it is preferable to remove or eliminate the noises from the optical output signal by the first optical filter51in order to avoid deterioration in signal-to-noise ratio due to provision of the excitation light switch41. Similarly, a second optical filter52is provided on the third optical transmission line121and positioned closer to the output side than the second output side optical coupler23. If the second erbium doped fiber EDF12is excited, then this second erbium doped fiber EDF12also serves as an optical power amplifier which, however, amplifies not only the divided optical signal from the first optical transmission line110but also noises included in the optical signals, for which reason it is preferable to remove or eliminate the noises from the optical output signal by the second optical filter52in order to avoid deterioration in signal-to-noise ratio due to provision of the excitation light switch41.

If the excitation light switch41is operated to switch to supply the excitation light to the first erbium doped fiber EDF11, then the first erbium doped fiber EDF11is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF11without any optical absorption. The optical output signal is then fed to the first optical filter51to remove or eliminate the noises from the optical output signal by the first optical filter51in order to avoid deterioration in signal-to-noise ratio due to provision of the excitation light switch41. Therefore, the optical signal filtered in wavelength and having an intensity of 0 dBm is outputted from the second optical transmission line120. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the excitation light switch41to be fed through the first excitation light transmission line111and the first output side optical coupler22to the first erbium doped fiber EDF11. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the excitation light switch41whereby a leaked excitation light is then fed through the second output side optical coupler23to the second erbium doped fiber EDF12. However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF12, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF12. As a result, an optical output signal from the third optical transmission line121is free of any substantive noise and has an intensity of −60 dBm or less. The excitation light switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

If the excitation light switch41is operated to switch to supply the excitation light to the second erbium doped fiber EDF12, then the second erbium doped fiber EDF12is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF12without any optical absorption. The optical output signal is then fed to the second optical filter52to remove or eliminate the noises from the optical output signal by the second optical filter52in order to avoid deterioration in signal-to-noise ratio due to provision of the excitation light switch41. Therefore, the optical signal filtered in wavelength and having an intensity of 0 dBm is outputted from the second optical transmission line120. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the excitation light switch41to be fed through the second excitation light transmission line112and the second output side optical coupler23to the second erbium doped fiber EDF12. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the excitation light switch41whereby a leaked excitation light is then fed through the first output side optical coupler22to the first erbium doped fiber EDF11. However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF11, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF11. As a result, an optical output signal from the third optical transmission line121is free of any substantive noise and has an intensity of −60 dBm or less. The excitation light switch41causes an insertion loss of 2 dBm and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

As a modification to the above second embodiment, the above excitation light switch41may be replaced by a polymer optical switch similarly to the first embodiment.

If the polymer optical switch41is operated to switch to supply the excitation light to the first erbium doped fiber EDF11, then the first erbium doped fiber EDF11is excited whereby the divided optical signal with the wavelength of 1550 nanoseconds is transmitted through the first erbium doped fiber EDF11without any optical absorption. The optical output signal is then fed to the first optical filter51to remove or eliminate the noises from the optical output signal by the first optical filter51in order to avoid deterioration in signal-to-noise ratio due to provision of the polymer optical switch41. Therefore, the optical signal filtered in wavelength and having an intensity of 0 dBm is outputted from the second optical transmission line120. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the polymer optical switch41to be fed through the first excitation light transmission line111and the first output side optical coupler22to the first erbium doped EDF11. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the polymer optical switch41whereby a leaked excitation light is then fed through the second output side optical coupler23to the second erbium doped fiber EDF12. However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF12, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF12. As a result, an optical output signal from the third optical transmission line121is free of any substantive noise and has an intensity of −60 dBm or less. The polymer optical switch41causes an insertion loss of 20 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

If the polymer optical switch41is operated to switch to supply the excitation light to the second erbium doped fiber EDF12, then the second erbium doped fiber EDF12is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF12without any optical absorption. The optical output signal is then fed to the second optical filter52to remove or eliminate the noises from the optical output signal by the second optical filter52in order to avoid deterioration in signal-to-noise ratio due to provision of the polymer optical switch41. Therefore, the optical signal filtered in wavelength and having an intensity of 0 dBm is outputted from the second optical transmission line120. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the polymer optical switch41to be fed through the second excitation light transmission line112and the second output side coupler23to the second erbium doped fiber EDF12. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the polymer optical switch41whereby a leaked excitation light is then fed through the first output side optical coupler22to the first erbium doped fiber EDF11. However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF11, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF11. As a result, an optical output signal from the third optical transmission line121is free of any substantive noise and has no intensity of −60 dBm or less. The polymer optical switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

As a further modification to the above second embodiment, the excitation light has a wavelength of 980 nanometers in order to shorten the wavelength for a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

In the above embodiment, the number of the wavelength multiplexing on each optical transmission line is one. Notwithstanding, 8, 16, 32, 64-wavelength multiplexing are available, wherein the batch-switching operation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal at not only 1550 nanometers but also other wavelengths, for example, 1330 nanometers.

It is also possible to set the wavelength of the excitation light at not only 1480 nanometers or 980 nanometers but also other wavelengths provided that such wavelength is capable of exciting the impurity doped fiber. It is preferable to set the wavelength of the excitation light in consideration of both the wavelength of the optical input signal and the kind of the impurity doped fiber.

The above excitation light switch may also be replaced by an acousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical output signal by controlling an optical power of the excitation light to be fed to the impurity doped fiber. It is possible to control the optical power of the excitation light to be fed to the impurity doped fiber by controlling an injection current to the excitation light source or by use of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rear earth doped fiber such as tellurium doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rare earth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing to different excitation lights emitted separated from plural different excitation light sources in order to input the polarization-multiplexed excitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical devices at the optical coupler in accordance with the various design choices.

It is still further possible to freely set the transmission-band width in accordance with the number of the optical signals to be transmitted through the optical switch.

It is yet further possible to provide optical filters and optical isolators since the excitation light and returned light provide no influence to input and output sides of the optical switch.

The provisions of the smaller number of the excitation light source and the single excitation light switch permit ON-OFF switching operations of the plural gate switches by a simple structure. The above switch exhibits such a gain property as a sharp rising, for which reason there is substantially no influence due to a leaked light from the excitation light switch. This makes the switch available to switches having relatively large crosstalk levels such as a polymer type switch or LiNbO.sub.3 switch, thereby realizing a low crosstalk and low insertion loss optical switch. In addition, the use of the impurity doped fiber serving as an optical power amplifier can obtain a gain as the optical switch.

Third Embodiment

A third embodiment according to the present invention will be described in detail with reference toFIG. 3which is a diagram illustrative of a third novel optical switch having a single input and two outputs. A structural difference of the third novel optical switch from the first novel optical switch is in further providing first and second optical isolators as well as first and second optical mirrors in order to increase an efficiency of excitation of the erbium doped fiber with allowance of a sufficient optical absorption.

The optical switch has an input side coupler21which is connected to a first optical transmission line110on which an optical switch signal is transmitted and then inputted into the optical switch. The optical input signal has a wavelength of 1550 nanometers and an intensity of 0 dBm. The optical input signal is divided by the input side coupler21into two parts. The optical switch has second and third optical transmission lines120and121which are connected to the input side coupler21. The two divided optical signals are then transmitted through the second and third optical transmission lines120and121for output thereof. The second optical transmission line120is connected to a first output side coupler22. The third optical transmission line121is connected to a second output side optical coupler23. A first erbium doped fiber EDF11is provided on the second optical transmission line120between the input side coupler21and the first output side coupler22. A second erbium doped fiber EDF12is provided on the third optical transmission line120between the input side coupler21and the second output side coupler23. The first and second erbium doped fibers EDF11and EDF12have a length of 50 meters. The first and second erbium doped fibers EDF11and EDF12may be replaced by rare earth doped fibers. The two divided optical signals are transmitted through the first and second erbium doped fibers EDF11and EDF12respectively. The optical switch further has an excitation light switch41which is connected through a first excitation light transmission line111to the first output side coupler22as well as which is connected through a second excitation light transmission line112to the second output side coupler23. The optical switch further has an excitation light source31which is connected to the excitation light switch41. The excitation light source31emits an excitation light with a wavelength of 1480 nanometers. The excitation light switch41is operated to switch the excitation light to any one of the first and second excitation light transmission line111and112to supply any one of the first and second erbium doped fibers EDF11and EDF12.

In addition, a first optical isolator61is provided on the second optical transmission line120and positioned between the input side optical coupler21and the first erbium doped fiber EDF11. The first optical isolator61permits only a unidirectional transmission of the optical signal from the input side optical coupler21to the first erbium doped fiber EDF11, however, preventing an opposite direction transmission of the optical signal from the first erbium doped fiber EDF11to the input side optical coupler21. A second optical isolator62is provided on the third optical transmission line121and positioned between the input side optical coupler21and the second erbium doped fiber EDF12. The second optical isolator62permits only a unidirectional transmission of the optical signal from the input side optical coupler21to the second erbium doped fiber EDF12, however, preventing an opposite direction transmission of the optical signal from the second erbium doped fiber EDF12to the input side optical coupler21. Moreover, a first optical reflective mirror71is provided on a first terminal of the second optical transmission line120so that the divided optical signal having passed through the first erbium doped fiber EDF11is reflected by the first optical reflective mirror71toward the first erbium doped fiber EDF11, whereby the divided optical signal passes through the first erbium doped fiber EDF11two times. If the first erbium doped fiber EDF11is excited, then this first erbium doped fiber EDF11serves as an amplifier. This two times transmissions of the divided optical signal by the first optical reflective mirror71increases the efficiency of the excitation of the first erbium doped fiber EDF11even if the power of the excitation light emitted from the excitation light source31is not so high. The reflected optical signal is thus transmitted through the first erbium doped fiber EDF11and divided into two parts, wherein one of the further divided parts of the reflected optical signal is outputted from an output terminal of a fourth optical transmission line122whilst transmission of the remaining one of the further divided parts of the reflected optical signal is discontinued by the first optical isolator61so that no light is transmitted back to the first optical transmission line110. Furthermore, a second optical reflective mirror72is provided on a second terminal of the third optical transmission line121so that the divided optical signal having passed through the second erbium doped fiber EDF12is reflected by the second optical reflective mirror72toward the second erbium doped fiber EDF12, whereby the divided optical signal passes through the second erbium doped fiber EDF12two times. If the second erbium doped fiber EDF12is excited, then this second erbium doped fiber EDF12serves as an amplifier. This two times transmissions of the divided optical signal by the second optical reflective mirror72increases the efficiency of the excitation of the second erbium doped fiber EDF12even if the power of the excitation light emitted from the excitation light source31is not so high. The reflected optical signal is thus transmitted through the second erbium doped fiber EDF12and divided into two parts, wherein one of the further divided parts of the reflected optical signal is outputted from an output terminal of a fifth optical transmission line123whilst transmission of the remaining one of the further divided parts of the reflected optical signal is discontinued by the second optical isolator62so that no light is transmitted back to the first optical transmission line110.

If the excitation light switch41is operated to switch to supply the excitation light to the first erbium doped fiber EDF11, then the first erbium doped fiber EDF11is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF11without any optical absorption and then the optical signal is reflected by the first optical reflective mirror71for subsequent returning to the first erbium doped fiber EDF11. This two times transmissions of the divided optical signal by the first optical reflective mirror71increases the efficiency of the excitation of the first erbium doped fiber EDF11, even if the power of the excitation light emitted from the excitation light source31is not so high. The reflected optical signal is thus transmitted through the first erbium doped fiber EDF11and divided by an optical coupler into two parts, wherein one of the further divided parts of the reflected optical signal is outputted from an output terminal of a fourth optical transmission line122whilst transmission of the remaining one of the further divided parts of the reflected optical signal is discontinued by the first optical isolator61so that no light is transmitted back to the first optical transmission line110. The optical signal with an intensity of 0 dBm is outputted from the output terminal of the fourth optical transmission line122. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the excitation light switch41to be fed through the first excitation light transmission line111and the first output side optical coupler22to the first erbium doped fiber EDF11. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the excitation light switch41whereby a leaked excitation light is then fed through the second output side optical coupler23to the second erbium doped fiber EDF12. However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF12, for which reason the divided optical signal with the wavelength of 1550 nanometers in absorbed into the second erbium doped fiber EDF12. A leaked divided optical signal is also reflected by the second optical reflective mirror72and the reflected leaked optical signal is again transmitted through the second erbium doped fiber EDF12. As a result, an optical output signal from the fifth optical transmission line123has an intensity of −80 dBm or less. The excitation light switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

If the excitation light switch41is operated to switch to supply the excitation light to the second erbium doped fiber EDF12, then the second erbium doped fiber EDF12is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF12without any optical absorption and then the optical signal is reflected by the second optical reflective mirror72for subsequent returning to the second erbium doped fiber EDF12. This two times transmissions of the divided optical signal by the second optical reflective mirror72increases the efficiency of the excitation of the second erbium doped fiber EDF12even if the power of the excitation light emitted from the excitation light source31is not so high. The reflected optical signal is thus transmitted through the second erbium doped fiber EDF12and divided by an optical coupler into two parts, wherein one of the further divided parts of the reflected optical signal is outputted from an output terminal of a fifth optical transmission line123whilst transmission of the remaining one of the further divided parts of the reflected optical signal is discontinued by the second optical isolator62so that no light is transmitted back to the first optical transmission line110. The optical signal with an intensity of 0 dBm is outputted from the output terminal of the fourth optical transmission line122. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the excitation light switch41to be fed through the second excitation light transmission line112and the second output side optical coupler23to the second erbium doped fiber EDF12. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the excitation light switch41whereby a leaked excitation light is then fed through the first output side optical coupler22to the first erbium doped fiber EDF11. However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF11, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF11. A leaked divided optical signal is also reflected by the first optical reflective mirror71and the reflected leaked optical signal is again transmitted through the first erbium doped fiber EDF11. As a result, an optical output signal from the fourth optical transmission line122has an intensity of −80 dBm or less. The excitation light switch41causes an insertion loss of 2 dBm and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

As a modification to the above third embodiment, the above excitation light switch41may be replaced by a polymer optical switch.

If the polymer optical switch41is operated to switch to supply the excitation light to the first erbium doped fiber EDF11, then the first erbium doped fiber EDF11is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF11without any optical absorption and then the optical signal is reflected by the first optical reflective mirror71for subsequent returning to the first erbium doped fiber EDF11. This two times transmissions of the divided optical signal by the first optical reflective mirror71increases the efficiency of the excitation of the first erbium doped fiber EDF11even if the power of the excitation light emitted from the excitation light source31is not so high. The reflected optical signal is thus transmitted through the first erbium doped fiber EDF11and divided by an optical coupler into two parts, wherein one of the further divided parts of the reflected optical signal is outputted from an output terminal of a fourth optical transmission line122whilst transmission of the remaining one of the further divided parts of the reflected optical signal is discontinued by the first optical isolator61so that no light is transmitted back to the first optical transmission line110. The optical signal with an intensity of 0 dBm is outputted from the output terminal of the fourth optical transmission line122. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the polymer optical switch41to be fed through the first excitation light transmission line111and the first output side optical coupler22to the first erbium doped fiber EDF11. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the polymer optical switch41whereby a leaked excitation light is then fed through the second output side optical coupler23to the second erbium doped fiber EDF12. However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF12, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF12. A leaked divided optical signal is also reflected by the second optical reflective mirror72and the reflected leaked optical signal is again transmitted through the second erbium doped fiber EDF12. As a result, an optical output signal from the fifth optical transmission line123has an intensity of −80 dBm or less. The polymer optical switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

If the polymer optical switch41is operated to switch to supply the excitation light to the second erbium doped fiber EDF12, then the second erbium doped fiber EDF12is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF12without any optical absorption and then the optical signal is reflected by the second optical reflective mirror72for subsequent returning to the second erbium doped fiber EDF12. This two times transmissions of the divided optical signal by the second optical reflective mirror72increases the efficiency of the excitation of the second erbium doped fiber EDF12even if the power of the excitation light emitted from the excitation light source31is not so high. The reflected optical signal is thus transmitted through the second erbium doped fiber EDF12and divided by an optical coupler into two parts, wherein one of the further divided parts of the reflected optical signal is outputted from an output terminal of a fifth optical transmission line123whilst transmission of the remaining one of the further divided parts of the reflected optical signal is discontinued by the second optical isolators62so that no light is transmitted back to the first optical transmission line110. The optical signal with an intensity of 0 dBm is outputted from the output terminal of the fourth optical transmission line122. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the polymer optical switch41to be fed through the second excitation light transmission line112and the second output side optical coupler23to the second erbium doped fiber EDF12. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the polymer optical switch41whereby a leaked excitation light is then fed through the first output side optical coupler22to the first erbium doped fiber EDF11. However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF11, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF11. A leaked divided optical signal is also reflected by the first optical reflective mirror71and the reflected leaked optical signal is again transmitted through the first erbium doped fiber EDF11. As a result, an optical signal from the fourth optical transmission line122has an intensity of −80 dBm or less. The polymer optical switch41causes an insertion loss of 2 dBm and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

As a further modification to the above third embodiment, the excitation light has a wavelength of 980 nanometers in order to shorten the wavelength for a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

In the above embodiment, the number of the wavelength multiplexing on each optical transmission line is one. Notwithstanding, 8, 16, 32, 64-wavelength multiplexing are available, wherein the batch-switching operation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal at not only 1550 nanometers but also other wavelengths, for example, 1330 nanometers.

It is also possible to set the wavelength of the excitation light at not only 1480 nanometers or 980 nanometers but also other wavelengths provided that such wavelength is capable of exciting the impurity doped fiber. It is preferable to set the wavelength of the excitation light in consideration of both the wavelength of the optical input signal and the kind of the impurity doped fiber.

The above excitation light switch may also be replaced by an acousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical output signal by controlling an optical power of the excitation light to be fed to the impurity doped fiber. It is possible to control the optical power of the excitation light to be fed to the impurity doped fiber by controlling an injection current to the excitation light source or by use of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rare earth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing to different excitation lights emitted separately from plural different excitation light sources in order to input the polarization-multiplexed excitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at the optical coupler in accordance with the various design choices.

It is still further possible that the optical input and output transmission lines are used commonly or separately according to the required optical system.

It is yet further possible to replace the input side coupler21and the first and second optical isolators61and62by a circulator.

It is additionally possible to provide optical reflective mirrors having fixed or variable reflectivity as the first and second optical reflective mirrors71and72. If the variable reflectivity type optical reflective mirrors are provided, it is possible to control the optical powers of the output signals.

The provisions of the smaller number of the excitation light source and the single excitation light switch permit ON-OFF switching operations of the plural gate switches by a simple stricture. The above switch exhibits such a gain property as a sharp rising, for which reason there is substantially no influence due to a leaked light from the excitation light switch. This makes the switch available to switches having relatively large crosstalk levels such as a polymer type switch or LiNbO.sub.3 switch, thereby realizing a low crosstalk and low insertion loss optical switch. In addition, the use of the impurity doped fiber serving as an optical power amplifier can obtain a gain as the optical switch.

Fourth Embodiment

A fourth embodiment according to the present invention will be described in detail with reference toFIG. 4which is a diagram illustrative of a fourth novel optical switch having a single input and two outputs. A structural difference of the fourth novel optical switch from the first novel optical switch is in positions of first and second erbium doped fibers so that first and second erbium doped fibers receive an excitation light in the same direction as receipt of the optical signals, whilst in the first embodiment the first and second erbium doped fibers receive the excitation light in the opposite direction to the receipt of the optical signals.

The optical switch has an input side coupler21which is connected to a first optical transmission line110on which an optical input signal is transmitted and then inputted into the optical switch. The optical input signal has a wavelength of 1550 nanometers and an intensity of 0 dBm. The optical light signal is divided by the input side coupler21into two parts. The optical switch has second and third optical transmission lines120and121which are connected to the input side coupler21. The two divided optical signals are then transmitted through the second and third optical transmission lines120and121for output thereof. The second optical transmission line120is connected to a first output side coupler22. The third optical transmission line121is connected to a second output side optical coupler23. A first erbium doped fiber EDF11is provided on the second optical transmission line120and positioned between the first output side coupler22and the output terminal of the second optical transmission line120. A second erbium doped fiber EDF12is provided on the third optical transmission line120and positioned between the second output side coupler23and the output terminal of the third optical transmission line121. The first and second erbium doped fibers EDF11and EDF12have a length of 50 meters. The first and second erbium doped fibers EDF11and EDF12may be replaced by rare earth doped fibers. The two divided optical signals are transmitted through the first and second erbium doped fibers EDF11and EDF12respectively. The optical switch further has an excitation light switch41which is connected through a first excitation light transmission line111to the first output side coupler22as well as which is connected through a second excitation light transmission line112to the second output side coupler23. The optical switch further has an excitation light source31which is connected to the excitation light switch41. The excitation light source31emits an excitation light with a wavelength of 1480 nanometers. The excitation light switch41is operated to switch the excitation light to any one of the first and second excitation light transmission lines111and112to supply any one of the first and second erbium doped fibers EDF11and EDF12, so that the selected one of the first and second erbium doped fibers EDF11and EDF12receives the excitation light in the same direction as receipt of the optical signal.

If the excitation light switch41is operated to switch to supply the excitation light to the first erbium doped fiber EDF11so that the first and second erbium doped fiber EDF11receives the excitation light in the same direction as receipt of the optical signal, then the first erbium doped fiber EDF11is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF11without any optical absorption and then the optical signal with an intensity of 0 dBm is outputted from the second optical transmission line120. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the excitation light switch41to be fed through the first excitation light transmission line111and the first output side optical coupler22to the first erbium doped fiber EDF11. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the excitation light switch41whereby a leaked excitation light is then fed through the second output side optical coupler23to the second erbium doped fiber EDF12. However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF12, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF12. As a result, an optical output signal from the third optical transmission line121has an intensity of −60 dBm or less. The excitation light switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

If the excitation light switch41is operated to switch to supply the excitation light to the second erbium doped fiber EDF12so that the second erbium doped fiber EDF12receives the excitation light in the same direction as receipt of the optical signal, then the second erbium doped fiber EDF12is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF12without any optical absorption and then the optical signal with an intensity of 0 dBm is outputted from the third optical transmission line121. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the excitation light switch41to be fed through the second excitation light transmission line112and the second output side optical coupler23to the second erbium doped fiber EDF12. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the excitation light switch41whereby a leaked excitation light is then fed through the first output side optical coupler22to the first erbium doped fiber EDF11. However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF11, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF11. As a result, an optical output signal from the third optical transmission line121has an intensity of −60 dBm or less. The excitation light switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

As a modification to the above first embodiment, the above excitation light switch41may be replaced by a polymer optical switch.

If the polymer optical switch41is operated to switch to supply the excitation light to the first erbium doped fiber EDF11so that the first and second erbium doped fiber EDF11receives the excitation light in the same direction as receipt of the optical signal, then the first erbium doped fiber EDF11is excited whereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the first erbium doped fiber EDF11without any optical absorption and then the optical signal with an intensity of 0 dBm is outputted from the second optical transmission line120. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the polymer optical switch41to be fed through the first excitation light transmission line111and the first output side optical coupler22to the first erbium doped fiber EDF11. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the polymer optical switch41whereby a leaked excitation light is then fed through the second output side optical coupler23to the second erbium doped fiber EDF12. However, the leaked excitation light is incapable of exciting the second erbium doped fiber EDF12, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the second erbium doped fiber EDF12. As a result, an optical output signal from the third optical transmission line121has an intensity of −60 dBm or less. The polymer optical switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

If the polymer optical switch41is operated to switch to supply the excitation light to the second erbium doped fiber EDF12so that the second erbium doped fiber EDF12receives the excitation light in the same direction as receipt of the optical signal, then the second erbium doped fiber EDF12is excited hereby the divided optical signal with the wavelength of 1550 nanometers is transmitted through the second erbium doped fiber EDF12without any optical absorption and then the optical signal with an intensity of 0 dBm is outputted from the third optical transmission line121. Accurately, the majority part of the excitation light emitted from the excitation light source31is switched by the polymer optical switch41to be fed through the second excitation light transmission line112and the second output side optical coupler23to the second erbium doped fiber EDF12. On the other hand, the minority part of the excitation light emitted from the excitation light source31might be leaked through the polymer optical switch41whereby a leaked excitation light is then fed through the first output side optical coupler22to the first erbium doped fiber EDF11. However, the leaked excitation light is incapable of exciting the first erbium doped fiber EDF11, for which reason the divided optical signal with the wavelength of 1550 nanometers is absorbed into the first erbium doped fiber EDF11. As a result, an optical output signal from the third optical transmission line121has an intensity of −60 dBm or less. The polymer optical switch41causes an insertion loss of 2 dB and a crosstalk of 20 dB which allow the optical switch to be free from any substantive insertion loss and a low or reduced crosstalk.

As a further modification to the above first embodiment, the excitation light has a wavelength of 980 nanometers in order to shorten the wavelength for a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

In the above embodiment, the number of the wavelength multiplexing on each optical transmission line is one. Notwithstanding, 8, 16, 32, 64-wavelength multiplexing are available, wherein the batch-switching operation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal at not only 1550 nanometers but also other wavelengths, for example, 1330 nanometers.

It is also possible to set the wavelength of the excitation light at not only 1480 nanometers or 980 nanometers but also other wavelengths provided that such a wavelength is capable of exciting the impurity doped fiber. It is preferable to set the wavelength of the excitation light in consideration of both the wavelength of the optical input signal and the kind of the impurity doped fiber.

The above excitation light switch may also be replaced by an acousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical output signal by controlling an optical power of the excitation light to be fed to the impurity doped fiber. It is possible to control the optical power of the excitation light to be fed to the impurity doped fiber by controlling an injection current to the excitation light source or by use of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rare earth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing to different excitation lights emitted separately from plural different excitation light sources in order to input the polarization-multiplexed excitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at the optical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source and the single excitation light switch permit ON-OFF switching operations of the plural gate switches by a simple structure. The above switch exhibits such a gain property as a sharp rising, for which reason there is substantially no influence due to a leaked light from the excitation light switch. This makes the switch available to switches having relatively large crosstalk levels such as a polymer type switch or LiNbO.sub.3 switch, thereby realizing a low crosstalk and low insertion loss optical switch. In addition, the use of the impurity doped fiber serving as an optical power amplifier can obtain a gain as the optical switch.

Fifth Embodiment

A fifth embodiment according to the present invention will be described in detail with reference toFIG. 5which is a diagram illustrative of a fifth novel optical switch having two inputs and two outputs. The fifth novel optical switch comprises a pair of the above first novel optical switches described in the first embodiment. The two first novel optical switches are connected to each other through a first optical transmission line110as a common line. If the left side one of the paired first novel optical switches is in input side and the right side one of the paired first novel optical switches is in output side, then the switching operation of the left side one of the paired first novel optical switches is carried out to select or switch any one of the two inputs of the fifth novel optical switch having the two inputs and the two outputs, whilst the switching operation of the right side one of the paired first novel optical switches is carried out to select or switch any one of the two outputs of the fifth novel optical switch having the two inputs and the two outputs, whereby the switching operations of the paired first novel optical switches realize the fifth novel optical switch having the two inputs and the two outputs.

Each of the paired first novel optical switches is exactly the same as described in the first embodiment, for which reason duplicate descriptions to the first novel optical switches will be omitted.

As a modification to the above fifth novel optical switch, it is also possible that the fifth novel optical switch comprises a pair of the above fourth novel optical switches described in the fourth embodiment. The two fourth novel optical switches are connected to each other through a first optical transmission line110as a common line. If the left side one of the paired fourth novel optical switches is in input side and the right side one of the paired fourth novel optical switches is in output side, then the switching operation of the left side one of the paired fourth novel optical switches is carried out to select or switch any one of the two inputs of the fifth novel optical switch having the two inputs and the two outputs, whilst the switching operation of the right side one of the paired fourth novel optical switches is carried out to select or switch any one of the two outputs of the fifth novel optical switch having the two inputs and the two outputs, whereby the switching operations of the paired fourth novel optical switches realize the fifth novel optical switch having the two inputs and the two outputs.

Each of the paired fourth novel optical switches is exactly the same as described in the fourth embodiment, for which reason duplicate descriptions to the fourth novel optical switches will be omitted.

In the above embodiment, the number of the wavelength multiplexing on each optical transmission line is one. Notwithstanding, 8, 16, 32, 64-wavelength multiplexing are available, wherein the batch-switching operation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal at not only 1550 nanometers but also other wavelengths, for example, 1330 nanometers.

It is also possible to set the wavelength of the excitation light at not only 1480 nanometers or 980 nanometers but also the wavelengths provided that such wavelength is capable of exciting the impurity doped fiber. It is preferable to set the wavelength of the excitation light in consideration of both the wavelength of the optical input signal and the kind of the impurity doped fiber.

The above excitation light switch may also be replaced by an acousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical output signal by controlling an optical power of the excitation light to be fed to the impurity doped fiber. It is possible to control the optical power of the excitation light to be fed to the impurity doped fiber by controlling an injection current to the excitation light source or by use of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rare earth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing to different excitation lights emitted separately from plural different excitation light sources in order to input the polarization-multiplexed excitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at the optical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source and the single excitation light switch permit ON-OFF switching operations of the plural gate switches by a simple structure. The above switch exhibits such a gain property as a sharp rising, for which reason there is substantially no influence due to a leaked light from the excitation light switch. This makes the switch available to switches having relatively large crosstalk levels such as a polymer type switch or LiNbO.sub.3 switch, thereby realizing a low crosstalk and low insertion loss optical switch. In addition, the use of the impurity doped fiber serving as an optical power amplifier can obtain a gain as the optical switch.

Sixth Embodiment

A sixth embodiment according to the present invention will be described in detail with reference toFIG. 6which is a diagram illustrative of a sixth novel optical switch having two inputs and two outputs. A structural difference of the sixth novel optical switch from the fifth novel optical switch is in providing a single or common excitation light source to a pair of modified first novel optical switches excluding individual excitation light sources described in the first embodiment.

The sixth novel optical switch comprises a pair of the first novel optical switches described in the first embodiment. The two first novel optical switches are connected to each other through a first optical transmission line110as a common line. The two first novel optical switches are connected are also connected to the single and common excitation light source31to reduce the number of the required excitation light source. If the left side one of the paired first novel optical switches is in input side and the right side one of the paired first novel optical switches is in output side, then the switching operation of the left side one of the paired first novel optical switches is carried out to select or switch any one of the two inputs of the sixth novel optical switch having the two inputs and the two outputs, whilst the switching operation of the right side one of the paired first novel optical switches is carried out to select or switch any one of the two outputs of the sixth novel optical switch having the two inputs and the two outputs, whereby the switching operations of the paired first novel optical switches realize the sixth novel optical switch having the two inputs and the two outputs.

Each of the paired first novel optical switches is the same as described in the first embodiment except for excluding the individual excitation light sources, for which reason duplicate descriptions to the first novel optical switches will be omitted.

As a modification to the above sixth novel optical switch, it is also possible that the sixth novel optical switch comprises a pair of the above fourth novel optical switches described in the fourth embodiment. The two fourth novel optical switches are connected to each other through a first optical transmission line110as a common line. The two first novel optical switches are connected are also connected to the single and common excitation light source31to reduce the number of the required excitation light source. If the left side one of the paired fourth novel optical switches is in input side and the right side one of the paired fourth novel optical switches is in output side, then the switching operation of the left side one of the paired fourth novel optical switches is carried out to select or switch any one of the two inputs of the sixth novel optical switch having the two inputs and the two outputs, whilst the switching operation of the right side one of the paired fourth novel optical switches is carried out to select or switch any one of the two outputs of the sixth novel optical switch having the two inputs and the two outputs, whereby the switching operations of the paired fourth novel optical switches realize the sixth novel optical switch having the two inputs and the two outputs.

Each of the paired fourth novel optical switches is exactly the same as described in the fourth embodiment, for which reason duplicate descriptions to the fourth novel optical switches will be omitted.

In the above embodiment, the number of the wavelength multiplexing on each optical transmission line is one. Notwithstanding, 8, 16, 32, 64-wavelength multiplexing are available, wherein the batch-switching operation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal at not only 1550 nanometers but also other wavelengths, for example, 1330 nanometers.

It is also possible to set the wavelength of the excitation light at not only 1480 nanometers or 980 nanometers but also other wavelengths provided that such wavelength is capable of exciting the impurity doped fiber. It is preferable to set the wavelength of the excitation light in consideration of both the wavelength of the optical input signal and the kind of the impurity doped fiber.

The above excitation light switch may also be replaced by an acousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical output signal by controlling an optical power of the excitation light to be fed to the impurity doped fiber. It is possible to control the optical power of the excitation light to be fed to the impurity doped fiber by controlling an injection current to the excitation light source or by use of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rare earth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing to different excitation lights emitted separately from plural different excitation light sources in order to input the polarization-multiplexed excitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at the optical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source and the single excitation light switch permit ON-OFF switching operations of the plural gate switches by a simple structure. The above switch exhibits such a gain property as a sharp rising, for which reason there is substantially no influence due to a leaked light from the excitation light switch. This makes the switch available to switches having relatively large crosstalk levels such as a polymer type switch or LiNbO3 switch, thereby realizing a low crosstalk and low insertion loss optical switch. In addition, the use of the impurity doped fiber serving as an optical power amplifier can obtain a gain as the optical switch.

Seventh Embodiment

A seventh embodiment according to the present invention will be described in detail with reference toFIG. 7which is a diagram illustrative of a seventh novel optical switch having four separate optical transmission lines for separately switching optical signal transmission on the four separate optical transmission lines.

The seventh novel optical switch has first, second, third and fourth optical transmission lines1,2,3, and4on which separate optical signals are transmitted. The seventh novel optical switch also has an excitation light source31for emitting an excitation light. The seventh novel optical switch also has an excitation light switch41having a single input connected to the excitation light source31and four outputs. The excitation light switch41is capable of separately ON-OFF switching operations to transmissions of the excitation lights from the four outputs.

The first optical transmission line1comprises a first input side optical transmission line211and a first output side optical transmission line221, wherein the first input side optical transmission line211is connected through a first erbium doped fiber11EDF to the first input side optical transmission line211. A first optical coupler21is provided on the first input side optical transmission line211. The first optical coupler21is connected to a first output of the excitation light switch41. A first optical signal is transmitted on the first input side optical transmission line211through the first erbium doped fiber11EDF to the first input side optical transmission line211. If the excitation light switch41is operated to switch ON to allow transmission of the excitation light emitted from the excitation light source31through the first optical coupler21to the first erbium doped fiber11EDF, then the first erbium doped fiber11EDF is excited to allow that the transmission of the first optical signal having been transmitted on the first input side optical transmission line211is transmitted through the first erbium doped fiber11EDF to the first output side optical transmission line221.

The second optical transmission line1comprises a second input side optical transmission line212and a second output side optical transmission line222, wherein the second input side optical transmission line212is connected through a second erbium doped fiber12EDF to the second input side optical transmission line212. A second optical coupler22is provided on the second input side optical transmission line212. The second optical coupler22is connected to a second output of the excitation light switch41. A second optical signal is transmitted on the second input side optical transmission line212through the second erbium doped fiber12EDF to the second input side optical transmission line212. If the excitation light switch41is operated to switch ON to allow transmission of the excitation light emitted from the excitation light source31through the second optical coupler22to the second erbium doped fiber12EDF, then the second erbium doped fiber12EDF is excited to allow that the transmission of the second optical signal having been transmitted on the second input side optical transmission line212is transmitted through the second erbium doped fiber12EDF to the second output side optical transmission line222.

The third optical transmission line1comprises a third input side optical transmission line213and a third output side optical transmission line223, wherein the third input side optical transmission line213is connected through a third erbium doped fiber13EDF to the third input side optical transmission line213. A third optical coupler23is provided on the third input side optical transmission line213. The third optical coupler23is connected to a third output of the excitation light switch41. A third optical signal is transmitted on the third input side optical transmission line213through the third erbium doped fiber13EDF to the third input side optical transmission line213. If the excitation light switch41is operated to switch ON to allow transmission of the excitation light emitted from the excitation light source31through the third optical coupler23to the third erbium doped fiber13EDF, then the third erbium doped fiber13EDF is excited to allow that the transmission of the third optical signal having been transmitted on the third input side optical transmission line213is transmitted through the third erbium doped fiber13EDF to the third output side optical transmission line223.

The fourth optical transmission line1comprises a fourth input side optical transmission line214and a fourth output side optical transmission line224, wherein the fourth input side optical transmission line214is connected through a fourth erbium doped fiber14EDF to the fourth input side optical transmission line214. A fourth optical coupler24is provided on the fourth input side optical transmission line214. The fourth optical coupler24is connected to a fourth output of the excitation light switch41. A fourth optical signal is transmitted on the fourth input side optical transmission line214through the fourth erbium doped fiber14EDF to the fourth input side optical transmission line214. If the excitation light switch41is operated to switch ON to allow transmission of the excitation light emitted from the excitation light source31through the fourth optical coupler24to the fourth erbium doped fiber14EDF, then the fourth erbium doped fiber14EDF is excited to allow that the transmission of the fourth optical signal having been transmitted on the fourth input side optical transmission line214is transmitted through the fourth erbium doped fiber14EDF to the fourth output side optical transmission line224.

The excitation light switch41is capable of separate ON-OFF switching operations to the four outputs from which the excitation lights are outputted. If the excitation light switch41is operated to switch ON to the four outputs, then the excitation lights are red through the first, second, third and fourth couplers21,22,23and24to the first, second, third and fourth erbium doped fibers11EDF,12EDF,13EDF and14EDF, whereby the first, second, third and fourth optical signals are transmitted through the first, second, third and fourth erbium doped fibers11EDF,12EDF,13EDF and14EDF to the first, second, third and fourth output side optical transmission lines221,222,223and224. If the excitation light switch41is operated to switch ON to the first, second and third outputs, then the excitation lights are fed through the first, second and third couplers21,22and23to the first, second and third erbium doped fibers11EDF,12EDF and13EDF, whereby the first, second and third optical signals are transmitted through the first, second and third erbium doped fibers11EDF,12EDF and13EDF to the first, second and third output side optical transmission lines221,222and223, whilst the fourth optical signal is absorbed by the fourth erbium doped fiber14EDF. If the excitation light switch41is operated to switch ON to the first and second outputs, then the excitation lights are fed through the first and second couplers21and22to the first and second erbium doped fibers11EDF and12EDF, whereby the first and second optical signals are transmitted through the first and second erbium doped fibers11EDF and12EDF to the first and second output side optical transmission lines221and222, whilst the third and fourth optical signals are absorbed by the third and fourth erbium doped fibers13EDF and14EDF. If the excitation light switch41is operated to switch ON to the first output, then the excitation lights are fed through the first coupler21to the first erbium doped fiber11EDF, whereby the first optical signal is transmitted through the first erbium doped fiber11EDF to the first output side optical transmission line221, whilst the second, third and fourth optical signals are absorbed by the second, third and fourth erbium doped fibers12EDF,13EDF and14EDF.

In the above embodiment, the number of the wavelength multiplexing on each optical transmission line is one. Notwithstanding, 8, 16, 32, 64-wavelength multiplexing are available, wherein the batch-switching operation to the plural number wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal at not only 1550 nanometers but also other wavelengths, for example, 1330 nanometers.

It is also possible to set the wavelength of the excitation light at not only 1480 nanometers or 980 nanometers but also other wavelengths provided that such wavelengths is capable of exciting the impurity doped fiber. It is preferable to set the wavelength of the excitation light in consideration of both the wavelength of the optical input signal and the kind of the impurity doped fiber.

The above excitation light switch may also be replaced by an acousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical output signal by controlling an optical power of the excitation light to be fed to the impurity doped fiber. It is possible to control the optical power of the excitation light to be fed to the impurity doped fiber by controlling an injection current to the excitation light source or by use of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rare earth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing to different excitation lights emitted separately from plural different excitation light sources in order to input the polarization-multiplexed excitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at the optical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source and the single excitation light switch permit ON-OFF switching operations of the plural gate switches by a simple structure. The above switch exhibits such a gain property as a sharp rising, for which reason there is substantially no influence due to a leaked light from the excitation light switch. This makes the switch available to switches having relatively large crosstalk levels such as a polymer type switch or LiNbO3 switch, thereby realizing a low crosstalk and low insertion loss optical switch. In addition, the use of the impurity doped fiber serving as an optical power amplifier can obtain a gain as the optical switch.

Eighth Embodiment

An eighth embodiment according to the present invention will be described in detail with reference toFIG. 8which is a diagram illustrative of an eighth novel optical switch having four separate optical transmission lines for separately switching optical signal transmissions on the four separate optical transmission lines. A structural difference of the eighth novel optical switch from the seventh novel optical switch is in providing double excitation light sources and an optical cross connector serving as a switch.

The eighth novel optical switch has first, second, third and fourth optical transmission lines1,2,3, and4on which separate optical signals are transmitted. The seventh novel optical switch also has first and second excitation light sources32and33for emitting excitation lights. The eighth novel optical switch also has an optical cross connector81serving as a switch having two inputs connected to the first and second excitation light sources32and33and four outputs. The optical cross connector81is capable of separate switching operations of the four outputs for each of the excitation lights emitted from the first and second excitation light sources32and33. The dual excitation lights sources32and33increases the excitation power to be fed to the individual erbium doped fibers on the first to fourth optical transmission lines1,2,3, and4.

The first optical transmission line1comprises a first input side optical transmission line211and a first output side optical transmission line221, wherein the first input side optical transmission line211is connected through a first erbium doped fiber11EDF to the first input side optical transmission line211. A first optical coupler21is provided on the first input side optical transmission line211. The first optical coupler21is connected to a first output of the optical cross connector81. A first optical signal is transmitted on the first input side optical transmission line211through the first erbium doped fiber11EDF to the first input side optical transmission line211. If the optical cross connector81is operated to switch ON to allow transmission of the excitation light emitted from the excitation light source31through the first optical coupler21to the first erbium doped fiber11EDF, then the first erbium doped fiber11EDF is excited to allow that the transmission of the first optical signal having been transmitted on the first input side optical transmission line211is transmitted through the first erbium doped fiber11EDF to the first output side optical transmission line221.

The second optical transmission line1comprises a second input side optical transmission line212and a second output side optical transmission line222, wherein the second input side optical transmission line212is connected through a second erbium doped fiber12EDF to the second input side optical transmission line212. A second optical coupler22is provided on the second input side optical transmission line212. The second optical coupler22is connected to a second output of the optical cross connector81. A second optical signal is transmitted on the second input side optical transmission line212through the second erbium doped fiber12EDF to the second input side optical transmission line212. If the optical cross connector81is operated to switch ON to allow transmission of the excitation light emitted from the excitation light source31through the second optical coupler22to the second erbium doped fiber12EDF, then the second erbium doped fiber12EDF is excited to allow that the transmission of the second optical signal having been transmitted on the second input side optical transmission line212is transmitted through the second erbium doped fiber12EDF to the second output side optical transmission line222.

The third optical transmission line1comprises a third input side optical transmission line213and a third output optical transmission line223, wherein the third input side optical transmission line213is connected through a third erbium doped fiber13EDF to the third input side optical transmission line213. A third optical coupler23is provided on the third input side optical transmission line213. The third optical coupler23is connected to a third output of the optical cross connector81. A third optical signal is transmitted on the third input side optical transmission line213through the third erbium doped fiber13EDF to the third input side optical transmission line213. If the optical cross connector81is operated to switch ON to allow transmission of the excitation light emitted from the excitation light source31through the third optical coupler23to the third erbium doped fiber13EDF, then the third erbium doped fiber,13EDF is excited to allow that the transmission of the third optical signal having been transmitted on the third input side optical transmission line213is transmitted through the third erbium doped fiber13EDF to the third output side optical transmission line223.

The fourth optical transmission line1comprises a fourth input side optical transmission line214and a fourth output side optical transmission line224, wherein the fourth input side optical transmission line214is connected through a fourth erbium doped fiber14EDF to the fourth input side optical transmission line214. A fourth optical coupler24is provided on the fourth input side optical transmission line214. The fourth optical coupler24is connected to a fourth output of the optical cross connector81. A fourth optical signal is transmitted on the fourth input side optical transmission line214through the fourth erbium doped fiber14EDF to the fourth input side optical transmission line214. If the optical cross connector81is operated to switch ON to allow transmission of the excitation light emitted from the excitation light source31through the fourth optical coupler24to the fourth erbium doped fiber14EDF, then the fourth erbium doped fiber14EDF is excited to allow that the transmission of the fourth optical signal having been transmitted on the fourth input side optical transmission line214is transmitted through the fourth erbium doped fiber14EDF to the fourth output side optical transmission line224.

The optical cross connector81is capable of separate ON-OFF switching operations to the four outputs from which the excitation lights are outputted. If the optical cross connector81is operated to switch ON to the four outputs, then the excitation lights are fed through the first, second, third and fourth couplers21,22,23and24to the first, second, third and fourth erbium doped fibers11EDF,12EDF,13EDF and14EDF, whereby the first, second, third and fourth optical signals are transmitted through the first, second, third and fourth erbium doped fibers11EDF,12EDF,13EDF and14EDF to the first, second, third and fourth output side optical transmission lines221,222,223and224. If the optical cross connector81is operated to switch ON to the first, second and third outputs, then the excitation lights are fed through the first, second and third couplers21,22and23to the first, second and third erbium doped fibers11EDF,12EDF and13EDF, whereby the first, second and third optical signals are transmitted through the first, second and third erbium doped fibers11EDF,12EDF and13EDF to the first, second and third output side optical transmission lines221,222and223, whilst the fourth optical signal is absorbed by the fourth erbium doped fiber14EDF. If the optical cross connector81is operated to switch ON to the first and second outputs, then the excitation lights are fed through the first and second couplers21and22to the first and second erbium doped fibers11EDF and12EDF, whereby the first and second optical signals are transmitted through the first and second erbium doped fibers11EDF and12EDF to the first and second output side optical transmission lines221and222, whilst the third and fourth optical signals are absorbed by the third and fourth erbium doped fibers13EDF and14EDF. If the optical cross connector81is operated to switch ON to the first output, then the excitation lights are fed through the first coupler21to the first erbium doped fiber11EDF, whereby the first optical signal is transmitted through the first erbium doped fiber11EDF to the first output side optical transmission line221, whilst the second, third and fourth optical signals are absorbed by the second, third and fourth erbium doped fibers12EDF,13EDF and14EDF.

The duel excitation light sources32and33increases the excitation power to be fed to the individual erbium doped fibers on the first to fourth optical transmission lines1,2,3and4. This means it possible to further increase the number of the separate optical transmission lines to increase the size of the optical switch.

In the above embodiment, the number of the wavelength multiplexing on each optical transmission line is one. Notwithstanding, 8, 16, 32, 64-wavelength multiplexing are available, wherein the batch-switching operation to the plural member wavelength multiplexing is carried out.

It is also possible to set the wavelength of the optical input signal at not only 1550 nanometers but also other wavelengths, for example, 1330 nanometers.

It is also possible to set the wavelength of the excitation light at not only 1480 nanometers or 980 nanometers but also other wavelengths provided that such wavelengths is capable of exciting the impurity doped fiber. It is preferable to set the wavelength of the excitation light in consideration of both the wavelength of the optical input signal and the kind of the impurity doped fiber.

The above excitation light switch may also be replaced by an acousto-optical switch, or a quartz-based switch.

It is further possible to control an intensity of the optical output signal by controlling an optical power of the excitation light to be fed to the impurity doped fiber. It is possible to control the optical power of the excitation light to be fed to the impurity doped fiber by controlling an injection current to the excitation light source or by use of variable or fixed attenuator.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

It is moreover possible to input the excitation light into the rare earth doped fiber in either directions or in both directions.

It is still more possible to conduct a polarization-multiplexing to different excitation lights emitted separately from plural different excitation light sources in order to input the polarization-multiplexed excitation light into the rare earth doped fiber to obtain a high gain.

It is yet more possible to set freely a ratio of optical division at the optical coupler in accordance with the various design choices.

The provisions of the smaller number of the excitation light source and the single excitation light switch permit ON-OFF switching operations of the plural gate switches by a simple structure. The above switch exhibits such a gain property as a sharp rising, for which reason there is substantially no influence due to a leaked light from the excitation light switch. This makes the switch available to switches having relatively large crosstalk levels such as polymer type switch or LiNbO3 switch, thereby realizing a low crosstalk and low insertion loss optical switch. In addition, the use of the impurity doped fiber serving as an optical power amplifier can obtain a gain as the optical switch.

Ninth Embodiment

A ninth embodiment according to the present invention will be described in detail with reference toFIG. 9which is a diagram illustrative of a ninth novel optical switch provided in a novel first optical add-drop multiplexer performing optical addition, drop and transmission of said optical signals.

The ninth novel optical switch comprises a first optical transmission line110for transmitting an optical signal having a wavelength of 1550 nanometers, an optical reflectivity variable mirror50connected to the first optical transmission line110for reflecting the optical signal at a controlled reflectivity, a second optical transmission line120connected to the optical reflectivity variable mirror50, and an optical transmitter81connected through the second optical transmission line120to the optical reflectivity variable mirror50for transmitting an optical signal having a wavelength of 1550 nanometers.

In order to form the novel first optical add-drop multiplexer, a first optical coupler11is provided on the first optical transmission line110. Further, a third optical transmission line121is connected to the first optical coupler11. Furthermore, an optical receiver71is also connected with the third optical transmission line121so that the optical receiver71is also connected through the third optical transmission line121to the first optical coupler11. The optical input signal is divided by the first optical coupler11so that one of the divided optical input signals is transmitted through the third optical transmission line121to the optical receiver71, whilst the remaining one of the divided optical input signals is transmitted to the optical reflectivity variable mirror50whereby the remaining one of the divided optical input signals is reflected by the optical reflectivity variable mirror50at a controlled reflectivity. The optical reflectivity variable mirror50is capable of varying a reflectivity in the range of from 0% to 100%. If the reflectivity of the optical reflectivity variable mirror50is set 0%, then the optical reflectivity variable mirror50is a transmission state which allows an optical signal transmission. In this case, the optical signal transmitted from the optical transmitter81is transmitted through the optical reflectivity variable mirror50to the first optical transmission line.

A signal transmission operation of the novel first optical add-drop multiplexer will subsequently be described. An optical input signal having a wavelength of 1550 nanometers is transmitted on the first optical transmission line110and then reflected by the optical reflectivity variable mirror50before the reflected optical signal is then transmitted on the first optical transmission line110.

A signal drop operation of the novel first optical add-drop multiplexer will subsequently be described. An optical input signal having a wavelength of 1550 nanometers is transmitted on the first optical transmission line110and then divided into two parts by the optical coupler11. One of the divided optical input signals is then transmitted through the third optical transmission line121to the optical receiver71. It is possible to set a low ratio of a first optical division for the optical receiver71to a second optical division for the optical reflectivity variable mirror50, in order to suppress an optical loss by the optical division by the optical coupler11.

A signal add operation of the novel first optical add-drop multiplexer will subsequently be described. The optical reflectivity variable mirror50is capable of varying a reflectivity in the range of from 0% to 100%. If the reflectivity of the optical reflectivity variable mirror50is set 0%, then the optical reflectivity variable mirror50is in a transmission state which allows an optical signal transmission. In this case, the optical signal transmitted from the optical transmitter81is transmitted through the optical reflectivity variable mirror50to the first optical transmission line.

The above novel first optical add-drop multiplexer does require no optical coupler for signal adding, thereby realizing a low optical loss.

As a modification to this embodiment, it is possible to provide any one of the above first to fourth optical switches ofFIGS. 1 to 4in the first to fourth embodiments, in place of the above optical coupler11and the optical reflectivity variable mirror50.

It is possible that the input and output ports are commonly used or that the input and output ports are separated from each other by use of an optical coupler and an optical isolator or by use of a circulator.

It is also possible that the optical reflectivity variable mirror50may be replaced by an optical switch for switching an transmission and a reflection, or by an optical reflectivity switching mirror for switching 0% reflectivity and 100% reflectivity, provided that if the reflectivity is 0%, then the switch is capable of transmission of the optical signal.

It is also possible to change the number of the gate arrays from eight.

The optical multiplexer, the optical demultiplexer or the optical multiplexer/demultiplexer may comprise an array waveguide grating, a wavelength router having substantially the same grating structure as the array waveguide grating, or a wavelength MUX coupler having substantially the same grating structure as the array waveguide grating.

Since insertion loss is different among the optical multiplexer, the optical demultiplexer and the optical multiplexer/demultiplexer, it is possible to use optical attenuators in individual waveguides for control of the optical power levels.

It is also possible to control a gain of the erbium doped fiber amplifier gate or control reflectivity of the reflective mirror for control of the optical power levels for every wavelengths separately.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber, or an aluminum doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

The excitation light may have a wavelength of 980 nanometers in order to shorten the wavelength for a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

Tenth Embodiment

A tenth embodiment according to the present invention will be described in detail with reference toFIG. 10which is a diagram illustrative of a tenth novel optical switch as an optical gate switch.

The tenth novel optical switch is an optical gate switch. The tenth novel optical switch comprises an optical input signal transmission line110for transmitting an optical input signal, an optical output signal transmission line120for transmitting an optical output signal, an optical transmission line130connected through an optical coupler11to said input and output signal transmission lines110and120, an optical isolator91provided on said optical input signal transmission line110for permitting a unidirectional transmission of said optical input signal toward said optical transmission line130, an erbium doped fiber41provided on said optical transmission line130, a wavelength band selective optical reflecting mirror25provided on said optical transmission line130, and an excitation light source31connected to said wavelength band selective optical reflecting mirror25. The optical input signal has a wavelength of 1550 nanometers. The excitation light source31is capable of emitting an excitation light having a wavelength of 1480 nanometers. The wavelength band selective optical reflecting mirror25is capable of selecting a reflecting wavelength band of an optical signal to be reflected by the wavelength band selective optical reflecting mirror25.

In this case, the wavelength band selective optical reflecting mirror25so sets the reflecting wavelength band that the optical input signal with the wavelength of 1550 nanometers is total-reflected by the wavelength band selective optical reflecting mirror25, whilst the excitation light emitted from the excitation light source31is transmitted through the wavelength band selective optical reflecting mirror25to the erbium doped fiber41, whereby the erbium doped fiber41is excited by the excitation light. The excited erbium doped fiber41is capable of amplifying the optical input signal. The amplified input signal is then total-reflected by the wavelength band selective optical reflecting mirror25. The reflected input signal is then transmitted again through the erbium doped fiber41, whereby the reflected signal is further amplified. The further amplified optical signal is divided by the optical coupler11into two parts, one of which is transmitted to the optical isolator91. However, the transmission of the divided optical signal is prevented by the optical isolator91. On the other hand, the other divided part of the optical signal is transmitted through the output signal transmission line120. In the above state, the above optical gate switch is in ON state.

If no excitation light is emitted from the excitation light source31, the erbium doped fiber41receives no excitation light and is unexcited, whereby the input optical signal is absorbed by the erbium doped fiber41. No optical signal is outputted from the output signal transmission line120. In the above state, the optical gate switch is in OFF state.

The above novel optical gate switch is capable of reducing an insertion loss and also reducing the number of the required optical couplers.

It is also possible to integrate the wavelength band selective optical reflecting mirror25and the excitation light source31.FIG. 11is a schematic view illustrative of an integration of the above novel optical gate switch. An erbium doped fiber amplifier gate array300has an integration of eight sets of an excitation light source31, a wavelength band selective optical reflecting mirror25, an erbium doped fiber41, and an optical transmission line100.

As a modification to this embodiment, it is possible to provide any one of the above first to fourth optical switches ofFIGS. 1 to 4in the first to fourth embodiments, in place of the above optical coupler11and the optical reflectivity variable mirror50.

It is possible that the input and output ports are commonly used or that the input and output ports are separated from each other by use of an optical coupler and an optical isolator or by use of a circulator.

It is possible to change the positions of the optical couplers provided that the functions of the optical add-drop multiplexer can be ensured.

The optical multiplexer, the optical demultiplexer or the optical multiplexer/demultiplexer may comprise an array waveguide grating, a wavelength router having substantially the same grating structure as the array waveguide grating, or a wavelength MUX coupler having substantially the same grating structure as the array waveguide grating.

Since insertion loss is different among the optical multiplexer, the optical demultiplexer and the optical multiplexer/demultiplexer, it is possible to use optical attenuators individual waveguides for control of the optical power levels.

It is also possible to control a gain of the erbium doped fiber amplifier gate or control reflectivity of the reflective mirror for control of the optical power levels for every wavelengths separately.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber, or an aluminum doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

The excitation light may have a wavelength of 980 nanometers in order to shorten the wavelength of a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

Eleventh Embodiment

An eleventh embodiment according to the present invention will be described in detail with reference toFIG. 12which is a diagram illustrative of a novel optical add-drop multiplexer using an optical gate switch ofFIG. 10for performing optical addition, drop and transmission of said optical signals.

The novel optical add-drop multiplexer using an optical gate switch comprises an optical input signal transmission line110for transmitting an optical input signal, an optical output signal transmission line120for transmitting an optical output signal, an optical transmission line130connected through an optical coupler11to said input and output signal transmission lines110and120, and optical isolator91provided on said optical input signal transmission line110for permitting a unidirectional transmission of said optical input signal toward said optical transmission line130, an erbium doped fiber41provided on said optical transmission line130, a wavelength band selective optical reflecting mirror25provided on said optical transmission line130, an excitation light source31connected to said wavelength band selective optical reflecting mirror25, an optical receiver71connected through a second optical coupler12to the optical transmission line130and positioned between the first optical coupler11and the erbium doped fiber41and an optical transmitter81connected through a third optical coupler13to the output signal optical transmission line130. The optical input signal has a wavelength of 1550 nanometers. The excitation light source31is capable of emitting an excitation light have a wavelength of 1480 nanometers. The wavelength band selective optical reflecting mirror25is capable of selecting a reflecting wavelength band of an optical signal to be reflected by the wavelength band selective optical reflecting mirror25.

In this case, the wavelength band selective optical reflecting mirror25so sets the reflecting wavelength band that the optical input signal with the wavelength of 1550 nanometers is total-reflected by the wavelength band selective optical reflecting mirror25, whilst the excitation light emitted from the excitation light source31is transmitted through the wavelength band selective optical reflecting mirror25to the erbium doped fiber41, whereby the erbium doped fiber41is excited by the excitation light. The excited erbium doped fiber41is capable of amplifying the optical input signal. The amplified input signal is then total-reflected by the wavelength band selective optical reflecting mirror25. The reflected input signal is then transmitted again through the erbium doped fiber41, whereby the reflected signal is further amplified. The further amplified optical signal is divided by the optical coupler11into two parts, one of which is transmitted to the optical isolator91. However, the transmission of the divided optical signal is prevented by the optical isolator91. On the other hand, the other divided part of the optical signal is transmitted through the output signal transmission line120. In the above state, the above optical gate switch is in ON state.

A signal transmission operation of the above novel optical add-drop multiplexer will subsequently be described. The excitation light is emitted from the excitation light source31and then supplied to the erbium doped fiber41, whereby the erbium doped fiber41is excited. The optical input signal is transmitted through the erbium doped fiber41and amplified by the excited erbium doped fiber41. The amplified optical input signal is total-reflected by the wavelength band selective optical reflecting mirror25. The reflected optical signal is then transmitted again through the erbium doped fiber41, whereby the reflected signal is further amplified. The further amplified optical signal is divided by the first optical coupler11into two parts, one of which is transmitted to the optical isolator91. However, the transmission of the divided optical signal is prevented by the optical isolator91. On the other hand, the other divided part of the optical signal is transmitted through the output signal transmission line120.

A signal drop operation of the above novel optical add-drop multiplexer will subsequently be described. The optical input signal is divided by the second optical coupler into two parts, one of which is transmitted to the optical receiver71.

It is possible to set the ratio of first optical division for the optical receiver71to second optical division for the erbium doped fiber41is small in order to reduce the optical loss due to the second optical coupler12.

A signal add operation of the above novel optical add-drop multiplexer will subsequently be described. In this case, no excitation light is emitted from the excitation light source31, for which reason the erbium doped fiber41receives110excitation light and is unexcited, whereby the input output signal is absorbed by the erbium doped fiber41. No optical signal is outputted from the output signal transmission line120. On the other hand, the optical transmitter81emits a second optical signal with a wavelength of 1550 nanometers which is then transmitted on the output signal optical transmission line120, whereby the second optical signal is outputted from the output signal optical transmission line120.

The above novel optical add-drop multiplexer is capable of reducing an insertion loss and also reducing the number of required optical couplers.

It is also possible to integrate the wavelength band selective optical reflecting mirror25and the excitation light source31.FIG. 13is a schematic view illustrative of an integration of the above novel optical add-drop multiplexer. An erbium doped fiber amplifier gate module202is provided formed on a package210. The erbium doped fiber amplifier gate module202has an erbium doped fiber amplifier gate array300. The erbium doped fiber amplifier gate array300has an integration of eight sets of an excitation light source31, a wavelength band selective optical reflecting mirror25, an erbium doped fiber41, and an optical transmission line100. The erbium doped fiber amplifier gate module202has an input port110and an output port120which are provided in the same side of the package201. This allows a high density integration of the package.

As a modification to this embodiment, it is possible to provide any one of the above first to fourth optical switches ofFIGS. 1 to 4in the first to fourth embodiments, in place of the above optical gate switch.

It is possible that the input and output ports are commonly used or that the input and output ports are separated from each other by use of an optical coupler and an optical isolator or by use of a circulator.

It is also possible that the erbium doped fibers are packaged in the same array, or that the reflective mirrors are incorporated into the erbium doped fiber amplifier gate module, or that all of the above elements are packaged onto a PLC board.

The optical multiplexer, the optical demultiplexer or the optical multiplexer/demultiplexer may comprise an array waveguide grating, a wavelength router having substantially the same grating structure as the array waveguide grating, or a wavelength MUX coupler having substantially the same grating structure as the array waveguide grating.

Since the insertion loss is different among the optical multiplexer, the optical demultiplexer and the optical multiplexer/demultiplexer, it is possible to use optical attenuators in individual waveguides for control of the optical power levels.

It is also possible to control a gain of the erbium doped fiber amplifier gate or control reflectivity of the reflective mirror for control of the optical power levels for every wavelengths separately.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber, or an aluminum doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

The excitation light may have a wavelength of 980 nanometers in order to shorten the wavelength for a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

Twelfth Embodiment

A twelfth embodiment according to the present invention will be described in detail with reference toFIG. 14which is a diagram illustrative of a novel wavelength-multiplexed optical add-drop multiplexer using four sets of the above novel optical add-drop multiplexer ofFIG. 9.

The novel wavelength-multiplexed optical add-drop multiplexer comprises a single optical circulator60connected with optical transmission lines110,120and121, an optical multiplexer/demultiplexer410connected through said optical transmission line120to said optical circulator60and first to fourth optical add-drop multiplexer1,2,3and4. The first optical add-drop multiplexer is operable for a signal having a wavelength of 1548 nanometers. The second optical add-drop multiplexer is operable for a signal having a wavelength of 1550 nanometers. The third optical add-drop multiplexer is operable for a signal having a wavelength of 1552 nanometers. The fourth optical add-drop multiplexer is operable for a signal having a wavelength of 1554 nanometers.

The optical input signal having four wavelength compositions of 1548 nanometers, 1550 nanometers, 1552 nanometers, and 1554 nanometers is transmitted from the optical transmission line110through the optical circulator60to the optical multiplexer/demultiplexer410, so that the optical input signal is wavelength-demultiplexer by the optical multiplexer/demultiplexer410whereby the optical input signal is divided into a first signal having a wavelength of 1548 nanometers, a second signal having a wavelength of 1550 nanometers, a third signal having a wavelength of 1552 nanometers, and a fourth signal having wavelength of 1554 nanometers. The first, second, third and fourth optical signals are inputted into the first, second, third and fourth optical add-drop multiplexers1,2,3and4respectively.

The first optical add-drop multiplexer1comprises a first main optical transmission line131for transmitting the first optical signal, a first optical reflectivity variable mirror51provided on the first main optical transmission line131for reflecting the first optical signal at a controlled reflectivity, and an optical transmitter81with an end of the first main optical transmission line131, a first subordinate optical transmission line135connected through a first optical coupler11to the first main optical transmission line, and a first optical receiver71connected with said first subordinate optical transmission line135. The first optical input signal is divided by the first optical coupler11so that one of the divider first optical input signal is transmitted the first subordinate optical transmission line135to the optical receiver71, whilst the remaining one of the divided first optical input signal is transmitted to the optical reflectivity variable mirror51whereby the remaining one of the divided first optical input signal is reflected by the first optical reflectivity variable mirror51at a controlled reflectivity. The first optical reflectivity variable mirror51is capable of varying a reflectivity in the range of from 0% to 100%. If the reflectivity of the first optical reflectivity variable mirror51is set 0%, then the first optical reflectivity variable mirror51is in a transmission state which allows an optical signal transmission. In this case, the first optical signal transmitted from the first optical transmitter81is transmitted through the first optical reflectivity variable mirror51to the first main optical transmission line131.

A signal transmission operation of the first optical add-drop multiplexer will subsequently be described. The first optical input signal is transmitted on the first main optical transmission line131and then reflected by the first optical reflectivity variable mirror51before the reflected optical signal is then transmitted on the first main optical transmission line131.

A signal drop operation of the first optical add-drop multiplexer will subsequently be described. The first optical input signal is transmitted on the first main optical transmission line131and then divided into two parts by the first optical coupler11. One of the divided first optical input signals is then transmitted through the first subordinate optical transmission line135to the optical receiver71.

A signal add operation of the first optical add-drop multiplexer will subsequently be described. If the reflectivity of the first optical reflectivity variable mirror51is set 0%, then the first optical reflectivity variable mirror51is in a transmission state which allows an optical signal transmission. In this case, a first substitute optical signal transmitted from the optical transmitter81is transmitted through the first optical reflectivity variable mirror51to the first main optical transmission line131.

The second optical add-drop multiplexer2comprises a second main optical transmission line132for transmitting the second optical signal, a second optical reflectivity variable mirror52provided on the second main optical transmission line132for reflecting the second optical signal at a controlled reflectivity, and an optical transmitter82with an end of the second main optical transmission line132, a second subordinate optical transmission line136connected through a second optical coupler12to the second main optical transmission line, and a second optical receiver72connected with said second subordinate optical transmission line136. The second optical input signal is divided by the second optical coupler12so that one of the divided second optical input signal is transmitted through the second subordinate optical transmission line136to the optical receiver72, whilst the remaining one of the divided second optical input signal is transmitted to the optical reflectivity variable mirror52whereby the remaining one of the divided second optical input signal is reflected by the second optical reflectivity variable mirror52at a controlled reflectivity. The second optical reflectivity variable mirror52is capable of varying a reflectivity in the range of from 0% to 100%. If the reflectivity of the second optical reflectivity variable mirror52is set 0%, then the second optical reflectivity variable mirror52is in a transmission state which allows an optical signal transmission. In this case, the second optical signal transmitted from the second optical transmitter82is transmitted through the second optical reflectivity variable mirror52to the second main optical transmission line132.

A signal transmission operation of the second optical add-drop multiplexer will subsequently be described. The second optical input signal is transmitted on the second main optical transmission line132and then reflected by the second optical reflectivity variable mirror52before the reflected optical signal is then transmitted on the second main optical transmission line132.

A signal drop operation of the second optical add-drop multiplexer will subsequently be described. The second optical input signal is transmitted on the second main optical transmission line132and then divided into two parts by the second optical coupler12. One of the divided second optical input signals is then transmitted through the second subordinate optical transmission line136to the optical receiver72.

A signal add operation of the second optical add-drop multiplexer will subsequently be described. If the reflectivity of the second optical reflectivity variable mirror52is set 0%, then the second optical reflectivity variable mirror52is in a transmission state which allows an optical signal transmission. In this case, a second substitute optical signal transmitted from the optical transmitter82is transmitted through the second optical reflectivity variable mirror52to the second main optical transmission line132.

The third optical add-drop multiplexer3comprises a third main optical transmission line133for transmitting the third optical signal, a third optical reflectivity variable mirror53provided on the third main optical transmission line133for reflecting the third optical signal at a controlled reflectivity, and an optical transmitter83with an end of the third main optical transmission line133, a third subordinate optical transmission line137connected through a third optical coupler13to the third main optical transmission line, and a third receiver73connected with said third subordinate optical transmission line137. The third optical input signal is divided by the third optical coupler13so that one of the divided third optical input signal is transmitted through the third subordinate optical transmission line137to the optical receiver73, whilst the remaining one of the divided third optical input signal is transmitted to the optical reflectivity variable mirror53whereby the remaining one of the divided third optical input signal is reflected by the third optical reflectivity variable mirror53at a controlled reflectivity. The third optical reflectivity variable mirror53is capable of varying a reflectivity in the range of from 0% to 100%. If the reflectivity of the third optical reflectivity variable mirror53is set 0%, then the third optical reflectivity variable mirror53is in a transmission state which allows an optical signal transmission. In this case, the third optical signal transmitted from the third optical transmitter83is transmitted through the third optical reflectivity variable mirror53to the third main optical transmission line133.

A signal transmission operation of the third optical add-drop multiplexer will subsequently be described. The third optical input signal is transmitted on the third main optical transmission line133and then reflected by the third optical reflectivity variable mirror53before the reflected optical signal is then transmitted on the third main optical transmission line133.

A signal drop operation of the third optical add-drop multiplexer will subsequently be described. The third optical input signal is transmitted on the third main optical transmission line133and then divided into two parts by the third optical coupler13. One of the divided third optical input signals is then transmitted through the third subordinate optical transmission line137to the optical receiver73.

A signal add operation of the third optical add-drop multiplexer will subsequently be described. If the reflectivity of the third optical reflectivity variable mirror53is set 0%, then the third optical reflectivity variable mirror53is in a transmission state which allows an optical signal transmission. In this case, a third substitute optical signal transmitted from the optical transmitter83is transmitted through the third optical reflectivity variable mirror53to the third main optical transmission line133.

The fourth optical add-drop multiplexer4comprises a fourth main optical transmission line134for transmitting the fourth optical signal, a fourth optical reflectivity variable mirror54provided on the fourth main optical transmission line134of reflecting the fourth optical signal at a controlled reflectivity, and an optical transmitter84with an end of the fourth main optical transmission line134, a fourth subordinate optical transmission line138connected through a fourth optical coupler14to the fourth main optical transmission line, and a fourth optical receiver74connected with said fourth subordinate optical transmission line138. The fourth optical input signal is divided by the fourth optical coupler14so that one of the divided fourth optical input signal is transmitted through the fourth subordinate optical transmission line138to the optical receiver74, whilst the remaining one of the divided fourth optical input signal is transmitted to the optical reflectivity variable mirror54whereby the remaining one of the divided fourth optical input signal is reflected by the fourth optical reflectivity variable mirror54at a controlled reflectivity. The fourth optical reflectivity variable mirror54is capable of varying a reflectivity in the range of from 0% to 100%. If the reflectivity of the fourth optical reflectivity variable mirror54is set 0%, then the fourth-optical reflectivity variable mirror54is in a transmission state which allows an optical signal transmission. In this case, the fourth optical signal transmitted from the fourth optical transmitter84is transmitted through the fourth optical reflectivity variable mirror54to the fourth main optical transmission line134.

A signal transmission operation of the fourth optical add-drop multiplexer will subsequently be described. The fourth optical input signal is transmitted on the fourth main optical transmission line134and then reflected by the fourth optical reflectivity variable mirror54before the reflected optical signal is then transmitted on the fourth main optical transmission line134.

A signal drop operation of the fourth optical add-drop multiplexer will subsequently be described. The fourth optical input signal is transmitted on the fourth main optical transmission line134and then divided into two parts by the fourth optical coupler14. One of the divided fourth optical input signals is then transmitted through the fourth subordinate optical transmission line138to the optical receiver74.

A signal add operation of the fourth optical add-drop multiplexer will subsequently be described. If the reflectivity of the fourth optical reflectivity variable mirror54is set 0%, then the fourth optical reflectivity variable mirror54is in a transmission state which allows an optical signal transmission. In this case, a fourth substitute optical signal transmitted from the optical transmitter84is transmitted through the fourth optical reflectivity variable mirror54to the fourth main optical transmission line134.

First, second, third and fourth output signals are multiplexed by the optical multiplexer/demultiplexer410to form a single output signal which is then transmitted through the circulator60to the optical transmission line121.

The above novel optical add-drop multiplexers do require no optical coupler for signal adding, thereby realizing a low optical loss.

As a modification to this embodiment, it is possible to provide any one of the above first to fourth optical switches ofFIGS. 1 to 4in the first to fourth embodiments, in place of the above optical couplers and the optical reflectivity variable mirrors.

It is possible that the input and output ports are commonly used or that the input and output ports are separated from each other by use of an optical coupler and an optical isolator or by use of a circulator.

It is also possible to change the number of the wavelength-multiplexing from four into, for example, eight, sixteen, thirty two or sixty four.

It is also possible to change the wavelength of the optical signals and also change a bit rate or a signal rate to 2.5 Gbps, 5 Gbps, 100 Gbps or set a bit-rate free.

The optical multiplexer, the optical demultiplexer or the optical multiplexer/demultiplexer may comprise an array waveguide grating, a wavelength router having substantially the same grating structure as the array waveguide grating, or a wavelength MUX coupler having substantially the same grating structure as the array waveguide grating.

Since insertion loss is different among the optical multiplexer, the optical demultiplexer and the optical multiplexer/demultiplexer, it is possible to use optical attenuators in individual waveguides for control of the optical power levels.

It is also possible to control a gain of the erbium doped fiber amplifier gate or control reflectivity of the reflective mirror for control of the optical power levels for every wavelengths separately.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber, or an aluminum doped fiber. The length of the rare earth doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

The excitation light may have a wavelength of 980 nanometers in order to shorten the wavelength for a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

Thirteenth Embodiment

A thirteenth embodiment according to the present invention will be described in detail with reference toFIG. 15which is a diagram illustrative of a novel wavelength-multiplexed optical add-drop multiplexer using four sets of the above novel optical add-drop multiplexer ofFIG. 12.

The novel wavelength-multiplexed optical add-drop multiplexer comprises a single optical circulator60connected with optical transmission lines110,120and121, an optical multiplexer/demultiplexer410connected through said optical transmission line120to said optical circulator60and first to fourth optical add-drop multiplexers1,2,3and4. The first optical add-drop multiplexer is operable for a signal having a wavelength of 1548 nanometers. The second optical add-drop multiplexer is operable for a signal having a wavelength of 1550 nanometers. The third optical add-drop multiplexer is operable for a signal having a wavelength of 1552 nanometers. The fourth optical add-drop multiplexer is operable for a signal having a wavelength of 1554 nanometers.

The optical input signal having four wavelength compositions of 1548 nanometers, 1550 nanometers, 1552 nanometers, and 1554 nanometers is transmitted from the optical transmission line110through the optical circulator60to the optical multiplexer/demultiplexer410, so that the optical input signal is wavelength-demultiplexer by the optical multiplexer/demultiplexer410whereby the optical input signal is divided into a first signal having a wavelength of 1548 nanometers, a second signal having a wavelength of 1550 nanometers, a third signal having a wavelength of 1552 nanometers, and a fourth signal having a wavelength of 1554 nanometers. The first, second, third and fourth optical signals are inputted into the first, second, third and fourth optical add-drop multiplexers1,2,3and4respectively.

The first optical add-drop multiplexer1comprises a first optical transmission line131for transmitting an optical input signal, a first erbium doped fiber41provided on said first optical transmission line131, a first wavelength band selective optical reflecting mirror25provided on said first optical transmission line131, a first excitation light source31connected to said first wavelength band selective optical reflecting mirror25, a first optical receiver71connected through a first receiver side optical coupler11to the first optical transmission line131and an optical transmitter81connected through a first transmitter side optical coupler15to the first signal optical transmission line131. The excitation light source31is capable of emitting an excitation light having a different wavelength from the first optical signal. The first wavelength band selective optical reflecting mirror25is capable of selecting a reflecting wavelength band of the first optical signal to be reflected by the first wavelength band selective optical reflecting mirror25. The first optical input signal is total-reflected by the first wavelength band selective optical reflecting mirror25, whilst the first excitation light emitted from the first excitation light source31is transmitted through the first wavelength band selective optical reflecting mirror25to the first erbium doped fiber41, whereby the first erbium doped fiber41is excited by the first excitation light. The first excited erbium doped fiber41is capable of amplifying the first optical input signal. The amplified input signal is then total-reflected by the first wavelength band selective optical reflecting mirror25. The reflected input signal is then transmitted again through the first erbium doped fiber41, whereby the reflected signal is further amplified. The further amplified optical signal is transmitted through the first output signal transmission line131.

A signal transmission operation of the above first optical add-drop multiplexer will subsequently be described. The first excitation light is emitted from the first excitation light source31and then supplied to the first erbium doped fiber41, whereby the first erbium doped fiber41is excited. The optical input signal is transmitted through the first erbium doped fiber41and amplified by the excited first erbium doped fiber41. The amplified optical input signal is total-reflected by the first wavelength band selective optical reflecting mirror25. The reflected optical signal is then transmitted again through the first erbium doped fiber41, whereby the reflected signal is further amplified. The further amplified optical signal is transmitted through the first signal transmission line131to the optical multiplexer/demultiplexer410.

A signal drop operation of the above first optical add-drop multiplexer will subsequently be described. The first optical input signal is divided by the first receiver side optical coupler11into two parts, one of which is transmitted to the first optical receiver71.

A signal add operation of the above first optical add-drop multiplexer will subsequently be described. In this case, no excitation light is emitted from the first excitation light source31, for which reason the first erbium doped fiber41receives no excitation light and is unexcited, whereby the first input optical signal is absorbed by the first erbium doped fiber41. No optical signal is outputted from the first signal transmission line131. On the other hand, the first optical transmitter81emits a first substitute optical signal which is then transmitted through the first transmitter side optical coupler15on the first optical transmission line131, whereby the first substitute optical signal is outputted from the first optical transmission line131.

The second optical add-drop multiplexer2comprises a second optical transmission line132for transmitting an optical input signal, a second erbium doped fiber42provided on said second optical transmission line132, a second wavelength band selective optical reflecting mirror26provided on said second optical transmission line132, a second excitation light source32connected to said second wavelength band selective optical reflecting mirror26, a second optical receiver72connected through a second receiver side optical coupler12to the second optical transmission line132and an optical transmitter82connected through a second transmitter side optical coupler16to the second signal optical transmission line132. The excitation light source32is capable of emitting an excitation light having a different wavelength from the second optical signal. The second wavelength band selective optical reflecting mirror26is capable of selecting a reflecting wavelength band of the second optical signal to be reflected by the second wavelength band selective optical reflecting mirror26. The second optical input signal is total-reflected by the second wavelength band selective optical reflecting mirror26, whilst the second excitation light emitted from the second excitation light source32is transmitted through the second wavelength band selective optical reflecting mirror26to the second erbium doped fiber42, whereby the second erbium doped fiber42is excited by the second excitation light. The second excited erbium doped fiber42is capable of amplifying the second optical input signal. The amplified input signal is then total-reflected by the second wavelength band selective optical reflecting mirror26. The reflected input signal is then transmitted again through the second erbium doped fiber42, whereby the reflected signal is further amplified. The further amplified optical signal is transmitted through the second output signal transmission line132.

A signal transmission operation of the above second optical add-drop multiplexer will subsequently be described. The second excitation light is emitted from the second excitation light source32and then supplied to the second erbium doped fiber42, whereby the second erbium doped fiber42is excited. The optical input signal is transmitted through the second erbium doped fiber42and amplified by the excited second erbium doped fiber42. The amplified optical input signal is total-reflected by the second wavelength band selective optical reflecting mirror26. The reflected optical signal is then transmitted again through the second erbium doped fiber42, whereby the reflected signal is further amplified. The further amplified optical signal is transmitted through the second signal transmission line132to the optical multiplexer/demultiplexer420.

A signal drop operation of the above second optical add-drop multiplexer will subsequently be described. The second optical input signal is divided by the second receiver side optical coupler12into two parts, one of which is transmitted to the second optical receiver72.

A signal add operation of the above second optical add-drop multiplexer will subsequently be described. In this case, no excitation light is emitted from the second excitation light source32, for which reason the second erbium doped fiber42receives no excitation light and is unexcited, whereby the second input optical signal is absorbed by the second erbium doped fiber42. No optical signal is outputted from the second signal transmission line132. On the other hand, the second optical transmitter82emits a second substitute optical signal which is then transmitted through the second transmitter side optical coupler16on the second optical transmission line132, whereby the second substitute optical signal is outputted from the second optical transmission line132.

The third optical add-drop multiplexer3comprises a third optical transmission line133for transmitting an optical input signal, a third erbium doped fiber43provided on said third optical transmission line133, a third wavelength band selective optical reflecting mirror27provided on said third optical transmission line133, a third excitation light source33connected to said third wavelength band selective optical reflecting mirror27, a third optical receiver73connected through a third receiver side optical coupler13to the third optical transmission line133and an optical transmitter83connected through a third transmitter side optical coupler17to the third signal optical transmission line133. The excitation light source33is capable of emitting an excitation light having a different wavelength from the third optical signal. The third wavelength band selective optical reflecting mirror27is capable of selecting a reflecting wavelength band of the third optical signal to be reflected by the third wavelength band selective optical reflecting mirror27. The third optical input signal is total-reflected by the third wavelength band selective optical reflecting mirror27, whilst the third excitation light emitted from the third excitation light source33is transmitted through the third wavelength band selective optical reflecting mirror27to the third erbium doped fiber43, whereby the third erbium doped fiber43is excited by the third excitation light. The third excited erbium doped fiber43is capable of amplifying the third optical input signal. The amplified input signal is then total-reflected by the third wavelength band selective optical reflecting mirror27. The reflected input signal is then transmitted again through the third erbium doped fiber43, whereby the reflected signal is further amplified. The further amplified optical signal is transmitted through the third output signal transmission line133.

A signal transmission operation of the above third optical add-drop multiplexer will subsequently be described. The third excitation light is emitted from the third excitation light source33and then supplied to the third erbium doped fiber43, whereby the third erbium doped fiber43is excited. The optical input signal is transmitted through the third erbium doped fiber43and amplified by the excited third erbium doped fiber43. The amplified optical input signal is total-reflected by the third wavelength band selective optical reflecting mirror27. The reflected optical signal is then transmitted again through the third erbium doped fiber43, whereby the reflected signal is further amplified. The further amplified optical signal is transmitted through the third signal transmission line133to the optical multiplexer/demultiplexer430.

A signal drop operation of the above third optical add-drop multiplexer will subsequently be described. The third optical input signal is divided by the third receiver side optical coupler13into two parts, one of which is transmitted to the third optical receiver73.

A signal add operation of the above third optical add-drop multiplexer will subsequently be described. In this case, no excitation light is emitted from the third excitation light source33, for which reason the third erbium doped fiber43receives no excitation light and is unexcited, whereby the third input optical signal is absorbed by the third erbium doped fiber43. No optical signal is outputted from the third signal transmission line133. On the other hand, the third optical transmitter83emits a third substitute optical signal which is then transmitted through the third transmitter side optical coupler17on the third optical transmission line133, whereby the third substitute optical signal is outputted from the third optical transmission line133.

The fourth optical add-drop multiplexer4comprises a fourth optical transmission line134for transmitting an optical input signal, a fourth erbium doped fiber44provided on said fourth optical transmission line134, a fourth wavelength band selective optical reflecting mirror28provided on said fourth optical transmission line134, a fourth excitation light source34connected to said fourth wavelength band selective optical reflecting mirror28, a fourth optical receiver74connected through a fourth receiver side optical coupler14to the fourth optical transmission line134and an optical transmitter84connected through a fourth transmitter side optical coupler18to the fourth signal optical transmission line134. The excitation light source34is capable of emitting an excitation light having a different wavelength from the fourth optical signal. The fourth wavelength band selective optical reflecting mirror28is capable of selecting a reflecting wavelength band of the fourth optical signal to be reflected by the fourth wavelength band selective optical reflecting mirror28. The fourth optical input signal is total-reflected by the fourth wavelength band selective optical reflecting mirror28, whilst the fourth excitation light emitted from the fourth excitation light source34is transmitted through the fourth wavelength band selective optical reflecting mirror28to the fourth erbium doped fiber44, whereby the fourth erbium doped fiber44is excited by the fourth excitation light. The fourth excited erbium doped fiber44is capable of amplifying the fourth optical input signal. The amplified input signal is then total-reflected by the fourth wavelength band selective optical reflecting mirror28. The reflected input signal is then transmitted again through the fourth erbium doped fiber44, whereby the reflected signal is further amplified. The further amplified optical signal is transmitted through the fourth output signal transmission line134.

A signal transmission operation of the above fourth optical add-drop multiplexer will subsequently be described. The fourth excitation light is emitted from the fourth excitation light source34and then supplied to the fourth erbium doped fiber44, whereby the fourth erbium doped fiber44is excited. The optical input signal is transmitted through the fourth erbium doped fiber44and amplified by the excited fourth erbium doped fiber44. The amplified optical input signal is total-reflected by the fourth wavelength band selective optical reflecting mirror28. The reflected optical signal is then transmitted again through the fourth erbium doped fiber44, whereby the reflected signal is further amplified. The further amplified optical signal is transmitted through the fourth signal transmission line134to the optical multiplexer/demultiplexer440.

A signal drop operation of the above fourth optical add-drop multiplexer will subsequently be described. The fourth optical input signal is divided by the fourth receiver side optical coupler14into two parts, one of which is transmitted to the fourth optical receiver74.

A signal add operation of the above fourth optical add-drop multiplexer will subsequently be described. In this case, no excitation light is emitted from the fourth excitation light source34, for which reason the fourth erbium doped fiber44receives no excitation light and is unexcited, whereby the fourth input optical signal is absorbed by the fourth erbium doped fiber44. No optical signal is outputted from the fourth signal transmission line134. On the other hand, the fourth optical transmitter84emits a fourth substitute optical signal which is then transmitted through the fourth transmitter side optical coupler18on the fourth optical transmission line134, whereby the fourth substitute optical signal is outputted from the fourth optical transmission line134.

First, second, third and fourth output signals are multiplexed by the optical multiplexer/demultiplexer410to form a single output signal which is then transmitted through the circulator60to the optical transmission line121.

The above novel optical add-drop multiplexer is capable of reducing an insertion loss and also reducing the number of the required optical couplers.

It is also possible to integrate the wavelength band selective optical reflecting mirrors and the excitation light sources.

It is possible that the input and output ports are commonly used or that the input and output ports are separated from each other by use of an optical coupler and an optical isolator or by use of a circulator.

It is also possible to change the number of the wavelength-multiplexing from four into, for example, eight, sixteen, thirty two or sixty four.

It is also possible to change the wavelength of the optical signals and also change a bit rate or a signal rate to 2.5 Gbps, 5 Gbps, 100 Gbps or set a bit-rate free.

The optical multiplexer, the optical demultiplexer or the optical multiplexer/demultiplexer may comprise an array waveguide grating, a wavelength router having substantially the same grating structure as the array waveguide grating, or a wavelength MUX coupler having substantially the same grating structure as the array waveguide grating.

Since insertion loss is different among the optical multiplexer, the optical demultiplexer and the optical multiplexer/demultiplexer, it is possible to use optical attenuators in individual waveguides for control of the optical power levels.

It is also possible to control a gain of the erbium doped fiber amplifier gate or control reflectivity of the reflective mirror for control of the optical power levels for every wavelengths separately.

It is furthermore possible to replace the erbium doped fiber by rare earth doped fiber such as tellurium doped fiber and a doping concentration thereof may be set in accordance with the required specifications of the optical switch.

The excitation light may have a wavelength of 980 nanometers in order to shorten the wavelength for a remarkable reduction in noise factor of the optical output signal. In this case, the optical switch is also free from any substantive insertion loss and a low or reduced crosstalk.

Fourteenth Embodiment

A fourteenth embodiment according to the present invention will be described in detail with reference toFIG. 16which is a diagram illustrative of a novel wavelength-multiplexed optical add-drop multiplexer having four looped optical transmission paths.

The novel wavelength-multiplexed optical add-drop multiplexer comprises an optical multiplexer/demultiplexer410having an input port110and an output port120, and first to fourth optical transmission lines131,132,133and134connected to the optical multiplexer/demultiplexer410. The first optical transmission line131is provided for transmitting a signal having a wavelength of 1530 nanometers. The second optical transmission line132is provided for transmitting a signal having a wavelength of 1540 nanometers. The third optical transmission line133is provided for transmitting a signal having a wavelength of 1550 nanometers. The fourth optical transmission line is provided for transmitting a signal having a wavelength of 1560 nanometers.

The optical input signal having four wavelength compositions of 1530 nanometers, 1540 nanometers, 1550 nanometers, and 1560 nanometers is transmitted from the optical transmission line110to the optical multiplexer/demultiplexer410, so that the optical input signal is wavelength-demultiplexed by the optical multiplexer/demultiplexer410whereby the optical input signal is divided into a first signal having a wavelength of 1530 nanometers, a second signal having a wavelength of 1540 nanometers, a third signal having a wavelength of 1550 nanometers, and a fourth signal having a wavelength of 1560 nanometers. The first, second, third and fourth optical signals are inputted into the first, second, third and fourth optical transmission lines131,132,133and134respectively.

The first optical transmission line131has a first receiver side optical coupler31which is connected to a first optical receiver71, and a first transmitter side optical coupler44which is connected to a first optical transmitter84. The first optical transmission line131also has a first optical multiplexer/demultiplexer151.

FIG. 17is a diagram illustrative of the first optical multiplexer/demultiplexer151used in the wavelength-multiplexed optical add-drop multiplexer ofFIG. 16. The first optical multiplexer/demultiplexer151performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals having 1.55 micrometers and 1.54 micrometers which are outputted from two output ports. The first optical multiplexer/demultiplexer151is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the first optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The second optical transmission line132has a second receiver side optical coupler32which is connected to a second optical receiver72, and a second transmitter side optical coupler41which is connected to a second optical transmitter81. The second optical transmission line132also has a second optical multiplexer/demultiplexer152. The first optical multiplexer/demultiplexer151is also connected through a series connection of a first optical amplifier51and a first isolator91to the second optical multiplexer/demultiplexer152. One of the wavelength-demultiplexed optical signals is transmitted from the first optical multiplexer/demultiplexer151through the first optical amplifier51and the first isolator91to the second optical multiplexer/demultiplexer152.

The second optical multiplexer/demultiplexer152performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals which are outputted from two output ports. The second optical multiplexer/demultiplexer152is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the second optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The third optical transmission line133has a third receiver side optical coupler33which is connected to a third optical receiver73, and a third transmitter side optical coupler42which is connected to a third optical transmitter82. The third optical transmission line133also has a third optical multiplexer/demultiplexer153. The second optical multiplexer/demultiplexer152is also connected through a series connection of a second optical amplifier52and a second isolator92to the third optical multiplexer/demultiplexer153. One of the wavelength-demultiplexed optical signals is transmitted from the second optical multiplexer/demultiplexer152through the second optical amplifier52and the second isolator92to the third optical multiplexer/demultiplexer153.

Third optical multiplexer/demultiplexer153performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals which are outputted from two output ports. The third optical multiplexer/demultiplexer153is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the third optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The fourth optical transmission line134has a fourth receiver side optical coupler34which is connected to a fourth optical receiver73, and a fourth transmitter side optical coupler43which is connected to a fourth optical transmitter82. The fourth optical transmission line134also has a fourth optical multiplexer/demultiplexer154. The third optical multiplexer/demultiplexer153is also connected through a series connection of a third optical amplifier52and a third isolator93to the fourth optical multiplexer/demultiplexer154. One of the wavelength-demultiplexed optical signals is transmitted from the third optical multiplexer/demultiplexer153through the third optical amplifier52and the third isolator93to the fourth optical multiplexer/demultiplexer154.

The fourth optical multiplexer/demultiplexer154performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals which are outputted from two output ports. The fourth optical multiplexer/demultiplexer154is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the fourth optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The fourth optical multiplexer/demultiplexer154is also connected through a series connection of a fifth optical amplifier54and a fifth optical attenuator94to the first optical multiplexer/demultiplexer151.

The above wavelength-multiplexed optical add-drop multiplexer performs signal transmission operation, signal drop operation and signal add operation.

The signal transmission operation of the wavelength-multiplexed optical add-drop multiplexer will be described. The first input signal is transmitted through the first optical multiplexer/demultiplexer151to the first optical amplifier51, whereby the signal is amplified by the first optical amplifier51. The amplified signal is then transmitted through the first optical isolator91to the second optical multiplexer/demultiplexer152. Since the second optical multiplexer/demultiplexer152has a multiplexing function, the amplified signal is transmitted through the second optical transmission line132to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The second input signal is transmitted through the second optical multiplexer/demultiplexer152to the second optical amplifier52, whereby the signal is amplified by the second optical amplifier52. The amplified signal is then transmitted through the second optical isolator92to the third optical multiplexer/demultiplexer153. Since the third optical multiplexer/demultiplexer153has a multiplexing function, the amplified signal is transmitted through the third optical transmission line133to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The third input signal is transmitted through the third optical multiplexer/demultiplexer153to the third optical amplifier53, whereby the signal is amplified by the third optical amplifier53. The amplified signal is then transmitted through the third optical isolator93to the fourth optical multiplexer/demultiplexer154. Since the fourth optical multiplexer/demultiplexer154has a multiplexing function, the amplified signal is transmitted through the fourth optical transmission line134to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The fourth input signal is transmitted through the fourth optical multiplexer/demultiplexer154to the fourth optical amplifier54, whereby the signal is amplified by the fourth optical amplifier54. The amplified signal is then transmitted through the fourth optical isolator94to the first optical multiplexer/demultiplexer151. Since the first optical multiplexer/demultiplexer151has a multiplexing function, the amplified signal is transmitted through the first optical transmission line131to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120.

The signal drop operation of the wavelength-multiplexed optical add-drop multiplexer will be described. The first input signal is transmitted from the first optical transmission line131through the first receiver side optical coupler31into the first optical receiver71. The second input signal is transmitted from the second optical transmission line132through the second receiver side optical coupler32into the second optical receiver72. The third input signal is transmitted from the third optical transmission line133through the third receiver side optical coupler33into the third optical receiver73. The fourth input signal is transmitted from the fourth optical transmission line134through the fourth receiver side optical coupler34into the fourth optical receiver74.

The signal add operation of the wavelength-multiplexed optical add-drop multiplexer will be described. The first optical amplifier51turns OFF, whereby the transmission of the first optical signal through the first optical transmission line131and the first optical multiplexer/demultiplexer151is discontinued by the first optical amplifier51, whereby no signal is transmitted through the second optical multiplexer/demultiplexer152to the second optical transmission line132. On the other hand, a first substitute signal is transmitted from the second optical transmitter81so that the first substitute signal is then transmitted through the second optical transmission line132to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The second optical amplifier52turns OFF, whereby the transmission of the second optical signal through the second optical transmission line132and the second optical multiplexer/demultiplexer152is discontinued by the second optical amplifier52, whereby no signal is transmitted through the third optical multiplexer/demultiplexer153to the third optical transmission line133. On the other hand, a second substitute signal is transmitted from the third optical transmitter82so that the second substitute signal is then transmitted through the third optical transmission line133to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The third optical amplifier53turns OFF, whereby the transmission of the third optical signal through the third optical transmission line133and the third optical multiplexer/demultiplexer153is discontinued by the third optical amplifier53, whereby no signal is transmitted through the fourth optical multiplexer/demultiplexer154to the fourth optical transmission line134. On the other hand, a third substitute signal is transmitted from the fourth optical transmitter83so that the third substitute signal is then transmitted through the fourth optical transmission line134to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The fourth optical amplifier54turns OFF, whereby the transmission of the fourth optical signal through the fourth optical transmission line134and the fourth optical multiplexer/demultiplexer154is discontinued by the fourth optical amplifier54, whereby no signal is transmitted through the first optical multiplexer/demultiplexer151to the first optical transmission line131. On the other hand, a fourth substitute signal is transmitted from the first optical transmitter84so that the fourth substitute signal is then transmitted through the first optical transmission line131to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120.

The use of the optical wavelength-multiplexer/demultiplexer to serve as the same function as the optical coupler reduces the optical power loss by not less than 5 dB as compared to the 1:1 optical coupler.

Fifteenth Embodiment

A fifteenth embodiment according to the present invention will be described in detail with reference toFIG. 18which is a diagram illustrative of a novel wavelength-multiplexed optical add-drop multiplexer having four looped optical transmission paths.

The novel wavelength-multiplexed optical add-drop multiplexer comprises an optical multiplexer/demultiplexer410having an input port110and an output port120, and first to fourth optical transmission lines131,132,133and134connected to the optical multiplexer/demultiplexer410. The first optical transmission line131is provided for transmitting a signal having a wavelength of 1530 nanometers. The second optical transmission line132is provided for transmitting a signal having a wavelength of 1540 nanometers. The third optical transmission line133is provided for transmitting a signal having a wavelength of 1550 nanometers. The fourth optical transmission line is provided for transmitting a signal having a wavelength of 1560 nanometers.

The optical input signal having four wavelength composition of 1530 nanometers, 1540 nanometers, 1550 nanometers, and 1560 nanometers is transmitted from the optical transmission line110to the optical multiplexer/demultiplexer410, so that the optical input signal is wavelength-demultiplexed by the optical multiplexer/demultiplexer410whereby the optical input signal is divided into a first signal having a wavelength of 1530 nanometers, a second signal having a wavelength of 1540 nanometers, a third signal having a wavelength of 1550 nanometers, and a fourth signal having a wavelength of 1560 nanometers. The first, second, third and fourth optical signals are inputted into the first, second, third and fourth optical transmission lines131,132,133and134respectively.

The first optical transmission line131has a first receiver side optical coupler31which is connected to a first optical receiver71, and a first transmitter side optical coupler44which is connected to a first optical transmitter84. The first optical transmission line131also has a first optical circulator61.

The second optical transmission line132has a second receiver side optical coupler32which is connected to a second optical receiver72, and a second transmitter side optical coupler41which is connected to a second optical transmitter81. The second optical transmission line132also has a second optical circulator62. The first optical circulator61is also connected through a first optical amplifier51to the second optical circulator62. One of the wavelength-demultiplexed optical signals is transmitted from the first optical circulator61through the first optical amplifier51to the second optical circulator62.

The third optical transmission line133has a third receiver side optical coupler33which is connected to a third optical receiver73, and a third transmitter side optical coupler42which is connected to a third optical transmitter82. The third optical transmission line133also has a third optical circulator63. The second optical circulator62is also connected through a second optical amplifier52to the third optical circulator63. One of the wavelength-demultiplexed optical signals is transmitted from the second optical circulator62through the second optical amplifier52to the third optical circulator63.

The fourth optical transmission line134has a fourth receiver side optical coupler34which is connected to a fourth optical receiver73, and a fourth transmitter side optical coupler43which is connected to a fourth optical transmitter82. The fourth optical transmission line134also has a fourth optical circulator64. The third optical circulator63is also connected through a third optical amplifier52to the fourth optical circulator64. One of the wavelength-demultiplexed optical signals is transmitted from the third optical circulator63through the third optical amplifier52to the fourth optical circulator64.

The fourth optical circulator64is also connected through a series connection of a fifth optical amplifier54and a fifth optical attenuator94to the first optical circulator61.

The above wavelength-multiplexed optical add-drop multiplexer performs signal transmission operation, signal drop operation and signal add operation.

The signal transmission operation of the wavelength-multiplexed optical add-drop multiplexer will be described. The first input signal is transmitted through the first optical circulator61to the first optical amplifier51, whereby the signal is amplified by the first optical amplifier51. The amplified signal is then transmitted to the second optical circulator62. The amplified signal is transmitted through the second optical transmission line132to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The second input signal is transmitted through the second optical circulator62to the second optical amplifier52, whereby the signal is amplified by the second optical amplifier52. The amplified signal is then transmitted to the third optical circulator63. The amplified signal is transmitted through the third optical transmission line133to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The third input signal is transmitted through the third optical circulator63to the third optical amplifier53, whereby the signal is amplified by the third optical amplifier53. The amplified signal is then transmitted to the fourth optical circulator64. The amplified signal is transmitted through the fourth optical transmission line134to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The fourth input signal is transmitted through the fourth optical circulator64to the fourth optical amplifier54, whereby the signal is amplified by the fourth optical amplifier54. The amplified signal is then transmitted to the first optical circulator61. The amplified signal is transmitted through the first optical transmission line131to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120.

The signal drop operation of the wavelength-multiplexed optical add-drop multiplexer will be described. The first input signal is transmitted from the first optical transmission line131through the first receiver side optical coupler31into the first optical receiver71. The second input signal is transmitted from the second optical transmission line132through the second receiver side optical coupler32into the second optical receiver72. The third input signal is transmitted from the third optical transmission line133through the third receiver side optical coupler33into the third optical receiver73. The fourth input signal is transmitted from the fourth optical transmission line134through the fourth receiver side optical coupler34into the fourth optical receiver74.

The signal add operation of the wavelength-multiplexed optical add-drop multiplexer will be described. The first optical amplifier51turns OFF, whereby the transmission of the first optical signal through the first optical transmission line131and the first optical circulator61is discontinued by the first optical amplifier51, whereby no signal is transmitted through the second optical circulator62to the second optical transmission line132. On the other hand, a first substitute signal is transmitted from the second optical transmitter81so that the first substitute signal is then transmitted through the second optical transmission line132to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The second optical amplifier52turns OFF, whereby the transmission of the second optical signal through the second optical transmission line132and the second optical circulator62is discontinued by the second optical amplifier52, whereby no signal is transmitted through the third optical circulator63to the third optical transmission line133. On the other hand, a second substitute signal is transmitted from the third optical transmitter82so that the second substitute signal is then transmitted through the third optical transmission line133to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The third optical amplifier53turns OFF, whereby the transmission of the third optical signal through the third optical transmission line133and the third optical circulator63is discontinued by the third optical amplifier53, whereby no signal is transmitted through the fourth optical circulator64to the fourth optical transmission line134. On the other hand, a third substitute signal is transmitted from the fourth optical transmitter83so that the third substitute signal is then transmitted through the fourth optical transmission line134to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The fourth optical amplifier54turns OFF, whereby the transmission of the fourth optical signal through the fourth optical transmission line134and the fourth optical circulator64is discontinued by the fourth optical amplifier54, whereby no signal is transmitted through the first optical circulator61to the first optical transmission line131. On the other hand, a fourth substitute signal is transmitted from the first optical transmitter84so that the fourth substitute signal is then transmitted through the first optical transmission line131to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120.

The use of the optical isolator to serve as the same function as the optical coupler reduces the optical power loss by not less than 5 dB as compared to the 1:1 optical coupler.

Sixteenth Embodiment

A sixteenth embodiment according to the present invention will be described in detail with reference toFIG. 19which is a diagram illustrative of a novel wavelength-multiplexed optical amplifier having four looped optical transmission paths.

The novel wavelength-multiplexed optical amplifier is structurally different from the above wavelength-multiplexed optical add-drop multiplexer ofFIG. 16in view of no provision of optical receivers and optical transmitters.

The novel wavelength-multiplexed optical amplifier comprises an optical multiplexer/demultiplexer410having an input port110and an output port120, and first to fourth optical transmission lines131,132,133and134connected to the optical multiplexer/demultiplexer410. The first optical transmission line131is provided for transmitting a signal having a wavelength of 1530 nanometers. The second optical transmission line132is provided for transmitting a signal having a wavelength of 1540 nanometers. The third optical transmission line133is provided for transmitting a signal having a wavelength of 1550 nanometers. The fourth optical transmission line is provided for transmitting a signal having a wavelength of 1560 nanometers.

The optical input signal having four wavelength composition of 1530 nanometers, 1540 nanometers, 1550 nanometers, and 1560 nanometers is transmitted from the optical transmission line110to the optical multiplexer/demultiplexer410, so that the optical input signal is wavelength-demultiplexed by the optical multiplexer/demultiplexer410whereby the optical input signal is divided into a first signal having a wavelength of 1530 nanometers, a second signal having a wavelength of 1540 nanometers, a third signal having a wavelength of 1550 nanometers, and a fourth signal having a wavelength of 1560 nanometers. The first, second, third and fourth optical signals are inputted into the first, second, third and fourth optical transmission lines131,132,133and134respectively.

The first optical transmission line131has a first optical multiplexer/demultiplexer151.

The first optical multiplexer/demultiplexer151performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals having 1.55 micrometers and 1.54 micrometers which are outputted from two output ports. The first optical multiplexer/demultiplexer151is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the first optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The second optical transmission line132has a second optical multiplexer/demultiplexer152. The first optical multiplexer/demultiplexer151is also connected through a series connection of a first optical amplifier55and a first isolator91to the second optical multiplexer/demultiplexer152. One of the wavelength-demultiplexed optical signals is transmitted from the first optical multiplexer/demultiplexer151through the first optical amplifier55and the first isolator91to the second optical multiplexer/demultiplexer152.

The second optical multiplexer/demultiplexer152performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals which are outputted from two output ports. The second optical multiplexer/demultiplexer152is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the second optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The third optical transmission line133has a third optical multiplexer/demultiplexer153. The second optical multiplexer/demultiplexer152is also connected through a series connection of a second optical amplifier56and a second isolator92to the third optical multiplexer/demultiplexer153. One of the wavelength-demultiplexed optical signals is transmitted from the second optical multiplexer/demultiplexer152through the second optical amplifier56and the second isolator92to the third optical multiplexer/demultiplexer153.

The third optical multiplexer/demultiplexer153performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals which are outputted from two output ports. The third optical multiplexer/demultiplexer153is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the third optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The fourth optical transmission line134has a fourth optical multiplexer/demultiplexer154. The third optical multiplexer/demultiplexer153is also connected through a series connection of a third optical amplifier56and a third isolator93to the fourth optical multiplexer/demultiplexer154. One of the wavelength-demultiplexed optical signals is transmitted from the third optical multiplexer/demultiplexer153through the third optical amplifier56and the third isolator93to the fourth optical multiplexer/demultiplexer154.

The fourth optical multiplexer/demultiplexer154performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals which are outputted from two output ports. The fourth optical multiplexer/demultiplexer154is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the fourth optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The fourth optical multiplexer/demultiplexer154is also connected through a series connection of a fifth optical amplifier58and a fifth optical attenuator94to the first optical multiplexer/demultiplexer151.

The above wavelength-multiplexed optical amplifier performs signal transmission operation.

The signal transmission operation of the wavelength-multiplexer optical amplifier will described. The first input signal is transmitted through the first optical multiplexer/demultiplexer151to the first optical amplifier55, whereby the signal is amplified by the first optical amplifier55. The amplified signal is then transmitted through the first optical isolator91to the second optical multiplexer/demultiplexer152. Since the second optical multiplexer/demultiplexer152has a multiplexing function, the amplified signal is transmitted through the second optical transmission line132to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The second input signal is transmitted through the second optical multiplexer/demultiplexer152to the second optical amplifier56, whereby the signal is amplified by the second optical amplifier56. The amplified signal is then transmitted through the second optical isolator92to the third optical multiplexer/demultiplexer153. Since the third optical multiplexer/demultiplexer153has a multiplexing function, the amplified signal is transmitted through the third optical transmission line133to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The third input signal is transmitted through the third optical multiplexer/demultiplexer153to the third optical amplifier57, whereby the signal is amplified by the third optical amplifier57. The amplified signal is then transmitted through the third optical isolator93to the fourth optical multiplexer/demultiplexer154. Since the fourth optical multiplexer/demultiplexer154has a multiplexing function, the amplified signal is transmitted through the fourth optical transmission line134to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The fourth input signal is transmitted through the fourth optical multiplexer/demultiplexer154to the fourth optical amplifier58, whereby the signal is amplified by the fourth optical amplifier58. The amplified signal is then transmitted through the fourth optical isolator94to the first optical multiplexer/demultiplexer151. Since the first optical multiplexer/demultiplexer151has a multiplexing function, the amplified signal is transmitted through the first optical transmission line131to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120.

The use of the optical wavelength-multiplexer/demultiplexer to serve as the same function as the optical coupler reduces the optical power loss by not less than 5 dB as compared to the 1:1 optical coupler.

Seventeenth Embodiment

A seventeenth embodiment according to the present invention will be described in detail with reference toFIG. 20which is a diagram illustrative of a novel wavelength-multiplexed optical equalizer having four looped optical transmission paths.

The novel wavelength-multiplexed optical equalizer is structurally different from the above wavelength-multiplexed optical amplifier ofFIG. 19in view of further provision of an optical amplifier55on an input port and replacing optical amplifiers by attenuators181,182,183and184.

The novel wavelength-multiplexed optical equalizer comprises an optical multiplexer/demultiplexer410having an input port110and an output port120, and first to fourth optical transmission lines131,132,133and134connected to the optical multiplexer/demultiplexer410. The first optical transmission line131is provided for transmitting a signal having a wavelength of 1530 nanometers. The second optical transmission line132is provided for transmitting a signal having a wavelength of 1540 nanometers. The third optical transmission line133is provided for transmitting a signal having a wavelength of 1550 nanometers. The fourth optical transmission line is provided for transmitting a signal having a wavelength of 1560 nanometers.

The optical input signal having four wavelength compositions of 1530 nanometers, 1540 nanometers, 1550 nanometers, and 1560 nanometers is transmitted from the optical transmission line110to the optical multiplexer/demultiplexer410, so that the optical input signal is wavelength-demultiplexed by the optical multiplexer/demultiplexer410whereby the optical input signal is divided into a first signal having a wavelength of 1530 nanometers, a second signal having a wavelength of 1540 nanometers, a third signal having a wavelength of 1550 nanometers, and a fourth signal having a wavelength of 1560 nanometers. The first, second, third and fourth optical signals are inputted into the first, second, third and fourth optical transmission lines131,132,133and134respectively.

The first optical transmission line131has a first optical multiplexer/demultiplexer151.

The first optical multiplexer/demultiplexer151performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals having 1.55 micrometers and 1.54 micrometers which are outputted from two output ports. The first optical multiplexer/demultiplexer151is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the first optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The second optical transmission line132has a second optical multiplexer/demultiplexer152. The first optical multiplexer/demultiplexer151is also connected through a series connection of a first optical attenuator181and a first isolator91to the second optical multiplexer/demultiplexer152. One of the wavelength-demultiplexed optical signals is transmitted from the first optical multiplexer/demultiplexer151through the first optical attenuator181and the first isolator91to the second optical multiplexer/demultiplexer152.

The second optical multiplexer/demultiplexer152performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals which are outputted from two output ports. The second optical multiplexer/demultiplexer152is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the second optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The third optical transmission line133has a third optical multiplexer/demultiplexer153. The second optical multiplexer/demultiplexer152is also connected through a series connection of a second optical attenuator182and a second isolator92to the third optical multiplexer/demultiplexer153. One of the wavelength-demultiplexed optical signals is transmitted from the second optical multiplexer/demultiplexer152through the second optical attenuator182and the second isolator92to the third optical multiplexer/demultiplexer153.

The third optical multiplexer/demultiplexer153performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals which are outputted from two output ports. The third optical multiplexer/demultiplexer153is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the third optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The fourth optical transmission line134has a fourth optical multiplexer/demultiplexer154. The third optical multiplexer/demultiplexer153is also connected through a series connection of a third optical attenuator182and a third isolator93to the fourth optical multiplexer/demultiplexer154. One of the wavelength-demultiplexed optical signals is transmitted from the third optical multiplexer/demultiplexer153through the third optical attenuator182and the third isolator93to the fourth optical multiplexer/demultiplexer154.

The fourth optical multiplexer/demultiplexer154performs a wavelength demultiplexing so as to divide the first optical input signal into two different wavelength optical signals which are outputted from two output ports. The fourth optical multiplexer/demultiplexer154is used in place of the optical coupler so that the wavelength different two optical signals has a total optical power which is higher than the fourth optical signal, thereby to solve a problem with remarkable optical power loss caused when the signal is transmitted through a plurality of optical couplers.

The fourth optical multiplexer/demultiplexer154is also connected through a series connection of a fifth optical attenuator184and a fifth optical attenuator94to the first optical multiplexer/demultiplexer151.

The above wavelength-multiplexed optical equalizer performs signal transmission operation.

The signal transmission operation of the wavelength-multiplexed optical equalizer will be described. The first input signal is transmitted through the first optical multiplexer/demultiplexer151to the first optical attenuator181, whereby the signal is attenuated by the first optical attenuator181. The attenuated signal is then transmitted through the first optical isolator91to the second optical multiplexer/demultiplexer152. Since the second optical multiplexer/demultiplexer152has a multiplexing function, the attenuated signal is transmitted through the second optical transmission line132to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The second input signal is transmitted through the second optical multiplexer/demultiplexer152to the second optical attenuator182, whereby the signal is attenuated by the second optical attenuator182. The attenuated signal is then transmitted through the second optical isolator92to the third optical multiplexer/demultiplexer153. Since the third optical multiplexer/demultiplexer153has a multiplexing function, the attenuated signal is transmitted through the third optical transmission line133to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The third input signal is transmitted through the third optical multiplexer/demultiplexer153to the third optical attenuator183, whereby the signal is attenuated by the third optical attenuator183. The attenuated signal is then transmitted through the third optical isolator93to the fourth optical multiplexer/demultiplexer154. Since the fourth optical multiplexer/demultiplexer154has a multiplexing function, the attenuated signal is transmitted through the fourth optical transmission line134to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120. The fourth input signal is transmitted through the fourth optical multiplexer/demultiplexer154to the fourth optical attenuator184, whereby the signal is attenuated by the fourth optical attenuator184. The attenuated signal is then transmitted through the fourth optical isolator94to the first optical multiplexer/demultiplexer151. Since the first optical multiplexer/demultiplexer151has a multiplexing function, the attenuated signal is transmitted through the first optical transmission line131to the optical multiplexer/demultiplexer141, whereby the signal is multiplexed with other signal to output an output signal from the output port120.

The use of the optical wavelength-multiplexer/demultiplexer to serve as the same function as the optical coupler reduces the optical power loss by not less than 5 dB as compared to the 1:1 optical coupler.

Eighteenth Embodiment

An eighteenth embodiment according to the present invention will be described in detail with reference toFIG. 21which is a diagram illustrative of a novel optical gate switch utilizing optical wavelength multiplexer/demultiplexer and an erbium doped fiber.

An optical transmission line121is provided for transmitting an optical signal having a wavelength of 1550 nanometers. The optical transmission line121has first and second optical wavelength multiplexer/demultiplexers155and156, and an erbium doped fiber141between the first and second optical wavelength multiplexer/demultiplexers155and156. An excitation light source161is provided for emitting an excitation light having a wavelength of 1480 nanometers. The excitation light source161is connected through a first subordinate optical transmission line122to the first wavelength multiplexer/demultiplexer155. A second subordinate optical transmission line123extends from the second optical wavelength multiplexer/demultiplexer156. The excitation light is emitted from the excitation light source161and then transmitted through the first subordinate optical transmission line122to the first wavelength multiplexer/demultiplexer155. The excitation light is multiplexed with the optical signal by the first wavelength multiplexer/demultiplexer155and further fed to the erbium doped fiber141to excite the erbium doped fiber141, whereby the optical signal transmitted on the optical transmission line121is amplified by the erbium doped fiber141and then amplified signal is transmitted to the second wavelength multiplexer/demultiplexer156. The excitation of the erbium doped fiber141is caused by absorption of the 1480 nanometers wavelength composition of the multiplexed signal into the erbium doped fiber141. The 1480 nanometers wavelength composition of the multiplexed signal may partially be unabsorbed into the erbium doped fiber141. The multiplexed signal is then transmitted to the second wavelength multiplexer/demultiplexer156so that the remaining 1480 nanometers wavelength composition is demultiplexed from the 1550 nanometers wavelength composition by the second wavelength multiplexer/demultiplexer156, whereby the remaining 1480 nanometers wavelength composition is transmitted through the second subordinate optical transmission line123whilst the 1550 nanometers wavelength composition is then transmitted through the optical transmission line121.

The excitation light has a large intensity for causing an excitation of the erbium doped fiber141. A majority part of the excitation light is absorbed into the erbium doped fiber141for excitation of the erbium doped fiber141, whilst a minority part of the excitation light is not absorbed into the erbium doped fiber141and then transmitted through the erbium doped fiber141. This transmitted excitation light remains to have the large intensity. Actually, this remaining excitation light is multiplexed with the optical signal. However, the second wavelength multiplexer/demultiplexer156is operated for demultiplexing the signal into the optical signal having the wavelength of 1550 nanometers and the remaining excitation light having the wavelength of 1480 nanometers, whereby the second wavelength multiplexer/demultiplexer156sends the optical signal having the wavelength of 1550 nanometers on the optical transmission line121and also sends the remaining excitation light having the wavelength of 1480 nanometers on the second subordinate optical transmission line123to avoid the transmission of the remaining excitation light on the optical transmission line121. The optical transmission line121may be connected to an optical multiplexer/demultiplexer. However, the optical multiplexer/demultiplexer receives no excitation light, whereby the optical multiplexer/demultiplexer is free from any damage by the excitation light. If the optical transmission line121is connected to other optical device, then the optical device receives no excitation light, whereby the optical device is free from any damage by the excitation light.

Nineteenth Embodiment

A nineteenth embodiment according to the present invention will be described in detail with reference toFIG. 22which is a diagram illustrative of a novel optical gate switch utilizing optical wavelength multiplexer/demultiplexer and an erbium doped fiber.

An optical transmission line121is provided for transmitting an optical signal having a wavelength of 1550 nanometers. The optical transmission line121has first and second optical wavelength multiplexer/demultiplexers157and158, and an erbium doped fiber141between the first and second optical wavelength multiplexer/demultiplexers157and158. An excitation light source163is provided for emitting an excitation light having a wavelength of 1480 nanometers. The excitation light source163is connected through a first subordinate optical transmission line122to the first wavelength multiplexer/demultiplexer157. A second subordinate optical transmission line123extends from the second optical wavelength multiplexer/demultiplexer158. The second subordinate optical transmission line123has an optical reflective mirror25and a monitor200. The optical reflecting mirror may comprise a wavelength band selective optical reflecting mirror which is capable of selecting a wavelength band of a light to be reflected. The excitation light is emitted from the excitation light source163and then transmitted through the first subordinate optical transmission line122to the first wavelength multiplexer/demultiplexer157. The excitation light is multiplexed with the optical signal by the first wavelength multiplexer/demultiplexer157and further fed to the erbium doped fiber141to excite the erbium doped fiber141, whereby the optical signal transmitted on the optical transmission line121is amplified by the erbium doped fiber141and then amplified signal is transmitted to the second wavelength multiplexer/demultiplexer158. The excitation of the erbium doped fiber141is caused by absorption of the 1480 nanometers wavelength composition of the multiplexed signal into the erbium doped fiber141. The 1480 nanometers wavelength composition of the multiplexed signal may partially be unabsorbed into the erbium doped fiber141. The multiplexed signal is then transmitted to the second wavelength multiplexer/demultiplexer158so that the remaining 1480 nanometers wavelength composition is demultiplexed from the 1550 nanometers wavelength composition by the second wavelength multiplexer/demultiplexer158, whereby the remaining 1480 nanometers wavelength composition is transmitted through the second subordinate optical transmission line123whilst the 1550 nanometers wavelength composition is then transmitted through the optical transmission line121. The remaining 1480 nanometers wavelength composition corresponds to a transmitted minority part of the excitation light having a large intensity, for which reason the transmitted minority part of the excitation light having a large intensity is transmitted through the second subordinate optical transmission line to the wavelength-band optical reflecting mirror25. The transmitted minority part of the excitation light is thus reflected by the wavelength-band optical reflecting mirror25and then transmitted through the second wavelength multiplexer/demultiplexer158to the erbium doped fiber141again whereby the transmitted minority part of the excitation light is further used to excite the erbium doped fiber141. As a result, the efficiency of the excitation of the erbium doped fiber141is high.

The excitation light has a large intensity for causing an excitation of the erbium doped fiber141. A majority part of the excitation light is absorbed into the erbium doped fiber141for excitation of the erbium doped fiber141, whilst a minority part of the excitation light is not absorbed into the erbium doped fiber141and then transmitted through the erbium doped fiber141. This transmitted excitation light remains to have the large intensity. Actually, this remaining excitation light is multiplexed with the optical signal. However, the second wavelength multiplexer/demultiplexer158is operated for demultiplexing the signal into the optical signal having the wavelength of 1550 nanometers and the remaining excitation light having the wavelength of 1480 nanometers, whereby the second wavelength multiplexer/demultiplexer158sends the optical signal having the wavelength of 1550 nanometers on the optical transmission line121and also sends the remaining excitation light having the wavelength of 1480 nanometers on the second subordinate optical transmission line123to avoid the transmission of the remaining excitation light on the optical transmission line121. The optical transmission line121may be connected to an optical multiplexer/demultiplexer. However, the optical multiplexer/demultiplexer receives no excitation light, whereby the optical multiplexer/demultiplexer is free from any damage by the excitation light. If the optical transmission line121is connected to other optical device, then the optical device receives no excitation light, whereby the optical device is free from any damage by the excitation light.

Further, a slight amount of the optical signal is transmitted through the second wavelength multiplexer/demultiplexer158to the second subordinate optical transmission line123. Since the wavelength-band optical reflecting mirror25sets the reflecting wavelength band at 1448 nanometers for reflecting the excitation light component, then the slight amount of the optical signal having the wavelength of 1550 nanometers is transmitted through the wavelength-band optical reflecting mirror25to the monitor200. The monitor200monitors the intensity of the leaked optical signal for controlling optical power levels and device damage monitoring.

Twentieth Embodiment

A twentieth embodiment according to the present invention will be described in detail with reference toFIG. 23which is a diagram illustrative of a novel optical gate switch utilizing optical wavelength multiplexer/demultiplexer and an erbium doped fiber.

An optical transmission line121is provided for transmitting an optical signal having a wavelength of 1550 nanometers. The optical transmission line121has first and second optical wavelength multiplexer/demultiplexers157and158, and an erbium doped fiber141between the first and second optical wavelength multiplexer/demultiplexers157and158. A first excitation light source163is provided for emitting a first excitation light having a wavelength of 1480 nanometers. The first excitation light source163is connected through a first subordinate optical transmission line122to the first wavelength multiplexer/demultiplexer157. A second excitation light source164is provided for emitting a second excitation light having a wavelength of 1480 nanometers. The second excitation light source164is connected through a second subordinate optical transmission line123to the second wavelength multiplexer/demultiplexer158. The first excitation light is emitted from the excitation light source163and then transmitted through the first subordinate optical transmission line122to the first wavelength multiplexer/demultiplexer157. The first excitation light is multiplexed with the optical signal by the first wavelength multiplexer/demultiplexer157and further fed to the erbium doped fiber141to excite the erbium doped fiber141, whereby the optical signal transmitted on the optical transmission line121is amplified by the erbium doped fiber141and then amplified signal is transmitted to the second wavelength multiplexer/demultiplexer158. The second excitation light is emitted from the excitation light source164and then transmitted through the second subordinate optical transmission line123to the second wavelength multiplexer/demultiplexer158. The second excitation light is multiplexed with the optical signal by the first wavelength multiplexer/demultiplexer158and further fed to the erbium doped fiber141to excite the erbium doped fiber141, whereby the optical signal transmitted on the optical transmission line121is amplified by the erbium doped fiber141and then amplified signal is transmitted to the second wavelength multiplexer/demultiplexer158.

The first excitation light has a large intensity for causing an excitation of the erbium doped fiber141. A majority part of the first excitation light is absorbed into the erbium doped fiber141for excitation of the erbium doped fiber141, whilst a minority part of the first excitation light is not absorbed into the erbium doped fiber141and then transmitted through the erbium doped fiber141. This transmitted first excitation light remains to have the large intensity. Actually, this remaining first excitation light is multiplexed with the optical signal. However, the second wavelength multiplexer/demultiplexer158is operated for demultiplexing the signal into the optical signal having the wavelength of 1550 nanometers and the remaining first excitation light having the wavelength of 1480 nanometers, whereby the second wavelength multiplexer/demultiplexer158sends the optical signal having the wavelength of 1550 nanometers on the optical transmission line121and also sends the remaining first excitation light having the wavelength of 1480 nanometers on the second subordinate optical transmission line123to avoid the transmission of the remaining first excitation light on the optical transmission line121. The optical transmission line121may be connected to an optical multiplexer/demultiplexer. However, the optical multiplexer/demultiplexer receives no first excitation light, whereby the optical multiplexer/demultiplexer is free from any damage by the first excitation light. If the optical transmission line121is connected to other optical device, then the optical device receives no first excitation light, whereby the optical device is free from any damage by the first excitation light.

The second excitation light has a large intensity for causing an excitation of the erbium doped fiber141. A majority part of the second excitation light is absorbed into the erbium doped fiber141for excitation of the erbium doped fiber141, whilst a minority part of the second excitation light is not absorbed into the erbium doped fiber141and then transmitted through the erbium doped fiber141. This transmitted second excitation light remains to have the large intensity. Actually, this remaining second excitation light is multiplexed with the optical signal. However, the first wavelength multiplexer/demultiplexer157is operated for demultiplexing the signal into the optical signal having the wavelength of 1550 nanometers and the remaining second excitation light having the wavelength of 1480 nanometers, whereby the first wavelength multiplexer/demultiplexer157sends the optical signal having the wavelength of 1550 nanometers on the optical transmission line121and also sends the remaining second excitation light having the wavelength of 1480 nanometers on the second subordinate optical transmission line123to avoid the transmission of the remaining second excitation light on the optical transmission line121. The optical transmission line121may be connected to an optical multiplexer/demultiplexer.

However, the optical multiplexer/demultiplexer receives no second excitation light, whereby the optical multiplexer/demultiplexer is free from any damage by the second excitation light. If the optical transmission line121is connected to other optical device, then the optical device receives no second excitation light, whereby the optical device is free from any damage by the second excitation light.

The use of the first and second excitation light sources reduces the required intensity of the individual excitation light.

Twenty First Embodiment

A twenty first embodiment according to the present invention will be described in detail with reference toFIG. 24which is a novel wavelength-multiplexed optical add-drop multiplexer including the above optical multiplexer/demultiplexers utilizing the above novel optical gate switches having the same structure as illustrated inFIG. 22, wherein band-pass filters171and172are used in place of wavelength band selective optical reflecting mirror25ofFIG. 22as well as utilizing the above novel optical gate switches having the same structure as illustrated inFIG. 21.

Twenty Second Embodiment

A twenty second embodiment according to the present invention will be described in detail with reference toFIG. 25which is a novel wavelength-multiplexed optical add-drop multiplexer which is modified from the above novel wavelength-multiplexed optical add-drop multiplexer ofFIG. 15by utilizing the above novel optical gate switches having the same structure as illustrated inFIG. 21.

Twenty Fourth Embodiment

A twenty fourth embodiment according to the present invention will be described in detail with reference toFIG. 26which is a novel wavelength-multiplexed optical add-drop multiplexer which is modified from the above novel wavelength-multiplexed optical add-drop multiplexer ofFIG. 16by utilizing the above novel optical gate switches having the same structure as illustrated inFIG. 22, wherein band-pass filters171and172are used in place of wavelength band selective optical reflecting mirror25ofFIG. 22as well as utilizing the above novel optical gate switches having the same structure as illustrated inFIG. 21.

Twenty Fourth Embodiment

A twenty fourth embodiment according to the present invention will be described in detail with reference toFIG. 27which is a novel wavelength-multiplexed optical add-drop multiplexer utilizing the above novel optical gate switches having the same structure as illustrated inFIG. 22, wherein band-pass filters171and172are used in place of wavelength band selective optical reflecting mirror25ofFIG. 22.

Twenty Fifth Embodiment

A twenty fifth embodiment according to the present invention will be described in detail with reference toFIG. 28which is a novel wavelength-multiplexed optical add-drop multiplexer which is modified from the above novel wavelength-multiplexed optical add-drop multiplexer ofFIG. 15by eliminating optical transmitters and replacing optical receivers by a combination of band pass filters171,172,173and174with a monitor200.

Twenty Sixth Embodiment

A twenty sixth embodiment according to the present invention will be described in detail with reference toFIG. 29which is a novel wavelength-multiplexed optical add-drop multiplexer which is modified from the above novel wavelength-multiplexed optical add-drop multiplexer ofFIG. 19by replacing optical amplifiers with the above novel optical gate switches having the same structure as illustrated inFIG. 22, wherein band-pass filters171and172are used in place of wavelength band selective optical reflecting mirror25ofFIG. 22.

Whereas modifications of the present invention will be apparent to a person having ordinary skill in the art, to which the invention pertains, it is to be understood that embodiments as shown and described by way of illustrations are by no means intended to be considered in a limiting sense. Accordingly, it is to be intended to cover by claims all modifications which fall within the spirit and scope of the present invention.