Optical amplifying transmission system and optical amplifying repeater

An optical amplifying transmission system comprising a first optical amplifying transmission line including a first optical amplifier, a second optical amplifying transmission line including a second optical amplifier, a pumping light generator for generating pumping lights to be supplied to said first and second amplifiers, the powers of the pumping lights being variable, and first and second terminal stations which connect respectively to both ends of said first and second optical amplifying transmission lines.

FIELD OF THE INVENTION
 This invention is related to an optical amplifying transmission system and
 an optical amplifying repeater, and more specifically, to an optical
 amplifying transmission system in which the transmission capacity of two
 optical transmission lines is alterable and an optical amplifying repeater
 therefor.
 BACK GROUND OF THE INVENTION
 A main method for international communication, until quite recently, has
 been a telephone and the volume of communication traffic between two
 countries has been mutually the same in both transmission directions.
 Therefore, up and down lines of a transmission system are designed
 symmetrically, namely as the capacity of both directions becomes
 equivalent.
 However, due to the spread of an internet, a phenomenon has been occurring
 in which the volume of communication traffic of both transmission
 directions is greatly one-sided (asymmetrical traffic). Furthermore, the
 volume of communication traffic or the degree of asymmetry is subject to
 change with the passage of time.
 In that circumstances, an extremely uneconomic means has been usually
 employed in which a transmission system capable of realizing the estimated
 maximum transmission capacity of both up and down transmission lines is
 constructed in the first place and only part of the system is used at the
 beginning.
 Therefore, a transmission line has been greatly expected whose transmission
 capacity can be set up or altered more flexibly.
 SUMMARY OF THE INVENTION
 A purpose of this invention is to provide an optical amplifying
 transmission system in which the transmission capacity of two transmission
 lines is alterable even after completion of its construction and an
 optical amplifying. repeater used for the optical amplifying transmission
 system.
 According to the invention, pumping powers can be varied flexibly, which
 are applied to a first amplifier disposed on a first optical amplifying
 transmission line and a second optical amplifier disposed on a second
 optical amplifying transmission line. Consequently, the transmission
 capacities of the first and second optical amplifying transmission lines
 can be set asymmetrically and, thus, there is no need to prepare an
 excessive transmission capacity in advance.
 Preferably, the power distribution rate of each pumping power for being
 supplied to the first and second optical amplifiers is variable.
 Accordingly, the transmission capacities of the first and second optical
 amplifying transmission lines is easily controlled to be asymmetrical and
 its controlling structure is also simply realized. Furthermore, it is easy
 to insert the controlling structure as an optical amplifying repeater into
 a submarine optical cable.
 By providing a divider for dividing pumping power control command light
 transmitted to the first optical amplifying transmission line from a first
 terminal and a controller for controlling the powers of pumping lights
 supplied to the respective first and second amplifiers by a pumping light
 generator according to the pumping power control command light divided by
 the divider, the pumping powers to be supplied to the first and second
 optical amplifiers or the power distribution rate can be
 remote-controlled.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiments of the invention are explained below in detail with reference
 to the drawings.
 FIG. 1 is a schematic block diagram showing an embodiment of an optical
 amplifying transmission system of the invention. An up optical
 transmission line 14 for transmitting signal light from a terminal station
 10 to a terminal station 12 and a down optical transmission line 16 for
 transmitting signal light from the terminal station 12 to the terminal
 station 10 are connected between the terminal stations 10 and 12. The
 terminal station 10 comprises an optical sender 10S for sending the signal
 light to the up optical transmission line 14 and an optical receiver 10R
 for receiving and processing the signal light input from the down optical
 transmission line 16. The terminal station 12 comprises an optical sender
 12S for sending the signal light to the down optical transmission line 16
 and an optical receiver 12R for receiving and processing the signal light
 input from the up optical transmission line 14.
 The up optical transmission line 14 is an optical amplifying/repeating
 transmission line in which a plurality of transmission fibers 18 are
 connected through optical amplifiers 20. The down optical transmission
 line 16 is also an optical amplifying/repeating transmission line in which
 a plurality of transmission fibers 22 are connected through optical
 amplifiers 24.
 Each of the optical amplifiers 20 and 24 comprises, for example, an
 erbium-doped optical amplifying fiber and a pumping light generator 26
 generates pumping light for pumping the erbium-doped optical amplifying
 fibers. In a long distance optical transmission line system such as a
 submarine optical cable, the up optical transmission line 14 and the down
 optical transmission line 16 are kept in a single cable and the optical
 amplifiers 20 and 24 located within the same distance are stored in the
 same joint part. Each pumping light generator 26 supplies the pumping
 light to the optical amplifiers 20 and 24 stored in the same joint part.
 The signal light output from the optical sender lOS of the terminal station
 10 is optically amplified by the optical amplifier 20 while transmitting
 on the transmission optical fiber 18 of the up optical transmission line
 14 and enters the optical receiver 12R of the terminal station 12. The
 signal light output from the optical sender 12S of the terminal station 12
 is optically amplified by the optical amplifier 24 while transmitting on
 the transmission optical fiber 22 of the down optical transmission line 16
 and enters the optical receiver 10R of the terminal station 10.
 In the conventional art, the signal light level of the up optical
 transmission line 14 and that of the down optical transmission line 16 are
 designed to be exactly the same. That is, the pumping light having
 basically the same power is supplied to both of the optical amplifier 20
 of the up optical transmission line 14 and the optical amplifier 24 of the
 down optical transmission line 16 so that both amplifiers operate in the
 same state. As a result, optical signal levels of the up and down
 transmission lines become approximately equivalent and the transmission
 capacity of the up and down transmission lines is also the same.
 However, according to the invention, the pumping light generator 26
 comprises a distributor, the details are described later, which is
 externally controllable of the power rates to be distributed to the
 amplifiers 20 and 24, and, therefore, the power of the pumping light
 supplied to the optical amplifier 20 and that of the pumping light
 supplied to the optical amplifier 24 can be varied freely. The pumping
 light generator 26, therefore, supplies the pumping light of more power to
 an optical amplifier (for example, the optical amplifier 20 or 24)
 requiring a greater transmission capacity and the pumping light of less
 power to an optical amplifier (for example, the optical amplifier 24 or
 20) requiring less transmission capacity.
 In an optical transmission system designed satisfactorily, the transmission
 capacity is almost determined by an optical signal level of an optical
 transmission line. Further, the optical signal level is proportional to
 the pumping light power supplied to an optical amplifier. Therefore, by
 altering the distribution rate of the pumping light power, the desirable
 transmission capacity can be distributed to the up optical transmission
 line 14 and the down optical transmission line 16 without changing the
 total transmission capacity of the up and down optical transmission lines
 14 and 16.
 If only the distribution rate is altered keeping the fixed total power of
 the pumping light generated from the pumping light generator 26, the
 feeding power to the pumping light generator 26, namely the feeding power
 to the repeating amplifier becomes regular and, as a result, there is no
 need to make an investment in equipment for an unnecessary transmission
 capacity.
 As a whole transmission system, it is required that the optical senders
 10S, 12S and the optical receivers 10R, 12R of the terminal stations 10
 and 12 are capable of corresponding to the increase and decrease of the
 communication volume. However, in an international long distance
 communication system in which an optical submarine cable, which is
 difficult to be altered after construction, is employed as a part of
 transmission line, the utility value of the whole communication system
 remarkably increases since the form of utilization is extremely flexible
 as shown in the embodiment.
 FIG. 2 is a schematic block diagram of an embodiment of an optical
 amplifying repeater comprising the optical amplifiers 20 and 24, the
 pumping light generator 26 and their accompanying optical elements and
 circuits.
 On the up optical transmission line 14, an erbium-doped optical amplifying
 fiber 30 and an optical isolator 32 are connected in series between a
 transmission optical fiber 18-1 on the upper course and a transmission
 optical fiber 18-2 on the lower course. The optical isolator 32 prevents
 Rayleigh scattering light from entering the optical amplifying fiber 30
 from the transmission optical fiber 18-2 on the lower course. An WDM
 coupler 34 for introducing pumping light to the optical amplifying fiber
 30 in the opposite direction from signal light is arranged between the
 optical amplifying fiber 30 and the optical isolator 32.
 On the down optical transmission line 16, the structure is basically the
 same with that on the up optical transmission line 14. That is, an
 erbium-doped optical amplifying fiber 36 and an optical isolator 38 are
 connected in series between a transmission optical fiber 22-1 on the upper
 course and a transmission optical fiber 22-2 on the lower course. The
 optical isolator 38 prevents Rayleigh scattering light from entering the
 optical amplifying fiber 36 from the transmission optical fiber 22-2 on
 the lower course. A WDM coupler 40 for introducing pumping light to the
 optical amplifying fiber 36 in the opposite direction from signal light is
 arranged between the optical amplifying fiber 36 and the optical isolator
 38.
 Furthermore, on the up optical transmission line 14, a monitor coupler 42
 for forwarding control command light sent from the terminal station 10 to
 a controller 44 is arranged between the optical isolator 32 and the
 transmission optical fiber 18-2. The control command light is used to
 control the distribution rate of the pumping light toward the optical
 amplifying fiber 30 on the up optical transmission line 14 and the pumping
 light toward the optical amplifying fiber 36 on the down optical
 transmission line 16. The signal light and the control command light
 transmitting on the up optical transmission line 14 enter the controller
 44 through the monitor coupler 42, and the controller 44 detects the
 control command light and generates a distribution control voltage signal.
 If it is also desired to control the distribution rate from the terminal
 station 12, the same kind of monitor coupler is disposed between the
 optical isolator 38 and optical fiber 22-2 on the down optical
 transmission line 16 and output light of the monitor coupler is applied to
 the controller 44.
 Laser diodes 46 and 48 generate laser light having a wavelength for pumping
 the erbium to be doped to the optical amplifying fibers 30, 36 and apply
 to a pumping light combiner/divider 50. The wavelengths of output light of
 the laser diodes 46 and 48 are slightly different each other, for example,
 approximately one nm or more, so as not to interfere mutually. The pumping
 light combiner/divider 50 combines the laser lights from the laser diodes
 46, 48 and divides the combined light in two at a distribution rate
 according to the distribution control voltage signal from the controller
 44. Each divided light is supplied to the optical amplifying fibers 30 and
 36 through the WDM couplers 34 and 40 respectively.
 The operation for distributing pumping light in the optical amplifying
 repeater shown in FIG. 2 is explained below. The signal light (containing
 the control command light for controlling the pumping light distribution
 rate) outputted from the optical transmitter lOS of the terminal station
 10 transmits on the up optical transmission line 14 and enters the optical
 amplifying fiber 30 from the transmission optical fiber 18-1. As to be
 described later, pumping light from the WDM coupler 34 is introduced to
 the optical amplifying fiber 30 in the opposite direction from the signal
 light and the optical amplifying fiber 30 optically amplifies the signal
 light with the pumping light. The signal light optically amplified by the
 optical amplifying fiber 30 transmits the optical isolator 32 at low loss.
 The optical signal and the control command light output from the optical
 isolator 32 enter the controller 44 through the monitor coupler 42. The
 controller 44 detects the control command light from the input light. The
 output light of the optical isolator 32 also enters the next transmission
 optical fiber 18-2 and transmits toward the optical receiver 12R of the
 terminal station 12.
 The controller 44 generates the distribution control voltage signal by
 photoelectric-converting the control command light from the monitor
 coupler 42 and applies it to a control input of the pumping light
 combiner/divider 50. The output laser lights of the laser diodes 46 and 48
 enter to the two inputs of the pumping light combiner/divider 50. The
 pumping light combiner/divider 50 combines the two laser inputs and then
 divides into two portions at the distribution rate according to the
 distribution control voltage signal from the controller 44. The two
 portions of divided laser light are applied as pumping light to the
 optical amplifying fibers 30 and 36 through the WDM couplers 34 and 40
 respectively.
 As mentioned above, in the optical amplifying repeater shown in FIG. 2, the
 respective optical amplifying fibers 30 and 36 are pumped by the pumping
 light divided at the distribution rate determined by the distribution
 control voltage signal output from the controller 44. The gains of the
 optical amplifiers 20 and 24 at each repeating position of the
 transmission system shown in FIG. 1 are respectively controlled similarly
 on the up and down optical transmission lines 14 and 16. That is, signal
 levels on the up and down optical transmission lines 14 and 16 can be
 remote-controlled from the terminal station 10 (or 12).
 The pumping light combiner/divider 50 comprises, for example, an asymmetric
 X optical switch as shown in FIG. 3. FIG. 3 shows a plan view of the
 asymmetric X optical switch. The asymmetric X optical switch is formed,
 for example, as a waveguide on a crystal of lithium niobate. Two
 waveguides 60 and 62 on the input side of laser light and two waveguides
 64 and 66 on the output side intersect at a point showing a shape of X.
 The output light of the laser diode 46 enters to the waveguide 60 and the
 output light of the laser diode 48 enters the waveguide 62. For
 stabilizing the distributing operation of the optical switch, the
 waveguide structures of the waveguides 60 and 62 are made to be different
 each other.
 The laser lights input to the waveguides 60 and 62 are transmitted to the
 junction with the waveguides 64 and 66, combined there and distributed to
 the wavelengths 64 and 66. At the upper parts of the waveguides 64 and 66,
 electrodes 68 and 70 are respectively disposed. The electrode 68 is
 connected to the ground and, on the other hand, the control voltage from
 the controller 44 is applied to the electrode 70. Electric fields of
 mutually opposite directions are applied to the waveguides 64 and 66 to be
 located beneath the electrodes 68 and 70. With this operation, the applied
 voltage (the control voltage signal from the controller 44) of the
 electrode 70 can control the distribution rate for distributing the laser
 lights from the waveguides 60 and 62 to the waveguides 64 and 66. The
 laser lights transmitted the waveguides 64 and 66 are respectively applied
 to the WDM couplers 34 and 40. The laser lights from the laser diodes 46
 and 48 is distributed equally to the waveguides 64 and 66 when no voltage
 is applied to the electrode 70.
 Although the asymmetric X optical switch shown in FIG. 3 can realize the
 two functions of combining and dividing with the single optical element,
 it is also applicable to realize the two functions separately using
 different elements. For instance, a Y branch optical switch can be used
 for dividing wavelength-multiplexed pumping light or
 polarization-multiplexed pumping light at a desired distribution rate.
 FIG. 4 is a schematic block diagram showing another embodiment of the
 pumping light combiner/divider 50. The output light of the laser diodes 46
 and 48 is multiplexed in the same polarization state by a WDM coupler 72
 and applied to a polarization rotator 74. The polarization rotator 74
 rotates a polarization plane of the input light according to the control
 voltage signal from the controller 44. Output light of the polarization
 rotator 74 is applied to a polarization beam splitter 76 and split into
 two elements of mutually orthogonal polarization planes. The light of one
 polarization plane is applied to the WDM coupler 34 and the light of the
 other polarization plane is applied to the WDM coupler 40.
 As mentioned above, by using the polarization rotator 74 for rotating the
 polarization plane at an angle in accordance with the control voltage and
 the polarization beam splitter 76 together, the distribution rate, namely
 the rate of the pumping lights entering to the optical amplifying fibers
 30 and 36 can be remote-controlled.
 The simpler structure is that output lights of individual laser diodes pump
 the optical amplifiers 30, 36 and the terminal station 10 (or 12) controls
 the driving currents of the laser diodes. FIG. 5 shows a schematic block
 diagram of this embodiment. Identical elements are labelled with reference
 numerals common to those in FIG. 1.
 A controller 80 converts the control command light from the monitor coupler
 42 into electric signal and applies the control signal showing the
 distribution of driving currents toward an LD driver 82. The LD driver 82
 supplies the driving currents to the respective laser diodes 84 and 86 at
 the distribution rate in accordance with the control signal from the
 controller 80. That is, the LD driver 82 varies the distribution rate of
 driving currents to the laser diodes 84 and 86 according to the control
 signal from the controller 80 keeping a specific amount of feeding power.
 Generally, output of a laser diode is stabilized by an APC control and,
 therefore, its output power can be easily altered by changing a reference
 voltage of the APC circuit. The output laser lights of the laser diodes 84
 and 86 are supplied to the optical amplifying fibers 30 and 36 through the
 WDM couplers 34, 40 and pump the optical amplifying fibers 30 and 36
 respectively.
 The structure shown in FIG. 5 also can vary the pumping light powers to be
 supplied to the up and down optical amplifying fibers 30 and 36 keeping
 the amount of feeding power for being supplied to the repeater and as a
 result the transmission capacity of the up and down lines can be made
 asymmetry.
 Although the two laser diodes are employed in the abovementioned
 embodiment, this invention is naturally applicable to the structures of
 using more than two laser diodes.
 As readily understandable from the above description, according to the
 invention, it is possible to easily provide an economical optical
 transmission system to meet the needs of asymmetric communication traffic.
 That is, it is no longer necessary to construct an optical transmission
 line having an excessive transmission capacity.
 While the invention has been described with reference to the specific
 embodiment, it will be apparent to those skilled in the art that various
 changes and modifications can be made to the specific embodiment without
 departing from the spirit and scope of the invention as defined in the
 claims.