Source: http://www.google.com/patents/US6025947?dq=7,177,838
Timestamp: 2014-04-16 10:42:40
Document Index: 297617012

Matched Legal Cases: ['art 1000', 'art 2000', 'art 1000', 'art 1000', 'art 2000', 'art 1000', 'art 1000', 'art 2000', 'art 2000', 'art 1000', 'art 1000', 'art 2000', 'art 3000', 'art 3000', 'art 3000', 'art 2000', 'art 1000', 'art 3000', 'art 2000', 'art 3000', 'art 2000', 'art 3000', 'art 1000', 'art 3000', 'art 3000', 'art 3000', 'art 3000', 'art 3000', 'art 3000', 'art 3000', 'art 1000', 'art 2000', 'art 1000']

Patent US6025947 - Controller which controls a variable optical attenuator to control the power ... - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inAdvanced Patent SearchPatentsAn optical amplifying apparatus which includes an optical amplifier, an optical attenuator and a controller. The optical amplifier amplifies a light signal having a variable number of channels. The optical attenuator passes the amplified light signal and has a variable light transmissivity. Prior to...http://www.google.com/patents/US6025947?utm_source=gb-gplus-sharePatent US6025947 - Controller which controls a variable optical attenuator to control the power level of a wavelength-multiplexed optical signal when the number of channels are variedAdvanced Patent SearchPublication numberUS6025947 APublication typeGrantApplication numberUS 08/845,847Publication dateFeb 15, 2000Filing dateApr 28, 1997Priority dateMay 2, 1996Fee statusPaidAlso published asCN1121626C, CN1167269A, CN1406016A, CN1406017A, CN1406018A, CN1406019A, CN100477563C, CN100477571C, CN100477572C, CN100477573C, DE69737190D1, DE69737190T2, DE69737802D1, DE69737802T2, DE69737813D1, DE69737813T2, DE69739010D1, EP0805571A2, EP0805571A3, EP0805571B1, EP0902565A2, EP0902565A3, EP0902565B1, EP0902566A2, EP0902566A3, EP0902566B1, EP0902566B9, EP0902567A2, EP0902567A3, EP1578047A1, EP1578047B1, EP2296303A1, EP2296303B1, US5966237, US5995274, US6144485, US6157481, US6198572, US6377395, US6646791, US6865016, US7227681, US7477447, US7969649, US8553319, US20010017729, US20020067538, US20040036958, US20050046927, US20070201876, US20090086310, US20110205620, US20140043675Publication number08845847, 845847, US 6025947 A, US 6025947A, US-A-6025947, US6025947 A, US6025947AInventorsSusumu Kinoshita, Yasushi SugayaOriginal AssigneeFujitsu LimitedExport CitationBiBTeX, EndNote, RefManPatent Citations (12), Non-Patent Citations (6), Referenced by (57), Classifications (38), Legal Events (4) External Links: USPTO, USPTO Assignment, EspacenetController which controls a variable optical attenuator to control the power level of a wavelength-multiplexed optical signal when the number of channels are variedUS 6025947 AAbstract An optical amplifying apparatus which includes an optical amplifier, an optical attenuator and a controller. The optical amplifier amplifies a light signal having a variable number of channels. The optical attenuator passes the amplified light signal and has a variable light transmissivity. Prior to varying the number of channels in the light signal, the controller varies the light transmissivity of the optical attenuator so that a power level of the amplified light signal is maintained at an approximately constant level that depends on the number of channels in the light signal prior to the varying the number of channels. While the number of channels in the light signal is being varied, the controller maintains the light transmissivity of the optical attenuator to be constant. Subsequent to varying the number of channels in the light signal, the controller varies the light transmissivity of the optical attenuator so that a power level of the amplified light signal is maintained at an approximately constant level that depends on the number of channels in the light signal subsequent to the varying the number of channels.
What is claimed is: 1. An apparatus comprising:an optical amplifier which amplifies a light signal having a variable number of channels associated with different wavelengths; and a controller which receives an indicator signal indicating initiation of changing the number of channels in the light signal, and which, upon receipt of the indicator signal, controls the optical amplifier to amplify the light signal with an approximately constant gain during a process of changing the number of channels. 2. An apparatus as in claim 1, wherein the indicator signal is on a channel included in the light signal, and the controller extracts the indicator signal from the light signal.
3. An apparatus comprising:an optical amplifier which amplifies a light signal having a variable number of channels associated with different wavelengths; and a controller which receives an indicator signal indicating completion of changing the number of channels in the light signal, and which controls the optical amplifier to amplify the light signal with an approximately constant gain until the indicator signal is received. 4. An apparatus as in claim 3, whereinthe controller transmits downstream a ready signal indicating that a power level of the amplified light signal is ready to be controlled in response to the variation in the number of channels after receiving the indicator signal. 5. An apparatus as in claim 3, wherein the indicator signal is on a channel included in the light signal, and the controller extracts the indicator signal from the light signal.
6. An apparatus for amplifying a light signal having a variable number of channels associated with different wavelengths, comprising:a controller for preparing a variation of the number of channels in the light signal; and an optical amplifier which is controlled by the controller to amplify the light signal with an approximately constant gain during the preparation of the variation of the number of channels in the light signal. 7. An apparatus as in claim 6 further comprising an optical attenuator with a variable optical attenuation, which gives an attenuation to the light signal,wherein, while the number of channels in the light signal is being varied, the controller passes the light signal with a constant optical attenuation. 8. An apparatus as in claim 6, further comprising an optical attenuator with a variable optical attenuation, which gives an attenuation to the light signal, whereinthe controller varies the optical attenuation of the optical attenuator so that the power level of the amplified light signal is maintained within a predetermined range in accordance with a variation in the number of channels in the light signal, and while the number of channels in the light signal is being varied, the controller passes the amplified light signal through the optical attenuator with a constant optical attenuation. 9. A method for amplifying a light signal having a variable number of channels associated with different wavelengths by an optical amplifier, the method comprising the steps of:receiving a signal indicating initiation of changing the number of channels in the light signal; and upon receiving the signal, controlling the optical amplifier to amplify the light signal with an approximately constant gain during a process of changing the number of channels. 10. An apparatus for amplifying a light signal having a variable number of channels associated with different wavelengths, comprising:a controller which, in response to receiving a signal indicating initiation of changing the number of channels in the light signal, prepares a variation of the number of channels in the light signal; and an optical amplifier which is controlled by the controller to amplify the light signal with an approximately constant gain during the preparation of the variation of the number of channels in the light signal. 11. An apparatus comprising:an optical amplifier which amplifies a light signal having a variable number of channels associated with different wavelengths; and a controller which controls the optical amplifier to amplify the light signal with an approximately constant gain while the number of channels is being varied, and controls the optical amplifier to amplify the light signal with a gain which is varied when the number of channels is constant. Description
FIG. 2 is a diagram illustrating an optical amplifying apparatus for a fiber optic communication system which uses wavelength division multiplexing, and is similar to that disclosed in related to U.S. patent application Ser. No. 08/655,027, which is incorporated herein by reference.
First part 1000 includes a rare-earth-doped optical fiber (EDF) 34, optical branching couplers 36.sub.1 and 36.sub.2, optical isolators 38.sub.1 and 38.sub.2, photodiodes 40.sub.1 and 40.sub.2, an optical wavelength multiplexing coupler 42, a pump laser diode (LD) 44 and an automatic optical gain control circuit (AGC) 46.
Second part 2000 includes optical branching coupler 36.sub.3, an electrically-controlled variable optical attenuator (ATT) 48, a photodiode (PD) 40.sub.3 and an automatic level control circuit (ALC) 50. Optical attenuator 48 is, for example, constructed of a magnetooptical element. However, many different types of variable optical attenuators can be used.
A wavelength-multiplexed optical signal is fed to rare-earth-doped optical fiber 34 via optical branching coupler 36.sub.1, optical isolator 38 and optical wavelength multiplexing coupler 42. A pump light beam is supplied by pump laser diode 44 to rare-earth-doped optical fiber 38 via optical wavelength multiplexing coupler 42. The wavelength-multiplexed optical signal is amplified by rare-earth-doped optical fiber 34 and input to optical attenuator 48 via optical isolator 38.sub.2 and optical branching coupler 36.sub.2.
A portion of the wavelength-multiplexed optical signal branched by optical branching coupler 36.sub.1 is converted into an electrical signal by photodiode 40.sub.1 and input to automatic optical gain control circuit 46. A portion of the amplified wavelength-multiplexed optical signal branched by optical branching coupler 36.sub.2 is converted into an electrical signal by photodiode 40.sub.2 and input to automatic optical gain control circuit 46. Pump laser diode 44 is controlled so as to maintain a ratio between a level of the input wavelength-multiplexed optical signal and a level of the amplified wavelength-multiplexed optical signal at a predetermined level.
More specifically, optical gain control circuit 46 controls pump laser diode 44 so as to maintain, at a constant level, the ratio between the level of the input wavelength-multiplexed optical signal as converted into an electrical signal by the photodiode 40.sub.1 and the level of the amplified wavelength-multiplexed optical signal as converted into an electrical signal by the photodiode 40.sub.2. In this manner, first part 1000 conserves the wavelength dependence by controlling the optical gain at a constant level.
A portion of an output wavelength-multiplexed optical signal branched by optical branching coupler 36.sub.3 is converted into an electrical signal by photodiode 40.sub.3 and input to automatic level control circuit 50. Optical attenuator 48 is controlled so as to maintain the wavelength-multiplexed optical signal at a predetermined level.
More specifically, automatic level control circuit 50 controls optical attenuator 48 using the electrical signal derived by photodiode 40.sub.3 from the wavelength-multiplexed optical signal, so as to maintain the output level of the wavelength-multiplexed optical signal at a constant level.
For example, a predetermined output optical power of an amplifier is generally required for each wavelength (channel) so as to ensure a desired S/N ratio in a receiver. Assuming there are a total of N channels, the total optical output Pc of a rare-earth-doped optical fiber amplifier for amplifying a wavelength-multiplexed optical signal is controlled to be N number of channels N, switching control is effected so that the total optical power is (N.+-.α)P. Because the optical power for individual wavelengths (channels) varies due to the switching control, non-linear degradation or signal-to-noise (S/N) degradation may result.
FIG. 3 is a diagram illustrating an optical amplifying apparatus, according to an embodiment of the present invention. The optical amplifying apparatus includes a first part 1000 and a second part 2000. First part 1000 includes a rare-earth-doped optical fiber (EDF) 52.sub.1, optical branching couplers 54.sub.1 and 54.sub.2, optical isolators 55.sub.1 and 55.sub.2, an optical wavelength multiplexing coupler 56.sub.1, photodiodes (PD) 58.sub.1 and 58.sub.2, a pump laser diode (LD) 59.sub.1, and an automatic gain control circuit (AGC) 60.sub.1. First part 1000 amplifies a wavelength-multiplexed optical signal while conserving wavelength dependance.
As an example, a wavelength-multiplexed optical signal is typically in the 1.5 μm band. An erbium-doped optical fiber is known to amplify optical signals in this band, and is therefore used as rare-earth-doped optical fiber (EDF) 52.sub.1. Moreover, to appropriately amplify a wavelength-multiplexed optical signal in the 1.5 μm band travelling through an erbium-doped optical fiber, it is known to use pump light of a 0.98 μm or 1.48 μm pump band. Therefore, pump laser diode (LD) 59.sub.1 provides pump light in the 0.98 μm or 1.48 μm pump band.
Moreover, FIG. 3 shows a forward pumping construction in which a pump light beam emitted by pump laser diode 59.sub.1 travels through rare-earth-doped optical fiber 52.sub.1 in the same direction as the wavelength-multiplexed optical signal. However, a backward pumping construction could also be used, where a laser diode provides a pump light beam which travels through rare-earth-doped optical fiber 52.sub.1 in the opposite direction as the wavelength-multiplexed optical signal. Further, a bi-directional pumping construction could be used, where two laser diodes provide pump light which travels through rare-earth-doped optical fiber 52.sub.1 in both directions through rare-earth-doped optical fiber 52.sub.1. Thus, the present invention is not intended to be limited to any specific type of directional pumping.
Second part 2000 includes an electrically-controlled variable optical attenuator (ATT) 64, an automatic level control circuit (ALC) 66, optical branching coupler 54.sub.3 and a photodiode (PD) 58.sub.3. Second part 2000 controls the total optical output of a wavelength-multiplexed optical signal to be at a constant level, without conserving wavelength dependence. More specifically, automatic level control circuit 66 varies the attenuation, or light transmissivity, of optical attenuator 64 so that the power of the wavelength-multiplexed optical signal, as output from first part 1000, is maintained at a constant power level corresponding to the number of channels in the wavelength-multiplexed optical signal.
More specifically, the wavelength-multiplexed optical signal input to the optical amplifying apparatus is branched by an optical branching coupler 68.sub.1. The branched portion is provided to a photodiode (PD) 58.sub.4. Photodiode (PD) 58.sub.4 converts the branched portion into an electrical signal and provides the electrical signal to monitor signal processing circuit 70.
A control signal, which warns of a variation in the number of channels in the wavelength-multiplexed optical transmission system, is superimposed on the wavelength-multiplexed optical signal preferably as a low-speed signal through an amplitude modulation process. However, other methods can be used to superimpose the control signal. Monitor signal processing circuit 70 extracts and identifies the control signal. Monitor signal processing circuit 70 then controls optical attenuator 64 or automatic level control circuit 66 in accordance with the extracted control signal. If amplitude modulation is used, it is relatively easy to extract the control signal by demodulating the electrical signal obtained by photodiode 58.sub.4.
Alternatively, the control signal may be transmitted to monitor signal processing circuit 70 on a dedicated control channel (wavelength). If a dedicated control channel is used, an optical branching filter (not illustrated) should extract the control signal out of the wavelength-multiplexed optical signal (as branched by optical branching coupler 68.sub.1). For example, by feeding the optical signal extracted by the optical branching filter to photodiode 58.sub.4 so as to be converted into an electrical signal, it is possible to extract the control signal.
Therefore, a portion of the wavelength-multiplexed optical signal branched by optical branching coupler 68.sub.1 is converted into an electrical signal by photodiode 58.sub.4 and fed to monitor signal processing circuit 70. Monitor signal processing circuit 70 "freezes" an operation of optical attenuator 64, when a control signal warning of a variation in the number of channels is extracted and identified.
FIGS. 4(A) and 4(B) are graphs illustrating the operation of the optical amplifying apparatus in FIG.3, wherein the number of channels, N, in an optical signal is changed from, for example, four channels to eight channels. Referring now to FIGS. 4(A) and 4(B), optical attenuator 64 has a variable light transmissivity, or attenuation, which is controlled by automatic level control circuit 66 an monitor signal processing circuit 70.
FIG. 5 is a diagram illustrating automatic gain control circuit 60.sub.1, for controlling an optical gain to be at a constant level. Referring now to FIG. 5, automatic gain control circuit 60.sub.1 includes a divider 72, an operational amplifier 74, a transistor 76 and resistors R1-R6. V.sub.cc is a power supply voltage, V.sub.ref is a reference voltage, and G is the earth or ground.
As illustrated in FIG. 5, photodiode (PD) 58.sub.1 converts a portion of the wavelength-multiplexed optical signal into an electrical signal which is provided to divider 72. Photodiode (PD) 58.sub.2 converts a portion of the amplified wavelength-multiplexed optical signal into an electrical signal which is provided to divider 72. In this manner, divider 72 obtains a ratio between the input and the output of rare-earth-doped optical fiber (EDF) 52.sub.1. The pump light beam emitted by pump laser diode 59.sub.1 can then be controlled to produce a constant ratio, thereby providing a constant gain. The configuration of automatic gain control circuit 60.sub.1 in FIG. 5 is just one example of many possible configurations for an automatic gain control circuit.
FIG. 6 is a diagram illustrating automatic level control circuit 66, for controlling an optical output at a constant level. Referring now to FIG. 6, automatic level control circuit 66 includes resistors R7-R9, an operational amplifier 78, a transistor 80, a switching circuit (SWC) 82 and a reference voltage circuit 84. V.sub.cc is the power supply voltage, V.sub.ref is a reference voltage, G is the earth or ground, and cs1 and cs2 are control signals provided by monitor signal processing circuit 70. A control element 86 is a control element of optical attenuator 64 for controlling the transmissivity of optical attenuator 64.
A portion of the optical signal output from optical attenuator 64 (see FIG. 3) is branched by optical branching coupler 54.sub.3 and converted into an electrical signal by photodiode (PD) 58.sub.3. Then, in FIG. 6, operational amplifier 78 compares the electrical signal with the reference voltage (set voltage) V.sub.ref supplied by reference voltage circuit 84 in accordance with control signal CS1. A difference obtained as a result of the comparison is used to drive transistor 80. By controlling a current supplied to control element 86, the attenuation provided by optical attenuator 64 is controlled so that the optical output is maintained at a constant level.
More specifically, switching circuit 82, coupled with operational amplifier 78 (see FIG. 6) and resistors R7 (see FIG. 6) and R9 (see FIG. 6), forms a primary low-pass filter. The cut-off frequency, f.sub.c, of this primary low-pass filter is:
f.sub.c =1/(2&#960;R9
where C.sub.SWC is the selected capacitor C.sub.1 or C.sub.2. Therefore, by increasing the value of the capacitance C.sub.SWC, the control circuitry shown in FIG. 6 is operated at a lower frequency. That is, the response thereof is slowed down.
For example, when monitor signal processing circuit 70 extracts and identifies a control signal which warns of a variation in the number of channels, control signal cs2 is supplied to switching circuit 82 so that the frequency characteristic of automatic level control circuit 66 is switched to a low frequency zone. As a result, the following performance for following a variation in the signal detected by photodiode (PD) 58.sub.3 is lowered. That is, the constant-level control of the optical output is temporarily frozen (for example, the light transmissivity of optical attenuator 64 is maintained to be constant). Further, control signal cs1 corresponds to the number of channels to be included in the optical signal, and monitor signal processing circuit 70 supplies the control signal cs1 to reference voltage circuit 84. Reference voltage circuit 84 then supplies a reference voltage V.sub.ref corresponding to the number of channels. Therefore, the total optical output power assumes a level matching the number of channels after the variation in the number of channels. For example, the reference voltage V.sub.ref is changed such that, when a total of α channels are added to the total of N original channels, the total optical output becomes (N+α)
Referring again to FIGS. 6 and 7, the value of the capacitance C.sub.SWC may be large enough to freeze the operation of optical attenuator 64. Generally, this purpose may be achieved if, for example, the cut-off frequency f.sub.c is dropped from 10 kHz to 0.01 Hz, thereby requiring a drop in the cut-off frequency f.sub.c by the factor of 10,000 to 100,000. Such a large drop can be difficult to achieve.
More specifically, FIGS. 8 and 9 are diagrams illustrating automatic level control circuit 66, according to additional embodiments of the present invention. Referring now to FIG. 8, a filter 90 for cutting off high frequencies (f.sub.c :�10 KHz) and constructed of a capacitor and a resistor is provided between a switch 92 and transistor 80 so that the response of the automatic level control becomes adequate. For example, the time constant, typically on the order of sub-milliseconds, may be changed to the time constant on the order of 10-100 milliseconds.
When the cut-off frequency f.sub.c, is switched to the high-frequency zone, the filter response becomes quick so that a comparatively high-speed variation, such as a polarization variation, can be cancelled and the output of optical attenuator 64 is maintained constant.
More specifically, in FIG. 8, a latch circuit 94 which has a low-pass filter (f.sub.c :�0.01 Hz) stores a voltage corresponding to an average level of the current in control element 86. During an ALC operation, switching of the control loop occurs so that the control loop for controlling the drive current at a constant level is initiated. That is, when the switching of the control loop occurs, the voltage corresponding to the average level of the current is latched in latch circuit 94 so as to serve as a reference voltage. The term "average level" is used because the bias current has a time-dependent variation in order to maintain the level of the beam input to photodiode (PD) 58.sub.3 at a constant level. More specifically, the voltage obtained by integration using a more extended integral time than that provided by the time constant of the normal control loop is latched in latch circuit 94.
FIG. 9 is a combination of FIGS. 6 and 8. Referring now to FIG. 9, the capacitance C.sub.SWC is switched by switching circuit 82 to cause the cut-off frequency f.sub.c, to be shifted to a low-frequency zone, to thereby slow the filter response. Thereupon, latch circuit 94 controls the attenuation to the average based on a monitored value.
The control signal cs2 returns switching circuit 82 to the original frequency characteristic after the control signal for reporting a completion of a variation in the number of channels is received, or after a predetermined period of time has passed. Thereupon, the constant optical output control is resumed in accordance with the new reference voltage V.sub.ref set by reference voltage circuit 84.
The control for maintaining the total optical output at a constant level that corresponds to the number of channels may be resumed in a gradual manner. For example, the output signal of photodiode (PD) 58.sub.3 may be input to operational amplifier 78 via a time constant circuit 96, or reference voltage V.sub.ref may be gradually varied to assume a level that corresponds to the number of channels.
While the above-described arrangement ensures that the frequency characteristic is switched as a result of the control effected by switching circuit 82 so that the constant-level control of the optical output is frozen, it is also possible to hold the signal output by photodiode (PD) 58.sub.3 when the control signal for giving warning of a variation in the number of channels is extracted and identified. In this instance, the held value is input to operational amplifier 78 so that the constant-level control of the optical output is frozen. Other arrangements for freezing the constant-level control of the optical output are also possible. While it is assumed that the electrically-controlled optical device part is constructed using optical attenuator 64, a semiconductor optical amplifier can be used instead of optical attenuator 64. The semiconductor optical amplifier should have a small wavelength dependence. By controlling the semiconductor optical amplifier, the total optical output may be controlled at a constant level.
FIG. 10 is a diagram illustrating an optical amplifying apparatus, according to an additional embodiment of the present invention. Referring now to FIG. 10, the optical amplifying apparatus includes first part 1000, second part 2000 and a third part 3000. Third part 3000 includes a rare-earth-doped optical fiber (EDF) 52.sub.2, an optical branching coupler 54.sub.4, an optical wavelength multiplexing coupler 56.sub.2, optical isolators 55.sub.3 and 55.sub.4, a photodiode (PD) 58.sub.5, a pump laser diode (LD) 59.sub.2 and an automatic gain control circuit (AGC) 60.sub.2. Third part 3000 also shares optical branching coupler 54.sub.3 and the photodiode (PD) 58.sub.3 with second part 2000.
Therefore, pump laser diode 59.sub.1 of first part 1000 and pump laser diode 59.sub.2 of third part 3000 can each have a relatively small capacity, thereby reducing the cost and stabilization of the amplifying apparatus.
Although FIG. 10 shows second part 2000 and third part 3000 sharing optical branching coupler 54.sub.3 and photodiode (PD) 58.sub.3, it is also possible to provide a separate optical branching coupler and a separate photodiode in each of the second part 2000 and the third part 3000.
Automatic gain control circuits 60.sub.1 and 60.sub.2 may have the same configuration. Moreover, the optical gains provided by first part 1000 and third part 3000 may be identical. Alternatively, the gains may be varied according to the characteristics of a transmission optical fiber used in third part 3000.
More specifically, a portion of the wavelength-multiplexed optical signal output by optical attenuator 64 is branched by optical branching coupler 54.sub.3, converted into an electrical signal by photodiode (PD) 58.sub.3 and input to automatic level control circuit 66. Automatic level control circuit 66 controls optical attenuator 64 so that the total optical output power of the wavelength-multiplexed optical signal is maintained at a constant level. However, the optical output power of the output wavelength-multiplexed optical signal in third part 3000 is not fed to automatic level control circuit 66. Therefore, it cannot be ensured that the total optical output in the third part 3000 is maintained within a predetermined range.
Accordingly, a portion of the output wavelength multiplexed optical signal in the third part 3000 is converted into an electrical signal by photodiode (PD) 58.sub.5 and input to ALC correction circuit 98 as well as to automatic gain control circuit 60.sub.2. ALC correction circuit 98 determines whether or not the total optical output power is maintained within the predetermined range. If the total optical output power is not within the predetermined range, ALC correction circuit 98 controls automatic level control circuit 66 which, in turn, controls optical attenuator 64 to maintain the total optical output power within the predetermined range. If a semiconductor optical amplifier is used in place of optical attenuator 64, automatic level control circuit 66 controls the gain of the semiconductor optical amplifier so that the total optical output in third part 3000 is maintained within the predetermined level.
FIG. 13 is a diagram illustrating an optical amplifying apparatus, according to an additional embodiment of the present invention. The optical amplifying apparatus in FIG. 13 operates in a similar manner as previously described embodiments of the present invention, but also includes an optical branching coupler 54.sub.5, a photodiode (PD) 58.sub.6, a dispersion compensation fiber (DCF) 100 and a dispersion compensation fiber (DCF) loss correction circuit 102. Optical branching coupler 54.sub.5 and photodiode (PD) 58.sub.6 can be considered to be included in third part 3000.
Therefore, a portion of the wavelength-multiplexed optical signal output by dispersion compensation optical fiber 100 and branched by optical branching coupler 54.sub.5 is converted into an electrical signal by photodiode (PD) 58.sub.6. The electrical signal is input to DCF loss correction circuit 102 as well as to automatic gain control circuit 60.sub.2. DCF loss correction circuit 102 determines whether or not the level of the wavelength-multiplexed optical signal output by dispersion compensation fiber 100 is within a predetermined range. If the level is outside the predetermined range, DCF loss correction circuit 102 supplies a correction signal to automatic level control circuit 66. For example, the reference voltage (set voltage) for constant control of the optical output is corrected such that the optical output power is within the predetermined range. Therefore, a variation in insertion loss that results in a construction where dispersion compensation fiber 100 compensates for the dispersion in the transmission optical fiber is corrected, and a predetermined output level of the amplified wavelength-multiplexed optical signal is obtained.
FIG. 17 is a diagram illustrating modification to the optical amplifying apparatus illustrated in FIG. 16, according to an embodiment of the present invention. More specifically, in FIG. 17, an optical filter A1 is provided between the output of optical isolator 55.sub.2 and optical branching coupler 54.sub.2, at the input of photodiode (PD) 58.sub.2. Also, an optical filter A2 is provided between the output of optical isolator 55.sub.4 and optical branching coupler 54.sub.4, at the input of photodiode (PD) 58.sub.5. Optical filters A1 and A2 are optical filters as disclosed, for example, in U.S. patent application Ser. No. 08/655,027, which is incorporated herein by reference, for correcting wavelength dependency of the gain.
FIG. 18 (A) is a graph illustrating gain versus wavelength characteristics of rare-earth-doped optical fiber (EDF) 52.sub.2 in FIG. 17, FIG. 18(B) is a graph illustrating the transmissivity versus wavelength of optical filter A2 in FIG. 17, and FIG. 18(C) is a graph illustrating overall gain of rare-earth-doped optical fiber (EDF) 52.sub.2 and optical filter A2 in FIG. 17, according to an embodiment of the present invention.
If, for example, rare-earth-doped optical fiber (EDF) 52.sub.2 has a wavelength-dependent gain characteristic as shown in FIG. 18(A), wherein the gain is higher in the long wavelength range, providing a gain correction optical filter A2 at the input of photodiode (PD) 58.sub.5 ensures that the amplifier has an even gain with respect to wavelength. Providing optical filter A2 ensures that photodiode (PD) 58.sub.5 receives the corrected multi-wavelength signal so that the unfavorable sensitivity characteristic, wherein the signal sensitivity is low in the short wavelength range and high in the long wavelength range, is corrected. Optical filters A1 and/or A2 may or may not be provided, depending on the use of rare-earth-doped optical fibers (EDF) 52.sub.1, and 52.sub.2.
More specifically, an input wavelength-multiplexed optical signal is transmitted to optical attenuator 64. The wavelength-multiplexed optical signal output from optical attenuator 64 is transmitted to rare-earth-doped optical fiber 52.sub.1 via optical isolator 55.sub.1 and optical wavelength multiplexing coupler 56.sub.1. The amplified wavelength-multiplexed optical signal is output via optical isolator 55.sub.2 and optical branching coupler 54.sub.2.
A portion of the wavelength-multiplexed optical signal branched by optical branching coupler 54.sub.1 is converted into an electrical signal by photodiode 58.sub.1 and fed to automatic level control circuit 66 and automatic gain control circuit 60.sub.1. Automatic level control circuit 66 controls the optical attenuation provided by optical attenuator 64 so that the wavelength-multiplexed optical signal has its level controlled to be within a predetermined range and is then transmitted to first part 1000.
A portion of the wavelength-multiplexed optical signal branched by optical branching coupler 54.sub.2 is converted into an electrical signal by photodiode 58.sub.2 and transmitted to automatic gain control circuit 60.sub.1. Automatic gain control circuit 60.sub.1 controls pump laser diode 59.sub.1 so that a ratio between a level of the wavelength-multiplexed optical signal input to, and output from, rare-earth-doped optical fiber 52.sub.1 is maintained at a constant level.
Therefore, second part 2000 causes the power level of the wavelength-multiplexed optical signal to be constant even when a signal input via a transmission optical fiber varies greatly. As a result, a wavelength-multiplexed optical signal having a constant level is input to first part 1000. Accordingly, automatic gain control circuit 60.sub.1 may have a small control zone and a relatively simple construction. Further, since the power level of the optical signal input to rare-earth-doped optical fiber 52.sub.1 is prevented from exceeding a predetermined level, it is not necessary to raise the level of the pump laser beam supplied by pump laser diode 59.sub.1. That is, pump laser diode 59.sub.1 may have a small capacity.
FIG. 20 is a diagram illustrating an optical amplifying apparatus, according to an additional embodiment of the present invention. The optical amplifying apparatus illustrated in FIG. 20 is similar to the optical amplifying apparatus in FIG. 19, but also includes optical branching coupler 54.sub.3, photodiode (PD) 58.sub.3 and monitor signal processing circuit 70.
Referring now to FIG. 20, a wavelength-multiplexed optical signal supplied via a transmission optical fiber is input to variable optical attenuator 64 and has a portion branched by optical branching coupler 54.sub.3, converted into an electrical signal by photodiode 58.sub.3 and input to monitor signal processing circuit 70.
Therefore, in FIG. 23, a switch 104 is controlled by monitor signal processing circuit 70 to switch between automatic level control provided by automatic level control circuit 66 and automatic gain control provided by an automatic gain control circuit 60.sub.3. More specifically, for example, as illustrated in FIG. 4(A), monitor signal processing circuit 70 causes switch 104 to select automatic level control circuit 66 prior to, and subsequent to, a variation in the number of channels. While the number of channels is being varied, monitor signal processing circuit 70 causes switch 104 to select automatic gain control circuit 60.sub.3.
(2) When monitor signal processing circuit 70 receives a signal warning of a change in the number of channels, a gain monitoring signal 107 of automatic gain control circuit 60.sub.3 is read so that an average gain (attenuation) with respect to a time constant on the order of 10-100 ms is determined.
(3) A reference voltage V.sub.AGC corresponding to the average gain determined in (2) is output from monitor signal processing circuit 70 to automatic gain control circuit 60.sub.3.
(4) Switch 104 then selects automatic gain control circuit 60.sub.3.
(6) Monitors signal processing circuit 70 provides to automatic level control circuit 66 a reference voltage V.sub.ALC corresponding to the new number of channels.
FIGS. 4(A) and 4(B) are graphs illustrating the operation of the optical amplifying apparatus in FIG.3, wherein the number of channels, N, in an optical signal is changed, according to an embodiment of the present invention.
CROSS-REFERENCE TO RELATED APPLICATIONS This application is based on, and claims priority to, Japanese patent application 08-111447, filed May 2, 1996, in Japan, and which is incorporated herein by reference.
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ClassificationH04B10/2942, H04B10/296, H04B10/291, H04B10/2912, H04B10/2931, H01S3/13A, H04J14/02B, H01S3/067G2Legal EventsDateCodeEventDescriptionJul 13, 2011FPAYFee paymentYear of fee payment: 12Jul 20, 2007FPAYFee paymentYear of fee payment: 8Jul 21, 2003FPAYFee paymentYear of fee payment: 4Oct 31, 1997ASAssignmentOwner name: FUJITSU LIMITED, JAPANFree format text: ASSIGNMENT OF ASSIGNORS INTEREST;ASSIGNORS:SUGAYA, YASUSHI;KINOSHITA, SUSUMU;REEL/FRAME:008832/0082Effective date: 19971027RotateOriginal ImageGoogle Home - Sitemap - USPTO Bulk Downloads - Privacy Policy - Terms of Service - About Google Patents - Send FeedbackData provided by IFI CLAIMS Patent Services©2012 Google