Optical communication system and optical amplifier

Abstract: An optical communication system having a transmitting station for outputting WDM(wavelength-division multiplexing) signal light, an optical fiber transmission line, a receiving station, and an optical repeater including an optical amplifier. The transmitting station includes a supervisory circuit for detecting the number of channels of the WDM signal light and transmitting supervisory information including the number of channels to the optical repeater. The optical repeater further includes a circuit for controlling the optical amplifier so that the output level of the optical amplifier becomes a target level. The target level is set according to the supervisory information. According to the structure, it can be possible to provide a system which can easily respond to a change in the number of WDM channels.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates generally to an optical communication system and an optical amplifier suitable for long-haul and large-capacity transmission, and more particularly to an optical communication system applicable to WDM (wavelength-division multiplexing) and an optical amplifier suitable for WDM. Description of the Related Art In recent years, research and development on application of an optical amplifier to an optical communication system have been intensively pursued. For example, importance of a booster amplifier, optical repeater, and preamplifier having an EDFA (erbium doped fiber amplifier) has become apparent. Conventionally known is an optical amplifier comprising an optical amplifying medium for amplifying signal light and means for pumping the optical amplifying medium so that the optical amplifying medium has an amplification band including the wavelength of the signal light. In the case that the optical amplifying medium is an EDF (erbium doped fiber) having a first end and a second end, the pumping means includes a pump light source for outputting pump light having a proper wavelength, and means for supplying the pump light into the doped fiber from at least one of the first end and the second end. In the case that the optical amplifying medium is provided by a semiconductor chip, the pumping means includes means for injecting a current into the chip. To greatly increase a transmission capacity, a WDM system (wavelength-division multiplexing system) has been proposed. The WDM system includes a first terminal station for outputting WDM signal light (wavelength-division multiplexed signal light) obtained by wavelength-division multiplexing a plurality of optical signals having different wavelengths, an optical transmission line for transmitting the WDM signal light output from the first terminal station, and a second terminal station for receiving the WDM signal light transmitted through the optical transmission line. To increase a transmission distance in the WDM system, one or more optical repeaters each having an optical amplifier are provided in the optical transmission line. In applying the optical amplifier to the WDM system, gain tilt occurring in the optical amplifier must be considered. The gain tilt is based on the wavelength dependence of gain. In an EDFA, for example, the gain tilt changes with a change in total input power because of characteristics of homogenous broadening of an EDF. Accordingly, in operating the WDM system or the optical repeater, it is desirable to grasp the gain tilt of the optical amplifier and maintain a constant gain tilt. In the optical amplifier or the optical repeater, a feedback loop for ALC (automatic level control) is usually adopted, so as to maintain the output level constant. In applying the optical amplifier adopting ALC to the WDM system, a target level in ALC for maintaining output power per channel constant changes with a change in the number of channels of WDM signal light. Accordingly, the conventional optical communication system cannot easily respond to a change in the number of channels. Publication "1.55 UM FLUORIDE-BASED EDFA WITH GAIN-FLATNESS CONTROL FOR MULTIWAVELENGTH APPLICATIONS" by BAYART D ET AL, ELECTRONICS LETTERS, vol. 30, no. 17, 18 August 1994, pages 407-1409, XP000472066, describes gain control of erbium-doped fibre amplifiers by monitoring the power of the backward-propagating amplified spontaneous emission (ASE). Publication "GAIN MONITORING OF ERBIUM-DOPED FIBER AMPLIFIERS BY DETECTING SPONTANEOUS EMISSION" by MASUDA H ET AL, IEEE PHOTONICS TECHNOLOGY LETTERS, vol. 5, no. 9, 1 September 1993, pages 1017-1019, XP000414168, shows gain monitoring of EDFA's by detection of the spontaneous emission (SE) that radiates laterally out of the fibres. SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an optical communication system which can easily respond to a change in the number of channels in WDM. It is another object of the present invention to provide an optical amplifier which can maintain a constant gain tilt. It is a further object of the present invention to provide an optical amplifier which can maintain a constant gain tilt and allows automatic level control. In accordance with an aspect of the present invention, there is provided an optical amplifier comprising an optical amplifying medium having a first end and a second end, the first end receiving signal light; a first means for pumping the optical amplifying medium so that the optical amplifying medium has an amplification band including a wavelength of the signal light; a second means operatively connected to the first end of the optical amplifying medium, for monitoring spectral characteristics of amplified spontaneous emission propagating in a direction opposite to a propagation direction of the signal light in the optical amplifying medium; and a third means for controlling a gain in the amplification band so that the spectral characteristics are maintained. In accordance with another aspect of the present invention, there is provided an optical amplifier comprising an optical amplifying medium having an optical waveguide structure into which signal light is supplied; a means for pumping the optical amplifying medium so that the optical amplifying medium has an amplification band including a wavelength of the signal light; a means for extracting spontaneous emission leaked sideways from the optical waveguide structure; a means for monitoring spectral characteristics of the spontaneous emission; and a means for controlling a gain in the amplification band so that the spectral characteristics are maintained. The above and other objects, features and advantages of the present invention and the manner of realizing them will become more apparent, and the invention itself will best be understood from a study of the following description and appended claims with reference to the attached drawings showing some preferred embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram showing a first basic configuration of an optical amplifier; FIGS. 2A and 2B are graphs for illustrating preferred embodiments for removing the influence of accumulated ASE; FIG. 3 is a block diagram showing a first preferred embodiment of the optical amplifier; FIG. 4 is a graph for illustrating a gain tilt; FIG. 5 is a block diagram of a spectrum monitor; FIG. 6 is a block diagram of another spectrum monitor; FIG. 7A is a block diagram showing a concrete example of the optical amplifier shown in FIG. 12; FIG. 7B is a block diagram showing another concrete example of the optical amplifier shown in FIG. 12; FIG. 8 is a block diagram showing a second preferred embodiment of the optical amplifier; FIG. 9 is a block diagram showing a third preferred embodiment of the optical amplifier; FIG. 10 is a block diagram showing a fourth preferred embodiment of the optical amplifier; FIG. 11 is a block diagram showing a second basic configuration of the optical amplifier; FIG. 12 is a block diagram of a spectrum monitor that can be used in the second basic configuration shown in FIG. 19; and FIG. 13 is a block diagram showing a third basic configuration of the optical amplifier. DESCRIPTION OF THE PREFERRED EMBODIMENTS Some preferred embodiments of the present invention will now be described in detail with reference to the attached drawings. FIG. 1 is a block diagram showing a first basic configuration of the optical amplifier according to the present invention. Like the optical amplifier included in the optical repeater described above, the optical amplifier shown in FIG. 1 has an optical amplifying medium 38 and pumping means 40. When signal light 130 is supplied to a first end 38A of the optical amplifying medium 38 being pumped, amplified signal light 132 is output from a second end 38B of the optical amplifying medium 38. In such a condition that the optical amplifying medium 38 is being pumped so as to have an amplification band, ASE (amplified spontaneous emission) is generated in the optical amplifying medium 38. The ASE is output not only from the second end 38B in the same direction as the propagation direction of the signal light 132, but also from the first end 38A in the direction opposite to the propagation direction of the signal light 132 as shown by 134. The ASE 134 propagating opposite to the signal light 132 is extracted by ASE extracting means 136. According to the extracted ASE 134, monitoring means 138 monitors spectral characteristics giving the wavelength dependence of the power of the ASE 134. Parameter control means 140 controls a parameter on which the gain tilt in the amplification band of the optical amplifying medium 38 is dependent (or the gain itself) so that the spectral characteristics monitored above are maintained. As the optical amplifying medium 38, a doped fiber doped with a rare earth element, such as an EDF, may be used. Alternatively, a semiconductor chip may be used (semiconductor optical amplifier). In the latter case, the pumping means 40 includes means for injecting a current into the chip. Specifically, a pumping voltage is applied across a pair of electrodes of the semiconductor optical amplifier. The pumping means 40 suitable for the doped fiber includes a pump light source for outputting pump light, and optical coupling means operatively connected to at least one of the first end 38A and the second end 38B of the optical amplifying medium 38 to supply the pump light to the optical amplifying medium 38. In this specification, the wording that optical components are operatively connected to each other includes the case that the optical components are directly connected together by fiber connection or spatial connection using a collimated beam, and further includes the case that the optical components are connected through another optical component such as an optical filter. In the case that the pumping means 40 includes the pump light source, the power of the pump light may be adopted as the parameter to be controlled by the parameter control means 140. In this case, the pump light source cannot be included in a feedback loop of ALC for making constant the power of the amplified signal light 132 (total gain of the optical amplifier). Therefore, in performing the ALC, a feedback loop including an optical attenuator having a variable attenuation factor may be provided. In the case that this optical amplifier includes a compensation light source 142 for supplying to the optical amplifying medium 38 compensation light having a wavelength included in the amplification band of the optical amplifying medium 38, the parameter to be controlled by the parameter control means 140 may be the power of the compensation light. In this case, the pump light source can be included in the feedback loop for the ALC. The wavelength of the compensation light is set different from the wavelength of the signal light. In the case that this optical amplifier is applied to a WDM system, WDM signal light is supplied into the optical amplifying medium 38 from the first end 38A. Gain characteristics of the optical amplifying medium 38, i.e., the gain tilt, are reflected on the spectral characteristics of the ASE 134. Since the ASE 134 propagates opposite to the signal light in the optical amplifying medium 38, the spectral characteristics of the ASE 134 are not influenced by the number of channels of WDM signal light, input level, and accumulated ASE in principle. Accordingly, by controlling the parameter on which the gain tilt depends so that the spectral characteristics of the ASE 134 are maintained, a constant gain tilt can be easily obtained. Specific embodiments of a monitoring method for the spectral characteristics will be hereinafter described. Preferably, the first basic configuration of the optical amplifier shown in FIG. 1 has an optical band-pass filter 143 operatively connected to the second end 38B of the optical amplifying medium 38. The effectiveness of the filter 143 will now be described. The spectral characteristics of forward ASE propagating in the same direction as the propagation direction of the signal light in the optical amplifying medium 38 are influenced by the input level of the signal light and accumulated ASE. To the contrary, the spectral characteristics of backward ASE propagating in the direction opposite to the propagation direction of the signal light in the optical amplifying medium 38 are not influenced by these factors in principle. However, in actual, if there is any little reflection on the output side of the optical amplifying medium 38, there is a possibility that accumulated ASE may be reflected, and this reflected accumulated ASE may then be amplified in the optical amplifying medium 38 to mix into the backward ASE. Accordingly, in the case that such mixing of the accumulated ASE into the backward ASE becomes a problem, the optical band-pass filter 143 having a proper pass band is used. Referring to FIG. 2A, there is shown a preferable pass band of the optical band-pass filter 143. The shortest wavelength λ L in the pass band is set slightly shorter than the shortest wavelength of the WDM signal light, and the longest wavelength λ H in the pass band is set slightly longer than the longest wavelength of the WDM signal light. With this setting, the power of accumulated ASE can be effectively reduced. Preferably, the monitoring means 138 shown in FIG. 1 has two optical band-pass filters having different pass bands (e.g., optical band-pass filters 170 and 172 of a spectrum monitor shown in FIG. 5). In this case, as shown in FIG. 2B, the shortest wavelength and the longest wavelength in the pass band of one of the two filters are set to λ L - Δλ and λ L , respectively, wheres the shortest wavelength and the longest wavelength in the pass band of the other filter are set to λ H and λ H + Δλ, respectively. With this setting, even if a reflected component of the accumulated ASE is mixed into the backward ASE, no influence of such mixing appears to a result of monitoring. FIG. 3 is a block diagram showirg a first preferred embodiment of the optical amplifier according to the present invention. Signal light to be amplified is supplied through an optical coupler 144 into an optical amplifying medium 38 from its first end 38A. The ASE propagating in the direction opposite to the signal light in the optical amplifying medium 38 is extracted by the optical coupler 144. The extracted ASE is supplied to a spectrum monitor 146. As the optical coupler 144, a fiber fused type of optical coupler, a WDM coupler that is a special form of this type of optical coupler, or an optical circulator may be used. A laser diode 148 as a pump light source is used to pump the optical amplifying medium (e.g., doped fiber) 38. Pump light output from the laser diode 148 is supplied to the optical amplifying medium 38 through an optical coupler 150 connected to a second end 38B of the optical amplifying medium 38. The laser diode 148 is supplied with a bias current from a drive circuit 152. The power of the pump light can be controlled according to the bias current. The ASE spectral characteristics monitored by the spectrum monitor 146 is supplied to a control circuit 154. The control circuit 154 controls the bias current to be supplied from the drive circuit 152 to the laser diode 148 so that the spectral characteristics from the spectrum monitor 146 are maintained. In this preferred embodiment, the bias current for the laser diode 148 for outputting the pump light is included in a feedback loop for maintaining the gain tilt. Accordingly, ALC cannot be performed by using the bias current for the laser diode 148. To perform ALC, the amplified signal light output from the second end 38B of the optical amplifying medium 38 through the optical coupler 150 is input into an optical attenuator 156. The attenuation factor of the optical attenuator 156 is variable. The light output from the optical attenuator 156 is branched into first branch light and second branch light by an optical coupler 158. The first branch light from the optical coupler 158 is output to an optical transmission line (not shown). The second branch light from the optical coupler 158 is supplied to an optical band-pass filter 160 having a pass band including the wavelength of the signal light. Output light from the filter 160 is converted into an electrical signal by an O/E converter 162. An ALC circuit 164 controls the attenuation factor of the optical attenuator 156 so that the output level of the O/E converter 162 becomes constant. FIG. 4 is a graph for illustrating an example of the gain tilt in the optical amplifier shown in FIG. 3. There are shown in FIG. 4 the spectra of output beams when WDM signal beams each having four channels of wavelengths of 1548 nm, 1551 nm, 1554 nm, and 1557 nm are input with the same input power (-35 dBm/ch) into an EDF being pumped. In FIG. 4, the vertical axis represents output power (dBm) and the horizontal axis represents wavelength (nm). The spectrum shown by A corresponds to the case where the power of pump light is relatively large. In this case, a negative gain tilt occurs. That is, the differential of gain with respect to wavelength is negative (dG/dλ < 0). The spectrum shown by C corresponds to the case where the power of pump light is relatively small. In this case, a positive gain tilt is obtained (dG/dλ > 0). The spectrum shown by B corresponds to the case where the power of pump light is optimum such that no gain tilt occurs. In this case, the differential of gain with respect to wavelength is 0 (dG/dλ = 0). Each spectrum shown in FIG. 4 has such a shape that four sharp spectra corresponding to the four channels of each WDM signal light are superimposed on a spectrum of ASE. In the optical amplifier shown in FIG. 3, the ASE output from the first end 38A of the optical amplifying medium 38 is extracted, so that the spectrum of the WDM signal light is not superimposed on the spectrum of the ASE. Accordingly, the spectrum monitor 146 can monitor the ASE spectrum with a high accuracy without the influence of the power of the WDM signal light. Such ASE propagating in the direction opposite to signal light will be hereinafter referred to as backward ASE. Letting P ASE (λ) denote the power of backward ASE, it is given by Eq. (1), which is a function of wavelength λ. P ASE (λ 0 ) = 2n SP (λ 0 )h(C/λ 0 )[G(λ 0 ) - 1]Δλ where n SP (λ 0 ) is the spontaneous emission factor, h is the Planck constant, C is the velocity of light in a vacuum, λ 0 is the center wavelength in a band to be monitored, and Δλ is the bandwidth of the band to be monitored. Usually, the wavelength dependence of each parameter is substantially constant in the range of Δλ, so that λ 0 is used as a representative. The spontaneous emission factor n SP (λ 0 ) has wavelength dependence, and a method of improving a monitoring accuracy so as to cope with this wavelength dependence will be hereinafter described. In Eq. (1), G(λ 0 ) represents the gain to be given as a function of wavelength. In this manner, gain characteristics (wavelength dependence of gain) are reflected in the spectrum of backward ASE. Accordingly, gain characteristics can be evaluated by cutting out two or more narrow bands included in an amplification band, individually detecting the powers in these narrow bands, and obtaining a deviation between detected values. Specifically, the spectrum monitor 146 shown in FIG. 3 includes means for branching backward ASE into first branch light and second branch light, a first optical band-pass filter having a first narrow pass band included in an amplification band, for receiving the first branch light, a second optical band-pass filter having a second narrow pass band included in the amplification band but different from the first pass band, for receiving the second branch light, first and second photodetectors for respectively receiving lights passed through the first and second optical band-pass filters, and means for detecting a deviation between output levels of the first and second photodetectors. This configuration will now be described more specifically. FIG. 5 is a block diagram showing a preferred embodiment of the spectrum monitor 146 shown in FIG. 3. The backward ASE generated from the optical amplifying medium 38 (see FIG. 3) is supplied through an optical isolator 166 to an optical coupler 168. If reflection from a backward ASE monitoring system is low, the optical isolator 166 is unnecessary. The optical coupler 168 branches the input backward ASE into first branch beam and second branch beam. A branching ratio between the first branch beam and the second branch beam is set to 1:1, for example. The first and second branch beams are respectively supplied to optical band-pass filters 170 and 172. In the case that the backward ASE has a spectrum similar to the ASE spectrum as shown in FIG. 4, the center wavelengths in the pass bands of the filters 170 and 172 are respectively set to 1541 nm and 1559 nm, for example. The beams passed through the filters 170 and 172 are respectively supplied to photodiodes 174 and 176. Since output signals from the photodiodes 174 and 176 are current signals, I/V converters (current/voltage converters) 178 and 180 respectively corresponding to the photodiodes 174 and 176 are used. Output voltage signals from the I/V converters 178 and 180 are respectively supplied to a minus input port and a plus input port of an operational amplifier 182. As a result, an output signal from the operational amplifier 182 reflects a deviation between the output levels of the photodiodes 174 and 176. Accordingly, by feeding back the output signal from the operational amplifier 182 to the bias current for the laser diode 148 (see FIG. 3), the spectral characteristics of the backward ASE generated in the optical amplifying medium 38 can be maintained, so that the gain tilt can be maintained constant. By suitably setting a target value of the deviation in the feedback loop, the gain tilt can be made flat as shown by B in FIG. 4, for example. Referring to FIG. 6, there is shown another spectrum monitor applicable to the present invention. In this preferred embodiment, output signals from I/V converters 178 and 180 are taken through an I/O port 184 into an MPU (microprocessing unit) 186. The MPU 186 is connected through the I/O port 184 to a memory 188. The MPU 186 receives the output levels of the converters 178 and 180, calculates a deviation between the output levels, and outputs the calculated deviation through the I/O port 184. As described above, the spontaneous emission factor n SP (λ 0 ) in Eq. (1) has wavelength dependence, that is, depends on the wavelength λ 0 to be monitored. Accordingly, in the case that higher monitoring accuracies are required, a data table of the spontaneous emission factor n SP (λ 0 ) using wavelength as a parameter may be previously stored in the memory 188 to obtain accurate spectral characteristics according to the data table. For example, the gain G(λ) can be accurately calculated according to a calculated value of the deviation. In the spectrum monitor shown in FIG. 5 or FIG. 6, the two narrow bands (first and second pass bands) are cut out from the amplification band. However, three or more optical band-pass filters may be used to cut out three or more narrow bands corresponding to the number of the optical band-pass filters from the amplification band. In this case, the accuracy of monitoring of the backward ASE to be calculated by the MPU 186 can be improved, for example. FIG. 7A is a block diagram showing a concrete example of the optical amplifier shown in FIG. 3. In this example, an EDF(erbium doped fiber) 39 is used as the optical amplifying medium 38. In order to prevent a resonance in the EDF 39, an optical isolator 145A is provided on the input side of the optical coupler 144, and another optical isolator 145B is provided between the optical coupler 150 and the optical attenuator 156. The pump light from the laser diode 148 propagates in the direction opposite to the signal light in the EDF 39. That is, backward pumping is adopted. Alternatively, forward pumping may be adopted such that the pumping light propagates in the same direction as the signal light. Further, pump light beams may be supplied into the EDF 39 from both ends thereof, thereby improving a pumping efficiency. FIG. 7B is a block diagram showing an optical amplifier to which the forward pumping is adopted. In place of the optical coupler 150 (shown in FIG. 7A), an optical coupler 150' is provided on the input side of the optical isolator 145A. The pump light from the laser diode 148 end the signal light to be amplified are supplied to the EDF 39 through the optical coupler 150', the optical isolator 145A, and the optical coupler 144. FIG. 8 is a block diagram showing a second preferred embodiment of the optical amplifier according to the present invention. In contrast with the optical amplifier shown in FIG. 3, the optical amplifier shown in FIG. 8 is characterized in that an optical attenuator 156' for ALC is provided upstream of the optical amplifying medium 38. That is, signal light to be input into the optical amplifying medium 38 from its first end 38A is preliminarily attenuated rather than attenuating amplified signal light. The attenuation factor of the optical attenuator 156' is controlled by an ALC circuit 164 so that an output level of an O/E converter 162 corresponding to an output level of this optical amplifier becomes constant. According to the optical amplifier shown in FIG. 3 or FIG. 8, a constant gain tilt can be maintained, and ALC can also be performed. FIG. 9 is a block diagram showing a third preferred embodiment of the optical amplifier according to the present invention. This optical amplifier employs a compensation light source for supplying compensation light to the optical amplifying medium 38. The power of the compensation light is controlled so that the spectral characteristics of backward ASE are maintained. With this change, the power of pump light is subjected to ALC. A laser diode 190 is used as the compensation light source. The compensation light from the laser diode 190 is supplied through an optical coupler 192 to the optical amplifying medium 38 from its first end 38A. Signal light to be amplified is supplied through an optical coupler 144 for extracting the backward ASE and the optical coupler 192 for the compensation light in this order to the optical amplifying medium 38 from its first end 38A. The backward ASE generated in the optical amplifying medium 38 is supplied through the optical coupler 192 and the optical coupler 144 in this order to a spectrum monitor 146. The laser diode 190 is supplied with a bias current from a drive circuit 194. The bias current to be supplied to the laser diode 190 is controlled by a control circuit 154. The control circuit 154 controls the bias current for the laser diode 190 so that the spectral characteristics of the backward ASE monitored by the spectrum monitor 146 are maintained. Accordingly, the power of the compensation light to be output from the laser diode 190 is controlled to maintain constant the gain characteristics of this optical amplifier. In this preferred embodiment, the power of pump light is not used in the control for maintaining the gain characteristics constant. Accordingly, a pump light source can be included in the feedback loop for ALC. Since the compensation light is used for maintenance of the spectral characteristics, the compensation light is removed by an optical filter 160, and output light from the optical filter 160 is converted into an electrical signal by an O/E converter 162. A laser diode 148 as the pump light source is supplied with a bias current from a drive circuit 152. The bias current is controlled by an ALC circuit 164. Also according to the third preferred embodiment, a constant gain tilt can be maintained, and ALC can be performed. Further, an optical attenuator for ALC is unnecessary in the third preferred embodiment. FIG. 10 is a block diagram showing a fourth preferred embodiment of the optical amplifier according to the present invention. In this preferred embodiment, reflecting mirrors 196 and 198 are operatively connected to the second end 38B of the optical amplifying medium 38, so as to improve the accuracy of monitoring of the spectral characteristics of backward ASE in the spectrum monitor 146. In the case that the spectrum monitor 146 is configured as shown in FIG. 5, the reflecting mirror 196 reflects light having a wavelength included in the pass band of the optical band-pass filter 170 and transmits other light, and the reflecting mirror 198 reflects light having a wavelength included in the pass band of the optical band-pass filter 172 and transmits other light. By providing the reflecting mirrors 196 and 198, a part of the forward ASE output from the second end 38B of the optical amplifying medium 28 falling within a specific band can be reciprocated in the optical amplifying medium 38. Accordingly, the input powers into the photodiodes 174 and 176 shown in FIG. 5 can be increased to thereby improve the spectrum monitoring accuracy. In the case that three or more narrow bands are cut out from the ASE spectrum, three or more reflecting mirrors corresponding to the number of the narrow bands are used. In a doped fiber such as an EDF, SE (spontaneous emission) leaks sideways. The gain characteristics in the doped fiber are reflected in the SE. Further, the SE leaking sideways is not influenced by the number of channels of WDM signal light, input level, and accumulated ASE. This fact has been reported by Aida et al. in the International Conference, 1991 (Optical Amplifiers and their Applications; OAA, FE3), in which it has become apparent that gain G(λ) can be obtained from an integral P SE (λ) of SE from a side surface of a doped fiber over the fiber length L in accordance with Eqs. (2) and (3). P SE (λ)={ln[G(λ)] + α s (λ)L}/C(λ) C(λ)=η(λ){σ e (λ) + σ a (λ)}τ/{h(C/λ)π(γ Er ) 2 } where σ e (λ), σ a (λ), and α s (λ) are the emission cross section at λ, the absorption cross section at λ, and the loss at λ, respectively, and τ and γ Er are the spontaneous emission lifetime and the radius of a region doped with Er, respectively. Further, η(λ) is the coefficient having wavelength dependence. Accordingly, by monitoring the spectral characteristics of SE leaking sideways, gain characteristics (gain tilt) can be grasped. FIG. 11 is a block diagram showing a second basic configuration of the optical amplifier according to the present invention. An optical amplifying medium 38 has an optical waveguide structure to which signal light is supplied. Pumping means 40 pumps the optical amplifying medium 38 so that the optical amplifying medium 38 has an amplification band including the wavelength of the signal light. SE extracting means 200 extracts SE light leaking sideways from the optical waveguide structure of the optical amplifying medium 38. Monitoring means 138 monitors spectral characteristics giving the wavelength dependence of the power of the extracted SE. Parameter control means 140 controls a parameter on which the gain tilt in the amplification band of the optical amplifying medium 38 depends (or gain itself) so that the spectral characteristics monitored are maintained. In the configuration shown in FIG. 11, the parameter to be controlled by the parameter control means 140 is the power of pump light in the pumping means 40. Alternatively, the parameter to be controlled by the parameter control means 140 may be the power of compensation light like in FIG. 1. FIG. 12 is a block diagram of a spectrum monitor that can be used as the SE extracting means 200 and the monitoring means 138 shown in FIG. 11. An EDF 202 is used as the optical amplifying medium 38 (see FIG. 11). Means for pumping the EDF 202 is not shown. The EDF 202 is accommodated in a case 204 such as an integrating sphere configured so as to block entering of eternal light. A cover of the EDF 202 is partially removed, and SE leaks sideways from an uncovered portion of the EDF 202. The SE is supplied to optical band-pass filters 206 and 208. The filters 206 and 208 have their pass bands similar to the pass bands of the optical band-pass filters 170 and 172 shown in FIG. 16, respectively. The beams passed through the filters 206 and 208 are converted into current signals by photodiodes 210 and 212, respectively. The current signals from the photodiodes 210 and 212 are next converted into voltage signals by I/V converters 214 and 216, respectively. The voltage signals from the I/V converters 214 and 216 are supplied to an operational amplifier 218. As described above, the gain characteristics of the optical amplifier are reflected on the SE spectrum. Accordingly, by controlling the power of pump light according to the SE spectral characteristics monitored, the gain tilt of the optical amplifier can be maintained constant. The spectrum monitor shown in FIG. 12 may be modified according to the configuration shown in FIG. 6. That is, an MPU is used for monitoring of the spectral characteristics. In this case, by previously storing C(λ ), α s , and L in Eqs. (2) and (3) in a memory, the monitoring accuracy of the spectral characteristics can be improved. Further, three or more narrow bands may be cut out from the SE spectrum, and optical power in each band may be detected to perform various controls according to the result of detection. FIG. 13 is a block diagram showing a third basic configuration of the optical amplifier according to the present invention. This optical amplifier is configured by cascading a first optical amplifier unit 220 and a second optical amplifier unit 222. Each of the optical amplifier units 220 and 222 has the first basic configuration shown in FIG. 1. Light amplified in the first optical amplifier unit 220 is attenuated by an optical attenuator 224 having a variable attenuation factor, and next transmitted through a dispersion compensating fiber (DCF) 226 to the second optical amplifier unit 222. The DCF 226 has a dispersion value so as to cancel chromatic dispersion influenced the signal light in a transmission line. The light output from the second optical amplifier unit 222 is branched into first branch light and second branch light by an optical coupler 228. The first branch light from the optical coupler 228 is output to an optical transmission line (not shown). The second branch light from the optical coupler 228 is converted into an electrical signal by an O/E converter 230. An ALC circuit 232 controls the attenuation factor of the optical attenuator 224 so that the output level of the O/E converter 230 is maintained constant. Such a two-stage configuration of the optical amplifier in this preferred embodiment is due to the following reasons. The first reason is that a loss in a DCF is large in general, and it is therefore necessary to raise the level of the signal light to some extent on the upstream side of the DCF 226. The second reason is that if an optical amplifier gain on the upstream side of the DCF 226 is made excess to increase the power of the signal light, nonlinear effects are prone to occur in the DCF 226. If four-wave mixing (FWM) as one of the nonlinear effects occurs in the DCF 226 in a system employing WDM, interchannel crosstalk is increased. Further, self-phase modulation (SPM) also invites a deterioration in signal quality. According to the third basic configuration, a constant gain tilt can be maintained, and ALC can also performed. As described above, according to an aspect of the present invention, it is possible to provide an optical communication system which can easily respond to a change in the number of WDM channels. According to another aspect of the present invention, it is possible to provide an optical amplifier which can maintain a constant gain tilt. According to a further aspect of the present invention, it is possible to provide an optical amplifier which can maintain a constant gain tilt and allows automatic level control. The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.

Claims: An optical amplifier comprising: an optical amplifying medium (38) having a first end (38A) and a second end (38B), and amplification band including a wavelength of signal light (130), said first end (38A) being adapted for receiving said signal light (130), characterised in that it further comprises a monitor (138) operatively connected to said first end (38A) of said optical amplifying medium (38), for monitoring spectral characteristics of amplified spontaneous emission propagating in a direction opposite to a propagation direction of said signal light (130) in said optical amplifying medium (38); and a controller (140) for controlling a gain in said amplification band so that said spectral characteristics are maintained. An optical amplifier according to claim 1, further comprising a pump light source (40) for outputting pump light, and a first optical coupler (150; 144) operatively connected to at least one of said first end (38A) and said second end (38B) of said optical amplifying medium (38), for supplying said pump light to said optical amplifying medium (38), wherein said gain is controlled by power of said pump light. An optical amplifier according to claim 2, further comprising: a second optical coupler (158) for branching amplified signal light output from said second end (38B) of said optical amplifying medium (38) into first branch light and second branch light; a photodetector for receiving said first branch light; and an automatic level control means (164) for controlling a total gain of said optical amplifier so that an output level of said photodetector becomes constant. An optical amplifier according to claim 3, wherein: said automatic level control means (164) includes an optical attenuator (156) having a variable attenuation factor; said optical attenuator (156) being operatively connected to said second end (38B) of said optical amplifying medium (38) and said second optical coupler (158), for attenuating said amplified signal light output from said optical amplifying medium (38). An optical amplifier according to claim 3, wherein: said automatic level control means (164) includes an optical attenuator having a variable attenuation factor; said optical attenuator (156) being operatively connected to said first end (38A) of said optical amplifying medium (38), for attenuating said signal light to be supplied to said optical amplifying medium (38). An optical amplifier according.to claim 2, wherein: said optical amplifying medium (38) comprises a doped fiber doped with a rare earth element. An optical amplifier according to claim 1, further comprising: a first light source (142) operatively connected to said first'end (38A) of said optical amplifying medium (38), for supplying to said optical amplifying medium (38) compensation light having a wavelength different from the wavelength of said signal light; said gain is controlled by power of said compensation light. An optical amplifier according to claim 7, further comprising: a second light source (40) for outputting pump light; a first optical coupler (150; 144) operatively connected to at least one of said first end (38A) and said second end (38B) of said optical amplifying medium (38), for supplying said pump light to said optical amplifying medium (38); a second optical coupler (158) for branching amplified signal light output from said second end (38B) of said optical amplifying medium (38) into first branch light and second branch light; a photodetector for receiving said first branch light; and a means (164) for controlling power of said pump light so that an output level of said photodetector becomes constant. An optical amplifier according to claim 1, wherein: said monitor (138) comprises: a means (168) for branching said amplified spontaneous emission into first branch light and second branch light; a first optical band-pass filter (170) for receiving said first branch light, said first optical band-pass filter (170) having a first pass band included in said amplification band; a second optical band-pass filter (172) for receiving said second branch light, said second optical band-pass filter (172) having a second pass band included in said amplification band but different from said first pass band; first and second photodetectors (174; 176) for receiving lightwaves passed through said first and second optical band-pass filters, respectively; and a means for detecting a deviation between output levels of said first and second photodetectors. An optical amplifier according to claim 9, further comprising: a third optical band-pass filter operatively connected to said second end (38B) of said optical amplifying. medium (38); said third optical band-pass filter having a third pass band including the wavelength of said signal light but not including said first and second pass bands. An optical amplifier according to claim 9, further comprising: first and second reflecting means (196; 198) operatively connected to said second end (38B) of said optical amplifying medium (38); said first and second reflecting means (196; 198) reflecting lightwaves having wavelengths included in said first and second pass bands, respectively, to supply said lightwaves reflected to said optical amplifying medium (38) from said second end (38B). An optical amplifier according to claim 1, further comprising: an optical band-pass filter (143) operatively connected to said second end (38B) of said optical amplifying medium (38); said optical band-pass filter (143) having a pass band including the wavelength of said signal light. An optical amplifier according to claim 1, wherein: said signal light comprises a plurality of wavelength-division multiplexed optical signals. An optical amplifier comprising: first and second optical amplifier units (220; 222); and a means (226) for cascading said first and second optical amplifier units (220; 222); each of said first and second optical amplifier units (220; 222) comprising an optical amplifier according to claim 1; said optical amplifier further comprising: an optical attenuator (224) having a variable attenuation factor, for attenuating amplified signal light output from said first optical amplifier unit (220); a means (228) for branching amplified signal light output from said second optical amplifier unit (222) into first branch light and second branch light; a photodetector for receiving said first branch light; and a means (232) for controlling said attenuation factor of said optical attenuator (224) so that an output level of said photodetector becomes constant. An optical amplifier according to claim 14, wherein: said cascading means (226) comprises a dispersion compensating fiber having a preset dispersion value so as to cancel wavelength dispersion influenced said signal light. An optical amplifier comprising: an optical amplifying medium (38) having an optical waveguide structure into which signal light can be supplied; a means (40) for pumping said optical amplifying medium (38) so that said optical amplifying medium (38) has an amplification band including a wavelength of said signal light; a means (200) for extracting spontaneous emission leaked sideways from said optical waveguide structure; characterised in that it further comprises a means (138) for monitoring spectral characteristics of said spontaneous emission; and a means (140) for controlling a gain in said amplification band so that said spectral characteristics are maintained.

Inventors: KINOSHITA SUSUMU, KINOSHITA, SUSUMU, Kinoshita, Susumu, c/o Fujitsu Ltd.
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1996
Main classification: H04J14
IPC codes: H01S3, H04B10, H04J14