JP3917605B2 - Optical communication system and optical amplifier - Google Patents

Description

  The present invention generally relates to an optical communication system and an optical amplifier suitable for long-distance large-capacity transmission, and more particularly to an optical communication system applied to WDM (wavelength division multiplexing) and an optical amplifier suitable for WDM.

  In recent years, research and development relating to the application of optical amplifiers to optical communication systems have been vigorously advanced. For example, the importance of booster amplifiers, optical repeaters and preamplifiers with EDFAs (erbium-doped fiber amplifiers) has become apparent.

  There is known an optical amplifier including 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.

  When 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 that outputs pump light of an appropriate wavelength, and the pump light is transmitted to the first end and the second end. And means for supplying the doped fiber from at least one of the above.

  If the optical amplification medium is provided by a semiconductor chip, the pumping means includes means for injecting current into the chip.

  In order to dramatically increase the transmission capacity, a WDM system (wavelength division multiplexing system) has been proposed. The WDM system includes a first terminal that sends out a WDM signal light obtained by wavelength division multiplexing a plurality of signal lights having different wavelengths, an optical transmission line that transmits the sent WDM signal light, and a transmitted WDM signal A second terminal for receiving light. In order to increase the transmission distance in the WDM system, one or more optical repeaters having optical amplifiers are provided in the middle of the optical transmission path.

  When applying an optical amplifier to a WDM system, a consideration is the gain tilt found in the optical amplifier. The gain tilt is based on the wavelength dependence of gain.

  For example, in an EDFA, the gain tilt changes due to a change in total input power due to the characteristic of uniform broadening of the EDF. Therefore, when operating a WDM system or an optical repeater, it is desirable to keep track of the gain tilt of the optical amplifier so that a constant gain tilt is maintained.

  On the other hand, in an optical amplifier or an optical repeater, a feedback loop for normal ALC (automatic level control) is employed so that the output level is kept constant.

  When an optical amplifier employing ALC is applied to a WDM system, the target level of ALC for keeping the output power per channel constant changes in accordance with the increase or decrease in the number of channels of wavelength division multiplexed signal light. As a result, there is a problem that it is difficult to easily cope with an increase or decrease in the number of channels.

  Therefore, an object of the present invention is to provide an optical communication system that can easily cope with an increase or decrease in the number of channels in WDM.

  Another object of the present invention is to provide an optical amplifier capable of maintaining a constant gain tilt.

  Still another object of the present invention is to provide an optical amplifier capable of maintaining a constant gain tilt and capable of automatic level control.

According to the present invention, a first terminal station including a plurality of optical transmitters each outputting signal light and means for wavelength division multiplexing the signal light, and the wavelength division multiplexed signal connected to the first terminal station. An optical transmission line for transmitting the signal light, a second terminal connected to the optical transmission line for receiving the wavelength division multiplexed signal light, and provided in the middle of the optical transmission line for the wavelength division multiplexing. An optical repeater including an optical amplifier that amplifies and outputs the signal light, and the signal light output from each of the plurality of optical transmitters in operation is modulated by the main signal and is in operation The signal light output from each of the optical amplifiers is a continuous wave, and the optical repeater includes means for detecting the output level of the optical amplifier and the optical amplifier so that the detected output level matches the target level. Means for controlling the target level. Optical communication system but wherein the to be constant irrespective of the number of light transmitters being the operational power of the continuous wave, each of the optical transmitter that is not the operation is output is controlled Is provided.

  As described above, according to an aspect of the present invention, there is an effect that it is possible to provide an optical communication system that can easily cope with an increase or decrease in the number of channels in WDM.

  According to another aspect of the present invention, there is an effect that it is possible to provide an optical amplifier capable of maintaining a constant gain tilt.

  According to still another aspect of the present invention, it is possible to provide an optical amplifier capable of maintaining a constant gain tilt and capable of automatic level control.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a block diagram of a WDM system to which the present invention can be applied. This system includes a transmitting station 2 that outputs WDM signal light, an optical fiber transmission line 4 that transmits the WDM signal light, and a receiving station 6 that receives the transmitted WDM signal light.

  The transmitting station 2 has a plurality of optical transmitters 8 (# 1 to # 5). Each optical transmitter 8 has a terminal 10 for receiving a drive signal. Each optical transmitter modulates CW light (continuous wave light) from the laser diode, for example, by direct modulation of the laser diode or the like, and outputs signal light having a predetermined wavelength.

  The status signal 12 from the optical transmitter 8 (# 1 to # 4) is supplied to the SV circuit (monitoring circuit) 14. The status signal includes a flag indicating whether each optical transmitter 8 is operating. The SV circuit 14 outputs monitoring information including the number of optical transmitters 8 in operation, that is, the number of channels of WDM signal light.

  The monitoring information from the SV circuit 14 is input to the terminal 10 of the optical transmitter 8 (# 5), and the signal light having the wavelength λSV modulated by the monitoring information is output from the optical transmitter 8 (# 5). The optical transmitters 8 (# 1 to # 4) output signal lights having wavelengths λ1 to λ4 modulated by transmission data (main signals) of the respective channels.

  The signal light from each optical transmitter 8 is added by a multiplexer (MUX) 16 and sent out to the optical fiber transmission line 4.

  Two optical repeaters 18 are provided in the middle of the optical fiber transmission line 4. More optical repeaters 18 may be provided, or one optical repeater 18 may be provided.

  Each optical repeater 18 includes an optical amplifier 20 that amplifies and outputs WDM signal light, and an SV circuit (monitoring circuit) 22 that exchanges monitoring information transmitted from the transmission station 2 with the optical amplifier 20. Yes. The optical repeater 18 further includes an optical coupler 24 for bypassing the signal light having the wavelength λSV modulated by the monitoring information and supplying the signal light to the SV circuit 22, and an update output from the SV circuit 22. The transmitted signal light (wavelength λSV) is joined to the optical fiber transmission line 4 by the optical coupler 26.

  The receiving station 6 has a demultiplexer (DEMUX) 28 for branching the WDM signal light transmitted through the optical fiber transmission line 4 and an optical receiver for demodulating transmission data of each channel based on the branched WDM signal light. Machine 30 (# 1 to # 4). Transmission data demodulated by each optical receiver 30 is output from a terminal 32.

  FIG. 2 is a block diagram showing a first embodiment of the optical repeater. This optical repeater can be used as the optical repeater 18 of FIG. Throughout the drawings, substantially the same parts are denoted by the same reference numerals.

  The signal light of wavelength λSV branched from the optical fiber transmission line 4 by the optical coupler 24 is modulated by the monitoring information. This signal light is input to an O / E converter (optical / electrical converter) 34 built in the SV circuit 22, and monitoring information is reproduced based on an output signal of the O / E converter 34.

  The WDM signal light having the wavelength λj (j = 1 to 4) that has passed through the optical coupler 24 is bifurcated by the optical coupler 36, and one branched light is supplied to the optical amplification medium 38.

  The optical amplifying medium 38 is pumped by the pumping means 40, so that the optical amplifying medium 38 has an amplification band including the wavelength of the WDM signal light.

  As the optical amplifying medium 38, a doped fiber doped with a rare earth element, such as EDF, can be used. In this case, the pumping means 40 includes a pump light source that is operatively connected to at least one of the first end and the second end of the doped fiber and supplies pump light to the doped fiber.

  A semiconductor chip can also be used as the optical amplification medium 38 (semiconductor optical amplifier). In this case, the pumping means 40 includes means for injecting a current by applying a pumping voltage to the electrode pair of the chip.

  In the following description, the optical amplifying medium 38 is a doped fiber, and the pumping means 40 includes a pump light source.

  The signal light amplified by the optical amplifying medium 38 is branched into two by the optical coupler 42, and one branched light is transmitted to the downstream optical fiber transmission line 4 through the optical coupler 26.

  The other branched light branched by the optical coupler 42 is supplied to the optical bandpass filter 44. The filter 44 has a pass band including the wavelength of the WDM signal light.

  The light that has passed through the filter 44 is supplied to the O / E converter 46, and the output signal is supplied to the ALC circuit (automatic level control circuit) 48 and the SV circuit 22.

  The ALC circuit 48 controls the pumping means 40 so that the output level of the O / E converter 46 becomes constant. Specifically, when the pump light source included in the pumping means 40 is a laser diode, its drive current (bias current) is controlled.

  Of the monitoring information reproduced in the SV circuit 22, the signal SC giving the number of channels is supplied from the SV circuit 22 to the ALC circuit 48.

  Since the ALC circuit 48 performs the feedback control described above, the output level of the optical amplifier is stabilized so as to coincide with the target level. This target level should be set so that the output power per channel is constant.

  Therefore, the target level should be set according to the number of channels of WDM signal light.

  In this embodiment, the target level is set based on the signal SC that gives the number of channels. Specifically, it is as follows.

  FIG. 3 is a block diagram showing a specific example of the ALC circuit 48 of FIG. The ALC circuit 48 has an operational amplifier 56 for comparing the output signal (output voltage) of the O / E converter 46 with the reference voltage VREF.

  The output voltage of the O / E converter 46 is supplied to the negative input port of the operational amplifier 56, and the reference voltage VREF is supplied to the positive input port. The operational amplifier 56 outputs a voltage signal corresponding to the level difference between both input ports. This voltage signal is converted into a current signal by the V / I converter 58 and fed back to, for example, a driving current of the pump light source in the pumping means 40 (see FIG. 2).

  In order to set the reference voltage VREF according to the number of channels, an MPU (microprocessing unit) 60 is used. The signal SC giving the number of channels is taken into the MPU 60 through the I / O circuit 62, and the reference voltage VREF obtained based on the calculation result here is supplied to the operational amplifier 56 through the I / O circuit 62. .

  The calculation in the MPU 60 is performed, for example, by referring to a data table having a signal SC giving the number of channels as an address. This data table is stored in a memory 64 connected to the MPU 60 via the I / O circuit 62.

  For example, a large reference voltage (VREF) is set for a signal SC giving a large number of channels, and a small reference voltage (VREF) is set for a signal SC giving a small number of channels.

  As described above, in the present embodiment, the target value in ALC is set according to the number of channels of the optical transmitter 8 operating in the transmitting station (see FIG. 1). Therefore, the power per channel output from the optical amplifier can be kept constant. Therefore, by using the optical repeater as shown in FIG. 2, the WDM system of FIG. 1 can be easily adapted to increase / decrease in the number of channels in WDM.

  With reference to FIG. 2 again, further functions of the SV circuit 22 will be described. The other branched light branched by the optical coupler 36 is supplied to an optical bandpass filter 50 having a passband including the wavelength of the WDM signal light.

  The light output from the filter 50 is input to the O / E converter 52, and the output signal is supplied to the SV circuit 22.

  The output signal of the output side O / E converter 46 is also supplied to the SV circuit 22. Therefore, in the SV circuit 22, the level and gain of the input light and output light of this optical amplifier can be obtained.

  Therefore, by adding such information, the monitoring information sent from the transmitting station 2 is updated, and the updated monitoring information is sent to another optical repeater 18 or receiving station 6 on the downstream side. is there.

  The updated monitoring information is converted into signal light of wavelength λSV by an E / O converter 54 built in the SV circuit 22, and this signal light is sent out to the optical fiber transmission line 4 by the optical coupler 26.

  FIG. 4 is a block diagram of another WDM system to which the present invention is applicable. This system includes a transmission station 66 that outputs WDM signal light, an optical fiber transmission line 4 that transmits the WDM signal light, and a reception station 6 that receives the transmitted WDM signal light.

  Two optical repeaters 68 are provided in the middle of the optical fiber transmission line 4. There may be more optical repeaters 68 or one.

  Each optical repeater 68 includes an optical amplifier 70 and an SV circuit 72 that exchanges monitoring information directly with the optical amplifier 70.

  The optical transmitter 8 (# 5) for mainly sending monitoring information in the system of FIG. 1 is not used in this embodiment. That is, the optical transmitters 8 (# 1 to # 4) included in the transmission station 66 can output signal light modulated by transmission data of wavelengths λ1 to λ4, respectively.

  A status signal indicating the operating state of each optical transmitter 8 is supplied to the SV circuit 74. The SV circuit 74 generates a tone signal based on monitoring information including the number of channels of WDM signal light. This tone signal has a frequency sufficiently lower than the main signal (transmission data in each optical transmitter 8).

  The tone signal is supplied from the SV circuit 74 via the low pass filter 76 to the optical transmitter 8 (# 4). This tone signal is superimposed on the modulation signal (transmission data) supplied from the terminal 10 to the optical transmitter 8 (# 4).

  Referring to FIG. 5, a waveform diagram of the tone signal is shown. A tone signal 80 that is sufficiently slower than the main signal 78 is superimposed on one of the WDM signal lights. The tone signal 80 can be obtained by performing modulation based on monitoring information using a low-frequency tone component as a subcarrier. The frequency of the tone component is set to, for example, 1 KHz to 1 MHz so that each frequency component is not attenuated by the optical amplifier.

  FIG. 6 is a block diagram showing a second embodiment of the optical repeater. This optical repeater can be used as the optical repeater 68 of FIG.

  This optical repeater has a feedback loop including an ALC circuit 48. Since this loop is the same as that shown in FIG. 2, its description is omitted.

  The WDM signal light supplied from the upstream optical transmission line 4 is branched into two by the optical coupler 82. One branched light is supplied to the optical amplifying medium 38. The WDM signal light amplified in the optical amplifying medium 38 is sent to the downstream optical fiber transmission line 4 through the optical coupler 42.

  The other branched light branched by the optical coupler 82 is supplied to the optical bandpass filter 84. The filter 84 has a pass band including the wavelength of the WDM signal light.

  The light output from the filter 84 is supplied to the O / E converter 86. The output signal of the O / E converter 86 is supplied to the band pass filter 88.

  The filter 88 has a pass band including the carrier frequency of the tone signal. Accordingly, a tone signal is extracted by the filter 88 and this tone signal is supplied to the SV circuit 90.

  In the SV circuit 90, the monitoring information is reproduced based on the tone signal, and a signal SC giving the number of channels obtained based on the monitoring information is supplied from the SV circuit 90 to the ALC circuit 48.

  In the ALC circuit 48, the reference voltage VREF (see FIG. 3) is set based on the signal SC that gives the number of channels, so that the optical output power per channel can be kept constant regardless of the increase or decrease in the number of channels.

  In the system of FIG. 4, the tone signal is superimposed only on the signal light having the wavelength λ 4 output from the optical transmitter 8 (# 4). However, even if the tone signal is superimposed on all channels of the WDM signal light. Good. In this case, an optical modulator is provided between the optical multiplexer 16 and the optical transmission line 4, whereby the tone signal is superimposed on the WDM signal light.

  For example, the number of active channels can be transmitted according to the frequency of the tone signal. That is, when only one channel is operating, a 10 KHz tone signal is superimposed. When two channels are operating, an 11 KHz tone signal is superimposed. When three channels are operating, a 12 KHz tone signal is superimposed. The number of active channels is detected based on the frequency of the tone signal.

  Further, the frequency of the tone signal may be assigned to each channel, and each tone signal may be superimposed on each channel before performing wavelength division multiplexing. In this case, the number of active channels can be detected based on the number of frequency components of the tone signal.

  FIG. 7 is a block diagram of still another WDM system to which the present invention can be applied. This system includes a transmitting station 92 that outputs WDM signal light, an optical fiber transmission line 4 that transmits the WDM signal light, and a receiving station 6 that receives the transmitted WDM signal light.

  Two optical repeaters 94 are provided in the middle of the optical fiber transmission line 4. There may be more optical repeaters 94 or one.

  Each optical repeater 94 includes an optical amplifier 96 that amplifies the WDM signal light, and an ALC circuit 100 that controls the output level of the optical amplifier 96 to match the target level.

  A part of the WDM signal light output from the optical amplifier 96 is branched by the optical coupler 98, and the ALC circuit 100 controls the optical amplifier 96 so that the power of the branched light becomes constant.

  The transmitting station 92 includes five optical transmitters 8 (# 1 to # 5) that can generate signal lights having different wavelengths, and an optical multiplexer 16 that wavelength-division-multiplexes the signal lights into WDM signal lights. Have.

  In this embodiment, the three optical transmitters 8 (# 1 to # 3) are operated, and the remaining optical transmitters 8 (# 4 and # 5) are not operated.

  That is, a pulse signal corresponding to the main signal is supplied to each drive terminal 10 of the optical transmitter 8 (# 1 to # 3), and is supplied to the drive terminal 10 of the optical transmitter 8 (# 4, # 5). Is supplied with a DC bias.

  As a result, the signal light output from the optical transmitters 8 (# 1 to # 3) is modulated by the main signal, and the light output from the optical transmitters (# 4 and # 5) is CW light (continuous wave light). )

  The optical transmitters 8 (# 4, # 5) that are not operated in this way output CW light that is not related to the main signal because of the WDM signal light supplied to each optical repeater 94. This is to keep the total power constant. When the total power is constant, the target level of the ALC circuit 100 can be made constant in each optical repeater 94 regardless of the number of operating channels.

  Therefore, according to this embodiment, since it is not necessary to change the target level of ALC in each optical repeater 94, the ALC circuit can be simplified.

  Note that when the optical amplifier 96 is an EDFA, the control target of the ALC circuit 100 can be the power of pump light supplied to the EDF. When the power of pump light is used for gain tilt control, the control target of the ALC circuit 100 may be an attenuation factor of an optical attenuator provided on the upstream side or the downstream side of the optical amplifier 96.

  FIG. 8 is a block diagram showing a third embodiment of the optical repeater. This optical repeater can be used in place of the optical repeater 68 in the WDM system of FIG.

  This optical repeater has a feedback loop for ALC. The ALC circuit 102 included in this loop controls the power of the pump light in the pumping means 40 so that the output level of the optical amplifier matches the target level. In this embodiment, the target level is constant regardless of the number of channels of the WDM signal light.

  For this purpose, the compensation light source 104 that outputs compensation light that is included in the amplification band but has a wavelength different from the wavelength of the WDM signal light is used. One branched light branched by the optical coupler 82 and the compensation light from the light source 104 are added together by the optical coupler 106 and supplied to the optical amplification medium 38.

  The other branched light branched by the optical coupler 82 is supplied to an optical bandpass filter 84 having a passband including the wavelength of the WDM signal light. The light output from the filter 84 is supplied to the O / E converter 86.

  As described in the system of FIG. 4, the output signal of the O / E converter 86 includes a tone signal modulated by the monitoring information. This tone signal is extracted by the band pass filter 88 and supplied to the SV circuit 108.

  The SV circuit 108 controls the power of the compensation light output from the compensation light source 104 based on the number of channels of the WDM signal light determined by the reproduction of the monitoring information, and the target level in the ALC circuit 102 is irrespective of the number of operating channels. Make it constant.

  As described above, in this embodiment, compensation light is supplied to the optical amplifying medium 38 in addition to the WDM signal light, so that the ALC circuit 102 is set by setting the power of this compensation light according to the number of operating channels. No need to change the target level.

  FIG. 9 is a block diagram showing a fourth embodiment of the optical repeater. This optical repeater can be applied to the WDM system shown in FIG. 4 in place of the optical repeater 68 shown in FIG.

  This optical repeater is provided with another feedback loop in addition to the feedback loop including the ALC circuit 102 similar to FIG.

  Compensation light from the light source 104 is added to the WDM signal light in the optical coupler 106, and output light from the optical coupler 106 is supplied to the optical amplification medium 38 through the optical coupler 110.

  In the optical coupler 110, part of the WDM signal light and the compensation light is branched, and the branched light is supplied to the optical bandpass filter 112. The filter 112 has a pass band including the wavelength of the WDM signal light and the wavelength of the compensation light.

  The output light from the filter 112 is input to the O / E converter 114. The SV circuit 116 controls the power of the compensation light so that the output level of the O / E converter 114 becomes constant.

  The total power of the WDM signal light and compensation light supplied to the optical amplification medium 38 is reflected in the output level of the O / E converter 114. Therefore, by providing such a feedback loop on the upstream side of the optical amplifying medium 38, the total power of the WDM signal light and the compensation light supplied to the optical amplifying medium 38 can be kept constant. If the total power is kept constant, the target level in the ALC circuit 102 becomes constant regardless of the number of channels of the WDM signal light, and the ALC circuit 102 can be easily configured.

  By providing the feedback loop including the SV circuit 116, in this embodiment, it is not necessary for the optical repeater to receive information regarding the number of channels of the WDM signal light. Therefore, when the optical repeater of FIG. 9 is applied, the SV circuit 74 in the transmission station 66 of FIG. 4 may be omitted.

  8 and 9, the ALC circuit 102 controls the power of the pump light in the pumping means 40, but the power of the pump light is used for gain tilt control. In this case, the ALC circuit 102 may control the attenuation factor of an optical attenuator (not shown) provided on the upstream side or the downstream side of the optical amplification medium 38.

  FIG. 10 is a block diagram showing a first basic configuration of the optical amplifier of the present invention. This optical amplifier has an optical amplifying medium 38 and pumping means 40 as in the optical amplifier included in the optical repeater described so far.

  When the signal light is supplied to the first end 38 </ b> A of the pumped optical amplification medium 38, the amplified signal light 132 is output from the second end 38 </ b> B of the optical amplification medium 38.

  Thus, in the state where pumping is performed so that the optical amplifying medium 38 has an amplification band, ASE light (amplified spontaneous emission light) is generated in the optical amplifying medium 38. The ASE light is not only output from the second end 38B in the same direction as the signal light, but is also output from the first end 38A in the opposite direction to the signal light.

  The ASE light 134 propagating in the direction opposite to the signal light is extracted by the ASE extraction means 136. Based on the extracted ASE light, the monitoring means 138 monitors the spectral characteristics that give the wavelength dependence of the power of the ASE light.

  Then, the parameter control unit 140 controls a parameter (or gain itself) on which the gain tilt of the amplification band in the optical amplifying medium 38 depends so that the monitored spectral characteristic is maintained.

  As the optical amplifying medium 38, a doped fiber doped with a rare earth element such as EDF can be used. An optical amplification medium 38 made of a semiconductor chip can also be used (semiconductor optical amplifier). In this case, the pumping means 40 includes means for injecting current into the medium. Specifically, a pumping voltage is applied to the electrode pair of the semiconductor optical amplifier.

  The pumping means 40 suitable for the doped fiber is a pump light source that outputs pump light, and is operatively connected to at least one of the first end 38A and the second end 38B of the optical amplifying medium 38, and the pump light is optically amplifying medium. 38 to the optical coupling means.

  In this specification, optical components are “operably connected” to each other, including a case where they are directly connected by a fiber connection or a spatial connection using a collimated beam, and other optical components such as an optical filter. Including the case of connection via

  When the pumping means 40 includes a pump light source, the power of pump light can be adopted as a parameter to be controlled by the parameter control means 140.

  In this case, since the pump light source cannot be included in the feedback loop of ALC for making the power of the amplified signal light 132 (total gain of the optical amplifier) constant, in order to perform ALC, the attenuation factor is variable. A feedback loop including an optical attenuator may be provided.

  When this optical amplifier has a compensation light source 142 for supplying compensation light having a wavelength included in the amplification band of the optical amplification medium 38 to the optical amplification medium 38, the parameter to be controlled by the parameter control means 140 is The power of compensation light may be used. In this case, the pump light source can be included in the feedback loop for ALC. The wavelength of the compensation light is set to be different from the wavelength of the signal light.

  When this optical amplifier is applied to a WDM system, WDM signal light is supplied from the first end 38A to the optical amplifying medium 38.

  The spectral characteristic of the ASE light 134 reflects the gain characteristic of the optical amplifying medium 38, that is, the gain tilt. Since the ASE light 134 propagates in the opposite direction to the signal light in the optical amplifying medium 38, the spectral characteristics are not influenced by the number of channels of WDM signal light, the input level, and the accumulated ASE in principle.

  Therefore, a constant gain tilt can be easily obtained by controlling the parameter on which the gain tilt depends so that the spectral characteristic of the ASE light 134 is maintained. A specific example of the spectral characteristic monitoring method will be described later.

  The first basic configuration of the optical amplifier of FIG. 10 desirably includes an optical bandpass filter 143 operatively connected to the second end 38B of the optical amplification medium 38. The effectiveness of the filter 143 will be described below.

  The spectral characteristics of forward ASE propagating in the same direction as the signal light in the optical amplifying medium 38 are affected by the input level of the signal light and the accumulated ASE, whereas in the optical amplifying medium 38, the spectral characteristics propagate in the opposite direction to the signal light. The spectral characteristics of backward ASE are not affected by these in principle.

  However, in reality, if there is a slight reflection on the output side of the optical amplifying medium 38, the accumulated ASE is reflected, and the reflected accumulated ASE is amplified in the optical amplifying medium 38 and mixed into the backward ASE. is there. Therefore, when mixing of the accumulated ASE into the backward ASE becomes a problem, the optical band pass filter 143 having an appropriate pass band is used.

  Referring to FIG. 11A, a desirable pass band of the optical band pass filter 143 is shown. The shortest wavelength λL of the passband is set slightly shorter than the shortest wavelength of the WDM signal light, and the longest wavelength λH of the passband is set slightly longer than the longest wavelength of the WDM signal light. Thereby, the power of the cumulative ASE can be effectively reduced.

  The monitoring means 138 of FIG. 10 desirably includes two optical bandpass filters each having a different passband (eg, optical bandpass filters 170 and 172 of the spectrum monitor of FIG. 14).

  In this case, as shown in FIG. 11B, the shortest wavelength and the longest wavelength of the passband of one filter are set to λL−Δλ and λL, respectively, and the shortest wavelength and the longest wavelength of the passband of the other filter are set. Are set to λH and λH + Δλ, respectively. By doing so, even if the reflection component of the cumulative ASE is mixed into the backward ASE, the monitoring result is not affected.

  FIG. 12 is a block diagram showing a first embodiment of the optical amplifier of the present invention. The signal light to be amplified is supplied from the first end 38A to the optical amplifying medium 38 through the optical coupler 144.

  The ASE light propagating in the opposite direction to the signal light in the optical amplification medium 38 is extracted by the optical coupler 144. The extracted ASE light is supplied to the spectrum monitor 146. As the optical coupler 144, a fiber fusion type optical coupler, a WDM coupler as a special form thereof, or an optical circulator can be used.

  In order to pump the optical amplifying medium (for example, doped fiber) 38, a laser diode 148 as a pump light source is used. The pump light output from the laser diode 148 is supplied to the optical amplifying medium 38 through the optical coupler 150 connected to the second end 38B of the optical amplifying medium 38.

  A bias current is applied to the laser diode 148 by the drive circuit 152. The power of the pump light can be controlled by a bias current.

  The spectrum characteristic of the ASE light monitored by the spectrum monitor 146 is supplied to the control circuit 154. The control circuit 154 controls the bias current that the drive circuit 152 supplies to the laser diode 148 so that the spectral characteristics from the spectrum monitor 146 are maintained.

  In this embodiment, since the bias current of the laser diode 148 that outputs the pump light is included in the feedback loop for maintaining the gain tilt, ALC cannot be performed by the bias current of the laser diode 148.

  Therefore, the amplified signal light output from the second end 38 </ b> B of the optical amplifying medium 38 through the optical coupler 150 is input to the optical attenuator 156. The attenuation factor of the optical attenuator 156 is variable.

  The light output from the optical attenuator 156 is branched into two by an optical coupler 158, and one branched light is sent to an optical transmission path (not shown).

  The other branched light in the optical coupler 158 is supplied to the optical bandpass filter 160 having a passband including the wavelength of the signal light. The output light of the filter 160 is converted into an electric signal by the O / E converter 162, and the ALC circuit 164 controls the attenuation rate of the optical attenuator 156 so that the output level of the O / E converter 162 becomes constant.

  FIG. 13 is a diagram for explaining an example of gain tilt in the optical amplifier of FIG. A spectrum of output light when four channels of WDM signal light having wavelengths of 1548, 1551, 1554 and 1557 nm are input to the EDF with the same input power (−35 dBm / ch) is shown.

  The vertical axis represents output power (dBm), and the horizontal axis represents wavelength (nm).

  The spectrum indicated by A corresponds to when the power of the pump light is relatively large, and a negative gain tilt occurs. That is, the wavelength differentiation of the gain is negative (dG / dλ <0).

  The spectrum indicated by C corresponds to when the power of the pump light is relatively small, and a positive gain tilt is obtained (dG / dλ> 0).

  The spectrum indicated by B corresponds to the optimum pump light power for preventing the gain tilt, and the wavelength differentiation of the gain is 0 (dG / dλ = 0).

  Each spectrum has a shape in which four sharp spectra corresponding to the signal light of each channel are superimposed on the spectrum of ASE light.

  In the optical amplifier of FIG. 12, since the ASE light output from the first end 38A of the optical amplifying medium 38 is extracted, the spectrum of the WDM signal light is not superimposed on the spectrum. Therefore, the spectrum monitor 146 can monitor the spectrum of the ASE light with high accuracy without being affected by the power of the WDM signal light.

  Hereinafter, such ASE light propagating in the opposite direction to the signal light is referred to as backward ASE light.

  When the power of the backward ASE light is PASE (λ), the value is given by the following equation and is a function of the wavelength λ.

PASE (λ0) = 2nsp (λ0) h (C / λ0) [G (λ0) -1] Δλ (1)
Here, nsp (λ0) is the spontaneous emission coefficient, h is the Planck constant, C is the speed of light in vacuum, λ0 is the center wavelength of the band to be monitored, and Δλ is the bandwidth of the band to be monitored. Here, since the wavelength dependence of each parameter is generally constant in the range of Δλ, λ0 is used as a representative value.

  The spontaneous emission coefficient nsp (λ0) has a wavelength dependency, and a method for improving the monitoring accuracy by dealing with this wavelength dependency will be described later.

  In the equation (1), G (λ0) represents a gain given as a function of wavelength. Thus, the gain characteristics (the wavelength dependence of the gain) are reflected in the spectrum of the backward ASE light.

  Therefore, it is possible to evaluate the gain characteristics by cutting out two or more narrow bands included in the amplification band, separately detecting their powers, and obtaining the deviation of the detected values.

  Specifically, the spectrum monitor 146 of FIG. 12 includes a means for branching backward ASE light into first and second branched lights, and a narrow first passband included in the amplification band receiving the first branched light. A second optical bandpass filter having a narrow second passband that is included in the amplification band but is different from the first passband, The first and second photodetectors supplied with the light passing through the first and second optical bandpass filters, respectively, and means for detecting a deviation in output levels of the first and second photodetectors. More specifically, it is as follows.

  FIG. 14 is a block diagram illustrating a configuration example of the spectrum monitor 146 of FIG. The backward ASE light generated in the optical amplifying medium 38 (see FIG. 12) is supplied to the optical coupler 168 through the optical isolator 166. If the reflection from the backward ASE monitor system is small, the optical isolator 166 is unnecessary. The optical coupler 168 branches the received backward ASE light into first and second branched lights. The branching ratio of the first and second branched lights is, for example, 1: 1.

  The first and second branched lights are supplied to the optical bandpass filters 170 and 172, respectively.

  When the backward ASE light has a spectrum substantially similar to the spectrum of the ASE light as shown in FIG. 13, the center wavelengths of the passbands of the filters 170 and 172 are set to 1541 nm and 1559 nm, respectively.

  Light passing through the filters 170 and 172 is supplied to the photodiodes 174 and 176, respectively.

  Since the output signals of the photodiodes 174 and 176 are current signals, I / V converters (current / voltage converters) 178 and 180 are used corresponding to the photodiodes 174 and 176, respectively.

  The output voltage signals of the converters 178 and 180 are supplied to the negative input port and the positive input port of the operational amplifier 182, respectively. As a result, the output level deviation of the photodiodes 174 and 176 is reflected in the output signal of the operational amplifier 182.

  Therefore, by feeding back the output signal of the operational amplifier 182 to the bias current of the laser diode 148 (see FIG. 12), the spectral characteristics of the backward ASE light generated in the optical amplifying medium 38 can be maintained, and as a result, the gain tilt Can be kept constant. By appropriately setting the target value of the deviation in the feedback loop, the gain tilt can be made flat, for example, as indicated by B in FIG.

  Referring to FIG. 15, another spectrum monitor applicable to the present invention is shown. Here, the output signals of the I / V converters 178 and 180 are taken into the MPU (microprocessing unit) 186 via the I / O port 184.

  The MPU 186 is connected to the memory 188 via the I / O port 184. The MPU 186 receives the output levels of the converters 178 and 180, calculates the deviation, and outputs the deviation via the I / O port 184.

  As described above, the spontaneous emission light coefficient nsp (λ0) in the equation (1) has wavelength dependency, that is, depends on the wavelength λ0 to be monitored. Therefore, when higher monitoring accuracy is required, a data table of the spontaneous emission light coefficient nsp (λ0) with the wavelength as a parameter is stored in the memory 188, and an accurate spectral characteristic is obtained based on the data table. do it. For example, the gain G (λ) can be accurately calculated based on the calculated value of the deviation.

  In the spectrum monitor shown in FIG. 14 or 15, it is assumed that there are two narrow bands (first and second pass bands) to be cut out from the amplification band, but three or more optical band pass filters are used. A narrow band corresponding to the number may be cut out from the amplification band. By doing so, for example, the accuracy of the spectral characteristics of the backward light calculated by the MPU 186 can be improved.

  FIG. 16 is a block diagram showing a second embodiment of the optical amplifier of the present invention. This optical amplifier is characterized in that an optical attenuator 156 ′ for ALC is provided on the upstream side of the optical amplifying medium 38 as compared with the optical amplifier of FIG. 12.

  In other words, the amplified signal light is not attenuated, but the signal light input from the first end 38A to the optical amplification medium 38 is attenuated in advance. The attenuation factor of the optical attenuator 156 'is controlled by the ALC circuit 164 so that the output level of the O / E converter 162 corresponding to the output level of this optical amplifier becomes constant.

  According to the optical amplifier of FIG. 12 or FIG. 16, a constant gain tilt can be maintained, and ALC is also possible.

  FIG. 17 is a block diagram showing a third preferred embodiment of the optical amplifier according to the present invention. Here, a light source that supplies compensation light to the optical amplification medium 38 is used, and the power of the compensation light is controlled so that the spectral characteristics of the backward ASE light are maintained. Along with this, the power of the pump light is supplied to the ALC.

  A laser diode 190 is used as a compensation light source. Compensation light from the laser diode 190 is supplied from the first end 38A to the optical amplifying medium 38 through the optical coupler 192.

  The signal light to be amplified is supplied from the first end 38A to the optical amplification medium 38 through the optical coupler 144 for extracting backward ASE light and the optical coupler 192 for compensation light in this order.

  The backward ASE light generated in the optical amplifying medium 38 is supplied to the spectrum monitor 146 through the optical couplers 192 and 144 in this order.

  A bias current is supplied to the laser diode 190 from the drive circuit 194, and this bias current is controlled by the control circuit 154.

  The control circuit 154 controls the bias current of the laser diode 190 so that the spectral characteristic of the backward ASE light monitored by the spectrum monitor 146 is maintained. As a result, the power of the compensation light output from the laser diode 190 is controlled, and the gain characteristic of the optical amplifier is maintained constant.

  In this embodiment, since the power of the pump light is not used for the control for maintaining the gain characteristic constant, the pump light source can be included in the feedback loop for ALC. Since the compensation light is used for maintaining the spectral characteristics, in this case, after the compensation light is removed by the optical filter 160, the O / E converter 162 performs O / E conversion. A bias current is supplied from the drive circuit 152 to the laser diode 148 as a pump light source, and this bias current is controlled by the ALC circuit 164.

  As described above, according to the third embodiment, it is possible to provide an optical amplifier capable of maintaining a constant gain tilt and capable of ALC. Moreover, in the third embodiment, an optical attenuator for ALC is not necessary.

  In the embodiment of the optical amplifier described above, the laser diode 148 as a pump light source is operatively connected to the second end 38B of the optical amplifying medium 38, and the pump light in the optical amplifying medium 38 is in the direction opposite to the signal light. Propagated. That is, backward pumping is employed.

  A pump light source is operatively connected to the first end 38A of the optical amplifying medium 38 so that the pump light and the signal light to be amplified propagate in the same direction in the optical amplifying medium 38 so as to perform forward pumping. Also good.

  Further, the pumping light may be supplied to the optical amplifying medium 38 from both the first end 38A and the second end 38B so as to improve the pumping efficiency.

  FIG. 18 is a block diagram showing a fourth preferred embodiment of the optical amplifier according to the present invention. Here, in order to improve the monitoring accuracy of the spectral characteristics of the backward ASE light in the spectrum monitor 146, the reflecting mirrors 196 and 198 that are operatively connected to the second end 38B of the optical amplifying medium 38 are provided.

  When the spectrum monitor 146 is configured as shown in FIG. 14, the reflecting mirror 196 reflects light having a wavelength included in the passband of the optical bandpass filter 170 and transmits other light. The reflecting mirror 198 reflects light having a wavelength included in the passband of the optical bandpass filter 172 and transmits other light.

  By providing such reflecting mirrors 196 and 198, light in a specific band can be reciprocated in the optical amplifying medium 38 among the forward ASE light output from the second end 38 </ b> B of the optical amplifying medium 38. The input power of the photodiodes 174 and 176 in FIG. 14 is increased, and the spectrum monitoring accuracy is improved.

  In addition, when the narrow band cut out from the spectrum of ASE light is 3 or more, the number of reflectors according to the number is used.

  By the way, in a doped fiber such as EDF, SE light (spontaneously emitted light) leaks to the side, and the gain characteristics of the doped fiber are reflected in the SE light. Further, the SE light leaking to the side is not affected by the number of channels of WDM signal light, the number of input levels, and the accumulated ASE. This was reported by Aida et al. At the 1991 International Conference (Optical Amplifiers and their Applications; OAA) (FE3), which is a value obtained by integrating SE light from the side of the doped fiber over the fiber length PSE. From (λ), it is clear that the gain G (λ) can be obtained from the following equation.

PSE (λ) = [ln [G (λ)] + α S (λ) L] / C (λ) (2)
C (λ) = η (λ) [σe (λ) + σa (λ)] τ / [h (C / λ) π (γEr) 2] (3)
Where σe (λ), σa (λ), and αS (λ) are the emission cross section, absorption cross section, and loss at wavelength λ, respectively, and τ and γEr are the spontaneous emission lifetime and Er doped regions. Represents the radius. Η (λ) is a coefficient having wavelength dependency.

  Therefore, the gain characteristic (gain tilt) can be grasped by monitoring the spectral characteristic of SE light leaking to the side.

  FIG. 19 is a block diagram showing a second basic configuration of the optical amplifier of the present invention. The optical amplification medium 38 has an optical waveguide structure to which signal light is supplied. The 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.

  The SE extraction means 200 extracts SE light leaking to the side from the optical waveguide structure of the optical amplifying medium 38. The monitoring means 138 monitors the spectral characteristic that gives the wavelength dependence of the power of the extracted SE light.

  The parameter control unit 140 controls a parameter (or gain itself) on which the gain tilt of the amplification band in the optical amplifying medium 38 depends so that the monitored spectral characteristics are maintained.

  In the illustrated example, the parameter to be controlled is the power of pump light in the pumping means 40.

  Similarly to the case shown in FIG. 11, the parameter control means 140 may control the power of the compensation light.

  FIG. 20 is a configuration diagram of a spectrum monitor that can be used as the SE extraction unit 200 and the monitoring unit 138 of FIG.

  An EDF 202 is used as the optical amplification medium 38 (see FIG. 19). The means for pumping the EDF 202 is not shown.

  The EDF 202 is accommodated in a case 204 such as an integrating sphere configured to prevent light from entering from the outside. The coating of EDF 202 is partially removed, from which SE light leaks to the side.

  SE light is supplied to optical bandpass filters 206 and 208.

  The filters 206 and 208 have passbands similar to the optical bandpass filters 170 and 172 in FIG.

  The light transmitted through the filters 206 and 208 is converted into a current signal by the photodiodes 210 and 212, respectively. The respective current signals are converted into voltage signals by the I / V converters 214 and 216 and supplied to the operational amplifier 218.

  Since the spectrum of the SE light reflects the gain characteristics of the optical amplifier as described above, the gain tilt of the optical amplifier is kept constant, for example, by controlling the power of the pump light based on the monitored spectrum characteristics of the SE light. Can be maintained.

  The spectrum monitor of FIG. 20 can be changed according to the configuration of FIG. That is, the MPU is used for monitoring the spectral characteristics.

  In this case, by storing C (λ) and αs L in the equations (2) and (3) in the memory, the spectral characteristic monitoring accuracy can be improved. Note that the number of narrow bands cut out in the spectrum of SE light may be set to 3 or more, and the optical power in each band may be detected, and various controls may be performed based thereon.

  FIG. 21 is a block diagram showing a third basic configuration of the optical amplifier of the present invention. This optical amplifier is configured by cascading a first optical amplification unit 220 and a second optical amplification unit 222.

  Each of the optical amplification units 220 and 222 has the first basic configuration shown in FIG.

  The light amplified by the first optical amplification unit 220 is attenuated by an optical attenuator 224 having a variable attenuation factor, and is sent to the second optical amplification unit 222 by a dispersion compensation fiber (DCF) 226.

  The DCF 226 has a dispersion value that cancels the chromatic dispersion received by the signal light in the transmission path.

  The light output from the second optical amplification unit 222 is branched by the optical coupler 228, and one branched light is sent out to an optical transmission path (not shown).

  The other branched light in the optical coupler 228 is converted into an electric signal by the O / E converter 230.

  The ALC circuit 232 controls the attenuation factor of the optical attenuator 224 so that the output level of the O / E converter 230 is kept constant.

  In this embodiment, the first reason that the optical amplification unit has a two-stage configuration is that the loss of DCF is generally large and the level of the signal light needs to be raised to some extent on the upstream side of DCF 226.

  The second reason is that if the gain of optical amplification on the upstream side of the DCF 226 is excessively increased to increase the power of the signal light, a nonlinear effect is likely to occur in the DCF 226.

  In a system to which WDM is applied, if four-wave mixing (FWM), which is one of nonlinear effects, occurs in the DCF 226, cross-channel crosstalk deteriorates. Self-phase modulation (SPM) also causes signal quality degradation.

  According to the third basic configuration, it is possible to provide an optical amplifier capable of maintaining a constant gain tilt and capable of ALC.

1 is a block diagram of a WDM system to which the present invention can be applied. It is a block diagram which shows 1st Embodiment of an optical repeater. It is a block diagram of an ALC circuit. It is a block diagram of the other WDM system which can apply this invention. It is explanatory drawing of a tone signal. It is a block diagram which shows 2nd Embodiment of an optical repeater. It is a block diagram of the other WDM system which can apply this invention. It is a block diagram which shows 3rd Embodiment of an optical repeater. It is a block diagram which shows 4th Embodiment of an optical repeater. It is a block diagram which shows the 1st basic composition of an optical amplifier. It is explanatory drawing of embodiment for eliminating the influence of accumulation ASE. 1 is a block diagram illustrating a first embodiment of an optical amplifier. FIG. It is explanatory drawing of a gain tilt. It is a block diagram of a spectrum monitor. It is a block diagram of another spectrum monitor. It is a block diagram which shows 2nd Embodiment of an optical amplifier. It is a block diagram which shows 3rd Embodiment of an optical amplifier. It is a block diagram which shows 4th Embodiment of an optical amplifier. It is a block diagram which shows the 2nd basic composition of an optical amplifier. FIG. 20 is a configuration diagram of a spectrum monitor that can be used in the second basic configuration of FIG. 19. It is a block diagram which shows the 3rd basic composition of an optical amplifier.

Explanation of symbols

2 Transmitting station 4 Optical fiber transmission line 6 Receiving station 8 Optical transmitter 18 Optical repeater 20 Optical amplifier 30 Optical receiver

Claims (5)

A first terminal including a plurality of optical transmitters each outputting signal light and means for wavelength division multiplexing the signal light;
An optical transmission line connected to the first terminal station for transmitting the wavelength division multiplexed signal light;
A second terminal station connected to the optical transmission line and receiving the wavelength division multiplexed signal light;
An optical repeater including an optical amplifier provided in the middle of the optical transmission line and amplifying and outputting the wavelength division multiplexed signal light;
The signal light output from each of the plurality of optical transmitters being operated is modulated by the main signal, and the signal light output from each of those not being operated is a continuous wave,
The optical repeater further includes means for detecting the output level of the optical amplifier, and means for controlling the optical amplifier so that the detected output level matches a target level.
The power of the continuous wave output from each of the optical transmitters not in operation is controlled so that the target level is constant regardless of the number of optical transmitters in operation. Optical communication system.   The optical amplifier according to claim 1, wherein the optical amplifier includes an erbium-doped fiber (EDF), and the optical amplifier is controlled by controlling power of pump light supplied to the EDF. Communications system. A first terminal including a plurality of optical transmitters each outputting signal light and means for wavelength division multiplexing the signal light;
An optical transmission line connected to the first terminal station for transmitting the wavelength division multiplexed signal light;
A second terminal station connected to the optical transmission line and receiving the wavelength division multiplexed signal light;
An optical repeater provided in the middle of the optical transmission line and including an optical amplifier that amplifies and outputs the wavelength division multiplexed signal light and an optical attenuator provided upstream or downstream of the optical amplifier. Prepared,
The signal light output from each of the plurality of optical transmitters being operated is modulated by the main signal, and the signal light output from each of those not being operated is a continuous wave,
The optical repeater further includes means for detecting the output level of the optical repeater and means for controlling the optical attenuator so that the detected output level matches a target level,
The power of the continuous wave output from each of the optical transmitters not in operation is controlled so that the target level is constant regardless of the number of optical transmitters in operation. Optical communication system.   4. The optical communication system according to claim 3, wherein the optical amplifier has an erbium-doped fiber (EDF), and gain tilt is controlled by controlling power of pump light supplied to the EDF. Connected to the transmission line,
A plurality of optical transmitters each outputting signal light;
A multiplexing means for wavelength division multiplexing the signal light and sending it to the optical transmission line,
The signal light output from each of the plurality of optical transmitters being operated is modulated by the main signal, and the signal light output from each of those not being operated is a continuous wave,
The power of the continuous wave output from each of the optical transmitters that are not operated is controlled so that the output intensity of the multiplexing means is constant regardless of the number of optical transmitters that are operated. Terminal equipment characterized.

Images