JP5938007B2 - Optical receiving apparatus and optical receiving method - Google Patents

Description

  The present invention relates to a device and an optical reception method provided in an OLT (Optical Line Terminal) which is a terminal device on the station side in a PON (Passive Optical Network), and particularly to an optical reception device and an optical reception method using a coherent detection method. is there.

  In recent years, as demands for increasing transmission capacity, energy saving, and economy in optical access networks have increased, research on PON has been promoted as a means for realizing it. The PON is an optical communication system that achieves economy by sharing one station side device and a part of a transmission line with a plurality of users by using an optical multiplexer / demultiplexer using an optical passive element. In the PON, since the distance between the OLT that is the terminal device on the station side and each ONU (Optical Network Unit) that is the terminal device on the subscriber side is different, the optical loss between the ONU and the OLT differs depending on the path. In addition, since the transmitter output also varies depending on the individual, as a result, the intensity of the signal level of the optical signal received by the OLT varies greatly. At the same time, in the case of using TDM-PON that multiplexes signals in a time division manner, for example, when an ONU is transmitting time and when no ONU is transmitting, there occurs intermittently. The incident burst signal. From the above, the OLT optical receiver is required to have a wide dynamic range for receiving burst signals having different intensities.

  Currently, with the rapid development of digital coherent communication technology in recent years, as a configuration that realizes further economy by widening the area, the momentum to apply the coherent detection method to the access network has increased again, and various reports have been made. In a conventional access network, an intensity modulation direct detection method (IM-DD method), which is a method of linearly changing the intensity of light of a carrier wave and superimposing information, is used. The coherent detection method uses coherent light with a narrow oscillation beam width for the optical signal to be transmitted, detects the mixed light with the local oscillation light, reduces the effects of various noises such as phase noise, It is possible to superimpose information on the phase. As a result, multi-level phase and amplitude modulation becomes possible. In addition, the increase in feasibility by incorporating ultra-high-speed digital signal processing technology, which has been remarkably developed in recent years, is expected to increase the speed of application to coherent detection access networks and to realize higher speeds in future networks. ing. At the same time, in the coherent detection method, the receiver sensitivity is improved by amplifying the optical signal with the local oscillation light. From the above, it is expected that the network will be widened and extended. The above system is more complicated in configuration than the IM-DD system described above, and the cost of a single device increases. However, the effect of reducing OPEX by widening and extending the length is sufficient to compensate for the above problem. It is thought that.

However, the following points become problems when applied in an access network.
As described above, in the PON, burst signals having different intensities are received by the optical receiving unit of the station side device. Due to the difference in the intensity of the optical signal, when the signal is quantized, the sample value of each signal with respect to the set threshold value is relatively different, resulting in a detection error.

  In a current optical receiver in a PON using intensity modulation and direct detection, for example, as described in Non-Patent Document 1, a current signal optically / electrically converted by a photodiode is converted into a transimpedance amplifier (differential amplifier). TIA) is used to convert and amplify it into a voltage signal, and then the limiter amplifier is used to control the amplification factor according to the received signal level, and to shape the output signal so that the output signal becomes a constant level. It has been solved. In addition, as described in Non-Patent Document 2, a signal level detector and a semiconductor optical amplifier (SOA) are installed in front of the optical receiver, and the amplification factor of the amplifier is changed according to the detected signal level. Therefore, a method for controlling the optical signal incident on the photodiode to a constant level has been proposed.

  In Non-Patent Document 1, control is performed to keep the signal level constant in the electric stage in Non-Patent Document 2, and in the optical stage, and the applied system may be different depending on the modulation speed, the PON configuration, and the like. . However, when the coherent detection method is applied in an access network, an optical signal having a lower signal level is incident on the coherent optical receiver as compared with the conventional method due to an improvement in reception sensitivity. In the form of waveform shaping in the electrical stage as described, a wide dynamic range and linearity of amplification factor are required for the transimpedance amplifier and limiter amplifier. Therefore, realization is difficult, and a system that performs constant level control in the optical stage as described in Non-Patent Document 2 is desired.

  In the case of using a coherent detection method, a method for realizing a constant level control function of an optical stage is described in Non-Patent Document 2, in which a signal level of an optical signal is detected and an ALC (Auto Level Control) function is provided. A method of reducing the level difference of the optical signal incident on the coherent optical receiver with an amplifier and a method of controlling the input level of the local oscillation light incident on the coherent optical receiver to reduce the level difference of the received signal. Conceivable. In either method, the dynamic range of the coherent optical receiver is improved, and burst signals having intensity differences can be received. By applying the above technology, it is expected that the access network will be economicalized by widening and extending the length of the PON.

Makoto Nakamura, Yotaro Hamada, Jun Endo, Yuji Akatsu, “Optical Receiver Circuit Technology for Burst Transmission for PON” NTT Technical Journal, July 2006 Benn C.I. Thomsen, Benjamin J. et al. Putman, Polina Bayvel “Optically qualified 10 Gb / s NRZ digital burst-mode receiver for dynamic optical networks” 23 July 2007 / vol. 15, no. 15 / OPTICS EXPRESS Kazuro Kikuchi, Satoshi Tsukamoto, “Evaluation of Sensitivity of the Digital Coherent Receiver”, JOURNAL OF LIGHTWAVE TECHNOLOGY. 26, no. 13, 2008

  Transmission of each ONU in an optical receiver that reduces the level difference of optical signals received by the coherent optical receiver by controlling the input level of the local oscillation light to the coherent optical receiver included in the OLT by a variable optical attenuator. It is conceivable that an optical signal having a very low signal level is incident on the signal level detector that detects the signal level of the optical signal. In this case, it is assumed that the dynamic range of the optical receiver is limited by the minimum light receiving sensitivity limit of the signal level detector.

  The present invention has been made in view of the above problems, and the level difference between optical signals received by a coherent optical receiver by controlling the input level of local oscillation light to the coherent optical receiver by a variable optical attenuator. An object of the present invention is to provide an optical receiving apparatus and an optical receiving method that prevent a dynamic range from being limited by a minimum light receiving sensitivity limit of a signal level detector.

The optical receiver of the present invention is
An optical amplifier that amplifies a burst-like optical signal incident intermittently at a predetermined amplification rate;
An optical branching device for branching the optical signal amplified by the optical amplifier;
A coherent optical receiver that converts one of the optical signals branched by the optical splitter into an electrical signal using a coherent detection method;
A local oscillation light source for supplying local oscillation light used in the coherent optical receiver to the coherent optical receiver;
A signal level detector for detecting a signal level of an optical signal input to the coherent optical receiver using the other of the optical signals branched by the optical splitter;
A variable optical attenuator that attenuates the local oscillation light in accordance with a signal level detected by the signal level detector so that a reception current of the coherent optical receiver becomes constant;
Is provided.

  In the optical receiver of the present invention, the optical amplifier amplifies the optical signal at an amplification factor such that the signal level of the optical signal input to the signal level detector is within a predetermined range. Also good.

  In the optical receiver of the present invention, the optical amplifier may be a gain clamp type optical amplifier that amplifies at a constant amplification factor.

  In the optical receiver of the present invention, the optical amplifier is an optical fiber amplifier that amplifies the optical signal using pump light, and when an optical signal having a signal level exceeding a predetermined threshold is input, The light level of the pump light may be increased to a predetermined light level.

The optical receiving method of the present invention is:
An optical amplification procedure for amplifying intermittently incident bursty optical signals at a predetermined amplification rate;
The signal level of the optical signal amplified by the optical amplification procedure is detected, and the reception current of the coherent optical receiver that converts it into an electrical signal using a coherent detection method is constant according to the signal level of the optical signal. Light level adjustment procedure to attenuate the oscillation light,
An optical reception procedure in which the local oscillation light attenuated by the optical level adjustment procedure is incident on the coherent optical receiver, and the optical signal amplified by the optical amplification procedure is converted into an electric signal using the coherent optical receiver; ,
In order.

  In the optical reception method of the present invention, even in the optical amplification procedure, even if the optical signal is amplified at an amplification factor such that the signal level of the optical signal input to the signal level detector is within a predetermined range. Good.

  In the optical receiving method of the present invention, in the optical amplification procedure, an optical signal may be amplified using a gain clamp type optical amplifier that amplifies at a constant amplification factor.

  In the optical receiving method of the present invention, in the optical amplification procedure, when the optical signal is amplified using an optical fiber amplifier that amplifies the optical signal using pump light, and the optical signal exceeds a predetermined threshold, The light level of the pump light may be increased to a predetermined light level.

  According to the present invention, since the optical amplifier adjusts the signal level input to the signal level detector and the variable optical attenuator makes the reception current of the coherent optical receiver constant, the signal level detector has a minimum light receiving sensitivity limit. It is possible to prevent the dynamic range from being limited. As a result, the present invention realizes an optical receiving apparatus having a high-sensitivity burst-corresponding constant level control function by improving the light receiving sensitivity of the signal level detector, and making the access network economical by widening and extending the length of the PON. Can be realized.

An example of the optical receiver which concerns on the 1st Embodiment of this invention is shown. An example of a coherent optical receiver is shown. An example of the structure of the optical frame signal which ONU transmits is shown. It is explanatory drawing of the detection sensitivity in a signal level detector. An example of operation | movement of the optical receiver which concerns on 1st Embodiment is shown. An example of the optical receiver which concerns on the 2nd Embodiment of this invention is shown. An example of the configuration of the ALC optical amplifier 11 _ALC is shown. ALC optical amplifier 11 A control example of _ALC is shown. An example of operation | movement of the optical receiver which concerns on 2nd Embodiment is shown. An example of the structure of the optical amplifier which concerns on the 3rd Embodiment of this invention is shown. An example of the structure of the optical amplifier which concerns on the 4th Embodiment of this invention is shown.

  Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiments described below are examples of the present invention, and the present invention is not limited to the following embodiments.

<First embodiment>
A characteristic of the coherent detection method is that the receiver sensitivity is very high as described above. At this time, in order to realize a high receiver sensitivity such as a shot noise limit, it is necessary to input local oscillation light at a very high power level. However, sufficient reception sensitivity may not be obtained when there is a limit on the power level of the local oscillation light due to the equipment configuration or the influence of the overload limit of the coherent optical receiver. In such a case, amplifying an optical signal input to the coherent optical receiver with a preamplifier is an effective method for improving coherent reception sensitivity (Non-patent Document 3).

  A first embodiment of the present invention is shown in FIG. The optical receiving device 91 according to the present invention functions as an optical signal receiving unit in an OLT that is a station side device in an access network. The optical receiver 91 of the present invention has a configuration for coherently receiving an optical signal transmitted from an ONU. For example, the optical receiver 91 of the present invention includes a coherent optical receiver 12 and a local oscillation light source 13. The local oscillation light source 13 outputs local oscillation light that is input to the coherent receiver 12. The coherent optical receiver 12 receives an optical signal coherently using local oscillation light.

The optical receiver 91 of the present invention further includes a configuration for controlling the intensity of the optical signal received by the coherent optical receiver 12 to a certain level. For example, the optical receiver 91 of the present invention includes an AGC optical amplifier 11 _AGC , an optical splitter 14, a signal level detector 15, a level measuring device 16, a local oscillation light control circuit 17, and a variable optical attenuator 18. And comprising.

AGC optical amplifier 11 The _AGC amplifies the burst-like upstream optical signal transmitted from the ONU with a predetermined amplification factor. The burst optical signal is, for example, an optical frame signal with a preamble on top.
The optical splitter 14 splits the optical signal from the AGC optical amplifier 11 _AGC into two. One of the branches is input to the coherent optical receiver 12, and the other of the branches is input to the signal level detector 15.
The signal level detector 15 detects the light intensity of the optical signal from the optical splitter 14.
The level measuring device 16 measures a signal level corresponding to the light intensity detected by the signal level detector 15.
The variable optical attenuator 18 attenuates the light intensity of the local oscillation light from the local oscillation light source 13.
The local oscillation light control circuit 17 changes the attenuation amount of the local oscillation light of the variable optical attenuator 18 according to the signal level measured by the level measuring device 16.

In this configuration, as the signal level detector 15, for example, a light detection means such as PIN-PD or APD can be used. As the configuration of the optical branching device 14, for example, an N: 1 asymmetric coupler can be used. In addition, an AGC optical amplifier 11 _AGC is provided in the preceding stage of the coherent optical receiver 12 as a highly sensitive receiving means for measuring the signal level. The AGC optical amplifier 11 _AGC has a constant amplification factor within the range of the optical signal input level assumed by the optical receiver 91.

The optical reception method according to the present invention includes an optical amplification procedure, an optical level adjustment procedure, and a coherent optical reception procedure in this order.
In the optical amplification procedure, the AGC optical amplifier 11 _AGC connected to the preceding stage of the coherent optical receiver 12 amplifies the burst optical signal with a predetermined amplification factor.
In the optical level adjustment procedure, the local oscillation light is attenuated according to the signal level of the optical signal so that the reception current of the coherent optical receiver 12 becomes constant. For example, the local oscillation light control circuit 17 controls the intensity of light incident on the coherent optical receiver 12 to a certain level.
In the coherent light reception procedure, the local oscillation light attenuated by the light level adjustment procedure is incident on the coherent light receiver 12, and the optical signal amplified by the light amplification procedure is converted into an electrical signal using the coherent light receiver 12. .

  The present invention controls the intensity of light incident on the coherent optical receiver 12 to a constant level in the light level adjustment procedure. As a result, the present invention can reduce false detection caused by variations in signal strength in upstream signals, which is a problem when the coherent detection method is applied to an access network.

(Local oscillation light level control)
As the coherent optical receiver 12, for example, the polarization diversity coherent receiver shown in FIG. 2 can be used. The polarization diversity coherent receiver includes a polarization beam splitter (PBS) 21S, a beam splitter (BS) 21L, 90-degree optical hybrids 22X and 22Y, and balanced photodetectors 23 _XI , 23 _XQ , 23 _YI , 23 _YQ. And comprising.

PBS21S is the X polarization and the Y polarization of the optical signal _{P S} is separated and outputs the X polarization to the 90-degree optical hybrid 22X, and outputs the Y polarization 90 degree optical hybrid 22Y. The BS 21L branches the local oscillation light into 90-degree optical hybrids 22X and 22Y. The 90-degree optical hybrids 22X and 22Y separate the phases of the optical signals. The balanced photodetectors 23 _XI , 23 _XQ , 23 _YI , and 23 _YQ are the X-polarized I-axis component, the X-polarized Q-axis component, the Y-polarized I-axis component, and the Y-polarized Q-axis, respectively. Receive the component. Thereby, the sensitivity of the coherent optical receiver 12 does not depend on the polarization state of the optical signal.

式中において、αはＸＹ偏波間の信号レベルの比率、δは位相差である。上記の受信電流はそれぞれ、各ＸＹ偏波のＩＱ成分に対応し、この値を基に、信号処理により復号を行う。
以上の式より、各バランス型光検出器２３_ＸＩ、２３_ＸＱ、２３_ＹＩ、２３_ＹＱで検出される受信電流は、Ｐ_ｓとＰ_Ｌｏを乗じたものとなる。 Here, when the signal level of the optical signal input to the coherent optical receiver 12 is P _s and the input level of the local oscillation light is P _Lo , the balanced photodetectors 23 _XI , 23 _XQ , 23 _YI , The reception current detected by 23 _YQ is expressed by the following equation.

In the equation, α is a signal level ratio between XY polarized waves, and δ is a phase difference. Each of the reception currents corresponds to the IQ component of each XY polarization, and decoding is performed by signal processing based on this value.
From the above formula, the reception current detected by each of the balanced photodetectors 23 _XI , 23 _XQ , 23 _YI , and 23 _YQ is obtained by multiplying P _s and P _Lo .

At this time, as described above, in the access network, the signal level P _s of the optical signal transmitted from each ONU has a difference in value because the value of the path loss is different. In the technique of performing waveform shaping using a limiter amplifier on the electric signal after photoelectric conversion output from the coherent optical receiver 12, a limiter amplifier that compensates for the linearity of the amplification factor in a wide dynamic range is required. Realization is difficult. On the other hand, in the optical receiver 91 according to the present embodiment, the local oscillation light control circuit 17 controls the input level of the local oscillation light so as to satisfy, for example, P _s × P _Lo ≈A. The value A of the product of the optical signal serving as the control target value and the local oscillation light differs depending on the configuration of the coherent optical receiver 12 and the like.

(Signal level)
The burst optical signal transmitted by the ONU is as follows.
An example of the configuration of an optical frame signal transmitted by the ONU is shown in FIG. The optical frame signal is composed of three sections: preamble, data, and end-of-burst. The preamble is arranged at the head of the optical frame signal. When the level measurement unit 16 detects the preamble signal level (P _s ), the local oscillation light control circuit 17 determines the attenuation amount of the variable optical attenuator 18 based on the preamble signal level. Here, as described above, the attenuation is set so as to satisfy, for example, P _s × P _Lo ≈A.

  When the level measurement unit 16 detects the falling edge of the optical frame signal, the local oscillation light control circuit 17 preferably cancels the setting of the attenuation amount of the variable optical attenuator 18 and waits for detection of a new preamble. Thereby, signal level control is possible for an arbitrary optical frame signal.

  As described above, the optical receiving apparatus 91 according to the present embodiment controls the input level of the local oscillation light with respect to the preamble signal level of each optical frame signal, for example, when intensity modulation is used. However, it is possible to receive an optical signal at a constant optical level without damaging the information superimposed on the optical signal, and it is possible to reduce false detection caused by variations in optical signal intensity.

(Signal level of the optical signal incident on the signal level detector 15)
FIG. 4 shows an example of reception sensitivity when an optical signal using a 10 Gbps BPSK (Binary Phase Shift Keying) modulation method is received by a polarization diversity coherent optical receiver. In the figure, SNL indicates a shot noise limit, PLA indicates a case where PLo = 10 dBm, PLB indicates a case where PLo = 5 dBm, PLC indicates a case where PLo = 0 dBm, and PLD indicates a case where PLo = −5 dBm. Indicates.

  When the coherent detection method is used, the reception sensitivity can be improved to the shot noise limit SNL by increasing the input level of the local oscillation light. In the case shown in FIG. 4, −52 dBm is the minimum light receiving sensitivity when detection is performed with the shot noise limit SNL. As described above, by using the coherent detection method, the light receiving sensitivity is greatly improved, but the signal level detector 15 is also required to have the same detection sensitivity.

(Operation example when receiving signal constant level control)
FIG. 5 shows an operation example at the time of received signal constant level control in this configuration. Here, as an example, a configuration using a 3 dB coupler for the optical branching device 14 is shown. For the coherent optical receiver 12, for example, a polarization diversity coherent receiver is used. At this time, the reception current detected by each of the balanced photodetectors 23 _XI , 23 _XQ , 23 _YI, and 23 _YQ is expressed by equations (1) to (4).

The gain of the AGC optical amplifier _{11 AGC} and G, the signal level of the optical signal input to the AGC optical amplifier _{11 AGC} provided in front of the optical splitter 14 and _{P in,} and the loss of the optical branching unit 14 beta Then, the signal level P _s of the optical signal input to the coherent optical receiver 12 can be expressed by the following.

Is input to the signal level detector 15 coherent light receiver 12 detected by a signal level P _S of first one of the optical signal is b, than the signal level P _S of the second optical signal b with a small a Suppose there is. Further, it is assumed that the light level of the local oscillation light output from the local oscillation light source 13 is c, and the light level of the local oscillation light attenuated by the variable optical attenuator 18 is d. In this case, the local oscillation light control circuit 17 performs control so that a × d = b × c≈A.

  As described above, since the optical signal input to the coherent optical receiver 12 is amplified, the input level of the local oscillation light required when the signal level is controlled to a certain level is reduced. Therefore, as described above, this configuration works particularly effectively with respect to improving the sensitivity of the optical receiver 91 when there is a limit to the input level of the local oscillation light.

  As a modification of the first embodiment, the signal level detector 15 may have a coherent detection type reception function including the local oscillation light source 13. As in the previous embodiment, this configuration realizes a high-sensitivity digital coherent receiver corresponding to burst signals having a difference in intensity by increasing the sensitivity of the signal level detector 15 and widening and extending the length of the PON. The access network will be economicalized.

<Second Embodiment>
A second embodiment of the present invention is shown in FIG. The optical receiver 91 according to the present invention includes an ALC (Auto Level Control) optical amplifier 11 _ALC instead of the AGC optical amplifier 11 _AGC of the first embodiment. ALC optical amplifier 11 The _ALC controls the amplification factor so as to output an optical signal having a signal level within a certain range within the range of the input signal level assumed by the optical receiver 91.

An example of the configuration of the ALC optical amplifier 11 _ALC is shown in FIG. ALC Optical Amplifier 11 _ALC includes a branching device 111, a light receiver 112, an amplification factor control circuit 113, and an optical semiconductor amplifier 114. The light receiver 112 receives the optical signal branched by the branching device 111. The amplification factor control circuit 113 switches the gain of the optical semiconductor amplifier 114 according to the signal level of the optical signal received by the light receiver 112. Thus, the ALC optical amplifier 11 _ALC switches the gain according to the signal level of the input optical signal.

FIG. 8 shows a control example of the ALC optical amplifier 11 _ALC . R _ALC ′ is a value attenuated from R _ALC by the loss caused by the optical splitter 14. A certain threshold is set in the amplification factor control circuit 113 for the input signal level. The amplification factor control circuit 113 amplifies the input signal with a high amplification factor for an optical signal having a level lower than the threshold and with a low amplification factor for an optical signal with a level higher than the threshold. As a result, the output signal level from the optical semiconductor amplifier 114 falls within a predetermined range R _ALC .

FIG. 9 shows an operation example at the time of receiving signal constant level control in this configuration. Even when the signal level difference of the optical signal incident on the optical receiver 91 is large, the signal level difference (ab) of the optical signal output from the ALC optical amplifier 11 _ALC is small. Therefore, the signal level detector 15 can detect the signal level of the optical signal with high sensitivity.

According to the present embodiment, the ALC optical amplifier 11 _ALC and the signal level detection have the high dynamic range required for the coherent optical receiver 12 using the coherent detection method that receives intermittently incident burst-like optical signals. The dynamic range required for each signal device is reduced as compared with the first embodiment.

  At the same time, the optical signal input to the coherent optical receiver 12 of the optical receiver 91 is also amplified, so that the local oscillation light necessary for controlling the signal level of the optical signal to a certain fixed level that is the control target value is obtained. Input level is reduced. For example, the difference (cd) in the light level of the local oscillation light can be made smaller than in the first embodiment. Therefore, as described above, this configuration works particularly effectively with respect to improving the sensitivity of the optical receiver 91 when there is a limit to the input level of the local oscillation light.

  From the above, this configuration, as in the first embodiment, simultaneously increases the sensitivity of the signal level detector 15, expands the dynamic range of the optical receiver 91, and achieves high sensitivity corresponding to burst signals having intensity differences. The digital coherent receiving apparatus is realized. By applying the present invention, the access network can be made more economical by widening and extending the length of the PON.

<Third Embodiment>
A third embodiment of the present invention will be described below. In this configuration, instead of the AGC optical amplifier of the first embodiment or the ALC optical amplifier of the second embodiment, for example, a gain clamp type optical fiber amplifier 11 _CLAMP as shown in FIG. 10 is provided. The gain clamp type optical fiber amplifier 11 _{CLAMP according} to the present embodiment includes a pump light source 123, a WDM coupler 122, an amplification optical fiber 124, and a circulator 126. The WDM coupler 122 combines the pump light and the optical signal. The amplification optical fiber 124 amplifies the optical signal using pump light.

  The WDM coupler 125 cuts the wavelength component of the pump light. The circulator 126 causes the gain clamp light to enter the amplification optical fiber 124. Thereby, in the gain clamp type optical fiber amplifier, the range of the signal level of the optical signal that can be amplified with a constant amplification factor can be expanded. For this reason, an optical surge caused by a change in the incident signal level can be suppressed within the range of the signal level assumed by the system.

  From the above, according to the present embodiment, it is possible to obtain a constant amplification factor at the signal level assumed by the optical receiver 91. As a result, the signal level detector 15 has a wide dynamic range, and this configuration expands the dynamic range of the optical receiver 91 at the same time as increasing the sensitivity of the signal level detector 15 in the same manner as in the previous embodiment. A highly sensitive digital coherent receiving apparatus corresponding to a burst signal having a difference is realized. By applying the present invention, the access network can be made more economical by widening and extending the length of the PON.

<Fourth Embodiment>
A fourth embodiment of the present invention will be described below. In this configuration, instead of the AGC optical amplifier of the first embodiment or the ALC optical amplifier of the second embodiment, for example, as shown in FIG. An optical fiber amplifier 11 _FIBER is provided which controls the input level of pump light according to the signal level of the signal. The optical fiber amplifier 11 _{FIBER according} to the present embodiment includes an optical coupler 131, a pump light source 134, a variable optical attenuator 135, a pump light control circuit 133, a signal level detector 132, a WDM coupler 136, and an amplifier. An optical fiber 137 is provided.

  The WDM coupler 136 combines the optical signal and the pump light from the pump light source 134. The amplification optical fiber 137 amplifies the optical signal using pump light. The variable optical attenuator 135 attenuates the pump light. The signal level detector 132 detects the signal level of the optical signal branched by the optical coupler 131. The pump light control circuit 133 determines whether or not the signal level detected by the signal level detector 132 is a strong signal level that induces an optical surge, and in such a case, the variable optical attenuator Decrease the attenuation factor of 135. The determination of the pump light control circuit 133 is performed by determining whether or not a predetermined threshold value is exceeded, for example.

In this embodiment, when an optical signal having a strong signal level that induces an optical surge is incident, the pump light control circuit 133 increases the input level of the pump light. Thereby, it is possible to suppress an optical surge caused by a change in the signal level of the optical signal incident on the optical fiber amplifier 11 _FIBER, and to obtain a constant amplification factor at the input optical signal level assumed by the optical receiver 91. It becomes. From the above, the signal level detector 15 has a wide dynamic range, and this configuration expands the dynamic range of the optical receiver 91 at the same time as increasing the sensitivity of the signal level detector 15 as in the above embodiment. This realizes a highly sensitive digital coherent receiving apparatus corresponding to burst signals having intensity differences. By applying the present invention, the access network can be made more economical by widening and extending the length of the PON.

  The present invention can be applied to the information communication industry.

11 _AGC : AGC optical amplifier 11 _ALC : ALC optical amplifier 11 _CLAMP : gain clamp type optical fiber amplifier 11 _FIBER : optical fiber amplifier 12: coherent optical receiver 13: local oscillation light source 14: optical splitter 15: signal level detector 16 : Level measuring device 17: Local oscillation light control circuit 18: Variable optical attenuator 21S: Polarization beam splitter (PBS)
21L: Beam splitter (BS)
22X, 22Y: 90-degree optical hybrid 23 _XI , 23 _XQ , 23 _YI , 23 _YQ : Balanced photodetector 91: Optical receiver 111: Branch device 112: Light receiver 113: Amplification control circuit 114: Optical semiconductor amplifier 121 : Isolators 122 and 125: WDM coupler 123: Pump light source 124: Amplifying optical fiber 126: Circulator 131: Optical coupler 132: Signal level detector 133: Pump light control circuit 134: Pump light source 135: Variable optical attenuator 136: WDM Coupler 137: amplification optical fiber

Claims (8)

An optical amplifier that amplifies a burst-like optical signal incident intermittently at a predetermined amplification rate;
An optical branching device for branching the optical signal amplified by the optical amplifier;
A coherent optical receiver that converts one of the optical signals branched by the optical splitter into an electrical signal using a coherent detection method;
A local oscillation light source for supplying local oscillation light used in the coherent optical receiver to the coherent optical receiver;
A signal level detector for detecting a signal level of an optical signal input to the coherent optical receiver using the other of the optical signals branched by the optical splitter;
A variable optical attenuator that attenuates the local oscillation light in accordance with a signal level detected by the signal level detector so that a reception current of the coherent optical receiver becomes constant;
An optical receiver comprising:   The optical amplifier amplifies the optical signal at an amplification factor such that the signal level of the optical signal input to the signal level detector falls within a predetermined range. The optical receiver described.   The optical receiver according to claim 1, wherein the optical amplifier is a gain-clamped optical amplifier that amplifies at a constant amplification factor. The optical amplifier is
An optical fiber amplifier that amplifies the optical signal using pump light,
2. The optical receiver according to claim 1, wherein when an optical signal having a signal level exceeding a predetermined threshold is input, the optical level of the pump light is increased to a predetermined optical level. An optical amplification procedure for amplifying intermittently incident bursty optical signals at a predetermined amplification rate;
The signal level of the optical signal amplified by the optical amplification procedure is detected, and the reception current of the coherent optical receiver that converts it into an electrical signal using a coherent detection method is constant according to the signal level of the optical signal. Light level adjustment procedure to attenuate the oscillation light,
Coherent light receiving procedure in which the local oscillation light attenuated by the optical level adjusting procedure is incident on the coherent optical receiver, and the optical signal amplified by the optical amplification procedure is converted into an electric signal using the coherent optical receiver. When,
A coherent light receiving method in order.   The optical signal is amplified at an amplification factor such that the signal level of the optical signal input to the signal level detector is within a predetermined range in the optical amplification procedure. Coherent light receiving method.   6. The coherent optical reception method according to claim 5, wherein, in the optical amplification procedure, an optical signal is amplified using a gain clamp type optical amplifier that amplifies at a constant amplification factor. In the optical amplification procedure, the optical signal is amplified using an optical fiber amplifier that amplifies the optical signal using pump light,
6. The coherent light receiving method according to claim 5, wherein when the optical signal exceeds a predetermined threshold, the light level of the pump light is increased to a predetermined light level.

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