JP5583631B2 - Digital coherent receiver - Google Patents

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JP5583631B2
JP5583631B2 JP2011104996A JP2011104996A JP5583631B2 JP 5583631 B2 JP5583631 B2 JP 5583631B2 JP 2011104996 A JP2011104996 A JP 2011104996A JP 2011104996 A JP2011104996 A JP 2011104996A JP 5583631 B2 JP5583631 B2 JP 5583631B2
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JP2012238941A (en
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福太郎 濱岡
剛志 関
俊哉 松田
明 那賀
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Nippon Telegraph and Telephone Corp
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本発明は、偏波多重信号をデジタルコヒーレント検波方式を用いて受信処理するデジタルコヒーレント受信装置に関する。   The present invention relates to a digital coherent receiver that receives and processes polarization multiplexed signals using a digital coherent detection method.

100 Ｇbps を超える光伝送を実現するために、コヒーレント光通信技術とデジタル信号処理技術とを組み合わせたデジタルコヒーレント技術の研究が活発に行われている。デジタルコヒーレント技術を用いた伝送方式では、受信側でコヒーレントレシーバを用いたコヒーレント検波技術により高感度に偏波多重信号を分離して電気信号として出力し、それをＡＤ（Analog to Digital ）コンバータでデジタル信号に変換した後に、デジタルシグナルプロセッサ（ＤＳＰ： Digital Signal Processor ）を用いたデジタル信号処理により伝送路で歪んだ受信波形をデジタル領域で補償する（非特許文献１）。この受信技術により、偏波分離と波形の歪みの補正が簡易な構成で行えるため、偏波多重方式を適用した大容量かつ高速な伝送システムが実現可能である。   In order to realize optical transmission exceeding 100 Gbps, research on digital coherent technology combining coherent optical communication technology and digital signal processing technology has been actively conducted. In the transmission method using digital coherent technology, the polarization multiplexed signal is separated with high sensitivity by the coherent detection technology using a coherent receiver on the receiving side and output as an electrical signal, which is digitally output by an AD (Analog to Digital) converter. After conversion to a signal, a received waveform distorted in the transmission path by digital signal processing using a digital signal processor (DSP) is compensated in the digital domain (Non-Patent Document 1). With this reception technique, polarization separation and waveform distortion correction can be performed with a simple configuration, so that a large-capacity and high-speed transmission system using the polarization multiplexing method can be realized.

また、偏波多重光信号の偏波分離を、ＤＳＰを用いずに偏波コントローラを自動制御して実現する手法もある（非特許文献２）。この手法では、偏波多重光信号を偏波ビームスプリッタに入力し、その一方の偏波成分出力をフォトダイオードで光電気変換後にＲＦパワーモニタで監視して、そのパワーが最小になるように自動制御系を用いて偏波コントローラを制御することで偏波分離を実現している。   There is also a technique for realizing polarization separation of a polarization multiplexed optical signal by automatically controlling a polarization controller without using a DSP (Non-Patent Document 2). In this method, a polarization multiplexed optical signal is input to a polarization beam splitter, and one polarization component output is monitored by an RF power monitor after photoelectric conversion by a photodiode, and automatically adjusted so that the power is minimized. Polarization separation is realized by controlling the polarization controller using a control system.

S. J. Savory, “Digital filters for coherent optical receivers, ”Optics Express, vol.16, no.2, pp.804-814, Jan. 2008.S. J. Savory, “Digital filters for coherent optical receivers,” Optics Express, vol.16, no.2, pp.804-814, Jan. 2008. M.Yagi, S. Satomi, and S. Ryu,“Field Trial of 160-Gbit/s, Polarization-Division Multiplexed RZ-DQPSK Transmission System using Automatic Polarization Control, ”OFC/NFOEC, OThT7, California, USA, Feb. 2008.M.Yagi, S. Satomi, and S. Ryu, “Field Trial of 160-Gbit / s, Polarization-Division Multiplexed RZ-DQPSK Transmission System using Automatic Polarization Control,” OFC / NFOEC, OThT7, California, USA, Feb. 2008. 濱岡福太郎，関剛志，松田俊哉，那賀明，織田一弘，“デジタルコヒーレント方式における偏波状態変動耐力の評価”，信学技報，OCS2010-100 ，2010．Fukutaro Takaoka, Takeshi Seki, Toshiya Matsuda, Akira Naka, Kazuhiro Oda, “Evaluation of Polarization State Fluctuation Resistance in Digital Coherent Systems”, IEICE Technical Report, OCS2010-100, 2010. L. Liu, Z. Tao, W. Yan, S. Oda, T. Hoshida, and J. C. Rasmussen,“Initial Tap Setup of Constant Modulus Algorithm for Polarization De-multiplexing in Optical Coherent Receivers, ”OSA/OFC/NFOEC, OMT2, California, USA, Mar. 2009.L. Liu, Z. Tao, W. Yan, S. Oda, T. Hoshida, and JC Rasmussen, “Initial Tap Setup of Constant Modulus Algorithm for Polarization De-multiplexing in Optical Coherent Receivers,” OSA / OFC / NFOEC, OMT2 , California, USA, Mar. 2009. S. Hinz, D. Sandel, F. Wust, and R. Noe,“Interface detection enabling 2×20Gbit/s RZ polarization division multiplex transmission, ”Electronics Letters, vol.37, no.8, pp.511-512, Apr. 2001.S. Hinz, D. Sandel, F. Wust, and R. Noe, “Interface detection enabling 2 × 20 Gbit / s RZ polarization division multiplex transmission,” Electronics Letters, vol.37, no.8, pp.511-512, Apr. 2001. S. Hinz, D. Sandel, R. Noe, and F. Wust,“Optical NRZ 2 ×10Gbit/s polarization division multiplex transmission with endless polarization control driven by correlation signals,”Electronics Letters, vol.36, no.16, pp.1402-1403, Aug. 2000.S. Hinz, D. Sandel, R. Noe, and F. Wust, “Optical NRZ 2 × 10 Gbit / s polarization division multiplex transmission with endless polarization control driven by correlation signals,” Electronics Letters, vol.36, no.16, pp.1402-1403, Aug. 2000.

デジタルコヒーレント受信装置では、ＤＳＰでデジタル信号処理を施すことにより、偏波多重光信号の偏波分離および伝送路で生じた波形歪みの補償を同時に実現する。しかし、偏波多重光信号が伝送路を通過することによって、偏波状態（ＳＯＰ： State of Polarization）変化や偏波依存性損失（ＰＤＬ： Polarization Dependent Loss）が生じてしまうと、ＤＳＰ部の信号処理の不安定性増加や計算負荷増大の要因となり得る。例えば、伝送路中の高速なＳＯＰ変動に起因したデジタル信号処理の追従性低下によるビットエラーレートの悪化（非特許文献３）や、偏波多重信号がＰＤＬを有するデバイスを通過することにより偏波間に光強度差が生じるとデジタル信号処理の不安定性増加を招くこと（非特許文献４）が知られている。これらの問題はデジタル信号処理のアルゴリズムを改良することによって改善可能であるが、計算アルゴリズムの複雑化は、ＤＳＰ回路規模の増大や、ＤＳＰの処理速度低下を招いてしまう。   In the digital coherent receiver, digital signal processing is performed by the DSP, thereby simultaneously realizing polarization separation of the polarization multiplexed optical signal and compensation for waveform distortion generated in the transmission path. However, if the polarization multiplexed optical signal passes through the transmission path, a change in the polarization state (SOP: State of Polarization) or a polarization dependent loss (PDL) occurs, and the signal of the DSP unit This may increase processing instability and calculation load. For example, the bit error rate deteriorates due to a decrease in the followability of digital signal processing due to high-speed SOP fluctuations in the transmission path (Non-Patent Document 3), or the polarization multiplexed signal passes through a device having PDL. It is known that when a difference in light intensity occurs in a digital signal processing, the instability of digital signal processing increases (Non-Patent Document 4). These problems can be improved by improving the digital signal processing algorithm, but the complexity of the calculation algorithm leads to an increase in the DSP circuit scale and a decrease in the DSP processing speed.

また、非特許文献２のような偏波コントローラの自動制御のみによる偏波分離手法では、偏波モード分散（ＰＭＤ： Polarization Mode Dispersion ）は補償できず、制御系にＰＭＤ値に応じた遅延素子が必要となる。そして、ＰＭＤに高次の成分が存在する場合には、補償系に分散補償器が必要となる等、制御系が複雑なものとなってしまう。   In addition, the polarization separation method using only the automatic control of the polarization controller as in Non-Patent Document 2 cannot compensate for polarization mode dispersion (PMD), and a delay element corresponding to the PMD value is provided in the control system. Necessary. When a higher-order component exists in PMD, the control system becomes complicated, for example, a dispersion compensator is required for the compensation system.

本発明は、簡単な構成で、コヒーレント受信器に入力する偏波多重光信号の偏波状態を最適化し、後段のＤＳＰの負荷軽減および安定性向上を図ることができるデジタルコヒーレント受信装置を提供することを目的とする。   The present invention provides a digital coherent receiver capable of optimizing the polarization state of a polarization multiplexed optical signal input to a coherent receiver and reducing the load and improving the stability of a subsequent DSP with a simple configuration. For the purpose.

第１の発明は、偏波多重光信号の偏波状態を変化させる偏波コントローラと、偏波コントローラから出力される偏波多重光信号とローカル光を入力し、偏波分離してコヒーレント検波を行い、Ｘ偏波およびＹ偏波に対応する電気信号を出力するコヒーレント受信器と、コヒーレント受信器から出力されるＸ偏波およびＹ偏波に対応する電気信号をアナログ−デジタル変換して出力するＡＤコンバータと、ＡＤコンバータから出力されるデジタル信号をデジタル信号処理により復調するデジタルシグナルプロセッサと、コヒーレント受信器から出力されるＸ偏波の電気信号からＸ偏波のＲＦパワーを検出し、該Ｘ偏波のＲＦパワーに含まれるＹ偏波のクロストーク成分を除去するために、該Ｘ偏波のＲＦパワーが最小になるように偏波コントローラを制御する偏波コントローラ制御装置とを備える。 In the first invention, a polarization controller that changes the polarization state of a polarization multiplexed optical signal, a polarization multiplexed optical signal output from the polarization controller, and local light are input, and polarization separation is performed for coherent detection. A coherent receiver that outputs an electrical signal corresponding to the X polarization and the Y polarization, and an electrical signal corresponding to the X polarization and the Y polarization output from the coherent receiver is analog-digital converted and output. detecting an AD converter, a digital signal processor for demodulating the digital signal by the digital signal processing output from the AD converter, the RF power of the X polarization from the electrical signals of X polarization output from the coherent receiver, the X in order to remove the crosstalk component of the Y polarization contained in the RF power of the polarization, the polarization controller as RF power of the X polarized wave is minimized And a polarization controller control unit that controls the LA.

本発明のデジタルコヒーレント受信装置は、コヒーレント受信器から出力されるＸ偏波およびＹ偏波に対応する電気信号のＲＦパワーをモニタし、コヒーレント受信器へ入力される偏波多重光信号の偏波状態を光領域で最適化して各偏波におけるクロストーク成分を最小化することにより、デジタルシグナルプロセッサによるデジタル信号処理の負荷の軽減および安定性の向上を図ることができる。   The digital coherent receiver of the present invention monitors the RF power of the electrical signal corresponding to the X polarization and Y polarization output from the coherent receiver, and the polarization of the polarization multiplexed optical signal input to the coherent receiver. By optimizing the state in the optical region and minimizing the crosstalk component in each polarization, it is possible to reduce the load of digital signal processing by the digital signal processor and improve the stability.

本発明の実施例１の構成例を示す図である。It is a figure which shows the structural example of Example 1 of this invention. 本発明の実施例２の構成例を示す図である。It is a figure which shows the structural example of Example 2 of this invention. 本発明の実施例３の構成例を示す図である。It is a figure which shows the structural example of Example 3 of this invention.

図１〜図３は、本発明の実施例１〜実施例３の構成例を示す。
図１〜図３において、偏波多重光信号を偏波コントローラ１１に入力し、その出力と局発光源１３から出力されるローカル光をコヒーレント受信器１２に入力する。コヒーレント受信器１２は、偏波コントローラ１１の出力とローカル光をそれぞれ偏波ビームスプリッタ（ＰＢＳ）でＸ偏波とＹ偏波に分離し、各偏波ごとにコヒーレント検波する構成であり、Ｘ偏波のＩチャネルおよびＱチャネルの電気信号出力Ｉx ，Ｑｘと、Ｙ偏波のＩチャネルおよびＱチャネルの電気信号出力Ｉy ，Ｑy が取り出される。これらの電気信号出力は、ＡＤコンバータ１４でデジタル信号に変換した後に、ＤＳＰ１５によるデジタル信号処理により偏波多重信号が復調される。 1 to 3 show configuration examples of Embodiments 1 to 3 of the present invention.
1 to 3, the polarization multiplexed optical signal is input to the polarization controller 11, and the output and local light output from the local light source 13 are input to the coherent receiver 12. The coherent receiver 12 is configured to separate the output of the polarization controller 11 and the local light into X polarization and Y polarization by a polarization beam splitter (PBS), respectively, and perform coherent detection for each polarization. Wave I-channel and Q-channel electrical signal outputs Ix and Qx and Y-polarized I-channel and Q-channel electrical signal outputs Iy and Qy are taken out. These electrical signal outputs are converted into digital signals by the AD converter 14 and then the polarization multiplexed signal is demodulated by digital signal processing by the DSP 15.

実施例１では、コヒーレントレシーバ１２から出力されるＸ偏波のＩチャネルおよびＱチャネルの電気信号出力Ｉx ，Ｑｘを分岐し、偏波コントローラ制御装置１５へ入力する。偏波コントローラ制御装置１５は、Ｘ偏波のＩチャネルおよびＱチャネルの電気信号出力Ｉx ，ＱｘからＸ偏波のＲＦパワーを検出する。   In the first embodiment, the X-polarized I-channel and Q-channel electrical signal outputs Ix and Qx output from the coherent receiver 12 are branched and input to the polarization controller controller 15. The polarization controller control device 15 detects the RF power of the X polarization from the electric signal outputs Ix and Qx of the X polarization I channel and the Q channel.

実施例２では、コヒーレントレシーバ１２から出力されるＹ偏波のＩチャネルおよびＱチャネルの電気信号出力Ｉy ，Ｑy を分岐し、偏波コントローラ制御装置１５へ入力する。偏波コントローラ制御装置１５は、Ｙ偏波のＩチャネルおよびＱチャネルの電気信号出力Ｉy ，Ｑy からＹ偏波のＲＦパワーを検出する。   In the second embodiment, the Y-polarized I-channel and Q-channel electrical signal outputs Iy and Qy output from the coherent receiver 12 are branched and input to the polarization controller 15. The polarization controller controller 15 detects the Y-polarized RF power from the Y-polarized I channel and Q-channel electrical signal outputs Iy and Qy.

実施例３では、コヒーレントレシーバ１２から出力されるＸ偏波のＩチャネルおよびＱチャネルの電気信号出力Ｉx ，ＱｘおよびＹ偏波のＩチャネルおよびＱチャネルの電気信号出力Ｉy ，Ｑy を分岐し、偏波コントローラ制御装置１５へ入力する。偏波コントローラ制御装置１５は、Ｘ偏波のＩチャネルおよびＱチャネルの電気信号出力Ｉx ，ＱｘからＸ偏波のＲＦパワーを検出し、Ｙ偏波のＩチャネルおよびＱチャネルの電気信号出力Ｉy ，Ｑy からＹ偏波のＲＦパワーを検出する。   In the third embodiment, the X-polarized I-channel and Q-channel electrical signal outputs Ix and Qx output from the coherent receiver 12 and the Y-polarized I-channel and Q-channel electrical signal outputs Iy and Qy are branched to depolarize. Input to the wave controller controller 15. The polarization controller controller 15 detects the X-polarized RF power from the X-polarized I channel and Q-channel electrical signal outputs Ix and Qx, and the Y-polarized I-channel and Q-channel electrical signal outputs Iy, The RF power of Y polarization is detected from Qy.

非特許文献５，６によると、偏波多重光信号を偏波ビームスプリッタで分離した後に、フォトダイオードで光電気変換して得られる光電流ｉ1 ，ｉ2 は、
ｉ1 ∝ｂ1cos2ψ/2＋ｂ2sin2ψ/2＋２ｂ1ｂ2cosφsinψ …(1)
ｉ2 ∝ｂ1sin2ψ/2＋ｂ2cos2ψ/2−２ｂ1ｂ2cosφsinψ …(2)
で表される。ここで、φは送信機におけるＸＹ偏波間の位相差、ｂ1 およびｂ2 はＸ偏波およびＹ偏波の情報ビット（ｂ1,ｂ2 ∈｛0, 1｝）、ψは偏波ビームスプリッタの軸に対する入射角度を示している。 (1)式の第２項および第３項は、Ｘ偏波に対応する光電流ｉ1 におけるＹ偏波のクロストーク成分を示す。 (2)式の第１項および第３項は、Ｙ偏波に対応する光電流ｉ2 におけるＸ偏波のクロストーク成分を示す。 According to Non-Patent Documents 5 and 6, photocurrents i 1 and i 2 obtained by photoelectrically converting a polarization multiplexed optical signal using a photodiode after being separated by a polarization beam splitter are:
i 1 ∝b 1 cos 2 ψ / 2 + b 2 sin 2 ψ / 2 + 2b 1 b 2 cos φsinψ (1)
i 2 ∝b 1 sin 2 ψ / 2 + b 2 cos 2 ψ / 2−2 b 1 b 2 cos φsinψ (2)
It is represented by Where φ is the phase difference between the XY polarizations in the transmitter, b 1 and b 2 are the information bits (b 1 , b 2 ε {0, 1}) of the X and Y polarizations, and ψ is the polarization beam The incident angle with respect to the axis of the splitter is shown. The second term and the third term of the equation (1) indicate the crosstalk component of the Y polarization in the photocurrent i 1 corresponding to the X polarization. The first term and the third term in the equation (2) indicate the crosstalk component of the X polarization in the photocurrent i 2 corresponding to the Y polarization.

したがって、偏波多重光信号の偏波分離におけるＸＹ偏波間のクロストーク成分（ (1)式の第２項および第３項、および (2)式の第１項および第３項）を除去するために、ψ＝０となるように、すなわち実施例１ではＸ偏波のＲＦパワー（ (1)式の光電流ｉ 1 に比例）が最小となるように、偏波コントローラ制御装置１５が偏波コントローラ１１を用いて入力側の偏波状態を最適化すれば、各偏波におけるクロストーク成分が最小化してｉ1 ＝ｂ1 、ｉ2 ＝ｂ2 となる。
Therefore, the crosstalk component between the XY polarizations in the polarization separation of the polarization multiplexed optical signal (the second and third terms in the equation (1) and the first and third terms in the equation (2)) is removed. Therefore, the polarization controller control device 15 is biased so that ψ = 0, that is, in the first embodiment, the RF power of the X polarization ( proportional to the photocurrent i 1 in equation (1) ) is minimized. If the polarization state on the input side is optimized using the wave controller 11, the crosstalk component in each polarization is minimized so that i 1 = b 1 and i 2 = b 2 .

これにより、コヒーレント受信器１２へ入力される偏波多重光信号の偏波状態を光領域で最適化することができるので、ＤＳＰ１５のデジタル信号処理の負荷の軽減および安定性の向上を図ることができる。また、偏波コントローラ１１の自動制御単独では補償することのできないＰＭＤによる波形歪みは、ＤＳＰ１５を用いて補償することにより、簡易で高性能なデジタルコヒーレント受信装置を実現することができる。   As a result, the polarization state of the polarization multiplexed optical signal input to the coherent receiver 12 can be optimized in the optical region, so that the digital signal processing load of the DSP 15 can be reduced and the stability can be improved. it can. Moreover, a simple and high-performance digital coherent receiver can be realized by compensating for waveform distortion caused by PMD, which cannot be compensated for by automatic control alone of the polarization controller 11, by using the DSP 15.

１１ 偏波コントローラ
１２ コヒーレント受信器
１３ 局発光源
１４ ＡＤコンバータ
１５ ＤＳＰ
１６ 偏波コントローラ制御装置 11 Polarization Controller 12 Coherent Receiver 13 Local Light Source 14 AD Converter 15 DSP
16 Polarization controller controller

Claims (1)

偏波多重光信号の偏波状態を変化させる偏波コントローラと、
前記偏波コントローラから出力される偏波多重光信号とローカル光を入力し、偏波分離してコヒーレント検波を行い、Ｘ偏波およびＹ偏波に対応する電気信号を出力するコヒーレント受信器と、
前記コヒーレント受信器から出力されるＸ偏波およびＹ偏波に対応する電気信号をアナログ−デジタル変換して出力するＡＤコンバータと、
前記ＡＤコンバータから出力されるデジタル信号をデジタル信号処理により復調するデジタルシグナルプロセッサと、
前記コヒーレント受信器から出力されるＸ偏波の電気信号からＸ偏波のＲＦパワーを検出し、該Ｘ偏波のＲＦパワーに含まれるＹ偏波のクロストーク成分を除去するために、該Ｘ偏波のＲＦパワーが最小になるように前記偏波コントローラを制御する偏波コントローラ制御装置と
を備えたことを特徴とするデジタルコヒーレント受信装置。 A polarization controller that changes the polarization state of the polarization multiplexed optical signal;
A coherent receiver that inputs a polarization multiplexed optical signal and local light output from the polarization controller, performs polarization separation to perform coherent detection, and outputs an electrical signal corresponding to X polarization and Y polarization;
An analog-to-digital converter that outputs an electrical signal corresponding to the X polarization and the Y polarization output from the coherent receiver;
A digital signal processor that demodulates a digital signal output from the AD converter by digital signal processing;
Wherein detecting the RF power of the X polarized wave from the electrical signals of X polarization output from the coherent receiver, in order to remove the crosstalk component of the Y polarization contained in the RF power of the X polarization, the X A digital coherent receiver comprising: a polarization controller controller that controls the polarization controller so that the RF power of polarization is minimized.
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