JP2005123934A

JP2005123934A - Optical communication method, optical transmitter, optical receiver and optical communication system

Info

Publication number: JP2005123934A
Application number: JP2003357399A
Authority: JP
Inventors: Shigeru Kuwano; 茂桑野
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 2003-10-17
Filing date: 2003-10-17
Publication date: 2005-05-12
Anticipated expiration: 2023-10-17
Also published as: JP4247784B2

Abstract

<P>PROBLEM TO BE SOLVED: To easily realize optical fiber transmission of a polarized wave modulation signal which is conventionally difficult to be realized by a simple constitution without being influenced by polarization turbulence in a transmission path. <P>SOLUTION: In an optical transmitter 100, a polarization modulator 130 modulates the polarized wave state of the light outputted by a light source 110 by using a signal formed by differentially coding an inputted binary data signal by a precoder 120, and transmits the resultant light to the transmission path 200. In an optical receiver 300, an optical filter 310 receives light inputted from the transmission path 100 and light formed by delaying the inputted light by one bit. An optical receiving circuit 320 converts the obtained light photoelectrically, and differentially combines it to demodulate the data signal. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

本発明は、偏波状態に情報を重畳する光通信方法、光送信機、光受信機、および光通信システムに関するものである。 The present invention relates to an optical communication method for superimposing information on a polarization state, an optical transmitter, an optical receiver, and an optical communication system.

光の偏波状態に情報を重畳する光通信方法（偏波変調方法）は光通信技術の黎明期から提案されており、信号エネルギーを有効に活用できる方法として主に空間伝送系で注目されてきた（例えば、非特許文献１，２）。また、光ファイバ通信方法においても、長距離伝送時等において非線形光学効果の影響を受けにくい方法として注目されてきた。図１３に偏波変調方法を用いた従来の光通信システムの基本構成を示す。 An optical communication method (polarization modulation method) that superimposes information on the polarization state of light has been proposed since the dawn of optical communication technology, and has attracted attention mainly in spatial transmission systems as a method that can effectively use signal energy. (For example, Non-Patent Documents 1 and 2). Also, optical fiber communication methods have attracted attention as methods that are less susceptible to nonlinear optical effects during long distance transmission and the like. FIG. 13 shows a basic configuration of a conventional optical communication system using the polarization modulation method.

図１３において、光送信機１００の光源１１０からの出力光は、偏波変調器１３０において２つの直交する偏波状態に２値データがマッピングされ、偏波変調信号となる。ここで、２つの直交する偏波状態とは、ポアンカレ球上の対蹠点を示し、直交する直線偏波、逆回りの円偏波などが挙げられる。この偏波変調信号は光伝送路２００を伝搬され、光受信機３００に入力される。光受信機３００では、偏波分離器３４１で受信光を直交する２つの偏波に分離された後、それぞれ独立に受光器３４２，３４３に入力され、差動回路３４４でそれらの差をとることでもとのデータが復調される。 In FIG. 13, the output light from the light source 110 of the optical transmitter 100 is mapped to binary data in two orthogonal polarization states by the polarization modulator 130 to become a polarization modulation signal. Here, the two orthogonal polarization states indicate opposite points on the Poincare sphere, and examples include orthogonal linear polarization and reverse circular polarization. This polarization modulation signal propagates through the optical transmission line 200 and is input to the optical receiver 300. In the optical receiver 300, the received light is separated into two orthogonal polarizations by the polarization separator 341, and then separately input to the light receivers 342 and 343, and the difference between them is obtained by the differential circuit 344. But the original data is demodulated.

ところで、偏波変調方法を光ファイバ通信に適用する場合、ファイバ伝送中に生じる偏波面の変動（偏波面の回転ならびに直交する偏波間での位相差の発生）により、光受信機３００の偏波分離器３４１による分離が不十分となり、通信品質が時間的に変動してしまう。この偏波面の変動は機械的な振動や温度変化によって生じるものであり、その変化の速度は一般に光通信方法で用いられる情報伝送速度に対して十分に低い。 By the way, when the polarization modulation method is applied to optical fiber communication, the polarization of the optical receiver 300 is caused by fluctuations in the polarization plane (rotation of the polarization plane and generation of a phase difference between orthogonal polarizations) that occur during fiber transmission. Separation by the separator 341 becomes insufficient, and communication quality varies with time. The fluctuation of the polarization plane is caused by mechanical vibration or temperature change, and the speed of the change is generally sufficiently lower than the information transmission speed used in the optical communication method.

この問題を解決するために従来２つの方法が考えられている。第１の方法は図１４に示すように、受信機３００の前段部分に偏波コントローラ３４５を配置し、受信偏波状態を制御して良好な受信状態を保つものである（例えば、非特許文献２）。この場合、制御回路３４６において復調データ信号から偏波状態変動を推定し、その結果に基づいて偏波コントローラ３４５を制御するものである。 In order to solve this problem, two conventional methods have been considered. As shown in FIG. 14, the first method is to arrange a polarization controller 345 at the front stage of the receiver 300 and control the reception polarization state to maintain a good reception state (for example, non-patent document). 2). In this case, the control circuit 346 estimates the polarization state fluctuation from the demodulated data signal, and controls the polarization controller 345 based on the result.

第２の方法は受信された信号から偏波状態を電気的な信号処理により推定するものである（図１５、例えば、非特許文献３）。この場合、受信された光信号を３分波器３４７で３分波して偏波分離器３４１ａ、３４１ｂに入力し、また1/4波長板３４８を介して偏波分離器３４１ｃに入力し、それぞれの偏波分離器３４１ａ，３４１ｂ，３４１ｃの出力を受光器３４２ａと３４３ａ、３４２ｂと３４３ｂ、３４２ｃと３４３ｃに入力し、それぞれの出力信号を差動回路３４４ａ〜３４４ｃにより差をることによって、偏波状態を特徴付けるストークスパラメータｓ１，ｓ２，ｓ３を抽出し、その値に基づいてストークスパラメータ演算回路３４９で電気的な信号処理により元のデータを復調するものである。
塚本、桑野、森永著「二つの偏光面を用いた差動位相変調／へテロダイン検波方式」、電子情報通信学会論文誌 B-I,vol.J77-B-I,No.10,pp.629-639,1994年10月 A.Fukuchi et a1., "Polarization Shift Keying-Direct Detection (PolSK-DD) Scheme for Fiber Nonlinear Effect Insensitive Communication System",Proc. ECOC92, pp.169-172, 1994. S. Benedetto et al., "Direct Detection of Optical Digital Transmission Based on Polarization Shift Keying Modulation", IEEE JSAC, vol.13, No.3, pp.531-542,Apr. 1995. T. Miyano et al., "Suppression of degradation induced by SPM/XPM + GVD in WDM transmission using a bit-synchronous intensity modulated DPSK Signal",in Technical Digest of the 5th Optoelectronics and Communications Conference (OECC), Paper 14D3-3, 2000. The second method estimates the polarization state from the received signal by electrical signal processing (FIG. 15, for example, Non-Patent Document 3). In this case, the received optical signal is demultiplexed by the third demultiplexer 347 and input to the polarization separators 341a and 341b, and is input to the polarization separator 341c via the quarter wavelength plate 348. The outputs of the respective polarization beam splitters 341a, 341b, 341c are input to the light receivers 342a and 343a, 342b and 343b, 342c and 343c, and the respective output signals are differentiated by the differential circuits 344a to 344c. The Stokes parameters s1, s2, and s3 that characterize the wave state are extracted, and the original data is demodulated by electrical signal processing in the Stokes parameter calculation circuit 349 based on the extracted values.
Tsukamoto, Kuwano, Morinaga, “Differential phase modulation / heterodyne detection using two polarization planes”, IEICE Transactions BI, vol.J77-BI, No.10, pp.629-639,1994 October A. Fukuchi et a1., "Polarization Shift Keying-Direct Detection (PolSK-DD) Scheme for Fiber Nonlinear Effect Insensitive Communication System", Proc. ECOC92, pp.169-172, 1994. S. Benedetto et al., "Direct Detection of Optical Digital Transmission Based on Polarization Shift Keying Modulation", IEEE JSAC, vol.13, No.3, pp.531-542, Apr. 1995. T. Miyano et al., "Suppression of degradation induced by SPM / XPM + GVD in WDM transmission using a bit-synchronous intensity modulated DPSK Signal", in Technical Digest of the 5th Optoelectronics and Communications Conference (OECC), Paper 14D3-3 , 2000.

第１の方法の場合、偏波コントローラ３４５ならびに制御回路３４６が必要となり、回路規模が大きくなるとともに、コスト増大の要因となる。また、第２の方法の場合、回路構成が光学的ならびに電気的に複雑となり、高速伝送への適用が困難である。したがって、これら従来の方法は、いずれも実用的ではなく、現状で偏波変調方法は実用には供されていない。 In the case of the first method, the polarization controller 345 and the control circuit 346 are required, which increases the circuit scale and increases the cost. In the case of the second method, the circuit configuration is optically and electrically complicated, and it is difficult to apply to high-speed transmission. Therefore, none of these conventional methods are practical, and the polarization modulation method is not practically used at present.

本発明は、以上の問題を解決するため、簡易な方法で偏波変動の影響を受けることなく偏波変調信号を受信することが可能な光通信方法、光送信機、光受信機、および光通信システムを提供することを目的とする。 In order to solve the above problems, the present invention provides an optical communication method, an optical transmitter, an optical receiver, and an optical communication device capable of receiving a polarization-modulated signal without being affected by polarization fluctuations by a simple method. An object is to provide a communication system.

請求項１にかかる発明の光通信方法は、入力される２値のデータ信号を差動符号化した信号により光源の出力光の偏波状態を変調して伝送路に送出し、該伝送路からの入力光と該入力光を１ビット遅延させた光を合波させて前記データ信号を復調することを特徴とする。
請求項２にかかる発明は、請求項１に記載の光通信方法において、前記変調は、前記差動符号化した２値のデータを直交する２つの偏波状態に割り当てることにより行うことを特徴とする。
請求項３にかかる発明は、請求項１に記載の光通信方法において、前記変調は、前記入力される２値のデータ信号を２回差動符号化してから直並列変換して２つの信号とし、且つ前記光源の光を偏波状態が直交する２つの光に分波し、該２つの光の一方を前記２つの信号の一方により位相変調し、前記２つの光の他方を前記２つの信号の他方を１／２シンボル遅延した信号により位相変調し、該両位相変調された２つの光を合波して前記伝送路に送出することを特徴とする。
請求項４にかかる発明は、前記変調は、前記入力される２値のデータ信号を２回差動符号化してから相補関係にある２つのデュオバイナリ信号に変換し、且つ前記光源の光を偏波状態が直交する２つの光に分波し、該２つの光の一方を前記２つのデュオバイナリ信号の一方により位相変調し、前記２つの光の他方を前記２つのデュオバイナリ信号の他方により位相変調し、該両位相変調された２つの光を合波して前記伝送路に送出することを特徴とする。
請求項５にかかる発明は、請求項１乃至４のいずれか１つに記載の光通信方法において、前記光源として、前記２値のデータのシンボル周期に同期したパルス光を出力するパルス光源を使用することを特徴とする。
請求項６にかかる発明は、請求項１に記載の光通信方法において、前記復調は、前記伝送路から入力する入力光をマッハツェンダ干渉計で１シンボル分の光路長差をもつ２つの相補な光とし、該相補関係にある光信号を電気信号に変換してその差分をとることを特徴とする。
請求項７にかかる発明は、請求項１に記載の光通信方法において、前記復調を、前記伝送路からの入力光をヘテロダイン検波してから２つの偏波成分に分離して光電変換することにより２つの中間周波数信号を生成し、該各中間周波数信号を１ビット遅延検波した後に加算して前記データ信号を復調する復調方法に代えたことを特徴とする。
請求項８にかかる発明の光送信機は、光源と、入力される２値のデータ信号を差動符号化するプリコーダと、該プリコーダの出力信号により前記光源の出力光の偏波状態を変調する偏波変調器とを具備することを特徴とする。
請求項９にかかる発明は、請求項８に記載の光送信機において、前記偏波変調器は、前記プリコーダにより差動符号化した２値のデータを前記光源の出力光の直交する２つの偏波状態に割り当てることを特徴とする。
請求項１０にかかる発明は、請求項８に記載の光送信機において、前記プリコーダは、入力される２値のデータを差動符号化する第１の差動符号化器と、該第１の差動符号化器の出力データを再度差動符号化する第２の差動符号化器と、該第２の差動符号化器の出力信号を直並列変換して２つの信号に分離する直並列変換器と、該直並列変換器の一方の出力信号を１／２シンボル遅延させる遅延器とを具備し、前記偏波変調器は、前記光源の出力光が直交する２つの偏波成分に分離された各光を入力する第１および第２の位相変調器と、該第１および第２の位相変調器の出力光を合波する偏波合波器とを具備し、前記第１の位相変調器は前記直並列変換器の他方の出力信号により前記光源の出力光の一方の偏波成分を位相変調し、前記第２の位相変調器は前記遅延器の出力信号により前記光源の出力光の他方の偏波成分を位相変調することを特徴とする。
請求項１１にかかる発明は、請求項８に記載の光送信機において、前記プリコーダは、入力される２値のデータを差動符号化する第１の差動符号化器と、該第１の差動符号化器の出力データを再度差動符号化して相補関係にある２つの信号を出力する第２の差動符号化器と、該第２の差動符号化器から出力する一方の信号をデュオバイナリ信号に変換する第１のデュオバイナリフィルタと、前記第２の差動符号化器から出力する他方の信号をデュオバイナリ信号に変換する第２のデュオバイナリフィルタとを具備し、前記偏波変調器は、前記光源の出力光が直交する２つの偏波成分に分離された各光を入力する第１および第２の位相変調器と、該第１および第２の位相変調器の出力光を合波する偏波合波器とを具備し、前記第１の位相変調器は前記第１のデュオバイナリフィルタの出力信号により前記光源の出力光の一方の偏波成分を位相変調し、前記第２の位相変調器は前記第２のデュオバイナリフィルタの出力信号により前記光源の出力光の他方の偏波成分を位相変調することを特徴とする。
請求項１２にかかる発明の光受信機は、光伝送路からの入力光と該入力光を１シンボル遅延させた光とが合波されるインパルス応答を持った光フィルタと、該光フィルタの出力光を光電変換する光受信回路とを具備することを特徴とする。
請求項１３にかかる発明は、請求項１２に記載の光受信機において、前記光フィルタはマッハツェンダ干渉計からなり、前記光受信回路は該マッハツェンダ干渉計の２つの出力光を個別に光電変換する２つの受光器と、該両受光器の出力信号の差分を出力する差動回路とを具備することを特徴とする。
請求項１４にかかる発明は、光伝送路からの入力光を該入力光と異なる周波数の光で合波してヘテロダイン検波する光合波器と、該光合波器の出力光の直交する偏波成分を分離する偏波分離器と、該偏波分離器からの一方の出力光を光電変換する第１の受光器と、前記偏波分離器からの他方の出力光を光電変換する第２の受光器と、前記第１の受光器の出力信号を検波する第１の１ビット遅延検波器と、前記第２の受光器の出力信号を検波する第２の１ビット遅延検波器と、該第１および第２の１ビット遅延検波器の出力信号を加算する加算器とを具備することを特徴とする。
請求項１５にかかる発明の光通信システムは、請求項８乃至１１のいずれか１つに記載の光送信機と、請求項１２乃至１４のいずれか１つに記載の光受信機とを具備することを特徴とする。 The optical communication method of the invention according to claim 1 modulates the polarization state of the output light of the light source with a signal obtained by differentially encoding the input binary data signal and sends it to the transmission line. The data signal is demodulated by combining the input light and light obtained by delaying the input light by 1 bit.
According to a second aspect of the present invention, in the optical communication method according to the first aspect, the modulation is performed by assigning the differentially encoded binary data to two orthogonal polarization states. To do.
According to a third aspect of the present invention, in the optical communication method according to the first aspect, the modulation is performed by differentially encoding the input binary data signal twice and then performing serial-parallel conversion to obtain two signals. The light from the light source is demultiplexed into two lights whose polarization states are orthogonal, one of the two lights is phase-modulated with one of the two signals, and the other of the two lights is converted into the two signals. The other of the two is phase-modulated by a signal delayed by 1/2 symbol, and the two lights modulated in both phases are combined and transmitted to the transmission line.
In the invention according to claim 4, in the modulation, the input binary data signal is differentially encoded twice, converted into two duobinary signals having a complementary relationship, and the light from the light source is polarized. Demultiplexing into two lights whose wave states are orthogonal, one of the two lights is phase-modulated by one of the two duobinary signals, and the other of the two lights is phase-shifted by the other of the two duobinary signals Modulating, combining the two phase-modulated lights and sending them to the transmission line.
According to a fifth aspect of the present invention, in the optical communication method according to any one of the first to fourth aspects, a pulsed light source that outputs a pulsed light synchronized with a symbol period of the binary data is used as the light source. It is characterized by doing.
According to a sixth aspect of the present invention, in the optical communication method according to the first aspect, in the demodulation, the input light input from the transmission path is two complementary lights having an optical path length difference of one symbol by a Mach-Zehnder interferometer. The optical signal having the complementary relationship is converted into an electrical signal and the difference is obtained.
According to a seventh aspect of the present invention, in the optical communication method according to the first aspect, the demodulation is performed by performing heterodyne detection on the input light from the transmission path, and then separating the two polarization components into photoelectric conversion. Instead of the demodulation method, two intermediate frequency signals are generated and added after the intermediate frequency signals are subjected to 1-bit delay detection, and the data signals are demodulated.
An optical transmitter according to an eighth aspect of the present invention modulates the polarization state of the output light of the light source by a light source, a precoder that differentially encodes an input binary data signal, and an output signal of the precoder. And a polarization modulator.
According to a ninth aspect of the present invention, in the optical transmitter according to the eighth aspect of the present invention, the polarization modulator is configured to convert the binary data differentially encoded by the precoder into two orthogonal polarizations of the output light of the light source. It is characterized by assigning to a wave state.
According to a tenth aspect of the present invention, in the optical transmitter according to the eighth aspect, the precoder includes a first differential encoder for differentially encoding input binary data, and the first differential encoder. A second differential encoder for differentially encoding the output data of the differential encoder again, and a serial converter for converting the output signal of the second differential encoder into two signals by serial-parallel conversion. A parallel converter and a delay unit that delays one output signal of the serial-parallel converter by 1/2 symbol, and the polarization modulator converts the output light of the light source into two polarization components orthogonal to each other. 1st and 2nd phase modulator which inputs each separated light, and a polarization beam combiner which combines the output light of the 1st and 2nd phase modulator, The phase modulator phase-modulates one polarization component of the output light of the light source by the other output signal of the series-parallel converter, and the second modulator Phase modulator is characterized by phase-modulating the other polarization component of the output light of the light source by the output signal of the delay device.
According to an eleventh aspect of the present invention, in the optical transmitter according to the eighth aspect, the precoder includes a first differential encoder that differentially encodes input binary data, and the first differential encoder. A second differential encoder for differentially encoding the output data of the differential encoder and outputting two complementary signals, and one signal output from the second differential encoder And a second duobinary filter for converting the other signal output from the second differential encoder into a duobinary signal. The wave modulator is configured to input first and second phase modulators that receive the light beams separated into two orthogonal polarization components, and outputs from the first and second phase modulators. A polarization multiplexer for multiplexing light, and the first phase modulator. One polarization component of the output light of the light source is phase-modulated by the output signal of the first duobinary filter, and the second phase modulator outputs the output of the light source by the output signal of the second duobinary filter. The other polarization component of the light is phase-modulated.
An optical receiver according to a twelfth aspect of the invention includes an optical filter having an impulse response in which input light from an optical transmission line and light obtained by delaying the input light by one symbol are combined, and an output of the optical filter And a light receiving circuit for photoelectrically converting light.
According to a thirteenth aspect of the present invention, in the optical receiver according to the twelfth aspect, the optical filter includes a Mach-Zehnder interferometer, and the optical receiver circuit individually photoelectrically converts two output lights of the Mach-Zehnder interferometer. And a differential circuit that outputs a difference between output signals of the two light receivers.
According to the fourteenth aspect of the present invention, there is provided an optical multiplexer for combining the input light from the optical transmission line with light having a frequency different from that of the input light and performing heterodyne detection, and orthogonal polarization components of the output light of the optical multiplexer. , A first light receiver that photoelectrically converts one output light from the polarization separator, and a second light receiving that photoelectrically converts the other output light from the polarization separator A first 1-bit delay detector for detecting the output signal of the first light receiver, a second 1-bit delay detector for detecting the output signal of the second light receiver, and the first And an adder for adding the output signals of the second 1-bit delay detector.
An optical communication system according to a fifteenth aspect of the present invention includes the optical transmitter according to any one of the eighth to eleventh aspects and the optical receiver according to any one of the twelfth to fourteenth aspects. It is characterized by that.

本発明によれば、伝送路中での偏波擾乱の影響を受けることがなくなるので、従来実現が困難であった偏波変調信号の光ファイバ伝送を簡易な構成で容易に実現することが可能となる。 According to the present invention, since it is not affected by the polarization disturbance in the transmission path, it is possible to easily realize the optical fiber transmission of the polarization modulation signal, which has been difficult to realize conventionally, with a simple configuration. It becomes.

本発明では、光送信機においては、光源からの出力光に対して、プリコーダによって差動符号化されたデータ信号を用いて偏波変調器においてその偏波状態に情報を重畳され、光伝送路へと出される。光伝送路において信号光は、機械的振動や温度変化などによって偏波状態への擾乱を受けてしまうが、２値データに対する偏波状態の直交性はほぼ保たれている。ここで、データ伝送速度（＞１００Ｍｂｐｓ）に対して、擾乱の周波数は極めて低い（〜ｋＨｚ）ため、連続するシンボル間で受ける偏波の擾乱は同一であるとみなせる。このため、連続するシンボル間で相関を取ることにより、偏波変動の影響を受けることなく差動的に情報を復元することが可能となる。すなわち、光受信機において、１シンボル遅延させた信号と合波するインパルス応答を有する光フィルタに信号光を入力し、その出力を光検波器に入力することにより、情報を復調することが可能となる。 In the present invention, in the optical transmitter, information is superimposed on the polarization state in the polarization modulator using the data signal differentially encoded by the precoder for the output light from the light source, and the optical transmission line It is put out. In the optical transmission line, the signal light is disturbed to the polarization state due to mechanical vibration or temperature change, but the orthogonality of the polarization state with respect to the binary data is almost maintained. Here, since the frequency of the disturbance is extremely low (˜kHz) with respect to the data transmission rate (> 100 Mbps), it can be considered that the polarization disturbance received between consecutive symbols is the same. For this reason, by obtaining a correlation between consecutive symbols, it is possible to restore information differentially without being affected by polarization fluctuations. That is, in an optical receiver, it is possible to demodulate information by inputting signal light to an optical filter having an impulse response that is combined with a signal delayed by one symbol and inputting the output to an optical detector. Become.

本発明において、偏波変調に関しては、差動符号化された２値データを２つの互いに直交する偏波状態（ポアンカレ球上の対鍍点）にマッピングすれば十分であり、直交する２つの直線偏波や互いに逆回りの２つの円偏波といった特定の状態にする必要は無い。 In the present invention, with respect to polarization modulation, it is sufficient to map the differentially encoded binary data to two mutually orthogonal polarization states (opposite points on the Poincare sphere). There is no need for a specific state such as polarization or two circularly polarized waves that are opposite to each other.

また、差動符号化された２値データを直並列変換して２分岐した後、一方を１ビット（２分の１シンボル）だけ遅延させ、これらの信号を偏波変調器に入力し、２つの直交する偏波状態に独立に位相変調を行うことによって偏波変調を行うこともできる。 In addition, after differentially encoded binary data is serial-parallel converted and branched into two, one of them is delayed by 1 bit (1/2 symbol), and these signals are input to the polarization modulator. Polarization modulation can also be performed by independently performing phase modulation on two orthogonal polarization states.

さらに、差動符号化されたデータをデュオバイナリフィルタによってデュオバイナリ符号化し、この信号を偏波変調器に入力し、２つの直交する偏波状態に相補的に位相変調を行い、偏波変調を行うこともできる。 Furthermore, the differentially encoded data is duobinary encoded by a duobinary filter, and this signal is input to a polarization modulator, and phase modulation is performed in a complementary manner to two orthogonal polarization states. It can also be done.

本発明において光源として、シンボルレートに応じたパルス光源を用いることも可能でありその際における変調方法は上述の各方法が利用可能である。 In the present invention, a pulse light source corresponding to a symbol rate can be used as the light source, and the above-described methods can be used as the modulation method at that time.

本発明における光フィルタとしては、１シンボル分の光路長差を有するマッハツェンダ干渉計が利用可能である。マッハツェンダ干渉計では光カプラ部に２つの相補的に出力が得られるため、それぞれを独立に光電変換し、その出力差をとることにより、より効率的な受信を行うことが可能となる。 As the optical filter in the present invention, a Mach-Zehnder interferometer having an optical path length difference for one symbol can be used. In the Mach-Zehnder interferometer, two complementary outputs can be obtained in the optical coupler unit. Therefore, each of the Mach-Zehnder interferometers can independently perform photoelectric conversion and take an output difference thereof to perform more efficient reception.

本発明はコヒーレント光通信方法にも適用可能であり、光受信機において、伝送路からの入力光と周波数の異なる局部発振光源からの局部発振光とを合波し、これを偏波分岐器で分岐した後に受光器に入力し、出力として得られる中間周波数帯信号をそれぞれ１ビット遅延検波し、それらの和をとることにより復調を行うことができる。その際における変調方法は上述の各方法が利用可能である。 The present invention can also be applied to a coherent optical communication method. In an optical receiver, input light from a transmission path and local oscillation light from a local oscillation light source having different frequencies are combined, and this is combined with a polarization splitter. After branching, the signal is input to the optical receiver, and each intermediate frequency band signal obtained as an output is subjected to 1-bit delay detection, and demodulation can be performed by taking the sum thereof. As the modulation method at that time, the above-described methods can be used.

図１に本発明の実施例１の光通信システムの構成を示す。光送信機１００は、光源１１０と、２値のデータ信号を差動符号化するプリコーダ１２０と、そのプリコーダ１２０により差動符号化した２値のデータを直交する２つの偏波状態に割り当てることにより光源１１０の出力光の偏波状態を変調させて光伝送路２００に送出する偏波変調器１３０から構成される。光受信機３００は、光伝送路２００からの出力光とその出力光を１ビット遅延させた光とを合波するインパルス応答を有する光フィルタ３１０と、その光フィルタ３１０からの出力光を光電変換する光受信回路３２０とから構成される。 FIG. 1 shows a configuration of an optical communication system according to a first embodiment of the present invention. The optical transmitter 100 assigns light sources 110, a precoder 120 that differentially encodes a binary data signal, and binary data that is differentially encoded by the precoder 120 to two orthogonal polarization states. It comprises a polarization modulator 130 that modulates the polarization state of the output light of the light source 110 and sends it to the optical transmission line 200. The optical receiver 300 includes an optical filter 310 having an impulse response that combines output light from the optical transmission line 200 and light obtained by delaying the output light by 1 bit, and photoelectric conversion of the output light from the optical filter 310. And an optical receiving circuit 320.

以下、実施例１における変復調動作を説明する。プリコーダ１２０で差動符号化されたｎビット目のデータｄ_Tnにより変調された偏波変調光電界Ｅ_0nは、Jonesベクトルを用いて次式のように表すことができる。

ここで電界振幅Ｅは信号光電力で規格化している。なお、ここでは直交する２つの直線偏波に情報を重畳する場合を想定しているが、他の場合についても同様である。 Hereinafter, the modulation / demodulation operation in the first embodiment will be described. The polarization-modulated optical electric field E _0n modulated by the n-th bit data d _Tn differentially encoded by the precoder 120 can be expressed by the following equation using the Jones vector.

Here, the electric field amplitude E is normalized by the signal light power. Here, it is assumed that information is superimposed on two orthogonal linearly polarized waves, but the same applies to other cases.

この偏波変調光は光伝送路２００の伝搬中に偏波の擾乱を受ける。この擾乱Ｍは、偏波面の回転と直交偏波成分間の位相差であり、以下のJones行列によって記述される。

ここでθは回転角であり、φは位相差である。したがって、光受信機３００が受信する受信信号光Ｅ_1nは次式となる。

This polarization-modulated light is subjected to polarization disturbance during propagation through the optical transmission line 200. This disturbance M is a phase difference between the rotation of the polarization plane and the orthogonal polarization component, and is described by the following Jones matrix.

Here, θ is a rotation angle, and φ is a phase difference. Therefore, the received signal light E _1n received by the optical receiver 300 is expressed by the following equation.

光フィルタ３１０として図２に示すような３dBカップラ３１１，３１３および１ビット遅延器３１２をもつマッハツェンダフィルタ（マッハツェンダ干渉計）を用いるとすると、その出力Ｅ_Mnは、

となる。ここで連続する２ビット間で偏波擾乱に変化は無いと仮定している。また、±はマッハツェンダフィルタの２つの出力ポートに対応している。＋側の出力に対応する光強度Ｉ_1nは、

となり、伝送路中での偏波擾乱の影響を受けることなく差動的にデータが再生できていることが分かる。 If a Mach-Zehnder filter (Mach-Zehnder interferometer) having 3

dB couplers

311 and 313 and a 1-bit delay device 312 as shown in FIG. 2 is used as the optical filter 310, its output E _Mn is

It becomes. Here, it is assumed that there is no change in the polarization disturbance between two consecutive bits. Further, ± corresponds to the two output ports of the Mach-Zehnder filter. The light intensity I _1n corresponding to the output on the + side is

Thus, it can be seen that the data can be reproduced differentially without being affected by the polarization disturbance in the transmission line.

一方、マッハツェンダフィルタの−側の出力に対応する光強度I_2nは

となり、情報データに関して相補的な強度が得られる。したがって、マッハツェンダフィルタの２つの出力Ｉ_1n，I_2nをそれぞれ独立の受光器３２１，３２２で受光検出し、差動回路３２３でそれらの出力差をとると、

となり、より効率的な受信を行うことが可能となる。 On the other hand, the light intensity I _2n corresponding to the negative output of the Mach-Zehnder filter is

Thus, complementary strength can be obtained with respect to the information data. Accordingly, when the two outputs I _{1n and} I _2n of the Mach-Zehnder filter are detected by the independent

light receivers

321 and 322 and the output difference between them is obtained by the differential circuit 323,

Thus, more efficient reception can be performed.

図３に本実施例１における変復調動作例を示す。同図における各観測点(a)〜(e)の場所は図１に示しており、偏波状態の“Ｈ”および“Ｖ”は直線偏波での変調を想定して水平偏波と垂直偏波を示している。このように、差動符号化を行うことにより、データを再生できることが分かる。 FIG. 3 shows an example of modulation / demodulation operation in the first embodiment. The locations of the observation points (a) to (e) in the figure are shown in FIG. 1, and “H” and “V” in the polarization state are perpendicular to the horizontal polarization assuming modulation with linear polarization. The polarization is shown. Thus, it can be seen that data can be reproduced by performing differential encoding.

図４に数値シミュレーションによる受信波形を示す。同図より偏波擾乱を受けている場合においても上式で説明した通り、良好に信号が受信できていることが分かる。 FIG. 4 shows a received waveform by numerical simulation. From the figure, it can be seen that the signal can be received satisfactorily as described in the above equation even when the polarization disturbance is received.

図５に本発明の実施例２の光通信システムの構成を示す。本実施例２が実施例１と相違する点は、光送信機１００が、第１および第２の差動符号化器１２１，１２２、直並列変換回路１２３、および１／２シンボルの遅延器１２４を具備するプリコーダ１２０と、直交する偏波成分に独立した位相変調を行う第１および第２の位相変調器１３１，１３２および偏波合成器１３３を具備する偏波変調器１３０とから構成される点にある。本実施例２における光伝送路２００および光受信機３００は実施例１と同一である。 FIG. 5 shows the configuration of an optical communication system according to the second embodiment of the present invention. The second embodiment is different from the first embodiment in that the optical transmitter 100 includes first and second differential encoders 121 and 122, a serial-parallel converter circuit 123, and a 1/2 symbol delay unit 124. And a polarization modulator 130 including first and second phase modulators 131 and 132 that perform phase modulation independent of orthogonal polarization components, and a polarization beam combiner 133. In the point. The optical transmission line 200 and the optical receiver 300 in the second embodiment are the same as those in the first embodiment.

以下、図５と図６を用いて本実施例における変調動作を説明する。図６における各観測点(a)〜(e)の場所は図５に示す。第２の差動符号化器１２２の出力のデータ列は直並列変換回路１２３に入力され、２つのデータ列ｄｘ，ｄｙに分離される。偏波変調器１３０では、光源１１０からの入力光を直交する２つの偏波成分に等しく分割し、それぞれの偏波成分に対して位相変調器１３１，１３２でデータ列ｄｘ，ｄｙを用いて独立に位相変調（０，π）を行う。この際、データ列ｄｙによる変調タイミングを、遅延器１２４においてデータ列ｄｘによる変調タイミングに対して１／２シンボル（原データの１ビット）分だけ遅延させる。このように位相変調された２つの偏波光を偏波合成器１３３で合波することにより、１ビットごとに直交偏波成分間の位相差が変化する偏波変調信号を得ることができる。 Hereinafter, the modulation operation in this embodiment will be described with reference to FIGS. The locations of the observation points (a) to (e) in FIG. 6 are shown in FIG. The data string output from the second differential encoder 122 is input to the serial-parallel conversion circuit 123 and separated into two data strings dx and dy. In the polarization modulator 130, the input light from the light source 110 is equally divided into two orthogonal polarization components, and the phase modulators 131 and 132 independently use the data strings dx and dy for each polarization component. Phase modulation (0, π) is performed. At this time, the modulation timing by the data sequence dy is delayed by 1/2 symbol (one bit of the original data) by the delay unit 124 with respect to the modulation timing by the data sequence dx. By combining the two polarization lights thus phase-modulated by the polarization beam combiner 133, it is possible to obtain a polarization modulation signal in which the phase difference between the orthogonal polarization components changes for each bit.

ここで、変調時の信号帯域は各偏波成分の変調速度が１シンボルごとであることから、実施例１における変調に対して１／２と狭帯域にすることができる。図７に本実施例２における光スペクトルと受信波形（光受信回路３２０の差動出力のアイダイアグラム）を示す。同図より、本実施例２においても良好に信号が受信できていることが分かる。なお、本実施例２では変調時ならびに復調時に差動動作が必要になるため、プリコーダ１２０での差動符号化は２段にわたって行う必要がある。 Here, since the modulation speed of each polarization component is one symbol, the signal band at the time of modulation can be narrowed to 1/2 as compared with the modulation in the first embodiment. FIG. 7 shows an optical spectrum and a received waveform (eye diagram of the differential output of the optical receiving circuit 320) in the second embodiment. From the figure, it can be seen that the signal can be received well in the second embodiment. In the second embodiment, a differential operation is required at the time of modulation and demodulation, and therefore the differential encoding at the precoder 120 needs to be performed in two stages.

図８に本発明の実施例３の光通信システムの構成を示す。本実施例３が前記した実施例１，２と相違する点は、光送信機１００が、相補的な２出力を行う２重の差動符号化器１２１，１２２Ａ、およびデュオバイナリフィルタ１２５，１２６を具備するプリコーダ１２０と、直交する偏波成分に独立な位相変調を行う第１および第２の位相変調器１３１，１３２および偏波合成器１３３からなる偏波変調器１３０とから構成される点にある。 FIG. 8 shows a configuration of an optical communication system according to the third embodiment of the present invention. The third embodiment is different from the first and second embodiments in that the optical transmitter 100 has dual differential encoders 121 and 122A that perform two complementary outputs, and duobinary filters 125 and 126. And a polarization modulator 130 composed of first and second phase modulators 131 and 132 that perform independent phase modulation on orthogonal polarization components and a polarization beam combiner 133. It is in.

以下、図８と図９を用いて本実施例における変調動作を説明する。図９における各観測点(a)〜(e)の場所は図８に示す。差動符号化器１２２Ａの相補的な２出力はそれぞれデュオバイナリフィルタ１２５，１２６に入力され３値のデュオバイナリ信号となる。偏波変調器１３０では、光源１１０からの入力光を直交する２つの偏波成分に等しく分割し、それぞれの偏波成分に対して相補的なデュオバイナリ信号を用いて２つの位相変調器１３１，１３２で位相変調（-1/2π、０、1/2π）を行う。このように位相変調された偏波成分を偏波合成器１３３で合波することにより、１ビットごとに直交偏波成分間の位相差が変化する偏波変調信号を得ることができる。 Hereinafter, the modulation operation in this embodiment will be described with reference to FIGS. The locations of the observation points (a) to (e) in FIG. 9 are shown in FIG. The two complementary outputs of the differential encoder 122A are input to the duobinary filters 125 and 126, respectively, and become ternary duobinary signals. In the polarization modulator 130, the input light from the light source 110 is equally divided into two orthogonal polarization components, and two phase modulators 131, 131 using a duobinary signal complementary to each polarization component. At 132, phase modulation (-1 / 2π, 0, 1 / 2π) is performed. By combining the polarization components thus phase-modulated by the polarization beam combiner 133, a polarization-modulated signal in which the phase difference between the orthogonal polarization components changes for each bit can be obtained.

ここで、変調時の信号帯域はデュオバイナリ信号を用いていることから、実施例１における変調に対して１／２と狭帯域にすることができる。図１０に本実施例における光スペクトルと受信波形（光受信回路３２０の差動出力のアイダイアグラム）を示す。同図より、本実施例３においても良好に信号が受信できていることが分かる。なお、本実施例３ではデュオバイナリ符号化時ならびに復調時に差動動作が必要になるため、プリコーダ１２０での差動符号化は２段にわたって行う必要がある。 Here, since a duobinary signal is used as a signal band at the time of modulation, the band can be narrowed to 1/2 as compared with the modulation in the first embodiment. FIG. 10 shows an optical spectrum and a received waveform (eye diagram of the differential output of the optical receiving circuit 320) in the present embodiment. From the figure, it can be seen that signals can be received well in the third embodiment. In the third embodiment, a differential operation is required at the time of duobinary encoding and at the time of demodulation. Therefore, the differential encoding by the precoder 120 needs to be performed in two stages.

図１１に本発明の実施例４の光通信システムの構成を示す。本実施例４が前記した実施例１〜３と相違する点は、光源にクロック信号を入力し、シンボル周期に同期したパルス光源１１１とした点である。パルス光源１１１としては、モードロックレーザのように光源を直接変調するものならびに外部変調器によって強度変調を行うものがある。本実施例４のようにシンボル周期に同期したパルス光源１１１を用いる場合、信号光強度はＲＺ−ＤＰＳＫ正方法（例えば、非特許文献４）と同様にシンボル周期で変化する。このため、非線形光学効果による相互位相変調ひずみがシンボル繰り返し周波数成分に集中し、その影響を電気フィルタにより簡易に抑圧することができる。 FIG. 11 shows the configuration of an optical communication system according to the fourth embodiment of the present invention. The fourth embodiment is different from the first to third embodiments in that a clock signal is input to the light source and the pulse light source 111 is synchronized with the symbol period. Examples of the pulsed light source 111 include those that directly modulate the light source, such as a mode-locked laser, and those that perform intensity modulation with an external modulator. When the pulse light source 111 synchronized with the symbol period is used as in the fourth embodiment, the signal light intensity changes with the symbol period as in the RZ-DPSK positive method (for example, Non-Patent Document 4). For this reason, the cross-phase modulation distortion due to the nonlinear optical effect concentrates on the symbol repetition frequency component, and the influence can be easily suppressed by the electric filter.

図１２に本発明の実施例５の光通信システムの構成を示す。本実施例５が前記した実施例１〜４と相違する点は、光受信機３００を光コヒーレント受信機としている点である。すなわち、光受信機３００は、局発光源３３１、ヘテロダイン検波を行う光合波器３３２、直交する偏波成分を分離する偏波分離器３３３、光電変換を行う受光器３３４ａ，３３４ｂ、バンドパスフィルタ３３５ａ，３３５ｂ、１ビット遅延検波回路３３６ａ，３３６ｂ、ローパスフィルタ３３７ａ，３３７ｂ、ならびに加算回路３３８から構成される。 FIG. 12 shows the configuration of an optical communication system according to the fifth embodiment of the present invention. The fifth embodiment is different from the first to fourth embodiments in that the optical receiver 300 is an optical coherent receiver. That is, the optical receiver 300 includes a local light source 331, an optical multiplexer 332 that performs heterodyne detection, a polarization separator 333 that separates orthogonal polarization components, light receivers 334a and 334b that perform photoelectric conversion, and a bandpass filter 335a. , 335b, 1-bit delay detection circuits 336a and 336b, low-pass filters 337a and 337b, and an adder circuit 338.

以下、本実施例５での受信動作を説明する。受信光信号は局発光源３３１からの局発光と光合波器３３２で混合されてヘテロダイン検波され、局発光成分を等しく分配する偏波分離器３３３において直交する２つの偏波成分に分離される。各成分は受光器３３４ａ，３３４ｂでそれぞれ独立に光電変換され、バンドパスフィルタ３３５ａ，３３５ｂにより中間周波数（ＩＦ）の成分が抽出される。各中間周波成分i_xn、ｉ_ynの複素振幅は次式となる。

Hereinafter, the reception operation in the fifth embodiment will be described. The received optical signal is mixed with the local light from the local light source 331 and the optical multiplexer 332, subjected to heterodyne detection, and separated into two orthogonal polarization components in the polarization separator 333 that equally distributes the local light components. Each component is independently photoelectrically converted by the light receivers 334a and 334b, and an intermediate frequency (IF) component is extracted by the bandpass filters 335a and 335b. The complex amplitude of each intermediate frequency component i _xn , i _yn is given by

２つの偏波に対応するＩＦ成分は１ビット遅延検波回路３３６ａ，３３６ｂでそれぞれ独立に１ビット遅延検波され、ローパスフィルタ３３７ａ，３３７ｂで抽出される。それらの低域成分Ｉ_xn、Ｉ_ynは、

(12)
となり、加算器３３８において２つの偏波に対応する成分の和をとることによって

として偏波変動の影響を受けることなく信号が復調される。なお、本実施例における変調方法としては実施例１〜４に記載の各変調方法が適用可能である。 The IF components corresponding to the two polarizations are independently detected by 1-bit delay detection by the 1-bit

delay detection circuits

336a and 336b and extracted by the low-

pass filters

337a and 337b. Their low frequency components I _xn and I _yn are

(12)
By adding the components corresponding to the two polarizations in the adder 338,

As a result, the signal is demodulated without being affected by the polarization fluctuation. Note that each modulation method described in the first to fourth embodiments can be applied as a modulation method in the present embodiment.

実施例１の光通信システムの構成を示すブロック図である。1 is a block diagram illustrating a configuration of an optical communication system according to a first embodiment. マッハツェンダフィルタを用いた光受信機の構成のブロック図である。It is a block diagram of the structure of the optical receiver using a Mach-Zehnder filter. 実施例１の変復調動作の説明図である。It is explanatory drawing of the modulation / demodulation operation | movement of Example 1. FIG. 実施例１の受信波形の説明図である。It is explanatory drawing of the received waveform of Example 1. FIG. 実施例２の光通信システムの構成を示すブロック図である。It is a block diagram which shows the structure of the optical communication system of Example 2. FIG. 実施例２の変復調動作の説明図である。It is explanatory drawing of the modulation / demodulation operation | movement of Example 2. FIG. 実施例２の光スペクトルと受信波形の説明図である。It is explanatory drawing of the optical spectrum of Example 2, and a received waveform. 実施例３の光通信システムの構成を示すブロック図である。FIG. 6 is a block diagram illustrating a configuration of an optical communication system according to a third embodiment. 実施例３の変復調動作の説明図である。It is explanatory drawing of the modulation / demodulation operation | movement of Example 3. FIG. 実施例３の光スペクトルと受信波形の説明図である。It is explanatory drawing of the optical spectrum of Example 3, and a received waveform. 実施例４の光通信システムの構成を示すブロック図である。FIG. 10 is a block diagram illustrating a configuration of an optical communication system according to a fourth embodiment. 実施例５の光通信システムの構成を示すブロック図である。FIG. 10 is a block diagram illustrating a configuration of an optical communication system according to a fifth embodiment. 従来の光通信システムの構成を示すブロック図である。It is a block diagram which shows the structure of the conventional optical communication system. 従来の偏波変動対策をもつ光受信機の構成を示すブロック図である。It is a block diagram which shows the structure of the optical receiver with the conventional polarization fluctuation countermeasure. 従来の別の偏波変動対策のもつ光受信機の構成を示すブロック図である。It is a block diagram which shows the structure of the optical receiver with another conventional countermeasure against polarization fluctuation.

Explanation of symbols

１００：光送信機
１１０：光源
１１１：パルス光源
１２０：プリコーダ
１２１，１２２，１２２Ａ：差動復号化器
１２３：直並列変換器
１２４：遅延器
１２５，１２６：デュオバイナリフィルタ
１３０：偏波変調器
１３１，１３２：位相変調器
１３３：偏波合成器
２００：光伝送路
３００：光受信機
３１０：光フィルタ
３１１：３ｄＢカップラ
３１２：１ビット遅延器
３１３：３ｄＢカップラ
３２０：光受信回路
３２１，３２２：受光器
３３１：局発光源
３３２：光合波器
３３３：偏波分離器
３３４ａ，３３４ｂ：受光器
３３５ａ，３３５ｂ：バンドパスフィルタ
３３６ａ，３３６ｂ：１ビット遅延検波回路
３３７ａ，３３７ｂ：ローパスフィルタ
３３８：加算回路
３４１，３４１ａ，３４１ｂ，３４１ｃ：偏波分離器
３４２，３４２ａ，３４２ｂ，３４２ｃ：受光器
３４３，３４３ａ，３４３ｂ，３４３ｃ：受光器
３４４，３４４ａ，３４４ｂ，３４４ｃ：差動回路
３４５：偏波コントローラ
３４６：制御回路
３４７：３分波器
３４８：1/4波長板
３４９：ストークスパラメータ演算回路 DESCRIPTION OF SYMBOLS 100: Optical transmitter 110: Light source 111: Pulse light source 120: Precoder 121,122,122A: Differential decoder 123: Series-parallel converter 124: Delay device 125, 126: Duobinary filter 130: Polarization modulator 131 132: Phase modulator 133: Polarization synthesizer 200: Optical transmission line 300: Optical receiver 310: Optical filter 311: 3 dB coupler 312: 1-bit delay device 313: 3 dB coupler 320: Optical receiving circuit 321, 322: Light reception 331: Local light source 332: Optical multiplexer 333: Polarization separator 334a, 334b: Light receiver 335a, 335b: Band pass filter 336a, 336b: 1-bit delay detection circuit 337a, 337b: Low pass filter 338: Adder circuit 341 , 341a, 341b, 341c: polarization separator 3 2, 342a, 342b, 342c: Light receivers 343, 343a, 343b, 343c: Light receivers 344, 344a, 344b, 344c: Differential circuit 345: Polarization controller 346: Control circuit 347: Third duplexer 348: 1 / 4-wavelength plate 349: Stokes parameter calculation circuit

Claims

The polarization state of the output light of the light source is modulated by a signal obtained by differentially encoding the input binary data signal and sent to the transmission line, and the input light from the transmission line and the input light are delayed by 1 bit. An optical communication method, wherein the data signal is demodulated by multiplexing the received light.

The optical communication method according to claim 1,
The optical communication method according to claim 1, wherein the modulation is performed by assigning the binary data subjected to differential encoding to two orthogonal polarization states.

The optical communication method according to claim 1,
In the modulation, the input binary data signal is differentially encoded twice and then serial-parallel converted into two signals, and the light from the light source is split into two lights having orthogonal polarization states. Then, one of the two lights is phase-modulated by one of the two signals, the other of the two lights is phase-modulated by a signal obtained by delaying the other of the two signals by 1/2 symbol, and the both phase modulation is performed. An optical communication method comprising: combining two transmitted lights and sending them to the transmission line.

In the modulation, the input binary data signal is differentially encoded twice and then converted into two duobinary signals having a complementary relationship, and the light from the light source is converted into two lights having orthogonal polarization states. And phase-modulating one of the two lights with one of the two duobinary signals, phase-modulating the other of the two lights with the other of the two duobinary signals, An optical communication method characterized by combining two light beams and sending them to the transmission line.

The optical communication method according to any one of claims 1 to 4,
An optical communication method using a pulsed light source that outputs pulsed light synchronized with a symbol period of the binary data as the light source.

The optical communication method according to claim 1,
In the demodulation, the input light input from the transmission path is converted into two complementary lights having an optical path length difference of one symbol by a Mach-Zehnder interferometer, the optical signal having the complementary relationship is converted into an electric signal, and the difference is obtained. An optical communication method comprising:

The optical communication method according to claim 1,
In the demodulation, input light from the transmission path is heterodyne detected, and then separated into two polarization components and photoelectrically converted to generate two intermediate frequency signals, and each intermediate frequency signal is subjected to 1-bit delay detection. An optical communication method according to claim 1, wherein the data signal is replaced by a demodulation method for demodulating the data signal.

Comprising: a light source; a precoder that differentially encodes an input binary data signal; and a polarization modulator that modulates a polarization state of output light of the light source by an output signal of the precoder. Optical transmitter.

The optical transmitter according to claim 8, wherein
The optical transmitter, wherein the polarization modulator assigns binary data differentially encoded by the precoder to two orthogonal polarization states of output light of the light source.

The optical transmitter according to claim 8, wherein
The precoder includes a first differential encoder that differentially encodes input binary data, and a second difference that differentially encodes output data of the first differential encoder again. A dynamic encoder, a serial-parallel converter that performs serial-parallel conversion on the output signal of the second differential encoder and separates it into two signals, and outputs one of the output signals of the serial-parallel converter to 1/2 A delay unit for delaying symbols,
The polarization modulator includes first and second phase modulators for inputting respective lights separated into two polarization components orthogonal to each other, and the first and second phase modulators. A polarization multiplexer that combines the output light of
The first phase modulator phase-modulates one polarization component of the output light of the light source with the other output signal of the series-parallel converter, and the second phase modulator uses the output signal of the delay device. An optical transmitter characterized in that the other polarization component of the output light of the light source is phase-modulated.

The optical transmitter according to claim 8, wherein
The precoder is complementary to a first differential encoder that differentially encodes input binary data, and differentially encodes output data of the first differential encoder again. A second differential encoder that outputs two signals; a first duobinary filter that converts one of the signals output from the second differential encoder into a duobinary signal; and A second duobinary filter that converts the other signal output from the differential encoder into a duobinary signal;
The polarization modulator includes first and second phase modulators for inputting respective lights separated into two polarization components orthogonal to each other, and the first and second phase modulators. A polarization multiplexer that combines the output light of
The first phase modulator phase-modulates one polarization component of the output light of the light source according to an output signal of the first duobinary filter, and the second phase modulator includes the second duobinary filter. An optical transmitter characterized in that the other polarization component of the output light of the light source is phase-modulated by the output signal.

An optical filter having an impulse response in which input light from an optical transmission line and light obtained by delaying the input light by one symbol are combined, and an optical receiving circuit that photoelectrically converts the output light of the optical filter. An optical receiver characterized by that.

The optical receiver according to claim 12, wherein
The optical filter comprises a Mach-Zehnder interferometer,
The optical receiver circuit includes two light receivers that individually photoelectrically convert two output lights of the Mach-Zehnder interferometer, and a differential circuit that outputs a difference between output signals of the two light receivers. Optical receiver.

An optical multiplexer for combining the input light from the optical transmission line with light having a frequency different from that of the input light and performing heterodyne detection; and a polarization separator for separating orthogonal polarization components of the output light of the optical multiplexer; A first light receiver that photoelectrically converts one output light from the polarization separator, a second light receiver that photoelectrically converts the other output light from the polarization separator, and the first light reception. A first 1-bit delay detector for detecting the output signal of the detector, a second 1-bit delay detector for detecting the output signal of the second optical receiver, and the first and second 1-bit delay detectors An optical receiver comprising an adder for adding the output signals of the receiver.

An optical communication system comprising the optical transmitter according to any one of claims 8 to 11 and the optical receiver according to any one of claims 12 to 14.