JP4562657B2

JP4562657B2 - Demodulator circuit

Info

Publication number: JP4562657B2
Application number: JP2006004705A
Authority: JP
Inventors: 寛和久保田; 宮本　　裕
Original assignee: Nippon Telegraph and Telephone Corp
Current assignee: Nippon Telegraph and Telephone Corp
Priority date: 2006-01-12
Filing date: 2006-01-12
Publication date: 2010-10-13
Anticipated expiration: 2026-01-12
Also published as: JP2007189402A

Description

本発明は、非線形光学効果による伝送特性劣化を軽減する技術に関する。 The present invention relates to a technique for reducing deterioration of transmission characteristics due to a nonlinear optical effect.

ＤＱＰＳＫ−ＯＦＤＭなどの位相変調信号を受信する際に、受信した信号の補償や多重伝送路特性の推定を行う方法が知られている（例えば、特許文献１、非特許文献１または２参照）。図９に従来例の模式構成図を示す。 When receiving a phase-modulated signal such as DQPSK-OFDM, there are known methods for compensating the received signal and estimating multiple transmission path characteristics (for example, refer to Patent Document 1, Non-Patent Document 1 or 2). FIG. 9 shows a schematic configuration diagram of a conventional example.

図９の従来例では、受信信号にフーリエ変換（ＦＦＴ）を適用して、信号空間ダイヤグラムの位置に対応した信号（Ｉ，Ｑ信号）とし、この信号に位相補償を行い、位相変動補償後の受信信号に対してキャリア間の位相ずれを補償して送信シンボルを推定する。 In the conventional example of FIG. 9, the Fourier transform (FFT) is applied to the received signal to obtain a signal (I, Q signal) corresponding to the position of the signal space diagram, and phase compensation is performed on this signal. A transmission symbol is estimated by compensating for a phase shift between carriers with respect to the received signal.

特開２００３−２３４０８号公報JP 2003-23408 A ＰｒｏｃｅｅｄｉｎｇｓｏｆＯＦＣ，ＴｈＥ７（２００３）Proceedings of OFC, ThE7 (2003) ＰｒｏｃｅｅｄｉｎｇｓｏｆＯＦＣ，ＴｈＥ５（２００３）Proceedings of OFC, ThE5 (2003)

上述したような従来技術は、光伝送を用いて行われる通信であっても、光信号を電気信号に変換した後、その電気信号に対して信号の補償や多重伝送路特性の推定を行う技術であるため、ＦＦＴなどの負荷の高い計算が必要である。よって、光伝送を用いて行われる通信に対し、光信号の段階で受信信号の補償や多重伝送路特性の推定を行うことにより、ＦＦＴなどの負荷の高い計算を省くことができる復調回路が望まれる。 The conventional technology as described above is a technology for compensating an optical signal and estimating a multiplex transmission line characteristic after converting the optical signal into an electric signal even in communication performed using optical transmission. Therefore, calculation with high load such as FFT is required. Therefore, it is desirable to provide a demodulation circuit capable of omitting a heavy calculation such as FFT by performing compensation of received signals and estimation of multiplex transmission line characteristics at the optical signal stage for communication performed using optical transmission. It is.

本発明は、このような背景の下に行われたものであって、受信信号の補償や多重伝送路特性の推定を、光信号のまま行うことにより、従来と比較して負荷の高い計算を不要とすることができる復調回路を提供することを目的とする。 The present invention has been performed under such a background. Computation of a received signal and estimation of multiplex transmission line characteristics are performed as they are in an optical signal, so that a calculation with a higher load than that in the past is performed. An object of the present invention is to provide a demodulation circuit that can be made unnecessary.

伝送される光信号は、雑音等でランダムにばらつくと共に、光ファイバ中を伝搬する光信号の自己位相変調効果によって、強度が強い信号ほど位相変化が大きくなる。強度変調・直接検波（ＩＭ／ＤＤ）方式においては、この位相の乱れは復調時に問題とならないが、位相変調においては伝送特性を劣化させる要因となる。自己位相変調効果とは、光ファイバ中を伝搬する光信号が伝送媒質の持つ非線形屈折率の影響で、信号自身の光強度によって位相変調を生じる効果のことである。 The transmitted optical signal varies randomly due to noise or the like, and due to the self-phase modulation effect of the optical signal propagating through the optical fiber, the stronger the signal, the larger the phase change. In the intensity modulation / direct detection (IM / DD) system, this phase disturbance does not cause a problem at the time of demodulation, but the phase modulation causes a deterioration in transmission characteristics. The self-phase modulation effect is an effect that an optical signal propagating through an optical fiber is influenced by the nonlinear refractive index of the transmission medium and causes phase modulation by the light intensity of the signal itself.

光信号の光ファイバ等誘電体中の伝搬はマクスウェルの方程式で記述されるが、用いる信号周波数が光の周波数（１．５μｍ帯の光通信においては約２００テラヘルツ）の１／１０程度以下である場合には、包絡線近似を適用することにより、非線形シュレディンガ方程式で表すことができる。二次の分散（群速度分散）までを考慮し、また、高次の非線形効果および伝送損失を無視すると式１のように表される。 Propagation of optical signals in dielectrics such as optical fibers is described by Maxwell's equations, but the signal frequency used is about 1/10 or less of the optical frequency (about 200 terahertz in 1.5 μm optical communication). In some cases, it can be expressed by a nonlinear Schrodinger equation by applying envelope approximation. Considering up to second-order dispersion (group velocity dispersion), and ignoring higher-order nonlinear effects and transmission loss, the following expression is obtained.

ここでｕは信号波形、ｔは時間、ｚは伝搬距離、ｂ₂は分散定数、γは非線形定数、ｉは虚数単位である。これより、光信号の伝搬による位相変化は強度により異なることがわかる。 Here, u is a signal waveform, t is time, z is a propagation distance, b ₂ is a dispersion constant, γ is a nonlinear constant, and i is an imaginary unit. From this, it is understood that the phase change due to the propagation of the optical signal varies depending on the intensity.

本発明は、非線形光学効果による伝送特性劣化を軽減する復調回路であり、光伝送においては、自己位相変調効果により、強度の変動が位相の変動になるため、復調時に強度変動に応じて逆位相の位相変化を与える、もしくは、位相弁別器の弁別位相を変化させることで復調特性を改善する。 The present invention is a demodulation circuit that reduces transmission characteristic degradation due to a nonlinear optical effect. In optical transmission, the fluctuation in intensity becomes a fluctuation in phase due to the self-phase modulation effect. The demodulation characteristic is improved by changing the phase of the phase discriminator or changing the discrimination phase of the phase discriminator.

図１に非線形光学効果による伝送後の光信号の位相変化を示す。横軸が光信号の複素振幅の実数成分、縦軸が同虚数成分である。すなわち、原点からの距離が強度を表し、回転角度が位相を表す。式（１）より、強度が高い信号ほど位相回転が大きくなる。図１（ａ）は光雑音の無い理想的な場合であり、強度により位相回転量は一意に決まる。図１（ｂ）は光雑音が存在する場合であり、楕円型に信号点が分布する。 FIG. 1 shows a phase change of an optical signal after transmission due to a nonlinear optical effect. The horizontal axis is the real component of the complex amplitude of the optical signal, and the vertical axis is the imaginary component. That is, the distance from the origin represents the intensity, and the rotation angle represents the phase. From equation (1), the higher the intensity, the greater the phase rotation. FIG. 1A shows an ideal case without optical noise, and the amount of phase rotation is uniquely determined by the intensity. FIG. 1B shows a case where optical noise exists, and signal points are distributed in an elliptical shape.

自己位相変調効果による位相回転によってもたらされる伝送特性劣化を軽減するためには、位相弁別器の前段に自己位相変調によってもたらされた位相変化と逆方向に位相変化を与える方法がある。なお、位相変化量は基準となる任意の信号強度の位相変化を基準とし、それとの差を与えればよい。例えば、連続する信号列の最初の信号強度を基準とすることができる。 In order to reduce the transmission characteristic deterioration caused by the phase rotation due to the self-phase modulation effect, there is a method of giving a phase change in the opposite direction to the phase change caused by the self-phase modulation before the phase discriminator. Note that the phase change amount is based on a phase change of an arbitrary signal intensity as a reference, and a difference from the phase change may be given. For example, the first signal strength of a continuous signal sequence can be used as a reference.

また、ＤＰＳＫ、ＤＱＰＳＫ方式などのように、１タイムスロット前との位相差を検出する方式において自己位相変調効果による位相回転によってもたらされる伝送特性劣化を軽減するためには、位相弁別器の弁別位相をこの位相変位の差と同じ位相差としてもよい。これらの方式では１タイムスロット前との位相差を検出するため、１タイムスロット前の信号強度が強い場合には、位相差が減る方向に作用する。 In addition, in order to reduce transmission characteristic deterioration caused by phase rotation due to the self-phase modulation effect in a method of detecting a phase difference from the previous time slot, such as DPSK and DQPSK methods, a discrimination phase of a phase discriminator is used. May be the same phase difference as this phase displacement difference. In these methods, since the phase difference from the previous time slot is detected, when the signal strength before the previous time slot is strong, the phase difference is reduced.

すなわち、本発明は、位相変調光伝送の復調に用いる復調回路であって、本発明の特徴とするところは、光伝送における自己位相変調効果により光信号の強度の変動と位相の変動とが同方向に連動する関係に基づき光信号の復調に先立って当該光信号の強度変動とは逆方向となる位相変化を当該光信号に与える位相変調器を備えたところにある。 That is, the present invention is a demodulation circuit used for demodulation of phase-modulated optical transmission. The feature of the present invention is that the fluctuation of the intensity of the optical signal and the fluctuation of the phase are the same due to the self-phase modulation effect in the optical transmission. A phase modulator is provided that gives a phase change to the optical signal in a direction opposite to the intensity fluctuation of the optical signal prior to demodulation of the optical signal based on the relationship linked to the direction.

あるいは、光伝送における自己位相変調効果により光信号の強度の変動と位相の変動とが同方向に連動する関係に基づき光信号の復調に先立って受光器の前段に備えられた位相弁別器の弁別位相を、被復調光信号の強度変動に応じて変化させる弁別位相調整手段を備えたことを特徴とする。 Alternatively, the discrimination of the phase discriminator provided in the front stage of the optical receiver prior to the demodulation of the optical signal based on the relationship in which the fluctuation of the intensity of the optical signal and the fluctuation of the phase are linked in the same direction due to the self-phase modulation effect in the optical transmission. It is characterized by comprising a discrimination phase adjusting means for changing the phase according to the intensity fluctuation of the demodulated optical signal.

この弁別位相調整手段は、例えば、前記弁別位相を、被復調光信号の現タイムスロットにおける強度と１タイムスロット前における強度との差に応じて変化させる手段を備える。あるいは、前記位相弁別器の一部に前記被復調光信号が自己位相変調を生じ易い高非線形領域部を備え、この高非線形領域部により生じた自己位相変調により当該位相弁別器の弁別位相を変化させる。 For example, the discrimination phase adjusting means includes means for changing the discrimination phase according to a difference between the intensity of the demodulated optical signal in the current time slot and the intensity of one time slot before. Alternatively, a part of the phase discriminator is provided with a highly nonlinear region part in which the demodulated optical signal is likely to cause self-phase modulation, and the discrimination phase of the phase discriminator is changed by the self-phase modulation generated by the highly nonlinear region part. Let

本発明によれば、受信信号の補償や多重伝送路特性の推定を、光信号のまま行うことにより、従来と比較して負荷の高い計算を不要とすることができる。 According to the present invention, it is possible to eliminate the need for a calculation with a higher load than in the prior art by performing compensation of received signals and estimation of multiplex transmission path characteristics as they are.

（第一実施例）
第一実施例を図２を参照して説明する。図２に第一実施例の復調回路の模式構成図を示す。受信した光信号の一部を分岐し、その一方を高速光検出器４で強度を測定し、位相変調器２によって該当光信号に、前記強度の逆方向に対応した位相変化により位相補償を与えた後、復調器３によって電気信号に戻す。なお、遅延器１は、分岐した光信号の他方に対し、高速光検出器４を通過する一方の光信号とほぼ等しい遅延を与えることにより、位相変調器２に入力される双方の光信号のタイミングを合わせるためのものである。この位相変化の補正前後の模式図を図３に示す。図３（ａ）は補正前、図３（ｂ）は補正後である。図３に示すように、本方法により、位相マージンを増加させることができることがわかる。 (First Example)
A first embodiment will be described with reference to FIG. FIG. 2 shows a schematic configuration diagram of the demodulation circuit of the first embodiment. A part of the received optical signal is branched, one of which is measured for intensity by the high-speed photodetector 4, and the phase modulator 2 gives phase compensation to the corresponding optical signal by a phase change corresponding to the opposite direction of the intensity. After that, the demodulator 3 returns the electric signal. The delay device 1 gives a delay substantially equal to one optical signal passing through the high-speed photodetector 4 to the other one of the branched optical signals, so that both of the optical signals input to the phase modulator 2 are transmitted. It is for adjusting timing. A schematic diagram before and after correction of the phase change is shown in FIG. FIG. 3A shows before correction, and FIG. 3B shows after correction. As shown in FIG. 3, it can be seen that the phase margin can be increased by this method.

（第二実施例）
第二実施例を図４を参照して説明する。図４にＤＰＳＫ復調に使用する第二実施例の復調回路の模式構成図を示す。光信号の一部を分岐し、その一方を高速光検出器４で強度を測定し、その強度によって位相弁別器５の弁別位相を変化させる。弁別位相は、非線形効果によって生じる位相回転の半分だけ変化させればよい。この模式動作を図５に示す。信号強度によって位相弁別器５の弁別位相を変化させることにより、位相マージンが拡大できることがわかる。なお、遅延器１の役割は第一実施例と同じである。 (Second embodiment)
A second embodiment will be described with reference to FIG. FIG. 4 shows a schematic configuration diagram of a demodulating circuit of the second embodiment used for DPSK demodulation. A part of the optical signal is branched, and the intensity of one of the optical signals is measured by the high-speed photodetector 4, and the discrimination phase of the phase discriminator 5 is changed according to the intensity. The discrimination phase may be changed by half of the phase rotation caused by the non-linear effect. This schematic operation is shown in FIG. It can be seen that the phase margin can be increased by changing the discrimination phase of the phase discriminator 5 according to the signal intensity. The role of the delay device 1 is the same as in the first embodiment.

（第三実施例）
第三実施例を図６および図７を参照して説明する。図６および図７に第三実施例の復調回路の模式構成図を示す。第二実施例と比較して、さらに、位相マージンを拡大させるために、現タイムスロットにおける信号強度と１タイムスロット前における信号強度との差によって弁別位相を変化させる。この際、１タイムスロット前の信号の影響は逆方向であることに注意すると、強度差によって弁別位相を変化させればよいことがわかる。強度差による弁別位相の変化は、図６のように、１スロット遅延器τおよび位相差検出器７を用いて電気的に現タイムスロットにおける信号強度と１タイムスロット前における信号強度との差分をとり位相弁別器５の片方の腕の位相変調器Ｍに加えてもよく、また、図７のように、１スロット遅延器τを用いて遅延したものと遅延しないものを位相弁別器５の各腕に位相変調器Ｍを用いて加えてもよい。 (Third embodiment)
A third embodiment will be described with reference to FIGS. 6 and 7 are schematic configuration diagrams of the demodulating circuit of the third embodiment. Compared with the second embodiment, in order to further expand the phase margin, the discrimination phase is changed by the difference between the signal strength in the current time slot and the signal strength in the previous time slot. At this time, if it is noted that the influence of the signal one time slot before is in the reverse direction, it can be seen that the discrimination phase may be changed depending on the intensity difference. As shown in FIG. 6, the change in the discrimination phase due to the intensity difference is obtained by electrically calculating the difference between the signal intensity in the current time slot and the signal intensity in the previous time slot using the 1-slot delay device τ and the phase difference detector 7. In addition to the phase modulator M of one arm of the phase discriminator 5, each of the phase discriminators 5 may be delayed or not delayed using the 1-slot delay device τ as shown in FIG. It may be added to the arm using a phase modulator M.

（第四実施例）
第四実施例を図８を参照して説明する。図８に第四実施例の復調回路の模式構成図を示す。第四実施例は、弁別位相を変化させるために、受信信号の自己位相変調を利用する。これは、位相弁別器５内に自己位相変調を効率よく発生させる高非線形領域部Ｎを持たせることにより実現できる。 (Fourth embodiment)
A fourth embodiment will be described with reference to FIG. FIG. 8 shows a schematic configuration diagram of a demodulation circuit of the fourth embodiment. The fourth embodiment uses self-phase modulation of the received signal in order to change the discrimination phase. This can be realized by providing the phase discriminator 5 with a highly nonlinear region N that efficiently generates self-phase modulation.

すなわち、位相弁別器５内の高非線形領域部Ｎで、受信信号が、伝送路の光ファイバ中を伝搬するときとほぼ等しい自己位相変調を生じると推定できるので、高非線形領域部Ｎで生じた自己位相変調を利用して位相弁別器５の弁別位相を変化させることにより、位相弁別器５は、受信信号が伝送路の光ファイバ中を伝搬するときに生じた自己位相変調に応じた弁別位相の変化を得ることができる。 That is, since it can be estimated that the received signal causes self-phase modulation substantially equal to that when propagating in the optical fiber of the transmission path in the highly nonlinear region portion N in the phase discriminator 5, this occurred in the highly nonlinear region portion N. By changing the discrimination phase of the phase discriminator 5 using the self-phase modulation, the phase discriminator 5 can discriminate the discrimination phase according to the self-phase modulation generated when the received signal propagates in the optical fiber of the transmission line. Change.

図８は各腕に高非線形領域部Ｎを持たせているが、片方でもよい。さらに、１タイムスロット遅延部Ｔが高非線形領域部Ｎを兼ねてもよい。第四実施例によれば、高速光検出器４を省略できるため、回路構成を簡単化（安価化）できる。 In FIG. 8, each arm has a highly nonlinear region portion N, but one arm may be provided. Further, the one time slot delay portion T may also serve as the highly nonlinear region portion N. According to the fourth embodiment, since the high-speed photodetector 4 can be omitted, the circuit configuration can be simplified (inexpensive).

これらの第二〜第四実施例において受光器はバランス型でも、単一型でもよい。また、ＤＱＰＳＫ、８ＰＳＫなど、多相の位相変調においても同様に構成できる。 In these second to fourth embodiments, the light receiver may be a balanced type or a single type. Further, the same configuration can be applied to multiphase phase modulation such as DQPSK and 8PSK.

本発明によれば、受信信号の補償や多重伝送路特性の推定を、光信号のまま行うことにより、従来と比較して負荷の高い計算を不要とすることができる復調回路を実現することができる。 According to the present invention, it is possible to realize a demodulation circuit that can eliminate the need for calculation with a higher load than in the past by performing compensation of received signals and estimation of multiplex transmission path characteristics as they are, with optical signals. it can.

非線形光学効果による位相変化を説明するための図。The figure for demonstrating the phase change by a nonlinear optical effect. 第一実施例の復調回路の模式構成図。The schematic block diagram of the demodulation circuit of a 1st Example. 第一実施例の復調回路の補正の効果を説明するための図。The figure for demonstrating the effect of the correction | amendment of the demodulation circuit of a 1st Example. 第二実施例の復調回路の模式構成図。The schematic block diagram of the demodulation circuit of a 2nd Example. 第二実施例の復調回路の模式動作を示す図。The figure which shows the model operation | movement of the demodulation circuit of a 2nd Example. 第三実施例の復調回路の模式構成図。The schematic block diagram of the demodulation circuit of a 3rd Example. 第三実施例の復調回路の模式構成図。The schematic block diagram of the demodulation circuit of a 3rd Example. 第四実施例の復調回路の模式構成図。The schematic block diagram of the demodulation circuit of 4th Example. 従来例の復調回路の模式構成図。The schematic block diagram of the demodulation circuit of a prior art example.

符号の説明Explanation of symbols

１遅延器
２、Ｍ位相変調器
３復調器
４高速光検出器
５位相弁別器
６受光器
７位相差検出器
Ｎ高非線形領域部
Ｔ１スロット遅延部
τ １スロット遅延器 DESCRIPTION OF SYMBOLS 1 Delay device 2, M Phase modulator 3 Demodulator 4 High-speed photodetector 5 Phase discriminator 6 Light receiver 7 Phase difference detector N High nonlinear area | region T 1 Slot delay part (tau) 1 slot delay device

Claims

位相変調光伝送の復調に用いる復調回路において、
受信した光信号を分岐する分岐回路と、
分岐された一方の光信号の強度を検出する光検出器と、
分岐された他方の光信号を遅延させ、前記一方の光信号と等しい遅延を与える遅延器と、
光信号を電気信号に変換する受光器の前段に設けられ、前記遅延器で遅延された光信号に対して、前記光検出器で検出した光信号の強度に応じて、光伝送における自己位相変調効果により光信号の強度の変動に応じて生ずる位相回転に対して位相回転の半分だけ変化させる弁別位相調整手段と、
を備えたことを特徴とする復調回路。 In a demodulation circuit used for demodulation of phase-modulated optical transmission,
A branch circuit for branching the received optical signal;
A photodetector for detecting the intensity of one of the branched optical signals;
A delay device for delaying the other optical signal branched and providing a delay equal to the one optical signal;
Self-phase modulation in optical transmission according to the intensity of the optical signal detected by the photodetector with respect to the optical signal delayed by the delay device provided in the front stage of the optical receiver that converts the optical signal into an electrical signal A discriminating phase adjusting means for changing only half of the phase rotation with respect to the phase rotation generated according to the fluctuation of the intensity of the optical signal due to the effect ;
A demodulation circuit comprising:

前記位相変調は差動位相変調方式であり、
前記弁別位相調整手段は、前記弁別位相を、被復調光信号の現タイムスロットにおける強度と１タイムスロット前における強度との差に応じて、強度差が弱い場合には位相差を小さく、強度差が大きい場合には位相差を大きく変化させる手段を備えた請求項１記載の復調回路。 The phase modulation is a differential phase modulation method,
The discriminating phase adjusting means determines the discriminating phase according to the difference between the intensity in the current time slot of the demodulated optical signal and the intensity in the previous time slot , and reduces the phase difference when the intensity difference is weak. 2. The demodulation circuit according to claim 1, further comprising means for greatly changing the phase difference when the difference is large .