JP3661966B2 - AFC circuit for phase diversity receiver of optical PSK signal - Google Patents

Description

【００１７】
上式（１）において、隣接するシンボル間で位相変化がない場合には、ψ−φとｃｏｓ項の括弧内は共に零となるので、周波数弁別特性は下記の式（２）となる。
【００１８】
【数２】

【００１９】
ところが、位相変化がπの場合にはｔ＋τｄからｔ＋τｓの間では上式（２）と同じであるが、ｔからｔ＋τｄの間では上式（１）のｃｏｓの括弧内がπ／２または−π／２となるため、ｆ（ω）が零となり、上式（２）のＡＦＣ特性が劣化する。位相変化がπ／２の場合には、ｔからｔ＋τｄの間ではｃｏｓ項の括弧内はπ／２または−π／２とはならないため、変調データが周波数弁別特性に混入してＡＦＣ特性が劣化する。一方、τｄをτｓと比べて小さくすれば、ｆ（ω）が零となってしまう時間率を減らせるので、周波数弁別器の出力をＬＰＦ４６で平滑することで劣化の程度を抑えることはできるが、局部発光源と信号光の周波数差を零とすることはできない。また、前述のように、同調時における感度低下と広帯域な差動増幅器が必要となるという問題がある。
【００２０】
本発明は、上述のような課題を解決するために成されたもので、その目的は光ＰＳＫ信号の位相ダイバーシティ受信機用ＡＦＣ回路において変調データによるＡＦＣ特性の劣化をなくすことにある。
【００２１】
【課題を解決するための手段】
上記目的を達成するため、請求項１の発明は、光ＰＳＫ変調された光波を位相ダイバーシティ方式により遅延検波して復調するコヒーレント光伝送の受信機の局発光源の発振周波数を信号光の周波数に追従させるためのＡＦＣ回路において、遅延線と差動増幅器、乗算器、減算器を用いて直交する２つの中間周波数信号から該中間周波数信号の周波数差に対応した電圧を得る周波数弁別器と、前記２つの中間周波数信号を遅延検波して得たＰＳＫ復調信号から隣接シンボル間で位相変化が無い場合はサンプル動作、有る場合はホールド動作となるようなキィ信号を生成するキィ信号発生回路と、前記キィ信号により前記周波数弁別器の出力信号に対して隣接シンボル間で位相変化が無い場合はサンプル動作、有る場合はホールド動作となる選択制御を行って該選択結果を前記局発光源に帰還させるサンプルホールド回路とを組み合わせたことを特徴とする。
【００２２】
ここで、前記ＰＳＫがＢＰＳＫであるとすることができる。
【００２３】
また、前記ＰＳＫがＱＰＳＫであるとすることができる。
【００２４】
【発明の実施の形態】
以下、図面を参照して本発明の実施形態を詳細に説明する。
【００２５】
（第１の実施形態）
最初に、本発明の一実施形態におけるＢＰＳＫ信号の受信機の構成を図１に示す。ここで、１は位相ダイバーシティ方式の光ＢＰＳＫ信号の受信機であり、下記の構成要素２〜１５を用いて、送信機側（図示しない）で光ＢＰＳＫ変調された光ＢＰＳＫ信号を位相ダイバーシティ方式により遅延検波してＢＰＳＫ変調以前の原信号に復調する。２は光ＢＰＳＫ変調されたＢＰＳＫ信号光（入力信号光）を導入する入力光ファイバであり、例えば偏波保存光ファイバを用いることができる。３は９０°光ハイブリッドであり、５の局発光源から供給される無変調の局発光と上記入力信号光とからそれらを９０°偏移した光波を生成する。４は平衡型光受光器であり、例えばＰＩＮ- ＰＤ（ピン・フォトダイオード）からなり、光ハイブリット３の直交した出力光を受光して、局発光源５により周波数変換された直交した２つの中間周波数信号を出力する。
【００２６】
６はそれぞれの平衡型光受光器４から供給される上記中間周波数信号Ｉ，Ｑを遅延量τｓで遅延する遅延線である。７は乗算器であり、それぞれの平衡型光受光器４から供給される上記中間周波数信号Ｉ，Ｑとその信号を遅延線６でτｓ遅延した信号とを乗算する。乗算器７としては、例えば上記文献３で開示されたような比較的良好な直線性が得られるGilbert Cell型広帯域乗算器を用いることができる。８は加算器であり、両乗算器７の出力を加算する。９は識別判定器であり、加算器８から供給される信号から位相変化０の場合を識別することにより１１のＢＰＳＫ復調信号を生成する。
【００２７】
１０は図４のタイミングチャートに示す信号処理動作を行うキィ信号発生器であり、２つの中間周波数信号を遅延検波して得たＢＰＳＫ復調信号１１から隣接シンボル間で位相変化が無い場合はサンプル動作、有る場合はホールド動作となるようなキィ信号１０ａを生成する。
【００２８】
１２は位相ダイバーシティ用受信機１の局発光源５の発振周波数を信号光の周波数に追従させるためのＡＦＣ回路であり、周波数弁別器１３、サンプルホールド回路１４、ＬＰＦ１５、および局発光源５を含む。周波数弁別器１３は図３で上述した構成の周波数弁別器であり、遅延線４２、４７と差動増幅器４３、乗算器４４、減算器４５およびＬＰＦ４６を用いて直交する２つの中間周波数信号Ｉ，Ｑからその中間周波数信号の周波数差に対応した電圧の信号を得る。サンプルホールド回路１４は、周波数弁別器１３と局発光源５の間に挿入されて、上記キィ信号１０ａのレベルに応じて、周波数弁別器１３の出力信号に対して隣接シンボル間で位相変化が無い場合はサンプル動作、有る場合はホールド動作となる選択制御を行って、その選択結果をＬＰＦ１５を介して局発光源５に帰還させる。ＬＰＦ１５はサンプルホールド回路１４と局発発光源５との間に挿入されて、サンプルホールド回路１４の出力を平滑する。局発光源５は送信機と同じ周波数特性を持つ。
【００２９】
次に、受信機１の全体の動作を信号の流れに従って説明する。
【００３０】
入力光ファイバ１を通って入力する光ＢＰＳＫ信号は、９０°光ハイブリット３を通って２分されると共に、一方は９０°偏移され、両者はそれぞれの平衡型受光器４に受光される。受光器４は信号光を局発光源５の無変調の局発光と共に受光すると、信号光は両者のうなりの周波数（ビート周波数）のＩ、Ｑに変換される。このように受光器４で直交検波したＩとＱには、交互に信号が現れるので、両者Ｉ、Ｑを遅延量τｓの遅延線６と乗算器７とを用いて遅延検波して加算器８により加算することで、ＢＰＳＫ信号を復調できる。加算器８の出力は識別判定器９を通ってＢＰＳＫ復調信号１１となり、受信機１から出力される。
【００３１】
さらに、上記信号Ｉ、ＱとＢＰＳＫ復調信号１１はＡＦＣ回路１２の周波数弁別器１３に供給される。また、ＢＰＳＫ復調信号１１を基にキィ信号発生器１０で生成されたキィ信号１０ａはＡＦＣ回路１２のサンプルホールド回路１４に供給される。
【００３２】
図４はＢＰＳＫ復調信号１１とキィ信号１０ａとの対応を示す。本発明の特徴は、ＢＰＳＫ復調信号１１の位相変化の有無により周波数弁別器１３の出力を選択制御することにある。すなわち、光ＢＰＳＫ変調波の遅延検波による復調信号１１から、図４に示すような位相変化０の場合のみ、識別判定器９の識別判定のタイミングでサンプル動作が能動状態となるキィ信号１０ａをキィ信号発生器１０で生成する。このキィ信号１０ａを周波数弁別器１３の後段のサンプルホールド回路１４に加える。そして、サンプルホールド回路１４で選択制御された周波数弁別信号をＬＰＦ１５で平滑して局発光源５の電流に帰還する。位相変化がπの時にはサンプルホールド回路１４の出力は零にはならないため、同調時に高い感度を得るためにはτｄはτｓと等しく選ぶと有利である。
【００３３】
上述のように、本実施形態では、遅延検波したＢＰＳＫ復調信号１１から位相変化の無い場合のみ能動となるキィ信号１０ａを作り、サンプルホールド回路１４で周波数弁別器１３の出力をキィ信号１０ａを基にサンプルホールドしてから局発光源５に帰還する選択制御型のＡＦＣを行っているので、変調データによるＡＦＣ特性の劣化をなくすことができ、局部発光源５の光波と信号光の周波数差を零にすることができる。
【００３４】
（第２の実施形態）
次に、本発明の他の実施形態として位相ダイバーシティ方式の光ＱＰＳＫ信号の受信機の構成を図５に示す。ここで、１６は位相ダイバーシティ方式の光ＱＰＳＫ信号の受信機であり、下記の構成要素１７〜３２を含む。１７は図１の２と同様の入力光ファイバ、１８は図１の３と同様の９０°光ハイブリッドである。１９は図１の４と同様の平衡型光受光器であって全体で２個あり、２０の局発光源により周波数変換された直交した２つの中間周波数信号Ｉ、Ｑを出力する。２１は図１の６と同様の遅延量τｓの遅延線であって全体で４個あり、２２は図１の７と同様の乗算器であって全体で４個ある。２３は加算器、２４は減算器であり、図５に示すようにそれぞれ２個づつ直列的に接続されている。２５は図１の９と同様の識別判定器であって全体で２個あり、２６は論理素子、２７は図１の１０と同じキィ信号発生器、２８はＱＰＳＫの復調信号である。論理素子２６は一対の識別判別器２５からそれぞれ出力される一対のＱＰＳＫ復調信号２８に対して論理演算を下記のように行って、その演算結果をキィ信号発生器２７に出力する。
【００３５】
２９は図１の１２と同様の構成と機能を有するＡＦＣ回路であり、図３で述べた周波数弁別器３０、サンプルホールド回路３１、ＬＰＦ３２、および局発光源２０を有する。
【００３６】
本発明のＡＦＣ方式は許容周波数差がＢＰＳＫより一桁厳しいＱＰＳＫでは特に有効である。図５の２８のＱＰＳＫの復調信号Ｉ′、Ｑ′は下記の式（３）のように表わすことができる。
【００３７】
【数３】

【００３８】
ここで、△Φは０、π／２、π、３π／２を取り得る。位相変化のない場合（△Φ＝０）に図４と同じキィ信号を得るためには、論理素子２６の入出力の関係をＩ′が正、Ｑ′が負の時のみ出力が正とすれば良い。
【００３９】
ＱＰＳＫでは位相変化が起こらない確率はＢＰＳＫの半分となるので、３３のキィ信号の数も半分となるが、キィ信号間でのビート周波数の変動は無視できる程度に小さいと考えられる。
【００４０】
【発明の効果】
以上説明したように、本発明によれば、遅延検波により復調された信号をＡＦＣのサンプルホールド回路に帰還し、位相変化が無かった場合のみ、識別判定のタイミングでサンプル動作を行わせるので、変調データによるＡＦＣ特性の劣化をなくすことができる。
【００４１】
また、本発明は許容周波数差がＢＰＳＫより一桁厳しいＱＰＳＫでは特に有効である。
【図面の簡単な説明】
【図１】本発明の一実施形態における光ＢＰＳＫ信号の受信機の構成を示すブロック図である。
【図２】従来例における光ＰＳＫ信号の位相ダイバーシティ方式受信機の周波数弁別器の原理を示すブロック図である。
【図３】本発明の実施形態において用いた微分器を遅延線と差動増幅器で構成した周波数弁別器を示すブロック図である。
【図４】本発明の一実施形態の図１で生成されるＢＰＳＫ復調信号とキィ信号との対応を示す波形図である。
【図５】本発明の他の実施形態における光ＱＰＳＫ信号の受信機の構成を示すブロック図である。
【符号の説明】
１ 位相ダイバーシティ方式の光ＢＰＳＫ信号の受信機
２ 入力光ファイバ
３ ９０°光ハイブリッド
４ 平衡型光受光器
５ 局発光源
６ 遅延量τｓの遅延線
７ 乗算器
８ 加算器
９ 識別判定器
１０ キィ信号発生器
１１ ＢＰＳＫ復調信号
１２ ＡＦＣ回路
１３ 周波数弁別器
１４ サンプルホールド回路
１５ ＬＰＦ
１６ 位相ダイバーシティ方式の光ＱＰＳＫ信号の受信機
１７ 入力光ファイバ
１８ ９０°光ハイブリッド
１９ 平衡型光受光器
２０ 局発光源
２１ 遅延量τｓの遅延線
２２ 乗算器
２３ 加算器
２４ 減算器
２５ 識別判定器
２６ 論理素子
２７ キィ信号発生器
２８ ＱＰＳＫの復調信号
２９ ＡＦＣ回路
３０ 周波数弁別器
３１ サンプルホールド回路
３２ ＬＰＦ
４１ 微分器
４２ 遅延線
４３ 差動増幅器
４４ 乗算器
４５ 減算器
４６ ＬＰＦ
４７ 遅延線[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an AFC (Automatic Frequency Control) circuit of a local oscillation light source of a coherent optical transmission receiver that receives an optical PSK (Phase Shift Keying) signal by a phase diversity method, In particular, the present invention relates to the configuration of an AFC circuit designed to allow the local oscillation frequency to follow the frequency of signal light without being affected by modulation by data.
[0002]
[Prior art]
Conventionally, research on coherent optical transmission has been focused on large-capacity long-distance transmission of point-to-point communication systems that take advantage of high light reception sensitivity. However, in recent years, the advantages of light receiving sensitivity have been halved by the realization of optical amplifiers.
[0003]
On the other hand, multipoint-to-multipoint, for example, transmission of broadcast material signals within a broadcasting station, although the distance is short, signals are transmitted between many distributed studios and adjustment rooms. A system using optical wave multiplexing and optical branching is effective for arbitrary transmission. Optical FSK (Frequency Shift Keying) and optical PSK are mainly studied as modulation methods suitable for such applications for coherent optical transmission. The optical PSK can be transmitted with a narrower bandwidth than the optical FSK, and the optical PSK is more advantageous when paying attention to the number of multiplexing. There are three optical PSK detection methods: heterodyne, homodyne, and phase diversity.
[0004]
The homodyne method does not interfere with the image, but when a commercially available semiconductor LD (Laser Diode) is used as the light source, from the viewpoint of the configuration of an optical PLL (Phase-Locked Loop) for carrier wave reproduction. Realization is difficult.
[0005]
Of the reception systems for coherent optical transmission of optical BPSK (Binary Phase Shift Keying; two-phase PSK) signals, in the heterodyne system that converts signal light to an intermediate frequency by local light, transmission data is multiplied by multiplying the intermediate frequency signal. The AFC which removes the modulation component due to the above and then returns to the local light source after frequency discrimination is reported in the following paper 1.
[0006]
Paper 1: Alberto Bononi: “Analysis of the Automatic Frequency Control in Heterodyne Optical Receivers”, J. Lightwave Technol. Vol.10, No.6, pp.794-803 (1992).
[0007]
In the heterodyne method, the intermediate frequency is set to a frequency that is several times higher than the symbol frequency of the transmission data, so whether the frequency difference (beat frequency) between the signal light and the local light is higher or lower than the set intermediate frequency. A frequency discriminator capable of obtaining a voltage corresponding to a frequency difference including polarity can be easily configured. However, the heterodyne system has a drawback that image interference occurs in channel selection, so that the advantage of the narrow spectrum of the optical PSK signal cannot be fully utilized.
[0008]
On the other hand, the phase diversity method is a method that can solve the problem of homodyne carrier recovery, and does not require phase synchronization and can be expected to have a high density similar to that of homodyne. However, in the phase diversity method in which the intermediate frequency is zero, it is possible to obtain a voltage corresponding to the absolute value of the frequency difference by multiplying the signal of the intermediate frequency to remove the modulation component, but know the above polarity. Can not. In the phase diversity reception, if the frequency difference is within about 3% of the symbol frequency of the transmission data, the frequency that operates at a low center frequency that satisfies this can be utilized by utilizing the fact that the demodulation BER characteristics of the BPSK signal can be ignored. AFC using a discriminator is reported in the following paper 2.
[0009]
Paper 2: AW Davis: “Phase Diversity Techniques for Coherent Optical Receivers”, J. Lightwave Technol. Vol.5, NO.4, pp.561-572 (1987).
[0010]
The AFC described in the paper 2 cannot strictly reduce the frequency difference to zero. In particular, when demodulating an optical QPSK (Quadrature Phase Shift Keying; 4-phase PSK) signal, the difference between the frequency of the local light source and the frequency of the signal light is within about 0.3% of the transmission symbol frequency. A characteristic that is one order of magnitude severer than that is required. If the AFC is configured with a frequency discriminator having a lower center frequency in the prior art, the frequency control polarity of the local light source is inverted due to the frequency variation of the transmitted light, and it is difficult to realize the required characteristics.
[0011]
As a frequency discriminator operating with zero as the center frequency, it can be easily analogized to apply the demodulation circuit technology of the phase diversity receiver of the FSK signal. As shown in FIG. 2, the frequency discriminator differentiates two orthogonal components I and Q by a pair of differentiators 41 to obtain information on the frequency difference, and information on these frequency differences and the two orthogonal components I and The subtractor 45 subtracts the signal obtained by multiplying the Q by the pair of multipliers 44 to subtract the signal by the subtractor 45, thereby obtaining a voltage proportional to the frequency difference including the polarity described above. It is reported in the following paper 3 and is a known technique. Here, ω and Φ are a frequency difference and a phase difference between the signal light and the local light, respectively. Reference numeral 46 denotes an LPF (Low Pass Filter) that smoothes the output of the subtracter 45.
[0012]
Paper 3: Maeda et al., “Phase Diversity DPSK Coherent Optical Transmission Experiment”, IEICE Technical Report, OCS 96-126 (1997).
[0013]
However, the known frequency discrimination technique as shown in the paper 3 is that the transmission data (modulation data) is mixed into the frequency discrimination characteristics and the AFC characteristics are not improved as described in [Problems to be solved by the invention] in the next section. There is a problem of deterioration.
[0014]
[Problems to be solved by the invention]
As an operation of differentiation of the differentiator 41 shown in FIG. 2, a method of dividing the input signal into two, delaying one signal and subtracting it from the other signal is conceivable. When this delay amount is τd, if τd is selected to be sufficiently smaller than the reciprocal τs of the symbol frequency of the optical PSK signal, it can be made less susceptible to the influence of data modulation. However, if this is done, the sensitivity of the frequency discriminator at a zero frequency difference (during tuning) is reduced, and a wideband differential amplifier is required. On the contrary, when the delay amount τd is selected to be as large as τs, the sensitivity of the frequency discriminator increases, but when the phase changes between adjacent symbols, the output of the AFC circuit is ω (the frequency difference between the signal light and the local light). There is a problem that it becomes zero regardless of. These problems will be described in detail below with reference to FIG.
[0015]
FIG. 3 shows a frequency discriminator in which the differentiator 41 of FIG. 2 is constituted by a delay line 42 and a differential amplifier 43. Reference numeral 47 denotes a delay line 47 having a delay amount τd / 2. Here, the phase of the signal light at time t + τd is ψ. The I and Q signals are delayed by τd / 2 by a pair of delay lines 47 and then multiplied by a multiplier 44, and the phase at time t + τd / 2 is θ. The output signal f (ω) of this frequency discriminator is expressed by the following equation (1).
[0016]
[Expression 1]

[0017]
In the above equation (1), when there is no phase change between adjacent symbols, both ψ-φ and the parentheses in the cos term are zero, so the frequency discrimination characteristic is the following equation (2).
[0018]
[Expression 2]

[0019]
However, when the phase change is π, it is the same as the above equation (2) between t + τd and t + τs, but between t and t + τd, the parenthesis of cos in the above equation (1) is π / 2 or −π Therefore, f (ω) becomes zero, and the AFC characteristic of the above equation (2) deteriorates. When the phase change is π / 2, since the parenthesis in the cos term is not π / 2 or −π / 2 between t and t + τd, the modulation data is mixed into the frequency discrimination characteristic and the AFC characteristic is deteriorated. To do. On the other hand, if τd is made smaller than τs, the time rate at which f (ω) becomes zero can be reduced. Therefore, the degree of deterioration can be suppressed by smoothing the output of the frequency discriminator with the LPF 46. The frequency difference between the local light source and the signal light cannot be made zero. In addition, as described above, there is a problem that sensitivity is lowered during tuning and a wideband differential amplifier is required.
[0020]
The present invention has been made to solve the above-described problems, and an object thereof is to eliminate deterioration of AFC characteristics due to modulation data in an AFC circuit for a phase diversity receiver of an optical PSK signal.
[0021]
[Means for Solving the Problems]
In order to achieve the above object, the invention of claim 1 uses the oscillation frequency of the local light source of a coherent optical transmission receiver that demodulates an optical wave modulated by optical PSK modulation using a phase diversity method as the frequency of the signal light. In the AFC circuit for tracking, a frequency discriminator that obtains a voltage corresponding to a frequency difference between the intermediate frequency signals from two orthogonal intermediate frequency signals using a delay line, a differential amplifier, a multiplier, and a subtractor; A key signal generating circuit for generating a key signal that performs a sample operation when there is no phase change between adjacent symbols from a PSK demodulated signal obtained by delay detection of two intermediate frequency signals, and a hold operation when there is any; When there is no phase change between adjacent symbols with respect to the output signal of the frequency discriminator by the key signal, a sampling operation is selected, and if there is, a selection operation is performed. Performs control, characterized in that the combination of the sample and hold circuit for feeding back the selection result to the local light source.
[0022]
Here, the PSK may be BPSK.
[0023]
The PSK may be QPSK.
[0024]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0025]
(First embodiment)
First, FIG. 1 shows the configuration of a BPSK signal receiver according to an embodiment of the present invention. Here, reference numeral 1 denotes a receiver for a phase diversity optical BPSK signal. By using the following components 2 to 15, an optical BPSK signal optically BPSK modulated on the transmitter side (not shown) is converted by the phase diversity method. Delay detection is performed to demodulate the original signal before BPSK modulation. Reference numeral 2 denotes an input optical fiber for introducing optical BPSK-modulated BPSK signal light (input signal light). For example, a polarization maintaining optical fiber can be used. Reference numeral 3 denotes a 90 ° optical hybrid, which generates light waves obtained by shifting them by 90 ° from unmodulated local light supplied from the local light source 5 and the input signal light. Reference numeral 4 denotes a balanced optical receiver, which is composed of, for example, a PIN-PD (pin photodiode), receives the orthogonal output light of the optical hybrid 3, and converts the two orthogonally frequency converted by the local light source 5. Output frequency signal.
[0026]
Reference numeral 6 denotes a delay line for delaying the intermediate frequency signals I and Q supplied from the respective balanced optical receivers 4 by a delay amount τs. A multiplier 7 multiplies the intermediate frequency signals I and Q supplied from the respective balanced optical receivers 4 and signals obtained by delaying the signals by a delay line 6 by τs. As the multiplier 7, for example, a Gilbert Cell type wideband multiplier that can obtain relatively good linearity as disclosed in the above-mentioned document 3 can be used. An adder 8 adds the outputs of both multipliers 7. Reference numeral 9 denotes an identification / determination unit, which generates 11 BPSK demodulated signals by identifying the case of phase change 0 from the signal supplied from the adder 8.
[0027]
A key signal generator 10 performs the signal processing operation shown in the timing chart of FIG. 4, and performs a sample operation when there is no phase change between adjacent symbols from the BPSK demodulated signal 11 obtained by delay detection of two intermediate frequency signals. If there is, the key signal 10a is generated so that the hold operation is performed.
[0028]
Reference numeral 12 denotes an AFC circuit for causing the oscillation frequency of the local light source 5 of the phase diversity receiver 1 to follow the frequency of the signal light, and includes the frequency discriminator 13, the sample hold circuit 14, the LPF 15, and the local light source 5. . The frequency discriminator 13 is a frequency discriminator having the configuration described above with reference to FIG. 3, and includes two intermediate frequency signals I, Q, which are orthogonal to each other using delay lines 42 and 47, a differential amplifier 43, a multiplier 44, a subtractor 45 and an LPF 46. A signal having a voltage corresponding to the frequency difference of the intermediate frequency signal is obtained from Q. The sample hold circuit 14 is inserted between the frequency discriminator 13 and the local light source 5, and there is no phase change between adjacent symbols with respect to the output signal of the frequency discriminator 13 according to the level of the key signal 10a. In this case, selection control is performed such that a sampling operation is performed, and a hold operation is performed if there is, and the selection result is fed back to the local light source 5 via the LPF 15. The LPF 15 is inserted between the sample hold circuit 14 and the local light emission source 5 to smooth the output of the sample hold circuit 14. The local light source 5 has the same frequency characteristics as the transmitter.
[0029]
Next, the overall operation of the receiver 1 will be described according to the signal flow.
[0030]
The optical BPSK signal input through the input optical fiber 1 is divided into two through the 90 ° optical hybrid 3 and one is shifted by 90 °, and both are received by the respective balanced light receivers 4. When the light receiver 4 receives the signal light together with the unmodulated local light of the local light source 5, the signal light is converted into I and Q of the beat frequency of both. Thus, since signals appear alternately in I and Q that have been orthogonally detected by the light receiver 4, both I and Q are subjected to delay detection using the delay line 6 and the multiplier 7 of the delay amount τs, and the adder 8. BPSK signal can be demodulated. The output of the adder 8 passes through the discrimination / determination unit 9 to become a BPSK demodulated signal 11 and is output from the receiver 1.
[0031]
Further, the signals I and Q and the BPSK demodulated signal 11 are supplied to the frequency discriminator 13 of the AFC circuit 12. The key signal 10 a generated by the key signal generator 10 based on the BPSK demodulated signal 11 is supplied to the sample hold circuit 14 of the AFC circuit 12.
[0032]
FIG. 4 shows the correspondence between the BPSK demodulated signal 11 and the key signal 10a. A feature of the present invention resides in that the output of the frequency discriminator 13 is selectively controlled depending on whether or not the phase of the BPSK demodulated signal 11 is changed. That is, the key signal 10a in which the sampling operation is activated at the timing of the discrimination determination of the discrimination determiner 9 only from the demodulated signal 11 by the delay detection of the optical BPSK modulated wave as shown in FIG. It is generated by the signal generator 10. This key signal 10a is applied to the sample hold circuit 14 at the subsequent stage of the frequency discriminator 13. The frequency discrimination signal selected and controlled by the sample and hold circuit 14 is smoothed by the LPF 15 and fed back to the current of the local light source 5. Since the output of the sample and hold circuit 14 does not become zero when the phase change is π, it is advantageous to select τd equal to τs in order to obtain high sensitivity during tuning.
[0033]
As described above, in the present embodiment, a key signal 10a that is active only when there is no phase change is generated from the delay-detected BPSK demodulated signal 11, and the output of the frequency discriminator 13 is based on the key signal 10a by the sample and hold circuit 14. Since the selective control type AFC is performed to return to the local light source 5 after being sampled and held, the deterioration of the AFC characteristic due to the modulation data can be eliminated, and the frequency difference between the light wave of the local light source 5 and the signal light can be reduced. Can be zero.
[0034]
(Second Embodiment)
Next, FIG. 5 shows the configuration of a phase diversity optical QPSK signal receiver as another embodiment of the present invention. Here, reference numeral 16 denotes a phase diversity optical QPSK signal receiver, which includes the following components 17-32. 17 is an input optical fiber similar to 2 in FIG. 1, and 18 is a 90 ° optical hybrid similar to 3 in FIG. Reference numeral 19 denotes a balanced optical receiver similar to 4 in FIG. 1, and there are two in total, and outputs two orthogonal intermediate frequency signals I and Q that are frequency-converted by 20 local light sources. Reference numeral 21 denotes a delay line having a delay amount τs similar to 6 in FIG. 1, and there are four as a whole. Reference numeral 22 denotes a multiplier similar to 7 in FIG. Reference numeral 23 denotes an adder, and 24 denotes a subtracter, and two are connected in series as shown in FIG. 25 is a total of two discriminating / determining units similar to 9 in FIG. 1, 26 is a logic element, 27 is the same key signal generator as 10 in FIG. 1, and 28 is a demodulated signal of QPSK. The logic element 26 performs a logical operation on the pair of QPSK demodulated signals 28 output from the pair of identification discriminators 25 as follows, and outputs the calculation result to the key signal generator 27.
[0035]
29 is an AFC circuit having the same configuration and function as 12 in FIG. 1, and includes the frequency discriminator 30, the sample hold circuit 31, the LPF 32, and the local light source 20 described in FIG.
[0036]
The AFC system of the present invention is particularly effective in QPSK where the allowable frequency difference is one order of magnitude more severe than that of BPSK. The demodulated signals I ′ and Q ′ of 28 QPSK in FIG. 5 can be expressed as the following equation (3).
[0037]
[Equation 3]

[0038]
Here, ΔΦ can be 0, π / 2, π, 3π / 2. In order to obtain the same key signal as in FIG. 4 when there is no phase change (ΔΦ = 0), the output of the logic element 26 should be positive only when I ′ is positive and Q ′ is negative. It ’s fine.
[0039]
In QPSK, the probability that a phase change does not occur is half that of BPSK, so the number of 33 key signals is also halved, but the fluctuation of the beat frequency between the key signals is considered to be negligibly small.
[0040]
【The invention's effect】
As described above, according to the present invention, the signal demodulated by the delay detection is fed back to the AFC sample and hold circuit, and only when there is no phase change, the sampling operation is performed at the timing of discrimination determination. Degradation of AFC characteristics due to data can be eliminated.
[0041]
The present invention is particularly effective in QPSK where the allowable frequency difference is one order of magnitude more severe than that of BPSK.
[Brief description of the drawings]
FIG. 1 is a block diagram illustrating a configuration of an optical BPSK signal receiver according to an embodiment of the present invention.
FIG. 2 is a block diagram showing the principle of a frequency discriminator of a phase diversity receiver of an optical PSK signal in a conventional example.
FIG. 3 is a block diagram showing a frequency discriminator in which a differentiator used in the embodiment of the present invention is configured by a delay line and a differential amplifier.
FIG. 4 is a waveform diagram showing the correspondence between the BPSK demodulated signal and the key signal generated in FIG. 1 according to the embodiment of the present invention.
FIG. 5 is a block diagram showing a configuration of an optical QPSK signal receiver according to another embodiment of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Receiver of optical diversity BPSK signal 2 Input optical fiber 3 90 ° optical hybrid 4 Balanced optical receiver 5 Local light source 6 Delay line τs delay line 7 Multiplier 8 Adder 9 Discriminator / determiner 10 Key signal Generator 11 BPSK demodulated signal 12 AFC circuit 13 Frequency discriminator 14 Sample hold circuit 15 LPF
16 Phase diversity optical QPSK signal receiver 17 Input optical fiber 18 90 ° optical hybrid 19 Balanced optical receiver 20 Local light source 21 Delay line τs delay line 22 Multiplier 23 Adder 24 Subtractor 25 Discriminator 26 Logic element 27 Key signal generator 28 QPSK demodulated signal 29 AFC circuit 30 Frequency discriminator 31 Sample hold circuit 32 LPF
41 Differentiator 42 Delay Line 43 Differential Amplifier 44 Multiplier 45 Subtractor 46 LPF
47 Delay line

Claims (3)

In an AFC circuit for making the oscillation frequency of a local light source of a coherent optical transmission receiver that demodulates optical PSK-modulated light waves by delay detection by a phase diversity method, follow the frequency of the signal light,
A frequency discriminator that obtains a voltage corresponding to the frequency difference between the intermediate frequency signals from two orthogonal frequency signals using a delay line and a differential amplifier, a multiplier, and a subtractor;
A key signal generating circuit that generates a key signal that performs a sample operation when there is no phase change between adjacent symbols from a PSK demodulated signal obtained by delay detection of the two intermediate frequency signals, and a hold operation when there is a phase change;
When there is no phase change between adjacent symbols with respect to the output signal of the frequency discriminator by the key signal, selection control is performed such that a sampling operation is performed, and if there is a holding operation, the selection result is fed back to the local light source. An AFC circuit for a phase diversity receiver of an optical PSK signal, which is combined with a sample hold circuit. The AFC circuit according to claim 1, wherein the PSK is BPSK. The AFC circuit according to claim 1, wherein the PSK is QPSK.

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