JP4905937B2 - PDF measuring method and apparatus - Google Patents

PDF measuring method and apparatus Download PDF

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JP4905937B2
JP4905937B2 JP2006216695A JP2006216695A JP4905937B2 JP 4905937 B2 JP4905937 B2 JP 4905937B2 JP 2006216695 A JP2006216695 A JP 2006216695A JP 2006216695 A JP2006216695 A JP 2006216695A JP 4905937 B2 JP4905937 B2 JP 4905937B2
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広人 川上
英二 吉田
宮本　　裕
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入力光を光路長の異なる２つの光路に分岐した後、再び合波させるタイプの光干渉計（以下、２光束干渉計）の偏波依存性を測定する方法に適する。特に、高速な位相変調信号の復調に用いるマッハツェンダ型干渉計（ＭＺＩ）の偏波依存性を精密に測定するのに適する。   This method is suitable for a method of measuring the polarization dependence of an optical interferometer (hereinafter referred to as a two-beam interferometer) of a type in which input light is split into two optical paths having different optical path lengths and then multiplexed again. In particular, it is suitable for precisely measuring the polarization dependence of a Mach-Zehnder interferometer (MZI) used for demodulation of a high-speed phase modulation signal.

高速な光位相変調を復調する受信器にはＭＺＩが広く用いられている。この目的でＭＺＩを用いる場合には、異なるｂｉｔ間で干渉を行い、かつまた干渉強度が最大または最小となる光周波数が光位相変調信号のキャリア周波数に一致しなければならない。   MZI is widely used for receivers that demodulate high-speed optical phase modulation. When MZI is used for this purpose, the optical frequency at which interference occurs between different bits and the interference intensity is maximized or minimized must match the carrier frequency of the optical phase modulation signal.

ところで、光導波路の光学的な異方性を完全に無くすことは難しく、特にＰＬＣでは、基板の方向によって定まる異方性が生じる。入力する光の偏波変動に伴い、干渉計の作る遅延時間が変動するため、干渉強度のピーク波長がシフトすることになる。一方、伝送路を伝播する光信号の偏波状態は、伝送路の曲げその他の原因によって容易に変化してしまう。安定した送受信を行なうためには、受信器内のＭＺＩの偏波依存性を小さくすることが極めて重要となる。   By the way, it is difficult to completely eliminate the optical anisotropy of the optical waveguide. In particular, in the PLC, anisotropy determined by the direction of the substrate occurs. As the input light changes in polarization, the delay time produced by the interferometer changes, and the peak wavelength of the interference intensity shifts. On the other hand, the polarization state of the optical signal propagating through the transmission line easily changes due to the bending of the transmission line and other causes. In order to perform stable transmission and reception, it is extremely important to reduce the polarization dependence of the MZI in the receiver.

紫外線の照射により光路の複屈折率を調整し偏波依存性を小さくしたＭＺＩの構成が特許文献１に開示されている。あるいは、偏波を回転させる光学素子をＰＬＣ内に配置し、偏波依存性をキャンセルするＭＺＩの構成が特許文献２に開示されている。現実のＭＺＩでは、精度の限界または光学素子の波長依存性により、光導波路の偏波依存性を完全に無くすことは困難であるため、作成されたＭＺＩの偏波依存性を精密に測定し、許容値以下であるか否かを判定することが必要とされる。特許文献３では、プローブ光の偏波状態をスキャンすることにより光フィルタの中心波長の偏波依存性を測定する方法が開示されている。   Patent Document 1 discloses a configuration of MZI in which the birefringence of an optical path is adjusted by irradiation of ultraviolet rays to reduce polarization dependency. Alternatively, Patent Document 2 discloses an MZI configuration in which an optical element that rotates polarization is arranged in a PLC and polarization dependency is canceled. In actual MZI, it is difficult to completely eliminate the polarization dependence of the optical waveguide due to the limit of accuracy or the wavelength dependence of the optical element. Therefore, the polarization dependence of the created MZI is precisely measured, It is necessary to determine whether or not it is below the allowable value. Patent Document 3 discloses a method of measuring the polarization dependence of the center wavelength of an optical filter by scanning the polarization state of probe light.

特許３０９０２９３号公報Japanese Patent No. 3090293 特許２６１４３６５号公報Japanese Patent No. 2614365 特許３２５５２５９号公報Japanese Patent No. 3255259

しかしながら、位相変調信号の復調に用いられるＭＺＩは受光器その他に接続されており、単体での評価は必ずしも容易ではない。プローブ光を入力することは可能でも、ＭＺＩから出力された時点でのプローブ光の状態をパワメータまたは光スペクトルアナライザで測定することは常に可能ではない。   However, the MZI used for demodulating the phase modulation signal is connected to a light receiver and the like, and it is not always easy to evaluate alone. Although it is possible to input the probe light, it is not always possible to measure the state of the probe light at the time when it is output from the MZI with a power meter or an optical spectrum analyzer.

本発明は、このような背景の下に行なわれたものであって、復調器内に配置されている２光束干渉計、特に、復調器内で光電気変換手段に接続されているＭＺＩについても実現可能なＰＤＦ(Polarization Dependent Frequency/２光束干渉計の干渉周波数の偏波による変動量)測定方法を提供することを目的とする。   The present invention has been carried out under such a background, and also relates to a two-beam interferometer disposed in a demodulator, particularly an MZI connected to photoelectric conversion means in the demodulator. An object of the present invention is to provide a feasible PDF (Polarization Dependent Frequency / 2 fluctuation amount due to polarization of interference frequency of a two-beam interferometer) measurement method.

本発明は、２光束干渉計の干渉周波数の偏波によるＰＤＦを測定するＰＤＦ測定方法であって、本発明の特徴とするところは、２値以上のディジタル信号パターンを生成する信号生成ステップと、前記ディジタル信号パターンで光を変調することによりシンボル周期Ｔの位相変調信号光を生成する位相変調光送信ステップと、前記位相変調信号光の偏波を変更して前記２光束干渉計に入力する偏波変更ステップと、前記２光束干渉計内から出力される強度変調信号光を受光する受光ステップと、前記強度変調信号光を識別し、前記ディジタル信号パターンと比較することによりＢＥＲ(Bit Error Rate)を測定するＢＥＲ測定ステップと、前記位相変調信号光の中心光周波数Ｆcを変更する波長変更ステップと、前記Ｆcと前記ＢＥＲとの関係を分析するデータ解析ステップとを有し、前記シンボル周期Ｔは、前記２光束干渉計の有する２つの光路の時間差τに等しく、前記偏波変更ステップは、前記ＢＥＲ測定ステップの結果を参照しつつ前記ＢＥＲが最良となる偏波状態ＳＯＰ１および最悪となる偏波状態ＳＯＰ２の２種を検出するステップを含み、前記ＢＥＲ測定ステップは、前記偏波状態ＳＯＰ１におけるＢＥＲであるＢＥＲ１と前記偏波状態ＳＯＰ２におけるＢＥＲであるＢＥＲ２とを測定するステップを含み、前記データ解析ステップは、Ｆc対ＢＥＲ１の傾斜部とＦc対ＢＥＲ２の傾斜部との光周波数軸方向のシフト量ΔＦcを求め、ＰＤＦ＝ΔＦcの関係によりＰＤＦを求めるステップを含むことを特徴とする。 The present invention is a PDF measurement method for measuring a PDF due to polarization of an interference frequency of a two-beam interferometer, and a feature of the present invention is that a signal generation step for generating a digital signal pattern of two or more values; A phase-modulated light transmission step for generating a phase-modulated signal light having a symbol period T by modulating light with the digital signal pattern; and a polarization that is input to the two-beam interferometer by changing the polarization of the phase-modulated signal light. BER (Bit Error Rate) by discriminating the intensity modulation signal light, receiving the intensity modulation signal light output from the two-beam interferometer, and comparing the intensity modulation signal light with the digital signal pattern a BER measurement step of measuring the wavelength changing step of changing the central optical frequency F c of the phase-modulated optical signal, analyzing the relationship between the F c the BER Wherein and a that the data analysis step, the symbol period T, the two-beam interferometer two optical paths of the time difference equal properly to τ possessed by the polarization changing step, with reference to the result of the BER measurement step The method includes detecting two types of a polarization state SOP1 where the BER is the best and a polarization state SOP2 where the BER is the worst. The BER measurement step includes BER1 which is a BER in the polarization state SOP1 and the polarization state SOP2. wherein the step of measuring the BER2 and a BER, the data analyzing step obtains the shift amount [Delta] F c of the optical frequency axis direction and the inclined portion of the inclined portion and the F c pair BER2 of F c pair BER1, PDF = [Delta] F a step of obtaining a PDF according to the relationship of c .

あるいは、前記位相変調信号光の中心光周波数Ｆcを変更する波長変更ステップに代えて、前記Ｆcを一定として前記２つの光路の幾何学的光路差Ｌを変更することにより位相差φを変更する位相差変更ステップを有し、前記Ｆcと前記ＢＥＲとの関係を分析するデータ解析ステップに代えて、前記Ｆcを一定として前記Ｌと前記ＢＥＲとの関係を解析するデータ解析ステップを有し、このデータ解析ステップは、φ対ＢＥＲ１の傾斜部とφ対ＢＥＲ２の傾斜部との位相方向のシフト量Δφを求め、Δφ／２πに前記２光束干渉計のＦＳＲ(Free Spectrum Range) をかけることによりＰＤＦを求めるステップを含むこともできる。例えば、前記位相変調信号光はＤＭＰＳＫ(Differential Multiple-Phase-Shift Keying)である。 Alternatively, in place of the wavelength changing step of changing the center optical frequency F c of the phase-modulated signal light, the phase difference φ is changed by changing the geometric optical path difference L between the two optical paths while keeping the F c constant. And a data analysis step for analyzing the relationship between the L and the BER with the F c constant, instead of the data analysis step for analyzing the relationship between the F c and the BER. In this data analysis step, the shift amount Δφ in the phase direction between the inclined portion of φ vs. BER1 and the inclined portion of φ vs. BER2 is obtained, and Δφ / 2π is multiplied by the FSR (Free Spectrum Range) of the two-beam interferometer. Thus, a step of obtaining the PDF can be included. For example, the phase-modulated signal light is DMPSK (Differential Multiple-Phase-Shift Keying).

また、本発明をＰＤＦ測定装置の観点からみると、本発明は、２値以上のディジタル信号パターンを生成する信号生成手段と、前記ディジタル信号パターンで光を変調することによりシンボル周期Ｔの位相変調信号光を生成する位相変調光送信手段と、前記位相変調信号光の偏波を変更して前記２光束干渉計に入力する偏波変更手段と、前記２光束干渉計内から出力される強度変調信号光を受光する受光手段と、前記強度変調信号光を識別し、前記ディジタル信号パターンと比較することによりＢＥＲを測定するＢＥＲ測定手段と、前記位相変調信号光の中心光周波数Ｆcを変更する波長変更ステップと、前記Ｆcと前記ＢＥＲとの関係を分析するデータ解析手段とを有し、前記シンボル周期Ｔは、前記２光束干渉計の有する２つの光路の時間差τに等しく、前記偏波変更手段は、前記ＢＥＲ測定ステップの結果を参照しつつ前記ＢＥＲが最良となる偏波状態ＳＯＰ１および最悪となる偏波状態ＳＯＰ２の２種を検出する手段を含み、前記ＢＥＲ測定手段は、前記偏波状態ＳＯＰ１におけるＢＥＲであるＢＥＲ１と前記偏波状態ＳＯＰ２におけるＢＥＲであるＢＥＲ２とを測定する手段を含み、前記データ解析手段は、Ｆc対ＢＥＲ１の傾斜部とＦc対ＢＥＲ２の傾斜部との光周波数軸方向のシフト量ΔＦcを求め、ＰＤＦ＝ΔＦcの関係によりＰＤＦを求める手段を含むことを特徴とする。 Further, when the present invention is viewed from the viewpoint of a PDF measuring apparatus, the present invention includes a signal generating means for generating a digital signal pattern of two or more values, and a phase modulation of a symbol period T by modulating light with the digital signal pattern. Phase modulated light transmitting means for generating signal light, polarization changing means for changing the polarization of the phase modulated signal light and inputting it to the two-beam interferometer, and intensity modulation output from within the two-beam interferometer Light receiving means for receiving signal light, BER measuring means for measuring the BER by identifying the intensity modulated signal light and comparing it with the digital signal pattern, and changing the center optical frequency F c of the phase modulated signal light A wavelength changing step; and data analysis means for analyzing a relationship between the F c and the BER, wherein the symbol period T is a time of two optical paths of the two-beam interferometer. Etc. properly, the polarization changing means the difference τ includes means for detecting the two polarization states SOP2 said BER with reference to the result of the BER measuring step becomes polarization state SOP1 and worst becomes the best The BER measuring means includes means for measuring BER1 which is BER in the polarization state SOP1 and BER2 which is BER in the polarization state SOP2, and the data analysis means includes an inclined portion of F c versus BER1 It includes means for obtaining a shift amount ΔF c in the optical frequency axis direction with respect to the inclined portion of F c vs. BER 2 and obtaining PDF by the relationship of PDF = ΔF c .

あるいは、前記位相変調信号光の中心光周波数Ｆcを変更する波長変更手段に代えて、前記Ｆcを一定として前記２つの光路の幾何学的光路差Ｌを変更することにより位相差φを変更する位相差変更手段を有し、前記Ｆcと前記ＢＥＲとの関係を分析するデータ解析手段に代えて、前記Ｆcを一定として前記Ｌと前記ＢＥＲとの関係を解析するデータ解析手段を有し、このデータ解析手段は、φ対ＢＥＲ１の傾斜部とφ対ＢＥＲ２の傾斜部との位相方向のシフト量Δφを求め、Δφ／２πに前記２光束干渉計のＦＳＲをかけることによりＰＤＦを求める手段を含むこともできる。例えば、前記位相変調信号光はＤＭＰＳＫである。 Alternatively, the phase difference φ is changed by changing the geometric optical path difference L between the two optical paths with the F c constant, instead of the wavelength changing means for changing the center optical frequency F c of the phase-modulated signal light. In place of the data analysis means for analyzing the relationship between the F c and the BER, there is provided a data analysis means for analyzing the relationship between the L and the BER while keeping the F c constant. Then, this data analysis means obtains the shift amount Δφ in the phase direction between the inclined portion of φ vs. BER1 and the inclined portion of φ vs. BER2, and obtains PDF by multiplying Δφ / 2π by the FSR of the two-beam interferometer. Means can also be included. For example, the phase modulation signal light is DMPSK.

本発明によれば、２光束干渉計、特にＭＺＩの有する偏波依存性を精密に測定することが可能となる。   According to the present invention, it is possible to accurately measure the polarization dependence of a two-beam interferometer, particularly an MZI.

本発明実施例のＰＤＦ測定方法を説明するのに先立って、本発明の作用について説明する。まず、干渉計外部の各光学パーツを省略した簡単な状態について説明する。２光束干渉計の模式図を図１に示す。   Prior to describing the PDF measurement method of the embodiment of the present invention, the operation of the present invention will be described. First, a simple state in which each optical part outside the interferometer is omitted will be described. A schematic diagram of a two-beam interferometer is shown in FIG.

図１に示すように、２つの光導波路は、紙面に垂直な方向がｓｌｏｗ軸、紙面に水平な方向がｆａｓｔ軸であるとする。ここでは簡単のため、分岐から合波に至るまで、ｆａｓｔ軸は常に平行であると仮定している。第１の光導波路の、最も大きい遅延時間をτ1s、最も小さい遅延時間をｔ1fとする。第２の光導波路の遅延時間も同様にτ2s、τ2fと定義する。 As shown in FIG. 1, in the two optical waveguides, the direction perpendicular to the paper surface is the slow axis and the direction horizontal to the paper surface is the fast axis. Here, for simplicity, it is assumed that the fast axis is always parallel from the branching to the multiplexing. The largest delay time of the first optical waveguide is τ 1s , and the smallest delay time is t 1f . Similarly, the delay time of the second optical waveguide is defined as τ 2s and τ 2f .

ＤＭＰＳＫでは、直前のビットとの位相比較が必要であるため、
τ1s−τ2s
または
τ1f−τ2f
はシンボル周期Ｔとほぼ等しくするが、厳密にはＴとやや異なる値に設定する。必要とされる差分は多重度によって決まるが、ここでは詳細は省く。２光束干渉計内部の、分波に伴う位相の変動は上記の遅延時間に含まれるものとする。パワの分散および合波は５０％対５０％と仮定している。入力光がキャリア周波数Ｆであるとき、その電場は、 Since DMPSK requires a phase comparison with the previous bit,
τ 1s −τ 2s
Or τ 1f −τ 2f
Is substantially equal to the symbol period T, but strictly, it is set to a value slightly different from T. The required difference depends on the multiplicity, but details are omitted here. It is assumed that the phase fluctuation accompanying demultiplexing inside the two-beam interferometer is included in the delay time. Power dispersion and multiplexing is assumed to be 50% vs. 50%. When the input light is at the carrier frequency F, the electric field is

Figure 0004905937
で表すことができる。光の強度は電場の内積に比例するが、ここでは内積が１となるよう正規化してある。θinやφinは入力光の偏波状態を表す。出力時の電場は、
Figure 0004905937
 Can be expressed as The intensity of light is proportional to the inner product of the electric field, but here it is normalized so that the inner product is 1. θ in and φ in represent the polarization state of the input light. The electric field at the time of output is

Figure 0004905937
であり、出力時の光の強度は、
Figure 0004905937
 And the intensity of light at the time of output is

Figure 0004905937
となる。
Figure 0004905937
 It becomes.

以上の結果から、以下のことがわかる。
（ａ）干渉強度のカーブの最大値（または最小値）は入力のθinに依存するが、入力がθin＝０またはθin＝π／２の直線偏波であるとき、両者の最大値（または最小値）は等しくなる。
（ｂ）入力がθin＝０のとき、干渉強度のピークは最も低周波数側にシフトし、入力がθin＝π／２のとき、干渉強度のピークは最も高周波数側にシフトする。
（ｃ）したがって入力がθin＝０のときとθin＝π／２のときの光学特性の光周波数軸方向のシフト量２ΔＦはＰＤＦに等しい。 From the above results, the following can be understood.
The maximum value of the curve (a) interference intensity (or minimum value) is dependent on the input of theta in, when the input is linearly polarized wave of theta in = 0 or θ in = π / 2, both of the maximum value (Or minimum) are equal.
(B) When the input is θ in = 0, the interference intensity peak shifts to the lowest frequency side, and when the input is θ in = π / 2, the interference intensity peak shifts to the highest frequency side.
(C) Therefore, when the input is θ in = 0 and θ in = π / 2, the shift amount 2ΔF in the optical frequency axis direction of the optical characteristics is equal to PDF.

次に、この２光束干渉計（以下、単に干渉計という）を用いて、Ｎ値のディジタル位相変調信号の復調を行なうことを考える。正しく復調を行なうためには、位相変調信号の中心光周波数Ｆcと干渉計の光学特性とを正確に調整しなければならず、ミスマッチがあると復調された信号のアイ開口率は劣化する。 Next, it is considered to demodulate an N-value digital phase modulation signal using this two-beam interferometer (hereinafter simply referred to as an interferometer). In order to perform demodulation correctly, the center optical frequency F c of the phase modulation signal and the optical characteristics of the interferometer must be adjusted accurately. If there is a mismatch, the eye opening ratio of the demodulated signal deteriorates.

図２に示した干渉計の山と信号光の中心周波数Ｆcとの最適な関係は、変調方式に依存するので簡単には言えないが、Ｎ＝２のＤＭＰＳＫ信号であればＦcが干渉強度の山に一致するときに最良のアイ開口が得られる。両者がずれたとき、アイ開口は劣化し、ＢＥＲは直ちに劣化する。 The optimum relationship between the peak of the interferometer shown in FIG. 2 and the center frequency F c of the signal light depends on the modulation method and cannot be said easily. However, if the N = 2 DMPSK signal, F c interferes. The best eye opening is obtained when matching the intensity peak. When they deviate, the eye opening deteriorates and the BER immediately deteriorates.

ずれの量とＢＥＲの劣化量とは送信系の特性や信号光に重畳するランダム雑音に強く依存するので自明ではないが、模式的に両者の関係を図３に示す。干渉計の光学特性は図２に示したものと同じとする。信号光の中心周波数Ｆcを随時変更したときに得られるＢＥＲ（θin＝０、θin＝π／４、θin＝π／２）を縦軸にプロットしているが、模式図であるからこのスロープの角度は厳密なものではない。 The amount of deviation and the amount of BER degradation are not obvious because they strongly depend on the characteristics of the transmission system and random noise superimposed on the signal light, but the relationship between them is schematically shown in FIG. The optical characteristics of the interferometer are the same as those shown in FIG. The BER (θ in = 0, θ in = π / 4, θ in = π / 2) obtained when the center frequency F c of the signal light is changed as needed is plotted on the vertical axis. Because of this slope angle is not exact.

上記の（ｃ）の性質から、θin＝０のときとθin＝π／２のときのＢＥＲのカーブが、光周波数軸上で最も乖離し、この乖離量がＰＤＦに等しくなる。θin＝π／４の場合は、ＢＥＲのカーブは先述の２つのカーブの間に入る（図３）。ＢＥＲはＭＺＩの消光比に依存し、消光比はθinに依存するので、θin≠π／２かつθin≠０のときのＢＥＲのカーブは、θin＝π／２またはθin＝０のときのＢＥＲのカーブとやや異なる。 Due to the above property (c), the BER curves when θ in = 0 and θ in = π / 2 are most dissimilar on the optical frequency axis, and this dissociation amount is equal to the PDF. When θ in = π / 4, the BER curve falls between the two curves described above (FIG. 3). Since BER depends on the extinction ratio of MZI and the extinction ratio depends on θ in , the curve of BER when θ in ≠ π / 2 and θ in ≠ 0 is θ in = π / 2 or θ in = 0. Slightly different from the BER curve.

しかし、θin＝π／２のときとθin＝０のときとでは消光比が等しくなるため、θin＝０におけるＢＥＲのカーブを、光周波数軸上で平行移動するとθin＝π／２におけるＢＥＲのカーブに一致する。 However, since the extinction ratio is equal between θ in = π / 2 and θ in = 0, when the BER curve at θ in = 0 is translated on the optical frequency axis, θ in = π / 2 It matches the BER curve at.

次に、θin＝０のときにＢＥＲが最良となる光周波数をＦ１、θin＝π／２のときにＢＥＲが最良となるＦcをＦ２とする。図３より明らかなように、ＦcがＦ１よりやや低い場合には、ＢＥＲが最良となるのはθin＝０となる偏波であり、最悪となるのはθin＝π／２となる偏波である。ＦcがＦ２よりやや高い場合はその逆となる。

したがって、Ｆcを掃引しながらθinをランダムに変更し、最良または最悪のＢＥＲを記録してゆけば、θin＝０およびθin＝π／２のＢＥＲのカーブをプロットすることが可能となる。これら２つのカーブの光周波数方向のシフト量は、θin＝０およびθin＝π／２における干渉計の光学特性の光周波数方向のシフト量、すなわちＰＤＦに等しくなる。 Next, the optical frequency at which the BER is best when θ in = 0 is F1, and F c at which the BER is best when θ in = π / 2 is F2. As is clear from FIG. 3, when F c is slightly lower than F1, the best BER is the polarization with θ in = 0, and the worst is θ in = π / 2. Polarization. If F c is slightly higher than F2 becomes the opposite.

Therefore, if θ in is randomly changed while sweeping F c and the best or worst BER is recorded, it is possible to plot BER curves of θ in = 0 and θ in = π / 2. Become. The shift amount in the optical frequency direction of these two curves is equal to the shift amount in the optical frequency direction of the optical characteristics of the interferometer at θ in = 0 and θ in = π / 2, that is, PDF.

以上の議論はＦcを掃引した例であるが、Ｆcを一定とし干渉計の位相差φを掃引しても同様のＢＥＲ曲線を描くことができる。最良および最悪のＢＥＲ曲線のシフト量Δφを求め、干渉計のＦＳＲをΔφ／２πにかけることによりＰＤＦを求めることができる。位相差φの掃引は、干渉計の幾何学的光路差Ｌを熱で変更することにより容易に実現できる。 The above discussion is an example in which sweeping the F c, can be drawn the same BER curves by sweeping the phase difference φ of the interferometer to the F c is constant. The PDF can be obtained by obtaining the shift amount Δφ of the best and worst BER curves and multiplying the FSR of the interferometer by Δφ / 2π. The sweep of the phase difference φ can be easily realized by changing the geometric optical path difference L of the interferometer with heat.

（第一実施例）
第一実施例の２光束干渉計の偏波依存性測定方法に用いる測定系の基本構成を図４に示す。信号生成手段１は、２値以上のディジタル信号パターンを生成する。位相変調光発生手段２は、信号生成手段１により生成されたディジタル信号パターンを入力し、シンボル周期Ｔが、測定対象の２光束干渉計の有する２つの光路の時間差τに等しい、ディジタル位相変調信号光（例えば、ＤＭＰＳＫ）を出力する。偏波変更手段３は式１で記したＥinのθinを変更することができる。ＤＵＴ４は、測定対象となる２光束干渉計である。ＤＵＴ４の出力は位相変調信号光を強度変調信号光に変換する。光電気変化手段５は、強度変調信号光を電気に変換する。 (First Example)
FIG. 4 shows the basic configuration of the measurement system used in the polarization dependence measurement method of the two-beam interferometer of the first embodiment. The signal generation means 1 generates a digital signal pattern having two or more values. The phase-modulated light generating means 2 receives the digital signal pattern generated by the signal generating means 1 and has a symbol period T equal to the time difference τ between two optical paths of the two-beam interferometer to be measured. Output light (for example, DMPSK). The polarization changing means 3 can change the θ in of E in expressed by Equation 1. The DUT 4 is a two-beam interferometer to be measured. The output of the DUT 4 converts the phase modulated signal light into intensity modulated signal light. The photoelectric change means 5 converts the intensity-modulated signal light into electricity.

このとき、ＤＵＴ４がＭＺＩのように特性が相補的な２つの出力ポートを持つのであれば、バランス型受信器を用いて電気変換を行なってもよい。また、位相変調光発生手段２およびＢＥＲ測定手段６は、ＣＬＫ発生手段９が発生するクロックに従って動作する。   At this time, if the DUT 4 has two output ports with complementary characteristics such as MZI, electrical conversion may be performed using a balanced receiver. The phase-modulated light generating means 2 and the BER measuring means 6 operate according to the clock generated by the CLK generating means 9.

さらに、位相変調光発生手段２が発生する位相変調信号光の中心光周波数Ｆcを変更する波長変更手段７と、前記Ｆcと前記ＢＥＲとの関係を分析するデータ解析手段８とを有し、前記シンボル周期Ｔは、前記２光束干渉計の有する２つの光路の時間差τに実質等しく、偏波変更手段３は、ＢＥＲ測定手段６の結果を参照しつつ前記ＢＥＲが最良となる偏波状態ＳＯＰ１および最悪となる偏波状態ＳＯＰ２の２種を検出する手段（図示省略）を含み、ＢＥＲ測定手段６は、前記偏波状態ＳＯＰ１におけるＢＥＲであるＢＥＲ１と前記偏波状態ＳＯＰ２におけるＢＥＲであるＢＥＲ２とを測定する手段（図示省略）を含み、データ解析手段８は、Ｆc対ＢＥＲ１のカーブとＦc対ＢＥＲ２のカーブとの光周波数軸方向のシフト量ΔＦcを求め、ＰＤＦ＝ΔＦcの関係によりＰＤＦを求める手段（図示省略）を含む。 Furthermore, it has wavelength changing means 7 for changing the center optical frequency F c of the phase modulated signal light generated by the phase modulated light generating means 2 and data analyzing means 8 for analyzing the relationship between the F c and the BER. The symbol period T is substantially equal to the time difference τ between the two optical paths of the two-beam interferometer, and the polarization changing means 3 refers to the result of the BER measuring means 6 and the polarization state where the BER is the best. It includes means (not shown) for detecting two types of SOP1 and worst polarization state SOP2, and the BER measurement means 6 is BER1 which is BER in the polarization state SOP1 and BER2 which is BER in the polarization state SOP2. comprises means for measuring (not shown) the door, the data analysis unit 8 obtains the optical frequency axis direction of the shift amount [Delta] F c of the curve of the curve and F c pair BER2 of F c pair BER1, PD = Including means for determining the PDF (not shown) by the relationship of [Delta] F c.

（第二実施例）
第二実施例の２光束干渉計の偏波依存性測定方法に用いる測定系の基本構成を図５に示す。第二実施例の２光束干渉計の偏波依存性測定方法に用いる測定系の基本構成では、第一実施例の波長変更手段７に代えて、前記Ｆcを一定として前記２つの光路の幾何学的光路差Ｌを変更することにより位相差φを変更する位相差変更手段１７を有し、ＦcとＢＥＲとの関係を分析するデータ解析手段８に代えて、前記Ｆcを一定として前記Ｌと前記ＢＥＲとの関係を解析するデータ解析手段１８を有し、このデータ解析手段１８は、φ対ＢＥＲ１の傾斜部とφ対ＢＥＲ２の傾斜部との位相方向のシフト量Δφを求め、Δφ／２πにＤＵＴ４のＦＳＲをかけることによりＰＤＦを求める。 (Second embodiment)
FIG. 5 shows the basic configuration of the measurement system used in the polarization dependence measurement method of the two-beam interferometer of the second embodiment. In the basic configuration of the measurement system used in the polarization dependence measurement method of the two-beam interferometer of the second embodiment, the geometry of the two optical paths is set with the F c constant, instead of the wavelength changing means 7 of the first embodiment. The phase difference changing means 17 for changing the phase difference φ by changing the optical path difference L is provided, and instead of the data analyzing means 8 for analyzing the relationship between F c and BER, the F c is constant. The data analysis means 18 analyzes the relationship between L and the BER, and the data analysis means 18 obtains a shift amount Δφ in the phase direction between the inclined portion of φ vs. BER1 and the inclined portion of φ vs. BER2, and Δφ The PDF is obtained by multiplying / 2π by the FSR of DUT4.

（ＰＤＦ測定結果）
実際に得られた最良および最悪のＢＥＲ曲線を図６に示す。この測定では、図４に示した第一実施例の２光束干渉計の偏波依存性測定に用いる測定系の基本構成を用い、光周波数Ｆcを掃引した。ここではＦcの絶対値は重要ではないので、Ｆcの相対的な変更量を横軸にプロットしている。白抜きのシンボルはＢＥＲが最悪となるよう偏波を選んだＢＥＲ曲線であり、色付きのシンボルはＢＥＲが最良となるよう偏波を選んだＢＥＲ曲線である。実線および破線は前述のθin＝０とθin＝π／２のときのＢＥＲ曲線を理論的に計算し、実測値にフィッティングしたものである。両者の乖離量は０．９５ＧＨｚであり、これがＰＤＦに相当する。なお、信号光のＳＮ比を高くするとＢＥＲ曲線が急峻になるため、より精度の高い測定結果が得られる。 (PDF measurement result)
The best and worst BER curves actually obtained are shown in FIG. In this measurement, the optical frequency Fc was swept using the basic configuration of the measurement system used for the polarization dependence measurement of the two-beam interferometer of the first embodiment shown in FIG. Since the absolute value of F c is not important here, the relative change amount of F c is plotted on the horizontal axis. A white symbol is a BER curve in which the polarization is selected so that the BER is worst, and a colored symbol is a BER curve in which the polarization is selected so that the BER is best. The solid line and the broken line are obtained by theoretically calculating the BER curve when θ in = 0 and θ in = π / 2, and fitting them to the actual measurement values. The difference between the two is 0.95 GHz, which corresponds to PDF. In addition, since the BER curve becomes steep when the SN ratio of the signal light is increased, a more accurate measurement result can be obtained.

本発明によれば、２光束干渉計、特にＭＺＩの有する偏波依存性を精密に測定することが可能となるため、例えば、ＤＰＳＫ伝送方式等の光通信システムにおける通信品質の測定精度の向上に寄与することができる。   According to the present invention, it is possible to precisely measure the polarization dependence of a two-beam interferometer, particularly an MZI. For example, to improve the measurement accuracy of communication quality in an optical communication system such as the DPSK transmission method. Can contribute.

２光束干渉計の模式図。The schematic diagram of a two-beam interferometer. 干渉計の山と信号光の中心周波数Ｆcとの関係を模式的に示す図。Diagram schematically illustrating the relationship between the center frequency F c of the mountain of the interferometer and the signal light. ずれの量とＢＥＲの劣化量との関係を模式的に示す図。The figure which shows typically the relationship between the amount of deviation | shift, and the deterioration amount of BER. 第一実施例の２光束干渉計の偏波依存性測定方法に用いる測定系の基本構成示す図。The figure which shows the basic composition of the measurement system used for the polarization dependence measuring method of the two-beam interferometer of a 1st Example. 第二実施例の２光束干渉計の偏波依存性測定方法に用いる測定系の基本構成を示す図。The figure which shows the basic composition of the measurement system used for the polarization dependence measuring method of the two-beam interferometer of a 2nd Example. 実際に得られた最良および最悪のＢＥＲ曲線を示す図。The figure which shows the best and worst BER curve actually obtained.

符号の説明Explanation of symbols

１ 信号生成手段
２ 位相変調光発生手段
３ 偏波変更手段
４ ＤＵＴ
５ 光電気変換手段
６ ＢＥＲ測定手段
７ 波長変更手段
８、１８ データ解析手段
９ ＣＬＫ発生手段
１７ 位相差変更手段 DESCRIPTION OF SYMBOLS 1 Signal generation means 2 Phase modulation light generation means 3 Polarization change means 4 DUT
5 Photoelectric conversion means 6 BER measuring means 7 Wavelength changing means 8, 18 Data analyzing means 9 CLK generating means 17 Phase difference changing means

Claims (6)

２光束干渉計の干渉周波数の偏波によるＰＤＦ(Polarization Dependent Frequency)を測定する方法において、
２値以上のディジタル信号パターンを生成する信号生成ステップと、
前記ディジタル信号パターンで光を変調することによりシンボル周期Ｔの位相変調信号光を生成する位相変調光送信ステップと、
前記位相変調信号光の偏波を変更して前記２光束干渉計に入力する偏波変更ステップと、
前記２光束干渉計内から出力される強度変調信号光を受光する受光ステップと、
前記強度変調信号光を識別し、前記ディジタル信号パターンと比較することによりＢＥＲ(Bit Error Rate)を測定するＢＥＲ測定ステップと、
前記位相変調信号光の中心光周波数Ｆcを変更する波長変更ステップと、
前記Ｆcと前記ＢＥＲとの関係を分析するデータ解析ステップと
を有し、
前記シンボル周期Ｔは、前記２光束干渉計の有する２つの光路の時間差τに等しく、
前記偏波変更ステップは、前記ＢＥＲ測定ステップの結果を参照しつつ前記ＢＥＲが最良となる偏波状態ＳＯＰ１および最悪となる偏波状態ＳＯＰ２の２種を検出するステップを含み、
前記ＢＥＲ測定ステップは、前記偏波状態ＳＯＰ１におけるＢＥＲであるＢＥＲ１と前記偏波状態ＳＯＰ２におけるＢＥＲであるＢＥＲ２とを測定するステップを含み、
前記データ解析ステップは、Ｆc対ＢＥＲ１の傾斜部とＦc対ＢＥＲ２の傾斜部との光周波数軸方向のシフト量ΔＦcを求め、ＰＤＦ＝ΔＦcの関係によりＰＤＦを求めるステップを含む
ことを特徴とするＰＤＦ測定方法。 In a method for measuring PDF (Polarization Dependent Frequency) by polarization of interference frequency of a two-beam interferometer,
A signal generation step of generating a digital signal pattern of two or more values;
A phase-modulated light transmission step of generating phase-modulated signal light having a symbol period T by modulating light with the digital signal pattern;
A polarization changing step of changing the polarization of the phase-modulated signal light and inputting it to the two-beam interferometer;
A light receiving step of receiving intensity modulated signal light output from within the two-beam interferometer;
A BER measurement step of measuring the BER (Bit Error Rate) by identifying the intensity-modulated signal light and comparing it with the digital signal pattern;
A wavelength changing step for changing the center optical frequency F c of the phase-modulated signal light;
A data analysis step of analyzing a relationship between the F c and the BER,
The symbol period T, two equal properly to the time difference of the optical path τ possessed by the two-beam interferometer,
The polarization changing step includes a step of detecting two types of polarization state SOP1 where the BER is best and worst polarization state SOP2 while referring to the result of the BER measurement step,
The BER measurement step includes a step of measuring BER1 which is BER in the polarization state SOP1 and BER2 which is BER in the polarization state SOP2,
The data analyzing step obtains the optical frequency axis direction of the shift amount [Delta] F c of the inclined portion of the inclined portion and the F c pair BER2 of F c pair BER1, including the step of obtaining the PDF by the relationship of PDF = [Delta] F c A characteristic PDF measurement method. 前記位相変調信号光の中心光周波数Ｆcを変更する波長変更ステップに代えて、前記Ｆcを一定として前記２つの光路の幾何学的光路差Ｌを変更することにより位相差φを変更する位相差変更ステップを有し、
前記Ｆcと前記ＢＥＲとの関係を分析するデータ解析ステップに代えて、前記Ｆcを一定として前記Ｌと前記ＢＥＲとの関係を解析するデータ解析ステップを有し、
このデータ解析ステップは、φ対ＢＥＲ１の傾斜部とφ対ＢＥＲ２の傾斜部との位相方向のシフト量Δφを求め、Δφ／２πに前記２光束干渉計のＦＳＲ(Free Spectrum Range)をかけることによりＰＤＦを求めるステップを含む
請求項１記載のＰＤＦ測定方法。 In place of the wavelength changing step of changing the center optical frequency F c of the phase-modulated signal light, the phase difference φ is changed by changing the geometric optical path difference L between the two optical paths while keeping the F c constant. A phase difference changing step,
Instead of a data analysis step for analyzing the relationship between the F c and the BER, a data analysis step for analyzing the relationship between the L and the BER with the F c constant.
In this data analysis step, a shift amount Δφ in the phase direction between the inclined portion of φ versus BER 1 and the inclined portion of φ versus BER 2 is obtained, and Δφ / 2π is multiplied by the FSR (Free Spectrum Range) of the two-beam interferometer. The PDF measurement method according to claim 1, further comprising a step of obtaining a PDF. 前記位相変調信号光はＤＭＰＳＫ(Differential Multiple-Phase-Shift Keying)である請求項１記載のＰＤＦ測定方法。   2. The PDF measuring method according to claim 1, wherein the phase-modulated signal light is DMPSK (Differential Multiple-Phase-Shift Keying). ２光束干渉計の干渉周波数の偏波によるＰＤＦを測定する装置において、
２値以上のディジタル信号パターンを生成する信号生成手段と、
前記ディジタル信号パターンで光を変調することによりシンボル周期Ｔの位相変調信号光を生成する位相変調光送信手段と、
前記位相変調信号光の偏波を変更して前記２光束干渉計に入力する偏波変更手段と、
前記２光束干渉計内から出力される強度変調信号光を受光する受光手段と、
前記強度変調信号光を識別し、前記ディジタル信号パターンと比較することによりＢＥＲを測定するＢＥＲ測定手段と、
前記位相変調信号光の中心光周波数Ｆcを変更する波長変更ステップと、
前記Ｆcと前記ＢＥＲとの関係を分析するデータ解析手段と
を有し、
前記シンボル周期Ｔは、前記２光束干渉計の有する２つの光路の時間差τに等しく、
前記偏波変更手段は、前記ＢＥＲ測定ステップの結果を参照しつつ前記ＢＥＲが最良となる偏波状態ＳＯＰ１および最悪となる偏波状態ＳＯＰ２の２種を検出する手段を含み、
前記ＢＥＲ測定手段は、前記偏波状態ＳＯＰ１におけるＢＥＲであるＢＥＲ１と前記偏波状態ＳＯＰ２におけるＢＥＲであるＢＥＲ２とを測定する手段を含み、
前記データ解析手段は、Ｆc対ＢＥＲ１の傾斜部とＦc対ＢＥＲ２の傾斜部との光周波数軸方向のシフト量ΔＦcを求め、ＰＤＦ＝ΔＦcの関係によりＰＤＦを求める手段を含む
ことを特徴とするＰＤＦ測定装置。 In an apparatus for measuring PDF by polarization of the interference frequency of a two-beam interferometer,
Signal generating means for generating a digital signal pattern of two or more values;
Phase-modulated light transmitting means for generating phase-modulated signal light having a symbol period T by modulating light with the digital signal pattern;
Polarization changing means for changing the polarization of the phase-modulated signal light and inputting it to the two-beam interferometer;
A light receiving means for receiving intensity modulated signal light output from within the two-beam interferometer;
BER measuring means for measuring the BER by identifying the intensity-modulated signal light and comparing it with the digital signal pattern;
A wavelength changing step for changing the center optical frequency F c of the phase-modulated signal light;
Data analysis means for analyzing the relationship between the F c and the BER;
The symbol period T, two equal properly to the time difference of the optical path τ possessed by the two-beam interferometer,
The polarization changing means includes means for detecting two types of polarization state SOP1 where the BER is best and worst polarization state SOP2 while referring to the result of the BER measurement step,
The BER measurement means includes means for measuring a BER that is a BER in the polarization state SOP1 and a BER2 that is a BER in the polarization state SOP2.
The data analyzing means determines the optical frequency axis direction of the shift amount [Delta] F c of the inclined portion of the inclined portion and the F c pair BER2 of F c pair BER1, to include means for obtaining the PDF by the relationship of PDF = [Delta] F c A characteristic PDF measuring device. 前記位相変調信号光の中心光周波数Ｆcを変更する波長変更手段に代えて、前記Ｆcを一定として前記２つの光路の幾何学的光路差Ｌを変更することにより位相差φを変更する位相差変更手段を有し、
前記Ｆcと前記ＢＥＲとの関係を分析するデータ解析手段に代えて、前記Ｆcを一定として前記Ｌと前記ＢＥＲとの関係を解析するデータ解析手段を有し、
このデータ解析手段は、φ対ＢＥＲ１の傾斜部とφ対ＢＥＲ２の傾斜部との位相方向のシフト量Δφを求め、Δφ／２πに前記２光束干渉計のＦＳＲをかけることによりＰＤＦを求める手段を含む
請求項４記載のＰＤＦ測定装置。 In place of the wavelength changing means for changing the center optical frequency F c of the phase-modulated signal light, the phase difference φ is changed by changing the geometric optical path difference L between the two optical paths while keeping the F c constant. Having phase difference changing means,
In place of the data analysis means for analyzing the relationship between the F c and the BER, the data analysis means for analyzing the relationship between the L and the BER with the F c constant.
This data analysis means is means for obtaining a PDF by obtaining a shift amount Δφ in the phase direction between the inclined portion of φ vs. BER1 and the inclined portion of φ vs. BER2, and multiplying Δφ / 2π by the FSR of the two-beam interferometer. The PDF measuring apparatus according to claim 4. 前記位相変調信号光はＤＭＰＳＫである請求項４記載のＰＤＦ測定装置。   The PDF measuring apparatus according to claim 4, wherein the phase-modulated signal light is DMPSK.
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