JP2011226930A - Optical complex-amplitude waveform measurement device and measurement method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately measure optical complex amplitude of a signal light even if a phase fluctuation of a local oscillation light occurs.SOLUTION: An input signal light and a local oscillation light having a spectrum within a frequency band of the input signal light are divided by an optical divider 11, 16, respectively, into a first line and a second line. The signal light and the local oscillation light of the first line are mixed by a first 90° optical hybrid 12, and the signal light and the local oscillation light of the second line are mixed by a second 90° optical hybrid 14. Then, outputs of the first and the second 90° optical hybrids 12, 14 are photoelectrically converted to generate first and second complex amplitude signals, respectively. In this configuration, the signal light of the second line is delayed by an optical delay unit 13 for a predetermined period of time. In addition, optical path lengths of the first and the second local oscillation lights are adjusted to agree with each other. From this state, the first and the second complex amplitude signals are input into a waveform analysis unit 23 to measure a phase shift due to delay time difference therebetween.

Description

  The present invention relates to an optical complex amplitude waveform measuring instrument for measuring an optical complex amplitude waveform of an optical signal subjected to optical amplitude and phase modulation and a measuring method thereof in a technical field such as optical communication.

  In recent years, in large-capacity optical fiber communication systems, modulation schemes using phase modulation of light waves have become mainstream. When evaluating such an optical fiber communication system, it is an essential requirement to measure a modulation waveform including the amplitude and phase of signal light, that is, a waveform of an optical complex amplitude.

Here, the optical complex amplitude is the time waveform of the electric field of the optical signal obtained by modulating the laser beam having the center frequency ω ₀ as a (t) · e ^{jω0 t} = | a (t) | exp [jφ (t )] · E ^{jω0 t} means a (t) = | a (t) | exp [jφ (t)]. Here, | a (t) | represents optical wave amplitude modulation, and φ (t) represents phase modulation.

As a conventional optical complex amplitude waveform measuring device, it is known that an optical complex amplitude waveform can be measured by using a configuration used in a so-called coherent receiver as it is. 1.
In the measuring instrument described in this document, the signal light under measurement modulated in amplitude or phase is input to the optical 90-degree hybrid and mixed with continuous (unmodulated) local light, and the light from the optical 90-degree hybrid is mixed. The optical output is photoelectrically converted to obtain a current proportional to the real part and the imaginary part of the optical complex amplitude of the signal light under measurement, thereby measuring the optical complex amplitude waveform of the signal light under measurement.

However, in the measurement method described above, the phase component of the local light is included in the phase component of the waveform to be measured, so that the accurate optical complex amplitude of the signal light cannot be measured.
That is, as is well known, the physical quantity measured here is not exactly the optical complex amplitude of the signal light, but the optical complex amplitude of the signal light to be measured as a _S (t) = | a _S (t) When | exp [jφ _S (t)] and the light complex amplitude of local light is a _L (t) = | a _L | exp [jφ _L (t)],
| a _L || a _S (t) | exp [j {φ _S (t) −φ _L (t)}] (1)
It is known that However, | a _S (t) | and | a _L | are the amplitudes of the signal light and the local light, respectively, and φ _S (t) and φ _L (t) are the phases of the signal light and the local light, respectively. Since local light is not modulated, its amplitude does not depend on time, but its phase has phase noise due to the finite spectral line width of the light source and changes as a function of time t.

As is apparent from the equation (1), the phase component of the waveform measured by the conventional technique includes the local light emission phase φ _L (t). That is, what is measured by the conventional technique is not exactly the optical complex amplitude a _S (t) = | a _S (t) | exp [jφ _S (t)] of the signal light. It only measures the relative phase based on the emission phase φ _L (t).

In order to accurately measure the optical complex amplitude of signal light in such a situation, the phase fluctuation φ _L (t) of the local light is made extremely small by using a laser having a very small spectral line width as the local light source. However, it is extremely difficult to realize this with a normal laser, and it has been extremely difficult to accurately measure the optical complex amplitude of signal light using conventional techniques.

Dany-Sebastien Ly-Gagnon, Satoshi Tsukamoto, Kazuhiro Katoh, and Kazuro Kikuchi, “Coherent Detection of Optical Quadrature Phase-Shift Keying Signals With Carrier Phase Estimation,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 24, NO. 1, JANUARY 2006.

As described above, in the conventional optical complex amplitude waveform measuring instrument, the phase component of the local light is included in the phase component of the waveform to be measured, and therefore the optical complex amplitude of the signal light cannot be measured accurately. It was.
The present invention has been made in view of the above circumstances, and an optical complex amplitude waveform measuring instrument capable of accurately measuring the optical complex amplitude of signal light even when there is a phase fluctuation of local light emission and its measurement. It aims to provide a method.

In order to achieve the above object, the optical complex amplitude waveform measuring instrument according to the present invention has the following configuration.
(1) first branching means for branching signal light; local light generator for generating local light having a spectrum within the frequency band of the signal light; and second branching means for branching the local light; A first optical 90-degree hybrid that inputs and mixes one signal light branched by the first branching means and one local light branched by the second branching means; and the first light A second optical 90-degree hybrid that inputs and mixes the other signal light branched by the branching means and the other local light branched by the second branching means; and the first optical 90-degree hybrid A first signal generating means for photoelectrically converting the output of the second optical signal to generate a first complex amplitude signal; and a second signal for generating a second complex amplitude signal by photoelectrically converting the output of the second optical 90-degree hybrid. Signal generating means and one or the other branched by the first branching means Delaying means for delaying the signal light for a certain time, optical path length adjusting means for adjusting the optical path length of one or the other local light branched by the second branching means to make the optical path lengths coincide with each other, and An aspect comprising waveform analysis means for inputting the first and second complex amplitude signals generated by the first and second signal generation means and measuring a phase change due to a time difference by the delay means to analyze the waveform; To do.

(2) In the configuration of (1), the waveform analyzing means converts complex signals obtained by receiving the outputs of the first and second optical 90-degree hybrids to I ₁ , Q ₁ , I ₂ , Q _{2, respectively.} If the square of the amplitude of signal light | a _S (t) |
| a _S (t) | ² = I ₁ ² (t) + Q ₁ ² (t)
The phase change φ _S (t) -φ _S (t-τ _S ) in the signal light delay time τ _S
φ _S (t) −φ _S (t−τ _S ) = tan ⁻¹ {Q ₁ (t) / I ₁ (t)} − tan ⁻¹ {Q ₂ (t) / I ₂ (t)}
The mode obtained by

(3) In the configuration of (1), the waveform analyzing means integrates the phase change φ _S (t) −φ _S (t−τ _S ) in the delay time τ _S of the signal light, thereby The phase φ _S (t) is obtained.
Moreover, the optical complex amplitude waveform measuring method according to the present invention has the following configuration.

(4) The signal light is branched by the first branching means,
The local light generated by the local light generator and having a spectrum within the frequency band of the signal light is branched by the second branching means,
One signal light branched by the first branching means and one local light branched by the second branching means are inputted to the first light 90-degree hybrid and mixed,
The other signal light branched by the first branching means and the other local light branched by the second branching means are input to a second light 90-degree hybrid and mixed,
Photoelectrically converting the output of the first light 90-degree hybrid to generate a first complex amplitude signal;
Photoelectrically converting the output of the second light 90-degree hybrid to generate a second complex amplitude signal;
Delaying one or the other of the signal light branched by the first branching means for a fixed time;
Adjusting the optical path length of one or the other local light branched by the second branching means to make the optical path lengths coincide with each other;
The first and second complex amplitude signals are input, the phase change due to the delay time difference is measured, and the waveform is analyzed.

(5) In the configuration of (4), the waveform analysis is performed by converting complex signals obtained by receiving the outputs of the first and second optical 90-degree hybrids to I ₁ , Q ₁ , I ₂ , Q ₂ , respectively. The square of the signal light amplitude | a _S (t) |
| a _S (t) | ² = I ₁ ² (t) + Q ₁ ² (t)
The phase change φ _S (t) -φ _S (t-τ _S ) in the signal light delay time τ _S
φ _S (t) −φ _S (t−τ _S ) = tan ⁻¹ {Q ₁ (t) / I ₁ (t)} − tan ⁻¹ {Q ₂ (t) / I ₂ (t)}
The mode obtained by

(6) In the configuration of (4), the waveform analysis is performed by integrating the phase change φ _S (t) −φ _S (t−τ _S ) in the delay time τ _S of the signal light. This is a mode for obtaining φ _S (t).

  As described above, according to the present invention, there is provided an optical complex amplitude waveform measuring device and a measuring method thereof capable of accurately measuring the optical complex amplitude of signal light even when there is a phase fluctuation of local light. be able to.

The block block diagram which shows one Embodiment of the optical complex amplitude waveform measuring device which concerns on this invention. The wave form diagram which shows the spectrum example of the local light with respect to the spectrum of the signal light of embodiment shown in FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 is a block diagram showing an embodiment of an optical complex amplitude waveform measuring instrument according to the present invention. In the figure, signal light under measurement (hereinafter simply referred to as signal light) is split into first and second system signal lights S1 and S2 by an optical splitter 11, and the first system signal light S1 is the first system signal light S1. The second signal light S2 is sent to the second optical 90-degree hybrid 14 after being delayed by a certain period (τ _S ) by the optical delay device 13.

  On the other hand, the local light generator 15 generates local light having a spectrum within the frequency band of the signal light. The local light generated here is generated by the optical branching device 16 in the first and second systems. Branched into L 1 and L 2, the first system local light L 1 is sent to the first light 90-degree hybrid 12, and the second system local light L 2 is sent via the optical path length adjuster 17 to the second light 90 degrees. Sent to the hybrid 14.

  Here, the optical path length adjuster 17 includes an optical path length from the output end A of the optical branching device 16 to the local light emitting input end B of the first optical 90-degree hybrid 12 and a second output from the output end A of the optical branching device 16. Are adjusted so that the optical path lengths to the local light input terminal C of the 90-degree hybrid 14 are equal.

It is known that the first and second light 90-degree hybrids 12 and 14 have the same configuration, and are realized by, for example, a spatial optical system using a half mirror and a polarization separation element or an integrated optical circuit. Yes. The first optical 90-degree hybrid 12 mixes the first signal light S1 and the local light L1 to generate a complex IQ signal light, and its output is supplied by a pair of balanced light receiving elements 181 and 182 as a current. Converted to a signal. Similarly, the second optical 90-degree hybrid 14 generates a complex type IQ signal light by mixing the second system signal light S2 and the local light L2, and its output is a pair of balanced light receiving elements 191, 191. It is converted into a current signal by 192.
A pair of current signals obtained by the light receiving elements 181 and 182 are sent to the digitizers 201 and 202, and a pair of current signals obtained by the light receiving elements 191 and 192 are sent to the digitizers 211 and 212. These digitizers 201, 202, 211, and 212 sample and digitize the input current signal based on the sampling clock generated by the clock generator 22, and the output is sent to the waveform analyzer 23. The waveform analysis device 23 is a data processing device using, for example, a computer. The waveform analysis device 23 obtains output waveforms of the optical 90-degree hybrids 12 and 14 obtained by the digitizers 201, 202, 211, and 212. The phase change of the complex amplitude signal is measured.

Here, for the following explanation, the spectrum of the signal light is assumed as shown in FIG. 2 without losing generality. That is, the spectrum of the signal light has a spread corresponding to the band B on both sides with the optical frequency ω ₀ as the center. The local light is continuous light, and its frequency is located at the frequency ω _L within the band of the signal light. This configuration is called homodyne detection or intradyne detection in the coherent detection technique.

In the above configuration, a method for measuring an optical complex signal will be described below.
First, the optical complex amplitudes of signal light and local light are expressed as follows.
Signal light: a _S (t) = | a _S (t) | exp [jφ _S (t)] ・ e ^{jω0 t} (1)
Local light: a _L (t) = | a _L | exp [jφ _L (t)] ・ e ^{jωL t} (2)
In the signal light represented by the expression (1), both the amplitude | a _S (t) | and the phase φ _S (t) change depending on time. An object of the present invention is to measure the optical complex amplitude | a _S (t) | exp [jφ _S (t)] of signal light including the amplitude | a _S (t) | and the phase φ _S (t). It is in. On the other hand, the local light in equation (2) is continuous light, and its amplitude | a _L | does not depend on time. However, the phase φ _L (t) is a function that depends on time because it has temporal fluctuations due to the spectral line width that the laser beam inevitably has.

The operation of the present invention will be described with reference to FIG. The signal light Sin is branched into the first and second systems by the optical splitter 11, the first system signal light S1 is sent directly to the optical 90-degree hybrid 12, and the second system signal light S2 is optically delayed. A relative time delay is imparted to the other by the device 13 and sent to the second optical 90-degree hybrid 14. In the following description, the time delay of the signal light applied at this time is represented by τ _S.

The time delay amount τ _S to be inserted is determined as follows. As will be described later, in measuring the optical complex amplitude of the signal light, first, the phase change φ _S (t) −φ _S (t−τ _S ) generated during the time τ _S of the signal light is calculated. In order to uniquely calculate this phase change, it is clear that the phase change amount during the time τ _S should not exceed 2π at the maximum. For this purpose, it is necessary that 2π × 2Bτ _S <1 where B is a band represented by the angular frequency of the signal light shown in FIG. Therefore, τ _S needs to be set so as to satisfy the following condition.
τ _S <1/2 (2πB) (2-1)
In this way, τ _S needs to be set smaller than 1/2 (2πB), but if τ _S is set too small, there is a concern that the phase change within that time will be extremely small and the measurement error will increase. is there. Therefore, tau in determining the set value of _S is in the range of requests of the formula (2-1), be employed large tau _S as possible desirable.

  Next, the local light supplied from the local light generator 15 is branched into the first and second systems by the optical branching device 16 and sent to the optical 90-degree hybrids 12 and 14 via the optical path length adjuster 17, respectively. It is done. The optical path length adjuster 17 is inserted in order to adjust the optical path lengths of the two branch station lights L1 and L2 so as to coincide with each other. That is, a path (AB in the figure) from the output end A of the optical branching device 16 inserted on the local light emission path to the input end B of the first optical 90-degree hybrid 11, and the second light 90. The route (AC in the figure) to the input terminal C of the hybrid 14 coincides. This local light emission path length adjustment operation is the main point of the present invention, and its meaning will be clarified in the following description.

With the above-described configuration, the complex light amplitude of the signal light and the local light input to the optical 90-degree hybrids 12 and 14 is expressed as follows.
Signal light: a _S ⁽¹⁾ (t) ＝ | a _S ⁽¹⁾ (t) | exp [jφ _S (t)] ・ e ^{jω0 t} (3)
Local light: a _L ⁽¹⁾ (t) ＝ | a _L | exp [jφ _L (t)] ・ e ^{jω0 t} (4)
The signal light _S 2 input to the second optical 90-degree hybrid 14 is delayed in arrival by the delay τ _S given by the optical delay device 13, whereas the local light input to the second optical 90-degree hybrid 14 is transmitted. L2 becomes exactly the same as the local light input to the first light 90-degree hybrid 12 by the optical path length adjustment operation by the optical path length adjuster 17. Therefore, the optical complex amplitude of the signal light and the local light input to the second optical 90-degree hybrid 14 is expressed as follows.
Signal light: a _S ⁽²⁾ (t) = | a _S (t−τ _S ) | exp [jφ _S (t−τ _S )] ・ e ^{jω0 t} (5)
Local light: a _L ⁽²⁾ (t) ＝ | a _L | exp [jφ _L (t)] ・ e ^{jω0 t} (6)
Light waves output from the light 90-degree hybrids 12 and 14 are converted into currents by balanced light receiving elements 181, 182, 191 and 192. If the current values observed from the light 90 degree hybrid 12 are I ₁ and Q ₁ , and the current values observed from the light 90 degree hybrid 14 are I ₂ and Q ₂ , these current values are respectively 90 degrees light. Using the expressions (3), (4), (5), and (6) of the optical complex amplitudes input to the hybrids 12 and 14, they can be expressed as follows.

I ₁ (t) = Re [a _S ⁽¹⁾ (t) a _L ^{(1) *} (t)]
＝ Re {| a _L || a _S (t) | ・ exp [j {φ _S (t) −φ _L (t)}]}
＝ | a _L || a _S (t) | ・ cos {φ _S (t) −φ _L (t)}
(7)
Q ₁ (t) = Im [a _S ⁽¹⁾ (t) a _L ^{(1) *} (t)]
＝ Im {| a _L || a _S (t) | ・ exp [j {φ _S (t) −φ _L (t)}]}
＝ | a _L || a _S (t) | ・ sin {φ _S (t) −φ _L (t)}
(8)
I ₂ (t) = Re [a _S ⁽¹⁾ (t) a _L ^{(1) *} (t)]
= Re {| a _L || a _S (t−τ _S ) | ・ exp [j {φ _S (t−τ _S ) −φ _L (t)}]}
＝ | a _L || a _S (t−τ _S ) | ・ cos {φ _S (t−τ _S ) −φ _L (t)}
(9)
Q ₂ (t) = Im [a _S ⁽¹⁾ (t) a _L ^{(1) *} (t)]
＝ Im {| a _L || a _S (t−τ _S ) | ・ exp [j {φ _S (t−τ _S ) −φ _L (t)}]}
＝ | a _L || a _S (t−τ _S ) | ・ sin {φ _S (t−τ _S ) −φ _L (t)}
(Ten)
The above current values are digitized by the digitizers 201, 202, 211, and 212, respectively, and are input to the waveform analyzer 23. The digitizers 201, 202, 211, and 212 can be realized by, for example, an A / D converter that converts a current value into a digital signal. As shown in FIG. 1, a clock signal is supplied from the clock generator 22 to the digitizers 201, 202, 211, and 212 at a constant cycle, and a digitization operation is performed at a constant cycle using this as a trigger. Shall. This period T needs to satisfy the following relationship from the request of the sampling theorem.
T <1 / 2πB (11)
The digitizers 201, 202, 211, and 212 digitize the current values I ₁ , Q ₁ , I ₂ , and Q ₂ every time T and send them to the waveform analyzer 23. The waveform analyzer 23 uses the received current values I ₁ , Q ₁ , I ₂ , and Q ₂ to calculate the optical complex amplitude of the signal light at the time. By repeating this every time T, the optical complex amplitude at all times can be calculated, and an optical complex amplitude waveform can be constructed.

Using the digitized current values I ₁ , Q ₁ , I ₂ , and Q ₂ obtained above, the waveform analyzer 23 calculates the optical complex amplitude of the signal light as follows.
First, to find the amplitude of signal light | a _S (t) |
I ₁ ² (t) + Q ₁ ² (t)
＝ [| a _L || a _S (t) | ・ cos {φ _S (t) −φ _L (t)}] ² + [| a _L || a _S (t) | ・ sin {φ _S (t ) −φ _L (t)}] ²
= | A _L | ² | a _S (t) | ²
Since | a _L | ² is a constant, if it is ignored, the amplitude | a _S (t) | of the signal light can be obtained as follows.
| a _S (t) | ² = I ₁ ² (t) + Q ₁ ² (t) (12)
Also, by dividing both sides of equation (8) by equation (7) and dividing both sides of equation (10) by equation (9),

を得る。これより、

Get. Than this,

を得る。
式(15)，(16)の差を取ることにより、

Get.
By taking the difference between equations (15) and (16),

を得る。式(17)によって、時間差τ_Sでの信号光の位相変化を求められることがわかる。
ここで注目すべきは、式(15)，(16)から式(17)を導くときに、局発光の位相揺らぎφ_L(t)を完全に消去できることである。これは、図１の構成において光路長調整器１７を用いて、分岐後の２つの局発光の光路長を同一とし、光90度ハイブリッド１２，１４に入力される局発光の位相を全く同一としたことによる効果に他ならない。このように、局発光の位相が如何に揺らごうとも、その影響を全く受けることなく信号光の時間τ_Sにおける位相変化を抽出できることは、本発明で初めて開示される最大の利点であり、従来のいかなる測定法を用いても実現不可能であった効用である。

Get. It can be seen from Equation (17) that the phase change of the signal light at the time difference τ _S can be obtained.
It should be noted here that the phase fluctuation φ _L (t) of the local light can be completely eliminated when the equation (17) is derived from the equations (15) and (16). This is because, using the optical path length adjuster 17 in the configuration of FIG. 1, the optical path lengths of the two local lights after branching are made the same, and the phases of the local lights inputted to the optical 90-degree hybrids 12 and 14 are exactly the same. It is nothing but the effect of doing. As described above, it is the greatest advantage disclosed in the present invention for the first time that the phase change at the time τ _S of the signal light can be extracted without being affected at all by the influence of the local light phase. This is a utility that could not be realized using any measurement method.

Dividing both sides of equation (17) by τ _S

を得るが、式(18)の左辺はφ_S(t)微分に等しいので、式(18)を積分することによって以下のようにφ_S(t)を求めることができる。

However, since the left side of the equation (18) is equal to the φ _S (t) derivative, the equation (18) can be integrated to obtain φ _S (t) as follows.

このようにして、式(12)及び式(19)から、信号光の振幅と位相、即ち光複素振幅を求めることができる。
説明を終えるにあたり、光路長調整器１７によって同一に調整される局発光の光路長に許容される誤差について考察しておく。上記に説明したように、本来、図１の経路Ａ−ＢとＡ−Ｃは完全に同一になるように調整されるべきであり、この点が本発明を構成する重要な要素である。仮に、Ａ−ＣがＡ−Ｂに対して時間τ_Lだけ長く設定されたと仮定する。この場合には、信号光の振幅が式(12)により与えられる点はいささかも変わらないが、信号光の位相を求めるために用いる式(13)，(14)は、局発光の光路長時間差τ_Lの影響により次のように変形される。

In this way, the amplitude and phase of the signal light, that is, the optical complex amplitude can be obtained from the equations (12) and (19).
When the description is finished, an error allowed for the optical path length of the local light that is adjusted in the same way by the optical path length adjuster 17 will be considered. As described above, the paths A-B and A-C in FIG. 1 should be adjusted to be completely the same, and this is an important element constituting the present invention. Suppose that AC is set longer than AB by time τ _L. In this case, the point where the amplitude of the signal light is given by Equation (12) does not change a little, but Equations (13) and (14) used to obtain the phase of the signal light are different from the long-time difference in the optical path of the local light. It is deformed as follows due to the influence of τ _L.

これに式(17)を得たときと同様の演算を付すと以下の式を得る。

If the same calculation as that for obtaining the equation (17) is added to this, the following equation is obtained.

式(22)から分かるように、局発光の光路長時間差τLが存在する場合にはもはやφ_S(t)−φ_S(t−τ_S)は電流値I₁、Q₁、I₂、Q₂だけの関数とはならず、局発光の時間τ_Lの間の位相揺らぎφ_L(t−τ_L)−φ_L(t)が誤差として含まれることになる。

As can be seen from equation (22), when there is a local light path long-time difference τL, φ _S (t) −φ _S (t−τ _S ) is no longer the current values I ₁ , Q ₁ , I ₂ , Q It does not become a function of only _two, stations phase fluctuation between the emission time _{τ L φ L (t-τ} L) -φ L (t) will be included as an error.

According to the coherence theory, when the coherence time of local light is τc, the statistical dispersion of phase fluctuation φ _L (t−τ _L ) −φ _L (t) during time τ _L

は、以下のようになることが知られている。

Is known to be as follows.

または、局発光のスペクトル線幅（角周波数の広がり）をΔω（Hz）として、

Or, the spectral line width of the local light (the spread of the angular frequency) is Δω (Hz),

であることが知られている。従って、測定誤差の指標としてその標準偏差を考えることにすれば、局発光の位相揺らぎによってもたらされる信号光の位相の測定誤差は、

It is known that Therefore, if the standard deviation is considered as an index of the measurement error, the measurement error of the phase of the signal light caused by the phase fluctuation of the local light is

となる。
局発光発生器１５として半導体レーザを用いると仮定すると、その典型的なスペクトル線幅（角周波数の広がり）は2π×1MHz程度である。また、許容される位相の測定誤差を0.01radと仮定すれば、式(25)より、許容される時間差τ_Lは、約3.2×10^-11s、即ち約32psとなり、光速度を3×10⁸m/sを乗算することによりこれを自由区間の距離に換算すると、約1cmとなる。したがって、一例として上記の条件での測定を実現するために光路長調整器１７が備えるべき精度は約1cmであり、通常入手可能な機械的な遅延調整器によって十分に実現可能な精度であることが分かる。更に精度のよい考慮長の調整を行えば、局発光の位相揺らぎによる測定誤差はいくらでも小さくすることが可能であり、極めて小さな測定誤差で信号光の光複素振幅を求めることができる。

It becomes.
Assuming that a semiconductor laser is used as the local light generator 15, the typical spectral line width (angular frequency spread) is about 2π × 1 MHz. Assuming that the measurement error of the allowable phase is 0.01 rad, the allowable time difference τ _L is about 3.2 × 10 ⁻¹¹ s, that is, about 32 ps, from Equation (25), and the speed of light is 3 × 10 ^When this is converted into the distance of the free section by multiplying by ⁸ m / s, it is about 1 cm. Therefore, as an example, the accuracy that the optical path length adjuster 17 should have in order to realize the measurement under the above-mentioned conditions is about 1 cm, and the accuracy can be sufficiently realized by a normally available mechanical delay adjuster. I understand. Further, if the consideration length is adjusted with high accuracy, the measurement error due to the phase fluctuation of the local light can be reduced as much as possible, and the optical complex amplitude of the signal light can be obtained with an extremely small measurement error.

As described above, according to the present invention, a means for measuring the optical complex amplitude of the signal light with high accuracy is provided, and the optical complex amplitude of the signal light is extremely reduced while avoiding the measurement error caused by the phase fluctuation of the local light. It becomes possible to measure with high accuracy.
Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the constituent elements without departing from the scope of the invention in the implementation stage. Moreover, various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiment. In addition, some components may be deleted from all the components shown in the embodiment. Furthermore, you may combine the component covering different embodiment suitably.

  DESCRIPTION OF SYMBOLS 11 ... Optical branching device, 12 ... 1st optical 90 degree hybrid, 13 ... Optical delay device, 14 ... 2nd optical 90 degree hybrid, 15 ... Local light generator, 16 ... Optical branching device, 17 ... Optical path length adjustment , 181, 182, 191, 192... Balanced light receiving element, 201, 202, 211, 212, digitizer, 22, clock generator, 23, waveform analyzer.

Claims (6)

First branching means for branching the signal light;
A local light generator for generating local light having a spectrum in the frequency band of the signal light;
Second branching means for branching the local light;
A first optical 90-degree hybrid that inputs and mix-outputs one signal light branched by the first branching means and one local light branched by the second branching means;
A second optical 90-degree hybrid that inputs and mixes the other signal light branched by the first branching means and the other local light branched by the second branching means;
First signal generation means for photoelectrically converting the output of the first light 90-degree hybrid to generate a first complex amplitude signal;
Second signal generating means for photoelectrically converting the output of the second light 90-degree hybrid to generate a second complex amplitude signal;
Delay means for delaying one or the other of the signal light branched by the first branch means for a fixed time;
An optical path length adjusting means for adjusting the optical path length of one or the other local light branched by the second branching means so as to make the optical path lengths of the two coincide with each other;
Waveform analysis means for inputting the first and second complex amplitude signals generated by the first and second signal generation means, measuring a phase change due to a time difference by the delay means, and analyzing the waveform;
An optical complex amplitude waveform measuring instrument comprising: When the complex signals obtained by receiving the outputs of the first and second optical 90-degree hybrids are denoted by I ₁ , Q ₁ , I ₂ , and Q ₂ , respectively, the waveform analysis means has an amplitude of signal light | a _s (t) | squared
| a _S (t) | ² = I ₁ ² (t) + Q ₁ ² (t)
The phase change φ _S (t) -φ _S (t-τ _S ) in the delay time τ _S of the signal light is obtained as φ _S (t) −φ _S (t−τ _S ) = tan ⁻¹ {Q ₁ ( t) / I ₁ (t)} − tan ⁻¹ {Q ₂ (t) / I ₂ (t)}
2. The optical complex amplitude waveform measuring device according to claim 1, wherein Said waveform analyzing means, by integrating the phase change phi _s in the delay time tau _s of the signal light _{(t) -φ s (t-} τ s), an optical signal phase phi _s to seek (t) The optical complex amplitude waveform measuring instrument according to claim 1, wherein: Branching the signal light by the first branching means;
The local light generated by the local light generator and having a spectrum within the frequency band of the signal light is branched by the second branching means,
One signal light branched by the first branching means and one local light branched by the second branching means are inputted to the first light 90-degree hybrid and mixed,
The other signal light branched by the first branching means and the other local light branched by the second branching means are input to a second light 90-degree hybrid and mixed,
Photoelectrically converting the output of the first light 90-degree hybrid to generate a first complex amplitude signal;
Photoelectrically converting the output of the second light 90-degree hybrid to generate a second complex amplitude signal;
Delaying one or the other of the signal light branched by the first branching means for a fixed time;
Adjusting the optical path length of one or the other local light branched by the second branching means to make the optical path lengths coincide with each other;
An optical complex amplitude waveform measuring method, comprising: inputting the first and second complex amplitude signals, measuring a phase change due to the delay time difference, and analyzing the waveform. In the waveform analysis, when the complex signals obtained by receiving the outputs of the first and second optical 90-degree hybrids are I ₁ , Q ₁ , I ₂ , and Q ₂ , respectively, the amplitude of the signal light | a _S the square of (t) |
| a _S (t) | ² = I ₁ ² (t) + Q ₁ ² (t)
The phase change φ _S (t) -φ _S (t-τ _S ) in the delay time τ _S of the signal light is obtained as φ _S (t) −φ _S (t−τ _S ) = tan ⁻¹ {Q ₁ ( t) / I ₁ (t)} − tan ⁻¹ {Q ₂ (t) / I ₂ (t)}
5. The optical complex amplitude waveform measuring method according to claim 4, wherein: The waveform analysis is performed by integrating the phase change φ _S (t) −φ _S (t−τ _S ) in the delay time τ _S of the signal light to integrate the phase φ _S (t) of the optical signal. The optical complex amplitude waveform measuring method according to claim 4, wherein:

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