JP2013152118A - Measurement method and device of laser beam characteristics - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for more accurately measuring the degree of coherence of laser beams.SOLUTION: A linearly frequency-swept laser beam is split into 2 systems. One of the split beams is injected into an optical fiber 5 with an optical circulator 4. Rayleigh scattered light which is generated and reflected at the optical fiber 5 is collected so that the Rayleigh scattered light and the other split beam are multiplexed (3-2). A light receiving element 6 detects a photocurrent from the multiplexed light. The current value of the detected photocurrent is Fourier transformed, so that the frequency spectrum thereof is analyzed (7-11). In the analysis, the full scope of the absolute values of degree of coherence of the laser beam is uncovered as a function of delay time τ. The characteristics of a laser beam are thus measured for more details.

Description

  The present invention relates to a laser beam characteristic measuring method and a measuring apparatus for measuring a coherence degree of a laser beam and a laser center frequency change.

  As a conventional laser beam characteristic measuring method, a laser beam spectrum measuring method described in Non-Patent Document 1 is known. In this method, a laser to be measured is branched into two, and one of them is transmitted to a sufficiently long optical fiber, and the other is passed through an acoustooptic device or the like to shift the frequency by a certain amount. Thereafter, both are combined, received by a light receiving element, and the beat spectrum width of the received light signal is measured.

  It is known that 1 / √2 of the beat spectrum width is an oscillation spectrum line width of the laser beam (hereinafter referred to as a laser beam spectrum line width), and the laser beam spectrum line width is measured from the measured beat spectrum width. be able to. If this laser beam spectral line width is Δν, the coherence time of the laser beam is 1 / Δν. Also, the coherence length can be obtained by multiplying the coherence time by the light velocity constant.

  However, in the above method, it is necessary to pass one of the branched laser beams through a sufficiently long optical fiber to give a sufficient time delay and then combine the other with the other, which is longer than the coherence length of the laser beam to be measured. A sufficiently long delay is required. For this reason, it is a precondition that a delay longer than the coherence length of the laser beam, which is unknown at the time of measurement, is obtained. Therefore, although the measurement result enables a constant estimation of the spectral line width of the laser beam, the measurement result has to be uncertain. This concern is particularly noticeable when laser light with a narrow spectral line width (long coherence time) is measured. In recent years, lasers having a spectral line width of about several kilohertz, such as fiber lasers, have been commercialized. Then, the ambiguity of the measurement is a problem.

T. Okoshi, K. Kikuchi, and A. Nakayama, “Novel method for high resolution measurement of laser output spectrum,” Electronics Letters, Vol. 16, No. 16, pp. 630-631, 1980 Y. Koshikiya, X. Fan, and F. Ito, “Long range and cm-level spatial resolution measurement using coherent optical frequency domain reflectmetry with SSB-SC modulator and narrow linewidth fiber laser”, IEEE / OSA J. Lightwave Technol. Vol 26, No. 18, pp. 3287-3294, 2008

As described above, the conventional laser light characteristic measurement technique is premised on that a delay longer than the coherence length of the laser light is obtained, so that it is possible to estimate the laser light spectral width. Uncertainty is included in the measurement results.
The present invention has been made in view of the above circumstances, and a laser capable of more accurately measuring the degree of coherence, the coherence time, and the laser center frequency of a laser beam having a narrow spectral line width and a long coherence time. An object of the present invention is to provide an optical property measurement method and a measurement apparatus.

The laser light characteristic measuring method according to the present invention has the following configuration.
(1) A laser light characteristic measuring method for measuring a coherence degree γ (τ) of a laser beam to be measured that is a function of a delay time τ, wherein the frequency of the laser beam to be measured is linearly swept and the frequency sweep is performed. The laser beam is branched into two systems, one of the branched laser beams is incident on an optical fiber, the Rayleigh scattered light generated in the optical fiber is captured, and the Rayleigh scattered light and the other branched laser beam are And the photocurrent generated by the multiplexing is detected, the current value of the detected photocurrent is Fourier transformed to calculate its frequency spectrum I _q (τ _i ), and similarly, the photocurrent is continuously detected. And the phase conjugate I _s ^* (τ _k ) of the frequency spectrum

が成り立つことを利用して、|γ(τ)|²exp{j[ω_q - ω_s]τ_i}の絶対値の平方根を算出することにより前記コヒーレンス度の絶対値|γ(τ)|を求める態様とする。
（２）（１）において、前記被測定レーザ光のコヒーレンス度の絶対値が1/eになるτをもってコヒーレンス時間を求める態様とする。

The absolute value of the coherence degree | γ (τ) | is calculated by calculating the square root of the absolute value of | γ (τ) | ² exp {j [ω _q −ω _s ] τ _i }. It is set as the aspect which calculates | requires.
(2) In (1), the coherence time is obtained with τ at which the absolute value of the degree of coherence of the laser beam to be measured is 1 / e.

(3) A laser beam characteristic measuring method for measuring a laser center frequency change of a laser beam to be measured that is a function of a delay time τ, wherein the frequency of the laser beam to be measured is swept linearly, and the frequency-swept laser beam Is split into two systems, one of the branched laser beams is incident on an optical fiber, Rayleigh scattered light generated by the optical fiber is taken in, and the Rayleigh scattered light and the other branched laser beam are combined. The photocurrent generated by the multiplexing is detected, the current value of the detected photocurrent is Fourier transformed to calculate the frequency spectrum I _q (τ _i ), and the photocurrent is detected continuously in the same manner. And the phase conjugate I _s ^* (τ _k ) of the frequency spectrum

が成り立つことを利用して、|γ(τ)|²exp{j[ω_q - ω_s]τ_i}の偏角が、遅延時間τにおける２回の測定の間に変化したレーザ中心周波数差に比例することを利用してレーザ中心周波数変化を求める態様とする。
本発明に係るレーザ光特性測定装置は、以下のような態様の構成とする。

The deviation of | γ (τ) | ² exp {j [ω _q −ω _s ] τ _i } is changed between the two measurements at the delay time τ. It is set as the aspect which calculates | requires a laser center frequency change using that it is proportional to.
The laser light characteristic measuring apparatus according to the present invention has the following configuration.

(4) A laser beam characteristic measuring apparatus for measuring a characteristic coherence degree γ (τ) of the laser beam to be measured which is a function of the delay time τ, the sweeping means for linearly sweeping the frequency of the laser beam to be measured, A branching unit for branching the frequency-swept laser light into two systems; a capturing unit for entering one of the branched laser beams into the optical fiber and capturing the Rayleigh scattered light generated in the optical fiber; and the Rayleigh scattered light; A multiplexing means for multiplexing the other branched laser beam, a detecting means for detecting a photocurrent generated by the multiplexing, and a frequency spectrum I _q ( τ _i ), and similarly, a calculating means for continuously detecting the photocurrent and calculating the phase conjugate I _s ^* (τ _k ) of the frequency spectrum, and

が成り立つことを利用して、|γ(τ)|²exp{j[ω_q - ω_s]τ_i}の絶対値の平方根を算出することにより前記コヒーレンス度の絶対値|γ(τ)|を求める解析手段とを具備する態様とする。
（５）（４）の構成において、前記解析手段は、前記被測定レーザ光のコヒーレンス度の絶対値が1/eになるτをもってコヒーレンス時間を求める態様とする。

The absolute value of the coherence degree | γ (τ) | is calculated by calculating the square root of the absolute value of | γ (τ) | ² exp {j [ω _q −ω _s ] τ _i }. And an analysis means for obtaining.
(5) In the configuration of (4), the analyzing means obtains the coherence time with τ at which the absolute value of the coherence degree of the laser beam to be measured is 1 / e.

(6) A laser light characteristic measuring apparatus for measuring a change in the laser center frequency of the laser beam to be measured, which is a function of the delay time τ, and sweeping means for linearly sweeping the frequency of the laser beam to be measured; Branching means for branching the laser light into two systems, taking-in means for entering one of the branched laser lights into the optical fiber and capturing Rayleigh scattered light generated in the optical fiber, and branching the Rayleigh scattered light A combining means for combining the other laser light, a detecting means for detecting a photocurrent generated by the combining, a Fourier transform of the current value of the detected photocurrent, and its frequency spectrum I _q (τ _i ), and similarly calculating means for continuously detecting the photocurrent and calculating the phase conjugate I _s ^* (τ _k ) of its frequency spectrum, and

が成り立つことを利用して、|γ(τ)|²exp{j[ω_q - ω_s]τ_i}の偏角が、遅延時間τにおける２回の測定の間に変化したレーザ中心周波数差に比例することを利用してレーザ中心周波数変化を求める解析手段とを具備する態様とする。

The deviation of | γ (τ) | ² exp {j [ω _q −ω _s ] τ _i } is changed between the two measurements at the delay time τ. And an analyzing means for obtaining a change in the laser center frequency by utilizing the proportionality to.

  As described above, the present invention is characterized by removing the ambiguity of the measurement conditions of the conventional method and improving the measurement reliability by performing statistical processing on the measurement values. Specifically, by measuring the degree of coherence in the vicinity of the delay time τ and analyzing the degree of coherence of the photocurrent in the vicinity of the delay 0, for example, when decreasing to | γ (τ) | ≈1 / e The coherent time is obtained from τ.

According to the present invention, it is possible to obtain the degree of coherence with high accuracy even when τ is smaller than the round-trip time of the full length of the prepared optical fiber.
Therefore, according to the present invention, a laser beam characteristic measuring method and a measuring apparatus capable of more accurately measuring the degree of coherence, the coherence time, and the laser center frequency of a laser beam having a narrow spectral line width and a long coherence time. Can be provided.

It is a block diagram which shows the structure of the measuring apparatus to which the laser beam characteristic measuring method which concerns on this embodiment is applied. It is a figure showing the waveform of the frequency modulation provided by the frequency sweep apparatus of the measuring apparatus shown in FIG.

Embodiments of the present invention will be described with reference to the accompanying drawings. The embodiment described below is an example of the configuration of the present invention, and the present invention is not limited to the following embodiment.
FIG. 1 is a block diagram showing the configuration of the laser light characteristic measuring apparatus of this embodiment. In FIG. 1, reference numeral 1 denotes a laser light source to be measured, and the laser light to be measured generated here is subjected to a frequency sweep of a specified width within a predetermined period as shown in FIG. The light is branched into two systems by the optical branching device 3-1. The branched transmission light of one system is guided to the optical fiber 5 by the optical circulator 4, and the transmission light of the other system is guided to the optical multiplexer 3-2. Subsequently, the reflected light that has undergone Rayleigh scattering in the optical fiber 5 is guided to the optical multiplexer 3-2 through the optical circulator 4, and is combined with the other transmitted light, and then received by the balanced light receiving element 6. Is done.

  The light reception signal obtained by the balanced light receiving element 6 is converted into data at a predetermined sampling period by the data acquisition device 7, and then the beat spectrum width is analyzed by the frequency spectrum analysis device 8 and sequentially stored in the memory device 9. . The laser beam characteristic analyzer 10 reads beat spectrum width data stored in the memory device 9 at a predetermined period, analyzes the coherence degree of the laser beam to be measured, and obtains the coherence length of the laser beam from the analysis result. Both the sweep period of the frequency sweep device 2 and the data acquisition period of the data acquisition device 7 are determined according to instructions issued from the control device 11.

In the above configuration, the measurement method and procedure will be described below.
First, the electric field amplitude E ′ (t) of the laser beam of time length T emitted from the laser light source 1 to be measured is expressed as follows.

ここで、式（１）のE_i(t)は

Where E _i (t) in equation (1) is

と表され、Ａは一定値を持つ振幅、ω_iは中心周波数、ｉは事象（アンサンブル）の番号を表す。すなわち、式（１）はレーザ光から取り出される電界振幅のｉ番目のアンサンブルを意味している。また、θ_i(t)は、レーザの位相雑音を表す確率変数である。

A is an amplitude having a constant value, ω _i is a center frequency, and i is an event (ensemble) number. That is, Equation (1) means the i-th ensemble of the electric field amplitude extracted from the laser light. Θ _i (t) is a random variable representing the phase noise of the laser.

  The coherence function γ (τ) of the laser beam measured in this embodiment is

で与えられる量である。かっこ記号の＜ ＞はアンサンブル平均を意味し、多くのアンサンブルｉに対する平均を表す。尚、＊は位相共役を意味する。
ここで、式（２）で表される量が持つ意味を考察しておく。もともとのレーザ光の電界は式（１）で表わされるが、これは中心周波数ω_iを中心として位相揺らぎθ_i(t)を持っている。しかるに、式（２）においては、中心周波数ω_iは現われておらず、位相揺らぎθ_i(t)のみに関係する量であることがわかる。よって本実施形態で測量される値は、レーザ光が一定時間Ｔだけ発振した時の位相もしくは周波数の揺らぎ幅を意味しており、これはレーザ光のスペクトルの広がりを評価する際には極めて有用な尺度になると考えられる。

Is the amount given by. The parenthesis <> means an ensemble average, which represents the average for many ensembles i. Note that * means phase conjugation.
Here, the meaning of the quantity represented by the formula (2) will be considered. The original electric field of the laser beam is expressed by equation (1), which has a phase fluctuation θ _i (t) with the center frequency ω _i as the center. However, in the equation (2), it can be seen that the center frequency ω _i does not appear and is an amount related only to the phase fluctuation θ _i (t). Therefore, the value measured in this embodiment means the fluctuation width of the phase or frequency when the laser beam oscillates for a certain time T, which is extremely useful for evaluating the spread of the spectrum of the laser beam. It is thought that it becomes a proper scale.

The frequency of the light emitted from the laser light source 1 to be measured is swept linearly for T seconds with respect to time by the frequency sweep device 2 as shown in FIG. The frequency sweep width is represented by ΔF. The frequency sweep device 2 can be realized by using, for example, a single sideband modulator described in Non-Patent Document 2. That is, the electric field amplitude (ensemble) E ″ _i (t) of the light wave swept by the frequency sweep device 2 is expressed by the following equation (4).

ここで、ｇは周波数掃引速度（Hz/s）である。
周波数掃引されたレーザ光は、光分岐器３−１により２分岐され、一方は光サーキュレータ４を介して光ファイバに入射される。光ファイバ５内ではレイリー散乱と呼ばれる光散乱が生じ、その散乱光は光ファイバを逆方向に伝搬してサーキュレータ４に戻り、光合波器３−２に向かって進行する。分岐されたもう一方のレーザ光は、そのまま光合波器３−２に向かって進行する。光合波器３−２ではこれらのレーザ光が合波される。

Here, g is a frequency sweep speed (Hz / s).
The frequency-swept laser light is branched into two by an optical branching device 3-1, and one is incident on an optical fiber via an optical circulator 4. Light scattering called Rayleigh scattering occurs in the optical fiber 5, and the scattered light propagates in the opposite direction through the optical fiber, returns to the circulator 4, and travels toward the optical multiplexer 3-2. The other branched laser light travels toward the optical multiplexer 3-2 as it is. In the optical multiplexer 3-2, these laser beams are multiplexed.

This configuration is an application of the configuration of a reflection distribution measuring device called a coherent optical frequency domain reflectometer (hereinafter referred to as C-OFDR), which is also described in Non-Patent Document 2, If the round-trip time of light up to one reflection point z _m is τ _m , the ensemble E ″ _i (t) of the i-th optical electric field is scattered from this reflection point and the light generated in the balance light receiving element 6 The current I _{i, m} (t) is expressed by Equation (5).

ここで複素数r_ｍは反射点z_ｍのレイリー反射係数であり、ランダムな値をとる確率変数である。
式（２）より、

Here, the complex number r _m is the Rayleigh reflection coefficient of the reflection point z _m and is a random variable that takes a random value.
From equation (2)

ここで、複素数r_ｍは反射点z_ｍのレイリー反射係数であり、ランダムな値をとる確率変数である。実際の光電流は反射点z_ｍの近傍の多数の反射点からの散乱光の和によって生じるから、観測される光電流の大きさI_i,total(t)は、

Here, the complex number r _m is the Rayleigh reflection coefficient of the reflection point z _m and is a random variable that takes a random value. Since the actual photocurrent is generated by the sum of scattered light from a number of reflection points near the reflection point z _m , the magnitude of the observed photocurrent I _{i, total} (t) is

と表される。データ取得装置７は、式（７）で表される光電流I_i,total(t)を数値化して取得し、データとして格納する。
C-OFDRでは、光ファイバの距離zからの散乱光の強度は、式（７）で表現される光電流により算出される。C-OFDRの距離分解能は、遅延時間τ_ｍに換算して1/ΔFであり、典型値として例えばΔF＝10GHzと仮定すると、距離分解能に相当する遅延時間差は100psとなる。これは今考えているレーザのコヒーレンス時間（典型的には１μsまたはそれ以上）と比べて非常に小さいものである。C-OFDRが観測する距離z_ｍにおける散乱光強度は、式（７）において、この距離分解能に相当する遅延時間差1/ΔFにわたる範囲で、ｍについて和を取ったものである。

It is expressed. The data acquisition device 7 converts the photocurrent I _{i, total} (t) expressed by the equation (7) into a numerical value, acquires it, and stores it as data.
In C-OFDR, the intensity of scattered light from the distance z of the optical fiber is calculated from the photocurrent expressed by Equation (7). The distance resolution of C-OFDR is 1 / ΔF in terms of the delay time τ _m , and assuming that, for example, ΔF = 10 GHz as a typical value, the delay time difference corresponding to the distance resolution is 100 ps. This is very small compared to the coherence time of the laser we are currently considering (typically 1 μs or more). The scattered light intensity at the distance z _m observed by the C-OFDR is the sum of m in the range over the delay time difference 1 / ΔF corresponding to the distance resolution in the equation (7).

The frequency spectrum analyzer 8 calculates the power spectrum of the photocurrent I _i (τ _m ). That is,

とすると、

Then,

と表される。
タイミング制御装置１１により、上記の一連の測定を２回連続して行う。この連続測定により、上記のI_q(τ_i)およびI_s(τ_i)（但し、qとsの測定間隔はT_m以上開ける）を取得し、この値から、I_q(τ_i)およびI_s(τ_i)の相互相関として、式（１０）が求まる。

It is expressed.
The above-described series of measurements are continuously performed twice by the timing control device 11. By this continuous measurement, the above I _q (τ _i ) and I _s (τ _i ) (however, the measurement interval between q and s is opened by T _m or more), and from this value, I _q (τ _i ) and Equation (10) is obtained as the cross-correlation of I _s (τ _i ).

ここで、上記において求められた光電流の相互相関と、本実施形態で測定される光源のコヒーレンス度との関係を考察する。
まず式（８）より、I_i(τ_m)はE_i(t)E^* _i(t-τ_m)の時間平均であることがわかる。よって、それらのアンサンブル平均は、適当な比例係数を除きお互いに等しいから、式（１１）のように表される。

Here, the relationship between the cross-correlation of the photocurrent obtained in the above and the degree of coherence of the light source measured in the present embodiment will be considered.
First, it can be seen from equation (8) that I _i (τ _m ) is a time average of E _i (t) E ^* _i (t−τ _m ). Therefore, these ensemble averages are equal to each other except for an appropriate proportionality coefficient, and therefore are expressed as in Expression (11).

ここで、γ(τ_m)は求めるべきコヒーレンス度である。
Ｋは適当な比例係数であるが、以下の説明には影響しないため、以後Ｋ＝１とおくことにする。このことより、I_q(τ_i)およびI_s ^＊(τ_k)は、式（１２）のように表すことができる。

Here, γ (τ _m ) is the degree of coherence to be obtained.
K is an appropriate proportionality coefficient, but does not affect the following description, and hence K = 1. From this, I _q (τ _i ) and I _s ^* (τ _k ) can be expressed as in Expression (12).

ここで、n_i,q，n^* _k,sは、そのアンサンブル平均（ｉおよびｋについての平均）が0であるような、相関をもたない複素数である。すなわち、

Here, n _{i, q and} n ^* _{k, s} are complex numbers having no correlation such that the ensemble average (average for i and k) is zero. That is,

である。
一方、γ(τ_i),γ^*(τ_k)はqやsに依存しない、すなわち繰り返し測定の間で一定の値をとる複素数である。τ_mの範囲ではコヒーレント度はほとんど変化しないとすると、

It is.
On the other hand, γ (τ _i ) and γ ^* (τ _k ) are complex numbers that do not depend on q or s, that is, take a constant value between repeated measurements. If the coherence is almost unchanged in the range of τ _m ,

である。
式（１２）および式（１４）を式（１０）に代入し、Ａの値は何であっても以下の議論に影響しないので、Ａ＝１とおくと、式（１５）が求まる。

It is.
Substituting Equation (12) and Equation (14) into Equation (10), and any value of A does not affect the following discussion, so if A = 1, then Equation (15) is obtained.

ここで、ｎ_i,qのランダム性から<ｎ_i,q>＝0と想定し、ｎ_i,qとｎ^* _k,s (q ≠ s)およびr_iとr^* _k (i ≠ k)は統計的に互いに独立と仮定すると、クロネッカーのデルタ関数を用いて式（１６）、式（１７）が成り立つ。

_{Here, n i, <n i,} q> from the random nature of the _q = 0 and the _assumption, n i, _q and _{^{n * k, s (q ≠}} s) and r _i and r ^* _k (i ≠ _k) Are statistically independent of each other, equations (16) and (17) are established using the Kronecker delta function.

上記式を用いて、τ近傍におけるγ(τ)は同一とみなし、τ近傍の光電流の相互相関のアンサンブル平均を求めると、式（１８）となる。

Using the above equation, it is assumed that γ (τ) in the vicinity of τ is the same, and the ensemble average of the cross-correlation of photocurrents in the vicinity of τ is obtained as Equation (18).

よって、事象のサンプル数が大きければ大きいほど精度よくアンサンブル平均を求めることがわかるが、現実的には測定回数には限度があり、許容される測定時間等の制約から決定されるものである。
また、

Therefore, it can be seen that the larger the number of event samples, the more accurately the ensemble average is obtained. However, in practice, there is a limit to the number of times of measurement, and it is determined based on the allowable measurement time and the like.
Also,

と表わされるので、光電流の相互相関とコヒーレンス度には、式（１９）の関係があることがわかる。

Therefore, it can be seen that the cross-correlation between the photocurrents and the degree of coherence have the relationship of equation (19).

ここで、複素数の絶対値|γ(τ)|²は、遅延時間τごとにコヒーレンス度の２乗を示す。また、偏角[ω_q−ω_ｓ]τ_iは、測定ｑ回目とｓ回目の中心周波数の差と遅延時間の積を示している。
以上から、レーザ光特性解析装置１０は、遅延時間τの付近でのコヒーレンス度を計測し、遅延０の近傍の光電流のコヒーレンス度と解析することで、例えば|γ(τ)| ≒ 1/eに減少する時のτからコヒーレンス時間を求めることができる。

Here, the absolute value of complex numbers | γ (τ) | ² represents the square of the degree of coherence for each delay time τ. Further, the declination [ω _q −ω _s ] τ _i represents the product of the difference between the center frequency of the q-th measurement and the s-th center frequency and the delay time.
From the above, the laser beam characteristic analyzer 10 measures the degree of coherence in the vicinity of the delay time τ and analyzes it as the degree of coherence of the photocurrent in the vicinity of the delay 0, for example, | γ (τ) | The coherence time can be obtained from τ when decreasing to e.

Further, by analyzing the declination [ω _q −ω _s ] τ _i of the measurement interval T, it becomes possible to measure the center frequency change (drift) of the laser. This is because when the delay time at which ω _q −ω _s changes by 2π is 50 μs, the reciprocal of the delay time, that is, the change in the laser center frequency, and the center frequency of the laser may drift by 20 kHz / s. Recognize. From this, the absolute value γ (τ) of the degree of coherence and the laser center frequency change (drift) at an arbitrary delay time τ can be quantitatively determined using Equation (19).

As described above, the laser light characteristic measuring method according to the present embodiment has the following advantages over the conventional technology.
First, in the present embodiment, measurement can be performed with an optical fiber length comparable to the coherence time, and if the coherence time is much longer than that, it will be clarified by measurement. Therefore, it is possible to take measures such as replacing the optical fiber with a longer optical fiber and starting measurement again. Therefore, according to the present embodiment, it is possible to obtain a measurement result that is more reliable than the related art.

Secondly, according to the present embodiment, not only the coherence time is measured, but the entire absolute value of the coherence degree is grasped as a function of the delay time τ. This makes it possible to investigate the properties of the laser light in more detail.
Third, according to the present embodiment, the degree of coherence can be determined from two different photocurrents. Therefore, measurement results can be obtained in real time.

Fourth, according to the present embodiment, it is possible to evaluate the difference in laser center frequency that has changed between two measurements as a function of the delay time τ. This makes it possible to quantitatively measure the center frequency change (drift) of the laser.
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. Various inventions can be formed by appropriately combining a plurality of constituent elements disclosed in the embodiments. For example, some configurations may be deleted from all the components shown in the embodiment. Furthermore, constituent elements over different example embodiments may be combined as appropriate.

  DESCRIPTION OF SYMBOLS 1 ... Measuring laser light source, 2 ... Frequency sweep apparatus, 3-1 ... Optical branching device, 3-2 ... Optical multiplexer, 4 ... Optical circulator, 5 ... Optical fiber, 6 ... Balance type light receiving element, 7 ... Data acquisition Device: 8 ... Frequency spectrum analysis device, 9 ... Memory device, 10 ... Laser light characteristic analysis device, 11 ... Timing control device.

Claims (6)

A laser beam characteristic measuring method for measuring a coherence degree γ (τ) of a laser beam to be measured that is a function of a delay time τ,
The frequency of the laser beam to be measured is swept linearly,
Branching the frequency-swept laser light into two systems;
One of the branched laser beams is incident on an optical fiber, and Rayleigh scattered light generated in the optical fiber is captured.
Combining the Rayleigh scattered light and the other branched laser light;
Detecting the photocurrent generated by the multiplexing;
Fourier transform the current value of the detected photocurrent to calculate its frequency spectrum I _q (τ _i ),
Similarly, the detection of photocurrent and the phase conjugate I _s ^* (τ _k ) of its frequency spectrum are calculated continuously,
Next formula

Using the fact that
| γ (τ) | ² The absolute value of the coherence degree | γ (τ) | is obtained by calculating the square root of the absolute value of | γ (τ) | ² exp {j [ω _q −ω _s ] τ _i } Optical property measurement method.   2. The laser light characteristic measuring method according to claim 1, wherein the coherence time is obtained from τ where the absolute value of the degree of coherence of the laser light to be measured is 1 / e. A laser beam characteristic measuring method for measuring a laser center frequency change of a laser beam to be measured that is a function of a delay time τ,
The frequency of the laser beam to be measured is swept linearly,
Branching the frequency-swept laser light into two systems;
One of the branched laser beams is incident on an optical fiber, and Rayleigh scattered light generated in the optical fiber is captured.
Combining the Rayleigh scattered light and the other branched laser light;
Detecting the photocurrent generated by the multiplexing;
Fourier transform the current value of the detected photocurrent to calculate its frequency spectrum I _q (τ _i ),
Similarly, the detection of photocurrent and the phase conjugate I _s ^* (τ _k ) of its frequency spectrum are calculated continuously,
Next formula

Using the fact that
| γ (τ) | ² exploiting the fact that the angle of exp {j [ω _q -ω _s ] τ _i } is proportional to the laser center frequency difference changed between two measurements at the delay time τ. A method for measuring laser light characteristics, characterized in that a change in laser center frequency is obtained. A laser beam characteristic measuring device that measures a characteristic coherence degree γ (τ) of a laser beam to be measured that is a function of a delay time τ,
Sweeping means for linearly sweeping the frequency of the laser beam to be measured;
Branching means for branching the frequency-swept laser light into two systems;
Capturing means for entering one of the branched laser beams into an optical fiber and capturing Rayleigh scattered light generated in the optical fiber;
A multiplexing means for multiplexing the Rayleigh scattered light and the other branched laser beam;
Detecting means for detecting a photocurrent generated by the multiplexing;
The frequency value I _q (τ _i ) of the detected photocurrent is Fourier transformed to calculate the frequency spectrum I _q (τ _i ). Similarly, the photocurrent is detected and the phase conjugate I _s ^* (τ _k ) Calculating means,
Next formula

Using the fact that
| γ (τ) | ^{2 The} analysis means for obtaining the absolute value | γ (τ) | of the degree of coherence by calculating the square root of the absolute value of ² exp {j [ω _q −ω _s ] τ _i }. A laser light characteristic measuring apparatus characterized by the above.   5. The laser light characteristic measuring apparatus according to claim 4, wherein the analyzing means obtains a coherence time from τ where the absolute value of the degree of coherence of the laser light to be measured is 1 / e. A laser beam characteristic measuring device for measuring a laser center frequency change of a laser beam to be measured that is a function of a delay time τ,
Sweeping means for linearly sweeping the frequency of the laser beam to be measured;
Branching means for branching the frequency-swept laser light into two systems;
Capturing means for entering one of the branched laser beams into an optical fiber and capturing Rayleigh scattered light generated in the optical fiber;
A multiplexing means for multiplexing the Rayleigh scattered light and the other branched laser beam;
Detecting means for detecting a photocurrent generated by the multiplexing;
The frequency value I _q (τ _i ) of the detected photocurrent is Fourier transformed to calculate the frequency spectrum I _q (τ _i ). Similarly, the photocurrent is detected and the phase conjugate I _s ^* (τ _k ) Calculating means,
Next formula

Using the fact that
| γ (τ) | ² exploiting the fact that the angle of exp {j [ω _q -ω _s ] τ _i } is proportional to the laser center frequency difference changed between two measurements at the delay time τ. An apparatus for measuring laser light characteristics, comprising: an analyzing means for obtaining a change in laser center frequency.

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