JP2012112657A - Laser beam measuring method and measurement device thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately measure a coherence time of laser beam having a narrow spectral line width (long coherence time).SOLUTION: When a coherence function γ(τ) of the laser beam to be measured, which is the function of a delay time τ, is measured, an optical frequency of the laser beam (11) to be measured is linearly swept (12), the swept laser beam is branched (131), one side of the branched laser beam is incident to a fiber optic (15) by a circulator (14) to generate a Rayleigh scattered light, the Rayleigh scattered light generated in the fiber optic (15) and branched laser beam of another side are combined (132), the combined beams are received to detect a light current (16), the detected light current is digitized and subjected to Fourier transformation (17, 18), a standard deviation of an amplitude on the delay 0 and a standard deviation of an amplitude on delay τ are calculated, and an absolute value of the coherence function on the delay τ is obtained by a ratio of both deviations (19).

Description

  The present invention relates to a laser beam measurement method and a measurement apparatus for accurately measuring a coherence time of a laser beam.

Conventionally, a method for measuring the coherence time of a laser beam as a function has been proposed in order to accurately measure the coherence time of a laser beam having a relatively narrow spectral line width (long coherence time).
As a method related to the measurement of the coherence function of the laser beam, a laser beam spectrum measurement method described in Non-Patent Document 1 is known. In this measurement method, the laser beam to be measured is split into two, one is incident on a sufficiently long optical fiber, and the other is shifted in optical frequency by a certain amount by an acoustooptic device or the like. Then, both are combined and a beat spectrum is measured with a light receiving element.

  Since it is known that 1 / √2 of the beat spectrum width is the oscillation spectrum width of the laser beam, the spectral line width of the laser beam can be measured from the beat spectrum width. If the spectral line width of the laser beam is Δν, the coherence time of the laser beam is 1 / Δν, and from this, the coherence time of the laser beam can be obtained. If this is multiplied by the speed of light, the coherence length can be obtained.

  However, the following problems are known in the above measurement method. That is, in the above description, one of the branched laser beams needs to be combined with the other one after giving a sufficient time delay by using a sufficiently long optical fiber, which is longer than the coherence length of the laser beam to be measured. A sufficiently long delay is required. In other words, the conventional measurement method is just an amount to be measured, and thus is effective on the assumption that a delay longer than the coherence length of the unknown laser beam is obtained at this stage. Therefore, although the measurement result obtained by the conventional measurement method enables a certain estimation of the laser beam spectrum width, it has to be uncertain.

  The above-mentioned concerns are particularly noticeable when measuring laser light with a narrow spectral line width (long coherence time), and in recent years, lasers having a spectral line width of about several kHz such as fiber lasers have been commercialized. In the situation, the ambiguity of the measurement was 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

  However, in the conventional measurement method as described above, one of the branched laser beams is transmitted through a sufficiently long optical fiber, and a delay sufficiently longer than the coherence length of the laser beam to be measured is generated. The measurement amount here is the coherence length generated by the optical fiber prepared to cause the delay, and a delay longer than the coherence length of the laser light that is unknown at the time of measurement is obtained. This is a prerequisite. Therefore, although it is possible to estimate the spectrum width of the laser beam, there is a problem that the measurement result includes uncertainty.

  The present invention has been made by paying attention to the above-described circumstances, and its object is to perform laser beam measurement capable of accurately measuring the coherence time of a laser beam having a narrow spectral line width (long coherence time). It is to provide a method and a measuring device thereof.

The laser beam measurement method according to the present invention has the following configuration.
(1) A method of measuring a coherence function γ (τ) of a laser beam to be measured that is a function of a delay time τ, wherein the optical frequency of the laser beam to be measured is swept linearly, and the optical frequency is swept A laser beam is split, and the branched one laser beam is incident on an optical fiber to generate Rayleigh scattered light. The Rayleigh scattered light generated in the optical fiber is combined with the other branched laser beam. And detecting the photocurrent by receiving the combined light, digitizing and Fourier transforming the detected photocurrent, and calculating the standard deviation of the amplitude at delay 0 from the Fourier-transformed photocurrent value. The standard deviation of the amplitude at the delay τ is calculated, and the absolute value of the coherence function at the delay τ is obtained by the ratio between the two.

(2) In the configuration of (1), the coherence time is further determined by τ where the absolute value of the coherence function is 1 / e.
The laser beam measurement apparatus according to the present invention has the following configuration.
(3) An apparatus for measuring a coherence function γ (τ) of a laser beam that is a function of the delay time τ, an optical frequency sweeping means for linearly sweeping the optical frequency of the laser beam to be measured, and the optical frequency sweep A branching means for branching the laser beam, an optical fiber that causes one of the branched laser beams to enter to generate Rayleigh scattered light, a Rayleigh scattered light generated in the optical fiber, and the other branched one. Light that is combined with laser light, detection means that receives the combined light and detects photocurrent, and digitizes the detected photocurrent and performs Fourier transform, and light subjected to Fourier transform An arithmetic means is provided that calculates the standard deviation of the amplitude at the delay 0 and the standard deviation of the amplitude at the delay τ from the current value, and obtains the absolute value of the coherence function at the delay τ by the ratio of the two.

  (4) In the configuration of (3), the coherence time is further determined by τ at which the absolute value of the coherence function is 1 / e.

  As described above, the present invention is characterized in that the reliability is improved as compared with the prior art by performing statistical processing on the measured value. Specifically, the standard deviation of the absolute value of the Rayleigh scattering intensity obtained by measuring the Rayleigh scattering intensity by applying C-OFDR to the laser light to be measured that has been swept in frequency is calculated, and the coherence function at τ (coherence time) The absolute value of is obtained.

  According to the present invention, the absolute value of the coherence function can be obtained even when τ is smaller than the round-trip time of the full length of the prepared optical fiber. Further, by obtaining τ whose value is reduced to 1 / e, it is possible to obtain the coherence time of the laser beam.

  In short, according to the configuration of the present invention, it is possible to provide a laser beam measurement method and a measurement apparatus capable of accurately measuring the coherence time of a laser beam having a narrow spectral line width (long coherence time).

The block diagram which shows the structure of one Embodiment of the laser beam measuring apparatus which concerns on this invention. The characteristic view which shows the change of the optical frequency given by the frequency sweep of embodiment shown in FIG.

Hereinafter, embodiments of the present invention will be described with reference to the drawings.
FIG. 1 is a block diagram showing a configuration of an embodiment of a laser beam measuring apparatus according to the present invention. In FIG. 1, reference numeral 11 denotes a laser light source to be measured, and reference numeral 12 denotes an optical frequency sweeping device having a function of making the laser light generated by the laser light source 11 to be measured incident and sweeping the optical frequency. The laser light whose optical frequency has been swept by the optical frequency sweeping device 12 is sent to the optical transmission path and branched into two by the optical branching device 131.

  One of the branched laser beams is incident on the optical fiber 15 via the optical circulator 14. Light scattering called Rayleigh scattering occurs in the optical fiber 15. The scattered light travels backward through the optical fiber 15 and is transmitted toward the optical multiplexer 132 by the optical circulator 14. The other laser beam branched by the optical splitter 131 is sent to the optical multiplexer 132 as it is, and after being combined with the laser beam from the optical circulator 14, it is again distributed to two systems and received by the balanced light receiving element 14. The surface is irradiated.

The balanced light receiving element 14 individually receives the two optical combined outputs and generates a photocurrent corresponding to the amount of received light. Two photocurrent outputs obtained by the balanced light receiving element 14 are sent to the data acquisition unit 17.
The data acquisition unit 17 includes a digitization processing unit such as an A / D converter and a memory for storing the digitized data. The data acquisition unit 17 digitizes the photocurrent generated in the balanced light receiving element 16 and records it in the memory. Keep it.

On the other hand, the Fourier transform unit 18 and the coherence function analysis unit 19 are realized by a numerical computation unit such as a personal computer, and perform a coherence function analysis by Fourier transforming the photocurrent.
In the above configuration, the processing operation will be described below.
First, the electric field amplitude E (t) of the laser light emitted from the laser light source 11 to be measured is expressed as shown in equation (1).

ここでθ(t)は、レーザ光の位相雑音を表す確率変数である。
レーザ光のコヒーレンス関数γ(τ)は、（２）式で与えられる。

Here, θ (t) is a random variable representing the phase noise of the laser beam.
The coherence function γ (τ) of the laser beam is given by equation (2).

ここで、かっこ記号の＜＞は統計平均を意味するが、通常のレーザ光においてはこれを時間平均で置き換えることができる。その場合、コヒーレンス関数は（３）式により定義される。

Here, <> of the parenthesis sign means a statistical average, but this can be replaced with a time average in a normal laser beam. In that case, the coherence function is defined by equation (3).

で表される。また、＊は位相共役を意味する。
被測定レーザ光源１１が発した被測定レーザ光は、光周波数掃引装置１２によって、図２に示すようにその光周波数が時間に対してＴ秒間線形に掃引される。光周波数掃引装置１２としては、例えば非特許文献２に記載の単一側波帯変調器を用いることにより実現できる。

It is represented by * Means phase conjugation.
The measured laser light emitted from the measured laser light source 11 is swept linearly for T seconds with respect to time by the optical frequency sweep device 12 as shown in FIG. The optical frequency sweeping device 12 can be realized by using, for example, a single sideband modulator described in Non-Patent Document 2.

The electric field amplitude E _SW (t) of the light wave whose optical frequency has been swept by the optical frequency sweeping device 12 is expressed by equation (4).

ここでｇは光周波数の掃引速度（Hz/s）である。
周波数掃引されたレーザ光は、光分岐器１３１により２分岐され、一方は光サーキュレータ１４を介して光ファイバ１５に入射される。光ファイバ１５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１５を逆方向に伝搬して光サーキュレータ１４に戻り、このサーキュレータ１４の機能によって光合波器１５に送り出される。分岐されたもう一方のレーザ光は、そのまま光合波器１３２に向かって進行する。光合波器１３２ではこれらのレーザ光が合波される。

Here, g is the sweep speed (Hz / s) of the optical frequency.
The frequency-swept laser light is branched into two by the optical branching device 131, and one of the laser light is incident on the optical fiber 15 through the optical circulator 14. Light scattering called Rayleigh scattering occurs in the optical fiber 15, and the scattered light propagates in the reverse direction through the optical fiber 15, returns to the optical circulator 14, and is sent out to the optical multiplexer 15 by the function of this circulator 14. The other branched laser beam travels toward the optical multiplexer 132 as it is. In the optical multiplexer 132, 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 (C-OFDR), which is also described in Non-Patent Document 2, one When round trip time tau _m of light to the reflection point Z _m of photocurrent i _m generated in balanced light-receiving element 16 by the scattered light from the reflection point (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。
（５）式のフーリエ変換を（６）式に変換すると、

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.
When the Fourier transform of equation (5) is converted into equation (6),

（６）式に（５）式を代入してｆ＝ｇτ_m と置くことにより（７）式を得る。

By substituting equation (5) into equation (6) and setting f = gτ _m , equation (7) is obtained.

ここでＴが十分に大きく、

Where T is large enough,

が成立すると、（７）式は（８）式に示すようになる。

(7) becomes as shown in (8).

一般に、光ファイバにコヒーレントなレーザ光を入射すると、そのレイリー散乱光はランダムな強度揺らぎを持つことが知られているが、上記の装置構成においてもそれは同様である。このことから、（８）式は光ファイバの微小体積からのコヒーレントな散乱量を表し、コヒーレントな散乱の全散乱強度Ω(gτ_m )はそれらの無数の和で決まる。すなわち（９）式に示すようになる。

In general, when coherent laser light is incident on an optical fiber, the Rayleigh scattered light is known to have random intensity fluctuations, but the same applies to the above apparatus configuration. From this, the equation (8) represents the amount of coherent scattering from a minute volume of the optical fiber, and the total scattering intensity Ω (gτ _m ) of the coherent scattering is determined by their innumerable sum. That is, as shown in equation (9).

尚、バランス型受光素子１６に生じる全電流（あらゆる反射点からの電流の和）をｉ(t)とすれば、

If the total current (sum of currents from all reflection points) generated in the balanced light receiving element 16 is i (t),

であるので、Ω(gτ_m )は全電流ｉ(t)のフーリエ変換によって以下のように求められることは明らかである。

Therefore, it is clear that Ω (gτ _m ) can be obtained as follows by Fourier transform of the total current i (t).

（８）式によれば、遅延時間がτ_m の場合には、微小体積からのコヒーレントな散乱の大きさがγ(τ_m )倍に小さくなる（コヒーレンス関数γ(τ)は複素数であるが、その絶対値は０以上１以下である）ので、その近傍でのレイリー散乱強度の揺らぎの大きさもγ(τ_m )倍に小さくなることになる。

According to the equation (8), when the delay time is τ _m , the magnitude of coherent scattering from a small volume is reduced by γ (τ _m ) times (although the coherence function γ (τ) is a complex number). Therefore, the magnitude of the fluctuation of the Rayleigh scattering intensity in the vicinity thereof is also reduced by γ (τ _m ) times.

Using this, in order to calculate the coherence function from the above observation, the statistic of the absolute value | Ω (gτ _m ) | of the Rayleigh scattering intensity, which is an observable amount, is calculated. First, at τ = 0, γ (τ) = γ (0) = 1, and γ (τ _m ) is considered to be almost constant near τ _m , so the standard deviation of the absolute value of the scattering intensity Ω (0) Is

の標準偏差σ|γ|に等しい。すなわち（１０）式に示すようになる。

Is equal to the standard deviation σ | γ |. That is, as shown in equation (10).

（９）式において、ｒ_m とγ(τ_m )はいずれも複素数であり、かつｒ_m はランダムな確率複素変数であって、その偏角は０〜２πまで均一に分布する。よって、γ(τ_m )の偏角をｒ_m に組み入れて新しい変数ｒ’_m を定義し、γ(τ_m )はその絶対値のみを残す形に表記しても問題ない。すなわち

In equation (9), both r _m and γ (τ _m ) are complex numbers, and r _m is a random random complex variable whose declination is uniformly distributed from 0 to 2π. Therefore, there is no problem even if a new variable r ′ _m is defined by incorporating the argument of γ (τ _m ) into r _m and γ (τ _m ) is left in its absolute value. Ie

が成立する。ｒ’_m はｒ_m と全く同じ性質の確率変数である。遅延τ_m におけるΩ(gτ_m )の絶対値の標準偏差は、（１１）式より直ちに

Is established. r ′ _m is a random variable having exactly the same properties as r _m . The standard deviation of the absolute value of Ω (gτ _m ) at the delay τ _m is immediately calculated from the equation (11).

と求めることができる。（１０）式、（１２）式の比をとってσ_|γ|を消去すれば、以下のようにコヒーレンス関数を求めることができる。

It can be asked. If σ _{| γ |} is eliminated by taking the ratio of Equations (10) and (12), the coherence function can be obtained as follows.

標準偏差を求めるには多数の標本を必要とする。C-OFDRの理論によれば、観測されるレイリー散乱強度は、光周波数１／Ｔごとに独立である。したがって、例えばΩ(0)の標準偏差を求めるに当たっては、τ=０近傍において、Δｆ＝１／Ｔごとに標本

A large number of samples are required to obtain the standard deviation. According to the C-OFDR theory, the observed Rayleigh scattering intensity is independent for each optical frequency 1 / T. Therefore, for example, when obtaining the standard deviation of Ω (0), in the vicinity of τ = 0, every Δf = 1 / T

を収集し、これらの標準偏差を求めればよい。Ω(gτ_m )の標準偏差を求める場合も同様である。
以上の知見にもとづき、図１の構成において次の手順でコヒーレンス関数を求める。
データ取得部１７は、例えばＡ／Ｄコンバータによる数値化装置と数値化された値を格納するメモリによって構成され、バランス型受光素子１６に生じた光電流ｉ(t)を数値化し、メモリに記録する。フーリエ変換部１８は、パーソナルコンピュータ等の数値演算処理装置で構成され、上記光電流ｉ(t)をフーリエ変換し、

Are collected, and these standard deviations may be obtained. The same applies when obtaining the standard deviation of Ω (gτ _m ).
Based on the above knowledge, a coherence function is obtained by the following procedure in the configuration of FIG.
The data acquisition unit 17 includes, for example, a digitizing device using an A / D converter and a memory that stores the digitized value. The data acquisition unit 17 digitizes the photocurrent i (t) generated in the balanced light receiving element 16 and records it in the memory. To do. The Fourier transform unit 18 is composed of a numerical calculation processing device such as a personal computer, and Fourier transforms the photocurrent i (t).

を得る。
最後に、コヒーレンス関数解析部１９は、Ω(0)近傍及びΩ(gτ)近傍でのそれぞれの標準偏差を算出し、

Get.
Finally, the coherence function analysis unit 19 calculates respective standard deviations in the vicinity of Ω (0) and Ω (gτ),

によりτにおけるコヒーレンス関数の絶対値を求める。
この作業は、使用する光ファイバ１５の全長の光の往復時間よりも小さいτの場合であっても可能であり、その範囲でのコヒーレンス関数の絶対値を求めることができる。その値が1/eに減ずるτを求めれば、光源のコヒーレンス時間を求めることもできる。

To obtain the absolute value of the coherence function at τ.
This operation can be performed even when τ is smaller than the round-trip time of light of the entire length of the optical fiber 15 to be used, and the absolute value of the coherence function in that range can be obtained. If the value τ whose value is reduced to 1 / e is obtained, the coherence time of the light source can be obtained.

However, the coherence function can also be obtained by using a variation coefficient (standard deviation / number of points used for calculating the standard deviation) instead of the standard deviation.
The coherence function measurement method and the coherence time measurement method according to the above embodiment have the following advantages over the prior art.

  First, measurement is possible with an optical fiber length comparable to the coherence time, and if the coherence time is much longer than the optical fiber length, the measurement itself is revealed by the measurement. It is possible to take measures such as re-measurement by replacing with a long optical fiber. Therefore, it can be said that this embodiment provides a measurement result that is more reliable than the prior art.

Second, not only is it possible to measure the coherence time, but the entire absolute value of the coherence function is captured. Thereby, it is considered that the properties of the laser beam can be measured in more detail.
As described above, according to the configuration described in the embodiments, a laser beam measurement method and a measurement apparatus that can accurately measure the coherence time of a laser beam having a narrow spectral line width (long coherence time) are realized.

  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. In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the embodiment. 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 11 ... Laser light source to be measured, 12 ... Optical frequency sweep device, 131 ... Optical splitter, 132 ... Optical multiplexer, 14 ... Optical circulator, 15 ... Optical fiber, 14 ... Balance type light receiving element, 17 ... Data acquisition unit, 18 ... Fourier transform unit, 19 ... coherence function analysis unit.

Claims (4)

A method for measuring a coherence function γ (τ) of a laser beam to be measured, which is a function of a delay time τ,
The optical frequency of the laser beam to be measured is swept linearly,
Branching the laser light whose optical frequency has been swept,
One of the branched laser beams is incident on an optical fiber to generate Rayleigh scattered light,
Combining the Rayleigh scattered light generated in the optical fiber and the other branched laser light,
Receiving the combined light and detecting a photocurrent;
The detected photocurrent is digitized and Fourier transformed. From the Fourier transformed photocurrent value, the standard deviation of amplitude at delay 0 and the standard deviation of amplitude at delay τ are calculated. A method for measuring a laser beam, characterized by obtaining an absolute value of a coherence function.   2. The laser beam measurement method according to claim 1, wherein the coherence time is obtained from τ where the absolute value of the coherence function is 1 / e. An apparatus for measuring a coherence function γ (τ) of laser light that is a function of a delay time τ,
An optical frequency sweeping means for linearly sweeping the optical frequency of the laser beam to be measured;
Branching means for branching the laser light subjected to the optical frequency sweep;
An optical fiber that injects one of the branched laser beams to generate Rayleigh scattered light;
Multiplexing means for multiplexing the Rayleigh scattered light generated in the optical fiber and the other branched laser beam;
Detecting means for receiving the combined light and detecting a photocurrent;
The detected photocurrent is digitized and Fourier transformed. From the Fourier transformed photocurrent value, the standard deviation of amplitude at delay 0 and the standard deviation of amplitude at delay τ are calculated. A laser beam measuring apparatus comprising: an arithmetic means for obtaining an absolute value of a coherence function.   4. The laser beam measuring apparatus according to claim 3, wherein the arithmetic means further obtains a coherence time from τ where the absolute value of the coherence function is 1 / e.

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