JP2004069333A - Wavelength measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a wavelength measuring method which can realize measurement of a continuous wave light source capable of more precise wavelength measurement, and wavelength measurement of a pulse light source with high precision by measuring indirectly measuring wavelength of the pulse light source, by measuring wavelength difference between the continuous light source and the pulse light source. <P>SOLUTION: In the pulse laser light source, a continuous light source exists in a wavelength band produced in a wavelength conversion process necessary for forming an oscillation wavelength or a final output wavelength. This wavelength measuring method is characterized by measuring the wavelength of the pulse laser by measuring wavelength difference between the wavelength of the pulse laser and the wavelength of the continuous light source from a beat signal, when the pulse laser and the continuous light source having the same wavelength are made to interfere with each other. <P>COPYRIGHT: (C)2004,JPO

Description

ここで、発振器１のパルス電場をＥ１０ｅｘｐ（−ａｔ＾２−ｉω１ｔ）、連続波光源の電場をＥ２０Ｅｘｐ（−Ｉω２ｔ）であり、両者の位相差をφ１２としている。
【００２１】
数式１で表される時間干渉波形は、ファイバ結合器６の出力に設けられた高速光検出器８により、電気信号へと変換される。得られた電気信号は、任意時間中に入力されるある閾値以上の電圧信号の数をカウントするカウンター９によりビートの計数を行う。ここでビートの計数とは、連続波同士のビート周波数検出が、ある時間間隔内に検出されるビートを計数し、単位時間間隔あたりのカウント数を算出するのに対して、１パルスあたりのカウント数を算出することを意味する。
前記した発振器１のパルスと連続波光源の位相差のランダム性により、同一の光周波数差における時間干渉波形もパルスごとにランダムに変化し、結果としてカウント値もばらつく事になる。
【００２２】
このばらつきの影響を避けて高精度な測定を行うためには、カウント値のサンプル数を増やして平均化すればよく、図４に示す様に１０００パルス程度の平均化で十分な精度のビートカウントを行えるようになる。本実施例の発振器１は５ｋＨｚの繰り返し周波数でパルスを生成するため２００ｍｓｅｃ程度の平均時間でよい。
ここで、連続波光源用波長計５により得られた光源２の波長測定値と、発振器１のパルス電場に対する連続波光源の電場の光周波数差から、パルスレーザ装置１乃至３の出力波長を測定する手順について説明する。
１．連続波光源の波長設定
まず、連続波光源７の波長の設定を行う。連続波光源７は発振器１の波長とほぼ同一に設定するが、ビートの計測のため、微小な波長オフセット量を正及び負に与えた値とする。
【００２３】
ここで、波長オフセット量としては、発振器１の出力パルス幅においてビート信号が観測できる、即ち一周期以上のビート周波数となる必要性と、高速光検出器８及びカウンター９の帯域制限から光周波数にして数百ＭＨｚ程度、波長換算で〜１ｐｍ程度に設定する。具体的には１９２．９９９ｎｍと１９３．００１ｎｍを用いる。また、連続波用波長計５の絶対精度が不十分な場合には、発振器１の波長帯域において吸収構造を有するヨウ素等の吸収線に波長を固定してもよい。この場合には前記設定波長の前後にある２つの吸収線に波長を固定することになる。
２．連続波光源の波長測定及びビートカウント
波長の設定が完了したら、連続波光源用波長計５の計測を開始すると同時に、カウンター９によるビートカウントの積算を行う。この時、カウンター９では計測中の発振器１のパルス数をカウントする。パルス数をカウントするためには励起光源２のＱスイッチ等に設けられた同期出力を用いれば良い。また、連続波用波長計５の絶対精度が不十分な場合には、発振器１の波長帯域において吸収構造を有するヨウ素等の吸収線に波長を固定してもよい。１で吸収線へのロックを行った場合には波長計測値としては吸収線の波長値を用いる。
【００２４】
測定終了後、連続波光源波長値、ビートカウント積算値、パルスカウント積算値を制御コンピュータに送信する。
３．連続波ダイオードレーザの波長切り替え
１及び２の手順が終了したら再び１の手順を行う。ここで２乃至１に以降する際には連続波光源７の波長オフセットの正負を切り替える。この波長オフセットの切替は、一般の制御におけるディザーに相当し、周波数差の符号を判別するためのものである。
４．レーザ装置１の第１波長の計算
前記した１乃至３の手順により、連続波光源波長値、ビートカウント積算値、パルスカウント積算値のそれぞれがレーザ制御コンピュータに記録される。
【００２５】
これらを用いて発振器１の波長λ１は次式で表される。
【００２６】
【数２】

ここで各記号はそれぞれ、λ２：連続波光源の正側オフセット波長測定値、λ３：連続波光源の負側オフセット波長測定値、ＢＣ（λｉ）：波長λｉにおけるビートカウント積算値、ＰＣ（λｉ）：λｉにおけるパルスカウント積算値、ｋ：ビートカウント対ビート周波数敏感度を表す。
【００２７】
ｋのビートカウント対ビート周波数敏感度は、発振器１の出力、連続波光源出力、カウンターの閾値により変化してしまうため、同一測定条件のもと、予め測定しておく必要がある。測定方法としては、連続波光源をλ２、λ３近傍の複数波長に設定したうえで２の手順を実行し、連続波光源波長の変化量に対するビートカウント値の変化の割合として求めればよい。図３に示したように、敏感度ｋは発振器１のスペクトル幅より大きなビート周波数の領域ではほぼ直線で表されるため、前述した方法で得られた値を直線でフィッティングすれば精度良く求めることが可能である。
５．発振器１へのフィードバック
手順４で算出したλ１の測定値と、手順１における波長１の設定値の差分を補正するために、制御コンピュータからレーザ装置の波長制御手段へ帰還信号を与える。発振器１では、前記帰還信号により共振器長を変化させ、λ１を補正する。これにより、第１波長から波長変換過程を経て出力される出力波長が設定値に安定化される。
以上の手順１から手順４を繰り返すことにより、波長変換機１からの出力波長を高精度に測定可能であるとともに、その測定結果を下に帰還信号を加える事で設定波長に固定を行う事が可能となる。
【００２８】
本発明の第２の実施例として、ビート周波数の特定をその周波数分布から直接特定する方法を示す。本実施例の場合は、平均化が不要のため、単一パルスでの波長測定が可能となる。図４に実施例２の構成を示す。
【００２９】
発振器１と連続波光源５の干渉信号を高速受光素子８で電気信号に変換するまでの過程は実施例１と同様であるため省略する。本実施例ではカウンタの変わりに高速のＡ／Ｄ変換器１１に前記電気信号を入力する。Ａ／Ｄ変換機１１に必要な帯域及びサンプリング周波数は、発振器１と連続波光源間で想定される波長差および、算出するビート周波数の大きさによって決定される。
まず、Ａ／Ｄ変換器によってデジタル化した信号に対し周波数解析を行う。この方法としては高速いフーリエ変換などの手法を用いればよい。信号のフーリエ変換後の波形は図で示されるように、２つのピークを持つ。電気信号を式１で近似した場合のフーリエ変換が次式であらわされる。
【００３０】
【数３】

数式３から分るように、周波数０にピークを持つ分布ははビート信号の包絡線を示す周波数分布であり、もう一方がビートそのものを示す周波数分布となるため、第２のピークの周波数の値を計算する事でビート信号の周波数の計算が可能である。
実測定における制約としては、よく知られているＦＦＴの周波数分解能Δｆ、及び周波数の最大値ｆｍａｘはサンプリング間隔Δｔとｆｍａｘ＝１／Δｔ、Δｆ＝１／（Δｔ×ｎ）に加え、図の周波数分布の二つのピークが分離できることの条件としては、発振器１の周波数線幅をｆｗｉｄｔｈ、ビート周波数ｆｂｅａｔとして、ｆｂｅａｔ＞ｆｗｉｄｔｈの必要がある。
【００３１】
上記の条件により、ビート周波数の下限値は発振器１の線幅、上限は受光素子の帯域及びＡ／Ｄ変換器のサンプリング周波数によって制限される事になる。本実施例ではこれらを満たす条件として、ビート周波数として出力パルス線幅の５倍程度の値となるように連続波光源の波長λ４を設定する。つまり、連続波光源の設定波長は、発振器１のをλ１、線幅を周波数表記でΔｆとして、以下に示す数式に設定を行う。
【００３２】
【数４】

測定中は、設定値λ４からのずれ量を連続波光源用波長計によって高精度に測定を行い、その結果と算出されるビート周波数の値により、波長変換ユニット２からの出力波長の算出を行う。ここで、ビート周波数の符号の不確定性は、連続波光源の波長を設定値から既知の少量変化させた際のビート周波数変化を測定することにより判断が可能である。
【００３３】
【発明の効果】
本発明によれば、高精度な測定が比較的容易な連続波光源において波長絶対値の測定を行い、パルス光源からの連続波光源との波長差をパルス光源と連続波光源のビート信号から計測することにより、高価かつ大型の分光器等を使用せずに、容易に高精度なパルス光源の波長計測が可能となるとともに、波長計測部も小型で済むため、常時計測の必要のある干渉計用光源などへの組み込みが可能となる。
【図面の簡単な説明】
【図１】本発明の第１の実施の形態を示す概略図である。
【図２】パルス光源と連続波光源のビート信号を示したグラフである。
【図３】ビートカウントと周波数差の敏感度を示したグラフである。
【図４】本発明の第２の実施の形態を示す概略構成図である。
【図５】ビート信号のスペクトルを示したグラフである。
【図６】従来例を示す概略構成図である。
【符号の説明】
１　　　　　　　発振器
２　　　　　　　励起光源
３　　　　　　　波長変換ユニット
４　　　　　　　ハーフミラー
５　　　　　　　波長計
６　　　　　　　ファイバ結合器
７　　　　　　　連続波光源
８　　　　　　　高速受光素子
９　　　　　　　カウンター
１０　　　　　　制御コンピュータ
１１　　　　　　Ａ／Ｄ変換機[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a high-precision wavelength measuring method for a pulse laser device.
[0002]
[Prior art]
Since the pulse light source has a short coherence distance and a temporally discrete intensity distribution, it cannot perform highly accurate wavelength measurement generally used in a continuous wave light source. As a measurement method typified by the former, a continuous wave light source is split into two light beams, and the light and dark of interference fringes generated when the optical axis is matched again after an optical path length difference is added is counted over a certain optical path length difference. This is a so-called fringe scan wavelength meter that obtains a wavelength measurement value by performing the measurement, and it is possible to measure the wavelength with high reproducibility of 0.1 pm or less using a small device. As a typical example of the latter, there is a method of fixing a wavelength to an absorption line of a substance having an absorption structure in a wavelength band of a continuous wave light source, and wavelength measurement with high absolute accuracy is possible.
As a method of measuring the wavelength of a pulse light source that cannot use these methods, measurement has been performed using a pulse wavelength meter that measures the wavelength from the distribution using spatial interference fringes.
[0003]
A conventional wavelength measurement method and a wavelength control method of a pulse light source will be described with reference to FIG.
[0004]
The light beam emitted from the pulse light source 51 is split by the half mirror 59 into an optical path toward the interferometer and an optical path toward the wavelength measuring device 50. This is because, in an interferometer for measuring a wavefront shape and the like, it is extremely important to accurately manage a measurement wavelength, and thus it is necessary to measure a light source wavelength simultaneously with measurement by an interferometer.
The light beam incident on the wavelength measuring device 50 is reflected by the convex mirror 52 to spread the light beam, and then reflected by the concave mirror 53 again to become a parallel light beam. The light is incident on a so-called Fizeau etalon 54 composed of a wedge substrate formed with high precision, and forms spatial interference fringes by the effect of multiple interference. The spatial interference fringes are photographed by a light receiving element such as a line sensor 55 disposed after the Fizeau etalon 54. In addition, a wavelength-stabilized HeNe laser is introduced into the same optical path as the light flux, and the spatial distribution of the same interference fringes is measured. The measurement of the wavelength is calculated from the optical thickness d at a location of the Fizeau etalon and the interval between the interference fringes of the pulsed light source and the HeNe laser.
[0005]
In the case where higher accuracy is to be obtained, the distribution of interference fringes when a light beam is diffused and incident on a so-called Fabry-Perot etalon 56 formed of a parallel substrate at the same time as the measurement of the Fizeau etalon is measured by a line sensor 57. Increases the accuracy of the measurement. In this case, an etalon having an optical thickness about 10 times that of the Fizeau etalon 54 is used for the purpose of high-precision measurement after rough interference order determination by the Fabry-Perot etalon 54 is performed. Also in this case, a wavelength-stabilized HeNe laser is introduced on the same optical axis, the Fizeau etalon interval is calculated from the obtained interference fringe interval, and the wavelength is calculated from the interference fringe interval of the pulsed light based on the calculation result.
[0006]
The wavelength calculation is performed by the calculation unit 58 provided in the wavelength measuring device 50, and the measured wavelength value is transmitted to the control computer 60 by communication.
[0007]
The control computer 60 uses the obtained wavelength measurement values to control actuators and the like provided in the pulse light source 51 to control the diffraction grating and the resonator length in order to keep the wavelength of the pulse light source 51 at a desired value. By doing so, wavelength control was performed.
[0008]
[Problems to be solved by the invention]
However, in the case of the pulse light source, since the wavelength is measured from the spatial interference fringe interval, it is easily affected by the intensity change of the light source, the directional stability, and the like, and also easily affected by the sensitivity characteristics of the light receiving element. However, there is a problem that the wavelength measurement accuracy is low.
[0009]
Although the absolute value of the wavelength depends on the wavelength stabilized HeNe, the reliability of the absolute value of the wavelength is determined by the optical adjustment of the wavelength meter and the wavelength dispersion of the glass material used for the etalon together with the absolute wavelength accuracy of the HeNe itself. Was low.
[0010]
Therefore, the present invention measures the wavelength of the pulse light source indirectly by measuring the wavelength difference between the continuous wave light source and the pulse light source, together with the measurement of the continuous wave light source that enables more accurate wavelength measurement. It is an exemplary object to provide a wavelength measurement method capable of realizing highly accurate pulse light source wavelength measurement.
[0011]
[Means for Solving the Problems]
In order to achieve the above object, a wavelength measuring method according to an exemplary aspect of the present invention provides a continuous wave light source in an oscillation wavelength or a wavelength band generated in a wavelength conversion process necessary for producing a final output wavelength. In the pulse laser light source that exists, the pulse laser and the pulse signal by measuring the wavelength difference between the continuous wave light source and the wavelength of the pulse laser by a beat signal when causing a continuous wave light source of the same wavelength to interfere. It is characterized in that the wavelength of the laser is measured. When the wavelength difference between the pulse laser light source and the continuous wave light source is measured, the wavelength measurement or the wavelength control of the continuous wave light source is performed at the same time. A beat signal having a certain threshold value or more in a temporal interference signal waveform of each pulse is counted for a plurality of pulses, and a beat frequency is specified from the count value. The beat frequency is specified from the interval between two spectral peaks of the frequency spectrum of the temporal interference signal waveform of each pulse. A single longitudinal mode continuous wave laser is used as the continuous wave light source. The continuous wave light source is split into two light beams, the light path of the interference fringe generated when the optical axis is matched again after the optical path length difference is added, and the wavelength measurement value is obtained by counting over a certain optical path length difference. The measurement is performed using a so-called fringe scan wavelength meter. The wavelength of the continuous wave light source is measured by transmitting a gas cell having an absorption structure in the wavelength band of the continuous wave light source and fixing the wavelength to an absorption line based on the transmission intensity.
[0012]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. FIG. 1 shows a pulse light source wavelength measuring device according to an embodiment of the present invention.
[0013]
The light beam emitted from the oscillator 1 having the fundamental wavelength enters the wavelength conversion unit 3 for performing wavelength conversion to an output wavelength by a nonlinear optical effect. The oscillator 1 of the fundamental wavelength emits pulse light in the near infrared region using titanium sapphire crystal as a laser medium. For excitation of the titanium sapphire crystal, an excitation light source 2 using Nd: YAG or Nd: YLF using Nd ions is used. The oscillator 1 is a so-called improved Littman resonator having an actuator for controlling a diffraction grating and a resonator length so that single longitudinal mode oscillation can be performed independently without using an injection light source. As a result, the output spectrum is narrowed, and a coherent distance of several meters is realized. Further, the oscillator 1 is provided with a wavelength control means for controlling the oscillation wavelength by driving the mirror from outside. Since the improved Littman resonator does not use an injection light source, the structure is simple, the size and reliability of the light source device can be improved, and a wide wavelength tunable width can be obtained by using the wide gain band of the titanium sapphire. Is an advantage.
[0014]
In this embodiment, the output wavelength is 193 nm, which is generated by creating a fourth harmonic of a pulse light beam of 772 nm emitted from the oscillator 1. Here, since the output wavelength is exactly に times the wavelength of the oscillator 1, by measuring the wavelength of the oscillator 1, the output wavelength, that is, the wavelength of 193 nm can be measured.
[0015]
First, a continuous wave light source 7 capable of oscillating at the same wavelength as the fundamental wavelength is prepared. As a continuous wave light source in the near infrared region, a semiconductor laser can be used. In the present embodiment, an external resonator type semiconductor laser capable of high-accuracy wavelength control and having a wide wavelength variable band is used. The continuous wave light source 7 has an actuator for driving a wavelength selection element such as a diffraction grating in a resonator or a means for changing an injection current to a semiconductor element which is a laser medium as a means for externally controlling the wavelength. are doing.
[0016]
The emitted light from the continuous wave light source 7 is split into two by the half mirror 4, one of which is used for performing wavelength measurement by the continuous wave light source wavelength meter 5, and the other light flux is output from the oscillator 1. Used to measure the wavelength difference from light. Here, as the continuous wave light source wavelength meter 5, a wavelength meter that divides an optical path inside the wavelength meter and counts the number of interference fringes of two light beams generated when each optical path length difference is scanned to specify the wavelength. Is used. The measurement reproducibility by the wavelength meter 5 is 0.1 pm or less.
[0017]
The wavelength value measured by the wavelength meter 5 is transmitted to the control computer 10 via the interface of the wavelength meter 5.
[0018]
The light beam reflected by the half mirror 4 enters the incident fiber on one side of the fiber coupler 6. The output from the oscillator 1 is incident on the other incident fiber of the fiber coupler. Since the fiber coupler couples two fiber inputs to one output fiber, interference from two light sources on the same optical axis is observed at the output from the fiber coupler. Here, since the two light sources have different oscillators, the phase between the two light beams is random for each pulse emitted from the oscillator 1, so that the time interference waveform is also random.
[0019]
FIG. 2 shows a typical time interference waveform. The emission pulse width in the oscillator 1 is about 10 ns. Since the interference waveform occurs only during the time when this pulse exists, it does not become a complete sine wave like a beat signal between continuous wave lasers. When the pulse waveform of the oscillator 1 is approximated by a normal distribution, the interference waveform in FIG. 2 is expressed by the following equation.
[0020]
(Equation 1)

Here, the pulse electric field of the oscillator 1 is E10exp (−at ＾ 2-iω1t), the electric field of the continuous wave light source is E20Exp (−Iω2t), and the phase difference between the two is φ12.
[0021]
The time interference waveform represented by Expression 1 is converted into an electric signal by the high-speed photodetector 8 provided at the output of the fiber coupler 6. The obtained electric signal is beat-counted by a counter 9 which counts the number of voltage signals having a certain threshold value or more input during an arbitrary time. Here, the counting of beats means that the beat frequency detection between continuous waves counts the beats detected within a certain time interval and calculates the number of counts per unit time interval, whereas the count per pulse It means calculating a number.
Due to the randomness of the phase difference between the pulse of the oscillator 1 and the continuous wave light source, the time interference waveform at the same optical frequency difference also changes randomly for each pulse, and as a result, the count value also varies.
[0022]
In order to perform high-precision measurement while avoiding the influence of this variation, it is only necessary to increase the number of samples of the count value and perform averaging, and as shown in FIG. Can be performed. Since the oscillator 1 of the present embodiment generates a pulse at a repetition frequency of 5 kHz, the average time may be about 200 msec.
Here, the output wavelengths of the pulse laser devices 1 to 3 are measured from the measured wavelength of the light source 2 obtained by the wavelength meter 5 for the continuous wave light source and the optical frequency difference between the electric field of the continuous wave light source and the pulse electric field of the oscillator 1. The following describes the procedure.
1. First, the wavelength of the continuous wave light source 7 is set. The wavelength of the continuous wave light source 7 is set to be substantially the same as the wavelength of the oscillator 1, but a small wavelength offset amount is set to a value given positively and negatively for beat measurement.
[0023]
Here, as the wavelength offset amount, the beat signal can be observed in the output pulse width of the oscillator 1, that is, the optical frequency is determined based on the necessity of a beat frequency of one cycle or more and the band limitation of the high-speed photodetector 8 and the counter 9. About several hundred MHz, and about 1 pm in terms of wavelength. Specifically, 192.999 nm and 193.001 nm are used. If the absolute accuracy of the continuous wave wavelength meter 5 is insufficient, the wavelength may be fixed to an absorption line such as iodine having an absorption structure in the wavelength band of the oscillator 1. In this case, the wavelengths are fixed to the two absorption lines before and after the set wavelength.
2. When the measurement of the wavelength of the continuous wave light source and the setting of the beat count wavelength are completed, the measurement by the continuous wave light source wavelength meter 5 is started, and at the same time, the beat count is accumulated by the counter 9. At this time, the counter 9 counts the number of pulses of the oscillator 1 being measured. In order to count the number of pulses, a synchronous output provided in a Q switch or the like of the excitation light source 2 may be used. If the absolute accuracy of the continuous wave wavelength meter 5 is insufficient, the wavelength may be fixed to an absorption line such as iodine having an absorption structure in the wavelength band of the oscillator 1. When locking to the absorption line is performed in step 1, the wavelength value of the absorption line is used as the wavelength measurement value.
[0024]
After the measurement is completed, the continuous wave light source wavelength value, the beat count integrated value, and the pulse count integrated value are transmitted to the control computer.
3. When the procedures for wavelength switching 1 and 2 of the continuous wave diode laser are completed, the procedure 1 is performed again. Here, when switching from 2 to 1, the positive / negative of the wavelength offset of the continuous wave light source 7 is switched. This switching of the wavelength offset corresponds to dither in general control, and is for determining the sign of the frequency difference.
4. Calculation of First Wavelength of Laser Device 1 According to the above-described procedures 1 to 3, each of the continuous wave light source wavelength value, the beat count integrated value, and the pulse count integrated value is recorded in the laser control computer.
[0025]
Using these, the wavelength λ1 of the oscillator 1 is expressed by the following equation.
[0026]
(Equation 2)

Here, each symbol is λ2: the positive offset wavelength measured value of the continuous wave light source, λ3: the negative offset wavelength measured value of the continuous wave light source, BC (λi): the beat count integrated value at the wavelength λi, PC (λi). : Pulse count integrated value at λi, k: beat count versus beat frequency sensitivity.
[0027]
Since the beat count vs. beat frequency sensitivity of k changes depending on the output of the oscillator 1, the output of the continuous wave light source, and the threshold value of the counter, it is necessary to measure in advance under the same measurement conditions. As a measuring method, after setting the continuous wave light source to a plurality of wavelengths in the vicinity of λ2 and λ3, the procedure of 2 may be executed to obtain the ratio of the change of the beat count value to the change amount of the continuous wave light source wavelength. As shown in FIG. 3, the sensitivity k is substantially represented by a straight line in a region of a beat frequency larger than the spectrum width of the oscillator 1, so that the value obtained by the above-described method can be accurately obtained by fitting a straight line. Is possible.
5. In order to correct the difference between the measured value of λ1 calculated in feedback procedure 4 to oscillator 1 and the set value of wavelength 1 in procedure 1, a feedback signal is provided from the control computer to the wavelength control means of the laser device. In the oscillator 1, the resonator length is changed by the feedback signal, and λ1 is corrected. As a result, the output wavelength output from the first wavelength through the wavelength conversion process is stabilized at the set value.
By repeating the above steps 1 to 4, the output wavelength from the wavelength converter 1 can be measured with high accuracy, and the measurement result can be fixed to the set wavelength by adding a feedback signal below. It becomes possible.
[0028]
As a second embodiment of the present invention, a method of directly specifying a beat frequency from its frequency distribution will be described. In the case of the present embodiment, since averaging is not required, wavelength measurement with a single pulse becomes possible. FIG. 4 shows the configuration of the second embodiment.
[0029]
The process of converting the interference signal between the oscillator 1 and the continuous wave light source 5 into an electric signal by the high-speed light receiving element 8 is the same as that in the first embodiment, and thus the description is omitted. In this embodiment, the electric signal is input to the high-speed A / D converter 11 instead of the counter. The band and the sampling frequency required for the A / D converter 11 are determined by the wavelength difference assumed between the oscillator 1 and the continuous wave light source and the magnitude of the calculated beat frequency.
First, frequency analysis is performed on the signal digitized by the A / D converter. As this method, a method such as a fast Fourier transform may be used. The waveform of the signal after the Fourier transform has two peaks as shown in the figure. The Fourier transform when the electric signal is approximated by Expression 1 is expressed by the following expression.
[0030]
[Equation 3]

As can be seen from Equation 3, the distribution having a peak at frequency 0 is a frequency distribution indicating the envelope of the beat signal, and the other is a frequency distribution indicating the beat itself. Is calculated, the frequency of the beat signal can be calculated.
As restrictions in actual measurement, the well-known frequency resolution Δf of FFT and the maximum value fmax of the frequency are obtained in addition to the sampling interval Δt and fmax = 1 / Δt, Δf = 1 / (Δt × n), As a condition that the two peaks of the distribution can be separated, it is necessary that the frequency line width of the oscillator 1 is fwidth and the beat frequency fbeat is fbeat> fwidth.
[0031]
Under the above conditions, the lower limit of the beat frequency is limited by the line width of the oscillator 1, and the upper limit is limited by the band of the light receiving element and the sampling frequency of the A / D converter. In the present embodiment, as a condition satisfying these conditions, the wavelength λ4 of the continuous wave light source is set so that the beat frequency has a value of about five times the output pulse line width. That is, the setting wavelength of the continuous wave light source is set to the following equation, where λ1 is the oscillator 1 and the line width is Δf in frequency notation.
[0032]
(Equation 4)

During the measurement, the amount of deviation from the set value λ4 is measured with high accuracy by a continuous wave light source wavelength meter, and the output wavelength from the wavelength conversion unit 2 is calculated based on the result and the calculated beat frequency value. . Here, the uncertainty of the sign of the beat frequency can be determined by measuring the change in the beat frequency when the wavelength of the continuous wave light source is changed from the set value by a known small amount.
[0033]
【The invention's effect】
According to the present invention, the absolute value of the wavelength is measured in a continuous wave light source that is relatively easy to measure with high accuracy, and the wavelength difference between the pulse light source and the continuous wave light source is measured from the beat signals of the pulse light source and the continuous wave light source. By doing so, it is possible to easily and accurately measure the wavelength of a pulsed light source without using an expensive and large spectroscope, and the interferometer that needs to be constantly measured because the wavelength measurement unit can be small. It can be incorporated into a light source or the like.
[Brief description of the drawings]
FIG. 1 is a schematic diagram showing a first embodiment of the present invention.
FIG. 2 is a graph showing beat signals of a pulse light source and a continuous wave light source.
FIG. 3 is a graph showing the sensitivity between a beat count and a frequency difference.
FIG. 4 is a schematic configuration diagram showing a second embodiment of the present invention.
FIG. 5 is a graph showing a spectrum of a beat signal.
FIG. 6 is a schematic configuration diagram showing a conventional example.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Oscillator 2 Excitation light source 3 Wavelength conversion unit 4 Half mirror 5 Wavelength meter 6 Fiber coupler 7 Continuous wave light source 8 High-speed light receiving element 9 Counter 10 Control computer 11 A / D converter

Claims (7)

In a pulse laser light source where a continuous wave light source exists in a wavelength band generated in a wavelength conversion process required for creating an oscillation wavelength or a final output wavelength,
By measuring the wavelength of the pulse laser and the wavelength difference of the continuous wave light source, by measuring the wavelength difference between the pulse laser and the continuous wave light source by a beat signal when causing the continuous wave light source of the same wavelength to interfere with the pulse laser, Characteristic wavelength measurement method. 2. The wavelength measuring method according to claim 1, wherein when measuring the wavelength difference between the pulse laser light source and the continuous wave light source, the wavelength measurement or the wavelength control of the continuous wave light source is performed at the same time. 3. The beat signal according to claim 1, wherein a beat signal of a certain threshold value or more in a temporal interference signal waveform of each pulse is counted for a plurality of pulses, and a beat frequency is specified from the count value. Wavelength measurement method. 3. The wavelength measurement method according to claim 1, wherein a beat frequency is specified based on an interval between two spectrum peaks of a frequency spectrum of a temporal interference signal waveform of each pulse. The wavelength measuring method according to claim 1, wherein a single longitudinal mode continuous wave laser is used as the continuous wave light source. The continuous wave light source is split into two light fluxes, and the lightness of the interference fringes generated when the optical axis is matched again after the optical path length difference is added is counted over a certain optical path length difference to obtain a wavelength measurement value. The wavelength measurement method according to any one of claims 1 to 5, wherein the measurement is performed using a so-called fringe scan wavelength meter. The wavelength of the continuous wave light source is measured by transmitting a gas cell having an absorption structure in the wavelength band of the continuous wave light source and fixing the wavelength to an absorption line based on the transmission intensity. The wavelength measurement method according to any one of the preceding claims.

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