JP3597946B2 - Single pulse autocorrelator - Google Patents

Description

【数５】

【数６】

なる条件式を満足していることが好適である。
【００１６】
【発明の実施形態】
本発明にかかるシングルパルスオートコリレータは、前述したように、相対的な光学的遅延時間差を零とした２つのパルスを非線形結晶の結晶面に対して垂直に入射させることとしたので、非線形結晶の屈折率が大きくても、２つのパルスの入射光線間の角度を位相整合条件から決まる角度に一致させ、小さくすることができるので、装置を小型化することが可能となり、光学系の調整も容易となる。
【００１７】
しかも、入射光が非線形結晶の入射表面で反射することをかなり防ぐことができるので、この散乱光が迷光となって光検出器に入射することを大幅に低減することができ、被測定光のパルス幅を正確に測定することが可能となる。
以下、図面に基づき本発明の一実施態様について説明する。
【００１８】
図４には本発明の一実施態様にかかるシングルパルスオートコリレータの概略構成が示されている。
なお、本実施態様においては、被測定光にシングルパルスを想定し、このシングルパルスのパルス幅を計測する場合について説明する。
同図に示すようにシングルパルスオートコリレータ４４は、ビームスプリッタＢＳと、可変遅延系と、レンズＬ１と、非線形結晶４６と、レンズＬ２と、フィルタＦと、撮像検出器Ｄと、記録計３０とを備える。
【００１９】
ビームスプリッタＢＳは、シングルパルス４８を反射光５０と透過光５２の２つのパルスに分波する。
可変光学的遅延系は、反射器Ｍ２〜Ｍ５よりなり、Ｍ３とＭ４をＭ２とＭ５に対し例えばステッピングモータ（図示省略）により駆動して平行移動し、遅延時間差を変化させ、２つのパルスの中心部が非線形結晶４６中で時間的かつ空間的に一致させるものである。
【００２０】
非線形結晶４６は、反射器Ｍ２〜Ｍ５により遅延時間差が与えられた２つのパルスが結晶面に対して垂直に入射するように、その結晶面がプリズム状にカットされている。
２つのパルス５０，５２が非線形結晶４６中で空間的かつ時間的に完全に一致するように、反射器Ｍ２〜Ｍ５により遅延時間差を調整し、これが空間的に一致すると、非線形結晶４６はＳＨＧ光を発生する。
生じたＳＨＧ光５４の像がレンズＬ２で基本フィルタＦを通過した後、撮像検出器Ｄに結像する。
【００２１】
したがって、撮像検出器Ｄにより検出されたＳＨＧ光強度の空間的像幅は、光学的遅延量に対応するので、記録計３０により遅延時間差の関数として被測定パルスの自己相関関数がプロットされる。
この記録計３０にプロットされた相関関数に基づき、入射レーザパルス５０，５２の関数形を仮定して、シングルパルス４８のパルス幅を推定する。
なお、非線形結晶４６としては、負の一軸性結晶と正の一軸性結晶が例として挙げられ、この負の一軸性結晶には、波長が１μｍから１０μｍの赤外スペクトル域においては、例えばＡｇＧａＳｅ_２、ＡｇＧａＳ_２等、正の一軸性結晶には例えばＣｄＧｅＡｓ_２等を用いることができる。
【００２２】
本発明の実施態様にかかるシングルパルスオートコリレータ３６は概略以上のように構成され、以下にその作用について説明する。
シングルパルス４８はビームスプリッタＢＳにより分波されると、一方の透過光５２は、反射器Ｍ２〜Ｍ５を通過した後、レンズＬ１で非線形結晶４６に入射する。
他方の反射光５０は、反射器Ｍ１で反射した後、レンズＬ１で非線形結晶４６に入射する。
【００２３】
このように２つのパルス５０，５２は非線形結晶４６に非共軸（ノンコリニア）で入射するので、適当なアパーチャＡを使用すれば、入射レーザパルス５０，５２が直接撮像検出器Ｄに入射することを防ぐことができる。
ここで、２つのパルス５０，５２が非線形結晶４６中で空間的かつ時間的に完全に一致するように、反射器Ｍ２〜Ｍ５により遅延時間差を調整し、これが空間的に一致すると、非線形結晶４６は２倍波（ＳＨＧ光５４）を発生する。
このように、非線形結晶４６は、２つのパルス５０，５２が結晶面に対して垂直に入射するようにプリズム形状にカットされており、非線形結晶４６の屈折率が大きくてもパルス５０，５２の入射光線間の角度θを位相整合角ψと同じにでき、小さくすることができるので、装置を小型化することができる。
【００２４】
しかも、前述のように２つのパルス５０，５２が結晶面に対して垂直に入射するようにすることにより、入射光５０，５２が非線形結晶４６の表面にて反射することを防ぐことができるので、これが迷光となって撮像検出器Ｄに入射することをより防ぐことができる。
そして、撮像検出器Ｄにより検出されたＳＨＧ光強度の空間的像幅は、光学的遅延量に対応しているので、記録計３０により遅延時間差の関数として入射パルスの自己相関関数がプロットされる。この記録計３０にプロットされた相関関数に基づき、シングルパルス４８のパルス幅を推定する。
なお、アパーチャＡは、基本波の大部分をカットし、それによってバックグラウンドフリー測定が行える。
【００２５】
以下、本実施態様において特徴的事項である非線形結晶の設計方法について詳細に説明する。なお、本実施態様においては、非線形結晶に負の一軸性結晶を想定し、その場合の位相整合条件について説明する。
まず、周波数がω_１、波動ベクトルがｋ_１，ｋ_１ ^’の２つのパルスを非線形結晶に入射させるとき、周波数がω_２＝２ω_１、波動ベクトルがｋ_２のＳＨＧ光を最も効率的に発生させるための条件式は、次式（１）で表される。
ｋ_２−（ｋ_１＋ｋ_１’）＝０ … （１）
【００２６】
前記２つのパルス、ＳＨＧ光の計３つの光に対する非線形結晶の屈折率を、ｎ_１α（ω_１，ｋ_１），ｎ_１β（ω_１，ｋ_１ ^’），ｎ_２γ（ω_２＝２ω_１，ｋ_１）としたとき、式（１）は次式（２）で表せる。
２ｎ_２γκ_２−（ｎ_１ακ_１＋ｎ_１βκ_１’）＝０ … （２）
【００２７】
ここで、α，β，γは偏光の種類、κ_１，κ_１’，κ_２は波動ベクトルｋ_１，ｋ_１ ^’，ｋ_２方向の単位ベクトルである。
入射光ｋ_１とｋ_１ ^’が同じ偏りをもち（α＝β）、入射光κ_１とκ_１ ^’をＳＨＧ光κ_２と±ψ／２の角度をなすように入射させたとき、κ_２方向にＳＨＧ光（ω_２＝２ω_１）を効率的に発生させるための条件式は次式（３）で表される。
ｎ_２γ（ω_２＝２ω_１）＝ｎ_１α（ω_１）ｃｏｓ（ψ／２） … （３）
【００２８】
前記屈折率を速度で置き換えると、式（３）は次式（４）で表される。
ｖ_１α（ω_１）＝ｖ_２γ（ω_２＝２ω_１）ｃｏｓ（ψ／２） … （４）
この式（３）または式（４）を満足する結晶方位を負の一軸性結晶に対して探索する。
【００２９】
ここで、本実施態様においては、図５に示す光学配置を想定する。
すなわち、同図に示すように、基本波は図中ＸＹ面上にあり、Ｙ軸に対して±ψ／２の角度をなすものとする。
また、負の一軸性結晶は図中原点Ｏにあるものとする。ＳＨＧ光はｋ_２方向、つまり図中Ｙ軸方向に発生するものとする。
そして、同図に示す光学配置で負の一軸性結晶の光軸方向をいろいろと変化させて、式（３）または式（４）を満足する結晶方位を探索する。
【００３０】
ところで、一軸性結晶を光学的に取り扱うときには、その結晶を屈折率楕円体または法線速度面で置き換えると便利である。
そこで、本実施態様においては、図６に示すように法線速度面を考える。
同図に示すように、負の一軸性結晶の直交軸方向をＯＡ，ＯＢ，ＯＣ、光軸をＯＣとすると、法線速度面は同図に示すように、ＯＣ軸を軸とする回転楕円面５６と、この回転楕円面５６とＯＣ軸方向で内接する球面５８の二重面となる。
この内接球面の半径はＯＣ＝ｖ^ｏであり、常光線速度を与える。
【００３１】
この回転楕円面５６とＯＣ軸方向で内接する球面５８上の点Ｐを通る速度ＯＰ（＝ｖ^Ｏ）の光の偏光方向は、直線ＯＣと直線ＯＰによりつくられる平面と垂直である。
回転楕円面５６の長軸はＯＡ＝ＯＢ＝ｖ^ｅであり、異常光光線速度を与える。原点Ｏからこの回転楕円面５６上に向かう光の速度は、一般的には伝播方向によって変わるが、ＯＡＢ面内に限り等方的である。
したがって、仮に非偏光がＯＡＢ面上を図中ｋ方向に進むとすると、水平偏光成分がｖ^Ｏ、垂直偏光成分がｖ^ｅで伝播することとなる。
【００３２】
ここで、図８に示すようにｖe＞ｖOであり、ｖe，ｖOは角周波数ωに依存する。また、これを屈折率ｎで表現すると、図７に示すようになる。
そして、非線形結晶を回転して光軸ＯＣが空間軸ＸＹＺと平行になる各々の場合について、式（３）または式（４）がどのような偏光に対して成立するかを調べる。
【００３３】
［１］光軸ＯＣがＯＺ軸に平行な場合について
このとき、ＯＡ／／ＯＸ，ＯＢ／／ＯＹである。
水平面内における光の伝播の様子と、ω_１およびω_２（＝２ω_１）の光線速度面は、図９に示すようになる。
同図より明らかなように、式（３）または式（４）を満足するのは、次式（５）を満足する場合のみである。
ｖ_１ ^ｏ（ω_１）＝ｖ_２ ^ｅ（ω_２＝２ω_１，／／ｋ_２）ｃｏｓ（ψ／２） … （５）
【００３４】
速度を屈折率で置き換えると、式（５）は次式（６）で表される。
ｎ_２ ^ｅ（ω_２＝２ω_１）＝ｎ_１ ^Ｏ（ω_１）ｃｏｓ（ψ／２） … （６）
すなわち、水平偏光のω_１の２つのパルスを式（５）または式（６）を満足する角度（±ψ／２）で入射させると、非線形結晶３８は垂直偏光のＳＨＧ光を発生する。
【００３５】
しかしながら、垂直偏光を入射させた場合、
ｖ_１ ^ｅ（ω_１）＞ｖ_２ ^ｅ（ω_２＝２ω_１） … （７）
ｖ_１ ^ｅ（ω_１）＞ｖ_２ ^Ｏ（ω_２＝２ω_１） … （８）
となり、式（３）または式（４）を満足しないので、非線形結晶４６はＳＨＧ光５４を発生しない。
【００３６】
このように、光軸ＯＣがＯＺ軸に平行な場合、ＳＨＧ光５４を得ることができるのは、非線形結晶４６に式（５）または式（６）を満足するように入射させたときである。
【００３７】
［２］光軸ＯＣがＯＸ軸に平行な場合について
このとき、水平面の光線速度面は図１０のようになる。
同図より明らかなように、この場合も式（３）または式（４）を満足する角度（ψ／２）は存在し、次式（９）または（１０）で表される。
ｖ_１ ^Ｏ（ω_１）＝ｖ_２ ^ｅ（ω_２＝２ω_１，／／ｋ_２）ｃｏｓ（ψ／２） … （９）
ｎ_２ ^ｅ（ω_２＝２ω_１）＝ｎ_１ ^Ｏ（ω_１）ｃｏｓ（ψ／２） … （１０）
【００３８】
つまり、垂直偏光のω_１のパルスを式（９）または式（１０）を満足するように角度（ψ／２）で入射すると、水平偏光のＳＨＧ光が得られる。
しかしながら、この場合は、水平偏光を入射しても、非線形結晶４６はＳＨＧ光を発生しない。
このように、光軸ＯＣがＯＸ軸に平行な場合、ＳＨＧ光を得ることができるのは、式（９）または式（１０）を満足するときのみである。
【００３９】
［３］光軸ＯＣがＯＹ軸に平行な場合について、図１１に基づき説明する。
この場合は、次式（１１）と（１２）で表される
ｖ_２ ^ｏ（２ω_１）ｃｏｓ（ψ／２）＜ｖ_１ ^ｏ，ｅ（ω_１） … （１１）
ｖ_２ ^ｅ（２ω_１）ｃｏｓ（ψ／２）＜ｖ_１ ^ｏ，ｅ（ω_１） … （１２）
したがって、光軸ＯＣがＯＹ軸に平行な場合、式（３）または式（４）を満足する角度ψ／２は存在しない。
【００４０】
このように、負の一軸性結晶は、［１］光軸ＯＣがＯＺ軸に平行な場合では式（５）または式（６）を満足するように、［２］光軸ＯＣがＯＸ軸に平行な場合では式（９）または式（１０）を満足するように、２つのパルスを入射させると、ＳＨＧ光を発生することとなる。
【００４１】
したがって、例えば光軸ＯＣがＯＺ軸に平行な場合、非線形結晶は図１２、図１３で示すようにプリズム形状にカットすればよい。すなわち、非線形結晶が設計できる。
この場合、位相整合角をψとすると、非線形結晶の頂角はπ−ψとなる。図１２はＳＨＧプリズムの平面図、図１３は全体図を示す。
なお、ｌ_２、ｌ_１、ｈはそれぞれ非線形結晶の幅、奥行き方向の幅、高さである。
【００４２】
まず、２つのレーザパルス５０，５２のビーム径をｘとすると、このビーム５０，５２の径ｘが入射する非線形結晶の平面の大きさより小さい必要があり、次式が得られる。
【００４３】
【数７】

つぎに、レーザパルスの非線形結晶中での群速度をｕ、時間的なパルス幅をΔｔとしたとき、２つのパルスの中心部が非線形結晶中で空間的かつ時間的に一致したときの横方向の幅Ｗ_１と奥行き方向の幅Ｗ_２は、それぞれ次式で表せる。
Ｗ_１＝ｕ・Δｔ／ｓｉｎ（ψ／２）
Ｗ_２＝Ｗ_１・ｔａｎ（ψ／２）
このうち、横方向の幅Ｗ_１に沿っての空間プロファイルがパルスの相関関数を与える。
したがって、横方向の幅Ｗ_１が非線形結晶内部に生じるための条件として次式が得られる。
【００４４】
【数８】

また、２つのパルスの中心部の交差点が非線形結晶内部に生じる必要があるので、横方向の幅ｌ_１に関しては、次式を満たす値のうち、大きい方の値を選択すれば良いことがわかる。
【００４５】
【数９】

このようにして非線形結晶の寸法を決定することができる。
図１１〜図１２に示すように、非線形結晶の頂角は光軸に対してπ−ψの角度をなすようにカットされている。
同様にして、光軸ＯＣがＯＸ軸に平行な場合に対しても、位相整合条件を満足するように非線形結晶を設計することができる。
【００４６】
このように、本発明者らは、非線形結晶がＳＨＧ光を最も効率的に発生させるための条件について検討を行い、式（３）または式（４）を導出した。
そして、例えば光軸ＯＣがＯＺ軸に平行な場合、式（５）または式（６）に基づき、シングルパルス４８をＢＳで分波し、反射器Ｍ２〜Ｍ５により相対的な光学的遅延時間差を零とした２つのパルス５０，５２が結晶面に対して垂直に入射するように、その結晶面ををプリズム形状に設計するすることにより、非線形結晶の屈折率が大きくても、２つのパルスの入射光線間の角度を位相整合角に一致させて小さくすることができるので、装置を小型化することができる。
【００４７】
しかも、入射光が非線形結晶の表面で反射することををかなり防ぐことができることにより、この散乱光が迷光となって光検出器に入射することを大幅に低減することができるので、被測定光のパルス幅を正確に測定することができる。
さらには、入射光が結晶で屈折することがなくなるので、位相整合等の光学調整が格段に容易になる。
なお、本実施態様においては、非線形結晶の設計方法について、非線形結晶に負の一軸性結晶を用いた例について説明したが、これに限られるものではなく、正の一軸性結晶にも適用することができる。
【００４８】
また、非線形結晶を入射光が垂直入射するようにプリズム形状にカットする方法は、本実施態様のシングルパルスオートコリレータのみならず、全ての非線形光学現象を利用した計測技術に応用可能であることは自明である。
【００４９】
【発明の効果】
以上説明したように、本発明にかかるシングルパルスオートコリレータによれば、被測定光を分波し、相対的な光学的遅延時間差を零とした２つのパルスを非線形結晶の結晶面に対して垂直に入射するようにプリズム形状としたので、非線形結晶の屈折率が大きくても、２つのパルスの結晶に対する入射光線間の角度を小さくすることができるので、装置を小型化することができる
しかも、入射光が非線形結晶により反射することをかなり防ぐことができることにより、この散乱光が迷光となって光検出器に入射することを大幅に低減することができるので、被測定光のパルス幅を正確に測定することができる。
さらには、入射光が光学的調整が容易に行えるという利点を有する。
【図面の簡単な説明】
【図１】従来のマイケルソン干渉計タイプのオートコリレータの説明図である。
【図２】従来のビーム交差方式のシングルパルスオートコリレータの説明図である。
【図３】図１において用いられる非線形結晶の設計方法の説明図である。
【図４】本発明の一実施態様にかかるシングルパルスオートコリレータの概略構成の説明図である。
【図５】図４において用いられる非線形結晶の設計方法の説明図である。
【図６】図４において用いられる非線形結晶の設計方法の説明図である。
【図７】図４において用いられる非線形結晶の設計方法の説明図である。
【図８】図４において用いられる非線形結晶の設計方法の説明図である。
【図９】図４において用いられる非線形結晶の設計方法の説明図である。
【図１０】図４において用いられる非線形結晶の設計方法の説明図である。
【図１１】図４において用いられる非線形結晶の設計方法の説明図である。
【図１２】図４において用いられる非線形結晶の設計方法の説明図である。
【図１３】図４において用いられる非線形結晶の設計方法の説明図である。
【符号の説明】
３０ 記録計
４４ シングルパルスオートコリレータ
４６ 非線形結晶
４８ 被測定光
ＢＳ ビームスプリッタ
Ｍ２〜Ｍ５ 可変光学的遅延系
Ｌ１ レンズ
Ｄ 撮像検出器[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a single pulse autocorrelator, and more particularly to a single pulse autocorrelator suitable for use in a second harmonic (second harmonic) generation (SHG) method.
[0002]
[Prior art]
Accurate measurement of the pulse width of ultrashort light pulses generated by pulsed lasers is indispensable for the evaluation of material properties of optical fibers in the field of optical communication, the study of physical properties in various fields, or the evaluation of laser characteristics. is there. The pulse width of this pulse light ranges from pico (10 ⁻¹² ) seconds to femto (10 ⁻¹⁵ ) seconds, and the oscillation wavelength ranges from ultraviolet to infrared.
Therefore, various measurement methods have been attempted in the past. Examples of the measurement methods include a direct method of performing photoelectric conversion at a speed following a pulse waveform and a correlation method of measuring an autocorrelation of a measured pulse. Can be
[0003]
First, the former direct method directly measures the intensity waveform of a pulse. The simplest form is to measure the output of a high-speed semiconductor detector with a sampling oscilloscope.
There is also a form using a streak camera.
This streak camera is an electronic panning shot. That is, a pulse is made incident on a photocathode through a slit, generated photoelectrons are accelerated, and then a fluorescent electrode is swept at high speed by a deflection electrode.
However, in any case of the direct method, the time resolution of measurement is governed by the speed of photoelectric conversion, and thus the latter correlation method is widely used at present.
[0004]
The correlation is obtained by providing a relative delay time difference between the two pulses and superimposing them while shifting the time. The delay time difference at this time is obtained by dividing the optical path length of each of the two pulses by the speed of light.
The accuracy of the optical path length difference is 1 μm even with a simple system. This corresponds to a delay time difference accuracy of about 3 femtoseconds, and a high time resolution can be obtained by the correlation method.
Examples of the correlation method include a two-photon fluorescence method, a second harmonic generation method, a two-photon ionization method, and a two-photon photoelectric effect method.
[0005]
For example, single-pulse measurement can be easily performed by the two-photon fluorescence method, but the usable wavelength range is limited, and when the pulse becomes short, the deformation of the pulse due to dispersion when propagating through the fluorescent medium can be ignored. Gone.
Among them, the second harmonic generation (SHG) method is widely used at present. A single pulse autocorrelator using the SHG method will be described with reference to the principle diagram of FIG.
[0006]
The autocollimator 10 shown in FIG. 1 includes a beam splitter 12, a variable delay system including a fixed mirror 14 and a movable mirror 16, a lens 18, a nonlinear crystal 20, a lens 22, a fundamental wave cut filter 24, and a photomultiplier. The photodetector includes a photomultiplier tube 26, an amplifier 28, and a recorder 30.
Then, the measured light 32 is demultiplexed by the beam splitter 12, passes through a Michelson interferometer type variable delay system, and is incident on the nonlinear crystal 20 by the lens 18 coaxially (colinear).
[0007]
The nonlinear crystal 20 has a substantially rectangular parallelepiped shape and is cut so as to satisfy a phase matching condition with respect to incident laser light.
The generated second harmonic (SHG light) passes through a fundamental wave cut filter 24 by a lens 22 and is detected by a photodetector 26.
The movable mirror 16 on one side of the delay time system is driven in parallel by, for example, a stepping motor 34 (driver), and measures the SHG light intensity while changing the delay time difference.
[0008]
The step of the optical path length difference is determined by the step angle of the stepping motor 34 and the pitch of the lead screw of the moving stage, and this is divided by the speed of light to calculate the step of the delay time difference.
After the output current of the photomultiplier tube 26 is amplified by the amplifier 28, the output current is plotted by the recorder 30 as a function of the delay time difference, whereby the autocorrelation function of the SHG light intensity is recorded.
The pulse width of the measured light is estimated by assuming the function form of the incident laser pulse from the autocorrelation function plotted by the recorder 30.
[0009]
However, this method has the following problems.
(1) Because of the collinear arrangement, the background light of the fundamental wave is easily mixed into the detector.
(2) Single pulse measurement cannot be performed.
In particular, the point (2) that a single pulse cannot be measured is a common problem of the conventional method.
For this reason, a method of intersecting a laser beam in a crystal as described below has been proposed. This method is temporarily called a beam crossing method.
FIG. 2 shows a principle diagram of the beam crossing method.
[0010]
In the figure, the ultrashort pulse laser beam 36 is split into two by a beam splitter BS, and the optical delay is adjusted by the reflectors M2 to M5 in the SHG crystal 38 so as to completely coincide spatially and temporally.
The coincident laser pulse emits SHG light, and an image thereof is detected by the imaging detector D via the fundamental wave cut filter F by the lens L2.
With this system, the correlation function of the intensity of a single pulse is obtained at once without sweeping the variable mirror.
[0011]
Since most of the fundamental wave is cut by the aperture A, background-free measurement can be performed. At this time, the SHG light phase matching condition is different from the collinear arrangement. The detailed description will be described below.
Also in this system, the detector D is a single detector, and the reflectors M3 to M4 are moved in the direction of the arrow in the drawing to give an optical delay, so that the detector output is a function of the optical delay amount. Is plotted as a conventional autocorrelator, and there is no background component.
However, single pulse measurement cannot be performed.
[0012]
[Problems to be solved by the invention]
However, in the beam crossing type autocorrelator having the conventional configuration, since the nonlinear crystal 38 is cut into a substantially rectangular parallelepiped shape as shown in FIG. The angle θ between them becomes large, and the optical system becomes large.
[0013]
When two pulses are made incident on the rectangular parallelepiped nonlinear crystal shown in the same figure, the incident light is specularly reflected on the surface of the nonlinear crystal 38, and becomes incident light loss or stray light and enters the photodetector D, There is a possibility that the pulse width of the light to be measured cannot be measured accurately.
Further, for phase matching, it is necessary to precisely adjust the crossing angle (phase matching angle ψ) of the two pulses, but the incident light rays 40 and 42 are refracted by the nonlinear crystal 38 as shown in FIG. Therefore, this adjustment becomes difficult.
[0014]
SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the related art, and has as its object a single-pulse auto-tuning device that is small in size, can easily adjust an optical system, and can accurately measure a pulse width of light to be measured. It is to provide a correlator.
[0015]
[Means for Solving the Problems]
In order to achieve the above object, a single-pulse autocorrelator according to the present invention includes a beam splitter, an optical system, a nonlinear crystal, a detector, and a delay system, and includes a second harmonic obtained by the detector. In a single-pulse autocorrelator that measures the pulse width of the measured light based on the spatial image width of the wave,
The optical system generates the two pulses so that the two pulses intersect approximately crosswise in the nonlinear crystal, and a second harmonic from the nonlinear crystal is emitted in the optical axis direction of the detector. With respect to the non-linear crystal, it is made to be incident on the non-coaxial and oblique directions with respect to the optical axis direction of the detector, respectively.
In the nonlinear crystal, a cross section of a crystal surface on a side where a pulse from the optical system is incident is formed in a substantially prism shape corresponding to a vertex portion of a substantially triangular shape, and two crystal surfaces forming the vertex angle portion are formed. Each of them forms a predetermined angle that is not perpendicular to the optical axis direction of the detector around the optical axis direction of the detector, and pulses from the optical system are incident on the crystal plane at the predetermined angle from orthogonal directions. It is characterized by being performed.
Here, the beam splitter separates a pulse, which is light to be measured.
The optical system causes the two pulses obtained by the beam splitter to enter a nonlinear crystal.
The nonlinear crystal generates a second harmonic by two pulses from the optical system.
The detector measures a spatial image width of a second harmonic emitted from the nonlinear crystal.
The delay system is for adjusting a relative optical delay difference between two pulses incident on the nonlinear crystal.
Note that, when the beam diameter of the two pulses is x,
(Equation 4)

(Equation 5)

(Equation 6)

It is preferable that the following conditional expression is satisfied.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
As described above, the single-pulse autocorrelator according to the present invention is such that two pulses having a relative optical delay time difference of zero are perpendicularly incident on the crystal plane of the nonlinear crystal. Even if the refractive index is large, the angle between the incident light beams of the two pulses can be made smaller by matching the angle determined by the phase matching condition, so that the device can be downsized and the optical system can be easily adjusted. It becomes.
[0017]
Moreover, since the incident light can be considerably prevented from being reflected on the incident surface of the nonlinear crystal, it is possible to greatly reduce the possibility that the scattered light becomes stray light and is incident on the photodetector. The pulse width can be measured accurately.
Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
[0018]
FIG. 4 shows a schematic configuration of a single-pulse autocorrelator according to one embodiment of the present invention.
In this embodiment, a case where a single pulse is assumed as the light to be measured and the pulse width of the single pulse is measured will be described.
As shown in the figure, the single-pulse auto-correlator 44 includes a beam splitter BS, a variable delay system, a lens L1, a nonlinear crystal 46, a lens L2, a filter F, an imaging detector D, and a recorder 30. Is provided.
[0019]
The beam splitter BS splits the single pulse 48 into two pulses, a reflected light 50 and a transmitted light 52.
The variable optical delay system includes reflectors M2 to M5, and drives M3 and M4 in parallel with each other by driving, for example, a stepping motor (not shown) with respect to M2 and M5, thereby changing the delay time difference and changing the center of the two pulses. The parts are matched in time and space in the nonlinear crystal 46.
[0020]
The crystal plane of the nonlinear crystal 46 is cut into a prism shape so that two pulses having a delay time difference given by the reflectors M2 to M5 are perpendicularly incident on the crystal plane.
The delay time difference is adjusted by the reflectors M2 to M5 so that the two pulses 50 and 52 are completely spatially and temporally coincident in the nonlinear crystal 46. When the difference is spatially coincident, the nonlinear crystal 46 becomes an SHG light. To occur.
After the generated image of the SHG light 54 passes through the basic filter F by the lens L2, it forms an image on the imaging detector D.
[0021]
Therefore, since the spatial image width of the SHG light intensity detected by the imaging detector D corresponds to the optical delay amount, the recorder 30 plots the autocorrelation function of the measured pulse as a function of the delay time difference.
Based on the correlation function plotted on the recorder 30, the pulse width of the single pulse 48 is estimated assuming the function form of the incident laser pulses 50 and 52.
Note that examples of the nonlinear crystal 46 include a negative uniaxial crystal and a positive uniaxial crystal. The negative uniaxial crystal includes, for example, AgGaSe ₂ in an infrared spectrum region having a wavelength of 1 μm to 10 μm. , ₂ etc. AgGaS, the positive uniaxial crystal can be used, for example CdGeAs ₂ or the like.
[0022]
The single-pulse autocorrelator 36 according to the embodiment of the present invention is configured as described above, and its operation will be described below.
When the single pulse 48 is split by the beam splitter BS, one transmitted light 52 passes through the reflectors M2 to M5 and then enters the nonlinear crystal 46 by the lens L1.
The other reflected light 50 is reflected by the reflector M1, and then enters the nonlinear crystal 46 by the lens L1.
[0023]
As described above, since the two pulses 50 and 52 are incident on the nonlinear crystal 46 non-coaxially, the incident laser pulses 50 and 52 can be directly incident on the imaging detector D if an appropriate aperture A is used. Can be prevented.
Here, the delay time difference is adjusted by the reflectors M2 to M5 so that the two pulses 50 and 52 are completely spatially and temporally coincident in the nonlinear crystal 46, and when these are spatially coincident, the nonlinear crystal 46 Generates a second harmonic (SHG light 54).
As described above, the nonlinear crystal 46 is cut into a prism shape so that the two pulses 50 and 52 are perpendicularly incident on the crystal plane. Even if the refractive index of the nonlinear crystal 46 is large, the pulses 50 and 52 Since the angle θ between the incident light beams can be made the same as the phase matching angle 、 and can be reduced, the size of the device can be reduced.
[0024]
Moreover, by making the two pulses 50 and 52 perpendicularly incident on the crystal plane as described above, it is possible to prevent the incident lights 50 and 52 from being reflected on the surface of the nonlinear crystal 46. This can be further prevented from entering the imaging detector D as stray light.
Since the spatial image width of the SHG light intensity detected by the imaging detector D corresponds to the optical delay amount, the recorder 30 plots the autocorrelation function of the incident pulse as a function of the delay time difference. . The pulse width of the single pulse 48 is estimated based on the correlation function plotted on the recorder 30.
In addition, the aperture A cuts most of the fundamental wave, so that background-free measurement can be performed.
[0025]
Hereinafter, a method for designing a nonlinear crystal, which is a feature of the present embodiment, will be described in detail. In this embodiment, a negative uniaxial crystal is assumed as the nonlinear crystal, and the phase matching condition in that case will be described.
First, when two pulses having a frequency of ω ₁ and wave vectors of k ₁ and k ₁ ^′ are incident on a nonlinear crystal, SHG light with a frequency of ω ₂ = 2ω ₁ and a wave vector of k ₂ is most efficiently generated. The conditional expression for this is expressed by the following expression (1).
k ₂ − (k ₁ + k ₁ ′) = 0 (1)
[0026]
The refractive indices of the nonlinear crystal with respect to a total of three lights of the two pulses and the SHG light are defined as n ₁ α (ω ₁ , k ₁ ), n ₁ β (ω ₁ , k ₁ ^′ ), and n ₂ γ (ω ₂ = (2ω ₁ , k ₁ ), equation (1) can be expressed by the following equation (2).
_{2n 2 γκ 2 - (n 1} ακ 1 + n 1 βκ 1 ') = 0 ... (2)
[0027]
Here, α, β, and γ are types of polarization, and κ ₁ , κ ₁ ′, and κ ₂ are unit vectors in the wave vectors k ₁ , k ₁ ^′ , and k ₂ directions.
Incident light k ₁ and k ₁ ^'have the same bias (α = β), κ ₁ between the incident light kappa ^_1' when illuminating at an angle of SHG light kappa ₂ and ± ψ / _2, κ 2 A conditional expression for efficiently generating SHG light (ω ₂ = 2ω ₁ ) in the direction is expressed by the following expression (3).
n ₂ γ (ω ₂ = 2ω ₁ ) = n ₁ α (ω ₁ ) cos (ψ / 2) (3)
[0028]
When the refractive index is replaced by speed, equation (3) is expressed by the following equation (4).
v ₁ α (ω ₁ ) = v ₂ γ (ω ₂ = 2ω ₁ ) cos (ψ / 2) (4)
A crystal orientation satisfying the formula (3) or (4) is searched for a negative uniaxial crystal.
[0029]
Here, in the present embodiment, the optical arrangement shown in FIG. 5 is assumed.
That is, as shown in the figure, the fundamental wave is on the XY plane in the figure, and forms an angle of ± ψ / 2 with respect to the Y axis.
It is assumed that the negative uniaxial crystal is at the origin O in the figure. SHG light is assumed to occur k ₂ direction, that is, the Y-axis direction in the drawings.
Then, by changing the optical axis direction of the negative uniaxial crystal in various ways with the optical arrangement shown in the figure, a crystal orientation satisfying the expression (3) or (4) is searched.
[0030]
By the way, when handling a uniaxial crystal optically, it is convenient to replace the crystal with an index ellipsoid or a normal velocity plane.
Therefore, in the present embodiment, a normal velocity plane is considered as shown in FIG.
As shown in the figure, if the orthogonal axis direction of the negative uniaxial crystal is OA, OB, OC, and the optical axis is OC, the normal velocity plane is a spheroid about the OC axis as shown in FIG. The surface 56 and the spherical surface 58 inscribed in the OC axis direction with the spheroid 56 are formed as a double surface.
The radius of this inscribed sphere is OC = ^vo , giving the ordinary ray velocity.
[0031]
The polarization direction of light at a speed OP (= v ^O ) passing through a point P on a spherical surface 58 inscribed in the OC axis direction with the spheroid 56 is perpendicular to a plane formed by the straight line OC and the straight line OP.
The major axis of the spheroid 56 is OA = OB = ^{v e,} gives the abnormal light beam velocity. The speed of light traveling from the origin O onto the spheroid 56 generally depends on the propagation direction, but is isotropic only in the OAB plane.
Therefore, assuming that the non-polarized light travels on the OAB plane in the k direction in the drawing, the horizontal polarized light component propagates as v ^O and the vertical polarized light component propagates as v ^e .
[0032]
Here, as shown in FIG. 8 , ve> vo, and ve and v0 depend on the angular frequency ω. Also, when expressed by the refractive index n of this, as shown in FIG.
Then, for each case in which the optical axis OC is parallel to the space axis XYZ by rotating the nonlinear crystal, it is examined what kind of polarization the expression (3) or the expression (4) holds.
[0033]
[1] Regarding the case where the optical axis OC is parallel to the OZ axis, OA // OX and OB // OY at this time.
The state of light propagation in the horizontal plane and the ray velocity planes of ω ₁ and ω ₂ (= 2ω ₁ ) are as shown in FIG.
As is clear from the figure, Expression (3) or Expression (4) is satisfied only when Expression (5) is satisfied.
v ₁ ^o (ω ₁ ) = v ₂ ^e (ω ₂ = 2ω ₁ , // k ₂ ) cos (ψ / 2) (5)
[0034]
When the velocity is replaced by the refractive index, equation (5) is expressed by the following equation (6).
n ₂ ^e (ω ₂ = 2ω ₁ ) = n ₁ ^O (ω ₁ ) cos (ψ / 2) (6)
That is, when two pulses of horizontally polarized ω ₁ are incident at an angle (± ψ / 2) that satisfies Equation (5) or Equation (6), the nonlinear crystal 38 generates vertically polarized SHG light.
[0035]
However, when vertically polarized light is incident,
v ₁ ^e (ω ₁ )> v ₂ ^e (ω ₂ = 2ω ₁ ) (7)
v ₁ ^e (ω ₁ )> v ₂ ^O (ω ₂ = 2ω ₁ ) (8)
And does not satisfy Expression (3) or Expression (4), so that the nonlinear crystal 46 does not generate the SHG light 54.
[0036]
As described above, when the optical axis OC is parallel to the OZ axis, the SHG light 54 can be obtained when the nonlinear crystal 46 is incident so as to satisfy the expression (5) or the expression (6). .
[0037]
[2] Regarding the case where the optical axis OC is parallel to the OX axis At this time, the horizontal ray velocity surface is as shown in FIG.
As is clear from FIG. 11, there is an angle (ψ / 2) that satisfies Expression (3) or Expression (4) also in this case, and is expressed by Expression (9) or (10).
v ₁ ^O (ω ₁ ) = v ₂ ^e (ω ₂ = 2ω ₁ , // k ₂ ) cos (ψ / 2) (9)
n ₂ ^e (ω ₂ = 2ω ₁ ) = n ₁ ^O (ω ₁ ) cos (ψ / 2) (10)
[0038]
That is, when the omega ₁ pulse of vertically polarized light incident at an angle ([psi / 2) so as to satisfy the equation (9) or formula (10), SHG light of horizontal polarization is obtained.
However, in this case, the nonlinear crystal 46 does not generate SHG light even if horizontal polarized light is incident.
Thus, when the optical axis OC is parallel to the OX axis, SHG light can be obtained only when the expression (9) or the expression (10) is satisfied.
[0039]
[3] A case where the optical axis OC is parallel to the OY axis will be described with reference to FIG.
In this case, v ₂ ^o (2ω ₁ ) cos (ψ / 2) <v ₁ ^{o, e} (ω ₁ ) expressed by the following equations (11) and (12). … (11)
v ₂ ^e (2ω ₁ ) cos (ψ / 2) <v ₁ ^{o, e} (ω ₁ ) (12)
Therefore, when the optical axis OC is parallel to the OY axis, there is no angle ψ / 2 that satisfies Expression (3) or Expression (4).
[0040]
As described above, the negative uniaxial crystal has the following characteristics: [1] When the optical axis OC is parallel to the OZ axis, [2] the optical axis OC is aligned with the OX axis so as to satisfy Expression (5) or Expression (6). In the case of parallel, if two pulses are incident so as to satisfy Expression (9) or Expression (10), SHG light will be generated.
[0041]
Therefore, for example, when the optical axis OC is parallel to the OZ axis, the nonlinear crystal may be cut into a prism shape as shown in FIGS. That is, a nonlinear crystal can be designed.
In this case, if the phase matching angle is ψ, the apex angle of the nonlinear crystal is π-ψ. FIG. 12 is a plan view of the SHG prism, and FIG. 13 is an overall view.
Here, l ₂ , l ₁ , and h are the width, the width in the depth direction, and the height of the nonlinear crystal, respectively.
[0042]
First, assuming that the beam diameter of the two laser pulses 50 and 52 is x, the diameter x of the beams 50 and 52 needs to be smaller than the plane size of the incident nonlinear crystal, and the following equation is obtained.
[0043]
(Equation 7)

Next, when the group velocity of the laser pulse in the nonlinear crystal is u and the temporal pulse width is Δt, the lateral direction when the center of the two pulses spatially and temporally coincides in the nonlinear crystal width W ₁ and the width W ₂ in the depth direction of each expressed by the following equation.
W ₁ = u · Δt / sin (ψ / 2)
W ₂ = W ₁ · tan (ψ / 2)
Among them, the space profile along the width W ₁ in the transverse direction gives the correlation function of the pulse.
Therefore, the width W ₁ in the transverse direction following equation is obtained as a condition for generating the internal nonlinear crystal.
[0044]
(Equation 8)

In addition, since the intersection point between the center portions of the two pulses needs to be generated inside the nonlinear crystal, it can be understood that the larger value among the values satisfying the following expression should be selected for the width l _{1 in the} horizontal direction. .
[0045]
(Equation 9)

In this way, the dimensions of the nonlinear crystal can be determined.
As shown in FIGS. 11 to 12, the apex angle of the nonlinear crystal is cut so as to form an angle of π-ψ with respect to the optical axis.
Similarly, even when the optical axis OC is parallel to the OX axis, a nonlinear crystal can be designed so as to satisfy the phase matching condition.
[0046]
As described above, the present inventors have studied the conditions for the nonlinear crystal to generate SHG light most efficiently, and have derived Equation (3) or Equation (4).
Then, for example, when the optical axis OC is parallel to the OZ axis, the single pulse 48 is split by the BS based on the formula (5) or (6), and the relative optical delay time difference is determined by the reflectors M2 to M5. By designing the crystal plane to have a prism shape so that the two pulses 50 and 52 set to zero are perpendicular to the crystal plane, even if the refractive index of the nonlinear crystal is large, the two pulses Since the angle between the incident light beams can be reduced to match the phase matching angle, the device can be downsized.
[0047]
Moreover, since it is possible to considerably prevent the incident light from being reflected on the surface of the nonlinear crystal, it is possible to greatly reduce the possibility that the scattered light becomes stray light and enters the photodetector. Can be accurately measured.
Furthermore, since the incident light is no longer refracted by the crystal, optical adjustment such as phase matching becomes much easier.
Note that, in the present embodiment, the example of using a negative uniaxial crystal as the nonlinear crystal has been described as a method of designing a nonlinear crystal, but the present invention is not limited to this, and the present invention is also applicable to a positive uniaxial crystal. Can be.
[0048]
Further, the method of cutting the nonlinear crystal into a prism shape so that the incident light is perpendicularly incident is applicable not only to the single-pulse autocorrelator of the present embodiment, but also to a measurement technique using all nonlinear optical phenomena. It is obvious.
[0049]
【The invention's effect】
As described above, according to the single-pulse autocorrelator according to the present invention, the light to be measured is demultiplexed, and two pulses having a relative optical delay time difference of zero are perpendicular to the crystal plane of the nonlinear crystal. Is formed into a prism shape so that the angle between the incident light beams of the two pulses with respect to the crystal can be reduced even if the refractive index of the nonlinear crystal is large. Since the incident light can be considerably prevented from being reflected by the nonlinear crystal, this scattered light can be greatly reduced from entering the photodetector as stray light. Can be measured.
Further, there is an advantage that optical adjustment of incident light can be easily performed.
[Brief description of the drawings]
FIG. 1 is an explanatory diagram of a conventional Michelson interferometer type autocorrelator.
FIG. 2 is an explanatory view of a conventional beam crossing type single pulse autocorrelator.
FIG. 3 is an explanatory diagram of a method for designing a nonlinear crystal used in FIG. 1;
FIG. 4 is an explanatory diagram of a schematic configuration of a single pulse autocorrelator according to one embodiment of the present invention.
FIG. 5 is an explanatory diagram of a method for designing a nonlinear crystal used in FIG. 4;
6 is an explanatory diagram of a method for designing a nonlinear crystal used in FIG.
FIG. 7 is an explanatory diagram of a method for designing a nonlinear crystal used in FIG. 4;
FIG. 8 is an explanatory diagram of a method for designing a nonlinear crystal used in FIG. 4;
FIG. 9 is an explanatory diagram of a method of designing a nonlinear crystal used in FIG.
FIG. 10 is an explanatory diagram of a method for designing a nonlinear crystal used in FIG.
FIG. 11 is an explanatory diagram of a method for designing a nonlinear crystal used in FIG. 4;
FIG. 12 is an explanatory diagram of a method for designing a nonlinear crystal used in FIG. 4;
FIG. 13 is an explanatory diagram of a method for designing a nonlinear crystal used in FIG.
[Explanation of symbols]
Reference Signs List 30 Recorder 44 Single pulse autocorrelator 46 Nonlinear crystal 48 Light under measurement BS Beam splitters M2 to M5 Variable optical delay system L1 Lens D Imaging detector

Claims (2)

A beam splitter that splits a pulse that is the light to be measured , an optical system that causes the two pulses obtained by the beam splitter to enter a nonlinear crystal, and a second harmonic generated by two pulses from the optical system A detector for measuring a spatial image width of the nonlinear crystal, a second harmonic emitted from the nonlinear crystal, and a relative optical delay difference between two pulses incident on the nonlinear crystal. A delay system, comprising: a single-pulse autocorrelator that measures a pulse width of the measured light based on a spatial image width of a second harmonic obtained by the detector ;
The optical system generates the two pulses so that the two pulses intersect approximately crosswise in the nonlinear crystal, and a second harmonic from the nonlinear crystal is emitted in the optical axis direction of the detector. With respect to the non-linear crystal, it is made to be incident on the non-coaxial and oblique directions with respect to the optical axis direction of the detector, respectively.
In the nonlinear crystal, a cross section of a crystal surface on a side where a pulse from the optical system is incident is formed in a substantially prism shape corresponding to a vertex portion of a substantially triangular shape, and two crystal surfaces forming the vertex angle portion are formed. Each of them forms a predetermined angle that is not perpendicular to the optical axis direction of the detector around the optical axis direction of the detector, and pulses from the optical system are incident on the crystal plane at the predetermined angle from orthogonal directions. Single pulse autocorrelator, characterized in that the. The single-pulse autocorrelator according to claim 1,
The nonlinear crystal has a beam diameter of the two pulses x,

A single pulse autocorrelator characterized by satisfying the following conditional expression.

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