JP2004037165A - Interferometer device - Google Patents
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【０００１】
【発明の属する技術分野】
本発明は、被検面からの波面形状を観察する干渉計装置、特にフィゾー型等の不等光路型干渉計装置に関し、詳しくは、ガラス板などの平行な２つの面を有する被検体についていずれか一方の面を被検面とした場合に、被検面の干渉縞に重畳される非被検面からの干渉縞ノイズを抑制し得る干渉計装置に関するものである。
【０００２】
【従来の技術】
例えば、平行平面ガラスの表面形状を測定する手法として、可干渉性の良いレーザ光源を搭載したフィゾー型干渉計を用いることが従来より知られているが、この手法においては、可干渉性の良いレーザ光を使用しているために、平行平面ガラスの被検面の干渉縞のみならず被検面とは反対側の非被検面（以下、単に非被検面と称する）の干渉縞が同時に発生してしまう。すなわち、参照光と物体光の光路長差が互いに異なるフィゾー型干渉計（不等光路干渉計）においては、可干渉性の良いレーザ光束を使用することが必須となるため、基準面と被検面、基準面と非被検面、および被検面と非被検面からの光干渉が各々生じてしまう。通常、所望される干渉縞は基準面と被検面からの光干渉のみであるため、その他の面相互の光干渉によって生じる干渉縞はノイズとなり、被検面の形状を高精度で測定することが困難となってしまう。
【０００３】
そこで従来、このような干渉縞ノイズを抑制する手法として、非被検面に屈折率マッチングオイルを塗り、その上から光散乱シートを張り付けることで、非被検面からの反射光を散乱させ、非被検面と他の面との相互の光干渉による干渉縞の発生を防止することが行なわれている。
【０００４】
しかしながら、このような干渉縞ノイズ抑制手法では、非被検面とはいえ、被検体の一方の面にオイルを塗らなくてはならず、手間がかかるだけではなく、被検体が汚れるという不都合があり、さらに、厚みが薄い被検体においては、オイルを塗ったり、光散乱シートを張り付ける等の処理により、被検面自体の形状が変化するおそれがあった。
【０００５】
そこで、このような問題を解決する手法として、フィゾー型干渉計の光源に、基準面位置を中心として光軸方向に周期的に干渉縞が生じるようなコヒーレンス関数を有する光を発振するものが知られている。なお、この光源としては、多モードの半導体レーザまたは複数本の半導体レーザが用いられる（米国特許公報 ５，４５２，０８８）。
【０００６】
この干渉計装置によれば、被検面位置では縞が発生し、被検面とは光軸方向に所定距離だけ離れた非被検面位置では縞が発生しないようにすることで被検面からの干渉縞のみを観察することができる。
【０００７】
【発明が解決しようとする課題】
しかしながら、上記米国特許公報記載の手法では、半導体レーザ光源毎に縦モードが決定されており、その光源毎にその干渉縞の縞周期が決定されてしまうので、被検体の光学的厚みが上記周期と一致する場合は、被検面についての干渉縞を良好なものとしつつ、非被検面についての干渉縞ノイズを除去することは困難となってしまう。
【０００８】
本発明は、上記事情に鑑みなされたものであり、透明な被検体の表面と裏面のように互いに対向する２つの面のうち、一方が被検面で、他方が被検面ではない面（非被検面）である場合に、該被検面についての干渉縞のコントラストは良好とし、かつ該被検面ではない面についての干渉縞の抑制を図り得る、高精度かつ安価な干渉計装置を提供することを目的とするものである。
【０００９】
【課題を解決するための手段】
本発明の干渉計装置は、単一縦モードのレーザ光を発振する波長走査が可能な光源を用い、透明な被検体の非被検面からの干渉縞ノイズを抑制する干渉計装置において、
干渉縞を受光する素子の１光蓄積期間に対し十分短い周期で、前記光源からのレーザ光を複数の波長に変調し、
該複数の波長に変調されたレーザ光を用いて前記被検体からの物体光と前記干渉計装置の基準面から参照光により生成される干渉光を前記素子により受光して、該干渉光を前記１光蓄積期間で積分するように構成し、
前記複数の波長の変調は、前記１光蓄積期間で積分された各波長の光が前記被検体の非被検面からの干渉縞ノイズを抑制し得る所定の光強度となるように、該各波長に対する変調期間が設定されていることを特徴とするものである。
【００１０】
また、前記光源が、前記波長変調を注入電流の変化に基づいて行なうレーザ光源であって、前記波長変調を行なうための注入電流の信号波形は、少なくとも、該注入電流と前記レーザ光源の出力との関係および時間軸に対する前記光強度の所望されるスペクトラム形状の２つの要素に基づいて決定されることが好ましい。
【００１１】
また、前記波長変調を行なうための注入電流の信号波形は、前記光源から前記素子までの前記レーザ光の光量の減衰率についての要素を加味して決定されることがさらに好ましい。
【００１２】
また、前記注入電流の信号波形は、前記各要素に基づいて決定されたデューティー比を有する矩形波形状をなすように形成することが好ましい。
【００１３】
また、前記複数の波長が２波長の場合において、これら２波長のレーザ光に対する前記変調期間は、前記１光蓄積期間で積分された各レーザ光の光強度が互いに略等しくなるように設定することができる。
【００１４】
また、前記レーザ光源は、半導体レーザ光源とすることが好ましい。
【００１５】
さらに、前記干渉計装置はフィゾー型等の不等光路型とすることができ、この場合に特に有用である。
なお、上記「非被検面」とは、被検体のうち被検面以外の光学面をいうものとする。
【００１６】
【発明の実施の形態】
以下、本発明の実施形態に係る干渉計装置について図面を用いて説明する。
図１は実施形態に係る干渉計装置の構成を示す概略図である。
【００１７】
図示のように、この干渉計装置１は、透明な平行平面ガラス板（被検体）１７の被検面１７ａの表面形状を干渉縞により観察するフィゾー型の干渉計本体１０と、コンピュータ２０と、モニタ２１と、半導体レーザ光源（ＬＤ）１１の電源（ＬＤ電源）２２と、この電源（ＬＤ電源）２２からの出力電流値を制御する制御信号を発生するファンクション・ジェネレータ２３とを備えてなる。
【００１８】
上記干渉計本体１０は、半導体レーザ光源１１からの可干渉光を平行光とするコリメータレンズ１２、発散レンズ１３、ビームスプリッタ１４、コリメータレンズ１５、被検体１７との間にワークスペースを介して対向する、基準面１６ａを有する基準板１６、ならびに光干渉により得られた干渉縞を撮像する撮像レンズ１８およびＣＣＤ撮像装置１９とを備えてなる。
【００１９】
この干渉計本体１０においては、半導体レーザ光源１１からのレーザ光３０を基準板１６の基準面１６ａに入射させて、該基準面１６ａにおいて透過光束と反射光束とに２分割し、透過光束を平行平面ガラス１７の被検面１７ａに入射させてその反射光を物体光とするとともに基準面１６ａにおける反射光を参照光とし、これら物体光および参照光の光干渉により生じる干渉光をコリメータレンズ１５、ビームスプリッタ１４、撮像レンズ１８を介してＣＣＤ撮像装置１９に導き、このＣＣＤ撮像装置１９において干渉縞を撮像するようになっている。
【００２０】
撮像された干渉縞はコンピュータ２０において解析され、これにより被検面１７ａの表面形状を測定し得るようになっている。なお、撮像された干渉縞および解析された被検面１７ａの表面形状はモニタに表示されるようになっている。
【００２１】
なお、基準板１６ａは、図示されないＰＺＴ駆動回路に接続されたピエゾ素子２４を介して図示されない基準板支持部材に支持されている。そして、コンピュータ２０からの指示にしたがい、ピエゾ素子２４に所定電圧が印加され該ピエゾ素子２４が駆動され、これにより基準板１６ａが光軸Ｚ方向に所定位相分だけ移動せしめられる。この移動により変化する干渉縞の画像データはコンピュータ２０に出力され、これら複数枚の画像データに対して、縞画像解析が行なわれる。
【００２２】
上記半導体レーザ光源１１は、温度制御機能が施されたものを用い、前述したように単一縦モードのレーザ光（例えば波長λが６５０ｎｍ付近）を発振し得るようになっている。さらに、注入電流を変化させると、出力されるレーザ光の波長と光強度が変化するという、一般の半導体レーザ光源としての特徴を有している。
【００２３】
また、上記ＣＣＤ撮像装置１９は、１光蓄積期間が１／３０（秒）のＣＣＤを用いている。
また、上記ファンクション・ジェネレータ２３から出力される上記制御信号は、矩形波（階段状矩形波を含む）とされており、その周波数は、例えば２００Ｈｚ程度で、ＣＣＤにより撮像された画像情報を再生する際にフリッカが生じない程度の速さに設定されている。
【００２４】
ところで、フィゾー型等の不等光路の干渉計装置１においては、基準面１６ａからの参照光と被検面１７ａからの物体光の光路長が互いに異なるため、可干渉性の良好な光を使用する必要があり、一般には、本実施形態のように照明光としてレーザ光を用いる。しかしながら、このように可干渉性の良好な光を照明光として用いると、図１において、基準面１６ａと被検面１７ａのみならず、基準面１６ａと被検体裏面１７ｂ、および被検面１７ａと被検体裏面１７ｂからの光干渉も生じてしまう。したがって、基準面１６ａと被検面１７ａからの光干渉以外の光干渉によって生じる干渉縞はノイズとなり、被検面１７ａの形状を高精度で測定することが困難となってしまうことから、この干渉縞ノイズの発生を阻止するために、本実施形態の干渉計装置１においては、以下のような特徴点を備えている。
【００２５】
すなわち、本実施形態の干渉計装置１の構成の主要な特徴点は、▲１▼単一縦モードの半導体レーザ光源１１を用い、干渉縞を受光する素子（ＣＣＤ撮像装置１９のＣＣＤ）の１光蓄積期間に対し十分短い周期で光源１１から出力されるレーザ光３０を複数の波長に変調し、被検体１７からの干渉光を上記素子により受光することで、その干渉光を上記１光蓄積期間に亘って積分する点、および▲２▼波長の変調は、上記１光蓄積期間に亘って積分された各波長の光が被検体１７の被検体裏面（非被検面）１７ｂからの干渉縞ノイズを抑制し得る所定の光強度と各々なるように、この各波長に対する変調期間が設定されている点にある。
【００２６】
ところで、半導体レーザ光源は、注入電流を変化させることで波長が変化するという特徴を有する。干渉縞を受光する素子は、所定の光蓄積期間を有しているため、その１光蓄積期間よりも十分速い速度で波長を走査すれば、多波長の光を同時に出力する光源を用いて干渉縞を観察する場合と同様の結果が得られることになる。このような知見に基づき、例えば１９９５年５月光波センシング予稿集第７５〜８２頁にコヒーレンス関数を合成する手法が示されている。この手法によれば、矩形波を基準レベル（ＤＣレベル）を中心として上下に振幅させながら、ランプ状に変化させてなる制御信号により注入電流を制御する（図１１参照；注入電流の変化を表わす）ことで、所望とするコヒーレンス関数を生成している。これは、本実施形態の特徴点として挙げた上記▲１▼に相当する。しかしながら、上記▲１▼の特徴点のみを有するこのような手法によっては、前述した干渉縞ノイズのコントラストが０とならず、干渉縞ノイズを良好に抑制することができていない。
【００２７】
本願発明者は、上述した手法において干渉縞ノイズを良好に抑制することができないのは、上述した矩形波の形状（互いに隣接する上向矩形波と下向矩形波のペア）が、図１１に示すように、基準レベル（ＤＣレベル）に対し上下で互いに等しい高さとなっており、注入電流が変化すれば光強度も変化することについて何ら考慮されていないことにあるとの見解に達し、注入電流の変化に対する光強度の変化についての関係を考慮して（例えば、ウィナー・キンチンの定理に基づき）、この矩形波の形状を決定すれば、干渉縞ノイズを良好に抑制することができることを見出した。
【００２８】
このことを図８（Ａ）、（Ｂ）を用いて概念的に説明する。まず、上述した予稿集に記載された技術について図８（Ａ）を用いて説明すると、２つの波長λＡ、λＢの光が被検面１７ａの位置では互いに同位相となって干渉縞コントラストが最高となるが、被検体裏面１７ｂの位置では互いに逆相となって干渉縞のコントラストが弱められるものの、２つの光の振幅が互いに異なるために、干渉縞を完全に消去することはできない。これに対し、本発明の干渉計装置１においては、図８（Ｂ）に示すように、被検面１７ａの位置では２つの光が同相となって、干渉縞コントラストが最高となり、その一方、被検体裏面１７ｂの位置では、２つの光が逆相となり、しかも２つの光の振幅（光強度）が略等しく設定されているので、干渉縞は完全に消去されることになる。
【００２９】
次に、干渉縞のコントラストの変化を表す図１２を用いて、本実施形態の干渉計装置１の作用効果をさらに説明する。図１２に示すように、本実施形態の干渉計装置１においては、干渉縞のコントラストが基準面１６ａおよび被検面１７ａで“山”の状態となるように、かつ被検体裏面１７ａで“谷”の状態となるように設定している。したがって、“山”と“山”が重ね合わされる、基準面１６ａと被検面１７ａからの干渉光はコントラストの良好な干渉縞を形成する。一方、“山”と“谷”が重ね合わされる、基準面１６ａと被検体裏面１７ａからの干渉光、および被検面１７ａと被検体裏面１７ａからの干渉光はコントラストが０の干渉縞を形成することから、干渉縞ノイズは消去されることになる。
【００３０】
以下、本実施形態による干渉計装置１について、計算式を用い、被検面１７ａについての干渉縞のコントラストは良好とし、かつ被検体裏面１７ｂについての干渉縞は消去する条件について説明する。なお、ここでは、簡単のため２つの波長を発生する場合について説明する。
【００３１】
ここで説明する干渉計装置１は、上述した実施形態の構成を基本とし、さらに以下の如き条件を満足している。
【００３２】
（１）半導体レーザ光源１１の注入電流を時間軸に対し矩形波信号（ＣＣＤの１光蓄積期間に対して十分に短い周期を有する）を用いて変調することで２種類の波長の光が交互に出力されるように切り替える。
（２）ＣＣＤの１光蓄積期間において積分された２種類の波長の光の強度が互いに略等しくなるように矩形波信号のデューティー比が調整されている。
【００３３】
これらの条件を満足していることから、１光蓄積期間が１／３０（秒）のＣＣＤにおいては、受光された２種類の波長の光を上記期間に亘り積分し、これら両者の１光蓄積期間における光強度を互いに略等しくすることで、光強度が互いに等しく、波長が互いに異なる２つの光を出力する、一般の２波長レーザ光源（常時２種のレーザ光を出力する光源）を用いた場合と同様の結果が得られることになる。また、上述した如き半導体レーザ光源１１を用いた場合、注入電流を変調する矩形波信号（ファンクション・ジェネレータ２３から出力される制御信号）の振幅を変化させることで、上記２種類の波長の差を変化させたり、ＤＣレベルを変化させたりすることが可能である。
【００３４】
ところで、半導体レーザ光源１１は、注入電流を変化させると波長が変化することは前述したとおりであるが、本実施例のものでは注入電流を１ｍＡ変化させると、モードホップ点以外では、波長が８ｐｍ程度変化する。また、前述したように、矩形波信号により注入電流を変調し、かつ、ＣＣＤの１光蓄積期間よりも十分短い周期で注入電流を走査して２つの波長を生じさせた場合には、いわゆる２波長レーザ光源を用いて干渉縞を生じさせる場合と同様の結果が得られる。このような前提の下、上記２つの波長をλ１、λ２とし、被検面１７ａと被検体裏面１７ｂの光路長差を２Ｌ（＝２ｎＴ、ｎは屈折率、Ｔは厚み）とすると、２種類の波長の光による２つの面からの反射光により生じる干渉縞の強度Ｉ（ｘ，ｙ）は、各光によって生じる干渉縞の強度Ｉ０１（ｘ，ｙ）、Ｉ０２（ｘ，ｙ）の和となり、下式（１）で表される。
【００３５】
【数１】
【数２】
【数３】
【数４】
【数５】
【数６】
【数７】
したがって、上式（８）の条件を満足する場合には、コントラストが良好な干渉縞を得ることができることになる。
【００４３】
【数８】
したがって、上式（１０）の条件を満足する場合には、干渉縞を消すことができることになる。
【００４５】
このことから、波長λと面間隔Ｌが既知の場合、２つの波長の差がΔλとなるように、かつ、ＣＣＤの１光蓄積期間内で得られる２つの波長の光の強度が互いに略等しくなるように設定するだけで、被検体裏面１７ｂからの反射光によって生じる干渉縞を略消去することが可能である。
【００４６】
ここで、被検体１７である平行平面ガラスの屈折率ｎおよび厚みＴを各々、１．５および６ｍｍとし、また、一方の光の波長λ１を６５０ｎｍであるとすれば、
ｍ＝０の場合、
Δλ＝１２ｐｍ
となる。
【００４７】
また、注入電流をＡ（ｍＡ）とした場合、波長差Δλ（ｐｍ）と注入電流の関係式は、近似的に、
Δλ＝８Ａ
となる（ただし、モードホップ点は除く）。
【００４８】
すなわち、１．５ｍＡの振幅の矩形波信号で注入電流を変調すれば、被検体裏面１７ｂからの干渉縞のコントラストを低下できる。ただし、この干渉縞を消去するためには、ＣＣＤに取り込まれた２つの波長の光の強度を略等しくすることが必要となる。上述したように、２つの波長が６５０ｎｍと６５０．０１２ｎｍであるとすれば、これら２つの光の強度比は、略２：３であるから、この場合には矩形波信号のＨレベルとＬレベルのデューティー比を、上記強度比の逆数である３：２程度とし、２つの波長の光の、ＣＣＤにより積分された光強度を略等しくする。なお、上記Ｈレベルとは、注入電流が大きいレベルであることを意味するものであり、逆に、上記Ｌレベルとは、注入電流が小さいレベルであることを意味するものである。
【００４９】
上述した説明は、被検体裏面１７ｂで干渉縞が生じないようにするための条件についての説明であるが、この干渉縞が生じていないことを確認するためには、図１に示す如きフィゾー型干渉計において、基準面１６ａを光軸Ｚに対し傾けたり、基準面１６ａを光路中から取り除く等し、基準面１６ａからの反射光がＣＣＤに到達しないようにした状態で干渉縞を観察すればよい。
【００５０】
なお、上述したようにして各条件を設定した後、基準面１６ａからの反射光の軸と被検体１７の両面１７ａ、１７ｂからの反射光の軸を一致させた後、ＣＣＤ上に干渉縞を生じさせる。
【００５１】
この際には、基準面１６ａから被検面１７ａまでの距離Ｌを上式（８）にしたがって設定しておくことにより、被検面１７ａの形状を表す干渉縞のみを観察することが可能となる。
【００５２】
なお、本発明の干渉計装置としては上記実施形態のものに限られるものではなく、その他の種々の態様の変更が可能である。例えば、上記実施形態と異なり、被検体１７の基準面１６ａとは反対側の面（上記では被検体裏面１７ｂ）を被検面１７ａとすることも可能である。
【００５３】
また、前記干渉計装置１の基準面がフィゾー型の球面参照面を有するように構成することも可能であり、これにより被検体が光学レンズ等の球面形状を有するものについても本発明を適用することが可能となる。
また、上記干渉計装置１が斜入射型の干渉計装置であっても適用可能である。
【００５４】
また、本発明の干渉計装置としてはフィゾー型等の不等光路干渉計装置のみならず、マイケルソン型やマッハツェンダー型等の等光路型干渉計装置に適用することも可能である。
【００５５】
さらに、上記実施形態においては、２種類の波長の光を用いているが、３種類以上の波長の光を用いるようにすれば、被検面についての干渉縞のコントラストをより向上させつつ、非被検面についての干渉縞の消去をより確実に行なうことができる。
さらに、光源としては半導体レーザ光源に限られるものではなく、他のレーザ光源を用いることも可能である。
【００５６】
以下、具体的な実施例について図面を用いて説明する。
【００５７】
＜実施例１＞
実施例１において使用した半導体レーザ光源１１の特性は、以下のようになっている。図２および図３はこの特性を示すグラフである。
【００５８】
（１）波長が連続的に変化し（注入電流の変化１ｍＡに対し波長の変化は６〜９ｐｍ）、かつ注入電流が４ｍＡ以上となるような幅を有する、互いに分離された２つの領域（低波長領域および高波長領域）が存在する。
（２）上記分離された領域の間に１つのモードホップ領域が存在し、そのモードホップ領域の波長変化幅が約０．２５ｎｍである。
【００５９】
この半導体レーザ光源１１の波長特性および光強度特性に関する数値マップを下記表１に示す。
【００６０】
【表１】
被検体１７の板厚を５．６７ｍｍ、屈折率を１．５とすると、被検面１７ａと被検体裏面１７ｂとの光学的厚みｔは８．５ｍｍとなる。
図２に示すように、分離された２つの領域のうち低波長領域の一部についての波長特性および光強度特性に関する数値マップを表２に示す。
【００６２】
【表２】
すなわち、この付近の領域（低波長領域）では、下式が成り立つ。
波長＝７．６（Ａ−３４）＋６５３９０７．６ ｐｍ
【００６４】
ここで、光学的厚みｔが８．５ｍｍである被検体１７の両面からの反射光による干渉縞をキャンセルするには、下記条件を満足することが必要となる。
８．５×２＝（λ２／２）／Δλ
すなわち、波長差Δλは
Δλ＝０．６５３９０７６×０．６５３９０７６／（４×８５００×１０００×１０００）＝１２．６ｐｍ
とすれば、上記干渉縞をキャンセルできる。
【００６５】
これを注入電流に換算すると、
１２．６／７．６＝１．６ｍＡ
となる。すなわち、注入電流を１．６ｍＡ上げれば良いことになる。
よって、条件を満足する２つの波長を得るための注入電流は、
３４ｍＡと３５．６ｍＡ
を選択すれば良いことになる。
【００６６】
また、光強度は、注入電流１ｍＡ上昇する毎に約０．４ｍＷだけ上昇するから、注入電流３４ｍＡのときの光強度が２．７６６であれば、注入電流３５．６ｍＡのときの光強度は３．４０６である。
【００６７】
すなわち、両者のデューティー比を、
１：１．２３
とすればよい。
【００６８】
被検面１７ａの位置は、基準面１６ａから、
１７ｍｍ
の位置に設定すればよい。
【００６９】
なお、入力された、被検体１７の厚みｔおよび屈折率ｎの値に基づき、コンピュータ２０によって被検面１７ａの位置を自動演算し、その演算値に応じて被検体１７が所定の位置に自動的に移動するようなシステムを構築することも可能である。
【００７０】
＜実施例２＞
（モードホップ領域を間に挟む；２波長レーザ光）
次に、下記表３に示す、モードホップ領域を挟んだ２点での干渉縞のキャンセルについて検討する。
【００７１】
【表３】
すなわち、低波長領域に含まれる注入電流３８ｍＡの点と高波長領域に含まれる注入電流４１ｍＡの点との間で干渉縞がキャンセルされる場合の光学距離Ｌは、以下のように表される。
Ｌ＝６５３．９３７３×６５４．２１４／（０．２７６７×４×１０００×１０００）
＝０．３８６ｍｍ
【００７３】
次に、モードホップ領域を挟んだ２つの波長領域の点間での最小波長差（注入電流をｍＡ単位の整数値毎とした場合）は、下記表４に示すものとなる。
【００７４】
【表４】
したがって、低波長領域に含まれる注入電流３９ｍＡの点と高波長領域に含まれる注入電流４０ｍＡの点との間で干渉縞がキャンセルされる場合の光学距離Ｌは、上記と同様に算出して以下のように表される。
Ｌ＝０．４１２ｍｍ
【００７６】
これは、モードホップ領域を挟んだ２点で干渉縞をキャンセルし得る、最大の光学距離となる。ここで、屈折率を１．５とすると、被検体１７の板厚ｔは、
ｔ＝０．２７５ｍｍ
となる。
【００７７】
次に、上記表１において、モードホップ領域を挟んだ２点で干渉縞をキャンセルし得る最小の光学距離を求める。上記表１において、注入電流の最も離れた２点は下記表５に示すものとなる。
【００７８】
【表５】
この場合も、上記と同様にして光学距離Ｌを求めると以下のように表される。
Ｌ＝０．３２３ｍｍ
【００８０】
ここで、屈折率を１．５とすると、被検体１７の板厚ｔは、
ｔ＝０．２１５ｍｍ
となる。
【００８１】
すなわち、上記表１における、モードホップ領域を挟んだ２点で測定できる厚みｔは、下記範囲または下記範囲の２ｍ＋１（ｍ＝０、１、２、３……）倍となる。
ｔ＝０．２１５ｍｍ〜０．２７５ｍｍ
【００８２】
＜実施例３＞
（モードホップ領域なし；２波長レーザ光）
実施例３は、表１に示されるモードホップ領域が存在しない範囲（高波長領域あるいは低波長領域）中の２点を選択して示すものである。
【００８３】
まず、最大の光学距離Ｌは、例えば、注入電流の大きい領域（高波長領域）では、以下のようにして求められる。
まず、下記表６に示すように、注入電流が４１ｍＡと４２ｍＡとなる２点を選択する。
【００８４】
【表６】
すなわち、注入電流４１ｍＡの点と注入電流４２ｍＡの点との間で干渉縞がキャンセルされる場合の光学距離Ｌは、上述した実施例２と同様にして以下のように表される。
Ｌ＝１４．２７ｍｍ
【００８６】
ここで屈折率を１．５とすると、被検体１７の板厚ｔは、
ｔ＝９．５ｍｍ
となる。
【００８７】
一方、最小の光学距離Ｌは、例えば、注入電流の大きい領域（高波長領域）では、以下のようにして求められる。
まず、下記表７に示すように、注入電流が４０ｍＡと４４ｍＡとなる２点を選択する。
【００８８】
【表７】
すなわち、注入電流４０ｍＡの点と注入電流４４ｍＡの点との間で干渉縞がキャンセルされる場合の光学距離Ｌは、上述した場合と同様にして以下のように表される。
Ｌ＝３．２５ｍｍ
【００９０】
ここで屈折率を１．５とすると、被検体１７の板厚ｔは、
ｔ＝２．１７ｍｍ
となる。
【００９１】
すなわち、上記表１における、モードホップ領域を挟まない２点で測定できる厚みｔ（注入電流１ｍＡを最小変調幅とした場合）は、下記範囲または下記範囲の２ｍ＋１（ｍ＝０、１、２、３……）倍となる。
ｔ＝２．１７〜９．５０ｍｍ
【００９２】
＜実施例４＞
（モードホップ領域なし；多波長（３波長以上）レーザ光；各レーザ光の光量は互いに等しい）
実施例４は、表１に示されるモードホップ領域が存在しない範囲（高波長領域あるいは低波長領域）中の３点以上を選択して下記演算を行なうものである。
【００９３】
まず、最大の光学距離Ｌは、例えば、注入電流の小さい領域（低波長領域）では、以下のようにして求められる。
まず、注入電流が３４ｍＡ、３５ｍＡ、３６ｍＡ、３７ｍＡ、３８ｍＡとなる５点を選択する。
この５点についての、この半導体レーザ光源１１の波長特性および光強度特性に関する数値マップを下記表８に示す。
【００９４】
【表８】
上記表８に示すように、上記５つの注入電流により各々互いに異なる波長のレーザ光が発生することになるが、それらの光強度比は１：１．１５：１．２８：１．４３：１．５６となるから、１光蓄積期間内にＣＣＤにおいて受光される各レーザ光の強度を等しくしたい場合には、デューティ比は１．５６：１．４３：１．２８：１．１５：１とすればよい。
【００９６】
図４は、このようにして決定されたデューティ比により階段状に変化する注入電流の変化を示すものであり、図５はＣＣＤの１光蓄積期間内で積分された５つの波長の光の強度が互いに等しくなることを示すものである。
【００９７】
このように注入電流を変化させることにより、不用な干渉縞を良好に消去することができる。
【００９８】
＜実施例５＞
（モードホップ領域なし；多波長（３波長以上）レーザ光；１光蓄積期間に亘り積分して得られる光強度が各レーザ光毎に異なる）
次に、上記５点の中心となる、注入電流３６ｍＡの点についての光強度を３とおき、その両脇の点である注入電流３５ｍＡおよび３７ｍＡの光強度を２、さらにその両脇の点である注入電流３４ｍＡおよび３８ｍＡの光強度を１というようにして、全体として三角波状のスペクトル分布をとるようにした場合には、各注入電流についてのデューティー比は、下記表９に示す如くなる。
【００９９】
【表９】
図６は、この場合におけるＣＣＤの１光蓄積期間内で積分された５つの波長の光の強度を示すものである。
図７は、このようにして決定されたデューティ比により階段状に変化する注入電流の変化を示すものである。
このように、３波長以上の光を用いる場合には、注入電流を変化させることによっても、不用な干渉縞を良好に消去することができる。
【０１０１】
＜得られた干渉縞画像＞
図９は、本実施形態（実施例１、２）を用いて得られた干渉縞画像を示すものであり、図１０は、従来技術を用いて得られた干渉縞画像を示すものである。これら２つの干渉縞画像の比較から明らかなように、本実施形態においては、干渉縞ノイズのないコントラストの良好な干渉縞画像を得ることができる。
【０１０２】
【発明の効果】
本発明の干渉計装置においては、以下の特徴点を備えている。
すなわち、▲１▼単一縦モードの半導体レーザ光源を用い、干渉縞を受光する素子の１光蓄積期間に対し十分短い周期で光源から出力される光を複数の波長に変調し、被検体からの干渉光を上記素子により受光することで、その干渉光を上記１光蓄積期間に亘って積分する点、および▲２▼波長の変調は、上記１光蓄積期間に亘って積分された各波長の光が被検体の非被検面からの干渉縞ノイズを抑制し得る所定の光強度と各々なるように、この各波長に対する変調期間が設定されている点、である。
【０１０３】
したがって、干渉縞を受光する素子は、所定の光蓄積期間を有しているため、その１光蓄積期間よりも十分速い速度で波長を走査すれば、多波長の光を同時に出力する光源を用いて干渉縞を観察する場合と同様の結果が得られる。
しかも、注入電流が変化すれば光強度も変化することに着目し、注入電流の変化に対する光強度の変化についての関係を考慮して光源の注入電流を制御するようにしているから、被検面についての干渉縞のコントラストを最高としつつ、非被検面についての干渉縞（干渉縞ノイズ）の消去をより確実に行なうことができる。
【図面の簡単な説明】
【図１】本発明の実施形態に係る干渉計装置の概略図
【図２】本発明の実施形態に係る半導体レーザ光源の注入電流と出力光波長の関係を示すグラフ
【図３】本発明の実施形態に係る半導体レーザ光源の注入電流と出力光の強度の関係を示すグラフ
【図４】実施例４における半導体レーザ光源の注入電流の変化を示すグラフ
【図５】実施例４におけるＣＣＤの１光蓄積期間に亘って積分された５つの波長の光の強度を示すグラフ
【図６】実施例５におけるＣＣＤの１光蓄積期間に亘って積分された５つの波長の光の強度を示すグラフ
【図７】実施例５における半導体レーザ光源の注入電流の変化を示すグラフ
【図８】公知文献記載の干渉計装置を用いた場合の作用効果を概念的に示す図（Ａ）および本実施形態の干渉計装置を用いた場合の作用効果を概念的に示す図（Ｂ）
【図９】本実施形態の干渉計装置を用いて得られた干渉縞画像
【図１０】従来技術の干渉計装置を用いて得られた干渉縞画像
【図１１】公知文献記載の干渉計装置の半導体レーザ光源の注入電流の変化を示すグラフ
【図１２】本実施形態の干渉計装置の作用を示す図
【符号の説明】
１ 干渉計装置
１０ 干渉計本体
１１ 半導体レーザ光源（ＬＤ）
１２ コリメータレンズ
１３ 発散レンズ
１４ ビームスプリッタ
１５ コリメータレンズ
１６ 基準板
１６ａ 基準面
１７ 平行平面ガラス板（被検体）
１７ａ 被検面
１７ｂ 被検体裏面
１８ 撮像レンズ
１９ ＣＣＤ撮像装置
２０ コンピュータ
２１ モニタ
２２ 電源（ＬＤ電源）
２３ ファンクション・ジェネレータ
２４ ピエゾ素子
３０ レーザ光[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an interferometer apparatus for observing a wavefront shape from a surface to be inspected, and more particularly to an unequal optical path type interferometer apparatus such as a Fizeau type, and more particularly, to an object having two parallel surfaces such as a glass plate. The present invention relates to an interferometer device capable of suppressing interference fringe noise from a non-test surface superimposed on interference fringes of a test surface when one of the surfaces is a test surface.
[0002]
[Prior art]
For example, it has been conventionally known to use a Fizeau interferometer equipped with a laser light source having good coherence as a technique for measuring the surface shape of parallel plane glass. Since the laser beam is used, not only the interference fringes on the test surface of the parallel flat glass but also the interference fringes on the non-test surface opposite to the test surface (hereinafter, simply referred to as non-test surface) are generated. It happens at the same time. That is, in a Fizeau interferometer (unequal optical path interferometer) in which the optical path length difference between the reference light and the object light is different from each other, it is essential to use a laser beam having good coherence, so Light interference from the surface, the reference surface and the non-test surface, and the light interference from the test surface and the non-test surface occur. Normally, the desired interference fringes are only light interference from the reference surface and the test surface, so that interference fringes caused by optical interference between other surfaces become noise, and the shape of the test surface must be measured with high accuracy. Becomes difficult.
[0003]
Therefore, conventionally, as a method of suppressing such interference fringe noise, a non-test surface is coated with a refractive index matching oil, and a light scattering sheet is stuck thereon, thereby scattering the reflected light from the non-test surface. It has been practiced to prevent the occurrence of interference fringes due to mutual optical interference between the non-test surface and other surfaces.
[0004]
However, such an interference fringe noise suppression method has a disadvantage that, although it is a non-test surface, oil must be applied to one surface of the test object, which is not only troublesome, but also causes the test object to become dirty. In addition, in the case of a thin subject, there is a possibility that the shape of the test surface itself may be changed by a process such as applying oil or attaching a light scattering sheet.
[0005]
Therefore, as a method of solving such a problem, a light source of a Fizeau interferometer that oscillates light having a coherence function such that interference fringes periodically occur in the optical axis direction around a reference plane position is known. Have been. As the light source, a multimode semiconductor laser or a plurality of semiconductor lasers is used (US Pat. No. 5,452,088).
[0006]
According to this interferometer apparatus, fringes occur at the position of the test surface, and fringes do not occur at the position of the non-test surface that is separated from the test surface by a predetermined distance in the optical axis direction. Can be observed.
[0007]
[Problems to be solved by the invention]
However, in the method described in the above-mentioned US Patent Publication, the longitudinal mode is determined for each semiconductor laser light source, and the fringe period of the interference fringe is determined for each light source. In the case of the above, it is difficult to remove the interference fringe noise on the non-test surface while improving the interference fringe on the test surface.
[0008]
The present invention has been made in view of the above circumstances, and one of two surfaces facing each other, such as a front surface and a back surface of a transparent object, is a surface to be inspected, and the other surface is not an object surface ( (Non-test surface), a high-precision and inexpensive interferometer device capable of improving the contrast of interference fringes on the test surface and suppressing interference fringes on a surface that is not the test surface. The purpose is to provide.
[0009]
[Means for Solving the Problems]
The interferometer apparatus of the present invention uses a light source capable of wavelength scanning that oscillates laser light in a single longitudinal mode, and in an interferometer apparatus that suppresses interference fringe noise from a non-test surface of a transparent subject,
Modulating the laser light from the light source into a plurality of wavelengths in a cycle sufficiently short for one light accumulation period of the element that receives the interference fringes;
Using the laser light modulated into the plurality of wavelengths, the object light from the subject and interference light generated by reference light from a reference plane of the interferometer device are received by the element, and the interference light is received by the element. It is configured to integrate in one light accumulation period,
The modulation of the plurality of wavelengths is performed such that the light of each wavelength integrated in the one light accumulation period has a predetermined light intensity capable of suppressing interference fringe noise from a non-test surface of the subject. A modulation period for a wavelength is set.
[0010]
Further, the light source is a laser light source that performs the wavelength modulation based on a change in the injection current, and the signal waveform of the injection current for performing the wavelength modulation is at least the injection current and the output of the laser light source. Is determined based on two factors of the desired spectrum shape of the light intensity with respect to the relationship and the time axis.
[0011]
Further, it is more preferable that a signal waveform of an injection current for performing the wavelength modulation is determined in consideration of an element regarding an attenuation rate of a light amount of the laser light from the light source to the element.
[0012]
In addition, it is preferable that the signal waveform of the injection current is formed to have a rectangular wave shape having a duty ratio determined based on each of the elements.
[0013]
Further, when the plurality of wavelengths are two wavelengths, the modulation period for the laser light of the two wavelengths is set so that the light intensity of each laser light integrated in the one light accumulation period is substantially equal to each other. Can be.
[0014]
Preferably, the laser light source is a semiconductor laser light source.
[0015]
Further, the interferometer device can be of an unequal optical path type such as a Fizeau type, which is particularly useful in this case.
Note that the “non-test surface” refers to an optical surface other than the test surface in the subject.
[0016]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, an interferometer device according to an embodiment of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic diagram illustrating a configuration of the interferometer apparatus according to the embodiment.
[0017]
As shown in the figure, the interferometer apparatus 1 includes a Fizeau-
[0018]
The
[0019]
In the interferometer
[0020]
The imaged interference fringes are analyzed by the
[0021]
The reference plate 16a is supported by a reference plate support member (not shown) via a
[0022]
The semiconductor
[0023]
The
The control signal output from the
[0024]
By the way, in the interferometer apparatus 1 having an unequal optical path such as a Fizeau type, since the optical path lengths of the reference light from the reference surface 16a and the object light from the test surface 17a are different from each other, light having good coherence is used. In general, laser light is used as illumination light as in the present embodiment. However, when the light having good coherence is used as the illumination light in FIG. 1, not only the reference surface 16a and the test surface 17a, but also the reference surface 16a and the test object back surface 17b, and the test surface 17a Light interference from the subject back surface 17b also occurs. Therefore, interference fringes caused by light interference other than the light interference from the reference surface 16a and the test surface 17a become noise, and it becomes difficult to measure the shape of the test surface 17a with high accuracy. In order to prevent the occurrence of fringe noise, the interferometer apparatus 1 of the present embodiment has the following features.
[0025]
That is, the main features of the configuration of the interferometer apparatus 1 of the present embodiment are as follows: (1) One of the elements (CCD of the CCD imaging device 19) that uses the semiconductor
[0026]
Incidentally, the semiconductor laser light source has a feature that the wavelength changes by changing the injection current. Since the element that receives the interference fringes has a predetermined light accumulation period, if the wavelength is scanned at a speed sufficiently faster than the one light accumulation period, interference using a light source that outputs light of multiple wavelengths simultaneously is performed. The same result as when observing fringes is obtained. Based on such knowledge, a method of synthesizing a coherence function is shown in, for example, May 1995, Lightwave Sensing Proceedings, pp. 75-82. According to this method, the injection current is controlled by a control signal obtained by changing a rectangular wave up and down around a reference level (DC level) while changing the injection current in a ramp shape (see FIG. 11; representing the change in the injection current). Thus, a desired coherence function is generated. This corresponds to (1) described above as a feature of the present embodiment. However, according to such a method having only the feature point (1), the contrast of the interference fringe noise described above does not become 0, and the interference fringe noise cannot be satisfactorily suppressed.
[0027]
The inventor of the present invention cannot appropriately suppress interference fringe noise in the above-described method because the shape of the above-described rectangular wave (a pair of an upward rectangular wave and a downward rectangular wave adjacent to each other) is shown in FIG. As shown, the heights are equal to each other above and below the reference level (DC level), and it has been concluded that there is no consideration that the light intensity changes when the injection current changes. It has been found that interference fringe noise can be satisfactorily suppressed by determining the shape of this rectangular wave in consideration of the relationship between the change in light intensity and the change in current (for example, based on Wiener-Kinchin's theorem). Was.
[0028]
This will be conceptually described with reference to FIGS. First, the technology described in the above-mentioned proceedings will be described with reference to FIG. A , Λ B Are in phase with each other at the position of the test surface 17a and have the highest interference fringe contrast. However, at the position of the subject back surface 17b, they have opposite phases and the contrast of the interference fringes is weakened. Are different from each other, interference fringes cannot be completely eliminated. On the other hand, in the interferometer apparatus 1 of the present invention, as shown in FIG. 8B, at the position of the test surface 17a, the two lights are in phase, and the interference fringe contrast becomes the highest. At the position of the subject back surface 17b, since the two lights have opposite phases and the amplitudes (light intensities) of the two lights are set to be substantially equal, the interference fringes are completely eliminated.
[0029]
Next, the operation and effect of the interferometer apparatus 1 of the present embodiment will be further described with reference to FIG. 12 showing a change in contrast of interference fringes. As shown in FIG. 12, in the interferometer apparatus 1 of the present embodiment, the contrast of the interference fringes is in a “peak” state on the reference surface 16a and the test surface 17a, and is “valley” on the back surface 17a of the test object. "Is set. Therefore, the interference light from the reference surface 16a and the test surface 17a, where the “mountains” and the “mountains” overlap, forms interference fringes with good contrast. On the other hand, the interference light from the reference surface 16a and the back surface 17a of the subject and the interference light from the test surface 17a and the back surface 17a of the subject, where the "peak" and "valley" overlap, form interference fringes having a contrast of 0. Therefore, the interference fringe noise is eliminated.
[0030]
Hereinafter, conditions of the interferometer apparatus 1 according to the present embodiment, which use a calculation formula, make the contrast of the interference fringe on the test surface 17a good, and erase the interference fringe on the back surface 17b of the subject will be described. Here, a case where two wavelengths are generated will be described for simplicity.
[0031]
The interferometer device 1 described here is based on the configuration of the above-described embodiment, and further satisfies the following conditions.
[0032]
(1) Light of two wavelengths is alternated by modulating the injection current of the semiconductor
(2) The duty ratio of the rectangular wave signal is adjusted such that the intensities of the two wavelengths of light integrated during one light accumulation period of the CCD are substantially equal to each other.
[0033]
Since these conditions are satisfied, in a CCD in which one light accumulation period is 1/30 (second), received light of two kinds of wavelengths is integrated over the above period, and one light accumulation of these two is performed. A general two-wavelength laser light source (a light source that constantly outputs two types of laser light) that outputs two lights having the same light intensity and different wavelengths by making the light intensities in the periods substantially equal to each other was used. The same result as in the case is obtained. When the semiconductor
[0034]
As described above, the semiconductor
[0035]
(Equation 1)
(Equation 2)
[Equation 3]
(Equation 4)
(Equation 5)
(Equation 6)
(Equation 7)
Therefore, when the condition of the above equation (8) is satisfied, it is possible to obtain an interference fringe with good contrast.
[0043]
(Equation 8)
Therefore, when the condition of the above equation (10) is satisfied, interference fringes can be eliminated.
[0045]
From this, when the wavelength λ and the surface interval L are known, the difference between the two wavelengths is set to Δλ, and the intensities of the two wavelengths of light obtained within one light accumulation period of the CCD are substantially equal to each other. The interference fringes generated by the reflected light from the back surface 17b of the subject can be substantially eliminated simply by setting such that.
[0046]
Here, the refractive index n and the thickness T of the parallel flat glass as the subject 17 are 1.5 and 6 mm, respectively, and the wavelength λ of one light 1 Is 650 nm,
If m = 0,
Δλ = 12 pm
It becomes.
[0047]
When the injection current is A (mA), the relational expression between the wavelength difference Δλ (pm) and the injection current is approximately:
Δλ = 8A
(However, the mode hop point is excluded).
[0048]
That is, if the injection current is modulated with a rectangular wave signal having an amplitude of 1.5 mA, the contrast of interference fringes from the back surface 17b of the subject can be reduced. However, in order to eliminate the interference fringes, it is necessary to make the intensities of the two wavelengths of light taken into the CCD substantially equal. As described above, if the two wavelengths are 650 nm and 650.12 nm, the intensity ratio of these two lights is approximately 2: 3. In this case, the H level and the L level of the rectangular wave signal are used. Is set to about 3: 2, which is the reciprocal of the above intensity ratio, to make the light intensities of the light of the two wavelengths integrated by the CCD substantially equal. The H level means that the injected current is at a high level, and the L level means that the injected current is at a low level.
[0049]
The above description is about the conditions for preventing the occurrence of interference fringes on the back surface 17b of the subject. In order to confirm that the interference fringes do not occur, a Fizeau type as shown in FIG. In the interferometer, if the reference plane 16a is inclined with respect to the optical axis Z, or the reference plane 16a is removed from the optical path, and the interference fringes are observed in a state where the reflected light from the reference plane 16a does not reach the CCD, Good.
[0050]
After setting the conditions as described above, the axis of the reflected light from the reference surface 16a is made to coincide with the axis of the reflected light from both surfaces 17a and 17b of the subject 17, and then interference fringes are formed on the CCD. Cause.
[0051]
At this time, by setting the distance L from the reference surface 16a to the test surface 17a according to the above equation (8), it is possible to observe only the interference fringes representing the shape of the test surface 17a. Become.
[0052]
Note that the interferometer device of the present invention is not limited to the above-described embodiment, and other various aspects can be changed. For example, unlike the above-described embodiment, the surface of the subject 17 opposite to the reference surface 16a (the subject back surface 17b in the above example) may be the test surface 17a.
[0053]
Further, the reference surface of the interferometer device 1 may be configured to have a Fizeau-type spherical reference surface, whereby the present invention is also applied to an object having a spherical shape such as an optical lens. It becomes possible.
Further, the present invention is applicable even when the interferometer device 1 is a grazing incidence type interferometer device.
[0054]
The interferometer device of the present invention can be applied not only to an unequal optical path interferometer device such as a Fizeau type but also to an equal optical path interferometer device such as a Michelson type or a Mach-Zehnder type.
[0055]
Further, in the above embodiment, light of two types of wavelengths is used. However, if light of three or more types of wavelengths is used, the contrast of interference fringes on the surface to be measured can be further improved, Elimination of interference fringes on the test surface can be performed more reliably.
Further, the light source is not limited to the semiconductor laser light source, and other laser light sources can be used.
[0056]
Hereinafter, specific examples will be described with reference to the drawings.
[0057]
<Example 1>
The characteristics of the semiconductor
[0058]
(1) Two regions separated from each other (the low region where the wavelength changes continuously (the change in the wavelength is 6 to 9 pm with respect to the change in the injection current of 1 mA) and the injection current is 4 mA or more). Wavelength region and high wavelength region).
(2) One mode hop region exists between the separated regions, and the wavelength change width of the mode hop region is about 0.25 nm.
[0059]
Table 1 below shows a numerical map relating to the wavelength characteristics and the light intensity characteristics of the semiconductor
[0060]
[Table 1]
Assuming that the plate thickness of the subject 17 is 5.67 mm and the refractive index is 1.5, the optical thickness t of the test surface 17a and the subject back surface 17b is 8.5 mm.
As shown in FIG. 2, Table 2 shows a numerical map relating to the wavelength characteristics and the light intensity characteristics of a part of the low wavelength region of the two separated regions.
[0062]
[Table 2]
That is, in the area near this (low-wavelength area), the following equation is established.
Wavelength = 7.6 (A-34) +653907.6 pm
[0064]
Here, in order to cancel interference fringes due to reflected light from both surfaces of the subject 17 whose optical thickness t is 8.5 mm, the following conditions must be satisfied.
8.5 × 2 = (λ 2 / 2) / Δλ
That is, the wavelength difference Δλ is
Δλ = 0.6539076 × 0.6539076 / (4 × 8500 × 1000 × 1000) = 12.6 pm
Then, the interference fringes can be canceled.
[0065]
Converting this to injection current,
12.6 / 7.6 = 1.6 mA
It becomes. That is, it is sufficient to increase the injection current by 1.6 mA.
Therefore, the injection current for obtaining two wavelengths satisfying the condition is:
34mA and 35.6mA
You just have to select
[0066]
Since the light intensity increases by about 0.4 mW every time the injection current increases by 1 mA, if the light intensity at the injection current of 34 mA is 2.766, the light intensity at the injection current of 35.6 mA becomes 3 .406.
[0067]
That is, the duty ratio of both
1: 1.23
And it is sufficient.
[0068]
The position of the test surface 17a is determined from the reference surface 16a.
17mm
Should be set to the position.
[0069]
The position of the test surface 17a is automatically calculated by the
[0070]
<Example 2>
(The mode hop region is sandwiched between the two wavelength laser beams)
Next, cancellation of interference fringes at two points sandwiching the mode hop region shown in Table 3 below will be examined.
[0071]
[Table 3]
That is, the optical distance L when interference fringes are canceled between the point of the injection current 38 mA included in the low wavelength region and the point of the injection current 41 mA included in the high wavelength region is expressed as follows.
L = 653.9373 × 654.214 / (0.2767 × 4 × 1000 × 1000)
= 0.386mm
[0073]
Next, the minimum wavelength difference between the points of the two wavelength regions sandwiching the mode hop region (when the injection current is set to an integer value in mA units) is as shown in Table 4 below.
[0074]
[Table 4]
Therefore, the optical distance L when the interference fringe is canceled between the point of the injection current 39 mA included in the low wavelength region and the point of the injection current 40 mA included in the high wavelength region is calculated in the same manner as described above. Is represented as
L = 0.412mm
[0076]
This is the maximum optical distance at which interference fringes can be canceled at two points across the mode hop region. Here, assuming that the refractive index is 1.5, the plate thickness t of the subject 17 is
t = 0.275 mm
It becomes.
[0077]
Next, in Table 1 above, the minimum optical distance at which interference fringes can be canceled at two points across the mode hop region is determined. In Table 1 above, the two farthest points of the injection current are as shown in Table 5 below.
[0078]
[Table 5]
Also in this case, when the optical distance L is obtained in the same manner as described above, it is expressed as follows.
L = 0.323mm
[0080]
Here, assuming that the refractive index is 1.5, the plate thickness t of the subject 17 is
t = 0.215 mm
It becomes.
[0081]
That is, in Table 1, the thickness t that can be measured at two points sandwiching the mode hop region is the following range or 2m + 1 (m = 0, 1, 2, 3,...) Times the following range.
t = 0.215mm to 0.275mm
[0082]
<Example 3>
(No mode hop region; dual wavelength laser light)
In the third embodiment, two points in the range (high wavelength region or low wavelength region) where the mode hop region shown in Table 1 does not exist are selected and shown.
[0083]
First, the maximum optical distance L is obtained as follows, for example, in a region where injection current is large (high wavelength region).
First, as shown in Table 6 below, two points where the injection current is 41 mA and 42 mA are selected.
[0084]
[Table 6]
That is, the optical distance L in the case where the interference fringe is canceled between the point of the injection current 41 mA and the point of the injection current 42 mA is expressed as follows in the same manner as in the second embodiment.
L = 14.27 mm
[0086]
Here, assuming that the refractive index is 1.5, the plate thickness t of the subject 17 is
t = 9.5 mm
It becomes.
[0087]
On the other hand, the minimum optical distance L is obtained as follows, for example, in a region where injection current is large (high wavelength region).
First, as shown in Table 7 below, two points where the injection current is 40 mA and 44 mA are selected.
[0088]
[Table 7]
That is, the optical distance L when the interference fringe is canceled between the point of the injection current of 40 mA and the point of the injection current of 44 mA is expressed as follows in the same manner as in the above case.
L = 3.25 mm
[0090]
Here, assuming that the refractive index is 1.5, the plate thickness t of the subject 17 is
t = 2.17mm
It becomes.
[0091]
That is, in Table 1 above, the thickness t that can be measured at two points not sandwiching the mode hop region (when the injection current of 1 mA is the minimum modulation width) is in the following range or 2m + 1 (m = 0, 1, 2,. 3 ...)).
t = 2.17 to 9.50 mm
[0092]
<Example 4>
(No mode hop region; multi-wavelength (three or more wavelengths) laser light; the amount of each laser light is equal to each other)
In the fourth embodiment, the following calculation is performed by selecting three or more points in the range (high wavelength region or low wavelength region) where the mode hop region shown in Table 1 does not exist.
[0093]
First, the maximum optical distance L is obtained as follows, for example, in a region where injection current is small (low wavelength region).
First, five points at which the injection current is 34 mA, 35 mA, 36 mA, 37 mA, and 38 mA are selected.
Table 8 below shows numerical maps relating to the wavelength characteristics and the light intensity characteristics of the semiconductor
[0094]
[Table 8]
As shown in Table 8, the five injection currents generate laser beams having different wavelengths, respectively, and their light intensity ratios are 1: 1.15: 1.28: 1.43: 1. .56, the duty ratio is 1.56: 1.43: 1.28: 1.15: 1 to make the intensity of each laser beam received by the CCD within one light accumulation period equal. do it.
[0096]
FIG. 4 shows the change of the injection current which changes stepwise according to the duty ratio determined in this manner. FIG. 5 shows the intensity of light of five wavelengths integrated within one light accumulation period of the CCD. Are equal to each other.
[0097]
By changing the injection current in this way, unnecessary interference fringes can be satisfactorily erased.
[0098]
<Example 5>
(No mode hop region; multi-wavelength (three or more wavelengths) laser light; light intensity obtained by integrating over one light accumulation period differs for each laser light)
Next, the light intensity at the point of the injection current of 36 mA, which is the center of the above five points, is set to 3, the light intensity of the injection currents of 35 mA and 37 mA, which are both sides thereof, is set to 2, and the points at both sides thereof are further set. When the light intensity at certain injection currents of 34 mA and 38 mA is set to 1 to obtain a triangular wave spectrum distribution as a whole, the duty ratio for each injection current is as shown in Table 9 below.
[0099]
[Table 9]
FIG. 6 shows the light intensities of five wavelengths integrated within one light accumulation period of the CCD in this case.
FIG. 7 shows a change in the injection current that changes stepwise according to the duty ratio determined in this way.
As described above, when light having three or more wavelengths is used, unnecessary interference fringes can be satisfactorily eliminated by changing the injection current.
[0101]
<The obtained interference fringe image>
FIG. 9 shows an interference fringe image obtained using the present embodiment (Examples 1 and 2), and FIG. 10 shows an interference fringe image obtained using the conventional technique. As is apparent from the comparison between these two interference fringe images, in the present embodiment, it is possible to obtain an interference fringe image with good contrast without interference fringe noise.
[0102]
【The invention's effect】
The interferometer device of the present invention has the following features.
That is, (1) a semiconductor laser light source of a single longitudinal mode is used, and light output from the light source is modulated into a plurality of wavelengths in a period sufficiently short for one light accumulation period of an element for receiving an interference fringe, so that the light is emitted from the subject. And (2) the wavelength modulation is performed by receiving the interference light of the above-mentioned elements by the above-mentioned element and integrating the interference light over the above-mentioned one light accumulation period. The modulation period for each of the wavelengths is set so that each of the light beams has a predetermined light intensity capable of suppressing interference fringe noise from the non-test surface of the subject.
[0103]
Therefore, since the element that receives the interference fringes has a predetermined light accumulation period, if the wavelength is scanned at a speed sufficiently faster than the one light accumulation period, a light source that outputs light of multiple wavelengths at the same time is used. To obtain the same result as when observing interference fringes.
In addition, focusing on the fact that the light intensity changes when the injection current changes, the injection current of the light source is controlled in consideration of the relationship between the change in the light intensity and the change in the injection current. , The interference fringes (interference fringe noise) on the non-test surface can be more reliably eliminated while maximizing the contrast of the interference fringes.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of an interferometer apparatus according to an embodiment of the present invention.
FIG. 2 is a graph showing a relationship between an injection current and an output light wavelength of the semiconductor laser light source according to the embodiment of the present invention.
FIG. 3 is a graph showing a relationship between an injection current and an output light intensity of the semiconductor laser light source according to the embodiment of the present invention.
FIG. 4 is a graph showing a change in injection current of a semiconductor laser light source in Example 4.
FIG. 5 is a graph showing light intensities of five wavelengths integrated over one light accumulation period of a CCD in Example 4.
FIG. 6 is a graph showing the intensity of light of five wavelengths integrated over one light accumulation period of a CCD in Example 5.
FIG. 7 is a graph showing a change in injection current of a semiconductor laser light source in Example 5.
FIG. 8A is a diagram conceptually showing the operation and effect when an interferometer device described in a known document is used, and FIG. )
FIG. 9 is an interference fringe image obtained using the interferometer apparatus of the present embodiment.
FIG. 10 shows an interference fringe image obtained using a conventional interferometer apparatus.
FIG. 11 is a graph showing a change in injection current of a semiconductor laser light source of an interferometer device described in a known document.
FIG. 12 is a view showing the operation of the interferometer device of the embodiment.
[Explanation of symbols]
1 Interferometer device
10 Interferometer body
11 Semiconductor laser light source (LD)
12 Collimator lens
13 Divergent lens
14 Beam splitter
15 Collimator lens
16 Reference plate
16a Reference plane
17 parallel flat glass plate (subject)
17a Test surface
17b Back side of subject
18 Imaging lens
19 CCD imaging device
20 computers
21 Monitor
22 Power supply (LD power supply)
23 Function Generator
24 Piezo element
30 Laser light
Claims (7)
干渉縞を受光する素子の１光蓄積期間に対し十分短い周期で、前記光源からのレーザ光を複数の波長に変調し、
該複数の波長に変調されたレーザ光を用いて前記被検体からの物体光と前記干渉計装置の基準面からの参照光により生成される干渉光を前記素子により受光して、該干渉光を前記１光蓄積期間で積分するように構成し、
前記複数の波長の変調は、前記１光蓄積期間で積分された各波長の光が前記被検体の非被検面からの干渉縞ノイズを抑制し得る所定の光強度となるように、該各波長に対する変調期間が設定されていることを特徴とする干渉計装置。In an interferometer device that uses a light source capable of wavelength scanning that oscillates laser light in a single longitudinal mode and suppresses interference fringe noise from a non-test surface of a transparent subject,
Modulating the laser light from the light source into a plurality of wavelengths in a cycle sufficiently short for one light accumulation period of the element that receives the interference fringes;
Interfering light generated by the object light from the subject and reference light from the reference plane of the interferometer device is received by the element using the laser light modulated to the plurality of wavelengths, and the interference light is received. A configuration in which integration is performed in the one light accumulation period,
The modulation of the plurality of wavelengths is performed such that the light of each wavelength integrated in the one light accumulation period has a predetermined light intensity capable of suppressing interference fringe noise from a non-test surface of the subject. An interferometer apparatus wherein a modulation period for a wavelength is set.
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