レンズやプリズムといった光学素子の様々な機能は波面形状の変化でも説明することができます。
これは、一般的に多くの光学素子は光線の向きを変化させて光の進行方向を変える機能を持ちますが、この光線は波面に対して垂直な線として定義できるためです。
LCOS-SLMでは、出力光の波面形状を任意に制御できるため、様々な光学素子と同じ機能を実現できます。また、動的な波面制御も可能なので、複数の光学素子の機能を組み合わせたり、異なる機能に変更したりすることもできます。このような波面制御のためには、目的に応じた位相分布パターンをLCOS-SLMに入力することが必要です。
ここではLCOS-SLMによる光の波面制御で実現できる、代表的な光学機能の例を紹介します。
通常、レーザ光はレンズを用いると1点に集光されますが、LCOS-SLMを使用することで、レーザ光を分岐させて複数の異なる位置に集光できます。この機能によって、レーザ光を様々な位置に同時照射したり、任意の形状で広範囲に照射する「パターン化照射」を実現できます。レーザ加工では、この機能を活用して複数箇所や広範囲を同時に加工することができます。また、顕微鏡では異なる部位の同時計測にも応用可能です。
LCOS-SLMの液晶部分に、照射したい形状に対応するようにコンピュータで計算した「計算機生成ホログラム」(CGH:Computer Generated Hologram)を表示することで、1点に集光されるはずのレーザ光を任意形状のパターンで照射することができます。
LCOS-SLMを用いたビーム分岐の模式図
パターン化照射の例
LCOS-SLMに入射したレーザ光に球面波を加えることで集光位置を光軸方向に、傾斜波面を加えることで光軸と垂直な方向に移動させることができます。これらの波面を組み合わせることで、3次元的な集光位置の制御も可能です。
また、入射光に円錐状の位相分布を加えると、光軸方向に長く伸びた集光形状が得られます。この集光形状は、レーザ加工で細長い穴を成形することが期待できます。らせん状の位相分布を加えると、集光形状がドーナツ状になります。STED顕微鏡では、中心穴が非常に小さなドーナツ状のレーザ光によって、通常の光学システムを超える解像度が実現されています。
LCOS-SLMを用いたビーム形状制御の模式図
ビーム形状制御の例
光学素子は理想的な状態からずれているため、光が通過すると位相が不均一になり、波面が歪むことがあります。この歪みは「収差」と呼ばれます。収差が発生するとレンズによる集光点がきれいな点にならなかったり、投影したパターンが不鮮明になる場合があります。しかし、LCOS-SLMで収差を打ち消すように波面を補正することで、理想的な波面状態に戻すことができます。この機能により、高精度・高効率なレーザ微細加工や精緻な顕微鏡観察が可能になります。
LCOS-SLMを用いた収差補正の模式図
側面から観察した集光ビームにおける収差補正前後の比較
眼底カメラでの収差補正前後の比較
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