JP2004157404A

JP2004157404A - Diffraction optical element and method for manufacturing diffraction optical element, and optical system using diffraction optical element

Info

Publication number: JP2004157404A
Application number: JP2002324288A
Authority: JP
Inventors: Toru Nakamura; 徹中村
Original assignee: Nikon Corp
Current assignee: Nikon Corp
Priority date: 2002-11-07
Filing date: 2002-11-07
Publication date: 2004-06-03

Abstract

<P>PROBLEM TO BE SOLVED: To provide a diffraction optical element which has high diffraction efficiency and little flare light even if the shapes of crests of peaks or bottoms of valleys of grating surfaces are rounded, a method for manufacturing the same, and an optical system having the diffraction optical element. <P>SOLUTION: The diffraction optical element has diffraction gratings and at least either of the peaks 5 or valleys 6 of the grating surfaces of the diffraction gratings are rounded. When the use wavelength is defined as λ, the refractive index of the diffraction gratings as n1, the refractive index of a medium adjacent to the grating surfaces as n2, and (m) as an integer, a difference S in level between the crests of the peaks and the bottoms of the valleys of the grating surfaces is given by equation (1)1. The equation (1) is S=mxλ/(n1-n2). <P>COPYRIGHT: (C)2004,JPO

Description

【０００１】
【発明の属する技術分野】
本発明は、カメラ等の光学装置に用いられる回折格子を具えた回折光学素子、及び回折光学素子の製造方法、及びこの回折光学素子を具えた光学系に関する。
【０００２】
【従来の技術】
図８はブレーズ型の回折光学素子の理想的な形状を示す。図２はこのブレーズ型回折光学素子が具えるブレーズ型回折格子の理想的な格子面の断面を示す。図２に示されるように、ブレーズ型回折格子は、格子面の断面形状が鋸歯状である。その山の頂上と谷の底とは共に刃先状とされ、理想的には山の頂上と谷の底の曲率半径は共に零である。このような理想形状のブレーズ型回折格子は、特定次数に対して使用波長の光の回折効率を理論上１００％にすることができ、最大の回折効率は、式２が成立するときに得られることが知られている。
（式２）Ｌ×（ｎ１−ｎ２）＝ｍ×λ
ここで、λは使用波長であり、通常は回折効率を最適化して最大にする対象の光の波長とされる。Ｌは理想形状のブレーズ型回折格子の格子面の格子高、ｎ１は回折格子の基板の屈折率、ｎ２は格子面に隣接する媒質の屈折率、ｍは回折次数を表す整数である。
【０００３】
従って、従来、ブレーズ型回折光学素子のブレーズ型回折格子の製造に当たっては、先ず、式２に所望の使用波長λ、製造対象の回折格子の基板の屈折率ｎ１、使用媒質の屈折率ｎ２、所望の回折次数ｍを代入することにより理想形状の格子高Ｌを求める。次に、格子高がＬになるように、ブレーズ型回折格子の格子面の形状を設計する。
【０００４】
従来は、回折光学素子の格子面が、以上のように設計された理想形状の格子面に極力近い形状を持つように、ブレーズ型回折光学素子の製造が行なわれる。（例えば、特許文献１参照。）
【０００５】
【特許文献１】
特開平１０−２６８１１６号公報
【０００６】
【発明が解決しようとする課題】
しかしながら、ブレーズ型回折光学素子の格子面を、図２で示されるように山や谷が尖った理想的な断面形状に製造することは困難である。特に、成形型の反転形状を転写する成形法によりブレーズ型回折光学素子を製造する場合、山や谷を実質的に完全に尖らせることは困難である。
【０００７】
成形法によりブレーズ型回折光学素子を製造する場合、前述のようにブレーズ型回折格子を設計した後に、設計形状の格子面の反転形状の型面を有する成形型を作製する。この型面の形成は、例えば、先端径が小さい切削バイトで切削加工により行なわれる。そのために、成形型の型面では、少なくとも谷の形状が切削バイトの先端の径に依存した丸みを帯びてしまう。この様子を図４に示す。
【０００８】
次に、この成形型を未硬化の樹脂に接触させて、硬化後に成形型から樹脂を離型して、ブレーズ型回折光学素子を製造する。製造されたブレーズ型回折光学素子の格子面の山は成形型の型面の谷の丸みを転写して丸みを帯びる。更に、成形時に発生する樹脂の流動摩擦等により成形型の型面の一部に樹脂が充填されにくいために、一般に型面を完全に転写することは困難である。その結果、型面が仮に完全に尖っていたとしても成形された格子面の山の頂上や谷の底が丸みを帯びることは避けることが出来ない。そのために、成形された回折格子の格子面の形状は理想形状からずれてしまう。この理想形状の格子面の格子高Ｌと、成形して得られた格子面の山の頂上と谷の底との段差Ｓ’との関係は図３に模式的に示される。一般にＳ’はＬよりも小さい。
【０００９】
図３に示された山や谷が丸みを帯びたブレーズ型回折光学素子は、所望の波長λの光、所望の回折次数に対して回折効率が著しく低下してしまう。この回折効率が低下した分だけ、回折光学素子へ入射した光の内でフレア光として意図しない方向へ進む光は増え、その結果、回折光学素子の光学性能は低下する。
【００１０】
本発明は、格子面の山の頂上または谷の底の形状が丸みを帯びている場合でも、回折効率が高く、且つフレア光の少ない回折光学素子及びその製造方法、並びに本回折光学素子を具えるカメラ等の光学系を提供することを課題とする。
【００１１】
【課題を解決するための手段】
以上の問題を解決する為に、本発明の回折光学素子は、回折格子を具えた回折光学素子であり、前記回折格子の格子面の山又は谷の少なくとも一方が丸みを帯びており、使用波長をλ、前記回折格子の基板の屈折率をｎ１、前記格子面に隣接する媒質の屈折率をｎ２、ｍを整数としたとき、前記格子面の山の頂上と谷の底との段差Ｓが式１で与えられることを特徴とする。
（式１）Ｓ＝ｍ×λ／（ｎ１−ｎ２）
又、本発明の回折光学素子において、前記回折格子の基板の材料がガラスであることが好ましい。
【００１２】
本発明の他の回折光学素子は、回折格子と前記回折格子の格子面に隣接する媒質を有する媒質部とを具えた回折光学素子であり、前記媒質が流体又は樹脂から成り、前記格子面の山又は谷の内少なくとも一方が丸みを帯びており、使用波長をλ、前記回折格子の基板の屈折率をｎ１、前記格子面に隣接する媒質の屈折率をｎ２、ｍを整数としたとき、前記格子面の山の頂上と谷の底との段差Ｓが式１で与えられることを特徴とする。
（式１）Ｓ＝ｍ×λ／（ｎ１−ｎ２）
又、本発明の他の回折光学素子において、前記回折格子の基板の材料がガラスであることが好ましい。
【００１３】
本発明の回折光学素子の製造方法は、所望の使用波長、所望の回折次数に対して最適化され、且つ、山又は谷の内少なくとも一方が丸みを帯びた格子面を持つ回折格子を有する回折光学素子を、母材の成形により製造する方法であって、前記所望の使用波長λ、前記所望の回折次数ｍ、回折格子の基板の屈折率ｎ１、該回折格子の格子面に隣接する媒質の屈折率ｎ２を次式、
Ｌ＝ｍ×λ／（ｎ１−ｎ２）
に代入して理想形状の格子面の格子高Ｌを求め、前記母材に格子面を転写する型面における山の頂上と谷の底との段差が、前記格子高Ｌより大きい成形型を準備し、前記母材に前記成形型を接触させ、前記型面の略反転形状で且つ段差が前記格子高Ｌに等しい格子面を前記母材に転写する段階と、前記母材を前記成形型から離型する段階とを具えることを特徴とする。
【００１４】
又、本発明の回折光学素子の製造方法において、更に、前記製造対象の回折光学素子の回折格子の格子面の山の丸みの曲率半径をＲ１とするとき、前記型面の谷の丸みの曲率半径Ｒ２を、Ｒ２＜Ｒ１とすることが好ましい。
【００１５】
又、本発明の回折光学素子の製造方法において、前記回折格子の基板の材料がガラスであることが好ましい。
【００１６】
又、本発明の回折光学素子の製造方法において、更に、前記格子面に流体又は樹脂を接触させ、前記接触させた状態を保持するようにする段階を具えることが好ましい。
【００１７】
本発明の光学系は、前記回折光学素子を何れか一つは具えることを特徴とする。
【００１８】
【発明の実施の形態】
図６は本発明の単層型回折光学素子１を、そして図７は本発明の密着複層型回折光学素子２を示す。また、図１は本発明の単層型回折光学素子と密着複層型回折光学素子に共通する回折格子３の格子面４の断面を示す。
【００１９】
本発明の単層型回折光学素子１は、回折格子３を具える。７は回折格子の基板であり、８は媒質である。本発明の単層型回折光学素子１の格子面４の山５と谷６は少なくともどちらか一方が丸みを帯びている。そして回折光学素子１の使用波長をλ、回折格子３の基板の屈折率をｎ１、格子面４に隣接する媒質の屈折率をｎ２、所望の回折の次数をｍとしたときに、隣接する山５と谷６の段差Ｓは以下の式１、
（式１）Ｓ＝ｍ×λ／（ｎ１−ｎ２）
で表される。
【００２０】
本発明の密着複層型回折光学素子２は、回折格子３を具える。７は回折格子の基板であり、９は媒質部であり、媒質部９の少なくとも格子面に接する部分は媒質である。本発明の回折光学素子２の格子面４の山５と谷６は少なくともどちらか一方が丸みを帯びている。そして回折光学素子２の使用波長をλ、回折格子３の基板の屈折率をｎ１、格子面４に隣接する媒質の屈折率をｎ２、所望の回折の次数をｍとしたときに、隣接する丸みのある山５の頂上と丸みのある谷６の底との段差Ｓは式１で表される。
【００２１】
即ち、本発明の単層型回折光学素子と密着複層型回折光学素子（以下、特に明記の無い限り、回折光学素子によってこれらを総称する）は、所望の回折次数がｍ次であり、使用波長がλのときには、式１を満足するような段差Ｓと回折格子の基板の屈折率ｎ１と媒質の屈折率ｎ２とを持つように調整されている。
【００２２】
具体的に、例えば１次の回折光を使用し、所望の使用波長がλのときには、本発明の回折光学素子では、Ｓ＝λ／（ｎ１−ｎ２）を満足するような段差Ｓと回折格子の基板の屈折率ｎ１と媒質の屈折率ｎ２とを持つように調整されている。また、例えば１次の回折光を使用し、所望の使用波長がλ、回折格子の基板の屈折率がｎ１、媒質の屈折率がｎ２のときには、本発明の回折光学素子では、段差Ｓが、Ｓ＝λ／（ｎ１−ｎ２）を満足するように調整されている。
【００２３】
以上の説明にて、段差Ｓは、丸みを帯びている山又は丸みを帯びている谷を有する格子面において、隣接し合う山の頂上と谷の底との段差である。
【００２４】
本発明の使用波長λは、本発明の回折光学素子で使用される光の波長を意味し、使用する光がレーザ発振光のような狭帯域光の場合は、通常その帯域の中心の波長が使用波長として選ばれ、使用する光が白色光のような広帯域光の場合には、通常その帯域の中心付近の特定波長またはその帯域の中の特に重要な波長、等が使用波長として選ばれる。
【００２５】
本発明の回折光学素子は、それが具える回折格子が、位相型回折格子のときに好ましい効果を発揮する。このとき、回折格子の格子面の形状に特別な制約はないが、格子面の形状が鋸歯状であるブレーズ型回折格子のときに、特に好ましい効果を発揮する。
【００２６】
本発明の回折光学素子の回折格子の基板７の材料としては、使用波長の光を透過し、式１を満足する屈折率を持っていさえすれば、特に限定されるものではなく、ガラスや樹脂から選ばれた材料が使用される。
【００２７】
本発明で基板７の材料として用いられるガラス材料としては、石英ガラス、蛍石、ＢＫ７が好ましく使用される。特に軟化点が低い低融点ガラスと呼ばれる物質はガラスモールド法により回折格子の格子面を成形できるので、製造コストを低くできるために、好ましく使用される。又、回折格子の基板がガラスの場合、ガラスは格子面の山の頂上の曲率半径を微少な刃先状にすることが樹脂の場合よりも困難であり、基板がガラスの回折格子の格子面の山又は谷の曲率半径は、基板が樹脂のものよりも一般に大きい。従って、格子面の製造された形状と所望形状との差異、更には、製造された格子面の山の頂上と谷の底との段差と理想の格子高との差異は、樹脂よりもガラスにおいて大きい。その結果、従来の基板がガラスの回折光学素子は基板が樹脂のものよりも回折効率が一般に低かった。本発明では、丸みを見込んだ段差を設計に取り入れているので、回折格子の基板にガラスを用い、格子面の山の丸みや谷の丸みが比較的大きい場合でも、回折効率が高く、フレアが少ない。
【００２８】
本発明の回折光学素子の回折格子の基板７の材料に用いる樹脂としては、射出成形用の熱可塑性樹脂のうち、使用波長の光を透過するものが広く用いられる他、ウレタンアクリレートやエポキシアクリレートなどの紫外線硬化型あるいは熱硬化型樹脂などが用いられる。
【００２９】
本発明の回折光学素子の格子面４に隣接する媒質８は、回折光学素子として機能させるために、その屈折率ｎ２が基板の屈折率ｎ１と少なくとも異ならなければならない。図６に示す単層型光学素子の場合、媒質は一般には空気であり、屈折率ｎ２は１．０である。
【００３０】
図７に示す本発明の密着複層型回折光学素子の媒質９としては、樹脂または流体が用いられる。樹脂としては、ウレタンアクリレートやエポキシアクリレートなどの紫外線硬化型樹脂又は熱硬化型樹脂などが用いられ、流体としては空気またはシリコンオイルなどが用いられる。媒質が樹脂の場合、樹脂そのものが媒質部９を構成し、媒質が流体の場合、流体を格子面に接触させながら保持するために保持容器に入れて使用され、流体と保持容器とが媒質部を構成する。
【００３１】
本発明の密着複層型回折光学素子は、媒質９の屈折率ｎ２と回折格子の基板７の屈折率ｎ１が式１を満足するようにすると共に、媒質９の分散と回折格子の基板７の分散との関係を適当に組み合わせることにより、使用波長のみならず広い波長域で高い回折効率を実現することができる。その結果、カメラの撮像光学系等にも使用可能となる。
【００３２】
本発明の回折光学素子の回折格子の基板７を、低融点ガラス等のガラスにすると、ガラスモールド法を用い、低い製造コストで回折格子を製造することができ、又、樹脂にすると、射出成形などの成形を用い低い製造コストで回折格子を製造できるので、回折格子の基板７はガラス又は樹脂とすることが好ましい。本発明の密着複層型回折光学素子２の格子面４に隣接する媒質９としては樹脂を用いると加工上有利であり、特に紫外線硬化型樹脂を用いると未硬化状態で液体であり、製造しやすく製造コストが低いので、特に好ましい。
【００３３】
図６、図７に示される本発明の回折光学素子の回折格子３は直線格子であるが、本発明はその他に、同心円状格子、等の場合を含み、回折格子の形状や配置には限定されないことは言うまでもない。
【００３４】
図９は本発明の回折光学素子とその製造方法の説明に用いる、回折格子の格子面と成形型の型面の断面の拡大図であり、１ピッチ分のみ示されている。図９にて実線は製造対象の格子面の、破線は理想形状の格子面の、そして点線は成形型の型面の断面の図である。Ｓは製造対象の格子面の段差であり、これは理想格子面の格子高Ｌに等しい。Ｓ０は成形型の型面の段差であり、Ｓ０はＳよりも大きい。
【００３５】
本発明の回折光学素子の製造方法を以下に示すが、本発明は本手順に限定されるものではない。
（１）所望の使用波長λ、回折次数ｍ、回折格子の基板の材料、媒質の種類を決定する。媒質は、単層型回折光学素子の場合、通常空気とされ、密着複層型回折光学素子の場合、樹脂又は流体である。
（２）λ、ｍ、回折格子の基板の屈折率ｎ１、媒質の屈折率ｎ２を式２に代入して、理想の回折格子の格子高Ｌを求める。
（３）段差Ｓが格子高Ｌに等しく、Ｓ＝Ｌであり、且つ所望の形状の格子面を決定する。この格子面は山５又は谷６の少なくとも一方が丸みを帯びており、所定の格子ピッチ、格子面の傾き等を有する。
（４）成形型の型面の段差が（３）で決定された段差Ｓよりも大きいＳ０であり、且つ、型面の形状が（３）で決定された格子面の反転形状とほぼ同じ形状の成形型を準備する。
【００３６】
成形型の型面の好ましい段差Ｓ０は事前の成形実験により知られている。
（５）回折格子の基板となる母材に（４）で準備された成形型の型面を接触させ、成形型の型面の反転形状を母材に転写する。
（６）母材を成形型から離型し、格子面の段差がＳの回折格子を有する回折格子基板（単層型回折光学素子）を得る。
【００３７】
このようにして単層型回折光学素子が製造される。
【００３８】
密着複層型回折光学素子を製造するためには、（６）に引き続いて（７）の手順を経る。
（７）更に、（６）で作製された回折格子基板の格子面に媒質として紫外線硬化型樹脂を接触させた状態で樹脂に紫外線を照射することにより樹脂を硬化させる。
このようにして回折格子基板とその上に密着させた紫外線硬化型樹脂とを具える密着複層型回折光学素子が製造される。媒質の樹脂として、紫外線硬化型樹脂でなくて熱硬化型樹脂を用いる場合は、樹脂を加熱することにより硬化させれば良い。また、媒質として液体や気体を用いる場合は、回折格子基板上の格子面を取り囲む位置に、予め容器としての側壁を形成しておき、液体又は気体を注入後に容器を封止すれば良い。
【００３９】
尚、従来の成形では、例えば成形型の型面の谷の形状が、図９の１０に示すように、曲率半径０の刃先状の理想形状であるという前提の基に製造される。一方本発明では、図９の１１に示すように、型面の谷が有限の曲率半径の丸みを持つという前提で回折格子の成形が行なわれる。この型面の谷１１の曲率半径は所望の格子面の山５の曲率半径よりも小さくされる。
【００４０】
型面の谷１１の曲率半径が小さすぎると、成形された格子面の山の曲率半径が所望の値よりも小さくなる。その結果、所望形状の格子面が得られず、成形される回折格子の格子面の段差が所望の値Ｓよりも大きくなるばかりでなく、成形型の作製に必要以上のコストを掛けることになる。一般に、刃先状の型面の谷の曲率半径が小さいほど、成形型の製造コストが高いからである。
【００４１】
逆に、型面の谷１１の曲率半径が大きすぎると、格子面の山の曲率半径が所望の値よりも大きくなり、その結果、所望形状の格子面が得られず、成形される回折格子の格子面の段差が所望の値Ｓよりも小さくなる。
【００４２】
回折格子の格子面を成形法で形成すると、前に述べたように、成形型の格子の切削加工上の問題と成型時の樹脂の摩擦の為に、山又は谷が特に丸みを帯びやすいので、本発明は成形法で作製された回折光学素子に対して特に有効である。成形法には、熱可塑性樹脂を成形する射出成形法、加熱して軟化したガラスをプレス成形するガラスモールド法、未硬化で液体状態の紫外線硬化樹脂又は熱硬化型樹脂を硬化させる樹脂成形法、等があるが、特に射出成形法とガラスモールド法は格子面の先端（山と谷）の丸みが大きい。丸みが大きいほど、山の頂上と谷の底との段差と理想の格子高との差異は大きくなり、従来方法では回折効率が低くなる。従って、本発明は、射出成形法とガラスモールド法に於いて特に有効である。
【００４３】
以上のように、本発明の回折光学素子は、丸みのある山と丸みのある谷との段差を、式１を基に決めているために、回折光学素子の格子面の山、谷の丸みが比較的大きい場合でも回折効率が高く、且つフレアが少ない。
【００４４】
以下、実施例１〜４を開示するが、これら実施例の説明のために共通して図９を合わせ用いる。
［実施例１］
本実施例では、ガラスモールド法で、図６で示される３０ｍｍ×３０ｍｍの単層型回折光学素子を作製した。この単層型回折光学素子の基板７の材料は低融点ガラス（住田光学ガラス社製Ｐ−ＳＫ６０：ｄ線屈折率ｎｄ＝ｎ１＝１．５９１９）であり、回折格子３としてブレーズ型の直線格子を具える。
（１）単層型回折光学素子の使用波長をｄ線波長５８７．６ｎｍ、回折次数を１０、媒質を空気（ｎ２＝１．０）とした。
（２）λ＝５８７．６ｎｍ、ｍ＝１０、ｎ１＝１．５９１９、ｎ２＝１．０を前記式２に代入して、理想の回折格子の格子高９．９μｍを求めた。
（３）作製対象の回折格子の格子面の所望の形状を、作製対象の単層型回折光学素子の所望の光学特性から決定した。この所望の形状は格子面は山と谷とが丸みを帯びており、格子面の山の頂上と谷の底との段差Ｓは（２）で決定された格子高９．９μｍに等しく、格子面の斜面が直線状であり、ピッチは１５０μｍである。
（４）成形型の型面の形状を決定した。型面は、段差が（３）で決定された段差９．９μｍよりも２．５μｍ大きい１２．４μｍであり、且つ、（３）で決定された格子面の反転形状を谷の方向（格子面では山の方向に対応する）に２．５μｍだけ引き延ばした形状に決定された。型面の谷と山の曲率半径は、この谷と山に各々対応する所望の格子面の山と谷の曲率半径よりも小さくされた。
（５）（４）で決定された形状の型面の成形型を以下のようにして作製した。
炭素綱製の３０ｍｍ×３０ｍｍのブランク上にＮｉ及びＰを主成分とする無電解メッキ層を形成し、この無電解メッキ層を先端径が３μｍのダイヤモンドバイトで切削加工し、所望の型面を形成することにより成形型を作製した。
（６）（５）で作製した成形型を上部成形型として用い、以下のようにガラスモールド法で回折格子を成形した。尚下部成形型の型面は平面とした。
【００４５】
成形機（加熱加圧状態での成形型の近傍を図１２に示す）にセットした上部成形型２１と下部成形型２２と上記低融点ガラスの３０ｍｍ×３０ｍｍの平板の母材２０とを、窒素雰囲気中で４１５℃に加熱しながら、成形型２１、２２を母材２０に接触させ、加圧して、押し当てて、成形型の型面２４、２５を母材２０の面に転写し、冷却、離型後、母材（基板）２０の上面に格子面が、下面に平面が成形された回折光学素子を成形機から取り出した。
【００４６】
このようにして製造された単層型回折光学素子は、図９に示すように、格子面の山と谷とが成形型の対応する谷と山よりも丸みを帯びており、山の頂上と谷の底との段差Ｓが所望の９．９μｍだった。
［実施例２］
本実施例では図１０で示される５０ｍｍφの密着複層型回折光学素子をガラスモールド法により作製した。図１０にて４は格子面、７は基板、１４は樹脂層である。この密着複層型回折光学素子の回折格子の基板の材料は低融点ガラス（住田光学ガラス社製Ｐ−ＳＫ６０：ｄ線屈折率ｎｄ＝ｎ１＝１．５９１９）、媒質である樹脂はウレタンアクリレート系の紫外線硬化型樹脂（ｎｄ＝ｎ２＝１．５５５）である。回折格子は同心円状のブレーズ型であり、そのピッチは中心から外側に向かうに従い小さくなり、この同心円状格子は平面上に配列される。また、使用波長はｄ線の波長である５８７．６ｎｍ、回折次数を１とする。
（１）λ＝５８７．６ｎｍ、ｍ＝１、ｎ１＝１．５９１９、ｎ２＝１．５５５を式２に代入して、理想の回折格子の格子高１６．３μｍを求めた。
（２）作製対象の回折格子の格子面の形状を決定した。この作製対象の回折格子の格子面は山５と谷６とが丸みを帯びており、格子面の山の頂上と谷の底との段差Ｓは（１）で決定された格子高１６．３μｍに等しく、格子面の斜面は曲線状であり、ピッチは中央部で２ｍｍ、周辺部で０．２ｍｍの同心円状格子である。
（３）成形型の型面の形状を決定した。型面は、段差が（２）で決定された段差１６．３μｍよりも２．５μｍ大きい１８．８μｍであり、且つ、（２）で決定された格子面の反転形状を谷の方向（格子面では山の方向に対応する）に２．５μｍだけ引き延ばした形状である。型面の谷と山の曲率半径は、この谷と山に各々対応する所望の格子面の山と谷の曲率半径よりも小さくされた。
（４）上で決定された形状の型面の成形型を以下のようにして作製した。
炭素綱製の５０ｍｍφのブランク上にＮｉ及びＰを主成分とする無電解メッキ層を形成し、この無電解メッキ層を先端径が３μｍのダイヤモンドバイトで切削加工し、所望の型面を形成することにより成形型を作製した。
（５）（４）で作製した成形型を上部成形型として用い、以下のようにガラスモールド法で回折格子を成形した。尚下部成形型の型面は平面とした。
成形機（加熱加圧状態での成形型の近傍を図１２に示す）にセットした上部成形型２１と下部成形型２２と上記低融点ガラスの５０ｍｍφの平板の母材２０とを、窒素雰囲気中で４１５℃に加熱しながら、成形型２１、２２を母材２０に接触、加圧して、押し当てて成形型の型面２４、２５を母材２０の面に転写し、冷却、離型後、母材（基板）２０の上面に同心円状の格子面が、下面に平面が成形された回折格子基板を成形機から取り出した。
【００４７】
このようにして作製された回折格子基板は、図９に示すように、格子面の山と谷とが成形型の対応する谷と山よりも丸みを帯びており、山の頂上と谷の底との段差Ｓが所望の１６．３μｍだった。
（６）回折格子基板の格子面にウレタンアクリレート系の紫外線硬化型樹脂を滴下した後、格子面上の樹脂に、型面が平面であるステンレス製の金型を近接させ、樹脂を押し広げ、樹脂層を所定厚みに調整した。この状態で高圧水銀灯の紫外線（中心波長３６５ｎｍ）を回折格子基板の側から照射し、樹脂を光硬化させた。紫外線の積算照射量は２０００ｍＪ／ｃｍ^２だった。
（７）光硬化後、金型を樹脂から離型することにより、図１０のような所望の回折格子を具えた密着複層型回折光学素子が得られた。
【００４８】
このようにして製造された密着複層型回折光学素子は、図９に示すように、格子面の山と谷とが成形型の対応する谷と山よりも丸みを帯びており、山の頂上と谷の底との段差Ｓが所望の１６．３μｍだった。
［実施例３］
本実施例では、射出成形法で、図６で示される３０ｍｍ×３０ｍｍの単層型回折光学素子を作製した。この単層型回折光学素子の母材は脂環式オレフィン系熱可塑性樹脂（ＪＳＲ社製ＡＲＴＯＮ：ｎｄ＝ｎ１＝１．５１０）であり、回折格子としてブレーズ型の直線格子を具える。
（１）実施例１の（１）と同様である。
（２）λ＝５８７．６ｎｍ、ｍ＝１０、ｎ１＝１．５１０、ｎ２＝１．０を式２に代入して、理想の回折格子の格子高１１．５μｍを求めた。
（３）作製対象の回折格子の格子面の所望の形状を決定した。この所望の形状は山５と谷６とが丸みを帯びており、格子面の山の頂上と谷の底との段差は（２）で決定された格子高１１．５μｍに等しく、格子面の斜面が直線状であり、ピッチ１５０μｍの直線状である。
（４）射出成形用の成形型の型面の形状を決定した。型面は、段差が（３）で決定された段差１１．５μｍよりも２．０μｍ大きい１３．５μｍであり、且つ、（３）で決定された格子面の反転形状を谷の方向（格子面では山の方向に対応する）に２．０μｍだけ引き延ばした形状に決定した。型面の谷と山の曲率半径は、この谷と山に各々対応する所望の格子面の山と谷の曲率半径よりも小さくされた。
（５）上で決定された形状の型面を有する射出成形用の成形型を作製し、射出成形することにより、所望の形状の格子面を有する回折格子を具える単層型回折光学素子を作製した。
【００４９】
このようにして製造された単層型回折光学素子は、図９に示すように、格子面の山と谷とが成形型の対応する谷と山よりも丸みを帯びており、山の頂上と谷の底との段差Ｓが所望の１１．５μｍだった。
［実施例４］
本実施例では図５で示される５０ｍｍφの密着複層型回折光学素子をガラスモールド法により作製した。図５にて４は格子面、７は基板、１４は樹脂層である。この密着複層型回折光学素子の基板の材料は低融点ガラス（住田光学ガラス社製Ｐ−ＳＫ６０：ｄ線屈折率ｎｄ＝ｎ１＝１．５９１９）、媒質である樹脂はウレタンアクリレート系の紫外線硬化型樹脂（ｎｄ＝ｎ２＝１．５５５）である。回折格子は同心円状のブレーズ型であり、そのピッチは中心から外側に向かうに従い小さくなり、この同心円状格子は上に凸の９３ｍｍＲの球面上に配列される。図５では同心円状のブレーズ型の回折格子は明示されていない。基板の中心厚は１２ｍｍ、基板の格子面に対向する面は下に凹の９３ｍｍＲとされる。また、樹脂層の厚みは０．１ｍｍ、樹脂層の格子面に対向する面は上に凸の９３ｍｍＲとされる。また、使用波長はｄ線の波長である５８７．６ｎｍ、回折次数を１とする。
（１）実施例２の（１）と同様に求めた。
（２）作製対象の回折格子の格子面の形状を実施例２の（２）と同様な形状に決定した。但し、同心円状格子は平面上にではなくて、上に凸の９３ｍｍＲの球面上に配列するようにした。
（３）上部成形型の型面の形状を実施例２の（３）と同様な形状に決定した。但し、型面を全体的に平面状にするのではなくて、下に凹の９３ｍｍＲの球面状にした。また、基板の格子面に対向する面を成形するための下部成形型の型面の形状を上に凸の９３ｍｍＲに決定した。
（４）上部成形型を、実施例２の（４）と同様な方法で、（３）で決定された形状の型面をブランクに形成することにより作製した。下部成形型は、上で形状が決定された型面を炭素綱製の５０ｍｍφのブランクに形成し、その上にＮｉ及びＰを主成分とする無電解メッキ層を形成した後、切削加工することにより作製した。
（５）（４）で作製した成形型を用い、実施例２の（５）と同様なガラスモールド法で、回折格子と対向の球面を成形した。
このようにして作製された回折格子基板は、図９に示すように、格子面の山と谷とが成形型の対応する谷と山よりも丸みを帯びており、山の頂上と谷の底との段差Ｓが所望の１６．３μｍだった。
（６）回折格子基板の格子面にウレタンアクリレート系の紫外線硬化型樹脂を滴下した後、格子面上の樹脂に、型面が上に凹の９３ｍｍＲの球面であるステンレス製の金型を近接させ、樹脂を押し広げ、樹脂層の層厚を０．１ｍｍに調整した。この状態で高圧水銀灯の紫外線（中心波長３６５ｎｍ）を回折格子基板の側から照射し、樹脂を光硬化させた。紫外線の積算照射量は２０００ｍＪ／ｃｍ^２だった。
（７）光硬化後、樹脂を金型から離型することにより、図５のような所望の回折格子と所望の球面とを具えた密着複層型回折光学素子が得られた。
【００５０】
以上、実施例１〜４で説明した本発明の回折光学素子は、格子面の山の頂上または谷の底が丸みを帯びている場合でも、丸みを帯びた山の頂上と丸みを帯びた谷の底との段差を考慮して設計されているので、回折効率が高く、且つフレア光が少なく、高性能である。更に、本発明の回折光学素子は、製造に当たって、成形型の型面の形状をブレーズ型の回折格子の理想的な形状である山と谷との曲率半径が極小の刃先状にする必要がない上に、回折格子の格子面の形状を成形型の型面の正確な反転形状にする必要もないので、製造が容易であり、従って製造コストが低い。
［実施例５］
図１１は本発明のカメラ用光学系であり、この光学系の中に本発明の実施例４の密着複層型回折光学素子１５を用いている。
【００５１】
本発明の光学系は、回折効率が高く、フレア光が少なく、且つ製造コストが低い本発明の回折光学素子を用いているので、透過率が高く、フレアが少なく、且つ製造コストが低い。また、本発明の回折光学素子の特性を有効に用いているので、コンパクトである。
【００５２】
【発明の効果】
本発明の回折光学素子は、格子面の山の頂上または谷の底が丸みを帯びている場合でも、丸みを帯びた山の頂上と丸みを帯びた谷の底との段差を考慮して設計されるので、回折効率が高く、且つフレア光が少ない。又、本発明の回折光学素子を用いたカメラ等の光学系は、フレアが少なくて、透過率が高い。
【図面の簡単な説明】
【図１】本発明の単層型回折光学素子と密着複層型回折光学素子の回折格子３の格子面４の断面を示す。
【図２】ブレーズ型回折格子の理想的な格子面４の断面形状を示す。
【図３】理想形状格子面の格子高Ｌと、成形して得られた格子面の段差Ｓ’との関係を示す。
【図４】格子の切削加工の様子を示す。
【図５】本発明の実施例４の密着複層型回折光学素子である。
【図６】本発明の単層型回折光学素子１を示す。
【図７】本発明の密着複層型回折光学素子２を示す。
【図８】ブレーズ型回折光学素子の理想的な形状を示す。
【図９】本発明の実施形態及び実施例の、回折光学素子及びその製造方法の説明に共通に用いる、回折格子の格子面と成形型の型面の断面の拡大図を示す。
【図１０】本発明の実施例２の密着複層型回折光学素子を示す。
【図１１】本発明の回折光学素子を用いたカメラ用の光学系を示す。
【図１２】成形機の成形型の近傍を示す。
【符号の説明】
１単層型回折光学素子
２密着複層型回折光学素子
３回折格子
４格子面
５格子面の山
６格子面の谷
７回折格子の基板
８媒質
９媒質部
１０理想的な成形型の谷の頂点
１１本発明に用いる成形型の丸みのある谷
１４樹脂層
１５実施例４の密着複層型回折光学素子
２０母材
２１上部成形型
２２下部成形型
２３スリーブ
２４、２５型面[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a diffractive optical element having a diffraction grating used for an optical device such as a camera, a method for manufacturing the diffractive optical element, and an optical system having the diffractive optical element.
[0002]
[Prior art]
FIG. 8 shows an ideal shape of a blaze type diffractive optical element. FIG. 2 shows an ideal cross section of a blazed diffraction grating provided in the blazed diffraction optical element. As shown in FIG. 2, the blazed diffraction grating has a sawtooth-shaped cross section on the grating surface. Both the top of the hill and the bottom of the valley have a cutting edge shape, and ideally, the radii of curvature of both the top of the hill and the bottom of the valley are zero. Such a blazed diffraction grating having an ideal shape can theoretically make the diffraction efficiency of light of a used wavelength for a specific order 100%, and the maximum diffraction efficiency is obtained when Expression 2 is satisfied. It is known.
(Equation 2) L × (n1-n2) = m × λ
Here, λ is a used wavelength, which is usually a wavelength of light to be optimized and maximized in diffraction efficiency. L is the grating height of the grating surface of the ideal blazed diffraction grating, n1 is the refractive index of the substrate of the grating, n2 is the refractive index of a medium adjacent to the grating surface, and m is an integer representing the diffraction order.
[0003]
Therefore, conventionally, in manufacturing a blazed diffraction grating of a blazed diffractive optical element, first, a desired working wavelength λ, the refractive index n1 of the substrate of the diffraction grating to be manufactured, the refractive index n2 of the working medium, Of the ideal shape is obtained by substituting the diffraction order m. Next, the shape of the grating surface of the blazed diffraction grating is designed so that the grating height becomes L.
[0004]
Conventionally, a blazed diffractive optical element is manufactured such that the grating surface of the diffractive optical element has a shape as close as possible to the ideally designed grating surface designed as described above. (For example, refer to Patent Document 1.)
[0005]
[Patent Document 1]
JP-A-10-268116
[0006]
[Problems to be solved by the invention]
However, it is difficult to manufacture the grating surface of the blazed diffractive optical element to have an ideal cross-sectional shape with peaks and valleys as shown in FIG. In particular, when a blazed diffractive optical element is manufactured by a molding method for transferring an inverted shape of a molding die, it is difficult to make the peaks and valleys substantially completely sharp.
[0007]
When a blazed diffractive optical element is manufactured by a molding method, after a blazed diffraction grating is designed as described above, a mold having a mold surface having an inverted shape of a lattice surface of a designed shape is manufactured. The mold surface is formed by, for example, cutting with a cutting tool having a small tip diameter. For this reason, at least the shape of the valley on the mold surface of the mold is rounded depending on the diameter of the tip of the cutting tool. This is shown in FIG.
[0008]
Next, the mold is brought into contact with an uncured resin, and after curing, the resin is released from the mold to manufacture a blaze-type diffractive optical element. The peak of the grating surface of the manufactured blazed diffractive optical element is rounded by transferring the roundness of the valley of the mold surface of the mold. In addition, it is generally difficult to completely transfer the mold surface because a part of the mold surface of the mold is hardly filled with the resin due to the flow friction of the resin generated during molding. As a result, even if the mold surface is completely sharp, it is unavoidable that the peaks and valleys of the formed lattice surface are rounded. Therefore, the shape of the grating surface of the formed diffraction grating deviates from the ideal shape. FIG. 3 schematically shows the relationship between the lattice height L of the lattice plane having the ideal shape and the step S ′ between the top of the mountain and the bottom of the valley of the lattice plane obtained by molding. Generally, S 'is smaller than L.
[0009]
In the blazed diffractive optical element shown in FIG. 3 in which the peaks and valleys are rounded, the diffraction efficiency is remarkably reduced for light having a desired wavelength λ and a desired diffraction order. Due to the decrease in the diffraction efficiency, the amount of light that travels in an unintended direction as flare light out of the light incident on the diffractive optical element increases, and as a result, the optical performance of the diffractive optical element decreases.
[0010]
The present invention provides a diffractive optical element having high diffraction efficiency and low flare light, a method for manufacturing the same, and a diffractive optical element, even when the shape of the top of the peak or the bottom of the valley of the lattice plane is rounded. An object of the present invention is to provide an optical system such as a camera which can be used.
[0011]
[Means for Solving the Problems]
In order to solve the above problems, the diffractive optical element of the present invention is a diffractive optical element having a diffraction grating, wherein at least one of the peaks or valleys of the grating surface of the diffraction grating is rounded, and the wavelength used is Is λ, the refractive index of the substrate of the diffraction grating is n1, the refractive index of the medium adjacent to the grating surface is n2, and m is an integer, the step S between the peak top and the valley bottom of the grating surface is It is characterized by being given by Equation 1.
(Equation 1) S = m × λ / (n1-n2)
In the diffractive optical element of the present invention, it is preferable that the material of the substrate of the diffraction grating is glass.
[0012]
Another diffractive optical element of the present invention is a diffractive optical element including a diffraction grating and a medium portion having a medium adjacent to a grating surface of the diffraction grating, wherein the medium is made of a fluid or a resin, and At least one of the peaks or valleys is rounded, the wavelength used is λ, the refractive index of the substrate of the diffraction grating is n1, the refractive index of the medium adjacent to the grating surface is n2, and m is an integer, The step S between the peak of the ridge and the bottom of the valley of the lattice plane is given by Equation 1.
(Equation 1) S = m × λ / (n1-n2)
In another diffractive optical element according to the present invention, it is preferable that the material of the substrate of the diffraction grating is glass.
[0013]
The method of manufacturing a diffractive optical element according to the present invention is a diffraction grating having a diffraction grating optimized for a desired wavelength to be used and a desired diffraction order, and having at least one of a peak and a valley having a rounded grating surface. A method for manufacturing an optical element by molding a base material, the method comprising the step of using the desired wavelength λ, the desired diffraction order m, the refractive index n1 of the substrate of the diffraction grating, and the medium adjacent to the grating surface of the diffraction grating. The refractive index n2 is given by the following equation:
L = m × λ / (n1-n2)
To obtain the lattice height L of the lattice plane of the ideal shape, and prepare a mold in which the step between the top of the hill and the bottom of the valley in the mold surface for transferring the lattice plane to the base material is larger than the lattice height L. Contacting the molding die with the base material, and transferring a lattice surface having a substantially inverted shape of the mold surface and a step equal to the lattice height L to the base material, and transferring the base material from the molding die. Releasing the mold.
[0014]
In the method for manufacturing a diffractive optical element according to the present invention, when the radius of curvature of the roundness of the peak of the grating surface of the diffraction grating of the diffraction optical element to be manufactured is R1, the curvature of the roundness of the valley of the mold surface is further defined. It is preferable that the radius R2 satisfy R2 <R1.
[0015]
In the method for manufacturing a diffractive optical element according to the present invention, it is preferable that a material of the substrate of the diffraction grating is glass.
[0016]
Preferably, the method for manufacturing a diffractive optical element according to the present invention further comprises a step of bringing a fluid or a resin into contact with the lattice surface and maintaining the contact state.
[0017]
The optical system according to the present invention is characterized by including any one of the diffractive optical elements.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 6 shows a single-layer diffractive optical element 1 of the present invention, and FIG. 7 shows a contact multilayer diffractive optical element 2 of the present invention. FIG. 1 shows a cross section of a grating surface 4 of a diffraction grating 3 which is common to the single-layer diffractive optical element and the contact multilayer diffractive optical element of the present invention.
[0019]
The single-layer diffractive optical element 1 of the present invention includes a diffraction grating 3. Reference numeral 7 denotes a diffraction grating substrate, and reference numeral 8 denotes a medium. At least one of the peaks 5 and the valleys 6 of the grating surface 4 of the single-layer diffractive optical element 1 of the present invention is rounded. When the wavelength used of the diffractive optical element 1 is λ, the refractive index of the substrate of the diffraction grating 3 is n1, the refractive index of the medium adjacent to the grating surface 4 is n2, and the desired diffraction order is m, the adjacent peaks are determined. The step S between the valley 5 and the valley 6 is given by the following equation 1,
(Equation 1) S = m × λ / (n1-n2)
Is represented by
[0020]
The contact multilayer diffractive optical element 2 of the present invention includes a diffraction grating 3. Reference numeral 7 denotes a substrate of the diffraction grating, 9 denotes a medium part, and at least a part of the medium part 9 which is in contact with the lattice plane is a medium. At least one of the peaks 5 and the valleys 6 of the grating surface 4 of the diffractive optical element 2 of the present invention is rounded. When the use wavelength of the diffractive optical element 2 is λ, the refractive index of the substrate of the diffraction grating 3 is n1, the refractive index of the medium adjacent to the grating surface 4 is n2, and the desired diffraction order is m, the adjacent roundness is obtained. The step S between the top of the peak 5 having a valley and the bottom of the rounded valley 6 is represented by the following equation (1).
[0021]
That is, the single-layer diffractive optical element and the contact multilayer-type diffractive optical element of the present invention (hereinafter collectively referred to by the diffractive optical element unless otherwise specified) have a desired diffraction order of the m-th order. When the wavelength is λ, the refractive index is adjusted so as to have the step S, the refractive index n1 of the substrate of the diffraction grating, and the refractive index n2 of the medium that satisfy Expression 1.
[0022]
Specifically, for example, when the first-order diffracted light is used and the desired wavelength to be used is λ, in the diffractive optical element of the present invention, the step S and the diffraction grating satisfying S = λ / (n1−n2) Are adjusted so as to have the refractive index n1 of the substrate and the refractive index n2 of the medium. Further, for example, when the first-order diffracted light is used, the desired wavelength to be used is λ, the refractive index of the substrate of the diffraction grating is n1, and the refractive index of the medium is n2, in the diffractive optical element of the present invention, the step S is It is adjusted to satisfy S = λ / (n1−n2).
[0023]
In the above description, the step S is a step between the top of an adjacent peak and the bottom of the valley in a lattice plane having a rounded mountain or a rounded valley.
[0024]
The wavelength λ used in the present invention means the wavelength of light used in the diffractive optical element of the present invention. When the light to be used is a narrow band light such as a laser oscillation light, the wavelength at the center of the band is usually used. If the light to be used is a broadband light such as white light, a specific wavelength near the center of the band or a particularly important wavelength in the band is usually selected as the used wavelength.
[0025]
The diffractive optical element of the present invention exerts a favorable effect when the diffraction grating provided therein is a phase type diffraction grating. At this time, although there is no particular limitation on the shape of the grating surface of the diffraction grating, a particularly preferable effect is exhibited when the blazed diffraction grating has a sawtooth-shaped grating surface.
[0026]
The material of the substrate 7 of the diffraction grating of the diffractive optical element of the present invention is not particularly limited as long as it transmits light of the used wavelength and has a refractive index that satisfies the expression 1, and is not limited to glass or resin. The material selected from is used.
[0027]
As the glass material used as the material of the substrate 7 in the present invention, quartz glass, fluorite, and BK7 are preferably used. Particularly, a substance called a low-melting glass having a low softening point is preferably used because the grating surface of the diffraction grating can be formed by a glass molding method, so that the manufacturing cost can be reduced. In addition, when the substrate of the diffraction grating is glass, it is more difficult to make the glass with a small radius of curvature at the top of the peak of the grating surface than in the case of resin, and the substrate is formed of a glass surface of the diffraction grating of the glass. The radius of curvature of the peak or valley is generally larger than that of a substrate made of resin. Therefore, the difference between the manufactured shape and the desired shape of the grating surface, and furthermore, the difference between the step height between the top of the peak and the bottom of the valley and the ideal grating height of the manufactured grating surface is more in glass than in resin. large. As a result, the diffraction efficiency of the conventional diffractive optical element having a glass substrate is generally lower than that of a resin having a resin substrate. In the present invention, since a step that allows for roundness is incorporated into the design, glass is used for the substrate of the diffraction grating, and diffraction efficiency is high and flare is reduced even when the peaks and valleys of the grating surface are relatively large. Few.
[0028]
As the resin used as the material of the substrate 7 of the diffraction grating of the diffractive optical element of the present invention, among the thermoplastic resins for injection molding, those which transmit light of the used wavelength are widely used, and urethane acrylate, epoxy acrylate, etc. UV-curable or thermosetting resin is used.
[0029]
The medium 8 adjacent to the grating surface 4 of the diffractive optical element of the present invention must have a refractive index n2 at least different from the refractive index n1 of the substrate in order to function as a diffractive optical element. In the case of the single-layer type optical element shown in FIG. 6, the medium is generally air, and the refractive index n2 is 1.0.
[0030]
As the medium 9 of the contact multilayer diffractive optical element of the present invention shown in FIG. 7, a resin or a fluid is used. As the resin, an ultraviolet curable resin such as urethane acrylate or epoxy acrylate or a thermosetting resin is used, and as the fluid, air or silicone oil is used. When the medium is a resin, the resin itself constitutes the medium portion 9, and when the medium is a fluid, the medium is used in a holding container for holding the fluid while making contact with the lattice surface. Is composed.
[0031]
The contact multilayered diffractive optical element of the present invention is configured such that the refractive index n2 of the medium 9 and the refractive index n1 of the substrate 7 of the diffraction grating satisfy Expression 1, and the dispersion of the medium 9 and the refractive index n By appropriately combining the relationship with the dispersion, high diffraction efficiency can be realized not only in the wavelength used but also in a wide wavelength range. As a result, it can be used for an imaging optical system of a camera and the like.
[0032]
When the substrate 7 of the diffraction grating of the diffractive optical element of the present invention is made of glass such as low melting point glass, the diffraction grating can be manufactured at a low manufacturing cost by using a glass molding method. The substrate 7 of the diffraction grating is preferably made of glass or resin, because the diffraction grating can be manufactured at a low manufacturing cost using molding such as molding. The use of a resin as the medium 9 adjacent to the lattice surface 4 of the close-contact multilayered diffractive optical element 2 of the present invention is advantageous in processing, and the use of an ultraviolet curable resin is a liquid in an uncured state. It is particularly preferable because it is easy and the production cost is low.
[0033]
Although the diffraction grating 3 of the diffractive optical element of the present invention shown in FIGS. 6 and 7 is a linear grating, the present invention also includes concentric gratings and the like, and the shape and arrangement of the diffraction grating are limited. Needless to say, it will not be done.
[0034]
FIG. 9 is an enlarged view of the cross section of the grating surface of the diffraction grating and the mold surface of the mold used for describing the diffractive optical element of the present invention and its manufacturing method, and shows only one pitch. In FIG. 9, a solid line is a cross section of a lattice plane to be manufactured, a broken line is a cross section of an ideal lattice plane, and a dotted line is a cross section of a mold plane of a molding die. S is the step of the lattice plane to be manufactured, which is equal to the lattice height L of the ideal lattice plane. S0 is a step of the mold surface of the mold, and S0 is larger than S.
[0035]
The method for producing the diffractive optical element of the present invention will be described below, but the present invention is not limited to this procedure.
(1) Determine the desired wavelength λ, the diffraction order m, the material of the substrate of the diffraction grating, and the type of medium. The medium is usually air in the case of a single-layer diffractive optical element, and is a resin or a fluid in the case of a contact multilayer diffractive optical element.
(2) The grating height L of the ideal diffraction grating is obtained by substituting λ, m, the refractive index n1 of the substrate of the diffraction grating, and the refractive index n2 of the medium into Equation 2.
(3) The step S is equal to the lattice height L, S = L, and a lattice plane having a desired shape is determined. In this lattice plane, at least one of the peaks 5 and the valleys 6 is rounded, and has a predetermined lattice pitch, inclination of the lattice plane, and the like.
(4) The step of the mold surface of the molding die is S0 larger than the step S determined in (3), and the shape of the mold surface is substantially the same as the inverted shape of the lattice plane determined in (3). Prepare a molding die.
[0036]
The preferable step S0 of the mold surface of the mold is known by a preliminary molding experiment.
(5) The mold surface of the mold prepared in (4) is brought into contact with the base material serving as the substrate of the diffraction grating, and the inverted shape of the mold surface of the mold is transferred to the base material.
(6) The base material is released from the molding die to obtain a diffraction grating substrate (single-layer type diffractive optical element) having a diffraction grating having a step of a grating surface of S.
[0037]
Thus, a single-layer diffractive optical element is manufactured.
[0038]
In order to manufacture a contact multilayer diffractive optical element, the procedure of (7) follows the procedure of (6).
(7) Further, the resin is cured by irradiating the resin with ultraviolet rays in a state in which the medium is in contact with an ultraviolet-curable resin as a medium on the grating surface of the diffraction grating substrate manufactured in (6).
In this manner, a contact multilayered diffractive optical element including the diffraction grating substrate and the ultraviolet curable resin adhered thereon is manufactured. When a thermosetting resin is used as the medium resin instead of the ultraviolet curable resin, the resin may be cured by heating. When a liquid or gas is used as the medium, a side wall as a container may be formed in advance at a position surrounding the grating surface on the diffraction grating substrate, and the container may be sealed after injecting the liquid or gas.
[0039]
In the conventional molding, for example, the valleys on the mold surface of the molding die are manufactured on the premise that the shape is a cutting edge-like ideal shape having a curvature radius of 0 as shown in FIG. On the other hand, in the present invention, as shown at 11 in FIG. 9, the diffraction grating is formed on the assumption that the valley of the mold surface has a roundness with a finite radius of curvature. The radius of curvature of the valley 11 of the mold surface is made smaller than the radius of curvature of the peak 5 of the desired lattice surface.
[0040]
If the radius of curvature of the valley 11 of the mold surface is too small, the radius of curvature of the peak of the formed lattice surface will be smaller than a desired value. As a result, a grating surface having a desired shape cannot be obtained, and the step of the grating surface of the diffraction grating to be formed becomes not only larger than a desired value S, but also costs more than necessary for manufacturing a mold. . Generally, the smaller the radius of curvature of the valley of the cutting edge-shaped mold surface, the higher the manufacturing cost of the mold.
[0041]
Conversely, if the radius of curvature of the valleys 11 of the mold surface is too large, the radius of curvature of the peaks of the lattice surface becomes larger than a desired value. As a result, a lattice surface of a desired shape cannot be obtained, and the formed diffraction grating Is smaller than the desired value S.
[0042]
If the grating surface of the diffraction grating is formed by a molding method, as described above, the peaks or valleys are particularly likely to be rounded due to problems in cutting the grating of the molding die and friction of the resin during molding. The present invention is particularly effective for a diffractive optical element manufactured by a molding method. Examples of the molding method include an injection molding method for molding a thermoplastic resin, a glass molding method for press-molding heated and softened glass, a resin molding method for curing an uncured liquid-state ultraviolet curable resin or a thermosetting resin, In particular, in the injection molding method and the glass molding method, the tips (peaks and valleys) of the lattice surface have large roundness. The larger the roundness, the greater the difference between the step between the peak of the mountain and the bottom of the valley and the ideal grating height, and the lower the diffraction efficiency in the conventional method. Therefore, the present invention is particularly effective in the injection molding method and the glass molding method.
[0043]
As described above, in the diffractive optical element of the present invention, since the step between the rounded peak and the rounded valley is determined based on Equation 1, the peaks and valleys of the grating surface of the diffractive optical element are rounded. Is relatively large, the diffraction efficiency is high, and the flare is small.
[0044]
Hereinafter, Examples 1 to 4 will be disclosed, and FIG. 9 is used in common for the description of these Examples.
[Example 1]
In this example, a 30 mm × 30 mm single-layer diffractive optical element shown in FIG. 6 was produced by a glass molding method. The material of the substrate 7 of this single-layer type diffractive optical element is a low melting point glass (P-SK60 manufactured by Sumita Optical Glass Co., Ltd .: d-line refractive index nd = n1 = 1.919). With.
(1) The wavelength used for the single-layer diffractive optical element was d-line wavelength 587.6 nm, the diffraction order was 10, and the medium was air (n2 = 1.0).
(2) By substituting λ = 587.6 nm, m = 10, n1 = 1.5919, and n2 = 1.0 into the above equation 2, a grating height of an ideal diffraction grating of 9.9 μm was obtained.
(3) The desired shape of the grating surface of the diffraction grating to be manufactured was determined from the desired optical characteristics of the single-layer diffractive optical element to be manufactured. In this desired shape, the lattice plane has rounded peaks and valleys, and the step S between the peak top and the valley bottom of the lattice plane is equal to the lattice height 9.9 μm determined in (2). The slope of the surface is linear, and the pitch is 150 μm.
(4) The shape of the mold surface of the mold was determined. The mold surface is 12.4 μm in which the step is 2.5 μm larger than the step difference of 9.9 μm determined in (3), and the inverted shape of the lattice plane determined in (3) is the direction of the valley (grating plane). (Corresponding to the direction of the hill). The radii of curvature of the valleys and ridges of the mold surface were made smaller than the radii of curvature of the ridges and valleys of the desired lattice plane respectively corresponding to the valleys and ridges.
(5) A mold having a mold surface having the shape determined in (4) was produced as follows.
An electroless plating layer mainly composed of Ni and P is formed on a 30 mm × 30 mm blank made of carbon steel, and the electroless plating layer is cut with a diamond bit having a tip diameter of 3 μm to form a desired mold surface. By forming the mold, a mold was produced.
(6) Using the mold prepared in (5) as an upper mold, a diffraction grating was formed by a glass mold method as follows. The mold surface of the lower mold was a flat surface.
[0045]
An upper mold 21 and a lower mold 22 set in a molding machine (the vicinity of the mold in a heated and pressurized state is shown in FIG. 12) and the base material 20 of a 30 mm × 30 mm flat plate of the low melting point glass are mixed with nitrogen. While heating to 415 ° C. in an atmosphere, the molds 21 and 22 are brought into contact with the base material 20, pressurized and pressed, and the mold surfaces 24 and 25 of the mold are transferred to the surface of the base material 20 and cooled. After the mold release, the diffractive optical element having the lattice surface formed on the upper surface of the base material (substrate) 20 and the flat surface formed on the lower surface was taken out of the molding machine.
[0046]
In the single-layer diffractive optical element manufactured in this manner, as shown in FIG. 9, the peaks and valleys of the lattice plane are more rounded than the corresponding valleys and peaks of the molding die, and The step S from the bottom of the valley was the desired 9.9 μm.
[Example 2]
In this embodiment, a contact multilayered diffractive optical element having a diameter of 50 mm shown in FIG. 10 was produced by a glass molding method. In FIG. 10, 4 is a lattice plane, 7 is a substrate, and 14 is a resin layer. The material of the substrate of the diffraction grating of this contact multilayer diffractive optical element is a low-melting glass (P-SK60 manufactured by Sumita Optical Glass Co., Ltd .: d-line refractive index nd = n1 = 1.919), and the resin as a medium is a urethane acrylate-based resin. (Nd = n2 = 1.555). The diffraction grating is a concentric blaze type, and its pitch becomes smaller from the center to the outside, and the concentric grating is arranged on a plane. The wavelength used is 587.6 nm, which is the wavelength of the d-line, and the diffraction order is 1.
(1) By substituting λ = 587.6 nm, m = 1, n1 = 1.519, and n2 = 1.555 into Expression 2, the ideal grating height of 16.3 μm was obtained.
(2) The shape of the grating surface of the diffraction grating to be fabricated was determined. The grating surface of the diffraction grating to be manufactured has rounded peaks 5 and valleys 6, and the step S between the top of the peak and the bottom of the valley of the grating surface has a grating height of 16.3 μm determined in (1). The slope of the lattice surface is curved, and the pitch is 2 mm at the center and 0.2 mm at the periphery.
(3) The shape of the mold surface of the mold was determined. The mold surface has a step of 18.8 μm, which is 2.5 μm larger than the step of 16.3 μm determined in (2), and sets the inverted shape of the lattice plane determined in (2) to the valley direction (grating plane). (Corresponding to the direction of the mountain). The radii of curvature of the valleys and ridges of the mold surface were made smaller than the radii of curvature of the ridges and valleys of the desired lattice plane respectively corresponding to the valleys and ridges.
(4) A mold having a mold surface having the shape determined above was produced as follows.
An electroless plating layer mainly composed of Ni and P is formed on a 50 mmφ blank made of carbon steel, and the electroless plating layer is cut with a diamond bit having a tip diameter of 3 μm to form a desired mold surface. Thus, a molding die was prepared.
(5) Using the mold prepared in (4) as an upper mold, a diffraction grating was formed by a glass mold method as follows. The mold surface of the lower mold was a flat surface.
An upper mold 21 and a lower mold 22 set in a molding machine (the vicinity of the mold in a heated and pressurized state is shown in FIG. 12) and a base material 20 of a 50 mmφ flat plate of the low melting glass are placed in a nitrogen atmosphere. While heating to 415 ° C., the molds 21, 22 are brought into contact with the base material 20, pressurized and pressed to transfer the mold surfaces 24, 25 to the surface of the base material 20, and then cooled and released. A diffraction grating substrate having a concentric lattice surface formed on the upper surface of the base material (substrate) 20 and a flat surface formed on the lower surface was taken out of the molding machine.
[0047]
In the diffraction grating substrate manufactured in this manner, as shown in FIG. 9, the peaks and valleys of the grating surface are more rounded than the corresponding valleys and peaks of the mold, and the peaks and the bottoms of the valleys are formed. Was the desired 16.3 μm.
(6) After a urethane acrylate-based UV-curable resin is dropped on the grating surface of the diffraction grating substrate, a stainless steel mold having a flat mold surface is brought close to the resin on the grating surface, and the resin is spread. The resin layer was adjusted to a predetermined thickness. In this state, ultraviolet rays (center wavelength: 365 nm) of a high-pressure mercury lamp were irradiated from the side of the diffraction grating substrate, and the resin was light-cured. The total amount of UV irradiation is 2000mJ / cm ² was.
(7) After the photo-curing, the mold was released from the resin to obtain a contact multilayered diffractive optical element having a desired diffraction grating as shown in FIG.
[0048]
In the contact multilayered diffractive optical element manufactured in this manner, as shown in FIG. 9, the peaks and valleys of the lattice surface are more rounded than the corresponding valleys and peaks of the mold, and And the step S between the bottom of the valley was the desired 16.3 μm.
[Example 3]
In this example, a single-layer type diffractive optical element of 30 mm × 30 mm shown in FIG. 6 was produced by an injection molding method. The base material of the single-layer diffractive optical element is an alicyclic olefin-based thermoplastic resin (ARTON manufactured by JSR: nd = n1 = 1.510), and has a blazed linear grating as a diffraction grating.
(1) Same as (1) of the first embodiment.
(2) By substituting λ = 587.6 nm, m = 10, n1 = 1.510, and n2 = 1.0 into Equation 2, a grating height of an ideal diffraction grating of 11.5 μm was obtained.
(3) The desired shape of the grating surface of the diffraction grating to be fabricated was determined. In this desired shape, the peaks 5 and the valleys 6 are rounded, and the step between the peaks of the lattice planes and the bottoms of the valleys is equal to the lattice height 11.5 μm determined in (2), and The slope is linear, with a pitch of 150 μm.
(4) The shape of the mold surface of the mold for injection molding was determined. The mold surface has a step of 13.5 μm, which is 2.0 μm larger than the step of 11.5 μm determined in (3), and sets the inverted shape of the lattice plane determined in (3) in the direction of the valley (grating plane). (Corresponding to the direction of the mountain) was determined to have a shape elongated by 2.0 μm. The radii of curvature of the valleys and ridges of the mold surface were made smaller than the radii of curvature of the ridges and valleys of the desired lattice plane respectively corresponding to the valleys and ridges.
(5) A single-layer diffractive optical element having a diffraction grating having a grating surface of a desired shape is prepared by preparing a mold for injection molding having a mold surface having the shape determined above and performing injection molding. Produced.
[0049]
In the single-layer diffractive optical element manufactured in this manner, as shown in FIG. 9, the peaks and valleys of the lattice plane are more rounded than the corresponding valleys and peaks of the molding die, and The step S with the bottom of the valley was the desired 11.5 μm.
[Example 4]
In this embodiment, a contact multilayered diffractive optical element having a diameter of 50 mm shown in FIG. 5 was produced by a glass molding method. In FIG. 5, 4 is a lattice plane, 7 is a substrate, and 14 is a resin layer. The material of the substrate of this contact multilayer diffractive optical element is a low-melting glass (P-SK60 manufactured by Sumita Optical Glass Co., Ltd .: d-line refractive index nd = n1 = 1.919), and the resin as a medium is a urethane acrylate-based ultraviolet curable resin. Mold resin (nd = n2 = 1.555). The diffraction grating is a concentric blaze type, and its pitch becomes smaller from the center toward the outside, and the concentric grating is arranged on a 93 mmR spherical surface that is convex upward. In FIG. 5, the concentric blazed diffraction grating is not shown. The center thickness of the substrate is 12 mm, and the surface facing the lattice surface of the substrate is 93 mmR concave downward. The thickness of the resin layer is 0.1 mm, and the surface of the resin layer facing the lattice plane is 93 mmR which is upwardly convex. The wavelength used is 587.6 nm, which is the wavelength of the d-line, and the diffraction order is 1.
(1) Determined in the same manner as (1) of Example 2.
(2) The shape of the grating surface of the diffraction grating to be fabricated was determined to be the same shape as (2) of Example 2. However, the concentric lattices were arranged not on a plane but on a spherical surface of 93 mmR convex upward.
(3) The shape of the mold surface of the upper mold was determined to be the same shape as (3) of Example 2. However, the mold surface was not made flat as a whole, but was made into a spherical shape with a concave recess of 93 mmR. In addition, the shape of the mold surface of the lower mold for molding the surface facing the lattice surface of the substrate was determined to be 93 mmR which is convex upward.
(4) An upper mold was produced by forming a mold surface having the shape determined in (3) on a blank in the same manner as in (4) of Example 2. For the lower mold, the mold surface whose shape is determined above is formed into a 50 mmφ blank made of carbon steel, and an electroless plating layer mainly composed of Ni and P is formed thereon, followed by cutting. Produced by
(5) Using the mold prepared in (4), a spherical surface facing the diffraction grating was formed by the same glass molding method as in (5) of Example 2.
In the diffraction grating substrate manufactured in this manner, as shown in FIG. 9, the peaks and valleys of the grating surface are more rounded than the corresponding valleys and peaks of the mold, and the peaks and the bottoms of the valleys are formed. Was the desired 16.3 μm.
(6) After a urethane acrylate-based ultraviolet curable resin is dropped on the grating surface of the diffraction grating substrate, a stainless steel mold having a 93 mmR spherical surface with a concave upper surface is brought close to the resin on the grating surface. The resin was spread out, and the thickness of the resin layer was adjusted to 0.1 mm. In this state, ultraviolet rays (center wavelength: 365 nm) of a high-pressure mercury lamp were irradiated from the side of the diffraction grating substrate, and the resin was light-cured. The total amount of UV irradiation is 2000mJ / cm ² was.
(7) After the photo-curing, the resin was released from the mold to obtain a contact multilayered diffractive optical element having a desired diffraction grating and a desired spherical surface as shown in FIG.
[0050]
As described above, the diffractive optical elements according to the present invention described in the first to fourth embodiments have the rounded peaks and the rounded valleys even when the peaks or the valleys of the lattice planes are rounded. Since it is designed in consideration of the step with the bottom, the diffraction efficiency is high, the flare light is small, and the performance is high. Further, in the production of the diffractive optical element of the present invention, it is not necessary to make the shape of the mold surface of the molding die into a cutting edge having a minimum radius of curvature between peaks and valleys, which is an ideal shape of a blazed diffraction grating. In addition, since it is not necessary to make the shape of the grating surface of the diffraction grating an exact inverted shape of the mold surface of the mold, the manufacturing is easy and the manufacturing cost is low.
[Example 5]
FIG. 11 shows an optical system for a camera according to the present invention, in which the contact multilayered diffractive optical element 15 of Example 4 of the present invention is used.
[0051]
Since the optical system of the present invention uses the diffractive optical element of the present invention having high diffraction efficiency, low flare light, and low manufacturing cost, it has high transmittance, low flare, and low manufacturing cost. Further, since the characteristics of the diffractive optical element of the present invention are effectively used, the device is compact.
[0052]
【The invention's effect】
The diffractive optical element of the present invention is designed in consideration of the step between the top of the rounded hill and the bottom of the rounded valley, even when the peak or the bottom of the valley of the lattice plane is rounded. Therefore, the diffraction efficiency is high and the flare light is small. An optical system such as a camera using the diffractive optical element of the present invention has a low flare and a high transmittance.
[Brief description of the drawings]
FIG. 1 shows a cross section of a grating surface 4 of a diffraction grating 3 of a single-layer diffractive optical element and a contact multilayer diffractive optical element of the present invention.
FIG. 2 shows a cross-sectional shape of an ideal grating surface 4 of a blazed diffraction grating.
FIG. 3 shows a relationship between a lattice height L of an ideally-shaped lattice plane and a step S ′ of the lattice plane obtained by molding.
FIG. 4 shows a state of cutting of a lattice.
FIG. 5 is a close-contact multi-layer diffractive optical element of Example 4 of the present invention.
FIG. 6 shows a single-layer diffractive optical element 1 of the present invention.
FIG. 7 shows a contact multilayer diffractive optical element 2 of the present invention.
FIG. 8 shows an ideal shape of a blazed diffractive optical element.
FIG. 9 is an enlarged view of a cross section of a grating surface of a diffraction grating and a mold surface of a mold, which are commonly used for describing a diffractive optical element and a method of manufacturing the same according to the embodiments and examples of the present invention.
FIG. 10 shows a contact multilayer diffractive optical element of Example 2 of the present invention.
FIG. 11 shows an optical system for a camera using the diffractive optical element of the present invention.
FIG. 12 shows the vicinity of a molding die of a molding machine.
[Explanation of symbols]
1 Single-layer diffractive optical element
2 Contact multilayer diffractive optical element
3 Diffraction grating
4 Lattice surface
5 Mountains on the grid surface
6 trough of lattice plane
7 Diffraction grating substrate
8 Medium
9 Medium part
10 Top of valley of ideal mold
11. Round valley of the mold used in the present invention
14 Resin layer
15 Adhesive multilayer diffractive optical element of Example 4
20 Base material
21 Upper mold
22 Lower mold
23 sleeve
24, 25 mold surface

Claims

A diffractive optical element having a diffraction grating, wherein at least one of a peak and a valley of a grating surface of the diffraction grating is rounded, a wavelength used is λ, a refractive index of a substrate of the diffraction grating is n1, and the grating surface is Wherein the refractive index of the medium adjacent to the lattice plane is n2 and m is an integer, and the step S between the top of the ridge and the bottom of the valley of the lattice plane is given by Equation 1.
(Equation 1) S = m × λ / (n1-n2)

2. The diffractive optical element according to claim 1, wherein the material of the substrate of the diffraction grating is glass.

A diffractive optical element comprising a diffraction grating and a medium portion having a medium adjacent to a grating surface of the diffraction grating, wherein the medium is made of a fluid or a resin, and at least one of peaks or valleys of the grating surface has a rounded shape. When the wavelength used is λ, the refractive index of the substrate of the diffraction grating is n1, the refractive index of the medium adjacent to the grating surface is n2, and m is an integer, the peaks and valleys of the grating surface are A diffractive optical element, wherein a step S with respect to the bottom is given by Expression 1.
(Equation 1) S = m × λ / (n1-n2)

4. The diffractive optical element according to claim 3, wherein the material of the substrate of the diffraction grating is glass.

A diffractive optical element having a diffraction grating optimized for a desired wavelength and a desired diffraction order and having a grating surface with a rounded peak or valley is manufactured by molding a base material. A method wherein the desired working wavelength λ, the desired diffraction order m, the refractive index n1 of the substrate of the diffraction grating, and the refractive index n2 of the medium adjacent to the grating surface of the diffraction grating are represented by the following formulas:
L = m × λ / (n1-n2)
To obtain the lattice height L of the lattice plane of the ideal shape, and prepare a mold in which the step between the top of the hill and the bottom of the valley in the mold surface for transferring the lattice plane to the base material is larger than the lattice height L. Contacting the molding die with the base material, and transferring a lattice surface having a substantially inverted shape of the mold surface and a step equal to the lattice height L to the base material, and transferring the base material from the molding die. Releasing the mold from the mold.

Further, when the mold surface of the molding die has a radius of curvature of a rounded peak of the grating surface of the diffraction grating of the diffraction optical element to be manufactured as R1 and a radius of curvature of the rounded valley of the mold surface as R2, R2 The method of manufacturing a diffractive optical element according to claim 5, wherein <R1 is satisfied.

7. The method according to claim 5, wherein a material of the substrate of the diffraction grating is glass.

8. The method for manufacturing a diffractive optical element according to claim 5, further comprising a step of bringing a fluid or a resin into contact with the grating surface to maintain the contact state. .

An optical system comprising the diffractive optical element according to claim 1.