JP3829356B2

JP3829356B2 - Optical head device

Info

Publication number: JP3829356B2
Application number: JP09440596A
Authority: JP
Inventors: 弘樹保高; 譲田辺; 友紀郡島
Original assignee: Asahi Glass Co Ltd
Current assignee: AGC Inc
Priority date: 1996-04-16
Filing date: 1996-04-16
Publication date: 2006-10-04
Anticipated expiration: 2016-04-16
Also published as: JPH09281332A

Description

【０００１】
【発明の属する技術分野】
本発明は、ＣＤ（コンパクト・ディスク）、ＣＤ−ＲＯＭ、ビデオディスク等の光ディスク及び光磁気ディスク等に光学的情報を書き込んだり、光学的情報を読み取るための光ヘッド装置に関する。
【０００２】
【従来の技術】
従来、光ディスク及び光磁気ディスク等に光学的情報を書き込んだり、光学的情報を読み取る光ヘッド装置としては、ディスクの記録面から反射された信号光を検出部へ導光（ビームスプリット）する光学部品としてプリズム式ビームスプリッタを用いたものと、回折格子又はホログラム素子を用いたものとが知られていた。
【０００３】
従来、光ヘッド装置用の回折格子又はホログラム素子は、ガラスやプラスチック基板上に、矩形断面を有する矩形格子（レリーフ型）をドライエッチング法又は射出成形法によって形成し、これによって光を回折しビームスプリット機能を付与していた。
【０００４】
また、光の利用効率が１０％程度の等方性回折格子よりも光の利用効率を上げようとする場合、偏光を利用することが考えられる。偏光を利用しようとすると、プリズム式ビームスプリッタにλ／４板を組み合わせて、往き（光源から記録面へ向かう方向）及び帰り（記録面から検出部へ向かう方向）の効率を上げて往復効率を上げる方法があった。
【０００５】
しかし、プリズム式偏光ビームスプリッタは高価であり、他の方式が模索されていた。一つの方式としてＬｉＮｂＯ₃ 等の複屈折結晶の平板を用い、表面に異方性回折格子を形成し偏光選択性をもたせる方法が知られている。しかし、複屈折結晶自体が高価であり、民生分野への適用は困難である。また通常、プロトン交換法によって格子を形成するため、細かいピッチの格子を形成するのが困難である問題もあった。
【０００６】
等方性回折格子は前述のように、往き（光源から記録面へ向かう方向）の利用効率が５０％程度で、帰り（記録面から検出部へ向かう方向）の利用効率が２０％程度であるため、往復で１０％程度が限界である。
【０００７】
それに対して、透明基板上に格子状凹凸部を形成し、そこに液晶組成物を充填することによって光の利用効率の高いホログラム（回折素子）を利用した光ヘッド装置が、本出願人により提案されている。
【０００８】
しかし、液晶ディスプレイ等で工業的かつ一般的に使用されている液晶組成物材料では、屈折率の異方性が小さくかつ使用温度範囲が狭いという欠点があった。屈折率の異方性が小さいと、所望の回折効率を得るために、格子深さを深くする必要があり、加工そのものがきわめて困難である。また、加工に多大の費用がかかる問題があった。
【０００９】
また高温側の相転移温度が低いと、光ヘッド装置として使用する最高温度の６０〜７０℃で仮に液晶性（光学異方性）を示しても、常温付近で回折効率が大きく低下する問題があり、実用には適さなかった。また高温での信頼性にも問題があった。
【００１０】
また低温側の相転移温度が高いと、光ヘッド装置として使用する最低温度の０℃付近で仮に液晶性を示しても、常温付近で大きく回折効率が低下するという問題があり、実用には適さなかった。また低温での信頼性にも問題があった。
【００１１】
【発明が解決しようとする課題】
本発明は、前記問題を解消し広い温度範囲で高い光利用効率を有する光ヘッド装置を提供することを目的とする。
【００１２】
【課題を解決するための手段】
本発明は、光源からの光を光学異方性回折格子と位相差フィルムとを備えた回折素子を通して光記録媒体上に照射することにより情報の書き込み及び／又は情報の読み取りを行う光ヘッド装置において、前記回折素子は、透明基板の表面に格子状の凹凸部が形成され前記凹凸部に光学異方性を有する液晶組成物が充填されている光学異方性回折格子を備えてなり、前記液晶組成物は、温度２０℃、光波長５８９ｎｍにおける常光屈折率と異常光屈折率との差Δｎが０．２７〜０．３５であり、ネマチック相からアイソトロピック相への相転移温度が８０℃以上であり、ネマチック相からスメクチック相又は固体液晶相への相転移温度が−１０℃以下であり、下記一般式で表される化合物を６０重量％以上含有してなるネマチック液晶組成物であることを特徴とする光ヘッド装置を提供する。
【００１３】
【化２】

【００１４】
ただし、Ａはフェニレン基又はトランス−１，４−シクロヘキシレン基、ｍは０又は１、Ｘはフッ素原子又は水素原子、Ｙはシアノ基、フッ素原子又は塩素原子、Ｚはフッ素原子又は水素原子、Ｒは炭素数２〜８の直鎖状アルキル基又は炭素数２〜８の直鎖状アルコキシル基である。
【００１５】
【発明の実施の形態】
本発明では、前記凹凸部は光学的に等方的であってもよく、非等方的であってもよい。前記凹凸部が光学的に等方的である場合、前記透明基板として、屈折率が１．５程度のガラス基板、プラスチック基板等を使用し、その透明基板に前記凹凸部を直接形成したものを使用でき好ましい。
【００１６】
また一般に液晶組成物の常光屈折率は異常光屈折率より低く、常光屈折率が１．５付近のものが多いため、基板の屈折率と液晶組成物の常光屈折率が等しいという条件が実現しやすく、好ましい。
【００１７】
液晶分子の長軸方向は、格子状の前記凹凸部のストライプ方向（長手方向）に対して平行に配向すると考えられる。そのため、図１でみた場合紙面と平行な方向に偏光した光（Ｐ波）に対しては液晶組成物の常光屈折率が対応し、前記常光屈折率と透明基板の屈折率はほぼ等しく、回折格子として機能しない。一方、紙面と垂直に偏光した光（Ｓ波）に対しては液晶の異常光屈折率が対応し、前記異常光屈折率と基板の屈折率は異なり、回折格子として機能する。
【００１８】
したがって、本発明の光学異方性回折格子は、Ｐ波に対しては光学的に透明であり、Ｓ波に対しては回折格子として機能する。
【００１９】
前記光学異方性回折格子を設けた透明基板の上に（光記録媒体側に）、ポリカーボネート、ポリビニルアルコール等の材料からなる位相差フィルム（λ／４板）を積層すると、下方（光源側）から入射したＰ波は光学異方性回折格子をほぼ１００％透過し、位相差フィルムで円偏光となる。その後非球面レンズ（対物レンズ）を通過し、光記録媒体の記録面で反射し、再び非球面レンズを透過し、再度位相差フィルムを透過するとＳ波に変換され、光学異方性回折格子に入射する。Ｓ波に対して光学異方性回折格子は回折格子として機能する。
【００２０】
例えばその長手方向に垂直な断面において左右対称な矩形状凹凸部とし、その深さが適切に設定された場合には、原理的には１次回折光方向に４０％程度の回折効率、−１次回折光方向に４０％程度回折効率で、光を回折できる。
【００２１】
前記凹凸部の長手方向に垂直な面における断面形状は、長方形、正方形等の左右対称の矩形形状でもよく、階段状、のこぎり状等の左右非対称の形状でもよい。左右非対称の形状の場合、光学異方性回折格子による±１次回折光のうちいずれか一方の回折効率が高くなり、回折効率の高い方の回折光のみを検出すればよく、検出器が１つで高い光の利用効率が得られるため好ましい。
【００２２】
さらに前記凹凸部については、凹凸部と凹凸部の間隔に分布を付与する、左右対称のものと左右非対称のものとを混在させる、凹凸部と凸部を混在させる等の変更もできる。
【００２３】
前記の構成及び作用効果からして、本発明の回折素子は集積化が容易で高効率な偏光ビームスプリッタとして機能する。このような回折素子の使用により、光利用効率の高い光ピックアップを実現できる。
【００２４】
しかし、前記の格子状の凹凸部の深さは、液晶組成物の異常光屈折率と基板の屈折率（ほぼ液晶組成物の常光屈折率に等しい）の差に凹凸部の深さを乗じた値が、光波長の半分に等しいときに、原理的に最も高い回折効率が得られる。
【００２５】
そのため量産上有利な比較的浅い凹凸部として、高い回折効率を実現するためには、基板の屈折率と液晶組成物の異常光屈折率の差、実質的には液晶組成物の常光屈折率と異常光屈折率との差であるΔｎが大きいことが必要となる。
【００２６】
一方、液晶表示素子等で通常用いられている液晶組成物材料では、液晶組成物の常光屈折率と異常光屈折率との差が大きいと、表示素子としての特性が液晶組成物が充填される空間（セルギャップ）の変動に対して敏感になるため、必ずしも好まれない。そのため、液晶表示素子用の液晶組成物としては、Δｎが０．２０以下のものが通常使用されている。
【００２７】
本発明では、Δｎは０．２７以上であるので、量産が容易な浅い凹凸部とでき、また所望の高い回折効率を実現できる。一方、Δｎが０．３６より大きいと、液晶組成物自体が紫外線等に対して不安定になり信頼性の点で問題となる。そのため、Δｎは０．３５以下とする。
【００２８】
液晶組成物としては、広い温度範囲で高い屈折率差を保持でき、かつその変動率の小さい液晶組成物が好ましい。また変動率とも関連して広い温度範囲で液晶性を示す液晶組成物が好ましい。本発明における回折素子の場合、使用温度範囲は通常０〜６０℃である。したがって、液晶相であるネマチック相から非液晶相であるアイソトロピック相への相転移温度が８０℃以上のときに、０〜６０℃の範囲で特に異常光屈折率の変動が小さく、回折効率の温度変化を小さくできる。
【００２９】
また低温側では、最低０℃程度までの温度で、安定した動作及びより低温での保存時の信頼性を確保するために、ネマチック液晶相からスメクチック液晶相又は固体液晶相への相転移温度は−１０℃以下が好ましい。
【００３０】
本発明では、前記のような諸特性、すなわち高いΔｎ、低い温度変化率、広いネマチック液晶相の温度範囲、高い信頼性を実現する材料として、下記一般式で表される化合物を６０重量％以上含有してなる液晶組成物を使用する。
【００３１】
【化３】

【００３２】
ただし、Ａはフェニレン基（以下、−Ｐｈ−と略記する）又はトランス−１，４−シクロヘキシレン基（以下、−Ｃｈ−と略記する）、ｍは０又は１、Ｘはフッ素原子又は水素原子、Ｙはシアノ基、フッ素原子又は塩素原子、Ｚはフッ素原子又は水素原子、Ｒは炭素数２〜８の直鎖状アルキル基又は炭素数２〜８の直鎖状アルコキシル基である。
【００３３】
また前記液晶組成物には、４’−トランス−ｎ−プロピル−４−シクロヘキシル−１−シアノベンゼン、４’−トランス−ｎ−プロピル−４−シクロヘキシル−１−フルオロベンゼン等を適宜混合してもよい。
【００３４】
本発明の回折素子は、さらに前記光源側の面に他の回折格子を形成してもよく、その場合３ビーム法によるトラッキングエラー検出ができ好ましい。
【００３５】
本発明の凹凸部（光学異方性回折格子）のパターンは、光記録媒体からの戻り光のビーム形状が所望の形状になるように、回折格子面内で曲率をつけたり、格子間隔に分布をつけたりもできる。
【００３６】
前記光学異方性回折格子は、表面の回折格子パターンに形成された凹凸部を各々有する２枚の透明基板を、前記凹凸部が対面した状態で凹凸部に液晶組成物を充填し、積層して形成してもよい。その場合、各々の凹凸部の深さは浅くてよく、そのため作製が容易になり好ましい。また、２つの対面する凹凸部により液晶組成物の配向性が向上する点でも好ましい。
【００３７】
前記２枚の透明基板に形成された凹凸部が、積層面に対して非対称となるように積層されている場合、断面形状が非対称な回折格子を容易に作製でき、±１次回折光のいずれか一方の回折効率を大きくし、回折効率の大きい方の光を１つの検出器で検出できるという効果があり好ましい。
【００３８】
本発明の回折素子の光源側の面か光記録媒体側の面の少なくともいずれか一方の面に、ＵＶ硬化型アクリル樹脂等の被膜を設けた場合、λ／４板やガラス基板の表面の凹凸に起因する波面収差を低減でき好ましい。さらに前記ＵＶ硬化型アクリル樹脂等の被膜の上に、平坦度のよいガラス基板やプラスチック基板等を積層することにより、格段に波面収差を低減でき好ましい。したがって、回折素子の光の入出射面が平坦化されていることにより、結果的に波面収差が低減化される。
【００３９】
本発明の光源としては半導体レーザ、ＹＡＧレーザ等の固体レーザ、Ｈｅ−Ｎｅ等の気体レーザが使用でき、半導体レーザが小型軽量化、連続発振、保守点検等の点で好ましい。また、光源部に半導体レーザ等と非線形光学素子を組み込んだ高調波発生装置（ＳＨＧ）を使用し、青色レーザ等の短波長レーザを用いると、高密度の光記録及び読み取りが可能になる。
【００４０】
本発明における光記録媒体とは、光により情報を記録及び／又は読み取ることができる媒体である。光記録媒体の例としては、ＣＤ（コンパクトディスク）、ＣＤ−ＲＯＭ、ＤＶＤ（デジタルビデオディスク）の光ディスク、及び光磁気ディスク、相変化型光ディスク等が挙げられる。
【００４１】
【実施例】
１０ｍｍ縦×１０ｍｍ横×０．５ｍｍ厚、屈折率１．５２の第１のガラス基板１上に、フォトリソグラフィ法及びドライエッチング法により、深さ１．２μｍ、ピッチ（周期）１０μｍの矩形状断面を有する格子状の凹凸部２を形成した。
【００４２】
１０ｍｍ縦×１０ｍｍ横×０．５ｍｍ厚、屈折率１．５２の第２のガラス基板３を用意し、その液晶組成物４と接する側の面にポリイミド配向膜５を形成した。前記ポリイミド配向膜５のラビング方向を前記凹凸部２のストライプ方向に沿うようにして、第２のガラス基板３を第１のガラス基板１に積層し接着した。
【００４３】
２つのガラス基板の積層接着は具体的には以下のように行った。直径４μｍの球状スペーサを含むエポキシ樹脂８を第１のガラス基板１の周辺部に塗布し、第２のガラス基板３を載置し、上部より押圧し接着した。その際、２つのガラス基板の周辺部にはエポキシ樹脂８を一部塗布しない領域を設けておき、液晶組成物注入用の開口部とした。このとき、第１のガラス基板１の凹凸部（液晶組成物充填部）２と、第２のガラス基板３のポリイミド配向膜５とを対面させて接着した。
【００４４】
その後、減圧した雰囲気中で混合液晶組成物（ネマチック液晶組成物、Δｎ＝０．２８９６、常光屈折率＝１．５２６５、固体液晶相への相転移温度≦−２０℃、アイソトロピック相への相転移温度＝１０８℃）を、前記開口部から注入した。前記開口部を封止用の樹脂で塞ぎ、シールを完了した。前記混合液晶組成物は表１に示すように、前記化３の一般式に含まれる５種類の化合物を主成分（８６重量％）として含む。その他の液晶成分として、アルコキシシアノビフェニル系液晶及びアルキルシアノターフェニル系液晶等を１４重量％含む。
【００４５】
第２のガラス基板３のポリイミド配向膜５とは反対側の面に、透明接着剤を用いてポリカーボネート製の位相差フィルム（λ／４板）６を接着した。さらに位相差フィルム６の上部にＵＶ硬化型アクリル樹脂を塗布し、その上に第３のガラス基板７を押し当て紫外線を照射して第３のガラス基板７を接着し、回折素子１０を作製した。回折素子１０の光の入射面（第１のガラス基板の凹凸部と反対側の面）１２及び光の出射面（第３のガラス基板のＵＶ硬化型アクリル樹脂と反対側の面）１１には、それぞれ光源からの光に対する反射防止膜１３を形成した。
【００４６】
以上の結果、回折素子１０は、半導体レーザ（図示せず）からの波長６７８ｎｍのＰ波（図１において紙面に平行な偏光方向を持つ光）に対して９６％の透過率であった。また、光ディスク（図示せず）から反射してきたＳ波（図１において紙面に垂直な偏光方向を持つ光）に対しては、１次回折光の回折効率が３９．４％で、−１次回折光の回折効率が３６．７％であった。したがって往復効率は７３％となった。また透過光の波面収差は、回折素子の光の入出射面の中心部（直径２ｍｍの円形部）で０．０１４λ_rms （ｒｍｓ：自乗平均）以下であった。
【００４７】
【表１】

【００４８】
【発明の効果】
本発明の光ヘッド装置は、ネマチック液晶相として存在する温度範囲が広く、Δｎが大きい液晶組成物を用いているため、温度変化に対して安定しており、また浅い格子状の凹凸部で高い光の利用効率を実現できるという効果を有する。
【図面の簡単な説明】
【図１】本発明の実施例の光ヘッド装置の側断面図。
【符号の説明】
１：第１のガラス基板
２：凹凸部
３：第２のガラス基板
４：液晶組成物
５：ポリイミド配向膜
６：位相差フィルム
７：第３のガラス基板
８：エポキシ樹脂
１０：回折素子
１１：入射面
１２：出射面
１３：反射防止膜[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an optical head device for writing optical information on an optical disk such as a CD (compact disk), a CD-ROM, a video disk, and a magneto-optical disk, and for reading the optical information.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, as an optical head device that writes optical information on an optical disk, a magneto-optical disk, or the like, or reads optical information, an optical component that guides (beam splits) signal light reflected from the recording surface of the disk to a detection unit In particular, the one using a prism type beam splitter and the one using a diffraction grating or a hologram element are known.
[0003]
Conventionally, a diffraction grating or hologram element for the optical head device, a glass or plastic substrate, thus forming a rectangular grid of (relief type) in a dry etching method or an injection molding method having a rectangular cross-section, whereby diffracted light A beam split function was added.
[0004]
In addition, it is conceivable to use polarized light when attempting to increase the light utilization efficiency as compared with an isotropic diffraction grating having a light utilization efficiency of about 10%. When using polarized light, a prism beam splitter is combined with a λ / 4 plate to increase the efficiency of reciprocation (direction from the light source to the recording surface) and return (direction from the recording surface to the detection unit) to increase the reciprocal efficiency. There was a way to raise.
[0005]
However, prismatic polarization beam splitters are expensive, and other methods have been sought. As one method, a method is known in which a flat plate of a birefringent crystal such as LiNbO ₃ is used, and an anisotropic diffraction grating is formed on the surface to provide polarization selectivity. However, the birefringent crystal itself is expensive and difficult to apply to the consumer field. In addition, since the lattice is usually formed by the proton exchange method, there is a problem that it is difficult to form a fine pitch lattice.
[0006]
As described above, the use efficiency of the isotropic diffraction grating is about 50% in the forward direction (direction from the light source to the recording surface), and the utilization efficiency in the return direction (direction from the recording surface to the detection unit) is about 20%. Therefore, about 10% is the limit in the round trip.
[0007]
On the other hand, the present applicant proposes an optical head device that uses a hologram (diffraction element) with high light utilization efficiency by forming a lattice-shaped uneven portion on a transparent substrate and filling it with a liquid crystal composition. Has been.
[0008]
However, the liquid crystal composition materials used industrially and generally in liquid crystal displays and the like have the disadvantages that the refractive index anisotropy is small and the operating temperature range is narrow. When the anisotropy of the refractive index is small, it is necessary to increase the grating depth in order to obtain a desired diffraction efficiency, and the processing itself is extremely difficult. In addition, there is a problem that the processing is very expensive.
[0009]
Also, if the phase transition temperature on the high temperature side is low, even if liquid crystallinity (optical anisotropy) is exhibited at the maximum temperature of 60 to 70 ° C. used as an optical head device, the diffraction efficiency is greatly reduced near room temperature. Yes, it was not suitable for practical use. There was also a problem with reliability at high temperatures.
[0010]
Moreover, if the phase transition temperature on the low temperature side is high, there is a problem that even if liquid crystallinity is exhibited at around 0 ° C., the lowest temperature used as an optical head device, there is a problem that the diffraction efficiency is greatly reduced at around room temperature, which is suitable for practical use. There wasn't. There was also a problem with reliability at low temperatures.
[0011]
[Problems to be solved by the invention]
It is an object of the present invention to provide an optical head device that solves the above problems and has high light utilization efficiency over a wide temperature range.
[0012]
[Means for Solving the Problems]
The present invention relates to an optical head device for writing information and / or reading information by irradiating light from a light source on an optical recording medium through a diffraction element including an optical anisotropic diffraction grating and a retardation film . The diffraction element comprises an optically anisotropic diffraction grating in which a lattice-shaped uneven portion is formed on the surface of a transparent substrate, and the uneven portion is filled with a liquid crystal composition having optical anisotropy. The composition has a difference Δn between ordinary light refractive index and extraordinary light refractive index at a temperature of 20 ° C. and a light wavelength of 589 nm of 0.2 7 to 0.35, and a phase transition temperature from a nematic phase to an isotropic phase of 80 ° C. A nematic liquid crystal composition having a phase transition temperature from a nematic phase to a smectic phase or a solid liquid crystal phase of −10 ° C. or lower and containing 60% by weight or more of a compound represented by the following general formula: An optical head device is provided.
[0013]
[Chemical 2]

[0014]
Where A is a phenylene group or trans-1,4-cyclohexylene group, m is 0 or 1, X is a fluorine atom or hydrogen atom, Y is a cyano group, fluorine atom or chlorine atom, Z is a fluorine atom or hydrogen atom, R is a linear alkyl group having 2 to 8 carbon atoms or a linear alkoxyl group having 2 to 8 carbon atoms.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
In the present invention, the concavo-convex portion may be optically isotropic or anisotropic. When the concavo-convex portion is optically isotropic, a glass substrate or a plastic substrate having a refractive index of about 1.5 is used as the transparent substrate, and the concavo-convex portion is directly formed on the transparent substrate. It can be used and is preferable.
[0016]
In general, the ordinary refractive index of the liquid crystal composition is lower than the extraordinary refractive index, and the ordinary refractive index is often around 1.5, so that the condition that the refractive index of the substrate and the ordinary refractive index of the liquid crystal composition are equal is realized. Easy and preferable.
[0017]
The major axis direction of the liquid crystal molecules is considered to be aligned in parallel to the stripe direction (longitudinal direction) of the lattice-shaped uneven portions. Therefore, when viewed in FIG. 1, the ordinary light refractive index of the liquid crystal composition corresponds to the light polarized in the direction parallel to the paper surface (P wave), and the ordinary light refractive index and the refractive index of the transparent substrate are substantially equal. Does not function as a grid. On the other hand, the extraordinary refractive index of the liquid crystal corresponds to light (S wave) polarized perpendicular to the paper surface, and the extraordinary refractive index and the refractive index of the substrate are different and function as a diffraction grating.
[0018]
Therefore, the optically anisotropic diffraction grating of the present invention is optically transparent for P waves and functions as a diffraction grating for S waves.
[0019]
When a retardation film (λ / 4 plate) made of a material such as polycarbonate or polyvinyl alcohol is laminated on the transparent substrate (on the optical recording medium side) provided with the optical anisotropic diffraction grating, the lower side (light source side) The P wave incident from the light passes through the optical anisotropic diffraction grating almost 100% and becomes circularly polarized light by the retardation film. Thereafter, the light passes through an aspheric lens (objective lens), is reflected by the recording surface of the optical recording medium, is transmitted through the aspheric lens again, and is transmitted through the retardation film again. Incident. The optical anisotropic diffraction grating functions as a diffraction grating for the S wave.
[0020]
For example, in the case of a rectangular concavo-convex portion that is symmetrical in the cross section perpendicular to the longitudinal direction and the depth is set appropriately, in principle, the diffraction efficiency is about 40% in the first-order diffracted light direction, and the next time Light can be diffracted in the folding direction with a diffraction efficiency of about 40%.
[0021]
The cross-sectional shape of the surface of the concavo-convex portion perpendicular to the longitudinal direction may be a rectangular shape such as a rectangle or a square, or a left-right asymmetric shape such as a step shape or a saw shape. In the case of a left-right asymmetric shape, either one of the ± 1st-order diffracted lights by the optical anisotropic diffraction grating has a higher diffraction efficiency, and it is only necessary to detect the diffracted light having the higher diffraction efficiency, and one detector. It is preferable because high light utilization efficiency can be obtained.
[0022]
Further, the uneven portion can be changed such as providing a distribution in the interval between the uneven portion, making the left-right symmetrical and left-right asymmetric mixed, and making the uneven portion and the convex mixed.
[0023]
Due to the above-described configuration and operational effects, the diffraction element of the present invention functions as a highly efficient polarizing beam splitter that is easy to integrate. By using such a diffraction element, an optical pickup with high light utilization efficiency can be realized.
[0024]
However, the depth of the rugged portion of the lattice shape is obtained by multiplying the difference between the extraordinary refractive index of the liquid crystal composition and the refractive index of the substrate (approximately equal to the ordinary refractive index of the liquid crystal composition) by the depth of the concavo-convex portion. In principle, the highest diffraction efficiency is obtained when the value is equal to half the light wavelength.
[0025]
Therefore, in order to realize high diffraction efficiency as a relatively shallow uneven part advantageous for mass production, the difference between the refractive index of the substrate and the extraordinary light refractive index of the liquid crystal composition, substantially the ordinary light refractive index of the liquid crystal composition It is necessary that Δn which is a difference from the extraordinary light refractive index is large.
[0026]
On the other hand, in a liquid crystal composition material normally used in a liquid crystal display element or the like, if the difference between the ordinary light refractive index and the extraordinary light refractive index of the liquid crystal composition is large, the liquid crystal composition is filled with characteristics as a display element. This is not always preferred because it becomes sensitive to changes in space (cell gap). Therefore, as a liquid crystal composition for a liquid crystal display element, one having Δn of 0.20 or less is usually used.
[0027]
In the present invention, [Delta] n is because 0.2 7 above, mass production can be an uneven portion easily shallow, also possible to realize a desired high diffraction efficiency. Hand, [Delta] n is larger than 0.36, the liquid crystal composition itself is a problem in terms of reliability becomes unstable to ultraviolet light or the like. Therefore, Δn is set to 0.35 or less.
[0028]
As the liquid crystal composition, a liquid crystal composition that can maintain a high refractive index difference in a wide temperature range and has a small variation rate is preferable. Further, a liquid crystal composition exhibiting liquid crystallinity in a wide temperature range in relation to the variation rate is preferable. In the case of the diffraction element in the present invention, the operating temperature range is usually 0 to 60 ° C. Therefore, when the phase transition temperature from the nematic phase which is a liquid crystal phase to the isotropic phase which is a non-liquid crystal phase is 80 ° C. or higher, the fluctuation of the extraordinary optical refractive index is particularly small in the range of 0 to 60 ° C. Temperature change can be reduced.
[0029]
On the low temperature side, the phase transition temperature from the nematic liquid crystal phase to the smectic liquid crystal phase or the solid liquid crystal phase is assured in order to ensure stable operation at temperatures up to around 0 ° C and storage reliability at lower temperatures. -10 degrees C or less is preferable.
[0030]
In the present invention, a compound represented by the following general formula is 60% by weight or more as a material that realizes the above-mentioned characteristics, that is, high Δn, low temperature change rate, wide temperature range of nematic liquid crystal phase, and high reliability. The liquid crystal composition formed is used.
[0031]
[Chemical 3]

[0032]
However, A is a phenylene group (hereinafter abbreviated as -Ph-) or trans-1,4-cyclohexylene group (hereinafter abbreviated as -Ch-), m is 0 or 1, and X is a fluorine atom or a hydrogen atom. Y represents a cyano group, a fluorine atom or a chlorine atom, Z represents a fluorine atom or a hydrogen atom, and R represents a linear alkyl group having 2 to 8 carbon atoms or a linear alkoxyl group having 2 to 8 carbon atoms.
[0033]
Further, 4′-trans-n-propyl-4-cyclohexyl-1-cyanobenzene, 4′-trans-n-propyl-4-cyclohexyl-1-fluorobenzene and the like may be appropriately mixed with the liquid crystal composition. Good.
[0034]
The diffraction element of the present invention may further be formed with another diffraction grating on the light source side surface, and in that case, tracking error detection by the three beam method is preferable.
[0035]
The pattern of the concavo-convex portion (optically anisotropic diffraction grating) of the present invention has a curvature in the diffraction grating plane or a distribution in the grating spacing so that the beam shape of the return light from the optical recording medium becomes a desired shape. You can also put it on.
[0036]
The optically anisotropic diffraction grating is formed by laminating two transparent substrates each having an uneven portion formed in a diffraction grating pattern on a surface, filling the uneven portion with a liquid crystal composition in a state where the uneven portion faces each other. May be formed. In that case, the depth of each concavo-convex portion may be shallow, which is preferable because the fabrication becomes easy. Moreover, it is preferable also from the point which the orientation of a liquid-crystal composition improves by the two uneven | corrugated | grooved part which faces.
[0037]
When the concavo-convex portions formed on the two transparent substrates are laminated so as to be asymmetric with respect to the laminated surface, a diffraction grating having an asymmetric cross-sectional shape can be easily produced, and any of ± first-order diffracted light One of the diffraction efficiencies is increased, and light having a higher diffraction efficiency can be detected by one detector, which is preferable.
[0038]
When a coating such as a UV curable acrylic resin is provided on at least one of the light source side surface and the optical recording medium side surface of the diffraction element of the present invention, the surface of the λ / 4 plate or glass substrate is uneven. This is preferable because it can reduce the wavefront aberration caused by. Further, it is preferable that a wavefront aberration can be remarkably reduced by laminating a glass substrate or a plastic substrate having a good flatness on the coating film of the UV curable acrylic resin or the like. Accordingly, since the light incident / exit surface of the diffraction element is flattened, the wavefront aberration is reduced as a result.
[0039]
As the light source of the present invention, a solid-state laser such as a semiconductor laser or a YAG laser, or a gas laser such as He-Ne can be used. Further, when a harmonic generator (SHG) in which a semiconductor laser or the like and a nonlinear optical element are incorporated in a light source unit and a short wavelength laser such as a blue laser is used, high-density optical recording and reading can be performed.
[0040]
The optical recording medium in the present invention is a medium that can record and / or read information by light. Examples of the optical recording medium include CD (compact disc), CD-ROM, DVD (digital video disc) optical disc, magneto-optical disc, phase change optical disc and the like.
[0041]
【Example】
A rectangular cross section having a depth of 1.2 μm and a pitch (period) of 10 μm on a first glass substrate 1 having a length of 10 mm × 10 mm × 0.5 mm and a refractive index of 1.52 by photolithography and dry etching. A grid-like concavo-convex portion 2 having s was formed.
[0042]
A second glass substrate 3 having 10 mm length × 10 mm width × 0.5 mm thickness and a refractive index of 1.52 was prepared, and a polyimide alignment film 5 was formed on the surface in contact with the liquid crystal composition 4. The second glass substrate 3 was laminated and bonded to the first glass substrate 1 so that the rubbing direction of the polyimide alignment film 5 was along the stripe direction of the uneven portion 2.
[0043]
Specifically, the lamination adhesion of the two glass substrates was performed as follows. An epoxy resin 8 including a spherical spacer having a diameter of 4 μm was applied to the peripheral portion of the first glass substrate 1, and the second glass substrate 3 was placed and pressed and bonded from above. In that case, the area | region which does not apply | coat a part of epoxy resin 8 was provided in the peripheral part of two glass substrates, and it was set as the opening part for liquid-crystal composition injection | pouring. At this time, the uneven | corrugated | grooved part (liquid crystal composition filling part) 2 of the 1st glass substrate 1 and the polyimide alignment film 5 of the 2nd glass substrate 3 were faced, and were adhere | attached.
[0044]
Thereafter, a mixed liquid crystal composition (nematic liquid crystal composition, Δn = 0.2896, ordinary refractive index = 1.5265, phase transition temperature to solid liquid crystal phase ≦ −20 ° C., phase to isotropic phase in a reduced pressure atmosphere. Transition temperature = 108 ° C.) was injected from the opening. The opening was closed with a sealing resin to complete sealing. As shown in Table 1, the mixed liquid crystal composition contains five kinds of compounds included in the general formula of Chemical Formula 3 as main components (86% by weight). Other liquid crystal components include 14% by weight of alkoxycyanobiphenyl liquid crystal, alkyl cyanoterphenyl liquid crystal, and the like.
[0045]
A polycarbonate retardation film (λ / 4 plate) 6 was bonded to the surface of the second glass substrate 3 opposite to the polyimide alignment film 5 using a transparent adhesive. Further, a UV curable acrylic resin was applied to the upper part of the retardation film 6, and the third glass substrate 7 was pressed onto it and irradiated with ultraviolet rays to adhere the third glass substrate 7, thereby producing a diffraction element 10. . The light incident surface (surface opposite to the concave and convex portion of the first glass substrate) 12 and the light output surface (surface opposite to the UV curable acrylic resin of the third glass substrate) 11 of the diffraction element 10 are provided. The antireflection film 13 for the light from each light source was formed.
[0046]
As a result, the diffractive element 10 has a transmittance of 96% with respect to a P-wave having a wavelength of 678 nm (light having a polarization direction parallel to the paper surface in FIG. 1) from a semiconductor laser (not shown). Further, for S waves reflected from an optical disk (not shown) (light having a polarization direction perpendicular to the paper surface in FIG. 1), the diffraction efficiency of the first-order diffracted light is 39.4%, and the −1st-order diffracted light. The diffraction efficiency of was 36.7%. Therefore, the round trip efficiency was 73%. The wavefront aberration of the transmitted light was 0.014λ _rms (rms: root mean square) or less at the central portion (circular portion having a diameter of 2 mm) of the light incident / exit surface of the diffraction element.
[0047]
[Table 1]

[0048]
【The invention's effect】
Since the optical head device of the present invention uses a liquid crystal composition having a wide temperature range and a large Δn as a nematic liquid crystal phase, it is stable with respect to temperature changes, and is high in a shallow lattice-shaped uneven portion. This has the effect of realizing light utilization efficiency.
[Brief description of the drawings]
FIG. 1 is a side sectional view of an optical head device according to an embodiment of the present invention.
[Explanation of symbols]
1: first glass substrate 2: uneven portion 3: second glass substrate 4: liquid crystal composition 5: polyimide alignment film 6: retardation film 7: third glass substrate 8: epoxy resin 10: diffraction element 11: Incident surface 12: Output surface 13: Antireflection film

Claims

In an optical head device for writing information and / or reading information by irradiating light from a light source on an optical recording medium through a diffraction element having an optical anisotropic diffraction grating and a retardation film , the diffraction element Is provided with an optically anisotropic diffraction grating in which a lattice-shaped uneven portion is formed on the surface of a transparent substrate, and the uneven portion is filled with a liquid crystal composition having optical anisotropy, The difference Δn between the ordinary light refractive index and the extraordinary light refractive index at a temperature of 20 ° C. and a light wavelength of 589 nm is 0.2 7 to 0.35, and the phase transition temperature from the nematic phase to the isotropic phase is 80 ° C. or higher. A nematic liquid crystal composition having a phase transition temperature from a nematic phase to a smectic phase or a solid liquid crystal phase of −10 ° C. or lower and containing 60% by weight or more of a compound represented by the following general formula. An optical head device.

Where A is a phenylene group or trans-1,4-cyclohexylene group, m is 0 or 1, X is a fluorine atom or hydrogen atom, Y is a cyano group, fluorine atom or chlorine atom, Z is a fluorine atom or hydrogen atom, R is a linear alkyl group having 2 to 8 carbon atoms or a linear alkoxyl group having 2 to 8 carbon atoms.