JP3789692B2 - High density optical memory system - Google Patents

Description

【００７１】
反射光は、空間内を次式（３．２）に従い伝搬する。
【００７２】
【数２】

【００７３】
製作したプログラムにより遠視野像での回折パターンを計算した。図１に解析結果を示す。（図１０に解析モデルを示す）。なお、伝搬距離Ｌ＝１００μｍとした。図１はピット幅一定の時、回折効果と複屈折膜による位相差の効果の両方を考慮した光検出器出力をピット深さに対してとったグラフである。
【００７４】
〔３．３〕回折波の検出
一般に光ディスクの光信号検出系は、対物レンズの開口内に放射された回折光を集光し、その強度をフォトディテクタで電気信号に変換する。
【００７５】
いま、図１１に示すように、参照対物レンズ４１の開口数をＮＡとすると、光検出器４２の出力特性に関与するのは対物レンズ４１の光軸から±ｓｉｎ^-1（ＮＡ）以内の角度に入射する成分のみである。実際に、光検出器Ｉ₁ ，Ｉ₂ に入射する光電力は、次式（３．５），（３．６）で表される。
【００７６】
【数３】

【００７７】
複屈折膜により２つの直交偏光成分が受けた位相差を干渉計を用いて検出するときの、ピットの深さに対する光強度を解析した。解析結果は、前記した図１に示されている。なお解析結果は、ピット幅ｌ＝０．６μｍ（現在、ＣＤ等で使用されているピット幅）複屈折率Δｎ＝０．１で行い、波長で規格化した。
【００７８】
この結果から、対物レンズ４１のＮＡが小さいと回折による影響が大きいが、ＮＡが０．８のレンズを使用することにより、単調減少の特性が得られ、多値記録が可能であることが分かる。また、この解析結果から読み取り誤差に一番強いと思われる深さ０．１５，０．６０，０．８５μｍに設定して基板を製作した。
【００７９】
〔３．４〕複屈折のないピットでの回折パターン
複屈折のないピットでの回折パターンを計算して、ＮＡ＝０．８のレンズを用いて集光した場合の光検出器への入射光電力を計算した。その結果を、図１２に示す。この図１２において、横軸はピット深さ（μｍ）、縦軸は正規化されたパワーを示している。
【００８０】
その結果から、複屈折なしのピットにおいては、深さにより強度分布がそれほど変化しないことが分かった。つまり、複屈折の効果は、複屈折をもつピットと複屈折をもたないピットを測定して比較すると分かる。
【００８１】
〔４〕干渉計による偏光状態の測定
〔４．１〕干渉計による偏光状態の測定（１）
マッハツェンダー干渉計により、複屈折膜の測定を行った。
【００８２】
最初に入射面に複屈折膜を設置せず、全反射ミラーを置いて検出器側で干渉縞が観測できることを確認した。
【００８３】
次に、ウエッジ基板で光路長を変化させ、干渉縞の移動を確認した。
【００８４】
次に、複屈折膜を設置して直交偏光間位相差を測定した。その際、完成した複屈折膜の片側の面にアルミニウムを蒸着した。
【００８５】
アルミニウムを蒸着した複屈折膜をＳｉ基板に張り付けて、測定を行った。
【００８６】
まず、測定に際して、片側の光路（１／２波長板側）のみに光を通し、ＰＢＳからの出射光をビームエクスパンダーを通して、レンズに入射させ、光検出器によりガウス分布を測定する。
【００８７】
この時、測定したガウス分布のスポットサイズが０．６３３μｍ程度になるようビームエクスパンダーを調整する。
【００８８】
次に、被測定物を設置して、光検出器によりパワーの強度変化を確認（ピットに光が入射すると、強度変化が生じる）する。そこで、２つの光路を干渉させて、光検出器での強度が最大になるようにウエッジ基板により調整する。そして、ピットの存在する付近をスキャン〔手動式マイクロメータ（分解能１μｍ）により〕させ干渉光の強弱を測定する。その結果を図１３、図１４に示す。これらの図１３、図１４において、横軸は位置（ｍｍ）、縦軸は正規化されたパワーを示している。図１５はピットパターンのモデルとして用いた溝パターンを示している。なお、この図１５において、黒色の矢印は、走査方向を示している。
【００８９】
この結果と解析結果を比較すると、ほぼ一致していることが分かる。また、複屈折を持たないピットにおいても光検出器での強度変化が生ずるため、この測定結果が複屈折による効果であるかどうかを次の方法により確認した。
【００９０】
ウエッジ基板を用いて、光検出器での強度を最大にして測定を始めたが、複屈折による効果を確認するため、光検出器での強度を最大の約７０％、６０％（位相をずらしていることに相当する）に設定して測定を開始した。
【００９１】
位相と干渉後の強度分布のグラフを図１６に示す。この図１６において、横軸は位相差、縦軸は正規化されたパワーである。また、それぞれの測定結果を、図１７と図１８に示す。これらの図１７、図１８において、横軸は位置（ｍｍ）、縦軸は正規化されたパワーを示している。
【００９２】
この結果から、位相変化により光検出器出力が変化していることが確認された。
【００９３】
しかしながら、この測定方法では、ピット幅０．６μｍと比較して、スキャンのステップ幅が１μｍと大きく、分解能が不十分である。
【００９４】
そこで、Ｄ８０ステッピングモーターコントローラ（駿河精機製）を用いての４０ｎｍステップ幅のスキャンで再度測定を行うことにした。
【００９５】
〔４．２〕スポットサイズの測定
複屈折膜への入射光のスポットサイズは、ピットでの回折現象がスポットサイズに大きく依存するため、設計値通りの０．６μｍに設定しなければならない。そこで、光軸合わせおよび高さ調整などを行い、複屈折膜への入射光のスポットサイズの測定を以下の方法により行った。
【００９６】
複屈折膜への入射光Ψ（ｘ′，ｙ′）を知るには、スポットサイズｗを測定しなければならない。測定方法を、図１９に示す。レンズから十分な距離Ｌだけ離れた所の光強度分布を光検出器により順次測定する。この結果から、スポットサイズＷを知ることができる。
【００９７】
また、レンズで集光されたビームのビームウェストでのスポットサイズｗと遠視野像でのスポットサイズＷ（空間周波数軸上でのスポットサイズ）とには、次なる関係がある。
【００９８】
ｗ＝２／Ｗ （４．１）
これにより実際に複屈折膜への入射光のスポットサイズは、ｗ＝１．１４μｍと求められた。この値は、本来複屈折膜へ入射すべきスポットサイズ０．６３３μｍに比べて、２倍ほど大きくなっている。しかしながら、測定で使用しているビームエクスパンダーには限界があり、これが最小のスポットである。これからの測定では、このビームを使用することにした。
【００９９】
〔４．２．１〕集光用レンズに要求されるＮＡの値
フレネル数をＮ、測定領域までの距離をＺ、ビームのスポットサイズをｗとすると次なる関係がある。
【０１００】
Ｎ＝ｗ² ／λＺ …（４．２）
上式に、ｗ＝１μｍ、λ＝０．６３３μｍ、フランホーファ領域になるための条件Ｎ＝１を代入するとＺ＝０．７９μｍとなり、この距離以上離してスポットサイズの測定をする必要がある。また、ピットに入射される光のスポットサイズｗを０．６３３μｍにするにはどれぐらいのビーム径の光をレンズに入射させるべきかを計算した。
【０１０１】
次に、レンズのＮＡ（開口数）には、次なる関係がある。
【０１０２】
ＮＡ＝ｓｉｎθ＝λ／（πｗ₀ ） …（４．３）
ｗ₀ ＝λ／２よって、ＮＡ＝ｓｉｎθ＝２／π …（４．４）
また、図２０より次なる関係がある。
【０１０３】
ＮＡ_max ＝ｓｉｎθ_max ＝Ｄ／√［Ｌ² ＋（Ｄ）² ］ …（４．５）
ＮＡ＝ｓｉｎθ＝Ｗ／√［Ｌ² ＋Ｗ² ］ …（４．６）
本測定で使用したレンズ５１は、ＮＡ＝０．８，２Ｄ＝６ｍｍであるので上記式（４．５）に代入すると、Ｌ＝２．２５ｍｍとなる。次に、上記式（４．６）に先程述べたＬ＝２．２５ｍｍを代入して、入射ビームのスポットサイズＷ_B を求めると２Ｗ_B ＝３．７ｍｍとなり、レンズ５１へ入射させるべきビーム径が求められた。そこで、レンズ５１へ入射させるベきビーム径を得るため、ビームエクスパンダーによりビーム径を大きくしてレンズ５１に入射させた。
【０１０４】
なお、本発明は上記実施例に限定されるものではなく、本発明の趣旨に基づいて種々の変形が可能であり、これらを本発明の範囲から排除するものではない。
【０１０５】
【発明の効果】
以上、詳細に説明したように、本発明によれば、以下のような効果を奏することができる。
【０１０６】
（Ａ）光の位相の検出に利用するもので、複屈折膜を用いて、多段階の深さのピットを設け、その多値情報を得ることができる。
【０１０７】
（Ｂ）その多値情報は、複屈折膜に直交主軸に対し４５°の直線偏光又は円偏光を入射し、膜面からの反射光を干渉計でもって、ピットからの反射光の直交する偏光間の位相差をアナログ的に検出することができる。
【図面の簡単な説明】
【図１】本発明の実施例を示すピットの深さに対する干渉計出力の計算値を示す図である。
【図２】ピット幅Ｗ：０．６μｍ，深さ０．４４μｍのピット製作用基板の断面図である。
【図３】直線偏光の直交主軸分解図である。
【図４】位相差Δに対する偏光状態の変化を示す図である。
【図５】本発明の実施例を示す複屈折膜基板での反射を示す模式図である。
【図６】本発明の実施例を示す測定用干渉計の構成図である。
【図７】本発明の実施例を示す複屈折膜の製作工程図である。
【図８】本発明の実施例を示す複屈折膜製作用の基板の製作工程図である。
【図９】光ビームの空間内伝搬の模式図である。
【図１０】解析モデルを示す図である。
【図１１】光検出器での検出を示す図である。
【図１２】複屈折がないピットでの回折による光検出器への入射光電力を示す図である。
【図１３】干渉計による偏光状態の測定結果（Ｉ）を示す図である。
【図１４】干渉計による偏光状態の測定結果（ＩＩ）を示す図である。
【図１５】ピットパターンのモデルとして用いた溝パターンを示す図である。
【図１６】位相変化と干渉後の強度分布の関係を示す図である。
【図１７】測定結果（７０％）を示す図である。
【図１８】測定結果（６０％）を示す図である。
【図１９】スポットサイズの測定方法を示す図である。
【図２０】レンズに入射させるべきビーム径の説明図である。
【符号の説明】
１ 複屈折記録膜を保持するための基板
２ 複屈折記録膜
３ 直線偏光
４ 楕円偏光
１１ レーザ光源〔Ｈｅ−Ｎｅレーザ又は半導体レーザ〕
１２ ハーフミラー
１３ ウエッジ基板
１４，１５ 全反射ミラー
１６ １／２波長板
１７ 偏光ビームスプリッター（ＰＢＳ）
１８ 光記録媒体（光ディスク）
１９ 光検出器
２１，３１ Ｓｉウエハ
２２，３５ ピットパターンを含むＳｉＯ₂ 膜
２３，２３′ ポリイミド膜
２４ 金属フレーム
２５ Ａｌ膜
３２ ＳｉＯ₂ 膜
３３ Ｃｒ膜
３４ レジスト膜
４１ 対物レンズ
４２ 光検出器
５１ レンズ[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-density optical memory system.
[0002]
[Prior art]
Conventionally, an optical memory has a large storage capacity and a high access speed, so it has been used for CDs and DVDs and also as a computer memory.
[0003]
All of these are systems in which 0 or 1 information is recorded and read in correspondence with the presence or absence of pits on the substrate. For this reason, the storage capacity is naturally determined by the wavelength of light and its recordable area.
[0004]
Here, in place of the binary code, a multi-level code is used to increase the storage capacity.
[0005]
By the way, although an optical recording system for multilevel signals has already been proposed, the conventional system simply attempts to use the depth as a parameter by making the pit depth direction multistage.
[0006]
[Problems to be solved by the invention]
However, the conventional multi-level optical recording system has a problem that it is difficult to control the edge and depth of the pits, and it is not easy to increase the multi-level detection accuracy.
[0007]
An object of the present invention is to provide a high-density optical memory system that can eliminate the above-described problems and improve the multi-level detection accuracy.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides
[1] In a high-density optical memory system, a birefringent recording film having pits with multiple levels of depth and reading means for reading out the pit information with multiple levels of depth as a phase difference between orthogonal polarizations are provided. It is characterized by.
[0009]
[2] The high-density optical memory system according to [1], wherein the birefringent recording film is made of a material having a birefringence of 0.1 or more.
[0010]
[3] In the high-density optical memory system according to [1], the reading unit is incident on the birefringent recording film with linearly polarized light or circularly polarized light having an angle of 45 ° with respect to an orthogonal main axis, and interferes with reflected light from the film surface. The phase difference between the orthogonal polarization main axes between the optical paths is detected in an analog manner.
[0011]
[4] The high-density optical memory system according to [1], wherein the numerical aperture NA of the condenser lens of the readout optical system is 0.8 or more.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
[0013]
FIG. 1 is a diagram showing calculated values of the interferometer output with respect to the pit depth according to the embodiment of the present invention, where the horizontal axis indicates the pit depth (μm) and the vertical axis indicates the normalized power. .
[0014]
The high-density optical memory of the present invention is a high-density optical memory using a birefringent recording film (hereinafter simply referred to as a birefringent film) and multi-level encoding, and development of the high-density optical recording technology using multi-level encoding technology. This is a system in which pits having multiple levels of depth are provided in a birefringent film (for example, fluorinated polyimide), and depth information is read out as a phase difference between orthogonally polarized light.
[0015]
FIG. 1 shows the calculation result of the light intensity with respect to the pit depth when the phase difference received by the two orthogonal polarization components by the birefringent film is detected using an interferometer.
[0016]
When the NA of the condensing lens of the reading optical system is small, the influence of diffraction is large. However, it can be seen that by using a lens with NA of 0.8, a substantially monotonically decreasing characteristic is obtained and multi-value recording is possible.
[0017]
Further, pits having a recording pit width W = 0.6 μm and a depth of 0.44 μm were formed on the Si substrate. A sectional view of the pit is shown in FIG. Further, polyimide was applied to the substrate, and the reference [(1) S.E. Ando, T .; Sawada, and Y.S. Inoue ,: Electron. Lett. , Vol. 29, no. 24, p. 2143, (1993). The birefringent film was formed by the same method as disclosed in the above and succeeded in obtaining a birefringence of 0.12.
[0018]
Hereinafter, detailed examples of the present invention will be described.
[0019]
First, the principle of multilevel optical recording using a birefringent film will be described.
[0020]
[1] Principle of multi-level optical recording using a birefringent film [1.1] Coherent beam light propagating in the electromagnetic field distribution and polarization state space (infinitely steady-state sinusoidal vibration) is , E (electric field) and H (magnetic field) with vibration perpendicular to the traveling direction.
[0021]
Here, when the directions of E and H are constant regardless of time, the directions are kept constant in the space where the wave propagates, and E and H are in a pair of orthogonal planes (this plane vibrates). (That is, the direction of vibration is one direction). At this time, when the observer sees the traveling light, the E and H vector tips appear to be orthogonal. Such light is called linearly polarized light. Here, linearly polarized light is decomposed into orthogonal principal axes x and y as shown in FIG.
[0022]
Therefore, using arbitrary orthogonal coordinates (x, y) with this linearly polarized light fixed in space,
E _x = A _x cos (ωt + Ψ _x ) (1.1)
E _y = A _y cos (ωt + Ψ _y ) (1.2)
A _x = | E ₀ | cos θ, A _y = | E ₀ | sin θ (1.3)
φ _x = φ _y = φ (1.4)
It can be expressed as Here, θ is an angle formed by the vibration surface with the x axis. That is, an arbitrary linearly polarized light can be expressed as the sum of two orthogonal linearly polarized lights, and these two linearly polarized lights have the same phase and a constant amplitude ratio. Here, when the linearly polarized light components E _x and E _y have the following relationship, the tip of E draws an ellipse. Such a light wave is called elliptically polarized light.
[0023]
A _x / A _y = const. ... (1.5)
φ _y −φ _x = Δφ = const. ... (1.6)
As a special case, linear polarization is obtained when Δφ = 0 and Δφ = π.
[0024]
When Δφ = ± π / 2, circularly polarized light is obtained. That is, arbitrary elliptically polarized light is displayed as the sum of two orthogonally polarized light components orthogonal to each other.
[0025]
Here, the change of the polarization state with respect to the phase difference Δφ is shown in FIG. 4A is linearly polarized light (Δφ = 0), FIG. 4B is clockwise elliptically polarized light (Δφ = 0 to π / 2), and FIG. 4C is clockwise circularly polarized light (Δφ = π). / 2), FIG. 4D shows clockwise elliptically polarized light (Δφ = π / 2 to π), and FIG. 4E shows linearly polarized light (Δφ = π).
[0026]
Here, the signal is multivalued by utilizing the fact that the polarization state changes as the phase difference Δφ changes.
[0027]
This phenomenon is called optical anisotropy or birefringence when light is split into two different refracted lights when propagating through the medium. The refracted light propagates through the crystal by being divided into two polarized light beams whose vibration directions are orthogonal to each other.
[0028]
This birefringence occurs because the phase velocities, i.e. the refractive indices, in the mediums of two polarized light that are orthogonal to each other are different. Such a substance is called an anisotropic medium. Conversely, a substance that does not exhibit birefringence is called an isotropic medium. The direction of propagation and phase velocity of the refracted light of the substance exhibiting birefringence (anisotropic medium) changes depending on the incident angle.
[0029]
The characteristics of fluorinated polyimide are shown below as an example of the material exhibiting birefringence described above.
[0030]
(1) It is strong with a thin yellow film.
[0031]
(2) There is flexibility.
[0032]
(3) High transparency with infrared rays.
[0033]
(4) Stable to temperature and humidity.
[0034]
(5) It has a very large birefringence of 0.08 to 0.12.
[0035]
There are various types of fluorinated polyimide, but OPI-2005 (manufactured by Hitachi Chemical Co., Ltd.) was used here as an example. This material has a large birefringence of about 0.13. ing.
[1.2] Principle of Multilevel Optical Recording FIG. 5 is a schematic diagram showing reflection on a birefringent film substrate according to the present invention.
[0036]
In this figure, 1 is a substrate for holding a birefringent film, 2 is a birefringent film, 3 is linearly polarized light as incident light, and 4 is elliptically polarized light as reflected light.
[0037]
In optical transmission and optical recording using a multilevel signal, attention is paid to the phase that is a characteristic of light as an electromagnetic wave, in addition to the conventionally used light amplitude (or light intensity).
[0038]
Therefore, in order to utilize the phase of light, a birefringent film (polyimide) 2 is employed as the birefringent recording film 2 of the optical disk. When pits having different depths are formed in the substrate 1 of this optical disc, and linearly polarized light 3 of 45 ° with respect to the orthogonal main axis is incident on the birefringent film 2, the incident light is a birefringent film (polyimide). Is used as the recording film, and the refractive index in the directions (1) and (2) in FIG. At this time, if the phase change in the (1) direction is φ _e and the wave phase change in the (2) direction is φ _o ,
φ _e = k ₀ n _e 2L (1.7)
φ _o = k ₀ n _o 2L (1.8)
It can be expressed. Here, k ₀ is the wave number in vacuum and L is the pit depth.
[0039]
In other words, the wave in direction (1) and the wave in direction (2)
_{Δφ = φ e -φ o = k} 0 (n e -n o) 2L ... (1.9)
Will be out of phase only. For this reason, as described in [1.1], the reflected light becomes circularly polarized light or elliptically polarized light 4 due to the property of light. Further, it can be seen from the above equation (1.9) that the phase difference Δφ changes depending on the depth of the pits.
[0040]
Therefore, the phase difference Δφ can be detected in an analog manner by using an interferometer using a polarized beam splitter and a wave plate for the reflected light. Thereby, information can be multi-valued.
[0041]
[1.3] Principle of Interferometer for Detecting Phase Difference FIG. 6 shows the principle and configuration of the Mach-Zehnder interferometer for detecting the phase difference between the incident light and the reflected light. Here, FIG. 6A shows the case of incidence, and FIG. 6B shows the case of reflection.
[0042]
In these figures, 11 is a laser light source (He-Ne laser or semiconductor laser), 12 is a half mirror, 13 is a wedge substrate, 14 and 15 are total reflection mirrors, 16 is a half-wave plate, and 17 is a polarization beam splitter. (PBS), 18 is an optical recording medium (optical disk), and 19 is a photodetector.
[0043]
First, the incident light is x-polarized light [(here, laser light source (He-Ne laser) 11]) as shown in FIG. The incident light is first divided into two optical paths by the half mirror 12 so as to be 0.6 μm.
[0044]
One of the two divided lights is directly incident on the deflecting beam splitter (PBS) 17 through the total reflection mirror 14. The other light passes through the half-wave plate 16, rotates its deflection direction by 90 ° (becomes y-polarized light), and enters the PBS 17.
[0045]
In this way, the combined wave of x-polarized light and y-polarized light coming from the two optical paths is incident on the optical disk 18 (45 ° linearly polarized light or circularly polarized light).
[0046]
Next, as shown in FIG. 8B, the light reflected by the birefringent film and subjected to the phase change returns again into the Mach-Zehnder interferometer. The reflected light (combined wave of x′-polarized light and y′-polarized light) returns to the interferometer again by the PBS 17. One x′-polarized light returns directly to the laser light source 11 side and the photodetector 19 side.
[0047]
On the other hand, the y′-polarized light passes through the λ / 2 wavelength plate 16 and is converted into x′-polarized light, and returns to the laser light source 11 side and the photodetector 19 side. The returned x ′ polarized light is made to interfere. Due to this interference, the phase difference received by two orthogonally polarized components in the birefringent film can be known.
[0048]
Here, the wedge substrate 13 is inserted into the interferometer in order to control two optical path differences.
[0049]
[2] Manufacturing Method of Birefringent Film A manufacturing process of the birefringent film is shown in FIG.
[0050]
(1) First, as shown in FIG. 7A, a 3-inch Si wafer 21 is cleaved to 5 cm × 5 cm, and a pit pattern is formed by an electron beam (EB) drawing apparatus and a reactive ion etching apparatus. A SiO ₂ film 22 is formed.
[0051]
(2) Thereafter, as shown in FIG. 7B, a polyimide film 23 is formed on the manufactured substrate by spin coating.
[0052]
(3) Next, as shown in FIG. 7C, after the formed polyimide film 23 is heated in an oven (at 80 ° C. for 4 minutes 30 seconds), the polyimide film 23 ′ is peeled off from the substrate.
[0053]
(4) Next, as shown in FIG. 7 (d), the uniaxial direction is fixed by the metal frame 24 so that the peeled polyimide film 23 'has birefringence, and 300 ° C. at a rate of 5 ° C./min in an oven. The temperature is raised to 0 ° C. and then heated for 1 hour. In addition, the black arrow in FIG.7 (d) has shown the shrinkage | contraction direction of polyimide film 23 '.
[0054]
(5) Next, as shown in FIG. 7E, an Al film 25 is deposited as a reflective film on the back surface of the manufactured birefringent film.
[0055]
[2.1] Pit Formation Process [2.1.1] Manufacturing Conditions FIG. 8 shows a manufacturing process of a substrate for producing a birefringent film.
[0056]
(1) First, as shown in FIG. 8A, a 3-inch Si wafer 31 is prepared.
[0057]
(2) Next, as shown in FIG. 8B, a SiO ₂ film 32 is formed on the 3-inch Si wafer 31 by sputtering.
[0058]
(3) Next, as shown in FIG. 8C, a Cr film 33 is deposited by a vacuum vapor deposition device, and a resist film 34 is coated.
[0059]
(4) Next, as shown in FIG. 8D, a pit pattern 35 is formed by an electron beam (EB) drawing apparatus.
[0060]
(5) Next, as shown in FIG. 8E, the Cr film 33 is etched.
[0061]
(6) Next, as shown in FIG. 8F, the resist film 34 is peeled off.
[0062]
(7) Thereafter, as shown in FIG. 8G, the SiO ₂ film 32 is etched by the RIE apparatus.
[0063]
(8) Finally, as shown in FIG. 8H, the Cr film 33 is peeled off.
[0064]
In this way, for example, a substrate having a cross section as shown in FIG. 2 can be obtained. Further, pit patterns having different depths are obtained by repeating FIGS. 8C to 8H.
[0065]
[3] Next, the diffraction pattern of the reflected wave will be described.
[0066]
[3.1] Diffraction phenomenon The intensity distribution in the cross section perpendicular to the optical axis of the lowest order mode is a so-called Gaussian distribution with a bell shape, and about 87% of the energy is localized in the spot size. In order to perform high-density recording and reproduction of information on an optical disc, the light is condensed to a spot size of about a wavelength by a lens.
[0067]
At that time, the light is also diffracted outside the condenser lens upon reflection from the pits. Therefore, the signal attenuated by this diffraction phenomenon is detected by the photodetector. Here, we analyzed how the diffraction phenomenon at the pits affects the light intensity at the photodetector and what kind of pits are actually formed to enable multi-values.
[0068]
FIG. 9 is a schematic diagram of propagation in space.
[0069]
[3.2] When a far-field image light beam (spot size w: 0.633 μm, wavelength λ: 0.633 μm) of a diffracted wave is incident into a pit, it is expressed by the following equation (3.1) due to the diffraction phenomenon. Reflected light.
[0070]
[Expression 1]

[0071]
The reflected light propagates in the space according to the following equation (3.2).
[0072]
[Expression 2]

[0073]
The diffraction pattern in the far-field image was calculated by the produced program. FIG. 1 shows the analysis results. (Analysis model is shown in FIG. 10). Note that the propagation distance L = 100 μm. FIG. 1 is a graph in which the photodetector output taking into account both the diffraction effect and the phase difference effect due to the birefringent film when the pit width is constant is taken with respect to the pit depth.
[0074]
[3.3] Detection of diffracted wave In general, an optical signal detection system of an optical disc collects diffracted light radiated into an aperture of an objective lens and converts the intensity thereof into an electrical signal by a photodetector.
[0075]
As shown in FIG. 11, if the numerical aperture of the reference objective lens 41 is NA, the output characteristic of the photodetector 42 is related to the angle within ± sin ⁻¹ (NA) from the optical axis of the objective lens 41. It is only the component which injects into. Actually, the optical power incident on the photodetectors I ₁ and I ₂ is expressed by the following equations (3.5) and (3.6).
[0076]
[Equation 3]

[0077]
The light intensity relative to the pit depth when the phase difference received by the two orthogonal polarization components by the birefringent film was detected using an interferometer was analyzed. The analysis result is shown in FIG. The analysis result was performed with a pit width l = 0.6 μm (a pit width currently used in CDs) birefringence Δn = 0.1 and normalized by wavelength.
[0078]
From this result, it can be seen that if the NA of the objective lens 41 is small, the influence of diffraction is large, but by using a lens with an NA of 0.8, a monotonous decrease characteristic can be obtained and multi-value recording is possible. . In addition, the substrate was manufactured with the depths set to 0.15, 0.60, and 0.85 μm, which are considered to be the most resistant to reading errors from the analysis results.
[0079]
[3.4] Diffraction pattern at pits without birefringence Diffraction pattern at pits without birefringence is calculated, and incident light to the photodetector when condensed using a lens with NA = 0.8 The power was calculated. The result is shown in FIG. In FIG. 12, the horizontal axis represents the pit depth (μm), and the vertical axis represents the normalized power.
[0080]
From the results, it was found that in the pits without birefringence, the intensity distribution did not change so much with the depth. In other words, the effect of birefringence can be understood by measuring and comparing pits with birefringence and pits without birefringence.
[0081]
[4] Measurement of polarization state by interferometer [4.1] Measurement of polarization state by interferometer (1)
The birefringent film was measured with a Mach-Zehnder interferometer.
[0082]
First, it was confirmed that interference fringes could be observed on the detector side without placing a birefringent film on the incident surface and placing a total reflection mirror.
[0083]
Next, the movement of the interference fringes was confirmed by changing the optical path length on the wedge substrate.
[0084]
Next, a birefringent film was installed, and the phase difference between orthogonal polarizations was measured. At that time, aluminum was deposited on one surface of the completed birefringent film.
[0085]
Measurement was performed by pasting a birefringent film on which aluminum was deposited on a Si substrate.
[0086]
First, at the time of measurement, light is allowed to pass through only one side of the optical path (half wavelength plate side), and light emitted from the PBS is incident on a lens through a beam expander, and a Gaussian distribution is measured by a photodetector.
[0087]
At this time, the beam expander is adjusted so that the spot size of the measured Gaussian distribution is about 0.633 μm.
[0088]
Next, an object to be measured is installed, and a power intensity change is confirmed by a photodetector (intensity change occurs when light enters the pit). Therefore, the two optical paths are made to interfere with each other and adjusted by the wedge substrate so that the intensity at the photodetector is maximized. Then, the vicinity of the pit is scanned [with a manual micrometer (resolution: 1 μm)] to measure the intensity of the interference light. The results are shown in FIGS. 13 and 14, the horizontal axis indicates the position (mm), and the vertical axis indicates the normalized power. FIG. 15 shows a groove pattern used as a pit pattern model. In FIG. 15, black arrows indicate the scanning direction.
[0089]
Comparing this result with the analysis result, it can be seen that they are almost the same. In addition, since the intensity change at the photodetector occurs even in a pit having no birefringence, whether or not this measurement result is an effect due to birefringence was confirmed by the following method.
[0090]
Measurement was started using a wedge substrate with the maximum intensity at the photodetector, but in order to confirm the effect of birefringence, the maximum intensity at the photodetector was about 70% and 60% (shifting the phase). The measurement was started at the same setting.
[0091]
FIG. 16 shows a graph of the intensity distribution after the phase and interference. In FIG. 16, the horizontal axis represents the phase difference, and the vertical axis represents the normalized power. Moreover, each measurement result is shown in FIG. 17 and FIG. 17 and 18, the horizontal axis indicates the position (mm), and the vertical axis indicates the normalized power.
[0092]
From this result, it was confirmed that the photodetector output changed due to the phase change.
[0093]
However, in this measurement method, the step width of the scan is as large as 1 μm as compared with the pit width of 0.6 μm, and the resolution is insufficient.
[0094]
Therefore, it was decided to perform the measurement again by scanning with a step width of 40 nm using a D80 stepping motor controller (manufactured by Suruga Seiki).
[0095]
[4.2] Measurement of spot size The spot size of the incident light on the birefringent film must be set to 0.6 μm as designed because the diffraction phenomenon at the pits greatly depends on the spot size. Therefore, optical axis alignment and height adjustment were performed, and the spot size of incident light on the birefringent film was measured by the following method.
[0096]
In order to know the incident light Ψ (x ′, y ′) incident on the birefringent film, the spot size w must be measured. The measurement method is shown in FIG. The light intensity distribution at a position away from the lens by a sufficient distance L is sequentially measured by a photodetector. From this result, the spot size W can be known.
[0097]
The spot size w at the beam waist of the beam condensed by the lens and the spot size W (spot size on the spatial frequency axis) in the far field image have the following relationship.
[0098]
w = 2 / W (4.1)
As a result, the spot size of the incident light on the birefringent film was actually determined to be w = 1.14 μm. This value is about twice as large as the spot size of 0.633 μm that should be incident on the birefringent film. However, the beam expander used in the measurement has its limits and this is the smallest spot. We decided to use this beam for future measurements.
[0099]
[4.2.1] The value of NA required for the condenser lens When the Fresnel number is N, the distance to the measurement region is Z, and the beam spot size is w, the following relationship is established.
[0100]
N = w ² / λZ (4.2)
Substituting into the above formula w = 1 μm, λ = 0.633 μm, and the condition N = 1 for becoming a Franhofer region, Z = 0.79 μm, and it is necessary to measure the spot size apart from this distance. In addition, it was calculated how much light beam diameter should be incident on the lens in order to make the spot size w of the light incident on the pits 0.633 μm.
[0101]
Next, the NA (numerical aperture) of the lens has the following relationship.
[0102]
NA = sin θ = λ / (πw ₀ ) (4.3)
Since w ₀ = λ / 2, NA = sin θ = 2 / π (4.4)
Further, there is the following relationship from FIG.
[0103]
NA _max = sin θ _max = D / √ [L ² + (D) ² ] (4.5)
NA = sin θ = W / √ [L ² + W ² ] (4.6)
Since the lens 51 used in this measurement has NA = 0.8 and 2D = 6 mm, L = 2.25 mm is obtained by substituting into the above formula (4.5). Next, when L = 2.25 mm described above is substituted into the above formula (4.6) to obtain the spot size W _B of the incident beam, 2W _B = 3.7 mm is obtained, and the beam diameter to be incident on the lens 51 is obtained. Was requested. Therefore, in order to obtain a beam diameter to be incident on the lens 51, the beam diameter is increased by a beam expander and is incident on the lens 51.
[0104]
In addition, this invention is not limited to the said Example, A various deformation | transformation is possible based on the meaning of this invention, and these are not excluded from the scope of the present invention.
[0105]
【The invention's effect】
As described above in detail, according to the present invention, the following effects can be obtained.
[0106]
(A) It is used for detecting the phase of light, and a multi-level information can be obtained by providing pits with multi-stage depths using a birefringent film.
[0107]
(B) The multi-value information is obtained by making a linearly polarized light or a circularly polarized light having an angle of 45 ° with respect to the orthogonal principal axis incident on the birefringent film, and using the reflected light from the film surface with an interferometer, The phase difference between them can be detected in an analog manner.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a calculated value of an interferometer output with respect to a pit depth according to an embodiment of the present invention.
FIG. 2 is a cross-sectional view of a pit manufacturing substrate having a pit width W of 0.6 μm and a depth of 0.44 μm.
FIG. 3 is an exploded view of orthogonal principal axes of linearly polarized light.
FIG. 4 is a diagram showing a change in polarization state with respect to a phase difference Δ.
FIG. 5 is a schematic diagram showing reflection on a birefringent film substrate according to an embodiment of the present invention.
FIG. 6 is a configuration diagram of a measurement interferometer showing an embodiment of the present invention.
FIG. 7 is a manufacturing process diagram of a birefringent film showing an embodiment of the present invention.
FIG. 8 is a production process diagram of a birefringent film-producing substrate showing an embodiment of the present invention.
FIG. 9 is a schematic diagram of propagation of a light beam in space.
FIG. 10 is a diagram showing an analysis model.
FIG. 11 is a diagram showing detection by a photodetector.
FIG. 12 is a diagram showing the incident optical power to the photodetector due to diffraction at a pit without birefringence.
FIG. 13 is a diagram showing a measurement result (I) of a polarization state by an interferometer.
FIG. 14 is a diagram showing a measurement result (II) of a polarization state by an interferometer.
FIG. 15 is a diagram showing a groove pattern used as a model of a pit pattern.
FIG. 16 is a diagram showing the relationship between phase change and intensity distribution after interference.
FIG. 17 is a diagram showing measurement results (70%).
FIG. 18 is a diagram showing measurement results (60%).
FIG. 19 is a diagram illustrating a spot size measuring method.
FIG. 20 is an explanatory diagram of a beam diameter to be incident on a lens.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Substrate for holding birefringent recording film 2 Birefringent recording film 3 Linearly polarized light 4 Elliptical polarized light 11 Laser light source [He-Ne laser or semiconductor laser]
12 Half mirror 13 Wedge substrate 14, 15 Total reflection mirror 16 1/2 wave plate 17 Polarizing beam splitter (PBS)
18 Optical recording media (optical disc)
19 photodetectors 21 and 31 Si wafers 22, 35 SiO ₂ film 23, 23 'polyimide film 24 the metal frame 25 Al film 32 SiO ₂ film 33 Cr film 34 resist film 41 objective lens 42 optical detector 51 lenses comprising pit pattern

Claims (4)

(A) a birefringent recording film having pits with multiple levels of depth;
(B) A high-density optical memory system comprising reading means for reading the pit information of the multi-stage depth as a phase difference between orthogonally polarized light. 2. The high-density optical memory system according to claim 1, wherein the birefringent recording film is made of a material having a birefringence of 0.1 or more. 2. The high-density optical memory system according to claim 1, wherein the reading means is incident on the birefringent recording film with 45 ° linearly polarized light or circularly polarized light with respect to an orthogonal main axis, and reflects light from the film surface with an interferometer, A high-density optical memory system characterized in that a phase difference between orthogonal polarization main axes between optical paths is detected in an analog manner. 2. The high-density optical memory system according to claim 1, wherein the numerical aperture NA of the condenser lens of the readout optical system is 0.8 or more.

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