JPS61271636A - Optical disc - Google Patents

Abstract

PURPOSE:To decrease the effect on the aberration due to variation in the refractive index and thickness by selecting the refractive index of a transparent parallel plate shaped base material to a specific value. CONSTITUTION:As the refractive index (n) of a transparent parallel plate base 1 constituting an optical disc, a tolerance of + or -0.02 with respect to n=3<1/2>=1.73, that is, 1.7 approx.<n approx.<1.75 (3<1/2>-0.02<=n<=3+0.02) is selected. Thus, the effect of aberration due to the variation in the refractive index and the thickness is minimized and the reproduction with small aberration and excellent compatibility is attained by using, e.g., the same optical reproducer.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an optical disc such as a digital audio disc such as a compact disc or a video disc, which is made of a transparent parallel plate-shaped substrate and in which information is recorded or read out optically.

As shown in FIG. 1, optical discs such as video discs and compact discs have a signal surface (2) formed by forming information pits on one side of a transparent parallel plate-like substrate, and a reflective layer (3) on top of this. ) is deposited on top of which a protective layer (4
) is coated.

To read information from this optical disk, a laser beam is usually applied to the signal surface (2) from the back side of the transparent parallel plate substrate (11) opposite to the side having the signal surface (2) via a lens system (5). This is done by condensing light and detecting the amount of reflected light based on the difference in diffraction phenomenon due to the information bits on the signal surface (2).

In this case, the optical system containing the optical disc of this optical reproducing device, specifically the lens system (5), is operated under preset conditions such as the refractive index and thickness of the optical disc substrate (11). The lens is designed to reduce spherical aberration.

Video discs are made of polymethyl methacrylate, for example.
The compact disc is made of, for example, polycarbonate (PC) and has a thickness of 1.2 m. Regarding optical discs, an optical system including this is designed.

However, in reality, these optical discs suffer from variations in refractive index due to variations in their materials, variations in thickness, and
If the same playback device is used due to uneven thickness, etc., and especially large differences between manufacturers, the spherical aberration in optical playback may become large and the signal amount may drop significantly. be.

That is, an optical disk is a parallel flat plate having a refractive index n and basically a constant thickness t, and when a convergent light beam passes through the parallel plate, spherical aberration occurs.

The numerical aperture NA of this convergent light beam is expressed as NA=sinθ (N
A-n' sin θ (n + is the refractive index of air and rl'
~1) ), then the spherical aberration δ2 (longitudinal aberration) is
(Smith, “Modern Optical Engineering (W, J, Ss+ith,” Mo
dern OptialEngineering″Mc
(See Graw Hill 1966, published by N, Y).

(1) is Taylor-expanded for NA=sinθ,
The third-order term is taken as the aberration.

When this δ2 is defined as a longitudinal aberration and multiplied by sin θ (˜tan θ) to convert it into a lateral aberration δy, the following is obtained.

On the other hand, the relationship between the wavefront aberration E and the lateral aberration δy is, so
The wavefront aberration E is obtained by taking the corresponding fourth-order wavefront aberration as follows: E”W4o(ξ2+η2) 2 ... (5
), and will be evaluated using the fourth-order wavefront aberration below.

The spherical aberration W40 is becomes.

Usually, in this type of optical disc playback device, the spherical aberration in the disc is canceled out by the spherical aberration of the condensing lens system of the optical playback device, so that the optical system including the optical disc has the minimum spherical aberration. The condensing lens system (5) is designed. For example, the PM
In a video disc made of MA, its refractive index n
is n −1,49, and the optical system is designed with the thickness t = 1.25. In other words, the thickness t is now set as shown schematically by the solid line in FIG.
o For example, even if the spherical aberration is minimized at 1.2511, and in this case the spherical aberration is within the allowable amount W & with a predetermined thickness change ±Δto, if the refractive index n changes, the second As shown by the broken line in the figure, since the thickness of the disk that obtains the minimum spherical aberration differs, for example, even +Δto in the figure exceeds the allowable amount of spherical aberration W&, causing a significant drop in the reproduced signal.

Now, when the refractive index n = 1.49 and the thickness t = 1.25, the equal aberration curve of the spherical aberration occurring in the disk is expressed as (6
) is obtained by integrating the equation, as shown in Fig. 3. According to this, when either the refractive index n or the thickness t changes, in order to maintain a constant aberration, it is necessary to greatly change the other t or n, respectively. Become.

In fact, when an optical disk made of transparent parallel flat plates such as PC1 or PMMA as described above is used, spherical aberration due to variations in refractive index and thickness becomes a problem.

The present invention aims to improve such problems.

That is, in the present invention, by configuring the optical disk as a transparent parallel plate whose refractive index is close to a specific value, an optical device including the optical disk, such as an optical reproduction device, can be used. The influence on spherical aberration due to variations in refractive index in a pre-designed optical system, and furthermore the influence on spherical aberration due to variations in thickness, can be effectively kept small.

In the following, the present invention will be described in detail.
In particular, the refractive index n of the transparent parallel plate substrate (1) constituting the optical disk explained in FIG. 1 is n-/T-
With 1,73 as the most preferable value, -/T-o, o2≦
It is selected that n≦n+0.02, that is, 1.7≦n≦1.75.

In other words, regarding the spherical aberration (formula (6)) in the parallel plate substrate (1) of the optical disk having a thickness of t and a refractive index of n described above, the spherical aberration when the thickness changes by Δt and the refractive index changes by Δn. Looking at the change in , the total differentiation of equation (6)...(7) According to equation (7), when there is no change in the refractive index, that is, when Δ11 x Q, the change in spherical aberration is , Δt
n=-1 d W4Q = = --sin'θ B n3 (8).

Also, since Δn is actually a small value, the third term in equation (7) can be ignored, so when Δn≠0, n-, /T =
When it is 1.732, the formula (8) is obtained. In other words, Δ
When n≠0, the change in spherical aberration takes a minimum value at n=5. In other words, there is a part indicating an inflection point on the isoaberration line in Fig. 3, and this inflection point is considered to be gentle. According to the optical device for which the system was designed, for example, an optical reproducing device, the amount of spherical aberration due to changes in the refractive index, and therefore the increase in the amount of spherical aberration due to changes in thickness as explained in Fig. 2, can be kept small. be able to. In practice, the refractive index n may be set as follows: 1 - 0,025 n ≦ f - 0.02. This is because within this range, there is virtually no increase in the amount of spherical aberration.

The constituent material of the substrate (1) that meets this refractive index condition is optical glass SF 13 (n = 1.
73).

Next, for example, in an optical playback device whose optical system is designed so that the spherical aberration is minimized for an optical disk with a refractive index of n -1,73, playback is performed using an optical disk with a refractive index of n+α. Let us consider how much α can be tolerated in this case. (7) Formula % Formula % When the refractive index n changes by α, that is, at Δn−α, the change in spherical aberration is Tonal. Now, Δn = α, n = r = 1.73, t = 1
．． 2 and sinθ=0.6, = 3.74X 10-10-3ex2(-4,8α
2 (λ), (λ = 0.78 μm) 2 of this wavefront aberration
The root mean (RMS) is v = 0.0745 l W4
Since it is given by o 1, ΔV= 0.357α2([
? MS-λ) Now, set the tolerance range to ΔV < 0.02 (R
MS・λ), then 1αl < 0.23, and the refractive index n of the optical disc in the same playback device is 1.5 <
It becomes usable when n < 1.96. In other words, if the isospherical aberration line is drawn by curve (41) in Fig. 4, ΔV −0,02λ
The following range is indicated by diagonal lines, and the usable margin is the range indicated by Ml.

By the way, if we compare the case where an n -1.5 optical disc is used, for example, if the optical system is designed so that the spherical aberration is minimized for an n = 1.5 optical disc, When reproducing in a reproducing apparatus using another optical disk having a refractive index of n+α, similarly considering how much α can be tolerated, the following is obtained. Now, if Δn=α, n=1.5, t=1.2, and sinθ=0.6, then =5.76xlO-3(0,5α-1,67α2), and dW40=7.38 (0,5α−1,67α2)λ
ΔV=0.551 0.5α-1゜67α21<0.0
From 2, when α≧0, 1.67α' −0,5α10
．． Solving 038≧O and solving 0≦α<0.123 α 0, ■, 67α'-0,5α-0,036< 0 Solving -0,06<α 0 (11, (2) Therefore, -0.06<α<0.123 Therefore, it can be used with 1.44<n<1.62.

In other words, the usable refractive index margin in FIG. 4 is M2.

Therefore, the above-mentioned lens designed according to the present invention according to n = 1.73 has a usable range about 2.5 times that of the lens designed according to n -1.5.

In the present invention, as described above, the refractive index n is specified in the range of ±0.02 around n-J〒; 1.73, that is, 1.71≦n≦1.75. However, this is because, as is clear from FIG. 4, it has been recognized that this range is a range in which a margin M1 comparable to n-1,73 can be obtained.

As is clear from the above, according to the present invention, the influence of variations in refractive index and thickness on aberrations can be kept small, so that, for example, the same optical reproduction device can reproduce highly compatible reproductions with small aberrations. It can be performed.

Although the above-mentioned example mainly refers to reading information from an optical disk, the optical disk according to the present invention can also achieve similar effects in various optical devices related to optical disks that form information bits optically using laser light, for example. be able to.

[Brief explanation of the drawing]

FIG. 1 is a linear sectional view of an optical disc, FIG. 2 is an explanatory diagram of its spherical aberration, and FIGS. 3 and 4 are equal aberration curve diagrams. (1) is the substrate, and (2) is the signal surface. 9〒Sisk m-viewing surface 1st figure Sufu 1〒1〒〒〒〒〒〒〒〒〒〒〒〒

Claims (1)

[Scope of Claim] An optical disc comprising a transparent parallel plate-shaped substrate, characterized in that the refractive index n of the substrate material is selected to be 1.71≦n≦1.75.