JPS63124252A - Optical device for optical information recording medium - Google Patents

Abstract

PURPOSE:To reduce the noise level of a detection signal due to the influence of retardation of a transparent substrate by adjusting within + or -5 deg. the inflection surface of an information read light with a position parallel vertical or at 45 deg. to the main refractive index axis of an optical information recording medium as a center. CONSTITUTION:A relation between an angle between the deflection surface of the information read light P-P and the main refractive index axis of the optical information recording medium m-m, and the size of a noise component included in a read signal can be searched. With adjusting within + or -5 deg., the deflection surface of the information read light P-P with the position parallel, vertical or at 45 deg. to the main refractive index axis of the optical information recording medium m-m as the center, noise can be adjusted to below permissible level. Thus, the influence of retardation of the transparent substrate 4a is optically removed, and information can be read with a high S/N from the optical information recording medium using the transparent substrate 4a with high retardation.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a magneto-optical fi medium drive device, for example,
The present invention relates to an optical device for an optical information recording medium that uses linearly polarized light as ffl information reading light.

[Prior art]

In recent years, magneto-optical recording media have attracted attention as optical information recording media on which information can be repeatedly erased and rewritten, similar to magnetic recording media.

This magneto-optical recording medium has a magnetic film formed on one side of a transparent substrate, and linearly polarized light is incident on the surface of the transparent substrate opposite to the surface on which the magnetic film is formed and is focused on the magnetic film. Information is read by detecting a change in the Kerr rotation angle (rotation direction of the polarization plane) between a portion where a 1-inversion magnetic domain is formed and a portion where it is not formed.

If the transparent substrate is completely optically isotropic and does not exhibit birefringence, no matter which direction the polarization plane of the linearly polarized light that is the information readout light is directed, there will be no special effect for reading out the information. However, if the transparent substrate is optically anisotropic or has birefringence, the transparent Linearly polarized light incident on the substrate becomes elliptically polarized light while passing through the transparent substrate, and the S/N ratio of the detection signal read out by the detector deteriorates due to the influence of retardation (phase delay 9).

As described above, in a magneto-optical recording medium that uses linearly polarized light as information readout light, the effect of the transparent substrate thickness on the S/N ratio of the detection signal is particularly large, so it is difficult to reduce or eliminate the effect of retardation. This is one of the most important technical issues in putting magneto-optical recording media into practical use.

Conventionally, transparent substrates have been made of ceramics such as glass, thermosetting resins such as epoxy resins, etc.

Research is being conducted in the direction of reducing the effects of retardation by selectively using materials with extremely low retardation.

[Problems with conventional technology]

However, ceramics such as glass are difficult to handle during manufacturing, use, and transportation because they are prone to cracking and chipping, and they cannot be used to transfer signal patterns such as guide tracks that correspond to tracking signals or pre-pits that correspond to address signals. When doing so, use the 112P method (photo poly+*e
Therefore, there is a problem that productivity is poor. In addition, thermosetting resins such as epoxy resins cannot be molded without using the casting method (a method in which resin in a fluid state is injected into a mold of a predetermined shape and cured by heating). There is a problem in that productivity is significantly lower than that of thermoplastic resins to which injection molding can be applied.

In addition, no matter how low retardation material is used, it is virtually impossible to create a transparent material with zero retardation. The essential drawback is that the signal-to-noise ratio inevitably deteriorates depending on the size of the material turbulence.

[Means for solving problems]

The present invention has been made in order to solve the problems of the prior art described above, and is capable of optically removing the influence of adhesive tarpsion on a transparent substrate and recording optical information using a transparent substrate with high retardation. It is an object of the present invention to provide an optical device for an optical information recording medium that can read information from the medium with a high signal-to-noise ratio.

In order to achieve this object, the optical device for an optical information recording medium of the present invention is an optical device for an optical information recording medium that uses linearly polarized light as information readout light, in which the polarization plane of the information readout light is set to It is adjusted within ±5 degrees with respect to a position parallel to or perpendicular to the refractive index axis or 45 degrees.

[Effect]

According to the research of the inventor of the present application, there is a close relationship between the polarization plane of the information readout light and the angle formed by the principal refractive index axis of the optical information recording medium, and the polarization plane of the information readout light is By adjusting it so that it is parallel to, perpendicular to, or 45 degrees to the principal refractive index axis of the information recording medium, the noise component contained in the readout signal can be reduced to zero regardless of the size of the transparent substrate tarpsion. .

In addition, since it is possible to determine the relationship between the angle between the polarization plane of the information readout light and the principal refractive index axis of the optical information recording medium and the magnitude of the noise component included in the readout signal, the polarization plane of the information readout light can be determined. parallel or perpendicular to the principal refractive index axis of the optical information recording medium.

Or m within ±5 degrees around the 45 degree position.
By adjusting the noise level, the noise level can be adjusted to below an acceptable level.

〔Example〕

1st page! ! 1 shows an optical circuit diagram of the optical device for optical information recording media of the present invention. As shown in this figure, the optical circuit of the optical device for optical information recording media of the present invention mainly consists of a laser 1
, an incident optical path 5 consisting of a half prism 2, a condensing lens 3, and an optical information recording medium 4, and the optical information recording medium 4.
, a focusing lens 3 , a half prism 2 , and a polarizing beam splitter 6 .

7.7a, and a reflection optical path 8 consisting of a detector 7.7a.

As the laser 1, a semiconductor laser or a laser equipped with a linear polarizer is used, and the optical information recording medium 4 is irradiated with linearly polarized light through the incident optical path 5.

On the other hand, the optical information recording medium 4 is formed by forming a recording I1g (magnetic film) 4b on one side of a transparent substrate 4a.

The transparent substrate 4a is arranged on the incident optical path S so as to face the condensing lens 3, and the @M4b is on the back side of the condensing lens 3. The transparent substrate 4a may be made of materials with extremely low retardation such as glass or epoxy resin, as well as polycarbonate resin, acrylic resin layer, methacrylate resin, etc.
It is also possible to use injection molded thermoplastic #I resin with large retardation.

The condenser lens 3 is arranged so as to focus the linearly polarized light incident from the incident optical path 5 onto the recording film 4b of the optical information recording medium 4.

In the incident optical path 5 configured as described above, the polarization plane of the linearly polarized light irradiated from the laser 1 is determined by considering the direction of the main refractive index axis of the transparent substrate 4a, and the noise of the detection signal due to retardation. The component is adjusted to become smaller. That is, preferably, the polarization plane of the linearly polarized light is adjusted to be parallel to, perpendicular to, or 45 degrees to the main refractive index axis of the transparent substrate 4a. In this case.

The noise component of the detection signal due to retardation can be reduced to zero. Incidentally, even in high-end machines, the intensity ratio of the noise component is practically allowed up to 20 dBa degrees, so in this case, the plane of polarization of the linearly polarized light is parallel to, perpendicular to, or perpendicular to the principal refractive index axis of the transparent substrate 4a. Alternatively, the angle can be adjusted within ±3 degrees around the 45 degree position. Furthermore, in medium and low-grade machines, the intensity ratio of the noise component is allowed up to about 40 dB, so in this case it can be adjusted to within ±5 degrees.

For example, when the transparent substrate 4a is disk-shaped and made of thermoplastic resin by injection molding, the radial direction of the disk ( x-x direction), and the circumferential direction perpendicular to this (y-
It is known that the main refractive index axes are oriented in the thickness direction (z-direction) and the thickness direction (z-z direction). In this case, the polarization plane of the linearly polarized light is set to the x-x direction or the y-y direction.

Alternatively, adjustment can be made using an angle of 45 degrees with respect to the x-x direction as a reference.

Note that the angle of the polarization plane of the linearly polarized light with respect to the principal refractive index axis can be determined by measuring the principal refractive index axis of the applied transparent substrate 4a in advance, so that the angle of the polarization plane of the linearly polarized light with respect to the principal refractive index axis can be determined by measuring the principal refractive index axis of the applied transparent substrate 4a in advance. Can be applied to substrates.

The polarizing beam splitter 6 includes an analyzer, and is set at an appropriate angle with respect to the reflected optical path 8. Further, the detector 7.7a is arranged on two optical paths divided by the polarizing beam splitter 6.

Hereinafter, the reason why the above-described optical device for an optical information recording medium of the present invention can reduce the noise component of the detection signal detected by the detector 7.7a regardless of the magnitude of the retardation of the transparent substrate 4a will be explained.

First, as shown in FIG. 3, the angle between the plane r - r, which is perpendicular to the polarization plane pp of the linearly polarized light, and the transmitted light surface s - s of the polarizing beam splitter is α, the polarization plane of the linearly polarized light. p
−r and the principal refractive index axis m of the transparent substrate
-m is the angle β, the detector 7
．． The intensity of light incident on 7a is expressed by the above equation (1). In the formula, θ is the Kerr rotation angle, and δ is the retardation.

1 = (s1nLacos'0<+cos'ccs
i total θ<)+5in2β5in(2α+2β)gin'
δCO8'θ-(1/2) ・cos 55in2
a gin20K... (1) The first of the above equation (1)
term is a DC component whose sign does not depend on the Kerr rotation angle θ,
The second term is a noise component due to beam tarpsion, and the third term is a signal component from which a zero scale change can be extracted as a signal. Therefore, gin(2α+2β)20, or sin
The noise component of the second term can be removed by adjusting the polarization plane of the information reading light so that 2β=0.

As is clear from the third term of equation (1) above, the signal strength can be maximized by adjusting the angle α to 45 degrees.

Furthermore, if we take the difference between the intensities l(α) and I(-α) of the light incident on the two detectors 7.7a shown in FIG. 1, we get the DC component as shown in equation (2) above. can be removed.

■(α)-I(-α) ==sin'δ5in2 a 5in4βcosJg-
cos δ1in2 a 5in20K...(2) Note that the above equations (1) and (2) hold true for the component light that passes through the center of the lens 3 and is incident perpendicularly to the transparent substrate 4a. The effect of retardation on the peeled incident light component will be discussed below, which does not hold true for light obliquely incident on the substrate 4 from the periphery of the lens 3.

As shown in FIGS. 4 and 5, the retardation for the obliquely incident light component that obliquely enters the transparent substrate 4a from the periphery of the condensing lens 3 is determined by It can be determined by imagining a refractive index ellipsoid (normal ellipsoid) given by a function of refractive index. That is, the main refractive index in the circumferential direction of the transparent substrate 4a is nx
, the principal refractive index in the radial direction is ny, and the principal refractive index in the vertical direction is nz
In this case, the refractive index ellipsoid is given by equation (3).

As shown in FIG.
As shown in FIG. Origin 0
Retardation can be expressed as the difference in length between the major axis and the minor axis of the ellipse e formed on the cut surface when the refractive index ellipsoid E is cut with a plane passing through. Note that the retardation value varies depending on the angle between the polarization direction of the incident light and the main axis of the ellipse e of the cut plane, and its magnitude is determined by the above-mentioned θ
It can be determined by the function 0 (O9φ) of and ψ.

By the way, when focusing on the circumferential direction of the condensing lens 3 (the incident angle O in FIG. )
It is estimated that the intensity of each obliquely incident light is approximately constant. Since the signal detected by detector 7.78 is obtained by the sum of these,
It is expected that the noise as a noise component to the detection signal of retardation due to obliquely incident light may be canceled out in the circumferential direction of the condenser lens 3.

Below is the 6th [! The validity of the above assumption will be examined using l. In FIG. 6, 3 is a condenser lens, E and E' are incident polarization planes, E is an index ellipsoid imagined on a transparent substrate placed perpendicular to the incident light (in the direction of the paper), x - x indicates the principal refractive index axis of the transparent substrate. Now, as shown in FIG. 6, the incident polarization plane E is aligned with the main refractive index axis X -
Suppose that linearly polarized light is irradiated parallel to .

In this case, the point on the condensing lens 3 and the principal refractive index axis x
The light that is obliquely incident on the transparent substrate through two points P and Q located symmetrically with respect to -x has the same incident angle 0 and the same absolute value of the incident angle φ, so it is refracted perpendicularly to this. When the index ellipsoid E is cut by a plane passing through the origin of the index ellipsoid E, an ellipse e1. ez are isomorphic and the retardations δ are equal.

By the way, when light passes through the condensing lens 3, the seventh
As shown in the figure, it is known that the plane of incident polarization changes from E to E'. As shown in Figure 7, the wavefront of the linearly polarized light before incidence is Σ, the polarization plane of the linearly polarized light before incidence is E, the angle between the polarization plane E on the wavefront Σ and the direction Ep of the P wave is φ, and the transmitted The polarization plane of the subsequent linearly polarized light is Σ', 7! The polarization plane of the linearly polarized light after passing i is E', the angle between the polarization plane E' and the p-wave direction EP' on the wavefront Σ' is φ', and the angle between the perpendicular to the incident plane and the direction of incidence of the linearly polarized light is 01. The angle between the perpendicular to the incident surface and the traveling direction of the refracted linearly polarized light is 02. The angle between the perpendicular to the exit surface and the linearly polarized light traveling inside the condensing lens is θ, and the angle between the perpendicular to the exit surface and the direction after transmission is Assuming that the angle formed by the linearly polarized light is 04, the variation of the incident Hi light surface can be determined by equation (.1).

F, anφ'-? :anφcos (θ1−θ2)C
oS(O5-04)...(4) Since points P and Q are on the same circumference on the lens, 01 in FIG. 7 is equal, so 02. θ! .

OJ will also be equal. Therefore, in equation (4)? ,a
The coefficients applied to nφ become equal. Furthermore, since the angle φ is represented by the angle Δ in FIG. 6 and has the same absolute value and opposite sign, φ' in equation (4) also has the same absolute value and opposite sign (±β).

When the angles β and -β have such a relationship, the first term of equation (2) becomes zero when the respective sums are taken. As described above, the light focused by the focusing lens 3 includes obliquely incident components from all directions.

Moreover, since the intensity of each obliquely incident light is estimated to be approximately constant, the relationship between the points P and Q holds true for all light transmitting parts on the condenser lens 3.
It can be seen that the first term of equation (2) becomes zero even when integrated for all the light transmitting parts above, and that the obliquely incident component and the noise component due to the turbulence are removed.

On the other hand, if the incidence is non-parallel to the principal refractive index axis as shown by E' in Figure 6, the absolute values of angles β and -β will not be equal as shown at 81°8° in the same figure. 2nd (2nd
) cannot make the sum of the first term zero.

Using Equations (1) 2 to (4) above and assuming that the intensity distribution of the information readout light is Gaussian, the magnitude of the noise due to the optical circuit arithmetic operation of the present invention is calculated in the case of the differential detection method. When the calculation was performed, the results shown in FIG. 8 were obtained.

In this graph, the horizontal axis shows the angle between the polarization plane of linearly polarized information readout light and the principal refractive index axis of the transparent substrate, and the vertical axis shows the detected signal level, and the analyzer angle of the polarizing beam splitter is set to 45 degrees. The case where the polarizing beam splitter is set to 70 degrees is shown as a solid line, and the case where the analyzer angle of the polarizing beam splitter is set to 70 degrees is shown as a broken line.

As is clear from this graph, regardless of the analyzer angle, by setting the angle between the polarization plane of the information readout light and the principal refractive index axis of the transparent substrate to 0 degrees, 45 degrees, or 90 degrees.

It has been found that the noise component of the detection signal can be reduced to zero.

〔Effect of the invention〕

As explained above, the optical device for an optical information recording medium of the present invention is an optical device for an optical information recording medium that uses linearly polarized light as information readout light, and the polarization plane of the information RFI readout light is set to the optical information recording medium. parallel or perpendicular to the principal refractive index axis,
Alternatively, since the adjustment is made within ±3 degrees around the 45 degree position, the noise level of the detection signal due to the influence of retardation can be reduced regardless of the size of the transparent substrate adhesive tarpsion. Therefore, an injection-molded transparent substrate with high retardation but low cost and high productivity can be used as a transparent substrate for a magneto-optical distribution medium.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an optical circuit of an optical device for optical information storage medium according to the present invention, and FIG. 2 is a circuit diagram showing the principal refraction 4! which is usually seen in a transparent substrate that is injection molded. FIG. 3 is an explanatory diagram showing each parameter of a calculation formula; FIGS. 4 and 5 are explanatory diagrams explaining a refractive index ellipsoid; FIG.
Figure 6 is an explanatory diagram showing the relationship between the incident point on the lens and the tarpaulin, Figure 7 is an explanatory diagram explaining the change in the polarization plane due to convergence, and Figure 8 is the polarization plane of the information readout light and the transparency. 7 is a graph showing the relationship between the angle formed between the substrate and the principal refractive index axis and the noise caused by the turbulence. 1: Laser, 2-half mirror, 3: Condensing lens, 4:
Optical information recording medium, 4a: transparent substrate, 4b: recording film, 5:
Incident optical path, 6: Polarizing beam splitter, 7, 7A: Detector, 8: Reflected optical path 8 Figure 1! : Rede 5: Induction of people 2: Half mirror 6: Knitting beam spree 1
．． Ta 3: Certain Fan Lens' 7.70:
Day Inactor 4: Hikari 41 Limited Edition 8: Anti-I! R 9th grade or 4a: Transparent, 54th anti-4b, 2 precepts, 1 cough, Figure 2, Figure 4, Figure 6, Figure 7, Σ Σ' Figure 8

Claims (1)

[Claims] In an optical device for an optical information recording medium that uses linearly polarized light as information readout light, the polarization plane of the information readout light is parallel to, perpendicular to, or perpendicular to the principal refractive index axis of the optical information recording medium.
1. An optical device for an optical information recording medium, characterized in that the optical device is adjusted within ±5 degrees around a position of 5 degrees.