JPH0276143A - magneto-optical reproducing device - Google Patents

Abstract

PURPOSE:To make the polarized light beams of all the partial luminous fluxes into linear polarized light beams and to improve a C/N by correcting the turbulence of the polarized light beams to occur when laser beams are converged on a disk substrate having optical anisotropy. CONSTITUTION:The title device is equipped with a transparent substrate 3, for example, a LiNbO3 substrate, to change the shape of its index ellipsoid to indicate the optical anisotropy of the medium when an electric field is impressed to it, and further, a phase difference correcting means 9 composed by providing an electrode so that the electric field can be impressed to the surface of the substrate and the side surface of the substrate is provided in convergent light composed of light beams from a laser beam 5 to be made incident to an optical information recording medium 2. Further, the device guides reflected light beams from the optical information recoding medium 2 to light detectors 19a and 19b, controls the phase difference correcting means 9 using a control means so that phase difference to be generated in the phase difference correcting means 9 and phase difference to be generated in the optical information recording medium 2 can be offset with each other, and corrects the turbulence of the polarized light beams in the luminous flux. Thus, the fluctuation of DC light quantity and the decrease of AC light quantity can be prevented, and the C/N can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a magneto-optical reproducing device, and particularly to an optical head suitable for canceling a phase difference having a distribution due to birefringence of a disk substrate.

[Conventional technology]

The conventional magneto-optical reproducing device is located between the objective lens and the analyzer, as described in the publication of 1986-226045.

A phase difference correction means is provided for correcting the disturbance of polarization caused by the disk alone or by the optical components of the disk and the playback device.

[Problem to be solved by the invention]

First, a description will be given of the polarization disturbance that the above-mentioned prior art and the present invention attempt to correct, particularly the polarization disturbance that occurs on the disk.

In Fig. m2, a parallel light beam 300 is emitted from the objective lens IIJC onto a disk 1 having a recording film 1 and a disk substrate 1.

This shows how the results are narrowed down. Moreover, the third south shows the polarization state of each partial light beam of the parallel light beam 300. The polarization state of each partial light beam in this figure is linearly polarized light with a constant polarization direction. Parallel light beam 3 shown in Figure 3
00 is focused on the disk 1 by the objective lens 11, and after being reflected by the recording film 1, it is again focused on the objective lens 11Vc.
Disturbance of polarization when the beam becomes more parallel can be analyzed as follows.

First, when light enters a medium with a different refractive index.

The rotation of the plane of polarization, which is caused by the difference in transmittance between the P-polarized light component and the S-polarized light component, differs for each partial light beam in the convergent light 301 that is narrowed down to the substrate 3 by the objective lens IIK. Y301, as shown in FIG. 4 (1), the polarization is disturbed due to the difference in the polarization direction of each partial beam in the convergent light 301. )K, since the convergent light is transmitted, a different phase difference δ occurs in each partial beam, resulting in disturbance of polarization. As a result, the light beam 300 reflected by the recording film and made into parallel light by the objective lens 11 is polarized light with a different polarization direction and phase difference δ of each partial light beam, as shown in FIG. 4 (■). This causes a disturbance.

As described above, it can be said that the disturbance of polarization that occurs on the disk 1 is the result of two types of disturbances of polarization overlapping.

However, in the above conventional technology, the convergent light 3 is placed on the disk substrate 3.
Although the disturbance of polarization that occurs when 01 is incident has been described, the disturbance of polarization K that occurs during the process of convergent light transmitting through a disk substrate having monological anisotropy has not been considered.

As a result, there was a problem in that disturbances in the original polarized light reflected from the disk 1 using a substrate actually having optical anisotropy could not be sufficiently corrected.

Therefore, the second object of the present invention is to solve the above-mentioned problems of the prior art and to provide a magneto-optical reproducing device having a phase difference correction means capable of correcting the disturbance t of polarization in the light flux generated on the disk 1. be.

[Means to solve the problem]

The above objectives are 1. For example, when an electric field is applied, the shape of the refractive index ellipsoid representing the optical anisotropy of the medium changes. 1. A transparent substrate, for example, a LiNbO3 substrate, and an electric field can be applied to the surface and side surfaces of the substrate. A phase difference correction means constituted by providing an electrode is installed in the convergent light beam incident on the optical information recording medium from the laser source, and guides the reflected light from the optical information recording medium to the photodetector, and corrects the phase difference. This is achieved by controlling the phase difference correction means using the control means so that the phase difference produced by the correction means and the phase difference produced in the optical information recording medium cancel each other out.

[Effect]

When an electric field is applied to an electro-optic crystal such as L 1Nb03, the refractive index ellipsoid representing the optical anisotropy of the crystal changes depending on the strength of the electric field. Therefore, a phase difference correction means having an electrode group configured to apply an electric field to the surface of the IJbOs substrate is provided in the convergent light between the objective lens and the disk substrate. Then, by controlling the refractive index ellipsoid t representing the optical anisotropy of the L1Nbo3 substrate of the phase difference correction means, and the voltage applied to the phase difference correction means, the convergent light is incident on the original disk. It is possible to generate a phase difference distribution that is approximately the same as the resulting polarization disturbance (phase difference distribution).

The control means guides a part of the laser beam reflected from the optical information recording medium to a photodetector using a beam splitter, and controls a phase difference generated by the phase difference correction means from the photoelectrically converted detection signal. The control signal for the following control method.

Supplied to the phase difference correction means.

The wobbling frequency component is wobbled at a constant frequency in the direction in which the phase difference generated by the phase difference correction means changes, and the wobbling frequency component is detected from the detection signal using a band pass filter, and the result of multiplying this signal and the wobbling signal is ,
1. The phase difference generated by the phase difference correction means is controlled so that the signal level obtained through the low-pass filter becomes zero.

by the control signal obtained by the above control method.

The phase difference generated by the disk and the phase difference generated by the phase difference correction means are controlled so as to cancel each other out, and as a result, fluctuations in the amount of DC light that cause noise and decreases in the amount of AC light are prevented, and the CN This can be done by improving the ratio.

〔Example〕

Embodiments of the present invention will now be described with reference to one drawing.

FIG. 1 is a block diagram showing an original magnetic reproducing device as an embodiment of the present invention. This embodiment will be explained by taking as an example the case where the original is reflected on the disk.

In this figure, reference numeral 1 denotes a magneto-optical disk having a one-dimensional reflective magnetic recording film 2 and a cover glass 3, and is rotated by a motor 4. To reproduce a recorded signal, laser light emitted from a semiconductor laser 5, which is a linearly polarized light source, is collimated by a collimating lens 6, and then passed through a beam shaping prism 7.
After the light is shaped into a substantially circular light intensity distribution, it enters a beam splitter (polarizer) 8. Here, the polarizer 8 is.

The polarization direction of the linearly polarized light emitted from the laser light source 5 is P(
or S) polarized light, the incident original S (or P)
The polarized light component is arranged so that almost total reflection occurs. Therefore, part of the P (or S) polarized light component is reflected and the rest is transmitted. Of these, the transmitted laser beam is further condensed by an objective lens 11 via a mirror 10, transmitted through a phase difference correcting means 9 and a disk substrate 3, and irradiated onto a recording film 2.

At this point, due to the Kerr effect, the polarization plane of the laser beam irradiated onto the recording film 2 is at an angle θk (or −θ
k) changes.

Further, the reflected light from the recording film 2 passes through the disk substrate 32 and the phase difference correction means 9 again.

It returns to parallel light through the objective lens 11 and passes through the mirror 10'.

At the first polarizer 8, the S (or P) polarized light component is totally reflected, a part of the P (or S) polarized light component is reflected, and the rest is transmitted. In this reflected light, almost all of the S (or P) polarized light component is reflected by the second beam splitter (polarizer) 12, similar to the polarizer 8, and a portion of the P (or S) polarized light component is reflected. and the rest are passed through.

Here, the transmitted laser light is focused by a convex lens 13 and enters a servo detector 15 via a cylindrical lens 14. This servo photodetector 15 is divided into a plurality of parts depending on the detection method, and preamplifiers 29, 305-
Processing their output signals 70'Ik via.

Obtain a focus error signal and a tracking error signal [so that the laser beam is accurately positioned on the disk l]
Objective lens ttV control.

On the other hand, the laser beam reflected by the second polarizer 12.

1/2 wavelength plate 16. The light enters a polarizing beam splitter (analyzer) 18 through a convex lens 17 and is split into two.

Of the two branched laser beams, one beam is incident on the first photodetector 19aK, and the other beam is incident on the second photodetector 19b, and the respective photoelectric conversion systems 20,
21, it is converted into an electrical signal. moreover.

The signals detected by these photodetectors 19a and 19b are as follows.

The signal is amplified by preamplifiers 22 and 23 and input to a differential circuit 24. In the differential circuit 24, components of the same phase of each signal inputted by the preamplifier 22 or 23 are subtracted, and at the same time components of opposite phase are added. Therefore, the amplitude of the reproduced signal is equal to that of the signal inputted by the preamplifier 22. The sum of the amplitude and the amplitude of the signal manually input by the preamplifier 23 is output 25.

Next, how the information recorded on the magneto-optical disk 1 is detected will be explained using FIG. 2.

FIG. 5 (at is the recording film 2 of the one-source magnetic disk 1. When the laser beam incident on the polarizing beam splitter 18 and the recording film 2 of the one-source magnetic disk 1 is linearly polarized light (when there is no birefringence in the disk substrate or optical components), the reproduced signal is This shows the light intensity amplitude.

In this figure, 101 and 102 indicate P (or S) polarization and S (or P) polarization direction of laser light. Further, 103 and 104 indicate the P (or S) polarization and the S (or P) polarization direction in the polarizing beam splitter 18 in FIG.
and 104 is the transmission axis of the polarizing beam splitter 18.

As shown in the figure, when laser light is irradiated onto the recording film 2 as linearly polarized light 105, the reflected laser light rotates at an angle θ and partially rotates the plane of polarization in the orientation state of the magnetic moment. The resulting two-linearly polarized light 106a.

106b. This phenomenon is the Kerr effect, and this rotation angle θ is called the Kerr rotation angle. Furthermore, in this embodiment, since a differential detection configuration is adopted as shown in FIG. 1, P (or S) polarized light and S
(or P) The polarization directions 103 and 104 are summed to obtain reproduced signals with the same amplitude in the polarization plane, and the polarization directions 103 and 104 are -c with respect to the polarization directions 101 and 102 of the laser beam, with an approximately 4f slope tj<5Kl/2 A wave plate 16 is provided. Therefore, the reflected laser beam reflected as linearly polarized light 106a e106b is split into two by the analyzer 1B, so that
The linearly polarized light 106a has a light intensity amplitude of al e at as shown in the light intensity and light intensity diagram, and the light intensity, that is, the light intensity is 4. al, obtained as linearly polarized light 10
6b is obtained as the element quantity bF and the element quantity. °Therefore, the amount of light detected by the one-element detectors 19a and 19b is:
On the other hand, is detected as a change in light intensity from F to b,
The other is detected as a change in light amount from b; to a;. Here, if the polarization of the reflected laser beams reflected as linearly polarized lights 106a and 106b is maintained as linearly polarized light until it is detected by the analyzer 18, then the reproduced signal will be detected at the light amounts d and d:. , in the DC original light intensity and c
: are equal.

Next, birefringence exists in the one-dimensional magnetic disk 1, and the recording film 2 of the magneto-optical disk 1. The case where the laser beam incident on the polarizing beam splitter 18 is elliptically polarized light will be explained using @5 (b) t'.

FIG. 3(5) shows the light intensity amplitude of the reproduced signal in such a case. In this figure, parts with the same numbers as in FIG. 3(a) are the same parts.

As shown in the figure, the laser beam is elliptically polarized 107g and 107g.
In case b, the P (or S) polarized light component is reduced compared to the linearly polarized lights 106a and 106b shown in FIG. 5(a).

Accordingly, a new [8 (or P) polarization component is generated, and the polarization plane is further rotated. Therefore, the amount of signal light detected d
:' and dr are smaller than the signal amplitudes d and d:PC detected at the end of the linearly polarized light in FIG. 5(a). Therefore, the carrier component decreases.

A DC element amount difference, which is a noise component, occurs, resulting in a decrease in the CN ratio. These elliptically polarized lights 107a and 107b are.

This occurs when the laser beam is incident on and reflected from the disk substrate 31C of the magneto-optical disk 1. P (or S) polarized light and S
(or P) This is mainly due to the phase difference with the polarized light, and when a plastic such as polycarbonate (hereinafter referred to as PC) with a large one-dimensional elastic constant is used as the material of the disk substrate 3, Generates a phase difference. Furthermore, as shown in FIG. 3, this phase difference is caused by laser light (parallel light 3
00) is.

Since the objective lens 11 narrows down the disk 11C, the light passing through the disk substrate 3 converges 'yt
, 301, the polarization is disturbed as shown in FIG. 4(1).

Next, regarding the above-mentioned phase difference light, 1. Figure 6, Figure 7, @
This will be explained in more detail using FIG.

FIG. 6 shows the direction of incidence of a certain partial light beam U on the disk substrate in the convergent light of the parallel light beam 300 focused onto the disk l by the objective lens 11.

In this figure, the X axis is the radial direction of the disk substrate 3, the y axis is the circumferential direction of the disk substrate 3, and the two axes are the substrate thickness direction. Further, the partial light flux U in this figure is incident on the substrate 3 in the direction of the incident projection angle ψ from the X axis and at an incident angle θ tilted from the two axes.

In FIG. 7, a parallel light beam 300 is generated by the objective lens 11.

It is narrowed down as a disc IK minute spot, and the recording film 2
, and again passes through the objective lens lit as a parallel beam of light 30
The luminous flux when it reaches 0 is expressed above.

In this figure, D is the original east diameter, and 0 is the center of the luminous flux.

ψ is the incident projection angle in FIG. 6, and the coordinates are.

It is shown in the same coordinate system as FIG. 6.

FIG. 8 shows the phase difference J between A and A and B and B' of the light beam in FIG. In this figure, one curve h (ψ=
θ°) is between A and A', one curve v (91=90') is between B and 8, and δ0 indicates the phase difference at the center of the light beam. As shown in this figure, the phase difference of the partial light beams located on the outer periphery of the light beam increases rapidly.

The reason for this is as follows.

The phase difference δ is given by the equation , when convergent light is incident on a disk substrate with optical anisotropy, ■
...The optical path length d becomes longer as θ increases. In the case of biaxial anisotropy, where the principal refractive index Nz in the plate thickness direction is larger than the principal refractive index (Nx, Ny) IC in the two in-plane directions, the refractive index difference Δn depends on the incident angle θ. .

As θ increases, it becomes larger.

Therefore, θ becomes large. That is, the phase difference of the partial light beams located on the outer periphery of the light beam 300 increases.

The phase difference δ has a distribution shown in FIG.

Moreover, the curves v and h in FIG. 8 do not completely match. The cause of this is the principal refractive index N of the refractive index ellipsoid of the PC board.
This is because x and Ny are not equal.

As explained in FIG. 4 (1) and FIG. 1. By correcting the phase difference by the phase difference correction means 9, linearly polarized light as shown in FIG. 3 is always obtained, and a good reproduced signal can be obtained.

Therefore, next to this phase difference correction means 9, FIG.

This will be explained using FIGS. 10 and 11.

In this embodiment, as the first phase difference correction means 9, an electro-optic crystal whose refractive index changes when an electric field is applied to the crystal is used. An example using a substrate will be shown.

FIG. 9 is a perspective configuration diagram of the phase difference correction means 9 of this embodiment. 400 is a LiNb0a substrate. 401 is.

This is a transparent electrode for applying an electric field between the back surface and the front surface of the substrate 400, and a voltage Ein is applied between the transparent electrodes. here,! , the shape of the refractive index ellipsoid representing the optical anisotropy of the substrate 400 can be changed by varying the pressures Ein'Y.

The phase difference correction means 9 having such a 5&C configuration is shown in FIG. K is provided in the convergent light between the objective lens 11 and the disk substrate 3, and by controlling the applied voltage Ein, a phase difference distribution with the opposite sign and the same 1 distribution state as the phase difference distribution occurring in the disk 1 is generated. By doing so, the phase differences are mutually cancelled, and the disturbance of polarization occurring on the disk 1 is corrected.

In other words, as shown in Fig. 811, the sign of the phase difference %Note (curved, v) occurring on the disk l is opposite (in Fig. 1, the negative phase difference/characteristic (curve ah, v) v, phase difference correction By generating the signal in the means 9, the eight phase differences can be canceled out.

Although a &LiNbO3 substrate was used in this embodiment, similar effects can be obtained with other electro-optic crystals.

Next, a phase difference correction method used with the phase difference correction means 9 explained in FIGS. 9, 10, and 11 will be explained.

In FIG. 1, 36 is a wobbling signal generation circuit;
Reference numeral 7 denotes an excitation circuit which wobbles the applied voltage Et of the phase difference correction means 9 in the direction in which the phase difference generated by the phase difference correction means 9 is generated. 38 is an envelope detector which performs envelope detection of the amplitude of the reproduced signal 25, that is, the AC component in the signal, ie, d in FIG. 5 from the output of the differential circuit 24.

Further, 26 is a band pass filter, and 33 is a multiplier, which performs analog multiplication of the output signal of the band pass filter 26 and the wobbling signal output from the wobbling signal generation circuit 36. Furthermore, 34 is a low-pass filter;
Reference numeral 27 denotes a phase difference correction driving circuit, which performs one phase difference correction by varying the voltage EY of the one phase difference correction means 9.

These operations will be further explained with reference to the drawings. FIG. 12 shows the relationship between the reproduced signal amplitude d, that is, the AC component in the signal, and the phase difference δ in the laser beam.

As shown in the curve 44 in the figure, the phase difference δ in the laser beam is 0.
．． In other words, when δ0, the amplitude d reaches its maximum value, but when one phase difference occurs, that is, after δ1 or δ2,
The amplitude d decreases.

When the curve 44 follows such a trajectory, if there is a phase difference of δ0 to δ2 in the laser beam, the phase difference correction means 9 converts the wobbling signal generation circuit 36. When excited by the wobbling signal 45 generated by the vibrating circuit 37, wobbling frequency components such as 46 to 48 are generated in the amplitude of the reproduced signal. That is, when the phase difference in the laser beam is (δ0), the reproduced signal amplitude d is maximum at the amplitude center of the wobbling signal 45.91 Regardless of whether the phase difference δ changes to positive or negative, the amplitude d becomes As a result, a signal such as 46 having twice the frequency of the wobbling signal 45 is detected. On the other hand, the edges with one phase difference between δ1 and δ2 are at the same frequency as the wobbling signal 45, as shown at 47 and 48.

An amplitude and phase modulated signal is detected.

Signals 47 and 48 are at positions where the slope of one curve is different.

In other words, since the signals are detected when the signs of the phase difference δ are different, they match around one circumference, but the phases are reversed.

Incidentally, the envelope waveform of the reproduced signal amplitude d that is actually detected is modulated by the wobbling frequency component, as shown in FIG. 13(1). In this figure, reference numeral 111 indicates the AC component in the signal, which is the reproduced signal amplitude. This AC component 111 is envelope-detected by the envelope detector 38, and then passed through the band-pass filter 26 to generate a high-frequency component, which is a reproduced signal.
and tracking.

By removing low frequency components such as focusing,
7. (A signal 112 whose amplitude and phase are modulated as shown in FIG.
) becomes O, and the phase is reversed depending on the sign of δ.

This signal 112 is further analog multiplied by a wobbling signal 113 outputted by a wobbling signal generation circuit 25 shown in IC (IID) in a 1 multiplier 33, and as a result, a multiplication output shown in (■), that is, a phase detection output 114 is obtained. When the wobbling frequency component is further removed from this multiplication output 114 by the low-pass filter 34, a signal 115 as shown in (V) is obtained.This signal 115 has an output of 0 and a phase difference of δ=0(ao). 'Ie
When the phase difference δ is positive or negative, the polarity is different and the output increases according to one phase difference amount. Therefore, phase correction can be performed using these 115 signals. here.

Not necessarily the above-mentioned δ0 baldness, but an integral multiple of lso',
That is, n ・180' (n=o, ±1.±2,
・) is sufficient. That is, this phase control signal 11
If the phase difference correction means 9 is controlled by the phase difference correction drive circuit 27, the phase difference δ in one beam can be corrected to an integral multiple of 180°.

As described above, according to the present invention, the recording film 2. of the magneto-optical disk l. Since the laser light incident on the analyzer 17 is corrected to linearly polarized light, the reproduced signal amplitude d can be maximized and the CN ratio can be improved. Furthermore, since the laser beam corrected to be substantially linearly polarized is also incident on the polarizer 8, predetermined polarization characteristics can be obtained in the polarizers 8 and 12.

Moreover, when the frequency of the phase difference to be corrected is low, the band pass filter 26 is used as a low pass filter.

Further, if necessary, after the low-pass filter 34.

Furthermore, one more low-pass filter may be provided.

1. In this embodiment, the phase difference correction means is installed between the objective lens and the disk substrate, but this is not restrictive.This phase difference correction means can be installed in the convergent light to achieve the effect. Since this occurs, a convergent light neutralizer may be installed within the objective lens.

For example, in the case of an objective lens with a three-lens configuration as shown in Fig. 14, it is sufficient to install it between these lenses.Furthermore, as shown in Fig. 15, the constituent lenses themselves can be used as the phase difference correction means. Good too.

〔Effect of the invention〕

As described above in detail, according to the present invention, the optical anisotropy of the disk substrate occurs when the laser source is narrowed down. The correction button for polarization disturbance makes the laser beam incident on the optical information recording medium and the analyzer uniformly polarized, that is, the polarization of all partial beams within the two beams is linearly polarized.

Since the reproduced signal amplitude can be maximized, the CN ratio can be improved.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of a magneto-optical reproducing device showing an embodiment of the present invention, Figs. 2, 3, and 4 are explanatory diagrams of disturbance of polarization, and Fig. 5 shows that the phase difference is a magneto-optical signal. 6 and 7 are explanatory diagrams of the influence on Figure 8 is a diagram showing the disturbance of polarized light due to light aperture. FIGS. 9, 10, and 11 are explanatory diagrams of the phase difference correction means of the embodiment, FIGS. 12 and 13 are explanatory diagrams of the phase correction control means, and FIGS. 14 and 15 are explanatory diagrams of the phase difference correction means. It is an explanatory view of an example of a correction means. 1... Magneto-optical disk, 2... Recording film. 3... Disc substrate, 9... Phase difference correction means, 1
1... Objective lens, 17... Analyzer. 105, 106... linearly polarized light, 107, 108... elliptically polarized light. 300... Parallel light flux, 301... Convergent light. 400...TJ I NbO3 substrate. Figure 1 L...Nine flower spirits 〒4 Sufu 5-41 bodies and 12-...
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Claims (1)

[Claims] 1. A linearly polarized light source, an optical information recording medium, an objective lens that irradiates the optical information recording medium with the light beam emitted from the linearly polarized light source, and a light beam transmitted or reflected by the optical information recording medium. A beam splitter that separates a light beam into a photodetection system, a photodetector into which the light beam separated by the beam splitter is incident, and at least two or more partial beams of the light beam emitted from the linearly polarized light source have different positions. In an optical information reproducing device having a phase difference correction means for providing a phase difference, the phase difference correction means is configured using an optical crystal having optical anisotropy in which the generated phase difference differs depending on the incident angle and direction of the light beam. A magneto-optical reproducing apparatus characterized in that the phase difference correcting means is provided in a convergent beam between the objective lens and the optical information recording medium.