JPH0212620A - Optical head device - Google Patents

Abstract

PURPOSE:To make an optical head device compact and to reduce the costs of the optical head device by utilizing a total reflection prism as a phase compensating means. CONSTITUTION:A total reflection prism 60 to compensate the phase difference between polarizations of the light incident on a read signal detecting system is provided on the reflection light axis from a recording medium at the section of a polarizing prism 23. When light is made incident on an interface with a low refraction factor at an angle of >=a critical angle from a high refraction factor side, the total refraction prism 60 causes the total refraction, phase jumping caused by the total refraction is generated by respective polarizations P and S, since values are different, a phenomenon to generate the phase difference between the polarizations after the reflection is utilized. Further, the phase difference can be controlled by the refraction factor of the prism and the incident angle to the interface. Thus, the compact optical head device can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical head device used for recording and reproducing in a so-called rewritable (E-DRAW) magneto-optical disk device.

(Prior Art) There are various types of conventional optical head devices for magneto-optical disks, and FIG. 2 shows an optical head device provided so far by the present inventor. Emitted light 2 from a semiconductor laser 1, which is a light source, is converted into collimated light 4 by a collimating lens 3, transmitted through a polarizing beam splitter 5, and totally reflected by a total reflection prism 6, bending the optical path by 90 degrees, and passing through a converging lens. 7, it is converged on the disk surface 8. The reflected light from the optical disk surface is reflected by the polarizing beam splitter 5 on the opposite path. The reflected light is converted into convergent light by the converging lens 9 and enters the reflective grating lens 22 . This incident light is diffracted by the grating lens 22, and the 8-split photodetector 2
The light is incident on the first group of photodetecting elements, which consists of six photodetecting elements on four. The O-th order light is orthogonal to 2 by the polarizing prism 23.
The second polarized light is divided into two polarized lights and is composed of two photodetecting elements.
The light is incident on the group light detection element. FIG. 3 is a partial perspective view for explaining the relationship between the grating lens 22 and the eight-divided photodetector 24 shown in FIG. In FIG. 3, the converging lens 7 and the disk surface 8 are shown simultaneously through the snow line 25 to show the directional relationship between the divided areas within the grating lens and the track. Furthermore, the two photodetectors 29.30 are divided into four-part detectors 31.
32, 33, and 34. They can also be placed one above the other as shown in Figure 2, and there is no functional difference.

The reflective grating lens 22 consists of four grating lens regions, including an A-region grating lens (first region) 3 with different focal lengths and diffraction directions with a line 35 intersecting the optical axis of the converging lens 9 as the boundary.
6 and B area grating lens (second area) 37,
Further, on the dividing line 35, there is a C area grating lens 36 and a C area grating lens 37, which have different focal lengths and diffraction directions from the A area grating lens 36 and the B area grating lens 37.
A region grating lens (third region) 38 and a D region grating lens (fourth region) 39 are formed, respectively. The A-area grating lens 36 has a grating pattern corresponding to interference fringes between a spherical wave diverging from the convergence point of the 0th-order diffracted light 59 and a spherical wave diverging from a point 40 on the division line of the 8-split photodetector 24. The B-region grating lens 37 has a grating pattern corresponding to interference fringes between a spherical wave diverging from the convergence point of the 0th-order diffracted light 59 and a spherical wave diverging from a point 41 on the division line of the 8-split photodetector 24. The C area grating lens 38 has a pattern corresponding to interference fringes between a spherical wave diverging from the convergence point of the 0th order diffracted light 59 and a spherical wave diverging from the point 42 on the photodetecting element 30 of the 8-split photodetector. There is. The D area grating lens 39 has a pattern corresponding to interference fringes between the spherical wave diverging from the convergence point of the 0th order diffracted light 59 and the spherical wave diverging from the point 43 on the photodetection element 29 of the 8-split photodetector. There is.

In Figure 3, the grid pitch is drawn larger than it actually is to make the arrangement easier to understand. Since such a reflective grating lens 22 is used, the light reflected from the disk surface 8 and incident on the reflective grating lens is diffracted light 55.56.2.
Points 40, 41.4 on the 8-split photodetector as 6 and 27
3 and 42, respectively. Further, the O-order diffracted light 59 that has not undergone diffraction is split into two orthogonal polarized lights by the polarizing prism 23, and convergently reaches the photodetecting elements 57 and 58 on the 8-split photodetector 24.

FIG. 4 is a diagram for explaining the state of diffracted light on the 6-split detection element in the center of the 8-split photodetector 24.

FIG. 4(a) is a diagram showing a focused state in which the light beam is converged on the disk surface 8, a diffracted light 55 from the A-area grating lens 36 and a diffracted light 56 from the B-area grating lens 37.
converge on the second dividing line 45 of the eight-divided photodetector 24, with the second dividing line 45 in between. FIG. 4(b) is a diagram showing the diffracted light in a defocused state where the disk surface 8 has been displaced and has moved away from the converging lens 7. Diffracted light 55.56 is 8
Photodetection element 33 and photodetection element 3 of split photodetector 24
2 and not incident on the photodetecting element 31 and the photodetecting element 34, respectively.

FIG. 4(e) is a diagram showing the diffracted light in a defocused state when the disk surface 8 is displaced and approaches the converging lens 7. The diffracted lights 55 and 56 are detected by the photodetecting element 31 of the 8-split photodetector 24.
and incident on the photodetecting element 34, but not on the photodetecting element 33 and the photodetecting element 32. Therefore, the four photodetecting elements 31, 32, 33° 3 in the center of the 8-divided photodetector 24
4 is 81.8283.84, the focus error signal is obtained from (81+84)-(82+83).

On the other hand, the tracking error signal utilizes the fact that when the focus spot on the disk surface 8 shifts from the center of the track, the intensity distribution of the returning light becomes unbalanced. A line connecting the center of the C-area grating lens 38 and the center of the D-area grating lens 39 of the grating lens 22 includes the point where the optical axis of the convergent lens 9 and the grating lens intersect, and when viewed from the optical axis direction of the convergent lens 9 In this case, it should be parallel to the direction of the tracking error of the disk. When a tracking error occurs, a difference occurs between the amount of light incident on the C area grating lens 38 and the amount of light incident on the D area grating lens 39. This light amount difference can be detected as the output difference between the two photodetectors 29 and 30, and the direction of the tracking error can also be detected depending on the sign of this signal.

Detection is performed using the RF multiplied signal differential detection method. Magneto-optical disk devices read information using the Kerr effect, in which the plane of polarization rotates. FIG. 5 shows the state of polarized light before and after it enters the disk.

The plane of polarization of the reflected light from the disk rotates with respect to the plane of polarization of the incident light. This rotation direction differs depending on the presence or absence of a recording signal. This rotation angle is very small, about 0.2 degrees, and the signal polarization component 15.16 generated by this rotation is small. In order to obtain a high C/N ratio of the reproduced signal from this small signal polarization component, it is necessary to guide this signal polarization component to the signal system with low loss and an efficient detection method. Ru. The surface of the grating lens is covered with a metal film, and the diffraction efficiency has polarization dependence. For example, a grating lens whose surface is coated with gold with a grating pitch of 1.4 pm and a grating depth of 0.14 pm has an incident angle of 60 pm.
The polarized light component perpendicular to the grating groove can be diffracted by 20%, and the polarized light component parallel to the other grating groove can be reflected as zero-order light with low loss without being diffracted. Therefore, by making the signal polarization component parallel to the grating groove, it is possible to reach the two-split photodetector element 57, 58 with low loss, and the error signal is detected by the 20% diffracted light from the polarization component perpendicular to it. . A differential detection method is an effective detection method.

In the differential detection method, by inserting a polarizing prism whose optical axis is inclined by 45 degrees from the The Rp multiplied signal is obtained from the difference between these two signal voltages by the method of guiding the light to the split photodetecting elements 57 and 58 and detecting it. This detection method has the advantage of being able to efficiently extract signals and remove in-phase component noise. When the surface of a grating lens is covered with metal, polarized light parallel to the direction of the grooves of the grating lens and polarized light perpendicular thereto have different phase changes after reflection, resulting in a phase difference. Figure 6 shows that the metal film is made of Au.
The figure shows the phase difference between polarized lights with respect to the incident angle of the reflecting surface. For a grating lens, when the angle of incidence is 60 degrees, it is slightly larger than when it is a simple metal surface.
A phase difference of ° occurs. When a grating lens is used in an optical head, the signal polarization component 15 caused by the Kerr effect shown in FIG.
16 and the polarization component orthogonal thereto are made to coincide with the above polarization direction, the C/N ratio of the reproduced signal deteriorates due to the phase difference caused by reflection. Figure 7 shows the relationship between the phase difference between polarized lights and the amount of signal deterioration.
At this time, the reproduced signal cannot be detected at all. Therefore, the deterioration of the C/N ratio has been solved by inserting a phase compensating plate 10 that can compensate only for the phase difference caused by the metal reflection of the grating lens.

(Problems to be Solved by the Invention) The optical head device described above uses a transmission type phase compensation plate using a birefringent crystal such as quartz as a phase compensation means, so the optical system of the optical head device can be made compact. It is difficult to summarize.

An object of the present invention is to eliminate the above-mentioned drawbacks and provide a compact optical head device.

(Means for Solving the Problems) An optical head device of the present invention includes a light source, an imaging lens system that focuses the light from the light source onto a recording medium, and a light reflected from the recording medium into the imaging lens system. a polarizing beam splitter that takes out the reflected light off-axis; a metal reflective grating lens that mainly diffracts a specific polarized light of the reflected light taken out by the polarizing beam splitter; and a metal reflective grating lens that consists of multiple regions with different characteristics; The detection system includes a polarizing prism and a two-split photodetector for detecting a readout signal, and a six-split photodetector for detecting a focus error signal and a tracking error signal from the diffracted light of the grating lens. The optical head device is characterized in that a total reflection prism that compensates for the phase difference between polarizations of light incident on the readout signal detection system is provided on the optical axis of reflection from the recording medium up to the polarization prism. .

(Operation) The operation and principle of the present invention are as follows. The optical head device of the present invention realizes the transmission type phase compensation plate used in the conventional optical head device using a total reflection prism. In this total reflection prism, when light is incident from the high refractive index side to the interface with the low refractive index at an angle greater than the critical angle, total reflection occurs.
Since a phase jump occurs in each of the P and S light beams due to total reflection and the values thereof are different, the phenomenon in which a phase difference occurs between the polarized lights after reflection is utilized. This phase difference is caused by the refractive index of the prism and
It can be controlled by the angle of incidence on the interface. - As an example, the 8th
The figure shows the relationship between the phase difference and the refractive index when the angle of incidence is constant at 45°, and Figure 9 shows the relationship between the phase difference and the angle of incidence when the refractive index is 1.5. It is something. Since this total reflection prism is a reflection type, the optical system can be folded and made compact.

(Example) Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a plan view showing a first embodiment of the present invention, in which the same parts as in the prior art are designated by the same reference numerals. In this example,
The reflected light from the optical disk taken out from the polarizing beam splitter is reflected by a total reflection prism 60 to compensate for the phase difference between polarized lights. This phase compensation means uses the former method described in the operation section. Glass with a refractive index of 1.55 is used to obtain a phase difference of 45° caused by the grating lens when the incident angle is 45°. FIG. 10 is a diagram showing a total reflection prism 61 used in the second embodiment of the present invention, in which the latter method is used as the phase compensation means. Glass with a refractive index of 1.5 is used, and the angle of incidence is 50° to obtain a phase difference of 45°.

These phase compensation means include a converging lens 7 and a polarizing prism 2.
The effect can be demonstrated by installing it between 3.

(Effects of the Invention) The optical head device of the present invention can be made compact by using a total reflection prism as a phase compensation means. In addition, inexpensive glass can be used as the total reflection prism, and an inexpensive optical head device can also be provided.

[Brief explanation of the drawing]

1 is a plan view showing a first embodiment of the present invention, FIG. 2 is a perspective view showing an example of a conventional optical head device, FIG. 3 is a partial perspective view of a conventional optical head device, and FIG. 4 is a perspective view showing an example of a conventional optical head device. Figure 5 is a diagram for explaining the state of diffracted light on the 8-split detector, Figure 5 is a diagram for explaining the state of polarized light before and after reflection on the magneto-optical disk, Figure 6 is a diagram for explaining the state of polarized light before and after reflection on the magneto-optical disk.
Figure 7 shows the phase difference caused by metal reflection, Figure 7 shows the signal degradation due to the phase difference between polarized lights, Figure 8 shows the relationship between the refractive index of the prism and the phase difference, and Figure 9 shows the relationship between the refractive index of the prism and the phase difference. Relationship between angle and phase difference 1... Semiconductor laser, 2... Radiation beam, 3... Collimating lens, 4... Collimated beam, 5... Polarizing beam splitter, 6...
Reflector, 7... Converging lens, 8... Optical disk surface,
9... Converging lens, 10... Phase compensation plate, 60.6
1... Total reflection prism, 12... Polarized light when there is a signal, 13... Polarized light when there is no signal, 14...
・Polarization direction of incident light, 15.16... signal polarization component,
17... Direction parallel to the optical axis of the polarizing prism, 18.
...Direction perpendicular to the optical axis of the polarizing prism, 22.36.
37.38.39... Reflective grating lens, 23...
Polarizing prism, 26,2755.56... Diffraction light, 5
9...0th order diffracted light, 24...8-divided photodetector, 29
．． 30.31.32°33, 34.57.58...Photodetection element, 40.41.42.43...Convergence point, 25
・・・Omitted line◇

Claims (1)

[Claims] a light source; an imaging lens system that focuses the light from the light source onto a recording medium; a polarizing beam splitter that takes out the reflected light from the recording medium outside the optical axis of the imaging lens system; Detection consisting of a metal reflective grating lens consisting of a plurality of regions with different characteristics that mainly diffracts a specific polarization of the reflected light, a polarizing prism and a two-split photodetector that detects a readout signal from the reflected light of the grating lens. and a six-segment photodetector that detects a focus error signal and a tracking error signal from the diffracted light of the grating lens. An optical head device characterized in that a total reflection prism for compensating a phase difference is provided on the optical axis of reflection from the recording medium up to the polarizing prism.