JP2888280B2 - Optical head device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an optical head device for a magneto-optical disk.

[0002]

2. Description of the Related Art FIG. 5 shows a conventional example of an optical head device for a magneto-optical disk described in JP-A-6-308309. The light emitted from the semiconductor laser 1 is reflected by a polarizing beam splitter layer 3 formed on the surface of a substrate 10, collimated by a collimating lens 4, and then converged by a converging lens 5.
Converges on the optical disk 6. The return light from the optical disc 6 travels in the same optical path in the opposite direction, and a part of the original polarized light component is transmitted through the polarization beam splitter layer 3, while the polarized light component generated by the Kerr effect is almost transmitted. The light transmitted through the polarizing beam splitter layer 3 has a pattern divided into four regions as shown in FIG.
4. Phase compensation film 15, and metal or dielectric reflection film 1
Xy by the birefringent diffraction grating layer 16 composed of
In the plane, the polarization component in the direction of −45 ° or + 45 ° with respect to the y axis is reflected, and the polarization component in the direction of + 45 ° or −45 ° is diffracted and received by the photodetector 12. Conditions for defining the depth of the proton exchange layer 14 and the thickness of the phase compensation film 15 are expressed by the following equation (1).

[0003]

(Equation 1) Here, [Delta] n _o, [Delta] n _e is ordinary in the proton exchange layer 14, the refractive index variation with respect to extraordinary light, n _d is the refractive index of the phase compensation film 15, T _p, T _d is the depth of the proton exchange layer 14, the phase The thickness λ of the compensation film 15 is the wavelength of light. By this equation, T _p and T _d are obtained as in the following equation (2).

[0004]

(Equation 2) The optical axis of the substrate 10 lies in the plane of the substrate, and
It exists in a direction forming an angle of 5 °.

FIG. 7 is a detailed view of the photodetector 12. FIG.
A80, B81, C82, D8
The + 1st-order diffracted lights from the four areas 3 are focused on the first photodetector 63 in FIG.
2, and the -1st-order diffracted lights are respectively condensed spots 100, 102, 103, and 10 on the second photodetector 64 in FIG.
Form one. The reflected light forms a focused spot 104 on the third photodetector 65 of FIG. If the light is focused on the optical disk 6, the dividing line 54 between the photodetector 94 and the photodetector 95 and between the photodetector 96 and the photodetector 97
A converging spot 90 and a converging spot 93 exist on the upper side. When the focal point is located in front of the optical disk 6, the amount of light received by the photodetector 97 and the photodetector 94 is smaller than that of the photodetector 9.
5, and the amount of light received by the photodetector 96 becomes larger,
When the focal point is shifted to the back of the optical disk 6, the amount of light received by the photodetector 95 and the photodetector 96 is reduced.
7, and the amount of light received by the photodetector 94 is larger.
In FIG. 7, the photodetector 94, the photodetector 95, and the photodetector 9
6. If the outputs of the photodetector 97 are A, B, C, and D, respectively, the focus error signal is represented by the calculation of A + D- (B + C). The track error signal is detected by the push-pull method from the difference between the amounts of light received by the photodetectors 98 and 99. The RF signal is detected from the difference between the sum of the amounts of light received by the first photodetector 63 and the second photodetector 64 and the amount of light received by the third photodetector 65.

[0006]

SUMMARY OF THE INVENTION The optical head device for a magneto-optical disk having the above-mentioned structure has a structure in which the direction of the optical axis of the substrate, the depth of the proton exchange layer, and the thickness of the phase compensation film are perpendicularly incident. , The extinction ratio of the birefringent diffraction grating layer is reduced with respect to 45 ° incidence, and when light is reflected by the reflective film, the light between the P-polarized component and the S-polarized component In this case, a phase difference is generated, and the extinction ratio of the birefringent diffraction grating layer is further reduced, so that there is a problem that a signal amplitude required for detecting an RF signal cannot be obtained.

An object of the present invention is to provide an optical head device which prevents such a decrease in the extinction ratio of the birefringent diffraction grating layer.

[0008]

According to the present invention, there is provided an optical head device comprising: a light source; a triangular prism having a polarizing beam splitter layer on a part of an inclined surface; A collimating lens for collimating the light reflected by the beam splitter layer, an imaging lens for converging the light collimated by the collimating lens onto a recording medium, and a polarizing hologram layer on a part of the surface; A substrate having an optical anisotropy whose surface on the hologram layer side is adhered to the inclined surface of the triangular prism, and reflected by the recording medium and transmitted through the polarizing beam splitter layer, transmitted and diffracted by the polarizing hologram layer And a photodetector that receives light reflected on the surface of the substrate opposite to the surface on which the polarizing hologram layer is formed, and wherein the optical axis of the substrate is Light that exists in a direction such that the angle between the projection of the optical axis onto a plane perpendicular to the direction of the incident light and the polarization direction of the incident light is substantially 45 °, and that passes through the line portion of the polarizing hologram layer. And the phase difference between the light passing through the space portion is 2nπ for one of the polarization component perpendicular to the projection of the optical axis and the parallel polarization component, and (2n + 1) π (n is an integer) for the other. It is characterized by being defined as follows.

[0009]

According to the optical head device of the present invention, the direction of the optical axis of the substrate is defined such that the projection onto a plane perpendicular to the direction of the incident light is at 45 ° to the polarization direction of the incident light. The phase difference between the light passing through the line portion and the light passing through the space portion of the hologram layer is 2nπ for one of a polarization component perpendicular to the projection of the optical axis and a polarization component parallel thereto, and (2nπ) for the other.
n + 1) π (n is an integer). Thereby, the polarization direction of the incident light is + 45 ° or −45 °.
The polarization component in the direction of ° is almost completely transmitted, and the polarization component in the direction of -45 ° or + 45 ° is almost completely diffracted, thereby preventing a decrease in the extinction ratio of the polarizing hologram layer. Further, by eliminating the reflection film, it is possible to prevent the extinction ratio of the polarizing hologram layer from decreasing, thereby preventing the signal amplitude of the RF signal from decreasing.

[0010]

FIG. 1 shows an embodiment of an optical head device according to the present invention. Light emitted from the semiconductor laser 1 enters a triangular prism 2, is partially reflected by a polarization beam splitter layer 3 provided on the slope of the triangular prism 2, is collimated by a collimating lens 4, and then converges (an imaging lens). ) 5 converges on the optical disk 6.
The reflected light from the optical disk 6 travels in the same optical path in the opposite direction, enters the triangular prism 2 again, transmits a part of the original polarized light component in the polarized beam splitter layer 3, and almost transmits the polarized light component generated by the Kerr effect. The light transmitted through the polarizing beam splitter layer 3 passes through an adhesive 7 having the same refractive index as the triangular prism 2 and is bonded to the slope of the triangular prism 2 with almost no reflection loss by the antireflection film 8. The light is incident on the polarizing hologram layer 9 provided on the incident surface side of 10 and composed of the proton exchange layer 14 and the phase compensation film 15. The polarizing hologram layer 9 has a pattern divided into four regions as shown in FIG. When light is incident on the polarizing hologram layer 9, if the traveling direction of the incident light in the substrate 10 is the z 'direction, x' on the x'-y 'plane in the x', y ', z' coordinate system. The polarization component in the + 45 ° direction with respect to the axis is transmitted, and the polarization component in the −45 ° direction is diffracted. The light transmitted through the polarizing hologram layer 9 and the diffracted light are totally reflected on the back surface of the substrate 10, and the reflection of the adhesive 7 with little reflection loss between the substrate 10 and the adhesive 7 by the antireflection film 8. , Through the triangular prism 2 and received by the photodetector 11, respectively.

When the optical axis is in the plane of the substrate, the x ', y', z 'components of the optical axis can be expressed by the following equation (3).

[0012]

(Equation 3) Here, φ is the angle between the y′-z ′ plane and the optical axis, and θ is the refraction angle with respect to 45 ° incidence from the adhesive 7 to the polarizing hologram layer 9. At this time, the projection of the optical axis onto the x'-y 'plane is 45 degrees with respect to the polarization direction (x' direction) of the incident light.
The condition of ° is as in the following equation (4).

[0013]

(Equation 4) From Snell's law for incident light,
The formula holds.

[0014]

(Equation 5) Here, n ₁ and n ₂ are the refractive index of the adhesive 7 and the refractive index of the substrate 10. Therefore, φ is expressed by the following equation (6).

[0015]

(Equation 6) When transmitting a polarized light component (+ 45 ° component) perpendicular to the projection of the optical axis and diffracting a parallel polarized light component (−45 ° component), the light transmitted through the line portion of the polarizing hologram layer and the light transmitted through the space portion are transmitted. The phase difference of light is 2n for the former
π, and for the latter, (2n + 1) π (n is an integer). At this time, if n = 0, the proton exchange layer 1
The condition for defining the depth of 4 and the thickness of the phase compensation film 15 is expressed by the following equation (7).

[0016]

(Equation 7) Here, the following 1 is a polarization component parallel to the projection of the optical axis of the proton exchange layer 14 onto the x'-y 'plane, a refractive index change amount with respect to a vertical polarization component, and T _p ' and T _d 'are protons. the depth of the exchange layer 14, the thickness of the phase compensation film 15, n _d is the refractive index of the phase compensation film 15. From this equation,
T _p ′ and T _d ′ are obtained as in the following equation (8).

[0017]

[Outside 1]

[0018]

(Equation 8) The following (2) is expressed by the following equation (9).

[0019]

[Outside 2]

[0020]

(Equation 9) Here, n _o, ordinary of n _e is the substrate 10, the refractive index with respect to extraordinary _{light, n p, O, n p} , e is ordinary proton exchange layer 14,
It is a refractive index for extraordinary light.

The material of the triangular prism 2 is glass such as BK7, the material of the adhesive 7 is the same as that of the triangular prism 2, the material of the substrate 10 is lithium niobate, and the material of the phase compensation film is Nb ₂ O _5. , The wavelength of the incident light is λ = 685 nm
Then, n ₁ = 1.5, n ₂ = 2.2, n _d = 2.
_{2, n o = 2.27, n} e = 2.18, n p, O = 2.2
3, n _{p, e} = 2.3.

Therefore, the direction of the optical axis is θ = 29 °, φ =
It is defined by 41 °, and the depth of the proton exchange layer 14 and the thickness of the phase compensation film 15 are given by the following equation (10) according to the above equation.

[0023]

(Equation 10) FIG. 3 is a detailed view of the photodetector 11. The + 1st-order diffracted light from the four regions 21, 22, 23, and 24 shown in FIG.
, The focused spot 39 on the track error detector A31, the focused spot 40 on the track error detector B32, the focused spot 38 on the focus error detector 30,
37, and the -1st order diffracted light is
Focused spot 41 on RF signal detector B34, Focused spot 42 on RF signal detector A33, RF signal detector A
A condensed spot 43 is formed on 33 and a condensed spot 44 is formed on the RF signal detector B34. The transmitted light forms a focused spot 45 on the RF signal detector C35. FIG. 4 is a detailed view of the focus error detector 30. When the light is focused on the optical disc 6, a condensed spot is formed on a dividing line 54 between the focus error detector A50 and the focus error detector B51 and between the focus error detector C52 and the focus error detector D53. Exists. When the focus is shifted to the front of the optical disc 6, the light amount received by the focus error detector B51 and the focus error detector C52 is changed to the focus error detector A50 and the focus error detector D5.
3, the amount of light received is larger than the amount of light received. When the focus shifts to the back of the optical disk 6, the light amount received by the focus error detector A50 and the focus error detector D53 is larger than the light amount received by the focus error detector B51 and the focus error detector C52. More. In FIG. 4, a focus error detector A50,
Assuming that outputs of the focus error detector B51, the focus error detector C52, and the focus error detector D53 are A, B, C, and D, respectively, the focus error signal is represented by an operation of A + D- (B + C). The track error signal is the track error detector A3 in FIG.
It is detected by the push-pull method from the difference between 1 and the amount of light received by the rack error detector B32. RF signal is RF signal detector A3
3 and the difference between the sum of the amounts of light received by the RF signal detector B34 and the amount of light received by the RF signal detector C35.

[0024]

According to the present invention, the direction of the optical axis of the substrate is defined so that the projection onto a plane perpendicular to the direction of the incident light is at 45 ° to the polarization direction of the incident light. The phase difference between the light transmitted through the line portion and the light transmitted through the space portion of the hologram layer is 2nπ for one of a polarization component perpendicular to the projection of the optical axis and a polarization component parallel thereto, and (2n
+1) + 45 ° or −45 ° with respect to the polarization direction of the incident light by defining it to be π (n is an integer).
The polarization component in the −45 ° or + 45 ° direction is almost completely diffracted, and the extinction ratio of the polarizing hologram layer can be prevented from lowering.
Further, by eliminating the reflection film, an optical head device in which the extinction ratio of the polarizing hologram layer does not decrease and the signal amplitude of the RF signal is prevented from decreasing can be obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing one embodiment of an optical head device of the present invention.

FIG. 2 is a detailed view of a polarizing hologram layer used in the optical head device of FIG.

FIG. 3 is a detailed view of a photodetector used in the optical head device of FIG.

FIG. 4 is a detailed view of a focus error detector in the photodetector of FIG. 3;

FIG. 5 is a diagram illustrating a conventional example of an optical head device for a magneto-optical disk.

FIG. 6 is a detailed view of a birefringent diffraction grating layer used in the optical head device of FIG.

FIG. 7 is a detailed view of a photodetector used in the optical head layer of FIG.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Semiconductor laser 2 Triangular prism 3 Polarization beam splitter layer 4 Collimating lens 5 Converging lens 6 Optical disk 7 Adhesive 8 Antireflection film 9 Polarizing hologram layer 10 Substrate 11 Photodetector 12 Photodetector 14 Proton exchanger 15 Phase compensation film 16 Birefringent diffraction grating layer 17 Reflective film 21, 22, 23, 24 Area 30 Focus error detector 31 Track error detector A 32 Track error detector B 33 RF signal detector A 34 RF signal detector B 35 RF signal detector C 36 mirror 37, 38, 39, 40, 41, 42, 43, 44, 4
5 Focus Spot 50 Focus Error Detector A 51 Focus Error Detector B 52 Focus Error Detector C 53 Focus Error Detector D 54 Division Line 63 First Photo Detector 64 Second Photo Detector 65 Third Photo Detector 80 Area A 81 Area B 82 Area C 83 Area D 90, 91, 92, 93 Focused spots 94, 95, 96, 97, 98, 99 Photodetectors 100, 101, 102, 103, 104 Focused spots

Claims (4)

(57) [Claims] 1. A light source, a triangular prism having a polarizing beam splitter layer on a part of an inclined surface, and a collimator for emitting light from the light source, entering the triangular prism, and collimating light reflected by the polarizing beam splitter layer. A lens, an imaging lens for focusing light collimated by the collimating lens on a recording medium, and a polarizing hologram layer on a part of the surface, and the surface on the polarizing hologram layer side is formed on an inclined surface of the triangular prism. A bonded substrate having optical anisotropy, reflected by the recording medium and transmitted through the polarizing beam splitter layer, transmitted by the polarizing hologram layer,
And a photodetector that receives light that is diffracted and reflected on the surface of the substrate opposite to the surface on which the polarizing hologram layer is formed, wherein the optical axis of the substrate is perpendicular to the direction of incident light. The light that passes through the line portion and the space portion of the polarizing hologram layer exists in a direction such that the angle between the projection of the optical axis onto the plane and the polarization direction of the incident light is substantially 45 °. Is defined to be 2nπ for one of the polarization component perpendicular to and parallel to the projection of the optical axis and (2n + 1) π (n is an integer) for the other. An optical head device comprising: 2. The optical head device according to claim 1, wherein said substrate is made of lithium niobate. 3. The optical head device according to claim 1, wherein said polarizing hologram layer is composed of a proton exchange layer and a phase compensation film. 4. The optical head device according to claim 1, wherein said substrate is made of lithium niobate, and said polarizing hologram layer is composed of a proton exchange layer and a phase compensation film.