JPH03225636A - Optical head device - Google Patents

Abstract

PURPOSE:To make a device small in size and light in weight and to enhance the utilization factor of light by providing a 1/4 wavelength plate between a diffraction grating and a hologram element which are constituted of double refraction diffraction grating type element and a recording medium and receiving diffracted light separated out of an optical axis. CONSTITUTION:The diffraction grating 11 and the hologram element 5 are constituted of the double refraction diffraction grating type element and their crystal optical axes are set in parallel. The hologram element 5 is set so that it may hardly diffract radiated light 2 from a semiconductor laser 1 but mostly diffract returning light from an optical disk 8. Then, the diffraction grating 11 is set so that it may partially dif fract the radiated light 2 but hardly diffract the returning light from the optical disk 8. The radiated light 2 is converted to collimated light by a collimating lens 3, converted to circularly polarized light by a lambda/4 plate 6, and converged on the optical disk 8 by a converging lens 4. The returning light from the optical disk 8 passes through a reverse path and the 1st order diffracted light of the hologram element 5 is made incident on a six-division photodetector 7 to detect an error signal and a recording signal, and it is made incident on a photodetector 12 to detect the record ing signal.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical recording device used for recording and reproducing optical discs.

[Conventional technology]

Conventional optical head devices have various configurations depending on the tracking error signal and focus error signal detection methods, and are configured using a large number of lenses and prisms. An example is shown in FIG.

In this conventional optical head device, a three-beam method is used to detect a tracking error signal, and an astigmatism method is used to detect a focus error signal. Emitted light 2 from a semiconductor laser 1, which is a light source, is diffracted by a diffraction grating 16, and is divided into a main beam of 0th order light, and sub-beams of +1st order light and 11th order light.

After passing through the beam splinter 13, the converging lens 4 forms a main beam spot 26 and sub-beam spots 27 and 28 on the surface of the optical disk 8, as shown in FIG.

Note that in FIG. 3, 24 indicates a bit. The light reflected by the optical disk 8 passes through the opposite optical path, is reflected by the beam splinter 13, and then passes through the convex lens 17 and the cylindrical lens 14.
As a result, the light becomes light with astigmatism and enters the 6-split photodetector 15.

FIG. 4 is a diagram showing the condensing state of the light incident on the six-divided photodetector 15. (b) shows the condensed state when the light is focused on the surface of the optical disk, and (a) and (C) show the condensed state when the optical disk approaches and moves away from the converging lens 4 from the focused state. From this, the focus error signal is detected by the photodetector element 21.2.
2.23.24 output V(21), V(22).

Assuming V(23) and V(24), V(21) +
It is detected from V (24) -V (22) -V (23). On the other hand, the tracking error signal is detected by utilizing the fact that when the main beam spot 26 deviates from the pit 24 on the surface of the optical disk 8, an imbalance occurs in the amount of reflected light from the two sub-beam spots 27 and 28.

That is, each of the sub-beams 27.28 on the 8th surface of the optical disk is detected by the photodetecting element 19.2 on the 6-divided photodetector 15.
Since the light is focused at 0, the respective outputs are ν(19) and V
(20), the tracking error signal is V (19)
−V (20). Also, the recording signal is the total light intensity of the main beam, that is, V (21) + V (22)
+ V (23) + V (24).

[Problems to be Solved by the Invention] In the conventional optical head device described above, it is difficult to reduce the size and weight of the device because of the two-wheel system configuration that directs the return light from the optical disk off the optical axis.

An object of the present invention is to provide an optical head device that is small, lightweight, and has a high light utilization rate.

[Means to solve the problem]

The optical head device of the present invention includes: a light source; a diffraction grating that substantially divides the light emitted from the light source into three beams; an imaging lens system that focuses an image of the light source onto a recording medium; a hologram element that diffracts and separates the returned light of the image to off the optical axis of the imaging lens system; a hologram element arranged between the diffraction grating and the hologram element and the recording medium;
a half-wave plate and a photodetector that receives the diffracted light separated off the optical axis, the diffraction grating and the hologram element are birefringent diffraction grating type elements made of birefringent crystal, and The optical axes of the respective crystals are parallel to each other, the hologram element hardly diffracts the light emitted from the light source, and almost diffracts the return light from the recording medium, and the diffraction grating diffracts the light emitted from the light source. It is characterized in that it partially diffracts and hardly diffracts the return light from the recording medium.

Further, the optical head device of the present invention includes a light source, a diffraction grating on the front side that substantially divides the light emitted from the light source into three beams, and a diffraction grating on the back side that diffracts and separates the return light from the recording medium off the optical axis. a hologram element having a grating; an imaging lens system that focuses an image of the light source onto the recording medium; a quarter-wave plate disposed between the hologram element and the recording medium; and a photodetector that receives diffracted light separated into two, the hologram element being a birefringent diffraction grating type element made of a birefringent crystal, and a diffraction grating on the back of the hologram element detecting the emitted light from the light source. The diffraction grating on the front side of the hologram element partially diffracts the light emitted from the light source and hardly diffracts the return light from the recording medium. It is characterized by

[Effect]

The operation and principle of the present invention are as follows. In the present invention, in order to miniaturize the apparatus, a single hologram element is used for the beam splitter and the focus error signal detection optical system of the conventional optical head apparatus, and the optical head is configured as a uniaxial system. In the case of this -axis system configuration, the light emitted from the light source passes through a hologram element and a three-beam generation diffraction grating used for detecting a tracking error signal, so that unnecessary diffracted light components are also generated. Therefore, in order to realize an optical head with high light utilization efficiency, the hologram element and the diffraction grating for generating three beams are made into a birefringent diffraction grating type structure having polarization properties.

As a method for realizing a birefringent diffraction grating, there is a method using a birefringent crystal described below. As shown in Figure 5, the X plate of lithium niobate 52, which is a birefringent crystal, or the Y
When the plate is subjected to proton exchange with benzoic acid, the refractive index for extraordinary light, which is light polarized parallel to the optical axis of the crystal, decreases, for example, for light with a wavelength of 0.83 μm, which is commonly used in optical disk devices. increases by about 0.11, and the refractive index for ordinary light, which is light polarized perpendicular to the optical axis, is about 0.04.
Decrease.

Therefore, if a grating is formed in which exchange regions 53 subjected to proton exchange and non-exchange regions 54 not subjected to proton exchange are arranged periodically, it acts as a diffraction grating.

On this grating, a phase compensation film 55 of an appropriate thickness is formed on the exchange area as shown in FIG. 5 in order to cancel the phase difference between the ordinary light passing through the exchange area 53 and the ordinary light passing through the non-exchange area 54. Then, for ordinary light, this grating does not function as a diffraction grating and can be transmitted without being diffracted. In other words, this grid looks like a simple transparent substrate. By changing the depth of the exchange region 53 while satisfying the above phase difference canceling conditions for ordinary light, the phase difference for extraordinary light can be set to an appropriate value, and any diffraction efficiency can be set for extraordinary light. can get. For example, when the phase difference is π, the extraordinary light is completely diffracted. On the other hand, by increasing the thickness of the phase compensation film 55, it appears as a transparent substrate to extraordinary light and can diffract only ordinary light. For example, by adding a phase compensation film with a phase difference of π, the phase distribution state is such that the phase difference is O1 for the above ordinary light, and π for the extraordinary light, and 7 for the ordinary light. The phase difference is π, and for extraordinary light, the phase distribution state is 2π, that is, the phase difference is O. At this time, the ordinary light is completely diffracted and the extraordinary light passes through. The application of such birefringent diffraction gratings as polarizers is being considered, and is reported in Technical Digest No. 16 of the Second Optoelectronics Conference.
rBIREFR written by Tsune and others published on pages 7 to 169
TNGENT GRATING POLARIZERJ
This is stated in the paper.

In the above, a method of imparting anisotropy to the diffraction efficiency of the birefringent crystal material lithium niobate using a proton exchange method was mentioned as a birefringent diffraction grating type structure of the diffraction grating and hologram element. This can also be achieved by filling the lattice grooves of a birefringent medium with lattice grooves as described in the specification of No. 170244 with a substance having the same refractive index for either ordinary light or extraordinary light of the medium. Furthermore, the patent application 1986-9
It is also possible to realize a birefringent diffraction grating type structure in which an isotropic medium with grating grooves is filled with a birefringent material such as liquid crystal as described in the 8854 specification.

[Example] Next, an example of the present invention will be described with reference to the drawings.

FIG. 1 is a diagram showing a first embodiment of the present invention, in which the same parts as in the prior art shown in FIG. 2 are designated by the same reference numerals. This optical head device includes a semiconductor laser 1 as a light source, a diffraction grating 11 having a birefringent diffraction grating structure that essentially divides light emitted from the semiconductor laser into three beams, and an image of the light source on an optical disk 8. A collimating lens 3 and a converging lens 4 constituting an imaging lens system for narrowing down, a hologram element 5 having a birefringent diffraction grating structure that diffracts and separates the return light from the optical disk 8 to off the optical axis of the imaging lens system, and a diffraction grating. 11 and 4 arranged between the hologram element 5 and the optical disk 8
It is composed of a quarter-wavelength plate (λ/4 plate) 6, and a six-divided photodetector 7 and a photodetector 12 that receive diffracted light separated off the optical axis.

The optical head device of this embodiment uses the birefringent diffraction grating type hologram element 5 and the diffraction grating 11 described in the operation section. The hologram element 5 has a birefringent diffraction grating type structure that allows ordinary light to pass through and most of the extraordinary light is diffracted, while the diffraction grating 11 has a birefringent diffraction grating type structure that allows ordinary light to slightly diffract but allows extraordinary light to pass through. I'm taking it.

Furthermore, the optical axes of the respective crystals are set parallel to the polarization plane of the linearly polarized emitted light 2 from the semiconductor laser 1 serving as the light source so that it enters the hologram element 5 and the diffraction grating 11 as ordinary light. Therefore, the emitted light 2 is transmitted through the hologram element 5 without being diffracted, and is diffracted only by the diffraction grating 11. The phase difference of the diffraction grating 11 with respect to ordinary light is
The sub-beam used for tracking error signal detection is set to be slightly diffracted. The emitted light 2 accompanied by the sub-beams is converted into collimated light 10 by a collimating lens 3, and then converted into circularly polarized light by a λ/4 plate 6. and,
The convergent lens 4 converges the light onto the surface of the optical disk 8, as in the conventional optical head device (as shown in FIG. 3).

The return light reflected by the optical disk 8 is passed through the converging lens 4. λ
/4 board 6. It passes through the collimating lens 3 in the opposite direction. λ/
In the four plates 6, the polarization state of the returned light is converted from circularly polarized light to linearly polarized light perpendicular to the polarization plane of the emitted light 2. Therefore, the returned light enters the hologram element 5 and the diffraction grating 11 as extraordinary light, passes through the diffraction grating 11 without being diffracted, and is mostly diffracted by the hologram element 5. Of this diffracted light, the +1st order diffracted light enters the 6-split photodetector 7 and is used for error signal detection and recording signal detection, and the 11st order diffracted light of the main beam enters the photodetector 12 and is used for recording signal detection. .

As shown in FIG. 6, this hologram element 5 is formed of two A hologram areas 36 and a B hologram 2 placement 37, which have different diffraction directions and are separated by a dividing line 44 perpendicular to the track direction 45. As shown in FIG. 7, the diffracted light is focused onto photodetectors 7 and 12. FIG. 7(b) shows when the convergent light is focused on the optical disc 8, and among the returned light from the optical disc that has entered the A hologram area 36 and the B hologram area 37, the diffracted light of the main beam is detected as a 6-split light beam. It converges at points 40 and 41 on the dividing line of vessel 7.

The A hologram area 36 has a hologram pattern corresponding to interference fringes between a spherical wave emitted and diverging from the semiconductor laser l and a spherical wave diverging from the convergence point 40 of the diffracted light. On the other hand, the B hologram area 37 has a hologram pattern corresponding to interference fringes between the spherical wave diverging from the semiconductor laser 1 and the spherical wave diverging from the convergence point 41 of the diffracted light. FIGS. 7(a) and 7(C) show cases where the optical disk 8 is displaced and moved away from the converging lens 4, and when it approaches it.

Therefore, the four photodetecting elements 3L 3 in the 6-divided photodetector 7
2.33.34 output as V(31), V(32),
V(33LV(34)), the focus error signal is (V(
31) +V (34)) (V (32) + V (3
3)"). On the other hand, the tracking error signal uses the three-beam method as in the conventional technology, and the sub-beams that have returned from the optical disk 8 along with the main beam are each sent to the photodetector elements 29 and 30. It converges, but the two photodetector elements 29°30(7) outputs are V(29), V
(30), it is obtained from V (29) −V (30). Moreover, the recording signal is the sum of the returned light of the main beam, that is, the photodetecting elements 31 and 32 in the 6-divided photodetector 7.
．． 33. The sum of the outputs of 34 and the output of photodetector 12 V(31) +V(32) 10V(33) +V(34
) +V(12).

As described above, in the optical head device of this embodiment, the efficiency of light utilization is increased by preventing the occurrence of diffraction of the outgoing light by the hologram element and diffraction of the returning light by the diffraction grating.

Furthermore, the optical head device of the present invention can be configured with a hologram element that diffracts most of the ordinary light and allows the extraordinary light to pass through, and a diffraction grating that allows the ordinary light to pass through but slightly diffracts the extraordinary light. The polarization plane is set perpendicular to the crystal optical axis of the hologram element and the diffraction grating. That is, the light emitted from the semiconductor laser is made to enter as extraordinary light.

FIG. 8 shows a second embodiment of the present invention, in which the collimating lens 3 and converging lens 4 of the first embodiment are replaced with one imaging lens 9.

FIG. 9 shows a third embodiment of the present invention, in which a hologram element 5, which is a flat plate of the second embodiment, a diffraction grating 11, and a λ
This is an optical head device in which a /4 plate 6 is bonded together to form a single element.

FIG. 10 shows a fourth embodiment of the present invention, which is an optical head in which the six-segment photodetector 7, photodetector 12, and semiconductor laser l of the third embodiment are housed in one package. It is a device.

FIG. 11 shows a fifth embodiment of the present invention, in which a hologram element 60 has a birefringent diffraction grating type structure of the hologram element 5 and the diffraction grating 11 shown in the above embodiments 1 to 4. These are elements on the front and back sides of the substrate. The operating principle of the optical head device of this embodiment is the same as that of the first to fourth embodiments.

〔Effect of the invention〕

In the optical head device of the present invention, since a hologram element is used, the structure can be simplified, and furthermore, a -axis system structure can be adopted, so that the device can be made smaller and lighter. moreover,
By forming the hologram element and the diffraction grating into a birefringent diffraction grating type structure, signals from an optical disc can be reproduced with high efficiency.

[Brief explanation of drawings]

FIG. 1 is a diagram showing a first embodiment of the present invention, FIG. 2 is a basic configuration diagram of a conventional optical head device, and FIG. 3 is a condensed spot state on an optical disk surface of a conventional optical head device. FIG. 4 is a diagram for explaining the state of incident light on a 6-split photodetector in a conventional optical head device, and FIG. 5 is a diagram for explaining the structure of the hologram element and diffraction grating used in the present invention. FIG. 6 is a diagram showing the configuration of the hologram element used in the optical head device of the present invention, and FIG. 7 is a diagram showing the condensation of diffracted light to be focused on the photodetector of the optical head device of the present invention. 8 is a diagram showing the second embodiment of the present invention, FIG. 9 is a diagram showing the third embodiment of the present invention, and FIG. 1O is a diagram showing the fourth embodiment of the present invention. Diagrams showing examples,
FIG. 11 is a diagram showing a fifth embodiment of the present invention. 1... Semiconductor laser 2... Synchrotron radiation 3... Collimating lens 4... Converging lens 5.60... Hologram element 6... S/4 plate 7.15... Six-segment photodetector 8... Optical disk 9... Imaging lens 10... Collimated light 1116... Diffraction grating 12... Photodetector 13...Beam splinter 14...Cylindrical lens 17...Convex lens 24...Pit 26...Main beam 27, 28...Sub beam 19, 20.21 .22.23.24゜29, 30.3
1.32.33.34...Photodetection elements 36, 37...
- Lattice areas 40, 41... Convergence point 44... Parting line 45... Track direction 52... Lithium niobate 53... Exchange area 54... Non-exchange area 55...Phase compensation film

Claims (2)

[Claims] (1) a light source; a diffraction grating that substantially divides the light emitted from the light source into three beams; an imaging lens system that focuses the image of the light source onto a recording medium; a hologram element for diffraction separation off the optical axis of the imaging lens system; a hologram element arranged between the diffraction grating and the hologram element and the recording medium;
a half-wave plate and a photodetector that receives the diffracted light separated off the optical axis, the diffraction grating and the hologram element are birefringent diffraction grating type elements made of birefringent crystal, and The optical axes of the respective crystals are parallel to each other, the hologram element hardly diffracts the light emitted from the light source, and almost diffracts the return light from the recording medium, and the diffraction grating diffracts the light emitted from the light source. An optical head device characterized in that it partially diffracts and hardly diffracts the returning light from the recording medium. (2) a hologram element having a light source, a diffraction grating on the front side that substantially divides the light emitted from the light source into three beams, and a diffraction grating on the back side for diffracting and separating the return light from the recording medium off the optical axis; , an imaging lens system that focuses an image of the light source onto the recording medium; a quarter-wave plate disposed between the hologram element and the recording medium; and the diffracted light separated off the optical axis. a photodetector that receives light from the light source; the hologram element is a birefringent diffraction grating type element made of a birefringent crystal; the diffraction grating on the back of the hologram element hardly diffracts the light emitted from the light source; An optical head characterized in that most of the return light from the recording medium is diffracted, the diffraction grating on the front side of the hologram element partially diffracts the emitted light from the light source, and almost no return light from the recording medium is diffracted. Device.