JPH0750532B2 - Optical head device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an optical head device used for recording / reproduction of a so-called optical disc, digital audio disc, video disc, or the like.

(Prior Art) A conventional optical head device for a video disc, a digital audio disc, and an optical disc (hereinafter collectively referred to as an optical disc) has a semiconductor laser 1 as a light source and a semiconductor laser 1 as shown in FIG. Collimating lens 4 for converting emitted light 2 of collimated light 3 to converging light 3 and converging lens 5
In addition to the beam splitter prism 6, a focus error detecting means and a tracking error detecting means are provided.

There are various types of focus error detecting means, and the wedge prism type can be cited as the type most closely related to the type of the present invention. The wedge prism type focus error detecting means is, as shown in FIG. 2, wedge prisms 7 and 8, a two-divided photodetector including photodetectors 9 and 10, and
It is composed of a two-divided photodetector including photodetectors 11 and 12. When the converging beam 14 is just focused on the disk surface 13, the light beam from the wedge prism
15 and 16 are between the photodetectors 9 and 10 and the photodetector 11 respectively.
, And 12 but when the focused beam 14 is defocused with respect to the disk surface 13, the light beams 15 and 16
Are defocused in the direction away from each other or in the direction approaching each other, the focus error signal is obtained by taking the differential output of the photodetectors 9 and 10 or the differential output of the photodetectors 11 and 12. can get.

There are various types of tracking error detecting means, but the push-pull method can be cited as the method most closely related to the method of the present invention. The push-pull method is a method of detecting the reflected light from the disk surface using a two-division photodetector, and is the sum of the outputs of the photodetectors 9 and 10 shown in FIG. 2 and the outputs of the photodetectors 11 and 12. The tracking error signal is obtained by taking the difference from the sum of The optical head device of the prior art shown in FIG. 2 is the Philips Technical Review Volume 40 (issued in 1982).
No. 6, pp. 151-156.

(Problems to be Solved by the Invention) The conventional optical head device described above has a size of about 40 × 40 × 30 mm ³ even though it has been put to practical use, and therefore it is heavy and the optical disc device is downsized. However, it is an obstacle to the realization of weight reduction or stack type large capacity optical disc. One of the reasons for this is that the reflected light from the optical disk is bent 90 ° by a half prism or a polarizing beam splitter prism to separate it from the light source, and a photodetector is placed behind it. Therefore, it is difficult to make the optical system uniaxial.

For such a problem, a small-sized optical head has been proposed that utilizes the so-called SCOOP effect in which the oscillation output increases due to the self-coupling effect when the light is returned to the light emitting portion of the semiconductor laser light source.

However, it is pointed out that the self-coupling effect is the instability of the oscillation phenomenon of semiconductor lasers, and in digital audio discs, video discs, etc. that have been put to practical use in the last few years, as a noise leaking into the reproduction signal and the positioning signal. On the contrary, the technology for suppressing this self-coupling effect is being developed. The self-coupling result of a semiconductor laser must be considered in the configuration of a resonator composed of three mirrors in which a reflection surface called an optical disk is added to the resonator of the semiconductor laser itself. During the rotation of the disk, because of the fluttering in the optical axis direction of the disk, the distance between the semiconductor laser and the optical disk fluctuates with a width of about 1 μm even when the focus servo is applied, resulting in a resonator configuration with extremely poor stability. It's gone. Therefore,
Due to such SCOOP effect, it is too difficult to reproduce a signal on an optical disc.

An object of the present invention is to provide an optical head device capable of solving the above drawbacks and realizing a compact optical head.

(Means for Solving the Problems) An optical head device according to the present invention includes a light source, an imaging lens that narrows down an image of the light source on a recording medium, and first and second orthogonal to each other.
A photodetector whose light receiving surface is divided into four by a dividing line, and a boundary line which is provided between the light source and the imaging lens and which intersects the optical axis of the imaging lens and is parallel to the first dividing line. The second division line of the photodetector, which has different convergence distances at the boundaries and splits the reflected light from the recording medium that has passed through the imaging lens as the first-order diffracted light at the boundaries. And a grating lens that guides the light from the light source as 0th-order diffracted light to the imaging lens, respectively.

Furthermore, in the optical head device of the second invention, in the above configuration,
The grating lens is a reflective grating lens.

The optical head device according to a third aspect of the present invention has a configuration in which, in the first or second configuration, the light source and the photodetector are hybridly mounted in the same package.

(Operation) The operation and principle of the present invention are as follows. In the optical head device of the present invention, a grating lens is used to guide the reflected light from the optical disk surface to the photodetector in order to achieve the uniaxial optical system. In addition to the 1st-order diffracted light, the grating lens includes 0th-order diffracted light directly transmitted through the grating lens. Therefore, this grating lens is placed between the semiconductor laser light source and the imaging lens, and for light going from the semiconductor laser to the disk surface,
Using the 0th order diffracted light is the same as inserting a transparent plate equal to the thickness of the grating lens substrate.

On the other hand, with respect to the reflected light from the disk surface, the information light can be taken out of the optical axis by using the first-order diffracted light without using the half prism or the polarization beam splitter prism. That is, the grating lens acts as a beam splitter. As a result, a compact and lightweight optical head device can be constructed.

Further, in the present invention, in order to extract the focus error signal and the tracking error signal in addition to the signal from the information light as the first-order diffracted light extracted outside the optical axis, a line intersecting the grating direction of the grating lens with the optical axis of the imaging lens is used. By making the boundaries different from each other, an effect equivalent to the wedge prism in the conventional optical head device shown in FIG. 2 is exerted, and the light beam is converted into a light beam almost equivalent to the wedge prism system.

(Example) Next, this invention is demonstrated with reference to drawings.

FIG. 1 is a perspective view showing the basic configuration of the first embodiment of the present invention. The emitted light 2 of the semiconductor laser 1 is a grating lens 17
Through 0 as diffracted light and is converged on the disk surface 13 by the imaging lens 18. The reflected light from the disk surface 13 is converged by the imaging lens 18, diffracted by the grating lens 17, and reaches the four-division photodetector 25 beside the semiconductor laser as diffracted light 19 and diffracted light 20. 4-split photodetector 25
Consists of photodetectors 21, 22, 23, 24. The grating lens 17
It is composed of an upper grating lens 27 and a lower grating lens 28 having different focal lengths and different diffraction directions with a line 26 intersecting the optical axis of the imaging lens 18 as a boundary. The upper grating lens 27 has a grating pattern corresponding to the interference fringe of the spherical wave diverging from the semiconductor laser 1 and the spherical wave diverging from the point 29. On the other hand, the lower grating lens 28 is a spherical wave and a point diverging from the semiconductor laser 1.
It has a lattice pattern corresponding to the interference fringe with the spherical wave diverging from 30. In FIG. 1, the pitch of the grating is written larger than it actually is in order to make the arrangement easy to understand. The light diffracted by the upper grating lens 27 converges on the point 29, becomes semi-circular divergent light, and reaches the dividing line 31 of the point detectors 21 and 22. On the other hand, the light diffracted by the lower grating lens 28 becomes convergent light that converges on the point 30, and the light detector 23 and
A half circle spot is reached on the dividing line 31 of 24. Therefore, when the convergent light 14 is focused on the disk surface 13, the spot diameters of both diffracted lights on the four-division photodetector 25 are equal and the incident light intensities on the photodetectors 21, 22, 23, 24 are equal. To be 4
By disposing the divided photodetector 25, a focus error signal, a tracking error signal, and a reproduction signal can be obtained as described below.

FIG. 4 is a diagram for explaining the state of diffracted light on the four-division photodetector 25. FIG. 4 (a) is a diagram showing a focused state in which the light beam 14 is focused on the disk surface 13, and the diffracted lights 33 and 34 have the same spot diameter and are formed on the 4-division photodetector 25. It has reached on the 1st division line 31. FIG. 4B is a diagram showing diffracted light in a defocused state in which the disk surface 13 is out of focus and approaches the imaging lens 18. Since the convergent points of the diffracted lights 33 and 34 are farther from the grating lens 27 than when focused,
As shown in FIG. 4B, the spot diameter of the diffracted light 33 from the lower grating lens 28 becomes large and the spot diameter of the diffracted light 34 from the upper diffraction grating 27 becomes small on the four-division photodetector. . However, the position of the boundary line 35 of the semicircular spot corresponding to the boundary line 26 of the grating lens does not change. Therefore, the outputs of the photodetectors 22 and 24 increase and the outputs of the photodetectors 21 and 23 decrease. On the contrary, when the disk surface 13 moves away from the imaging lens 18, the converging points of the diffracted lights 33 and 34 are closer to the grating lens 17 than when focused, so that the disk is divided into four as shown in FIG. 4 (c). On the photodetector, the spot diameter of the diffracted light 33 becomes smaller and the spot diameter of the diffracted light 34 becomes larger. Also in this case, the position of the boundary line 35 of the semicircular spot does not change. Therefore, the outputs of the photodetectors 22 and 24 decrease and the photodetector 21
And the output of 23 is increased. Based on the above consideration, the photodetector 2
The output voltage of 1,22,23,24 is V (21), V (22), V (2
3), V (24), the focus error signal is V (22) + V
(24) -V (21) -V (23) can be used to detect the direction and amount of defocus of the disc.

On the other hand, the tracking error signal utilizes that an imbalance occurs in the intensity distribution of the returning light when the narrowed spot on the disk surface 13 deviates from the track position. Since the intensity ratio of the diffracted lights 19 and 20 in FIG. 1 changes due to the track shift, a difference occurs between the sum of the output signals of the photodetectors 21 and 22 of the four-division photodetector 25 and the sum of the output signals of 23 and 24. Therefore, the tracking signal is V (21) + V (22) -V (23) -V (2
It can be detected by 4), and the direction of track deviation can also be detected by the positive or negative of this signal.

The reproduction signal from the disc can be detected by taking the sum V (21) + V (22) + V (23) + V (24) of the light quantity of the four-division photodetector 25. In focus error detection and tracking error detection using a grating lens that is a diffractive element, when the wavelength of the semiconductor laser changes, the diffraction angle changes and the position shift of the diffracted light on the photodetector occurs. Although it is necessary to take measures against fluctuations in the oscillation wavelength, the present invention provides the following solution in this respect. Now, the positional deviation of the diffracted light due to the change of the diffraction angle will be considered on the four-division photodetector 25 by dividing it into two directions, that is, the direction parallel to the first division line 31 and the direction orthogonal thereto. With respect to the positional fluctuation in the direction parallel to the first dividing line 31, there is no problem because the signal strength is not changed unless it exceeds the second dividing line 32.
It should be noted that the output of the photodetectors 21, 22, 23, 24 changes with respect to the position variation in the direction orthogonal to the first dividing line, but the grating lens 28 of the present invention uses the spatial frequency component in this direction. Since it has almost no, the position variation of the diffracted light in this direction can be ignored.

FIG. 3 is a perspective view showing the basic structure of the second embodiment of the present invention. In this embodiment, the grating lens of the first embodiment is replaced with a reflection type grating lens 36, and the other structures are the same. By introducing such a reflective grating lens, the optical head can be made thin without increasing the number of optical elements.

(Effects of the Invention) The optical head device of the present invention requires only the imaging lens and the grating lens as the optical parts, and it is possible to significantly reduce the parts of the optical head device which have used many parts until now.
It is possible to downsize the entire optical disc device or reduce the size of the optical head, which has been a bottleneck in the stack type optical disc device. Further, according to the present invention, an optical head having high mass productivity and reliability can be realized by hybridizing the semiconductor laser and the four-division photodetector in the same package.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view showing a basic configuration of a first embodiment of the present invention, and FIG. 2 is a sectional view showing an example of a conventional optical head device.
FIG. 3 is a perspective view showing the basic structure of the second embodiment of the present invention, and FIGS. 4 (a), (b), and (c) are for explaining the state of diffracted light on the four-division photodetector. FIG. 1 ... Semiconductor laser, 2 ... Radiation beam, 3 ... Collimated beam, 4 ... Collimating lens, 5 ... Converging lens, 6 ... Beam splitter prism 7,8 ... Wedge prism, 9,10, 11 , 12,21,22,23,24 …… Photodetector 13 …… Disk surface, 14 …… Convergent beam, 15,16,19,20,33,34 …… Diffracted light, 17 …… Grating lens, 18 ...... Imaging lens, 25 ...... 4-division photodetector, 26 ...... Boundary line, 27 ...... Upper lattice lens, 28 ...... Lower lattice lens, 29, 30 ...... Convergence point, 31 ...... First division Line, 32 …… Second dividing line, 35 …… Semicircular spot boundary line, 36 …… Reflective grating lens

Claims (3)

[Claims] 1. A light source, an imaging lens for narrowing down an image of the light source on a recording medium, a photodetector whose light receiving surface is divided into four by first and second dividing lines which are orthogonal to each other, and the light source. Is provided between the image forming lens and the optical axis of the image forming lens, intersects with the optical axis of the image forming lens, and has different convergence distances at a boundary line parallel to the first dividing line. The reflected light from the recording medium is divided as a first-order diffracted light with the boundary line as a boundary, and the divided light is guided to the first division line on both sides of the second division line of the photodetector, and the light from the light source is emitted. An optical head device comprising at least a grating lens that guides 0-th order diffracted light to the imaging lens. 2. The optical head device according to claim 1, wherein the grating lens is a reflective grating lens. 3. The optical head device according to claim 1 or 2, wherein the light source and the photodetector are hybridly mounted in the same package.