JPH02101640A - Optical head device - Google Patents

Abstract

PURPOSE:To form an optical head device small in size and with high performance by polarizing and separating incident light to 1st-order diffracted light and 0th-order diffracted light of polarization intersecting orthogonally with each other, and obtaining the read signal of an optical disk setting them as difference signals with respective output strength. CONSTITUTION:The emitting light 2 of a light source 1 is converted to collimate light 4 by a collimating lens 3, and transmits a beam splitter 5, and is reflected on a total reflection prism 6, then, its optical path is bent by 90 deg., and converged on the plane 8 of the optical disk by a convergence lens 7. The reflected light is returned on a reverse path, and reflected on the splitter 5. Then, the polarizing plane of the reflected light is rotated by around 45 deg. at a 1/2-wave plate 22, and after that, the S-polarization of the light is diffracted mainly by a transmissive grating lens 23 with wavelength/grating pitch of around 1.6, and made incident on a sexenary photodetector 31. Here, the read signal can be obtained by taking the difference between a sum signal in which the signals of two outside elements 29 and 30 are added on the sum signals of four center elements 25-28 of the detector 31 and the output signal of a photodetector 24 for detecting 0th-order light.

Description

[Detailed description of the invention] [Industrial application field] This invention is applicable to recording on optical discs and disc devices.

The present invention relates to an optical head device used for reproduction.

[Conventional technology]

The 9th conventional optical head device used in optical disk devices
As shown in the figure. Emitted light 2 from a semiconductor laser 1 as a light source is converted into collimated light 4 by a collimating lens 3,
The light passes through the beam splitter 5, is totally reflected by the total reflection prism 6, bends the optical path by 90 degrees, and is converged onto the optical disk surface 8 by the converging lens 7. The reflected light from the optical disk surface is reflected by the beam splitter 5 on the opposite path. The polarization direction of the reflected light is rotated by 90 degrees by the 172-wave plate 22, and then converted into convergent light by the lens 9. The polarizing beam splitter 10 separates the transmitted light 11 and the reflected light 1 with mutually orthogonal polarization.
It is divided into 2. The transmitted light 11 is incident on a two-split photodetector 13, and a tracking error signal is obtained using a push-pull method using a difference signal between photodetecting elements 14 and 15.

On the other hand, the reflected light 12 becomes an astigmatic wavefront by the cylindrical lens 16, and a focus error signal is obtained by the astigmatism method by the 4-split photodetector 17. That is, the photodetector element 1B,
The output voltages of 19, 20, and 21 are respectively V(18).

When V(19), V(20), and V(21), the focus error signal is V(18) 10V(20)
-V (19) -V (21).

The read signal (RF multiplied signal is the polarizing beam splitter 10
Since the signal is obtained as a difference signal between orthogonal polarized light intensities divided by
, V(15), then the RF multiplied signal V(14)+
V (15) -V (18) -V (19) -
V (20) −V (21).

[Problem to be solved by the invention]

The above-mentioned conventional optical head devices, even those that have been put into practical use, have a size of approximately 40 x 40 x 30 mm or more, and are therefore heavy, which has been an obstacle to reducing the size and weight of the entire optical disk.

In addition, since the push-pull method is adopted for tracking error detection, when the converging lens 7 is moved in a direction perpendicular to the optical axis by an actuator (not shown) based on the tracking error signal, the optical axis of the converging lens A shift occurs in the dividing line of the two-split photodetector 13 for tracking error detection,
The incident light i on the photodetecting elements 14 and 15 becomes unbalanced. As a result, a direct current offset occurs in the tracking error signal, resulting in a disadvantage that the control range of tracking error control becomes narrow.

Furthermore, since the above-described conventional optical head device uses a large number of optical parts that require optical polishing, it has the disadvantage that adjustment is difficult and costs are high.

SUMMARY OF THE INVENTION An object of the present invention is to eliminate the above-mentioned drawbacks and provide a compact and low-cost optical head device.

[Means to solve the problem]

The present invention provides an optical head device for writing and reading information by converging a laser beam onto a disk surface by a converging optical system via a beam splitter, in which the beam splitter converges a laser beam onto a disk surface by a converging optical system. a grating element consisting of a plurality of regions with different characteristics that separates the reflected light taken out from the disk surface into diffracted light mainly having a specific polarization and non-diffracted light mainly having a polarized light perpendicular to the polarized light; The present invention is characterized in that it includes a detection system that detects a readout signal from the difference between the diffracted light and the undiffracted light, and a detection system that detects a focus error signal and a tracking error signal from the diffracted light.

[Effect]

The working principle of the present invention is as follows. In the optical head device of the present invention, a grating lens is used to make the optical system for receiving light light and simple. There are two types of grating lenses: transmissive grating lenses and reflective grating lenses. Transmissive grating lenses have O-order diffracted light that is directly transmitted in addition to the 1st-order diffracted light, and reflective grating lenses have O-order diffracted light that is directly transmitted in addition to the 1st-order diffracted light. There is reflected O-order diffracted light.

In the present invention, by utilizing the polarization dependence of the diffraction efficiency of the grating lens, the grating lens can be used as a polarizing beam splitter (PBS).
The incident light is polarized at right angles to each other.
The polarization is separated into the second-order diffracted light and the O-th-order diffracted light, and an optical disk read signal is obtained as a difference signal between the respective output intensities. That is, a differential reading method is used. In order to provide the grating lens with such a PBS function, in the present invention, as described below, in the transmission type, by using a grating with a relatively small grating pitch d and deep grooves with respect to the wavelength λ,
In the reflective type, this is achieved by using a grating with deep grooves and a grating pitch d that is approximately equal to the wavelength λ.

FIG. 2 shows the dependence of the diffraction efficiency on λ/d of a transmission type diffraction grating, in the case of a sinusoidal cross-sectional grating where the grating groove depth h is h/d=2.0. λ/a 2t, e is P
The diffraction efficiency of polarized light becomes almost O, and the diffraction efficiency of S-polarized light becomes almost 100%. This property allows it to function as a PBS.

FIG. 3 shows the dependence of the diffraction efficiency on the groove depth (h) of a reflection type diffraction grating, in the case of a rectangular cross-sectional grating with a grating pitch d of d-λ. At h/λ~0.9, the diffraction efficiency of P-polarized light is almost O, and the diffraction efficiency of S-polarized light is almost 10.
It becomes 0%. Due to this characteristic, the reflection type grating can also function as a PBS like the above-mentioned transmission type diffraction grating.

Furthermore, in the present invention, in order to extract the focus error signal using the first-order diffracted light, the grating lens is divided into two regions with different characteristics, and the boundary line between the two regions is made to function equivalent to the knife in the conventional optical head device. .

In addition, in order to extract the tracking error signal, two regions with different grating directions slightly separated from each other are formed in the grating lens around the point where the optical axis of the collimating lens intersects with the grating lens. By comparing the diffracted light intensities from the two regions, a tracking error signal is obtained using the push-pull principle.

〔Example〕

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

FIG. 1 is a perspective view showing the basic configuration of an embodiment of the present invention. In the optical head device of this embodiment, in the conventional optical head device shown in FIG.
9, the two-split photodetector 13 is replaced with a zero-order photodetector 24, and the cylindrical lens 16 and four-split photodetector 17 are replaced with a six-split photodetector 31. The other configuration is shown in FIG. It is the same as that of .

Therefore, the same elements as in FIG. 9 are designated with the same numbers.

In the optical head device having the above configuration, the emitted light 2 of the light source 1 is
is converted into collimated light 4 by a collimating lens 3, transmitted through a beam splitter 5, and then reflected by a total reflection prism 6.
The light is totally reflected by the light beam, the optical path is bent by 90 degrees, and the light is focused onto the optical disk surface 8 by the converging lens 7. The reflected light from the optical disk surface returns along the opposite path and is reflected by the beam splitter 5.

After the polarization plane of the reflected light is rotated by about 45 degrees by the 172 wavelength plate 22, mainly S-polarized light is diffracted by the transmission grating lens 23 of λ/d~1.6 shown in FIG. The light enters the 6-divided photodetector 31. The 0th-order diffracted light, which is mainly P-polarized light, enters the 0th-order photodetector 24 .

Four elements 25, 26, 27 at the center of the 6-split photodetector 31.
A read signal is obtained by calculating the difference between the sum signal obtained by adding the signals of the two outer elements 29 and 28 to the output signal of the zero-order light photodetector 24.

FIG. 4 is a partial perspective view for explaining the relationship between the transmission grating lens 23, the six-segment photodetector 31, and the zero-order light photodetector 24 shown in FIG. In FIG. 4, omitted line 3 is used to show the directional relationship between the divided areas in the grating lens and the track.
2, the converging lens 7 and the optical disk surface 8 are shown simultaneously.

The grating lens 23 consists of four grating lens regions, an A region grating lens (first region) 36 and a B region having different focal lengths and diffraction directions with respect to a line 35 intersecting the optical axis of the converging lens 7.
It is further divided into a region grating lens (second region) 37, and furthermore, on the dividing line 35, a region A grating lens 36 and a region B grating lens 37 have different focal lengths and diffraction directions.
area grating lens (third area) 38, D area grating lens (
A fourth region) 39 is formed respectively. Furthermore, the fourth
In the figure, the grid pitch is drawn larger than it actually is to make the arrangement easier to understand.

The A-region grating lens 36 connects the spherical wave diverging from the convergence point of the O-order diffracted light 48 to the point 4 on the dividing line of the 6-divided photodetector 310.
It has a grating pattern that corresponds to interference fringes with a spherical wave that diverges from zero.

The B area grating lens 37 connects the spherical wave diverging from the convergence point of the 0th order diffracted light 48 to the point 4 on the division line of the 6-split photodetector 31.
It has a lattice pattern corresponding to interference fringes with a spherical wave that diverges from 1.

The CeM region grating lens 38 forms a pattern corresponding to interference fringes between a spherical wave diverging from the convergence point of the 0th-order diffracted light 48 and a spherical wave diverging from a point 42 on the photodetecting element 30 of the 6-split photodetector 31. have.

The D area grating lens 39 detects the spherical wave diverging from the convergence point of the 0th order diffracted light 48 and the photodetecting element 2 of the 6-split photodetector 31.
9 has a pattern corresponding to interference fringes with a spherical wave diverging from point 43 on point 9.

The transmission grating lens 23 having such a configuration is an element with an uneven surface, and is therefore manufactured using a photolithography method.

In this embodiment, since the grating lens 23 as described above is used, the light reflected from the optical disk surface 8 and incident on the grating lens 23 is detected as diffracted light 46, 47, 33, and 34 by the 6-split photodetector. Convergence is reached at points 40, 41, 42, and 43 on 31, respectively. In addition, the O-order diffracted light 48 that did not undergo diffraction
converges and reaches the photodetector 24 for zero-order light.

The RF multiplied signal is obtained as a difference signal between the sum signal of each element of the 6-split photodetector 31 and the photodetector 24.

FIG. 5 is a diagram for explaining the state of diffracted light on the 6-split light detection 2S31. FIG. 5(a) shows the optical disc surface 8.
The figure shows a focused state in which the light beam is converged on the top, and the diffracted light 46 from the A-area grating lens 36 and the diffracted light 47 from the B-area grating lens 37 are at the first dividing line of the six-divided photodetector 31. 44 with the second division 4145 in between.

FIG. 5(b) shows that the optical disk surface 8 is displaced and the converging lens 7
It is a figure which shows the diffracted light of the defocused state which moved away from. The diffracted lights 46 and 47 enter the photodetecting element 27 and the photodetecting element 25 of the 6-split photodetector 31, respectively, but do not enter the photodetecting element 28 and the photodetecting element 26.

FIG. 5(C) shows that the optical disk surface 8 is displaced and the converging lens 7
FIG. 3 is a diagram showing diffracted light in a defocused state approaching .

The diffracted lights 46 and 47 enter the photodetecting element 28 and the photodetecting element 26 on the 6-split photodetector 31, and
7 and the light does not enter the light output element 25.

Therefore, the output of the four photodetecting elements 28, 25, 27, 26 (7) at the center of the 6-divided photodetector 31 is V(28),
V (25).

If V(27) and V(26), the focus error signal is V C2B) + V C26) −V (25
) −V (27).

On the other hand, the detection of the tracking error signal is performed on the optical disc surface 8.
If the top aperture spot shifts from the center of the track,
It takes advantage of the fact that the intensity distribution of the returning light is unbalanced. C region of the grating lens 23 The center of the grating lens 38 and I
A line connecting the centers of the IJf region grating lens 39 is the convergent lens 7
It includes the point where the optical axis of the grating lens 23 intersects with the grating lens 23, and is parallel to the direction of the tracking error of the disk. When a tracking error occurs, the C area grating lens 3
A difference occurs between the amount of light incident on the D area grating lens 39 and the amount of light incident on the D area grating lens 39. This difference in light amount can be detected as a difference in the outputs of the two photodetectors 29 and 30, and the direction of the tracking error can also be detected depending on the sign of this signal.

In focus error detection and tracking error detection using a grating lens, which is a diffraction element, when the wavelength of the semiconductor laser changes, the diffraction angle changes, causing a positional shift of the diffracted light on the photodetector. It is necessary to take measures against fluctuations in the oscillation wavelength, and the present invention provides the following solution to this problem. First, we will consider two directions on the 6-divided photodetector 31: a direction parallel to the first dividing line 44 and a direction perpendicular to it. Regarding the positional fluctuation in the direction parallel to the first dividing line 44, it crosses the second dividing line 45, but there is no problem as long as the diffracted light does not deviate from the photodetector. Regarding the positional fluctuation in the direction perpendicular to the first dividing line 44, 6
Four light detection elements 25.26.2 in the center of the split photodetector 31
Care must be taken because the output of 7.28 changes, but since the grating lens A+BSJr region of this embodiment has almost no spatial frequency in this direction, the positional fluctuation of the diffracted light in this direction can be ignored.

In the method of adopting a push-pull method as a tracking error detection method and moving a converging lens to correct the error,
A misalignment occurs between the optical axis of the optical system on the error detection side and the optical axis on the lens side, and a difference in the amount of light incident on the tracking error detection photodetector occurs, resulting in an offset in the error signal. In order to solve this problem, the present invention takes the following measures.

The grating lenses 38 and 39 in the C area and the D area are symmetrical about the optical axis and are arranged so as to detect light incident on an equal area. Therefore, even if the optical axis of the converging lens deviates from the optical axis of the error detection system in the direction perpendicular to the track, there is no change in the amount of light incident on the grating lenses 38 and 39 in areas C and D, and a tracking error offset occurs. is suppressed.

In this embodiment, the first dividing line 44 of the 6-split photodetector 31 is perpendicular to the disk track direction, but the 6-split photodetector 31 is arranged around the optical axis of the light incident on the grating lens 23. It is also possible to rotate the grating lens 23 by an arbitrary angle along with the dividing line 35 from the arrangement shown in FIG. Also, a photodetector 29 for trunking error detection.
30, the diffracted light 34 from the C region grating lens 38
If the diffracted light 33 from the D-area grating lens 39 can be separated and detected independently, the position can be set to any desired position.

FIG. 6 is a diagram showing another example of the structure of the grating lens. This grating lens 49 is obtained by rotating the grating lens boundary line 35 by 90 degrees with respect to the grating lens 23 shown in FIG. The relationship with the 6-split photodetector is exactly the same as the embodiment shown in FIG. 4, and as shown in FIG. The operation in which the diffracted light from the grating lenses 38 and 39 converges to convergence points 42 and 43 on the six-split photodetector is exactly the same as in the embodiment shown in FIG.

FIG. 7 is a diagram showing still another example of the structure of the grating lens. This grating lens 50 is divided into four areas by two boundary lines 52 and 53, including A, A, and A in the embodiment shown in FIG.
Areas 54, 55, and 56 correspond to areas B, C, and D.
．． It is 57. Therefore, the track direction is shown in the diagram 51
By setting in the direction of the arrow, the same operation as the embodiment shown in FIG. 4 can be obtained.

FIG. 8 is a perspective view showing the basic configuration of another embodiment of the present invention. The embodiment shown in FIG. 1 is a transmission grating lens 23.
In contrast, in this embodiment, a reflective grating lens 5 was used.
The only difference is that 8 is used, and the other configurations are the same. As the reflective grating lens 58, a grating with an average grating pitch d=λ=0.83 μm and a depth of 0.75 μm as explained in FIG. there was. It goes without saying that reflective types of the grating lenses shown in FIGS. 6 and 7 can also be used as the grating lens.

〔Effect of the invention〕

The optical head device of the present invention requires only a grating lens as the optical component of the light receiving system, and it is possible to significantly reduce the number of parts in the optical head device, which conventionally used a large number of parts.
It becomes possible to reduce the size of the optical head, which has hitherto been a bottleneck in downsizing the entire optical disk device.

Furthermore, since the grating lens used in the present invention is an element with a convex-concave surface, a replica can be easily obtained by a heat press method or a photopolymer method after making a mold, so it can be mass-produced at low cost. .

Furthermore, in the present invention, since regions (areas C and D) for obtaining tracking signals are provided, no track offset occurs due to movement of the converging lens.

[Brief explanation of the drawing]

Fig. 1 is a perspective view showing an embodiment of the present invention, Fig. 2 is a diagram showing the diffraction efficiency of the transmission grating lens versus λ/d, and Fig. 3 is a diagram showing the diffraction efficiency of the reflection grating against h/λ. Diagrams showing diffraction efficiency; FIG. 4 is a partial perspective view of the embodiment shown in FIG. 1; FIG.
A diagram for explaining the state of diffracted light on the split photodetector, FIG. 6 is a diagram showing another example of the configuration of the grating lens, FIG. 7 is a diagram showing still another example of the configuration of the grating lens, FIG. 8 is a perspective view showing another embodiment of the present invention, and FIG. 9 is a perspective view showing an example of a conventional optical head device. 1... Semiconductor laser 2... Radiation beam 3... 4... 5... 6... 7...... 8...・ ・ 9 ・ ・ ・ ・ ・ 22・ ・ ・ ・ ・ 23, 36. 37°50, 54. 55゜24・ ・ ・ ・ ・ 31・ ・ ・ ・ ・ 33, 34. 46°35, 52. 53. 4 sail 41.42° 44. . . . . . 45. . . . . 48. . . . 51. . . . . 58. . Lens optical disk surface lens 172 wave plate 38, 39. 49° 56, 57...Transmission type grating lens 0th order photodetector 6-divided photodetector 47...Diffracted light...Boundary line 43...Convergence point 1st dividing line 2nd dividing line 0 Arrow indicating the direction of the next diffracted light track Reflective grating lens Fig. 1 616 minutes Figure 8 Figure 9

Claims (1)

[Claims] (1) In an optical head device that writes and reads information by converging a laser beam onto a disk surface by a converging optical system via a beam splitter, the disk is taken out from the optical axis of the converging optical system by the beam splitter. a grating element consisting of a plurality of regions with different characteristics that separates reflected light from a surface into diffracted light mainly having a specific polarization and non-diffracted light mainly having a polarization perpendicular to the polarized light; An optical head device comprising: a detection system that detects a read signal from a difference in undiffracted light; and a detection system that detects a focus error signal and a tracking error signal from the diffracted light.