JPH0622063B2 - Optical head device - Google Patents

Description

The present invention relates to an optical head device used for recording / reproducing of so-called optical disc, digital audio disc, video disc and the like.

[Conventional technology]

As shown in FIG. 2, 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) collimates a semiconductor laser 1 which is a light source and a light 15 emitted from the semiconductor laser 1. light
16 collimating lens 17 and converging lens 18
In addition to the beam splitter prism 19, a focus error detecting unit and a tracking error detecting unit 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. As shown in FIG. 2, the focus error detecting means of the wedge prism type includes wedge prisms 20 and 21, a two-divided photodetector composed of photodetection elements 22 and 23, and a two-divided light composed of photodetection elements 24 and 25. It is composed of a detector. When the converging beam 5 is just focused on the disk surface 6, the light beams 26 and 27 from the wedge prisms 20 and 21 are between the light detecting elements 22 and 23 and the light detecting element 24 and However, when the focused beam 5 is defocused with respect to the disk surface 6, the light beams 26 and 27 are defocused in a direction away from each other or in a direction approaching each other. Detection element
A focus error signal can be obtained by taking the differential output of 22 and 23 or the differential output of the photodetecting elements 24 and 25.

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 reflected light from the disk surface using a two-division photodetector, and is the sum of the outputs of the photodetection elements 22 and 23 shown in FIG.
The tracking error signal is obtained by taking the difference from the sum of the outputs of the photodetection elements 24 and 25. The prior art optical head device shown in FIG. 2 is described in detail in Philips Technical Review Vol. 40 (issued in 1982) No. 6, pages 151 to 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 if it has been put to practical use, and therefore it is heavy, and the overall size and weight of the optical disc device is reduced, or a stack type large capacity optical disc is realized. Has become an obstacle. 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 the semiconductor laser, and in digital audio discs, video discs, etc., which have been put into practical use in the last few years, it appears as 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 effect of a semiconductor laser must be considered in the configuration of a resonator composed of three mirrors in which an anti-slope surface called an optical disk is added to the resonator of the semiconductor laser itself. During the rotation of the disc, fluttering along the optical axis of the disc causes
Even when the focus servo is applied, the distance between the semiconductor laser and the optical disk fluctuates within a width of about 1 μm, resulting in a resonator structure with extremely poor stability. Therefore, due to such SCOOP effect, it is too difficult to reproduce the signal on the optical disk.

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 problems]

In the optical head device of the present invention, a semiconductor laser light source, an imaging lens that narrows down the image of the light source on a recording medium, and a light receiving surface that is arranged beside the light source and is divided into four by parallel dividing lines. A boundary line that is provided between the photodetector, the light source, and the imaging lens, transmits the outward light as 0th-order diffracted light, and is parallel to the dividing line that intersects the optical axis of the imaging lens. On the two left and right split lines of the photodetector, the reflected light on the return path from the recording medium that has passed through the imaging lens and has a different convergence distance at the boundary is divided at the boundary as a + 1st-order diffracted light. And a grating lens having a beam splitter function of guiding the respective beams to

[Action]

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. The grating lens has 0th-order diffracted light in addition to the desired + 1st-order diffracted light. The 0th-order diffracted light is light that has directly passed through the grating lens. Therefore, if this grating lens is arranged between the semiconductor laser light source and the imaging lens and the 0th-order diffracted light is used for the light that goes from the semiconductor laser to the disk surface, it is simply transparent to the thickness of the substrate of the grating lens. It will be the same as the board was inserted.

On the other hand, for the light reflected from the disk surface, the desired +1
When the secondary diffracted light is used, the information light can be taken out of the optical axis without using a half prism or a 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 addition to the signal from the information light extracted outside the optical axis, the focus error signal and the tracking error signal are also extracted, so that the convergence distance of the grating lens is different from each other with the line intersecting the optical axis of the imaging lens as a boundary. By letting the second
The same effect as the wedge prism in the conventional optical head device shown in the figure is applied, and the light beam is converted into a light beam almost equivalent to the wedge prism system.

〔Example〕

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

FIG. 1 is a front sectional view showing the basic structure of an embodiment of the present invention. The emitted light 2 of the semiconductor laser 1 is generated by the grating lens 3
Is passed as the 0th-order diffracted light and is converged by the imaging lens 4 to a point 7 on the disk surface 6. The reflected light from the disk surface 6 is converged by the imaging lens 4 and diffracted by the grating lens 3 to be diffracted light 8 and diffracted light 10 from the photodetector elements 32, 33, 34, 35 beside the semiconductor laser. It reaches the four-division photodetector 28.

FIG. 3 is a right side view of the optical head device shown in FIG. As shown in FIG. 3, the grating lens 3 is an off-axis lens and has converging points 11 and 9, respectively, and guides the returning light from the disk surface 6 to the four-division photodetector 28 placed on the off-axis portion.

FIG. 4 schematically shows the arrangement of the gratings when the grating lens 3 is viewed from the direction of the semiconductor laser 1. The pitch and direction of the grating lens are written larger than they actually are to make the arrangement easier to understand. The grating lens 3 is a left-side grating lens having a different convergence distance from the line intersecting the optical axis of the imaging lens 4.
29 and a right-side grating lens 30. The + 1st-order diffracted light of the left-side grating lens 29 is the point 9 shown in FIGS. 1 and 3.
Converge to. On the other hand, the + 1st order diffracted light of the right grating lens 30 converges on the point 11 shown in FIGS. 1 and 3. Therefore,
As shown in FIG. 1, the left-side diffracted light 8 is detected by the light-detecting elements 32 and 33 and the right-side diffracted light 10 is detected by the light-detecting elements 34 and 35 as shown in FIG. By detecting and arranging so that the spot diameters of both diffracted lights on the four-division photodetector 28 become equal in the focused state where the light beam 5 is converged on the disk surface 6, as will be described next. In addition, a focus error signal, a tracking error signal, and a reproduction signal can be obtained.

FIG. 5 is a diagram for explaining the state of the diffracted light on the four-division photodetector 28. FIG. 5 (a) is a diagram showing a focused state in which the light beam 5 is converged on the disk surface 6. The left diffracted light 8 and the right diffracted light 10 have the same spot diameter, and the four-division photodetector is used. It has reached the two dividing lines on the left and right of 28. In FIG. 5 (b), the disc surface 6 is deviated, and the imaging lens 4
It is a figure which shows the diffracted light of the defocused state which approached to.
The converging point of the diffracted lights 8 and 10 is at the grating lens 3 more than at the time of focusing.
Therefore, as shown in FIG. 5 (b), the spot diameter of the left diffracted light 8 becomes large and the spot diameter of the right diffracted light 10 becomes small on the four-division photodetector 28. But,
Semicircular spot boundary line 31 corresponding to the boundary line of the grating lens
The position of does not change. Therefore, the photodetectors 32 and 35
Output increases and the outputs of the photodetection elements 33 and 34 decrease. On the contrary, when the disk surface 6 is moved away from the imaging lens, the converging points of the diffracted lights 8 and 10 are closer to the grating lens 3 than when focused, so that as shown in FIG. On the split photodetector, the spot diameter of the left diffracted light 8 is small and the spot diameter of the right diffracted light 10 is large. Also in this case, the position of the boundary line 31 of the semicircular spot does not change. Therefore, the outputs of the photodetectors 32 and 35 decrease and the outputs of the photodetectors 33 and 34 increase.

From the above consideration, if the output voltages of the photodetector elements 32, 33, 34, and 35 are V (32), V (33), V (34), and V (35), the focus error signal is V ( 32) + V (35) -V (33) -V (34), and the direction and amount of defocus of the disk can be detected.

On the other hand, the tracking deviation signal uses the fact that the narrowed spot on the disk of the emitted light 2 from the semiconductor laser 1 deviates from the track position and causes an imbalance in the intensity distribution of the returning light. Assuming that the track extends in the direction of, the intensity ratio of the diffracted light 8 and 10 changes due to the track shift, so the 4-division photodetector
The sum of the output signals of the 28 photodetector elements 32 and 33 and the photodetector element
There is a difference in the sum of the output signals of 34 and 35. Therefore, the tracking signal can be detected by V (32) + V (33) -V (34) -V (35), and the direction of track deviation can also be detected by the positive / negative of this difference signal.

The signal from the disk can be detected by taking the sum V (32) + V (33) + V (34) + V (35) of the light quantity of the four-division photodetector 28.

In focus error detection and tracking error detection using a grating lens that is a diffractive element, the diffraction angle changes when the wavelength of the semiconductor laser changes, and the position of the diffracted light on the photodetector shifts. However, the present invention provides the following solution in this respect.

Now, the positional deviation due to the diffraction angle will be considered on the photodetector 28 by dividing it into two directions, a direction parallel to the dividing line and a direction orthogonal to the dividing line. Positional fluctuations in the direction parallel to the dividing line as shown by the dotted line in FIG. 6 cause no problem because no change is caused in the signal strength. It is necessary to pay attention to the positional fluctuation in the direction orthogonal to the dividing line as shown by the dotted line in FIG. 7 because the outputs of the photodetector elements 32 to 35 change. Since the tracking error signal is V (32) + V (33) -V (34) -V (35) and the information signal is V (32) + V (33) + V (34) + V (35), no change occurs. Absent. Regarding the focus error signal, in the present invention, as shown in FIG. 1, the fluctuations in the directions orthogonal to the dividing lines of the two diffracted lights on the 4-division photodetector are the same as shown in FIG.
The split photodetector 28 is connected to the boundary line of the grating lens 3 and the imaging lens 4.
By arranging it on one side of a plane including the optical axis of, and by making the interval between the diffracted lights 8 and 10 on the 4-part photodetector 28 equal to the interval between the two left and right parting lines of the 4-part photodetector, As described below, a focus error is prevented from occurring with respect to wavelength fluctuation. FIG. 8 shows the output of each photodetector element 32 to 35 in such an arrangement. Curves 36 and 3 in Fig. 8 (a)
7 is the output voltage V = V (34) for the focus error ΔZ
-V (35) and curves 38 and 39 show V = V (32) -V (33).
The dotted and solid lines correspond to the solid and dotted lines in FIG.
As described above, V (34) -V (35) or V (32) -V (33) produces an offset voltage as shown in FIG. However, in the present invention, the focus error signal is V (32) + V (35) -V (33).
-V (34), so curves 40 and 41 are shown in FIG. 8 (b).
As shown by, even if the position of the diffracted light changes, almost no offset occurs. In FIG. 8 (b), ΔZ is a focus error signal, VF is a focus error signal, and dotted lines 41 and solid lines 40 correspond to the solid line and dotted line in FIG. In the above, the solution when the wavelength of the semiconductor laser changes and the diffraction angle changes has been described. In the grating lens, when the wavelength changes, not only does the diffraction angle change, that is, the lateral aberration occurs, but also the longitudinal aberration, that is, the focal length of the grating lens also changes, but the present invention provides a solution as described below. It has been subjected.

In the present invention, the imaging lens 4 has a main converging function of converging the light reflected from the disk surface 6, and the grating lens 3 has a slight lens function for varying the converging position before and after the photodetector. It has only a lens function of correcting astigmatism caused by diffraction. Referring to FIG. 1, the right-side grating lens 30 has a convex lens function for making the convergence point of the imaging lens 4 converge to a convergence point 11 closer to the imaging lens, and the left-side grating lens 29 converges by the imaging lens 4. It has a concave lens action for converging a point to a converging point 9 farther from the imaging lens. Now, if the wavelength of the semiconductor laser changes to the long wavelength side, the action of both grating lenses becomes strong. That is, the absolute value of the focal length of the grating lens becomes small. As a result, the convergence point 11 changes in the direction closer to the imaging lens 4, and the convergence point 9 changes in the direction further away from the imaging lens 4. Therefore, the light beams on the photodetector are both large, V (32) -V (33) and V (34) -V (3
Each of 5) produces an offset. However, in the present invention, the focus error signal is V (32) + V (35) −.
Since it is V (33) -V (34), almost no offset occurs.

〔The invention's effect〕

In the optical head device of the present invention, the optical component includes an imaging lens,
Since only a grating lens is needed, it is possible to drastically reduce the parts of the optical head device that used many parts up until now, downsizing the entire optical disk device until now, or becoming the neck of the stack type optical disk device. It is possible to reduce the size of the conventional optical head.

Further, according to the present invention, by hybridizing the semiconductor laser and the four-division photodetector in the same package, it is possible to realize an optical head device having high mass productivity and reliability.

[Brief description of drawings]

1 is a sectional view showing the basic structure of an embodiment of the present invention, FIG. 2 is a sectional view showing an example of a conventional optical head device, FIG. 3 is a right side view of the embodiment shown in FIG. FIG. 4 is a plan view showing the structure of the grating lens of the embodiment shown in FIG. 1, and FIGS. 5, 6 and 7 are views for explaining the state of diffracted light on the 4-split photodetector. FIG. 8 is a diagram for explaining changes in the focus error signal with respect to the focus error. 1 ... Semiconductor laser 2,15 ... Synthetic light 3 ... Grating lens 4 ... Imaging lens 5 ... Convergent beam 6 ... Disk surface 7,9,11 ... Convergence point 8,10 ... Diffracted light 22 , 23,24,25,32,33,34,35 …… Photodetector 16 …… Collimated light 17 …… Collimating lens 18 …… Converging lens 19 …… Beam splitter prism 20,21 …… Wedge prism 26 , 27 …… Split light beam 28 …… 4-split photodetector 29 …… Left-side grating lens 30 …… Right-side grating lens 31 …… Semicircular spot boundary line 36,37,38,39 …… Difference between photodetectors Signal 40,41 …… Focus error signal

Claims (2)

[Claims] 1. A semiconductor laser light source, an imaging lens for narrowing down an image of the light source on a recording medium, and a photodetector which is arranged beside the light source and whose light-receiving surface is divided into four parallel division lines. And provided between the light source and the imaging lens,
The light that has passed through the image forming lens passes through the image forming lens and transmits the light on the outward path as 0th-order diffracted light, and has different convergence distances at a boundary line parallel to the dividing line that intersects the optical axis of the image forming lens. A light including a grating lens having a beam splitter function, which splits the reflected light on the return path from the recording medium at the boundary line and guides it as + 1st-order diffracted light onto the two left and right split lines of the photodetector, respectively. Head device. 2. The photodetector is arranged on one side of a plane including the boundary line and the optical axis, and the distance between the reflected light on the photodetector and the distance between the left and right dividing lines. 2. The optical head device according to claim 1, wherein the values are equal to each other.