JPH0636312A - Defocus detection method for optical heads for optical disks - Google Patents

Abstract

(57) [Abstract] [Purpose] To provide a defocus detection method capable of simplifying assembly and adjustment of an optical head. [Structure] The reflected light from the disk recording medium 8 is a zero-order light,
At least one detector 17b, which receives the + 1st-order light and the -1st-order light individually by the detectors 17a to 17c, and receives the ± 1st-order light, has a size smaller than the incident beam diameter, and is used for defocus detection. The output value when the information signal amplitude is maximum 23 or the output value when the scoop signal of the monitor output of the semiconductor laser is detected is stored, and the stored value is subtracted from the output value of the defocus detection detector. By doing so, the offset is canceled and the subtraction result is used as the defocus signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a defocus detection method for an optical head for an optical disk.

[0002]

2. Description of the Related Art A conventional focus shift detecting method for an optical head for an optical disk will be described with reference to FIGS.

The laser beam emitted from the semiconductor laser 48 is collimated by the collimator lens 49, is shaped by the beam shaping prism 50, passes through the beam splitter 51, and is then passed through the corner mirror (or galvano mirror) 52 to change its optical path. In other words, the light is focused on the disc recording film surface 54, which is a disc recording medium, as a light spot by the focus lens 53.

The laser beam reflected by the disk recording film surface 54 is reflected by the beam splitter 51, changes its optical path by the corner mirror 55, and becomes a convergent light by the convex lens 56. This focused light is separated into a transmitted beam and a reflected beam by a polarization beam splitter 57, the transmitted beam focused light is received by a photodetector 58a after the focus (or before the focus), and the reflected beam focused light is before the focus (or after the focus). The light is received by the photodetector 58b. In this case, the detector (light receiving element) of the photodetector 58a is divided into the light receiving surfaces a1 to a4, and the photodetector 58a is also divided.
The detector b is divided into light receiving surfaces b1 to b4.

When the laser beam focused as a light spot is focused on the disk recording film surface 54,
As shown in (1), the light beam diameters incident on both photodetectors 58a and 58b are equal. However, if the disc recording film surface 54 shifts toward the focus lens 53, (2) in FIG.
As shown in FIG. 5, the light flux diameter of the transmission side photodetector 58a becomes small, and the light flux diameter of the reflection side photodetector 58b becomes large. On the contrary, when the disc recording film surface 54 shifts in the direction away from the focus lens 53, the light flux diameter of the transmission side photodetector 58a increases and the light flux of the reflection side photodetector 58b increases as shown in (3) of FIG. The diameter becomes smaller.

As described above, since the amount of light received by the photodetectors 48a and 48b changes depending on the direction of defocus and the amount of defocus, defocus can be detected. For example, the defocus amount can be calculated by the following calculation.

Defocus amount = (a1 + a2)-(b1 + b2)

[0008]

However, in the conventional defocus detection method, the light receiving surface of the detector that receives the reflected beam from the disk recording medium is divided into a plurality of parts. Therefore, it is necessary to adjust so that the laser beam is properly incident on each of the divided light receiving surfaces (for example, the amount of light received by each of the divided light receiving elements is equal).
Moreover, since the size of the divided light receiving surface is very small (about several tens of μm), the amount of light received changes greatly with a deviation of several μm of the incident laser beam. Therefore, the adjustment is extremely difficult and a long time is spent on the adjustment.

Further, since the light receiving surface is divided into a plurality of parts, it is necessary to adjust at least two axes (up and down, left and right), and it is difficult to automate the adjustment. Furthermore, even if it is fixed by adhesion or screwing after adjustment, it is easy to make a slight deviation due to temperature changes or changes over time, and these deviations (including adjustment deviations) cause offsets in the detected servo signal, resulting in unstable servo operation. In some cases, it becomes impossible to accurately record and reproduce the information signal.

An object of the present invention is to solve the above-mentioned drawbacks of the prior art and to provide a novel defocus detection method for an optical head for an optical disk, which can simplify the assembly and adjustment of the optical head.

[0011]

In order to achieve the above object, the defocus detection method of the present invention uses a laser beam from a semiconductor laser as a 0th order light, a + 1st order light and a -1st order light by a diffraction grating.
As a light spot of the next light, it is coupled on the disk recording medium, and the reflected light from the disk recording medium is individually received by the detector for each of the 0th order light, the + 1st order light, and the −1st order light. At least one detector for receiving the primary light is formed to have a size smaller than the diameter of the beam incident on the detector and is used for defocus detection (claim 1). The defocus detector is installed in front of or behind the focal position of the signal detection system lens (claim 2).

In the present invention, the semiconductor laser is caused to emit light, the disk recording medium is further rotated, and the focus servo operation is turned off. Then, the point where the information signal amplitude becomes maximum is detected, and the information signal amplitude becomes maximum. It is preferable to provide a circuit for storing the output value of the defocus detection detector at that time, and to cancel the offset of the defocus detection detector by the stored value (claim 3). To cancel this offset, the semiconductor laser is caused to emit light, the disk recording medium is further rotated, and the focusing servo operation is turned off. The monitor output of the semiconductor laser is detected, and the focus at the time of detecting the scoop signal is detected. A circuit for storing the output value of the shift detection detector may be provided, and the offset of the focus shift detection detector may be canceled by the stored value (claim 4). In any case, the cancellation can be performed by subtracting the stored value from the output value of the defocus detection detector and using the subtraction result as the defocus signal.

[0013]

When the focus is achieved (FIG. 2 (1)), the light beam incident on the convex lens is parallel light, and the light beam is a little larger than the size of the defocus detection detector. However, if it deviates from the disk recording film surface from this position (Fig. 2 (2)
(3)), the amount of light received by this detector increases or decreases according to the direction of displacement and the amount of displacement. Therefore, the focus position can be detected based on this phenomenon. In this case, since the detector is not divided, the assembly and adjustment of the optical head is simplified (claims 1 and 2).

However, when the disc shift is zero (in focus), the detector has an output (offset), which is an offset canceling method using an envelope (RF signal) of the information signal amplitude, or a semiconductor laser. It can be removed by the offset canceling method using the scoop signal of (5).

[0015]

An embodiment of the present invention will be described in detail below with reference to the accompanying drawings.

In FIG. 1 showing the first embodiment, 1 is a semiconductor laser which is a light source. A laser beam emitted from this semiconductor laser 1 is collimated by a collimator lens 2 and further shaped into a circle by a beam shaping prism 3. To be done. afterwards,
After being diffracted by the diffraction grating 4 into a plurality of (± first-order light and zero-order light) beams and passing through the beam splitter 5, the corner mirror (or galvano mirror) 6 changes the optical path, and the focus lens 7 serves as a disk recording medium. The disc recording film surface 8 is narrowed down. At this time, since it is diffracted by the diffraction grating 4, the disc recording film surface 8 has
Three spots of the 0th-order light spot 9a, the + 1st-order light spot 9b, and the -1st-order light spot 9c are formed.

The respective light spots reflected by the disk recording film surface 8 follow the reverse direction of the outward path, are reflected by the beam splitter 5, change the optical path by the corner mirror 10, and become a convergent light flux through the convex lens 11. Then, the polarized beam splitter 12 separates the P-polarized component and the S-polarized component into P-polarized component and P-polarized component.
The polarization component is received by the photodetector 13a, and the S polarization component is received by the photodetector 13b. Incidentally, the photodetectors 13a, 13
b is arranged in front of the focal position of the convex lens 11 which is the signal detection system lens.

Here, the 0th-order light beam 14a, the + 1st-order light beam 14b, and the -1st-order light beam 14c of the P-polarized component are incident on the photodetector 13a. Therefore, as shown in FIG.
The photodetector 13a includes 0th order, + 1st order, −1th order of the P polarization component.
Detectors 15a, 15b, 1 for receiving the next light beam
5c are arranged in the order of + first order, 0th order and -1st order.
These detectors 15a, 15b, 15c are all larger in size than the light beams 14a, 14b, 14c, and are capable of receiving the total amount of light. still,
The light receiving surface of each detector is not divided.

On the other hand, the 0th-order light beam 16a, the + 1st-order light beam 16b, and the -1st-order light beam 16c of the S polarization component are incident on the photodetector 13b. Therefore, in the photodetector 13b,
The detectors 17a, 17b, and 17c that receive the 0th-order, + 1st-order, and -1st-order light beams of the S-polarized component are
They are arranged in the order of next and first order. The light-receiving surface of each detector is also not divided. However, the + 1st order light beam 16
The size of the detector 17b that receives b is smaller than that of the + 1st order light beam 16b, and is used as a defocus detection detector. As for the other detectors 17a and 17c for receiving the 0th and -1st order light beams, the dimensions thereof are smaller than those of the 0th order light beam 16a and the -1st order light beam 16c as in the case of the photodetector 13a for the P polarization component. The size is increased so that the total amount of light can be received.

Next, the principle of defocus detection will be described with reference to FIG.

First, a light spot 9 is formed on the disk recording film surface 8.
When a, 9b, 9c (0th order light, ± 1st order light) are in focus (FIG. 2 (1)), the light beam incident on the convex lens 11 is parallel light, which is before the focus position 19 of the convex lens. The light beam 16b is a little larger than the size of the defocus detection detector 17b of the photodetector 13b placed at.

On the other hand, when the disk recording film surface 8 is displaced in the direction away from the focus lens 7 (FIG. 2 (2)), the light beam incident on the convex lens 11 is convergent and the focal point 20 of the convex lens 11 is on the convex lens 11. It shifts toward you. For this reason, the luminous flux of the light beam 16 incident on the defocus detection detector 17b becomes small, and the detector 17
Since the amount of light that is vignetting and loss is reduced, the amount of light received by the detector 17b is increased as compared with the time of focusing.

On the other hand, when the disk recording film surface 8 is displaced in the direction of approaching the focus lens 7 (FIG. 2 (3)).
), The light beam incident on the convex lens 11 becomes a divergence, and the focal point 21 of the convex lens 11 shifts in a direction away from the convex lens 11. For this reason, the luminous flux of the light beam 16 that enters the defocus detection detector 17b becomes large, so that the amount of light that does not enter the detector 17b increases, and the amount of light received by the detector 17b decreases as compared with the time of focusing. In this way, the amount of light received by the defocus detection detector 17b increases or decreases depending on the direction of displacement of the disk recording film surface 8.

FIG. 3 shows a shift direction of the disc recording film surface 8 and changes in the amount of light received by the focus shift detector 17b. In this signal, the output (offset) 22 of the detector 17b is present when the disc displacement is zero (at the time of focusing), and it is not suitable for application for focusing servo.

Therefore, a method of canceling the offset 22 will be described next.

FIG. 4 is a block diagram showing a method of canceling the offset 22 of the detector 17b using the envelope of the information signal amplitude (RF signal). That is, the point where the information signal amplitude is maximum is detected when the semiconductor laser is emitted, the disk recording medium is rotated, and the focusing servo operation is turned off. A circuit for storing the output value of the defocus detection detector 17b having a small size is provided, and the offset is canceled by subtracting the stored value from the output value of the defocus detection detector 17b to obtain a desired defocus signal. It is a thing.

More specifically, as shown in FIG. 5A, the maximum information signal amplitude is 23 when the optical spot 9a (0th order light) is on the disc recording film surface 8 at the focused point. On the other hand, when the light spot 9a is displaced from the disc recording film surface 8, the information signal amplitude is reduced, and when the defocus is about 5 μm or more, the information signal amplitude becomes zero 24. Therefore, by obtaining the maximum point 23 of the information signal amplitude, it is possible to know that it is the in-focus position.

Therefore, in FIG. 4, the DC component of the information signal is cut by the circuit 25 and the RF signal is obtained through the low-pass filter 26. Next, when the circuit 27 cuts one side wave (here, the minus side), the waveform shown in FIG. 5 (2) is obtained. By differentiating this waveform with the differentiating circuit 28, the waveform shown in FIG. 5 (3) is obtained. The zero cross point 29 of this differential waveform indicates the in-focus position. This differential waveform is converted into a square wave 31a by the comparison circuit 30 (FIG. 5 (3)).

On the other hand, the output of the defocus detecting detector 17b is input to the sample hold circuit 31, and the output 36 of the detector 17b is held at the falling edge of the square wave pulse. This hold value is a value when the square wave pulse falls, that is, at the time of focusing, and corresponds to the offset 22 in FIG.

This hold (offset) value 22 is set to A
It is digitized by the / D conversion circuit 32 and stored in the memory 33. Then, from the output of the detector 17b, a value obtained by converting the value of the memory 33 into an analog value by the D / A conversion circuit 34 (which corresponds to the offset value 22) is calculated by the subtraction circuit 35, so that the focus without offset is obtained. The shift signal 37 (FIG. 6) can be obtained. Focusing servo can be realized using this signal.

The tracking servo can be performed by using, for example, a known sample servo method. This is to use a change in the sum of the output of the detector 15a in the sample pit and the output of the defocus detection detector 17a as a tracking error signal.

Although the photodetectors 13a and 13b are located before the convex lens focal point 19, they may be provided after the convex lens focal point 19. However, at this time, the change in the size of the light beam on the detector 17b due to the displacement direction of the disk recording film surface 8 is reversed, so the defocus signal is opposite to the polarity in FIG.

FIG. 7 shows the scoop signal 3 of the semiconductor laser 1.
8 is a block diagram showing a method of canceling the offset 22 of the detector 17b using 8; FIG. This makes the semiconductor laser emit light and further rotates the disk recording medium,
When the focusing servo operation is turned off, the monitor output of the semiconductor laser is detected, and when a scoop signal is detected, a circuit for storing the output value of the defocus detection detector 17b having a size smaller than the incident beam diameter is provided. The offset is canceled by subtracting the stored value from the output value of the deviation detection detector 17b, and a desired defocus signal is obtained.

More specifically, a large amount of reflected light from the disk recording film surface 8 returns to the semiconductor laser 1 at the time of focusing. Then, the oscillation mode of the semiconductor laser 1 changes. Therefore, a peak called a scoop signal 38 appears in the monitor output of the semiconductor laser 1 (FIG. 8). Therefore, the monitor output of the semiconductor laser 1 is monitored, and when the scoop signal 38 appears, it is known that the disk recording film surface 8 is at the in-focus position.

Therefore, the scoop signal 3 of the semiconductor laser 1
8 using the comparison circuit 40 to generate a square wave pulse 39 (FIG. 9).
Convert to. On the other hand, the output 36 of the defocus detection detector 17b is input to the sample hold circuit 41, and the output 36 of the detector 17b is held at the trailing edge of the square wave pulse 39. This hold value is a value when the square wave pulse 39 falls, that is, at the time of focusing, and corresponds to the offset 22 in FIG.

This hold (offset) value 22 is set to A
It is digitized by the / D conversion circuit 42 and stored in the memory 43. Then, from the output 36 of the detector 17b, a value obtained by converting the value of the memory 43 into an analog value by the D / A conversion circuit 44 (this corresponds to the offset value 22) is calculated by the subtraction circuit 45, so that there is no offset. The defocus signal 46 (FIG. 5) can be obtained. Focusing servo can be realized using this signal 46.

According to the above configuration, the photodetectors 13a, 13a
Each of the beams 14a to 14c and 16a to 16c incident on b
Since each detector for receiving the light is not divided into a plurality, the light amount adjustment becomes easy. Since the detector 17b of the photodetector 13b is smaller in size than the light beam 16b, it is necessary to adjust the light amount (position adjustment), but high-precision (several μm) positioning is not necessary. Because the position adjustment is rough and the output (offset) 22 of the detector 17b varies,
This is because the offset cancellation of the defocus signals 37 and 46 becomes zero by the offset cancellation described in the embodiment.
Therefore, the position adjustment and processing accuracy of each optical component may be rough, and the simplification of the assembly adjustment of the optical head and the cost reduction of the optical head can be realized.

Further, even if the output (offset) 22 of the detector 17b fluctuates due to the shift of the optical parts due to a change with time after the optical head is assembled, the defocus signal 37 can be obtained by performing the offset cancellation described in the embodiment. , 46, the offset is zero, so that it is not necessary to readjust the optical components.

In the embodiment of FIG. 1 described above, the detector 17b of the photodetector 13b has a size smaller than that of the + 1st order light beam 16b, but the detector 15b of the photodetector 13a has +1.
The size may be smaller than that of the next light beam 14b.

Although the polarization beam splitter 12 is used in the embodiment shown in FIG. 1, if a Wollaston prism is used instead of the polarization beam splitter 12, the P-polarized light and the S-polarized light are emitted in the same direction. It is good with the individual. However, in this case, the photodetector is equipped with a detector for detecting the 0th order light and the ± 1st order light of the P polarization component and the 0th order light and the ± 1st order light of the S polarization component.

The example of the magneto-optical head has been described above, but the present invention can also be applied to a phase change optical head. In the case of the phase change type, since the information signal is read by the intensity of light, not the polarized light,
The polarization beam splitter 12 is not necessary, and the number of photodetectors is only one as long as it has the detector structure of the photodetector 13b. The method of detecting the defocus signal and the tracking error signal is as described in this embodiment.

[0042]

In summary, according to the present invention, the following excellent effects are exhibited.

1) According to the constructions of claims 1 and 2, since the detectors for receiving the incident beam are not divided into a plurality of pieces, the light quantity adjustment becomes easy.

2) According to the constitutions of claims 3 to 5, since the detector is smaller in size than the light beam, it is necessary to adjust the light amount (position adjustment), but high-precision (several μm) positioning is not necessary. . This is because even if the position adjustment is rough and the output (offset) of the detector fluctuates, the offset of the defocus signal becomes zero due to the offset cancellation. Therefore, the position adjustment and processing accuracy of each optical component may be rough, and the simplification of the assembly adjustment of the optical head and the cost reduction of the optical head can be realized. Even after the optical head is assembled, even if the optical components deviate due to changes with time and the output (offset) of the detector fluctuates, the offset cancellation cancels the defocus signal offset to zero.
There is no need to readjust the optical components.

[Brief description of drawings]

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

FIG. 2 is a diagram showing the principle of defocus detection of the present invention.

FIG. 3 is a diagram showing changes in the amount of light received by the detector due to defocus.

FIG. 4 is a block diagram showing a defocus detection method according to an embodiment of the present invention.

5 is a diagram for explaining the operation of FIG.

FIG. 6 is a diagram showing a focus shift signal of an optical head.

FIG. 7 is a block diagram showing a defocus detection method according to another embodiment of the present invention.

FIG. 8 is a diagram showing a scoop signal in a monitor output of a semiconductor laser.

FIG. 9 is a diagram after converting a scoop signal into a square wave.

FIG. 10 is a configuration diagram of a conventional optical head optical system.

FIG. 11 is a diagram showing the principle of conventional defocus detection.

[Explanation of symbols]

1 Semiconductor Laser 2 Collimating Lens 3 Beam Shaping Prism 4 Diffraction Grating 5 Beam Splitter 6 Corner Mirror (or Galvano Mirror) 7 Focus Lens 8 Disc Recording Medium Disk Recording Film Surface 9a 0th Order Light Spot 9b + 1st Order Light Spot 9c -1st Order Light spot 10 Corner mirror 11 Convex lens 12 Polarization beam splitter 13a P-polarization component photodetector 13b S-polarization component photodetector 14a P-polarization component 0th-order light beam 14b P-polarization component + 1st-order light beam 14c P-polarization component photodetector -1st order light beam 15a, 15b, 15c Detector of P polarization component 16a 0th order light beam of S polarization component 16b + 1st order light beam of S polarization component 16c -1st order light beam of S polarization component 17a 0th order light beam detector 17b + 1st order light beam detector ( Defocus detection) 17c, Detector for -1st-order light beam 19, 20, 21 Focus position of convex lens 22 Output of detector (offset) 23 Maximum point of information signal amplitude 24 Zero point of information signal amplitude 25 DC component cut circuit 26 Low pass filter 27 Single side wave cut circuit 28 Differentiation circuit 29 Zero cross point 30,40 Comparison circuit 31a Square wave 32,42 A / D conversion circuit 33,43 Memory 34,44 D / A conversion circuit 35,45 Subtraction circuit 36 Detector output 37,46 Defocus signal 38 Scoop signal 39 Square wave pulse 41 Sample and hold circuit

Claims (5)

[Claims] 1. A laser beam from a semiconductor laser is combined on a disk recording medium as a light spot of 0th order light, + 1st order light and −1st order light by a diffraction grating, and reflected light from the disk recording medium is a 0th order light. Each of the light, the + 1st order light, and the −1st order light is individually received by the detector, and ±
A defocus detection method for an optical head for an optical disc, wherein at least one detector for receiving the primary light is formed to have a size smaller than a beam diameter incident on the primary light and used for defocus detection. 2. A defocus detection method for an optical head for an optical disk according to claim 1, wherein the defocus detection detector is installed in front of or behind the focal position of the signal detection system lens. 3. When the semiconductor laser emits light, the disk recording medium is further rotated, and the focusing servo operation is turned off, the point where the information signal amplitude becomes maximum is detected, and when the information signal amplitude becomes maximum. 2. A circuit for storing the output value of the defocus detecting detector in 1 is provided, and the offset of the defocus detecting detector is canceled by the stored value.
2. A defocus detection method for an optical head for an optical disc as described in 2. 4. The focus when the semiconductor laser monitor output is detected and the scoop signal is detected in a state in which the semiconductor laser is caused to emit light, the disk recording medium is rotated, and the focusing servo operation is turned off. 4. A defocus detecting method for an optical head for an optical disc according to claim 1, wherein a circuit for storing an output value of the defocus detecting detector is provided, and the stored value cancels the offset of the defocus detecting detector. . 5. The optical disc according to claim 3, wherein the cancellation is performed by subtracting the stored value from the output value of the defocus detection detector, and the subtraction result is used as a defocus signal. Deviation detection method for optical heads.