JP4723114B2 - Focus control device and focus pull-in method - Google Patents

Description

[0001]
[Industrial application fields]
The present invention relates to a focus control device and a focus pull-in method applied to an initialization device such as a phase change optical disk and an inspection device, and more particularly to a focus control device and a focus pull-in method suitable for a recording medium whose reflectivity changes.
[0002]
[Prior art]
In general, an apparatus for recording / reproducing data on a CD or DVD using an optical disk as a recording medium requires a focus control apparatus that focuses a laser beam applied to a rotating optical disk on the disk recording film surface. Many methods such as a knife edge method, an astigmatism method, and a beam size method have been proposed as methods by which the focus control device generates a focus error signal for detecting the amount of deviation between the recording film surface and the focus. .
[0003]
Ideally, the focus error signal should have linearity in a range that is greater than the amount of surface deflection or deflection caused by the mechanical characteristics of the optical disk. However, in actuality, any of the above methods has linearity only in a narrow range in the vicinity of the in-focus position and generally draws an S-shaped curve. Therefore, before performing the focusing control, the focusing control including the in-focus position is performed. A focus pull-in operation for detecting a possible range, that is, between P and P of the S-shaped curve and closing the focus loop within the detection range is required.
[0004]
The focus pull-in method according to the prior art is based on the premise that the amplitude of the S-curve is output with good reproducibility, and a threshold value (absolute value) is provided at a position slightly lower than the peak of the S-curve, thereby providing a focusing controllable range. The focus control system, which is detected and becomes a preset stable system, has shifted to the focus follow-up mode. This conventional focus pull-in method will be described with reference to FIG. 6 where all the binarized signals according to the present description are active Lo and the in-focus position is a position indicated by reference numeral 4 in the figure.
[0005]
As shown in FIG. 6, when the focus 3 of the laser beam 2 irradiated to the optical disc 1 is moved from the position farther from the optical disc 1 to the laser beam incident side as shown in FIG. The S-curve 6 is drawn as the focus error signal for 5 approaches / crosses / separates the in-focus position 4. The focus error signal (S-shaped signal 6) is generated by the above-mentioned various methods by detecting the reflected light of the optical disk with an optical detector.
[0006]
In the conventional focus pull-in method, a threshold value 8 is provided at a position slightly lower than the peak 7 of the S-curve 6, and the focus 3 is moved from the position farther from the optical disk 1 as shown in the focal track 5 as described above. When the focus error signal falls below the threshold value after exceeding the threshold value 8, it is recognized that the focus 3 has entered the focus controllable range 10, and the focus loop is closed by the focus on signal 9 (AFON-N). Control is performed to shift to the focus tracking mode.
[0007]
As this control method, for example, in Japanese Patent Laid-Open No. 10-31828, when the amplitude of the S-curve changes mainly by the optical disc reflectivity and the laser output, the amplitude of the S-curve is measured in advance and the measurement result is used as a basis. In the technique of performing the focus pull-in operation after adjusting the loop loop gain of the servo system to a predetermined value, or in JP-A-11-203691, the RF signal level in the vicinity of the S-curve zero cross point is set while keeping the focal position away from the optical disk. A technique has been proposed in which the type of disk is measured, the type of the disk is discriminated based on the measured value, the focus gain is switched, and then the focus tracking mode is switched to the focus tracking mode in the vicinity of the in-focus point while keeping the focal position away from the optical disk.
[0008]
On the other hand, a phase change optical disk used for CD-RW / DVD-RAM / DVD-RW / DVD + RW is in an amorphous state (as depo state) in which the recording film has a low reflectivity in the state after the recording film is generated. Yes, it is necessary to perform initialization in order to change to a crystalline state in which the recording film has a high reflectivity so that it can be used in a recording / reproducing apparatus. This initialization apparatus is called a phase change optical disk initialization apparatus. It is.
[0009]
[Problems to be solved by the invention]
This phase change optical disk initialization apparatus requires the same focus control as described above, and forms an elliptical laser spot capable of covering 100 tracks or more on an optical disk recording film using a high-power semiconductor laser (2 W) as a heat source for recording. The entire surface of the optical disk is crystallized by scanning the laser spot while keeping the film at the crystallization temperature or higher. The laser wavelength of this phase change optical disk initialization apparatus is about 810 nm, which is different from the laser wavelength of the apparatus for recording / reproducing the phase change optical disk.
[0010]
Accordingly, the reflectivity of the laser beam having a wavelength of about 810 nm detected by the focus device incorporated in the phase change optical disk initialization apparatus varies greatly depending on the disk type, and even in the phase change optical disk currently in practical use, it is 0 in the as depo state. If the reflectance is several percent or more in the as depo state, the laser output that does not crystallize can be used as in the above-mentioned Japanese Patent Laid-Open No. 10-31828. Although the focus pull-in by the method is possible, the reflectivity in the as depo state is generally 1% or less, and the S / N is extremely lowered, so that the focus pull-in is difficult.
[0011]
On the other hand, it is conceivable to perform a focus pull-in operation with a laser output to be crystallized in such a phase change optical disk initialization apparatus, but the laser spot formed on the recording film surface is narrowed near the zero cross in the S-shaped curve. Only when the recording film is crystallized and the reflectance is increased, the S-curve is in an amplified state. In such a state, the peak of the S-curve cannot be detected correctly, and the focus is determined from the amplitude value of the S-curve. It has been difficult to use conventional control methods such as making the control system stable.
[0012]
Similarly, when the focus pull-in operation is performed with the laser output to be crystallized, and the method described in Japanese Patent Laid-Open No. 11-203691 is adopted, the RF signal level near the S-shaped curve zero cross point can be measured, Even if the optimum gain is calculated and set based on the measured value, the laser spot scans on the same circumference, so the crystallized part is detected again, and the focus is set at a position outside the focusing control range. There was a problem that it malfunctioned to switch to the follow-up mode.
[0013]
Further, as a document related to the focusing device and focusing control of the phase change optical disc initialization apparatus, a technique for slightly shifting the wavelength of the initialization laser from the laser wavelength used for recording / reproducing / erasing is disclosed in, for example, Japanese Patent Laid-Open No. 10-241161. However, the wavelength of the high-power semiconductor laser (2W) in the current technology is in the far infrared band (800 nm band) and has not been realized.
[0014]
As described above, the focus pull-in method using the focus device according to the prior art has a problem that the focus pull-in is unstable and the success rate is low, and even if the focus pull-in retry operation is performed, it takes a long time to pull the focus. It was.
[0015]
An object of the present invention is to eliminate the above-described disadvantages of the prior art, and in an optical disc whose reflectivity varies depending on the state or a phase change optical disc whose reflectivity changes due to the heat of irradiated light, it is possible to focus reliably in a short time. To provide a focus control device and a focus pull-in method for performing a pull-in operation.
[0016]
[Means for Solving the Problems]
In order to achieve the above object, the present invention generates an S-shaped focus error signal by receiving reflected light from a focus control target surface irradiated from a light source, and causes the focus control target surface to generate the S-shaped focus error signal. In a focus control device comprising a control means for positioning a focused spot of a light source, the apparatus includes a plurality of photodetectors that detect the reflected light before and after the focal position, and relative output signals from the plurality of photodetectors According to a first aspect of the present invention, a focus signal generation circuit that opens a focus gate indicating a focus controllable range of the S-shaped focus error signal based on a level difference is provided.
[0017]
Furthermore, the present invention is characterized in that the control means comprises means for measuring the signal level of the focus sum signal from a plurality of photodetectors, and stabilization means for stabilizing the focus control system based on the measurement result. The second feature is that the gate is opened and the focus control system is stabilized by the stabilizing means in the vicinity of the zero cross of the S-shaped focus error signal, then the focus loop is closed and the focus tracking mode is entered.
[0018]
According to a third aspect of the present invention, the control means recognizes that the focus is out of focus when the focus gate is closed in the focus follow-up mode.
[0019]
Furthermore, the present invention generates an S-shaped focus error signal by receiving the reflected light from the focus control target surface irradiated from the light source, and condensing the light source on the focus control target surface by this focus error signal. In the focus pull-in method of focus control for positioning a spot, the reflected light is detected by a plurality of photodetectors arranged before and after the focal position, and an S-shape is formed based on a relative level difference between the plurality of detection signals. Open the focus gate that indicates the focus control range of the focus error signal and measure the signal level of the focus sum signal near the zero cross of the S-shaped focus error signal, and stabilize the focus control system based on this measurement result After setting the system, close the focus loop and switch to focus follow mode. It is a feature of the.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 is a block diagram of a focus control device according to an embodiment of the present invention, FIG. 2 is a diagram showing an optical system for detecting reflected light of the focus control device, and FIG. 3 is a focus signal of the focus control device. FIG. 4 is a diagram illustrating an example of a generation circuit, FIG. 4 is a diagram illustrating a relationship between a focus position and an FE signal / FT signal / FG signal, and FIG. 5 is a diagram illustrating a focus pull-in method of the focus control device according to the present embodiment.
[0021]
As shown in FIG. 1, the focus control apparatus according to the present embodiment irradiates an optical disk 11 such as a phase change optical disk with a laser beam 12 and detects an optical system 13 having a plurality of optical detectors that detect reflected light from the disk 11. And an output signal from the optical detector, a focus error signal (hereinafter abbreviated as FE signal), a focus sum signal (hereinafter abbreviated as FT signal), and a focus gate signal (indicated by the focus control signal within the S-curve) (Hereinafter, abbreviated as FG signal) 14, a gain adjustment circuit 15 that varies the degree of amplification of the FE signal, an analog switch 16 that closes the focus loop, and phase compensation that stabilizes the focus system. The circuit 17, the lens coil 18 that moves the focal position in the vertical direction of the optical disk, and the lens coil are operated. Drive circuit 19 for converting the FT signal into digital signals, an AD converter 20 for digitizing the FT signal, a zero-cross latch circuit 21 for generating a timing at which the focus gate opens and the FE signal becomes a negative voltage, and an up-down movement for linearly moving the focal position A circuit 22 and an MPU 23 capable of programming an output signal by an input signal are provided.
[0022]
As shown in FIG. 2, the basic configuration of the optical system 13 includes a half mirror 25 that reflects and partially transmits reflected light from a recording film 24 of the optical disk 11 through a lens (not shown), and the half mirror 25. A front light detector 27 that receives the reflected light transmitted through the half mirror 25, and a rear light detector 28 that receives the reflected light reflected by the half mirror 25. The front light detector 27 receives the light before the focal position 26 of the reflected light, The rear light detector 28 is arranged to receive light at a position after the focal position 26.
[0023]
As shown in FIG. 3, the focus signal generation circuit 14 captures the reflected light from the recording film and outputs a voltage obtained by subjecting the output current from the photodetector to current-voltage conversion and outputs a front light detector 27 and a rear light detector 27. An optical detector 28, a comparator 35 that inputs output signals A and B from the detector 27 and outputs a difference, a comparator 36 that inputs output signals C and D from the detector 28 and outputs a difference, and the comparator An AND element 37 that receives the output signals 35 and 36 and takes an AND condition, an arithmetic element 33 that inputs the outputs B and D and calculates a difference, and an arithmetic element that inputs and adds the outputs B and D 34. The light receiving surfaces (light receiving areas) of the photodetectors 27 and 28 have the same configuration, and the light receiving surfaces are divided into a central portion 29 (31) and an outer peripheral portion 30 (32), and output signals A, B, C and D are output.
[0024]
Further, the photodetectors 27 and 28 receive a spot slightly smaller than the outer peripheral detector size at the in-focus position, as shown by a dotted line 31, and when the focal position shifts, the spot enters one of the photodetectors. The size of the spot becomes smaller, and the size of the spot entering the other optical detector becomes larger.
[0025]
The focus signal generation circuit 14 configured in this way obtains a difference (BD) between the output signal of the outer peripheral portion 30 of the front light detector 27 and the output signal of the outer peripheral portion 32 of the rear light detector 28 by the arithmetic element 33. An FE signal (focus error signal) is generated, and a sum (B + D) is obtained by the arithmetic element 34 to generate an FT signal (focus sum signal). Further, the center portions 29 and 31 and the outer peripheral portions 30 and 32 of the optical detector are generated. the output difference binarization Ri by the comparators 35 and 36, the output value is higher than the outer peripheral portion 30 and 32 state of the central portion 29 and 31 of the respective optical detectors by the aND element 37 [(a-B)> 0 and (C -D)> 0], the FG signal (focus gate signal) is generated.
[0026]
In particular focus signal generating circuit 14 according to this embodiment, since the relative detecting an output value of the light detector 27 and 28, it is possible to reliably detect the FG signal without depending on the amount of reflected light. In addition, although the center part and the outer peripheral part are each comprised by one piece, the center part and the outer peripheral part may be divided | segmented into plurality, respectively. Further, when the light receiving element is divided as described above, an FG signal may be generated by processing an output signal from each light receiving element by an arithmetic element. In the astigmatism method and the knife edge method, the FG signal may be generated by the same method.
[0027]
Next, the relationship between the focal position using the focus signal generation circuit according to the present embodiment and the FE signal / FT signal / FG signal will be described with reference to FIG. In the present embodiment, when the focal point 40 of the laser beam 39 irradiated on the optical disc 38 is moved from the position farther from the optical disc 38 toward the laser beam incident side as indicated by the focal orbit 41, the FE signal for the focal orbit 41 becomes the in-focus position. The S curve 42 is drawn as approaching / intersecting / separating. On the other hand, the FG signal 43 is at the Lo level only in the focusing controllable range 45 in the S-shaped curve including the zero-cross point 44 of the S-shaped curve that is the focal position. In the present embodiment, the state where the FG signal 43 is at the Lo level is the state where the focus gate is open. The signal level of the FT signal 45 increases around the in-focus position 44.
[0028]
Next, the focus pull-in operation and method according to the present embodiment will be described with reference to FIG. FIG. 5 shows a case where focus pull-in control is performed with a laser output for crystallizing a phase change optical disk in a non-crystallized state (as depo state: hatched portion), which will be described together with each part of the focus device in FIG.
[0029]
Focus control device according to the present embodiment is increased to output an initialization laser spot 47 is irradiated to the phase change optical disc 46 by controlling the recording layer surface so as to scan 48 to crystallize the initialization laser, the focal position 50 Is moved from the back side of the recording film 51 to the front side as shown by the track 49 by the up-down circuit 22.
[0030]
With this movement, the focus signal generation circuit causes the FT signal level to rise as the focus position 50 of the initialization spot approaches the in-focus position 52, and the first rise of the S curve 53 of the FE signal appears. When the peak is exceeded, the focus gate opens (FG signal Lo), and when the initialization spot 47 is narrowed closer to the in-focus point 52, the recording film which has been in the as-depo state is crystallized 54, and the reflectance increases rapidly. .
[0031]
At this time, in this circuit, the amplitude 55 of the FE signal (S-curve) and the FT signal level 56 also increase rapidly in proportion to the increase in reflectance, and then the S-curve 53 passes through the zero cross point and immediately after the passage. When the zero cross latch circuit 21 is activated, the zero cross signal (ZC signal) becomes Lo to inform the MPU 23 that the in-focus position 52 has been reached. Upon receiving this notification, the MPU 23 reads the FT signal 57 AD-converted by the AD converter 20 at the fall of the ZC signal, and calculates a gain necessary for the focus loop to take a stable system based on the value. The value is set in the gain adjustment circuit 15. Thereafter, the MPU 23 sets the focus ON signal (AFON signal) to Lo, closes the focus loop by the analog switch 16, and shifts to the focus follow-up mode. The AD conversion and gain calculation setting are performed at high speed so that the time 58 until the ZC signal becomes Lo and the AFON signal becomes Lo is much shorter than the time for drawing the S-curve 53. With the above operation, the focus pull-in operation is completed.
[0032]
Here, in order to prevent malfunction due to electrical noise, local disk defects, dust adhesion, etc., the FT signal value used by the MPU for gain calculation is the time when the S-curve passes the zero-cross point (the rise of the ZC signal). It is desirable that the FT signal level 57 is read a plurality of times and averaged. In this example, the gain calculation for the focus control system to take a stable system is performed by the MPU 23. However, the gain adjustment circuit 15 is replaced with a division element, and the FT signal is directly input to the division element, so that the focus control system The stable system may be taken. In the above-described embodiment, the case where the reflectance change occurs after the focus gate is opened is not limited to this. The reflectance change occurs before the focus gate opens or before the peak of the S-curve. May be. Furthermore, although this example showed the case where the reflectance increases near the in-focus point, it can be applied even when the reflectance decreases near the in-focus point or when the reflectance does not change, and stable focus pull-in is possible. .
[0033]
In the focus pull-in method according to the present embodiment, the focus pull-in is completed in a short time of about 1/4 cycle without reciprocating (one cycle) the focal position across the recording film surface in the direction perpendicular to the recording film surface. .
[0034]
Further, in this focus control device, it is possible to reliably detect out-of-focus by constantly monitoring the level of the focus gate signal during focus tracking.
[0035]
【The invention's effect】
As described above, the present invention includes a focus signal generation circuit that outputs a focus gate signal indicating a focusing controllable range without depending on the amount of reflected light from the optical disc, and the focus control system is based on the FT signal level at the in-focus position. By providing a control circuit that calculates and sets a stable system, it is possible to reliably perform a focus pull-in operation in a short time on optical disks with different reflectivities or phase change optical disks whose reflectivity changes due to the heat of irradiated light. it can. In addition, it is possible to reliably grasp out-of-focus during focus tracking.
[Brief description of the drawings]
FIG. 1 is a block diagram of a focus control device according to the present invention.
FIG. 2 is a diagram showing an optical system for detecting reflected light of the focus control device of the embodiment.
FIG. 3 is a diagram illustrating an example of a focus signal generation circuit of the focus control apparatus according to the embodiment.
FIG. 4 is a diagram showing a relationship between a focal position and an FE signal / FT signal / FG signal.
FIG. 5 is a diagram illustrating a focus pull-in method of the focus control device according to the present invention.
FIG. 6 is a diagram for explaining a conventional focus pull-in method.

Claims (3)

Control of irradiating light reflected from the focus control target surface and generating an S-shaped focus error signal, and positioning the focused spot of the light source on the focus control target surface by this focus error signal in focus control device comprising means, said about the reflected light each photodetector before after the focal position, the output difference between the output signals of the outer peripheral portion of the central portion of the light detector is binarized, the binary A focus signal generation circuit that opens a focus gate indicating a focus controllable range of the S-shaped focus error signal by taking an AND condition of the converted signal, and the control means includes focus signals from a plurality of optical detectors. A means for measuring the signal level of the sum signal and the focus control based on this measurement result. A stabilization means for stabilizing the system, and the focus control system is made stable by the stabilization means in the vicinity of the zero cross of the S-shaped focus error signal generated by opening the focus gate and moving the focus position. After that, the focus control device is characterized by closing the focus loop and shifting to the focus follow-up mode.   2. The focus control apparatus according to claim 1, wherein the control means recognizes out of focus when the focus gate is closed in the focus follow mode. A focus that irradiates light from the light source and receives reflected light from the focus control target surface to generate an S-shaped focus error signal, and positions the focused spot of the light source on the focus control target surface by the focus error signal in focus pull method of the control, the on reflected light each photodetector before after the focal position, the output difference between the output signals of the outer peripheral portion of the central portion of the light detector is binarized, the binarized Based on the AND condition of the signal, the focus gate showing the focus controllable range of the S-shaped focus error signal generated by moving the focus position is opened, and near the zero cross of the S-shaped focus error signal, The signal level of the focus sum signal is measured, and the focus control system is A focus pull-in method characterized by closing a focus loop and shifting to a focus follow-up mode after making a stable system.

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