JPH0384737A - Focus control method - Google Patents

Abstract

PURPOSE:To stably attain focus control with high accuracy by devising the method such that a 1st photodetector detects a luminous intensity of an area having a large intensity of light beam so as to discriminate the deviation of a focal position and a 2nd photodetector detects a luminous intensity of an area having a small intensity of light beam so as to discriminate the deviation of a focal position. CONSTITUTION:A 1st photodetector 11 is provided in an optical path of a reflected light from a recording medium 6, and the intensity of part of the reflected light including the maximum strength when viewing a face orthogonal in the optical axis direction of the reflected light is detected and its output is stored in a luminous intensity storage section 13. Moreover, a 2nd photodetector 12 is provided at a different position from that of the 1st photodetector 11 to detect the intensity of a light beam of the lower strength area. Then an objective lens 5 is controlled based on an output signal of the 1st and 2nd photodetectors 11, 12 and the difference between the luminous intensity before and after the movement of the objective lens 5 is controlled to be nearly 0 thereby applying focus control. Thus, the focus control method operated stably is obtained with simple constitution.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a focus control method in an optical disk device in which information is recorded and reproduced optically.

[Conventional technology]

In recent years, optical disk devices capable of recording and reproducing large amounts of information have been put into practical use. In optical disk devices,
It is necessary to irradiate the information recording track with a laser beam focused on a minute spot diameter, with the minimum spot diameter, and for this purpose, focus control is important to make the optical head follow the disc fluctuations. It has become a technology. Many methods of focus control have been proposed and put into practical use. Typical methods are the Naifetsu method, the astigmatism method, and the critical angle method.

FIG. 2 shows an example of the configuration of an optical head of an optical disk device when the Knifezi method is used for focus control. The linearly polarized laser beam emitted from the semiconductor laser 1 is reflected by the polarization beam splitter (PBS) 2 and
After being converted into circularly polarized light by the four-wavelength plate 3, it is converted into parallel light by the collimator lens 4, condensed into a minute spot diameter by the objective lens 5, and irradiated onto the information recording medium of the disk B. The laser beam reflected by the information recording medium of the disk 6 passes through the objective lens 5, the collimator lens 4, and the 1/4 wavelength plate 6, is converted into linearly polarized light again, and is transmitted through the polarization beam splinter 2, where it is then transmitted through the polarization beam splinter 2 and is then converted into linearly polarized light. incident on the prism 7. The laser beam transmitted through the half prism 7 passes through the knife lens 21.
The laser beam enters a focus control section 20 which also includes a two-split photodetector 22 and a differential amplifier 23, and is reflected by a half prism 7, and enters a tracking control section 8.

Here, the push-pull method is often used for tracking control and is a well-known technique, so its explanation will be omitted in this specification.

Based on the focus error information detected by the focus control unit 20, the objective lens actuator 1. Focus control is performed by exciting the focus coil 9 constituting the ether to move the objective lens 5 in the optical axis direction.The operation of focus error detection using the Knifezi method will be described with reference to FIG. In FIG. 3, only optical members directly involved are shown in order to explain the operating principle. The laser beam reflected by the information recording medium on the disk 6 becomes a convergent beam when it passes through the collimator lens 4, but the beam waist position of the convergent light when the information recording medium on the disk 6 comes to the focused position, that is, the collimator lens 4 A knife 21 is provided at a focal point position 300. In this case, since equal amounts of reflected light are incident on the light receiving surfaces 22a and 22b of the two-split photodetector 22 installed behind the knife, the output of the differential amplifier 23 becomes O.

However, when the disk 6 approaches the objective lens 5,
The beam waist position of the convergent light moves away from the point 300, the light is eclipsed by the knife 21, and more light enters the light receiver 22b, so the output of the differential amplifier 26 becomes negative. Conversely, when the disk 6 moves away from the objective lens 5, more light enters the photoreceiver 22a, so the output of the differential amplifier 26 becomes positive. In this manner, focus control is performed by differentially detecting the two-split photodetector.

[Solution 1-Problem 3 of the Invention] The conventional focus control method using the Knifetzge method described above suffers from a large amount of light loss due to blocking a part of the light beam, and also requires the use of a two-split photodetector at the time of focusing. The adjustment for setting the knife position in order to make an equal amount of light beam incident on the two-split photodetector 22 is complicated, and the differential amplification according to the difference in the light amount of the light beam incident on the two-split photodetector 22 is required. Therefore, there are various problems such as complicated gain adjustment of the differential amplifier 26.

It is an object of the present invention to solve the above-mentioned problems (12) and to provide a focus control method that has a simple configuration, does not require highly accurate adjustment, and operates stably.

[Means to solve the problem]

A first photodetector is provided in the convergence optical path of the reflected light from the information recording medium to detect a portion of the reflected light that includes the maximum intensity part of the light intensity distribution seen in a plane orthogonal to the optical axis direction of the reflected light. detecting the intensity L, storing the detection output in a light intensity storage section, providing a second photodetector at a position different from the first photodetector, and detecting an area with the first photodetector. detecting the intensity of the light beam in a lower intensity region than the first optical detector I;
When the objective lens is controlled based on the output signals of the B detector and the second photodetector, the difference intensity between the light intensities stored in the light intensity storage section before and after the movement of the objective lens is O. Focus control is performed to maintain a viewable state.

[Effect]

Generally, the light intensity distribution of a laser beam has a Gaussian distribution, with the light beam having a high intensity at the center and a low intensity at the periphery. Among the two photodetectors installed in the convergence optical path of the O reflected light from the information recording medium, the first photodetector detects the light intensity of the region where the light beam intensity is high and determines the amount of shift in the focus position. judge. The second photodetector detects light intensity in a relatively low intensity region to determine the direction of focus position shift. Based on the output signals of both the first photodetector and the second photodetector, when performing focus control in the direction of correcting out-of-focus, detecting changes in reflected light intensity before and after movement of the objective lens. . As the focus position is approached, the light intensity detected by the first photodetector increases, and the difference in intensity between before and after the movement of the objective lens decreases. Therefore, if the above-mentioned difference intensity becomes O or a magnitude that can be considered as O, that position is determined to be the pest focus position.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of the configuration of an optical disk device using the focus control device of the present invention.

Numbers 1 to 9 have the same configuration as explained in FIG. The laser beam reflected by the information recording medium of the disk 6 is focused by the collimator lens 4 to become convergent light, passes through the quarter-wave plate 6 and the polarizing beam splitter 2, and further enters the half prism 7.

The laser beam reflected by the half prism 7 is incident on a tracking control section 8 to perform tracking control. The push-pull method is often used for tracking control, and since it is a well-known technique, its explanation will be omitted in this specification. The laser beam transmitted through the half prism 7 enters the focus control device 10 according to the present invention.

The focus control device 10 in the optical disk device having the above configuration is comprised of the constituent members described below.

11 is a first photodetector, and 12 is a second photodetector, both of which are installed in the convergence optical path of the collimator lens 4.

Although this example shows an example of convergent light using the collimator lens 4, an optical head with a different configuration may use another condensing lens to converge the reflected light instead of using the collimator lens 4. .

The light intensity distribution of the laser beam has a Gaussian intensity distribution, and the first photodetector 11 includes the peak intensity of the Gaussian intensity distribution and detects a portion of the light intensity near the peak intensity.

Therefore, the first photodetector 11 is set at the beam waist position of the converging reflected light or at a position close to the beam waist when the objective lens 5 is at the focused position. The first photodetector 11 is used to detect the light intensity of a region where the intensity of reflected light is strong and to determine the magnitude of defocus.

The second photodetector 12 is set at a different position from the first photodetector 11, and detects light intensity in a region where the light intensity is relatively low, excluding the vicinity of the peak intensity described above.

The second photodetector 12 performs an auxiliary operation of the first photodetector 11, and the direction of focus position shift...
. . . It is determined whether the objective lens 5 is shifted closer to the disk 6 or farther away.

Reference numeral 16 denotes a light intensity storage unit that stores the light intensity detected by the first photodetector 11. When the reflected light intensity is stored digitally, the light intensity signal, which is an analog quantity detected by the first photodetector 11, may be A/D converted and stored in the memory circuit.

In this case, the sampling period JTs for performing A/D conversion is
Perform memorization in sync with When the reflected light intensity is stored in analog form, the light intensity signal, which is an analog quantity detected by the first photodetector 11, may be applied to the charge pump circuit and stored as the quantity of charge stored in the capacitor. When the objective lens 5 is far from the focused position of the disk 6, the light intensity detected by the first photodetector 11 is low, and increases as it approaches the focused position. 14 is an objective lens drive control unit which creates a control signal 105 for driving the objective lens 5 based on the detection information of both the first photodetector 11 and the second photodetector 12, and controls the objective lens 5. An excitation current is applied to a focus coil 9 constituting a lens actuator to drive the objective lens 5. Here, a control signal having a magnitude corresponding to the light intensity of the first photodetector 11 is set, and a control signal in a control direction corresponding to the light intensity of the second photodetector 12 is set.

Reference numeral 15 denotes a differential speed detection section which detects the intensity of the difference in light intensity detected and stored by the photodetector 11 before and after the movement of the objective lens 5. For example, the objective lens 5
, the light intensity detected in the later state is 1tx
Detect Itz Ilt+ for (signal 115). This difference intensity is detected for the entire driving period of the objective lens 5. Reference numeral 16 denotes a focus state determination section, which determines the focus state based on the change in the magnitude of the difference intensity based on the output signal of the difference speed detection section 15.

If the objective lens 5 is far from the in-focus position, the first
Although the light intensity detected by the photodetector 11 is small, the difference in intensity between before and after the movement of the objective lens 5 is large when the objective lens 5 is controlled in the direction of the focal point. In this case, focus control is continued using the signal 120.

As the focus position approaches, the light intensity detected by the first photodetector 11 increases, and the difference in intensity before and after the movement of the objective lens 5 described above decreases. At the focal point position, the light intensity detected by the first photodetector 11 is maximum, and the above-mentioned difference intensity is 1, which can be regarded as 0 or almost O. When the focus position is determined, a signal 125 is applied to the objective lens drive control unit 14 to maintain the drive signal to the focus coil 9 in a constant control state and stop the movement of the objective lens 5. Note that in order to make the focus state determination section 160 perform the foot operation more reliably, the output signals of the first photodetector 11 and the second photodetector 12 are input in addition to the output signal of the differential speed detection section 15. death.

The focus state may be determined based on three pieces of information.

The detection operation of the photodetector installed in the convergence optical path of reflected light will be explained with reference to FIG. FIG. 4(a) shows the relationship between the converging reflected light from the disk 6 and the arrangement of the photodetectors.

The convergent light 40 is in a focused state, and the convergent light 41 is in a state where the disk 6 approaches the objective lens 5 (hereinafter abbreviated as N state).
, the convergent light 42 is in a state where the disk 6 is far away from the objective lens 5 (hereinafter abbreviated as F state ε). The beam waist position of the convergent light corresponds to the convergent light 40, 41, and 42.1. The position of the f point is 400.410.420.

The first photodetector 11 is set at a beam waist position 400 of the convergent light 40, and the second photodetector 12 is set near the beam waist position 420 of the convergent light 42. Figure 4 (
Curves 45 (b) and (c) show the Gaussian intensity distribution of the light intensity distribution of the laser beam, where the horizontal axis of the graph is the position d in the beam diameter direction perpendicular to the optical axis direction of the light beam, and the vertical axis is the light intensity distribution of the laser beam. It is strength. Peak intensity 450 at the center position of d20
has.

A shaded area 46 in FIG. 4(b) is a light intensity region detected by the first photodetector 11, which includes the peak intensity and detects the light intensity of a portion of the light intensity distribution.

The shaded area 47 in FIG. 4 is a light intensity region detected by the second photodetector 12, which detects light intensity in a low intensity region excluding the peak intensity region. Here, if the second photodetector 12 is installed near the e-m waist position 420 of the convergent light 42, the light beam incident on the second photodetector 12 is 0 in the state of the convergent light 42, and the convergent light The light beam enters the second photodetector 12 in the state of 40 and convergent light 41 . Therefore, the direction of the no-orccus deviation can be detected based on the light intensity detected by the second photodetector 12. In this example, the second photodetector 12 is installed near the beam waist position 420,
Although the configuration for detecting the convergent light 40 and the convergent light 41 has been shown,
In this case, the light beam detection light receiving surface is made smaller and the convergent light 41
It is also possible to detect a certain light intensity, and furthermore, the second photodetector 12 may be installed between the beam waist positions 400 and 410.

FIG. 5 shows an example of the configuration of a photodetector used in the focus control device of the present invention. FIGS. 5(a) and 5(b) show configuration examples of the first photodetector 11. FIG.

If the reproduction of information on the information recording medium of disk B is obtained from the light intensity received by the tracking control section 8, the light intensity received by the focus control section 10 may be small.

FIG. 5(a) shows the configuration of the first photodetector 11 in the above case. For example, with respect to the light beam receiving surface 50 having a square shape, only the central portion 520 is used as the light receiving surface. , the peripheral part 51 is masked as indicated by diagonal lines, and the peak intensity part of the light beam is received in the central part 52, and the light beam is cut in the masked part. good.

FIG. 5(b) shows a second configuration example of the first photodetector 11, which is designed to reduce loss in the amount of light of the light beam incident on the focus control unit 10 and utilize the amount of light effectively. This is advantageous for performing various information processing from the focus control signal. The central portion 5 with respect to the light beam receiving surface 50
2 is provided in the same way as in the case of (a) above, but narrow shading bands 56 and 54 are provided on both sides of the central portion 52,
The light beams are received in regions 55 and 56 outside the light shielding zone.

The total intensity of the light beam is obtained by the total intensity received by the light receiving surfaces 52, 55, 56. In this case as well, the focus state is determined based on the magnitude of the light intensity detected at the central portion 52. Figure 5 (L is the central light receiving surface 5 □ is a single light receiving surface.
2 can be further divided into multiple parts. When the central light-receiving surface 52 is divided into n parts, the light-receiving cell with the strongest light intensity among each light-receiving cell may be determined, and the focus state may be determined from the intensity information at that time.

In this case, even if the peak intensity position of the reflected light shifts in a direction perpendicular to the optical axis, it is possible to stably detect the reflected light intensity in a region including the peak intensity without being affected by the shift. In this case, the light intensity detected by the single light-receiving cell i of the multi-divided cell becomes small, so for example, the light intensity can be determined by detecting the sum of the light intensities detected by the light-receiving cells i, i-1, and i+1. The focus state may be determined in a large state.

FIG. 5 (/1 shows an example of the configuration of the second photodetector 12. A narrow light-shielding band 57 is provided at the center of the light beam receiving surface 5o, and the light beam is reflected by the left and right light receiving surfaces 58 and 59. Light is detected. When the second photodetector 12 is installed at the position shown in FIG. If the light is not incident, it is determined that the convergent light 40 is present, and the light receiving surface 58.
If the light beams are incident on both the light beams 59 and 59, it can be determined that the light beams 41 are in a convergent state, which makes it easier to detect the direction of focus shift.

The relationship between the focus state by the focus control device 10 of the present invention and the light intensity detected by the photodetectors 11 and 12 will be explained with reference to FIG. FIG. 6(a) is a graph showing the relationship between the light intensity detected by the first photodetector 11 and the focus state, where the horizontal axis is the optical axis direction between the objective lens 5 and the information recording medium of the disk 6. At the distance, 2=0 is the focal point position, Z〉
0 corresponds to the N state when the disk 6 approaches, and z<o corresponds to the F state when the disk 6 moves away. The vertical axis of the graph is the light intensity, with maximum intensity at Z=O, and Z=
The strength decreases as it moves away from O. At this time, it changes in almost the same shape for Z>0 and Zku0. A curve 60 shows the change in light intensity at this time. The graph in FIG. 6 (b) shows the relationship between the light intensity detected by the second photodetector 12 and the focus state. 12 is installed, the light intensity becomes almost 0 because there is almost no light beam incident on the second photodetector 12 in the F state of Z
The light intensity increases as it approaches =0, Z>.

In the N state, the intensity is almost constant. A curve 61 shows the change in light intensity at this time. At time [, the light intensity detected by the first photodetector 11 is the intensity at point 620 of the curve 60, and the light intensity detected by the second photodetector 12 is at the point 610 of the curve 60.
If the intensity is at point 64, it is determined that it is in the F state and Z>o
control in the direction of curve 60 at time t+Jt.
The intensity shown at point 66 is detected. At time t, the first
The light intensity detected by the photodetector 11 is at the point 65 of the curve 6o.
If the intensity is 0 and the light intensity detected by the second photodetector 12 is the intensity at point 67 of the curve 61, it is determined that the N state is present and control is performed in the direction of Z 0, and at time t -1 -Jt, the intensity shown at point 66 of curve 60 is detected. Figure 6 (/
→ is a graph of differential intensity, which is equivalent to the differentiation of the curve 6o shown in (a). This difference intensity shows the intensity change with respect to the change in the focal length direction.If we assume that the difference intensity is positive in the F state of ZkuO, and negative in the N state of Z〈0, then an S-shaped curve 68 It is indicated by. In the curve 60, if the intensity before and after the movement of the objective lens 5 is at points 62 and 66, the differential intensity will be at point 620 of the curve 68, and if the intensity is at points 65 and 66, the differential intensity will be at the curve 68
It becomes 0 point 650. At the focused position, the maximum intensity 600 of the curve 60 is obtained, and the differential intensity in this case is the curve 6
8, it is O as shown at one point 610. Therefore, when performing focus direction defocus control, if a state where the differential intensity can be regarded as O is detected, the focus position Z? Can be controlled to 0. In addition to the differential intensity, curve 60 and curve 61
Focus control can be performed more reliably if information on each light intensity shown by is also used for determination at the same time.

The focus control sensitivity of the focus control device R10 of the present invention depends on the light intensity near the peak intensity detected by the first photodetector 11. Shown in Figure 4 (b)! .

The wider the detection area 46 is, the lower the focus control sensitivity is, so it is desirable that the detection area 46 be narrow. According to the experimental results, immediately after the reflected light beam (peak intensity of 136
5% intensity range) V vs. l-, center the peak position at L1. If the light intensity is detected in a region of ~20% or less, the sensitivity of focus control increases. Furthermore, in order to improve the detection accuracy of the focal point position, it is possible to detect a state in which the positive and negative signs of the differential intensity obtained with respect to the control direction are reversed from 1 to 1, and control the state to a state in which the sign is reversed by a certain amount from that state. good.

In the focus control device 10 described above, the focus state is determined by software calculation by the microprocessor 47. FIG. 7 shows a flowchart for explaining the focus determination operations 1 to 1 described above. This is a flowchart of operations from starting focus control to setting the focus position.

Step 700 is a first light intensity detection, in which the first photodetector 11 detects the light intensity, and in step 702 the detected first light intensity is stored in memory. Step 70
Reference numeral 4 denotes a second light intensity detection, in which the second photodetector 12 detects the light intensity.

Step 706 is to detect the direction of defocus, and it is determined from the magnitude of the second light intensity detected in step 704 whether the defocus is in the N state or the F state. In step 708, the objective lens 5 is moved in a direction that corrects the defocus. The amount of movement at this time is determined depending on the magnitude of the light intensity detected in step 700. Step 710 is the first light intensity detection, and step 712 is the first light intensity memory, which is the same operation as steps 700 and 702 described above.

Step 714 is differential velocity detection, in which the intensity of the difference between the first light intensities before and after the movement of the objective lens is detected. When the first light intensity before the objective lens is moved is Vn, and the first light intensity after the objective lens is moved is VA, then ■^-Va'& is detected. At this time, when the focus position is in the F state, the sign of VA V@ is positive, and when the focus position is in the N state, the sign of VA-■8 is negative ε.

Step 716 determines the sign reversal of the difference intensities.

When sign reversal has not occurred, the difference intensity has not reached the state where the difference intensity becomes 0, but the state is still in the F state or the N state, which is the same as the previous state, and it is necessary to continue focus control, so step At 718, the memory is replaced.

That is, after the objective lens is moved, the memory location of the first light intensity where IgI was detected is replaced with the memory location before the movement. Do it like this.

Then, the objective lens 5 is moved until the sign of VaV is reversed. If the sign is reversed, it means that the difference intensity has reached a state of 0 and has skipped over that state, so in step 720, the objective lens is moved by a certain amount in the opposite direction to the previous control direction and the °C correction is performed. do. This state is the best focus position, and focus control ends.

〔Effect of the invention〕

As is clear from the explanation below, according to the present invention, it is possible to install a detector for focus control in the convergence optical path of reflected light without requiring high position adjustment accuracy, and It is possible to make the head smaller and lower costs.

Furthermore, in the present invention, temporal differential detection of the light intensity within a short period of time is performed as well as pattern changes of the light beam of reflected light.
It is possible to achieve stable, high-precision focus control of 1°, which is less susceptible to disturbances caused by changes in the intensity of the laser beam. Furthermore,
Two photodetectors cover a wide area, so
The dynamic range of focus control is also wide. Note that the focus control device of the present invention can be applied not only to optical disk devices but also to the field of optical measurement.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing an example of the configuration of an optical disk device using the focus control method of the present invention, and FIG. 2 is an optical path diagram showing an example of the configuration of the optical disk device when the conventional Knifezi method is used for focus control. , FIG. 3 is an optical path diagram explaining the operation of the conventional knife method, and FIGS. Figure, Figure 5 (a)~←
No. 6 is an explanatory diagram showing an example of the configuration of a photodetector of a focus control device according to the present invention, and FIGS. An explanatory diagram showing the relationship between the intensities, and FIG. 7 is a flowchart diagram explaining the operation when performing focus control of the present invention. 1... Semiconductor laser, 4... Collimator lens, 5...Objective lens, 6...Disc, 7...Half prism, 11...First photodetector, 12... Second photodetector, 16... Light intensity storage unit, 14... Objective lens drive control unit, 15...
. . . Differential speed detection section, 16 . . . Focus state determination section. Figure 5 Figure 6 F state N method breath flute Figure 7

Claims (1)

[Claims] Laser light emitted from a laser light source is focused by an objective lens and irradiated onto an information recording medium, and the reflected light from the information recording medium is collected and received by a photodetector, and an output signal from the photodetector is generated. In the focus control device, the focus position of the objective lens is always set on the information recording medium based on detecting a portion of the reflected light including the maximum intensity part of the light intensity distribution viewed in a plane orthogonal to the optical axis direction of the reflected light, storing the detection output in a light intensity storage section, and storing the detected output in the first photodetector. A second photodetector is provided at a different position to detect the intensity of the reflected light in a lower intensity area than the area detected by the first photodetector, and the first photodetector and the second photodetector The objective lens is controlled based on the output signal of the photodetector, and the focus control is performed by controlling the difference intensity between the light intensities stored in the light intensity storage unit before and after the movement of the objective lens to be considered to be zero. A focus control method characterized by: