JPH0434211B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a focus adjustment device that automatically detects, for example, the focus of a laser optical system and automatically positions the focus at a desired position.

[Technical background of the invention and its problems]

Conventionally, a focus detection method for a laser optical system has been carried out by using a special lens such as a cylindrical lens or a prism, and detecting the laser light that has passed through this special lens with an optical sensor. Another method is to arrange a slit plate with a slit near the focal point of the laser optical system, and vibrate this slit plate in a direction perpendicular to the optical axis. There is a method of detecting a focus based on changes in the phase of laser light.

However, the former method has the disadvantage that precise focus detection cannot be performed if the positioning accuracy of the special lens is poor. On the other hand, the latter method requires positioning so that the optical axis of the laser beam accurately passes through the slit, which not only reduces work efficiency but also makes it difficult to obtain high detection accuracy.

[Purpose of the invention]

The present invention has been made in consideration of the above circumstances, and an object of the present invention is to provide a focus adjustment device that can detect and position a focus position automatically and with high efficiency.

[Summary of the invention]

The convergent light projected from the optical system is received by a two-split light receiving sensor via a mask plate with slits, and a sum signal indicating the sum of these two signals is generated based on the two signals output from the two-split sensor. and a difference signal indicating the difference between these two signals, and further, a peak value of the sum signal and a peak-to-peak value of the difference signal are determined, and a focusing operation is performed based on these peak values and peak-to-peak values. It is something.

[Embodiments of the invention]

Hereinafter, one embodiment of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows the focus adjustment device of this embodiment. This focus adjustment device includes a base 2 on which a laser optical system 1 is installed, a focus detection section 3 that detects the focal position of the laser optical system 1, and a laser beam on a slit plate 4 forming a part of this focus detection section 3. It consists of a positioning section 5 that positions the optical system 1 so that it is in focus, and an arithmetic control section 6 that controls the operations of the focus detection section 3 and the positioning section 5. As shown in FIG. 2, the laser optical system 1 includes a semiconductor laser 7, a collimator lens 9 that converts the laser beam 8 emitted from the semiconductor laser 7 into a parallel beam, and a collimator lens 9. Beam splitter 10 that reflects the passed laser beam 8 upward
A 1/4 wavelength plate 11 that converts the laser beam 8 that has passed through this beam splitter 10 into circularly polarized light, an objective lens 12 that converges the laser beam 8 that has passed through this 1/4 wavelength plate 11, and a A condensing lens 15 that condenses the laser beam 14 that is reflected by the disk 13, travels backward along the same optical path, and passes through the beam splitter 10, and a cylindrical lens that makes the laser beam 14 that has passed through the condensing lens 15 into a perfect circle. 16, and a divided sensor 17 in which a plurality of photoelectric elements for receiving and photoelectrically converting the laser beam 14 that has passed through the cylindrical lens 16 are equally distributed. That is, the main purpose of the apparatus of this embodiment is to detect the focal position of the objective lens 12, and the laser optical system 1 is installed on the base 2 so that its optical axis is in the vertical direction. Note that when the detection of the focal position of the objective lens 12 is completed, the slit plate 4 is removed and the objective lens 12 is removed.
A recording medium such as an optical disk is positioned at the focal point of the image by means not shown. That is, after the slit plate 4 has been positioned, it can be replaced with an object to be inspected, such as an optical disk. Therefore, the positioning part 5 is connected to the support column 18 erected on the base 2.
A table 2 is installed on this support column 18 so as to be slidable in the directions of arrows 19a and 19b (up and down).
0 and the feed screw 2 screwed onto this table 20.
1, a stepping motor 22 which is fixed to the base 2 and has a rotating shaft connected to the feed screw 21 to rotationally drive the feed screw 21, and a motor driver 23 which controls the stepping motor 22. The table 20 is moved up and down in the directions of arrows 19a and 19b in conjunction with the rotation of the stepping motor 22. On the other hand, the focus detection section 3 is mounted on the table 20 and is connected to arrows 19a, 1.
A vibrator 24, such as a piezo actuator, which applies vibration in the directions of arrows 24a and 24b orthogonal to the direction 9b, and a vibrator 24 at the tip of the vibrator 24 so as to be approximately coaxial with the optical axis 1a of the laser optical system 1. a cylindrical holder 25 connected to the holder 25, a slit plate 4 attached to the opening at the lower end of the holder 25, and a slit plate 4 attached to the lower end opening of the holder 25;
2 split light receiving sensor 2 attached to the upper end opening of 5
6, and a focus calculating circuit 27 electrically connected to the two-split light receiving sensor 26. As shown in FIGS. 3A and 3B, the slit plate 4 includes a transparent substrate 28 such as a glass plate, and a thickness of, for example, 2000 mm formed on one main surface of the transparent substrate 28.
and a mask 29 made of a Cr vapor-deposited film. This mask 29 has a plurality of parallel slits 30 at a constant pitch interval (for example, 10.5 μm).
...is being formed. The width of these slits 30 is set to, for example, 1 μm so as to be approximately the same as the diameter of the focused laser beam. Further, the longitudinal direction of the slits 30 is set to be orthogonal to the directions of arrows 24a and 24b. The vibrator 24 is set to vibrate the slit plate with an amplitude greater than the pitch of the slits 30 (for example, 15 μm). In addition, the two-split light receiving sensor 26
consists of a pair of light receiving elements 31 and 32.
Then, the two-split light receiving sensor 26 and the slit plate 4
The plate surface is disposed perpendicular to the optical axis 1a. Further, the straight boundary lines between the longitudinal direction of the slit 30 and the light receiving elements 31 and 32 are set to be parallel to each other. On the other hand, the focus calculation circuit 27 includes amplifiers 33 and 34 connected to the light receiving elements 31 and 32 respectively, and these amplifiers 33,
an adder 35 connected to the output side of the amplifier 34; and a subtracter 3 connected to the output side of the amplifiers 33 and 34.
6, a peak detector 37 connected to the output side of the adder 35, and a peak-to-peak detector 38 connected to the output side of the subtracter 36. The arithmetic control section 6 has a microcomputer as its main component, and has arithmetic, storage, and control functions. The arithmetic control section 6 receives detection signals from the peak detector 37 and the peak-to-peak detector 38, and clears each of these detectors 37 and 38 in a timely manner. Further, a work program is stored in this arithmetic control unit 6, and based on this work program, the motor driver 23 and the vibrator 2 are
A control signal is output to the terminal 4.

Next, the operation of the focus detection device having the above configuration will be described.

First, the laser beam 8 is emitted from the laser optical system 1. Then, the slit plate 4 is moved by the vibrator 24.
is vibrated in the directions of arrows 24a and 24b, for example, with an amplitude of 15 μm. Then, the slit 3 of the slit plate 4
The laser beam 8 that has passed through the light receiving elements 31, 3
The light is received at 2 and photoelectrically converted. Each of the light receiving elements 31 and 32 outputs signals SA and SB having voltages corresponding to the amount of light received. These signals SA and SB are amplified by amplifiers 33 and 34, respectively, and input to an adder 35 and a subtracter 36. Then, the adder 35 adds the signals SA and SB, and outputs a signal SC indicating the sum thereof to the peak detector 37. On the other hand, the subtracter 36 subtracts the signals SA and SB, and outputs a signal SD indicating the difference between them to the peak-to-peak detector 38. Therefore, the peak detector 37 sequentially detects the maximum peak value of the input signal SC, and
A sum signal SE indicating the detected peak value is output to the calculation control section 6. On the other hand, the peak-to-peak detector 38 outputs a difference signal SF indicating the difference between the maximum peak value and the minimum peak value of the input signal SD to the calculation control section 6. By the way, as shown in FIG. 4, when the focal point F position is on the slit plate 4, that is, when the focal point F position is at the proper position, the sum signal SE becomes maximum, and conversely, the difference signal SF becomes minimum. . In other words, when the focus F position is not at a proper position, that is, when a focus shift occurs, the sum signal SE tends to decrease from the maximum value, while the difference signal SF tends to increase from the minimum value. In other words, the sum signal SE is a normal distribution curve that takes its maximum value at the focal point F position, whereas the difference signal SF is a bimodal curve that has a saddle, or minimum value, at the focal point F position. . This point will be explained in further detail with reference to FIGS. 5 to 10. First, when the focal point F is below the slit plate 4, the receiving position of the laser beam changes as shown in FIGS. 5A, B, and C. That is, in FIG. 5A, only the light receiving element 32 receives the laser beam. In addition, in FIG. 5B, both light receiving elements 3
Laser light is received at 1 and 32. Further, in FIG. 5C, the laser beam is received only by the light receiving element 31. In FIG. Therefore, as shown in FIGS. 8A and B, the curve showing the time change of the sum signal SE has a mountain shape, and the curve showing the time change of the difference signal SF has a mountain shape and a valley shape that are point symmetrical. It becomes a curve. On the other hand, when the focal point F is on the slit plate 4, the sixth
As shown in Figures A, B, and C, the receiving position of the laser beam changes. That is, in FIG. 6A, the laser beam is blocked by the slit plate 4 and is not received by the light receiving elements 31 and 32. In FIG. Also, the 6th
In Figure B, the laser beam is transmitted to both light receiving elements 31,
The light is received at 32. The amount of light received at this time is the fifth
It is larger than the case in Figure B. Further, in FIG. 6C, the laser beam is blocked by the slit plate 4 and is not received by the light receiving elements 31 and 32.
Therefore, as shown in FIG. 9A and B, the sum signal
The curve showing the time change of SE has a peak value larger than that in Figure 8A, and on the other hand, the difference signal
The curve showing the time change of SF has a peak value much smaller than that shown in FIG.
Furthermore, when the focal point F is above the slit plate 4, the receiving position of the laser beam changes as shown in FIGS. 7A, B, and C. That is, in FIG. 7A, the laser beam is received only by the light receiving element 31. In FIG. Further, in FIG. 7B, the laser beam is received by both light receiving elements 31 and 32. Also,
In FIG. 7C, the laser beam is received only by the light receiving element 32. In FIG. Therefore, as shown in FIGS. 10A and 10B, the curve showing the time change of the sum signal SE is
The peak value is almost the same as that shown in FIG. 8A, and the curve showing the time change of the difference signal SF is a curve that is inversely symmetrical to that shown in FIG. 8B. Thus, the eighth
As shown in the figures through FIG. 10, the peak value of the sum signal SE is maximum when the focal point F is on the slit plate 4. On the other hand, the peak value of the difference signal SF is minimum when the focal point F is on the slit plate 4. Therefore, by utilizing these characteristics, the focusing point operation can be performed automatically as shown in the flowchart of FIG. 11. In other words, since the point where the sum signal SE has the maximum value and the difference signal SF has the minimum value is the in-focus position, it is sufficient to find a position that satisfies this condition.
Therefore, the table 20 is first raised in the direction of the arrow 19a (step 39). At this time, the calculation control unit 6 determines whether the voltage value of the sum signal SE has increased (step 40). And the sum signal
When the voltage value of SE increases, the table 20 is raised again (step 41). However, if the voltage value of the sum signal SE decreases, Table 20
is lowered in the direction of arrow 19b (step 42).
Then, in either case, the arithmetic control section 6
, the voltage value of the sum signal SE is the maximum value SE (max)
(See FIG. 4) is determined, and the table 20 is raised and lowered until it passes the maximum value SE (max) (steps 43 and 44). Then, the maximum value SE
(max), the table 20 is raised by a certain amount (step 45). Next, the arithmetic control unit 6 determines whether the voltage value of the difference signal SF has increased (step 46). Then, when the voltage value of the difference signal SF increases, Table 2
0 is lowered (step 47). But the difference signal
If the voltage value of SF decreases, Table 20
(Step 48). Then, in either case, the arithmetic control section 6 calculates the difference signal.
Has the voltage value of SF changed from decreasing to increasing? In other words, the minimum value SF (min) of the difference signal SF (see Figure 4)
A judgment is made as to whether or not the minimum value SF
The table 20 is raised and lowered until it passes (min) (steps 49 and 50). Then, the minimum value SF (min)
If step 49 is passed, table 2
0 is increased by a certain amount (step 51), and the difference between the difference signal SF and its minimum value SF (min) is made to be less than or equal to a preset value ε (step 51).
52). On the other hand, for step 50, table 20
is lowered by a certain amount (step 53), and the difference between the difference signal SF and its minimum value SF (min) is made to be equal to or less than the set value ε (step 54). Thus, the focusing point operation is completed. However, this time, after removing the slit plate 4 positioned at the focal position,
The object to be inspected is replaced with an object to be inspected, such as an optical disk, by means not shown, and the focus F of the laser optical system 1 is set to be on the recording surface of the optical disk, for example, and the next inspection operation is started.

In this way, the focus adjustment device of this embodiment can detect and position the focus of the laser optical system 1 automatically and with high precision. In particular, since a plurality of slits 30 are formed in the slit plate 4, complicated fine adjustments are not necessary, and the focusing efficiency is improved.

In addition, in the above embodiment, the laser optical system 1
and fix the slit plate 4 side as indicated by the arrows 19a and 19b.
The focal point is positioned by moving the
The slit plate 4 side may be fixed and the laser optical system 1 side may be moved. Furthermore, the focus adjustment device of the present invention is not limited to laser light, and can be applied to any light having a small beam diameter and shallow depth of focus.

〔Effect of the invention〕

The focus adjustment device of the present invention can detect and position the focus automatically and with high precision. In particular, since a plurality of slits are formed in the slit plate, complicated fine adjustments are not required, which contributes to improved efficiency.

[Brief explanation of drawings]

FIG. 1 is an overall configuration diagram of a focus adjustment device according to an embodiment of the present invention, FIG. 2 is a configuration diagram of a laser optical system, and FIG.
Figure A is a plan view of the slit plate shown in Figure 1, Figure 3B is a sectional view taken along the - line in Figure 3A, Figure 4 is a graph showing the relationship between the focal point position and the detected output value, and Figures 5 to 5. Figure 7 is a diagram showing the relationship between the focal point position and the amount of received light;
8 to 10 are graphs showing the relationship between the focal point position and the output voltage value, and FIG. 11 is a flowchart for explaining the operation of the focus adjustment device shown in FIG. 1. 1... Laser optical system, 2... Base (holding part),
3... Focus detection unit, 4... Slit plate, 5... Positioning unit, 6... Arithmetic control unit, 24... Vibrator,
26...Two-split light receiving sensor, 30...Slit,
31... Light receiving element (first -), 32... Light receiving element (second -), 35... Adder, 36... Subtractor, 37... Peak detector, 38... Peak-peak detection vessel.

Claims (1)

[Scope of Claims] 1. A focus adjustment device characterized by having the following configuration. (a) A holding part that holds the optical system that converges light to a focal position. (b) A slit plate for position adjustment, which is arranged so that the plate surface is substantially orthogonal to the optical axis of the light, and in which a slit is formed, and a slit plate on the opposite side of the optical system with the slit plate in between. a two-part light-receiving sensor that is arranged in a constant positional relationship with the plate and is composed of a first light-receiving element and a second light-receiving element; and an exciter that vibrates the slit plate in a direction intersecting the longitudinal direction of the slit. an adder connected to the output side of the first light receiving element and the second light receiving element to calculate the sum of the signals output from the first light receiving element and the second light receiving element; The first light receiving element and the second light receiving element are connected to the output sides of the light receiving element and the second light receiving element.
a subtracter that calculates the difference between the signals output from the light-receiving elements; a peak detector connected to the output side of the adder that detects and outputs the peak value of the sum signal output from the adder; a focus detection section comprising a peak-to-peak detector that detects and outputs a difference between a maximum peak value and a minimum peak value of a difference signal output from the detector; (c) A positioning unit that drives the slit plate forward and backward relative to the optical system. (d) Input the first detection signal output from the peak detection section and the second detection signal output from the peak-to-peak detector, output the control signal to the positioning section, and an arithmetic control unit that positions the slit plate and the optical system at a position where the detection signal is maximum and the second detection signal is minimum; 2. Claim 1, characterized in that the slit plate has a plurality of slits arranged in parallel.
Focus adjustment device as described in section.