JPH0660404A - Method and device for controlling optical axis of laser light - Google Patents

Abstract

PURPOSE:To improve the detection sensitivity of a displacement signal for the optical axis of a laser near a quadripartite center and to improve control accuracy. CONSTITUTION:By executing the analog calculation of a signal from a photodetector(quadripartite photodiode) 5 by an optical axis detecting circuit 6, a displacement signal (Vx, Vz) in the two axial directions is obtained. The displacement signal (Vx, Vz) passes through control circuits 7a, 7b and works so as to drive linear actuators 8a, 8b. Since a circular insensitive zone is provided on the central part of the relevant quadripartite photodiode, the sensitivity of displacement detection in the vicinity of the center is improved.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a laser optical axis control method and control apparatus, and more particularly to a laser optical axis stabilization method and stabilization apparatus in an optical disc master exposure apparatus using an Ar laser. For example, it is applied to stabilization of the optical axis in a laser processing device.

[0002]

2. Description of the Related Art FIG. 5 is a circuit diagram for obtaining a displacement signal of a conventional laser optical axis. The photoelectric conversion signals from the four-divided elements, which are the photodetectors, are subjected to analog calculation. As a result, V _Z = A + B- (C + D) in the Z direction and V _X = A + D- in the X direction.
(B + C). Although the photoelectric conversion signal is generally shown only for the element A in FIG. 6, it can be obtained by converting the photocurrent generated in the element A into a voltage by an operational amplifier. Therefore,
When increasing the detection sensitivity of V _Z and V _X , the current / voltage conversion resistance (R) is set so that the photoelectric conversion signal becomes maximum at the position where the displacement of the optical axis is maximum within the range where each operational amplifier is not saturated. It is determined. However, in the case of the four-division element as shown in FIG. 5, it is difficult to increase the current / voltage conversion resistance and increase the detection sensitivity. In addition, as previously proposed, the output laser light from the corner cube is 4
X-axis / Z
There is a servo circuit that obtains an axial variation signal and drives a corner cube in the X and Z axis directions.

[0003]

[Object] The present invention has been made in view of the above circumstances, and provides a laser optical axis control method and a control device for increasing the detection sensitivity of a laser optical axis displacement signal near the center of four divisions and improving the control accuracy. It was made for the purpose of providing.

[0004]

In order to achieve the above object, the present invention provides (1)
A part of the laser emission light that has passed through the corner cube is received by a photodetector (four-division photodiode), and the signal from the photodetector is fed back as a displacement signal in two axial directions by analog calculation. In the laser optical axis control method for driving a laser to stabilize the laser emission optical axis, the photodetector (four-division photodiode)
A circular dead zone is provided in the center of the
(2) A corner cube that is moved in two axial directions, a photodetector that receives a part of laser emission light that has passed through the corner cube, an arithmetic device that performs an analog operation on a signal from the photodetector, and the operation An actuator for driving the corner cube by feeding back displacement signals in two axial directions based on the calculation result by the device, and providing a circular dead zone in the central portion of the photodetector,
(3) In (2) above, the radius of the dead zone of the photodetector is set to approximately ½ of the laser beam diameter, and further,
(4) In (2) above, the radius of the dead zone of the photodetector can be changed to correspond to various laser beam diameters.
It is characterized by being insane. Hereinafter, description will be given based on examples of the present invention.

FIG. 1 is a block diagram for explaining an embodiment of a laser optical axis control device according to the present invention, in which 1 is A.
r lasers, 2 and 4 are deflection beam splitters (PB
S), 3 is a corner cube, 5 is a photodetector (4PD;
4-division photodiode), 6 is an optical axis detection circuit, 7a,
Reference numeral 7b is a control circuit (X axis, Z axis), and 8a, 8b are linear actuators (X axis, Z axis).

Even if the incident laser beam changes its optical axis, the corner cube 3 can be moved along the X and Z axes to suppress the fluctuation and stabilize the laser emission optical axis. The signals from the photodetector (4PD; 4-division photodiode) 5 are analog-calculated by the optical axis detection circuit 6 to obtain displacement signals (V _X , V _Z ) in the two axis directions. Displacement signal V
_X and V _Z are control circuits 7a and 7b (generally a phase compensation circuit,
It operates to drive the linear actuators (X axis, Z axis) 8a, 8b through an amplifier circuit. The linear actuator (X axis) 8a is for moving in the horizontal direction, and the linear actuator (Z axis) 8b is for moving in the vertical direction.

FIG. 3 is a diagram showing the structure of a photodetector, wherein FIG. 3A is a conventional photodetector of type (a) for comparison with the photodetector of the present invention, FIG. 3B and FIG. (C) and (d) are photodetectors of type (b), type (c), and type (d) having a dead zone at the center. First, FIG.
A method of obtaining the optical axis displacement signal by analog calculation using the conventional photodetector shown in (a) will be described. The relationship between the optical axis displacement and the displacement signal (Z-axis direction) when using this element is as shown in FIG. The displacement detection sensitivity near the center of the 4-division photodiode is S (mV / u
In the case of m), the displacement detection sensitivity must be improved when high control accuracy is targeted.

The photodetector shown in FIG. 3 (b) has an element shape aiming at an improvement in sensitivity, and is provided with a circular dead zone which is 1/2 of the laser beam diameter at the center of the four-divided element. Therefore, if the center of control is set to the center of the four-divided element, the photoelectric conversion signal of each element here becomes small, so that the detection sensitivity is improved by increasing the current / voltage conversion resistance and extracting the displacement signal largely. Further, since the dead zone diameter is w / 2 (w is the beam diameter), the detection sensitivity is improved as compared with the case of setting it to another size. The diameter of the dead zone is set to 1/2 of the beam diameter because the beam intensity distribution shows the maximum intensity gradient at w / 2. In FIG. 4, w is the beam diameter, 1 / e ² of Imax,
At w / 2, the intensity is Imax / √e, which is the maximum slope.
As shown in FIG. 2B, the displacement detection characteristic has a steep inclination.

FIG. 3 (c) can deal with a case where the diameter is larger than the laser beam diameter used in FIG. 3 (b). In this case, V _Z = A + B- (C + D), V _X = A + D- (B +
C). When the beam diameter is small (the same as in FIG. 3B), V _Z = A + B + E + F− (C + D + G +
_{H), V X = A +} D + E + H- use as (B + C + G). Further, if the number of divisions is further increased, various beam diameters can be supported. An example thereof is shown in FIG.

[0010]

As is apparent from the above description, the present invention has the following effects. (1) Since the circular dead zone is provided in the central portion of the photodetection element which is the center of control, the displacement detection sensitivity in the vicinity of the center is improved and high precision control becomes possible. (2) Since the radius of the circular dead zone is set to 1/2 of the beam diameter, the maximum detection sensitivity with respect to the beam displacement can be obtained. (3) Since the number of divided elements is increased and the radius of the circular dead zone can be changed, it is possible to cope with various beam diameters.

[Brief description of drawings]

FIG. 1 is a configuration diagram for explaining an embodiment of a laser optical axis control device according to the present invention.

FIG. 2 is a diagram showing a relationship between a displacement signal and an optical axis displacement of a photodetector.

FIG. 3 is a diagram showing a configuration of a photodetector.

FIG. 4 is a diagram showing a beam intensity distribution.

FIG. 5 is a circuit diagram for obtaining a displacement signal of a conventional laser optical axis.

FIG. 6 is a diagram showing a photoelectric conversion circuit.

[Explanation of symbols]

1 ... Ar laser, 2, 4 ... Deflection beam splitter (P
BS), 3 ... Corner cube, 5 ... Photodetector (4P
D: 4-division photodiode), 6 ... Optical axis detection circuit, 7
a, 7b ... Control circuit (X axis, Z axis), 8a, 8b ... Linear actuator (X axis, Z axis).

Claims (4)

[Claims] 1. A part of laser emitted light which has passed through a corner cube is received by a photodetector, and a signal from the photodetector is fed back as a displacement signal in two axial directions by analog calculation, so that the corner cube is moved. In the method of controlling the laser optical axis for driving and stabilizing the laser emission optical axis,
A method for controlling a laser optical axis, characterized in that a circular dead zone is provided in a central portion of the photodetector. 2. A corner cube that is moved in two axial directions, and one of laser emission light that has passed through the corner cube.
A photodetector that receives light from a light source, a computing device that performs analog computation of the signal from the photodetector, and an actuator that feeds back displacement signals in two axial directions based on the computation results of the computing device and drives the corner cube. Consists of
A laser optical axis control device, wherein a circular dead zone is provided in a central portion of the photodetector. 3. The laser optical axis control device according to claim 2, wherein a radius of a dead zone of the photodetector is set to be approximately ½ of a laser beam diameter. 4. The laser optical axis control device according to claim 2, wherein the radius of the dead zone of the photodetector is variable so as to correspond to various laser beam diameters.