Abstract:
An ion laser apparatus includes a laser tube, first and second mirrors, a mirror angle adjusting mechanism, and an alignment controller. The first and second mirrors are disposed to sandwich the laser tube. The mirror angle adjusting mechanism adjusts an angle of at least one of the mirrors while scanning the mirror within a predetermined angle width. The alignment controller determines a scan angle width of the mirror in accordance with a light intensity distribution of a laser beam such that a variation value of the laser beam output from the laser tube falls within a predetermined width. A mirror angle adjusting method for this ion laser apparatus is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to an ion laser apparatus and a mirror adjusting method therefor. 
     FIG. 5 shows the overall arrangement of a conventional ion laser apparatus. As shown in FIG. 5, an ion laser apparatus  101  is comprised of a laser oscillator  102 , alignment controller  103 , and power supply  127 . The laser oscillator  102  is comprised of a laser tube  104 , a support  105  for supporting the laser tube  104 , an output mirror  106 , a total-reflecting mirror  107 , and a mirror angle adjusting mechanism  108  for adjusting the mirror angle. The alignment controller  103  is comprised of an optical detector  118  for measuring the intensity of a monitor beam  117 , an A/D converter  121 , an arithmetic controller  122  for controlling stepping motors  120 , and motor drivers  123  for driving the stepping motors  120 . 
     The operation of the ion laser apparatus having the above arrangement will be briefly described. A laser beam  115  emitted by the laser tube  104  is reflected by the total-reflecting mirror  107  and transmitted through the output mirror  106 . Part of the transmitted laser beam  115  is guided by a beam splitter  116  to the optical detector  118  as the monitor beam  117 , while the remaining transmitted laser beam  115  emerges to the outside as the laser beam  115 . The optical detector  118  detects the optical intensity of the monitor beam  117 . A signal obtained by the optical detector  118  is A/D-converted by the A/D converter  121 , is processed by the arithmetic controller  122 , and drives the stepping motors  120  through the motor drivers  123  in order to correct the tilt of the total-reflecting mirror  107  or output mirror  106 . 
     The detailed structure and operation of the mirror angle adjusting mechanism  108  will be described. 
     FIG. 6 shows the mirror angle adjusting mechanism  108  in enlargement. 
     FIG. 7 shows a stationary plate seen from a side where the stepping motors  120  are disposed. The support  105  where the laser tube  104  is fixed is constituted by an invar rod  109  which is a metal having a low coefficient of thermal expansion, to suppress thermal expansion in the direction of optical path of the laser beam  115 . Stationary plates  110  and movable plates  111  are arranged on two ends of the support  105  with tension springs  112  and adjustment screws  114   a  to  114   c,  to be parallel to each other. The output mirror  106  and total-reflecting mirror  107  are fixed to the movable plates  111 , respectively, through mirror holders  113 . 
     The gap between each stationary plate and the corresponding movable plate is determined by the projecting lengths of the adjustment screws  114   a  to  114   c  from the movable plate. The lengths of the projecting portions of the three adjustment screws  114   a  to  114   c  and the positions of the three adjustment screws  114   a  to  114   c  which are determined by design determine the tilt of the mirror. When the adjustment screw  114   b  is rotated about, of the adjustment screws  114   a  to  114   c  arranged to form a shape L, the adjustment screw  114   a  located at the pivotal point as the fulcrum, the tilt of the mirror can be changed in the vertical direction. Similarly, when the adjustment screw  114   c  is rotated, the tilt of the mirror can be changed in the horizontal direction. The tilt of the mirror is adjusted to an arbitrary value by the two adjustment screws  114   b  and  114   c  while measuring a laser output, thereby adjusting the laser output to the maximum value. The adjustment screws  114  are driven in the following manner. The arithmetic controller  122  performs arithmetic operation based on an output signal of the monitor beam  117 . The obtained operation signal indicating the rotational direction and angle of each motor shaft is sent from the arithmetic controller  122  to the stepping motors  120   b  and  120   c  through the motor drivers  123  as the number of pulses necessary for the stepping motors  120   b  and  120   c.  As a result, the stepping motors  120   b  and  120   c  are rotated, thereby driving the adjustment screws  114 . 
     The procedure of adjusting the mirror angle of the ion laser apparatus having the above arrangement will be described. 
     FIG. 8 shows a conventional mirror angle adjusting procedure. In the following description, for the sake of descriptive convenience, left and right sides are those obtained when viewed from the reflection side to the exit direction. The stepping motors  120   b  and  120   c,  and the adjustment screws  114   b  and  114   c  may form reduction gear structures by means of gears. For easy description, a case wherein the reduction ratio is 1:1 will be described. Regarding the tilt angle of the mirror with respect to the rotational angle of the adjustment screw, for the sake of easy description, note that a rotational angle of 1° of the adjustment screw corresponds to a change of 0.01° of the tilt angle of the mirror. 
     In step S 200 , the ion laser apparatus  101  is started by a constant-current operation obtained by controlling a discharge current to a constant value, thereby performing laser oscillation. In step S 210 , of the laser beam, a monitor beam reflected by the beam splitter  116  is detected. In step S 220 , the detected data is A/D-converted. 
     The flow enters the coarse adjustment operation mode (S 230 ), which is the first step of automatic mirror adjustment. First, in step S 231 , coarse adjustment in the vertical direction is performed. In step S 231 , the vertical-direction adjustment screw  114   b  is rotated, and its output change data is acquired. For example, the vertical-direction stepping motor  120   b  is rotated counterclockwise through a ½ turn to rotate the adjustment screw  114   b  counterclockwise through a ½ turn. From this position, the stepping motor  120   b  is rotated clockwise through a ½ turn, while measuring the output data of the laser beam in units of specified angles (the angle is specified by variably changing the pulse count). In this conventional example, an angle of 3° is defined as one step (unit). When  60  data corresponding to a ½ turn are measured, the stepping motor  120   b  returns to the initial position. After that, the stepping motor  120   b  is further rotated clockwise through a ½ turn while measuring the output data. Hence, measurement of data on the ½ turn from the initial position in each of the clockwise and counterclockwise directions or a total of 1 turn, i.e.,  120  output data, is completed. This corresponds to 3.6° in mirror angle. FIG. 9 shows a measurement example of the output data. 
     FIG. 9 shows the characteristics of alignment sensitivity indicating the scan angle width and an output variation width. Generally, the change characteristics of the laser output with respect to a change in mirror angle are called alignment sensitivity characteristics. In FIG. 9, the initial position at the start of a laser is defined as the reference position, and the center of the axis of abscissa is defined as 0. If the width of these characteristics is large, the laser oscillator is not sensitive to a change in mirror angle; inversely, if it is small and forms a sharp shape, the laser oscillator is sensitive to a change in mirror angle. In the case of FIG. 9, the maximum value is located at 90° of the counterclockwise rotation of the motor shaft. This is due to the following reason. Since this state is immediately after the laser is started, the temperature in the oscillator has not reached a stable state, so that the maximum value is offset from the optimum angle of the mirror. 
     The adjustment screws are adjusted on the basis of the measured data. More specifically, the stepping motor  120   b  is so rotated as to return to the maximum angle of the measured data, and is stopped. In this example, the stepping motor  120   b  is rotated counterclockwise through 90°, and then stopped. Coarse adjustment in the vertical direction is thus completed. In this state, a position reached after rotation through 90° from the initial position serves as the reference position in the next step. Therefore, data is shifted such that the position of 90° in FIG. 9 comes to 0° at the center (see FIG.  10 ). Subsequently, in step S 232 , coarse adjustment in the horizontal direction is performed in the same manner as in step S 231 . More specifically, the horizontal-direction stepping motor  120   c  is rotated, and data is measured. The stepping motor  120   c  is rotated, on the basis of the measurement data, through an optimum mirror angle with which the maximum output can be obtained. Coarse adjustment in the horizontal direction is thus performed. 
     The coarse adjustment mode is thus completed. Consecutively, the fine adjustment operation mode in step S 240  is performed in order to maintain the optimum mirror angle so as to cope with a temperature rise in the oscillator or a change in the ambient temperature that occurs after the coarse adjustment mode. In both the vertical and horizontal directions, a position in FIG. 10 where the maximum output can be obtained is assumed as the reference  0 . 
     In the same manner as the coarse adjustment mode, the fine adjustment mode is performed by rotating the motor clockwise and counterclockwise from this position to a position where the maximum output within the rotation width can be obtained. If the rotational angle is large, the output varies largely. Thus, the rotational angle in the fine adjustment mode is smaller than that in the coarse adjustment mode. The fine adjustment mode is different from the coarse adjustment mode in this respect. 
     In the coarse adjustment mode, as shown by the alignment sensitivity characteristics of FIG. 9, the motor is rotated clockwise and counterclockwise through an angle equal to or more than the width of the laser oscillation angle (with a rotational angle of about 230° and a mirror angle of 2.3°). In this example, the motor is rotated clockwise and counterclockwise with a rotational angle of 360° and a mirror angle of 3.6°. In the fine adjustment mode, the motor is rotated in a trial-and-error manner in the vicinity of the peak of the alignment sensitivity characteristics to find the maximum point. 
     Conventionally, a scan angle width θ is set in advance, and the stepping motor is rotated within this range. 
     To perform fine adjustment in the vertical direction (S 241 ), the vertical-direction stepping motor  120   b  and adjustment screw  114   b  are rotated counterclockwise through θ, and are rotated clockwise through θ while measuring the output data of the laser beam in units of specified angles (specified by variably changing the pulse count). In this example, θ data corresponding to θ° are measured with reference to 1° as one step (unit), and the motor  120   b  is consecutively rotated clockwise through θ° while measuring the output data. Hence, measurement of 2θ pieces of output data corresponding to θ° in each of the clockwise and counterclockwise directions, i.e., a total of 2θ°, is completed. 
     Subsequently, the motor is rotated from the initial position through an angle corresponding to the maximum value of the measurement data of θ° in both the clockwise and counterclockwise directions, and is stopped, to adjust the adjustment screw. Thus, one cycle of vertical-direction fine adjustment is ended. When the adjustment screw is moved in this manner, the laser output varies accordingly. The magnitude of variations changes within an output variation width δ 1 , as indicated by the output characteristics with respect to the mirror angle shown in FIG.  11 . Then, horizontal-direction fine adjustment in step S 242  is performed by moving the horizontal-direction stepping motor  120   c  in the same manner as in step S 241 . The horizontal-direction motor  120   c  is rotated through an angle corresponding to the maximum value of the measurement data, to adjust the corresponding adjustment screw. Thus, one cycle of horizontal-direction fine adjustment is ended. 
     If the respective elements of the laser oscillator  102  do not vary, further adjustment is not needed. In practice, however, since an alignment error occurs accompanying a temperature change, the fine adjustment operation in step S 240  must be repeated until the laser apparatus  101  is stopped. With the above operation, an error in mirror angle caused by the temperature change is constantly corrected in order to set an optimum alignment state necessary for obtaining the maximum output at that point. The maximum output value accompanies an output variation width δ 1 . 
     The automatic mirror adjusting mechanism in the conventional ion laser apparatus has the following problems. The ion laser apparatus  101  is a laser unit that excites ions by performing discharge of several 10 amperes (to be described as A hereinafter) in a small hole called a thin pipe  124 , in the laser tube  104 , which has an inner diameter of several millimeters (to be described as mm hereinafter) and a length of several 100 mm, thereby producing laser oscillation. A discharge plasma of several 10 A sputters the inner surface of the thin pipe  124  to denature its material, accordingly changing the shape of the inner surface. As a material that can endure sputtering, beryllium oxide having excellent sputtering resistance is generally employed. Under the discharge current condition of as large as 50 A, however, the thin pipe  124  made of beryllium oxide is naturally denatured and deforms by sputtering. Also, a Brewster window  125  of the laser tube  104  is degraded by the ultraviolet rays emitted by the plasma discharge, and its transmission characteristics are accordingly decreased. 
     It is known that degradation of the component material over time as described above changes the alignment sensitivity to have a sharp peak. When the alignment sensitivity becomes sharp due to this change in the laser tube  104 , the conventional automatic mirror adjusting mechanism performs mirror angle adjustment while maintaining the constant scan angle width of θ° which is set initially. Since the scan angle width of θ° is excessively large, the output variation width increases from δ 1  to δ 2 , as shown in FIG.  12 . 
     Various types of automatic mirror adjusting mechanisms have been proposed to solve these problems. For example, according to Japanese Patent Laid-Open No. 5-37050, to accurately obtain the output value and to perform correct determination, the number of times of the sampling operation is increased. A plurality of data corresponding in number to the sampling operation times are averaged to determine the magnitude of the output value. In this invention, however, the number of times of the sampling operation is increased to merely improve the reliability of the data, and the alignment sensitivity itself stays sharp. Therefore, the problem of sharp alignment sensitivity described above cannot be solved. 
     Japanese Patent Laid-Open No. 9-153654 discloses an invention that enables adjustment with a very small angle by using an electrostrictive element. According to this technique, the very small displacement amount of the angle of the laser mirror is adjusted by adjusting the pulse width of the voltage pulse in advance. A very small displacement amount of the angle of the mirror is synonymous with the preset scan angle width θ. The adjustment range does not change in accordance with the alignment sensitivity that changes over time. Therefore, the problem described above cannot be solved. 
     According to the technique disclosed in Japanese Patent Laid-Open No. 5-21885, in the operation of light feedback mode or light mode generally referred to in the ion laser apparatus, a discharge current is used when performing detection and control. Referring to the graph of the operation characteristics shown in this reference, when the transverse mode is a single mode, the flat portion of the peak forms a sharp hill, while the discharge current forms a deep bottom. As a result, with the control operation of this technique, a small change in mirror angle causes a sharp increase/decrease in the discharge current, and discharge is sometimes discontinued, which is a problem. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above problems, and has as its main object to provide an ion laser apparatus and a mirror angle adjusting method therefor, with which the output variation width of a laser beam can be set within a predetermined range even when the characteristics of a constituent component such as a laser tube change over time. 
     In order to achieve the above object, according to the present invention, there is provided an ion laser apparatus comprising a laser tube, first and second mirrors disposed to sandwich the laser tube, a mirror angle adjusting mechanism for adjusting an angle of at least one of the mirrors while scanning the mirror within a predetermined angle width, and an alignment controller for determining a scan angle width of the mirror in accordance with a light intensity distribution of a laser beam such that a variation value of the laser beam output from the laser tube falls within a predetermined width. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the overall arrangement of an ion laser apparatus according to the present invention; 
     FIG. 2A is an enlarged view showing the structure of the mirror angle adjusting mechanism (when a stepping motor is used) of the ion laser apparatus; 
     FIG. 2B is a front view showing a stationary plate; 
     FIG. 2 is an enlarged view showing the structure of the mirror angle adjusting mechanism (when an electrostrictive element is used) of the ion laser apparatus; 
     FIG. 3 is a flow chart showing a mirror angle adjusting procedure according to an embodiment of the present invention; 
     FIG. 4 is a graph showing the alignment sensitivity characteristics obtained when vertical-direction coarse adjustment is completed; 
     FIG. 5 is a diagram showing the overall arrangement of a conventional ion laser apparatus; 
     FIG. 6 is an enlarged view showing the structure of the mirror angle adjusting mechanism of the conventional ion laser apparatus; 
     FIG. 7 is a front view showing a stationary plate; 
     FIG. 8 is a flow chart showing a conventional mirror angle adjusting procedure; 
     FIG. 9 is a graph of alignment sensitivity characteristics showing a scan angle width and an output variation width; 
     FIG. 10 is a graph of alignment sensitivity characteristics obtained after measurement of vertical-direction coarse adjustment is performed; 
     FIG. 11 is a graph of alignment sensitivity characteristics showing a conventional scan angle width and output variation width; and 
     FIG. 12 is a graph of alignment sensitivity characteristics showing the conventional scan angle width and output variation width. 
     FIG. 13 is a graph of alignment sensitivity characteristics showing the conventional scan angle width and output variation width. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     An ion laser apparatus according to one preferred embodiment of the present invention is as follows. This ion laser apparatus has a laser tube ( 4  in FIG.  1 ), a pair of output mirror ( 6  in FIG. 1) and total-reflecting mirror ( 7  in FIG.  1 ), an optical detector ( 18  in FIG.  1 ), screw assemblies ( 114   a,    114   b,  and  114   c  in FIG.  2 B), stepping motors ( 20   b  and  20   c  in FIG.  1 ), and an arithmetic controller ( 22  in FIG.  1 ). The pair of output mirror  6  and total-reflecting mirror  7  are arranged through the laser tube  4  to form an optical resonator. The optical detector  18  detects a laser output. The screw assemblies  14   a,    14   b,  and  14   c  tilt the mirrors in two different directions. The stepping motors  20   b  and  20   c  rotate the screws of the screw assemblies. The arithmetic controller  22  controls the rotational angles of the stepping motors  20   b  and  20   c.  In this ion laser apparatus, a scan angle width calculation unit ( 26  in FIG. 1) is provided to detect the characteristics of the laser output with respect to the tilt angle of the mirror and to repeatedly set the scan angle width of the mirror in accordance with the detected characteristics. 
     FIG. 1 shows the overall arrangement of the ion laser apparatus. As shown in FIG. 1, an ion laser apparatus  1  is comprised of a laser oscillator  2 , alignment controller  3 , and power supply  27 . The laser oscillator  2  is comprised of a laser tube  4 , a support  5  for supporting the laser tube  4 , an output mirror  6 , a total-reflecting mirror  7 , and a mirror angle adjusting mechanism  8  for adjusting the mirror angles of the output mirror  6  and total-reflecting mirror  7 . The alignment controller  3  is comprised of an optical detector  18 , an A/D converter  21 , a scan angle width calculation unit  26  as the characteristic feature of this embodiment, an arithmetic controller  22 , and motor drivers  23 . The optical detector  18  measures the intensity of a monitor beam  17 . The A/D converter  21  performs A/D conversion. The scan angle width calculation unit  26  calculates and updates the can angle width. The arithmetic controller  22  controls the stepping motors  20   b  and  20   c  in accordance with the intensity of the monitor beam  17 . The motor drivers  23  drive the stepping motors  20   b  and  20   c.    
     The operation of the ion laser apparatus having the above arrangement will be briefly described. A laser beam  15  emitted by the laser tube  4  is reflected by the total-reflecting mirror  7  and transmitted through the output mirror  6 . Part of the transmitted laser beam  15  is guided by a beam splitter  16  to the optical detector  18  as the monitor beam  17 , while the remaining transmitted laser beam  15  is emitted to the outside as the laser beam  15 . The optical detector  18  comprised of a solar cell, photodiode, and load circuit detects the optical intensity of the monitor beam  17 . A signal obtained by the optical detector  18  is A/D-converted by the A/D converter  21 . The scan angle width calculation unit  26  calculates and updates the scan angle width on the basis of the A/D-converted data. The scan angle width is then processed by the arithmetic controller  22 , and drives the stepping motors  20  through the motor drivers  23  in order to correct the tilt of the total-reflecting mirror  7  or output mirror  6 . 
     FIG. 2A shows the mirror angle adjusting mechanism  8  of FIG. 1 in detail. 
     Since FIGS. 6 and 7 show the parts of the mirror adjusting mechanism  8  of FIG. 1, they will be used wherever possible in the following description of the mirror adjusting mechanism  8 . 
     The basic arrangement of the mirror angle adjusting mechanism  8  is common between the output mirror  6  side and the total-reflecting mirror  7  side, and a description will thus be made on the total-reflecting mirror  7  side. The support  105  where the laser tube  104  is fixed is constituted by an invar rod  109  which is a metal having a low coefficient of thermal expansion, to suppress thermal expansion in the direction of optical path of the laser beam  15 . Stationary plates  110  are fixed to two ends of the invar rod  109 , and movable plates  111  are arranged outside the stationary plates  110 . The stationary plates  110  and movable plates  111  are arranged parallel to each other, and are connected to each other through tension springs  112 . The output mirror  6  and total-reflecting mirror  107  are fixed to the movable plates  111 , respectively, through mirror holders  113 . 
     In each movable plate, screw holes are formed at three positions which form a shape L when viewed from the total-reflecting mirror  107  side (when seeing the right side from the left side in FIG. 7) to the exit direction of the laser beam  15 , to be parallel to the optical axis. The adjustment screws  114   a  to  114   c  are screwed into the three screw holes respectively from the outside to extend through them. The distal ends of the three adjustment screws  114   a  to  114   c  project from the movable plate  111  toward the stationary plate  110  for a length equal to or more than the lengths of the tension springs  112 , and the springs  112  serve as the tension springs. The gap between each stationary plate and the corresponding movable plate is determined by the projecting lengths of the adjustment screws  114   a  to  114   c  from the movable plate. The lengths of the projecting portions of the three adjustment screws  114   a  to  114   c  and the positions of the three adjustment screws  114   a  to  114   c  which are determined by design determine the tilt of the mirror. 
     After the gap between the stationary plate  110  and the corresponding movable plate  111  is determined, of the adjustment screws  114   a  to  114   c  arranged to form a shape L, the adjustment screw  114   a  located at the pivotal point is fixed as it need not be rotated any longer. The adjustment screw  114   a  serves as the fulcrum for the two other adjustment screws  114   b  and  114   c  located at the two distal ends of the shape L. When the adjustment screws  114   a  to  114   c  are rotated clockwise, the projecting lengths of their distal ends increase, so that the gap between the stationary plate  110  and movable plate  111  increases. Inversely, when the adjustment screws  114   a  to  114   c  are rotated counterclockwise, the projecting lengths of their distal ends decrease, so that the gap between the stationary plate  110  and movable plate  111  decreases. 
     When the adjustment screw  114   b  at the upper end of the shape L is rotated, the gap between the stationary plate  110  and movable plate  111  at the upper portion changes about the adjustment screw  114   a  as the fulcrum, so the tilt of the mirror can be changed in the vertical direction. Similarly, when the adjustment screw  114   c  at the lower end of the shape L is rotated, the gap between the stationary plate  110  and movable plate  111  changes about the adjustment screw  114   a  as the fulcrum, so the tilt of the mirror can be changed in the horizontal direction. The tilt of the mirror can be adjusted to an arbitrary value by the two adjustment screws  114   b  and  114   c.  Usually, mirror angle adjustment is performed by rotating the adjustment screws  114   b  and  114   c  while measuring the laser output, so that the laser output is adjusted to the maximum value. 
     The adjustment screws  114   b  and  114   c  are driven by the stepping motors  120   b  and  20   c.  The shaft of the stepping motor  120   b  is connected to the adjustment screw  114   b  on the outer side through a coupling component  119 . As the motor used in this case, a stepping motor advantageous for quick start, rotation, and stopping, and high-precision positioning is selected. Calculation is performed by the arithmetic controller  22  on the basis of the output signal of the monitor beam  17 . As a result, an operation signal indicating the rotational direction and angle of the shaft is output from the arithmetic controller  22  to the motor drivers  23 . The motor drivers  23  output necessary pulse counts to the stepping motors  120   b  and  20   c,  thereby rotating the stepping motors  120   b  and  20   c.  Within the output range of the laser to be used, this electrical signal must keep linearity with respect to the output value of the laser beam. 
     FIG. 2 shows the structure of the mirror angle adjusting mechanism of FIG. 1 in detail using element number identical to FIG. 6 wherever possible, and indicates a case wherein electrostrictive elements are used in place of the stepping motors. As shown in FIG. 3, the same effect as that described above can be obtained if electrostrictive elements such as piezoelectric elements are used in place of the stepping motors. A composite structure in which coarse adjustment is performed by stepping motors and fine adjustment is performed by piezoelectric elements is also possible. Although not shown, the adjustment screw  114   a  located at the pivotal point may also be connected to a stepping motor or an electrostrictive element. 
     FIG. 3 explains a mirror angle adjusting method of this embodiment. In the following description, for the sake of descriptive convenience, left and right sides correspond to those obtained when viewed from the reflection side to the exit direction. When performing positioning, the motor is always rotated from a predetermined rotational direction and is stopped so that a backlash is obtained. This motion does not directly concern the present invention, and a description on the backlash operation will accordingly be omitted. The stepping motors  20   b  and  20   c,  and the adjustment screws  114   b  and  114   c  may form reduction gear structures by means of gears. In this embodiment, however, a case wherein the reduction ratio is 1:1 will be described, in the same manner as in the conventional example. The tilt angle of the mirror with respect to the rotational angle of the adjustment screw is determined by the screw pitches of the adjustment screws  114   b  and  114   c  and the distance between the adjustment screws  114   b  and  114   a,  and that between the adjustment screws  114   c  and  114   a.  Note that a rotational angle of 1° of the adjustment screw corresponds to a change of 0.01° of the tilt angle of the mirror, in the same manner as in the conventional example. 
     The ion laser apparatus  1  is started by a constant-current operation obtained by controlling a discharge current to a constant value (S 100 ), and after that, of the laser beam  15 , the monitor beam  17  reflected by the beam splitter  16  is detected by the optical detector  18  (S 110 ). The detected data is A/D-converted (S 120 ), and the flow enters the coarse adjustment operation mode, which is the first step of automatic mirror adjustment. In the coarse adjustment operation mode (S 130 ), first, coarse adjustment in the vertical direction is performed (S 131 ). The vertical-direction adjustment screw  114   b  is rotated, and its output data is acquired. For example, the vertical-direction stepping motor  20   b  is rotated counterclockwise through a ½ turn to rotate the adjustment screw  114   b  counterclockwise through a ½ turn. From this position, the stepping motor  20   b  is rotated clockwise through a ½ turn, while measuring the output data of the laser beam in units of specified angles (the angle is specified by variably changing the pulse count). In this embodiment, an angle of 3° is defined as one step (unit). When  60  data corresponding to a ½ turn are measured, the stepping motor  20   b  returns to the initial position. With the above steps, measurement of the output data corresponding to counterclockwise rotation through a ½ turn from the initial position is completed. The stepping motor  20   b  is further rotated clockwise through a ½ turn while measuring the output data. Hence, measurement of data on the ½ turn from the initial position in each of the clockwise and counterclockwise directions or a total of 1 turn, i.e.,  120  output data, is completed. This corresponds to 3.6° in mirror angle. A measurement example of this output data is the same as that shown in FIG.  9 . 
     The adjustment screws are then adjusted. More specifically, when output measurement of the ½ turn from the initial position in each of the clockwise and counterclockwise directions is completed, the stepping motor  20   b  is so rotated as to return to the maximum angle within the measured data, and is stopped. In this embodiment, the stepping motor  20   b  is rotated counterclockwise through 90°, and then stopped. Coarse adjustment in the vertical direction is thus completed. In this state, a position reached after rotation through 90° from the initial position serves as the reference position in the next step. Therefore, data is shifted as shown in FIG. 10, so that the peak falls at the center. Subsequently, coarse adjustment in the horizontal direction (S 132 ) is performed by rotating the horizontal-direction stepping motor  20   c,  in the same manner as in step S 131 . With this measurement, when the mirror angle has an error in the horizontal direction, the peak position can be confirmed in the same manner as in the vertical direction. The stepping motor  20   c  is rotated, on the basis of the measured data, through an optimum mirror angle with which the maximum output can be obtained, thereby adjusting the adjustment screws. 
     The coarse adjustment mode is thus completed. The alignment sensitivity characteristics change from time to time to cope with a temperature rise in the oscillator or a change in the ambient temperature that occurs after the coarse adjustment mode. 
     FIG. 4 shows the alignment sensitivity characteristics in two types of operation times (initial state and after a change over time). The outer curve indicates an example of alignment sensitivity obtained when the operation time is on the order of several hours (initial state), and the inner curve indicates an example of alignment sensitivity obtained when the operation time is on the order of several thousand hours (after a change over time). The latter curve has a sharper shape. 
     In the conventional case, mirror fine adjustment is performed with the predetermined scan angle width θ without considering the alignment sensitivity characteristics that change from time to time. In contrast to this, according to the characteristic feature of this embodiment, before entering the fine adjustment mode, the scan angle width is calculated in accordance with the alignment sensitivity characteristics (S 140 ). This method will be described hereinafter. 
     In step S 141 , the width of the alignment sensitivity characteristics is calculated. More specifically, the width of the spread of the curve is calculated from data on the alignment sensitivity obtained by coarse adjustment performed beforehand. In this embodiment, the full-width at half maximum of these characteristics will be calculated as a value representing the width of the span. This value is not limited to the full-width at half maximum, but no problem arises if a value such as {fraction (1/10)} the maximum value is used. For the later calculation, as the full-width at half maximum, σ 0  is set as a fixed value from the initial characteristics. Subsequently, as shown in FIG. 5, the maximum output obtained in the coarse adjustment process is defined as P 1 , and the rotational angle of the adjustment screw with which P 1  is obtained is defined as 0°. Since the full-width at half maximum is the width obtained between points corresponding to ½ the maximum value, a value P h  which is half the maximum output is calculated as: 
     
       
           P   h   =P   1 /2  (1) 
       
     
     Then, while comparing the magnitudes of the respective output data on the alignment sensitivity and P h , points (the angles of the adjustment screws) before and after a point where the magnitudes are inverted are obtained. Concerning the two points before and after the point where the magnitudes are inverted, assume that between these two points, the value changes linearly, and an angle corresponding to P h  is calculated from a ratio. To calculate the angle at the point P h , a simple method may be employed, for example, a larger one of angles after magnitude inversion may always be determined as the angle corresponding to P h . The width of the angle obtained in this case is defined as σ 0 , the full-width at half maximum of the alignment sensitivity with the operation time of several ten hours is defined as σ 1 , and the full-width at half maximum of the alignment sensitivity with the operation time of several thousand hours is defined as σ 2 . From the sharpness of the alignment sensitivity, σ 2 &lt;σ 1  is established. 
     In step S 142 , the scan angle width is calculated. assuming that the scan angle width representing the angle width (=the width of the angle through which the stepping motor is rotated) through which the mirror is oscillated in the fine adjustment stage is θ, the scan angle width determined by the initial characteristics is set as a fixed value θ 0 . Scan angle widths θ 1  and θ 2  of the respective operation times are calculated from the initial full-width at half maximum σ 0  and the full-widths at half maximum σ 1  and σ 2  of the respective operation times by proportional calculation: 
     
       
         scan angle width θ 1  after lapse of several ten hours: θ 1 =θ 0 ×σ 1 /σ 0   (2) 
       
     
     
       
         scan angle width θ 2  after lapse of several thousand hours: θ 2 =θ 0 ×σ 2 /σ 0   (3) 
       
     
     Calculation, reset, and resetting calculation of these scan angle widths (S 143 ) are performed automatically during coarse adjustment each time the operation is started. With the characteristics obtained after the lapse of several ten hours where the operation time does not substantially elapsed yet, the change amount in alignment sensitivity is small. Accordingly, the scan angle widths satisfy θ 1 ≈θ 0 . An output variation width δ 1  is small. 
     Since the scan angle width θ 2  calculated from the alignment characteristics obtained after the lapse of several thousand hours is set small to match the sharp alignment sensitivity, an output variation width δ 2  does not become larger than δ 1 , unlike in the conventional example. In this manner, an output variation width δ is not increased or decreased over time by the scan angle width θ updated upon each use. 
     After that, fine adjustment operation (S 150 ) identical to that in the conventional case is performed with θ which is to be updated. The fine adjustment mode is performed by rotating the motor clockwise and counterclockwise from this position until reaching a position where the maximum output within the width of this rotation is obtained, in the same manner as the coarse adjustment mode. If the rotational angle is large, the output varies largely. For this reason, the rotational angle is set smaller than that in the coarse adjustment. The fine adjustment mode is different from the coarse adjustment mode in this respect. Assume that updated θ is expressed as θ 2 . In fine adjustment in the vertical direction, first, the vertical-direction stepping motor  20   b  is rotated counterclockwise through θ 2 ° in order to rotate the adjustment screw  14   b  counterclockwise through θ 2 °. After that, the stepping motor  20   b  is rotated clockwise through θ 2 ° while measuring output data on the laser beam in units of specified angle (specified by variably changing the pulse count). In this embodiment, when θ data corresponding to θ 2 ° are measured with reference to 1° as one step (unit), the motor is returned to the initial position. 
     Hence, measurement of output data corresponding to θ 2 ° in the counterclockwise direction is completed. Furthermore, the motor is consecutively rotated clockwise through θ 2 ° while measuring the output data. Hence, measurement of 2θ output data corresponding to θ 2 ° in each of the clockwise and counterclockwise directions, i.e., a total of 2θ 2 °, is completed (S 131 ). 
     Subsequently, the motor is rotated from the initial position through an angle corresponding to the maximum value of the measurement data of θ 2 ° in both the clockwise and counterclockwise directions, and is stopped, to adjust the adjustment screw. Thus, one cycle of vertical-direction fine adjustment is ended. When the adjustment screw is moved in step S 151 , the laser output varies accordingly. The magnitude of variations changes within the output variation width δ 2 , as indicated by the output characteristics with respect to the mirror angle shown in FIG.  4 . Then, horizontal-direction fine adjustment in step S 152  is performed by moving the horizontal-direction stepping motor  20   c  in the same manner as in step S 151 . The horizontal-direction motor  20   c  is rotated through an angle corresponding to the maximum value of the measurement data, to adjust the corresponding adjustment screw. Thus, one cycle of horizontal-direction fine adjustment is ended. After this, fine adjustment in step S 150  is repeated while the ion laser apparatus operates. 
     In this manner, according to this embodiment, calculation and resetting of the scan angle width on the basis of the alignment sensitivity are always performed so that an optimum alignment state required for obtaining the maximum output at the time point in question is obtained. Therefore, an error in mirror angle caused by a temperature change can always be corrected, and the output of the laser beam can be maintained at substantially the maximum value with only the small output variation width δ 1 . 
     In this embodiment, the rotational angles and steps employ simple numbers to facilitate the description. Depending on the stepping motors, the reduction gears of the gears, and the components of the mechanism, the precision can be improved by defining 1 step on the order of 1/100°. In this embodiment, the mirror angle adjusting mechanism  8  is provided on the total-reflecting mirror  7  side, but it may be provided to the output mirror  6  side. Alternatively, mirror angle adjusting mechanisms  8  may be provided to the both sides to perform adjustment alternately. 
     As has been described above, according to the present invention, in automatic of mirror angle adjustment for an ion laser apparatus, variations in laser beam output over time can be suppressed. This is due to the following reason. With the arrangement of the present invention, each time coarse adjustment is performed, the scan angle width is automatically calculated and updated, and fine adjustment is performed by using the obtained value. An error in mirror angle caused by a temperature change can thus always be corrected, and the output of the laser beam can be maintained at substantially the maximum value.