Coherent light generators – Particular resonant cavity – Mirror support or alignment structure
Reexamination Certificate
2000-02-03
2002-04-23
Scott, Jr., Leon (Department: 2881)
Coherent light generators
Particular resonant cavity
Mirror support or alignment structure
C372S024000, C372S029011
Reexamination Certificate
active
06377601
ABSTRACT:
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°
Jr. Leon Scott
NEC Corporation
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