Note: Descriptions are shown in the official language in which they were submitted.
1303709
LASER DEVICE
The present invention relates to a laser device for
stabilizing the wavelength of the laser beam.
In the appended drawings:
Figure 1 is a diagram showing a conventional laser
device;
lQ Figure 2 is a structural diagram showing an embodiment
of the laser device according to the present invention;
Figure 3 is a diagram showing an intensity distribution
of a fringe on an image pickup element 22;
Figure 4, is a diagram showing another embodiment of the
present invention; and
Figure 5 is a diagram showing a still another embodiment
of the present invention.
Figure 1 is a diagram showing a conventional wavelength
tuning type laser device disclosed, for instance, in "Applied
Optics", July 1974, vol. 13, No. 7, pages 1625-1628. In
Fiyure 1, a reference numeral 1 designates a laser
oscillator, e.g., a dye laser in this case, a numeral 2
designates a partial reflection mirror, a numeral 3
designates a gap type Fabry-Perot etalon (hereinbelow,
referred to as an FP), a numeral 4 designates a sealed
container filled with gas and containing therein the FP 3, a
numeral 5 designates a laser beam, a numeral 6 designates a
pressure gauge to measure a gas pressure in the sealed
container 4~ a symbol G designates a gas bomb, numerals 7 and
8 designate valves and a numeral 9 designates a grating.
~303709
The operation of the conventional laser device will be
described. A wavelength of a laser emitted from the laser
oscillator 1 is selected by means of various elements in the
oscillator. In an example in the conventional device, the
width of the oscillation wavelength is narrowed by inserting
spectroscopic elements such as the grating 9 and the FP into
a resonator. By adjusting the spectroscopic elements, the
wavelength can be determined to be a desired wavelength
within the width of the originally existing oscillation
wavelength. In the conventional laser device, the selection
of the wavelength is performed by changing an angle of
inclination of the grating 9, or by changing the refraction
index of gas between a gap in the FP 3, the refraction index
being changed by changing a pressure of gas in the sealed
container 4. The wavelength can be roughly adjusted by
changing the angle of inclination of the grating 9. On the
other hand, the wavelength can be finely adjusted by
adjusting the pressure of gas between a gap in the FP 3. The
adjustment of the gas pressure can be performed by measuring
a pressure of gas by means of the pressure gauge 6 and by
opening/closing operations the valves 7 and 8 as a result of
the measurement.
In the conventional laser device having the construction
described above, there was the problem that a pressure gauge
capable of indicating a value of smaller than 0.1% in the
full scale was needed in order to precisely tune the
wavelength, with an accuracy of, for instance +0.01 nm.
It is an object of the present invention to provide a
laser device capable of tuning accurately the wavelength and
of stabilizing the wavelength.
~3~)3709
The foregoing and other objects of the present invention
have been attained by providing a laser device which
comprises a Fabry-Perot etalon for selecting a wavelength of
laser oscillation, a wavelength monitoring means for
monitoring a laser beam emitted from an oscillation
wavelength changing type laser oscillator and means for
adjusting a pressure of gas in a gap in the Fabry-Perot
etalon by the output signal of the wavelength monitoring
means.
Preferred embodiments of the laser device according
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to the present invention will be described.
In Figure 2, reference numerals 1 through 4
designate the same elements as in Figure 1. However, the
wavelength tuning type Fabry-Perot etalon (FP) 3 is
received in the sealed container 4. A total reflection
mirror 10 is disposed opposite the partial reflection
mirror 2 with respect to the FP 3. A volume adj~stable
Carn~q~ n/cat~
means 11 which may be a bellows is comminuo~tcd with the
sealed container 4. A driving means 12 is connected to
the volume adjustahle means 11 so that it can expand and
contract the volume of the volume adjustable means 11.
A numeral 13 designates a laser beam generated fro~
an oscillation means, which comprises the laser
oscillator 1, the total reflection mirror 10, the partial
reflection mirror 2 and the FP 3. A mirror 14 is
disposed in a path for laser beam 13 to take out a part
of the laser beam 13.
A numeral 15 designates a wavelength monitoring
means which has a spectroscopic function for the laser
beam taken out through the beam separating mirror 140
The wavelength monitoring means 15 is constituted by an
interference filter 16 which permits only the laser beam
13 to pass therethrough, a light strength adjusting
filter 17, an integrator 18 for defusing the laser beam
13, a monitoring FP 19 having a gap, a sealed container
20 which sealingly receives the FP 19 and a lens 21.
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A numeral 22 designates an image pickup element for
observing fringes produced by the FP 19. The image
pickup element 19 may be a first-dimensional image
sensor. A numeral 23 designates a light shielding box
which receives the above-mentioned elements 16-22 and
shields light from the outside, and in which the
interference filter 16 is disposed to allow the laser
beam from the beam separating mirror 1~ to enter into the
box 23. A numeral 24 designates a temperature adjusting
means which keeps the temperature of the FP 19 to be
constant, and a numeral 25 designates a picture image
processing means whcih analyzes the fringes and outputs a
signal to the driving means 12.
The operation of the above-mentioned embodiment of
the present invention will be described.
The wavelength of the laser beam emitted from the
laser oscillator 1 is selected by the various elements in
the oscillator. In case of an excimer laser, the width
of the wavelength can be narrower than that of the
originally produced wavelength, i.e. a width of several
angstrom, by installing the spectroscopic elements such
as a prism, a grating, an FP and so fourth in the
resonator. Further, by adjusting the spectroscopic
elements, the wavelength can be determined to be a
desired wavelength within the width of the originally
exsisting wavelength.
A part of the thus obtained laser beam 13 is
130:~709
introduced in the wavelength monitoring means 15 which
utilizes the FP 19 to determine the wavelength.
In this embodiment, circular fringes which appear
when the light is transmitted through the FP 19 is
utilized. The diameters o~ the fringes are related to a
value ~ in the following equation (1), and a wavelength
~m can be obtained from the equation (1) by obtaining the
value ~. The FP is so constructed that two mirrors
having a high degree of flatness face with a gap, and the
wavelength at the center of the light which passes
through the mirrors at an angle of ~ gives a specified
wavelength which can be expressed by:
2nd cos ~
~m = . .. (1)
m
where n is a refraction index of the gap and m is an
integer.
The intensity of the light having ~m in the
distribution of the wavelength of the oscillated laser
beam is obtainable by using an FP having a high
resolution. Since a laser beam generally has an angle of
divergence, only a component of the laser beam which
satisfies the above-mentioned equation transmits and
forms ring-like interference fringes around the center of
the optical axis of the laser beam.
The wavelength monitoring means 15 has the
integrator 18 which weakens or diffuses the laser beam,
1303709
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the FP 19 and the lens 21. Of divergence components
produced by the integrator 18, only the light having ~
which satisfies the above-mentioned equation reaches the
lens 21 through the FP 19. When the focal distance of
the lens is represented by f, the light having the
component of ~ is focussed on the point apart from the
axis of the lens by f~. Then, the value ~ is obtainable
by measuring the light focussing point by means of the
image pickup element 22; thus the value A can be
calculated.
The light intensity distribution on the image pickup
element 22 is as shown in Figure 3, wherein the ordinate
represents a distance x from the center of the fringes.
Each peak corresponds to the number of power of the FP.
The space between adjacent peaks is called a free
spectrum range, and the wavelength can be primarily
determined in this range. Since the free spectrum range
can be determined when an FP is designed, it is so
determined as to be broader than a value in estimation of
a shift of wavelength.
Each of the peaks has a light intensity distribution
corresponding to the wavelength distribution of a laser
beam. Accordingly, the picture image processing means 25
is needed to obtain the value ~ by processing the light
intensity distribution. Further, in this embodiment, the
wavelength A at present is calculated and then, the
volume adjustable means 11 is actuated by the driving
13~3~
means 12 depending on a result of the calculation of the
wavelength A, whereby the wavelength of the oscillator i~
adjusted by adjusting a pressure in the sealed container
4.
In the above-mentioned embodiment, description has
been made as to the wavelength monitoring means in which
the fringes of the FP are measured by the image pickup
element. However, the same effect can be attained by
using another type of wavelength monitoring means.
Figure 4 is a diagram showing another embodiment of
the laser device according to the present invention. In
Figure 4, the same reference numerals as in Figure 2
designate the same elements. A numeral 26 designates a
gas bomb for supplying clean gas in the sealed container
4, numerals 27 and 28 respectively designate adjusting
valves provided at the inlet and the outlet of the sealed
container 4 respectively to adjust a flow rate of gas,
numerals 29 and 30 respectively designate stop valves,
and a numeral 31 designates a control device which
receives an output signal from the wavelength monitoring
means 101 to thereby control the adjusting valve 27.
The control device 31 is such that it receives the
output of the picture image processing means 25 to obtain
the wavelength of the laser beam 13 at present, and it
controls the adjusting valve 27 for adjusting a flow rate
of gas so that a ~as pressure around the FP 3 is
1303709
adjusted, whereby the wavelength of the laser beam 13 has
a predetermined value. The adjusting valve 27 is
manually adjusted at the initial step of adjustmentO
The function of selecting the wavelength by the FP is
the same as that of the FP 19 for monitoring the
wavelength.
In this embodiment, the adjusting valve 27 at the
inlet side of the sealed container 4 is controlled by the
control device 31. ~owever, it is possible that only the
adjusting valve 28 at the outlet side of the sealed
container 4 is controlled. Orl the both adjusting valves
27, 28 may be controlled to thereby adjust a gas pressure
in the sealed container 4. The adjusting valves 27, 28
may be replaced by orifices or a mass-flow controller or
the like to thereby control a flow rate of gas, hence a
gas pressure.
Figure 5 is a diagram showing another embodiment of
the laser device according to the present inventionc The
construction of this embodiment is the same as that as
shown in Figure 4 provided that a pressure sensor 32 and
a control device 33 for keeping a pressure constant are
added. Namely, the pressure sensor 32 is provided at the
sealed container 4 so that the output signal of the
pressure sensor 32 and an output signal having
information of the wavelength which is generated from the
control device 20 are inputted in the control device 33
"`" 1303709
-- 10 -- -
for keeping the pressure to be constant, whereby the
adjusting valve 27 for adjusting a flow rate of the gas
can be controlled through the device 33. Thus, a gas
pressure in the sealed container 4 can be easily
adjusted.
In the above-mentioned embodiments, the FP 19 for
monitoring the wavelength is not provided in the
atmosphere where the clean gas is supplied, in the same
manner as the FP 3 for selecting the wavelength. It is
because the FP 19 is so constructed that only small part
of the laser beam 13 enters in the FP 19. However,
further excellent result is obtainable when the FP 19 is
disposed in the atmosphere in which clean gas is
supplied.
In this embodiment, description has been made as to
the wavelength monitoring means 101 in which the fringes
resulted by the light transmitted through the FP 19 are
measured by the image pickup element 22. However, the
wavelength may be measured by another method.
The present invention i5 useful for stabilizing the
exC ~n~r
wavelength of a laser device such as an oximor laser
device.
