Solid state laser

Details Solid state laser Solid state laser

1998-09-25

2001-09-11

Davie, James W. (Department: 2881)

Coherent light generators

Particular beam control device

Optical output stabilization

C372S033000, C372S036000

Reexamination Certificate

active

06289029

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a solid state laser such as a laser diode-pumped solid state laser, and more particularly to a solid state laser in which fluctuation in the oscillation wavelength and/or the output power due to change in the length of the resonator is prevented.
2. Description of the Related Art
As disclosed, for instance, in Japanese Unexamined Patent Publication No. 62(1987)-189783, there has been known a solid state laser in which a solid state laser medium doped with a rare earth metal such as neodymium is pumped by a laser beam emitted from a semiconductor laser or the like.
In such a laser diode-pumped solid state laser, the oscillation wavelength is shifted toward the longer wavelength side as the environmental temperature becomes higher and toward the shorter wavelength side as the environmental temperature becomes lower, which changes the optimal drive condition of the laser, e.g., the temperature to which the temperature of the resonator is to be controlled, and causes fluctuation in the output power of the laser.
In order to overcome this problem, the applicant has proposed to contain the solid state laser crystal, the resonator and the like in an enclosed casing which is filled with gas the refractive index of which is closer to 1 than air or which is evacuated to lower than the atmospheric pressure. See Japanese Unexamined Patent Publication No. 9(1997)-266337.
Though these approaches can overcome the problem described above, the former is disadvantageous in that the manufacturing cost is increased and the latter is disadvantageous in that the properties of the solid state laser vary as the state of parts about the resonator varies from the time the casing is sealed to the time the solid state laser is assembled and adjusted.
SUMMARY OF THE INVENTION
In view of the foregoing observations and description, the primary object of the present invention is to provide a solid state laser which can be manufactured at low cost and in which fluctuation in the oscillation wavelength and/or the output power due to change in the environmental temperature can be surely prevented.
In accordance with a first aspect of the present invention, there is provided a solid state laser comprising a solid state laser crystal, a pumping source for pumping the laser crystal, a resonator, an enclosed casing which is filled with gas and in which the resonator is contained, and a temperature control means which keeps the resonator at a predetermined temperature, wherein the improvement comprises that
the ratio of the optical length of the gas layer in the resonator to the oscillation wavelength of the solid state laser is not larger than 13600.
Preferably the ratio is not larger than 6800 and more preferably not larger than 3400.
In accordance with a second aspect of the present invention, there is provided a solid state laser comprising a solid state laser crystal, a pumping source for pumping the laser crystal, a resonator, an enclosed casing which is filled with gas and in which the resonator is contained, and a temperature control means which keeps the resonator at a predetermined temperature, wherein the improvement comprises
a cover which is provided in the enclosed casing and envelops at least the resonator and at least 80% of the gas in the enclosed casing.
In accordance with a third aspect of the present invention, there is provided a solid state laser comprising a solid state laser crystal, a pumping source for pumping the laser crystal, a resonator, an enclosed casing which is filled with gas and in which the resonator is contained, and a temperature control means which keeps the resonator at a predetermined temperature, wherein the improvement comprises that
at least the optical elements forming the resonator are fixed to a support member of aluminum or aluminum alloy in the enclosed casing.
In accordance with a fourth aspect of the present invention, there is provided a solid state laser comprising a solid state laser crystal, a pumping source for pumping the laser crystal, a resonator, an enclosed casing which is filled with gas and in which the resonator is contained, and a temperature control means which keeps the resonator at a predetermined temperature, wherein the improvement comprises that
the temperature control means comprises a Peltier element fixed to a bottom plate of the enclosed casing, and
the portion of the bottom plate at which the Peltier element is fixed to the bottom plate is not larger than 3 mm in thickness.
The reason why the oscillation wavelength of a solid state laser varies according to the environmental temperature Te will be discussed in detail, hereinbelow. For this purpose, a model shown in
FIG. 7
is imagined. In
FIG. 7
, reference numeral
1
denotes an enclosed casing which keeps hermetic the inside thereof from the exterior and reference numeral
2
denotes the gas around the temperature controlled optical elements. In the following description, the volume and the number of particles of the enclosed gas near the temperature controlled part are respectively represented by V
1
and N
1
, the volume and the number of particles of the enclosed gas near the wall surface of the casing
1
are respectively represented by V
2
and N
2
, and the pressure of the gas and the gas constant are respectively represented by P and R.
Since the wall surface of the enclosed casing
1
is substantially at the environmental temperature Te, the temperature of the enclosed gas near the wall surface is T
2
close to the environmental temperature Te. On the other hand, the temperature of the enclosed gas near the temperature controlled elements is T
1
close to the controlled temperature Tc. For example, if the environmental temperature Te is higher than the controlled temperature Tc, Te≈T
2
>T
1
≈Tc.
When Te=Tc and there is no temperature distribution, the enclosed gas is distributed uniformly and accordingly, the particle density in the casing
1
is N
0
/V
0
(the number of the whole particles/the volume of the casing
1
) and constant. When Te≠Tc assuming that V
2
=V
1
for the purpose of simplicity of description, the particle density of the enclosed gas near the temperature controlled part is N
1
/V
1
=P/R/T
1
and the particle density of the enclosed gas near the wall surface is N
2
/V
2
=P/R/T
2
.
Accordingly when the environmental temperature Te is higher than the controlled temperature Tc, the particle density of the enclosed gas in the resonator is increased and the refractive index of the gas in the resonator increases, whereby the oscillation wavelength becomes longer.
The relation between the optical length lgas of the gas layer in the resonator and the oscillation wavelength will be described next. When the refractive index of the gas in the resonator is represented by ngas and the optical length of the resonator is represented by L,
L=&Sgr;
(
ni·li
)=
n
gas·
l
gas+
L
0.
The oscillation wavelength &lgr;=2L/m
0
wherein m
0
represents the order of the longitudinal mode. Change in the oscillation wavelength with change in the refractive index of the gas layer is as follows.
∂&lgr;/∂
n
gas=2
·l
gas/
m
0
=
l
gas·&lgr;/
L
The proportion of the change in the oscillation wavelength to the separation between longitudinal modes (=&Dgr;&lgr;LM=&lgr;
2
/2L) is 2·lgas/&lgr; and is proportional to the optical length lgas of the gas layer.
Now change in the length of the resonator according to the temperature distribution in the part to which the resonator is fixed will be described with reference to
FIGS. 8A and 8B
. In
FIGS. 8A and 8B
, reference numeral
3
denotes a resonator portion and reference numeral
4
denotes, for instance, a Peltier element for controlling the temperature of the resonator portion
3
.
Generally, the upper portion
4
a
of the Peltier element
4
where the resonator portion
3
is fixed is held at the controlled temperature Tc while the lower portion
4
b
of the Peltier

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