Coherent light generators – Particular temperature control
Reexamination Certificate
2000-12-11
2003-06-24
Leung, Quyen (Department: 2828)
Coherent light generators
Particular temperature control
Reexamination Certificate
active
06584127
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a light emitting device using a solid-state laser, and more particularly to a light emitting device which is provided with an automatic light output control function together with a function of operating the solid-state laser and/or the pumping light source at an optimal temperature.
2. Description of the Related Art
There has been known a solid-state laser in which a solid laser medium doped with a rare-earth metal such as neodymium is pumped with pumping light emitted from a pumping light source such as a semiconductor laser. Further, there has been known a light emitting device obtained by combining such a solid-state laser with a nonlinear optical crystal so that the fundamental wave oscillated by the solid-state laser is converted to a second harmonic which is a half of the fundamental wave in wavelength.
In the light emitting devices described above, the properties of the solid laser medium, the nonlinear optical crystal and the semiconductor laser have great temperature-dependency and accordingly, at least a part of these components must be held at a constant temperature in order to stabilize the light output of the light emitting device.
Specifically, as shown in
FIG. 3
, the light emitting device is generally provided with the following first and second means in addition to a solid-state laser unit
1
comprising a solid laser medium
1
a
, a resonator mirror
1
b
, a nonlinear optical crystal
1
c
and the like and a pumping light source
2
such as a semiconductor laser which emits pumping light
2
a.
The first means is an automatic temperature control means comprising a temperature control element
3
such as a Peltier element on which important components such as the solid-state laser unit
1
, the pumping light source
2
and like are placed, a temperature sensor
4
which detects the temperature of the important components and a temperature control circuit
5
which controls the temperature control element
3
on the basis of the temperature detected by the temperature sensor
4
.
The second means is an automatic output control means comprising a photodetector
6
which detects a part of a laser beam
11
emitted from the solid-state laser unit
1
(split from the laser beam
11
by a half-silvered mirror
6
a
) and converts the intensity of the part of the laser beam
11
to an electric signal, a light output control circuit
7
which generates a feedback signal for changing the energy to be injected into the pumping light source
2
according to the level of the electric signal so that a desired light output of the solid-state laser unit
1
can be obtained, and a drive circuit
8
which provides energy to the pumping light source
2
under the control of the feedback signal (in this example, a circuit for applying an electric current to a semiconductor laser).
In this example, the laser beam
11
is a second harmonic which is obtained by wavelength conversion of a laser beam emitted from the solid laser medium
1
a
by the nonlinear optical crystal
1
c.
The reason the automatic temperature control means and the automatic light output control means are required to stabilize the light output of the light emitting device shown in
FIG. 3
will be described in detail hereinbelow.
In
FIG. 4
, change of the light output of the light emitting device when the temperature of the temperature control element
3
is changed with energy of a given level injected into the pumping light source
2
is shown by the solid line. As can be seen from
FIG. 4
, the light output of the light emitting device is often maximized at a particular working temperature. This is because the oscillating wavelength of the semiconductor laser, the wavelength conversion efficiency by the nonlinear optical crystal, and the like have great temperature-dependency.
Accordingly, unless the light emitting device is operated at an optimal temperature, larger energy must be injected into the pumping light source in order to obtain a light output of a given level, which can shorten the service life of the light emitting device. This is a reason the first means is required.
The relation between the working temperature and the light output of the light emitting device can change as shown by the broken line in
FIG. 4
with age and/or with the ambient temperature. Accordingly, in order to compensate for the change with age and/or with the ambient temperature, the second means is required.
However, even with the first and second means, an attempt to obtain a stabilized high quality laser beam for a long period encounters the following difficulties.
That is, for example, the temperature of the temperature control element
3
at which the output of the light emitting device is maximized (temperature
1
) changes with change of the ambient temperature and/or change with age of the components. The working temperature-light output properties can change as shown by the broken line in
FIG. 5
, and the temperature at which the output of the light emitting device is maximized can change from the temperature
1
to a temperature
2
.
In such a case, when the automatic output control means is simply operated with the temperature of the temperature control element
3
fixed at the temperature
1
, a larger drive current is apparently required to increase the output of the pumping light in order to obtain a desired light output of the light emitting device, which is greatly disadvantageous not only from the viewpoint of power consumption but also from the viewpoint of the service life of the light emitting device.
Accordingly, it is necessary to search for a new optimal temperature (the second temperature
2
) somehow and to operate the automatic output control means at the new optimal temperature. There is disclosed in Japanese Unexamined Patent Publication No. 8(1996)-17106 a method of searching for a new optimal temperature and controlling the temperature of the important components to the new optimal temperature.
The method will be described in detail with reference to
FIG. 6
which shows a light emitting device for carrying out the method. As shown in
FIG. 6
, the light emitting device comprises a solid-state laser unit
30
formed of a solid laser medium
30
a
, a resonator mirror
30
b
, a nonlinear optical crystal
30
c
and the like, a pumping light source
31
such as a semiconductor laser which emits pumping light
31
a
for pumping the solid laser medium
30
a
, a temperature control element
32
such as a Peltier element which controls the temperature of the solid-state laser unit
30
and/or the pumping light source
31
, a temperature sensor
33
which detects the temperature of the solid-state laser unit
30
and/or the pumping light source
31
, a temperature control circuit
34
which controls the temperature control element
32
on the basis of the temperature detected by the temperature sensor
33
according to a temperature signal
40
representing a target temperature, a photodetector
35
which detects a part of a laser beam
43
emitted from the solid-state laser unit
30
(split from the laser beam
43
by a half-silvered mirror
35
a
) and converts the intensity of the part of the laser beam
43
to an electric signal, a light output control circuit
36
which generates a signal for controlling the energy to be supplied to the pumping light source
31
according to the level of the electric signal so that the output of the laser beam
43
is fixed, and a drive circuit
37
which supplies energy to the pumping light source
31
under the control of the signal from the light output control circuit
36
.
The light emitting device is further provided with a target temperature calculating unit
38
which sweeps the temperature signal
40
, stores the detecting signal of the photodetector
35
during the sweep of the temperature signal
40
, calculates an optimal temperature on the basis of the relation between the temperature and the level of the detecting signal of the photodetector
35
(usually the temperatur
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