Coherent light generators – Particular pumping means – Pumping with optical or radiant energy
Reexamination Certificate
1999-04-30
2003-09-30
Ip, Paul (Department: 2828)
Coherent light generators
Particular pumping means
Pumping with optical or radiant energy
C372S071000, C372S075000, C372S050121
Reexamination Certificate
active
06628692
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a solid-state laser device for use as a light emission device or a similar device and a solid-state laser amplifier provided therewith, and more particularly to a solid-state laser device of a reduced size and a solid-state laser amplifier provided therewith.
2. Description of the Related Art
A so-called semiconductor laser pumped solid-state laser device (LD pumped solid-state laser device) using a semiconductor laser (laser diode) as an excitation light source has such a desired characteristic for industrial use as a long service life, a high efficiency and high brightness. For the reason, the LD pumped solid-state laser device has been actively developed. Particularly, the LD pumped Nd:YAG laser has been developed so that its output is 10W level on the market or kW level on development stage.
By using a laser crystal having a high absorption peak in a pump wavelength range, an output of the solid-state laser device can be provided in a very small volume. Thus, attention has been paid to development on reduction of the size of a laser head size. As such a solid-state laser device, for example, so-called microchip-type solid-state laser device is currently available.
FIG. 1
is a schematic diagram showing an existing microchip-type solid-state laser device. In the existing microchip-type solid-state laser device, one end face
102
of a laser crystal
101
in which a thickness thereof is about several hundreds &mgr;m, generally about one or two times absorption length which is a reciprocal of the absorption coefficient, is treated with high reflection coating, while the other end face
103
is treated with partial reflection coating.
If laser beam impinges upon the end face
102
on which the high reflection coating is applied from inside of the laser crystal
101
, the laser beam is fully reflected. However, excitation beam impinging on the end face
102
from outside of the laser crystal
101
is not reflected but passes through. If the laser beam impinges upon the end face
103
on which the partial reflection coating is applied from inside of the laser crystal
101
, part of the laser beam is reflected, and the other part thereof is not reflected but passes through.
A focusing lens
104
and a collimate lens
105
are disposed in parallel to the end face
102
on a side of the end face
102
of the laser crystal
101
. A semiconductor laser
106
is disposed at a position sandwiching the focusing lens
104
with the collimate lens
105
. Excitation laser beam
107
is emitted from an emission portion
106
a
of the semiconductor laser
106
to the collimate lens
105
.
In a existing microchip-type solid-state laser device having such a structure, the excitation laser beam
107
, which is emitted from the emission portion
106
a
of the semiconductor laser
106
and has a divergence angle, is converted to parallel beam by the collimate lens
105
. After converted to parallel beam, the excitation laser beam
107
is refracted by the focusing lens
104
so that its focal point is made on the end face
102
. The laser crystal
101
is excited by the beam impinging upon the end face
102
and the beam is reflected between the end face
102
and end face
103
so as to generate resonator internal circulating beam
107
a
. After that, laser oscillation beam
109
is emitted from the end face
103
.
Table 1 shows absorption coefficients and absorption lengths of typical solid-state laser crystals (Nd:YVO
4
crystal, Nd:YAG crystal and Yb:YAG crystal) having an oscillation wavelength band of 1 &mgr;m.
TABLE 1
Pump
Absorption
Doping
Absorption
Laser
wavelength
coefficient
density
length
crystal
(nm)
(cm
−1
)
(atomic %)
(mm)
Nd:YAG
809
8
1
1.25
Nd:YVO
4
809
25(&sgr;)
1.5
0.4
110(&pgr;)
0.1
Yb:YAG
940
50
50
0.2
Nd:YVO
4
having an excessively large absorption coefficient of Nd series has been often used as a laser crystal for the microchip-type solid-state laser device. Further, Yb:YAG having a quasi-three-level energy structure can be also formed to a microchip. By that microchip-type structure, it is possible to integrate a laser resonator comprising a solid-state laser crystal and a reflector sandwiching it with the solid-state laser crystal. As a result, a small solid-state laser device having a high resistance to mechanical vibration and thermal variation can be achieved.
For example, a solid-state laser device having a structure shown in FIG.
1
and employing Nd:YVO
4
crystal as the laser crystal
101
has been described in “Optics Letters, vol.16, p.1955, 1991”. The thickness of the Nd:YVO
4
crystal which is used as the laser crystal of the solid-state laser device described in the document is 500 &mgr;m. By exciting the Nd:NVO
4
with a semiconductor laser (wavelength: 809 nm) having 500 mW output, a laser output of 160 mW (wavelength: 1064 nm) can be obtained. At this time, in the excitation optical system, a single lens having a focal length of 4.5 mm, which functions as a collimate lens and a focusing lens at the same time, is used. The reflection factor of the partial reflector is 99% with respect to the laser oscillation wavelength.
However, in a conventional LD pumped solid-state laser device as described previously, as shown in
FIG. 1
, an optical element (lens, mirror, prism and such) having a limit in miniaturization of the excitation optical system for introducing the excitation laser beam
107
into the laser crystal
101
has been often used. Therefore, in the conventional microchip-type solid-state laser device, even if the laser crystal can be thinned, the entire laser head including an excitation optical system and a LD is not always of small size. In such an excitation optical system having a micro optical element, after each optical element is adjusted to an accurate position on the optical axis, it needs to be held strictly. Further, there is such a problem that a change of the semiconductor laser focusing position and a change of the laser output thereby is caused by thermal expansion of the case due to temperature change, mechanical vibration and such, because of two reasons that a propagation distance of the excitation beam is large and that the macro optical element is spatially separated from the laser crystal for not integral type.
Therefore, a solid-state laser device not having the excitation optical system provided with a lens and such has been proposed in “Optics Letters, vol.17, p.1201, 1992”.
FIG. 2
is a schematic diagram showing a conventional solid-state laser device described in “Optics Letters, vol.17, p.1201, 1992”. In the conventional solid-state laser device, a partial reflector
112
a
is disposed on an end face of the laser crystal
111
and a full reflector
113
is disposed on the other end face. Further, a Q switch element
120
is connected to the partial reflector
112
a
and a partial reflector
112
b
is disposed on the other end face of the Q switch element
120
. Further, an electrode
120
a
on which a source voltage is applied and an electrode to be grounded are provided for the Q switch element
120
. Then, a semiconductor laser
116
is disposed on the end face in which the full reflector
113
of the laser crystal
111
is provided.
Because the solid-state laser device does not contain any excitation optical system like the collimate lens, miniaturization thereof can be achieved more easily than the above-mentioned conventional solid-state laser device. However, because the divergence angle of the excitation laser beam
117
is large, it is necessary to dispose the semiconductor laser
116
and the laser crystal
111
near each other. Therefore, it is difficult to connect plural semiconductor lasers for excitation so that there is an obstacle in enhancement of the output.
A solid-state laser device in which the excitation laser beam is transmitted via multi-mode fiber has been described in “Conference on Lasers and Electro-optics (CLEO), p.176, paper CWC6, 1995”.
FIG. 3
is a schematic diagram showing a co
Ip Paul
McGinn & Gibb PLLC
Menefee James
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