Diode-laser side-pumped solid-state laser device

Details Diode-laser side-pumped solid-state laser device Diode-laser side-pumped solid-state laser device

2001-02-26

2003-03-11

Paladini, Albert W. (Department: 2827)

Coherent light generators

Particular pumping means

Pumping with optical or radiant energy

C372S034000, C372S071000

Reexamination Certificate

active

06532248

ABSTRACT:

BACKGROUND OF THE INVENTION
(a) Field of the Invention
The present invention relates to a diode-laser side-pumped solid-state laser device and, more particularly, to a solid-state laser device pumped by a semiconductor laser diode to generate a solid-state laser beam with a higher brightness and a higher efficiency.
(b) Description of the Related Art
As a pumping (exciting) scheme for a solid-state laser device such as having a Nd:YAG laser medium, pumping of the solid-state laser device by using a semiconductor laser diode (referred to as simply “laser diode” hereinafter) is now highlighted due to a higher absorbing efficiency thereof by the laser medium compared to the lamp-pumping scheme for the solid-state laser device. This type of solid-state laser device (SSLD) uses the laser diode as a pumping light source having a longer lifetime, smaller dimensions and higher efficiency.
A variety of SSLDs using a side-pumping scheme have been proposed heretofore, wherein a large number of laser diodes are arranged in an array on the side surface of an elongate solid laser medium, such as a cylindrical solid laser rod, along the lasing axis of the solid laser rod. The laser diode has inherently a linear brightness distribution, which suitably matches with the side-pumping scheme.
FIG. 1
shows a sectional view of a conventional SSLD using the side-pumping scheme, as described in “IEEE Journal of Quantum Electronics” 1992, Vo. 28, No.4, pp977-985. The SSLD has a cylindrical solid laser rod (Nd:YAG laser rod)
1
extending normal to the sheet of FIG.
1
. The laser rod
1
is encircled with a cooling tube
3
having an inner diameter larger than the outer diameter of the laser rod
1
. The space between the laser rod
1
and the cooling tube
3
is filled with a cooling medium
2
flowing therebetween for cooling the laser rod
1
.
In the vicinity of the outer surface of the cooling tube
3
, a large number of laser diodes
100
a
to
100
h
are arranged, with four laser diodes
100
a
to
100
d
being arranged for a unit length of the laser rod
1
and separated from one another by a uniform angular distance with respect to the central axis of the laser rod
1
. Other four laser diodes
100
e
to
100
h
in another array are deviated from the array of laser diodes
100
a
to
100
h
by 45 degrees as viewed along the axis of the laser rod
1
. This configuration provides eight pumping directions to improve the axial symmetry of the energy absorption distribution for the pumping laser by the laser rod
1
.
In the SSLDs using the side-pumping scheme and described in Patent Publications JP-A-10-326927 and —10-84150, the laser beam emitted by each of the laser diodes diverges to a whole angle as large as 30 degrees in the direction normal to the active layer of the laser diode. Thus, the laser diodes should be disposed in close proximity with the laser rod
1
in order to efficiently emit the laser beam toward the laser crystal.
In the exemplified SSLD of
FIG. 1
, the distance between the emission end surface of each of the laser is diodes
100
a
to
100
h
and the cooling tube
3
is as small as 1 mm. Although the laser diode has a small chip size, the overall dimensions of the laser diode are equivalent to the diameter of the laser rod, because the laser diode has a mount member for the chip and a cooling device such as a Peltier element or cooling water path. This prevents a large number of laser diodes from being disposed for a unit length of the laser rod, and impedes a higher output power of the SSLD.
For alleviating the difficulty of arrangement of a large number of laser diodes in close proximity of the laser rod, it is considered to prevent the divergence of the laser beam from the laser diode by using an optical unit such as a lens, thereby efficiently emitting the laser beams from the laser diodes toward the side surface of the laser rod. In an alternative, it is also known that an elongate optical waveguide encircling the laser rod is provided for guiding the laser beams emitted from a large number of laser diodes toward the laser crystal of the laser rod.
The SSLD of
FIG. 1
has a disadvantage in that the power efficiency of the pumping laser beam is relatively low because some of the laser beam passes the laser rod without being absorbed by the laser rod.
FIG. 2
shows another conventional SSLD using the side-pumping scheme, described in “Optics Letter” 1995, vol. 20, No. 10, pp1148-1150. In the SSLD, a cylindrical lens (collimate lens)
101
a
, for example, disposed in close proximity of the laser diode
100
a
collimates a laser beam component (advanced-phase-axis component), which is normal to the thickness direction of the active layer. This alleviates divergence of the pumping laser beam
106
emitted from the laser diode
100
, and allows the pumping laser beam
106
to transmit in the space toward the side surface of the laser rod
1
. Thus, a large number (nine at a maximum in this example) of laser diodes
100
can be disposed around the circumference of the laser rod
1
for a unit length thereof due to a large distance between the laser diode
100
and the laser rod
1
.
In the SSLD of
FIG. 2
, a portion of the pumping laser beam
106
(
106
a
or
106
b
) not absorbed in the laser rod
1
and passing the same is reflected by mirrors
104
(
104
a
or
104
b
) surrounding the cooling tube
3
. For example, the pumping laser beam
106
a
is irradiated onto the laser rod
1
through a slit formed between adjacent mirrors
104
f
and
104
g
, and is absorbed or passed by the laser rod
1
. The laser beam passed by the laser rod
1
is then reflected by a corresponding mirror
104
a
toward the laser rod
1
.
The SSLD of
FIG. 2
has a disadvantage in that a portion of the laser beam which is not absorbed by the laser rod
1
passes the slit of the mirror member and thus is not recovered for absorption. More specifically, when a parallel ray of the pumping laser beam is incident onto the cylindrical laser rod
1
, some of the laser beam not absorbed and passed by the laser rod
1
is focused and then diverged. The diverged laser beam is more likely to pass through the slit without being reflected by the mirror member.
Patent Publications JP-A-11-284256 and —11-284253 describe SSLDs having reflecting mirrors similarly to the SSLD of FIG.
2
.
FIG. 3
shows the SSLD described in JP-A-11-284256, wherein the advanced-phase-axis component of the pumping laser beam
6
collimated by a rod lens (not shown) is irradiated through a slit
135
formed in a mirror member
134
, which is located on the outer periphery of the cylindrical body
133
encircling the solid laser rod
1
. The space between the laser rod
1
and the cylindrical member
133
is filled with a cooling medium, and the cylindrical member
133
alleviates the convex lens function of the laser rod
1
. A portion of the pumping laser not absorbed by the laser rod
1
is focused at a focal point in the vicinity of the laser rod
1
.
The focused laser beam portion
6
t
is then reflected by a corresponding mirror
134
toward the laser rod
1
after a moderate divergence. The moderate divergence, effected by the alleviation of the convex lens function of the laser rod
1
and shown by a small diameter “d” of the laser beam
6
, allows the effective reflection area of the mirror member
134
to be maintained larger irrespective of the presence of a number of the slits
135
formed therein.
The SSLD of
FIG. 3
has a disadvantage in that the cylindrical body has a larger thickness and thus has a larger weight and a higher equipment cost. In addition, a larger number of laser beams in different directions reduces effective reflection of the mirror member due to the larger number of openings disposed for introducing the laser beams.
Patent Publication JP-A-11-284253 describes the SSLD shown in
FIG. 4
, wherein a reflecting layer
144
is provided on the outer surface of the cooling tube
3
for reflecting laser beams passed by the laser rod
1
toward the laser rod
1
again. The laser beams are irradiated from

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