Semiconductor laser-pumped solid state laser system and optical coupling system coupling semiconductor laser with optical fiber

Light beams output from active layer stripes of a semiconductor laser array are individually collimated in a GRIN lens array and are converged, in a aspheric lens, into a beam spot, to thereby end-pump a solid state laser with high efficiency.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a semiconductor laser-pumped solid state 
laser system wherein an optical power from a semiconductor laser array as 
a pump source is optically coupled with a solid state laser element in 
high efficiency, and an optical coupling system for coupling an optical 
power from a semiconductor laser array with an optical fiber with high 
efficiency. 
2. Description of the Related Art 
A solid state laser using a semiconductor laser as a light source for 
optical pumping has attracted much attention because of its high 
efficiency, long life, and its potential miniaturization. Among the 
pumping methods of the solid state laser with the semiconductor laser, an 
end-pumping method wherein the solid state laser is pumped in the 
direction of its optical axis (e.g., Japanese Unexamined Patent 
Publication (Kokai) No. 58-528), realizes single fundamental transverse 
mode oscillation with high efficiency by matching the pump space of an 
optical power from the semiconductor laser with a spatial mode of the 
oscillation in the solid laser. 
To obtain high output power from the semiconductor laser-pumped solid state 
laser, a light beam of the semiconductor laser must be sufficiently 
condensed and a semiconductor laser generating high optical power must be 
used. 
Meanwhile, the optical power of a semiconductor laser, which is supplied 
through an optical fiber, has been widely used, for example, in the field 
of medical science. The optical power supplied through the optical fiber 
is obtained by optically coupling the semiconductor laser with the optical 
fiber. Again, to obtain high optical power from the optical fiber, the 
condensing of the optical power from the semiconductor laser and high 
output power from the semiconductor laser is important. 
However, it is not easy to condense the optical power from the 
semiconductor laser because the divergence angle of the light beam of the 
semiconductor laser is large, and therefore, a condensing system must be 
located close to the semiconductor laser. 
Furthermore, in the semiconductor laser, the laser beam is generated in a 
striped active layer, and since the optical power from a single striped 
laser is limited, a plurality of stripes having an array construction 
i.e., a semiconductor laser array has to be used to obtain the higher 
optical power. 
Since the width of the semiconductor laser array generating an optical 
power sufficient for effecting the end-pumping amounts to 1 cm, a 
plurality of light beams cannot be condensed at a spot using a 
conventional lens system, and therefore the semiconductor laser array has 
not been employed in the end-pumping method efficiently, but has only been 
employed in a side-pumping method (for example, R. Burnham and A. D. Hays, 
Opt. Lett., 14, 27(1989); M. K. Reed, W. J. Kozlovsky, R. L. Byer, G. L. 
Harnagel, and P. S. Cross, Opt. Lett., 13. 204(1988).) 
SUMMARY OF THE INVENTION 
Therefore, it is an object of the present invention to provide a 
semiconductor laser array-pumped solid state laser system, wherein a 
plurality of light beams having a large divergence angle generated in a 
semiconductor laser array having multiple stripes are condensed so that a 
pump space of an optical power from the semiconductor laser array matches 
a space mode of oscillation of the solid state laser, to thereby generate 
a highly efficient optical power in the solid state laser. 
It is another object of the present invention to provide an optical 
coupling system for coupling an optical power from the semiconductor laser 
array with an optical fiber of high efficiency. 
In accordance with the present invention, there is provided a semiconductor 
laser-pumped solid state laser system comprising a resonator means 
including a solid state laser, a semiconductor laser means having a 
plurality of active portions for outputting a plurality of light beams, 
respectively, a condensing means, optically coupled with the semiconductor 
laser means, for condensing the respective light beams from the respective 
active portions of the semiconductor laser means, and a converging means, 
optically coupled with the condensing means, for converging the light 
beams condensed in the condensing means, to thereby optically pump the 
solid state laser. 
In accordance with the present invention there is also provided an optical 
coupling system for coupling a semiconductor laser with an optical fiber; 
the semiconductor laser element having a plurality of active portions for 
outputting a plurality of light beams, respectively, comprising a 
condensing means, optically coupled with the semiconductor laser element 
for condensing the respective light beams from the respective active 
portions of the semiconductor laser elements, and a converging means 
optically coupled with the condensing means and coupled with the optical 
fiber for converging the light beams condensed in the condensing means and 
for supplying the optical fiber with the converged light beams.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a schematic diagram of a semiconductor laser array-end-pumped 
solid state laser system according to an embodiment of the present 
invention. As shown in FIG. 1, a resonator is constructed by a solid state 
laser 10 including a Nd:YAG laser, and an output mirror 12. A dychroic 
coating is applied to one end face 11 of the solid state laser 10 to 
render the face highly reflective (HR) at a wavelength 1064 nm of 
oscillation of the Nd:YAG laser and to make the face anti-reflective (AR) 
at a wavelength 808 nm of the semiconductor laser array. The end face 11 
is used as a pump face. A used semiconductor laser array 14 has 20 active 
layer stripes 16 having a width of 100 .mu.m and spaced at intervals of 
500 .mu.m. A gradient index lens (GRIN lens) array 18 as a condenser lens 
array is constructed by 20 GRIN lenses having widths of 500 .mu.m to 
condense light beams from the 20 stripes to form 20 parallel beams, 
respectively. The 20 parallel laser beams are converged by an aspherical 
lens 20, and are superimposed on the end face 11 to end-pump the Nd:YAG 
laser 10. 
Characteristics of a transverse mode in a semiconductor laser-pumped solid 
state laser is determined by the shape of the pump space inside a solid 
state laser element. Therefore, it is important to approach the intensity 
distribution of the condensed pumping light to the Gaussian distribution 
and stably form a beam spot having a constant size the inside the solid 
state laser element, in order to obtain a fundamental transverse mode. 
Since the GRIN lens, which is optical glass having a refractive index 
distribution where the refractive index is gradually reduced from the 
central axis to periphery, is used as an optical coupler for the 
semiconductor laser-pumped solid state laser, the laser beam having the 
large divergence angle from the semiconductor laser can be easily 
condensed. And, since the condensed laser beams are converged into a beam 
spot by the aspherical lens, which does not cause spherical aberration, to 
pump the solid state laser, a high quality fundamental transverse mode 
light beam is obtained. 
In the semiconductor laser array-pumped solid state laser system, which is 
end-pumped in the aforementioned condition, a fundamental transverse mode 
oscillation of the Nd:YAG laser (wavelength 1064 nm) is obtained through 
the output mirror 12 with high output power of 1.5 W, under 5 W of output 
power of the semiconductor laser array (wavelength 808 nm). 
FIG. 2 is a schematic diagram of a semiconductor laser array 
stack-end-pumped solid state laser system according to another embodiment 
of the present invention. The used semiconductor laser array stack 22 is 
made by stacking two semiconductor laser arrays 14, as described with 
reference to FIG. 1, at intervals of 300 .mu.m. A GRIN lens array stack 24 
as a condenser lens array stack is made by stacking two GRIN lens arrays 
14 (FIG. 1) under central axes thereof coincident with that of the 
semiconductor laser array stack 22. The GRIN lens array stack 24 condenses 
light beams from 40 stripes 16 of the semiconductor laser array stack 22, 
to form 40 parallel light beams, respectively. 
In the semiconductor laser array-pumped solid state laser system of FIG. 2, 
a fundamental transverse mode oscillation of the Nd:YAG laser (wavelength 
1064 nm) has been obtained with high output power of 3 W, under 10 W of 
output power from the semiconductor laser array stack (wavelength 808 nm). 
FIG. 3 is a schematic diagram of a semiconductor laser array-end-pumped 
solid state laser system according to another embodiment of the present 
invention. 
In this embodiment, a polarization beam splitter 26 is used for beam mixing 
two light beam groups from two semiconductor laser arrays 14a and 14b for 
optical pumping. As the light beam from the semiconductor laser is 
polarized, the polarization beam splitter 26 can mix the two polarized 
light beams. As denoted by an arrow 28a and a mark 28b, oscillation of a 
light beam group from the first semiconductor laser array 14a is polarized 
in a direction parallel to the surface of the drawing and oscillation of 
another light beam group from the second semiconductor laser array 14b is 
polarized in a direction normal to the surface of the drawing. Two light 
beam groups from the first and the second semiconductor laser array 14a 
and 14b are condensed by first and second GRIN lens arrays 18a and 18b, 
respectively, and are mixed in the polarization beam splitter 26. The 
mixed light beam group is converged into a beam spot by the aspherical 
lens 20 inside the Nd: YAG laser 10 as a solid state laser element. 
Note that, if a halfwave plate inserted between one of the semiconductor 
laser arrays 14a and 14b and the polarization beam splitter 26 is used, 
required polarization directions of the two semiconductor arrays 14a, 14b 
become the same. Such a construction is convenient, for example, in the 
case where spatial interference must be avoided in assembling. 
The used semiconductor arrays 14a and 14b are the same as the semiconductor 
array 14 of FIG. 1. Also, the GRIN lens arrays 18a, 18b are the same as 
the GRIN lens array 18 of FIG. 1. 
In the above-mentioned semiconductor laser array-end-pumped solid state 
laser system, a fundamental transverse mode oscillation of the Nd:YAG 
laser has been obtained with a high output power of 3 W, under 10 W of 
output power of the semiconductor laser. 
FIG. 4 is a schematic diagram of a semiconductor laser array 
stack-end-pumped solid state laser system according to another embodiment 
of the present invention. 
Used semiconductor laser array stacks 22a, 22b and GRIN lens array stacks 
24a and 24b are the same as the semiconductor laser array stack 22 and the 
GRIN lens array stack 24 of FIG. 2, respectively. Polarization directions 
of the semiconductor laser array stack 22a and 22b are the same as those 
of the semiconductor laser arrays 14a and 14b of FIG. 3, respectively. 
In the above semiconductor laser array stack-end-pumped solid state laser 
system, an output power of 6 W is obtained with 20 W of output power from 
the semiconductor laser. 
Although the Nd:YAG laser element is used as the solid state laser element 
in the aforementioned embodiments, the present invention is, of course, 
not limited to this. If a solid state laser element having a different 
absorption wavelength from that of the Nd:YAG laser element is used, a 
semiconductor laser element having a wavelength suitable for maximum 
absorption wavelength of the solid state laser element is used. 
Nonreflective coatings in the wavelength of the semiconductor laser are 
applied on both surfaces of the lenses and the polarization beam 
splitters. In addition, the number of semiconductor laser arrays and the 
GRIN lens arrays in the semiconductor laser array stack 22 and the GRIN 
lens array stack 24 of FIG. 2 and 4 are not limited to two. 
FIG. 5 is a schematic diagram of a semiconductor laser array-end-pumped 
solid laser system according to another embodiment of the present 
invention. 
In this embodiment, an anamorphic prism pair 30 is used between the GRIN 
lens array 18 and the aspherical lens 20 for shaping cross sections of the 
light beams from the GRIN lens array 18. 
As previously mentioned, it is preferable to approach the intensity 
distribution of the condensed pumping light to the Gaussian distribution 
to obtain the fundamental transverse mode. In addition, it is preferable 
that the distribution is a rotation symmetrical Gaussian distribution. 
Nevertheless, since the divergence angle of the light beam from the active 
layer stripe of the semiconductor laser is larger in a direction normal to 
the active layer than in a direction parallel to the active layer, namely, 
since the cross section of the light beam generally has an elongated 
elliptic shape, it is difficult to approach the intensity distribution of 
the condensed pumping light to the rotation symmetrical Gaussian 
distribution. 
Using the anamorphic prism pair 30 having wedge-shaped cross sections 
according to the present invention, the cross section of the light beam is 
expanded in only one direction. Therefore, the anamorphic prism pair 30 
can expand the light beam having an elliptic cross section in a direction 
of its minor axis to approach a complete round. Thus, by shaping the light 
beams from the GRIN lens array 18 in the anamorphic prism pair 30 to 
approach the intensity distribution of the rotation symmetrical Gaussian 
distribution, by converging the light beams in the aspherical lens 20 to a 
single beam spot, and by pumping the solid state laser element 10 with the 
converged beam spot, a high quality transverse mode light beam is 
obtained. 
In the semiconductor laser array-pumped solid state laser system shown in 
FIG. 5, an 1.11 W of YAG output (wavelength 1064 nm) is obtained under a 
3.48 W of pumping input (wavelength 808 nm). The threshold level of 
oscillation was low, i.e., 270 mW and slope efficiency was high, i.e., 
maximum 42%. 
FIG. 6 is a schematic diagram of an optical coupling system for coupling a 
semiconductor laser array with an optical fiber according to another 
embodiment of the present invention. Used semiconductor laser array 14, 
GRIN lens array 18, and aspherical lens 20 are the same as those of FIG. 
1. 
The light beams from the active stripes 16 of the semiconductor laser array 
14 are collimated in the GRIN lens array 18, are converged in the 
aspherical lens 20 into a light beam, and then enter an optical fiber 32. 
As the shape of the light beam is corrected by multiple reflection in the 
optical fiber 32, output light 34 of the optical fiber 32 becomes a 
transverse mode light having a high quality with high efficiency. In the 
optical coupling system of FIG. 6, output power of the semiconductor laser 
array 14 was coupled with optical fiber 32 with a high efficiency of 54%. 
FIG. 7 is a schematic diagram of a semiconductor laser array-end-pumped 
solid state laser system according to another embodiment of the present 
invention. 
In this embodiment, the semiconductor laser array 14, the GRIN lens array 
18, the aspherical lens 20, and the optical fiber 32 are the same as those 
of FIG. 6. Also the solid state laser 10 and the output mirror 12 are the 
same as those of FIG. 1. 
As described with reference to FIG. 6, the optical power from stripes 16 of 
the semiconductor laser array 14 is coupled with the optical fiber 32 with 
high efficiency. The output light 34 which is shaped by the optical fiber 
32 is collimated by a lens 36 and is converged by a lens 38 to end-pump 
the Nd:YAG solid state laser 10. 
As shown in FIG. 8, in the semiconductor laser array-pumped solid state 
laser system, 810 mW of Nd:YAG output power is obtained under pumping 
power of 2.27 W. 
FIG. 9 is a schematic diagram of a semiconductor laser array-end-pumped 
solid state laser system according to another embodiment of the present 
invention. 
In FIG. 9, semiconductor laser arrays 14.sub.1 to 14.sub.n, GRIN lens 
arrays 18.sub.1 to 18.sub.n, aspherical lenses 20.sub.1 to 20.sub.n and 
optical fibers 32.sub.1 to 32.sub.n are the same as the semiconductor 
laser array 14, the GRIN lens array 18, the aspherical lens 20 and the 
optical fiber 32 of FIG. 7. 
Other ends 40 not coupled with the semiconductor laser arrays 14.sub.1 to 
14.sub.n in the optical fibers 32.sub.1 to 32.sub.n are bundled and the 
two-dimensionally arranged lens bundles 42 are located so that the optical 
axis of each optical fiber coincides with optical axis of each lens. Light 
beams output from the ends 40 of the optical fibers 32.sub.1 to 32.sub.n 
are collimated in the lens bundle 42 and are converged in a lens 44 to 
thereby pump the solid state laser 10.