A laser-diode-pumped solid-state laser including a solid-state laser rod doped with a rare-earth material such as neodymium, a semiconductor laser for emitting a laser beam to pump said solid-state laser rod to oscillate a beam, and a resonator including a bulk single crystal of organic nonlinear optical material for converting the wavelength of the beam which is oscillated by said solid-state laser rod. The organic nonlinear optical material is preferably PRA, MNA, NPP, NPAN, MAP, m-NA, or the like. The semiconductor laser may comprise a single-transverse-mode, or a single-longitudinal-mode semiconductor laser.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to a laser-diode-pumped solid-state 
laser which comprises a solid-state laser rod pumped by a semiconductor 
laser (laser diode), and more particularly to a laser-diode-pumped 
solid-state laser which includes an optical wavelength converter device 
disposed in a resonator for converting the wavelength of a laser beam 
which is oscillated by a solid-state laser rod. 
2. Description of the Prior Art 
U.S. Pat. No. 4,656,635, for example, shows a laser-diode-pumped 
solid-state laser in which a solid-state laser rod doped with a rare-earth 
material such as neodymium is pumped by a semiconductor laser. In order to 
obtain a laser beam having a shorter wavelength, the laser-diode-pumped 
solid-state laser includes a bulk monocrystal of nonlinear optical 
material disposed in a resonator for converting the wavelength of a laser 
beam which is oscillated by the solid-state laser into the wavelength of a 
second harmonic or the like. 
As disclosed in the above publication, the nonlinear optical material used 
is an inorganic optical material such as KTP, LiNbO.sub.3, or the like. 
The inorganic optical material is however problematic in that the 
efficiency with which the wavelength is converted thereby is low, and 
hence the efficiency with which the energy is utilized is also low. If a 
highly intensive laser beam having a converted wavelength (i.e., a shorter 
wavelength) is desired, then an expensive semiconductor laser of a very 
high output power of such as 200 mW or more is required as the pumping 
source. If such a high-output-power semiconductor laser is employed, a 
large and expensive system for radiating the heat from and hence cooling 
the semiconductor laser is also needed, since a large amount of heat is 
radiated by the semiconductor laser. 
The wavelength conversion efficiency may be increased by using a large 
crystal which provides a long path for the laser beam, as the bulk single 
crystal of nonlinear optical material. However, it is technically 
difficult and highly costly to produce such a large crystal. 
An increased wavelength conversion efficiency may also be achieved by using 
a nonlinear optical material having a larger nonlinear optical constant. 
Inorganic optical materials having nonlinear optical constants which are 
larger than that of KTP include LiNbO.sub.3, BNNB, and KNbO.sub.3 which is 
disclosed in Optics Letters, Vol. 13, page 137 (1988), for example. These 
inorganic nonlinear optical materials, however, fail to provide a stable 
wavelength conversion efficiency over a wide temperature range because the 
phase matching angle of these materials tends to shift due to temperature 
change. 
If the efficiency with which the solid-state laser is oscillated by the 
semiconductor laser is high, then the intensity of the oscillating laser 
beam that is applied to the nonlinear optical material becomes high, 
resulting in a wavelength conversion beam of a high intensity. However, 
the conventional laser-diode-pumped solid-state laser general employs an 
array laser as the pumping source. Since the spectral line width of the 
array laser is as large as 10 nm, the efficiency with which the 
solid-state laser is oscillated is low and the energy utilization 
efficiency is also low. 
A single-transverse-mode, single-longitudinal-mode semiconductor laser as a 
semiconductor laser having a small spectral line width (which is normally 
as large as about 0.1 nm) is known. The oscillation efficiency of the 
solid-state laser can be increased by controlling the temperature of the 
single-transverse-mode, single-longitudinal-mode semiconductor laser with 
a Peltier device so that the oscillation wavelength of the laser will 
match the absorption peak value of the solid-state laser. However, the 
presently available single-transverse-mode, single-longitudinal-mode 
semiconductor laser has a lower output power than the array laser. In 
order to produce a wavelength conversion laser beam of a certain high 
intensity, the laser beams emitted by a plurality of 
single-transverse-mode, single-longitudinal-mode semiconductor lasers must 
be combined into a pumping laser beam. Such a system is costly to 
manufacture and low in reliability. 
SUMMARY OF THE INVENTION 
In view of the aforesaid drawbacks of the conventional laser-diode-pumped 
solid-state lasers, it is the object of the present invention to provide a 
laser-diode-pumped solid-state laser which has a high wavelength 
conversion efficiency, is highly stable with respect to temperature 
changes, and has a good energy utilization efficiency. 
According to the present invention, a laser-diode-pumped solid-state laser 
includes a solid-state laser rod doped with a rare-earth material such as 
neodymium or the like and pumped by a semiconductor laser, and a bulk 
single crystal of nonlinear optical material which is disposed in a 
resonator for converting the wavelength of a laser beam oscillated by the 
solid-state laser rod, the bulk single crystal comprising a bulk crystal 
of organic linear optical material. 
The organic nonlinear optical material may be MNA 
(2-methyl-4-nitroaniline), as disclosed in Japanese Unexamined Patent 
Publication No. 60(1985)-250334, NPP (N-(4-nitrophenyl)-L-prolinol), as 
disclosed in J. Opt. Soc. Am. B, NPAN 
(N-(4-nitrophenyl)-N-methylaminoacetonitrile), MAP, m-NA, or the like. 
These organic nonlinear optical materials can provide a high wavelength 
conversion efficiency since their nonlinear optical constants are very 
high as compared with inorganic nonlinear optical materials such as 
LiNbO.sub.3 and KTP. The organic nonlinear optical materials are also 
advantageous in that their damage thresholds are higher and their optical 
damage is smaller than the inorganic nonlinear optical materials. 
A nonlinear optical material (3,5-dimethyl-1-(4-nitrophenyl) pyrazole: 
hereinafter referred to as "PRA") represented by the following molecular 
diagram: 
##STR1## 
is preferably used as the organic nonlinear optical material. The PRA is 
disclosed in U.S. patent application No. 263,977, and is known to have a 
very large nonlinear optical constant. 
The various organic nonlinear optical materials referred to above, like 
KTP, have a phase matching angle which shifts or varies only very slightly 
in response to a temperature change. Since any change in the refractive 
index of an organic material is a reduction in the refractive index due 
primarily to a volumetric expansion, the dependency of the birefringence 
on temperature is near zero, and as a result, any shift of the phase 
matching angle is almost eliminated. 
Preferably, a single-transverse-mode, single-longitudinal-mode 
semiconductor laser is used as the pumping semiconductor laser of the 
laser-diode-pumped solid-state laser. Since the wavelength of a laser beam 
is converted by the organic nonlinear optical material which has a high 
wavelength conversion efficiency, as described above, a sufficiently 
intensive wavelength-converted beam can be produced even if the intensity 
of the laser beam which is oscillated by the solid-state laser rod and is 
applied as a fundamental wave to the nonlinear optical material is 
relatively low. Accordingly, the presently available 
single-transverse-mode, single-longitudinal-mode semiconductor laser, 
which can produce only a relatively low output power, is sufficiently 
effective to generate wavelength-converted beam having a sufficiently high 
intensity. When the single-transverse-mode, single-longitudinal-mode 
semiconductor laser which has a small spectral line width is employed, the 
oscillation efficiency of the solid-state laser rod is increased, and 
hence the energy utilization efficiency is also increased. 
The above and other objects, features and advantages of the present 
invention will become more apparent from the following description when 
taken in conjunction with the accompanying drawings in which preferred 
embodiments of the present invention are shown by way of illustrative 
example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIGS. 1 shows a laser-diode-pumped solid-state laser 10 according to a 
first embodiment of the present invention. The laser-diode-pumped 
solid-state laser 10 comprises a semiconductor laser (phased-array laser) 
12 for emitting a laser beam 11 as a pumping beam, a collimator lens 13 
for converting the laser beam 11 which is divergent into a parallel laser 
beam, a condenser lens 14 for focusing the parallel laser beam 11, a YAG 
rod 15 doped with neodymium (Nd) (hereinafter referred to as an "Nd:YAG 
rod 15"), a resonator 16 disposed forward (rightward in FIG. 1) of the 
Nd:YAG rod 15, and an optical wavelength converter device 17 disposed 
between the resonator 16 and the Nd:YAG rod 15. These elements 12 through 
17 are mounted as a unitary assembly in a common casing (not shown). 
The phased-array laser 12 is arranged to emit the laser beam 11 which has a 
wavelength of 809 nm. The Nd:YAG rod 15 has a rear end surface 15a which 
is partly spherical and positioned where the laser beam 11 is focused by 
the condenser lens 14. The Nd:YAG rod 15 emits a laser beam 18 having a 
wavelength of 1064 nm when the neodymium atoms are excited by the laser 
beam 11. The rear end surface 15a of the Nd:YAG rod 15 is coated with a 
layer which readily transmits the laser beam 11, but reflects the laser 
beam 18. The resonator 16 has a partly spherical surface 16a which faces 
the Nd:YAG rod 15 and is also coated with a layer that reflects the laser 
beam 18 but readily transmits therethrough a second harmonic 18' having a 
wave-length of 532 nm. The laser beam 18 is therefore confined between the 
surface 16a of the resonator 16 and the rear end surface 15a of the Nd:YAG 
rod 15, and causes laser oscillation. The laser beam 18 is applied to the 
optical wavelength converter device 17 by which the wavelength thereof is 
converted to 1/2. Therefore, the laser beam 18 which has a wavelength of 
1064 nm is converted by the optical wavelength converter device 17 into 
the laser beam 18' which has a wavelength of 532 nm. 
The optical wavelength converter device 17 will be described in detail 
below. The optical wavelength converter device 17 is made of a bulk single 
crystal of PRA. The crystal structures of the PRA along the c-, a-, and 
b-axes are illustrated in FIGS. 2A, 2B, and 2C, and the bulk crystal 
structure of the PRA is illustrated in FIG. 3. The optical wavelength 
converter device 17 may be fabricated by the usual Bridgeman process. More 
specifically, PRA in a molten state is poured into a suitable mold and 
quenched so that the PRA is polycrystallized. Thereafter, the PRA is 
gradually pulled from a furnace, which is kept at a temperature (e.g., 
102.5.degree. C.) higher than the melting point (102.degree. C.) of PRA, 
into an outer space which is kept at a temperature lower than that melting 
point, thereby causing the molten PRA to be single-crystallized 
continuously at the point where it is withdrawn from the furnace. The PRA 
single crystal thus obtained is of a highly long single-crystalline form, 
50 mm long or longer, and has a uniform crystal orientation. The optical 
wavelength converter device 17 is thereby made sufficiently long. Since, 
as is well known in the art, the wavelength conversion efficiency of an 
optical wavelength converter device of this type is proportional to the 
length of the device, the longer the optical wavelength converter device, 
the greater the practical value of the device. 
The PRA single crystal is then cut along a Y-Z plane containing the optical 
axis Y and the Z-axis (i.e., the crystal b- and a-axes), and cut to a 
thickness of 5 mm along the X-axis (i.e., the crystal c-axis), thereby 
producing the optical wavelength converter device 17 of bulk single 
crystal. 
As shown in FIG. 1, the laser beam 18 is applied to the optical wavelength 
converter device 17 along a direction normal to the Z-axis of the device 
17 and at an angle .theta. of 15.degree. in the crystal with respect to 
the X-axis, the angle .theta. being displaced from the X-axis toward the 
Y-axis. When the laser beam 18 is thus applied to the optical wavelength 
converter device 17, phase matching of type I is achieved between the 
laser beam 18 which is a fundamental wave and the second harmonic 18'. The 
optical wavelength converter device 17 emits a mixture of the laser beam 
18 and the second harmonic 18'. The phase matching will be described in 
detail later on. Since the surface of the resonator 16 is coated with the 
layer for reflecting the laser beam 18, only the second harmonic 18' which 
has the wavelength of 532 nm is extracted by the resonator 16. 
It has been confirmed that the phase matching condition in the optical 
wavelength converter device 17 of PRA is sufficiently stable in a 
temperature range exceeding .+-.30.degree.. The performance index of PRA 
is 100 times greater than the performance index of KTP. Therefore, the 
wavelength conversion efficiency of the optical wavelength converter 
device 17 is sufficiently high. For example, when the pumping laser beam 
11 had an output power of 100 mW, the output powers of the laser beam 18 
and the second harmonic 18' were 30 mW and about 20 mW, respectively, and 
when the pumping laser beam 11 has an output power of 200 mW, the output 
powers of the laser beam 18 and the second harmonic 18' were 60 mW and 
about 40 mW, respectively. 
As described above, PRA, which is an organic non-linear optical material, 
has a high damage threshold and suffers small optical damage. 
Consequently, it is possible to produce a laser-diode-pumped solid-state 
laser 10 capable of producing a high-output-power laser beam. 
Comparative examples will be described below. The optical wavelength 
converter device 17 shown in FIG. 1 was replaced with an optical converter 
device made of a bulk crystal of KTP, and the resultant solid-state laser 
was tested for the generation of a second harmonic. When the pumping laser 
beam 11 had an output power of 100 mW, the output powers of the laser beam 
18 and the second harmonic 18' were 30 mW and about 2.5 mW, respectively, 
and when the pumping laser beam 11 had an output power of 200 mW, the 
output powers of the laser beam 18 and the second harmonic 18' were 60 mW 
and about 10 mW, respectively. The output powers of about 20 mW and 40 mW 
of the second harmonic 18', produced by the inventive solid-state laser, 
are much higher than the output values of about 2.5 mW and 10 mW of the 
second harmonic 18', produced by the comparative solid-state laser. The 
results of the comparison indicate that the optical wavelength converter 
device 17 according to the present invention has a high wavelength 
conversion efficiency. 
The phase matching between the laser beam 18 and the second harmonic 18' 
will hereinafter be described in detail. The PRA crystal is of an 
orthorhombic system, and its point group is mm2. Therefore, the tensor of 
the nonlinear optical constants is as follows: 
##EQU1## 
If it is assumed that optical axes X, Y, Z are determined with respect to 
crystal axes a, b, c as shown in FIG. 3, then d.sub.31 is a nonlinear 
optical constant at the time a second harmonic, which is Z-polarized, is 
extracted, when light which is linearly polarized in the direction X 
(X-polarized light) is applied as a fundamental wave. Similarly, d.sub.32 
is a nonlinear optical constant at the time a second harmonic, which is 
Z-polarized, is extracted, when light, which is linearly polarized in the 
direction Y (Y-polarized light), is applied as a fundamental wave. 
Likewise, d.sub.33 is a nonlinear optical constant at the time a second 
harmonic which is Z-polarized is extracted, when light which is linearly 
polarized in the direction Z (Z-polarized light) is applied as a 
fundamental wave. d.sub.24 is a nonlinear optical constant at the time a 
Y-polarized second harmonic is extracted, when Y- and Z-polarized 
fundamental waves are applied. d.sub.15 is a nonlinear optical constant at 
the time an X-polarized wavelength-converted wave is extracted, when X- 
and Z-polarized fundamental waves are applied. The magnitudes of the 
respective nonlinear optical constants are given in the following table: 
______________________________________ 
(1) (2) 
______________________________________ 
d.sub.31 26 -- 
d.sub.32 160 240 .+-. 140 
d.sub.33 67 70 .+-. 10 
d.sub.15 26 -- 
d.sub.24 160 -- 
______________________________________ 
The values in column (1) are obtained from an X-ray crystal structure 
analysis and the values in column (2) are measured by the Marker Fringe 
process (both columns give values with respect to a fundamental wave 
having a wavelength of 1.064 .mu.m, and the units are [.times.10.sup.-9 
esu] in both columns. 
The optical linear constant d.sub.32 of PRA is 260 times larger than the 
optical linear constant d.sub.31 of LiNbO.sub.3, and about 100 times 
larger than an effective nonlinear constant d.sub.eff of KTP. 
Since PRA is of an orthorhombic system and also a biaxial crystal, it has a 
refractive index n.sub.x when the plane of polarization extends along the 
optical X-axis (i.e., the crystal c-axis), a refractive index n.sub.y when 
the plane of polarization extends along the optical Y-axis (i.e., the 
crystal b-axis) normal to the X-axis, and a refractive index n.sub.z when 
the plane of polarization extends along the optical Z-axis (the crystal 
a-axis) normal to the X- and Y-axes. The wavelength-dependent dispersion 
of these refractive indexes n.sub.x, n.sub.y, n.sub.z is shown in FIG. 4. 
The lower limit wavelength for achieving the angular phase matching at 
normal temperature is 950 nm. However, this lower limit wavelength varies 
by about 50 nm when the temperature of the bulk crystal is varied. With 
PRA used in the laser-diode-pumped solid-state laser according to the 
present invention, the lower limit wavelength value for a laser beam which 
is oscillated by the solid-state laser rod and applied as a fundamental 
wave to the optical wavelength converter device is 900 nm. If the 
wavelength of the fundamental wave ever exceeded 4000 nm, then it would 
reach the vibration level of PRA molecules, and the fundamental wave would 
be absorbed by the PRA molecules. According to the present invention, 
therefore, the upper limit wavelength value for the fundamental wave is 
4000 nm. 
The phase matching process for a biaxial crystal is described in detail in 
an article written by Yao et al., page 65, J. Appl. Phys. Vol. 55 (1984). 
More specifically, it is assumed as shown in FIG. 9 that an angle .phi. is 
formed between the direction in which a fundamental wave travels in the 
crystal and the optical Z-axis of the crystal, and an angle .theta. is 
formed between the optical X-axis and the direction of the fundamental in 
the crystal in a plane containing the optical X- and Y-axes, and assumed 
that the crystal has refractive indexes n.sup.w, n.sup.2w with respect to 
the fundamental wave applied at a desired angle and the second harmonic, 
and refractive indexes n.sub.x.sup.w, n.sub.y.sup.w, n.sub.z.sup.w, 
n.sub.x.sup.2w, n.sub.y.sup.2w, and n.sub.z.sup.2w with respect to the 
fundamental and the second harmonics along the respective axes. If 
k.sub.x =sin.phi..cos.theta. 
k.sub.y =sin.phi..sin.theta. 
k.sub.z =cos.phi., 
then the solutions to the following equations (1-1), (1-2) give the phase 
matching condition: 
##EQU2## 
If 
B.sub.1 =-k.sub.x.sup.2 (b.sub.1 +c.sub.1)-k.sub.y.sup.2 (a.sub.1 
+c.sub.1)-k.sub.z.sup.2 (a.sub.1 +b.sub.1) 
C.sub.1 =k.sub.x.sup.2 b.sub.1 c.sub.1 +k.sub.y.sup.2 a.sub.1 c.sub.1 
+k.sub.z.sup.2 a.sub.1 b.sub.1 
B.sub.2 =-k.sub.x.sup.2 (b.sub.2 +c.sub.2)-k.sub.y.sup.2 (a.sub.2 
+c.sub.2)-k.sub.z.sup.2 (a.sub.2 +b.sub.2) 
C.sub.2 =k.sub.x.sup.2 b.sub.2 c.sub.2 +k.sub.y.sup.2 a.sub.2 c.sub.2 
+k.sub.z.sup.2 a.sub.2 b.sub.2 
a.sub.1 =(n.sub.x.sup.w).sup.-2 a.sub.2 =(n.sub.x.sup.2w).sup.-2 
b.sub.1 =(n.sub.y.sup.w).sup.-2 b.sub.2 =(n.sub.y.sup.2w).sup.-2 
c.sub.1 =(n.sub.z.sup.w).sup.-2 c.sub.2 =(n.sub.z.sup.2w).sup.-2, 
then the solutions to the equations (1-1), (1-2) are given as follows: 
##EQU3## 
(the .+-. sign becomes + when i=1, and - when i=2) 
The phase matching condition of the type I is 
EQU n.sup.w, .sub.2 =n.sup.2w, .sub.1 (1-3) 
The phase matching condition of the type II is 
EQU 1/2(n.sup.w, .sub.1 +n.sup.w, .sub.2)=n.sup.2w, .sub.1 
When there exist angles .phi., .theta. which satisfy the equation (1-3), 
the phase matching condition of the type I can be achieved. 
To simplify the explanation, it is assumed that .phi.=90.degree.. As shown 
in FIG. 4, the refractive indexes of the PRA always have the following 
relationship in the wavelength range from 900 to 4000 nm: 
EQU n.sub.x &lt;n.sub.z &lt;n.sub.y 
If the plane of polarization is inclined in an intermediate direction 
between the X- and Y-axes and an intermediate refractive index 
n.sub.xy.sup.w between the refractive indexes n.sub.x.sup.w, n.sub.y.sup.w 
with respect to the fundamental wave applied is equalized to the 
refractive index n.sub.z with respect to the second harmonic, then the 
angular phase matching can be achieved. That is, the following equation 
should be met: 
EQU n.sub.z.sup.2w =n.sub.xy.sup.w (1) 
Let the angle formed between the direction of travel of the fundamental 
wave in the X-Y plane and the optical X-axis be represented by .theta. 
(the angle being displaced from the X-axis toward the Y-axis). Then, the 
following equation is met: 
##EQU4## 
In FIG. 4, the refractive indexes n.sub.x, n.sub.y when the wavelength of 
the fundamental wave is 950 nm are 1.521 and 1.775, respectively, and the 
refractive index n.sub.z at the 1/2 wavelength of 475 nm is 1.775. Using 
these values for the refractive indexes, the angle .theta. which meets the 
above equations (1) and (2) is about 0.degree.. The relationship between 
the refractive indexes n.sub.x.sup.w, n.sub.y.sup.2, n.sub.xy.sup.w, and 
n.sub.z.sup.2w in this case is illustrated in FIG. 5. 
In FIG. 4, the refractive indexes n.sub.x, n.sub.y when the wavelength of 
the fundamental wave is 1200 nm are 1.519 and 1.767, respectively, and the 
refractive index n.sub.z at the wavelength of 600 nm is 1.725. Using these 
values for the refractive indexes, the angle .theta. which meets the above 
equations (1) and (2) is about 22.degree.. The refractive indexes remain 
substantially unchanged when the wavelengths of the fundamental wave are 
1200 nm and 4000 nm. Therefore, the angle .theta. which meets the 
equations (1) and (2) exists at all times. The relationship between the 
refractive indexes n.sub.x.sup.w, n.sub.y.sup.w, n.sub.xy.sup.w, and 
n.sub.z.sup.2w in this case is illustrated in FIG. 6. The value of the 
angle .theta. varies by about 3.degree. depending on the temperature of 
the PRA crystal. 
Considering the fact that the lower limit wavelength for the angular phase 
matching varies by 50 nm depending on the temperature of the PRA crystal, 
the angle .theta. which can achieve the phase matching of type I exists at 
all times in the fundamental wavelength range from 900 to 4000 nm. 
Likewise, even at a certain angle .phi. other than 90.degree., there 
exists a combination of angles .theta., .phi. which achieves the phase 
matching of the type I in the fundamental wavelength range from 900 to 
4000 nm. Therefore, it is possible to produce a second harmonic of a 
fundamental wave having such a wavelength range. 
The angle .theta.=15.degree. referred to above is determined based on the 
equations (1) and (2) and the wavelength-dependent dispersion of the 
refractive indexes n.sub.x, n.sub.y, n.sub.z shown in FIG. 4. The 
relationship between the refractive indexes n.sub.x.sup.w, n.sub.y.sup.w, 
n.sub.xy.sup.w, and n.sub.z.sup.2w in this case is illustrated in FIG. 7. 
FIG. 8 shows a transmission spectrum of a thin film of PRA which has a 
thickness of 200 .mu.m. As shown, the PRA does not absorb much light near 
the wavelength of 400 nm. Therefore, an optical wavelength converter 
device made of a bulk crystal of the PRA can efficiently generate a second 
harmonic in a blue range. 
FIG. 11 illustrates a laser-diode-pumped solid-state laser 20 according to 
a second embodiment of the present invention. Those components shown in 
FIG. 11 which are identical to those shown in FIG. 1 are denoted by 
identical reference numerals and will not be described in detail. The 
laser-diode-pumped solid-state laser 20 differs from the 
laser-diode-pumped solid-state laser 10 shown in FIG. 1 in that a 
single-transverse-mode, single-longitudinal-mode semiconductor laser 
(single stripe laser) 22 is employed as a pumping source in place of the 
phased-array laser 12. The single-transverse-mode, 
single-longitudinal-mode semiconductor laser 22 is mounted on a Peltier 
device 26 to which a heat sink 25 is fixed. The Peltier device 26 is 
driven by a temperature control circuit 28. The temperature of the 
single-transverse-mode, single-longitudinal-mode semiconductor laser 22 is 
detected by a temperature sensor 27 which feeds a temperature signal back 
to the temperature control circuit 28. The temperature control circuit 28 
controls the operation of the Peltier device 26 depending on the 
temperature indicated by the temperature signal so that the temperature of 
the single-transverse-mode, single-longitudinal-mode semiconductor laser 
22 will be kept exactly at a predetermined level. A power supply 29 is 
connected to the temperature control circuit 28. A laser beam 21 emitted 
by the single-transverse-mode, single-longitudinal-mode semiconductor 
laser 22 has a sufficiently small spectral line width of, for example, 
about 1 nm. With the temperature of the single-transverse-mode, 
single-longitudinal-mode semiconductor laser 22 being controlled as 
described above, the wavelength of the laser beam emitted thereby can be 
equalized precisely to the absorption peak wavelength (809 nm) of the 
Nd:YAG rod 15. In this manner, the oscillation efficiency of the YAG laser 
which emits the laser beam 18 is increased. 
According to the laser-diode-pumped solid-state laser 20, when a laser beam 
21 emitted by the laser 22 had an output power of 30 mW, the output powers 
of the laser beam 18 and the second harmonic 18' were 12 mW and about 4 
mW, respectively, and when the laser beam 21 had an output power of 50 mW, 
the output powers of the laser beam 18 and the second harmonic 18' were 20 
mW and about 10 mW, respectively. When the laser beam 21 had an output 
power of 100 mW, the output powers of the laser beam 18 and the second 
harmonic 18' were 40 mW and about 30 mW, respectively. The oscillation 
efficiency of the YAG laser is 40%, a value which is higher than the 
oscillation efficiency of the YAG laser in the first embodiment (30%). 
In the above embodiments, the optical wavelength converter device is made 
of a bulk single crystal of PRA. However, the optical wavelength converter 
device may be made of a bulk single crystal of MNA, NPP, MAP, or the like. 
In cases where a single-transverse-mode, single-longitudinal-mode 
semiconductor laser is employed as a pumping source, it may be a DFB 
laser, a laser with an external resonator, or the like, rather than the 
single stripe laser 22. 
With the present invention, the laser-diode-pumped solid-state laser 
includes an optical wavelength converter device for converting the 
wavelength of a laser beam which is oscillated by a solid-state laser rod, 
the optical wavelength converter device being made of a bulk single 
crystal of organic nonlinear optical material. The optical wavelength 
converter device has a high wavelength conversion efficiency and produces 
a highly intensive laser beam of a short wavelength. The optical 
wavelength converter device is also stable against temperature changes. 
Since the wavelength conversion efficiency of the optical wavelength 
converter device is high, the laser-diode-pumped solid-state laser can 
emit a highly intensive laser beam of a short wavelength even if it 
employs as a pumping source a single-transverse-mode, 
single-longitudinal-mode semiconductor laser which has a relatively low 
output power. If such a single-transverse-mode, single-longitudinal-mode 
semiconductor laser is used as a pumping source, then the oscillation 
efficiency of the solid-state laser rod is high, and hence the energy 
utilization efficiency is also high. 
Inasmuch as a sufficiently high intensive laser beam of a short wavelength 
can be produced even if a semiconductor laser of a relatively low output 
power is used as a pumping source, the laser-diode-pumped solid-state 
laser of the present invention can use a less costly semiconductor laser 
of lower output power and can be manufactured more inexpensively than the 
conventional laser-diode-pumped solid-state laser, provided a 
wavelength-converted beam of the same intensity is to be generated. 
Although certain preferred embodiments have been shown and described, it 
should be understood that many changes and modifications may be made 
therein without departing from the scope of the appended claims.