Optical wavelength converting apparatus

An optical wavelength converting apparatus comprises an optical wavelength converting device, which is constituted of a crystal of a nonlinear optical material and converts a wavelength of an incident fundamental wave into a different wavelength, a holder, which is constituted of a metal and to which the optical wavelength converting device is adhered and secured, and a device for adjusting the temperature of the region containing the optical wavelength converting device. The optical wavelength converting device is secured to the holder by adhering only the surface of the optical wavelength converting device to the holder, which surface is normal to the direction of a crystallographic axis that has a coefficient of thermal expansion most different from the coefficient of thermal expansion of the metal constituting the holder among the coefficients of thermal expansion of the crystallographic axes of the optical wavelength converting device. The optical wavelength converting device is thus reliably secured to the holder and prevented from breaking or being distorted due to a change in environmental temperature, and the accuracy, with which the temperature of the optical wavelength converting device is adjusted, is kept high.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an optical wavelength converting apparatus 
provided with an optical wavelength converting device for converting a 
fundamental wave into its second harmonic, or the like. This invention 
particularly relates to an optical wavelength converting apparatus, which 
is provided with an improved structure for mounting the optical wavelength 
converting device on a holder. 
2. Description of the Prior Art 
Various attempts have heretofore been made to convert the fundamental wave 
of a laser beam into its second harmonic, or the like, e.g. to shorten the 
wavelength of a laser beam, by using a crystal of a nonlinear optical 
material. As optical wavelength converting devices for carrying out such 
wavelength conversion, there have heretofore been known a bulk crystal 
type of optical wavelength converting device, an optical waveguide type of 
optical wavelength converting device, and the like. 
By way of example, the optical wavelength converting device of this type is 
combined with a laser diode pumped solid laser or is employed in a 
structure comprising a semiconductor laser, which serves as a fundamental 
wave source, and an external resonator. In such cases, the optical 
wavelength converting device is ordinarily located in the region inside of 
the resonator. In general, the temperature of such a resonator is very 
accurately adjusted such that the resonator length may be kept constant. 
In such cases, the temperature of the optical wavelength converting 
device, which is located in the region inside of the resonator, is also 
adjusted. 
The optical wavelength converting device described above is mounted on a 
holder and is secured at a predetermined position such that its optical 
axis may align with the optical axis of a laser, which serves as a 
fundamental wave source, a condensing lens, or the like. As the structure 
for mounting the optical wavelength converting device on the holder, two 
types of structures have heretofore been known. In one of the structures, 
the optical wavelength converting device is mounted on the holder by using 
a crystal pushing member, which is constituted of a metal, a plastic 
material, or the like. In the other structure, the optical wavelength 
converting device is adhered to the holder. 
However, with the structure in which the crystal pushing member is 
utilized, the problems occur in that the optical wavelength converting 
device moves slightly due to a change in the environmental temperature, or 
the like, and cannot be secured at the correct position for a long period. 
In cases where the optical wavelength converting device is located in the 
region inside of the resonator, if the optical wavelength converting 
device thus moves to an incorrect position, the resonating conditions of 
the resonator will vary. As a result, the problems occur in that the 
intensity and the beam shape of the wavelength-converted wave fluctuate, 
and noise occurs. 
With the structure in which the optical wavelength converting device is 
adhered to the holder, in cases where the holder is constituted of a 
metal, such as copper, the problems described below are encountered. 
Specifically, when a change in the environmental temperature occurs, a 
large stress is generated in the optical wavelength converting device due 
to a difference in the coefficient of thermal expansion between the 
optical wavelength converting device and the holder constituted of the 
metal. As a result, the optical wavelength converting device breaks or is 
distorted. A technique for adhering and securing a optical wavelength 
converting device to a metal material having a coefficient of thermal 
expansion close to the coefficient of thermal expansion of the optical 
wavelength converting device is disclosed in, for example, U.S. Pat. No. 
5,150,376. However, even if the disclosed technique is applied to a 
structure for securing the optical wavelength converting device to the 
holder described above, satisfactory results cannot always be obtained. 
Therefore, an attempt has been made to make the holder described above from 
glass, which has a coefficient of thermal expansion approximately equal to 
the coefficient of thermal expansion of the optical wavelength converting 
device. However, such a holder made from the material other than a metal 
has a low thermal conductivity. Therefore, in cases where the holder made 
from the material other than a metal is located in the region inside of 
the resonator, the accuracy, with which the temperature of the optical 
wavelength converting device is adjusted, cannot be kept high. As a 
result, the performance of the optical wavelength converting apparatus 
fluctuates. 
SUMMARY OF THE INVENTION 
The primary object of the present invention is to provide an optical 
wavelength converting apparatus, wherein an optical wavelength converting 
device is reliably secured to a holder and prevented from breaking or 
being distorted due to a change in environmental temperature, and the 
accuracy, with which the temperature of the optical wavelength converting 
device is adjusted, is kept high. 
The present invention provides an optical wavelength converting apparatus 
comprising: 
i) an optical wavelength converting device, which is constituted of a 
crystal of a nonlinear optical material, and which converts a wavelength 
of an incident fundamental wave into a different wavelength, 
ii) a holder, to which the optical wavelength converting device is adhered 
and secured, and 
iii) a means for adjusting the temperature of the region containing the 
optical wavelength converting device, 
wherein the holder is constituted of a metal, and 
the optical wavelength converting device is adhered and secured to the 
holder by adhering only the surface of the optical wavelength converting 
device to the holder, which surface is normal to the direction of a 
crystallographic axis that has a coefficient of thermal expansion most 
different from the coefficient of thermal expansion of the metal 
constituting the holder among the coefficients of thermal expansion of the 
crystallographic axes of the optical wavelength converting device. 
In many cases, the coefficient of thermal expansion of a nonlinear optical 
material varies for different crystallographic axis of the nonlinear 
optical material. As described above, when the technique disclosed in U.S. 
Pat. No. 5,150,376 is applied to the structure for securing an optical 
wavelength converting device to a holder, satisfactory results cannot 
always be obtained. This is because the anisotropy of the coefficient of 
thermal expansion is not taken into consideration. Specifically, in cases 
where a material, which is considered as having a coefficient of thermal 
expansion close to the coefficient of thermal expansion of the optical 
wavelength converting device, is selected as the material for the holder, 
if the anisotropy of the coefficient of thermal expansion is not taken 
into consideration, it will often occur that the coefficient of thermal 
expansion of the optical wavelength converting device and the coefficient 
of thermal expansion of the holder are far different from each other with 
respect to the directions, which are included in the plane of adhesion 
between the optical wavelength converting device and the holder. In such 
cases, if a change in the environmental temperature occurs, a large stress 
will be generated in the optical wavelength converting device due to a 
difference in the coefficient of thermal expansion between the optical 
wavelength converting device and the holder. As a result, the optical 
wavelength converting device will break or will be distorted. 
With the optical wavelength converting apparatus in accordance with the 
present invention, the optical wavelength converting device is adhered and 
secured to the holder by adhering only the surface of the optical 
wavelength converting device to the holder, which surface is normal to the 
direction of the crystallographic axis that has a coefficient of thermal 
expansion most different from the coefficient of thermal expansion of the 
metal constituting the holder among the coefficients of thermal expansion 
of the crystallographic axes of the optical wavelength converting device. 
Therefore, the problems do not occur in that the coefficient of thermal 
expansion of the optical wavelength converting device and the coefficient 
of thermal expansion of the holder are far different from each other with 
respect to the directions, which are included in the plane of adhesion 
between the optical wavelength converting device and the holder. 
Accordingly, even if a change in the environmental temperature occurs, a 
large stress will not be generated in the optical wavelength converting 
device due to a difference in the coefficient of thermal expansion between 
the optical wavelength converting device and the holder. As a result, the 
optical wavelength converting device can be prevented from breaking or 
being distorted. 
Also, with the optical wavelength converting apparatus in accordance with 
the present invention, the holder is constituted of a metal. Therefore, 
heat conduction can be effected sufficiently between the optical 
wavelength converting device and the means for adjusting the temperature 
of the region containing the optical wavelength converting device. 
Accordingly, the accuracy, with which the temperature of the optical 
wavelength converting device is adjusted, can be kept high.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will hereinbelow be described in further detail with 
reference to the accompanying drawings. 
FIG. 2 is a schematic side view showing a first embodiment of the optical 
wavelength converting apparatus in accordance with the present invention. 
With reference to FIG. 2, the first embodiment of the optical wavelength 
converting apparatus in accordance with the present invention is built in 
a laser diode pumped solid laser. The laser diode pumped solid laser is 
provided with a semiconductor laser 11, which produces a laser beam 10 
serving as a pumping beam, and a condensing lens 12, which condenses the 
laser beam 10 having been produced as divergent light and which may be 
constituted of a rod lens, or the like. The laser diode pumped solid laser 
is also provided with a YVO.sub.4 crystal 13, which is a solid laser 
medium doped with neodymium (Nd). The YVO.sub.4 crystal 13 doped with 
neodymium (Nd) will hereinafter be referred to as the Nd:YVO.sub.4 crystal 
13. The laser diode pumped solid laser is further provided with a 
resonator mirror 14, which is located on the side downstream (i.e. the 
right side in FIG. 2) from the Nd:YVO.sub.4 crystal 13. A crystal 15 of 
KTP, which serves as a nonlinear optical material, is located between the 
Nd:YVO.sub.4 crystal 13 and the resonator mirror 14. 
The Nd:YVO.sub.4 crystal 13, the resonator mirror 14, and the KTP crystal 
15 are respectively secured to a barrel 19 by approximately disk-like 
holders 16, 17, and 18, which are constituted of copper having a high 
thermal conductivity. The barrel 19 is mounted on a Peltier device 20. The 
temperatures of the Nd:YVO.sub.4 crystal 13, the resonator mirror 14, and 
the KTP crystal 15 are kept at a predetermined temperature by the Peltier 
device 20 and a temperature adjusting circuit (not shown). 
The semiconductor laser 11 produces the laser beam 10 having a wavelength 
of .lambda..sub.1 =809 nm. The neodymium atoms contained in the 
Nd:YVO.sub.4 crystal 13 are stimulated by the laser beam 10, and the 
Nd:YVO.sub.4 crystal 13 thereby produces a solid laser beam 21 having a 
wavelength of .lambda..sub.2 =1,064 nm. The solid laser beam 21 impinges 
upon the KTP crystal 15 and is thereby converted into its green second 
harmonic 22 having a wavelength of .lambda..sub.3 =.lambda..sub.2 /2=532 
nm. 
A rear end face 13a and a front end face 13b of the Nd:YVO.sub.4 crystal 
13, a rear end face 15a and a front end face 15b of the KTP crystal 15, 
and a concave mirror surface 14a of the resonator mirror 14 are provided 
with coatings, which have the characteristics shown below with respect to 
the wavelength of .lambda..sub.1 =809 nm, the wavelength of .lambda..sub.2 
=1,064 nm, and the wavelength of .lambda..sub.3 =532 nm. AR represents no 
reflection (a transmittance of at least 99%), and HR represents high 
reflection (a reflectivity of at least 99.9%). 
______________________________________ 
809 nm 1,064 nm 532 nm 
______________________________________ 
End face 13a AR HR -- 
End face 13b -- AR HR 
End face 15a -- AR AR 
End face 15b -- AR AR 
Mirror surface 14 
-- HR AR 
______________________________________ 
Because the coatings described above are provided, the laser beam 21 
resonates between the end face 13a of the Nd:YVO.sub.4 crystal 13 and the 
mirror surface 14a. The laser beam 21 impinges in the resonating state 
upon the KTP crystal 15. Therefore, the laser beam 21 is sufficiently 
absorbed by the KTP crystal 15, and the second harmonic 22 can be produced 
efficiently. The second harmonic 22 passes through the resonator mirror 14 
directly or after being reflected from the end face 13b of the 
Nd:YVO.sub.4 crystal 13 towards the resonator mirror 14. 
FIG. 3 is an explanatory view showing the relationship between 
crystallographic axes a, b, and c of the KTP crystal 15, which has been 
cut into a rectangular parallelepiped shape, and the direction of 
incidence of the solid laser beam 21 serving as the fundamental wave in 
the first embodiment of the optical wavelength converting apparatus in 
accordance with the present invention. The coefficient of thermal 
expansion .alpha. of copper, which is employed as the material of the 
holder 18 for securing the KTP crystal 15, the coefficient of thermal 
expansion .alpha..sub.a of the KTP crystal 15 in the direction of the 
crystallographic axis a, the coefficient of thermal expansion 
.alpha..sub.b of the KTP crystal 15 in the direction of the 
crystallographic axis b, and the coefficient of thermal expansion 
.alpha..sub.c of the KTP crystal 15 in the direction of the 
crystallographic axis c take values shown below (units:.times.10.sup.-6 
/.degree.C.). 
EQU .alpha.=16.7 
EQU .alpha..sub.a =8.7, .alpha..sub.b =10.5, .alpha..sub.c =-0.2 
As illustrated in detail in FIG. 1, the holder 18 is made from an 
approximately disk-like member and provided with a cutaway portion 18a, 
through which the crystal is to be inserted, and crystal support surfaces 
18b and 18c, which make an angle of 90.degree. with respect to each other. 
The KTP crystal 15 is secured to the holder 18 by adhering only an end 
face 15c of the KTP crystal 15 to the crystal support surface 18c of the 
holder 18. The end face 15c is normal to the direction of the 
crystallographic axis c that has the coefficient of thermal expansion 
.alpha..sub.c most different from the coefficient of thermal expansion 
.alpha. of copper among the coefficients of thermal expansion of the 
crystallographic axes a, b, and c of the KTP crystal 15. An end face 15d 
of the KTP crystal 15 is merely in close contact with the other crystal 
support surface 18b of the holder 18. By way of example, the adhesion is 
effected by using a known resin. An adhesive layer 25 is formed between 
the end face 15c of the KTP crystal 15 and the crystal support surface 18c 
of the holder 18. 
With the embodiment wherein the KTP crystal 15 is adhered to the holder 18 
in the manner described above, the problems do not occur in that the 
coefficient of thermal expansion of the KTP crystal 15 and the coefficient 
of thermal expansion of the holder 18 are far different from each other 
with respect to the directions, which are included in the plane of 
adhesion between the KTP crystal 15 and the holder 18. Specifically, the 
coefficient of thermal expansion of the KTP crystal 15 with respect to an 
arbitrary direction, which is included in the plane of adhesion between 
the KTP crystal 15 and the holder 18, does not contain the component of 
the coefficient of thermal expansion .alpha..sub.c, and takes a value 
falling within the range of .alpha..sub.a to .alpha..sub.b. Therefore, the 
coefficient of thermal expansion of the KTP crystal 15 with respect to an 
arbitrary direction, which is included in the plane of adhesion between 
the KTP crystal 15 and the holder 18, takes a value close to the 
coefficient of thermal expansion .alpha. of copper. Accordingly, even if a 
change in the environmental temperature occurs, a large stress will not be 
generated in the KTP crystal 15 due to a difference in the coefficient of 
thermal expansion between the KTP crystal 15 and the holder 18. As a 
result, the KTP crystal 15 can be prevented from breaking or being 
distorted. 
Also, with this embodiment, the holder 18 is constituted of copper having a 
high thermal conductivity. Therefore, heat conduction can be effected 
sufficiently between the KTP crystal 15 and the Peltier device 20 for 
adjusting the temperature of the region containing the KTP crystal 15. 
Accordingly, the accuracy, with which the temperature of the KTP crystal 
15 is adjusted, can be kept high. 
In experiments for confirming the effects of the first embodiment of the 
optical wavelength converting apparatus in accordance with the present 
invention, the holder 18, to which the KTP crystal 15 had been adhered and 
secured in the manner described above, was subjected to a storage 
temperature cycle, in which the temperature was changed within the range 
of -20.degree. C. to +60.degree. C. Thereafter, the state of the KTP 
crystal 15 was investigated. In the experiments, no sign of cracking and 
distortion of the KTP crystal 15 was observed. 
Also, in Comparative Example 1, as illustrated in FIG. 4, a sample was 
prepared by adhering the two end faces 15c and 15d of the KTP crystal 15 
to the holder 18. The sample was subjected to the same storage temperature 
cycle as that described above. After the KTP crystal 15 had been subjected 
to the storage temperature cycle, cracks and distortion occurred in the 
KTP crystal 15. 
A second embodiment of the optical wavelength converting apparatus in 
accordance with the present invention will be described hereinbelow with 
reference to FIGS. 5, 6, and 7. In FIGS. 5, 6, and 7, similar elements are 
numbered with the same reference numerals with respect to FIGS. 1, 2, and 
3. As illustrated in FIG. 5, the second embodiment is substantially 
different from the first embodiment in that a Nd:YAG crystal 33 is 
employed in lieu of the Nd:YVO.sub.4 crystal 13, and in that a KN crystal 
35 is employed in lieu of the KTP crystal 15. 
The Nd:YAG crystal 33 is a solid laser medium doped with neodymium (Nd). 
The neodymium atoms contained in the Nd:YAG crystal 33 are stimulated by 
the laser beam 10, which has been produced by the semiconductor laser 11 
and has a wavelength of .lambda..sub.1 =808 nm, and the Nd:YAG crystal 33 
thereby produces a solid laser beam 31 having a wavelength of 
.lambda..sub.2 =946 nm. The solid laser beam 31 impinges upon the KN 
crystal 35, which serves as a nonlinear optical material, and is thereby 
converted into its blue second harmonic 32 having a wavelength of 
.lambda..sub.3 =.lambda..sub.2 /2=473 nm. 
FIG. 6 is an explanatory view showing the relationship between 
crystallographic axes a, b, and c of the KN crystal 35, which has been cut 
into a rectangular parallelepiped shape, and the direction of incidence of 
the solid laser beam 31 serving as the fundamental wave in the second 
embodiment of the optical wavelength converting apparatus in accordance 
with the present invention. The coefficient of thermal expansion .alpha. 
of copper, which is employed as the material of the holder 18 for securing 
the KN crystal 35, the coefficient of thermal expansion .alpha..sub.a of 
the KN crystal 35 in the direction of the crystallographic axis a, the 
coefficient of thermal expansion .alpha..sub.b of the KN crystal 35 in the 
direction of the crystallographic axis b, and the coefficient of thermal 
expansion .alpha..sub.c of the KN crystal 35 in the direction of the 
crystallographic axis c take values shown below (units:.times.10.sup.-6 
/.degree.C.). 
EQU .alpha.=16.7 
EQU .alpha..sub.a =5.0, .alpha..sub.b =14.0, .alpha..sub.c =0.5 
In the second embodiment, the KN crystal 35 is secured to the holder 18 by 
adhering only an end face 35c of the KN crystal 35 to the crystal support 
surface 18c of the holder 18. The end face 35c is normal to the direction 
of the crystallographic axis c that has the coefficient of thermal 
expansion .alpha..sub.c most different from the coefficient of thermal 
expansion .alpha. of copper among the coefficients of thermal expansion of 
the crystallographic axes a, b, and c of the KN crystal 35. Therefore, the 
problems do not occur in that the coefficient of thermal expansion of the 
KN crystal 35 and the coefficient of thermal expansion of the holder 18 
are far different from each other with respect to the directions, which 
are included in the plane of adhesion between the KN crystal 35 and the 
holder 18. Accordingly, even if a change in the environmental temperature 
occurs, a large stress will not be generated in the KN crystal 35 due to a 
difference in the coefficient of thermal expansion between the KN crystal 
35 and the holder 18. As a result, the KN crystal 3 can be prevented from 
breaking or being distorted. 
Also, with the second embodiment, the holder 18 is constituted of copper 
having a high thermal conductivity. Therefore, heat conduction can be 
effected sufficiently between the KN crystal 35 and the Peltier device 20 
for adjusting the temperature of the region containing the KN crystal 35. 
Accordingly, the accuracy, with which the temperature of the KN crystal 35 
is adjusted, can be kept high. 
In experiments for confirming the effects of the second embodiment of the 
optical wavelength converting apparatus in accordance with the present 
invention, the holder 18, to which the KN crystal 35 had been adhered and 
secured in the manner described above, was subjected to a storage 
temperature cycle, in which the temperature was changed within the range 
of -20.degree. C. to +60.degree. C. Thereafter, the state of the KN 
crystal 35 was investigated. In the experiments, no sign of cracking and 
distortion of the KN crystal 35 was observed. 
Also, in Comparative Example 2, as illustrated in FIG. 8, a sample was 
prepared by adhering the two end faces 35c and 35d of the KN crystal 35 to 
the holder 18. The sample was subjected to the same storage temperature 
cycle as that described above. After the KN crystal 35 had been subjected 
to the storage temperature cycle, cracks and distortion occurred in the KN 
crystal 35. 
In the embodiments described above, the optical wavelength converting 
apparatus in accordance with the present invention is applied to the laser 
diode pumped solid laser for converting the fundamental wave into its 
second harmonic. The optical wavelength converting apparatus in accordance 
with the present invention is also applicable when fundamental waves are 
converted into a wave having a frequency equal to the difference between 
or the sum of the frequencies of the fundamental waves. Also, nonlinear 
optical materials and holder materials, which are other than those 
described above, may be employed.