Laser having frequency conversion element

In a laser having a frequency conversion element or a non-linear optical element such as a second-harmonic generation (SHG) element arranged on an optical axis of a laser light and outputting a frequency-converted laser light, laser incident and emitting faces of the SHG element have such convex form as cylindrical lenses with a common center axis of curvature thereof and orthogonal to the optical axis of the laser light. The SHG element is rotatable about the common center axis of curvature so that an optimum control of phase matching angle is possible. Since the common center axis of curvature of the laser incident and emitting faces of the SHG element is coincident with the rotational axis thereof, an optical axis of a fundamental wave of the laser light source is not deviated when the SHG element is rotated and thus reduction of fundamental wave laser output can be restricted. Further, a control mechanism for always maintaining wavelength conversion efficiency of the SHG element at maximum may be added, in which changes of a second-harmonic wave output light, caused by minute variation of rotation angle given to the SHG element, is detected to automatically control the phase matching angle.

BACKGROUND OF THE INVENTION 
The present invention relates to a laser having a frequency conversion 
element and, in particular, an ultraviolet laser using a non-linear 
optical element. 
A conventional ultraviolet laser utilizes a non-linear optical element such 
as a second-harmonic generation (SHG) element, which in response to a 
visible laser light emits ultraviolet rays by wavelength conversion. An 
example of a conventional ultraviolet laser which uses an argon laser tube 
as a visible laser light source and .beta. barium borate crystal 
(.beta.-BaB.sub.2 O.sub.4 ; referred to as BBO crystal, hereinafter) as an 
SHG element is disclosed in "High Efficiency Frequency Multiplication of 
Continuous Wave Ion-Laser", Applied Physics, Vol. 61, No. 9 (1992), pp. 
931-934. In the disclosed ultraviolet laser which is shown schematically 
in FIG. 1, an argon laser tube 101, a condenser lens 104 and an SHG 
element 105 disposed between a pair of cylindrical lenses 106 are arranged 
in that order between a reflecting mirror 102 and an output mirror 103 to 
form an optical resonator. A fundamental wave having wavelength of 514 nm 
from the argon laser tube 101 is converted into a second-harmonic wave 
(wavelength of 257 nm) by passing through the SHG element 105 of the 
optical resonator and ultraviolet light is emitted from the output mirror 
103. Since wavelength conversion efficiency of the SHG element is 
proportional to a square of laser field density, it is improved by 
condensing laser light by the condenser lens 104. In this laser, since 
tolerance width of angular phase matching (incident laser light deviation 
at which wavelength conversion efficiency becomes a half) of the BBO 
crystal 105 which is the SHG element in a first phase matching in which an 
optical axis of the BBO crystal 105 is in a horizontal direction in the 
drawing sheet (polarizing direction of argon laser) is larger than that in 
a direction perpendicular to the horizontal direction, the laser beam is 
enhanced by the cylindrical lenses 106 to thereby improve the conversion 
efficiency. 
In this prior art, however, there is a tendency of the actual incident 
angle of the fundamental wave to deviate with respect to the phase 
matching angle of the SHG element due to vibration, mechanical shock 
and/or environmental temperature variation during its use, since the SHG 
element is arranged between the cylindrical lenses and such deviation is 
difficult to be regulated. For these reasons, the wavelength conversion 
efficiency thereof is degraded. Particularly, since wavelength conversion 
efficiency of the BBO crystal which is a typical SHG element for 
ultraviolet light is reduced by half upon incident angle deviation of only 
0.1 degree, an oscillation of such conventional laser as shown in FIG. 1 
which includes many optical parts to be optically regulated may easily go 
undetected. 
Further, in the example shown in FIG. 1, reflection loss between the 
cylindrical lenses and the SHG element is large and highly efficient 
wavelength conversion is difficult. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a laser having a 
non-linear optical element constructed such that reflection loss is 
minimized. 
Another object of the present invention is to provide an ultraviolet laser 
which has high wavelength conversion efficiency and in which disorder of 
phase matching due to increase of temperature of the non-linear optical 
element can be easily regulated. 
A further object of the present invention is to provide an ultraviolet 
laser capable of always maintaining high wavelength conversion efficiency 
by automatically detecting and easily regulating improper phase matching 
due to increase of temperature of the non-linear optical element. 
According to the present invention, in a laser having a non-linear optical 
element such as the SHG element arranged on an optical axis thereof and 
outputting a laser light which frequency being converted, the laser light 
,emitting surfaces of the non-linear optical element are formed as convex 
surfaces of a cylindrical lens with center axes of curvatures thereof 
being coincident with each other and being orthogonal to the optical axis 
of the laser. 
Further, according to the present invention, an optimum control of phase 
matching angle, becomes possible by making the non-linear optical element 
rotatable about the center axis of the curvature. Since the center axis of 
the curvature of the laser light emitting surface of the non-linear 
optical element is coincident with the rotation center thereof and 
therefore there is no deviation of optical axis of a fundamental wave of a 
laser light source even if the non-linear optical element is rotated, 
reduction of output of fundamental laser wave can be restricted. 
According to the present invention, it is further possible to add a 
mechanism with which wavelength conversion efficiency of a non-linear 
optical element can be always maintained at its maximum, by detecting a 
variation of the output laser light such as the secondary harmonic output 
light while varying the rotation angle of the non-linear optical element 
slightly and automatically controlling the phase matching angle on the 
basis of the detected laser light variation.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 2A, a fundamental wave oscillator portion is constituted 
of an argon laser tube 1, and an optical resonator composed of a 
reflection mirror 2, an output mirror 3 and an etalon 4. A wave length 
converter portion is composed of an SHG element 5 having an incident 
surface and an emitting surface which are convex. An output stabilizer 
portion is constituted of a turntable 11 supporting the SHG element 5, a 
drive portion for rotationally vibrating the turntable 11 slightly and an 
output light monitor portion. The drive portion is constituted of an 
electrostrictive element 6, a metal rod 7 and a Peltier effect element 8 
for heating or cooling the metal rod 7. The monitor portion is constituted 
of a beam splitter 9 for splitting a monitor light and a light detecting 
element 10 for monitoring an output light. 
In the fundamental wave oscillator portion, a gain is provided by the argon 
laser tube 1 and a laser oscillation at wavelength of 514 nm is obtained 
by reflecting an output laser light of the argon laser tube 1 between the 
reflecting mirror 2 and the output mirror 3. The etalon 4 is arranged on 
an axis of the laser light to realize a single longitudinal mode. 
The SHG element 5 which constitutes the wavelength converter portion is 
arranged within the optical resonator of the fundamental wave oscillator 
portion and converts the fundamental wave having wavelength of 514 nm into 
a second-harmonic wave having wavelength of 257 nm. As shown in FIGS. 2B 
and 2C, the SHG element 5 is formed from a cylindrical BBO crystal having 
opposing side faces (x-z plane) cut parallel to a center line of the 
cylinder. Therefore, an incident face and an emitting face of the SHG 
element 5 for laser light form cylindrical lenses having centers of 
curvature which are coincident. An optical axis of the SHG element 5 is 
regulated such that the fundamental wave .omega. passes through the 
centers of curvature as shown in FIG. 2B. In this case, the optical axis 
of laser light is always maintained constant even when the SHG element 5 
is rotated about the center axis OX in order to match phase of the SHG 
element 5. Therefore, the output of the fundamental wave .omega. which is 
influenced by change of optical axis is not reduced. The phase matching is 
performed at an angle .theta. at which the normal light refractive index 
of the fundamental wave .omega. coincides with abnormal light refractive 
index of the second-harmonic wave 2.omega., as shown in FIG. 2D. For BBO 
crystal, this angle is 46 degrees. The angle regulation must be done in 
two directions, .theta..sub.zx direction and .theta..sub.zy direction 
shown in FIG. 2C. However, since, when the BBO crystal having the 
above-mentioned configuration is used, light in .theta..sub.zx direction 
is not reduced in cross section and an alignment of its construction is 
hardly changed due to the fact that the optical system is constructed with 
a single part, it is possible to remove the necessity of optical axis 
regulation in practical use, provided that it is regulated at an initial 
setting time. On the other hand, for optical axis regulation in the 
.theta..sub.zy direction, since light in that direction is reduced in 
cross section and thus it falls at an angle, tolerance of angle change is 
small although its tolerance is generally large when the cross section is 
not reduced. Therefore, it is preferable in view of stability of laser 
output to continuously regulate the angle in that direction based on 
deviations of the optical axis of laser light due to change of ambient 
temperature. 
Stabilization of the second-harmonic wave output will be described with 
reference to FIGS. 3A-3C. The turntable 11 is provided with a protrusion 
11a which is coupled with the electrostrictive element 6 and the metal rod 
7 of stainless steel by means of adhesive, as shown in FIG. 3B. Basically, 
the rotation angle of the turntable 11 is changed by expansion/contraction 
of the electrostrictive element 6 and the metal rod 7 determines an 
initial value of rotation angle. Therefore, the protrusion 11a is moved 
vertically in FIG. 3B correspondingly to expansion/contraction of these 
members and the turntable 11 is rotated by a very small angle. In this 
case, when the rotational axis of the turntable 11 is made coincident with 
the center OX of the cylindrical SHG element 5, it becomes possible to 
regulate angle of the SHG element 5 and to match phase of the SHG element 
5 by monitoring the second-harmonic wave output by means of the light 
receiving element and feedback-controlling the drive portion on the basis 
of the monitored value of the output. The metal rod 7 is preferably made 
of such material having a large thermal expansion coefficient. 
In a block diagram of a driving system electric circuit shown in FIG. 3C, 
the electrostrictive element 6 is expanded/contracted by a drive pulse 
signal from a modulator 12. Therefore, the second-harmonic wave output 
from the SHG element 5 is changed and the change is detected by the 
light-receiving element 10. An output signal of the light-receiving 
element 10 is supplied to a phase sensitive detector (referred to as PSD, 
hereinafter) 13. The PSD 13 is further supplied with the pulse signal from 
the modulator 12. The PSD 13 calculates a differential output of the 
second-harmonic wave 2.omega. from the output signals of the 
light-receiving element 10 and the modulator 12 and controls a temperature 
control power source 14 to cause the Peltier effect element 8 to generate 
or absorb heat. 
The operation of this circuit is performed qualitatively as follows: in a 
case where, when the electrostrictive element 6 slightly increases the 
incident angle of the fundamental wave .omega. to the SHG element 5 by a 
small value (on the order of 0.001 degree) from the initial value thereof, 
an increase of the output of the second-harmonic wave 2.omega. is 
detected, the metal rod 7 is heated by the Peltier effect element 8 and 
expanded. Thus, by repeating this operation by which the reference 
position of rotation of the SHG element 5 is increased and the incident 
angle of the fundamental wave .omega. is increased, the output of the 
second-harmonic wave 2.omega. is made closer to the maximum value. When 
the output exceeds the maximum value, the differential signal from the PSD 
13 is inverted, so that the laser is controlled in a direction in which 
the output of the second-harmonic wave 2.omega. is increased again by 
cooling the Peltier effect element 8 to thereby contract the metal rod 7 
and reduce the incident angle of the fundamental wave .omega. of the SHG 
element 5. By repeating this operation successively at a period on the 
order of 500 Hz, the output of the second-harmonic wave 2.omega. is 
continuously kept at the maximum value regardless of ambient temperature 
variation and thus the condition of phase matching is satisfied. Since the 
basic control circuit for stabilizing a laser output using the PSD 13 is 
well known, as disclosed for example in U.S. Pat. No. 3,555,453, details 
thereof are omitted in this description. 
In the ultraviolet laser constructed as mentioned above, the number of 
parts is reduced due to the fact that the SHG element 5 itself has the 
role of both cylindrical lenses and optical loss between optical parts is 
reduced. Furthermore, since the fundamental wave passes through the center 
of the SHG element, the fundamental wave passes through the SHG element 
without refraction and the optical axis is not changed even if the SHG 
element 5 is rotated about its center axis. Therefore, a stable operation 
becomes possible and the phase matching condition can be easily satisfied. 
On the other hand, for regulation in .theta..sub.zx direction, it is 
possible to obtain an ultraviolet laser having stabilized output without 
regulating the angle in this direction since the wavelength conversion is 
performed by the single optical part and the incident light is conducted 
without reducing the cross sectional area thereof. 
FIG. 4 schematically shows a construction of the second embodiment of the 
present invention. In the second embodiment, the fundamental wave .omega. 
is generated by an optical resonator composed of a reflection mirror 2, an 
output mirror 3 and an argon laser tube 1, and the fundamental wave 
.omega. having wavelength of 514 nm is emitted from the output mirror 3. 
The emitted fundamental wave .omega. is directed to a reflection mirror 
15a provided externally of the optical resonator. The fundamental wave 
.omega. incident on the external reflection mirror 15a passes along a 
triangular path defined by the reflection mirror 15a, an output mirror 16 
and another reflection mirror 15b. An SHG element 5 is disposed on an 
optical axis of the triangle path and converts the fundamental wave 
.omega. into a second-harmonic wave 2 having wavelength of 257 nm. The 
second-harmonic wave .omega. is divided by a beam splitter 9 and a portion 
of the second-harmonic wave is detected by a light-receiving element 10. 
The SHG element 5 is disposed on a turntable 11 with which the angle 
regulation and phase matching are performed as in the first embodiment. 
According to the second embodiment, it is possible to obtain an ultraviolet 
laser whose noise is smaller than in the first embodiment since the 
fundamental wave oscillator portion is provided separately from the 
wavelength converter portion and the propagating direction of light in the 
wavelength converter portion is limited to one direction. 
FIG. 5 shows the third embodiment of the present invention. The third 
embodiment differs from the second embodiment in that the fundamental wave 
.omega. is generated by a solid state laser excited by a laser diode. That 
is, an exciting light having wavelength of 1064 nm from a laser diode 17 
is condensed by a condenser lens 18 to excite a solid state laser medium 
19 (for example, Er:YVO.sub.4) to thereby generate visible light using 
multi-layered films formed on both surfaces of the solid state laser 
medium 19 as an optical resonator. The visible light is used as the 
fundamental wave .omega. and wavelength conversion is performed in a 
similar manner to that used in the second embodiment. 
FIG. 6 shows the fourth embodiment of the present invention. The fourth 
embodiment differs from the third embodiment in that an SHG element 5 is 
arranged in an optical resonator. 
Exciting light emitted from a laser diode 17 is condensed by a condenser 
lens 18 to a solid state laser medium 19 (for example, Er:YVO.sub.4) to 
optically excite the latter. An optical resonator for the fundamental wave 
.omega. is constructed with an exciting light incident side face of the 
solid state laser medium 19 and an output mirror 3. The SHG element 5 is 
arranged in the optical resonator as in the first embodiment and the angle 
regulation is performed in the same manner as in the first embodiment. 
Although the present invention has been described with reference to the 
preferred embodiments, the present invention is not limited thereto and 
various modifications of them are possible within the scope of the present 
invention which is defined by the appended claims. For example, the solid 
state laser media such as Nd:YAG and Nd:YLF can be used. Further, instead 
of the electrostrictive element, a magnetostrictive element or a 
displacement regulator having a step motor, known by the name 
"motor-micro", may be used as means for giving a small rotation or pivotal 
movement to the turntable on which the SHG element is mounted. 
As described hereinbefore, according to the present invention in which a 
visible light is wavelength-converted by the cylindrical SHG element, it 
is possible to reduce the number of parts of the wavelength converter 
portion and to reduce optical loss occurring between optical parts. 
Furthermore, according to the present invention, the incident angle of the 
fundamental wave .omega. with respect to the SHG element is always 
maintained perpendicular thereto even when the SHG element is rotated 
about the center axis thereof and therefore there is no misalignment due 
to refraction of the fundamental wave .omega.. Consequently, the angle 
regulation in rotational direction about the center axis of the 
cylindrical SHG element is facilitated.