Laser light beam generating apparatus

A laser light beam generating apparatus includes at least one light beam source, first and second reflectors, a non-linear optical crystal element and an actuator. The light beam source emits a light beam. The non-linear optical crystal element is provided between the first reflector and the second reflector. A light beam emitted from the light beam source is incident on the non-linear optical crystal element through the first reflector. The actuator actuates at least one of the first and second reflectors along an optical axis of the light beam emitted from the light beam source.

BACKGROUND 
1. Field of the Invention 
This invention relates to a laser light beam generating apparatus. More 
particularly, the present of invention relates to a laser light beam 
generating apparatus in which a laser light beam converted wavelength is 
generated by a non-linear optical crystal element. 
2. Background of the Invention 
It has hitherto been proposed to realize wavelength conversion by taking 
advantage of the high power density within a resonator. For example, 
second harmonic generation (SHG) is often achieved by placing a non-linear 
optical crystal in an external resonator in an attempt to improve the 
efficiency of the wavelength conversion. 
As an SHG used the non-linear optical crystal element providing the 
resonator, the resonator which includes at least a pair of mirrors, a 
laser medium and a non-linear optical crystal element is known. In this 
resonator, the laser medium and the non-linear optical crystal element are 
provided between the pair of mirrors. With this type of the laser light 
beam generating apparatus, the second harmonic laser light beam is taken 
out efficiently by phase matching the second harmonic laser flight beam 
with respect to the laser light beam of the fundamental wavelength by a 
non-linear optical crystal element provided within the resonator. 
There is also known an external resonant method according to which a laser 
light beam from a laser light source is introduced into an external 
resonator as laser light beam of a fundamental wavelength and propagated 
through a non-linear optical crystal element back and forth for a resonant 
operation to generate a second harmonic laser light beam. In the external 
resonant method, the finesse value of the resonator, corresponding to a 
Q-value of resonation, is set to a larger value of about 100 to 1000 to 
set the light density within the resonator to a value hundreds of times as 
large as the incident light density. As a result, this type resonator can 
take advantage effectively of non-linear effects of the non-linear optical 
crystal element within the resonator. 
Meanwhile, for producing laser light beams of second or higher harmonics or 
so-called sum frequency according to the external resonant method, it is 
necessary to realize extremely fine position control of limiting changes 
or errors of the optical path length of the resonator to less than 1/1000 
or 1/10000 of the wavelength, that is less than 1 .ANG.. 
The conventional practice in limiting the resonator length has been to have 
the reflective mirrors of the resonator supported by stacked piezoelectric 
elements by so-called PZT and to feed an error signal proportional to 
changes in the resonator length back to the stacked piezoelectric elements 
to complete a servo loop for automatically controlling and stabilizing the 
resonator length. 
In general, piezoelectric elements have multiple resonance at intervals of 
several to tens of kilohertz frequencies and have phase delay over the 
entire frequency range due to self capacitance. As a result, it is 
difficult to spread frequency range of the servo range to several 
kilohertz. Since the stacked piezoelectric elements are in need of a high 
driving voltage of hundreds to thousands of volts, the driving electric 
circuit is complicated and expensive. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a laser 
light beam generating apparatus to resolve above-described problem. 
It is an another object of the present invention to provide a laser light 
beam generating apparatus to improve a control operation of limiting 
changes or errors of the optical path length of the resonator. 
According to a first embodiment of the present invention, there is provided 
laser light beam generating apparatus including at least one light beam 
source, a first reflector, a second reflector, a non-linear optical 
crystal element and an actuator. The light beam source emits a light beam. 
The non-linear optical crystal element is provided between the first and 
second reflectors. A light beam is incident on the non-linear optical 
crystal element through the first reflector. The actuator actuates at 
least one of the first and second reflectors along an optical axis of the 
light beam emitted from the light beam source. 
According to a second embodiment of the present invention, there is 
provided laser light beam generating apparatus having at least one light 
beam source, a first resonator, a second resonator and an actuator. The 
first resonator includes first and second reflectors and a laser medium 
into which the pumping light beam is incident from the light beam source 
through the first reflector. The second resonator includes third and 
fourth reflectors and a non-linear optical crystal element in which the 
light beam from the first resonator is incident through the third 
reflector. The actuator actuates at least one of the first, second, third 
and fourth reflectors along an optical axis of the light emitted from the 
first resonator. 
Since an electromagnetic actuator is employed as a driver for controlling 
the optical path length of the resonator with high accuracy, the servo 
range may be increased to tens of kilohertz to permit stable control and 
highly efficient wavelength conversion. Since a low driving current for 
the electromagnetic actuator suffices, it becomes possible to simplify the 
circuitry and to reduce production costs.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows, in a schematic block diagram of an embodiment of the laser 
light beam generating apparatus according to the present invention. 
Referring to FIG. 1, a laser light beam of a fundamental wavelength is 
emitted from a laser light source 11, such as a semiconductor laser 
device, e.g. a laser diode, or a second harmonic generating (SHG) laser 
light source device. The laser light beam of the fundamental wavelength is 
phase modulated by a phase modulator 12 employing an electro-optical (EO) 
device or an acoustic-optical (AO) device before being incident on an 
external resonator 15 via an optical element 13 for detecting the 
reflected light beam from the resonator 15 and a light converging lens 14. 
The external resonator 15 is made up of a reflecting surface 16 of a 
concave mirror, a reflecting surface 17 of a plane mirror and a non-linear 
optical crystal element 18 interposed therebetween. The state of resonance 
is produced when the optical path length L.sub.R between the reflecting 
surfaces 16, 17 of the resonator 15 is a preset length and the optical 
path phase difference .DELTA. is an integer number times 2.pi. with the 
reflection and the phase of reflection being acutely changed near the 
resonance phase. At least one of the reflective surfaces 16, 17 of the 
resonator 15, for example, the reflective surface 17, is adapted for being 
driven along the optical axis by electromagnetic actuator 19. 
If an SHG laser light source device is used as the laser light source 11 
for generating a single-mode laser light beam of the wavelength of 532 nm 
which is supplied to the external resonator 15, the non-linear optical 
crystal element 18 of barium borate (BBO) is used in the resonator 15 and, 
by taking advantage of its non-linear optical effects, a laser light beam 
of the wavelength of 266 nm, which is the second harmonic wave of the 
input laser light beam of 532 nm (or the fourth harmonic wave if the input 
light beam is the SHG laser light beam) is generated. The reflective 
surface 16 of the concave mirror of the external resonator 15 is a 
dichroic mirror which reflects substantially all of the input light beam 
of the wavelength of 532 nm, while the reflective surface 17 of the plane 
mirror is a dichroic mirror reflecting substantially all of the input 
light beam and transmitting substantially all of the output light beam 
having the wavelength of 266 nm. 
An oscillator 21 outputs a modulating signal with e.g. a frequency fm=10 
MHz for driving the optical phase modulator 12 to phase modulator 12 via 
driver 22. The reflected or return laser light beam transmitted to 
resonator 15 is detected via reflective surface 13 and a photodetector 23, 
such as a photodiode. The reflected light beam detection signal is 
transmitted to a synchronous detection circuit 24. Modulating signals from 
oscillator 21 are supplied, if necessary, after waveshaping, phase 
delaying, etc. to the synchronous detection circuit 24, and multiplied by 
the reflected light detection signal, for synchronous detection. Detected 
output signals from the synchronous detection circuit 24 are supplied via 
a low-pass filter (LPF) 25, an output of which is a resonator optical 
length error signal as later explained. This error signal is transmitted 
to a driver 26, a driving output signal of which actuates the actuator 19 
for shifting the reflective surface 17 along the optical axis by way of a 
servo control for reducing the error signal to zero. In this manner, the 
optical path length L.sub.R of the external resonator 12 is controlled to 
be a length corresponding to a local minimum of a reflection curve 
(resonant point). 
The electromagnetic actuator 19 may be a so-called voice coil driving type 
actuator and the double resonance frequency can be rendered equal to tens 
of KHz to 100 KHz or higher. As the servo loop resonance frequency is 
raised, and phase deviations are minimized, the servo range (cut-off 
frequency) can be increased to e.g. 20 KHz or tens of KHz. Since a low 
driving current of tens to hundreds of milliamperes suffices for driving 
the electromagnetic actuator 19, the driving electric circuit may be 
simplified and rendered inexpensive. Consequently, it becomes possible to 
provide, in a method for effectively utilizing non-linear effects 
employing the external laser resonator method, an inexpensive system for 
stably suppressing changes in the resonator length to less than 1/1000 to 
1/10000 of a wavelength, that is to less than 1 .ANG.. 
The principle of introduction of a laser light into the external resonator 
15, or a so-called Fabry-Perot resonators, and error detection, is 
explained. Such a resonator is brought into a resonant state when the 
optical path phase difference .DELTA. is equal to an integer number times 
2.pi. with the reflection phase being acutely changed near the resonant 
phase. Frequency control of the resonator by taking advantage of the phase 
changes is disclosed for example in "Laser Phase and Frequency 
Stabilization Using an Optical Resonator" by R. W. P. Drever et al., 
Applied Physics B 31.97-105 (1983). The principle of detection of the 
error signal by this technique is hereinafter explained. 
In general, if a non-linear optical element having a refractive index n and 
a thickness L is present within a Fabry-Perot resonator, the optical path 
phase difference .DELTA. is 4.pi.nL/.lambda.. If the single-pass 
transmittance is T, the single-pass SHG conversion efficiency is .eta., 
the reflectance at the incident surface is R1 and the reflection at the 
outgoing surface is R.sub.2, the complex reflection r becomes 
##EQU1## 
where Rm=R.sub.2 (T (1-.eta.)).sup.2. The absolute value of r (power 
reflection) and the phase (reflection phase) are shown in FIGS. 2 and 3, 
respectively. By taking advantage of these phase changes, the values of 
the resonant frequency fo of the external resonator 15 and the frequency 
fc of the fundamental wavelength laser light source 11 are brought into a 
relationship of an integer number times multiple relative each other. 
The laser light beam of the laser light source 11 having the frequency fc 
of e.g. about 500 to 600 THz is phase-modulated by phase modulator 12 with 
the frequency fm of 10 MHz, such that a side band fc.+-.fm is produced. An 
error signal exhibiting polarities is obtained by detecting the beat 
between the frequencies of fc and fc.+-.fm of the return light from the 
external oscillator having the resonant frequency of f.sub.0. 
That is, with the electric field E of the fundamental wavelength laser 
light source 11 of E.sub.0 exp(i .omega.c t), the electrical field after 
the modulation becomes E.sub.0 exp(i (.omega.c t+sin (.omega.m t))), where 
.omega.c is an angular frequency of the fundamental wavelength laser 
light, .omega.m is an angular frequency of the modulation signal of the 
phase modulator 12 and .beta. is the modulation index. If the modulation 
index is sufficiently small such that .beta.&lt;0.2, it suffices to take 
account of .omega.c and two sidebands .omega.c.+-..omega.m. Consequently, 
we obtain the following formula (2) 
EQU .EPSILON.=.EPSILON..sub.0 [J.sub.0 (.beta.)e.sup.i.omega..sbsp.c.sup.t 
+J.sub.1 (.beta.)e.sup.i(.omega..sbsp.c.sup.+.omega..sbsp.m)t -J.sub.1 
(.beta.)e.sup.i(.omega..sbsp.c.sup.-.omega..sbsp.m.sup.)t ](2) 
where J0 (.beta.) and J1(.beta.) are Bessel functions of the first and 
second orders, respectively. 
Since the complex reflections for .omega.c and two sidebands 
.omega.c.+-..omega.m modify the respective terms, the electric field of 
the reflected light from the external resonator 15 becomes 
##EQU2## 
Since B&lt;0.2, J0 (.beta.).apprxeq..sqroot.(1-.beta..sup.2 /2) and J1 
(.beta.).apprxeq..beta./2, the following formula is true (4) 
##EQU3## 
Therefore, if the terms of the second and higher orders of .beta. 
disregarded, the intensity .vertline.E.vertline..sup.2 becomes 
##EQU4## 
Synchronous detection of the reflected light with a suitable phase being 
given to the original modulation signal (with the angular frequency 
.omega..sub.m) gives the above formulas (6) and (7) which are the 
coefficients of cos (.omega..sub.m t) and sin (.omega..sub.m t). The 
above-mentioned error signal may be obtained from the formula (7) which is 
the coefficient of sin (.omega..sub.m t). 
That is, FIG. 4 shows a detection signal of the return light (reflected 
light) from resonator 16 in FIG. 1 as detected by the photodetector 23 in 
FIG. 1. This detection signal is a signal component of FIG. 5 as an 
intensity signal of the reflected light superimposed on a signal component 
of FIG. 6 corresponding to the modulation signal. The modulation signal 
component of FIG. 6 may be taken out by transmission through a band-pass 
filter having a center transmission frequency of 10 MHz which is the 
above-mentioned modulation signal frequency. If the modulation signal 
component of FIG. 6 is multiplied by a signal which affords a suitable 
phase to the original modulation signal, and synchronous detection is 
performed, the signal component sin (.omega..sub.m t) as shown in FIG. 7 
is obtained. If the signal is freed of the modulation carrier frequency of 
10 MHz by the low-pass filter, the error signal shown by a thick line of 
FIG. 7, that is the signal of the formula (7), is obtained. Meanwhile, 
FIG. 8 shows, for reference sake, the signal component of the cos 
(.omega..sub.m t) and the signal of formula (6). 
FIG. 9 shows, in a perspective view, a typical structure of the 
electromagnetic actuator 19 in FIG. 1. 
Referring to FIG. 9, the reflective surface 17 of FIG. 1 is formed, such as 
by coating, on a reflective mirror 31, which is fitted on a ring-shaped or 
cylindrically-shaped coil bobbin 32 formed of a ceramic or the like 
insulating material. A coil (so-called voice coil) 33 is wound in the form 
of a solenoid around the coil bobbin 32. This coil bobbin 32 is mounted on 
spirally-shaped spring plates 34, as shown in FIG. 10. The spirally-shaped 
spring plates 34 are secured to and supported by a ring-shaped yoke 36 via 
a permanent magnet 35. The magnet 35 is mounted for encircling the 
cylindrically-wound coil 33 of the coil bobbin 32 and is magnetized so 
that its inner periphery is the N pole and its outer periphery is the S 
pole. The magnet 35 has its outer periphery secured to a yoke 36 of iron 
or the like ferromagnetic material. The spring plates 34 are secured, such 
as by adhesion, to the upper and lower surfaces of the coil bobbin 32. The 
outer periphery of each of the spring plates 34 has its outer periphery 
supported by the yoke 36. The above-mentioned components are sandwiched 
between shield plates 37, 38 of iron or the like ferromagnetic material. 
These shield plates 37, 38 also play the part of a return path for the 
magnetic flux from the magnet 35 in cooperation with the yoke 36. The 
totality of the components are surrounded by the shield plates 37, 38 for 
ease of handling. 
With the electromagnetic actuator, arranged and constructed as shown in 
FIGS. 9 and 10, the magnetic circuit has a substantially closed magnetic 
path, despite the fact that an electrically conductive material or a 
magnetic material is not provided within the coil 33. Characteristics 
exhibiting a large thrust (driving force) along the optical axis and less 
phase deviations may be obtained. On the other hand, the coil bobbin 32 is 
formed of ceramics to diminish the weight of the moving components, so 
that the double resonant frequency may be set to 100 kHz or higher. 
FIGS. 11 and 12 are Bode diagrams showing transmission characteristics of a 
tentatively produced electromagnetic actuator. Specifically, FIGS. 11 and 
12 show the gain and the phase, respectively. A mirror holder (coil bobbin 
32) of the actuator is formed of ceramics, with the resistance, inductance 
and weight of the actuator being 8 .OMEGA., 570 .mu.H and 1.25 g, 
respectively, and the spring constant and viscosity coefficient of the 
spring plate 34 being 570 Nm/rad and 0.057 Nm/sec. In these figures, 
resonance is not noticed at 100 KHz and up to nearly 100 KHz of f.sub.0. 
Phase deviations in the higher frequency range are caused by coil 
inductances. 
FIG. 13 is a block diagram of a servo control system. In this figure, 
initial position or desired position setting signals are supplied at an 
input terminal 41 so as to be transmitted to a subtractor 42. An output 
signal from subtractor 42 is servo-phase-compensated at a phase 
compensator circuit 43 and converted at a driver 44 into a driving signal 
at a driver 44 which is supplied to the electromagnetic actuator 45. The 
driver 44 and the electromagnetic actuator 45 correspond to the driver 26 
and the electromagnetic actuator 19, respectively, both in FIG. 1. The 
position of the reflective surface 17 in FIG. 1 of the resonator 15 in 
FIG. 1 along the optical axis is controlled by the electromagnetic 
actuator 45 and a position detection signal for the reflective surface 
position is transmitted as a subtraction signal to a subtractor 42 where 
it is subtracted from the desired position signal to produce a position 
error signal corresponding to the error signal shown in FIG. 7. 
FIG. 14 shows, in a block diagram, a typical arrangement for detecting the 
error signal. In this figure, a reflected light detection signal from a 
photodetector 23 shown in FIG. 4 is supplied at an input terminal 46 to a 
low-pass filter (LPF) 47 where it is freed of the above-mentioned 
modulation carrier component. An output signal from the LPF 47 is supplied 
to an additive node 48 where it is added to an offset DC level from an 
offset output circuit 49 to produce a reflection signal (reflected light 
intensity signal) as shown in FIG. 5 so as to be taken out at an output 
terminal 50. 
On the other hand, the reflected light detection signal, supplied to the 
input terminal 46, is transmitted through a band-pass filter (BPF) 51 
where the phase-modulated carrier frequency, such as fm=10 MHz, is taken 
out and supplied to a sample-and-hold circuit 52 where a processing 
comparable to synchronous detection is performed to take out the term of 
sin (.omega..sub.m t) in formula (5). Besides, the modulation carrier 
component is removed by the low-pass filter (LPF) so that the component of 
the coefficient of sin (.omega..sub.m t) as shown in FIG. 7 is output at 
an output terminal 54. The modulating signal (fm=10 KHz) from the 
oscillator 21 supplied to the input terminal 55 is a waveform shaped by a 
clock generator 56 into pulse signals which are delayed by a predetermined 
phase of, for example, 90 degrees, and supplied to the sample-and-hold 
circuit 52. The carrier frequency component from the BPF 51 is sample-held 
by the phase-delayed modulation signal to perform a synchronous detection 
of taking out the above-mentioned sin (.omega..sub.m t) signal component. 
FIG. 15 is a Bode diagram showing closed-loop characteristics of an entire 
system inclusive of the servo circuit shown in FIG. 13 when the 
electromagnetic actuator explained in connection with FIGS. 9 to 12 is 
employed. In this figure, curves A and B represent the gain and the phase, 
respectively. The cut-off frequency may be raised to 20 KHz, by adjusting 
the gain in the electric circuit. The phase margin at this time is about 
34 meaning that a stable closed loop system may now be realized. 
FIG. 16 shows an error signal (A) and a reflected light detection signal 
(B) when the electromagnetic actuator is driven without servo control for 
deviating the reflecting surface 66 along the optical axis, with a 
peak-to-peak distance of the error signal (A) being about 1 .ANG.. FIG. 17 
shows the error signal (A) and the reflecting light detection signal (B) 
when the closed loop servo is applied. It is seen that fluctuations of the 
error signal (A) are suppressed to not more than .+-.0.1 .ANG. while the 
reflected light detection signal (B) is approximately zero so that 
substantially all of the laser light beam has been introduced into the 
external resonator 15. 
FIG. 18 shows a modification of a laser light emitting apparatus according 
to the present invention, in which the laser light beam of the fundamental 
wavelength, radiated from a laser light source 61, is phase-modulated by a 
phase modulator 62 so as to be incident via a light converging lens 64 to 
an external resonator 65. The external resonator 65 is made up of a 
reflective surface 66 of a concave mirror, a reflective surface 67 of a 
concave mirror 67, and a non-linear optical crystal element 68 arranged 
therebetween, so that an optical path of a resonator 65 is defined by 
these reflective surfaces 66, 67 and the reflective surface 63 of the 
plane mirror. The resonator is operated in resonance when the optical path 
length L.sub.R of the resonator 65 is changed such that the optical path 
phase difference becomes equal to an integer number times 2.pi. so that 
the reflection and the reflection phase are changed acutely. The 
reflective surface 66 of the resonator 65 is driven along the optical path 
by the electromagnetic actuator 69. 
The arrangement from the oscillator 21 to the driver 26 is the same as that 
of the embodiment shown in FIG. 1, so that description is omitted for 
brevity. The electromagnetic actuator 69 may be arranged and constructed 
as shown in FIGS. 9 and 10. The operation of the various components is 
similar to that of the above-described embodiment and hence the 
description is again omitted for brevity. 
The laser light beam generating apparatus according to the present 
invention may be designed in many ways other than in the above-described 
embodiments. Several basic arrangements of the laser light generating 
apparatus according to the present invention are hereinafter explained by 
referring to FIGS. 19 to 23. 
FIG. 19 shows a first basic arrangement of the present invention in which a 
so-called SHG laser resonator as a solid-state laser resonator is employed 
as a laser light source 11 shown in FIG. 1. Referring to FIG. 19, a 
resonator 91 for SHG laser light beam generation includes a laser medium 
94, such as Nd:YAG, and a non-linear optical crystal element 95, such as 
KTP (KTiOPO.sub.4), arrayed between a pair of reflecting surfaces 92, 93. 
An excitation light beam, radiated from an excitation light source, such 
as a semiconductor laser 101, is converged via a light converging lens 102 
on the laser medium 94 of the resonator 91. The laser light beam having 
the fundamental wavelength of 1064 nm, for example, is radiated from the 
laser medium 94 and transmitted through the non-linear optical crystal 
element 95 for resonation within the resonator 91 for generating the SHG 
laser light beam of the wavelength of 532 nm. The SHG laser light beam is 
phase-modulated by a phase modulator 12 shown in FIG. 1 and caused to be 
incident via a reflecting surface 13 for detecting the reflected light 
beam from the resonator and via the light converging lens 14 into an 
external resonator 75. One of the reflecting surfaces 76, 77 of the 
external resonator 75, for example, the reflecting surface 76, is driven 
along the optical axis in a controlled manner by the electromagnetic 
actuator 79. Within the external actuator 75, a laser light beam having 
the wavelength of 266 nm, which is the second harmonic of the incident 
laser light beam, that is the fourth harmonic of the original laser light 
beam with the wavelength of 1064 nm, is generated and taken out of the 
external resonator 75. The arrangement of the oscillator 21, the driver 
22, the photodetector 23, the synchronous detection circuit 24, the 
low-pass filter (LPF) 25 and the driver 26 is the same as the 
above-described first embodiment and hence the explanation is omitted for 
simplicity. 
FIG. 20 shows a second basic arrangement of the present invention in which 
a solid-state laser resonator having a pair of reflective surfaces 72, 73 
and a laser medium 74, of such as Nd:YAG etc arranged therebetween is 
employed as the above-mentioned laser light source. In this resonator, the 
laser light beam of the fundamental wavelength of 1064 nm, for example, is 
introduced from the laser light source through a non-linear optical 
crystal element 78, such as lithium niobate (LiNbO.sub.3) arranged between 
the reflective surfaces 76, 77 of the external resonator 75 for generating 
second harmonics having the wavelength of 532 nm. One of the reflective 
surfaces of the external resonator 75, such as the reflective surface 75, 
is position-controlled along the optical axis by the above-mentioned 
electromagnetic actuator 79. 
FIG. 21 shows a third basic arrangement of the present invention in which a 
solid-state laser resonator having a pair of reflective surfaces 82, 83 
and a laser medium 84 of such as Nd:YAG etc arranged therebetween is 
employed as the above-mentioned laser light source, and in which the laser 
light beam of the fundamental wavelength of 1064 nm, for example, is 
introduced from the laser light source through a non-linear optical 
crystal element 88, such as lithium niobate (LiNbO.sub.3) arranged between 
the reflective surfaces 86, 87 of the external resonator 85 for generating 
second harmonics having the wavelength of 532 nm. One of the reflective 
surfaces of the resonator 81, such as the reflective surface 83, is 
position-controlled along the optical axis by the above-mentioned 
electromagnetic actuator 89. With the present third basic arrangement, 
reflection of the laser light beam with respect to the external resonator 
85 is changed by the oscillation frequency of the laser light beam of the 
fundamental laser light beam from the laser light source being changed, 
thereby establishing a stable state in which laser light beam introduction 
into the external resonator 85 is increased. 
In these basic arrangements, shown in FIGS. 20 and 21, Nd:YVO.sub.4, LNP, 
Nd:BEL, etc. may be used as the laser media 74, 84, in addition to Nd:YAG. 
The non-linear optical crystal elements 78, 88 may also be KTP, QPM LN, 
LBO or BBO besides LN. 
Although not shown, one of the reflective mirrors of the SHG laser 
resonator as a laser light source may be driven by the electromagnetic 
actuator as in the case of the above-mentioned first basic arrangement. If 
the second harmonic generating type laser resonator generating the second 
harmonic laser light beam within the resonator is employed as a laser 
light source, and the laser oscillator is of the homogeneous line 
broadening as is the solid-state laser resonator, an oscillation of the 
polarization of the mode closest to the peak of the gain curve (gain 
frequency characteristic curve) is produced and the gain is saturated so 
that the single mode oscillation is produced. However, in effect, 
multi-mode oscillation is produced due to the hole burning effects. This 
is because the standing wave is present within the laser resonator 13 and 
the gain is not fully saturated at the node of the standing wave, as a 
result of which oscillations having a different mode are produced. Should 
longitudinal multi-mode be present in the same polarization mode of the 
laser light beam of the fundamental wavelength, there is a risk that the 
mode hop noise due to mode coupling in one and the same polarization mode 
tends to be produced within the same polarization mode. 
In the specification and drawings of Japanese Patent Application No. 
2-125854, the present Assignee has proposed arranging an optical device 
inhibiting coupling of two polarization modes of the laser light beam of 
the fundamental wavelength due to generation of sum frequency, or a 
so-called etalon, within the laser resonator, or arranging the laser 
medium 16 in proximity to the quarter wave plate 15, for inhibiting the 
multi-mode oscillation due to the above-mentioned hole-burning effect. In 
the specification and drawing of the Japanese Patent Application No. 
3-17068, the present Assignee has also proposed providing an optical 
element inhibiting coupling of the two intrinsic polarization modes of the 
laser light of the fundamental wavelength, and an adjustment device or 
adjusting polarization so that the laser light beam of the fundamental 
wavelength propagated back and forth in the laser medium will become 
circular polarization. It is preferred to inhibit hole burning effects in 
the SHG laser resonator or to prevent the mode hop noise from being 
produced by the techniques disclosed in these Publications. 
By setting the optical path length of the SHG laser light source so as to 
be an integer number times as large as the optical path length of the 
external resonator, the SHG laser light beam can be introduced efficiently 
into the external oscillator. This arrangement is required in order that 
the longitudinal modes of the SHG laser light beam, which are based on the 
two intrinsic polarization modes of the fundamental wavelength laser light 
beam produced by introducing a double refraction device such as a quarter 
wave plate in the resonator of the SHG laser light source adapted for 
establishing the so-called type II phase matching conditions between the 
fundamental wave laser light beam and the SHG laser light beam, will be 
introduced in their entirety into the external resonator. The multi-modes 
may be efficiently introduced by setting the optical path length of the 
SHG laser light source so as to be an integer number times that of the 
external resonator. 
That is, by introducing the SHG laser light beam from the SHG laser 
resonator into the external resonator having an internal non-linear 
optical crystal element, in which the frequency difference of the 
longitudinal modes within the two polarization modes of the resonator 
having the internal non-linear optical crystal element is equal to an odd 
number multiple of one half the interval of the longitudinal resonance 
modes, and by setting the optical path length of the external resonator so 
as to be an integer number times the optical path length of the SHG laser 
resonator, two or more modes of the laser light beam from the SHG laser 
resonator may be simultaneously introduced into the external resonator to 
improve the multi-stage wavelength conversion efficiency. 
FIG. 22 shows a fourth basic arrangement of the solid-state laser resonator 
according to the present invention in which two external resonators 75a, 
75b are arranged in series with each other. In the embodiment shown in 
FIG. 22, a laser light beam from the resonator 71 of the fundamental wave 
laser light beam having the wavelength of e.g. 1064 nm is introduced into 
a first external resonator 75a for converting the laser light beam into 
the SHG laser light beam having the wavelength of 532 nm by the non-linear 
optical crystal element 78a such as LiNbO.sub.3. The SHG laser light beam 
thus produced is introduced into a second external resonator 75b for 
converting the SHG laser light beam into the laser light beam of the 
fourth harmonic of 266 nm wavelength (FHG) by the non-linear optical 
effects of the non-linear optical crystal element 78b, such as BBO. One of 
the reflective surfaces 76a, 77a of the first external resonator 75a, such 
as the reflective surface 76a, is shifted along its optical axis by the 
electromagnetic actuator 79a, while one of the reflective surfaces 76b, 
77b of the second external resonator 75b, such as the reflective surface 
76b, is shifted along its optical axis by the electromagnetic actuator 
79b, until the conditions concerning the optical paths of the resonators 
71, 75a and 75b are satisfied. 
FIG. 23 shows a fifth basic arrangement of the present invention in which 
wavelength conversion is performed by so-called sum frequency mixing. That 
is, the SHG laser light beam of the wavelength of 532 nm from a laser 
resonator 91 as the aforementioned SHG laser light source as explained 
with reference to FIG. 19 is transmitted via a wave combining mirror 97, 
such as a dichroic mirror, to an external resonator 85. One of reflecting 
surfaces 92, 93 of the resonator 91 of the SHG laser light source, for 
example, the reflecting surface 93, is shifted along the optical axis by 
an electromagnetic actuator 96 such as the above-described electromagnetic 
actuators. The laser light beam from a laser resonator 81 as shown in FIG. 
21 is transmitted to an external resonator 85 via a wave combining mirror 
97 after deflection by a mirror (reflective surface) 98. In the external 
resonator 85, the laser light beam of 532 nm wavelength and the laser 
light beam of the 1064 nm wavelength are sum frequency mixed by the 
non-linear optical effect of the non-linear optical crystal element 88, 
such as an MMO element, for producing a laser light beam of, for example, 
the wavelength of 355 nm, which is outputted. 
The present invention is not limited to the above-described embodiments. 
For example, the wavelength of the fundamental laser light from the laser 
medium of Nd:YAG may be 956 nm or 1318 nm, besides 1064 nm. The laser 
light source may also be a semiconductor laser, such as a laser diode, or 
a gas laser, such as He-Ne laser, besides the solid-state laser. The laser 
light beam from the light sources for sum frequency mixing as shown in 
FIG. 23 may also be the laser light beam from the external resonator as 
shown in FIG. 22.