Method for selecting wavelength in wavelength-tunable lasers and laser oscillators capable of selecting wavelengths in wavelength-tunable lasers

In order to eliminate a mechanically movable section, such as a rotation mechanism of a diffraction grating, to achieve a compact fabrication of the whole apparatus, and to realize stable action for selecting a wavelength, a laser oscillator selecting a wavelength in a wavelength-tunable laser is composed of a laser resonator consisting of a mirror on the input side and a mirror on the output side; a laser medium which is placed in the laser resonator and can oscillate in a predetermined range of wavelengths; a crystal to which is piezoelectric element is attached, the crystal receiving acoustic waves from the piezoelectric element in accordance with a desired wavelength; and a polarizing plate which is placed in the laser resonator and transmits only the output light beam having a prescribed plane of polarization or having a prescribed direction of light propagation among the output light beams from the laser medium. The apparatus thereby outputs only the desired wavelength.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for selecting a wavelength in a 
wavelength-tunable laser and a laser oscillator which can select a 
wavelength in the wavelength-tunable laser. Applications of such a tuned 
laser included, for example, selecting the wavelength of a laser beam to 
coincide with the absorption spectrum of a desired isotope with a high 
precision in isotope separation, or selecting the wavelength of a laser 
beam to coincide with the absorption spectrum of a substance in the 
atmosphere with a high accuracy in order to measure a concentration of 
trace amounts of molecules of the substance as a laser radar. 
2. Description of the Related Art 
As a wavelength-tunable laser, solid state tunable lasers, for example, a 
Ti:Al.sub.2 O.sub.3 (titanium-sapphire) crystal laser and the like have 
attracted many applications because they can oscillate in a wide 
wavelength range. 
Heretofore, as a method for selecting a wavelength to oscillate such 
wavelength-tunable oscillator at a desired wavelength, there is, for 
example, a known wavelength selecting method for which, for example 
diffraction grating mirrors or birefrigent plates are arranged in the 
laser resonator containing a wavelength-tunable laser. Such diffraction 
grating mirrors or birefrigent plates are mechanically rotated, whereby 
the output light beam having only a desired wavelength is taken out from 
the possible wide wavelength-tunable range of oscillation with tunable 
lasers. As a result of mechanical rotation, the mirrors or plates from an 
optical cavity at a desired wavelength for laser oscillation. 
However, in the case where such a conventional wavelength selecting method 
as described above is executed, the method involves such problems that it 
is required to constitute a rotary mechanism for rotating mechanically the 
diffraction grating mirrors or the birefringent plates in the laser 
resonator. This rotation requirement results in the whole apparatus of the 
mechanism becoming inevitably large-sized. Further, the reliability of 
selecting a wavelength is always restricted by a backlash inherent to the 
rotary mechanism. 
OBJECT AND SUMMARY OF THE INVENTION 
The present invention has been made in view of a variety of the problems 
involved in the prior art. An object of the present invention is to 
provide a method for selecting a wavelength in a wavelength-tunable laser 
and a laser oscillator capable of selecting a wavelength electronically in 
the wavelength-tunable laser by which there is no need of providing a 
mechanically operating section such as a rotating mechanism. The 
eliminator of such a rotating mechanism results in a compact design of the 
whole apparatus and provides a stable action for selecting a wavelength. 
In order to attain the above described object, the method for selecting a 
wavelength in a wavelength-tunable laser and the laser oscillator capable 
of selecting a wavelength in a wavelength-tunable laser according to the 
present invention have been made in a quite different point of view from a 
conventional manner, wherein diffraction grating mirrors or birefringent 
plates are employed. More specifically, the present invention has been 
made as a result of noticing the fact that when acoustic waves are 
produced in a crystal, the plane of polarization or the angle of a 
propagating light beam at a specified wavelength tunes its plane of 
propagation or angle according to the frequency of the acoustic waves is 
altered. 
FIG. 1 is a conceptual view illustrating a wavelength selecting function 
which utilizes a polarization action of the light beam having a specified 
wavelength by means of acoustic waves wherein incident light 102 having a 
wavelength .lambda..sub.i and an angular frequency .omega..sub.i is input 
into a crystal 100 having birefringent property, and in this condition 
when acoustic waves 104 each having a frequency .omega. are applied into 
the crystal 100 to obtain diffracted light 106. 
More specifically, when the incident light 102 and the acoustic waves 104 
are applied into the crystal 100 as described above, the wavelength 
.omega..sub.i of the incident light 102 shifts to the value ".omega..sub.o 
=.omega..sub.e +.omega..sub.i " as a result of the interaction between the 
incident light 102 and the acoustic waves 104, and thus, the diffracted 
light 106 having the angular frequency (.omega..sub.o =.omega..sub.e 
+.omega..sub.i) is obtained. 
Furthermore, in this case, the phase relation ".DELTA.k=k.sub.o -k.sub.e 
-k.sub.i " is simultaneously given wherein k.sub.e represents the 
wavenumber vector of the acoustic waves 104, k.sub.i represents a 
wavenumber vector of the incident light 102, and k.sub.o represents a 
wavenumber vector of the diffracted light 106 and which controls direction 
of the acoustic waves 104 to be applied into the diffracted light 106 is 
perpendicular to the incident light 102. 
In the above described phase relation ".DELTA.k=k.sub.o -k.sub.e -k.sub.i 
", an intensive diffraction appears to convert the k.sub.i wave to the 
k.sub.o wave by a high efficiency when the phase matching condition is 
satisfied, that is, .DELTA.k=0, so that the frequency of the acoustic 
waves 104 is adjusted so as to be .DELTA.k=0. 
In the meantime, the phase matching condition ".DELTA.k=0" varies when the 
wavelength of the incident light 102 changes. Accordingly, when a light 
beam extending over a wide range of wavelength band is input, as the 
incident light 102, into the crystal 100, the plane of polarization is 
rotated in the vertical direction to the incident light 102 in respect of 
only the component of the incident light 102 having the frequency wherein 
.DELTA.k=0 is realized in a relationship with the frequency of the 
acoustic waves 104. Hence, when the diffracted light 106 the plane of 
polarization of which has been rotated in the vertical direction with 
respect to the incident light 102 is taken out by means of a polarizing 
plate 108, only the diffracted light 106 having a desired wavelength can 
selectively be taken out from the incident light 102 extending over the 
wide wavelength range. 
The method for selecting a wavelength in a wavelength-tunable laser and the 
laser oscillator being capable of selecting a wavelength in the 
wavelength-tunable laser according to the present invention have been made 
on the basis of the above described principle. The method for selecting a 
wavelength in a wavelength-tunable laser capable of oscillating a laser in 
a predetermined range of wavelengths is characterized by placing a crystal 
into which have been input acoustic waves in a laser resonator; and 
changing the angle of a polarized light beam or an output light beam with 
respect to a specified wavelength in the wavelength tunable range of the 
wavelength-tunable laser in response to the frequency of the aforesaid 
acoustic waves to oscillate a laser with respect to the specified 
wavelength thereby to change a wavelength to be selected in response to 
the frequency of the acoustic waves. 
The method for selecting a wavelength in a wavelength-tunable laser 
according to the present invention may be altered in a case in which a 
solid crystal is used as a laser active medium by a method for selecting a 
wavelength in a wavelength-tunable laser capable of oscillating a laser in 
a predetermined range of wavelengths is characterized by inputting 
acoustic waves into the crystal of the wavelength-tunable laser capable of 
oscillating a laser in the predetermined range of wavelengths; and 
constituting a laser resonator so as to resonate only with respect to a 
prescribed wavelength wherein the angle of a polarized light beam or an 
output light beam has been changed in response to the aforesaid acoustic 
waves thereby to oscillate the laser with respect to a specified 
wavelengths. 
Furthermore, the laser oscillator capable of selecting a wavelength in a 
wavelength-tunable laser according to the present invention comprises a 
laser resonator being composed of opposed mirrors each having a prescribed 
transmissivity; a wavelength-tunable laser crystal which is placed in said 
laser resonator and can oscillate a laser in a predetermined range of 
wavelengths; a crystal which is also placed in said laser resonator and to 
which is inputted an output light beam from the wavelength-tunable laser; 
an acoustic wave input attached to the latter crystal for inputting 
acoustic waves to the crystal; and a polarizing means which is placed in 
the laser resonator and transmits only the output light beam having a 
prescribed plane of polarization among the output light beams from the 
crystal. 
Furthermore, the laser oscillator capable of selecting a wavelength in a 
wavelength-tunable laser according to the present invention may be altered 
by the one which comprises a laser resonator being composed of opposed 
mirrors each having a prescribed transmissivity; a wavelength-tunable 
laser which is placed in said laser resonator and can oscillate a laser in 
a predetermined range of wavelengths, besides to which is attached an 
acoustic wave inputting means; and a polarizing means which is placed in 
said laser resonator and transmits only the output light beam having a 
prescribed plane of polarization among the output light beams from said 
wavelength-tunable laser.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In addition to the general concept of the present invention discussed above 
in relation to FIG. 1, embodiments of the method for selecting a 
wavelength in a wavelength-tunable solid state laser and the laser 
oscillator capable of selecting a wavelength in the wavelength-tunable 
solid state laser according to the present invention will be described in 
detail hereinbelow in conjunction with the accompanying drawings. 
In FIG. 2, illustrated in a schematic constitutional diagram for explaining 
the laser oscillator capable of selecting a wavelength in a 
wavelength-tunable solid-state laser (hereinafter referred to simply as 
"laser oscillator") for embodying the method for selecting a wavelength in 
the wavelength-tunable solid state laser according to the first embodiment 
of the present invention. 
In the lasers oscillator, a laser resonator is composed of a mirror 10 on 
the input side and a mirror 12 on the output side, both of the mirrors 
each having a prescribed transmissivity. 
In the laser resonator, a Ti:Al.sub.2 O.sub.3 laser crystal 14 as the 
wavelength-tunable solid state laser, a crystal for selecting a wavelength 
16, and a polarizing plate 18 are successively placed in this order from 
the mirror 10 on the inputting side towards the mirror 12 on the 
outputting side. 
Further, to the crystal for selecting a wavelength 16 is attached a 
piezoelectric element 22 driven by an RF power source as an acoustic wave 
input. Accordingly, when the piezoelectric element 22 is driven by the RF 
power source 20 to produce a strain, acoustic waves each having a 
frequency corresponding to the strain of the piezoelectric element are 
input to the crystal for selecting a wavelength 16 on the basis of the 
strain of the piezoelectric element 22. 
Moreover, the polarizing plate 18 is arranged in such that only the light 
beam having a prescribed plane of polarization is transmitted through. 
Furthermore, the piezoelectric element 22 inputs acoustic waves to the 
crystal for selecting a wavelength 16 in such a manner that the plane of 
polarization of the output light beam having the wavelength of an output 
laser beam 26 which is intended to output from the mirror 12 on the 
outputting side becomes a prescribed plane of polarization which transmits 
through the polarizing plate 18. 
In the above arrangement, the Ti:Al.sub.2 O.sub.3 laser crystal 14 is 
excited by using the second harmonic of a Nd:YAG laser as excitation laser 
beam 24. On the other hand, the frequency of the RF power source 20 is 
controlled in response to the wavelength of the output laser beam 26 which 
is intended to output from the mirror 12 on the output side based on the 
above described principle, whereby the piezoelectric element 22 is driven. 
According to the above described arrangement, the plane of polarization of 
an output light beam having the wavelength in response to the frequency of 
the RF power source 20 is rotated among light beams input to the crystal 
for selecting a wavelength 16, i.e. the output light beams extending over 
a wide range of wavelengths which are output from the Ti:Al.sub.2 O.sub.3, 
laser crystal 14, whereby the prescribed plane of polarization is 
attained, and the resulting light beam is outputted from the crystal for 
selecting a wavelength 15. Thus, only the output light beam having the 
above described prescribed plane of polarization among the output light 
beams output from the crystal for selecting a wavelength 16 is transmitted 
through the polarizing plate 18, the light beam thus transmitted is 
reflected by the mirror 12 on the outputting side, and as a result the 
light beam reciprocates in the laser resonator. Consequently, only the 
light beam having the aimed frequency is amplified to produce a laser 
oscillation, whereby only the output laser beam 26 having a desired 
wavelength can be outputted from the laser resonator. 
Furthermore, illustrated in FIG. 3 is a schematic constitutional diagram 
for explaining the laser oscillator according to the second embodiment of 
the present invention wherein the same components as that of the first 
embodiment are designated by the same reference numerals as that of the 
former embodiment. 
More specifically, in also the laser oscillator according to the second 
embodiment, a laser resonator is composed of a mirror 10 on the input side 
and a mirror 12 on the output side, both of the mirrors each having a 
prescribed transmissivity as same as in the laser oscillator of the first 
embodiment. 
In the laser resonator, a Ti:Al.sub.2 O.sub.3 laser crystal 14 as the 
wavelength-tunable solid state laser and a polarizing plate 18 are 
successively placed from the mirror 10 on the input side towards the 
mirror 12 on the output side. 
Further, to the Ti:Al.sub.2 O.sub.3 laser crystal 14 is attached a 
piezoelectric element 22 driven by an RF power source as an acoustic wave 
inputting means. Accordingly, when the piezoelectric element 22 is driven 
by the RF power source 20 to produce a strain, acoustic waves each having 
a frequency corresponding to the strain of the piezoelectric element are 
inputted to the Ti:Al.sub.2 O.sub.3 laser crystal 14 on the basis of the 
strain of the piezoelectric element 22. 
Moreover, the polarizing plate 18 is arranged as in the first embodiment in 
such that only the light beam having a prescribed plane of polarization is 
transmitted to constitute a laser cavity wherein the piezoelectric element 
22 inputs acoustic waves to the Ti:Al.sub.2 O.sub.3 laser crystal 14 in 
such a manner that the plane of polarization of the output light beam 
having the wavelength of an output laser beam 26 which is intended to 
output from the mirror 12 on the outputting side becomes a prescribed 
plane of polarization which transmits through the polarizing plate 18. 
In the above arrangement, the Ti:Al.sub.2 O.sub.3 laser crystal 14 is 
excited by using the second harmonic of a Nd:YAG laser as excitation laser 
beam 24. On the other hand, the frequency of the RF power source 20 is 
controlled in response to the wavelength of the output laser beam 26 which 
is intended to output from the mirror 12 on the output side based on the 
above described principle, whereby the piezoelectric element 22 is driven. 
According to the above described arrangement, only the plane of 
polarization of the output light beam having a wavelength in response to 
the frequency of the RF power source is rotated among the output light 
beams extending over a wide range of wavelengths which are output from the 
Ti:Al.sub.2 O.sub.3 laser crystal 14, whereby the resulting light beam is 
output. Thus, only the output light beam which is outputted from the 
Ti:Al.sub.2 O.sub.3 laser crystal 14 and the plane of polarization of 
which has been rotated in transmitted through the polarizing plate 18, the 
light beam thus transmitted is reflected by the mirror 12 on the output 
side, and as a result the light beam reciprocates in the laser resonator. 
Consequently, only the light beam having the aimed frequency is amplified 
to produce a laser oscillation, whereby only the output laser beam 26 
having a desired wavelength can be output from the laser resonator. 
FIG. 4 is a graphical representation showing the results of an experiment 
which was conducted by employing the laser oscillator illustrated in the 
first embodiment under the following experimental conditions. The graph 
indicates a relationship between the outputs and the wavelengths of the 
output laser beam 26 in the case when a frequency of the RF power source 
20 is changed. As is apparent from the experimental results shown in FIG. 
4, the laser oscillator according to the present invention can effect 
laser oscillation with an arbitrarily selected wavelength so far as the 
wavelength is within a range of from about 700 nm to about 800 nm. 
(Experimental Conditions) 
Excitation Laser Beam 24: Second Harmonic of a pulsed Nd:YAG 
Laser, Wavelength 532 nm, Energy 100 mJ, 
Pulse Width 8 ns 
Mirror on Input Side: 99% Reflection 
Mirror on Reflecting Side: 30% Reflection at 755 nm Wavelength 
RF Power Source: Frequency Variable Range 40 MHz.about.150 MHz Input 
Electric Power 1 W 
While the explanation has been made on the case where the plane of 
polarization is rotated by means of acoustic waves produced by the 
piezoelectric element 22 in the above described embodiment, it may be 
modified in such that the direction of acoustic waves produced by the 
piezoelectric element 22 and which are to be applied to the crystal for 
selecting a wavelength 16 or the Ti:Al.sub.2 O.sub.3 laser crystal 14 is 
controlled, whereby an output angle of the output light beam having a 
specified frequency in response to the wavelength of the acoustic waves is 
changed among the output light beams outputted from the crystal for 
selecting a wavelength 16 or the Ti:Al.sub.2 O.sub.3 laser crystal 14. 
Furthermore, it is to be noted that the present invention is also 
applicable to the other wavelength-tunable lasers such as a dye laser. 
Since the present invention is constructed as described above, the 
wavelength of an output laser beam can be selected with provision of no 
mechanically operation section, such as a rotating mechanism. Therefore, 
in accordance with the present invention, down-sizing of the whole 
apparatus can be achieved and stable operation for selecting a wavelength 
can be realized. 
It will be appreciated by those of ordinary skill in the art that the 
present invention can be embodied in other specific forms without 
departing from the spirit or essential characteristics thereof. 
The presently disclosed embodiments are therefore considered in all 
respects to be illustrative and not restrictive. The scope of the 
invention is indicated by the appended claims rather than the foregoing 
description, and all changes that come within the meaning and range of 
equivalents thereof are intended to be embraced therein.