The present invention relates to a laser oscillation apparatus using a semiconductor laser as an excitation source.
First, general features of a laser oscillation apparatus will be described referring to FIG. 11.
FIG. 11 is a laser oscillation apparatus 1, schematically showing a laser oscillation apparatus using a second harmonic, and it comprises a laser light source 2, a condenser lens 3, a laser crystal 4, a nonlinear optical medium 5, an output mirror 6, and laser driving means 9.
The laser light source 2, the condenser lens 3, the laser crystal 4, the nonlinear optical medium 5, and the output mirror 6 having a concave reflection surface are arranged on a same optical axis 10. A first dielectric reflection film 7 is formed on a surface of the laser crystal 4 facing to the condenser lens 3, and a second dielectric reflection film 8 is formed on a concave reflection surface of the output mirror 6 facing to the nonlinear optical medium 5.
The laser light source 2 generates a laser beam. In the present embodiment, a semiconductor laser is used, and the laser light source 2 has a function as a pump light generator for generating a fundamental wave. The laser light source 2 is driven by the laser driving means 9, and the laser driving means 9 can drive the laser light source 2 by pulse driving.
The laser crystal 4 is a medium of negative temperature and it is used for amplification of the light. As the laser crystal 4, YAG (yttriumxe2x80x94aluminumxe2x80x94garnet), etc. doped with Nd3+ ions is adopted. YAG has oscillation lines of 946 nm, 1064 nm, 1319 nm, etc. In addition to YAG, Nd with oscillation line at 1064 nm (YVO4), or Ti with oscillation lines at 700 to 900 nm (Sapphire), etc. is used as the laser crystal 4.
The first dielectric reflection film 7 is highly transmissive to the laser light source 2 and is highly reflective to an oscillation wavelength of the laser crystal 4. It is also highly reflective to a second harmonic. The second dielectric reflection film 8 is highly reflective to the oscillation wavelength of the laser crystal 4 and is highly transmissive to the second harmonic.
As described above, it may be designed in such manner that the laser crystal 4 is combined with the output mirror 6, the laser beam from the laser light source 2 enters through the condenser lens 3, and the entered laser beam is reflected between the first dielectric reflection film 7 and the second dielectric reflection film 8 and is pumped to the laser crystal 4. The laser beam can be confined for a long time between the first dielectric reflection film 7 and the second dielectric reflection film 8 through the nonlinear optical medium 5. As a result, the laser beam can be resonated and amplified, and a laser beam of the second harmonic can be projected through the output mirror 6.
Brief description will be given on the nonlinear optical medium 5.
When an electric field is applied on a substance, electrical polarization occurs. When the electric field is small, the polarization is proportional to the electric field. However, in case of strong coherent light such as the laser beam, proportional relationship between the electric field and the polarization is impaired, and nonlinear polarization components proportional to square or cube of the electric field become prominent.
Therefore, in the nonlinear optical medium 5, the polarization generated by the laser beam contains a component proportional to square of the light wave electric field. By the nonlinear polarization, bonding occurs between light waves with different frequencies, and a second harmonic to double the light frequency is generated. The generation of the second harmonic is generally called xe2x80x9cSHG (second harmonic generation)xe2x80x9d.
In the conventional example as described above, the nonlinear optical medium 5 is placed in an optical resonance unit, which comprises the laser crystal 4 and the output mirror 6, and this is called as internal type SHG. Conversion output is proportional to square of fundamental wave opto-electric power, and high light intensity in the optical resonance unit can be directly utilized.
As the nonlinear optical medium 5, for instance, KTP (KTiOPO4; titanyl potassium phosphate), BBO (xcex2-BaB2O4; xcex2-lithium borate), LBO (LiB3O5; lithium triborate), etc. are used. Primarily, it is converted from 1064 nm to 532 nm.
KNbO3 (potassium niobate), etc. is also adopted. Primarily, it is converted from 946 nm to 473 nm. In FIG. 11, xcfx89 is an angular frequency of the optical fundamental wave, and 2xcfx89 is an angular frequency of the second harmonic.
In the laser oscillation apparatus using the second harmonic, for the purpose of generating a higher harmonic from the optical fundamental wave oscillating in the optical resonance unit using a nonlinear crystal (KTP crystal), the following conditions are needed:
(1) Temperature control of the nonlinear crystal (phase coordination temperature at constant level of 25xc2x0 C.)
(2) Phase coordinating conditions of the nonlinear crystal which is satisfied by adjusting a nonlinear crystal axis with respect to a fundamental wave oscillation axis in the optical resonance unit.
Therefore, the laser oscillation apparatus of conventional type has a cooling mechanism and an aligning mechanism of nonlinear crystal axis.
Referring to FIG. 12, description will be given now on the cooling mechanism and the aligning mechanism of the nonlinear crystal axis used in the past.
On an optical resonator block 11 made of a material with high heat transfer property, a recessed portion 12 for accommodating a nonlinear optical medium 5 is formed. An optical path hole 13 is provided, which passes through the recessed portion 12 and has an axis aligned with an optical axis 10 of the laser oscillation apparatus. The optical path hole 13 is cut by the recessed portion 12. A laser crystal 4 is disposed on a part of the optical path hole 13 closer to an incident side, and an output mirror 6 is provided on an exit side end of the optical path hole 13. On the lower surface of the optical resonator block 11, a Peltier element 14 is fixed.
The nonlinear optical medium 5 is placed on and closely fitted to the bottom surface of the recessed portion 12. The nonlinear optical medium 5 is held at the lower end of an angle adjusting jig 15. The angle adjusting jig 15 has a knob 15a extended in a direction of xcex8 axis 16 running perpendicularly to the optical axis 10. The knob 15a is protruded from the recessed portion 12, and an angle xcex8 for the nonlinear optical medium 5 can be adjusted around the xcex8 axis 16 by the knob 15a. 
The non linear optical medium 5 is placed so that a nonlinear crystal axis of the nonlinear optical medium 5 is aligned with the optical axis 10. Because the nonlinear optical medium 5 is cut out along the nonlinear crystal axis, when the nonlinear optical medium 5 is closely fitted to the bottom surface of the recessed portion 12, the position of the nonlinear crystal axis is determined within a horizontal plane with respect to the optical axis 10. The nonlinear optical medium 5 is rotated by turning the knob 15a while it is pressed against the bottom surface of the recessed portion 12, and the angle xcex8 is adjusted so that the optical axis 10 and the nonlinear crystal axis run in parallel to each other within the same plane.
When the adjustment has been completed, the nonlinear optical medium 5 is fixed on the optical resonator block 11 by adequate means such as bonding or by a screw.
An excitation light passing through the first dielectric reflection film 7 is absorbed by the laser crystal 4. A fundamental wave oscillated by the laser crystal 4 is reflected between the first dielectric reflection film 7 and the second dielectric reflection film 8, and a second harmonic generated from the nonlinear optical medium 5 is projected through the output mirror 6.
As described above, the nonlinear optical medium 5 is a medium of negative temperature. In order to obtain predetermined stable second harmonic output, it is necessary to perform temperature control of the nonlinear optical medium 5. The nonlinear optical medium 5 is cooled down by the Peltier element 14 via the optical resonator block 11.
The temperature of the nonlinear optical medium 5 is detected by a thermister 17 installed in the optical resonator block 11. Based on the detected temperature of the thermister 17, electric current to the optical resonator block 11 is controlled, and the temperature of the nonlinear optical medium 5 is controlled via the optical resonator block 11.
The output of the laser beam becomes higher in recent years, and the laser light source 2 comprises now a plurality of semiconductor lasers instead of a single semiconductor laser. As a method in order to bundle the laser beams emitted from a plurality of semiconductor lasers to a single luminous flux, such method is adopted that the laser beams emitted from the semiconductor lasers are received separately through corresponding optical fibers, and the optical fibers are bundled together to form a single cable. By this cable, the laser beams are guided to the condenser lens 3, and a single luminous flux is projected.
In the conventional example as described above, the nonlinear optical medium 5 is cut out in such manner that the nonlinear crystal axis of the nonlinear optical medium 5 is in parallel to the bottom surface of the recessed portion 12. Actually, however, there are errors during cut-out operation, and the axes are not always accurately parallel to each other. In case the required output is low or in case there is some surplus in the output, it is not necessary to adjust the angle of the nonlinear optical medium 5 to the bottom surface of the recessed portion 12. However, this does not provide sufficiently high accuracy when higher output efficiency is required or in case the output near the theoretical limit is needed.
The nonlinear optical medium 5 is placed on and is closely fitted to the optical resonator block 11 via its one surface, and a heat transfer rate of the nonlinear optical medium 5 to the optical resonator block 11 is strongly influenced by contact condition of the surfaces.
When the angle adjusting jig 15 is removed after once angle adjustment of the nonlinear optical medium 5 is completed, phenomenon such that the second harmonic output changes occurs. When the angle adjusting jig 15 has been separated from the nonlinear optical medium 5, which had been integrated with the angle adjusting jig 15 at the angle adjustment, transfer of heat to the angle adjusting jig 15 is lost, and the pressure from the angle adjusting jig 15 to press the nonlinear optical medium 5 is lost. As a result, changes occur in the heat transfer rate of the optical resonator block 11, and temperature distribution (temperature gradient) inside the nonlinear optical medium 5 is changed.
It is an object of the present invention to provide a laser oscillation apparatus, by which it is possible to align a nonlinear crystal axis of a nonlinear optical medium with higher accuracy, to strictly satisfy phase coordinating conditions of the nonlinear crystal of the nonlinear optical medium, and to prevent the change of output conditions after the alignment of the nonlinear crystal axis.
To attain the above object, the laser oscillation apparatus according to the present invention comprises a laser beam emitting unit for emitting an excitation light, an optical resonance unit at least having a laser crystal and an output mirror, a nonlinear optical medium which is placed in the optical resonance unit and which generates a second harmonic from a fundamental wave oscillated by the laser crystal, and a driving means for driving the laser beam emitting unit, wherein the nonlinear optical medium is held by a nonlinear optical medium holder, and the nonlinear optical medium holder can be rotated with respect to axes of at least two directions crossing to an optical axis passing through the optical resonance unit. Also, the present invention provides the laser oscillation apparatus as described above, wherein a nonlinear optical medium for generating a sum frequency wave is used instead of the nonlinear optical medium for generating the second harmonic. Further, the present invention provides the laser oscillation apparatus as described above, wherein a nonlinear optical medium for generating a difference frequency wave is used instead of the nonlinear optical medium for generating the second harmonic. Also, the present invention provides the laser oscillation apparatus as described above, wherein a Peltier element is integrally incorporated in the nonlinear optical medium holder. Further, the present invention provides the laser oscillation apparatus as described above, wherein the optical resonance unit comprises an optical resonator block, the nonlinear optical medium holder is rotatably mounted on a spherical seat of the optical resonator block and is tiltably mounted on the optical resonator block via the spherical seat.