Method and Device for Laser Radiation Modulation

Abstract

The proposed method and device relate to acousto-optics and laser technology and can be attributed, in particular, to acousto-optical (AO) laser resonator Q-switches, AO devices for extra-cavity control of single-mode (collimated) and multimode (uncollimated) monochromatic and non-monochromatic laser radiation, i.e, AO modulators, AO frequency shifters, and dispersion delay lines for visible and middle IR wavelengths (0.4-5.5 μm). The object of the method and device is providing a geometry of AO interaction in laser resonator Q-switches so that to optimize the preset parameters of the Q-switch in accordance with the system requirements to the laser operation mode depending on the intended use of the laser, more specifically, lower control RF power and capability of operation without additional efficiency loss with multimode or uncollimated laser radiation.

Claims

1. Laser radiation modulation method comprising excitation in a KRE(WO.sub.4).sub.2 group single crystal of a amplitude-modulated traveling quasi-shear acoustic wave with the polarization orthogonal to the N.sub.p axis and propagating in the N.sub.mN.sub.g plane of the crystal, wherein the laser beam has the polarization of the eigenwave in the crystal and propagates at Bragg angles from 0.15 to 8 arc deg relative to the acoustic wavefront and the acoustic wave frequency in the acousto-optic prism meets the phase matching condition for laser beam diffraction.

2. Acousto-optic Q-switch comprising an acousto-optic prism made from a KRE(WO.sub.4).sub.2 group single crystal the acoustic surface of which is parallel to the N.sub.p axis of the crystal and is at an angle of 0 to −40 arc deg to the N.sub.m axis, an opposite surface which is at an arbitrary angle to said acoustic surface, an acoustic absorber attached to said opposite surface, an input optical surface with an antireflection coating, an output optical surface with an antireflection coating, and a shear piezotransducer made from a lithium niobate plate with a thickness of 15 to 200 μm attached to said acoustic surface.

3. Acousto-optic Q-switch of claim 2 wherein said KRE(WO.sub.4).sub.2 group single crystal is a potassium gadolinium tungstate KGd(WO.sub.4).sub.2 crystal or a potassium yttrium tungstate KY(WO.sub.4).sub.2 crystal or a potassium lutetium tungstate KLu(WO.sub.4).sub.2 crystal or a potassium ytterbium tungstate KYb(WO.sub.4).sub.2 crystal.

4. Acousto-optic Q-switch of claim 2 wherein said piezotransducer is attached to said acousto-optic prism using glue attachment or using direct dielectric bonding or using vacuum diffusion bonding with the formation of binary alloys or using atomic diffusion bonding of similar alloys.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0022] The invention is illustrated with the following drawings.

[0023] FIG. 1. Polar projection of AO figure of merit of non-collinear geometry of isotropic AO diffraction for quasi-shear (QS) acoustic wave propagating in the N.sub.mN.sub.g plane of potassium yttrium tungstate.

[0024] FIG. 2. AO figure of merit of isotropic AO diffraction for quasi-longitudinal (QL) and quasi-shear (QS) acoustic waves propagating in the N.sub.mN.sub.g plane of potassium yttrium tungstate.

[0025] FIG. 3. Vector diagram of diffraction in AO Q-switch.

[0026] FIG. 4. Phase velocity of ultrasound and deflection angle in the N.sub.mN.sub.g plane of potassium yttrium tungstate.

[0027] FIG. 5. AO prism orientation relative to crystal symmetry axes.

[0028] FIG. 6. AO Q-switch design.

[0029] FIG. 7. Photo of experimental KY(WO.sub.4).sub.2 crystal AO Q-switch.

[0030] The notations in FIGS. 5 and 6 are as follows: (1) potassium yttrium tungstate AO prism; (2) crystal acoustic surface; (3) crystal surface opposite to acoustic one; (4) crystal input optical surface; (5) crystal output optical surface; (6) shear piezotransducer; (7) acoustic absorber; (8) input laser beam; (9) input beam polarization vector; and (10) quasi-shear elastic wave in crystal.

[0031] The technical result of the first object of the invention is achievable because an amplitude-modulated traveling acoustic wave is generated in a single crystal with large acoustic anisotropy in a direction other than the crystal's symmetry axis. As a result the directions of the phase and group acoustic wave velocities differ and the acoustic beam cross-section becomes smaller than the area of the piezotransducer, therefore the AO Q-switch operation becomes faster. The laser beam has the polarization of the proper wave in the crystal and propagates at the Bragg angle, and the acoustic wave frequency meets the phase matching condition.

[0032] The single crystal belongs to the KRE(WO.sub.4).sub.2 group, the acoustic wave is a quasi-shear one, propagates in the N.sub.mN.sub.g plane of the crystal and is polarized orthogonally to the N.sub.p axis of the crystal, and the laser beam direction which is polarized parallel to the N.sub.g axis of the crystal is at a Bragg angle of 0.15 to 8 arc deg relative to the acoustic wavefront.

[0033] The technical result of the second object of the invention is achievable because the Q-switch is operated with a quasi-shear acoustic wave propagating along the crystal's symmetry axis. Here N.sub.m and N.sub.g form a Cartesian coordinate system related to the dielectric axes of the crystal. The second order symmetry axis N.sub.p is directed perpendicular to the drawing plane. The AO figure of merit M.sub.2 of the crystal for the quasi-shear acoustic wave is shown by a solid line for two proper polarizations of light wave in the crystal (solid line: polarization along N.sub.m, dashed line: polarization along N.sub.g). The elastic, photo-elastic and optical constants of the KRE(WO.sub.4).sub.2 group crystals are close. Hereinafter the calculations are performed for yttrium tungstate KY(WO.sub.4).sub.2.

[0034] It can be seen from FIGS. 1 and 2 that if the light is polarized along the N.sub.g axis the AO figure of merit M.sub.2 of the crystal is as high as 22×10.sup.−15 s.sup.3/kg at a quasi-shear acoustic wave propagation angle of −12 arc deg relative to the N.sub.m axis which is only 35% smaller than the AO figure of merit M.sub.2 for classic orientation AO Q-switch for fast longitudinal wave in paratellurite which has been used in industrial AO Q-switches for more than 50 years. In the 0 to 28 arc deg range the AO figure of merit is above 15×10.sup.−15 s.sup.3/kg, i.e., it is by more than 10 times higher than the maximum AO figure of merit of fused silica. The AO figure of merit M.sub.2 of the prototype for quasi-longitudinal ultrasonic wave along the N.sub.g axis is within 10×10.sup.−15 s.sup.3/kg. Thus, the present invention eliminates the first disadvantage of the prototype, i.e., the relatively high control HF power.

[0035] FIG. 3 schematically shows the geometry of AO interaction in an isometric projection as per the present invention. The birefringence and the Bragg angle are shown oversized for demonstrativeness. Dashed lines show the sections of the light wave normal surface by the N.sub.mN.sub.g and N.sub.pN.sub.g planes and the diffraction plane which is parallel to the N.sub.p axis and is at a −12 arc deg angle to the N.sub.m axis.

[0036] A specific essential feature of the invention is that the piezotransducer plate made from a lithium niobate crystal is attached to the acoustic surface of the AO prism made from a KRE(WO.sub.4).sub.2 crystal by a unique vacuum nanotechnology with the formation of binary alloys (RU Patent 2646517C1 05.03.2018) which reduces conversion losses for HF electric power conversion to acoustic power as compared with other attachment technologies.

[0037] The other disadvantage of the prototype which hinders the operation of the AO Q-swtich with multimode laser radiation is the reduced AO Q-switch diffraction efficiency for operation with divergent radiation the divergence of which is comparable with or exceeds the diffraction divergence of the acoustic wave generated by the piezotransducer.

[0038] The physical origin of this phenomenon is that in this case the high-frequency components of the light wave angular spectrum do not meet the Bragg phase matching condition with the angular spectrum of the acoustic wave and therefore their participation in diffraction is little if any. The diffraction divergence of the acoustic wave generated by the homogeneous piezotransducer is described by the formula v/Lf, where v is the velocity of the acoustic wave, L is the length of the piezotransducer and f is the frequency.

[0039] The technical result of the invention is particularly illustrated on FIG. 4, and achieved because the velocity of the quasi-shear acoustic wave corresponding to the maximum AO figure of merit M.sub.2 is reached at an angle of −12 arc deg and is equal to 2.4×10.sup.3 m/s; the velocity of the quasi-longitudinal acoustic wave of the prototype at −90 arc deg is 4.8×10.sup.3 m/s. Thus, other conditions being the same, the acoustic angular spectrum of this invention is 2 times broader compared to that of the prototype. Therefore, other conditions being the same, the AO Q-switch provided herein unlike the prototype can be operated without compromise in efficiency with multimode or uncolimated laser radiation the divergence of which is 2 times greater than the divergence of collimated radiation.

[0040] The acoustic anisotropy of the crystal shows itself, in particular, in that the angle ψ between the direction of the wave vector K and the group velocity S of the quasi-shear acoustic wave in the N.sub.mN.sub.g crystallographic plane of the potassium yttrium tungstate crystal polarized orthogonally to the N.sub.p axis may exceed 30 arc deg by absolute value, as shown in FIG. 4. In particular, in the −12 arc deg direction relative to the N.sub.m axis in which the AO figure of merit M.sub.2 is the maximum for the light wave polarized parallel to the N.sub.g axis, the angle ψ is approximately −23 arc deg.

[0041] The KRE(WO.sub.4).sub.2 group crystals have high laser-induced damage threshold and sufficiently high AO effect which makes them the most promising material for acousto-optic Q-switches, dispersion delay lines and AO frequency shifters for visible and middle IR wavelengths. For example, the minimum laser damage threshold of KGd(WO.sub.4).sub.2 crystals is 50 GW/cm.sup.2 for 20 ns pulses at 1064 nm (I. V. Mochalov, “Laser and nonlinear properties of the potassium gadolinium tungstate laser crystal KGd(WO.sub.4).sub.2:Nd.sup.3+-(KGW:Nd)”, Optical Engineering 36 (1997) 1660-1669). KRE(WO.sub.4).sub.2 group materials have high optical and acoustic anisotropy which depends largely on the crystal orientation relative to the crystallographic axes.

PREFERRED EMBODIMENTS OF THE INVENTION

[0042] The present invention is implemented as follows. The acousto-optic Q-switch comprises an AO prism 1 made from a KRE(WO.sub.4).sub.2 group single crystal and having an acoustic surface 2 which is parallel to the N.sub.p axis of the AO prism 1 crystal, its normal being at an angle of 0 to −30 arc deg relative to the N.sub.m axis, an opposite surface 3, an input optical surface 4 which is orthogonal to the N.sub.p axis, an output optical surface 5 which is orthogonal to the N.sub.p axis, a piezotransducer 6 attached to said acoustic surface 2, and an acoustic absorber 7 attached to said opposite surface 3. Said piezotransducer 6 made from a lithium niobate plate with a thickness of 15 to 200 μm excites a quasi-shear acoustic wave 10 in said AO prism 1. Said acoustic absorber 7 is attached to the surface 6 of said AO prism 1 which is at an arbitrary angle to said acoustic surface 2 thus providing a traveling acoustic wave in said AO prism 1. The input laser beam 8 has the polarization 9 parallel to the N.sub.g axis of the crystal and propagates at a Bragg angle of 0.5 to 1.5 arc deg relative to the normal in the diffraction plane formed by the N.sub.p axis of the crystal and the normal to said acoustic surface 2 of said AO prism 1.

[0043] For reducing the control HF power said piezotransducer can be attached using the unique vacuum technology with the formation of binary alloys to said acoustic surface 3 of said AO prism 1. Said piezotransducer alternatively can be attached to said acoustic surface 3 of said AO prism 1 using glue attachment or using atomic diffusion bonding of similar metals (T. Shimatsu and M. Uomoto, “Atomic diffusion bonding of wafers with thin nanocrystalline metal films”, J. Vac. Sci. Technol. B 28 (2010) 706-704) or using direct bonding (K. Eda, K. Onishi, H. Sato, Y. Taguchi, and M. Tomita, “Direct Bonding of Piezoelectric Materials and Its Applications”, Proc. 2000 IEEE Ultrasonics Symposium (2000) 299-309), providing for an acoustic contact between the bonded surfaces.

[0044] Said acoustic wave absorber 7 can be fabricated using the unique vacuum technology on the basis of a binary alloy with indium excess for efficient absorption of the traveling shear acoustic wave.

[0045] The present invention was tested experimentally. The instant inventors fabricated an experimental AO Q-switch from a potassium yttrium tungstate crystal for operation with horizontally polarized input laser radiation, and confirmed our calculation data. FIG. 7 shows a photo of the fabricated experimental AO Q-switch. The active aperture of the AO Q-switch was 2.0 mm, the piezotransducer length was 14.0 mm, and the working frequency of the ultrasound was 100 MHz. The measurements were carried out at 532 nm. The maximum diffraction efficiency was 96% at a control power of 15 W. The main parameters of the AO Q-switch if recalculated for a 1064 nm wavelength were as follows: efficiency in excess of 95% at a control power of 2.0 W and a piezotransducer length of 40 mm