Bhar, Gopal C

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Indian J .  Phys. 6 6 B  (5  &  6 ). 563 -  567  (1992 )Tunable ultraviolet and far infrared lasers for plasma diagnosticsGopal CBharLaser“Laboratoiy. Department of lliytics. University of Burdwan, Burdwan-713 104,West Bengal, IndiaAbstract : Generation of tunable laser radiation is considered m the ultraviolcl for thermonuclear fusion research and in the far infrared for nl^^ma diacnosucs. Both calctoncs vif laser radiation is generated by nonlinear interaction of laser radiation in a suitable material medium using K d : Y A G  laser as the pnmary source. For the ultraviolet sum-frequency mixing is employed tn Barium Borate crystal while for the far infrared self-healing of dye laser radiation is used.K e y w o rd s  : 1’unable laser, non-lmear malcnals, parametric devices, frequency conversion.P A C S  Nos. : 62.65. +k, 42.70.Nq, 42..55.Rz1. IntroductionLasers play an important role in plasma physics. These include generation, confinement, diagnostics, thermonuclear fusion and so on. Generation by existing high power high energy Nd : Glass and CO2 lasers is being pursued in numerous laboratories all over the world. Characterisation of the generated plasma is routinely carried out by microwave radiation and far infrared lasers. However, lasers most suitable for excitation and absorption of energy by the generated plasma and conducive to realise Lawson criterion should operate in the wavelength range 250-400 nm [1], The development of strong coherent source in the UV is also important for application in photochemisu'y and phoiobiology. But there are few lasers as such, in the ultraviolet and in Uic far infrared as required for purposes ol {)lasma diagnostics and controlled thermonuclear fusion research. Out of the few methods available for generation of laser radiation in such wavelength range the techniques of nonlinear optibs f2J is attractive and is potentially capable of providing large efficiency and wide tuning range. These arc described in Sections 2 and 3.2. Generation of tunable ultraviolet radiationThere are few primary lasers as .such in the UV range. It is only through frequency-mixing techniques such as, harmonic generation and sum-lrcqucncy mixing of primary lasers in the visible and near infrared in appropriate non-linear crystals such radiations are generated. The crystals of importance arc : Barium borate (BBO), Lithium iriboralc (LBO), Potassium tilanyl pho.sphaie (KTP), Urea and Potassium niobalc. Table 1 illustrates some relevant properties of the crystals as compared to the Jamiliar KDP crystal. Each crystal is associated© 1992 lACS564 Gopal C Bharwith some advantages and disadvantages. The desirable properties are : wide possible transmission range, high laser damage threshold, adequate birefringence to allow efficient interaction and low susceptibility to temperature sensitivity. However, a small temperatureTabic 1 . Useful properties of some important nonlinear crystals.Crystal 'IVansnussion range (nm)Relative nonlinear figure of meritDamage threshold (GW / cm^)T(X<m)KDP 200  - 12 0 0 1 0 .20 7.0LiNb0 3 400 - 5000 23 0.05 0.6UJO3 300 - 5500 40 0.50 50.0BBO 190 - 3500 26 10 .0 0 37.0Urea 210 - 1400 10 1.50IB O 165 - 3200 7 6 .00 4.2KTP 350 - 4500 215 1.0 0 25.0KNbOs 400 - 5500 270 0.35 0.3width would allow in addition to angular tunability temperature tuning in non-linear devices. Out of these BBO is near to idealism and that is why it has been the subject matter of various investigation. And various nonlinear devices have already been successfully demonstrated [3-5].Tabic 2 . Generation of UV-Visible-ND^ radiation by some nonlinear techniques.SI Crystal InlcracUon Input / s Generated Special featureNo laser (nm) wavelengths (nm) efficiency (%)1 . BBO SIIG Dye 205 - 3 10 36.02 . BBO OK) 308 354 - 2370 10 .03. BBO OIK) 360 406 - 3170 8.54. BBO SFG Nd. Dye 217 - 291 4.05. UNb03 OPO 1064 600 - 37006 . UNb03 OIK) 1060, 532 1300 - 48007. LiNb03 DFG Dyes 1500 - 48008 . KTP DFG 1064, Dye 1400 - 16009. Kl'P OIK) 1064 1800 - 24001 0 . Ur6a OPO 1064 1800 . 2400Tunable ultraviolet, visible and near infrared radiation have been generated by harmonic generation of dye laser, by sum-frequency mixing of different primary la.sers and by optical parametric oscillators pumped by different lasers. The latter include second and third harmonic of Nd laser, excimer laser, alexandrite lasers. Generation of longest infrared wavelength till its transmission limit 3.4 micron has also recently been reported by difference-Tunable ultraviolet andfar infrared lasers etc 565frequency mixing [5]. Table 2 summarizes some of the demonstrated techniques for generation of tunable ultraviolet radiation in some nonlinear crystals. The shortest ultraviolet transmission of 160 nm is provided by LBO crystal. However, in BBO deep ultraviolet tuning 189 - 197 nm is obtained by sum-frequency mixing of excimer 248.5 nm with dye laser 788 to 950 nm [6J. By second harmonic generation of dye laser, the shortest obtainable tuning limit is 204 nm.We have reported sum-frequency mixing of Nd laser and its harmonics with same pumped dye laser in BBO. The phase matched short wavelength tuning obtainable by sum- frequency mixing of second harmonic 532 nnn and dye laser is 268 nm while that by thirdF igu re  1. Schematic experimental set up for tunable ultraviolet generation by sum-frequency mixing. M ] and M 2 arc the dielectric coated mirrors and N L X  stands for nonlinear crystal (BBO). W  | and W 2 aie the Nd . Y A G  and dye laser radiations while W , is the generated ultraviolet radiationF igu re  2. Angular tuning characteristics for generation of tunable ultraviolet fn a B B O  cry.stal using tunable dye In.ser radiation and 532 nm la.scr radiation as input sources The smooth curve is theoretically predicted while dots —  represents the expenmental points The experiment is done ai a noncollinear angle a  = 3.3°harmonic 355 nm and dye laser is 203 nm. Sum-frcqucncy mixing with the fundamental 1064 nm and dye lasers provide shortest wavelength 360 nm. The experimental arrangement is shown in Figure 1. The major parr of the Nd : YAG laser beam is used to pump dye laser while the remaining part directly or by harmonic generation in a DCDA crystal is combined with the generated dye laser beam in a 22.8  ^  ^ BBO crystal for sum-frcqucncymixing. With this crystal cut we have been able to tunc as short as 246 nm in the ultraviolet end. The tuning characteristics is shown in the Figure 2. Using a noncollincar interaction the generation is made in crystal with near collincar interaction volume.3. Generation of far infrared radiationNonlinear optical technique is attractive because it is potentially wide tunable in contrast to fixed frequency optically pumped lasers and raman lasers in the far hifrared. Dillcrencc-R SjCrA/^ l566 Gopal C  Bharfrequency mixing in an appropriate nonlinear crystal excited by strong input pump lasers is capable of providing suitable FIR source. The input pump lasers may be dye laser, Ruby laser, Nd Glass laser or CO 2 laser [7-9]. Unlike the common nonlinear processes where all the three interacting frequencies lie within the crystals defined transmission range, only the two input laser wavelength lie here while the generated signal lie in the far infrared. As such crystal with good far infrared transmission is required. Various common nonlinear crystals have been proposed. Out of these LiNbOa, ZnGeP2, AgGaS2 and AgGaSe2 ai*e important as regards availability and matching with pump lasers [10]. Experiments have been reported in ZnGcP2 with CO2 laser and also in LiNbOa with Ruby and Nd Glass lasers. We report self­beating of second harmonic of Nd : YAG laser radiation in LiNbO^. Experimental arrangement is shown ii\ Figure 3. Far infrared radiation is generated by beating of two modes within the laser oscillation line width. A polyethylene black sheet is used to block the unconverted green pump radiation from reaching the detector.F igu re  3. Experimental setup for submillimeter/FIR generation with dual frequency dye laser. M  |, M 2, M-mirrors. D C  |, DCi-dye cells. G T -G LA N  Thomson Prism. G ,, G^ Graiings FD-Frequency Doubler.The far infrared radiation is detected in a pyro-electric detector array followed by an amplifier. The nonlinear crystal is rotated and generation is obtained at an angle 9.4  ^ to optic axis. The wavelength of the generated far infrared radiation is not measured but estimated (to be 800 - 900 micron) from the input pump wavelengths near its half-power points (0.53185 micron and 0.53220 micron). This is near the predicted crystal phase­matching angle as calculated from its refractive index data. To avoid uncertainty of the mixing wavelengths a dual frequency dye laser is designed capable of producing radiations at two clear wavelengths simultaneously. The cavity grating controls the dye laser wavelength which in turn determined generated far infrared wavelength. The dye laser was pumped by the second harmonic of Nd : YAG laser. The polarisations corresponding to the two wavelengths are made orthogonal by placing an intracavity Glan-Thomson polariser. The output of two dye laser wavelength are accurately measured with a wavemeier. Rhodamin 640 in methanol is used. With dye laser wavelengths at 0.6162 and 0.6157 micron farTunable ultraviolet and fa r  infrared lasers etc 567infrared output at 758.7 micron is generated. With one of the dye lasers changed from0.6157 micron to 0.6155 micron keeping the other fixed at 0.6162 micron the generated output wavelength at 541.8 micron is observed.FIjjure 4 (a). V anaijon of phase m atching angle wiih input wavelength.Figure 4(b). Normalizcil subnini ouiput vs angular detuning in both theoretica l and cx[x:nmcnial situations, e.1, Ih co rc tica l« 11.02 deg, cxpenmefual - 10 9 deg.The ihcorciical tuning curve along with the two experimental points is shown in the Figure 4(a). Also shown in the Figure 4 (b) is tlie angular acccplance.Referencesf ll A F Gilson 1982 Contemp Phys, 23 285[21 C L Tang 1979 Methods of Experimental Physics Vol 15B (New York . Academic)[31 R S Adhav, S R Adhav and J M Pclaprai 1987 lxi\er Focus 23 88[41 Y X Fan, R C Eckardt and R L Bycr 1989 IEEE J , Quantum Electron QE-25 196[5] G O B har, U Chauerjec and S Das 1991 Appl Phyy Lett 58 231[6] W M uckhcnheim , P U)kai, Burghardi and I) Basting 1988 Appl Phys B45 259|7) K H Yang, J R M om s, P L Richards and Y R Shen 1974 Appl Phys Lett 23 069[8) G D Boyd, T  J Bridges, E Buchlcr and C K N Patel 1972 Appl Phys Lett 2 1 ,553|9J M Berg, C B H am s, Kenny and P L Richards 1985, Appl Phys Lett 43 20b110] R K Tyagi, V V Rampal and G C Bhar 1986 Infrared Physics 26 29

Tunable Ultraviolet and Far Infrared Lasers for Plasma Diagnostics

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Tunable Ultraviolet and Far Infrared Lasers for Plasma Diagnostics

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