Far infrared generation by CO 2 lasers frequencies subtraction in a ZnGeP 2 crystal. Yu.A.Shakir V.V.Apollonov A.M.Prokhorov A.G.Suzdal tsev General Physics Institute of RAS, 38 Vavilov st., Moscow 117333, Russia. A.I.Gribenyukov United Institute of Atmospheric Optics RAS SD, 1 Akademicheskiy av., Tomsk 634055, Russia. R.Bocquet LSH Universite' des Sciences te techniques de Lille 59655 Villeneuve d'ascq, Cedek, France. ABSTRACT A pulsed radiation power of ~1 W was reached for the first time, with the aid of a ZnGeP 2 crystal, at the difference frequency of CO 2 lasers radiation in the submillimeter spectral range (102-110 µm). Keywords: nonlinear crystal, pulsed laser, subtraction of frequencies, far infrared radiation. 1 INTRODUCTION At far infrared (FIR) spectrum region a generation of CO 2 lasers difference frequencies is possible by means of certain nonlinear crystals 1. ZnGeP 2 crystal has following advantages: it's use does not require neither cryogenic temperature nor magnetic fields 2. Also the crystal is characterized by low values of absorption at infrared (IR) and FIR spectrum regions 3,4. Investigation of high power pulsed CO 2 lasers frequencies subtraction by ZnGeP 2 crystal is represented in this paper. For radiation interaction a collinear synchronism condition was maintained in the nonlinear crystal for ordinary and extraordinary IR rays. For ordinary wave FIR spectral characteristics (Fig. 1) computed according to article 4 SPIE Vol. 2842 0-8194-2230-4/96/$6.00 163
Figure 1. Dependence of refraction n o (1) and absorption α o (2) of ZnGeP 2 on radiation wavelength λ. 164
θ ( o ) 200 400 600 800 1000 λ (µm) Figure 2. Dependence of phase-matching angle θ for ZnGeP 2 on difference wavelength λ. Figure 3. Schematic diagram of the apparatus. 165
for ZnGeP 2 have allowed to define phase-matching angle values at the same wavelength region (Fig. 2) and to estimate FIR pulse energy. For an example, estimation of IR pulses conversion (0.1 and 0.4 J energies, ~100 ns duration) yielded ~3.6 µj pulse energy at 100 µm radiation wavelength. 2 EXPERIMENT Two pulsed tunable CO 2 lasers with shared active medium 1 have been designed and manufactured (Fig. 3). The medium was excited (240J/2.25 litre) by a high voltage generator of the type proposed by authors. 5 At two independent tuned frequencies lasing was excited in two cavities including gratings 2,3 and common output mirror 5 (Ge plate with antireflection coating). Both Brewster's windows and gratings (100 and 150 mm -1 ) orientation was suited to have perpendicular rays polarization. Cavities length was L 5.8 m. Adjustable diaphragms 6 selected the fundamental mode TEMoo. Radiation energy of each beam was measured by calorimeter IMO-2N, their values were 0.2-0.7 J (± 7%) depending on radiation frequency. Laser pulse duration was observed by drag detectors and storage 50 MHz oscilloscope C8-14, usually it's value at half maximum was ~ 100 ns. Energy portion of pulse tail (~ 1.5 s ) didn't exceed 30 % of total value. Laser pulses had the nanosecond spike structure due to longitudinal modes beats with the modulation depth 60-80 %. Pulse structure was observed by the 350 MHz oscilloscope C 1-108. As it was shown in 6 this temporal structure of pulses had to decrease the threshold power value of plasma formation on crystal (10) surface and therefore total laser energy density on the crystal face was limited 0.6 J/cm 2. Spatial coincidence of rays was provided by a reflector pair: ZnSe transparent plate 8 and Cu mirror 9. After passing this combination a parallelism value of two rays was estimated to be not worse than 0.001 rad. Some temporal delay was usually observed between two pulses because of gain rate difference for whichever CO 2 lines. This delay value was reduced by placing additional Teflon attenuators 7 inside the leading line's cavity. Laser parameters stability observed for 90 pulses series: temporal coincidence jitter of two laser pulses didn't exceed 20 nsec. Apparatus locking was provided by a special pulsed generator optical guide-coupled with laser spark gap, this generator had ~2 ns delay jitter 7. For FIR radiation detection a receiver 12 was used with pyroelectric detector ELTEC 420M3 type and charge preamplifier CAMAC 1005A type. IR radiation was suppressed by attenuators 11 (1 mm Teflon film). Signal amplitude at oscilloscope screen was proportional to radiation energy value and leading edge of signal was equal to radiation duration. During nonlinear conversion of various laser lines combinations FIR radiation has been detected with difference frequencies, their corresponded wavelength values have been calculated: 102.60, 106.58, 108.79 and 110.76 µm, Laser lines were determined by CO 2 Laser Spectrum Analyzer. FIR radiation pulse energy was 180 ± 100 nj. For FIR pulse duration measured (~100 ns) it has been possible to estimate the pulse power value ~1.8 ±1.0 W. Experimental energy value differs from one computed (3.6 µj) primarily due to losses resulting from mismatch between crystal dimensions (8x8x11 mm) and laser ray cross section (diameter 10 mm). Phase-matching angles were close to computed values for wavelengths observed. 3 CONCLUSION Investigation results have made it possible to recommend ZnGeP2 as efficient nonlinear crystal for FIR radiation generation without recourse to both cryogenic conditions and magnetic field. For the first time with this crystal a FIR radiation power value has been in excess of 1 W. This fact allows to plan the creation of a tuned FIR radiation source operating at pulse repetition with high average power, reasonable for thermal properties of crystal ZnGeP 2. 166
4 ACKNOWLEDGMENTS Authors are thankful to Yu.M.Andreev for useful discussions and A.A.Volkov, V.V.Voronov, G.A.Komandin, S.P.Lebedev for spectroscopic assessment of components of our apparatus. Work has been done in General Physics Institute of Russian Academy of Sciences. 5 REFERENCES 1. R.L.Aggarwal and B.Lax, In: "Topics in Applied Physics. V.16. Nonlinear Infrared Generation", p. 19-80, Springer Verlag, N.Y., 1977. 2. G.D.Boyd, T.J.Bridges, C.K.N.Patel, "Phase-matched submillimeter wave generation by difference-frequency mixing in ZnGeP 2 ", Appl.Phys.Lett., Vol. 21, pp. 553-555, Dec. 1972. 3. G.D.Boyd, E.Buehler, F.G.Storz, J.H.Wernick, "Linear and Nonlinear Optical Properties of Ternary A 11 B IV C V Chalcopyrite Semiconductors", IEEE J. Quant. Electron., Vol. QE-8, pp.419-426, April 1972. 4. V.V.Voicehovskij, A.A.Volkov, G.A.Komandin, Yu.A.Shakir, "Dielectric Properties of a ZnGeP 2 at Far Infrared Wavelength Range", Solid State Physics, Vol. 37, pp. 2199-2202, 1995. 5. V.V.Apollonov, G.G.Baisur, B.B.Kudabaev, A.M.Prokhorov, B.V.Seomkin, E.A.Trefilov, K.N.Firsov, B.G.Shubin, "Possibilities of increase of volume discharge interelectrode gap by means of discharge gap filling by electrons", Kvantovaya Elektronika, Vol. 19, pp. 2139-2142, 1987. 6. V.V.Apollonov, K.N.Firsov, V.I.Konov, P.I.Nikitin, A.M.Prokhorov, A.S.Silenok, V.R.Sorochenko, "Plasma formation by the train of nanosecond CO 2 laser pulses", Lett. Sov. Journ. Techn. Phys., Vol. 11, N17, pp. 1034-1039, 1985. 7. V.V.Apollonov, V.V.Brytkov, S.I.Zienko, S.V.Murav'ev, Yu.A.Shakir, "Receiver-transmitter of optical pulses synchronization", Instruments and Experimental Techniques, No 4, pp. 125-126, 1987. 167