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Continuous-Wave Infrared Laser Spectrometer Based on Difference Frequency Generation in Aggas2 for High-Resolution Spectroscopy

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Continuous-Wave Infrared Laser Spectrometer Based on Difference Frequency Generation in Aggas2 for High-Resolution Spectroscopy Canarelli et al. Vol. 9, No. 2/February 1992/J. Opt. Soc. Am. B 197 Continuous-wave infrared laser spectrometer based on difference frequency generation in AgGaS2 for high-resolution spectroscopy P. Canarelli, Z. Benko, R. Curl, and F. K. Tittel Department of Electrical and Computer Engineering, Rice Quantum Institute, Rice University, R0. Box 1892, Houston, Texas 77251 Received May 7, 1991; revised manuscript received September 26, 1991 A high-resolution cw spectrometer based on difference frequency generation (DFG) in a 20-mm-long AgGaS2 crystal pumped by two stabilized single-frequency cw dye-Ti:sapphire lasers is described. Experiments per- formed with a Rhodamine 6G and DCM dye laser combination pumped by a 20-W argon laser are reported. In- frared radiation is generated from 7 to 9 Atm with an ultranarrow linewidth (<0.5 MHz) and an output power of 1 W by making use of 90° type I phase matching. The performance of the DFG laser spectrometer is evaluated by using a portion of the v2 band of NH3 near 1177 cm-'. LiIO , Wellegehausen INTRODUCTION linewidth (<10-4 cm'1). Using 3 et al.7 obtained coherent infrared radiation from 2.3 to High-resolution laser spectroscopy in the infrared region 4.6 gm at -0.5 ,uW. A combination of both crystals was requires a tunable coherent cw radiation that is capable of used by Bawendi et al.' for high-resolution spectroscopy precise wavelength resolution, high spectral brightness, between 1.9 and 5.3 ,m. and good frequency and amplitude stability. A wave- The optical and physical properties of the nonlinear- length resolution of better than 1 MHz and a power optical material is critical to generation of cw infrared greater than 1 ,uW are desirable for high sensitivity and radiation by difference frequency mixing. The nonlinear time-resolved absorption spectroscopy. For wavelengths crystal must be transparent in the infrared and visible at beyond -2 Am the only practical tunable laser sources are all three mixing frequencies. For achieving the highest color-center lasers, ternary salt diode lasers, and sources possible output by avoiding walk-off effects, collinear 900 based on difference frequency generation (DFG). The phase matching over the entire wavelength region used is color-center lasers operate extremely well with single- desirable. Other important desidered characteristics are frequency power levels up to 100 mW but only in a limited a high nonlinear coefficient, a large damage threshold, spectral range from 1.1 to 3.5 m and at liquid-nitrogen good chemical stability, and commercial availability. temperatures. Diode lasers, on the other hand, can cover Silver thiogallate (AgGaS2) meets these requirements. a wide wavelength range (from 2 to 30 m) based on the It exhibits low optical absorption from 0.5 to 12 ,m (typi- but the tuning range of any cally -0.04 cm-', except near the ends of this wavelength semiconductor composition, 9 single diode is very small (-100 cm-'). Moreover, range) and is 900 phase matchable from 2 to 10 Am. The commercially available diode lasers that are currently nonlinear polarizability of AgGaS2 was first investigated available do not cover their nominal wavelength ranges by Chemla et al.' and Boyd et al."; its nonlinear coeffi- continuously and also require low temperatures (<100 K). cient was found to be four times larger than that of In addition, power and coverage tend to degrade on tem- LiNbO3 . However, to date the only mixing experiments perature cycling. Tunable infrared sources based on using that crystal have been carried out with pulsed dye DFG require suitable nonlinear-optical materials and tun- lasers employing non-90' phase matching. For instance, 2 of the appropriate wavelengths, such as Hanna et al.' generated infrared radiation from 4.6 to able pump sources 3 dye or solid-state lasers. 12 Am, and Seymour et al.1 extended the spectrum to 4 5 Generation of tunable infrared radiation by DFG is re- 18.3 Atm. Elsaesser et al. 1 and more recently Yodh et al. ported by many groups, 3 but cw sources are reported less achieved tuning ranges from 3.9 to 9.4 ,.m and from 3.4 to source offers significantly 7 Am, respectively. Pulsed operation of an optical para- frequently. A cw pump 6 smaller spectral linewidths than a pulsed source, better metric oscillator based on AgGaS2 was also reported.' frequency stability, and continuous scan possibilities at The current availability of AgGaS2 crystals of high opti- the expense, however, of a lower conversion efficiency. cal quality in large dimensions (>45 mm), along with sta- Pine 4 described cw DFG for a spectrometer based on bilized cw tunable dye-Ti:sapphire sources, now makes it possible to design an infrared coherent radiation source, LiNbO 3 , covering the region from 2.2 to 4.2 m at a power level of 0.5 /LW with a resolution of 5 x 10-4 cm'1 suitable for high-resolution spectroscopy, that is tunable and a scan precision of 2 x 10-3 cm-'. Recently two from 3 to 9 ,m at the 10-AW power level. groups5'6 reported improved performance of such a laser Results of a 90° phase-matching calculation carried out spectrometer in terms of output power (-10 ,W) and for AgGaS2 with the refractive-index data given in Ref. 16 0740-3224/92/020197-06$05.00 © 1992 Optical Society of America 198 J. Opt. Soc. Am. B/Vol. 9, No. 2/February 1992 Canarelli et al. 1100 and higher power levels can be achieved by increasing this dimension. Increasing 1000 the crystal length also increases .O the diameter of the beam waist of the pump lens propor- 900 tionally, permitting the use of higher pump powers with- lf out crystal damage. al 800 a ZD1 700 SPECTROMETER DESCRIPTION 600 Irv A schematic of the DFG spectrometer system is shown in Fig. 4. A 20-W argon-ion laser is used to pump both a 500 computer-controlled, frequency-stabilized, tunable Coher- ent 899-29 3 4 5 6 7 8 9 ring dye-Ti:sapphire laser and a Coherent 699-21 dye laser. A beam splitter is employed to separate Infrared wavelength (m) the all-lines Ar+ pump beam into two beams of equal Fig. 1. 900 phase-matching curves of AgGaS2 based on the power. In order to prevent degradation of the pump refractive-index data given in Ref. 15. beam quality because of interference effects of the beam splitter, the beam splitter is wedged. The range of wave- 0.4 lengths covered by the two tunable lasers extends from 580 to 1100 nm and from 580 to 900 nm, respectively, with a 0.3 single-frequency output power higher than 0.5 W when they are pumped with a 6-W all-lines argon laser beam. Even higher pump powers can be used effectively 0.2 if the dye flow is cooled below room temperature. Each ring laser operates stably in a single mode selected by a three-plate birefringent filter and two intracavity talons. 0.1 The output frequency of each laser is actively stabilized to 0.5 MHz by feedback locking to an external reference cavity. The 899-29 ring laser also possesses a built-in 0.0 0.01 0.1 10 1.5 4=/k.w2 Fig. 2. Plot of the focusing function h(IL, ) versus the = /b parameter for Atequal to 0.7, 0.8, and 0.9 ( = k/kp). 1.0 E are shown in Fig. 1. It can be seen that infrared radia- a tion can be generated from 3 to 9 m provided that the CL IL0 input beams are tuned from 580 to 800 nm (pump) and C] 0.5 from 620 to 1100 nm (signal). This tuning can be achieved by using two dyes, Rhodamine 6G and DCM plus a Ti:sapphire laser for the pump and the signal beams. 0.0 -1-Without AR coatings An analysis was performed in order to estimate the cw 0.0 I infrared DFG output power based on Refs. 17 and 18 with 3 4 5 8 9 Gaussian input beams and a lossless crystal assumed. In DFGwavelength (gm) this case the DFG power can be expressed as Fig. 3. Calculated DFG output power as a function of the wave- length. The case of focused Gaussian 2 2 beams in AgGaS2 is con- 4w i ksdeff l sidered in the calculation. A value of 31 pm/V (Ref. 19) is used Pi = + Pc~npnn(lP Ph(p,), 8 (1) for d36. where the subscripts p, s, and i refer to the pump, signal, and infrared beams, respectively, k is the wave vector, deff the effective nonlinear coefficient, I the crystal length, and n the refractive index; u is the ratio of the input wave vectors ( = k/kp) and, is the ratio of the crystal length Wedged to the confocal parameter (assumed to be equal for both B.S. input beams). The dependence of the focusing function h(,u, 4)on 4is depicted in Fig. 2 for three different values of . The maximum dependence corresponds to optimum focusing. For the case of a 2-cm-long AgGaS 2 crystal pumped by two equal-power beams of 1 W total power and focused op- Polar. Polar. - ' IR cell rotator cube Oven 1 0-cmfocal timally, an infrared power larger than 50 ,W over most of I lengthBaF2 Clloppor lona the spectral range (Fig. 3) is predicted. It is worth noting Fig. 4. Schematic diagram of the cw DFG infrared laser spec- that the output power is proportional to the crystal length, trometer system.
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