Broadly Tunable Mid-Infrared Radiation By Difference Frequency Mixing

Abstract

1250 Charleston Road, Mountain View, California 94043AbstractHigh power tunable infrared radiation has been generated by difference frequency mixingin the non -linear crystals LiNbO3 and AgGaS2. A commercially available system in whichone to ten millijoules per pulse in the spectral region of 1.5 to 4.5 microns is described.Also discussed is work being performed to extend the range of the instrument to 12microns.IntroductionThe theory of non -linear mixing to generate infrared radiation has been treated indetail by several authors.1'2 Furthermore, there have been a variety of architecturesreported for generating both cw3 and pulsed4,5 output. Recently a new class of nearinfrared laser dyes developed by K. Kato has made difference frequency mixing an attractiveprocess for pulsed Nd:YAG laser systems.6 These dyes exhibit high efficiency when pumpedby the second harmonic of the Nd:YAG fundamental and show good long term stability.The mixing process is implemented by collinearly propagating two coherent beams, one ofwhich is tunable, through a non -linear crystal. The infrared output has a frequency equalto the difference of the frequencies of the two input beams. The choice of non -linearcrystal is chiefly dependent on material parameters. The transmission range of the devicemust be broad enough to pass both input waves and IR output wave. The crystal should havea high non -linear coefficient for good efficiency and must be of good optical quality.Most importantly the crystal must satisfy the phasematching reouirements for the desiredinteraction.In the case of Nd:YAG based systems the crystals LiIO3, LiNbO3, AgGaSe2, andAgGaS2 have exhibited efficient performance for infrared applications. This paper willreport on two of these devices. First a system incorporating a LiNbO3 crystal whichgenerates tunable output from 1.5 to 4.5 microns is described. This instrument is capableof scanning over the entire range of the dye laser by automatically tracking and lockingthe crystal phasematching angle with the input dye wavelength. Also to be discussed iswork being done using the crystal AgGaS2. This device allows infrared output to 12microns. It can be utilized either by subtracting the 1.06 micron fundamental from thenear infrared dye frequency or by using 1.06 micron pumped dyes the interaction of 1.06micron minus dye can be realized.Laser system descriptionThe laser system for difference frequency mixing is shown in Figure 1. The pump sourceis a standard Quanta -Ray DCR -2A Nd:YAG laser. This laser provides Q- switched energy inexcess of 740 millijoules per pulse at a repetition rate of ten hertz. The pulse width isapproximately 8.5 ns. The unstable resonator produces a near diffraction limited annularbeam of 6.5 mm diameter and approximately .5 mrad divergence. The fundamental Nd:YAGenergy is frequency doubled in a temperature controlled KDP crystal yielding greater than350 mJ per pulse at 532 nm. Dichroic beam splitters separate the fundamental and secondharmonic beams. The 532 nm energy pumps the Quanta -Ray PDL -1 dye laser and the fundamentalpropagates through the dye laser into the IR mixer system.The dye laser oscillator is approximately 30 cm long consisting of a Littrow mounted 600lines /mm grating, a 50x prism beam expander, an air -spaced etalon, a dye cell and an outputcoupler. The oscillator energy is amplified first by a side pumped preamplifier and thenby a longitudinally pumped amplifier. Using DCM dye in this configuration output energiesof 80 mJ per pulse have been attained. The laser has a linewidth of .25 cm -1 withoutthe etalon and less than .05 cm -1 with the etalon. The pulse width of the dye laser istypically 5.5 ns. The oscillator grating, etalon, and beam expander are housed in apressure box enabling the laser to be pressure tuned.