Widely Tunable Efficient Intra-cavity Quasi-phase-matched Mid-lR Generation

Abstract

Recent developments in electric field poling have made possible the production of periodically poled lithium niobate (PPLN) with nearly ideal domain structures for the generation of mid-infrared radiation by optical parametric oscillation’ and difference-frequency generation (DFG).’ When periodic poling is combined with waveguide fabrication, one can produce a laser source capable of generating tens to hundreds of microwatts of tunable mid-IR radiation using the DFG of two hundred-milliwatt diode lasers. The fundamental absorption band of carbon dioxide at 4.3 p is one possible target for practical devices based on such a laser source. In this work we demonstrate, for the first time to our howl edge, the production of4.3 p radiation by DFG in PPLN waveguide structures. Domain reversal with periods between 17 p and 21 p was accomplished through the application of a high-voltage pulse to photolithographically patterned wafers of 0.5-mm-thick LiNbO,., Waveguides were formed by proton exchange for 24 h at 17OoC followed by annealing for 20 h at 340°C. DFG of two cw lasers was carried out at room temperature. Tunable radiation at h, = 715-840 nm was provided by a Ti:Sapphire laser. Fixed-frequency radiation at A, = 1009 nm was provided by a grating-stabilized tunable diode laser. The combined beams from these two lasers were focussed using a 20X microscope objective into the waveguides. The generated mid-IR radiation at X, was collimated with a CaF, lens, separated from the input beams with a dichroic filter, and measured with an indium antimonide detector. In some experiments the combined beams were focussed into the bulk of the sample with an f = 20 cm lens. Figure 1 shows the generated power as a function of X, in a 10-p-wide waveguide with a quasi-phase-matched (QPM) period of 18.3 p. The phase-matching curve deviates from the ideal sinc-squared curve seen in bulk DFG because of the excitation of multiple waveguide modes. The peak at X, = 817 nm corresponds