Broadly tunable mid- infrared femtosecond optical parametric oscillator using all-solid-state-pumped periodically poled lithium niobate

We describe a high-repetition-rate femtosecond optical parametric oscillator (OPO) that was broadly tunable in the mid infrared. The all-solid-state-pumped OPO was based on quasi-phase matching in periodically poled lithium niobate. The idler was tunable from approximately 1.7 mm to beyond 5.4 mm, with maximum average power levels greater than 200 mW and more than 20 mW of average power at 5.4 mm. We used interferometric autocorrelation to characterize the mid-infrared idler pulses, which typically had durations of 125 fs. This OPO had a pumping threshold as low as 65 mW of average pump power, a maximum conversion efficiency of .35% into the near-infrared signal, a slope efficiency for the signal of approximately 60%, and a maximum pump depletion of more than 85%. © 1997 Optical Society of America Mid-IR femtosecond pulses have potential applications for the study of dynamics in a variety of materials. Vibrational relaxations in molecules, intersubband transitions in quantum wells, and materials for use in lasers and detectors operating in the 3-5-mm atmospheric transmission window can all be studied effectively with ultrashort mid-IR pulses. In this Letter we report on the generation of mid-IR pulses with an all-solid-state-pumped high-repetition-rate femtosecond (fs) optical parametric oscillator (OPO). The most-common techniques for the generation of high-repetition-rate mid-IR fs pulses are differencefrequency mixing and the use of a synchronously pumped OPO. Difference frequency of the signal and the idler output from Ti:sapphire-pumped OPO's produced fs pulses of between 2.5 and 5.5 mm (Ref. 1) but with maximum average power levels of only 500 mW . Mid-IR synchronously pumped fs OPO's based on KTiOPO 4 and its isomorphs 2 -5 were demonstrated, but they were limited to wavelengths shorter than ϳ4.1 mm. Average power levels greater than 20 mW at 5.2 mm were obtained with KNbO 3 . 6 The all-solid-state-pumped broadly tunable mid-IR fs OPO described in this Letter is based on quasi-phase matching 7 (QPM) in periodically poled lithium niobate (PPLN) and produced more than 20 mW of average power at 5.4 mm. The experimental arrangement was similar to that described in Ref. 8, except that a ring cavity was used instead of a linear cavity. We used a pumping geometry with a small noncollinear angle between the pump and the signal so that the long-wavelength idler beam did not have to be transmitted through any of the cavity optics and optical isolation of the pump laser was unnecessary. The PPLN OPO was synchronously pumped at a repetition rate of 81 MHz by a mode-locked fs Ti:sapphire laser powered by a diodepumped cw frequency-doubled Nd : YVO 4 laser (SpectraPhysics Millennia). The Ti:sapphire laser produced nearly transform-limited pulses of ϳ 90-fs duration, with average power levels of as much as 850 mW over the 790-815-nm wavelength range used in this experiment. We used two optics sets and two PPLN crystals, which were antiref lection coated on both sides with single layers of SiO 2 , in the OPO to cover the tuning range. In both cases the signal was resonated in a ring cavity consisting of two 15-cm radius-of-curvature mirrors, a f lat high ref lector, a f lat output coupler, and a four-prism (SF-14) sequence for dispersion compensation. Several different output couplers were used, with transmission varying from ϳ1% to ϳ9% over the tuning range of the OPO. In a noncollinear fs OPO both temporal walk-off owing to the group-velocity mismatch (GVM) of the interacting pulses and spatial walk-off owing to the noncollinearity of the interacting beams can limit the effective interaction length in the crystal. Near degeneracy 9 the GVM between the pump pulses and signal and idler pulses was of the order of 300 fs͞mm (signal-and-idler-leading pump), 10 resulting in a temporal walk-off length 11 of the order of 330 mm for ϳ100-fs pulse widths. Far from degeneracy, the GVM between pump and signal was reduced to as little as ϳ110 fs͞mm, but the GVM between signal and idler became as large as ϳ260 fs͞mm (signal-leading idler), resulting in a temporal walk-off length of ϳ900 mm between the pump and the signal and ϳ 380 mm between the signal and the idler. For the focusing parameters (ϳ35-mm signal and pump waist radius) and the noncollinear angle (varied from ϳ1.0 ± to ϳ1.6 ± between pump and signal measured internal to the crystal) used in this experiment, near degeneracy the spatial walk-off length of ϳ 550 mm between signal and idler was much larger than the temporal walk-off length and, therefore, did not limit the interaction length. When we were trying to reach long idler wavelengths, however, the increasingly large angle between signal and idler (as large as 9 ± measure

A review of image-warping methods

Image warping is a transformation which maps all positions in one image plane to positions in a second plane. It arises in many image analysis problems, whether in order to remove optical distortions introduced by a camera or a particular viewing perspective, to register an image with a map or template, or to align two or more images. The choice of warp is a compromise between a smooth distortion and one which achieves a good match. Smoothness can be ensured by assuming a parametric form for the warp or by constraining it using differential equations. Matching can be specified by points to be brought into alignment, by local measures of correlation between images, or by the coincidence of edges. Parametric and non-parametric approaches to warping, and matching criteria, are reviewed.