Frequency doubling and compression of chirped femtosecond pulses using aperiodically poled lithium niobate

Summary form only given. The technique of periodic poling has made possible the design of nonlinear crystals with novel phase-matching characteristics based on quasi-phase-matching (QPM). This technique has permitted frequency doubling of a wide range of wavelengths at high conversion efficiencies within the picosecond and femtosecond regimes. When chirped pump pulses are used it is possible to reduce the pulse duration of the second-harmonic pulses using discrete compression schemes such as those based on prisms-pairs or diffractive elements. Self-compression of the second-harmonic pulses is made possible by designing crystals with gratings, which are not periodic along the propagation direction. In particular, by fabricating frequency-doubling crystals with a linearly varying grating period one can define the position in the crystal at which any given frequency is converted to its second harmonic. We present the results of frequency doubling and compressing pulses directly obtained from an OPO using crystal of AP-LN with eight different linearly chirped grating periods. In each chirped grating, the initial (or input) period (/spl Lambda//sub i/) and final (or output) period (/spl Lambda//sub f/) are different while the average (or central) grating period [/spl Lambda//sub av/=(/spl Lambda//sub i/+/spl Lambda//sub f/)/2] is left constant. To effectively compress the chirped pulses delivered by the PPLN-based OPO, the crystal was oriented for positive crystal chirp /spl Lambda//sub i/>/spl Lambda//sub f/ (longest period facing the input pulses).