Simultaneous generation of sum, difference, and harmonics of two laser frequencies by spread spectrum phase matching

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The intensive recent activity in nonlinear parametric processes is motivated by their unique offer in providing light oscillations with very large tuning ranges. The study and much of the advances benefitted from many new nonlinear crystals with large nonlinear coefficients. The phasematching requirement,r however, remains a critical and a limiting factor in such nonlinear processes. The common method to solve it is to use the crystal birefringence, where the difference between the refractive indices for beams with different polarizations that propagate along specitic crystal directions compensate the refractive index mismatch due to dispersion. Then, a degree of wavelength tunability of the input and the generated beams can be obtained by changing the angle of the beam propagation in the crystal (and to some extent by changing the temperature). Another way to overcome the problem is quasiphase matching, where “artit?cial” spatial modulation of the optical nonlinearity1 or the refractive index provides the missing k vector. This method restricts the phase matching to a specitic input wavelength. In a recent work,’ we have shown a new method for obtaining broadband second harmonic generation (SHG) by controllable spread spectrum quasiphase matching. It was accomplished by forming quasirandom domain gratings in Sr0~61Ba0~39Nb206 (SBN) crystals. The broad spectrum of the domain grating period provided the broadband capability. We have also demonstrateds second harmonic generation for prespecified discrete or continuous beam wavelengths by tailored quasiphase matching. In this work we demonstrate the simultaneous generation of sum, difference and harmonics of two laser frequencies by spread spectrum phase matching in Sr0.61Ba0.s$b,0,. The