We demonstrate a method that enables in situ modification of the spectral shape of the parametric-gain profile in quasi-phase-matching crystals. In our experiment we used the electro-optic effect to modify the phase-matching profile for second-harmonic generation in a 57-mm-long nonuniformly poled crystal. In the direction of beam propagation the crystal is divided into three segments, where the first and the third segments have an equal length of 17 mm. Both segments are periodically poled with the same period of 21.6 µm, in order to obtain quasi phase matching for frequency doubling a fundamental wavelength of 1653 nm. The center segment is single-domain whose index of refraction is changed by the electro-optic effect by applying a voltage. Using a continuously tunable, single-frequency, single-stripe, distributed-feedback diode laser as the fundamental source, we recorded the parametric phase-matching profile for second-harmonic generation as a function of the laser wavelength and investigated the modification of the profile in dependence of the voltage applied to the crystal center segment. The measured phase-matching spectra are in excellent agreement with the theoretical prediction. The demonstrated method opens the possibility of rapidly changing the parametric-gain profile for all types of nonlinear conversion processes.
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