Characterization of submicron-period periodically-segmented ion-exchanged KTiOPO/sub 4/ waveguides based on backward second-harmonic generation

Abstract

Summary form only given. Periodically-segmented ion-exchanged KTiOPO/sub 4/ (KTP) waveguides can be used for not only quasi-phase-matched (QPM) second-harmonic generation (SHG), but also acting as a distributed Bragg reflector (DBR) due to index grating to enhance conversion efficiency for SHG. In this case, a submicron-period index grating is desirable because it can reach a given reflectivity in a short length. One way to characterize the waveguide containing a submicron-period index grating is to measure reflection spectrum of an incident beam. Due to the presence of the index grating, it is also possible to use SHG to characterize the grating. However, in such a short-period grating, the forward SHG can never be phase-matched. Therefore, backward SHG can be used as a novel and unique technique to characterize a short-period grating. This technique has advantage for determining interface roughness, fluctuation of index layer thickness from one period to the next, and duty cycle. In addition, one can use this technique to determine whether the domain is inverted after ion exchange. Previously, it was demonstrated that domain inversion can still occur for the period as short as 3 /spl mu/m. Here, we report our results of achieving QPM backward SHG in periodically-segmented ion-exchanged KTP waveguide with the period of 0.7 /spl mu/m. We have fabricated 4-/spl mu/m-wide periodically-segmented KTP waveguides on a z-cut flux-grown KTP substrate by ion-exchange in a RbNO/sub 3//Ba(NO/sub 3/)/sub 2/ melt at 350/spl deg/C for 45 minutes. The diffused segments were 0.4-/spl mu/m-long RTP separated by 0.3-/spl mu/m-long KTP. There are 3200 periods in the center of the waveguide, which corresponds to a total length of 2.24 mm for the segment portion. A mode-locked Ti:sapphire laser with a pulse width of /spl sim/130 fs and repetition rate of 76 MHz was used as the pump source. The polarization of the fundamental beam is always parallel to the z-axis of the crystal for utilizing d/sub 33/. We first measured the average power of the backward SH beam while scanning the fundamental wavelength from 750 to 910 nm with a constant average pump power across the spectrum. For each center pump wavelength, we measured the SH spectrum. We then determined the peak power and corresponding SH wavelength. By varying the center pump wavelength, we obtained a spectrum for backward SH beam.