Large nonlinear phase modulation in quasi-phase-matched KTP waveguides as a result of cascaded second-order processes.

It has been known for many years that a nonlinear phase shift can occur with non-phase-matched second-harmonic generation in particular and with parametric processes in general.1·2 Frequently it is an undesirable factor that leads to instabilities in parametric oscillators. Such a nonlinear phase shift has been measured experimentally in bulk KTP at 1.06 11-m with Z-scan techniques, 3 which measure the phase distortion directly, and earlier in bulk cesium dihydrogen arsenate by observation of the temporal modulation on the pulses. 4 Spectral modulation of the fundamental has also been reported in bulk samples. 5 In general, the effective third-order nonlinearities have been small (10-14 cm/W), and the required powers have been large (kilowatts to megawatts). However, with the development of quasi-phase-matching (QPM) techniques, which allow large second-order nonlinear coefficients to be phase matched in waveguide geometries, large nonlinear phase shifts are potentially possible at relatively low powers. It has recently been shown theoretically that such phase shifts could prove useful for low-power all-optical switching in waveguides at watt power levels, provided that phase shifts of '1T or more can be obtained. 6 However, to our knowledge no measurements of this nonlinear phase shift have been performed in waveguides. In this Letter we report the measurement of large nonlinear phase shifts (>?T), using 3-ps pulses at 850 nm in KTP quasi-phase-matched channel waveguides. Experiments were performed in a Z-cut sample of hydrothermally grown KTP with segmented, domaininverted waveguides formed by ion exchange. 7 Ion exchange was performed in a 95 mol.% RbN03/5 mol.% Ba(N03 ) 2 molten salt bath for 45 min at 325 oc. The y-propagating waveguides are 2.8 mm long, with an asymmetric QPM period of 4 11-m consisting of 2. 7 -11-m exchanged regions followed by 1.3 11-m of bulk material. The waveguides used had