Picosecond Optical Pulse Autocorrelator Using Second-Harmonic Generation in ALGaAS Waveguides

Abstract

An optical pulse autocorrelation detection scheme based on the second-harmonic generation (SHG) of counter-propagating beams¹ has been recently demonstrated in AIGaAs waveguides^2,3,4 and applied to sensors and OTDR measurements.⁵ Since the SHG light is emitted perpendicular to the surface, the autocorrelation trace can be observed directly, without scanning delay lines. High SHG efficiencies can be obtained by quasi-phase matching, alternating layers of AIGaAs and GaAs in the waveguide core region. While larger numbers of repetitions can produce better optical coupling, they require higher accuracies on individual layer thickness and Al content. An optimum waveguide design for specific wavelength ranges and growth parameter tolerances is achieved by modelling the second harmonic generation in single-mode waveguides. The standard (100) substrate orientation is appropriate in double-input applications, where the positions of pulse overlaps are monitored. However, in single-input autocorrelator applications, the SHG output of (100) orientation waveguides is plagued by oscillations due to the unavoidable interaction between TE and TM modes. The (211)B orientation corrects that problem and offers both high SHG efficiency and cleavable facets.