On-chip polarization rotator for type I second harmonic generation

We demonstrate a polarization rotator integrated at the output of a GaAs waveguide producing type I second harmonic generation (SHG). Form-birefringent phase matching between the pump fundamental transverse electric (TE) mode near 2.0 mu m wavelength and the signal fundamental transverse magnetic (TM) mode efficiently generates light at 1.0 mu m wavelength. A SiN waveguide layer is integrated with the SHG device to form a multifunctional photonic integrated circuit. The polarization rotator couples light between the two layers and rotates the polarization from TM to TE or from TE to TM. With a TE-polarized 2.0 mu m pump, type I SHG is demonstrated with the signal rotated to TE polarization. Passive transmission near 1.0 mu m wavelength shows similar to 80% polarization rotation across a broad bandwidth of similar to 100 nm. By rotating the signal polarization to match that of the pump, this SHG device demonstrates a critical component of an integrated self-referenced octave-spanning frequency comb. This device is expected to provide crucial functionality as part of a fully integrated optical frequency synthesizer with resolution of less than one part in 10(14).