# On-chip polarization rotator for type I second harmonic generation

## TL;DR

This paper presents an integrated on-chip polarization rotator enabling efficient type I second harmonic generation in GaAs waveguides, advancing integrated photonic frequency comb technology.

## Contribution

It introduces a novel polarization rotator integrated with SHG in GaAs waveguides, achieving broad bandwidth polarization control for frequency comb applications.

## Key findings

- Achieved ~80% polarization rotation over 100 nm bandwidth.
- Demonstrated efficient type I SHG with polarization control.
- Enabled integration of polarization rotation with frequency comb components.

## Abstract

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 multi-functional 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 ~80 % polarization rotation across a broad bandwidth of ~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$^{14}$.