Genetic-algorithm-optimized wideband on-chip polarization rotator with an ultrasmall footprint.

Polarization control of light waves is an important technique in optical communication and signal processing. On-chip polarization rotation from the fundamental transverse-electric (TE00) mode to the fundamental transverse-magnetic (TM00) mode is usually difficult because of their large effective refractive index difference. Here, we demonstrate an on-chip wideband polarization rotator designed with a genetic algorithm to convert the TE00 mode into the TM00 mode within a footprint of 0.96 μm ×4.2  μm. In simulation, the optimized structure achieves polarization rotation with a minimum conversion loss of 0.7 dB and the 1-dB bandwidth of 157 nm. Experimentally, our fabricated devices have demonstrated the expected polarization rotation with a conversion loss of ∼2.5  dB in the measured wavelength range of 1440-1580 nm, where the smallest value reaches ∼2  dB. The devices can serve as a generic approach and standard module for controlling light polarization in integrated photonic circuitry.