Asymmetrically Strained Ridge Waveguide for Passive Polarization Conversion

A novel passive polarization converter is presented using the elasto-optic effect in a ridge waveguide. A compressively stressed Si ₃N ₄ layer is deposited onto one side of the waveguide while a tensile layer is deposited on the opposing side. The average stress of the tensile and compressive layers is used to cancel the modal transverse-electric-transverse-magnetic (TE-TM) birefringence of the ridge waveguide. The difference in the tensile and compressive stresses induces birefringence with principal axes rotated 45deg from the TE-TM orientations. The method is demonstrated on an AlGaAs-GaAs waveguide at 1550 nm. We measure the orientation of the axes to be 46deg plusmn 1deg, demonstrating that complete mode conversion can be obtained without using periodic phase matching. Complete conversion is obtained in 1.16 mm with a waveguide loss of 0.1 dB/cm. Stability over temperature is also demonstrated.