Design of a low-loss non-volatile optical phase shifter based on the lossy phase change material Ge₂Sb₂Te₅

We present a low-loss, non-volatile optical phase shifter leveraging the phase change material (PCM) Ge 2 Sb 2 Te 5 (GST) operating at a 1550 nm wavelength. The design is based on the coupling mechanism between a primary silicon strip waveguide and a GST-loaded hybrid waveguide, minimizing direct interaction between the waveguide mode and the PCM. In the amorphous, low-loss state of GST, phase matching between the waveguides enables efficient coupling, allowing light to re-couple into the primary waveguide after a certain length. Upon transitioning to the crystalline state, the GST induces phase mismatch, confining the light within the primary waveguide and circumventing the lossy crystalline GST, thereby preserving low-loss performance. At the central wavelength, the phase shifter exhibits a loss of ~1 dB in both states and achieves a phase shift of π upon phase change. A Mach-Zehnder interferometer incorporating this phase shifter demonstrates optical switching with a low insertion loss of ∼0.6 dB and minimal crosstalk below −20 dB over a 30 nm bandwidth, underscoring the potential of GST-based phase shifters for low-power, high-performance photonic circuits.