High-resolution, low-loss multilevel phase shifters based on electrically driven phase-change materials integrated...

Multilevel phase shifters are key components in photonic integrated circuits. A major requirement in many applications is achieving non-volatile operation and low insertion loss simultaneously. Electrically, multiple phase levels can be encoded by controlling the heater power and employing different microheater architectures to induce varying degrees of phase-change material (PCM) amorphization, thereby modulating the device's optical properties and the amplitude and phase of the propagating field. Here, we explore a platform based on the integration of a low-loss PCM, namely GeSe, sandwiched between microheaters and silicon waveguides. To achieve a large number of levels, we modify standard straight microheaters and propose a segmented heater design that breaks the heater's symmetry along the propagation direction. We numerically demonstrate under pulse-width/pulse-amplitude modulation (PWM/PAM) that multilevel phase shifts can be achieved due to non-uniform heating in the GeSe PCM layer. However, the resulting phase levels for the basic configuration are highly abrupt because the constant power dissipation along the light propagation direction, associated with a uniform cross-section, does not permit a wide range of amorphization patterns. The proposed segmented heater, whose width gradually increases in steps along the light propagation direction, allows overcoming this limitation. This configuration enables the encoding of 164 well-spaced phase levels between 0 and π (>7-bit resolution), facilitated by smoother amorphization arising from the combined effects of non-uniform heating across segments and within each segment, while maintaining an insertion loss of only 0.6 dB in the fully crystalline state (worst case).