Experimental Demonstration of a Resolution-Enhanced Loop-Terminated Mach–Zehnder Interferometer Narrow Band Fourier Transform Spectrometer

We experimentally demonstrate a compact on-chip narrowband Fourier Transform spectrometer (FTS) based on spatially heterodyned array of loop-terminated Mach-Zehnder interferometers (LT-MZIs) in a foundry fabricated silicon-on-insulator (SOI) platform for chip-integrated sensing applications. We demonstrate that LT-MZIs with the same progressive geometric path length difference between the spatially heterodyned arrayed interferometer arms, as in Mach-Zehnder interferometers (MZIs), double the optical phase delay and thus double the wavelength resolution compared to MZIs. Our proof-of-concept device demonstrates one method to address the bandwidth-resolution-compactness tradeoff inherent in on-chip FTSs. The resolution enhancement is significant for optical sensing applications in biosensing and chemical sensing requiring single digit picometers resolution within a narrow wavelength bandwidth in compact on-chip form factors. We discuss the challenges arising from fabrication imperfections in spatially heterodyned FTS in foundry fabricated chips. We propose a method to compensate for phase errors arising from fabrication imperfections within a narrow wavelength bandwidth in the FTS using the refractive index changes in the amorphous to crystalline phase transformations of phase change materials (PCMs), which would enable zero active power consumption during on-chip narrowband FTS operation.