Interferometric modal splitting enables broadband, dual-polarization on-chip spectroscopy

The modal dispersion of waveguides typically limits integrated photonic devices to operation with a single polarization state. In this work, we propose a generic mode separation technique we call “interferometric mode splitting” (IMS), which enables guided modes to be separated over wide bandwidths with a large extinction ratio. To demonstrate the general principle of IMS, we show that an unmodified thermally driven silicon photonic Fourier transform spectrometer exhibits a polarization-separating effect in the frequency domain, even though only one polarization-insensitive detector is used. Using this effect, we experimentally demonstrate a simple on-chip spectrometer capable of extracting two-polarization spectra over a wide 1480–1630 nm bandwidth with a greater than 20 dB polarization extinction ratio. These specifications would be highly challenging to achieve using existing, conventional on-chip polarization-splitting techniques. Though we focus on this specific realization of IMS, we also show that IMS is general to various on-chip spectrometer architectures, other spatial modes, and technologies other than thermally driven Fourier transform spectrometers. Interferometric mode splitting shows promise as a general approach for robust and fundamentally broadband detection of orthogonal modes in guided-wave sensing.