Thermo-optically tuned spatial heterodyne Fourier-transform spectrometer

Photonics integration in the mid-Infrared (mid-IR) spectral range, and more specifically the fingerprint region between 5 and 20 μm wavelength has garnered a great interest as it provides an immense potential for applications in spectroscopy and sensing. The unique vibrational and rotational resonances of the molecules at these wavelengths can be exploited for non-intrusive, unambiguous detection of the molecular composition of a broad variety of gases, liquids or solids, with a great interest for many high-impact applications. Fourier-transform spectrometers (FTS) are a particularly interesting solution for the on-chip integration due to their superior robustness against fabrication imperfections. However, the performance of current on-chip FTS implementations is limited by tradeoffs between bandwidth and resolution, for a given footprint. In this work we propose and experimentally demonstrate a new FTS approach that gathers the advantages of spatial heterodyning and optical path tuning by thermo-optic effect. The high resolution is provided by spatial multiplexing among different interferometers with increasing imbalance length, while the broadband operation is enabled by fine sampling interval of the optical path delay in each interferometer harnessing the thermo-optic effect. This novel approach overcomes the bandwidth-resolution tradeoff in conventional counterparts. The fabricated device enables a bandwidth as wide as 603 cm-1 (instead of 74 cm-1 with no-thermal tuning) near 7.7 μm wavelength, keeping a resolution better than 15 cm-1 with the same footprint. This device is fabricated in a Ge-rich graded-index SiGe platform with experimentally proven low loss operation up to 8.5 μm wavelength.