In static SHS, an array of interferometers with linearly increasing optical path delays is implemented to yield a spatial interferogram that allows on-chip spectral detection without any moving parts 12, 14, 15, 16, 21. The resolution of this approach is seriously limited by practical constraints in the photodetector array, e.g. This interference pattern is diffracted off-chip and monitored by an array of photodetectors. In SWI an stationary wave pattern is generated by the interference of two counter- or co-propagating waves 13, 17. Current on-chip FTS rely on one of the three following operation schemes: i) stationary wave interferometry (SWI), ii) static spatial heterodyne spectroscopy (SHS), and iii) scanning interferometry. On-chip Fourier transform spectrometers (FTS) implement simple yet effective calibration algorithms that compensate phase and amplitude impairments, thereby providing a superior robustness against fabrication imperfections, compared to dispersive counterparts. While commercial mid-IR spectrometers currently available are often based on an assembly of discrete elements 7, on-chip photonic integration offer key advantages to develop fast, low-cost, low-power consumption, high-reliability and high performances systems.Ī myriad of integrated spectroscopic systems has been demonstrated, based on dispersive devices such as array waveguide gratings (AWG) 8, 9, echelle grating 10, 11 or Fourier transform spectroscopy 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22. The unique vibrational and rotational resonances of molecules at these wavelengths can be used to univocally discern and quantify the molecular composition of a broad variety of gases, liquids or solids, with application in environmental monitoring 1, astronomy 2, hazard detection 3, industrial process control 4 and non-invasive medical diagnostics 5, 6.
Photonics integration in the mid-Infrared (mid-IR) spectral range, and more specifically in the fingerprint region between 5 and 20 μm wavelength has garnered a great interest due to its immense potential for applications in spectroscopy and sensing. This is a resolution comparable to state-of-the-art on-chip mid-infrared spectrometers with a 4-fold bandwidth increase with a footprint divided by a factor two. Capitalizing on this concept, we experimentally demonstrate a mid-infrared SiGe FTS, with a resolution better than 15 cm −1 and a bandwidth of 603 cm −1 near 7.7 μm wavelength with a 10 MZI array. The high resolution is provided by spatial multiplexing among different interferometers with increasing imbalance length, while the broadband operation is enabled by fine tuning of the optical path delay in each interferometer harnessing the thermo-optic effect. Here, we propose a new FTS approach that gathers the advantages of spatial heterodyning and optical path tuning by thermo-optic effect to overcome this tradeoff. However, the performance of current on-chip FTS implementations is limited by tradeoffs in bandwidth and resolution. Fourier-transform spectrometers (FTS) are a particularly interesting solution for the on-chip integration due to their superior robustness against fabrication imperfections. Indeed, optical spectrometers working in the mid-infrared spectral range have garnered a great interest for their singular capability to monitor the main absorption fingerprints of a wide range of chemical and biological substances.
Miniaturized optical spectrometers providing broadband operation and fine resolution have an immense potential for applications in remote sensing, non-invasive medical diagnostics and astronomy.
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