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1. A compact Michelson interferometer based on-chip Fourier transform spectrometer

   https://doi.org/10.1117/12.2663712  

   Donnelly, Daniel; Shen, Jianhao; Chakravarty, Swapnajit (June 2023, SPIE)

   Crocombe, Richard A.; Profeta, Luisa T. (Ed.)

   We experimentally demonstrate a compact Fourier Transform on-chip spectrometer based on spatially heterodyned array of Michelson interferometers (MIs) in a silicon-on-insulator (SOI) platform. We demonstrate that with the same progressive geometric path length difference between the spatially heterodyned arrayed interferometer arms, MIs double the optical phase delays and thus double the wavelength resolution (δλ=0.8nm) compared to Mach-Zehnder interferometers (MZIs) (δλ=1.6nm). Our proof-of-concept device demonstrates one method to address the bandwidth-resolution tradeoff inherent in on-chip FTIRs, which gains in significance for optical sensing applications requiring single digit picometers resolution in compact on-chip form factors 

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2. Experimental Comparison of Slow Light and Subwavelength Waveguide Interferometer Sensors

   https://doi.org/10.1109/JSEN.2024.3450186  

   Shen, Jianhao; Perera, Asela; Rai, Arushi; Kango-Singh, Madhuri; Chakravarty, Swapnajit (October 2024, IEEE Sensors Journal)

   We experimentally demonstrate slow light photonic crystal waveguide (PCW) and subwavelength waveguide (SWWG) loop terminated Mach-Zehnder interferometer (LT-MZI) sensors in a foundry-fabricated silicon-on-insulator (SOI) platform. We compare the experimental results on sensitivity and limit of detection (LOD) on the interferometer sensors with microcavity-type sensors. We show experimentally that 2-D PCW interferometers have higher phase sensitivities than SWWGs of the same length. Based on experimental results, 20- μ m-long 2-D PCW LT-MZI sensors and 200- μ m-long SWWG LT-MZI sensors achieve an LOD of 3.4×10−4 and 2.3×10−4 RIU, respectively, with nearly the same insertion losses in foundry-fabricated devices. We show that by considering the various sources of loss in our benchtop fiber-to-fiber photonic integrated circuit measurement system, it will be possible to reach 10−7 LOD in both slow light PCW and SWWG-based LT-MZI sensors with on-chip integrated light sources and detectors. We show via simulations and experiment that the LOD of a 20- μ m-long slow light PCW LT-MZI is equivalent to that of a 100- μ m-long SWWG LT-MZI, thus enabling more compact LT_MZI sensors when using slow light PCWs versus SWWGs 

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3. Mastering Silicon Photonics Device Design for Scalable and Robust Optical Neural Networks

   https://doi.org/10.1364/IPRSN.2024.IW2B.1  

   Ghanaatian, Zahra; Shafiee, Amin; Nikdast, Mahdi (January 2024, Optica Publishing Group)

   We demonstrate a design optimization approach for Mach–Zehnder Interferometers (MZIs) in optical neural networks under fabrication-process variations (FPVs). Our results show increased inferencing accuracy in the network under realistic FPVs by using optimized MZIs. 

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4. On-Chip Si3N4 Spatial Heterodyne Fourier Transform Spectrometer for the Optical Window in Biological Tissue

   https://doi.org/10.1364/CLEO_AT.2021.JTu3A.122  

   Yoo, Kyoungmin; Chen, Ray T. (June 2021, Cleo)

   Optica CLEO program Committee (Ed.)

   We designed and demonstrated an on-chip Fourier transform spectrometer on Si3N4-on-SiO2 using an array of Mach-Zehnder interferometers (MZIs) for 𝜆 = 600~1000 nm. The retrieval of an input spectrum is demonstrated by the interconnect simulation. 

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5. Dual-Polarization Bandwidth-Bridged On-Chip Bandpass Sampling Fourier Transform Spectrometer from Visible to Near-Infrared on a Silicon Nitride Platform

   https://doi.org/0000-0002-9181-4266  

   Yoo, Kyoung Min; Chen, Ray T. (July 2022, ACS photonics)

   Jelena Vuckovic (Ed.)

   On-chip broadband optical spectrometers that cover the entire tissue transparency window (λ = 650–1050 nm) with high resolution are highly demanded for miniaturized biosensing and bioimaging applications. The standard spatial heterodyne Fourier transform spectrometer (SHFTS) requires a large number of Mach–Zehnder interferometer (MZI) arrays to obtain a broad spectral bandwidth while maintaining high resolution. Here, we propose a novel type of SHFTS integrated with a subwavelength grating coupler (SWGC) for the dual-polarization bandpass sampling on the Si3N4 platform to solve the intrinsic trade-off limitation between the bandwidth and resolution of the SHFTS without having an outrageous number of MZI arrays or adding additional active photonic components. By applying the bandpass sampling theorem, the continuous broadband input spectrum is divided into multiple narrow-band channels through tuning the phase-matching condition of the SWGC with different polarization and coupling angles. Thereby, it is able to reconstruct each band separately far beyond the Nyquist criterion without aliasing error or degrading the resolution. We experimentally demonstrated the broadband spectrum retrieval results with the overall bandwidth coverage of 400 nm, bridging the wavelengths from 650 to 1050 nm, with a resolution of 2–5 nm. The bandpass sampling SHFTS is designed to have 32 linearly unbalanced MZIs with the maximum optical path length difference of 93 μm within an overall footprint size of 4.7 mm × 0.65 mm, and the coupling angles of SWGC are varied from 0° to 32° to cover the entire tissue transparency window. 

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