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

   https://doi.org/10.1109/JLT.2024.3447349  

   Shen, Jianhao; Donnelly, Daniel; Chakravarty, Swapnajit (January 2025, Journal of Lightwave Technology)

   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. 

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2. 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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3. Thermo-optic properties of silicon-rich silicon nitride for on-chip applications

   https://doi.org/10.1364/OE.396969  

   Nejadriahi, Hani; Friedman, Alex; Sharma, Rajat; Pappert, Steve; Fainman, Yeshaiahu; Yu, Paul (August 2020, Optics Express)

   We demonstrate the thermo-optic properties of silicon-rich silicon nitride (SRN) films deposited using plasma-enhanced chemical vapor deposition (PECVD). Shifts in the spectral response of Mach-Zehnder interferometers (MZIs) as a function of temperature were used to characterize the thermo-optic coefficients of silicon nitride films with varying silicon contents. A clear relation is demonstrated between the silicon content and the exhibited thermo-optic coefficient in silicon nitride films, with the highest achievable coefficient being as high as (1.65±0.08) ×10⁻⁴K^-1. Furthermore, we realize an SRN multi-mode interferometer (MMI) based thermo-optic switch with over 20 dB extinction ratio and total power consumption for two-port switching of 50 mW. 

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4. 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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5. Reconfigurable nonlinear optical element using tunable couplers and inverse-designed structure

   https://doi.org/10.1515/nanoph-2023-0152  

   Nikkhah, Vahid; Mencagli, Mario Junior; Engheta, Nader (March 2023, Nanophotonics)

   Abstract In recent years, wave-based analog computing has been at the center of attention for providing ultra-fast and power-efficient signal processing enabled by wave propagation through artificially engineered structures. Building on these structures, various proposals have been put forward for performing computations with waves. Most of these proposals have been aimed at linear operations, such as vector-matrix multiplications. The weak and hardly controllable nonlinear response of electromagnetic materials imposes challenges in the design of wave-based structures for performing nonlinear operations. In the present work, first, by using the method of inverse design we propose a three-port device, which consists of a combination of linear and Kerr nonlinear materials, exhibiting the desired power-dependent transmission properties. Then, combining a proper arrangement of such devices with a collection of Mach–Zehnder interferometers (MZIs), we propose a reconfigurable nonlinear optical architecture capable of implementing a variety of nonlinear functions of the input signal. The proposed device may pave the way for wave-based reconfigurable nonlinear signal processing that can be combined with linear networks for full-fledged wave-based analog computing. 

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