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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. 

Changes in the real and imaginary parts of the waveguide effective index in the presence of analytes have been used in various microcavity and slow light devices for on-chip sensing and absorption spectroscopy respectively in diverse applications. Periodically patterned waveguide sensors in interferometer configurations can lead to small interferometer sizes comparable in dimensions to microcavity resonator sensors, and/or significantly higher sensitivities compared to resonator type sensors. We show our work with compact silicon photonic interferometer devices for on-chip biosensing and absorbance sensing, overcoming fabrication tolerances with post-fabrication phase trimming. 

We experimentally demonstrated slow-wave-enhanced phase and spectral sensitivity in asymmetric Michelson interferometer (MI) sensors. Compared to Mach–Zehnder interferometers (MZI) that experimentally demonstrated a phase sensitivity of 84,000 rad/RIU-cm, the reflected path enhancement of the optical path length coupled with slow light enhancement with photonic crystal waveguides in on-chip slow light Michelson interferometer sensors resulted in experimentally demonstrated phase sensitivity of 277,750 rad/RIU-cm with theoretical phase sensitivity as high as 461,810 rad/RIU-cm, at the same form factor as the MZI of identical interferometer arm lengths. 

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 

Evanescent wave sensors in photonic integrated circuits have been demonstrated for gas sensing applications. While some methods rely on the distinctive response of certain polymers for sensing specific gases, absorption spectroscopy identifies any gas uniquely from their unique vibration signatures. Based on the Beer-Lambert principle, the sensitivity of absorption by a gas on chip relies on the length of the sensing region, the optical overlap integral with the analyte gas and the absorption cross-section at the wavelength with the fundamental vibration signature. The overlap of the optical mode with the analyte has been enhanced in photonic devices by combining slot waveguide confinements with photonic crystal slow light effects. While the absorption cross-section is a property of the gas, the length of the sensing region is limited by the available area on a chip and waveguide propagation losses that limit the minimum signal to noise ratio. In this paper, we show that by incorporating reflecting loop mirrors, the absorption path length can be doubled for the same geometric length of the absorption sensing waveguide. Light from a waveguide is split into two paths, each with a slow light photonic crystal waveguide, by a 2×2 multimode interference (MMI) power splitter. Each path is terminated by a loop mirror that causes the light to retrace its path back down the sensing arms thereby doubling the optical path length over which light interacts with the analyte. Results on the enhancement of phase sensitivity and absorbance sensitivity in the interferometric configuration are presented 

The benefits of photonics over electronics in the application of optical transceivers and both classical and quantum computing have been demonstrated over the past decades, especially in the ability to achieve high bandwidth, high interconnectivity, and low latency. Due to the high maturity of silicon photonics foundries, research on photonics devices such as silicon micro ring resonators (MRRs), Mach-Zehnder modulators (MZM), and photonic crystal (PC) resonators has attracted plenty of attention. Among these photonic devices, silicon MRRs using carrier depletion effects in p-n junctions represent optical switches manufacturable in the most compact magnitude at high volume with demonstrated switching energies ~5.2fJ/bit. In matrix multiplication demonstrated with integrated photonics, one approach is to couple one bus straight waveguide to several MRRs with different resonant wavelengths to transport signals in different channels, corresponding to a matrix row or column. However, such architectures are potentially limited to ~30 MRRs in series, by the limited free-spectral range (FSR) of an individual MRR. We show that PC switches with sub-micron optical mode confinements can have a FSR >300nm, which can potentially enable energy efficient computing with larger matrices of ~200 resonators by multiplexing. In this paper, we present designs for an oxide-clad bus-coupled PC switch with 1dB insertion loss, 5dB extinction, and ~260aJ/bit switching energy by careful control of the cavity geometry as well as p-n junction doping. We also demonstrate that air-clad bus-coupled PC switches can operate with 1dB insertion loss, 3dB extinction, and ~80aJ/bit switching energy. 

Diverse chip-based sensors utilizing integrated silicon photonics have been demonstrated in resonator and phase shifter/interferometer configurations. Till date, interferometric techniques with the Mach-Zehnder Interferometer (MZI) and Young’s interferometer have shown the lowest mass detection limits (in pg/mm2). Slow light in photonic crystal waveguides integrated with MZIs enables compact geometries due to enhanced optical path lengths as light propagates with high group index. In a typical MZI, light propagating in the signal arm overlaps with analytes and undergo a relative phase change with respect to the light in the reference arm which leads to measured output intensity changes. In this paper, using integrated photonic methods, we demonstrate a slow light enhanced Michelson interferometer (MI) biosensor, wherein the reference and signal arms are traversed twice by the propagating optical mode. As a result, the analyte interaction length is effectively doubled since the propagating optical mode undergoes twice the phase shift as would be observed in a MZI. In an asymmetric MI configuration, the resultant doubling of the phase shift is observed as a doubling of the resonance wavelength shift for a fixed change in the analyte concentration. The device sensitivity is thus doubled with respect to a conventional MZI while also effectively halving the geometric length compared to the MZI sensor