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1. High-resolution and compact serpentine integrated grating spectrometer

   https://doi.org/10.1364/JOSAB.423968  

   Brand, Michael ; Zhang, Bohan ; Onural, Deniz ; Al_Qubaisi, Kenaish ; Popović, Miloš ; Dostart, Nathan ; Wagner, Kelvin ( June 2021 , Journal of the Optical Society of America B)

   Integrated astrophotonic spectrometers are integrated variants of conventional free-space spectrometers that offer significantly reduced size, weight, and cost and immunity to alignment errors, and can be readily integrated with other astrophotonic instruments such as nulling interferometers. Current integrated dispersive astrophotonic spectrometers are one-dimensional devices such as arrayed waveguide gratings or planar echelle gratings. These devices have been limited to${10}^4$resolving powers and$<<\#comment/>1000$spectral bins due to having limited total optical delay paths and 1D detector array pixel densities. In this paper, we propose and demonstrate a high-resolution and compact astrophotonic serpentine integrated grating (SIG) spectrometer design based on a 2D dispersive serpentine optical phased array. The SIG device combines a 5.2 cm long folded delay line with grating couplers to create a large optical delay path along two dimensions in a compact integrated device footprint. Analogous to free-space crossed-dispersion high-resolution spectrometers, the SIG spectrometer maps spectral content to a 2D wavelength-beam-steered folded-raster emission pattern focused onto a 2D detector array. We demonstrate a SIG spectrometer with$\sim <\#comment/>100\mathrm{k}$resolving power and$\sim <\#comment/>6750$spectral bins, which are approximately an order of magnitude higher than previous integrated photonic designs that operate over a wide bandwidth, in a$0.4{\mathrm{m}\mathrm{m}}^2$footprint. We measure a Rayleigh resolution of$1.93\pm <\#comment/>0.07\mathrm{G}\mathrm{H}\mathrm{z}$and an operational bandwidth from 1540 nm to 1650 nm. Finally, we discuss refinements of the SIG spectrometer that improve its resolution, bandwidth, and throughput. These results show that SIG spectrometer technology provides a path towards miniaturized, high-resolution spectrometers for applications in astronomy and beyond.

    

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2. Vector beams in planar photonic crystal cavities with rotating air holes

   https://doi.org/10.1364/OL.381458  

   Gao, Xiaomei ; Yang, Lechen ; Bo, Fang ; Li, Jiafang ; Zhang, Guoquan ; Xu, Jingjun ( March 2020 , Optics Letters)

   We report a method to generate angularly polarized vector beams with a topological charge of one by rotating air holes to form two-dimensional photonic crystal (PC) cavities. The mode volume and resonance wavelength of these cavities are tuned from$0.33(\mathrm{\lambda <\#comment/>}/n{)}^3$to$12(\mathrm{\lambda <\#comment/>}/n{)}^3$and in a wide range of 400 nm, respectively, by controlling the range of fixed air holes near the center of the structure. As a benefit, the half-maximum divergence angles of the vector beam can be widely changed from 90° to$\sim <\#comment/>{60}^{\circ <\#comment/>}$. By adjusting the shift direction of the air holes in the PC cavities, optical vector beams with different far-field morphology are obtained. The scheme provides not only an alternative method to generate optical vector beams, but also an effective strategy to control far-field morphology and polarizations, which holds promising applications such as optical microscopy and micro-manipulation.

    

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3. Wavelength beam-combining of terahertz quantum-cascade laser arrays

   https://doi.org/10.1364/OL.420398  

   Chen, Ji ; Jin, Yuan ; Gao, Liang ; Reno, John_L ; Kumar, Sushil ( April 2021 , Optics Letters)

   Wavelength beam-combining of four terahertz (THz) distributed-feedback quantum-cascade lasers (QCLs) is demonstrated using low-cost THz components that include a lens carved out of a plastic ball and a mechanically fabricated blazed grating. Single-lobed beams from predominantly single-mode QCLs radiating peak power in the range of$50-<\#comment/>170\mathrm{m}\mathrm{W}$are overlapped in the far field at frequencies ranging from$3.31-<\#comment/>3.54\mathrm{T}\mathrm{H}\mathrm{z}$. Collinear propagation with a maximum angular deviation of${0.3}^{\circ <\#comment/>}$is realized for the four beams. The total power efficiency for the focused and beam-combined radiation is as high as$25\mathrm{\%<\#comment/>}$. This result could pave the way for future commercialization of beam-combined monolithic THz QCL arrays for multi-spectral THz sensing and spectroscopy at standoff distances.

    

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4. Measurement of sub-fm/Hz 1/2 displacement spectral densities in ultrahigh-Q single-crystal microcavities with hertz-level lasers

   https://doi.org/10.1364/PRJ.449782  

   Jang, Yoon-Soo ; Lim, Jinkang ; Wang, Wenting ; Kim, Seung-Woo ; Savchenkov, Anatoliy ; Matsko, Andrey B. ; Wong, Chee Wei ( April 2022 , Photonics Research)

   Tracing a resonance frequency of a high quality factor (Q) optical cavity facilitates subpicometer displacement measurements of the optical cavity via Pound–Drever–Hall (PDH) locking scheme, tightly synchronizing a laser frequency to the optical cavity. Here we present observations of subfemtometer displacements on a ultrahigh-Qsingle-crystal${\mathrm{MgF}}_2$whispering-gallery-mode microcavity by frequency synchronization between a 1 Hz cavity-stabilized laser and a resonance of the${\mathrm{MgF}}_2$cavity using PDH laser-cavity locking. We characterize not only the displacement spectral density of the microcavity with a sensitivity of$1.5\times {10}^{-16}\mathrm{m}/{\mathrm{Hz}}^{1/2}$over the Fourier offset frequency ranging from 15 mHz to 100 kHz but also a 1.77 nm displacement fluctuation of the microcavity over 4500 s. Such measurement capability not only supports the analysis of integrated thermodynamical and technical cavity noise but allows for minute displacement measurements using laser-cavity locking for ultraprecise positioning, metrology, and sensing.

    

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5. Serpentine optical phased arrays for scalable integrated photonic lidar beam steering

   https://doi.org/10.1364/OPTICA.389006  

   Dostart, Nathan ; Zhang, Bohan ; Khilo, Anatol ; Brand, Michael ; Al_Qubaisi, Kenaish ; Onural, Deniz ; Feldkhun, Daniel ; Wagner, Kelvin_H ; Popović, Miloš_A ( June 2020 , Optica)

   Optical phased arrays (OPAs) implemented in integrated photonic circuits could enable a variety of 3D sensing, imaging, illumination, and ranging applications, and their convergence in new lidar technology. However, current integrated OPA approaches do not scale—in control complexity, power consumption, or optical efficiency—to the large aperture sizes needed to support medium- to long-range lidar. We present the serpentine OPA (SOPA), a new OPA concept that addresses these fundamental challenges and enables architectures that scale up to large apertures. The SOPA is based on a serially interconnected array of low-loss grating waveguides and supports fully passive, 2D wavelength-controlled beam steering. A fundamentally space-efficient design that folds the feed network into the aperture also enables scalable tiling of SOPAs into large apertures with a high fill-factor. We experimentally demonstrate, to the best of our knowledge, the first SOPA using a 1450–1650 nm wavelength sweep to produce 16,500 addressable spots in a$27\times <\#comment/>610$array. We also demonstrate, for the first time, far-field interference of beams from two separate OPAs on a single silicon photonic chip, as an initial step towards long-range computational imaging lidar based on novel active aperture synthesis schemes.

    

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