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1. Side-lobe reduction by cascading Bragg grating filters on a Si-photonic chip

   https://doi.org/10.1364/OE.446588  

   Kumar, Sushant; Alshamrani, Naif; Grieco, Andrew; Fainman, Yeshaiahu (January 2022, Optics Express)

   Bragg-grating based cavities and coupler designs present opportunities for flexible allocation of bandwidth and spectrum in silicon photonic devices. Integrated silicon photonic devices are moving toward mainstream, mass adoption, leading to the need for compact Bragg grating based designs. In this work we present a design and experimental validation of a cascaded contra-directional Bragg-grating coupler with a measured main lobe to side-lobe contrast of 12.93 dB. This level of performance is achieved in a more compact size as compared to conventional apodized gratings, and a similar design philosophy can be used to improve side-lobe reduction in grating-based mirror design for on-chip lasers and other cavity-based designs as well. 

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2. Interferometric modal splitting enables broadband, dual-polarization on-chip spectroscopy

   https://doi.org/10.1364/OPTICA.577600  

   Johnson, Karl; Fedorov, Vladimir; Belogolovskii, Dmitrii; Grieco, Andrew; Rubin, Noah A; Fainman, Yeshaiahu (December 2025, Optica)

   The modal dispersion of waveguides typically limits integrated photonic devices to operation with a single polarization state. In this work, we propose a generic mode separation technique we call “interferometric mode splitting” (IMS), which enables guided modes to be separated over wide bandwidths with a large extinction ratio. To demonstrate the general principle of IMS, we show that an unmodified thermally driven silicon photonic Fourier transform spectrometer exhibits a polarization-separating effect in the frequency domain, even though only one polarization-insensitive detector is used. Using this effect, we experimentally demonstrate a simple on-chip spectrometer capable of extracting two-polarization spectra over a wide 1480–1630 nm bandwidth with a greater than 20 dB polarization extinction ratio. These specifications would be highly challenging to achieve using existing, conventional on-chip polarization-splitting techniques. Though we focus on this specific realization of IMS, we also show that IMS is general to various on-chip spectrometer architectures, other spatial modes, and technologies other than thermally driven Fourier transform spectrometers. Interferometric mode splitting shows promise as a general approach for robust and fundamentally broadband detection of orthogonal modes in guided-wave sensing. 

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3. Over 100 million Q silicon nitride Fabry–Perot Bragg nanoresonators enabled by advanced tapered gratings

   https://doi.org/10.1364/OL.578449  

   Zhang, Yang; Heidari_Khouzani, Masoud; Yu, ChengDao; Veilleux, Sylvain; Dagenais, Mario (January 2025, Optics Letters)

   We report new, to the best of our knowledge, advances in Fabry–Perot Bragg grating nanoresonators on low-loss silicon nitride (Si₃N₄). Building on a previous demonstration of an intrinsic quality factor Q_inof 19.3 million with 1.4 dB/m propagation loss, we now achieve Q_inup to 121 million on a 100-nm Si₃N₄platform with an ultralow propagation loss of 0.19 dB/m. Even on a 300-nm Si₃N₄platform, Q_inof 58 million is obtained with 0.45 dB/m loss. Crucially, an advanced tapered grating design is introduced to minimize mode mismatch between the waveguide and Bragg mirror. These improvements pave the way for compact ultrahigh-Q photonic integrated resonators for applications in nonlinear optics and precision photonics. 

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4. Fabry-Perot Bragg grating nanoresonator with ultrahigh intrinsic Q based on low-loss silicon nitride

   https://doi.org/10.1364/OE.499930  

   Zhang, Yang; Veilleux, Sylvain; Dagenais, Mario (January 2023, Optics Express)

   Photonic integrated circuits based on ultralow loss silicon nitride waveguides have shown significant promise for realizing high-performance optical systems in a compact and scalable form factor. For the first time, we have developed a Fabry-Perot Bragg grating nanoresonator based on silicon nitride on silicon dioxide platform with an ultra-high intrinsic quality factor of 19.3 million. By combining the introduction of tapered grating between cavity and periodic Bragg grating, increasing the width of cavity to multi-mode region and optimized annealing strategy for Si₃N₄film, the propagation loss is reduced to around 0.014 dB/cm. Fabry-Perot Bragg grating nanoresonator can be easily implemented in a simple straight waveguide occupying a minimal amount of space. Therefore, it is a key component to build a high performance photonic integrated circuit for many applications. 

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5. Low-loss fiber-to-chip interface for lithium niobate photonic integrated circuits

   Lingyan He, Mian Zhang (January 2019, Optics letters)

   Integrated lithium niobate (LN) photonic circuits have recently emerged as a promising candidate for advanced photonic functions such as high-speed modulation, nonlinear frequency conversion, and frequency comb generation. For practical applications, optical interfaces that feature low fiber-to-chip coupling losses are essential. So far, the fiber-to-chip loss (commonly >10  dB/facet) has dominated the total insertion losses of typical LN photonic integrated circuits, where on-chip losses can be as low as 0.03–0.1 dB/cm. Here we experimentally demonstrate a low-loss mode size converter for coupling between a standard lensed fiber and sub-micrometer LN rib waveguides. The coupler consists of two inverse tapers that convert the small optical mode of a rib waveguide into a symmetrically guided mode of a LN nanowire, featuring a larger mode area matched to that of a tapered optical fiber. The measured fiber-to-chip coupling loss is lower than 1.7 dB/facet with high fabrication tolerance and repeatability. Our results open the door for practical integrated LN photonic circuits efficiently interfaced with optical fibers. 

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