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1. Electrically empowered microcomb laser

   https://doi.org/10.1038/s41467-024-48544-2  

   Ling, Jingwei; Gao, Zhengdong; Xue, Shixin; Hu, Qili; Li, Mingxiao; Zhang, Kaibo; Javid, Usman A; Lopez-Rios, Raymond; Staffa, Jeremy; Lin, Qiang (December 2024, Nature Communications)

   Abstract Optical microcomb underpins a wide range of applications from communication, metrology, to sensing. Although extensively explored in recent years, challenges remain in key aspects of microcomb such as complex soliton initialization, low power efficiency, and limited comb reconfigurability. Here we present an on-chip microcomb laser to address these key challenges. Realized with integration between III and V gain chip and a thin-film lithium niobate (TFLN) photonic integrated circuit (PIC), the laser directly emits mode-locked microcomb on demand with robust turnkey operation inherently built in, with individual comb linewidth down to 600 Hz, whole-comb frequency tuning rate exceeding 2.4 × 10¹⁷ Hz/s, and 100% utilization of optical power fully contributing to comb generation. The demonstrated approach unifies architecture and operation simplicity, electro-optic reconfigurability, high-speed tunability, and multifunctional capability enabled by TFLN PIC, opening up a great avenue towards on-demand generation of mode-locked microcomb that is of great potential for broad applications. 

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2. Low-noise frequency-agile photonic integrated lasers for coherent ranging

   https://doi.org/10.1038/s41467-022-30911-6  

   Lihachev, Grigory; Riemensberger, Johann; Weng, Wenle; Liu, Junqiu; Tian, Hao; Siddharth, Anat; Snigirev, Viacheslav; Shadymov, Vladimir; Voloshin, Andrey; Wang, Rui Ning; et al (December 2022, Nature Communications)

   Abstract Frequency modulated continuous wave laser ranging (FMCW LiDAR) enables distance mapping with simultaneous position and velocity information, is immune to stray light, can achieve long range, operate in the eye-safe region of 1550 nm and achieve high sensitivity. Despite its advantages, it is compounded by the simultaneous requirement of both narrow linewidth low noise lasers that can be precisely chirped. While integrated silicon-based lasers, compatible with wafer scale manufacturing in large volumes at low cost, have experienced major advances and are now employed on a commercial scale in data centers, and impressive progress has led to integrated lasers with (ultra) narrow sub-100 Hz-level intrinsic linewidth based on optical feedback from photonic circuits, these lasers presently lack fast nonthermal tuning, i.e. frequency agility as required for coherent ranging. Here, we demonstrate a hybrid photonic integrated laser that exhibits very narrow intrinsic linewidth of 25 Hz while offering linear, hysteresis-free, and mode-hop-free-tuning beyond 1 GHz with up to megahertz actuation bandwidth constituting 1.6 × 10¹⁵Hz/s tuning speed. Our approach uses foundry-based technologies - ultralow-loss (1 dB/m) Si₃N₄photonic microresonators, combined with aluminium nitride (AlN) or lead zirconium titanate (PZT) microelectromechanical systems (MEMS) based stress-optic actuation. Electrically driven low-phase-noise lasing is attained by self-injection locking of an Indium Phosphide (InP) laser chip and only limited by fundamental thermo-refractive noise at mid-range offsets. By utilizing difference-drive and apodization of the photonic chip to suppress mechanical vibrations of the chip, a flat actuation response up to 10 MHz is achieved. We leverage this capability to demonstrate a compact coherent LiDAR engine that can generate up to 800 kHz FMCW triangular optical chirp signals, requiring neither any active linearization nor predistortion compensation, and perform a 10 m optical ranging experiment, with a resolution of 12.5 cm. Our results constitute a photonic integrated laser system for scenarios where high compactness, fast frequency actuation, and high spectral purity are required. 

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3. Cavity-enhanced narrowband spectral filters using rare-earth ions doped in thin-film lithium niobate

   https://doi.org/10.1038/s44310-024-00023-8  

   Zhao, Yuqi; Renaud, Dylan; Farfurnik, Demitry; Jiang, Yuxi; Dutta, Subhojit; Sinclair, Neil; Lončar, Marko; Waks, Edo (December 2024, npj Nanophotonics)

   Abstract On-chip optical filters are fundamental components in optical signal processing. While rare-earth ion-doped crystals offer ultra-narrow optical filtering via spectral hole burning, their applications have primarily been limited to those using bulk crystals, restricting their utility. In this work, we demonstrate cavity-enhanced spectral filtering based on rare-earth ions in an integrated nonlinear optical platform. We incorporate rare-earth ions into high quality-factor ring resonators patterned in thin-film lithium niobate. By spectral hole burning at 4 K in a critically coupled resonance mode, we achieve bandpass filters ranging from 7 MHz linewidth, with 13.0 dB of extinction, to 24 MHz linewidth, with 20.4 dB of extinction. By reducing the temperature to 100 mK to eliminate phonon broadening, we achieve an even narrower linewidth of 681 kHz, which is comparable to the narrowest filter linewidth demonstrated in an integrated photonic device, while only requiring a small device footprint. Moreover, the cavity enables reconfigurable filtering by varying the cavity coupling rate. For instance, as opposed to the bandpass filter, we demonstrate a bandstop filter utilizing an under-coupled ring resonator. Such versatile integrated spectral filters with high extinction ratio and narrow linewidth could serve as fundamental components for optical signal processing and optical memories on-a-chip. 

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4. Visible light photonic integrated Brillouin laser

   https://doi.org/10.1038/s41467-021-24926-8  

   Chauhan, Nitesh; Isichenko, Andrei; Liu, Kaikai; Wang, Jiawei; Zhao, Qiancheng; Behunin, Ryan O.; Rakich, Peter T.; Jayich, Andrew M.; Fertig, C.; Hoyt, C. W.; et al (December 2021, Nature Communications)

   Abstract Narrow linewidth visible light lasers are critical for atomic, molecular and optical (AMO) physics including atomic clocks, quantum computing, atomic and molecular spectroscopy, and sensing. Stimulated Brillouin scattering (SBS) is a promising approach to realize highly coherent on-chip visible light laser emission. Here we report demonstration of a visible light photonic integrated Brillouin laser, with emission at 674 nm, a 14.7 mW optical threshold, corresponding to a threshold density of 4.92 mW μm −2 , and a 269 Hz linewidth. Significant advances in visible light silicon nitride/silica all-waveguide resonators are achieved to overcome barriers to SBS in the visible, including 1 dB/meter waveguide losses, 55.4 million quality factor (Q), and measurement of the 25.110 GHz Stokes frequency shift and 290 MHz gain bandwidth. This advancement in integrated ultra-narrow linewidth visible wavelength SBS lasers opens the door to compact quantum and atomic systems and implementation of increasingly complex AMO based physics and experiments. 

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5. High-power diode laser spectrally narrowed with prism–etalon feedback

   https://doi.org/10.1063/5.0203666  

   Muller, A (September 2024, Review of Scientific Instruments)

   A simple method for reducing the linewidth of a diode laser while maintaining high output power is described. It is based on a dispersive prism and a thin etalon for retroreflective feedback. The etalon creates two weak external cavities that provide spectral selectivity that is periodic with a period equal to the etalon’s free spectral range. The method was applied to a multimode blue laser diode, which in the absence of feedback features a linewidth of several nanometers. The spectral properties of the laser were investigated for different etalon thicknesses and operating currents and tested in the presence of temperature fluctuations. With a SF11 equilateral uncoated prism near Brewster’s angle and a 0.3 mm-thick uncoated fused silica etalon, the linewidth was reduced 20-fold to 70 pm (3.6 cm−1) with an output power of 3 W at a current of 2.15 A. The largest diode current probed was 2.75 A, which resulted in a linewidth of 100 pm (5.1 cm−1) and an output power of 4 W. In contrast to the use of, for example, a volume Bragg grating, a high degree of flexibility is afforded as the same prism–etalon pair can be used across the visible and near infrared. 

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