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1. Relative timing jitter in a counterpropagating all-normal dispersion dual-comb fiber laser

   https://doi.org/10.1364/OPTICA.458339  

   Prakash, Neeraj ; Huang, Shu-Wei ; Li, Bowen ( June 2022 , Optica)

   The counterpropagating all-normal dispersion (CANDi) fiber laser is an emerging high-energy single-cavity dual-comb laser source. Its relative timing jitter (RTJ), a critical parameter for dual-comb timing precision and spectral resolution, has not been comprehensively investigated. In this paper, we enhance the state-of-the-art CANDi fiber laser pulse energy from 1 nJ to 8 nJ. We then introduce a reference-free RTJ characterization technique that provides shot-to-shot measurement capability at femtosecond precision. The measurement noise floor reaches$1.6\times <\#comment/>{10}^{-<\#comment/>7}{\mathrm{f}\mathrm{s}}^2/\mathrm{H}\mathrm{z}$, and the corresponding integrated measurement precision is only 1.8 fs (1 kHz, 20 MHz). With this characterization tool, we are able to study the physical origin of the CANDi laser’s RTJ in detail. We first verify that the cavity length fluctuation does not contribute to the RTJ. Then we measure the integrated RTJ to be 39 fs (1 kHz, 20 MHz) and identify the pump relative intensity noise (RIN) to be the dominant factor responsible for it. In particular, pump RIN is coupled to the RTJ through the Gordon–Haus effect. Finally, solutions to reduce the free-running CANDi laser’s RTJ are discussed. This work provides a general guideline to improve the performance of compact single-cavity dual-comb systems such as the CANDi laser, benefitting various dual-comb applications.

    

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2. Surface-enhanced Raman scattering sensing platform for detecting amyloid-β peptide interaction with an aggregation inhibitor

   https://doi.org/10.1364/AO.399307  

   de Oliveira, Marcos A. Soares ; Hilt, Silvia ; Chang, Che-Wei ; Lee, Changwon ; Voss, John C. ; Chan, James W. ( August 2020 , Applied Optics)

   Soluble, small amyloid-$\mathrm{\beta <\#comment/>}$oligomers ($\mathrm{A}\mathrm{\beta <\#comment/>}\mathrm{O}$) are recognized as significant contributors to the pathology of Alzheimer’s disease (AD). Although drugs for treating AD symptoms have been approved, no therapy targeting amyloid-$\mathrm{\beta <\#comment/>}$($\mathrm{A}\mathrm{\beta <\#comment/>}$) capable of modifying the course of the disease is available. In an effort to develop a label-free method for screening new anti-AD therapeutic agents, we show the use of a surface-enhanced Raman scattering (SERS) active substrate for detecting the interactions between$\mathrm{A}\mathrm{\beta <\#comment/>}$peptides and spin-labeled fluorine (SLF), a peptide aggregation inhibitor. Changes in the peak positions and intensity ratios of two spectral peaks near$1600\mathrm{c}{\mathrm{m}}^{-<\#comment/>1}$and$2900\mathrm{c}{\mathrm{m}}^{-<\#comment/>1}$can be used to monitor the molecular interactions between SLF and$\mathrm{A}\mathrm{\beta <\#comment/>}$. This study demonstrates the potential of SERS spectroscopy for rapidly screening and identifying new anti-$\mathrm{A}\mathrm{\beta <\#comment/>}$therapeutic agents.

    

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3. Higher-order QAM data transmission using a high-coherence hybrid Si/III–V semiconductor laser

   https://doi.org/10.1364/OL.383137  

   Zou, Kaiheng ; Zhang, Zhewei ; Liao, Peicheng ; Wang, Huolei ; Cao, Yinwen ; Almaiman, Ahmed ; Fallahpour, Ahmad ; Alishahi, Fatemeh ; Satyan, Naresh ; Rakuljic, George ; et al ( March 2020 , Optics Letters)

   We experimentally demonstrate the use of a high-coherence hybrid silicon (Si)/III–V semiconductor laser as the light source for a transmitter generating 20 Gbaud 16- and 64- quadrature amplitude modulated (QAM) data signals over an 80 km single-mode fiber (SMF) link. The hybrid Si/III–V laser has a measured Schawlow–Townes linewidth of$\sim <\#comment/>10\mathrm{k}\mathrm{H}\mathrm{z}$, which is achieved by storing modal optical energy in low-loss Si, rather than the relatively lossy III–V materials. We measure a received bit error rate (BER) of$4.1\times <\#comment/>{10}^{-<\#comment/>3}$when transmitting the 64-QAM data over an 80 km SMF using the hybrid Si/III–V laser. Furthermore, we measure a BER of$<<\#comment/>1\times <\#comment/>{10}^{-<\#comment/>4}$with the Viterbi–Viterbi digital carrier phase recovery method when transmitting the 16-QAM data over an 80 km SMF using the hybrid Si/III–V laser. This performance is achieved at power penalties lower than those obtained with an exemplary distributed feedback laser and slightly higher than those with an exemplary narrow-linewidth external cavity laser.

    

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4. Emissions in short-gated ns/ps/fs-LIBS for fuel-to-air ratio measurements in methane-air flames

   https://doi.org/10.1364/AO.418453  

   Gragston, Mark ; Hsu, Paul ; Jiang, Naibo ; Roy, Sukesh ; Zhang, Zhili ( May 2021 , Applied Optics)

   A study of short-gated 10 nanosecond (ns), 100 picosecond (ps), and 100 femtosecond (fs) laser induced breakdown spectroscopy (LIBS) was conducted for fuel-to-air ratio (FAR) measurements in an atmospheric Hencken flame. The intent of the work is to understand which emission lines are available near the optical range in each pulse width regime and which emission ratios may be favorable for generating equivalence ratio calibration curves. The emission spectra in the range of 550–800 nm for ns-LIBS and ps-LIBS are mostly similar with slightly elevated atomic oxygen lines by ps-LIBS. Spectra from fs-LIBS show the lowest continuum background and prominent individual atomic lines, though have significantly weaker ionic emission from nitrogen. A qualitative explanation based on assumed local thermodynamic equilibrium and electron temperatures calculated by the${\mathrm{N}}_{\mathrm{I}\mathrm{I}}(565\mathrm{n}\mathrm{m})$and${\mathrm{N}}_{\mathrm{I}\mathrm{I}}(594\mathrm{n}\mathrm{m})$emissions is presented. In studying line emission ratios for FAR calculation, it is found that${\mathrm{H}}_{\mathrm{\alpha <\#comment/>}}(656\mathrm{n}\mathrm{m})/{\mathrm{N}}_{\mathrm{I}\mathrm{I}}(568\mathrm{n}\mathrm{m})$is best for FAR measurements with ns-LIBS and remains viable for ps-LIBS, while${\mathrm{H}}_{\mathrm{\alpha <\#comment/>}}(656\mathrm{n}\mathrm{m})/{\mathrm{O}}_{\mathrm{I}}(777\mathrm{n}\mathrm{m})$is optimal for the ps-LIBS and fs-LIBS cases. Due to low continuum background and short time delay for spectra collection, fs-LIBS is very promising for high-speed FAR measurements using short-gated LIBS.

    

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5. Wake dynamics of air filaments generated by high-energy picosecond laser pulses at 1 kHz repetition rate

   https://doi.org/10.1364/OL.439232  

   Higginson, A. ; Wang, Y. ; Chi, H. ; Goffin, A. ; Larkin, I. ; Milchberg, H. M. ; Rocca, J. J. ( October 2021 , Optics Letters)

   We investigated the filamentation in air of 7 ps laser pulses of up to 200 mJ energy from a 1.03 μm-wavelength Yb:YAG laser at repetition rates up to$f=1\mathrm{k}\mathrm{H}\mathrm{z}$. Interferograms of the wake generated show that while pulses in a train of repetition rate$f=0.1\mathrm{k}\mathrm{H}\mathrm{z}$encounter a nearly unperturbed environment, at$f=1\mathrm{k}\mathrm{H}\mathrm{z}$, a channel with an axial air density hole of$\sim <\#comment/>20\mathrm{\%<\#comment/>}$is generated and maintained at all times by the cumulative effect of preceding laser pulses. Measurements at$f=1\mathrm{k}\mathrm{H}\mathrm{z}$show that the energy deposited decreases proportional to the air channel density depletion, becoming more pronounced as the repetition rate and pulse energy increase. Numerical simulations indicate that contrary to filaments generated by shorter duration pulses, the electron avalanche is the dominant energy loss mechanism during filamentation with 7 ps pulses. The results are of interest for the atmospheric propagation of joule-level picosecond pulses from Yb:YAG lasers, of which average powers now surpass 1 kW, and for channeling other directed energy beams.

    

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