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1. Spectroscopic study of the 4 f 7 6 s 2 8 S 7/2∘−4 f 7 ( 8 S ∘ )6 s 6 p ( 1 P ∘ ) 8 P 9/2 transition in neutral europium-151 and europium-153: absolute frequency and hyperfine structure

   https://doi.org/10.1364/JOSAB.467968  

   Herd, M. T. ; Maruko, C. ; Herzog, M. M. ; Brand, A. ; Cannon, G. ; Duah, B. ; Hollin, N. ; Karani, T. ; Wallace, A. ; Whitmore, M. ; et al ( September 2022 , Journal of the Optical Society of America B)

   We report on spectroscopic measurements on the$4f^{7}6s^2{}^8{S}_{7/2}^{\circ <\#comment/>}\to <\#comment/>4f^7{(}^8{S}^{\circ <\#comment/>})6s6p{(}^1{P}^{\circ <\#comment/>}){}^8{P}_{9/2}$transition in neutral europium-151 and europium-153 at 459.4 nm. The center of gravity frequencies for the 151 and 153 isotopes, reported for the first time in this paper, to our knowledge, were found to be 652,389,757.16(34) MHz and 652,386,593.2(5) MHz, respectively. The hyperfine coefficients for the$6s6p{(}^1{P}^{\circ <\#comment/>}){}^8{P}_{9/2}$state were found to be$\mathrm{A}(151)=-<\#comment/>228.84(2)\mathrm{M}\mathrm{H}\mathrm{z}$,$\mathrm{B}(151)=226.9(5)\mathrm{M}\mathrm{H}\mathrm{z}$and$\mathrm{A}(153)=-<\#comment/>101.87(6)\mathrm{M}\mathrm{H}\mathrm{z}$,$\mathrm{B}(153)=575.4(1.5)\mathrm{M}\mathrm{H}\mathrm{z}$, which all agree with previously published results except for A(153), which shows a small discrepancy. The isotope shift is found to be 3163.8(6) MHz, which also has a discrepancy with previously published results.

    

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2. Mid-IR spectroscopy of Er 3+ doped low-phonon CsCdCl 3 and CsPbCl 3 crystals

   https://doi.org/10.1364/JOSAB.470152  

   Brown, Ei Ei ; Fleischman, Zackery D. ; McKay, Jason ; Hommerich, Uwe ; Kabir, Al Amin ; Riggins, Jazmine ; Trivedi, Sudhir ; Dubinskii, Mark ( September 2022 , Journal of the Optical Society of America B)

   The mid-IR spectroscopic properties of${\mathrm{E}\mathrm{r}}^{3+}$doped low-phonon${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$and${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$crystals grown by the Bridgman technique have been investigated. Using optical excitations at$\sim <\#comment/>800\mathrm{n}\mathrm{m}$and$\sim <\#comment/>660\mathrm{n}\mathrm{m}$, both crystals exhibited IR emissions at$\sim <\#comment/>1.55$,$\sim <\#comment/>2.75$,$\sim <\#comment/>3.5$, and$\sim <\#comment/>4.5\text{µ<\#comment/>}\mathrm{m}$at room temperature. The mid-IR emission at 4.5 µm, originating from the${}^4{\mathrm{I}}_{9/2}\to <\#comment/>{}^4{\mathrm{I}}_{11/2}$transition, showed a long emission lifetime of$\sim <\#comment/>11.6\mathrm{m}\mathrm{s}$for${\mathrm{E}\mathrm{r}}^{3+}$doped${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$, whereas${\mathrm{E}\mathrm{r}}^{3+}$doped${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$exhibited a shorter lifetime of$\sim <\#comment/>1.8\mathrm{m}\mathrm{s}$. The measured emission lifetimes of the${}^4{\mathrm{I}}_{9/2}$state were nearly independent of the temperature, indicating a negligibly small nonradiative decay rate through multiphonon relaxation, as predicted by the energy-gap law for low-maximum-phonon energy hosts. The room temperature stimulated emission cross sections for the${}^4{\mathrm{I}}_{9/2}\to <\#comment/>{}^4{\mathrm{I}}_{11/2}$transition in${\mathrm{E}\mathrm{r}}^{3+}$doped${\mathrm{C}\mathrm{s}\mathrm{C}\mathrm{d}\mathrm{C}\mathrm{l}}_3$and${\mathrm{C}\mathrm{s}\mathrm{P}\mathrm{b}\mathrm{C}\mathrm{l}}_3$were determined to be$\sim <\#comment/>0.14\times <\#comment/>{10}^{-<\#comment/>20}{\mathrm{c}\mathrm{m}}^2$and$\sim <\#comment/>0.41\times <\#comment/>{10}^{-<\#comment/>20}{\mathrm{c}\mathrm{m}}^2$, respectively. The results of Judd–Ofelt analysis are presented and discussed.

    

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3. 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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4. Ultralow-threshold thin-film lithium niobate optical parametric oscillator

   https://doi.org/10.1364/OPTICA.418984  

   Lu, Juanjuan ; Al Sayem, Ayed ; Gong, Zheng ; Surya, Joshua B. ; Zou, Chang-Ling ; Tang, Hong X. ( April 2021 , Optica)

   Materials with strong second-order (${\mathrm{\chi <\#comment/>}}^{(2)}$) optical nonlinearity, especially lithium niobate, play a critical role in building optical parametric oscillators (OPOs). However, chip-scale integration of low-loss${\mathrm{\chi <\#comment/>}}^{(2)}$materials remains challenging and limits the threshold power of on-chip${\mathrm{\chi <\#comment/>}}^{(2)}$OPO. Here we report an on-chip lithium niobate optical parametric oscillator at the telecom wavelengths using a quasi-phase-matched, high-quality microring resonator, whose threshold power ($\sim <\#comment/>30\text{µ<\#comment/>}\mathrm{W}$) is 400 times lower than that in previous${\mathrm{\chi <\#comment/>}}^{(2)}$integrated photonics platforms. An on-chip power conversion efficiency of 11% is obtained from pump to signal and idler fields at a pump power of 93 µW. The OPO wavelength tuning is achieved by varying the pump frequency and chip temperature. With the lowest power threshold among all on-chip OPOs demonstrated so far, as well as advantages including high conversion efficiency, flexibility in quasi-phase-matching, and device scalability, the thin-film lithium niobate OPO opens new opportunities for chip-based tunable classical and quantum light sources and provides a potential platform for realizing photonic neural networks.

    

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5. Simultaneous turbulence mitigation and channel demultiplexing for two 100  Gbit/s orbital-angular-momentum multiplexed beams by adaptive wavefront shaping and diffusing

   https://doi.org/10.1364/OL.383714  

   Zhang, Runzhou ; Song, Hao ; Zhao, Zhe ; Song, Haoqian ; Du, Jing ; Liu, Cong ; Pang, Kai ; Li, Long ; Zhou, Huibin ; Willner, Ari N. ; et al ( January 2020 , Optics Letters)

   We experimentally demonstrate simultaneous turbulence mitigation and channel demultiplexing in a 200 Gbit/s orbital-angular-momentum (OAM) multiplexed link by adaptive wavefront shaping and diffusing (WSD) the light beams. Different realizations of two emulated turbulence strengths (the Fried parameter$r_0=0.4,1.0\mathrm{m}\mathrm{m}$) are mitigated. The experimental results show the following. (1) Crosstalk between OAM$l=+1$and$l=-<\#comment/>1$modes can be reduced by$><\#comment/>10.0$and$><\#comment/>5.8\mathrm{d}\mathrm{B}$, respectively, under the weaker turbulence ($r_0=1.0\mathrm{m}\mathrm{m}$); crosstalk is further improved by$><\#comment/>17.7$and$><\#comment/>19.4\mathrm{d}\mathrm{B}$, respectively, under most realizations in the stronger turbulence ($r_0=0.4\mathrm{m}\mathrm{m}$). (2) The optical signal-to-noise ratio penalties for the bit error rate performance are measured to be$\sim <\#comment/>0.7$and$\sim <\#comment/>1.6\mathrm{d}\mathrm{B}$under weaker turbulence, while measured to be$\sim <\#comment/>3.2$and$\sim <\#comment/>1.8\mathrm{d}\mathrm{B}$under stronger turbulence for OAM$l=+1$and$l=-<\#comment/>1$mode, respectively.

    

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