Polarization-dependent neutral nitrogen fluorescence induced by long-distance laser filamentation
Yuezheng Wang, Lu Sun, Zhiwenqi An, Zeliang Zhang... and Weiwei Liu|Show fewer author(s)
Femtosecond laser filamentation has attracted significant attention due to its applications in remote sensing of atmospheric pollutants and artificial weather intervention. Nitrogen is the most abundant gas in the atmosphere, and its stimulated ultraviolet emission is remarkably clean, distinctly different from the fluorescence obtained through electron impact or laser breakdown. While numerous experiments and mechanism analyses have been conducted on its characteristic fluorescence excited by laser filamentation, they predominantly focused on short-distance filamentation (less than 1 m). Contrary to previous reports, we find that at long distances (30 m), the fluorescence intensity of neutral nitrogen molecules excited by linearly polarized laser pulses is approximately 7 times that excited by circularly polarized pulses with the same energy. This enhancement is caused by the enhanced tunneling ionization rate, 3.7 times that under circular polarization, and the elongated filament length, 1.85 times that under circular polarization, when using linear polarization. Additionally, after comparing existing theories for N2(C3Πu)) excitation, the dissociation-recombination model is found to be more appropriate for explaining the formation of N2(C3Πu)) excited states during long-distance filamentation.Femtosecond laser filamentation has attracted significant attention due to its applications in remote sensing of atmospheric pollutants and artificial weather intervention. Nitrogen is the most abundant gas in the atmosphere, and its stimulated ultraviolet emission is remarkably clean, distinctly different from the fluorescence obtained through electron impact or laser breakdown. While numerous experiments and mechanism analyses have been conducted on its characteristic fluorescence excited by laser filamentation, they predominantly focused on short-distance filamentation (less than 1 m). Contrary to previous reports, we find that at long distances (30 m), the fluorescence intensity of neutral nitrogen molecules excited by linearly polarized laser pulses is approximately 7 times that excited by circularly polarized pulses with the same energy. This enhancement is caused by the enhanced tunneling ionization rate, 3.7 times that under circular polarization, and the elongated filament length, 1.85 times that under circular polarization, when using linear polarization. Additionally, after comparing existing theories for
- May. 30, 2025
- Photonics Research
- Vol. 13, Issue 6, 1691 (2025)
- DOI：10.1364/PRJ.550756
Collisions of heteronuclear dichromatic soliton compounds in a passively mode-locked fiber laser
Yuansheng Ma, Ziyang Zhang, Yu Ning, Jiangyong He... and Zhi Wang|Show fewer author(s)
The complexity of multi-dimensional optical wave dynamics arises from the introduction of multiple degrees of freedom and their intricate interactions. In comparison to multimode spatiotemporal mode-locked solitons, expanding the wavelength dimension is also crucial for studying the dynamics of multi-dimensional solitons, with simpler characterization techniques. By inserting a section of zero-dispersion highly nonlinear fiber (HNLF) into a passively mode-locked fiber laser, two heteronuclear dichromatic soliton compounds with different group velocities (GVs) are formed within the resonant cavity of the laser. The cross-phase modulation effect leads to the formation of a robust fast-GV compound (FGC), consisting of a partially coherent dissipative soliton bunch (PCDSB) and dispersion waves (DWs), while a conventional soliton (CS) and a narrow spectral pulse (NSP) form a slow-GV compound (SGC). Multiple SGCs can further interact to form an SGC loosely bound complex. These two types of compounds with different GVs continuously collide and exchange energy through the four-wave mixing (FWM) effect in the HNLF, promoting the annihilation, survival, and regeneration of the SGC complex. This exploration of interactions between asynchronous compounds broadens the study of soliton dynamics in multi-dimensions and offers insights for potential applications in areas such as high-throughput optical communication and optical computing.The complexity of multi-dimensional optical wave dynamics arises from the introduction of multiple degrees of freedom and their intricate interactions. In comparison to multimode spatiotemporal mode-locked solitons, expanding the wavelength dimension is also crucial for studying the dynamics of multi-dimensional solitons, with simpler characterization techniques. By inserting a section of zero-dispersion highly nonlinear fiber (HNLF) into a passively mode-locked fiber laser, two heteronuclear dichromatic soliton compounds with different group velocities (GVs) are formed within the resonant cavity of the laser. The cross-phase modulation effect leads to the formation of a robust fast-GV compound (FGC), consisting of a partially coherent dissipative soliton bunch (PCDSB) and dispersion waves (DWs), while a conventional soliton (CS) and a narrow spectral pulse (NSP) form a slow-GV compound (SGC). Multiple SGCs can further interact to form an SGC loosely bound complex. These two types of compounds with different GVs continuously collide and exchange energy through the four-wave mixing (FWM) effect in the HNLF, promoting the annihilation, survival, and regeneration of the SGC complex. This exploration of interactions between asynchronous compounds broadens the study of soliton dynamics in multi-dimensions and offers insights for potential applications in areas such as high-throughput optical communication and optical computing.
- May. 30, 2025
- Photonics Research
- Vol. 13, Issue 6, 1680 (2025)
- DOI：10.1364/PRJ.554554
Efficient on-chip waveguide amplifiers in GeSbS-loaded etchless erbium-doped lithium niobate thin film
Chunxu Wang, Jingcui Song, Zhaohuan Ao, Yingyu Chen... and Zhaohui Li|Show fewer author(s)
In this paper, an efficient Ge25Sb10S65 (GeSbS)-loaded erbium-doped lithium niobate waveguide amplifier is demonstrated. By dimensional optimization of the waveguide, an internal net gain of approximately 28 dB and a maximum on-chip output power of 8.2 dBm are demonstrated upon 1480 nm bidirectional pumping. Due to the improved optical mode field distribution within the active erbium-doped lithium niobate film and the mode overlap ratio between the pump and signal sources, a 15% high conversion efficiency can be achieved at a modest pump power of 45 mW. Furthermore, the noise figure of the amplifier can be maintained below 6 dB for low-input-signal power levels. Compared to state-of-the-art erbium-doped waveguide amplifiers (EDWAs), this heterogeneously integrated device shows superior gain performance at the desired optical C-band while avoiding the complex plasma etching process of lithium niobate, providing an inspirative solution for power compensation in the optical telecommunications.In this paper, an efficient
- May. 30, 2025
- Photonics Research
- Vol. 13, Issue 6, 1674 (2025)
- DOI：10.1364/PRJ.555158
High-precision spatiotemporal profiler of femtosecond laser pulses
Zegui Wang, Qijun You, Yun Gao, Peixiang Lu, and Wei Cao
The precise spatiotemporal characterization of broadband ultrafast laser beams is essential for accurate laser control and holds significant potential in photochemistry and high-intensity laser physics. Existing methods for spatiotemporal characterization, such as frequency-resolved optical gating (FROG) and compressed ultrafast photography (CUP), are often spatially averaged or suffer from limited spatial resolution. Recent advances in imaging techniques based on multiplexed ptychography have enabled high-spatial-resolution diagnostics of ultrafast laser beams. However, the discrete spectral assumption inherent in multiplexed ptychographic algorithms does not align with the continuous spectral structure of ultrafast laser pulses, leading to significant crosstalk between different wavelength channels (WCs). This paper presents a method to reduce the bandwidth of each wavelength channel through spectral modulation, followed by the discretization of the continuous spectrum using interference techniques, which significantly improves the convergence and accuracy of the reconstruction. Using this method, the experiment accurately measured chromatic dispersion, spatial chirp, and other spatiotemporal coupling effects in femtosecond laser beams, achieving a spatial resolution of 9.4 μm, close to the pixel size resolution limit of the angular spectrum method.The precise spatiotemporal characterization of broadband ultrafast laser beams is essential for accurate laser control and holds significant potential in photochemistry and high-intensity laser physics. Existing methods for spatiotemporal characterization, such as frequency-resolved optical gating (FROG) and compressed ultrafast photography (CUP), are often spatially averaged or suffer from limited spatial resolution. Recent advances in imaging techniques based on multiplexed ptychography have enabled high-spatial-resolution diagnostics of ultrafast laser beams. However, the discrete spectral assumption inherent in multiplexed ptychographic algorithms does not align with the continuous spectral structure of ultrafast laser pulses, leading to significant crosstalk between different wavelength channels (WCs). This paper presents a method to reduce the bandwidth of each wavelength channel through spectral modulation, followed by the discretization of the continuous spectrum using interference techniques, which significantly improves the convergence and accuracy of the reconstruction. Using this method, the experiment accurately measured chromatic dispersion, spatial chirp, and other spatiotemporal coupling effects in femtosecond laser beams, achieving a spatial resolution of 9.4 μm, close to the pixel size resolution limit of the angular spectrum method.
- May. 30, 2025
- Photonics Research
- Vol. 13, Issue 6, 1666 (2025)
- DOI：10.1364/PRJ.559295
170 Gbps PDM underwater visible light communication utilizing a compact 5-
Zhilan Lu, Zhenhao Li, Xianhao Lin, Jifan Cai... and Nan Chi|Show fewer author(s)
The next generation of mobile communication is committed to establishing an integrated three-dimensional network that encompasses air, land, and sea. The visible light spectrum is situated within the transmission window for underwater communication, making visible light laser communication a focus of intense research. In this paper, we design and integrate a compact 5-λ transmission module based on five laser diodes with different wavelengths, utilizing a self-developed narrow-ridge GaN blue laser. With this transmitter, we have developed a polarization division multiplexing (PDM) 5-λ underwater visible light laser communication (UVLLC) system based on this transmission module. To enhance the transmission quality of the system, we designed a dual-branch ResDualNet network as a reciprocal differential receiver that incorporates common-mode noise cancellation and equalization functions for post-processing the received signals. With the combined contribution of the devices and algorithms, we achieved a total transmission rate of 170.1 Gbps, which represents a 16.1 Gbps increase compared to systems that do not utilize ResDualNet. To the best of our knowledge, this is the highest communication rate currently achievable in a UVLLC system using a single laser transmission module.The next generation of mobile communication is committed to establishing an integrated three-dimensional network that encompasses air, land, and sea. The visible light spectrum is situated within the transmission window for underwater communication, making visible light laser communication a focus of intense research. In this paper, we design and integrate a compact
- May. 30, 2025
- Photonics Research
- Vol. 13, Issue 6, 1654 (2025)
- DOI：10.1364/PRJ.551924
High-speed and versatile ONN through parametric-based nonlinear computation
Xin Dong, Yuanjia Wang, Xiaoxiao Wen, Yi Zhou, and Kenneth K. Y. Wong
Neural networks (NNs), especially electronic-based NNs, have been rapidly developed in the past few decades. However, the electronic-based NNs rely more on highly advanced and heavy power-consuming hardware, facing its bottleneck due to the slowdown of Moore’s law. Optical neural networks (ONNs), in which NNs are realized via optical components with information carried by photons at the speed of light, are drawing more attention nowadays. Despite the advantages of higher processing speed and lower system power consumption, one major challenge is to realize reliable and reusable algorithms in physical approaches, particularly nonlinear functions, for higher accuracy. In this paper, a versatile parametric-process-based ONN is demonstrated with its adaptable nonlinear computation realized using the highly nonlinear fiber (HNLF). With the specially designed mode-locked laser (MLL) and dispersive Fourier transform (DFT) algorithm, the overall computation frame rate can reach up to 40 MHz. Compared to ONNs using only linear computations, this system is able to improve the classification accuracies from 81.8% to 88.8% for the MNIST-digit dataset, and from 80.3% to 97.6% for the Vowel spoken audio dataset, without any hardware modifications.Neural networks (NNs), especially electronic-based NNs, have been rapidly developed in the past few decades. However, the electronic-based NNs rely more on highly advanced and heavy power-consuming hardware, facing its bottleneck due to the slowdown of Moore’s law. Optical neural networks (ONNs), in which NNs are realized via optical components with information carried by photons at the speed of light, are drawing more attention nowadays. Despite the advantages of higher processing speed and lower system power consumption, one major challenge is to realize reliable and reusable algorithms in physical approaches, particularly nonlinear functions, for higher accuracy. In this paper, a versatile parametric-process-based ONN is demonstrated with its adaptable nonlinear computation realized using the highly nonlinear fiber (HNLF). With the specially designed mode-locked laser (MLL) and dispersive Fourier transform (DFT) algorithm, the overall computation frame rate can reach up to 40 MHz. Compared to ONNs using only linear computations, this system is able to improve the classification accuracies from 81.8% to 88.8% for the MNIST-digit dataset, and from 80.3% to 97.6% for the Vowel spoken audio dataset, without any hardware modifications.
- May. 30, 2025
- Photonics Research
- Vol. 13, Issue 6, 1647 (2025)
- DOI：10.1364/PRJ.553388
Hand-held laser for miniature photoacoustic microscopy: triggerable, millimeter scale, cost-effective, and functional
Hanjie Wang, Xingyu Zhu, Xiaobin Weng, Lanxin Deng... and Hongsen He|Show fewer author(s)
Miniaturization of photoacoustic microscopy (PAM) to portable and wearable levels requires special design of scanning, detection, acquisition, and excitation units. Now the first three can be minimized to gram and millimeter levels, but the excitation sources usually remain bulky and also face different challenges, including low pulse energy, wide pulse width, limited wavelength, or high cost. Here, we propose a high-performance laser source specially designed for a miniature PAM system, that is, the pulse-pumped passively Q-switched solid-state laser (PQS-SSL). Its kilohertz repetition rate, nanosecond pulse width, microjoule pulse energy, and UV to NIR spectra are exactly within the requirements of functional PAM imaging, together with the merits of millimeter scale and low cost, originating from the all-crystal-based configuration. The pulsed pump technique empowers the laser with frequency lock and trigger-in ability for system synchronization, overcoming the conventional free-running drawbacks, and the senior multi-pulse pump is also feasible to further compress the laser size and cost. We showcase its PAM performance on the USAF1951, carbon fiber, zebrafish, and lipid (wavelength extension to ∼1.2 μm). The novel, to our knowledge, pulse-pumped PQS-SSL is not only promising for general PAM, but also paves the way to develop miniature PAM systems, such as hand-held or brain-wearable modalities.Miniaturization of photoacoustic microscopy (PAM) to portable and wearable levels requires special design of scanning, detection, acquisition, and excitation units. Now the first three can be minimized to gram and millimeter levels, but the excitation sources usually remain bulky and also face different challenges, including low pulse energy, wide pulse width, limited wavelength, or high cost. Here, we propose a high-performance laser source specially designed for a miniature PAM system, that is, the pulse-pumped passively Q-switched solid-state laser (PQS-SSL). Its kilohertz repetition rate, nanosecond pulse width, microjoule pulse energy, and UV to NIR spectra are exactly within the requirements of functional PAM imaging, together with the merits of millimeter scale and low cost, originating from the all-crystal-based configuration. The pulsed pump technique empowers the laser with frequency lock and trigger-in ability for system synchronization, overcoming the conventional free-running drawbacks, and the senior multi-pulse pump is also feasible to further compress the laser size and cost. We showcase its PAM performance on the USAF1951, carbon fiber, zebrafish, and lipid (wavelength extension to
- May. 30, 2025
- Photonics Research
- Vol. 13, Issue 6, 1637 (2025)
- DOI：10.1364/PRJ.553607
Generation of arbitrary vector vortex beams on the hybrid-order Poincaré sphere from cholesteric liquid crystals
Tiegang Lin, Ben Niu, Furong Liu, Xianglin Ye... and Yufang Liu|Show fewer author(s)
In this Letter, by exploiting the spin-to-orbital angular momentum conversion capability and the polarization selectivity characteristic of cholesteric liquid crystal planar optical elements, an approach of combing two cholesteric liquid crystal layers with opposite handedness and independent surface patterns is proposed and investigated for generating arbitrary vector vortex beams on the hybrid-order Poincaré sphere. Furthermore, the intensity profiles and polarization distributions of typical vector vortex beams are experimentally demonstrated and analyzed, which shows good agreement with the theoretical prediction. The approach also suggests its advantages of operating light in the polychromatic spectral region of Bragg reflection. Our method presents a simple and direct way of phase and polarization manipulation, which also provides promising opportunities for developing advanced applications in structured light, high-resolution imaging, and information processing.In this Letter, by exploiting the spin-to-orbital angular momentum conversion capability and the polarization selectivity characteristic of cholesteric liquid crystal planar optical elements, an approach of combing two cholesteric liquid crystal layers with opposite handedness and independent surface patterns is proposed and investigated for generating arbitrary vector vortex beams on the hybrid-order Poincaré sphere. Furthermore, the intensity profiles and polarization distributions of typical vector vortex beams are experimentally demonstrated and analyzed, which shows good agreement with the theoretical prediction. The approach also suggests its advantages of operating light in the polychromatic spectral region of Bragg reflection. Our method presents a simple and direct way of phase and polarization manipulation, which also provides promising opportunities for developing advanced applications in structured light, high-resolution imaging, and information processing.
- May. 30, 2025
- Chinese Optics Letters
- Vol. 23, Issue 6, 063603 (2025)
- DOI：10.3788/COL202523.063603
39 Influence of defect anisotropy on luminescence properties in Pr:YAP crystals
Lu Zhang, Bowen Jiang, Mingyan Pan, Weiguo Ji... and Hongji Qi|Show fewer author(s)
This study aims to investigate the anisotropic properties of Pr:YAP on (100), (010), and (001) crystal planes. Raman spectroscopy shows anisotropy in vibrational modes, but absorption spectra display no significant anisotropy. X-ray excited luminescence (XEL) and photoluminescence (PL) spectra reveal anisotropy in Pr3+ and F+ luminescence intensities. The PL decay time (∼7 ns) indicates similar luminescence mechanisms. The anisotropic defect distribution observed in thermoluminescence analysis can be explained using areal ion density and the offset parameter of Al atoms. Ultimately, it is inferred that shallow-level defects compete with Pr3+ ions, leading to variations in anisotropic luminescence intensity.This study aims to investigate the anisotropic properties of Pr:YAP on (100), (010), and (001) crystal planes. Raman spectroscopy shows anisotropy in vibrational modes, but absorption spectra display no significant anisotropy. X-ray excited luminescence (XEL) and photoluminescence (PL) spectra reveal anisotropy in Pr3+ and F+ luminescence intensities. The PL decay time (∼7 ns) indicates similar luminescence mechanisms. The anisotropic defect distribution observed in thermoluminescence analysis can be explained using areal ion density and the offset parameter of Al atoms. Ultimately, it is inferred that shallow-level defects compete with Pr3+ ions, leading to variations in anisotropic luminescence intensity.
- May. 30, 2025
- Chinese Optics Letters
- Vol. 23, Issue 6, 061601 (2025)
- DOI：10.3788/COL202523.061601
Channel modeling for underwater scattered light communication based on Gaussian and Bessel beams
Yi Gao, Zhitong Huang, Jie Xu, Hongcheng Qiu... and Yuefeng Ji|Show fewer author(s)
Underwater scattered light communication (USLC) utilizes the strong scattering properties of seawater to achieve a non-line-of-sight (NLOS) communication channel, which is a promising solution to the stringent alignment requirements and random transmission path blocking in underwater wireless optical communication. In this Letter, we model the channel for USLC based on Gaussian and Bessel beams, in which we comprehensively explore the effects of different water types and transceiver configurations on USLC and simulate the photon propagation in seawater by Monte Carlo (MC) method. Specifically, we analyze the fluctuation in received signal strength as a function of the communication distance across three distinct water types, in which the model considers the influence of different optical wavelengths and spatial modes at the transmitter as well as various optical lens configurations at the receiver. Modeling and experiments validate blue Gaussian beams for short-range, low-turbidity cases; green Bessel beams for long-range, high-turbidity conditions; and the receiver antenna’s utility which is restricted to short-range applications. The conclusion obtained can be used for the selection of transceiver devices in USLC systems.Underwater scattered light communication (USLC) utilizes the strong scattering properties of seawater to achieve a non-line-of-sight (NLOS) communication channel, which is a promising solution to the stringent alignment requirements and random transmission path blocking in underwater wireless optical communication. In this Letter, we model the channel for USLC based on Gaussian and Bessel beams, in which we comprehensively explore the effects of different water types and transceiver configurations on USLC and simulate the photon propagation in seawater by Monte Carlo (MC) method. Specifically, we analyze the fluctuation in received signal strength as a function of the communication distance across three distinct water types, in which the model considers the influence of different optical wavelengths and spatial modes at the transmitter as well as various optical lens configurations at the receiver. Modeling and experiments validate blue Gaussian beams for short-range, low-turbidity cases; green Bessel beams for long-range, high-turbidity conditions; and the receiver antenna’s utility which is restricted to short-range applications. The conclusion obtained can be used for the selection of transceiver devices in USLC systems.
- May. 30, 2025
- Chinese Optics Letters
- Vol. 23, Issue 6, 060606 (2025)
- DOI：10.3788/COL202523.060606
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Editor (s): Yuanjie Yang, Yangjian Cai, Qiwen Zhan
Innovative Optical Sensor Systems (2025)
Submission Open:15 January 2025; Submission Deadline: 30 April 2025
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