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We report on a linear optical-feedback cavity ring-down spectrometer (LOF-CRDS) operating under ultra-high vacuum (UHV) conditions, designed for high-sensitivity detection of light hydrocarbons. The system employs an interband cascade laser (ICL) operating at 3 µm, in coincidence with the strong absorption features of the asymmetric C-H stretching vibrational mode. Taking advantage of the passive optical feedback from the high-finesse linear cavity, the ICL emission linewidth is drastically narrowed, thus facilitating light coupling into the cavity. An empty-cavity ring-down time of about 18 µs was measured, which translates into a cavity finesse of ∼37000 and a cavity-mode width of ∼9 kHz. As a result of an Allan-Werle analysis, performed at a fixed ICL frequency, we determined a minimum detectable absorption coefficient of ∼2.5×10^-10 cm^-1 for an integration time of approximately 100 s. The spectrometer was tested on the R(25) component of the acetylene ν₃ vibrational band at 3352.2874 cm^-1. In this regard, we report on spectroscopic measurements of the C₂H₂ partial pressure in the UHV chamber. For the selected line, the sensitivity leads to a limit of detection of 2.1×10^-8 mbar.

Astrocombs are promising calibration sources for high-resolution astronomical spectrographs, offering stable, uniformly spaced calibration lines whose positions are traceable to an atomic frequency reference. Most spectrograph pixels fall in the gaps between astrocomb lines, remaining uninterrogated by the calibration light; however, it is becoming clear that detector inhomogeneities and intra-order variations in the spectrograph performance make it important to characterize the entire detector area to obtain the best radial-velocity precision. Here, this requirement is fulfilled by the demonstration of an astrocomb architecture in which lines can be swept in any chosen increment in frequency. Based on Fabry-Pérot filtering of a primary comb with a 1 GHz spacing, the astrocomb employs feed-forward locking to overlap a single-frequency laser onto a chosen primary comb line, a process that precisely selects which subset of primary comb modes will be transmitted by the Fabry-Pérot cavity. Operating from 650-1030 nm, and exhibiting continuous performance for >12 hours, the system demonstrates the long-term stability required by a practical astrocomb, while providing the fine sampling of the spectrograph instrument function likely to be necessary for achieving radial-velocity precisions supporting Earth-like exoplanet detection.

Human activity recognition has been a prominent research focus for several decades. While computer vision-based passive sensing is desirable for many applications, traditional RGB-based methods face significant limitations, including high computational cost, sensitivity to ambient lighting conditions, and privacy concerns. Single-photon LiDAR (light detection and ranging) is emerging as a robust alternative, offering efficient and high-resolution 3D imaging over long ranges and in difficult conditions while preserving privacy. In this study, we combine an eye-safe single-photon LiDAR system with a deep learning pipeline to achieve fast, long-range human activity recognition. To address the issue of limited available data for training, we generate synthetic datasets by combining real motion capture data with virtual models in a 3D modeling environment. A state-of-the-art recurrent neural network is trained on short-duration depth-image sequences of six activities. We also contribute two long-range, video frame rate single-photon LiDAR datasets for human activity recognition, recorded at distances of 325 m and 1.4 km, which exhibit markedly different noise levels. When evaluated on these data, the network maintains more than 80% accuracy even in the most challenging scenario, while supporting continuous and fast inference.

In this study, we aim to further advance solutions to critical challenges in physical layer secure key generation and distribution (PLSKGD) for optical fiber communication, particularly the limitations of traditional information reconciliation (IR) protocols in mitigating initial key inconsistencies caused by noise and interference. We introduce two innovative approaches: a check node correlation coefficient-based early termination (CNCC-ET) scheme and an elastic reconfigurable (ER) scheme to enhance error correction performance and accelerate IR. The CNCC-ET scheme dynamically monitors the correlation coefficient of messages between check and variable nodes, enabling early termination of iterations when decoding is likely to fail, thus significantly reducing decoding complexity. In high bit error rate (BER) regions, this scheme achieves up to a 51.8% reduction in average iteration counts while maintaining low frame error rates (FER). Concurrently, our ER scheme adapts to varying BER conditions by dynamically adjusting the ratio of information to check bits, achieving a remarkable 90.26% FER reduction in high-BER scenarios. Hardware implementation on a Xilinx Virtex-7 FPGA validates the feasibility of these methods, demonstrating a 47.2% reduction in IR time overhead and significantly improved error correction capability. This study advances the development of more secure and efficient optical fiber communication systems, with potential applications extending to other physical layer security contexts.

Simultaneously reconstructing quantitative phase and birefringence information from a single exposure remains challenging, as existing multimodal imaging methods typically rely on multi-angle illumination or optical path switching, severely limiting temporal resolution. In this study, we propose a single-shot dual-modality microscopy system that combines circularly polarized illumination with color-multiplexed differential phase contrast (cDPC). By employing illumination encoded with color and polarization, together with a reconstruction algorithm, the system synchronously extracts quantitative phase and birefringence parameters from a single intensity image. Imaging experiments on phase targets, sodium urate crystals, potato starch granules, malaria-infected red blood cells, and live paramecia verify the system's high accuracy in phase quantification, birefringence reconstruction, and video-rate dual-modality imaging. The system achieves an imaging speed that is 4 times faster than previous methods. This approach effectively eliminates temporal misalignment and motion artifacts, providing a powerful tool for dynamic live-cell observation, pathological crystal identification, and label-free biomedical imaging.

Quantifying ocean primary productivity (OPP) in the South China Sea (SCS) is crucial for understanding regional carbon sequestration processes. In this study, an airborne dual-wavelength light detection and ranging (lidar) (ADWL: 532/486 nm) methodology is introduced to resolve vertical phytoplankton absorption and productivity dynamics. A full-chain inversion framework was developed to obtain OPP profiles from raw ADWL backscattering signals, and the lidar radiative transfer equation was integrated with spectral decoupling algorithms to isolate phytoplankton absorption, given interference from color dissolved organic matter (CDOM) and detritus. We establish a functional relationship between the total absorption coefficient at 486 nm and the chlorophyll a (Chla) concentration using in situ data and retrieve the vertical profiles of the Chla concentration along the ADWL flight track. The ADWL system effectively detected the subsurface maximum phytoplankton absorption and Chla concentration layer at depths ranging from 20 to 70 m and OPP profiles extending below the conventional euphotic zone depth. Validation at Stations C1 and C2 demonstrated high accuracy, with Chla concentration retrieval achieving R² = 0.89 (RMSE=0.13 mg m^-3) and OPP estimates yielding R² = 0.68 (RMSE=1.62 mg C m^-3 d^-1). Finally, we discuss the effects of the euphotic zone depth definition on the precision of OPP estimation, demonstrating that integration to the depth of zero productivity (PP∼0) is essential for reliable OPP quantification. Overall, the proposed methodology represents an important step toward future global-scale and high-precision ocean carbon monitoring using spaceborne ocean lidar system.

Frequency modulated continuous wave (FMCW) LiDAR offers strong interference immunity, eye safety, and simultaneous distance-velocity measurement. However, long-range and low-reflectivity detection is often limited by polarization fading, reduced return power, bandwidth constraints, and system noise. This work presents a silicon-photonic polarization-diverse coherent receiver that enables both polarization states of the return signal to participate in coherent detection, reducing polarization-induced fading. A pipeline real-time constant false-alarm rate (CFAR) is further incorporated to robustly identify beat-frequency peaks under long-range and high-frequency-attenuated conditions. Experiments show that for a 200 m target with 10% reflectivity, the detection probability increases from 43.5% to 97% after applying CFAR and polarization-diverse detection. The system achieves real-time 4D imaging at 100,000 points per second, demonstrating improved detection reliability for integrated FMCW LiDAR.

Particularly for curved mirrors, nonplanar illumination with its shape matching the curved mirrors under test in phase measuring deflectometry (PMD) has the advantages of enlarging the measurement range, decreasing the depth range of the reflected virtual screen. In this paper, a monocular deflectometry with nonplanar illumination is proposed, which contains a camera and a curved screen. Firstly, a method of modelling the curved screen is introduced to obtain a high-accuracy model of the screen, making the transformation of the screen free from the displayed feature points. Then, in a system of monocular PMD, vision ray calibration (VRC) is used to calibrate the ray distributions in the PMD with two separate steps and an integrated step. Meanwhile, two models of the screen are introduced to the PMD. Finally, with the calibrated ray distributions and the law of reflection, the slope distributions of the specular surfaces under test (SUT) are obtained to integrally reconstruct the SUT. In simulated measurements, VRC-based mono PMDs with the two screen models are verified, respectively. In actual measurements, two specular balls with different radii and a flat mirror as SUTs are measured to verify the proposed methods. And the measurement results show high accuracy, repeatability, and stability.

We present a topology-optimization framework for engineering photonic band gaps with TM- and TE-polarized sources based on computation of the photonic density of states with a uniform source substituting for the standard Dirac delta function sources. Optimization is performed in a manner that allows for targeting of a given midgap frequency and minimum band gap. We apply this framework in formalisms analogous to Γ-point integration and to integration over a full Brillouin zone. We also demonstrate how our approach can be generalized to the treatment of the frequency-dependent optical response of materials and could potentially be employed to invert photonic and phononic bandstructures. Finally, we show that we can recover known two-dimensional photonic crystals for the TM and TE polarizations, and we demonstrate waveguiding applications of our designs. Additionally, our full Brillouin zone formalism for the uniform-source approach inherently encourages binarized designs even in gradient descent.

Synchronized three-color mode-locked lasers offer a powerful new dimension to vibrational microscopy, particularly in the realm of broadband coherent Raman techniques like Stimulated Raman Scattering (SRS) and coherent anti-Stokes Raman scattering. We report on the realization of a synchronized three-color all-polarization-maintaining (PM) fiber laser system operating in a mode-locking regime with emission wavelengths of 1.56, 1.03, and 0.92 μm. The different wavelengths are passively synchronized, using the cross-phase modulation effect in common passive PM-fibers between the three laser resonators, to a temporal pulse jitter lower than 400 fs over an integration time of 1 s. A comprehensive characterization of the synchronization mechanism and laser relative intensity noise is also reported. This three-color laser system, based on Er-, Yb-, and Nd-doped fiber lasers, represents the core technology for a new concept of multiplex SRS microscopy, which enables simultaneous fast acquisition of broad spectra in both the C-H stretching (2800-3100 cm^-1) and the fingerprint (1000-1700 cm^-1) regions.