Search Results (111)

Continuous-Variable Quantum Key Distribution (CV-QKD) offers practical advantages for secure communication, but laser linewidth-induced phase noise remains a critical performance limitation. This work presents a comprehensive simulation-based analysis quantifying the impact of laser linewidth on secret key rate (SKR) in Gaussian-modulated coherent-state CV-QKD systems. We develop a detailed noise model incorporating detector electronics, Raman scattering, phase recovery, ADC quantization, and laser relative intensity noise. Through systematic parameter sweeps spanning linewidths from 10 Hz to 250 kHz, modulation variances from 1 to 20 SNU, and fiber distances up to 100 km, we identify three distinct operational regimes and optimization strategies for both transmitted local oscillator (TLO) and local–local oscillator (LLO) configurations under homodyne and heterodyne detection. Results show that metropolitan-scale links (50 km) require linewidths below 5 kHz to maintain secure operation, with performance decreasing beyond 25 kHz. We demonstrate that modulation variance must be jointly optimized with laser quality, with optimal values decreasing from 3–4 SNU at narrow linewidths to 2–2.5 SNU at moderate linewidths. The analysis reveals asymmetric sensitivity in LLO systems where local oscillator linewidth degrades performance more strongly than signal laser linewidth. These quantitative findings provide practical design guidelines for achieving secure CV-QKD operation over metropolitan distances with realistic hardware constraints, supporting deployment of defense-grade quantum communication networks. Full article

Dual-frequency lasers with narrow linewidth and high coherence serve as essential light sources for systems such as heterodyne detection, LiDAR, and precision interferometry. However, existing technologies cannot directly separate the two frequency components at MHz-scale differences, which remains a persistent bottleneck in this field. In this paper, we present a dual-frequency fiber laser based on a bidirectional dual-path ring cavity. The proposed laser supports flexible switching between single-frequency and dual-frequency operation while allowing straightforward physical separation of the two outputs via intrinsic beam routing. In single-frequency mode, the two beams exhibit Lorentzian linewidths of 1.1 kHz and 1.16 kHz, respectively. In dual-frequency operation, the laser produces a beat signal at 470 MHz with a 3-dB linewidth of 340.2 Hz and a signal-to-noise ratio (SNR) exceeding 70 dB. This dual-frequency fiber laser provides a novel and practical source for heterodyne detection and LiDAR-based measurement systems. Full article

A robust and cost-effective fiber-optic ultrasound sensor based on a slope-symmetric Fabry–Perot interferometer (FPI) is presented, employing dual-channel quadrature-biased heterodyne interrogation with an acousto-optic modulator (AOM). By introducing a 200 MHz frequency shift that yields an effective π/2 phase offset between the direct (unshifted) and frequency-shifted optical paths, the system ensures complementary sensitivity: when one channel operates at zero slope on the FPI transfer function (minimum sensitivity), the other resides at maximum slope, providing inherent immunity to laser wavelength drift and environmental perturbations. Experimental validation demonstrates reliable ultrasound detection across varying operating points. At quadrature extremes, one channel achieves peak amplitudes of ±2 V while the other is quiescent, whereas intermediate points enable simultaneous detection with amplitudes of ±1.5 V (AOM channel) and ±0.05–0.1 V (direct channel), accompanied by corresponding DC levels ranging from ~0.4 V to 1.6 V. The AOM channel utilizes simple envelope detection after 9.5–11.5 MHz bandpass filtering, maintaining low cost, though coherent mixing is suggested for enhanced weak-signal performance. The angle-symmetric FPI design, combined with gold-disk reflector adaptations and potential femtosecond laser micromachining, further reduces fabrication costs without sacrificing finesse or sensitivity. This quadrature-biased approach offers superior stability compared to single-channel systems, making it highly suitable for practical applications in photoacoustic imaging, nondestructive testing, and structural health monitoring. Full article

The spatial response of rectangular pulse heterodyne phase-sensitive optical time-domain reflectometry ($\varphi$-OTDR) to an acoustic event is characterized by a windowing function rather than a point-like sensitivity. This effect degrades the system’s spatial resolution and introduces systematic errors in array signal processing. This work presents modeling analysis and a mitigation strategy for this fundamental limitation. The spatial windowing effect is modeled as a point spread function (PSF) derived from physical mechanisms and system parameters, including the pulse width, gauge length, and intra-pulse intensity dynamics. The PSF model is validated against measurements under near-ideal conditions using a fiber-coupled tuning fork. A Wiener filter-based deconvolution method is utilized to invert the windowed spatial response towards a point-like response. The effectiveness of this inversion is demonstrated through enhanced spatial resolution and accurate reconstruction of two-dimensional wavefront geometry. Furthermore, the impact of this effect on array signal processing is quantitatively evaluated. The results demonstrate that the proposed method effectively suppresses systematic errors in wavefield analysis, and specifically enhances the accuracy and confidence of steered response power—phase transform (SRP-PHAT) spatial spectrum estimation. This study provides a systematic framework for understanding, quantifying, and inverting the spatial response in $\varphi$-OTDR, enabling accurate and interpretable acoustic field sensing. Full article

This paper proposes a distributed vibration sensing system based on dual-chirped pulses and weak fiber Bragg gratings. Compared with conventional dual-pulse heterodyne detection techniques, the proposed approach utilizes the time-frequency characteristics of chirp signals, effectively relaxing the stringent requirements on the signal pulse width. In addition, when chirp signals with different chirp rates are adopted, frequency-division multiplexing technology can be realized to enhance the system’s response bandwidth. Last but not least, the sensing probe signal is generated in the optical domain using microwave photonic technology, which theoretically helps alleviate the dependence on arbitrary waveform generators. A simulation study on the system was conducted. Using a chirp signal of 200 MHz/μs and with a grating spacing of 50 m, the desired vibration signal is phased modulated onto a beat signal with a frequency of 100 MHz, and the allowable pulse width can be up to 500 ns. To verify frequency-division multiplexing capability further, an additional pair of chirped pulses with a chirp rate of 400 MHz/μs was introduced into the system, which was generated by frequency multiplication. The results demonstrate that the system response bandwidth is increased to twice the original bandwidth. The proposed scheme provides a new solution for performance enhancement in a distributed fiber optic vibration sensing system. Full article

We demonstrate a distributed acoustic sensing (DAS) system based on phase-sensitive optical time-domain reflectometry (Φ-OTDR) that employs I/Q demodulation and heterodyne detection. The proposed DAS system utilizes a 90° optical hybrid to obtain in-phase (I) and quadrature (Q) signals. By applying undersampling theory, the system significantly reduces the required analog-to-digital sampling rate. In an experimental demonstration, a 200 MHz heterodyne beat signal is successfully recovered at a sampling rate of 110 MSa/s without any loss of phase information. The system achieves a spatial resolution of 10 m, a signal-to-noise ratio of approximately 63.54 dB at a demodulation frequency of 200 Hz, and a background noise level of −52.27 dB·rad2/Hz. In addition, an amplitude-based analysis of the I/Q data is used to locate vibration events and estimate their effective length, so that an adaptive differential gauge length can be chosen to suppress common-phase fluctuations and restrict phase demodulation to a short fiber segment. This approach effectively reduces data throughput and system complexity while maintaining high sensitivity and resolution, illustrating the potential for more efficient real-time DAS implementations. Full article

Raman spectroscopy is essential for the in situ identification of lunar minerals, yet weak signals and stringent payload constraints demand instruments with high throughput and mechanical robustness. Here a microscope-coupled spatial heterodyne Raman spectrometer (SHRS) is developed for stable, adjustment-free operation, with performance set by an explicit sampling analysis that links magnification, pixel pitch, and detector format to achievable spectral resolution and range. The interferometer geometry is fixed in service and is established using removable alignment blocks referenced to the Littrow condition during integration and then removed from the optical path, which mitigates backlash, creep, and dust sensitivity while preserving reinstallability for verification. Guided by the sampling analysis, the laboratory prototype meets a 100–3600 cm⁻¹ spectral range with an effective resolution better than 10 cm⁻¹, further corroborated by the narrow FWHM of the diamond Raman line. Representative minerals are recovered at the expected wavenumber, and a broad-scan of gypsum retrieves the sulfate fundamentals and the O–H stretching envelope near 3400 cm⁻¹, indicating maintained coverage and sensitivity into the high-wavenumber region relevant to bound water. A comparative study of sampling magnification confirms the sampling-limited predictions and shows that higher magnification improves effective SNR and peak visibility with only minor changes in width, providing practical guidance for compact SHRS design under low-signal conditions. The results support a compact, slit-free SHRS as a credible basis for future lunar and other planetary deployments. Full article

In this study, we statistically analyze the performance of a threshold-based multiple optical signal selection scheme (TMOS) for wavelength division multiplexing (WDM) and adaptive coded modulation (ACM); this is achieved using free space optical (FSO) communication between mobile platforms in maritime environments with fog and 3D pointing errors. Specifically, we derive a new closed-form expression for a composite probability density function (PDF) that is more appropriate for applying various algorithms to FSO systems under the combined effects of fog and pointing errors. We then analyze the outage probability, average spectral efficiency (ASE), and bit error rate (BER) performance of the conventional detection techniques (i.e., heterodyne and intensity modulation/direct detection). The derived analytical results were cross-verified using Monte Carlo simulations. The results show that we can obtain a higher ASE performance by applying TMOS-based WDM and ACM and that the probability of the beam being detected in the photodetector increased at a low signal-to-noise ratio, contrary to conventional performance. Furthermore, it has been confirmed that applying WDM and ACM is suitable, particularly in maritime environments where channel conditions frequently change. Full article

The spatial heterodyne spectrometer, used in gas and mineral detection, requires phase error correction for data accuracy. Traditionally, this needs ground-based system calibration. In space, however, environmental changes can alter instrument parameters, making recalibration of the phase error surface difficult. Therefore, there is an urgent need to develop a novel phase correction method that does not require measuring the phase error surface. This study focuses on the phase errors in the interferogram and establishes a predictive model using a recurrent neural network by analyzing the relationship between spectra with errors and those without errors, thereby achieving correction of the error-containing spectra. The results indicate that, in the absence of known phase errors, the recurrent neural network can effectively perform phase error correction, yielding corrected spectra that align closely with the profiles of error-free spectra, while significantly reducing residuals and standard deviations. Compared to convolutional methods, the recurrent neural network approach demonstrates superior correction efficacy and good applicability. Therefore, the recurrent neural network method can be effectively applied to phase correction for various types of spatial heterodyne interferograms. Full article

Phase unwrapping is the decisive factor for achieving dimensional accuracy in phase-shifting profilometry, yet unavoidable phase jumps occur at discontinuities. Existing dual-frequency heterodyne techniques suffer from a narrow measurement range and overly coarse projected fringes due to grating superposition requirements, leading to large errors when scanning objects with hole-like features. To address these issues, this paper proposes an edge-oriented phase-unwrapping error-compensation method based on fringe projection phase profilometry. First, the wrapped phase of the measured object is acquired via phase-shifting profiling. The wrapped phase map is then smoothed at multiple scales using Gaussian filters, and parallel Canny edge detection combined with phase gradient thresholding is applied to comprehensively capture both coarse and fine discontinuities. Morphological closing fills in breakpoints, followed by skeleton thinning and connectivity reconstruction to generate an edge band of defined width. Within this band, edge-preserving smoothing is performed using guided filtering or bilateral filtering, and the result is fused with the original phase through Gaussian weighting based on the distance to the skeleton. Finally, an ordered multi-frequency heterodyne unwrapping restores the absolute phase, maximally preserving true discontinuities while effectively correcting noise and detection errors. Experiments show that this method overcomes edge-induced phase jumps—with jump-error correction rates exceeding 96.7%—exhibits strong noise resilience under various conditions, and achieves measurement precision better than 0.06 mm. Full article

Millimeter-wave technology helps achieve antenna miniaturization and high gain, but it is limited by factors such as short wavelength, high transmission loss, and high signal-to-noise ratio, which put higher requirements on the accuracy and computing speed of signal processing methods. The weak signal detection method based on the Duffing oscillator is suitable for detecting and estimating the parameters of such signals, but its intermittent chaotic state brings difficulties in phase determination and limited frequency detection accuracy. This article proposes a Heterodyne Duffing equation, which analyzes system properties through bifurcation diagrams, timing diagrams, and phase diagrams. Based on this, signal detection and frequency estimation models are designed, and frequency detection accuracy and calculation time are discussed. The analysis and simulation results show that the phase state discrimination speed and accuracy of the Heterodyne Duffing oscillator (HDO) are superior to the traditional Duffing equation-based intermittent chaotic state method. It has adjustable frequency resolution, overcomes the inherent 0.03ω frequency detection error limitation of the traditional Duffing oscillator, and has a significant advantage in phase state discrimination speed. The frequency estimation method based on the proposed HDO can better meet the frequency resolution and real-time requirements of millimeter-wave sensor signals. Full article

In the paper, a heterodyne quartz-enhanced photoacoustic spectroscopy (H-QEPAS)-based integrated methane (CH₄) sensor prototype is reported. The CH₄ absorption line located at 1650.96 nm was selected as the target spectral line. The design features an integrated, 3D-printed gas chamber for reduced size and weight. To realize the coordinated operation of each hardware component, a control program was designed based on LabVIEW platform, enabling the adjustment of various hardware parameters. The piezoelectric signal generated by the quartz tuning fork (QTF) was amplified via a trans-impedance amplifier (TIA), acquired by a data acquisition card (DAQ), and then transmitted to a virtual lock-in amplifier (LIA) on the PC terminal for processing. The dimensions of the integrated CH₄ sensor prototype are 33 cm in length, 27 cm in width, and 15 cm in height. The final test results demonstrate that the sensor prototype exhibits an excellent concentration linear response, with a detection limit of 26.72 ppm and a short detection time of approximately 4 s. Full article

The thermosphere serves as a pivotal region for Sun–Earth interactions, and thermospheric winds are of great scientific importance for deepening insights into atmospheric dynamics, climate formation mechanisms, and space environment evolution. This study designed and developed a Ground-based Doppler Asymmetric Spatial Heterodyne Interferometer (GDASHI). Targeting the nightglow of the oxygen atomic red line (OI 630.0 nm), this instrument enables high-precision observation of thermospheric winds. The GDASHI was deployed at Gemini Astronomical Manor (26.7°N, 100.0°E), and has obtained one year of nighttime meridional and zonal wind data. To verify the reliability of GDASHI-derived winds, a collocated observation comparison was performed against the Dual-Channel Optical Interferometer stationed at Binchuan Station (25.6°N, 100.6°E), Yunnan. The winds of the two instruments are basically consistent in both their diurnal variation trends and amplitudes. Further Deming regression and correlation analysis were conducted for the two datasets, with the meridional and zonal winds yielding fitting slopes of 0.808 and 0.875 and correlation coefficients of 0.754 and 0.771, respectively. An uncertainty analysis of the inter-instrument comparison was also carried out, incorporating instrumental measurement uncertainties, instrumental parameter errors, and small-scale perturbations induced by observational site differences; the synthesized total uncertainties of zonal and meridional winds are determined to be 20.24 m/s and 20.77 m/s, respectively. This study not only verifies the feasibility and reliability of GDASHI for ground-based thermospheric wind detection but also provides critical observational support for analyzing the spatiotemporal variation characteristics of mid-low latitude thermospheric wind fields and exploring their underlying physical mechanisms. Full article

As one of the key payloads for space-borne gravitational wave detection (SGWD), the phasemeter is primarily responsible for conducting phase measurements of heterodyne signals. A phase extraction precision at the micro-radian level constitutes a crucial performance metric for intersatellite heterodyne interferometry. In this work, an ultra-stable hexagonal optical bench was developed using hydroxide-catalysis bonding technology. Different beat-notes were generated in accordance with the requirements of four experimental stages, which were applied to simulate the main beat-note of the inter-satellite scientific interferometer, thereby verifying the phase measurement performance of the phasemeter for beat-notes. Experimental results demonstrate that the phase extraction precision meets the index requirement of $2\pi \mu \mathrm{rad}/\sqrt{\mathrm{Hz}}$ for SGWD missions. Based on the test environment of the ultra-stable hexagonal optical bench, the feasibility of the phasemeter’s core phase measurement function was verified, laying a solid foundation for subsequent research on its auxiliary functions and extended tests. Full article

The dual-pulse heterodyne demodulation distributed acoustic sensing (HD-DAS) system has superior performance but is fundamentally limited by the short sensing range, which poses a significant obstacle to its application in long-distance monitoring. This paper proposes and experimentally demonstrates a novel binary-tree structure DAS (BTS-DAS) aimed at overcoming this critical limitation. By physically decoupling the long-distance transmission fiber from the final sensing part, this structure effectively expands the system’s remote sensing capability without compromising the high pulse repetition rate for high-performance measurement. We identified modulation instability (MI), rather than stimulated Brillouin scattering (SBS), as the dominant nonlinear noise source in the extended fiber chain. Through careful power management, we established an optimal launch power window. The practical feasibility of the system was verified during on-site testing, where vibrations were successfully detected over a 10 km transmission link with sensing occurring in the 250 m sensing fiber segment, achieving a low background noise of −59.79 dB ref rad/$\sqrt{\mathrm{Hz}}$. This work presents a robust and scalable solution for long-range, high-performance acoustic sensing. Full article