Search Results (604)

CO₂ gas-line absorption is emerging as a major L-band impairment in low-loss hollow-core fiber (HCF) links. We compare two transponder-side mitigation strategies—spectral pre-emphasis and spectral avoidance—over span lengths of 100–300 km and transmission reach of up to 3000 km. The preferred strategy depends on reach, launch power, span length, and the stability of the live-link absorption comb. Pre-emphasis is favored at short reach and for short spans, whereas spectral avoidance is superior at moderate to long reach, with a peak capacity gain of about 4 Tb/s. Pre-emphasis is also more sensitive to mismatch between the design-time and live-link absorption combs: increasing the live absorption peak from 0.10 to 0.35 dB/km reduces capacity by up to 8.5 Tb/s, while tripling the CO₂ absorption linewidth reduces capacity by up to 10.3 Tb/s. We further review implementation options for both methods: DGFF-based pre-emphasis at the WSS sites, and DSP-based avoidance via digital subcarrier multiplexing (DSCM) or entropy-loaded orthogonal frequency-division multiplexing (OFDM). These results provide a concise framework for selecting mitigation strategy under realistic operating conditions. Full article

High-power 355 nm ultraviolet (UV) lasers, leveraging their short wavelength, high photon energy, and high absorption across a broad range of materials, have become indispensable light sources for precision manufacturing, semiconductor processing, and laser direct imaging (LDI). In this paper, we demonstrate a high-power 355 nm UV laser system based on a narrow-linewidth polarization-maintaining (PM) Yb-doped fiber laser and cascaded frequency conversion. A single-frequency semiconductor laser is employed as the seed source, with its spectral linewidth broadened to 0.32 nm (full width at half maximum, FWHM) via phase modulation to suppress stimulated Brillouin scattering (SBS). Through a PM master oscillator power amplifier (MOPA) architecture, a maximum average output power of 899 W at 1064 nm is achieved with a beam quality factor of M² = 1.12 (M²_x = 1.11, M²_y = 1.13). By employing lithium triborate (LiB₃O₅, LBO) crystals for extracavity cascaded second-harmonic generation (SHG) and sum-frequency generation (SFG), a maximum green output power of 613.7 W at 532 nm is obtained, corresponding to a SHG conversion efficiency of 68.2%, and a maximum UV output power of 227.1 W at 355 nm is achieved, with a total conversion efficiency of 25.2%. At the maximum output power, the UV beam quality factors are M² = 1.16 (M²_x = 1.24 and M²_y = 1.09), and the power fluctuation is better than ±1.5% root-mean-square (RMS) over 8 h of continuous operation. These results indicate that the cascaded frequency conversion approach based on narrow-linewidth PM fiber lasers possesses the capability for further scaling to higher-power single-path high-brightness UV output and can provide high-brightness UV sources for applications such as flexible printed circuit (FPC) laser cutting, flat-panel display laser direct imaging, and semiconductor wafer scribing. Full article

There is a growing demand for extreme miniaturization and enhanced sensitivity in next-generation sensing systems, including wearable devices and bioelectronics. Such advanced platforms require highly conductive, biocompatible, and mechanically robust architectures capable of conforming to dynamic surfaces. Conventional metallic thin-film fabrication techniques have reached their fundamental physicochemical limits, often suffering from suboptimal mechanical strength, complex multi-step processing, and high costs. In contrast, additive manufacturing methodologies offer streamlined microfabrication, yet traditional printing methods frequently struggle with low-viscosity constraints, insufficient metal loading, and significant material losses. This paper covers the morphological fidelity, mechanical resilience, and electrical performance of rheologically tailored, high-concentration (above 90%) gold nanoparticle paste deposited via Ultra-Precise Dispensing (UPD) technology. The capability of the UPD system to print complex, high-density fractal geometries with linewidths down to 5 μm is evaluated on both rigid and flexible substrates, glass and polyimide, respectively. The mechanical structural integrity of these conductive traces is characterized under initial 360-degree bending tests. Finally, the electrical stability and thermal response of a printed proof-of-concept temperature sensor are evaluated. The printed fractal microstructures exhibit good resolution and the fabricated sensor demonstrates good stability, displaying a linear thermal response with a temperature coefficient of resistance of 1.98·10⁻³ °C⁻¹, validating this combined material-deposition approach for microelectronics. Full article

Single-frequency fiber lasers (SFFLs) are essential for applications such as gravitational wave detection, high-precision spectroscopy, and inertial confinement fusion, requiring narrow linewidth, low noise, and high output power. Here, we present a comparative study of 1 μm waveband distributed Bragg reflector (DBR) SFFLs with varying cavity parameters. Numerically, we investigate the effects of key cavity parameters on laser performance by plotting contour maps of output power versus grating reflectivity and lasing wavelength. We also simulate intensity noise transfer functions from pump fluctuations. Increasing pump power shifts the relaxation oscillation peak to higher frequency and reduces its amplitude, which originates from the higher intracavity photon density that speeds up the damping of perturbations. Experimentally, we construct two lasers using 6.5 mm and 10.5 mm YDFs spliced between FBG pairs. These lasers employ low-reflectivity FBGs centered at 1053 nm and 1064 nm, with reflectivities of 74% and 55%, respectively. The corresponding maximum output powers are 29.7 mW and 197 mW. The 1053 nm SFFL exhibits a relative intensity noise (RIN) of −102 dBc/Hz at 2.07 MHz, a linewidth of 12.52 kHz, and a mode-hop-free tuning range of 0.64 nm. Although increasing the pump power suppresses the relaxation oscillation peak, it broadens the linewidth due to laser phase noise degradation caused by pump noise-induced temperature fluctuations in the gain fiber. For SFFLs, the output powers should be selected according to the specific application, as a higher output power inherently leads to a broader linewidth. These insights are essential for optimizing such lasers and underscore their strong potential for future applications. Full article

Spin squeezing can suppress quantum projection noise via interparticle entanglement, therefore enabling measurement sensitivities beyond the standard quantum limit. In practice, however, the Gaussian and finite intensity profiles of the optical probe beam induce spatially inhomogeneous atom-light interactions. As polarized atoms move within a vapor cell, they experience position-dependent optical intensities, generating transit noise that limits spin squeezing performance. Here, we investigate the transit noise in a coated rubidium vapor cell through combined theoretical analysis and experimental measurements. By varying the probe beam diameter, we quantify the dependence of transit noise on beam size and atomic Larmor frequency. Our results show that, for a vapor cell with fixed dimensions, the transit noise increases as the probe beam spot area decreases. Moreover, when the Larmor frequency is below the characteristic linewidth of the transit noise, the noise contribution becomes larger. We further calculated and measured spin squeezing for different beam sizes and found an experimental difference of $2.7\pm 0.2$ dB between 2 mm and 0.6 mm, similar to the theoretical prediction of $3.0\pm 0.3$ dB. Theoretical analysis under conditions of stronger squeezing shows that transit noise becomes an even more critical limiting factor. These results provide practical guidance for optimizing probe beam parameters and suppressing transit noise in spin squeezing experiments. Full article

Vertical-cavity surface-emitting lasers (VCSELs) have emerged as essential light sources for atomic-precision measurement, quantum-secure communication, high-speed optical transmission, and laser coherent scanning detection, owing to their low power consumption, high-quality beam characteristics, and ease of two-dimensional integration. However, the fundamental limitation on linewidth narrowing in VCSELs arises from their inherently short resonator, resulting in a natural linewidth on the order of 50–100 MHz. This limitation prevents conventional VCSELs from meeting the stringent requirements of advanced applications, making the ultra-narrow linewidth a key focus in optoelectronics research. This review analyzes representative achievements and application scenarios of narrow-linewidth VCSELs, evaluates the merits and limitations of industrial-grade devices, and envisions future directions in next-generation optoelectronic systems. Distinct from existing reviews, it integrates key single-mode fabrication techniques, quantitative linewidth requirements across applications, silicon photonic integration, and scalable manufacturing trends, establishing a complete mechanism–technology–application–industry analytical framework. Full article

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

We demonstrate a sideband Pound–Drever–Hall (SPDH) locking scheme that enables the simultaneous narrow-linewidth stabilization and continuous broadband frequency tuning of a laser referenced to an ultra-stable cavity. The method employs dual-frequency modulation applied to a fiber electro-optic modulator, where high-frequency modulation generates tunable sidebands and low-frequency modulation provides the error signal. We experimentally stabilize a 922 nm seed laser to the cavity and achieve a laser linewidth of 85(1) kHz with frequency noise suppression of up to 25 dB. The residual amplitude modulation (RAM) remains below 0.08% across the full tuning range. In addition, we demonstrate a continuous frequency tuning range of 1.4 GHz for a frequency-doubled 461 nm laser, with scan rates up to 317 MHz/s, while preserving stable locking to the cavity. This approach avoids complex waveform generation and provides a simple and robust solution for broadband laser frequency control. Full article

In the fields of atomic physics, quantum sensing, and precision measurement, 852 nm lasers are essential for the resonant excitation and manipulation of the cesium (Cs) D₂ transition ($6S_{1/2}\to 6P_{3/2}$). While significant global progress has been made in developing 852 nm laser based on distributed feedback (DFB) lasers and external cavity diode lasers (ECDL), the burgeoning demand for portable and integrated quantum instruments imposes stringent requirements on miniaturization and long-term, maintenance-free operation. To address the challenge of mode competition in Faraday lasers, this work demonstrates a frequency-stabilized semiconductor laser based on an atomic frequency-selective architecture. By utilizing a customized Faraday Anomalous Dispersion Optical Filter (FADOF) for frequency selection, the laser wavelength automatically corresponds to the Cs 852 nm D₂ transition, offering “Plug-and-play” operation. To further enhance integration, we propose and demonstrate a miniaturized Faraday laser architecture that resolves the instability caused by the mismatch between the FADOF transmission bandwidth and the free spectral range (FSR) of the external cavity. By employing a 7000 Gs magnetic field, the FADOF bandwidth is actively broadened to ∼15 GHz, while the cavity length is concurrently compressed to 30 mm to maximize FSR to effectively suppressing unstable mode competition. The resulting laser achieves a highly compact dimension of $102\times 109\times 96{\mathrm{mm}}^3$. Performance testing demonstrates a Lorentzian fitted linewidth of $16.4\mathrm{kHz}$ and a 1-s frequency stability of $3.05\times {10}^{-13}$ after modulation transfer spectroscopy (MTS)-based frequency locking. This robust and autonomous 852 nm laser source provides a critical technological foundation for the miniaturization of high-performance quantum sensors. Full article

Vertical-External-Cavity Surface-Emitting Lasers (VECSELs) have gained significant attention over the past two decades due to their versatility in a wide range of photonic applications. This review focuses on VECSEL configurations for dual-wavelength emission, highlighting their use in high-resolution spectroscopy, terahertz (THz) generation, and advanced optical communication. We explore recent developments in VECSEL designs, including systems utilizing birefringent crystals for polarization-based frequency separation and configurations with dual-VECSEL chips or dual-gain regions within a single cavity. These two-wavelength VECSELs enable diverse operation modes, including narrow-linewidth, pulsed, multimode, and frequency-converted emission, with high-brightness output, excellent beam quality, and tunable wavelengths. Additionally, the review discusses advancements in dual-frequency VECSELs, with applications in LIDAR systems for environmental monitoring, highly stable optical clocks, and fiber sensors. We examine improvements in cavity design, semiconductor structures, and power stabilization, which have enhanced frequency stability and spectral purity, making VECSELs suitable for precision metrology and sensing applications. Full article

Realizing high-quality-factor (high-Q) plasmonic resonances in the visible regime is critical for enhancing light-matter interactions and advancing biochemical sensing. However, traditional localized surface plasmon resonances (LSPRs) typically suffer from broad spectral linewidths due to severe radiative damping. In this work, we propose a simple two-dimensional symmetric gold nanohole-array metasurface that supports a symmetry-protected bound state in the continuum (SP-BIC) at normal incidence. By introducing extrinsic symmetry breaking via oblique incidence, this non-radiative dark state is successfully transformed into an observable high-Q quasi-BIC Fano resonance. Cartesian multipole decomposition reveals that this sharp mode ($\lambda \approx 688$ nm) is predominantly driven by a tightly confined Magnetic Dipole (MD) excitation, which drastically suppresses radiative leakage compared to the highly damped Electric Dipole (ED)-dominated LSPR. Consequently, the quasi-BIC mode exhibits an ultra-narrow spectral linewidth ($\mathrm{FWHM}\approx 17.4$ nm). While its bulk sensitivity ($236.9$ nm/RIU) is slightly lower than that of the LSPR mode, the exceptionally sharp resonance yields a remarkably low Limit of Detection (LOD) of $7.35\times {10}^{-3}$ RIU, achieving a nearly five-fold improvement over the traditional LSPR. Furthermore, the quasi-BIC mode maintains an outstanding Figure of Merit (FOM up to ∼19.7 ${\mathrm{RIU}}^{-1}$) across the entire sensing range. By eliminating the need for complex asymmetric nanofabrication, this robust angle-tuned design strategy provides a highly promising platform for the development of high-resolution, low-cost optical biosensors. Full article

To tackle the drawbacks of small effective area and limited incident angle in conventional Fabry–Pérot (F-P) etalons, this paper presents a high-utilization laser frequency-selective ultra-narrow band F-P filter module, along with systematic investigations into its operating principle, simulation, design, and verification. A simulation model with a central wavelength of 532.20 nm (±0.1 nm) and a free spectral range of 300 pm is developed, and a prototype filter is fabricated accordingly. The prototype exhibits a transmission peak linewidth better than 0.037 nm and a peak transmittance over 68%, matching well with the simulation. Simulations also reveal symmetric high-transmittance peaks at ±2.9° with stable frequency selection performance. On this basis, an integrated module is proposed using field-of-view angle conversion and optical path multiplexing. Under combined incidence at 0° and ±2.9°, the module achieves 1.67 times the energy of single-angle incidence while satisfying wavelength and bandwidth requirements. The proposed structure breaks through the conventional normal-incidence restriction and offers a novel approach for high-efficiency, multi-angle multiplexing applications of F-P etalons in precision optical systems. Full article

This paper introduces a laser interferometric elastography (LIE) system that uses a narrow linewidth laser and a single photodetector to measure mechanical displacements induced by surface acoustic waves (SAWs) generated by an electrically driven piezoelectric transducer. The method relies on phase delay analysis of the resulting interference signal to determine displacement within the medium, thereby eliminating the need for complex interferometers and broadband light sources. By substantially reducing optical hardware requirements, the system provides a compact and cost-effective platform for elasticity mapping in biological samples. Quantitative assessment of mechanical properties is achieved through controlled mechanical excitation and phase-resolved signal collection, demonstrating the practicality of simplified LIE for real-world applications. Full article

Experimental results suggest a feasible strategy for tuning the superconducting properties of MgB₂ through the incorporation of an electroluminescent inhomogeneous phase. By introducing GaP electroluminescent inhomogeneous phases into MgB₂, the effects of emission intensity variation on the sample structure, superconducting transition temperature, electrical transport behavior, and magnetic properties were systematically investigated. The results show that, at a fixed GaP addition level, the superconducting transition temperature T_c increases steadily from 38.2 K to 39.6 K with increasing emission intensity of the inhomogeneous phase, corresponding to a maximum enhancement of approximately 1.4 K. Meanwhile, the zero-resistance temperature shifts upward synchronously, indicating that the entire superconducting transition region moves toward higher temperatures. Raman measurements show that the peak position and linewidth of the E_2g phonon mode evolve systematically with emission intensity, while the electron–phonon coupling parameter λ exhibits a trend consistent with that of T_c. In addition, the nanoscale dispersed distribution of the GaP inhomogeneous phase, together with the interface/defect structures it introduces, appears to promote sample densification and enhance flux pinning, resulting in an increase in the critical current density J_c by approximately 69% at 20 K in self-field and an enhancement of the irreversibility field H_irr by about 31.5%. These results suggest that, beyond the effect of static inhomogeneous-phase incorporation, the luminescence-activated state under bias excitation is likely involved in modulating the superconducting response of MgB₂. This work provides a new experimental perspective for synergistically regulating the properties of conventional superconductors through the combined effects of inhomogeneous phases and excited states. Full article

Axions and axion-like particles are compelling candidates for ultralight bosonic dark matter, forming coherent oscillating fields that can be probed by experiments known as haloscopes. A broad range of haloscope concepts has been developed, including resonant cavity haloscopes, lumped-element circuit detectors, and spin-based experiments, each sensitive to different axion couplings and mass ranges. Rather than attempting an exhaustive survey of all existing approaches, this comparative review provides a unified framework for the major haloscope classes, establishing a common language for the descriptions of signal generation, noise properties, analytical methodologies, and scanning strategies. Key properties of ultralight bosonic dark matter relevant for detection are summarized first, including coherence time, spectral linewidth, and stochasticity under the standard halo model. The discussion then compares cavity, Earth-scale, lumped-element, and spin haloscopes, focusing on expected signal shapes, dominant noise sources, and statistical frameworks for axion searches. Particular emphasis is placed on consistent definitions of signal-to-noise ratio and on how detector bandwidth, axion coherence, and noise characteristics determine optimal scan strategies. By systematically comparing operating principles and performance metrics across these detector families, this framework clarifies shared concepts as well as the essential differences that govern sensitivity in different mass and coupling regimes. The resulting perspective synthesizes current search methodologies and offers guidance for optimizing future haloscope experiments. Full article