Optics letters最新文献

With the advent of higher baud rates and modulation orders in optical fiber communication systems, device nonlinearities have emerged as a critical limiting factor. In this Letter, we propose a simplified probability-maintained (PM) notch signal generation technique, along with its associated post-processing method, to enable accurate nonlinearity measurement. The feasibility of the proposed method is verified through comprehensive simulations and experimental comparisons with the existing methods. This method offers the advantages of low-complexity probing signal generation and can be readily implemented using a real-time spectrum analyzer (RTSA). It experimentally achieves an estimation error within 0.5 dB when the noise power ratio (NPR) is around -20 dB, thereby facilitating precise nonlinearity characterization at frequencies of interest.

In this Letter, we report a single-pixel imaging (SPI) method for optical edge detection with an extended depth-of-field (DOF). To realize enhanced DOF in optical imaging, our method exploits a sequence of propagation-invariant sinusoidal fringes (PISFs) to illuminate the object. Furthermore, the spiral phase is encoded into the sinusoidal fringes in k-space to acquire the Fourier spectrum of the edge-enhanced image. Experimentally, the phase-encoded PISFs are generated and scanned by a digital micromirror device (DMD). A single-pixel detector synchronously measures the total intensity of the signal light in sequence for image reconstruction. Our method achieves a DOF of 3800 μm for NA = 0.1, achieving a 60-fold improvement over the conventional counterpart. The superior performance is demonstrated by imaging the USAF resolution target and living algal cells.

We propose a method to split isolated attosecond pulses (IAPs) using phase-mismatch-induced spectral minima in high-order harmonic generation driven by single-cycle mid-infrared laser pulses. We demonstrate that the spectral minimum shifts to higher harmonic orders with increasing gas pressure but is insensitive to the laser's carrier-envelope phase. At higher laser intensities, the double-minimum harmonics can reach the hundred-eV range. The resulting IAP profiles, obtained through spectral synthesis and spatial integration of low-divergence far-field high harmonics, exhibit clear splitting. This work offers a tunable scheme for shaping IAPs in the extreme ultraviolet to soft X-ray spectral regions.

Hafnium oxide (HfO₂) thin films are widely used in high-power laser coatings. However, enhancing their laser-induced damage resistance remains challenging, primarily due to intrinsic defects such as oxygen vacancies, which can lead to increased absorption. This study aims to enhance the ultraviolet (UV) laser-induced damage resistance of HfO₂ thin films deposited on fused silica substrates through femtosecond laser conditioning. The films were conditioned with femtosecond laser at fluences of 1 J/cm², 2 J/cm², and 3 J/cm², respectively. The optical properties of the femtosecond laser-conditioned films were subsequently characterized. After 2 J/cm² femtosecond laser conditioning, the 355 nm nanosecond laser-induced damage threshold (LIDT) increased significantly by ~70%, from 3.2 J/cm² to 5.4 J/cm². Photoluminescence (PL) spectroscopy revealed a noticeable reduction in defect-related emission, suggesting that the reduction of defect-induced absorption is an important contributing factor to the enhanced LIDT. This work provides a promising strategy for improving the durability and performance of HfO₂-based optical coatings in high-power laser environments.

Accurate in-situ volume measurement of small (1 mm-10 cm) drifting underwater particles is critical for marine ecology and pollutant monitoring, yet it demands snapshot 3D imaging to avoid motion artifacts. Existing imaging techniques-including digital holography and conventional light field imaging-face a fundamental limitation in recovering the complete surface geometry of opaque and semi-transparent particles due to optical occlusion and limited perspective sampling. We overcome this challenge with a face-to-face dual light field camera (F2F-DLFC) system, which simultaneously captures both sides of a target under incoherent dark-field illumination. This dual-side snapshot strategy enables full 3D reconstruction of opaque particles, with experimental results showing volume errors below 6% for targets such as live fish and irregular pellets. While semi-transparent objects still present reconstruction challenges, this work establishes a foundational methodology for in-situ volumetric instrument development, providing a viable approach for accurate volumetry of a wide range of underwater particles.

Integrating vertical-cavity surface-emitting lasers (VCSELs) on flexible substrates offers significant opportunities for developing smart light sources and multifunctional photonic platforms. In this study, AlGaN-based deep ultraviolet VCSELs on a flexible substrate were demonstrated. The AlGaN quantum well heterojunction was separated from the sapphire substrate by selectively removing the thin n-GaN sacrificial layer using electrochemical etching and subsequently transferred onto a flexible substrate. Meanwhile, two dielectric distributed Bragg reflectors were deposited to construct the vertical resonant cavity. Single-mode lasing at 294.2 nm with a threshold power density of 7.4 MW/cm² and a linewidth of 0.39 nm was achieved at room temperature. Furthermore, multimode lasing attributed to non-uniformities within the distributed Bragg reflectors cavity was observed. This work opens up possibilities for advancing flexible VCSELs, as well as for the flexible photonic integration in the deep ultraviolet spectrum.

We demonstrate the thermal stability of femtosecond laser-inscribed fiber Bragg gratings (FBGs) in all-glass fibers fabricated via the molten-core method (MCM), using aluminosilicate, alumino-zirconate, and YAG-based core compositions. These novel fibers exhibit sustained FBG reflection up to 1200°C, which is an improvement of 100°C over standard SMF-28. Thermal degradation of void FBGS in molten-core fiber was analyzed via the Variable Reaction Pathways (VAREPA) framework, yielding activation energy distributions through a master curve formalism. These results highlight the potential of Al₂O₃-rich fibers for high-temperature sensing. This work provides a foundation for the development of robust fiber-based sensors for use in aerospace, nuclear, and industrial high-temperature environments.

In this Letter, we propose what we believe to be a novel technique by combining the multi-laser pulses frequency beating and coherent undulator amplification for generating high-power multi-color terahertz (THz) radiation with tunable frequency. Numerical simulations indicate that the proposed technique can produce multi-color THz radiation with three to six distinguished colors and a peak power up to hundreds of MW, and the temporally separated two-color pulses can also be produced by employing undulators with different resonance. Due to the intrinsic properties of the proposed technique, the THz frequencies, the color number, and the frequency interval can be effectively controlled by simply adjusting the beating laser. This method paves the way for advanced application of THz pump-THz probe experiments for selective excitation of atomic multi-level systems and molecular fingerprint recognition.

Miniaturized lenses with a large depth of field and high imaging quality are desirable for compact optical systems, as they eliminate the need for lens switching and repeated refocusing. Metalenses, composed of flat, subwavelength nanostructures, are well suited to this demand due to their ultra-thin profile and design flexibility. However, miniaturized metalenses typically require larger numerical apertures (NA), which lead to strong chromatic dispersion and resolution degradation. To address this limitation, we propose a Metalens Depth-of-Field Generative Adversarial Network tailored for restoring full-color images captured by a high-NA (0.447) millimeter-scale metalens. It achieves a 35% increase in peak signal-to-noise ratio and a 57.7% reduction in perceptual loss, while maintaining reconstruction quality across over 17.5 cm depth of field without additional training. This network provides a practical and scalable solution for enhancing image quality in miniaturized imaging systems.

Large Sagnac interferometers in the form of active ring lasers have emerged as unique rotation sensors in the geosciences, where their sensitivity allows one to detect geodetic and seismological signals. The passive laser gyroscope variant, however, is still at a stage of development, and thus far, only the Pound-Drever-Hall frequency stabilization technique has been explored, a method limited by residual amplitude modulation. Here, as an alternative method, we present the first Hänsch-Couillaud locked passive laser gyroscope. We find that this method is limited by flicker noise, and we introduce a cost-effective lock-in scheme to overcome this limitation. We achieve a sensitivity of 3.1nrad/s, corresponding to a fraction of 7.7×10^-5 in the Earth's rotation rate.