Optics express最新文献

Coherent heterodyne lidars are typically used for windspeed and attenuated backscattering measurements. The lack of molecular backscattering detection capability has limited the calibrated backscattering measurements until recent advances in coherent lidar technology. In this work, the simultaneous detection of aerosol and molecular backscattering is demonstrated with coherent heterodyne lidar, and the results are compared with a state-of-the-art Raman lidar PollyXT as a reference in a long-range for the first time. The molecular scattering is measured up to 3 km altitude and the extinction of laser power is calculated based on the attenuation. The essential corrections for the molecular scattering data are described and discussed. The backscattering coefficients are calculated with simple high spectral resolution lidar algorithms and the low altitude overlap is calibrated with on-site ceilometer data. A successful high spectral resolution measurement within coherent heterodyne lidar enables the self-calibration of the measured data and removes guessing of the backscatter to extinction ratio and initial values for the typical inversion algorithms. The self-calibration also enables basic aerosol classification based on the measured backscatter to extinction ratios.

Laser ablation propulsion is an important micro-propulsion system for microsatellites. Polymers with carbon added and carbon-based nanomaterial have been demonstrated as propellants with high impulse coupling coefficient (C_m). Among them, the carbon nanotube film exhibits a low ablation threshold fluence of 25 mJ/cm², which shows its potential for propulsion under low laser fluence. In this study, we investigate carbon nanotube papers (CNTPs) as propellants for laser ablation propulsion. Here four types of CNTPs have been included: S-CNTP (composed of single-walled carbon nanotubes, SWCNTs) and M-CNTP1 (composed of multi-walled carbon nanotubes, MWCNTs) and polymer composited CNTP of M-CNTP2 (30% MWCNTs) and M-CNTP3 (8% MWCNTs). SEM shows that S-CNTP and M-CNTP1 feature a network structure of carbon nanotubes while M-CNTP2 and M-CNTP3 have polymer-filled solid surfaces. Notably, M-CNTP3 exhibited a high C_m of 58.1 µN/W under a laser fluence of 1.09 J/cm². Time-resolved plasma spectroscopy revealed a reduced C₂ Swan band emission for M-CNTP3. Thermogravimetric analysis (TGA-DSC) further showed that the polymer's decomposition temperature contributes to the enhanced C_m value for M-CNTP3. These findings suggest that the performance of CNTP-based composite materials as propellants is closely related to the type and quantity of carbon nanotubes, providing an alternative propellant for microsatellite propulsion under low laser fluence conditions.

The significant absorption and scattering of light during its propagation in water severely degrade the quality of underwater imaging, presenting challenges for developing high-precision 3D imaging techniques based on optical methods. Polarization imaging has demonstrated effectiveness in mitigating the effects of scattering, making it a valuable approach for underwater imaging. Additionally, the polarization state of reflected light can be utilized for surface normal estimation and 3D shape reconstruction. This paper presents a learning-based method for 3D shape reconstruction of underwater targets using shape from polarization techniques. To address the lack of publicly available datasets for underwater polarization 3D imaging, we have developed a data acquisition system that simulates Jerlov Type I water conditions, creating a dataset of underwater polarized images along with corresponding ground truth surface normal images. Furthermore, we propose a network framework based on Attention U²Net for the 3D reconstruction of underwater polarized images. This framework is designed to capture detailed texture information of underwater targets and incorporates an effective polarization representation to resolve azimuthal ambiguity, thus enhancing the accuracy of underwater 3D imaging. Experimental results demonstrate that our method effectively addresses azimuthal ambiguity, reduces texture loss during reconstruction, and improves the accuracy of surface normal estimation, achieving superior performance compared to existing methods.

Optical properties of InGaN/GaN red quantum well(QW) and their microcavities were studied and compared under optical pumping. Incidence of the excitation laser from the p-side was employed for both structures in order to acquire better emission characteristics. The QW structure was grown on sapphire substrate by metalorganic vapor-phase epitaxy(MOVPE) with a blue pre-layer QW. X-ray and scanning transmission electron microscopy(STEM) measurements demonstrate the good crystalline quality. Emissions from both blue and red QWs were observed and demonstrated to be dominated by radiative recombination. For red InGaN microcavity with two dielectric distributed Bragg reflector(DBR) mirrors, a high Q factor of 2355 at the longitudinal mode of 612.3 nm was achieved. Discrete higher-order modes were also clearly observed, being attributed to the lateral confinement on the photons in the microcavity caused by change in the refractive index of the laser-irradiation area because of the increase of carrier density. The Purcell effect accelerates the radiation recombination rate, leading to the fast decay process in the red InGaN microcavity which does exist for QWs only. Compared with the red QW sample, the emission of red microcavities is much purer and more stable. The above results lay a foundation for the realization of InGaN-based red vertical-cavity surface-emitting lasers(VCSELs) in the future.

The digital back-propagation (DBP) is an algorithm that can equalize the chromatic dispersion and nonlinearity in the coherent optical fiber communication system. However, the nonlinear equalization effect of traditional split-step Fourier method (SSFM)-based DBP is limited. This paper replaces the SSFM in DBP algorithm with the fourth-order Runge-Kutta in the interaction picture (RK4IP) method, and employs the Bayesian optimization algorithm (BOA) to optimize the coefficients in RK4IP-based DBP algorithm, then compares it with SSFM-based DBP algorithm, which is also optimized using BOA. The experimental results of 9×100 km standard single-mode fiber (SSMF) 20 Gbaud 16QAM transmission system demonstrate that the coefficient-optimized RK4IP (CO-RK4IP)-based DBP algorithm can achieve the maximum improvement of 0.89 dB in the Q-factor compared to the coefficient-optimized SSFM (CO-SSFM)-based DBP algorithm at equivalent complexity, proving that CO-RK4IP is an effective recursive method for DBP algorithm.

We report a high peak power mid-infrared (MIR) source via efficient optical parametric generation (OPG) in a piece of 50-mm-long MgO: PPLN crystal pumped by using a near-infrared (NIR) narrow-band picosecond laser source. The highest peak power of the idler pulse can reach ∼109.86 kW with a duration of ∼7.3 ps and wavelength of ∼2924 nm. Both the signal and idler pulses can be agilely tunable in repetition rate, from 500 kHz to 4 MHz, depending on the pump pulses. Through adjusting the operation temperature and switching the applied grating of the MgO: PPLN crystal, the emission wavelengths of the signal and idler can be broadband-tunable across the NIR range of 1430-1741nm and the MIR range of 2773-4156 nm, respectively. The root-mean-square errors of the delivered signal and idler average power are less than 1% for all operational parameters. Our result provides a simplified yet agile solution of a coherent MIR source featuring a high pulse peak power, customizable emission wavelength, exceptional operation stability, etc.

A polarization-independent dual-peak narrow-band filter is proposed and demonstrated theoretically and experimentally, which is realized by using a helical long-period fiber grating (HLPG) but with a period small down to tens of micrometers. Unlike those excessively tilted fiber gratings (Ex-TFGs) or the conventional long-period fiber gratings (LPGs) but with a small period down to tens of micrometers where the generated dual-peak pairs (DPPs) are all of the strong polarization-dependence, the DPPs obtained in this study are of the polarization-independent, which is the first time, to the best of our knowledge, that the underlying mechanism for generation of the polarization-independent DPPs in transmission spectrum of the helical small-period fiber grating (HSPFG) has been revealed both theoretically and experimentally.

Event cameras only report changes in brightness when thresholds in individual pixels relative to previous levels are crossed. They output sparse streams of events that quantify these changes spatially and temporally. We have developed a measurement system using two event cameras in a stereo configuration with a specialized projector for 3D measurements of static objects. We show that optimal contrast in the structured illumination yields high-fidelity measurements, reaching 3D root-mean-square deviations of 0.6 mm (≈0.3% of the measurement volume's diagonal). This work opens up the possibility of precise event-based, highly dynamic 4D measurements in the near future.

Fluorescent antennas (FAs) exhibit considerable promise in optical wireless communication (OWC), primarily due to their advantages over conventional optical systems in terms of optical gain and field of view (FoV). This paper presents a COMSOL-based model designed to optimize external light-concentrating structures for FAs, with its accuracy validated through both qualitative and quantitative comparisons. Leveraging refractive index modulation and the conservation of optical étendue, two distinct light-concentrating structures are developed. The first structure couples the FA with an array of optical fibers, achieving up to a 1.5-fold increase in optical power density at the emitting surface. The second structure integrates the FA with a Compound Parabolic Concentrator (CPC), incorporating media of varying refractive indices. When refractive index matching is achieved with commonly used silicon-based detectors, this structure theoretically enhances optical power density by approximately 2.5 times. Compared to prior designs aimed at improving FA performance, the external light-concentrating structures proposed here improve the system's SNR without significantly affecting other device performance metrics.

The misalignment between the geometric and optical axes, combined with rotational asymmetry, poses significant challenges for achieving high-accuracy measurement of the off-axis aspheric mirror during the fabrication and polishing processes. To address this issue, this paper presents a method based on stereo deflectometry for measuring the figure of the off-axis aspheric mirror. In this method, point cloud of the off-axis aspheric mirror is first obtained using stereo deflectometry. Subsequently, the point cloud is transformed into the parent mirror coordinate system using the final-optimized transformation matrix via the nonlinear least-squares algorithm, and the figure of the off-axis aspheric is determined by subtracting the conic formula. To verify the feasibility and high accuracy of the proposed method, measurements are conducted in both simulations and experiments on an off-axis parabolic mirror with a diameter of 142 mm. The simulation results indicate that the difference between the calculated results of the proposed method and the ground truth is 3.01 nm PV (peak-to-vally), and the experimental results are consistent with those obtained using the interferometer, demonstrating the feasibility and high accuracy of the proposed method.