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This paper advances an in situ method to measure the spectral linewidth directly from the currently generated wavelength-modulated signals in frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) systems, diverging from traditional methods that focus on the linewidth of the original unmodulated laser source. Our approach, employing a self-heterodyne technique with a short-delay line, specifically targets the modulated signal's linewidth in real-time, which is vital for the operational fidelity of FMCW LiDAR systems. Crucially, our method leverages the unique capabilities of an optical hybrid for accurate phase noise and linewidth measurements, distinguishing it from conventional beat frequency extraction techniques. For the evaluation of the spectral linewidth measurement, a frequency-modulated laser source based on an optical phase-locked loop configuration was first described where the laser achieves linear optical frequency modulation by controlling the injection current of an external cavity diode laser (ECDL). The phase error measured from a Mach–Zehnder interferometer signal is used to detect the frequency deviation error from the target value, which is then fed back to the driving current of the ECDL to compensate it. Utilizing the proposed method, the laser's linewidth for the fabricated FMCW LiDAR was measured to be 287 kHz, exhibiting a clear Lorentzian spectrum shape, where the spectral modulation bandwidth and sweep time were 2.91 GHz and 50 µs, respectively. The results clearly demonstrate that the proposed in situ spectral linewidth measurement provides an efficient method for performance monitoring of FMCW LiDAR. [ABSTRACT FROM AUTHOR]

Full-Stokes polarimetric imaging light detection and ranging (LiDAR) provides rich information about distance, materials, texture, surface orientations, and profiles of objects, and it is an important remote-sensing technology. One major challenge to reach a long distance is to efficiently collect and detect the echo photons, as for long-range LiDAR, echo photons may become sparse. Here, we demonstrate a full-Stokes polarimetric imaging LiDAR, working at the eye-safe, telecommunication wavelength of 1560 nm, that can reach a range of 4 km. The key enabling technology is a four-channel system with multimode-fiber-coupled, large-area fractal superconducting nanowire single-photon detectors. Furthermore, we also explore faster imaging (e.g., pixel-dwell time of 1 ms) of the objects at a shorter distance, approximately 1 km. Our demonstration has significantly extended the working range of full-Stokes polarimetric imaging LiDAR and represents an important step toward practical systems that may enable many applications in remote sensing and the detection and recognition of targets. [ABSTRACT FROM AUTHOR]

Tunable and narrowband light detection provides a means of selectively detecting optical signals at a specific wavelength, enabling a precise tool for object identification, machine vision, spectroscopy, and so on. Simultaneous tunable and narrowband response in shortwave infrared (SWIR) detectors is critical yet still challenging. This work utilizes dual-Fano resonance enhanced sum-frequency generation in a two-layer structure comprising a silicon metasurface and a two-dimensional (2D) nonlinear GaSe layer to realize tunable and narrowband light detection in the SWIR range. The silicon metasurface affords a high-quality-factor dual-Fano resonance in the SWIR regime, which enhances the near-field optical density of the two resonant wavelengths (pump and signal) when passing through the 2D nonlinear layer, leading to drastically enhanced sum-frequency generation. The sum-frequency light at a visible wavelength that contains the information of the SWIR signal light, can then be detected by a low-noise visible detector. The tunability and selectivity in the response spectrum stem from the geometry-dependent dual-Fano resonance in the silicon metasurface, covering the 1200–1550 nm range. The upconversion detector exhibits a sub-nanometer narrowband detection with a full width at half maximum of down to ∼0.1 nm, owing to the high quality factor of the Fano resonances. This SWIR narrowband detection is one of the best performances reported so far, much narrower than commercial filter products. The peak value of the specific detectivity of 1.5 × 1012 Jones at 1256.3 nm is achieved, comparable to broadband commercial InGaAs detectors. The detector designs in this work open up the opportunity of upconversion sensors for delicate spectroscopic applications. [ABSTRACT FROM AUTHOR]

The ability of the projectile to strike accurately in the air depends on the accurate measurement of its flight distance. Due to the influence of the external environment, it is very difficult to measure the distance of high-speed projectiles in real time with ranging equipment such as radar and laser. Based on this, a real-time measurement model of the horizontal flight distance of the projectile was designed, which combines trajectory equation and discrete acceleration. First, the angle of inclination of the projectile at each position was calculated according to the six-degree-of-freedom rigid outer trajectory model. Then, two kinds of distance measurement models based on discrete acceleration integration were introduced, and the accuracy simulation was carried out. Simulation results show that the Simpson integral method is more accurate. The flight arc length of the projectile was calculated by the Simpson integral method. Finally, the horizontal flight distance of the projectile was obtained by combining the flight arc length and inclination angle of the projectile. Finally, the flight distance measurement model was simulated, and the simulation results show that the model has high accuracy. The simulation results show that the horizontal flight distance of the projectile can be measured by the trajectory equation and discrete acceleration according to the set conditions. This method can be used to measure the distance of the projectile by carrying the sensor and is less affected by the external environment. It can provide a theoretical basis and reference for the measurement of the horizontal distance of the projectile. [ABSTRACT FROM AUTHOR]

The intrinsic detection performance of superconducting nanowire single photon detectors (SNSPDs) is highly dependent on the superconducting properties of underlying thin films. This report outlines the enhancement of detection performance for single telecom wavelength photons in disordered NbTiN SNSPD at 4.2 K. By increasing the nitrogen content and deposition pressure, the NbTiN films show suppression in critical temperature and an increase in sheet resistance. Notably, the resulting SNSPDs display a broader saturation plateau at 2.2 K, leading to superior detection performance at 4.2 K. With the disordered 7-nm-thick NbTiN films, we fabricated SNSPDs with system detection efficiency up to 83% for 1550 nm photons at 4.2 K. Moreover, these devices also show saturated intrinsic detection efficiency for 2000 nm photons. With the features outlined, the devices can be integrated into the idle 4.2 K stage of the dilution refrigerator for applications in optical quantum information processing or utilize for detecting laser radar signals in airborne platforms. [ABSTRACT FROM AUTHOR]

In this work, a hybrid integrated tunable external cavity laser (ECL) with a sub-10 Hz intrinsic linewidth is demonstrated. In this hybrid ECL, two Si3N4-based subwavelength hole defect assisted microring resonators, acting as high-Q laser reflectors, are butt-coupled with a chip-scale semiconductor optical amplifier at the two edges, respectively. The experimental results show that the hybrid ECL achieves a sub-10 Hz intrinsic linewidth across a wavelength tuning range from 1525 to 1565 nm, featuring a record lowest value of 6.06 Hz. The demonstrated hybrid ECL opens up opportunities toward the applications of high-speed optical communications and high-resolution light detection and ranging. [ABSTRACT FROM AUTHOR]

High-sensitivity avalanche photodiodes (APDs) are used to amplify weak optical signals in a wide range of applications, including telecommunications, data centers, spectroscopy, imaging, light detection and ranging, medical diagnostics, and quantum applications. This paper reports antimony-based separate absorption, charge, and multiplication structure APDs on InP substrates. Al0.7In0.3As0.79Sb0.21 is used for the multiplier region, and InGaAs is used as the absorber. The excess noise is comparable to that of silicon APDs; the k-value is more than one order of magnitude lower than that of APDs that use InP or InAlAs for the gain region. The external quantum efficiency without an anti-reflection coating at 1550 nm is 57%. The gradient of the temperature coefficient of avalanche breakdown voltage is 6.7 mV/K/μm, which is less than one-sixth that of InP APDs, presenting the potential to reduce the cost and complexity of receiver circuits. Semi-insulating InP substrates make high-speed operation practical for widely reported AlxIn1−xAsySb1−y-based APDs. [ABSTRACT FROM AUTHOR]

Position-sensitive detector (PSD) plays a vital role in various applications, such as motion tracking, pilotless automobile, laser radars, and precision machining. However, limited by the detection designs of the lateral photovoltaic effect and segmented sensors, the state-of-the-art PSD suffers from complicated architecture, slow response, and narrow waveband. Herein, we propose a conceptually distinct PSD operated in short-wave infrared (SWIR, 0.8–2.3 μm), an important optical communication waveband and atmosphere window, in single crystalline lead salts thin film. The SWIR PSD present self-driven (0 V bias), fast response (590 ns), and high position resolution (45.8 nm/ H z) with a position sensitivity of 257.8 mV/mm. By combining with the numerical simulation, the underlying physics of lateral thermalized carrier diffusion driven by temperature gradient is proposed to explain the ultrafast and high-resolved SWIR PSD. Finally, we demonstrate its applications in infrared target real-time tracking, indicating its great potential in infrared guidance, trajectory tracking, and microrobots. [ABSTRACT FROM AUTHOR]

Integrated photonic systems require fast modulators to keep up with demanding operation speeds and increasing data rates. The silicon nitride integrated photonic platform is of particular interest for applications such as datacom, light detection and ranging (LIDAR), quantum photonics, and computing owing to its low losses and CMOS compatibility. Yet, this platform inherently lacks high-speed modulators. Heterogeneous integration of lithium niobate on silicon nitride waveguides can address this drawback with its strong Pockels effect. We demonstrate the first high-speed lithium niobate modulator heterogeneously integrated on silicon nitride using micro-transfer printing. The device is 2 mm long with a half-wave voltage Vπ of 14.8 V. The insertion loss and extinction ratio are 3.3 and 39 dB, respectively. Operation beyond 50 GHz has been demonstrated with the generation of open eye diagrams up to 70 Gb/s. This proof-of-principle demonstration opens up possibilities for more scalable fabrication of these trusted and performant devices. [ABSTRACT FROM AUTHOR]

The increasing demand for high pixel acquisition rates in the fields of augmented reality, autonomous driving, and robotics has led to a growing interest in solid-state beam scanning solutions that do not rely on moving parts. As a result, modern photonics has been driving the development of passive beam steering technology to meet these demands. Recently demonstrated imagers based on focal plane arrays, nanophotonic metasurfaces, and optical phased arrays have enabled unprecedented pixel resolutions and measurement speeds. However, the parallelization of > 100 lasers and detectors, which has been successfully implemented in commercial time-of-flight sensors, has not been widely adopted for passive scanning approaches. In this work, we demonstrate both inertia-free and parallel light detection and ranging (LiDAR) with microresonator frequency combs. We use 40 independent channels of a continuously scanned microresonator frequency comb operated in the chaotic regime in combination with optical dispersive elements to perform random modulation LiDAR with 2D passive beam steering. [ABSTRACT FROM AUTHOR]

Photonic-crystal surface-emitting lasers (PCSELs) with modulated photonic crystals have attracted much attention for their unrivaled capabilities, such as broad area coherent resonance, and lens-free beam scanning and flash illumination. In this paper, we first explain the principles and the development of PCSELs with modulated photonic crystals toward non-mechanical two-dimensional (2D) beam-scanning applications. Then, we show PCSELs with modulated photonic crystals, whose modulation is designed based on an inverse Fourier transform to enable the emission of various beam patterns, such as flash patterns and multi-dot patterns, from a single photonic crystal without using external optical elements. This demonstration underscores the flexibility of PCSELs with modulated photonic crystals as compact, highly functional light sources for a wide range of applications, including not only beam-scanning-type, flash-type, and multidot-type light detection and ranging but also advanced object recognition and adaptive illumination. [ABSTRACT FROM AUTHOR]

Light detection and ranging is a key technology for a number of applications, from relatively simple distance ranging to environmental monitoring. When dealing with low photon numbers, an important issue is the improvement of the signal-to-noise-ratio, which is severely affected by external sources whose emission is captured by the detection apparatus. In this paper, we present an extension of the technique developed in Cohen et al. [Phys. Rev. Lett. 123, 203601 (2019)] to the effects caused by the propagation of light through a turbulent media as well as the detection through photon counting devices bearing imperfections in terms of efficiency and number resolution. Our results indicate that even less performing technology can result in a useful detection scheme. [ABSTRACT FROM AUTHOR]

In this paper, we propose a procedure for quantifying and reducing uncertainties that impact numerical simulations involved in the estimation of the fatigue of a wind turbine structure. The present study generalizes a previous work carried out by the authors proposing to quantify and to reduce uncertainties that affect the properties of a wind turbine model by combining a global sensitivity analysis and a recursive Bayesian filtering approach. We extend the procedure to include the uncertainties involved in the modeling of a synthetic wind field. Unlike the model properties having a static or slow time-variant behavior, the parameters related to the external solicitation have a non-explicit dynamic behavior, which must be taken into account during the recursive inference. A non-parametric data-driven approach to approximate the non-explicit dynamic of the inflow related parameters is used. More precisely, we focus on data assimilation methods combining a nearest neighbor or an analog sampler with a stochastic filtering method such as the ensemble Kalman filter. The so-called data-driven data assimilation approach is used to recursively reduce the uncertainties that affect the parameters related to both model properties and wind field. For the approximation of the non-explicit dynamic of the wind inflow related parameters, in situ observations obtained from a light detection and ranging system and a cup-anemometer device are used. For the data-assimilation procedure, synthetic data simulated from the aero-servo-elastic numerical model are considered. The next investigations will be to verify the procedure with real in situ data. [ABSTRACT FROM AUTHOR]

This article discusses the use of LiDAR-enabled smartphones to study the dynamics and constants of a damped spring pendulum. Traditional experiments using smartphone accelerometers have limitations in analyzing the damping of the movement. LiDAR technology, which measures distances with high precision, provides a more intuitive alternative for studying the spring pendulum. The article describes the theoretical background, experimental setup, data collection, and data analysis process. The results show that the integration of smartphone-based LiDAR technology is valuable in measuring the oscillation of the damped spring pendulum and analyzing its intrinsic parameters. The article concludes by highlighting the potential of smartphone technology and LiDAR for investigating classical physics phenomena. [Extracted from the article]

Chip-scale lasers with a wide tunable range and a narrow linewidth have rich applications in optical communications, sensing, and light detection and ranging systems. External cavity lasers (ECLs) have the advantage of a wide tuning range and a narrow linewidth compared with their counterparts such as distributed feedback lasers and distributed Bragg reflector lasers because the passive filter separated from the gain section can be widely tuned and the low-loss external cavity extends the photon lifetime. However, so far, the tuning range achieved by the chip-scale ECLs is in the range of 80–120 nm, smaller than that of the bulk free-space ECL (typically >160 nm). The bottleneck in reaching an ultrabroad tuning range for chip-scale ECLs lies in the unsuppressed side modes of the external cavity. Here, we demonstrate a hybrid-integrated ECL consisting of a broadband III–V reflective semiconductor optical amplifier and a low-loss Si3N4 wavelength-selective reflector. A record 172-nm lasing wavelength tuning range together with a more than 40 dB side-mode suppression ratio is achieved beyond the free spectral range of the Vernier ring filter due to the enhanced lasing mode selection from a tunable Sagnac loop reflector. The Si3N4 platform enables a low-loss external cavity, facilitating laser linewidth reduction below 4 kHz over the full tuning range. The on-chip output power reaches 26.7 mW at a wavelength of 1550 nm. [ABSTRACT FROM AUTHOR]

Light detection and ranging (LiDAR) with scanning beam has been widely used in autonomous driving and large-scale manufacturing, where the fast-changing scene is of great interest. However, the acquisition rate of current LiDAR is limited by beam steering speed and the round-trip delay, hindering the development of video-rate LiDAR. Here, we uniquely combine all-optical encoders with wavelength-division multiplexing devices to implement spectro-temporal encoding on the illumination light. Parallel detection and fast spectral scanning can be achieved with only one laser and one single photodetector. Our result shows the fastest single-pixel LiDAR to date, and the detection speed can be multiplied with scalability. We demonstrate 3.56-fold improvement of speed for a maximum of 75 m detection range compared with a serial LiDAR. A record acquisition rate of 21.38 MHz for a maximum of 25 m detection range is obtained. This approach has the potential to break the limitation on current ultrafast single-pixel LiDAR and opens a new paradigm for ultrafast-frame-rate imaging systems. [ABSTRACT FROM AUTHOR]

Recent results from our electron-injection detectors as well as other heterojunction phototransistors with gain suggest that these devices are useful in many applications including medical imaging, light detection and ranging, and low-light level imaging. However, there are many parameters to optimize such structures. Earlier, we showed a good agreement between experimental results and our models. In this paper, we provide detailed analytical models for rise time, gain, and dark current that very accurately evaluate key parameters of the device. These show an excellent agreement with detailed three-dimensional numerical simulations. We also explore a figure of merit that is useful for low-light-detection applications. Based on this figure of merit, we examine the ultimate sensitivity of the device. Furthermore, we explore the effects of variations in some of the key parameters in the device design and present an optimum structure for the best figure of merit. Our models suggest ways to improve the existing devices that we have, and may be a guideline for similar phototransistors. [ABSTRACT FROM AUTHOR]

Light detection and ranging (LiDAR) measurements of isolated wakes generated by wind turbines installed at an onshore wind farm are leveraged to characterize the variability of the wake mean velocity and turbulence intensity during typical operations, which encompass a breadth of atmospheric stability regimes and rotor thrust coefficients. The LiDAR measurements are clustered through the k-means algorithm, which enables identifying the most representative realizations of wind turbine wakes while avoiding the imposition of thresholds for the various wind and turbine parameters. Considering the large number of LiDAR samples collected to probe the wake velocity field, the dimensionality of the experimental dataset is reduced by projecting the LiDAR data on an intelligently truncated basis obtained with the proper orthogonal decomposition (POD). The coefficients of only five physics-informed POD modes are then injected in the k-means algorithm for clustering the LiDAR dataset. The analysis of the clustered LiDAR data and the associated supervisory control and data acquisition and meteorological data enables the study of the variability of the wake velocity deficit, wake extent, and wake-added turbulence intensity for different thrust coefficients of the turbine rotor and regimes of atmospheric stability. Furthermore, the cluster analysis of the LiDAR data allows for the identification of systematic off-design operations with a certain yaw misalignment of the turbine rotor with the mean wind direction. [ABSTRACT FROM AUTHOR]

Next-generation models of wind farm flows are increasingly needed to assist the design, operation, and performance diagnostic of modern wind power plants. Accuracy in the descriptions of the wind farm aerodynamics, including the effects of atmospheric stability, coalescing wakes, and the pressure field induced by the turbine rotors are necessary attributes for such tools as well as low computational costs. The Pseudo-2D RANS model is formulated to provide an efficient solution of the Navier–Stokes equations governing wind-farm flows installed in flat terrain and offshore. The turbulence closure and actuator disk model are calibrated based on wind light detection and ranging measurements of wind turbine wakes collected under different operative and atmospheric conditions. A shallow-water formulation is implemented to achieve a converged solution for the velocity and pressure fields across a farm with computational costs comparable to those of mid-fidelity engineering wake models. The theoretical foundations and numerical scheme of the Pseudo-2D RANS model are provided, together with a detailed description of the verification and validation processes. The model is assessed against a large dataset of power production for an onshore wind farm located in North Texas showing a normalized mean absolute error of 5.6% on the 10-min-averaged active power and 3% on the clustered wind farm efficiency, which represent 8% and 24%, respectively, improvements with respect to the best-performing engineering wake model tested in this work. [ABSTRACT FROM AUTHOR]

Photonic crystal waveguide slow-light grating emits a free-space optical beam and steers it widely by changing the optical wavelength or waveguide refractive index. In the reverse process, returned light is coupled into the device again. We have proposed to use this optical transmission and reception antenna as a beam scanner for light detection and ranging (LiDAR). Ideally, a large-aperture antenna can narrow the transmission beam and enhance the reception efficiency. Actually, however, the transmission and reception performance is not scalable owing to waveguide loss even though the waveguide is simply lengthened. A serial array configuration in which the waveguide is divided into multiple antennas is effective for mitigating this problem. In this study, we fabricated such a device using Si photonics technology and obtained a small beam divergence of 0.02° at a telecom wavelength. Then, we observed the ranging operation by adding an optical setup of frequency-modulated continuous-wave (FMCW) LiDAR and confirmed that the divided antenna device improved the reception intensity by 12 dB. Moreover, we fabricated a FMCW LiDAR chip in which the serial array antennas were integrated in parallel with switch trees and Ge photodiodes and obtained point cloud images by two-dimensional beam scanning. [ABSTRACT FROM AUTHOR]

Light detection and ranging (lidar) is an attractive technique to measure wind speed via the Doppler effect, thanks to its flexibility. Although stand-alone deployment is growing, current standards for wind energy applications often require wind lidars to be verified against an in situ met mast with cup anemometers, which deliver point-like measurements. Analyzing 38 lidar-to-cup verification campaigns at eight accredited sites, we investigate the difference between these techniques during turbulence influenced atmospheric conditions. We demonstrate that strong turbulence results in the lidar 10-min scalar average wind speed overestimate the cup value, whereas the lidar vector average results in an underestimate. Although scalar and vector averaging provide acceptable measurements, we show that a linear combination, with appropriate weighting, reduces the lidar turbulence sensitivity by more than one order of magnitude. The resulting overall lidar-to-cup bias is within the one-standard-deviation comparison uncertainty at all sites. The resulting ensemble mean bias is (− 0.14 ± 0.42) % , where the uncertainty stems from variations between sites. We explain the experimental observations by a model derived from first principles. The developed hybrid averaging is implemented in the Leosphere WindCube v2.1 wind lidar. [ABSTRACT FROM AUTHOR]

In recent years, wind turbine yaw misalignment that tends to degrade the turbine power production and impact the blade fatigue loads raises more attention along with the rapid development of large-scale wind turbines. The state-of-the-art correction methods require additional instruments such as laser imaging detection and ranging to provide the ground truths and are not suitable for long-term operation and large-scale implementation due to the high costs. In the present study, we propose a framework that enables the effective and efficient detection and correction of static and dynamic yaw errors by using only turbine supervisory control and data acquisition data, suitable for a low-cost regular inspection for large-scale wind farms in onshore, coastal, and offshore sites. This framework includes a short-period data collection of the turbine operating under multiple static yaw errors, a data mining correction for the static yaw error, and ultra-short-term dynamic yaw error forecasts with machine learning algorithms. Three regression algorithms, i.e., linear, support vector machine, and random forest, and a hybrid model based on the average prediction of the three, have been tested for dynamic yaw error prediction and compared using the field measurement data from a 2.5 MW turbine. For the data collected in the present study, the hybrid method shows the best performance and can reduce the total yaw error by up to 85% (on average of 71%) compared to the cases without static and dynamic yaw error corrections. In addition, we have tested the transferability of the proposed method in the application of detecting other static and dynamic yaw errors. [ABSTRACT FROM AUTHOR]

Light detection and ranging (LiDAR) technology has recently been attracting considerable attention, but it is still difficult to achieve distributed reflectivity sensing and vibration detection simultaneously at high speed. To tackle this issue, we develop a new LiDAR configuration by extending fiber-optic correlation-domain reflectometry to the spatial system and demonstrate its fundamental operations. We experimentally show that the random accessibility unique to this configuration enables high-speed measurement of the vibration frequency and the position of a mirror oscillating at up to 100 kHz. This LiDAR may be applicable to visualization of flow velocity distributions, especially to distributed detection of turbulence, which induces vibration of dust particles in air. [ABSTRACT FROM AUTHOR]

Near-infrared transparent electrodes constitute an essential component of light-emitting and photovoltaic devices widely employed in short- and long-range communication, light detection and ranging, biodiagnostics, security, virtual and augmented reality, night vision, gas sensing, and solar cells. However, the efficiency of all these devices and related applications suffers from significantly reduced transmittance of the indium tin oxide electrode compared to the visible wavelength range. Here, we explore the potential of randomly and uniformly arranged silver, gold, and aluminum nanopore and nanowire films for the near-infrared optoelectronics. We show that these metallic nanopatterned layers, except for randomly arranged nanoporous configurations, exhibit considerably higher performance than the commonly used indium tin oxide. Furthermore, silver layers possess higher transmittance and lower haze than gold and aluminum ones, while the nanowire configuration overperforms the nanoporous one. The obtained results offer a means for deeper analysis of metallic nanopatterned transparent electrodes for many near-infrared optoelectronic applications. [ABSTRACT FROM AUTHOR]

Designed to outperform conventional computers when performing machine-learning tasks, neuromorphic computation is the principle whereby certain aspects of the human brain are replicated in hardware. While great progress has been made in this field in recent years, almost all input signals provided to neuromorphic processors are still designed for traditional (von Neumann) computer architectures. Here, we show that a simple photosensitive capacitor will inherently reproduce certain aspects of biological retinas. We found that capacitors based on metal halide perovskites will output a brief voltage spike in response to changes in incident light intensity, but output zero voltage under constant illumination. Such a sensor is not only optimized for use with spiking neuromorphic processors but also anticipated to have broad appeal from fields such as light detection and ranging, autonomous vehicles, facile recognition, navigation, and robotics. [ABSTRACT FROM AUTHOR]

This paper presents the implementation of a pulse-type LAser Detection And Ranging (LADAR) system based on heterodyne detection for long-range measurement. A pulse-type LADAR based on an intensity direct-detection is certainly simple and mature, but it requires a high peak-power laser and a low-noise avalanche photodiode for long-range measurement, which restricts the scope of the application due to the weight, power consumption, and cost of the laser and the photodetector. In this work, heterodyne detection using a PIN photodiode is implemented to increase receiver sensitivity instead of using a low-noise avalanche photodiode. An optical phase-locked loop is adopted to generate an optical local oscillator signal for heterodyne detection. The proposed heterodyne detection scheme achieves a minimum detectable signal level of −52.6 dBm at a bandwidth of 1.2 GHz, and it is adopted in a pulse-type LADAR system for long-range measurement. The pulse-type LADAR system can measure a distance of 2.77 km at a repetition rate of 40 kHz, and it demonstrates great advantages for realizing real-time 3D imaging for long-range measurement with a high frame rate. [ABSTRACT FROM AUTHOR]

Modulated photonic-crystal surface-emitting lasers (M-PCSELs) are new semiconductor lasers that can emit a laser beam in arbitrary directions. Therefore, they can be regarded as a next-generation light source for light detection and ranging. In this manuscript, we numerically investigate the characteristics of M-PCSELs, including one- and two-dimensional optical coupling constants and radiation constants, by using three-dimensional coupled wave theory. We also carry out lattice point designs and show that the two-dimensional optical coupling can be enhanced by more than four times over our previous M-PCSEL devices by employing elliptical lattice points. Moreover, we find that two-dimensional optical coupling can be maintained even when large lattice point position modulations are introduced for large radiation constants. These results indicate that more stable two-dimensional oscillation and a higher slope efficiency are expected in M-PCSELs with elliptical lattice points. [ABSTRACT FROM AUTHOR]

Based on the generating function formalism, we investigate broadband photon statistics of emission for single dimers and trimers driven by a continuous monochromatic laser field. In particular, we study the first and second moments of the emission statistics, which are the fluorescence excitation line shape and Mandel's Q parameter. Numerical results for this line shape and the Q parameter versus laser frequency in the limit of long measurement times are obtained. We show that in the limit of small Rabi frequencies and laser frequencies close to resonance with one of the one-exciton states, the results for the line shape and Q parameter reduce to those of a two-level monomer. For laser frequencies halfway the transition frequency of a two-exciton state, the photon bunching effect associated with two-photon absorption processes is observed. This super-Poissonian peak is characterized in terms of the ratio between the two-photon absorption line shape and the underlying two-level monomer line shapes. Upon increasing the Rabi frequency, the Q parameter shows a transition from super- to sub- to super-Poissonian statistics. Results of broadband photon statistics are also discussed in the context of a transition (frequency) resolved photon detection scheme, photon tracking, which provides a greater insight in the different physical processes that occur in the multi-level systems. [ABSTRACT FROM AUTHOR]

Lidar (light detection and ranging) depolarization measurements were performed simultaneously at three wavelengths using a coherent white light continuum. The white light continuum ranging from the ultraviolet to the infrared region was generated by a focused 0.5 TW/100 fs pulse in a krypton gas cell at the atmospheric pressure. The lidar system consisted of three depolarization channels at 450, 550, and 800 nm. The observed depolarization ratios for each wavelength showed differences for clouds. These depolarization features in wavelength are discussed in terms of particle size and shape using T-matrix computations. The depolarization ratios at three wavelengths can be used to infer information on the size of the particles in clouds, assuming an ice crystal shape a priori. Also, the obtained size distributions from the three-wavelength depolarization ratios correspond with the changes in particle size indicated by changes in the Angstrom coefficient obtained by multiwavelength lidar techniques. This study demonstrated that the three-wavelength depolarization ratios provided additional information about the size and shape of clouds and aerosols. [ABSTRACT FROM AUTHOR]

In the present paper, a comprehensive self-consistent three-dimensional model is used to analyze physical aspects of the operation of oxide-confined vertical-cavity surface-emitting diode lasers (VCSELs) and to optimize their structures. The impact of the built-in radial confining mechanisms created by oxide apertures, i.e., the influence of their diameters and localizations on radial confinements of both the current injection into VCSEL active regions and electromagnetic fields of successive cavity modes, has been investigated. Basically, there are two extreme cases: the index-guided (IG) VCSELs with the aperture localized at the antinode position of the optical standing wave, characterized by a very low lasing threshold but exhibiting rather poor mode selectivity, and the gain-guided (GG) VCSELs with the aperture shifted to the node position, ensuring usually the single-fundamental-mode operation, but at the expense of much higher lasing threshold. In the present paper, the separate-confinement-oxidation VCSEL structure has been proposed exhibiting simultaneously advantages of both the IG VCSELs (the low lasing threshold) and the GG ones (the single-fundamental-mode operation). [ABSTRACT FROM AUTHOR]

We introduce the concept of optical dynamic range compression and discuss its utilities in the non-uniform quantization, enhancing the signal-to-noise ratio as well as reshaping signal's statistical distribution and extending the detection range in light detection and ranging systems. The technology represents a photonics hardware accelerator that reduces the burden on the dynamic range of the photodetection and the data acquisition including the required number of bits of the analog-to-digital converter. The energy of photons that are intentionally blocked can be harvested using a two-photon photovoltaic effect. Implementations using other approaches are also discussed. [ABSTRACT FROM AUTHOR]

Presents a study which examined double scattering lidar events involving long cylindrical particles. Geometry of double scattering from long cylinders; Discussion of double scattering intensity; Results of the double scattering calculations.

The avalanche-photodiode-array (APD-array) laser detection and ranging (LADAR) system has been continually developed owing to its superiority of nonscanning, large field of view, high sensitivity, and high precision. However, how to achieve higher-efficient detection and better integration of the LADAR system for real-time three-dimensional (3D) imaging continues to be a problem. In this study, a novel LADAR system using four linear mode APDs (LmAPDs) is developed for high-efficient detection by adopting a modulation and multiplexing technique. Furthermore, an automatic control system for the array LADAR system is proposed and designed by applying the virtual instrumentation technique. The control system aims to achieve four functions: synchronization of laser emission and rotating platform, multi-channel synchronous data acquisition, real-time Ethernet upper monitoring, and real-time signal processing and 3D visualization. The structure and principle of the complete system are described in the paper. The experimental results demonstrate that the LADAR system is capable of achieving real-time 3D imaging on an omnidirectional rotating platform under the control of the virtual instrumentation system. The automatic imaging LADAR system utilized only 4 LmAPDs to achieve 256-pixel-per-frame detection with by employing 64-bit demodulator. Moreover, the lateral resolution is 15 cm and range accuracy is 4 cm root-mean-square error at a distance of ~40 m. [ABSTRACT FROM AUTHOR]

As a wind turbine generates power, induced velocities, lower than the freestream velocity, will be present upstream of the turbine due to perturbation of the flow by the rotor. In this study, the upstream induction zone of a 225kW horizontal axis Vestas V27 wind turbine located at the Danish Technical University's Risø campus is investigated using a scanning Light Detection and Ranging (lidar) system. Three short-range continuous-wave "WindScanner" lidars are positioned in the field around the V27 turbine allowing detection of all three components of the wind velocity vectors within the induction zone. The time-averaged mean wind speeds at different locations in the upstream induction zone are measured by scanning a horizontal plane at hub height and a vertical plane centered at the middle of the rotor extending roughly 1.5 rotor diameters (D) upstream of the rotor. Turbulence statistics in the induction zone are studied by more rapidly scanning along individual lines perpendicular to the rotor at different radial distances from the hub. The mean velocity measurements reveal that the longitudinal velocity reductions become greater closer to the rotor plane and closer to the center of the rotor. Velocity deficits of 1%-3% of the freestream value were observed 1 D upstream of the rotor, increasing at the rotor plane to 7.4% near the edge of the rotor and 18% near the center of the rotor while the turbine was operating with a high estimated mechanical coefficient of power (CP) of 0.56 yielding an estimated axial induction factor of 0.25. The velocity reductions relative to the freestream velocity become smaller when the turbine's coefficient of power decreases; for a low CP of 0.16 resulting in an estimated induction factor of 0.04, the velocity deficits are 1% of the freestream value 1 D upstream of the rotor and only 6% at the rotor plane near the center of the rotor. Additionally, the mean radial wind speeds were found to increase close to the edge of the rotor disk indicating an expansion of the incoming flow around the rotor. Radial velocity magnitudes at the edge of the rotor disk of approximately 9% and 3% of the freestream longitudinal wind speed were measured for the abovementioned high and low CP values, respectively. Turbulence statistics, calculated using 2.5-min time series, suggest that the standard deviation of the longitudinal wind component decreases close to the rotor, while the standard deviation of the radial wind component appears to increase. When the turbine was operating with a high CP of 0.54 resulting in an estimated induction factor of 0.22, standard deviation decreases of up to 22% of the estimated freestream value and increases of up to 46% were observed for the longitudinal and radial components, respectively, near the center of the rotor. [ABSTRACT FROM AUTHOR]

A chopper wheel construct is used to shorten the duration of a molecular beam to 13 µs. Molecular beams seeded with NO or with Br2 and an initial pulse width of ≥200 µs were passed through a spinning chopper wheel, which was driven by a brushless DC in vacuo motor at a range of speeds, from 3000 rpm to 80 000 rpm. The resulting duration of the molecular-beam pulses measured at the laser detection volume ranged from 80 µs to 13 µs and was the same for both NO and Br2. The duration is consistent with a simple analytical model, and the minimum pulse width measured is limited by the spreading of the beam between the chopper and the detection point as a consequence of the longitudinal velocity distribution of the beam. The setup adopted here effectively eliminates buildup of background gas without the use of a differential pumping stage, and a clean narrow pulse is obtained with low rotational temperature. [ABSTRACT FROM AUTHOR]