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At microwave frequencies, radar cross-section (RCS) measurements are usually performed by placing the target in the far-field region of the antenna. The wavefront of the radiating field from the antenna can be approximated as planar, ensuring that the incident field and the power interact with the target independently of the antenna. However, for electrically large targets, the required distance becomes significant, posing challenges for implementation. Scaled-model RCS measurements offer an alternative solution. RCS measurements at terahertz and optical frequencies typically require a collimated beam as the source, where the intercepted power and RCS become dependent on the excitation. To address this dependency, researchers have proposed modifying the RCS definition to account for the intercepted power and to analytically formulate the scattering problem. However, such modifications require prior knowledge of the target's geometry and material properties, which are often not readily available in practice. This also limits the study to only canonical targets. In this paper, we propose an alternative approach for modelling the intercepted power. The Gaussian beam is decomposed into a number of plane waves travelling to different directions using the theory of plane wave spectrum. The scattering problem is solved using the full-wave method of moment. Through theoretical proofs and numerical examples involving spheres and a non-canonical target, with a scaled-model aircraft, we demonstrate that the original RCS definition can serve as a good approximation for scaled measurements, provided that the beam waist is approximately four times the target's dimensions. These findings provide valuable guidelines for radar engineers when performing scaled measurements using collimated beams. The results, which match those obtained from full-model measurements, enable us to predict the RCS of full-scale targets. This capability facilitates various target-related applications, such as target characterization, classification, detection, and even recognition. [ABSTRACT FROM AUTHOR]

This work concerns the assessment of mold parts with multi-view scanning point cloud data. With the soaring demand for complex geometry parts, multi-axis electrical discharge machining (EDM) manufacturing also faces the challenge of high-cost and the requirement of high-accuracy. As a part of EDM, well-judged life-time and wear condition of mold parts are critical to ensuring product quality. In this work, we present a novel non-contact surface inspection solution for mold part 3D reconstruction and inspection based on light-section scanning. After calibration and registration, multi-view stereo (MVS) point cloud data for the parts can be obtained, which can then be used to generate 2D section profiles. Comparison method is designed to examine if the design tolerance is met by comparing the section profiles against those based on the CAD model of the parts. Our method is verified by assessing worn electrode parts, which are commonly used in machining centers. The method proposed in this paper ensures the detection accuracy and improves the detection speed. It overcomes the problem of slow detection speed of mold part wear using 3D point cloud. [ABSTRACT FROM AUTHOR]

Tunnel dislocation affects the stability and waterproof of the structure and endangers its service life. This paper presents a dislocation calculation method of shield tunnel based on cross sectional point clouds, which contain three main steps. First, a longitudinal joint detection method is proposed based on the circumferential joints detected by the existing gradient accumulation method. The point clouds containing capping block in each ring are converted into gray image to obtain the center line position of the block, and the longitudinal joints are calculated through combining with the tunnel design data. Second, tunnel appendages are removed quickly through unfolding the tunnel point clouds by cylinder projection and adjusting the parameters of the cloth simulation filtering algorithm. Finally, the circumferential dislocations are calculated by selecting multiple denoised sections on both sides of the joint and clustering the points at each angle. Meanwhile, the longitudinal dislocations are calculated through fitting the segments in each ring separately. Experimental results show that the automatic extraction rate of longitudinal joints is higher than 94%. The point clouds filtering method can quickly separate the very long tunnel lining from the close appendages attached to it. Meanwhile, the RMSE of the repeated circumferential and longitudinal dislocation are evaluated to be 1.56 mm and 0.57 mm respectively. Compared with the existing state of art methods, these values reach 1.75 mm and 0.63 mm. Through the method proposed, the dislocation value at any mileage and any angle of tunnel can be displayed intuitively. [ABSTRACT FROM AUTHOR]

A standard method for distance determination is light detection and ranging (LiDAR), which relies on the emission and detection of reflected laser pulses. When LiDAR systems become common for every vehicle, many simultaneous laser signals will produce mutual LiDAR interference between LiDAR systems. In this paper, we analyze the possibility to recognize mutual interference in time-correlated single photon counting (TCSPC) LiDAR with particular focus on flash systems. We evaluate the LiDAR interference appearance by deriving the expected event distribution for ego and aggressor signal. From that, we calculate the probability of photon detection within each measured signal. This paper shows the high potential of different pulse repetition frequencies to reduce LiDAR interference. Using signal-to-noise ratio (SNR), we define the extinction distance, beyond which the aggressor signal completely extinguishes the ego signal. Applied on different background and laser event rates, we find the connection between ideal LiDAR system designs and lowest probability for unrecognized LiDAR interference. Furthermore, we show the relationship to a specific LiDAR design, which must fulfill eye safety condition and receives lower intensities with increasing target distances. Finally, we present different solutions for the recognition and reduction of LiDAR interference based on our previous results. [ABSTRACT FROM AUTHOR]

The accurate measurement of rail wear is critical in track quality inspection. Usually, by matching the measured profile with the standard one based on the unworn rail waist double circle segment (DCS), and comparing their railhead differences, we obtain the rail wear. However, in the complex field environment, this task becomes tricky. For profile registration, one is that the location of rail waist becomes difficult with lots of outliers mixed in the profile, and the other is that the obtained rail waist could be polluted or incomplete. For wear measurement, the outliers scattered on the railhead could cause serious errors. This paper is devoted to solving the above problems. For problem 1, firstly, the rail waist is located after a preprocessing procedure. Then, utilizing the proposed hybrid model called R-H-ICP, we realize the profile registration with a process from coarse to fine. For problem 2, by using the standard profile and the reconstructed one with wear constraints to serve as the template separately, and computing the distance from each point of railhead on measured profile to the template, both the distinct and the inapparent outliers are detected precisely. The efficiency and superiority of proposed methods were verified by vast experiments. For the former, compared with DCS and ICP, the R-H-ICP not only improves the profile utilization ratio obviously, also guarantees the registration accuracy as far as possible. For the latter, the F1-Measure average score of outlier detection reaches 0.998, which outperforms some classical models markedly. [ABSTRACT FROM AUTHOR]

This paper addresses the evaluation of object detection performance for non-contact safety-related sensors in low-visibility environments due to adverse weather conditions. An evaluation metric, MOT (Minimum Object-detectable Transmittance), is proposed to quantitatively evaluate the influence of the low-visibility environments on the object detection performance. Traditionally, the Meteorological Optical Range (MOR) has been used for this purpose. However, MOR is not appropriate for the sensor evaluation because it is a meteorological metric which is optimized for human eyes to estimate the distance at which a person can see clearly through an atmosphere. On the other hand, MOT is a physical and optical measure that quantifies the relationship between the spatial transmittance and the distance at which a sensor can detect a specified object. It is specialized for the sensor evaluation and can express the sensor characteristics in the low-visibility environments at each measurement distance. The usefulness of MOT will be demonstrated by presenting the experimental results of MOT measurements for various sensor devices in a fog space reproduced with environmental simulator equipment. [ABSTRACT FROM AUTHOR]

While remote inspection of industrial structures, such as nuclear reactors, using robotic crawlers currently presents significant advantages in terms of safety, accuracy and cost, other challenges emerge due to poor context-awareness and positional accuracy. This results in a lack of visibility for path planning and difficulty in precise localization of NDE (Non-Destructive Evaluation) inspection data. LIDAR (Light Detection and Ranging) are one form of sensors that estimate distances at various angles to map the surrounding environment using optical techniques. Existing commercial LIDARs offer a long range of measurement, allowing mapping of the surroundings. However, such sensors often have centimeter accuracy and a minimum scan range, resulting in a blind area and are generally unsuitable for compact spaces and areas with high density of neighboring objects. This paper presents a framework for using a customized 2D laser scanner, an IMU (Inertial Measurement Unit) and a data fusion approach for localization inside high-density volumes such as nuclear reactors. The laser scanner offers precise measurements with submillimeter accuracy for items located in the short range. The IMU calculates the robot attitude angles, which are critical for inclination angle corrections. The facilities are often made of metallic materials with highly reflective surfaces, which remains problematic for the laser scanner. A mock-up nuclear dome, of realistic material construction, was utilized to benchmark the performance of this framework. The distance and orientation error observed were below 2 mm and 1°, respectively. The framework will be further processed to produce a close-range environment mapping. [ABSTRACT FROM AUTHOR]

This paper presents direct time-of-flight image LiDAR to realize a reliable autopilot system, which consists of hardware architecture and processing algorithm based on field programmable logic array (FPGA). The processing algorithm consists of multi-channel time-to-digital converters (TDCs), equivalent sampling technology consisting of equivalent sampling circuit (ESC) and amplitude fitting algorithm (AFA), correlation slope fitting algorithm (CSFA), and stray light suppression algorithm (SLSA). The TDCs are used to measure the flight time. The ESC is used to record waveforms based on time discrimination circuits, dynamic threshold adjustment circuits, and TDCs. The AFA is used to achieve the amplitude of the echo signal whether it is saturated or not. The CSFA based on the Gauss function is used to eliminate walk error, which does not need to calibrate the compensation coefficient in advance. And the stary light suppression algorithm based on laser pulse signal modulation technology is used to suppress the interference of background light and interference within different LiDAR systems. The LiDAR system can achieve 100 m measurement range with the laser peak power of 120 W. A measurement precision of ~+/−15 mm was achieved with non-cooperative targets when the measurement range was less than 10m. And the measured relative precision was less than 1% for a distance up to 100 m with a 100-klux direct sunlight. [ABSTRACT FROM AUTHOR]

Simultaneous phases detection in multi-wavelength interferometry (MWLI) is essential for high-accuracy dynamic absolute distance measurement in high-end equipment manufacturing. To address this problem, a simultaneous phase detection scheme based on frequency-division multiplexing (FDM) technique is proposed in this paper. High-frequency sinusoidal phase modulation with non-overlapping frequencies were applied to multiple lasers individually using electro-optic modulators (EOMs), and interference phases of multiple wavelengths were extracted in parallel using phase generated carrier (PGC) demodulation technique from the FDM interference signal. Principles of FDM multi-wavelength interferometry and simultaneous phase detection scheme were illustrated. A FDM dual-wavelength interferometer was constructed to verify the performance of the proposed phase detection scheme by simulation analysis and experiments. Results of simulations show that the resolution, accuracy and nonlinear error of the two demodulated phases are all better than 0.01°. Experiments of nanometer displacement measurement further verified the good performance of the phase detection scheme and its feasibility in dynamic measurement. [ABSTRACT FROM AUTHOR]

This paper presents a digital processing system for terrestrial laser range finding systems. It is mainly comprised of a commercial digital signal processor with 400 MHz. For the signal digitization, an analog-to-digital converter with a 12-bit resolution is sampled with 50 MS/s. Such a digital system architecture has several advantages: automatic gain control, coherent multi-pulse averaging, digital signal filtering, upsampling, and advanced peak detection methods. When combined with a suitable laser and detector unit, the digital system outperforms the classic analog threshold-based laser finders with regard to required laser output power, multitarget discrimination, and peak detection accuracy. The best results were achieved using a matched filter, compared to wavelet denoising techniques and classic Savitzky-Golay filtering. Finally, this system architecture allows for a small-sized, lightweight, battery-operated, and energy-efficient, class 1 eye-safe laser range finder, capable of measuring distances up to multiple km with accuracy and precision of better than 1 m to uncooperative targets. [ABSTRACT FROM AUTHOR]

As a critical sensor for high-level autonomous vehicles, LiDAR’s limitations in adverse weather (e.g. rain, fog, snow, etc.) impede the deployment of self-driving cars in all weather conditions. However, studies in literature on LiDAR’s performance in harsh conditions are insufficient. In this paper, based on a dataset collected with a popular Near-InfraRed (NIR) ToF LiDAR in a well-controlled artificial fog chamber, we statistically model the LiDAR ranging process in fog conditions through a data-driven approach. Specifically, giving an object at a known distance, our model is able to predict LiDAR measures (range and intensity) under various fog conditions. For a transmitted laser under fog, we first model and predict the minimum visibility required to detect its true range or not. Then, the noisy range and intensity measures are sampled from the probabilistic measurement distributions inferred from the dataset. The performance of the proposed method has been quantitatively and qualitatively evaluated. Experimental results show that our approach can provide a promising performance prediction of the utilized NIR ToF LiDAR under fog, which opens a new gate to the quantitative assessment of adverse weather and contributes to the specification of relevant Operational Domain Designs (ODDs). The developed ROS package is available at: https://github.com/cavayangtao/lanoising. [ABSTRACT FROM AUTHOR]

The rotational inertial navigation system (RINS) could greatly improve navigation performance by rotating the inertial measurement unit with gimbals. And the self-calibration of error parameters could be achieved conveniently. Among the rate-biased Ring laser gyro (RLG), the input angular velocity can reach tens of degrees per second. Therefore, the attitude errors caused by scale factor is dozens of times that of the other RINS. In this paper, gyro scale factor errors are calibrated through optimal estimation with attitude errors in dual-axis laser RINS. During the movement of vehicle, the dynamic gyro scale factor errors calibration can be performed online by adding parking points. Moreover, the parking points can be accurately determined by the statistical characteristics of accelerometers and gyros output information, without extern sensors. The experiment results indicate that the calibration repeatability scale factor parameters are less than 1ppm, and the attitude errors cause by the scale factor error is reduce from tens of arc seconds to 10 arc seconds, which verifies the effectiveness and accuracy of the method proposed in this paper. Through the compensation of calibration results, the attitude error of RINS is further improved in both the static and dynamic experiment. [ABSTRACT FROM AUTHOR]

Terahertz (THz, $0.3\!\!-\!\!3$ THz) wireless access is nowadays considered as a major enabling technology for sixth generation (6G) cellular systems. To compensate for high propagation losses, these systems will utilize antenna arrays with extremely directional beams. The performance of such systems will thus be heavily affected by micromobility such as shakes and rotations of user equipment (UE) even when user is in stationary position. The ultimate impact of micromobility is spontaneous degradation of signal-to-noise (SNR) level leading to outages. In this paper, we measure and statistically investigate the micromobility process of various applications including video viewing, phone calling, virtual reality viewing and racing game. Particularly, we characterize occupancy distributions and first-passage time (FPT) to outage for various antenna configurations. We also assess the radial symmetry in micromobility patterns and characterize distance-dependent velocity and drift to the origin parameters. The obtained results are essential for developing mathematical models of micromobility patterns that needs to be further used in system-level analysis of THz cellular systems. To this end, we also illustrate that Markov models are only suitable for applications with low and purely random micromobility dynamics such as video viewing and phone calling. When a user is controlled by the application, as in the case of gaming, Markov models overestimate FPT to outage. [ABSTRACT FROM AUTHOR]

Oxides of Nitrogen (NOx) are among the most critical pollutants, leading to a variety of environmental concerns. This paper describes the demonstrates a high-temperature Quantum Cascade Laser-based (QCL) probe-based system for in-situ measurement of the pollutants in the flue gas. The system demonstrates a sensitivity of less than one ppm and a fast system response time of 60 seconds at high temperature (>300°C) and high dust-load (70 g/m3) flue gas from a coal-fired boiler. The system uses a pulsed 2f wavelength modulation spectroscopy technique for the NO concentration measurement at the core. The paper also elaborates on a novel optical design using hollow-core fibres to couple the laser and launch the laser beam into the measurement zone. A unique fibre cooling mechanism was adopted to shield the fibres from excessive temperatures. The filter arrangements and shielding mechanism on the measurement chamber prevent the dust from entering the measurement zone and maintain the stoichiometry of the flue gas during measurement. The probe was tested in a lab setup at 300°C for its sensitivity, response time and linearity. Field-test demonstrates designed performance and repeatability in a coal-fired power plant at an air-preheater (APH) inlet. The gas temperature was below 350°C. In conclusion, this paper demonstrats the useof a QCL based probe for in-situ gas concentration measurement in harsh conditions. [ABSTRACT FROM AUTHOR]

Generation of optical frequency combs (OFC) using semiconductor laser sources has been an active research topic for years, since they offer good performances for many applications without the bulky and expensive aspects of ion doped fiber or crystal based laser sources. In this paper, we demonstrate the generation of low free spectral range mode-locked laser using hybrid III-V/Si3N4 butt coupling. We start by describing the design and the fabrication process of the active R-SOA chip made at our laboratory, and the passive Si3N4 chip having low losses measured to be around 6 dB/m. We then report the integration process and the results of passive mode locking. Our mode-locked laser has a repetition rate of 360 MHz, which is, to the best of our knowledge, the lowest repetition rate achieved for a hybrid integrated mode-locked laser. The frequency comb generated has a 30 dB bandwidth of 2.8 nm (349 GHz). Finally, we show the demonstration of active and harmonic mode locking up to the sixth harmonic. [ABSTRACT FROM AUTHOR]

In this paper, we present the demonstration of a 10-W level picosecond Yb3+-doped master oscillator and power amplifier at 976 nm in all-fiber configuration. A linear-cavity Yb3+-doped fiber laser mode-locked by a semiconductor-saturable-absorber-mirror was developed as the seed laser with a pulse width of 10.9 ps operating at a repetition rate of 34.1 MHz. The average power of the picosecond mode-locked laser was amplified to 8.1 W by two core-pumped pre-amplifiers and two cladding-pumped power amplifiers. The pulse width of the output mode-locked laser was measured to be 15.6 ps and its pulse energy is 237.5 nJ, corresponding to a peak power of 15.2 kW. The polarization extinction ratio and beam quality (M2) of this 10-watt-level mode-locked laser were measured to be 13 dB and 1.2, respectively. [ABSTRACT FROM AUTHOR]

The performance of optical-pulse-coding (OPC) phase-sensitive optical time-domain reflectometry ($\boldsymbol {\Phi }$ -OTDR) has been improved significantly in recent years. However, laser frequency drift is still a crucial issue, as it reduces the peak to side-lobe ratio (PSR) in Golay coding $\boldsymbol {\Phi }$ -OTDR. In this paper, a novel and simple distributed frequency-drift compensation method is proposed. As the phase accumulations of the reference fiber and the sensing fiber are consistent through theoretical analysis and experimental verification, the frequency drift can be compensated by injecting the modulated Golay coding pulses into two fibers at the same time, and subtracting the phase drift of the sensing fiber and the reference fiber. In order to verify the distributed compensation effect, a two-point perturbation measurement experiment is carried out in 9.7 km sensing range. As a result, the far-end signal-to-noise ratio (SNR) is enhanced and the demodulation perturbation SNRs are increased by 2.8 dB and 18.9 dB separately compared with the case without frequency drift compensation. Furthermore, this scheme can also be used in other complex coding systems, providing more possibilities for system designs. [ABSTRACT FROM AUTHOR]

In this paper, we demonstrate a hybrid Laser-LED transmitter module for indoor optical wireless communication with closed-loop, non-mechanical beam steering capability. The hybrid transmitter module consists of a near infrared laser diode for data communication and white LED array for illumination, combined on a diffuser surface. Dual-axis non-mechanical beam steering of the laser beam is implemented using two off-centered liquid lenses. The diffused laser beam directed towards the receiver is steered over an angular range of −7.6o to 7.6o (−1.7o to 2.6o) along the horizontal (vertical) axes spanning −200 to 200 mm (−44 to 67 mm) at the receiver placed 1.5-meter from the transmitter. M-QAM/OFDM in combination with adaptive bit-and power-loading is utilized to achieve a total data throughput of 5.15 Gbps for the diffused laser beam with steering. Laser intensity levels as measured at the receiver plane are kept below the maximum permissible exposure limit for indoor usage across the entire beam steering range. Closed-loop beam steering is also demonstrated by scanning the transmitted laser beam horizontally, measuring the signal strength using a low bandwidth photodetector and locking the laser beam to the receiver position for data-communication. Such hybrid transmitters offer the benefit of decoupling the data communication and illumination requirements of the indoor optical link, thereby tailoring the individual light emitter's performance to specific use-case. [ABSTRACT FROM AUTHOR]

Circuit failure analysis based on optical fault isolation (OFI) techniques is widely used for Si debugging at the early phase of product development or failure analysis to improve yield and reliability issues at mass production stage. Three of OFI techniques are usually used for failure analysis. Dynamic laser stimulation affects $V_{th}$ shift due to thermal heating or electron-hole pair generation in local area depending on the wavelength of the laser used. Laser voltage image and probing provide toggle image of signal and direct waveform differences between good and failure circuits, respectively. This paper introduces the fundamentals of Si level circuit analysis including digital and analog circuit using optical fault isolation method based on theoretical and experimental results. [ABSTRACT FROM AUTHOR]

In the paper, a hybrid Mach-Zehnder interferometer (MZI) and Rayleigh backscattering (RB) injection is designed and applied in the erbium-doped fiber (EDF) ring laser configuration. The MZI scheme and RB-induced signal can accomplish the single-longitudinal-mode (SLM) oscillation and narrow the linewidth to kHz target for each generated wavelength over the available wavelength-tuning bandwidth. In the measurement, the output power, optical signal to noise ratio (OSNR) and wavelength linewidth of the designed fiber laser are also executed and discussed. [ABSTRACT FROM AUTHOR]

Transmission matrix can be used as an elegant tool to represent the light transmission characteristics of a scattering medium. In this paper, we propose a novel common path interferometer technique to measure the transmission matrix utilizing the polarization rotation of incident beam. Specifically, the horizontally polarized and vertically polarized components of the incident beam are used as signal light and reference light, respectively. By rotating the polarization of the incident beam, the ratio between the powers of signal and reference light can be freely adjusted and balanced. The phase shift method is employed to retrieve the transmission matrix of scattering medium. Once the transmission matrix is known, the polarization of the incident light beam can be rotated to switch off the reference and only the signal will remain. Focusing and manipulation of light through a ground glass has been experimentally demonstrated with the measured transmission matrix. An optical focus enhancement reaches 83% of the theoretical maximal value is obtained, which is about 30% improvement compared to the previous co-propagation method. The transmission matrix characterization method has the potential to become a general tool to harness the multiple scattering of light through complex medium. [ABSTRACT FROM AUTHOR]

This paper reports a complete solution for short-acquisition-time, flood-illumination, and non-scanning 3D imaging. Based on an InGaAs/InP 64×64 Geiger-mode avalanche photodiode array, we developed a single-photon imaging Lidar with a field of view of 19.2 mrad. A spatiotemporal dominant imaging method was also applied for depth estimation under high daytime background noise conditions. In the field tests, 3D imaging information was effectively extracted from objects with strong solar background light. An acquisition time of only 0.5 ms to 4 ms was needed for a building target approximately 1300 m away. The experimental results show that the Lidar and its data processing method have the potential for dynamic scenarios that require high performance 3D imaging, such as vehicle navigation, airborne mapping, and moving target monitoring. [ABSTRACT FROM AUTHOR]

Computer-generated hologram (CGH) is a diffractive optical element that has been commonly used as wavefront corrector in the interferometer and can be accurately manufactured to meet the requirements of high-precision detection. But the transmitted wavefront error (TWE) of the CGH substrate and the adjustment errors of cylindrical surface are influence factors of high accuracy measurement. In this paper, we propose the method of calibrating the CGH substrate TWE. After calibrating, the substrate TWE is 0.0052λ (RMS). Meanwhile, we obtain the general expression for the misalignment aberrations by describing the relationship between the wavefront of the measured cylindrical mirror and the corresponding adjustment errors. Then the adjustment errors of the test cylindrical surface can be completely separated by subtracting the fitting function from the measurement on the basis of least-square algorithm. After that, the PV value changes from 3.0565λ to 0.9825λ, and the RMS value changes from 0.5278λ to 0.1273λ, the measurement results have been greatly improved. [ABSTRACT FROM AUTHOR]

A noise-like pulse (NLP) thulium-doped fiber laser based on single-wall carbon nanotubes (SWCNTs) has been proposed and demonstrated in this paper, which the microfiber decorated with SWCNTs work as a saturable absorber to generate an ultrafast pulse. The length of the cavity is 12 m, corresponding to the fundamental frequency of 17.2 MHz. NLP with a duration of 643 fs can be obtained at 1965 nm under the signal-to-noise ratio of 56 dB. This work may assist us to better understand the dynamic characteristic of NLP with the help of materials and offer a fine laser source to the subsequent applications just like the generation of supercontinuum. [ABSTRACT FROM AUTHOR]

The presence of liquid slightly changes the local properties of submerged transmission line. A dedicated sensor is presented in this paper to estimate the level of liquid, through two different determinations: measurement of the overall capacitance of a bi-wires and time of flight estimation of electromagnetic radiation. Since two measurement methods are physically independent, the proposed sensor has an intrinsic redundancy. The level of liquid is measured through phase shift: the signals are sampled at a sufficiently high frequency and then phase differences is evaluated by calculating the coefficients of the digital Fourier transform. Both measurements are performed directly in baseband without requiring frequency conversions. In contrast with typical TDR system where the end of the cable is open or short-circuited, the bi-wires fold and return to the measurement electronics. The characterization of the proposed sensor is determined experimentally for water and diesel fuel, in good agreement with simulation results. As results show, both phase-shifting measurements are linear with the liquid level, providing a resolution in the order of 0.1 mm through a compact and low-cost realization. [ABSTRACT FROM AUTHOR]

This paper investigates a novel method for the in-situ evaluation of the low-frequency magnetic field fluctuation in an atomic comagnetometer (ACM). An in-situ magnetic field measurement model is first established for the hybrid atomic spin ensembles in the ACM based on the simplified Bloch equations. Then, the ACM is transformed into an in-situ magnetometer by employing a large longitudinal magnetic field and a transversal alternating current (AC) magnetic field, respectively. This in-situ magnetometer is sensitive to the magnetic field fluctuation along the detection laser direction according to the established measurement model and can realize the magnetic field measurement sensitivity of 88fT/Hz1/2 at 0.1Hz. Finally, the low-frequency magnetic field fluctuation is evaluated with this in-situ magnetometer, whose influence on the stability of the ACM is also analyzed by the Allan variance method. The experimental results show that the contribution of the magnetic field fluctuation along the detection laser direction to the stability of the ACM is about 0.0068 deg/h, which means that the magnetic field error still has a non-negligible effect on the stability of the system. This new in-situ magnetic field measurement method has the advantages of simple operation and no need to replace the atomic vapor cell, which provides an effective means of the magnetic field error evaluation for the ACM. [ABSTRACT FROM AUTHOR]

Efficient millimeter wave (mmWave) beam selection in vehicle-to-infrastructure (V2I) communication is a crucial yet challenging task due to the narrow mmWave beamwidth and high user mobility. To reduce the search overhead of iterative beam discovery procedures, contextual information from light detection and ranging (LIDAR) sensors mounted on vehicles has been leveraged by data-driven methods to produce useful side information. In this paper, we propose a lightweight neural network (NN) architecture along with the corresponding LIDAR preprocessing, which significantly outperforms previous works. Our solution comprises multiple novelties that improve both the convergence speed and the final accuracy of the model. In particular, we define a novel loss function inspired by the knowledge distillation idea, introduce a curriculum training approach exploiting line-of-sight (LOS)/non-line-of-sight (NLOS) information, and we propose a non-local attention module to improve the performance for the more challenging NLOS cases. Simulation results on benchmark datasets show that, utilizing solely LIDAR data and the receiver position, our NN-based beam selection scheme can achieve 79.9% throughput of an exhaustive beam sweeping approach without any beam search overhead and 95% by searching among as few as 6 beams. In a typical mmWave V2I scenario, our proposed method considerably reduces the beam search time required to achieve a desired throughput, in comparison with the inverse fingerprinting and hierarchical beam selection schemes. [ABSTRACT FROM AUTHOR]

Optical frequency domain reflectometry measurements in internally clad single crystal sapphire fiber have received attention in recent years due to their high temperature distributed sensing potential. As work with these fibers has proceeded, there have been some inconsistencies in the results. Deeper investigation and testing has identified two critical considerations for the proper functioning of these fibers. First, users must address the propagation of multimode light along the outer surface of the fiber. By observing the far-field image of an internally clad sapphire fiber when adding index matching fluid to the outer fiber surface, we demonstrate the effects of removing the higher order modes from these fibers. The addition of index matching fluid resulted in nearly single mode performance where multimode performance was previously observed. Second, users must address the effect that coupling the fiber to the interrogator via silica based fiber has on the internally clad sapphire fiber's performance. Direct fusion splicing of silica to sapphire, as has been used in the recent work with these fibers, has a mode filtering effect which can be beneficial towards the modal behavior of the fibers. However, in this paper we demonstrate that the splicing can cause a sensing failure due to little or no low order mode light, that is useful for sensing, returning to the detector. The positive results from recent years have demonstrated that optical frequency domain reflectometry sensing performance will be successful in clad sapphire fiber; but only when the considerations described herein are addressed properly. [ABSTRACT FROM AUTHOR]

Swept source basedcoherent detection system (CDS) is a common method to measure weak signals in optical sensing and imaging systems. The nature of the phase noise of swept source based CDS however has attracted little attention. In this paper, the statistical properties of the phase noise in a CDS based on Fourier domain mode locked (FDML) laser are investigated theoretical and experimental. The fast spectral evolution of the FDML laser is characterized by using the time-varying phase. The probability density function (PDF) of the measured residual phase jitter (RPJ) is found to be close to the Gaussian distribution. The variance of the RPJ increases quadratically with the optical path difference of the CDS. The PDF of the RPJ and the variation of the variance of the RPJ and the frequency-noise power spectral density are studied numerically using stationary white noise. The experimental results agree well with simulations. The study provides insight into the noise property of FDML laser based CDS and evaluates the temporal coherence of swept lasers. [ABSTRACT FROM AUTHOR]

This paper reports on the investigation of weather influence on a one channel direct time-of-flight (dToF) Lidar operating at 905nm. The weather-dependent performance degradation is continuously monitored throughout one year at the Bosch Research campus in Renningen close to Stuttgart, Germany. The collected data is analyzed in terms of extinction (i.e. range of vision) and a long-term measurement data base is created. We derive the atmospheric extinction by signal reflection from a reference target as well as direct line of sight measurements with transmitter and receiver being separated by a distance of approximately 100m. Using these findings as a baseline, we furthermore show, that the extinction can be estimated by analyzing the direct backscattering occurring at very close ranges in front of the sensor. This method works independently of the cause for the signal degradation (i.e. rain, hail, snow, fog). For fog scenarios, we fit the backscattering signature with established models known from literature and find a good agreement. Overall, the extinction estimation method based on backscattering shows itself very promising and could be a candidate for implementation in a Lidar sensor system for e.g. automated driving scenarios. [ABSTRACT FROM AUTHOR]

The use of Terrestrial Laser Scanning (TLS) data for structural analysis is gaining growing interest. This paper is focused on a new method for detecting local structural changes in masonry walls investigated with TLS technology, which is accurate and capable to obtain dense 3D coordinates of point clouds for the measured objects. The kernel of the method is the definition of a structural change descriptor based on a baseline network. Baselines connect feature points of each brick with its adjacent bricks within one scan. To analyze changes, the baseline vectors in the baseline network of each brick are compared with the unique standard baseline network matrix, computed on those bricks without apparent damage. This paper also describes intermediate steps of the procedure, namely the TLS data acquisition, a coordinate transformation for the point cloud, and the segmentation of bricks and mortar for further extraction of individually labelled bricks. Furthermore, it illustrates the performance of the proposed baseline-based method with a validation experiment at the Stevin laboratory of Delft University of Technology. From the analysis of this experiment, interesting results were obtained: the changes along the X axis are more significant than those along the Z axis, while the changes along the Y axis only happened within the same rows of bricks of the wall. By comparing the results obtained by manually picking the brick centers, the accuracy of the proposed methodology was proved with the maximum difference value of ~4 mm. [ABSTRACT FROM AUTHOR]

The problem of recovering a pair of signals from their blind phaseless short-time Fourier transform measurements arises in several important phase retrieval applications, including ptychography and ultra-short pulse characterization. In this paper, we prove that in order to determine a pair of generic signals uniquely, up to trivial ambiguities, the number of phaseless measurements one needs to collect is, at most, five times the number of parameters required to describe the signals. This result improves significantly upon previous papers, which required the number of measurements to be quadratic in the number of parameters rather than linear. In addition, we consider the simpler problem of recovering a pair of generic signals from their blind short-time Fourier transform, when the phases are known. In this setting, which can be understood as a special case of the blind deconvolution problem, we show that the number of measurements required to determine the two signals, up to trivial ambiguities, equals exactly the number of parameters to be recovered. As a side result, we study the classical phase retrieval problem—that is, recovering a signal from its Fourier magnitudes—when some entries of the signal are known a priori. We derive a bound on the number of required measurements as a function of the size of the set of known entries. Specifically, we show that if most of the signal’s entries are known, then only a few Fourier magnitudes are necessary to determine a signal uniquely. [ABSTRACT FROM AUTHOR]

We investigateon a two-fiber short-reach self-homodyne coherent transmission system without optical amplification,where the same transmission laser is used to generate a modulated signal carrying useful data and a continuous wave signal, which serves as a local oscillator at the receiver side. Target of the work is to determine by experiments and theoretical models under which conditions DFB lasers can be used instead of more expensive ECL lasers. After careful characterization of lasers phase noise in terms of linewidth as a function of the mismatch between the optical paths of the signal and of the local oscillator, the performance of two laser technologies is investigated in the proposed transmission setup, showing that commercial DFB laser can be used, provided that the optical path mismatch between the two fibers is kept below 1.8 m for 28 $GBaud$ PM-QPSK and 0.8 m for PM-16QAM modulation format in combination with a soft-decision forward error correction algorithm. After an experimental demonstration, we theoretically investigate the scalability laws of the proposed systems in different configuration flavours. [ABSTRACT FROM AUTHOR]

A novel flexible capacitive pressure sensor based on hybrid micro-structured (HM) polydimethylsiloxane (PDMS) dielectric layer was developed. The HM-PDMS with M-tooth structured patterns was fabricated using a laser engraved acrylic master mold. The top and bottom electrodes were fabricated by depositing silver (Ag) on flexible polyethylene terephthalate PET) substrate using additive screen-printing process. The pressure sensor was assembled by sandwiching two HM-PDMS based dielectric layers between the top and bottom electrodes. The capability of the developed pressure sensor to detect a wide range of pressure was investigated by applying varying pressure from 0 to 1 kPa, 1 to 10 kPa, and 10 kPa to 40 kPa. A sensitivity of 51.36% kPa−1, 8.37% kPa−1, and 2.46% kPa−1 was obtained for the pressure ranges of 0 to 1 kPa, 1 to 10 kPa, and 10 to 40 kPa, respectively. The capacitive pressure sensor response, hysteresis, and repeatability of the fabricated pressure sensor is investigated and presented in this paper. To demonstrate the wide sensing range of the pressure sensor, the sensor was attached to face mask for breath monitoring applications that require low pressure sensing of ~20 Pa, and to the hand glove for fingertip pressure monitoring applications that require relatively high pressure sensing of ~3 kPa. [ABSTRACT FROM AUTHOR]

Free space optical communication has been applied in many scenarios because of its security, low cost and high rates. In such scenarios, a tracking system is necessary to ensure an acceptable signal power. Free space optical links were considered unable to support fast moving nodes, such as unmanned aerial vehicles, cars or pedestrians, in optical mobile communication because existing tracking schemes fail to track the nodes accurately and rapidly. In this paper, we propose a novel tracking system exploiting multiple beacon lasers. Each beacon laser’s power is measured to estimate the orientation of the target. Unlike existing schemes which drive servo motors multiple times based on consecutive measurements and feedback, our scheme can directly estimate the next optimal targeting shift for the servo motors based on a single measurement, allowing the tracking system to converge much faster. Closed-form outage probability expression is derived for the optical mobile communication system with ideal tracking, where pointing error and moving statistics are considered. To maintain sufficient average power and reduce the outage probability, the recommended beam divergence of the main laser is expressed in closed form as a function of the target’s statistics of random shift, providing insights to the system design. [ABSTRACT FROM AUTHOR]

Size and absolute concentration of suspensions of nanoparticles are important information for the study and development of new materials and products in different industrial applications spanning from biotechnology and pharmaceutics to food preparation and conservation. Laser Transmission Spectroscopy (LTS) is the only methodology able to measure nanoparticle size and concentration by performing a single measurement. In this paper we report on a new variable gain calibration procedure for LTS-based instruments allowing to decrease of an order of magnitude the experimental indetermination of the particle size respect to the conventional LTS based on the double ratio technique. The variable gain calibration procedure makes use of a specifically designed tunable-gain, dual-channel, dual-phase Lock-In Amplifier (LIA) whose input voltage signals are those ones generated by two Si photodiodes that measure the laser beam intensities passing through the sample containing the nanoparticles and a reference optical path. The LTS variable gain calibration procedure has been validated by firstly using a suspension of NIST standard polystyrene nanoparticles even 36 hours after the calibration procedure was accomplished. The paper reports in detail the LIA implementation describing the design methodologies and the electronic circuits. As a case example of the characterization of biological nanostructures, we demonstrate that a single LTS measurement allowed to determine size density distribution of a population of extracellular vesicles extracted from orange juice (25 nm in size) with the presence of their aggregates having a size of 340 nm and a concentration smaller than 3 orders of magnitude. [ABSTRACT FROM AUTHOR]

We study improvements of the Self-Injection Locked (SIL) distributed feedback (DFB) semiconductor laser phase noise as well as integral linewidth on the amount of the SIL feedback and show that SIL reduces the laser noise nearly equally in a broad frequency range, spanning from 1 Hz to 10 MHz. In our experiment SIL operation is supported by a tunable resonant reflector consisting of a high quality factor (high-Q) monolithic whispering gallery mode resonator (WGMR) supplied with a controllable attenuator. Using the experimental setup we validated theoretical formula describing the SIL spectral purity and identified practical parameters of the system allowing reaching the phase noise limit resulting from the fundamental thermodynamic noise of the resonator. We show that the linewidth improvement due to SIL effect is drastically impacted by the frequency spectral dependence of the phase noise of the laser and that it drops significantly in a flickering laser. We also confirmed that the thermorefractive coefficient as well as mode volume of the WGMR imposes the major limitations on the SIL performance. Low intracavity absorption is needed to suppress the attenuation-related thermal effects in the cavity. [ABSTRACT FROM AUTHOR]

In this paper, we propose SCALAR, a calibration method to simultaneously calibrate the kinematic parameters of a 6-DoF robot and the extrinsic parameters of a 2-D laser range finder (LRF) attached to the robot’s flange. The calibration setup requires only a flat plate with two small holes carved on it at a known distance from each other and a sharp tool-tip attached to the robot’s flange. The calibration is formulated as a nonlinear optimization problem where the laser and the tool-tip are used to provide the planar and distance constraints, and the optimization problem is solved using the Levenberg–Marquardt algorithm. We demonstrate through experiments that SCALAR can reduce the mean and the maximum tool position error from 0.44 to 0.19 mm and from 1.41 to 0.50 mm, respectively. Note to Practitioners—Many industrial robotic applications require the robotic system to be calibrated in order to achieve the demanded accuracy. Unfortunately, the existing calibration processes are often cumbersome or require expensive measurement systems. This paper presents a novel calibration method to simultaneously calibrate a 6-DoF industrial robot and a 2-D laser scanner attached to the robot’s flange. The method only uses the data from the robot and the laser, so there is no need for external measurement systems. The laser scanner can also be used after the calibration for subsequent robot tasks such as scanning a workpiece to accurately determine its location. The calibration setup only requires a flat plane and a sharp tool-tip attached on the robot’s flange. The proposed method is easy to deploy and is more cost-effective than existing calibration methods. [ABSTRACT FROM AUTHOR]

In this paper, a new thermally stable ionic-liquid (IL)-type lubricant has been developed. Since heat is applied intensively on the surface of magnetic disks in heat-assisted magnetic recording (HAMR) systems, a thermally stable lubricant is expected to prolong the life of HAMR drives. To verify this concept experimentally, the depletion of ultrathin film lubricants was evaluated for IL- and perfluoropolyether (PFPE)-type lubricants by far-field laser irradiation. For PFPE, lubricant depletion decreased with increasing bond ratio, whereas lubricant depletion for IL did not depend on its bond ratio. The lubricant depletion quantities of ILs were small and comparable to that of PFPEs with the highest bond ratio. [ABSTRACT FROM AUTHOR]

Photovoltaic cells are detrimentally impacted by exposure to ionizing radiation, a consideration of particular significance for applications in outer space. In this paper, we demonstrate that vertical epitaxial heterostructure architecture (VEHSA) laser power converters are particularly resilient toward radiation-induced degradation; in particular, we observed reductions in efficiency of 1.9% and 6.4% for 5- and 20-junction monolithic devices, respectively, at near 3 W of input power ($V_{\textrm {oc}}$ of 5.78 and 23.32 V and peak response near 850 nm). This contrasts markedly with the 16.9% and 25.4% reductions in efficiency for the same 5- and 20-junction devices when using a detuned source at 808 nm, which we attribute to a reduction in nonradiative recombination lifetimes leading to a suppression of radiative recombination driven photon recycling. [ABSTRACT FROM AUTHOR]

Real time surface topography measurement in the paper and paperboard industries is a challenging research field. The existing online techniques measure only a small area of paper surface and estimate topographical irregularities in a narrow scale as a single predictor. Considering the limitations and complications in measuring the surface at high speed, a laser line triangulation technique is explored to measure surface topography in a wide scale. The developed technique is new for the paper and paperboard application that scans a line onto the paper-web surface up to 210 mm in length in the cross machine direction. The combination of a narrow laser linewidth imaging, a subpixel resolution, and the selection of a unique measurement location has made it possible to measure roughness and simultaneously characterize paper surface topography from 0.1 to 30 mm spatial wavelength. This spatial range covers wide scale surface properties such as roughness, cockling, and waviness. The technique clearly distinguishes and characterizes the surface of newspaper, and lightweight coated, coated, and uncoated paperboard in real time during the paper manufacturing process. The system temporal noise for the average roughness is estimated as 37 dB. The signal to noise ratio found is from 5.4 to 8.1 in the short spatial wavelength up to 1 mm, whereas it is more than 75 in the long spatial wavelength from 5 to 10 mm. [ABSTRACT FROM PUBLISHER]

The W-band pulsed traveling-wave tubes (TWTs) are known to play a crucial role in the high-resolution radars due to their wide bandwidth and high output power. This paper presents some experimental results of an ultrawide bandwidth W-band pulsed TWT with folded-waveguide circuits fabricated by ultraviolet lithography, electroplating, and molding. A prototype tube has been tested, showing a gain of over 30 dB from 94 to 110 GHz and saturated output power of over 100 W from 94 to 101.8 GHz, with an electronic efficiency of 2.86%–4.08%. Limited by the drive power, the prototype’s unsaturation output power is over 60 W from 101.8 to 110 GHz. [ABSTRACT FROM AUTHOR]

Factory machine diagnosis is important for preventive maintenance. Diagnosis equipment needs to be reliable and easy to handle, and the diagnostic results should be easily recognizable. An equipment that meets the above conditions is developed; it comprises a laser beam source, leaf spring with a mirror and a magnet as a position-adjustable mass, a copper plate, and a remote switch, if necessary. The principle is based on optical lever and mechanical vibration resonance. The laser spot vibration trajectory is irradiated onto the wall, ceiling, floor of a factory, or the housing of a machine as a screen. An equipment (29 mm $\times35$ mm $\times22$ mm) placed on the vibrating machine irradiates a laser beam and the magnified vibration is displayed on the screen. In this paper, the theoretical model and the equipment are presented, and the measured vibrations for a test vibrator and an actual machine are shown. A peak-to-peak vibration of 0.006 mm measured by a vibrometer is magnified to the 115 mm laser beam spot trajectory. The magnification is 38,333 times (91.6 dB), which is even larger on screens placed farther away. The calculation results by the theoretical model are in good agreement with the measurements. [ABSTRACT FROM AUTHOR]

To study the performance of the glide path scan wind shear alerting algorithm (GLYGA) for wind shear detection with wind LiDAR sensors, this paper constructed two types of simulated wind fields for wind shear detection experiments based on a MATLAB toolbox: uniform wind field and shear wind field. The uniform wind field is used to study the influence of the relative position between the wind LiDAR system and the glide path on the wind detection results; the shear wind field is used to study the effectiveness of the GLYGA. The shear wind field consists of four types: frontal, downburst, low-level jet and turbulence. The results show that both the distance from the wind LiDAR system to the runway and the angle between the laser beam and the glide path should be minimized to ensure better wind detection results while avoiding the scenario in which the laser beam runs perpendicular to the glide path; otherwise, the wind speed measured by LiDAR is 0, which will introduce great error. At the same time, the GLYGA can adequately identify the position and intensity of wind shear along the glide path of aircraft. [ABSTRACT FROM AUTHOR]

This paper presents the integration of an InGaAs avalanchephoto diode (APD)with the firstamplificationstage, comprised of a low capacitance silicon N-channel dual-gate MOS-FET and associated passive components, in the APD case. This combined linear mode optical hybrid detector is used in the field of terrestrial laser range finding. In this case, a reduction of input capacitance by 0.7 pF and an improvement of signal-to-noise ratio (SNR) at lowbackground noise conditions by 32 percent when compared to the equivalent discrete detector was measured. When integrated in a laser range finding system at very strong daylight conditions an improvement in SNR by approximately 10 percent was achieved. Furthermore a significant improvement in immunity to electromagnetic interference (EMI) is shown. Finally, when used in an eye-safe mobile laser range finding system, ranging beyond 10 km with an accuracy of ±2 m on non-cooperative targets using this hybrid detector is demonstrated. [ABSTRACT FROM AUTHOR]

Free-electron lasers are used to generate laser radiation using an accelerated electron beam. In this paper, an electrostatic accelerator was used with a thermionic cathode. Lasing pulses of the order of tens of microseconds were generated by passing the current pulses through a wiggler and resonator combination. The change in lasing power as a result of postsaturation variation of the energy of the electron beam during the lasing period is presented in this paper in the form of experimental measurements and simulations. In the experiments, the initial electron beam energy was ~1.4 MeV and the current was 1.13 A, whilst the lasing frequency was always close to 102 GHz. It is shown that postsaturation, positively ramping the beam energy increases the radiation extraction efficiency by around 50%. Whilst a falling beam energy results in a reduction in extraction efficiency. [ABSTRACT FROM PUBLISHER]

The processing quality of laser cladding is a topic of interest to laser machine manufacturers. The management of various experimental data and process quality of the laser machine can effectively guide the customer to better adjust the processing parameters. This study finds that the processing quality of laser cladding is related to the signal of the coaxial image. Therefore, this study uses a machine learning method to establish a model of coaxial image and laser processing quality. The study does not merely implement a single machine learning method but also compares various machine learning algorithms. Convolutional neural networks and autoencoders are implemented as algorithms for the feature extraction phase. Linear regression, random forest, support vector machine, and SoftMax neural networks are implemented as algorithms for classification. The receiver operating characteristic curve and the accuracy rate are the result indicators of this paper. The experimental results show that there is indeed a correlation between the laser processing quality and the coaxial image, and the algorithm in this study can effectively supervise the processing quality of laser cladding. [ABSTRACT FROM AUTHOR]

This paper presents an enhanced microscopic vision-based approach supporting fast and high-precision measurement on planar motions of a multi-degree of freedom (DOF) compliant stage with sub-micrometer motion accuracy. The proposed method is based on an improved particle swarm optimization (PSO) algorithm, including two phases. First, an inverted pyramid structure is employed to divide the solution space into a top-level solution space, such that the expected solution can be quickly located into a smaller range from bottom to top. Then, the spacial three search step (S-TSS) algorithm is applied to achieve an accurate result. The effectiveness of the proposed method is evaluated on a three-DOF micro stage with translational and rotational motions, where comparative studies with existing results are also conducted. The experimental results demonstrate significant improvement on algorithm efficiency without sacrificing the measurement accuracy, which indicates good potential for visual servo control of such micro stages with the proposed approach. [ABSTRACT FROM AUTHOR]