Pulsed Time-of-flight Research Articles

Modelling and Analysis of Vector and Vector Vortex Beams Reflection for Optical Sensing

Light Detection and Ranging (LiDAR) sensors can precisely determine object distances using the pulsed time of flight (TOF) or amplitude-modulated continuous wave (AMCW) TOF methods and velocity using the frequency-modulated continuous wave (FMCW) approach. In this paper, we focus on modelling and analysing the reflection of vector beams (VBs) and vector vortex beams (VVBs) for optical sensing in LiDAR applications. Unlike traditional TOF and FMCW methods, this novel approach uses VBs and VVBs as detection signals to measure the orientation of reflecting surfaces. A key component of this sensing scheme is understanding the relationship between the characteristics of the reflected optical fields and the orientation of the reflecting surface. To this end, we develop a computational model for the reflection of VBs and VVBs. This model allows us to investigate critical aspects of the reflected field, such as intensity distribution, intensity centroid offset, reflectance, and the variation of the intensity range measured along the azimuthal direction. By thoroughly analysing these characteristics, we aim to enhance the functionality of LiDAR sensors in detecting the orientation of reflecting surfaces.

A Progress Review on Solid‐State LiDAR and Nanophotonics‐Based LiDAR Sensors

AbstractLight detection and ranging (LiDAR) sensors enable precision sensing of an object in 3D. LiDAR technology is widely used in metrology, environment monitoring, archaeology, and robotics. It also shows high potential to be applied in autonomous driving. In traditional LiDAR sensors, mechanical rotator is used for optical beam scanning, which brings about limitations on their reliability, size, and cost. These limitations can be overcome by a more compact solid‐state solution. Solid‐state LiDAR sensors are commonly categorized into the following three types: flash‐based LiDAR, microelectromechanical system (MEMS)‐based LiDAR, and optical phased array (OPA)‐based LiDAR. Furthermore, advanced optics technology enables novel nanophotonics‐based devices with high potential and superior advantages to be utilized in a LiDAR sensor. In this review, LiDAR sensor principles are introduced, including three commonly used sensing schemes: pulsed time of flight (TOF), amplitude‐modulated continuous wave TOF, and frequency‐modulated continuous wave. Recent advances in conventional solid‐state LiDAR sensors are summarized and presented, including flash‐based LiDAR, MEMS‐based LiDAR, and OPA‐based LiDAR. The recent progress on emerging nanophotonics‐based LiDAR sensors is also covered. A summary is made and the future outlook on advanced LiDAR sensors is provided.

Investigation of transport properties of perovskite single crystals by pulsed and DC bias transient current technique

Time-of-flight (ToF) transient current method is an important technique to study the transport characteristics of semiconductors. Here, both the direct current (DC) and pulsed bias ToF transient current method are employed to investigate the transport properties and electric field distribution inside the MAPbI3 single crystal detector. Owing to the almost homogeneous electric field built inside the detector during pulsed bias ToF measurement, the free hole mobility can be directly calculated to be about 22 cm2⋅V−1⋅s−1, and the hole lifetime is around 6.5 μs–17.5 μs. Hence, the mobility-lifetime product can be derived to be 1.4 × 10−4 cm2⋅V−1–3.9 × 10−4 cm2⋅V−1. The transit time measured under the DC bias deviates with increasing voltage compared with that under the pulsed bias, which arises mainly from the inhomogeneous electric field distribution inside the perovskite. The positive space charge density can then be deduced to increase from 3.1 × 1010 cm−3 to 6.89 × 1010 cm−3 in a bias range of 50 V–150 V. The ToF measurement can provide us with a facile way to accurately measure the transport properties of the perovskite single crystals, and is also helpful in obtaining a rough picture of the internal electric field distribution.

Temporal and Spatial Focusing in SPAD-Based Solid-State Pulsed Time-of-Flight Laser Range Imaging

The relation between signal and background noise strengths in single-photon avalanche diode (SPAD)-based pulsed time-of-flight 3-D range imaging is analyzed on the assumption that the SPAD detector is operating in the single photon detection mode. Several practical measurement cases using a 256-pixel solid-state pulsed time-of-flight (TOF) line profiler are presented and analyzed in the light of the resulting analysis. It is shown that in this case it is advantageous to concentrate the available optical average power in short, intensive pulses and to focus the optical energy in spatial terms. In 3-D range imaging, this could be achieved by using block-based illumination instead of the regularly used flood illumination. One modification of this approach could be a source that would illuminate the system FOV only in narrow laser stripes. It is shown that a 256-pixel SPAD-based pulsed TOF line profiler following these design principles can achieve a measurement range of 5–10 m to non-cooperative targets at a rate of ~10 lines/s under bright sunlight conditions using an average optical power of only 260 µW.

Optimization and High-performance Hardware Implementation of Multipath Interference Compensation Algorithm

Our previous sparse regularization method for multipath interference (MPI) compensation in pulsed Time-of-Flight (ToF) depth camera works well on desktop computer. However, pulsed ToF depth cameras are mainly applied in fields such as autopilot vehicles and robots. In these situations, exporting depth images to a computer and running MPI compensation in MATLAB every time are not realistic. Therefore, according to the hardware structure of the multipath interference real-time compensation platform, we use QR decomposition to optimize the sensing matrix in our algorithm for better hardware implementation, while maintaining the accuracy of our algorithm. For the optimized algorithm, we designed an efficient parallel data processing hardware structure that matches the features of our algorithm to optimize the calculation efficiency. Then, we implemented it on the realtime compensation platform. The experimental results show that this algorithm architecture runs stably under the clock frequency and it runs faster than desktop computer.

Pulsed light time-of-flight measurement based on a differential hysteresis timing discrimination method.

In the pulsed light time-of-flight (ToF) measurement, the timing point generated in the receiver channel is very important to the measurement accuracy. Therefore, a differential hysteresis timing discrimination method is proposed to generate timing points of the receiver channel. This method is based on utilizing the unbalanced characteristics of the fully differential operational amplifier circuit as well as introducing extra hysteresis levels to achieve the stable generation of timing points. With this method, fewer circuit components are consumed and the dynamic range of the receiver channel is not limited by its linear range. The experiments demonstrate that a receiver channel applying the proposed discrimination reaches better single shot accuracy compared to that using leading-edge timing discrimination. This method is also suitable for the timing walk error compensation by means of pulse width. Finally, these results verify the effectiveness of the proposed method in pulsed light ToF measurement.

A CMOS Peak Detect and Hold Circuit With Auto-Adjust Charging Current for NS-Scale Pulse ToF Lidar Application

This paper presents a novel CMOS peak detect and hold (PDH) circuit scheme for pulsed time of flight (ToF) Lidar application. The proposed PDH circuit, which is one part of analog front end (AFE) circuit of Lidar receiver, is used to widen the narrow input pulse width, aiming to easily digitize the pulse amplitude through a low-speed and low-cost ADC in pulsed ToF Lidar application. The reset voltage clamped to the common-mode level of the input pulse voltage is beneficial to reduce the pedestal error voltage. Meanwhile, the auto-adjust charging current scheme is employed to decrease the peak error through rejecting the overshoot voltage in the proposed PDH circuit. The circuit was implemented and fabricated in a 65-nm CMOS technology. The proposed PDH circuit can detect the pulse voltage with a pulse amplitude range from ~20 mV to ~500 mV and a minimum pulse width of 5 ns. The measured results show that the maximum absolute and relative errors are less than 16 mV and 4.5%, respectively. The layout area of the proposed PDH circuit is equal to 0.17 × 0.14 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Highly non-linear solitary wave imaging method for detecting cylindrical defects in the metal plate of an adhesive composite metal structure

In this paper, a non-destructive testing (NDT) method based on highly non-linear solitary waves (HNSWs) was employed to detect defects in the metal plate of an adhesive composite metal structure. Firstly, HNSWs were generated by applying impulses to a one-dimensional granular chain of steel spheres. Proof-of-concept finite element (FE) simulations were then performed using Abaqus software to investigate the reflection behaviour of HNSWs at the interface of the chain and the two-layer material with defects. The time-of-flight (TOF) of primary reflected solitary waves (PSWs) was confirmed as a good indicator for defect localisation and sizing. A HNSW generator and detector device was used in the experiment and HNSW B-scan and C-scan inspections of adhesive composite steel specimens were achieved. The TOF imaging results accurately indicated the locations of cylindrical defects in the bottom steel block. The linear correlation between the peak value of the pulsed TOF profile and the defect diameter was determined. The contours of TOF with different values were introduced to reconstruct the shape of the circular defective region. Through selecting the correct threshold of TOF, the relative error of the area surrounded by the contour compared to the actual area of the five investigated defects could be less than 6.5%.

A high-performance CMOS FDMA for pulsed TOF imaging LADAR system

This paper presents a high bandwidth and low noise fully differential main amplifier (FDMA) for pulsed time-of-flight (TOF) imaging laser detection and ranging application (LADAR), which serves to amplify the small pulse echo signal. The cascaded architecture and active inductor technology are used to enlarge the bandwidth of the circuit and reduce the chip area. The cascaded gain stages, which adopted DC offset isolation circuit, are more robust to the alteration of process. A large bandwidth amplifier (LBA) and an output buffer (OB) structure have been designed to enhance the drive capabilities. Besides, in order to adapt the demand of the LADAR system, the amplifier receiver's bandwidth has been limited by using an inter-stage bandpass filter. Implemented in CSMC CMOS technology, the FDMA chip realizes the -3 dB bandwidth of 730.6 MHz, and an open loop gain of 23.5 dB with the bandpass filter worked. The input-referred noise voltage is 2.7 nV/sqrt(Hz), which effectively reduces the system noise. This chip that occupies 0.25 mmc×0.25 mm in area consumes a power dissipation of 102.3 mW from the 3.3 V power supply. As a part of the integrated chip of the laser radar system, it can better meet the requirements of system.

A CMOS Receiver–TDC Chip Set for Accurate Pulsed TOF Laser Ranging

An integrated receiver-time-to-digital converter (TDC) chip set is developed for pulsed time-of-flight (TOF) laser rangefinding. The receiver detects the current pulse from the optical detector and produces a timing mark for the TDC. The receiver uses time mode walk error compensation scheme achieving <; ±2.5-mm residual timing-walk error within a dynamic range of ~1:40 000. The multichannel TDC measures the time position, width, and rise time of the echo pulses simultaneously with ~10-ps precision. Both chips are manufactured in 0.35-μm complementary metal-oxide-semiconductor (CMOS) technology. The functionality of the chip set was demonstrated in a laser radar platform using 12 W and 3-ns optical pulses produced by a laser diode (LD). Measurement accuracy of ~ ±3 mm was achieved with noncooperative targets at a distance range of a few tens of meters within an amplitude range of received echoes of 1:40 000.

The Design of Clocked-Comparator-Based Time-Interval Measurement Circuit for Pulse ToF Measurement

A clocked-comparator-based time-interval measurement approach is proposed for the measurement of the time-of-flight (ToF) of wave pulses in a threshold-crossing-detection-based pulse ToF range-finding application. As compared to the conventional TDC-based pulse ToF measurement approach, the proposed approach does not require the use of extra channels in the case where multiple echoes can be expected for every wave pulse launched. As compared to the conventional (flash) analog-to-digital converter-based pulse ToF measurement approach, the proposed approach allows for a simpler backend digital processing and does not require the comparators used in the measurement to be skew-less. Meanwhile, the approach leverages on the strength of a clocked comparator circuit, which can be designed with a very wide sampling bandwidth. A clocked-comparator-based time-interval measurement circuit with a ~7-GHz bandwidth was assembled using low cost commercial-of-the-shelf components. The measurements done using the measurement circuit agree to within 10 ps with the same measurements done using a 13-GHz, 40-GSa/s oscilloscope and are precise to within ±15 ps in terms of their $2\sigma $ range.

Recovering temporal PSF using ToF camera with delayed light emission

Recovering temporal point spread functions (PSFs) is important for various applications, especially analyzing light transport. Some methods that use amplitude-modulated continuous wave time-of-flight (ToF) cameras are proposed to recover temporal PSFs, where the resolution is several nanoseconds. Contrarily, we show in this paper that sub-nanosecond resolution can be achieved using pulsed ToF cameras and an additional circuit. A circuit is inserted before the illumination so that the emission delay can be controlled by sub-nanoseconds. From the observations of various delay settings, we recover temporal PSFs of the sub-nanosecond resolution. We confirm the effectiveness of our method via real-world experiments.

A compressive sensing algorithm using truncated SVD for three-dimensional laser imaging of space-continuous targets

For traditional array 3-D laser radars, the resolution of the intensity image and range profile is limited by the number and accuracy of sensors. Moreover, for a space-continuous target, peak detection in the pulsed time of flight is no longer suitable for super-resolution reconstruction algorithms. Hence, a compressive sensing algorithm for 3-D laser imaging is proposed. A range observation matrix composed of time interval basis vectors is constructed to acquire the range information regarding a target. However, the range observation matrix is generally ill-posed owing to the spatial continuity of the target. To address this shortage, truncated singular value decomposition is utilized to extract the peak values of echo pulses for image reconstruction. Simulation results demonstrate the effectiveness and performance of the proposed algorithm.

Bayesian Time-of-Flight for Realtime Shape, Illumination and Albedo.

We propose a computational model for shape, illumination and albedo inference in a pulsed time-of-flight (TOF) camera. In contrast to TOF cameras based on phase modulation, our camera enables general exposure profiles. This results in added flexibility and requires novel computational approaches. To address this challenge we propose a generative probabilistic model that accurately relates latent imaging conditions to observed camera responses. While principled, realtime inference in the model turns out to be infeasible, and we propose to employ efficient non-parametric regression trees to approximate the model outputs. As a result we are able to provide, for each pixel, at video frame rate, estimates and uncertainty for depth, effective albedo, and ambient light intensity . These results we present are state-of-the-art in depth imaging. The flexibility of our approach allows us to easily enrich our generative model. We demonstrate this by extending the original single-path model to a two-path model, capable of describing some multipath effects. The new model is seamlessly integrated in the system at no additional computational cost. Our work also addresses the important question of optimal exposure design in pulsed TOF systems. Finally, for benchmark purposes and to obtain realistic empirical priors of multipath and insights into this phenomena, we propose a physically accurate simulation of multipath phenomena.

Multi-resolution model correction for improving the accuracy of flying laser ranging sensor

More and more unexpected incidents come out currently. Static sensor networks can not satisfy the increasing need for emergency response due to lack of adaptivity and limited coverage, thus mobile sensor networks become the research focus for overcoming above drawbacks. With excellent performances including autonomy, flexibility, stability and low-cost, micro-unmanned air vehicles (MUAVs) play a greater pole in mobile sensor networks. Ranging is one of the most critical sensing functions for mobile sensor networks. In this paper, a flying laser ranging sensor is proposed, essentially, an airborne pulsed time-of-flight (TOF) laser rangefinder (LRF) is realized. Because of the influence from walk error, time jitter, temperature drift and other factors, there is still room to improve the measurement precision of pulsed TOF LRFs. So the more important contribution of the paper is to present a novel interval time amendment approach for higher ranging precision. Based on leading-edge time discrimination, this method first employs wavelet multi-resolution analysis to compute compensation time values, which are computed mainly from local modulus maxima (LMMs) position differences. With approximations signals similarity and measurement relative error (MRE), a fusion evaluation is built to select the best wavelet parameters, and optimal corrected measurement model is obtained. Results show that MRE of the improved model is reduced by 73.41%. Therefore, the performance of flying laser ranging sensor is improved significantly with multi-resolution model correction.

A continuous wavelet transform-based modulus maxima approach for the walk error compensation of pulsed time-of-flight laser rangefinders

Semiconductor pulsed time-of-flight (TOF) laser rangefinders (LRFs) have been the main distance measurement components equipped by many radar systems of Micro Unmanned Air Vehicles (MUAVs). A novel approach is proposed to improve the measurement accuracy of pulsed TOF LRFs. This approach adopted traditional single-threshold leading edge detection firstly, then employed continuous wavelet transform (CWT)-based local modulus maxima (LMMs) to detect singularities of emitting pulses and receiving echoes of LRFs, and obtained the compensation time with detected singularities to revise the measurement model. A series of experiments were done in a proposed pulsed TOF LRF prototype, and the measurement model is built by linear fitting. With the compensation values of various wavelets, the optimum revised measurement model is obtained. It is demonstrated that the single-shot precision of the pulsed TOF LRF achieves 195ps (∼29mm) with SNR=7, and the absolute error is reduced from 9.25 to 6.32mm. Therefore, the proposed approach can better compensate the walk error of the existing pulsed TOF LRFs and improve the performance of TOF LRFs.

A Direct-Sampling Pulsed Time-of-Flight Radar With Frequency-Defined Vernier Digital-to-Time Converter in 65 nm CMOS

This paper presents a direct-sampling pulsed radar with a high-resolution digital-to-time converter (DTC) for estimating the time of flight (TOF), which is to identify the distance between a target and radar. The implemented direct-sampling radar can reconstruct the scanning waveforms in digital domain. The link budget of the radar transceivers is analyzed for the overall scanning range. The scanning range of the radar is dependent on the TOF between radar transmitter and receiver. The range resolution of the pulsed TOF radar is determined by DTC. With the help of exquisite DTC, a high resolution radar can be achieved. The vernier concept has been adopted to achieve an accurate timing resolution design in the DTC. The vernier time steps are defined by the oscillating frequency of the phase-locked loops (PLL), and therefore the DTC with a high resolution in the order of picosecond and with high immunity to PVT variation was developed and demonstrated. The proposed radar was fabricated using 65 nm CMOS technology and occupies a chip area of 2 mm $^{2}$ , consumes 88.4 mW of DC power. The receiver has a 10 GHz instantaneous front-end bandwidth for capturing all scattering reflected waveforms and a 666 GS/s equivalent sampling rate for recording all received signals for subsequent digital signal processing (DSP) analysis.

On Laser Ranging Based on High-Speed/Energy Laser Diode Pulses and Single-Photon Detection Techniques

This paper discusses the construction principles and performance of a pulsed time-of-flight (TOF) laser radar based on high-speed (FWHM $\sim$ 100 ps) and high-energy ( $\sim$ 1 nJ) optical transmitter pulses produced with a specific laser diode working in an “enhanced gain-switching” regime and based on single-photon detection in the receiver. It is shown by analysis and experiments that single-shot precision at the level of 2…3 cm is achievable. The effective measurement rate can exceed 10 kHz to a noncooperative target (20% reflectivity) at a distance of $> 50 \hbox{m}$ , with an effective receiver aperture size of $2.5 \hbox{cm}^{2} $ . The effect of background illumination is analyzed. It is shown that the gating of the SPAD detector is an effective means to avoid the blocking of the receiver in a high-level background illumination case. A brief comparison with pulsed TOF laser radars employing linear detection techniques is also made.

ALR—a laser altimeter for the first Brazilian deep space mission. Modeling and simulation of the instrument and its operation

Among the scientific instruments that will fly on the upcoming mission ASTER, the first Brazilian deep space mission, the laser altimeter will play an important role in the surveys regarding shapes, topographies and masses of the asteroids of the triple system. The development of the instrument in partnership among universities (UNICAMP, UFABC) and companies of the Brazilian aerospace sector is in progress. This paper describes and presents the set of studies that were conducted aiming at the creation of software to embark on the control and signal processing unit of the instrument, with respect to the modeling and simulation of its operation and regarding the emission of laser pulses along with the detection and processing of return pulses. As the main result, a package of computer programs was created to simulate the operation of a pulsed laser altimeter with principle of operation based on the measurement of the pulse time of flight. The simulator software was named ALR_Sim. The results obtained with the ALR_Sim helped to better understand some key features and parameters of the operation of the future instrument, such as the sampling time of the detected signal and the definition of the type of detector suitable for the expected intensity of the return signal. The simulator software was successfully tested with respect to the most common expected situations.

Recent advances in dental optics – Part II: Experimental tests for a new intraoral scanner

The object of this paper is testing the performance of a new device for 3D oral scanning: a two channel PTOF (pulsed time-of-flight) laser scanner, designed for dental and industrial applications in the measurement range of zero to a few centimetres. The application on short distances (0–10cm) has entailed the improvement of performance parameters such as single shot precision, average precision and walk error up to mm-level and to µm-level respectively.The single-shot precision (σ-value) has resulted to range from 43 to 63ps (9–10mm), having considered the measurement range (6.5–10mm) corresponding to 1–2V signal; this result agrees well with estimates made from simulations. The average precision has resulted to be dependent on the number of measurements and can reach a value equal to ±25µm, whenever the measurements frequency is sufficiently high. For example, if the required scanning speed is 1000 points/s and the required average precision is ±25µm, then a pulses frequency of 30–50MHz is needed, considering signal amplitude varying between 1–2V.On the whole, the performance of this new device, based on PTOF has proven to be adequate to its employment in the field of restorative dentistry.