Frequency-modulated Continuous-wave Lidar Research Articles

3D imaging with large range dynamics and simultaneous accurate speed measurement

In this paper, we describe 3D imaging experiments performed using a frequency modulated continuous wave (FMCW) LIDAR. A detailed study of noise and signal statistics is provided in order to analyze the sensitivity of our FMCW LIDAR, and its capability to image complex scenes over a broad range (up to 3 km) is demonstrated. An image example is also given to illustrate that simultaneous measurement of the speed for each pixel gives additional information that can be used to help segmentation algorithms and target classification. Although 3D imaging using the FMCW system has already been demonstrated, the demonstrated range is generally limited to a few 100 m. In this paper, 3D imaging up to 3 km is achieved while preserving compactness and ruggedness to the various environmental conditions.

Enhancing Signal Recognition Accuracy in Delay-Based Optical Reservoir Computing: A Comparative Analysis of Training Algorithms

To improve the accuracy of signal recognition in delay-based optical reservoir computing (RC) systems, this paper proposes the use of nonlinear algorithms at the output layer to replace traditional linear algorithms for training and testing datasets and apply them to the identification of frequency-modulated continuous wave (FMCW) LiDAR signals. This marks the inaugural use of the system for the identification of FMCW LiDAR signals. We elaborate on the fundamental principles of a delay-based optical RC system using an optical-injected distributed feedback laser (DFB) laser and discriminate four FMCW LiDAR signals through this setup. In the output layer, three distinct training algorithms—namely linear regression, support vector machine (SVM), and random forest—were employed to train the optical reservoir. Upon analyzing the experimental results, it was found that regardless of the size of the dataset, the recognition accuracy of the two nonlinear training algorithms was superior to that of the linear regression algorithm. Among the two nonlinear algorithms, the Random Forest algorithm had a higher recognition accuracy than SVM when the sample size was relatively small.

Development of hybrid photonic integrated wavelength-tunable laser at 2 µm and its application to FMCW LiDAR

This paper reports on the experimental demonstration of a fully integrated frequency-modulated continuous-wave (FMCW) LiDAR sensing system, operating at 2.0 µm. It makes use of a widely tunable hybrid external cavity laser based on the combination of GaSb gain chip and silicon waveguide circuits. The single-frequency laser operation over the full spectral bandwidth of the gain chip is secured using a frequency-selective filter, consisting of two sequential microring resonators in a Vernier configuration. To increase the mode-hop free wavelength tuning range while preserving the linewidth of the laser, the heater of the phase section placed along the bus waveguide is synchronously controlled with two independent heaters placed on each microring resonator. This laser is then implemented for the development of an FMCW LiDAR, consisting of all-optical fiber-based two independent unbalanced Mach-Zehnder interferometers: k-space interferometer for the linearization of continuously swept laser frequency and main interferometer for the measurement of the distributed back-reflection over the distance. The optical frequency of the laser is continuously swept over a ∼100 GHz range (or Δλ=1.47 nm at the operating wavelength) at a modulation speed of 100 Hz. Using this wavelength tunable laser, a light detection and ranging system (LiDAR) is experimentally demonstrated, showing a very high axial resolution of 1.36 mm in air with an extremely high precision of ∼9 µm at a 100 Hz measurement rate.

Enhancing the field-of-view of spectral-scanning FMCW LiDAR by multipass configuration with an echelle grating.

We present a spectral-scanning frequency-modulated continuous wave (FMCW) 3D imaging system capable of producing high-resolution depth maps with an extended field of view (FOV). By employing a multipass configuration with an echelle grating, the system achieves an FOV of 5.5° along the grating axis. The resulting depth maps have a resolution of 70 × 40 pixels, with a depth resolution of 5.1 mm. The system employs an echelle grating for beam steering and leverages the multipass configuration for angular FOV magnification. Quantitative depth measurements and 3D imaging results of a static 3D-printed depth variation target are demonstrated. The proposed approach offers a promising solution for enhancing the FOV of spectral-scanning FMCW LiDAR systems within a limited wavelength-swept range, thereby reducing system complexity and cost, paving the way for improved 3D imaging applications.

Decoupling and Parameter Extraction Methods for Conical Micro-Motion Object Based on FMCW Lidar.

Micro-Doppler time-frequency analysis has been regarded as an important parameter extraction method for conical micro-motion objects. However, the micro-Doppler effect caused by micro-motion can modulate the frequency of lidar echo, leading to coupling between structure and micro-motion parameters. Therefore, it is difficult to extract parameters for micro-motion cones. We propose a new method for parameter extraction by combining the range profile of a micro-motion cone and the micro-Doppler time-frequency spectrum. This method can effectively decouple and accurately extract the structure and the micro-motion parameters of cones. Compared with traditional time-frequency analysis methods, the accuracy of parameter extraction is higher, and the information is richer. Firstly, the range profile of the micro-motion cone was obtained by using an FMCW (Frequency Modulated Continuous Wave) lidar based on simulation. Secondly, quantitative analysis was conducted on the edge features of the range profile and the micro-Doppler time-frequency spectrum. Finally, the parameters of the micro-motion cone were extracted based on the proposed decoupling parameter extraction method. The results show that our method can effectively extract the cone height, the base radius, the precession angle, the spin frequency, and the gravity center height within the range of a lidar LOS (line of sight) angle from 20° to 65°. The average absolute percentage error can reach below 10%. The method proposed in this paper not only enriches the detection information regarding micro-motion cones, but also improves the accuracy of parameter extraction and establishes a foundation for classification and recognition. It provides a new technical approach for laser micro-Doppler detection in accurate recognition.

Highly‐Linear and Wavelength‐Tunable Frequency‐Modulated Continuous‐Wave Hybrid‐Integrated Laser

AbstractFrequency‐sweeping laser sources play a critical role in the accurate 3D mapping of frequency‐modulated continuous‐wave (FMCW) LiDAR systems, whose chirp linearity greatly impacts the measurement precision. Although various pre‐distortion algorithms have been proposed to reduce chirp nonlinearity, these open‐loop methods can rapidly suppress the chirp nonlinearity but they are susceptible to environmental variations and often exhibit subpar performance in long‐range detection scenarios. In contrast, the electro–optical phase‐locked loop (EO‐PLL) serves as a real‐time linearizing solution for FMCW LiDAR, enabling high‐precision, high‐resolution, and long‐distance measurement capabilities. In this study, a III/V‐Si3N4 hybrid‐integrated external cavity laser (ECL) is used to generate an FMCW signal over a large wavelength tuning range of 68 nm. The maximum output optical power is 15.8 mW, and the intrinsic linewidth is 0.9 kHz. By modulating the phase shifter, an FMCW signal with a chirp bandwidth of 0.7 GHz is generated at a repetition rate of 1 kHz. The pre‐distortion algorithm is combined with the gain‐tunable EO‐PLL system to suppress the chirp nonlinearity and frequency noise across multiple wavelengths, leading to a remarkable improvement in range precision from 202 to 4.2 cm when targeting a distance of 200 m.

Wide range linearization calibration method for DFB Laser in FMCW LiDAR

In this paper, a solution based on a hybrid electro-optic phase-locked loop (HEO-PLL) is proposed to address the nonlinearity of the continuous sweep frequency of the light source in Frequency Modulated Continuous Wave (FMCW) LiDAR. This nonlinearity leads to large measurement errors, low accuracy, and poor system adaptability. The proposed solution uses a HEO-PLL to control the frequency sweep linearity of the distributed feedback (DFB) laser when it undergoes continuous, rapid, and wide-range frequency modulation through current tuning. The system is designed to allow the DFB laser to be used as a high-quality source for FMCW LiDAR. This enhances the resolution and ranging capability of the FMCW LiDAR with the DFB laser as the source, while avoiding issues such as difficult tuning of the loop bandwidth and central frequency and being highly susceptible to temperature and voltage variations caused by the charge pump phase-locked loop.

Integrated nonlinearity calibration optical-electrical engine for FMCW LiDAR application.

We demonstrate an integrated optical-electrical calibration module for improving the nonlinearity of the optical source for frequency-modulated continuous-wave (FMCW) LiDAR applications. The linearity of the light source has a considerable influence on FMCW LiDAR range performance, and calibration is typically necessary. However, a majority of existing calibration techniques are based on separate devices, resulting in high cost and limited integration. Our module is made up of a silicon photonic chip with a long optical delay line, a tunable phase shifter, two balanced photodetectors, and some passive components. For this module, we also built the aided amplification and voltage bias circuits. After packaging this module, we used it with our nonlinearity calibration algorithm to analyze the laser's relative nonlinearity. After nonlinearity calibration, the laser relative nonlinearity 1-r2 could be improved to 10-6∼10-7. In the future, the calibration result of nonlinearity could be enhanced further by increasing the length of the on-chip optical delay line.

Dynamic Frequency-Modulated Continuous-Wave LiDAR Coupled Through a Rotary Interface

Dynamic Frequency-Modulated Continuous-Wave LiDAR Coupled Through a Rotary Interface

Massively parallel FMCW lidar with cm range resolution using an electro-optic frequency comb.

Frequency-modulated continuous wave (FMCW) light detection and ranging (lidar) is a promising solution for three-dimensional (3D) imaging and autonomous driving. This technique maps range and velocity measurement to frequency counting via coherent detection. Compared with single-channel FMCW lidar, multi-channel FMCW lidar can greatly improve the measurement rate. A chip-scale soliton micro-comb is currently used in FMCW lidar to enable multi-channel parallel ranging and significantly increase the measurement rate. However, its range resolution is limited due to the soliton comb having only a few-GHz frequency sweep bandwidth. To overcome this limitation, we propose using a cascaded modulator electro-optic (EO) frequency comb for massively parallel FMCW lidar. We demonstrate a 31-channel FMCW lidar with a bulk EO frequency comb and a 19-channel FMCW lidar using an integrated thin-film lithium niobate (TFLN) EO frequency comb. Both systems have a sweep bandwidth of up to 15 GHz for each channel, corresponding to a 1-cm range resolution. We also analyze the limiting factors of the sweep bandwidth in 3D imaging and perform 3D imaging for a specific target. The measurement rate achieved is over 12 megapixels per second, which verifies its feasibility for massively parallel ranging. Our approach has the potential to greatly benefit 3D imaging in fields where high range resolution is required, such as in criminal investigation and precision machining.

Long-range frequency-modulated continuous-wave LiDAR employing wavelength-swept optical frequency comb

We propose a FMCW LiDAR using a wavelength-swept optical frequency comb, which overcomes the ranging limitation caused by the receiver bandwidth. We have established the fundamental part of our proposal and conducted proof-of-concept ranging experiments. With optical fiber transmission for emulating long distance free-space propagation, we have succeeded in ranging corresponding to 1605-m free-space propagation with a 10-MHz FMCW receiver.

Si Photonics FMCW LiDAR Chip with Solid-State Beam Steering by Interleaved Coaxial Optical Phased Array.

LiDAR has attracted increasing attention because of its strong anti-interference ability and high resolution. Traditional LiDAR systems rely on discrete components and face the challenges of high cost, large volume, and complex construction. Photonic integration technology can solve these problems and achieve high integration, compact dimension, and low-cost on-chip LiDAR solutions. A solid-state frequency-modulated continuous-wave LiDAR based on a silicon photonic chip is proposed and demonstrated. Two sets of optical phased array antennas are integrated on an optical chip to form a transmitter-receiver interleaved coaxial all-solid-state coherent optical system which provides high power efficiency, in principle, compared with a coaxial optical system using a 2 × 2 beam splitter. The solid-state scanning on the chip is realized by optical phased array without a mechanical structure. A 32-channel transmitter-receiver interleaved coaxial all-solid-state FMCW LiDAR chip design is demonstrated. The measured beam width is 0.4° × 0.8°, and the grating lobe suppression ratio is 6 dB. Preliminary FMCW ranging of multiple targets scanned by OPA was performed. The photonic integrated chip is fabricated on a CMOS-compatible silicon photonics platform, providing a steady path to the commercialization of low-cost on-chip solid-state FMCW LiDAR.

Noise suppression of FMCW lidar

Aiming at the problem that the ranging accuracy of the frequency-modulated continuous wave (FMCW) lidar decreases in the strong noise environment, experiments are carried out to verify the feasibility of the Faraday anomalous dispersion filter (FADOF) in the FMCW lidar system. The transmitter of the system uses the potassium atom vapor cell to stabilize the frequency and uses the field programmable gate array (FPGA) as the servo system. K-FADOF is used to suppress the additive noise at the receiving end of the lidar system. The results of this experiment show that the use of FADOF can successfully eliminate the interference of noise at the receiver. This experiment verifies the effectiveness of FADOF in suppressing noise in the field of FMCW lidar.

A Fully Solid-State Beam Scanner for FMCW LiDAR Application

We demonstrate a fully solid-state beam scanner that incorporates a Si <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> photonic integrated switch array, a collimator lens, and a lithographically patterned diffraction grating. The off-chip components enable nearly circular output beams under the reduced system complexity. By manipulating the optical switch array and tuning the input wavelength, two-dimensional beam steering with 32 × 23 = 736 resolvable spots across a 14.3°×9.9° field of view (FOV) is realized. Furthermore, the coherent ranging within 18 m is demonstrated experimentally by utilizing the triangular frequency modulation. The tested results validate the feasibility of the proposed beam scanner, which might offer a flexible, scalable, and easily-implemented solution for the solid-state frequency modulated continuous-wave (FMCW) LiDAR.

Real-time high-precision FMCW laser range extraction method based on a hardware multiplier array.

Frequency-modulated continuous wave (FMCW) laser interferometry is an ideal large-scale absolute distance measurement method. It has advantages of high precision and noncooperative target measurement capability, with no blind spot for ranging. To meet the requirements of high-precision, high-speed 3D topography measurement technologies, a faster measurement speed of FMCW LiDAR at each measurement point is required. To solve the shortcomings of the existing technology, a real-time high-precision hardware solution method (including but not limited to FPGA and GPU) for lidar beat frequency signals is provided here based on hardware multiplier arrays to reduce lidar beat frequency signal processing time and to save energy and resource consumption during processing. A high-speed FPGA architecture was also designed for the frequency-modulated continuous wave lidar range extraction algorithm. The whole algorithm was designed and implemented in real time based on the principle of full-pipelines and parallelism. The results show that the processing speed of the FPGA system is faster than that of current top-performing software implementations.

High-Speed Two-Dimensional Spectral-Scanning Coherent LiDAR System Based on Tunable VCSEL

Non-mechanical LiDAR systems have attracted extensive research interests due to their advantages of robustness and high beam-steering speed. Recently, spectral scanning has emerged as a promising solid-state beam-steering method, which combines dispersive elements with frequency-swept laser sources to perform ultrafast beam-steering. However, swept light sources used in LiDAR systems, such as semiconductor lasers and supercontinuum sources, are usually suffering from low sweep rate, bulky and high-cost that severely hinder the development of spectral-scanning LiDAR. Here, we propose a non-mechanical spectral-scanning frequency-modulated continuous-wave (FMCW) LiDAR system based on a simple compact tunable vertical-cavity surface-emitting laser (VCSEL). The fast-swept VCSEL together with wavelength division multiplexing (WDM) technology is employed for high-speed beam-steering. Millimeter-level ranging accuracy and a lateral resolution of 0.286° with an overall depth voxel acquisition rate of 128 kHz are achieved experimentally.

Picking up Speed: Continuous-Time Lidar-Only Odometry Using Doppler Velocity Measurements

Frequency-Modulated Continuous-Wave (FMCW) lidar is a recently emerging technology that additionally enables per-return instantaneous relative radial velocity measurements via the Doppler effect. In this letter, we present the first continuous-time lidar-only odometry algorithm using these Doppler velocity measurements from an FMCW lidar to aid odometry in geometrically degenerate environments. We apply an existing continuous-time framework that efficiently estimates the vehicle trajectory using Gaussian process regression to compensate for motion distortion due to the scanning-while-moving nature of any mechanically actuated lidar (FMCW and non-FMCW). We evaluate our proposed algorithm on several real-world datasets, including publicly available ones and datasets we collected. Our algorithm outperforms the only existing method that also uses Doppler velocity measurements, and we study difficult conditions where including this extra information greatly improves performance. We additionally demonstrate state-of-the-art performance of lidar-only odometry with and without using Doppler velocity measurements in nominal conditions. Code for this project can be found at: <uri xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">https://github.com/utiasASRL/steam_icp</uri> .

High-Resolution Frequency-Modulated Continuous-Wave LiDAR Using Multiple Laser Sources Simultaneously Scanning

We propose a high-resolution frequency-modulated continuous-wave (FMCW) ranging system using multi-source stitching, which can effectively solve the contradiction between detection depth and scanning bandwidth. The system only needs a photodetector to simultaneously detect the measurement signal corresponding to each laser source, and can effectively correct the frequency modulation nonlinearity, the system structure is simplified while reduces the amount of data acquisition and enhances real-time. In this paper, ten tunable laser sources are set up for simulation measurements, which increases the resolution of the system by a factor of 10 and increases the signal-to-noise ratio (SNR). Demonstration experiments are performed with two lasers, the results show that the measurement error at 2.6 m was usually less than 10 μm and the maximum error is 20 μm. In addition, this system is very promising to be combined with silicon photonics to achieve chip-scaled LiDAR, which will enhance its application potential greatly.

Low-pixel-count imaging FMCW lidar.

We demonstrate the imaging capability of a frequency modulated continuous wave (FMCW) lidar based on a fiber bundle. The lidar constructs velocity and range images for hard targets at a rate of 60Hz. The sensing range is up to 30m with 20mW output power. The instrument employs custom electronics with seven parallel heterodyne receivers. An example of image recovery is presented on 6-pixel "pictures" of a spinning disk and a drone hovering in the air. In experiments, we also tested the laser tuning linearity correction with a phase-locked loop. We see the practicality of such a low-pixel-count system as a boost in scanning rate of conventional lidars or for direct target imaging with a further upgrade of pixel count.

Simultaneous measurement of distance and speed via frequency-modulated continuous-wave LIDAR system based on H13C14N gas cell

• Simultaneous and accurate measurement of distance and speed with frequency-modulated continuous-wave laser. • The traditional method of finding the measurement signal's frequency by Fourier transform is replaced. • Accurately fitting the relationship between the phase of the beat signal and the frequency of tunable laser according to the characteristics of the gas absorption cell. • Up and down-scanning frequency rate are obtained by Fabry Perot cavity. • The measurement signal's frequency is obtained by fitting relationship result and frequency rate. Traditional frequency-modulated continuous-wave lidar is to measure the distance of the target through the spectrogram of the beat signal. In this paper, we propose a lidar system to obtain distance and speed at the same time. The distance and speed of the moving target are extracted by product of the delay time obtained by H 13 C 14 N standard gas cell and the fitting frequency modulation rate obtained by Fabry Perot cavity, which can avoid spectral broadening. The experimental results show that the speed resolution of the system was 10µm/s, the mean error of the speed measurement from 0.1mm/s to 100mm/s was less than 162µm/s, and the mean standard deviation was less than 523µm/s. When the speed was less than 10mm/s and the distance was about 4.5m, the mean standard deviation of the distance was less than 163µm. With the advantages of light computation requirements, a simple optical path and measurement results with good repeatability and accuracy, this system has high practical value in precision industrial manufacturing.