A Spurless and Wideband Continuous-Time Electro-Optical Phase Locked Loop (CT-EOPLL) for High Performance LiDAR

An integrated continuous-time electro-optic phase-locked loop (CT-EOPLL) is presented that features image and harmonic spur suppression, and is used in a frequency-modulated continuous-wave (FMCW) LIDAR. The proposed EOPLL has its loop bandwidth equal to its reference frequency, which enables it to relax the trade-off between chirp bandwidth and Mach-Zehnder (MZ) delay and consequently reduce the area and loss associated with the silicon-photonic delay implementation by 10×. Image and harmonic spurs are rejected through single-sideband (SSB) and harmonic-reject (HR) mixing techniques. This EO-PLL is integrated in 65nm CMOS technology, suppresses the highest spur by more than 25dB, and is used in a LIDAR system that can detect an object at ranges exceeding 3.3 meters with an RMS depth precision of 558µm at 2m distance and 9.4mm depth resolution.

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.

We demonstrate a closed-loop Electro-Optical Phase-Locked Loop (EO-PLL) designed to increase the linearity of frequency chirps in Frequency-Modulated Continuous-Wave (FMCW) LiDAR systems, resulting in improved metrological accuracy. The system is fully implemented on an easily accessible FPGA-based digital electronic platform (Red Pitaya STEMlab 125-14). A PC-based UI was developed to facilitate remote real-time control over key system parameters. The proposed closed-loop control reduces the Full-Width Half Maximum (FWHM) in frequency domain to 11 kHz, corresponding to a range resolution of 19.9 mm at a beat frequency of 2.45 MHz. Additionally, the system effectively suppresses disturbances in the modulation signal for frequencies up to 150 kHz, with stabilization settling within a single modulation period. This approach enables the use of lasers with nonlinear modulation slopes in FMCW LiDAR applications, even under fluctuating ambient conditions.

In industrial sites, large-scale advanced manufacturing equipment such as optical diamond cutting lathes require stable operation during processing and are very sensitive to operating speed. However, the vibration generated by the environment or equipment will interfere with its operation and machining accuracy. A precise laser interference measurement system is required to monitor the running speed and position of the three-axis tool of the lathe in real time, providing accurate parameters for the closed-loop control system of the equipment, and improving the stability of lathe operation. Frequency-modulated continuous-wave (FMCW) lidar is widely used in the field of industrial measurement due to its non-contact, high accuracy, and fast dynamic response. The basic principle of FMCW lidars is to measure the velocity of a moving object through the Doppler frequency shift phenomenon. But the vibration generated by the moving object will cause the spectrum to broaden and the precision and repeatability of measurement to decrease. Therefore, we propose a large-bandwidth triangular wave-modulated lidar structure with Fabry-Pérot(F-P) cavity to achieve real-time high-precision measurement of the speed and distance of moving targets. This structure is based on the F–P resonance peaks generated by changing the length of the F-P cavity to accurately split the beat signal generated in the sweep frequency range of 1545-1565nm to obtain 40 sets of data with equal frequency intervals of 62GHz, effectively solve the problem of excessive data volume when measuring the continuous moving target speed and reduces the complexity of the algorithm. Splitting the measured beat signal based on the resonance peaks signal of the F-P cavity, which reduces the phase delay of the beat signal corresponding to the up- and down-scanning, thus reducing the signal spectrum broadening caused by frequency deviation and nonlinear, and increasing the target measurement resolution, accuracy and range. The experimental results show that for speeds of up to 250mm/s, the mean standard deviation was less than 152μm/s, the mean error was less than 183μm/s, the relative error of the mean value of speed measurement does not exceed 0.22%, and the standard deviation of the distance measurement results within a range of 4m under various speed conditions does not exceed 17μm, the error does not exceed 14μm, it has good accuracy and repeatability for the speed and distance measurement. The lidar architecture solution we proposed has important application value for large-scale industrial equipment measurement and operation monitoring.

Frequency-modulated continuous-wave (FMCW) LIDAR is a promising technology for next-generation integrated 3D imaging systems. However, it has been considered difficult to apply FMCW LIDAR for long-distance (> 100m) targets, such as those in automotive and airborne applications. Maintaining coherence between the reflected beam from the target and locally forwarded beam becomes a significant challenge for tunable laser design. This paper demonstrates the possibility of extending the detection range of FMCW LIDAR beyond the coherence range of its laser by improving the spectral estimation algorithm. By exploiting the Lorentzian prior of the received signal in the spectral domain, > 10x improvement in ranging accuracy is achieved compared to traditional algorithms that do not consider phase noise in the signal model. In light of this finding, the end-to-end modeling framework is presented to examine true system-level trade-offs of FMCW LIDAR and the feasibility of long -distance measurement.

Frequency tuning nonlinearities in semiconductor lasers constitute a critical factor that degrades measurement precision and spectral resolution in frequency-modulated continuous-wave (FMCW) LiDAR systems. This study systematically investigates the influence of nonlinear beat signal phase distortions on spectral peak broadening and develops a phase-fitting-based pre-correction algorithm. To further enhance system performance, an electro-optic phase-locked loop architecture combined with non-uniform discrete Fourier transform signal processing is implemented, establishing a comprehensive solution for tuning nonlinearity suppression. Experimental validation demonstrates a sub-18 µm standard deviation in absolute distance measurements at a 19 m target range. This integrated approach represents a significant advancement in coherent frequency-sweep detection methodologies, offering considerable potential for high-precision photonic radar applications.

This work presents a novel scheme for a 4-dimensional (D) frequency modulated continuous wave (FMCW) LiDAR, which demonstrates an unprecedentedly high velocity sensitivity. This is achieved by utilizing the driving waveform to minimize the phase noise originating from the distributed feedback (DFB) laser during the wavelength sweeping process on the transmitting side, while on the receiving end, we use a combination of a self-injection-locked oscillator (SILO) and an avalanche photodiode (APD) with a cascaded multiplication layer, to ensure excellent responsivity and saturation current performance. Comparison is made with a reference LiDAR system comprised of the traditional radio-frequency (RF) oscillator with a p-i-n PD at the receiving-end. The results show that our SILO + APD design provides much better quality 4-D images of slow-moving objects (5 μm/sec) and state-of-the-art velocity sensitivity. This can be attributed to a reduction in the phase noise of the down-converted baseband signals, which is beneficial to the resolution of a small Doppler frequency shift. Furthermore, the incorporation of a high-performance APD also improves the contrast ratio between the object and the background pixels. These improvements offer new possibilities for the development of the next generation of 4-D FMCW LiDARs capable of simultaneously providing the absolute 3-D geometric size and small vibration information.

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.

<div class="section abstract"><div class="htmlview paragraph">In pursuit of safety validation of automated driving functions, efforts are being made to accompany real world test drives by test drives in virtual environments. To be able to transfer highly automated driving functions into a simulation, models of the vehicle’s perception sensors such as lidar, radar and camera are required. In addition to the classic pulsed time-of-flight (ToF) lidars, the growing availability of commercial frequency modulated continuous wave (FMCW) lidars sparks interest in the field of environment perception. This is due to advanced capabilities such as directly measuring the target’s relative radial velocity based on the Doppler effect. In this work, an FMCW lidar sensor simulation model is introduced, which is divided into the components of signal propagation and signal processing. The signal propagation is modeled by a ray tracing approach simulating the interaction of light waves with the environment. For this purpose, an ASAM Open Simulation Interface (OSI) object list referencing virtual 3D objects provides the input for the ray tracer. The divergence of the continuous laser beam is approximated by super-sampling the beam with multiple rays, the calculation of the received power is supported by the future ASAM OpenMATERIAL standard. Subsequently, the output of the ray tracer serves as the input of the signal processing that adapts the so-called Fourier tracing from the field of radar sensor simulation. This approach uses the range and velocity information of the individual rays to estimate the frequency spectrum of the intermediate frequency signal. A subsequent peak detection algorithm determines the output of the model, which is provided in the form of OSI lidar detections. Verification scenarios are tested to check the plausibility of the output and the source code of the signal processing is made available as open source.</div></div>

Frequency-modulated continuous-wave (FMCW) lidar offers high precision and strong interference resistance, capable of synchronously measuring target motion speed and vibration information. However, extracting target information using single-photon signal levels poses a critical challenge in extreme conditions. In this study, we propose a single-photon level FMCW lidar scheme. Quantum compressed sensing (QCS) is employed to simultaneously extract target distance and vibration information. Experimental results demonstrate successful synchronous detection at a beat frequency of 27.304 kHz and a vibration frequency of 500 Hz within an integration time of 0.125 s at a photon counting rate of 9 kcps. This approach provides a new, to the best of our knowledge, solution for FMCW radar application in extreme environments or long-range scenarios.

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.

A 24 GHz self-calibrated all-digital frequency-modulated continuous-wave (FMCW) synthesizer is presented in this article. A multi-bank digitally controlled oscillator (DCO) is employed to provide adequate frequency tuning range and high frequency resolution. A full background self-calibration scheme is proposed to linearize the DCO tuning curve for highly linear and fast chirp generation. An overlap compensator based on an adaptive lookup table (LUT) is utilized to eliminate the frequency overlaps between adjacent DCO tuning bands. The compensation words in the LUT are altered by least-mean-square (LMS) algorithm for robust and precise matching. A ramp tracker, which features real-time ramp tracking with zero steady-state phase error, is employed to estimate and stabilize the frequency ramp. Implemented in 40 nm bulk CMOS, the FMCW synthesizer prototype consumes 28 mW with a 0.26 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> core area. The maximum chirp bandwidth is 3.2 GHz, corresponding to 13.33% of the center frequency. Measurement results show that the root-mean-square (RMS) frequency error is reduced by 1/10 with self-calibration after 2 ms convergence time. The maximum chirp slope is up to 320 MHz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> with 309 kHz RMS frequency error. The minimum RMS frequency error is 7.35 kHz under 5 MHz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\mu \text{s}$ </tex-math></inline-formula> chirp slope, corresponding to 0.0002% of the chirp bandwidth.

The first 77 GHz frequency modulated continuous wave (FMCW) radar transceiver IC with an accurate FMCW chirp signal generator using a 90 nm CMOS process is presented. To realize accurate FMCW radar system in CMOS, a PLL synthesizer based FMCW generator with chirp smoothing technique that is able to output linear FMCW frequency chirp using a nonlinear reference chirp signal supplied from a low spec/cost digital-oriented frequency reference is applied. The fabricated IC consists of an LNA, a down-conversion mixer with an output buffer, a driver amplifier, a power amplifier with power combiner, an LO distributer and an FMCW synthesizer. The measured FMCW signal from the proposed FMCW generator achieves 93 kHz frequency error (nonlinearity) at the 77 GHz chip signal. Radar performance of the IC has less than 1% ranging error from 1 m to 8 m distance from the measurement in a laboratory. These results show the transceiver achieves a fundamental function for radar applications with 520 mW power consumptions.

We present a high SNR signal processing system for coherent solid-state LIDAR. A receiving frequency processing system is initially developed. In this LIDAR system, a frequency-modulated continuous wave (FMCW) laser is used as the transmitter, and balanced detectors array based on silicon photonic technology is used as the laser echo receiver. The receiving processing system includes multi-channel low-noise transimpedance amplifier, band-pass filter, high resolution ADC and output buffers. Based on the signal processing system, frequency signal processing with a high signal-to-noise ratio is realized, and the distance detection is realized to confirm on-chip balanced-photodetector-based coherent ranging. The system can be integrated by CMOS technology in the future and realizing three-dimensional integration through through-silicon-via (TSV) with the silicon photonic chip to get low integration complexity, low power consumption, low optical loss, and large array integration.

The calibration and traceability of high-resolution frequency modulated continuous wave (FMCW) ladar sources is a requirement for their use in length and volume metrology. We report the calibration of FMCW ladar length measurement systems by use of spectroscopy of molecular frequency references HCN (C-band) or CO (L-band) to calibrate the chirp rate of the FMCW sources. Propagating the stated uncertainties from the molecular calibrations provided by NIST and measurement errors provide an estimated uncertainty of a few ppm for the FMCW system. As a test of this calibration, a displacement measurement interferometer with a laser wavelength close to that of our FMCW system was built to make comparisons of the relative precision and accuracy. The comparisons performed show <10 ppm agreement, which was within the combined estimated uncertainties of the FMCW system and interferometer.