Adaptive digital Electro-Optical Phase-Locked Loop for frequency modulation linearization of FMCW LiDAR system

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.

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.

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.

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.

<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 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.

Frequency-modulated continuous-wave (FMCW) LiDAR systems are drawing increasing interest due to their potential applications in autonomous driving, machine perception, rapid prototyping, and medical diagnostics. The nonlinearity of a laser’s input-output transfer function can degrade the performance of an FMCW LiDAR. However, traditional discrete-time electro-optical phase-locked loops (DT-EOPLLs) face an unfavorable trade-off between chirp bandwidth and Mach-Zehnder delay. We present an integrated continuous-time electro-optic phase-locked loop (CT-EOPLL) to address this problem. The proposed EOPLL is very wideband, with its loop bandwidth equal to its reference frequency. This feature enables it to relax the trade-off between chirp bandwidth and Mach-Zehnder (MZ) delay by 10× in dB scale, which consequently reduces the area and loss associated with the silicon photonic delay implementation. It also does not suffer from the challenging issue of spurs in wideband PLLs because it features image and harmonic spur suppression in the loop using single-sideband (SSB) and harmonic-reject (HR) mixing techniques. The electrical part of this EOPLL is implemented in 65nm CMOS technology, and its optical integrated circuit is fabricated using a silicon photonic process. Featuring more than 25dB of suppression of the highest spur, this EOPLL is utilized in a high precision LiDAR sensor that shows an RMS depth precision of 558μm at 2m distance, and a 9.4mm RMS depth resolution at ranges exceeding 3.3m.

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.

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.

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.

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.

The realization of a coherent frequency-modulated continuous-wave LIDAR aimed for the accurate measurement of short distances employing a distributed feedback tunable twin-guide laser diode is demonstrated. Theoretical calculations on the ultimate limits in accuracy of distance measurements as determined by the laser phase noise are carried out. A relative accuracy of 8x 10-5 (8 μm) for a single-shot measurement has been achieved at a distance of 10 cm, which is significantly better than expected from the laser linewidth. The results presented here prove the predictions calculated from theory.

Nonmechanical beam-steering devices are of importance to achieve fast, compact, and reliable LiDAR. We propose a 2D nonmechanical beam-steering device based on a virtually imaged phased array (VIPA) for frequency-modulated continuous-wave (FMCW) LiDAR. In the design, 2D nonmechanical beam steering and high-resolution FMCW ranging can be achieved at the same time by wavelength tuning. The design formulas of the VIPA-based 2D disperser are greatly simplified by introducing appropriate approximation, and a feasible design procedure is proposed for the first time, to the best of our knowledge. Based on the proposed method, several design examples with different optimal properties are exhibited.

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.

Frequency modulated continuous wave (FMCW) LiDAR -based 3D imaging has significantly advanced in resolution and frame rate. External modulation provides superior frequency-swept slopes but relies on high-quality RF signals. While Arbitrary Waveform Generators (AWGs) meet performance requirements, their high cost and bulkiness limit practical application. This paper proposes a hybrid optoelectronic system using coherent optical injection into a semiconductor laser. Exploiting the period-one oscillation state enables frequency-tunable microwave generation. To counteract nonlinearities, an adaptive predistortion algorithm was evaluated via numerical simulations. Results show linearity improved by four orders of magnitude, reducing nonlinearity from 10^-3 to 10^-7. This system provides a compact, competitive alternative to expensive digital equipment for LiDAR applications.