Frequency-swept Laser Research Articles

Symmetrical dual-sideband oppositely chirped differential FMCW LiDAR.

A differential FMCW LiDAR for high-precision distance measurements of remote non-stationary targets is proposed and demonstrated experimentally. The required positive and negative symmetrically oppositely chirped laser beams are generated synchronously through a fixed-frequency laser by employing externally unified broadband optical phase modulation and symmetrical dual-sideband optical filtering. After coaxial transmission and reception, orthogonally polarized optical beat signals containing target distance and vector velocity data are de-chirped separately by optical in-phase and quadrature demodulations and then synchronously received by four-channel photoelectric balance detectors. After differential processing of the received beat signals and a fast Fourier transform, it is possible to implement real-time simultaneous range and vector velocity measurements. The inherent symmetrically oppositely chirped optical frequency make it possible to measure the target distance immune to the internal random phase noise introduced by the spectral linewidth of the frequency-swept laser and the external random phase noise introduced by atmospheric turbulence, speckle, and vibration. Meanwhile, the measurement of the target velocity is immune to the nonlinearity of the frequency-swept laser. These results encourage an approach to overcome the barriers of coherence length, nonlinearity, and external noise, and implement simultaneous real-time ranging and velocimetry of long-range, rapid-moving targets.

High-Linear Frequency-Swept Lasers with Data-Driven Control

The frequency-swept laser (FSL) is applied widely in various sensing systems in the scientific and industrial fields, especially in the light detection and ranging (Lidar) area. However, the inherent nonlinearity limits its performance in application systems, especially in the broadband frequency-swept condition. In this work, from the perspective of data-driven control, we adopt the reinforcement learning-based broadband frequency-swept linearization method (RL-FSL) to optimize the control policy and generate the modulation signals. The nonlinearity measurement system and the system simulator are established. Since the powerful learning ability of the reinforcement learning algorithm, the linearization policy is optimized off-line and the generated modulation signals reduce the nonlinearity almost 20 times, compared to the case without control. In the long-term operation, the regular updated modulation signals perform better than the traditional iteration results, demonstrating the efficiency of the proposed data-driven control method in application systems. Therefore, the RL-FSL method has the potential to be the candidate of optical system control.

Precise chirp control with model-based reinforcement learning for broadband frequency-swept laser of LiDAR.

Artificial intelligence (AI) has been widely used in various fields of physics and engineering in recent decades. In this work, we introduce model-based reinforcement learning (MBRL), which is an important branch of machine learning in the AI domain, to the broadband frequency-swept laser control for frequency modulated continuous wave (FMCW) light detection and ranging (LiDAR). With the concern of the direct interaction between the optical system and the MBRL agent, we establish the frequency measurement system model on the basis of the experimental data and the nonlinearity property of the system. In light of the difficulty of this challenging high-dimensional control task, we propose a twin critic network on the basis of the Actor-Critic structure to better learn the complex dynamic characteristics of the frequency-swept process. Furthermore, the proposed MBRL structure would stabilize the optimization process greatly. In the training process of the neural network, we apply a delaying strategy to the policy update and introduce a smoothing regularization strategy to the target policy to further enhance the network stability. With the well-trained control policy, the agent generates the excellent and regularly updated modulation signals to control the laser chirp precisely and an excellent detection resolution is obtained eventually. Our proposed work demonstrates that the integration of data-driven reinforcement learning (RL) and optical system control gives an opportunity to reduce the system complexity and accelerate the investigation and optimization of control systems.

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.

Influence of the linewidth enhancement factor on the signal pattern of Fourier domain mode-locked lasers

Fourier domain mode-locked (FDML) lasers are frequency-swept lasers that operate in the near-infrared region and allow for the attainment of a large sweep-bandwidth, high sweep-rate, and a narrow instantaneous linewidth, all of which are usually quite desirable characteristics for a frequency-swept laser. They are used in various sensing and imaging applications but are most commonly noted for their practical use in optical coherence tomography (OCT). An FDML laser consists of three fundamental components, which are the semiconductor optical amplifier (SOA), optical fiber, and the wavelength-swept optical bandpass filter. Due to the complicated nonlinear dynamics of FDML lasers that stems from the coaction of these three components, often the output signal of an FDML laser is corrupted by frequent power-dips of varying depth and duration. The frequent recurrence of these dips in the FDML laser signal pattern lowers the quality of imaging and detection. This study examines the role of the linewidth enhancement factor (LWEF) of an SOA in reducing both the strength and the number of power-dips throughout the FDML laser operation. The results are obtained using numerical computations that are in agreement with experimental data. The study aims to show that using SOAs with low LWEFs, the number of power-dips can be reduced for a better detection and imaging quality.

Narrow Linewidth External Cavity Laser Capable of High Repetition Frequency Tuning for FMCW LiDAR

A light source with well-rounded performance including the narrow linewidth, wide frequency tuning range, high repetition rate and good linearity, is highly sought-after for frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR). In this work, we report a directly frequency modulated external cavity laser with the linewidth of 5.06 kHz. The frequency tuning range of 1.1 GHz, 445.5 MHz, 228.9 MHz are demonstrated at the repetition rate of 1 kHz, 10 kHz and 100 kHz, respectively. The frequency sweep linearization is achieved by iterative learning pre-distortion. The ranging performance with the repetition rate up to 100 kHz is investigated. The signal-to-noise ratio is still higher than 35 dB while the length of delay fiber is 156 m, corresponding to a target 110 m away in free space. These results indicate that the proposed swept-frequency laser has great potential for autonomous driving applications.

Silicon Photonics FMCW LiDAR Chip With a Slow-Light Grating Beam Scanner

Photonic crystal slow-light gratings fabricated using Si photonics enable high-speed, high-resolution, and wide field-of-view two-dimensional beam scanning via the thermo-optic effect. In this paper, we built a frequency-modulated continuous-wave light detection and ranging system on a chip by combining a beam scanner with Ge photodiodes for delay homodyne coherent detection. Emitting and scanning frequency-swept laser beam, point cloud images of 154 × 32 = 4928 points were obtained. The real-time operation and velocity imaging were also demonstrated. This device is expected to detect Lambertian targets over long distances in the 100-m class by reasonably reducing chip and optics losses and suppressing internal noise components.

Rapid Linear Frequency Swept Frequency-Modulated Continuous Wave Laser Source Using Iterative Pre-Distortion Algorithm

We present a simple iterative pre-distortion algorithm for achieving a rapid linear frequency sweep of semiconductor lasers. The algorithm achieves the desired frequency swept linearity with only four iterations. We derive a general formula for iterative pre-distortion by establishing the relationship between the laser output frequency and the drive current. The linear frequency-swept laser source obtained by this algorithm can be used in FMCW LiDAR systems. Experimentally, we investigated the algorithm using a 1550 nm distributed feedback (DFB) laser, achieving frequency swept excursion of 30.26 GHz, and frequency swept slope of 504 THz/s. We analyzed the linearity of the frequency swept results for the fourth iteration, achieving less than 5 MHz root mean square (RMS) value of frequency swept nonlinearity.

Directly modulated optical negative feedback lasers for long-range FMCW LiDAR.

Frequency sweep operation of directly modulated optical negative feedback lasers is numerically and experimentally investigated for frequency-modulated continuous-wave (FMCW) light detection and ranging (LiDAR) with a high signal-to-noise ratio (SNR), particularly over a long range. Low FM noise corresponding to a spectral linewidth of ∼2.0 kHz is sustained even with injection current modulation of an optical negative feedback laser through optical feedback from a Fabry-Perot etalon, and a beat note spectrum with a 30-dB SNR is achieved even when a 300-m delay fiber is used as a ranging sample. These results encourage an approach to provide directly modulated frequency-swept lasers for long-range FMCW LiDAR.

FMCW lidar multitarget detection based on skeleton tree waveform matching.

Frequency-modulated continuous-wave lidar realizes 4D (three-dimensional space and velocity) imaging of the scene by emitting positive and negative frequency sweep laser signals. The premise of it is to identify the frequency points corresponding to the same target in the positive and negative sweep echo signals. For dechirp receiving, there is usually one peak in the frequency spectrum of the positive and negative sweep signals, respectively. Therefore, it is easy to identify and match the peaks. But in a complex environment, the laser beam will irradiate multiple targets at the same time. In addition, beam scanning and target motion cause the echo spectrum to broaden. The above reasons make it extremely difficult to identify and match peaks in practice. To solve this problem, the waveform-matching algorithm based on the skeleton tree is first applied to multitarget echo pairing. The basic idea of the algorithm is to quantify the target echo hierarchically to generate a skeleton tree. The generation of nodes is based on the relative amplitude of waveform peaks and reflects the characteristics of wave crests nesting. Then the similarity of the signal is determined by comparing the distance between the two signal waveform feature trees. Finally, the waveforms are matched in terms of similarity. To further substantiate the role of the proposed algorithm, imaging experiments and related comparative data for different targets have been completed. The results show that the accuracy of matching processed by the algorithm exceeds 90%, which is improved by about 50% compared with not using the algorithm for the target whose overlapping part accounts for a large proportion of itself.

Frequency-Swept Fiber Laser Based on Fourier-Domain Mode-Locking: A Case Study on Erbium-Doped Fiber Laser

Frequency-Swept Fiber Laser Based on Fourier-Domain Mode-Locking: A Case Study on Erbium-Doped Fiber Laser

基于光学滤波器的扫频光纤激光器研究进展

基于光学滤波器的扫频光纤激光器研究进展

Auxiliary interferometer in an optoelectronic swept-frequency laser and its application to the measurement of the group refractive index.

An optoelectronic swept-frequency laser (SFL) is an optoelectronic feedback system that includes an auxiliary interferometer that can exert precise control over the optical frequency sweep. The arm-length difference (ALD) of the auxiliary interferometer directly affects the performance of the whole system. We established a theoretical model to choose the optimal ALD of an auxiliary interferometer in an optoelectronic SFL system using a frequency-modulated continuous-wave reflectometry experimental setup. The experimental results indicated that, based on our system, the optimal ALD was 7 m, which agreed with the theoretical analysis. As an example application, we implemented the proposed system for measurement of the group refractive index of a glass sample. A minimum measurement error of 0.12% was obtained with the ALD of 7 m.

Microwave Photonic Interrogation of a High-Speed and High-Resolution Temperature Sensor Based on Cascaded Fiber-Optic Sagnac Loops

A high-speed and high-resolution temperature sensor based on two cascaded fiber-optic Sagnac loops (FSLs) interrogated using a microwave photonic (MWP) method is proposed and experimentally demonstrated. In the proposed system, an FSL is used as a sensor element. When it is experiencing temperate change, its spectrum is shifted, which leads to the shift in the spectrum of the cascaded FSLs. Instead of measuring the wavelength shift of the spectrum by using an optical spectrum analyzer (OSA), which has a low wavelength resolution and a slow scanning speed, we convert the optical spectrum to the time domain based on spectral shaping and wavelength-to-time (SS-WTT) mapping, and use a digital signal processor (DSP) to extract the sensing information. In our experiment, two cascaded FSLs with two different free spectrum ranges (FSRs) are employed. Due to the Vernier effect, multiple dips in the envelope of the optical spectrum are introduced. By passing a broadband frequency-chirped optical pulse generated by a frequency-swept laser source to the two cascaded FSLs, the sensing information is encoded in the optical spectrum and it is converted to the time domain due to SS-WTT mapping. By detecting the optical waveform at a photodetector (PD), a temporal microwave waveform with its shape identical to the optical spectrum is generated. By using a DSP, the time shift of a dip is measured and the sensing information is precisely demodulated. Experimental result shows that the proposed fiber-optic temperature sensor can provide a temperature resolution of 1.33 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−5</sup> °C at a sensing speed of 23.497 kHz.

Simultaneous generation of ultra-wideband LFM and phase-coded LFM microwave waveforms based on an improved frequency-sweeping OEO

Simultaneous generation of linear frequency-modulated (LFM) and phase-coded LFM microwave waveform with a large time-bandwidth-product (TBWP) based on an improved frequency-sweeping optoelectronic oscillator (OEO) is proposed and experimentally demonstrated. The key significance of the frequency-sweeping OEO is that a near-zero-dispersion single-mode-fiber (SMF) is introduced into the loop, which can not only avoid the limitation of high frequency oscillation caused by dispersion, but also construct long OEO delay loop to realize large temporal duration. A fiber-Bragg-grating Fabry–Perot (FBG-FP) with large reflection bandwidth is also adopted to construct microwave photonic filter (MPF) with wide tuning range of central frequency. Thus, the limitations on temporal duration and oscillation frequency are solved and a LFM microwave waveform with large TBWP can be generated. Simultaneously, by modulating the optical sideband from the MPF and beating the phase-modulated signal with a portion of frequency-sweeping laser light, a phase-coded LFM microwave waveform can be generated. In the experimental demonstration, a LFM microwave waveform with a chirp rate of 0.736 GHz∕μs and a large TBWP of 173,856 is generated, and reconfiguring central frequency and bandwidth is also completed. Simultaneously, a phase-coded LFM microwave waveform with a bandwidth of 8 GHz and a TBWP of 173,856 is also experimentally demonstrated.

High-Resolution Static Strain Sensor Based on Random Fiber Laser and Beat Frequency Interrogation

We proposed a high-resolution static strain sensor using random fiber lasers (RFLs) and the sweep-beat frequency interrogation technique. An RFL with 3-dB linewidth of 548 Hz and frequency jitter of ~±70 kHz is established by random distributed gratings and a high-precision π-phase shifted fiber Bragg grating (π-FBG) ring mirror. Two RFLs act as probes and are interrogated by frequency-swept laser. One RFL is used for strain sensing, the other is used for compensating temperature and frequency shift of swept laser. By using sweep-beat frequency demodulation, a static strain resolution of 196 pico-strain (pε) is obtained in 160 s. To the best of our knowledge, this is the first time that RFLs have been used in static strain sensing based on sweep beat-frequency interrogation.

Three-Dimensional Object Profiling Using Highly Accurate FMCW Optical Ranging System

We have developed three-dimensional object profiling system using a highly accurate frequency-modulated continuous-wave optical ranging system. A DFB laser and a vertical-cavity surface-emitting laser (VCSEL) are used as the laser source and the optical frequency is swept by the injection current modulation with a symmetric triangular wave. The influence of nonlinearity in the optical frequency sweep is canceled by utilizing $k$ -sampling technique. Since the optical frequency sweep range of a VCSEL is larger than that of a DFB laser, high spatial resolution and high ranging accuracy are achieved by using a VCSEL. The spatial resolution evaluated by the full-width at half-maximum (FWHM) is 460 $\mu$ m and the standard deviation of the ranging accuracy is $\sigma = \text{2.7} \, \mu$ m when a VCSEL is used as the frequency-swept laser source, and the FWHM is 2.3 mm and $\sigma = 14.8 \, \mu$ m when a DFB laser is used as the frequency-swept laser source. Accordingly, fine and clear object profiling of a coin and a printed circuit board is achieved by using a VCSEL as the frequency-swept laser source. Finally, we speed up the measurement time by increasing the repetition frequency of the injection current modulation and optimizing timing of the data acquisition, the galvano mirror scan and the fast Fourier transform analysis by taking account of the transient response of the galvano mirror, and the fastest measurement time of 22.6 s is achieved for $201 \times 201$ measurement points, which is four times faster than our previous results. We also discuss the effect of moving average filtering and median filtering to improve profiling quality.

Coherent imaging using laser feedback interferometry with pulsed-mode terahertz quantum cascade lasers.

We report a coherent terahertz (THz) imaging system that utilises a quantum cascade laser (QCL) operating in pulsed-mode as both the source and detector. The realisation of a short-pulsed THz QCL feedback interferometer permits both high peak powers and improved thermal efficiency, which enables the cryogen-free operation of the system. In this work, we demonstrated pulsed-mode swept-frequency laser feedback interferometry experimentally. Our interferometric detection scheme not only permits the simultaneous creation of both amplitude and phase images, but inherently suppresses unwanted background radiation. We demonstrate that the proposed system utilising microsecond pulses has the potential to achieve 0.25 mega-pixel per second acquisition rates, paving the pathway to video frame rate THz imaging.

Experimental electric field visualisation of multi-mode dynamics in a short cavity swept laser designed for OCT applications.

An experimental study into the modal dynamics of a short cavity, fast frequency-swept laser is presented. This commercially available external cavity swept source is designed for use in optical coherence tomography (OCT) applications and displays a number of dynamic lasing regimes during the course of the wavelength sweep. Interferometric full electric field reconstruction is employed, allowing for measurement of the laser operation in a time-resolved, single-shot manner. Recovery of both the phase and intensity of the laser output across the entire sweep enables direct visualization of the laser instantaneous optical spectrum. The electric field reconstruction technique reveals the presence of multi-mode dynamics, including coherent mode-locked pulses. During the main part of the imaging sweep, the laser is found to operate in a second harmonic sliding frequency mode-locking regime. Examination of the modal evolution of this coherent regime reveals evidence of previously unobserved frequency switching dynamics.

Self-adaptive method for performance enhancement of sweep-velocity-locked lasers

Optical frequency-domain reflectometry and frequency-modulated continuous wave (FMCW)-based sensing technologies, such as LiDAR and distributed fiber sensors, fundamentally rely on the performance of frequency-swept laser sources. Specifically, frequency-sweep linearity, which determines the level of measurement distortion, is of paramount importance. Sweep-velocity-locked semiconductor lasers (SVLLs) controlled via phase-locked loops (PLLs) have been studied for many FMCW applications owing to its simplicity, low cost, and low power consumption. We demonstrate an alternative, self-adaptive laser control system that generates an optimized predistortion curve through PLL iterations. The described self-adaptive algorithm was successfully implemented in a digital circuit. The results show that the phase error of the SVLL improved by around 1 order of magnitude relative to the one without using this method, demonstrating that this self-adaptive algorithm is a viable method of linearizing the output of frequency-swept laser sources.