Lidar Transmitter Research Articles

Experimental studies of the characteristics of a resonant leader emitter with a single-pass amplifier

The results of experimental studies of the energy and spatial characteristics of the lidar transmitter, built according to the generator-amplifier scheme based on an organic dye with tube pumping, are presented. The choice of a single-pass traveling wave amplifier used in the experiments was determined by preserving the spectral purity of the radiation. When constructing a lidar emitter according to the generator-amplifier scheme under conditions of constant pumping density, the problem arises of choosing the ratio between the length of the active element of the generator and the length of the active medium of the traveling wave amplifier, which would ensure the maximum efficiency of the entire emitter. The main task of the work was the experimental verification of the results of theoretical studies of the narrow-band emitter of the resonant lidar in order to determine the factors affecting the choice of the ratio of the lengths of the active elements of the generator and the amplifier based on the organic dye rhodamine 6Zh with tube pumping with limited total length. The amount of effective radiated energy was used as the main criterion for evaluating the efficiency of the generator-amplifier system. The experimental results confirm the theoretical conclusions that there are optimal ratios of the lengths of the generator and the amplifier, at which the effective radiation energy is maximal. The limiting length of the amplifier and the energy of the emitter built according to the generator-amplifier circuit are limited by the increase in the intensity of the radiation amplified along the active element, as well as the increase in the intensity of the amplified noise.

Self-homodyne coherent Lidar system for range and velocity detection.

A high resolution FMCW Lidar system based on a phase-diverse self-homodyne coherent receiver is demonstrated. Using the same linearly chirped waveform for both the transmitted lidar signal and the local oscillator, the self-homodyne coherent receiver performs frequency de-chirping in the photodiodes which significantly simplifies the task of signal processing, and the required receiver bandwidth can be much lower than the signal chirping bandwidth. While only amplitude modulation is required in the lidar transmitter, phase-diverse coherent receiver allows simultaneous detection of target range and velocity through the spectrum of the de-chirped complex waveform. Multi-target detection is also demonstrated experimentally.

The All-Solid-State Narrowband Lidar Developed by Optical Parametric Oscillator/Amplifier (OPO/OPA) Technology for Simultaneous Detection of the Ca and Ca+ Layers

We report an all-solid-state narrowband lidar system for the simultaneous detection of Ca and Ca+ layers over Yanqing (40.41°N, 116.01°E). The uniqueness of this lidar lies in its transmitter, which is based on optical parametric oscillation (OPO) and optical parametric amplification (OPA) techniques. The injection seeded OPO and the OPA are pumped by the second harmonic of an injection-seeded Nd:YAG laser, which can generate coherent light at the wavelength of 786 nm or 846 nm lasers, whose second harmonics in turn generate the 393 nm or 423 nm pulses, respectively, for the detection of thermospheric and ionospheric Ca+ and Ca layers. Compared to the conventional dye-based system, this lidar transmitter is a narrowband system (bandwidth < 200 MHz), which has produced a factor of two more output power with higher stability and reliability. The lidar system in Yingqing demonstrated Ca+ detection sensitivity of 0.1 atoms-cm−3 for long-term observation and reached a height of ~300 km. Potential applications and further improvements in this lidar technique are also discussed in this paper.

Compact all-fiber quantum-inspired LiDAR with over 100 dB noise rejection and single photon sensitivity

Entanglement and correlation of quantum light can enhance LiDAR sensitivity in the presence of strong background noise. However, the power of such quantum sources is fundamentally limited to a stream of single photons and cannot compete with the detection range of high-power classical LiDAR transmitters. To circumvent this, we develop and demonstrate a quantum-inspired LiDAR prototype based on coherent measurement of classical time-frequency correlation. This system uses a high-power classical source and maintains the high noise rejection advantage of quantum LiDARs. In particular, we show that it can achieve over 100dB rejection (with 100ms integration time) of indistinguishable (with statistically identical properties in every degree of freedom) in-band noise while still being sensitive to single photon signals. In addition to the LiDAR demonstration, we also discuss the potential of the proposed LiDAR receiver for quantum information applications. In particular, we propose the chaotic quantum frequency conversion technique for coherent manipulation of high dimensional quantum states of light. It is shown that this technique can provide improved performance in terms of selectivity and efficiency as compared to pulse-based quantum frequency conversion.

Study of 256 fiber array biaxial LiDAR optical assembly measurements.

This paper presents a method for measuring the optical assembly results based on multi-beam biaxial LiDAR. This method analyzes the optical assembly parameters of a LiDAR system affecting the LiDAR operation, and an experimental measurement system is built using a collimator to simulate the infinity imaging field. An InGaAs infrared camera is used to take pictures of the laser spot from the LiDAR transmitter and receiver, and then fit the laser spot images with Gaussian equations to calculate the biaxial LiDAR optical assembly results. Finally, the possible effecting factors of LiDAR alignment results are analyzed. This method is experimentally proven to achieve the measurement of the optical assembly results of a large scale multi-beam LiDAR. The possibility of further optimizing the measurement method by shaping the transmit laser is also reported.

The physical mechanism underpinning superfast switching of GaAs S-diode

Recently a physical phenomenon termed “collapsing field domains” (CFDs) was discovered, which manifests itself in several avalanching gallium arsenide (GaAs) structures. The phenomenon consists of the formation of a comb of ultra-high-amplitude field domains, which generate a dense electron-hole (e-h) plasma. Each domain surrounded by the plasma shrinks down to a nanometre scale in width, followed by collapse. Those domains cause different peculiar transient features from superfast (picosecond range) high-current avalanche switching in the voltage-blocking layers to sub-THz pulsed electromagnetic emission, filaments with extreme (∼10 MA/cm2) current density, hot-photon radiation, etc. The physics of those moving domains differs drastically from the Gunn effect having in common only a requirement of negative differential mobility (NDM) of the electrons and the fact that both phenomena have so far been observed in n-type GaAs layers. Here we demonstrate the CFD phenomenon in a p-layer formed by a deep acceptor. This finding contributes to the interpretation of superfast switching in so-called S-diodes. Besides proving the CFD nature underpinning superfast switching, we show the unique parameters of a simple, S-diode-based optical transmitter that achieves high-power (∼160 W) sub-nanosecond output. This performance is relevant to automotive and flight guidance applications of lidar. A brief review of alternative solutions presenting previous state-of-the-art for miniature lidar transmitters is given for comparison.

Research on the Performance of an Active Rotating Tropospheric and Stratospheric Doppler Wind Lidar Transmitter and Receiver

This paper investigates the transmitter and receiver performance of an active rotating tropospheric stratospheric Doppler wind Lidar. A 532 nm laser was determined as the detection wavelength based on transmission and scattering aspects. A ten-fold Galileo beam expander consisting of spherical and aspherical mirrors was designed and produced to compress the outgoing laser’s divergence angle using ZEMAX simulation optimization and optical-mechanical mounting means. The structure and support of the 800 mm Cassegrain telescope was redesigned. Additionally, the structure of the receiver was optimized, and the size was reduced. Meanwhile, the detectors and fiber mountings were changed to improve the stability of the received optical path. A single-channel atmospheric echo signal test was used to select the best-performing photomultiplier tube (PMT). Finally, the atmospheric wind field detection results of the original and upgraded systems were compared. The results show that after optimizing the transmitter and receiver, the detection altitude of the system is increased to about 47 km, and the wind speed and wind direction profiles match better with radiosonde measurements.

Fully Integrated Solid-State LiDAR Transmitter on a Multi-Layer Silicon-Nitride-on-Silicon Photonic Platform

We report a LiDAR transmitter incorporating both a hybrid-integrated tunable external cavity laser and a high-resolution 2-D optical phased array beam-steerer. Widely tunable single-mode lasing over a span of approximately 100 nm is achieved with >42 dB side-mode-suppression-ratio, 18 mW output power, and 2.8 kHz linewidth. Two-dimensional beam steering in a field-of-view of 140° × 16° exhibits a beam divergence of 0.051° × 0.016° measured in full width at half maximum. Leveraging the low propagation loss, negligible nonlinear loss, and low thermal sensitivity of silicon nitride, and combining the high mode confinement and efficient thermal tuning of silicon, the device affirms the feasibility of high-power on-chip lasing, power bottleneck elimination, and low on-chip insertion loss. It represents the first demonstration of a fully integrated LiDAR transmitter on the multi-layer silicon-nitride-on-silicon photonic platform, revealing the potential of complementary integration in the effort toward a LiDAR transmitter of sufficient optical power budget.

All-MEMS Lidar Using Hybrid Optical Architecture with Digital Micromirror Devices and a 2D-MEMS Mirror.

In a lidar system, replacing moving components with solid-state devices is highly anticipated to make a reliable and compact lidar system, provided that a substantially large beam area with a large angular extent as well as high angular resolution is assured for the lidar transmitter and receiver. A new quasi-solid-state lidar optical architecture employs a transmitter with a two-dimensional MEMS mirror for fine beam steering at a fraction of the degree of the angular resolution and is combined with a digital micromirror device for wide FOV scanning over 37 degree while sustaining a large aperture area of 140 mm squared. In the receiver, a second digital micromirror device is synchronized to the transmitter DMD, which enables a large FOV receiver. An angular resolution of by was achieved with 0.588 fps for scanning 1344 points within the field of view.

Differential Absorption Lidar Transmitter Based on an Indium Phosphide Photonic Integrated Circuit for Carbon Dioxide Sensing

We demonstrate carbon dioxide sensing using a random-modulation continuous-wave differential absorption lidar transmitter based on an indium phosphide photonic integrated circuit. We have designed and characterized the photonic circuit that has been fabricated through an open access generic integration platform based on standard building blocks. It consists of three four-section distributed Bragg reflector lasers, two fast photodiodes, two electro-absorption modulators and five semiconductor optical amplifiers integrated together with several couplers and waveguides. The circuit contains two interrelated subsystems, one for performing the differential absorption lidar measurement, and the other for stabilizing the emission wavelength of the different lasers. We characterize the individual integrated devices, especially lasers, photodiodes and modulators. The carbon dioxide sensing is done by measuring a gas cell in a fiber setup emulating a lidar configuration. Our promising results pave the way to miniaturized differential absorption lidar systems, while highlighting some of the main challenges to overcome.

Silicon nonlinear switch as a conditional circulator for monostatic LiDAR systems

All-optical silicon-photonics-based LiDAR systems allow for desirable features in scanning resolution and speed, as well as leverage other advantages such as size, weight, and cost. Implementing optical circulators in silicon photonics enables bidirectional use of the light path for both transmitters and receivers, which simplifies the system configuration and thereby promises low system cost. In this work, to the best of our knowledge, we present the first experimental verification of all-passive silicon photonics conditional circulators for monostatic LiDAR systems using a nonlinear switch. The proposed silicon nonlinear interferometer is realized by controlling signal power distribution with power-splitting circuits, allowing the LiDAR transmitter and receiver to share the same optical path. Unlike the traditional concept requiring a permanent magnet, the present device is implemented by using common silicon photonic waveguides and a standard foundry-compatible fabrication process. With several additional phase shifters, the demonstrated device exhibits considerable flexibility using a single chip, which can be more attractive for integration with photodetector arrays in LiDAR systems.

An Eye-Safe, SBS-Free Coherent Fiber Laser LIDAR Transmitter with Millijoule Energy and High Average Power

We report on an eye-safe, transform-limited, millijoule energy, and high average power fiber laser. The high gain and short length of the NP phosphate-glass fibers enable the SBS-free operation with kW level peak power. The output energy is up to 1.3 mJ, and the average power is up to 23 W at an 18 kHz repetition rate with 600 ns pulses (peak power > 2.1 kW). The PER is ≈16 dB and the M2 of the beam is 1.33 × 1.18. The coherent LIDAR Figure Of Merit (FOM) is 174 mJ*sqrt(Hz), which to our knowledge is the highest reported for a fiber laser. We also report 0.75 mJ energy and >3.7 kW peak power with down to 200 ns pulses and up to 1.21 mJ energy with a 3–5 kHz repetition rate operation of the current system.

Initial results of Rayleigh scattering lidar observations at Zhongshan station, Antarctica

A Rayleigh scattering lidar for measuring the atmospheric density and temperature has been deployed at Zhongshan Station (69.4° S, 76.4° E), Antarctica. Lidar transmitter was a frequency doubled Nd:YAG laser with ~400 mJ pulse energy and 30 Hz repetition rate. A telescope with 0.8 m diameter pointing to the zenith direction served as the lidar receiver. This lidar was capable of profiling the density and temperature in the Upper Stratosphere and Lower Mesosphere (USLM) region. At the vertical resolution of 300 m and the temporal resolution of 30 min, the lidar measurement uncertainties, mainly due to the photon noise, were calculated to be within 1.5% and 1 K for density and temperature, respectively. Since March 2020, this lidar has been routinely operated at Zhongshan station for exploring the atmospheric density and temperature variations and wave propagation characteristics in the polar USLM region.

Avalanche Delay and Dynamic Triggering in GaAs-Based S-Diodes Doped With Deep Level Impurity

The article is concerned with a detailed switching delay effect exhibited by avalanche S-diodes-superfast GaAs closing switches doped with deep Fe centers. The current and voltage time dependences are simulated in a simplified generator. The dynamic electric field and charge profiles in the structures are calculated. This article describes an impact that Fe capture cross sections of free charge carriers have on delayed switching. The simulation results show that delayed switching is associated with deep center recharging in a double injection mode due to three different processes. There are two different delay mechanisms to be herewith distinguished. A delay effect is experimentally viewed to control the dynamic switching voltage (and the avalanche breakdown voltage) using constant voltage adjustment capability enabled by a triggering circuit supply. The authors demonstrate the way it is possible to adjust the amplitude of current nanosecond pulses in the range of 20-45 A through a lidar transmitter circuit with a semiconductor laser and nonoptimized S-diode. The findings are consistent with the results of numerical simulation.

Investigation of the possibilities of increasing the resolution of lidars in the infrared wavelength range for monitoring methane in the atmosphere

The paper deals with the resolution of an optical lidar for measuring the background concentration of methane in the atmosphere both on surface horizontal paths with reflection of an optical signal from natural objects and on vertical paths, when an optical signal is reflected from cloud layers with extrapolation to low space orbits. It was shown using simulations that in the case of measurements from a low space orbit, the signal accumulation time can be reduced to 1 second or less due to the large signal attenuation at the center of the absorption line in the atmospheric air column. For a signal accumulation time of 1 s, the horizontal resolution along the horizontal coordinate will be ∼8 km at a satellite orbital velocity of ∼8 km / s. The vertical resolution for measurements from space orbit can be ∼ 1 km if the accuracy of measuring the absorption line broadening is ∼0.006 nm (10%). On the horizontal paths, the experimental results are in good agreement with the simulation results. Broadening the emission line of the lidar transmitter by a factor of ∼3 makes it possible to increase the radiated power at the considered wavelength ∼ 1653 nm, improve the energy potential of the device, and bring the horizontal resolution to 5-10 meters on the speed of lidar beam, corresponding to the speed of the helicopter.

A simulation comparison of calibration functions for different sets of spectral filter passbands in the traditional pure rotational Raman lidar technique

The accuracy of tropospheric temperature measurements with pure rotational Raman (PRR) lidars is affected by the collisional broadening of N2 and O2 PRR lines. In this paper, we intercompare nine calibration functions (CFs) in the traditional PRR lidar technique via simulation. Taking into account the PRR line broadening, the simulation is performed for five sets of spectral filters (SFs) with different passbands in a PRR lidar receiving system. For simplicity of calculations, the transmission function of each SF is approximated by a rectangular function on the wavenumber interval, within which a SF passes the bulk of the backscattered signal intensity in the corresponding PRR lidar channel. A narrow-linewidth laser operating at wavelengths of 354.67 and 532 nm is considered as a lidar transmitter. The CF best suited for tropospheric temperature retrievals from raw PRR lidar data for each set of SFs and laser wavelength is determined by comparative analysis of calibration errors ΔT produced using these CFs. The absolute error |ΔT| does not exceed the value of 3 × 10–3 K when using the best three-coefficient CF, while |ΔT| < 4 × 10–4 K for the best four-coefficient CF regardless of the laser wavelength and SF set used.

Modeling and Experimental Study of Lidar Resolution to Determine Methane Concentration in the Earth’s Atmosphere

This article discusses the resolution of an optical lidar for measuring the background concentration of methane in the Earth’s atmosphere. Using simulation and experimental studies, it has been shown that a resolution the width of the absorption line of which is ~0.006 nm and that has a concentration of methane of ~6% of the background can be obtained with a signal-to-noise ratio for received signals of ~200. When extrapolating the results to orbital measurements, the horizontal resolution will be ~8 km and the vertical resolution will be ~1.5 km. A broadening of the emission line of the lidar transmitter by a factor of 2–3 does not significantly reduce the resolution of the lidar, which makes it possible to increase the radiated power at the considered wavelength of ~1653 nm and improve the energy potential of the device as a whole.

Multichannel fiber-optic amplifier at the wavelength of 1653 nm for lidar remote sensing of atmospheric methane concentrations

We have developed and tested a multichannel fiber-optic Raman-effect amplifier with peak output power of the order of 10 W operating at the wavelength of 1653 nm. This amplifier is intended for use in a remote-sensing lidar for atmospheric methane and consists of three power amplifiers, each with an output power of more than 3 W. The output from these amplifiers was combined in a fiber-optic multiplexer and then fed to the lidar transmitter collimator. Our modeling of a single power amplifier indicated that the Raman conversion efficiency is improved when thin-core optical fibers are used; a thin-core optical fiber can be used to decrease both the pump power and the length of the optical fiber. We describe the seed light source spectrum and the multichannel amplifier output spectrum. The output power can be increased beyond 20 W by multiplexing in another four power amplifiers, which would provide sufficient lidar transmitter power to measure gas concentrations from a spacecraft in low-Earth orbit.

Underwater 3D Imaging Utilizing 520 nm Chaotic Lidar

We present a chaotic lidar for underwater ranging and three-dimension (3D) imaging applications. The lidar transmitter based on a laser diode with free space optical feedback is designed to generate 520 nm wavelength wideband chaotic signal. This transmitter is used with a correlation-based lidar receiver to perform a ranging demonstration in clean water with a measured attenuation coefficient of 0.06 m−1 at 520 nm. The experimental results prove that 1.2 cm mean absolute error (MAE) and 3.4 cm range resolution can be achieved and kept in a range of 140 cm at least. Moreover, employing a two-axis mirror for laser scanning, the possibility of the designed chaotic lidar system for 3D imaging is also verified.

Wide-angle MEMS-based imaging lidar by decoupled scan axes.

We present an optical architecture for a scanning lidar in which a digital micromirror device (DMD) is placed at an intermediate image plane in a receiver to decouple the trade-offs between scan angle, scan speed, and aperture size of the lidar's transmitter and receiver. In the architecture, the transmitter with a galvo mirror and the receiver with a DMD scan the horizontal and vertical fields of view, respectively, to enable an increased field of view of 50°, centimeter transmitter beam diameter, and video frame rate range finding captures. We present our optimized system and discuss the adjustable parameter trade-offs.