Coherent ranging, also known as frequency-modulated continuous-wave (FMCW) laser-based light detection and ranging (lidar)1 is used for long-range three-dimensional distance and velocimetry in autonomous driving2,3. FMCW lidar maps distance to frequency4,5 using frequency-chirped waveforms and simultaneously measures the Doppler shift of the reflected laser light, similar to sonar or radar6,7 and coherent detection prevents interference from sunlight and other lidar systems. However, coherent ranging has a lower acquisition speed and requires precisely chirped8 and highly coherent5 laser sources, hindering widespread use of the lidar system and impeding parallelization, compared to modern time-of-flight ranging systems that use arrays of individual lasers. Here we demonstrate a massively parallel coherent lidar scheme using an ultra-low-loss photonic chip-based soliton microcomb9. By fast chirping of the pump laser in the soliton existence range10 of a microcomb with amplitudes of up to several gigahertz and a sweep rate of up to ten megahertz, a rapid frequency change occurs in the underlying carrier waveform of the soliton pulse stream, but the pulse-to-pulse repetition rate of the soliton pulse stream is retained. As a result, the chirp from a single narrow-linewidth pump laser is transferred to all spectral comb teeth of the soliton at once, thus enabling parallelism in the FMCW lidar. Using this approach we generate 30 distinct channels, demonstrating both parallel distance and velocity measurements at an equivalent rate of three megapixels per second, with the potential to improve sampling rates beyond 150 megapixels per second and to increase the image refresh rate of the FMCW lidar by up to two orders of magnitude without deterioration of eye safety. This approach, when combined with photonic phase arrays11 based on nanophotonic gratings12, provides a technological basis for compact, massively parallel and ultrahigh-frame-rate coherent lidar systems. A massively parallel coherent light detection and ranging (lidar) scheme using a soliton microcomb—a light source that emits a wide spectrum of sharp lines with equally spaced frequencies—is described.

Massively parallel coherent laser ranging using a soliton microcomb

Disclosed is a laser radar device having a wide two-dimensional field of view, a large receiving aperture, and responsivity to short pulse light. Specifically disclosed is a laser radar device provided with a laser light source which generates pulse laser light, a scanner which transmits a transmission pulse to a target while two-dimensionally scanning a pencil-shaped transmission beam and outputs scan angle information, a reception lens which receives reception light that has arrived, a long light-receiving element array which converts the reception light to reception signals and outputs the reception signals, a transimpedance amplifier array which amplifies the reception signals, an adder circuit which adds the reception signals from respective elements of the transimpedance amplifier array, a distance detection circuit which measures the light round-trip time to the target of an output signal from the adder circuit, and a signal processing unit which obtains the distances to multiple points on the target on the basis of the light round-trip time and the light speed by causing the scanner to perform a two-dimensional scan operation while associating the two-dimensional scan operation with the scan angle information to thereby measure the three-dimensional shape of the target.

Laser radar device

Lidar imaging systems are one of the hottest topics in the optronics industry. The need to sense the surroundings of every autonomous vehicle has pushed forward a race dedicated to deciding the final solution to be implemented. However, the diversity of state-of-the-art approaches to the solution brings a large uncertainty on the decision of the dominant final solution. Furthermore, the performance data of each approach often arise from different manufacturers and developers, which usually have some interest in the dispute. Within this paper, we intend to overcome the situation by providing an introductory, neutral overview of the technology linked to lidar imaging systems for autonomous vehicles, and its current state of development. We start with the main single-point measurement principles utilized, which then are combined with different imaging strategies, also described in the paper. An overview of the features of the light sources and photodetectors specific to lidar imaging systems most frequently used in practice is also presented. Finally, a brief section on pending issues for lidar development in autonomous vehicles has been included, in order to present some of the problems which still need to be solved before implementation may be considered as final. The reader is provided with a detailed bibliography containing both relevant books and state-of-the-art papers for further progress in the subject.

An Overview of Lidar Imaging Systems for Autonomous Vehicles

We present the demonstration of an integrated frequency modulated continuous wave LiDAR on a silicon platform. The waveform calibration, the scanning system, and the balanced detectors are implemented on a chip. Detection and ranging of a moving hard target at upto 60 m with less than 5 mW of output power is demonstrated in this paper.

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Photonic Integrated Circuit-Based FMCW Coherent LiDAR

Coherent ranging, also known as frequency-modulated continuous-wave (FMCW) laser based ranging (LIDAR) is currently developed for long range 3D distance and velocimetry in autonomous driving. Its principle is based on mapping distance to frequency, and to simultaneously measure the Doppler shift of reflected light using frequency chirped signals, similar to Sonar or Radar. Yet, despite these advantages, coherent ranging exhibits lower acquisition speed and requires precisely chirped and highly-coherent laser sources, hindering their widespread use and impeding Parallelization, compared to modern time-of-flight (TOF) ranging that use arrays of individual lasers. Here we demonstrate a novel massively parallel coherent LIDAR scheme using a photonic chip-based microcomb. By fast chirping the pump laser in the soliton existence range of a microcomb with amplitudes up to several GHz and sweep rate up to 10 MHz, the soliton pulse stream acquires a rapid change in the underlying carrier waveform, while retaining its pulse-to-pulse repetition rate. As a result, the chirp from a single narrow-linewidth pump laser is simultaneously transferred to all spectral comb teeth of the soliton at once, and allows for true parallelism in FMCW LIDAR. We demonstrate this approach by generating 30 distinct channels, demonstrating both parallel distance and velocity measurements at an equivalent rate of 3 Mpixel/s, with potential to improve sampling rates beyond 150 Mpixel/s and increase the image refresh rate of FMCW LIDAR up to two orders of magnitude without deterioration of eye safety. The present approach, when combined with photonic phase arrays based on nanophotonic gratings, provides a technological basis for compact, massively parallel and ultra-high frame rate coherent LIDAR systems.

Massively parallel coherent laser ranging using soliton microcombs

A high-performance airborne UV Rayleigh lidar system was developed within the European project DELICAT. With its forward-pointing architecture it aims at demonstrating a novel detection scheme for clear air turbulence (CAT) for an aeronautics safety application. Due to its occurrence in clear and clean air at high altitudes (aviation cruise flight level), this type of turbulence evades microwave radar techniques and in most cases coherent Doppler lidar techniques. The present lidar detection technique relies on air density fluctuations measurement and is thus independent of backscatter from hydrometeors and aerosol particles. The subtle air density fluctuations caused by the turbulent air flow demand exceptionally high stability of the setup and in particular of the detection system. This paper describes an airborne test system for the purpose of demonstrating this technology and turbulence detection method: a high-power UV Rayleigh lidar system is installed on a research aircraft in a forward-looking configuration for use in cruise flight altitudes. Flight test measurements demonstrate this unique lidar system being able to resolve air density fluctuations occurring in light-to-moderate CAT at 5 km or moderate CAT at 10 km distance. A scaling of the determined stability and noise characteristics shows that such performance is adequate for an application in commercial air transport.

/pdf/airborne-forward-pointing-uv-rayleigh-lidar-for-remote-clear-3tqenb1c3c.pdf

Airborne forward pointing UV Rayleigh lidar for remote clear air turbulence (CAT) detection: system design and performance

In this paper, we present the development of a laser source for a LIDAR application. This sensor is proposed as a standby instrument to provide a way to measure some aircraft air data such as the air speed. Although such systems already exist, none of them are based on an optical measurement. Thus, the use of a LIDAR would provide a backup channel with different failure modes than existing systems. Our LIDAR system allows determining the air speed through Doppler measurement at a wavelength of 1.55 µm on aerosol particles present around the aircraft. The core of this device is a glass integrated optics continuous DFB laser. Its performances in term of single-frequency, stability, noise and linewidth are assessed in order to ensure the correct operation of the LIDAR system.

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Development of a ion-exchanged glass integrated optics DFB laser for a LIDAR application

The general field of the invention is that of optical lidars comprising an optical porthole and operating at a first wavelength. The optical porthole of the lidar according to the invention comprises a layer or a sheet made of an optical material. The lidar comprises means for illuminating said layer or said sheet at a second wavelength different from the first wavelength, said material being transparent at the first wavelength and absorbent at the second wavelength, said second wavelength being located in the visible spectrum.

Lidar comprising optical means for deicing or preventing icing

High power single-mode pump laser diodes operating around 980nm are key components for Erbium-doped
devices. Much effort is still currently devoted to improve both their wavelength stability and their achievable
output power, while maintaining a stable single-mode operation.
Usually, the emission wavelength is stabilized by an external Fiber Bragg Grating (FBG). This configuration
requires free-space optics between the laser diode output facet and the fiber or a lensed fiber to ensure an
efficient coupling efficiency. This constraint increases fabrication costs, dimensions and mechanical instabilities.
Moreover, the maximum achievable output power is limited because a high optical power density can damage
the laser facets. To increase the achievable output power, a solution consists in using Broad-Area Laser Diodes
(BALD), which are multimode emitters that are composed of large active ribbons with width of some hundreds
of micrometers. The objective is then to improve the beam quality by locking the BALD emission on its
transverse fundamental mode. We propose in this article to insert an integrated adiabatic transition between the
multimode laser and a single-mode FBG. This taper, made by ion-exchange in glass, provides a coupling
efficiency of -22.0dB from the multimode laser emission to the single-mode fiber. An optical feedback of -34dB
demonstrates the stabilization of the BALD spectrum at the Bragg wavelength. The spectrum of the device is
characterized by a maximum side-mode suppression ratio of 35dB, a RMS spectral width of (0.16 ± 0.04) nm
and a frequency shift with current of -12GHz/100mA.

Broad-area laser diode with stable single-mode output and wavelength stabilization

An important aeronautics application of lidar is the airborne remote detection of Clear Air Turbulence which cannot be performed with onboard radar. We report on a DLR-developed lidar system for the remote detection of such turbulent areas in the flight path of an aircraft. The lidar, consisting of a high-power UV laser transmitter and a direct detection system, was installed on a Dutch research aircraft. Flight tests executed in 2013 demonstrated the performance of the lidar system to detect local subtle variations in the molecular backscatter coefficient indicating the turbulence some 10 to 15 km ahead.

/pdf/flight-tests-of-the-delicat-airborne-lidar-system-for-remote-52aqk5axxt.pdf

Philippe Rondeau

Papers

Airborne forward pointing UV Rayleigh lidar for remote clear air turbulence (CAT) detection: system design and performance

Development of a ion-exchanged glass integrated optics DFB laser for a LIDAR application

Lidar comprising optical means for deicing or preventing icing

Broad-area laser diode with stable single-mode output and wavelength stabilization

Flight tests of the delicat airborne lidar system for remote clear air turbulence detection