Multi-beam single-photon LiDAR with hybrid multiplexing in wavelength and time

Highlights

• Multi-beam single-photon LiDAR at 1550 nm.

• Hybrid multiplexing technology of wavelength and time based on FPGA.

• Coaxial detection optical path of single-beam transceiver and multi-beam detection based on a blazed grating.

• Suitable for LiDAR measurement in the field of human eye safety, such as vehicle LiDAR.

Abstract

We report a multi-beam single-photon light detection and ranging (LiDAR) in eye-safe band at 1.5 µm with hybrid multiplexing in wavelength and time. 16 independent laser diodes with equal frequency spacing in dense wavelength division multiplexing (DWDM) were combined with a blazed grating to generate multi-beam dispersing with equal angle in space as the LiDAR laser source. Meanwhile, time division multiplexing (TDM) was applied on the 16 laser diodes as well. A single-pixel single-photon detector (SP-SPD) was used to detect the echo photon signal of all laser beams by TDM. In this way, we constituted a compact optical system without multi-beam transceiver alignment. The LiDAR system was simple and robust with this hybrid multiplexing technology. An outdoor photon-counting imaging within 75-m range was achieved, with the laser power lower than 1/10 of the human eye safety threshold. It has high application potential in the field of automatic driving with strict eye safety requirements.

Introduction

In recent years, LiDAR has been developing rapidly in the fields of laser mapping [1], [2], industrial measurement [3], [4], [5], [6], and automatic driving [7], [8]. Multi-beam LiDAR can effectively improve the measurement field of view and speed, which makes Velodyne’s LiDARs widely used in the field of automatic driving [9]. However, due to the limitation of total laser power, the energy of single laser beam decreases linearly with the increase of the beam number, which greatly reduces the ranging distance of multi-beam LiDAR. Single-photon LiDAR (SPL) improves the detection sensitivity to sparse photons by using single-photon detector and is suitable for long-distance or low-power applications [10], [11], [12], [13], [14], [15], [16], which makes the SPL have great advantages in multi-beam LiDAR. The Leica SPL100 splits a 532 nm laser into 10 × 10 laser beams and receives the echo photons by a 10 × 10 single-photon detection array of photomultiplier tube (PMT). It reached to 6 million points per second with a 60 kHz pulse repetition rate, which was the fastest LiDAR as far as we known [17]. In 2017, we realized a multi-beam SPL [18]. A 532-nm laser split into 1 × 100 beams through a diffractive optical element, and their echo photons were collected into a 1 × 100 fiber array and detected by 100 silicon avalanche photodiode single-photon detectors one-on-one. Both SPLs required precise multi-beam transceiver alignment, which was not conducive to mass production. In order to simplify the system, frequency multiplexing photon-counting method was proposed [19]. Sixteen beams of individually triggered laser diodes output through fiber array, and their echo photons detected by a single-pixel single-photon detector (SP-SPD). Recently, a wavelength-time coding method has been applied to single-photon detection. A pulsed supercontinuum laser source conjunction with an acousto-optic tunable filter (AOTF) were used to output discrete pulse trains of different central frequencies and delay. This method improved the spectral response and depth resolution by simultaneously detecting the echoes of the multiple-wavelength lasers with a SP-SPD [20]. In addition, a time-stretch LiDAR modulated broadband light source by spectro-temporal encoded to generate multi-wavelength light beam with controllable time delay [21]. Then, the multi-wavelength laser was diffracted into multi beams, and their echo photons were also detected by a SP-SPD. In this way, the scanning range and measuring distance of the SPL were greatly extended. These LiDARs developed multiplexing technologies to simplify the detection. However, in order to realize the long-distance and multi-beam measurement, these wavelength-time coding method usually need the extra time delay modulation device, which will increase the complexity of the SPL.

In this paper, we designed a simple and robust SPL system with hybrid multiplexing technique in wavelength and time. 16 independent laser diodes with equal frequency spacing in DWDM were combined with a blazed grating to generate multi-beam dispersing with equal angle in space as the LiDAR laser source. Meanwhile, TDM was applied on the 16 laser diodes as well. A field programmable gate array (FPGA) board was used to freely adjust the time delay between different wavelength laser diodes to realize flexible adjustment of the measurement distance. The optical path of single-beam transceiver and multi-beam detection was constructed based on a blazed grating and a multi-wavelength laser source. The 16-beam scanning imaging was realized in the field of view of 45° × 20° within the range of 75 m outdoor. In addition, the main components of the SPL come from the fiber communication industry, which is mature, reliable, and cheap, and is conducive to large-scale production.