Liang Wei of Suzhou Institute of Nanotechnology and others made progress in the

With the development of automatic driving, frequency modulated continuous wave lidar (FMCW LiDAR) has attracted more and more attention. The FMCW scheme uses a continuous scanning laser light source. The principle is that the target reflected light and reference light are mixed at the detector to beat, and the target distance corresponds to the beat signal frequency. Compared with the pulse LiDAR based on time of flight (TOF), the FMCW LiDAR using coherent detection can well resist the interference of direct sunlight and other lidars. The resolution of FMCW lidar is related to the sweep frequency range. It can achieve high resolution without using high bandwidth devices. More importantly, it can obtain range and velocity information simultaneously through a single measurement using Doppler effect. Narrow linewidth linear sweep light source is the key device of FMCW LiDAR. The linewidth of the laser will affect the detection range and measurement sensitivity, while the repetition frequency will affect the point cloud density of the lidar.

Recently, Liang Wei, a researcher team from Suzhou Institute of Nanotechnology, developed an external cavity narrow linewidth sweep laser with a linewidth of 5.06kHz and a repetition rate of 100kHz, which provides an effective light source for realizing long distance FM CW lidar ranging.

The schematic diagram of the linewidth measurement system is shown in Figure 1, and the illustration is a packaged laser. The linewidth of the laser is measured by heterodyne interferometer. The 20dB Lorentz linewidth is 71.6 kHz, and the corresponding laser linewidth is 5.06 kHz.

Fig. 1 Block Diagram of Delay Heterodyne Linewidth Measurement System

Fig. 2 Laser delay heterodyne spectrum measured with 20 km optical fiber delay line

The ranging performance of the laser is evaluated by building a fiber interferometer. When the repetition frequency of the driving current is 1 kHz, the continuous linear sweep frequency range exceeds 1 GHz. When the sweep repetition frequency is increased to 100 kHz, the sweep range is reduced to 228.9 MHz, and the corresponding free space resolution is 0.7129 m. It can be seen from Figure 3 that when the fiber length is 156 meters (corresponding to 110 meters of free space reflection), the SNR is still higher than 35 dB.

Figure 3 Test Results of Different Optical Fiber Lengths at FM Repetition Rate of 100 kHz

The team used the external cavity to compress the laser linewidth, changed the drive current to achieve frequency tuning, and developed an external cavity narrow linewidth sweep laser with a linewidth of 5.06 kHz and a repetition rate of 100 kHz, which can meet the requirements of automatic driving for hundreds of meters ranging. In the latest research results, by optimizing the frequency modulation mechanism, the continuous linear sweep frequency range has reached 4GHz, and the corresponding ranging spatial resolution is about 3.75cm.

Relevant achievements were published in the international journal IEEE Photonics Technology Letters under the title of "Narrow linewidth external activity class capable of high repetition frequency tuning for FMCW LiDAR". Suzhou Institute of Nanotechnology, Chinese Academy of Sciences was the first unit to complete the paper, Wu Ying, a postdoctoral fellow in the Nanodevice Research Department, and Deng Lihua, a doctoral student, were the co first authors of the paper, and Liang Wei, a researcher in the Nanodevice Research Department, was the corresponding author of the paper.

The above work has been supported by the National Natural Science Foundation of China.

Source: Suzhou Institute of Nanotechnology and Nanobionics, Chinese Academy of Sciences