Frequency feedback control of a FMCW LiDAR for a high position accuracy

Abstract

The LiDAR (Light Detection and Ranging) is a sensor that measures the shape of an object over a long distance and can measure the distance without lighting. Therefore, LIDAR is expanding various application areas by grasping information on buildings and surrounding roads as 3D image data. Among them, the pulsed TOF(Time-of-Flight) LiDAR sensor is the most typical one in market. it uses a method of measuring the time difference between the transmitted pulse wave and the received pulse signal and converting it into a distance by using the speed of light. However, pulsed LiDAR is weak against noise in the solar wavelength band, and has the disadvantage of inaccurate distance measurement in harsh environments like rain, snow, or fog.

On the other hand, the frequency-modulated continuous wave(FMCW) LiDAR is a method of measuring a distance by measuring a frequency of an optical interference signal between a transmitted laser beam and received laser beam which is reflected from the object. In this method, the light interference effect, which is the wave characteristic of the laser, is used, so that the transmittance is excellent in particles such as rain, snow, and fog. So that the measuring the target in a long distance can be measured more accurately against the other way. And also, by the interference effect, interruption of the signal does not occur even in an environment under sunlight or external light. Due to such advantages, it is in the spotlight as a sensor used in various environments such as autonomous driving or traffic infrastructure.

In FMCW LiDAR, the optical frequency of the laser source should be modulated into a sweeping shape that linearly increases or decreases over time to produce an interference frequency proportional to the distance. However, even if the driving signal of the laser is linearly input over time, due to variables such as laser characteristics or external temperature, there is a problem that the actual frequency modulation is nonlinear.

To solve this non-linearity, there are methods such as feed-forward method, Pre-distortion, and feedback method, PLL(Phase locked loop). However, the Pre-distortion method is vulnerable to changes in the external environment such as temperature. And because of the slow repetition rate of the PLL method, it is not suitable for LiDAR, which requires high scanning rate at least several kHz.

Due to these problems, the above methods are not suitable for 3D LiDAR, which needs to quickly measure the distances of several outer points. Therefore, this study proposes to control laser frequency linearity using a Frequency-Voltage converter (FVC) to achieve frequency modulation at a high speed of several kHz or more.