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Swept-Frequency Source for Automotive Light Detection and Ranging (LiDAR)
Tool Used: OptSim
This application note illustrates a basic setup to achieve a swept frequency light source commonly used for LiDAR applications in automotive cruise control (ACC) and automotive collision avoidance systems (ACAS) [1-2].
Since chirped frequency scanning requires lower power and can transmit and receive at the same time, it is more popular than pulsed scanning in automotive LiDAR systems. However, compared to pulsed radar, chirped frequency scanning is complex to implement.
Since the chirp is precisely linear, a scatterer with round-trip delay τ (and a corresponding distance cτ/2 from the source, c is speed of light) results in constant frequency difference ξt between the launched and reflected waves.
Such a scanning would require an optical frequency modulated continuous-wave (FMCW) source [2]. The purpose of this application is to show how to create such a source in OptSim.
The topology layout is shown in Figure 1 below.
Fig.1 Topology layout for this example
The idea is to phase modulate light by an electrical drive of ξt2/2. Since the modulated optical signal’s instantaneous frequency is the time-derivative of the argument, we get a swept frequency light source.
Figure 2 shows a parabolic sweep at the expression generator (left) and corresponding optical frequency sweep at the phase modulator output (right).
Fig.2 Parabolic electrical sweep (left) used to obtain linear optical frequency sweep (right)
The reflection is modeled by the lower arm at the coupler with attenuator representing the reflectance (loss) and the time-delay block representing a two-way delay. This round trip delay times the sweep range (frequency excursion) gives a constant frequency difference between the transmitted and reflected waves.
Figure 3 shows frequency offsets between launched and reflected optical signals for two different round-trip delays.
Fig.3 Frequency offsets between transmitted and reflected signals for two different values of round-trip delay
The coefficients in the coupler model are set such that the optical fields constructively (upper port) and destructively (lower port) interfere for balanced photodetection, if desired.
References
1. Tong, Z., Reuter, R., and Fujimoto, M., “Fast chirp FMCW radar in automotive applications,” Proc. Of the IET International Radar Conference, pp. 7-11, 2015.
2. Zheng, J., "Analysis of optical frequency-modulated continuous-wave interference," Applied Optics, vol. 43, no. 21, pp. 4189-4198, July 2004.