Time-of-flight (ToF) Resolution and Measurement from Received RF Spectra in Optical Coherence Tomography (OCT) and Light Detection and Ranging (LiDAR) Applications

Tools Used: OptSim

A swept frequency light source is commonly used for automotive time-of-flight (ToF) LiDAR applications in automotive cruise control (ACC) and automotive collision avoidance systems (ACAS) [1-2] as well as in swept source optical coherence tomography (SS-OCT) [3]. In this application note, we study receiver considerations for resolution sensitivity and measurement of time of flight from the detected radio-frequency (RF) spectral tones.

Figure 1 shows OptSim schematic of the setup.

Figure 1. Topology layout for this example

The variables used in the design are defined in OptSim symbol table. Variable “sweep_slope” sets the value of the frequency sweep rate, which is set to 1.25E15 Hz/s (equivalently, 10,000 μm/s wavelength sweep rate). The value of bit rate parameter affects the sweep rate because the slope of the parabolic sweep is evaluated over one bit period.

Figure 2 shows frequency sweep at the transmitter.

Figure 2. Frequency sweep at the transmitter (left), one sweep period zoomed-in (right)

The value of sweep interval (i.e., time duration of one sweep at the transmitter output, as shown in the right Fig. 2) determines spectral (or Fourier) resolution, which for our case is: Fourier Resolution = 1/ sweep interval = 1/2000ns = 0.5 MHz

The time-delay resolution (below which we can’t detect) is Fourier resolution divided by the sweep rate, i.e., the round-trip time-delay resolution = 0.5MHz/(1.25E15Hz/s) = 0.4ns.

In order to observe RF tones corresponding to different values of round-trip delays, a petameter scan is setup for variable “Reflection_delay” which represents round-trip delay from detected target.

Click on the “Scan” button to run the project file, and after the simulation, compare detected RF spectra at the balanced receiver. The tone frequency and round-trip delay are related by: RF Tone frequency = round-trip delay x sweep rate

RF tones at four of the parameter scan values are shown in Figure 3.

Figure 3. RF tones at (i) 125.6MHz (0.1μs) (ii) 276.8MHz (0.3μs) (iii) 628MHz (0.5μs) and (iv) 879.6MHz (0.7μs)

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, July 2004, pp. 4189-4198.

3.       Thomas DiLazaro, and George Nehmetallah, “Large-volume, low-cost, high-precision FMCW tomography using stitched DFBs,” Optics Express, vol. 26, n0. 3, Feb. 2018, pp. 2891-2904.