# An Optical Interference Suppression Scheme for TCSPC Flash LiDAR Imagers
## 6 Experimental Results
The TDC implements a coarse measurement block that counts the periods of the time base signal (giving a least significant bit (LSB) resolution of 1.6 ns) and a fine measurement block realized as a 32-stage phase interpolator that brings the LSB resolution down to the target 50 ps. A small digital interface block merges the coarse and fine measurements to produce a 13-bit, binary coded timestamp representing the time of arrival of the last detected photon and a 6-bit address identifying the pixel that fired last. Figure 7 shows the histogram of the victim in absence of any aggressor activity. Under the measurement conditions indicated in Table 1 and with an integration time of \(T_{\text{int}}=100\) ms, the victim expresses a single histogram peak with \(T_{\text{FWHM},\sigma}=1.65\) ns located at \(T_{\text{ToF},\rho}=132.1\) ns. The aggressor was then activated and configured to run at the same PRF as the victim. The plot in Figure 8 shows the histogram measured by the victim in this case. A second peak at \(T_{\text{ToF},a}=204.8\) ns appears due to the aggressor interfering with the victim. In the following measurement, the optical interference suppression characteristics of the FLISS method were investigated. In all previous measurements, the time interval between two consecutive laser pulses was always and unconditionally equal to \(\frac{1}{\text{PRF}}\). Now, each laser pulse of the victim is delayed by a random number of system clock cycles compared to its scheduled time. An LFSR with a  
Figure 8: Once the aggressor is activated and running at the same PRF as the victim, the victim will build a histogram with multiple peaks. Without external information, it is not possible to identify the source of each signal. During this measurement, the position and width of the victim peak was measured to be very stable (less than 0.5% fluctuation over the entire experiment) and identical to the previous measurement. The aggressor peak appears later in the histogram (not a condition) at \(T_{\text{ToF},a}=204.8\) ns with a width of \(T_{\text{FWHM},a}=1.65\) ns. Both emitters operate at the nominal PRF of 1 MHz. Figure 7: Output of the victim before activating the aggressor. Measurements have been taken indoors to limit the effect of ambient light and focus on the behavior of the TCSPC peaks. The victim measures a consistent time-of-flight of \(T_{\text{ToF},\rho}=132.1\) ns (not calibrated, therefore affected by a constant offset that is immaterial to this work) with a peak width of \(T_{\text{FWHM},\rho}=1.65\) ns. The measurement plot has been zoomed in to the timestamp range \((2000,4500)\) from the original \((0,8191)\) to better appreciate the shape of the pulse. primitive polynomial is used to generate a maximum-length pseudorandom sequence of delays. Four LFSRs were used, with length \(L\in[3,4,5,6]\) bits, and the suppression ratio was measured for three different values of integration time (Figure 9). The suppression ratio is defined as  
\[S=10\log\frac{P_{v}}{P_{a}}\quad[\mathrm{dB}], \tag{10}\]  
where \(P_{v}\) is the histogram peak value of the victim, \(P_{a}\) is that of the aggressor, and \(S\) is the suppression ratio expressed in dB. Figure 10 illustrates the effect of the suppression on the histogram of the victim. Figure 10: This plot illustrates the histogram acquired by the victim when the suppression was active, for 4 different lengths of the LFSR. All the measurements were acquired at \(T_{\mathrm{int}}=100\) ms integration time. In all four subplots, the major peak is the victim’s peak. The minor peaks represent the aggressor signal. In each subplot, the red line indicates the highest aggressor peak, while the green line highlights the victim’s peak. It is shown that aggressor peaks get progressively more numerous and smaller as the victim’s LFSR gets longer. Figure 9: Characterization of the interference suppression performance. The three lines show the suppression ratio (in dB) as a function of the length of the linear feedback shift register (LFSR) for three different integration times.