# An Optical Interference Suppression Scheme for TCSPC Flash LiDAR Imagers
## 6 Experimental Results
With a maximum LFSR length of 6 bits and using only complete sequences, we were able to suppress the aggressor peak by 35 dB. The plot shows clearly a direct correlation between the length of the LFSR and the suppression ratio. This is expected, as shorter LFSRs produce shorter sequences containing fewer possible values; therefore, the aggressor cannot be efficiently diluted across the entire time base of the victim. Another notable point is that changing the integration time seems to have marginal effects on the suppression. It is shown that the suppression ratio improves with the increasing length of the LFSR, reaching a maximum measured suppression of 35 dB using complete sequences. This is expected, as longer LFSRs produce longer sequences with a larger set of delay steps. The suppression appears to be worst for short LFSRs and long integration times. This occurs when \(R_{L}\cdot F_{\text{clk}}^{-1}<<T_{\text{int}}\), where \(R_{L}\) is the length of the m-sequence generated by the LFSR of length \(L\), \(F_{\text{clk}}\) is the system clock frequency, and \(T_{\text{int}}\) is the integration time. In this case, the repetition of the same pseudorandom sequence of delays during the same acquisition introduces deterministic systematic patterns in the scrambling of the aggressor. The use of the FLISS technique is shown to have minimal impact on the characteristics of the victim's output. Figure 11 illustrates the variation of three key quantities (the intensity of the TCSPC peak \(P_{v}\), the measured time-of-flight \(T_{\text{TOF},v}\), and the width of the peak \(T_{\text{FWHM},v}\)) relative to a reference acquisition taken in absence of interference. The experiment shows that applying FLISS does not substantially impact the measurement of the time-of-flight. Similarly, the full-width at half-maximum (FWHM) of the peak shows a maximum deviation of 3%, which is very tolerable from the point of view of signal integrity. The relative variation of the TCSPC peak intensity \(P_{v}\) stays small and close to 0 for \(L\leq 4\). For larger values of \(L\), a consistent and expected decline has been observed. The reason is to be found in the probability of accidentally shifting aggressor activity right before the time of arrival of the signal peak. Given the parameters of the system, including the distance between victim and aggressor and the mutual phase shift of their PRFs, there will be a value of \(L\) after which the activity of the aggressor will start to be shifted in the vicinity of the time of arrival of the signal. When that happens, the shifted aggressor events will likely occupy the sensor and compete with signal photons. It is exactly the same effect we observe when the LiDAR imager interacts with uncorrelated signals such as sunlight. Figure 10 provides a good example to visualize this concept. This is another evidence that for \(L\geq 4\), the aggressor will lose its correlation to the victim and will start behaving like ambient light for all intents and purposes. Figure 11: Variation of key signal parameters during the application of the interference suppression technique, relative to a reference acquisition performed in the absence of interference. The relative variation of the measured time-of-flight is negligible (<0.15%) and practically constant, and it does not seem to depend on the integration time \(T_{\text{int}}\) or on the LFSR length \(L\). The relative variation of the peak width is mostly small and marginally dependent on the length of the LFSR, with all the curves being contained within 3% of the reference measurement. The relative variation of the TCSPC peak height \(P_{v}\) provides an indirect indication of the changing statistical properties of the suppressed aggressor. The longer the LFSR, the more likely it is for aggressor events to be shifted right before the time of arrival of the signal. What follows is a reduction of the number of detectable signal photons due to competition between the regular signal photons and the shifted aggressor.