FPGA high rate pseudo-random single photon counting correlator

Random modulation continues wave (RM-CW) LIDAR was first applied for aerosol detection in km range [1]. It is capable of obtaining range resolved back-scattering information comparable to pulsed techniques. In RM-CW LIDAR, a pseudorandom binary sequence (PRBS) is transmitted, and the received signal correlated with the original PRBS code giving range resolved response with non-ambiguous range determined by the length of the PRBS. Although the range resolution is dictated by the bit-rate, our previous work [2] demonstrated that cm distance precision can be achieved using interpolation techniques. In contrast to traditional RM-CW LIDAR that uses linear mode detector and analogue to digital converters we combine single photon counting detectors and pseudo-random modulation following [3,4]. While most systems perform post processing (of multi-channel scalar data or single photon time tags), real-time implementations of photon counting RM-CW LIDAR with long PRBS sequence are uncommon. In this work, a FPGA-based real-time correlator clocked at 100MHz for photon counting RM-CW LIDAR is presented. Recently, a few FPGA hardware correlators [5] have been described in the literature. Our implementation deviate from these prior studies, for it performs correlation of a random sequence with bipolar version of that sequence, hence zeroing the offset of the correlation result. The simple architecture can be applied for various other photon correlation applications including differential absorption LIDAR DIAL, dynamic light scattering and with the next generation GHz clock rate FPGA’s, fluorescent correlation time imaging FLIM. The correlator schematic is illustrated in Figure 1(a). The top section of the schematic illustrates the linear feedback shift registers LFSR that produces the pseudo-random binary sequences PRBS a(t). a(t) is used to modulate the transmitted light. The middle section oversamples a(t) to a[i] ∈ {0,1}. with a faster clock to provide sub-bit resolution. From the single photon detector, photon incidents are converted into electrical pulses, the logics in the lower part of the schematic performs the correlation. The rising edge of the photon arrival b[i], are linked to all counters’ clock pin in the correlator, these counters respond by either counting up/or down dependent upon a[i] (1 count-up and 0 count-down representing the bipolar version a′ [i] = a[i] − a̅[i] ∈ {−1,1}). The outputs of the counters then register the correlation result, that is c[i] = d[i] ⊕ a′[i], and readout every integration time. Some correlation results of a LIDAR for range finding are included in the Figure 1(b), demonstrating the ranging capability.