Solutions to this problem have gene

Solutions to this problem have generally fallen under two broad categories. The first utilizes a pulsed source and range-gated receiver. In this approach, only photons that arrive within the receiver gate on time are collected [2, 3, 4, 5, 6]. The gate timing can be varied to coincide with the expected target location, and the target range is determined by measuring the pulse time of flight. The backscatter component is attenuated while the receiver is gated off. Additionally, since most practical blue/green laser sources have been frequency doubled from the IR, the conversion efficiency may be higher for pulsed laser sources due to the higher peak powers. The drawback of the pulsed range-gated technique is that it requires a stable, high energy, short-pulse (less than 5 ns) laser source, which can be difficult to synthesize at blue/green wavelengths. Additionally, gating a large area, high gain receiver like a photomultiplier tube (PMT) is difficult electronically due to the large voltages needed, as well as significant impedance mismatch between driver circuit and PMT dynodes. The use of short pulses also requires a wideband receiver, which can introduce more noise. However, the pulsed/range-gated technique in theory provides a relatively straightforward method for suppressing backscatter while providing target range information.The other approach aims to reduce backscatter by intensity modulating a CW laser source at RF frequencies (more than 0.01–1 GHz) [7, 8, 9]. In this scenario, range is determined by calculating the phase of the detected RF envelope relative to the modulation drive signal using a standard RF coherent receiver. The major motivation of the intensity modulated method lies in its ability to separate target photons from backscatter photons. Here, nonscattered and minimally forward-scattered target-reflected photons retain their modulation. However, backscattered returns, distributed in range between the source and target, destructively interfere with each other, causing the modulation of the backscattered light to wash-out at high frequencies [10, 11, 12]. The target (AC) component and backscatter (DC) component can be separated at the receiver with a bandpass filter centered at the modulation frequency.