Random-modulation CW Laser Ceilometer Signal Processing Based on Compound PN Sequence

LIDAR can be well used to measure structure of atmosphere, visibility, cloud height et al. Two kinds of LIDARs have been used to obtain the response function: micro pulse and continuous wave. A new type of LIDAR with continuous-wave operation is proposed in this paper, which is called random modulation continuous-wave (RM-CW) LIDAR based on compound pseudo noise sequence modulation. Compared with more sophisticated LIDAR systems commonly used for similar investigations it has several advantages, including the low peak power, ability to operate eye-safe, small size and low price. The RM-CW technique is a typical way to recover the weak signal buried in random noise. A specific diode laser (DL) was selected for such a compact LIDAR system because of its small size and high lasing efficiency. Although it works reliably at a CW power level of several tens of milliwatts, this power level is smaller than that of the pulse laser system by as much as several orders of magnitude. The generated new sequence is more suitable for the situation that calls for the accumulation of energy by plenty of pulses to improve SNR.