Photon counting pseudorandom noise code laser altimeters

Abstract

We have characterized the performance of photon counting pseudorandom noise (PN) code laser ranging system at both 1064 and 1570 nm. The PN code modulation is a binary signal which is widely used in telecommunications and RF ranging applications. When used in lidar PN code modulation allows the transmitter to operate with a peak power, equal to only twice the average power which is compatible with the optical fiber communication technology. Our photon counting receiver approach accumulates the received photon stream into a histogram, then cross correlates the histogram with the original PN code sequence. The time of flight is given by the location of the peak in the correlation function. For multiple targets within the laser beam footprint, each target produce a peak in the correlation function at the delay corresponding to the target range. Our approach for altimetry also uses receiver range bin much shorter than the PN code bit period. This permits the correlation to be calculated with higher time resolution than the PN code bit period and substantially improves the time and range resolution. Our laboratory measurements with a breadboard receiver shows a greater than 90% probability of detection in a 0.5 msec integration time with 50 fW average received optical power at 1064 nm when using an InGaAsP hybrid photomultiplier tube detector. We have also achieved a less than 5cm rms ranging precision, when using a 250 psec range bin and at about 100 KHz detected signal photon rate.