A fiber-optic-based 1550-nm laser radar altimeter with RF pulse compression

The authors are developing a laser radar (lidar) altimeter that uses commercial off-the-shelf fiber-optic components and traditional RF and digital signal processing techniques to achieve fine-range accuracy despite a low transmit peak power. A 1550-nm operating wavelength was selected so that available erbium-doped fiber amplifiers (EDFAs) could be used to provide optical gain. The transmitted signal, an amplified, gated CW optical carrier intensity modulated with a linear-FM (chirp) RF signal, is launched into free-space via a fiber-optic collimator. An optical telescope serves as the receive aperture and couples the backscattered signal into a single-mode optical fiber prior to photodetection. The RF signal is then de-chirped and digitized for subsequent digital signal processing. Significant signal processing gains are achieved through coherent integration and pulse compression (FFT), resulting in fine-range accuracy. The authors have also performed preliminary design simulations for their pulse-compression laser radar for a spaceborne altimeter application and have compared its performance with that of the ICESAT (Ice, Cloud and land Elevation Satellite) GLAS (Geoscience Laser Altimeter System) lidar. Their simulations show that a pulse-compression laser radar with an operating wavelength around 1310 nm, 1 W of peak power, 5 GHz of transmitter bandwidth and a PRF of 4 kHz could provide 200 elevation samples per second with an accuracy comparable to that of the GLAS lidar, which transmits 15 MW peak power with a 40-Hz PRF and obtains 40 elevation samples per second. The five-fold increase in elevation sample rate could then be used to provide elevation measurements across a swath, increasing the elevation mapping rate of the polar ice sheets. Notable advances demonstrated in this proof-of-concept system include the ability to trade signal bandwidth for transmitter power in a laser radar while achieving a constant range accuracy, and the joining of modern radar processing techniques with fiber-optic technology in a laser radar. This is the first stage of a program to develop a near-infrared laser altimeter for spaceborne polar region monitoring.