The building blocks for a two-frequency laser LIDAR-RADAR: Concept and preliminary experimental results

A new principle of lidar-radar is theoretically and experimentally investigated. The proposed architecture is based on the use of a RF modulation of the emitted light beam and direct detection of the backscattered intensity. The use of a radar processing chain allows to obtain range and Doppler measurements with the advantages of lidar spatial resolution. The maximum range of this device is calculated, taking different possible improvements into account. In particular, it is shown that the use of a pulsed two-frequency laser and a spatially multimode optical pre-amplification of the backscattered light leads to calculated ranges larger than 20 km, including the possibility for both range and Doppler measurements. The building blocks of this lidar-radar are tested experimentally : the radar processing of an RF modulated backscattered cw laser beam is demonstrated at 532 nm, illustrating the Doppler and identification capabilities of the system. Besides, signal-to-noise ratio improvement by optical pre-amplification and filtering of the ASE noise is demonstrated at 1.55 μm. Finally, a two-frequency passively Q-switched Nd:YAG laser is developed. This laser then permits to obtain two-frequency pulses with tunable pulse duration (from 18 to 240 ns) and beat frequency (from 0 to 2.65 GHz).