Coherent beam combination of seven 1.5 µm fiber amplifiers through up to 1 km atmospheric turbulence: near- and far-field experimental analysis.

A laser testbed based on active coherent beam combination (CBC) of seven 1.5 µm, 3 W fiber amplifiers was developed for applications requiring high power such as power density deposition on targets or free space laser communication. For the first time to our knowledge, the frequency-tagging locking of optical coherence by single-detector electronic-frequency tagging technique was implemented in the field in real atmospheric turbulence conditions in a target-in-the-loop configuration. Successful combination was achieved after horizontal propagation of 311 m and 1 km, at 1.5 m above the ground, while the estimated average turbulence strength was Cn2∼4.10-14m-2/3. We present the CBC laser bench and an embedded near-field interferometer called PISTIL (PISton and TILt) able to measure the relative phase shift of each emitter. We show that this measurement can provide information on relative turbulence-induced phase variation of the combined laser beams. In particular, the far-field beam envelope wandering can be estimated through this diagnosis. Results are supported by an analytical model and confirmed by numerical post-analysis of measured far-field interference. This additional interferometer may improve CBC beam pointing through turbulence.

Raman lidar for hydrogen gas concentration monitoring and future radioactive waste management.

A multi-channel Raman lidar has been developed, allowing for the first time simultaneous and high-resolution profiling of hydrogen gas and water vapor. The lidar measures vibrational Raman scattering in the UV (355 nm) domain. It works in a high-bandwidth photon counting regime using fast SiPM detectors and takes into account the spectral overlap between hydrogen and water vapor Raman spectra. Measurement of concentration profiles of H2 and H2O are demonstrated along a 5-meter-long open gas cell with 1-meter resolution at 85 meters. The instrument precision is investigated by numerical simulation to anticipate the potential performance at longer range. This lidar could find applications in the French project Cigéo for monitoring radioactive waste disposal cells.