Fiber lasers: new effective sources for coherent lidars

摘要

Originally developed for telecommunications, fiber lasers are now becoming new effective sources for coherent lidars allowing new instruments to be designed. The advent of the double clad fiber, along with advances in semiconductor pump diode sources, have allowed rapid power scaling of both pulsed and CW fiber sources. The unique capabilities of fiber sources, coupled with significant commercial and academic progress in implementation, have driven fiber technology to enter active remote sensing markets as signal sources and amplification stages for direct detection lidars and coherent lidars as well. Some interesting fiber lasers benefit from a good transmission in the near infrared spectral band: Ytterbium lasers (1.0-1.06μm), Erbium lasers (1.48-1.62μm) and Thulium lasers (1.8-2.1μm). However, useful wavelengths have to be tuned between absorption H20 and CO2 lines. Eye safety may be an issue for atmospheric lidars. Above 1.4 μm, the maximum permitted exposure (MPE) is 3 orders of magnitude higher than for shorter wavelengths and an eye-safe operation is possible even with multi-watt lasers. Low power fiber lasers using single mode fibers have a good spatial quality. However, higher power lasers and amplifiers need larger fiber cores, to store enough energy and to avoid non linear effects. Trade-off between high power, single mode operation, stable polarization and spectral quality need to be considered for coherent lidars.