Eye-Safe Semiconductor Lasers for Lidar: Experimental Studies of Coherence and Atmospheric Propagation

Shorter-wavelength lasers (with λ of order 1.5-2.5 μm), both semiconductor and solid-state, are showing increasing promise for lidar applications, with improved power levels and frequency stability. Advantages over existing long-wavelength systems include: compactness and robustness, compatibility with cheap components and optical fibres from the telecommunications industry, and the lack of need for cooled detectors. It is thus becoming important to investigate the primary aspects of such lasers to establish their performance characteristics. The potential for semiconductor lasers in lidar/rangefinder systems is currently limited by their relatively low output power and short coherence length (or equivalently, their broad spectral bandwidth). This reduces the level of performance in range and velocity resolution, as well as leading to a degraded signal-to-noise ratio. In fact, the two considerations of power and linewidth are closely linked, because attempts to increase the output by turning up the laser drive current eventually lead to a significant increase in bandwidth. This "rebroadening" effect is poorly understood; it imposes a limit on the coherence performance of this type of laser, and hence also on its capabilities in laser radar applications. The work reported here could benefit understanding of the serious problem of rebroadening. Further limitations to the use of semiconductor lasers may result from the increased effects of atmospheric turbulence at these wavelengths.