1.5 microns: the future of unattended aerosol lidar?

Unattended lidars for clouds and aerosols are currently deployed at tens of locations in the U.S. and in other countries. These lidars operate in the mid-visible region. The micro-pulse lidar known as MPL is a very successful instrument in terms of numbers deployed, and it is also very sophisticated, because its operating wavelength of 523 nm combines two challenges: it is at the peak of the eye's sensitivity, so eye safety can only be achieved with very low pulse energy (on the order of 10 muJ); and it is at the peak of the solar spectrum, so minimizing sky background radiation is a major problem. In order to operate during daytime, micro-pulse lidars must have an extremely narrow field of view (FOV) and a very small optical bandpass. They are consequently not inexpensive, they tend to suffers from mechanical instability, and they are not field serviceable when certain types of failures occur. In order to establish the optimum wavelength region for an unattended aerosol lidar, the spectral dependencies of eye safety standards, sky radiance, laser availability, detector performance, atmospheric optical properties, and optical materials are presented. In particular, eye safety standards allow a fluence of 1 J/cm² at 1.5 micron, which is 10⁷ times the fluence allowed in the mid-visible. Pulse energies on the order of 10 mJ are sufficient to make daytime operation easy and low-cost. A conventional bistatic lidar configuration can then be used with a field of view on the order of milliradians, which eliminates the problem of mechanical instability, and the optical bandpass can be limited with an inexpensive interference filter. In addition, the InGaAs detectors used at 1.5 microns are much less susceptible to optical damage than the Geiger-mode Silicon APDs used in visible-light lidars