Power requirements for Rayleigh beacon generation in laser beam projection systems

The task of delivering sufficient level of airborne laser energy to ground based targets is of high interest. To overcome the degradation in beam quality induced by atmospheric turbulence, it is necessary to measure and compensate for the phase distortions in the wavefront. Since, in general, there will not be a cooperative beacon present, an artificial laser beacon is used for this purpose. In many cases of practical interest, beacons created by scattering light from a surface in the scene are anisoplanatic, and as a result provide poor beam compensation results when conventional adaptive optics systems are used. Three approaches for beacon creation in a down-looking scenario have been developed and commonly used in simulating laser beam projection systems utilizing down-looking scenarios. In the first approach, the entire volume of the atmosphere between transmitter and the target is probed by scattering an initially focused beam from the surface of the target. The second approach utilizes generation of an uncompensated Rayleigh beacon at some intermediate distance between the transmitter and the target and allows compensation for only part of the atmospheric path. Lastly, a more advanced technique of bootstrap beacon generation that allows achieving dynamic wavefront compensation creating a series of compensated beacons along the optical path, with the goal of providing a physically smaller beacon at the target plane. For all case sceneries discussed above, it is crucial to estimate the power requirements for single Rayleigh beacon generation as a function of the distance from the transmitting laser source. Sufficient amount of energy is required to allow for wavefront sensor measurements with satisfactory signal-to-noise ratio. In this paper, the calculations conducted in order to estimate the power requirements for a single Rayleigh beacon as a function of the laser altitude and the slant range between the transmitter and the generated beacon are presented. To fully understand the results presented here, some physical understanding of nature of scattering is required. There are four main types of scattering the transmitting light can experience: Rayleigh, Raman, Mie, and resonance scattering. Raman scattering is very weak typically, only one photon out of 107 is Raman scattered. Resonance scattering requires tuning the laser to the frequency closely comparable to the internal rotational or vibrational frequency present in the specific atom or molecule. Presence of dust, fog, haze, or clouds cause the Mie scattering and may vary unpredictably. In practice, it is important to produce a stable and constant intensity beacon, and therefore the generation of a Reileigh beacon for laser beam projection systems should not rely on surrounding atomic and molecular content, and unpredictable events such as presence of clouds, haze, dust or fog. Therefore, to properly estimate power requirements for Rayleigh beacon generation it is reasonably to only rely on always present elastic Rayleigh scattering excluding impact of other types of scattering. The results presented in this paper form a lower bound on power requirement for Rayleigh beacon generation, and occasional presence of other types of scattering may only improve the final result by boosting the scattered energy.