Propagation of finite cross-section laser beams in sea water.

An expression is derived for the mean irradiance distribution of a finite cross-section laser beam propa: gating in sea water. The effects of large-scale refractive variations on beam quality due to temperature and salinity fluctuations and the incoherent scattering of suspended biological particles with indices of refraction close to that of sea water are included. The analysis is general; it is valid in both the near and the far field of the laser transmitting aperture and also for an arbitrary complex electromagnetic optical wavefunction in the exit pupil of the transmitting aperture. Analytic results are presented for the beam pattern and spot size of a Gaussian laser beam. In particular, the manner in which image resolution, beam spread, and the parameters that describe the inhomogeneities in the water are quantitatively related is discussed. Furthermore, we present a general procedure for estimating beam spot size. Finally some comments regarding focused-beam propagation and limiting angular resolution are given. It is shown that when underwater inhomogeneities dominate the small-angle scattering, the limiting angular resolution is independent of wavelength.