EFFECTS OF A MODIFIED SPECTRAL MODEL ON THE SPATIAL COHERENCE OF A LASER-BEAM

The influence of a modified (bump) spectrum of refractive index fluctu ations on the spatial coherence of an optical wave is studied here and compared with that based on a von Karman spectrum. Analytical express ions are derived for the mutual coherence function (mcF) and wave stru cture function (WSF) of a lowest-order Gaussian beam wave from which t he beam spot size and degree of coherence are deduced. The qualitative behaviour of beam spreading and coherence length is basically the sam e for both spectral models. Also, when the radius of the Fresnel zone and initial beam radius are of comparable size, the presence of a spec tral bump appears to have minimal effect on spatial coherence for all beams. However, the choice of spectral model is important for certain ranges of parameters. In particular, the implied spatial coherence len gth for a collimated beam based on the modified spectrum is significan tly smaller than that based on the von Karman spectrum whenever the Fr esnel zone is either much larger or much smaller than the initial beam radius, whereas for a focused beam the predicted coherence length bas ed on the modified spectrum is slightly larger when the Fresnel zone s ize is much smaller than the initial beam radius.