SPECTRUM INVARIANCE FOR A PARTICULAR CLASS OF OPTICAL-FIELDS PROPAGATED IN FAR ZONE

'This paper describes invariance of the normalized optical spectrum for a particular class of optical fields propagated in the far zone from a secondary, spatially, partially coherent source. The optical field across the secondary planar source is inhomogeneous to give the complex degree of spectral coherence such that mu=h{alpha omega(rho(1)-rho(2))}exp{i epsilon omega(rho(1)(2)-rho(2)(2))}, where alpha, epsilon are constants, omega, is optical frequency, and rho(1), rho(2) denote two points in the secondary source. This expression for mu is the same as obtained in the Fresnel zone from a primary, spatially incoherent source. The invariance law does not hold for the spectrum of the light propagated from the primary source.