Comment on “Application of Fresnel diffraction to nondestructive measurement of the refractive index of optical fibers”

We examine the validity of Eq. (5) in [A. Sabatyan and M. T. Tavassoly, “Application of Fresnel diffraction to nondestructive measurement of the refractive index of optical fibers,” Opt. Eng., 2007, Vol. 46(12), pp. 128001-1–128001-7] to describe the diffraction pattern of an optical fiber. We show that Eq. (5) must be changed to account for the phase introduced by the fiber correctly. In particular, we show that, to agree with the implicit criterion for representing traveling waves, given when using the Fresnel–Kirchoff integral [Eq. (4)], the phase introduced by the object must be carried by positive exponentials. The results obtained correcting Eq. (5) are compared with those obtained by the rigorous solution of Maxwell equations in the optical fiber. We demonstrate the importance of adequately taking into account the criterion implicitly assumed in the Fresnel–Kirchoff integral to represent traveling waves.