Refractive index profiling of a GRIN optical fiber using a modulated speckled sheet of light

A method for calculating the refractive index of GRIN optical fiber From it s transverse interference pattern is presented. In this method the transver se interference fringe pattern through an optical fiber using a sheet of li ght is applied to get the refractive index profile of it. The optical fiber is not immersed in a matching liquid as used in different techniques [Bara kat N, El-Hennawi HA, El-Zaiat SY, Hassan R. Pure Appl Opt 1996;5:27]. In this method a sheet of He-Ne laser light is allowed to illuminate the fi ber. The light sheet is divided into two parts, the first is refracted thro ugh the fiber while the second is used as a reference beam. Interference pa ttern will be obtained between these two rays displaying the refractive ind ex variation along the fiber radius. The fringe shift of such interference pattern has been measured and used to calculate the deflection angle of light refracted by the fiber and the cla dding. An equation is derived to calculate the refractive index profile dif ference ratio delta n at different positions across the fiber cross section in terms of the corresponding deflection angle and is verified experimenta lly. The optical path difference between these two rays (refracted and reference beam divided by the fiber) has been derived and the fringe shift obtained has been used to calculate the refractive index profile of the fiber. Introducing a ground glass screen on the passage of the two light beams (re fracted and reference beam divided by the fiber), two superimposed identica l speckle patterns are formed leading to the formation of a third speckle p attern modulated by a grid structure displaying the optical thickness of th e fiber. (C) 2000 Elsevier Science Ltd. All rights reserved.