Twisting Effect on the Mean Spot Radius out of Optical Fiber

In this work, the effect of variation of twist angle from (0 °- 810°) by step of(90°) on the mean radius ofthe output spot from a single modes optical fiber was studied. The twist had been affected at four different positions from (85-100) em far from the end of the fiber by step of (5) em. A comparable study had been made also, when a chopper was used at the entrance of the optical fiber. It was found that twisting the fiber at further distances from the output end shows only relatively small decrease in the relative diameter of the spot. The relative radius of the spot remained nearly constant for most twisting angles less than ( 400 °) especially with presence of the chopper. We found that the asymmetry in stress level in the fiber induced by twisting, affects the mode coupling, and with the presence of a chopper, enlarge the relative geometrical diameter of the spot.