What Is a Step Reflex?

Step-index fiber is a type of fiber with a step-type refractive index profile.

Optical fibers are classified according to the refractive index distribution: step-type and graded-type fibers.

Optical fibers can have different refractive index profiles. The simplest refractive index profile is rectangular, except for optical fibers that transmit light through the air-glass section. The core's refractive index is higher than the cladding's refractive index, so that the input light can be at the core-cladding interface The total reflection continues to move forward. The refractive index of this fiber core is uniform, and the refractive index of the cladding is slightly lower.

The assumption of step refractive index distribution is usually used in the calculation of fiber optics. However, the refractive index distribution of fiber obtained by standard manufacturing techniques is more complicated. In particular, the preferential evaporation of dopants during the preparation of a preform usually results in a depression in the center of the refractive index distribution (assuming that the preform is made by a chemical vapor deposition method). For a refractive index distribution fiber that is far away from the step refractive index distribution, an effective step refractive index distribution can be defined, and a mode property similar to the actual distribution can be obtained.

The propagation mode of a step-index fiber can be described by a Bessel function, and then multiplied by an exponential phase factor exp (i z) to characterize the longitudinal phase change. If the radial variation is taken into account, the field strength at the core is proportional to the zero-order Bessel function of the first type, and the cladding part is described by the modified Bessel equation of the second type. The mode function and its first derivative are usually continuous at the core-cladding interface.

Many fiber parameters, especially the numerical aperture and V value, were originally only for step-index fibers. Of course, other types of fibers can use corresponding effective values. When V is large, the number of modes is proportional to V.

Some techniques in fiber manufacturing may cause the refractive index to deviate from the step refractive index profile. In some cases, it is thought to use this property to achieve specific guided wave characteristics. For example, a decrease in the refractive index of a region between the core and the cladding can introduce additional cut-off wavelengths, above which transmission losses become large ^[2] .