Step index and graded index in singlemode fiber
Basically optical fibers can be classified according to the number of modes they can propagate. Single mode fibers will propagate only the fundamental mode. Multimode fibers will propagate many number of modes say hundreds of modes. Though this is the case, classification of optical fibers depends on more than the number of modes that a fiber can propagate.
Refractive index profile and core size of an optical fiber are used to distinguish single mode and multimode fibers. The refractive index profile of an optical fiber describes the numerical value of refractive index as a function of radial distance at any fiber diameter. By using the refractive index profiles the classification of single mode and multimode fibers can be done as shown below:
Single mode step-index fibers Single mode graded-index fibers Multimode step-index fibers Multimode graded-index fibers
For a step-index fiber (both single mode and multimode optical fibers), the refractive index of the core is uniform throughout the cladding region and undergoes an abrupt change at the core-cladding boundary. Step-index fibers obtain their name from this abrupt change called the step change in refractive index. If we look at the profile drawn from the measured data of a step index fiber, it looks like a step up and step down on a staircase as shown below:
For a graded-index fiber, if we draw the refractive index from the cladding region to the core, we can see it varies gradually as a function of radial distance from the fiber center as shown in the diagram below:
Both Single mode and multimode optical fibers can have a step-index or graded-index refractive index profile. For a multimode fiber, the performance of graded-index fibers is superior compared to the step index multimode fibers.
Anyway, depending on the intended application, both type of multimode fibers improve system design and operation Performance. As far as single mode fibers are concerned, advantages for single mode graded-index fibers compared to single mode step-index fibers are relatively small. Almost all Single mode fibers have step index profile. Step index single mode fibers are denoted by SM MC or Single mode matched clad optical fiber.