In a graded-index Optical fiber, how is the refractive index distributed?
Decreasing continuously from center to edge
This question asks about the refractive-index profile of a graded-index optical fiber. An optical fiber guides light by total internal reflection, which requires the core to have a higher refractive index than the surrounding cladding. Fibers are classified by how the core index varies with radial position.
The two main profiles are:
- Step-index fiber: the core has a uniform (constant) refractive index, and there is an abrupt step down to the lower cladding index at the core–cladding boundary.
- Graded-index fiber: the refractive index is highest along the central axis of the core and decreases smoothly and continuously outward toward the cladding, typically following a near-parabolic profile.
Hence the correct description is that the index is decreasing continuously from center to edge.
The purpose of this gradual grading is to reduce intermodal (modal) dispersion. In a graded-index fiber, rays that travel near the axis pass through the high-index central region and therefore move more slowly, while rays that stray off-axis spend part of their path in lower-index outer regions where they travel faster and also follow curved (sinusoidal) paths. These effects compensate so that different modes arrive at the far end at nearly the same time, sharpening pulses and allowing higher bandwidth than step-index multimode fiber.
Why the other options are wrong:
- Higher in the cladding than the core: this would prevent total internal reflection and the fiber could not guide light; the core is always the higher-index region.
- Increasing from center to edge: this is the reverse of the actual graded profile and would not confine light to the axis.
- Uniform throughout the core: a constant core index describes a step-index fiber, not a graded-index one.
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In a multimode fiber (step index), number of modes passing at an operating wavelength of 1300 nm are 1000, the refractive index of the core is 1.50 and that of the cladding is 1.48. The value of core diameter is:
