The core of an optical fiber has
a higher index of refraction than the cladding
Guidance in a fibre rests entirely on total internal reflection, and total internal reflection requires the core to be the denser medium — option 3.
\(n_{1}\gt n_{2}\qquad\text{core}\ n_{1},\ \text{cladding}\ n_{2}\)
Why the inequality must run that way. Snell's law at the core-cladding boundary gives
\(n_{1}\sin\theta_{1}=n_{2}\sin\theta_{2}\)
A critical angle exists — an angle beyond which no refracted ray can be constructed, so all the energy is reflected — only if the refracted ray bends away from the normal, that is only if \(n_{2}\lt n_{1}\). Then
\(\theta_{c}=\sin^{-1}\dfrac{n_{2}}{n_{1}}\)
and every ray striking the wall at more than \(\theta_{c}\) is reflected without loss, bouncing along the fibre indefinitely. Reverse the indices and light simply leaks into the cladding at every bounce.
| Option | Verdict |
|---|---|
| Lower than cladding | ✗ Light escapes; no guidance |
| Lower than air | ✗ Impossible — air is n = 1, the lowest of ordinary media |
| Higher than cladding | ✓ |
| Equal to air | ✗ No boundary contrast at all |
How the contrast is made. Both core and cladding are silica; the difference is created by doping — germanium or phosphorus raises the core index, fluorine or boron lowers the cladding index. The difference is deliberately tiny,
\(\Delta=\dfrac{n_{1}-n_{2}}{n_{1}}\approx0.003\)
for a typical \(n_{1}=1.48,\ n_{2}=1.46\), because a small contrast keeps the range of guided ray angles narrow and so keeps intermodal dispersion low.
Why a cladding is needed at all, given that air has a still lower index: a bare glass rod would guide light, but every speck of dust or drop of moisture touching the surface would destroy the total internal reflection at that point. The cladding keeps the guiding boundary buried inside the glass, safe from contamination and from the mechanical damage that surface scratches would cause.
Hence, the core has a higher index of refraction than the cladding.
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Losses in optical fibers are caused by
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2. Microbending
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4. Step index profileOf these statements :
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Reason (R) : Low power LASER beams are considered to be very powerful as compared to high power ordinary light beams.
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2. The refractive index undergoes sudden change at the cladding boundary.
3. It provides better bandwidth and the data rate than the multimode step index.
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1. Splicing
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3. Scattering
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