The core diameter of single mode fiber is in the order of
10 µm
A single-mode core is about 8 to 10 µm across — option 2 — and the figure is not arbitrary; it is fixed by the condition for single-mode operation.
The number of guided modes is governed by the normalised frequency,
\(V=\dfrac{2\pi a}{\lambda}\sqrt{n_{1}^{2}-n_{2}^{2}}=\dfrac{2\pi a}{\lambda}\,NA\)
and only one mode propagates when
\(V\lt2.405\)
the first zero of the Bessel function \(J_{0}\). Put the numbers in for the 1.55 µm window with NA = 0.12:
\(a\lt\dfrac{2.405\times1.55}{2\pi\times0.12}=4.9\ \mu\text{m}\)
a radius of about 5 µm, so a diameter near 10 µm. The answer follows from the operating wavelength and nothing else — which is why the core cannot be shrunk indefinitely.
| Option | Size | Verdict |
|---|---|---|
| 100 µm | Multimode / plastic fibre | ✗ V ≈ 24, many modes |
| 10 µm | Single mode | ✓ |
| 1 Å | 0.1 nm — an atom | ✗ Absurdly sub-wavelength |
| 1 nm | A few atoms | ✗ Same objection |
Options 3 and 4 fail on physical grounds, not just numerically. A waveguide much smaller than the wavelength — 1550 nm here — guides nothing; the field is not confined but radiates away. An Ångström is roughly one atomic diameter, so such a "core" could not be built even in principle.
What the small core buys. Only one path exists, so intermodal dispersion vanishes entirely and the bandwidth-distance product rises from a few tens of MHz·km for step-index multimode fibre to tens of THz·km. What it costs is handling: a 9 µm core demands a laser rather than an LED, fusion splicing with sub-micron alignment, and connectors held to comparable tolerance — the reason multimode fibre with its 50 or 62.5 µm core survives for short in-building links.
Hence, the core diameter is of the order of 10 µm.
Similar Questions
If numerical aperture and fractional refractive index of an optical fibre are 0.22 and 0.012, respectively. The refractive index of core (µ1) and cladding (µ2) will be
Which of the following is not a usual classification of optical fibre ?
Assertion (A) : In the propagation of light along multimode graded index fibre, the rays moving toward the cladding travel longer path with greater velocity than the rays travelling shorter path near the axis of fibres. These cause less spreading as compared to spreading caused by multimode step index fibre.
Reason (R) : The velocity varies because refractive index of the multimode graded index fibre increases with radial distance from the centre (axis).
An optical fibre has numerical aperture (NA) of 0.3 and refractive index $\eta_2$ of cladding material is 1.6. What is the refractive index of core material?
Match the following lists :
List – I List – II a. \(\pi a\,NA/\lambda\) i. attenuation factor (dB/km) b. \(10\log_{10}\left(\dfrac{P_{in}}{P_{out}}\right)\) ii. Intermodal time delay c. \(\dfrac{l}{L}\dfrac{dt_g}{d\lambda}\) iii. Dispersion causing pulse spreading d. \(\dfrac{L(n_1-n_2)n_1}{n_2c}\) iv. Number of modes produced by an optical fibre Assertion (A) : Attenuation and dispersion have negative effects on the propagation of signal in the optical fibres.
Reason (R) : Optical signal degradation is caused due to structural imperfections of the fibre material.
Select your answer using the codes given below.
A fiber has a core radius of 6 μm, operating wavelength = 1550 nm. The V-number of the fiber is given by :
In linearly polarized modes traversing in the optical fibers the LP01 is exactly equal to :
The value of Numerical Aperture in case of optical fiber is
The core of an optical fiber has
Important Questions from Optical Fiber
The material used for making optic-fibre cable in general is-
Multimode step-index fiber with a core diameter of 80 μm and a relative index difference of 1.5% is operating at a wavelength of 0.85 μm. If the core refractive index is 1.48, then the normalized frequency for the fiber is
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:
In optical fibers, the Rayleigh scattering is proportional to:
A graded indexed optical fiber has a parabolic refractive index profile (α = 2). If the fiber has a numerical aperture = 0.22 the total number of guided modes at a wavelength of 1310 nm is given by:
