(A) Multimode fibre is less lossy than single mode Choose the most appropriate answer from the options given below :
(B) The bandwidth of step index fibre is 50 MHz
(C) The graded index fibre has theoretically infinite bandwidth
(D) The step index fibre has numerical aperture of 0.2 to 0.5
(E) The graded index fibre has numerical aperture of 0.46 to 0.99
(B), (C), (D) Only
Statements (B), (C) and (D) are the accepted figures — option 2 — while (A) and (E) each state the opposite of the truth.
(A) is false, and the reason is instructive. Single-mode fibre is the less lossy of the two, reaching about 0.2 dB/km at 1550 nm against 2 to 3 dB/km for multimode. Its very small core carries the light almost entirely in the purest central region of the glass, and it suffers none of the mode-coupling and mode-dependent losses that a multimode core does. The common intuition — that a bigger core must be better — is exactly backwards.
(D) — the numerical aperture of step-index fibre.
\(NA=\sqrt{n_{1}^{2}-n_{2}^{2}}\)
For a multimode step-index fibre with a comparatively large index contrast this lies in the range 0.2 to 0.5, which is what allows an inexpensive LED to be coupled into it. The quoted band is the standard one.
(E) is false because the numbers are far too high. A graded-index fibre's NA is smaller than a step-index fibre's, typically about 0.2, and it varies across the core because the index does. A value approaching 0.99 would need an index contrast no glass fibre possesses — and since NA is a sine, values close to 1 imply an acceptance angle approaching 90°, which is physically absurd for a guided structure.
(C) — the "infinite bandwidth" claim, properly understood. Grading the index equalises the transit times of the different modes: rays taking the longer off-axis paths spend that extra distance in lower-index glass where they travel faster. For an ideal parabolic profile the compensation is exact to first order, so intermodal dispersion vanishes altogether and the theoretical bandwidth is unlimited. In practice profile errors and material dispersion cap it near 1 GHz·km — but the statement is about the theoretical limit, and in that sense it stands.
(B) — the 50 MHz figure is the conventional textbook value for the bandwidth-distance product of multimode step-index fibre, whose large intermodal dispersion of tens of nanoseconds per kilometre restricts it severely. It is the contrast with (C) that the question is drawing.
| Fibre | NA | Bandwidth | Loss |
|---|---|---|---|
| Multimode step index | 0.2 – 0.5 | ~50 MHz·km | 2 – 3 dB/km |
| Multimode graded index | ~0.2 | ~1 GHz·km | ~1 dB/km |
| Single mode | 0.1 – 0.15 | > 100 GHz·km | 0.2 dB/km |
Hence, the correct statements are (B), (C) and (D).
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:
