Which of the following is not a usual classification of optical fibre ?
Single mode graded index
The two independent labels. A fibre is described by how many modes it supports and by how the refractive index varies across the core. In principle that gives four combinations, but only three are found in practice:
| Type | Core diameter | Used for |
|---|---|---|
| Single mode step index | 8–10 μm | long-haul, highest bandwidth |
| Multimode step index | 50–200 μm | short links, cheap sources |
| Multimode graded index | 50–62.5 μm | LAN links, reduced modal dispersion |
| Single mode graded index | — | not a usual class |
Why the missing combination makes no sense. A graded index profile exists for one purpose: to equalise the transit times of the many modes in a multimode fibre. Rays that stray towards the cladding travel further, but the index there is lower so they travel faster, and all modes arrive together — which cuts modal dispersion dramatically.
A single mode fibre, by definition, carries only one mode. With just one path there are no differing transit times to equalise, so grading the index would serve no purpose. Single mode operation is obtained instead by shrinking the core until the normalised frequency falls below cut-off:
\(V=\dfrac{2\pi a}{\lambda}\,NA \lt 2.405\)
and a step profile does that perfectly well.
Note on the boundary of the rule. Specialised dispersion-shifted and dispersion-flattened single mode fibres do use tailored index profiles — but their purpose is to move the zero-dispersion wavelength by controlling material and waveguide dispersion, not to equalise modal delays, so they are not called "graded index" in this classification.
Hence, the classification that does not usually exist is single mode graded index.
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
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
The core diameter of single mode fiber is in the order of
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:
