Modes of Propagation and Types of Optical Fibers - Civil Engineering (CE) PDF Download

Modes of Propagation 
Though optical fiber should support any numbers of rays for propagation practically it is found that it allow only certain restricted number of rays for propagation. The maximum number of rays supported by the fiber is called Modes of propagastion. Modes of propagation can be determined using parameter called V-number.

V-number (Normalised Frequency of the fiber):
An Optical fibre may be characterized by one more parameter called V-number. This determines the Number of modes supported for propagation in the fibre.

where “d is the diameter of the core, λ is wavelength, n₁ is the refractive index of the core and n₂ is the refractive index of the cladding. NA is numerical Aperture. If the fiber is surrounded by a medium of refractive index n₀ then

If  , the number of modes supported by fiber can be determined using the formula

Number of Modes ≈ V² /2

Types of Optical Fibers
Based on R I profile and Core size and Modes of Propagation Optical fibers are classified into 3 types,
(1) Single mode step index fiber.
(2) Multi mode step index fiber.
(3) Graded index multi mode or Grain index fiber.

Single mode step index fiber:
A single mode step index fiber consists of a very fine thin

core of uniform RI surrounded by Cladding of RI lower than that of Core. Since there is abrupt change of RI of Core and Cladding at the interface it is called Step index fiber. Since the Core size is small the Numerical aperture is also small. They accept light from laser source. Splicing is difficult. They are used in submarine cables.

Multimode step index fiber:
This is similar to single mode step index fiber with the exception that it has a larger core diameter. The core diameter is very large as compared to wavelength of light transmitted. A typical multimode step index fiber is as shown above. The numerical aperture is large because of large core size. They accept light from both laser as well as from LED. They are used in data links.

Graded index multimode fiber(GRIN fiber):
The multimode fiber, which has concentric layers of RI, is called GRIN fiber. It means

the R I of the Core varies with distance from the fiber axis. It has high R I at Center and falls rapidly as radial distance increases from the axis. The R I profile is as shown in fig. In GRIN fibers the acceptance angle and numerical aperture diminishes with radial distance. The light transmission is as shown above. They accept light from both laser as well as from LED. They are used for medium distance communication for example telephone link between central offices.

Attenuation and Fiber losses
An Optical signal passing through a fiber will get progressively reduced. This reduction or attenuation of signal may be defined as the ratio of the optical output power from a fibre of length L to the input optical power. It is expressed in terms of decibel / km.

Where P1 → Power of optical signal at launching end (input power)
P2 → Power of optical signal at receiving end (output power)

The attenuation is wavelength dependent and so the wavelength must also be specified.

The attenuation in fibers gives rise to the following three losses
(1) Absorption losses
(2) Geometric Effects
(3) Rayleigh Scattering.

Absorption:
Even very pure glass absorbs light of a specific wavelength. Strong electronic absorption occurs in UV region and vibrational absorption occurs in IR region of wavelength 7 micrometer to 12 micrometer. These losses are attributed due to inherent property of the glass and are called intrinsic absorption. However, this loss is insignificant.

Impurities are major extrinsic source of losses in fiber. Hydroxyl radical ions (OH) and transition metals like Nickel, Chromium, Copper, Manganese etc. have electronic losses near visible range of spectrum. These impurities should be kept as minimum as possible in the fiber. Intrinsic as well as extrinsic losses are found to be minimum at about 1.3 micrometer.

Geometric effects:
These may occur due to manufacturing defects like irregularities in fibre dimensions during drawing process or during coating, cabling or insulation processes.

The microscopic bends are the bends with radii greater than fiber diameter. The micro bends couple light between the various guided modes of the fiber and some of them then leak through the fiber.

Rayleigh Scattering: 
As glass has disordered structure having local microscopic variation in density which may also cause variation in RI. So light traveling through these structures may suffer scattering losses due to Rayleigh ie., scattering α = 1/λ⁴ . It means Rayleigh scattering sets a lower limit on wavelength that can be transmitted by a glass fibre at 0.8 micrometer below which scattering loss is appreciably high.

Dispersion: 
A pulse launched with a fibre gets attenuated due to losses in fibre. Moreover the incoming pulse also spreads during the transit through the fibre. So a pulse at the output is wider than the pulse at the input i.e., the pulse gets distorted as it moves through the fibre. This distortion of pulse is due to dispersion effects, which is measured in terms nanoseconds per km. There are three phenomena that may contribute towards the distortion effect.
(1) Material dispersion
(2) Wave guide dispersion
(3) Intermodal dispersion.

In optical fibers the Cladding material has uniform RI. But the RI of Core may remain constant or subjected to variation in a particular way. The curve representing the variation of RI with the Radial distance from the axis of the Core is called Refractive index profile. Modes of Propagation is the number paths of light rays along which the waves are in phase inside the fibre. The number of modes, a fibre can support depends on the ratio d/λ where d is the diameter of the core and λ is the wavelength of the wave transmitted.