Intra modal and Overall Dispersion

Submitted to Chetan sir

Submitted byMonika Tanwani M.Tech DC II Sem

Types of Dispersion Two types 1.Intermodal or Modal dispersion. 2.Intramodal or Chromatic dispersion. Each type of dispersion mechanism leads to pulse spreading. As a pulse spreads, energy is overlapped. The spreading of the optical pulse as it travels along the fiber limits the information capacity of the fiber.

Intra modal Dispersion

Occur in all types of optical fibers and results from finite spectral line width of optical source.

Since all optical source do not emit just a single frequency but a band of frequencies.

Due to this there may be propagation delay differences between the different spectral components of the transmitted signal.

The delay differences caused by : 1.Dispersive properties of waveguide

material this type of dispersion is known a material dispersion.

2.Guidance effects within the fiber structure then this type of dispersion is known as Waveguide dispersion.

This cause broadening of each transmitted mode and hence intra modal dispersion.

Occurs because different colors of light travel through different materials and different waveguide structures at different speeds.

A light Pulse, although propagating as a fundamental mode, has number of spectral components, and the group velocity of the fundamental mode varies with frequency.

Therefore different spectral components in the pulse propagate with slightly different group velocitys, resulting in pulse broadning.

1. Sometimes called intramodal or colour dispersion.

2. Result from different group velocities of various spectral components launch into fiber from optical source.

3.It occurs when phase velocity of a plane wave propagating in the dielectric medium varies non linearly with the wavelength.

4.Material said to exibit material dispersion when the second differential of the refractive index w. r . t wavelength is not zero.{[d2n/d λ 2] ≠ zero}.

3. Typical optical source has optical output that spreads over a range of wavelength.

4.spectral width can defined as either ‘rms’ value.

5.In an optical fibre,the propagation velocity varies with wavelenth.

6.Thus a pulse made up of many wavelength will be spread out in time as its propagates.

(i) Group velocity is given by:

(ii) The time delay per unit length L of a medium is called Group delay of a medium and it is given by:

(iii) In a medium that is susceptible to material dispersion, the refractive index itself is a function of wavelength.

(iv) Free space propagation constant k is given by 2 π/ λ.

(v) The propagation constant in medium is given by

(VI) Using Eq 2 & 3 we will determine Group Delay.

(VII) Assume Propagation distance L.

(VIII) Group delay is given by

Where n1 is the refractive index of the core material.

(IX) Using Eq.4 we can also determine the time taken by pulse to propagate a distance L in fibre is given by

(X) The rms pulse broadening due to material dispersion is given by

Eq.6 (XI) Using Eq6 the rms spread is given by

(XII) Using Eq 8 several other parameters can be defined.

Caused by wavelength dependence

of distribution of energy for fundam-ental mode in fibre.

Mainly a problem for single mode, in multimode mode penetration into the cladding is very small relatively.

As wavelength increases an increasing

proportion of the mode energy propagates in the cladding. Altering the refractive index profile will alter the waveguide dispersion. Magnitude of waveguide dispersion is independent of wavelength.

1.Waveguide dispersion is chromatic dispersion which arises from waveguide effects: The dispersive phase shifts for a wave in a waveguide.

NOTE: The dispersive phase shifts for a wave in a waveguide differ from those which the wave would experience in a homogeneous medium.

2.The origin of waveguide dispersion can be understood by considering that a guided wave has a frequency-dependent distribution of wave vectors (k vectors), whereas a plain wave (as the reference case) has only a single wave vector, which points exactly in the propagation direction.

Waveguide Dispersion

3.Waveguide dispersion may be tailored via the fiber design to obtain the desired dispersion properties.

4.For fibers with large mode areas, waveguide dispersion is normally negligible, and material dispersion is dominant.

5. Waveguide dispersion occurs when the speed of a wave in a waveguide (such as an optical fiber) depends on its frequency for geometric reasons, independent of any frequency dependence of the materials from which it is constructed. More generally, "waveguide" dispersion can occur for waves propagating through any inhomogeneous structure (e.g., a photonic crystal), whether or not the waves are confined to some region.

Dispersion SpecificationsMultimode Fibre • For multimode fibre dispersion is both chromatic

and modal • Dispersion rarely specified, instead fibre

bandwidth is specified • Maximum bit rate can be found from bandwidth

Singlemode Fibre • For singlemode fibres chromatic dispersion is

present (also PMD?) • Chromatic dispersion is specified as ps/nm-km • ITU defines limits for various types and

wavelengths • Total dispersion must be calculated

Multimode Fibres (1) The overall dispersion in multimode fibers

comprises both chromatic and intermodal terms.

(2).The total rms pulse broadening σT is given by:

σT=(σc2+σn

2) where

σc is the chromatic broadening

σn is the intermodal broadening

(3).Since waveguide dispersion is generally negligible compared with material dispersion in multimode fibers, then σc =σn.

Single Mode Fibre

1.In this the expression can be separated into 3 composite dispersion components.

2.The dominating effects are:

(a).The material dispersion parameter DM defined by

λ / c |d2n/dλ2| where n=n1 or n2 for the core or cladding.

(b). The waveguide dispersion parameter DW, which is defined as:

  DW= -[(n1-n2)/ λc]V[d2Vb/dV2] where V is the normalized frequency for the fiber. Since the

normalized propagation constant b for a specific fiber is only dependent on V , then the normalized waveguide dispersion coefficient also depend on V.

(c). A profile dispersion parameter Dv.