Dispersion:
Dispersion is the broadening of actual time-width of the pulse due to material properties and imperfections.
As pulse travels down the fiber, dispersion causes pulse spreading. This limits the distance travelled by the pulse and the bit rate of data on optical fiber.
In a fiber three distinct types of distortion are observed:
1. Intramodal dispersion:
Pulse broadening within a single mode is called as intramodal dispersion or chromatic dispersion.
Since this phenomenon is wavelength dependent and group velocity is a function of wavelength, it is also called as group velocity dispersion (GVD).
The two main causes of intramodal dispersion are as follows:
a. Material dispersion:
- It is the pulse spreading due to the dispersive properties of material.
- It arises from variation of refractive index of the core material as a function of wavelength.
- Material dispersion is a property of glass as a material and will always exist irrespective of the structure of the fiber.
b. Waveguide dispersion:
It occurs because a single mode fiber confines only about 80% of the optical power to the core.
Dispersion thus arises since the 20% light propagating in the cladding travels faster than light confined to the core.
The amount of waveguide dispersion depends on the structure of the fiber and can be varied by altering the parameters such as NA, core radius etc.
2. Intermodal dispersion:
Dispersion caused by multipath propagation of light energy is referred to as intermodal dispersion.
Signal degradation occurs due to different values of group delay for each individual mode at a single frequency.
In digital transmission, we use light pulse to transmit bit 1 and no pulse for bit 0.
When the light pulse enters fiber it is breakdown into small pulses carried by individual modes. At the output individual pulses are recombined and since they are overlapped receiver sees a long pulse causing pulse broadening.
Dispersion affect the transmission bandwidth:
For no overlapping of light pulse down on an optical fiber link, the Digital bit rate $(B_T)$
$${B_T} = \frac{1}{2τ}$$
where $2\tau$ is the pulse duration
Maximum bit rate $B_{T_{(max)} } = \frac{0.2}{σ}$ bits
where $\sigma$ represents the rms impulse response for the channel
For RZ; $\ \ \ $ $B_T$ = B.W
where B.W is the bandwidth
For NRZ; $ \ \ \ $ $B_T$ = 2 B.W
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