If we want to send or receive information through optical fiber cables then we have to attach some light signals with the information. This can be achieved either by digital or analog signals.
Basically light modulations are of two types:
1. Direct Modulation
2. Indirect Modulation
Indirect modulation: System degradations do not affect the indirect modulation. In this an external modulator is used. It contains some amount of lithium niobate in it. Lithium niobate is present in the crystal form. Its refractive index can be controlled easily by controlling the electric field. As the refractive index will come under control, the incoming signals will also come under control. When we pass the light signals through the space then there will be change in the cumulative phase.
Usually external optical modulators are used for two reasons. One is for switching of optical signals and other is for time division multiplexing.
Direct Modulation: In this technique the direct current which is supplying the light source is modulated. This is done by making small changes to biased current of the laser. In this the value of the biased current is maintained below the threshold value. It is done in order to make the laser on and off according to the situation. The wavelength of the circuit depends upon the type of modulation technique used. The frequency modulation of laser can also be done by using the laser bias current.
As we all know that wavelength of the circuit also depends upon the frequency. So, when the laser frequency is more than the threshold frequency then its wavelength can be changed up to a range of 2 X 10-1m. This change is done by controlling the bias current.
In brief we can say that in direct modulation by changing the bias current we can create multiple channels.
