17-07-2014, 10:12 AM
Modulation characteristics of LED and LASER
Modulation characteristics of LED and LASER.ppt (Size: 548 KB / Downloads: 12)
MODULATION CHARACTERISTICS OF LED AND LASER
In fiber optic communication the carrier is light.
For sending an information through a fiber optic system, the information is encoded on the light wave by changing the light irradiance of a function of time. such a process of varying light level with time is known as modulation.
Modulation done in two ways.
1.Analog modulation
2. Digital modulation
In analog modulation the light band is a continuous manner.
Modulation bandwidth [BW]:
Modulation bandwidth is the range of modulating frequencies with in which detected electric power declines at -3dB BW=0.35/tr Bandwidth is inversely proportional to the lifetime
Rate equation:
Rate equation describes how electron density and photon density change with respect to time
Light emission based on two coupled rate equations
The carrier density of excited electrons (N)
The photon density (Nph)
Switching characteristics of laser
For digital modulation ,we apply a step like input. The resulting electron and photon densities will display a damping-oscillation behavior.
There is a delay time before a laser start to generate light (1.5ns)
It takes about 8 ns for laser oscillation settle down.
Limitations of direct modulation:
*Turn on delay and resonance frequency are the two major factors that limit the speed of digital laser modulation
*Saturation and clipping introduces nonlinear distortion with analog modulation (especially in multi carrier systems)
*Nonlinear distortions introduce second and third order inter modulation products
*Chirp: Laser output wavelength drift with modulating current is also another issue