5. Quantum efficiency in terms of carrier
lifetime
where Δn(0) is the initial injected excess electron density and τ represents the total carrier
recombination lifetime. The excess density of electrons is Δn
13. Modulation bandwidth
• This corresponds to the electrical 3 dB point or the frequency at which the
output electric power is reduced by 3 dB with respect to the input electric
power.
• As optical sources operate down to d.c. level we only consider the high-
frequency 3 dB point, the modulation bandwidth being the frequency
range between zero and this high-frequency 3 dB point.
15. Disadvantages of LED
(a) generally lower optical power coupled into a
fiber (microwatts);
(b) usually lower modulation bandwidth;
(c) harmonic distortion.
