Q$3.$

$($a$)$ Draw $3$ diagrams showing the mechanisms of absorption$,$ spontaneous emission and stimulated emission in a $2-$level electronic excitation scheme, and briefly describe the differences between photons emitted via stimulated and spontaneous emission.

$4$ marks

$($b$)$ What is the attenuation limited link length for the system parameters shown below?

Launched power $=4mW$

Fibre attenuation $=0\mathrm{.}25d\frac{B}{k}m$

Fibre dispersion $=4\mathrm{.}0$ ps$/$km$.$nm

Joint loss, power penalties and system margin $=10dB$

Receiver sensitivity $=-36dBm$

Maximum tolerable pulse spreading $=0\mathrm{.}2T$ where $T$ is the period of the data transmission.

$3$ marks

$($c$)$ What is the dispersion$-$limited link$-$length of the system in $($b$)$ if the required bit$-$rate is $500$ MHz and the optical source linewidth is $2$ nm $?$

$3$ marks