8. Optical Modulation

Modulation Techniques Direct modulation of laser diode –Vary the current supply to the laser diode –Directly modulates the output power of the laser External modulation –Change the transmission characteristics –Change the power of a continuous wave laser

Rate Equations of Laser Diodes The semiconductor laser is essentially a two-level laser Light emission based on two coupled rate equations –The carrier density of excited electrons (N) –The photon density (N ph )

Carrier Density Rate Equation v g : group velocityL: confinement factor a: gain constant  g : gain suppression coef.

Photon Density Rate Equation  sp : percentage of spontaneous emission coherent and in phase with stimulated emission (~10E-5)  sp : photon decay constant  tot : total cavity loss

Steady State Solution Steady state requires the carrier density and the photon density to b constant The photon density rate equation yields N ph must be positive which requires

Threshold Condition The carrier threshold condition is where Since the gain is also given by Resulting in a threshold carrier density of The photon density then becomes

Steady State This means that in steady state N th >N High photon flux occurs when N~N th With N~N th Resulting in The total power is

DC Laser Diode Response

Initial Photon Density Rate of increase of photon density (dN ph /dt) is essentially zero when N ph is small –It will not become significant until the net gain is positive –This is equivalent to –When the laser diode is initially turned on the photon density stays essentially zero until N reached N th

Initial Carrier Density Rate of increase of N (dN/dt) is positive when N ph is small –Causing an increase in the carrier density

Exceeding Threshold When N>N th –Optical gain becomes positive –Photon density increases rapidly –Exceeds the steady state value The increase in N ph causes –decrease in the dN/dt because of the stimulated emission term is negative When N ph reaches a certain value dN/dt becomes negative –N starts to decrease

Relaxation Oscillations When N drops below N th –N starts increasing again –The process repeats itself as a damped oscillation N stays very close to N th

Final Pulse Response When the laser turns off –N decreases –When N<N th the photon density drops to essentially zero