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8. Optical Modulation

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.

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8. Optical Modulation

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  1. 8. Optical Modulation

  2. 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

  3. 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 (Nph)

  4. Carrier Density Rate Equation vg: group velocity L: confinement factor a: gain constant hg: gain suppression coef.

  5. Photon Density Rate Equation bsp: percentage of spontaneous emission coherent and in phase with stimulated emission (~10E-5) tsp: photon decay constant atot: total cavity loss

  6. Steady State Solution • Steady state requires the carrier density and the photon density to b constant • The photon density rate equation yields • Nph must be positive which requires

  7. 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

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

  9. DC Laser Diode Response

  10. Initial Photon Density • Rate of increase of photon density (dNph/dt) is essentially zero when Nph 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 Nth

  11. Initial Carrier Density • Rate of increase of N (dN/dt) is positive when Nph is small • Causing an increase in the carrier density

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

  13. Relaxation Oscillations • When N drops below Nth • N starts increasing again • The process repeats itself as a damped oscillation • N stays very close to Nth

  14. Final Pulse Response • When the laser turns off • N decreases • When N<Nth the photon density drops to essentially zero

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