On chaotic pulsation in semiconductor laser diodes, the optical systems used in determining the occurrence of chaotic pulsation, and the application of this to the encryption of data signals

Luke Pomfrey, Mohammed Moussa, Alexander Shalashilin, Philip Smith, Wai Cheong Tou, Gihan Weerasinghe

Self-pulsation is the term commonly used to describe the pulsed emission from semiconductor laser diodes operated under DC bias. Self-pulsating lasers have the advantage that their short coherence length, due to the pulsation, makes them less sensitive to optical feedback and gives them a low relative noise intensity. These features are important in the use of semi-conductor lasers in Compact Disc (CD) players and other optical storage systems.

The behaviour of these lasers can be described using a rate equation model. It can be seen that, if a suitable sinusoidal electrical modulation is added to the rate equations the frequency of free-running self-pulsations will become mode-locked to the frequency of the modulation. By moving the modulation frequency away from the natural resonant frequency of the self-pulsation then the two frequencies can be made to compete, resulting in chaotic self-pulsation.

In this paper we investigate the effects of changing the electrical modulation frequency applied to the laser, and the impact it has on the chaotic pulsation of the laser. We also investigate methods of discerning whether the laser is in fact pulsating chaotically using an auto-correlator with a second harmonic generation crystal.

We will be looking at the application of the chaotic self-pulsation of the laser to forms of hardware encryption in network interfaces.

L Pomfrey, M Moussa, A Shalashilin, P Smith, Wai Cheong Tou, G Weerasinghe. On chaotic pulsation in semiconductor laser diodes, the optical systems used in determining the occurrence of chaotic pulsation, and the application of this to the encryption of data signals. March, 2008. http://lukepomfrey.org/group-project-report