Modeling a semiconductor laser by a two level system. Study of the first optical instabilities

Eugenio Roldán

M. Ganesh Madhan

Taciano Amaral Sorrentino

We analyze the dynamics of semiconductor lasers radiation frequency when these lasers are subject to different conditions of optical feedback, and we demonstrate how it is possible to control the emission frequency using the different configurations explored. In the first system studied a semiconductor laser is fed back with light which polarization is orthogonal to the output polarization, spectrally filtered by a cesium atomic vapor. This system, besides presenting an effective technique to stabilize the laser frequency and reduce the laser linewidth, was the first to exhibit frequency bistable and multistable optical regimes with amplitude practically constant. This unique feature opens the way for applications in all-optical FM logical devices. The bistable and multistable regimes are interpreted through a phenomenological model, and, for the bistable regime, we discuss a rate equation model, built taking in account thermal effects and gain saturation. We also present an all-optical frequency switch, the first of his kind. The second system is used to investigate the behavior of the emission frequency of a semiconductor laser with an external cavity, inside which is placed an atomic filter. For different values of the injection current we observe the laser frequency locking in the atomic line, bistability and reproducible frequency instabilities.

Lloyd W. Hillman

Fluctuations in the amplitude, frequency, and polarization of the light emitted by semiconductor laser diodes limit the performance and reliability of many optical systems, such as communication links, optical radar, and optical disk read/write memories. Although it seems logical to explain this noise as random or stochastic, such erratic behavior can arise from completely deterministic models of lasers that retain the dynamics of the coherently induced polarization. This has been shown to be true for the simple model of a laser based on a two-level resonance, which results in the coupled Maxwell-Bloch equations. These equations are termed coherent because the Bloch equations describe, not only the inversion, but also the induced polarization of the gain media. The Maxwell-Bloch equations predict an array of instabilities and novel operation conditions for a laser: the output light may oscillate regularly or pulsate chaotically. Such behavior is not found in the incoherent population rate equation that adiabatically eliminated the dynamics of the polarization. However, the Maxwell-Bloch equations apply only to dilute materials in which the total gain is described by an ensemble average over many noninteracting molecules, such as gas lasers.

Claus F. Klingshirn, J. Grohs, M. Wegener

Dominic F. G. Gallagher, Ian H. White, J.E. Carroll

A new form of optical Instability in semiconductor-injection lasers is reported in which the position of the optical pulse, generated by a gain switched laser, can take one of two stable positions within a clock cycle. Unlike some forms of optical bistability,1–3 the contrast ratio between the light levels for the two states is high (>20 dB). The optical power required to trigger from one state to the other is typically a few percent of the laser output power, and the duration of the trigger pulse needs be only a fraction of the clock cycle. Trigger energies can, therefore, be in the femtojoule range. Both electrical and optical trigger mechanisms are discussed.