INJECTION LOCKING OF SEMICONDUCTOR LASERS

Abstract

We have developed a theoretical model for the study of an injection-locked semiconductor laser. In our model, we consider the slave laser injection-locked when the carrier density of the slave laser goes below the free-running threshold or equivalently, where the output intensity of the slave laser is greater than its free-running output intensity. The static locking bandwidth is asymmetrical about zero detuning. The upper static locking boundary is a linear function of injection ratio and is independent of biasing current and linewidth enhancement factor. The lower static locking boundary is a linear function of the injection ratio only at low injection levels when the output intensity of the injection-locked laser is approximately equal to the free-running value. The lower static locking boundary can be extended further from the zero detuning, thereby increasing the static locking range, by increasing the biasing current or the value of linewidth enhancement factor. Only part of the static locking region is dynamically stable. The stable locking bandwidth is also asymmetrical about zero detuning and is dependent on the injection level, linewidth enhancement factor and biasing condition. The lower boundaries of the stable and static locking regions coincide but coincidence between the upper stable and static boundaries only exist for injection levels below a certain injection ratio. This is the threshold injection level for dynamic instability below which the entire static locking range is dynamically stable. When the detuning frequency between the master and slave lasers is decreased below the lower boundary of stable locking, momentary unlockings coupled with 360-degree phase jumps occur. These momentary unlockings and phase jumps occur in greater regularity when the detuning frequency is further decreased, until the laser goes into chaotic pulsation eventually. A period-doubling route to chaos can also be found within the unstable locking region by increasing the injection level while the detuning frequency remain fixed. The intensity modulation bandwidth of an injection-locked laser operating in the dynamically stable region can be increased beyond the intrinsic value achievable with the free-running laser. With proper control of the injection level and detuning frequency we can obtain a wider bandwidth and flattened response curve. The chirp-to-power ratio is also always lower in the presence of optical injection. Besides, it can be further decreased by increasing the injection level or detuning frequency. The inclusion of non-linear gain in the injection locking model will result in two different non-linear gain models depending on how the optical frequency term is expanded. With non-linear gain, the static locking range is decreased but the stable locking range is increased. The presence of non-linear gain also sets a minimum limit of injection level below which the slave laser cannot be locked to the master laser.