Modulation detuning characteristics of actively mode-locked diode lasers

Existing frequency domain models for the active mode locking of diode lasers are appropriate for antireflection- (AR-) coated devices. This paper presents the first frequency domain model for non-AR-coated devices. The use of uncoated devices permits the independent measurement of all of the important laser parameters required for the model, so that no adjustable parameters are needed. An additional novel feature of this model is that the effect of gain saturation on the exact shape of the gain waveform is allowed for in a completely self-consistent fashion. Experimental studies were carried out on uncoated undegraded commerical laser diodes. Threshold reductions of 25-30 percent were achieved for external cavity operation. The mode-locking studies reported in this paper were carried out above the external threshold current, with a sinusoidal modulation near 1 GHz at a modulation index of 4 percent. The results of these studies showed good agreement with the predictions of the model presented here, in contrast with the predictions of the existing models. Specifically, the modulation frequency corresponding to the shortest most intense mode-locked pulses was found to occur at a fraction of 1 percent below the frequency matched to the external cavity roundtrip time. As well, the predicted effects of gain saturation were observed in the detailed structure of the mode-locked pulses, and in their detuning characteristics.