Output power and coherence length of stripe-geometry double-heterostructure semiconductor lasers in incoherent feedback

We studied the power characteristic and the coherence length of stripe-geometry double-heterostructure semiconductor lasers with an external cavity. The length of the external cavity was made larger than the coherence length of the laser at threshold, measured without feedback. Some of the experimental results could be explained by a model in which a single laser mode was considered as a van der Pol oscillator. The effect of the external cavity was simulated by adding an external signal to the oscillator. The behavior of the coherence length depends on the mode spectrum and cannot be explained fully by a single-mode model.