Feedback effects at stable and unstable resonant external cavity lengths

External feedback applied to a high-power semiconductor laser is found to be responsible for a drop in output power and an increased noise-level when the optical external cavity length is an integral multiple of the optical internal cavity length. The power drops and extra noise originates from a scan of the quasi-single-mode laser over its internal cavity modes, caused by a lack of interferometric stability. In the case of interferometric stability, the wavelength of the laser exhibits a staircase-like dependence on the external cavity length. All experimental results can be described by a simple rate equation model.