Optical feedback in a vertical-cavity surface-emitting laser due to a fiber facet of varying position is studied in experiment and theory. Measured spectra and light-current curves show periodic variations in resonant wavelength, threshold current, differential quantum efficiency, and output power as a function of fiber position. Theoretical results were obtained using a 2 2 vector propagation matrix method which models the laser and fiber in a single structure and shows good agreement with

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... rimental results. A novel method for determining the linewidth enhancement factor by analyzing the wavelength variation as a function of fiber position is presented and a value of = 2.8 was obtained. Index Terms-Linewidth enhancement factor, optical feedback, optical fiber, vertical-cavity surface-emitting laser.