We present measurements of the intensity autocorrelation function of the output of a free-running AlGaAs diode laser. To our knowledge these are the first such measurements performed on the output of a semiconductor laser. Our data display large structure at the relaxation oscillation frequency and reveal features that depend on the number of secondary modes lasing. This provides evidence that, even in so-called single-mode diode lasers, these side modes have a significant effect on the fluctuations of the total output of the laser. The autocorrelation function in fact can be used to measure how many modes contribute to the fluctuating intensity. The major features in the measured intensity autocorrelation function are in good agreement with the predictions of a simple multimode phase-diffusion model.
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