Abstract

Measurement of the intensity autocorrelation function of the output of an AlGaAs diode laser reveals that the presence of many small secondary modes which accompany the predominant mode in so-called “single-mode” diode lasers has a significant effect on the intensity of the laser. The total intensity of the output varies at frequencies due to mode beating between the main mode and the many tiny side modes. We have shown that these variations (at frequencies as large as 1000 GHz) are many times larger than that of the dynamical fluctuations due to noise processes in the laser.¹ This is supported by a multimode-phase-diffusion-model which we developed from the theories of Henry² and Agrawal.³ In contrast, the intensity of individual modes should fluctuate differently than that of the total output due to the partition of noise among the modes. The fluctuations of a single mode of a diode laser can thus be greater than that of the total intensity. We filter out the side modes with a monochrometer and measure the intensity autocorrelation function of the main mode alone. Our results also show that intensity autocorrelation measurements are a sensitive indicator of the presence and magnitude of external optical feedback.

© 1993 Optical Society of America