DEPENDENCE OF THE LINEWIDTH OF A SEMICONDUCTOR-LASER ON THE MODE DISTRIBUTION

A theory as well as measurements are presented that show the dependence of the frequency noise spectrum and, therefore, of the semiconductor laser linewidth, on the mode distribution. The described theory is based on the rate equations (1), where the linewidth is strongly influenced by the gain saturation coefficients (nonlinear gain). Analytical results are restricted to two modes. Measurements show, that the linewidth change of the main mode depends on the wavelength side of the side mode (related to the wavelength mode of the main mode) and that the effect decreases with increasing mode separation. Furthermore, the theoretical and measured frequency noise spectrum of a three-mode laser are depicted showing a characteristic resonance peak for certain mode distributions. This property of the frequency noise spectrum is also described by the theory.