2ND-MOMENT APPROACH TO THE TIME-AVERAGED SPATIAL CHARACTERIZATION OF MULTIPLE-TRANSVERSE-MODE LASER-BEAMS

Analytical expressions are derived for the time-averaged propagation c haracteristics of multimode light beams made up of a superposition of paraxial transverse modes. The transverse-mode distributions are not r estricted to the usual Hermite-Gaussian or Laguerre-Gaussian functions and may thus have arbitrary characteristics. It is shown that the pro pagation characteristics of multimode beams can be expressed in simple form in terms of those of the constituent transverse modes, which can be obtained directly from the modal complex-amplitude profiles in any given transverse plane. These expressions may simplify considerably t he calculation of the second-moment characteristics of multimode beams because the numerical propagation of each modal field in free space i s no longer required. The expressions are used to investigate numerica lly the degradation of the spatial quality of the output beam radiated by a broad-area semiconductor laser as its fundamental-lateral-mode o peration ceases.