In many VCSEL applications, it is essential to know the transverse beam characteristics. This paper reports an experiment of transverse modal characterization based on intensity measurement. The beam form a VCSEL is imaged by a microscope objective and intensity profiles are recorded by scanning an apertured detector. The second moment of the intensity profile is found to vary quadratically with the distance along the direction of beam propagation. An effective Rayleigh range is extracted by means of quadratic data fitting. Once this parameter is obtained, Fourier analysis of one intensity profile yields the relative weights of the Hermite-Gaussian (HG) modes, provided that the beam is indeed a superposition of independent HG modes. It is found that a VCSEL driven at low current generates a beam that is approximately HG or a superposition of independent HG modes. At high drive current, however, the transverse modal structure becomes more complicated. The experiment demonstrates that intensity-based Fourier analysis is a convenient method to assess the closeness of approximating the outputs form semiconductor lasers by superposition of independent HG modes without using sophisticated spatial modal filters. The experiment also measures the M$+2$/ modes without using sophisticated spatial modal filters. The experiment also measures the M$+2$/ parameter of beam quality versus the drive current.
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