For nearly Gaussian beams the characterization of changes in the beam parameters with variation of the operating conditions using a Shack-Hartmann sensor is a well-known technique. For broad-area semiconductor lasers the beam analysis based on wavefront detection is still a field of high innovation potential for quality control since the wavefront gives an additional insight to the modal composition of the laser which is strongly influenced by the processes inside the resonator. The diode lasers used for investigation are based on the material system of GaAs (λ=808nm-980nm). They show a multimodal behavior strongly affected by thermal and electrical effects inside the active medium resulting in a complex structure of the intensity and wavefront pattern. The investigations in this paper have been carried out using a so called Gaussian telescope which allows a magnification of the beam cross section as well as the investigation of the spatial evolution of the intensity and wavefront distribution in vicinity to the beam waist. Furthermore the pattern of the detected wavefront is associated with Legendre polynomials to obtain a quantitative expression of the pattern. Additionally simulation software is used to connect the modal composition of the intensity with a set of Hermite Gaussian modes. The aim of our work is to combine these two tracks of information to find a way to forecast whether the laser under test shows a stable or instable intensity distribution over the whole operation current range.
In recent years wavefront measurements using a Shack-Hartmann Sensor became a fast and easy way to analyze the change of laser beam characteristics over a wide range of parameters. This method is well known for nearly Gaussian laser beams while the wavefront analysis of broadarea semiconductor lasers is still an open field of current research. Detailed analysis of the wavefront gives an additional path to get insight into the modal composition of semiconductor lasers, which has a dominant impact on the output parameters of the devices. For our investigations we utilize lasers based on the material system of GaAs emitting light in the near infrared. These types of laser emit typically more than one optical mode. The composition of these modal structures is highly affected by thermal and electric effects inside the active medium. By using a simulation software the intensity distribution at various diode currents can be associated with an assembly of Hermite Gaussian modes and thus gives insight into the basic modal structure. Additionally the change of modal composition can be recorded within the wavefront deflection. This delivers an extra track of information to the light emission. The aim of our research is to associate the wavefront with the modal structure gained by measuring the intensity distribution under changing working conditions. Furthermore we use a lens system to receive a magnified image of the beam and investigate the spatial evolution of the intensity and wavefront distribution of the laser emission along the propagating axis.
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