This dissertation explains our method of finding the structure and modal weights of spatial modes in lasers by analyzing the spatial coherence of the laser beam. Most previous methods for determining modal weights have relied on assumed shapes for the spatial modes.
Using a twin-fiber interferometer we measured correlation data from each laser. The two arms of a single-mode fiber coupler sample the laser beam and send the summed output onto a detector. The detected intensity is a correlation measurement, giving the spatial coherence of the two sample points. With computer control of fiber positioning and automated data acquisition we were able to measure a matrix (or partial matrix) of correlation data.
The correlation data is processed using an algorithm based on Jacobian control. This algorithm extracts the basis modes and weights from the matrix. The algorithm is iterative and uses information from the data for its initial guess instead of a start based on prior knowledge of the laser.
We tested eight semiconductor lasers which varied from having a single lateral mode to four lateral spatial modes. The lasers were ridge-waveguide, Fabry-Perot cavity, quantum well (or multiple quantum well) GaAs lasers that varied in stripe width from 3 to 30 micron. We verified that the wider confinement layers in one MQW laser design allowed two or three transverse modes compared to one transverse mode for the other design. We noted that the wider stripe lasers had more dominant higher-order modes but there were multiple lateral spatial modes even for the smallest width. One important consideration in determining the spatial modes and weights was whether the laser was operating in a single longitudinal mode. If not, the total coherence was lowered, the modal weights were not accurately determined, and there may not have been enough stability to retrieve some of the lower weighted spatial modes.