Although it is not generally possible to observe directly the optical field distribution in optical waveguiding devices, one approach to obtaining the information is to examine the spatial coherence function (SCF) of the emitted beam. Very little information has been published on the spatial coherence of optoelectronic devices due to the cumbersome nature of the usual measurement technique (i.e. Young's double slit), and the relatively low optical powers in these devices. A new fiber- based interferometric technique has been developed by the principal investigator, which, for the first time, allows the direct and repeatable measurement of spatial coherence of low-power light beams. It is planned in this Research Initiation Award to use the measured SCF to determine the mode distribution in the cavities of laser diodes. Three possible approaches are proposed: 1) a non-rigorous first order analysis based on invoking the Van Cittert- Zernike Theorem (not strictly applicable to coherent sources such as lasers, but coherence effects can be removed by time-averaging); 2) an approach following that of Spano, i.e. writing down expressions for the first few modes in an optical cavity and predicting the SCF, then using the measured SCF to extract the weight for each mode; and 3) adapting the algorithm of Tervonen et al, who presents a method for computing the mode distribution from the SCF for Hermite-Gaussian modes (fiber), to planar waveguides such as those used in laser diodes and optoelectronic integrated circuits.
