Modal noise simulations due to high frequency VCSEL numerical aperture fluctuations

We have experimentally identified and characterized two dynamic components of a VCSEL/multimode optical fiber link: 1) a high temporal frequency fluctuation of the VCSEL's numerical aperture (NA) and, 2) source power spectrum shift in time (chirp) both upon ac modulation. We use these variables of the excitation irradiance distribution at a fiber input to simulate the resulting modal noise; without the typical assumptions that all fiber modes are equally filled and all relative fiber mode delays are equally probable. Results indicate that fluctuations in the NA can generate significant modal noise at the fiber output. On the other hand, source chirp, in the presence of mode selective loss, is only moderately effective in generating modal noise. Moreover, since chirping increases with the VCSEL's series resistance, it's significance will diminish as design efforts to decrease the potential drop across the VCSEL are successful.