Numerical analysis of modal instability onset in fiber amplifiers

Numerical analysis of the onset of modal instability in fiber amplifiers is presented. Specifically calculations of the evolution of the intensity fluctuation spectrum along the fiber for a sampling point offset from the core center are presented for different instability onset conditions. These include seeding with LP01 only, seeding with LP01 and LP11 at the same frequency, seeding with LP01 and LP11 at offset frequencies, and seeding taking quantum shot noise into account. The position dependent spectra are shown to be very similar for each of these cases suggesting a common instability mechanism.