Mode coupling and output beam quality of 100-400 μm core silica fibers.

Propagation and mode coupling within relatively short (∼1-10 m) large core, nominally multimode, fibers are of interest in a number of applications. In this research, we have studied the output beam quality and mode coupling in various fibers with core diameters of 100-400 μm and lengths of 2 m. Output beam quality (M2) and mode-coupling coefficients (D) have been studied for different clad dimensions, numerical apertures, and wavelengths. The mode-coupling coefficients have been determined based on modal power diffusion considerations. The results show that D scales approximately as the inverse square of the clad dimension and inverse square root of the wavelength. Output from a 2 m length fiber of 100 μm core and 660 μm clad fiber is close to single mode (M2=1.6), while output from a 200 μm core and 745 μm clad fiber also has high beam quality.