Excitation of the dominant mode of a round fiber by a Gaussian beam

The excitation of the dominant HE11 mode of a round optical fiber by a gaussian beam has been calculated. The calculation is based on the assumption that reflected waves can be neglected. It is thus applicable only to fibers with low index difference between core and cladding. It is found that optimum excitation of the HE11 mode is achieved for loosely guided beams if the product of the beam half-width w times the radial decay constant γ of the HE11 mode outside of the guide is unity, γw = 1. For tightly coupled modes 2^½w must be equal to the core radius in order to achieve optimum excitation. As much as 99 percent of the power can be transferred to the HE11 mode. Also investigated are the effects of an off-set or tilted beam on the mode excitation. The mode excitation drops to 36 percent if the amount of off-set equals the beam half-width. The effect of tilts depends on the parameter kd, free space propagation constant times core radius of the fiber. For small values of kd or loosely guided modes, the mode excitation is very sensitive to tilts of the gaussian beam. As long as the HE11 mode is the only mode that can propagate, increasing values of kd lead to less sensitivity with respect to tilts. For multimode operation of the fiber, the sensitivity to tilts increases with increasing values of kd. The minimum of tilt sensitivity coincides with the minimum spot size of the guided mode.