Alignment tolerances for plane-wave to single-mode fiber coupling and their mitigation by use of pigtailed collimators.

We discuss the efficiency with which coherent plane waves can be coupled to single-mode fibers in the presence of deterministic or stochastic misalignments of the fiber relative to the focal point of a lens. We point out how the alignment demands can be relaxed by means of graded-index-lens fiber-pigtailed collimators.