Single-mode optical fibers are widely used in different applications, and they play a crucial role in long-distance optical communication. Launching light into such kind of single-mode fibers can be a challenging task in practice. In this example, we select one commercially available lens, and show how to find the optimal working distance to achieve maximum coupling efficiency. Particularly, we demonstrate that the optimal working distance found by field tracing differs from the focal distance predicted by ray optics.
