Fiber couplers are also used for fiber-to-fiber coupling: Light from the first fiber is collimated with a fiber collimator and then focused into the second fiber by another collimator. Another application is the combination with a back-reflecting mirror and some. Thorlabs offers a variety of fiber collimation and coupling solutions. They can also be used in reverse to focus light into a fiber. In essence, a simple collimation lens is all that is needed for this purpose.