Photopolymer microtips for efficient light coupling between single-mode fibers and small-core photonic crystal fibers

A novel method for light coupling between single mode fibers and small-core photonic crystal fibers is demonstrated. The method is based on growing photopolymer micro-tips directly on the end face of single mode fibers. The advantages of this micro-tip fabrication method are its simplicity, controllability, reproducibility and being inexpensive. The shape and the size of the tips can be controlled, by adjusting the laser power, the exposure time and the oxygen diffusion concentration for polymerization, to match its mode field to the small-core photonic crystal fibers. A photopolymer micro-tip integrated on the end face of a single mode fiber is used to reduce the mode field diameter and increase the numerical aperture of the light beam coming out from the single mode fiber, so that there is a better match to the small mode field diameter and the large numerical aperture of small-core photonic crystal fibers. A 5 dB improvement in coupling efficiency between a single mode fiber and a commercial small-core, highly nonlinear photonic crystal fiber is experimentally demonstrated. This compact and efficient butt-coupling method is particularly suitable for photonic crystal fiber gas sensor applications where holes in the photonic crystal fiber need to be kept open at the joint for easier access to the evanescent field, This method also may be suitable for the connecting a single mode fiber to a hollow-core photonic crystal fiber with the central hole filled with a liquid sample for liquid sensor applications.