Fabrication and implementation of a multi-to-single mode converter based on a tapered multimode fiber

In this paper, an efficient multi-to-single mode converter based on a tapered multimode fiber is presented. This novel tapered multimode fiber has a multimode input end and a single mode output end, which is fabricated by a reproducible chemical etching process. The core diameter of the thicker end of the fiber is around 150 μm (a typical multimode fiber case) while the core diameter of the thinner end of the fiber is around 10 μm (a typical single mode fiber case). By launching the light into the tapered multimode fiber from the thicker end, the multimode light field distribution at the thicker input end becomes a single mode light field distribution at the thinner output end. Thus, a unique multi-to-single mode conversion is achieved. The major advantages of this mode converter include: (1) high conversion efficiency (>95% based on theoretical analysis), (2) compact size and (3) low cost, which make it very useful for a variety of applications, such as: (1) efficiently coupling the light from a laser, in particular a semiconductor diode laser, into a single mode fiber; (2) highly sensitive evanescent wave based fiber optic sensors and (3) effective mode conversion and connection between multimode and single mode fibers. Experimental results have confirmed this theoretical prediction (i.e., a multi-to-single mode conversion can be realized via a special tapered multimode fiber).