Selective excitation and coupling of high-order optical modes of a microstructured optical fiber by using a fiber-end microtip

We present the selective excitation and coupling of high-order optical modes in microstructured optical fibers (MOFs) by using a fiber-end microtip. After the self-growing of a microtip on the end face of a standard single-mode fiber, it is demonstrated that a fine tuning of the relative displacement between the microtip and MOF can greatly improve the excitation and coupling efficiency of high-order optical modes of the MOF. A 6.85 dB improvement in coupling efficiency of the second-order mode of a homemade small-hole MOF is experimentally achieved, and a sensitivity-enhanced measurement of the refractive index in MOF microholes is demonstrated.