All-optical tunable multi-channel mode conversion based on a modal interferometer using two-mode fiber

Abstract

This paper presents all-optical tunable multi-channel mode conversion based on a Norland Optical Adhesive (NOA)-coated modal interferometer constructed by two-mode fiber. The modal interferometer consists of a tapered two-mode fiber (TTMF) sandwiched between two pieces of single-mode fibers. The TTMF provides a uniform modal interferometer with LP01 and LP11 modes as well as a strong evanescent field on its surface. The NOA is illuminated by a 405 nm laser which changes the refractive index of the NOA, thus the output spectrum of the modal interferometer can be controlled. Due to the strong light-matter interaction between NOA and the evanescent field around the TTMF, a large wavelength shift of the output spectrum can be observed under external pump light excitation. The experimental results show the transmission spectrum exhibits a blue shift with increasing pump power. A wavelength variation rate of 289 pm/mW is obtained under external pump light excitation. The interference spectrum has multiple dips where the input LP01 mode is converted to the LP11 mode. Thus, the modal interferometer acts as a tunable multi-channel mode converter. The proposed device might have potential applications in all-optical controllable devices and mode division multiplexing systems, etc.