Abstract
The transverse asymmetry of the index modulation profile in the asymmetric few-mode fiber Bragg grating (FM-FBG) was investigated. The transverse asymmetry of the index modulation profile will lead to mode conversion between modes with the different azimuthal orders, and this asymmetry is characterized by the attenuation coefficient α. We evaluated that the value of attenuation coefficient α was 0.2 μm−1, and grating amplitude χ was 2.8 × 10−4 for FM-FBG inscribed by UV single-side illumination. We found that the optimized value of α was 0.16 μm−1, at which the maximum mode conversion efficiency of LP01–LP11 can be achieved. The results of this paper provide great potential application in few-mode fiber (FMF) devices and mode division multiplexing (MDM) optical communication.
Highlights
In recent years, few-mode fiber Bragg grating (FM-FBG) has attracted considerable attention, owing to the advantages of simple structure, flexible operation, versatility, low loss, and low crosstalk [1]
The FM-FBGs can realize the coupling of the forward propagating mode and the phase-matched backward propagating mode, which are widely used in various applications such as fiber filters [2], fiber lasers [3,4,5], fiber sensors [6,7], mode division multiplexing (MDM) communication systems [8], and mode converters (MCs) [9,10,11,12]
A high-order vector mode conversion approach was proposed based on asymmetric fiber Bragg grating (AFBG), and the influence of the attenuation coefficient α on vector mode conversion was theoretically analyzed, and the maximum conversion efficiency at specific α was achieved for each vector mode [15]
Summary
Few-mode fiber Bragg grating (FM-FBG) has attracted considerable attention, owing to the advantages of simple structure, flexible operation, versatility, low loss, and low crosstalk [1]. When FM-FBG has an asymmetric transverse index profile, the mode can convert into the higher azimuthal order modes [13,14]. Yao et al [18] designed experiments to realize high-order excitation modes by a lateral core offset splicing spot (OSS), and mode conversion from LP01 to LP01, LP01 to LP11, LP11 to LP11, LP11 to LP21, and LP11 to LP21 was demonstrated through UV single-side illumination. With the single-side illumination of UV light, the refractive index modulation profile across the fiber core in FM-FBG could change asymmetrically [19]. When FM-FBG was fabricated by UV single-side illumination, and the fiber core on the side close to the UV beam had the higher refractive index change because of the oafb8sorption of UV light. The refractive index profile over the fiber core became asymmetric after UV single-side illumination [19].
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