In this paper, the effects of the asymmetric refractive index change profile on the reflection spectra of multimode fiber Bragg gratings (MMFBGs) are experimentally and theoretically investigated. Different guided modes in a multimode fiber (MMF) are coupled with each other and the sub-reflection peaks are generated if the refractive index change profile of the MMFBG is asymmetric in the cross section of the fiber core. It is found that by increasing the UV exposure, the reflectivities of the main reflection peaks are decreased, the reflectivities of the sub-peaks are increased, even higher than the main reflection peaks, which indicate the grating becomes more asymmetric with increasing the UV exposure. It is further shown that an MMFBG is much more sensitive to the polarization state of the injected light compared with an SMFBG. The numerical simulations are carried out to explain the excitation condition dependence of the reflection spectra of MMFBG, which agree well with the experimental results.
Recently, multimode fiber (MMF) and components based on MMF have attracted much attention due to their potential applications in future optical access networks. Fiber Bragg gratings (FBGs) are considered to be key components in both telecommunication and sensing applications. Although single-mode fiber based FBGs (SMFBG) have been studied thoroughly, few studies have been reported on MMFBGs, mainly because of the complexity and multiple mode nature of the MMF.
In this paper, transmission and reflection spectra of MMFBGs are studied systematically. Relationships between transmission/reflection spectra and the excitation conditions are clearly demonstrated by observing the far-field pattern. Different launching methods including the lateral offset launching and angular offset launching and light sources with different spectrum width are used in experiments. Furthermore, the transmission/reflection spectra dependence on the polarization state of excitation light and asymmetric refractive index perturbation profile are studied in detail. Theoretical simulations are used to compare these experimental results.
Fiber Bragg gratings (FBGs) have emerged as important components and received intensive research attention in both fiber telecommunication and sensing fields. Bragg gratings in single mode fiber structure (SMFBGs) have been studied extensively. On the other hand, fewer studies have been reported on multimode fiber Bragg gratings (MMFBGs) despite of their potential applications in future optical access networks. In this paper, MMFBGs are studied in detail both theoretically and experimentally. A comprehensive numerical model is developed for MMFBGs based on the coupled mode theory and applied to analyze measured transmission and reflection spectra from MMFBGs with reflectivities ranging from 78% to 99%. It is found that the spectra of MMFBGs depend strongly on fabrication conditions (e.g. modulation depth of the grating) and experimental conditions (e.g. mode excitation). Good agreement is obtained between the theoretical simulations and experimental measurements. Our simulations based on the developed MMFGB numerical model can provide quantitative explanations for the observed experimental phenomena. These explanations give a complete understanding of the nature of the interaction between the wave propagation and multimode fiber gratings. It is considered that the spectral simulations provide a theoretical guidance in design MMFBG based devices.