Simulation of a chirped fiber Bragg grating in a few-mode step-index optical fiber under axisymmetric excitation

In this paper, a multimode fiber Bragg grating variant inscribed in a few-mode step-index fiber and operating in dual-mode LP₀₁-LP₀₂ regime has been proposed. The commercially available Thorlabs FG025LJA fiber with a 25-μm core diameter and a numerical aperture of 0.1, as a suitable optical fiber was selected (this fiber exhibits only two axisymmetric modes in the O-band). The self-coupling and cross-coupling coefficients of the modes in the grating and the reflectance spectra of homogeneous and chirped gratings were calculated. The effects of saturation, broadening, and resonance interference, as well as the generation of a single reflection peak with a maximum reflectivity of up to 98% were demonstrated.