We investigate a slab waveguide with periodic structures on the both sides of the core to study the optical mode interaction behaviors in the presence of structure periodicity. As the variation is taking place in both substrate and coating at the same time, the refractive index can be formulated as nsb(z) = ncl(z) = n0(1 + εW(z)), where W(z) = W(z +T) is a periodic function with period T, and ε is a small parameter. Considering a two-dimension mathematical problem, the Maxwell’s equations have been solved by assuming the Bloch modes in transverse direction. Particularly, the Ey component have been solved when the function W(z) is selected as a sine function. The field distributions of both straight and periodic core waveguide have been compared to get a better insight of modes generation. The waveguide mode field in a non-periodic waveguide is centered in the core layer, and exponentially decays outside the core, and the size of the mode field is only related to the lateral distance x. Introducing a variation in the refractive index by a sine function, influences the propagation of modes. The mode field distribution of the core layer remains unchanged, but the pattern field distributions in the cladding layer and the substrate have changed significantly. The TE0 and TE1 modes in the cladding and substrate are not continuous and decay gently, but periodically fluctuate along the z-axis. Changing the distribution of the mode field in the periodic structure destroys the orthogonality of the modes and leads to the interaction between a series of modes, referred as resonance.
While much research has been developed to achieve very large mode areas (LMA) fibers, many difficulties arise, such as
bending losses, mode deformation, and high order modes suppression. The main obstacle which remains difficult to
confront in LMA fibers is bending distortion. When a conventional LMA fiber is coiled, it will generally suffer large
bending distortion, and the mode area will contract accordingly, which would significantly affect laser or amplifier
performances for some LMA fibers. In this report, we proposed a simple and efficient way for bending compensation in
LMA fiber. A periodically etching structure is proposed to compensate the deformation, bending loss, and mode-coupling
effects in large mode area fibers. The numerical simulation results showed that the design not only efficiently
improves the effective area, but also the fundamental mode bending loss is resistant. Without the structure, the bending
losses are very low at large bending radius. When the bending radius reduces, there is a rapid increase of losses. In
particular, for smaller bending radius (less than 20 cm), the fundamental mode deforms severely, and cannot normally
transmit in the core. The numerical simulation results showed that the design not only efficiently improves the effective
mode area, but also the fundamental mode bending loss is resistant. Even at bending radius as small as 5cm, the
fundamental mode can transmit normally in the fiber with an increased mode area scaling. Furthermore, the LMA fiber
with the structure can be flexibly manufactured by conventional fiber manufacturing approaches and recent etching
technologies.
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