Wavelength Selective Mode Conversion in Few-mode Fiber with Cascaded Long-Period Gratings

We propose a wavelength selective mode converter based on cascaded long-period gratings (CLPFGs) in few-mode fiber (FMF). The CLPFGs consist of two identical LPFGs with period of 1180 μm and period number of 9, and a separation between these two LPFGs. The transmission characteristics of the proposed decice were studied therotically and experimentally. In theory, with the separation distance of CLPFGs increasing, the resonance wavelengths of the CLPFGs increase, and the resonance wavelengths in the transmission spectrum drift. In the experiment, CLPFGs with different separation distances were fabricated in FMF using CO
2
laser. With the separation distance increasing, the dual resonances firstly shifted toward to shorter wavelength, and then the third resoances come out at longer wavelength, accompanied by the wavelength separation increasing. Moreover, the mode conversion efficiency of 92.825%, 99%, and 94.4% could be obtained at 1444 nm, 1533 nm and 1634 nm in experiment, respectively, with the separation distance of 2.5 cm. Thus, the proposed CLPFGs could be regarded as a wavelength mode converter, which would be potentical in mode-division multiplexing systems.