Bandwidth optimization of cascaded long-period gratings for broadband mode conversion over 1.0-2.2 µm waveband.

We investigated theoretically and experimentally the cascaded long-period fiber gratings (c-LPFGs) in a few-mode fiber (FMF) for the generation of LP core mode in a broad wavelength range. The dependence of the transmission spectra of the c-LPFGs on the spacing between the gratings, and grating periods are studied in detail. The c-LPFGs experimentally generate LP core mode in a 10-dB bandwidth of 193.6 nm in 1.55 µm waveband and 447.5 nm in 2 µm waveband, respectively. The first-order orbital angular momentum mode can be converted by the c-LPFGs with the same broadband wavelength range. The 10-dB bandwidth and corresponding wavelength range for mode conversion can be adjusted by changing the grating spacing and grating periods.