Analysis of transmission spectra and cladding mode evolution of etched double-cladding fiber long-period grating.

We demonstrate the inscription of long-period gratings (LPGs) in a fluorine-doped double-cladding fiber (DCF) with a low refractive index trench. The grating has a single resonance dip in the wide wavelength range from 1200 nm to 2400 nm. The transmission spectra and cladding mode evolution of the DCF-LPG during the cladding etching process are investigated theoretically and experimentally. The mode order of the cladding modes decreases with one order of every 20 μ m diameter etching, while the resonance wavelength of the DCF-LPG barely shifts when the fiber diameter decreases gradually. Only LP 11 mode can be excited in the etched DCF-LPG when the fiber diameter is less than 40 μ m. The characteristics of the DCF-LPGs are investigated experimentally. The torsion sensitivity was measured to be −0.154 nm/(rad/m) and −0.271 nm/(rad/m) at 1. 55 μ m and 2. 0 μ m wavebands, respectively, which are one order of magnitude higher than that of the conventional LPG. The proposed DCF-LPG has potential application as a tunable band-rejection filter in broad wavebands. • Only one resonance dip of the DCF-LPG can be observed in the wavelength range from 1200 nm to 2400 nm. • The mode order of the cladding modes decrease with diameter etching, while the resonance wavelength barely shifts. • The twist sensitivity of the DCF-LPGs at 1. 55 μ m and 2 μ m wavebands was one order of magnitude higher than that of the conventional LPG. [ABSTRACT FROM AUTHOR]