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 mu m diameter etching, while the resonance wavelength of the DCF-LPG barely shifts when the fiber diameter decreases gradually. Only LP11 mode can be excited in the etched DCF-LPG when the fiber diameter is less than 40 mu 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 mu m and 2.0 mu 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.

Automated summary

This study demonstrates the inscription of long-period gratings (LPGs) in a fluorine-doped double-cladding fiber (DCF) with a low refractive index trench. The research investigates the transmission spectra and cladding mode evolution of the DCF-LPG during the cladding etching process. It is found that the mode order of the cladding modes decreases while the resonance wavelength of the DCF-LPG remains stable. Experimental results show that the proposed DCF-LPG has a higher torsion sensitivity and potential application as a tunable band-rejection filter in broad wavebands.