The spectral sensitivity of long-period gratings fabricated in elliptical core D-shaped optical fiber

Abstract

Long period gratings (LPGs) were written into a D-shaped optical fibre, which has an elliptical core with a W-shaped refractive index profile. The LPG's attenuation bands were found to be sensitive to the polarisation of the interrogating light with a spectral separation of about 15nm between the two orthogonal polarisation states. In addition, two spectrally overlapping attenuation bands corresponding to orthogonal polarisation states were observed; modelling successfully reproduced this spectral feature. The spectral sensitivity of both orthogonal states was experimentally measured with respect to temperature, surrounding refractive index, and directional bending. These LPG devices produced blue and red wavelength shifts of the stop-bands due to bending in different directions. The measured spectral sensitivities to curvatures, d /dR, ranged from -3.56nm m to +6.51nm m. The results obtained with these LPGs suggest that this type of fibre may be useful as a shape/bend sensor. It was also demonstrated that the neighbouring bands could be used to discriminate between temperature and bending and that overlapping orthogonal polarisation attenuation bands can be used to minimise error associated with polarisation.

Bending characteristics of fiber long-period gratings with cladding index modified by femtosecond laser

“…Example of the spectral sensitivity of the induced attenuation band at 1610 nm before (left) and after (right) the femtosecond laser modification of LPG2. which is low compared with the polarization dependence of the other previously reported bend sensors such as LPGs in E-core D-shaped fibers, where the shift is approximately 15 nm [11]. The observed spectral behavior is explained by the introduction of the asymmetry as seen previously in the response to the bending of LPGs in D-shaped fiber [4].…”