Abstract
In this paper, we studied the basic characteristics of tilted fiber Bragg gratings (TFBGs), inscribed line-by-line. Experimental results showed that if the TFBGs were located within different planes parallel to the fiber axis, the spectra performed differently. For 2°TFBG, if it was located near the central plane, the Bragg resonance was stronger than ghost mode resonance, and the order reversed if it was located near the boundary between core and cladding. As the tilted angle increased, the range of cladding mode resonance increased. When the tilted angle was larger than 12°, the birefringence effect was observed. Based on the birefringence phenomenon, torsion characteristics were experimentally studied; the sensitivity was about 0.025 dB/degree in the linear variation range. The harmonic order of TFBGs also affected the transmission spectrum. Leaky mode resonance was observed in the 8th order TFBG, and torsion (or polarization) influenced the spectrum of the 8th order TFBG. Our research represented the theory of line-by-line inscribed TFBGs and provided an inscription guidance for TFBGs.
Highlights
In excessively tilted fiber grating (Ex-TFG), the coupling occurs between core mode and cladding modes transmitted in the same direction, which behaves like long period fiber grating
The birefringence decreased because the refractive index modulation (RIM) was more uniform than tilted fiber Bragg gratings (TFBGs) inscribed with phase mask (PM) [17], but insertion loss increased in the direct writing process
We investigated the torsional effects of 18◦ TFBG
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. The birefringence decreased because the RIM was more uniform than TFBGs inscribed with PM [17], but insertion loss increased in the direct writing process. These methods could only fabricate non-localized TFBGs, in which the transmission spectrum was only influenced by the tilted angle. The RIM in the nonlocalized condition covered the whole core region and the coupling process was relatively stationary [19,20] Another femtosecond laser direct writing method—namely, line-by-line inscription [21,22]—was a potential method of fabricating TFBGs. In the year of this study’s writing, Liu et al. Our research works improved the theory of line-by-line inscribed TFBGs and presented guidance for a femtosecond laser direct writing technique
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