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Analysis of optical response of long period fiber gratings to nm-thick thin-film coatings

Zhiyong Wang, J. R. Heflin, Rogers H. Stolen, and Siddharth Ramachandran

Open Access Open Access

Abstract

We have theoretically and experimentally demonstrated that the resonant wavelength of long period fiber gratings (LPG) can be shifted by a large magnitude by coating with only a nm-thick thin-film that has a refractive index higher than that of the glass cladding. The resonant wavelength shift can result from either the variation of the thickness of the film and/or the variation of its refractive index. These results demonstrate the sensitivity of LPG-based sensors can be enhanced by using a film of nm-thickness and refractive index greater than silica. This coating schematic offers an efficient platform for achieving high-performance index-modulating fiber devices and high-performance index/thickness-sensing LPG-based fiber sensors for detecting optical property variations of the thin-film coating.

©2005 Optical Society of America

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Figures (5)
Fig. 1.
Fig. 1. (a) Illustrative schematic of LPG with an nm-thick thin-film coating (b) Index profile of the thin-film coated LPG, n 1 is index of fiber core, n 2 is index of fiber cladding, n 3 is index of the thin-film, nair is index of air, and n 3>n 1>n 2>nair . Thickness d of the film is denoted d=b-a.

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Fig. 2.
Fig. 2. Phase Matching Curves (PMC) shifts as functions of index n 3 and thickness d of nm-thick thin-film (a) PMC of LP0,4 mode shifts with d (0, 20, 40, 60, 80, 100nm) at n 3=1.8, (b) PMC of LP0,12 mode shifts with d (0, 20, 40, 60, 80, 100nm) at n 3=1.8, (c) PMC of LP0,4 mode shifts with n 3 (1.5, 1.6, 1.7, 1.8) at d=40nm and d=100nm, (d) PMC of LP0,12 mode shifts with n 3 (1.5, 1.6, 1.7, 1.8) at d=40nm and d=100nm.

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Fig. 3.
Fig. 3. LPG spectra shifts as functions of index n 3 and thickness d of nm-thick thin-film (a) spectra of LP0,4 mode shifts with d (0, 20, 40, 60, 80, 100nm) at n 3=1.8, (b) spectra of LP0,12 mode shifts with d (0, 20, 40, 60, 80, 100nm) at n 3=1.8, (c) spectra of LP0,4 mode shifts with n 3 (1.5, 1.6, 1.7, 1.8) at d=40nm and d=100nm, (d) spectra of LP0,12 mode shifts with n 3 (1.5, 1.6, 1.7, 1.8) at d=40nm and d=100nm.

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Fig. 4.
Fig. 4. (a) Comparison between experimental and simulated LPG spectrum without ISAM coating (b) Experimental LPG spectra with 0, 5, 10, 15 and 20 PAH/PCBS bilayers of ISAM films for PAH at pH=7.5 and PCBS at pH=8.0. One bilayers≈1.3 nm in thickness.

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Fig. 5.
Fig. 5. Experimental results of Δλres of ISAM–coated LPG as a function of thickness d of ISAM films (different pH combinations of PAH and PCBS [(7.5,6), (7.5,8), (9,6), (9,8)] offer corresponding index n 3 [1.7107, 1.6912, 1.6887, 1.6715], respectively).

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