BENT LONG PERIOD FIBE R GRATINGS FOR REFRA CTIVE INDEX SENSING [612500]

BENT LONG PERIOD FIBE R GRATINGS FOR REFRA CTIVE INDEX SENSING
Alecsandra Rusu1, Jean -Louis Auguste2, Georges Humbert2, Lorant Szolga1, Ramona
Galatus1
1 Technical University of Cluj -Napoca, 28 Street Memorandumului, Cluj -Napoca 400114,
Roumania
2 XLIM Research Institute, UMR 7252 CNRS / Université de Limoges, 123 av. A. Thomas, 87060
Limoges, France
[anonimizat]
RÉSUM É
We demonstrate that bending a Long Period Fibre Grating ( LFPG ) improve s the
sensitivity of the resonance wavelength s to variation s of the refractive index of the
external medium. LPFG are written on a standard single mode fibre with the electric
discharge method . The sensitivity of LPFG to water or ethanol is studied for different
bend radius. We report that bending a LPFG can improve the sensiti vity to water by 20
times compared to measurement s done with str aight LPFG.
MOTS-CLEFS : Long period grating; optical fiber sensor , refractive index sensing
1. INTRODUCTION
A LPFG create s co-directional couplings between modes propagated in an optical fibre.
Usually, a LPFG is written on a portion of a standard single -mode fibre with beforehand removed
polymer coating for enabling ligh t coupling from the fundamental mode to higher order modes ,
providing that the refractive index of the external index is lower than silica one. Co -directional
coupling from the fundamental mode to a specific high -order mode is obtained at a resonance
wavele ngth where the phase difference of both modes matches the pitch of the LPFG. The position
of the resonance wavelength is therefore very sensitive to small phase variations of the modes,
making LPFG attractive components as optical fibres sensors.
LPFG pro perties have been extensively studied for sensing variations of temperature, refractive
index (RI), strain, bend, twist [1-3]. Detection of specific species (in liquid or gas) realised by
adding a chemical sensitive layer around the LPFG have also been reported [3-4]. Besides,
fabrication techniques and optical fibre topologies have been widely investigated for improving
LPFG performances as sensors or filters. However, even if the modifications of LPFG properties
induced by bending it have been widely studied, sensing sensitivity of bent LPFG has not been
reported (to the best of our knowledge). In this communication, we study experimentally the
evolution the RI sensitivity of bent LPFG versus different bend radius

2. FABRICATION OF LPFG
We have fabricated several LPF G with the electric -discharge technique [ 5]. This technique
is very simple, does not require special optical fibres and specific preparations (as hydrogen
loading). The polymer coating has been removed on a portion of a standard single -mode fibre (ex.
SMF 28). This portion is placed in between the electrodes of a commercial fibre fusion splicer. A
white light source and an optical spectrum analyser (OSA) are connected in both ends of the fibre
sample for monitoring LPFG’s growth. After each electric di scharge, the fibre is translated by a
translation stage to a distance equal of the grating pitch (here  = 600 µm). Distinct dips are formed
in the transmission spectrum corresponding to resonance wavelength s where the fundamental mode
is coupled to a specific higher order mode . As shown in Fig. 1(a), we have stopped this fabrication
process when the isolation of the dip at 1521.8 nm is maximal .

3. REFRACTIVE INDEX SENS ITIVITY OF BENT LPFG
We have used this LPFG for investigatin g the effect of bend s on the sensitivity of the
resonance wavelength position (at 1521.80 nm) to RI variation s of the external medium . We used
water and ethanol as external medium with refractive index of 1.33 and 1.36 respectively . In first,
we have immersed (in these liquids ) the LPFG without bending it , for measuring a reference. As
shown in Fig. 1(a) and ( b), the resonance wavelength (at 1521.8 nm) slightly shifts to shorter
wavelength by 1 nm and 1.4 nm respectively for water and ethanol. These shifts are in good
agreement with reported similar experiments. Then, we have coiled the LPFG around a cone shaped
structure w hich permits a pr ecise control of the fibre loop diameter . This LPFG bent with a radius
of 30 mm is illustrated in Fig. 1(c -d). As expected, bending the LPFG leads to a shift of the
resonance wavelengths to longer wavelengths with broadening and isolation reduction of the dips
(Cf. black curve of Fig. 1(c-d) compared to Fig. 1(a -b)). When, the LPFG is immersed in water or
ethanol , the resonance wavelength (at 1521.8 nm) shifts to shorter wavelength by 6.4 nm and 8.8
nm respectively , correspond ing to a RI sensit ivity improvement by more than 6 times .

(a) (b)

(c) (d)
Fig. 1: Transmission spectrum of a LPFG immersed or not in water (a) without bend or (c) with bent with a
bend radiu s of 30 mm. Transmission spectrum of a LPFG immersed or not in ethanol (b) without bend or (d)
with bent with a bend radiu s of 30 mm.

Smaller and larger bend radii have been tested in order to determine the largest value of RI
sensitivity improvement . As shown in Fig. 2, smaller bend radius leads to larger wavelength shift
but also to stronger degradations of the LPFG spectrum, i.e . broader dips with smaller isolation s.
These spectral degradations associated with the fibre fragility under very small bend radius have
limited this study and their sensing applications to a bend radius of 15 mm (Cf. Fig. 2(b) , largest
shift of 19.8 nm of the resonance wavelength at 1539 nm ).

(a) (b)
Fig. 2: (a) Transmission spectrum of a LPFG bent with a bend radius of 21 mm, immersed or not in water , or
in ethanol . (b) Transmission spectrum of a LPFG bent with a bend radius of 15 mm, immersed or not in water .

The evolution of the resonance wavelength shift () when immersed in water or ethanol, versus
the bend radius is plotted in Fig. 3. The res onance wavelength shift () is normalised to the
resonance wavelength () in air to compensate the shift of the resonance wavelength induced by
bending the LPFG. Bend radius smaller than 30 mm leads to significant higher sensitivity with a
maximum improvement factor 20 and 17.8 respectively for water and ethanol.

Fig. 3: Evolu tion of the normalised resona nce shift induced by immersing the LPFG in water or ethanol,
versus the diameter of the fibre loop (i.e. 2 * bend radius applied on the LPFG ).

CONCLUSION
We report for the first time to our knowledge that bending a LFPG improve s the sensitivity of the
resonance wavelength to variations of the refractive index of the ext ernal medium. We demonstrate
that bending a LPFG can improve the sensiti vity to water by 20 times compared to measurement s
done with str aight LPFG. These result s could offer new prospects for sensing refractive index
variation in common fluids ( water or ethanol ) that a re difficult to sense due to the large index
contrast with silica . Further simulation studies are in progress for testing our assumption that this
sensitivity improvement is induced by the optical field delocalisation of higher order modes in the
bend.
RÉFÉRENCES
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[2] V. Bhatia et al Opt. Lett. 22, 648 -650 (1997)
[3] S. W James et al. Meas . Sci. Technol. 14 (2003) R49 –R61
[4] Z. Gu, et al. Opt. Lett. 31, 2405 -2407 (2006)
[5] G. Humbert et al. J. of Optics A: Pure and applied optics, n°4, pp. 194, 2002.