Long-term stable device for tuning fiber Bragg gratings

Long-term stable device for tuning fiber Bragg gratings

Erik Bélanger,* Bernard Déry, Martin Bernier, Jean-Philippe Bérubé, and Réal ValléeCentre d’optique, photonique et laser, Université Laval, Québec, Québec G1K 7P4, Canada

*Corresponding author: [email protected]

Received 14 December 2006; accepted 31 January 2007;posted 1 February 2007 (Doc. ID 77870); published 15 May 2007

It is demonstrated that with a proper choice of embedding material, the composite beam bending methodconstitutes an effective and reliable approach for tuning fiber Bragg gratings. A long-term stable deviceis presented with a dynamic range of 80 nm, which exhibits insertion losses smaller than 0.28 dB andsmall variations of the full width at half-maximum. © 2007 Optical Society of America

OCIS codes: 230.1480, 060.2340, 060.2370, 140.3510, 160.5470, 140.3600.

1. Introduction

Fiber Bragg gratings (FBGs) can be used as filtersin optical communications or in optical sensing ap-plications, as couplers in fiber lasers, or as disper-sion compensation devices [1]. These optical sensorsmay involve temperature [2], strain�force [3], orpressure [4] sensing, and offer multiple advantages[5]. In addition, they are compact, lightweight, andsmall enough to be coated or embedded into structuresor substances to create smart materials operating inextreme environments. Also, there has been a genuineinterest recently in tuning the Bragg wavelength ofFBGs in the field of tunable fiber lasers [6–10]. In thepast, many tuning devices have been investigated.Tunable filters based on dilatation or elongation of thegrating due to temperature change have been pro-posed [11,12], but their tuning rate from 0.4 nm�K to10 pm�K appeared too restrictive for several applica-tions. A second method with a sensitivity of the orderof 0.2 nm�K is based on the use of piezoelectric ac-tuators and transducers [13]. However, this approachrequires high voltage and is therefore not ideal interms of compactness and weight. The third tech-nique frequently encountered is based on the appli-cation of mechanical stress. There are several waysto induce a mechanical stress to an optical fiber butthe best results to date were obtained by the appli-cation of a compression�tension strain by way of thebeam bending technique. Based on this approach itwas demonstrated by Goh et al. [14,15] that a wave-

length shift at approximately 90 nm could be ob-tained at an operating wavelength of 1.5 �m.However, to our knowledge, some aspects related tothe fabrication of these devices, in particular thechoice of embedding material, have never been spe-cifically addressed. Furthermore, the performances oftuning filters strongly depend on the choice of embed-ding material.

In this paper, we propose to demonstrate that theso-called beam bending technique, when implementedwith a proper choice of material, constitutes an effec-tive approach for the development of long-term stabledevices performing FBG tuning. Two types of embed-ding material have been tested. The first one involvesreadily available fast drying epoxies whereas the sec-ond relies on ballistic polymers (DuPont Surlyn). TheSurlyn polymers have been selected not only becausethey possess the appropriate mechanical propertiesbut also because they show a very high adherence toboth glass and metal. Thus they allow the fabricationof tuning devices in a single-step process eliminatingthe need for additional adhesives to bond the embed-ding material to the stiff substrate [14].

2. Fabrication of the Bending Devices

Our experimental setup is depicted in Fig. 1. The stiffsubstrate is a high quality spring steel blade of lengthL � 15.3 cm, width w � 3.2 cm, and thickness h �0.1 cm. The stiff substrate is held between two endstops with holding grooves at 4 cm high. One end ismovable while the other is fixed. The movable end isdisplaced by a micrometer screw that has a 5 cmtravel. The FBGs used in the experiment have beenwritten in Corning SMF-28 fiber. Hydrogen loaded

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1 June 2007 � Vol. 46, No. 16 � APPLIED OPTICS 3189

fiber samples were exposed to 244 nm radiation froma cw Sabre frequency-doubled (Fred) laser (CoherentGroup Lasers, Santa Clara, California) through aphase mask. The holographic phase mask and theFBGs were both fabricated in our laboratory. TheBragg wavelength of the FBGs used in our experi-ments was approximately 1550.0 nm with a FWHMof 0.3 nm and reflectivities ranging from 3 to 20 dB.The FBGs were subsequently embedded into a flexi-ble material having a low Young modulus (�0.4 to2 GPa). This material adheres to the stiff substrate,which has a high Young modulus (�200 GPa), as wellas to the fiber to prevent longitudinal movement dur-ing the tuning process. Note that owing to the largedifference between the Young modulus of the twomaterials the neutral axis is essentially located in themiddle of the steel blade so that the d factor appear-ing in Eqs. (4) and (5) is essentially given by thegeometric distance between the fiber core and themiddle of this steel blade [15]. Different types of flex-ible material have been tested: fast drying epoxiesand Surlyn polymers. Teflon molds were used toachieve the molding of the optical fiber inside theflexible material. The length of the molds is L �13.0 cm, the width is w � 2.4 cm, and the d factorvaried from 4.0 to 10.0 mm. The fast drying epoxieshave been poured directly into the mold. In the caseof the Surlyn polymers small pellets were put insidethe mold with the optical fiber and heated in an ovenusing a multiple step heating process, starting withan initial drying of the Surlyn polymers followed by4 h of heating at smoothly increasing temperature.Heating longer to a lower temperature was foundpreferable to obtain a uniform and defect-free moldedpolymer. In particular, one had to pay attention thatno residual air bubbles remained in the polymer,which would eventually result in a fiber break at thetuning step. It is also important to note that thegrating has to be positioned in the middle of the moldand the spring steel blade, otherwise nonuniform de-formation is applied to the FBG, which induces anasymmetric chirp. Following the molding process, theFBGs were optically characterized in reflection or in

transmission with the use of an optical spectrum an-alyzer (Ando, Newnan, Georgia, AQ6315E) in con-junction with a supercontinuum light source emittingfrom 0.5 to 1.8 �m.

3. Theory and Principles of the Beam BendingTechnique

Strain and temperature are the two main factors in-ducing a change in the Bragg wavelength. As thefiber is strained, the grating spacing and the refrac-tive index both vary, allowing a different wavelengthto be reflected. The variation in the Bragg wave-length caused by strain and temperature fluctuationsmay be described in terms of the photoelastic andthermo-optic constants

�� 1 � �neff2

2 ��p12 � ��p11 � p12��

��a �

dneff

dtneff

�T�B, (1)

where neff is the effective index of the fiber, p11 and p12are the photoelastic constants of the stress-optic ten-sor, � is the Poisson ratio, is the coefficient of ther-mal expansion of the fiber, T is the temperaturechange, � is the strain variation, and �B is the Braggwavelength. The refractive index, photoelastic con-stants, and Poisson ratio determined the photoelasticcoefficient �e�. Using bulk values of fused silica forthe strain-optic coefficients, e is approximately 0.22.Neglecting temperature variation Eq. (1) becomes

�� 1 � e��B � 0.78��B. (2)

When the beam bending technique is used to tune aFBG, the strain has to be related to the physicalparameters involved. An axial force is applied to abeam of length L by a micrometer screw. This beamcan be bent upward or downward when the microme-ter screw moves from the rest position. This displace-ment of the micrometer screw, normalized by thelength of the beam, is related to the arc angle � by

�zL � �1 �

sin��

2��

2� . (3)

The plane (parallel to the beam) where no strain isfelt even if the beam is bent is called the neutralplane. Therefore the strain that is applied to a FBGplaced at a distance d from the neutral plane is

� � �d�

L , (4)

where positive values represent the compressivestrain and negative values represent the tensile

Fig. 1. Experimental setup.

3190 APPLIED OPTICS � Vol. 46, No. 16 � 1 June 2007

strain. Compressive strain is applied when the beamis bent downward and tensile strain is applied whenthe beam is bent upward. The FBG is embedded atdistance d from the neutral plane in a flexible mate-rial, which adheres to the stiff substrate. The Braggwavelength shift of the FBG is given by

�� 0.78d�

L �B. (5)

By measuring the horizontal displacement of themicrometer screw and the physical length of thebeam, Eq. (3) can be solved for �. It is thereforepossible to relate a wavelength shift to linear dis-placement �z.

4. Experimental Results and Discussion

For each fabricated device, different characteristicshave been measured. The maximum tuning range ofthe device was first determined. The maximum tun-ing range obtained in our experiment was of theorder of 80 nm. Also note that for each device thetransmission curves and the reflectivity curveswere both measured in order to assess the variousoptical properties, namely, the insertion losses andthe spectral deformations. Thus, following the tun-ing range measurements, transmission losses wereanalyzed with respect to the normalized displace-ment. The device stability with respect to its Braggwavelength was also carefully measured over aworking day. Finally, the device longevity over aperiod of time of several months was assessed. Toour knowledge, these longevity curves have neverbeen reported and constitute our main result.

A. Maximum Tuning Range

Several flexible materials have been used to embed theFBGs. The first family of materials that we used forreference was fast drying epoxies, selected because oftheir ease of use and availability. The FBGs were 1 cmlong with center wavelengths of 1549.5 nm, and the dfactor was set to 5 mm. The corresponding Braggwavelength shift versus the normalized displace-ment, �z�L, is plotted in Fig. 2. The maximum tuningrange achieved is 25 nm with the Tra-Con F112 glue.Tra-Con 2110T offers 18 nm of tuning, and the lasttwo glues offer 10 and 3 nm of tuning, respectively.As the normalized displacement increases, the spec-trum of the FBGs was deformed. The peak value ofthe reflection and transmission curves varied, sec-ondary peaks appeared, and the FWHM changedconsiderably. In addition, the signal was strongly at-tenuated because of increased insertion losses and inmost cases limited the practical tuning range. Forthese reasons, it is impossible to use the beam bend-ing technique with these epoxies as embedding ma-terials for any applications such as optical filters,optical sensing, or couplers in fiber lasers to name afew.

The second type of embedding material that weused was Surlyn polymers. These polymers allow fab-

rication of the tuning device in a single step andeliminate the use of glue to bond the embedding ma-terial to the stiff substrate [14]. The FBG was 1 cmlong with a center wavelength of 1554.24 nm. Thepolymer selected for this experiment was Surlyn8940, and d was set to 4.5 nm. The experimental andtheoretical shifts of the Bragg wavelength against thenormalized displacement are illustrated in Fig. 3(a).First note that with this polymer the normalized dis-placement, �z�L, is significantly broader, both incompression and tension. Accordingly, the tuningrange is now of the order of 80 nm, with 55 nm incompression and 25 nm in tension corresponding to�4.51% of compressive strain and 2.06% of tensilestrain. Note that the FBG used for this test was sub-sequently broken in compressive strain. When theadhesion between the fiber and the polymer was nolonger intact, the fiber broke. Figure 3(b) shows thetypical evolution of the reflectivity curves and thevariation of the FWHM of the tunable FBG filteracross a tuning range in excess of 50 nm, �40 nm incompression and �10 nm in tension. The spectrum ofthe FBG remained nearly the same across the wholetuning range, the peak value in reflection does notchange, no secondary peaks appeared, and the FWHMvaried slightly from 0.3 to 0.45 nm.

B. Optical Losses

We have evaluated the optical losses for a setup madewith Surlyn 8940 polymer with d � 4.0 mm. TheFBG was 1 cm long with a center wavelength of1554.37 nm. The insertion losses of the correspondingtuning device as a function of the normalized displace-ment are shown in Fig. 4. They were calculated bysubtracting the baselines of the transmission spectraof the compressed–stretched FBGs for various normal-ized displacements to that of the unstrained spectra.The general trend is that the losses increase more orless linearly with the absolute value of the normalizeddisplacement. Thus for a maximum negative normal-ized displacement of �0.122 (strain of �4.59%), the

Fig. 2. (Color online) Tuning curves (C, compression and T,traction) for different materials (fast drying glues). Diamonddotted curve is Lepage epoxy 5 min, square dotted curve isTra-Con F112, triangle dotted curve is Tra-Con 813J01, andcircle dotted curve is Tra-Con 2170T (�B � 1549.5 nm, d �

5 mm, and L � 15.3 cm).


losses are approximately 0.28 dB; for a positive max-imum normalized displacement of 0.010 (strain of1.18%) the losses are 0.11 dB. Those results are sim-ilar to those observed, for instance, by Mokhtar et al.[16] for large compressive strains. However, up tonow there has been, to our knowledge, no report ad-dressing the issue of the physical origin of theselosses. Apparently, these losses would not be caused

by macrobending since the critical radius of curva-ture for bending losses is �1 cm whereas the typicalfiber curvature radius in our setup varied from 20 to30 cm. In fact, based on our observations, these losseswould more likely be caused by microbending losses.Indeed, we noted that, when the optical fiber moldedin the embedding material is not perfectly straightand is presenting tiny local deviations or local buck-ling, additional losses appear as the normalized dis-placement is increased. At the idle position, both theglobal radius of curvature and the local radius ofcurvature are too large to cause significant losses.When the normalized displacement is increased, thelocal radius of curvature approaches the critical ra-dius, and microbending losses appear even if theglobal radius of curvature is too large to cause losses.Still, insertion losses lower than 0.28 dB have beenobtained over 70 nm of tuning, and this confirms thatFBG tuning devices made of this material show lowinsertion losses on a large range of compressive�tensilestrain and are therefore suitable for many applica-tions.

C. Stability of the Bragg Wavelength Over a Working Day

The stability of the Bragg wavelength has beenstudied for Surlyn 8940 with d � 4.5 mm during aperiod of time corresponding to a working day. TheFBG was 1 cm long with a center wavelength of1554.84 nm. Similar measurements have been re-ported once previously but only for a tuning devicebased on stretching�compression of metal coatedFBGs with a very limited tuning range ��10 nm�.According to this report by Rocha et al. [17] the Braggwavelength actually deviates from its original posi-tion by 1.5 nm in 8.5 h at approximately 6 nm incompression whereas for 5 nm in traction it deviatesfrom 0.5 nm in 8.5 h. To better establish the stabilityof the Bragg wavelength in our tuning device duringa working day, we set it to a fixed displacement, andthe deviation of the Bragg wavelength has been mea-sured every 30 min. The fixed displacements corre-spond to wavelength shifts of 10, 20, and 30 nm incompression and to 5, 10, and 15 nm in traction. Theresults are shown in Fig. 5. A negative deviationmeans that the Bragg wavelength shifts toward theoriginal (uncompressed) wavelength and inversely fora positive deviation. The Bragg wavelength shiftedby 0.26, �0.31, and �1.56 nm during a working dayfor 10, 20, and 30 nm in compression mode, respec-tively. In traction mode, the Bragg wavelengthshifts were 0.29 nm, �0.10 nm, and �0.19 nm dur-ing a working day for 5, 10, and 15 nm, respectively.For the displacement that corresponds to 30 nm ofwavelength shift in compression, the Bragg wave-length moved by �1.56 nm toward its original posi-tion over 8.5 h. As compared to the results reportedby Rocha et al. [17], the threshold in terms of wave-length shift before observing such a significant devi-ation of the Bragg wavelength in compression hasbeen multiplied by a factor of 6. Those deviations, ifneeded, can be overwhelmed by moving the microme-

Fig. 3. (Color online) (a) Solid line corresponds to the theoreticalcurve and the square dotted is the experimental results (C,compression and T, traction) with Surlyn 8940 as the flexiblematerial (�B � 1554.24 nm, d � 4.5 mm, and L � 15.3 cm). (b)Reflectivity curves of the FBG tuning device over 50 nm (C, com-pression and T, traction) with Surlyn 9950 as the flexible material(�B � 1549.5 nm, d � 4.5 mm, and L � 15.3 cm).

Fig. 4. (Color online) Insertion losses versus the normalized dis-placement for Surlyn 8940 (�B � 1554.37 nm, d � 4.0 mm, andL � 15.3 cm).


ter screw forward or backward. When the device isbrought back to the z � 0 position, the Bragg wave-length perfectly returns to its original position withina recovery time of less than a few minutes if the

spring steel blade and the polymer have not been setbeyond their limit of elasticity.

D. Device Longevity

The long-term behavior of the device has been ana-lyzed for both fast drying epoxies (�B � 1549.5 nm andd � 5 mm) and Surlyn polymers (�B � 1554.15 nmand d � 4.5 mm). The main observation is that epoxybased devices have a very limited longevity. Figure6(a) shows the tuning curves of Tra-Con F112 epoxy(which produced the best results among the variousepoxies) at three different times as a function of thenormalized displacement. Over a 45 day interval, theBragg wavelength is shown to shift by 4 nm, whichrepresents a relative variation of 40% with respect tothe initial Bragg wavelength shift. Our second deviceinvolved Surlyn 9950 polymer. Based on the longev-ity plot of Fig. 6(b) the corresponding device appearsto show no variation of its tuning curve over a timeinterval of 263 days. In fact, the tuning curve wasmeasured several times (at 11, 19, 37, 60, 160, and263 days after the fabrication) during this period, andthe observed slight deviations are actually related tovariations in the experimental conditions as they donot show any trend as a function of time. Only thetuning curves of 11, 160, and 263 days are plotted inFig. 6(b) to keep it clear and readable. These smallvariations are caused mainly by the uncertainty inthe identification of the rest (idle) position.

5. Analysis

The long-term stability of the embedding material isobviously one of the most crucial points to consider inchoosing the ideal material for the fabrication of astable device for FBG tuning. The observed degrada-tion of the optical characteristics of the epoxy baseddevice is most likely attributable to the slow evapo-ration of the solvents found in the epoxy hardener.This evaporation causes the epoxy to harden, which isgenerally the desired result, but actually degradesthe tuning performances. Thus although epoxies andother solvent based cements present interesting me-chanical properties they are intrinsically unstable intime and are therefore not appropriate. We present inTable 1 comparative data between epoxies and com-mon plastics with respect to their relevant mechani-cal properties. We first note that most of thesematerials present similar properties with respect totheir Young’s modulus and hardness. However, con-trary to epoxies, plastics such as polymethyl methac-rylate (PMMA), high density polyethylene (HDPE),and polypropylene copolymer (PPC) possess moder-ate adhesion to glass and poor adhesion to metal.Surlyn is the only material showing excellent adhe-sion to both glass and metal. Now, good adhesion toglass is essential in ensuring that the fiber will notslip inside the embedding material whereas adhesionto metal permits one step fabrication. Moreover, as aballistic plastic, Surlyn also possesses very good prop-erties in term of elongation at break, which is essen-tial in relation to the device’s maximum tunabilityrange as well as to its performance in long-term re-

Fig. 6. (Color online) Longevity (tuning) curves for different ma-terials. (a) Tra-Con F112 epoxy (�B � 1549.5 nm, d � 5 mm, andL � 15.3 cm). Dotted circles correspond to 2 days after curing,dotted squares 34 days after curing, and dotted diamonds 45 daysafter curing. (b) Surlyn 9950 polymer (�B � 1554.15 nm, d �

4.5 mm, and L � 15.3 cm). Dotted circles after 11 days, dottedsquares 160 days, and dotted diamonds 263 days after thefabrication.

Fig. 5. (Color online) Bragg wavelength deviation for a fixed dis-placement versus time for Surlyn 8940 (�B � 1554.84 nm, d �

4.5 mm, and L � 15.3 cm). Dotted empty squares, 10 nm; dottedempty circles, 20 nm, and dotted empty diamonds, 30 nm in com-pression. Dotted full circles, 5 nm; dotted full squares, 10 nm; anddotted full diamonds, 15 nm in traction.


peatability. In fact, we have shown that good perfor-mances could be repeatedly obtained provided thelimit of plastic deformation of the material was notreached. On that basis, it appears that, among thevarious polymers commercially available, the Surlynare the most appropriate for the fabrication of anefficient and long-lived FBG tuning device. Since Sur-lyn polymers possess a large elongation at break andare especially malleable, they firmly hold the fiberduring the stretching–compressing process. In con-trast, the epoxies have a small elongation at break,are brittle, and have lower adhesion to glass thanSurlyn, which gives rise to some major problems. Asthe normalized displacement increases, the fiber canmove slightly or microfissures appear in the flexiblematerial. The fiber breaks or high insertion lossesappear when those effects happen. This severely lim-its the use of those materials to develop high-qualitytunable optical filters.

6. Conclusion

We have shown that the proper choice of embeddingmaterial is important in developing FBG tuning de-vices based on the beam bending method. Using Sur-lyn polymers as embedding materials, we have shownthat temporally stable devices tunable over 80 nmand with small FWHM variations could readily befabricated. According to our results such deviceswould be stable over a period of several years if elasticdeformation of the polymer is avoided, which does notprevent tunability over a broad wavelength region. Itwas also shown that the optical losses could be lessthan 0.28 dB for a corresponding tuning range of70 nm. Altogether, these characteristics demonstratethat, owing to a proper choice of the embedding ma-terial, reliable and low-cost FBG tuning devices canbe designed for most applications.

This research was supported by the Canadian In-stitute for Photonics Innovations (CIPI), the NationalSciences and Engineering Research Council of Can-ada (NSERC), and the Fonds Québécois de Recherchesur la Nature et les Technologies (FQRNT). The au-thors thank Anna Ritcey and Simon Leduc.

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Table 1. Mechanical Properties of Epoxies and Common Plastics

MaterialAdhesion to

MetalAdhesion to

Glass

Young’sModulus

(GPa)

Elongation atBreak

(%)Hardness(Shore D)

EpoxiesTra-Con F112 Good Good �3.5 �4 86Tra-Con F112 heated Good Good �3.5 �4 112Tra-Con 813J01 Good Good �3.5 �4 75Tra-Con 2170T Good Good �3.5 �4 65Lepage epoxy 5 min Good Good �3.5 �4 98

Polymers�PlasticsHDPE None Good 0.7 100 68PPC None Good 0.9 200–300 73PMMA Poor Poor 4.0 10 85Surlyn Excellent Excellent 0.4–2.0 480 65


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Long-term stable device for tuning fiber Bragg gratings

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