We report on the effects of liquids at high temperature and pressure on silica optical fibres, sensors and gratings. We propose that the diffusion of molecules into the silica and the resultant expansion of the network are responsible for observed fibre expansions of up to 0.2% and Bragg wavelength increases of 2nm at 1525nm. Amorphous carbon hermetic coating has shown a reduction of these effects by an order of magnitude at 300°C. These results have strong implications for the deployment of fibre sensors in oil wells

Clowes, J.R.

McInnes, J.

Payne, D.N.

Zervas, M.N.

English

Southampton (e-Prints Soton)

1Southampton SO17 1BJ,  UKTelephone: +44 1703 593150e-mail:   light@orc.soton.ac.ukOptoelectronics Research CentreUniversity of SouthamptonEffects of High Temperature and Pressure on Silica Optical Fibre Sensors J. R.Clowes , J.McInnes , M.N.Zervas, D.N.Payne (b)Sensor Dynamics, Abbas Business Centre, Itchen Abbas,  Winchester, Hampshire, UK.AbstractWe report on the effects of liquids at high temperature and pressure on silica optical fibres, sensorsand gratings.  We propose that the diffusion of molecules into the silica and the resultant expansionof the network are responsible for observed fibre expansions of up to 0.2% and Bragg wavelengthincreases of 2nm at 1525nm. Amorphous carbon hermetic coating has shown a reduction of theseeffects by an order of magnitude at 300 C.  These results have strong implications for the deploymentBof fibre sensors in oil wells.  Introduction:Many optical fibre sensors rely on the fact that  the measurand modulates the optical path lengthand/or refractive index of the “stable” silica waveguide.  If the long-term stability of silica isquestionable, the accuracy of such sensors is dubious.In this paper, we present recent results which demonstrate some of the problems of silica opticalfibres as sensors in high temperature and pressure environments and report on the need forsuitable “hermetic” coatings for such devices.  By monitoring the pressure-induced birefringence of a side-hole optical fibre [1] under conditionsof high temperature and pressure, we have found that the birefringence drifts considerably.  Thisdrift increases non-linearly with increasing temperature and the effect is permanent.The high temperature instabilities have been further investigated using Hi-Bi fibre pressuresensors, fibre Bragg gratings and standard elecommunications single-mode fibre.  Similar resultsfor the drift in all cases show that the observed effects are inherent to silica and are not due togeometry.  It is therefore expected that similar effects will be observed for any silicainterferometric or polarimetric sensor in high temperature fluid environments.Side-Hole Polarimetric Fibre SensorSide-hole fibre was proposed by Xie et al [1] as a possible sensor for hydrostatic or acousticpressure.  In the presence of hydrostatic pressure acting on the outside of the fibre, an anisotropicstress is induced in its core due to the fibre geometry.  The resulting birefrigence is therefore ameasure of the applied pressure.   The pressure sensor consists of an input polariser, a length of side-hole fibre and a mirror.  Theinput polariser is aligned at 45to the birefringent axes of the side-hole fibre to coupleBapproximately equal amounts of light into each axis.  Under the application of hydrostaticpressure, a change in the refractive index in each a is is produced through the photoelastic effect.The result is  the generation of a  fast and slow axis, where nand n are the fast and slow indicesf s 2respectively.  The magnitude of  )n, where )n = n - n , is dependent upon the applied pressures f and also upon the fibre geometry. The pressure-induced optical path difference between the birefringent fibre axes is monitoredusing a dynamically-matched, low-coherence interferometer in which the path difference ismatched in a reference Hi-Bi fibre scanned in temperature. Measurements are made at 1300nm.Hydrostatic pressure is applied to the fibre by means of a dead-weight tester through a liquidmedium within high-pressure steel tubing.  The chamber temperature is varied as required usingheating cord.Figure 1 shows the results of two 0.15m long sensors at temperatures of 200 and 300C, whereBwater has been used as the pressure medium.  A pressure of 4000 psi was applied to the fibres.The resulting birefringence and corresponding pressure measurement are shown.   The effect oftemperature on the drift is very noticeable and increasing temperature has a disproportionateeffect on the drift rate. The drift is therefore not observed at room temperature as the rate ofchange inbirefringence is extremely low.  For fibres at 300C, however, there is a zero crossingBon the birefringence/pressure axis indicating that there has been an exchange of the birefringentfibre axes with the fast axis becoming slow and vice versa.  This large drift effect is permanent andtherefore, as the initial zero pressure offset of the fibre is  low, on removal of the pressure, theresulting side-hole fibre is very highly birefringent (equivalent to a pressure of 6000psi) with atypical beat length of the order of  1.5mm at 633nm.  It was found that this can be reducedthrough HF etching of the fibre or by heat treating in an oven at high temperature.  As well as the temperature effect on the fibre drift, the effect of the liquid medium has also beenstudied.  Figure 2 shows some of these results, all at approximately 300C.   Other liquids haveBbeen used in an attempt to remove all trace amounts of water from the pressure chamber.  It isbelieved that liquid ingress into the outer layers of the fibre cladding results in the developmentof a highly stressed layer which exerts an opposing “pressure” on the core.  This has beenconfirmed through the use of a finite element model (FEM) of the fibre in which the introductionof a swollen surface layer is capable of inducing such a highly birefrefringent fibre.  It is interestingto notice that the water has by far the greatest effect on the fibre and, for this reason, water hasbeen used for the majority of recent experiments.Effect on Silica FibresIn order to investigate the effect of the fibre side-holes on the drift phenomenon, the pressuresensor experiment has been repeated using Bow Tie and Panda Hi-Bi fibres as pressure sensors.The resulting permanent-induced birefringence, normalised to pressure, was consistent with thatfor side-hole fibre, indicating that the fibre geometry and composition do not make a significantcontribution to the drift rate.  To investigate this further, measurements have been made using single-mode fibre and negative-index fibre Bragg gratings (to prevent high temperature decay [2]).  The postulated swelling ofa surface layer on the fibre is anticipated to produce an overall lengthening of the fibre.  Thelength of a fibre and also the Bragg wavelength of a grating have, therefore, be n monitored undersimilar conditions to the side-hole fibre.  Lengths of carbon-coated fibre  have also been measuredas a comparison to uncoated fibre and as a test of the coating hermeticity.3A length of carbon-coated fibre, a length of standard telecommunications fibre and a negativeindex grating were spliced to high-pressure seals in both a heated air chamber and a chambercontaining water.  The lengths were  measured using low-coherence  interferometry and thegratings were interrogated using an optical spectrum analyser and subsequent averaging of thspectrum to achieve a single Bragg wavelength value.  The lengths of fibre wit in the pressurechamber were all 1.8m with mirrored endfaces.  However, only approximately 0.6m of the fibrewas in the hot area of the chamber.  Figure 3 shows the results for the length cha e of the fibre with time and Figure 4 is that for thechange in Bragg wavelength with time.  Both curves indicate that the same process is occurringfor the two fibres.  The sudden increase in fibre length and Bragg wavelength after 7 daysoccurred when the temperature of the chamber was lowered to 200C and then returned to 300C,B      Ba procedure followed because of the fading of the grating reflection.  However, after thetemperature cycle had been made, the full grating reflection had returned and the fibre length andBragg wavelength experienced sudden increases.  This occurrence is being investigated further.These effects were not noticed in the carbon-coated fibre or the fibres in the dry chamber. On removal of the fibres from the pressure chamber after the experiment, the lengths of fibre werephysically measured and were observed to have increased in length more than the optical patlength measurement of 1.1mm suggested.  The difference between the length change and optical-path-length change indicates that there has been a decrease in the fibre index during the process.This effect is to be expected when a swollen cladding layer exerts a tensil  stress on the fibre core.The curve for the carbon-coated fibres in Figure 3 demonstrates a solution to the problem ofliquid ingress into optical fibres, since it shows a reduction of the effect by a factor ofapproximately 10.  Theory of Drift ProcessWe postulate that the drift phenomenon in optical fibres at high temperatures and pressure is dueto the ingress of liquid molecules into the silica structure and the resultant disruption of the silicanetwork.  This disruption gives rise to an expansion of the glass and hence results in a tensilestress on the fibre core.  In thecase of the side-hole pressure sensor, this is shown as a change inbirefringence of the fibre due to the fibre geometry.  For lengths of fibre and fibre Bragg gratings,the result is the combination of a longitudinal strain and a decrease in the refractive index of thewaveguide, which has the effect of masking the true magnitude of the fibre extension.  Modelling of this process using FEM shows that all the observed changes in the fibres can beexplained by the swelling of a thin outer layer of the fibre.  Also, etching experiments on thetreated grating and side-hole fibres showed that th  changes can be reversed.  Figure 5 shows theresults of etching the two Bragg gratings in a dilute solution of HF down to a radius of45approximately 15µm.  In untreated fibres little change in Bragg wavelength is expected down tothis fibre diameter [3].  Similar behaviour was observed with the treated “dry” grating. However,the Bragg wavelength of the “wet” grating has shifted considerably from its offset value of1526.39nm back towards its init al (untreated) wavelength of 1524.49nm.  The observed changecontinued to occur  until the end of the etch experiment when the fibres failed at a measureddiameter of 30µm.  This would suggest that the water (or OH groups) had diffused almost 50µminto the glass, which is much greater than has been previously shown in bulk glasses duringautoclave xperiments [4] where a value for the diffusion coefficient of approximately 5x10-cm/s has been given for 300C.  From this diffusion coefficient, the average diffusion distance13 2      Bof a water molecule (or hydroxyl group) would be approximately 8µm.  ConclusionsWe have shown results which question the stability of silica fibres in high temperature andpressure conditions and have attributed this insability to the diffusion of the surrounding mediumconstituents into the glass.  This has great implications for the use any optical fibre sensors,especially in harsh conditions such as down-hole sensing in oil wells. Long-term drifts at lowertemperatures are also therefore inevitable, althought the effects are considerably less at reducedtemperatures.   Finally, we present a solution to this problem with the use of hermetic coatings,in this case carbon, which has reduced the effects by an order of magnitude at 300C.  BWe acknowledge useful discussions with Dr. M. Varnham and Dr. E. Kluth.  We would like tothank Sensor Dynamics and Chevron for sponsorship of this work and also Dr. K. Walker ofLucent Technologies for the donation of carbon-coated hermetic fibre.References1. H.M.Xie et al, “Side-hole fiber for fiber-optic pressure sensing”,  Optics Letters, Vol. 11,No.5, 1986.2. W.F.Liu et al, “Comparison of the Decay Behaviour of Negative and Positive IndexChange Fibre Bragg Gratings”, IEE Colloq. Optical Fibre Gratings, London, 7/2/1997.3. L.Dong et al,  “Tuning and Chirping Fiber Bragg Gratings by Deep Etching”, IEEEPhotonics Technology Letters, Vol. 7, No.12, 1995.  4. H.Wakabayashi and M.Tomozawa, “Diffusion of Water into Silica Glass at LowTemperature”, J. Am. Ceram. Soc, Vol. 72, No.10, pp. 1850-55, 1989.  

Comparison of the Decay Behaviour of Negative and Positive Index Change Fibre Bragg Gratings”, IEE Colloq. Optical Fibre Gratings,

M.Tomozawa, “Diffusion of Water into Silica Glass at Low Temperature”,

Side-hole fiber for fiber-optic pressure sensing”,

Tuning and Chirping Fiber Bragg Gratings by Deep Etching”,

Effects of high temperature and pressure on silica optical fibre sensors

Effects of high temperature and pressure on silica optical fibre sensors

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