The present invention concerns a method for application of a fibre-optic Bragg grating as an hydrostatic pressure sensor.

Background A fibre Bragg grating (FBG) is a permanent, periodic refractive index modulation in the core of a single-mode optical silica glass fibre over a length of typically 1-100mm, formed by transversely illuminating the fibre with a periodic interference pattern generated by ultra- violet laser light, e. g. from a so called Eximer laser, either by using a two-beam interferometer, as disclosed by G. Meltz et. al. in"Formation of Bragg gratings in optical fiber by a transverse holographic method,"Opt. Lett., Vol. 14, pp. 823-825,1989, or by illuminating the fibre through a periodic silica phase mask, as disclosed by K. O. Hill et. al. in"Bragg gratings fabricated in monomode photosensitive optical fiber by UV exposure through a phase-mask,"Appl. Phys. Lett., Vol. 62, pp. 1035-1037,1993. An FBG reflects light within a narrow bandwidth, centred at the Bragg wavelength, AB = 2neffA, where neff is the effective refractive index seen by the light propagating in the fibre, and A is the physical period of the refractive index modulation. Outside the FBG bandwidth light will pass with negligible loss. It is known that the reflected Bragg wavelength from an FBG will change with any external perturbation which changes the effective refractive index seen by the propagating light and/or the physical grating period (fibre length), such as temperature and strain. By measuring the reflected Bragg wavelength, using for example a broadband light source and a spectrometer, an FBG can be used as a sensor for measuring such external perturbations. The bandwidth of the reflection spectrum from an FBG sensor is typically 0.1-0.3nm (-10-30GHz).

An FBG can also be used as a pressure sensor by measuring the shift in Bragg wavelength caused by hydrostatic pressure induced compression of the silica glass fibre. This provides a very simple sensor design with small dimensions and good reproducibility and long-term stability provided by the all-silica construction of the sensor. However, owing to the low compressibility of a silica glass sylinder the sensitivity is low, at 1550nm the Bragg wavelength changes by only-0.4pm/bar. With a high-resolution spectrometer the wavelength resolution can be < lpm, in which case the pressure resolution will be 2.5bar.

too low for many applications, and a higher sensitivity sensor is desirable. Important in making a practical FBG pressure sensor is to compensate for temperature fluctuations. The temperature sensitivity at 1550nm is typically-10pm/°C, implying that a change in pressure of 2.5bar, yielding a shift in Bragg wavelength of lpm, corresponds to a change in temperature of only 0.1 °C. One way to temperature compensate the FBG pressure sensor is to use a second FBG, which is isolated from the pressure, to measure the temperature. Because of the low pressure-relative to the temperature-sensitivity, an all-fibre pressure sensor will put strong requirements on the accuracy of the temperature and pressure measurements and on the temperature difference between the two FBGs. Hence, a reduction in temperature sensitivity and a minimisation of the temperature difference between the pressure and the temperature sensor is desirable.

One known technique to enhance the pressure sensitivity and reduce the temperature sensitivity of fibre-optic pressure sensors, and maintain the all-silica construction, is to use a side-hole fibre, which has two open channels symmetrically posititions at each side of the fibre core, as disclosed by Xie et. al., in"Side-hole fiber for fiber-optic pressure sensing," Optics Letters, Vol. 11, pp. 333-335,1986, where an external hydrostatic pressure is converted into an anisotropic stress in the core region of the fibre, which through the elasto- optical effect induces a birefringence B = nu-nu where nx and ny are the refractive indices seen by the two orthogonally polarised fibre eigenmodes in the fibre. The total birefringence is given by the sum of the inherent fibre birefringence and the birefringence B induced by the differential pressure between the surroundings and the side-holes. A stress concentration, and hence changes in refractive indices, by a factor of typically 3-5 can be achieved by suitable choice of the geometry of the side-holes, compared to a standard fibre. Side-hole fibres have been used to make dual-path polarimetric interferometers for high pressure sensing, as described by K. Jansen and Ph. Dabkiewicz in"High pressure fiber-optic sensor with side-hole fiber,"SPIE Proceedings, Fiber Optic Sensors II, Vol. 798, pp. 56-60,1987.

The use of an FBG written in a birefringent side-hole fibre for pressure sensing has been suggested in Norwegian Patent Application No 951052, March 20,1995 and US Patent Application No. 08/618,789, March 20,1996, but here only in a distributed feedback fibre laser construction, where the fibre core is doped with one or more of the rare-earths to provide gain when pumped with an external pump source, and the FBG provides feedback

and laser operation at the two orthogonally polarised Bragg wavelengths of the FBG with very narrow optical bandwidths (-10-100kHz). The patent also suggests using measurements of both wavelengths to provide simultaneous measurements of two measurands, such as pressure and temperature. The basic principle of such a two-parameter measurement in a birefringent FBG was first presented in US Patent No. 5,399,854, March 21,1995, where it was shown that birefringence in an FBG causes a splitting in the reflection spectrum, with one reflection peak corresponding to each of the two polarisation eigenmodes of the fibre.

When fibre-optic pressure sensors are operated under conditions of high temperature and pressure, such as in oil wells, there might be considerable drift effects, as taught us by J. R.

Clowes et. a/. in"Effects of high temperature and pressure on silica optical fibre sensors," Proceedings of 12* Conference on Optical Fiber Sensors, Williamsburg, VA, pp. 626-629, 1997. The drift effect occurs when the fibre is surrounded by a liquid, such as water, and increases with increasing temperature. The effect is belived to be due to ingress of liquid molecules into the outer layers of the fibre cladding resulting in the development of a highly stressed layer and consequently a tensile stress on the fibre core. It was shown that the use of a hermetic, carbon coating, reduced the effect significantly. In addition, diffusion of gases, such as hydrogen, into the fibre, will cause changes in effective refractive index associated with changes in the light absorption spectrum of the fibre, as disclosed by Malo et al. in "Effective index drift from molecular hydrogen diffusion in hydrogen-loaded optical fibres and its effect on Bragg grating fabrication", Electornics Letters, Vol. 30, pp. 442-444,1994.

Finally, diffusion of gases into side-holes will change the pressure inside the holes, and cause drift in measurements of the external hydrostatic pressure.

Objects The main object of the invention is to provide a method for application of a practical all- fibre/all-silica FBG pressure sensor with enhanced pressure sensitivity and reduced temperature sensitivity compared to a standard FBG, i. e. an FBG written into a standard, single-mode silica fibre. A second object is to provide temperature compensated pressure measurements using one FBG only. A third object is to ensure that the sensor is not subject to drift at high temperatures and pressures.

The Invention The object of the invention is achieved with a method having features as stated in the characterising part of Claim 1. Further features are stated in the dependent claims.

The main part of the invention comprises an FBG written into the core of a birefringent side-hole fibre, with a birefringence in the operating pressure range i) sufficient to split the two reflection peaks (or transmission dips), with bandwidths of typically 0.1-0.3nm, corresponding to each of the two orthogonal polarisation eigenmodes of the fibre, and ii) with the same sign such that the two peaks do not cross each other under operation. The reflection spectrum of the FBG sensor is measured with a device for measurement of optical wavelengths enable to resolve the peak wavelengths of the two reflection peaks such that by measuring one of the wavelengths and the wavelength separation simultaneous pressure and temperature measurements are achieved owing to the different pressure sensitivity and similar temperature sensitivity of the two peaks. The invention also includes an FBG as described above coated with an hermetic coating, such as a carbon, a ceramic, or a metal coating, to prevent penetration of gases, vapours and liquids in the surrounding environment to ingress/diffuse into the fibre and cause drift in the measured Bragg wavelengths.

Examples In the following, the invention will be described with reference to illustrations, where Fig. 1 shows the cross-section of a side-hole fibre with hermetic coating, Fig. 2 shows an FBG pressure sensor in a birefringent side-hole fibre, with input, output and reflected optical spectra, and

Fig. 3 shows the principle operation of an FBG side-hole fibre pressure sensor, showing the changes in reflected spectrum with an increase in pressure and temperature.

Fig. 1 shows the cross-section of a side-hole fibre with a core 1, which contains a fibre Bragg grating (FBG), a cladding 2, and two side-holes 3 with diameter 2ro, and a coating 4, which can be hermetic. The coating 4 can be a carbon or a metal coating, such as a gold coating, to prevent penetration of molecules in the surrounding environment, which could be water or hydrogen to ingress/diffuse into the fibre glass and cause changes in the internal stresses and drift in the measured Bragg wavelengths.

The fibre diameter is 2r. The side-holes are oriented along the x-axis. The pressure outside the fibre is Po, while the internal pressure inside the side-holes is Pi.

Fig. 2 shows the operation of the side-hole FBG sensor in principle. The FBG 5 is written into the core of the side-hole fibre with side-holes 3, which is spliced between two standard single-mode fibres 6 with splices 7. One way to interrogate the sensor is, as illustrated, to launch into the standard fibre 6 unpolarised, broadband light 8 covering at least the wavelength range of the Bragg reflection from the FBG which is reflected at two orthogonally polarised wavelengths 9 and 10 with a separation AA. = 2BA, with individual bandwidths tiB of typically 0.1-0.3nm. The input light which is not reflected is transmitted with a spectrum 11, with dips at the Bragg wavelengths.

The birefringence in the fibre can be expressed as: B = BT + (A/2X) (SoPo-SjPj), where BT is the static birefringence of the fibre, which is a combination of stress induced and geometrically induced birefringence, X is the wavelength, and S. and Si is the phase sensitivity of the birefringence to inner and outer pressure pressures Po and Pi, respectively.

K. Jansen and Ph. Dabkiewicz in"High pressure fiber-optic sensor with side-hole fiber," SPIE Proceedings, Fiber Optic Sensors II, Vol. 798, pp. 56-60,1987 measured an S,, = 16.8 rad/barm at X = 633nm. This corresponds to an expected wavelength splitting sensitivity 6AA./6P,, = 1.8pm/bar, which is a factor of-4.5 times the sensitivity of a standard FBG. The measured phase sensitivity of the birefringence to temperature at 633nm was measured to be 0.18rad/°Cm, which correspond to an expected wavelength splitting sensitivity oA/oT = 0.020pm/°C, which is a factor of-0. 002 smaller than the temperature sensitivity of a standard FBG.

From Y. Namihira, J. Lightwave Technol., Vol. LT-3, pp. 1078-1083,1985 is known that the change in refractive index due to stresses cj,,, ay and az in the fibre core can be expressed as: onX = C1 QX + C2 (#y+#x), and #ny = C, 0y + C2 where C, and C2 denote the direct and tranverse opto-elastic constant, respectively, where C, =-0. 74#10-12Pa-1 and C2 # -4.1#10- '2Pa~', c. f. K. Hayata et. al., J. Lightwave Technol., Vol. LT-4, pp. 601-607,1986. Xie et. al presented in"Side-hole fiber for fiber-optic pressure sensing,"Optics Letters, Vol. 11, pp.

333-335,1986 approximate estimates of ax ~ 0 and ay # -Por/# (and cy, = 0), where U = r-2ro is the centre wall thickness along the x-axis, with and external pressure Po. This leads to the following estimates for the shifts in the individual wavelengths ABX and iBy with a change in external pressure PO: ##Bx/#Po # 1. 2####/#Po and 0.2####/#Po.##By/#Po# Fig. 3 shows the change in reflected Bragg wavelengths 9 and 10 by increasing the differential pressure AP = Po-Pi from #P0 to #P1. keeping a constant temperature. Also shown is the effect of increasing the temperature from To to TI, shifting both peaks by almost equally, with a typical sensitivity of ##Bx/#T # ##By/#T # 10pm/°C. By measuring i) the first wavelength, #By, which as a relatively small pressure sensitivity, and ii) the wavelength splitting #Bx - #By, both differential pressure AP and the temperature can be measured with high accuracy, owing to the low cross-sensitivity.