12 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 41, NO. I , FEBRUARY 1992

Fiber-optic Strain-Gauge Manometer up to lo0 MPa Wojtek J. Bock, Senior Member, IEEE, Roland Wisniewski, and Tomasz R. Wolinski

Abstract-The paper describes a practical realization of a novel pressure transducer, utilizing a fiber-optic strain sensor and an active element configured to simulate an infinite cylin- der with free ends. The deformation of such a cylinder depends uniquely on pressure acting from its inside and is independent of the stress resulting from the attachment of the device to the pressure system. The fiber-optic strain sensor is permanently bonded to the external surface of the cylinder and as such is fully isolated from the high pressure region. The sensing ele- ment of the device consists of a highly birefringent (HB) polar- ization-maintaining optical fiber strain gauge. The device was characterized at ambient temperatures for pressures up to 100 MPa using a Harwood DWT-35 deadweight tester with a read- ing accuracy of at least 0.1%, traceable to the NIST. The de- scribed sensor has inherent advantages such as immunity to electromagnetic interference, safety in electrically dangerous, hazardous or explosive environments, direct compatibility with optical data transmission systems, simplicity and cost-effective- ness. It does not require any fiber-optic leadthrough, since it is located outside the pressure region and has significantly in- creased sensitivity (around 0.1 MPa- ’) over similar devices based on electrical strain gauges.

I. INTRODUCTION ESSURE transducers based on strain gauges fixed to p” an active mechanical element that undergoes a defor-

mation (dilatation, deflection, etc.) under the influence of pressure have been known for a long time. This type of transducer is particularly well suited for measurements of elevated or high pressures inside pipelines, storage ves- sels, or processing chambers. Up to now, however, they have been based uniquely on bonded resistance strain gauges, characterized by the gauge factor GF:

where R, is the electrical resistance of a strain gauge, and E is the longitudinal strain A l / l , . The actual deformation of the element and the strain resulting from it at the lo- cation of the strain gauge will obviously depend on the design of the active element. For higher pressures up to 100 MPa this element most often takes the form of a di- lating cylinder. The cylinder is usually closed at one end and has its other end directly connected to a pressure ap- paratus, an arrangement that may generate some un- wanted stress in the active area of the cylinder.

Manuscript received May 14, 1991; revised September 20, 1991. The authors are with DCpartement d’informatique, Laboratoire d’opto-

electronique, UniversitC du QuCbec a’Hull, Quebec, J8X 3x7 Canada. R. Wisniewski and T. R. Wolinski are on leave from Instytut Fizyki,

Politechnika Warszawa, Poland. IEEE Log Number 9105564.

11. FIBER-OPTIC STRAIN SENSING The main idea described in this paper is the use of a

fiber-optic device similar to the one that we proposed re- cently [ 13 to replace the standard electrical resistance strain gauge in the strain-gauge pressure manometer based on a dilating cylinder. Electrical strain gauges, although widely used, suffer from significant temperature drift (thermally induced voltages caused by thermocoupling and temperature effects on gauge resistance and gauge factor). In addition, the low electrical output level of such a strain gauge makes it extremely susceptible to electro- magnetic interference (EMI) , especially in noisy indus- trial environments. Desensitization of these gauges to EM1 is very difficult and not always possible, and the proce- dure is actually more costly than resistance-strain sensing technology itself.

The currently emerging fiber-optic sensors based on the measurand-modulated effects on the properties of light propagating through the optical fibers offer important ad- vantages over many presently used transducers or mea- suring devices. As their main component is a quartz waveguide, they are inherently immune to EMI, safe in electrically dangerous or explosive environments, and have a significantly greater sensitivity. They are usually very small, configurable into arbitrary shapes, directly compatible with optical data transmission systems and op- tical multiplexing technology, simple, and cost-effective.

Fiber-optic sensing of longitudinal strain has been ex- plored for some time by several groups of researchers. Although extensive theoretical studies have been under- taken on various effects occumng in axially strained op- tical fibers [2], [3], few practical devices designed to measure longitudinal strain have been developed and pub- lished. Perhaps the most mature fiber-optic strain sensor published to date is the one reported by Weiss [4], which was developed essentially as a spin-off of a well-known microbending pressure sensor [ 5 ] . It was built using a plastic fiber containing permanent microbends bonded to a surface undergoing strain. Diminution in the amplitude of these deformations that occurs when the surface is being strained improves the fiber’s transmission properties and increases the output signal in accordance with externally applied strain. The sensitivity of this device is superior to that of metallic foil gauges and comparable to that of semiconductor gauges.

The idea of using the axial stress in highly birefringent (HB) fibers as a transduction mechanism was first theo- retically suggested by Rashleigh [6]. The feasibility of a

001 8-9456/92$03 .OO 0 1992 IEEE

BOCK et al. : FIBER-OPTIC STRAIN-GAUGE MANOMETER

strain gauge based on this effect was later demonstrated by Vamham [7], using one sample in a very simple ar- rangement and without presenting any measurement char- acteristics. During the last 2 years, several researchers [8]-[ 101 have published initial results of their investiga- tions of polarimetric strain sensors structurally embedded in composite materials. Two kinds of devices have been reported-single-mode [9] and two-mode [8]-both of which are essentially similar to the configuration pre- sented in this paper from the optical point of view. The utility of polarimetric sensors of this kind was problem- atic until very recently, due to their enormous temperature drift. For instance, using experimental data presented by Hogg [9], we find that gauge thermal output, resulting in a false apparent strain reading, is equal to 50 ~ E / O C for this type of gauge. This is more than one order of mag- nitude over the apparent strain coefficient for standard metallic foil strain gauges and as such is unacceptable for measurement purposes. However, recently two of us pre- sented new experimental evidence that at least partial (if not complete) compensation of the temperature effects is feasible. The configuration we proposed to this end [14] would be particularly well suited for the application pre- sented in this paper, but not very useful for the embedded structures discussed by Murphy [8].

111. CONSTRUCTION OF THE TRANSDUCER The active element of the fiber-optic strain gauge ma-

nometer (FOSGM) was designed in the form of a cylinder (1) having both its ends free, thus emulating the behavior of an infinite cylinder. The construction of this trans- ducer, together with the pressure inlet (4), closing ele- ment (6) (which can, as well, be replaced with an output pressure tube) and the external protective cover (2), is shown in Fig. 1. This concept of a free dilating cylinder pressure transducer was first demonstrated by Wisniewski

The deformation of the free cylinder depends exclu- sively on the value of the internal pressure delivered from outside the transducer and is totally independent of the stress induced by connecting it to the pressure system. The longitudinal E / and circumferential E , strains can be found on the basis of Lame theory using the following expressions:

(2)

mi.

VPD E [ = -~ 2dE

PD E , = -

2dE (3)

where

E is the Young modulus, v is the Poisson ratio, d is the thickness of the cylinder, D is the inside diameter of the cylinder.

It should be noted that longitudinal strain in the case of a free cylinder active element is negative, contrary to the case of a classical cylinder fixed at one end.

13

5 4 3 1 2 6

Fig. 1 . Fiber-optic strain gauge manometer with a free cylinder: 1) free dilating cylinder; 2) external protective cover; 3) strain-sensitive optical fiber; 4) pressure inlet; 5) inputioutput optical fibers; 6 ) closing element.

The active part of a HB optical fiber serving as a strain- sensitive element (3) was epoxied to the outer wall of the free dilating cylinder and thus was totally isolated from the region of high pressure. The axial strain to which this element is exposed will influence the relative phase retar- dation A+ = +, - +2 between the two perpendicular ei- genmodes guided by the fiber according to the equation:

(4) dA+ = 2a ( dL dAn) 2a dAn A n - + L - = - L -

de h de de h de

where h is the wavelength of the light, and An is the dif- ference between the effective indices of the two polariza- tion eigenstates of the HB fiber (An = n, - ny) defined as fiber birefringence B.

No high-pressure leadthrough is needed in this case, which obviously simplifies the transducer construction and avoids the uncertainties and false readings usually asso- ciated with such a leadthrough. Before being permanently fixed to the dilating cylinder, the sensing element was fu- sion-spliced with input and output optical quartz fibers (5 ) . To optimize the propagation conditions for the strain- modulated output light signal, we investigated three dif- ferent configurations of these leading fibers

1. an input HB fiber spliced at 45 O to the HB sensing fiber and an output single-mode (SM) fiber;

2. an input HB fiber spliced at 45" to the HB sensing fiber and an output low-birefringence (LB) fiber;

3. input and output LB fibers spliced to the HB sensing fiber.

The most successful configuration involved an input HB fiber of same type as in the sensing element but having its birefringence axes precisely aligned at 45" relative to the axes of the sensing fiber in order to allow the incoming linearly polarized light to equally excite both perpendic- ular polarization eigenmodes inside the sensor. We were able to align both fibers with high accuracy since our splicing facility has an angular resolution of the rotation stage better than 0.1 O. To output the optical sensor signal we used a LB fiber capable of guiding any polarization state with practically no perturbations. The third config- uration (with LB fibers for both leads) is very interesting from the practical point of view since it does not require any angularly precise splices to the sensing HB fiber.

The length of the HB optical fiber serving as a strain- sensitive element could obviously not exceed the length

. - -

14 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 41, NO. 1, FEBRUARY 1992

of the cylinder [l] in Fig. 1 which was about 50 mm. Assuming that for the cylinder used, E / v equals 2 - lop6 MPa-I, and D / ( 2 d ) is about 1, according to (2), we can estimate the maximum value of the longitudinal strain due to the deformation caused by an internal pressure of 100 MPa at q - -200 P E . The sensing element was a HB 600 York polarization-preserving bow-tie fiber having a diameter of 125 pm and a length of 40 mm (alternatively, 48 mm). For this fiber, the longitudinal strain required to induce a 27r phase shift in the polarized light observed at the output (strongly dependent on fiber’s length) can be calculated based on our data [12] and will amount to T, - 1400 LE. As a measure of the periodicity of the phase shift with strain, T, will be twice diminished when the output fiber signal is monitored with a Wollaston prism combined with a two-photodetector system instead of a single analyzer/detector configuration. Additionally, the total operation range of the FOSGM (100 MPa, corre- sponding to 100 pe in the strain scale) can be easily ad- justed with the help of a Soleil-Babinet compensator to a quasi-linear steep region of a sin-like characteristic (Fig. 2).

IV. INSTRUMENTATION SYSTEM FOR THE FOSGM Fig. 3 shows an optical instrumentation system de-

signed to deliver a controlled polarization light signal and to detect the pressure-modulated output signal of the sens- ing element. A linearly polarized light was launched from a 3 mW HeNe laser emitting at 633 nm into an input York HB 600 (IF) with a cut-off wavelength of 550 nm, which assured a single-mode operation of the device. To pre- cisely align the polarization plane of the light parallel to one of its two principal birefringence axes, a combination of polarization controllers was used, including a quarter- wave plate (X/4) and a polarizer (P). Such alignment is necessary to avoid any influence of environmental param- eters on light propagation through the leading fiber. The output fiber (OF) signal was monitored with a Soleil-Ba- binet compensator (SBC) and an appropriate analyzer (Wollaston prism WP) combined with a two-detector sys- tem (Dl and D2), configured using a synchronous detec- tion scheme based on a lock-in amplifier. If an output sig- nal (beam irradiance) is monitored using only one of these detectors, it can be correspondingly described by the equations:

IO I, = - (1 + sin A+) 4

10 I2 = - (1 - sin A+). 4

The use of the difference-over-sum processing, as shown in Fig. 3, allows significant reduction of system fluctuations introduced by nonstable optical power Io emitted by the source, according to the formula:

-- - sin A+. I, - 1 2

]I + 1 2

1.2

HB-600 sensor range

STRAIN [mlcrostrainsl

Fig. 2. The FOSGM’s range of operation in comparison with the full char- acteristic (strain-induced phase shift equal to 2 ~ ) of an HB 600 fiber-based strain-sensor of the same length (40 mm).

P U4

l a m -

U I TRANSDUCER pL I

Fig. 3 . Optical instrumentation and processing system for the FOSGM.

The final stage of the analog signal recovery system was a computer-controlled (C) digital voltmeter (VM) and a plotter (PL).

A Harwood DWT-35 deadweight tester was used to in- vestigate the output characteristics of the FOSGM. It can generate and calibrate high pressure up to 105 MPa fed into the characterized transducer, with attainable accu- racy of 0.01% traceable to NIST. If required, the dead- weight tester could be disconnected and the transducer fed directly from a high pressure pump equipped with a sec- ondary Bourdon gauge.

V. RESULTS The FOSGM has been characterized at a constant tem-

perature for pressures up to 100 MPa in the three config- urations described above and involving different combi- nations of input/output optical fibers fusion-spliced at both ends of the sensing fiber. Interesting results were obtained in the second configuration for which Fig. 4 shows pres- sure characteristics of two sensor elements having 40 mm and 48 mm in length. However, the stable and repeatable results were also obtained in the third configuration (Fig. 5 ) when lead-ins were made of LB fibers. Application of LB fibers allowed for more precise control of the state of polarization in the system than would be possible using a sensor equipped with a standard SM output fiber, thus di- minishing signal fluctuations, while at the same time sig- nificantly decreasing the cost of the device compared to that of a sensor equipped with a HB fiber input. Shown in Fig. 5 are two characteristics A and B of the same sensor which were displaced using a controlled phase shift intro-

BOCK et al. : FIBER-OPTIC STRAIN-GAUGE MANOMETER

d l . I

7s

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0

PRESSURE [MPa]

Fig. 4. The characteristics of FOSGM in the second configuration (HB/ 45" splice/HB-LB) for two lengths of sensing element.

0 2 0 4 0 6 0 8 0 1 0 0

PRESSURE [MPa]

Fig. 5 . The FOSGM in the third configuration (LB-HB-LB). The sharp minimum in the B characteristics (same as A but displaced in phase using SBC) corresponds to a circular polarization state of the sensor output signal ( I , = l2 at the output of the Wollaston prism).

duced by a Soleil-Babinet compensator. The sharp mini- mum in the B characteristics corresponds to an output cir- cular polarization which is separated into both linear- polarization directions (II = Z2) by passing through a Wollaston prism. By combining [4] and the following one which results from (2):

(7) de uD dp 2dE' _ -

we can establish the final expression for the phase pres- sure sensitivity of the proposed device. This expression clearly shows the sensor design criterion to achieve spec- ified sensitivity in the predetermined pressure range:

(8)

One could increase this sensitivity simply by increasing the length L of the sensing element (and the length of the free cylinder), but this might be difficult without at the same time compromising the usual requirement for mini- aturization of the sensor head. Other means, however, such as the choice of a shorter wavelength, optimal ge- ometry ( D / d ) , and/or appropriate dilating cylinder (U, E ) and fiber (An) materials would allow sufficient flexibility in sensor design to cover most of the potential applica- tions.

Initial results of pressure cycling on the metrological properties of the FOSGM are shown in Fig. 6 . It appears that pressure-induced hysteresis of the FOSGM is resid- ual, and generally is due to the presence of adhesives. It is well known that the mechanical properties of the optical

dA+ auD dAn dP DEh de

L-. - = - -

0 2 0 4 0 6 0 8 0 1 0 0

PRESSURE [MPa]

Fig. 6 . Hysteresis cycle of the FOSGM in the third configuration (LB-HB-LB).

quartz fibers themselves are excellent and do not contrib- ute to the eventual sensor hysteresis. We found that the hysteresis diminished as the number of pressure cycles increased, and in the future it can certainly be optimized through an appropriate choice of adhesive materials.

Temperature compensation of the FOSGM's fiber-optic strain sensor based on HB fibers can be achieved using a procedure that we reported recently [14]. The compen- sated sensor consists of two identical (sensing and com- pensating) parts of a HB polarization maintaining fiber, spliced at 90" in relation to their polarization axes. If both parts remain at the same temperature but only one is strained, their total temperature-induced phase retardation will cancel out. The degree of cancellation depends heav- ily on precise angular alignment and exact lengths of both parts of the sensor. Although the prototype FOSGM pre- sented in this paper was not specifically designed to op- erate using a compensation version of the fiber-optic strain gauge, the appropriate conversion is now under way and will soon be completed.

VI. CONCLUSIONS We have demonstrated a practical realization of a novel

pressure transducer, a FOSGM which utilizes a fiber-op- tic strain sensor and an active element configured in the form of an infinite-like cylinder with free ends. The de- formation of such a cylinder depends entirely on the pres- sure acting from its inside and is independent of the stress resulting from the attachment of the device to the pressure system. The fiber-optic strain sensor is permanently bonded to the external surface of the cylinder and fully isolated from the high pressure region. The sensing ele- ment of the device consists of a HB polarization-main- taining optical fiber strain gauge.

The FOSGM was characterized at stabilized tempera- tures in the range 20-30°C for pressures up to 100 MPa using a Hanvood DWT-35 deadweight tester with a read- ing accuracy at ambient temperatures of at least 0.1 %, traceable to the NIST. Three different configurations of the FOSGM were investigated in order to minimize dis- turbing influences of external parameters on the input/out- put fibers. The described sensor has inherent advantages such as immunity to electromagnetic interference, safety in electrically dangerous, hazardous or explosive environ- ments, direct compatibility with optical data transmission

76 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 41, NO. 1. FEBRUARY 1992

systems, simplicity, and cost-effectiveness. It does not re- quire any fiber-optic leadthrough since it is located out- side the pressure region, thereby simplifying the trans- ducer construction and avoiding the uncertainties and false readings usually associated with such a leadthrough.

Preliminary investigations of the proposed device’s characteristics clearly demonstrate that the FOSGM has significantly increased sensitivity (at least 0.1 MPa- ’) over similar devices based on electrical strain gauges. This high sensitivity together with a good linearity makes it particularly well suited for measurements of elevated or high pressures inside pipelines, storage vessels, or pro- cessing chambers.

Future developments will include an all-fiber version of the FOSGM similar to that which we have recently dem- onstrated for a fiber-optic high-pressure sensor [ 131. An optimal selection of adhesives, optical fibers, and optical components, as well as the materials used for mechanical elements, will have to be made in order for the sensor to satisfy the most stringent requirements that may be placed on its metrological properties in the case of its proposed application as a secondary measuring instrument of high pressure. More detailed studies on the metrological prop- erties including calibration data of FOSGM and precision measurements of a temperature shift in a compensated configuration of the device are in progress.

REFERENCES [ l ] W. J . Bock and T. R. Wolinski, “Temperature-compensated fiber-

optic strain sensor based on polarization-rotated reflection,” Proc.

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[3] S.-Y. Huang, J . N. Blake, and B. Y. Kim, “Perturbation effects on mode propagation in highly elliptical core two-mode fibers,” J . Lightwave Technol., vol. 8 , pp. 23-33, 1990.

[4] J . D. Weiss, “Fiber-optic strain gauge,” J. Lightwave Technol., vol.

[5] J. W. Berthold et al . , “Design and characterization of a high tem- perature fiber-optic pressure transducer,” J . Lightwave Technol., vol.

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