Temperature compensated self-referenced fiber optic microbend pressure transducer

What is claimed is:

1. A temperature compensated, self-referenced fiber optic sensor, comprising:

a pair of plates with corrugated surfaces having facing and offset teeth, at least one of said plates being in contact with a parameter to be sensed;

a first signal optical fiber clamped between said corrugated plates for being bent to a greater or less extent depending on a parameter exerted on said corrugated plates;

a second reference optical fiber located adjacent said plates to be exposed to a similar environment, said second reference optical fiber being formed in a loop with at least one turn and secured to itself by at least one point with securing means, said optical fiber loop having a predetermined radius r constructed to produce a controlled degree of microbending loss for mode conversion to compensate for thermal offset in addition to cable and connector offsets and light source fluctuations;

a light source for supplying light to said signal and reference optical fibers; and

optical detecting means for receiving light from said signal and reference optical fibers, said optical detecting means measuring changes in transmitted light therethrough for determining a sensed parameter, said optical detecting means further including light transmission determining means for measuring a change in transmission in said reference fiber to compensate for thermal offset in the sensed parameter.

2. A temperature compensated, self-referenced fiber optic sensor, comprising:

a pair of plated with corrugated surfaces having facing and offset tooth, at least one of said plates being in contact with a parameter to be sensed;

a first signal optical fiber clamped between said corrugated plates for being bent to a greater or less extent depending on a parameter exerted on said corrugated plates, said signal fiber having an optical fiber loop situated past said corrugated plates, said optical fiber loop having a predetermined radius r with at least one turn and secured to itself with securing means, said optical fiber loop being constructed to produce a controlled degree of microbending loss for mode conversion to compensate for thermal offset;

a second reference optical fiber located adjacent said plates to be exposed to a similar environment;

a light source for supplying light to said signal and reference optical fibers; and

optical detecting means for receiving light from said signal and reference optical fibers, said optical detecting means measuring changes in transmitted light therethrough for determining a sensed parameter, said optical detecting means further including light transmission determining means for measuring a change in transmission in said reference fiber to compensate for thermal offset in the second parameter.

3. A fiber optic sensor as recited in claim 1, further comprising:

a second light source for supplying light at a second wavelength .lambda..sub.2 together with said first light source supplying light at a first wavelength .lambda..sub.1, said light at the first wavelength .lambda..sub.1 being supplied to said signal optical fiber and said light at the second wavelength .lambda..sub.2 being supplied to said reference optical fiber; and

optical detecting means including at least two photodetectors for receiving light intensities at the two wavelengths from said signal and reference fibers to produce corresponding output signals, and optical detecting means further receiving light intensities directly from said first and second light sources to provide source signals at both wavelengths, said optical detecting means comparing said output signals with source signals to determine the sensed parameter.

4. A fiber optic sensor as recited in claim 2, further comprising:

a second light source for supplying light at a second wavelength .lambda..sub.2 together with said first light source supplying light at a first wavelength .lambda..sub.1, said light at the first wavelength .lambda..sub.1 being supplied to said signal optical fiber and said light at the second wavelength .lambda..sub.2 being supplied to said reference optical fiber; and

optical detecting means including at least two photodetectors for receiving light intensities at the two wavelengths from said signal and reference fibers to produce corresponding output signals, said optical detecting means further receiving light intensities directly from said first and second light sources to provide source signals at both wavelengths, said optical detecting means comparing said output signals with source signals to determine the sensed parameter.

5. A fiber optic sensor, as recited in claim 1, wherein said reference fiber includes a delay coil to create a separation in time of light intensity, said light source being pulsed to provide pulses of light for separation.

6. A fiber optic sensor, as recited in claim 2, wherein said reference fiber includes a delay coil to create a separation in time of light intensity, said light source being pulsed to provide pulses of light for separation.

7. A fiber optic sensor, as recited in claim 5, wherein said optical detecting means provides a ratiometric measurement of signal fiber intensity to reference fiber intensity based upon time domain intensity.

8. A fiber optic sensor, as recited in claim 6, wherein said optical detecting means provides a ratiometric measurement of signal fiber intensity to reference fiber intensity based upon time domain intensity.

9. A temperature compensated, self-referenced fiber optic sensor, comprising:

a pair of plates with corrugated surfaces having facing and offset teeth, at least one of said plates being in contact with a parameter to be sensed, one plate of said pair of plates being positioned centrally on a diaphragm which deflects in proportion to a pressure difference, said other plate being opposite and offset thereof, said diaphragm being circumferentially sealed inside a cap and body;

a first signal optical fiber clamped between said corrugated plates for being bent to a greater or less extent depending on a parameter exerted on said corrugated plates;

a second reference optical fiber located adjacent said plates to be exposed to a similar environment, said second reference optical fiber being formed in a loop with at least one turn and secured to itself by at least one point with securing means, said optical fiber loop having a predetermined radius r constructed to produce a controlled degree of microbending loss for mode conversion to compensate for thermal offset in addition to cable and connector offsets and light source fluctuations;

a light source for supplying light to said signal and reference optical fibers; and

optical detecting means for receing light from said signal and reference optical fibers, said optical detecting means measuring changes in transmitted light therethrough for determining a sensed parameter, said optical detecting means further including light transmission determining means for measuring a change in transmission in said reference fiber to compensate for thermal offset in the sensed parameter.

10. A fiber optic sensor as recited in claim 9, wherein said cap includes an aperture for producing a vacuum within the body.

11. A fiber optic sensor as recited in claim 10, wherein said body includes a greater for absorbing outgassed and in-diffused gases for maintaining the vacuum.

12. A temperature compensated, self-referenced fiber optic sensor, comprising:

a pair of plates with corrugated surfaces having facing and offset teeth, at least one of said plates being in contact with a parameter to be sensed;

a first signal optical fiber clamped between said corrugated plates for being bent to a greater or less extent depending on a parameter exerted on said corrugated plates;

a second reference optical fiber located adjacent said plates to be exposed to a similar environment, said second reference optical fiber being formed in a loop with at least one turn and secured to itself by at least one point with securing means, said optical fiber loop having a predetermined radius r constructed to produce a controlled degree of microbending loss for mode conversion to compensate for thermal offset in addition to cable and connector offsets and light source fluctuations;

a light source for supplying light to said signal and reference optical fibers; and

optical detecting means for receiving light from said signal and reference optical fibers, said optical detecting means measuring changes in transmitted light therethrough for determining a sensed parameter, said optical detecting means further including light transmission determining means for measuring a change in transmission in said reference fiber to compensate for thermal offset in the sensed parameter, said optical fiber loop including one end of the optical fiber forming the loop having reflecting means at or near the point of being secured, said reflecting means reflecting light supplied to the optical fiber back to said optical detecting means.

13. A fiber optic sensor as recited in claim 3, wherein said optical detecting means provides a ratiometric measurement of signal fiber intensity to reference fiber intensity based upon wavelength domain intensity.

14. A fiber optic sensor as recited in claim 4, wherein said optical detecting means provides a ratiometric measurement of signal fiber intensity of reference fiber intensity based upon wavelength domain intensity.

15. A temperature compensated, self-referenced fiber optic sensor, comprising:

a pair of plates with corrugated surfaces having facing and offset teeth, at least one of said plates being in contact with a parameter to be sensed, one plate of said pair of plates being positioned centrally on a diaphragm which deflects in proportion to a pressure difference said other plate being opposite and offset thereof, said diaphragm being circumferentially sealed inside a cap and body;

a first signal optical fiber clamped between said corrugated plates for being bent to a greater or less extent depending on a parameter exerted on said corrugated plates, said signal fiber having an optical fiber loop situated past said corrugated plates, said optical fiber loop having a predetermined radius r with at least one turn and secured to itself with securing means, said optical fiber loop being constructed to produce a controlled degree of microbending loss for mode conversion to compensate for thermal offset;

a second reference optical fiber located adjacent said plates to be exposed to a similar environment;

a light source for supplying light to said signal and reference optical fiber; and

optical detecting means for receiving light from said signal and reference optical fibers, said optical detecting means measuring changes in transmitted light therethrough for determining a sensed parameter, said optical detecting means further including light transmission determining means for measuring a change in transmission in said reference fiber to compensate for thermal offset in the sensed parameter.

16. A fiber optic sensor as recited in claim 15, wherein said cap includes an aperture for producing a vacuum within the body.

17. A fiber optic sensor as recited in claim 16, wherein said body includes a getter for absorbing outgassed and in-diffused gases for maintaining the vacuum.

18. A fiber optic sensor as recited in claim 2, wherein said optical fiber loop includes one end of the optical fiber forming the loop having reflecting means at or near the point of being secured, said reflecting means reflecting light supplied to the optical fiber back to said optical detecting means.

19. A fiber optic sensor as recited in claim 12, wherein said reflecting means includes a silicon film deposited on one end of the optical fiber forming the loop.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general, to fiber optic sensors, and in particular, to a temperature compensated self-referenced fiber optic microbend pressure transducer.

2. Description of the Related Art

Numerous applications exist for pressure measurement in high-temperature environments. These applications include pipe line pressure in petrochemical refineries, gas path pressure in aircraft engines and main steamline pressure in turbine generators. Traditional diaphragm type pressure transducers are not suited for these high-temperature applications for several reasons. First, the diaphragm material may creep and cause output offset error. Second, the strain or capacitance gages used to measure diaphragm deflection exhibit large non-repeatable and unpredictable changes in gauge outputs at temperatures greater than 300.degree. C. These changes are caused by such effects as alloy segregation, phase changes, selective oxidation, and diffusion. Ultimately, they lead to premature failure of the gage or lead-wire.

One attempt to eliminate the effects of diaphragm creep and hysteresis used dimensionally stable fused silica or other ceramic for the diaphragm material. Another attempt to enable measurement of diaphragm deflection at high temperatures employed fiber optic sensors with a fused silica optical fiber.

While there are several fiber optic sensors available for sensing diaphragm deflection, most of these sensors have been configured for dynamic applications such as the detection of acoustic signals. Since pressure changes occur over long periods of time in most process applications, pressure transducers and manometers must be designed to measure static or dc pressure. Consequently, it is desirable that these devices have low drift and insensitivity to environmental changes other than pressure, for example, temperature.

In addition, there are practical problems associated with the design of fiber optic sensors to operate at elevated temperatures with adequate sensitivity within a transducer configuration exhibiting low drift and environmental insensitivity.

Microbend sensors exhibit zero offset as a function of temperature. The zero offset causes an apparent error in applications such as pressure transducers where the fiber optic microbend sensor is used to measure diaphragm deflection. It is known that microbend sensor offset is a linear, repeatable function of temperature. Thus, it is desirable that an independent, linear fiber optic measurement of temperature provide a signal which could be subtracted from the microbend sensor to provide a simple method for compensation of temperature offset.

SUMMARY OF THE INVENTION

The present invention solves the aforementioned problems with the prior art as well as others by providing a temperature compensated, self-referenced fiber optic sensor.

The temperature compensated, self-referenced fiber optic sensor comprises a pair of plates with corrugated surfaces having facing and offsetting teeth. At least one of the plates is in contact with a parameter to be sensed or measured. A signal optical fiber is clamped between the corrugated plates so that when the parameter applies a force thereon, it causes a change in separation of the plates and microbending of the signal fiber. At least one reference optical fiber is located in a similar environment to be exposed to the same conditions. The reference optical fiber is formed in a loop with a predetermined radius to compensate for thermal offset. A light source supplies light to the fibers and optical detecting means receives the light and measures the changes in light intensity for determining the sensed parameter. The change in light transmission in the reference fiber compensates for thermal offset in the sensed parameter.

One embodiment of the present invention employs wavelength division multiplexing where light at two different wavelengths is propagated through a common fiber and then separated so that one wavelength is propagated through the sensor fiber and the other wavelength through the reference fiber. After passing through the sensor, the light is combined in an output fiber and later separated at the optical detecting means. The optical detecting means includes photodetectors for comparing light intensities of the two wavelengths with source signals directly from the light source.

The preferred embodiment of the present invention uses time domain intensity normalization. In this embodiment the reference fiber includes a delay line to create a separation in time between light intensity pulses traveling in the sensor and reference fibers.

The present invention advantageously employs a first optic loop for temperature compensation. The fiber optic loop may be incorporated directly in the reference fiber or it may be serially connected to the sensor optical fiber so that the slope change due to temperature compensates the slope of thermal offset in the microbend sensor.

Accordingly, one aspect of the present invention is to provide an optical fiber loop temperature sensor to compensate for thermal offset.

Another aspect of the present invention is to provide a temperature compensated, self-referenced fiber optic sensor.

Still another aspect of the present invention is to provide a temperature compensated self-referenced fiber optic microbend pressure transducer.

A further aspect of the present invention is to provide a fiber optic sensor which is simple in design, rugged in construction, and economical to manufacture.

The various features of novelty which characterize the present invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the present invention, its operating advantages and specific objects attained by its use, reference is made to the accompanying drawings and descriptive matter in which the preferred embodiments of the invention are illustrated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a fiber optic pressure transducer;

FIG. 2(a) is an exploded cross-sectional view of the preferred embodiment of a temperature compensated, self-referenced fiber optic pressure transducer;

FIG. 2(b) is an assembled view of FIG. 2(a);

FIG. 3 is a close-up view of the microbend sensor employed in FIGS. 1, 2(a) and 2(b);

FIG. 4 is a graph of sensitivity versus tooth pitch illustrating optimum corrugation spacing;

FIG. 5 is a graph of applied load versus displacement using corrugated sensor plates;

FIG. 6 is a graph of measured microbend sensor output voltage versus displacement at ambient temperature;

FIG. 7 is a graph of calculated diagram deflection versus pressure with 27N point load at center of diaphragm;

FIG. 8 is a schematic representation of an embodiment of the present invention employing time domain referencing;

FIGS. 9(a) and 9(b) are graphs showing transmitted intensity and received intensity employing time domain referencing;

FIG. 10 is a schematic representation of another embodiment employing wavelength division multiplexing;

FIG. 11 is a schematic illustration of the signal processing employed in the embodiment depicted in FIG. 10;

FIG. 12 is a schematic representation of the optical fiber loop temperature sensor;

FIGS. 13(a) and 13(b) depict different fiber loop geometries;

FIG. 14 depicts a single ended fiber optic loop;

FIG. 15 is a calibration curve for a 4.8 dB loop;

FIG. 16 depicts typical pressure response of a microbend fiber optic transducer at two different temperatures;

FIG. 17 is a plot of throughput (dB) versus temperature over the temperature range of -60.degree. C. to 80.degree. C.; and

FIG. 18 is a calibration curve for a silicon etalon temperature sensor.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, and FIG. 1 in particular, there is shown a diaphragm pressure transducer containing a fiber optic microbend sensor (2). These transducers use a microbend fiber optic sensing principle where a multimode step-index optical fiber (10) with a buffer coating is squeezed between a pair of plates (12, 14) with corrugated surfaces having facing and offsetting teeth (16). One of the plates (12) is positioned approximately at the center of a diaphragm (18) which deflects in proportion to a pressure exerted upon it. This deflection causes a change in the amplitude for the periodic distortion of the optical fiber (10) squeezed between plates (12, 14). Optical light transmitted through the optical fiber (10) is attenuated in proportion to the amplitude of these mechanical distortions (microbends).

Thus, measurement of the change in transmitted light provides a signal proportional to pressure applied to the diaphragm (18).

In FIG. 1, the transducer consists of a 1.4 mm. thick, 12.7 mm. diameter diaphragm (18) fastened to a cylinder (4) with bolts (6). The cylinder (4) is fastened to a flange (3) by welds (5). This assembly is designed to be inserted directly into a high pressure pipeline (not shown) which contains a mating flange. Pressure seals (7, 9) ensure a tight fit. The transducer diaphragm (18) and cylinder (4) are made from a high strength steel super alloy such as Inconel (a Registered Trademark of The International Nickel Company, Inc.). This alloy is chosen because of its low creep at high temperatures and the relative ease in machining this material compared to fused silica.

Next, referring to FIG. 2, there is shown another embodiment of the microbend pressure transducer.

Diaphragm (18) is sealed around its circumference by suitable means such as an electron beam weld to a cap (20) and a body (22). The body (22) has a step-bored cavity (24) for receiving the diaphragm (18), and the second plate (14) which may be affixed by any suitable means such as a post (26) with bellows (28) circumferentially surrounding the post (26) immediately below plate (14). A second aperture (30) in the side of the body (22) in communication with the first aperture (24) allows for a getter (32) and a plug (34). Openings (36, 38) allow the optical fiber to enter the body (22) and are sealed with Torr Seal vacuum epoxy commercially available through Varian Associates. Protection tubes (40) may be employed to protect and secure optical fiber (10) as it enters the body (22). An opening (42) in cap (20) allows introduction of the unknown pressure to the assembled transducer. The volume surrounding the microbend sensor and bellows is evacuated to create a vacuum with opening (30) being plugged thereafter. The getter (32) absorbs outgassed and in-diffused gases to ensure that a vacuum reference is maintained for extended time periods behind the diaphragm (18). This embodiment provides an absolute pressure reference.

The multi-mode step index optical signal fiber (10) is clamped between a pair of plates (12, 14) as best seen in FIG. 3. The first plate (12) is located at approximately the diaphragm (18) center and the second plate (14) is positioned on a fixed reference surface of the transducer body such as post (26). When a pressure is applied to the flexible diaphragm (18), the optical fiber (10) is spatially deformed into a sinusoidal shape. The deformation induces loss in the optical fiber (10) proportional to an applied pressure. The loss sensitivity may be optimized by matching the sinusoidal deformation period to the difference between the propagation constants of adjacent modes in the optical fiber (10). The power loss from the core to radiation modes is optimum when the fiber spatial-bend frequency equals the difference in propagation constants (.DELTA.B) between propagating and radiation modes as follows: ##EQU1## where: .LAMBDA. is the corrugation spacing. For step index fibers, .DELTA..beta. is given by the following formula: ##EQU2## where: a=the fiber core radius

m=the mode number

M=the total number of modes

.DELTA.=(n.sub.core -n.sub.clad)/n.sub.clad

The higher-order modes are preferentially coupled to adjacent higher-order modes and radiation modes by the periodic distortion applied to the fiber by the corrugations. For these higher-order modes the mode number is about equal to the total number of modes, and by combining Equations, 1 and 2, the optimum corrugation spacing is calculated as follows for a fiber with 0.15 mm core and 0.18 mm clad diameters. ##EQU3##

Although the result in Equation 3 is close to the optimum value, the corrugation spacing providing best sensitivity for a 0.15 mm core diameter fiber has been found to be about 0.15 mm, as shown in FIG. 4.

FIG. 5 shows the applied load versus displacement measured using corrugated sensor plates that apply two spatial bends to the optical fiber. The spring constant is the slope of the curve in FIG. 5. For small changes, the displacement changes linearly with load.

FIG. 6 plots the inverted sensor output voltage signal versus displacement. The electronics bandwidth employed was 100 Hz. This microbend sensor calibration curve was generated using a piezoelectric transducer (PZT) (driven at 100 Hz) and corrugated plates similar to those in the sensor. The PZT was in turn calibrated using a Brown and Sharpe displacement gage whose calibration is traceable to the National Bureau of Standards. The piezoelectric transducer retracts with applied voltage at 20 nm/volt. It is apparent from FIG. 6 that the microbend sensor output signal is linear with displacement over almost four orders of magnitude. The plotted signal voltage is directly proportional to light intensity transmitted through the optical fiber squeezed between the corrugated plates (12, 14).

The graph in FIG. 7 is the calculated deflection of a 12.7 mm diameter, 1.4 mm thick diaphragm (18) versus applied pressure with a 27N point load at the diaphragm center. The 27N point load is a non-critical value and represents a typical preload on the optical fiber (10). The calculation represents the configuration diagrammed in FIG. 1. The deflection W.sub.r is given by the following formula: ##EQU4## where: .mu.=Poisson's ratio

E=Young's modulus

t=diaphragm thickness

a=diaphragm radius

q=pressure load

Q is the force due to compression of the optical fiber with a preload (L.sub.o =27N) according to the following equation:

Q=-[(W.sub.T -W')K+L.sub.o ] (5)

where:

K=fiber spring constant

W'=diaphragm deflection at q=0

FIGS. 4-7 provide sufficient information to analyze and predict the performance of the microbend sensor with the following assumptions:

a) two periodic distortions of fiber;

b) spring constant as calculated from FIG. 5;

c) a light-emitting diode light sensor with a nominal output wavelength of 830 nm;

d) 100 .mu.W of optical power (P.sub.o) propagating in unloaded fiber;

e) 27N static load (L.sub.o) on fiber at diaphragm center; and

f) 53% static light transmission (T.sub.o) caused by static load.

The values of preload and quiescent light transmission are noncritical because of the extremely wide sensor linearity range (see FIGS. 5 and 6). These values chosen for calculation below are typical, but vary with machining tolerances on transducer parts and mating surfaces. Since the light transmission, T, is log linear with displacement, .DELTA.h (FIG. 6), and the displacement is linear with applied pressure, q (FIG. 7), the light transmission is log linear with applied pressure. The change in light transmission, .DELTA.T, with displacement is expressed as follows: ##EQU5##

From FIG. 7, deflection .DELTA.h is 10 .mu.m at 22.8 MPa full-scale pressure, and resolves 0.1% of full scale in a 1 Hz electrical bandwidth. This corresponds to a minimum detectable displacement of 10 nm. Substituting the appropriate value into Equation 6, as follows: ##EQU6##

The static load causes a static loss in optical power (P) given by:

P=P.sub.o T.sub.o =53 .mu.W (8)

Thus, the minimum detectable power is (combining Equations 7 and 8):

.DELTA.P.sub.min =P.DELTA.T/T.sub.o =3.5 nW (9)

Silicon photodiodes are available with two to three orders magnitude better noise equivalent power (NEP) and linear dynamic range than what is required. In addition, there is ensured shot-noise-limited operation with 53 .mu.W of background light level. Consequently, the microbend sensor signal-to-noise ratio (S/N) is more than adequate to achieve excellent diaphragm deflection resolution and dynamic range.

Compensation of the microbend sensor is performed to ensure that temperature, vibration, and light level changes do not introduce errors. To compensate the microbend sensor, one of two self-referencing methods is employed in addition to a fiber optic loop for temperature compensation.

Microbend fiber optic pressure transducer outputs may be dc coupled and errors are introduced by random changes in coupling loss when fiber optic connectors are mated, demated, and remated. This random loss results in output signal offset, which can be zeroed out with an adjustment potentiometer in the electronics. However, if cable bending results in similar random offsets, the magnitude of these offsets could lead to random unknown errors. To eliminate these potential offset errors, time domain or wavelength referencing methods are employed.

The time domain referencing preferred embodiment is depicted in FIG. 8. Light from a pulsed source (44) is transmitted to a sensor head (46) containing a fiber optic intensity sensor (48) such as a microbend pressure sensor, delay coil (50) and 2.times.2 power splitter (52). Each input light pulse is divided at the splitter (52) between the sensor tap (54) and the delay coil tap (56) acting as the reference fiber. The ends of the taps are mirrored (58, 60). If the round trip time through the delay coil (50) is long enough, the received light pulses from the sensor, I.sub.S, and delay coil, I.sub.R, will be separated in time as shown in FIG. 9. Any cable bending or connector mating and demating introduce offsets which affect the received pulses similarly. Thus, a ratiometric measurement I.sub.S /I.sub.R provides an output signal free of any cable or connector dependent offsets, and free of errors due to fluctuations in the average light level from the source.

The wavelength referencing embodiment is shown in FIG. 10. It uses two light sources operating at different wavelengths (.lambda.1 and .lambda.2). With the dual wavelength approach, the light intensities at each wavelength are combined into a common fiber (66), which travels to the sensor (68) and directly to the detectors (82). A diffraction grating or wavelength division multiplexor (WDM) (70) separates the wavelengths between the signal and reference fibers (72, 74), respectively, in the sensor head (68). The signals are recombined by combiner (76) into an output fiber (78). At the signal pr