Abstract:
A temperature correcting pressure gauge which has a substrate having at least one surface coupled to a source of pressure to be measured, the substrate first surface having a first fiber Bragg grating from a first optical fiber attached in an appropriately sensitive region of the substrate, a fiber Bragg grating from a second optical fiber attached to the opposite surface from the first fiber Bragg grating, the first and second fiber Bragg gratings reflecting or transmitting optical energy of decreasing or increasing wavelength, respectively, in response to an applied pressure. The first and second fiber Bragg gratings have nominal operating wavelength ranges that are adjacent to each other but are exclusive ranges and the fiber Bragg gratings also have closely matched pressure coefficients and temperature coefficients.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a pressure sensor. In particular, the invention relates to a temperature-compensated pressure sensor using fiber optic fiber Bragg gratings applied to opposite surfaces of a substrate. 
     BACKGROUND OF THE INVENTION 
       FIG. 1  shows a prior art differential pressure transducer  100 . A first pressure port couples into a first chamber  102 , and a second pressure port couples to a second chamber  104 . The differential pressure is thereby transferred to diaphragm  108  in the form of a displacement which is measured by displacement sensor  114 . 
       FIG. 2  shows a prior art single-ended pressure transducer  200 , which has a pressure inlet  210  coupled to a first chamber  202  which includes generally rigid walls which surround diaphragm  208 . A spring  212  provides a resistive pressure, or alternatively, chamber  204  may be filled with a non-hysteresis, temperature neutral fluid, or it may be opened to a neutral pressure environment compared to the pressure to be measured by inlet  210 . Displacement sensor  214  measures the diaphragm  208  movement. 
     In higher pressure applications, the diaphragm  108  of  FIG. 1  and diaphragm  208  of  FIG. 2  may have a suitable thickness which provides a suitable modulus for the differential pressure in use. 
     A problem arises in pressure sensors and transducers which utilize fiber optic Bragg gratings, also known as fiber Bragg gratings (FBG), for displacement measurement where the fiber Bragg grating has response coefficients such that the FBG is responsive not only to pressure but also to temperature. This becomes a serious problem in oil and gas exploration, where temperature variations from 25 degrees C. to 200 degrees C. or more are not uncommon. In prior art pressure sensor systems, a per-transducer calibration characteristic has been stored, and a separate temperature sensor is used in combination with the strain reading to compensate for this temperature effect on the pressure measurement. 
     OBJECTS OF THE INVENTION 
     A first object of the invention is a differential pressure sensor having a substrate with a first pressure applied to one surface, a second pressure applied to a second surface opposite the first surface, and a first optical fiber with its fiber Bragg grating zone attached to a region of optimum deflection on the first surface, a second optical fiber with its fiber Bragg grating zone attached to the second surface opposite the first surface, a source of optical energy applied to the fiber Bragg gratings, and a wavelength interrogator for determination of pressure and temperature based on the reflected or transmitted wavelengths of the fiber Bragg gratings of the first and second optical fibers. 
     A second object of the invention is a pressure sensor having a substrate coupled to a source of pressure, the substrate having a first optical fiber with its Bragg grating zone located on a region of one surface of the substrate and a second optical fiber with its fiber Bragg grating zone located in a region on the opposite surface from the first optical fiber Bragg grating, the fiber Bragg gratings reflecting or transmitting applied optical energy such that an increase in applied pressure causes one fiber Bragg grating to reflect or transmit a longer wavelength and the other fiber Bragg grating to reflect or transmit a shorter wavelength. 
     A third object of the invention is a process for measurement of pressure applied to a substrate having fiber Bragg gratings on opposite surfaces and in the same region of the substrate, the fiber Bragg gratings operating in reflection mode or transmission mode, the fiber Bragg gratings having an optical source coupled to them in a series configuration, or independently, and a wavelength interrogator for conversion of reflected or transmitted wavelengths into pressure data. 
     SUMMARY OF THE INVENTION 
     A transducer substrate has a first surface with a first optical fiber having a first fiber Bragg grating zone attached in a measurement region, the substrate having a second surface opposite the first surface and having a second optical fiber having a second fiber Bragg grating zone attached in the same measurement region as the first fiber Bragg grating. The substrate first surface measurement region fiber Bragg grating is responsive to λ 1 , and is formed on a first optical fiber, and a second optical fiber having a second fiber Bragg grating responsive to λ 2  is positioned opposite the first fiber Bragg grating. The first optical fiber Bragg grating has a first temperature coefficient k 1 =Δλ 1 /ΔT which is closely matched to a second optical fiber Bragg grating temperature coefficient k 2 =Δλ 2 /ΔT. Additionally, one of the sensor fiber Bragg gratings is responsive to a wavelength which is slightly above or below the responsive wavelength of the other grating. Each of the sensor gratings generates a range of responses, and in the best mode of the invention, these range of responses are in separate ranges such that each response can be associated with a particular grating, however other embodiments of the invention may utilize fiber Bragg gratings with responses which include overlapping ranges. When a pressure is applied to the substrate, one fiber Bragg grating undergoes an incremental compression which lowers the response wavelength, and the other grating undergoes an incremental expansion which increases the response wavelength. A wavelength interrogator converts the first fiber Bragg grating response and the second fiber Bragg grating response into a pressure measurement. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross sectional diagram of a prior art pressure transducer. 
         FIG. 2  shows a cross sectional diagram of a prior art single ended pressure transducer. 
         FIG. 3  shows a single ended pressure transducer according to the present invention. 
         FIG. 4  shows a detailed view of the pressure transducer of  FIG. 3 . 
         FIG. 5  shows a block diagram for a wavelength interrogator. 
         FIG. 6  is a plot of an excitation source with wavelength responses which are to be applied to a sine optical filter and detector. 
         FIG. 7  shows the waveforms for a wavelength interrogator. 
         FIG. 8A  shows a cross section view of a single fiber sensor. 
         FIG. 8B  shows the block diagram of a wavelength interrogator for use with the sensor of  FIG. 8A . 
         FIG. 9A  shows a cross section view of a dual operation fiber sensor, for use with either the redundant interrogator of  FIG. 9B  or transmission interrogator of  FIG. 9C . 
         FIG. 9B  shows the block diagram for a redundant interrogator for use with the sensor of  FIG. 9A . 
         FIG. 9C  shows the block diagram for a transmission interrogator for use with the sensor of  FIG. 9A . 
         FIG. 10  shows the block diagram for a pressure transducer having multiple sensors. 
         FIG. 11  shows a response plot for a fiber Bragg grating. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention is a pressure sensor which may be operated in a single port mode, or a differential (two port) mode for detecting a pressure difference. The invention may be used or adapted for any pressure measuring purpose, including oil or gas exploration, aerospace, or any known application requiring measurement of pressure. The invention has the additional advantage that series strings of sensors may be placed along a single fiber, and the responses may be individually and concurrently read using a wavelength interrogator, or the responses may be time division multiplexed (TDM) and read sequentially. 
       FIG. 3  shows a single-ended pressure transducer  300  according to the present invention. A pressure to be measured enters port  312  and into chamber  302  which is separated from a second chamber  304  optionally containing a reference pressure by a transducer substrate  308  which separates the two chambers. In the single ended case, a pressure to be measured is coupled into one of the chambers  302  with a reference pressure applied to the other chamber  304  opposite the substrate  308 . In the differential pressure sensing case, a first pressure and second pressure are provided to the chambers  302  and  304  on opposite sides of the substrate  308 . Using the single-ended case of  FIG. 3  as an example, an increase of pressure causes a small deflection in substrate  308 . A wavelength interrogator  320  provides appropriate optical energies to first fiber  322  and second optical fiber  324 , then optical energies are reflected by a first fiber Bragg grating located on first fiber  322  and also a second fiber Bragg grating located on second fiber  324 , each fiber Bragg grating on opposite surfaces of substrate  308  and located in region  310 . Each fiber Bragg grating is formed into the optical fiber over a finite extent known as a grating zone, or simply zone, here the grating zone is typically attached to the substrate&#39;s measurement region as the grating zone has very high sensitivity to strain and translates that strain into a shift in reflected or transmitted wavelength. The first optical fiber  322  reflects a particular wavelength λ 1  back to wavelength interrogator  320 , and second optical fiber  324  similarly reflects a particular wavelength λ 2  back to interrogator  320 . The first and second fiber Bragg gratings are positioned on opposite surfaces of a pressure substrate  308 , preferably over a region of maximum deflection, with the first grating and second grating positioned directly over and under each other, and oriented in the same direction. In this arrangement, the temperature coefficient of the first grating and second grating cause the reflected wavelengths of each grating to offset in the same direction, such that a similar directional offset in wavelength related to the temperature change occurs for both sensors. The fiber Bragg gratings are attached to substrate  308  on opposite sides in any manner which minimizes hysteresis (also known as deflection memory, or creep). An optional temperature sensor (shown as  812  of  FIG. 8A , or  912  and  913  of  FIG. 9A ) may be included in an unstressed zone of one or more of the fibers, or placed on a separate fiber, if desired, for a redundant temperature measurement. As will be described later, each of the sensors may have a transmission or reflection mode response which provides unique wavelength response regions, and provides for the estimation of both pressure and temperature. 
       FIG. 4  shows a detailed view of region  310  of  FIG. 3 . First fiber  322  is attached to one surface of substrate  308 , and second fiber  324  is attached to the opposite surface of substrate  308  and in the same region and grating orientation. First fiber Bragg grating  402  is preferably placed over a centerline region  410  of the substrate, which is an area of maximum sensitivity, and second fiber Bragg grating  404  is placed on the opposite surface and an equal distance from centerline  412  of the substrate  308 . The attachment of grating  402  and  404  to the substrate  308  may be achieved using any method which minimizes or eliminates hysteresis, and may include metallization of the exterior surface of the fibers  322 ,  324  for subsequent metallic bonding to the substrate  308  using high temperature structural adhesives, or placing the fiber into a groove in the substrate  308  for mechanical attachment. Any means of attachment of the grating zone of the fiber to the substrate which provides for coupling of the deflection of the substrate into a wavelength shift of the grating while minimizing creep would provide for satisfactory operation according to the objects of the invention. Additionally, any prior art means for sealing the region  302  or  304  where fibers  322  and  324  penetrate enclosure  306  is required for satisfactory device operation. Many such sealing techniques are available including a pressurized-side gasket fitting into a conical counterbore, where the seal is driven deeper into the conical counterbore surrounding the fiber by pressure in the enclosure  302 , and the sealed fiber exit port would be located in a region of the enclosure  306  which would not interfere with the operation of the substrate  308 . 
       FIG. 5  shows one embodiment of a separate sensor wavelength interrogator for use in the separate fiber sensor system of  FIG. 3 . During a first measurement interval of arbitrary time duration, a first broadband source SRC_ 1   504  is enabled with second broadband source SRC_ 2   502  disabled, and during the first measurement interval, SRC_ 1  couples optical energy through circulator  506  to the first fiber Bragg grating strain sensor (operative initially at λ 1 ), and narrowband reflected energy (initially at λ 1 ) from the first sensor is coupled through circulator  506  to combiner  514  (with no optical energy returned from circulator  508  as SRC_ 2   502  is not enabled during the interval that SRC_ 1  is enabled), which couples optical energy into wavelength detector  515  which in one embodiment includes a splitter  516  and to a means for discriminating wavelength such as sine filter A  518  and sine filter B  520 , which are coupled to first detector DET_A  522  and second detector DET_B  524 , respectively. The output from the two detectors are fed to a pressure calculator  526  which computes the pressure from the amplitude responses (the amplitudes presented to the detectors derived from the wavelength-dependent transfer function of the sine filter), of the two detectors DET_A  522  and DET_B  524 . During a second measurement interval of arbitrary time duration following the first measurement interval, the first source SRC_ 1   504  is disabled and the second broadband source SRC_ 2   504  is enabled. During the second measurement interval, the second circulator  508  couples broadband optical energy to the second fiber Bragg grating strain sensor (operative initially at λ 2 ) through bidirectional port  512 , and narrowband optical energy (initially at λ 2 ) reflected from the second sensor is coupled through circulator  508 , through combiner  514 , and to wavelength detector  515 , through splitter  516 , and to first sine filter  518  and second sine filter  520 , which generate optical outputs related to wavelength as will be described for  FIG. 6 , and the optical outputs of sine filters  518  and  520  are converted to an electrical signal by first detector  522  and second detector  524 , after which the electrical outputs of first and second detectors  522  and  524  are converted to a pressure measurement using pressure calculator  526 . The first time interval and second time interval are typically established from the time-of-flight interval for the reflected wavelength from the strain sensor fiber Bragg grating to reach the interrogator, and for the detectors to respond thereafter. For a broadband source illuminating the fiber Bragg gratings, it is possible for a wavelength interrogator separated from the measurement gratings by a 10 km fiber length, with an index of refraction of 1.48 for the fiber core (resulting in a 97 us round-trip delay), and a detector with a 2 us response, to therefore operate at a repetition rate of up to 10,000 unambiguous samples per second. In this manner, the repetition rate for any length of fiber and detector response time can be calculated. 
       FIG. 6  shows the characteristics of the first sine filter (SINE_A)  604  (of filter  518  of  FIG. 5 ) and second sine filter (SINE_B)  606  (of filter  520  of  FIG. 5 ), as well as the broadband source  602  (of source  502  or  504  of  FIG. 5 ). A reflected optical signal from a fiber Bragg grating sensor at a first wavelength λ 1   608  produces an output L 1 _DetA at response point  612  with the first sine characteristic  604  and L 1 _DetB  614  from the second sine characteristic  606 . An optical signal at a second wavelength λ 2   610  generates a first sine characteristic  604  output L 2 _DetA  618  and second sine filter characteristic  606  output L 2 _DetB  616 . 
       FIG. 11  shows a wavelength shift characteristics (Y axis) of a fiber Bragg grating having a strain applied (X axis). The wavelength shift is shown with reference to an unspecified starting wavelength associated with the unstressed fiber Bragg grating after mounting into a surface such as the substrate  308  of  FIG. 4 . The relationship between wavelength shift and fiber Bragg grating strain can be described as a linear equation, shown for  FIG. 11  as Y=0.7328X (ignoring temperature effects for simplicity of illustration). 
     In a linear system, an increased pressure at port  312  of  FIG. 3  causes grating  404  of corresponding  FIG. 4  to stretch and grating  402  to compress. Additionally, the two gratings are each responsive to a temperature, as expressed below:
 
λ1 =L 1 −C 1 *P+K 1 *T  
 
λ2 =L 2 +C 2 *P+K 2 *T  
 
     Ideally, if the coefficient of temperature response K is matched between the two fibers such that K 1 =K 2 =K, and the coefficient of pressure response C is matched between the two fibers such that C 1 =C 2 =C, and first grating  402  has an unstressed or starting reflection wavelength of L 1 , and second grating  404  has an unstressed or starting reflection wavelength of L 2 , the system of equations which govern the system is:
 
λ1 =L 1 −C*P+K*T  
 
λ2 =L 2 +C*P+K*T  
 
and calculated pressure is therefore:
 
 P ={(λ2−λ1)+( L 1 −L 2)}/2 C   (Eq. 1)
 
wherein the temperature dependence drops out. For a more typical case where K 1 ≠K 2  and C 1 ≠C 2 , the governing system of equations would be:
 
 P =({[λ2−λ1]+[( K 1 −K 2)* T]}+{L 1 −L 2})/( C 2 +C 1)  (Eq. 2)
 
 T =({[λ2−λ1]−[( C 1 +C 2)* P]}+{L 1 −L 2})/( K 2 −K 1)  (Eq. 3)
 
     From the above relationships, it can be seen that the pressure and temperature can be derived from the two wavelength measurements, when coupled with independent constant temperature and constant pressure calibration profiles, respectively. 
     In a preferred embodiment of the invention, the reflection wavelengths λ 1  and λ 2  are distinct and non-overlapping over the combinations of temperature and pressure, as shown in the x-axis of  FIG. 6  corresponding to a single cycle of sine filter response for uniqueness of y-axis response. This may be expressed as the following criteria: 
     1) λ 2 &gt;λ 1  such that (λ 2 −λ 1 )=|λ 2 λ 1 | 
     2) λ 2  and λ 1  are always in non-overlapping ranges. 
     There are several motivations for the best mode of non-overlapping ranges of wavelengths produced by the pair of fiber Bragg gratings of a particular pressure transducer. One motivation is to provide a clear association between a particular response wavelength and a given sensor FBG, such that λ 1  and λ 2  are not indeterminate in the equations. Another advantage of using separate wavelength response ranges is to prevent the “shadowing” of a downstream reflection-mode sensor or additive superposition of a downstream transmission-mode sensor, which would cause two sensors responses to appear as a single sensor response. While it is possible to operate the two sensors in overlapping ranges, a disadvantage is the inability of the wavelength discriminator to distinguish between a single sensor response caused by two separate sensors operating in the same wavelength and a failure in the fiber which interconnects the two FBG sensors, resulting in a single sensor reflection response. By tracking each sensor response for association to a particular sensor, and detection of same-wavelength sensor response, it is possible for the two sensors to operate in overlapping response ranges. 
       FIG. 7  shows a timing sequence diagram for the operation of the wavelength interrogator of  FIG. 5 . Waveform  702  shows the sequence of first broadband source SRC_ 1  measurements during a first interval previously described interleaved with a second broadband source SRC_ 2  which is enabled during a second interval of time. Each detector DetA and DetB generates part of the differential output which can be concurrently read and converted into a pair of values and converted thereafter by pressure calculator  526  into a pressure value  528 , such as the use of stored pre-deployment calibration data profiles which converts sensed pressure P as that of Equations 1 or 2 into corrected pressure. 
       FIG. 8A  shows a single fiber pressure sensor having a pressure chamber  822  coupled to a pressure to be measured through aperture  824  which provides deflection of a substrate  814  having fiber Bragg gratings applied on opposite sides in region  818 , as was shown for  FIGS. 3 and 4 . The sensor of  FIG. 8A  has the top and bottom sensors tied together in series such that the two fiber Bragg grating sensors are formed onto a single optical fiber  808  in conduit  806  which is also housing a support cable  804  tied to a support  802  on one end, and the pressure transducer enclosure  816  on the other end. The optical fiber  808  and support cable  804  may have any length, shown as 10 km, and the end of fiber  808  opposite to sensor region  818  is coupled at port  826  to a single fiber interrogator  800 . 
       FIG. 8B  shows an example embodiment of a single fiber interrogator  800 . A broadband source  852 , which may operate continuously, couples broadband optical power to circulator  850 , which couples broadband optical power to port  826 , and to the gratings in region  818  of  FIG. 8A  which reflect superimposed optical energy as λ 1  and λ 2  through circulator  850  and to filter  854 , which splits the wavelengths from each reflection grating into separate channels and provides each to wavelength detectors  515 A and  515 B which are each operative such as was described for  515  of  FIG. 5 , and which may operate according to the wavelength discrimination principles described in  FIG. 6 . The pressure calculator  864  which receives the detected wavelengths for each sensor may perform the pressure and temperature calculations based on Equations 1, 2, or 3, in combination with stored calibration data, or any other means for converting measured wavelengths into pressure and temperature. Additional measurement channels may be added by placing additional sensors which are operative within unique wavelengths which also couple out of filter  854  and are coupled to additional wavelength detectors  515 C,  515 D, etc (not shown) operative at each unique wavelength to detect additional measurement phenomenon such as optional temperature sensor  812  of  FIG. 8A . 
       FIG. 9A  shows a diagram for a double ended sensor, which may be operated in at least two configurations. A redundant configuration which protects against a fiber failure provides redundancy protection and is used with reflection gratings on opposite surfaces of substrate  914  in region  918  using the interrogator of  FIG. 9B . An alternative use of the double ended sensor of  FIG. 9A  is a non-redundant configuration with transmission mode fiber Bragg gratings on opposite substrate surfaces and located in region  918  and using the interrogator of  FIG. 9C . For either mode of operation, the pressure transducer has a housing  916  with a sealed substrate  914  forming a pressure chamber coupled to a pressure source through aperture  924 , and the gratings are located in region  918 , as was described previously. 
       FIG. 9B  shows dual ended sensor interrogator  900  for redundancy operation, where the interrogator can recover from a break in one of the two optical fibers  908  and  909  which travel in the conduit  906 . Broadband source  952  is coupled to either a first (primary) optical fiber  927 , or to a second (secondary) optical fiber  926  as selected by optical switch  970 . The first and second fibers of  FIG. 9A  are coupled to reflection mode fiber Bragg gratings, which return optical energy at a first and second wavelength, respectively. The reflected optical energy is coupled through circulator  950  to wavelength filter  954 , which separates and delivers the response wavelengths to a first wavelength detector  515 A and second wavelength detector  515 B, which are coupled to pressure calculator  964 . Wavelength detectors  515 A and  515 B also detect the absence of reflected optical energy from a first fiber  927 , such as from a fiber break, which causes optical switch  970  to deselect primary fiber  927  and select secondary fiber  926  for coupling to broadband source  952  and which also directs reflected optical energy through circulator  950  to filter  954 . As the order of the first grating or second grating along the fiber path does not affect the reflected optical energy, by virtue of their unique operating ranges, either the first optical fiber  927 , or second optical fiber  926  may be exclusively selected by optical switch  970 . 
       FIG. 9C  shows a double ended sensor interrogator operating with transmission fiber Bragg gratings for use with the double ended sensor of  FIG. 9A  where gratings in region  918  are utilized in transmission mode with co-propagating fiber Bragg grating wavelength signals. For this type of operation, a broadband optical source  982  is coupled to one of the optical fibers  927 , and the other optical fiber  926  contains a superposition of the wavelengths associated with the first and second gratings. As was described previously, the wavelength filter  972  separates them into two bands, which are resolved into particular wavelengths by wavelength detectors  515 A and  515 B, as was described previously, and fed to pressure calculator  984  to generate computed pressure  986 . 
     In another embodiment shown in  FIG. 10 , a plurality of n pressure transducers  1004 ,  1006 , . . . ,  1008 , each functioning as previously described for  FIGS. 3 and 4 , may be placed in a series configuration, with each pressure transducer generating respective optical responses λ 1   a  and λ 1   b  of sensor  1004 , λ 2   a  and λ 2   b  of sensor  1006 , and λna and λnb of sensor  1004 . Filter  1020  separates the wavelength pairs associated with each particular pressure transducer, and applies this to a respective pressure/temperature computer  1010 ,  1012 , and  1014 , each of which computes the pressure for a particular transducer. In this manner, the wavelengths of each of the pressure transducers are received by a single wavelength interrogator  1002  which separates the wavelengths associated with each sensor  1004 ,  1006 ,  1008  and computes for each pressure transducer a respective pressure and temperature measurement. The wavelength interrogator of  FIG. 10  shows the use of reflection fiber Bragg gratings with a multi-channel interrogator, and it is possible to combine the series transducer configuration of  FIG. 10  with the double-ended multi-channel interrogator of  FIG. 9B  modified to provide multi-channel response by replacing the filter  954  and successive components of  FIG. 9B  with the filter  1020  and successive components of  FIG. 10 . In another embodiment, a plurality of pressure transducers are connected in series, with each pressure transducer having a pair of transmission fiber Bragg gratings. The optical fibers on opposite ends of the series string of transducers can be coupled to a modified multi-channel sensor of  FIG. 9C , where the filter  972  is replaced by the filter  1020  and following components, each of which is coupled to a pressure/transducer computer for each respective pressure transducer  1004 ,  1006 ,  1008  operative using transmission fiber Bragg gratings. 
     The examples provided herein are for illustration only, and are not intended to limit the invention to only the particular embodiments used for explanation.