Sapphire Sensor for Measuring Pressure and Temperature
A sensor for measuring pressure, temperature or both may be provided. The sensor may include a diaphragm that may respond to a change in temperature or pressure, a base connected to the diaphragm, a cavity, and an optical fiber that may conduct light reflected off of a surface of the diaphragm. The diaphragm and base may be sapphire elements. An interrogator may be provided for detecting a deflection of the diaphragm.
- 1-37. -37. (canceled)
- 38. A system for measuring a pressure, a temperature, or both, the system comprising
a diaphragm configured to respond to a change in temperature or pressure, the diaphragm comprising at least one sapphire element; a base located adjacent to the diaphragm, the base comprising at least one sapphire element; a cavity defined between the diaphragm and the base; an optical fiber that is configured to conduct light reflected off of surfaces adjacent said cavity; and an interrogator for detecting a deflection and an optical path distance (OPD) of the cavity based on at least two light signals having similar wavelengths and coherence lengths reflected off of surfaces of said diaphragm and said base, wherein said coherence lengths at of intermediate length between an OPD of said cavity and OPDs of other pairs of surfaces of said diaphragm and said base; wherein a quadrature phase detection unit is provided, the quadrature phase detection unit being configured to demodulate one or more signals received by the interrogator.
- View Dependent Claims (39, 40, 41, 51, 52, 53, 54, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65)
- 42-50. -50. (canceled)
- 55. A sensor apparatus for measuring a pressure, a temperature or both, the sensor comprising:
a diaphragm configured to respond to a change in temperature or pressure, the diaphragm comprising at least one sapphire element; a base located adjacent to the diaphragm, the base comprising at least one sapphire element; a cavity between the diaphragm and the base defined by at least one of a recess in at least one of said diaphragm and said base and a sapphire spacer wherein boundary surfaces of said diaphragm a boundary surface of said base and boundary surfaces of said cavity are partially reflecting surfaces and respective pairs of said partially reflecting surfaces define optical path distances (OPDs) that produce preferential reflections and interference patterns at different optical wavelengths; an optical fiber that is configured to conduct light signals of similar wavelengths and coherence lengths reflected from boundary surfaces of said base, said diaphragm and said cavity, wherein said coherence lengths of said light signals are intermediate between an OPD of said cavity and OPDs formed by other pairs of surfaces of said diaphragm, said base and said cavity whereby said reflections from surfaces of said diaphragm and said base do not contribute to interference fringes from the OPD formed by surfaces of said cavity.
This application claims priority to U.S. Provisional Patent Application No. 61/816,146 filed on Apr. 25, 2013 and entitled DIRECT-BOND ENABLED SINGLE CRYSTAL SAPPHIRE SENSORS FOR MEASUREMENT OF PRESSURE AND TEMPERATURE, the entire contents of which are hereby incorporated by reference.
Pressure and temperature measurements are two of the most common parameters that need to be measured in nearly every industrial sector. Most of the current pressure sensors and measurement devices are based on the use of semiconductors, such as silicon. However, some pressure measurement needs are difficult to be fulfilled by the existing pressure sensor technologies due to various measurement environment challenges, such as high temperatures, electromagnetic interference (EMI) and remote signal transmission as required in oil/gas downhole measurement.
Single-crystal sapphire has a melting point of 2045° C. and is known to exhibit excellent resistance to chemical corrosion. In addition, it is transparent to a broad range of optical spectrum from ultra-violate (UV) to infrared (IR) and is relatively low cost. Besides the capability of high temperature operation and resistance against chemical corrosion, single-crystal sapphire is also known to offer excellent stability in other harsh environments. For example, many amorphous materials, such as glasses, show creep under high pressure especially at elevated temperatures. Further other foreign chemical species, such as gases and water can gradually diffuse into these materials under high pressure. The diffusion rate will pick up as temperature increases. The sensors built with amorphous materials will therefore exhibit drifts under these operating conditions. In contrast, single-crystal sapphire may exhibit minimal material creep and foreign material diffusion even under high pressure and elevated temperature. Therefore, single-crystal sapphire is an attractive material for construction of sensors for excellent long-term stability under high temperatures or high pressure or both even with presence of other diffusive species, such as various gases and water.
To build a pressure sensor, a hermetically sealed hollow cavity that can change in response to an externally applied pressure may be provided. Construction of such a hollow cavity may include bonding between different mechanical parts. For a sapphire pressure sensor, sapphire-to-sapphire direct bonding may provide benefits. The resulting sensor may offer high long-term stability. Additionally, the sensor may also have ultra-high temperature operation capability.
Two major sapphire-to-sapphire direct bond methods are known. One method was reported by A. Sugiyama, et al. [A. Sugiyama et al., “Direct bonding of Ti:sapphire laser crystals,” Appl. Opt., vol. 37, p2407, 1998]. This method consists of two steps. The first is to pre-bond two sapphire elements at a temperature around 200° C. The second step is to bake the pre-bonded sapphire assembly at a temperature above 1000° C.
The other method is plasma assisted bonding, described in U.S. Patent Application Publication No. 2012/0024073. Plasma assisted bonding may substantially reduce the baking or anneal temperature. Using a method similar to the one reported by Sugiyama et al., Virginia Tech researchers lately constructed a hermetically sealed sapphire Fabry-Perot (FP) cavity and demonstrated pressure measurement at room temperature [J. Yi, et al., “Demonstration of an all-sapphire Fabry-Perot cavity for pressure sensing,” IEEE Photon. Tech. Lett., vol23, p9, 2011]. In this work, two a-cut sapphire wafers were used. One was etched to form an approximately 6 μm circular pit using a reactive ion etching (RIE) process. This etched wafer was then bonded to another wafer based on sapphire to sapphire direct bond. The FP cavity was then glued to a ceramic tube. A multimode fiber was inserted into the ceramic tube to the FP cavity for the sensor interrogation. The FP cavity was demodulated using whitelight interferometry. Because of the shallow FP cavity, a very broadband spectrum halogen lamp was used as the source along with an Ocean Optics spectrometer. In their test, the whole sensor including the FP cavity and the ceramic tube were placed in a pressure chamber and the fiber ran through a fiber feedthrough.
This sensor structure is not ideal for real applications. A practical pressure sensor usually has a metal casing with mechanical threads for convenient pressure-sealed sensor installation to a pressure vessel. Further, due to the significant mismatch in the coefficients of thermal expansion (CTEs) between the sapphire FP cavity and the ceramic tube, this sensor structure may not survive at high temperatures. Also, the thermal stresses induced by the CTE mismatch will introduce significant thermal dependence of the FP cavity distance and this dependence may not be repeatable due to the gradual release of the stresses trapped in the adhesive during its cure.
In one embodiment, a sensor apparatus for measuring pressure, temperature or both may be provided. The sensor may include a diaphragm that may respond to a change in temperature or pressure, a base connected to the diaphragm, a cavity defined by the diaphragm and the base, and an optical fiber that may conduct light reflected off of a surface of the diaphragm. The diaphragm and base may be sapphire elements.
In another embodiment, a system for measuring pressure, temperature or both may be provided. The system may include a diaphragm that may respond to a change in temperature or pressure, a base connected to the diaphragm, a cavity defined by the diaphragm and the base, and an optical fiber that may conduct light reflected off of a surface of the diaphragm. The diaphragm and base may be sapphire elements. An interrogator may be provided for detecting a deflection of the diaphragm.
Advantages of embodiments of the present invention will be apparent from the following detailed description of the exemplary embodiments. The following detailed description should be considered in conjunction with the accompanying figures in which:
Aspects of the invention are disclosed in the following description and related drawings directed to specific embodiments of the invention. Alternate embodiments may be devised without departing from the spirit or the scope of the invention. Additionally, well-known elements of exemplary embodiments of the invention will not be described in detail or will be omitted so as not to obscure the relevant details of the invention. Further, to facilitate an understanding of the description discussion of several terms used herein follows.
As used herein, the word “exemplary” means “serving as an example, instance or illustration.” The embodiments described herein are not limiting, but rather are exemplary only. It should be understood that the described embodiment are not necessarily to be construed as preferred or advantageous over other embodiments. Moreover, the terms “embodiments of the invention”, “embodiments” or “invention” do not require that all embodiments of the invention include the discussed feature, advantage or mode of operation.
Further, many of the embodiments described herein are described in terms of sequences of actions to be performed by, for example, elements of a computing device. It should be recognized by those skilled in the art that the various sequences of actions described herein can be performed by specific optical components, devices, and circuits (e.g. application specific integrated circuits (ASICs)) and/or by program instructions executed by at least one processor. Furthermore, the sequence of actions described herein can be embodied in a combination of hardware and software. Thus, the various aspects of the present invention may be embodied in a number of different forms, all of which have been contemplated to be within the scope of the claimed subject matter.
In an exemplary embodiment, a pressure sensor may be fabricated such that a pressure sensitive hollow cavity on a mechanical support that is made of the same material as that of the cavity. The construction may be such that there is no direct physical contact between the sensor metal casing or sensor housing and the proximity of the hollow cavity. The pressure sensitive cavity may be fabricated on a relatively long base. In another embodiment, a relatively shorter or smaller base may be provided. In this case, the pressure sensor may be mounted on another relatively long mechanical support that may be made of the same material.
A sensor probe may contain a pressure sensitive hermetically sealed hollow FP cavity. A hermetic seal may be realized by direct bonding of two, three or four single-crystal sapphire elements. Greater numbers of single-crystal sapphire elements may be combined as will be understood by those skilled in the art. One of the elements may be a diaphragm that can deflect under an externally applied pressure. The sapphire elements that form and surround the cavity may be directly bonded to one another such that the elements may collectively form a homogenous sapphire structure. The elements may form a substantially monolithic sapphire structure. The elements may be bonded via direct or fusion bonding without the use of any foreign materials which may have different coefficients of thermal expansion. Thus, the structure surrounding the cavity may consist essentially of sapphire.
In the exemplary embodiment shown in
The length L of the sapphire base may be long, generally greater than the diaphragm outer diameter. The length of the base may be at least 2 times or 3 times the diaphragm outer diameter. Alternatively, other dimensions may be employed. As a result of the relatively long base 204, an influence on the cavity by stresses induced by a sensor mounting in sensor installation or ambient temperature varations may be negligible or substantially zero.
A relatively thin base 205 may be provided that is relatively thin compared to the outer diameter of the diaphragm. In this case, the thin base 205 may be bonded to another relatively long sapphire support 204 so the physical contact of the sapphire sensor probe with a sensor metal casing can be designed to be sufficiently distanced from the FP cavity. The support may be a solid cylinder, or may have a center through-hole 212 or partial hole 216 whose diameter may be constant or may vary along the hole as shown in
The FP cavities may be optically interrogated using an optical fiber 300. By the separation between the FP cavity and the interrogating fiber 300, the interrogation systems may be generally divided into two classes, namely close up interrogation and standoff interrogation.
The sensor or FP cavity interrogation may be realized by the use of an optical fiber 302, which may be a singlemode or a multimode fiber. The fiber may be or may be not connected to another fiber.
A graded index quarter pitch fiber 304 may be spliced via thermal fusion to an interrogation fiber 302. The interrogation fiber 302 may be made of glass. The interrogation fiber 302 may also be connected to a segment of a single-crystal sapphire fiber 306 as shown in
By the spatial separation between the interrogation fiber or fiber/collimation lens assembly, and the FP cavities, the sensor interrogation methods may be divided into close-up and standoff interrogations. Using one of the four cases presented in
The fiber 300 may also be mounted in the base 204 by the use of a fiber ferrule 322 as shown in
The base 204 may also have a partial center hole 316 as shown in
The fiber or the collimator end may have an anti-reflection (AR) coating or simply bare glass without any coating. An index-matching optical adhesive may also be used between the fiber collimator or the fiber/ferrule end and the bottom of the base partial hole. The optical adhesive may be defined to be transparent to the wavelength of the light used in the sensor interrogation. The index of refraction of the adhesive may match that of the fiber or the sapphire. The index of the adhesive may also be between the indices of the fiber and the sapphire. For a given index of the adhesive, by controlling the geometrical thickness of the adhesive on the front end of the fiber/ferrule assembly or the fiber collimator, the optical reflection from the adhesive layer can be increased or decreased to best support the sensor interrogation. The adhesive applied to the cylindrical surface of the fiber/ferrule assembly or the fiber collimator may be or may be not transparent adhesive for the wavelengths of the light used in the sensor interrogation.
When the sapphire is thin (L is comparable to or smaller than the diaphragm outer diameter), the thin base 205 may be bonded to a relatively long sapphire support 204. As shown in
Exemplary embodiments of the formation of the hollow FP cavity defined between the diaphragm and the end of the interrogation fiber may be shown in
For applications where the sensor may be placed in a high temperature environment, the interrogation fiber 302 may be connected to a segment of sapphire fiber 306 as shown in
For the different close-up and standoff interrogation schemes as previously described, multiple reflections from the interfaces between different elements may be generated.
To realize simultaneous measurement of pressure and temperature, at least two OPDs including the one between R2 and R3 (the OPD of the hollow FP cavity) may be demodulated. Two exemplary methods by which to demodulate these OPDs will be discussed. The first is whitelight interferometry.
Whitelight interferometry (WLI) allows the demodulation of a fiber Fabry-Perot interferometer. A WLI system may use either a tunable laser as the source or uses a broadband source such as a light emitting diode (LED) along with an optical spectrometer. The optical spectrum returned from the FP cavity may be measured. This spectrum may be modulated by the FP cavity so fringes with peaks and valleys may be observed in the detected optical spectrum. The peaks and valleys may correspond to constructive and destructive optical interference between the reflections from the FP cavity. By detecting the phase changes of the fringes in response to the FP cavity variation, the cavity OPD can be determined. When more than two reflections are returned from a composite FP cavity structure, fast Fourier transform (FFT) may be performed first. In the FFT spectrum, multiple peaks may appear which correspond to the interference between any pair of two reflections. When the OPDs of the composite FP structure are sufficiently different, these peaks may appear at different frequencies without overlap. These peaks may then be separated by the use of digital or analog bandpass filters. The interference fringes for each pair of reflections can be reconstructed and demodulated to determine the OPD between the two reflections [C. Ma, et al., “Optimization of single-/Multi-/single-mode intrinsic Fabry-Perot fiber sensors,” J. Lightwave Tech., 30, p2281, 2012; C. Ma and A. Wang, “Signal processing of white-light interferometric low-finesse fiber-optic Fabry-Perot sensors,” Appl. Opt., 52, p127, 2013].
To mathematically explain how each of the sensor OPDs can be determined using the WLI, we assign these OPDs to be the OPDd between R1 and R2, the OPDp between R2 and R3, and the OPDb between R3 and R4. As discussed previously OPDp is primarily sensitive to pressure. Due to the thermal expansion of the diaphragm shoulders that define the FP cavity distance and the thermal dependence of the Young'"'"'s modulus of the diaphragm material, the OPDp is also temperature dependent. In the meantime, OPDd and OPDb are primarily sensitive to temperature but still show some degree sensitivity to pressure. To the first order of approximation, these three OPDs may be expressed as
where Api (i=1, 2 and 3) are the dependence coefficients of OPDp, OPDd, and OPDb on pressure, respectively, and ATi are the dependence coefficients of OPDp, OPDd, and OPDb on temperature. Generally, we have Ap1>>Ap2 and Ap3 and AT2 and AT3>>AT1. By solving Eqs. (1), (2) and (3), both pressure and temperature can be simultaneously determined. Here both OPDd and OPDb are mainly sensitive to temperature variations but insensitive to pressure. For some applications where the sensor probe is immersed into a medium, such as oil, whose index of refraction may be relatively close to that of sapphire, R1 may be much weaker that R3 and R4, the temperature measured from OPDb may be more accurate.
When an optical spectrometer is used to measure optical spectrum from the sensor, the response time may be limited below several kilohertz. Although high speed tunable lasers are available, they are generally expensive. However, many applications require cost-effective high-speed dynamic pressure measurement. In the meantime, the response time for temperature measurement may not be a strong requirement. This is also partially because the thermal mass of the sensor tip may prevent the temperature of the sensor tip to vary rapidly. In these cases, different sensor interrogation techniques may be used.
Quadrature phase detection is a technique to measure relative changes of the OPD of an optical interferometer such as an FP cavity. The method may provide a low implementation cost and high speed signal demodulation. In addition, whitelight interferometry may require a minimum OPD for a given optical spectral range of detection. For example, for an LED at 1550 nm that has a spectral width of 50 nm, a minimum OPD may be more than 100 μm to warrant a high demodulation accuracy. In contrast, the quadrature phase detection does not have this contingent requirement.
The principle of quadrature phase detection is to inject two light beams at two different wavelengths λ1 and λ2. These two beams may also have a broadband spectrum. In this case the two wavelengths are effective center wavelengths of the two broadband spectra. For a given FP cavity, these two wavelengths are chosen such that their optical phases are different by Nπ±π/2 where N is an integer. Using a standard quadrature phase detection, any change in the OPD can be determined [P. L. M. Heydemann, “Determination and correction of quadrature fringe measurement errors in interferometers,” Applied Optics, 20(19), 3382, 1981]. For the quadrature detection, it is preferred for the interrogation light to see interference fringes from only one FP cavity, which in the sapphire sensor case is the pressure sensitive hollow FP cavity. However, as described earlier, there are additional reflections from the sensor besides the two from the hollow FP cavity, such as R1 and R4 as shown in
OPDp<Lc<OPDd and OPDb (4)
where Lc is the coherence length of the sources. The coherence length of a source with a Gaussian spectrum distribution is given by Lc=λo2/Δλ where λ0 is the center wavelength and Δλ is the spectral width [Principle of Optics by M. Born and E. Wolf, 7th Edition, Cambridge University Press, Cambridge, UK, 1999]. Here we assume the two sources have similar coherence lengths. When the condition in Eq. (4) is met, effective optical interference between R2 and R3 occurs but the other reflections namely R1 and R4 do not contribute to the generation of interference fringes.
The sources in the exemplary embodiment of
Quadrature detection may be useful for high-speed sensor signal processing.
The spatial separation between two neighboring FBGs may be denoted as D. D may be constant or varying. In the present explanation, D may be assumed to be constant. The FBGs may be disposed in one fiber or in different fibers. If the FBGs are disposed in different fibers, another l×N, N×N fiber coupler or any other type of optical beam splitter, as would be reasonably understood by a person having ordinary skill in the art, may be used to split the reflections from the sensor into N channels of fiber. The optical distances between the FBGs and the fiber coupler may be different from each other.
The optical signal reflected from the sensor may be reflected by the serial FBGs. Each FBG may reflect only a portion of the incident optical spectrum. The reflections from the serial FBGs may then be detected by a light detector (DET). The photoelectric signal may be amplified by an electronic amplifier (AMP) and digitized by an analog-to-digital converter (A/D) for further signal processing.
Since the reflections from the serial FBGs may be delayed by different amounts of time, successive light pulses may appear at the DET. The magnitude of each pulse from each of the FBGs may offer a sampling of the optical spectrum of the signal reflected from the sensor. The sensor OPDs may then be determined by the application of an interferometric signal processing technique. The interferometric signal processing technique may include, but is not limited to, quadrature detection, whitelight interferometry, or any other technique as would reasonably be understood by a person having ordinary skill in the art.
The temporal separation between neighboring pulses reflected from the serial FBGs may be designed to be relatively large by choosing a large D. As a result, the light source pulse width may be large and the requirement on the speed of the DET and AMP may be relaxed. Additionally, the requirement on the speed of the A/D may be reduced. This combination of features may allow high-speed sensor signal demodulation at a low cost.
The sensing schemes shown in
The foregoing description and accompanying figures illustrate the principles, preferred embodiments and modes of operation of the invention. However, the invention should not be construed as being limited to the particular embodiments discussed above. Additional variations of the embodiments discussed above will be appreciated by those skilled in the art.
Therefore, the above-described embodiments should be regarded as illustrative rather than restrictive. Accordingly, it should be appreciated that variations to those embodiments can be made by those skilled in the art without departing from the scope of the invention as defined by the following claims.