Abstract:
A high-temperature pressure sensor is provided. The sensor includes a quartz substrate with a cavity etched on one side. A reflective coating is deposited on at least a portion of the cavity. The sensor further includes a ferrule section coupled to the quartz substrate with the cavity therebetween. The cavity exists in a vacuum, and cavity gap is formed between the reflective metal coating and a surface of the ferrule. The sensor also includes an optical fiber enclosed by the ferrule section and extending from the cavity gap to an opposing end of the ferrule section and a metal casing surrounding the ferrule section and the quartz substrate with an opening for said optical fiber extending therefrom. The pressure applied to the quartz substrate changes the dimensions of the cavity gap and a reflected signal from the reflective coating is processed as a pressure.

Description:
BACKGROUND 
     Pressure sensors are used in a wide range of industrial and consumer applications. Pressures of many different magnitudes may be measured using various types of pressure sensors, such as Bourdon-tube type pressure sensors, diaphragm-based pressure sensors and piezoresistive pressure sensors on silicon or silicon on insulator (SOI). Several variations of the diaphragm-based pressure sensor have been utilized to measure different ranges of pressure, such as by utilizing cantilever-based pressure sensors, optically read pressure sensors and the like. 
     Fiber optic sensors utilizing a Fabry-Perot cavity have been demonstrated to be attractive for the measurement of temperature, strain, pressure and displacement, due to their high sensitivity. The major advantages of fiber optic sensors over conventional electrical sensors include immunity to electromagnetic interference (EMI), compatibility with harsh environments and potential for multiplexing. 
     Microelectro-mechanical systems (MEMS) fabrication techniques make Fabry-Perot sensors more attractive by the potential precision in achieving specific Fabry-Perot cavity depths, diaphragm thicknesses, and diameters. This reduces potential yield loss from “out of specification” parts and reduces the necessary accuracy of the interrogation optics. In comparison to electronic high temperature pressure sensors, Fabry-Perot optical sensors are ideal for use in harsh environments because they do not require electronics to be located in the high temperature, harsh environment. Typically, a piezoresistive or piezoelectric pressure sensor require electronics to be located in close proximity to reduce noise by amplifying the signal. At temperatures greater than 200° C., commercially available high temperature electronics are not available limiting the use of these sensors due to poor signal to noise ratios. For a Fabry Perot optical sensor, the electronics and optics for reading and converting the optical signal to an output voltage can be located in a cool region, allowing the use of commercially available components which can enable reduced cost and high accuracy. 
     Fiber optic sensors are also of great interest for application in avionics and aerospace applications because their immunity to EMI provides significant weight savings through the elimination of cable shielding and surge protection electronics. In the biomedical field, fiber optic sensors have also proven successful resulting from their reliability, biocompatibility and the simplicity of the sensor-physician interface. 
     BRIEF DESCRIPTION 
     In accordance with one exemplary embodiment of the present invention, a high-temperature pressure sensor is provided. The sensor includes a quartz substrate with a cavity etched on one side and a reflective coating deposited on at least a portion of the cavity. Further, a ferrule section is coupled to the quartz substrate with the cavity therebetween, wherein said cavity exists in a vacuum. A cavity gap is formed between the reflective metal coating and a surface of the ferrule. The sensor further includes an optical fiber enclosed by the ferrule section and extending from the cavity gap to an opposing end of the ferrule section; and a metal casing surrounding the ferrule section and the quartz substrate with an opening for the optical fiber extending therefrom. Further, the pressure applied to said quartz substrate changes the dimensions of the cavity gap and a reflected signal from the reflective coating is processed as a pressure. 
     In accordance with another exemplary embodiment of the present invention, a high-temperature pressure sensor is provided. The sensor includes a first quartz substrate with a cavity etched on one side and a reflective coating deposited on at least a portion of the cavity. The sensor also includes, a second quartz substrate bonded to the first quartz substrate with the cavity therebetween. The cavity exists in a vacuum and a cavity gap is formed between said reflective metal coating and a surface of the second quartz substrate. Further, a ferrule section is coupled to the second quartz substrate and an optical fiber is enclosed by the ferrule section and extending from the second quartz substrate to an opposing end of the ferrule section. A metal casing is provided surrounding the ferrule section and the first and the second quartz substrates with an opening for said optical fiber extending therefrom. Further, the pressure applied to said quartz substrate changes the dimensions of the cavity gap and a reflected signal from the reflective coating is processed as a pressure. 
     In accordance with another exemplary embodiment of the present invention, a method of forming a pressure sensor is provided. The method includes providing a quartz substrate having a top side and a bottom side and etching the quartz substrate to form a cavity. The method further includes depositing a reflective coating on at least a portion of the cavity and attaching a ferrule section to the quartz substrate with the cavity therebetween wherein a cavity gap is formed between the reflective metal coating and a surface of the ferrule. The method also includes enclosing an optical fiber inside the ferrule section and extending from the cavity gap to an opposing end of the ferrule section and placing a metal casing around the ferrule section and the quartz substrate with an opening for said optical fiber extending therefrom. 
     In accordance with another exemplary embodiment of the present invention, a method of forming a pressure sensor is provided. The method includes providing a first quartz substrate having a top side and a bottom side and etching the first quartz substrate to form a cavity. The method further includes depositing a reflective coating on at least a portion of the cavity and bonding a second quartz substrate to the first quartz substrate with the cavity therebetween, wherein a cavity gap is formed between the reflective metal coating and a surface of the second substrate. The method also includes attaching a ferrule section to the second quartz substrate, enclosing an optical fiber inside the ferrule section and extending from the cavity gap to an opposing end of the ferrule section and placing a metal casing around the ferrule section and the quartz substrate with an opening for said optical fiber extending therefrom. 
    
    
     
       DRAWINGS 
       These and other features, aspects, and advantages will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein: 
         FIG. 1  is a diagrammatical representation of an extrinsic Fabry-Perot interferometer based pressure sensor; 
         FIG. 2  is a diagrammatical representation of a basic demodulation system using a single wavelength interrogation; 
         FIG. 3  is a diagrammatical representation of a sensor system for measuring pressure, in accordance with one embodiment of the present invention; 
         FIG. 4  is a diagrammatical representation of another high temperature sensor system of measuring pressure, in accordance with an embodiment of the present invention; 
         FIG. 5  is a diagrammatical representation of an exemplary process of manufacturing the pressure sensor of  FIG. 4 , in accordance with one embodiment of the present invention; 
         FIG. 6  is a diagrammatical representation of an exemplary process of manufacturing the pressure sensor of  FIG. 3 , in accordance with one embodiment of the present invention; 
         FIG. 7  is a diagrammatical representation of an exemplary process of manufacturing an optical fiber-ferrule structure, in accordance with one embodiment of the present invention; and 
         FIG. 8  is a diagrammatical representation of 3-dimensional view of the pressure sensor of  FIG. 3 , in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present description relates generally to pressure sensors, and more particularly to pressure sensors for measuring pressures at high temperatures. Embodiments of the invention include a novel high temperature optical sensor based on an extrinsic Fabry-Perot interferometer (EFPI) and method of manufacturing the same. 
       FIG. 1  is a conceptual view of an EFPI based pressure sensor  10 . The pressure sensor  10  uses a distance measurement technique based on the formation of a low-finesse Fabry-Perot cavity  12  between a polished end face  14  of a fiber  16  and a reflective surface of a diaphragm  18 . A light signal  20  is passed through the fiber  16 , where a first portion of the light R 1  is reflected off the fiber/air interface. The remaining light propagates through the air gap between the fiber and the reflective surface and a second portion of the light R 2  is reflected back into the fiber  16 . 
     In one embodiment, a light emitting diode (LED) (not shown) may generate the light signal to interrogate the sensor. In another embodiment, other light sources, such as super-luminescent leds (SLEDS), lasers or broadband light sources may also be used. The interaction between the two light waves R 1  and R 2  in the Febry-Perot cavity is modulated by the path length of each wave. As will be appreciated by those skilled in the art, the path length is defined as the distance that a light wave travels in the cavity  12 . The reflected light waves are detected by a detector (not shown) where the signals are demodulated to produce a distance measurement. As the cavity distance  12  changes, the demodulated signal from the detector determines the pressure. The cavity distance  12  can change if the diaphragm  18  translates due to an external force such as, but not limited to, an external pressure. Several different demodulation methods exist to convert the return signal into a distance measurement. 
       FIG. 2  shows a basic demodulation system  30  using single wavelength interrogation. A light source  32  supplies coherent light  34  to the sensor head  36  through an optical fiber cable  38  and the reflected light  40  is detected at the second leg of an optical fiber coupler  42  by a detector  44 . To illustrate the concept, a simplified analysis will ignore multiple reflections and the output of the detector is approximated as a low finesse Fabry-Perot cavity in which the intensity at the detector I r  is given by
 
 I   r   =|A   1   +A   2 | 2   =A   1   2   +A   2   2 +2 A   1   A   2  cos Δφ  (1);
 
where, A 1  and A 2  are the amplitudes of light waves R 1  and R 2 , and Δφ is the phase difference between them. A more detailed analysis will account for multiple reflections within the cavity, and the need for this analysis is dictated by the relative magnitude of the reflections involved.
 
     The output I r  is a sinusoid with a peak-to-peak amplitude and offset that depends on the relative intensities of R 1  and R 2 . A phase change of 360 degrees in the sensing reflection corresponds to one fringe period. In one embodiment, if a source wavelength of 1.3 μm is used, the change in gap for one fringe period is 0.65 μm. Thus, by tracking the output signal, minute displacements are determined. As will be appreciated by those skilled in the art, the above demodulation approach is just an exemplary one and other demodulation schemes such as dual wavelength interrogation may also be used. A potential disadvantage of this type of demodulation system is the non-linearity of the sinusoidal transfer function. If the sensor gap is not biased at the zero crossing of the sinusoid, but is incorrectly biased near a peak or valley, the sensitivity of the detection system may be severely degraded. 
       FIG. 3  is a sensor system  60  for measuring pressure in accordance with one embodiment. The sensor system  60  consists of a sensor section  62  and a sensor assembly  64 . An optical fiber  66  is fixed inside a ferrule  68  such as by using laser welding, a direct bonding process, a high temperature adhesive or another high-temperature compatible process. In one example, the fiber diameter is 125 microns. One side  70  of the fiber-ferrule structure is then polished using standard fiber polishing processes. The polishing ensures a planar surface for mounting the sensor assembly  64 . The sensor section  62  is attached to the signal detection system  64  on the polished surface  70  of the fiber-ferrule structure. 
     The sensor section  62  is composed of a first substrate  72  and a second substrate  74 . In one embodiment, the first substrate  72  is made of quartz or fused silica material and the second substrate  74  is made of quartz or fused silica. A cavity gap  76  is formed between the first substrate  72  and the second substrate  74 . The use of quartz or fused silica for the second substrate  74  by itself may lead to a low intensity light reflection from the substrate and consequently, a low signal-to-noise ratio. Hence, in one embodiment, a metal coating  78  is disposed on the second substrate  74  and into the cavity gap  76  to increase the reflectivity of the Fabry-Perot cavity allowing a higher percentage of light to be reflected back. Another advantage of the metal coating is it eliminates “ghost” or secondary reflections from the back of the sensor. In one embodiment, a roughened surface, a curved surface, an absorbing surface or an anti-reflective (AR)-coated surface may be placed on the back of the sensor to eliminate secondary reflections. In one exemplary embodiment, the metal coating  78  may be a gold metal coating. In yet another embodiment, the material used for metal coating  78  comprises platinum, titanium, chrome, silver or any other high temperature compatible metal. 
     The second substrate  74  of sensor section  62  acts as diaphragm and translates the applied force or pressure into a variation in the cavity gap depth  76 . In one embodiment, the cavity gap is formed by etching the second substrate. In another embodiment, oxide wet etching or reactive ion etching is used for etching the second substrate. The first substrate  72  and the second substrate  74  are then attached to one another through a bonding process to create a vacuum in the cavity gap. In one embodiment, the vacuum bonding process includes a laser melting process or surface activated bonding process. The vacuum bond ensures that the expansion effects of any residual gas inside the cavity gap due to increasing temperature do not result in unwanted variations in cavity gap. Further, the vacuum bond isolates the cavity gap  76  from the applied pressure such that there is a differential pressure that results in a deflection of the diaphragm. 
     The signal detection system  64  further includes a strain buffer material  80  attached to the opposite side of the ferrule  68 . The strain buffer material  80  is attached to the ferrule  68  by using a metal bond material  82 . In one embodiment the strain buffer  80  and the ferrule  68  are bonded using thermocompression bonding, diffusion bonding, or other welding processes with or without the bond material  82 . The strain buffer material  80  is typically a high temperature compatible material with a coefficient of thermal expansion (CTE) between that of the low CTE ferrule  68  and a high CTE outer metal casing  84 . The outer metal casing  84  encloses the signal detection system  64  and forms the sensor system  60 . In one embodiment, the metal casing  84  may extend to entire perimeter of the signal detection system  64  and the sensor section  62 . In one embodiment, the strain buffer material  80  is silicon nitride. The metal bond material  82  in one embodiment is deposited on the strain buffer material  80  and the ferrule  68  through a standard metallization process. In one example, the metallization process may be evaporation, sputtering or electroplating. In another embodiment, the composition of the bond material  82  is gold, platinum or alloys containing one high melting point element. The strain buffer material  80  in this example is attached to the metal casing through a braze layer  86 . In one embodiment, the braze layer  86  may be an active brazing alloy. In another embodiment, the strain buffer  80  is metallized to facilitate brazing. In yet another embodiment, a material such as nickel may be used for metallization. 
     It should be noted here that material choices for the substrates and coatings are important for limiting the cavity gap variation due to temperature. If the cavity gap changes due to temperature, it becomes more difficult to differentiate between the cavity gap variations due to pressure and the cavity gap variations due to temperature. Thus, in one embodiment, low coefficient of thermal expansion (CTE) materials such as quartz/fused silica may be used for substrates and coatings to form the cavity gap. This minimizes the intrinsic temperature coefficient of the cavity gap over the extended operating range of the sensor. 
       FIG. 4  is another embodiment of the high temperature pressure sensor system  100 . The high temperature pressure sensor  102  of sensor system  100  is similar to the sensor  62  of  FIG. 3 . However, the middle interface substrate  72  of  FIG. 3  is eliminated in sensor  102 . The advantage of this sensor design is it reduces the divergence of light and minimizes the possibility of creating a second fabry-perot cavity. It also removes a bonding step from the assembly process of the sensor. In this embodiment, a hole  104  is formed inside the ferrule  68 . The hole  104  is covered during the vacuum bonding process where the strain buffer material  80  is attached to the ferrule  68 . It should be noted here that in one embodiment, the sensor  102  may be used to measure an absolute pressure. In one embodiment, the vacuum bonding process is performed to eliminate temperature expansion of any gas trapped in cavity gap  76 . In another embodiment, the cavity gap is about 1.25 microns and the thickness of the metal coating is about 175 nm. In yet another embodiment, the thickness of the second substrate  74  is 300 microns and the diameter of the etched cavity  76  is 1800 microns to create a 100 nm deflection at a pressure of 250 pounds per square inch (psi). It should be noted here that these are exemplary parameters and can be modified depending on the desired pressure range of the sensor. 
     As described earlier in  FIG. 1 , in one embodiment a light signal is passed through the optical fiber  66 . A part of the light signal hits the glass fiber—air interface and returns back as the first reflected signal R 1 . A second part of the light further hits the cavity gap—metal coating interface and returns back as the second reflected signal R 2 . The cavity gap  76  varies with the diaphragm  74  or the quartz substrate deflection, which in turn varies with applied pressure. The second reflected signal R 2  changes according to the variation in the cavity gap. The reflected signals are detected by a detector and analyzed to measure the pressure. 
       FIG. 5  describes a process  120  of manufacturing the sensor  102  of  FIG. 4 . At step  122 , a first quartz wafer  124  of the sensor  102  is formed. The thickness of the quartz wafer depends on the relationship between the pressure and defection. At step  126 , the wafer  124  is patterned such as by using standard photoresist and lithographic processes with the geometry of the diaphragm. The quartz wafer  124  is then etched to define the diaphragm diameter and the cavity gap depth. In one embodiment, a buffered oxide etching or reactive ion etching is used to etch the quartz wafer  124 . At step  128 , a thin metal reflective coating  130  is deposited onto the wafer diaphragm covering at least a portion of the diaphragm and in some examples the entire diaphragm surfaces including the sidewalls. In one embodiment, the deposition process includes an evaporation process or a sputtering process. The metal coating in one example is patterned such that it only remains in the center portion of the diaphragm. In one embodiment, the patterning of the metal coating is performed by reactive ion etching process or by a lift-off process. As will be appreciated by those skilled in the art, the lithography process or the etching processes described here are exemplary one and other similar processes are in scope of the present invention. 
       FIG. 6  describes the process  150  of manufacturing the sensor  62  of  FIG. 3 . The process  150  is similar to the earlier process  120  of  FIG. 5 . However, an additional step  152  is incorporated in this process. As described earlier, the sensor  102  is formed at step  128 . A second quartz substrate  154  is then thermally bonded to the quartz wafer using chemically activated quartz bonding techniques or quartz laser welding techniques. Thus, forming the sensor  62  comprising the first quartz substrate  124 , the metal coating  130  and the second quartz substrate  154 . 
       FIG. 7  describes one exemplary process  170  for assembly of the optical fiber-ferrule structure. At step  172 , the fiber  66  is inserted into the ferrule  68  such that a small distance protrudes from the opposing front end. A laser welding or an adhesive attachment process is then used to fix the fiber  66  to the rear portion of ferrule  68 . The protective strain buffer  80  is slipped over the fiber and temporally attached to the rear surface of the ferrule. The protective strain buffer  80  strengthens the assembly for subsequent steps in the process. In step  174 , a laser is used to form a ball of melted glass  175  from the protruding fiber and attach the fiber  66  to the front face of the ferrule. In one embodiment, instead of melting the fiber, a bonding glass with similar coefficient of expansion and refractive index may be melted to form the joint between the protruding fiber and ferrule. In step  176 , the front face  177  of the ferrule is polished such that ferrule surface and fiber surface are coincident. In step  178 , the sensor section  62  is attached to the fiber-ferrule structure through a laser welding process or a chemically activated bonding process. 
       FIG. 8  is one example of a 3-dimensional view of the entire assembly  180  of the high temperature pressure sensor system  60  of  FIG. 3 . An optical fiber  182  is passed through a flexible conduit  184  to a sensor assembly section  186 . The sensor assembly section  186  includes a quartz diaphragm  188  that is fixed to a top side  190  of ferrule  192 . As described earlier in  FIG. 3  the fiber  182  is fixed inside ferrule  192  and a strain buffer  194  is attached to one side of the ferrule  192 . The strain buffer  194  is then attached to a metal housing  196  through a braze layer  198 . 
     While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.