Abstract:
An improved optical pressure sensor determines the pressure of the fluid to be monitored by the deflection of a diaphragm in the pressure chamber of the sensor which has an inlet from the measured vessel. The deflection of the diaphragm is determined by monitoring the interference of diode light reflected from the diaphragm and a silicon grating structure superimposed over the diaphragm, at critical positions. Intensity detectors are placed at critical positions such as the specific orders of the diffraction grating to measure the interference intensity of the reflected light. The interferometric accuracy with which the pressure measurement is made with the present invention far exceeds that obtained with optical pressure sensors described in the prior art.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 60/094,533, filed Jul. 29, 1998. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of micromachined pressure sensors, and particularly to micromachined pressure sensors that incorporate diffractive optics as a means of sensing the pressure. 
     BACKGROUND OF THE INVENTION 
     Several varieties of optical pressure sensors are comprised of micromachined sensors or components that serve as prior art to this invention. For example, U.S. Pat No. 4,620,093 describes an optical pressure sensor designed for rocket engine applications. This prior art optical pressure sensor incorporates a micromachined diaphragm etched with a diffraction grating to measure the changes in central order and first order diffracted beams of impinging laser light to monitor the effects of pressure and temperature. Additionally, U.S. Pat. No. 4,932,262 describes a pressure sensor comprising two micromachined elements, one of which is a diaphragm, which sandwich an optical fiber coupled to an optical grating. The variations monitored in the output of the fiber reflect the pressure variations induced on the diaphragm. 
     U.S. Pat. No. 5,101,664 describes a prior art optical pressure transducer than is comprised of a micromachined flexible silicon diaphragm fixed to a silicon bridge. The bridge is driven at its resonant frequency which is dependent upon the pressure applied to the silicon diaphragm. Measurements of the bridge&#39;s vibrations are made using interferometric techniques. U.S. Pat. No. 5,105,665 also describes prior art based on the vibrating bridge fixed to a diaphragm style of optical sensor. This prior art also uses optical means to interpret the pressure on the diaphragm from the vibration of the bridge. U.S. Pat. No. 5,152,173 also provides prior art of an optically excited and detected micromachined pressure sensor comprised of a vibrating beam mounted on a diaphragm. 
     The prior art listed above are all optically based pressure sensors which rely on simple intensity reflectance measurements to determine the pressure opposite the deflecting diaphragm. This limits the sensitivity of pressure measurements which can be obtained, in contrast to significantly more sensitive optical pressure measurements that can be obtained by detecting and analyzing optical interference patterns. 
     U.S. Pat. No. 5,808,210 describes a prior art resonant microbeam pressure sensor that provides frequency based measurements corresponding to strain on the diaphragm supported microbeam. Although the microbeam is integrated into the diaphragm to which external pressure to be measured is applied, the sensing mechanism relies on detecting and accurately measuring the resonant frequency of the microbeam. 
     In addition to the optically and frequency based sensors described above, there are a variety of micropressure sensors based in semiconductor materials which rely on piezo electric or piezo resistive properties to measure pressure. The limiting factor in piezo based pressure sensors is their inability to be used in applications in which there are electromagnetic fields present or in applications which risk explosion due to their electrical nature. It is in these areas that an improvement to the prior art is clearly needed. 
     OBJECTS OF THE INVENTION 
     It is therefore desirable to provide an improved optical pressure sensor system which utilizes micromachined diffraction grating components capable of significantly more sensitive pressure measurements. 
     It is also desirable to provide an improved optical pressure sensor system which utilizes micromachined diffraction grating components capable of application in the presence of electromagnetic fields and explosion risks. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved optical pressure sensor over the prior art, by relying on a significantly more sensitive optical interference measurement to determine the desired pressure. The pressure of a fluid in a vessel for which pressure is to be monitored (measured vessel) is related in the present invention to the deflection of a diaphragm in a pressure chamber of the sensor which has an inlet for fluid transfer from the measured vessel. The deflection of the diaphragm is determined by monitoring the interference at critical positions of coherent light reflected from the diaphragm and from a grating structure superimposed over the diaphragm. Intensity detectors are placed at critical positions such as the specific orders of the diffraction grating to measure the interference of the reflected light. The interferometric accuracy with which the pressure measurement is made with the present invention far exceeds that obtained with optical pressure sensors described in the prior art. The interferometric detection is sensitive to motion equivalent to fractions of the wavelength of light used in the system. 
     The present invention has direct application to sensitive pressure measurements in a variety of industrial situations. The present invention is a viable option as a pressure monitoring system for pipelines and storage tanks, whether remote or local. The digital nature of the measurement results could easily allow for remote monitoring or local storage of results for periodic downloading. In addition, the present invention is an ideal pressure monitor for automated packaging and manufacturing systems such as in the pharmaceutical and chemical industries. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features of the invention believed to be novel are set forth in the associated claims. The invention, however, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a perspective view of the diffractive pressure sensor system. 
     FIG. 2 is a perspective view of the micromachined diffractive pressure sensor. 
     FIG. 3 is a cross-sectional view of the micromachined diffractive pressure sensor. 
     FIG. 4 is an exploded view of the micromachined diffractive pressure sensor. 
     FIG. 5 is an orthogonal view of the diaphragm grating device. 
     FIG. 6 is a perspective view of the diaphragm grating device. 
     FIG. 7 is a cross-sectional view of the diffractive pressure sensor showing the deflection of the micromachined diaphragm. 
     FIG. 8 a  is a close up cross-sectional view of the diaphragm grating device in an undeflected position, and the diffractive light pathways. 
     FIG. 8 b  is a close up cross-sectional view of the diaphragm grating device with a deflected diaphragm, and the diffractive light pathways. 
     FIG. 9 a  is a close up cross-sectional view of an alternative embodiment of the diaphragm grating device, with undeflected rulings, an undeflected diaphragm and diffractive light pathways. 
     FIG. 9 b  is a close up cross-sectional view of an alternative embodiment of the diaphragm grating device, with deflected rulings, an undeflected diaphragm and diffractive light pathways. 
     FIG. 9 c  is a close up cross-sectional view of an alternative embodiment of the diaphragm grating device, with undeflected rulings, a deflected diaphragm and diffractive light pathways. 
     FIG. 9 d  is a close up cross-sectional view of an alternative embodiment of the diaphragm grating device, with deflected rulings, a deflected diaphragm and diffractive light pathways. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A micromachined diffractive pressure sensor system in accordance with the invention, is described below with reference to FIG. 1. A diffractive pressure sensor  100 , is comprised of a pressure inlet  102 , a pressure chamber  104 , a diaphragm grating device  106  described in detail below, a gradient index medium  108 , a primary output fiber  110 , a secondary output fiber  112 , and an input fiber  116 . The diffractive pressure sensor  100 , is connected to a multiple channel photodetector  114 , via primary and secondary output fibers  110  and  112 , respectively. Additionally, a diode laser source  120 , is connected to input fiber  116 , via a fiber optic connector  118 . Primary output fiber  110 , secondary output fiber  112 , and input fiber  116  will be collectively referred to as lightwave input and output means, since some embodiments of the invention permit the function of any or all of these to be carried out by a single multimode fiber or similar light carriage means, rather than by separate, discrete fibers. Similarly, while these are referred to as “fiber” in this disclosure, it is understood that any form of light carriage means suitable to the light carriage functions to be described are considered to be within the scope of this disclosure and its associated claims. FIGS. 2,  3 , and  4  show the details of the diffractive pressure sensor  100  with a perspective view, cross-sectional view, and exploded view, respectively. Referring to FIGS. 2,  3 , and  4 , diffractive pressure sensor  100  is comprised of pressure inlet  102 , which serves to connect the measured vessel/medium (not shown) for which pressure of its fluidic contents is to be measured, to diffractive pressure sensor  100 . Pressure inlet  102  transfers the pressure of the measured fluid medium (i.e. air, gas, or liquid) to pressure chamber  104  of diffractive pressure sensor  100 . Pressure chamber  104  toward the lower region of diffractive pressure sensor  100  is a sealed chamber, aside from pressure inlet  102 , shown to be cylindrical with a circular or oval cross-section in the preferred embodiment, but which can alternatively be constructed in any other cross-sectional shape such as a square or rectangle. The pressure chamber is topped by diaphragm grating device  106 . 
     Diaphragm grating device  106  is best described with reference to the exploded view of diffractive pressure sensor  100  in FIG. 4 as well as the plane and perspective views of diaphragm grating device  106  in FIGS. 5 and 6. Diaphragm grating device  106  is constructed of a material such as silicon. Its base is a micromachined diaphragm  422 . A micromachined grating support frame  426  extends up from, over the top of, and substantially parallel to micromachined diaphragm  422 . A micromachined diffraction grating  424  is substantially centered in support frame  426 , and is also substantially parallel to the diaphragm  422 . 
     The operating principles of the diffractive pressure sensor system are best described initially with reference to FIG.  1 . As described above, the pressure of the fluid to be measured is introduced from the measured vessel (not shown) to pressure chamber  104  via pressure inlet  102 . Since it is a sealed cavity, the pressure inside pressure chamber  104  will be identical to that in the measured vessel (not shown). The pressure of the fluid transferred into chamber  104  deflects micromachined diaphragm  422  of diaphragm grating device  106 , as seen in FIG.  7 . To measure the pressure, light from diode laser  120  is introduced through input fiber  116 , and through gradient index medium  108 , to impinge on diffraction grating  424 , as well as diaphragm  422 , of diaphragm grating device  106 . Alternatively, any coherent light source may be used in place of the diode laser  120 . 
     The preferred embodiment of the present invention includes an integral path from input fiber  116  to diaphragm grating device  106  and back to primary and secondary output fibers,  110  and  112  respectively, by way of gradient index medium  108 . In the preferred embodiment, gradient index medium  108  is fabricated to provide the necessary imaging pathways among input fiber  116 , output fibers  110  and  112 , and the diaphragm grating device  106 . Gradient index media  108  is an integrated optical component that utilizes ion induced changes in index of refraction to create optical channels or buried waveguides through a solid optical component. This technique allows for the construction of simple to sophisticated optical circuits in a single integrated optical component. Alternative embodiments or substitutes for gradient index medium  108  may include but is not limited to a separate discrete optics lens system. However, the gradient index medium  108  is preferred due to its low cost, reliability, and robust construction. Integral construction of gradient index medium  108  creates a more durable, and therefore, more reliable sensor system. 
     Referring to FIG. 7, the fundamental optical principle on which the sensor system operation is based is to utilize the interference of light reflected from the grating rulings  728  of micromachined diffraction grating  424 , and from its underlying surface—namely the top surface of micromachined diaphragm  422 —to yield a diffraction pattern that is used to obtain a diaphragm deflection measurement with interferometric accuracy. In particular, diaphragm  422  is deflected towards or away from diffraction grating  424  in response to positive or negative pressure variations in pressure chamber  104 , introduced via pressure inlet  102  from the measured vessel. (FIG. 7 illustrates deflection toward the grating.) The diffraction pattern will thus change in a quantifiable way as micromachined diaphragm  422  is so-deflected toward or away from grating  424  because of these pressure variations. 
     In the preferred embodiment of the present invention, primary output fiber  110  is placed in a location to collect the interference between the central order of the diffraction pattern and the light reflected from the top surface of diaphragm  422 . Similarly, secondary output fiber  112  is placed in a location to collect the interference between the first order of the diffraction pattern and the light reflected from the top surface of diaphragm  422 . The primary and secondary output fibers  110  and  112  respectively, transmit the collected light interference to multiple channel photodetector  114 , to sense the interference intensity at the two positions. By comparison of the relative interference intensities measured at the specific order positions, a diaphragm displacement value and subsequently an applied pressure measurement can be determined. There are established relationships known in the art to determine the pressure on various shaped diaphragms based on their measured center deflections, as a function of Young&#39;s modulus and Poisson&#39;s ratio, such as are described in H. Chau and K. D. Wise, IEEE Trans. Electr. Dev., v. ED-34, #4, 1987, p. 850-858. 
     The design parameters of diaphragm grating device  106 , including the thickness of micromachined diaphragm  422 , the distance between diaphragm  422  and diffraction grating  424 , the width of grating rulings  728 , and the spacing between grating rulings  728 , all contribute to design parameters of the entire diffractive pressure sensor  100  and its overall performance. Specifically, the positioning of primary and secondary output fibers  110  and  112  respectively, are dependent upon the expected location of the central and first order of diffracted light produced by diffraction grating  424 , which is wholly dependent upon the design parameters of diffraction grating  424  as mentioned above. 
     FIGS. 8 a  and  8   b  specifically show the lightpaths produced by the components of the diaphragm grating device  106 . Incoming light will strike and be reflected from either an individual grating ruling  728 , or diaphragm  422 . An illustrative light ray striking grating ruling  728  is denoted by  830 , and an illustrative light ray striking diaphragm  422  is denoted by  832 . The light  830  striking grating rulings  728  is reflected into ruling-reflected zero order light  834  and ruling-reflected first order light  838 . Similarly, the light  832  striking diaphragm  422  is reflected into diaphragm-reflected zero order light  836  and diaphragm-reflected first order light  840 . FIG. 8 a  shows the light paths for an undeflected diaphragm  422  and FIG. 8 b  shows the light paths for a deflected diaphragm  422 . Although the paths appear similar, the interference intensity at zero order and first order positions changes with the deflection of diaphragm  422 , and this is what is used as the basis for very precise pressure measurements. 
     More specifically, the displacement of diaphragm  422  defines the specific interference intensity of light that is collected at the central and first order positions by primary and secondary output fibers  110  and  112 , respectively. At these locations, reflections of light from grating rulings  728  and reflections of light from diaphragm  422  will interfere with each other. By the nature of the design, the angle of incidence, and operating principles of diffraction gratings in general, when the distance between the reflecting surface of rulings  728  and the reflecting surface of diaphragm  422  is an even number of quarter-wavelengths of impinging light  830 ,  832  from diode laser  120 , the interference intensity in the zero order is at a maximum. Similarly, when the distance between the reflecting surface of rulings  728  and the reflecting surface of diaphragm  422  is an odd number of quarter-wavelengths of impinging light from diode laser  120 , the interference intensity in the zero order is at a minimum as the intensity is redistributed to higher orders. To minimize error induced by fluctuations of diode laser  120 , in the practical implementation of this pressure sensor system, the relative intensities, I R , of the interference at the primary and secondary fibers,  110  and  112  respectively, which collect the interference intensity at the central and first order positions, respectively, are both measured and compared. The most accurate comparison is made when the relationship I R =(I 0 −I 1 )/(I 0 +I 1 ) is calculated. Where I 0  is the measured interference intensity at the central or zero order location, as collected by the primary output fiber, and I 1  is the measured interference intensity at the first order location, as collected by the secondary output fiber. Typically, the sum of I 0  and I 1  is a constant, regardless of the degree of deflection of the membrane. 
     Since the majority of the light will be reflected into the central and first order positions in any configuration used, the preferred embodiment described above is generally sufficient for most applications and is the most useful, cost-effective design for the present invention. However, it would be obvious to someone of ordinary skill, and is considered to be within the scope of this disclosure and its associated claims, to expanding the output measurement to include higher diffractive orders as well. To do so, alternative blazes of diffraction grating  424  are incorporated into the design of the sensor in order to include the measurement of the higher diffractive orders, wherein “blazing” a grating constitutes reshaping individual beam elements in order to maximize the intensity diffracted into a specific order. 
     An alternative embodiment of the present invention includes alternative configurations of diaphragm  422  of diaphragm grating device  106 . To broaden the pressure range measurement capability of pressure sensor  100 , diaphragm  422  is alternatively constructed with variations in thickness. This thickness variation can be embodied as an incremental increase in thickness across the entire diaphragm  422 , or as individual localized elements embedded in the diaphragm  422 . Such thickness variations in diaphragm  422  would increase the pressure measurement range since a thinner diaphragm  422  will deflect more than a thicker diaphragm  422  under the same pressure. This extension of the range would be accounted for in the calculation of pressure by a calibration factor specific to the area of diaphragm  422  used for the measurement. 
     Another alternative embodiment of the invention involves replacing the multiple output fibers ( 110  and  112 ) with a single multimode output fiber through which the interference intensity collected at each order position can be transmitted simultaneously. The plane of the grating is imaged onto the input end of the multimode fiber to allow the multiple order transmission. Multimode fiber transmits light along different modes or pathways within the fiber based on angle of input into the fiber. At the output end of the fiber, the various modes can be differentiated by the angle of output. This configuration minimizes the effect of temperature, stress and bending of the fiber on the results since only a single fiber is used. 
     Alternatively, the multimode fiber described above can be used as a single lightwave input and output means to replace all of the fibers, both input and output (namely,  116 ,  110 , and  112 ). In this configuration, the multimode fiber would provide the illumination and collect the output simultaneously. 
     An additional alternative embodiment of the invention provides an alternative means of implementing the diaphragm grating device portion of the system utilizing an electronically reconfigurable diffraction grating. This alternative embodiment only requires the use of a single output fiber. This alternative configuration replaces the interference intensity measurements collected from the zero order and first order positions with successive interference intensity measurements of any single order position with the grating in an undeflected position and also in a deflected position. This configuration simplifies the measurement process by allowing the interference intensity at any order to be collected by the single output fiber, but the sensitivity of the measurement remains the same since it is still an interference based measurement. In addition, the measurement remains differential, insensitive to fluctuations of the laser power and properties of the fiber used for transporting the light from the laser to the sensor. Referring to FIGS. 9 a ,  9   b ,  9   c  and  9   d , the reconfigurable grating in this embodiment comprises the same series of rulings  728  as the preferred embodiment, but also a corresponding set of electrodes  944  directly under the rulings on the surface of the diaphragm  422 . In this configuration, as in the preferred embodiment, impinging light  946  strikes grating rulings  728 . Simultaneously, diaphragm-impinging light  948  strikes the surface of diaphragm  422 . Ruling-diffracted light  950  in any single diffraction order interferes with diaphragm diffracted light  952  at the face of the output collection fiber. The interference of this collected light is transferred to the photodetector, as shown in FIG. 1 for the preferred embodiment. 
     The relative interference intensity, I R , collected at the selected order position is calculated according the identical equation as in the preferred embodiment, I R =(I 0 −I 1 )/(I 0 +I 1 ). However, in this embodiment, I 0  is equal to the interference intensity collected by a single output fiber in the specified order position when the grating rulings  728  are in a initial, undeflected position (as represented in FIGS. 9 a  and  9   c ). Similarly, I 1  is equal to the interference intensity collected by the single output fiber in the specified order position when the grating rulings  728  are in a deflected position (represented in FIGS. 9 b  and  9   d ). This deflection, for each grating ruling, is caused by application of a predetermined DC voltage to its corresponding electrode  944 . This relative intensity measurement, I R , is taken when diaphragm  422  is in an initial, undeflected or unpressurized position (represented by FIGS. 9 a  and  9   b ), and compared to the relative intensity measurement, I R , taken when the diaphragm  422  is at a secondary, deflected or pressurized position (represented by FIGS. 9 c  and  9   d ). This provided the basis for calculating the deflection of diaphragm  422 , and hence fluidic pressure in pressure chamber  104 . 
     This alternative embodiment can be implemented with several variations. For example, the electrodes  944  under the grating rulings  728  can be segmented, as shown in FIGS. 9 a  through  9   d , or a continuous solid electrode that is under all rulings and the spacing between (not shown in Figures). Alternatively, the electrodes  944  can be under a periodic series of rulings  728  (for example, every other ruling, or every third ruling, etc.) to create a desired diffraction pattern for a specific application. The final alternative includes a variation in which a solid electrode is under all rulings and a sound frequency AC voltage is applied to the electrode. This would cause similar oscillations in the intensity and thus output to the photodetector  114  as the separate DC voltage application described above. 
     While only certain preferred features of the invention have been illustrated and described, many modifications, changes and substitutions will occur to those skilled in the art. It is, therefore, to be understood that this disclosure and its associated claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.