Abstract:
Ultra-miniature surface-mountable optical pressure sensor is constructed on an optical fiber. The sensor design utilizes an angled fiber tip which steers the optical axis of the optic fiber by 90°. The optical cavity is formed on the sidewall of the optic fiber. The optical cavity may be covered with a polymer-metal composite diaphragm to operate as a pressure transducer, Alternatively, a polymer-filled cavity may be constructed which does not need a reflective diaphragm. The sensor exhibits a sufficient linearity over the broad pressure range with a high sensitivity. The sensitivity of the sensor may he tuned by controlling the thickness of the diaphragm. Methods of batch production of uniform device-to-device optical pressure sensors of co-axial and cross-axial configurations are presented.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This Utility patent application is a Continuation-In-Part of application Ser. No. 12/849,436 filed on 3 Aug. 2010, currently pending. 
    
    
     The U.S. Government has certain rights to the invention. The work was funded by NSF Contract No. CMMI0644914. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to microsensors, and more in particular, to ultra-miniature pressure sensors formed on optical fibers. 
     In overall concept, the present invention is directed to an ultra-miniature pressure sensor system in which a sensor element (sensing head) features an optical cavity structure formed on an optical fiber. 
     Additionally, the present invention relates to a surface-mountable fiber-optic ultra-miniature optical pressure sensor that utilizes internal reflection at an angled fiber end surface to steer the optical axis by 90° in order to redirect the light travelling along the optical guide channel to impinge on the fiber sidewalls at a location where an optical cavity structure is formed. The optical cavity structure may include a Fabry-Perot cavity, or any other air-filled cavity covered with a polymer-metal composite diaphragm serving as a pressure transducer to detect pressure whose direction is perpendicular to a surface of interest. Alternatively, the optical cavity structure may be fabricated in the form of a polymer-filled optical cavity in which the interface between the polymer and the air functions as the diaphragm of the air-filled cavity. 
     The present invention is further directed to ultra-miniature pressure sensors which are suitable for space-constrained biomedical applications that require minimally invasive, in vivo pressure monitoring of body fluids, and are envisioned in a broad range of highly sensitive pressure measurements in a single sensing head structure or as sensing networks capable of interrogating an array of sensing heads in question. 
     Additionally, the present invention is directed to a fabrication method for a surface-mountable ultra-miniature fiber-optic pressure sensor system having high sensitivity which may be used as miniature microphones for various surveillance and industry applications as well as for aerodynamic measurements without disturbing a measurand while being electromagnetic interference resistant. 
     The present invention is further directed to a simple and inexpensive batch production of pressure microsensors which yields high device-to-device uniformity. 
     BACKGROUND OF THE INVENTION 
     Fiber optic sensors are widely used due to their light weight, miniature dimensions, low power consumption, electromagnetic interference resistance, high sensitivity, wide bandwidths and environmental ruggedness in combination with low cost and well developed fabrication techniques. Among fiber optic sensors, Fabry-Perot based pressure transducers are widely used for localized measurements free of a measurand disturbance. This type of sensor detects changes in optical path length induced by a change in the refractive index or a change in physical length of the Fabry-Perot cavity. The Fabry-Perot sensors are attractive for their miniature size, low cost of the sensing element, and compatibility with low coherence light sources, such as light emitting diodes. 
     In Fabry-Perot cavity based sensors, pressure is measured by detecting deflection of a membrane to which the pressure is applied. By using the optical measurements, a remote data acquisition may be achieved without loss of signal-to-noise ratio. 
     Shown in  FIG. 1 , is a co-axial configuration of fiber pressure sensor which has a Fabry-Perot cavity  10  formed at the end of the optical fiber  12 . The cavity  10  is surrounded by a housing  14  and is covered by a diaphragm  16 . In this co-axial configuration, the optical characteristics of the light traveling along the fiber optical guide channel  18  are responsive to the pressure field having a direction parallel to the optical axis of the fiber. These co-axially configured fiber sensors are not surface-mountable. 
     It would be highly desirable to combine the attractive characteristics of the fiber based ultra-miniature pressure sensors with the ability of being surface-mountable to sense and measure pressure fields directed perpendicularly to the surface. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a surface-mountable ultra-miniature fiber-optic pressure sensor system capable of optical measurements of pressure fields directed perpendicular to the surface of interest. 
     It is a further object of the present invention to provide a unique fabrication technique for a surface-mountable ultra-miniature fiber-optic pressure sensor system where each sensor element features an optical cavity structure formed externally to the fiber sidewall in cross-axial relationship with the optical axis of the fiber. 
     It is another object of the present invention to provide a fiber optic pressure sensor system in which a surface-mountable sensing head has an angled fiber end surface utilized to redirect the light traveling within the optical fiber to impinge onto the sidewall at a location where the optical cavity structure is formed for sensing a pressure field applied thereto. 
     It is also an object of the present invention to provide a fabrication technique which permits batch production of ultra-miniature fiber-optic pressure sensors, both co-axial and surface-mountable, that yields high device-to-device uniformity in an efficient fashion. 
     It is another object of the present invention to provide a biologically compatible ultra-miniature fiber-optic pressure sensor system suitable for low-invasive measurements in medical in vivo applications. 
     It is an additional object of the present invention to provide a Spatial-Division-Multiplexing (SDM) sensor network capable of high performance interrogation of a plurality of ultra-miniature fiber-optic surface-mountable pressure sensors. 
     In one aspect, the present invention is directed to a pressure sensor system, which utilizes one or more ultra-miniature surface-mountable sensing heads, each formed on an optical fiber with the tip contoured with an end surface angled relative to the longitudinal axis of the optical fiber to provide total internal reflection of the incidence light. The angled end surface, optionally, may be covered with a reflective material to enhance the reflection. 
     The operation of the sensing head is based on an optical cavity structure which is located externally on the sidewall of the optical fiber and may have a diaphragm covering the air-filled cavity. A specific location of the optical cavity structure at the sidewall of the fiber is defined at a spot where the light traveling along the optical guide channel of the optical fiber and being redirected by the angled end surface, impinges on the sidewall of the optical fiber. 
     The pressure applied to the diaphragm on the top of the optical cavity structure changes the optical characteristics of the light signal traveling in the optical fiber, and thus may be detected by a light detector coupled to the fiber output optical signal. The detected output light signal is further processed to transform the changes in the optical signal characteristics into an applied pressure. 
     The optical cavity structure may include an air-filled optical cavity defined in a cavity confining layer formed externally of the fiber sidewall and a polymer-metal composite diaphragm covered on the top of the optical cavity, or a polymer filled cavity which does not require a reflective diaphragm since the interface between the polymer and the air ambient serves the purpose. 
     The composite polymer-metal diaphragm on the air-filled optical cavity is a thin membrane of nanometer-scale to micrometer-scale uniform thickness which may be covered with one or more additional polymer layers to enhance mechanical stability, provide biological compatibility of the sensing head, as well as to fine tune the sensitivity of the sensing head by controlling the number of additional polymer layers on the diaphragm. In the embodiment based on a polymer-filled optical cavity, the interface between the optical cavity and ambient air acts as a reflective layer due to the refractive index difference therebetween. Additional metal layer or optical coating can be used to increase reflectivity at the interface, and to block unwanted light from ambient. 
     The cavity confining layer formed on the sidewall of the optical fiber may be of a photoresistive nature. Alternatively, the optical cavity may be formed in a jacket of the optical fiber, or in a UV-curable composition deposited on the sidewall. A reflective layer is preferably positioned at the bottom of the optical cavity to enhance visibility of an optical signal by increasing reflectance of the outer surface of the optical fiber. 
     The sensing head is surface-mountable and may be installed on a substrate to sense the pressure field applied to the diaphragm in a direction perpendicular to the substrate surface. A light source generating an optical input signal coupled to the input end of the optical fiber, a light detector to detect the optical signal emanating from the optical fiber, and a signal processing unit coupled to the light detector for determining the characteristics of the optical output signal are provided in systems using a single sensing head or in sensor networks. 
     Another aspect of the present invention constitutes a method for manufacturing a surface-mountable ultra-miniature fiber-optic pressure sensor, which is carried out by the steps of: 
     contouring a tip of an optical fiber with an end surface angled relative to the optical guide channel; 
     optically coupling a light beam to an input end of the optical fiber in order that the light beam travels from the input end to the tip of the optical fiber along the optical guide channel and is re-directed at the reflective angled end surface toward the sidewall of the optical fiber; and 
     fabricating an optical cavity structure at the spot where the redirected light impinges on the sidewall of the fiber. The optical cavity structure may be formed as an air-filled cavity with a diaphragm covering a top of the cavity, or as a polymer-filled cavity which does not necessitate a diaphragm. 
     There are several alternative approaches, to carrying out the fabrication of the subject sensing head. In one of the approaches the cavity may be formed within the fiber jacket. In this case, the laser beam is transmitted from the input end of the optical fiber to the angled reflective surface, and the location of the optical cavity structure is found by sensing the laser beam emanating through the sidewall of the optical fiber. At the location where the laser beam is sensed, the optical cavity structure is formed by a subtraction technique applied to the fiber jacket, including laser ablation, wet and dry etching, or photoresist developing technique. 
     Alternatively, the optical cavity structure may be formed in a photoresist covering the sidewall of the optical fiber. In this embodiment, prior to the contouring the tip end of the optical fiber with the reflective angled surface, the fiber jacket is removed, the exposed sidewall is covered with a reflective material, and a photoresist layer is deposited on the sidewall reflective layer. Subsequently, the UV light beam is coupled to the input end of the optical fiber, and, being redirected at the reflective angled end surface, is incident on the sidewalls of the optical fiber, in order to UV pattern the photoresist layer. The photoresist layer is a positive resist, and when exposed to light, becomes soluble to a photoresist developer. The patterned photoresist layer is further exposed to a developer to remove the soluble matter, thereby creating a cavity within the photoresist. 
     In a further embodiment, the fiber jacket is peeled from the optical fiber, a reflective layer is deposited on the exposed sidewall, and a predetermined volume of a UV-curable material is deposited on the reflective layer. A mold contoured oppositely to the shape of the desired optical cavity structure is then aligned with the intended cavity location and is lowered into the UV-curable material. After the UV-curing is performed, the mold is removed. 
     Upon forming a cavity by any of these techniques, a composite diaphragm formed of a polymer diaphragm layer and a reflective diaphragm layer is covered on the top opening of the optical cavity. The polymer diaphragm layer is pre-formed separately to have a uniform thickness in a nanometer-scale range. 
     The present invention further is directed to a method of batch fabrication of a plurality of ultra-miniature fiber-optic pressure sensors, yielding high uniformity of device-to-device dimensional and operational characteristics. The batch fabrication includes the steps of:
         securing a plurality of optical fibers in a fiber holder (wafer) in aligned disposition each with respect to the other, and   polishing the tips of the optical fibers to form end surfaces.       

     The subject batch production method is applied to fabrication of both the cross-axial configuration and co-axial configuration of the sensing heads. For the cross-axial configuration, the tips of the optical fibers are polished to contour the tips with angled end surfaces. For the co-axial configuration, the tips of the optical fibers are polished to form end surfaces perpendicular to the optical axis of the fibers. 
     After the tips are polished, the subsequent steps of the subject method include:
         optionally depositing a reflective layer on the angled end surfaces of the optical fibers,   optionally depositing a sidewall reflective layer on the fibers sidewalls,   forming a photoresist layer on the sidewall (or the sidewall reflective layer) of the fibers,   coupling a light beam into the optical fibers at the input ends to pattern the photoresist at a location of the optical cavity structure to be formed, and   developing the patterned photoresist on fibers, thus simultaneously forming optical cavities at the sidewalls of the plurality of fibers.       

     Alternatively, for the mass production of the cross-axial and co-axially configured sensing heads, after polishing the end face either perpendicular to the optical axis of the fibers (for the co-axial configuration), or angled relative to the optical axis of the fibers (for the cross-axial configuration), the method further proceeds in the following fashion: 
     UV-curable material is deposited on the wafer to cover the polished tips of the optic fibers, 
     a mold contoured oppositely to the shape of the desired optical cavities is aligned with desired cavities locations and is lowered into the UV-curable material. 
     After the UV-exposure, the mold is removed, and the cavities formed either in the end surface of each fiber, or at the sidewall thereof, are covered with a polymer diaphragm, which is subsequently punched to result in a plurality of separate structures, each of which is covered with a reflection coating in a subsequent step. Finally, the fabricated sensors are released from the wafer. 
     Alternatively, upon forming the cavities at the fibers (either on the sidewalls or at the end face), the method further may include the steps of:
         pre-forming a thin polymer film of a uniform thickness (the thickness may range in nm to μm scale),   attaching the polymer film on the cavities of the plurality of optical fibers, thus forming polymer diaphragm layer, and   coating the reflective coating, i.e. the reflective diaphragm layer, on the patches of the polymer diaphragm layer on the cavities.       

     The reflective layer may be coated on the angled end surface, as well as on the sidewall and on the polymer diaphragm layer by any technique including sputtering, evaporation, electroplating, bonding or sticking. 
     These and other features and advantages of the present invention will be apparent from the following detailed description taken in conjunction with accompanying Patent Drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of a coaxial configuration of a conventional fiber-optic pressure sensor; 
         FIG. 2  is a schematic representation of a surface-mountable sensing head with an air-filled optical cavity structure of a cross-axial configuration of the present invention; 
         FIG. 3A-3B  represent schematically fabrication of a surface-mountable sensing head with a polymer-filled optical cavity structure of the present invention; 
         FIGS. 4A-4I  represent a sequence of the manufacturing steps of the present method in one embodiment thereof; 
         FIGS. 5A-5G  show in sequence the manufacturing steps of the present method in an alternative embodiment; 
         FIGS. 6A-6D  show in sequence the manufacturing steps of the present method in yet another alternative embodiment; 
         FIGS. 7A-7B  are prospective views of fabrication tools for batch production of the sensors of the present invention; 
         FIGS. 8A-8H  show in sequence the manufacturing steps of an alternative embodiment of the batch fabrication method in accordance with the present invention; 
         FIGS. 9A-9F  represent schematically a technique for compensation for deviation of the end surface from a predetermined angle; 
         FIGS. 10A-10B  show, respectively, a spectrum-based measurement scheme and a diagram representing cavity length vs. pressure of the manufactured pressure sensor; 
         FIG. 11  is a schematic representation of a sensor system using the sensing head of the present invention; and 
         FIG. 12  is a schematic representation of an integrated sensor network using an array of the sensing elements of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 2 , an ultra-miniature pressure sensor system  20  includes a sensing head  22  which due to its cross-axial configuration is uniquely designed to permit surface mountability on a substrate  24 . The sensing head  22  features an optical cavity structure  26 , for example, a low finesse Fabry-Perot cavity structure, formed externally at the sidewall  28  of the optical fiber  30 . 
     A diaphragm  32  of nanometer-scale—micrometer-scale thickness is covered on the air-filled optical cavity  34  of the optical cavity structure  26 . The diaphragm  32  may be formed as a thin polymer layer or as a composite of a polymer diaphragm layer  58  and reflective diaphragm layer  60 , shown in  FIGS. 4H-4I ,  5 G, and  6 D, as well as  8 G- 8 H, and  9 D- 9 F. The reflectivities of the polymer diaphragm layer  58  and the reflective diaphragm layer  60  are approximately 4% and 90%, respectively. As one example, a thin UV-curable polymer layer (e.g., negative photoresist) and a metal layer formed of Ag, Cu, Au, or Ti may be used to form the composite pressure sensing diaphragm  32 . The shape of the diaphragm  32  may be concave, convex, flat, aspherical, or combinations thereof. 
     The optical cavity  34  is formed within a cavity confining layer  36  as will be presented in detail in following paragraphs. The sensing head  22  is mounted on the surface of the substrate  24  to detect pressure  38  applied to the diaphragm  32  in a direction perpendicular to the substrate surface. 
     The interface between the optic fiber sidewall  28  and the air-filled optical cavity  34  provides about 4% reflection. In order to increase the reflectivity in this region (for a better performance of the sensor, e.g., a better visibility), a thin metal layer (less than 50 nm in the thickness), or an optical coating (i.e., alternative layers of dielectric materials)  72 , shown in  FIGS. 4D-4H ,  5 A- 5 G, and  6 A- 6 D, may be used to attain an increased level of reflectivity at the sidewall  28 . 
     The diaphragm thickness, cavity diameter and cavity length may be adjusted during the manufacturing process to fulfill the requirements of different sensitivity and measurement range in various applications. For example, the resulting cavity length may vary from several micrometers to several hundreds of micrometers, which may be adjusted based on the intended application of the sensor by controlling the thickness of the cavity confining layer  36 . 
     Alternatively, as shown in  FIG. 3A-3B , the surface-mountable sensor  20 ′ can be formed with an optical cavity structure  26 ′ which is based on a polymer-filled optical cavity  150 . In this embodiment, a polymer layer  152  is deposited on the optic fiber sidewall  28 . The polymer layer  152  may be made from a UV- or thermally-curable polymer with low elastic modulus. 
     The reflection  156  at the interface  158  between the optic fiber sidewall  28  and the polymer layer  152  is insignificant due to a low (&lt;0.1) refractive index difference between the materials of the sidewall  28  and polymer layer  152 . An optical coating  154 , such as a thin metal layer, or dielectric layers is used to obtain a desired reflectivity at the interface  158 . 
     An interface  160  between the optical cavity material  152  and the ambient air  162  may have a concave, flat, convex, aspherical, or combinations thereof shape, and acts as a reflective layer due to a high (˜0.3-0.5) refractive index difference therebetween. By changing the shape of the mold bottom surface  168 , the polymer layer can be shaped accordingly. 
     The embodiment of the sensor which is based on the polymer filled cavity is not required to be formed with a diaphragm as is provided for the sensor head shown in  FIG. 2 . Alternatively, a reflective layer  163 , for example, a metal layer, may be formed on the top of the polymer layer  152 . In this case, reflected light intensity may be increased. As an additional benefit, light incident to the polymer layer  152  from the ambient, can be blocked by the layer  163 . The layer  163  also can have flat, convex, concave, aspherical shape, or a combination thereof. 
     The subject ultra-miniature fiber-optic pressure sensor  22  may be used in array systems applicable in a wide variety of technology areas. For example, biologically compatible diaphragm materials may be used to produce the sensor suitable for space-constrained medical diagnosis and treatment applications that require minimally invasive, in vivo, monitoring of the pressures of blood, bones, joints, and bladder, etc. Further, the sensor system may be used for aerodynamic measurements without disturbing the measurand and being free of electromagnetic interferences/disturbances. 
     Sensors with a thin diaphragm in the range of nanometers have high sensitivity and may be used as miniature microphones for various surveillance and industry applications. The unique fabrication technique for the sensor development presented in detail infra, includes simple processes, safe procedures, and inexpensive materials, and does not require a clean room environment and associated equipment. 
     Referring to  FIG. 2 , the cross-axial configured sensing head  22  has an end surface  40  which is angled relative to the optical axis  42  of the optical fiber  30 , e.g. relative the optical guide channel  44  extending between an input end  46  and a tip  48  of the optical fiber  30 . 
     The end surface  40  is angled or inclined to provide the critical incidence angle for the light beam  52  in order to ensure a full reflection from the end surface. Preferably, the end surface  40  is angled at a 45° angle relative to the optical axis  42 . However, other angles are also envisioned in the subject sensor between the end surface  40  and the optical axis  42  as long as a critical incidence angle is provided for the light beam  52 . 
     A thin reflective coating  50  preferably in a film thickness range, optionally may be deposited at the end surface  40  to enhance reflection of light  52  traveling along the optical guide channel  44 . Since the end surface is angled relative to the optical axis  42 , the light beam  52  is redirected by 90° relative to the optical axis  42 . The redirected light beam  54  impinges on the sidewall  28  of the optical fiber at a location  56  which is defined as the location where the optical cavity structure  26  is formed, as will be described herein in further paragraphs. 
     As shown in  FIGS. 4H-4I ,  5 G and  6 D, the diaphragm  32  includes a polymer diaphragm layer  58  and a reflective diaphragm layer  60  covering the polymer diaphragm layer  58 . As shown in  FIG. 2 , when the redirected light beam  54  impinges on the diaphragm  32 , it reflects (as shown by the arrow  62 ) and is returned to the end surface  40  to be redirected to the input end of the optical fiber along the optical guide channel  44 . 
     The produced light signal constitutes an output optical signal  64  which is interrogated by a spectrometer  122  (or an optical spectrum analyzer), shown in  FIG. 10A , or a photo detector  123 , shown in  FIG. 11 , and discussed infra herein. The photo detector may be any detector which covers the wavelength range of the output optical signal. For example, a GaAs or InGaAs photo detector with the working range of 600-800 nm and 800-1600 nm may be used. 
     The optical characteristics of the output optical signal  64 , including intensity of the signal and spectrum parameters, depend on the length of the optical cavity. When the pressure  38  is applied to the composite diaphragm  32 , the diaphragm bends into the cavity  34 , thereby reducing the length L of the cavity, and thereby changing the optical path of the light. The change of the optical path will change the phase of the reflected signal  62 , and subsequently the optical characteristics of the output optical signal  64  detected by the spectrometer (or optical spectrum analyzer)  122  or the photo diode  123  and determined by a microcontroller (or microprocessor)  68  coupled to the photo diode, as shown in  FIG. 11 . The output optical signal characteristics are processed to produce the corresponding measure of the applied pressure. 
     With respect to the embodiment shown in  FIGS. 3A-3B , the variation in the thickness of the polymer layer  152  with respect to the pressure applied onto the interface  160  is measured by demodulating the interference signal of the light reflected from both interfaces,  158  and  160 . In terms of signal processing, the method used for the optical pressure sensor with a diaphragm detailed in further paragraphs may be applied. 
     When the pressure measurements require an absolute cavity length measurement, i.e., optical cavity length measurements through using wavelength spectrum, the thickness variation across the polymer layer  152  does not affect the measurements result. In this case, the optical length can be loosely controlled by controlling the volume of the polymer material  152 , shown in  FIG. 3A . 
     However, when using an optical measurement technique which needs a precise optical path length control, such as, for example, the intensity monitoring at the quadrature point, a molding method shown in  FIG. 3B  may be used. In accordance with which the optical cavity length, i.e., the thickness of the polymer layer  152 , is monitored while the mold  166  moves vertically. In this method, the interference of the light reflected from the reflective layer  154  and the bottom surface  168  of the mold  166  is used. 
     Referring to  FIGS. 4A-4I , in one of the embodiments of the current fabrication process, a tip portion of the optical fiber  30  ( FIG. 4A ) is peeled to remove the fiber jacket  70  as shown in  FIG. 4B . Subsequently, as shown in  FIG. 4C , the fiber  30  is polished to create an end surface  40 , and a metal layer  50  which is deposited on the polished and cleaned end surface  40 , as presented in  FIG. 4D , to enhance reflectivity at the boundary of the polished surface  40 . The sidewalls  28  of the fiber are covered with a thin reflective layer  72  (further referred to herein as the sidewall reflective layer) to enhance visibility of the optical signal by increasing the reflectance of the outer surface of the sidewall of the optical fiber. As shown in  FIG. 4E , a positive photoresist layer  74  is subsequently deposited on the reflective material  72  on the sidewalls  28  and the end surface  40 . The photoresist  74  may be coated by a dipping technique with a desired thickness. The thickness of the cavity  34  depends on the viscosity of the photoresist and the dipping conditions. Subsequently, the photoresist layer is soft baked and is exposed from the inside of the optical fiber to the UV light  76  which is focused on the input end  46  of the optical fiber  30 . 
     The optical fiber, specifically the optical guide channel  44 , and the angled end surface  40  are used to guide the UV light toward the sidewall  28 . As shown in  FIG. 4F , the UV light  76  is coupled to the input end  46  of the optical fiber to travel along the optical guide channel  44  towards the reflective angled end surface  40  where the UV light is steered 90°, and the redirected light  77  impinges on the sidewall  28  and the positive photoresist layer  74 . 
     Being exposed to the UV light  77 , the photoresist layer  74  changes its chemical characteristics at the area of exposure and becomes soluble to a photoresist developer which is subsequently applied to the photoresist layer  74  to remove the photoresist material (either by wet or dry etching), thereby forming the cavity  34  through the thickness of the photoresist layer  74  of predetermined length and diameter. As shown in  FIG. 4G , the cavity  34  is formed in the photoresist layer  74  which is further hard baked to solidify the remaining photoresist in order to produce a durable cavity confining layer. The diameter of the cavity is defined by a beam spot size and may range in μm range. The depth of cavity, depending on the photoresist may be, for example, in the range of 15-24 μm. 
     Referring to  FIG. 4H , the polymer diaphragm layer  58  having a uniform thickness is covered on the top  78  of the cavity  34 . The polymer diaphragm layer  58  is preformed by the process described further herein, and at the time of deposition on the top of the cavity  34  has adhesive properties sufficient to permit the securement of the diaphragm to the outside wall  28  of the optical fiber without contractions. The polymer diaphragm layer  58  is then cured by ultra violet light and is fixed firmly on the sidewall of the fiber. The polymer diaphragm layer  58  is then coated with a reflective layer  60 , shown in  FIG. 4H , by either sputtering, evaporation, electroplating, bonding, or sticking, thus completing the fabrication of the sensing head  22 . The reflective layer  60  (as well as reflective layers  50  and  72 ) is typically made of a metal material such as, but not limited to, gold, aluminum, silver, copper, nickel, chromium, etc. 
     As shown in  FIG. 4I , the sensor element, e.g., the Fabry-Perot cavity structure  26 , may be covered with additional polymer layers  80 . These additional layers of polymer render additional protection from the external environment and improve the biocompatibility of the sensor especially in biomedical applications which expose the sensors to the body fluids such as blood, urine, etc. Additional layers  80  also provide the ability to tune the stiffness of the multi-layered diaphragm  32  to address different requirements of sensitivities and pressure measurement ranges. The stiffness of the diaphragm  32  is mainly determined by the stiffness of the reflective diaphragm layer  60  since the metal material has much higher mechanical strength than the polymer material of the polymeric diaphragm layer  58 . However, adding more polymer layers  80  increases the stiffness in small steps and thus offers fine tuning of the sensitivity. 
     Referring to  FIG. 5A-5G , the subject surface-mountable optical pressure sensor  22  may be fabricated by a laser ablation. This is an alternative method of forming the cavity  34  on the outer surface of the optical fiber  30  in a simplified manner. Initially the optical fiber  30  having the jacket  70  (shown in  FIG. 5A ) is polished together with the jacket to form the angled end surface  40 , as shown in  FIG. 5B . The end surface  40  is subsequently covered with the reflective coating  50  shown in  FIG. 5C . Further, as shown in  FIG. 5D , the laser light  84  is guided through the optical fiber  30 . At the end surface  40  the laser light is redirected toward the sidewalls  28  and impinges thereon at the location  85  where the cavity  34  is to be fabricated. This location is found by sensing the redirected laser light  82  coming from the sidewall of the fiber by a photo detector  83  compatible with the laser used. Any laser producing light in the wavelength range of approximately 600 nm-800 nm may be used. For example, commercially available laser diodes generating in the range of 635-705 nm or 700-800 nm are suitable for this application. 
     Further, referring to  FIG. 5E , the cavity  34  is formed by removing the polymer material of the fiber jacket  70  selectively using laser ablation. As shown in  FIG. 5F , after contouring the cavity  34  by laser ablation, the reflective layer  72  is deposited which covers the surface of the fiber jacket  70  and the exposed sidewall  28  on the bottom of the cavity  34 . 
     In the following step, the polymer diaphragm layer  58  and the reflective diaphragm layer  60  are covered on the top  78  of the cavity  34 , as shown in  FIG. 5G , thus completing the formation of the sensing head  22 . 
     Referring further to  FIGS. 6A-6D , another alternative fabrication technique is carried out by making the cavity  34  by a UV molding process. The fiber jacket of the optical fiber is removed and cleaved, and the polishing and metal layer deposition are performed respectively to form the reflective end surface  40  and the sidewall reflecting layer  72  as is discussed supra and shown in  FIGS. 4A-4D . Subsequently, as shown in  FIG. 6A , the controlled volume of the UV-curable polymer  86  is dispensed at the sidewall in proximity to the tip of the fiber. The UV-curable material may be a polymer based on urethane acrylate, acrylate, epoxy, etc. 
     A mold  88  contoured opposite to the shape of the intended cavity is used in this process. The light  89  (for example, laser light) is introduced through the input end of the fiber which redirects at the end surface  40  covered with metal  50 , impinges on the sidewalls  28 , and escapes to the outside of the fiber through the UV-curable material  86 , as it is shown in  FIG. 6A . 
     The light beam emanating through the sidewall of the optical fiber is used for precise positioning of the mold  88 . The mold is fabricated with Si, glass, polymer, or metal with an anti-adhesion surface treatment, and is mounted on a high precision stage. If the mold  88  is formed of a transparent material, such as polymer or a glass, a reflective layer is coated on the mold to facilitate reflection of light. As shown in  FIG. 6A , the mold  88  is positioned to be in axial alignment with the emanating laser light by monitoring the intensity of the light  92  reflected from the end facet  90  of the mold  88 . When the mold  88  is precisely aligned relative to the optical axis of the emanating laser light at the position where the cavity is to be formed, the reflective signal  92  reaches its maximum that is detected by the photo detector (Power Meter)  66 . 
     After finishing the axial alignment, the mold  88  is lowered into the UV-curable polymer  86  to bring the end facet  90  in contact with the sidewall reflective layer  72 , as shown in  FIG. 6B . Subsequently, the UV-curable polymer  86  is cured by UV light. After the polymer  86  has been UV-cured, the mold  88  is removed, as shown in  FIG. 6C , thereby leaving the cavity  34  formed in the material  86 . As further presented in  FIG. 6D , the polymer diaphragm layer  58  and the reflective diaphragm layer  60  are deposited on the top  78  of the cavity  34 . 
     The fabrication technique for the subject pressure sensor embedded on the optical fiber is easily adapted for batch production that yields high device-to-device uniformity. 
     Referring to  FIGS. 7A-7B , a fiber holder  100  may be used to hold and align a plurality of optical fibers  30 . The fiber holder  100  may be used which includes both a bare silicon wafer  106  and a silicon wafer  102  wherein a plurality of grooves  104  are etched. In use, a plurality of optical fibers  30  with their tip portions cleared of the jackets  70 , are positioned in the grooves  104  in aligned and substantially parallel relationship each to the other and covered with the bare silicon wafer  106 . An adhesive, for example, a wax  108  may be used to stably secure the fibers  30  within the fiber holder  100 . The optical fibers are then polished at their tips to form angled end surfaces, for example, a 45° angle, which are cleaned and deposited with the metal layer. Further, the manufacturing steps shown in  FIGS. 4E-4G  are simultaneously performed for all the optical fibers held in the fiber holder  100  by dipping the tips of the fibers in the photoresist, UV-patterning and developing the photoresist in sequential order. 
     In order to form the polymer diaphragm layer  58  at the top of the cavities  34  formed externally on the sidewalls of the optical fibers, the polymer layer is pre-formed in a separate process. Deionized water is placed in a Petri dish. A hard ring holder  110  (shown in  FIG. 7B ) is placed in the Petri dish under the surface of the water. A controlled volume of a UV-curable polymer is dispensed onto the surface of the deionized water. The polymer floats over the water surface and spreads to form a thin film of polymer on the water surface. By observing the coloration of the polymer film and controlling the spreading time, the film thickness may be controlled with high accuracy. Using this technique, a very thin layer of the polymer is obtained. 
     The polymer layer is then pre-cured by UV light at a predetermined power density and for a predetermined time period sufficient to accomplish the pre-curing process, thereby forming the polymer layer  112 , that has a uniform thickness across the entire area. At this stage, the polymer layer  112  is strong enough to be removed from the water. 
     The hard ring holder  110  is lifted to be contagious with the polymer layer and then is removed from the water surface at a predetermined angle to lift the pre-cured polymer layer  112  out of the water and cover the fabricated cavities at the end of the fibers, as shown in  FIG. 7B . The hard ring holder  110  with the polymer film  112  is moved downwardly towards the fiber holder  100  while the fibers are pushed up to break the polymer film. As a result, each fiber end is covered with a patch of the polymer film. Since the polymer film has a uniform thickness across its entire area, the cavities  34  formed on the optical fibers, are covered with the polymer diaphragm layer of identical thickness. The polymer film at this stage has a viscosity which is sufficient to attach the polymer diaphragm to the sidewalls of the optical fibers without contractions. 
     Further, the diaphragms are cured by the UV light and thus are secured firmly on the sidewalls of the fibers. The polymer diaphragms are further coated with the reflective diaphragm layer  60  by sputtering, evaporation, electroplating, bonding, or sticking for all optical fibers simultaneously. The reflective material is a metal which includes gold, aluminum, silver, copper, nickel, chromium, etc. The batch production of the pressure sensors embedded on optical fibers includes well developed simple processes, safe procedures and inexpensive materials and does not require the use of a clean room environment and equipment. 
     An alternative batch fabrication technique is presented in  FIGS. 8A-8H . Although illustrated for the fabrication of co-axially configured sensors, this method is also applicable for batch production of the cross-axial configured sensors. Also, although illustrated for three fiber based sensors, the principles of the method presented in  FIGS. 8A-8H  are also applicable to a batch production of a larger number of sensors. 
     As shown in  FIG. 8A , the optical fibers  30  are secured in a holder  101  with mounting wax; and the end faces  31  of the fibers  30  are being polished. The fiber jacket may be left on the fibers. 
     Subsequently, as shown in  FIG. 8B , a controlled volume of UV-curable polymer (photoresist)  103  is dispensed on the wafer  101 . 
     A preformed mold  33  is contoured “opposite” to the shape of a plurality of cavities to be formed at the fibers  30 . The mold  33  may be fabricated with Si, glass, polymer, or a metal with an anti-adhesion surface treatment. The mold  33  is mounted on a high-precision stage (not shown) to provide precise alignment of “anti-cavity” structures  35  with locations of the end faces  31  of the fibers  30  in the wafer  101 . 
     After axially (horizontally) aligning the mold  33  with the end faces of the fibers, and lowering the mold into the polymer (photoresist)  103 , the latter is exposed to the UV light via a shadow mask  37  to pattern the photoresist  31  as shown in  FIG. 8C . 
     After the photoresist  103  is cured and the mold is removed, as shown in  FIG. 8D , coaxial cavities  34  are formed in the end faces of the fibers  30 . Subsequently, hard baking may be performed. 
     As shown in  FIG. 8E , a polymer diaphragm film  112  is covered on the tops of the cavities  34 . A micromachined punch (not shown) having a plurality of openings aligned with the end faces of the fibers is pressed down to remove the unnecessary portions of the polymer film  112  as shown in  FIG. 8F . Further, as shown in  FIG. 8G , a metal layer  114  is deposited on the diaphragms  112 , thus forming cavities  34  each covered with the composite diaphragm  32 . 
     Finally, the mounting wax is dissolved with heat or a solvent to complete the fabrication process to release the fabricated sensors  20 , as shown in  FIG. 8H . 
     As described in previous paragraphs, the end surface  40  of the optic fiber is angled relative to the axis of the optical guide channel in the optic fiber by a predetermined angle α (shown in  FIG. 2 ), to provide the critical incidence angle for the light incident on the end surface  40  to redirect the light with the least loss towards the fiber sidewall  28 . It is preferred that the end surface forming the angle α of substantially 45° relative to the axis  42  of the optic fiber, which enables a 90° redirection of the light towards the fiber sidewalls, thereby attaining optimal performance and surface mountability of the subject pressure sensor. 
     During the fabrication process, errors in the polishing of the end surface may lead to the angle α deviations from 45°. If the deviation of the angle α from 45° is too large (over several degrees), the power of the light emanating from the fiber sidewall diminishes exponentially due to large internal reflections. Also, with the angle deviation, the reflected light does not return to the end of the fiber core and be guided to optical waveguide. Therefore the reflected intensity will be diminished drastically due to this dominant factor. Therefore measures are taken to prevent a large deviation of the angle α from 45°. 
     However, small deviation (several degrees) from 45° may be compensated for in order that the overall performance of the resulting sensor does not degrade substantially. 
     As shown in  FIG. 9A , for the end surface angled at 45°, the mold  88  (used in the process shown in  FIGS. 6A-6D ), has the optical axis  91  extending in perpendicular to the optical axis  42  of the optic fiber  30 . However, when it is determined that the end surface  40  deviates from 45° (for example, the angle is 40°, as shown in  FIG. 9B , or 50°, as shown in  FIG. 9C ), and the light  93  reflected therefrom travels in the direction other than 90° relative to the optical axis  42 , in the cavity fabrication process, the mold  88  should be tilted, as shown in  FIGS. 9B-9C  to align the mold&#39;s axis  91  with the direction of the light  93  reflected from the end surface  40 . 
     The fabrication of the composite diaphragm  32  on the top of the cavity  34  for the case scenarios of the angle α of 45°, 40°, and 50°, is presented in  FIGS. 9D ,  9 E, and  9 F, respectively. It is important to ensure that the bottom  56  and the top  57  of the cavity  34  are strictly parallel for optional sensor performance, i.e. for high intensity level. If the parallelism between the bottom and top of the cavity is provided, then the light  93  reflected from the end surface  40  is directed to the cavity&#39;s bottom  56  and top  57  in the same direction thus attaining the highest reflectance level. 
     Referring further to  FIG. 2 , the cavity length L of the optical pressure sensor depends on the pressure  38  applied perpendicularly to the diaphragm  32  formed on the top of the cavity  34 . When the pressure is applied, the diaphragm deflects and the length of the optical cavity changes. The deflection of the diaphragm  32  is proportional to the pressure and may be measured by several optical signal processing methods including spectrum based measurements and intensity-based measurements. 
     The principles of the spectrum based measurements are shown in  FIG. 10A  where white light may be used as an input light source  116 . The white light  117  travels to the sensing head  22  through a 2×1 coupler  118 . In the fiber  30 , as shown in  FIG. 2 , the light travels along the optical guide channel towards the reflective angled end surface and is redirected (beam  54 ) toward the sidewall  28  of the optical fiber. At the sidewall reflective layer  72 , a portion  120  of the light beam is reflected, while another portion of the redirected light beam  54  passes through the sidewall reflective layer  72  and travels along the length of the optical cavity  34  and reflects (as shown by the arrow  62 ) at the boundary between the top  78  of the cavity  34  and the diaphragm  32 . Both reflected light beams,  120  and  62 , interfere with each other on their return inside the fiber, and when the spectrum of the output light signal  64  is scanned by a spectrometer (or an optical spectrum analyzer)  122 , some peaks would appear on the spectrum diagram. The cavity length is calculated by (Eq. 1): 
                   L   =         λ   1     ⁢     λ   2         2   ⁢     (       λ   2     -     λ   1       )                 (     Eq   .           ⁢   1     )               
where L is the cavity length, and λ 1 , λ 2  represent the wavelengths of two adjacent peaks found on the spectrum diagram.
 
     Since the wavelength measurement has high resolution (˜1 pm), the deflection of the diaphragm  32  may be measured with the resolution of 1 nm. Such a high resolution ensures that very small deflection of the diaphragm is detected with high precision.  FIG. 10B  represents a diagram of the cavity length vs. pressure. The spectrum based measurements have enhanced sensitivity, rather large dynamic range, and are insensitive to light source fluctuations. 
     An intensity-based measurement scheme (not shown in the drawings), uses a light source generating the light whose coherence length is longer than the cavity length. The light from the light source passes through a coupler into the input end of the sensing head. The interference between the light beams,  120  and  62 , shown in  FIG. 2 , causes intensity variation of the output optical signal with respect to the optical phase difference between lights  120  and  62 . The intensity variation is calculated by (Eq. 2),
 
 I=I   1   +I   2 +√{square root over (2 I   1   I   2 )}cos φ  (Eq. 2)
 
where
 
φ=4π nL/λ   (Eq. 3)
 
where L is the cavity length, I 1  and I 2  represent the intensity of the light reflected from the outer sidewall of the fiber and the diaphragm reflective layer, respectively, φ is a phase shift defined by refractive index n of the cavity medium, the cavity length L, and λ is the wavelength of the light generated by the light source. The intensity of the output optical signal  64  is measured by a power meter (photodiode). The deflection of the diaphragm  32  due to the outside pressure  38  thus may be measured with high accuracy.
 
     In order to correlate the applied pressure field with the optical characteristics of the output optical signal  64 , the sensor calibration is performed with the use of a reference sensor. To calibrate the manufactured sensor, any type of commercially available well calibrated pressure sensor may be used. In the present system, a Kulite sensor was used as a reference for static pressure calibration. Pressure reading was made from the reference sensor, and the cavity length was measured at the same time with the optical interrogation system shown in  FIG. 10A . From the calibration, the relationship between the cavity length and the pressure was attained. This relation was exploited in calculating actual pressure from the cavity length values. 
     Sensor calibration was performed on the fabricated cavity having length L approximately 14 μm with Ni/Ti coating on the diaphragm of 0.3 μm thickness. Linear relationship was obtained between optical power and static external pressure in the pressure range for calibration between 0-20 psi. A sensitivity of 0.009 μm/psi was attained with the sensor calibration. 
     The sensing heads  22  are envisioned as a part of a variety of sensor systems  20  using one or an array of the subject sensing heads. For example, as shown in  FIG. 11 , the sensor system  20  uses a single sensing head  22  on the substrate  24  optically coupled through a 1×2 coupler  134  to the optical MEMS (microelectromechanical) chip  135 . The MEMS chip  135  carries a SLED chip  132  to produce a light signal  133  which passes through a Fabry-Perot filter  136  and the coupler  134  prior to being optically coupled to the fiber  30  (light  52 ). The output optical signal  64 , which carries information on the pressure applied to the diaphragm  32 , passes through the coupler  134  to the photo diode  123 , and is processed by the microcontroller (or microprocessor)  68  to generate measurements of the pressure of interest. 
     The sensing head  22  is also envisioned as a part of integrated sensor networks, including spatial-division-multiplexing (SDM) systems with a plurality of sensing heads  22  and the optical MEMS (microelectromechanical systems) chip which integrates a signal processing element. 
     Shown in  FIG. 12 , is a low coherence fiber optic interferometer (LCFOI) system that may be used to measure dynamic response of the Fabry-Perot pressure sensor. Light from a broadband source  132  (such as, for example, a super-luminescent light emitting diode (SLED)) is first sent to a 1×N coupler  134  via a tunable Fabry-Perot filter  136  that serves as a reference interferometer. The light from each of the output ports of the 1×N coupler  134  is then sent to several (depending on the number N of sensors) 1×2 couplers  138 . At the output of each coupler  138 , the light beam is directed to a Fabry-Perot (FP) interferometer based pressure sensing head  22 , acting as the sensing interferometer. The reflected light from each FP sensor  22  is then coupled back to a photo detector  140 . 
     Maximum sensitivity may be achieved when the initial differential optical path difference (OPD) is at the vicinity of quadrature points, i.e. 
                   OPD   =         L   s     -     L   r       =           2   ⁢   m     -   1     4     ⁢   λ               (     Eq   .           ⁢   4     )               
where L r  and L s  are the cavity length of the reference interferometer and the sensing interferometer, respectively, and m=0, ±1, ±2 . . . .
 
     The obtained signal which is proportional to the measured pressure is sent to an oscilloscope for display or to a data acquisition board  142  for data analysis. This configuration is a tree topology spatial-division multiplexing sensor network, which may be used to interrogate an array of the miniature pressure sensors  22 . This scheme is applicable both with the cross-axial sensing heads  22 , and co-axial sensing heads. 
     In the integral approach on a single substrate shown in  FIG. 11 , the resulting footprint may be about 1.5×2. Total sensor height of less than 200 μm has been achieved with the present design. 
     The miniature surface-mountable fiber-optic pressure sensor of the present invention is mounted on a surface and measures with high precision dynamic pressure directed perpendicular to the surface. By using the optical fiber and the angled end surface as the light guide to expose photoresist for forming the cavity, an accurate position of the cavity may be attained which is satisfied without the use of high accuracy masks or alignment systems. In the design using a fiber jacket as a cavity confining layer, a relatively thick cavity may be obtained by the laser machining, wet etching, and dry etching in an efficient and inexpensive way. The versatility of the subject method allows the use of the UV imprint lithography for formation of the cavity where a mold is used made of Si, glass or metal with anti-adhesion surface treatment. 
     The formed sensor is fully bio-compatible since it is covered with protective polymer layers. Also, the coarse and fine tuning of the diaphragm stiffness may be attained by adjusting metal layer thickness and/or the diameter of cavity, to address different applications, which have different pressure ranges and require different sensitivities, by controlling the number of polymer layers on the composite diaphragm and the diameter of the Fabry-Perot cavity. 
     Although this invention has been described in connection with specific forms and embodiments thereof, it will be appreciated that various modifications other than those discussed above may be resorted to without departing from the spirit or scope of the invention as defined in the appended claims. For example, equivalent elements may be substituted for those specifically shown and described, certain features may be used independently of other features, and in certain cases, particular applications of elements may be reversed or interposed, all without departing from the spirit or scope of the invention as defined in the appended claims.