Patent Publication Number: US-2004047535-A1

Title: Enhanced fiber-optic sensor

Description:
BACKGROUND OF INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The invention relates generally to methods and devices for sensing and detecting stimuli. More specifically, the invention relates to a fiber-optic sensor having enhanced sensitivity.  
       [0003] 2. Background Art  
       [0004] Fiber-optic sensors can be used to sense and detect stimuli in various applications, e.g., chemical applications such as in-situ reactor monitoring of chemical reactions, acidity measurements, and gas analysis (especially of explosive or flammable gases), and physical applications such as temperature, pressure, voltage, and current monitoring, particle measurement, motion monitoring, and imaging. Fiber-optic sensors offer the advantages of immunity to hostile environments, wide bandwidth, compactness, and high sensitivity as compared with other types of sensors.  
       [0005] Typically, a fiber-optic sensor can have one or more optical fibers, a light source, a light detector, and one or more couplers for coupling the light source and light detector to an optical fiber. The light source generates the light that is transmitted to the environment to be sensed (or monitored), and the light detector detects and analyzes light received from the sensed environment. The optical fibers are used to transmit light to and from the sensed environment.  
       [0006] A fiber-optic sensor may be classified as an extrinsic or intrinsic sensor depending on how the sensing and detecting are performed. In an extrinsic sensor, sensing takes place outside of the fiber, and the fiber is used to transmit light to and from the sensing region. In an intrinsic sensor, physical properties of the fiber change, and this change is detected by monitoring amplitude, phase, frequency, or polarization state of the light transmitted through the fiber.  
       [0007] Existing fiber-optic sensors are based on using an optical fiber that is modified in some way. One approach involves applying a sensing material to the probe part of the fiber and allowing the sensed environment to be monitored by changes in the optical properties of the sensing material. This approach is typically used for monitoring a chemical environment. FIG. 1A shows the probe part  1  of a chemical sensor, including an optical fiber  2 . A sensing material  3 , i.e., a reagent whose light transmission properties, e.g., fluorescence, refractive index, or transmission at wavelength(s) being monitored, changes upon reacting with a target compound, is applied at a terminal end of the optical fiber  2 .  
       [0008] Another approach involves removing cladding from a section of an optical fiber and allowing the sensed environment to be monitored by total internal reflection in the unclad region. FIG. 1B shows an unclad region  4  at a terminal end of an optical fiber  5 . FIG. 1C shows an unclad region  6  in the middle of an optical fiber  7 . For the configuration shown in FIG. 1B, light is transmitted to and detected from the same end  5   a  of the optical fiber  5 . For the configuration shown in FIG. 1C, light is transmitted into the input end  8  of the optical fiber  7  and detected at the output end  9  of the optical fiber  7 . In general, this approach lacks robustness and sensitivity because detection is done via evanescent wave only.  
       [0009] Another approach involves making lateral deformations called microbends in the fiber and allowing the sensed environment to be monitored by changes in intensity of light radiating from the microbends. This approach can be used for both chemical and physical sensing.  
       SUMMARY OF INVENTION  
       [0010] In one aspect, the invention relates to a fiber-optic sensor probe which comprises an optical fiber terminated with a lens.  
       [0011] In another aspect, the invention relates to a fiber-optic sensor probe which comprises an optical fiber, a lens, and an elongated region formed between the optical fiber and the lens for evanescent probing.  
       [0012] In another aspect, the invention relates to a fiber-optic sensor which comprises a lensed fiber, a light source optically coupled to the lensed fiber so as to send light into the lensed fiber, and a light detector optically coupled to the lensed fiber so as detect light reflected into the lensed fiber.  
       [0013] In another aspect, the invention relates to a fiber-optic sensor which comprises a sensor probe having an optical fiber, a lens, and an elongated region formed between the optical fiber and lens for evanescent probing. The fiber-optic sensor further includes a light source that sends light into the optical fiber, a light detector that detects light reflected into the lens and elongated region, and a coupler for coupling the light source and the light detector to the optical fiber.  
       [0014] In another aspect, the invention relates to a fiber-optic sensor which comprises a first lensed fiber, a second lensed fiber optically coupled to the first lensed fiber, a light source optically coupled to the first lensed fiber so as to send light into the first lensed fiber, and a light detector optically coupled to the second lensed fiber so as to detect light transmitted through the second lensed fiber.  
       [0015] In another aspect, the invention relates to a chemical sensor which comprises an optical fiber terminated with a lens, a light source and a light detector coupled to the optical fiber, and a reagent situated in an optical path of the lens, the reagent having an optical property that changes in response to a chemical stimulus.  
       [0016] In another aspect, the invention relates to a chemical sensor which comprises a pair of sensor probes, each sensor probe having a lens for sensing and an optical fiber for transmitting a light signal, wherein the lenses are optically coupled. The chemical sensor further comprises a light detector coupled to one of the sensor probes, a light source coupled to the other of the sensor probes, and a reagent situated in an optical path of the sensor probes, the reagent having an optical property that changes in response to a chemical stimulus.  
       [0017] In another aspect, the invention relates to a temperature sensor which comprises an optical fiber terminated with a lens, a light source and a light detector coupled to the optical fiber, and a temperature-sensitive material proximate the lens, the temperature-sensitive material having a different refractive index and dn/dT than the lens, where n is refractive index and T is temperature.  
       [0018] In another aspect, the invention relates to an electrical sensor which comprises an optical fiber terminated with a lens, a light source and a light detector coupled to the optical fiber, and a birefringent material proximate the lens, the birefringent material having a polarization state that changes in response to changes in an electrical stimulus. In one embodiment, the electrical stimulus is change in voltage. In another embodiment, the electrical stimulus is change in current.  
       [0019] In another aspect, the invention relates to a motion sensor which comprises an optical fiber terminated with a lens, a light source coupled to the optical fiber so as to send light into the optical fiber, and a transducer coupled to the optical fiber so as to measure an intensity and a frequency of light reflected into the optical fiber.  
       [0020] In another aspect, the invention relates to a mechanical sensor which comprises an optical fiber terminated with a lens, a light source and a light detector coupled to the optical fiber, and an optical cavity having an optical path difference that changes in response to a physical stimulus. In one embodiment, the physical stimulus is change in pressure. In another embodiment, the physical stimulus is change in force. In another embodiment, the physical stimulus is change in acceleration.  
       [0021] Other features and advantages of the invention will be apparent from the following description and the appended claims. 
     
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
     [0022] FIGS.  1 A- 1 C show prior-art fiber-optic sensors.  
     [0023]FIG. 2 shows a fiber-optic sensor probe having a convex surface for sensing and/or probing in accordance with one embodiment of the invention.  
     [0024]FIG. 3 shows the sensor probe of FIG. 2 in transmission configuration.  
     [0025]FIG. 4 shows a fiber-optic sensor probe having a convex surface and an extended guiding region for sensing and/or probing in accordance with another embodiment of the invention.  
     [0026]FIG. 5 shows a graph of back-reflection loss as a function of lens thickness and radius of curvature for a diverging lens operated in reflection mode.  
     [0027]FIG. 6A shows an aligning step of a method for making a sensor probe.  
     [0028]FIG. 6B shows a fusion-splicing step of a method for making a sensor probe.  
     [0029]FIG. 6C shows a taper-cutting step of a method for making a sensor probe.  
     [0030]FIG. 6D shows the glass fiber of FIG. 6C after taper-cutting.  
     [0031]FIG. 6E shows a melting-back step of a method for making a sensor probe.  
     [0032] FIGS.  7 A- 7 C show a fiber-optic chemical sensor incorporating the sensor probe of FIG. 2 in a reflection configuration.  
     [0033] FIGS.  8 A- 8 C show a fiber-optic chemical sensor incorporating the sensor probe of FIG. 4 in a reflection configuration.  
     [0034] FIGS.  9 A- 9 C show a fiber-optic chemical sensor incorporating the sensor probe of FIG. 2 in a transmission configuration.  
     [0035]FIG. 10A shows a fiber-optic temperature sensor incorporating the sensor probe of FIG. 2 in a reflection configuration.  
     [0036]FIG. 10B shows a graph of reflection coefficient as a function of temperature for a silica lens having an infinite radius of curvature and embedded in a polymer material.  
     [0037]FIG. 11A shows a voltage/current sensor incorporating the sensor probe of FIG. 2 in a transmission configuration.  
     [0038]FIG. 11B shows a voltage/current sensor incorporating the sensor probe of FIG. 4 in a reflection configuration.  
     [0039]FIG. 12 shows a motion sensor incorporating the sensor probe of FIG. 2 in a reflection configuration.  
     [0040]FIG. 13 shows a mechanical sensor incorporating the sensor probe of FIG. 2 in a reflection configuration.  
     [0041]FIG. 14 shows an alternate arrangement of sensor probes in a transmission configuration.  
    
    
     DETAILED DESCRIPTION  
     [0042] Embodiments of the invention provide a fiber-optic sensor probe having enhanced sensitivity as compared with conventional fiber-optic sensor probes. Embodiments of the invention also provide sensors incorporating the fiber-optic sensor probe of the invention. The enhanced sensitivity of the fiber-optic sensor probe is achieved by use of a lensed fiber. A lensed fiber is an optical fiber terminated with a lens. The sensitivity of the fiber-optic sensor probe is tuned by tailoring the lens geometry and/or coating the lens with a reflective or anti-reflective coating.  
     [0043] Various embodiments of the invention will now be described with reference to the accompanying drawings.  
     [0044]FIG. 2 shows a fiber-optic sensor probe  10  according to one embodiment of the invention. The sensor probe  10  is a lensed fiber having a plano-convex lens  12  attached to, or formed at, the end of an optical fiber  14 . The convex surface  16  of the lens  12  is used for sensing and/or probing. The optical fiber  14  has a core  18  and a clad  20  surrounding the core  18 , where the core  18  is for transmitting light to or from the convex surface  16 . The optical fiber  14  can be any single-mode fiber, including polarization-maintaining fiber (PM fiber), or a multimode fiber. The lens  12  can be made from a material having transparency at the wavelength(s) of interest. Preferably, the lens  12  has a refractive index similar to that of the fiber core  18 . For robustness, i.e., protection from fire, explosion, and corrosion, the lens  12  is preferably made of silica or doped silica, e.g., B 2 O 3 —SiO 2  and GeO 2 —SiO 2 .  
     [0045] In the reflection mode, the sensor probe  10  is used to transmit light to and detect light from the environment to be sensed. The detected light is decoded to determine the changes in the sensed environment. In the transmission mode, a pair of the sensor probes  10  are needed. FIG. 3 illustrates sensor probes  10   a ,  10   b  in transmission configuration. The lenses  12   a ,  12   b  of the sensor probes  10   a ,  10   b  are optically coupled. The sensor probe  10   a  is used to transmit light to the sensed environment, and the sensor probe  10   b  is used to detect light from the sensed environment.  
     [0046]FIG. 4 shows a fiber-optic sensor probe  22  according to another embodiment of the invention. The sensor probe  22  includes an optical fiber  26  with a core  27 . The optical fiber  26  is spliced to a coreless optical fiber  28  that is terminated with a lens  24 . The lensed fiber  28  provides an extended surface area for evanescent probing. The lensed fiber  28  could be formed from a larger-diameter fiber so that the active area where evanescent probing occurs is increased in comparison to that of the sensor probe ( 10  in FIG. 2). The lensed fiber  28  could also be formed from a fiber having a diameter that is the same as or smaller than the diameter of the optical fiber  26 . The sensor probe  22  has a high back-reflection, e.g., greater than −10 dB, which results in improved sensitivity in comparison to the sensor probe ( 10  in FIG. 2) in the reflection mode.  
     [0047] The sensor probes  10 ,  22  (see FIGS. 2, 4) provide several advantages when compared with conventional fiber-optic sensor probes. One advantage provided is that a wide range of lens geometries are possible, and the lenses  12 ,  24  (see FIGS. 2, 4) can be coated, as needed, with reflective or anti-reflective coating. Thus, the sensitivity of the sensor probes  10 ,  22  can be tuned by tailoring the geometry of the lenses  12 ,  24  and/or coating the lenses  12 ,  24 . Another advantage provided is that the convex surfaces  16 ,  30  (see FIGS. 2, 4) create a high surface area for interaction with the sensed environment. The sensor probe  22  (see FIG. 4) provides an extended surface area for evanescent probing in comparison to the sensor probe  10  (see FIG. 2). Another advantage provided is that in the reflection mode, the properties of the lenses  12 ,  24  can be used to tailor back-reflection to a desired value without use of reflective coating.  
     [0048] In general, the lenses  12 ,  24  (see FIGS. 2, 4) can be designed to be collimating, focusing, or diverging, depending on the sensing configuration and sensed environment. Typically, for the reflection mode, it is desirable to maximize back-reflection at the convex surfaces  16 ,  30  (see FIGS. 2, 4). A diverging lens is most efficient for the reflection mode. The diverging lens can be used to tailor back-reflection to a desired value with or without using reflective coating. FIG. 5 shows a graph of back-reflection as a function of lens thickness and radius of curvature for a diverging lens operated in reflection mode without reflective coating. The calculations are for a wavelength of 1550 nm and silica-air interface. In the case of probing by focusing on a substrate, the lenses  12 ,  24  can be focusing lenses.  
     [0049] Typically, for the transmission mode, it is desirable to minimize back-reflection at the convex surfaces  16 ,  30  (see FIGS. 2, 4). The geometry of the lenses  12 ,  24  (see FIGS. 2, 4) can be selected to limit back-reflection to a desired value. In addition, an anti-reflective coating applied on the lenses  12 ,  24  can be used to further reduce back-reflection. Typically, for the transmission mode, it is desirable to maximize coupling between the transmitting sensor probe, i.e., the sensor probe carrying light to the sensed environment, and the detecting sensor probe, i.e., the sensor probe receiving light from the sensed environment. Thus, when the sensor probes  10 ,  22  (see FIGS. 2, 4) are used in a transmission mode, the lenses  12 ,  24  are preferably collimating or focusing lenses. Preferably, the lens geometries are selected to maximize coupling and anti-reflective coating is used to minimize back-reflection.  
     [0050] The sensor probes  10 ,  22  (see FIGS. 2, 4) are monolithic devices. One method for fabricating a monolithic sensor probe will now be described.  
     [0051] A monolithic sensor probe can be fabricated in three or four steps. In the first step, called the aligning step, an optical fiber and a glass fiber are aligned in opposing relation. FIG. 6A shows an optical fiber  32  aligned with a glass fiber  34 . Preferably, the glass fiber  34  is a coreless glass fiber. Preferably, the refractive index of the glass fiber  34  is similar to that of the core of the optical fiber  32 . The diameter of the glass fiber  34  can be smaller than, equal to, or greater than the diameter of the optical fiber  32 . The second step, called the fusion-splicing step, involves fusing the glass fiber  34  to the optical fiber  32 . FIG. 6B shows the glass fiber  34  being fused to the optical fiber  32 . The process involves bringing the opposing ends of the glass fiber  34  and optical fiber  32  together and using a heater  36 , e.g., a tungsten filament, to heat and fuse the opposing ends.  
     [0052] After joining the glass fiber  34  to the optical fiber  32 , the glass fiber  34  is then shaped into a lens. Thus, the third step, called taper-cutting, involves shaping the glass fiber  34  into a lens. As shown in FIG. 6C, taper-cutting involves moving the heater  36  along the glass fiber  34  to taper-cut the glass fiber  34 . While moving the heater  36  along the glass fiber  34 , the glass fiber  34  is pulled in a direction away from the optical fiber  32  to accomplish the taper-cut. FIG. 6D shows the glass fiber  34  after taper-cutting. The glass fiber  34  is taper-cut such that the desired lens thickness and radius of curvature is achieved. In general, the radius of curvature obtained by taper-cutting is small. To make a lens with a larger radius of curvature, an additional step, called melting-back, is needed. In the melting-back step, illustrated in FIG. 6E, the heater  36  is moved toward the taper-cut end of the glass fiber  34  to form a larger radius of curvature, as shown by the dotted lines.  
     [0053] The following are various examples of fiber-optic sensors incorporating the sensor probes described above.  
     Chemical Sensors  
     [0054]FIG. 7A shows a chemical sensor  40  incorporating the sensor probe  10 . The chemical sensor  40  includes a light source  42 , a light detector  44 , and a coupler  46 , e.g., a bifurcated fiber, for coupling the light source  42  and light detector  44  to the sensor probe  10 . If multiple wavelengths are to be transmitted through the sensor probe  10 , the light source  42  may include a wavelength-division multiplexer (WDM). In this case, the detector  44  should have the capability to analyze multiple wavelengths.  
     [0055] In reflection mode, light is transmitted from the light source  42  to the sensor probe  10 . The light exits the sensor probe  10 , enters into the chemical environment to be monitored or analyzed, and is reflected back into the sensor probe  10 . In this embodiment, either the chemical environment will modify the reflected light in some way, or the physical properties of the sensor probe  10  will change in response to changes in the chemical environment. The reflected light travels to the light detector  44 , where it is detected and decoded to determine the changes in the chemical environment.  
     [0056] The chemical sensor  40  may optionally include a sensing material or reagent ( 48  in FIG. 7B) whose light transmission properties, e.g., fluorescence, refractive index, or transmission at wavelength(s) being monitored, change upon reacting with a target compound. The reagent ( 48  in FIG. 7B) may be applied on the lens  12  so that the light reflected back into the sensor probe  10  is modified as the chemical environment being monitored and/or analyzed changes.  
     [0057] Alternatively, as shown in FIG. 7C, the chemical sensor  40  may be inserted in a reaction cell  50  containing a reagent  52 , such as described above. The cell  50  includes a semi-permeable membrane  53  through which a chemical being detected can flow into the cell  50 .  
     [0058] Another modification that can be made to the chemical sensor  40  is to replace the sensor probe  10  with the sensor probe  22 , as shown in FIGS.  8 A- 8 C. The sensor probe  22  provides increased surface area for interaction with the sensed environment. The sensor probe  22  is also better suited for the reflection mode because it has a high return loss.  
     [0059]FIG. 9A shows a chemical sensor  54  in transmission configuration. In this configuration, the chemical sensor  54  includes a pair of sensor probes  10 , one for transmitting and the other for detecting. For convenience, the characters referencing the transmitting sensor probe or parts of the transmitting sensor probe will have the suffix “a.” Similarly, the characters referencing the detecting sensor probe or parts of the receiving sensor probe will have the suffix “b.” 
     [0060] The chemical sensor  54  includes a light source  56  coupled to the sensor probe  10   a  and a light detector  58  coupled to the sensor probe  10   b . The light source  56  can include a WDM if using multiple wavelength. In this case, the detector  58  can be a spectrum analyzer or other suitable detector for detecting multiple wavelengths. The sensor probes  10   a ,  10   b  are arranged such that their optical axes are substantially aligned and their lenses  12   a ,  12   b  are spaced apart, allowing light to be coupled between the lenses  12   a ,  12   b.    
     [0061] In transmission mode, light is transmitted from the light source  56  to the sensor probe  10   a . The light exits the sensor probe  10   a  into the chemical environment being monitored and/or analyzed. In this embodiment, either the chemical environment will modify the light in some way, or the physical properties of the sensor probe  10   b  will change in response to changes in the chemical environment. The light is then transmitted through the sensor probe  10   b  to the light detector  58 , where it is detected and decoded to determine the changes in the chemical environment.  
     [0062] The chemical sensor  54  may optionally include a sensing material or reagent ( 60  in FIG. 9B) whose light transmission properties, e.g., fluorescence, refractive index, or transmission at wavelength(s) being monitored, change upon reacting with a target compound. The reagent ( 60  in FIG. 9B) may be applied on the lens  12   b  so that the light entering into the sensor probe  10   b  is modified as the chemical environment being monitored and/or analyzed changes. (The reagent may also be applied to the lens  12   a .)  
     [0063] Alternatively, as shown in FIG. 9C, a reaction cell  62  containing a reagent  64  may be positioned in between the lenses  12   a ,  12   b . The windows  62   a ,  62   b  of the reaction cell  62  are transparent at the wavelengths of interest, allowing light to be transmitted from the sensor probe  10   a  into the cell  62  and out of the cell  62  into the sensor probe  10   b . Alternatively, the lenses  12   a ,  12   b  can be embedded in the cell  62 , eliminating the need for transparent windows  62   a ,  62   b . The reaction cell  62  includes a semi-permeable membrane  63  through which a chemical being detected can flow into the cell.  
     [0064] Another modification that can be made to the chemical sensor  54  is to replace the pair of sensor probes  10  with a pair of the sensor probe  22  (shown in FIG. 4). The sensor probe  22  provides increased surface area for interaction with the sensed environment.  
     Temperature Sensor  
     [0065]FIG. 10A shows a fiber-optic temperature sensor  70  incorporating the sensor probe  10 . The temperature sensor  70  includes a light source  72 , a light detector  74 , and a coupler  76 , e.g., a bifurcated fiber, for coupling the light source  72  and light detector  74  to the sensor probe  10 . The lens  12  is embedded in a temperature-sensitive material  78 . The material  78  has a different refractive index and different dn/dT than the lens material, where n is refractive index and T is temperature. As an example, the material  78  can be a polymer, which typically has a negative dn/dT, or an inorganic material, such as sol-gel with a positive dn/dT.  
     [0066] In operation, light is transmitted from the light source  72  to the sensor probe  10 . The light exits the convex surface  16  into the material  78  and is reflected back into the sensor probe  10  for detection at the light detector  74 . The light reflected back into the sensor probe  10  is affected by changes in refractive index of the material  78 , where the refractive index of the material  78  changes with temperature of the sensed environment. FIG. 10B shows an example of change in reflection coefficient due to temperature variation at a silica lens (n=1.457, dn/dT=10 −3 /° C.) having an infinite radius of curvature and embedded in a polymer material (n=1.55; dn/dT=−10 −3 /° C.).  
     Voltage/Current Sensor  
     [0067]FIG. 11A shows a voltage/current sensor  80  in transmission configuration. The voltage/current sensor  80  includes a pair of sensor probes  10  (a pair of the sensor probes  22  in FIG. 4 can also be used): one for transmitting and the other for detecting. For convenience, the characters referencing the transmitting sensor probe or parts of the transmitting sensor probe will have the suffix “a.” Similarly, the characters referencing the detecting sensor probe or parts of the receiving sensor probe will have the suffix “b. The voltage/current sensor  80  includes a light source  82  coupled to the sensor probe  10   a  and a light detector  84  coupled to the sensor probe  10   b . The sensor probes  10   a ,  10   b  are arranged such that their optical axes are substantially aligned and their lenses  12   a ,  12   b  are spaced apart.  
     [0068] In one embodiment, the light source  82  is a polarized light source, the optical fibers  14   a ,  14   b  are PM fibers, and the detector  84  is a polarization analyzer. The lenses  12   a ,  12   b  are submerged in a cell  85  filled with a sensing material  86  that is birefringent, e.g., ferroelectric or liquid crystal. Changes in current and/or voltage will change the polarization state of the sensing material  86 . This change in polarization will be sensed by the detector  84  as a reduction in light intensity compared to a reference state where there is no applied electromagnetic field. Alternatively, an unpolarized light source can be used, and the sensor  80  can evaluate the relative ratio of two polarizations.  
     [0069]FIG. 11B shows a voltage/current sensor  88  in a reflection configuration. The voltage/current sensor  88  includes a light source  90  coupled to the sensor probe  22 , and a light detector  92  coupled to the sensor probe  22  (the sensor probe  10  in FIG. 2 can also be used, but the sensor probe  22  generally provides enhanced sensitivity in the reflection mode.) The lensed fiber  28  is inserted into a cell  94  filled with a birefringent material  95 . The light detector  92  could be a polarization analyzer for analyzing the polarization state of the light reflected from the cell  94  into the sensor probe  22 .  
     Motion Sensor  
     [0070]FIG. 12 shows a motion sensor  96  in reflection mode with a light source  98  and light detector  100  coupled to the sensor probe  10  by a coupler  102 . Typically, the light detector  100  is a transducer. The sensor probe  10  detects motion of a moving part  104  that is encoded and that modulates the light coming out of the sensor probe  10 . The light is retro-reflected back and passed through the coupler  102 , such as a 3 dB directional coupler, into the transducer  100 . The output of the transducer  100 , i.e., intensity vs. frequency plot, is shown in the figure.  
     [0071] The fiber  14  and lens  12  can be made of high silica glass so that the motion sensor  96  can be exposed to harsh environment. The coupler  102  can be made of polymer, because it is away from the lens  12 , thus reducing the cost of the sensor. The sensor probe ( 22  in FIG. 4) can also be used instead of the sensor probe  10 . The sensor probe ( 22  in FIG. 4) generally provides enhanced sensitivity in comparison to the sensor probe  10  when used in the reflection mode.  
     Mechanical Sensor  
     [0072]FIG. 13 shows a mechanical sensor  106  in reflection mode with a light source  108  and detector  110  coupled to the sensor probe  10  by a coupler  112 . The sensing is based on monitoring optical path difference changes in a Fabry-Perot cavity  114  that is made of two mirrors  116 ,  118 . Low-reflectance coatings  116   a ,  118   a  are applied on the glass or other substrate (e.g., polymer)  116 ,  118 , respectively. The changes in optical path difference  120  are monitored using intereferometric fringe pattern analysis. Fringes can be analyzed using spectral domain or phase domain processing (using either temporal fringe formation or spatial fringe formation). By measuring the round-trip phase shift of the reflected optical power in the Fabry-Perot cavity  114 , optical path difference  120  can be calculated.  
     [0073] As shown in the figure, the mirror  116  is mounted on a pressure sensing diaphragm  122 , which moves along with mirror  116  in response to pressure. Thus, the mechanical sensor  106  senses change in pressure. Alternatively, if the diaphragm  122  is replaced by a weight, the cavity  114  can sense acceleration, or force in general.  
     Other Modifications  
     [0074] Several modifications can be made to the sensors described above which are within the scope of the invention. The underlying principle of the invention is the use of a lensed fiber to achieve enhanced sensitivity. One example of a modification that can be made is the way the lensed fibers or sensor probes are arranged in the transmission mode, i.e., the optical axes of the sensor probes do not have to be always aligned. FIG. 14 shows an alternative configuration where the optical axes of the optical fibers  124   a ,  126   a  of the sensor probes  124 ,  126  are intentionally misaligned with respect to the center of curvature of the lenses  124   b ,  126   b  to induce field angle. This type of configuration is particularly suitable for monitoring changes in surface properties of an element, such as an element that needs to be monitored for wear and tear.  
     [0075] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.