Patent Publication Number: US-2007098323-A1

Title: Reflection-mode fiber sensing devices

Description:
REFLECTION-MODE FIBER SENSING DEVICES  
      This application claims the benefit of U.S. Provisional Application No. 60/448,940 filed Feb. 21, 2003.  
      This application is also a continuation-in-part application of a co-pending U.S. patent application Ser. No. 10/714,503 filed on Nov. 12, 2003 which further claims the benefits of U.S. Provisional Application Nos. 60/425,991 filed Nov. 12, 2002, 60/431,026 filed Dec. 4, 2002.  
      The entire disclosures of the above patent applications are incorporated herein by reference as part of this application. 
    
    
     BACKGROUND  
      This application relates to optical sensing devices based on evanescent optical coupling through a side-polished surface in an optical waveguide such as fibers and planar waveguides.  
      Optical fibers can be used to transmit or process light in a variety of applications, including delivering light to or receiving light from integrated optical components or devices formed on substrates, transmitting information channels in wavelength-division multiplexed optical communication devices and systems, forming fiber optic switch matrix devices or fiber array to array connector, and producing optical gain for optical amplification or laser oscillation. Optical fibers essentially operate as “light pipes” to confine light within the fiber boundary and transfer light from one point to another.  
      A typical fiber may be simplified as a fiber core and a cladding layer surrounding the fiber core. The refractive index of the fiber core is higher than that of the fiber cladding to confine the light. Light rays that are coupled into the fiber core within a maximum angle with respect to the axis of the fiber core are totally reflected at the interface of the fiber core and the cladding. This total internal reflection provides a mechanism to spatially confine the optical energy of the light rays in one or more selected fiber modes to guide the optical energy along the fiber core. Similarly, optical waveguides on substrates such as planar and other waveguides may also operate as light pipes to confine and transfer port light is and may be used in integrated optical devices where optical elements, opto-electronic elements, or MEMS elements are integrated on one or more substrates.  
      The guided optical energy in the fiber or waveguide, however, is not completely confined within the core of the fiber or waveguide. In a fiber, for example, a portion of the optical energy can “leak” through the interface between the fiber core and the cladding via an evanescent field that essentially decays exponentially with the distance from the core-cladding interface. The distance for a decay in the electric field of he guided light is less than or on the order of one wavelength of the guided optical energy. This evanescent leakage may be used to couple optical energy into or-out of the fiber core, or alternatively, to perturb the guided optical energy in the fiber core.  
     SUMMARY  
      This application describes examples of fiber sensing devices based on evanescent optical coupling. According to one implementation, a fiber is provided to include a side surface formed on fiber cladding where an evanescent field of guided light in the fiber exists. A waveguide is formed over the side surface and is exposed to an external medium to cause a change at the side surface. A wavelength shift in a spectral peak in optical loss of light guided in the fiber is monitored and information about the external medium is extracted based on the wavelength shift.  
      In another implementation, a fiber sensing device includes a fiber having a side surface formed on fiber cladding within a reach of an evanescent field of guided light in the fiber. In addition, a waveguide is formed over the side surface and has a refractive index greater than an effective refractive index of the fiber. An optical detector is used to receive guided light in the fiber transmitting through a section with the side surface to produce a detector output to represent a measurement of an external medium in contact with the waveguide.  
      In the above and other fiber sensing devices based on the evanescent coupling at a side surface, a reflective Bragg grating may be formed at or above the side surface to reflect light back so that reflected light can be measured to extract information about the external medium. Two or more reflective fiber sensors may be formed in a single fiber and the reflected signals from the sensors may be distinguished by the timings of arrival at an optical detector.  
      These and other implementations are described in greater detail in the drawings, the detailed description, and the claims.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  shows one exemplary implementation of a fiber device that integrates or engages a fiber to a substrate with a groove for positioning the fiber and openings for holding the fiber.  
       FIGS. 2A and 2B  show a cross sectional view of the device in  FIG. 1  along the direction AA′ and a side view of the device in  FIG. 1  along the direction BB′, respectively.  
       FIGS. 2C and 2D  show examples of two different cross sections for grooves shown in  FIG. 1 .  
       FIG. 2E  shows one example of a V groove with varying depth and width.  
       FIG. 3A  shows a design to engage a fiber on to a substrate by using an elongated groove with a single through hole, where a portion of the fiber cladding is removed and polished to form a side-polished evanescent coupling port.  
       FIG. 3B  shows another way of engaging a fiber onto a substrate without using through holes shown in  FIG. 1 , where a portion of the fiber cladding is removed and polished to form a side-polished evanescent coupling port.  
       FIG. 4  shows one exemplary fiber sensing device formed over a side-polished fiber.  
       FIGS. 5 and 6  illustrate optical properties of the device in  FIG. 4 .  
       FIG. 7A  shows an exemplary fiber sensing device with a fiber grating in the fiber.  
       FIG. 7B  shows an optical property of the device in  FIG. 7B .  
       FIG. 8A  shows another exemplary fiber sensing device that measures presence of selected materials.  
       FIGS. 8B and 8C  illustrate optical properties of the device in  FIG. 8A .  
       FIG. 9  shows a fiber sensing device that measures presence of water and oil.  
       FIG. 10A  shows an exemplary fiber pressure sensing device.  
       FIG. 10B  shows optical properties of the device in  FIG. 10A .  
       FIG. 11  shows an exemplary device configuration for a pressure sensing device shown in  FIG. 10A .  
       FIG. 12  shows another example of a fiber pressure sensing device.  
       FIGS. 12A and 12B  show the sensing device in  FIG. 12  with a linear polarizer in the input under two different configurations.  
       FIG. 13A  shows an exemplary fiber sensing device with a waveguide overlay and a liquid overlay.  
       FIG. 13B  illustrates the optical properties of the device in  FIG. 13A .  
       FIG. 14A  shows an example of a sensing device with two fiber sensors for measurements of both temperature and pressure.  
       FIG. 14B  shows optical properties of the two sensors in  FIG. 14A .  FIGS. 15A and 15B  show two examples of multiple fiber sensors in a single fiber.  
       FIGS. 16 and 17  illustrate two exemplary reflective fiber sensors based on evanescent coupling.  
       FIGS. 18A, 18B , and  18 C shows one example of a waveguide formed over a polished side surface of a fiber to have a tapered transition region at each end for gradual transformation of the mode.  
       FIG. 19  shows the simulated transverse Mode profile of the guided mode at the location in the waveguide shown in  FIG. 17  where the guided mode is shifted away from the fiber core towards the waveguide and the grating.  
       FIG. 20  shows the dependence of the wavelength of the reflection peak on the index n(P,T) of the overlay layer for sensors shown in  FIGS. 16 and 17 .  
       FIG. 21  shows one example of a multi-sensor system having multiple reflection fiber sensors in a single fiber.  
    
    
     DETAILED DESCRIPTION  
      The optical sensing devices under various implementations of this application are in part based on the recognition that the power of the evanescent light of the guided light in the fiber or waveguide may be used to represent the power of the guided light. A small amount of the evanescent light may be accessed from a side-polished fiber or waveguide and then may be coupled into an optical detector. When the percentage of the received evanescent light out of the total guide light in the fiber is known, the power of the detected evanescent light can be used to measure the absolute power within the fiber. In particular, the location at which the evanescent coupling may be selected so that only a desired small percentage of the guided light, e.g., a few percent or less (i.e., a fraction of one percent), is coupled into the optical detector. Such a device essentially does not change the original polarization state of the guided light when the fiber is the polarization-maintaining type.  
      Notably, the evanescent coupling is sensitive to the boundary conditions at or near the side-polished coupling port of the fiber or waveguide. For example, if the environment around the side-polished coupling port changes the boundary conditions for the evanescent coupling, the evanescent coupling can change accordingly. This change can be reflected in the remaining guided light in the fiber or waveguide. Hence, a measurement of this change in the remaining guided light in the fiber or waveguide may be calibrated and used to measure the change in the environment. Therefore, this evanescent coupling mechanism may be used to provide optical sensing of the environment. As described in the examples in this application, this evanescent coupling mechanism may provide optical sensing in real time for a range of sensing applications, including measurements of temperature, pressure, presence of selected materials, and others.  
      The fiber in the sensing devices of this application may be integrated on a substrate. One or more fibers may be integrated on or engaged to the substrate fabricated with one or more grooves. One portion of the cladding of each fiber is removed and polished to form a fiber coupling port as a part of the sensor. In general, the polished surface on the fiber cladding is sufficiently close to the fiber core so that optical energy can be coupled via evanescent fields out of the fiber core for optical monitoring. Two or more such fiber coupling ports may be formed at different positions in each fiber when needed. The following sections first describe the basic structures for integrating fibers onto substrates for forming side-polished fiber coupling ports based on evanescent coupling. Exemplary implementations of fiber sensors based on such structures are then described in detail.  
       FIG. 1  shows one exemplary implementation of a fiber device  100  where a fiber  140  is integrated or engaged to a substrate  110 . The fiber device  100  may be used as a building block to construct a variety of fiber devices, including but not limited to, fiber optical monitors, fiber couplers, fiber attenuators, fiber modulators, fiber beam splitters, optical fiber switches, and fiber frequency-division multiplexers.  FIGS. 2A and 2B  show additional details of the fiber device  100 .  
      The substrate  110  may be formed of various materials, such as semiconductors, insulators including dielectric materials (e.g., a glass, a quartz, a crystal, etc), metallic materials, or any other solid-state materials that can be processed to form the device features such as grooves and through holes disclosed herein. Two parallel and opposing substrate surfaces,  112  and  114 , are generally flat and may be polished. An elongated groove  120  is formed in the substrate  110  on the surface  112  and is essentially a recess from the surface  112 . The groove  120  may be fabricated by removing a portion of the material from the substrate  110  through etching or other processes.  
      The geometry of the groove  120  is generally elongated along a straight line as illustrated or along a curved line. Unless otherwise indicated, the following description will use straight-line grooves as examples. Some exemplary implementations are described with specific reference to groove with V-shaped cross sections as shown by the groove  220  in  FIG. 2D . The cross sections are generally not so limited and may also be other shapes as well, including rectangular as shown in  FIG. 2A , U-shaped as shown by the groove  210  in  FIG. 2C , a circularly shape or other suitable shapes. Unless specifically indicated otherwise, the techniques, structures, and applications disclosed in this application are generally applicable to grooves of different shapes.  
      The width, W, of the groove  120  is generally greater than the diameter, d, of the fiber  140  and may either remain a constant or vary spatially along the groove  120 , e.g., increasing from the center towards the two ends ad illustrated in the V groove  220  in  FIG. 2E . The length, L, of the groove  120  may vary from one grove to another and can be determined based on specific requirements of applications. The depth D of the groove  120  may be a constant or may vary along the groove  120 , e.g., increasing from the center towards the two ends as shown in  FIG. 2E . In general, at least a portion- of the groove  120  has a depth D to expose a portion of the fiber cladding of the fiber  140  above the surface  112  while still keeping the fiber core below the surface  112 . Sometimes, the depth D of the groove  120  may also be selected to expose the fiber core. Other portions of the groove  120  may have a different depth so that the fiber can be placed within the groove  120  under the substrate surface  112 . Depending on the geometry of the groove  120  (e.g., the apex angle of a V-shaped groove), the depth D of the entire groove  120  ma be greater than fiber diameter d. For a groove with a rectangular cross section as shown in  FIG. 2A , at least a portion of the groove  120  has a depth D less than the fiber diameter d but greater than the sum of the fiber radius r=d/2 and radius of the fiber core r c =d c /2. This portion of the groove  120  exposes partial fiber cladding of the fiber  140  above the surface  112  while still keeping the fiber core below the surface  112 . Other portions of the groove  120  may have a depth that is at least the fiber diameter d so that the fiber can be essentially placed in the groove  120  below the surface  112 . However, in certain applications, the depth D of the entire groove  120  may be greater than fiber diameter d to avoid evanescent coupling of a guided mode. Unless otherwise indicated, the following description will assume that at least a portion of a groove  120  to expose a portion of the fiber cladding above the surface  112  and adjacent portions sufficiently deep to keep the fiber below the surface  112 . In case of the rectangular groove  120 , the central portion of the groove  120  may have a depth D less than d but greater than (d+d c  )/2 while the portions on either sides of the central portion may have a depth equal to or greater than the fiber diameter d.  
      Notably, the fiber device  100  includes two openings  131  and  132  that are respectively formed at the two ends of the groove  120  and penetrate through the substrate  110 . Hence, the openings  131  and  132  are through holes extending between the two surfaces  112  and provide access from one surface ( 112  or  114 ) to another. The spacing between the openings  131  and  132  essentially determines the length L of the groove  120 . The aperture of the openings  131  and  132  should be sufficiently large to receive the fiber  140 , e.g., with a diameter greater than the diameter of the fiber  140 . The shape of the holes  131  and  132  may generally be in any suitable geometry.  
      A portion of the fiber  140  is placed in the groove  120  near the surface  112 . The remaining portions  141 ,  142  of the fiber  140  on both sides of the portion in the groove  120  are respectively fed through the first and second openings  131 ,  132  to the other side  114  of the substrate  110 . After being placed in the substrate  110  as shown in  FIG. 1 , the fiber  140  may be slightly pulled by moving the fiber portions  141  and  142  in opposite directions so that the portion of the fiber  140  in the groove  120  is in substantially full contact with the groove  120 .  
      Since a portion of the groove  120  has a depth D less than the fiber diameter d, the cladding of the fiber  140  in, this portion protrudes out of the surface  112 . The fiber core in this portion of the fiber is generally kept under the surface  112 . For example, the cladding of a central portion of the fiber  140  between the holes  131  and  132  may be exposed. This protruded or exposed cladding is then removed and polished to form a flat surface  144  of a length L c  that is above the fiber core  143  and is substantially coplanar with the surface  112  of the substrate  110  as illustrated in  FIG. 2B . When the spacing, h, between the flat surface  144  and the fiber core  143  is sufficiently small (e.g., on the order of or less than one wavelength of optical energy), the flat surface  144  can be used to couple optical energy into or out of the fiber core  144  through the evanescent fields outside the fiber core. Hence, the length, L c , of the flat surface  144  approximately represents the optical coupling length for the fiber device  100 . This coupling surface  144  may also be non-flat, e.g., curved to a certain extent, as long as it can transmit evanescent signals.  
      Alternatively, only one through hole  132  in the substrate  110  may be needed to engage the fiber  140  to form the fiber module for coupling with a waveguide module. As shown in the design  301  in  FIG. 3A , the groove  120  may extend to one end side  310  of the substrate  110  so that one end  141  of the fiber  140  leaves the groove  120  without going through a through hole. In addition,  FIG. 3B  shows a conventional design  302  in which the groove  120  may extend to two opposing end sides  310  and  330  of the substrate  110  so that the fiber  140  is engaged to the groove  120  without relying on any through holes.  
      Notably, the through holes in the substrate  110  shown in  FIGS. 1 and 3 A, may be used to engage a single fiber on both sides of a substrate to form two or more side-polished coupling ports for evanescent coupling. For example, two grooves may be formed on opposite sides of the substrate  110  to share a common through hole at ends. A fiber may be threaded through the substrate  110  to have one fiber portion in the groove on one side and another fiber portion in the groove on the opposite side of the substrate  110 . Hence, fiber coupling ports may be is formed in the same fiber on both sides of the substrate  110 . This structure may be use to construct a variety of fiber devices, including stacking two substrates to provide optical coupling from a fiber in one substrate to another fiber in another substrate. The fabrication of this double-sided fiber structure may be implemented by polishing the substrate and the fiber on both sides as described  
       FIG. 4  shows one exemplary implementation of a fiber sensing device  400 . A fiber  140  with a core  140 A and a cladding  140 B has one portion whose cladding is partially removed to form a surface  144 . The surface  144  is within the extent of the evanescent field of the guided light in the fiber core  140 A. The surface  144  is polished to operate as the fiber coupling port. The amount of evanescent light at the surface  144  may be set at a desired percentage of the total guide ling in the fiber  140  by controlling the distance between the fiber core  140 A and the surface  144  during the fabrication phase. The evanescent light decays in magnitude exponentially with the distance. Hence, the closer the surface  144  to the fiber core  144 A, the higher the percentage of the evanescent light being coupled out of the fiber.  
      In the device  400 , the substrate  110  is shown to operate as a fiber support that holds the fiber  140 . The substrate  110  has two opposing surfaces  112  and  114 . A depth-varying groove  120  may be formed on the surface  112  of the substrate  110 . When the fiber  140  is placed in the groove  120 , the cladding of the fiber portion where the surface  144  is formed protrudes above the surface  112 . The protruded cladding is then removed to form the surface  144  which is approximately coplanar with the surface  112 . Other portions of the fiber  140  in the groove  120  stay under the surface  112 . As described above, different ways may be used to engage the fiber  140  to the substrate  110  to form the fiber coupling port  144  for evanescent coupling.  
      Notably, a high-index transparent overlay layer  420  is formed over the surface  144 . The overlay  420  may have an index higher than the effective index that of the fiber  140  to assist extraction of the evanescent light out of the guide mode of the fiber  140 . The property of the overlay layer  420 , such as the index, the thickness, the order of the waveguide of the overlay  420 , its mechanical properties including Young&#39;s modulus and Poisson ratio may be selected to meet the specific sensing operations. More details on this aspect of the sensors are described at later sections of this application. The top of the overlay layer  420  is exposed to the external medium as the sensing area for the sensing device  400 .  
      The fiber  140  generally may be any fiber, including single-mode fibers, multi-mode fibers, and birefringent fibers. In particular, the fiber  140  may be a polarization maintaining (PM) fiber to preserve the polarization state of light to be transmitted.  
      A light source  410  such as a laser diode or other suitable light-emitting device is provided to supply input light as the probe light to the sensor  400 . The fiber sensing device  400  further includes an optical detector  440  that is optically coupled to receive a portion or the entirety of the transmitted light in the fiber  140  that passes through the fiber section with the port  144  and the overlay  144 . The received transmitted light is converted into a detector signal  442 . A signal processor  460  is used to process the detector signal  442  to extract the desired information about the parameter measured by the sensing device  400 , such as the pressure or temperature at the waveguide  420  and the port  144 . The processor  460  has the processing logic that correlates a change in the evanescent coupling, such as a wavelength shift for the maximum evanescent coupling, at the port  144  in the transmitted light received by the detector  440  and the parameter to be measured.  
       FIG. 5  shows the optical loss in the guided light through the side-polished coupling port  144  and the overlay layer  420  (i.e., the evanescently coupled light) as a function of the refractive index of the overlay  420 . This relationship between the index of the overlay  420  and the optical loss in the guided light may be used for sensing. When the overlay  420  is an optical waveguide, such as a planar waveguide formed above the surface  144 , the mode matching condition dictates that only certain modes can be coupled out of the fiber into the overlay waveguide  420 . As indicated in  FIG. 5 , a change in the index of the overlay layer  420  causes a change in the evanescent coupling. At a particular value for the overlay index, the optical loss, i.e., the evanescent coupled signal, reaches a maximum. Accordingly, the remaining guided light in the fiber reaches a minimum power level under this condition.  
      This evanescent coupling is sensitive to at least the wavelength of the guided light in the fiber.  FIG. 6  shows the optical loss in such a waveguide overlay structure as a function of the wavelength of the guided light. For a fixed overlay index value, the evanescent coupling reaches a maximum at a particular wavelength. As described below, as the index of the overlay layer  420  changes, the wavelength for the maximum evanescent coupling changes and this change in wavelength may be used as one parameter to measure the change in the overlay index upon calibration. In one implementation, an optical wavemeter or an optical spectrum analyzer may be used to measure the shift in the transmission peak to determine the change in the index due to the variation in, e.g., the pressure or temperature at the location of the location of the overlay  420  and the port Em  144 .  
       FIG. 7A  shows a fiber device  700  where a fiber Bragg grating (FBG)  710  is formed in the fiber  140 , e.g., in the fiber core, and is located at the side-polished portion. The presence of the grating  710  requires a mode matching condition on evanescent coupling. As a result, the coupling is wavelength sensitive. In addition, as the index of the external medium  720  changes, the mode matching condition changes. The grating  710  may be designed to reflect a portion of the incoming light energy of a specific wavelength back into the fiber and allow the light energy of other wavelengths to pass through. The selection of the reflection wavelength is dependent on the index of the external medium  720 . Therefore, as the index of the external medium  720  changes, the reflection peak wavelength or transmission dip wavelength changes.  FIG. 7B  illustrates this feature by showing the shift in the transmission dip wavelength due to the variation in the index of the medium  720 . This relationship, again, may be used for sensing applications where the transmitted or reflected light through the sensor in the fiber is measured to extract information such as a variation in the pressure applied to the external medium  720  or a change in temperature.  
       FIG. 8A  shows a fiber sensor  800  for sensing the external medium above the waveguide  810  formed over the side-polished fiber  140 . A protection layer  820  may be formed on the waveguide  810  to prevent the external medium  830  under measurement from being in direct contact with the waveguide  810 . This protection layer  820  should be sufficiently thin so that the layer  820  does not optically isolate the waveguide  810  from the external medium  830  and the property of the external medium  830  still affects the waveguiding operation of the waveguide  810 . The optical loss at the fiber evanescent coupling port, hence, varies with the index of the external medium  830 . This variation in the optical loss may be calibrated and used to measure the presence and relative volume traction of a particular substance in the-medium  830 .  
       FIG. 8B  shows the relative optical loss of gas, water, and oil in a mixture under measurement. The measured ratio P 2 /P 1  is between the optical loss (P 2 ) at the sensing port when air is present at the sensing area and the optical loss (P 1 ) at the sensing port when oil is present at the sensing area. The optical loss P 3  is the optical loss measured when water is present at the sensing area.  FIG. 8C  shows the transmission spectra in the fiber for the gas (air), water, and oil, respectively. The transmission spectra for the air, water, and oil are different. Air and water show prominent optical loss peaks at different wavelengths λ 2  and λ 3 .  
      A sensing device may be configured to include multiple sensors for respectively measuring different materials. Each sensor may be configured to have a structure for sensing one particular substance and multiple such sensors designed for respectively sensing different materials may be integrated on a single substrate to form a multi-phase sensor.  
       FIG. 9  shows an example of such a 3-phase sensor that has 3 sensors for respectively detecting gas, water and oil in a mixture flow. As illustrated in  FIG. 8A , the ratios of optical losses measured at the 3 different sensors may be used detect presence of air, water, and oil.  
       FIG. 10A  shows an exemplary optical pressure sensor  1000 . An overlay waveguide  1010  is formed over the side-polished coupling port of the fiber  140  to measure the pressure on the waveguide  1010 . This device  1000  operates based on the shift in the resonance wavelength for the evanescently-coupled light caused by the pressure. The resonance wavelength can be calculated using the eigenvalue equation of the planar waveguide and fiber waveguide:  
         λ   =       2   ⁢   d   ⁢         n   0   2     -     n   eff   2           m       ,         
 where the planar waveguide is a symmetric structure, n 0  is the index of the planar waveguide, d is the thickness of the planar waveguide, m is the mode order of the waveguide mode for the guided light, n eff  is the effective index of the fiber mode. The free spectral range(FSR) is  
         Δ   ⁢           ⁢     λ   FSR       =         2   ⁢   d   ⁢         n   0   2     -     n   eff   2             m   ⁡     (     m   +   1     )         .           
 If d=20 μm, n eff =1.447,n 0 =1.51,m=1, then the free spectral range is 2.9 μm. The axial stain along the planar waveguide to an applied pressure P is given by 
 ε=− P (1−2μ)/ E,    
 where μ and E are the Poisson ratio and Young&#39;s modulus of waveguide material. The shift of the resonance wavelength to the applied pressure P is give by  
           Δ   ⁢           ⁢   λ     =         -       2   ⁢     d   ⁡     (     1   -     2   ⁢   μ       )       ⁢         n   0   2     -     n   eff   2           mE       ⁢   P     =       S   p     ⁢   P         ,         
 where S p  is the pressure sensitivity of the sensor. The sensitivity of the sensor depends on the material properties of waveguide, waveguide thickness, waveguide index and working wavelength (defined by the mode order m). For an example, assuming n eff =1.447, n 0 =1.51, m=1, d=20 μm, μ=0.16, and E=0.7 Gpa, then the associated sensitivity of the sensor is calculated to be about 1 pm/psi if the waveguide material is BK7 glass. This sensitivity is higher than some other optical pressure sensors by at least one order of magnitude. Therefore, a sensitive optical pressure sensor can be constructed based An this sensing mechanism. 
 
       FIG. 10B  shows the shift in the peak of the optical loss in wavelength caused by the variation in the pressure on the waveguide  1010 .  
       FIG. 11  further shows one implementation of the above pressure sensor  1100  where a housing unit  1101  is used to package the sensor  1110  located at a location in the fiber  140 . A chamber  1102  is formed in the housing to receive a flexible diaphragm  1120  upon which a pressure port  1130  is used to receive the external medium such as a liquid, gas, or a mixture of both to measure the pressure in the external medium. In this design, the external medium is in direct contact with the upper side of the diaphragm  1120  to exert the pressure to the fiber sensor via the diaphragm  1120 .  
      In the sensor  1000  in  FIG. 10A , the overlay waveguide  1010  is in direct contact with the external medium in which the external pressure is applied. Hence, the sensing operation by the sensor  1000  is affected by a change in the optical properties of the external medium, such as its index of refraction. This is undesirable in this particular application when the pressure is the parameter to be measured.  
       FIG. 12  illustrates another sensor  1200  which includes an overlay layer  1210  to eliminate this effect. More. specifically, an overlay layer  1210  is formed between the top surface of the waveguide  1010  and the external medium. The thickness of the overlay layer  1210  is sufficiently large that the optical field of the light coupled from the fiber  140  into the waveguide  1010  does not reach the external medium. Hence, under this condition, the layer  1210  operates as an optical insulator to optically “insulate” the waveguide  1010  from the external medium. As a result, the evanescent coupling in the sensor  1200  mainly varies with the pressure applied to the waveguide  1010  through the layer  1210 .  
      In another aspect of this application, evanescent optical coupling may be used to sense both pressure and temperature in a given environment.  FIG. 13A  illustrates one exemplary implementation of such a sensor  1300 . An overlay liquid  1320 , whose index of refraction changes in response to a pressure, is applied over and is in direct contact with the waveguide  1010 . The external pressure under measurement is applied to the overlay liquid  1320 . When the index of the liquid  1320  changes, the mode coupling condition at the liquid-waveguide boundary changes. This change also alters the evanescent coupling from the fiber  140  to the waveguide  1010  through the evanescent coupling port in the fiber  140 . As a result, the pressure can be measured.  
      The sensor  1300  includes a sensor package and liquid container  1310  to hold the substrate  110  with the side-polished fiber  140  and the overlay liquid  1320 . The container  1310  has an opening through which the liquid  1320  exposes to the environment where the pressure and temperature are measured. The material for the overlay liquid  1320  may be any suitable liquid or a mixture of liquids, such as water, water-based solutions, or oils. The sensor element, which includes the polished fiber  140 , the waveguide overlay  1010  and the liquid overlay  1320 , is placed in a sensor container package which is strong enough where no significant change in shape will occur under pressure. The waveguide  1010  may be made of suitable materials, such as semiconductors (Si, Ge, etc.), dielectric materials (glasses, SiN; SiO, etc.), or metals (Cr, Gold and others).  
      In operation, the external pressure under measurement is applied to the liquid  1320  to cause a change in the liquid  1320 . In practice, this pressure is applied through a diaphragm  1330  on top of the liquid  1320  that seals the liquid  1320  at the opening of the container  1310 . The diaphragm  1330  may be made of a thin sheet of metal such as steel, rubber or other suitable materials. The optical index of liquid  1320  can change under pressure, thus affecting the boundary condition of the overlay waveguide  1010  and also the optical coupling between fiber and waveguide  1010  through the side-polished fiber coupling port.  
       FIG. 13B  illustrates operations of the sensor  1300  in  FIG. 13A  by showing a shift in the resonance wavelength of the peak in the optical loss caused by the variation in pressure. Under a normal condition, the fiber  140  and waveguide  1010  have a strong coupling at a certain wavelength that satisfies the mode coupling condition (resonance condition). As the pressure changes, the strong coupling wavelength is shifted to a different resonance wavelength. By measuring the shift in the peak wavelength of the transmission dip or the peak in the optical loss, the external pressure applied to the liquid overlay  1320  can be determined.  
      Notably, the index of the liquid  1320  can also change with the temperature and thus, the change in the evanescent coupling can also reflect the temperature in the surrounding environment. In order to determine the pressure applied to liquid  1320 , it is desirable to measure temperature precisely as well to account for the change in the coupling contributed by the change in temperature.  
      In designing a transmission sensor described above in  FIGS. 12 and 13 A, the parameters of the overlay waveguide  1010  should be designed so that the resonance condition for evanescent coupling from the fiber to the waveguide  1010  is sensitive to the change in the index of the overlay layer  1210  above the waveguide  1010 . The design parameters of the waveguide  1010  include its refractive index and the thickness d. Assume that the boundary phase conditions at the interface between the side-polished fiber and the waveguide  1010  and the interface between the overlay layer  1210  and the waveguide  1010  are φ 1  and φ 2 , respectively, the resonance condition for the evanescent coupling is 
 
2 k   x   d= 2(2π/λ) √{square root over (n o   2 −n eff   2 )}   d =φ1+φ2+2 mπ,  
 
 where k x  is the wavevector of light along the vertical direction that is perpendicular to the fiber, n o  is the refractive index of the waveguide  1010  and m is an integer. This condition is sensitive to wavelength and this wavelength dependence can be made sensitive with properly selected values for the indices of the layers  1210 ,  1010 , and the fiber  140 . For example, the boundary phase condition φ 2  may be approximately an arctangent function of  √{square root over (n eff   2 −n 1210   2 )}/√{square root over (n   1010   2 −n eff   2 )}. Hence, for the case of small m such as m=3, the index of the layer  1210  (n 1210 ) may be designed to be near the value of the effective index n eff  to obtain a strong dependence of the resonance wavelength on the pressure- or temperature-caused change of the index n 1210  of the overlay layer  1210 . In particular, it is recognized that the TM mode coupling is more sensitive than the TE mode coupling. Hence, the polarization of light is controlled to be in the TM mode. 
 
      Hence, the coupling port  144  with the layers  1010  and  1210  may be configured to be sensitive to one of two orthogonal polarizations, the TM mode and TE mode. This sensitivity to the light polarization for the evanescent coupling may be advantageously used to reduce noise in the sensor  1200 . In general, sensors described in this application can be designed to exhibit such sensitivity to polarization. Accordingly, an optical linear polarizer may be implemented in the sensor to substantially reduce or eliminate one polarization while maintaining light in the orthogonal polarization in the sensor.  
      For example, in the sensor  1200  which is more sensitive to the TM mode, an in-line polarizer may be formed in the fiber  140  to control the light in the sensor  1200  to be in the TM mode by eliminating the light in the TE mode. Alternatively, a linear polarizer may be spliced to the input end of the fiber  140  to select the preferred polarization. A sensor configured to operate in the TE mode may use the in-line polarizer or a polarizer at the input end to select light in the TM mode by rejecting light in the TM mode.  FIGS. 12A and 12B  illustrate the sensor  1200  with an in-line linear polarizer and an input linear polarizer, respectively.  
       FIG. 14A  illustrates one exemplary sensor  1400  having two separate evanescent sensors  1410  and  1420  in the same fiber that are respectively used to measure temperature and pressure at the same location. The two sensors  1410  and  1420  may be built in the same way such as the design in  FIG. 13A  but with different resonance peak positions in wavelength as illustrated in  FIG. 14B . For example, the sensor  1410  may be designed to have resonance wavelengths in a first wavelength range for its temperature sensing range while the sensor  1420  may be designed to have resonance wavelengths in a second wavelength range for its temperature and pressure ranges. The first and second resonance wavelength ranges do not overlap with each other. This feature allows for separate detection of the optical signals from the same fiber. Both sensors  1410  and  1420  are exposed to external temperatures, but only one sensor  1420  is designed to expose to the external pressure through the liquid  1320 . The sensor  1410  is based on the sensor  1300  but adds a rigid sealing cap  1412  to seal off the opening so that the liquid  1320  does not receive the external pressure. The sensor  1420  is designed according to  FIG. 13A  to expose the liquid  1320  to the external pressure. Under this twin-sensor design, the sensor  1410  is responsive to the temperature only and can be used to calibrate out the temperature effect on the second sensor  1420 . The output signals from both sensors  1410  and  1420  can be processed in a way to extract the pressure information from the signal produced by the sensor  1420 . Accordingly, the sensor system  1400  in  FIG. 14A  can be used to obtain both temperature and pressure measurements.  FIG. 14B  illustrates the output transmission signals of the two sensors  1410  and  1420  during operation.  
      Using the above sensor designs, multiple sensors may be multiplexed to a single fiber where each sensor can work at a wavelength band different from other sensors.  FIGS. 15A and 15B  illustrate two examples where transmission sensors ( 1510 ,  1520 , etc.) are fabricated in or coupled to a single fiber to operate at different bands with different center wavelengths λ 1 , λ 2 , etc. The sensor  1510 , for example, is designed to couple and attenuate only light in the first band centered at λ 1  while transmitting light in other bands, e.g., in the band centered at λ 2 , without attenuation.  
      Two different output designs may be implemented. In  FIG. 15A , WDM couplers  1511  and  1521  for coupling light at different bands are locally coupled to the common fiber at the outputs of the respective sensors  1510  and  1520 , respectively. Photodetectors  1512 ,  1522 , etc. are coupled to receive the outputs of the WDM couplers  1511  and  1521 , etc., respectively and are used to measure the attenuated output beams at different bands. A signal processor  1530  is coupled to receive the is detector output s from the detectors  1512  and  1522  and is programmed to process the detector outputs to extract the measurements at different sensors  1510  and  1520 .  
      Alternatively,  FIG. 15B  shows WDM couplers  1511 ,  1521 , etc. for coupling light at different bands are coupled to the fiber at an output section and are spatially located away from the sensors  1510  and  1520 , respectively, to output beams in different bands for measurements in detectors  1512 ,  1522 , etc. This design separates the sensors from the detectors to allow for “remote” sensing.  
      In addition to the above transmission sensors, an evanescent-coupled sensor may also be designed to operate in a reflection mode. Under this reflection mode design, a reflective grating can be formed either in the fiber core or outside the fiber core within the reach of the evanescent field of the guided light so that the grating can interact with the guided light to produce a Bragg reflection. The reflective grating is designed to make the Bragg condition depend on the index of an overlay layer above the grating to sense either the pressure or temperature or both. Different from the above transmission sensors, such a reflection sensor reflects back the light in the Bragg resonance condition so that the detection is is performed at the same fiber location where the input light is coupled into the fiber.  
      For example,  FIG. 16  shows one implementation of a reflection sensor  1600  where a reflective Bragg grating  1610  is formed in the fiber core  140 A of the side-polished fiber  140  by physical grating grooves. This grating  1610  may be formed by first removing the fiber cladding to expose the fiber core and then etching grating grooves on the exposed part of the fiber core. An overlay layer  1620  with a different index n(P,T) is then filled over the grating grooves. The difference between the index of the fiber core, n core , and n(P,T) effectuates the grating  1610 . This grating  1610  is designed to have a Bragg resonance condition to couple a forward-propagating mode to a backward-propagating mode. When the index n(P,T) of the overlay layer  1620  changes, the Bragg resonance condition of the grating  1610  changes and thus the wavelength of the reflected light changes. This change in the reflected light, under proper calibration, can then be used to measure the pressure P, or temperature T that causes the change in n(P,T). In addition, when the overlay layer has an index n(P,T) lower than that of the fiber core  140 A, the grating  1610  formed on the edge of the fiber core  140 A may interact with only a fraction of the guided mode so the reflected signal may-be insensitive to the change of index n(P,T) for certain applications.  
      In order to increase the sensitivity of reflected signal in response to the change of n(P,T), a thin film with index higher than that of the fiber core  140 A can be added to cover the grating  1610  so as to increase the fraction of guide mode on the grating  1610 . The index difference in the grating may be designed to be large to produce a strong grating coupling. This strong grating coupling may produce a broad bandwidth in the reflection peak and thus may reduce the detection spectral resolution in the wavelength domain. As a result, the measurement accuracy in the shift of wavelength of the reflection peak may be reduced.  
      In implementation, a high-index thin dielectric layer may be formed between the grating  1610  and the overlay layer  1620  to cover the etched grating on one side of the fiber core. This layer may have an index comparable to or greater than the index of the fiber core  140 A and thus operates to increase the portion of the mode on the fiber grating so that the shift of reflection wavelength can be more sensitive to the index change in n(P,T).  
       FIG. 17  shows another exemplary implementation of a reflection sensor  1700  where the reflective Bragg grating  1710  is formed outside the fiber core  140 A on the top of a high-index slab or ridge waveguide  1720  over the exposed fiber core. An additional layer with index very close to that of the overlay layer  1620  is added on the top of the high-index slab/ridge waveguide  1720 . The waveguide  1720  has one surface in contact with the exposed fiber core and another opposing surface processed with grating grooves (e.g., by etching). The index of the waveguide  1720 , n s , may be greater than the index n core  of the fiber core  140 A to shift the center of the guided mode from the center of the fiber core  140 A towards the high-index waveguide  1720  so that the grating  1710  in the top surface of the waveguide  1720  can interact with a greater portion of the guided mode than the sensor  1600  in  FIG. 16 . On the other hand, the difference between the index of the grating layer, n g , and the overlay layer  1620 &#39;s index n(P,T) may be designed to be small to effectuate a weak grating coupling to achieve a narrow bandwidth in the reflection peak.  
      The thickness of the high-index waveguide  1720  may be small so that the grating  1710  is within the reach of the evanescent field of the guided mode in the fiber  140 . In practice, the thickness of the waveguide  1720  is less than one wavelength of the guided light, usually only a fraction of the wavelength of the guided light but is sufficiently thick to support at least one guided mode. The slab/waveguide  1720  may be designed to have a desired index and thickness to allow for two different operating configurations. In the first configuration, the thickness of the slab/waveguide  1720  is sufficiently small to barely support one mode in the slab/waveguide  1720  for interaction with the grating  1710  so that the change in the index n(P,T) of the overlay layer  1620  effectively turns on or off the optical reflection caused by the grating  1710  or to change the reflected peak wavelength abruptly. In the second configuration, the thickness of the slab/waveguide  1720  is sufficiently large to support at least one mode for interaction with the grating  1710  so that there is always a grating-caused reflection signal but the strength of the reflection signal changes with the index n(P,T) of the overlay layer  1620 .  
      [In one implementation, the high-index slab/waveguide  1720  may be formed of a dielectric layer such as an aluminum oxide (AlOx) with an index around 1.75. This thickness of the slab  1720  may be approximately in the range from 80 nm to about 150 nm. The grating  1710  on top of the slab  1720  may be formed by, e.g., forming a dielectric layer such as SiOx over the slab  1720  and then etching the layer to form the grating grooves. The overlay layer  1620  over the grating  1710  with the index n(P,T) may use a variety of materials such as liquids like oil, alcohol and water. To achieve a narrow band reflection, the index of the grating material should be close to the index of the overlay layer  1620  above the grating  1710 . Materials such as SiO 2  or similar materials whose refractive indices are close to that of the overlay layer  1620  such as 1.424 for standard oil or 1.38 for alcohol, etc. may be used to achieve a low index contrast in the grating  1710 . This low index contrast of grating results in a much narrower FWHM of the reflection peak, for example, a FWHM of about 0.3 nm.  
      The slab  1720  over the side-polished fiber core  140 A provides a physical discontinuity of the fiber  140  for guiding light confined in a guided mode. This physical discontinuity can cause the guided light to scatter and thus some optical loss. To reduce this optical loss, a transition region may be provided at the two ends of the waveguide  1720  to gradually transfer the mode initially guided by the fiber core  140 A to the mode guided in the combination structure of the waveguide  1720  and the fiber core  140 A.  
       FIGS. 18A, 18B , and  18 C show one implementation of a slab design with two tapered end regions. Each tapered end region gradually transforms the mode to reduce optical loss.  FIG. 18A  shows the top view,  FIG. 18B  the sectional view along the line BB, and  FIG. 18C  the sectional view along the line CC. The tapered end regions are designed to change their geometrical dimension in an optically gradual manner so that guided mode can adiabatically transform without an abrupt change. An optically adiabatic change reduces the optical loss in comparison to an abrupt change that does not satisfy the adiabatic condition.  
       FIG. 19  shows the simulated transverse mode profile of the guided mode at the location in the waveguide  1720  where the guided mode is shown to shift towards the waveguide  1720  and the grating  1710 .  
       FIG. 20  shows the dependence of the wavelength of the reflection peak on the index n(P,T) of the overlay layer  1620 . A shift of 6 nm in wavelength is illustrated for a change in the index from 1.415 to 1.435.  
      The reflection sensors may be used to place the optical terminal for injecting the probe light and the optical detector for receiving reflected probe light at the same location. In this aspect, the reflection sensors are different from the transmission sensors. Notably, when multiple reflection sensors are formed at different locations in a single fiber, the reflected signals from different sensors arrive at the same detection location in the fiber with different time delays. This feature may be used to distinguish signals from different sensors based on signal delays in time without relying on differences in wavelengths at different sensors as described above in the transmission sensors in a single fiber.  
       FIG. 21  shows a fiber sensing system having at least two fiber sensors  2110  and  2120  formed at different locations in a single fiber  2100 . A light source  2101  such as a diode laser is coupled to one end of the fiber  2100  to inject a probe beam into the fiber  2100 . A first portion of the probe beam is reflected back at the first sensor  2110  and a second portion of the probe beam is reflected back at the second sensor  2120  at a later time. Sensors  2110  and  2120  may be configured to operate at the same wavelength. Optical reflections from different sensors propagate in the opposite direction of the original probe beam. The reflected signals may be coupled out of the fiber  2100  by using a fiber coupler or an optical circulator  2130  at a location in the fiber  2100 . The optical output from the coupler or circulator  2130  is sent to an optical detector  2140 . A signal processor  2150  is used to receive and process the detector output from the detector  2140  to produce the measurements at the sensors  2110  and  2120 . The fiber coupler/circulator  2130 , the diode laser  2101 , and the optical detector  2140  may be located at the same side of the fiber  2100 . The signal processor  2150  may be designed to distinguish signals from different reflection sensors based on the timings of arrival for different signals. Hence, a single optical detector  2140  may be sufficient in this multi-sensor system to measure signals from different sensors in the fiber  2100 .  
      Only a few exemplary implementations are disclosed. However, variations and enhancements may be made.