Abstract:
The invention provides a fiber-optic sensing system, utilizing a fiber-grating-based sensor, for a physical parameter, e.g., a pressure or a temperature. Different kinds of fiber-grating-based sensors may be used for this purpose but in-fiber gratings such as Fiber Bragg Grating, Long Period Grating and Surface Corrugated Long Period Fiber Grating are particularly suitable. Due to the small size of the optical fiber and the fact that same fiber acts as the sensing element as well as the signal conducting medium, it is possible to install the sensor in a small diameter needle which is commonly used for medical diagnosis and treatment. As a result, when the fiber-optic sensing system of the invention is used for in-vivo measurement of a biological parameter, such a sensing needle can be used for different in-vivo pressure or temperature sensing applications without causing too much harm and discomfort to the subject tested.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the sensing of physical parameter, such as pressure or temperature, using in-fiber sensors, e.g., short period fiber Bragg grating, long period fiber grating or surface corrugated long period fiber grating. Since signal sensing and signal transfer can both occur in the same fiber, it is possible to derive miniature pressure or temperature sensing systems or transducers for use in medical diagnoses in the areas of intradiscal, intracranial, intramuscular, intra-articular, ventricular pressure or temperature monitoring with minimum invasiveness. For example, for intra-articular pressure measurement, the small size of the needle means that the pressure of small joint cavities like the temporo-mandibular joint, the facet joints of the vertebral column, the carpal joints of the wrist and the tarsal joints of the midfoot region can also be assessed by means of this device. With respect to the technology background of the invention, please refer to the following references:
     [1] Pollintine P, Przybyla A S, Dolan P, Adams M A. Neural arch load-bearing in old and degenerated spines.  J Biomech  2004;37:197–204.;   [2] Sato K, Kikuchi S, Yonezawa T. In Vivo intradiscal pressure measurement in healthy individuals and in patients with ongoing back problems. Spine 1999;24:2468–74;   [3] Wilke H J, Neef P, Caimi M, Hoogland T, Claes L E. New in-vivo measurements of pressures in the intervertebral disc in daily life. Spine 1999;24:755–62;   [4] McNally D S, Shackleford I M, Goodship A E, Mulholland R C. In vivo stress measurement can predict pain on discography. Spine 1996;21(22):2580–7;   [5] Kane; James, “Optical pressure sensor for measuring blood pressure”, U.S. Pat. No. 4,691,708;   [6] Wallace L. Knute, Wilber H. Bailey, “Fiber-optic transducer apparatus”, U.S. Pat. No. 5,107,847;   [7] Alderson; Richard, “Fiber optic coupled pressure transducer using single fiber and method of fabrication”, U.S. Pat. No. 4,711,246;   [8] U.S. Pat. No. 4,924,870;   [9] U.S. Pat. No. 5,275,053;   [10] U.S. Pat. No. 5,385,053; and   [11] U.S. Pat. No. 5,422,478.   

     2. Description of the Prior Art 
     Pressure measurement is important in engineering, medical diagnosis and research and development in many fields. Technology used in conventional pressure measurement may be broadly differentiated into mechanical, electrical and fiber-optic categories. A diaphragm that can deform under the application of pressure is normally employed as the primary transducer for pressure. The deformation of the diaphragm is converted into the movement of a dial pointer through suitable mechanisms in the mechanical pressure gage. The use of purely mechanical components makes this type of gages very bulky. In the electrical category, resistive strain gages are normally employed to convert the diaphragm deformation into electrical signals. Although this type of pressure transducer can be made much smaller than the mechanical ones, the size of the strain gages and the need to lead out a number of wires for electrical excitation and signal measurement make it difficult to reduce their size to below the millimeter level. Typical small-sized pressure probe of this category used for in-vivo medical measurement has a diameter from 1.5 to 3 mm, please refer to references [1] to [4]. At these sizes, the implantation of the pressure transducer to make in-vivo measurement is a rather invasive procedure and could mean much discomfort to the subject concerned. This type of transducer is also susceptible to electromagnetic interference and measurement accuracy may be affected if there are other medical instruments nearby. 
     Referring to references [5], [6] and [7], the typical fiber-optic pressure transducer system comprises two sets of optical fibers. One set of fibers transmits a light beam to shine on the deformable diaphragm and the other set of fibers returns a modulated light beam reflected from the diaphragm. It is also possible to use a single optical fiber for the two-way light traffic, please refer to reference [7]. Pressure variation will deform the diaphragm, thereby varying the proximity of the diaphragm to the fiber ends, thus modulating the intensity of the reflected light. By measuring the reflected light intensity using a photo-sensor, the pressure can be deduced. Good alignment of the fibers and the reflecting surface of the diaphragm is needed and high precision manufacturing process make this kind of sensor expensive to produce. This type of transducers have not been entirely satisfactory as the intensity of light transmitted in an optical fiber can be reduced by bending of any part of the fiber, movement of the pressure probe and a faulty connector. Moreover, temperature fluctuation may also affect the accuracy of the measurement. In fact, there are a number of inventions made to combat these problems, please refer to references [8], [9], [10] and [11], but this often means packing some more optical fibers into the transducer for reference purposes. 
     SUMMARY OF THE INVENTION 
     In view of the limitation of related conventional arts, one objective of this invention is to provide a low cost yet simple and robust fiber-optic sensing system for measurement of pressure or temperature, especially for in-vivo pressure or temperature, whose measurement accuracy is independent of the bending of the fiber and temperature fluctuations. Pressure or temperature measurement at multiple sites can also be achieved by having a number of fiber-grating-based sensors along the optical fiber. Simultaneous temperature may also be made in the vicinity of the pressure measuring point. 
     Another objective of this present invention is to provide a miniature pressure or temperature transducer suitable for in-vivo measurement with minimal invasiveness. The miniature transducer will also be useful for pressure measurement in engineering structures and compartments too small to house a conventional sensor. The transducer can be used in adverse environment that involves magnetic field, electromagnetic interference, and ionizing radiation. 
     As aforementioned, this present invention provides a fiber-optic pressure transducer, comprising: an outer sheath with closed distal end, one or more windows on the sheath, diaphragms covering and sealing each of the windows, an optical fiber running from the distal end of the sheath to a data processing and read-out instrument, and a designated number of in-fiber sensing elements along the length of the optical fiber. These sensing element can be fiber Bragg grating, long period grating or periodic surface corrugation that exhibits long period grating effect under deformation. When the flexible diaphragm deform under pressure, the attached in-fiber sensor will be deformed as well, modulating the wavelength of the reflected and/or transmitted light. It should be pointed out that with the current design, in addition to fluid pressure, pressure exerted by soft tissue such as muscle or tissue between vertebrate discs can also be monitored. 
     The present invention also provides a fiber-optic pressure transducer, comprising: an outer sheath with closed distal end, one or more windows on the sheath, an optical fiber fixed at and running from the distal end of the sheath to a data processing and read-out instrument and a flexible diaphragm downstream of the windows that fix the fiber to the sheath. An in-fiber sensing elements along the length of the optical fiber between the fixed distal end and the diaphragm is used to sense the fluid pressure when the sheath is put into a fluid environment. This sensor will not be able to monitor pressure from soft tissue or fluid that is too viscous to pass into the interior of the sheath through the windows. 
     By using optical fiber as the sensing and signal conducting element, the overall size of the sensor can be greatly reduced. Standard glass optical fiber has an outer diameter of 125 μm. Smaller diameter fibers are available commercially and it can also be achieved by etching standard fibers. The size of the sheath needs only be slightly (say 50 μm) larger than the fiber although larger sheath can also be used if the other considerations call for it. For in-vivo applications, by integrating the diaphragm/fiber sensor assembly with the spinal needle or other needles, the resulting instrument is robust enough to be deployed by direct insertion to the organ concerned without using an additional catheter. The insertion of such small needle into the body will constitute minimum invasiveness and discomfort. 
     By employing fiber gratings as sensor, the parameter modulated by the measurand is coded in the wavelength and so intensity fluctuation of the source or random losses caused by bending will not affect the accuracy of measurement. Temperature induced drift in the measurement can be compensated by an independent grating in the same fiber that is fixed to the sheath, exposed to the same temperature but not to the pressure. This additional grating also enabled the local temperature to be monitored alongside with the pressure. 
     The advantage and spirit of the invention may be understood by the following recitations together with the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE APPENDED DRAWINGS 
         FIG. 1A  is an outside perspective view of a sheath  12  and an optical fiber  16 , disposed in the sheath  12 , of a fiber-optic sensing system  1  according to a preferred embodiment of the invention. 
         FIG. 1B  is a cross section view of the sheath  12  and the optical fiber  16  of  FIG. 1A  along the A—A line. 
         FIG. 2  shows the measured variation in pressure inside the space between two vertebral discs using an embodiment of  FIGS. 1A and 1B  when the vertebrate segment is subjected to different axial loading. 
         FIG. 3  is schematic drawing showing another embodiment of the invention using a long period grating as the fiber-grating-based sensor. 
         FIG. 4  shows an improvement on the basic structure in  FIG. 1B  to obtain a better sensitivity by moving the fiber core sensor farther away from the flexible diaphragm. 
         FIG. 5  shows another way to improve sensitivity by adding a low stiffness fiber between the diaphragm and the optical fiber in  FIG. 1B . 
         FIG. 6  shows yet another way to increase sensitivity by introducing some notches to the optical fiber in  FIG. 1B  to induce strain concentration effect. 
         FIG. 7  shows additional fiber-grating-based sensor fixed to the sheath for monitoring the local temperature as well as providing temperature drift correction to the original fiber-grating-based sensor as pressure sensor. 
         FIG. 8A  shows a different layout of the fiber-grating-based sensor in  FIG. 1B  as pressure sensor wherein the opening is not sealed. 
         FIG. 8B  shows a modification of the embodiment of  FIG. 8A  wherein the opening is sealed with an additional diaphragm. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A description will now be given of the preferred embodiments of the present invention with reference to the drawings. 
     In the drawings, the same numeral notation refers to the same element. The drawings and the following detailed descriptions show specific embodiments of the invention. In the preferred embodiment, polymeric adhesive was employed to manufacture the flexible diaphragm and spinal needle was employed as the sheath. Numerous specific details including materials, dimensions, and products are provided to illustrate the invention and to provide a more thorough understanding of the invention. However, it will be obvious to one skilled in the art that the present invention may be practiced using other materials for the sheath and flexible diaphragm and without these specific details. 
       FIG. 1A  is an outside perspective view of a needle  12  and an optical fiber  16 , disposed in the needle  12 , of a fiber-optic sensing system  1  according to a preferred embodiment of the invention. 
     Referring to  FIGS. 1A and 1B , the basic structure of the fiber-optic sensing system  1  according to a preferred embodiment of the invention is schematically illustrated.  FIG. 1A  is a sectional outside perspective view of the fiber-optic sensing system  1 . In  FIG. 1A , the essentials of the fiber-optic system  1  including a sheath  12  and an optical fiber  16 , disposed in the sheath  12 , are shown. In this case, the outer sheath  12  is a spinal needle.  FIG. 1B  is a cross section view of the sheath  12  and the optical fiber  16  of  FIG. 1A  along A—A line. 
     As shown in  FIG.1B , the sheath  12  has a sealed tip  122 , a main body  124  and a formed-through opening  126  formed on the main body  124  and sealed with a diaphragm  14 . In this case, an original opening at distal end (needle tip)  122  is sealed with a polymeric adhesive. Also in this case, the opening  126  is machined near the needle tip and is sealed by a flexible polymeric diaphragm  14 . 
     The optical fiber  16  has a distal end  162  and a head end (not shown). The optical fiber  16  thereon includes a fiber-grating-based sensor  18   a . In this case, the fiber-grating-based sensor  18   a  is a fiber Bragg grating (FBG). The optical fiber  16  with the FBG  18   a  is inserted into the interior of the needle  12 . The portion of the optical fiber  16  with the FBG  18   a  written to the core of the optical fiber  16  is stuck to the inside surface of the flexible diaphragm  14 . 
     The fiber-optic sensing system  1  also includes an optical device and a signal processing device (not shown). The optical device functions emitting a sensing light signal into the second end of the optical fiber  16  and receiving a first reflected light signal resulting from the sensing light signal reflected by the fiber-grating-based sensor  18   a . When the needle  12  is inserted into a region, for example, a fluid medium or soft tissue, where a physical parameter needs to be measured, the region affects the fiber-grating sensor  18   a  through the diaphragm  14  to induce a variation on the first reflected light signal. The signal processing device is coupled to the optical device, and functions interpreting the variation on the first reflected light signal into the physical parameter. 
     Taking pressure as example, pressure in the region will cause a deformation of the diaphragm  14 . The FBG  18   a  will be deformed as well and the characteristic Bragg wavelength will be shifted away from its initial position. The amount of shift is proportional to the pressure acting on the diaphragm  14 . By measuring the shift in the reflected Bragg wavelength using a suitable signal processing device, the pressure can be deduced. 
       FIG. 2  shows the variation in pressure measured when a pressure transducer was inserted inside the space between two vertebral discs and the vertebrate segment is subjected to different axial loading. The pressure transducer was obtained by employing the embodiment illustrated in  FIGS. 1A and 1B  using a 26-G (0.45 mm outer diameter) spinal needle as the outer sheath. 
     Besides using a short period fiber Bragg grating, long period grating (LPG) can also be used as the fiber-grating-based sensor, e.g., long period fiber grating or surface corrugated long period fiber grating.  FIG. 3  shows another embodiment using the LPG as the fiber-grating-based sensor  18   b . The LPG  18   b  will attenuate a characteristic spectrum when a broad spectrum light is passed through it. This characteristic spectrum will shift with strain applied to the LPG  18   b . However, such a characteristic attenuation spectrum is only evident from the transmitted light. To allow this spectrum to be measured at the proximal end, a mirror coating  164  is plated at the distal end  162  of the optical fiber  16  to reflect the transmitted spectrum back. This is illustrated in the embodiment in  FIG. 3 . 
     Since the flexible diaphragm  14  as well as the optical fiber  16  deform by bending, the induced strain in the in-fiber sensor (the fiber-grating-based sensor)  18   a  can be amplified by moving the sensor region further away from the neutral axis (i.e. the axis without extension or contraction under bending). Since the in-fiber sensor  18   a  essentially situated at the core of the optical fiber  16 , the above requirement can be achieved by moving the fiber core as far from the flexible diaphragm  14  as possible.  FIG. 4  shows yet another embodiment that employs an optical fiber  16  with off-centered core to achieve this purpose. Such an off-centered core may be achieved during the manufacturing of the optical fiber  16 . It can also be obtained by selective etching of the cladding on a standard fiber. 
       FIG. 5  shows yet another embodiment to improve sensitivity by moving the core of the optical fiber  16  as far from the flexible diaphragm  14  as possible. It is achieved by bonding a low stiffness fiber  166  between the diaphragm  14  and the optical fiber  16 . The stiffness of the additional fiber  166  is chosen to be low so as keep the flexural rigidity of the whole diaphragm/fibers structure low to ensure a higher strain at the fiber core. 
       FIG. 6  shows yet another embodiment to increase the pressure sensitivity by introducing some notches  168  in the cladding of the optical fiber  16  in the vicinity of the in-fiber sensor  18   a . These notches  168  will induce strain concentration and amplify the strain at the sensor region. 
     For person skilled in the art, there will be other similar ways to increase the strain and thus the sensitivity of the pressure sensor. For clarity of explanation, a separate technique is employed in each of the above embodiments to increase the sensitivity of the pressure sensor. There is no reason that the different techniques cannot be combined together and applied to the same transducer to obtain the maximum increase in sensitivity. Moreover, in the above embodiments, only one opening and one sensor have been employed. In practice, more openings with multiple in-fiber sensors in the same or multiple optical fibers may be employed to allow the pressure or temperature at multiple sites to be measured. 
     It is well known that fiber-grating-based sensor is sensitive to strain as well as temperature. If temperature fluctuation occurs during measurement, the resulting change in the characteristic spectra will be the combined effect of temperature and pressure variations.  FIG. 7  shows an embodiment that may be used to compensate for the temperature induced drift in the characteristic spectra. An additional fiber-grating-based sensor  20  in the optical fiber  16  in the vicinity of the original fiber-grating-based sensor  18   a  underneath the diaphragm  14  is employed. This additional fiber-grating-based sensor  20  is fixed to the sheath  12  and so is isolated from the pressure of the surrounding environment (the region) such that the physical parameter is shielded by the sheath  12  and will not affect the additional fiber-grating-based sensor  20 . However, another physical parameters, such as temperature, that cannot be shielded by the sheath  12  will still affect the additional fiber-grating-based sensor  20 . Thus variation in the local temperature will cause shift in the characteristic spectrum of the additional fiber-grating-based sensor  20 . This enables the local temperature to be monitored. The latter can be used both as additional information as well as to provide temperature drift correction to the pressure sensor (the original fiber-grating-based sensor)  18   a.    
       FIG. 8A  shows yet another embodiment of the fiber-grating-based sensor  18   a  that uses a slightly different layout as the above embodiments. In this embodiment, the opening  126  is not sealed so that fluid under pressure may flow into the distal part of the sheath  12 . A flexible diaphragm  14   a  is situated inside the sheath downstream of the opening  126  to isolate any fluid from going into the proximal end of the sheath  12 . The optical fiber  16  is fixed at the distal end  162  using an adhesive  32  upstream of the opening  126 . The diameter of the optical fiber  16  near the diaphragm  14   a  is enlarged by attaching additional material (enlarged section)  34  such as polymeric adhesive to the optical fiber  16 . The enlargement is made as large as the inside diameter of the sheath  12  can accommodate but still allows smooth axial motion should the optical fiber  16  extend under pressure. This enlarged section  34  is attached to the interior of the sheath  12  through the flexible diaphragm  12 . As the pressure of the fluid acts on the enlarged section  34 , the optical fiber  16  will be elongated, straining the fiber-grating-based sensor  18   a  and modulating the characteristic light spectrum reflected. The amount of elongation or the pressure sensitivity can be controlled by choosing the ratio of diameters of the enlarged section  34  and that of the optical fiber  16 . An 60 μm optical fiber with a 300 μm diameter enlargement will give a wavelength shift of about 330 pm for 1 MPa pressure change. 
       FIG. 8B  shows a modification of the embodiment of  FIG. 8A , wherein the opening  126  is sealed with another flexible diaphragm  14   b  to form a closed space in the sheath  12  between the sheath downstream and upstream. The closed space is previously filled with a fluid. Since the diaphragm  14   b  and the fluid inside the sheath  12  are flexible, thus they will still respond to pressure fluctuation outside the sheath  12 . 
     To sum up, the description of the above-mentioned preferred embodiments is for providing a better understanding on the strengths and spirits of this present invention, not for limiting the domain of the invention. Moreover, it aims to include various modification and arrangement parallel in form into the domain of the patent applied by this present invention. Due to the above mentioned, the domain of the patent applied by the invention should be explained in a macro view to cover all kinds of possible modification and arrangement of equal form.