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 . fig1 a 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 fig1 a and 1b , the basic structure of the fiber - optic sensing system 1 according to a preferred embodiment of the invention is schematically illustrated . fig1 a is a sectional outside perspective view of the fiber - optic sensing system 1 . in fig1 a , 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 . fig1 b is a cross section view of the sheath 12 and the optical fiber 16 of fig1 a 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 . fig2 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 fig1 a 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 . fig3 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 fig3 . 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 . fig4 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 . fig5 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 . fig6 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 . fig7 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 . fig8 a 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 μm for 1 mpa pressure change . fig8 b shows a modification of the embodiment of fig8 a , 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 .
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 .