Patent Document

BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to optical fibers and, more particularly, to optical fiber sensors to measure deformation. 
     2. Background Art 
     Optical fiber systems offering sensing functions have been used in numerous applications. One known sensor technology for optical fibers is the Bragg grating sensor. The Bragg grating is a periodic variation of the refractive index along the fiber axis and it is photoimprinted in the core of the fiber by using UV light. The main property of a Bragg grating is that part of the light going along the fiber is reflected back by the grating. This process is wavelength selective and the wavelength with the highest reflectivity is called the Bragg wavelength (typically located at the center of the spectral response). Any temperature variation or strain applied to the optical fiber might result in a Bragg wavelength shift and/or spectral deformation, depending on how perturbations are applied to the fiber. 
     United States patent application Publication No. US 2004/0234218, by inventors Tao et al. discloses an optical fiber device that is applied for the measurement of multiple parameters, such as deformation. More specifically, the optical fiber device described in Publication No. US 2004/0234218 has at least two cores within a cladding, with the two cores respectively provided with Bragg grating sensors. The optical fiber of the Tao et al. reference describes a fiber configuration in which four cores, each with different characteristics, are provided (FIG. 1 of US 2004/0234218), with a central core, and three peripheral cores, at an equal radial distance from the central core and 120 degrees apart. Such an optical fiber is suitable for decoupling the effect of bending from the effect of temperature on the Bragg gratings of the fiber. However, an orientation of the fiber must be considered in such decoupling, but this publication provides no detail as to how the fiber may be oriented in a desired manner. Moreover, it is noted that in embodiments each fiber core in this publication has different characteristics (optical, thermal and mechanical) due to different chemical compositions. 
     SUMMARY OF INVENTION 
     It is therefore an aim of the present invention to provide an optical fiber sensor that addresses issues associated with the prior art. 
     It is a further aim of the present invention to provide novel optical fiber sensor and optical fiber sensor system. 
     Therefore, in accordance with the present invention, an optical fiber sensor for detecting curvature of a body/structure is described, comprising: a cladding having an outer periphery; a central core for receiving and transmitting light, the central core having Bragg gratings and being positioned in a first neutral plane of the cladding, such that the Bragg gratings of the central core are insensitive to bending about a first axis associated with the first neutral plane and sensitive to temperature variations; at least one peripheral core for receiving and transmitting light, the at least one peripheral core having Bragg gratings and being peripherally positioned in the cladding with respect to the first neutral plane such that the Bragg gratings of the at least one peripheral core are sensitive to bending about the first axis and to temperature variations; and a connection configuration in the outer periphery of the cladding to attach the optical fiber sensor to a body/structure such that the central core and the at least one peripheral core are in a predetermined position and orientation with respect to the body/structure, so as to measure curvature of the body/structure about at least the first axis independently of the effect of temperature variations by associating the Bragg wavelengths of the central core and of the at least one peripheral core. 
     Further in accordance with the present invention, there is provided a method for measuring curvature in a body, comprising the steps of: providing an optical fiber sensor having at least two cores in a cladding, a plurality of longitudinal sets of Bragg gratings being provided in the cores at known locations along the optical fiber sensor, each Bragg grating having a different Bragg wavelength; positioning the optical fiber sensor on a body in a known position and orientation; emitting light in the optical fiber sensor; receiving light from the optical fiber sensor; measuring the Bragg wavelength shifts from the light received from the optical fiber sensor; and calculating the curvature of the body by associating the measured Bragg wavelength shifts at the known locations in the cores to the curvature of the optical fiber sensor with respect to the known position and orientation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Having thus generally described the nature of the invention, reference will now be made to the accompanying drawings, showing by way of illustration a preferred embodiment thereof and in which: 
         FIG. 1  is an end elevation view of an optical fiber sensor in accordance with a first embodiment of the present invention; 
         FIG. 2  is a perspective schematic line-wire view of the optical fiber sensor of  FIG. 1 ; 
         FIG. 3  is a graph illustrating a Bragg spectral response reacting to strains on a reflectivity scale; 
         FIG. 4  is an end elevation view of an optical fiber sensor in accordance with a second embodiment of the present invention; 
         FIG. 5A  is a schematic view illustrating the optical fiber sensor of  FIG. 4  being bent in a first direction about one axis; 
         FIG. 5B  is a schematic view illustrating the optical fiber sensor of  FIG. 4  without being bent; 
         FIG. 5C  is a schematic view illustrating the optical fiber sensor of  FIG. 4  being bent in a second direction about the axis; 
         FIG. 6  is a schematic view of an optical fiber sensor system having one of the optical fiber sensors of  FIG. 1  and  FIG. 4 ; and 
         FIG. 7  is an end elevation view of an optical fiber sensor in accordance with a third embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings and, more particularly, to  FIG. 1 , an optical fiber sensor in accordance with a first embodiment is generally shown at  10 . The optical fiber sensor  10  is an optical fiber having a cladding  11 , a central core  12 , and peripheral cores  13  and  14 . 
     The cladding  11  has a generally circular section, but has a flat edge, defining a flat surface  15  on the full length of the cladding  11 . The cladding  11  is made of a material having an effective index of refraction smaller than that of the cores  12 ,  13  and  14 . 
     The central core  12  is generally centrally positioned within the cladding  11 , as is visible in  FIG. 1 . 
     The peripheral cores  13  and  14  are respectively spaced apart from the central core  12  by distances A and B. In the first embodiment of  FIGS. 1 and 2 , the central core  12  and the peripheral core  13  lie in a first neutral plane N x  generally parallel to the flat surface  15 . The central core  12  and the peripheral core  14  lie in a second neutral plane N y  generally perpendicular to the flat surface  15 . The central core  12  is therefore at the intersection of the first and second neutral planes. 
     Referring to  FIG. 2 , the optical fiber sensor  10  is shown having sets of Bragg gratings in the cores  12 ,  13  and  14 . More specifically, the central core  12  has gratings  20 ,  21 ,  22 , etc. The peripheral core  13  has gratings  30 ,  31 ,  32 , etc. The peripheral core  14  has gratings  40 ,  41 ,  42 , etc. For illustrative purposes, only a portion of the optical fiber sensor  10  is illustrated, as the optical fiber sensor  10  typically has a plurality of other Bragg gratings. 
     The gratings are regrouped by sets of longitudinally aligned gratings, namely a first set of gratings  20 ,  30  and  40  at location L 1  along the fiber, a second set of gratings  21 ,  31 , and  41 , at a location L 2  along the fiber, etc. Each set of gratings represents a point of detection in a known location along the optical fiber sensor  10 . 
     The optical fiber sensor  10  is provided with the flat surface  15  so as to be installed in a predetermined way (i.e., known position and orientation) on a body whose curvature must be detected. The optical fiber sensor  10  is typically associated with a support that will be secured to the body, and the predetermined way by which the optical fiber sensor  10  will be secured to the body through the support will enable to reference the two axes of curvature (illustrated as axis X and axis Y in  FIG. 1 ) about which curvature can be measured. 
     Moreover, in the first embodiment, the optical fiber sensor  10  is secured to the support in such a way that the optical fiber sensor  10  is prevented from being elongated or stretched. 
     Bragg gratings are generally sensitive to elongation or compression that might result from bending or strain and from temperature variations. Referring to  FIG. 3 , a graph  50  illustrates the reflectivity spectrum of the Bragg grating as a function of the wavelength of light reflected by the Bragg grating, although a similar graph would have been obtained by capturing the light transmitted by the Bragg grating. From an initial position  51 , a compression (for instance, resulting from a temperature decrease) of the Bragg grating results in a decrease of the Bragg wavelength, as illustrated as position  52 . On the other hand, an elongation (for instance, from a temperature increase) of the Bragg grating results in an increase of the Bragg wavelength, as illustrated at position  53 . 
     The central core  12  is neutrally positioned within the optical fiber sensor  10  and its support so as not to be sensitive to bending. More specifically, the central core  12  lies in the first neutral plane N x , so as to not be sensitive to bending about the X-axis. Similarly, the central core  12  lies in the second neutral plane N y , so as not to be sensitive to bending about the Y-axis. In the preferred embodiment, the peripheral core  13  lies in the first neutral plane N x , and is therefore not sensitive to bending about the X-axis (sensitive only to bending about the Y-axis), whereas the peripheral core  14  lies in the second neutral plane N y , so as not to be sensitive to bending about the Y-axis (sensitive only to bending about the X-axis). 
     The optical fiber cross-section temperature at a specific location should be almost uniform. This implies that all gratings at a known location will have the same temperature, such that the gratings in all cores will have the same Bragg wavelength shift at that known location. Accordingly, the combination of the central core  12  with the peripheral cores  13  and  14  enables to separate bending-induced wavelength shifts from temperature-induced wavelength shifts in the Bragg gratings. Hence, the optical fiber sensor  10  enables to measure the curvature of a body independently of the effect of temperature on the optical fiber sensor  10 . 
     Referring to  FIG. 4 , an optical fiber sensor in accordance with a second embodiment is generally shown at  10 ′. The optical fiber sensor  10 ′ is generally similar to the optical fiber sensor  10  ( FIG. 1 ), in that it has a cladding  11 , a central core  12 , and peripheral cores  13  and  14 . The optical fiber sensor  10 ′ additionally has peripheral cores  13 ′ and  14 ′, diametrically opposed to the peripheral cores  13  and  14 , respectively. 
     The peripheral cores  13 ′ and  14 ′ are provided to increase the sensitivity of the optical fiber sensor  10 ′. More specifically, the longitudinally aligned sets of gratings of the peripheral cores  13 ′ and  14 ′ are respectively combined with that of the peripheral cores  13  and  14 , to provide two gratings per axis of curvature (e.g., axes X and Y of  FIG. 1 ). For example, when the grating in the peripheral core  13  is compressed, the corresponding grating in the peripheral core  13 ′ is elongated, giving twice the total spectral shift, in opposite directions, of this Bragg gratings set compared to the central core and the peripheral core  13  Bragg gratings set. 
     Referring to  FIGS. 5A to 5C , the optical fiber sensor  10 ′ is shown as being bent in a first direction about a first axis in  FIG. 5A , and in a second direction about the first axis in  FIG. 5C , while the optical fiber sensor  10 ′ in  FIG. 5B  is not bent. It is therefore seen in  FIG. 5A  that the gratings  30  and  30 ′ (respectively of the peripheral cores  13  and  13 ′) undergo compressive and tensile strains, respectively, whereas in  FIG. 5C  the gratings  40  and  40 ′ (respectively of the peripheral cores  14  and  14 ′) undergo tensile and compressive strains, respectively, while the centrally-positioned grating  20  generally remains unstrained. 
     In fact, due to the central position of the core  12 , the grating  20  and any other grating in the central core  12  only undergo strains (within the operative curvature range of the sensors  10  and  10 ′) caused by temperature. As all gratings of a same set are generally subjected to the same temperature, the strain induced by bending can be isolated from the strain induced by temperature by relating the gratings of the central core  12  to the gratings of the peripheral cores  13 ,  13 ′,  14  and/or  14 ′. 
     Referring to  FIG. 6 , an optical fiber sensor system in accordance with the first and second embodiments is generally shown at  100 . The optical fiber sensor system  100  has one of the optical fiber sensor  10 / 10 ′/ 10 ″ (with optical fiber sensor  10 ″ being described hereinafter for  FIG. 7 ) secured to a support  101  in a predetermined way (i.e., known position and orientation), in which the optical fiber sensor  10 / 10 ′/ 10 ″ is oriented for curvature measurement about two reference axes ( FIGS. 1 and 2 ). 
     A light source  102  is provided with coupling optics  104  so as to multiplex light signals into the optical fiber sensor  10 / 10 ′/ 10 ″. Therefore, light from the light source  102  is coupled to the central core  12 , and the peripheral cores  13 ,  13 ′,  14  and/or  14 ′. As the optical fiber sensor  10 / 10 ′/ 10 ″ is mounted to a body by way of the support  101 , movements in the body will cause strain in the Bragg gratings of the peripheral cores  13 ,  13 ′,  14  and/or  14 ′. Accordingly, Bragg wavelength shifts can be determined in order to qualify (compressive or tensile strain) and quantify (angular value) the curvature. 
     In order to couple the light source  102  to the optical fiber sensor  10 / 10 ′/ 10 ″ via the coupling optics  104 , a first approach known in the art provides a multicore-to-single-core fiber fan-out to feed each core of the multicore fiber independently. 
     In such an arrangement for the coupling optics  104 , the diameters of four single-core single-mode fibers are reduced using hydrofluoric acid. The fibers are then arranged in a square shape to match the core spacing of the optical fiber sensor  10 / 10 ′/ 10 ″. The fiber  10 / 10 ′/ 10 ″ is then connected to the fan-out by an adhesive bonded splice. 
     In one embodiment of the coupling optics  104 , the fan-out arrangement includes five fibers, with one in the middle surrounded by four other fibers arranged in square shape in order to match the core distribution of the configuration of  FIG. 4  of the fiber  10 / 10 ′/ 10 ″. An efficient coupling is assured by reducing the fibers&#39; cladding until the fiber core distances are matched. 
     In a second approach, as indicated in the US Publication No. US 2004/0234218, a refractive plano-concave lens is used to couple and extract the light from free-space beams. The light beams from different cores of the multicore fiber are separated in different directions by the plano-concave lens. Of course, the same configuration can be used to couple separated light beams into the fiber  10 / 10 ′/ 10 ″. 
     At the outlet end of the optical fiber sensor  10 / 10 ′/ 10 ″, the optical fiber sensor  10 / 10 ′/ 10 ″ is coupled to a light analyzer so as to receive the transmitted light signals in an embodiment in which transmitted light signals are analyzed. It is, of course, considered to analyze reflected light signals in an alternative embodiment. The light analyzer  106  is typically an optical spectrum analyzer measuring shifts in the Bragg wavelengths. 
     A processor  108  associated with the light analyzer  106  performs the calculation of curvatures along the two axes of reference (e.g., axes X and Y in  FIG. 1 ), as a function of the shift magnitude in Bragg wavelengths as provided by the light analyzer  106 . The Bragg wavelength readings obtained from the Bragg gratings of the central core  12  are used to factor out the effect of temperature on the optical fiber sensor  10 / 10 ′/ 10 ″. Hence, the processor  108  provides curvature magnitude and orientation over time, for instance in the form of angular values with respect to reference axes such as axes X and Y. 
     According to a first configuration, the Bragg wavelengths of each Bragg grating are different. This simplifies the detection of the wavelengths for the subsequent analysis of the results by both the light analyzer  106  and the processor  108  as each transmitted (or reflected) wavelength is directly associated with a specific Bragg grating and location in a specific core. The variation of Bragg wavelength of the Bragg gratings of the central core  12  will be used to determine the effect of temperature on the optical fiber sensor  10 / 10 ′/ 10 ″. 
     According to a second configuration, the Bragg wavelengths at each location (e.g., L 1 , L 2 , etc., of  FIG. 2 ) of the optical fiber sensor  10  are the same for an uncurved optical fiber sensor  10  at that given location. The light analyzer  106  must receive the transmitted (or reflected) light signals from each core of the optical fiber sensor  10 / 10 ′/ 10 ″ separately. The Bragg gratings are then identified as a function of the wavelengths detected and the selected core. 
     The processor  108  will read the Bragg wavelengths provided by the optical analyzer  106 , and will determine shifts of the Bragg wavelengths, whereby curvature is calculable with respect to the reference axes (i.e., axes X and Y of  FIG. 1 ). The reading of the Bragg wavelengths and calculation of curvature by the processor  108  is optionally performed over time. 
     The support  101  is defined as a function of the type of body/structure upon which the optical fiber sensor  10 / 10 ′/ 10 ″ will be installed for curvature measurement. For instance, in one contemplated use of the optical fiber system  100 , the optical fiber sensor  10 / 10 ′/ 10 ″ is used to calculate curvature on various parts of the human body (e.g., back, spine, or the like). Therefore, the support  101  is some clothing that will marry the shape of the body part, and keep the optical fiber sensor  10 / 10 ′/ 10 ″ in the predetermined orientation. As examples of suitable clothing are undershirts, tights, gloves, arm and leg sleeves, and the like. 
     Accordingly, the flat surface  15  (or flat surfaces  15  and  15 ′ as will be described hereinafter for  FIG. 7 ) represent one configuration, among other connection configurations, by which the orientation of the optical fiber sensor  10 / 10 ′/ 10 ″ can be maintained throughout the use of the optical fiber sensor system  100 . The flat surface  15  is machined into a glass (or polymeric) preform which will be melted and drawn to get an optical fiber. Therefore this technique represents an efficient solution for a quick and precise axes orientation determination. Alternatively, connection holes, peripheral depressions or the like can be used to connect the optical fiber sensor  10 / 10 ′/ 10 ″ to a body in a connection configuration. 
     In accordance with the contemplated use of the optical fiber sensor system  100 , a calibration is typically performed at a constant temperature to obtain an initial position and orientation of the optical fiber sensor  10 / 10 ′/ 10 ″ with the body whose curvature is to be detected. For instance, it is contemplated to perform a calibration of the initial value of Bragg wavelengths of each grating with respect to the body, such that a given curvature at a specific location of the optical fiber sensor  10 / 10 ′/ 10 ″ is associated with a position on the body. 
     In the embodiments of  FIGS. 1-2  and  45 , the peripheral cores  13 ,  13 ′,  14  and/or  14 ′ are positioned to provide curvature about the axes X and Y. More specifically, strain sustained by the peripheral cores  13 / 13 ′ will represent curvature about the Y-axis (normal to the plane of the body at the location of the set of gratings), whereas strain sustained by the peripheral cores  14 / 14 ′ will represent curvature about the X-axis (parallel to the plane of the body at the location of the set of gratings). It is pointed out that other positions are contemplated for the peripheral cores  13 ,  13 ′,  14  and  14 ′, within the cladding  11 , and with respect to the connection configuration (i.e., the flat surface  15 ). 
     Moreover, the distances A and B between the central core  12  and the peripheral cores  13  and  14  ( FIG. 1 ) can be adjusted at the design time, as a function of the required sensitivity and the flexibility of the optical fiber sensor  10 / 10 ′/ 10 ″. Greater distances A and B (and thus greater diameter of the fiber  10 / 10 ′/ 10 ″) will result in increased sensitivity of the sensor and decreases flexibility of the fiber, and vice-versa. 
     Although the optical fiber sensor  10 / 10 ′/ 10 ″ is illustrated as being generally circular, it is pointed out that the other cross-section shapes (square, rectangular, trapezoidal, etc . . . ) are considered for the optical fiber sensors  10 / 10 ′/ 10 ″. Moreover, curvature detection can be performed about a single axis, such as axis X ( FIG. 1 ). In such a case, only one peripheral core is necessary, such as peripheral core  14  for curvature measurement about the X-axis. 
     Referring to  FIG. 7 , an optical fiber sensor in accordance with a third embodiment is generally shown at  10 ″ The optical fiber sensor  10 ″ is similar to the optical fiber sensor  10 ′ of  FIG. 4 , whereby like reference numerals will designate like elements. 
     The cladding  11  of the optical fiber sensor  10 ″ has a generally circular section, but with a pair of flat edges, defining flat surfaces  15  and  15 ′ on the full length of the cladding  11 . Advantageously, the fiber sensor  10 ″ is symmetrical along both the X- and Y-axes. The planes of symmetry are therefore coplanar with the first neutral plane N x  and the second neutral plane N y . The presence of a pair of flat surfaces  15  and  15 ′ facilitates the securing of the optical fiber sensor  10 ″ in a desired orientation, and ensures that the central core  12  is n the neutral planes for both axes. 
     Amongst contemplated uses for the optical fiber sensors  10 / 10 ′/ 10 ″ and the optical fiber sensor system  100  are posture detection (e.g., health clubs) and posture correction, ergonomic studies, physical rehabilitation. Other uses are virtual-reality movement detection, computer animation (e.g., reproduction of body movements), air-bag deployment control, movement-detecting prosthesis, auto-adjusting seating devices. Other uses are contemplated, whereby the list of above-described uses is non-exclusive.

Technology Category: g