Abstract:
The present invention relates generally to an image tracking device in an optical communication system, and in particular, to a device and method for measuring the transverse characteristics, including the refractive index or residual stress of an optical fiber or a fiber preform.

Description:
CLAIM OF PRIORITY 
     This application claims priority to an application entitled, “Image Tracking Device and Method for Transverse Measurement of Optical Fiber,” filed in the Korean Industrial Property Office on Aug. 22, 2000 and there duly assigned Ser. No. 2000-48506. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an image tracking device in an optical communication system, and in particular, to a device and method for measuring the transverse characteristics, including the refractive index or residual stress, of an optical fiber or a fiber preform. 
     2. Description of the Related Art 
     Hereinafter, the term, “transverse section,” relating to an optical fiber (or preform) will refer to the section perpendicular to “the lengthwise section” of the optical fiber (or preform), whereas the term, “longitudinal section” will refer to the section that is parallel to “the length direction” of the optical fiber (or preform). 
     Various types of image-tracking techniques for tracking the transverse and longitudinal images of the optical fiber (or preform) are available. The transverse measuring device is typically utilized to detect the residual stress and refractive index of an optical fiber (or fiber preform). Despite having a low-image resolution, the transverse measuring device allows a non-destructive testing environment compared to other measurement techniques that are available in the industries related to fiber and fiber fabrication device. Moreover, unlike the longitudinal measuring technique, a polarization distribution effect can be measured accurately using the transverse measuring technique. Therefore, the transverse measurement is more preferred for measuring the characteristics of an optical fiber (or preform). 
     FIG. 1 illustrates a conventional measuring device for enabling the longitudinal measurement of an optical fiber. For the purpose of illustration, an optical fiber  14  is arranged along the z-axis direction, and the longitudinal section  15  of the optical fiber  14  is aligned in the x-y plane. Parallel light  13 , emitted from a light source  11 , is focused on the longitudinal section  15  of the optical fiber  14  by a first lens  12 . Some portion of the light  13  incident on the transverse section  15  is transmitted into the optical fiber  14 , while the other portion of the light  13  is reflected. The reflected light  13  from the fiber  14  is coupled into a second convex lens  16  in backward direction and thereafter determined by an optical detector  17  as light power. Accordingly, the optical detector  17  measures the power of the reflected light received thereon, and the measured power is used to obtain information about the refractive index of the parallel light  13  at the beam spot on the transverse section  15  of the fiber  14 . Hence, by implementing this type of parallel light measuring device, the refractive index distribution of the transverse section  15  of the fiber  14  can be derived using the power of the light detected at the detector  17 . 
     FIG. 2 illustrates another conventional measuring device for detecting the transverse characteristics of an optical fiber. As shown in FIG. 2, the optical fiber  23  is arranged along the z-axis direction, and the longitudinal section  24  of the optical fiber  23  is aligned along the x-z plane,. Light  22  emitted from a light source  21  passes through the fiber surface and the longitudinal section  24  of the optical fiber  23  and eventually arrives at the image sensor  25 . Accordingly, the image of the longitudinal section  24  can be measured and can determine whether the optical fiber  23  contains beam deflecting sections, which result from different refractive indexes by dopants. To achieve this, the conventional measuring device also includes an image sensor (i.e., CCD)  25  to measure the intensity distribution of light passing through the fiber component  24 . Hence, the image of the intended longitudinal section of an optical fiber, including the center of the optical fiber as well as the sectional profiles of an optical fiber, is detected. 
     However, there are some drawbacks with the above-identified conventional systems, which rely on the diffraction of an optical fiber image. Typically, the transverse image of an optical fiber is detected not by projecting light onto a sample but by transversely radiating the optical fiber with light and then detecting refracted light therefrom. When light is projected onto the outer circumferential surface of the optical fiber, the cylindrical core structure acts as a lens. That is, an optical fiber composed of a core and a cladding with different refractive indices and with a symmetrical cylinder shape has equivalent function as a lens. Thus, the light focusing effect and the light diffraction effect are generated when light passes before and behind the center of the core, respectively. Currently, there is no way to numerically analyze these focusing and diffraction effects caused by the fiber in the conventional image sensor. Therefore, there is a need for a new image-tracking method that is capable of detecting the focusing and diffraction effects caused by the fiber core member. 
     SUMMARY OF THE INVENTION 
     The present invention relates to an image-tracking device that can minimize measurement errors caused by the fiber core member in the transverse measurement method. 
     Accordingly, an image-tracking device that is capable of detecting the transverse characteristics of an optical fiber is provided and includes a linear object; a light source for emitting light onto the light object; a first convex lens for projecting the light received via the linear object onto the outer circumferential surface of the optical fiber and forming a primary image of the linear object penetrating the optical fiber; a second convex lens for converging the light received via the optical fiber and forming a secondary image of the linear object; an image sensor for detecting the secondary image; and, a controller for calculating the distance between the primary image and the center of the optical fiber based on the distortion degree on the detected secondary image. 
     The present invention provides a method for measuring the transverse characteristics of an optical fiber, a linear object is arranged to be inclined at an angle other than 90° with respect to the transverse direction of the optical fiber. Light is projected onto the linear object and the light that passes through the linear object is focused thereafter. A primary image of the linear object is generated to be within the optical fiber. A secondary image of the linear object is generated by focusing the light that has passed through the optical fiber with a lens  70 . As a consequence, the secondary image is detected and a differential curve is derived from the detected secondary image. Finally, the distance between the primary image and the center of the optical fiber is calculated according to the length of a distortion region and distortion peaks on the differential curve. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
     FIG. 1 illustrates a conventional device for measuring the longitudinal characteristics of an optical fiber; 
     FIG. 2 illustrates a conventional device for measuring the transverse characteristics of an optical fiber; 
     FIGS. 3,  4 , and  5  illustrate the principles to which the embodiments of the present invention are applied thereto; 
     FIG. 6 illustrates an image-tracking device for measuring the transverse characteristics of an optical fiber according to a preferred embodiment of the present invention; and, 
     FIG. 7 is a flowchart illustrating an image-tracking method for measuring the transverse characteristics of an optical fiber according to a preferred embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     A preferred embodiment of the present invention will be described hereinbelow with reference to the accompanying drawings. For the purpose of simplicity and clarity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail. 
     FIGS. 3,  4 , and  5  illustrate the principle of the embodiments of the present invention by which the present invention relies on to measure the transverse characteristics of an optical fiber. As shown in FIG. 3, the lengthwise direction of an optical fiber  31  is aligned parallel to the z-axis, whereas the longitudinal section of the optical fiber  31  is aligned along the x-z plane. A sheet of white paper with a straight line  32  drawn thereon is disposed behind the fiber along the x-z plane. If the line  32  is aligned perpendicular to the fiber and there was no distortion in the fiber, the line  32  running along x-axis and perpendicular to the central axis of the fiber  31  would be projected as a straight line. That is, the image of the line  32  via the optical fiber  31  would be linear. However, if the line  32  is aligned obliquely at an angle other than 90°, then the line  32  would be distorted. The length of a distorted region is smaller than the diameter of the optical fiber  31 . As shown in FIG. 3, the image of the line  32  via the optical fiber  31  will be curved. The distortion is severe at the boundary between the overlap portion of the line  32  and the optical fiber  31 . In particular, the distorted image of the line  32  is anti-symmetrical with respect to the center of the image and includes two distortion peaks near both edges of the fiber  31 . If a differential algorithm is applied to the distorted line  32 , a differential curve  33  can be obtained. 
     Referring to FIG. 4, if the fiber  41  is spaced a part longer in the y-axis direction away from the white sheet compared to the fiber  31  of FIG. 3, the line  42  will be distorted more than the distorted line  32  shown in FIG. 3 due the lens effect by the fiber  31 . Similarly, a differential curve  43  of the distorted line image can be obtained. Here, the differential curve is symmetrical with respect to the center of the image and has two distortion peaks at both edges near the distorted regions of the fiber  42 . As shown in FIG. 4, the distortion peaks are higher than those shown in FIG.  3 . 
     Referring to FIG. 5, if the fiber  51  is spaced longer in the y-axis direction away from the white sheet than the fiber  41  of FIG. 4, the line  52  is seen to be more distorted than the image of the line  42  shown in FIG. 4 due to lens effect by the fiber  41 . A differential curve  53  of the distorted line image is symmetrical with respect to the center of the image and has two distortion peaks at both edges of a distortion region. The distortion peaks are higher than those of the differential line  43  shown in FIG. 4, and the length of the distortion region equals the diameter of the optical fiber  51 . 
     As noted from FIGS. 3,  4 , and  5 , the image of a line has different distortion peaks and a distortion region depending on the distance between the displacement of an optical fiber relative to the straight line image being projected therefrom. In the present invention, the distance between the optical fiber relative the line image is tracked based on the distortion degree of the line image relying on the above-described principle. Then, the detected distance is measured by another testing condition with no distortion effect. By comparing these two conditions—one with the distortion effect and the other without distortion effect—the transverse characteristics of the fiber component can be derived therefrom. 
     Now, FIG. 6 illustrates an image-tracking device for the transverse measurement of an optical fiber according to the preferred embodiment of the present invention using the principle as described in the preceding paragraphs. The image-tracking device according to the exemplary embodiment of the present invention includes, in succession: a light source  61 , a rotating diffuser  63 , a collimator lens  64 , a blade  65  for providing an image of a linear object  66 , a first convex lens  67 , an optical fiber  68 , a second convex lens  70 , an image sensor  71 , and a controller  73 . As shown in FIG. 6, the y-z plane view of the image-tracking device is arranged along the y-axis direction, except for the controller  73 . 
     Referring to FIG. 6, the light  62 , emitted from the light source  61 , is incident on the rotating diffuser  63 , which can be constructed by using a grounded glass. Here, a light emitting device, such as a laser diode (LD), a light emitting diode (LED), a He—Ne laser, a nitrogen laser, or a lamp can be used as the light source  61 . The light  61  travels along the y-axis direction. The rotating diffuser  63  then scatters the incident light  62  to the collimator lens  64 . The collimator lens  64  converts the incident light into parallel light beams. Here, a convex lens may be used as the collimator lens  64 . The distance between the collimator lens  64  and the rotating diffuser  63  is set to be equal to the focusing distance of the collimator lens  64 . The edge of the blade  65  is disposed to receive the output light beam from the collimator lens  64 . Here, the blade  56  comprises a corner of an object like a mask slit, a wire, or a thread so that an image of a linear object  66  can be generated. To obtain the rotated primary image  69  from the linear object  66 , the image of the linear object  66  is projected, via a first convex lens  67 , to penetrate the optical fiber  68  at an angle other than 90° with respect to the diameter direction of the optical fiber  68 . 
     The first convex lens  67  projects light with the image of the linear object  66  onto the outer circumferential surface of the optical fiber to generate a primary image  69  of the linear object  66 . In this manner, the primary image  69  of the linear object  66  is penetrated through the optical fiber  68 . Meanwhile, a second convex lens  70  converges the light passing through the optical fiber  68  to form a secondary image  72  on the light-receiving surface of the image sensor  71 . As a consequence, the image sensor  71  detects the secondary image  72  of the linear object  66 . 
     As noted from the above description, the primary image  69  of the linear object  66  is in a conjugate relationship with the secondary image  72 . The secondary image  72  detected from the image sensor  71  is distorted and this distortion varies depending on the distance between the primary image  69  and the center of the optical fiber  68 . Accordingly, the controller  73  derives a differential curve for the secondary image  72  represented by the image information received from the image sensor  71  similar to FIGS. 3,  4 , and  5 . Hence, the controller  73  calculates the distance between the primary image  69  and the center of the optical fiber  68  according to the length of the distortion region and distortion peaks shown on the differential curve. The primary image of a linear object by a lens  67  is located in the optical fiber with a certain distance from the center of the fiber. The main purpose of calculating the distance of the length of distortion region on peak is to make the location of the primary image coincide with the center of the fiber. 
     After the image-tracking process, the image-tracking device may be used as a device for measuring the residual stress or refractive index distribution of the optical fiber  68  by removing the blade  65 . In general, a polarizer (not shown) is required to measure the residual stress of the optical fiber  68 . In the preferred embodiment, the polarizer may be disposed between the light source  61  and the rotating diffuser  63  and a waveplate (not shown) between the second convex lens  70  and the optical fiber  68 . For measuring the refractive index of the optical fiber  68 , a chopper (not shown) or an edge filter (not shown) may be inserted between the second convex lens  70  and the optical fiber  68 . 
     FIG. 7 is a flowchart illustrating an image-tracking method for measuring the transverse characteristics of an optical fiber according to the preferred embodiment of the present invention. 
     Referring to FIGS. 6 and 7, the image-tracking methods include the alignment of the linear object (step  81 ), formation of a non-distorted secondary linear object image (not shown) step  82 ), formation of the distorted secondary linear object image  72  (step  83 ), and distance calculation (step  84 ). 
     In step  81 , the image of a linear object  66  is aligned on the optical axis at an angle between 0° and 90° with respect to the diameter direction of the optical fiber  68 . Here, the optical axis is parallel to the propagation direction of the light  62  emitted from the light source  61 . The optical fiber  68  must be deviated from the optical axis. After the non-distorted secondary image is formed, the optical fiber  68  is inserted to form the distorted secondary linear object image  72  in step  83 . It is noted that to keep the center of the optical fiber  68  at the position of the primary image  69  of the linear object  66 , the image of the linear object  66  and its non-distorted secondary image must be in the complete conjugate relationship. 
     In step  82 , the linear object  66  is set in the conjugate relationship with the non-distorted secondary image formed on the light receiving surface of the image sensor  71  using the first and second convex lenses  67  and  70 , shown in FIG.  6 . This can be done by controlling the linear object  66  along the optical axis and thus positioning the primary image  69  of the linear object  66  along the optical axis via the first convex lens  67 . Alternatively, the second convex lens  70  may be controlled or the image sensor  71  may be defocused to achieve the same effect. 
     In step  83 , the light  62  that has passed through the optical fiber  68  is converged and he distorted-secondary image  72  of the linear object  66  is generated. That is, the distorted secondary image  72  for the primary image  69  as an object is formed using the second convex lens  70 . The center of the optical fiber  68  is controlled to be at the position of the primary image  69  by reflecting the analysis result of the distorted secondary image  72  in positioning the optical fiber  68  on the optical axis. 
     In step  84 , the distorted secondary image  72  is detected using the image sensor  71  and the distance between the primary image  69  and the center of the optical fiber  68  is calculated from the distortion degree of the detected secondary image  72 . This is done to obtain the movement value of the optical fiber  68  before the non-distorted secondary image was formed and after the distorted secondary image was formed in response to the insertion of fiber  68 . Here, the distortion degree of the distorted secondary image  72  can be calculated by deriving a differential curve for the secondary image  72  to determine the length of a distortion region and distortion peaks. That is, the distance between the primary image  69  and the center of the optical fiber  68  is calculated from the values indicative of the distortion degree of the secondary image  72 . 
     As described above, the image-tracking device and method for the transverse measurement of an optical fiber according to the present invention can minimize measurement errors and automate the transverse measurement of an optical fiber by evaluating the alignment state of an optical fiber using images of a linear object. 
     While the invention has been shown and described with reference to a certain preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and the scope of the invention as defined by the appended claims.