Method and apparatus for controlling internal rotation of principal axes in fabrication of birefringement fibers

Abstract

A technique and apparatus for detecting the internal rotation of the principal axes in the fabrication of birefringent fibers is based upon a new method for producing and interpreting the interference fringe pattern backscattered power from a birefringent fiber side-illuminated with a laser light source to determine the internal orientation of the strain-inducing axes. An apparatus employing this technique for controlling internal rotation of the principal axes provides for adjustment of the orientation of at least one of the traction surfaces pulling the fiber during drawing fabrication to maintain parallel alignment of the planes of rotation of the surfaces to reduce the fiber rotation-inducing motion which leads to internal rotation of the fiber's principal axes.

What is claimed is: 
     
       1. A method for detecting the internal rotation of the principal axes of a birefringent fiber comprising: directing a beam of laser light toward said fiber in a direction normal to the longitudinal axis of said fiber to generate a backscattered interference fringe pattern determined by the fiber cross section;   visually displaying said interference fringe pattern; and   providing relative parallel movement between said fiber and the point of incidence of said beam of collimated light on said fiber;   wherein said visual displaying of said interference fringe pattern is done effective to exhibit motion related to the angle between the incident beam of light and either of the principal axes, the fringes of said pattern evolving effective to cause, at the angle where the front motion changes from receding to covering, an indication that one of the principal axis is aligned with the axis of said beam of light.   
     
     
       2. A method as set forth in claim 1, wherein said beam of collimated light is provided by a low power laser source. 
     
     
       3. A method as set forth in claim 1, wherein the relative parallel movement is provided by moving the beam of collimated light with respect to said fiber. 
     
     
       4. A method as set forth in claim 1, wherein the relative parallel movement is provided by moving said fiber relative to a stationary beam of collimated light. 
     
     
       5. A method as set forth in claim 4, wherein movement of said fiber is provided by movement of said fiber in a drawing fabrication technique for said fiber. 
     
     
       6. A method as set forth in claim 1, wherein said interference fringe pattern is visually displayed on a screen positioned to intercept said pattern backscattered from said fiber. 
     
     
       7. A method as set forth in claim 1, wherein said interference fringe pattern is visually displayed by a photodetection device. 
     
     
       8. An apparatus for detecting the internal rotation of the principal axes of a birefringent fiber comprising: a source of a beam of collimated light;   means to direct said beam of light toward said fiber in a direction normal to the longitudinal axis of said fiber to generate a backscattered interference fringe pattern determined by the fiber cross section;   means to provide a visual display of said interference fringe pattern; and means to provide relative parallel movement between said fiber and the point of incidence of said beam of collimated light on said fiber,   said visual display of said interference fringe pattern exhibiting motion related to the angle between the incident beam of light and either of the principal axes, the fringes of said pattern evolving effective to cause, at the angle where the fringe motion changes from receding to converging, an indication that one of the principal axes is aligned with the axis of said beam of light.   
     
     
       9. An apparatus as set forth in claim 8, wherein said source of the beam of light is a low power laser. 
     
     
       10. An apparatus as set forth in claim 9, wherein said laser is positioned to direct said beam of light in a direction parallel to the length of said fiber, and further comprising a reflector located to intercept said beam of light and direct said beam normal to said fiber.   
     
     
       11. An apparatus as set forth in claim 10, wherein the means to provide relative parallel movement comprises means to move said laser light and said reflector with respect to said fiber. 
     
     
       12. An apparatus as set forth in claim 11, wherein said means to provide a visual display comprises a screen positioned to intercept said fringe pattern backscattered from said fiber, and said means to provide relative movement comprises a member of the group consisting of: (a) means to move said laser and (b) reflector and said screen in unison parallel to the length of said fiber. 
     
     
       13. An apparatus as set forth in claim 11, wherein said means to provide a visual display comprises a photodetection device positioned to intercept said fringe pattern backscattered from said fiber, and said means provide relative movement comprises means to move said laser, said reflector and said photodetection device in unison parallel to the length of said fiber. 
     
     
       14. An apparatus as set forth in claim 10, wherein the means to provide relative parallel movement comprises means to move said fiber with respect to a stationary light source. 
     
     
       15. An apparatus as set forth in claim 14, wherein said means to move said fiber comprises traction means for pulling said fiber in a drawing fabrication of said fiber. 
     
     
       16. An apparatus as set forth in claim 8, wherein the means to provide relative parallel movement comprises means to move said light source with respect to said fiber. 
     
     
       17. In a mechanism for drawing fabrication of a birefringent fiber including coacting means for pulling a length of said fiber, an apparatus for detecting the internal rotation of the principal axes of said fiber induced by said drawing mechanism, comprising: a low power laser source for directing a beam of collimated light parallel to the length of said fiber;   a reflector positioned to direct said beam of light toward said fiber normal to the length of said fiber so as to generate a backscattered interference fringe pattern determined by the fiber cross section; and   display means positioned to intercept and provide a visual display of said interference fringe pattern;   said visual display of said interference fringe pattern exhibiting motion relative to the angle between the beam of light incident on said fiber and either of the principal axes, the fringes of said pattern evolving effective to cause, at the angle where the fringe motion changes from receding to converging, an indication that one of the principal axis is aligned with the axis of said beam of light.   
     
     
       18. The mechanism of claim 17, wherein said coacting pulling means comprises contacting rotating surface which apply a traction force to pull said fiber, said mechanism further comprises: control means for said rotating surfaces includes means for adjusting the orientation of at least one of said rotating surface effective to cause the planes of rotation of said surfaces to be adjusted parallel with respect to each other to reduce induced rotation of said fiber due to misalignment of said rotating surfaces.   
     
     
       19. The mechanism of claim 18, wherein said control means comprises means supporting said at least one rotating surface for pivotal movement relative to the other rotating surface to permit said parallel alignment of the planes of rotation. 
     
     
       20. The mechanism of claim 19, wherein said display means comprises a screen positioned substantially normal to said light beam and having an apertures to pass said light beam to said fiber. 
     
     
       21. The mechanism of claim 19, wherein said display means comprises a photodetection device positioned to intercept said backscattered interference fringe pattern. 
     
     
       22. In combination with a mechanism for drawing fabrication of a birefringent fiber having coating rotary surfaces for pulling a length of said fiber through the mechanism, an apparatus for detecting and controlling the internal rotation of the principal axes of said fiber induced by misalignment of said rotary surfaces, comprising: a low power laser source for directing a beam of collimated light parallel to the length of said fiber;   a reflector positioned to direct said beam of light toward said fiber in a direction normal to the length of said fiber so as to generate an interference fringe pattern determined by the fiber cross section and backscattered by said fiber;   display means positioned to intercept and provide a visual display of said interference fringe pattern,   said visual display of said interference fringe pattern exhibiting motion related to the angle between the beam of light incident on said fiber and either of the principal axes, the fringes of said pattern effective to cause, at the angle where the fringe motion changes from receding to converging, an indication that one of the principal axis is aligned with the axis of said beam of light; and   control means for said coacting rotary surface including means for adjusting the orientation of at least one of said rotary surface effective of cause the planes of rotation of said surfaces to be adjusted parallel with respect to each other to reduce induced rotation of said fiber due to misalignment of said rotary surfaces.   
     
     
       23. The combination of claim 22, wherein said control means comprises means supporting said at least one rotary surface for pivotal movement relative to the other rotary surface to permit said parallel alignment of the planes of rotation. 
     
     
       24. The combination of claim 23, wherein said display means comprises a screen positioned substantially normal to said light beam and having an aperture to pass said light beam to said fiber. 
     
     
       25. The combination of claim 23, wherein said display means comprises a photodetection device positioned to intercept said backscattered interference fringe pattern. 
     
     
       26. A method for determining the orientation of the principal birefringent axes at a point in a birefringent fiber, comprising steps for: directing a beam of laser light toward said fiber in a direction normal to the longitudinal axis of said fiber to generate a backscattered interference fringe pattern determined by the fiber cross section;   visually displaying said interference fringe pattern;   causing rotation of said axes with respect to said light effective to observe convergent or divergent motion of said fringe pattern;   determining the point of said rotation where there is a reversal of said motion, so as to identify that one of said birefringent axes is aligned with said light;   determining which of said axes is aligned with the laser light.