Abstract:
The present invention relates to a quadrangular-pyramid-shaped lensed fiber. One end of the fiber is ground to become quadrangular-pyramid-shaped. Small volume of the tip of the quadrangular-pyramid-shaped fiber is heated to form a semi-ellipsoidal microlens, thereby forming the quadrangular-pyramid-shaped lensed fiber. The advantage of the present invention is that the shape of the semi-ellipsoidal microlens can be controlled by adjusting the angles of the quadrangular-pyramid-shaped fiber according to the aspect ratio of the diode laser so as to enhance the coupling efficiency between an optical fiber and a diode laser.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates to a lensed fiber and the method of making the same, particularly to a quadrangular-pyramid-shaped lensed fiber and the method of making the same.  
       DESCRIPTION OF THE RELATED ART  
       [0002]     For optimal performance in fiber-optic communication system, efficient coupling between diode laser and fiber is essential. In order to enhance the coupling efficiency between diode laser and fiber, various types of lensed fibers are provided as follows.  
         [0003]     Referring to  FIG. 1 , U.S. Pat. No. 4,671,609 disclosed a method of making a lensed fiber comprising the following steps. A fiber  10  was pulled to form a tapered end that has a flat end face  12  or a rounded tip. A lens  14  is formed by immersing the tapered end of the fiber  10  in molten glass and then withdrawing the tapered end from the molten glass. The dimensions and the shape of the lens  14  can be influenced by the immersion depth, the angles of the tapered end, the shape of the tapered end and the temperature of the molten glass, which cause the manufacturing process to be complicated, time consuming and difficult to control, which are the disadvantages of this method. In addition, the rounded lensed fiber fabricated by this method is only suitable for the laser of low aspect ratio.  
         [0004]     Referring to  FIG. 2 , U.S. Pat. No. 5,037,174 disclosed a method for making a tapered fiber comprising the following steps. A fiber was drawn to be separated into two parts by jerking separation and little heat of arc energy, and a tapered extension  22  and a nipple-like extension  24  were formed on the end of one part. Then, the application of a burst of arc softened the nipple-like extension  24  to form a hyperbolic shaped fiber lens  26 . The disadvantage of this method is that the dimensions and the shapes of the tapered extension  22  and nipple-like extension  24  are difficult to control and unstable during the manufacturing process. In addition, the rounded lensed fiber fabricated by this method is only suitable for the laser of low aspect ratio.  
         [0005]     Referring to  FIG. 3 , U.S. Pat. No. 5,256,851 disclosed a method for making a tapered fiber comprising the following steps. A fiber  30  was rotated along the axis thereof, and then a CO 2  laser controlled by computer program was applied to the fiber  30  to form a lens consisting of a hyperbolical portion  32  on an axis and a spherical portion  34  on another axis. Such a fiber lens has high coupling efficiency, but it is very difficult to fabricate a fiber lens having an asymmetric curve.  
         [0006]     Referring to  FIG. 4 , U.S. Pat. No. 5,256,851 disclosed an optical fiber having a lens of a wedge-shaped external form having two-stage tapered portions and with different angles of θ 1  and θ 2  between the two slants and the axis  42 , respectively, wherein the intersection of the two slants must be controlled to be within the scope of the core of the fiber. Such wedge-shaped fiber is most widely used as a lensed fiber for coupling between 980-nm laser diode and single-mode fiber. However, the fabricating process of the wedge-shaped fiber lens only controls one axial curvature. Therefore, it is difficult to form any different aspect ratios of elliptical curvatures to match the far field of high power diode lasers. In addition, as the radius of the core of the fiber is usually 4 to 6 μm, it is very difficult to control the intersection of the two slants to be within the scope of the core of the fiber. Furthermore, the lensed fiber fabricated by this method is only suitable for the laser of high aspect ratio.  
         [0007]     Consequently, there is a need for improved quadrangular-pyramid-shaped lensed fiber and the method of making the same to solve the above-mentioned problem.  
       SUMMARY OF THE INVENTION  
       [0008]     The primary objective of the present invention is that the shape of the optical fiber can be controlled by adjusting the angles of the quadrangular-pyramid-shaped fiber according to the aspect ratio of the diode laser so as to enhance the coupling efficiency between an optical fiber and a diode laser.  
         [0009]     Another objective of the present invention is to provide a quadrangular-pyramid-shaped lensed fiber, which is easy to fabricate, and the fabricating method is polishing the tip of an optical fiber to form four slants and an apex, and then fusing the apex.  
         [0010]     To achieve the above method, the present invention provides a quadrangular-pyramid-shaped lensed fiber comprising an optical fiber and a tapered region. The optical fiber has a central axis and an end. The tapered region is at the end of the optical fiber. The tapered region has four slants, four edges and a fiber lens. Two of the four slants intersect each other to form the four edges, and the extension of the four edges cross at an intersection point on the central axis. Two separate edges of the four edges and the central axis are on the same plane. The fiber lens is at the tip of the tapered region, and the geometric center of the fiber lens is on the central axis.  
         [0011]     Additionally, the present invention provides a method for making a quadrangular-pyramid-shaped lensed fiber, comprising: 
        (a) providing an optical fiber having a central axis and an end;     (b) cutting the end of the optical fiber to form a flat end face;     (c) forming a tapered region at the end of the optical fiber, wherein the tapered region has four slants, four edges and an apex, two of the four slants intersect each other to form the apex with the four edges, the apex is on the central axis, and two separate edges of the four edges and the central axis are on the same plane; and     (d) fusing the apex to form a fiber lens.       
 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]      FIG. 1  shows the conventional fiber lens of U.S. Pat. No. 4,671,609;  
         [0017]      FIG. 2  shows the typical method disclosed in U.S. Pat. No. 5,037,174, in which the tapered fiber is fabricated by arc welding;  
         [0018]      FIG. 3  shows the conventional asymmetric fiber lens of U.S. Pat. No. 5,256,851;  
         [0019]      FIG. 4  shows the conventional wedge fiber lens of U.S. Pat. No. 5,455,879;  
         [0020]      FIG. 5   a  is a perspective view of a quadrangular-pyramid-shaped fiber according to the first embodiment of the present invention;  
         [0021]      FIG. 5   b  is a side view of the quadrangular-pyramid-shaped fiber of  FIG. 5   a;    
         [0022]      FIG. 5   c  is a top view of the quadrangular-pyramid-shaped fiber of  FIG. 5   a;    
         [0023]      FIG. 5   d  is a front view of the quadrangular-pyramid-shaped fiber of  FIG. 5   a;    
         [0024]      FIG. 6   a  is a perspective view of a quadrangular-pyramid-shaped lensed fiber according to the second embodiment of the present invention;  
         [0025]      FIG. 6   b  is a side view of the quadrangular-pyramid-shaped lensed fiber of  FIG. 6   a;    
         [0026]      FIG. 6   c  is a top view of the quadrangular-pyramid-shaped lensed fiber of  FIG. 6   a;    
         [0027]      FIG. 6   d  is a front view of the quadrangular-pyramid-shaped lensed fiber of  FIG. 6   a;    
         [0028]      FIG. 7  shows the machining apparatus of the present invention;  
         [0029]      FIG. 8  shows the relative position between a laser and a optical fiber; and  
         [0030]      FIG. 9  shows the relationship between the coupling efficiency and the working distance. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]     Referring to  FIG. 5   a , a quadrangular-pyramid-shaped fiber according to the first embodiment of the present invention is shown. In the embodiment, the quadrangular-pyramid-shaped fiber  50 , fabricated by polishing an optical fiber  54 , comprises an optical fiber  54  and a tapered region.  
         [0032]     The optical fiber  54  has a central axis  56  extending in the longitudinal direction thereof. The tapered region is at one end of the optical fiber  54  and has four slants  51   a ,  51   b ,  51   c ,  51   d , four edges  52   a ,  52   b ,  52   c ,  52   d  and an apex  55 . The four slants are a first slant  51   a , a second slants  51   b , a third slants  51   c  and a fourth slant  51   d . The four slants  51   a ,  51   b ,  51   c ,  51   d  intersect each other to form four edges which are a first edge  52   a , a second edge  52   b , a third edge  52   c  and a fourth edge  52   d , wherein the first slant  51   a  intersect the fourth slant  51   d  to form the first edge  52   a , the first slant  51   a  intersect the second slant  51   b  to form the second edge  52   b , the second slant  51   b  intersect the third slant  51   c  to form the third edge  52   c , and the third slant  51   c  intersect the fourth slant  51   d  to form the fourth edge  52   d.    
         [0033]     The four edges  52   a ,  52   b ,  52   c ,  52   d  intersect at the apex  55 , which is on the central axis  56 . Two separate edges of the four edges  52   a ,  52   b ,  52   c ,  52   d  and the central axis  56  are on the same plane. For example, referring to  FIG. 5   b , the first edge  52   a , the third edge  52   c  and the central axis  56  are on a first plane, and the central axis 56 divides the inclination angle α (α is 10 degrees to 170 degrees) between the first edge  52   a  and the third edge  52   c  equally. Hence, the first inclination angle between the first edge  52   a  and the central axis  56  is α/2, and the third inclination angle between the third edge  52   c  and the central axis  56  is also α/2.  
         [0034]     Referring to  FIG. 5   c , the second edge  52   b , the fourth edge  52   d  and the central axis  56  are on a second plane, and the central axis  56  divides the inclination angle β (β is 10 degrees to 170 degrees) between the second edge  52   b  and the fourth edge  52   d  equally. Hence, the second inclination angle between the second edge  52   b  and the central axis  56  is β/2, and the fourth inclination angle between the fourth edge  52   d  and the central axis  56  is also β/2.  
         [0035]     Referring to  FIG. 5   d , a front view of a quadrangular-pyramid-shaped fiber of  FIG. 5   a  is shown. In this embodiment, the first plane defined by the first edge  52   a  and the third edge  52   c  is perpendicular to the second plane defined by the second edge  52   b  and the fourth edge  52   d.    
         [0036]     Referring to  FIG. 6   a , a perspective view of a quadrangular-pyramid-shaped lensed fiber according to the second embodiment of the present invention is shown. In this embodiment, a quadrangular-pyramid-shaped lensed fiber  60  is formed by fusing the apex  55  of the quadrangular-pyramid-shaped fiber  50  of  FIG. 5   a . The elements in  FIGS. 6   a  to  6   d  are substantially same as those in  FIGS. 5   a  to  5   d , and are designated by the reference numbers of  FIGS. 5   a  to  5   d  plus 10. In the embodiment, the quadrangular-pyramid-shaped lensed fiber  60  comprises an optical fiber  64 , a tapered region and fiber lens  63 .  
         [0037]     The optical fiber  64  has a central axis  66  extending in the longitudinal direction thereof. The tapered region is at one end of the optical fiber  64  and has four slants  61   a ,  61   b ,  61   c ,  61   d  and four edges  62   a ,  62   b ,  62   c ,  62   d . The four slants are a first slant  61   a , a second slants  61   b , a third slants  61   c  and a fourth slant  61   d . The four slants  61   a ,  61   b ,  61   c,  61     d  intersect each other to form the four edges which are a first edge  62   a , a second edge  62   b , a third edge  62   c  and a fourth edge  62   d , wherein the first slant  61   a  intersect the fourth slant  61   d  to form the first edge  62   a , the first slant  61   a  intersect the second slant  61   b  to form the second edge  62   b , the second slant  61   b  intersect the third slant  61   c  to form the third edge  62   c , and the third slant  61   c  intersect the fourth slant  61   d  to form the fourth edge  62   d.    
         [0038]     The extension of the four edges  62   a ,  62   b ,  62   c ,  62   d  cross at a intersection point  65 , which is on the central axis  66 . Two separate edges of the four edges  62   a ,  62   b ,  62   c ,  62   d  and the central axis  56  are on the same plane. For example, referring to  FIG. 6   b , the first edge  62   a , the third edge  62   c  and the central axis  66  are on a first plane, and the central axis  66  divides the inclination angle γ ( γ is 10 degrees to 170 degrees) between the first edge  62   a  and the third edge  62   c  equally. Hence, the first inclination angle between the first edge  62   a  and the central axis  66  is γ/2, and the third inclination angle between the third edge  62   c  and the central axis  66  is also γ/2.  
         [0039]     Referring to  FIG. 6   c , the second edge  62   b , the fourth edge  62   d  and the central axis  66  are on a second plane, and the central axis  66  divides the inclination angle δ (δ is 10 degrees to 170 degrees) between the second edge  62   b  and the fourth  62   d  equally. Hence, the second inclination angle between the second edge  62   b  and the central axis  66  is δ/2, and the fourth inclination angle between the fourth edge  62   d  and the central axis  66  is also δ/2.  
         [0040]     Referring to  FIG. 6   d , a front view of a quadrangular-pyramid-shaped fiber of  FIG. 6   a  is shown. In this embodiment, the first plane defined by the first edge  62   a  and the third edge  62   c  is perpendicular to the second plane defined by the second edge  62   b  and the fourth edge  62   d.    
         [0041]     The fiber lens  63  is at the tip of the tapered region, and the geometric center of the fiber lens  63  is on the central axis  66 . The appearance of the fiber lens  63  can be semi-ellipsoidal or hemispherical.  
         [0042]     The present invention also relates to a method for making a quadrangular-pyramid-shaped lensed fiber, comprising the following steps: 
        (a) providing an optical fiber having a central axis and an end;     (b) cutting the end of the optical fiber to form a flat end face;     (c) machining (for example, lapping, polishing or grinding) the end of the optical fiber to form a tapered region like the above-mentioned quadrangular-pyramid-shaped fiber  50 , wherein the tapered region has four slants, four edges and a apex, two of the four slants intersect each other to form the apex with the four edges, the apex is on the central axis, and two separate edges of the four edges and the central axis are on the same plane; and     (d) fusing the apex by electric arcs so that the apex is melted to become liquid state and then forms a fiber lens by surface tension, wherein the appearance of the fiber lens is like the above-mentioned quadrangular-pyramid-shaped lensed fiber  60 .        
 
         [0047]     Referring to  FIG. 7 , the above-mentioned machining step of step (c) further comprises the following steps (taking the fabrication of the quadrangular-pyramid-shaped fiber  50  for example): 
        (c1) fixing the optical fiber  54  in a fixture  72  above a machining plate  73  (for example, lapping plate or polishing plate);     (c2) adjusting the inclination angle between the fixture  72  and the machining plate  73  to form a first angle θ between the optical fiber  54  and the surface of the machining plate  73 ;     (c3) machining (for example, lapping, polishing or grinding) the end of the optical fiber  54  to form the first slant  51   a;       (c4) rotating the optical fiber  54  along the central axis  56  with a second angle φ;     (c5) machining the optical fiber  54  to form the second slant  51   b  and the second edge  52   b;       (c6) rotating the optical fiber  54  along the central axis  56  with an angle of the supplementary angle of the second angle φ;     (c7) machining the optical fiber  54  to form the third slant  51   c  and the third edge  52   c;       (c8) rotating the optical fiber  54  along the central axis  56  with the second angle φ; and     (c9) machining the optical fiber  54  to form the fourth slant  51   d , fourth edge  52   d  and first edge  52   a.          
 
         [0057]     The advantage of the present invention is that the best coupling efficiency can be achieved by adjusting the inner angles α and β of the quadrangular-pyramid-shaped optical fiber  50  to control the shape of the fused fiber lens  63  of the quadrangular-pyramid-shaped lensed fiber  60  according to the aspect ratio of the laser. In a theoretical simulation, the coupling efficiency can reach 90% when the quadrangular-pyramid-shaped lensed fiber of the present invention matches the far field of laser.  
         [0058]     An example is described below. In the example, a 980-nm high-power diode laser with a typical far-field divergence of 8° (lateral)×40° (vertical) is used, and the fiber used in this example is Prime 980-nm step-index single-mode fiber with the mold field radius of 4.916 μm, while the refractive index of the core is 1.416.  
         [0059]     Then, the relative position between the laser and the fiber is defined. As shown in  FIG. 8 , the x direction is perpendicular to the paper, and the distance between the laser and the fiber along z direction is defined as the working distance d. Referring to the simulation result diagram of  FIG. 9 , the coupling efficiency is 95% when the working distance d is 13.5 μm.  
         [0060]     According to the theoretical deduction, the widths of the laser are W x =4.557 μm and W y =4.916 μm, wherein W x  is the width in the x direction and W y  is the width in the y direction, and the radii of the laser are R x =319.3 μm and R y =13.7 μm, wherein R x  is the curvature in the x direction and R y  is the curvature in the y direction.  
         [0061]     If the laser mode phase changed by the fiber lens can totally match the fiber mode phase, the two lens curvatures of the fiber lens in perpendicular are R lx =143.7 μm and R ly =6.4 μm, wherein R lx , is the curvature in the x direction and R ly  is the curvature in the y direction.  
         [0062]     The ratio of angles α and β can be derived by substituting R ly  and R lx  into the following equation:  
           R   lx       R   ly       =     (         1     sin   ⁢     α   2         -   1         1     sin   ⁢     β   2         -   1       )         
 
         [0063]     Therefore, if the value of α is determined, the corresponding value of β can be determined. Then the values of angles θ and φ can be derived by substituting α and β into the two following equations:  
             θ   =       ⁢       π   2     -       cos     -   1       ⁢       tan   ⁢     α   2     ⁢   tan   ⁢     β   2               tan   2     ⁢     α   2     ⁢     tan   2     ⁢     β   2       +       tan   2     ⁢     α   2       +       tan   2     ⁢     β   2                             ϕ   =       ⁢       cos     -   1       ⁢           tan   2     ⁢     β   2       -       tan   2     ⁢     α   2               tan   2     ⁢     β   2       +       tan   2     ⁢     α   2                       
 
         [0064]     The quadrangular-pyramid-shaped optical fiber  50  can be fabricated by applying the values of angles θ and φ to the above-mentioned method. Then, the quadrangular-pyramid-shaped lensed fiber  60  can be fabricated by fusing the apex  55  of the quadrangular-pyramid-shaped optical fiber  50  by electric arcs.  
         [0065]     While several embodiments of this invention have been illustrated and described, various modifications and improvements can be made by those skilled in the art. The embodiments of this invention are therefore described in an illustrative but not restrictive sense. It is intended that this invention may not be limited to the particular forms as illustrated, and that all modifications that maintain the spirit and scope of this invention are within the scope as defined in the appended claims.