Abstract:
An optical fiber collimator using a gradient index rod lens for securing a required long opposing distance and easy handling. The collimator includes a single mode fiber and a gradient index rod lens for receiving an incident light from the single mode fiber and converting the incident light into a collimated light, or condensing an incident light and coupling the condensed incident light to the single mode fiber. A meandering period (pitch) of a ray determined by a refractive index distribution of the rod lens is decided. The gradient index rod lens has a lens length larger by 0.5 meandering periods than a minimum lens length required to obtain a predetermined opposing distance between a pair of the rod lenses.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to an optical fiber collimator using a gradient index rod lens.  
           [0002]    [0002]FIG. 1 shows a conventional collimator optical device  50  having incident side and receiving side optical fiber collimators. The incident side optical fiber collimator includes an optical fiber  11  and a rod lens L 1 , and the receiving side optical fiber collimator includes an optical fiber  12  and a rod lens L 2 . The optical device  50  converts lights emitted from the single mode fiber  11  on the incident side into collimated lights by use of the collimator lens L 1 , and condenses the collimated lights by use of the collimator lens L 2  to couple them to the single mode fiber  12  on the receiving side. The collimator lenses L 1  and L 2  are gradient index rod lenses having a refractive index distribution in a radial direction.  
           [0003]    Various kinds of collimator optical devices (devices for optical communications)  50  are produced by inserting an optical function element (e.g., an optical filter, an optical isolator, an optical switch or an optical modulator) between the rod lenses L 1  and L 2 . The device for optical communications causes a predetermined function to a light having propagated through the optical fiber  11  by use of the optical function element, and then couples the light again to the optical fiber  12 . In order to use a function element (e.g., a large-sized matrix switch) requiring a long light path length and having a large size to cause the predetermined function, it is required to provide a device for optical communications having as great opposing distance (maximum collimation length Lmax) between the rod lenses L 1  and L 2  as possible, and as high coupling efficiency as possible.  
           [0004]    [0004]FIG. 2 shows an optical fiber collimator  10  used in the collimator optical device  50 . The optical fiber collimator  10  includes a gradient index rod lens  13 , a single mode fiber  14 , a capillary  15  for holding the optical fiber  14 , and a glass tube  16 . An incident side end face of the rod lens  13  and an end face of the optical fiber  14  are each inclined planes obliquely buffed. The rod lens  13  and the capillary  15  are fixed inside the glass tube  16  at a position where the incident side end face of the rod lens  13  and the end face of the optical fiber  14  are away from each other by a focal length of the rod lens  13 .  
           [0005]    In the optical fiber collimator  10 , it is necessary to increase the focal length of the rod lens  13  and enlarge a beam diameter, in order to increase the opposing distance. The focal length of the rod lens  13  can be changed by adjusting a lens length Z of the rod lens  13 . Here, the “lens length” is the length between both the end faces of the rod lens. In the case of the rod lens  13  having an inclined plane, the “lens length” is the distance from an intersection point of the inclined plane and a center axis to the incident side end face (see FIG. 6). Since the gradient index rod lens has a meandering period (pitch) of a ray determined by its refractive index distribution, the lens length Z is expressed by pitch as a unit.  
           [0006]    For example, in the case of a normal rod lens having a lens element diameter of φ 1.8 mm and a lens length Z of 0.25 pitches, the opposing distance is about 70 mm. On the contrary, if the lens length is changed to 0.1 pitches, the opposing distance extends up to about 200 mm. If the lens length Z of the rod lens having a lens element diameter of φ 0.1 mm is changed from 0.25 pitches to 0.1 pitches, the opposing distance extends from about 20 mm to about 70 mm.  
           [0007]    In the conventional optical fiber collimator  10 , it is necessary to decrease the lens length Z in order to increase the opposing distance. For example, if the lens element diameter of the rod lens  13  is φ 1.8 mm and the lens length Z thereof is 0.23 pitches, the actual lens length Z is 4.8 mm. If the lens element diameter of the lens  13  is φ 1.8 mm and the lens length Z thereof is 0.1 pitches, the actual lens length Z is about 2 mm. If the lens element diameter of the lens  13  is φ 1.0 mm and the lens length Z thereof is 0.1 pitches, the actual lens length Z is 1.2 mm. However, if the lens length Z is small, the following problems are caused.  
           [0008]    (1) As shown in FIG. 3, if a short rod lens  13 A having a length of, for example, 1.2 mm is set to the glass tube  16 , the rod lens  13 A might incline because an axial length of an outer circumferential surface (referential surface) of the rod lens  13 A is small. If the rod lens  13 A inclines, the collimated light (emitted light) emitted from the rod lens  13 A inclines with respect to the axial direction, which decreases the coupling efficiency. As a result, reliability might be decreased.  
           [0009]    (2) If the length of the lens is small, it is difficult to cut or buff the lens when the rod lens  13 A is manufactured. Especially, it is sometimes impossible to obliquely buff the end face of the lens. This is because it is difficult to hold the rod lens  13 A in the cutting and buffing processing.  
           [0010]    (3) It is difficult to handle the lens if the length of the lens is small.  
         SUMMARY OF THE INVENTION  
         [0011]    An object of the present invention is to provide an optical fiber collimator using a gradient index rod lens that secures a required long opposing distance and is easy to handle.  
           [0012]    To attain the aforementioned object, the present invention provides an optical fiber collimator including: a single mode fiber; and a gradient index rod lens for receiving an incident light from the single mode fiber and converting the incident light into a collimated light, or condensing an incident light and coupling the condensed incident light to the single mode fiber. A meandering period (pitch) of a ray determined by a refractive index distribution of the rod lens is decided. The gradient index rod lens has a lens length larger by 0.5 meandering periods than a minimum lens length required to obtain a predetermined opposing distance between a pair of the rod lenses.  
           [0013]    Furthermore, the present invention provides a gradient index rod lens optically coupled to an optical fiber. The rod lens has a refractive index distribution for deciding a meandering period (pitch) of a ray and a lens length larger by 0.5 meandering periods than a minimum lens length required to obtain a predetermined opposing distance between a pair of the rod lenses.  
           [0014]    Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0015]    The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which:  
         [0016]    [0016]FIG. 1 is a schematic constitution view showing a conventional collimator optical device;  
         [0017]    [0017]FIG. 2 is a schematic sectional view of a conventional optical fiber collimator;  
         [0018]    [0018]FIG. 3 is a schematic sectional view of another conventional optical fiber collimator;  
         [0019]    [0019]FIG. 4 is a schematic sectional view of an optical fiber collimator in accordance with a first embodiment of the present invention;  
         [0020]    [0020]FIG. 5 is a schematic sectional view of the conventional optical fiber collimator including a rod lens having a smaller lens length than a lens length of a rod lens of the collimator of FIG. 4;  
         [0021]    [0021]FIG. 6 is an enlarged view of the rod lens of the optical fiber collimator of FIG. 4;  
         [0022]    [0022]FIG. 7 is an explanatory view showing an imaging state of the rod lens of ½ pitch; and  
         [0023]    [0023]FIG. 8 is a schematic sectional view of the optical fiber collimator in accordance with a modification. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    In the drawings, like numerals are used for like elements throughout.  
         [0025]    [0025]FIG. 4 is a schematic sectional view of an optical fiber collimator  21  using a gradient index rod lens in accordance with a first embodiment of the present invention. The optical fiber collimator  21  includes a gradient index rod lens  22 , a single mode fiber  23 , a capillary  24  for holding the optical fiber  23 , and a glass tube  25 . An incident side end face  22   a  of the gradient index rod lens (hereinafter referred to as a rod lens)  22  and an end face  23   a  of the optical fiber  23  are each inclined planes obliquely buffed. The single mode fiber (hereinafter referred to as an optical fiber)  23  is inserted into a fiber insertion hole (not shown) of the capillary  24  and integrated with the capillary  24  by use of an adhesive agent. The rod lens  22  and the capillary  24  are fixed inside the glass tube  25  by use of, for example, an adhesive agent, at a position where the incident side end face  22   a  of the rod lens  22  and the end face  23   a  of the optical fiber  23  are away from each other by a focal length of the rod lens  22 .  
         [0026]    [0026]FIG. 5 is a schematic sectional view of a conventional optical fiber collimator  21 A including a rod lens  42  having a smaller lens length than the lens length of the rod lens  22 . The constitution of the optical fiber collimator  21 A except for the rod lens  42  is the same as that of the optical fiber collimator  21 .  
         [0027]    The lens element diameter of the rod lens  42  is φ 1.0 mm, and its actual lens length Z is 1.2 mm. The opposing distance of the rod lens  42  is about 70 mm.  
         [0028]    The lens element diameter of the rod lens  22  is φ 1.0 mm, and the actual lens length Z of the rod lens  22  is 7.2 mm (see FIG. 6). The lens length Z of the rod lens  22  is larger than the lens length (minimum lens length) Z of the rod lens  42  (e.g., 0.1 pitches) by 0.5 pitches (½ meandering periods). Therefore, the rod lens  22  makes it possible to obtain an opposing distance of about 70 mm equal to the opposing distance of the rod lens  42 .  
         [0029]    [0029]FIG. 7 shows the relation between a meandering period (pitch) P of a ray and the lens length Z. Normally, when the lens length Z of the gradient index rod lens is increased by ½ pitches, an image is only inverted at both ends having a length of ½ pitches (P 1 →Q 1 , P 2 →Q 2 : see FIG. 7), but the magnification of the lens is not changed. Therefore, the focal length of the lens is not changed. Owing to the characteristics of the gradient index rod lens, the rod lens  22  makes it possible to obtain the same opposing distance as that of the rod lens  42 , and the rod lens  22  can have a lens length Z about six times as large as that of the rod lens  42 .  
         [0030]    Hereinafter, the characteristics of the gradient index rod lens will be described using Equations.  
         [0031]    When a distance in a radial direction from the center of a section of the rod lens is r, a refractive index distribution n (r) of the gradient index rod lens is expressed by Equation (1) as follows:  
           n  ( r )= n   0  (1−Ar 2 /2)   (1)  
         [0032]    In this case, a focal length f of the lens is expressed by Equation (2) as follows:  
           f= 1/{ n   0    {square root}A ·sin({square root} A·Z )}  (2)  
         [0033]    In Equations (1) and (2), n 0  is the refractive index at the center of the rod lens, {square root}A is a refractive index distribution constant, and Z is the lens length. As apparent from Equation (2), the focal length f changes periodically with the lens length Z.  
         [0034]    The meandering period (pitch) P of the lens is expressed by Equation (3) as follows:  
         P=2π/{square root}A   (3)  
         [0035]    From Equations (2) and (3), the focal length f has the same value (absolute value) on a period of P/2 (0.5 pitches), with respect to the lens length Z. That is, the focal length f does not change even if the lens length Z is increased by P/2, so that the same lens characteristics can be obtained. In Equation (2), the sign of sin is inverted every P/2 periods, and the image is inverted in accordance with the inversion of the sign of sin.  
         [0036]    A maximum collimator length Lmax is expressed by Equation (4) as follows:  
           L max=1/{ n   0   {square root}A ·tan({square root} A·Z )}= f ·cos({square root} A·Z )   (4)  
         [0037]    Therefore, the maximum collimator length Lmax changes in the same period as that of the focal length f with respect to the lens length Z.  
         [0038]    The optical fiber collimator  21  in the first embodiment has the following advantages.  
         [0039]    (1) The lens length Z of the rod lens  22  is larger than the lens length (minimum lens length) Z of the rod lens  42  (0.1 pitches) by 0.5 pitches. Therefore, it is possible to obtain the same opposing distance (about 70 mm) as that of the rod lens  42 , and it is possible to use the rod lens  22  having a length of 7.2 mm, which is about six times as large as that of the rod lens  42 . In this way, the required long opposing distance can be secured, and an emitted light of the rod lens  22  can be prevented from inclining with respect to the axial direction of the lens, so that the coupling efficiency can be prevented from being decreased. Therefore, it is possible to improve the reliability while securing the required long opposing distance.  
         [0040]    (2) The lens length Z of the rod lens  22  is about six times as large as that of the rod lens  42 , so that it is easy to handle the lens  22 . Therefore, it is easy to hold the lens  22  in the buffing processing of, for example, cutting or obliquely buffing the rod lens  22 , thereby facilitating the cutting or oblique buffing when the lens  22  is manufactured.  
         [0041]    The gradient index rod lens used in the optical fiber collimator in accordance with a second embodiment of the present invention has a lens diameter of φ 1.8 mm and a lens length Z of about 12 mm. The opposing distance of the rod lens is about 200 mm. The lens length Z of the rod lens in the second embodiment is larger by 0.5 pitches than the lens length Z necessary to obtain an opposing distance of about 200 mm (0.1 pitches, about 2.0 mm).  
         [0042]    The optical fiber collimator in the second embodiment has the same advantages as the optical fiber collimator in the first embodiment.  
         [0043]    It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or scope of the invention. Particularly, it should be understood that the invention may be embodied in the following forms.  
         [0044]    In each embodiment, the minimum lens length required to obtain the opposing distance, which is increased by 0.5 pitches, is not limited to 0.1 pitches. In short, the rod lens may have a length increased by 0.5 pitches, with respect to the minimum lens length. Preferably, the minimum lens length is 0.1 pitches or more.  
         [0045]    The minimum lens length is preferably from about 0.7 or more to about 2 mm or less.  
         [0046]    In each embodiment, the lens element diameter is arbitrary.  
         [0047]    In each embodiment, any of the following methods may be applied as a “method of increasing the lens length (pitch) Z”. (1) A method of cutting a rod lens to let it have (0.1+0.5) pitches, out of a lens base material that is the same as a rod lens having a small lens length Z, for example, a rod lens of 0.1 pitches. (2) A method of cutting a rod lens having a small lens length, for example, a lens of 0.1 pitches, to let it have 0.5 pitches, out of a lens base metal having the same lens element diameter as a lens of 0.1 pitches, and then joining the cut rod lens to the lens of 0.1 pitches.  
         [0048]    The present invention can also be applied to an optical fiber collimator  21 B in which anti-reflection measures are taken as shown in FIG. 8. In the optical fiber collimator  21 B, anti-reflection films  31 ,  32  and  33  are formed on both end faces of the rod lens  22  and the end face of the optical fiber  23 , respectively.  
         [0049]    Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.