Abstract:
A microstructured optical fiber is structured with a core region and a cladding region surrounding the core region. A cross section of the fiber portion conforms to a translationally symmetric lattice, a center of each sub medium region in the outer region coinciding with respective lattice points, and a plurality of sub medium regions in the inner region having centers that are offset from respective lattice points.

Description:
FIELD OF THE INVENTION 
   The present invention relates to fiber optic waveguides, more particularly to arrangements of sub medium regions in microstructured optical fibers. 
   BACKGROUND 
   The recent development of the microstructured optical fiber, in which a high index core region is surrounded by cladding having a mix of silica and air, offers new fiber properties by virtue of the large refractive-index contrast that exists between glass and air. A cladding structure may have a spatially uniform average refractive index that can be adjusted to meet a desired relationship with the core index.  FIG. 1  illustrates a prior art microstructured optical fiber in cross section. In the optical fiber  1   b , a plurality of holes  3   b  are arranged in silica glass  2   b . The diameters of the holes  3   b  are substantially uniform in the cross section and the centers of the holes  3   b  substantially coincide with the lattice points  4   b  of a hexagonal lattice. With substantially periodically arranged sub medium regions in the cladding, the core region can be realized by substituting one or more sub medium regions with main medium (silica glass, for example). A lattice point  41   b  in the center of the fiber has no corresponding hole, so that the center of the fiber has a higher average refractive index than that of the surrounding region. As a result, a lightwave is localized in the center of the fiber and guided over the fiber. 
   It is difficult to realize a high yield of production and a low transmission loss in the production of microstructured optical fibers with zero or negative chromatic dispersion, zero or negative chromatic dispersion slope, and/or small effective core area. For realizing such characteristics in microstructured optical fibers, air holes with small diameters and accurate control of the dimension and arrangement of the air holes are necessary. However, decrease in the hole diameter causes increase in the surface tension at the surfaces of the holes during fiber drawing. The increase in surface tension causes excess shrinking of the holes resulting in increase in the uncontrollability of the optical characteristics of the drawn optical fiber. Although influence of surface tension can be decreased by lowering the drawing temperature, the drawing tension increases by decrease in drawing temperature, resulting in increased occurrence of fiber breaking during drawing, increase in transmission loss, and degradation in durability against UV radiation. 
   SUMMARY OF THE INVENTION 
   The present invention fulfills the above described fiber characteristic needs, while overcoming the drawbacks of the prior art, at least in part, by offsetting sub medium regions from the lattice points of periodical lattice in the innermost region of the fiber to form the core region. As a result, it becomes possible to realize valuable characteristics such as zero or negative chromatic dispersion, zero or negative chromatic dispersion slope, and small effective core area with a structure in which the curvature radii of sub medium regions are larger than those of the prior art. Increase in curvature radii of sub medium regions reduces the influence of surface tension during fiber drawing, resulting in a high controllability of the fiber structure and optical characteristics, a high yield of production, a low transmission loss, and a high durability against UV radiation. 
   The following terms are defined to aid description of microstructured fiber characteristics. 
   A main medium is a medium that can constitute an optical fiber by itself. On the other hand, a sub medium is not necessarily able to constitute an optical fiber by itself. For example, glasses and polymers can be used as a main medium or a sub medium, while liquids, gases and vacuum can be used as a sub medium but not a main medium. 
   The average refractive index of a region composed of several media i (i=1. . . M) is defined by the following formulas:
 
         n   avg     =       {       (       ∑     i   =   1     M     ⁢         n   2     ⁡     [   i   ]       ⁢     f   ⁡     [   i   ]           )     /     (       ∑     i   =   1     M     ⁢     f   ⁡     [   i   ]         )       }       1   /   2           
 
wherein n[i] and f[l] are respectively the refractive index and the volume of medium i.
 
   The lattice cell of a lattice point is the polygon region outlined by perpendicular bisectors between that lattice point and the neighboring lattice points. In a structure where holes are periodically arranged, the relative hole diameter is the ratio of the hole diameter d to the pitch L of the periodical lattice. If a structure possesses translational symmetry, an operation to translate the structure by a non-zero distance keeps the structure the same. The direction and distance of a translational operation can be represented by a vector. When there are N independent vectors representing the translational operation keeping the structure the same, that structure possesses N-dimensional translational symmetry. 
   Advantages of the present invention will become readily apparent from the following detailed description, simply by way of illustration of the best mode contemplated of carrying out the invention. The invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the invention. Accordingly, the drawing and description are illustrative in nature, not restrictive. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawing and in which like reference numerals refer to similar elements and in which: 
       FIG. 1  is an illustration of a cross section taken perpendicular to the fiber axis of a prior art microstructured optical fiber. 
       FIG. 2  is an illustration of a cross section taken perpendicular to the fiber axis of a microstructured optical fiber in accordance with the present invention. 
       FIGS. 3-5  are graphs showing various fiber characteristics for each fibers of the present invention and prior art fibers. 
       FIG. 6  is an illustration of a cross section taken perpendicular to the fiber axis of a microstructured optical fiber in accordance with a second embodiment of the present invention. 
       FIG. 7  shows graphs of the dispersion and mode field diameter (MFD) of the optical fiber  1   a  shown in FIG.  6 . 
       FIG. 8  shows is an illustration of a cross section taken perpendicular to the fiber axis of a microstructured optical fiber in accordance with a third embodiment of the present invention. 
       FIGS. 9-11  are graphs showing comparison of various fiber characteristics of fiber structures comporting to the embodiment of  FIG. 8  with other fiber structures. 
       FIG. 12  is illustrative of a process for fabricating a preform in accordance with the present invention. 
       FIG. 13  is illustrative of a process of drawing a preform in accordance with the present invention. 
       FIG. 14  is illustrative of another process of drawing a preform in accordance with the present invention. 
       FIG. 15  is illustrative of preform fabrication in accordance with another embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2  shows the cross section of an optical fiber  1  of a first embodiment of the present invention, in which a plurality of holes  3 , which are sub medium regions, are arranged in silica glass  2 , which is a main medium. The cross section has an inner region  11  and an outer region  12  surrounding the inner region. The arrangement of the holes is substantially a hexagonal lattice. In the outer region, the centers of the holes  32  substantially coincide with the outer lattice points  42 , which are a part of the lattice points  4  of a hexagonal lattice and are located in the outer region. On the other hand, in the inner region, the centers of the holes  31  are apart from the inner lattice points  41  by approximately 0.1 lattice period outward. The inner lattice points are a part of the lattice points  4  of the hexagonal lattice and are located in the inner region. The diameters of the holes  31  and  32  are substantially uniform in the cross section. For each lattice cell of the lattice point  41  or  42 , a single hole  31  or  32  has its center in the cell. In describing the present invention, the hole located in a lattice cell of a lattice point is called the hole corresponding to the lattice point. 
   The outer region is further surrounded by a jacket region  13  composed of silica glass. Although the jacket region is not essential for guidance of lightwave in the core of the optical fiber, it has effects to improve the mechanical strength of the optical fiber and to reduce microbending loss, which is one of the causes of transmission loss. Because of the arrangement of the holes described above, the area of the main medium region  21  between the holes  31  in the inner region  11  becomes larger than that of the main medium region  22  between the holes  32  in the outer region  12 . As a result, the inner region has a higher average refractive index than that of the outer region, and it becomes possible to localize a lightwave in the main medium region  21  in the inner region  11  by total reflection and guide it over the optical fiber. 
   Investigation of optical characteristics has been done on three structures ha8, ha7 and ha6 of the present invention and three structures hb8, hb7 and hb6 of the prior art. Relative hole diameters of these structures are given in the following table. 
                                           TABLE 1               Symbol   ha6   ha7   ha8   hb6   hb7   hb8                   Relative hole diameter   0.6   0.7   0.8   0.6   0.7   0.8                    
In the investigation, the hole diameter has been varied and other dimensions in the fiber have been varied proportionally.
 
     FIGS. 3-5  show various fiber characteristics for each of the listed fibers.  FIG. 3  shows the chromatic dispersion at 1550 nm wavelength as a function of the hole diameter. As shown in the figure, the increase in the relative hole diameter results in an increase in the hole diameter below which zero or negative dispersion can be obtained. The hole diameter below which zero or negative dispersion can be obtained is larger for the fibers of the present invention than in the prior art fibers having the same relative hole diameters.  FIG. 4  shows the chromatic dispersion slope at 1550 nm wavelength as a function of the hole diameter. As shown in the figure, the increase in the relative hole diameter results in an increase in the hole diameter below which zero or negative dispersion slope can be obtained. The hole diameter below which zero or negative dispersion slope can be obtained is larger for the fibers of the present invention than in the prior art fibers having the same relative hole diameters.  FIG. 5  shows the effective core area at 1550 nm wavelength as a function of the hole diameter. As shown in the figure, the increase in the relative hole diameter results in an increase in the hole diameter below which small (less than 5 μm 2 ) effective core area can be obtained. The hole diameter below which small (less than 5 μm 2 ) effective core area can be obtained is larger for the fibers of the present invention than in the prior art fibers having the same relative hole diameters. These graphs demonstrate advantages obtained with fibers of the present invention. 
   It is also preferable to add dopants such as germanium and fluorine to the main medium in the main medium region  21  in the inner region  11 , and decrease the viscosity therein. As a result, it becomes possible to reduce the concentration of the drawing tension to the main medium region in the inner region, in which a guided lightwave is to be localized, and reduce excess transmission loss due to residual tension. It is also preferable to add 1-35 mol % of germanium to the main medium in the main medium region  21  in the inner region  11 , and form a fiber grating by varying the refractive index along the fiber length by exposing the optical fiber to UV radiation. As a result, it becomes possible to realize a wavelength-selective transmission and a wavelength-selective directional coupler. It is also preferable to vary the cross-sectional structure along the fiber length. As a result, it becomes possible to change chromatic dispersion along the fiber length and form a dispersion-managed transmission line. It is also preferable to form a length of section in which no holes are included. By providing such sections in the fiber ends, it becomes possible to prevent the entrance of contaminants into the holes. It is also preferable to employ compound glass or polymer as main medium, or to fill the holes with gases or metals having optical amplifying properties. As a result, it becomes possible to enhance the efficiency of nonlinear optical effects, and/or to realize an optical amplifying function. 
     FIG. 6  shows the cross section of an optical fiber  1   a  of a second embodiment of the present invention. In this structure, a plurality of holes  3   a  are arranged in silica glass  2   a , and the holes  32   a  in the outer region  12   a  are arranged so that there centers coincide with respective lattice points  42   a , which are a part of the lattice points  4   a  of a hexagonal lattice and located in the outer region, and consequently the arrangement possesses a two-dimensional translational symmetry. Other media having appropriate refractive indices may be used in lieu of holes. Because of the two-dimensional translational symmetry, it is possible to localize a lightwave in the inner region  11   a  by Bragg reflection and guide it over the fiber. Since the arrangement of the holes  32   a  is periodical in the outer region  12   a , the outer region  12   a  can reflect a lightwave belonging to a certain range of frequencies and propagation constant by Bragg reflection. Although such a lightwave cannot propagate in the outer region  12   a , since the structural periodicity is broken in the inner region  11   a , it localizes and propagates in the inner region  11   a.    
   A waveguiding mechanism known as guidance by Bragg reflection, or guidance by photonic bandgap, is described in Stig E. Barkou et al., OFC &#39;99 FG5, and J. A. West et al., ECOC &#39;01 Th.A.2.2. However, these prior art disclosures do not recognize that it is possible to break the structural periodicity by offsetting the holes from the corresponding lattice points. As a result, although a dispersion with a positive dispersion slope was disclosed by J. A. West, a dispersion with a negative dispersion slope has not been known. In the present invention, a dispersion with a negative dispersion slope is realized by forming a region in which a lightwave is guided by offsetting the holes from the corresponding lattice points.  FIG. 7  shows the dispersion and mode field diameter (MFD) of the optical fiber  1   a  shown in FIG.  6 . In this example, the pitch of the hexagonal lattice is 1.68 μm, the hole diameter 0.84 μm. The holes  35   a  in the inner region  11   a  is located 0.15 lattice period outward from the inner lattice points  41   a . The hole  31   a  at the center of the inner region  11   a  is located on the inner lattice point  41   a . As in this example, it is also preferable to provide in the inner region holes having centers that are located apart from the corresponding lattice points in addition to holes whose centers are located on the corresponding lattice points. 
   As shown in  FIG. 7 , the lightwave from 1540 nm wavelength to 1570 nm is localized in the inner region  11   a  and has a small mode field diameter (MFD). In addition, the dispersion slope is negative in this wavelength range, which is favorable for application to compensation of dispersion with a positive slope. It is also possible to use the optical fiber  1   a  as a band-pass filter because Bragg reflection occurs in a finite frequency range and hence a wavelength-dependent transmission can be realized. 
     FIG. 8  shows the cross section of an optical fiber  1   c  of a third embodiment of the invention. In the optical fiber  1   c , a plurality of holes  3   c  are arranged in silica glass  2   c . The cross section has an inner region  11   c  and an outer region  12   c  surrounding the inner region. In this embodiment, the arrangement of the holes is substantially a square lattice. In the outer region  12   c , the centers of holes  32   c  are located at the outer lattice points  42   c , which are a part of lattice points  4   c  of the square lattice and are located in the outer region  12   c . On the other hand, in the inner region  11   c , the centers of holes  31   c  are located apart from the inner lattice points  41   c  by approximately 0.1 lattice period outward. The inner lattice points  41   c  are a part of lattice points  4   c  of square lattice and are located in the inner region  11   c . For each lattice cell of the lattice point  41   c  or  42   c , a single hole  31   c  or  32   c  has its center in the cell. The outer region  12   c  is further surrounded by a jacket region  13   c . Although the jacket region  13   c  is not essential for waveguiding, it has effects to improve the mechanical strength of the optical fiber  1   c  and to reduce microbending loss, which is a cause of transmission loss. 
   Because of the arrangement of the holes described above, the area of the main medium region  21   c  between the holes  31   c  in the inner region  11   c  is larger than that of the main medium region  22   c  between the holes  32   c  in the outer region  12   c . As a result, the inner region has a higher average refractive index than that of the outer region, and it becomes possible to localize a lightwave in the main medium region  21   c  in the inner region  11   c  by total reflection and guide it over the optical fiber. 
   An investigation has been made comparing optical characteristics of fibers having structures such as illustrated in  FIG. 8  with microstructure fibers having holes whose centers substantially coincide with all lattice points a square lattice but lack a hole in the center of the fiber, thereby being provided with a higher average refractive index in the center than that of the surrounding region. Relative hole diameters are given in the following table, wherein structures ra8, ra7 and ra6 represent fiber structures of the present invention as illustrated in FIG.  8  and the structures rb8, rb7 and rb6 have no holes offset from the lattice. 
                                           TABLE 2               Symbol   ra6   ra7   ra8   rb6   rb7   rb8                   Relative hole diameter   0.6   0.7   0.8   0.6   0.7   0.8                    
In the investigation, the hole diameter has been varied and other dimensions in the fiber have been varied proportionally.
 
     FIGS. 9-11  show various fiber characteristics for each of the listed fibers.  FIG. 9  shows the chromatic dispersion at 1550 nm wavelength as a function of the hole diameter. As shown in the figure, the increase in the relative hole diameter results in an increase in the hole diameter below which zero or negative dispersion can be obtained. The hole diameter below which zero or negative dispersion can be obtained is larger for the fibers of the present invention than in the other fibers having the same relative hole diameter.  FIG. 10  shows the chromatic dispersion slope at 1550 nm wavelength as a function of the hole diameter. As shown in the figure, the increase in the relative hole diameter results in an increase in the hole diameter below which zero or negative dispersion slope can be obtained. The hole diameter below which zero or negative dispersion slope can be obtained is larger for the fibers of the present invention than in the other fibers having the same relative hole diameter.  FIG. 11  shows the effective core area at 1550 nm wavelength as a function of the hole diameter. As shown in the figure, the increase in the relative hole diameter results in an increase in the hole diameter below which small (less than 5 or 6 μm 2 ) effective core area can be obtained. The hole diameter below which small (less than 5 or 6 μm 2 ) effective core area can be obtained is larger for the fibers of the present invention than in the other fibers that have the same relative hole diameters. These graphs demonstrate advantages obtained with fibers of the present invention. 
   The optical fibers of the embodiments of the invention described above can be fabricated as illustrated in FIG.  12 . First, a preform with a column shape and made of silica glass is fabricated by well-known methods such as VAD method, MCVD method, and OVD method. The silica glass for the preform can be free from impurities, or can include dopants such as germanium, fluorine, chlorine, boron, aluminum, phosphorus, rare earth elements, and transition metals. By appropriately choosing the kind and concentration of the dopants, it is possible to enhance the efficiency of nonlinear optical effects or to realize optical amplifying characteristics. 
   Second, a plurality of holes  64  extending along the preform length are formed by a boring means  63  in the preform  61 , which is secured by grabbing means  62 . The arrangement of the holes should be made proportional to that of the holes in the fiber to be made, such as those in  FIGS. 2 ,  6  and  8 . It is preferable that the tip  65  of the boring means  63  is made of an alloy including diamond particles or of metal with a surface equipped with diamond particles. It is also possible to enhance the efficiency of boring holes by applying an ultrasonic wave to the boring means  63 . Although the holes  64  can penetrate through the preform, it is also possible to end the holes in the midst of the preform so that enough length of the original preform length can be retained for handling in the later steps. Shortening the boring length also shortens the time for the boring step. The holes  64  may be bored, either by rotating the boring means  63  around the axis of a hole  64 , or by rotating the preform  61  around the axis of a hole  64 , or by rotating both the boring means and the preform. 
   After the boring step, further processing steps comprise elongating the preform  61 , inserting the elongated preform  61  in a glass tube with an inner diameter slightly larger than the outer diameter of the elongated preform  61 , and heating and collapsing the glass tube onto the elongated preform  61 . Also, it is possible to deposit soot of silica glass onto the outer surface of the preform  61  by vapor phase deposition, and heat and sinter the deposited soot. By employing such steps, it becomes possible to reduce the ratio of the hole diameter to the whole preform diameter, and to equivalently form holes with small diameter that are difficult to form by boring. 
   It is also possible to etch the inner surface of the holes  64  by hydrofluoric acid solution or SF 6  gas. As a result, it becomes possible to eliminate impurities such as hydroxyl group and transition metals on the surface or in the glass near the surface, and to reduce the surface roughness, resulting in a low transmission loss. 
   Finally, the preform  61  is drawn into an optical fiber as illustrated in FIG.  13 . The preform  61  is grabbed by a grabbing means (not shown). An end of the preform  61  having the holes  64  is connected to a pressure adjusting means  72  via a connecting means  71 , and thereby the pressure in the holes  64  is adjusted. The arrangement of the holes is same or proportional to that in the preform-febrication step shown in  FIG. 12  or  15  given below. From the other end, the preform is fed into a heating means  73  at a predetermined velocity. From the heated and softened portion of the preform, an optical fiber  76  is drawn. The optical fiber is pulled by a pulling means (not shown). The hole diameter in the optical fiber can be controlled by controlling the pressure of the pressure adjusting means  72 . 
   It is also possible to draw an optical fiber from the preform  61  in which the holes  64  are sealed, as shown in FIG.  14 . As a result, it becomes easier to prevent contaminants to enter into the holes  64 . The arrangement of the holes is same or proportional to that in the preform-febrication step shown in  FIG. 12  or  15  given below. 
   It is also possible to form the holes  64  in the preform  61  not by boring as in FIG.  12 .  FIG. 15  is illustrative of preform fabrication to obtain optical fibers using several different materials. A preform  61   a  can be formed by arranging a plurality of silica capillaries  67  around a silica rod  66  into a bundle, and insert the bundle into a silica jacket tube  68 . The silica rod  66  has a smaller diameter than the silica capillaries  67 . Then, the preform  61   a  is drawn in the step as shown in  FIG. 13  or  14 . The drawing temperature is chosen so that the interstitial gaps between the silica capillaries  67  shrink and/or close by surface tension. Since the flow amount of softened glass accompanying the shrink of the gap is small where the silica rod  66  exists, the distance between the holes becomes larger there than elsewhere in the cross section. As a result, an optical fiber with a cross section that is similar to that in embodiment  1  can be obtained. 
   The preform fabrication method of  FIG. 15  is suited to fabricate optical fibers using several different materials. For example, by employing silica with more than 10 mol % of germanium as the material of the silica rod  66 , the efficiency of the nonlinear optical effects can be enhanced. Also, by employing a silica rod  66  doped with rare earth elements, an optical fiber with optical amplifying characteristics can be obtained. 
   While this invention has been described in connection with what is presently considered to be the most practical and preferred embodiments, the invention is not limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.