Abstract:
The present invention relates to an optical fiber comprising a structure which yields a low optical transmission loss even when doped with a high concentration of Ge element and is excellent in reliability for strength. The optical fiber according to the present invention comprises a core region whose maximum value Δmax of relative refractive index difference with respect to silica glass non-intentionally doped with impurities is 0.8% or more, in its diameter direction, a cladding region comprising silica glass non-intentionally doped with impurities or silica glass doped with a predetermined amount of fluorine, and a hermetic coat mainly composed of carbon. In particular, the transmission loss α at a wavelength of 1.55 μm and the maximum value Δmax satisfy the relationship of: 
     
       
         α≦0.131×(Δmax) 2 −0.214×(Δmax)+0.284 
       
     
     in the optical fiber according to the present invention.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical fiber employable as a transmission line in optical communication systems. 
     2. Related Background Art 
     Usually, an optical fiber is made by drawing one end of an optical fiber preform comprising a plurality of regions with refractive indices different from each other while heating it. In a typical conventional drawing step, while the optical fiber preform is in a held state, it is heated and, at the same time, tension is applied in the gravity direction to its part softened upon heating. As one end of the optical fiber preform softened upon heating is drawn until a desirable fiber diameter is attained due to the application of tension in the gravity direction, an optical fiber is obtained. 
     For example, Japanese Patent Application Laid-Open No. HEI 9-127354 discloses a dispersion compensating optical fiber whose core is doped with a high concentration of GeO 2 , while stating that, when the optical fiber doped with such a high concentration of GeO 2  is drawn, a tension of 5 to 16 kg/mm 2  is added to its preform. In general, while an optical fiber yields a lower transmission loss as the drawing tension is greater, there is a possibility of increases in tension causing the optical fiber to break. Therefore, the above-mentioned publication indicates 5 to 16 kg/mm 2  as a preferable tension range at the time of drawing. 
     SUMMARY OF THE INVENTION 
     The inventors have studied the prior art and, as a result, have found problems as follows. Namely, in the case of an optical fiber whose core is doped with a high concentration of GeO 2  in order to attain a large refractive index difference between the core and cladding, such as dispersion compensating optical fiber and dispersion-shifted optical fiber, the increase in Rayleigh scattering loss caused by GeO 2  doping becomes problematic. For suppressing or reducing this increase in loss, the drawing tension may be increased. In this case, however, there is a possibility that the reliability of the optical fiber concerning its strength may lower as it breaks more often, for example. 
     In order to overcome such problems as mentioned above, it is an object of the present invention to provide an optical fiber comprising a structure which yields a low transmission loss even when doped with a high concentration of GeO 2  and is excellent in reliability for strength. 
     The optical fiber according to the present invention comprises a core region having a desirable refractive index profile, a cladding region provided on the outer periphery of the core region, and a hermetic coat provided on the outer periphery of the cladding region and mainly composed of carbon. Here, at least one of areas constituting the core region is doped with GeO 2  as a refractive index raising material, and the area having the maximum refractive index in the areas doped with GeO 2  is doped with GeO 2  of 8 mol % or more. 
     In a first embodiment of the optical fiber according to the present invention, the core region is set such that its maximum value (Δmax) of relative refractive index difference in the core region with respect to silica glass non-intentionally doped with impurities (hereinafter referred to as pure silica glass) is 0.8% or more, in its diameter direction. The above-mentioned cladding region is made of pure silica glass. 
     In the optical fiber according to the first embodiment as mentioned above, since the surface of the cladding region is covered with the carbon coat, the resulting optical fiber is hard to break even when the tension at the time of drawing is increased in order to suppress or reduce transmission loss, whereby high reliability for strength is obtained. 
     If Δmax is made smaller (the doping amount of GeO 2  is lowered), on the other hand, then an optical fiber having a transmission loss equal to or less than that in the first embodiment can be obtained even at a lower drawing tension. If the value of Δmax with reference to pure silica glass is lowered alone, however, then the shape of refractive index profile also changes, thereby making it hard to yield desirable optical characteristics. Therefore, a second embodiment according to the present invention characteristically comprises a structure in which the cladding region is doped with fluorine, which is a refractive index lowering material, so as to substantially lower Δ max with reference to pure silica glass without changing the maximum relative refractive index difference of the core region with respect to the cladding region (totally lower the refractive index without changing the shape of refractive index profile itself). Here, the doping amount of fluorine with respect to the cladding region is preferably 0.5 wt % or more but 2 wt % or less. 
     In the optical fiber according to the first embodiment, in particular, the transmission loss α at a wavelength of 1.55 μm and the maximum relative refractive index difference Δmax of the core region with respect to the cladding region (pure silica glass) satisfy the relationship of: 
     
       
         α≦0.131×(Δmax) 2 −0.214×(Δmax)+0.284. 
       
     
     Also, when the transmission loss α is given by a quartic function including (A·λ −4 +B) with respect to a wavelength λ, the coefficient A in (A·λ −4 +B) is given by: 
     
       
           A≦ 0.446×(Δmax) 2 −0.484×(Δmax)+1.072 
       
     
     in the range where Δmax&gt;0.8%. 
     In the optical fiber according to the second embodiment in which the cladding region is doped with fluorine, on the other hand, the transmission loss α at a wavelength of 1.55 μm and the maximum relative refractive index difference Δ max satisfy the relationship of: 
     
       
         α≦0.0846×(Δmax) 2 −0.147×(Δmax)+0.262. 
       
     
     Also, when the transmission loss α is given by a quartic function including (A·λ −4 +B) with respect to a wavelength λ, the coefficient A in (A·λ −4 +B) is given by: 
     
       
           A≦ 0.374×(Δmax) 2 −0.369×(Δmax)+1.003 
       
     
     in the range where Δmax&gt;0.8%. 
     Further, in each of the above-mentioned first and second embodiments, the above-mentioned hermetic coat in the optical fiber according to the present invention has a film thickness of 10 nm or more but 100 nm or less, and a resistivity of 0.5×10 −3  Ω·cm or more but 5×10 −3  Ω·cm or less. Also, the optical fiber according to the present invention is applicable to various optical fibers such as, for example, any of an optical fiber having, as characteristics at a wavelength of 1.55 μm, a dispersion of −5 ps/nm/km or more but +5 ps/nm/km or less and an effective area of 50 μm 2  or more; an optical fiber having, as characteristics at a wavelength of 1.55 μm, a dispersion of +6 ps/nm/km or more but +10 ps/nm/km or less and an effective area of 50 μm 2  or more; an optical fiber having, as characteristics at a wavelength of 1.55 μm, a dispersion of −70 ps/nm/km or more but −15 ps/nm/km or less and an effective area of 20 μm 2  or more; and an optical fiber having, as characteristics at a wavelength of 1.55 μm, a dispersion of −200 ps/nm/km or more but −75 ps/nm/km or less and an effective area of 15 μm 2  or more. 
     The optical fiber according to the present invention is also applicable to a dispersion management fiber in which signs of dispersion value at a wavelength of 1.55 μm alternate in a traveling direction of light signals. Such an optical fiber is also obtained when the drawing rate at the time of drawing is altered at predetermined time intervals, or when an optical fiber preform in which the diameter of a core material is altered at predetermined spacing in the longitudinal direction is drawn. 
     In the making of the optical fiber according to the present invention, a first step of drawing an optical fiber from an optical fiber preform with a predetermined tension applied thereto, a second step of applying a hermetic coat mainly composed of carbon to the optical fiber obtained by the first step, and a third step of covering the optical fiber obtained by the second step with a resin material are carried out in succession. 
     In the case where the optical fiber according to the first embodiment is to be made, it is preferred that a tension of 13 kg/mm 2  or more, more preferably 17 kg/mm 2  or more but 28.5 kg/mm 2  or less, be applied to the optical fiber preform; whereby transmission loss is reduced in thus obtained optical fiber according to the first embodiment even when doped with a high concentration of GeO 2 . Specifically, if the maximum doping amount of GeO 2  with respect to the core region is 15 mol %, for example, then the transmission loss of the resulting optical fiber becomes 0.3 dB/km or less. Also, the surface of the optical fiber obtained by the first step is provided with the carbon coat in the second step, and the carbon coat surface is covered with the resin material in the third step, so that the optical fiber is hard to break even though it is drawn at a high tension, whereby its reliability for strength improves. When the optical fiber according to the second embodiment is to be made, the tension applied to the optical fiber preform in the first step may be 13 kg/mm 2  or less. 
     The optical fiber according to the present invention manufactured as mentioned above has a fatigue parameter of 50 or more. 
     The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given by way of illustration only and are not to be considered as limiting the present invention. 
     Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view showing a first configuration of manufacturing apparatus for making an optical fiber according to the present invention; 
     FIGS. 2A to  2 C are views showing respective cross-sectional structures of the optical fiber at individual parts of the manufacturing apparatus shown in FIG. 1; 
     FIG. 3A is a view showing a cross-sectional structure in the first embodiment of the optical fiber according to the present invention, whereas 
     FIG. 3B is a refractive index profile thereof; 
     FIGS. 4A to  4 N are various refractive index profiles employable as the refractive index profile of the optical fiber according to the present invention; 
     FIG. 5 is a graph showing relationships between the drawing tension (kg/mm 2 ) and the transmission loss of the resulting optical fibers; 
     FIG. 6 is a graph showing relationships between the drawing tension (kg/mm 2 ) and the fatigue index (n value) of the resulting optical fibers; 
     FIG. 7 is a graph showing a relationship between the GeO 2  concentration (mol %) in the core region and the maximum relative refractive index difference Δmax of the core region with respect to the cladding region; 
     FIG. 8 is a graph showing relationships between the drawing tension (kg/mm 2 ) and the transmission loss of the resulting optical fibers at a wavelength of 1550 nm; 
     FIG. 9A is a refractive index profile of the first embodiment of the optical fiber according to the present invention, whereas 
     FIG. 9B is a refractive index profile of a modified example of the first embodiment; 
     FIG. 10 is a graph showing relationships between the transmission loss of optical fibers at a wavelength of 1550 nm and the maximum relative refractive index difference Δ max of the core region with respect to the cladding region; 
     FIG. 11 is a graph showing relationships between the maximum relative refractive index difference Δmax of the core region with respect to the cladding region and the coefficient A of the quartic function indicating the relationship between the transmission loss α and the wavelength λ; 
     FIG. 12 is a view showing a second configuration of manufacturing apparatus for making the optical fiber according to the present invention; 
     FIG. 13 is a view for explaining a spatial positional relationship between a rocking guide roller and a first fixed guide roller; and 
     FIG. 14 is a view for explaining a spatial positional relationship between a pair of guide rollers for suppressing the passive movement of optical fiber and a rocking guide roller. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, individual embodiments of the present invention will be explained with reference to FIGS. 1,  2 A to  4 N,  5  to  8 ,  9 A,  9 B, and  10  to  14 . In the explanation of the drawings, constituents identical to each other will be referred to with numerals identical to each other without repeating their overlapping descriptions. 
     FIG. 1 is a view showing a first configuration of manufacturing apparatus for making an optical fiber according to the present invention. 
     In the making of the optical fiber, as shown in FIG. 1, an optical fiber preform  100  to be drawn is initially prepared. This optical fiber preform  100  is mainly composed of silica glass and has a predetermined refractive index profile. The optical fiber preform  100  can be produced by vapor phase axial deposition method (VAD method), outside vapor deposition method (OVD method), inside vapor phase oxidation method (MCVD method), rod-in-tube method, or the like. 
     Subsequently, the optical fiber preform  100  is attached to a dummy rod  130 , and a preform leader  220  moves the dummy rod  130  toward a heater  230 , where by the optical fiber preform  100  attached to the dummy rod  130  is introduced into the heater  230 . Then, as the lower end of the optical fiber preform  100  heated by the heater  230  is drawn, a bare fiber  150  is obtained (first step). 
     The bare fiber  150  obtained by drawing subsequently passes through a reaction tube  250  for forming a carbon coat. Supplied into the reaction tube  250  is a mixed gas of halogenated carbon (such as CHCl 3 , CCl 4 , or the like) and hydrocarbon (such as C 2 H 4 , C 3 H 8 , C 6 H 6 , or the like), in which halogenated carbon and hydrocarbon react with each other on the surface of the bare fiber  150 , whereby the surface of the bare fiber  150  is covered with a hermetic coat (carbon coat)  151  mainly composed of carbon (second step). 
     The outside diameter of the carbon coated fiber  160  covered with the carbon coat  151  is measured by a laser outside diameter meter  300 . According to the result of measurement obtained by the laser outside diameter meter  300 , the heating temperature and drawing rate are controlled by a control system  400  such that the outside diameter of the carbon coated fiber  160  becomes a predetermined value (usually 125 μm). 
     The carbon coated fiber  160  having passed the laser outside diameter meter  300  further passes through a liquid resin  510  stored in a resin coating die  500 , whereby the resin attaches to the surface of the carbon coated fiber  160  (thus yielding a resin-attached fiber  170 ). Subsequently, the resin-attached fiber  170  passes a UV lamp  600 . Here, the resin attached to the surface of the carbon coated fiber  160  cures upon ultraviolet light irradiation from the UV lamp  600 . As a consequence, an optical fiber  180  (optical cord) in which the surface of the carbon coated fiber  160  is covered with a resin coating  161  is obtained (third step), and the optical fiber  180  is taken up by a drum  700 . 
     In the foregoing drawing step (first step), a tension of 13 kg/mm 2  or more, preferably 17 to 28.5 kg/mm 2 , is applied to the bare fiber  150 . Therefore, transmission loss is reduced in the optical fiber obtained by the above-mentioned manufacturing method even when doped with a high concentration of GeO 2 . 
     The surface of the drawn bare fiber  150  is provided with the carbon coat  151  in the second step, and further is covered with the resin material in the third step. Therefore, even when residual stress occurs in the optical fiber  180  finally obtained after cooling, the carbon coat  151  prevents vapor and hydroxide ion from diffusing from the resin coating  161  into the bare fiber  150 , thereby suppressing the growth of damages on the order of submicron in the bare fiber  150 . Consequently, the optical fiber made by way of the foregoing individual steps is hard to break even though it is drawn at a high tension, thereby attaining a high reliability for strength. 
     Specifically, the core region of the optical fiber  180  made by the above-mentioned manufacturing method includes an area doped with GeO 2  of 8 mol % or more. In this optical fiber  180 , the fatigue index (n value) indicative of the difficulty in breaking upon application of load is 50 or more. 
     Here, FIGS. 2A,  2 B, and  2 C are views showing respective cross sections of the prepared optical fiber preform  100 , the carbon coated fiber  160  in which the surface of the drawn bare fiber (including the core region and a cladding region)  150  is covered with the carbon coat  151 , and the optical fiber  180  as the final product in which the surface of the carbon coated fiber  160  is provided with the resin coating  161 . 
     First Embodiment 
     A first embodiment of the optical fiber according to the present invention will now be explained. 
     FIG. 3A is a view showing a typical cross-sectional structure of a dispersion compensating optical fiber as the optical fiber  180  according to the first embodiment, whereas FIG. 3B is a refractive index profile thereof. 
     As shown in FIG. 3A, the optical fiber  180  according to the first embodiment is constituted by a core region  110 , a cladding region  120  provided on the outer periphery of the core region  110 , a carbon coat  151  provided on the outer periphery of the cladding region  120 , and a resin coating  161  provided on the outer periphery of the carbon coat  151 . The core region  110  comprises a double structure comprising an inner area  110   a,  which is a silica glass area doped with 15 mol % of GeO 2 , having an outside diameter a and a maximum refractive index n 1 , and an outer area  110   b,  which is a silica glass area doped with fluorine, having an outside diameter b (&gt;a) and a refractive index n 2 . On the other hand, the cladding region  120  is constituted by silica glass which is non-intentionally doped with impurities (hereinafter referred to as pure silica glass), while having a refractive index n 3 . Also, the maximum refractive index difference Δ max (=(n 1 −n 3 )/n 3 ) of the inner area  110   a  doped with GeO 2  with respect to the cladding region is 1.5%. 
     The above-mentioned core region  110  and cladding region  120  constitute the bare fiber  150 ; the bare fiber  150  and the carbon coat  151  constitute the carbon coated fiber  160 ; and the carbon coated fiber  160  and the resin coating  161  constitute the optical fiber  180  (optical cord). 
     FIG. 3B is a refractive index profile  15  of the optical fiber  180  shown in FIG. 3A, indicating the refractive index of each part on a line L. In this refractive index profile  15 , areas  16 ,  17 , and  18  indicate refractive indices of individual parts on the line L in the inner area  110   a  in the core region  110 , the outer area  110   b  in the core region  110 , and the cladding region  120 , respectively. The structure of the core region  110  is not restricted to that shown in FIG.  3 A. For example, it may employ structures having various refractive index profiles as shown in FIGS. 4A to  4 N. 
     The optical fiber  180  according to the first embodiment was made as follows. Initially, a circular columnar inner core material made of silica glass doped with GeO 2 , a circular tubular outer core material made of silica glass doped with fluorine, and a circular tubular cladding material made of pure silica glass, each made by the VAD method, were prepared. The outer core material was inserted inside the cladding material, the inner core material was subsequently inserted inside the outer core material, and they are unified by the rod-in-tube method. Thus unified member was employed as the optical fiber preform  100 . 
     In the first step, the drawing rate for yielding the bare fiber  150  from the optical fiber preform  100  by drawing is set at 200 m/min. In the second step, the thickness of the carbon coat  151  covering the surface of the bare fiber  150  is set at 30 to 50 nm. In the third step, the outside diameter of the resin coating  161  covering the surface of the carbon fiber  160  is set at 250 μm. 
     FIG. 5 is a graph showing the results of measurement of transmission loss at a wavelength of 1550 nm in individual optical fibers  180  obtained while the drawing tension (kg/mm 2 ) was successively changed under the above-mentioned conditions, whereas FIG. 6 is a graph showing the results of measurement of fatigue index (n value) in the individual optical fibers  180  obtained while the drawing tension (kg/mm 2 ) was successively changed under the above-mentioned conditions. 
     As can be seen from the graph shown in FIG. 5, the greater is the tension at the time of drawing, the lower becomes the transmission loss of the optical fiber  180  manufactured. For example, in order for the transmission loss at a wavelength of 1550 nm to be 0.35 dB/km or less, the tension at the time of drawing is preferably 13 kg/mm 2  or greater. Since the transmission loss at a wavelength of 1550 nm is desired to be 0.3 dB/km or less in practice, it is further preferred that the tension at the time of drawing be 17 kg/mm 2 or greater. For preventing breakage from occurring, however, the tension at the time of drawing is preferably 28.5 kg/mm 2  or less. 
     In the graph of FIG. 6, n values of the carbon coated fiber  160  measured at respective drawing tensions are plotted with black circles, whereas n values of the bare fiber  150  (without carbon coat) are plotted with white circles. As can be seen from the graph of FIG. 6, the fatigue index of the carbon coated fiber  160  (black circle) is 50 or greater, whereas the fatigue index of the bare fiber  150  (white circle) without the carbon coat  151  is less than 20. Thus, the optical fiber  180  can attain a favorable reliability for strength when the surface of the bare fiber  150  is covered with the carbon coat  151 . 
     Second Embodiment 
     As in the foregoing, the optical fiber according to the first embodiment can suppress the increase in transmission loss and attain a favorable reliability for strength by enhancing the drawing tension in the manufacturing step thereof. However, if Δmax with reference to the refractive index of pure silica glass is lowered in the configuration of the first embodiment, then the transmission loss can be reduced even in the case where the same drawing tension is applied. 
     Lowering Δmax means a decrease in the amount of GeO 2  added into the core region. If the value of Δmax with reference to pure silica glass is simply lowered alone, on the other hand, then the shape of refractive index profile itself changes such that the maximum relative refractive index difference of the core region with respect to the cladding region becomes smaller and so forth, whereby desirable optical characteristics are harder to attain. 
     The second embodiment of the optical fiber according to the present invention is an optical fiber having a transmission loss equal to or less than that in the above-mentioned first embodiment even when the drawing tension at the time of making is made lower than that in the case of the first embodiment. Its refractive index profile comprises a configuration similar to that of the optical fiber according to the above-mentioned first embodiment (FIGS. 3A,  3 B, and  4 A to  4 N), but differs therefrom in that the cladding region  120  is doped with fluorine. Therefore, the refractive index profile of the optical fiber according to the second embodiment can be realized by various shapes as shown in FIGS. 4A to  4 N. Namely, the optical fiber according to the second embodiment characteristically has a structure in which the cladding region  120  is doped with fluorine, which is a refractive index lowering material, whereby Δmax with reference to pure silica glass is substantially lowered without changing the maximum relative refractive index difference of the core region with respect to the cladding region (thus totally lowering the refractive index without changing the shape of refractive index profile itself). 
     In general, a proportional relationship such as one shown in FIG. 7 holds between the doping amount of GeO 2  and the relative refractive index difference of a glass material doped with GeO 2 . Here, FIG. 7 is obtained from a Sellmeier&#39;s polynomial disclosed in S. Kobayashi, S. Shibata, N. Shibata, and T. Izawa, “Refractive-index dispersion of doped fused silica,” IOOC &#39;77. Also, the inventors have experimentally studied how the relationship between the drawing tension (kg/mm 2 ) at the time of making and the transmission loss (dB/km) changes in samples with different values of Δmax. FIG. 8 is a graph in which relationships between the drawing tension and the transmission loss at a wavelength of 1550 nm are measured in samples having different values of Δ max. The prepared samples are optical fibers according to the first embodiment having a refractive index profile shown in FIG. 9A, whereas the cladding region  120  is constituted by pure silica glass. In FIG. 8, G 100  is a curve concerned with a sample in which the maximum relative refractive index difference Δmax in the core region  110  with respect to pure silica glass (cladding region  120 ) is set at 2.5%, whereas G 200  is a curve concerned with a sample in which the maximum relative refractive index difference Δmax in the core region  110  with respect to pure silica glass (cladding region  120 ) is set at 1.5%. 
     As can be seen from the curves G 100 , G 200  in FIG. 8, each of these cases tends to lower the transmission loss at a wavelength of 1550 nm as the drawing tension increases as in the graph shown in FIG.  5 . It is also seen that, even at the same drawing tension, the sample with a smaller value of Δmax can yield a lower transmission loss. This result of measurement shows that, in view of the relationship in FIG. 7, a further decrease in transmission loss can be realized if the doping amount of GeO 2  is lowered (Δmax with reference to the refractive index of pure silica glass is reduced). 
     According to the foregoing findings, the optical fiber according to the second embodiment dopes the cladding region  120  with fluorine, so as to totally lower the refractive index, thereby maintaining the shape of refractive index profile, while comprising a structure in which Δmax with reference to the refractive index of pure silica glass is made lower than that in the first embodiment, i.e., structure in which the maximum doping amount of GeO 2  added into the core region  110  is smaller than that in the first embodiment. Preferably, the amount of fluorine added to the cladding region is 0.5 wt % or more but 2 wt % or less. 
     Concerning each of the optical fibers according to the first and second embodiments, the relationship between the maximum relative refractive index difference Δmax of the core region with respect to pure silica glass (silica glass non-intentionally doped with impurities) and the transmission loss will now be explained. The samples of the first and second embodiments prepared for the following explanations have the refractive index profiles shown in FIGS. 9A and 9B, respectively. Namely, FIG. 9A is a refractive index profile of the sample prepared as the optical fiber according to the first embodiment, comprising a core region  110  of a triple structure having two regions doped with GeO 2 , and a cladding region  120  made of pure silica glass. FIG. 9B is a refractive index profile of the sample prepared as the optical fiber according to the second embodiment, comprising a single core region  110  doped with GeO 2 , and a cladding region  120  doped with fluorine. 
     The curve G 300  shown in FIG. 10 indicates the relationship between Δmax (%) and the transmission loss (dB/km) at a wavelength of 1550 nm in the sample of optical fiber according to the first embodiment (whose cladding region  120  is pure silica glass) having the refractive index profile shown in FIG. 9A, whereas the curve G 400  indicates the relationship between Δmax (%) and the transmission loss (dB/km) at a wavelength of 1550 nm in the sample of optical fiber according to the second embodiment (whose cladding region  120  is silica glass doped with 1 wt % of fluorine) having the refractive index profile shown in FIG.  9 B. In each of the samples, the drawing tension at the time of making is 13 kg/mm 2 . 
     In FIG. 10, letting a be the transmission loss, the curve G 300  can be approximated by the following expression (1): 
     
       
         α=0.131×(Δmax) 2 −0.214×(Δmax)+0.284.  (1) 
       
     
     On the other hand, letting α be the transmission loss, the curve G 400  can be approximated by the following expression (2): 
     
       
         α=0.0846×(Δmax) 2 −0.147×(Δmax)+0.262.  (2) 
       
     
     In view of the foregoing, the first embodiment of the optical fiber according to the present invention is characterized by the relationship between transmission loss and Δmax represented by the above-mentioned expression (1). Also, the second embodiment of the optical fiber according to the present invention is characterized by the relationship between transmission loss and Δmax represented by the above-mentioned expression (2). 
     The curve indicating the wavelength dependence of transmission loss in a generally known optical fiber is similar to the shape of a quartic function, for example, such as (A·λ −4 +B). Therefore, the inventors have studied the wavelength dependence of transmission loss in the first and second embodiments of the optical fiber according to the present invention. FIG. 11 shows relationships between Δmax and the coefficient A in (A·λ −4 +B) in the range where Δmax&gt;0.8%. The curve G 500  shown in FIG. 11 indicates the relationship between Δmax (%) and the parameter A ((dB·μm 4 )/km) in the sample of optical fiber according to the first embodiment (whose cladding region  120  is pure silica glass) having the refractive index profile shown in FIG. 9A, whereas the curve G 600  indicates the relationship between Δmax (%) and the parameter A ((dB·μm 4 )/km) in the sample of optical fiber according to the second embodiment (whose cladding region  120  is silica glass doped with 1 wt % of fluorine) having the refractive index profile shown in FIG.  9 B. 
     In FIG. 11, the curve G 500  can be approximated by the following expression (3): 
     
       
           A =0.446×(Δmax) 2 −0.484×(Δmax)+1.072  (3) 
       
     
     in the range where Δmax≧0.8%. 
     On the other hand, the curve G 600  can be approximated by the following expression (4): 
     
       
           A =0.374×(Δmax) 2 −0.369×(Δmax)+1.003  (4) 
       
     
     in the range where Δmax≧0.8%. 
     In view of the foregoing, the first embodiment of the optical fiber according to the present invention is also characterized by the relationship between parameter A and Δmax represented by the above-mentioned expression (3). Similarly, the second embodiment of the optical fiber according to the present invention is characterized by the relationship between parameter A and Δmax represented by the above-mentioned expression (4). 
     The optical fibers according to the first and second embodiments explained in the foregoing are also obtained by rocking drawing as follows in order to improve polarization mode dispersion. 
     Namely, FIG. 12 is a view showing a second configuration of manufacturing apparatus for making the optical fiber according to the present invention. 
     The manufacturing apparatus of FIG. 12 is an apparatus for yielding an optical fiber by rocking drawing. In the rocking drawing, the optical fiber  180  having passed the UV lamp  600  initially passes between a pair of guide rollers  710  for suppressing the passive movement of optical fiber, which freely rotate without hindering the progress of the optical fiber  180 . Subsequently, the optical fiber  180  is successively guided by a rocking guide roller  720 , a first fixed guide roller  731  arranged at a stage next to the rocking guide roller  720 , and a second fixed guide roller  732  arranged at a stage next to the first fixed guide roller  731 . The optical fiber  180  having successively passed the rocking guide roller  720 , first fixed guide roller  731 , and second fixed guide roller  732  is taken up by a drum  700 . 
     Here, the pair of guide rollers  710  for suppressing the passive movement of optical fiber are arranged at a position separated from the rocking guide roller  720  in the directly upward direction (along the depicted Z axis) by a distance of 100 mm, whereas the gap between the pair of guide rollers  710  is 2 mm. The rocking guide roller  720  has a roller outside diameter of 150 mm, a roller width of 30 mm, and a roller surface made of aluminum which is the material of the roller itself; and is installed such that its axis of rotation can pivot about the depicted Z axis at a frequency of 100 rpm (to an angle of −θ from the Y axis orthogonal to the X axis indicating the direction by which the optical fiber  180  is drawn and to an angle of +θ from the Y axis). While the first fixed guide roller  731  is installed at a position separated from the rocking guide roller  720  in a directly sidewise direction (on the depicted X-Y plane where the guide roller  720  is installed) by a distance of 250 mm and has a roller outside diameter of 150 mm and a roller width of 30 mm as with the rocking guide roller  720 , its axis of rotation is fixed, and the center part of the roller surface is provided with a V-shaped narrow groove as means for inhibiting the rotation of optical fiber. The combination of the pair of guide rollers  710  for suppressing the passive movement of optical fiber, rocking guide roller  720 , and first fixed guide roller  731  arranged the foregoing condition adds a predetermined twist to the optical fiber  180  effectively, i.e., at a high efficiency with respect to the rocking rate of the rocking guide roller  720 . 
     A method of effectively adding a predetermined twist to the optical fiber  180  will now be explained with reference to FIGS. 13 and 14. Here, FIG. 13 is a view of the rocking guide roller  720  and first fixed guide roller  731  as seen from the side of a reaction furnace  250 . Also, FIG. 14 is a view of the pair of guide rollers  710  for suppressing the passive movement of optical fiber and rocking guide roller  720  as seen from the side of the reaction furnace  250 . For easier viewing of the spatial positional relationship between the pair of guide rollers  710  and rocking guide roller  720 , FIG. 14 is a view of the rollers  710 ,  720  as seen from a slightly slanting direction. 
     When the rocking guide roller  720  rotates about the Z axis from the Y axis as shown in FIG. 13, a force directed orthogonal to the Z axis is applied to the optical fiber  180  due to this rotation, whereby the optical fiber  180  rotates on the roller surface of the rocking guide roller  720 . Upon this rotation, a twist is given to the optical fiber  180 . Subsequently, the rocking guide roller  720  inversely rotates about the Z axis from the Y axis by an angle of −θ. Thus, as indicated by the depicted arrows, a symmetrical movement in which the rocking guide roller  720  pivots about the Z axis from an angle of +θ to an angle of −θ is repeated, whereby clockwise and counterclockwise twists are alternately applied to the optical fiber  180  with respect to its traveling direction. 
     Here, since the first fixed guide roller  731  at the stage next to the rocking guide roller  720  is installed directly aside the rocking guide roller  720  with the same roller outside diameter, the length by which the optical fiber  180  is in contact with the roller surface of the rocking guide roller  720  substantially equals the length of the roller circumference corresponding to a center angle of 90° in the rocking guide roller  720 . Namely, the optical fiber  180  is in contact with the rocking guide roller  720  from one roller side face to the bottom face thereof, and is liberated therefrom at the lowest bottom portion thereof. Thus inhibited is a situation where the rotation of the optical fiber  180  on one side face is impeded by the rotation of the optical fiber  180  occurring on the other side face, which causes the optical fiber  180  to slide. Therefore, as the optical fiber  180  rotates on one side face of the rocking guide roller  720 , a twist is applied to the optical fiber  180  at a high efficiency with respect to the rocking rate of the rocking guide roller  720 . 
     Also, the center part of the roller surface of the first fixed guide roller  731  is provided with a V-shaped narrow groove  750  as means for inhibiting the rotation of optical fiber, into which the optical fiber  180  to be guided by the first fixed guide roller  731  is inserted. Thus inhibited is a situation where the optical fiber  180  rotates on the roller surface of the first fixed guide roller  731 , which hinders the optical fiber  180  from rotating in the rocking guide roller  720  for attaining a twist. Therefore, as the V-shaped narrow groove  750  inhibits the optical fiber  180  from rotating on the roller surface of the first fixed guide roller  731 , a twist is applied to the optical fiber  180  at a high efficiency with respect to the rocking rate of the rocking guide roller  720 . 
     Then, as shown in FIG. 14, when the optical fiber  180  rotates on the roller surface of the rocking guide roller  720  as the latter rotates about the Z axis in FIG. 12 from the Y axis by an angle of +θ, the part of fiber located on the side of the reaction furnace  250  immediately upstream the rocking guide roller  720  passively moves in the rocking direction of the rocking guide roller  720  as the optical fiber  180  rotates. If the passive movement of the optical fiber  180  exceeds a predetermined range, then it causes the amount of twist applied to the optical fiber  180  to decrease and the part of optical fiber coated with the resin coating  161  to become uneven. Since a pair of guide rollers  710  are installed directly above the rocking guide roller  720  (at a position close thereto along the Z axis), however, the optical fiber  180  comes into contact with one of the pair of guide rollers  710  if the passive movement of the optical fiber  180  becomes a predetermined level or greater, whereby the optical fiber  180  is inhibited from passively moving more. Therefore, as the pair of guide rollers  710  suppress the passive movement of the optical fiber  180 , the decrease in the amount of twist applied to the optical fiber  180  and the unevenness in the part of optical fiber coated with the resin coating  161  are effectively suppressed. 
     Thus, since the pair of guide rollers for suppressing the passive movement of optical fiber, the rocking guide roller  720 , and the first fixed guide roller  731  are combined, the second configuration of manufacturing apparatus shown in FIG. 12 functions such that the rocking guide roller  720  alternately applies clockwise and counterclockwise twists to the optical fiber  180  by rotating the latter on the roller surface upon its rocking movement, while the pair of guide rollers  710  for suppressing the passive movement of optical fiber and the first fixed guide roller  731  provided with means for inhibiting the rotation of optical fiber assist the optical fiber  180  to rotate smoothly on the roller surface of the rocking guide roller  720 . As a consequence, the optical fiber  180  can be provided with twists efficiently with respect to the rocking rate of the rocking guide roller  720 . 
     In the manufacturing apparatus of FIG. 12, the passive movement of the optical fiber  180  is suppressed by the pair of guide rollers  710  for suppressing the passive movement of optical fiber when the optical fiber  180  is rotated on the roller surface of the rocking guide roller  720 , whereby the part of optical fiber coated with the resin coating  161  is effectively prevented from becoming uneven. 
     As a consequence, the optical fiber  180  made by the foregoing manufacturing apparatus comprises the core region  110  and the cladding region  120  covering the core region  110 , while clockwise and counterclockwise twists have alternately been applied thereto, whereby its polarization mode dispersion is suppressed in the elongated optical fiber as a whole in a manner equivalent to the case where the core region  110  and the cladding region  120  have truly concentric circular cross sections even if they are not. Also, since the part of optical fiber coated with the resin coating  161  is kept from becoming uneven, the optical fiber  180  is prevented from yielding asymmetrical stress distributions in the cross section of this part, whereby the strength of the optical fiber  180  upon cabling can be improved. 
     Though the rocking movement of the rocking guide roller  720  in the foregoing manufacturing apparatus capable of rocking drawing is a symmetrical reciprocation from an angle of −θ to an angle of +θ as shown in FIG. 13, it is not restricted thereto. For example, it may be an asymmetric movement of rocking from an angle of 0 to an angle of +θ. In this case, twists are intermittently applied to the optical fiber  180 . Alternatively, it may be a symmetrical reciprocation of rocking in the direction of axis of rotation of the rocking guide roller  720 . In this case, as with the movement explained above, the optical fiber  180  is alternately provided with clockwise and counterclockwise twists. Though the roller surface of the first fixed guide roller  731  is formed with the V-shaped narrow groove  750  as means for inhibiting the rotation of optical fiber in the manufacturing apparatus of FIG. 12, a U-shaped narrow groove or depressed narrow groove yields similar effects. 
     As in the foregoing, since the present invention comprises a structure in which the outer periphery of a bare fiber is provided with a carbon coat, it becomes an optical fiber having a favorable transmission loss while being kept from breaking even when the drawing tension at the time of making is set to 13 kg/mm 2  or greater. 
     Also, when the cladding region is doped with fluorine, so as to totally lower the refractive index without changing the shape of refractive index profile, thereby keeping a low level of Δmax with reference to the refractive index of pure silica glass, an optical fiber having a low transmission loss is obtained without increasing the drawing tension at the time of making. 
     From the invention thus described, it will be obvious that the embodiments of the invention may be varied in many ways. Such variations are not to be regarded as a departure from the spirit and scope of the invention, and all such modifications as would be obvious to one skilled in the art are intended for inclusion within the scope of the following claims. 
     The basic Japanese Applications No. 7-072372 filed on Mar. 6, 1995, No. 7-168298 filed on Jun. 9, 1995, No. 8-075343 on Mar. 5, 1996, No. 8-3129878 filed on Nov. 9, 1996 are hereby incorporated by reference.