Abstract:
The present invention provides an optical fiber having dispersion of −8 ps/nm/km or less at a wavelength of 1460 nm including a reference layer which is a reference of a refractive index profile and at least three glass layers that exist inside the reference layer, characterized in that when it is assumed that the maximum relative refractive index difference of the first glass layer formed innermost of the at least three glass layers with respect to the reference layer is Δ 1 , the relative refractive index difference of the second glass layer formed second from the inside with respect to the reference layer is Δ 2 , the relative refractive index difference of the third glass layer formed third from the inside with respect to the reference layer is Δ 3  and the relative refractive index difference of the reference layer with respect to pure quartz glass is ΔC, Δ 1&gt;Δ3&gt;Δ2, Δ1 ≧1.0% and ΔC&lt;0 are satisfied, and further provides an optical-module and a Raman amplifier using the optical fiber.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an optical fiber preferably used for high-speed optical communication, optical module and Raman amplifier using the optical fiber. 
   2. Related Background Art 
   A dispersion compensating technique is indispensable for realizing high-speed optical communication, and a dispersion compensating fiber (DCF) is generally used in the technique. The DCF is an optical fiber for compensating dispersion of an optical fiber in a used wavelength band, for example in a set wavelength band in a wavelength range of 1460 to 1625 nm. To provide a characteristic suitable for dispersion compensation, it is necessary to increase a relative refractive index difference of the center core of the DCF which is located in the center of the DCF, and therefore, the center core is normally doped with germanium of high concentration. 
   An optical signal processing technique which processes an optical signal as light itself without converting it to an electrical signal is required for high-speed optical communication systems. This technique requires an optical fiber that can produce a large nonlinear phenomenon, that is, a high nonlinear optical fiber. This high nonlinear optical fiber also needs a large relative refractive index difference of its center core, and therefore, the center core is doped with germanium of high concentration. 
   However, when the center core is doped with germanium of high concentration, the nonlinear refractive index increases, which results in a problem of provoking waveform distortion. The high nonlinear optical fiber has a characteristic in which waveform distortion occurs when the nonlinear coefficient is increased. But, since the high nonlinear optical fiber for a Raman amplifier has the problem of double Rayleigh scattering, the germanium concentration of the center core is now being questioned. 
   Furthermore, when the DCF or the high nonlinear optical fiber is drawn, many defects and structural inconsistency are caused in the center core during drawing because the glass softening point of the center core is considerably different from that of the cladding. Due to this fact, there is a problem that it is difficult to reduce the transmission loss of the optical fiber. 
   SUMMARY OF THE INVENTION 
   The present invention has been implemented under such circumstances, and it is an object of the present invention to provide an optical fiber that has a refractive index profile satisfying desired characteristics and realizes a low loss by reducing the refractive index of the cladding lower than that of pure quartz glass to reduce germanium concentration of the center core. The present invention further provides an optical module and Raman amplifier using this optical fiber. 
   In order to solve the above described problems, the present invention provides an optical fiber provided with a plurality of glass layers having dispersion of −8 ps/nm/km or less at a wavelength of 1460 nm, the composition of one of the plurality of glass layers is different from the composition of glass layers adjacent to the one glass layer, the plurality of glass layers comprise a reference layer to be referenced for a refractive index profile and at least three glass layers are provided on inside of the reference layer, and Δ 1 &gt;Δ 3 &gt;Δ 2 , Δ 1 ≧1.0% and ΔC&lt;0 are satisfied wherein Δ 1  denotes the maximum relative refractive index difference of the first glass layer formed innermost of the at least three glass layers with respect to the reference layer, Δ 2  denotes the relative refractive index difference of the second glass layer formed second from the inside with respect to the reference layer, Δ 3  denotes the relative refractive index difference of the third glass layer formed third from the inside with respect to the reference layer, and ΔC denotes the relative refractive index difference of the reference layer with respect to pure quartz glass. 
   The optical fiber of the present invention in the above described configuration more preferably satisfies 1.0%≦Δ 1 ≦3.0%, −1.0%≦Δ 2 ≦−0.4% and 0%&lt;Δ 3 ≦0.5%. 
   Further, the present invention provides an optical module characterized by comprising the above described optical fiber. 
   Furthermore, the present invention provides a Raman amplifier characterized in that a pumping light source for Raman amplification is connected to the above described optical module. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a refractive index profile of an optical fiber according to an embodiment of the present invention; 
       FIG. 2  illustrates a refractive index profile of an optical fiber according to another embodiment of the present invention; and 
       FIG. 3  illustrates an optical module constructed using the optical fiber according to an embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   With reference now to the attached drawings, various embodiments of the present invention will be explained below. 
   As it is known, a nonlinear coefficient can be defined by n 2 /Aeff and the waveform distortion is more likely to occur if the nonlinear coefficient is greater. Where n 2  is a nonlinear refractive index and Aeff is an effective core area. Furthermore, since n 2  is determined by germanium concentration of the center core, n 2  increases by increasing the germanium concentration. But, simultaneously, there is a problem that Rayleigh scattering also increases. In contrast, Aeff can be expanded by increasing the relative refractive index difference of the center core with respect to the cladding. Aeff can also be expanded by reducing the refractive index of the cladding even if the germanium concentration is reduced. 
   In this case, if fluorine is used as a dopant for reducing the refractive index of the cladding, the glass softening point of the cladding approaches the glass softening point of the center core simultaneously with the lowering of the refractive index of the cladding, and therefore it is possible to prevent the generation of defects or structural inconsistency which occurs during drawing. Therefore, an optical fiber having low loss and low nonlinear coefficient can be realized. 
   In this case, the glass softening point of the cladding is preferably 1200 to 1500° C., etc., in contrast to the glass softening point of 1600° C. of pure quartz glass. 
   To realize this, the amount of fluorine doped to the cladding is preferably an amount of doping which corresponds to the relative refractive index difference ΔC with respect to pure quartz glass of approximately −0.6 to −0.1%. 
   EXAMPLE 1 
   24 types of optical fibers (fibers A to X) having the optical characteristic in Tables 1 to 3 shown below were produced. The optical fibers have refractive index profiles of a W-shaped profile as shown in  FIG. 1  or a W-segment profile as shown in  FIG. 2 . 
   That is, the W-shaped profile shown in  FIG. 1  consists of a first glass layer  1  forming the center core, a second glass layer  2  surrounding the first glass layer  1  and a reference layer  3  forming the cladding surrounding the second glass layer  2 . The W-segment profile shown in  FIG. 2  consists of a first glass layer  1  forming the center core, a second glass layer  2  surrounding the first glass layer  1 , a third glass layer  4  surrounding the second glass layer  2  and a reference layer  3  forming the cladding surrounding the third glass layer  4 . 
   In these Tables, Δ 1  denotes the refractive index difference of the first glass layer  1  with respect to the cladding  3 , Δ 2  denotes the refractive index difference of the second glass layer  2  with respect to the cladding  3 , Δ 3  denotes the refractive index difference of the third glass layer  1  with respect to the cladding  3 , and ΔC denotes the refractive index difference of the cladding  4  with respect to the pure quartz glass. Each of Δ 1 , Δ 2 , Δ 3  and ΔC is defined by the following Expressions (1) to (4), respectively:
 
Δ1={( n   1   2   −n   c   2 )/2 n   1   2 }×100  (1)
 
Δ2={( n   2   2   −n   c   2 )/2 n   2   2 }×100  (2)
 
Δ3={( n   3   2   −n   c   2 )/2 n   3   2 }×100  (3)
 
Δ C ={( n   c   2   −n   s   2 )/2 n   c   2 }×100  (4)
 
where n 1  is the maximum refractive index of the first glass layer  1 , n 2  is the refractive index of the second glass layer  2 , n c  is the refractive index of the cladding  3  and n s  is the refractive index of the pure quartz glass. These values can be modified by adjusting the amount of germanium or amount of fluorine doped when a preform is synthesized.
 
   Further, GR denotes a Raman gain and DPS denotes a value obtained by dividing dispersion by a dispersion slope in these Tables. 
   In an optical fiber of the present invention, the DPS at a used wavelength is preferably a positive value smaller than 330 nm. Thus, by setting the DPS at a used wavelength to a positive value smaller than 330 nm, it is possible to achieve a compensation rate of 90% or more when this optical fiber is used as a dispersion compensating fiber for a conventional single-mode optical fiber (SMF) having zero dispersion wavelength within 1.3 μm wavelength band (1280–1330 nm). In the case of a non-zero dispersion-shifted fiber (NZ-DSF), a DPS of smaller than 330 nm is suitable and about 50 nm might be optimum. For this reason, by setting a DPS to at least equal to or smaller than 330 nm, the optical fiber can function as a dispersion compensating fiber for NZ-DSF and SMF. 
   
     
       
             
             
             
             
             
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 1 
             
             
                 
                 
             
             
                 
                 
                 
                 
                 
                 
                 
                 
                 
               Rayleigh 
                 
                 
                 
             
             
                 
                 
                 
                 
                 
               Transmission 
                 
               Dispersion 
               GR 
               scattering 
             
             
                 
                 
                 
                 
                 
               loss 
               Dispersion 
               slope 
               1450 nm 
               coefficient 
               Aeff 
                 
               DPS 
             
             
                 
               Δ1 
               Δ2 
               Δ3 
               ΔC 
               1550 nm 
               1550 nm 
               1550 nm 
               pump 
               1550 nm 
               1550 nm 
               λc 
               1550 nm 
             
             
                 
               % 
               % 
               % 
               % 
               dB/km 
               ps/nm/km 
               ps/nm2/km 
               (m/W) 
               (l/m) 
               um 2   
               nm 
               nm 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               A 
               2.0 
               −0.55 
               0.2 
               0 
               0.53 
               −100 
               −0.35 
               1.1e−13 
               1.0e−6 
               16 
               1500 
               286 
             
             
               B 
               2.0 
               −0.55 
               0.2 
               −0.2 
               0.44 
               −103 
               −0.36 
               1.1e−13 
               1.0e−6 
               16 
               1500 
               286 
             
             
               C 
               2.0 
               −0.55 
               0.2 
               −0.2 
               0.47 
               −132 
               −0.48 
               1.2e−13 
               1.0e−6 
               15 
               1520 
               275 
             
             
               D 
               2.0 
               −0.55 
               0 
               0 
               0.40 
               −70 
               −0.21 
               0.9e−13 
               1.0e−6 
               17 
               800 
               333 
             
             
               E 
               2.0 
               −0.55 
               0 
               −0.2 
               0.35 
               −70 
               −0.21 
               0.9e−13 
               1.0e−6 
               17 
               805 
               333 
             
             
               F 
               2.2 
               −0.55 
               0 
               −0.2 
               0.37 
               −90 
               −0.25 
               1.0e−13 
               1.0e−6 
               16 
               780 
               360 
             
             
               G 
               1.5 
               −0.4 
               0.2 
               0 
               0.35 
               −40 
               −0.39 
               0.5e−13 
               0.6e−6 
               18 
               1480 
               103 
             
             
               H 
               1.5 
               −0.4 
               0.2 
               −0.2 
               0.31 
               −40 
               −0.38 
               0.5e−13 
               0.5e−6 
               18 
               1480 
               105 
             
             
               I 
               1.7 
               −0.4 
               0.2 
               −0.2 
               0.33 
               −70 
               −0.61 
               0.6e−13 
               0.6e−6 
               17 
               1390 
               115 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
             
             
             
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 2 
             
             
                 
                 
             
             
                 
                 
                 
                 
                 
                 
                 
                 
                 
               Rayleigh 
                 
                 
                 
             
             
                 
                 
                 
                 
                 
               Transmission 
                 
               Dispersion 
               GR 
               scattering 
             
             
                 
                 
                 
                 
                 
               loss 
               Dispersion 
               slope 
               1500 nm 
               coefficient 
               Aeff 
                 
               DPS 
             
             
                 
               Δ1 
               Δ2 
               Δ3 
               ΔC 
               1600 nm 
               1600 nm 
               1600 nm 
               pump 
               1600 nm 
               1600 nm 
               λc 
               1600 nm 
             
             
                 
               % 
               % 
               % 
               % 
               dB/km 
               ps/nm/km 
               ps/nm2/km 
               (m/W) 
               (l/m) 
               um 2   
               nm 
               nm 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               J 
               1.8 
               −0.4 
               0.2 
               0 
               0.35 
               −110 
               −0.31 
               1.0e−13 
               0.8e−6 
               18 
               1540 
               355 
             
             
               K 
               1.8 
               −0.4 
               0.2 
               −0.2 
               0.30 
               −110 
               −0.31 
               1.0e−13 
               0.8e−6 
               18 
               1535 
               355 
             
             
               L 
               2.0 
               −0.4 
               0.2 
               −0.2 
               0.33 
               −120 
               −0.30 
               1.1e−13 
               0.8e−6 
               17 
               1500 
               400 
             
             
               M 
               2.4 
               −0.6 
               0 
               0 
               0.51 
               −100 
               −0.26 
               1.2e−13 
               1.0e−6 
               17 
               780 
               385 
             
             
               N 
               2.4 
               −0.6 
               0 
               −0.1 
               0.46 
               −100 
               −0.27 
               1.2e−13 
               1.0e−6 
               17 
               780 
               370 
             
             
               O 
               2.5 
               −0.6 
               0 
               −0.1 
               0.48 
               −120 
               −0.25 
               1.3e−13 
               1.0e−6 
               16 
               750 
               480 
             
             
               P 
               1.1 
               −0.5 
               0 
               0 
               0.27 
               −9 
               −0.12 
               0.2e−13 
               0.3e−6 
               18 
               800 
               75 
             
             
               Q 
               1.1 
               −0.5 
               0 
               −0.2 
               0.24 
               −10 
               −0.12 
               0.2e−13 
               0.3e−6 
               18 
               800 
               83 
             
             
               R 
               1.3 
               −0.5 
               0 
               −0.2 
               0.25 
               −18 
               −0.12 
               0.3e−13 
               0.3e−6 
               18 
               790 
               150 
             
             
                 
             
           
        
       
     
   
   
     
       
             
             
             
             
             
             
             
             
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
             
             
             
             
           
         
             
                 
               TABLE 3 
             
             
                 
                 
             
             
                 
                 
                 
                 
                 
                 
                 
                 
                 
               Rayleigh 
                 
                 
                 
             
             
                 
                 
                 
                 
                 
               Transmission 
                 
               Dispersion 
               GR 
               scattering 
             
             
                 
                 
                 
                 
                 
               loss 
               Dispersion 
               slope 
               1400 nm 
               coefficient 
               Aeff 
                 
               DPS 
             
             
                 
               Δ1 
               Δ2 
               Δ3 
               ΔC 
               1500 nm 
               1500 nm 
               1500 nm 
               pump 
               1500 nm 
               1500 nm 
               λc 
               1500 nm 
             
             
                 
               % 
               % 
               % 
               % 
               dB/km 
               ps/nm/km 
               ps/nm2/km 
               (m/W) 
               (l/m) 
               um 2   
               nm 
               nm 
             
             
                 
                 
             
           
           
             
                 
             
           
        
         
             
               S 
               2.4 
               −0.55 
               0.2 
               0 
               0.62 
               −110 
               −0.54 
               1.0e−13 
               1.0e−6 
               14 
               1355 
               204 
             
             
               T 
               2.4 
               −0.55 
               0.2 
               −0.1 
               0.54 
               −110 
               −0.53 
               1.0e−13 
               1.0e−6 
               14 
               1350 
               208 
             
             
               U 
               2.6 
               −0.55 
               0.2 
               −0.1 
               0.56 
               −140 
               −0.52 
               1.1e−13 
               1.0e−6 
               13 
               1300 
               269 
             
             
               V 
               1.0 
               −0.4 
               0 
               0 
               0.62 
               −8.3 
               −0.05 
               0.3e−13 
               0.3e−6 
               18 
               780 
               166 
             
             
               W 
               1.1 
               −0.4 
               0 
               −0.1 
               0.57 
               −8.5 
               −0.05 
               0.3e−13 
               0.3e−6 
               18 
               780 
               170 
             
             
               X 
               1.1 
               −0.4 
               0 
               −0.1 
               0.65 
               −11.2 
               +0.01 
               0.4e−13 
               0.3e−6 
               17 
               760 
               −1120 
             
             
                 
             
           
        
       
     
   
   The 24 types of optical fibers include fibers whose reference layers are pure quartz glass with. ΔC=0 and a plurality of fibers with fluorine-doped cladding with ΔC&lt;0. And they also include the optical fibers whose Δ 1  are the same though refractive indexes of reference layers (ΔC) are different and the optical fibers whose germanium concentrations are the same though Δ 1  are different. 
   Among these fibers, the fiber A and fiber B have the same Δ 1 , Δ 2  and Δ 3  and only ΔC is different. Since the fiber A and fiber B have the same refractive index profile, the fiber A and fiber B have substantially the same Aeff and dispersion characteristic, but the fiber B having the fluorine-doped cladding has a transmission loss lower than the fiber A. This is caused that fluorine is doped to the cladding, and thereby, the softening point of the cladding has lowered and low temperature drawing can be realized. 
   Therefore, an optical module using the fiber B can compensate for the same amount of dispersion with the same length as that of the optical module using the fiber A and provides an optical module with low transmission loss. 
   The fiber A and fiber C have the same germanium concentration of the center core. When these fibers are compared, the fiber C with the fluorine-doped cladding has a greater Raman gain, while these fibers have substantially the same Rayleigh scattering coefficient. This is because that irrespective of the same germanium concentration, the fiber C with the fluorine-doped cladding can increase Δ 1  and decrease Aeff. 
   Furthermore, since the fiber C has an increasing Raman gain and reduced transmission loss, a required gain with lower pumping power can be realized by a DCRA (dispersion compensating Raman amplifier) using an optical module including the fiber C. 
   Since the fiber A and fiber C have substantially the same Rayleigh scattering coefficient, the DCRA formed by the fiber C can obtain a desired Raman gain with lower pumping power, and can thereby realize low noise. This is also similar to the fiber D and fiber F, fiber G and fiber I, fiber J and fiber L, fiber M and fiber O, fiber P and fiber R, fiber S and fiber U, and fiber V and fiber X. 
   However, what is different among them is a cutoff wavelength. Since the cutoff wavelength of the fiber C is 1520 nm, that is, smaller than 1530 nm, it can be used in a C-band (1530 nm to 1565 nm) or longer. Furthermore, if the cutoff wavelength is set to 100 nm or shorter of the used wavelength band, that is, 1430 nm or shorter, it can be used as a Raman amplification medium in the C-band. 
   Likewise, when an L-band (1565 nm to 1625 nm) is set as the used wavelength band, it is necessary to set the cutoff wavelength to 1565 nm or shorter and further setting the cutoff wavelength to 1465 nm or shorter allows it to be used as a Raman amplification medium in the L-band. 
   Furthermore, when an S-band (1460 nm to 1530 nm) is set as the used wavelength, it is necessary to set the cutoff wavelength to 1460 nm or shorter and further setting the cutoff wavelength to 1360 nm or shorter allows it to be used as a Raman amplification medium in the S-band. 
   That is, the fiber T whose cutoff wavelength is 1350 nm can be used as a Raman amplifier in any one of the S-band to L-band. 
   In this context, the cutoff wavelength refers to a fiber cutoff wavelength λc which is defined in the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector) G.650.1. Other terms not specially defined in the present specification are in accordance with the definitions and measuring method of ITU-T G.650.1. 
   Furthermore, fiber B and fiber E have the same Δ 1  and Δ 2 , but have different magnitudes of Δ 3 . That is, while fiber E has Δ 3  of the third layer of 0% and has a W-shaped profile, fiber B has a W-segment profile. The characteristic difference between the two is the magnitude of dispersion. By providing the third layer and the W-segment profile, the cutoff wavelength is shifted to a longer wavelength, but a negative dispersion slope with a large absolute value can be realized. Therefore, fiber B can realize a sufficiently small DPS even if the absolute value of dispersion is increased. 
   Further decreasing the diameter of the center core of the fiber E having a W-shaped profile makes it possible to increase the absolute value of dispersion, but at the same time increases loss and prevents transmission in the C-band. 
   By providing the third layer and making the W-segment profile in this way, it is possible to increase the absolute value of dispersion and dispersion slope, and at the same time it becomes easy to adjust the profile. 
   This makes it possible to obtain the desired optical fiber characteristic and manufacture the optical fiber with high yield. 
   EXAMPLE 2 
   Using the fiber A and fiber B, a Raman amplifier for compensating SMF having the length of 50 km in the C-band was produced. Since these two fibers have substantially the same dispersion, two optical modules were produced using an 8 km fiber for both. The configuration is as shown in  FIG. 3 . 
     FIG. 3  shows the Raman amplifier  10  to which the optical fiber of the present invention is applied. The Raman amplifier  10  shown in  FIG. 3  includes the optical module  13 , couplers  15 , isolators  17  and pumping light  19 . 
   Since these two optical modules have the same fiber length and Raman gain of the optical fibers used but have different transmission losses, the pumping power required to realize loss-less modules was 55 mW (fiber A) and 49 mW (fiber B). For this reason, the DCRA using the fiber A has greater noise due to double Rayleigh scattering and the DCRA using the fiber B showed an improvement of noise index by 1 dB. Here, SMF was used as the fiber for the transmission line, but it is apparent that if the fiber H is used it is also applicable to an NZ-DSF. 
   Furthermore, using the fiber K and fiber T, they are applicable to not only the C-band but also the L-band, S-band or a plurality of wavelengths including them. 
   As shown above, the present invention makes it possible to produce a low loss optical fiber. Furthermore, with reduced germanium concentration of the center core, the present invention can realize an optical fiber with reduced double Rayleigh scattering. 
   Furthermore, adopting a W-segment profile, it makes possible to obtain the desired optical fiber characteristic easily and manufacture optical fibers with high yield. 
   The present invention can be effectively used for dispersion compensating optical fiber and high nonlinear optical fiber in particular. 
   It should be noted that the above disclosure of the embodiments are intended to be illustrative, rather than exhaustive, of the present invention. One skilled in the art will be able to make any additions and/or modification to the embodiment disclosed in the above without departing from the spirit of the invention or its scope, as defined by the following claims.