Patent Publication Number: US-2004052486-A1

Title: Optical fibers and modules for dispersion compensation with simultaneous raman amplification

Description:
CROSS REFERENCE TO RELATED APPLICATION  
     [0001] U.S. patent application Ser. No. 10/099,820 filed Mar. 16, 2002, and entitled “Raman Amplified Dispersion Compensating Modules”, which is assigned to the assignee of the present invention and application. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] 1. Field of the Invention  
       [0003] This invention relates to optical fibers for use in dispersion compensation, and to dispersion compensating modules that include such fibers.  
       [0004] 2. Discussion of the Known Art  
       [0005] Dispersion slope compensating modules or DSCMs including one or more lengths of dispersion compensating fibers (DCFs), are generally known as a means for compensating for chromatic dispersion of light signals when transmitted through a fiber of a fiber optic communication system or network. DSCMs enable existing systems to handle signals with bandwidths and wavelengths for which the systems were not originally designed, without the need for replacing long spans of installed fiber optic cable with newer, higher rated cables. Because DSCMs may themselves use several kilometers of fiber having a certain signal attenuation factor, it is useful to combine the functions of dispersion compensation and Raman amplification into a single, Raman amplified, dispersion compensating module. See the above mentioned &#39;820 application all relevant portions of which are incorporated by reference. DSCMs are also sometimes referred to simply as dispersion compensating modules or DCMs.  
       [0006] Dispersion compensating fibers happen also to be well suited for use as gain media for Raman amplification. This is because fibers used for dispersion compensation tend to have a high concentration of germanium in the core; the core has a Δn above 0.020 where Δn is the absolute index difference with respect to the index of the surrounding cladding; and the core has a small effective area typically less than 20 μm 2  at a wavelength of 1550 nanometers (nm). Accordingly, there is an advantage in using a DCF that is part of a dispersion compensating module also as a gain medium. Instead of introducing additional loss, light signals are amplified by the DSCM by a factor limited only by whatever pump power is available for Raman amplification.  
       [0007] The broad concept of Raman pumping a fiber in a DSCM to compensate for signal loss is known. See, e.g., U.S. Pat. No. 5,887,093 (Mar. 23, 1999); and S. A. E. Lewis, et al., Broadband high-gain dispersion compensating Raman amplifier, 36 Electronics Letters, No. 16 (Aug. 3, 2000) at page 1355; all relevant portions of which are incorporated by reference. A problem arises, however, in that the chromatic dispersion of the DCF in the module is often so high that the required dispersion is attained by a length of fiber which is too short to enable the fiber also to achieve effective gain when pumped for Raman amplification. Further, the use of a discrete Raman amplifier having a fiber that is optimized for Raman amplification in order to compensate for loss introduced by a DSCM would introduce additional dispersion to the system, and the added dispersion would then need to be compensated by a second DSCM introducing additional loss.  
       SUMMARY OF THE INVENTION  
       [0008] According to the invention, an optical fiber suitable for use for dispersion compensation with simultaneous Raman amplification has a central core, an outer cladding, and a refractive index profile through a cross section of the fiber with respect to the outer cladding. The core has a diameter of between 2 and 5 microns (μm), and a difference (Δn) between the refractive index of the core and the outer cladding of between 0.012 and 0.035. The fiber includes a trench region adjacent the core, a ring region adjacent the trench region, and an inner cladding intermediate the ring region and the outer cladding. The trench region has a width in the index profile of between 2 and 6 μm and a negative Δn of between −0.015 and −0.003. The ring region has a width in the index profile of between 1 and 5 μm and a Δn of between 0.001 and 0.015, and the inner cladding has a width in the profile of between 0 and 5 μm and a Δn of between −0.011 and 0.001.  
       [0009] According to another aspect of the invention, a dispersion slope compensating module includes a first fiber having a signal input end and an output end opposite the signal input end, and a pump light source coupled to the first fiber in such manner as to obtain Raman amplification with respect to a light signal applied to the signal input end of the first fiber. The first fiber has a central core, an outer cladding, and a refractive index profile through a cross section of the fiber with respect to the outer cladding. The core has a diameter of between 2 and 5 microns (μm), and a difference (Δn) between the refractive index of the core and the outer cladding that is between 0.012 and 0.035. The fiber includes a trench region adjacent the core, a ring region adjacent the trench region, and an inner cladding intermediate the ring region and the outer cladding. The trench region has a width in the index profile of between 2 and 6 μm and a negative Δn of between −0.015 and −0.003. The ring region has a width in the index profile of between 1 and 5 μm and a Δn of between 0.001 and 0.015, and the inner cladding has a width in the profile of between 0 and 5 μm and a Δn of between −0.011 and 0.001.  
       [0010] For a better understanding of the invention, reference is made to the following description taken in conjunction with the accompanying drawing and the appended claims.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
     [0011] In the drawing:  
     [0012]FIG. 1 is a cross sectional view of a dispersion compensating fiber (DCF) according to the invention;  
     [0013]FIG. 2 is a graph showing refractive index profiles of two DCFs according to the invention and that of an existing fiber;  
     [0014]FIG. 3 is a graph showing dispersion of the two inventive DCFs and that of the existing fiber;  
     [0015]FIG. 4 is a graph showing relative dispersion slope (RDS) of the two inventive DCFs and that of the existing fiber;  
     [0016]FIG. 5 is a schematic block diagram of a test measurement arrangement used to evaluate performance of three dispersion slope compensating modules (DSCMs) that were constructed using the inventive and existing fibers;  
     [0017]FIG. 6 is a graph showing net gain and noise figure measured for each of the three DSCMs;  
     [0018]FIG. 7 is a graph showing multi-path interference (MPI) measured for each of the three DSCMs; and  
     [0019]FIG. 8 is a block diagram of a fiber optic transmission system including a DSCM according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0020]FIG. 1 shows a cross section of an optical fiber  10  that is suited for dispersion compensation with simultaneous Raman amplification, according to the invention. As demonstrated further below, the fiber  10  achieves a significantly higher Raman gain than current DCFs for a given level of pump power.  
     [0021] The fiber  10  has a refractive index profile in terms of Δn (see FIG. 2) that defines five regions through the cross section of the fiber. The five regions include:  
     [0022] 1. A central core  12  which defines a raised (positive) index region with respect to an outer cladding  20 . The core  12  is formed of SiO 2  doped with an appropriate amount of GeO 2  to achieve a desired refractive index.  
     [0023] 2. A trench  14  surrounding the core  12 . The trench  14  defines a depressed (negative) index region and generally comprises SiO 2  doped with an appropriate amount of GeO 2  and/or F to achieve a desired index.  
     [0024] 3. A ring  16  surrounding the trench  14 . The ring  16  defines a raised index region and generally comprises SiO 2  doped with an appropriate amount of GeO 2  to achieve a desired index.  
     [0025] 4. A deposited inner cladding  18  surrounding the ring  16 . The inner cladding  18  is a region having close to the same refractive index as the outer cladding  20 .  
     [0026] 5. The outer cladding  20 , which is formed of SiO 2 . Only a portion of the entire cross section of the outer cladding  20 , adjacent the inner cladding, appears in FIG. 1.  
     [0027] The inventive fiber  10  was developed to obtain the following features:  
     [0028] A. The relative dispersion slope (RDS) of the fiber  10  is such as to match closely that of a transmission fiber of the kind typically encountered in systems to be upgraded.  
     [0029] B. The chromatic dispersion of the fiber  10  is of such a value that the length of the fiber  10  needed to compensate for dispersion in a span of a given transmission fiber, is consistent with an optimal length of the fiber  10  needed to achieve a desired Raman gain.  
     [0030] C. Desirable Raman gain properties, including minimum noise figure.  
     [0031] D. Minimal double Rayleigh scattering and multipath interference (MPI) when using the fiber as a Raman amplifier.  
     [0032] E. Desirable spectral properties in both pump- and signal-bands, i.e., relatively low attenuation at wavelengths from, e.g., 1400 to 1650 nm.  
     [0033] Experiments and simulations were conducted. It was discovered that an optical fiber having an index profile as described above and with the following parameters, is well suited for dispersion compensation with simultaneous Raman amplification when used alone or in combination with another fiber in a dispersion compensating module.  
     [0034] I. A Δn for the core  12  of between 0.012 and 0.035, and preferably between 0.015 and 0.022. The lower limit ensures a relatively high Raman gain coefficient, while the upper limit is chosen to keep the spectral attenuation low.  
     [0035] II. A depressed or negative Δn for the trench  14  of between −0.015 and −0.003, and preferably between −0.010 and −0.006 to obtain a low effective area and a high relative dispersion slope for the fiber  10 . The trench  14  is also relatively wide with a width of between 2 and 6 μm. As used herein, the term “width” as applied to a region in the index profile of the fiber  10  means the radial distance over which the region extends in FIG. 1.  
     [0036] III. A Δn for the ring  16  of between 0.001 and 0.015, and preferably between 0.003 and 0.005 to improve bending characteristics of the fiber.  
     EXAMPLE  
     [0037] Two fibers, designated herein as R-DCF-1 and R-DCF-2, were produced having index profiles in their cross sections within the above ranges. The dimensions and index values (in terms of Δn) of the two fibers are given in the following tables.  
                                               Index difference to outer       Region   Dimension   cladding                                    R-DCF-1                         Core 12   Radius = 1.8 μm   Δn = 0.0215       Trench 14   Width = 3.62 μm   Δn = −0.0078       Ring 16   Width = 2.34 μm   Δn = 0.0040       Cladding 18   Width = 1.65 μm   Δn = −0.0006       Outer Cladding 20   Radius = 62.5 μm   Δn = 0                 R-DCF-2                         Core 12   Radius = 1.56 μm   Δn = 0.0155       Trench 14   Width = 3.95 μm   Δn = −0.0078       Ring 16   Width = 2.39 μm   Δn = 0.0040       Cladding 18   Width = 1.38 μm   Δn = −0.0006       Outer Cladding 20   Radius = 62.5 μm   Δn = 0                  
 
     [0038] An optical fiber&#39;s Raman gain efficiency is given by a so-called Raman Figure-of-Merit (FOM). The Raman FOM is defined as the Raman gain coefficient measured at a certain pump wavelength and normalized to the effective area of a given fiber, divided by the fiber&#39;s spectral attenuation at the pump wavelength. The Raman FOM has units of [W −1  dB −1 ]. Further, Rayleigh backscattering in optical fibers is a primary cause of Multipath Interference (MPI), also referred to as Double Rayleigh Backscattering (DRB), in Raman amplification. Large backscattering results in a bigger penalty due to MPI. Raman FOM and Rayleigh backscattering values were measured for various fibers, including the inventive fibers R-DCF-1 and R-DCF-2, and are given below.  
                                                           Raman FOM   Rayleigh backscattering               @1453 nm   @1550 nm           Fiber   [W −1 dB −1 ]   [10 −6 m −1 ]                                                        SSMF   1.7   0.060           Truewave   2.5   0.095           RS           HS-DK   4.4   0.59           WB-DK   4.6   0.36           EHS-DK   4.9   0.56           Raman fiber   5.8   0.36           R-DCF 1   5.4   0.31           R-DCF 2   5.8   0.49                      
 
     [0039] In the above table, SSMF connotes a standard single mode transmission fiber. Truewave RS is a non-zero dispersion fiber (NZDF) typically used for transmission. HS-DK, WB-DK and EHS-DK are all existing dispersion compensating fibers available from OFS Fitel. The “Raman fiber” is a fiber that is especially well suited for Raman amplification, and is also available from OFS Fitel.  
     [0040]FIG. 2 is a graph showing relative index profiles labeled “1” and “2” for the inventive fibers R-DCF-1 and R-DCF-2, and a relative index profile, labeled “3”, for the HS-DK (or “High Slope” DK) fiber. FIG. 3 is a graph showing dispersion of the two fibers R-DCF-1 and R-DCF-2, and that of the HS-DK fiber as a function of wavelength, and FIG. 4 shows the RDS of all three fibers as a function of wavelength.  
     [0041] The HS-DK fiber is intended to match the dispersion slope of the Truewave RS transmission fiber in the L-band (1570 to 1605 μm). The two fibers R-DCF-1 and R-DCF-2 have substantially the same RDS at 1585 nm as the HS-DK fiber (see FIG. 4) but exhibit significantly improved Raman FOMs, as shown below.  
                                           Fiber   HS-DK   R-DCF-1   R-DCF-2                                                Spectral attenuation @1585 nm   0.57   0.34   0.42       [dB/km]       Dispersion @1585 nm [ps/nm/km]   −111   −71   −78       RDS @1585 nm [1/nm]   0.0065   0.0060   0.0067       Effective area @1580 nm [μm 2 ]   15.6   21.0   15.5       Raman gain efficiency with pump   3.6   2.1   3.2       @1453 nm [1/W/km]       Rainan FOM, pump @1453 nm   4.4   5.4   5.8       [1/W/dB]       Rayleigh backscattering [10 −6 m −1 ]   0.59   0.31   0.49       Polarization Mode Dispersion   &lt;0.2   &lt;0.1   &lt;0.1       (PMD) [ps/✓km]                  
 
     [0042] Three dispersion slope compensating modules intended to compensate for chromatic dispersion produced in a 50 km span of Truewave RS transmission fiber in L-band, were constructed. The three modules each included lengths of the EHS-DK compensating fiber, in combination with the HS-DK fiber (module DSCM-1), the inventive R-DCF-1 fiber (module DSCM-2), and the inventive R-DCF-2 compensating fiber (module DSCM-3), respectively. Simulations using realistic system input parameters were performed on the modules as discussed below with reference to FIGS.  5  to  7 .  
     [0043] Lengths of fibers in each of the three modules tested, were as follows:  
                                                   Module   Fiber lengths (meters)                          DSCM-1:   839 m of EHS-DK + 1333 m of HS-DK           DSCM-2:   916 m of EHS-DK + 2007 m of R-DCF-1           DSCM-3:   879 m of EHS-DK + 2084 m of R-DCF-2                      
 
     [0044]FIG. 5 is a schematic diagram of a test measurement configuration used to test for each of the three DSCMs. An input stage  30  included an array of laser diodes  32  for producing 16 equally spaced channels at wavelengths of from 1554 to 1608 nm. The input power was 1.68 mW per channel, and a combined output of the stage  30  was applied through an angled physical contact (APC) connector  34  to an input port of a circulator  36 .  
     [0045] A pump signal stage  38  produced five pump signals at wavelengths (and powers) of 1444 nm (190 mW), 1457 nm (190 mW), 1470 nm (110 mW), 1491 nm (215 mW) and 1508 nm (135 mW). The pump signals were applied to an input port of another circulator  40  an output port of which was coupled to an output of the DSCM under test, thus causing the DSCM to be counter or backward pumped. Another output of the circulator  40  was coupled to an optical spectrum analyzer (OSA)  42 .  
     [0046]FIG. 6 is a graph showing net gain and noise figure for each of the three modules as a function of wavelength, and FIG. 7 plots the MPI for each of the three modules. Because the dispersions of the inventive fibers R-DCF-1 and R-DCF-2 are numerically lower than that of HS-DK, the lengths (2007 m and 2084 m) of the inventive fibers needed to compensate for dispersion in the transmission fiber were greater than the length of HS-DK which would have been needed to achieve the same level of compensation. The greater lengths of the fibers  10  provide a longer gain medium for Raman amplification. Rayleigh backscattering and resulting MPI are lower and the Raman FOM is higher for both inventive fibers R-DCF-1 and R-DCF-2, making them better suited for Raman amplification than the HS-DK fiber. The module (DSCM-3) using the R-DCF-2 fiber realized an improvement in gain of up to 1.6 dB. The module (DSCM-2) with the R-DCF-1 fiber obtained a slight improvement in both gain and in MPI. Also, the RDS is substantially the same for all three fibers. It was also found that lengths of the inventive fibers greater than about three meters will behave as single mode fibers at a wavelength of 1550 nm.  
     [0047]FIG. 8 is a block diagram of a fiber optic transmission system  50 , including an optical transmitter  52 , an optical receiver  54 , and a dispersion slope compensating module  56  according to the invention. The DSCM  56  is connected in series with a span of transmission fiber  58  (which may be a non-zero dispersion fiber or NZDF) that forms part of a light signal path between the transmitter  52  and the receiver  54 . Both of the transmitter  52  and the receiver  54  may be included within corresponding optical transceivers currently provided in fiber optic communication systems or networks.  
     [0048] While the foregoing description represents preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention pointed out by the following claims.