Abstract:
A method of compensating for chromatic dispersion in an optical signal transmitted on a long-haul terrestrial optical communication system including a plurality of spans, including: allowing chromatic dispersion to accumulate over at least one of the spans to a first predetermined level; and compensating for the first pre-determined level of dispersion using a dispersion compensating fiber causing accumulation of dispersion to a second predetermined level. There is also provided a hybrid Raman/EDFA amplifier including a Raman portion and an EDFA portion with a dispersion compensating fiber disposed therebetween. An optical communication system and a method of communicating an optical signal using such a Raman/EDFA amplifier are also provided.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of the filing date of U.S. Provisional Application Nos. 60/249,347 and 60/249,346 filed Nov. 16, 2001, the teachings of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates in general to optical communication systems, and in particular to dispersion compensation in an optical communication system. 
     BACKGROUND OF THE INVENTION 
     Long-haul optical communication networks, e.g. networks of lengths greater than 600 kilometers, are particularly susceptible to the effects of chromatic dispersion. Chromatic dispersion results from wavelength-variation of the speed of travel for an optical signal on a fiber, and is manifested by pulse spreading in the transmitted signal and corresponding difficulties in signal detection. In long-haul terrestrial systems, the available fiber base is typically non-zero dispersion shifted fiber (NZ-DSF), which may exhibit a dispersion of 2–4 ps/nm/km. Significant dispersion may thus accumulate over long transmission distances. Long haul systems also suffer from signal attenuation resulting from a variety of factors, including scattering, absorption, and bending. 
     Compensation for dispersion and attenuation in long-haul systems has been accomplished on a per-span basis by inserting dispersion compensating fibers (DCFs) between stages of a multi-stage rare earth doped fiber amplifier, such as an erbium doped fiber amplifier (EDFAs). EDFA configurations are well known. In general, an EDFA operates by passing an optical signal through an erbium-doped fiber segment, and “pumping” the segment with light from another source such as a laser. The pump source excites erbium atoms in the doped segment, which then serves to amplify the optical signal passing therethrough. 
     Raman amplifiers and hybrid Raman/EDFA amplifiers are also known. Raman amplification occurs throughout an optical transmission fiber segment when it is pumped at an appropriate wavelength or wavelengths. Each Raman amplifier may contain one or more pumps. Gain is achieved over a spectrum of wavelengths longer than the pump wavelength through the process of Stimulated Raman Scattering. The difference between the Raman amplifier pumped wavelength and the peak of the associated amplified wavelength spectrum at the longer wavelength is referred to as a “Stokes shift.” The Stokes shift for a typical silica fiber is approximately 13 THz. Hybrid Raman/EDFA amplifiers combine the features of both Raman and EDFA amplifiers, typically in separate amplifier stages. 
     Although conventional amplifier configurations may be applied in a system for addressing signal attenuation, the conventional approach of providing dispersion compensation for every span through use of DCF between stages of a multi-stage EDFA has proven to be inefficient. For example, this approach does not consider system optimization through use of an optimum dispersion map. Also, non-linearities limit the amount of power that can be launched into the DCF, which complicates the EDFA design and potentially degrades performance. 
     Accordingly, there is a need for a system and method for providing improved dispersion compensation in long-haul optical networks. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided a method for compensating for chromatic dispersion in an optical signal transmitted on a long-haul terrestrial optical communication system including a plurality of spans. The method includes: allowing chromatic dispersion to accumulate over at least one of the spans to a first predetermined level; and compensating for the first pre-determined level of dispersion using a dispersion compensating fiber causing accumulation of dispersion to a second predetermined level. Dispersion compensation may be achieved using a dispersion compensating fiber in combination with a rare earth doped fiber amplifier, e.g. an EDFA. 
     According to another aspect of the invention, dispersion compensation may be achieved using a Raman/EDFA amplifier consistent with the invention, which includes a Raman portion, an EDFA portion and at least one dispersion compensating fiber disposed between the Raman portion and the EDFA portion. Use of a Raman/EDFA amplifier consistent with the invention allows improved noise performance and reduced system complexity. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, together with other objects, features and advantages, reference should be made to the following detailed description which should be read in conjunction with the following figures wherein like numerals represent like parts: 
         FIG. 1  is a block diagram of an exemplary multi-span optical communication system consistent with the present invention; 
         FIG. 2  is an exemplary dispersion map consistent with the present invention. 
         FIG. 3  is block diagram of one exemplary hybrid Raman/EDFA amplifier consistent with the invention; 
         FIG. 4  is a block diagram of an exemplary single-stage EDFA useful in connection with the optical amplifier of  FIG. 3 ; and 
         FIG. 5  is an exemplary plot of effective noise figure versus Raman gain for an exemplary Raman/EDFA amplifier consistent with the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Turning now to  FIG. 1 , there is illustrated an exemplary optical communication system  100  consistent with the present invention. Those skilled in the art will recognize that the system  100  has been depicted as a highly simplified point-to-point system for ease of explanation. It is to be understood the present invention may be incorporated into a wide variety of optical networks, systems, and optical amplifiers without departing from the spirit and scope of the invention. 
     The optical communication system  100  includes a transmitter  102  and a receiver  106  connected via an optical information channel  104 . At the transmitter, data may be modulated on a plurality of wavelengths for transmission over the optical information channel  104 . Depending on system characteristics and requirements, the optical information channel  104  may include an optical path  110 , e.g., optical fiber, optical amplifiers  108 - 1 ,  108 - 2 ,  108 - n - 1 ,  108 - n , optical filters, and other active and passive components. A variety of configurations for each of these elements will be known to those skilled in the art. For clarity, only optical amplifiers  108 - 1 ,  108 - 2 ,  108 - n - 1 ,  108 - n  and the optical path  110  are illustrated in the optical information channel  104 . 
     In general, the distance between optical amplifiers defines a span length. For example, in the illustrated exemplary embodiment the distance from the first amplifier  108 - 1  to the second amplifier  108 - 2  defines Span  1 . Those skilled in the art will recognize that span lengths may vary significantly in a particular system. In a long-haul terrestrial system, for example, some spans may be as short as 20 kilometers, while the average span may be about 70 kilometers to about 100 kilometers depending on system characteristics and requirements. In view of the span length variation, signal attenuation and dispersion vary from span-to-span. 
     Consistent with the present invention, dispersion compensation in a long-haul terrestrial network may be accomplished according to a dispersion map that allows accumulation of dispersion over a number of spans before providing compensation. Turning to  FIG. 2 , for example, there is illustrated an exemplary dispersion map, represented by plot  200 , for an exemplary system  100  consistent with the invention. In the illustrated embodiment, the plot  200  has several portions  202 ,  204 ,  206 . The first portion  202  indicates accumulation of dispersion over about 200 kilometers of the optical path  110 , typically NZ-DSF in terrestrial systems. The second portion  204  of the plot indicates compensation for the accumulated dispersion down to −1,000 ps, e.g. resulting from a DCF. Finally, the third portion  206  of the plot  200 , indicates accumulation of dispersion over the optical path until the accumulated dispersion returns to a desired predetermined level of about 0 ps at about 600 kilometers. With longer multi-span communication systems, such an exemplary dispersion plot can be repeated until the desired transmission distance is reached. 
     The dispersion map for a particular system may be selected, for example, by determining the configuration of the existing terrestrial fiber plant and allocating dispersion compensation only to relatively low-loss spans in the system. Dispersion may thus be allowed to accumulate over long spans (e.g., Span  1  and Span  2 ) but may be compensated on the occurrence of a relatively low loss span (e.g., Span n−1). For example, in a system where loss in longer spans is between about 15 to 25 dB, a relatively low-loss span would be a span exhibiting attenuation of from about 5 to 15 dB. 
     Thus, in contrast to conventional long-haul terrestrial systems wherein dispersion compensation is provided for every span, dispersion compensation may be strategically allocated to selected spans according to a predetermined dispersion map. Dispersion compensation may be accomplished by a variety of means. Compensation may be achieved, for example, by insertion of a multi-stage EDFA with one or more DCFs inserted between the stages. If the EDFA with the DCF is inserted into a low loss span, the amount of power launched into the DCF may be minimized, thereby optimizing performance of the EDFA. Allowing dispersion to accumulate over several spans according to a dispersion map consistent with the invention also reduces the cost and complexity of the system. 
     Dispersion compensation may also be accomplished through use of a hybrid Raman/EDFA amplifier consistent with the invention. An exemplary embodiment of a hybrid Raman/EDFA amplifier  108 - 2  consistent with the invention is illustrated in  FIG. 3 . In the illustrated exemplary embodiment, the optical amplifier  108 - 2  includes a Raman portion  302  and an EDFA portion  304 . The Raman portion  302  may include a fiber transmission path segment  306  in which Raman gain is generated for amplifying an optical signal propagating through the path  110 . Energy from a pump source  310  is coupled to the segment  306  of path  110  by a coupler  308 , e.g., a WDM. One or more Raman pump sources  310  may be coupled to the optical path  110  in a wide variety of configurations known to those skilled in the art. An exemplary Raman pump may include a grating stabilized Fabry-Perot laser with a pump power of 1.4 watts at a wavelength of 1450 nm to provide 10 dB of Raman gain. Alternatively, Raman portion  302  may also include DCF fiber (not shown) similar to the DCF  312 . Thus, within the Raman portion  302 , there may be provided DCF fiber, which is coupled to the existing transmission fiber  110 . 
     The EDFA portion  304  may be a single or multi-stage EDFA. An exemplary single-stage EDFA  400  is illustrated in  FIG. 4 . A single-stage EDFA may include an EDFA pump source  402 , a coupler  404 , one erbium-doped fiber segment  406 , and an isolator  408 . In contrast, a two-stage EDFA has two separate erbium doped segments and typically two separate pump sources. Those skilled in the art will recognize a variety of EDFA pump sources that may be controlled locally or remotely for use with the single-stage EDFA. Also, it will be recognized that the pump sources may be coupled to the optical path  110  in a wide variety of coupling configurations. 
     A Raman/EDFA amplifier consistent with the invention also includes a DCF  312  disposed between the Raman portion  302  and the EDFA portion  304 , as illustrated in  FIG. 3 . Placement of the DCF between the Raman and EDFA stages has significant advantages compared to conventional two-stage EDFAs with a DCF between EDFA stages. For example, use of a Raman/EDFA amplifier consistent with the invention allows for improved noise performance compared to conventional two-stage EDFAs. This can facilitate frequent insertion of shorter lengths of DCF in the optical path for maintaining desired dispersion levels. Also, difficulties associated with EDFA and system design for limiting launch power from an EDFA into a DCF are eliminated. 
     In operation, the Raman portion of an exemplary Raman/EDFA amplifier consistent with the invention may be pumped by pump source  310  in a counter-propagating fashion to provide, e.g., 10–15 dB of Raman gain. The Raman gain may be set depending on system characteristics in order to optimize the Raman gain while minimizing the effects of noise accumulation and multi-path interference (MPI).  FIG. 5  illustrates an exemplary plot  502  of Raman gain in dB versus effective noise figure in dB for the exemplary Raman/EDFA amplifier  108 - 2  consistent with the invention. The exemplary plot  502  illustrates exemplary performance for an amplifier span length of 80 kilometers. As shown, a Raman gain of about 15 dB is optimum in the exemplary embodiment. This gain results in the lowest effective noise figure of about 1.6 dB for the Raman/EDFA amplifier and also minimizes MPI. In this embodiment, therefore, a Raman assisted EDFA amplifier consistent with the present invention may improve the signal to noise ratio (SNR) by as much as about 2.4 dB compared to a typical EDFA with a good noise figure of 4 dB. (4.0 dB−1.6 dB=2.4 dB). 
     The EDFA portion  304  provides the remaining gain, e.g., an additional 5–15 dB, to compensate for the remaining fiber loss over any particular span. The total Raman/EDFA gain may thus be in the range from about 10–25 dB. For example, if the Raman gain for the system illustrated in  FIG. 5  were set to 15 dB to minimize noise figure, an EDFA providing gain of less than 10 dB would be appropriate for typical span losses. The level of EDFA and Raman gain may, of course, be varied depending on the particular system characteristics including losses over each particular span. The Raman and EDFA gains may be varied to properly compensate for fiber losses over a particular span but to optimize the Raman gain to minimize the effective noise figure for the Raman/EDFA amplifier and to minimize MPI. As known to those skilled in the art, Raman gain may be adjusted by varying pump powers and/or wavelengths from the Raman pump source  310 . 
     To provide a medium level of EDFA gain necessary in a Raman/EDFA amplifier consistent with the present invention, e.g., from about 5 db to about 15 dB, a single-stage EDFA amplifier with a high pump power may be used. This allows for vastly improved noise performance compared to a conventional two stage EDFA with a DCF in between the stages. Also, the DCF  312  allows adherence to a dispersion map as illustrated, for example, in  FIG. 2 . That is, dispersion may be a owed to accumulate for several spans before being overcompensated by a DCF  312  and finally allowed to accumulate back to near zero or some desired small path-average dispersion. 
     The embodiments that have been described herein, however, are but some of the several which utilize this invention and are set forth here by way of illustration but not of limitation. It is obvious that many other embodiments, which will be readily apparent to those skilled in the art, may be made without departing materially from the spirit and scope of the invention.