Patent Publication Number: US-2003235365-A1

Title: Multistage chromatic dispersion slope compensator

Description:
CROSS-REFERENCE OF RELATED APPLICATION(S)  
     [0001] This application claims the benefit of U.S. provisional application No. 60/ 390,918, filed on Jun. 24, 2002, the contents of which are incorporated herein by reference. 
    
    
     
       BACKGROUND OF THE INVENTION  
       [0002] As more operational bandwidths are being used at higher modulation rates in telecommunication transmission fibers, signal anomalies resulting from the characteristics of such fibers need to be more accurately compensated for. One such anomaly is chromatic dispersion. In chromatic dispersion, different wavelengths of light travel at different speeds down a fiber, thereby causing light pulses encoded on such wavelengths to smear and merge together. This smearing and merging results in the inability to distinguish neighboring bits in the optical data stream at the end of transmission and, if not corrected, results in bit errors.  
       [0003] A common method to correct for chromatic dispersion is to reverse its effects; that is, to somehow pass the smeared and merged data pulses through a material that negates the transmission fiber&#39;s chromatic dispersion. This undoing of chromatic dispersion by sending chromatically dispersed light through a material that has the reverse, or negative, amount of chromatic dispersion that the transmission fiber has is called dispersion compensation.  
       [0004] Dispersion compensation has presented significant technical challenges. For example, the chromatic dispersion induced by telecommunication transmission fibers is often wavelength-dependent. More particularly, chromatic dispersion changes roughly linearly with wavelength over an operational bandwidth, for example, an International Telecommunications Union (ITU) transmission channel, and this chromatic dispersion “slope” often persists over multiple such operational bandwidths. Such fibers may even have a zero-dispersion cross point wavelength whereat chromatic dispersion transitions from negative to positive.  
       [0005] As with correction of chromatic dispersion in general, a common method to correct for chromatic dispersion slope is to reverse its effects. Attempts to correct for chromatic dispersion slope using single-stage dispersion slope compensators have, however, met with certain problems, such as inability to provide an accurate compensating dispersion across multiple operational bandwidths, incurring too much loss, or exacting too high a cost.  
       SUMMARY OF THE INVENTION  
       [0006] In one aspect, the present invention provides a pair of chromatic dispersion slope compensators (CDSCs) used in tandem to create a “net CDSC” having a zero-dispersion crosspoint. The CDSCs may have similar compensating dispersion slopes but opposing average compensating dispersions across one or more operational bandwidths. The first CDSC may vary in dispersion from a large positive value to a small positive value across the operational bandwidths, while the second CDSC may vary in dispersion from a small negative value to a large negative value across the operational bandwidths. The CDSCs may be applied in either order to reduce the dispersion slope of chromatically dispersed light received over a transmission fiber. The CDSCs may be Gires-Tournois etalon (GTE) based.  
       [0007] In another aspect, the present invention provides a method for providing a multistage chromatic dispersion slope compensation having a zero-dispersion cross point, wherein the method comprises applying to chromatically dispersed light a first compensating chromatic dispersion varying from a large positive value to a small positive value across one or more operational bandwidths; and applying to the chromatically dispersed light a second compensating dispersion varying from a small negative value to a large negative value across the operational bandwidths. The steps may be applied in either order.  
       [0008] These and other aspects of the invention will be better understood by reference to the following detailed description, taken in conjunction with the accompanying drawings which are briefly described below. Of course, the actual scope of the invention is defined by the appended claims. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 shows an optical transmission system including a CDSC pair;  
     [0010]FIG. 2 shows a GTE-based CDSC;  
     [0011]FIG. 3 shows chromatic dispersion induced by the transmission fiber of an optical transmission system as a function of wavelength; and  
     [0012]FIG. 4 shows compensating dispersions applied by a CDSC pair in the optical transmission system as a function of wavelength. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
     [0013] In FIG. 1, an optical transmission system  10  includes a system input  20 , a transmission fiber  30 , a transmission fiber output  40 , a first CDSC  50 , a second CDSC  60  and a system output  70 , arranged in series. CDSCs  50 ,  60  are preferably GTE-based. Light pulses within one or more operational bandwidths, such as ITU channels, are applied to system input  20 . Upon application to system input  20 , the wavelength components of an isolated pulse transmitted on a particular channel are tightly lined up with one another in time, that is, the pulse is “sharp.” Due to the chromatic dispersion induced by transmission fiber  30 , however, certain wavelength components travel faster through transmission fiber  30  than other wavelength components. Thus, upon arrival at transmission fiber output  40 , the wavelength components of the isolated pulse are chromatically dispersed, and neighboring pulses may interfere with one another. Moreover, the chromatic dispersion induced by transmission fiber  30  is wavelength-dependent, that is, it exhibits a non-zero chromatic dispersion “slope.” The chromatically dispersed light pulses are subjected to first CDSC  50  and second CDSC  60 , which in tandem operate to reverse the chromatic dispersion induced by transmission fiber  30  within the one or more operational bandwidths, including reducing or eliminating the chromatic dispersion slope. The re-sharpened pulses are then applied to system output  70 .  
     [0014] Turning to FIG. 2, CDSC  200  is shown. CDSC  200  is representative of CDSCs  50 ,  60 . CDSC  200  is a GTE having a first mirror  210  which has a reflectivity R 1  which is less than 100% and a second mirror  220  which has a reflectivity R 2  which is 100%. Chromatically dispersed pulses  230  arriving from, for example, transmission fiber  30  enter and exit CDSC  200  through first mirror  210 . CDSC  200  subjects different wavelength components of pulses  230  to variable delay due to its resonant properties. That is, the partial reflectivity of first mirror  210  causes certain wavelength components to be restrained in the glass cavity  240  between first mirror  210  and second mirror  220  longer than others. More particularly, CDSC  200  imposes a wavelength-dependent time delay on the wavelength components of pulses  230  which, when implemented in tandem with its counterpart CDSC, negates the sloped chromatic dispersion induced on pulses  230  by transmission fiber  30 . Naturally, etalons are just one example of CDSCs with which the present invention may be implemented. Other CDSCs, such as ring resonators, may be used.  
     [0015] Turning to FIG. 3, the chromatic dispersion induced by transmission fiber  30  is plotted as a function of wavelength for transmission channels  1  through  5 , which may be, for example, ITU channels. Transmission channels  1  through  5  reflect operational bandwidths of optical transmission system  10 . The chromatic dispersion of fiber  30  is plotted along the vertical axis  310  of the graph while the wavelength components of the light traveling through fiber  30  are plotted along the horizontal axis  320  of the graph. At the short wavelength end of the graph, fiber  30  induces a strong negative chromatic dispersion. Near the center of the plotted wavelength span the chromatic dispersion of fiber  30  passes through zero. At the long wavelength end of the graph, fiber  30  induces a strong positive chromatic dispersion. Moreover, chromatic dispersion increases linearly with wavelength over channels  1  through  5  individually, and this linearity is persistent across the combination of channels as well.  
     [0016] To correct for the chromatic dispersion profile In FIG. 3, a compensating dispersion of the same magnitude but the opposite sign must be applied across the operational bandwidths. Adding the chromatic dispersion of fiber  30  and such compensating dispersion would thus result in a null chromatic dispersion across the operational bandwidths.  
     [0017] Turning now to FIG. 4, such a compensating dispersion applied by the paired CDSCs  50 ,  60  is plotted. Compensating dispersions are plotted along the vertical axis  410  while wavelength is plotted along the horizontal axis  420 . The compensating dispersion is applied in two stages. A first compensating dispersion is applied by first CDSC  50  and is plotted on the graph as  430 . This compensating dispersion varies, across the operational bandwidths of channels  1  through  5 , from a large positive dispersion at the short wavelength end of the graph to a small positive dispersion at the long wavelength end of the graph. A second compensating dispersion is applied by second CDSC  60  and is plotted on the graph as  440 . This compensating dispersion varies, across the operational bandwidths of channels  1  through  5 , from a small negative dispersion at the short wavelength end of the graph to large negative dispersion at the long wavelength end of the graph. The compensating dispersions applied in the two stages together result in a net compensating dispersion  450  which varies from a large positive value to a large negative value across the operational bandwidths. This net compensating dispersion  450  effectively “zeroes out” the chromatic dispersion of fiber  30 .  
     [0018] It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character hereof. For example, rather than eliminating chromatic dispersion induced by fiber  30 , it may be desirable in certain applications to retain some chromatic dispersion slope across one or more operational bandwidths. The present invention is therefore considered in all respects illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come within the meaning and range of equivalents thereof are intended to be embraced therein.