Abstract:
A method and system for dispersion compensation comprises a dispersion compensator (DC) for receiving a train of chromatically dispersed light pulses over a transmission fiber at multiple operational bandwidths and inducing on the train a compensatory dispersion having an adjustable broadband dispersion slope. In one approach, broadband dispersion slope is tuned using a pair of dispersion compensation blocks (DCBs) and mode hopping. In another approach, broadband dispersion slope is tuned using paired DCBs and symmetric intra-channel slope adjustment with mode mismatch. The DCBs are etalon-based. Slope tuning is induced by etalon tuning performed, by way of example, thermally or using microactuators.

Description:
CROSS REFERENCE TO RELATED APPLICATION(S)  
       [0001]    This application claims the benefit of U.S. provisional application No. 60/455,448, filed on Mar. 18, 2003, the contents of which are incorporated herein by reference. 
     
    
     
       BACKGROUND OF 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 signal anomaly is chromatic dispersion. In chromatic dispersion, different wavelengths of light travel at different speeds down a transmission 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 chromatic dispersion is to reverse its effects; that is, to 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]    A related signal anomaly arising from the characteristics of transmission fibers is chromatic dispersion slope. The chromatic dispersion induced by transmission fibers is often wavelength-dependent. More particularly, chromatic dispersion typically changes roughly linearly with wavelength over an operational bandwidth, for example, an International Telecommunications Union (ITU) transmission channel, and this chromatic dispersion slope generally persists over multiple operational bandwidths. In other words, a transmission fiber typically has associated with it both an intra-channel, or “in-band”, slope and an inter-channel, or “broadband”, slope.  
           [0005]    As with correction of chromatic dispersion, a common method to correct chromatic dispersion slope is to reverse its effects. However, known methods to correct broadband chromatic dispersion slope in particular have proven suboptimal. Known methods have included channel-by-channel approaches applied after wavelength demultiplexing, for example, using fiber Bragg gratings or electronics, and certain planar waveguide approaches. Channel-by-channel approaches have generally been suboptimal because they have required mux/demux overhead. Planar waveguide approaches have generally been unsuitable because they have not been able to provide sufficient flexibility in their wavelength dependent coupling constants to tune effectively across a broad spectral band.  
         SUMMARY OF INVENTION  
         [0006]    The present invention, in one feature, provides a method and system for broadband (i.e. inter-channel) chromatic dispersion slope tuning using a pair of dispersion compensation blocks (DCBs) and mode hopping. The DCBs are applied in series to a train of chromatically dispersed light pulses received over a transmission fiber on multiple operational bandwidths. The DCBs are arranged to apply a substantially equal and opposite intra-channel dispersion slope at the operational bandwidths, resulting in a net near-zero intra-channel dispersion slope at the operational bandwidths. Moreover, at least one of the DCBs is adjustable to change to a different mode number from the other, resulting in a net non-zero inter-channel dispersion slope across the operational bandwidths.  
           [0007]    The present invention, in another feature, provides a method and system for broadband chromatic dispersion slope tuning using paired DCBs and symmetric intra-channel slope adjustment with mode mismatch. The DCBs are applied in series to a train of chromatically dispersed light pulses received over a transmission fiber on multiple operational bandwidths. The DCBs are arranged to apply a substantially equal and opposite intra-channel dispersion slope at the operational bandwidths, resulting in a net near-zero intrachannel dispersion slope at the operational bandwidths, and are arranged to operate on different mode numbers, resulting in a net non-zero inter-channel dispersion slope across the operational bandwidths. Moreover, the DCBs are adjustable to change to a steeper or less steep substantially equal and opposite intrachannel dispersion slope at the operational bandwidths, retaining the net near-zero intra-channel dispersion slope while inducing a steeper or less steep net inter-channel dispersion slope.  
           [0008]    Each DCB preferably comprises a group of one or more etalons. The adjustments may be made through, for example, thermal, microactuator-driven, or electric field tuning.  
           [0009]    The present invention, in another feature, provides a method and system for dispersion compensation comprising a dispersion compensator (DC) for receiving a train of chromatically dispersed light pulses over a transmission fiber at multiple operational bandwidths and inducing on the train a compensatory dispersion having an adjustable inter-channel dispersion slope. The DC preferably comprises a DCB pair as generally described above.  
           [0010]    The present invention, in another feature, provides a method and system for dispersion compensation comprising a first DC for receiving a train of chromatically dispersed light pulses over a transmission fiber on multiple operational bandwidths and inducing on the train a first compensatory dispersion; and a second DC for receiving the train from the first DC and inducing on the train a second compensatory dispersion, wherein the second compensatory dispersion has an adjustable inter-channel dispersion slope. The first DC preferably comprises a dispersion compensating fiber (DCF). The second DC preferably comprises dispersion equalization module (DEM) having a DCB pair as generally described above.  
           [0011]    These and other features of the invention will be better understood by reference to the detailed description of the preferred embodiment, taken in conjunction with the drawings which are briefly described below. Of course, the invention is defined by the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 shows a dispersion compensation system having a DCF and a DEM, serially arranged on an optical path, in a preferred embodiment of the invention;  
         [0013]    [0013]FIG. 2A shows a thermally tunable etalon operative within a DCB in a preferred embodiment of the invention;  
         [0014]    [0014]FIG. 2B shows a microactuator tunable etalon operative within a DCB in a preferred embodiment of the invention;  
         [0015]    [0015]FIGS. 3A through 3C illustrate inter-channel dispersion slope adjustment through mode hopping, and in particular:  
         [0016]    [0016]FIG. 3A shows the chromatic dispersion profile of a first DCB (CD 1 ), a second DCB (CD 2 ), and the sum thereof (CD SUM) within each channel when the first DCB and second DCB are operative on the same mode (M);  
         [0017]    [0017]FIG. 3B shows the chromatic dispersion profile of a first DCB (CD 1 ), a second DCB (CD 2 ), and the sum thereof (CD SUM) within each channel when the first DCB is operative on a mode (M−m) and the second DCB is operative on a higher mode (M);  
         [0018]    [0018]FIG. 3C shows the chromatic dispersion profile of a first DCB (CD 1 ), a second DCB (CD 2 ), and the sum thereof (CD SUM) within each channel when the first DCB is operative on a mode (M+m) and the second DCB is operative on a lower mode (M);  
         [0019]    [0019]FIGS. 4A through 4C illustrate inter-channel dispersion slope adjustment through symmetric intra-channel dispersion slope adjustment with mode mismatch, and in particular:  
         [0020]    [0020]FIG. 4A shows the chromatic dispersion profile of a first DCB (CD 1 ), a second DCB (CD 2 ), and the sum thereof (CD SUM) within each channel when the first DCB is operative on a mode (M+m), the second DCB is operative on a lower mode (M), and the intra-channel dispersion slopes of the first and second DCBs are of medium magnitude;  
         [0021]    [0021]FIG. 4B shows the chromatic dispersion profile of a first DCB (CD 1 ), a second DCB (CD 2 ), and the sum thereof (CD SUM) within each channel when the first DCB is operative on a mode (M+m), the second DCB is operative on a lower mode (M), and the intra-channel dispersion slopes of the first and second DCBs are of large magnitude; and  
         [0022]    [0022]FIG. 4C shows the chromatic dispersion profile of a first DCB (CD 1 ), a second DCB (CD 2 ), and the sum thereof (CD SUM) within each channel when the first DCB is operative on a mode (M+m), the second DCB is operative on a lower mode (M), and the intrachannel dispersion slopes of the first and second DCBs are of small magnitude.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]    In FIG. 1, a dispersion compensation system having a DCF  110  and a DEM  120 , serially arranged along an optical path, is shown. DCF  110  is a dispersion compensating fiber for reversing bulk chromatic dispersion accumulated on light pulses during transmission on a telecommunication transmission fiber. DEM  120  is a dispersion compensator for reversing residual chromatic dispersion and chromatic dispersion slope remaining on the light pulses after application of DCF  110 . DEM  120  has a pair of DCBs  122 ,  124  serially arranged on the optical path. DCBs  122 ,  124  are Gires-Toumois etalon (GTE) based dispersion compensators each having a group of one or more GTEs. The group of GTEs within each DCB, where the DCB consists of more than one GTE, are also serially arranged along the optical path and provide a group dispersion and a group dispersion slope which are the effective dispersion and dispersion slope, respectively, for the DCB. The group of GTEs within each DCB also operate on group mode number which is the effective mode number for the DCB, although individual GTEs in the DCB may operate on different mode numbers.  
         [0024]    In operation, a train of light pulses at multiple operational bandwidths arrives at DCF  110  after transmission on a long haul dense wave division multiplexing (DWDM) transmission fiber with significant chromatic dispersion and a chromatic dispersion slope accumulated during transmission on the fiber. As the train traverses DCF  110 , DCF  110  eliminates most of the chromatic dispersion and partially compensates for the chromatic dispersion slope. However, some residual chromatic dispersion and chromatic dispersion slope remain. The train then traverses DEM  120 , where the residual chromatic dispersion is reduced to near zero and the chromatic dispersion slope is nearly fully compensated.  
         [0025]    At least one of DCBs  122 ,  124  is tunable to adjust the inter-channel dispersion slope induced by DEM  120  on the train incident from DCF  110 . This tuning may be achieved using various techniques alone or in combination. Such techniques include, without limitation, microactuator-driven changes to one or more GTEs within one or more of DCBs  120 ,  124 , or changes to the environment in which one or more GTEs operate induced thermally or through electric field manipulation.  
         [0026]    Turning to FIG. 2A, a thermally tunable GTE suitable for application within one of DCBs  122 ,  124  is shown. The GTE has a first mirror  210  that is partially reflective and a second mirror  220  that is fully reflective light  230  arriving from, for example, DCF  110  enters and exits the GTE through first mirror  210 . The GTE subjects different wavelength components of light  230  to variable delay due to its resonant properties. That is, the partial reflectivity of first mirror  210  causes certain wavelength components of light  230  to be restrained in the cavity  240  between first mirror  210  and second mirror  220  longer than others. More particularly, the GTE imposes a wavelength-dependent time delay on the wavelength components of light  230  which, when implemented with other GTEs in its group and its counterpart DCB, reverses the residual inter-channel dispersion slope of light  230 .  
         [0027]    Thermal tuning is accomplished by selective activation of temperature controller  200 , which raises or lowers the temperature of the GTE by a desired number of degrees (ΔT). Raising or lowering the temperature changes the length and the refractive index of cavity  240 , thereby inducing a resonance point shift on the GTE. This, in turn, changes the mode number and/or intra-channel dispersion slope of the DCB in which the GTE is operative.  
         [0028]    Turning to FIG. 2B, a microactuator tunable GTE suitable for application within one of DCBs  122 ,  124  is shown. The GTE has a first mirror  260  that is partially reflective and a second mirror  270  that is fully reflective of incident light  280 , and the partial reflectivity of first mirror  260  causes certain wavelength components of light  280  to be restrained in the cavity  290  longer than others. Here, however, tuning is accomplished by selective activation of microactuator  250 , which moves second mirror  270  horizontally and thereby changes the length of cavity  290  by a desired distance (Δd). Changing the length of cavity  290  induces a resonance point shift on the GTE. This, in turn, changes the mode number and/or the intra-channel dispersion slope of the DCB in which the GTE is operative.  
         [0029]    Other tuning methods are possible, such as the introduction of electric field into the environment in which one or more GTEs are operative to induce a change in the refractive index of one or more GTE cavities and a consequent resonance point shift.  
         [0030]    An important aspect of the present invention is to be able to translate changes in the mode number and/or intra-channel dispersion slope induced within Individual ones of DCBs  122 ,  124  by, for example, thermal, microactuator-based, or electric field tuning, into desired changes in the interchannel dispersion slope of DEM  120 . This is accomplished, in a preferred embodiment, through (i) mode hopping or (ii) symmetric intra-channel dispersion slope adjustment with mode mismatch. Mode hopping will be illustrated in a preferred embodiment by reference to FIGS. 3A through 3C. Symmetric intra-channel dispersion slope adjustment with mode mismatch will be illustrated in a preferred embodiment by reference to FIGS. 4A through 4C.  
         [0031]    In FIG. 3A, the individual chromatic dispersion profiles of DCB  122  (CD 1 ) and DCB  124  (CD 2 ), and the sum thereof (CD SUM), are shown when the DCB  122  and DCB  124  are operative on the same mode (M). As can be seen, the profiles of DCB  122  and DCB  124  apply a substantially equal and opposite dispersion slope to incident light at each operational bandwidth (i.e. channel), resulting in inducement of a net zero, or substantially zero, intrachannel dispersion slope on each operational bandwidth. Moreover, since the DCB  122  and DCB  124  are operative on a common mode number, the profiles of DCB  122  and DCB  124  also apply a substantially equal and opposite dispersion to incident light at each operational bandwidth, resulting in a net zero, or substantially zero, inter-channel dispersion slope across the operational bandwidths.  
         [0032]    In FIG. 3B, the individual chromatic dispersion profiles of DCB  122  (CD 1 ) and DCB  124  (CD 2 ), and the sum thereof (CD SUM), are shown when the DCB  122  and DCB  124  are operative on different mode numbers (M−m and M, respectively). In particular, DCB  122  has been tuned to, i.e. “hopped” to, a lower mode (M−m) such that DCB  122  and DCB  124  share only one resonance point. As can be seen, the profiles of DCB  122  and DCB  124  still induce a substantially equal and opposite dispersion slope on incident light at each operational bandwidth, resulting in inducement of a net zero, or substantially zero, intra-channel dispersion slope at each operational bandwidth. However, since the DCB  122  and DCB  124  are operative on different mode numbers, the profiles of DCB  122  and DCB  124  now induce an opposite but unequal dispersion at each operational bandwidth away from the shared resonance point, resulting in a positive inter-channel dispersion slope across the operational bandwidths.  
         [0033]    In FIG. 3C, the individual chromatic dispersion profiles of DCB  122  (CD 1 ) and DCB  124  (CD 2 ), and the sum thereof (CD SUM), are shown when the DCB  122  and DCB  124  are operative on different mode numbers (M+m and M, respectively). In particular, DCB  122  has been tuned to a higher mode (M+m) such that DCB  122  and DCB  124  share only one resonance point. As can be seen, the profiles of DCB  122  and DCB  124  still induce a substantially equal and opposite dispersion slope on incident light at each operational bandwidth, resulting in inducement of a net zero, or substantially zero, intra-channel dispersion slope at each operational bandwidth. However, since the DCB  122  and DCB  124  are operative on different mode numbers, the profiles of DCB  122  and DCB  124  now induce an opposite but unequal dispersion on each operational bandwidth away from the shared resonance point, resulting in a negative inter-channel dispersion slope across the operational bandwidths.  
         [0034]    Turning to FIG. 4A, the individual chromatic dispersion profiles of DCB  122  (CD 1 ), DCB  124  (CD 2 ), and the sum thereof (CD SUM), are shown when the DCB  122  is operative on a mode (M+m), the DCB  124  is operative on a lower mode (M), and the intra-channel dispersion slopes of DCBs  122 ,  124  are of medium magnitude. The situation resembles that shown in FIG. 3C. The profiles of DCB  122  and DCB  124  induce a substantially equal and opposite dispersion slope on incident light at each operational bandwidth, resulting in inducement of a net zero, or substantially zero, intra-channel dispersion slope at each operational bandwidth. However, since the DCB  122  and DCB  124  are operative on different mode numbers, i.e. there is “mode mismatch,” the profiles of DCB  122  and DCB  124  induce an opposite but unequal dispersion on each operational bandwidth away from the shared resonance point, resulting in a negative inter-channel dispersion slope across the operational bandwidths. Moreover, the steepness of the inter-channel dispersion slope may be characterized as medium owing to the medium magnitude of the individual intra-channel dispersion slopes of DCBs  122 ,  124 .  
         [0035]    In FIG. 4B, the individual chromatic dispersion profiles of DCB  122  (CD 1 ), DCB  124  (CD 2 ), and the sum thereof (CD SUM), are shown when the DCB  122  is operative on a mode (M+m), the DCB  124  is operative on a lower mode (M), and the intra-channel dispersion slopes of DCBs  122 ,  124  are of large magnitude. In particular, DCB  122  and DCB  124  have been symmetrically tuned to increase the steepness of their dispersion slopes equally and oppositely. The profiles of DCB  122  and DCB  124  still induce a substantially equal and opposite dispersion slope on incident light at each operational bandwidth, resulting in inducement of a net zero, or substantially zero, intra-channel dispersion slope on each operational bandwidth. However, since the DCB  122  and DCB  124  are operative on different mode numbers, the profiles of DCB  122  and DCB  124  induce an opposite but unequal dispersion at each operational bandwidth away from the shared resonance point, resulting in a negative inter-channel dispersion slope across the operational bandwidths. Moreover, the steepness of the inter-channel dispersion slope may be characterized as large owing to the large magnitude of the individual intra-channel dispersion slopes of DCBs  122 ,  124 .  
         [0036]    Finally, in FIG. 4C, the individual chromatic dispersion profiles of DCB  122  (CD 1 ), DCB  124  (CD 2 ), and the sum thereof (CD SUM), are shown when the DCB  122  is operative on a mode (M+m), the DCB  124  is operative on a lower mode (M), and the intra-channel dispersion slopes of DCBs  122 ,  124  are of small magnitude. In particular, DCB  122  and DCB  124  have been symmetrically tuned to decrease the steepness of their dispersion slopes equally and oppositely. The profiles of DCB  122  and DCB  124  still induce a substantially equal and opposite dispersion slope on incident light at each operational bandwidth, resulting in inducement of a net zero, or substantially zero, intra-channel dispersion slope on each operational bandwidth. However, since the DCB  122  and DCB  124  are operative on different mode numbers, the profiles of DCB  122  and DCB  124  induce an opposite but unequal dispersion at each operational bandwidth away from the shared resonance point, resulting in a negative inter-channel dispersion slope across the operational bandwidths. Moreover, the steepness of the inter-channel dispersion slope may be characterized as small owing to the small magnitude of the individual intra-channel dispersion slopes of DCBs  122 ,  124 .  
         [0037]    Although DEM  120  and its constituent DCBs  122 ,  124  have been described and illustrated as cooperative with DCF  110  within the dispersion compensation system shown in FIG. 1, DEM  120  may operate independently of any other dispersion compensation element. For example, DEM  120  may operate on incident light having zero dispersion and/or zero dispersion slope and generate a positive or negative dispersion and/or dispersion slope on the light “as needed.” It will therefore be appreciated by those of ordinary skill in the art that the invention may be embodied in other specific forms without departing from the spirit or essential character hereof. The present description is therefore considered in all respects to be 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.