PATENT ABSTRACT
An optical wavelength add/drop multiplexer (WADM) is configured to add or drop two or more signals each associated with one of a plurality of channels in a wavelength division multiplexed (WDM) signal. The WADM comprises an optical circulator that is optically coupled at one port to two or more serially interconnected fiber Bragg gratings (FBGs), and is optically coupled at another port to a thin film filter including two or more serially interconnected thin film filter elements. Each of the two or more FBGs is matched with a thin film filter element, both arranged to be responsive to signals associated with one of the plurality of channels. Bandwidth and dispersion properties for the FBGs are selected to permit operation of the WADM at two distinct signal data rates. To equalize associated insertion losses in embodiments of the invention arranged to add or drop two or more signals, the FBGs are matched to the thin film filter elements in inverse order with respect to their optical distance from the optical circulator.

PATENT DESCRIPTION
FIELD OF THE INVENTION 
     This invention relates to optical wavelength add/drop multiplexers. More specifically, it relates to an optical wavelength add/drop multiplexer operable to add or drop digital optical signals from optical channels that may each be operating at one of two or more data transmission rates. 
     BACKGROUND OF THE INVENTION 
     Broadband telecommunications networks are being configured to carry increasing volumes of voice, data and multimedia information. To meet these increasing volume demands, such networks are being implemented using optical communications systems technology. For example, optical wavelength-division multiplexed (WDM) technology may be used to support dozens of communications channels transported at different wavelengths on a single optical fiber. 
     In WDM optical networks, wavelength add/drop multiplexers (WADMs) have been used to selectively remove and reinsert WDM channels at intermediate points across these networks (see, e.g., C. Randy Giles et al., “The Wavelength Add/Drop Multiplexer for Lightwave Communications Networks,” Bell Labs Technical Journal, January-March 1999, pp. 207-229). For example, WADMs have been constructed using optical multiplexer/demultiplexer pairs that first demultiplex a multi-channel WDM optical signal into individual WDM channels on individual optical paths, and then re-multiplex signals on the individual optical paths back into a single multi-channel WDM optical signal. Single channel WDM signals may be dropped from or added to a selected number of the individual optical paths before the signals are re-multiplexed. 
     Alternatively, in order to avoid demultiplexing and re-multiplexing each of the channels in the WDM signal, a variety of optical filter technologies have been employed in WADM systems to drop signals from or add signals to selected channels in the multi-channel WDM optical signal. Such filter technologies include, for example, fiber Bragg gratings (FBGs), thin film filters and arrayed waveguide gratings. Use of such filter technologies in WADMs is preferred when only a few of many channels in a WDM signal are either being dropped or added. 
     Optical filter characteristics are largely dictated by associated WDM signal characteristics. For example, synchronous optical network (SONET) OC192 channels operating at 10 gigabits per second require filters with an effective bandwidth of at least 48 gigahertz, while SONET OC48 channels operating at 2.5 gigabits per second require filters with an effective bandwidth of at least 10 gigahertz. In addition, OC192 channels require filters that are selective among channels spaced at 100 gigahertz intervals, while OC48 channels require filters that are selective among channels spaced at 50 gigahertz intervals. As a result, WADM filters usable at one WDM data transmission rate are generally unusable at alternate data rates. 
     For increased flexibility, some current WDM systems allow individual channels to be operated at alternate data rates. For example, an OC192 channel with 100 gigahertz spacing may alternatively be replaced by two OC48 channels with 50 gigahertz spacing. This increased flexibility helps to maximize utilization of capacity in WDM systems. 
     To date, such flexible systems have used dedicated WADM filters to filter signals at each data rate. This approach adds cost and reduces inherent flexibility in the selection of channels for a given WADM signal. Accordingly, there is a need to provide a more flexible and cost-effective means for filtering optical channels in a WDM signal with varying data rates. 
     SUMMARY OF THE INVENTION 
     Flexibility is increased and cost is reduced in an optical wavelength add/drop multiplexer (WADM) configured to add or drop two or more WDM channels that may each be operating at one of either a first data rate or a second data rate. The WADM comprises an optical circulator that is coupled at one port to two or more serially interconnected FBGs, and at another port to a thin film filter including two or more serially interconnected thin film filter elements (TFFEs). Each of the FBGs and TFFEs has an effective bandwidth to filter signals from one of the two or more WDM channels. Bandwidth and dispersion characteristics for the FBGs are selected to minimize anticipated filter performance penalties for operation at both the first and second data rates. 
     FBGs and TFFEs contribute insertion loss to the filtered signals. According to the principles of the present invention, FBGs and TFFEs are configured to approximately equalize the amount of insertion loss associated with each added or dropped channel. Specifically, FBGs and TFFEs are configured such that optical channels are assigned to FBGs in order of the FBGs&#39; increasing optical distance from the circulator, and assigned to TFFEs in order of the TFFEs&#39; decreasing optical distance from the circulator. 
     In a preferred embodiment of the invention supporting a first data rate of no more than 2.5 gigabits per second and a second data rate of 10 gigabits per second, the WADM includes four FBGs and four thin film filters. In order to employ conventional thin film filter elements having an effective bandwidth of 200 gigahertz, each pair of adjacent FBGs and each pair of adjacent thin film filters are selected to have characteristic wavelengths spaced at 200 gigahertz intervals. Bandwidth and dispersion characteristics of the FBGs are selected to enable operation at both the first and second data rates. Specifically, each FBG is selected to have an effective bandwidth (i.e., reflected by a power difference over the bandwidth of no more than 10 dB) of about 0.45 nanometers. Each FBG is further selected with dispersion values that deviate by no more than approximately 150 picoseconds per nanometer from a predetermined reference value at wavelengths no more than 0.1 nanometers above and below a characteristic wavelength, and with deviation increasing above 150 picoseconds per nanometer at a rate no greater than approximately 20,000 picoseconds per square nanometer at wavelengths beyond 0.1 nanometers from the characteristic wavelength. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The invention will be more fully understood from the following detailed description taken in connection with the accompanying drawing, in which: 
     FIG. 1 depicts a first embodiment of the present invention for dropping optical channels from a WDM signal; 
     FIG. 2 depicts a second embodiment of the present invention for adding optical channels to a WDM signal; 
     FIG. 3 shows WADM employing both the first and second embodiments of FIGS. 1 and 2; 
     FIG. 4 illustrates how the WADM of FIG. 3 may be placed in a WDM network; 
     FIG. 5 illustrates a typical reflection and transmission spectrums for a fiber Bragg grating (FBG) used in the embodiments of FIGS. 1-4; 
     FIG. 6 shows a comparison of reflection spectrums for an FBG and a thin film filter used in the embodiments of FIGS. 1-4; 
     FIGS. 7A and 7B illustrate SPM/XPM penalties for an FBG used in the present invention at OC48 and OC192 data rates, respectively; 
     FIG. 8 illustrates how multiple WADMs may be used to add or drop a series of OC48 and OC192 channels; 
     FIG. 9 shows limits for FBG dispersion levels as a function of wavelength; and 
     FIG. 10 shows dispersion levels for typical FBGs used in the embodiments of FIGS. 1-4. 
     For consistency and ease of understanding, those elements of each figure that are similar or equivalent share identification numbers that are identical in the two least significant digit positions (for example, FBG  132  of FIG. 1 is equivalent to FBG  232  of FIG.  2 ). 
    
    
     DETAILED DESCRIPTION 
     Consistent with the principles of the present invention, FIG. 1 depicts a wavelength add/drop multiplexer (WADM)  100  configured to drop optical signals associated with a maximum of four channels in a multi-channel WDM signal. The WDM signal enters an optical circulator  130  in WADM  100  via input  102 . Optical circulator  130  functions to transport optical signals received at input  102  to link  116  and to transport optical signals received via link  116  to link  114 . Optical circulator  130  is an asymmetrical circulator, as it does not further function to transport optical signals received via link  114  to input  102 . Such asymmetrical circulators are well-known in the art and are commercially available, for example, from JDS Uniphase and others. 
     Circulator  130  of FIG. 1 transports the WDM signal from input  102  via link  116  to fiber Bragg gratings (FBGs)  132 ,  134 ,  136  and  138 . FBGs  132 ,  134 ,  136  and  138  are responsive to optical signals carried by channels approximately centered at wavelengths λ 1 , λ 3 , λ 5 , and λ 7 , respectively. 
     FBGs  132 ,  134 ,  136  and  138  are of a type that may be obtained commercially, for example, from JDS Uniphase, Corning, and Sumitomo Electric Lightwave Corp. In order to be suitable for application in the present invention, FBGs  132 ,  134 ,  136  and  138  are selected to exhibit the bandwidth and dispersion characteristics described further herein. 
     Upon receiving a WDM signal over link  116 , FBG  132  operates to substantially reflect a component of the multi-wavelength WDM signal carried by a channel approximately centered at wavelength λ 1 , and to substantially pass other WDM signal components over link  115  to FBG  134 . Similarly, FBG  134  operates to substantially reflect a component of the multi-wavelength WDM signal carried by a channel approximately centered at wavelength λ 3  and to pass other signal components over link  117  to FBG  136 . FBG  136  substantially reflects a component of the WDM signal carried by a channel approximately centered at wavelength λ 5  while passing other components over link  119  to FBG  138 , and FBG  138  substantially reflects a component of the WDM signal carried by a channel approximately centered at wavelength λ 7  while passing other components to output  104 . 
     As a result of the operation of FBGs  132 ,  134 ,  136  and  138 , signal components of the input WDM signal carried by channels approximately centered at wavelengths λ 1 , λ 3 , λ 5 , and λ 7  are substantially removed from the WDM signal reaching link  104 . These removed component signals are reflected by FBGs  132 ,  134 ,  136  and  138  back to circulator  130 , which directs the reflect signals over link  114  to thin film filter  120 . Other WDM signal components not substantially reflected by FBGs  132 ,  134 ,  136  and  138  are transmitted through the WADM  100  over output  104 . 
     Thin film filter  120  includes thin film filter elements (TFFEs)  122 ,  124 ,  126 , and  128 . TFFEs may be obtained commercially, for example, from JDS Uniphase, Corning, and DiCon Fiberoptics, Inc. One skilled in the art will readily recognize that other optical signal demultiplexing devices (for, example, such as a star coupler) may alternatively be employed in place of thin film filter  120  without deviating from the principles of the present invention. Low cost and insertion loss characteristics associated with thin film filter  120  suggest that it is particularly well-suited to be selected as the demultiplexing device. 
     TFFEs  122 ,  124 ,  126  and  128  are responsive to optical signals carried by channels approximately centered at wavelengths λ 1 , λ 3 , λ 5 , and λ 7 , respectively. For example, TFFE  128  receives the removed component signals in channels approximately centered at wavelengths λ 1 , λ 3 , λ 5 , and λ 7  over link  121 , and operates to substantially transmit the component associated with λ 7  over output  106  and to substantially reflect other remaining signal components over link  123  to TFFE  126 . Similarly, TFFE  126  operates to substantially transmit the component associated with wavelength λ 5  over output  110  and to substantially reflect other remaining components over link  125  to TFFE  124 . TFFE  124  substantially transmits the signal component associated with wavelength λ 3  over output  108 , and reflects the final remaining component associated with wavelength λ 1  over link  127  to TFFE  122 . TFFE  122  substantially transmits this final component associated with wavelength λ 1  over output  112 . Accordingly, WDM signal components in channels associated with wavelengths λ 1 , λ 3 , λ 5 , and λ 7  are dropped from the WDM input signal at outputs  112 ,  108 ,  110  and  106 , respectively. 
     In addition to reflecting signals in channels associated with wavelengths λ 1 , λ 3 , λ 5 , and λ 7 , FBGs  132 ,  134 ,  136  and  138  may each also reflect signal components associated with adjacent channels. For example, signal drift and jitter may cause signal components at wavelengths normally at the edge of adjacent channels to overlap signals at the edges of the reflected channel. These overlapping signal components introduce adjacent channel crosstalk, which degrades the reflected signal. 
     In the embodiment of FIG. 1, adjacent channel crosstalk is reduced as a result of next-stage filtering performed by TFFEs  122 ,  124 ,  126  and  128 . FIG. 6 shows a typical FBG reflection profile  632  and a typical TFFE transmission profile  636  consistent with the embodiment of FIG.  1 . Profiles.  632  and  636  are associated with a FBG and a TFFE, respectively, that are each intended to filter a signal at a characteristic wavelength  642  of 1533.6 nanometers. This example may be easily extended to other WDM signals at a variety of characteristic wavelengths. 
     Profile  634  illustrates a shift in FBG profile  634  as the result of signal drift or jitter that causes, for example, in an increase in adjacent channel crosstalk  649  of approximately 10 dB at channel edge  648  (50 gigahertz away from characteristic wavelength  642 ). Beyond channel edge  648 , TFFE profile  636  exhibits an increasing transmission loss. For example, attenuation levels of −10 dB and greater are exhibited by TFFE profile  636  at and beyond wavelengths  647  and  646  which lie approximately 1.4 nanometers away from characteristic wavelength  642  (or approximately 100 gigahertz away from characteristic wavelength  642 ). Thus, at and beyond channel edge  648 , adjacent channel crosstalk transmitted by a TFFE exhibiting profile  636  will be attenuated. 
     WDM signals dropped by, added by or passed through WADM  100  are also subject to insertion losses. For example, insertion losses of approximately 0.2 dB are incurred by WDM signals being reflected or transmitted by one of the FBGs  132 ,  134 ,  136  and  138 . Insertion losses of approximately 1.5 dB are incurred by WDM signals being transmitted by one of the thin film filter elements  128 ,  126 ,  124  and  122 . Insertion losses of approximately 0.7 dB are incurred by WDM signals being reflected by one of the thin film filter elements  128 ,  126 ,  124  and  122 . In addition, signal losses of approximately 0.6 dB per port are incurred by circulator  130 . 
     WADM  100  of FIG. 1 is arranged to minimize the insertion loss experienced by each of the dropped WDM signals by substantially equalizing the number of filter elements each dropped WDM signal interacts with. For a given characteristic wavelength λ 1 , λ 3 , λ 5 , or λ 7 , the position of a TFFE in the series of TFFEs  128 ,  126 , 124  and  122  and the position of an associated FBG in the series of FBGs  132 ,  134 ,  136  and  138  are inverted with respect to circulator  130 . For example, signals associated with wavelength λ 1  are reflected by FBG  132  and transmitted by TFFE  122 . As a result, the dropped signal associated with wavelength λ 1  interacts with five elements (FBG  132  and thin film elements  128 ,  126 ,  124  and  122  ) between circulator  130  and output  112 . The number of interactions and approximate insertion losses for each of the WDM signals dropped by WADM  100  of FIG. 1 is shown in Table 1. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 FBGs reflect- 
                   
                   
                 Approx- 
               
               
                 Character- 
                 ing or 
                 TFFEs reflect- 
                 Total number 
                 imate 
               
               
                 istic 
                 transmitt- 
                 ing or transmit- 
                 of affecting 
                 inser- 
               
               
                 Wavelength 
                 ing signal 
                 ting signal 
                 elements 
                 tion loss 
               
               
                   
               
             
             
               
                 λ 1   
                 (132) 
                 (128, 126, 124, 
                 five 
                 5.6 dB 
               
               
                   
                   
                 122) 
               
               
                 λ 3   
                 (132, 134, 132) 
                 (128, 126, 124) 
                 six 
                 5.3 dB 
               
               
                 λ 5   
                 (132, 134, 136, 
                 (128, 126) 
                 seven 
                 5.0 dB 
               
               
                   
                 134, 132) 
               
               
                 λ 7   
                 (132, 134, 136, 
                 (128) 
                 eight 
                 4.7 dB 
               
               
                   
                 138, 136, 134, 
               
               
                   
                 132) 
               
               
                   
               
             
          
         
       
     
     In the embodiment of FIG. 1, FBGs  132 ,  134 ,  136  and  138  and TFFEs  128 ,  126 ,  124  and  122  are selected to filter channels approximately centered at wavelengths λ 1 , λ 3 , λ 5 , and λ 7 . Wavelengths λ 1 , λ 3 , λ 5 , and λ 7  are selected with between-wavelength spacing of 200 gigahertz. As illustrated in FIG. 6, this spacing is consistent with thin film filter transmission profile  636 , which has an effective −10 dB bandwidth (bounded by wavelengths  646  and  647 ) of approximately 1.6 nanometers or 200 gigahertz. Filter spacing of 200 gigahertz also helps to minimize the effects of coherent crosstalk. 
     Coherent crosstalk may arise when two or more copies of a signal are combined in one signal. In WADM  100  of FIG. 1, for example, components of the signal centered at wavelength λ 5  may be reflected by FBGs  132  and  134  before the remaining signal is fully reflected by FBG  136 . Since FBGs  132  and  134  precede FBG  136  in the signal path, any signal component reflected by FBG  132  will have a phase advanced from the signal component reflected by FBG  134 , and any signal component reflected by FBG  134  will have a phase advanced from the signal component reflected by FBG  136 . As all three components recombine before reaching filter  120 , the recombined signal with its component signal phase differences exhibits coherent crosstalk. 
     By spacing FBGs  132 ,  134 ,  136  and  138  such that adjacent FBGs have characteristic wavelengths that are 200 gigahertz apart, very little of the signal associated with one of the FBGs  132 ,  134 ,  136  and  138  is reflected by an adjacent FBG. As illustrated in FIG. 5, signals reflected by an FBG as near as 50 gigahertz to the characteristic wavelength (for example, the distance of wavelength  515  from characteristic wavelength  513 ) are reduced in power by nearly −40 dB. 
     Although FBGs  132 ,  134 ,  136  and  138  in WADM  100  of FIG. 1 have characteristic frequency spacing of 200 gigahertz, WDM signals with characteristic frequency spacing of 50 gigahertz or 100 gigahertz may be effectively filtered by combining a plurality of WADMs in sequence with filters at appropriately selected characteristic wavelengths. FIG. 8 illustrates the effect of such a combination. 
     In FIG. 8, WDM signal  802  has characteristic wavelength spacing of 50 gigahertz, and WDM signal  804  has characteristic wavelength spacing of 100 gigahertz. With 50 gigahertz spacing, for example, WDM signal  802  over spectrum  803  generates sixteen channels centered at wavelengths λ 1 , through λ 16 . Alternatively, with 100 gigahertz spacing, WDM signal  804  over spectrum  813  generates eight channels centered at wavelengths λ 2 , λ 4 , λ 6 , λ 8 , λ 10 , λ 12 , λ 14 , and λ 16 . 
     WADMs  806 ,  808 ,  810  and  812  are employed to filter signals from WDM channels associated with spectrums  803  and  813 . Each of the WADMs  806 ,  808 ,  810  and  812  incorporate filters associated with WDM channels having 200 gigahertz characteristic wavelength spacing. For example, WADM  806  incorporates filters responsive to channel spectrum  807  associated with characteristic wavelengths λ 2 , λ 6 , λ 10  and λ 12 . Collectively, WADMs  806 ,  808 ,  810  and  812  incorporate filters that are responsive to respective channel spectra  807 ,  811 ,  805  and  809  that together include channel spectrum  813  centered at wavelengths λ 2 , λ 4 , λ 6 , λ 8 , λ 10 , λ 12 , λ 14 , and λ 16  and channel spectrum  803  centered at wavelengths λ 1  through λ 16 . 
     As shown in FIG. 8, channel spectra  803  and  813  are selected to include low frequency channels in WDM signals  802  and  804 . Because cladding-mode resonance produces FBG reflectances at wavelengths below the associated characteristic wavelength (see, e.g., Raman Kashyap,  Fiber Bragg Gratings , Academic Press, 1999, pg. 159), selection of these lowest channels helps to reduce the accumulation of cladding-mode resonances. 
     In a second embodiment of the present invention related to the embodiment of FIG. 1, FIG. 2 depicts a WADM  200  configured to add optical signals associated with a maximum of four channels in a WDM signal. WDM signals associated with channels approximately centered at wavelengths λ 1 , λ 3 , λ 5 , and λ 7  are added at inputs  212 ,  208 ,  210  and  206 , respectively. TFFEs  222 ,  224 ,  226  and  228  are respectively coupled to inputs  212 ,  208 ,  210  and  206  to transmit respective signals associated with wavelengths λ 1 , λ 3 , λ 5 , and λ 7 . TFFEs  222 ,  224 ,  226  and  228  function to reflect WDM signals not associated with respective wavelengths λ 1 , λ 3 , λ 5 , and λ 7 . 
     Accordingly, a WDM signal associated with wavelength λ 1  may be transmitted by the TFFE  222  over link  227  and reflected by the TFFEs  224 ,  226  and  228  over respective links  225 ,  223  and  221  to reach optical circulator  230  via link  214 . Similarly, a WDM signal associated with wavelength λ 3  may be transmitted by the TFFE  224  over link  225  and reflected by the TFFEs  226  and  228  over respective links  223  and  221  to reach circulator  230  via link  214 . In addition, WDM signals associated with wavelengths λ 5  and λ 7  may be transmitted by TFFEs  210  and  206 , respectively. In this case, the WDM signal transmitted by TFFE  210  will be further reflected by TFFE  206 , and WDM signals associated with wavelengths λ 5  and λ 7  will both travel over links  221  and  214  to reach optical circulator  230 . 
     Optical circulator  230  is an asymmetrical circulator of the same type noted for optical circulator  130  of FIG.  1 . As an asymmetrical circulator, optical circulator  230  does not function to transport any optical signals received at output  204  to link  214 . 
     Signals reflected by TFFEs  222 ,  224 ,  226  and  228  and forwarded to optical circulator  230  are next forwarded over link  216  to FBG  232 . FBGs  232 ,  234 ,  236  and  238  are configured to reflect signals associated with channels approximately centered at wavelengths λ 1 , λ 3 , λ 5  and λ 7  respectively. Upon reaching circulator  230 , the signal associated with wavelength λ 1  is transmitted to FBG  232  over link  216 , where it is reflected back by FBG  232  over link  216  through circulator  230  to output  204 . Similarly, the signal associated with wavelength λ 3  is transmitted over links  216  and  215  through FBG  232  to FBG  234 , where it is reflected back by FBG  234  over links  215  and  216  through FBG  232  and circulator  230  to output  204 . Signals associated with wavelengths λ 5 , and λ 7  are reflected by FBGs  236  and  238 , respectively. Signals associated with wavelength λ 7  are reflected by FBG  238  over link  219  through FBG  236 . Signals associated with wavelengths λ 5 , and λ 7  are further transmitted over link  217  through FBG  234 , over link  215  through FBG  232 , and over link  216  though circulator  230  to output  204 . 
     A WDM signal may be input to the WADM  200  at input  202 . As signals associated with wavelengths λ 1 , λ 3 , λ 5 , and λ 7  are intended to be added to the WDM input signal, the WDM signal at input  202  will typically not contain any signal components in channels centered at these wavelengths. As a result, the WDM input signal will pass essentially unaltered over links  219 ,  217 ,  215  and  216  through FBGs  238 ,  236 ,  234  and  232  and through circulator  230  to output  204 . However, if signals in channels associated with wavelengths λ 1 , λ 3 , λ 5 , or λ 7  are present in the WDM signal at input  202 , these signals will be essentially reflected back to input  202  by FBGs  232 ,  234 ,  236  or  238 , respectively, and thereby removed from the WDM signal originating at input  202 . Thus, in either case, signals in channels associated with wavelengths λ 1 , λ 3 , λ 5 , and λ 7  may be effectively added to the WDM signal at output  204  via thin film filter  220 . 
     Like WADM  100  of FIG. 1, WADM  200  of FIG. 2 is arranged to minimize maximum signal insertion loss for the added signals by equalizing the number of filter elements each added signal interacts with. The number of interactions and approximate insertion losses for each of the WDM signals added by WADM  200  of FIG. 2 is shown in Table 2. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 TFFEs reflect- 
                   
                   
                 Approx- 
               
               
                 Character- 
                 ing or 
                 FBGs reflect- 
                 Total number 
                 imate 
               
               
                 istic 
                 transmitt- 
                 ing or transmit- 
                 of affecting 
                 inser- 
               
               
                 Wavelength 
                 ing signal 
                 ting signal 
                 elements 
                 tion loss 
               
               
                   
               
             
             
               
                 λ 1   
                 (222, 224, 226, 
                 (232) 
                 five 
                 5.6 dB 
               
               
                   
                 228) 
               
               
                 λ 3   
                 (224, 226, 228) 
                 (232, 234, 232) 
                 six 
                 5.3 dB 
               
               
                 λ 5   
                 (226, 228) 
                 (232, 234, 236, 
                 seven 
                 5.0 dB 
               
               
                   
                   
                 234, 232) 
               
               
                 λ 7   
                 (228) 
                 (232, 234, 236, 
                 eight 
                 4.7 dB 
               
               
                   
                   
                 238, 236 234, 
               
               
                   
                   
                 232) 
               
               
                   
               
             
          
         
       
     
     By way of comparison, signals in through channels (i.e., neither dropped nor added to the signal stream) are transmitted, for example, through the four FBGs  238 ,  236 ,  234  and  232  as well as through two ports of circulator  230  to accumulate an insertion loss of approximately 2.0 dB. 
     It will be readily apparent to one skilled in the art that the embodiments of FIGS. 1 and 2 may be altered to include a greater or lesser number of FBGs and associated TFFEs. In addition, the WADM embodiments of FIGS. 1 and 2 may be used, for example, in combination to both add signals to and remove signals from the WDM network. 
     FIG. 3 illustrates one possible arrangement of a combination WADM  300 . Combination WADM  300  includes WADM  301  for adding WDM signals at inputs  306 A,  308 A,  310 A and  312 A, and WADM  303  for dropping WDM signals at outputs  306 D,  308 D,  310 D, and  312 D. WADM  300  also includes an optical amplifier (OA)  305  interposed between WADMs  301  and  303 . Because WADMs generally interconnect fiber media spans of significant length (for example, tens of kilometers), WDM signals reaching and traveling beyond WADM  300  may require amplification prior to further processing. OA  305  is employed to amplify through signals received from a predecessor span that travel through WADM  300  on to a next optical fiber span, signals added by WADM  301  that travel on to the next span, and signals received from the predecessor span that are dropped by WADM  303 . 
     In order to minimize the number of OAs required (and thereby decrease cost), a single OA  305  is interposed between WADM  301  and WADM  303 . In this preferred configuration, through signals, added signals and dropped signals are each amplified by OA  305  at an appropriate point in their transit. However, because WDM signals are added at WADM  301  upstream from WADM  303  where WDM signals are dropped, WDM channels associated with the added signals must generally be distinct from channels associated with the dropped signals. Otherwise, channels added by WADM  301  will be immediately dropped by WADM  303 . 
     In practice, this limitation may be overcome by adding an additional WADM SU  300  downstream from output  304 . Since downstream WADM SU  300  adds WDM signals after upstream WADM  303  drops WDM signals, downstream WADM SU  300  may add signals in channels associated with signals dropped by upstream drop WADM  303 . 
     FIG. 4 illustrates a WDM network  400  that employs the WADM SU  300  of FIG.  3 . WADM network  400  is delineated by WDM terminals  402  and  404 . Optical signals are multiplexed by optical multiplexer  401  of terminal  402  to form a WDM signal that is transported over fiber optical links  413  to terminal  404 . At terminal  404 , optical demultiplexer  403  demultiplexes the WDM signal received over links  413  in order to reproduce the optical signals multiplexed at terminal  402 . In addition, terminal  404  also includes an optical multiplexer  401  that multiplexes optical signals to form a WDM signal that is transported over optical links  415  to optical multiplexer  403  in terminal  402 . In this manner, optical signals are sent in two directions over separate fiber optical links  413  and  415 . 
     Optical terminal  402  also incorporates optical amplifiers  405  and  407  to amplify WDM signals sent by optical multiplexer  401  of terminal  402  and to amplify WDM signals received for optical de-multiplexer  403  of terminal  402 . Optical amplifiers  405  and  407  of terminal  404  perform similar functions for optical multiplexer  401  of terminal  404  and optical de-multiplexer  403  of terminal  404 , respectively. 
     Optical links  413  and  415  may each span tens of kilometers, over which significant signal losses will occur. As a result, one or more optical repeaters  408  are placed at prescribed span lengths (for example, of approximately 80 kilometers) along optical links  413  and  415  in order to regenerate WDM signals. Optical repeaters  408  include optical amplifiers  406 , which operate in analogy to optical amplifiers  405  and  407  of terminals  402  and  404 . 
     One or more WADM terminals  410  may also be placed along optical links  413  and  415  to selectively add and drop WDM signals from specified WDM channels. WADM terminals  410  include WADMs  412  for each of the optical fiber links  413  and  415 . Even with optical signal regeneration at, for example, optical repeater  407 , signal to noise degradation limits the absolute number of spans that may be used with WADM terminals  410 . Experience suggests that WADM terminals  410  may be used in WDM networks  400  at OC192 data rates having six or fewer spans along links  413  and  415 . For longer spans, additional hardware is required to convert optical signals to electronic signals which may be retimed and reconverted to optical form for further transmission. 
     Transmission of WDM signals over long distance optical fiber spans at high bit rates requires use of dispersion compensating techniques to mitigate the effects of signal dispersion inherent to optical fiber transmission. For example, for OC192 transmissions over network spans of at least 60 kilometers, signals should be treated to 95 percent span loss compensation (in other words, reducing signal dispersion arising in the transmitted signal by 95 percent). In the network  400 , for example, 95 percent span loss dispersion compensation along optical link  413  is provided by introducing 35 percent pre-compensation at OA  406  in terminal  402 , 95 percent compensation at OAs  405  in repeater  408  and in WADM terminal  412 , and  60  percent post-compensation at OA  407  in terminal  404 . 
     As illustrated in FIG. 3, OA  305  provides dispersion compensation by incorporating dispersion compensating fiber (DCF)  307  within its signal path. DCF  307  introduces negative signal dispersion to compensate for positive signal dispersion arising from transmission of the WDM signal over preceding and subsequent network spans. DCFs of the type employed in DCF  307  are well-known and commercially available, for example, from JDS Uniphase and Corning. 
     Because channels are added by WADM  301  only a short distance from OA  305  and channels are dropped by WADM  303  only a short distance from OA  305 , OA  305  overcompensates for dispersion in the added channels and dropped channels. In order to reduce the effects of this overcompensation, single mode fiber  309  in WADM  301  is positioned between thin film filter  320 A and circulator  330 A to introduce additional positive dispersion in the signal paths for the added channels. Similarly, single mode fiber  311  in WADM  303  is positioned between thin film filter  320 D and circulator  330 D to introduce additional positive dispersion in the signal paths for the dropped channels. Alternatively or additionally, FBGs  332 A,  334 A,  336 A and  338 A in WADM  301  and FBGs  332 D,  334 D,  336 D and  338 D in WADM  303  may be designed to add positive dispersion to the added and dropped WDM signals, respectively. 
     For example, for OC192 signals traveling over single mode optical fiber, WADM  301  of FIG. 3 should incorporate a positive dispersion of approximately 650 picoseconds per nanometer and WADM  303  should incorporate a positive dispersion of approximately 450 picoseconds per nanometer. Of these amounts, approximately 250 picoseconds per nanometer of dispersion may be generated by the FBGs  132 ,  134 ,  136  and  138  of FIG.  1  and FBGs  232 ,  234 ,  236  and  238  of FIG. 2 directly. In order to reach desired dispersion levels, an additional 400 picoseconds per nanometer of positive dispersion may be added by single mode fiber  309  and an additional 200 picoseconds per nanometer of positive dispersion may be added by single mode fiber  311 . 
     An objective of the present invention is to be capable of adding or dropping two or more WADM channels that may each carry optical signals transmitted at either a first data rate or a second data rate. Selected attributes of the FBGs and associated TFFEs employed in the present invention are key to achieving this objective. 
     For example, FIG. 5 illustrates a reflection and transmission profile for FBGs employed in the embodiments illustrated by FIGS. 1,  2  and  3 . The FBG represented by FIG. 5 can be used, for example, to filter OC48 WDM signals transmitted at a rate of approximately 2.5 gigabits per second in channels spaced at 50 gigahertz intervals as well as OC192 WDM signals transmitted at a rate of approximately 10 gigabits per second in channels spaced at 100 gigahertz intervals. This embodiment may also be used to filter signals transmitted at lesser data rates (for example, OC-12 signals operating at 622 megabits per second and OC-3 signals operating at 155 megabits per second) 
     In FIG. 5, FBG reflection profiles  502  and  504  are shown for equivalent FBGs associated with add WADM  301  and drop WADM  303  of FIG. 3, respectively. The profiles portray the relative power of reflected signals as compared to input signal power at selected wavelengths within and near the reflection bandwidth. Similarly, FGB transmission profiles  506  and  508 , associated with add WADM  301  and drop WADM  303 , respectively, portray the relative power of transmitted signals as compared to input signal power at selected wavelengths. 
     The effective signal bandwidth for WDM signals transmitted or reflected by the FBG depicted by the profiles of FIG. 5 is demarcated by wavelengths at which the reflected or transmitted power drops by 10 dB with respect to the power of an associated input signal. Accordingly, the effective transmission bandwidth  512  in FIG. 5 is approximately 0.45 nanometers and the effective reflection bandwidth  514  is approximately 0.4 nanometers. 
     In order to function at both OC48 and OC192 signal rates, these effective bandwidths must be sufficiently narrow to avoid adjacent channel crosstalk from closely spaced channels at lower data rates (for example, OC48 channels that are nominally spaced at approximately 0.4 nanometers). In addition, the bandwidths must be wide enough to capture a sufficient portion of signals at higher data rates (for example, OC192 signal carried in channels that are nominally spaced at approximately 0.8 nanometers). 
     An appropriate FBG bandwidth can be selected by analyzing signal power penalties for both signal rates at various effective FBG bandwidths. The use of power penalties in the analysis of signal quality is well-known in the art (see, e.g., Harry J. R. Dutton,  Understanding Optical Communications , Prentice-Hall, 1998, pp. 403, 404). Common measures of signal quality include signal to noise ration and inter-symbol interference. 
     FIGS. 7A and 7B illustrate FBG power penalties for OC48 and OC192 signal rates, respectively, at various effective bandwidths. The power penalties are influenced by various transmission impairments present in the WDM signal as it is input to the FBG. These input signal impairments may be characterized by self phase modulation/ cross phase modulation (SPM/XPM) penalty, a pre-FBG power penalty measure with respect to inter-symbol interference. SPM/XPM penalty is influenced, for example, by a variety of WDM system attributes including dispersion characteristics, system architecture, signal chirp and signal power. As illustrated in FIGS. 7A and 7B, FBG power penalty varies non-linearly with SPM/XPM penalty. 
     In FIGS. 7A and 7B, FBG power penalty is shown as a function of effective bandwidth and SPM/XPM penalty. The SPM/XPM penalty present in the input signal ranges from no penalty (“no chirp”) to a penalty of 2.0 dB. Increasing FBG power penalties shown in FIG. 7A for OC48 signal rates reflect the effects of cross talk from neighboring channels at bandwidth upper boundaries and the effects of loss of signal spectrum at the lower boundaries. Similarly, increasing FBG penalties shown in FIG. 7B for OC192 signal rates reflect the effects of loss of signal spectrum at the lower boundaries. 
     Assuming a SPM/XPM penalty of 2.0 dB, an effective bandwidth  722  of approximately 0.38 nanometers appears to minimize the overall power penalty at both OC48 and OC192 signal rates. Since OC192 signals tend to accumulate higher SPM/XPM penalties in a given WDM network than OC48 signals and FBGs may experience significant drift and jitter, our experience suggests that a somewhat larger effective bandwidth  724  of about 0.45 nanometers (approximately 54 gigahertz) provides better overall performance. 
     For OC192 signal rates, signal dispersion becomes a critical issue. As illustrated in FIG.  3  and as previously discussed, dispersion for signals introduced at add WADM  301  and for signals dropped at drop WADM  303  may be nominally adjusted by a variety of means. However, treating nominal conditions alone is insufficient, as FBGs typically exhibit a strongly varying dispersion over their reflection bandwidth. Our experience shows, for example, that dispersion increases dramatically at edge wavelengths as SPM/XPM penalties increase. 
     The effects of these variations must be appropriately limited. FIG. 9 presents a dispersion template with appropriate limits to satisfy requirements for the present invention. For an OC192 signal reflected by an FBG with an effective bandwidth of about 0.45 nanometers, the template graphs allowable limits in dispersion deviation from the nominal value over that bandwidth such that FBG power penalty (including the associated SPM/XPM penalty) is no greater than 2.0 dB. 
     Allowable dispersion limits are shown by limits  910 . Limits  910  define a region  908  applicable to reflected wavelengths within 0.05 nanometers of FBG characteristic wavelength  904 . Within region,  908 , dispersion may vary by no more than 150 picoseconds per nanometer from a nominal FBG dispersion value  902  (for example, 250 picoseconds per nanometer). For reflected wavelengths beyond region  908 , the limit of 150 picoseconds per nanometer may increase from the edges  907  and  909  of region  908  at a rate no greater than 20,000 picoseconds per square nanometer. 
     FBG dispersion variation may in fact increase over time as a result of various FBG aging effects. In order to maintain performance within the boundaries of limits  910 , guard band limits  912  may be established for newly-manufactured FBGs. In the example shown in FIG. 9, guard band limits  912  define a region  906  applicable to reflected wavelengths within 0.1 nanometers of center wavelength  904 . Within region  906 , dispersion may vary by no more than 150 picoseconds per nanometer from nominal dispersion value  902 . For reflected wavelengths beyond region  906 , the limit of 150 picoseconds per nanometer may increase from the edges  903  and  905  of region  906  at a rate no greater than 20,000 picoseconds per square nanometer. Various other guard band limits may be established according to actual experience with FBG aging effects. 
     FIG. 10 illustrates some sample dispersion profiles for FBGs that comply with the dispersion template of FIG.  9 . 
     The exemplary embodiment described above is but one of a number of alternative embodiments of the invention that will be apparent to those skilled in the art in view of the foregoing description. Accordingly, this description is to be construed as illustrative only, and is for the purpose of teaching those skilled in the art the best mode of carrying out the invention. It is therefore to be understood that changes may be made in the particular embodiments of the invention which are within the scope and spirit of the invention as outlined by the appended claims.