Abstract:
An optical drop structure comprising a multi-port optical circulator (MOC), a first reflection filter unit optically connected in series between a first port and a second port of the MOC, a second reflection filter unit optically connected in series between a third port and a fourth port of the MOC, and the optical drop structure is arranged, in use, in a manner such that, a first optical signal entering through a fifth port of the MOC is subjected to the first reflection filter unit and exits at a sixth port of the MOC and a reflected portion of the first optical signal exits at a seventh port of the MOC, and a second optical signal entering through the sixth port of the MOC is subjected to the second reflection filter unit and exits at the fifth port of the MOC and a reflected portion of the second optical signal exits at an eights port of the MOC.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention relates broadly to an add/drop structures for use in e.g. single-fibre bi-directional wavelength division multiplexing (WDM) ring networks.  
       BACKGROUND OF THE INVENTION  
       [0002]     There is a trend in the design of bi-directional WDM ring networks that optical signals travel along single-fibre connections in both directions. Typically, different groups of wavelengths propagate in opposite directions. The allocation of propagation directions to individual wavelengths may e.g. be interleaved in the spectral domain, i.e. adjacent wavelengths within the spectral domain propagate in opposite directions, resulting in symmetric traffic conditions.  
         [0003]     Bi-directional optical add/drop structures may be implemented with an N×N array waveguide grating in-line in the single fibre bi-directional link. However, such adding/dropping of wavelengths in a bi-directional mode can have the disadvantage of increased likelihood of cross talk between the different channels, i.e. wavelength signals, as the adding/dropping is performed on the bi-directional optical signal covering the entire bandwidth used on the single-fibre connection.  
         [0004]     In at least preferred embodiments, the present invention seeks to provide an optical add/drop structure with reduced likelihood of cross talk between the different channels.  
       SUMMARY OF THE INVENTION  
       [0005]     In the summary of invention and the claims components of the same name have been identified as e.g. “first”, “second”, “third” etc. This is intended to mean “first identified”, “second identified”, “third identified” etc. rather than being intended to define a total number of the same components in individual embodiments of the invention. For example, where an embodiment is defined with MOCs having a first, a second and a fifth port, this does not define that there must be a third and a fourth port. In other words, in such an embodiment each MOC has at least 3 ports.  
         [0006]     In accordance with a first aspect of the present invention, there is provided an optical drop structure comprising: 
        a multi-port optical circulator (MOC);     a first reflection filter unit optically connected in series between a first port and a second port of the MOC,     a second reflection filter unit optically connected in series between a third port and a fourth port of the MOC,     and the optical add/drop structure is arranged, in use, in a manner such that:     a first optical signal entering through a fifth port of the MOC is subjected to the first reflection filter unit and exits at a sixth port of the MOC and a reflected portion of the first optical signal exits at a seventh port of the MOC, and     a second optical signal entering through the sixth port of the MOC is subjected to the second reflection filter unit and exits at the fifth port of the MOC and a reflected portion of the second optical signal exits at an eights port of the MOC.        
 
         [0013]     Accordingly, the present invention can provide a bi-directional optical drop structure with reduced likelihood of crosstalk between different channels by implementing the dropping filtering in a uni-directional mode.  
         [0014]     Preferably, the optical drop structure is arranged, in use, in a manner such that a third optical signal entering at an ninth port of the MOC is added to the first optical signal prior to the first optical signal exiting the MOC, and such that a fourth optical signal entering at a tenth port of the MOC is added to the second optical signal prior to the second optical signal exiting the MOC.  
         [0015]     In one embodiment, the optical drop structure is arranged, in use, such that the third and fourth optical signals are reflected at the first and the second reflection filter units respectively for being added to the first and second optical signals respectively.  
         [0016]     Accordingly, the present invention can provide a bi-directional optical add/drop structure with reduced likelihood of crosstalk between different channels by implementing the adding/dropping filtering in a uni-directional mode.  
         [0017]     Advantageously, the optical drop structure further comprises: 
        third and fourth reflection filter units optically connected to an eleventh port and a twelfth port of the MOC respectively, and     wherein the optical drop structure is arranged, in use, in a manner such that:     the first optical signal is filtered at the third reflection filter unit prior to exiting the MOC, and     the second optical signal is filtered at the fourth reflection filter unit prior to exiting the MOC.        
 
         [0022]     Preferably, the optical drop structure further comprises a bi-directional amplifier structure disposed in series between a thirteenth port and a fourteenth port of the MOC, and the optical drop structure is arranged, in use, in a manner such that the first and second optical signals are amplified in a bi-directional amplifier structure prior to exiting the MOC.  
         [0023]     In one embodiment, the amplifier unit comprises a gain medium or a semiconductor amplifier or a Raman amplifier. The amplifier unit may comprise a gain medium, the amplifier structure comprises at least one pump laser coupled to the gain medium. The pump laser may be coupled to the gain medium via a wavelength coupler. The gain medium may comprise an active optical fibre or an active planar waveguide. The active optical fibre may comprise Erbium-doped fibre or rare earth doped fibre.  
         [0024]     Advantageously, the optical drop structure is arranged, in use, in a manner such that the first and second optical signals are subjected to the first and second reflection filter units prior to being subjected to the first and second reflection filter units respectively.  
         [0025]     Preferably, the optical drop structure is arranged, in use, in a manner such that the first and second optical signals are amplified prior to or after being filtered at the third and fourth reflection filter units respectively. In such an embodiment, the optical drop structure may further comprise: 
        fifth and sixth reflection filter units optically connected to a fifteenth port and a sixteenth port of the MOC respectively, and     wherein the optical drop structure is arranged, in use, in a manner such that the first and second amplified signals are filtered at the fifth and sixth reflection filter units respectively, prior to exiting the MOC.        
 
         [0028]     In one embodiment, the optical structure is implemented with two MOCs interconnected in series. The two MOCs may have opposite circulation directions.  
         [0029]     Advantageously, any one or all of the reflection filter units comprises a fibre Bragg grating structure or a Fabry-Perot filter.  
         [0030]     In accordance with a second aspect of the present invention, there is provided an optical add structure comprising: 
        a multi-port optical circulator (MOC);     a first reflection filter unit optically connected in series between a first port and a second port of the MOC,     a second reflection filter unit optically connected in series between a third port and a fourth port of the MOC,     and the optical add/drop structure is arranged, in use, in a manner such that:     a first optical signal entering through a fifth port of the MOC is subjected to the first reflection filter unit and exits at a sixth port of the MOC,     a second optical signal entering through the sixth port of the MOC is subjected to the second reflection filter unit and exits at the fifth port of the MOC,     a third optical signal entering at a seventh port of the MOC is reflected at the first reflection filter unit and exits at the sixth port of the MOC for adding to the first optical signal, and     a fourth optical signal entering at an eighth port of the MOC is reflected at the second reflection filter unit and exits at the fifth port of the MOC for adding to the second optical signal.        
 
         [0039]     In accordance with a third aspect of the present invention there is provided a method of adding/dropping signal portions from a bi-directional optical transmission path, the method comprising the steps of utilising at least one MOC to distinguish between first and second optical signals having opposite transmission directions on the transmission path, subjecting the first and second optical signals to first and second filtering units for adding/dropping said signal portions in a uni-directional mode, and utilising said at least one MOC to direct the first and second signal portions for continued propagation along the transmission path in their respective transmission directions.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]      FIG. 1  is a schematic drawing of a bi-directional optical add/drop structure embodying the present invention;  
         [0041]      FIG. 2  is a schematic drawing of a bi-directional optical add/drop structure, with amplification, embodying the present invention;  
         [0042]      FIG. 3  is a schematic drawing of a bi-directional optical add/drop structure, with amplification, embodying the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0043]     Preferred embodiments described provide a bi-directional optical add/drop structure with reduced likelihood of cross talk between different channels by implementing the adding/dropping filtering in a uni-directional mode.  
         [0044]      FIG. 1  shows a bi-directional optical add/drop structure  300  embodying the present invention. The add/drop structure  300  comprises a 12 port MOC  302  connected in line with a bi-directional single fibre connection  304 . A fibre Bragg grating  306  is connected to port  308  of the MOC  302  in series with an optical isolator  310 . Another fibre Bragg grating  312  is connected to port  314  of MOC  302  in series with another optical isolator  316 .  
         [0045]     Yet another fibre Bragg grating  318  is connected in series between ports  320  and  322  of MOC  302 , and a further fibre Bragg grating  324  is connected in series between ports  326  and  328  of MOC  302 .  
         [0046]     A bi-directional optical signal is travelling along the single-fibre connection  304 , with certain wavelengths in a set of wavelengths A and wavelengths in a set of wavelengths B travelling in opposing directions. In the following, it will be described how optical signals travel through the bi-directional add/drop structure  300  in each direction.  
         [0047]     An optical signal entering the MOC  302  at port  330  is initially filtered at fibre Bragg grating  306 . The fibre Bragg grating  306  is designed such that only the particular wavelengths intended for propagation in that propagation direction are reflected back into the MOC  302 , whilst unwanted signal is transmitted. Re-entering of the removed signal as a result of e.g. reflection events is inhibited by isolator  310 .  
         [0048]     The filtered signal then exits at port  320  and is filtered at the fibre Bragg grating  318 . Light that is not reflected at the fibre Bragg grating  318  re-enters the MOC  302  at port  322  and exits at port  332  for continued propagation along the bi-directional single-fibre connection  304 .  
         [0049]     A particular wavelength that is being reflected at the fibre Bragg grating  318  depending on the design of that fibre Bragg grating  318  re-enters the MOC  302  at port  320  and subsequently exits at port  334 . In other words, that particular wavelength is dropped from the optical signal.  
         [0050]     At the same time, an optical signal of the same wavelength(s) that is/are dropped at port  334  may enter at port  336  of MOC  302 . That signal will exit at port  322 , however, it will be reflected by the fibre Bragg grating  318  designed for that particular wavelength(s) (see above), thus re-entering MOC  302  at port  322 . Effectively, the additional signal is thus added to the optical signal and exits the MOC  302  through port  332  for continued propagation along the bi-directional single-fibre connection  304 .  
         [0051]     Similarly, an optical signal entering the MOC  302  at port  332  is filtered at fibre Bragg grating  312  designed to reflect only the particular wavelengths intended for propagation in that particular direction. Unwanted optical signals are transmitted and re-entering into the MOC  302  as a result of e.g. reflection events is inhibited by isolator  316 .  
         [0052]     The filtered signal exits at port  328  and is filtered at fibre Bragg grating  324 . The portion of the signal that is not reflected at the fibre Bragg grating  324  re-enters the MOC  302  at port  326  and exits at port  330  for continued propagation along the bi-directional single-fibre connection  304 .  
         [0053]     At the same time, a portion of the optical signal i.e. a particular wavelength reflected by the fibre Bragg grating  324  re-enters the MOC  302  at port  328  and subsequently exits at port  338 . Thus, that particular wavelength has been dropped from the optical signal.  
         [0054]     An optical signal of the same wavelength(s) may enter the MOC  302  at port  340 . That signal exits at port  326  but is reflected at fibre Bragg grating  324 , which has been designed at that particular wavelength(s) which are dropped at port  338  (see above). The reflected signal thus re-enters at port  326  and is effectively combined with the remainder of the optical signal which is transmitted through the fibre Bragg grating  324 , and exits the MOC  302  at port  330  for continued propagation along the bi-directional single-fibre connection  304 .  
         [0055]      FIG. 2  shows a bi-directional optical add/drop structure  400 , with amplification embodying the present invention. The add/drop structure  400  comprises a 16 port MOC  402  connected in line with a bi-directional single fibre connection  404 . A fibre Bragg grating  406  is connected to port  408  of the MOC  402  in series with an optical isolator  410 . Another fibre Bragg grating  412  is connected to port  414  of MOC  402  in series with another optical isolator  416 .  
         [0056]     Yet another fibre Bragg grating  418  is connected in series between ports  420  and  422  of MOC  402 , and a further fibre Bragg grating  424  is connected in series between ports  426  and  428  of MOC  402 .  
         [0057]     A further fibre grating  500  is connected to port  502  of MOC  402 , in series with another optical isolator  504 . Yet a further fibre Bragg grating  506  is connected to port  508  of MOC  402  in series with optical isolator  510 .  
         [0058]     A bi-directional optical signal is travelling along the single-fibre connection  404 , with certain wavelengths in a set of wavelengths A and wavelengths in a set of wavelengths B travelling in opposing directions. In the following, it will be described how optical signals travel through the bi-directional add/drop amplifier structure  400  in each direction.  
         [0059]     An optical signal entering the MOC  402  at port  430  is initially filtered at fibre Bragg grating  406 . The fibre Bragg grating  406  is designed such that only the particular wavelengths intended for propagation in that propagation direction are reflected back into the MOC  402 , whilst unwanted signal is transmitted. Re-entering of the transmitted signal as a result of e.g. reflection events is inhibited by isolator  410 .  
         [0060]     The filtered signal then exits at port  514  and is amplified in a bi-directional optical amplifier structure  512  connected between ports  514  and  516  of the MOC  402 . The amplified signal re-enters the MOC  402  at port  516 .  
         [0061]     The amplified signal is then filtered again at the fibre Bragg grating  506  to remove unwanted optical signal, such as out-of-band amplified spontaneous emission noise. Re-entering of the removed signal as a result of e.g. reflection events is inhibited by isolator  510 .  
         [0062]     The reflected (desired) signal is next filtered at the fibre Bragg grating  418 . Light that is not reflected at the fibre Bragg grating  418  re-enters the MOC  402  at port  422  and exits at port  432  for continued propagation along the bi-directional single-fibre connection  404 .  
         [0063]     A particular wavelength that is being reflected at the fibre Bragg grating  418  depending on the design of that fibre Bragg grating  418  re-enters the MOC  402  at port  420  and subsequently exits at port  434 . In other words, that particular wavelength is dropped from the optical signal.  
         [0064]     At the same time, an optical signal of the same wavelength(s) that is/are dropped at port  434  may enter at port  436  of MOC  402 . That signal will exit at port  422 , however, it will be reflected by the fibre Bragg grating  418  designed for that particular wavelength(s) (see above), thus re-entering MOC  402  at port  422 . Effectively, the additional signal is thus added to the optical signal and exits the MOC  402  through port  432  for continued propagation along the bi-directional single-fibre connection  404 .  
         [0065]     Similarly, an optical signal entering the MOC  402  at port  432  is filtered at fibre Bragg grating  412  designed to reflect only the particular wavelengths intended for propagation in that particular direction. Unwanted optical signals are transmitted and re-entering into the MOC  402  as a result of e.g. reflection events is inhibited by isolator  416 .  
         [0066]     The filtered signal exits at port  516  and is amplified in the bi-directional amplifier structure  512  before re-entering MOC  402  at port  514 . The amplified signal is then filtered at fibre Bragg grating  500  and unwanted signal such as out-of-band amplified spontaneous emission noise is removed. Re-entering of the unwanted optical signal into the MOC  402  as a result of e.g. reflection events is inhibited by isolator  504 .  
         [0067]     The reflected amplified signal is next filtered at fibre Bragg grating  424 . The portion of the signal that is not reflected at the fibre Bragg grating  424  re-enters the MOC  402  a port  428  and exits at port  430  for continued propagation along the bi-directional single-fibre connection  304 .  
         [0068]     At the same time, a portion of the optical signal i.e. a particular wavelength reflected by the fibre Bragg grating  424  re-enters the MOC  402  at port  426  and subsequently exits at port  438 . Thus, that particular wavelength has been dropped from the optical signal.  
         [0069]     An optical signal of the same wavelength(s) may enter the MOC  402  at port  440 . That signal exits at port  428  but is reflected at fibre Bragg grating  424 , which has been designed at that particular wavelength(s) which are dropped at port  438  (see above). The reflected signal thus re-enters at port  428  and is effectively combined with the remainder of the optical signal which is transmitted through the fibre Bragg grating  424 , and exits the MOC  402  at port  430  for continued propagation along the bi-directional single-fibre connection  404 .  
         [0070]     Turning now to  FIG. 3 , in a bi-directional add/drop structure  200 , with amplification, embodying the present invention, a gain medium in the form of an Erbium doped fibre  202  is connected between ports  204 ,  206  of two MOCs  208 ,  210  respectively. Two pump lasers  236 ,  238  are coupled to the Erbium doped fibre  202  by way of wavelength couplers  240 ,  242  respectively.  
         [0071]     A fibre Bragg grating  212  is connected to port  214  of the MOC  208  in series with an optical isolator  216 . Similarly, another fibre Bragg grating  218  is connected to port  220  of MOC  210  in series with an optical isolator  222 .  
         [0072]     A further fibre Bragg grating  224  is connected in series between ports  226  and  228  of MOC  208 . Another fibre Bragg grating  230  is also connected in series between ports  232  and  234  of MOC  210 .  
         [0073]     A bi-directional optical signal is travelling along the single-fibre connection  244 , with certain wavelengths in a set of wavelengths A and certain wavelengths in a set of wavelengths B travelling in opposing directions. In the following, it will be described how optical signals travel through the bi-directional add/drop amplifier structure  200  in each direction.  
         [0074]     An optical signal entering the MOC  208  at port  246  is firstly filtered at fibre Bragg grating  212 . The fibre Bragg grating  212  is designed such that only the particular wavelengths intended for propagation in that propagation direction are reflected back into the MOC  208 , whilst unwanted signal is transmitted. Re-entering of the transmitted signal as a result of e.g. reflection events is inhibited by isolator  216 .  
         [0075]     The filtered signal exits at port  204  and is then amplified in the Erbium doped fibre  202  prior to entering MOC  210  at port  206 . The amplified signal then exits at port  232  and is filtered at the fibre Bragg grating  230 . Light that is not reflected at the fibre Bragg grating  230  re-enters the MOC  210  at port  234  and exits at port  248  for continued propagation along the bi-directional single-fibre connection  244 .  
         [0076]     A particular wavelength that is being reflected at the fibre Bragg grating  230  depending on the design of that fibre Bragg grating  230  re-enters the MOC  210  at port  232  and subsequently exits at port  250 . In other words, that particular wavelength is dropped from the amplified signal.  
         [0077]     At the same time, an optical signal of the same wavelength(s) that is/are dropped at port  250  may enter at port  252  of MOC  210 . That signal will exit at port  234 , however, it will be reflected by the fibre Bragg grating  230  designed for that particular wavelength(s) (see above), thus re-entering MOC  210  at port  234 . Effectively, the additional signal is thus added to the amplified optical signal and exits the MOC  210  through port  248  for further propagation along the bi-directional single-fibre connection  244 .  
         [0078]     Similarly, an optical signal entering MOC  210  at port  248  is filtered at fibre Bragg grating  218  designed to reflect only the particular wavelengths intended for propagation in that particular direction. Unwanted optical signals are transmitted and their re-entering into the MOC  210  as a result of reflection events is inhibited by isolator  222 .  
         [0079]     The filtered signal exits at port  206  and is then amplified in the Erbium doped fibre  202  prior to entering MOC  208  at port  204 . The amplified signal exits at port  226  and is filtered at fibre Bragg grating  224 . The portion of the amplified signal that is not reflected at the fibre Bragg grating  224  re-enters the MOC  208  at port  228  and exits at port  246  for continued propagation along the bi-directional single-fibre connection  244 .  
         [0080]     At the same time, a portion of the amplified signal i.e. a particular wavelength reflected by the fibre Bragg grating  224  re-enters the MOC  208  at port  226  and subsequently exits at port  254 . Thus, that particular wavelength has been dropped from the amplified signal. An optical signal of the same wavelength(s) may enter the MOC  208  at port  256 . That signal exits at port  228  but is reflected at fibre Bragg grating  224 , which has been designed to reflect at that particular wavelength(s) which are dropped at port  254  (see above). The reflected signal thus re-enters at port  228  and is effectively combined with the remainder of the amplified optical signal which is transmitted through the fibre Bragg grating  224 , and exits the MOC  208  at port  246  for continued propagation along the bi-directional single-fibre connection  244 .  
         [0081]     It will be appreciated by the person skilled in the art that numerous modifications and/or variations may be made to the present invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.  
         [0082]     In the claims that follow and in the summary of the invention, except where the context requires otherwise due to express language or necessary implication the word “comprising” is used in the sense of “including”, i.e. the features specified may be associated with further features in various embodiments of the invention.