Abstract:
The present invention is generally directed to integrated optical add/drop multiplexer devices in which light of a specific wavelength (or specific wavelengths) can be added to or dropped from a main fiber optic line. An actuation mechanism (e.g., heat) is used to tune and/or trim the devices. Some proposed architectures provide for tuning of the adding and dropping of channels in a hitless manner, a manner that is non-disruptive to other wavelength channels.

Description:
PRIORITY INFORMATION 
   This application claims priority from a continuation of Ser. No. 10/075,138, filed Feb. 14, 2002, is now abandoned, and from provisional application Ser. No. 60/274,976 filed Mar. 12, 2002. 

   BACKGROUND OF THE INVENTION 
   The invention relates to the field of optical components. 
   One method of increasing the transportable bandwidth in optical communications networks is a technique known as wavelength division multiplexing (WDM). WDM is a technology that combines two or more wavelengths of light for transmission along a single optical waveguide. Each wavelength represents a channel that can carry a bit stream, i.e. content. Wavelength and channel are used herein interchangeably. Transporting two or more wavelengths on a waveguide effectively increases the aggregate bandwidth of the waveguide. For example, if 40 wavelengths, each capable of 10 Gb/s are used on a single fiber, the aggregate bandwidth of the fiber becomes 400 Gb/s. 
   A similar manner of increasing transportable bandwidth has been termed dense wavelength division multiplexing (DWDM). DWDM generally involves combining a larger number of wavelengths onto a fiber than WDM. While DWDM deals with more difficult issues associated with multiplexing a larger number of wavelengths on a fiber, such as cross-talk and non-linear effects, WDM and DWDM are typically used interchangeably. 
   A number of optical components are used in WDM networks, such as optical multiplexers (MUX), optical demultiplexers (DEMUX), optical add/drop multiplexers (OADM), wavelength selective switches (WSS) and optical cross connects (OXC). A MUX takes different channels from different waveguides and combines them as a WDM signal into one waveguide. A DEMUX divides a WDM signal received from a waveguide into its different channels and couples each channel into a different waveguide. An OADM selectively removes a subset of the total channels from a WDM signal and selectively adds in the same subset of the total channels with different content. A WSS selectively switches the contents of a subset of the total channels between WDM signals, i.e. amongst L WDM signals that have N channels, the contents of any M of the N channels are selectively switched. An OXC performs the same function as a WSS, except that all N channels are switchable, i.e. amongst L WDM signals that have N channels, the contents of the N channels can be selectively switched. 
   When one of these optical components can be tuned to operate on different channels by software control, or otherwise, it is generally referred to as being dynamic. If a dynamic optical component can be tuned from operating on a source channel (e.g., channel A) to operating on a destination channel (e.g., channel C) without dropping, switching or otherwise removing intermediate channels (e.g., channel B) from the WDM signal, the component is generally referred to as being hitless. Further, if the bit-error-rate (BER) of intermediate channels is not significantly affected during tuning, then the optical component is generally referred to as being errorless. Lastly, if the optical component contains trimmable elements that allow the component to be fine-tuned for optimal operation, then it is generally referred to as being trimmable. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention provides a dynamic optical add/drop multiplexer comprising a first optical circulator, a second optical circulator and a tunable reflective filter formed on a single substrate. The first optical circulator has a first port to receive a multiple wavelength optical signal, a second port to output the received multiple wavelength optical signal, and a third port. The tunable reflective filter is connected to the second port of the first circulator to receive the optical signal. The filter segregates a tuned wavelength from the optical signal and reflects the tuned wavelength back to the first circulator, which outputs the tuned wavelength via the third port. The second optical circulator has a first port to receive the optical signal from the filter, a second port to receive an add optical signal comprising the same wavelength as the tuned wavelength, and a third port to output the multiple wavelength optical signal and the add optical signal. 
   Another aspect of the present invention provides a trimmable Mach-Zehnder Interferometer (MZI) based dynamic optical add/drop multiplexer. The multiplexer has a first optical path with a tunable reflective filter formed therein and a second optical path having a tunable reflective filter formed therein. The first optical path has a first phase shifter associated with it. The second optical path has a second phase shifter associated with the second optical path. A first 3 dB coupler has a first port to receive a multiple wavelength optical signal and substantially evenly splits the signal into the first optical path and the second optical path. The tunable reflective filters of the first and second paths segregate a tuned wavelength from the split optical signal and reflect the tuned wavelength back to the first 3 dB coupler, which outputs the tuned wavelength via a second port. A second 3 dB coupler receives the split optical signal from the first and second optical paths and combines the split signal into a single signal that outputs via a first port. The second 3 dB coupler also has a second port to receive an add optical signal comprising the same wavelength as the tuned wavelength wherein the add optical signal is additionally output via the first port. The phase shifters balance the optical lengths of the first and second optical path. 
   Another aspect of the present invention provides an optical device comprising a first directive coupler, a tunable reflective filter, a second directive coupler and a switch. The first directive coupler has a first port to receive a multiple wavelength optical signal, a second port to output the received multiple wavelength optical signal, and a third port. The tunable reflective filter is connected to the second port of the first directive coupler to receive the optical signal. The filter segregates a tuned wavelength from the optical signal and reflects the tuned wavelength back to the first directive coupler, which outputs the tuned wavelength via the third port. The second directive coupler has a first port to receive the optical signal from the filter, a second port to output the multiple wavelength optical signal, and a third port. Any optical signals that input the third port, output the first port. The switch has an input port connected to the third port of the first directive coupler to receive the tuned wavelength, a first output port connected to the third port of the second directive coupler, and a second output port. The switch in a first state causes the tuned wavelength to be output to the third port of the second directive coupler via the first output port of the switch. The switch in a second state causes the tuned wavelength to be output via the second output port of the switch. 
   Another aspect of the present invention provides an optical device comprising first, second, third and fourth directive couplers and a cross bar switch. The first and second directive couplers have at least one tunable reflective filter connected therebetween. The first directive coupler has a first port to receive a multiple wavelength optical signal, a second port to output the received multiple wavelength optical signal to the at least on filter, and a third port. The filter segregates a tuned wavelength from the optical signal and reflects the tuned wavelength back to the first directive coupler, which outputs the tuned wavelength via the third port. The second directive coupler has a first port to receive the optical signal from the filter, a second port to output the multiple wavelength optical signal, and a third port, wherein any optical signals that input the third, output the first port. Similarly the third and fourth directive couplers have at least one tunable reflective filter connected therebetween. The third directive coupler has a first port to receive a multiple wavelength optical signal, a second port to output the received multiple wavelength optical signal to the at least on filter, and a third port. The filter segregates at least one tuned wavelength from the optical signal and reflects the tuned wavelength back to the third directive coupler, which outputs the tuned wavelength via the third port. The fourth directive coupler having a first port to receive the optical signal from the filter, a second port to output the multiple wavelength optical signal, and a third port, wherein any optical signals that input the third port, output the first port. The cross bar switch in a first state connects the third port of the first directive coupler to the third port of the fourth directive coupler and connects the third port of the third directive coupler to the third port of the second directive coupler. In a second state, the cross bar switch connects the third port of the first directive coupler to the third port of the second directive coupler and connects the third port of the third directive coupler to the third port of the fourth directive coupler. 
   Another aspect of the present invention provides a hitless errorless dynamic optical add/drop multiplexer. A first optical circulator has a first port to receive a multiple wavelength optical signal, a second port to output the received multiple wavelength optical signal, and a third port. A first switch has an input port, a first output port, and a second output, the input port is connected to the second port of the first circulator. A filter path comprising a tunable reflective filter is connected to the first output port of the first switch. A bypass path is connected to the second output port of the first switch. A second switch has a first input port connected to the filter path, a second input port connected to the bypass path, and an output port. In a first state, the first and second switch cause the optical signal to be directed along the filter path wherein the reflective filter segregates a tuned wavelength from the optical signal and reflects the tuned wavelength back to the first circulator, which outputs the tuned wavelength via the third port. In a second state, the first and second switch cause the optical signal to be directed along the bypass path which leaves the optical signal substantially unaffected. A second optical circulator has a first port connected to the output port of the second switch to receive the optical signal from the second switch, a second port to output the optical signal and a third port to receive an add optical signal, the add optical signal comprising the same wavelength as the tuned wavelength. The add optical signal is output to the second port of the second optical circulator with the optical signal. 
   Another aspect of the present invention provides a hitless errorless dynamic optical add/drop multiplexer. A first optical path has a tunable reflective filter formed therein and a second optical path has a tunable reflective filter formed therein. A first switch has an input port to receive a multiple wavelength optical signal, a first output port to output the optical signal when the first switch is in a first state, and a second output port to output the optical signal when the first switch is in a second state. A first 3 dB coupler has a first port connected to the second output port of the first switch to receive the multiple wavelength optical signal and substantially evenly split the signal into the first optical path and the second optical path. The tunable reflective filters of the first and second paths segregate a tuned wavelength from the split optical signal and reflect the tuned wavelength back to the first 3 dB coupler, which outputs the tuned wavelength via a second port. A second 3 dB coupler receives the split optical signal from the first and second optical paths and combines the split signal into a single signal that outputs a first port. The second 3 dB coupler has a second port to receive an add optical signal comprising the same wavelength as the tuned wavelength. The add optical signal is additionally output to the first port. A second switch has a first input port connected to the first port of the second 3 dB coupler to receive the optical signals, a second input port connected to the second output port of the first switch, and an output port to output optical signals from the second 3 dB coupler when the second switch is in a first state and to output optical signals from the bypass optical path when the second switch is in a second state. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIGS. 1 and 2   a–c  illustrate single-channel dynamic optical OADMs according to the principles of the present invention. 
       FIGS. 3   a–c  and  4   a–b  illustrate single-channel hitless dynamic optical add/drop multiplexers according to the principles of the present invention. 
       FIGS. 5   a–b  and  6  illustrate multi-channel hitless dynamic optical add/drop multiplexers according to the principles of the present invention. 
       FIGS. 7   a–b  and  8  illustrate hitless dynamic demultiplexers according to the present invention. 
       FIGS. 9   a – 9   c  and  10   a – 10   c  illustrate M-channel Hitless Dynamic wavelength selective switches according to the principles of the present invention. 
       FIGS. 11   a–b  and  12   a–b  illustrate single-channel errorless hitless dynamic optical add/drop multiplexers according to the present invention. 
       FIGS. 13   a–b  and  14  illustrate multi-channel errorless hitless dynamic demultiplexers according to the present invention. 
       FIG. 15  illustrates a polarization independent errorless hitless dynamic demultiplexer according to the present invention. 
       FIG. 16  illustrates a multi-stage polarization independent errorless hitless dynamic demultiplexer according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2   a–c  illustrate single-channel dynamic optical OADMs according to the principles of the present invention. As previously described, generally a dynamic OADM can be selectively tuned to remove a wavelength from a WDM signal and add in the same wavelength with different content. 
     FIG. 1  illustrates a dynamic OADM  100  of the present invention comprising a tunable reflective filter  106  and 3-port optical circulators  102  and  104  integrated on the same substrate  108 . Tunable reflective filter  106  is, for example, a tunable Bragg grating. Integrated circulators  102  and  104  are based, for example, on planar MZIs using either polarization splitting and nonreciprocal polarization conversion, or nonreciprocal phase shift within the interferometric arms. 
   Filter  106  has one end connected to the third port  118  of circulator  102  and its other end connected to the first port  120  of circulator  104 . First port  110  of circulator  102  is the In port of dynamic OADM  100 , while third port  116  of circulator  104  is the Pass port. The second port  112  of circulator  102  and the second port  114  of circulator  104  are, respectively, the Drop port and the Add port of dynamic OADM  100 . 
   During operation, a WDM signal comprising a plurality of channels (e.g., channel A, B, C and D) is input via the In port  110  of OADM  100 . Circulator  102  outputs this signal via its third port  118  to filter  106 . Filter  106  reflects the channel to which it is tuned (e.g., channel A) back to circulator  102 , while allowing the rest of the channels (e.g., B, C and D) to pass through. Circulator  102  outputs the reflected channel out Drop port  112 . The channels that are passed through are input to circulator  104  via its first port  120  and, consequently, exit out of Pass port  116 . 
   For each setting of tunable filter  106 , the same channel as the one dropped, except with different content (e.g., A′), can be added by inputting it into Add port  114 . The added channel is directed by circulator  104  out its first port  120  to filter  106 . Because the added channel is the same one that filter  106  is tuned to, filter  106  reflects it back to the first port  120  of circulator  104 . Circulator  104  then outputs it out Pass port  116  such that a WDM signal comprising the passed through channels and the added channel (e.g., channels A′, B, C and D) is output via Pass port  116 . 
     FIG. 2   a  illustrates a dynamic OADM  200  of the present invention based on a trimmable MZI configuration. The MZI configuration has two optical paths  203  and  205  with tunable filters  206  (e.g., Bragg gratings) formed in them. While a filter can be formed separately in each arm, filters  206  are preferably formed from a single Bragg grating that spans both paths  203  and  205 . The tuning of filters  206  to operate on different channels (e.g., by heating for thermo-optical tuning) is preferably done with a single element (e.g., a single thin film heater) that tunes them simultaneously. The MZI configuration is made trimmable by phase shifters  218   a  and  218   b  (e.g., thin film heaters), which are associated with optical paths  203  and  205  so that their optical lengths can be adjusted to insure they are balanced. Optical paths  203  and  205  are connected between two 3 dB directional couplers  202  and  204 . The arms of 3 dB directional couplers  202  and  204  can be symmetric or asymmetric in width, however, the asymmetric design enables a wavelength-flattened response. 
   During operation, a WDM signal comprising a plurality of channels (e.g., channel A, B, C and D) is input via the In port of dynamic OADM  200 , i.e. first input  210  of coupler  202 . Coupler  202  splits the input power of the WDM signal substantially evenly into optical paths  203  and  205 . The Bragg gratings in optical paths  203  and  205  reflect the channel they are tuned to (e.g., channel A) back into coupler  202 , while allowing the rest of the channels (e.g., B, C and D) to be transmitted through. The optical signal carrying the transmitted channels merges in second coupler  204 . When optical paths  203  and  205  are balanced, the optical signal of the transmitted channels is transferred to a first port  214  of second coupler  204  (i.e., the Pass port), with little signal being transmitted to a second port  216  of second coupler  204 . 
   Similar to the transmitted signal, the optical signal of the reflected channel merges in first coupler  202 . Like the transmitted channels, when paths  203  and  205  are balanced, the optical signal of the reflected channel is carried out a second port  212  of first coupler  202  (i.e. the Drop port) with little leakage to first port  210  of first coupler  202 . 
   The same channel as the one dropped, except with different content (e.g., A′), can be added by inputting it into Add port  216  of coupler  204 . Coupler  204  splits the input power of the added channel evenly into optical paths  203  and  205 . The Bragg gratings in optical paths  203  and  205  reflect the added channel back into coupler  204 , which transfers the optical signal of the added channel to Pass port  216 . As a result, a WDM signal comprising the passed through channels and the added channel (e.g., channels A′, B, C and D) is output via Pass port  214 . 
   As illustrated in  FIG. 2   b , the use of 3 dB couplers  202  and  204  based on Multimode Interference (MMI) couplers in place of directional couplers is within the scope of the present invention. Similarly, as illustrated in  FIG. 2   c , the use of 3 dB couplers  202  and  204  based on MZI couplers in place of directional couplers is within the scope of the present invention. MZI couplers can be symmetric or asymmetric in arm length, however, the asymmetric design enables a wavelength-flattened response. 
     FIGS. 3   a–c  and  4   a–b  illustrate single-channel HDOADMs (SHDOADMs) according to the principles of the present invention. As previously described, generally a hitless dynamic OADM can be selectively tuned to remove a wavelength from a WDM signal and add in the same wavelength with different content without effecting any intermediate wavelengths during tuning. 
     FIG. 3   a  illustrates one embodiment of SHDOADM  300  according to the principles of the present invention. SHDOADM  300  comprises a tunable reflective filter  306 , such as a tunable Bragg grating, on a substrate  308 , a first directive coupler  302 , such as a 3-port optical circulator, a second directive coupler  304 , such as a 3-port optical circulator, and a cross-bar switch  322  on a substrate  324 . Filter  306  has one end connected to the third port  318  of circulator  302  and its other end connected to the first port  320  of circulator  304 . First port  310  of circulator  302  is the In port of SHDOADM  300 , while third port  316  of circulator  304  is the Pass port. 
   The second port  312  of circulator  302  is connected to the second port  314  of circulator  304  through a bar arm of cross-bar switch  322 . The input side  326  and output side  328  of the other bar arm of cross-bar switch  322  are respectively the Add port and the Drop port of SHDOADM  300 . 
   During operation, a WDM signal comprising a plurality of channels (e.g., channel A, B, C and D) is input via the In port  310 . Circulator  302  outputs this signal via its third port  318  to filter  306 . Filter  306  reflects the channel to which it is tuned (e.g., channel A) back to circulator  302 , while allowing the rest of the channels (e.g., B, C and D) to pass through. The channels that are passed through are input to circulator  304  via its first port  320  and exit out of Pass port  304 . 
   Circulator  302  outputs the reflected channel to cross-bar switch  322  via its second port  312 . When filter  306  is not being tuned, cross-bar switch  322  is operated in the cross state causing the reflected channel to be dropped. The reflected channel is dropped because operation of cross-bar switch  322  in the cross state directs the reflected channel to Drop port  328 . Further, when filter  306  is not being tuned, the same channel with different content (e.g., A′) can be added by inputting it into Add port  326 . The added channel is directed by cross-bar switch  322  to the second port  314  of circulator  304 . Circulator  304  directs the added channel out its first port  320  to filter  306 . Because the added channel is the same one that filter  306  is tuned to, filter  306  reflects it back to the first port  320  of circulator  304 . Circulator  304  then outputs it out Pass port  316  such that a WDM signal comprising the passed through channels and the added channel (e.g., channels A′, B, C and D) is output via Pass port  316 . 
   When tunable filter  306  is being tuned between a source channel (e.g., channel A) and a destination channel (e.g., channel D), the adding and dropping is deactivated to avoid dropping intermediate channels (e.g., channels B and C) by switching cross-bar switch  322  to the bar state. By placing cross-bar switch  322  into the bar state, a reflected channel is directed to the second port  314  of circulator  304  rather than Drop port  328 . As such, the intermediate channels (e.g., B and C) that are reflected by filter  306  while it is being tuned from one channel to the other are added back into the signal that is output via Pass port  316 . This permits the output signal to contain all of the channels (e.g., A, B, C and D) during tuning, i.e. no intermediate channels are dropped during tuning of SHDOADM  300  from operation on one channel to another. 
   Another embodiment of SHDOADM  300  is illustrated in  FIG. 3   b . SHDOADM  300  is the same except the tunable reflective filter  306  and the cross-bar switch  322  are integrated on the same substrate  308 . 
   Another embodiment of SHDOADM  300  is illustrated in  FIG. 3   c . SHDOADM  300  is the same except the tunable reflective filter  306 , cross-bar switch  322 , and optical circulators  302  and  304  are integrated on the same substrate  308 . 
   Another embodiment of a SHDOADM  400  is illustrated in  FIG. 4   a . SHDOADM  400  is similar to SHDOADM  300 , except the tunable reflective filter is two optical paths with Bragg gratings formed across them  406  and the two directive couplers are 3 dB couplers  402  and  404  (e.g., MZI coupler, Multimode Interference Coupler, Directional Coupler). The two optical paths with Bragg gratings formed across them  406  and 3 dB couplers  402  and  404  form a balanced MZI structure with substantially identical Bragg gratings in the two MZI arms, i.e. the two optical paths. While preferably formed as a single filter that spans both arms, as one of skill in the art would recognize, the individual Bragg gratings can be formed separately in each arm. 
   SHDOADM  400  operates similarly to SHDOADM  300 . A WDM signal comprising a plurality of channels (e.g., channel A, B, C and D) is input via the In port of SHDOADM  400 , i.e. first input  410  of coupler  402 . Coupler  402  splits the input power of the WDM signal evenly into the two MZI arms. The Bragg gratings in the two MZI arms reflect the channel to which they are tuned (e.g., channel A) back into coupler  402 , while allowing the rest of the channels (e.g., B, C and D) to be transmitted through. The optical signal carrying the transmitted channels merges in second coupler  404 . When the optical paths are balanced, the optical signal of the transmitted channels is transferred to a first port  416  of second coupler  404  (i.e., the Pass port), with little signal being transmitted to a second port  414  of second coupler  404 . Similarly, the optical signal of the reflected channels merges in first coupler  402 . Like the transmitted channels, the optical signal of the reflected channel is carried out a second port  412  of first coupler  402  with little leakage to first port  410  of first coupler  402 . 
   Similar to SHDOADM  300 , when the filter  406  is not being tuned, cross-bar switch  422  is operated in the cross state causing the reflected channel to be dropped. The reflected channel is dropped because operation of cross-bar switch  422  in the cross state directs the reflected channel to drop port  428 . Further, when filter  406  is not being tuned, the same channel with different content (e.g., A′) can be added by inputting it into Add port  426 . The added channel is directed by cross-bar switch  422  to the second port  414  of coupler  404 . Coupler  404  splits the input power of the added channel evenly into the two MZ arms. The Bragg gratings in the two MZ arms reflect the added channel back into coupler  404 . The optical signal of the add channel is transferred to first port  416  of second coupler  404  such that a WDM signal comprising the passed through channels and the added channel (e.g., channels A′, B, C and D) is output via Pass port  416 . 
   When filter  406  is being tuned between a source channel (e.g., channel A) and a destination channel (e.g., channel D), the adding and dropping is deactivated to avoid dropping intermediate channels (e.g., channels B and C) by switching cross-bar switch  422  to the bar state. By placing cross-bar switch  422  into the bar state, a reflected channel is directed to the second port  414  of coupler  404  rather than drop port  428 . As such, the intermediate channels (e.g., B and C) that are reflected while the filter  406  is being tuned are added back into the signal that is output. This permits the output signal to contain all of the channels (e.g., A, B, C and D) during tuning, i.e. no intermediate channels are dropped during tuning of SHDOADM  400  from operation on one channel to another. 
     FIG. 4   b  illustrates another embodiment of a SHDOADM  400 . SHDOADM  400  is the same except reflective filters  430   a  and  430   b  replace the 180° bends used to connect the second port  414  of coupler  404  to cross-bar switch  422  and to connect the second port  412  of coupler  402  to cross-bar switch  422 . 
     FIGS. 5   a–b  and  6  illustrate multi-channel hitless dynamic optical add/drop multiplexers formed by a cascade of SHDOADMs according to the present invention. Generally a multi-channel HDOADM selectively removes more than one wavelength from a WDM signal and selectively adds in more than one of the same wavelengths with different content. 
     FIG. 5   a  illustrates one embodiment of an exemplary four-channel HDOADM  500  formed by a cascade of SHDOADMs according to the present invention. Four-channel HDOADM is formed by a cascade of SHDOADMs  502 ,  504 ,  506  and  508 . SHDOAMs  502 ,  504 ,  506  and  508  are single-channel HDOADMs according to any of the embodiments of  FIG. 3   a–c . Each of SHDOADMs  502 ,  504 ,  506  and  508  is tuned to operate on a different channel. When a SHDOADM is not being tuned and channels are to be add/dropped, its cross-bar switch is set to the cross state. This causes the SHDOADM to drop the input channel to which it is tuned and allows the same channel with different content to be added, as previously described in conjunction with  FIG. 3   a . The channels to which it is not tuned and the added channel are transmitted through to the next SHDOADM of the cascade. In contrast, when a SHDOADM is being tuned, or channels are not to be add/dropped, the cross bar switch of the SHDOADM is set to the bar state. This results in all of the channels being passed to the next SHDOADM in the cascade. 
   For example, a WDM signal comprising channels A, B, C, D, and E are input via the In port of four-channel HDOADM  500 . SHDOADM  502  is tuned to channel A, SHDOADM  504  is tuned to channel B, SHDOADM  506  is tuned to channel C and SHDOADM  508  is tuned to channel D. It is desired to drop channel A in the input WDM signal, while keeping the B, C, D and E channels. To accomplish this, the cross-bar switch of SHDOADM  502  is placed in the cross state, while the cross-bar switches of the other SHDOADMs are placed in the bar state. As a result, channel A is dropped and is output via the Drop 1  port, while channels B, C, D and E are output via the Pass port. When the same channel with different content, A′, is to be added, it is input via the Add 1  port. SHDOADM  502  then adds channel A′ and the signal output via the Pass port contains A′, B, C, D and E. 
   Further, in the case that SHDOADM  502  is to be re-tuned, for example, to channel E, the add/drop function is deactivated by switching the corresponding cross-bar switch of SHDOADM  502  to the bar state. As described above, this prevents SHDOADM  502  from dropping the intermediate channels B, C and D while it is being tuned from operating on channel A to channel E. 
     FIG. 5   b  illustrates another embodiment of an exemplary four-channel HDOADM  500 . HDOADM  500  is the same except that a 4-port circulator replaces the dual 3-port circulators at the intermediate connections between SHDOADMs  502 ,  504 ,  506 , and  508 . 
     FIG. 6  illustrates another embodiment of an exemplary four-channel HDOADM  600  formed by a cascade of SHDOADMs according to the present invention. HDOADM  600  is a four-channel HDOADM and is the same as multi-channel HDOADM  500  except that SHDOADMs  602 ,  604 ,  606  and  608  are MZI-based SHDOADMs as described in  FIG. 4   a.    
     FIGS. 7   a–b  and  8  illustrate hitless dynamic demultiplexers (HDDEMUXs) according to the present invention. As previously described, generally a DEMUX divides a WDM signal received from a waveguide into its different channels and couples each channel into a different waveguide. 
   The filters of the device do not have to be tunable since, generally, a DEMUX separates out all of the channels in a system. However, a hitless dynamic DEMUX according to the principles of the present invention is advantageous as it can be used in systems that have the same channel number and channel spacing but varying channel allocations. 
   In general, an N-channel HDDEMUX according to the principles of the present invention is formed using N-1 tunable reflective filters, such as tunable Bragg gratings, N-1 1×2 switches, and 2(N-1) directive couplers, such as 3-port circulators. During operation, the N-channel HDDEMUX receives a WDM signal that comprises at least the N channels on which the HDDEMUX is designed to operate and divides the N channels into different signals that are each coupled to a waveguide. 
     FIG. 7   a  illustrates an exemplary four-channel HDDEMUX  700  according to the principles of the present invention. As shown, exemplary HDDEMUX  700  is a four-channel HDDEMUX and, consequently, during operation it receives, via the In port, a WDM signal that comprises the four channels on which it is designed to operate (e.g., channels A, B, C and D). HDDEMUX  700  then divides each of the 4 channels on which it is designed to operate into four different signals and outputs each one on a separate one of the outputs Out 1 , Out 2 , Out 3  and Out 4  (e.g., channel A is output via Out 1 , channel B is output via Out 2 , channel C is output via Out 3  and channel D is output via Out 4 ). 
   Because HDDEMUX  700  is a 4-channel HDDEMUX, it comprises 3 tunable reflective filters  704 ,  712  and  720 , 3 1×2 switches  706 ,  714  and  722 , and 6 3-port circulators  702 ,  708 ,  710 ,  716 ,  718  and  724 . Circulators  702  and  708 , tunable reflective filter  704  and 1×2 switch  706  are arranged in a fashion similar to HDOADM  300  to form a first hitless optical divide module (HODM)  730 . HODM  730  operates similar to HDOADM  300 . Circulator  702  directs an input signal to reflective filter  704  and reflective filter  704  reflects a channel to which it is tuned back to circulator  702 , which then directs the reflected signal to switch  706 . When switch  706  is in a first position, the reflected channel is directed out the port Out 2 . When switch  706  is in the other position, the reflected channel is directed towards circulator  708 , which recombines it with the transmitted channels and outputs the WDM signal such that no channels are divided out. 
   Similarly, circulators  710  and  716 , tunable reflective filter  712  and 1×2 switch  714  are arranged to form second HODM  732  which is cascaded with HODM  730  and circulators  718  and  724 , tunable reflective filter  720  and 1×2 switch  722  are arranged to form a third HODM  734  which is cascaded with HODM  732 . HODM  730  divides out the channel on which it operates (e.g., channel B) and outputs it via the port Out 2 . HODM  732  divides out the channel on which it operates (e.g., channel C) and outputs it via the port Out 3 . HODM  734  divides out the channel on which it operates (e.g., channel D) and outputs it via the port Out 4 . The remaining channel (e.g., channel A) is output via the port Out 1 . 
     FIG. 7   b  illustrates another embodiment of a HDDEMUX  700  in which the intermediate 3-port circulators are replaced with 4-port circulators  708  and  714 . Thus, in this embodiment, a single 4-port circulator acts as two directive couplers to provide the 2(N-1) directive couplers. 
   In another embodiment of an N-channel HDDEMUX according to the principles of the present invention, the N-1 tunable reflective filters are MZI-based reflective filters and the 2(N-1) directive couplers are 3 dB couplers. Thus, in this embodiment, N-1 MZI-based reflective filters, 2(N-1) 3 dB couplers and N-1 1×2 switches are used for an N-channel HDDEMUX. Similar to the embodiment of  FIG. 1 , the couplers, reflective filters and 1×2 switches are arranged to form cascaded HODMs. 
   An exemplary 4-channel HDDEMUX  800  according to this embodiment is illustrated in  FIG. 8 . Couplers  802  and  808 , reflective filter  804  and 1×2 switch  806  are arranged to form a first HODM  830 . Couplers  810  and  816 , reflective filter  812  and 1×2 switch  814  are arranged to form a second HODM  832 , which is cascaded with HODM  830 . Couplers  818  and  824 , reflective filter  820  and 1×2 switch  822  are arranged to form a third HODM  834 , which is cascaded with HODM  832 . Each HODM  830 ,  832  and  834  operates in a like manner to the HODMs of the embodiment of  FIG. 7   a  to divide out the channel it is set to operate on from the input WDM signal. As such, 4-channel HDDEMUX  800  functions in the same fashion as 4-channel HDDEMUX  100  to demultiplex 4 channels in a WDM signal. 
     FIGS. 9   a – 9   c  and  10   a – 10   c  illustrate M-channel Hitless Dynamic WSSs (HDWSSs) according to the principles of the present invention. As previously described, a WSS provides for selectively switching amongst L WDM signals that have N channels, the contents of any M of the N channels. For example, two WDM signals, WDM 1  and WDM 2  have N channels each, including channels A and A′, respectively. Channels A and A′ are the same wavelength in both WDM signals, i.e. they are the same channel, but with different content. A WSS provides for one of two possibilities with respect to channels A and A′: (a) the channels are not switched between the WDM signals, such that WDM, continues to have the content of A and WDM 2  continues to have the content of A′ (b) the channels are switched between the WDM signals such that WDM 1  has the content of A′ and WDM 2  has the content of A. The same occurs for M-1 more channels. 
   One embodiment of a HDWSS according to the principles of the present invention uses M tunable reflective filters, such as tunable Bragg gratings, per each one of the L WDM signals, M pairs of directive couplers, such as 3-port optical circulators, per each one of the L WDM signals, and M L×L switches.  FIG. 9   a  illustrates an exemplary HDWSS  900  of this embodiment that switches amongst two WDM signals that have N channels, the contents of any four of the N channels. Consequently, HDWSS  900  comprises 4 tunable reflective filters  906 ,  920 ,  934  and  952  for the first WDM signal input via port In 1  and 4 tunable reflective filters  908 ,  922 ,  936  and  954  for the second WDM signal input via port In 2 . WSS  900  also comprises 8 3-port circulators  902 ,  912 ,  916 ,  926 ,  930 ,  942 ,  948  and  958  for the first WDM signal, 8 3-port circulators  904 ,  914 ,  918 ,  928 ,  932 ,  944 ,  950  and  960  for the second WDM signal and 4 2×2 switches  910 ,  924 ,  940  and  956 . 
   For M-channel HDWSSs of this embodiment, M wavelength switches are formed from the circulators, reflective filters and switches; one for each of the wavelengths in the subset to be switched. These wavelength switches are then placed in cascade. As shown, for four-channel HDWSS  900 , there are four wavelength switches  962 ,  964 ,  966  and  968  formed from the circulators, filters and switches. 
   For instance, wavelength switch  962  is formed from tunable reflective filters  906  and  908 , 3-port circulators  902 ,  912 ,  904  and  912 , and 2×2 switch  910 . Reflective filter  906  has one of its ends connected to the third port of 3-port circulator  902  and its other end connected to first port of 3-port circulator  912 . One of the bar arms of switch  910  is connected between the second ports of circulators  902  and  912 . The other bar arm of switch  910  is connected between the second ports of circulators  904  and  914 . Reflective filter  908  has one end connected to the third port of circulator  904  and its other end connected to the first port of circulator  914 . Reflective filter  908  and  906  are tuned to the same wavelength. 
   Circulator  902  and reflective filter  906  operates in the same manner as HDOADM  300  to direct the reflected channel that reflective filter  906  is tuned to towards switch  910 . When switch  910  is in the bar state, the reflected channel is directed by switch  910  to circulator  912 , resulting in it being output with the channels transmitted by reflective filter  906 . Correspondingly, the channel reflected by reflective filter  908  is directed by switch  910  to circulator  914 , resulting in it being output with the channels transmitted by reflective filter  914 . 
   When switch  910  is in the cross-state, however, the channel reflected by filter  906  is instead directed towards circulator  914 , which results in the reflected channel being output with the channels transmitted by reflective filter  908 . Correspondingly, the channel reflected by reflective filter  908  is output with the channels transmitted by reflective filter  906 . 
   Therefore, wavelength switch  962  is operative to switch the contents of a channel between two WDM signals. Each of the additional wavelength switches  966 ,  966  and  968  are formed in a like manner, however, are tuned to a different one of the four channels. Thus, by cascading each of the wavelength switches, the contents of any four out of the N channels carried by the WDM signals input into the In 1  port and the In 2  port can be switched between the WDM signals. If less than four channels need to be switched, one or more of the wavelength switches are effectively turned off by tuning them so that they filter either between channels or outside the frequency band used (e.g., outside the erbium “C” band). 
   Tuning the tunable reflective filters of the wavelength switches tunes HDWSS  900  to operate on different channels. Preferably, the tunable filters of a wavelength switch can be tuned simultaneously (indicated by the dashed lines linking the arrows). Further, placing a wavelength switch&#39;s corresponding switch in the bar-state operates HDWSS  900  in a hitless manner when that wavelength switch is tuned. 
     FIG. 9   b  illustrates another embodiment of a HDWSS  900  except that the intermediate 3-port circulators are replaced with 4-port circulators  912 ,  922 ,  932 ,  914 ,  924  and  934 . Thus, in this embodiment, a HDWSS according to the principles of the present invention is formed using M tunable reflective filters per each one of the L WDM signals, 2 3-port directive couplers, such as 3-port optical circulators, per each one of the L WDM signals, M-1 4-port directive couplers, such as 4-port optical circulators, per each one of the L WDM signals, and M L×L switches. 
   In another embodiment, a HDWSS according to the principles of the present invention is formed using M tunable reflective filters per WDM signal, 2 directive couplers, such as 3-port optical circulators, per WDM signal, and one L×L switch.  FIG. 9   c  illustrates an exemplary HDWSS  900  of this embodiment that switches amongst two WDM signals that have N channels, the contents of four of the N channels. As shown, four tunable reflective filters  906 ,  986 ,  987  and  988  are cascaded between a third port of 3-port circulator  981  and a first port of 3-port circulator  983 . One of the bar arms of switch  994  is connected between the second ports of circulators  981  and  983 . The other bar arm of switch  928  is connected between the second ports of circulators  982  and  984 . Four tunable reflective filters  989 ,  990 ,  991  and  992  are cascaded between a third port of 3-port circulator  982  and a first port of a 3-port circulator  984 . Reflective filter  985  and  989  are tuned to the same channel. Reflective filter  986  and  990  are tuned to the same channel. Reflective filter  987  and  991  are tuned to the same channel. Reflective filter  988  and  992  are tuned to the same channel. 
   Each of the reflective filters  985 ,  986 ,  987  and  988  operate to reflect their respective channel back to circulator  981 . Circulator  981  then directs the reflected channels towards switch  994 . When switch  994  is in the bar state, the reflected channels are directed by switch  994  to circulator  983 , resulting in it being output with the channels transmitted by reflective filters  985 ,  986 ,  987  and  988 . Correspondingly, the channels reflected by reflective filters  989 ,  990 ,  991  and  992  are directed by switch  994  to circulator  984 , resulting in them being output With the channels transmitted by reflective filters  989 ,  990 ,  991  and  992 . 
   When switch  994  is in the cross-state, however, the channels reflected by filters  985 ,  986 ,  987  and  988  are instead directed towards circulator  984 , which results in the reflected channels being output with the channels transmitted by reflective filters  989 ,  990 ,  991  and  992 . Correspondingly, the channels reflected by reflective filters  989 ,  990 ,  991  and  992  are output with the channels transmitted by reflective filters  985 ,  986 ,  987  and  988 . 
   Therefore, when switch  994  is in the cross-state, HDWSS  900  operates to switch the contents of four of the N channels carried by the WDM signals input into the In 1  port and the In 2  port between the WDM signals. If less than four channels need to be switched, one or more of the sets of filters tuned to the same channel are effectively turned off by tuning them so that they filter either between channels or outside the frequency band used (e.g., outside the erbium “C” band). 
   Tuning the reflective filters tunes HDWSS  900  to operate on different channels. Preferably, each one of the tunable filters that are tuned to the same channel (e.g., filters  906  and  908 ) can be tuned simultaneously (indicated by the dashed lines linking the arrows). Further, placing switch  928  into the bar-state operates HDWSS  900  in a hitless manner when any of the reflective filters is tuned. 
   In another embodiment, a HDWSS according to the principles of the present invention is formed using a cascade of M balanced MZI structures per WDM signal and M L×L switches.  FIG. 10   a  illustrates an exemplary HDWSS  1000  of this embodiment that switches amongst two WDM signals that have N channels, the contents of any four of the N channels. As can be seen, this embodiment is similar to the embodiment of  FIG. 9   a , except the tunable reflective filters  1002 ,  1006 ,  1010 ,  1014 ,  1004 ,  1008 ,  1012  and  1016  are MZI-based reflective filters, i.e. two optical paths with Bragg gratings formed across them, and the directive couplers  1030 – 1060  are 3 dB couplers. Consequently, HDWSS  1600  operates in the same manner as HDWSS  900 . 
   In another embodiment, a WSS is formed using M cascaded MZI-based filters per WDM signal, 2 3 dB couplers per WDM signal, and one L×L switch.  FIG. 10   b  illustrates an exemplary HDWSS  1080  of this embodiment that switches amongst two WDM signals that have N channels, the contents of four of the N channels. As can be seen, this embodiment is similar to the embodiment of  FIG. 9   c , except the tunable reflective filters  1085 – 1092  are MZI-based reflective filters, i.e. two optical paths with Bragg gratings formed across them, and the directive couplers are 3 dB couplers. Consequently, HDWSS  1000  operates in the same manner as HDWSS  980 . 
   In another embodiment, a HDWSS is formed using M cascaded MZI-based filters per WDM signal, 2 3 dB couplers per WDM signal, and one L×L switch, however, the M MZI-based reflective filters are built for all of the WDM signals by forming a single set of tunable Bragg gratings across all of the MZI arms.  FIG. 10   c  illustrates an exemplary HDWSS  1001  of this embodiment that switches amongst two WDM signals that have N channels, the contents of four of the N channels. As can be seen, a first MZI structure (arms  1019  and 3 dB couplers  1003  and  1007 ) for the first WDM signal is formed alongside a second MZI structure (arms  1021  and 3 dB couplers  1005  and  1009 ) for the second WDM signal. A single set of four tunable Bragg gratings  1011 – 1017  are formed across arms  1019  and  1021  to form the four tunable MZI-based filters for the first WDM signal and the four tunable MZI-based filters for the second WDM signal. A 2×2 switch  1023  is connected between the MZI structures in the same manner as switch  1094  of the embodiment of  FIG. 9   c . HDWSS  1001 , thus, operates in the same manner as the embodiment of  FIGS. 10   b , however, the use of a single set of gratings provides for synchronicity and easier control when tuning the filters. 
   The embodiments of  FIG. 9   a–b  and  FIG. 10   a  are advantageous in that each filter can be tuned separately without having to bar the switching of the other filters during tuning. Between the embodiments of  FIG. 9   a  and  FIG. 9   b , the advantage of the  FIG. 9   a  embodiment is that it uses only 3-port circulators, which might be easier to produce than 4-port circulators, and the advantage of the  FIG. 9   b  embodiment is that it uses fewer circulators, which is an advantage when 4-port circulators can be easily produced. The advantage of the  FIG. 9   c  embodiment is that it uses even fewer circulators than the embodiments of  FIGS. 9   a  and  9   b , but whenever one filter is being tuned, all the filters must be barred (i.e., no switching). 
   An OXC provides for selectively switching amongst L WDM signals that have N channels, the contents of any or all of the N channels. Therefore, any of the HDWSS embodiments above can be used as an OXC by making M equal to N. 
   When used as an OXC, the filters of the device do not have to be tunable since, by definition, the content of all the channels in the system can be switched. However, it is preferable that the filters are tunable so that the OXC can be used in different systems that have the same channel number and channel spacing but different channel allocations. In this case, the tuning elements would typically be set initially for filtering at a specific channel allocation, with no subsequent tuning needed in the system. Since the spacing between the channels remains constant, it is preferable that all the filters are tunable simultaneously, allowing for simpler electronic control of the device. However, it should be noted, that having independent tuning elements might be advantageous as it allows for individual trimming of the filters to compensate for individual characteristics possibly due to fabrication imperfections. 
     FIGS. 11   a–b  and  12   a–b  illustrate single-channel errorless hitless dynamic optical add/drop multiplexers (EHDOADMs). A hitless dynamic optical add/drop multiplexer is called errorless when the intermediate channels do not experience any significant change in the BER during tuning. 
   To exemplify this, while the embodiments of  FIGS. 3   a–c  and  4   a–b  are hitless dynamic OADMs, they are not errorless at high modulation speeds because, during the short time when the filter&#39;s edge is going through the signal spike of a channel, part of the signal goes straight through the filter to the pass port and part of the signal is dropped/added then goes through the pass port. When the modulation speed is low enough, the two segments of the signal rejoin relatively in sync, causing some broadening that affects the SNR (signal-to-noise ratio) and the duty cycle. When the modulation speed is high, the delayed segment of the signal joins the straight-through segment with a large delay, causing the delayed part of a bit to join a non-corresponding bit on the straight-through path, which typically (a) lowers the SNR when the reflected segment is weaker than the straight-through segment, (b) destroys the signal when the reflected segment and the straight-through segment have competing intensities, or (c) causes bit sequence destruction when the reflected segment is stronger than the straight-through segment. All of these effects result in a poorer BER (bit error rate) during the short time when the filter edge is going through the signal spike of a channel. Also, because the cross-bar switches do not switch instantaneously, errors result during the switching performed for tuning. 
   It is desirable to have an errorless hitless dynamic OADM, i.e. one in which none of the intermediate channels experiences any significant change in the BER during tuning. This effect is achieved by having a bypass path for the entire WDM signal and establishing a balanced MZI during the time when switching is occurring between the filter path and the bypass path. 
   One embodiment which achieves this functionality is illustrated in  FIG. 11   a . In this embodiment, a EHDOADM  1100  comprises two 3-port optical circulators  1102  and  1104 , two opposed 1×2 switches  1106  and  1108 , with the 2-port sides facing, and a filter path  1116  and bypass path  1118 , connected between switches  1108  and  1106 . A port  1120  of circulator  1102  is connected to the single-port side of switch  1106 . Likewise, a port  1122  of circulator  1104  is connected to the single-port side of switch  1108 . Filter path  1114  comprises a tunable reflective filter  1114 , such as a Bragg grating, which is tuned by an appropriate means, such as heater  1115 . Bypass path  1118  is simultaneously tuned by heater  1114 . A phase shifter  1112  is used to trim path  1118  so that a balanced MZI configuration is maintained. In the embodiment of  FIG. 11   a , all of the components except for circulators  1104  and  1102  are integrated on a single substrate  1110 . 
   During operation, a WDM signal comprising a plurality of channels (e.g., channel A, B, C and D) is input to circulator  1102  via the In port. This WDM signal is output to switch  1106  by port  1120 . When channels are being added and/or dropped, switches  1106  and  1108  are set so that the signals travel along filter path  1116 . The WDM signal, therefore, follows this path. Filter  1114  reflects the channel to which it is tuned (e.g., channel A) back to switch  1106 , while allowing the rest of the channels (e.g., B, C and D) to pass through. The channels that are passed through are directed by switch  1108  to circulator  1104  via port  1122 . The passed through channels exit out of the Pass output of circulator  1104 . The reflected channel is directed by switch  1106  back to circulator  1102 , which outputs it via the Drop port. 
   The same channel as the one dropped with different content (e.g., A′) can be added by inputting it into the Add port of circulator  1104 . The added channel is output from circulator  1104  via port  1122  to switch  1108 . Switch  1108  directs the added channel along filter path  1116 . Because the added channel is the same one that filter  1114  is tuned to, filter  1114  reflects it back to through switch  1108  to port  1122  of circulator  1104 . Circulator  1104  then outputs it out the Pass port such that a WDM signal comprising the passed through channels and the added channel (e.g., channels A′, B, C and D) is output by EHDOADM  1100 . 
   When tunable filter  1114  is to be tuned to operate on another channel, switches  1106  and  1108  are switched to direct signals along bypass path  1118 . The arrangement of the two paths  1116  and  1118  and switches  1106  and  1108  establishes a balanced MZI during the time when switching is occurring between the filter path  1116  and the bypass path  1118 . This prevents errors resulting from switching. Further, by directing all of the channels through bypass path  1118  during tuning of filter  1114 , filter  1114  does not operate on any of the channels while it is being tuned, which prevents problems resulting from segments of the WDM signal being delayed relative to the other segments during tuning. 
   Generally, switches  1106  and  1108  are required to switch substantially simultaneously for proper operation of EHDOADM  1100 . Since there is a need to simultaneously switch switches  1106  and  1108 , corresponding switch electrodes of each switch  1106  and  1108  are preferably connected for synchronicity and/or easier electronic control of the device. This is illustrated by the dashed lines linking the switch electrodes. 
   A device that is fully integrated on a single chip is less likely to have transient and synchronicity issues. As such, an alternative to the embodiment of  FIG. 11   a  has the circulators additionally integrated on the same substrate. This is illustrated in  FIG. 11   b . As shown, the embodiment of  FIG. 11   a  is the same as that of  FIG. 11   b , except for the integration of circulators  1102  and  1104  on substrate  1110 . Integrated circulators  1102  and  1104  are based, for example, on planar MZIs using either polarization splitting and nonreciprocal polarization conversion, or nonreciprocal phase shift within the interferometric arm. 
   Another integrated design uses an MZI-based design for the filter path. This is illustrated in  FIG. 12   a . EHDOADM  1200  comprises two opposed 1×2 switches  1206  and  1208  having the 2-port sides facing with a filter path  1216  and bypass path  1218  connected therebetween. Filter path  1216  comprises two optical paths with Bragg gratings formed across them  1206  and two 3 dB couplers  1202  and  1204 . This forms a balanced MZI structure with substantially identical Bragg gratings in the two MZI arms. Filter path  1216  is connected between switches  1206  and  1208  by one port of 3 dB coupler  1202  connected to switch  1206  and one port of 3 dB coupler  1204  connected to switch  1208 . The second port of 3 dB coupler  1202  outputs the dropped channel. Similarly, the other port of 3 dB coupler  1204  is used as an input port to input add channels. The single-port side of switch  1206  is used as the In port and the single-port side of switch  1208  is the Pass port. The Bragg gratings act as a tunable reflective filter, which is tuned by, for example, heater  1214 . Bypass path  1218  is simultaneously tuned by heater  1214 . Switch electrodes of each switch  1206  and  1208  are preferably connected for synchronicity and/or easier electronic control of the device as they are in EHDOADM  1100 . This is similarly illustrated by the dashed lines linking the switch electrodes. 
   EHDOADM  1200  operates similarly to EHDOADM  1100 . During operation, a WDM signal comprising a plurality of channels (e.g., channel A, B, C and D) is input via the In port. When channels are being added and/or dropped, switches  1206  and  1208  are set so that the signals travel along filter path  1216 . The WDM signal, therefore, follows this path. As such, the WDM signal input coupler  1202  by switch  1206 . Coupler  1202  splits the input power of the WDM signal evenly into the two MZI arms. The Bragg gratings in the two MZI arms reflect the channel they are tuned to (e.g., channel A) back into coupler  1202 , while allowing the rest of the channels (e.g., B, C and D) to be transmitted through. The optical signal carrying the transmitted channels merges in second coupler  1204 . When the optical paths are balanced, the optical signal of the transmitted channels is transferred to switch  1208  and are output via the Pass port. The optical signal of the reflected channel merges in first coupler  1202 . Like the transmitted channels, the optical signal of the reflected channel is carried out the Drop port, with little leakage to the port connected to switch  1206 . 
   The same channel as the one dropped with different content (e.g., A′) can be added by inputting it into coupler  1204  by the Add port. Coupler  1204  splits the input power of the added channel evenly into the two MZI arms. The Bragg gratings in the two MZI arms reflect the added channel back into coupler  1204 . The optical signal of the add channel is then transferred by coupler  1204  to switch  1208  such that a WDM signal comprising the passed through channels and the added channel (e.g., channels A′, B, C and D) is output via the Pass port. 
   Like EHDOADM  1100 , when the tunable filter is to be tuned to operate on another channel, switches  1206  and  1208  are switched to direct signals along bypass path  1218 . The arrangement of the two paths  1216  and  1218  and switches  1206  and  1208  also establishes a balanced MZI during the time when switching is occurring between the filter path  1216  and the bypass path  1218 , which prevents errors resulting from switching. Further, by directing all of the channels through bypass path  1218  during tuning likewise prevents problems resulting from segments of the WDM signal being delayed relative to the other segments. 
   While thermo-optic 1×2 digital optical switches (DOS) have been illustrated, MMI-based or MZI-based 1×2 switches can also be used. The use of MZI-based 1×2 switches is illustrated in  FIG. 12   b . The embodiment of  FIG. 12   b  is the same, except the 1×2 DOS thermo-optic switches have been replaced by MZI 1×2 switches  1202  and  1204 . MMI-based and MZI-based 1×2 switches can also be used in place of the 1×2 DOS thermo-optic switches in the embodiments of  FIGS. 11   a–b.    
   Similar to the multi-channel HDOADM formed from the single channel HDOADMs, multi-channel EHDOADMs can be formed by cascading multiple single-channel EHDOADMs.  FIG. 13   a  illustrates one embodiment of an exemplary four-channel HEDOADM  1300  formed using a cascade of single-channel EHDOADMs  1302 ,  1304 ,  1306 ,  1308  according to the embodiment of  FIG. 11   a . Each single-channel EHDOADMs  1302 ,  1304 ,  1306 ,  1308  is tuned to operate on a different channel. When a single-channel EHDOADM is not being tuned and channels are to be add/dropped, its switches are set to direct signals along the filter path. This causes the EHDOADM to drop the input channel to which it is tuned and allows the same channel with different content to be added, as previously described in conjunction with  FIG. 11   a . The channels to which it is not tuned and the added channel are transmitted through to the next EHDOADM of the cascade. In contrast, when a EHDOADM is being tuned, or channels are not to be add/dropped, the switches are set to direct signals along the bypass path. This results in all of the channels being passed to the next EHDOADM in the cascade. 
     FIG. 13   b  illustrates another embodiment of exemplary four-channel EHDOADM  1300  formed by a cascade of single-channel EHDOADM according to the embodiment of  FIG. 11   b . As shown, a 4-port circulator replaces the dual 3-port circulators at the intermediate connections between each single-channel EHDOADM  1302 ,  1304 ,  1306  and  1308 . 
     FIG. 14  illustrates another embodiment of an exemplary four-channel EHDOADM  1400  formed by a cascade of single-channel EHDOADM according to the embodiment of  FIG. 12   a . HDOADM  1400  is a four-channel HDOADM and is the same as multi-channel HDOADM  1300  except that single-channel EHDOADM  1402 ,  1404 ,  1406  and  1408  are MZI-based EHDOADMs as described in  FIG. 12   a.    
   Polarization independent EHDOADMs can be formed from EHDOADMs according to the present invention. Sometimes, tunable reflective filters operate slightly different on light depending upon the light&#39;s polarization state, i.e. they have polarization dependent behavior. Because of this behavior, difficulties can occur when a reflective filter is used to operate on light consisting of more than one polarization state. An EHDOADM that is polarization dependent, however, can be formed using polarization mode splitters/combiners with two EHDOADMs of the present invention. Generally, a polarization splitter is used to split incoming light signals into their two orthogonal components, the transverse electric (TE) and the transverse magnetic (TM) polarizations. Separate EHDOADMs are then used to operate on these two polarized signals. After the EHDOADMs operate on these polarized signals, the signals are recombined by the polarization combiners for output. 
   A polarization independent EHDOADM  1500  formed, for example, from two EHDOADMs according to the embodiment of  FIG. 12   b  is illustrated in  FIG. 15 . As illustrated, a polarization splitter  1508  is connected between the input ports of a first EHDOADM  1502  and a second EHDOADM  1504 . As shown, polarization splitter  1508  has one of its outputs connected to the input port  1516  of first EHDOADM  1502  and its other output connected to the input port  1518  of second EHDOADM  1504 . The input port of polarization splitter  1508  is used as the In port of polarization independent EHDOADM  1500 . Similarly, a polarization splitter  1510  is connected between the add ports of first and second EHDOADMs  1502  and  1504 . Polarization splitter  1510  has one of its outputs connected to the add port  1522  of first EHDOADM  1502  and its other output is connected to the add port  1524  of second EHDOADM  1504 . The input port of polarization splitter  1510  is used as the Add port of polarization independent EHDOADM  1500 . 
   A polarization combiner  1506  is connected between the drop ports of first and second EHDOADMs  1502  and  1504 . As shown, polarization combiner  1506  has one of its inputs connected to the drop port  1512  of first EHDOADM  1502  and its other input connected to the drop port of second EHDOADM  1504 . The output of polarization combiner  1506  is used as the Drop port for polarization independent EHDOADM  1500 . Similarly, a polarization combiner  1512  is connected between the pass ports of first and second EHDOADMs  1502  and  1504 . Polarization combiner  1512  has one of its inputs connected to the pass port  1522  of first EHDOADM  1502  and its other input connected to the pass port of second EHDOADM  1504 . The output of polarization combiner  1512  is used as the Pass port for polarization independent EHDOADM  1500 . 
   Both EHDOADMs  1502  and  1504  are tuned to operate on the same channel. Thus, when a WDM signal is input the In port of EHDOADM  1500 , it is split into two orthogonally polarized signals carrying the channels. One polarized signal is input to first EHDOADM  1502  via port  1516 , while the other polarized signal is input to second EHDOADM  1504  via port  1518 . Each EHDOADM  1502  and  1504  operates on its respective signal. When EHDOADMs  1502  and  1504  are not being tuned and channels are to be add/dropped, each EHDOADM&#39;s switches are set so that signals are directed signals along the filter path. Each EHDOADM  1502  and  1504  drops the tuned input channel from its polarized signal. First EHDOADM- 1502  drops the tuned input channel via port  1512  and second EHDOADM drops the tuned input channel via port  1518 . Polarization combiner combines the signals carrying the dropped channel and outputs it via the Drop port. The channels to which EHDOADMs  1502  and  1504  are not tuned are transmitted through to polarization combiner  1512 , which combines the signals carrying these channels, and outputs them via the Pass port. 
   An add channel is input the Add port and split into two orthogonally polarized signals by polarization splitter  1510 . Each polarized signal is input into a EHDOADM and reflected back to polarization combiner  1512 . The two polarized signals containing the add channel are combined and output via the Pass port. 
   When the EHDOADMs  1502  and  1504  are being tuned, or channels are not to be add/dropped, the switches are set to direct signals along the bypass path. This results in all of the channels being passed from the In port to the Pass port. 
     FIG. 16  illustrates a multi-stage polarization independent EHDOADM  1600 . At times, tunable reflective filters only have a dynamic range over a portion of the spectrum of the WDM signal. For instance, some tunable reflective filters only have a dynamic range over half of the Erbium C band. By using 1×2 switches and multiple EHDOADMs, a multi-stage polarization independent EHDOADM  1600  that operates over the entire Erbium C band can be formed. As shown, a first EHDOADM  1602  has its add port  1614  connected to one port of the two-port side of 1×2 switch  1606 . A second EHDOADM  1604  has its add port  1618  connected to the other port of the two-port side of switch  1606 . The port on the one-port side of switch  1606  is then used as the Add port for the multi-stage EHDOADM  1600 . Similarly, 1×2 switch  1608  is connected to the pass ports  1616  and  1620 , switch  1610  is connected to drop ports  1622  and  1628 , and switch  1612  is connected to in ports  1624  and  1626 . EHDOADMs  1602  and  1604  operate as described in conjunction with  FIG. 15 . Signals over one half of the spectrum are routed by 1×2 switches  1606 ,  1608 ,  1610 , and  1612  so as to be operated on by first EHDOADM  1602 , while the other half is routed by the 1×2 switches  1606 ,  1608 ,  1610 , and  1612  so as to be operated on by second EHDOADM  1604 . 
   Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention. For instance, in all of the embodiments, the tuning and switching can be performed using any known actuation method, such as thermo-optic, electro-optic, magneto-optic or stress-optic tuning, or any combination thereof. Couplers can take any form including directional couplers, MMI couplers, or MZI couplers and can be tunable (for trimming) or non-tunable. MZI couplers can be symmetric or asymmetric. MMI couplers can have any shape including rectangular and tapered (e.g., parabolic). Tunability as illustrated by arrows, and connections indicating simultaneous actuation of elements as illustrated by dashed lines, can be used or not. Reflective filters can be used in place of 180° bends. Switches can be based on any design including digital optical switches (based on Y-branches, X-junctions or other structures), MMIs, or MZIs. MZIs include Generalized MZIs (GMZIs), which consist of a pair of cascaded MMI couplers with thermal phase shifters on the connecting arms. Switches can be single-stage or multi-stage. Further, DEMUXs can be used as MUXs by using the input ports as output ports and the output ports as input ports.