Abstract:
A switchless optical add/drop module (OADM) includes: a first variable optical splitter (VOS) for splitting a composite optical signal including a plurality of channels into a first portion and a second portion; a first multi-channel variable optical attenuator (MCVOA) optically coupled to the first VOS, where the first MCVOA blocks dropped channels of the first portion, transmits express channels of the first portion, and balances power levels of each of the transmitted express channels of the first portion; a second VOS optically coupled to the first MCVOA opposite to the first VOS for combining the transmitted express channels of the first portion and added channels; and a second MCVOA optically coupled to the first VOS, where the second MCVOA blocks express channels of the second portion, transmits dropped channels of the second portion, and balances power levels of each of the transmitted dropped channels of the second portion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application claims priority under 35 U.S.C. §119(e) the benefit of U.S. Provisional Patent Application Ser. No. 60/336,856, entitled “Optical Add/Drop Multiplexer Utilizing Variable Optical Attenuator,” filed on Nov. 14, 2001. 

   FIELD OF THE INVENTION 
   The present invention relates to wavelength division multiplexers and de-multiplexers in optical communications networks and systems. More particularly, the present invention relates to such multiplexers and de-multiplexers that perform the additional functions of detecting and/or variably attenuating the optical power of each signal channel comprising a wavelength division multiplexed composite optical signal. 
   BACKGROUND OF THE INVENTION 
   Introduction 
   Fiber optic communication systems are becoming increasingly popular for data transmission due to their high speed and high data capacity capabilities. A number of very basic optical functions are required to permit the efficient transfer of large amounts of data over such systems and to maintain the operation of the system. Among these basic functions are those of wavelength division multiplexing and demultiplexing. Wavelength division multiplexing permits simultaneous transmission of a composite optical signal comprising multiple information-carrying signals, each such signal comprising light of a specific restricted wavelength range, along a single optical fiber. A multiplexer combines optical signals of different wavelengths from different paths onto a single combined path; a de-multiplexer separates combined wavelengths input from a single path onto multiple respective paths. Such wavelength combination and separation must occur to allow for the exchange of signals between loops within optical communications networks and to ultimately route each signal from its source to its ultimate destination. 
   Another basic function needed by fiber optic communication systems is that of independent control of the power levels of all signals comprising a wavelength division multiplexed optical transmission. Because of the power level expectations of receiver equipment within a fiber optic communication systems, all channels must be of a uniform power level. No channel can be significantly more intense than others. However, because of general non-uniform amplification by optical amplifiers and different routes traced by the various channels, re-balancing the channel powers is frequently required at various points. 
   Further, the exact form of the gain spectrum of the commonly utilized Erbium-Doped Fiber Amplifier (EDFA) type of optical amplifier can vary depending upon the amount of total optical power that is input to an EDFA. Because of such changing gain characteristics, the difference in amplification between channels may not be constant. Therefore, variable optical attenuator (VOA) apparatuses are generally utilized within optical communications networks so as to balance the powers carried by the various channels and to control the overall optical power of all channels. 
     FIG. 4  presents a known OADM architecture. A composite optical signal entering OADM  400  from an input fiber optic line is de-multiplexed into its component channels λ 1 , λ 2 , . . . , λ n  by de-multiplexer  402   a . Simultaneously, a set of channels to be added are input to OADM  400  from add line  404  and de-multiplexed into their component channels by de-multiplexer  402   c . The channels λ 1  and λ′ 1  (if present) are directed to the 2×2 switch  406 . 1 ; the channels λ 2  and λ′ 2  (if present) are directed to the 2×2 switch  406 . 2 ; and so on. In the example shown in  FIG. 4 , it is assumed that the add channels comprise only the two channels λ′ 1  and λ′ 2 . Since each add operation is always paired with a concurrent drop operation, this implies that the channels λ 1  and λ 2  are dropped. Each of the 2×2 switches  406 . 1 – 406 . n  can be in either one of two states—a “cross state” or a “bar” state. In the example shown in  FIG. 4 , since the channels λ′ 1  and λ′ 2  are added, the two switches  406 . 1 – 406 . 2 , which receive these channels, are in their “cross” states. Since no other channels are added, the switches  406 . n  (and all other switches) are in their “bar” states. Thus, the channels λ 1  and λ′ 1  and λ′ 2  and λ′ 2  are switched such that the channels λ 1  and λ 2  are “dropped” to the multiplexer  402   d  whilst the channels λ′ 1  and λ′ 2  are directed to the multiplexer  402   b . The non-dropped or “express” channels λ 3 –λ n  are all directed to the multiplexer  402   b . The multiplexer  402   b  multiplexes the “added” channels λ′ 1  and λ′ 2  together with the “express” channels λ 3 –λ n  so as to be output as a single composite optical signal along the output fiber optic line. The multiplexer  402   d  multiplexes the two channels λ′ 1  and λ′ 2  so as to be output as a composite optical signal along the drop line  408 . 
   Although the conventional OADM  400  performs its intended function adequately, it requires one 2×2 switch for each wavelength as well as four separate multiplexers. Further, the conventional OADM does not provide channel power balancing or overall power control. Additional components must be either incorporated into or interfaced to the conventional OADM  400  to provide these latter functions so as to prevent signal distortions which would otherwise arise from non-uniform power levels of signals propagating through optical amplifiers present within an optical communications network. 
     FIGS. 5A–5   b  illustrate two known OADM architectures based upon micro-mirror arrays. The OADM  500  ( FIG. 5A ) comprises a micro-mirror array  501   a  that comprises only one set of micro-mirrors  503 . 1 – 503 . 4  to facilitate both channel adding and dropping operations simultaneously; the OADM  550  ( FIG. 5B ) comprises a different micro-mirror array  501   b  that comprises a first set of mirrors  505 . 1 – 505 . 4  to facilitate channel dropping operations and a second set of mirrors  507 . 1 – 507 . 4  to facilitate channel adding operations. The micro-mirrors may be fabricated using either Micro-ElectroMechanical (MEMS) or micro-fluidic techniques, both of which are known in the art. Each of the mirrors  503 . 1 – 503 . 4 ,  505 . 1 – 505 . 4 ,  507 . 1 – 507 . 4  may assume one of only two positions or states—an “on” position whereby the mirror is disposed within the path of light comprising a channel so as to deflect the light and an “off” position whereby the channel light does not encounter the mirror. In  FIGS. 5A–5   b , mirrors in the “on” and “off” configurations are indicated by solid and dashed lines, respectively. 
   In both the OADM  500  ( FIG. 5A ) and the OADM  550  ( FIG. 5B ), a composite optical signal enters the respective OADM from an input fiber optic line is de-multiplexed into its component channels λ 1 , λ 2 , . . . , λ n  by de-multiplexer  502   a . Simultaneously, a set of channels to be added are input to the respective OADM from add line  514  and are de-multiplexed into their component channels by de-multiplexer  502   c . Each of the de-multiplexed channels leaving de-multiplexer  502   a  is collimated by a respective collimator lens  504  and is input to the adjacent micro-mirror array—array  501   a  in OADM  500  and array  501   b  in OADM  550 . 
   Each of the mirrors comprising the OADM  500  ( FIG. 5A ) and the OADM  550  ( FIG. 5B ), in its “on” position, deflects the path of one channel to be dropped and/or one channel to be added. The paths of one dropped channel λ 2  and of one added channel λ′ 2  are illustrated, respectively, by solid and dashed lines in  FIGS. 5A–5   b . In the OADM  500  ( FIG. 5A ), both of the channels λ 1  and λ′ 1  (if present) will encounter and be deflected by the mirror  503 . 1  if this mirror is in its “on” configuration. Likewise, both of the channels λ 2  and λ′ 2  (if present) will encounter and be deflected by the mirror  503 . 2 , both of the channels λ 3  and λ′ 3  (if present) will encounter and be deflected by the mirror  503 . 3  and both of the channels λ 4  and λ′ 4  (if present) will encounter and be deflected by the mirror  503 . 4 , if the respective mirror, in each case is “on”. Any mirror in an “on” position will cause one signal from the input line to be dropped to the drop line and/or will cause another signal from the add line to be added to the output line, wherein the added and dropped channels have the same wavelengths. In the OADM  550  ( FIG. 5B ), each pair of mirrors  505 . 1  and  507 . 1 ,  505 . 2  and  507 . 2 ,  505 . 3  and  507 . 3  and  505 . 4  and  507 . 4  functions in a coordinated fashion such that either both of or neither of the mirrors comprising each pair are in their “on” states. As shown in  FIG. 5B , the deflection caused by one mirror of each pair causes one signal to be dropped while the deflection caused by the other mirror causes a signal of the same wavelength to be added. 
   The collimated light of each channel exiting the apparatus  500  or the apparatus  550  to either the drop line or to the output line is collected by one of the focusing lenses  506  from which it is directed to either the multiplexer  502   d  or the multiplexer  502   b . Each multiplexer combines the various channels which it receives from the micro-mirror array  501   a – 501   b  into a single composite optical signal. 
   Although the micro-mirror based OADM&#39;s  500  and  550  utilize an elegant architecture and appear to perform their intended functions adequately, they suffer the drawback that the free-space path length through each apparatus increases proportionally to the total number of channels. The diameter of the collimated light of each channel increases as this free-space path length increases, thereby requiring high performance levels for the collimator lenses and tight tolerances for the mirror positions. This leads to difficulties in achieving and maintaining alignment between the various collimator and focusing lenses associated with each wavelength. Further, neither of these micro-mirror based OADM&#39;s provides channel power balancing or overall power control. Additional components must be either incorporated into or interfaced to the conventional OADM  500  or the OADM  550  to provide these latter functions. 
   Glossary 
   In this document, the individual information-carrying lights are referred to as either “signals” or “channels.” The totality of multiple combined signals in a wavelength-division multiplexed optical fiber, optical line or optical system, wherein each signal is of a different wavelength range, is herein referred to as a “composite optical signal.” 
   The term “wavelength,” denoted by the Greek letter λ (lambda) is used herein synonymously with the terms “signal” or “channel.” Although each information-carrying channel actually comprises light of a certain range of physical wavelengths, for simplicity, a single channel is referred to as a single wavelength, λ, and a plurality of n such channels are referred to as “n wavelengths” denoted λ 1 –λ n . Used in this sense, the term “wavelength” may be understood to refer to “the channel nominally comprised of light of a range of physical wavelengths centered at the particular physical wavelength, λ.” 
   Strictly speaking, a multiplexer is an apparatus which combines separate channels into a single wavelength division multiplexed composite optical signal and a de-multiplexer is an apparatus that separates a composite optical signal into its component channels. However, since many multiplexers and de-multiplexers ordinarily operate in either sense, the single term “multiplexer” is usually utilized to described either type of apparatus. Although this liberal usage of the term “multiplexer” is generally used in this document, the exact operation—either as a multiplexer or a de-multiplexer—of any particular apparatus should be clear from its respective discussion. 
   According to the above discussion, there is a need for an integrated optical component which can simultaneously perform optical demultiplexing, adding and dropping of multiple channels, power balancing among channels, and control of overall optical power levels. The present invention addresses such a need. 
   SUMMARY OF THE INVENTION 
   To address the above-mentioned need, a switchless OADM apparatus and an optical network system utilizing the same is herein disclosed. In an exemplary embodiment, a switchless OADM in accordance with the present invention comprises first and second variable optical splitters, a first multi-channel variable optical attenuator optically coupled between the first and second variable optical splitters, and a second multi-channel variable optical attenuator optically coupled to the first variable optical splitter. The first variable optical splitter splits a composite optical signal received from an input fiber optic line into two portions delivered to the first and second multi-channel variable optical attenuator, respectively. Each channel of each portion of the composite optical signal is either un-attenuated, partially attenuated or completely attenuated upon passing through one of the multi-channel variable optical attenuators, wherein the attenuation of each channel portion is independent of that of every other channel. The first multi-channel variable optical attenuator balances the optical power levels of a first set of channels comprising a first set of wavelengths, blocks transmission of the wavelengths comprising the remaining dropped channels and delivers the first set of channels to the second variable optical splitter. The second multi-channel variable optical attenuator blocks transmission of the wavelengths comprising the first set of channels, balances the optical power levels of the second set of channels and drops these channels to a dropped-channel line. A set of added channels, comprising the same wavelengths as the dropped channels are input to the second variable optical splitter from an added-channel line. The added channels and the first set of channels are combined by the second variable optical splitter into a single output composite optical signal that is output from the switchless OADM along the output fiber optic line. 
   In an exemplary embodiment, an optical network system in accordance with the present invention comprises a switchless OADM optically coupled to an input fiber line, an output fiber line comprising a first optical amplifier, a dropped-channel line comprising a second optical amplifier and an added-channel line. Such an optical network system in accordance with the present invention further comprises an analyzer/controller module which receives sample composite optical signals through optical couplings to the output line, to the dropped-channel line and to the added-channel line and which delivers electronic control signals to the switchless OADM and to the optical amplifiers. Such an optical network system in accordance with the present invention still further comprises a data input and/or computational device to receive configuration commands or information from network operators or from the network itself. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A  is an illustration of a first preferred embodiment of a switchless Optical Add-Drop Multiplexer (switchless OADM) apparatus in accordance with the present invention, wherein apparatuses combining multiplexing, de-multiplexing and variable optical attenuation functions are utilized. 
       FIG. 1B  is an illustration of a second preferred embodiment of a switchless OADM apparatus in accordance with the present invention, wherein interleaved channel separators are utilized together with apparatuses combining multiplexing, de-multiplexing and variable optical attenuation functions. 
       FIG. 1C  is an illustration of a third preferred embodiment of a switchless OADM apparatus in accordance with the present invention. 
       FIG. 1D  is an illustration of a fourth preferred embodiment of a switchless OADM apparatus in accordance with the present invention. 
       FIG. 2A  is an illustration of a first multifunctional apparatus suitable for use within a switchless OADM in accordance with the present invention, wherein the apparatus is a combined de-multiplexer, multiplexer and variable optical attenuator. 
       FIG. 2B  is an illustration of a second multifunctional apparatus suitable for use within a switchless OADM in accordance with the present invention, wherein the apparatus is a combined de-multiplexer and variable optical attenuator. 
       FIG. 2C  illustrates the attenuation of a single optical channel within the multifunctional apparatus of  FIG. 2A . 
       FIG. 2D  illustrates the attenuation of a single optical channel within the multifunctional apparatus of  FIG. 2B . 
       FIG. 3A  illustrates a first preferred embodiment of an optical network system, in accordance with the present invention, that utilizes a switchless OADM. 
       FIG. 3B  illustrates a second preferred embodiment of an optical network system, in accordance with the present invention, that utilizes a switchless OADM. 
       FIG. 3C  illustrates a third preferred embodiment of an optical network system, in accordance with the present invention, that utilizes a switchless OADM. 
       FIG. 4  is an illustration of a conventional OADM architecture utilizing separate multiplexers, de-multiplexers and a plurality of 2×2 switches. 
       FIGS. 5A–5   b  are illustrations of two prior-art OADM architectures utilizing micro-mirror arrays. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1A  illustrates a first preferred embodiment of a switchless OADM apparatus in accordance with the present invention. The switchless OADM  100  shown in  FIG. 1A  comprises a first variable optical splitter (VOS)  104   a , a second VOS  104   b , a first multi-channel variable optical attenuator (MC-VOA)  102   a  optically coupled between the first and second VOS&#39;s  104   a – 104   b , and a second MC-VOA  102   b  optically coupled to the first VOS  104   a . The first VOS  104   a  receives a composite optical signal comprising the channels λ 1 –λ n  as input from an input fiber optic line and splits the composite optical signal into two optical portions delivered to a first input/output optical fiber  206   a  optically coupled to the first MC-VOA  102   a  and a second input/output optical fiber  206   b  optically coupled to the second MC-VOA  102   b . The first and second optical portions comprise all the channels λ 1 –λ n  but the relative power levels of the two optical portions are variable and controlled by the first VOS  104   a.    
   Each of the MC-VOA&#39;s attenuates the optical power carried by each of the individual channels λ 1 –λ n  comprising the portion delivered to it. The construction of an MC-VOA is exhibited in  FIG. 2A  and is described in greater detail later in this document. Each channel comprising each optical portion is either un-attenuated, partially attenuated or completely attenuated (i.e., blocked) within one of the MC-VOA&#39;s. The attenuation of each channel within each optical portion is independent of that of every other channel. The first MC-VOA  102   a  outputs an attenuated composite optical signal to the second VOS  104   b  along first output fiber  207   a . The composite optical signal comprising the output of the first MC-VOA  102   a  comprises all the original channels from the input fiber optic line except for the dropped channels. These non-dropped channels are referred to as “express” channels. In the example provided in  FIG. 1A , the channels λ 1  and λ 2  are dropped; thus, in this example, the channels λ 3 –λ n  comprise the express channels. The dropped channels λ 1  and λ 2  comprising the first optical portion are blocked by the first MC-VOA  102   a . Further, the optical power levels of the express channels λ 3 –λ n  are balanced by the first MC-VOA  102   a.    
   The second MC-VOA  102   b  drops a set of drop channels to a Drop Port from the switchless OADM along the second output fiber  207   b . The second MC-VOA  102   b  blocks the express channels λ 3 –λ n  and balances the optical power levels of the dropped channels λ 1  and λ 2 . A second set of added channels are input to the switchless OADM  100  along the add line  114 . The set of wavelengths comprising the add channels input to the switchless OADM along add line  114  are a subset of the set of wavelengths dropped along the second output fiber  207   b . The wavelength of each added channel matches the wavelength of exactly one of the dropped channels and the number of added channels is less than or equal to the number of dropped channels. In the example illustrated in  FIG. 1A , there are two added channels λ′ 1  and λ′ 2  comprising the same physical wavelengths as the dropped channels λ 1  and λ 2 . The added channels and the express channels are combined by the second VOS  104   b  into a single output composite optical signal that is output from the switchless OADM  100  along the output fiber optic line. 
     FIG. 1B  illustrates a second preferred embodiment of a switchless OADM apparatus in accordance with the present invention. The switchless OADM  150  shown in  FIG. 1B  is utilized for situations in which the inter-channel wavelength spacing of the input composite optical signal is narrower than that which can be accepted directly by an MC-VOA. Therefore, in operation of the switchless OADM  150 , a composite optical signal is first split into a first subset of channels and a second subset of channels wherein the first and second subsets are interleaved with one another. The inter-channel spacing between channels comprising each subset is greater than that of the original composite optical signal and can be accepted by a an MC-VOA. The first subset of channels, e.g., the subset comprising “odd” channels, is directed to a first MC-VOA whilst the second subset, e.g., the subset comprising “even” channels, is directed to a second MC-VOA. After passing through the first and second MC-VOA&#39;s, the first and second subsets are recombined into a single composite optical signal. 
   The separation of a composite optical signal into a first subset of channels and a second subset of channels wherein the first and second subsets are interleaved with one another is performed by an interleaved channel separator. Suitable interleaved channel separator apparatuses are described in the following U.S. Pat. Nos. 6,263,126; 6,130,971; 6,215,926; 6,310,690; 6,263,129; 6,307,677; and 6,396,629 all of which are assigned to the assignee of the present application and which are incorporated herein by reference in their entirety. Additional suitable interleaved channel separator apparatuses are disclosed in a first co-pending U.S. patent application assigned to the assignee of the present application entitled “Multi-Functional Optical Device Utilizing Multiple Polarization Beam Splitters and Non-Linear Interferometers,” Ser. No. 09/630,891, filed on Aug. 2, 2000; and in a second co-pending U.S. patent application assigned to the assignee of the present application entitled “Method and Apparatus for Asymmetric Multiplexing and Demultiplexing of Optical Signals Utilizing a Diffraction Grating”, Ser. No. 09/894,057, filed Jun. 28, 2001. Applicant incorporates these co-pending U.S. patent applications herein by reference in their entirety. The re-multiplexing of a first set of channels and a second set of channels, wherein the first and second sets of channels are interleaved with one another is performed by an interleaved channel combiner. As a practical matter, the interleaved channel combiner may comprise an apparatus identical to the interleaved channel separator, but with the inputs and outputs interchanged from those of the interleaved channel separator. 
   In operation of the switchless OADM  150  ( FIG. 1B ), the first VOS  104   a  receives a composite optical signal as input from an input fiber optic line and splits the composite optical signal into two optical portions respectively delivered to the first interleaved channel separator  116   a  via a first optical coupling  106   a  and to the second interleaved channel separator  116   b  via a second optical coupling  106   c . Preferably, the optical coupling  106   a  and the optical coupling  106   c  comprise optical fibers but may comprise alternative or additional bulk optical components or other waveguides. The first interleaved channel separator  116   a  separates the composite optical signal comprising the first portion into a subset comprising the odd channels λ 1 , λ 3 , . . . λ n−1  delivered to the input/output fiber  206   a  of a first MC-VOA  102   a  and into a subset comprising the even channels λ 2 , λ 4 , . . . λ n  delivered to the input/output fiber  206   c  of a third MC-VOA  102   c . The first interleaved channel combiner  117   a  combines the odd channels exiting the first MC-VOA  102   a  via its output fiber  207   a  together with the even channels exiting the third MC-VOA  102   c  via its output fiber  207   c . Likewise, the second interleaved channel separator  116   b  separates the composite optical signal comprising the second optical portion into a subset comprising the odd channels λ 1 , λ 3 , . . . λ n−1  delivered to the input/output fiber  206   b  of a second MC-VOA  102   b  and into a subset comprising the even channels λ 2 , λ 4 , . . . λ n  delivered to the input/output fiber  206   d  of a fourth MC-VOA  102   d . The second interleaved channel combiner  117   b  combines the odd channels exiting the second MC-VOA  102   b  via its output fiber  207   b  together with the even channels exiting the fourth MC-VOA  102   d  via its output fiber  207   d.    
   The channels comprising the composite optical signal output by the second interleaved channel combiner  117   b  exit the switchless OADM  150  via the optical coupling  106   d  which leads to a Drop Port. The channels output from the first interleaved channel combiner  117   a  are combined together with added channels input along add line  114  by the second VOS  104   b . The second VOS  104   b  outputs a single output composite optical signal along the output fiber optic line. 
   In summary, the switchless OADM  150  ( FIG. 1B ) comprises a first VOS  104   a , a second VOS  104   b , a first interleaved channel separator  116   a  optically coupled to the first VOS  104   a , a first interleaved channel combiner  117   a  optically coupled to the second VOS  104   b , an added-channel line  114  optically coupled to the second VOS  104   b , a pair of MC-VOA&#39;s  102   a  and  102   c  optically coupled in parallel between the first interleaved channel separator  116   a  and first interleaved channel combiner  117   a , a second interleaved channel separator  116   b  optically coupled to the first VOS  104   a , a second interleaved channel combiner  117   b  optically coupled to a Dropped Port via optical coupling  106   d , and a second pair of MC-VOA&#39;s  102   b  and  102   d  optically coupled in parallel between the second interleaved channel separator and the second interleaved channel combiner. 
   In the example illustrated in  FIG. 1B , two channels λ 1  and λ 2  are dropped to the Drop Port via the optical coupling  106   d  and the two channels λ′ 1  and λ′ 2  are added from the added-channel line  114 . The channels comprising the first optical portion are separated between the first MC-VOA  102   a  and the third MC-VOA  102   c  as described above. The first MC-VOA  102   a  blocks the dropped channel λ 1  whilst the third MC-VOA  102   c  blocks the dropped channel  2 . Further, the power levels of the remaining channels, which are the express channels λ 3 –λ n , are balanced through the coordinated operation of the two MC-VOA&#39;s  102   a  and  102   c . The express channels λ 3 –λ n  are delivered to the second VOS  104   b  via the optical coupling  106   b . The channels comprising the second optical portion are separated between the second MC-VOA  102   b  and the fourth MC-VOA  102   d  as described above. The second MC-VOA  102   b  blocks the odd express channels λ 3 , λ 5 , . . . , λ n−1  whilst the fourth MC-VOA  102   d  blocks the even express channels λ 2 , λ 4 , . . . , λ n . The power levels of the dropped channels λ 1  and λ 2  are balanced the coordinated operation of the two MC-VOA&#39;s  102   b  and  102   d.    
     FIG. 1C  illustrates a third preferred embodiment of a switchless OADM apparatus in accordance with the present invention. The switchless OADM apparatus  160  shown in  FIG. 1C  is similar to the switchless OADM  100  shown in  FIG. 1A  except that the dropped channels enter a first Variable Optically Attenuating Multiplexer/Demultiplexer (MC-VOA-MUX)  103   a  instead of an MC-VOA and the added channels may pass through a second MC-VOA-MUX  103   b . The first MC-VOA-MUX  103   a  receives the channels comprising the second optical portion from the input/output fiber  206   b , demultiplexes these channels, transmits the dropped channels while blocking the non-dropped channels, and outputs each of the dropped channels to a different respective channel fiber. The channel fibers  205 . 1 ,  205 . 2 ,  205 . 3 , . . . ,  205 . n  carry the channels λ 1 , λ 2 , λ 3 , . . . , λ n , respectively, provided that such channels are dropped. The second MC-VOA-MUX  103   a  receives a unique channel or wavelength from a different one of the respective channel fibers  213 . 1 ,  213 . 2 ,  213 . 3 , . . . ,  213 . n , provided that such channels are present, and transmits the channels to be added while blocking the non-added channels, and multiplexes the added channels to a single input/output fiber  206   e . The construction of an MC-VOA-MUX is exhibited in  FIG. 2B  and is described in greater detail later in this document. 
     FIG. 1D  illustrates a fourth preferred embodiment of a switchless OADM apparatus in accordance with the present invention. The switchless OADM apparatus  170  shown in  FIG. 1D  is similar to the switchless OADM  150  shown in  FIG. 1B  except that a pair of MC-VOA-MUX&#39;s  103   a – 103   b  replace the two MC-VOA&#39;s within the pathways of the dropped channels and another pair of MC-VOA-MUX&#39;s  103   c – 103   d  replace the two MC-VOA&#39;s within the pathways of the added channels. The operation of the switchless OADM  160  ( FIG. 1C ) and the switchless OADM  170  ( FIG. 1D ) differs from that of the switchless OADM  100  ( FIG. 1A ) and the switchless OADM  150  ( FIG. 1B ), respectively, through the fact that, in operation of the apparatus  160  and the apparatus  170 , each of the dropped channels is output to a different respective one of the channel fibers  205 . 1 – 205 . n  and each of the added channels is received from a different respective one of the channel fibers  213 . 1 – 213 . n . Each channel fiber  205 . 1 – 205 . n  utilized for output in the operation of the apparatus  160  and the apparatus  170  may be optically coupled to a different respective optical receiver or detector. Each channel fiber  213 . 1 – 213 . n  utilized for input in the operation of the apparatus  160  and the apparatus  170  may be optically coupled to a different respective optical transmitter or other light source. 
     FIG. 2A  illustrates a multi-channel variable optical attenuator (MC-VOA) apparatus  102 . The MC-VOA  102  is disclosed in more detail in a co-pending U.S. patent application titled “Method and Apparatus for Simultaneous Multiplexing and Demultiplexing, Variable Attenuation and Power Detection of Wavelength Division Multiplexed Optical Signals”, Ser. No. 09/894,069, filed Jun. 28, 2001, assigned to the assignee of the present application. Applicant hereby incorporates this patent application by reference. The MC-VOA  102  shown in  FIG. 2A  comprises an input/output fiber  206 , an output fiber  207  adjacent to the input fiber, a collimating and focussing lens  208 , a diffraction grating  210 , and a mirror  218 . Additionally, the MC-VOA  102  comprises an array  204  of movable rods disposed between the lens  208  and the mirror  218  and slightly offset from the plane of the paths of channels  240 . 1 ,  240 . 2 , etc. There is exactly one moveable rod associated with and controlling the attenuation of each channel. Optionally, the MC-VOA  102  further comprises an array  209  of detectors also disposed between the lens  208  and the mirror  218  and on the opposite side of the plane of the channel paths from the moveable rod array  204 . There is exactly one optical detector within array  209  for each output channel. The fibers  206 – 207  and the mirror  218  are substantially disposed at the focal distance, f, from the lens  208 . 
   In operation of the MC-VOA  102 , a composite optical signal  240  emanates from the input/output fiber  206 . The diverging light of the composite optical signal  240  is collimated by the lens  208  from which it is directed onto the diffraction grating  210 . The diffraction grating  210  spatially disperses—that is, diffracts—the channels comprising the composite optical signal  240  according their respective wavelengths. The path of a first such channel  240 . 1  and of a second such channel  240 . 2  are respectively shown by dashed and dotted lines in  FIG. 2A . After diffraction by grating  210 , the collimated lights of channels  240 . 1 – 240 . 2  return to lens  208  from which they are focused to points  212 . 1  and  212 . 2 , respectively, on the mirror  218 . Other channels are diffracted such that they are focused to different respective points on mirror  218  roughly collinear with points  212 . 1  and  212 . 2 . 
   The mirror  218  causes these channels to reflect back through the lens  208  to the grating, and to diffract from the grating back through the lens to the single output fiber  207 , thereby substantially retracing their respective pathways and re-multiplexing the channels into a composite optical signal. The mirror  218  is tilted slightly from vertical such that, after reflection from the mirror  218 , the pathways of the reflected channels acquire a slight vertical component, causing them to return to the fiber  207 , instead of the fiber  206 . 
     FIG. 2B  illustrates a Variable Optically Attenuating Multiplexer/Demultiplexer (MC-VOA-MUX). The MC-VOA-MUX  103  shown in  FIG. 2B  is similar to the MC-VOA  102  shown in  FIG. 2A  except that the mirror  218  and the output fiber  207  comprising the MC-VOA  102  are eliminated; the mirror is replaced by a set of channel fibers  205 . 1 ,  205 . 2 ,  205 . 3 , . . . ,  205 . n  disposed such that an end of each such fiber is at a single respective focal point  212 . 1 ,  212 . 2 , etc. of the light of one of the channels. Each such fiber is referred to herein as a “channel fiber” because it carries the light of a single respective channel or wavelength, if present, from a composite optical signal potentially comprising the n different channels λ 1 –λ n . The channels may be either input to the apparatus  103  or output from the apparatus  103  via the channel fibers. In the first such case, the MC-VOA-MUX  103  operates as a multiplexer and the channels are combined, by diffraction by the grating  210 , so as to be output as a combined composite optical signal  210  through the input/output fiber  206 . In the second case, the MC-VOA-MUX  103  operates as a de-multiplexer such that a composite optical signal received from the input/output fiber  206  is spatially separated into its component channels, by diffraction by the grating  210 , so that each respective channel is focussed by lens  208  into a different respective channel fiber. The MC-VOA-MUX  103  ( FIG. 2B ) retains the moveable rod array  204  that also comprises the MC-VOA  102  ( FIG. 2A ). Each of the individual moveable rods  204 . 1 ,  204 . 2 ,  204 . 3 , etc. is disposed so as to either partially or completely intercept the light of a different respective channel just prior to its entry into just subsequent from its emanation from a channel fiber. 
     FIGS. 2C–2D  illustrate the operation of a single moveable rod, for instance moveable rod  204 . 1 , within the moveable rod array. The configuration illustrated in  FIG. 2C  and in  FIG. 2D  pertains to the MC-VOA  102  and to the MC-VOA-MUX  103 , respectively. The particular moveable rod  204 . 1  intercepts the optical path of only one particular channel  240 . 1  of the composite optical signal. Also shown in  FIGS. 2C–2D  is the relative position of one particular detector  209 . 1  of the detector array  209 , which is capable of receiving a portion of the light of channel  240 . 1  that is either reflected or scattered by the moveable rod  204 . 1 . It is to be kept in mind that a configuration similar to that shown in  FIGS. 2C–2D  exists for each channel and that each individual moveable rod and individual detector comprising the MC-VOA  102  or the MC-VOA-MUX  103  functions independently of the others. 
   The top drawing of each of  FIGS. 2C–2D  illustrates a situation in which the rod  204 . 1  is in a “null” position with respect to the light comprising the channel  240 . 1 . In this null position, a small proportion of the light of channel  240 . 1  is intercepted by the tip of rod  204 . 1  and is either scattered or reflected. This scattering or reflection occurs in the direction of the detector  209 . 1  (if present). The proportion of light that is intercepted by rod  204 . 1  in its null position sufficient to permit reliable detection by the detector  209 . 1  (if present) but is sufficiently small that the power level of channel  240 . 1  is not significantly degraded. In this null configuration, the detector measures the amount of light that is reflected or scattered out of the path of channel  240 . 1  by the rod  204 . 1 . Since the position of rod  204 . 1  is constant in this null position, the light reaching the detector  209 . 1  is proportional to the power level of channel  240 . 1 . By extracting an electrical signal from the detector  209 . 1 , the power level of channel  240 . 1  may be constantly monitored by a gauge, computer or other data apparatus (not shown). 
   The bottom drawing of each of  FIGS. 2C–2D  illustrates a situation in which the rod  204 . 1  is moved, rotated or bent into a position such that a significant proportion of the light comprising the channel  240 . 1  is intercepted. The proportion of the power of channel  240 . 1  that is intercepted is roughly proportional to the percentage of the cross sectional area of channel  240 . 1  that is intercepted by rod  204 . 1 , as projected onto a plane perpendicular to the light propagation direction. A proportion of the light intercepted by moveable rod  204 . 1  is either scattered or reflected in the direction of the detector  209 . 1 . Since the surface area of rod  204 . 1  that is illuminated by channel  240 . 1  varies with the depth of penetration of rod  204 . 1  into the light comprising channel  240 . 1 , the amount of light scattered or reflected to the detector  209 . 1  varies with the degree of attenuation. The signal produced by detector  209 . 1  may thus be used to monitor the degree of attenuation. Preferably, the tip of rod  204 . 1  is of a smooth curved shape—such as a sphere or cylinder—so that the scattered light observed by the detector varies predictably and regularly with the degree of attenuation. 
   When the rod  204 . 1  is moved out of its “null” position in the MC-VOA  102  ( FIG. 2C ), the light comprising the channel  240 . 1  is intercepted by the rod both prior to encountering the mirror  218  and after reflecting from the mirror. Thus, only the light of channel  240 . 1  occupying the shaded region  211 . 1  is capable of passing the moveable rod  204 . 1  in both the forward and reverse directions so as to complete its traverse through the MC-VOA  102 . The shaded region  211 . 1  represents a roughly conical volume. When the rod  204 . 1  is moved out of its “null” position in the MC-VOA-MUX  103  ( FIG. 2D ), the light comprising the channel  240 . 1  is intercepted by the rod only once—either just prior to entering the channel fiber  205 . 1  (if the apparatus  103  is utilized as a de-multiplexer) or just after emanating from the fiber (if the apparatus is  103  utilized as a multiplexer). Accordingly, only the light of channel  240 . 1  that occupies the shaded region  211 . 1  of  FIG. 2D  is capable of passing completely through the MC-VOA-MUX  103 . 
     FIG. 3A  illustrates a first optical system, in accordance with the present invention, that utilizes a switchless OADM. The system  300  shown in  FIG. 3A  comprises a switchless OADM  302  optically coupled to an input fiber line  306 , an output fiber line  308 , a drop line  310  and an add line  312 . Additionally, the system  300  comprises an optical amplifier  304   a  disposed along the output line and an optional second optical amplifier  304   b  disposed along the drop line. Preferably, the switchless OADM  302  comprises one of the aforementioned embodiments such as switchless OADM  100  ( FIG. 1A ), or switchless OADM  100   150  ( FIG. 1B ). In operation of the system  300 , an input composite optical signal comprising a plurality of channels is input to the switchless OADM  302  from the input line  306  and the switchless OADM  302  drops at least one selected channel to the output line  310  wherein it is amplified by optical amplifier  304   b  and passes through the remaining express channels to the output line  308 . Additionally, during operation of the system  300 , the switchless OADM  302  receives at least one channel to be added to the express channels, wherein the wavelength(s) of the at least one added channels correspond(s) to wavelength(s) of the dropped channels and outputs the added channel(s) together with the express channel(s) to the output line  308 , wherein they are amplified by optical amplifier  304   a . The optical amplifiers  304   a – 304   b  are utilized to restore the optical power of the output channels and of the dropped channels to nominal levels and are necessary because of the power loss, relative to the composite optical signal delivered from the input line  306 , caused by the action of the variable optical splitters comprising the switchless OADM  302 . 
     FIG. 3B  illustrates a second optical system, in accordance with the present invention, that utilizes a switchless OADM. The system  350  shown in  FIG. 3B  comprises all the same components as the system  300  ( FIG. 3A ). Additionally, the system  350  comprises an analyzer/controller module  314  which is optically coupled to the output line by optical taps  315   a – 315   b  and optical couplings  316   a – 316   b , to the drop line by optical taps  315   c – 315   d  and optical couplings  316   c – 316   d  and to the add line by optical tap  315   e  and optical coupling  316   e , respectively. The optical couplings  316   a – 316   e , which preferably comprise optical fibers, receive small sample proportions of the composite optical signals carried along the output line, the drop line and the add line from the optical taps  315   a – 315   e , respectively. In the example shown in  FIG. 3B , the two optical taps  315   a – 315   b  are disposed along the output line before and after, respectively, the optical amplifier  304   a  and the two optical taps  315   c – 315   d  are disposed along the drop line before and after, respectively, the optical amplifier  304   b . One or the other of each such pair of optical taps may be omitted, depending upon the needs of the operator. The system further comprises a data input and/or computational device  322 , such as a computer or data terminal, to receive configuration commands from network operators or to receive network status information from the network itself and to make decisions concerning which channels are to be dropped and which channels are to be added by the system  350 . The device  322  delivers this information to the analyzer/controller module  314  over electronic line  324 . 
   The analyzer/controller module  314  comprising the system  350  ( FIG. 3B ) receives the various sample proportions of the composite optical signals and analyzes for the presence of and the optical power level of each of the various wavelengths which may comprise each sampled composite optical signal. For instance, the analyzer/controller module  314  may comprise, in part, an optical channel analyzer, which is a known apparatus which performs such functions. The analyzer/controller module  314  further comprises a computer or device with other electronic control capability to send an adjustment signal or signals back to the switchless OADM  302  via electronic control line  320   c . The control signal delivered along the electronic control line  320   c  controls the multi-channel variable optical attenuators and the variable optical splitters comprising the switchless OADM  302  so as to balance the power levels of the individual channels propagating along the drop line and along the output line to common levels, to control the overall optical power along the drop line and the output line, to block express channels from the drop line and to block dropped channels from the output line. Optionally, the analyzer/controller module  314  further sends control signals to the optical amplifiers  304   a – 304   b  via the electronic control lines  320   a – 320   b  so as to increase or decrease the gain of these amplifiers as required by the needs of the network. 
     FIG. 3C  illustrates a third optical system, in accordance with the present invention, that utilizes a switchless OADM. The system  370  shown in  FIG. 3   c  comprises all the same components as the system  350  ( FIG. 3B ) except that the two optical taps  315   a  and  315   c  and the two optical couplings  316   a  and  316   c  comprising system  350  are replaced by the two electronic signal lines  326   a – 326   b . The electronic signal lines  326   a – 326   b  carry information from the detector arrays comprising the MC-VOA&#39;s  102   a – 102   b  with the switchless OADM  302  to the analyzer/controller module  314 . As described above, the output of the detector arrays carries information on the optical power level and/or the attenuation level of the various optical channels entering the MC-VOA of which the detector array is a part. With such information sent directly from the switchless OADM  302  to the analyzer/controller module  314 , the need for the separate optical couplings  316   a  and  316   c  is eliminated. 
   A switchless OADM apparatus and system have been disclosed. The apparatus and system of the present invention provide capabilities of multi-channel variable optical attenuation for channel power balancing that are not available from conventional OADM&#39;s. Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended claims.