Abstract:
An optical add/drop multiplexer (OADM) having reduced crosstalk is disclosed. The OADM uses an optical interleaver to separate channels of a wavelength division multiplexed signal into a plurality of branches. The branches then separately act on the widely spaced channels to add or drop channels. After the add/drop function is completed, the channels on the branches are recombined.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     This invention relates generally to design and manufacturing of multiple wavelength add/drop systems used in optical communications. Each system is comprised of wavelength selective optical switches and optical wavelength interleavers.  
         [0003]     2. Description of the Related Art  
         [0004]     Optical wavelength division multiplexing (WDM) is an important method used in modern optical fiber communication systems to drastically increase data transmission rate. In WDM systems, communication is by means of transmitting and receiving optical pulses consisting of signals with different wavelengths (wavelength channels). Each wavelength channel carries its own data information transmitted over optical fibers. The main advantage with WDM technology is, therefore, that a single optical fiber can be used to transmit a number of distinguishable optical signals simultaneously. The result is a significant increase of effective bandwidth of the optical fiber and data transmission rate of the communication system.  
         [0005]     In WDM networks of the past, adding, dropping or cross-connection of individual wavelengths involved conversion of optical signals into electronic signals first. After appropriate manipulations of the electronic signals, the electronic signals are converted back to optical signals before being delivered via optical fibers. These conversions became the bottleneck of the WDM networks. Development of all-optical switches for applications ranging from add-drop functionality to large-scale cross-connects is key to adding intelligence to the optical layer of, and thereby enhancing, the optical networking systems. However, with current technical limitations, all fiber network systems implemented with optical switches are still expensive.  
         [0006]     The current state of the art in optical switching and signal transmission systems, moreover, are limited to optical switching of an entire spectral range without wavelength differentiation or selection. As a result, an optical switch operation often requires a wavelength de-multiplexer and a multiplexer to achieve the transfer of optical signals of different wavelengths to different ports. This is interpreted into more complicated system configurations, higher manufacture and maintenance costs, and lower system reliability.  
         [0007]     Designs of all optical add/drop multiplexers (OADM) were proposed (see Okamoto, “Recent progress of integrated optics planar lightwave circuits,”  Optical and Quantum Electronics , Vol. 31, pp. 107-129, 1999). In conventional designs, optical signals undergo three basic steps within an OADM. First, all wavelength channels are demultiplexed. Optical signals are then dropped from, or added to, one or few chosen wavelength channels. Finally all channels are multiplexed back together. In this process, even if only signals from one channel are modified, signals in all channels are disturbed. After several passes through OADM&#39;s, signals in all channels are necessarily degraded. This presents a cascade problem.  
         [0008]     In U.S. patent application Ser. No. 10/188,955, an OADM utilizing optical switches based on a novel grating-assisted coupler had been suggested. In this structure, optical switches are chained one after another. The number of optical switches used is directly proportional to the number of channels of the OADM. As propagation loss of the optical signals is also proportional to the number of optical switches and the total optical path length, power loss can be significant if the OADM channel number is large.  
         [0009]     In this invention, a modified and improved architecture is suggested to rectify this disadvantage. 
     
    
     BRIEF DESCRIPTIONS OF THE DRAWINGS  
       [0010]     The present invention can be better understood with reference to the following drawings. The components within the drawings are not necessarily to scale relative to each other, emphasis instead being placed upon clearly illustrating the principles of the present invention.  
         [0011]      FIG. 1  is a graph illustrating the source of channel crosstalk in a conventional OADM when it is in the “off” state.  
         [0012]      FIGS. 2 and 4  are schematic diagrams showing the two different modes of operations of a prior art 1:2 optical wavelength interleaver.  
         [0013]      FIGS. 3A  to  3 C describe the input and output characteristics of the prior art 1:2 optical wavelength interleaver shown in  FIGS. 2 and 4 .  
         [0014]     FIGS.  5 A˜ 5 B are schematic diagrams showing the two different modes of operations of a prior art 1:4 optical wavelength interleaver.  
         [0015]     FIGS.  6 A˜ 6 E describe the input and output characteristics of the prior art 1:4 optical wavelength interleaver shown in  FIGS. 5A and 5B .  
         [0016]     FIGS.  7 A˜ 7 B are schematic diagrams showing, respectively, a two-to-one and a four-to-one optical waveguide combiner.  
         [0017]      FIG. 8  is a schematic diagram showing the function of a prior art optical circulator.  
         [0018]      FIG. 9  is a schematic diagram showing the function of a prior art optical isolator.  
         [0019]      FIG. 10A  is a schematic diagram showing the function of a wavelength selective optical “drop” switch when it is in the “off” state;  FIG. 10B  is a graph explaining the drastic reduction of channel crosstalk in this case.  
         [0020]     FIGS.  11 A˜ 11 B are schematic diagrams showing functions of, respectively, a wavelength selective (λ 3 ) optical “drop” switch and a similar “add” switch. The bold outline of the elements indicates that the switches are in the “on” state.  
         [0021]     FIGS.  12 A˜ 12 C are schematic diagrams showing the structure and operations of a 4-channel OADM which is based on the 1:2 optical wavelength interleavers as shown in  FIGS. 2 and 4  and the wavelength selective optical “drop” and “add” switches as shown in  FIGS. 11A and 11B .  
         [0022]      FIG. 13  is a schematic diagram showing the structure of a 4-channel OADM which is based on the 1:2 optical wavelength interleaver as shown in  FIG. 2 , the two-to-one optical waveguide combiner as shown in  FIG. 7A , and the wavelength selective optical “drop” and “add” switches as shown in FIGS.  11 A˜ 11 B.  
         [0023]      FIG. 14A  is a plot illustrating the channel crosstalk problem in the case that the bandwidth of the wavelength selective optical switches is too wide compared to the channel bandwidth.  
         [0024]      FIG. 14B  shows that the channel crosstalk problem can be avoided if the neighboring channels are farther apart.  
         [0025]     FIGS.  15 A˜ 15 C are schematic diagrams showing the structure and operations of an 8-channel OADM which is based on the 1:4 optical wavelength interleavers as shown in  FIGS. 5A and 5B  and the wavelength selective optical “add” and “drop” switches as shown in FIGS.  11 A˜ 11 B.  
         [0026]      FIG. 16  is a schematic diagram showing the structure of an 8-channel OADM which is based on the 1:4 optical wavelength interleavers as shown in  FIG. 5A , the four-to-one optical waveguide combiner as shown in  FIG. 7B , and the wavelength selective optical “add” and “drop” switches as shown in FIGS.  11 A˜ 11 B.  
         [0027]      FIG. 17  is a schematic diagram showing functions of a wavelength selective (λ 3 ) optical “add/drop” switch. The bold outline of the element indicates that the switch is in the “on” state.  
         [0028]     FIGS.  18 A˜ 18 B are schematic diagrams showing the structure and operations of a 4-channel OADM which is based on the 1:2 optical wavelength interleavers as shown in  FIGS. 2 and 4  and the wavelength selective optical “add/drop” switch as shown in  FIG. 17 .  
         [0029]      FIG. 19  is a schematic diagram showing the structure of a 4-channel OADM which is based on the 1:2 optical wavelength interleaver as shown in  FIG. 2 , the two-to-one optical waveguide combiner as shown in  FIG. 7A , and the wavelength selective optical “add/drop” switch as shown in  FIG. 17 .  
         [0030]     FIGS.  20 A˜ 20 B are schematic diagrams showing the structure and operations of an 8-channel OADM which is based on the 1:4 optical wavelength interleavers as shown in  FIGS. 5A and 5B  and the wavelength selective optical “add/drop” switch as shown in  FIG. 17 .  
         [0031]      FIG. 21  is a schematic diagram showing the structure of an 8-channel OADM which is based on the 1:4 optical wavelength interleaver as shown in FIG.  5 A, the four-to-one optical waveguide combiner as shown in  FIG. 7B , and the wavelength selective optical “add/drop” switch as shown in  FIG. 17 .  
         [0032]      FIG. 22  is a schematic diagram showing functions of a wavelength selective (λ k ) reflective optical switch. The bold outline of the element indicates that the switch is in the “on” state.  
         [0033]      FIG. 23  is a structural schematic showing a grating-based wavelength selective reflective optical switch when it is in the “on” state.  
         [0034]     FIGS.  24 A˜ 24 B are schematic diagrams showing the structure and operations of a 4-channel optical “drop” multiplexer which is based on the  1 : 2  optical wavelength interleavers as shown in  FIGS. 2 and 4 , the optical circulator as shown in  FIG. 8 , and wavelength selective reflective optical switches as shown in  FIG. 22 .  
         [0035]      FIG. 25  is a schematic diagram showing the structure of a 4-channel optical “drop” multiplexer which is based on the 1:2 optical wavelength interleaver as shown in  FIG. 2 , the two-to-one optical waveguide combiner as shown in  FIG. 7A , the optical circulator as shown in  FIG. 8 , and wavelength selective reflective optical switches as shown in  FIG. 22 .  
         [0036]     FIGS.  26 A˜ 26 B are schematic diagrams showing the structure and operations of an 8-channel optical “drop” multiplexer which is based on the  1 : 4  optical wavelength interleavers as shown in FIGS.  5 A˜ 5 B, the optical circulator as shown in  FIG. 8 , and wavelength selective reflective optical switches as shown in  FIG. 22 .  
         [0037]      FIG. 27  is a schematic diagram showing the structure of an 8-channel optical “drop” multiplexer which is based on the 1:4 optical wavelength interleaver as shown in  FIG. 5A , the four-to-one optical waveguide combiner as shown in  FIG. 7B , the optical circulator as shown in  FIG. 8 , and wavelength selective reflective optical switches as shown in  FIG. 22 .  
         [0038]      FIG. 28A  is a schematic diagram showing functions of a multi-wavelength optical blocking device, which is based on wavelength selective reflective optical switches as shown in  FIG. 22  and the optical isolator as shown in  FIG. 9 , when it is in the “off” state; the bold outline of the wavelength selective (λ 2 ) reflective optical switch in  FIG. 28B  indicates that the switch is in the “on” state.  
         [0039]     FIGS.  29 A˜ 29 B are schematic diagrams showing the structure and operations of a multi-wavelength optical blocking device which is based on the 1:2 optical wavelength interleavers as shown in  FIGS. 2 and 4 , the optical isolator as shown in  FIG. 9 , and wavelength selective reflective optical switches as shown in  FIG. 22 .  
         [0040]      FIG. 30  is a schematic diagram showing the structure of a multi-wavelength optical blocking device which is based on the 1:2 optical wavelength interleaver as shown in  FIG. 2 , the two-to-one optical waveguide combiner as shown in  FIG. 7A , the optical isolator as shown in  FIG. 9 , and wavelength selective reflective optical switches as shown in  FIG. 22 .  
         [0041]     FIGS.  31 A˜ 31 B are schematic diagrams showing the structure and operations of a multi-wavelength optical blocking device which is based on the 1:4 optical wavelength interleavers as shown in FIGS.  5 A˜ 5 B, the optical isolator as shown in  FIG. 9 , and wavelength selective reflective optical switches as shown in  FIG. 22 .  
         [0042]      FIG. 32  is a schematic diagram showing the structure of a multi-wavelength optical blocking device which is based on the 1:4 optical wavelength interleaver as shown in  FIG. 5A , the four-to-one optical waveguide combiner as shown in  FIG. 7B , the optical isolator as shown in  FIG. 9 , and wavelength selective reflective optical switches as shown in  FIG. 22 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]     Some Prior Art Optical Devices  
         [0044]     The present invention builds on previous work of the assignee of the present invention. For example, in co-pending U.S. patent application Ser. No. 10/188,955 filed Jul. 3, 2002 and herein incorporated by reference, an optical switching and routing system is shown that uses grating based wavelength selective switches. Similarly, in co-pending U.S. patent application Ser. No. 10/190,018 filed Jul. 5, 2002 and herein incorporated by reference, a Bragg grating switch is shown. These devices are used extensively throughout the present invention. However, for the sake of clarity, the details of those switches is described in detail. From a functional standpoint, the switches are operative to add or drop selected wavelengths from a multiplexed signal carried by an optical waveguide or fiber. The switching of the switches can be accomplished either thermally, electronically, or mechanically.  
         [0045]     Other types of “add/drop” switches are also suitable for use in the present invention. For example, one-ring resonators, such as those described in U.S. Pat. No. 6,411,752 are one alternative. The wavelength of the optical switch can be altered, for instance, via a heater placed above or in close proximity to the micro-ring. In the case that the micro-ring is made of semiconductor material, wavelength of the optical switch can be adjusted electrically by controlling the current injection level. Or if the micro-ring is moveable, wavelength of the optical switch can be adjusted mechanically.  
         [0046]     Similar optical “add/drop” switches based on multi-ring resonators, such as those suggested by Hryniewicz, et al in “Higher Order Filter Response in Coupled Microring Resonators,” IEEE Photonics Technology Letters, Vol. 12, No. 3, pp. 320-322, March 2000, are another alternative.  
         [0047]     In a conventional multi-wavelength (λ k ) optical switch when the switch is in the “off” state, there is the problem of channel cross-talk. The wavelength selective (λ k ) optical switch is said to be in the “off” state when the switch is tuned to a wavelength other than λ k , for example, at λ k″  (that is, λ k″ ≠λ k ). In order not to interfere with other channels, the wavelength λ k″  cannot be the same as any of the channel wavelengths. This means that λ k″  is necessarily in between channel wavelengths as shown in  FIG. 1 . If channel spacing is small compared with the bandwidth of the optical switch, this may result in (a) channel crosstalk and/or (b) unnecessary power loss among neighboring channels.  
         [0048]     An interleaver is a periodic optical filter that combines or separates a comb of WDM signals. The operations and functions of interleavers are well-known (see, for example, S. Cao et al, “Interleaver Technology: Comparisons and Applications Requirements,” IEEE Journal of Lightwave Technology, Vol. 22, No. 1, pp. 281-289, January 2004).  
         [0049]      FIG. 2  illustrates a prior art 1:2 optical wavelength interleaver  1101  with its input port  1104  and output ports  1102  and  1103 . In this configuration, it separates the set of WDM signals as shown in  FIG. 3A  into two separate sets as shown in  FIGS. 3B and 3C , respectively. Similarly,  FIG. 4  illustrates a prior art 2:1 interleaver  1111  with its input ports  1113  and  1114  and output port  1112 . It combines the two sets of WDM signals as shown in  FIGS. 3B and 3C  into one set as shown in  FIG. 3A . In other words, the input/output characteristics of an optical wavelength interleaver are reciprocated.  
         [0050]     The schematics of the two different modes of a prior art 1:4 optical wavelength interleaver are shown in FIGS.  5 A˜ 5 B. As in the case of the 1:2 optical wavelength interleaver, if the WDM signals at the input port  1301  of the 1:4 optical wavelength interleaver  1302  are as shown in  FIG. 6A , the interleaver separates the input signals into four streams such that the output WDM signals at ports  1303 ,  1304 ,  1305  and  1306  are as shown in  FIGS. 6B, 6C ,  6 D and  6 E, respectively.  
         [0051]     Conversely, in the case of the 4:1 optical wavelength interleaver  1311  as shown in  FIG. 5B , if the WDM signals at the input ports  1313 ,  1314 ,  1315  and  1316  are, respectively, as shown in  FIGS. 6B, 6C ,  6 D and  6 E, then the interleaver combines the input signals into one single stream such that the output WDM signals at port  1312  is as shown in  FIG. 6A . Thus, similar to the case of the 1:2 optical wavelength interleavers, the input/output characteristics of the 1:4 optical wavelength interleavers are also reciprocated.  
         [0052]     Another type of optical device is an optical waveguide combiner, which is shown in FIGS.  7 A˜ 7 B. The input/output characteristics of the 2:1 optical waveguide combiner  1501  ( FIG. 7A ) is such that if the input  1503  and  1504  are, respectively, as shown in  FIGS. 3B and 3C , then the output  1502  is as shown in  FIG. 3A . In this sense, the optical waveguide combiner functions similar to a 2:1 optical wavelength interleaver.  
         [0053]     Likewise, the input/output characteristics of a 4:1 optical waveguide combiner  1511  ( FIG. 7B ) is not too different from a 4:1 optical wavelength interleaver ( FIG. 5B ). If the input  1513 ,  1514 ,  1515  and  1516  are as shown in  FIGS. 6B, 6C ,  6 D and  6 E, respectively, than the output  1512  is as shown in  FIG. 6A .  
         [0054]     An optical circulator  1601  is shown in  FIG. 8 . Input signals at port  1602  are directed to port  1604 , whereas input signals at port  1604  are directed to port  1603 .  
         [0055]     An optical isolator  1701  is shown in  FIG. 9 . The device appears transparent to forward-propagating optical signals traversing from port  1702  to port  1703 . The device appears opaque for optical signals propagating in the opposite direction, however. Optical signals at port  1703  are blocked from reaching port  1702 .  
         [0056]     As will be seen below, the above prior art devices are used to implement an OADM device with significantly reduced crosstalk.  
         [0057]     OADM Architecture of the Present Invention  
         [0058]     A wavelength selective (λ 2 ) optical “drop” switch of the present invention is shown in  FIG. 10A , which is in the “off” state. Since the switch wavelength is different from wavelengths of all input signals, the switch is transparent to all input signals. Turning to  FIG. 10B , it can be seen that there is a substantial reduction of channel crosstalk.  
         [0059]     Notice that in the optical drop switch  1802  of  FIG. 10A , the spacing between adjacent channels is designed to be twice as much compared to the prior art. As a result, while the drop switch  1802  is in the “off” state, it can occupy the space of a channel that is not being used. This is illustrated in  FIG. 10B . Channel crosstalk and/or power loss between neighboring channels are thereby substantially reduced.  
         [0060]     A wavelength selective (λ 3 ) optical “drop” switch and a similar “add” switch is shown in FIGS.  11 A˜ 11 B, respectively. The bold outline of the elements indicates that they are in the “on” state (contrast to  FIG. 10A ).  
         [0061]     When the wavelength selective (λ 3 ) optical “drop” switch in  FIG. 11A  is “on”, signals with wavelength λ 3  from input port  1902  are directed to “drop” port  1904 . The switch is transparent to all other input wavelengths otherwise.  
         [0062]     Similarly, when the wavelength selective (λ 3 ) optical “add” switch in  FIG. 11B  is “on”, signals with wavelength λ 3  coming from the “add” port  1914  are directed to output port  1913 . The switch is transparent to all other input wavelengths otherwise.  
         [0063]      FIGS. 12A-12C  show a 4-channel OADM that is based on the 1:2 optical wavelength interleavers shown in  FIGS. 2 and 4  and the wavelength selective optical “drop” and “add” switches shown in FIGS.  11 A˜ 11 B. Elements with thin outline indicate that they are in the “off” state; those with thick (or bold) outline indicate that they are in the “on” state.  
         [0064]     Let the wavelengths at input  2001  be λ 1 , λ 2 , λ 3  and λ 4  in this illustrative example. Based on properties of 1:2 optical wavelength interleavers as explained via  FIGS. 2 and 4  and  3 A˜ 3 C, the input wavelengths are split into two sets such that the odd-numbered channel wavelengths (λ 1  and λ 3 ) propagate along the upper path  2005  and the even-numbered ones (λ 2  and λ 4 ) along the lower path  2006 . As the optical “drop” and the “add” switches along path  2005  are transparent to odd-numbered channel wavelengths, and likewise along path  2006 , these two sets of channel wavelengths are recombined via the 2:1 optical wavelength interleaver  2004 . It should be noticed that when all the wavelength selective optical switches are in the “off” state (as in  FIG. 12A ), the OADM is transparent to all channel wavelengths.  
         [0065]     As explained in FIGS.  10 A˜ 10 B, one of the advantages of this construction is the substantial crosstalk reduction between channels. Although the structure may be more complex than the prior art, the reduction in crosstalk in many applications justifies this tradeoff.  
         [0066]     Again, as an illustrative example, it is shown in  FIG. 12B  that when the λ 3  “drop” switch  2011  along the upper path is turned on, signals of the λ 3  channel from input are directed to port  2012 .  
         [0067]     Referring to  FIG. 12C , after signals of the λ 3  channel from the input are directed to port  2022  (with the λ 3  optical “drop” switch  2021  “on”), new signals (of the λ 3  channel) can be added through port  2024  with the λ 3  optical “add” switch  2023  “on”.  
         [0068]      FIG. 13  shows a schematic where the output 2:1 optical wavelength interleaver of the schematic in  FIG. 12A  is replaced by a two-to-one optical waveguide combiner. As mentioned earlier, given the same input the two-to-one optical waveguide combiner ( FIG. 7A ) functions similar to a 2:1 optical wavelength interleaver ( FIG. 4 ).  
         [0069]     In  FIG. 10B , it is assumed that the bandwidth of the wavelength selective optical switch (that is, width of the dotted curve) is comparable to that of the channels. In the case that the switch bandwidth is wider (see  FIG. 14A ), channel crosstalk may still occur.  
         [0070]     If the wavelength channels propagating along the same path are spaced even further apart, and if the “off” state of the wavelength selective optical switch occupies a channel midway in between, channel crosstalk can again be avoided. In the case illustrated in  FIG. 14B , as compared to the case in  FIG. 10B  the channel spacing is doubled. Even if the switch bandwidth (the dotted curve) is now wider, if the “off” state occupies far enough from either channels the channel crosstalk  2211  is small.  
         [0071]     To realize further channel separation along the same path, the schematics in  FIG. 1   5 A is suggested. Let the wavelengths at input  2301  be λ 1 , λ 2 , λ 3 , λ 4 , λ 5 , λ 6 , λ 7  and λ 8  in this illustrative example. FIGS.  15 A˜ 15 C are for explaining functions and operations of an 8-channel OADM, which is based on the 1:4 optical wavelength interleavers shown in FIGS.  5 A˜ 5 B and the wavelength selective optical “drop” and “add” switches shown in FIGS.  11 A˜ 11 B. Elements with thin outline indicate that they are in the “off” state; those with thick outline indicate that they are in the “on” state. Again, the advantage of this architecture is the substantial crosstalk reduction as illustrated in  FIG. 14B .  
         [0072]     As explained earlier with FIGS.  5 A˜ 5 B and  6 A˜ 6 E, with a 1:4 optical wavelength interleaver the input WDM signals are divided up such that the λ 1  and λ 5  channels are directed to path  2305 , the λ 2  and λ 4  channels are directed to path  2306 , the λ 3  and λ 7  channels are directed to path  2308 , and the λ 4  and λ 8  channels are directed to path  2307 . Thus, four branches are formed instead of two branches of  FIG. 12A .  
         [0073]     Notice that along each of the four paths, spacing between adjacent channels is quadrupled. While the wavelength selective optical switches are in the “off” state (as in  FIG. 15A ), each of them can occupy any wavelength which is not used. Channel crosstalk and/or power loss between channels are thereby further reduced.  
         [0074]     Referring to  FIG. 15B , in this example when the wavelength selective optical “drop” switch  2311  is turned “on”, input signals in the λ 5  channel are dropped at port  2312 .  
         [0075]     Referring to  FIG. 15C , moreover, while the wavelength selective optical “drop” switch  2321  is “on”, if the wavelength selective optical “add” switch  2323  is also turned “on”, new signals for the λ 5  channel can be added via port  2324 .  
         [0076]      FIG. 16  shows a schematic where the output 4:1 optical wavelength interleaver of the schematic in  FIG. 15A  is replaced by a four-to-one optical waveguide combiner. As mentioned earlier, given the same input, the four-to-one optical waveguide combiner ( FIG. 7B ) functions similar to a 4:1 optical wavelength interleaver ( FIG. 5B ).  
         [0077]     To generalize, the first embodiment of this invention relates to the design of an N-channel OADM utilizing:  
         [0078]     (1) two 1:M optical wavelength interleavers,  
         [0079]     (2) M optical paths,  
         [0080]     (3) P (where P times M is greater than or equal to N) wavelength selective “add” optical switches, and  
         [0081]     (4) the same number of wavelength selective “drop” optical switches on each path.  
         [0082]     FIGS.  12 A˜ 12 C and  13  illustrate the case when N=4 and M=2, and FIGS.  15 A˜ 15 C and  16  illustrate the case when N=8 and M=4. These are merely illustrative examples and the contemplated combinations are nearly endless for an N-channel OADM based on this method. In each design, signal degradation due to propagation loss and optical switches are reduced by a factor of M compared to the conventional design. This embodiment is advantageous, therefore, in cases where this factor-of-M reduction outweighs signal degradation due to the two interleavers.  
         [0083]     The architecture above can be adapted to use combination add/drop switches. Thus, a wavelength selective (λ 3 ) optical “add/drop” switch is shown in  FIG. 17 . The bold outline of the elements indicates that they are in the “on” state. When the wavelength selective (λ 3 ) optical “add/drop” switch in  FIG. 17  is “on”, input signals in the λ 3  channel coming from port  2503  are directed to “drop” port  2505 . Meanwhile, new signals with wavelength λ 3 *(λ k =λ k *) coming from the port  2501  are directed to output port  2504 . The switch is transparent to all other input wavelengths otherwise.  
         [0084]     Functions and operations of a 4-channel OADM are explained through FIGS.  18 A˜ 18 B. The switch is based on the 1:2 optical wavelength interleavers shown in  FIGS. 2 and 4  and the wavelength selective optical “add/drop” switch shown in  FIG. 17 .  
         [0085]     Let the wavelengths at input  2603  be λ 1 , λ 2 , λ 3  and λ 4  in this illustrative example. As in the previous case, the input wavelengths are split into two sets such that the odd-numbered channel wavelengths (λ 1  and λ 3 ) propagate along the upper path  2605  and the even-numbered ones (λ 2  and λ 4 ) along the lower path  2606 . Since the “add/drop” switches along path  2605  are transparent to odd-numbered channel wavelengths, and likewise along path  2606 , these two sets of channel wavelengths are recombined via the 2:1 optical wavelength interleaver  2601 . It should be noticed that when all the wavelength selective optical switches are in the “off” state (as in  FIG. 18A ) the OADM is transparent to all channel wavelengths.  
         [0086]     Elements with thin outline indicate that they are in the “off” state; those with thick outline indicate that they are in the “on” state. Again, advantage of this construction is the substantial crosstalk reduction.  
         [0087]     As described earlier with  FIG. 17 , when the wavelength selective optical “add/drop” switch  2611  is turned “on”, input signals in the λ 3  channel are dropped via port  2613  and new signals in the same channel can be added via port  2612 .  
         [0088]      FIG. 19  shows a schematic where the output 2:1 optical wavelength interleaver of the schematic in  FIG. 18A  is replaced by a two-to-one optical waveguide combiner. As mentioned earlier, given the same input the two-to-one optical waveguide combiner ( FIG. 7A ) functions similar to a 2:1 optical wavelength interleaver ( FIG. 4 ).  
         [0089]     As in the previous case, FIGS.  20 A˜ 20 B show the functions and operations of an 8-channel OADM that is based on the 1:4 optical wavelength interleavers as shown in FIGS.  5 A˜ 5 B, and the wavelength selective optical “add/drop” switch as shown in  FIG. 17 . Elements with thin outline indicate that they are in the “off” state; those with thick outline indicate that they are in the “on” state. It should be noticed that when all the wavelength selective optical switches are in the “off” state (as in  FIG. 20A ) the OADM is transparent to all channel wavelengths.  
         [0090]     Let the wavelengths at input  2803  be λ 1 , λ 2 , λ 3 , λ 4 , λ 5 , λ 6 , λ 7  and λ 8  in this illustrative example. As explained earlier with FIGS.  5 A˜ 5 B and  6 A˜ 6 E, with a 1:4 optical wavelength interleaver it is feasible to divide up the wavelength channels such that the λ 1  and λ 5  channels are directed to path  2805 , the λ 2  and λ 6  channels are directed to path  2806 , the λ 3  and λ 7  channels are directed to path  2808 , and the λ 4  and λ 8  channels are directed to path  2807 .  
         [0091]     Referring to description associated with  FIG. 17 , when the wavelength selective optical “add/drop” switch  2811  is turned “on”, input signals in the  5  channel are dropped via port  2813  and new signals in the same channel can be added via port  2812 . Again, advantage of this construction is the substantial crosstalk reduction between channels.  
         [0092]      FIG. 21  shows a schematic where the output 4:1 optical wavelength interleaver of the schematic in  FIG. 20A  is replaced by a four-to-one optical waveguide combiner. As mentioned earlier, given the same input the four-to-one optical waveguide combiner ( FIG. 7B ) functions similar to a 4:1 optical wavelength interleaver ( FIG. 5B ).  
         [0093]     To generalize, the second embodiment of this invention relates to the design of an N-channel OADM utilizing  
         [0094]     (1) two 1:M optical wavelength interleavers,  
         [0095]     (2) M optical paths,  
         [0096]     (3) P (where P times M is greater than or equal to N) wavelength selective “add/drop” optical switches, on each path.  
         [0097]     FIGS.  18 A˜ 18 C and  19  illustrate the case when N=4 and M=2, and FIGS.  20 A˜ 20 C and  21  illustrate the case when N=8 and M=4. These are merely illustrative examples and the contemplated combinations are nearly endless for an N-channel OADM based on this method. In each design, signal degradation due to propagation loss and optical switches are reduced by a factor of M compared to the conventional design. This embodiment is advantageous, therefore, in cases where this factor-of-M reduction outweighs signal degradation due to the two interleavers.  
         [0098]     The architecture described above can be adapted to use wavelength selective reflective optical switch as shown in  FIG. 22 . The thin outline of the element indicates that it is in the “off” state; thick outline when it is in the “on” state.  
         [0099]     As an example, shown in  FIG. 22  is a wavelength selective (λ k ) reflective optical switch  3001  such that when it is turned “on”, the reflected output signal  3004  propagates in a direction opposite to the input signals  3002 . The switch is transparent to all signals in other wavelengths.  
         [0100]      FIG. 23  shows the structure of a grating-based wavelength selective reflective optical switch. FIGS.  24 A˜ 24 B explain the functions and operations of a 4-channel optical “drop” multiplexer which is based on the 1:2 optical wavelength interleavers as shown in  FIGS. 2 and 4 , wavelength selective reflective optical switches as shown in  FIG. 22 , and the optical circulator as shown in  FIG. 8 . Elements with thin outline indicate that they are in the “off” state; those with thick outline indicate that they are in the “on” state.  
         [0101]     Let the wavelengths at input  3201  be λ 1 , λ 2 , λ 3  and λ 4  in this illustrative example. In this case, the input signals go through the optical circulator  3207 , enter the 1:2 optical wavelength interleaver  3203  and the channel wavelengths are split into two sets such that the odd-numbered channel wavelengths (λ 1  and λ 3 ) propagate along the upper path  3205  and the even-numbered ones (λ 2  and λ 4 ) along the lower path  3206 . Since the wavelength selective reflective optical switches along path  3205  are transparent to odd-numbered channel wavelengths, and likewise along path  3206 , these two sets of channel wavelengths are recombined via the 2:1 optical wavelength interleaver  3204 . It should be noticed that when all the wavelength selective optical switches are in the “off” state (as in  FIG. 24A ) the optical “drop” multiplexer is transparent to all channel wavelengths.  
         [0102]     As an illustrative example, it is shown in  FIG. 24B  that when the λ 3  wavelength selective reflective optical switch  3211  along the upper path is turned on, signals of the λ 3  channel from input are directed to drop port  3212 .  
         [0103]      FIG. 25  shows a schematic where the output 2:1 optical wavelength interleaver of the schematic in  FIG. 24A  is replaced by a two-to-one optical waveguide combiner. As mentioned earlier, given the same input the two-to-one optical waveguide combiner ( FIG. 7A ) functions similar to a 2:1 optical wavelength interleaver ( FIG. 4 ). Functions and operations of schematics as shown in  FIGS. 24A and 25  are expected to be identical. An advantage of this construction is the substantial crosstalk reduction between channels.  
         [0104]     Through FIGS.  26 A˜ 26 B, to explain the functions and operations of an 8-channel optical “drop” multiplexer which is based on the 1:4 optical wavelength interleavers as shown in  FIGS. 7A and 7B , the wavelength selective reflective optical switches as shown in  FIG. 22 , and the optical circulator as shown in  FIG. 8 . Elements with thin outline indicate that they are in the “off” state; those with thick outline indicate that they are in the “on” state.  
         [0105]     Let the wavelengths at input  3401  be λ 1 , λ 2 , λ 3 , λ 4 , λ 5 , λ 6 , λ 7  and λ 8  in this illustrative example. As explained earlier with FIGS.  5 A˜ 5 B and  6 A˜ 6 E, with a 1:4 optical wavelength interleaver it is feasible to divide up the wavelength channels such that the λ 1  and λ 5  channels are directed to path  3405 , the λ 2  and λ 6  channels are directed to path  3406 , the λ 3  and λ 7  channels are directed to path  3408 , and the λ 4  and λ 8  channels are directed to path  3407 .  
         [0106]     Considering  FIG. 26B  and referring to description associated with  FIG. 22 , when the wavelength selective optical reflective switch  3411  is turned “on”, input signals in the λ 5  channel are reflected and dropped via port  3412 . An advantage of this construction is the substantial crosstalk reduction between channels.  
         [0107]      FIG. 27  shows a schematic where the output 4:1 optical wavelength interleaver of the schematic in  FIG. 26A  is replaced by a four-to-one optical waveguide combiner. As mentioned earlier, given the same input the four-to-one optical waveguide combiner ( FIG. 7B ) functions similar to a 4:1 optical wavelength interleaver ( FIG. 5B ).  
         [0108]     To generalize, the third embodiment of this invention relates to the design of an N-channel optical “drop” multiplexer utilizing:  
         [0109]     (1) two 1:M optical wavelength interleavers,  
         [0110]     (2) M optical paths,  
         [0111]     (3) one optical circulator, and  
         [0112]     (4) P (where P times M is greater than or equal to N) wavelength selective reflective optical devices on each path.  
         [0113]     FIGS.  24 A˜ 24 C and  25  illustrate the case when N=4 and M=2, and FIGS.  26 A˜ 26 C and  27  illustrate the case when N=8 and M=4. These are merely illustrative examples and the contemplated combinations are nearly endless for an N-channel OADM based on this method. In each design, signal degradation due to propagation loss and optical multiplexers are reduced by a factor of M compared to the conventional design. This embodiment is advantageous, therefore, in cases where this factor-of-M reduction outweighs signal degradation due to the two interleavers.  
         [0114]      FIG. 28A  shows a schematic of an optical wavelength blocker. It consists of wavelength selective optical reflective switches as shown in  FIG. 22A ˜ 22 B and an optical isolator as shown in  FIG. 9 .  
         [0115]     When the optical wavelength blocker is “off” (as in  FIG. 28A ) all the wavelength selective optical reflective switches are tuned to wavelengths other than the input signal wavelengths, that is, none of the wavelengths λ 1″ , λ 2″ , . . . , λ N″  is the same as any of the wavelengths λ 1 , λ 2 , . . . , λ N .  
         [0116]     Consider the illustrative example as in  FIG. 28B  where the λ 2  optical reflective switch is “on”, input signals in the λ 2  channel are reflected but blocked by the optical isolator  3612 . All the λ 2  channel signals are blocked from reaching the output port  3613  as a result.  
         [0117]     FIGS.  29 A˜ 29 B explain the functions and operations of a multi-wavelength optical blocker which is based on the 1:2 optical wavelength interleavers as shown in  FIGS. 2 and 4 , wavelength selective reflective optical switches as shown in FIGS.  22 A˜ 22 B, and the optical isolator as shown in  FIG. 9 . Elements with thin outline indicate that they are in the “off” state; those with thick outline indicate that they are in the “on” state.  
         [0118]     Let the wavelengths at input  3701  be λ 1 , λ 2 , λ 3  and λ 4  in this illustrative example. In this case, the input signals go through the optical isolator  3705 , enter the 1:2 optical wavelength interleaver  3703  and the channel wavelengths are split into two sets such that the odd-numbered channel wavelengths (λ 1  and λ 3 ) propagate along the upper path  3706  and the even-numbered ones (λ 2  and λ 4 ) along the lower path  3707 . Since the wavelength selective reflective optical switches along path  3706  are transparent to odd-numbered channel wavelengths, and likewise along path  3707 , these two sets of channel wavelengths are recombined via the 2:1 optical wavelength interleaver  3704 . It should be noticed that when all the wavelength selective optical switches are in the “off” state (as in  FIG. 29A ) the multi-wavelength optical blocker is transparent to all channel wavelengths.  
         [0119]     As an illustrative example, it is shown in  FIG. 29B  that when the λ 3  wavelength selective reflective optical switch  3713  along the upper path is turned on, signals of the λ 3  channel from input  3711  are blocked from passing through to output port  3712 .  
         [0120]      FIG. 30  shows a schematic where the output 2:1 optical wavelength interleaver of the schematic in  FIG. 29A  is replaced by a two-to-one optical waveguide combiner. As mentioned earlier, given the same input the two-to-one optical waveguide combiner ( FIG. 7A ) functions similar to a 2:1 optical wavelength interleaver ( FIG. 4 ).  
         [0121]     Through FIGS.  31 A˜ 31 B, the functions and operations of a multi-wavelength optical blocker are explained. It is based on the 1:4 optical wavelength interleavers as shown in  FIGS. 7A and 7B , wavelength selective reflective optical switches as shown in  FIG. 22 , and the optical isolator as shown in  FIG. 9 . Elements with thin outline indicate that they are in the “off” state; those with thick outline indicate that they are in the “on” state.  
         [0122]     Let the wavelengths at input  3901  be λ 1 , λ 2 , λ 3 , λ 4 , λ 5 , λ 6 , λ 7  and λ 8  in this illustrative example. As explained earlier with FIGS.  5 A˜ 5 B and  6 A˜ 6 E, with a 1:4 optical wavelength interleaver it is feasible to divide up the wavelength channels such that the λ 1  and λ 5  channels are directed to path  3905 , the λ 2  and λ 6  channels are directed to path  3906 , the λ 3  and λ 7  channels are directed to path  3908 , and the λ 4  and λ 8  channels are directed to path  3907 .  
         [0123]     Considering  FIG. 31B  and referring to description associated with  FIG. 22 , when the wavelength selective optical reflective switch  3913  is turned “on”, input signals in the λ 5  channel are reflected and blocked at the optical isolator  3914 .  
         [0124]      FIG. 32  shows a schematic where the output 4:1 optical wavelength interleaver of the schematic in  FIG. 31A  is replaced by a four-to-one optical waveguide combiner. As mentioned earlier, given the same input the four-to-one optical waveguide combiner ( FIG. 7B ) functions similar to a 4:1 optical wavelength interleaver ( FIG. 5B ).  
         [0125]     To generalize, the fourth embodiment of this invention relates to the design of an N-channel optical wavelength blocker utilizing  
         [0126]     (1) two 1:M optical wavelength interleavers,  
         [0127]     (2) M optical paths,  
         [0128]     (3) one optical isolator, and  
         [0129]     (4) P (where P times M is greater than or equal to N) wavelength selective reflective optical devices on each path.  
         [0130]     FIGS.  29 A˜ 29 C and  30  illustrate the case when N=4 and M=2, and FIGS.  32 A˜ 32 C and  32  illustrate the case when N=8 and M=4. These are merely illustrative examples and the contemplated combinations are nearly endless for an N-channel OADM based on this method. In each of the design, signal degradation due to propagation loss and optical devices are reduced by a factor of M compared to the conventional design. This embodiment is advantageous, therefore, in cases where this factor-of-M reduction outweighs signal degradation due to the two interleavers.  
         [0131]     Although the present invention has been described in terms of the presently preferred embodiment, it is to be understood that such disclosure is not to be interpreted as limiting. Various alternations and modifications will no doubt become apparent to those skilled in the art after reading the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alternations and modifications as fall within the true spirit and scope of the invention.