Abstract:
An Optical Tapped Delay Line (OTDL) is combined with other known optical apparatuses to provide an add-drop multiplexer for a wavelength division multiplexing fiber optic network. Each output beam of the OTDL is spatially distinguishable in free space. This wavelength accessibility enables selection of one or more of the optical beams for adding or dropping. The system can be a fixed or tunable single channel add/drop system, a fixed or tunable multi-channel add/drop system, or a fully programmable add/drop system.

Description:
FIELD OF THE INVENTION 
   The present invention relates to fiber optic networks and the use of wavelength division multiplexing and dense wavelength division multiplexing techniques and, more specifically, to fiber optic wavelength add/drop systems. 
   BACKGROUND OF THE INVENTION 
   Wavelength division multiplexing (WDM) enables different wavelengths to be simultaneously carried over a common fiber optic waveguide. Each wavelength or light beam carries encoded data. WDM can separate the fiber bandwidth into multiple discrete channels with narrow channel spacing through a technique referred to as dense wavelength division multiplexing (DWDM). This technique provides a relatively low-cost method of substantially increasing long haul telecommunication capacity over existing fiber optic transmission lines. 
   Techniques and devices are required for multiplexing the discrete wavelengths in DWDM transmission systems. In other words, the individual optical signals must be combined onto a common fiber optic waveguide. Then, the optical signals must be separated again into the individual signals or channels at the opposite end of the fiber optic cable. Thus, the ability to effectively combine and then separate individual wavelengths (or wavelength bands) from a broad spectral source is of significant importance to the fiber optic telecommunications field. Similarly, this technique is important in many other fields employing optical networking devices. 
   Devices that couple multiple, closely-spaced carrier wavelengths within a single optical fiber are called multiplexers. Devices that separate the carrier wavelengths at the receiving end of a fiber are called demultiplexers or channelizers. 
   As fiber optic transmissions enter and leave metropolitan and local area networks (LANs), each data-carrying wavelength is usually switched through various points along the fiber optic network. These points are known as “nodes.” At node locations, optical signals can be forwarded to the next node or “dropped” towards their final destination via the best possible path. The best possible path may be determined by such factors as distance, cost, and the reliability of specific routes. In addition, specific data-carrying wavelengths may be recombined or “added” to the multiplexed optical signal at node sites. The devices that perform these functions in DWDM network systems are called add/drop multiplexers (ADMs). 
   The conventional way to drop a data signal from a DWDM fiber is to de-multiplex the signal into its constituent wavelengths. Next, the light is detected using a photodetector, thus converting the signals to an electronic form (OE conversion). The electronic signal is switched and/or routed, as appropriate. The remaining signals are converted back to an optical signal (EO conversion). The optical signal is then sent down the proper fiber. During this last step, a signal can be added to the remaining signals. Such OE and EO conversion operations are both protocol and data rate dependent. These operations also require inflexible devices that are costly and difficult to upgrade as system capacity demand is increased. 
   Optical add/drop multiplexers (OADMs) have several significant advantages. First, OADMs cost less because they eliminate the need for much of the expensive high-speed electronics in conventional devices. Second, OADMs require smaller packaging because removing the electrical conversion step results in a reduced component count within the switches. Finally, optical devices are relatively future-proof because the optics can accommodate any bit-rate, whereas electrical devices must always be customized for the bit-rate and protocol of the signals. 
   Optical add/drop systems are comprised of two major subsystems. The first subsystem is the demultiplexing and multiplexing subsystem for selecting and recombining the appropriate wavelength. The second subsystem is the add/drop apparatus for routing the wavelength to the desired optical fiber output. Existing techniques for wavelength separation from a multiplexed signal using optical architectures include thin film bandpass filters, Fabry-Perot filters, fiber Bragg or diffraction grating filters, and polarization controllers. Each of these optical filtering methods may have different forms. 
   Thin film bandpass filters have traditionally been used in OADM devices to select single wavelengths from a multi-channel optical signal. Although such filters have good channel isolation, they tend to exhibit a transmission light loss of approximately 10%. Such filters are also highly temperature-sensitive. Further, they often operate in only one direction. In addition, such filters are limited to a single, fixed wavelength. Thus, to construct a multi-channel OADM device, multiple filters must be combined. This results in increased complexity, optical loss, and cost. 
   In U.S. Pat. No. 5,751,456, Koonen disclosed an example of a solution to some of these issues wherein a narrow-bandpass Fabry-Perot filter was utilized in a bidirectional OADM. As Fabry-Perot filters can have a bandpass of 1–2 nm or less, they can provide better isolation and lower loss factors than other thin film interference filters.  FIG. 1  illustrates an example of the Koonen prior art. The device illustrated in  FIG. 1  is limited in that it can add/drop only a single wavelength. As illustrated in this example, a circulator  10  is used to pass four wavelengths λ 1 –λ 4  to a Fabry-Perot filter  11 . Filter  11  selects one wavelength λ 1  for continuation on to a receiver  12 . The remaining wavelengths λ 2 –λ 4  are reflected by the filter  11  back to a circulator  10 . A transmitter  13  sends a new wavelength λ 1 ′ to the filter  11 . The new wavelength λ 1 ′ is multiplexed with the original wavelengths λ 2 –λ 4 . The resulting wavelength is returned to the circulator  10  for continuation 
   The issue of such interference filter-based ADM devices being fixed in nature has been addressed in the prior art with the invention of “tunable” filters. Tunable filters can be selectively tuned to different wavelengths within a multi-channel optical signal. However, tuning thin-film optical filters requires that either the incident optical beam be repositioned with respect to the filter surface or that the filter itself be repositioned with respect to the input beam. Both scenarios require mechanical movement of components. These components include as actuators or stepper motors. The mechanical movement of these components makes these OADM devices active in nature. This results in increased complexity and cost. 
     FIG. 2  illustrates an example of a prior art tunable filter as disclosed in U.S. Pat. No. 6,292,299.  FIG. 2  illustrates the mechanical nature of selecting a single wavelength.  FIG. 2  also illustrates the potential complexity of matching the add/drop wavelengths to output fibers. An electronic controller  6  directs an optical filter  1  to move in the x and z directions to a specific location where a single wavelength from an incoming fiber  3  is intercepted. Once selected, the wavelength is passed or dropped to a fiber  2 . The unselected wavelengths are reflected to continue on a fiber  4 . A wavelength can be added from fiber  2  at the same time. As can be seen from the example illustrated in  FIG. 2 , the electronic controller must be mechanically manipulated to select a single wavelength. 
   Diffraction gratings and fiber Bragg grating filters (FBGs) offer alternative means of selecting and isolating single wavelengths from a multi-channel input beam in OADM devices. Diffraction gratings can be used in an OADM device to separate an input beam into its components in one direction, and recombine the wavelengths in the reverse direction. However, with diffraction grating systems, the component count can rise rapidly. Lenses, collimators, and focusing optics are required to refine, direct, and couple the light beams into fibers. 
   Because FBGs are constructed from optical fibers, rather than individual thin-film filter substrates, they allow for all-fiber systems to be constructed. Fiber Bragg grating systems offer high levels of selectivity. However, they are limited in that several fiber gratings must be combined, along with optical circulators, in order to handle a multiplexed optical signal with a high channel count. The result can be a very large device with a high component count, increased complexity, and a higher cost. In addition, the combination or cascading of multiple-fiber Bragg gratings can significantly reduce signal strength as the insertion loss of multiple devices is compounded throughout the system. 
   A recent development in the area of wavelength selectivity and separation of multiplexed optical signals has been the utilization of polarization controllers. As disclosed by U.S. Pat. No. 6,285,478, polarization-controlling elements can also be used within OADM devices to separate a multi-channel WDM input signal into odd and even channels. This is done, for example, by splitting the signal into its vertically and horizontally polarized components. When combined with birefringent beam displacing optics, the separated signals can then be directed to appropriate output paths. This method provides an add/drop device that can accommodate the high channel counts and narrow channel spacing of current DWDM networks, where channels are separated by 50 GHz or less. This channel-separation technique is expandable and can adapt to increasing channel counts. However, this technique is subject to very high optical component count. Included in the optical component count are multiple polarization controllers, birefringent elements and beam splitters. These components are required to manipulate dense multiplexed signals. Assembling and aligning these optical components within a device can be extremely expensive. This is particularly true when high levels of precision are required.  FIG. 3  illustrates an example of the prior art.  FIG. 3  illustrates an example of the large number of components required to separate eight-channels. 
   With the continued development of WDM fiber optic systems, it is becoming increasingly important to control the direction of wavelengths to desired output ports (i.e., routers). It is likewise important to permit a new signal to replace an existing signal at a specific wavelength (i.e., add/drop) using optical systems. Furthermore, since the development of DWDM sends hundreds and even thousands of wavelengths through a fiber, the ability to selectively control a single or several wavelengths without affecting the other wavelengths is very important. This ability is important because the optical to electrical to optical conversion process is expensive and uses significant power as well as space. In particular, optical add/drops are critical components in WDM regional-access ring or bus networks to provide broadband access to users. 
   Current optical subsystems that perform add/drop functions include mirrors and micro-electro-mechanical systems (MEMS) using movable and fixed mirrors and etalons. 
   In the prior art, it is known to use a reconfigurable switching matrix having front and back micromirrors. These micromirrors are a reconfigurable switching matrix capable of directing the output of wavelengths in multiple directions. 
   It is also known in the art to use a tunable optical add/drop that employs an optical filter device, such as a multi-layer dielectric wedge filter. This technique is successful using tunable Mach-Zehnder interferometers, acoustic tuning filters, tunable thin film interference filters, tunable Fabry-Perot etalons, and tunable Fabry-Perot interferometers. However, it is only possible to interact with a single wavelength at one time using this technique. 
   A wedged etalon with an actuator that moves the etalon to the position of the channel to be added or dropped may also be employed. However, this system can only accommodate adding or dropping a single channel simultaneously. Further, the added or dropped channel must be at the same frequency. 
   Accordingly, in light of the limitations of the prior art, it is desirable to have an optical add/drop system which is simpler then those known in the art, has low optical loss characteristics, operates on single or multiple channels, and is capable of adding or dropping finely-spaced channels with separations as close as 50 MHz. 
   SUMMARY OF THE INVENTION 
   Accordingly, an embodiment of the present invention provides an optical add/drop system for a wavelength division multiplexing fiber optic transmission system. More specifically, an embodiment of the present invention combines an Optical Tapped Delay Line (OTDL) with a subsystem to perform the adding or dropping of single or multiple wavelengths from an optical fiber. 
   The OTDL component of an add/drop system in accordance with one aspect of the present invention enables the simultaneous channelization of hundreds of discrete input beams into their constituent frequency components at independent spatial locations. Each of the output beams is a function of the frequency components of the corresponding input beams. The spatial separation of each wavelength enables the present invention, in this embodiment, to simultaneously add or drop one or more wavelengths from one or more input beams. Accordingly, this embodiment does not rely on grating and filters, which can only select single wavelengths and are not capable of dynamically selecting one or more wavelengths. 
   An OTDL can perform de-multiplexing and multiplexing even when the WDM channels are closely spaced. One evolutionary path of WDM is to pack more wavelengths into the same fiber using narrower channel spacings and bit rates such as 2.5 and 10 gigabits per second. Adding or dropping these narrowband channels using filters or grating technologies becomes very difficult or impossible. However, embodiments of the present invention do not rely on grating or filters. Therefore, these embodiments of the invention are capable of separating channels for adding or dropping even when the spacing between wavelengths is as narrow as 50 MHz (i.e., 0.4 pm at 1550 nm). More specifically, the spacing between wavelengths can be between 25 GHz and 50 MHz, for example. 
   According to one embodiment of the invention, once the wavelengths have been separated, a separate subsystem performs the adding and/or dropping of the wavelengths. This add/drop subsystem may include a mirror with a hole or a microelectro-mechanical system. Other known methods of adding and/or dropping wavelengths may be used. 
   According to an embodiment of the invention, a mirror with a hole may be used to add/drop the wavelengths. The location of the hole is at the same spatial location as the spatial location of the target wavelength. The target wavelength preferably passes through the hole, is coupled to a fiber and then is passed to an optical device, such as a circulator, for continuation on another fiber optic path. The target wavelength may also be passed on to a detector for conversion to an electronic form. According to an embodiment of the invention, to select multiple wavelengths, there can be multiple holes in the mirror at pre-determined locations. To dynamically select a wavelength, for example, the mirror can move up or down to position the hole or holes at the target wavelengths. 
   According to another embodiment of the invention, a Micro-Electro-Mechanical System (MEMS) with micro-mirrors may be utilized for the add/drop subsystem. The micro-mirrors are preferably positioned at the spatial location of every output light beam. To select one or more channels, one of the micro-mirrors is preferably canted at an angle to reflect the target wavelength to a fiber/circulator or detector for adding and dropping the information carrier to another destination. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A detailed description of some preferred embodiments of the present invention will be made with reference to the accompanying drawings, in which: 
       FIG. 1  illustrates an example of a prior art narrow-bandpass Fabry-Perot filter utilized in a bi-directional OADM; 
       FIG. 2  illustrates an example of a prior art tunable filter; 
       FIG. 3  illustrates an example of a prior art method of polarization controlling elements used to separate a multi-channel WDM input signal into odd and even channels; 
       FIG. 4  is a perspective view of an example of a two-dimensional OTDL according to an embodiment of the invention; 
       FIG. 5  is a side view of one input beam of an example of an OTDL according to an embodiment of the invention; 
       FIG. 6  is a side view of an example of a single mirror according to an embodiment of the present invention; 
       FIG. 7  is a top view of an example of a double mirror according to an embodiment of the invention; 
       FIG. 8  is a side perspective view of an example of a de-multiplexer side of a double mirror according to an embodiment of the invention; and 
       FIG. 9  is a top view of an example of a moveable mirror MEMS according to an embodiment of the invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 4 and 5  illustrate an example of an OTDL device according to an embodiment of the invention. As illustrated in  FIG. 4 , six collimated input beams  100 ( a )– 100 ( f ) preferably enter an optical cavity  112 . The optical cavity  112  may be a transparent plate having a desired thickness. The thickness of the cavity determines the free spectral range (FSR) of the device, i.e., the optical frequency ambiguity, or the optical frequency difference of wavelengths that appear at the same wavelength location in plane  144  as illustrated, for example, in  FIG. 5 . The origin of the beams may be, for example, the collimated outputs of six optical fibers (not shown) where each fiber typically carries multiple wavelength channels. The cavity  112  preferably has a first surface  114  that may be provided with a coating  116 , which is substantially 100% reflective. The coating  116  preferably covers the majority of the surface  114  with the exception of a transparent window where the input beams  110 ( a )–( f ) enter the device. The cavity  112  preferably has a second surface  118  that is opposed to the first surface  114 . The second surface  118  is preferably provided with a coating  120  that is partially reflective. 
   According to an embodiment of the invention, the partial reflectivity of the second surface coating  120  is spatially varying. In another embodiment, which is not illustrated, the partial reflectivity of the second surface coating  120  may be substantially uniform. 
   The reflective surface coatings  116  and  120  are preferably on opposite sides of the optical cavity  112 . The optical cavity  112  may be made of glass, other transparent materials or empty space. 
   The various output beams may then be directed to an anamorphic optical system that is preferably spaced apart from the optical cavity  112 . In the illustrated embodiment, the anamorphic optical system comprises a cylinder lens  140  and a spherical lens  142 . The anamorphic optical system  140 , 142  preferably performs a Fourier transformation of the output of the cavity  112  in the vertical dimension y, and images the output of the cavity  112  in the horizontal dimension x onto an output surface  144 . Although not illustrated in  FIG. 4 , it will be recognized that the optical system  140 ,  142  may have some form other than anamorphic, as described above, depending on the particular application of the OTDL device. 
     FIG. 5  illustrates an operational side view of an example of the device shown in  FIG. 4 . The single input beam  100 ( f ) illustrated in  FIG. 5  corresponds to the input beam  100 ( f ) illustrated as one of the multiple input beams  100 ( a )–( f ) in FIG.  4 . Although not illustrated in  FIG. 5 , it will be understood that the other multiple input beams  100 ( a )–( e ) reside behind the input beam  100 ( f ) in the view shown in  FIG. 5 . It will also be understood that the device of this embodiment is capable of processing and channelizing all of the multiple input beams simultaneously. 
   Referring to  FIG. 5 , the input beam  100 ( f ) preferably enters the cavity  112  as a collimated beam of light. After entering the cavity  112 , a portion of the collimated input beam may exit the cavity at a first location or “tap”  122 ( a ) as a collimated output beam. Another portion of the collimated input beam may be partially reflected by the coating  120  and then totally reflected by the coating  116 . In other words, a portion of the beam “bounces” from the coating  120  to the coating  116  and then back again. This reflection or “bounce” produces a collimated output beam that preferably exits at a second location or tap  122 ( b ). Tap  122 ( b ) may be slightly displaced spatially from the first tap  122 ( a ). As a result of the bounce, the distance traveled by the output beam  122 ( b ) may be greater than the distance traveled by output beam  122 ( a ). The width of the optical cavity  112  between reflective surfaces  116  and  120  may thereby introduce a time delay between adjacent taps. The reflective process continues, thereby preferably producing multiple additional collimated output beams  122 ( a )–( f ). The result may be a series of output beams that are distributed in the y-direction with a progressive time delay from beam to beam. 
   Although not illustrated in  FIG. 5 , it will be recognized that a similar series of output beams distributed in the y-direction may be simultaneously produced for each one of the input beams  100 ( a )–( f ). In other words, the device of this embodiment may be capable of operating on each one of the multiple collimated input beams independently of the other input beams. The device of this embodiment may therefore be referred to as a “two-dimensional” device. This is due to the fact that the device uses two different spatial dimensions to perform signal processing functions. A first dimension x preferably accommodates multiple independent collimated input beams that are all independently channelized along a second dimension y. 
   The various beams remain substantially collimated throughout the reflective process. Divergence of the beams and interference among the beams is minimized. Numerous internal reflections within the cavity  112  may be achieved without substantial divergence or interference. 
   Beam  122 ( a ) may pass through a lens system  142  performing the Fourier transform. Beam  122 ( a ) may illuminate the entire plane at  124 . Similarly, all of the remaining beams  122 ( b )–( f ) may pass through  142  and illuminate the entire plane at  124 . The totality of beams illuminating plane  124  may create an interference pattern which will preferably coalesce a single wavelength at  124 ( a ), a separate wavelength at  124 ( b ) and, similarly, at  124 ( c )–( f ). It will be understood that the number of wavelengths collected at plane  124  does not need to equal the number of beams exiting at plane  122 . The continuous spectrum will preferably be generated at plane  124  and the discrete wavelengths will be present only if discrete wavelengths, or more accurately, narrow wavelength bands, are present in the input beam  100 ( f ). 
   In  FIG. 5 , the OTDL subsystem of the present invention is represented as element  150 . Future references to  150  are intended to represent all of the functions illustrated within the area labeled  150  in  FIG. 5 . 
   The output surface  144  shown in the example illustrated in  FIG. 6  is two-dimensional. The horizontal dimension x of the output surface  144  may correspond to the input beam index. The vertical dimension y may correspond to the wavelength of the light in the input beam. There are a wide variety of devices that might be positioned at the output surface  144  to enable adding and dropping of a specific wavelength. 
   Referring to  FIG. 6 , a mirror  160  is preferably provided with a hole  161 . It should be understood that the location of the hole  161  may be at any of the output beam locations in any of the two dimensions. The separated wavelengths  124 ( a )–( f ) may strike the mirror  160 , except at the location of the hole  161 . Those wavelengths not striking the area where the hole is located are preferably reflected back into the OTDL device  150 . The wavelength striking the hole; i.e.,  124 ( e ) in this example, passes through and is preferably collected by a properly coupled fiber (e.g., with lenslet, waveguide, or other technique). This wavelength then preferably passes through a circulator  153 , for example. Then, the wavelength may be passed to a drop fiber  154 . The optical signal to be inserted at the same wavelength; i.e., the added channel  155 , is preferably coupled to the input port  156  of the circulator  153  and passed back to the hole  161  in the mirror  160 . It should be noted that the piece of fiber  152  between the hole  161  and the circulator  153  may carry both the dropped channel propagating left to right and the added channel propagating right to left. The added wavelength is then passed back through the OTDL  150  coupled to the output fiber  154 . The output fiber  154  now carries all of the original wavelengths. However, the information on the added/dropped wavelength is now different. 
   In alternative embodiments of the invention, a wavelength may be dropped without being re-added. Further, an unused wavelength may be added without being dropped. Similarly, a wavelength may be dropped and the same information added back at the same wavelength if desired; i.e., a broadcast mode. 
   This above-described embodiment of the invention relates to a single fixed wavelength add/drop. If two or more holes are present in the mirror  160 , then multiple fixed wavelengths may be added/dropped. If the mirror is movable such that the hole can be moved to any wavelength position, then the device is a tunable single channel add/drop demultiplexer. If the movable mirror has multiple holes, then it is a ganged wavelength tunable add/drop demultiplexer. The mirror movement/tuning may be manual (e.g., field settable) or automated. 
     FIG. 7  is a top view of an example of a double mirror according to an embodiment of the invention. The embodiment illustrated in  FIG. 7  utilizes two mirrors instead of one, as shown in the example illustrated in  FIG. 6 , and two OTDL channels (which may or may not be on the same device) instead of one. 
   Referring to  FIG. 7 , an input beam preferably enters an OTDL  150 ( a ) at  205  for de-multiplexing. After being separated into their respective wavelengths by the OTDL  150 ( a ), the wavelengths preferably arrive at mirror  200 . The mirror  200  has at least one hole (not shown) at the appropriate spatial location of the respective wavelength to be dropped. The wavelength, such as  124 ( e ), preferably passes through the hole for coupling to another fiber or to a detector. All wavelengths not passing through the hole or holes in mirror  200  are reflected to mirror  201  along path  215 . The mirror at  201  can also include a hole or a plurality of holes for adding new wavelengths. The optical signal carrying the frequency to be added preferably arrives at mirror  201  on a beam(s) at the proper wavelength(s) and passes through one or more holes in mirror  201 . Next, the beam(s) preferably continue on to the OTDL  150 ( b ) for multiplexing, i.e., recombining with the other wavelengths for output to a fiber at  206 . According to this embodiment, the adding/dropping is performed without the use of a circulator. 
     FIG. 8  is a side perspective view of an example of the de-multiplexer side of a double mirror according to an embodiment of the invention. More specifically,  FIG. 8  illustrates the de-multiplexing side of the OTDL  150 ( a ) discussed above in connection with  FIG. 7 .  FIG. 8  illustrates wavelengths  124 ( a )–( f ) arriving at a mirror  200  from the OTDL  150 ( a ). Wavelength  124 ( e ) preferably passes through the hole in mirror  200  for coupling to another fiber or to a detector. All the other wavelengths,  124 ( a )–( d ) and ( f ), are preferably reflected to a mirror  201 . 
   While  FIG. 6  showed an embodiment with one OTDL channel and one mirror and  FIG. 7  showed an embodiment with two OTDL channels and two mirrors, an intermediate embodiment exists which utilizes two OTDL channels and only one mirror. 
     FIG. 9  is a top view of a moveable mirror MEMS according to an embodiment of the invention. According to the embodiment shown in  FIG. 9 , mirror  160  discussed in connection with  160  may be replaced with a linear array of micromirrors using, for example, MEMS technology. All of the output wavelengths from the OTDL  150  preferably arrive at the MEMS device  160 . A micro-mirror preferably may exist at each spatial location of each output beam, i.e., there are six mirrors, one for each wavelength  124 ( a )–( f ), in this example. One of the micro-mirrors, shown as  161 ( e ), for example, is rotated to reflect one of the wavelengths to a fiber/circulator  153  and then to a drop fiber  154 . The optical signal to be inserted at the same wavelength (the added channel  155 ) is preferably coupled to the input port  156  of the circulator and passed back to the micro-mirror  161   e , where the added wavelength is then reflected to the proper location of the OTDL  150 . 
   It should be understood that while the above-description of the preferred embodiments of the invention as explained utilize a circulator. However, the present invention is not limited to using a circulator as described. Rather, any device which performs a function of separating/combining a bi-directionally propagating light beam into separate uni-directionally propagating beams may be used. 
   It should also be understood that while the embodiments described above use a mirror with a hole or holes for passing rather than light, any arrangement which performs a similar function, such as an optically, electrically or mechanically controlled port of any kind can be used so that light is selectively passed or reflected.