Abstract:
A multi-channel optical switching system particularly usable as a programmable optical add/drop multiplexer in a multi-wavelength communication system. The switching system uses a grating operating at Littrow that separates a multi-channel optical signal into a plurality of optical channels, and combines a plurality of optical channels into a multi-channel optical signal. The system also uses a plurality of optical ports optically coupled to the grating and a selecting device to select which optical channel is directed to which of the optical ports.

Description:
RELATED APPLICATION  
       [0001]    This application claims priority of U.S. Provisional Application No. 60/325,670 filed Sep. 26, 2001, No. 60/388,358 filed Jun. 12, 2002, and No. 60/397,944 filed Jul. 23, 2002, the disclosures of which are incorporated fully herein by reference. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates to the field of optical communications, and more particularly, to a reconfigurable optical add/drop system for use in optical multiplexing.  
         BACKGROUND OF THE INVENTION  
         [0003]    For several decades, fiber optics have been used for communication. Specifically, fiber optics are used for data transmission and other telecommunication applications. Despite the enormous information carrying capacity of fiber, as compared to conventional copper cable, the high cost of installing fiber optics presents a barrier to full implementation of fiber optics, particular as the “last mile”, from the central office to residences and businesses.  
           [0004]    One method of increasing carrying capacity without incurring additional installation costs has been to multiplex multiple signals onto a single fiber using various methods, such as time division multiplexing, where two or more different signals are carried over the same fiber, each sharing a portion of time. Another, more preferred multiplexing method is wavelength division multiplexing (WDM), where two or more different wavelengths of light are simultaneously carried over a common fiber.  
           [0005]    Until recently, typical fibers used for communications applications had preferred wavelength bands centered at 850 nm, 1300 nm, and 1550 nm, wherein each band typically had a useful bandwidth of approximately 10 to 40 nm depending on the application. Transmission within these bands was preferred by systems designers because of low optical attenuation. Recent advances in fiber design now provides fiber that have low attenuation over a very broad transmission range, from 1300-1620 nm.  
           [0006]    Wavelength division multiplexing can separate a fiber&#39;s bandwidth into multiple channels. Dividing bandwidth into multiple discreet channels, such as 4, 8, 16, 40, or even as many as 160 channels, through a technique referred to as dense channel wavelength division multiplexing (DWDM), is a relatively lower cost method of substantially increasing telecommunication capacity, using existing fiber optic transmission lines. Techniques and devices are required, however, for multiplexing the different discreet carrier wavelengths. That is, the individual optical signals must be combined onto a common fiber-optic line or other optical waveguide and then later separated again into the individual signals or channels at the opposite end or other point along the fiber-optic cable. Thus, the ability to effectively combine and then separate individual wavelengths (or wavelength sub-ranges) is of growing importance to the fiber-optic telecommunications field and other fields employing optical instruments.  
           [0007]    Optical multiplexers are known for use in spectroscopic analysis equipment and for the combination or separation of optical signals in wavelength division multiplexed fiber-optic telecommunications systems. Known devices for this purpose have employed, for example, diffraction gratings, prisms and various types of fixed or tunable filters.  
           [0008]    Approaches for selectively removing or tapping a channel, i.e., selective wavelengths, from a main trunk line carrying multiple channels, i.e., carrying optical signals on a plurality of wavelengths or wavelength sub-ranges, is suggested, for example, in U.S. Pat. No. 4,768,849 to Hicks, Jr. Hicks, shows filter taps, as well as the use of gangs of individual filter taps, each employing high performance, multi-cavity dielectric pass-band filters and lenses for sequentially removing a series of wavelength sub-ranges or channels from a main trunk line. The filter tap of Hicks, returns a multi-channel signal to the main trunk line as it passes the desired channel to a branch line. One known demux is disclosed in Pan et al., U.S. Pat. No. 5,652,814, FIG. 25. In Pan et al., the WDM input signal is cascaded through individual filter assemblies, consisting of a fiber collimator, thin film filter, and a fiber focusing lens. Each filter is set for a given wavelength. However, aligning the fibers for each wavelength is costly and errors in the alignment contribute significantly to the system losses. Further, FIG. 13 of Pan et al. teaches the use of a dual fiber collimator, thin film filter, and a dual fiber focusing lens to selectively DROP and ADD a single wavelength or range of wavelengths. As discussed above, aligning the collimators is expensive.  
           [0009]    Polarization dependent loss (PDL) is also a problem in WDM system because the polarization of the light drifts as it propagates through the fiber and furthermore this drift changes over time. Thus, if there is PDL in any component, the drifting polarization will change the signal level, which may degraded the system operation.  
           [0010]    Other multiplexer devices may be employed to add or drop channels in WDM systems. These systems are commonly known as optical add/drop multiplexers, or OADM. Another OADM, disclosed by Mizrahi U.S. Pat. No. 6,185,023, employs fiber Bragg gratings to demux and mux signals in a WDM system. This method requires optical circulators and multiple components.  
           [0011]    However, the multi channel OADM designs discussed above are not programmable by the end user. That is, each multiplexers is designed and manufactured to mux (add) specific channels; or when used in reverse each multiplexers is also designed and manufactured to demux (drop) specific channels. This limitation mandates that the optical system&#39;s parameters be fixed before installation. Changes are not possible without replacing the fixed optical multiplexers with different designed multiplexers. This is expensive.  
           [0012]    One known programmable OADM is discussed in Boisset et al, International Publication No. WO01/13151. In Boisset et al., the desired add/drop channel is programmed by translating a segmented filter. To achieve this translation however, a large mechanical mechanism is employed. A further limitation to Boisset et al. is that only a single channel may be added or dropped per device. Designers may employ multiple devices, deployed in series, and programmed as necessary to add/drop the correct channel; however, this approach requires multiple devices and has multiple points of failure. Furthermore, the size of such a device would be overly large and therefore not practical for many applications where space is limited.  
           [0013]    Two other programmable OADMs are disclosed by Tomlinson, U.S. Pat. No. 5,960,133, and Aksyuk, et al, U.S. Pat. No. 6,204,946, both use bulk optics and gratings to demultiplex and multiplex WDM input and output signal. While OADM&#39;s disclosed by Tomlinson and Aksyuk are both programmable, neither provides for discrete adding or dropping of an individual optical signal in a multi signal system. To achieve discrete adding or dropping of an individual optical signal in a multi signal system using the systems disclosed in Tomlinson and Aksyuk, additional components are required. All the Add wavelengths must be multiplexed onto a single fiber before it is sent to the OADM. Likewise, a demultiplexer must be added to the Drop port to access the individual wavelength channels. The additional components require additional space, add attenuation, and add cost to the system.  
           [0014]    It is an object of the present invention to provide improved optical multiplexing devices which reduce or wholly overcome some or all of the aforesaid difficulties inherent in prior known devices. Particular objects and advantages of the invention will be apparent to those skilled in the art, that is, those who are knowledgeable and experienced in this field of technology, in view of the following disclosure of the invention and detailed description of certain preferred embodiments.  
         SUMMARY OF THE INVENTION  
         [0015]    In accordance with a first aspect of the invention, a programmable Littrow grating based optical add/drop multiplexing device, programmed to add and/or drop one or more optical channels from/to a multi-channel light signal, comprises a focal plane, in combination with a lens in combination with a prism, a Littrow grating, and a plurality of programmable mirrors.  
           [0016]    The focal plane further comprises an IN port for receiving a multi-channel optical signal, a PASS port for transmitting a multi-channel optical signal, a plurality of ADD ports for receiving a plurality of optical channels, a plurality of DROP ports for transmitting a plurality of optical channels, and a plurality of programmable mirrors for directing light channels.  
           [0017]    The multi-channel light enters the device by way of the IN port and is directed through the Lens to the Littrow grating, where selected channels are dispersed and directed through the lens and focused onto to the plurality of programmable mirrors. The Littrow grating separates the multi-channel optical signal into its individual optical channels and directs the individual optical channels through the Prism, the Lens, and onto the programmable mirror that corresponds with that individual channel.  
           [0018]    Depending upon the programmed state of the mirrors, the channels are either directed through the lens, prism, and Littrow grating (or another wavelength separating medium) which combine the channels into a multi-channel light signal and directs it out of the system by way of the prism, lens and pass port, or the channels are directed through the lens, and mirror so as to exit the system by way of the Lens and one of the plurality of drop ports.  
           [0019]    In the instance where the programmed state of the mirrors directs one or more channels through one of the plurality of drop ports, one or more channels may enter the device by way of one of the plurality of add ports, and are directed through the lens, mirror, and lens, to the one of the plurality of programmable mirrors so as to exit the system by way of the lens, prism, and Littrow grating, which combines the channels into a multi-channel light signal and directs it out of the system by way of the prism, lens and pass port.  
           [0020]    To reduce polarization dependent loss (PDL) in the system a quarter-wave plate (QWP) may also be employed between the Lens and Littrow grating. The QWP rotates the polarization so that light that is s-polarized on the first pass is p-polarized on the second pass and there is no net polarization dependent loss (PDL) for light traveling between the IN and PASS ports. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 is a perspective view of a first embodiment of a Littrow grating based OADM detailing the various channel paths through the device.  
         [0022]    [0022]FIG. 2 is a perspective view of the Focal Plane of the embodiment of FIG. 1.  
         [0023]    [0023]FIG. 3 is a schematic view of a Focal Plane for an eight-channel embodiment of a Littrow grating based OADM.  
         [0024]    [0024]FIG. 4 is a schematic side view of a MEMS mirror in IN/PASS and DROP/ADD modes.  
         [0025]    [0025]FIG. 5 is a perspective view of the embodiment of FIG. 1 detailing the channel paths through the device for an PASS channel.  
         [0026]    [0026]FIG. 6 is a perspective view of the embodiment of FIG. 1 detailing the channel paths through the device for an DROP channel.  
         [0027]    [0027]FIG. 7 is a perspective view of the embodiment of FIG. 1 detailing the channel paths through the device for an ADD channel. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    The Littrow grating based OADM of the invention has numerous applications, including use in fiber optic telecommunications systems. For purposes of illustration, the preferred embodiments described below in detail multiplexing and demultiplexing, and adding and dropping channels, in wavelength division multiplexing and demultiplexing for a multi-channel fiber optic telecommunication systems. Exemplary references to an optical channel, or simply to a channel, should be understood to mean an optical signal with a centered wavelength and an upper and lower wavelength. Channel spacing is measured from the center of the first channel to the center of an adjacent channel.  
         [0029]    A three channel Littrow grating based OADM, employing one embodiment of the invention, is detailed in FIG. 1. It is of note that while only three channels are used in this example, a substantially larger number of channels/ports may be employed. The Littrow grating based OADM allows for demultiplexing and multiplexing separate optical channels onto or off of a multi-channel light signal. The OADM of FIG. 1 may be dynamically programmed to demultiplex and multiplex any combination of channels onto or off the multi-channel light signal.  
         [0030]    A first embodiment of the programmable OADM device of FIG. 1 comprises a focal plane  200  in combination with Lens  105 , a prism  107 , and a Littrow grating  109 . The device of FIG. 1 may be mounted within an enclosure optimized for optical transmission, including a gas-filled enclosure, or the like.  
         [0031]    A Littrow grating is a grating that operates at or near Littrow. Littrow is a special, but common case, in which the light is diffracted off the grating back toward the direction from which it came (i.e., a=b); this is called the Littrow configuration, for which the grating equation becomes:  
           ml= 2 d  sin( a )  
         [0032]    where a is the incident angle, b is the diffracted angle, m is the grating order, l is the wavelength, and d is the grating groove spacing. In one embodiment, the grating is used near the Littrow condition, so the same lens can be used for collimating and focusing the light. Further, using the grating near the Littrow condition takes advantage of the high diffraction efficiency of the grating near the Littrow condition.  
         [0033]    Lens  105  may be comprised of multiple lens elements  105   a ,  105   b  and  105   c . It is well known in the art that a lenses may be comprised of multiple lens elements to achieve a particular optical prescription.  
         [0034]    Prism  107  may optionally be used in any embodiment of the system. Temperature changes cause grating to expand and contract. As gratings expand and contract the wavelength-sized gradations that cause diffraction increase and decrease causing a change in the diffraction angle from a grating. Prism  107  may be used to minimize the thermal affects on Grating  109 . When Prism  107  and Grating  109  are properly designed and configured the effects of temperature on the system are greatly reduced. However, some embodiments of the system do not contain Prism  107 .  
         [0035]    Quarter-wave plate (QWP)  103  may also be employed between the Lens and Littrow grating to reduce polarization dependent loss (PDL) in the system a. The QWP  103  rotates the polarization so that light that is s-polarized on the first pass is p-polarized on the second pass and there is no net polarization dependent loss (PDL) for light traveling between the IN and PASS ports.  
         [0036]    The focal plane  200  of FIG. 2 further comprises an IN port  201  for receiving a multi-channel optical signal  101 , a PASS port  203  for transmitting a multi-channel optical signal, a plurality of ADD ports  213 ,  223 , and  233 , for receiving a plurality of optical channels, a plurality of DROP ports  215 ,  225 ,  235 , for transmitting a plurality of optical channels, and a plurality of Programmable Mirrors  211 ,  221 ,  231 , for directing light channels. Each DROP and ADD port is for a preassigned wavelength. All of these component are precisely aligned with each other, and mounted together so as to accommodate the entrance and exit of optical signals. Larger focal planes may be constructed and an eight channel system&#39;s focal plane is depicted in FIG. 3 comprising an IN port  301  for receiving a multi-channel optical signal  101 , a PASS port  303  for transmitting a multi-channel optical signal, a plurality of ADD ports  313 ,  323 ,  333 ,  343 ,  353 ,  363 ,  373 ,  383  for receiving a plurality of optical channels, a plurality of DROP ports  315 ,  325 ,  335 ,  345 ,  355 ,  365 ,  375 ,  385  for transmitting a plurality of optical channels, and a plurality of Programmable Mirrors  311 ,  321 ,  331 ,  341 ,  351 ,  361 ,  371 ,  381  for directing light channels.  
         [0037]    Turning again to FIG. 1, as well as to FIG. 2, a multi-channel light signal  101  enters the device through the IN port  201  on the focal plane  200 , and is directed through Lens  105 . The multi-channel light signal  101  is directed through the Lens  105 , QWP  103 , Prism  107 , and Littrow grating  109 . The Littrow grating  109  diffracts the individual channels of the multi-channel light signal  101  (hereafter channels) towards the Lens  105 , QWP  103 , to the channel&#39;s associated Programmable Mirror  211 ,  221 , or  231 .  
         [0038]    Depending upon the programmed state of the Programmable Mirrors channels received via the IN port  201  are either passed via the PASS port  203  or dropped via one of the plurality of DROP ports  215 ,  225 , or  235 . In the event one or more channels received via the IN port  201  are passed via PASS port  203 , the channel(s) are directed through the Lens  105 , QWP  103 , Prism  107 , and Littrow grating  109  which multiplexes the channel with other passed and added channels into a multi-channel light signal  111  and directs it out of the system by way of the Prism  107 , QWP  103 , Lens  105  and PASS port  203 .  
         [0039]    In the event one of more channels received via the IN port  201  are dropped via one of the plurality of DROP ports  215 ,  225 , or  235 , the channel(s) are directed through the Lens  105 , and mirror  117  so as to exit the system by way of the Lens  105  and one of the plurality of DROP ports  215 ,  225 , or  235  corresponding to the channel. Because the mirrors may be programmed individually, it will be clear to one skilled in the art that any channel may be dropped or passed.  
         [0040]    In the instance where one or more of the received via the IN port  201  are dropped via one of the plurality of DROP ports  215 ,  225 , or  235 , one or more channels corresponding channels may enter the device through one of the plurality of ADD ports  213 ,  223 , or  233 . These added channels enter the system by way of one of the plurality of ADD ports  213 ,  223 , or  233 , and are directed through the Lens  105 , Lens  105 , mirror  117 , Lens  105 , to the one of the plurality of Programmable Mirrors corresponding to the channel so as to exit the system by way of the Lens  105 , QWP  103 , Prism  107 , and Littrow grating  109 , which multiplexes the channel with other passed and added channels into a multi-channel light signal  111  and directs it out of the system by way of the Prism  107 , QWP  103 , Lens  105  and PASS port  203 .  
         [0041]    Turning to FIG. 4, in one embodiment the Programmable Mirrors  401  and  403  are constructed using Micro Electrical Mechanical Systems (MEMS). Programming of the Programmable Mirrors  401  and  403  is achieved by applying an electrical signal to the MEMS mirror. The Programmable Mirror  401  is programmed to reflect the IN port to the PASS port. The Programmable Mirror  403  is programmed to reflect the IN port to the DROP port, and to reflect the ADD port to the PASS port. A larger mirror may be employed by design to control more then one channel. Of course, other types of mirror actuators could be used.  
         [0042]    By engaging the channel mirrors, one or more separate channels may be dynamically routed onto or off of a multi-channel light signal. Further, by engaging the channel mirrors as a function of time and in synchronous conjunction with other system components, time-division multiplexing of optical signals may be achieved.  
         [0043]    One or more quarter-wave plates (QWP) may be employed in the system to reduce polarization dependent loss (PDL) in the system. The preferred location of the QWP is between Lens  105  and Grating  109 . QWP may be positioned such that they are substantially normal to the propagating light beam and the retardance axis is at 45 degrees to the light that is polarized parallel and perpendicular to the grating graduations. Passage through the QWP converts the parallel and perpendicular polarized components of the light into right and left circularly polarized states. Reflection off the grating converts changes the handedness of the polarization: right circularly polarized light into left circularly polarized light and visa versa. Passage through the QWP the second time converts the light back to a linearly polarized state, but it&#39;s departing polarization state is orthogonal to the input state. Thus, during one pass through the system the light is parallel and on the next is perpendicular leaving a substantially zero PDL for the system.  
         [0044]    Consider again the three channel system depicted in FIG. 1, where the multi-channel light signal contains:  
         [0045]    channel one—which is to be passed via PASS port  203 ;  
         [0046]    channel two—which is to be dropped via DROP port  225 ;  
         [0047]    no channel three comes into the system; and  
         [0048]    a channel three is added via ADD port  233  and passed via PASS port  203 .  
         [0049]    Table 1 details the desired channel operation (i.e., PASS, DROP, ADD, etc.) for each channel, as well as the Programmable Mirror&#39;s state.  
                               TABLE 1                                   CHANNEL   MODE   MIRROR STATE                           One   PASS   IN to PASS           Two   DROP   In to DROP           Three   ADD   ADD to PASS                      
 
         [0050]    An optical prescription for a three channel Littrow grating based OADM is provided in Table 2 in CODE V format. The numerical aperture of the lens is 0.17 to accommodate standard fiber and the grating has 600 lp/mm. The root mean square wavefront error is less than 0.03 waves in double pass over the temperature range of −20 to +70 degrees centigrade, when the mount is made of 416 Stainless Steel.  
                                                             TABLE 2                                   THICK-                   RADIUS       NESS   RMD   GLASS                                OBJ:   INFINITY       5.584779               1:   −62.78788       16.838678       SF11_SCHOTT       2:   −39.52723       96.862455       AIR       3:   −109.42245       1.700000       NSF15_SCHOTT       4:   76.61669       7.195070       NBAK1_SCHOTT       5:   −58.64552       0.100000       AIR       6:   520.40928       1.700000       NBK10_SCHOTT       7:   48.24900       6.885228       NBAK1_SCHOTT       8:   −199.75265       0.100000       AIR       9:   INFINITY       10.148101       NBK7_SCHOTT       10:     INFINITY       2.885689       AIR       ADE:   −22.806501   BDE:   0.000000   CDE:   0.000000 DAR       STO:   INFINITY       −50.000000   REFL   AIR       GRT:       GRO:   −1   GRS:   0.001667       GRX:   0.000000   GRY:   1.000000   GRZ:   0.000000       ADE:   −15.353235   BDE:   0.000000   CDE:   0.000000                  
 
         [0051]    Turning next to FIG. 5 and FIG. 2, the path of channel one of the three channel multi-channel collimated light signal  101  is more clearly illustrated. Recall that channel one is to be received and PASSED by the system as follows. The multi-channel light signal  101  enters the device through the IN port  201  and is directed through the Lens  105 , Prism  107 , and Liftrow grating  109 . Littrow grating  109  demultiplexes the channels of the Multi-channel light signal  101  and diffracts channel one  501  through the Prism  107  and Lens  105  to Programmable Mirror  211 . The state of Programmable Mirror  211  is set to “IN to PASS” and therefore reflects channel one  501  through Lens  105 , Prism  107  to Grating  109 . Grating  109  multiplexes channel one  501  with other passed and added channels into a multi-channel light signal  111  and directs multi-channel light signal  111  out of the system by way of the Prism  107 , Lens  105  and PASS port  203 .  
         [0052]    Turning next to FIG. 6 and FIG. 2, the path of channel two of the three channel multi-channel collimated light signal  101  is more clearly illustrated. Recall that channel two is to be received and dropped by the system. The multi-channel light signal  101  enters the device through the IN port  201  and is directed through the Lens  105 , Prism  107 , and Littrow grating  109 . Littrow grating  109  demultiplexes the channels of the Multi-channel light signal  101  and diffracts channel two  601  through the Prism  107  and Lens  105  to Programmable Mirror  221 . The state set to “IN to DROP” and therefore reflects channel two  601  through Lens  105 , to Mirror  117 . Mirror  117  reflects channel two  601  out of the system by way of Lens  105  and DROP port  225 .  
         [0053]    Turning next to FIG. 7 and FIG. 2, the path of channel three of the three channel multi-channel collimated light signal  101  is more clearly illustrated. Recall that the multi-channel light signal  101  does not contain a channel three, but instead, channel three is added multi-channel light signal  111  directed out of the system. Channel three  701  enters the device through ADD  233  and is directed through the Lens  105 , and Mirror  117 . Mirror  117  reflects channel three to Programmable Mirror  231  by way of Lens  105 . The state of Programmable Mirror  231  is set to “ADD to PASS” and therefore reflects channel three through Lens  105 , Prism  107  to Grating  109 . Grating  109  multiplexes the channel with other passed and added channels into a multi-channel light signal  111  and directs multi-channel light signal  111  out of the system by way of the Prism  107 , Lens  105  and PASS port  203 .  
         [0054]    Having thus described exemplary embodiments of the present invention, it should be understood by those skilled in the art that the above disclosures are exemplary only and that various other alternatives, adaptations and modifications may be made within the scope of the present invention. The presently disclosed embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention being indicated by the claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.