Abstract:
A method and device for optical add/drop is disclosed. The add/drop splits an input signal into two portions. The first portion is optically filtered to remove the channels, leaving an unmodulated carrier which is modulated with the newly added information. The second portion is split again into the through channels and a channel to be dropped. The dropped channel is detected or terminated and the through channels are recombined with the newly added channel to form an output optical signal. If desired, multiple channels may be dropped at the add/drop node. A dense wavelength division multiplexed (DWDM) optical communication system incorporating the add/drop node is also disclosed.

Description:
[0001]    This application claims priority from U.S. Provisional Patent Application Number 60/231,577, filed Sep. 11, 2000 and entitled “Optical Add/Drop Multiplexer and In-Band Wavelength Conversion”. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates generally to a method and apparatus for adding and dropping channels in an optical communications system.  
           [0004]    2. Description of Related Art and General Background  
           [0005]    In many applications of dense-wavelength division multiplexed (DWDM) optical systems (for example, optical computer networks, CATV (cable television) systems, and telecommunications networks), there exists a need to allow local dropping and adding of traffic carried by one or more wavelengths. Applications include when optical channels are sent to or dropped from an optical transmission line e.g., for sending optical channels to a local bus or for adding local channels to an incoming data signal. This form of optical routing may be generally referred to as “add-drop multiplexing.” 
           [0006]    While a basic optical add/drop multiplexer has been described by Taga, et al. (U.S. Pat. No. 5,822,095), it is suited to relatively wide channel spacing and is not suited to modern DWDM systems which tend to have a larger number of more narrow channels which are closely spaced. Mizrahi (U.S. Pat. No. 5,982,518) has proposed one solution for more narrow channels using sequential fiber gratings between optical circulators. In each case, however, a set of local transmitters is required to produce the added channels, increasing the cost and complexity of the add/drop node.  
           [0007]    In each of these devices, each transmitter requires the use of a wavelength locker in order to maintain the wavelength stability of the transmitted channels. Such wavelength lockers are available which allow wavelength stability of between about 2.5 GHz and 5 GHz. However, such tolerances are only effective for use in networks having a channel spacing of about 50 GHz. When channel spacing is below about 10 GHz, currently available wavelength lockers are too imprecise to allow transmission without any interference between channels.  
         SUMMARY OF THE INVENTION  
         [0008]    Embodiments of the present invention address the needs identified above and others by providing a method of optical data transmission including receiving an optical signal having a plurality of components, each component having a different wavelength, receiving an information signal, separating a first one of the components from the optical signal, dropping a second one of the components from the optical signal, modulating the first one of the components with the information signal to obtain a modulated component, and combining the modulated component with the optical signal.  
           [0009]    Another embodiment of the present includes an add/drop device for optical data transmission including an optical waveguide configured and arranged to receive a signal having a plurality of components, each component having a different wavelength, a splitter coupled to the optical waveguide and configured and arranged to produce a first output signal and a second output signal, each output signal having the plurality of components, a first filter configured and arranged to separate a first one of the components from the first output signal, a modulator configured and arranged to modulate the first component with an information signal, a second filter configured and arranged to drop a second one of the components, different from the first component, and a combiner configured and arranged to combine the modulated first component with the filtered second output signal.  
           [0010]    Yet another embodiment of the present invention includes a dense wavelength division multiplexed optical transmission system, including a transmitter, an optical waveguide configured and arranged to receive an optical signal from the transmitter through the transmission line, the signal having a plurality of components, each component having a different wavelength, a splitter coupled to the optical waveguide and configured and arranged to produce a first output signal and a second output signal, each output signal having the plurality of components, a first filter configured and arranged to separate a first one of the components from the first output signal, a modulator configured and arranged to modulate the first component with an information signal, a second filter configured and arranged to drop a second one of the components, different from the first component, a combiner configured and arranged to combine the modulated first component with the filtered second output signal, and a receiver in optical communication with the combiner. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]    The accompanying drawings, which are incorporated in and constitute a part of this specification illustrate an embodiment of the invention and together with the description, explains the objects, advantages, and principles of the invention.  
         [0012]    [0012]FIG. 1 is a schematic diagram illustrating an optical add/drop according to an embodiment of the present invention.  
         [0013]    [0013]FIG. 2 schematically illustrates a broad band optical signal.  
         [0014]    [0014]FIGS. 3 a  and  3   b  schematically illustrate a single band of an optical signal.  
         [0015]    [0015]FIG. 4 schematically illustrates a broad band optical signal.  
         [0016]    [0016]FIG. 5 is a schematic diagram illustrating an optical add/drop according to another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0017]    In the following description, for purposes of explanation and not limitation, specific details are set forth such as particular optical and electrical circuits, circuit components, techniques, etc. in order to provide a thorough understanding of the present invention. However, the invention may be practiced in other embodiments that depart from these specific details. In some instances, detailed descriptions of well-known devices and circuits may be omitted so as not to obscure the descriptions of the embodiments of the present invention with unnecessary details.  
         [0018]    Certain aspects of the description make mention of use of optical single sideband (OSSB) modulation or double sideband modulation. One method of optical single sideband transmission is disclosed in U.S. patent application Ser. No. 09/575,811 of Way et al., filed May 22, 2000, entitled “Method and Apparatus for Interleaved Optical Single Sideband Modulation”, and herein incorporated by reference. Other methods of optical single and double sideband modulation may be employed as appropriate.  
         [0019]    For purposes of this specification, some channels will be referred to as having a characteristic wavelength or frequency. This does not mean that the channel is restricted to the exact recited wavelength or frequency. If the channel has a width, then the characteristic wavelength or frequency is taken to be at approximately the center of the width. If a channel is substantially monochromatic, then the characteristic wavelength or frequency will be the wavelength or frequency of the monochromatic light source.  
         [0020]    An add/drop node  100  according to an embodiment of the present invention is shown in FIG. 1. The node  100  has an input  102  which is in optical communication with an optical path  104 . The optical path  104 , in most cases, will be a single mode fiber forming a part of an optical communication system. A transmitter  106  is in optical communication with the optical path  104 .  
         [0021]    The input  102  may include an optical amplifier  108 , such as a erbium doped fiber amplifier. Likewise, the amplifier  108  may be disposed along the optical path  104 , or in both locations. An optical pre-filter  110  is in optical communication with the input  102 . The optical pre-filter  110  may be, for example, a fixed or tunable filter and may have a bandwidth approximately equal to the bandwidth of a single ITU-grid window, for example, about 25-60 GHz. An optical circulator  112  in communication with the optical pre-filter  110  provides a first optical path leading to an optical filter  114 . The optical filter  114  may be, for example, a tunable Fabry-Perot filter or a fiber grating based filter. An optical isolator  116  and a polarization controller  118  are disposed in the optical path between the optical filter  114  and a modulator  120 . The modulator  120  may be, for example, an optical double sideband modulator, an optical single sideband modulator or an interleaved optical single sideband modulator. Note that the polarization controller  118  is of particular import only for those embodiments of the add/drop node  100  including a polarization dependent modulator  120 , such as a Mach-Zehnder modulator. The modulator is in communication with a source  121  of an information signal to be added to the optical signal.  
         [0022]    An output optical path  122  proceeds from the modulator  120  and may contain an optical amplifier  124 . A combiner  126  is disposed along the output optical path  122 . The combiner  126  may be any type of junction allowing optical signals from two fibers to be mixed, for example, the combiner  126  may be a 2×1 connector or a multiplexer.  
         [0023]    A second optical path leading from the circulator  112  contains a filter  130 , such as a notch filter. The filter  130  may be, for example, a band reject filter, or a pair of cascaded band reject filters, providing a deeper, narrower notch. A second circulator  132  is in optical communication with the filter  130  and is in optical communication with an optical filter  134 . An additional optical filter  136  is in optical communication with the optical filter  134  and the two are separated by an optical isolator  138 . The two optical filters  134 ,  136  may be, for example, tunable optical band pass filters. A photodetector  140  is optionally disposed in optical communication with the optical filters  134 ,  136 . If there is no need to detect a signal passed by the filters  134 ,  136 , a termination may be substituted for the photodetector  140 . The circulator  132  is further in optical communication with the optical output path  122 .  
         [0024]    As may be seen in FIG. 2, the optical filter  134  may have a characteristic which results in a phenomenon of repeated passbands  142 . Since it is undesirable to drop channels at upper or lower repeated passbands in an uncontrolled way, the optical pre-filter  110  is used to remove bands outside of the band to be processed in the add/drop node  100 . Thus, it is preferable to set the bandwidth of the optical pre-filter  110  to be less than the free spectral range  150  of the optical filter  134 . As a matter of convenience, the bandwidth of the optical pre-filter may be selected to be about equal to the ITU window passband of a conventional DWDM multiplexer/demultiplexer.  
         [0025]    In operation, the add/drop node  100  receives a signal including a plurality of channels λ 1  . . . λ n . The signal may be of a bandwidth greater than a single ITU-grid window as shown in FIG. 2. The signal includes an unmodulated carrier  200  as seen in FIGS. 3 a,    3   b  and  4 .  
         [0026]    [0026]FIG. 3 a  shows one band B of an interleaved optical single sideband signal in which upper sideband channels  204 ,  206  and lower sideband channels  208 ,  210  are interleaved. That is, an upper sideband channel  204  differs in wavelength from the carrier by a different amount than both lower sideband channels  208  and  210  such that a residual image of the upper sideband channel  204  will substantially not interfere with the lower channels  208  and  210 . The wavelength λ of each channel is expressed in terms of difference between the wavelength of the channel and the wavelength of the carrier. So if a channel is denoted λ 1 , that means that the channel has a wavelength of λ c +λ 1 . Thus, in the signal of FIG. 3 a,  an upper sideband channel has a wavelength λ 1  and neither lower sideband channel  208 ,  210  has a wavelength equal to the wavelength of the carrier  200  minus λ 1 . FIG. 3 b  shows one band B′ of a double optical single sideband signal in which each upper sideband channel  204 ′,  206 ′ has a corresponding lower sideband channel  208 ′,  210 ′. The signal  212  has a carrier  200 ′.  
         [0027]    [0027]FIG. 4 shows a plurality of channels λ 1  . . . λ 10  contained in two bands B 1 , B 2  or ITU-grid windows. The channels of each band B 1 , B 2  include four data channels on respective sub-carriers  220  and one continuous wave carrier  222  which is umnodulated. The signal  224  shown in FIG. 4, may represent, for example, a ultra-dense wavelength division multiplexed on-off key modulated signal (U-DWDM OOK). Though four channels are shown in each of FIGS. 3 a,    3   b  and  4 , different numbers of channels may be used, depending on the channel spacing and window size.  
         [0028]    In operation, referring to FIGS.  1 - 4 , add/drop node  100  an input optical signal from the transmitter  106  is received by the input  102 . The input optical signal may be, for example, similar to one of those signals shown in FIGS. 3 a,    3   b  and  4 . The input signal has a plurality of components, each component having a different wavelength from the other components. The input signal is amplified by the optical amplifier  108  and continues to the optical pre-filter  110 . As noted above, the optical pre-filter  110  preferably has a band pass bandwidth approximately the same as the width of a band B n  of the input optical signal, e.g. about 40-80 GHz, so that only one band of the input optical signal is processed by the add/drop node  100 .  
         [0029]    The input optical signal enters the circulator  112  and passes into the upper arm of the add/drop circuit. The optical filter  114  is a band pass filter which separates one component, the unmodulated carrier  200 ,  200 ′,  222 , from the signal. The signal proceeds through the optical isolator  116  and the polarization controller  118  before entering the modulator  120 . The modulator  120  receives an information signal and modulates the carrier  200 ,  200 ′,  222  with the information signal. Preferably, the information signal is modulated onto the carrier in a channel corresponding to the channel to be dropped; it may be placed in any empty channel, or even out of band, if there is not to be interference from an adjacent band.  
         [0030]    When the optical filter  114  passes the carrier into the upper arm, it also reflects a portion of the input optical signal, i.e. all but the optical carrier, back through the circulator  112  and into the lower arm. The notch filter  130  removes the carrier and the signal proceeds to the second circulator  132 . Even in the case where the filter  114  has removed most of the carrier, there may be residual components of the carrier which should be removed by the notch filter  130 . The two optical filters  134 ,  136  along with the optical isolator  138  act as a strong, narrow bandpass filter to extract a second component, the channel to be dropped, from the signal. The use of the two filters  134 ,  136  and the isolator  138  allows the dropped channel to be more completely removed from the signal, reducing residual components which might interfere with adding a new channel. The filters  134 ,  136  may be tunable so that any selected channel can be dropped.  
         [0031]    In most circumstances, the dropped channel will be dropped so that it may be locally received. In those cases, a photodetector  140  is used to receive the dropped channel after it passes through the filters  134 ,  136 . In the case that the dropped channel is being dropped simply to free bandwidth for a channel to be added and is not to be locally used, no photodetector  140  is required. A termination (not shown) should be used to ensure reflections are effectively eliminated, though this may not be necessary given the isolator  138 .  
         [0032]    The first filter  134  reflects the remaining optical signal, without the dropped channel, onward towards the output optical path  122 . The signal passes through the combiner  126  where it is combined with the carrier which has been modulated with the information signal, forming an output optical signal. The output optical signal is optionally amplified by the optical amplifier  124  and is output from the add/drop node  100  for further transmission to a receiver (not shown).  
         [0033]    Since the upper and lower arms of the circuit are recombined at the combiner  126 , it is desirable to match the optical path lengths so that the output optical signal components retain similar phase relationships to each other as they had prior to processing in the add/drop node  100 . This is of particular importance, for example, in a packet-switched network. In order to maintain phase relationships, delay loops, for example, can be added into whichever of the two arms has a shorter optical path.  
         [0034]    [0034]FIG. 5 shows an extension of the add/drop  100  of FIG. 1, adapted to add and drop up to N channels. The components of add/drop  300  are similar to add/drop  100 . The input  302  may include an optical amplifier  308  prior to the optical pre-filter  310 . The circulator  312  is in communication with an upper arm of the circuit including an optical filter  314  for separating the carrier from the optical signal. The optical filter  314  is in communication with a polarization controller  318  and a modulator  320 . The modulator  320  is in communication with a separate input  321  for inputting information signals to be modulated onto the carrier as in the add/drop  100  of FIG. 1.  
         [0035]    In the lower arm, a series of drop filters  334  and photodetectors  340  are used to drop each channel to be dropped. As can be seen, circulators  332  are used to direct the signal into each drop filter  334  seriatim. Though drop filters  334  are shown as a single component in FIG. 5, they may be understood to encompass an arrangement such as that shown in FIG. 1, each filter  334  including two bandpass filters in conjunction with an optical isolator. N sets  341  of filters  334  and photodetectors  340  are provided to drop N channels.  
         [0036]    At the junction of the two arms, a combiner  326  is provided to combine the two signals for output.  
         [0037]    The operation of the add/drop  300  may be understood from the operation of the add/drop  100  described above without further explanation. There is some upper limit on the number of channels N which can be added at each add/drop  300 . Though, in theory, the number of drops could be extended without a practical limit, the adding modulator generally has a maximum bandwidth. The maximum number of added channels may be derived from the maximum bandwidth, the channel width and the channel spacing and will depend on the application, as well as changes in standards and technologies.  
         [0038]    Embodiments of the present invention find uses, for example, in all-optical, packet-switched networks having fast optical switches and routers in the core or circuit-switched networks with relatively slow optical cross-connects for providing traffic re-routing or protection functions. Such networks may be used as telecommunications networks carrying voice and/or data, CATV networks or other such applications.  
         [0039]    While the invention has been described in connection with particular embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but on the contrary it is intended to cover various modifications and equivalent arrangement included within the spirit and scope of the claims which follow.