Abstract:
A method and apparatus is provided for power balancing an optical signal wavelength to be added to an OADM having at least one drop port and at least one add port. The method begins by monitoring a power level of a first signal wavelength being dropped on the drop port and a power level of a second signal wavelength being added on the add port. The power level of the first signal wavelength is compared to the power level of the second signal wavelength. Based on the step of comparing, the optical attenuation is adjusted along the add port so that the power level of the second signal wavelength becomes substantially equal to the power level of the first signal wavelength.

Description:
FIELD OF THE INVENTION 
   The present invention relates generally to optical add drop multiplexers, and more particularly to an optical add drop multiplexer in which power equalization is provided to a channel being added. 
   BACKGROUND OF THE INVENTION  
   An optical add drop multiplexer (OADM) is a device used to extract a set of optical signals (also called “wavelengths” or “channels” herein) from a wavelength division multiplexed (WDM) signal input to the OADM, and to subsequently reinsert the extracted set of wavelengths output by the OADM.  FIG. 1  shows a block diagram of an OADM  100 . A WDM optical signal comprises a plurality of wavelengths or channels. One of the wavelengths λ 2 , is extracted (also called “dropped” herein) from an input line side  102  via a drop port  104  and then subsequently reinserted (also called “added” herein) λ 2  onto an output line side  108  via an add port  106 . The purpose of adding and dropping wavelength(s) in this manner is to obtain information encoded on the dropped wavelength. New information may also be transmitted the added wavelengths. In most instances the carrier wavelength of the dropped wavelength is the same as carrier wavelength of the added wavelength. 
   OADMs may be implemented in a wide variety of different architectures and technologies. For example, one architecture involves arrayed waveguide grating routers and 2×2 optical switches. Another architecture involves a pair of interference filters that serve as multiplexers and demultiplexers. Depending on the architecture and the technology that is employed, an OADM may or may not be configurable, i.e., the determination of which wavelengths are dropped and added may or may not be fixed at the time of manufacture. 
   OADMs are employed in the network nodes of WDM transmission systems such as ring networks so that incoming data may be either passed through the node or dropped to a local receiver. If data from a particular wavelength is dropped, this wavelength is now available on the outbound direction, and hence new data can be added from a local transmitter. In a WDM system, when the optical signals are transmitted over long distances, periodic amplification of the optical signals is necessary to overcome fiber loss in the transmission path. Currently, amplification is accomplished by using optical amplifiers, e.g. Erbium Doped Fiber Amplifiers (EDFAs) or Raman amplifiers. 
   In general each of the wavelengths in a WDM transmission system employing optical amplifiers should have the same power. If the power levels of the wavelengths are not the same, those wavelengths having more power tend to be amplified more than other channels and take away gain that would otherwise be available for adjacent wavelengths. When such unbalanced wavelengths propagate through a series of optical amplifiers, deleterious effects may arise such as a high level of cross talk between adjacent wavelengths and nonlinearities in the fiber. 
   Accordingly, it would be desirable to provide an automatic power balancing arrangement for an OADM in which wavelengths or channels being added have the same power as the remaining wavelengths or channels in the WDM optical signal. 
   SUMMARY OF THE INVENTION  
   In accordance with the present invention, a method is provided for power balancing an optical signal wavelength to be added to an OADM having at least one drop port and at least one add port. The method begins by monitoring a power level of a first signal wavelength being dropped on the drop port and a power level of a second signal wavelength being added on the add port. The power level of the first signal wavelength is compared to the power level of the second signal wavelength. Based on the step of comparing, the optical attenuation is adjusted along the add port so that the power level of the second signal wavelength becomes substantially equal to the power level of the first signal wavelength. 
   In accordance with one aspect of the present invention, the step of monitoring the power level of the first signal wavelength includes the steps of tapping a portion of the power from the first signal wavelength as it traverses the drop port and generating a first electrical reference signal that corresponds to the tapped portion of power of the first signal wavelength. 
   In accordance with another aspect of the invention, the step of monitoring the power level of the second signal wavelength includes the steps of tapping a portion of the power from the second signal wavelength as it traverses the add port and generating a second electrical reference signal that corresponds to the tapped portion of power of the second signal wavelength. 
   In accordance with another aspect of the invention, the comparing step is performed in the electrical domain. 
   In accordance with another aspect of the invention, the step of adjusting the optical attenuation is performed by a variable optical attenuator coupled to the add port. 
   In accordance with another aspect of the invention, an optical add drop multiplexer (OADM) is provided that includes an input port for receiving a WDM optical signal having a plurality of signal wavelengths. At least one drop port is provided for extracting one of the plurality of signal wavelengths from the WDM optical signal. At least one add port is provided for inserting an add wavelength into the WDM optical signal. The OADM also includes an output port for transmitting to an external element the WDM optical signal with the add wavelength present and the extracted one of the plurality of wavelengths absent. A first monitoring arrangement monitors a power level of the extracted signal wavelength and a second monitoring arrangement monitors a power level of the add wavelength. A comparator compares the power level of the extracted signal wavelength to the power level of the add wavelength. A variable optical attenuator is coupled to the add port and adjusts optical attenuation of the add wavelength in response to a control signal received from the comparator. 
   In accordance with another aspect of the invention, the first monitoring arrangement includes a first optical tap located at the drop port and a first photodetector coupled to the optical tap for receiving a portion of the extracted signal wavelength. 
   In accordance with another aspect of the invention, the second monitoring arrangement includes a second optical tap located at the add port and a second photodetector coupled to the second optical tap for receiving a portion of the add wavelength. 
   In accordance with another aspect of the invention, the comparator is an electrical comparator electrically coupled to the first and second photodetectors. 
   In accordance with another aspect of the invention, the first and second photodetectors are photodiodes. 
   In accordance with another aspect of the invention, the control signal adjusts the optical attenuation of the add wavelength so that the power level of the add wavelength is substantially equal to the power level of the extracted signal wavelength. 
   In accordance with another aspect of the invention, an optical add drop multiplexer (OADM) is provided that includes an input port for receiving a WDM optical signal having a plurality of signal wavelengths and a plurality of drop ports each extracting one of the plurality of signal wavelengths from the WDM optical signal. A plurality of add ports each inserts an add signal wavelength into the WDM optical signal. An output port transmits to an external element the WDM optical signal with one or more add wavelengths present and one or more extracted wavelengths absent. A plurality of first monitoring arrangements is each associated with one of the drop ports. Each of the first monitoring arrangements monitors a power level of one of the extracted wavelengths and generates a first reference signal in response thereto. A plurality of second monitoring arrangements is each associated with one of the add ports. Each of the second monitoring arrangements monitors a power level of one of the add wavelengths and generates a second reference signal in response thereto. A processor receives the plurality of first reference signals and selects one of the plurality of first reference signals that represents a non-zero optical power level. A comparator arrangement compares the selected one of the first reference signals to each of the second reference signals. A plurality of variable optical attenuators is respectively coupled to the plurality of add ports for adjusting optical attenuation of the add wavelengths in response to control signals received from the comparator arrangement. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a block diagram of a conventional optical add drop multiplexer (OADM). 
       FIG. 2  shows a block diagram of one embodiment of an OADM constructed in accordance with the present invention. 
       FIGS. 3–6  show alternative embodiments of the OADM in accordance with the present invention. 
   

   DETAILED DESCRIPTION  
   The present invention provides an optical add drop multiplexer (OADM) in which the power level of the signal wavelength being added is monitored, as well as the power level of the wavelength being dropped. After comparing the power levels of the dropped wavelength to the power level of the added wavelength, the added wavelength can be sufficiently attenuated so that its power level is equal to the power level of the dropped wavelength. In some cases, instead of monitoring the power level of the dropped wavelength, the WDM signal itself may be monitored, from which an average power level per wavelength can be calculated. In this case the power level of the added wavelength can be compared to the average power per wavelength that has been calculated. 
     FIG. 2  shows a block diagram of one embodiment of an OADM  200  constructed in accordance with the present invention. The OADM  200  includes an input line side  202 , a drop port  204 , an output line side  208 , and an add port  206 . Optical taps  210  and  212  are located to receive a small portion of the power from the wavelengths traversing the drop and add ports  204  and  206 , respectively. A variable optical attenuator (VOA)  216  is also provided at the add port  206 . The wavelength to be added first traverses the VOA  216  before being received by port  206  via optical tap  210 . A photodiode  214  receives the power from optical tap  210  and sends an electric reference signal to an electrical comparator  218 . Likewise, a photodiode  220  receives the power from optical tap  212  and sends an electric reference signal to the electrical comparator  218 . The electrical comparator  218  generates an error signal representative of the power differential between the optical signal being dropped on drop port  204  and the optical signal being added on add port  206 . The error signal is used to adjust the attenuation of the VOA  216  so that the error (i.e., power differential) is reduced. That is, the VOA  216  attenuates the wavelength being added on port  206  so that its power level is substantially the same as the power of the wavelength being dropped on port  204 . 
     FIG. 3  shows an embodiment of the invention in which the OADM  300  has a multiplicity of add ports and drop ports, and specifically in this case, three drop ports  304   1 ,  304   2  and  304   3  and two add ports  304   4  and  304   5 . Optical taps  310   1 – 310   5  are located to receive a small portion of the power from the wavelengths traversing the ports  304   1 – 304   5 , respectively. Variable optical attenuators (VOAs)  316   1  and  316   2  are provided at the add ports  304   4  and  304   5 , respectively. Optical taps  310   1 – 310   5  direct a small portion of the optical power respectively traversing ports  304   1 – 304   5  to photodiodes  314   1 – 314   5 , respectively. The photodiodes  314   1 – 314   3  associated with the drop ports direct electric reference signals to a processor  320 . The photodiodes  314   4  and  314   5  send electric reference signals to comparators  318   1  and  318   2 , respectively, which reference signals represent the power of the wavelengths being added. The output of the processor  320  serves as the second input to both of the comparators  318   1  and  318   2 . 
   Processor  320  selects one of the electric reference signals received from photodiodes  314   1 – 314   3  and directs it to the input of the comparators  318   1  and  318   2 . The processor  320  ensures that the comparators  318   1 , and  318   2  receive a valid power level for a dropped channel. This is important because not every drop port will necessarily be dropping a wavelength at any given time. Thus, in order to provide a meaningful comparison between power levels, the processor  320  will only select a reference signal from a drop port on which there is a dropped wavelength at the time the comparators  318   1  and  318   2  are to generate error signals that adjust the attenuation of VOAs  316   1  and  316   2 . That is, the processor  320  will select a reference signal that represents a non-zero optical power level. For example, if processor  320  determines that no channel is being dropped on drop port  304   1  when a channel is being added on add port  304   4  or  304   5 , it will attempt to use a reference signal from drop port  304   2 . Similarly, if processor  320  determines that no channel is being dropped on drop port  304   2 , it will attempt to use a reference signal from drop port  304   2 . Moreover, if for some reason the WDM signal being received on the input port  302  of OADM  300  should fail so that no dropped channel is available, processor  320  can use previous reference signal values that it has stored in memory. In this way the processor  320  can be used to operate the system in the event of a failure at the OADM input  302 , which may occur as a result of a fiber break, for example. 
     FIG. 4  shows an alternative embodiment of the invention in which the channel power at the input port to the OADM  400  is monitored instead of the channel power at the drop ports. In this case the monitoring arrangement determines both the total power of the WDM signal received on input port  402  and the total number of wavelengths or channels in the WDM signal, from which an average power per channel can be determined. As shown, an optical tap  410  is provided at the input port  402 . The optical tap  410  directs a small portion of the WDM signal to an optical splitter  422 . The optical splitter  422 , in turn, directs a portion of the WDM signal to a photodiode  426 , the output of which is a reference signal that represents the total power of all the wavelengths that comprise the input WDM signal. The optical splitter  422  directs the remaining portion of the WDM signal to an optical tunable filter  424 . The tunable filter  424  can be tuned over the entire wavelength band occupied by the input WDM signal. For example, if the WDM signal is located in the C band that encompasses wavelengths between 1525 and 1565 nm, then the tunable filter can be tuned over this same range. The output from the tunable filter is directed to a photodiode  428 , which generates a reference signal in response thereto. 
   As the optical tunable filter  424  is swept across its waveband, the reference signal generated by photodiode  428  will go through peaks that correspond to the location of a channel and troughs that correspond to locations between channels. The number of channels employed in the input WDM signal corresponds to the number of peaks in the reference signal. The number of peaks can be counted by a register associated with an electrical circuit  430 . The electrical circuit  430  also receives the reference signal from photodiode  426 . Given the total power of the input WDM signal (as represented by the reference signal from photodiode  426 ) and the number of channels in the WDM signal (as represented by the number of peaks in reference signal from photodiode  428 ), the electrical circuit  430  can calculate the average power per wavelength or channel. The electric circuit  430  forwards this value to an input of the comparator  418 , which as in the previous embodiment of the invention, receives at its other input a signal representative of the channel being added on add port  406 . Once again, the comparator  418  adjusts the attenuation of the VOA  416  so that in this case the power of the added wavelengths is about equal to the average power per wavelength of the input WDM signal. 
     FIG. 5  shows an alternative embodiment of the invention that is similar to the embodiment depicted in  FIG. 4 , except that in  FIG. 5  the optical splitter  422  and photodiode  426  are eliminated. In  FIGS. 4 and 5 , as well as  FIG. 6  discussed below, like components are indicated by like reference numerals. In  FIG. 4 , the splitter  422  and photodiode  426  are used to determine the total power of the input WDM signal. In  FIG. 5 , this value can alternatively be determined by integrating the reference signal generated by photodiode  428  over the entire bandwidth of the optical tunable filter  424 . 
   One problem with the embodiments of the invention shown in  FIGS. 4 and 5  is that the WDM signal is tapped at the input port  402  to the OADM. This gives rise to optical losses in the OADM that must be taken into account when balancing the power of the added wavelengths. The optical losses can be accounted for by having the comparator  418  provide an offset voltage to the VOA  416  by the comparator  418  along with the error signal. However, the need for such an offset voltage can be avoided if the optical tap and tunable filter are located at the output port of the OADM instead of the input port. Such an arrangement is shown in  FIG. 6 . Similar to  FIG. 4 , in  FIG. 6  the optical splitter  422  and photodiode  426  are employed. In another embodiment of the invention, however, the optical splitter  422  and photodiode  426  of  FIG. 6  may be eliminated for the reason discussed in connection with  FIG. 5 .