Abstract:
The present invention includes an optical amplifier and method for reducing the impact of transient events within an optical transmission system. Specifically, a positive transient power or amplifier setting adjustment limit is applied based upon a number of active channels or a current “steady state” operating condition. This adjustment limit is described in terms of a number of channels to be amplified. By controlling the adjustment limit in this manner, automatic control of the amplifier is provided for a loss of channel condition as well as for the addition of a single or few channel condition. If a larger positive transient occurs, the control will not adjust the power limit, and the positive transient power will therefore be suppressed.

Description:
TECHNICAL FIELD 
     The invention relates to the field of optical telecommunications, and more particularly, to the suppression of positive optical amplifier power transients. 
     BACKGROUND OF THE INVENTION 
     In a wavelength division multiplexed (WDM) transmission system, it is possible for the power of an optical channel to be driven higher than a correct operating power level during amplification. For example, if a fiber cable is accidentally disconnected and then reconnected, a positive transient may occur during the reconnection due to the rapid increase in optical power at the input to the optical amplifier. 
     Generally speaking, a “primary” transient is caused by a loss of optical power in a first multiplexed group of optical channels, while “secondary” transients are caused by amplification mismatches between the surviving optical channels and subsequent portions of the network to which the surviving optical channels propagate. This mismatch may be due to errors in the transient suppression for amplifiers within the primary transient region. 
     SUMMARY OF THE INVENTION 
     In accordance with the principles of the invention, transient errors are avoided by selecting an initial level of amplification according to an anticipated number of optical channels within a multiplexed optical signal to be amplified and a desired power level of a resulting amplified multiplexed optical signal. The level of amplification is then incrementally adjusted in response to, for example, increases or decreases in the number of channels within the multiplexed optical signals. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       In the drawing: 
         FIG. 1  depicts a high-level block diagram of a portion of a mesh network of an optical communication system according to one embodiment of the present invention; 
         FIG. 2  depicts a high-level block diagram of a fiber amplifier according to one embodiment of the present invention; 
         FIG. 3  depicts a flow diagram of a method according to one embodiment of the present invention; and 
         FIG. 4  depicts a flow diagram of a method according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The following merely illustrates the principles of the invention. The invention is primarily described within the context of positive optical amplifier power transient suppression in a mesh network. However, those skilled in the art and informed by the teachings herein will realize that the invention is also applicable to any apparatus and method that involves gain control of amplifiers. 
       FIG. 1  depicts a high-level block diagram of a portion of a mesh network of an optical communication system according to one embodiment of the present invention. Specifically, the depicted portion of the mesh network  100  includes four Reconfigurable Optical Add/Drop Modules (ROADMs)  110 ,  120 ,  130  and  140 , which are coupled in series from WEST to EAST in the order named. As depicted in  FIG. 1 , there is a fiber cut in the communication path to the West of ROADM1  110 . ROADM2 has an additional communication path denoted as 2NORTH. ROADM3 also has an additional communication path on 3EAST. ROADM4 has an additional communication path denoted as 4NORTH. In various embodiments, each ROADM has additional optical communication paths. The connection between the ROADMs is composed of materials and components for transmitting optical signals such as optical fibers, light guides and the like. Each ROADM is structurally similar and allows individual multiplexed input signals to be switched onto various optical fibers forming additional optical communications paths. 
     In one embodiment depicted in  FIG. 1 , the received optical signals include groups of channels, illustratively, channels of group A, group B and group C. The channels of group A travel from the West communication path through ROADM1 (and ROADM2) to ROADM3. The channels of group B enter the mesh network at ROADM1 (at a different input than the West optical communications path), travel eastward and exit the mesh network at the communication path 2NORTH of ROADM2. The channels of group C also enter the mesh network at ROADM1 (at a different input than the West optical communications path), travel eastward and exit the mesh network at the communication path 4NORTH at ROADM4. Each ROADM has amplifiers for adjusting the signal strength of the individual channels. 
     The fiber cut  150 , when it occurs, results in a primary transient caused by the loss of channels in group A from ROADM1 to ROADM 3. As a result of the loss of group A, there is a positive gain error for the surviving channels of group B and group C at ROADM1, ROADM2 and ROADM 3. In this fiber cut example, secondary and positive transients occur on the 2NORTH, 3EAST, 4EAST, and 4NORTH paths. 
     Each ROADM includes at least one amplifier for adjusting the gain of optical signals passing therethrough. An automatic gain control (AGC) is used to control the gain of the amplifier. An initial amplifier gain is set to obtain a predefined channel power level based upon an expected number of received channels within a WDM signal to be amplified. A WDM signal having more optical channels is amplified by a greater amount than a WDM signal having fewer optical channels. When an expected number of received channels is greater than an actual number of received channels (e.g., due to an upstream loss of channel condition), the amplifier gain is normally lowered. Similarly, when an expected number of received channels is lower than an actual number of received channels (e.g., when one or more channels are added or switched onto a link), the amplifier gain is normally increased. However, for a positive transient condition, the amplifier does not increase its level of amplification because an adjustment due to the positive transient would lead to an incorrect setting of the amplifier. The channel number setting algorithm is explained in detail below. 
       FIG. 2  depicts a high-level block diagram of a fiber amplifier according to one embodiment of the present invention. The fiber amplifier  200  includes an optical amplifier  210  and a controller  220 . 
     The amplifier  210  receives a WDM signal and amplifies the received WDM signal according to a gain setting provided via the controller  220 . In one embodiment, individual channel gain is adjusted. In another embodiment, the gain across all channels is adjusted. In a further embodiment, the total power of the WDM signal output from the amplifier  210  is adjusted. Other adjustments to the amplifier  210  are possible in order to variably control the gain of the WDM signal. 
     The controller  220  includes a transient detector (T-detector)  230 , a processor  240  and memory  250 . The T-detector  230  monitors the transient events associated with the fiber amplifier. The T-detector determines a transient power level of the number of channels being received by the amplifier  210 . The processor  240  determines the gain of the amplifier  210  using an algorithm stored in memory  250 . The controller must allow for adding of channels by allowing for a predetermined amount of extra power for each channel. The controller  250  receives network information via the system controllers and/or node controllers (not shown) located in the mesh network. Using the received network information, the controller will store in its memory  250  the number of channels being received at the amplifier  210 . In one embodiment, the controller  250  sets a limit to the maximum amplifier output power, which limit is related to the number of received channels. The details of an algorithm according to an embodiment of the invention are described below. 
       FIG. 3  depicts a flow diagram of a method according to one embodiment of the present invention. The method  300  adapts the operation of an amplifier controller such as depicted above with respect to  FIG. 2 . 
     At step  310 , a system controller monitoring the mesh network transmits at least some of the monitored information to amplifier controller  220  as network information. In one embodiment, the network information includes the number and/or paths of the channels. In another embodiment, the network information includes error information of the network. Other supervisory information may be included to assist in the amplification of the signal power of the amplifier  210 . 
     At step  320 , a node controller monitoring the operational condition of a node transmits as least some of the monitored information to amplifier controller  220  as network information. In one embodiment, the node is a ROADM. In another embodiment, the node is an OADM. In a further embodiment, the node is a repeater. The network information transmitted from the node controller includes supervisory information and/or the current state of operations of the node. In one embodiment, the information includes number of channels of the WDM signal. In another embodiment, the information includes fault conditions encountered by the node. Any supervisory information and operational information can be included in the transmission. The network information in steps  310  and  320  may be transmitted via in-band or out-of-band signaling. Other signaling methods may be used. 
     At step  330 , the network information received by controller  220  is stored in the memory  250 . The network information includes the number of channels being amplified by the amplifier  210 . In one embodiment, the network information includes the number of channels being received by the node. In another embodiment, the network information includes the number of channels being added and dropped by the node. In a further embodiment, the network information also includes all the channels that are lost and did not arrive at the node. In other embodiments, additional network information such as the paths of the channels is stored in memory  250  and processed by processor  240  to improve the power output of the channels of the amplifier  210 . 
     At step  340 , the processor  240  determines the power limit of the output of the amplifier  210 . The maximum power for the amplifier is determined by adding a power level offset to a current power level of the amplifier. In one embodiment, if one channel is added or switched, the power level offset is the channel power for that one additional channel. Thus, if the amplifier is currently amplifying N channels, than the maximum power limit for the amplifier would be set at the amount of power needed to amplify N+1 channels. In another embodiment, the offset is based on the “steady state” of the operating point of the amplifier. In a further embodiment, the amplification is determined on a per channel basis. 
     At step  350 , the T-detector  230  obtains the signal power level of the amplifier  210 . The processor  240  determines if the required signal power level is above an output power limit determined at step  340 . When the required signal power is below the limit, the amplifier control algorithm is applied such that the gain of the amplifier is capped and the amplifier is not allowed to follow a positive transient to an incorrect amplification setting. The limit allows for the adding of channels by allowing a predetermined amount of extra power for each channel. When the required signal power is above the limit, then the current state of amplifier and signals is detected and a transient event is deemed to have occurred and the amplifier is not further adjusted. This is because allowing the amplification to follow the transient event will result in a positive gain error after amplification. 
     At step  360 , the controller applies the amplifier control algorithm and determines if the gain of the amplifier needs to be increased or decreased. In one embodiment, the gain adjustment is applied to the amplitude of the output signal. In another embodiment, the gain adjustment is applied to each channel individually as required. Any control method can be used to adjust the gain of the amplifier. 
     At step  370 , the controller adjusts the power level of the amplifier. The power of the amplifier is adjusted according to the calculations of the control algorithm. The control algorithm assists the controller to determine the output power of the amplifier. The control algorithm, using the network information, reduces the output power of the amplifier proportionate to the amount of channels that are dropped and increases the output power of the amplifier proportionate to the amount of channels that are added or switched. As previously noted, a maximum power of the amplifier output is provided to avoid undue influences on amplifier operation due to, for example, transient conditions in the network. 
     At step  380 , the controller obtains the current state of the amplifier and of the WDM signal being amplified. The controller uses the detected current state information with the network information to assist in determining whether a positive transient has occurred and/or whether the power level of the surviving channels needs to be adjusted. The current state is determined by a T-detector or by a processor and photodetectors. 
       FIG. 4  depicts a flow diagram of a method according to one embodiment of the present invention. The method  400  illustrates how a maximum power limit by using channel number information is determined in, for example, an amplifier controller such as depicted above with respect to  FIG. 2 . 
     At step  410 , a node receives a plurality of optical channels. Some of the channels are dropped at the node, while others are added and switched. The optical channels are transported in a WDM signal. The node amplifies the WDM signal that is being transported to the next node. The WDM signal includes N optical channels. In one embodiment, a controller at the amplifier monitors the WDM signal for the (N) number of channels. 
     At step  420 , the number of channels received by the node is determined. In one embodiment, the maximum power level of each channel is predetermined, and the maximum total power level of the amplifier is the total power level of all the channels. In another embodiment, the current gain of the amplifier is determined by the current “steady state” operation point of the amplifier by, for example, measuring the optical power levels of the channels at the input and output of the amplifier to calculate the current gain (e.g., in a decibel log scale). 
     At step  430 , every channel that is transmitting an optical signal is amplified. Because the controller includes the “steady state” power of each channel, the proper output power is determined by monitoring the N number of channels. If one channel is switched or added, the maximum level of the amplifier output is increased to a power level for N+1 channels. 
     At step  440 , the output power of the amplifier is limited to N+1 channels. If the output power is greater than the “steady state” power for N+1 channels, then a power spike resulting from a transient event is deemed to have occurred. Because a limit is set for the output power, the output power of the amplifier remains below the maximum power level for the N+1 channels. Thus, the transient power is suppressed. 
     At step  450 , in response to a loss or drop of a channel, the channel number or count is decreased by 1 and the output power limit of the amplifier is adjusted accordingly. In another embodiment, if a plurality of channels are dropped or are lost, then the channel number will decrease proportional to the number of channel lost or dropped. 
     At step  460 , in response to an addition of a channel or when a channel is switched onto the WDM signal, the channel number or count is increased by 1. Thus, the maximum power is adjusted accordingly. In another embodiment, if more than one channel is being added, the channel number is increased proportionally to ensure the amplifier has the proper output power limit. 
     In various embodiments, the amount of amplification increase (where channels are added) or decrease (where channels are dropped) may be a predetermined amount such that incremental changes in amplification are made according to the number of channels to be amplified after selection of an amplification level for an initial number of channels. The predetermined amount of amplification change may be calculated as, for example, (a) a fixed or predefined amount determined as the initial amplification level divided by N; (b) a fixed or predefined amount determined as the initial amplification level divided by N+/− a scaling factor; (c) a fixed or predefined amount that has been empirically derived and the like. Moreover, the ultimate level of amplification is bounded or clamped by a maximum amplification level such that propagation of an optical transient condition is avoided in the present network and/or a subsequent network. 
     While the foregoing is directed to various embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. As such, the appropriate scope of the invention is to be determined according to the claims, which follow.