Abstract:
Optical amplifying method and apparatus are proposed. Constant gain amplifiers operate such that the output power of the amplifier generally tracks the input power. However, optical systems are not perfect and the input to the optical amplifier stage includes not only the desired signal, but also includes accumulated effects of the imperfections. The imperfections include losses of the fiber sections, variations in laser powers, and drifts. Thus, simple amplification not only amplifies the desired signal, but also amplifies accumulated imperfections. Such imperfections occur over time and are generally small in magnitude. By operating the amplifier such that amplification of small variations is suppressed while allowing for tracking of large input variations, amplifying the accumulated imperfections is minimized.

Description:
PROVISIONAL APPLICATION 
   The present application claims priority under 35 U.S.C. § 119(e) of a provisional application 60/427,741, filed on Nov. 20, 2002 the entirety of which is hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The field of the invention generally relates to optical amplifying devices. More particularly, the invention relates to optical amplifying devices and methods with gain control. 
   BACKGROUND OF THE INVENTION 
   Conventional optical amplifiers may be operated under variable control to achieve an amplifier response that is flat over a wide gain range. Amplifiers may also be operated in constant gain mode in which the pumps are controlled such that the amplifier gain is constant with the result being that the output power closely tracks the input power. 
   In optical systems, it is desirable to launch a constant power per channel into the fiber. This may be achieved by operating the amplifier in a constant gain mode. In constant gain mode, any change in the input power of a constant gain amplifier produces a proportional change in the output power of the amplifier. Thus any change in the input due to change in channel count (i.e. addition and deletion of channels), will cause a proportional change in the output power of the amplifier, keeping the per channel launch power nominally constant. 
   While this method is useful for tracking changes in channel count, there are drawbacks. The amplifier has no means of differentiating between changes in input power caused by channel count changes and other events such as change in fiber loss, component losses etc. While these changes may be small over a single span, these changes will accumulate along multiple sections of fibers and amplifiers to a detrimental effect. 
   Consider a chain  100  of optical amplifiers  102 -X shown in the block diagram of  FIG. 1 . The input power per channel to the first amplifier  102 - 1  is P 1 . The average losses of the fiber sections  104 - 1  to  104 -N are shown as L 1 , L 2 , . . . L N . If the launch power per channel is assumed to be same at each fiber section (and same as at the output of the first amplifier), the gains G 2 , . . . G N  of the amplifiers  102 - 2  . . .  102 -N track the average loss of the fiber section preceding the amplifier (L 1 , L 2  . . . L N ). The time dependent variation in fiber loss given by ΔL 1 , ΔL 2 , . . . ΔL N  and the variation in input power is assumed to be ΔP 1 . 
   As noted above, conventional optical amplifiers operate in a simple constant gain mode. In this case, the control circuitry keeps the gain of the amplifier at target value G CG . A control mechanism for the conventional amplifier in constant gain mode is shown in the flow chart  200  of  FIG. 2 . In  FIG. 2 , P in  is the input power to an amplifier, and P out  is the output power from the amplifier. It is assumed that the power values are in a logarithmic scale for ease of calculation. 
   In step  210 , the target gain G CG  for the amplifier is set. This value is provisioned in the constant gain mode and typically corresponds to the gain of a particular stage or multiple stages of the chain of amplifiers. In step  220 , input and output powers P in  and P out  are measured and the actual gain G meas  is calculated from the measured power values. Step  230  determines whether or not a deviation G error  from the target gain G CG  exists. If so, the amplifier&#39;s output P out  is adjusted by G error  in step  240  and the method returns to step  220 . If the deviation is determined not to exist in step  230 , the output is not adjusted and the method returns to step  220 . 
     FIG. 3  graphically illustrates the tracking of the output power of the amplifier as a function of the input power. The amplifier is assumed to be operating at a target set gain of 25 dB. As shown, the output power of the amplifier tracks the input as closely as possible. 
   It can be seen that substantial output power variations occur if the constant gain control is utilized. Such power fluctuations accumulate over a chain of amplifiers to a detrimental effect as pointed out above. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Features of the present invention will become more fully understood to those skilled in the art from the detailed description given hereinbelow with reference to the drawings, which are given by way of illustrations only and thus are not limitative of the invention, wherein: 
       FIG. 1  is a block diagram illustrating a chain of conventional constant gain mode optical amplifiers; 
       FIG. 2  is a flow chart illustrating a control mechanism of a conventional constant gain optical amplifier; 
       FIG. 3  is a graphical illustration of tracking of input to output of a conventional constant gain optical amplifier; 
       FIG. 4  is a block diagram illustrating an exemplary control mechanism of an optical amplifying device according to an embodiment of the present invention; 
       FIG. 5  is a block diagram of an optical amplifying device according to an embodiment of the present invention; 
       FIG. 6  is a flow chart illustrating an exemplary control mechanism of an optical amplifying device according to an embodiment of the present invention; 
       FIG. 7  is an exemplary graphical illustration of power output of an optical amplifying device according to an embodiment of the present invention; and 
       FIG. 8  is a block diagram of an EDFA optical amplifying device according to an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
   For simplicity and illustrative purposes, the principles of the present invention are described by referring mainly to exemplary embodiments thereof. The same reference numbers and symbols in different drawings identify the same or similar elements. Also, the following detailed description does not limit the invention. The scope of the invention is defined by the claims and equivalents thereof. 
   As noted previously, a simple constant gain mode amplifier amplifies not only the desired input signal, but also amplifies the accumulated imperfections of the optical system. 
   Such imperfections in the input signal generally occur slowly over time and typically are low in magnitude (also known as signal drift or simply “drift”). For example, variations in span losses and variation in laser powers used for inputting signals to the amplifiers can occur due to polarization dependence of optical components. Also, mechanical and/or thermal effects can cause time dependent loss variations in optical fibers. Another source of noise that can couple into the system include any noise that can couple into the amplifier pump electronics and consequently into the amplifier output. 
   While all of these variations are usually fairly small, it is desirable that these variations are squelched at each amplifier and not allowed to accumulate along the system. For example, in order to minimize any noise coupling into the pump, it is desirable to operate the pumps using a constant current drive for the pumps. 
     FIG. 4  is a high level block diagram  400  of a control mechanism of an exemplary optical amplifying device of the present invention. As shown, the optical amplifying device operates in one of two modes—constant power mode  410  (gain threshold mode) or constant gain mode  420  (gain control mode). The explanation is as follows. 
   As noted above, variations in the power of the input to the amplifying device that is relatively small in magnitude are more likely due to the imperfections in the system rather than variations in the actual input signal. In this type of a situation, it is desired that the amplifying device be suppressed from reacting to the input variations. By operating the optical amplifying device in the constant power mode  410 , the suppression may be achieved. In the above constant power mode can mean both constant output power or constant pump power (by delivering constant pump current to pumps). For example, natural erbium doped fiber amplifier (EDFA) behavior may be utilized to suppress any low frequency, low magnitude changes to the input. 
   Conversely, large variations in power are more likely due to the changes in the actual input signal itself. For example, adding or dropping channels of a wavelength division multiplexed (WDM) signal will typically cause a large power variation. Switching operations may cause large power variation as well. Under this type of a circumstance, it is desired that the amplifying device track the input. By operating the optical amplifying device in the constant gain mode  420 , tracking may be achieved. 
   The optical amplifying device may switch from one mode to another appropriately through the use of a gain threshold. For example, assume that the optical amplifying device is currently operating in the constant power mode  410 . An actual gain G MEAS =P OUT −P IN  may be determined from the output and input powers of the amplifying device. Then, the gain error G ERROR =G CG −G MEAS  (where G CG  is the target gain) may be determined. If the absolute value of G ERROR  exceeds a predetermined threshold value G TH , i.e. if |G ERROR |−G TH &gt;0, then the amplifying device may switch to the constant gain mode  420  (arrow  430  in  FIG. 4 ). Otherwise, the amplifying device may remain in the constant power mode  410  (arrow  440 ). The constant power mode  410  may also be described as constant power with gain threshold mode or simply “gain threshold mode.” 
   In an embodiment of the present invention, the optical amplifying device operates in constant gain mode  420  for a brief lock out period (10 seconds for example) and returns to back to gain threshold (CP) mode  410 . It should be noted that the lock out period may be set at any period appropriate for the situation. 
   If no transient events occur during the lock out period, then the optical amplifying device may be switched to operate in the gain threshold mode (i.e. gain threshold can be re-enabled)  410  (arrow  450 ). In an embodiment, the transient event may be defined as the output power of the amplifying device deviating from a reference power level by more than a preset amount within prescribed time, such as the lock out period. In another embodiment, the transient event may be defined as the level of the output power fluctuating by more than a preset amount within the prescribed time. In general, the transient event is an indication that the output of the amplifying device is in a state of flux and the optical system has not reached a desired stability. 
   If one or more transient events do occur, which indicates that the optical system has not stabilized, then the lock out period may be reset and the optical amplifying device may remain in the constant gain mode  420  (arrow  460 ). 
   Other criteria can also be used to return the optical amplifying device to the gain threshold mode. For example, instead of detecting transient events as described above, an alternative may be simply set the lock out at a “long enough” period and return to the gain threshold mode after the lock out period without regard to transient events occurring. However, it is preferred that the optical amplifying device be returned to the gain threshold mode after determining that the optical system has stabilized. 
     FIG. 5  is a block diagram of an optical amplifying apparatus  500  according to an embodiment of the present invention. The apparatus includes an optical amplifying device  510 , a controlling device  520 , and a measuring device  530 . The optical amplifying device  510  receives input signal and outputs an output signal. The measuring device  530  is configured to measure power levels on a plurality of points within the optical amplifying device  510  including input and output power levels P IN  and P OUT . Both the optical amplifying device  510  and the measuring device  530  communicate with the controlling device  520 . 
   The controlling device  520  controls the operation of the optical amplifying device  510  based on the power levels measured by the measuring device  530 . The controlling device  520  controls the optical amplifying device  510  to operate either in the gain threshold mode ( 410  of  FIG. 4 ) or the constant gain mode ( 420  of  FIG. 4 ). 
   As an example, in the gain threshold mode, the controlling device  520  calculates the actual gain of the optical amplifying device  510  based on the input and output power levels P IN  and P OUT —more specifically, the gain may be determined as P OUT −P IN  (logarithmic scale). If the absolute value of the gain error does not exceed a preset threshold, then the controlling device  520  adjusts the gain of the optical amplifying device  510  so that output power level P OUT  is substantially equal to a preset level. 
   In the constant gain mode, the controlling device  520  determines whether or not a transient event occurred within a lock out period. If so, the controlling device  520  continues to operate the optical amplifying device  510  in the constant gain mode and resets the lock out period. 
   The controlling device  520  also switches the mode as necessary. As an example, the controlling device  520  switches the operation of the optical amplifying device  510  from the gain threshold mode to the constant gain mode when the absolute value of the gain error of the optical amplifying device  510  exceeds the preset gain threshold. The controlling device  520  switches the operation from the constant gain mode to the gain threshold mode if it determines that no transient event occurs within the lock out period. 
     FIG. 6  is a flow chart illustrating an exemplary control mechanism of an optical amplifying device according to an embodiment of the present invention. In step  610 , target gain G CG  and threshold gain G TH  are set. In step  620 , the optical amplifying device enters the gain threshold mode. In this step, input and output powers P IN  and P OUT  are measured and the gain G MEAS =P OUT −P IN  is determined. In step  630 , the gain error G ERROR =G CG −G MEAS  is determined and compared against the threshold gain G TH . 
   If the absolute value of the gain error G ERROR  exceeds the threshold gain G TH , then the constant gain mode is entered (see step  640 ). In step  640 , the input power P IN  the optical amplifying device is appropriately amplified by G CG  and output as P OUT . The optical amplifying device remains in the constant gain mode as long as one or more transient events occur within a preset lock out period as described above (see step  650 ). If no transient event occurs within the lock out period, then the optical amplifying device enters the gain threshold mode (see step  620 ). 
   If the absolute value of the gain error G ERROR  is within the threshold gain G TH  in step  630 , then the optical amplifying device remains in the gain threshold mode. 
     FIG. 7  is an exemplary graphical illustration  700  of power output of an optical amplifying device utilizing the gain threshold described above. In this non-limiting example, the target gain G CG  for the amplifying device is set at 25 dB and the gain threshold G TH  is set at 0.2 dB. It should be noted that the target gain G CG  and the gain threshold G TH  are not limited to the above values and may be set to any values as deemed appropriate. 
   In the gain threshold mode, for example between time t 0  and t 1 , the gain of the optical amplifying device varies according to the input power so that the output power P OUT  is substantially constant. 
   When the absolute value of the gain error G ERROR  exceeds the threshold G TH , i.e. when |G ERROR |−G TH &gt;0 becomes true, the optical amplifying device enters the constant gain mode, for example between times t 1  and t 2  and between times t 3  and t 4 . During these times, the output power P OUT  is appropriately amplified as shown. 
   Assuming no transient events occur, the optical amplifying device reenters gain threshold mode, for example between times t 2  and t 3  and beyond time t 4 . 
   As shown, during gain threshold mode, the gain is adjusted to provide the output power P OUT  at a substantially constant level. During the constant gain mode, the gain is kept substantially constant so that output power P OUT  tracks the input power P IN . 
   An optical amplifying devices may include a service channel, one or more variable optical attenuators (VOA), signal monitors, optical monitors, input isolator, output isolator, output back reflection monitor, gain flattening filter, pumps, on-board electrical control circuits including communication port, etc.  FIG. 8  is a block diagram of a three stage optical amplifying device  800  according to an embodiment of the present invention. For example, the amplifying device  800  may include EDFA amplifiers. Of course, it should be noted that the EDFA is but one of several amplifiers made of optically active materials that may be used. In addition, erbium doped waveguide amplifier (EDWA) may be used as well. 
   The device  800  includes various power monitoring points as shown that are monitored by the measuring device (see  FIG. 5 ). The device  800  may include first, second, and third optical amplifier stages  810 - 1 ,  810 - 2 , and  810 - 3  connected in series fashion. The device  800  may also include a VOA  820  and connected as shown. The VOA  820  may be manipulated to control the gain of the device  800 . While not shown, it should be noted that other VOAs may be added to finely tune the operation of the device  800 . 
   The device  800  may also include an optional dispersion compensation fiber (DCF)  830 . The DCF is desirable to provide periodic dispersion compensation for high frequency signals, typically 10 Gb/s or higher. 
   In general, an optical amplifying device may include a plurality of amplifier stages connected in series, one or more VOAs connected in series with the plurality of optical amplifier stages such that each VOA receives an output of one optical amplifier and outputs to a next optical amplifier. At least least one VOA is controlled by the controlling device. In addition, the optical amplifying device may include one or more DCFs, with each DCF receiving an output of an amplifier and outputting to a next amplifier. 
   A simpler implementation of an optical amplifying device would include one or more amplifier stages and a VOA. The gain of the amplifier may be controlled by adjusting the VOA. Alternatives include adjusting power supplied by the pump(s) and by controlling other VOA(s) of the amplifier. 
   While the invention has been described with reference to the exemplary embodiments thereof, it is to be understood that various modifications may be made to the described embodiments without departing from the spirit and scope of the invention thereof. The descriptions used herein are set forth by way of illustration only and are not intended as limitations.