Abstract:
Circuits and systems for use in monitoring and adjusting an optical signal strength of an optical data transmission system. A closed feedback loop which automatically controls an optical device&#39;s output uses an analog operational amplifier based circuit instead of a conventional microprocessor that uses complex calculations. The parameters of the operational amplifier circuit are monitored and controlled by a microcontroller to provide flexible operational settings. The analog circuit containing discrete analog components is used as a power meter for measuring an optical power signal strength of an optical signal. The optical signal is tapped by an optical coupler and the signal is received by a photodetector. The output of the photodetector is received by the analog circuit which produces an intermediate signal based on the level of optical power in the optical signal. The intermediate signal is used to control an optical device. Since the operating parameters of some of the analog components in the analog circuit may be controlled by the microcontroller, the analog circuit may therefore be calibrated or adjusted by the microcontroller. This allows one to take advantage of the speed of analog component based circuits while keeping the flexibility and communications capability of a digital circuit such as a microcontroller.

Description:
FIELD OF THE INVENTION 
   The present invention relates to systems for managing optical power in an optical data transmission system. More specifically, the present invention is particularly applicable for controlling a constant gain optical power management system. 
   BACKGROUND TO THE INVENTION 
   Optical communications is fast becoming the telecommunications industry&#39;s standard in terms of reliability and transmission capacity. To this end, optical communications equipment are continually being improved and updated to provide faster, cheaper, and more feature laden alternatives. To provide acceptable performance in optical communications networks, optical amplifiers and optical attenuators amplify or attenuate the optical signal as needed. However, while these amplifiers and attenuators generally provide the required change in optical power to the optical signal, it is standard practice in industry to use automatic output control devices to monitor the optical signal. 
   Such automatic output control devices, and the systems used in such devices, use a feedback loop to monitor the optical signal strength. If the optical signal strength falls below a certain threshold, a pump laser is activated and enough optical power is inserted into the optical signal to boost its optical power sufficiently to meet predetermined standards. These systems normally use expensive high sped microcontrollers and high speed components to provide fast response times. This approach is seen in the optical amplifier system as disclosed by Yang in U.S. Pat. No. 6,198,571. 
   In such an approach, a high speed microcontroller is required to perform the multiple calculations and decisions required to provide fast response times. Unfortunately, such an approach is not only complicated but is also quite expensive. The use of A/D (analog-digital) converters which convert the analog optical signal into a digital signal which can be read and used by the microcontroller introduces delays into the response times of the system. Also, such high speed microcontrollers can be quite expensive. Furthermore, the complexity of the software required for these microcontrollers increases the price for the system. 
   High speed microcontrollers are usually required as Erbium doped fiber amplifiers (EDFAs) may exhibit a transient problem when channels at the amplifier input suddenly increase in either power or number. The gain control for the EFDA must be very fast in order to compensate for the sudden change in input. The digital microcontroller must react within microseconds to be able to provide a stable gain to the other channels in the optical signal. The microcontroller must therefore execute a few iterations to not only detect but compensate for the sudden change. If the reaction time is not fast enough, large overshoots (over-compensation) or undershoots (under-compensation) will occur at the output of the amplifier. 
   To compensate for such problems, two major approaches are generally followed, both of which use digital circuits. The first is generally known as a feed forward compensation or a digital open loop. In this approach, a step in the gain control circuit is applied in response to a step change in the input power. Essentially, this approach measures the change in input and then calculates the amount by which the gain or output should be adjusted. The pump laser is then adjusted and the output is checked. If the desired level is not achieved, the steps in the loop are repeated. Such an approach, aside from being expensive due to the need for a high speed microcontroller, suffers from the problem of extraneous factors which may affect the performance of the system. Age, temperature effects, noise, and many other factors can degrade the performance of the digital open loop system. 
   A second approach, called a digital closed loop, compares the desired signal gain with the effective gain during the signal transient. Any difference or error between the two is used in a feedback loop to adjust the setting on the pump laser. 
   While digital control loops can be fast in terms of response times, they generally require expensive and complex digital components such as dedicated DSPs (digital signal processors) and high speed A/D and D/A converters. 
   Based on the above, a new approach is therefore needed that will not only provide the required fast response time but will simultaneously provide a solution that is inexpensive. Ideally, such a solution should also provide the flexibility of digital circuits while also providing the required fast response times. It is therefore an object of the present invention to provide alternatives which overcome or at least mitigate the drawbacks of the prior art. 
   SUMMARY OF THE INVENTION 
   The present invention provides circuits and systems for use in monitoring and adjusting an optical signal strength of an optical data transmission system. A closed feedback loop which automatically controls an optical device&#39;s output uses an analog operational amplifier based circuit instead of a conventional microprocessor that uses complex calculations. The parameters of the operational amplifier circuit are monitored and controlled by a microcontroller to provide flexible operational settings. The analog circuit containing discrete analog components is used as a power meter for measuring an optical power signal strength of an optical signal. The optical signal is tapped by an optical coupler and the signal is received by a photodetector. The output of the photodetector is received by the analog circuit which produces an intermediate signal based on the level of optical power in the optical signal. The intermediate signal is used to control an optical device. Since the operating parameters of some of the analog components in the analog circuit may be controlled by the microcontroller, the analog circuit may therefore be calibrated or adjusted by the microcontroller. This allows one to take advantage of the speed of analog component based circuits while keeping the flexibility and communications capability of a digital circuit such as a microcontroller. 
   In a first aspect, the present invention provides a system for managing optical power in an optical data transmission system, the system comprising:
         at least one analog feedback circuit for receiving an input from an optical means;   an optical device for controlling optical power characteristics of an optical signal in said optical data transmission system;   digital circuit means for controlling operational parameters of said at least one analog feedback circuit; and   an analog control circuit for controlling said optical device based on at least one intermediate signal from said at least one analog feedback circuit,
 
wherein
   an output of said optical device is controlled by said analog feedback circuit, said optical means is an optical means for directly receiving an optical input from an optical coupler coupled to said optical data transmission system.       

   In a second aspect, the present invention provides a method of managing optical power in an optical data transmission system, the method comprising:
         a) diverting an input portion of an input optical signal of said system;   b) diverting an output portion of an output optical signal of said system;   c) amplifying said input portion by a first predetermined amount to arrive at a first intermediate signal;   d) amplifying said input portion by a second predetermined amount to arrive at a second intermediate signal;   e) taking a difference between said first intermediate signal and said second intermediate signal;   f) using said difference to control an optical device to compensate for said difference,   wherein said first and second predetermined amounts are set by a microcontroller and steps c) and d) are performed by separate analog circuits.       

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the invention will be obtained by considering the detailed description below, with reference to the following drawings in which: 
       FIG. 1  is a block diagram of an optical power management system according to one aspect of the invention; 
       FIG. 2  is a circuit diagram of a combined analog power detector/amplifier subcircuit which may be used in an analog circuit in  FIG. 1 ; 
       FIG. 3  is a circuit diagram of a subcircuit which may be used as replacement for a section of the subcircuit of  FIG. 2 ; 
       FIG. 4  is a circuit diagram of a subcircuit which may be used in the control circuit as illustrated in  FIG. 1 ; 
       FIG. 5  is a circuit diagram of a second amplifier subcircuit which may be used in the control circuit in  FIG. 1 ; 
       FIG. 6  is a circuit diagram of a modified version of the second amplifier subcircuit of  FIG. 5 ; 
       FIG. 7  is a circuit diagram of a subsystem that uses the output of the circuit of  FIG. 4  to provide constant gain; and 
       FIG. 8  is a circuit diagram of a configuration of the circuit of  FIG. 4  with a controllable offset for ASE compensation. 
   

   DETAILED DESCRIPTION 
   Referring to  FIG. 1 , a feedback control mechanism for an optical data transmission system is illustrated. An optical fiber  10  carries an optical signal. Optical couplers  20 ,  30  each tap into the fiber  10  to divert a portion of the optical signal for the feedback mechanism. The portion of the optical signal is received by a photodiode  40 A,  40 B. The output of the photodiode is then received by an analog circuit  50 A,  50 B. Also illustrated in  FIG. 1  is a microcontroller  80  which controls the operating parameters of the analog circuits  50 A,  50 B. The microcontroller  80  is not programmed to assess the optical signal strength as in the digital control loops mentioned above. Instead, the microcontroller  80  merely adjusts the parameters of some of the components in the analog circuits  50 A,  50 B and, possibly, a mode of the control circuit  60 . Because the microcontroller  80  merely adjusts or perhaps calibrates the analog circuits  50 A,  50 B, a high speed microcontroller is not required. Instead, a low cost microcontroller will suffice. 
   In terms of function, the analog circuits  50 A,  50 B receive the output of the photodiodes  40 A,  40 B. Each of the analog circuits  50 A,  50 B then processes their respective input signals from their corresponding photodiodes  40 A,  40 B to arrive at an intermediate signal. These intermediate signals relate to the optical power levels received by the photodiodes  40 A,  40 B and are, in one embodiment, produced by amplifying these received power levels. The intermediate signals produced by the analog circuits  50 A,  50 B are then received by the control circuit  60 . Based on these intermediate signals, the control circuit  60  controls the output of the optical device  70 . The optical device  70  may be a device that controls the optical gain or power for the optical signal travelling through the optical fiber  10 . As such, the optical device  70  may be a pump laser for controlling the amplification of the optical signal, an optical attenuator for controllably attenuating the optical signal, or any other optical device capable of variably affecting the gain, power, or signal strength of an optical signal. 
   For clarity, it should be noted that photodiodes  40 A,  40 B receive input and output optical power respectively. Photodiode  40 A receives optical power from the input optical signal before any adjustments are made (if any) to this optical signal by the optical device  70 . Photodiode  40 B receives optical power from an output signal that results after adjustments (if any) are made to the input optical signal by the optical device  70 . 
   As noted above, the control circuit  60  controls the optical device  70  based on the intermediate signals produced by the analog modules  50 A,  50 B. Several modes of operation are possible for the control circuit  60 , each mode being for a different operating profile. As previously mentioned, the microcontroller  80  can control the mode of the control circuit  60 . One specific mode of operation is designed to maintain a constant gain between the input and the output power levels detected by the photodiodes  40 A,  40 B. In this mode, a desired ratio between the intermediate signals from analog circuits  50 A,  50 B is set and any difference between the intermediate signals is amplified. Once amplified, any difference is used to correct for such anomalies as they occur. 
   Regarding the analog circuits  50 A,  50 B,  FIG. 3  illustrates a block diagram of one possible configuration for its internal components. As can be seen, the photodiode  70  is included in the circuit as being coupled between ground  110  and a resistor  120  (with resistor value Rf). Also coupled to junction  130  along with the photodiode  70  and one end of resistor  120  is one input to an operational amplifier  130 . Another input to the operational amplifier  130  is coupled at junction  140  to one end of a digital potentiometer  150 . The other end of potentiometer  150  is coupled to ground  110  while the setting of the potentiometer  150  is coupled to the other end of resistor  120 . 
   The circuit in  FIG. 2  is linear yet has a controllable gain that does not sacrifice bandwidth in the same manner as a logarithmic amplifier does. For low level inputs, the digital potentiometer  150  (or a DAC) is set low. This setting keeps the output from being in the millivolt range. Only the operational amplifier  130  requires a very low offset voltage and not the whole feedback mechanism/system as a whole. As can be seen from the Figure, the input current is I=kpd Pin while the output is given as Output=(Gain)Pin where Gain=kpd RfG. 
   In the configuration of  FIG. 2 , optical power is detected and then amplified by an amount controlled by the setting of the digital potentiometer or DAC  150 . This configuration allows the implementation of constant gain mode without the use of logarithmic amplifiers or analog divider circuits that are inconsistent in accuracy and speed. Furthermore, having a controllable gain setting in the first stage of the overall system minimizes errors of later circuits. It should be clear that the setting for the digital potentiometer  150  may be controlled by the microcontroller  80 . 
   As a refinement to the circuit in  FIG. 2 ,  FIG. 3  illustrates a dual digital potentiometer configuration which may be used in place of the potentiometer  150  in the circuit of FIG.  2 . As can be seen in  FIG. 3 , the two potentiometers  160 ,  170  are cascaded. Both potentiometers are coupled to ground  110  and the output of potentiometer  170  controls the setting of the potentiometers  160 . The setting of potentiometer  170  is coupled to the other end of the resistor  120 . 
   While the circuits in  FIGS. 2 and 3  may be used for the analog circuits  50 A,  50 B, it is noted above that one of the possible modes of control circuit  60  provides constant gain. As such,  FIG. 4  illustrates a circuit which may be used for the control circuit  60 . In this circuit, a difference between the intermediate signals from the analog circuits  50 A,  50 B is taken and amplified. By judiciously selecting the gain settings for each of analog circuits  50 A,  50 B, the intermediate signals they produce could be made equal to one another. As mentioned above, constant gain requires a fixed ratio between input and output power of the optical system. If this fixed ratio is set and the gain settings for analog circuits  50 A,  50 B are set so that their intermediate signals signify a deviation from the desired fixed ratio, such a difference can then be used to compensate for the deviation that it signifies. 
   As can be seen from  FIG. 4 , the input from the analog circuit  50 A is received through resistor  180  which is coupled to junction  190 . Also coupled to junction  190  is one input to operational amplifier  200  and one end of resistor  210 . The other end of resistor  210  is coupled to the output of the operational amplifier  200 . The input from the analog circuit  50 B is received through resistor  220  which is coupled to junction  230 . Also coupled to junction  230  is the other input to operational amplifier  200  along with one end of resistor  240 . The other end of resistor  240  is coupled to ground  110 . 
   The constant gain control scheme explained above has the advantage of not requiring analog divider circuits. Such divider circuits are, as noted above, inconsistent in terms of accuracy and speed. Division is not required by the above scheme. All that is required is taking the difference between the voltages (intermediate signals) from the analog circuits  50   a,    50 B. 
   However, the above scheme does have a drawback. As signal levels drop (as optical signal levels get reduced), the loop bandwidth will also diminish. This is undesirable in that there is an optimum bandwidth for system performance and this optimum bandwidth should not significantly vary with signal level. To compensate for this drawback, an extra amplifier subcircuit may be coupled to the output of the operational amplifier  200  of FIG.  4 . 
   Referring to  FIG. 5 , a circuit diagram of a second amplifier circuit which may be used with the circuit of  FIG. 4  is illustrated. This second amplifier subcircuit may be located in the control circuit module  60  along with the circuit illustrated in FIG.  4 . Input from the circuit of  FIG. 4  is received at one end of a variable gain resistor  250 . The other end of resistor  250  is coupled to junction  260 . Also coupled to junction  260  is one input to operational amplifier  270  while the other input of operational amplifier  270  is coupled to ground  110 . Also coupled to junction  260  is, in series, resistor  280  and capacitor  290 . The output of the operational amplifier  270  is coupled to the other end of capacitor  290 . The variable gain resistor  250  may be implemented using a field effect transistor (FET) channel. The setting of the resistor  250  is adjusted to keep the bandwidth approximately constant. 
   Referring to  FIG. 6 , a refinement of the circuit in  FIG. 5  is illustrated. The circuit in  FIG. 6  is similar to that in  FIG. 5  except for a circuit arm  300  coupled to a junction  310  between resistor  280  and capacitor  290 . The circuit arm  300  has resistor  320  coupled to ground  110  at one end and, at the other end, to a pair of diodes  330 A,  330 B coupled in parallel to one another. The diodes  330 A,  330 B are coupled in parallel with one diode  330 A allowing the current to pass in one direction while the other diode  330 B allows current to pass in the other direction. As noted above, the parallel diode arrangement is coupled at one end to resistor  320  and coupled to junction  310  at the other end. 
   For the circuit in  FIG. 6 , the RC time constant is reduced, and hence the bandwidth is increased at high error output levels. At such levels, the diodes  330 A,  330 B turn on and shunt resistor  320  in the path. As such, the circuit allows the feedback system to respond quickly when significant errors are present but also allows the system to have a lower bandwidth and, consequently, lower noise when errors are small. 
   As a further refinement to the circuits of  FIGS. 5 and 6 ,  FIG. 7  illustrates an arrangement that achieves an approximately constant gain using an output of the operational amplifier  200  from FIG.  4 . As can be seen, although box  340  is virtually a copy of the circuit in  FIG. 5 , the circuit in  FIG. 6  can be inserted in the same spot. The main difference between the circuit in FIG.  5  and the circuit in box  340  is that the circuit in box  340  uses a field effect transistor (FET)  350  to implement the variable resistor  250  of FIG.  5 . The first input signal  360  in  FIG. 7  is from the operational amplifier  200  of  FIG. 4  while the second input signal  370  is the intermediate signal from the analog circuit  50 A which detects the optical input power to the system. The gates of FETs  350 ,  380  are both coupled to the output of operational amplifier  390  and, for both FETs  350 ,  380 , one lead is coupled to an input while the other is coupled to an input of an operational amplifier. For FET  350 , one lead is coupled to junction  260  (see FIG.  5 ). For FET  380 , one lead is coupled to the second input signal  370  while the other lead is coupled to junction  390 . Junction  390  is also coupled to a current source  400  which is, in turn, coupled to ground  110 . Junction  400  is also coupled to one input of operational amplifier  390 . The other input of operational amplifier  390  is coupled to ground  110 . As noted above, the output of operational amplifier  390  is coupled to the gates of FETs  350 ,  380   
   It should be noted that the circuit in  FIG. 7  has a few interesting characteristics. The voltage of the intermediate signal from analog circuit  50 A is proportional to the input optical power of the system. Also, the resistances represented by the FETs  350 ,  380  are each proportional to the input power as well. The method represented by the circuit in  FIG. 7  is that two matched FETs are used (in this case FETs  350 ,  380 ). Thus, one FET in a feed-back loop using the output of analog circuit  50 A results in the correct gate control voltage and compensates for any input power fluctuations. 
   It should be further noted that the design discussed above allows for easy ASE (amplified spontaneous emission) compensation. An offset to the operational amplifier  200  of  FIG. 4  can be provided to compensate for ASE. This is illustrated in FIG.  8 . As can be seen in  FIG. 8 , the circuit is the same as in  FIG. 4  except for the addition of a DAC  410 . The DAC  410  (Digital-Analog Converter) receives digital input from the microcontroller  80  and converts this into an analog signal. The output of the DAC  410  is coupled to junction  190  by way of resistor  420 . This DAC  410  output serves as an offset for the operational amplifier  200  and need only be changed when the gain settings for the system is changed. Clearly, changing the DAC setting is effected by changing the digital value of the digital input from the microcontroller  80 . Such an offset compensation is simple yet accurate due to the linear amplifier scheme employed. 
   While the analog circuit illustrated in  FIG. 1  does not include the photodiode  40 A, other optical means for receiving the optical signal from the coupler may be used and may be included in the analog circuit. 
   It should be noted that, while the microcontroller may not have a direct role in the power management role of the system, it can play a useful role. The microcontroller provides an interface by which the power management system acquires flexibility. Parameters in the system, such as the operating mode of the control circuit (e.g. constant gain, constant power), operational values of both the control circuit and of the analog circuits (e.g. gain, power), ASE compensation offset value, correction for optical or analog errors, may all be implemented by way of the microcontroller. As such, the microcontroller can be programmed to change values, settings, and other parameters in response to changing conditions or desired results. Clearly, other digital circuit means than a microcontroller may be used. A direct connection to a personal computer, programmable digital combinational circuits, and other digital circuit means may be used. 
   It should further be noted that the combination of the analog components in the analog circuits  50 A,  50 B and in the control module  60  with the digital circuit means, as embodied in the microcontroller, provides advantages unique to each one. The analog circuits provide very fast response times to changing conditions while the microcontroller provides an added dimension of flexibility and communication between the user and the power management system. The digital circuit means therefore provide the programmability and controllability of the system while the analog components provide the fast response times. 
   The above system may be used in any optical power management system as a means of controlling the optical power in an optical data transmission system. As such, optical amplifier systems, variable optical attenuator systems, and other systems which affect the optical power of a data transmission system are eminently suited to use the above system. It has been found that a variable optical attenuator using the above system has fast response times and flexibility while keeping overall costs down. 
   A person understanding this invention may now conceive of alternative structures and embodiments or variations of the above all of which are intended to fall within the scope of the invention as defined in the claims that follow.