Abstract:
Techniques to control an optical receiver having a control loop using Bit Error Rate (BER). In one implementation, a bit error rate (BER) associated with a received optical signal is determined. Indication of the BER to a control loop adapted is provided to adjust the optical signal in a manner tending to reduce the BER. The received optical signal is adapted in a manner tending to increase the BER such that the control loop operates within an active control region.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention is related to the field of optical telecommunications, and in particular, to method for optimizing optical receiver control loops.  
       BACKGROUND OF THE INVENTION  
       [0002]     It is well known that signals transported over an optical network suffer degradation between associated transmitters and receivers. There are many possible causes for the transmitted optical signals to degrade; among them are polarization mode dispersion (PMD), chromatic dispersion (CD), etc. Some of those effects might change the signal characteristic over time, some effects are temperature dependant. No matter what caused the degradation, the more the received signal is distorted, the more errors will be made at detection up to the point where the transmission becomes ineffective.  
         [0003]     There are many ways to compensate for errors at the receiver. For example, forward error correction (FEC) is commonly used in optical transmission networks to correct errors of the received signals. In FEC, the transmitted signals include redundant information used for reconstruction of the transmitted sequence (error correction). Another example is an adaptive receiver that allows for the receiver to adjust and/or modify optical and/or electrical components thereby reducing the amount of errors received at the receiver. A combination of the two examples is also possible.  
         [0004]     The adaptive receiver functions well in static optical networks where signals travel substantially the same path. Slow changes in the input signal characteristic at the receiver might be compensated by adjustments done by the adaptive receiver. The input signal characteristics can be compensated as long as the time constants of the adaptive receiver are faster than the signal changes. In particular, if some of the adjustments of the adaptive receiver are based upon the number of errors computed from the overhead information, then the time constants of the adaptive receiver are dependant on the signal quality. In that case, some of the adjustments might start to drift from the optimum for high quality input signals. This drift might be caused either by some offset in the control loop and/or by slowly changing characteristics of the input signal to the optical receiver. This will be termed ‘the receiver is outside the active control region’ henceforth.  
         [0005]     However, in an optically switched WDM network where the optical signals are constantly being switched onto different paths, the signal characteristic could change abruptly at the receiver. The latter is not limited to the switched signals but also applies to neighbor channels of the WDM link. The sudden change of signal characteristic will most probably result in drastic increase of errors at the receiver if the adaptive receiver is not at the optimum setting. As a result, the FEC might not be able to compensate for the errors at the receiver. If the settings of the adaptive receiver have drifted from the optimum settings, transmission faults will likely occur.  
         [0006]     An object of the present invention is, therefore, to enable the adaptive receiver to find and track the optimum setting thus enhancing the robustness against abrupt signal degradation.  
       SUMMARY  
       [0007]     Various deficiencies of the prior art are addressed by the present invention of methods for an optical receiver having a control loop using Bit Error Rate (BER) as a feedback. In one embodiment, the invention provides for a method of determining a bit error rate (BER) associated with a received optical signal and providing indication of said BER to a control loop adapted to adjust the optical signal in a manner tending to reduce the BER. In accordance with the method, the received optical signal is adapted in a manner tending to increase the BER such that the control loop operates within an active control region.  
         [0008]     Another embodiment of the invention provides a method for using an alternate feedback signal for an adaptive optical receiver normally based upon the bit error rate (BER). The feedback signal will be called ‘vertical eye opening’ and is based upon a combination of evaluating the BER feedback and actively controlling the optical receiver as described in the first embodiment.  
         [0009]     In another embodiment of the invention, a data transmission system wherein data transmitted to a receiver via a network is adapted via a transmitter control mechanism in response to a bit error rate (BER) signal provided by the receiver. The system adjusts, at said transmitter and/or receiver, at least one parameter of a received data signal to cause the BER associated with the received data signal to be within a range of BER values. In accordance with the invention, the range of BER values corresponds to an active control region.  
         [0010]     The invention further provides other methods and system elements that implement various aspects, embodiments, and features of the invention, as described in further detail below.  
         [0011]     The foregoing, together with other aspects of this invention, will become more apparent when referring to the following specification, claims, and accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]     The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:  
         [0013]      FIG. 1  depicts a block diagram of an optical transmission system;  
         [0014]      FIG. 2  depicts a block diagram of an optical receiver system according to an embodiment of the invention;  
         [0015]      FIG. 3  depicts a flow diagram of a method according to an embodiment of the invention; and  
         [0016]      FIG. 4  depicts a graphical representation useful in understanding the present invention. 
     
    
       [0017]     To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0018]     The invention will be primarily described within the context of an optical receiver in an optical switched 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 use control loops in a communications network.  
         [0019]      FIG. 1  depicts a block diagram of an optical transmission system. The optical communications network  100  of  FIG. 1  includes a transmitter with forward error correction (FEC) encoding  110 , an optical transmission link  120 , and a receiver system  130 . The receiver system includes a receiver with FEC decoding  132  and a controller  134 .  
         [0020]     The transmitter  110  may comprise a conventional device, group of devices or any system configured to receive an input data signal D in  and provide a corresponding modulated data signal. D in  may comprise an electrical input signal or an optical input signal. The output will be an optical signal which could be a wavelength division multiplexed (WDM) signal or other types of optical signals. The transmitter  110  also accepts a feedback control which allows the characteristics of the outputted optical signals, such as the output power, to be selectively varied.  
         [0021]     The optical link  120  may include conventional optical fiber or any part of an optical fiber network that transports optical signals. An example of the link could include optical fibers or the like. The optical link  120  could comprise optical components such as optical amplifiers (OA) or polarization mode dispersion (PMD) compensators (not shown). The optical link  120  is the conduit for the optical signals traveling from the transmitter  110  to the receiver  130 . This link also may accept feedback control signals from the receiver  130  which allows for the modification and adjustment of the characteristics of the optical signals being transmitted such as e.g. output power of the OA.  
         [0022]     The receiver system  130  includes a receiver with FEC  132  and a controller  134 . The receiver with FEC  132  may include any device, group of devices or system configured to receive and correct the errors of the received optical signals. Error correction will be performed on the received signal and the receiver  132  will output the inputted signals as electrical signals D out . The receiver  132  will monitor the number of received errors. The ratio of the number of error bits to total number of bits received is known as the bit error rate (BER). The BER is transmitted from the receiver  132  to the controller  134 . The BER is used by the controller  134  to determine if adjustments need to be made to certain components of the optical network. Depending on the received BER, the controller  134  will transmit a feedback signal to the transmitter  110 , the link  120 , or the receiver system  130  in order to increase the system margin against degradation in the received optical input signal.  
         [0023]      FIG. 2  depicts a block diagram of an optical receiver system  130  according to an embodiment of the invention. The optical receiver system  130  may include the following components: an optical amplifier  210 , a tunable dispersion compensator  220 , an optical to electrical (O/E) converter  230 , a clock and data recovery (CDR) circuit  240 , a data processing circuit  250  and a controller  134 .  
         [0024]     The optical receiver system  130  accepts as input the received optical signal which is received by the optical amplifier (OA)  210  which could be an erbium-doped fiber amplifier (EDFA). The OA might be able to adjust the signal power delivered to the dispersion compensator by using a variable optical attenuator (VOA) located in the OA. The VOA may be configured to respond to the feedback power control (PC) signal.  
         [0025]     The tunable dispersion compensator  220  is a standard component for compensating for the chromatic dispersion of the input optical signal. Compensator  220  receives an amplified optical signal from the output of the OA. The compensator  220  adjusts the received optical signal to compensate for chromatic dispersion (CD). A chromatic dispersion control (CDC) is used as feedback control for compensating the signal for CD by the tunable dispersion compensator  220 . The compensated optical signal is then outputted from the compensator  220 .  
         [0026]     The optical signal then passes to the O/E converter  230 . The O/E converter could be a photodiode or integrated photo receiver. This component receives the optical signal from the tunable dispersion compensator  220  and outputs the electrical equivalent of the received optical signal.  
         [0027]     The CDR  240  receives the outputted electrical signal from the O/E converter  230  and outputs the clock signal and the data signal. CDR uses decision threshold and sampling phase information in order to derive the clock and data signals from the received signal. CDR could consist of one or more components. Clock and data recovery control (CDRC) may be received by the CDR as feedback control signal. CDRC could be used to influence the decision threshold and/or sampling phase of the received signal.  
         [0028]     The data processing circuit  250  uses the clock and data signals and performs, among other functions, FEC decoding on the received signal to obtain the originally transmitted signal. It also informs the controller  134  of the BER at the optical receiver  132 . Determining BER is well known in the art. For example, some possible methods, alternative to using the information from the FEC overhead, include analyzing the eye diagram, Q-factor or the like.  
         [0029]     The feedback control signals mentioned above are generated by the controller  134  and transmitted back to the different components and devices of the system. The system may have some or all of the control loops shown in  FIG. 2 . The system might also have additional control loops based on the BER feedback not shown in  FIG. 2 . The receiver system  200  may have a clock and data recovery control (CDRC) loop that allows the controller  134  to adjust the decision threshold in the CDR  240  by using the information transmitted in a CDRC feedback signal to the CDR  240 . The controller  134  could also adjust the sampling phase of the received signal at the CDR  134  by transmitting another type of control information in the CDRC. Another possible control loop is the chromatic dispersion control (CDC) loop which allows the controller  134  to tune the dispersion compensator to adjust the receiver to compensate for CD of the optical channel. There could also be a control loop for adjusting the output power of the optical amplifier by transmitting a PC to the optical amplifier. That OA could be located either in the receiver or elsewhere in the optical transmission system. The controller  134  might also form a feedback control loops to the transmitter  110  and/or the transmission link  120  of  FIG. 1 . For example, the controller might send a feedback signal to adjust the PMD compensator which might be located in the transmitter  110  or in other parts of the optical link  120 . The controller  134  could also send a feedback signal to the transmitter  110  to adjust the output power of that transmitter  110  or other parameters that influence the characteristics of the optical signal, e.g. MZM bias or duty cycle.  
         [0030]     In one embodiment, the BER is used to optimize the components of an optical network by advising the controller to adjust for different parameters of the optical signals. The control loops using the BER information can optimize the parameters of the optical switched network. The controller  134  will receive a BER from the data processing circuit  250  which will allow the controller to know the quality of the received signal. Depending on the value of BER, the controller can selectively send feedback control information to certain components in the system. The control information can vary the parameters of the components and decrease BER that has been received at the receiver  130 .  
         [0031]     There are some parameters of the fiber link that can change over time. E.g. the amount of chromatic dispersion for the received optical signal is affected by the temperature of the transmission fiber and thus changes with temperature. If the BER of the transmission network is low, the receiver will not notice the dispersion of the fiber has changed as the receiver will not receive any bit errors. As a result, that control loop could be off the optimum. Thus, receiver will not receive an optimum signal.  
         [0032]     In an optical switching network, signal quality may degrade very fast. This is due to the physical nature of the optical network. For example, when a wavelength is added to a WDM signal, all other wavelengths in the signal will be affected. Depending on the system design target, adding an additional wavelength can take between 1 millisecond and 1 second.  
         [0033]     Keeping track and maintaining the input signal at optimum even when the BER at the moment is low is important to the present invention. In the case of the switched network, there could be sudden degradation of input signal. The receiver could have very good BER at one moment, then receiving an enormous amount of errors less than one second later. It is practically impossible to keep track of all the settings of all the receiver control loops in such a short time. The FEC might have problems correcting all the errors, and the control loop might not be able to adjust to all the error at the speed required. Due the physical nature of certain control loop, it might take up to 100 seconds for optimization to be obtained again at the receiver. Therefore, it is necessary to keep track of the optimum settings even if you have very good BER at the moment. As an example, at a BER of 1e-15 and a bit rate of 10 GBit/s, only approximately one error will happen every day. This is not sufficient to keep track of the optimum settings of all control loops. By intentionally degrading the optical input signal, the BER can be set to an arbitrary level above the current value, thereby enabling the controller  134  to keep track of the optimum settings.  
         [0034]      FIG. 3  depicts a flow diagram of a method according to an embodiment of the invention. As shown in  FIG. 3 , parameters of the components of the system are adjusted to keep the control loops of the optical communications system in an active mode of operation. The data processing circuit  250  will monitor and report the BER of the received signal at the receiver  310 . If the received signal contains too many errors  320 , then the controller  134  will send the required feedback signal to the transmitter  110 , transmission link  120 , or the receiver itself in order for the receiver to continue to optimize the settings thereby improving the signal quality and reducing the number of received errors. If the received signal contains not enough errors in a certain interval (i.e. the BER is below the acceptable range), then the controller will offset one parameter of a control loop  340  to degrade the BER  350 . Some parameters that could be offset include the decision threshold, sampling phase, chromatic dispersion, polarization dispersion, and input/output power  360 . The parameter will be selected such that the BER will increase at the receiver. As a result, this action of degrading the BER will force the control loops to check the network  310  and ensure all components of the optical communications system is still operating at optimum  330 .  
         [0035]     For example, one way to detect slow changes is to cause degradation using the CDRC. The controller can send a feedback signal modifying the decision threshold values at the CDR  240 . By changing the decision threshold to allow for more errors, the BER of the received signals will get worse. Now the amount of degradation needed to obtain a pre-set BER can be used for optimizing all other control loops (instead of using the BER itself). The controller  134  will perform its standard routine to ensure all the feedback control loop are optimized and allow for the optimal input signal to be received by the receiver. Through this process, acceptable minor drifts and other degradation will be compensated and/or corrected in a timely manner.  
         [0036]     In an embodiment, a decision threshold is changed to induce an acceptable high BER for optimizing the other control loops. It is also possible that any other one or more of the control loops can also be used by the controller  134  to degrade the BER in order to obtain information on the optimum settings of the control loops. However, decision threshold control loop is a good loop to vary because the decision threshold can be changed very fast with respect to the system. Even after the decision threshold is shifted away from the optimal setting, the decision threshold can be quickly adjusted back to the previous optimal setting, and the input signal will still be received at substantially the optimal setting. Because of the physical nature of the system, the controller can vary the decision threshold quickly so the receiver will continue receiving input signal with minimal errors after the check.  
         [0037]     Unlike the decision threshold control loop, a chromatic dispersion control loop may be a very slow control loop depending on the physical implementation. If that loop has drifted, then it might take up to 100 seconds to obtain the optimizing setting for the control loop. In a switched optical network, large amounts of errors could occur within the second right after switching. The FEC might not be able to correct all the errors received. The receiver will not know the new optimal chromatic dispersion setting. The FEC will fail and the transmission will be useless. The receiver does not have the luxury of up to 100 seconds to regain the optimizing settings for this control loop. The present invention will keep the chromatic dispersion control loop in an active state thereby preemptively tuning the parameters for chromatic dispersion.  
         [0038]     Another embodiment involves obtaining the settings for an optimized input signal by degrading the input signal to get bit error without influencing the other control loops. For example, by changing the decision threshold some other control loops might be influenced. The receiver might compensate for shift in decision threshold by introducing some dispersion. This compensation by the receiver will cause the controller  134  to transmit improper feedback control signals causes the control loops to drift away from optimal. Therefore, it is important to find ways to degrade input signal without influencing the other control loops. One way to achieve this objective is to move the decision threshold very fast from the upper rail to the lower rail and back. This adjustment can be done very fast relative to the system. The effect of the fast shifting decision threshold will average out over time, and the other control loops will not notice the variation of the decision threshold.  
         [0039]      FIG. 4  depicts a graphical representation useful in understanding the present invention. The graph shows a V-curve. The decision threshold is shown on the horizontal axis. It is an arbitrary value where zero is the optimal setting. On the negative side and positive side represent the degraded BER. On the vertical axis is the recorded BER. This graph represents the BER for a certain setting of the decision threshold, and at the same time, it shows a vertical eye opening. The vertical eye opening information, instead of the BER can be fed back to the other components of the system as feedback parameter to control the feedback control loops.  
         [0040]      FIG. 4  defines the margin of the system and quality of the input signal. The left half of the “V” represents the lower threshold which is a line from −2 arbitrary units at 10 −12  BER to −10 arbitrary units at 10 −3  BER. The right half of the “V” is the line representing the upper threshold which is the line from 2 arbitrary units at 10 −12  BER to 10 arbitrary units at 10 −3  BER. The difference between the upper threshold and lower threshold is known as “vertical eye opening.” This parameter can be taken as the amount of dithering the threshold for signal degradation. One advantage of dithering the threshold is that the vertical eye opening linearly represents the system margin generally given in the unit decibel [dB]. When dithering the decision threshold from the upper rail to the lower rail and back, it is important to move the decision threshold in a way with certain fixed bit error rate. For example, if the upper rail is at decision threshold  6 , then the lower rail should be shifted to a decision threshold of −6. The decision threshold can be shifted between +6 and −6 arbitrary units, and the BER is 10 −6 . Once the input signal is intentionally degraded to a fix BER, then the BER should not be used to optimize other control loop. The vertical eye opening as a measure of the intentionally introduced degradation should be used as a feedback signal for the other control loops in this case.  
         [0041]     In a different embodiment, the WDM signal received by the receiver may be degraded by loading the signal with optical noise. For example, EDFA could be used to introducing noise in the optical domain. Electrical noise could also be included to degrade the input signal. For example, this degradation could be accomplished by using an optical attenuator between the photodetector and the optical amplifier. The BER may also be degraded by setting one or more control loops on the receive side such as the already describe decision threshold. Also, degradation may be accomplished by adjusting to different sampling phase, offsets, chromatic dispersion, polarization mode dispersion, or other means to influence the optical signal on the receiver side. It is also envisioned that it might be advantageous to adjust a combination of the above parameters.  
         [0042]     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.