Abstract:
An optical receiver having compensation for signal level transients. The receiver includes an OE converter, a transient threshold compensator, a threshold combiner and a CDR system. The OE converter receives the incoming optical signal and provides a modulated electrical signal having a high speed response for tracking the modulation on the optical signal and an averaged electrical signal having a moderate speed response for tracking changes in the average level of the optical signal. The transient threshold compensator processes the averaged electrical signal for providing a transient feedforward adjustment. The threshold combiner combines the transient feedforward adjustment with a lower speed BER feedback threshold adjustment for providing a decision threshold signal. The CDR system uses the decision threshold signal for recovering a clock, providing the BER feedback threshold adjustment, and estimating the data carried by the modulation.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The invention relates generally to optical receivers and more particularly to an optical receiver having signal level transient compensation.  
         [0003]     2. Description of the Background Art  
         [0004]     Optical transmission systems include a transmitter for transmitting a modulated optical signal into a link and a receiver for receiving the signal from the link. The link may span a short distance or thousands of kilometers. Existing optical receivers have an optical to electrical (OE) converter, typically using a photodiode, for converting the incoming optical signal to an electrical signal. The electrical signal drives a clock data recovery (CDR) system that recovers the clock and then uses the clock for sampling the electrical signal. The modulated data is recovered by comparing the sampled signal to a fixed decision threshold level. A sampled signal level that is above the threshold level yields a bit sense of one and a level below the threshold level yields a bit sense of zero.  
         [0005]     When the decision threshold level is fixed, any variation or transient in average power of the incoming signal can lead to bit errors. Some workers have attempted to resolve this problem by leveling the variations and transients with optical devices in the link. This has been done with variable gain erbium-doped fiber amplifiers (EDFA)s and variable optical attenuators (VOA)s. However, these optical devices can be expensive for new systems and the installation cost for retrofitting into existing systems can be prohibitive.  
         [0006]     There is a need for an inexpensive way to minimize bit errors when variations and transients occur in optical signal power.  
       SUMMARY OF THE INVENTION  
       [0007]     Briefly, the present invention is an optical receiver having an electrical method for improving tolerance for the variations and transients on an incoming modulated optical signal. The receiver includes an optical to electrical (OE) converter, a transient threshold compensator, a threshold combiner and a clock data recover (CDR) system. The OE converter receives the incoming optical signal and provides a modulated electrical signal having a high speed response for tracking the modulation on the optical signal and an averaged electrical signal having a moderate speed response for tracking changes in the average level of the optical is signal. The transient threshold compensator processes the averaged electrical signal for providing a transient feedforward adjustment. The threshold combiner combines the transient feedforward adjustment with a low speed BER-based feedback adjustment for providing a decision threshold signal. The CDR system uses the decision threshold signal for recovering a clock, providing the BER-based feedback adjustment, and estimating the data carried by the modulation.  
         [0008]     The optical receiver of the present invention has the benefit of using low cost electrical circuitry contained within the receiver for improving bit error rate (BER) performance for an optical signal when the optical signal level has transients and/or variations.  
         [0009]     This and other benefits of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various figures.  
     
    
     IN THE DRAWINGS  
       [0010]      FIG. 1  is a block diagram of an optical receiver of the present invention;  
         [0011]      FIG. 2A  is a time chart showing a decision threshold signal for the receiver of  FIG. 1  for an incoming optical signal having a positive transient;  
         [0012]      FIG. 2B  is a time chart showing a decision threshold signal for the receiver of  FIG. 1  for an incoming optical signal having a negative transient; and  
         [0013]      FIG. 3  is flow chart of a method of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0014]      FIG. 1  is a block diagram of a receiver of the present invention referred to with a reference number  10 . The receiver  10  includes an optical to electrical (OE) converter  12 , a clock and data recovery (CDR) system  14  and a transient threshold compensator  16 . The CDR system  14  includes a clock recovery circuit  22 , a data estimator  24 , and an error detection/correction circuit  26 . It should be noted that a “circuit” typically includes both hardware and software. The receiver  10  may also include other components such as optical and electrical demultiplexers, amplifiers and filters.  
         [0015]     The OE converter  12  couples to an optical link  32  for receiving an incoming amplitude or intensity modulated optical signal  34  and converting the signal  34  to a modulated electrical signal  36 . The modulated electrical signal  36  may be baseband where the modulation is the signal  36 , or the modulation may be carried on an intermediate frequency carrier signal. In either case, the pattern and rate of the modulation on the electrical signal  36  is representative of the pattern and rate of the modulation on the optical signal  36 .  
         [0016]     The OE converter  12  also converts the optical signal  34  into an electrical signal  37  having a level proportional to the average of the optical signal  34  and passes the averaged signal  37  to the transient threshold compensator  16 . The averaged electrical signal  37  has a response time fast enough to follow transients in the optical signal  34  but not fast enough to track the modulation. The frequency response of the averaged electrical signal  37  may be about one-tenth to about one one-thousandth the frequency response of the signal  36 . The transient threshold compensator  16  uses the averaged electrical signal  37  for providing a dynamic transient feedforward adjustment  40 .  
         [0017]     One or more photodetectors may be used by the OE converter  12  for providing both the modulated electrical signal  36  and the averaged electrical signal  37 . The signal for the average of the level of the optical signal  34  can be the average current in a photodiode. Alternatively, the transient threshold compensator  16  filters the electrical signal  36  for providing the averaged electrical signal  37 .  
         [0018]     The OE converter  12  passes the electrical signal  36  to the clock recovery circuit  22  and the data estimator  24  in the CDR system  14 . The clock recovery circuit  22  uses the level of a decision threshold signal  42  for synchronizing a clock to the modulation on the electrical signal  36  and passes the clock to the data estimator  26 .  
         [0019]     The data estimator  24  uses the clock for sampling the electrical signal  36  and compares the samples to the level of the decision threshold signal  42  for providing estimated data having a sense or level of 1 (one) when the sampled signal is greater than the decision threshold signal  42  and a sense or level of 0 (zero) when the sampled signal is less than the decision threshold signal  42 . The senses of 1 and 0 are used for data words having single bit data estimation. It should be noted that multiple bits may be used for the estimated data. For example, for two bit data a word of “11” might indicate a high level one, a word of “10” might indicate a lower level one, a word of “01” a high level zero and a word “00” might indicate a lower level zero. The same idea can be extended to words having many bits.  
         [0020]     The data estimator  24  passes the ones and zeroes as estimated data to the error detection/correction circuit  26 . The error detection/correction circuit  26  uses the estimated data for detecting and correcting errors in the estimated data and then issues corrected estimated data as an output data signal  43 .  
         [0021]     The CDR system  14  optionally includes a BER estimator  44  and a BER-based threshold controller  46 . The BER estimator  44  estimates a bit error rate based on error detection information from the error detection/correction circuit  26 . The BER-based threshold controller  46  uses the estimated bit error rate for providing a BER feedback threshold adjustment  52  to a threshold combiner  54 . The threshold combiner  54  combines the BER feedback threshold adjustment  52  with transient feedforward adjustment  40  received from the transient threshold compensator  16  for providing the decision threshold signal  42 . It should be noted that the response time of the transient feedforward adjustment  40  may be about ten to a thousand or more times faster than the response time of the BER feedback threshold adjustment  52 .  
         [0022]     The electrical signals  36  and  37 , transient feedforward adjustment  40 , the decision threshold signal  42  and the BER feedback transient adjustment  52  are multi-level signals (more than two levels) when the incoming optical signal  34  is a multi-level signal.  
         [0023]      FIGS. 2A and 2B  are exemplary eye diagram time charts  60 A and  60 B showing positive and negative amplitude transients, respectively, for the modulated optical signal  34  and the responsive modulated electrical signal  36 . The drawing is scaled so that the signals  34  and  36  are shown with the same levels. In order to make the drawing easier to understand, the level  1  transients are shown in a compressed time scale as compared to the eye pattern modulation, that is, the transients are shown to occur with a faster rise and fall time (compared to the modulation) than is typical.  
         [0024]     The eye chart  60 A shows a level  1  and a level  0 . The level  1  has a first signal level  62  followed by a positive transient  63  having a higher signal level and settling at a second signal level  64 . The second level  64  may be greater, lesser, or the same as the first level  62 . A fixed decision threshold level  72  is shown at the mid level between the level  0  and the level  1  for the first signal level  62 . It can be seen by inspection that the fixed decision threshold level  72  is not at the mid level for the transient signal level  63  or the second signal level  64 . The transient threshold compensator  16  applies a scale factor to the averaged electrical signal  37  for providing a dynamic transient feedforward adjustment  40  that results in the decision threshold signal  42  shown as a level  74 .  
         [0025]     The eye chart  60 B also shows the level  1  and the level  0 . The level  1  has the first signal level  62  followed by a negative transient  66  having a lower signal level and settling at a third signal level  67 . The third level  67  may be greater, lesser, or the same as the first level  62 . The fixed decision threshold level  72  is shown at the mid level between the level  0  and the level  1  for the first signal level  62 . It can be seen by inspection that the fixed decision threshold level  72  is not at the mid level for the transient signal levels  66  or the third signal level  67 . The transient threshold compensator  16  applies a scale factor to the averaged electrical signal  37  for providing a dynamic transient feedforward adjustment  40  that results in the decision threshold signal  42  shown as a level  76 .  
         [0026]     For the exemplary cases  60 A and  60 B, the optimum decision threshold level is shown as the mid level between the level  1  and the level  0 . However, the scale factor that is applied by the transient threshold compensator  16  may be selected so that the levels  74  and  76  are higher or lower than the mid level.  
         [0027]     Without the transient feedforward adjustment  40  of the present invention, the BER feedback threshold adjustment  52  would eventually drive the decision threshold signal  42  ( FIG. 1 ) to an optimum level for minimizing errors. However, the BER feedback transient adjustment  52  necessarily requires errors to be detected over some period of time before the BER feedback threshold adjustment  52  can adjust the level of the decision threshold signal  42 . The transient feedforward adjustment  40  of the present invention can act more quickly than the BER feedback threshold adjustment  52  in order to reduce the number of bit errors that occur before the BER feedback threshold adjustment  52  has time to adjust.  
         [0028]     For a multi-level incoming optical signal  34  (two or more on-condition states and one zero state), the level  1  in the eye diagrams and the corresponding levels  62 - 76  are multi-level (corresponding in level to the two or more on-condition states). For example, for the optical signal  34  having three state modulation (two on-conditions states and one zero state), the level  1  and each of the corresponding levels  62 - 76  have two levels (corresponding to the two on-condition states); for five state modulation (four on-conditions states and one zero state), the level  1  and each of the corresponding levels  62 - 76  have four levels (corresponding to the four on-condition states); and so on.  
         [0029]      FIG. 3  is a flow chart of a method of the present invention for minimizing bit errors in the presence of transients and variations in a modulated optical signal. In a step  102  an amplitude or intensity modulated optical signal is received from an optical link. In steps  104  and  106  the optical signal is converted into a modulated electrical signal and an averaged electrical signal. The modulation on the electrical signal tracks the modulation on the optical signal. The level of the averaged electrical signal tracks the average level of an on-condition of the modulated optical signal. In a step  108  data is estimated for the modulated electrical signal by comparison to a level of a decision threshold signal.  
         [0030]     At least some of the errors on the estimated data are detected and corrected in a step  112 . In a step  114  the corrected estimated data is issued as an output data signal. A bit error rate is estimated in a step  122 . In a step  124  the bit error rate is used for generating a BER feedback threshold adjustment.  
         [0031]     A transient feedforward adjustment is generated in a step  132  from the averaged electrical signal. In a step  134  the decision threshold signal is determined from the transient feedforward adjustment and the BER feedback threshold adjustment. The decision threshold signal is used in the step  108  for estimating data.  
         [0032]     Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that such disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the true spirit and scope of the invention.