Abstract:
Optical receivers and a method for providing a diagnostic measurement of optical modulation amplitude at other than a signal output of an optical receiver are invented and disclosed. The method includes the steps of applying a representation of an output of an optical detector to a circuit that determines a difference between a first signal level and a second signal level that is different from the first signal level and buffering the difference between the first signal level and the second signal level received from the circuit. The optical receivers include an optical detector, a transimpedance amplifier that applies automatic gain control, a circuit that determines a difference between two signal values and a limiting post amplifier. The receivers provide a diagnostic measure that can be compared to certain thresholds relating to known bit error rates or applied in a calculation to generate an absolute optical modulation amplitude value.

Description:
BACKGROUND 
     A conventional optical communication system is illustrated in  FIG. 1 . The optical communication system  1  comprises a transmitter  10 , an optical medium  20  (e.g., an optical fiber, a waveguide, free space, etc.), and a receiver  30 . The optical communication system  1  receives a data input on connection  5  and generates a data output that is applied on connection  35 . The transmitter  10  includes a data input  12  and an optical output  14 . The receiver  30  includes an optical input  32  and a data output  34 . The optical medium  20  has a first end  22  that is coupled to the optical output  14  and a second end  24  that is coupled to the optical input  32 . The transmitter  10  receives data in an electrical format and couples an amplitude-modulated optical representation of the data on the optical medium  20 . The receiver  30  receives the amplitude modulated optical representation of the data from the optical medium  20  and converts the same to an electrical representation of the received data. 
     The optical modulation amplitude (OMA) of a data signal is an important parameter that is used in specifying the performance of optical links used in digital communication systems. At a given receiver noise floor, the OMA directly relates to the bit error ratio (BER) of a communication system. 
     In bipolar non-return to zero (NRZ) optical signaling schemes, only two discrete optical power levels are used. The higher level or P H  and the lower level or P L .  FIG. 2  includes a plot  200  of optical power versus time for both the transmitter  10  and the receiver  30  of  FIG. 1 . As illustrated in  FIG. 2 , OMA is defined as the difference between the high and low power levels, which can be represented mathematically as:
 
 OMA=P   H   −P   L    Equation 1
 
Average signal power is simply the average of the high and low power levels, i.e.,
 
                     P   AVG     =         P   H     +     P   L       2             Equation   ⁢           ⁢   2               
The extinction ratio (ER) is the ratio between the high and low power levels:
 
                   ER   =       P   H       P   L               Equation   ⁢           ⁢   3               
From Equation 1, Equation 2 and Equation 3, the following relationship can be derived:
 
     
       
         
           
             
               
                 
                   OMA 
                   = 
                   
                     2 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     
                       
                         P 
                         AVG 
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             ER 
                             - 
                             1 
                           
                           
                             ER 
                             + 
                             1 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
           
         
       
     
     OMA and ER by themselves are relative quantities, since they specify the difference and a ratio of power levels, respectively. To derive an absolute quantity from the OMA or ER an additional point of reference, such as P AVG , P H , or P L , is required. Each of the relationships defined in Equations 3 and 4 depend on one of these additional points of reference. 
     For example, an OMA of 100 μW can correspond to an infinite number of possible values for P AVG , P H , or P L . P H  could be 100 μW with P L  equal to 0 μW, or P H  could be 150 μW with P L  equal to 50 μW, or P H  could be 100 mW with P L  equal to 99.9 mW, etc. 
     In the alternate case of ER, a similar example using an ER=10 can correspond to an infinite number of possible values for P AVG , P H , or P L . P H  could be 100 μW with P L  equal to 10 μW, or P H  could be 150 μW with P L  equal to 15 μW, or P H  could be 100 mW with P L  equal to 10 mW, etc. 
     If in addition to OMA and ER a reference point of P AVG =100 μW is specified, then the ambiguity has been removed. With an OMA of 100 μW and P AVG =100 μW, P H  can only be 150 μW and P L  can only be 50 μW. If the ER is 10 and P AVG =100 μW, then P H  can only be 182 μW and P L  can only be 18.2 μW. 
     While it may seem apparent that OMA and ER are nearly equivalent, there are differences. One of these differences is how OMA and ER change as a signal propagates through an optical communication system. Assuming an optical communication system with linear attenuation between two points, the ER will stay constant as the signal is attenuated, while the OMA will change by a factor equal to the attenuation. For example, over 10 km of optical fiber with an attenuation of 0.3 dB/km, the total attenuation over 10 km is 3 dB, which is equivalent to a factor of 2. A signal transmitted through the optical fiber that starts with P H  of 1 mW and P L  of 0.1 mW, has an ER of 1/0.1=10 and an OMA=1−0.1=0.90 mW at the input to the optical fiber. At the output of the optical fiber, P H  is 0.5 mW and P L  of 0.05 mW (both are reduced by a factor of two). Therefore, ER is 0.5/0.05=10 and OMA=0.5−0.05=0.45 mW. Thus, ER is the same and OMA is reduced by a factor of two. Once the ER is known, an average power measurement from anywhere in the optical communication system will yield enough information to calculate P H , P L  and OMA. On the other hand, a measure of OMA at any point in the system does not provide enough information to determine the OMA at another point in the system without knowing the magnitude of the attenuation or measuring additional parameters (such as P AVG , P H , or P L ). 
     To optimize BER performance of an optical communication link, the OMA should be as large as possible. In optical communication links there are upper and lower limits on P AVG  and OMA. In an optical receiver, there is an upper limit on the optical power that can be received. When the received optical power exceeds this upper limit, saturation effects degrade BER performance For optimum receiver BER performance, the OMA should be as large as possible while avoiding the upper power limit, which occurs when P L  is zero and P H  is just below the upper power limit. For optical transmitters that use a laser as a light source, it is difficult to reduce P L  to zero. When a laser is switched from a completely off state to an on state, turn-on delay and relaxation oscillation negatively affect the communication link. If the laser is biased above its threshold level so that it is always on, problems with turn-on delay and relaxation oscillation decrease. For this reason, practical laser transmitters emit some optical power at P L . A complicating factor is that the laser threshold changes significantly with temperature, making it difficult to keep the difference between the bias and the threshold constant. Precise control of the bias current over a large temperature range adds significant complexity and cost to optical transmitters. 
     For conventional optical communication links that use relatively low-loss multimode fiber as the communication medium, a combination of the ER and the average power at the transmitter has provided an adequate measure of communication link quality. For optical communication applications that use large-core fiber (e.g., polymer optical fiber (POF)) the combination of ER and average power at the transmitter does not provide an adequate measure of optical communication link quality. While POF is inexpensive and easy to terminate with common tools and ordinary polishing paper, POF attenuates more and provides less bandwidth when compared to an optical fiber of similar length made from silica. Communication links using POF have been used in industrial control applications, robotics, and automotive applications where signaling rates are much lower than those used in high-speed telecommunication applications. The relatively low signaling rates, which enable simple and inexpensive light-emitting diode (LED) based transmitters, has proved to be a significant factor in market acceptance and penetration for POF communication based systems. However, there is a demand in industrial automation applications to use the Fast Ethernet data transfer protocol (100 Mbps) over POF links up to 50 meters long and hard cladded silica (HCS) links up to 100 meters long. Beyond these distances, the bandwidth of standard 0.5 numerical aperture (NA) POF and 0.37 NA HCS links will not support Fast Ethernet communications. The limited bandwidth of POF and HCS communication links, even at the desired maximum distances, renders the combination of ER and the average power at the transmitter ineffective as a measure of communication link quality. This is because the average received light power can be nominal but modal dispersion in the communication medium may reduce the difference between the high and low signal levels at the receiver. Such a reduction in the difference between the high and low signal levels can severely degrade BER performance of the communication link. 
     SUMMARY 
     An embodiment of an optical receiver system provides a diagnostic measure of OMA at other than a signal output of the receiver. The optical receiver system includes an input coupled to an optical detector that generates an electrical current that corresponds to an optical signal at the input. The optical receiver system further includes a transimpedance amplifier and a circuit. The transimpedance amplifier receives the electrical current and generates an amplified voltage. The transimpedance amplifier applies an automatic gain control to ensure that the output of the transimpedance amplifier is not limited. That is, the automatic gain control makes sure that the transimpedance amplifier does not become saturated. The circuit receives the amplified voltage and an indication of the gain applied by the transimpedance amplifier. The circuit generates a difference of a first signal level and a second signal level as a function of the electrical current and the gain applied by the transimpedance amplifier. The circuit provides a diagnostic measure of OMA. 
     An alternative embodiment of an optical receiver system provides a diagnostic measure of OMA at other than a signal output of the receiver. The optical receiver system includes an input coupled to an optical detector that generates an electrical current in response to an optical signal at the input. The optical receiver system further includes a signal mirror, a transimpedance amplifier and a circuit. The signal mirror is coupled to the electrical current and forwards first and second representations of the electrical current at respective outputs of the signal mirror. The transimpedance amplifier receives the electrical current and generates an amplified voltage. The circuit receives a representation of the electrical current at an input of the circuit and generates a difference between an average of a first signal level and a second average of a second signal level. The difference provides a diagnostic measure of OMA at an output of the circuit. 
     An embodiment of a method for providing a diagnostic measurement of OMA at an optical receiver that utilizes automatic gain control includes the steps of applying a representation of an output of an optical detector to a circuit that determines a difference between a first signal level and a second signal level and buffering the difference between the first signal level and the second signal level received from the circuit. 
     The figures and detailed description that follow are not exhaustive. The disclosed embodiments are illustrated and described to enable one of ordinary skill to make and use the optical receivers and methods for providing a measure of OMA. Other embodiments, features and advantages of the optical receivers and methods will be or will become apparent to those skilled in the art upon examination of the following figures and detailed description. All such additional embodiments, features and advantages are within the scope of the systems and methods as defined in the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The systems and methods for diagnostic monitoring of OMA at an optical receiver that uses automatic gain control can be better understood with reference to the following figures. The components within the figures are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of providing a measure of OMA at an output of an optical receiver that uses automatic gain control to prevent saturation of the transimpedance amplifier. Moreover, in the figures, like reference numerals designate corresponding parts throughout the different views. 
         FIG. 1  is a simplified functional block diagram of a conventional optical communication system. 
         FIG. 2  is an embodiment of an example plot of optical power versus time for the transmitter and the receiver of the optical communication system of  FIG. 1 . 
         FIG. 3  is a schematic diagram of an embodiment of an improved optical receiver. 
         FIG. 4  is a schematic diagram of an alternative embodiment of an improved optical receiver. 
         FIG. 5  is a flow chart illustrating a method for providing a diagnostic measure of OMA in a receiver that uses automatic gain control. 
         FIG. 6  is a flow chart of an embodiment of a method for providing a diagnostic measure of OMA in a receiver that uses automatic gain control. 
         FIG. 7  is a flow chart of an alternative embodiment of a method for providing a diagnostic measure of OMA in a receiver that uses automatic gain control. 
     
    
    
     DETAILED DESCRIPTION 
     An optical receiver monitors and outputs a voltage that represents the OMA of a received optical signal at the optical receiver. The voltage is a diagnostic measure of the quality of the optical communication link defined by an upstream transmitter, an optical medium and the optical receiver. The voltage provides a direct measure of signal strength, rather than an indirect calculation that uses one or more measures of a point of reference and the ER of the upstream transmitter to characterize the quality of the communication link. A direct measure of communication link quality provides the ability to track vertical eye closure due to inter-symbol interference (ISI) caused by increased transmitter rise/fall times, reduced transmitter ER and increased dispersion effects in a fiber medium, etc. As described above, diagnostic monitoring based on OMA is better correlated to BER performance of an optical communication link than monitors that use a measure of average optical input power. This is especially the case for optical communication links that use bandwidth-limited transmitters, bandwidth limited and/or relatively high-loss fiber media like polymer optical fiber (POF) and hard-cladded silica (HCS) fiber. 
     An optical receiver uses automatic gain control to support the dynamic range of POF and HCS communication media (1 mW to 1 μW or 0 dBm to −30 dBm) and provides a diagnostic measurement of OMA at other than a signal output. Two embodiments are presented. In a first embodiment, OMA, based on signal swing at an input to a signal-limiting amplifier (or post amplifier) in combination with an actual transimpedance gain setting is provided at a dedicated receiver output. A low-speed circuit (i.e., a circuit operating at a rate in the kHz range or slower) records an average of the high signal peaks and an average of the low signal peaks. Thus, the low-speed circuit provides a measure of the peak-to-peak signal swing at the input to the signal-limiting amplifier. The low-speed circuit combines the measure of the peak-to-peak signal swing with the gain of the transimpedance amplifier to generate an absolute measure of OMA at the receiver. 
     For example, if the light intensity at the optical detector is very low, the transimpedance gain setting will be at a maximum gain setting. Under these operating conditions, the signal swing at the output of the transimpedance amplifier will be a representation of the optical signal swing at the optical detector. When the light intensity increases to a point where the automatic gain control of the transimpedance amplifier starts to attenuate the signal to ensure that the output of the transimpedance amplifier is not limited (i.e., the transimpedance amplifier is not saturated), the output of the low-speed circuit is adjusted by the gain applied at the transimpedance amplifier to accurately track the optical signal swing at the optical detector. A driver provides an analog voltage at a non-data output of the receiver. The analog voltage can be related to a measure of link quality. Thus, for a particular receiver, a relationship between the analog voltage and OMA swing at the receiver can be established. In addition, once a measure of the receiver noise floor is recorded or characterized, a relationship between the OMA swing and BER for the communication link can be provided. 
     In a second embodiment, OMA, based on signal swing at an input to a transimpedance amplifier is provided at a dedicated receiver output. A signal or current mirror is inserted between the optical detector and a transimpedance amplifier that applies automatic gain control to ensure that the output of the transimpedance amplifier is not limited. A first output of the signal mirror is coupled to the transimpedance amplifier. A second output of the signal mirror is coupled to a fixed gain amplifier having a gain such that the output of the fixed gain amplifier will not saturate over the dynamic range of the receiver and a low-speed circuit that generates an average peak-to-peak signal swing. An optical receiver in accordance with this second embodiment will be preferably constructed with the signal mirror, transimpedance amplifier, fixed-gain amplifier, the low-speed circuit and the buffer being formed on a single substrate. The low-speed circuit provides a measure of the peak-to-peak signal swing at the output of the fixed gain amplifier. A driver coupled to an output of the low-speed circuit provides an analog voltage at a non-data output of the receiver. The analog voltage can be related to a measure of link quality. 
     Turning now to the drawings, wherein like reference numerals designate corresponding parts throughout the drawings, reference is made to  FIG. 3 , which includes a schematic diagram of an embodiment of an improved optical receiver  300 . The optical receiver  300  includes a series arrangement of an optical detector  310 , a pre-amplifier  320  and a post-amplifier  330 . The optical receiver  300  receives an optical signal at a second or output end  24  of an optical medium  20  and generates a first electrical signal labeled DATA on connection  333  that is an amplified version of the optical data signal received at input  302 . The first electrical signal on connection  333  and the complement signal on connection  335  are limited or clamped between the logic high voltage and the logic low voltage. In addition to the first and second electrical signals, the optical receiver  300  generates a voltage on connection  339  (labeled OMA) that is a measure of the optical modulation amplitude at the input  302 . 
     The optical detector  310  is coupled to the input  302  via an optical coupling mechanism  305 . The optical coupling  305  can be a butt coupling, a refractive coupling, a fiber stub, etc. The optical detector  310  is further coupled to the pre-amplifier  320  via connection  315 . The optical detector  310  is an optical-to-electrical signal converter. That is, the current on connection  315  is responsive to the time-varying light signal received via the input  302  and the optical coupling mechanism  305 . 
     The transimpedance amplifier  322  receives the current on connection  315  and converts the same to a time-varying voltage on pre-amplifier output connection  325 , which is coupled to a signal input of the post-amplifier  330 . An indication of the gain is provided on connection  323  to the post-amplifier  330 . The pre-amplifier  320  comprises a transimpedance amplifier  322  with automatic gain control. The pre-amplifier  320  is configured to dynamically apply the automatic gain control to ensure that the amplified voltage on connection  325  at the output of the pre-amplifier accurately reflects the time-varying optical signal swing at the input  302  without saturating the transimpedance amplifier  322 . 
     The signal limiting post-amplifier  332  receives the amplified voltage on connection  325  and generates the first electrical signal labeled DATA on connection  333  and its complement (i.e., the second electrical signal) on connection  335 . A relatively low-speed circuit  336  receives the amplified voltage on connection  325  and the indication of the gain applied by the transimpedance amplifier  322  on connection  323 . The low-speed circuit  336  generates the difference of a first signal level and a second signal level. The difference of the first signal level and the second signal level is forwarded via connection  337  to a driver or buffer  338 , which is coupled to the connection  339 . The low-speed circuit  336  operates in the kHz range or slower. The low-speed circuit  336  determines the average high signal level at the output of the pre-amplifier  320  and the average low signal level at the output of the pre-amplifier  320  and forwards the difference of these average signal levels at its output. To provide an accurate representation of the optical signal swing at the input  302  of the optical receiver  300 , the output of the low-speed circuit  336  is adjusted by the gain applied at the transimpedance amplifier  322 . For example, if a gain factor of 0.1 is applied at the transimpedance amplifier  322  to ensure that the output voltage is not limited and the average peak-to-peak voltage swing measured by the circuit  336  is 40 mV, the circuit  336  multiplies the inverse of the gain by the peak-to-peak voltage swing and generates an output signal on connection  337  of 400 mV. The buffer  338  is provided to ensure that external monitoring equipment does not adversely affect the diagnostic measure of OMA on connection  339 . 
       FIG. 4  is a circuit diagram of an alternative embodiment of an improved optical receiver. The optical receiver  400  includes an arrangement of an optical detector  310 , a signal mirror  410 , a pre-amplifier  420  and a post-amplifier  430 . The optical receiver  400  receives an optical signal at a second or output end  24  of an optical medium  20  and generates a first electrical signal labeled DATA on connection  433  that is an amplified version of the optical data signal received at the input  302 . The first electrical signal on connection  433  and the complement signal on connection  435  are limited or clamped between the logic high voltage and the logic low voltage. In addition to the first and second electrical signals, the optical receiver  400  generates a voltage on connection  429  (labeled OMA) that is a measure of the optical modulation amplitude at the input  302 . 
     The optical detector  310  is coupled to the input  302  via an optical coupling mechanism  305 . The optical detector  310  is further coupled to the signal mirror  410  via connection  315 . The optical detector  310  is an optical-to-electrical signal converter. That is, the current on connection  315  is responsive to the time-varying light signal received via the input  302  and the optical coupling mechanism  305 . 
     The signal or current mirror  410  provides a first pre-amplifier input on connection  413  and a second pre-amplifier input on connection  415 . The signal mirror  410  is a circuit designed to copy a current through one active device by controlling the current in another active device of the circuit. The signal mirror  410  keeps the output current on connection  413  and the output current on connection  415  constant regardless of pre-amplifier and post-amplifier loading (if any). The signal mirror  410  provides a representation of the current provided by the optical detector  310  on the connection  413  and the connection  415 . 
     The first pre-amplifier input on connection  413  is coupled to a transimpedance amplifier  422  that receives the current on connection  413  and converts the same to a time-varying voltage on pre-amplifier output connection  423 , which is coupled to a signal input of the post-amplifier  430 . The transimpedance amplifier  422  dynamically applies automatic gain control to ensure that the amplified voltage on connection  423  at the output of the pre-amplifier  420  accurately reflects the time-varying optical signal swing at the input  302  without saturating the transimpedance amplifier  422 . 
     An optical signal monitoring path within the pre-amplifier  420  includes an arrangement of a fixed gain amplifier  424 , a low-speed circuit  426 , and a driver or buffer  428 . The fixed gain amplifier  424  receives the second pre-amplifier input on connection  415  and forwards a time-varying amplified voltage on connection  425 . The fixed gain amplifier  424  is configured to provide a time-varying amplified voltage that is not limited over the dynamic range of the optical receiver  400 . The low-speed circuit receives the amplified voltage on connection  425  and generates the difference of an average high signal level and an average low signal level. The difference of the average high signal level and the average low signal level is forwarded via connection  427  to the buffer  428 , which is coupled to the connection  427 . The low-speed circuit  426  operates in the kHz range or slower. The buffer  428  provides a measure of the OMA at other than a signal output of the optical receiver  400 . The buffer  428  further ensures that any external monitoring equipment does not adversely affect the diagnostic measure of OMA on connection  429 . 
     The signal limiting post-amplifier  432  receives the amplified voltage on connection  423  and generates the first electrical signal labeled DATA on connection  433  and its complement (i.e., the second electrical signal) on connection  435 . In this way, the post amplifier  430  generates a limited or clamped version of the optical signal received at the input  302 . The first and second electrical signals on connection  433  and connection  435  are limited or clamped to the voltage levels corresponding to a logic high and a logic low, respectively. 
       FIG. 5  is a flow chart illustrating a method  500  for providing a diagnostic measure of OMA in an optical receiver that uses automatic gain control. The method  500  begins with block  502  where a representation of an output of an optical detector is applied to a circuit that determines a difference between a first signal level and a second signal level. Thereafter, as indicated in block  504 , the difference of the first signal level and the second signal level provided by the circuit is buffered. The buffered difference of the first signal level and the second signal level is a diagnostic measure of OMA. 
       FIG. 6  is a flow chart illustrating an embodiment of a method  600  for providing a diagnostic measure of OMA in an optical receiver that uses automatic gain control. The method  600  begins with block  602  where an output of an optical detector is applied to a transimpedance amplifier that uses automatic gain control. In block  604 , the optical receiver generates a measure of OMA by applying the output of the transimpedance amplifier to a circuit that determines a difference between a first signal level and a second signal level. In addition, the circuit generates a measure of OMA as a function of the gain applied at the transimpedance amplifier and the difference between the first signal level and the second signal level. As explained above, the product of a gain factor and the difference is a measure of OMA at the optical detector of the receiver. 
     Thereafter, as indicated by block  606 , the OMA as represented by an analog voltage, is buffered. In block  608 , the buffered OMA signal is coupled to an external apparatus that compares the buffered OMA signal to one or more thresholds. In block  610  calibration information that associates a voltage level with an absolute OMA is provided. 
       FIG. 7  is a flow chart of an alternative embodiment of a method  700  for providing a diagnostic measure of OMA in an optical receiver that uses automatic gain control. The method  700  begins as shown in block  702 , where an electrical signal responsive to the received light at an optical receiver is provided to a signal mirror that generates first and second signal mirror output signals. In block  704 , a first representation of the first signal mirror output signal is generated using a fixed gain amplifier that does not become saturated over the dynamic range of the optical receiver. In block  706 , a difference of an average high signal level and an average low signal level in the first representation of the first signal mirror is determined Thereafter, as indicated in block  708 , a measure of OMA is generated by buffering the difference of the average high signal level and the average low signal level. In block  710 , a second representation of the second signal mirror output signal is generated using a transimpedance amplifier that applies automatic gain control. In block  712 , differential output signals, responsive to the electrical signal are generated by applying the second representation to a signal limiting amplifier. In block  714 , the measure of OMA is coupled to an external device (i.e., an apparatus other than the optical receiver) that applies one or more thresholds to generate a measure of optical communication link quality. In block  716 , calibration information that associates the measure of OMA with an absolute OMA at the optical receiver is provided. The particular sequence of the steps or functions in blocks  702  through  712  is presented for illustration. It should be understood that the order of the steps or functions in blocks  702  through  712  can be performed in any other suitable order. The steps or functions in block  714  and block  716  are optional. 
     While various embodiments of the optical receiver systems and methods for providing a measure of OMA at an optical receiver that uses automatic gain control have been described, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible that are within the scope of this disclosure. Accordingly, the described optical receiver systems and methods for providing a diagnostic measure of OMA at an optical receiver that uses automatic gain control are not to be restricted or otherwise limited except in light of the attached claims and their equivalents.