Abstract:
An apparatus, circuit arrangement and method enable a host system to read a signal that is proportional to the peak-to-peak optical power incident on a transceiver at one end of a fiber optic link without breaking that link. By reading this signal and being aware of the characteristics of the laser, the “health” of the laser is known and the integrity of the link is maintained. Consequently, the host system has a predictive tool that is able identify substandard lasers so that such lasers can be replaced before serious, and potentially costly, data disruptions occur.

Description:
FIELD OF THE INVENTION 
     The invention is generally related to laser transceivers and fiber optic communication links. More specifically, the invention is related to monitoring operational performance of fiber optic communication links. 
     BACKGROUND OF THE INVENTION 
     Today, optical fibers form the backbone of a global telecommunication system. These strands of glass, each thinner than a human hair, are designed to carry huge amounts of data transmitted by tightly focused laser beams. Together, optical fibers and lasers have dramatically increased the capacity of the telephone and data systems. With equal improvements in computing, mankind has become dependant on this communication technology. 
     Thus, maintaining the integrity of a fiber optic communications link has become critical, particularly in these high capacity telephone and data systems. Failure to properly maintain a link can result in severed communications as well as data disruptions since both voice and data may be carried on the same fiber. This can lead to lost revenues since people often do not reestablish calls once they have been interrupted or “dropped.” Further, reestablished calls and data links must be rerouted by the host system over another link. This results in delays as well as the additional time and effort required for rerouting, not to mention reduced system capacity. 
     Often, breaks in these links can be traced to failures of laser diodes that transmit data over these fibers. These laser diodes function to transmit voice and data through modulation of their photonic emission. Laser diodes are “biased” by a DC current which causes them to emit radiation at a particular frequency. This emission frequency is then varied, or “modulated,” by an AC current in response to voice or data which is desired to be transmitted. Experience has shown that most laser diodes do not fail catastrophically but rather slowly deteriorate in performance, drawing more and more bias and modulation current to generate the amount of output power necessary to maintain the link. At some point, the amount of current required becomes so burdensome to the electronic driver that the link is broken. 
     Once a link breaks, a technician must be dispatched to diagnose and repair the broken link. Oftentimes, the technician must disconnect the optical fiber from the transceiver located at the opposite end of the link from the laser diode and connect it to an optical power meter in order to measure the emitted optical power from the transceiver. Based on the measured power, a diagnosis is made by the host system. Optical power transmitted from the laser diode must pass through an optical system of complex interconnections before reaching a transceiver. Many times, the optical output power is too low due to a failure of the laser diode in the laser module. Other times, the break in the link is due to a failure of a connection in the link itself. Optical fibers can be up to 10 km in length and can be spliced and optically switched as well. Optical attenuation along this path is a reality. Lasers experience optical attenuation along the network due to damaged or dirty optical interconnects. In either case, the end result is reduced modulated optical power reaching the transceiver disposed at the opposite end of the link. 
     This process of measuring the optical power incident on a transceiver can also be performed in an effort to predict potential failures in the future. However, the skill of the technicians which perform the measurements becomes paramount as the orientation of the fiber in relation to the optical power meter can significantly effect the amount of power measured. This along with the amount of time, effort, and manpower, as well as the reduced system capacity, that accompany this approach make service providers which use fiber optic links reluctant to use this process. 
     Therefore, a significant need exists in the art for a manner of monitoring the optical power incident on a transceiver in a fiber optical link without breaking the link. 
     SUMMARY OF THE INVENTION 
     The invention addresses these and other problems associated with the prior art by providing an apparatus, circuit arrangement, and method that monitor the optical power received by a transceiver in a fiber optical link without having to break the link. The optical power incident on a photo-detector configured to receive an optical from an optical fiber is monitored in part through the use of a receiver circuit coupled to the photo-detector and configured to generate a data signal representative of information encoded in the received optical signal and a monitor signal that is proportional to the peak-to-peak optical power of the received optical signal. By providing a signal which is proportional to incident peak-to-peak optical power, a host system such as a computer or other data processing system has the ability to read a signal and calculate, as desired, the peak-to-peak optical power received by a transceiver in a fiber optic link without breaking the link. Thereby, the host system is further able to determine laser performance and preempt laser failure of a laser disposed in another transceiver at the other end of a link that may not be detected by an average power meter. 
     In one embodiment consistent with the invention, a monitor voltage is presented directly to an analog-to-digital converter within the host system. In another embodiment of the invention, the monitor signal is digitized in a receiver circuit and delivered to the host system via a digital interconnect. In the embodiments, the host system is operative, using coefficients located in the transceiver nonvolatile memory, to calculate the optical power by reading the monitor signal. As such, the host system is capable of monitoring the peak-to-peak optical power received by a transceiver in a fiber optic link without disrupting the link. 
     These and other advantages and features, which characterize the invention, are set forth in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, and of the advantages and objectives attained through its use, reference should be made to the Drawings, and to the accompanying descriptive matter, in which there are described exemplary embodiments of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a photo-detector disposed in a fiber optic communication link transceiver supported by a host system, illustrating the signals used for monitoring the peak-to-peak optical power incident on the transceiver in a manner consistent with the present invention. 
     FIG. 2 is a block diagram of an embodiment of the receiver optical power meter circuit of FIG.  1 . 
     FIG. 3 is a block diagram of two systems, each incorporating a fiber optic communication link transceiver having the configuration shown in FIG. 1, connected by a duplex fiber optic link. 
     FIG. 4 is a flowchart illustrating the program flow of a process of monitoring the peak-to-peak optical power incident on the transceivers in the host systems of FIGS. 1 and 3. 
    
    
     DETAILED DESCRIPTION 
     Turning to the Drawings, wherein like numbers denote like parts throughout the several views, FIG. 1 illustrates a block diagram of a fiber optic communications link transceiver  10  supported by a host system  12  consistent with the present invention. As further shown in FIG. 1, a fiber optic data link transmitter  14  is linked to transceiver  10  through an optical fiber  16 . The transmitter  14  transmits data in the form of modulated light as is well known in the art, and may be part of a transceiver, incorporating an embodiment of the invention as will be shown in conjunction with FIG. 3 hereinafter. 
     The optical fiber  16  is coupled to the transceiver  10  via a lens  18  incorporated into an electro-optical package with a photodetector  20 , such as a photo-diode, and focuses modulated light from the transmitter  14  and carried by the fiber  16  onto the photodetector  20 . The light impinging on the photodetector  20  modulates the current through the photodetector  20  in response to variations in the light representing digital data carried by the fiber  16 . However, other electronic devices having functionality similar to the photodetector and packaged in a like or different manner could be used without departing from the scope of the invention. 
     The variations in the current through photodetector  20  cause a transimpedance amplifier  24  to sink more or less current from power supply (V DD ) in relation to the digital data. The general use and configuration of the transimpedance amplifier in the exemplary environment is described, for example, in U.S. patent application Ser. No. 09/761,526, filed on Nov. 16, 2000 by Stephen J. Ames et al., entitled “TRANSIMPEDANCE AMPLIFIER WITH AN IN-SITU OPTICAL POWER METER”, the disclosure of which is incorporated by reference herein. 
     Here, as in most systems, a photo-diode is used in the transceiver to demodulated the optical signal thus generating a data signal. As previously mentioned, the photo-diode is typically integrated into an electro-optical package and receives the light through a lens. Further, in certain implementations, a transimpedance amplifier may also be integrated into the electro-optical package. These electro-optical packages having a lens and containing a photo-diode and a transimpedance amplifier are generally limited in the number of pins available for external connection. This pin restriction limits the technology useable for power monitoring to simple AC signal conditioning. As such, these transceivers provide no externally readable monitor feature which describes the laser power incident on the transceiver. Rather, the only indicator available from the photo-diode is that which carries the data signal. The herein described embodiments process the data signal to overcome this pin limitation as well as provide additional benefits as described herein below. 
     One skilled in the art will recognize that the present invention is not limited those implementations in which a photo-diode and a transimpedance amplifier are disposed in the same electro-optical package; but rather, applies in the broader context of all implementations wherein a data signal is processed to provide a monitor signal that is proportional to the peak-to-peak optical power. 
     Included in the transimpedance amplifier  24  is a circuit that extracts the peak-to-peak content of the modulated input current (I IN ), which is designated as I P-P  in FIG.  1 . This circuit is known in the art as a full-wave rectifier and many take the form of a variety of circuit topologies including rectifiers, diodes, bridges, as well as other devices that may be configured to have similar functionality. Internal to the transimpedance amplifier  24 , the rectified peak-to-peak current (I P-P ) is used to control the common-mode voltage between the differential data outputs  38 . 
     Two resistors  26  are connected in series across the differential outputs  38  and develop a common-mode voltage (V CM ) at node  36 . The values of the resistors  26  are preferably selected so as to not excessively load the differential data outputs  38  of the transimpedance amplifier  24 . For example, and as is well know in the art, the values of the resistors  26  may be selected so their value when connected in series is at least ten times the differential output impedance of the transimpedance amplifier  24 . This ensures the signal integrity of the transimpedance amplifier  24  differential data outputs  38 . 
     Also present at the node  36  between the resistors  26  where the common-mode voltage (V CM ) is developed, are AC signal components associated with the digital data, the rectification process, imbalances between the differential data outputs  38 , etc. that must be filtered out so that the common-mode voltage (V CM ) is representative of the peak-to-peak optical power incident on the photo-detector. As such, the AC signal components present at the node between the resistors  26  are referred to as having frequency content that is “broadband” in nature, referring to the fact that both high frequency and low frequency components are present. 
     The filtering of these broadband components is accomplished through the use of a capacitor  28  in conjunction with resistors  26 . Together the resistors  26  and the capacitor  28  form a first order resistance/capacitance (RC) filter, as is also well known in the art. The value of the capacitor  28  is selected so that the RC time constant of the filter addresses the broadband nature of the AC signal components. 
     Both the high frequency and low frequency components are of concern. The selection of the RC time constant is primarily targeted at maintaining enough low frequency content to avoid a drop in the common mode voltage (V CM ). Since it is possible for the digital data to have repetitive sequences of logic “1 ” and “0,” the integrated power in the waveform will be reduced if the low frequency cut-off is too high. In addition, the RC time constant must be selected so that the filter “smooths” or “averages out” the high frequency content of the common mode voltage (V CM ). When the RC time constant is properly selected, the common-mode voltage (V CM ) is representative of the peak-to-peak optical power incident on the photo-detector  20 . The common-mode voltage (V CM ) is coupled to a receiver output power meter circuit (RX_OPM CKT)  30 . 
     The receiver output power meter circuit (RX_OPM CKT)  30  is functional to condition the common-mode voltage (V CM ) so that an analog output signal (RX_OPM), which is proportional to the peak-to-peak optical power incident on the photo-detector  20 , is presented on a signal path  40  to an analog to digital convertor in the host system  12 . In alternative embodiments of the present invention, the transceiver  10  may include an analog to digital convertor such that a digital representation of the output signal (RX_OPM) is presented to the host system  12  in either a serial or parallel data format. As described, a receiver circuit may include, either alone or in combination, a transimpedance amplifier, a filter, an output power meter circuit and an analog to digital convertor, as well as other components that will be apparent to one of ordinary skill in the art having the benefit of the instant disclosure. 
     The host system  12  is then functional, with coefficients downloaded from a memory  42  via a bus  44  (e.g., such as a serial bus), to calculate the received peak-to-peak optical power (P IN ) The coefficients are determined during the manufacturing process of the transceiver  10  and stored in memory  42 , such as non-volatile memory, thereby characterizing the performance of the photo-detector  20  used in conjunction with the transimpedance amplifier  24 . 
     Due to the non-linearities of the receiver optical power meter circuit  30  with respect to the received optical power (P IN ), a polynomial equation may be used to calculated the received peak-to-peak optical power (P IN ) For example, a polynomial equation which has the form: P IN =A 0 +A 1 *(RX_OPM)+A 2 *(RX_OPM) 2 , wherein the host system  12  has downloaded the coefficients A 0 , A 1 , and A 2 , may be used to calculate the peak-to-peak optical power (P IN ) 
     Alternatively, a threshold level for the output signal (RX_OPM) may be set whereby the host system  12  selects between the polynomial equation described hereinabove and a linear equation when calculating the peak-to-peak optical power (P IN ). In the later case, a linear equation may have the form: P IN =A 3 +A 4 *(RX_OPM), where the host system  12  has also downloaded the coefficients A 3  and A 4 . 
     Hence, by reading the instantaneous RX_OPM voltage and plugging it into an equation, a computed peak-to-peak optical power is accomplished. Thus, the present invention allows the fiber optic link to function as a data link while monitoring the peak-to-peak optical power at all times. 
     Further, the present invention provides a peak-to-peak optical power meter rather than the usual average power meter. The peak-to-peak optical power is more useful in that it directly measures the modulated optical character of a signal carried by an optical fiber. It is well know that laser transmitters having failing laser diodes attempt to compensate by increasing the DC bias current to the laser in order to maintain constant output power. However, once the modulated current limit is reached, further increases in DC bias current raise the DC optical output power, but not the modulated output power. This increase in the DC optical power may, in effect, compensate for the loss of modulated output power when measuring with an average power meter. Thus, when measuring with an average power meter a failing laser diode may not be detected. However, the present invention would detect the reduction in the modulated output power indicating a failure of the laser diode. 
     Turning now to FIG. 2, an embodiment of the receiver optical power meter circuit  30  of FIG. 1 is illustrated. The features of the embodiment in FIG. 2 include temperature compensation, power supply collection, and voltage offset correction or gain. However, other embodiments may provide any of these features alone or in combination without departing from the spirit of the invention. 
     For example, the transimpedance amplifier  24  from which the common mode voltage (V CM ) originates, as shown in FIG.  1  and incorporated by reference herein, may have thermal characteristics that affect the accuracy of the peak-to-peak optical power measured. Similarly, the transimpedance amplifier  24  may be subject to power supply variations that affect the accuracy of the peak-to-peak optical power measured. In addition, the common mode voltage (V CM ) may need to be amplified or offset so that it is able to be resolved by certain host analog-to-digital convertors. 
     Turning to the particulars of the circuit in FIG. 2, a commonly available integrated circuit having a thermal behavior characteristic that is inversely proportional to the transimpedance amplifier  24  thermal behavior is used to provide temperature compensation thereby flattening the overall thermal response of the common mode voltage (V CM ). As shown in FIG. 2, a National Semiconductor LM20 micro SMD temperature sensor  50  with a sensor gain of −11.7 mV/°C. is employed. However, other temperature sensors having suitable thermal characteristics for the referenced transimpedance amplifier  24  or other transimpedance amplifiers may be optionally used without departing from the spirit of the invention. 
     Another commonly available integrated circuit is used to compensate for variations in the transimpedance amplifier power supply (V DD ) by providing a precision voltage reference. As also shown in FIG. 2, a National Semiconductor LMV431A adjustable precision shunt regulator is configured as a precision voltage reference  52  thereby compensating for variations in the power supply (V DD ). Again, compensation for variation in the power supply may be used as desired without departing from the scope of the invention. 
     An operational amplifier  54  serves as the summing junction for the thermal compensation and supply voltage compensation for the common mode voltage (V CM ) and provides gain compensation. As shown, half of a National Semiconductor LMC6035 low power dual operational amplifier  54  raises the gain so that the minimum value of the signal RX_OPM  40  is of sufficient magnitude to be resolved by an analog to digital convertor in the host system  12 . As shown, the voltage gain of the operational amplifier  54  is set at 2 based on the values selected for resistors  68 ,  70 ,  72 , and  78 , as is well know in the art. Exemplary values for resistors  68 ,  70 ,  72 , and  78  are provided hereinafter in tabular format. 
     More specifically, the temperature sensor  50  biases the base of one of a differencing pair of transistors  56 , and along with resistors  60 ,  62 , and  64  sets the nominal voltage at the inverting input of the operational amplifier  54 . The base of the other differencing transistor  58 , preferably housed in the same package with matched gain and similar thermal characteristics as differencing transistor  56 , is biased by a voltage divider formed by resistors  74  and  76  and sets the nominal voltage at the non-inverting input of the operational amplifier  54  along with resistor  66 . Thus, when the temperature changes or the power supply voltage (V DD ) fluctuates, the nominal voltage seen at the inverting input of the operational amplifier  54  is adjusted to compensate so that the common mode voltage (V CM ) presented to the non-inverting input of the operational amplifier  56  is proportional to the peak-to-peak optical power incident on the photo-detector  20 . Exemplary values for resistors  60 ,  62 ,  64 ,  66 ,  74 , and  76 , as well  68 ,  70 ,  72 , and  78  are provided in the following table: 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 FIG. 2 Exemplary Resistor Values 
               
             
          
           
               
                   
                   
                 Value 
               
               
                   
                 Resistor 
                 (k ohms) 
               
               
                   
                   
               
               
                   
                 60, 62, 74, 76 
                 1  
               
               
                   
                 64 
                 16.2 
               
               
                   
                 66 
                 40.2 
               
               
                   
                 68, 70 
                 100   
               
               
                   
                 72, 78 
                 200   
               
               
                   
                   
               
             
          
         
       
     
     While certain values and types of components have been discussed in conjunction with the description of the receiver optical power meter circuit  30 , such values and components merely exemplify a variety of components and values that may be selected by one skilled in the art. As such, it will appreciated that different embodiments may use different components having different values without departing from the spirit of the invention. 
     Turning to FIG. 3, a block diagram of two systems  102 ,  104 , each containing the transceiver  10  described hereinbefore, connected by a duplex fiber optic link  100  is shown. In these systems  102 ,  104 , the hosts  12  download via a bus  44  the coefficients A 0 , A 1 , A 2 , A 3 , and A 4 , necessary to calculate the peak-to-peak optical power received from the duplex fiber optic link  100 . Once the downloads are complete, the hosts  12  may sample the RX_OPM voltages on lines  40 . The hosts  12  may then, using the respective downloaded coefficients (A 0 , A 1 , A 2 , A 3 , and A 4 ), utilize the afore-provided equations to calculate the peak-to-peak input power (P IN ) of each transceiver  10 . The hosts  12  may then be made operational using software to determine the “health” of the laser. Thus, an advantage of the present invention is that a host may be disposed to monitor the link  100  in “real-time” and predict laser failure. Notice of such potential failure could be provided to a system manager that could then schedule maintenance before the link failed. The present invention also provides for collection of historical performance data on lasers used in such links. As such, the systems  102 ,  104  have a predictive tool that is able identify substandard lasers and replace them before a serious, and potentially costly, data disruption occurs. 
     Once again, it will be appreciated that embodiments of the present invention discussed in conjunction with FIGS. 1 and 2 apply equally as well in the systems  102 ,  104  shown in FIG.  3 . Further, the systems  102 ,  104  in FIG. 3, could be a telephone and/or data system or a telecommunication system including any voice and data application. Similarly, the hosts  12  depicted FIG. 3 could be any computer or data processing system that has the ability to read and/or monitor a signal which is proportional to the input peak-to-peak optical power of the transceivers  10 . One skilled in the art will understand that although a host was shown for each system in FIG. 3, a single host could be configured to download coefficients, read signals, and calculate power from a plurality of transceivers or lasers. 
     Turning now to FIG. 4, a flowchart illustrating the program flow of a process  200  of monitoring the peak-to-peak input power of the hosts  12  of FIGS. 1 and 3 is shown. The process  200  begins by powering up a system  102 ,  104  and initializing the software as shown in block  80 . Control is then passed to block  82  which locally determines whether the host  12  can communicate with the transceiver  10 . If communication cannot be established, an error tag is set at block  92  and the process ceases. If communication is established, control of the process is passed to block  84 . Block  84  locally determines whether the transceiver  10  is in fault status. If the transceiver  10  is in fault status, once again, an error flag is set in block  92  and the process ceases. However, if the transceiver is not in fault status, control is passed to block  86 . In block  86 , the host  12  requests the coefficients (A 0 , A 1 , A 2 , A 3 , and A 4 ) necessary to calculate the peak-to-peak input power (P IN ). Next, block  88  continually loops control of the process to ensure that the host  12  receives the coefficients (A 0 , A 1 , A 2 , A 3 , and A 4 ). Once the coefficients (A 0 , A 1 , A 2 , A 3 , and A 4 ) are received by the host  12 , control is passed to block  90 . In block  90  the host  12  reads the RX_OPM voltage, be it represented in analog or digital format, and calculates the peak-to-peak input power (P IN ). Once the peak-to-peak input power (P IN ) has been calculated, control passes to block  96  wherein it is determined whether or not the peak-to-peak input power (P IN ) is acceptable. If the peak-to-peak input power (P IN ) is acceptable, control is returned to block  90  and the process proceeds as before. If the peak-to-peak input power (P IN ) is not acceptable, a warning flag is set in block  94  which signifies that a substandard laser diode should be replaced before a serious, and potentially costly, data disruption occurs. 
     Those skilled in the art will recognize that the exemplary environments illustrated in FIGS. 1,  2 ,  3 , and  4  are not intended to limit the present invention. Indeed, those skilled in the art will recognize that other alternative hardware and/or software environments may be used without departing from the scope of the invention.