Abstract:
The method and apparatus for monitoring a photo-detector generates a highly compliant mirror current across a broad range of photo-detector current levels. The apparatus for monitoring includes: a pair of bipolar transistors and a first non-linear isolation element. The pair of transistors are connected in a mirror configuration with a sense transistor one of the pair of transistors sensing a photo-detector current and with a mirror transistor one of the pair of transistors mirroring the photo-detector current with a mirror current. The first non-linear isolation element has at least two terminals a first of which couples to the collector of the mirror transistor. The first non-linear isolation element exhibits a non-linear voltage drop between the at least two terminals in response to varying levels of the mirror current to improve compliance between the mirror current and the detector current. Methods and means for monitoring a photo-detector are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of Invention 
   The field of the present invention relates in general to optical networks and more particularly to optical receivers. 
   2. Description of the Related Art 
   In communication systems light beams are increasingly used for transmitting information. The demand for communication bandwidth has resulted in a conversion of long and short haul communication trunk lines from copper to fiber optic (digital) communication. The wide spectral characteristics of fiber optics support broadband signals at very high data rates, gigabits per second. 
   Generally, an optical source, i.e. transmitter, converts an electrical signal, either digital or analog, to a modulated light beam which is then passed through an optical fiber to an optical detector, i.e. receiver, that extracts an electrical signal from the received light beam. A fiber may be shared with different communication channels using frequency, time or other forms of multiplexing. A typical optical link extends the range of a communication system with a transceiver unit that handles opto-electronic conversion between an optical fiber(s) and local area networks (LAN) on opposing ends of the fiber. Optical transceivers offer gigabit communication rates over long haul trans-oceanic cables or short range links in a metropolitan area. 
   Monitoring of optical transceivers is employed for diagnostic or preventive maintenance purposes. Monitored parameters include: laser bias current, transmit optical power, receive optical power, temperature, etc. A typical transmitter operates at a fixed power level. Since an optical link may range in distance from several meters to a hundred kilometers the optical receiver must function at a broad range of received signal strengths. Receiver monitoring is employed to assure the received signal is in appropriate range to ensure proper decoding of data at receiver. 
   What is needed are new means for monitoring optical receivers. 
   SUMMARY OF THE INVENTION 
   A method and apparatus is disclosed for monitoring a photo-detector which may be part of an optical receiver or transducer. The monitoring of the photo-detector may be used to determine the strength of a received optical signal during setup or normal operation of an optical communication system. During normal operation received signal strength can be used to determine component aging. The monitor generates a mirror current which is highly compliant with the photo-detector current across a broad current range. The linearity of the monitor circuit makes it particularly suited for optical networks such as telecommunication networks with a broad range of lengths between network transceiver nodes. 
   In an embodiment of the invention the apparatus for monitoring includes: a pair of bipolar transistors and a first non-linear isolation element. The pair of transistors are connected in a mirror configuration with a sense transistor one of the pair of transistors sensing a photo-detector current and with a mirror transistor one of the pair of transistors mirroring the photo-detector current with a mirror current. The first non-linear isolation element has at least two terminals a first of which couples to the collector of the mirror transistor. The first non-linear isolation element exhibits a non-linear voltage drop between the at least two terminals in response to varying levels of the mirror current to improve compliance between the mirror current and the detector current. 
   In alternate embodiments of the invention method and means for monitoring a photo-detector are also disclosed and claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other features and advantages of the present invention will become more apparent to those skilled in the art from the following detailed description in conjunction with the appended drawings in which: 
       FIG. 1  shows a plurality of optical transceivers coupled to one another to form a wide area network (WAN). 
       FIG. 2A  is a circuit diagram of an embodiment of the invention with an optical transceiver having a monitoring circuit coupled on the high side of the optical receiver to monitor the received signal strength. 
       FIG. 2B  is a circuit diagram of an embodiment of the invention with an optical transceiver having a monitoring circuit coupled on the low side of the optical receiver to monitor the received signal strength. 
       FIGS. 3A–B  are detailed circuit diagrams of alternate embodiments of the monitor circuit shown in  FIG. 2A . 
       FIGS. 4A–B  are detailed circuit diagrams of alternate embodiments of the monitor circuit shown in  FIG. 2B . 
       FIG. 5  is a graph showing the photo-detector current (Ip) in the optical receiver and the monitor current (Im) in the monitor circuit for various embodiments of the invention. 
   

   DETAILED DESCRIPTION OF THE EMBODIMENTS 
     FIG. 1  shows a plurality of optical transceivers coupled to one another to form a wide area optical network (WAN) which handles communications between a plurality of local area networks (LANS). LANS  112 ,  132  are shown in a corporate headquarters  100 . LAN  162  is shown in the corporate warehouse  150 . LAN  182  is shown in the corporate factory  170 . All LANS are linked by a high speed optical backbone network. Optical segments of the optical network may vary in length from a few meters to hundreds of kilometers. At each corporate location the corresponding LAN provides electrical communication links to networked workstations, servers, process machinery, etc. To handle the high bandwidth communication between LANs optical transceiver cards  110 ,  130 ,  160  and  180  are shown coupled to LANS  112 ,  132 , 162  and  182  respectively. These optical transceiver cards are optically linked together and form the nodes of the optical backbone network which handles high speed communications between the LANs. 
   In the embodiment shown each optical transceiver card comprises a pair of dual port transceivers and a router. Each transceiver card forms a node of the optical network. The optical network links the LANs. The transceivers cards are coupled in a dual ring topology with ring segments  190 , 192 ,  194 ,  196 . Each transceiver is shown receiving and transmitting optically modulated information from either of the dual rings. Each transceiver has a receive port and a transmit port coupled to the corresponding segments of each ring. Information can be thought of as passing clockwise in one ring and counterclockwise in the other of the dual rings. All transceivers perform an optical-to-electrical conversion on received data which is then analyzed by an integral router to determine its destination. If the information is determined by one of the routers to be destined for a LAN to which the transceiver card is coupled then the router offloads the information from the optical network to the corresponding LAN. 
   Optical card  110  includes two transceivers  118 ,  120  and a router  114 . The router  114  couples through a LAN interface  116  with LAN  112 . The transceivers couple via corresponding pairs of transmit and receive ports to the corresponding network segments which form the dual optical ring topology. Fiber optic cables  126  and  124  handle the transmission and reception respectively of information between line cards  110  and  180  via ring segments  196 . Another pair of fiber optic cables (not shown) handle the transmission and reception of information between line cards  110  and  130  via ring segments  190 . Monitoring of the transceivers  118 – 120  occurs via the monitor interface  122 . 
   Optical card  130  includes two transceivers  138 ,  140  and a router  134 . The router  134  couples through a LAN interface  136  with LAN  132 . The transceivers couple via corresponding pairs of transmit and receive ports to the corresponding network segments which form the dual optical ring topology. Fiber optic cables (not shown) handle the transmission and reception of information between line cards  130  and  160  via ring segments  192 . Monitoring of the transceivers  138 – 140  occurs via the monitor interface  142 . Line cards  160  and  180  are coupled to one another with ring segments  194 . 
   Monitoring is employed for diagnostic or preventive maintenance purposes such as determining when to replace a component based on monitored parameters. Component aging or life cycle stage can be determined based on monitored parameters, with the result that components are replaced before failure. Monitored parameters include: laser bias current, transmit optical power, receive optical power, temperature, etc. A typical transmitter operates at a fixed power level. Since an optical link may range in distance from several meters to a hundred kilometers the optical receiver must function at a broad range, e.g. 4–6 orders of magnitude, of received signal strengths. Receiver monitoring is best done without attaching a circuit to the same node of the photodiode, cathode or anode, from which the high speed modulated signal is extracted. Connecting both monitor circuit and high speed data circuit to the same node increases the complexity of the high speed data circuit without providing compensating benefits. Attaching the monitor circuit to photodiode node which is not connected to the high speed data circuit allows for a simple overall design. 
   The following circuits are designed to monitor received signals with strengths varying by 4–6 orders of magnitude without effecting received signal quality. 
     FIG. 2A  is a circuit diagram of an embodiment of the invention with an optical transceiver  118  (See  FIG. 1 ) with a transmitter  200 , a receiver  220  and a monitoring circuit  260 . The transmitter  200  includes one or more differential signal inputs  206 – 208  for high speed digital data input from the associated LAN and a photo-diode  202  which transmits an optical beam  204  modulated with the high frequency data into the corresponding fiber optic  126  which forms a segment of the optical network. 
   The receiver includes a photo-detector  242  optically coupled to the optical network via fiber optic  124  to receive an optically modulated information bearing signal there from. In an embodiment of the invention the photo-detector comprises a positive-intrinsic-negative (PIN) or an avalanche photo-detector (APD). A PIN operates at a 5 volt bias voltage level while an APD may operate at 40–60 volts. The signal strength of the received optical beam may vary over 4–6 orders of magnitude as represented by beams  244  and  246 . In the embodiment of the invention shown in  FIG. 2A  the cathode of the photo-detector is coupled to the input of a trans-impedance amplifier  236  which operates as a current sink for the photo-detector. The TIA has a wide dynamic range and exceptional linearity performance. A typical telecommunications application requires the TIA to maintain a linear trans-impedance characteristic for input currents ranging from less than 0.01 uA up to 2.5 mA. The high frequency modulated and amplified electrical data is output from the TIA on differential signal lines which are impedance matched and AC coupled with the rest of the receiver circuitry via AC coupler  224 . In the embodiment shown, the AC coupler includes high frequency coupling capacitors  230 – 232  which form together with series and parallel coupled resistors  234 ,  226 ,  228  an impedance matching network. The differential output of the AC coupler is subject to any post amplification in amplifier  222  and the opto-electrically converted signal is output by the receiver on one or more signal lines  246 – 248  to the rest of the receive path circuitry, e.g. the router  114  for example (See  FIG. 1 ). 
   A monitor circuit  260  couples on the high side of the receiver to monitor the received signal strength as measured by the photo-detector  242 . The monitor circuit includes a current mirror  276 . The current mirror has two legs, a.k.a a photo-detector leg and a mirror leg, through which pass the photo-detector current “Ip” and a mirror current “Im” respectively. Both legs of the current mirror couple on the positive side to a voltage source node  278  which in the example shown is a DC power supply  270 . The control leg of the current mirror couples via line  280  with the anode of the photo-detector  242 . In the example shown the supply voltage is 60 volts and the photo-detector is an APD. In alternate embodiments of the invention a PIN type photo-detector may be utilized with a corresponding reduction in the supply voltage level to 5 volts for example. The mirror leg  282  of the current mirror supplies the mirror current Im on line  282 , the level of which corresponds with the received optical signal level as detected by the photo-detector. In the embodiment shown, Im is monitored by conversion to a voltage proportional to current at monitor node  284 . This is accomplished by coupling the monitor node resistively to an electrical sink  286 . Resistor  274 , which couples the monitor node to ground, is used to perform this function. The monitor node  284  is coupled to the input of an operational amplifier  268 . Op-amp  268  provides an amplified output proportional to the mirror current to an analog-to-digital (A/D) converter  266 . A micro-controller  262  accepts the digital signal output of the A/D where it may be utilized as part of a diagnostic. The output of the micro-controller is supplied via monitor control line  290  to the monitor interface  116  (See  FIG. 1 ) for use by network administrator or system level diagnostic and or maintenance circuitry, not shown. 
     FIG. 2B  is a circuit diagram of an embodiment of the invention with an optical transceiver having the monitoring circuit  260  coupled on the low side of the optical receiver  220  to monitor the received signal strength. The receiver includes the photo-detector  242  optically coupled to the optical network via fiber optic  124  to receive the optically modulated information bearing signal there from. In an embodiment of the invention the photo-detector comprises a positive-intrinsic-negative (PIN) or an avalanche photo-detector (APD). The signal strength of the received optical beam may vary over 4–6 orders of magnitude as represented by beams  244  and  246 . In the embodiment of the invention shown in  FIG. 2B  the anode of the photo-detector is coupled to the input of a trans-impedance amplifier  236  which operates as a current source for the photo-detector. The TIA has a wide dynamic range and exceptional linearity performance over currents ranging from less than 0.01 uA up to 2.5 mA. The TIA has a Vsource input  240  coupled to the DC power supply  270  which is part of the monitor circuit. The Vsink input  238  of the TIA is coupled to a voltage sink at a level less than that of the power supply. The TIA supplies current to the anode of the photo-detector to which its input is coupled. The high frequency modulated and amplified electrical data is output from the TIA on differential signal lines to the AC coupler  224 . The output of the AC coupler is subject to amplification in post amplifier  222 . The output of the post amplifier is coupled to one or more high frequency data outputs  246 – 248 . 
   The monitor circuit  260  couples on the low side of the receiver to monitor the received signal strength as measured by the photo-detector  242 . The monitor circuit includes the current mirror  276 . The current mirror has two legs, a.k.a. a photo-detector leg and a mirror leg, through which pass the photo-detector current “Ip” and a mirror current “Im” respectively. Both legs of the current mirror couple on the negative side to a voltage sink at node  278  which in the example shown is an analog ground. The control leg of the current mirror couples via line  280  with the cathode of the photo-detector  242 . The mirror leg  282  of the current mirror supplies the mirror current Im on line  282 . The level of the mirror current corresponds with the received optical signal level as detected by the photo-detector. In the embodiment shown, Im is monitored by conversion to a voltage proportional to current at monitor node  284 . This is accomplished by resistor  274  which couples the monitor node to an electrical source, e.g. Vcc=5 Volts. The monitor node  284  is coupled to the input of the operational amplifier  268 . The op-amp provides an amplified output proportional to the mirror current to an analog-to-digital (A/D) converter  266 . The micro-controller  262  accepts the digital signal output of the A/D where it may be utilized as part of a diagnostic. The output of the micro-controller is supplied via monitor control line  290  to the monitor interface  116  (See  FIG. 1 ) for use by network administrator for system level diagnostic and/or maintenance. 
   The wide operational current range of the photo-detector places a significant demand on the current mirror in terms of linearity and range of performance, e.g. currents ranging over 4–6 orders of magnitude. The current mirrors shown in the following FIGS.  3 A–B and  4 A–B meet these requirements. 
     FIGS. 3A–B  are detailed circuit diagrams of alternate embodiments of the monitor circuit shown in  FIG. 2A  and specifically the current mirror  276  portion thereof. The current mirror includes a pair of back-to-back bipolar type transistors  302  and  304  configured as a current mirror. The sense transistor  302  defines the photo-detector (Pd) leg  330  of the current mirror in which flows the photo-detector current Ip reference  320 . The mirror transistor  304  defines the monitor leg  332  in which flows the mirror current Im reference  322 . The bases of the sense and mirror transistors are coupled to one another and to the collector of the mirror transistor. In the high side embodiment shown in  FIGS. 3A–B  the sense and mirror transistors comprise ‘pnp’ type bipolar transistors. 
   The performance of the current mirror formed by the pair of transistors  302 – 304  alone is unacceptable, because the mirror current Im generated by the combination of these transistors is limited for practical purposes to an upper range of 5 orders of magnitude. Even within that range the mirror formed by the sense and mirror transistors alone is highly non-linear. Ip and Im differ both in absolute magnitude across the range, e.g. more than 75% difference; as well as in the linearity of the relative magnitudes across the range, e.g. 10% variation. 
   The Ebers-Moll model of the bipolar transistor provides insight to and quantification of the source of non-linearity and is set forth in the following Equation 1: 
         Equation   ⁢           ⁢   1   ⁢     :       ⁢               
         I   c     =       I   o     (       ⅇ       cV   be     kT       -   1     )         
 
where Ic is the collector current, Vbe is the base to emitter voltage drop, Io is the reverse leakage current from the emitter to the base, c is the elementary unit of charge, k is the Boltzmann constant, and T is the absolute temperature (in Kelvin). With typical doping levels, the leakage current arising from the “intrinsic” behavior of the pure semiconductor is very small, and the second term −Io is negligible, giving a simple exponential dependence of Ic on Vbe.
 
   An extension to Ebers-Moll that must be considered in current mirrors is that of the Early effect. The Early effect describes the proportionate change in base-to-emitter voltage for bipolar transistors which occurs with changes in collector-to-emitter voltage. The non-linearity between the mirror current and the photo-detector current results from the differences in the collector-to-emitter voltage drops in the two transistors and the concomitant difference in the base-to-emitter voltages of the two transistors due to the Early effect. 
   Collector-to-emitter voltage differences can be 40 volts in an APD and 4 volts in a PIN implementation. The sense transistor is typically exposed to a voltage drop of 0.7 volts while the mirror transistor is subject to a voltage drop substantially equal to the full supply voltage. The collector currents in the sense and mirror transistors is very sensitive to differences in the base-to-emitter voltages between the two transistors. The disparity in base-to-emitter voltage drops between the sense and mirror transistors results in substantial and non-linear difference between the mirror current and the photo-detector current. 
   To reduce the difference in the collector-to-emitter voltage drops between each of the pair of transistors  302 ,  304  a non-linear isolation element is introduced into the mirror leg, with one terminal coupled to the collector of the mirror transistor  304  and an other terminal coupled to the monitor node  284 . Suitable non-linear isolation elements include: a Schmidt or Zener diode, a field effect transistor, and a bipolar transistor. Each of these non-linear isolation elements exhibit a non-linear voltage drop between the at least two terminals in response to varying levels of the mirror current. The voltage drop between the at least two terminals is substantially independent of mirror current. This characteristic improves compliance between the mirror current and the photo-detector current by reducing the collector-to-emitter and hence the base-to-emitter voltage differences between the sense and mirror transistors. The disparity in performance due to the Early effect is therefore substantially reduced. 
   Compliance is defined as the quotient of Ip/Im. In the embodiment shown in  FIG. 3A  the non-linear isolation element is a bipolar transistor  306  with the emitter terminal coupled to the collector of the mirror transistor and the collector coupled to the monitor node  284 . The base is coupled via signal line  312  to the collector of the sense transistor  302 . The compliance of this current mirror in an APD implementation is shown in  FIG. 5  line  520 . 
     FIG. 3B  shows an alternate embodiment of the high side current mirror  276  in which another non-linear isolation element is added photo-detector leg between the sense transistor  302  and the photo-detector  242 . Suitable non-linear isolation elements include: a Schmidt or Zener diode and a bipolar transistor. In the embodiment shown in  FIG. 3B  the non-linear isolation element is a bipolar transistor  308  with the emitter terminal coupled to the collector of the sense transistor and the collector coupled to the photo-detector  242 . The base is coupled to the collector of the sense transistor  302 . The compliance of this current mirror in an APD implementation is shown in  FIG. 5  line  530 . 
   In the embodiments shown in  FIGS. 3A–B  the emitters of the sense and mirror transistors couple to the voltage source  278  via a resistor  300 . This resistor is appropriate for embodiments of the invention in which the photo-detector comprises an APD type. Resistor  300  serves the function of varying the supply voltage inversely with respect to the strength of the received optical signal. Thus the supply voltage to the current mirror is reduced as the optical signal strength increases, thereby improving the performance of the APD, Such resistor would not be necessary in an embodiment of the invention in which a PIN type photo-detector was utilized. 
     FIG. 3B  further illustrates alternative embodiments of suitable non-linear isolation elements that can be used in the monitoring circuit  260 . The elements illustrated in  FIG. 3B  in the block  341 , like the bipolar transistor  306 , are examples of alternative non-linear isolation elements that may be used to improve compliance between the mirror current and the photo-detector current. Non-linear element  340  is a Schmidt or Zener diode, and element  342  is a field effect transistor. Each of the non-linear elements illustrated in block  341  may be connected between a terminal  346 , which may be a collector of the transistor  304 , and a terminal  348 , which may be the monitor node  284 . 
     FIGS. 4A–B  are detailed circuit diagrams of alternate embodiments of the monitor circuit shown in  FIG. 2B  with the monitor circuit  260  coupled on the low side of the receiver to monitor the received signal strength as measured by the photo-detector  242 . The monitor circuit includes the current mirror  276 . The current mirrors shown in  FIGS. 4A–B  are similar to those shown in  FIGS. 3A–B  respectively with the exception that the transistors are ‘npn’ bipolar types with the emitters of the sense and mirror transistors coupled to a voltage sink and with the monitor node  284  coupled through resistor  274  to a voltage source. 
     FIG. 5  is a graph showing the compliance between the photo-detector current (Ip) and the monitor current (Im) for various embodiments of the invention. The x axis is current over a range from 0.01 uA to 10,000 uA. They axis is compliance expressed as the ratio Ip/Im of the photo-detector current and the mirror current for the embodiments of the monitoring circuit shown in  FIGS. 3A–B . In these embodiments the photo-detector is an APD type with a 60 volt power supply. Similar results would be obtained for a PIN type photo-detector. Ideal behavior is represented by line  500  in which compliance between Ip and Im is 100% across the entire current range spanning 6 orders of magnitude. 
   Curve  502  corresponds to the performance of a current mirror formed by the sense and mirror transistors  302 – 304  and without non-linear isolation elements, e.g. transistors  306  and  308 . Ip is a fraction of Im thus there is no compliance in absolute terms. Additionally, there is substantial non-linearity, e.g. 10% or more over only 5 orders of magnitude and no functionality over a broader range. 
   Curve  520  corresponds to the compliance of the current mirror shown in  FIG. 3A  with a single non-linear isolation element on the mirror leg. The circuit exhibits acceptable matching of absolute current magnitudes as well as substantial linearity e.g. 2.4% total deviation in compliance between Ip and Im across the current range of 6 orders of magnitude. 
   Curve  530  corresponds to the compliance of the current mirror shown in  FIG. 3B  with two non-linear isolation element, one on mirror leg and the other on the photo-detector leg. The circuit exhibits acceptable matching of absolute current magnitudes as well as substantial linearity e.g. 0.4% total deviation in compliance between Ip and Im across the current range of 6 orders of magnitude. 
   The exceptional compliance of the current mirrors shown in FIGS.  3 A–B and  4 A–B allows improved monitoring of the photo-detector portion of the optical receiver for optical network setup, diagnostics, and preventive maintenance. 
   The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously many modifications and variations will be apparent to practitioners skilled in this art. It is intended that the scope of the invention be defined by the following claims and their equivalents.