Abstract:
An optical receiver includes a light receiving element such as an avalanche photodiode (APD) for converting an optical signal to an electrical photocurrent amplified by a first current gain value and a temperature sensor for measuring the temperature of the light receiving element. The optical receiver also includes a control unit configured to control a bias voltage applied to the light receiving element such that the first gain value is adjusted to a second gain value based at least in part on a predetermined relationship between the current gain, the temperature and the applied bias voltage. The second current gain value is based at least in part on one or more parameters characteristic of the optical receiver and a system in which the optical receiver is employed.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to optical receivers, and more particularly to an optical receiver with high dynamic range that employs an avalanche photodiode (APD) for analog applications. 
     BACKGROUND OF THE INVENTION 
     An optical receiver in which a photodetector serves as a receiver element is one of the key elements in an optical fiber transmission network. Optical receivers, in general, function to convert optical signals into electrical signals. A typical optical receiver includes a photodetector connected to the input of an amplifier (e.g., a transimpedance amplifier). The photodetector converts the optical signal it receives into an electric current, also called a photo-electric current (I P ), that is supplied to the amplifier. The amplifier then generates at its output a voltage that is proportional to the electric current. The photodetector is typically either an avalanche photodiode (APD) or a PIN (positive-intrinsic-negative) photodiode. 
     Optical receivers are used in both digital and analog applications. Analog applications generally require high linearity optical receivers, but some particular applications requires high sensitivity and high dynamic range as well. This is particularly true when the receivers are used in optical networks such as Fiber To The Home (FTTH) systems, examples of which include Passive Optical Networks (PONs) and Radio Frequency over Glass (RFoG) systems. These networks incur high optical splitting losses in order to serve a high number of customers, which makes them more cost effective. For instance, an optical receiver used in such networks, which receives upstream optical signals from Customer Premises Equipment (CPE), may need a sensitivity better than −25 dBm in order to overcome their typical 28 dB link budget requirement. 
     To enhance receiver sensitivity APDs are often preferred because of their superior power sensitivity in comparison to PIN photodiodes. APDs have the capability of internally multiplying the primary photocurrent by exploiting the phenomenon known as avalanche effect (impact ionization). Unfortunately, APDs also generally have relatively poor linearity. For this reason APD optical receivers are more often used in digital applications than analog applications since in digital systems sensitivity is typically more important than linearity. 
     Accordingly, it would be desirable to provide an optical receiver that employs an APD which is optimally biased for achieving an enhanced dynamic range and adequate linearity suitable for use in analog applications. 
     SUMMARY 
     In accordance with one aspect of the invention, an optical receiver is provided that includes a light receiving element for converting an optical signal to an electrical photocurrent amplified by a first current gain value and a temperature sensor for measuring the temperature of the light receiving element. The optical receiver also includes a control unit configured to control a bias voltage applied to the light receiving element such that the first gain value is adjusted to a second gain value based at least in part on a predetermined relationship between the current gain, the temperature and the applied bias voltage. The second current gain value is based at least in part on one or more parameters characteristic of the optical receiver and a system in which the optical receiver is employed. 
     In accordance with another aspect of the invention, a method is provided for converting an optical signal to an electrical signal. The method includes detecting an optical signal with a light receiving element and converting the optical signal to an electrical signal. A first bias voltage value and a first input current value applied to the light receiving element are both determined. A first value of the photocurrent generated by the light receiving element is obtained. A desired second value of the current gain is selected based at least in part on the first value of the photocurrent and one or more parameters characteristic of the optical receiver and a system in which the optical receiver is employed. A second value of the bias voltage is obtained which causes the light receiving element to impart the second value of the current gain to the optical signal. The second value of the bias voltage is applied to the light receiving element. 
     In accordance with yet another aspect of the invention, an optical communication network includes an optical transmitter unit for generating optical signals and at least one node for providing the optical signals to a plurality of optical network units. A first link optically couples the optical transmitter unit to the optical node. The optical node includes an optical receiver for receiving optical signals from the optical network units. The optical receiver includes a light receiving element for receiving the optical signals and a control unit configured to control a bias voltage applied to the light receiving element such that the light receiving element imparts a prescribed current gain to the optical signal based at least in part on one or more parameters characteristic of the optical receiver and the optical communication network. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an example of an optical communications network that may employ an analog optical receiver. 
         FIG. 2  is a simplified block diagram of an optical receiver. 
         FIG. 3  is a simplified block diagram of an optical APD receiver constructed in accordance with the methods and techniques described herein. 
         FIG. 4  is a flowchart showing one example of a process performed by the bias controller to apply the optimum current gain to the APD detector. 
         FIG. 5  shows one example of a simple arrangement that may be used to establish the relationship between the current gain M and the applied voltage V APD . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an example of an optical communications network that may employ an analog optical receiver. While a fiber to the home (FTTH) system is shown for illustrative purposes, those of ordinary skill in the art will recognize that the network may be any type in which an optical receiver may be employed. Examples of such networks include HFC networks and passive optical networks (PONs) such as Broadband PONs and Gigabit PONs. 
     A fiber to the home system (FTTH) such as a Radio Frequency over Glass (RFoG) system is depicted in  FIG. 1 . An RFOG system provides fiber or other optical connectivity directly to the premises while using existing provisioning systems such as cable modem termination system (CMTS) platforms, headend equipment, set top boxes, conditional access technology and cable modems. A headend  205  generates and transmits optical signals downstream through fiber links, such as fiber link  210 , and, optionally, to one or more optical nodes such as node  215 . When present, each node may serve a different geographic region, depending on network design. The node  215  receives the downstream optical signals from the headend  205  and passively distributes these signals to optical network units (ONUs) such as ONU  220  using optical splitters such as splitter  225 . The node  215  also receives upstream burst mode optical signals being transmitted by the ONU  220 . 
     The ONU  220  is generally located on the customer premises and terminates the fiber connection and converts the downstream optical signal traffic into Radio Frequency (RF) signal traffic. The ONU contains a downstream optical receiver which receives the downstream optical signal and converts it into an electrical signal that can be used by various devices found at the home, such as a cable modem, setup box, and telephone. The ONU also contains an upstream burst mode optical transmitter which transmits information to the headend  215  from the various devices found in the home. The upstream and the downstream signals are transmitted over optical fibers  260  and  265  that link the optical node  215  and the ONU  220 . More specifically, within the ONU  220  a wavelength division multiplexer (WDM)  250  separates the downstream optical signals from the upstream optical signals. The downstream optical signal is directed to an optical receiver  230  that converts the optical signals to a RF signal. A diplex filter  235  then isolates the downstream RF signals from the upstream path and provides the RF signals to the customer equipment on the premises. In the reverse or upstream path, RF signals emanating from the customer equipment are transmitted to the ONU  220 . The RF signals received from the customer equipment may be digital signals, or alternatively, analog signals employing any suitable modulation scheme such as AM-VSB or Quadrature Amplitude Modulation (QAM), for example. The diplex filter  235  isolates the upstream signals from the downstream path and provides the signals to an optical transmitter  245 , which converts the RF signals to optical signals so that they can be transmitted upstream via WDM  250 . 
     In an RFoG system such as shown in  FIG. 1 , analog optical receivers may be employed, for instance, in the headend  205 , node  215  and/or ONU  220   
       FIG. 2  is a simplified block diagram of an optical receiver. The light from an optical fiber  101  impinges on a light receiving element such as an APD detector  102 , producing a photocurrent I P . The avalanche effect of the APD multiplies the primary photocurrent I P  by a factor M and produces the final APD current I APD . Transimpedance amplifier (TIA)  103  converts the relatively small current generated by the APD detector  102  into a large signal voltage, V TIA , which may be further processed by an optional equalizing circuit  104  to produce a voltage V OUT . 
     The current gain M of the APD detector  102 , which is the ratio of the output APD current I APD  from the APD detector  102  to the primary photocurrent I P , is a function of both the bias voltage (V APD ) applied to the APD detector  102  and the device temperature (T) of the APD detector  102 . In a conventional APD receiver the bias circuitry for the APD detector  102  controls its gain and dynamic range by compensating for changes in temperature. However, as previously noted, this and other approaches do not provide a sufficiently large dynamic range and optimum bias point to the APD for many analog applications. 
     The current gain M of the APD detector  102  has an optimal value for which the dynamic range of the receiver is maximized while accounting for the linearity of the optical communication system in which it is employed. This value, M opt , can be calculated from the Noise Power Ratio (NPR) of the system, which can be expressed as: 
     
       
         
           
             
               
                 
                   NPR 
                   = 
                   
                     S 
                     
                       
                         ( 
                         
                           
                             N 
                             TH 
                           
                           + 
                           
                             N 
                             SN 
                           
                           + 
                           
                             N 
                             RIN 
                           
                           + 
                           
                             N 
                             CIN 
                           
                         
                         ) 
                       
                       · 
                       B 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     Where:
         S is the signal power (A 2 /Hz)   N TH  is the thermal noise (A 2 /Hz)   N S  is the shot noise (A 2 /Hz)   N RIN  is the relative intensity noise (A 2 /Hz)   N CIN  is the CIN equivalent noise (A 2 /Hz)   CIN is the composite intermodulation noise of the system   B is the channel bandwidth (Hz)
 
M opt  is the value of M for which the derivative of NPR with respect to M is zero. By performing this calculation Mopt is found to be:
       

     
       
         
           
             
               
                 
                   
                       
                   
                   ⁢ 
                   
                     
                       
                         
                           M 
                           opt 
                         
                         ≈ 
                         
                           
                             N 
                             TH 
                           
                           
                             
                               
                                 I 
                                 P 
                               
                               · 
                               q 
                               · 
                               x 
                             
                             + 
                             
                               N 
                               CIN 
                             
                           
                         
                       
                       ⁢ 
                       
                         
 
                       
                       ⁢ 
                       Where 
                       ⁢ 
                       
                         : 
                       
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       
                         N 
                         CIN 
                       
                     
                     = 
                     
                       
                         S 
                         · 
                         CIN 
                       
                       B 
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
         
         
           
             I P  is the primary photocurrent generated by the APD (A) 
             q is the electric charge (C or A/Hz) 
             x is the APD excess noise (dimensionless)
 
Except for Ip, all the remaining parameters on which M opt  depends are characteristics of either the optical communication system or the receiver components. That is, they are fixed, system-dependent parameters.
 
           
         
       
    
     The optimum current gain M opt  can thus be determined by characterizing the fixed, system-dependent parameters so that they are available when the APD receiver is in use. Once these parameters are known the optimum current gain M opt  can be determined from the above equation while the receiver is in operation by monitoring the photocurrent I p . 
     The primary photocurrent I p  can be determined from the equation
 
 I   P   =I   APD   /M   (3)
 
Where the APD current I APD  is the measurable input current to the APD detector. Thus, to determine I P , the input current I APD  needs to be measured and the current gain M needs to be determined.
 
     The APD current gain M can be determined by recalling that it is a function of the temperature T and the bias voltage V APD . Accordingly, a transfer function can be empirically derived relating the current gain M to T and V APD , which may then be stored in a memory in the form of an equation or look-up table. In this way the bias controller can measure T and V APD  and then simply look up the value of the current gain M. 
     Once the current gain M is known, the photocurrent I P  can be determined from equation 3 using the measured value of the input current I APD . Given the value for the photocurrent, the value for the optimal gain M opt  can be determined from equation 2. Finally, the bias controller can once again use the lookup table or equation relating M, T and V APD  to determine the value of the bias voltage V APD  necessary to produce the optimum current gain M opt  so that this value can be applied to the APD detector  102 . 
       FIG. 3  is a block diagram of an optical APD receiver constructed in accordance with the methods and techniques described herein. In  FIGS. 2 and 3 , as well as the figures that follow, like reference numerals denote like elements. As shown, the voltage  VAPD  is applied to the APD detector  102  by a DC-to-DC converter  120  that boosts the output of the control signal  Vin  from the bias controller  130 , which may be any suitable type of processor or microprocessor. The bias controller  130  uses the value of the APD temperature measured by the temperature sensor  140  and the transfer function data stored in memory  150  to calculate the current gain M that the APD detector  102  is generating. As in  FIG. 2 , transimpedance amplifier (TIA)  103  converts the current generated by the APD detector  102  into a large signal voltage,  VTIA , which may be further processed by an optional equalizing circuit  104  to produce a voltage  VOUT . 
       FIG. 4  is a flowchart showing one example of a process performed by the bias controller to apply the optimum current gain M opt  to the APD detector  102 . The method begins in step  405  where the system is initialized and the initial value of the voltage V APD  is applied to the APD detector  102 . The initial value of the voltage V APD  may be selected in any appropriate manner and may be, for example, a pre-determined value that is stored in memory. In step  410  the bias controller  130  measures or otherwise reads the current ambient temperature as detected by temperature sensor  140 . Next, the value of the voltage V APD  is measured in step  415  and the value of the input current I APD  to the APD detector  102  is measured in step  420 . Based on these values of V APD  and I AHD  the value of the current gain M is determined by the bias controller  130  from the transfer function or the look-up table stored in memory  150  in step  425 . Given the value of current gain M and the input current I M , the photocurrent I P  generated by the APD detector  102  is calculated in step  430 . In addition, given the value of the system parameters that were read in step  405  and the value of photocurrent I P , the optimum value of the current gain M opt  is determined from equation 2 in step  435 . The value of the voltage V APD  needed to set the gain of the APD detector  102  to the optimum value M opt  is next determined by the bias controller  130  in step  440  from the transfer function or the look-up table stored in memory  150 . Finally, in step  445  the value of the voltage V APD  applied to the APD detector  102  is adjusted to the value determined in step  440 , thereby setting the current gain to its optimal value M opt . The process may then return to step  410  to repeatedly adjust the voltage V APD  as necessary to maintain the current gain at its optimal value or if the optical received power, and consequently I P  has changed. 
     The transfer function or look-up table relating the current gain M to the applied voltage V APD  may be determined in any number of different ways.  FIG. 5  shows one example of a simple arrangement that may be used to establish this relationship in accordance with one such technique. In this example an optical transmitter  510  delivers a known optical power P in  over an optical fiber  520  to the optical APD receiver  530  that is being characterized. The accompanying table in  FIG. 5  shows the results of the measurements that are taken for the photocurrent I P , the input current I APD  to the photodetector in the receiver  430  and the voltage V APD  applied to the photodetector. From these values the value of the current gain M can be calculated for the single temperature (25° C.) and the varying optical powers P in  shown in  FIG. 5 . This process may be repeated for a range of different temperatures to fully characterize the relationship between the temperature T, the voltage V APD  and the current gain M for this particular optical receiver. Table 1 below shows an illustrative portion of a look-up table that may be obtained using the arrangement shown in  FIG. 5  for a temperature of 25° C. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 TEMP 
                   
                   
                   
                 CURRENT 
                   
               
               
                 [C.] 
                 P OPT  [dBm] 
                 I P  [μA] 
                 I APD  [μA] 
                 GAIN [M] 
                 V APD  [V] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 25 
                 −4 
                 398.11 
                 94.6 
                 0.24 
                 8.78 
               
               
                   
                 −6 
                 251.19 
                 94.4 
                 0.38 
                 8.85 
               
               
                   
                 −8 
                 158.49 
                 93.9 
                 0.59 
                 9.01 
               
               
                   
                 −10 
                 100.00 
                 79.5 
                 0.80 
                 13.77 
               
               
                   
                 −12 
                 63.10 
                 64.0 
                 1.01 
                 18.88 
               
               
                   
                 −14 
                 39.81 
                 48.0 
                 1.21 
                 24.16 
               
               
                   
                 −16 
                 25.12 
                 36.1 
                 1.44 
                 28.09 
               
               
                   
                 −18 
                 15.85 
                 26.9 
                 1.70 
                 31.12 
               
               
                   
                 −20 
                 10.00 
                 20.0 
                 2.00 
                 33.40 
               
               
                   
                 −22 
                 6.31 
                 15.1 
                 2.39 
                 35.02 
               
               
                   
                 −24 
                 3.98 
                 11.1 
                 2.79 
                 36.34 
               
               
                   
                 −26 
                 2.51 
                 8.3 
                 3.30 
                 37.26 
               
               
                   
                 −28 
                 1.58 
                 6.1 
                 3.85 
                 37.99 
               
               
                   
               
             
          
         
       
     
     In some implementations, instead of determining the optimal current gain M opt  to which the APD detector is set by the bias controller in accordance with equation 2, the optimal current gain M opt  may be determined in other ways. For instance, the optimal current gain M opt  may be the current gain that optimizes a figure of merit such as the signal-to-noise ratio (SNR) or the modulation error rate (MER). Similar to above, this optimal current gain may be determined for different temperatures and optical power levels. Regardless of how the optimal current gain is determined, data interpolation may be used to determine the value of the bias voltage for temperatures and optical power levels other than those that have been measured. 
     It should be noted that in some implementations the optical receiver&#39;s dynamic range can be further extended at high optical power levels (e.g., above −3 dBm) by reducing the APD bias voltage below the APD device breakdown voltage. In this way the avalanche effect ceases to take place and the photodetector operates in a manner similar to a PIN photodetector, with enhanced linearity characteristics. 
     The processes described above, including but not limited to those presented in connection with  FIG. 4  may be implemented in general, multi-purpose or single purpose processors. Such a processor will execute instructions, either at the assembly, compiled or machine-level, to perform that process. Those instructions can be written by one of ordinary skill in the art following the description of presented above and stored or transmitted on a computer readable storage medium. The instructions may also be created using source code or any other known computer-aided design tool. A computer readable storage medium may be any medium capable of carrying those instructions and include a CD-ROM, DVD, magnetic or other optical disc, tape, and silicon memory (e.g., removable, non-removable, volatile or non-volatile.