Abstract:
An optical signal receiver has an increased dynamic range for detecting optical signals whose intensity varies over a wide range. In one embodiment, the optical signal receiver includes a circuit operable to provide a reverse bias voltage and an avalanche photo-diode (APD) coupled to the circuit to receive the reverse bias voltage. The circuit is operable to lower the reverse bias voltage in response to an increase in power of the received optical signals. Since the current gain of the APD is a function of the reverse bias voltage, the circuit indirectly lowers the current gain of the APD in response to the increase in power of the received optical signals. As a result, the optical signal receiver can be used to detect optical signals whose intensity varies over a broad range.

Description:
[0001]    The present application claims priority to U.S. Provisional Patent Application serial No. 60/355,024, filed Feb. 8, 2002, which is incorporated herein by reference. 
     
    
     
       BRIEF DESCRIPTION OF THE INVENTION  
         [0002]    The present invention relates generally to optical signal receivers, and more particularly, to an improved optical signal receiver or transceiver for detecting optical signals whose intensity varies over a wide range.  
         BACKGROUND OF THE INVENTION  
         [0003]    Optical signal receivers, in general, function to convert optical signals into electrical signals. A typical optical signal receiver includes a photo-detector connected to the input of an amplifier (e.g., a transimpedance amplifier). The photo-detector converts the optical signal it has received into an electric current that is supplied to the amplifier. The amplifier then generates at its output a voltage or current that is proportional to the electric current. The photo-detector is typically either an avalanche photo-diode (APD) or a PIN (p-intrinsic-n) photo-diode.  
           [0004]    APDs are significantly better than PINs for detecting low-intensity optical signals. The avalanche effect in APDs magnifies the photo-current for a given intensity of input light, and the sensitivity of an APD receiver increases by an amount roughly equal to the current gain. Unfortunately, the maximum permissible input optical power to the APD receiver also drops by the same amount. This is because the photo-current generated by the APD may overload the pre-amplifier or other receiver circuits of the receiver. Thus, most conventional APD receivers have a lower maximum input optical power (or, overload power) than most conventional PIN receivers. Because of this low overload power, conventional APD&#39;s are not used in short haul optical links where the signal intensity is typically high.  
           [0005]    Accordingly, what is needed is an APD optical signal receiver that has a broader dynamic range than conventional APD receivers such that it can be used in both long haul and short haul optical links.  
         SUMMARY OF THE INVENTION  
         [0006]    An embodiment of the present invention is an optical signal receiver that has a high dynamic range to accommodate both low intensity and high intensity optical signals. In this embodiment, the high dynamic range is achieved by reducing the reverse bias voltage of the photo-diode of the optical signal receiver in response to strong optical signals. The reduced reverse bias voltage lowers the current gain and reduces the sensitivity of the photo-diode. When the incoming optical signals are weak, the reverse bias voltage of the photo-diode and its sensitivity is not significantly affected.  
           [0007]    In some embodiments, the reverse bias voltage of the photo-diode is provided by a resistor that is placed in series between a voltage source and the photo-diode. When the intensity of the incoming optical signals is low, a small photo-current will be generated. Since the same photo-current flows across the resistor, the voltage drop across the resistor will be small, and the reverse bias voltage is not significantly affected. As the intensity of the optical signals increases, the photo-current through the photo-diode increases. The increase in the photo-current results in a corresponding increase in the voltage drop across the resistor and a corresponding reduction in the reverse bias voltage. The reduction in the reverse bias voltage, in turn, reduces the current gain in the photo-diode.  
           [0008]    In other embodiments, a current sensor is coupled to the photo-diode to detect the photo-current. The current sensor is coupled to a voltage converter that provides the reverse bias voltage to the photo-diode. The voltage converter decreases the reverse bias voltage in response to increases in photo-current. The reduction in the reverse bias voltage in turn reduces the current gain in the photo-diode.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    For a better understanding of the invention, reference should be made to the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0010]    [0010]FIG. 1 depicts a portion of an optical signal receiver in accordance with a first embodiment of the present invention;  
         [0011]    [0011]FIG. 2 depicts a portion of an optical signal receiver in accordance with a second embodiment of the present invention; and  
         [0012]    [0012]FIGS. 3A and 3B depict a current sensor for use in a third embodiment of the present invention;  
         [0013]    [0013]FIG. 4 depicts a portion of an optical signal receiver in accordance with yet another embodiment of the present invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0014]    Preferred embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described. It will be appreciated that in the development of any such embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0015]    The current gain of an avalanche photo-diode (APD) increases non-linearly with its reverse bias voltage. The present invention takes advantage of this characteristic of the APD by varying its reverse bias voltage according to the intensity of received optical signals. More specifically, an embodiment of the present invention reduces the reverse bias voltage in response to a high intensity optical signal by decreasing the current gain of the APD. Reducing the current gain of the APD results in reduced sensitivity. In one embodiment, the current gain of the APD drops to near unity (at which point the APD operates like a photo-diode) when the photo-current approaches the maximum input current limit of a pre-amplifier circuit of the optical signal receiver. Thus, in that embodiment, the APD optical signal receiver has a similar overload power as a PIN optical signal receiver that shares similar amplifier and pre-amplifier circuitry.  
         [0016]    [0016]FIG. 1 is a block diagram depicting a portion of an optical signal receiver  100  in accordance with a first embodiment of the invention. The optical signal receiver  100  includes an APD  102  that is coupled to an amplifier  104 . When a reverse bias voltage is applied to the APD  102 , the APD  102  will generate a photo-current in response to an input optical signal. The photo-current generated by APD  102  is amplified by the amplifier  104  to generate an output signal (e.g., an output voltage or an output current). Also illustrated is a voltage source  106 . In operation, the voltage source  106  provides a constant DC voltage, V PS . A resistor  108  is coupled in series between the voltage source  106  and the APD  102  to provide a reverse bias voltage V PD  to the photo-diode  102 . The reverse bias voltage V PD  is preferably in the range of 30 to 70 volts, is more preferably in the range of 35 to 60 volts, and is approximately 50 volts, plus or minus 5 volts, in some implementations.  
         [0017]    Because the resistor  108  and the APD  102  are in series, the current through the resistor  108  is the same as the photo-current (i PD ) through the APD  102 . Accordingly, the voltage drop (V drop ) across the resistor  108  is equal to i PD ×R, where R denotes the resistance of the resistor  108 . The photo-current i PD  through the APD  102  is a function of the intensity of the input optical signals. As i PD  increases in response to strong incoming optical signals, V drop  increases correspondingly, and the reverse bias voltage applied to the APD  102  (V PD ) decreases. In other words, V PD  is determined by:  
         
       V 
       PD 
       V 
       PS 
       −i 
       PD 
       ×R  
     
         [0018]    Because the gain of the photo-current i PD  is a function of the reverse bias voltage V PD  applied to the APD  102 , the gain of the photo-current i PD  decreases as the input optical power level increases. The decrease in current gain limits the photo-current generated. At a certain light intensity, the current gain of i PD  approaches unity and the APD  102  behaves like a PIN photodiode. Thus, the optical signal receiver  100  can be used in short haul optical links, where the input optical power level tends to be relatively high.  
         [0019]    At low input optical power, the optical signal receiver  100  generates a small photo-current i PD . The reverse bias voltage of the APD  102  is not significantly affected. Thus, at low input optical power, the optical signal receiver  100  behaves like a conventional APD receiver and can be used in long haul optical links, as well as in short haul optical links.  
         [0020]    In one particular embodiment, the resistance of the resistor  108  is approximately 50K Ohms. The dynamic range of an APD receiver according to this embodiment is approximately 33 dB (e.g., between a maximum input optical power of one mwatt and a minimum input optical power of 0.5 μwatts). In comparison to some conventional optical signal receivers that have a dynamic range of approximately 20 dB (e.g., between a maximum input optical power of 50 μwatts and a minimum input optical power of 0.5 μwatts), this embodiment has a much higher dynamic range. In other embodiments of the receiver shown in FIG. 1, the resistance of the resistor  108  is between 10K Ohms and 200K Ohms.  
         [0021]    In one aspect, the resistance of the resistor  108  is chosen according to the saturation level of the amplifier circuit  104  of the optical signal receiver. Preferably, the resistance of the resistor  108  is chosen such that, when the photo-current approaches saturation level of the amplifier circuit  104 , the current gain of the photo diode  102  is near unity.  
         [0022]    A portion of an optical signal receiver  200  according to a second embodiment of the present invention is shown in FIG. 2. In this embodiment, the reverse voltage bias of the APD  102  is regulated by a current sensor  208 , control logic  210  and voltage converter  212 . A power supply  106  supplies a source voltage (V PS ) to the voltage converter  212 . The voltage converter  212  converts the source voltage V PS  to a reverse bias voltage (V PD ), which is provided to the APD  102 . The voltage converter may be a switching power supply that pumps charge onto a voltage supply node (e.g., the V PD  voltage node) until a feedback signal indicates that a specified voltage has been achieved. The feedback signal may be produced by a voltage divider (for example, a ladder or two or more resistors) having a top node at the V PD  voltage and an intermediate node from which the feedback signal is obtained. A capacitor  214  to ground is used to remove or reduce fluctuations in the reverse bias voltage (V PD ). In this embodiment, the resistor  108 , which is used as part of the current sensor  208 , has a resistance of approximately 50K Ohms. In other embodiments, the resistor  108  may have a smaller resistance (e.g.,10K Ohms), with the current sensor  208  being configured to have higher sensitivity to changes in the voltage across the resistor  108 .  
         [0023]    With reference still to FIG. 2, when an optical signal is detected by the APD  102 , a photo-current i PD  is generated. The current sensor  208  detects the increase in photo-current i PD  and generates a signal  209  proportional to or otherwise dependent on the photo-current. The control logic  210 , in response to the current sensor&#39;s output  209 , generates control signals  211  that cause the voltage converter  212  to reduce the reverse bias voltage (V PD ). When the reverse bias voltage V PD  is reduced, the current gain of the APD  102  is correspondingly limited. An even stronger optical signal will cause voltage converter  212  to further decrease the reverse bias voltage V PD . The result is a further decrease in the current gain of the APD  102 . When the received optical signals are sufficiently strong, the current gain of the APD approaches unity. In that event, the APD  102  behaves like a PIN photo-diode. Thus, at low input optical power, the optical signal receiver  200  behaves like a conventional APD receiver and is highly sensitive. And, at high input optical power, the optical signal receiver  200  behaves like a PIN-based receiver and overloads at a higher input optical power than APD receivers not implementing the present invention.  
         [0024]    The control logic  210  may be implemented, for example, in a microprocessor, a micro-controller, a programmable logic array (PLA), a field programmable logic array (FPGA) an application specific integrated circuit (ASIC) or any other computational device. The control logic  210  may include various means for correlating voltage target levels with monitored current levels. For example, the control logic may employ look-up tables to correlate output voltage with monitored current levels.  
         [0025]    The current sensor  208  may range in complexity from a series coupled resistor to a current mirror, for example. The current sensor  208  provides as an output a signal  209  proportional to the received signal strength. In an embodiment using a series resistor, this signal corresponds to the voltage drop across the resistor  108 , as described above in relation to the first described embodiment.  
         [0026]    In an embodiment using a current mirror, as shown in FIG. 3A, the current sensor  208  has two legs—a photo-detector leg  330  and a mirror leg  332 . The photo-detector current “I pd ” passes through the photo-detector “PD” leg  330 , and the mirror current “I m ” passes through the mirror leg  332 . The mirror current provides a signal proportional to (or approximately proportional to) the received signal strength. Both legs of the current mirror couple on the positive side to a voltage source node  334 . Voltage converter  212  controls the voltage on node  334  in accordance with a control signal from the control logic  210 . The photo-detector leg of the current mirror couples via line  340  with the high voltage terminal of the APD  102  (i.e., line  340  is coupled to n-doped portion of the APD  102 ). In the example shown, the supply voltage is controllable between 30 and 60 volts and the photo-detector  102  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 3-5 volts for example. The mirror leg  332  of the current mirror supplies the mirror current I m . The level of I m  corresponds to the received optical signal level as detected by the photo-detector  102 .  
         [0027]    FIGS.  3 A-B show alternate examples of current mirrors used in a third embodiment of the present invention, which is similar in many respects to the second embodiment. 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 I pd    320 . The mirror transistor  304  is in the mirror leg  332  in which flows the mirror current I m    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.  3 A-B the sense and mirror transistors comprise ‘pnp’ type bipolar transistors.  
         [0028]    In FIG. 3A, the sense and mirror transistors,  302  and  304 , are supplemented by an isolation transistor  306 , to form a Wilson mirror, which is a well known mirror circuit described in many text books. The isolation transistor  306  has an emitter coupled to the collector of mirror transistor  304 , a base coupled at node  312  to the collector of the sense transistor  302 , and an emitter coupled to monitor node  344 . The isolation transistor  306  helps to make the collector-to-emitter voltage drop across the mirror transistor  304  relatively constant at about 0.7 volts, even in the event of large changes in the mirror current. The collector-to-emitter voltage across the sense transistor  302  can vary considerably, depending on the amount of current drawn by the APD  102 . In other embodiments, the isolation transistor  306  could be replaced by a Schmidt or Zener diode.  
         [0029]    The current I m  flowing through the monitor leg  332  develops a voltage across resistor  348 , thereby generating a monitor signal on monitor node  344 . The resistance of resistor  348  is selected so as to provide a monitor signal with an appropriate voltage range, and is set to 10k ohm in one embodiment. Other appropriate resistance values would be used in other embodiments. Monitor node  344  provides a monitor signal that is proportional, or at least approximately proportional, to the photo-detector current and that is coupled to the control logic  210 .  
         [0030]    In the alternate embodiment shown in FIG. 3B, another non-linear isolation element is added to the photo-detector leg  330 - 2  between the sense transistor  302  and the photo-detector  102 . Suitable non-linear isolation elements include: a Schmidt or Zener diode, or a bipolar transistor. In the embodiment shown in FIG. 3B the non-linear isolation element is a bipolar transistor  308  with an emitter terminal coupled to the collector of the sense transistor  302  and a collector coupled to the photo-detector  102 . The base of transistor  308  is coupled to the collector of the sense transistor  302  as well as to the base of the other isolation transistor  306 . This embodiment has more linear operation than the embodiment shown in FIG. 3A because the collector-to-emitter voltages in both the sense and mirror transistors  302 ,  304  are relatively constant at about 0.7 volts, even when the currents in the photo-detector and mirror legs varies over a large range.  
         [0031]    In the embodiments shown in FIGS.  3 A-B the emitters of the sense and mirror transistors  302 ,  304  couple to the voltage source  212  via node  334  and resistors  300   a ,  300   b , respectively. These resistors  300   a ,  300   b  may be sized appropriately for embodiments of the invention in which the photo-detector  102  is an avalanche photodiode, or a PIN diode. Resistors  300   a ,  300   b  may have different resistance values. For instance, if the current sensor  208  is configured to provide a mirror current I m  that is one tenth the magnitude of the photo-detector current I pd , resistor  300   b  will have one tenth of the resistance (e.g., 100 ohms) of resistor  300   a  (e.g., 1000 ohms), and transistor  304  will be sized to pass one tenth as much current as transistor  302  when having identical terminal voltages. This configuration provides different but proportional currents to pass through the mirror and photo-detector legs. Having a unsymmetric current sensor  208  reduces the amount of power used to perform the current monitoring function.  
         [0032]    Referring to FIG. 3C, a current mirror in another embodiment may also be coupled on the “low side” of the receiver to monitor received signal strength from the photo-detector. In such a configuration, the mirror transistors  402 ,  404  are ‘npn’ bipolar types with the emitters of the sense and mirror transistors  402 ,  404  coupled to a voltage sink and with the monitor node coupled through a resistor to a voltage source.  
         [0033]    [0033]FIG. 4 is a block diagram depicting a portion of an optical signal receiver  400  in accordance with yet another embodiment of the invention. The optical signal receiver  400  includes voltage source  106 , APD  102  and resistor  108  coupled in series between the voltage source  106  and the APD  102 . In addition, the optical signal receiver  400  includes a transimpedance amplifier  410  coupled across the resistor  108 . The transimpedance amplifier  410 , in this embodiment, becomes saturated when the input photo-current exceeds a certain threshold level, at which the output voltage will cease to vary correspondingly with the photo-current i PD .  
         [0034]    In operation, in response to a weak optical signal (e.g., approximately 0.5 μwatt), a small photo-current i PD  is generated. The small photo-current i PD  causes a correspondingly small voltage drop across the resistor  108 . As a result, the gain of the photo-current is not greatly affected. The transimpedance amplifier  410  detects the small photo-current i PD , and generates an amplified voltage signal V out  as output. Thus, in response to a weak optical signal, the optical signal receiver  400  behaves like a conventional APD receiver.  
         [0035]    In response to a strong optical signal (e.g., approximately one milliwatt), the photo-diode will generate a very large photo-current i PD  if the reverse bias voltage V PD  remains the same. However, in the present embodiment, an increase in i PD  causes a corresponding increase in voltage drop across the resistor  108  and a corresponding decrease in photo-current gain. For instance, if the received optical signal has a power of approximately one mwatt, the current gain is approximately at unity. The optical signal receiver  400 , therefore, behaves like a PIN-based optical signal receiver.  
         [0036]    The foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed. Many modifications and variations suitable to the particular use contemplated are possible in view of the above teachings. For instance, it should be obvious to those skilled in the art having the benefit of this disclosure that the present invention can be applied to receiver parts of an optoelectronic transceiver.