Abstract:
A burst-mode optical receiver of a differential-output structure is disclosed. The burst-mode optical receiver includes a trans-impedance amplifier for converting currents indicating the burst-mode signals into voltage signals, a bottom-level detector for detecting the bottom level of signals outputted from the trans-impedance amplifier, an automatic gain controller for automatically adjusting a gain to prevent the output waveforms of the tran-simpedance amplifier from being distorted after receiving the bottom-level signals detected by the bottom-level detector, a top-level detector for detecting the top level of signals outputted from the trans-impedance amplifier, a pair of resistors for generating a signal-reference voltage from the bottom- and top-level voltages, one side of each resistor being connected to the bottom- and top-level detectors, respectively, and the other sides of each resistor being connected to each other, and a differential buffer for receiving outputs from the trans-impedance amplifier and the signal-reference voltage from the pair of resistors and for eliminating the offsets generated from the bottom- and top-level detectors in order to supply two differential outputs.

Description:
CLAIM OF PRIORITY  
         [0001]    This application claims priority to an application entitled “BURST-MODE OPTICAL RECEIVER OF DIFFERENTIAL OUTPUT STRUCTURE,” filed in the Korean Intellectual Property Office on Apr. 15, 2002 and assigned Ser. No. 2002-20489, the contents of which are hereby incorporated by reference.  
         BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a burst-mode optical receiver and, more particularly, to a burst-mode optical receiver for enhancing an available bit rate in a passive network.  
           [0004]    2. Description of the Related Art  
           [0005]    For the future generation of communications, optical subscriber networks, such as FTTH (Fiber to the Home), will be required to install optical fiber lines directly to the homes of subscribers for the transmissions/reception of information at a higher speed.  
           [0006]    The subscriber networks have been traditionally constructed with copper-based lines. As such, it would be very costly to replace them with the fiber-based lines. In this regard, a passive optical network (PON) has been taken into consideration to provide a more cost-effective optical-subscriber network.  
           [0007]    [0007]FIG. 1 illustrates a general PON system. As shown in FIG. 1, the PON is generally made up of an optical line termination (OLT) located in a central office, a 1×N passive optical splitter, a plurality of optical-network units (ONUs) corresponding to each subscriber. In this type of optical multi-access network, each node is designed to transmit data or packets to other nodes using a predetermined time slot. Typically, a plurality of subscribers can make use of a single optical line through which desired data are transmitted or received according to a time-division multiplexing scheme. Unlike the point-to-point link, burst-mode data are generated in which received data or packets have different sizes and phases from each other due to the optical loss or attenuation generated via different transmission routes. Each subscriber transmits data at the respective assigned time, but the packets received at the receiving ends are not uniform in size due to the path differences between the OLT and each subscriber.  
           [0008]    As each received packet has a different size and phase due to the optical loss or by different transmission routes between the nodes, an optical receiver must be employed to compensate the loss. To this end, a burst-mode optical receiver is used to enable the received packets to have the same sizes and phases. The conventional burst-mode optical receivers prevent the loss of burst data caused by a charging/discharging time of the capacitor in the receiver by removing the DC block capacitor. A threshold value is extracted from each received packet by the receiver which functions as a reference signal for the purpose of data discrimination, and the data is amplified using the extracted discrimination reference signal.  
           [0009]    For example, FIG. 2 is a circuit diagram of a conventional burst-mode optical receiver. The burst-mode optical receiver of FIG. 2 includes an optical detector  1  for converting input optical signals into current signals, and a trans-impedance amplifier (TIA)  2  for converting current signals passing through the optical detector  1  into voltage signals. Note that the TIA  2  is dc-coupled. Signals received by the optical detector  1  are amplified at the TIA  2  and then divided into two parts, of which one is dc-coupled to and inputted into a differential amplifier of a limiter amplifier  4  and the other is inputted into a circuit for an automatic threshold controller (ATC)  3 . The ATC  3  extracts discrimination thresholds of the respective packets received from the TIA  2 . The limiter amplifier  4  amplifies signals with a different optical intensity into signals having a constant amplitude using the extracted discrimination thresholds. The thresholds that vary according to the sizes of packets outputted from the ATC  3  are inputted into an input terminal as a reference voltage V ref  of the differential amplifier of the limiter amplifier  4  to be amplified and recovered.  
           [0010]    [0010]FIG. 3 is a circuit diagram of another conventional burst-mode optical receiver having a structure with a differential input/output feedback amplifier. The optical receiver of FIG. 3 includes an optical detector  8 , a differential preamplifier  10 , a peak detector  20 , and a limiting amplifier  30 . The peak detector  20  detects the peak value of an output signal to generate a reference voltage so as to set the discrimination thresholds of received packets. The limiting amplifier  30  amplifies recovered signals using the generated reference voltage. The differential preamplifier  10  is operative to receive current signals, which are detected at the optical detector  8 , as inputs and then outputs corresponding voltages. A ratio of the input current to the output voltage, i.e., a trans-impedance, is determined by a feedback resistor Z T . One side of the feedback resistor Z T  is connected to a “+” input terminal of amplifier  12  and the other is connected to a “−“ output terminal of amplifier  12 . The peak detector  20  is made up of an amplifier  22 , a drive transistor  24 , a buffer transistor  26 , a charging capacitor C PD , and a bias circuit  28 . Here, a reference voltage V ref , which is outputted from the peak detector  20 , is converted into a discrimination-threshold current by the feedback resistor Z T .  
           [0011]    During operation, the “+” input terminal of the amplifier  12  receives the current I IN  outputted from the optical detector  8 , and the “−” output terminal receives the reference voltage V ref   or a reference signal. Here, the reference signal inputted to the “−” output terminal is a discrimination-threshold current converted from the reference voltage V ref , which is detected from the peak detector  20 . Accordingly, the differential preamplifier  10  generates output voltages V o   +  and V o   −  depending on the difference between the two input currents.  
           [0012]    The output voltage V o   +  outputted from the “+” terminal of the amplifier  12  in the differential preamplifier  10  is inputted to a “+” terminal of an amplifier  22  of the peak detector  20 , whereas the reference voltage V ref  applied to the “−” terminal of the amplifier  12  of the differential preamplifier  10  is fed back to a “−” terminal of the amplifier  22  of the peak detector  20 . Therefore, when these two voltages are not the same at the amplifier  22  of the peak detector  20 , the drive transistor  24  is turned on and causes the charging capacitor C PD  to be charged with voltage until the “+” and “−” terminals of the amplifier  22  have the same voltage. Accordingly, when an optical-detection signal, first input I IN , flows into the differential preamplifier  10 , its output becomes ΔV o   + =ΔV o   − . Further, as the peak detector  20  is supplied with the output of ΔV o   +  at its “+” terminal, the voltage charged at the charging capacitor C PD  becomes the reference voltage V ref . This reference voltage V ref  is used as a threshold for discriminating data using a mean level of an output-data signal.  
           [0013]    Meanwhile, when the two voltages are the same at the amplifier  22  of the peak detector  20 , the drive transistor  24  is turned off, and thus the charging capacitor C PD  is discharged. With this discharge, the buffer transistor  26  is turned on, and thus the current flows through the bias circuit  28 . Thereafter, the reference voltage V ref  is applied to a node between the buffer transistor  26  and the bias circuit  28  and then converted into a discrimination-threshold current by the feedback resistor Z T , and finally fed back to the “−” terminal of the amplifier  12  of the differential preamplifier  10 . Thus, the current flowing to the “−” terminal of the amplifier  22  of the peak detector  20  corresponds to a middle value of the optical-detection signal I IN  current. Hence, the reference signal V ref  functions as the discrimination threshold of the differential preamplifier  10 .  
           [0014]    However, the actual reference signal V ref  is typically accompanied by an offset of the differential preamplifier  10 , resulting from device asymmetry as well as a structural offset caused by the turn-on voltages of transistors resulting from a circuit structure of the peak detector  20 . Thus the actual reference signal tends to deviate from a mean or middle level of the output data signal. A pulse width distortion is generated due to the change in the reference signal which in turn degenerates the sensitivity of the optical detector  8 .  
           [0015]    To minimize this pulse-width distortion, the conventional feedback burst-mode optical receiver employs a current source I ADJ , which is connected to the “+” input terminal and the resistor Z T  of the differential preamplifier  10 . The current source I ADJ  serves to compensate the offset generated by the differential preamplifier  10 , but does not compensate the structural offset generated by the turn-on voltages of the transistors within the peak detector  20 .  
           [0016]    Accordingly, there is a problem in that the reference signal generated from the peak detector  20  is not matched with the mean level of the output-data signal, thus still generates a pulse-width distortion and degrades the sensitivity of the optical detector.  
         SUMMARY OF THE INVENTION  
         [0017]    Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art and provides additional advantages, by providing a burst-mode optical receiver having a differential output structure capable of significantly reducing the pulse-width distortion while improving the reception sensitivity.  
           [0018]    One aspect of the present invention provides an additional circuit for precisely adjusting the offsets caused by a peak detector, so that a reset signal is generated automatically and a reference-voltage signal is set exactly to a middle level.  
           [0019]    Still another aspect is that the present invention may be realized in a simple, reliable, and inexpensive implementation.  
           [0020]    Another aspect of the present invention provides a burst-mode optical receiver having a differential output structure and includes: a trans-impedance amplifier for converting currents indicating burst-mode signals into voltage signals; a bottom level detector for detecting the bottom level of signals outputted from the trans-impedance amplifier; an automatic gain controller for automatically adjusting a gain to prevent the output waveforms of the trans-impedance amplifier from being distorted after receiving the bottom-level signals detected by the bottom-level detector; a top-level detector for detecting the top level of signals outputted from the trans-impedance amplifier, a pair of resistors for generating a signal-reference voltage from the bottom- and top-level voltages, one side of each resistor being connected to the bottom- and top-level detectors, respectively, and the other sides of each resistor being connected to each other; and, a differential buffer for receiving outputs from the trans-impedance amplifier and the signal reference voltage from the pair of resistors and for eliminating the offsets generated from the bottom- and top-level detectors in order to supply two differential outputs. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    The above features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0022]    [0022]FIG. 1 illustrates a passive optical-communication system;  
         [0023]    [0023]FIG. 2 is a circuit diagram of a conventional burst-mode optical receiver;  
         [0024]    [0024]FIG. 3 is a circuit diagram of another conventional burst-mode optical receiver having a structure of a differential input/output feedback amplifier;  
         [0025]    [0025]FIG. 4 is a circuit diagram of a burst-mode differential preamplifier of a differential-output structure according to the present invention;  
         [0026]    [0026]FIG. 5 is a circuit diagram of an automatic gain controller of FIG. 1;  
         [0027]    [0027]FIG. 6 is a circuit diagram of a differential buffer of FIG. 1;  
         [0028]    [0028]FIG. 7 is a diagram showing the output waveforms of a top-level voltage, a bottom-level voltage, and a reference voltage, all of which are detected at a burst-mode differential preamplifier having a differential-output structure according to the present invention;  
         [0029]    [0029]FIG. 8 is a diagram showing waveforms of differential-output voltages from a burst-mode differential preamplifier having a differential-output structure according to the present invention; and,  
         [0030]    [0030]FIG. 9 is a diagram showing waveforms of an output voltage of TIA, an LOS signal, an AGC control signal, and outputs of a differential buffer in a burst-mode differential preamplifier having a differential-output structure according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0031]    [0031]FIG. 4 is a circuit diagram of a burst-mode differential preamplifier of a differential-output structure according to the teachings of the present invention. As shown, the burstmode differential preamplifier includes an optical detector  108  for converting an in-putburst optical signal into a current signal according to the signal intensity of the in-putburst optical signal. The output of the optical detector  108  is connected to a trans-impedance amplifier (TIA)  110 . The TIA  110  amplifies the current received from the optical detector  108  and supplies the amplified output signal VE 2  to a bottom-level detector  120  and a top-level detector  130 .  
         [0032]    The bottom-level detector  120  detects a bottom level of the signal output from the TIA  110 , and the top-level detector  130  detects a top level of the signal output from the TIA  110 . The bottom voltage level detected from the bottom-level detector  120  is supplied to an automatic gain controller (AGC)  140  and, in response, the AGC  140  generates an AGC control signal. Note that a considerable signal distortion occurs if the input-current level received by the AGC  140  is beyond the range of −31 dBm to −16 dBm. To this end, the AGC operates to prevent the signal distortion, as explained hereinafter.  
         [0033]    [0033]FIG. 5 is a circuit diagram of an AGC  140  according to the embodiment of the present invention. As shown in the circuit diagram, the AGC  140  is constructed to generate the AGC control signal when a variation of input signal occurs and, in particular, when the transistors Q 1  and Q 2  are turned off by the bottom level of the output signal from the TIA  110 . Normally, the signal received in the bottom-level detector  120  begins to be distorted at its bottom level during the amplification operation by the TIA  110 . Thus, the AGC  140  begins to operate automatically whenever the output signal of the TIA  110  begins to be distorted. This way, the AGC  140  is able to compensate the gain characteristic based on the level of an input signal. To achieve this, the AGC  140  includes a signal-level-determining section  142  having a resistor R 3  and transistors Q 3 , Q 4 , and Q 5 . The signal-level-determining section  142  is configured to analyze the bottom level of a signal input to the base of the transistor Q 1  to determine whether the signal is present.  
         [0034]    If it is determined that a signal is present, the signal-level-determining section  142  maintains a gate voltage of the transistor Q 6  to be a “high” state and allows the transistor Q 6  to be turned on, thus allowing a capacitor C 1  to be charged to a predetermined voltage (i.e., 3× diode turn-on voltage). At the same time, a drain voltage of the transistor Q 7  becomes a “low” state, which indicates the presence of a signal.  
         [0035]    If it is determined that no signal is present, the signal-level-determining section  142  converts a gate voltage of the transistor Q 6  into a “low” state. As a result, the transistor Q 6  turns off and begins to be discharged. The transistor Q 6  is kept turned off until the discharge time, which is an internal time-constant circuit formed by the capacitor C 1  and the resistor R 4  sets, lapses(T(time constant)=1/(R*C)). Then, the drain voltage of the transistor Q 7  becomes a “high” state to show that no signal is present which also indicates the end of a packet transmission. This signal makes a transistor Q 8  to be discharged and generates an AGC signal reset, so that the AGC voltage level can be reset at the beginning of the next packet transmission. Accordingly, it is possible to prevent a signal distortion problem either when the AGC-operation standard level is too low and the output of the TIA is reduced excessively, or when the AGC-operation standard level is too high.  
         [0036]    Further, an initial AGC signal generated from a collector of the input transistor of the AGC  140  is set and maintained at a peak level by the peak detector  130  within the AGC  140 , thereby an actual AGC control signal is set to be a constant value at the beginning of the packet transmission, which is maintained during the packet transmission. As a result, a jitter is minimized which is generated with a change of the AGC control signal during the packet transmission.  
         [0037]    Referring to back to FIG. 4, the output terminal of the bottom-level detector  120  is connected to one side of a first resistor R 1 , and the other side of the resistor R 1  is connected to a second resistor R 2 . Similarly, the output terminal of the top-level detector  130  is connected to one side of the second resistor R 2 , and the other side of the second resistor R 2  is connected to the first resistor R 1 . These resistors R 1  and R 2  are used to get a precise middle-level voltage based on the outputs of the bottom-level voltage and the top-level voltage. These resistors R 1  and R 2  have their resistance values set to generate a middle-level voltage, i.e., a signal-reference voltage Sig-Ref, between the bottom-level voltage and the top-level voltage. A capacitor C is connected to the second resistor P 2  in parallel.  
         [0038]    Meanwhile, the resistors R 1  and R 2  have contacts connected to a differential buffer  150 , so that the mean or middle-level voltage between the bottom-level voltage and the top-level voltage is supplied to the differential buffer  150 . Note that in a burst-mode operation, the middle-level voltage functions as a reference signal for detecting data. Further, the differential buffer  150  is connected to the output terminal of the TIA  110 , so that it is supplied with the output signal VE 2 . The differential buffer  150  is operative to reduce offset errors, which are generated from the bottom- and top-level detectors  120  and  130 , as explained hereinafter.  
         [0039]    [0039]FIG. 6 is a circuit diagram of a differential buffer  150  according to the embodiment of the present invention. As shown, the output from the TIA  110  is applied to a base of the transistor Q 1 , and the signal reference voltage Sig-Ref indicative of the value between the bottom-level voltage and the top-level voltage is applied to a base of the transistor Q 2 . These transistors Q 1  and Q 2  are formed into a differential amplifier. A power source is supplied to the respective transistors Q 1  and Q 2  through a transistor Q 5 . Collectors of the transistors Q 1  and Q 2  are connected to bases of the transistors Q 3  and Q 4 , respectively. Further, the differential buffer  150  includes a current source  152  (not shown). Emitters of the transistors Q 6 , Q 7  and Q 8  are connected to the ground through resistors  220 ,  222  and  224 , respectively. The current source  152  functions to adjust the amount of current of an emitter terminal constantly. Transistors Q 3  and Q 4  allow two outputs of the differential amplifier, which is made up of transistors Q 1  and Q 2 , to be outputted through the respective emitters. In order to allow the differential buffer  150  to reduce the offset errors, which are generated from the bottom- and top-level detectors  120  and  130 , an offset adjustor  252  is provided which includes resistors  230 ,  232 ,  234  and  236 .  
         [0040]    Referring back to FIG. 4, a power-level detector  160  is provided for detecting the power level of an input signal. The power-level detector  160  determines whether or not a final output or the output-data (+,−) level is present from an output signal of the TIA  110  and provides the determined resultant to a pulse generator  170 . That is, the power-level detector  160  determines whether or not the final output signal is present and generates a signal related to a loss of signal (LOS). Then, the pulse generator  170  generates a pulse depending on the determined resultant provided from the power-level detector  160 .  
         [0041]    According to the present invention, the top- and bottom-level detectors are each designed to have a short time constant, so that the time constant can be automatically reset between the sequential packets and the two detectors can be operated without a separate reset signal, which was generated by the ATC circuit in the prior art. As a result, the capacitors in the chip have a reduced area when compared with those employed in the reset signal-generation circuit of the prior art. Moreover, the differential-output signals are generated by using the differential buffer with respect to a single input, thereby minimizing the additional circuits necessary to perform the same functions. Furthermore, a precise signal standard can be adjusted, because the circuit has an offset adjustment function.  
         [0042]    FIGS.  7 - 9  represent the signal output characteristics illustrating the advantages of the optical receiver according to the teachings of the present invention.  
         [0043]    In particular, FIG. 1 is a diagram showing the output waveforms of the top-level voltage, the bottom-level voltage, and a reference voltage, all of which are detected at a burst-mode differential output preamplifier according to the present invention. The longitudinal axis represents the voltage level and the transverse axis represents time (by ns).  
         [0044]    In this drawing, a symbol of Vin is a signal representing the current outputted from the optical detector  108  and inputted to the TIA  110 . The TIA  110  amplifies the inputted current to output the amplified current. The symbol of Vbot represents the bottom level that the bottom-level detector  120  detects from signals supplied from the TIA  110 , and a symbol of Sig_ REF represents the reference voltage generated by resistors R 1  and R 2  to which the bottom- and top-level voltages are applied. As shown in FIG. 7, the reference voltage corresponds almost exactly to the mean level between the top level and the bottom level.  
         [0045]    [0045]FIG. 8 is a diagram showing the waveforms of differential-output voltages from a burst-mode differential preamplifier of a differential-output structure according to the present invention. That is, these waveforms represent two output signals outputted through the differential buffer  150 . Note that these two output signals have different polarities which are inverse to each other.  
         [0046]    [0046]FIG. 9 is a diagram showing the waveforms of an output voltage of the TIA  110 , an LOS signal, an AGC control signal, and outputs of a differential buffer in a burst-mode differential preamplifier of a differential-output structure according to the present invention. As shown, the waveform shown on the lowest side represents the LOS signal, which is generated when no output is present from the TIA  110 . The waveform shown just above the LOS signal is the waveform representing the output voltage of the TIA  110 . Finally, two waveforms shown on the upper side represent the waveforms of differential-output voltages shown in FIG. 8. As can be seen from FIG. 9, the AGC control signal has a varying value according to the outputs of the TIA  110 .  
         [0047]    As can be seen from the foregoing, according to the present invention, the signal-reference voltage Sig_Ref is generated within the burst-mode optical receiver, and the differential-output signals are generated through the internal differential buffer. As a result, there is no need for a separate circuit for the automatic threshold controller (ATC) as in the prior art, thus the area of the whole circuit can be reduced and the whole system can be easily constructed.