Abstract:
An optical receiver includes: a converting unit that converts an optical signal into an electrical signal; an amplifying unit that amplifies the electrical signal; a regenerating unit that regenerates the amplified electrical signal; a correcting unit that performs correction of an error included in the regenerated electrical signal; a monitoring unit that performs monitoring of an optical current flowing through the converting unit; and a control unit that calculates a decision threshold based on a result of the correction and a result of the monitoring.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-296535, filed on Oct. 11, 2005, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an optical receiver that regenerates data from an optical signal based on an optimal decision threshold that is set dynamically according to the receiving power of the optical signal. 
         [0004]    2. Description of the Related Art 
         [0005]    With the popularization of the Internet in recent years, data traffic in communication networks has been significantly increasing. To cope with the increase of data traffic, an ultra-broadband photonic network employing a dense wavelength division multiplexing (DWDM) technology has been developed. An ultra-long-haul data communication can be performed with DWDM transmission, which uses an optical fiber including several tens of wavelength channels and a plurality of optical amplifiers connected in cascade on the optical fiber. In such ultra-long-haul data communication, however, the interference between wavelength channels significantly increases and the optical signal to noise ratio (OSNR) is seriously deteriorated due to optical noise from the optical amplifiers. Especially, data error due to the optical noise has become a bottleneck for DWDM transmission because it cannot be prevented by improving the sensitivity of an optical receiver. Therefore, to overcome this optical noise bottleneck an improvement of the error correction technology performed in the optical receiver is strongly needed. 
         [0006]    If the optical receiver corrects the data error using forward error correction (FEC), a bit error rate (BER) of the optical receiver can be obtained from a result of the error correction. On the other hand, the receiving characteristics of the optical receiver can be improved by optimizing its decision threshold that varies depending on the OSNR or a state of chromatic dispersion due to long-haul transmission. Therefore, the performance of the optical receiver can be improved by performing a feedback control based on the BER and by adjusting the decision threshold to the optimal level. 
         [0007]      FIG. 17  is a block diagram of a conventional optical receiver for DWDM transmission. As shown in  FIG. 17 , an optical receiver  1  includes a photodiode (PD)  2 , a trans-impedance amplifier (TIA) functioning as a preamplifier  3 , a variable-gain amplifier  4 , a gain-control amplifier  5 , a clock/data recovery (CDR)  6 , a forward error correction (FEC) unit  7 , a controller  8 , and a digital-to-analog converter (DAC)  9 . 
         [0008]    The PD  2  converts an optical input signal into an electrical signal. The preamplifier  3 , the variable-gain amplifier  4 , and the gain-control amplifier  5  perform reshaping of the electrical signal. The CDR  6  performs regeneration and retiming of the reshaped electrical signal. The FEC  7 , the controller  8 , and the DAC  9  are provided to adjust the decision threshold according to the amplitude of the reshaped electrical signal as shown in  FIG. 18  (see, for example, Japanese Patent Application Laid-Open No. H2-288640). 
         [0009]    However, the optical receiver  1  needs large circuit size and its control becomes complicated because it has to perform variable-gain control to keep constant reshaped electrical signal. Furthermore, the gain of the preamplifier  3  needs to be small to prevent saturation of amplitude when the input power of optical signal increases, thereby making it difficult to improve the sensitivity of the optical receiver  1 . 
         [0010]    On the other hand, another optical receiver achieving high sensitivity with a simple configuration has also been suggested. The optical receiver includes a high-gain limiting amplifier, and a direct current (DC) feedback circuit for controlling the DC level of the positive signal and the negative signal output from the limiting amplifier. The sensitivity of the optical receiver can be improved by increasing the gain of the preamplifier, while reducing the circuit size of the optical receiver. 
         [0011]    In such an optical receiver, however, the relation between the decision threshold of optical receiver and a feed-backed threshold control signal from an forward error correction (FEC) unit is not unique, because the condition of signal in the optical receiver greatly differs depending on, for example, the receiving power of the signal. The limiting amplifier performs a complex operation in the DC feedback control. Specifically, as long as the amplitude of an input signal is less than predetermined limiting amplitude, the limiting amplifier performs a linear operation and linearly amplifies the input signal. On the other hand, when the amplitude of the input signal reaches the limiting amplitude, the limiting amplifier performs a limiting operation and extracts a part of the input signal near cross points. The wide dynamic range of the receiving power makes it difficult to set an appropriate decision threshold, using the threshold control signal, for respective input power. As a result, a sufficient error correction cannot be achieved. 
       SUMMARY OF THE INVENTION 
       [0012]    It is an object of the present invention to at least solve the above problems in the conventional technology. 
         [0013]    An optical receiver according to an aspect of the present invention includes: a converting unit that converts an optical signal into an electrical signal; an amplifying unit that amplifies the electrical signal; a regenerating unit that regenerates the electrical signal amplified by the amplifying unit; a correcting unit that performs correction of an error included in the electrical signal regenerated by the regenerating unit; a monitoring unit that performs monitoring of an photo current flowing through the converting unit; and a control unit that calculates a decision threshold based on a result of the correction and a result of the monitoring. 
         [0014]    The other objects, features, and advantages of the present invention are specifically set forth in or will become apparent from the following detailed description of the invention when read in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0015]      FIG. 1  is a block diagram of an optical receiver according to a first embodiment of the present invention; 
           [0016]      FIG. 2  is a schematic illustrating an operation of the optical receiver shown in  FIG. 1 ; 
           [0017]      FIGS. 3 to 6  are waveform diagrams illustrating the output amplitude of a limiting amplifier shown in  FIG. 1 ; 
           [0018]      FIG. 7  is a flowchart of a decision threshold setting process according to the first embodiment; 
           [0019]      FIG. 8  is a block diagram of an optical receiver according to a second embodiment of the present invention; 
           [0020]      FIG. 9  is a block diagram of an optical receiver according to a third embodiment of the present invention; 
           [0021]      FIG. 10  is a block diagram of an optical receiver according to a fourth embodiment of the present invention; 
           [0022]      FIG. 11  is a block diagram of an optical receiver according to a fifth embodiment of the present invention; 
           [0023]      FIG. 12  is a block diagram of an optical receiver according to a sixth embodiment of the present invention; 
           [0024]      FIG. 13  is a block diagram of an optical receiver according to a seventh embodiment of the present invention; 
           [0025]      FIG. 14  is a block diagram of an optical receiver according to an eighth embodiment of the present invention; 
           [0026]      FIG. 15  is a block diagram of an optical receiver according to a ninth embodiment of the present invention; 
           [0027]      FIG. 16  is a flowchart of a decision threshold setting process according to the ninth embodiment; 
           [0028]      FIG. 17  is a block diagram of a conventional optical receiver; and 
           [0029]      FIG. 18  is a waveform diagram illustrating the output amplitude of the conventional optical receiver. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    Exemplary embodiments of the present invention will be explained in detail with reference to the accompanying drawings. 
         [0031]      FIG. 1  is a block diagram of an optical receiver according to a first embodiment of the present invention. An optical receiver  10  includes a power monitor  11 , a photodiode (PD)  12 , a preamplifier  13 , a limiting amplifier  14 , a direct current (DC) feedback amplifier  15 , a clock/data recovery (CDR)  16 , a forward error correction (FEC) unit  17 , and a controller  18 . 
         [0032]    The PD  12  converts an optical input signal into an electrical signal. The preamplifier  13  and the limiting amplifier  14  amplify the electrical signal. An output signal from the preamplifier  13  is input to one of the input terminals of the limiting amplifier  14 . The DC feedback amplifier  15  feedbacks an output signal from the limiting amplifier  14  back to the other input terminal of the limiting amplifier  14 . Thus, the DC feedback amplifier  15  controls the DC level of the positive signal and the negative signal output from the limiting amplifier  14 . The CDR  16  regenerates and retimes the output signal from the limiting amplifier  14 . 
         [0033]    The FEC  17  corrects data error included in the regenerated signal. The power monitor  11  monitors a photo current flowing through the PD  12 . The controller  18  calculates an optimal decision threshold according to the receiving power and the bit error rate. Specifically, the controller  18  calculates the optimal decision threshold based on a monitor signal from the power monitor  11 , which corresponding to the monitored reception power, and a threshold control signal from the FEC  17 , which corresponding to the bit error rate. The calculated decision threshold is converted into an analog signal in the controller  18 , and is set to the DC feedback amplifier  15 . 
         [0034]      FIG. 2  is a schematic illustrating an operation of the optical receiver  10 .  FIGS. 3 and 4  are waveform diagrams illustrating the output amplitude of the limiting amplifier  14  performing the linear operation with the decision threshold being set at 50% and 30%, respectively.  FIGS. 5 and 6  are waveform diagrams illustrating the output amplitude of the limiting amplifier  14  performing the limiting operation with the decision threshold being set at 50% and 30%, respectively. The above decision thresholds (%) are normalized with respect to the signal amplitude. 
         [0035]    As shown in  FIGS. 3 to 6 , the limiting amplifier  14  performs the linear operation and the limiting operation. In the linear operation, the decision threshold is changed in proportion to the reception power as shown in  FIG. 2  because the signal level of the positive signal and the negative signal changes due to the DC feedback control. On the other hand, in the limiting operation, the signal level does not change but the pulse width of the signal changes according to the rising edge timing and the falling edge timing of the signal. Therefore, as long as the rising and falling timings are stable in the signal, the decision threshold is kept substantially constant in the limiting operation as shown in  FIG. 2 . 
         [0036]    The controller  18  calculates an optimal decision threshold based on the above operations of the limiting amplifier  14 . The DC feedback amplifier  15  controls the DC level of the feedback signal to the limiting amplifier  14  based on the decision threshold set by the controller  18 , to control the DC level of the positive signal and the negative signal output from the limiting amplifier  14 . 
         [0037]      FIG. 7  is a flowchart of a decision threshold setting process performed by the controller  18 . The controller  18  receives the monitor signal indicating the receiving power of an optical signal from the power monitor  11 , and sets an initial value of the decision threshold (step S 1 ). Then, the controller  18  calculates an initial value of the error rate based on the initial value of the decision threshold and the threshold control signal from the FEC  17  (step S 2 ). The controller  18  determines whether the error rate satisfies a predetermined condition (step S 3 ). When the error rate satisfies the condition (“YES” at step S 3 ), the process is completed. 
         [0038]    On the other hand, when the error rate does not satisfy the condition (“NO” at step S 3 ), the controller  18  receives updated monitor signal from the power monitor  11 , and changes the decision threshold (step S 4 ). Then, the controller  18  calculates the error rate (step S 5 ), and determines whether the error rate satisfies the condition (step S 6 ). When the error rate does not satisfy the condition (“NO” at step S 6 ), the process returns to step S 4 . The process from step S 4  to step S 6  is repeated until an error rate that satisfies the condition is obtained. When the error rate satisfies the condition (“YES” at step S 6 ), the process is completed. 
         [0039]      FIG. 8  is a block diagram of an optical receiver according to a second embodiment of the present invention. An optical receiver  20  shown in  FIG. 8  performs a DC feedback control different from the DC feedback control explained in the first embodiment. Specifically, the optical receiver  20  includes a DC feedback amplifier  25  instead of the DC feedback amplifier  15  shown in  FIG. 1 . The output signals from the limiting amplifier  14  are input to the DC feedback amplifier  25 . The output signal from the DC feedback amplifier  25  controls a current source  22  connected to the PD  12  and the preamplifier  13 . 
         [0040]    In a similar manner as in the first embodiment, the decision threshold calculated by the controller  18  is set in the DC feedback amplifier  25 . The output signal from the preamplifier  13  is input to one of the input terminals of the limiting amplifier  14  as it is, and also input to the other input terminal through a low pass filter (LPF)  21  that extracts the DC level of the output signal of preamplifier. 
         [0041]    The DC feedback amplifier  25  performs a DC feedback control based on the decision threshold set by the controller  18 , to control the DC level of the positive signal and the negative signal that are output from the preamplifier  13  and input to the limiting amplifier  14 . 
         [0042]      FIG. 9  is a block diagram of an optical receiver according to a third embodiment of the present invention. An optical receiver  30  shown in  FIG. 9  performs a DC feedback control different from the DC feedback control explained in the second embodiment. Specifically, the optical receiver  30  includes a DC feedback amplifier  35  instead of the DC feedback amplifier  25  shown in  FIG. 8 . The output signal from the preamplifier  13  is input to the DC feedback amplifier  35 . The output signal from the DC feedback amplifier  35  controls the current source  22 . 
         [0043]    In a similar manner as in the second embodiment, the decision threshold calculated by the controller  18  is set in the DC feedback amplifier  35 . However, in the third embodiment, the output signal from the preamplifier  13  is subjected to a feedback control performed by the DC feedback amplifier  35 , to control the DC level of the positive signal and the negative signal to be input to the limiting amplifier  14 . 
         [0044]      FIG. 10  is a block diagram of an optical receiver according to a fourth embodiment of the present invention. An optical receiver  40  shown in  FIG. 10  controls, instead of performing the DC feedback control, a DC level of the output signal from the limiting amplifier  14  directly based on the decision threshold calculated the controller  18 . The limiting amplifier  14  and the CDR  16  are AC-coupled via capacitors  41  and  42 , and the decision threshold calculated by the controller  18  is input to one of the input terminals of the CDR  16  by an adder  43 . 
         [0045]      FIG. 11  is a block diagram of an optical receiver according to a fifth embodiment of the present invention. The configuration of an optical receiver  50  shown in  FIG. 11  is similar to that of the optical receiver  40  according to the fourth embodiment (see  FIG. 10 ). However, unlike the optical receiver  40 , the optical receiver  50  performs the same DC feedback control as that of the first embodiment (see  FIG. 1 ). Specifically, the DC feedback amplifier  15  of the optical receiver  50  feeds back the output signal from the limiting amplifier  14  to one of the input terminals of the limiting amplifier  14 . However, the decision threshold calculated by the controller  18  is not input to the DC feedback amplifier  15 . 
         [0046]      FIG. 12  is a block diagram of an optical receiver according to a sixth embodiment of the present invention. The configuration of an optical receiver  60  shown in  FIG. 12  is similar to that of the optical receiver  40  according to the fourth embodiment (see  FIG. 10 ). However, unlike the optical receiver  40 , the optical receiver  60  performs the same DC feedback control as that of the third embodiment (see  FIG. 9 ). Specifically, the DC feedback amplifier  35  of the optical receiver  60  controls the current source  22  connected to the PD  12  and the preamplifier  13  by inputting the output signal from the preamplifier  13  to the current source  22 . However, the decision threshold calculated by the controller  18  is not input to the DC feedback amplifier  35 . 
         [0047]      FIG. 13  is a block diagram of an optical receiver according to a seventh embodiment of the present invention. The configuration of an optical receiver  70  shown in  FIG. 13  is same as that of the optical receiver  50  according to the fifth embodiment (see  FIG. 11 ). However, in the optical receiver  70 , the decision threshold calculated by the controller  18  is input to the DC feedback amplifier  15  as in the optical receiver  10  according to the first embodiment (see  FIG. 1 ). In other words, in the optical receiver  70 , the DC level of the positive signal and the negative signal output from the limiting amplifier  14  is controlled at both sides of the limiting amplifier  14  (that is, the input side and the output side). According to the seventh embodiment, the decision threshold can be adjusted appropriately even when the relation between the reception power and the decision threshold is more complicated. 
         [0048]      FIG. 14  is a block diagram of an optical receiver according to an eighth embodiment of the present invention. The configuration of an optical receiver  80  shown in  FIG. 14  is similar to that of the optical receiver  10  according to the first embodiment (see  FIG. 1 ), except for including an analog operating unit  88 , such as an operational amplifier, instead of the controller  18 . The analog operating unit  88  performs an analog processing to set the decision threshold based on the monitor signal and the threshold control signal. With the above configuration, the decision threshold is output as an analog signal from the analog operating unit  88 . 
         [0049]      FIG. 15  is a block diagram of an optical receiver according to a ninth embodiment of the present invention. The configuration of an optical receiver  90  shown in  FIG. 15  is similar to that of the optical receiver  10  according to the first embodiment (see  FIG. 1 ), except for including a controller  91 , a calculator  92 , and a DAC  93  instead of the controller  18 . The controller  91  generates a normalized threshold control signal based on the threshold control signal input from the FEC  17 . The calculator  92  calculates an optimal decision threshold according to the reception power and the error rate. Specifically, the calculator  92  calculates the optimal decision threshold based on the normalized threshold control signal input from the controller  91  and the monitor signal input from the power monitor  11 . The DAC  93  converts the optimal decision threshold output from the calculator  92  from digital to analog, and set the decision threshold to the DC feedback amplifier  15 . 
         [0050]      FIG. 16  is a flowchart of a decision threshold setting process performed by the controller  91  and the calculator  92 . The controller  91  sets an initial value of the normalized threshold (step S 11 ). Then, the calculator  92  receives the monitor signal from the power monitor  11 , and sets an initial value of the decision threshold (step S 12 ). The calculator  92  calculates an initial value of the error rate based on the initial values of the normalized threshold and the decision threshold (step S 13 ), and determines whether the error rate satisfies a predetermined condition (step S 14 ). When the error rate satisfies the condition (“YES” at step S 14 ), the process is completed. 
         [0051]    On the other hand, when the error rate does not satisfy the condition (“NO” at step S 14 ), the controller  91  changes the normalized threshold (step S 15 ). The calculator  92  receives updated monitor signal from the power monitor  11 , and changes the decision threshold (step S 16 ). The calculator  92  recalculates the error rate based on the normalized threshold and the decision threshold (step S 17 ), and determines whether the error rate satisfies the condition (step S 18 ). 
         [0052]    When the error rate does not satisfy the condition (“NO” at step S 18 ), the process returns back to step S 15 , and the process from step S 15  to step S 18  is repeated until an error rate that satisfies the condition is obtained. When the error rate satisfies the condition (“YES” at step S 18 ), the process is completed. 
         [0053]    The configuration according to the ninth embodiment is suitable for a case in which the controller  91  and the calculator  92  are separately provided. For example, a module formed by the calculator  92 , the DAC  93 , and the PD can be mounted on a substrate provided with the controller  91 . The controller  18  or the analog calculator according to the first to the eighth embodiments may also be provided as two independent components of the controller and the calculator. 
         [0054]    According to the embodiments described above, an optimal decision threshold is set according to the receiving power varying in a wide range, thereby improving the performance of the error correction performed by an optical receiver. Moreover, a high-quality and error-free optical transmission can be achieved by applying a high-gain error correction technology to the highly-sensitive optical receiver with a limiting amplifier. 
         [0055]    Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.