Abstract:
Disclosed herein is an optical receiver including: a light receiving element configured to have an anode and a cathode and generate a photocurrent dependent on received signal light; a current-voltage conversion circuit configured to be connected to the anode of the light receiving element and convert the photocurrent to a voltage signal; and a capacitive passive element configured to have a first electrode and a second electrode. The cathode of the light receiving element is connected to the first electrode of the capacitive passive element, and the second electrode of the capacitive passive element is connected to a reference potential of the current-voltage conversion circuit and the second electrode is not coupled to objects other than a reference potential terminal of the current-voltage conversion circuit.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical receiver applied to an optical transmission system, and an optical transmission system. 
     2. Description of the Related Art 
     A transimpedance amplifier (TIA) to convert current to a voltage is used in an optical transmission system. 
     The optical transmission system refers to a system in which data arising from optical conversion of an electrical signal is transmitted from an optical transmitter (TX) and the optical data received by an optical receiver (RX) is converted to an electrical signal. 
     A photodiode (PD) converts the optical data sent from the optical transmitter (TX) to a current and the TIA is used to convert this current to a differential voltage. 
     The electrical signal output from a driver of the optical transmitter (TX) is converted to an optical signal by an electro-optical conversion element, a laser diode (LD) or a vertical cavity surface emitting laser (VCSEL). This optical signal passes through an optical fiber to be subjected to opto-electrical conversion by the PD of the optical receiver (RX). 
     In the communication from the optical transmitter (TX) to the optical receiver (RX), power loss at the connection part of the optical fiber and in the electrical/optical conversion and the optical/electrical conversion is large and the output current of the PD has significantly-low amplitude in some cases. Thus, it is preferable for the TIA to have a high signal-to-noise ratio (SNR). 
     SUMMARY OF THE INVENTION 
       FIG. 1  is a diagram showing a first configuration example of the optical receiver and is a diagram showing a method for connecting a PD and a TIA generally used. 
       FIG. 2  is a diagram showing a second configuration example of the optical receiver. 
       FIG. 3  is a diagram showing a third configuration example of the optical receiver. 
     An optical receiver  1  of  FIG. 1  has a photodiode (PD)  2  as a light receiving element, a capacitor (C)  3  as a capacitive passive element, a TIA  4  as a current-voltage conversion circuit, and a filter  5 . 
     In  FIG. 1 , L 1  to L 5  denote parasitic inductors and ND 1  denotes a node. 
     The TIA  4  in  FIG. 1  includes an input terminal “in,” a power supply terminal VDEF connected to a power supply potential VDD, a reference potential terminal VSFE connected to a reference potential VSS, and a front end (FE) part  41  connected to the input terminal “in,” the power supply terminal VDEF, and the reference potential terminal VSFE. 
     In a TIA  4 A in  FIG. 2  and a TIA  4 B in  FIG. 3 , a limiting amplifier (LA)  42  is disposed at the output stage of the FE part  41  in addition to the configuration of  FIG. 1 . 
     The cathode of the PD  2  is connected to the power supply potential VDD via the filter  5  and to a first electrode  31  of the capacitor  3 , and a second electrode  32  of the capacitor  3  is connected to the reference potential VSS. The node ND 1  is formed by these connection points. 
     The parasitic inductor L 1  exists between the cathode of the PD  2  and the node ND 1 . 
     The anode of the PD  2  is connected to the input terminal “in” of the TIA  4 . The parasitic inductors L 2  and L 3  exist between the anode of the PD  2  and the input terminal “in” of the TIA  4 . 
     The parasitic inductor L 4  exists between the power supply terminal VDFE of the TIA  4  and the power supply potential VDD, and the parasitic inductor L 5  exists between the reference potential terminal VSFE and the reference potential VSS. 
     In the optical receiver  1  of  FIG. 1 , the PD  2  receives an optical signal and the FE part  41  of the TIA  4  receives the current obtained by electrical conversion and converts it to a differential voltage. 
     The modulation current of the output of the PD  2  is only about several tens of microamperes (μA) when the power is weak. Therefore, if a 20-μA current is received by 50Ω as the input impedance of the FE part  41 , the modulation voltage of the terminal “in” is only about 1 mV and thus is easily buried in noise. 
     Because the TIA  4  amplifies low amplitude, the LA  42  is frequently provided in addition to the FE part  41  as shown in  FIG. 2 . 
     Self-generated noise is produced by this LA  42  and various circuits on the TIA chip and applied to a reference potential terminal VSFELA. 
     On the other hand, noise at the reference potential terminal VSFELA is not correctly transmitted to the terminal “in” due to the parasitic capacitance of the PD  2  and the FE part  41 , and so forth. Therefore, noise is added to the terminal “in” from the viewpoint of the reference potential terminal VSFELA as the basis and the SNR is deteriorated. 
     Therefore, a configuration in which the power supply and reference potentials are separated for only the FE part  41  as shown in  FIG. 3  is generally preferred. 
     However, because elements on the chip are coupled to each other by the board and interconnects actually, the noise propagated to the reference potential terminal VSFE is not completely removed. For the TIA, which should have a high SNR, this noise that is not completely removed is a problem. 
     Furthermore, as the number of channels increases, the amount of noise propagated to the reference potential terminal VSFE increases. In the case of an application with a larger number of channels, the influence of the noise increases and causes a more serious problem. 
       FIG. 4  is a diagram showing a fourth configuration example of the optical receiver and is a diagram showing a configuration in which the PD and the TIA chip are coupled to each other by a transmission line. 
     In an optical receiver  1 C of  FIG. 4 , the anode of the PD  2  is connected to the input terminal “in” of the TIA  4 B by a transmission line TL 1 . 
     In this case, similarly to the above description, if noise is applied to the reference potential terminal VSFE, the component that is not correctly propagated to the input terminal “in” acts as noise on the terminal “in” from the viewpoint of the reference potential terminal VSFE. 
     The existence of the transmission line TL 1  possibly causes a problem that the reflected component of noise propagated to the PD  2  is amplified to be imposed on the terminal “in” and the deterioration of the SNR increases. 
     There is a need for the present invention to provide an optical receiver and an optical transmission system capable of reducing noise on an input terminal from the viewpoint of the reference potential of the circuit and capable of decreasing a noise component superimposed on a signal to enhance the accuracy of the signal-to-noise ratio (SNR). 
     According to an embodiment of the present invention, there is provided an optical receiver including a light receiving element configured to have an anode and a cathode and generate a photocurrent dependent on received signal light, a current-voltage conversion circuit configured to be connected to the anode of the light receiving element and convert the photocurrent to a voltage signal, and a capacitive passive element configured to have a first electrode and a second electrode. The cathode of the light receiving element is connected to the first electrode of the capacitive passive element. The second electrode of the capacitive passive element is connected to a reference potential of the current-voltage conversion circuit and the second electrode is not coupled to objects other than a reference potential terminal of the current-voltage conversion circuit. 
     According to another embodiment of the present invention, there is provided an optical transmission system including an optical transmission line configured to transmit an optical signal, an optical signal transmitting device configured to transmit an optical signal to the optical transmission line, and an optical signal receiving device configured to include an optical receiver that receives an optical signal transmitted in the optical transmission line and converts the optical signal to an electrical signal. The optical receiver includes a light receiving element that has an anode and a cathode and generates a photocurrent dependent on received signal light, a current-voltage conversion circuit that is connected to the anode of the light receiving element and converts the photocurrent to a voltage signal, and a capacitive passive element having a first electrode and a second electrode. The cathode of the light receiving element is connected to the first electrode of the capacitive passive element. The second electrode of the capacitive passive element is connected to a reference potential of the current-voltage conversion circuit and the second electrode is not coupled to objects other than a reference potential terminal of the current-voltage conversion circuit. 
     According to these embodiments of the present invention, noise on an input terminal from the viewpoint of the reference potential of the circuit can be reduced. Thus, a noise component superimposed on a signal can be decreased and the accuracy of the signal-to-noise ratio (SNR) can be enhanced. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing a first configuration example of an optical receiver and is a diagram showing a method for connecting a PD and a TIA generally used; 
         FIG. 2  is a diagram showing a second configuration example of the optical receiver; 
         FIG. 3  is a diagram showing a third configuration example of the optical receiver; 
         FIG. 4  is a diagram showing a fourth configuration example of the optical receiver and is a diagram showing a configuration in which the PD and a TIA chip are coupled to each other by a transmission line; 
         FIG. 5  is a diagram showing the basic configuration of an optical transmission system according to embodiments of the present invention; 
         FIG. 6  is a diagram showing the configuration of an optical receiver according to a first embodiment of the present invention; 
         FIG. 7  is a diagram for explaining the influence of noise at a reference potential terminal in the TIA of the optical receiver of  FIG. 1  as a comparative example; 
         FIG. 8  is a diagram for explaining the influence of noise at the reference potential terminal in the TIA of the optical receiver according to the first embodiment; 
         FIGS. 9A to 9D  are diagrams showing simulation results about the influence of the noise at the reference potential terminal in the TIA of the optical receiver of  FIG. 1  as the comparative example; 
         FIGS. 10A to 10D  are diagrams showing simulation results about the influence of the noise at the reference potential terminal in the TIA of the optical receiver according to the first embodiment; 
         FIG. 11  is a diagram showing the configuration of an optical receiver according to a second embodiment of the present invention; 
         FIG. 12  is a diagram showing the configuration of an optical receiver according to a third embodiment of the present invention; 
         FIGS. 13A to 13D  are diagrams showing simulation results about the influence of noise at the reference potential terminal in the TIA of the optical receiver of  FIG. 4  as a comparative example; 
         FIGS. 14A to 14D  are diagrams showing simulation results about the influence of noise at the reference potential terminal in the TIA of the optical receiver according to the third embodiment; and 
         FIG. 15  is a diagram showing the configuration of an optical receiver according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below in association with the drawings. 
     The order of the description is as follows. 
     1. First Embodiment (first configuration example of optical receiver) 
     2. Second Embodiment (second configuration example of optical receiver) 
     3. Third Embodiment (third configuration example of optical receiver) 
     4. Fourth Embodiment (fourth configuration example of optical receiver) 
       FIG. 5  is a diagram showing the basic configuration of an optical transmission system according to the embodiments of the present invention. 
     This communication system  100  is configured with an optical signal transmitting device  200 , an optical signal receiving device  300 , and an optical transmission line  400 . 
     The optical signal transmitting device  200  includes an optical transmitter  210  and an electrical signal output from this transmitter  210  is converted to an optical signal by an electro-optical conversion element, an LD or a VCSEL. 
     This optical signal is transmitted in the optical transmission line  400  formed of an optical fiber and subjected to opto-electrical conversion by a PD of an optical receiver (RX)  310  of the optical signal receiving device  300 . 
     A specific description will be made below about the configuration and function of the optical receiver  310  of the optical signal receiving device  300  having characteristic configurations of the embodiments. 
     1. First Embodiment 
       FIG. 6  is a diagram showing the configuration of an optical receiver according to a first embodiment of the present invention. 
     As shown in  FIG. 6 , the optical receiver  310  has a photodiode (PD)  311  as a light receiving element, a capacitor (C)  312  as a capacitive passive element, a TIA  313  as a current-voltage conversion circuit, and a filter  314 . 
     In  FIG. 6 , L 311  to L 316  denote parasitic inductors and ND 311  denotes a node. 
     The TIA  313  in  FIG. 6  has input terminals in 1  (first terminal) and in 2  (second terminal), a power supply terminal VDFE 1  connected to a power supply potential VDD, and a reference potential terminal VSFE 1  connected to a reference potential VSS. 
     Furthermore, the TIA  313  includes a front end (FE) part  3131  connected to the input terminals in 1  and in 2 , the power supply terminal VDFE 1 , and the reference potential terminal VSFE 1 . 
     For the TIA  313 , the second terminal in 2  and the reference potential terminal VSFE 1  are connected to each other in the chip. 
     The cathode of the PD  311  is connected to the power supply potential VDD via the filter  314  and to a first electrode  3121  of the capacitor  312 , and the node ND 311  is formed by these connection points. 
     A second electrode  3122  of the capacitor  312  is connected to only the reference potential terminal VSFE 1  via the terminal in 2  of the TIA  313 . 
     The parasitic inductor L 311  exists between the cathode of the PD  311  and the node ND 311 . 
     The anode of the PD  311  is connected to the input terminal in 1  of the TIA  313 . The parasitic inductors L 312  and L 313  exist between the anode of this PD  311  and the input terminal in 1  of the TIA  313 . 
     The parasitic inductor L 314  exists between the second electrode  3122  of the capacitor  312  and the terminal in 2  of the TIA  313 . 
     The parasitic inductor L 315  exists between the power supply terminal VDFE 1  of the TIA  313  and the power supply potential VDD, and the parasitic inductor L 316  exists between the reference potential terminal VSFE 1  and the reference potential VSS (e.g. ground potential GND). 
     As a characteristic of the optical receiver  310  of the present embodiment, the reference potential of the PD  311  is supplied from the reference potential terminal VSFE 1  of the TIA  313  as the chip. 
     In the optical receiver  1  of  FIG. 1  as a comparative example, the return path between the PD  2  and the FE part  41  of the TIA  4  is continuous via the common GND. 
     In contrast, in the optical receiver according to the embodiment of the present invention, the return path between the PD  311  and the FE part  3131  is closed on the basis of the reference potential terminal VSFE 1 . 
     A consideration will be made below in association with  FIG. 7  and  FIG. 8  about the influence of noise at the reference potential terminal VSFE in the TIAs  313  and  4  in the optical receiver  310  according to the present embodiment and the optical receiver  1  of  FIG. 1  as the comparative example. 
       FIG. 7  is a diagram for explaining the influence of noise at the reference potential terminal VSFE in the TIA  4  of the optical receiver  1  of  FIG. 1  as the comparative example. 
       FIG. 8  is a diagram for explaining the influence of noise at the reference potential terminal VSFE in the TIA  313  of the optical receiver  310  according to the present embodiment. 
     Referring to  FIG. 7  and  FIG. 8 , in both of the FE parts  41  and  3131 , a resistive element Rin is formed between the signal line and the reference potential terminal VSFE, and a converter CNV (X) to convert a single signal to differential signals “out” and “outb” is disposed at the output stage. 
     In  FIG. 7  and  FIG. 8 , C 1  denotes the parasitic capacitance of the PDs  2  and  311  and C 2  denotes the parasitic capacitance of the FE parts  41  and  3131 . 
     In the optical receiver  1  of  FIG. 1  as the comparative example, the return path between the PD  2  and the FE part  41  of the TIA  4  is continuous via the common GND. 
     As a result, high-frequency noise HNZ is superimposed on a signal waveform SW in the TIA  4  of the optical receiver  1  of  FIG. 1  as the comparative example. 
     If the impedance of the parasitic capacitance C 1  of the PD  2  is defined as Z 1 , the impedance of the FE part  41  is defined as Z 2  and noise at the reference potential terminal VSFE is defined as VN, the amount NZ of noise transmitted from the side of the reference potential terminal VSFE to the input terminal “in” is represented by the following equation.
 
 NZ={Z 1/( Z 1+ Z 2)}× VN  
 
     As just described, in the optical receiver  1  of  FIG. 1  as the comparative example, the path between the terminals in-VSFE is affected by the VSFE noise VN by [Z 1 /(Z 1 +Z 2 )×VN]. 
     In contrast, in the optical receiver  310  according to the embodiment of the present invention, the return path between the PD  311  and the FE part  3131  is closed on the basis of the reference potential terminal VSFE. 
     As a result, the side of the input terminal “in” follows the VSFE noise VN and the path between the terminals in-VSFE is not affected by the noise in the TIA  313  of the optical receiver  310  according to the embodiment of the present invention. 
     As shown in  FIG. 8 , the high-frequency noise HNZ looks to be absent and the superposition thereof on the signal waveform SW is avoided. 
       FIGS. 9A to 9D  and  FIGS. 10A to 10D  show simulation results about the influence of noise at the reference potential terminal VSFE in the TIAs  313  and  4  in the optical receiver  310  according to the first embodiment and in the optical receiver  1  of  FIG. 1  as the comparative example. 
       FIGS. 9A to 9D  are diagrams showing the simulation results about the influence of the noise at the reference potential terminal VSFE in the TIA  4  of the optical receiver  1  of  FIG. 1  as the comparative example. 
       FIGS. 10A to 10D  are diagrams showing the simulation results about the influence of the noise at the reference potential terminal VSFE in the TIA  313  of the optical receiver  310  according to the first embodiment. 
       FIGS. 9A to 9D  and  FIGS. 10A to 10D  show the simulation results when noise is applied to the reference potential terminals VSFE and VSFE 1  in the configurations of  FIG. 1  and  FIG. 6 , respectively. 
     The waveform of the applied noise is a sine wave with amplitude of 1 mApp and a frequency of 761 MHz. The waveform of the input data is PRBS 7  with amplitude of 10 μApp and a frequency of 5 Gbps. The parasitic inductance is 1 nH. 
     The waveforms of  FIGS. 9A to 9C  and  FIGS. 10A to 10C  show the voltages of the reference potential terminal VSFE, the path in-VSFE between the input terminal and the reference potential terminal, and the differential output out-outb, respectively. 
     Each of  FIG. 9D  and  FIG. 10D  shows a so-called eye pattern that is the waveform pattern of the differential output out-outb. 
     The same circuits are used for the PDs and the FE parts in both of the configurations of  FIG. 1  and  FIG. 6 . 
     In the optical receiver  1  of  FIG. 1  as the comparative example, as shown in  FIG. 9D , the eye pattern looks to be completely closed and the data is deteriorated due to the influence of the variation of the reference potential terminal VSFE. 
     In contrast, in the optical receiver  310  of  FIG. 6  according to the first embodiment of the present invention, as shown in  FIG. 10D , the data is not deteriorated although the amount of variation of the reference potential terminal VSFE is the same as that in the optical receiver  1  of  FIG. 1 . 
     As described above, according to the first embodiment, the return path between the PD  311  and the FE part  3131  is closed on the basis of the reference potential terminal VSFE 1  in the optical receiver. 
     Due to this feature, the side of the input terminal “in” follows the VSFE noise VN and the path between the terminals in-VSFE is not affected by the noise. As a result, the noise component superimposed on the signal decreases and thus the accuracy of the signal-to-noise ratio (SNR) is enhanced. 
     The configuration of the first embodiment can be applied also to multiple channels. Whether the reference potential terminal VSFE 1  of the multiple channels is common or separated, the technique of this configuration holds as long as the reference potential of the PD  311  is the same as that of the reference potential terminal VSFE 1 . 
     If the reference potential terminal VSFE 1  of the multiple channels is separated for each one channel, noise is not imposed on the terminal “in” from the viewpoint of the reference potential terminal VSFE 1  as with the above description. 
     Even when the reference potential terminal VSFE 1  of the multiple channels is common, noise applied to the reference potential terminal VSFE 1  is equally transmitted to the PDs  311  of the respective channels. 
     As a result, the terminals “in” of the respective channels also equally vary and thus the noise is not superimposed (not imposed) on the respective terminals “in” from the viewpoint of the reference potential terminal VSFE 1 . 
     2. Second Embodiment 
       FIG. 11  is a diagram showing the configuration of an optical receiver according to a second embodiment of the present invention. 
     An optical receiver  310 A according to the second embodiment is different from the optical receiver  310  according to the above-described first embodiment in that a limiting amplifier (LA)  3132  as a limiting circuit is disposed at the output stage of the FE part  3131  in a TIA  313 A. 
     The LA  3132  and the FE part  3131  share a power supply terminal VDFE 1 A and a reference potential terminal VSFE 1 A. 
     In the optical receiver  310 A according to the second embodiment, even when self-generated noise of the LA  3132  and so forth is applied to the reference potential terminal VSFE 1 A, the cathode potential of the PD  311  also varies by the same amount as that of variation of the reference potential terminal VSFE 1 A, and the potential of the terminal “in” also varies by the same amount as well. 
     Because the noise applied to the reference potential terminal VSFE 1 A is transmitted to the terminal “in” without deterioration, the potential of the terminal “in” from the viewpoint of the reference potential terminal VSFE 1 A is not affected by the noise. 
     The second embodiment can achieve the same advantageous effects as those of the above-described first embodiment. 
     3. Third Embodiment 
       FIG. 12  is a diagram showing the configuration of an optical receiver according to a third embodiment of the present invention. 
     An optical receiver  310 B according to the third embodiment is different from the optical receiver  310 A according to the above-described second embodiment in the following point. 
     In the optical receiver  310 B, a transmission line TL 311  is formed between the PD  311  and the FE part  3131  of a TIA  313 B and the reference potential of the PD  311  is supplied from the reference potential terminal VSFE 1  of the TIA  313 B as the chip. 
     Furthermore, in the optical receiver  310 B, each of the FE part  3131  and the LA  3132  has the power supply terminal and the reference potential terminal separately. 
     The FE part  3131  is connected to the power supply potential VDD via the power supply terminal VDFE 1  and connected to the reference potential VSS via the reference potential terminal VSFE 1 . 
     The LA  3132  is connected to the power supply potential VDD via the power supply terminal VDLA 1  and connected to the reference potential VSS via the reference potential terminal VSLA 1 . 
     The parasitic inductor L 315  exists between the power supply terminal VDFE 1  of the TIA  313 B and the power supply potential VDD, and the parasitic inductor L 316  exists between the reference potential terminal VSFE 1  and the reference potential VSS (e.g. ground potential GND). 
     The parasitic inductor L 317  exists between the power supply terminal VDLA 1  of the TIA  313 B and the power supply potential VDD, and the parasitic inductor L 318  exists between the reference potential terminal VSLA 1  and the reference potential VSS (e.g. ground potential GND). 
     As a characteristic of the optical receiver  310 B of the present embodiment, the transmission line TL  311  exists between the PD  311  and the FE part  3131  and the reference potential of the PD  311  is supplied from the reference potential terminal VSFE 1  of the TIA  313 B as the chip. 
     In the optical receiver  1 C of  FIG. 4  as a comparative example, the return path between the PD  2  and the FE part  41  of the TIA  4 B is continuous via the common GND including the transmission line TL 1 . 
     In contrast, in the optical receiver  310 B according to the embodiment of the present invention, the return path between the PD  311  and the FE part  3131  is closed on the basis of the reference potential terminal VSFE 1  including the transmission line TL 311 . 
     In the optical receiver  310 B according to the third embodiment, if self-generated noise of the LA  3132  and so forth is applied to the reference potential terminal VSFE 1 , noise is propagated to the PD  311  from both of the reference potential terminal VSFE 1  and the terminal “in” via the transmission line TL 311 . Furthermore, the same amount of noise is imposed on the anode and cathode of the PD  311  and thus the noise is cancelled. 
     As a result, the terminal “in” from the viewpoint of the reference potential terminal VSFE 1  is not affected by the noise. 
       FIGS. 13A to 13D  and  FIGS. 14A to 14D  show simulation results about the influence of noise at the reference potential terminal VSFE in the TIAs  313 B and  4 B in the optical receiver  310 B according to the present embodiment and in the optical receiver  1 C of  FIG. 4  as the comparative example. 
       FIGS. 13A to 13D  are diagrams showing the simulation results about the influence of the noise at the reference potential terminal VSFE in the TIA  4 B of the optical receiver  1 C of  FIG. 4  as the comparative example. 
       FIGS. 14A to 14D  are diagrams showing the simulation results about the influence of the noise at the reference potential terminal VSFE 1  in the TIA  313 B of the optical receiver  310 B according to the third embodiment. 
       FIGS. 13A to 13D  and  FIGS. 14A to 14D  show the simulation results when noise is applied to the reference potential terminals VSFE and VSFE 1  in the configurations of  FIG. 4  and  FIG. 12 , respectively. 
     The waveform of the applied noise is a sine wave with amplitude of 1 mApp and a frequency of 761 MHz. The waveform of the input data is PRBS 7  with amplitude of 10 μApp and a frequency of 5 Gbps. The parasitic inductance is 1 nH. 
     The waveforms of  FIGS. 13A to 13C  and  FIGS. 14A to 14C  show the voltages of the reference potential terminal VSFE, the path in-VSFE between the input terminal and the reference potential terminal, and the differential output out-outb, respectively. 
     Each of  FIG. 13D  and  FIG. 14D  shows a so-called eye pattern that is the waveform pattern of the differential output out-outb. 
     The same circuits are used for the PDs and the FE parts in both of the configurations of  FIG. 4  and  FIG. 12 . 
     In the optical receiver  10  of  FIG. 4  as the comparative example, as shown in  FIG. 13D , the eye pattern looks to be completely closed and the data is deteriorated due to the influence of the variation of the reference potential terminal VSFE. 
     In contrast, in the optical receiver  310 B of  FIG. 12  according to the third embodiment of the present invention, as shown in  FIG. 14D , the data is not deteriorated although the amount of variation of the reference potential terminal VSFE 1  is the same as that in the optical receiver  10  of  FIG. 4 . 
     As described above, according to the third embodiment, the transmission line TL 311  exists between the PD  311  and the FE part  3131 , and the return path between the PD  311  and the FE part  3131  is closed on the basis of the reference potential terminal VSFE 1  including the transmission line TL 311  in the optical receiver. 
     As a result, noise is propagated to the PD  311  from both of the reference potential terminal VSFE 1  and the terminal “in” via the transmission line TL 311 . Furthermore, the same amount of noise is imposed on the anode and cathode of the PD  311  and thus the noise is cancelled. 
     Therefore, the terminal “in” from the viewpoint of the reference potential terminal VSFE 1  is not affected by the noise. 
     As a result, the noise component superimposed on the signal decreases and thus the accuracy of the signal-to-noise ratio (SNR) is enhanced. 
     The configuration of the third embodiment can be applied also to multiple channels. Whether the reference potential terminal VSFE 1  of the multiple channels is common or separated, the technique of this configuration holds as long as the reference potential of the PD  311  is the same as that of the reference potential terminal VSFE 1  including the transmission line. 
     If the reference potential terminal VSFE 1  of the multiple channels is separated for each one channel, noise is not imposed on the terminal “in” from the viewpoint of the reference potential terminal VSFE 1  as with the above description. 
     Even when the reference potential terminal VSFE 1  of the multiple channels is common, noise applied to the reference potential terminal VSFE 1  is propagated to the PD  311  from both of the reference potential terminal VSFE 1  and the terminal “in” via the transmission line. 
     As a result, the same amount of noise is imposed on the anode and cathode of the PD  311  and thus the noise is cancelled. Therefore, the terminal “in” from the viewpoint of the reference potential terminal VSFE 1  is not affected by the noise. 
     4. Fourth Embodiment 
       FIG. 15  is a diagram showing the configuration of an optical receiver according to a fourth embodiment of the present invention. 
     An optical receiver  310 C according to the fourth embodiment is different from the optical receiver  310 B according to the above-described third embodiment in the following point. 
     In the optical receiver  310 C of  FIG. 15 , which is the same as the optical receiver  310 B of  FIG. 12  as the equivalent circuit, transmission lines are formed by using a signal line SGL as a first layer and a floating plane  315  as a second layer. 
     Over a board  316 , the floating plane  315  and an ideal ground  317  are disposed in parallel. 
     Furthermore, the PD  311 , the capacitor  312 , and the transmission line TL 311  are formed on the floating plane  315  and an end part in which the terminals in 1  and in 2  of the TIA  313 B as the chip are formed is disposed on the floating plane  315 . 
     To the terminal in 1 , the transmission line TL 311  formed of the signal line SGL of the first layer is connected. 
     The second electrode  3122  of the capacitor  312  and the terminal in 2  are connected to the floating plane  315  of the second layer by a pad. 
     The fourth embodiment can achieve the same advantageous effects as those of the above-described third embodiment. 
     The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2010-139586 filed in the Japan Patent Office on Jun. 18, 2010, the entire content of which is hereby incorporated by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factor in so far as they are within the scope of the appended claims or the equivalents thereof.