Abstract:
An optical detecting device includes a light-detecting element for outputting an electrical signal, a pre-amplifier for amplifying the electrical signal, a signal line connected to an output of the pre-amplifier, and a resistor and a capacitor connected in series between the signal line and GND.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an optical receiving device used for optical communication. 
     2. Background Art 
     In optical receiving devices, a light-receiving element that receives an optical signal outputs an electric signal and a pre-amplifier amplifies the electric signal (e.g., see Japanese Patent Laid-Open No. 7-283711). In general, light-receiving sensitivity and a gain, and a passband have a trade-off relationship, and therefore a technique whereby the pre-amplifier applies peaking to extend the passband is adopted. 
     SUMMARY OF THE INVENTION 
     When peaking is applied excessively due to mounting conditions and circuit design of a light-receiving element and a pre-amplifier, manufacturing variations of the pre-amplifier, inductor of a wire connecting the pre-amplifier and the light-receiving element or the like, conventional optical receiving devices have a problem that a frequency response characteristic deteriorates and a reception sensitivity characteristic deteriorates. 
     In view of the above-described problems, an object of the present invention is to provide an optical receiving device which can improve the reception sensitivity characteristic. 
     According to the present invention, an optical receiving device includes: a light-receiving element outputting an electric signal; a pre-amplifier amplifying the electric signal; a signal line connected to an output of the pre-amplifier; and a resistor and a capacitor which are connected in series between the signal line and a GND. 
     The present invention makes it possible to improve the reception sensitivity characteristic. 
     Other and further objects, features and advantages of the invention will appear more fully from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram illustrating an optical receiving device according to a first embodiment of the present invention. 
         FIG. 2  is a diagram illustrating a frequency response characteristic of an output substrate according to the first embodiment of the present invention. 
         FIG. 3  is a diagram illustrating a frequency response characteristic of only the light-receiving element and the pre-amplifier. 
         FIG. 4  is a diagram illustrating a frequency response characteristic of the optical receiving device according to the first embodiment of the present invention. 
         FIG. 5  is a diagram illustrating an optical receiving device according to a second embodiment of the present invention. 
         FIG. 6  is a diagram illustrating a frequency response characteristic of an output substrate according to the second embodiment of the present invention. 
         FIG. 7  is a diagram illustrating a frequency response characteristic of the optical receiving device according to the second embodiment of the present invention. 
         FIG. 8  is a top view illustrating an optical receiving device according to a third embodiment of the present invention. 
         FIG. 9  is a cross-sectional view along I-II in  FIG. 8 . 
         FIG. 10  is a top view illustrating an optical receiving device according to a comparative example. 
         FIG. 11  is a top view illustrating an optical receiving device according to a fourth embodiment of the present invention. 
         FIG. 12  is a frequency response characteristic of an output substrate according to the fourth embodiment of the present invention. 
         FIG. 13  is a diagram illustrating a frequency response characteristic of the optical receiving device according to the fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Optical receiving devices according to the embodiments of the present invention will be described with reference to the drawings. The same components will be denoted by the same symbols, and the repeated description thereof may be omitted. 
     First Embodiment 
       FIG. 1  is a diagram illustrating an optical receiving device according to a first embodiment of the present invention. A light-receiving element  1  that receives an optical signal outputs an electric signal and a pre-amplifier  2  amplifies the electric signal. Signal lines  4  and  5 , a resistor  6  and a capacitor  7  are provided on an output substrate  3 . Impedance matching is achieved between the signal lines  4  and  5  and these signal lines are connected to the output of the pre-amplifier  2 . The resistor  6  and the capacitor  7  are connected in series between the signal lines  4  and  5  and a GND. 
       FIG. 2  is a diagram illustrating a frequency response characteristic of an output substrate according to the first embodiment of the present invention. The series circuit of the resistor  6  and the capacitor  7  has a high impedance when the frequency is low, and the impedance of the capacitor  7  decreases as the frequency increases, the impedance of the capacitor  7  can be ignored when the frequency is high and saturates at the resistance value of the resistor  6 . 
       FIG. 3  is a diagram illustrating a frequency response characteristic of only the light-receiving element  1  and the pre-amplifier  2 .  FIG. 4  is a diagram illustrating a frequency response characteristic of the optical receiving device according to the first embodiment of the present invention. By combining the light-receiving element  1  demonstrating the frequency characteristic in  FIG. 3  with the output substrate  3  demonstrating the frequency characteristic in  FIG. 2 , it is possible to obtain a frequency response characteristic with suppressed peaking. As a result, the reception sensitivity characteristic can be improved. 
     The resistor  6  and the capacitor  7  may be chip parts or may be configured with a wiring pattern on a ceramic substrate. 
     Second Embodiment 
       FIG. 5  is a diagram illustrating an optical receiving device according to a second embodiment of the present invention. An inductor  8  is added to the configuration of the first embodiment, which is connected in series to the resistor  6  and the capacitor  7  between the signal lines  4  and  5 , and the GND. 
     When the frequency response characteristic of only the light-receiving element  1  and the pre-amplifier  2  is as shown in  FIG. 3 , it is preferable to reduce peaking from a low frequency to the order of 20 GHz, but if the gain also drops in a frequency band of 20 GHz or higher likewise, the band deteriorates. Thus, the present embodiment adds the inductor  8 . 
       FIG. 6  is a diagram illustrating a frequency response characteristic of an output substrate  3  according to the second embodiment of the present invention. When the frequency increases, a joint impedance by the inductor  8  increases, and therefore the gain rises. 
       FIG. 7  is a diagram illustrating a frequency response characteristic of the optical receiving device according to the second embodiment of the present invention. By combining the light-receiving element  1  and the pre-amplifier  2  demonstrating the frequency response characteristic in  FIG. 3  with the output substrate  3  demonstrating the frequency response characteristic in  FIG. 6 , it is possible to suppress peaking and also suppress band deterioration. Furthermore, the amount of peaking of the pre-amplifier  2  greatly varies from one product to another, but this individual variation can also be compensated by the output substrate  3 . 
     The inductor  8  may be a chip part or may also be configured with a wiring pattern on a ceramic substrate. 
     Third Embodiment 
       FIG. 8  is a top view illustrating an optical receiving device according to a third embodiment of the present invention.  FIG. 9  is a cross-sectional view along I-II in  FIG. 8 . A light-receiving element  1 , a pre-amplifier  2  and an insulating substrate  10  are mounted on a conductive carrier  9 . The insulating substrate  10  is arranged between the light-receiving element  1  and the pre-amplifier  2 . 
     A GND electrode  11  and a signal electrode  12  making up a coplanar line are provided on a top surface of the light-receiving element  1 . Mutually electric field coupled signal input pad  13  and GND pad  14  are provided on a top surface of the pre-amplifier  2 . Electrodes  15  and  16  are provided on a top surface of the insulating substrate  10 . The electrode  15 , the carrier  9  and the insulating substrate  10  constitute a first capacitor. Similarly, the electrode  16 , the carrier  9  and the insulating substrate  10  constitute a second capacitor. 
     The signal electrode  12  is connected to the signal input pad  13  via a wire  17 . The GND pad  14  is connected to the electrode  15  via a wire  18 . That is, the GND pad  14  is connected to the carrier  9  via the wire  18  and the first capacitor. The GND electrode  11  is connected to the electrode  16  via a wire  19 . That is, the GND electrode  11  is connected to the carrier  9  via the wire  19  and the second capacitor. 
     Next, effects of the present embodiment will be described in comparison with a comparative example.  FIG. 10  is a top view illustrating an optical receiving device according to a comparative example. In the comparative example, a GND pad  14  of a pre-amplifier  2  is connected to a carrier  9  via a wire  18  between a light-receiving element  1  and the pre-amplifier  2 . In this case, to avoid interference between the wire mounting device and the light-receiving element  1  or the pre-amplifier  2 , the distance between the light-receiving element  1  and the pre-amplifier  2  needs to be increased up to, for example, on the order of 700 μm. Therefore, since the wire  17  that connects the signal electrode  12  and the signal input pad  13  is extended, peaking occurs between the light-receiving element  1  and the pre-amplifier  2 , and the characteristic deteriorates. 
     Furthermore, a voltage of, for example, 3.0 V needs to be applied to the GND electrode  11  of the light-receiving element  1 . On the other hand, the voltage of the GND pad  14  of the pre-amplifier  2  is 0 V. Therefore, since there is a potential difference between the two, the two cannot be directly connected. 
     On the other hand, according to the present embodiment, the GND pad  14  of the pre-amplifier  2  is connected to the electrode  15  on the insulating substrate  10  via a wire. Since the GND pad  14  and the electrode  15  have substantially the same height, the wire mounting device never interferes with the light-receiving element  1  or the pre-amplifier  2 . Therefore, since the distance between the light-receiving element  1  and the pre-amplifier  2  can be reduced, the reception sensitivity characteristic can be improved. Furthermore, there is a potential difference between the GND electrode  11  of the light-receiving element  1  and the GND pad  14  of the pre-amplifier  2 , but the two can be connected together if the first capacitor is interposed therebetween. 
     The electrodes  15  and  16  together with the insulating substrate  10  and the carrier  9  constitute a capacitor, but without being limited to this, a capacitor may also be configured between the electrodes  15  and  16 . Furthermore, the second capacitor is used as a bypass capacitor. The second capacitor may be individually provided beside the light-receiving element  1  as in the case of the comparative example, but providing the first capacitor and the second capacitor on one insulating substrate  10  can reduce the number of parts. 
     Fourth Embodiment 
       FIG. 11  is a top view illustrating an optical receiving device according to a fourth embodiment of the present invention. An impedance-matched signal line  20  is connected to the output of a pre-amplifier  2 . A signal line  21  is connected between the signal line  20  and the output of the pre-amplifier  2 . This signal line  21  has a higher impedance than that of the signal line  20 . 
       FIG. 12  is a frequency response characteristic of an output substrate  3  according to the fourth embodiment of the present invention. Adding the signal line  21  having a higher impedance than a matched impedance intentionally causes electrically multiplexed reflection to be generated. This produces a drop by a half cycle at 20 GHz and 1 dB in amplitude. The cycle and amplitude of multiplexed reflection are determined by the length of the signal line  21  and the amount of impedance deviation. 
       FIG. 13  is a diagram illustrating a frequency response characteristic of the optical receiving device according to the fourth embodiment of the present invention. By combining the light-receiving element  1  and the pre-amplifier  2  demonstrating the frequency characteristic in  FIG. 3  with the output substrate  3  demonstrating the frequency response characteristic in  FIG. 12 , it is possible to improve the frequency response characteristic. As a result, the reception sensitivity characteristic can be improved. 
     Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described. 
     The entire disclosure of Japanese Patent Application No. 2011-118767, filed on May 27, 2011, including specification, claims, drawings, and summary, on which the Convention priority of the present application is based, is incorporated herein by reference in its entirety.