Patent Publication Number: US-2011058819-A1

Title: Optical transmission device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a Continuation Application of International Application No. PCT/JP2009/054211, filed on Mar. 5, 2009, which claims priority to Japanese Patent Application No. 2008-54833 filed on Mar. 5, 2008 and Japanese Patent Application No. 2008-150364 filed on Jun. 9, 2008. The contents of the aforementioned applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Apparatuses and embodiments described herein relate to an optical transmission device including an optical semiconductor and a transmission line so as to transmit information by optical communication over a relatively short transmission distance in an apparatus or between apparatuses. This optical communication over a short distance will be applied to the fields of high-speed data transmission devices such as a server or a router, vehicles, mobile phones, business copiers, and game machines in the future. 
     BACKGROUND ART 
     An example of a known optical transmission device is shown in  FIG. 24 . 
     An optical transmission device  9  shown in  FIG. 24  includes an optical transmission unit  6 A having a light-emitting element  62 , an optical reception unit  7 A having a light-receiving element  73 , and a light-transmitting medium  8  optically coupling the light-emitting element  62  and the light-receiving element  72  to each other. A laser diode is an example of the light-emitting element  62  and a photo diode is an example of the light-receiving element  73 . An optical fiber or a polymer light waveguide is used as the light-transmitting medium  8 . 
     The optical transmission unit  6 A further includes a driving circuit  67  such as a laser driving IC controlling the emission of the light-emitting element  62 . The optical reception unit  7 A further includes a trans-impedance amplifier (TIA)  76  and a limiting amplifier  71 . 
     The optical transmission device  9  having the above-mentioned configuration basically operates as follows. 
     First, when an input signal is input from the outside, the driving circuit  67  of the optical transmission unit  6 A changes the supply current to the light-emitting element  62 . The light-emitting element  62  outputs light which varies with the variation in supply current. The light-receiving element  73  of the optical reception unit  7 A receives the light output from the light-emitting element  62 , generates current corresponding to the received light intensity, and outputs the generated current to the trans-impedance amplifier  76 . The trans-impedance amplifier  76  converts the input current into a voltage and amplifies and outputs the voltage to the limiting amplifier  71 . The limiting amplifier  71  amplifies the signal output from the trans-impedance amplifier  76  and outputs the amplified signal with a constant amplitude to the outside of the optical transmission device  9 . 
     In such an optical transmission device, since the optical power of the light-emitting element such as a laser diode deteriorates with the lapse of time and also varies with a variation in temperature, a variety of means has been employed to stabilize communications. 
     For example, in an optical transmission device disclosed in Patent Document 1, as shown in  FIG. 25 , an optical transmission unit  6 B further includes a monitoring light-receiving element  63  such as a photo diode, a trans-impedance amplifier (TIA)  66 , and a difference circuit  68  in addition to the driving circuit  67  and the light-emitting element  62 . The monitoring light-receiving element  63  is disposed in the vicinity of the light-emitting element  62  and serves to receive part of the light output from the light-emitting element  62 , to generate current corresponding to the received light intensity, and to output the generated current to the trans-impedance amplifier  66 . The trans-impedance amplifier  66  converts the input current into a voltage, amplifies the voltage, and outputs the amplified voltage to the difference circuit  68 . The voltage (received-light voltage) input to the difference circuit  68  is compared with a predetermined voltage by the difference circuit  68  and the difference voltage is output from the difference circuit  68 . 
     In the optical transmission device having the configuration disclosed in Patent Document 1, the received light intensity sensed by the monitoring light-receiving element  63  is fed back and the driving current for driving the light-emitting element  62  is varied, whereby the emission intensity of the light-emitting element  62  is maintained in a stable state. 
     In an optical transmission device disclosed in Patent Document 2, as shown in  FIG. 26 , an optical transmission unit  6 C further includes a temperature sensor  69  in addition to the driving circuit  67  and the light-emitting element  62 . The temperature sensor  69  is disposed in the vicinity of the light-emitting element  62  so as to sense the temperature around the light-emitting element  62 . The temperature sensor  69  compares the sensed temperature with information stored in advance to calculate a corrected current value, and changes the driving current based on the calculation result. 
     In the optical transmission device having this configuration disclosed in Patent Document 2, the emission intensity of the light-emitting element  62  is maintained in a stable state even when the temperature around the light-emitting element  62  varies. 
     In an optical transmission device disclosed in Patent Document 3, as shown in  FIG. 27 , an optical reception unit  7 B includes a level detector  79  and a difference circuit  78  instead of the limiting amplifier  71 . This is typically called an AGC (Auto Gain Control) circuit. 
     In the optical transmission device having this circuit configuration, the signal light intensity or the magnification factor of the trans-impedance amplifier  76  can be changed based on the received light intensity of the light-receiving element  73  sensed by the level detector  79 , whereby the output signal to the outside can be maintained stable even when the received light intensity is changed. 
     [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2005-012520 
     [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. H10-041575 
     [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2003-318681 
     However, in the optical transmission device disclosed in Patent Document 1, when a photoelectric composite substrate mounted with the light-emitting element  62  is employed as the polymer waveguide which is the light-transmitting medium, there is a problem in that an area for mounting the monitoring light-receiving element  63  cannot be guaranteed. Even when such an area can be guaranteed, a trans-impedance amplifier is further necessary. As a result, there are problems in that the configuration of the optical transmission unit  6 B is complicated, it is difficult to reduce the size of the device, and the cost increases. 
     In the optical transmission device disclosed in Patent Document 2, it is necessary to separately dispose the temperature sensor  69  in the optical transmission unit  6 C, thereby causing an increase in cost. With the variation in emission intensity due to the temporal deterioration of the light-emitting element  62 , there is a problem in that the communication cannot be stabilized. 
     In the optical transmission device disclosed in Patent Document 3, since an excessive gain should always be prepared to provide a wide dynamic range to the AGC circuit, there is a problem in that the power consumption is great. There are also problems in that the configuration of the optical reception unit  7 B is complicated, it is difficult to reduce the size of the device, and the cost increases. 
     A decrease in size and cost is required for the optical transmission device applied to a relatively-short transmission distance within an apparatus or between apparatuses, and various restrictions are applied to the shape. However, in the above-mentioned known optical transmission devices, sufficient countermeasures against the above-mentioned problems have not been prepared. 
     Aspects of exemplary embodiments of the present invention have been made in view of the above-mentioned situations. An aspect object of exemplary embodiments of the present invention is to provide an optical transmission device that can stably communicate while having a small size and a low cost. 
     SUMMARY OF THE INVENTION 
     Aspects of exemplary embodiments of the present invention employ the following configurations to solve the above-mentioned problems and to accomplish the object. 
     An optical transmission device according to an aspect of an exemplary embodiment of the present invention includes: an optical transmission unit including a light-emitting element; an optical reception unit including a static current source generating bias current for driving the light-emitting element; a light-transmitting medium optically connecting the light-emitting element and a light-receiving element to each other; and an electricity-transmitting medium transmitting the bias current from the static current source to the light-emitting element. 
     An optical transmission device according to another aspect of an exemplary embodiment of the present invention includes: an optical transmission unit including a light-emitting element; an optical reception unit including a light-receiving element and a current source generating bias current for adjusting an optical power of the light-emitting element based on an electrical signal into which light received by the light-receiving element is converted; a light-transmitting medium optically connecting the light-emitting element and the light-receiving element to each other; and an electricity-transmitting medium transmitting the bias current from the current source to the light-emitting element. 
     According to another aspect, the optical reception unit may further include an error detector measuring an intensity of the light received by the light-receiving element and controlling a magnitude of the bias current generated by the current source based on a measured value. 
     According to another aspect, the optical reception unit may further include a low-pass filter disposed between the error detector and the current source. 
     According to another aspect, the optical reception unit may further include a trans-impedance amplifier and a low-pass filter, and the light-receiving element, the trans-impedance amplifier, the low-pass filter, and the error detector may be disposed in this order. 
     According to another aspect, the optical reception unit may further include an average calculator calculating an average value of the intensity of the light received by the light-receiving element. 
     According to another aspect, the optical transmission unit may further include an impedance-matching element. 
     According to another aspect, the optical reception unit may further include an impedance-matching element. 
     According to another aspect, the optical transmission unit may further include a low-pass filter disposed between the electricity-transmitting medium and the light-emitting element. 
     According to another aspect, the optical transmission unit may receive a differential input signal as an input signal from an outside. 
     According to another aspect, the optical transmission unit may further include a protection circuit disposed between the electricity-transmitting medium and the light-emitting element. 
     According to another aspect, one of a photoelectric composite cable in which the light-transmitting medium and the electricity-transmitting medium are combined in a body, a photoelectric composite wiring board in which a light waveguide and the electricity-transmitting medium are disposed on a substrate, and a light waveguide coated with metal may be disposed as the light-transmitting medium and the electricity-transmitting medium. 
     According to another aspect, the optical transmission unit and the optical reception unit may each be air-tightly sealed by a conductive package, and the electricity-transmitting medium may be electrically connected between the package air-tightly sealing the optical transmission unit and the package air-tightly sealing the optical reception unit. 
     According to another aspect, the electricity-transmitting medium may transmit current wirelessly. 
     According to an aspect of an exemplary embodiment of the present invention, it is possible to reduce the size and cost of the optical transmission device and to reduce the power consumption. 
     By determining the emission intensity of the light-emitting element based on the received light intensity of the light-receiving element and performing a feedback control, it is possible to stabilize communications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram schematically illustrating the configuration of an optical transmission device according to a first exemplary embodiment of the present invention. 
         FIG. 2  is a diagram schematically illustrating the configuration of an optical transmission device according to a second exemplary embodiment of the present invention. 
         FIG. 3  is a diagram schematically illustrating the configuration of an optical transmission device according to a third exemplary embodiment of the present invention. 
         FIG. 4  is a diagram schematically illustrating the configuration of an optical transmission device according to a fourth exemplary embodiment of the present invention. 
         FIG. 5  is a diagram schematically illustrating the configuration of an optical transmission device according to a fifth exemplary embodiment of the present invention. 
         FIG. 6  is a diagram schematically illustrating the configuration of an optical transmission device according to a ninth exemplary embodiment of the present invention. 
         FIG. 7  is a diagram schematically illustrating the configuration of an optical transmission device according to an eleventh exemplary embodiment of the present invention. 
         FIG. 8  is a diagram schematically illustrating the configuration of an optical transmission device according to a fifteenth exemplary embodiment of the present invention. 
         FIG. 9  is a diagram schematically illustrating the configuration of an optical transmission device according to a seventeenth exemplary embodiment of the present invention. 
         FIG. 10  is a diagram schematically illustrating the configuration of an optical transmission device according to a nineteenth exemplary embodiment of the present invention. 
         FIG. 11  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-first exemplary embodiment of the present invention. 
         FIG. 12  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-third exemplary embodiment of the present invention. 
         FIG. 13  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-fifth exemplary embodiment of the present invention. 
         FIG. 14  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-seventh exemplary embodiment of the present invention. 
         FIG. 15  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-eighth exemplary embodiment of the present invention. 
         FIG. 16  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-ninth exemplary embodiment of the present invention. 
         FIG. 17  is a diagram schematically illustrating the configuration of an optical transmission device according to a thirtieth exemplary embodiment of the present invention. 
         FIG. 18  is a diagram schematically illustrating the configuration of an optical transmission device in which communications are stabilized by making a feedback control by bidirectional communication. 
         FIG. 19  is a diagram schematically illustrating the configuration of another optical transmission device in which communications are stabilized by making a feedback control by bidirectional communication. 
         FIG. 20  is a diagram schematically illustrating the configuration of another optical transmission device in which communications are stabilized by making a feedback control by bidirectional communication. 
         FIG. 21  is a diagram schematically illustrating the configuration of another optical transmission device in which communications are stabilized by making a feedback control by bidirectional communication. 
         FIG. 22  is a diagram schematically illustrating the configuration of an optical transmission device transmitting a large amount of data by parallel communication. 
         FIG. 23  is a diagram schematically illustrating the configuration of another optical transmission device transmitting a large amount of data by parallel communication. 
         FIG. 24  is a diagram schematically illustrating the configuration of a known optical transmission device. 
         FIG. 25  is a diagram schematically illustrating the configuration of an optical transmission unit in another known optical transmission device. 
         FIG. 26  is a diagram schematically illustrating the configuration of an optical transmission unit in another known optical transmission device. 
         FIG. 27  is a diagram schematically illustrating the configuration of an optical reception unit in another known optical transmission device. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, exemplary embodiments will be described in detail with reference to the accompanying drawings. 
     First Exemplary Embodiment 
       FIG. 1  is a diagram schematically illustrating the configuration of an optical transmission device according to a first exemplary embodiment. The optical transmission device  5  according to this exemplary embodiment includes an optical transmission unit  1  having a light-emitting element  12 , an optical reception unit  2  having a light-receiving element  23 , and a light-transmitting medium  3  optically connecting the light-emitting element  12  and the light-receiving element  23  to each other. 
     The optical reception unit  2  includes a static current source  29  generating bias current for driving the light-emitting element  12  of the optical transmission unit  1 . 
     The optical transmission device  5  further includes an electricity-transmitting medium  31  transmitting the bias current from the static current source  29  to the light-emitting element  12 . 
     The light-emitting element  12  may employ a known one and a specific example thereof may be a laser diode. The light-receiving element  23  may employ a known one and a specific example thereof may be a photo diode. 
     An optical-communication light waveguide such as an optical fiber or a substrate-type light waveguide can be used as the light-transmitting medium  3 . 
     Examples of the electricity-transmitting medium  31  include an electricity-transmitting medium for transmitting current in a wired manner as shown in the drawing and an electricity-transmitting medium for transmitting current in a wireless manner. These may all be known. 
     The optical transmission unit  1  receives a modulated signal from the outside and drives the light-emitting element  12 . A burst signal and a continuous signal can be selectively used as the modulated signal. 
     When an optical signal is transmitted from the light-emitting element  12  to the light-receiving element  23  of the optical reception unit  2  via the light-transmitting medium  3 , current based on the received light intensity is generated by the light-receiving element  23 . This current is converted into a voltage and amplified, for example, by a trans-impedance amplifier (hereinafter, abbreviated as “TIA”)  26 . Power for controlling the TIA  26  is supplied from the outside, although not shown herein. 
     The static current source  29  generates predetermined current. This current is transmitted to the light-emitting element  12  of the optical transmission unit  1  via the electricity-transmitting medium  31  and is used as bias current for driving the light-emitting element  12 . In this way, the light-emitting element  12  is driven with the external modulated signal and the bias current. 
     The solid arrow in  FIG. 1  indicates a flow of the signal for transmitting information and the dotted arrow indicates a flow of the bias current flowing from the optical reception unit  2  to the optical transmission unit  1 . 
     The optical transmission device  5  according to this exemplary embodiment is different from a known one and can maintain the optical power of the light-emitting element  12  at a level with which it can stably communicate for a long term by setting the current generated by the static current source  29  to a proper value, even when the monitoring light-receiving element, the temperature sensor, the AGC circuit, and the like are not particularly disposed in the vicinity of the light-emitting element  12  of the optical transmission unit  1  or when the limiting amplifier is not mounted on the optical reception unit  2 . The driving circuit which was disposed in the optical transmission unit  1  in the past is unnecessary. 
     Second Exemplary Embodiment 
       FIG. 2  is a diagram schematically illustrating the configuration of an optical transmission device according to a second exemplary embodiment of the present invention. Like elements described in the first exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. In an optical transmission device  5 A according to this exemplary embodiment, an optical transmission unit  1 A is provided as the configuration corresponding to the optical transmission unit  1  in the first exemplary embodiment and an optical reception unit  2 A is provided as the configuration corresponding to the optical reception unit  2 . 
     In the optical transmission device  5 A shown in  FIG. 2 , a variable current source  24  is provided instead of the fixed static current source  29  in the first exemplary embodiment, the current source  24  and the TIA  26  are electrically connected to each other, and a voltage signal is transmitted from the TIA  26  to the current source  24 . 
     The current source  24  generates current of an optimal value corresponding to a predetermined voltage signal based on the voltage signal output from the TIA  26 . This current is transmitted to the light-emitting element  12  of the optical transmission unit  1 A via the electricity-transmitting medium  31  and is used as bias current for driving the light-emitting element  12 . 
     In the optical transmission device  5 A according to this exemplary embodiment, since the optical driving current is supplied to the light-emitting element  12  based on the intensity of the light received by the light-receiving element  23 , it is possible to maintain the optical power of the light-emitting element  12  at an appropriate value. Since the value of the driving current is changed depending on the magnitude of the received light intensity, excessive current is not supplied and it is possible to effectively reduce the power consumption. 
     Third Exemplary Embodiment 
       FIG. 3  is a diagram schematically illustrating the configuration of an optical transmission device according to a third exemplary embodiment of the present invention. Like elements described in the second exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. In an optical transmission device  5 B according to this exemplary embodiment, an optical reception unit  2 B is provided as the configuration corresponding to the optical reception unit  2 A in the second exemplary embodiment. 
     In the optical transmission device  5 B shown in  FIG. 3 , an error detector  28  is disposed in the optical reception unit  2 A in the second exemplary embodiment. 
     The error detector  28  measures the intensity (received light intensity) of the light received by the light-receiving element  23 , calculates an error value between the received light intensity and reference intensity, and controls the magnitude of the bias current supplied from the current source  24  based on the error value. The error detector  28  may be set to regularly measure the received light intensity. 
     In the optical transmission device  5 B according to this exemplary embodiment, since the driving current of the light-emitting element  12  is controlled so as to keep the received light intensity of the light-receiving element  23  constant, it is possible to stably communicate for a long term. Even when characteristics of at least one of the light-emitting element  12  and the light-receiving element  23  varies with the variation in ambient temperature of the light-emitting element  12  or the light-receiving element  23 , it is possible to keep the voltage in the TIA  26  constant. 
     Fourth Exemplary Embodiment 
       FIG. 4  is a diagram schematically illustrating the configuration of an optical transmission device according to a fourth exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. In an optical transmission device  5 C according to this exemplary embodiment, an optical reception unit  2 C is provided as the configuration corresponding to the optical reception unit  2 B in the third exemplary embodiment. 
     In the optical transmission device  5 C shown in  FIG. 4 , an average calculator  25  is disposed between the TIA  26  of the optical reception unit  2 B and the error detector  28  in the third exemplary embodiment. 
     The average calculator  25  measures the intensity of the light received by the light-emitting element  23 , calculates the average, and outputs the calculated average. The average calculator  25  can be set to regularly measure the received light intensity. 
     In the optical transmission device  5 C according to this exemplary embodiment, since the average of the received light intensity of the light-receiving element  23  is acquired, it is possible to measure the received light intensity stably not depending on the existence of a modulated signal or the magnitude of a modulation speed. Accordingly, it is possible to improve the stability of the emission intensity of the light-emitting element  12 . 
     Fifth Exemplary Embodiment 
       FIG. 5  is a diagram schematically illustrating the configuration of an optical transmission device according to a fifth exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. In an optical transmission device  5 D according to this exemplary embodiment, an optical transmission unit  1 B is provided as the configuration corresponding to the optical transmission unit  1 A in the third exemplary embodiment. 
     In the optical transmission device  5 D shown in  FIG. 5 , an impedance-matching element  17  is disposed in the path through which the input signal is received by the optical transmission unit  1 A in the third exemplary embodiment. 
     The impedance-matching element  17  is of a fixed type. An element in which resistors are disposed in multi stages and multiple resistors are connected in parallel in accordance with desired impedance can be exemplified as the fixed impedance-matching element  17 . An element not including resistors may be employed as long as it can change the impedance. 
     In the optical transmission device  5 D according to this exemplary embodiment, the waveform deterioration of the optical signal from the light-emitting element  12  is suppressed, thereby preventing the characteristic deterioration due to the impedance difference of the light-emitting element  12  or the optical transmission unit  1 B. It is also possible to cope with the variation in impedance of the substrate connected to the optical transmission unit  1 B. 
     Although the optical transmission device  5 D including the error detector  28  has been described in this exemplary embodiment, the same advantageous effects can be obtained without disposing the error detector  28  in the optical reception unit  2 B, when the impedance-matching element  17  is disposed in the optical transmission unit  1 B. 
     Sixth Exemplary Embodiment 
     In an optical transmission device according to a sixth exemplary embodiment, an impedance-matching element is disposed in the path through which the input signal is received by the optical transmission unit  1 A in the fourth exemplary embodiment, but is not shown. 
     The impedance-matching element is of a fixed type. An element in which resistors are disposed in multi stages and multiple resistors are connected in parallel in accordance with desired impedance can be exemplified as the fixed impedance-matching element  17 . An element not including resistors may be employed as long as it can change the impedance. 
     In the optical transmission device according to this exemplary embodiment, the waveform deterioration of the optical signal from the light-emitting element  12  is suppressed, thereby preventing the characteristic deterioration due to the impedance difference of the light-emitting element  12  or the optical transmission unit  1 A. It is also possible to cope with the variation in impedance of the substrate connected to the optical transmission unit  1 A. Since the average calculator  25  and the impedance-matching element are used together, it is possible to stabilize both the DC component and the modulated component of the optical signal and to guarantee excellent communication quality for a long term. 
     Seventh Exemplary Embodiment 
     In an optical transmission device according to a seventh exemplary embodiment, a variable impedance-matching element is provided instead of the fixed impedance-matching element  17  in the fifth exemplary embodiment, but is not shown. An element controlling the impedance using a voltage or the like can be exemplified as the variable impedance-matching element, but the variable impedance-matching element is not limited to this element, as long as it can change the impedance. 
     In this exemplary embodiment, similarly to the fifth exemplary embodiment, it is possible to obtain the same advantageous effect, even when the error detector  28  is not disposed. 
     In the optical transmission device according to this exemplary embodiment, the impedance can be optimally controlled with the variation in the modulated signal input to the optical transmission unit  1 B. It is also possible to suppress the characteristic deterioration due to uneven impedance of components at the time of mounting the components. 
     Eighth Exemplary Embodiment 
     In an optical transmission device according to an eighth exemplary embodiment, a variable impedance-matching element is provided instead of the fixed impedance-matching element  17  in the sixth exemplary embodiment, but is not shown. An element controlling the impedance using a voltage or the like can be exemplified as the variable impedance-matching element, but the variable impedance-matching element is not limited to this element, as long as it can change the impedance. 
     In the optical transmission device according to this exemplary embodiment, the impedance can be optimally controlled with the variation in the modulated signal input to the optical transmission unit  1 A. It is also possible to suppress the characteristic deterioration due to uneven impedance of components at the time of mounting the components. In addition, since the average calculator and the impedance-matching element  27  are used together, it is possible to stabilize both the DC component and the modulated component of the optical signal and to guarantee excellent communication quality for a long term. 
     It is stated in the fifth to eighth exemplary embodiments that the impedance-matching element is disposed in the path through which the input signal is received by the optical transmission unit  1 A to  1 C, but the first exemplary embodiment may employ the similar configuration. That is, although not shown, the impedance-matching element may be disposed in the path through which the input signal is received by the optical transmission unit  1  in the first exemplary embodiment. As described above, a fixed type or a variable type may be used as the impedance-matching element. 
     Ninth Exemplary Embodiment 
       FIG. 6  is a diagram schematically illustrating the configuration of an optical transmission device according to a ninth exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. In an optical transmission device  5 E according to this exemplary embodiment, an optical reception unit  2 D is provided as the configuration corresponding to the optical reception unit  2 B in the third exemplary embodiment. 
     The optical transmission device  5 E shown in  FIG. 6  has a configuration in which the optical reception unit  2 B in the third exemplary embodiment includes an impedance-matching element  27 . 
     The impedance-matching element  27  is of a fixed type and is similar to the fixed impedance-matching element  17  in the fifth exemplary embodiment. 
     In the optical transmission device  5 E according to this exemplary embodiment, the waveform deterioration of the optical signal from the light-emitting element  12  is suppressed, thereby preventing the characteristic deterioration due to the impedance difference of the components used in the optical reception unit  2 D, such as the TIA  26 . It is also possible to cope with the variation in impedance of the substrate connected to the optical reception unit  2 D. 
     Here, the example where the error detector  28  is disposed is described, but the same advantageous effects can be obtained without disposing the error detector  28 . 
     Tenth Exemplary Embodiment 
     In an optical transmission device according to a tenth exemplary embodiment of the present invention, a variable impedance-matching element is provided instead of the fixed impedance-matching element  27  in the ninth exemplary embodiment, but is not shown. Like elements described in the seventh exemplary embodiment may be employed as the variable impedance-matching element. 
     In this exemplary embodiment, similarly to the ninth exemplary embodiment, the same advantageous effects can be obtained without disposing the error detector  28 . 
     According to this exemplary embodiment, the impedance can be optimally controlled with the variation in the modulated signal transmitted from the optical transmission unit  1 A. It is also possible to suppress the characteristic deterioration due to uneven impedance of components at the time of mounting the components. 
     It is stated in the ninth and tenth exemplary embodiments that the impedance-matching element  27  is disposed in the subsequent stage of the TIA  26  in the optical reception unit  2 D, but the first exemplary embodiment may employ the same configuration. That is, although not shown here, the impedance-matching element  27  may be disposed in the subsequent stage of the TIA  26  in the optical reception unit  2  in the first exemplary embodiment. Similarly to the ninth and tenth exemplary embodiments, a fixed type or a variable type can be used as the impedance-matching element  27 . 
     Eleventh Exemplary Embodiment 
       FIG. 7  is a diagram schematically illustrating the configuration of an optical transmission device according to an eleventh exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. In an optical transmission device  5 F according to this exemplary embodiment, an optical reception unit  2 E is provided as the configuration corresponding to the optical reception unit  2 B in the third exemplary embodiment. 
     In the optical transmission device  5 F, a low-pass filter (hereinafter, also abbreviated as “LPF”)  21  is disposed between the error detector  28  and the current source  24  in the third exemplary embodiment. 
     In the optical transmission device  5 F according to this exemplary embodiment, a high-frequency electrical signal transmitted as the modulated signal can be blocked and only a low-frequency electrical signal can be transmitted by the LPF  21 , and the low-frequency electrical signal can be transmitted from the current source  24  to the light-emitting element  12 . Accordingly, it is possible to make a stable communication with a low amount of noise, thereby improving the stability in emission intensity of the light-emitting element  12 . As described later, compared with a case where the optical transmission unit  1 A includes the low-pass filter, high-frequency current does not flow in the electricity-transmitting medium  31  (in the electricity-transmitting medium) in this exemplary embodiment. Accordingly, it is possible to prevent the generation of noise causing a bad influence on other devices. 
     Here, the example where the error detector  28  is disposed is described, but the same advantageous effects can be obtained without disposing the error detector  28 . 
     Twelfth Exemplary Embodiment 
     In an optical transmission device according to a twelfth exemplary embodiment of the present invention, an LPF is disposed between the error detector  28  and the current source  24  in the fourth exemplary embodiment, but is not shown. 
     In the optical transmission device according to this exemplary embodiment, the frequency of a high-frequency electrical signal is sufficiently lowered by the average calculator  25 , only the low-frequency electrical signal is transmitted by the LPF, and the low-frequency electrical signal can be transmitted from the current source  24  to the light-emitting element  12 . Accordingly, it is possible to make a stable communication with a smaller noise, thereby improving the stability in emission intensity of the light-emitting element  12 . 
     Thirteenth Exemplary Embodiment 
     In an optical transmission device according to a thirteenth exemplary embodiment of the present invention, the LPF  21  is disposed between the TIA  26  and the error detector  28  instead of between the error detector  28  and the current source  24  in the eleventh exemplary embodiment, but is not shown. That is, in the optical reception unit  2 E, the light-receiving element  23 , the TIA  26 , the LPF  21 , and the error detector  28  are arranged in this order. 
     In the optical transmission device according to this exemplary embodiment, similarly to the eleventh exemplary embodiment, the same advantageous effects can be obtained without disposing the error detector  28 . 
     Fourteenth Exemplary Embodiment 
     In an optical transmission device according to a fourteenth exemplary embodiment of the present invention, the LPF is disposed between the average calculator  25  and the error detector  28  instead of between the error detector  28  and the current source  24  in the twelfth exemplary embodiment, but is not shown. That is, in the optical reception unit  2 C, the light-receiving element  23 , the TIA  26 , the average calculator  25 , the LPF  21 , and the error detector  28  are arranged in this order. 
     In the optical transmission device according to this exemplary embodiment, the same advantageous effects as the twelfth exemplary embodiment. 
     Fifteenth Exemplary Embodiment 
       FIG. 8  is a diagram schematically illustrating the configuration of an optical transmission device according to a fifteenth exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. In an optical transmission device  5 G according to this exemplary embodiment, an optical transmission unit  1 C is provided as the configuration corresponding to the optical transmission unit  1 A in the third exemplary embodiment. 
     In the optical transmission device  5 G, an LPF  21  is disposed between the electricity-transmitting medium  31  and the light-emitting element  12  in the optical transmission unit  1 C according to the third exemplary embodiment. 
     In the optical transmission device  5 G according to this exemplary embodiment, a high-frequency electrical signal transmitted as the modulated signal can be blocked and only a low-frequency electrical signal can be transmitted from the current source  24  to the light-emitting element  12  in the optical transmission unit  1 C by the LPF  21 . Accordingly, it is possible to make a stable communication with a low amount of noise, thereby improving the stability in emission intensity of the light-emitting element  12 . 
     In this exemplary embodiment, the same advantageous effects can be obtained without disposing the error detector  28 . 
     Sixteenth Exemplary Embodiment 
     In an optical transmission device according to a sixteenth exemplary embodiment of the present invention, the LPF is disposed between the electricity-transmitting medium  31  and the light-emitting element  12  in the optical transmission unit  1 A in the fourth exemplary embodiment, but is not shown. 
     In the optical transmission device according to this exemplary embodiment, the frequency of a high-frequency electrical signal is sufficiently lowered by the average calculator  25 , only the low-frequency electrical signal is transmitted by the LPF, and the low-frequency electrical signal can be transmitted from the current source  24  to the light-emitting element  12  in the optical transmission unit  1 A. Accordingly, it is possible to make a stable communication with a low amount of noise, thereby improving the stability in emission intensity of the light-emitting element  12 . 
     In the fifteenth and sixteenth exemplary embodiments, it is stated that the LPF is disposed between the electricity-transmitting medium  31  and the light-emitting element  12 , but the first exemplary embodiment may employ the same configuration. That is, although not shown here, the LPF may be disposed between the electricity-transmitting medium  31  and the light-emitting element  12  in the optical transmission unit  1  in the first exemplary embodiment. In this case, the high-frequency electrical signal transmitted as the modulated signal can be blocked and only the low-frequency electrical signal can be transmitted from the static current source  29  to the light-emitting element  12  in the optical transmission unit  1 . 
     Seventeenth Exemplary Embodiment 
       FIG. 9  is a diagram schematically illustrating the configuration of an optical transmission device according to a seventeenth exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. In an optical transmission device  5 H according to this exemplary embodiment, an optical transmission unit  1 D is provided as the configuration corresponding to the optical transmission unit  1 A in the third exemplary embodiment. 
     The optical transmission device  5 H has a configuration in which the external input signal to the optical transmission unit  1 D is a differential input signal in the third exemplary embodiment. 
     The differential input signal input to the optical transmission unit  1 D may employ known signals and a preferable example thereof is an LVDS (Low Voltage Differential Signaling) signal. 
     In the optical transmission device  5 H according to this exemplary embodiment, it is possible to make a communication resistant to a noise by using the differential input signal. 
     Eighteenth Exemplary Embodiment 
     An optical transmission device according to an eighteenth exemplary embodiment of the present invention has a configuration in which the external input signal to the optical transmission unit  1 A in the fourth exemplary embodiment is a differential input signal, but is not shown. 
     In the optical transmission device according to this exemplary embodiment, it is possible to make a communication resistant to a noise by using the differential input signal. 
     In the seventeenth and eighteenth exemplary embodiments, it is stated that the external input signal to the optical transmission unit  1 A is the differential input signal, but the first exemplary embodiment may similarly have the configuration in which the external input signal to the optical transmission unit  1  is the differential input signal (not shown). The differential input signal is the same as described in the seventeenth and eighteenth exemplary embodiments and the same advantageous effects can be obtained. 
     Nineteenth Exemplary Embodiment 
       FIG. 10  is a diagram schematically illustrating the configuration of an optical transmission device according to a nineteenth exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. In an optical transmission device  5 I according to this exemplary embodiment, an optical transmission unit  1 E is provided as the configuration corresponding to the optical transmission unit  1 A in the third exemplary embodiment. 
     In the optical transmission device  5 I, a protection circuit  14  is disposed between the electricity-transmitting medium  31  and the light-emitting element  12  in the optical transmission unit  1 E according to the third exemplary embodiment. 
     In the optical transmission device  5 I according to this exemplary embodiment, even when a current signal for driving the light-emitting element  12  in the optical transmission unit  1 E rapidly varies, the light-emitting element  12  can be protected by the protection circuit  14 , thereby improving long-term reliability of communications. 
     Twentieth Exemplary Embodiment 
     In an optical transmission device according to a twentieth exemplary embodiment of the present invention, a protection circuit is disposed between the electricity-transmitting medium  31  and the light-emitting element  12  in the optical transmission unit  1 A in the fourth exemplary embodiment, but is not shown. 
     In the optical transmission device according to this exemplary embodiment, even when a current signal for driving the light-emitting element  12  in the optical transmission unit  1 A rapidly varies, the light-emitting element  12  can be protected by the protection circuit  14 , thereby improving the long-term reliability of communications. 
     In the nineteenth and twentieth exemplary embodiments, it is stated that the protection circuit (the protection circuit  14 ) is disposed between the electricity-transmitting medium  31  and the light-emitting element  12  in the optical transmission unit  1 A, but the first exemplary embodiment may employ the same configuration. That is, although not shown here, the protection circuit may be disposed between the electricity-transmitting medium  31  and the light-emitting element  12  in the optical transmission unit  1  in the first exemplary embodiment. 
     Twenty-First Exemplary Embodiment 
       FIG. 11  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-first exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. 
     In the optical transmission device  5 J according to this exemplary embodiment, instead of the light-transmitting medium  3  and the electricity-transmitting medium  31  in the third exemplary embodiment, a photoelectric composite cable  35  in which they are formed in a body is provided. 
     A cable in which an optical fiber as the light-transmitting medium and an electrical wire as the electricity-transmitting medium are combined can be exemplified as the photoelectric composite cable  35 . 
     In the optical transmission device  5 J according to this exemplary embodiment, since the light-emitting element  12  is coupled to the light-receiving element  23  and the current source  24  by a single medium, it is possible to improve the handleability and to reduce the mounting cost. 
     In this exemplary embodiment, the same advantageous effects can be obtained without disposing the error detector  28 . 
     Twenty-Second Exemplary Embodiment 
     An optical transmission device according to a twenty-second exemplary embodiment of the present invention has a configuration, which is not shown, in which a photoelectric composite cable in which these are foil ed in a body is provided instead of the light-transmitting medium  3  and the electricity-transmitting medium  31  in the fourth exemplary embodiment. 
     An example of the photoelectric composite cable is the same as described in the twenty-first exemplary embodiment. 
     In the optical transmission device according to this exemplary embodiment, it is possible to improve the handleability and to reduce the mounting cost. 
     Twenty-Third Exemplary Embodiment 
       FIG. 12  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-third exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. 
     In the optical transmission device  5 K according to this exemplary embodiment, a photoelectric composite wiring board  36  in which a light waveguide and the electricity-transmitting medium are disposed on a substrate is provided instead of the light-transmitting medium  3  and the electricity-transmitting medium  31  in the third exemplary embodiment. 
     A substrate in which a light waveguide as the light-transmitting medium is disposed on a substrate having an electrical wire as the electricity-transmitting medium formed therein can be exemplified as the photoelectric composite wiring board  36 . The light waveguide is not particularly limited and examples thereof include an optical fiber containing glass or plastics as a main component, a dielectric, a semiconductor, and a polymer. 
     In the optical transmission device  5 K according to this exemplary embodiment, since the light-emitting element  12  is coupled to the light-receiving element  23  and the current source  24  by a single medium, it is possible to improve the handleability and to reduce the mounting cost. 
     In this exemplary embodiment, the same advantageous effects can be obtained even without disposing the error detector  28 . 
     Twenty-Fourth Exemplary Embodiment 
     An optical transmission device according to a twenty-fourth exemplary embodiment of the present invention has a configuration, which is not shown, in which a photoelectric composite wiring board in which a light waveguide and the electricity-transmitting medium are formed on a substrate is provided as the light-transmitting medium  3  and the electricity-transmitting medium  31  in the fourth exemplary embodiment. 
     An example of the photoelectric composite wiring board is the same as described in the twenty-third exemplary embodiment. 
     In the optical transmission device according to this exemplary embodiment, it is possible to improve the handleability and to reduce the mounting cost. 
     Twenty-Fifth Exemplary Embodiment 
       FIG. 13  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-fifth exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. 
     In the optical transmission device  5 L according to this exemplary embodiment, a light waveguide  37  coated with metal is provided instead of the light-transmitting medium  3  and the electricity-transmitting medium  31  in the third exemplary embodiment. 
     An example in which an optical fiber as the light-transmitting medium is coated with metal as the electricity-transmitting medium can be used as the light waveguide  37  coated with metal. However, the light-transmitting medium may employ a material other than the optical fiber and the metal is not particularly limited as long as it has excellent electrical conductivity. 
     In the optical transmission device  5 L according to this exemplary embodiment, since the light-emitting element  12  is coupled to the light-receiving element  23  and the current source  24  by a single medium, it is possible to improve the handleability and to reduce the mounting cost. For example, since the optical fiber coated with metal is hardly changed in shape and is hardly bent at the time of mounting unlike a normal optical fiber, it is possible to stably position the optical fiber. Accordingly, the optical transmission device  5 L according to this exemplary embodiment is advantageous for mass production or cost reduction. 
     Twenty-Sixth Exemplary Embodiment 
     An optical transmission device according to a twenty-sixth exemplary embodiment of the present invention has a configuration, which is not shown, in which a light waveguide coated with metal is employed instead of the light-transmitting medium  3  and the electricity-transmitting medium  31  in the fourth exemplary embodiment. 
     An example of the light waveguide coated with metal is the same as described in the twenty-fifth exemplary embodiment. 
     In the optical transmission device according to this exemplary embodiment, it is possible to improve the handleability and to reduce the mounting cost. For example, since the optical fiber coated with metal is hardly changed in shape and is hardly bent at the time of mounting unlike a normal optical fiber, it is possible to stably position the optical fiber. Accordingly, the optical transmission device according to this exemplary embodiment is advantageous for mass production or cost reduction. 
     In the twenty-first to twenty-sixth exemplary embodiments, the configuration, instead of the light-transmitting medium  3  and the electricity-transmitting medium  31 , having the photoelectric composite cable in which they are formed in a body, the photoelectric composite wiring board in which the light waveguide and the electricity-transmitting medium are disposed on a substrate, or the light waveguide coated with metal is stated, but the first exemplary embodiment may have the same configuration. In this case, the light-emitting element  12  may be coupled to the light-receiving element  23  and the static current source  29  by a single medium. In this case, the same advantageous effects as described in the twenty-first to twenty-sixth exemplary embodiments can be obtained. 
     Twenty-Seventh Exemplary Embodiment 
       FIG. 14  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-seventh exemplary embodiment of the present invention. In the optical transmission device  5 M shown in the drawing, the optical transmission unit  1 F and the optical reception unit  2 F in the twenty-second exemplary embodiment are air-tightly sealed by packages  90  and  91  of which at least part has conductivity, respectively. The current source  24 , the electricity-transmitting medium, and the light-emitting element  12  are electrically connected to each other via the packages  90  and  91 . Specifically, the optical transmission unit  1 F is air-tightly sealed by the package  90  of which at least part has conductivity and the optical reception unit  2 F is air-tightly sealed by the package  91  of which at least part has conductivity. The electricity-transmitting medium in the photoelectric composite cable  35  is electrically connected to the light-emitting element  12  in the optical transmission unit  1 F via the package  90 . The electricity-transmitting medium is electrically connected to the current source  24  in the optical reception unit  2 F via the package  91 . 
     A specific example of the packages  90  and  91  is a package of which at least part is formed of metal, but the main material is not particularly limited. Examples thereof include iron, noniron metal, and precious metal. In some applications, a package formed by coating a resin with metal may be used. A resin package having conductivity may be used. 
     In the optical transmission device  5 M according to this exemplary embodiment, since the bias current is transmitted from the optical reception unit  2 F to the optical transmission unit  1 F via the packages  90  and  91  and the electricity-transmitting medium, a dedicated wiring pattern for supplying the bias current need not be disposed between the optical reception unit  2 F and the optical transmission unit  1 F, thereby reducing the size of the optical transmission device  5 M. 
     Although the optical transmission device having the average calculator  25  is stated in this exemplary embodiment, the same advantageous effects can be obtained even without disposing the average calculator  25 . The same advantageous effects can be obtained even without disposing both the error detector  28  and the average calculator  25 . 
     Twenty-Eighth Exemplary Embodiment 
       FIG. 15  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-eighth exemplary embodiment of the present invention. 
     In the optical transmission device  5 N shown in the drawing, the optical transmission unit  1 F and the optical reception unit  2 F in the twenty-fourth exemplary embodiment are air-tightly sealed by packages  90  and  91  of which at least part has conductivity, respectively. The current source  24 , the electricity-transmitting medium, and the light-emitting element  12  are electrically connected to each other via the packages  90  and  91 . Specifically, the optical transmission unit  1 F is air-tightly sealed by the package  90  of which at least part has conductivity and the optical reception unit  2 F is air-tightly sealed by the package  91  of which at least part has conductivity. The electricity-transmitting medium in the photoelectric composite wiring board  36  is electrically connected to the light-emitting element  12  in the optical transmission unit  1 F via the package  90 . The electricity-transmitting medium is electrically connected to the current source  24  in the optical reception unit  2 F via the package  91 . 
     An example of the packages  90  and  91  is the same as described in the twenty-seventh exemplary embodiment. 
     In the optical transmission device  5 N according to this exemplary embodiment, since the bias current is transmitted from the optical reception unit  2 F to the optical transmission unit  1 F via the packages  90  and  91  and the electricity-transmitting medium, a dedicated wiring pattern for supplying the bias current need not be disposed between the optical reception unit  2 F and the optical transmission unit  1 F, thereby reducing the size of the optical transmission device  5 N. 
     Although the optical transmission device having the average calculator  25  is stated in this exemplary embodiment, the same advantageous effects can be obtained even without disposing the average calculator  25 . The same advantageous effects can be obtained even without disposing both the error detector  28  and the average calculator  25 . 
     Twenty-Ninth Exemplary Embodiment 
       FIG. 16  is a diagram schematically illustrating the configuration of an optical transmission device according to a twenty-ninth exemplary embodiment of the present invention. 
     In the optical transmission device  5 P shown in the drawing, the optical transmission unit  1 F and the optical reception unit  2 F in the twenty-sixth exemplary embodiment are air-tightly sealed by packages  90  and  91  of which at least part has conductivity, respectively. The current source  24 , the electricity-transmitting medium, and the light-emitting element  12  are electrically connected to each other via the packages  90  and  91 . Specifically, the optical transmission unit  1 F is air-tightly sealed by the package  90  of which at least part has conductivity and the optical reception unit  2 F is air-tightly sealed by the package  91  of which at least part has conductivity. The electricity-transmitting medium in the light waveguide coated with metal (metal-coated light waveguide)  37  is electrically connected to the light-emitting element  12  in the optical transmission unit  1 F via the package  90 . The electricity-transmitting medium is electrically connected to the current source  24  in the optical reception unit  2 F via the package  91 . 
     An example of the packages  90  and  91  is the same as described in the twenty-seventh exemplary embodiment. 
     In the optical transmission device  5 P according to this exemplary embodiment, since the bias current is transmitted from the optical reception unit  2 F to the optical transmission unit  1 F via the packages  90  and  91  and the electricity-transmitting medium, a dedicated wiring pattern for supplying the bias current need not be disposed between the optical reception unit  2 F and the optical transmission unit  1 F, thereby reducing the size of the optical transmission device  5 P. 
     Although the optical transmission device having the average calculator  25  is stated in this exemplary embodiment, the same advantageous effects can be obtained even without disposing the average calculator  25 . The same advantageous effects can be obtained even without disposing both the error detector  28  and the average calculator  25 . 
     In the twenty-seventh to twenty-ninth exemplary embodiments, the configuration in which the light-transmitting medium and the electricity-transmitting medium are formed in a body is exemplified as the light-transmitting medium and the electricity-transmitting medium, the light-transmitting medium and the electricity-transmitting medium may be separately disposed without forming a body. 
     In the twenty-seventh to twenty-ninth exemplary embodiments, the optical transmission devices in which the optical transmission unit  1 F and the optical reception unit  2 F are air-tightly sealed by the packages  90  and  91  of which at least part has conductivity, respectively, and the current source  24 , the electricity-transmitting medium, and the light-emitting element  12  are electrically connected to each other via the packages  90  and  91  are described, but the first exemplary embodiment may employ the same configuration. That is, although not shown here, the optical transmission unit  1  in the first exemplary embodiment may be air-tightly sealed by the package  90  of which at least part has conductivity, the optical reception unit  2  may be air-tightly sealed by the package  91  of which at least part has conductivity, and the electricity-transmitting medium may be electrically connected to the light-emitting element  12  via the package  90  and may be electrically connected to the static current source  29  via the package  91 . Here, the electricity-transmitting medium is the same as described in the twenty-seventh to twenty-ninth exemplary embodiments. 
     Thirtieth Exemplary Embodiment 
       FIG. 17  is a diagram schematically illustrating the configuration of an optical transmission device according to a thirtieth exemplary embodiment of the present invention. Like elements described in the third exemplary embodiment are denoted by the same reference numerals and signs and description thereof is not repeated. In an optical transmission device  5 Q according to this exemplary embodiment, an optical transmission unit  1 G is provided as the configuration corresponding to the optical transmission unit  1 A in the third exemplary embodiment and an optical reception unit  2 G is provided as the configuration corresponding to the optical reception unit  2 B. 
     The optical transmission device  5 Q has the configuration in which current is transmitted wirelessly without using the electricity-transmitting medium  31  in the third exemplary embodiment. 
     The electricity-transmitting medium  32  transmitting the current wirelessly may have a configuration including a modulator and a second antenna which are disposed in the optical reception unit  2 G so as to transmit the bias current generated by the current source  24  as electric waves and a first antenna and a demodulation circuit which are disposed in the optical transmission unit  1 G so as to receive the electric waves and to demodulate the received electric waves into the bias current. 
     In the optical transmission device  5 Q according to this exemplary embodiment, the electrical wire for transmitting the bias current is not necessary. 
     In this exemplary embodiment, the same advantageous effects can be obtained even without disposing the error detector  28 . 
     It is stated in the thirtieth exemplary embodiment that the electricity-transmitting medium is constructed to transmit the current wirelessly, but the first exemplary embodiment may have the same configuration (not shown). The electricity-transmitting medium for transmitting the current wirelessly may employ the configuration including a modulator and a second antenna which are disposed in the optical reception unit  2  so as to transmit the bias current generated by the static current source  29  as electric waves and a first antenna and a demodulation circuit which are disposed in the optical reception unit  1  so as to receive the electric waves and to demodulate the received electric waves into the bias current, as described above. 
     The optical transmission device according to the present invention is not limited to the above-mentioned configurations, but partial configurations may be added thereto or deleted therefrom. 
     In the example where the optical reception unit includes the current source (for example, the second exemplary embodiment), two or more selected from a group consisting of the error detector, the average calculator, the impedance-matching element, and the LPF may be disposed in the optical reception unit in combination. 
     Other Exemplary Embodiments 
     Hitherto, the optical transmission device in which the light-receiving element of the optical reception unit generates the bias current from the received optical signal and transmits the bias current to the light-emitting element of the optical transmission unit to control the emission intensity of the light-emitting element has been described, but the communications may be stabilized with a configuration of making a feedback control using bidirectional communications. Examples of such an optical transmission device include the followings, all of which can allow a decrease in size and allow bidirectional communications. 
     An optical transmission device  50 A shown in  FIG. 18  includes a first optical transmission and reception device  10  including a first light-emitting element  120 , a first light-receiving element  130 , a first TIA  160 , a first error detector  180 , and a first current source  190 , a second optical transmission and reception device  20  including a second light-emitting element  220 , a second light-receiving element  230 , a second TIA  260 , a second error detector  280 , and a second current source  290 , a first light-transmitting medium  30  optically connecting the first light-emitting element  120  and the second light-receiving element  230  to each other, and a second light-transmitting medium  40  optically connecting the second light-emitting element  220  and the first light-receiving element  130  to each other. 
     The first TIA  160  converts the current generated based on the received light intensity of the first light-receiving element  130  into a voltage and amplifies the voltage. The first error detector  180  regularly measures the received light intensity of the first light-receiving element  130  based on the voltage signal acquired by the first TIA  160  and outputs a voltage signal for controlling the bias current to be transmitted to the second light-emitting element  220  based on the measured value. The first current source  190  converts the voltage signal output from the first error detector  180  into bias current and drives the first light-emitting element  120 . The second TIA  260  converts the current generated based on the received light intensity of the second light-receiving element  230  into a voltage and amplifies the voltage. The second error detector  280  regularly measures the received light intensity of the second light-receiving element  230  based on the voltage signal acquired by the second TIA  260  and outputs a voltage signal for controlling the bias current for driving the first light-emitting element  120  based on the measured value. The second current source  290  converts the voltage signal output from the second error detector  280  into bias current and drives the second light-emitting element  220 . 
     The first light-emitting element  120  and the second light-emitting element  220  are the same as the light-emitting element  12  shown in  FIG. 1 . 
     The first light-receiving element  130  and the second light-receiving element  230  are the same as the light-receiving element  23  shown in  FIG. 1 . 
     The first light-transmitting medium  30  and the second light-transmitting medium  40  are the same as the light-transmitting medium  3  shown in  FIG. 1 . 
     The first TIA  160  and the second TIA  260  are the same as the TIA  26  shown in  FIG. 1 . 
     The power for controlling the first TIA  160  and the second TIA  260  are not shown here, but both are supplied from the outside. 
     In the optical transmission device  50 A, a modulated signal is acquired from the outside to drive the first light-emitting element  120  and the second light-emitting element  220 . Here, the modulated signal may employ any of a burst signal and a continuous signal, and an appropriate one can be selected. As described above, the bias voltage for driving the second light-emitting element  220  is controlled by the first error detector  180  and the bias current is transmitted to the second light-receiving element  230  via the first light-transmitting medium  30  by the first current source  190 . Similarly, the bias voltage for driving the first light-emitting element  120  is controlled by the second error detector  280  and the bias current is transmitted to the first light-receiving element  130  via the second light-transmitting medium  40  by the second current source  290 . 
     The optical transmission device  50 A according to this exemplary embodiment can also obtain the same advantageous effects as the optical transmission device according to the second exemplary embodiment. 
     Here, it is stated that the first TIA  160  and the first error detector  180  are directly connected to each other and the second TIA  260  and the second error detector  280  are directly connected to each other, but the same advantageous effects can be obtained even when an average calculator is disposed between the TIA  160  and the error detector  180 . 
     In the optical transmission device  50 B shown in  FIG. 19 , a light-transmitting medium  34  in which the first light-transmitting medium  30  and the second light-transmitting medium  40  are combined into a single light-transmitting medium so as to allow single-core bidirectional communications is used instead of the first light-transmitting medium  30  and the second light-transmitting medium  40  in the optical transmission device  50 A shown in  FIG. 18 . The optical transmission device  50 B is the same as the optical transmission device  50 A, except for this point. For example, the light-transmitting medium  34  can employ an optical multiplexer/demultiplexer. 
     According to this configuration, since the number of light-transmitting mediums can be reduced, it is possible to further reduce the size of the optical transmission device, thereby improving the handleability. 
     Here, it is stated that the first TIA  160  and the first error detector  180  are directly connected to each other and the second TIA  260  and the second error detector  280  are directly connected to each other, but the same advantageous effects can be obtained even when an average calculator is disposed between the TIA  160  and the error detector  180 . 
     An optical transmission device  50 C shown in  FIG. 20  includes a first optical transmission and reception device  10  including a first light-emitting element  120 , a first light-receiving element  130 , a first TIA  160 , a first error detector  180 , and a first current source  190 , a second optical transmission and reception device  20  including a second light-emitting element  220 , a second light-receiving element  230 , a second TIA  260 , a second error detector  280 , and a second current source  290 , a first light-transmitting medium  30  optically connecting the first light-emitting element  120  and the second light-receiving element  230  to each other, a second light-transmitting medium  40  optically connecting the second light-emitting element  220  and the first light-receiving element  130  to each other, a first electricity-transmitting medium  31  electrically connecting the first current source  190  and the second light-emitting element  220  to each other, and a second electricity-transmitting medium  41  electrically connecting the second current source  290  and the first light-emitting element  120  to each other. The first TIA  160  converts the current generated based on the received light intensity of the first light-receiving element  130  into a voltage and amplifies the voltage. The first error detector  180  regularly measures the received light intensity of the first light-receiving element  130  based on the voltage signal acquired by the first TIA  160  and outputs a voltage signal for controlling the bias current to be transmitted to the second light-emitting element  220  based on the measured value. The first current source  190  converts the voltage signal output from the first error detector  180  into bias current and drives the second light-emitting element  220 . The second TIA  260  converts the current generated based on the received light intensity of the second light-receiving element  230  into a voltage and amplifies the voltage. The second error detector  280  regularly measures the received light intensity of the second light-receiving element  230  based on the voltage signal acquired by the second TIA  260  and outputs a voltage signal for controlling the bias current to be transmitted to the first light-emitting element  120  based on the measured value. The second current source  290  converts the voltage signal output from the second error detector  280  into bias current and drives the first light-emitting element  120 . 
     The optical transmission device  50 C is the same as the optical transmission device  50 A, except for the feedback control is performed by transmitting an electrical signal, not an optical signal. 
     The optical transmission device  50 C controls the emission intensity of the first light-emitting element  120  based on the received light intensity of the second light-receiving element  230  and controls the emission intensity of the second light-emitting element  220  based on the received light intensity of the first light-receiving element  130 . Accordingly, the controls can be independently performed, thereby allowing a more stable communication than that of the optical transmission device  50 A. 
     The optical transmission device  50 C can be suitably used in the fields of, for example, a USB (Universal Serial Bus) cable which is a standard of a high-speed transmission cable, an Infiniband cable, an intra-case wire in a mobile phone, a connecting wire between a game machine and a display, and a video wire between a display and a camera. 
     Here, it is stated that the first TIA  160  and the first error detector  180  are directly connected to each other and the second TIA  260  and the second error detector  280  are directly connected to each other, but the same advantageous effects can be obtained even when an average calculator is disposed between the TIA  160  and the error detector  180 . 
     In an optical transmission device  50 D shown in  FIG. 21 , a light-transmitting medium  34  in which the first light-transmitting medium  30  and the second light-transmitting medium  40  are combined into a single light-transmitting medium so as to allow single-core bidirectional communications is used instead of the first light-transmitting medium  30  and the second light-transmitting medium  40  in the optical transmission device  50 C shown in  FIG. 20 . The optical transmission device  50 D is the same as the optical transmission device  50 C, except for this point. For example, the light-transmitting medium  34  is the same as the optical transmission medium  50 B. 
     According to this configuration, since the number of light-transmitting mediums can be reduced, it is possible to further reduce the size of the optical transmission device, thereby improving the handleability. 
     The optical transmission device  50 D can be suitably used in the fields of, for example, a USB (Universal Serial Bus) cable which is a standard of a high-speed transmission cable, an Infiniband cable, an intra-case wire in a mobile phone, a connecting wire between a game machine and a display, and a video wire between a display and a camera. 
     Here, it is stated that the first TIA  160  and the first error detector  180  are directly connected to each other and the second TIA  260  and the second error detector  280  are directly connected to each other, but the same advantageous effects can be obtained even when an average calculator is disposed between the TIA  160  and the error detector  180 . 
     The optical transmission device may be constructed to transmit a large amount of data, in which the bias current is generated from the optical signal received by the light-receiving element of the optical reception unit and the bias current is transmitted to the light-emitting element of the optical transmission unit to control the emission intensity of the light-emitting element. Examples of such an optical transmission device include the followings, all of which can stably transmit a large amount of data and allow a decrease in size. 
     An optical transmission device  500 A shown in  FIG. 22  includes four sets of optical transmission devices  5 C according to the fourth exemplary embodiment in parallel and includes a photoelectric composite cable  700  in which four sets of light-transmitting mediums and four sets of electricity-transmitting mediums are formed in a body. That is, the optical transmission unit  1 H and the optical reception unit  2 H are coupled to each other by a single photoelectric composite cable  700 . The respective optical transmission devices of the four sets operate similarly to the fourth exemplary embodiment shown in  FIG. 4 . A cable in which an optical fiber as the light-transmitting medium and an electrical wire as the electricity-transmitting medium are combined can be exemplified as the photoelectric composite cable  700 . 
     In the optical transmission device  500 A, it is not necessary to dispose a driving circuit in the optical transmission unit. As a result, it is possible to reduce the size of the optical transmission unit. Since four sets of light-emitting elements are coupled to four sets of light-receiving elements and current sources by a single medium, it is possible to improve the handleability and to reduce the mounting cost. 
     The optical transmission device  500 A can be suitably used in the fields of, for example, an HDMI (High-Definition Multimedia Interface) cable which is a standard of a high-speed transmission cable, a display port cable, a connecting wire between a high-speed data transmission device, a business copier, or a game machine and a display, and a video wire between a display and a camera. 
     An optical transmission device  500 B shown in  FIG. 23  has a configuration in which a serializing IC  92  serializing data is disposed in the subsequent stage of the input signal in the optical transmission device  5 C shown in  FIG. 4  and a deserializing IC  93  parallelizing data is disposed in the subsequent stage of the TIA  26 . A photoelectric composite cable  710  in which the light-transmitting medium and the electricity-transmitting medium are formed in a body is further disposed. Multiple types of signals are input to the optical transmission unit  11  from the outside and multiple types of signals are output from the optical reception unit  21  to the outside. 
     According to the optical transmission device  500 B, it is possible to reduce the number of light-transmitting mediums and the number of electricity-transmitting mediums and thus to reduce the outer diameter of the photoelectric composite cable. 
     The optical transmission device  500 B can be suitably used in the fields of, for example, an HDMI (High-Definition Multimedia Interface) cable which is a standard of a high-speed transmission cable, a display port cable, a connecting wire between a high-speed data transmission device, a business copier, or a game machine and a display, and a video wire between a display and a camera. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be applied to the transmission of information using the optical communication over a relatively-short distance in the fields of high-speed data transmission devices such as a server or a router, vehicles, mobile phones, business copiers, and game machines.