Abstract:
A signal transmission system includes a first transmission unit for distributing and transmitting a downward optical signal from a downward signal transmitting unit to a plurality of downward signal receiving units, and a second transmission unit for multiplexing a plurality of upward optical signals applied thereto and transmitting the plurality of upward optical signals multiplexed, and for distributing and transmitting a downward signal applied thereto. The system also comprises a first WDM unit for applying the plurality of upward optical signals multiplexed and transmitted thereto by the second transmission unit to an upward signal receiving unit, and for applying an optical signal transmitted by an information transmitting unit, as a downward optical signal, to the second transmission unit, and a second WDM unit for applying a plurality of upward optical signal transmitted by a plurality of upward signal transmitting units to the second transmission unit, and for applying the optical signal from the information transmitting unit transmitted thereto by the second transmission unit to an information receiving unit.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a signal transmission system that employs an optical fiber as a communication medium.  
           [0003]    2. Description of the Prior Art  
           [0004]    [0004]FIG. 10 is a block diagram showing the structure of a prior art signal transmission system. In the figure, reference numeral  1  denotes a station apparatus, reference numeral  2  denotes customer premises equipment, reference numeral  11  denotes a laser diode that converts a downward electrical signal into a downward optical signal having a wavelength of 1480 nm to 1500 nm and transmits the downward optical signal, reference numeral  12  denotes a photo diode that receives an upward optical signal having a wavelength of 1260 nm to 1360 nm and converts the received upward optical signal into an electrical signal, and reference numeral  13  denotes a wavelength division multiplexer (WDM) that allows only downward optical signals to pass therethrough for a channel from a port thereof on the side of the laser diode  11  to a port thereof on the side of another WDM  16 , and that allows only upward optical signals to pass therethrough for a channel from the port thereof on the side of the other WDM  16  to a port thereof on the side of the photo diode  12 .  
           [0005]    Reference numeral  14  denotes a video signal transmitter that generates and transmits a video signal, and reference numeral  15  denotes a laser diode that converts the video signal (electrical signal) sent from the video signal transmitter  14  into an optical signal having a wavelength of 1550 nm to 1560 nm (referred to as video optical signal from here on) and that sends out the video optical signal. The WDM  16  allows only downward and upward optical signals to pass therethrough for a channel between a port thereof on the side of a star coupler  18  to a port thereof on the side of the WDM  13 , and that allows only video optical signals to pass therethrough for a channel from a port thereof on the side of the laser diode  15  to the port thereof on the side of the star coupler  18 .  
           [0006]    Reference numeral  17  denotes an optical fiber. The star coupler  18  distributes an optical signal from the WDM  16  to several locations. The star coupler  18  also multiplexes a plurality of optical signals received into an upward optical signal. Reference numeral  19  denotes a WDM that allows only downward and upward optical signals to pass therethrough for a channel between a port thereof on the side of the star coupler  18  to a port thereof on the side of another WDM  20 , and that allows only video optical signals to pass therethrough for a channel from the port thereof on the side of the star coupler  18  to a port thereof on the side of a photo diode  23 . The WDM  20  allows only downward optical signals to pass therethrough for a channel from a port thereof on the side of the WDM  19  to a port thereof on the side of a photo diode  22 , and that allows only upward optical signals to pass therethrough for a channel from a port thereof on the side of a laser diode  21  to the port thereof on the side of the WDM  19 . The laser diode  21  converts an upward electrical signal into an upward optical signal and then transmits the upward optical signal. The photo diode  22  receives a downward optical signal and then converts the received downward optical signal into an electrical signal. Reference numeral  23  denotes a photo diode that receives a video optical signal and converts the received video optical signal into an electrical signal, reference numeral  24  denotes a video signal receiver that receives a video signal which is the electrical signal from the photo diode  23 , and reference numeral  25  denotes a termination.  
           [0007]    In operation, when the laser diode  11  of the station apparatus  1  emits a downward optical signal, the downward optical signal travels through the optical fiber  17  and then reaches the star coupler  18  after it passes through the WDMs  13  and  16  included in the station apparatus. The star coupler  18  distributes the downward optical signal to only a number of pieces of customer premises equipment  2 . As a result, the downward optical signal is received and is converted into an electrical signal by the photo diode  22  of each customer premises equipment  2  after it passes through the WDMs  19  and  20  provided for each customer premises equipment  2 .  
           [0008]    When the laser diode  21  of each customer premises equipment  2  emits an upward optical signal, the upward optical signal travels through the optical fiber  17  and then reaches the star coupler  18  after it passes through the WDMs  20  and  19  provided for each customer premises equipment  2 . The star coupler  18  multiplexes a plurality of upward optical signals, the number of which corresponds to the number of pieces of customer premises equipment  2 . As a result, the plurality of upward optical signals multiplexed are received and are converted into an electrical signal by the photo diode  12  of the station apparatus  1  after it passes through the WDMs  16  and  13  included in the station apparatus  1 .  
           [0009]    When the laser diode  15  of the station apparatus  1  emits a video optical signal, the video optical signal travels through the optical fiber  17  and then reaches the star coupler  18  after it passes through the WDM  16  included in the station apparatus. The star coupler  18  distributes the video optical signal to only the number of pieces of customer premises equipment  2 . As a result, the video optical signal is received and is converted into an electrical signal by the photo diode  23  of each customer premises equipment  2  after it passes through the WDM  19  provided for each customer premises equipment  2 . However, some customers do not need the reception of the video optical signal. Therefore, some pieces of customer premises equipment  2  do not include a photo diode  23  and a video signal receiver  24 . In this case, the video optical signal is terminated by the termination  25  disposed for each customer premises equipment  2 .  
           [0010]    [0010]FIG. 11 is a graph showing insertion loss characteristics of the WDM  19 . In the figure, A shows the insertion loss characteristics for a channel from a port on the side of corresponding customer premises equipment  2  to a port on the side of the star coupler  18 , and B shows the insertion loss characteristics for a channel from the port on the side of the star coupler  18  to a port on the side of the video signal receiver  24 . It is apparent from FIG. 11 that those insertion loss characteristics change abruptly over a narrow range of wavelengths from 1500 nm and 1550 nm (the width of the wavelength range=50 nm). Therefore, it is difficult to design the WDM  19 , and hence the cost of the WDM  19  rises.  
           [0011]    A problem with a prior art signal transmission system constructed as above is that although it can transmit not only upward and downward optical signals but also a video optical signal, an expensive WDM  19  has to be provided regardless of whether each customer has a video signal receiver for receiving a video optical signal and therefore the cost of building the system rises and the transmission range of optical signals is shortened by the transmission loss in the WDM  19 .  
         SUMMARY OF THE INVENTION  
         [0012]    The present invention is proposed to solve the above-mentioned problems, and it is therefore an object of the present invention to provide a signal transmission system in which the number of WDMs can be reduced, thereby reducing the cost of building the system, and in which the transmission range of optical signals can be lengthened.  
           [0013]    In accordance with an aspect of the present invention, there is provided a signal transmission system comprising: a first transmission unit for distributing and transmitting a downward optical signal transmitted by a downward signal transmitting unit to a plurality of downward signal receiving units; a second transmission unit for multiplexing a plurality of upward optical signals applied thereto and transmitting the plurality of upward optical signals multiplexed, and for distributing and transmitting a downward signal applied thereto; an information transmitting unit for transmitting an optical signal that-Carries specific information; a first WDM unit for applying the plurality of upward optical signals multiplexed and transmitted thereto by the second transmission unit to an upward signal receiving unit, and for applying the optical signal transmitted by the information transmitting unit, as a downward optical signal, to the second transmission unit; a plurality of upward signal transmitting units each for transmitting an upward signal; an information receiving unit for receiving an optical signal that carries specific information; and a second WDM unit for applying a plurality of upward optical signals transmitted by the plurality of upward signal transmitting units to the second transmission unit, and for applying the optical signal from the information transmitting unit transmitted thereto by the second transmission unit to the information receiving unit. Accordingly, the cost of building the system can be reduced, and the transmission range of optical signals can be lengthened.  
           [0014]    In accordance with a preferred embodiment of the present invention, the information transmitting unit is a video signal transmitting unit for transmitting the optical signal based on a video signal. As an alternative, the information transmitting unit is a data signal transmitting unit for transmitting the optical signal based on a data signal. Preferably, the optical signal transmitted by the information transmitting unit has a wavelength longer than those of the plurality of optical signals transmitted by the plurality of upward signal transmitting units. Accordingly, each of the plurality of upward signal transmitting units can be prevented from malfunctioning even if no WDM is placed between the second transmission unit and each of the plurality of upward signal transmitting units.  
           [0015]    In accordance with another aspect of the present invention, there is provided a signal transmission system comprising: a downward signal transmitting unit for transmitting a downward optical signal; a first transmission unit for distributing and transmitting a downward optical signal applied thereto; a second transmission unit for multiplexing a plurality of upward optical signals transmitted by a plurality of upward signal transmitting units and for transmitting the plurality of upward optical signals multiplexed; an information transmitting unit for transmitting an optical signal that carries specific information; a first WDM unit for applying the downward optical signal transmitted by the downward signal transmitting unit and the optical signal transmitted by the information transmitting unit to the first transmission unit; an information receiving unit for receiving an optical signal that carries specific information; and a second WDM unit for applying the downward optical signal distributed and transmitted thereto by the first transmission unit to a plurality of downward signal receiving units, and for applying the optical signal from the information transmitting unit transmitted thereto by the first transmission unit to the information receiving unit. Accordingly, the cost of building the system can be reduced, and the transmission range of optical signals can be lengthened.  
           [0016]    In accordance with a preferred embodiment of the present invention, the information transmitting unit is a video signal transmitting unit for transmitting the optical signal based on a video signal. As an alternative, the information transmitting unit is a data signal transmitting unit for transmitting the optical signal based on a data signal. Preferably, the optical signal transmitted by the information transmitting unit has a wavelength longer than those of the plurality of optical signals transmitted by the plurality of upward signal transmitting units. Accordingly, each of the plurality of upward signal transmitting units can be prevented from malfunctioning even if no WDM is placed between the second transmission unit and each of the plurality of upward signal transmitting units.  
           [0017]    In accordance with a further aspect of the present invention, there is provided a signal transmission system comprising: a first transmission unit for distributing and transmitting a downward optical signal transmitted by a downward signal transmitting unit to a plurality of downward signal receiving units; a second transmission unit for multiplexing a plurality of upward optical signals transmitted by a plurality of upward signal transmitting units and transmitting the plurality of upward optical signals multiplexed, and for distributing and transmitting a downward optical signal applied thereto; an information transmitting unit for transmitting an optical signal that carries specific information; a WDM unit for applying the plurality of multiplexed upward optical signals transmitted by the second transmission unit to an upward signal receiving unit, and for applying the optical signal transmitted by the information transmitting unit, as a downward optical signal, to the second transmission unit; and an information receiving unit for receiving an optical signal that carries specific information, the second transmission unit transmitting the optical signal from the WDM unit to the information receiving unit. Accordingly, the cost of building the system can be reduced, and the transmission range of optical signals can be lengthened.  
           [0018]    In accordance with a preferred embodiment of the present invention, the information transmitting unit is a video signal transmitting unit for transmitting the optical signal based on a video signal. As an alternative, the information transmitting unit is a data signal transmitting unit for transmitting the optical signal based on a data signal. Preferably, the optical signal transmitted by the information transmitting unit has a wavelength longer than those of the plurality of optical signals transmitted by the plurality of upward signal transmitting units. Accordingly, each of the plurality of upward signal transmitting units can be prevented from malfunctioning even if no WDM is placed between the second transmission unit and each of the plurality of upward signal transmitting units.  
           [0019]    In accordance with another aspect of the present invention, there is provided a signal transmission system comprising: a downward signal transmitting unit for transmitting a downward optical signal; a first transmission unit for distributing and transmitting a downward optical signal applied thereto; a second transmission unit for multiplexing a plurality of upward optical signals transmitted by a plurality of upward signal transmitting units and for transmitting the plurality of upward optical signals multiplexed; an information transmitting unit for transmitting an optical signal that carries specific information; a WDM unit for applying the downward optical signal transmitted by the downward signal transmitting unit and the optical signal transmitted by the information transmitting unit to the first transmission unit; and an information receiving unit for receiving an optical signal that carries specific information, the first transmission unit transmitting the optical signal applied thereto from the WDM unit to the information receiving unit. Accordingly, the cost of building the system can be reduced, and the transmission range of optical signals can be lengthened.  
           [0020]    In accordance with a preferred embodiment of the present invention, the information transmitting unit is a video signal transmitting unit for transmitting the optical signal based on a video signal. As an alternative, the information transmitting unit is a data signal transmitting unit for transmitting the optical signal based on a data signal. Preferably, the optical signal transmitted by the information transmitting unit has a wavelength longer than those of the plurality of optical signals transmitted by the plurality of upward signal transmitting units. Accordingly, each of the plurality of upward signal transmitting units can be prevented from malfunctioning even if no WDM is placed between the second transmission unit and each of the plurality of upward signal transmitting units.  
           [0021]    Further objects and advantages of the present invention will be apparent from the following description of the preferred embodiments of the invention as illustrated in the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]    [0022]FIG. 1 is a block diagram showing the structure of a signal transmission system according to a first embodiment of the present invention;  
         [0023]    [0023]FIG. 2 is a graph showing insertion loss characteristics of a WDM included in the signal transmission system according to the first embodiment of the present invention of FIG. 1;  
         [0024]    [0024]FIG. 3 is a block diagram showing the structure of a signal transmission system according to a second embodiment of the present invention;  
         [0025]    [0025]FIG. 4 is a block diagram showing the structure of a signal transmission system according to a third embodiment of the present invention;  
         [0026]    [0026]FIG. 5 is a block diagram showing the structure of a signal transmission system according to a variant of the third embodiment of the present invention;  
         [0027]    [0027]FIG. 6 is a block diagram showing the structure of a signal transmission system according to a fourth embodiment of the present invention;  
         [0028]    [0028]FIG. 7 is a block diagram showing the structure of a signal transmission system according to a fifth embodiment of the present invention;  
         [0029]    [0029]FIG. 8 is a block diagram showing the structure of a signal transmission system according to a sixth embodiment of the present invention;  
         [0030]    [0030]FIG. 9 is a block diagram showing the structure of a signal transmission system according to a variant of the sixth embodiment of the present invention;  
         [0031]    [0031]FIG. 10 is a block diagram showing the structure of a prior art signal transmission system; and  
         [0032]    [0032]FIG. 11 is a graph showing insertion loss characteristics of a WDM included in the prior art signal transmission system of FIG. 10. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0033]    Embodiment 1.  
         [0034]    [0034]FIG. 1 is a block diagram showing the structure of a signal transmission system according to a first embodiment of the present invention. In the figure, reference numeral  31  denotes a station apparatus, reference numeral  32  denotes customer premises equipment, reference numeral  41  denotes a laser diode (downward signal transmitting means) that converts a downward electrical signal into a downward optical signal having a wavelength of 1260 nm to 1360 nm and transmits the downward optical signal, reference numeral  42  denotes an optical fiber, and reference numeral  43  denotes a star coupler that distributes the downward optical signal from the laser diode  41  to a number of pieces of customer premises equipment. A first transmission means consists of the optical fiber  42  and the star coupler  43 . Reference numeral  44  denotes a photo diode (downward signal receiving means) that receives a downward optical signal from the star coupler  43 .  
         [0035]    Reference numeral  45  denotes a photo diode (upward signal receiving means) that receives an upward optical signal having a wavelength of 1260 nm to 1360 nm and converts the received upward optical signal into an electrical signal, reference numeral  46  denotes a video signal transmitter that generates and transmits a video signal, reference numeral  47  denotes a laser diode (video signal transmitting means) that converts the video signal (electrical signal) sent from the video signal transmitter  46  into an optical signal (referred to as video optical signal from here on) having a wavelength of 1550 nm to 1560 nm and transmits the video optical signal, and reference numeral  48  denotes a WDM (first WDM means) that allows only upward optical signals to pass therethrough for a channel from a port thereof on the side of a star coupler  50  to a port thereof on the side of the photo diode  45 , and that allows only video optical signals to pass therethrough for a channel from a port thereof on the side of the laser diode  47  to the port thereof on the side of the star coupler  50 .  
         [0036]    Reference numeral  49  denotes an optical fiber. The star coupler  50  distributes the video optical signal from the WDM  48  to a number of pieces of customer premises equipment  32 . The star coupler  50  also multiplexes a plurality of upward optical signals. A second transmission means consists of the optical fiber  49  and the star coupler  50 . Reference numeral  51  denotes another WDM (second WDM means) that allows only upward optical signals to pass therethrough for a channel from a port thereof on the side of a laser diode  52  to a port thereof on the side of the star coupler  50 , and that allows only video optical signals to pass therethrough for a channel from the port thereof on the side of the start coupler  50  to a port thereof on the side of a photo diode  53 . The laser diode (upward signal transmitting means)  52  converts an upward electrical signal into an upward optical signal having a wavelength of 1260 nm to 1360 nm and transmits the optical signal, and the photo diode (information receiving means)  53  receives a video optical signal and then converts the received video optical signal into an electrical signal. Reference numeral  54  denotes a video signal receiver that receives a video signal which is the electrical signal from the photo diode  53 . One WDM  51  is provided for every customer who has a video signal receiver, as shown in FIG. 1.  
         [0037]    In operation, when the laser diode  41  of the station apparatus  31  emits a downward optical signal, the downward optical signal travels through the optical fiber  42  and then reaches the star coupler  43 . The star coupler  43  distributes the downward optical signal to only a number of pieces of customer premises equipment  32 . As a result, the downward optical signal is received and is converted into an electrical signal by the photo diode  44  of each customer premises equipment  32 .  
         [0038]    When the laser diode  52  of each customer premises equipment  32  emits an upward optical signal, the upward optical signal travels through the optical fiber  49  and then reaches the star coupler  50  after it passes through the WDM  51  if the WDM  51  is provided for each customer premises equipment  32 . The star coupler  50  then multiplexes a plurality of upward optical signals, the number of which corresponds to the number of pieces of customer premises equipment  32 . As a result, the plurality of upward optical signals multiplexed are received and are converted into an electrical signal by the photo diode  45  of the station apparatus  31  after it passes through the WDM  48  included in the station apparatus  31 .  
         [0039]    When the laser diode  47  of the station apparatus  31  emits a video optical signal, the video optical signal travels through the optical fiber  49  and then reaches the star coupler  50  after it passes through the WDM  48  included in the station apparatus. The star coupler  50  distributes the video optical signal to only the number of pieces of customer premises equipment  32 . As a result, the video optical signal is received and is converted into an electrical signal by the photo diode  53  provided for every customer who has a video signal receiver after it passes through the WDM  51 .  
         [0040]    [0040]FIG. 2 is a graph showing insertion loss characteristics of the WDM  51 . In the figure, A shows the insertion loss characteristics for a channel from a port on the side of corresponding customer premises equipment  32  to a port on the side of the star coupler  50 , and B shows the insertion loss characteristics for a channel from the port on the side of the star coupler  50  to a port on the side of the corresponding video signal receiver  54 . It is apparent from FIG. 2 that those insertion loss characteristics change gently over a wide range of wavelengths from 1360 nm and 1550 nm (the width of the wavelength range=190 nm). Therefore, it is easy to design the WDM  51 , and hence the cost of the WDM  51  can be reduced.  
         [0041]    As can be seen from the above description, in accordance with the first embodiment of the present invention, since the transmission path for downward signals and the transmission path for upward signals are so arranged that they are independent of each other, only the provision of the WDMs  48  and  51  as wavelength division multiplexer means makes it possible to transmit both upward and downward optical signals and a video optical signal. Accordingly, the present embodiment provides advantages of being able to reduce the cost of building the system, and to lengthen the transmission range of optical signals.  
         [0042]    Furthermore, in accordance with the first embodiment of the present invention, since the laser diode  47  is so constructed as to emit a video optical signal having a wavelength longer than that of an upward optical signal emitted out of the laser diode  52  of each customer premises equipment  32 , the laser diode  52  does not malfunction because of the video optical signal sent from the laser diode  47  even if no WDM is placed between the star coupler  50  and the laser diode  52  of each customer premises equipment  32 .  
         [0043]    Embodiment 2.  
         [0044]    [0044]FIG. 3 is a block diagram showing the structure of a signal transmission system according to a second embodiment of the present invention. In the figure, the same reference numerals as shown in FIG. 1 denote the same components as those of the first embodiment or like components, and therefore the explanation of those components will be omitted hereafter. Reference numeral  61  denotes a WDM (first WDM means) that allows only downward optical signals to pass therethrough for a channel from a port thereof on the side of a laser diode  41  to a port thereof on the side of a star coupler  43 , and that allows only video optical signals to pass therethrough for a channel from a port thereof on the side of a laser diode  47  to the port thereof on the side of the star coupler  43 , reference numeral  62  another WDM (second WDM means) that allows only downward optical signals to pass therethrough for a channel from a port thereof on the side of the star coupler  43  to a port thereof on the side of a photo diode  44 , and that allows only video optical signals to pass therethrough for a channel from the port thereof on the side of the start coupler  43  to a port thereof on the side of a photo diode  53 , and reference numeral  63  denotes a termination. One WDM  62  is provided for each customer premises equipment  32 .  
         [0045]    In the first embodiment, an upward optical signal and a video optical signal having different wavelengths are multiplexed and transmitted, as mentioned above. In contrast, in accordance with the second embodiment of the present invention, a downward optical signal and a video optical signal having different wavelengths are multiplexed and transmitted by using the WDMs  61  and  62 , as shown in FIG. 3. The second embodiment provides the same advantages as offered by the above-mentioned first embodiment.  
         [0046]    Embodiment 3.  
         [0047]    In the first and second embodiments, a video optical signal is applied to the photo diode  53  by using the WDM  51  of FIG. 1 or the WDM  62  of FIG. 3, as mentioned-above. In contrast, in accordance with a third embodiment of the present invention, a video optical signal is applied to a photo diode  53  by using a star coupler  50 , as shown in FIG. 4. As an alternative, a video optical signal is applied to the photo diode  53  by using a star coupler  43 , as shown in FIG. 5. Particularly, the structure of FIG. 4 makes it possible to eliminate the need for the WDM  51  of FIG. 1, and to simplify the system configuration. In FIGS. 4 and 5, the same reference numerals as shown in FIGS. 1 and 3 denote the same components as those of the first or second embodiment or like components, and therefore the explanation of those components will be omitted.  
         [0048]    Embodiment 4.  
         [0049]    [0049]FIG. 6 is a block diagram showing the structure of a signal transmission system according to a fourth embodiment of the present invention. In the figure, the same reference numerals as shown in FIG. 1 denote the same components as those of the first embodiment or like components, and therefore the explanation of those components will be omitted hereafter. Reference numerals  71  and  72  denote data signal transmitters that generate and transmit data signals, respectively, reference numeral  73  denotes a laser diode that converts the data signal, which is an electrical signal, sent from the data signal transmitter  71  into an optical signal (referred to as data optical signal from here on) having a wavelength of 1539 nm, the optical signal carrying data, and transmits the data optical signal, reference numeral  74  denotes a laser diode that converts the data signal, which is an electrical signal, sent from the data signal transmitter  72  into an optical signal (i.e., data optical signal) having a wavelength of 1565 nm, the optical signal carrying data, and transmits the data optical signal, reference numeral  75  denotes a dense wavelength division multiplexer or DWDM, reference numeral  76  denotes a fiber amplifier, and reference numeral  77  denotes a star coupler. A data signal transmitting means consists of the data signal transmitters  71  and  72 , the laser diodes  73  and  74 , the DWDM  75 , the fiber amplifier  76 , and the star coupler  77 .  
         [0050]    Reference numeral  78  denotes a WDM (first WDM means) that allows only upward optical signals to pass therethrough for a channel from a port thereof on the side of a star coupler  50  to a port thereof on the side of a photo diode  45 , and that allows only data optical signals to pass therethrough for a channel from a port thereof on the side of the start coupler  77  to the port thereof on the side of the star coupler  50 , and reference numeral  79  denotes a WDM (second WDM means) that allows only upward optical signals to pass therethrough for a channel from a port thereof on the side of a laser diode  52  to a port thereof on the side of the start coupler  50 , and that allows only data optical signals to pass therethrough for a channel from the port thereof on the side of the start coupler  50  to a port thereof on the side of a photo diode  80 . The photo diode (information receiving means)  80  receives a data optical signal having a wavelength of 1565 nm and then converts the received data optical signal into an electrical signal. Reference numeral  81  denotes a data signal receiver that receives a data signal which is the electrical signal from the photo diode  80 . One WDM  79  is provided for every customer who has a data signal receiver, as shown in FIG. 6.  
         [0051]    In the first embodiment, a video optical signal and an upward optical signal having different wavelengths are multiplexed and transmitted, as mentioned above. In contrast, in accordance with the fourth embodiment of the present invention, a data optical signal and an upward optical signal different wavelengths are multiplexed and transmitted by using the WDMs  78  and  79 , as shown in FIG. 6. The fourth embodiment provides the same advantages as offered by the above-mentioned first embodiment.  
         [0052]    Embodiment 5.  
         [0053]    [0053]FIG. 7 is a block diagram showing the structure of a signal transmission system according to a fifth embodiment of the present invention. In the figure, the same reference numerals as shown in FIG. 6 denote the same components as those of the fourth embodiment or like components, and therefore the explanation of those components will be omitted hereafter. Reference numeral  91  a WDM (first WDM means) that allows only downward optical signals to pass therethrough for a channel from a port thereof on the side of a laser diode  41  to a port thereof on the side of a star coupler  43 , and that allows only data optical signals that carry data to pass therethrough for a channel from a port thereof on the side of a star coupler  77  to the port thereof on the side of the star coupler  43 , and reference numeral  92  another WDM (second WDM means) that allows only downward optical signals to pass therethrough for a channel from a port thereof on the side of the star coupler  43  to a port thereof on the side of a photo diode  44 , and that allows only data optical signals to pass therethrough for a channel from the port thereof on the side of the start coupler  43  to a port thereof on the side of a photo diode  80 . One WDM  92  is provided for each customer premises equipment  32 .  
         [0054]    In the fourth embodiment, an upward optical signal and a data optical signal having different wavelengths are multiplexed and transmitted, as mentioned above. In contrast, in accordance with the fifth embodiment of the present invention, a downward optical signal and a data optical signal having different wavelengths are multiplexed and transmitted by using the WDMs  91  and  92 , as shown in FIG. 7. The fifth embodiment provides the same advantages as offered by the above-mentioned fourth embodiment.  
         [0055]    Embodiment 6.  
         [0056]    In the fourth and fifth embodiments, a data optical signal is applied to the photo diode  80  by using the WDM  79  of FIG. 6 or the WDM  92  of FIG. 7, as mentioned above. In contrast, in accordance with a sixth embodiment of the present invention, a data optical signal is applied to a photo diode  80  by using a star coupler  50 , as shown in FIG. 8. As an alternative, a data optical signal is applied to the photo diode  80  by using a star coupler  43 , as shown in FIG. 9. Particularly, the structure of FIG. 8 makes it possible to eliminate the need for the WDM  79  of FIG. 6, and to simplify the system configuration. In FIGS. 8 and 9, the same reference numerals as shown in FIGS. 6 and 7 denote the same components as those of the fourth or fifth embodiment or like components, and therefore the explanation of those components will be omitted.  
         [0057]    Many widely different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in the specification, except as defined in the appended claims.