Abstract:
An improvement is added to an optical time domain reflectometer for emitting pulsed light of invisible light to a measured optical fiber, receiving return light of the pulsed light by a light detection section, measuring the measured optical fiber, and emitting visible light for visible inspection of a fault point of the measured optical fiber to the measured optical fiber. The optical time domain reflectometer includes an incidence-emission port for emitting the invisible light and the visible light to the measured optical fiber and an output judgment section for judging that a communication light exists in the measured optical fiber based on the light power of the light detection section receiving light incident through the incidence-emission port in a state in which the pulsed light of the invisible light is not emitted.

Description:
TECHNICAL FIELD 
       [0001]    The present disclosure relates to an optical time domain reflectometer for emitting pulsed light of invisible light to a measured optical fiber, receiving return light of the pulsed light by a light detection section, measuring the measured optical fiber, and emitting visible light for visible inspection of a fault point of the measured optical fiber to the measured optical fiber. In particular, the present disclosure relates to an optical time domain reflectometer capable of improving the working efficiency of measurement using the return light and measurement using the visible light while suppressing occurrence of a communication failure. 
       RELATED ART 
       [0002]    In an optical communication system for conducting data communications, etc., using an optical signal, it is important to monitor an optical fiber for transmitting an optical signal. An optical time domain reflectometer (OTDR) is used in laying, maintenance, etc., of an optical fiber. 
         [0003]    The OTDR measures a state of a break, a loss, etc., of a measured optical fiber by repeatedly emitting pulsed light from an incidence-emission port (a connector end to which an optical fiber to be measured is connected) to the measured optical fiber and measuring the level and the light reception time of reflected light and backscattered light from the measured optical fiber. 
         [0004]    A function of finding the loss characteristic, a fault point, etc., of the measured optical fiber based on return light from the measured optical fiber (reflected light and backscattered light of the emitted pulsed light) is called OTDR function. The pulsed light in the OTDR function uses invisible light. For example, a light source for emitting light of a wavelength in a 1310 [nm], 1550 [nm] band, etc., and a light source for emitting light of a wavelength in a 1650 [nm] band as a monitor wavelength are used as the current line for domestic optical communications and generally the user measures at any desired wavelength in response to the use. In the description to follow, the invisible light for the OTDR function is OTDR measurement light and an invisible laser light source, an invisible laser element, etc., is an OTDR laser light source, an OTDR laser element. 
         [0005]    Since return light of pulsed light is used for measuring in the OTDR function, it is possible to conduct measurement at a long distance of several [km] to 100 [km] or more as the measurable measured optical fiber distance. The distance refers to the distance from the incidence-emission port of the OTDR. 
         [0006]    On the other hand, at a short distance or a close distance (to about 50 [m]) rather than at a long distance, if the user visually inspects the measured optical fiber directly and finds a fault point, conducts maintenance, etc., without using the OTDR function, the working efficiency may be higher. 
         [0007]    That is, when light is made incident on the measured optical fiber, if a fault point of a flaw, a broken part, etc., exists in the measured optical fiber, light leaks from the fault point and thus if the light is visible light, the user can easily recognize leakage light and can also find the fault point. 
         [0008]    Then, as an optional function of the OTDR, a measurement function including a visible light source for emitting visible light to a measured optical fiber from the OTDR is also available. In this case, return light need not be measured with the OTDR. That is, the visible light emitted to the measured optical fiber from the visible light source of the OTDR leaks from a fault point and thus the user can easily find the fault point by visibly inspecting the leakage light. The function using the visible light source and requiring no return light measurement is called visible light source function. 
         [0009]      FIG. 4  is a drawing to show the configuration of an OTDR in a related art. (For example, refer to patent document 1.) In FIG.  4 , a measured optical fiber F 1  is an optical fiber to be measured where a break, a loss characteristic, etc., is to be measured. An OTDR  100  has an incidence-emission port P 1  and an emission port P 2  to which the measured optical fiber F 1  is connected. 
         [0010]    The OTDR  100  emits pulsed light (invisible light) from the port P 1  to the measured optical fiber F 1  for the OTDR function and return light of the pulsed light (reflected light or backscattered light) is input to the OTDR  100  through the port P 1 . The OTDR  100  also emits intensity modulation light (visible light) from the port P 2  to the measured optical fiber F 1  for the visible light source function. 
         [0011]    The OTDR  100  further has an OTDR laser drive section  10 , an OTDR laser element  11 , an optical directional coupler  12 , a light detection section  13 , a signal processing section  14 , a display section  15 , a visible laser drive section  16 , a visible laser element  17 , a control section  18 , and a setting section  19 . 
         [0012]    The OTDR laser drive section  10  drives the OTDR laser element  11  in accordance with a command from the control section  18 . The OTDR laser element  11  emits pulsed light driven by the OTDR laser drive section  10 . The optical directional coupler  12  emits the pulsed light from the OTDR laser element  11  through the port P 1  to the measured optical fiber F 1  and emits return light from the measured optical fiber F 1  through the port P 1  to the light detection section  13 . 
         [0013]    The light detection section  13  receives the light from the optical directional coupler  12  and converts the light into an electric signal. The electric signal is input from the light detection section  13  to the signal processing section  14 . The display section  15  displays the processing result of the signal processing section  14 . 
         [0014]    The visible laser drive section  16  drives the visible laser element  17  in accordance with a command from the control section  18 . The visible laser element  17  emits intensity modulation light driven by the visible laser drive section  16  through the port P 2  to the measured optical fiber F 1 . 
         [0015]    The control section  18  performs timing control of emission of the pulsed light of the OTDR laser element  11  through the OTDR laser drive section  10  and timing control of emission of the visible laser element  17  through the visible laser drive section  16  and also causes the signal processing section  14  to perform processing. The setting section  19  sets the control section  18  as to which of the OTDR function and the visible light source function is to be used for measurement. 
         [0016]    The operation of the apparatus is as follows; 
         [0017]    To begin with, the operation of the OTDR function will be discussed. The user connects the measured optical fiber F 1  to the incidence-emission port P 1  of the OTDR  100 . The setting section  19  sets the control section  18  to the OTDR function as the operation mode based on operation of the user. 
         [0018]    The control section  18  issues a command to the OTDR laser drive section  10  and also issues a measurement start command to the signal processing section  14 . The signals output by the control section  18  to the OTDR laser drive section  10  and the signal processing section  14  are called timing signals. 
         [0019]    Accordingly, the OTDR laser drive section  10  causes the OTDR laser element  11  to emit pulsed light at a predetermined timing in accordance with the timing signal from the control section  18 . The pulsed light emitted from the OTDR laser element  11  is incident on the measured optical fiber F 1  through the optical directional coupler  12  and the incidence-emission port P 1 . In the measured optical fiber F 1 , Rayleigh scattering occurs and a part thereof proceeds in an opposite direction to the traveling direction of the pulsed light and returns to the OTDR  100  as backscattered light. Fresnel reflection light occurring at a connection point, a fault point, etc., of the measured optical fiber F 1  also returns to the OTDR  100 . 
         [0020]    The return light from the measured optical fiber F 1  is incident on the light detection section  13  through the port P 1  and the optical directional coupler  12 . Further, an OE conversion circuit (not shown) of the light detection section  13  converts the incident light into an electric signal (photoelectric current) responsive to the light power of the incident light. An IV conversion circuit (not shown) of the light detection section  13  converts the photoelectric current into a voltage and an amplification circuit (not shown) of the light detection section  13  amplifies the provided voltage too any desired level. 
         [0021]    An AD conversion circuit (not shown) of the signal processing section  14  converts an analog signal into a digital signal with the timing signal of the control section  18  as the time base. Further, the signal processing section  14  finds the time between the instant at which the OTDR laser element  11  is caused to emit the pulsed light and the instant at which the light detection section  13  receives the return light based on the input timing of the timing signal and the digital signal provided by the AD conversion circuit, measures the distance of the measured optical fiber F 1 , and the optical signal level of the return light, computes a loss characteristic and a fault point, and displays the measurement result, the loss characteristic, the fault point, etc., on the display section  15  with the distance on the horizontal axis and the optical signal level of the return light on the vertical axis. 
         [0022]    Since the signal level or the return light is very feeble, the pulsed light is repeatedly output to the measured optical fiber F 1  and the signal processing section  14  averages the measurement values, thereby reducing noise. 
         [0023]    Subsequently, the operation of the visible light source function will be discussed. The user removes the measured optical fiber F 1  from the port P 1  and connects the measured optical fiber F 1  to the emission port P 2  of the OTDR  100 . The setting section  19  sets the control section  18  to the visible light source function as the operation mode based on operation of the user. 
         [0024]    The control section  18  issues a command to the visible laser drive section  16 , which then outputs a drive signal of a modulation frequency as commanded to the visible laser element  17 . Accordingly, the visible laser element  17  emits modulation light intensity-modulated at a frequency of 2 [Hz], for example, to the measured optical fiber F 1  through the port P 2 . Visible light leaks from a fault point or the measured optical fiber F 1  and the fault point blinks. 
         [0025]    [Patent document 1] Japanese Patent Laid-Open No. 2001-066221 
         [0026]    Thus, to measure the measured optical fiber F 1  using the OTDR function, the measured optical fiber F 1  is connected to the incidence-emission port P 1 ; to measure the measured optical fiber F 1  using the visible light source function, the measured optical fiber F 1  is connected to the emission port P 2 . 
         [0027]    However, to switch the mode between the OTDR function and the visible light source function, it is always necessary to switch connection of the measured optical fiber F 1  (the incidence-emission port P 1  or the emission port P 2 ) and there is a problem of worsening the working efficiency. 
         [0028]    In maintenance, etc., of the measured optical fiber F 1 , the measured optical fiber F 1  may be measured in a state in which the communication light for actual optical communications is transmitted to the measured optical fiber F 1 . In measurement using the visible light source function, measurement of light from the measured optical fiber F 1  is not conducted and thus existence of the communication light of the measured optical fiber F 1  cannot be checked. Thus, there is a problem of a fear of occurrence of a serious accident causing a communication failure to occur in the optical communication system as visible light is emitted to the measured optical fiber F 1  although the communication light is transmitted. 
         [0029]    Exemplary embodiments of the present invention provide an optical time domain reflectometer capable of improving the working efficiency of measurement using return light and measurement using visible light while suppressing occurrence of a communication failure. 
         [0030]    According to a first invention, there is provided an optical time domain reflectometer for emitting pulsed light of invisible light to a measured optical fiber, receiving return light of the pulsed light by a light detection section, measuring the measured optical fiber, and emitting visible light for visible inspection of a fault point of the measured optical fiber to the measured optical fiber, the optical time domain reflectometer including; 
         [0031]    an incidence-emission port for emitting the invisible light and the visible light to the measured optical fiber; and 
         [0032]    an output judgment section for judging that a communication light exists in the measured optical fiber based on the light power of the light detection section receiving light incident through the incidence-emission port in a state in which the pulsed light of the invisible light is not emitted. 
         [0033]    A second invention is characterized by the fact that in the first invention, the optical time domain reflectometer further includes: 
         [0034]    a visible laser element for emitting the visible light; 
         [0035]    an invisible laser element for emitting the invisible light; and 
         [0036]    an optical directional coupler for emitting the visible light from the visible laser element and the invisible light from the invisible laser element through the incidence-emission port to the measured optical fiber and emitting light from the measured optical fiber through the incidence-emission port to the light detection section. 
         [0037]    A third invention is characterized by the fact that in the first or second invention, 
         [0038]    the light detection section includes a photodiode having almost no sensitivity to the wavelength of the visible light and having sensitivity to the wavelength of the invisible light. 
         [0039]    The invention provides the following advantages: 
         [0040]    The invisible light and the visible light are emitted to the measured optical fiber through the identical incidence-emission port. The output judgment section judges that the communication light exists in the measured optical fiber based on the output value of the light power from the light detection section before and while visible light is output or the like, for example. Accordingly, switching connection of the measured optical fiber to the optical time domain reflectometer is eliminated and the working efficiency is improved. In addition, if the communication light exists, output of visible light affecting optical communications of the measured optical fiber is suppressed and a communication failure of the optical communication system can be prevented. 
         [0041]    Other features and advantages may be apparent from the following detailed description, the accompanying drawings and the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]    In the accompanying drawings: 
           [0043]      FIG. 1  is a block diagram to show one embodiment of the invention; 
           [0044]      FIG. 2  is a block diagram of an optical directional coupler of the apparatus shown in  FIG. 1 ; 
           [0045]      FIG. 3  is a block diagram of another embodiment of the optical directional coupler of the apparatus shown in  FIG. 1 ; and 
           [0046]      FIG. 4  is a drawing to show the configuration of an optical time domain reflectometer in a related art. 
       
    
    
     DETAILED DESCRIPTION 
       [0047]    Referring now to the accompanying drawings, there is shown an embodiment of the invention. 
         [0048]      FIG. 1  is a block diagram to show one embodiment of the invention. Components identical with those previously described with reference to  FIG. 4  are denoted by the same reference numerals in  FIG. 1  and will not be discussed again. In  FIG. 1 , an optical directional coupler  20  is provided in place of the optical directional coupler  12  and a control section  21  is provided in place of the control section  18 . An output judgment section  22  and a comparison value storage section  23  are newly provided and the emission port P 2  is removed. 
         [0049]    The optical directional coupler  20  emits OTDR measurement light from an OTDR laser element  11  and visible light from a visible laser element  17  through an incidence-emission port P 1  to a measured optical fiber F 1 . The optical directional coupler  20  also emits light incident from the measured optical fiber F 1  through the incidence-emission port P 1  (return light, communication light, etc.,) to a light detection section  13 . That is, both the OTDR measurement light from the OTDR laser element  11  and the visible light from the visible laser element  17  are emitted from the identical incidence-emission port P 1  to the measured optical fiber F 1 . 
         [0050]    The control section  21  issues a command to an OTDR laser drive section  10  or a visible laser drive section  16  in accordance with setting from a setting section  19 . The control section  21  also issues a command of measurement start, etc., to a signal processing section  14 . 
         [0051]    The output judgment section  22  reads a comparison value in the comparison value storage section  23 , inputs data concerning light power received by the light detection section  13  from the signal processing section  14 , and outputs the determination result to the control section  21 . The comparison value storage section  23  stores a comparison value to judge the presence or absence of the communication light. As the comparison value, an appropriate value considering the specifications, etc., of an optical communication system is previously stored in the comparison value storage section  23 . 
         [0052]    As the OTDR laser element  11 , it is advisable to use an element for emitting light of a wavelength in a 1310 [nm], 1550 [nm] band, etc., as the current line for domestic optical communications and an element for emitting light of a wavelength in a 1650 [nm] band as a monitor wavelength in response to the use. As the visible laser element  17 , it is advisable to select an element for emitting light in a wavelength of a color to easily find a fault point of the measured optical fiber F 1  by visual inspection, for example, light of a wavelength of 630 [nm]. 
         [0053]    The operation of the apparatus is as follows: 
         [0054]    To begin with, the operation of the OTDR function will be discussed. The user connects the measured optical fiber F 1  to the incidence-emission port P 1  of an OTDR  100 . The setting section  19  sets the control section  21  to the OTDR function as the operation mode based on operation of the user. 
         [0055]    The control section  21  issues a command to the OTDR laser drive section  10  and also issues a measurement start command to the signal processing section  14 . The signals output by the control section  21  to the OTDR laser drive section  10  and the signal processing section  14  are called timing signals. 
         [0056]    Accordingly, the OTDR laser drive section  10  causes the OTDR laser element  11  to emit pulsed light at a predetermined timing in accordance with the timing signal from the control section  21 . The pulsed light emitted from the OTDR laser element  11  is incident on the measured optical fiber F 1  through the optical directional coupler  20  and the incidence-emission port P 1 . In the measured optical fiber F 1 , Rayleigh scattering occurs and a part thereof proceeds in an opposite direction to the traveling direction of the pulsed light and returns to the OTDR  100  as backscattered light. Fresnel reflection light occurring at a connection point, a fault point, etc., of the measured optical fiber F 1  also returns to the OTDR  100 . 
         [0057]    The return light from the measured optical fiber F 1  is incident on the light detection section  13  through the port P 1  and the optical directional coupler  20 . Further, an OE conversion circuit (not shown) of the light detection section  13  converts the incident light into an electric signal (photoelectric current) responsive to the light power of the incident light. An IV conversion circuit (not shown) of the light detection section  13  converts the photoelectric current into a voltage and an amplification circuit (not shown) of the light detection section  13  amplifies the provided voltage too any desired level. 
         [0058]    An AD conversion circuit (not shown) of the signal processing section  14  converts an analog signal into a digital signal with the timing signal of the control section  21  as the time base. Further, the signal processing section  14  finds the time between the instant at which the OTDR laser element  11  is caused to emit the pulsed light and the instant at which the light detection section  13  receives the return light based on the input timing of the timing signal and the digital signal provided by the AD conversion circuit, measures the distance of the measured optical fiber F 1  and the optical signal level of the return light, computes a loss characteristic and a fault point, and displays the measurement result, the loss characteristic, the fault point, etc., on a display section  15  with the distance on the horizontal axis and the optical signal level or the return light on the vertical axis. 
         [0059]    Since the signal level of the return light is very feeble, the pulsed light is repeatedly output to the measured optical fiber F 1  and the signal processing section  14  averages the measurement values, thereby reducing noise. 
         [0060]    Subsequently, the operation of the visible light source function will be discussed. The user connects the measured optical fiber F 1  to the incidence-emission port P 1  as with the OTDR function. Of course, if measurement is to be conducted using the visible light source function successively from the OTDR function, work of connection switching, etc., of the measured optical fiber F 1  does not occur. The setting section  19  sets the control section  21  to the visible light source function as the operation mode based on operation of the user. 
         [0061]    The control section  21  sends a measurement start command to the signal processing section  14  before visible light is emitted from the visible laser element  17 . Accordingly, the AD conversion circuit (not shown) of the signal processing section  14  converts an analog signal of voltage from the light detection section  13  into a digital signal. Of course, the light received at the light detection section  13  is only the communication light because visible light and OTDR measurement light are not emitted from the OTDR  100  to the measured optical fiber F 1 . The signal processing section  14  finds light power received at the light detection section  13  from the digital signal and outputs the light power to the output judgment section  22 . 
         [0062]    Further, the output judgment section  22  reads the comparison value (for example, −40 [dBm]) from the comparison value storage section  23  and makes a comparison between the output value of the light power from the signal processing section  14  and the comparison value. If the output value of the light power is larger than the comparison value, the output judgment section  22  judges that the communication light exists in the measured optical fiber F 1 , and outputs the determination result to the control section  21 . Based on the determination result, the control section  21  displays a warning on the display section  15  through the signal processing section  14  and prohibits the visible laser drive section  16  to drive the visible laser element  17 . 
         [0063]    On the other hand, if the output value of the light power is smaller than the comparison value, the output judgment section  22  judges that the communication light does not exist in the measured optical fiber F 1 , and outputs the determination result to the control section  21 . Based on the determination result, the control section  21  issues a command to the visible laser drive section  16 . The visible laser drive section  16  outputs a drive signal of a modulation frequency as commanded to the visible laser element  17 . Accordingly, the visible laser element  17  emits modulation light intensity-modulated at a frequency of 2 [Hz], for example, to the measured optical fiber F 1  through the optical directional coupler  20  and the incidence-emission port P 1 . Visible light leaks from a fault point of the measured optical fiber F 1  and the fault point blinks. Thus, the user can easily acknowledge the fault point of the measured optical fiber F 1  by visual inspection. 
         [0064]    Further, if the visible laser element  17  is emitting visible light, the control section  21  sends a measurement start command to the signal processing section  14  in a predetermined period. Accordingly, the AD conversion circuit (not shown) of the signal processing section  14  converts an analog signal of voltage from the light detection section  13  into a digital signal. The signal processing section  14  finds light power received at the light detection section  13  from the digital signal and outputs the light power to the output judgment section  22 . Further, the output judgment section  22  makes a comparison between the output value of the light power received during visible light output and the comparison value. If the output value of the light power is larger than the comparison value, the output judgment section  22  judges that the communication light exists in the measured optical fiber F 1 , and outputs the determination result to the control section  21 . Based on the determination result, the control section  21  displays a warning on the display section  15  through the signal processing section  14  and causes the visible laser drive section  16  to immediately stop driving the visible laser element  17 . 
         [0065]    If the visible laser element  17  is outputting visible light, the light detection section  13  receives return light of the visible light from the measured optical fiber F 1 . Then, it is advisable to use an element made of InGaAs, for example, as a light reception element (photodiode) of a kind of the OE conversion circuit of the light detection section  13 . That is, the photodiode made of InGaAs has almost no light reception sensitivity to the wavelength of the visible light of the visible laser element  17  and has sensitivity to OTDR measurement light of the OTDR laser element and therefore the light received at the light detection section  13  becomes the communication light only. The expression “almost no sensitivity” is used to mean that light is scarcely converted into photocurrent through the light reception element; for example, the sensitivity difference between the wavelength of the visible light and the wavelength of OTDR measurement light is 20 [dB] or more. 
         [0066]    A wavelength filter for allowing only invisible light to pass through may be provided between the optical directional coupler  20  and the light detection section  13 . 
         [0067]    Thus, the optical directional coupler  20  emits the OTDR measurement light from the OTDR laser element  11  and the visible light from the visible laser element  17  to the measured optical fiber F 1  through the identical incidence-emission port P 1 . Accordingly, the need for switching connection of the measured optical fiber F 1  to the port P 1  or P 2  between the OTDR function and the visible light source function as shown in  FIG. 4  is eliminated. Therefore, the working efficiency can be improved. 
         [0068]    In a state in which the pulsed light of OTDR measurement light is not emitted, namely, in the operation mode of the visible light source function, the output judgment section  22  judges that the communication light exists in the measured optical fiber F 1  based on the output value of the light power from the light detection section  13 , the signal processing section  14  before and while the visible laser element  17  outputs visible light. Based on the determination result, the control section  21  causes the visible laser drive section  16  to stop emission of the visible light of the visible laser element  17 . Accordingly, a communication failure of the optical communication system can be prevented. 
         [0069]    Subsequently,  FIG. 2  is a block diagram to show an embodiment of the optical directional coupler  20 . Components identical with those previously described with reference to  FIG. 1  are denoted by the same reference numerals in  FIG. 2  and will not be discussed again. In  FIG. 2 , an optical coupler  31  is an optical multiplexing section for multiplexing light of different wavelengths and is two inputs and two outputs. An optical coupler  32  is a coupling section for emitting the light provided by the optical coupler  31  of the optical multiplexing section to the measured optical fiber F 1  and transmitting light from the measured optical fiber F 1  to the light detection section  13  and is two inputs and two outputs. 
         [0070]    The optical coupler  31  is effective for maintaining the performance of the OTDR function by setting OTDR measurement light:visible light≦about 9:1 considering the wavelength difference between the laser elements  11  and  17  (for example, 1550 [nm] and 630 [nm]). 
         [0071]    The optical directional coupler will be discussed in detail with  FIG. 2 . 
         [0072]    Pulsed light of the OTDR laser element  11  is incident on one input end  31   a  of the optical coupler  31  and visible light of the visible laser element  17  is incident on another input end  31   b.  The light from the laser element  11  and the light from the laser element  17  are multiplexed and the resultant light is incident on one input end  32   a  of the optical coupler  32  from one output end  31   c  of the optical coupler  31 . Further, the light incident on the optical coupler  32  is emitted from one output end  32   c  of the optical coupler  32  through the incidence-emission port P 1  to the measured optical fiber F 1 . 
         [0073]    Return light, the communication light, etc., from the measured optical fiber F 1  is incident from the output end  32   c  of the optical coupler  32  through the incidence-emission port P 1  and is emitted from another input terminal  32   b  and is received at a light reception element (not shown) of the light detection section  13 . 
         [0074]    Another output end  31   d  of the optical coupler  31  and another output end  32   d  of the optical coupler  32  are treated so as to become non-reflected ends. One output end  32   c  of the optical coupler  32  and the measured optical fiber F 1  are connected by the incidence-emission port (optical fiber connector) P 1 . A lens for gathering the light from the laser element  11  and coupling at the input end  31   a  of the optical coupler  31 , a lens for gathering the light from the laser element  17  and coupling at the input end  31   b  of the optical coupler  31 , a lens for gathering the light from the input end  32   b  of the optical coupler  32  and coupling at the light reception element, and the like are not shown in the figure. 
         [0075]      FIG. 3  is a block diagram to show another embodiment of the optical directional coupler  20 . The optical couplers are used in  FIG. 2 ; space is used in  FIG. 3  in place of the optical couplers. Components identical with those previously described with reference to  FIG. 1  are denoted by the same reference numerals in  FIG. 3  and will not be discussed again. 
         [0076]    In  FIG. 3 , a lens  41  converts the pulsed light (1550 [nm] band) from the OTDR laser element  11  into parallel rays. A lens  42  converts the pulsed light (630 [nm] band) from the visible laser element  17  into parallel rays. An optical multiplexing-demultiplexing filter (optical multiplexing section)  43  multiplexes OTDR measurement light and visible light from the lenses  41  and  42 . A lens  44  gathers light from the optical multiplexing-demultiplexing filter  43 . A first optical fiber  45  has one end provided at the light gathering position of the lens  44  and an opposite end connected to the measured optical fiber F 1  through the incidence-emission port (optical fiber connector) P 1 . A beam splitter (BS)  46  is a coupling section and is provided between the optical multiplexing-demultiplexing filter  43  and the lens  44 . A lens  47  gathers light split through the BS  46 . A light reception element  13   a  of the light detection section  13  (for example, photodiode made of InGaAs) is provided at the light gathering position of the lens  47  and receives light from the measured optical fiber F 1  (return light, communication light, etc.,). 
         [0077]    The operation of the apparatus is as follows: 
         [0078]    The pulsed light from the OTDR laser element  11  becomes parallel rays through the lens  41 . Intensity modulation light of visible light from the visible laser element  17  becomes parallel rays through the lens  42 . The optical multiplexing-demultiplexing filter  43  allows the light from the OTDR laser element  11  to pass through and reflects the light from the visible laser element  17  and emits the light to the BS  46 . Further, the passing-through light (light from the OTDR laser element  11 ) and the reflected light (light from the visible laser element  17 ) from the optical multiplexing-demultiplexing filter  43  pass through the BS  46  and are gathered through the lens  44  and the gathered light is incident on one end of the first optical fiber  45 . 
         [0079]    The light incident on one end of the first optical fiber  45  is incident on the measured optical fiber F 1  through the opposite end of the first optical fiber  45  and the incidence-emission port P 1 . 
         [0080]    On the other hand, the return light, the communication light, etc., from the measured optical fiber F 1  is incident on the incidence-emission port P 1  and the opposite end of the first optical fiber  45 . Further, the return light, the communication light, etc., emitted from one end of the first optical fiber  45  becomes parallel rays through the lens  44  and the parallel rays are split by the BS  46 . Of the split light rays, the light reflected in the direction of the lens  47  is gathered through the lens  47  and is received at the light reception element  13   a.  Further, the light reception element  13   a  converts the incident light into an electric signal (photocurrent) responsive to the light power of the light and outputs the electric signal to the IV conversion circuit at the following stage. 
         [0081]    The invention is not limited to the apparatus described above and may be as follows: 
         [0082]    In the apparatus shown in  FIG. 1 , the visible laser element  17  outputs visible light intensity-modulated at a frequency of 2 [Hz], but the modulation frequency may be any value and visible light of constant light power on the time basis without undergoing intensity modulation may be output. 
         [0083]    In the apparatus shown in  FIG. 1 , in a state in which OTDR measurement light is not emitted, namely, in the operation mode of the visible light source function, it the output judgment section  22  judges that the communication light exists in the measured optical fiber F 1  before and while visible light is output, the control section  21  causes the visible laser drive section  16  to stop driving the visible laser element  17  so as not to emit the visible light. However, if it is judged that the communication light exists in the measured optical fiber F 1 , visible light may be emitted to the measured optical fiber F 1 . That is, the control section  21  causes through the visible laser drive section  16 , the visible laser element  17  to emit visible light at such an optical signal level to suppress occurrence of a communication failure in the optical communication system of the measured optical fiber F 1  (for example, optical signal level of about 10 [mW]). 
         [0084]    Thus, if the communication light exists, the control section  21  controls the visible laser element  17  for causing the visible laser element  17  to emit visible light at such an optical signal level not affecting the optical communication system. Accordingly, if the communication light enters the measured optical fiber F 1  before and while the visible laser element  17  emits light, while a communication failure is prevented, the user can find a fault point of the measured optical fiber F 1  by visual inspection and the working efficiency can be improved. 
         [0085]    In the apparatus shown in  FIG. 2 , the ratio of the optical coupler  31  as the optical multiplexing section is set to OTDR measurement light:visible light=about 9:1. However, to suppress the cost, the percentage of the visible light may be increased. 
         [0086]    In the apparatus shown in  FIG. 2 , an optical circulator may be used in place of the optical coupler  32  as the coupling section. 
         [0087]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.