Abstract:
optical fiber characteristic measuring device comprising a coherent light supply device, a light pulse generating device, a wave mixing device, a opto-electrical converting device, and a processing device, is provided such that parts have the frequency characteristic for corresponding to low frequency component for an opto-electrical converting device and a processing section so as to reduce the cost of the circuits.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to an optical fiber characteristic measuring device which produces a pulse of light incident to an optical fiber as an object to be measured and measures the characteristic of the optical fiber according to the returning light emitted from the optical fiber.  
           [0003]    2. Description of Related Art  
           [0004]    [0004]FIG. 5 is a block diagram showing a structure of an example of a conventional optical fiber characteristic measuring device. A light of constant frequency of ν  0  which is emitted from the light source  1  is incident on an incident port  4   i  of a first optical directional coupler  4 . The first optical directional coupler  4  has an incident port  4   i  and two emitting ports  4   t   1  and  4   t   2 . The first optical directional coupler  4  separates the light incident on the incident port  4   i  into 2 directions and emits the light from the two emitting ports  4   t   1  and  4   t   2 .  
           [0005]    The light which is emitted from the emitting port  4   t   1  of the first optical directional coupler  4  is incident on the light pulse generating device  5 . The light pulse generating device  5  is specifically an electro-optical switch. The light pulse generating device  5  extracts the light pulse from the incident light by turning the switch on and off and emits the extracted light pulse.  
           [0006]    The light pulse emitted from the light pulse generating device  5  is incident on a light amplifier  6 . The light amplifier  6  amplifies the incident light pulse to a predetermined level and emits the amplified light pulse. The light pulse emitted from the light amplifier  6  is incident on an incident port  7   i  of an optical switch  7 . The optical switch  7  has three ports such as an incident port  7   i , an emitting/incident port  7   ti , and an emitting port  7   t , and emits the light pulse which is incident on the incident port  7   i  from the emitting/incident port  7   ti . The optical switch  7  also emits the returning light which is incident on the emitting/incident port  7   ti  from the emitting port  7   t.    
           [0007]    The emitting/incident port  7   ti  of the optical switch  7  is connected to an end  9   a  of the optical fiber  9  as an object to be measured via an optical connector  8 . Therefore, the light pulse emitted from the emitting/incident port  7   ti  of the optical switch  7  is incident on an end  9   a  of the optical fiber  9  via an optical connector  8 . The returning light which is emitted from the end  9   a  of the optical fiber  9  is incident again on the emitting/incident port  7   ti  of the optical switch  7 , and is further emitted from the emitting port  7   t  of the optical switch  7 .  
           [0008]    The returning light emitted from the emitting port  7   t  of the optical switch  7  is incident to the incident port  10   i   1  of the second light directional coupler  10 . The second light directional coupler  10  has two incident ports such as  10   i   1  and  10   i   2  and two emitting ports such as  10   t   1  and  10   t   2 . To the incident port  10   i   2  of the second light directional coupler  10 , the light (hereinafter called “reference light”) emitted from the emitting port  4   t   2  of the first light directional coupler  4  is incident. Consequently, the second light directional coupler  10  combines the wave of the returning light which is incident from the incident port  10   i   1  and the wave of reference light which is incident from the incident port  10   i   2 . The second light directional coupler  10  further separates the combined light into two directions, and emits the lights from the two emitting ports  10   t   1  and  10   t   2 .  
           [0009]    Both of the combined lights emitted from the two emitting ports  10   t   1  and  10   t   2  of the second light directional coupler  10  are received by balance receiving photodiode PD 11 . The balance receiving photodiode PD 11  converts the combined lights which is received to an electric signal (beat signal) and outputs the converted electric signal (beat signal). The beat signal which is output by the balance light receiving photodiode PD 11  is input to an amplifier  12 . The amplifier  12  amplifies the input beat signal to a predetermined level and sends the amplified beat signal to a mixer  13 .  
           [0010]    The mixer  13  mixes the beat signal sent from the amplifier  12  and an RF signal generated by a signal generating circuit  14 , and outputs the mixed signal. A control circuit  15  controls the signal generating circuit  14  and determines the frequency νr of the RF signal generated by the signal generating circuit  14 . The frequency νr of the RF signal is set to a value which is close to 10.8 GHz as a shifting amount by the Brillouin scattering.  
           [0011]    A low pass filter  16  inputs the mixed signal which is output by the mixer  13 , removes high frequency component which is included in the mixed signal which is input, passes only low frequency component, and outputs a difference signal which is a low frequency component. The amplifier  17  amplifies the difference signal which is output by the low pass filter  16  to a predetermined level, and outputs the amplified difference signal. The signal process section  18  inputs the difference signal which is output by the amplifier  17 , performs various signal treatment on the inputted difference signa, and determines the characteristic of the optical fiber  9 .  
           [0012]    Next, the operation of the optical fiber characteristic measuring device is explained. The light with the frequency of ν 0  emitted from the light source  1  is sent to the light pulse generating circuit  5  via the light directional coupler  4 . Then, the light pulse generating circuit  5  extracts the light pulse with the frequency of ν 0  from the light which is sent.  
           [0013]    The light pulse emitted from the light pulse generating circuit  5  is incident on the end  9   a  of the optical fiber  9  via an optical amplifier  6 , an optical switch  7 , and an optical connector  8 . When the incident light pulse is transmitted in the optical fiber  9 , Brillouin scattering, Rayleigh scattering, and reflection occur at several points in the optical fiber  9 , then the returning light including the Brillouin scattered light, Rayleigh scattered light, and reflected light return to the end  9   a  from such several points. The returning light is emitted from the end  9   a.    
           [0014]    The returning light emitted from the end  9   a  of the optical fiber  9  and including the Brillouin scattered light is incident again on the emitting/incident port  7   ti  of the optical switch  7  via the optical connector  8 , and is further emitted from the emitting port  7   t . The returning light emitted from the emitting port  7   t  of the optical switch  7  and including the Brillouin scattered light is incident on the incident port  10   i   1  of the second light directional coupler  10 . Additionally, to another incident port  10   i   2  of the second light directional coupler  10 , the reference light emitted from the emitting port  4   t   2  of the first light directional coupler  4  with a frequency of ν 0 .  
           [0015]    The second light directional coupler  10  mixes the wave of the Brillouin scattering light with frequency of ν 0 ±νB and the wave of reference light with frequency of ν 0 . Consequently, resonance occur because the frequencies of these lights are so close that interference is caused. The frequency of the resonance is represented as the difference between the frequency of Brillouin scattering light such as ν 0 ±νB and the frequency of the reference light such as ν 0 . Therefore the frequency of the resonance becomes νB.  
           [0016]    When the mixed light in which the resonance of which frequency is νB occurs is received by the balance receiving photodiode PD 11 , the balance receiving photodiode PD 11  outputs the beat signal having the resonance of which frequency is νB. The beat signal which is output by the balance receiving photodiode PD 11  and has the resonance of which frequency is νB is input to the mixer  13  via the amplifier  12 . An RF signal of which frequency is νr which is generated by the signal generating circuit  14  is input into the mixer  13  together with the beat signal having the resonance of which frequency is νB, and these signals are mixed. Here, the frequency νr of the RF signal which is generated by the signal generating circuit  14  is set quite close to the frequency νB in advance. Then, the beat signal and the RF signal interfere; thus the resonance occurs. The frequency of the resonance is represented by a difference between the frequency νB of the beat signal and the frequency νr of the RF signal such as νB−νr. The frequency νr of the RF signal which is generated by the signal generating circuit  14  is set quite close to the frequency νB of the resonance of the beat signal.  
           [0017]    When the mixed signal in which the resonance of which frequency is νB−νr occurs is input to the low pass filter  16 , the low pass filter  16  cuts the high frequency signal (signal of which the frequency is νB or νr) included in the mixed signal, and outputs the difference signal having only frequency νB−νr of the resonance as a low frequency signal. The signal processing section  18  measures the frequency of the difference signal. Additionally, the signal processing section  18  calculates the frequency νB of the beat signal from the frequency νB−νr of the difference signal which is measured, and calculates the shifting amount νB due to the Brillouin scattering. Furthermore, the signal processing section  18  determines the distortion amount in a predetermined point in the optical fiber  9  from the shifting amount νB which is calculated.  
           [0018]    For a balance receiving photodiode PD 11 , and amplifier  12 , a mixer  13 , and a signal generating circuit  14  of the above described optical fiber characteristic measuring device, components having frequency characteristic so as to correspond to high frequencies such as 10.8 GHz of shifting amount νB due to the Brillouin scattering need be used; thus, the problem is that the cost for such components increases.  
         SUMMARY OF THE INVENTION  
         [0019]    The present invention was made in consideration of the above problem and provides an optical fiber characteristic measuring device which can reduce the cost of the above mentioned circuits and the like.  
           [0020]    The invention according of the first aspect  1  is an optical fiber characteristic measuring device comprising a coherent light supply device which supplies a coherent light with a second frequency which is almost equal to the frequency of coherent light with a first frequency and the frequency of returning light emitted from this optical fiber when the coherent light with first frequency is incident to the optical fiber as an object to be measured, a light pulse generating device which converts the coherent light with first frequency which is supplied by the coherent light supply device to light pulse and emits the light pulse which is converted, a wave mixing device which mixes the wave of returning light emitted from the optical fiber and the wave of the coherent light with second frequency supplied from the coherent light supply device when the light pulse emitted by the light pulse generating device is incident on the optical fiber as an object to be measured and emits the mixed light, a opto-electrical converting device which converts the mixed light emitted from the wave mixing device to an electrical signal and outputs the electrical signal which is converted, a processing device which calculates a shifting amount to the frequency of the returning light emitted from the optical fiber from the first frequency of coherent light which is incident to the optical fiber as an object to be measured according to the electric signal which is output from the opto-electrical converting device and determines characteristic of the optical fiber from the calculated shifting amount.  
           [0021]    The invention of the second aspect is an optical fiber characteristic measuring device according to the first aspect, wherein the coherent light supply device has a driving device which can output more than two kinds of driving current and a light source which can alter the frequency of the coherent light which is emitted corresponding to the driving current which is output by the driving device.  
           [0022]    The invention according to the third aspect is an optical fiber characteristic measuring device according to the second aspect, wherein the light source is a distributed-feedback laser diode.  
           [0023]    The invention according to the fourth aspect is an optical fiber characteristic measuring device according to the second aspect, wherein the returning light emitted from the optical fiber is Brillouin scattered light.  
           [0024]    According to the present invention, parts having the frequency characteristic for corresponding to low frequency component for an opto-electrical converting device (balance receiving photodiode PD 11  in the present embodiment) and a processing section (balance receiving photodiode PD 11 , amplifier  12 , mixer  13 , signal generating circuit  14  in the present embodiment); thus, the cost of opto-electrical converting device and the processing section can be reduced. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0025]    [0025]FIG. 1 is a block diagram showing the structure of an optical fiber characteristic measuring device in an embodiment of the present invention.  
         [0026]    [0026]FIGS. 2A to  2 D are diagrams showing the waveform at various points in an optical fiber characteristic measuring device in an embodiment of the present invention.  
         [0027]    [0027]FIG. 3 is a diagram showing the relationship between the frequency of the light pulse which is incident from the end  9   a  of the optical fiber  9  and the frequency of the Brillouin scattered light included in the returning light emitted from the end  9   a  at a predetermined timing t 1 .  
         [0028]    [0028]FIG. 4 is a diagram showing the relationship between the frequency ν 0  of the reference light and the frequency ν 1 ±νB of the Brillouin scattering light.  
         [0029]    [0029]FIG. 5 is a block diagram showing the structure of a conventional optical fiber characteristic measuring device. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0030]    [0030]FIG. 1 is a block diagram showing the structure of an optical fiber characteristic measuring device in an embodiment of the present invention. The light source  1  is specifically a DFB-LD (distributed-feedback laser diode), and emits a coherent light of which the line width is narrow, such as 1.55 μm bandwidth of wavelength λ 0 , that is, frequency ν 0  is a bandwidth of 193.55 THz). The wavelength (or the frequency) of the light emitted by the light source  1  slightly changes according to the driving current which is supplied to the light source  1 .  
         [0031]    The driving circuit  2  can supply two kinds of driving current to the light source  1 . That is, the driving circuit  2  can supply the driving current i 0  to the light source  1 , and can also supply the driving current i 1  to the light source  1 . These driving current values are, for example, i 0 =70 mA, and i 1 =80 mA. The control circuit  3  controls the driving circuit  2  and determines the driving current which is supplied to the light source  1  by the driving circuit  2 . That is, by the control circuit  3 , the driving current i 0  can be supplied to the light source  1  by the driving circuit  2 , and the driving current i 1  can also be supplied to the light source  1  by the driving circuit  2 .  
         [0032]    By doing this, it becomes possible for light source  1  to emit light of two wavelengths are such as λ 0  and λ 1 , which may also by referred to as ν 0  and ν 1 . That is, when the driving current i 0  is supplied, the light source  1  emits the light having wavelength ν 0 , when the driving current i 1  is supplied, the light source  1  emits the light having wavelength ν 1 . The driving current i 0  and i 1  are determined so that the difference ν 1 −ν 0  of the frequency of two lights emitted by the light source  1  become close value to the shifting amount νB of the frequency due to the below-mentioned Brillouin scattering. For example  1 , the frequency νB is 10.8 GHz, the driving current i 0  and i 1  are determined such that the difference ν 1 −ν 0  becomes 12.0 GHz.  
         [0033]    The light which is emitted by the light source  1  is incident to the incident port  4   i  of the first light directional coupler  4 . The light directional coupler  4  has an incident port  4   i  and two emitting ports  4   t   1  and  4   t   2 , and separates the incident light to the incident port  4   i  into two directions, and emits the lights from two emitting ports  4   t   1  and  4   t   2 . The emitted light from the emitting port  4   t   1  of the first light directional coupler  4  is incident on the light pulse generating circuit  5 . The light pulse generating circuit  5  is specifically an electro-optical switch, extracts the light pulse from the incident light by turning the switch on and off, and emits the extracted light pulses.  
         [0034]    The light pulse which is emitted by the light pulse generating circuit  5  is incident on the light amplifier  6 . The light amplifier  6 , specifically an electro-optical amplifier using an Er(Erbium) doped fiber, amplifies the incident light pulse to a predetermined level, and emits the amplified light pulse.  
         [0035]    The light pulse emitted by the light amplifier  6  is incident on the incident port  7   i  of the optical switch  7 . The optical switch  7  is specifically a light circulator, and has three ports such as an incident port  7   i , an emitting/incident port  7   ti , and an emitting port  7   t . The optical switch  7  emits the incident light pulse to the incident port  7   i  from the emitting/incident port  7   ti , and also emits the returning light which is incident to the emitting/incident port  7   ti  from the emitting port  7   t . The emitting/incident port  7   ti  of the optical switch  7  is connected to an end  9   a  of the optical fiber  9  as an object to be measured.  
         [0036]    Therefore, the light pulse which is emitted from the emitting/incident port  7   ti  of the optical switch  7  is incident to the end  9   a  of the optical fiber via the optical connector  8 . The returning light which is emitted from the end  9   a  of the optical fiber  9  is incident again into the emitting/incident port  7   ti  of the optical switch  7  again via the optical connector  8 , and is further emitted from the emitting port  7   t  of the optical switch  7 .  
         [0037]    The returning light which is emitted from the emitting port  7   t  of the optical switch  7  is incident to the incident port  10   i   1  of the second light directional coupler. The second light directional coupler  10  has two incident ports  10   i   1  and  10   i   2 , two emitting ports  10   t   1  and  10   t   2 . The light (hereinafter called a reference light) emitted from the emitting port  4   t   2  of the first light directional coupler  4  is incident on the incident port  10   i   2  of the second light directional coupler  10 . Then, the second light directional coupler  10  mixes the wave of the returning light which is incident from the incident port  10   i   1  and the wave of the reference light which is incident from the incident port  10   i   2 , separates the mixed lights into two directions, and emits the lights from two emitting ports  10   t   1  and  10   t   2 .  
         [0038]    Both of the mixed lights emitted from two emitting ports  10   t   1  and  10   t   2  of the second light directional coupler  10  are received by the balance receiving photodiode PD 11  (Balance receiving photodiode). The balance receiving photodiode PD 11  converts the received mixed light to an electric signal (beat signal), and outputs the converted electric signal (beat signal). Also, the balance receiving photodiode PD 11  removes noises in the process of balance receiving. The beat signal which is output by the balance receiving photodiode PD 11  is input to the amplifier  12 . The amplifier  12  amplifies the input beat signal to a predetermined level, and sends the amplified beat signal to the mixer  13 .  
         [0039]    The mixer  13  mixes the beat signal which is sent from the amplifier  12  and the RF signal (Radio Frequency signal) generated by the signal generating circuit  14 , and outputs the mixed signal. The control circuit  15  controls the signal generating circuit  14 , and determines the frequency fr of the RF signal generated by the signal generating circuit  14 . The frequency fr of the RF signal is determined according to which frequency light in the returning light should be detected. In the case of detecting Brillouin scattered light, the frequency fr of the RF signal is set to 1.2 GHz, and is set to 100 KHz in the case of detecting Rayleigh scattered light.  
         [0040]    The low pass filter  16  inputs the mixed signal which the mixer  13  outputs, removes the high frequency components included in the input mixed signal, passes only low frequency components, and outputs the difference signal as a low frequency component. The amplifier  17  amplifies the difference signal which is output by the low pass filter  16  to a predetermined level, and outputs the amplified difference signal. The signal processing section  18  inputs the difference signal which is output by the amplifier  17 , performs various signal processing to the input difference signal and determines the characteristic of the optical fiber  9 . More specifically, the signal processing section  18  firstly measures the frequency of the difference signal, next calculates the frequency of the beat signal from the frequency of the difference signal, and furthermore calculates the shifting amount as the difference between the frequency of the light pulse which is incident to the optical fiber  9  and the frequency of the returning light. Then, the characteristic of the optical fiber  9  is determined from the calculated shift amount. In this explanation of FIG. 1, an optical switch  7  may be replaced by an optical coupler. Also, balance receiving photodiode PD 11  may be replaced by a non balance type photodiode. In this case, optic sensitivity of photodiode may decrease.  
         [0041]    Next, the operation of the present embodiment is explained. FIGS. 2A to  2 D are diagrams of waveforms of various sections of the optical fiber characteristic measuring device in the present embodiment. A horizontal axis is time in this diagram. FIG. 2A is a diagram of the waveform of the driving current which the driving circuit  2  supplies to the light source  1 . In the waveform of the driving current, the direct current period in which the current value is i 0  and the pulse current period in which the current value is i 1  are repeated alternately, the period in which the current value is i 1  is 20 μs to 2 ms. The period of direct current is determined according to the length of the optical fiber  9  as an object to be measured. For example, if the length of the optical fiber  9  is 10 km, the period is 200 μs. If the length of the optical fiber  9  is 1 km, the period is 20 μs.  
         [0042]    When the driving circuit  2  supplies the driving current to the light source  1  as shown in FIG. 2A, the light source  1  emits light having the frequency shown in FIG. 2B. That is, the light source  1  emits light having the frequency ν 0  and light having the frequency ν 1  alternately corresponding to the waveform of the driving current.  
         [0043]    The emitted light form the light source  1  is sent to the light pulse generating circuit  5  via the light directional coupler  4 . Then, the light pulse generating circuit  5  emits the light pulse having the waveform as shown in FIG. 2C. That is, the light pulse generating circuit  5  extracts the light pulse having frequency ν 1  from the light which is sent. At this time, the period of emitting the light pulse is several ns to several μs.  
         [0044]    The light pulse which is emitted by the light pulse generating circuit  5  is incident to an end  9   a  of the optical fiber  9  via the light amplifier  6 , the optical switch  7 , and the optical connector  8 . When the light pulse which is incident is transmitted in the optical fiber  9 , the Brillouin scattering, Rayleigh scattering, and reflection occur at several points in the optical fiber  9 , and the returning light including the Brillouin scattered light, the Rayleigh scattered light, and the reflection light returns to the end  9   a  from several points. The returning light is emitted from the end  9   a.    
         [0045]    [0045]FIG. 2D is a diagram showing the waveform of the returning light which is emitted from the end  9   a . The Brillouin scattering or the likes occur at various points in the optical fiber  9 . The returning light returning from the close points to the end  9   a  is emitted from the  9   a  early because it does not take time for its transmission to the end  9   a . In contrast, the returning light returning from the distant points to the end  9   a  is emitted from the  9   a  late because it takes time for its transmission to the end  9   a . Additionally, this applies to the time taken for the transmission of the light pulse which is incident from the end  9   a  of the optical fiber  9  to various points in the optical fiber  9 . That is, the time taken during the incidence of the light pulse into the end  9   a  and the return of the returning light corresponds to the distance between the end  9   a  and the points where the Brillouin scattering and the like occur. Accordingly, if the returning light which is emitted from the end  9   a  is detected at a predetermined timing, the characteristic of the optical fiber  9  at a point having distance corresponding to the timing to can be detected. More specifically, if the frequency of the Brillouin scattered light included in the returning light is detected, the amount of the distortion at a predetermined point in the optical fiber  9  can be detected.  
         [0046]    [0046]FIG. 3 shows a relationship between the frequency of the light pulse which is incident from the end  9   a  of the optical fiber  9  and the frequency of the Brillouin scattered light included in the returning light which is emitted from the end  9   a  at a predetermined timing t 1 . In this drawing, a horizontal axis indicates the frequency of the light, and the vertical axis indicates the intensity of the light. The frequency of the Brillouin scattered light shifts to upward direction of the frequency and to downward direction of the frequency from the frequency ν 1  of the incident light pulse. The shift amount νB alters according to the amount of the distortion at predetermined points in the optical fiber  9 . Therefore, if the shift amount νB is detected, the amount of distortion at predetermined points in the optical fiber  9  can be detected.  
         [0047]    The returning light emitted from the end  9   a  of the optical fiber  9  and including the Brillouin scattering light is incident again to the emitting/incident port  7   ti  of the optical switch  7  via the optical connector  8 , and is further emitted from the emitting port  7   t . The returning light emitted from the emitting port  7   t  of the optical switch  7  and including the Brillouin scattering light is incident to the incident port  10   i   1  of the second light directional coupler  10 . On the other hand, the reference light which is emitted from the emitting port  4   t  of the first light directional coupler  4  is incident to the other incident port  10   i   2  of the second light directional coupler  10 .  
         [0048]    [0048]FIG. 4 is a diagram showing the relationship between the frequency of the reference light and the frequency ν 1 ±νB of the Brillouin scattered light. In this diagram, a horizontal axis indicates frequency of light, and a vertical axis indicates intensity of light. During the period in which the returning light including the Brillouin scattered light is incident on the incident port  10   i   1  of the second light directional coupler  10 , the frequency of the reference light is ν 0 . Accordingly, the second light directional coupler  10  mixes the wave of the Brillouin scattering light having frequency ν 1 ±νB and the wave of the reference light having frequency ν 0 . Then, these lights interfere because the frequencies of these lights are close; thus, the resonance occurs. The frequency of the resonance is a difference between the frequency ν 1 ±νB of the Brillouin scattered light and the frequency of the reference light ν 0 , such as ν 1 ±νB−ν 0 .  
         [0049]    When the mixed light in which the resonance having frequency ν 1 ±νB−ν 0  occurs is received by the balance receiving photodiode PD 11 , the balance receiving photodiode PD  11  outputs the resonance of which the frequency is lower such as beat signal having a resonance frequency of ν 1 +νB− 0 . The resonance of which the frequency is higher, such as a resonance having frequency ν 1 +νB−ν 0  is cut by the frequency characteristic of the balance receiving photodiode PD 11 .  
         [0050]    Here, the frequency ν 1 −νB−ν 0  of the resonance of the beat signal becomes low because the difference of the frequency ν 1 −ν 0  is set to the close value of shift amount νB due to the Brillouin scattering in advance. For example, when νB is 10.8 GHz and ν 1 −ν 0  is 12.0 GHz, the frequency of the resonance becomes ν 1 −νB−ν 0 =12.0 GHz−10.8 GHz=1.2 GHz. Therefore, for a balance receiving photodiode PD 11  which only should be able to output the beat signal and for an amplifier  12  to which the beat signal is input, and a mixer, parts which has frequency characteristic as to correspond to such level of frequency are only sufficient. Therefore, the cost of the balance receiving photodiode PD 11 , the amplifier  12 , an the mixer  13  can be reduced.  
         [0051]    The beat signal emitted from the balance receiving photodiode PD 11  and having the resonance of which frequency is ν 1 −νB−ν 0  is input to the mixer  13  via the amplifier  12 . The RF signal of frequency fr and generated by the signal generating circuit  14  is input to the mixer  13  with the beat signal having the resonance of which frequency is ν 1 −νB−ν 0 , and these signals are mixed. Here, the frequency fr of th RF signal generated by the signal generating circuit  14  is set to the value close to the frequency ν 1 −νB−ν 0  in advance. Then, the beat signal and the RF signal interfere, and the resonance occurs. The frequency of the resonance becomes the difference between the frequency ν 1 −νB−ν 0  and the frequency fr of the RF signal such as ν 1 −B−ν 0 −fr.  
         [0052]    The frequency fr of the RF signal generated by the signal generating circuit  14  is set quite close to the frequency of the resonance ν 1 −νB−ν 0  of the beat signal. Because the frequency ν 1 −νB−ν 0  becomes low, the frequency fr of the RF signal may also be low. Therefore, for a signal generating circuit  14 , it is only acceptable as long as the parts has the frequency characteristics for corresponding to such low frequencies. Thus, the cost of the signal generating circuit  14  can be reduced.  
         [0053]    When the mixed signal in which the resonance occurs with frequency ν 1 −νB−ν 0 −fr is input to the low pass filter  16 , the low pass filter  16  cuts th signal included in the mixed signal (signal having frequency of ν 1 −νB−ν 0  or fr), and outputs difference signal having only resonance frequency of ν 1 −νB−ν 0 −fr as a low frequency signal. In other words, high frequency ν 1 −νB−ν 0  of the beat signal decreased and become lower by a degree of frequency fr to be a low frequency difference signal.  
         [0054]    The signal processing section  18  measures the frequency of the difference signal. As mentioned above, the frequency of the difference signal decreased; thus the measurement becomes easy. The frequency of the beat signal ν 1 −νB−ν 0  is calculated from the frequency ν 1 −νB−ν 0 −fr of the measured difference signal, furthermore, the shifting amount νB due to the Brillouin scattering is calculated. Then, the amount of distortion at predetermined positions in the optical fiber  9  is determined from the calculated shifting amount νB.  
         [0055]    Additionally, although the above embodiment is an example of detecting the Brillouin scattering light in the returning light, the present invention is not limited to such case. The present invention can be applied to the detection of the Rayleigh scattered light, for example. Because, in the Rayleigh scattering, there is no shift of the frequency (νB= 0 ), for example, if ν 1 −ν 0  is 1.2 GHz, the frequency of the resonance of the beat signal is ν 1 −B−ν 0 =1.2 GHz−0=1.2 GHz. Therefore, in this case, for an amplifier  12  to which the beat signal is input and a mixer  13 , it is only acceptable as long as the parts has the frequency characteristics for corresponding to such a frequency. Thus, the cost of an amplifier  12  and a mixer  13  can be reduced.