Abstract:
The present apparatus includes: an intensity ratio calculation unit configured to calculate a first and a second intensity ratios which are ratios of Stokes light intensity to anti-Stokes light intensity obtained when a light pulse is output to a first end and a second end of an optical fiber, respectively; a temperature calculation unit configured to calculate a temperature distribution along the optical fiber based on a reference temperature, the first and the second intensity ratios, and a total length loss ratio, which is a loss ratio of Stokes light to anti-Stokes light with regard to a total length of the optical fiber; and a total length loss ratio calculation unit configured to calculate the total length loss ratio based on the first and the second intensity ratios with regard to a location in a predetermined section close to both ends of the optical fiber whose temperature is kept constant.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on Japanese Patent Application No. 2011-232838 filed with the Japan Patent Office on Oct. 24, 2011, the entire content of which is hereby incorporated by reference. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present disclosure relates to an apparatus for measuring temperature distribution along an optical fiber by using Raman scattered light. 
         [0004]    2. Related Art 
         [0005]    A DTS (Distributed Temperature Sensor) has been used for measuring temperature distribution along an optical fiber. The DTS is an apparatus for measuring temperature distribution along an optical fiber by using backscattered light generated in the optical fiber. The backscattered light includes Rayleigh scattered light, Brillouin scattered light and Raman scattered light, for example. The Raman scattered light, which has strong temperature dependence, is used for temperature measurement. In the case of the temperature measurement by using the Raman scattered light, anti-Stokes light (AS light) having a shorter wavelength than incident light and Stokes light (ST light) having a longer wavelength than the incident light are used. 
         [0006]    The DTS measures the intensity Ias of the AS light and the intensity Ist of the ST light to calculate the temperature based on the intensity ratio. Thus, the temperature distribution along the optical fiber is measured. The DTS is used in fields such as temperature management of a plant installation, disaster prevention, temperature management of an electric cable, and air conditioning of a server room or a building. 
         [0007]    The principle of the DTS is shown in  FIG. 4 . In measurement of the temperature distribution by the DTS, a temperature distribution measurement unit  100  outputs a light pulse to an optical fiber  101  (optical fiber for sensor). As shown in  FIG. 5 , the temperature distribution measurement unit  100  includes a pulse generation unit  102 , a light source  103 , a directional coupler  104 , an optical filter  105 , a first optical-electrical converter  106 , a second optical-electrical converter  107 , a first amplifier  108 , a second amplifier  109 , a first AD converter  110 , a second AD converter  111 , an averaging circuit  112 , a calculation unit  113 , a temperature correction unit  114 , and a reference temperature unit  115 . 
         [0008]    The pulse generation unit  102  causes the light source  103  to generate pulse light with timing in synchronization with the averaging circuit  112 . The generated pulse light is input to the optical fiber  101  through the directional coupler  104 . As shown in  FIG. 4 , Raman scattered light is generated almost everywhere in the optical fiber  101 . The generated Raman scattered light is input to the temperature distribution measurement unit  100 . The Raman scattered light input to the temperature distribution measurement unit  100  is guided by the directional coupler  104  to the optical filter  105 . 
         [0009]    The Raman scattered light is split by the optical filter  105  into anti-Stokes light (AS light) and Stokes light (ST light). The ST light is opto-electrically converted by the first optical-electrical converter  106 , amplified by the first amplifier  108 , and converted by the first AD converter  110  into a digital signal. The AS light is opto-electrically converted by the second optical-electrical converter  107 , amplified by the second amplifier  109 , and converted by the second AD converter  111  into a digital signal. 
         [0010]    The averaging circuit  112  performs averaging processing for noise reduction. The calculation unit  113  calculates the ratio of the AS light intensity Ias to the ST light intensity Ist (Raman intensity ratio). The Raman intensity ratio is proportional to the temperature of the optical fiber  101 . Therefore, the temperature distribution along the optical fiber  101  can be measured based on the Raman intensity ratio. 
         [0011]    As shown in  FIG. 5 , the reference temperature unit  115  is provided between the directional coupler  104  and the optical fiber  101 . An optical fiber is coiled in the reference temperature unit  115 . The reference temperature unit  115  is provided with a highly-accurate thermometer  115   s  typified by a platinum resistance temperature sensor. The thermometer  115   s  measures the reference temperature and outputs a measurement result to the calculation unit  113 . The calculation unit  113  calculates the temperature distribution along the optical fiber based on the reference temperature and the Raman intensity ratio. 
         [0012]    A signal on a time domain of the Raman intensity ratio can be obtained by inputting the light pulse to the optical fiber  101  as shown in  FIG. 4 . The time corresponds to a location along the optical fiber. The calculation unit  113  obtains the Raman intensity ratio of the whole length along the optical fiber  101 . The calculation unit  113  performs predetermined calculations by using the Raman intensity ratio and the reference temperature. As a result, the temperature distribution of the whole length of the optical fiber  101  can be obtained. The temperature correction unit  114  performs predetermined correction processing with respect to the temperature distribution along the optical fiber  101  obtained by the calculation unit  113 . 
         [0013]    As shown in  FIG. 4 , in a spectrum of the Raman scattered light, the anti-Stokes light (AS light) appears on the shorter-wavelength side (wavelength: λ 0 -λX) of the Rayleigh light (wavelength: λ 0 ). In addition, he Stokes light (ST light) appears on the longer-wavelength side (wavelength: λ 0 +Δλ′) of the Rayleigh light. 
         [0014]    In this manner, the temperature distribution measurement unit  100  uses the Raman scattered light to obtain the temperature with respect to each location along the optical fiber  101  corresponding to the time domain. As a result, the temperature distribution of the whole length of the optical fiber  101  is measured. If there is a hot section HT as shown in  FIG. 4 , the temperature of the optical fiber  101  is high in the vicinity of the hot section HT. Therefore, temperature distribution where the temperature of a part of the optical fiber  101  is increased is obtained as shown in  FIG. 4 . 
         [0015]    In the case of the configuration shown in  FIGS. 4 and 5 , only one end of the optical fiber  101  is connected to the temperature distribution measurement unit  100 . That is, the above-mentioned measurement is single-ended temperature distribution measurement with respect to the optical fiber  101 . Here, loss of the AS light intensity Ias and the ST light intensity Ist is caused within the optical fiber  101  while the Raman scattered light propagates from a location of generation to the near end of the optical fiber. Therefore, in the single-ended measurement, the Raman intensity ratio is corrected by using the loss ratio of Ias to Ist (Raman loss ratio (loss profile)) of the optical fiber. 
         [0016]    For this reason, the loss profile is recognized in advance in the single-ended measurement. However, if the optical fiber  101  includes a plurality of different kinds of optical fibers having different Raman loss ratios each of which is connected to each other, the Raman loss ratio is different depending on a location along the optical fiber  101 . In this case, an accurate loss profile is hard to recognize. 
         [0017]      FIG. 6  is a diagram illustrating an apparatus for performing double-ended measurement along the optical fiber temperature. In the double-ended measurement, either one of the near end and the far end of the optical fiber  101  is selected, and the selected one is connected to an optical switch  120  that is connected to the temperature distribution measurement unit  100 . 
         [0018]    The optical switch  120  has a channel A (CH-A) and a channel B (CH-B). The channel A is connected to one end of the optical fiber  101 , and the channel B is connected to the other end of the optical fiber  101 . In the double-ended measurement, both of measurement by using the channel A and measurement by using the channel B are performed in order to obtain Ias and Ist. Thereafter, measurement results are synthesized (obtain geometric mean). As shown in  FIG. 7 , a distance from the channel A to a location X in the optical fiber  101  is defined as X. The Raman intensity ratio that is measured with respect to the location X is defined as G(X). In this case, in the double-ended measurement, G(X) can be expressed by the following Equation 1. 
         [0000]    
       
         
           
             
               
                 
                   
                       
                   
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         [0019]    Here, IasA(X) is Ias at the location X responding to the light pulse from the channel A; 
         [0020]    IstA(X) is Ist at the location X responding to the light pulse from the channel A; 
         [0021]    IasB(m−X) is Ias at the location X responding to the light pulse from the channel B; 
         [0022]    IstB(m−X) is Ist at the location X responding to the light pulse from the channel B; 
         [0023]    m is the total length of the optical fiber  101 ; 
         [0024]    L(X) is the Raman loss ratio from the channel A to the location X; and 
         [0025]    L(m−X) is the Raman loss ratio from the channel B to the location X. 
         [0026]    Note that IasA(X)/IstA(X) and IasB(m−X)/IstB(m−X) in Equation 1 are equal to each other because they both are the pre-loss Raman intensity ratio in the optical fiber with regard to the same location. The above-mentioned same value is defined as G 0 (X). In this case, the following Equation 2 can be obtained. 
         [0000]      &lt;Equation 2&gt; 
         [0000]        G ( X )= G 0( X )×√{square root over ( L ( X )× L ( m−X ))}{square root over ( L ( X )× L ( m−X ))}  (Equation 2)
 
         [0027]    As mentioned above, L(X) is the Raman loss ratio from the channel A (one end of the optical fiber  101 ) to the location X, and L(m−X) is the Raman loss ratio from the location X to the channel B (other end of the optical fiber  101 ). The result of multiplication of L(X) and L(m−X) corresponds to the Raman loss ratio from the one end to the other end of the optical fiber  101  (i.e. the Raman loss ratio with regard to the total length of the optical fiber  101 ). Accordingly, the term “L(X)×L(m−X)” in Equation 2 takes a constant value independent of the location X. When “L(X)×L(m−X)” is denoted by “Ltotal”, the above-mentioned Equation 2 can be written as the following Equation 3. 
         [0000]      &lt;Equation 3&gt; 
         [0000]        G ( X )= G 0( X )×√{square root over ( L total)}  (Equation 3)
 
         [0028]    As mentioned above, the parameter Ltotal is the Raman loss ratio with regard to the total length of the optical fiber  101  and independent of the location X in the optical fiber  101 . Therefore, there is no need to recognize the above-mentioned loss profile in the double-ended measurement. The temperature distribution along the optical fiber is accurately measured as long as the parameter Ltotal being a fixed value is recognized. 
         [0029]    Such a technique that measures the temperature distribution along the optical fiber based on the intensity ratio of the Stokes light intensity Ist to the anti-Stokes light intensity Ias is disclosed in, e.g., JP-A-2008-249515. 
       SUMMARY 
       [0030]    An optical fiber temperature distribution measurement apparatus includes: a pulse output unit configured to output a light pulse to a selected one of a first end and a second end of an optical fiber; an intensity ratio calculation unit configured to calculate a first intensity ratio and a second intensity ratio, wherein the first intensity ratio is a ratio of Stokes light intensity to anti-Stokes light intensity obtained when the light pulse is output to the first end of the optical fiber, and the second intensity ratio is a ratio of Stokes light intensity to anti-Stokes light intensity obtained when the light pulse is output to the second end of the optical fiber, a reference temperature unit including a reference temperature optical fiber and a first reference thermometer configured to measure a temperature with a temperature of the reference temperature optical fiber being a reference temperature; a temperature calculation unit configured to calculate a temperature distribution along the optical fiber based on the reference temperature, the first intensity ratio, the second intensity ratio, and a total length loss ratio, wherein the total length loss ratio is a loss ratio of Stokes light intensity to anti-Stokes light intensity with regard to a total length of the optical fiber; a constant-temperature unit configured to keep a temperature of a predetermined section close to both ends of the optical fiber constant; and a total length loss ratio calculation unit configured to calculate the total length loss ratio based on the first intensity ratio and the second intensity ratio with regard to a location in the predetermined section. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0031]      FIG. 1  is a configuration diagram illustrating an optical fiber temperature distribution measurement apparatus according to the present embodiment; 
           [0032]      FIG. 2  is a diagram for describing double-ended measurement according to the present embodiment; 
           [0033]      FIG. 3  is a configuration diagram illustrating an optical fiber temperature distribution measurement apparatus provided with a reference thermometer; 
           [0034]      FIG. 4  is a diagram illustrating an optical fiber temperature distribution measurement apparatus used for single-ended measurement; 
           [0035]      FIG. 5  is a diagram illustrating a configuration of a temperature distribution measurement unit; 
           [0036]      FIG. 6  is a diagram illustrating an optical fiber temperature distribution measurement apparatus used for double-ended measurement; and 
           [0037]      FIG. 7  is a diagram for describing double-ended measurement. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    In the following detailed description, for purpose of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details. In other instances, well-known structures and devices are schematically shown in order to simplify the drawing. 
         [0039]    A phenomenon called “darkening” may be caused with respect to an optical fiber. If the darkening is caused, the Raman loss ratio in the optical fiber is changed. Accordingly, the above-described parameter Ltotal in Equation 3, namely, the Raman loss ratio with regard to the total length of the optical fiber  101  is changed. As a result, it becomes difficult to accurately measure the temperature distribution of the optical fiber  101  by the double-ended measurement. 
         [0040]    An object of the present disclosure is to accurately measure temperature distribution along an optical fiber by the double-ended measurement even if the parameter Ltotal of the optical fiber varies. 
         [0041]    Embodiments of the present disclosure will be described below with reference to the accompanying drawings. An optical fiber temperature distribution measurement apparatus (present apparatus) according to an embodiment performs double-ended measurement of temperature distribution along an optical fiber  2 . 
         [0042]    The present apparatus includes a temperature distribution measurement unit  1  and an optical switch  3 . The near end and the far end of the optical fiber (optical fiber for sensor)  2  are connected to the optical switch  3 . That is, the optical fiber  2  forms the double-ended configuration. The optical switch (pulse output unit)  3  has a channel A (CH-A) and a channel B (CH-B). The channel A is connected to the near end of the optical fiber  2 . The channel B is connected to the far end of the optical fiber  2 . The optical switch  3  connects the optical fiber  2  and the temperature distribution measurement unit  1  through either the channel A or the channel B. That is, the optical switch  3  connects a selected one of the near end (channel A) and the far end (channel B) of the optical fiber  2  to the temperature distribution measurement unit  1 . 
         [0043]    The temperature distribution measurement unit  1  includes a pulse generation unit  11 , a light source  12 , a directional coupler  13 , an optical filter  14 , a first optical-electrical converter  15 , a second optical-electrical converter  16 , a first amplifier  17 , a second amplifier  18 , a first ADC  19 , a second ADC  20 , an averaging circuit  21 , a calculation unit  22 , a temperature correction unit  23 , a reference temperature unit  24 , and an optical connector  25 . Note that, in the drawing, the optical-electrical converter is denoted by “O/E” and the ADC (Analog-Digital Converter) is denoted by “A/D”. 
         [0044]    The pulse generation unit (pulse output unit)  11  causes the light source  12  to output a light pulse with timing in synchronization with the averaging circuit  21 . The light pulse is input to the optical switch  3  through the directional coupler  13  and the optical connector  25 . In the example shown in  FIG. 1 , the optical switch  3  connects the channel A to the temperature distribution measurement unit  1 . In this case, the light pulse is output from the channel A and input to the optical fiber  2 . Of course, the optical switch  3  can switch the channel connected with the temperature distribution measurement unit  1  to the channel B instead. 
         [0045]    Due to the pulse light input to the optical fiber  2 , Raman scattered light is generated almost everywhere along the optical fiber  2 . The Raman scattered light is input to the temperature distribution measurement unit  1 . The Raman scattered light input to the temperature distribution measurement unit  1  is guided by the directional coupler (optical branching device)  13  to the optical filter  14 . The Raman scattered light is split by the optical filter  14  into anti-Stokes light (AS light) and Stokes light (ST light). 
         [0046]    The ST light is opto-electrically converted by the first optical-electrical converter  15  to be an electrical signal. The electrical signal is amplified by the first amplifier  17 . Then, the electrical signal is converted by the first ADC  19  from an analog signal to a digital signal. The digital signal corresponds to the intensity Ist of the ST light. The digital signal is input to the averaging circuit  21 . 
         [0047]    The AS light is opto-electrically converted by the second optical-electrical converter  16  to be an electrical signal. The electrical signal is amplified by the second amplifier  18 . Then, the electrical signal is converted by the second ADC  20  from an analog signal to a digital signal. The digital signal corresponds to the intensity Ias of the AS light. The digital signal is input to the averaging circuit  21 . 
         [0048]    The averaging circuit  21  performs averaging processing for noise reduction with respect to the digital signal corresponding to the intensity Ist and the digital signal corresponding to the intensity Ias. A synchronization signal is transferred between the averaging circuit  21  and the pulse generation unit  11 . The averaging circuit  21  and the pulse generation unit  11  are in synchronization with each other. 
         [0049]    The calculation unit (intensity ratio calculation unit, temperature calculation unit, total length loss ratio calculation unit)  22  calculates the ratio of the Stokes light intensity Ist to the anti-Stokes light intensity Ias (Raman intensity ratio). Moreover, the calculation unit  22  obtains temperature distribution along the optical fiber  2  based on the calculation result and the reference temperature obtained by the reference temperature unit  24 . The temperature correction unit  23  corrects the temperature obtained by the calculation unit  22  by using a parameter such as Raman shift frequency. 
         [0050]    The reference temperature unit  24  is provided between the directional coupler  13  and the optical connector  25 . A reference temperature optical fiber is coiled in the reference temperature unit  24 . The reference temperature unit  24  is provided with a highly-accurate temperature sensor (first reference thermometer)  24   s  typified by a platinum resistance temperature sensor. The temperature sensor  24   s  measures the temperature of the reference temperature optical fiber (reference temperature) and outputs a measurement result to the calculation unit  22  and the temperature correction unit  23 . 
         [0051]    As shown in  FIG. 1 , the near end of the loop part of the optical fiber  2  is connected to the channel A of the optical switch  3 . The far end of the loop part of the optical fiber  2  is connected to the channel B of the optical switch  3 . Two predetermined sections of the optical fiber  2  within a certain distance range from the two channels A and B are included in a constant-temperature unit  30 . The constant-temperature unit  30  keeps the temperature of the two predetermined sections of the optical fiber  2  constant. Therefore, the two predetermined sections are constant-temperature sections. 
         [0052]    Next, an operation of the present apparatus will be described. The intensity of the Stokes light and the anti-Stokes light generated in the optical fiber  2  is lost (attenuation) in optical the fiber  2  on the way back to the temperature distribution measurement unit  1 , as described above. As a result of the loss, the Raman intensity ratio (ratio of the Stokes light intensity Ist to the anti-Stokes light intensity Ias) at a location (location of scattering) where the Stokes light and the anti-Stokes light are generated in the optical fiber  2  differs from the Raman intensity ratio measured by the temperature distribution measurement unit  1 . The Raman intensity ratio at the location of scattering in the optical fiber  2  is hereinafter referred to as “pre-loss Raman intensity ratio”. 
         [0053]    According to the measurement of the temperature distribution along the optical fiber  2  by the present apparatus, a light pulse is first output from the channel A and the Raman intensity ratio (first intensity ratio) is measured. After that, a light pulse is output from the channel B and the Raman intensity ratio (second intensity ratio) is measured. 
         [0054]      FIG. 2  is a development diagram illustrating the optical fiber  2 . A location X 1  is located within the constant-temperature unit  30 . The distance from the channel A to the location X 1  is X 1 . The Raman intensity ratio G(X 1 ) that is the intensity ratio of Ias to Ist with regard to the location X 1  and measured by using the channel A is expressed by the following Equation 4. 
         [0000]        G ( X 1)= G 0( X 1)× L ( X 1)  (Equation 4)
 
         [0055]    Here, G 0 (X 1 ) is the pre-loss Raman intensity ratio at the location X 1  at the temperature T, and L(X 1 ) is the ratio of loss of Ias to Ist (Raman loss ratio) regarding the distance X 1  from the channel A to the location X 1 . 
         [0056]    Meanwhile, the total length of the optical fiber  2  (i.e. length from the channel A to the channel B) is expressed by “m”. In this case, the distance from the channel B to the location X 1  is expressed by “m−X 1 ”. Therefore, the Raman intensity ratio G(m−X 1 ) with regard to the location X 1  measured by using the channel B is expressed by the following Equation 5. 
         [0000]        G ( m−X 1)= G 0( X 1)× L ( m−X 1)  (Equation 5)
 
         [0057]    Dividing Equation 5 by Equation 4 yields the following Equation 6. 
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         [0058]    The total length m of the optical fiber  2  is several kilometers. On the other hand, the location X 1  is very close to the temperature distribution measurement unit  1  within the constant-temperature unit  30 . The distance X 1  from the channel A to the location X 1  is only several meters to several tens of meters. The distance X 1  is very short as compared with the total length m of the optical fiber  2 . Therefore, the Raman loss ratio with regard to the location X 1  is negligibly small, and we can assume “L(X 1 )≈1”. 
         [0059]    Moreover, since the distance X 1  from the channel A to the location X 1  is very short as compared with the total length m of the optical fiber  2 , we can assume that L(m−X 1 ) is almost equal to L(m). The parameter L(m) is the Raman loss ratio with regard to the total length of the optical fiber  2 , namely L(m)=Ltotal. Therefore, the following Equation 7 can be obtained. 
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         [0060]    Therefore, Ltotal can be obtained if G(X 1 ) and G(m−X 1 ) are obtained. As described above, G(X 1 ) is the Raman intensity ratio regarding the location X 1  that is measured by using the channel A (i.e. first intensity ratio with regard to the location X 1 ). On the other hand, G(m−X 1 ) is the Raman intensity ratio regarding the location X 1  that is measured by using the channel B (i.e. second intensity ratio with regard to the location X 1 ). Therefore, the calculation unit  22  can measure the Raman intensity ratios G(X 1 ) and G(m−X 1 ) by using the pulse light input to the optical fiber  2 . 
         [0061]    Accordingly, the calculation unit  22  can obtains Ltotal based on the Raman intensity ratios G(X 1 ) and G(m−X 1 ). Consequently, even if Ltotal of the optical fiber  2  varies due to darkening and the like, the Ltotal after variation can be easily obtained according to the present apparatus. 
         [0062]    The calculation unit  22  uses Ltotal to obtain G 0 (X) in the above-described Equation 3. The parameter G 0 (X) is the Raman intensity ratio at the location X and the Raman intensity ratio depends on the temperature. Therefore, the calculation unit  22  can obtain the temperature at the location X of the optical fiber  2  by using the reference temperature obtained by the reference temperature unit  24 . Accordingly, the calculation unit  22  can obtain the temperature distribution of the whole length of the optical fiber  2 . 
         [0063]    The above-mentioned G(X 1 ) and G(m−X 1 ) are the Raman intensity ratios with regard to the location X 1  within the constant-temperature unit  30 . The section where the temperature is kept constant included in the constant-temperature unit  30  (constant-temperature section) of the optical fiber  2  has a certain length. The parameter Ltotal may be measured for plural times by changing the location X 1  a little bit (e.g. several meters) at a time within the constant-temperature section, and an average value of the plurality of Ltotal with regard to a plurality of locations may be calculated as Ltotal. In this case, accuracy of Ltotal can be increased. As a result, accuracy of the measurement of the temperature distribution can be further improved. 
         [0064]    As shown in  FIG. 3 , the present apparatus may further have a reference thermometer (second reference thermometer)  41  for measuring the temperature of the optical fiber  2  within the constant-temperature unit  30 . The reference thermometer  41  measures the temperature of the optical fiber  2  in the constant-temperature unit  30  and outputs the measured temperature to the temperature correction unit  23 . The certain loss ratio exists to Ias and Ist in the optical switch  3 . 
         [0065]    The temperature correction unit  23  uses the temperature measured by the reference thermometer  41  to correct the temperature obtained by the calculation unit  22 . As a result, an error of the temperature measurement due to the loss ratio in the optical switch  3  can be corrected. Consequently, accuracy of the temperature measurement can be further improved. 
         [0066]    The optical fiber temperature distribution measurement apparatus according to the present disclosure may be any of the following first to third measurement apparatuses. The first measurement apparatus is an optical fiber temperature distribution measurement apparatus for measuring temperature distribution along an optical fiber by using Raman scattered light. The near end and the far end of the optical fiber are connected to an optical switch, and thus a double-ended optical fiber is formed. The first measurement apparatus includes a constant-temperature section and a calculation unit. The constant-temperature section whose temperature is constant is provided on the double-ended optical fiber at the same distance from the optical switch of the double-ended optical fiber. The calculation unit measures temperature distribution along the double-ended optical fiber based on the intensity ratio of anti-Stokes light and Stokes light with regard to the constant-temperature section. 
         [0067]    According to the first measurement apparatus, the double-ended optical fiber is provided with the constant-temperature section. As a result, the loss ratio regarding the whole length of the optical fiber can be obtained, and the temperature distribution of the whole length of the double-ended optical fiber can be measured based on the loss ratio regarding the whole length of the optical fiber. 
         [0068]    The second measurement apparatus further includes a temperature correction unit in addition to the first measurement apparatus. The temperature correction unit performs correction of the temperature distribution based on the temperature obtained by a reference thermometer that measures the temperature of the constant-temperature section. 
         [0069]    The Stokes light intensity and the anti-Stokes light intensity are lost due to the optical switch. Generally, the loss is different between the Stokes light and the anti-Stokes light. That is, the loss ratio corresponds to an error in the temperature measurement. Therefore, by measuring the temperature of the constant-temperature section and performing temperature correction, an error in the measurement of the temperature distribution caused by the loss ratio due to the optical switch can be avoided. 
         [0070]    The third measurement apparatus further has the following function in addition to the first measurement apparatus or the second measurement apparatus. That is, the calculation unit obtains a plurality of intensity ratios of the anti-Stokes light to the Stokes light within the constant-temperature section and averages the plurality of intensity ratios to measure the temperature distribution of the double-ended optical fiber. 
         [0071]    By obtaining a plurality of samples of the intensity ratio of the anti-Stokes light to the Stokes light within the constant-temperature section and averaging them, accuracy of the measurement of the temperature distribution can be improved. 
         [0072]    According to the first to third measurement apparatuses, the double-ended optical fiber is provided with the constant-temperature section and thereby the intensity ratio of the Stokes light to the anti-Stokes light can be calculated accurately even if variation in loss ratio is caused in the double-ended optical fiber. It is therefore possible to accurately measure the temperature distribution. 
         [0073]    The foregoing detailed description has been presented for the purposes of illustration and description. Many modifications and variations are possible in light of the above teaching. It is not intended to be exhaustive or to limit the subject matter described herein to the precise form disclosed. Although the subject matter has been described in language specific to structural features and/or methodological acts, it is to be understood that the subject matter defined in the appended claims is not necessarily limited to the specific features or acts described above. Rather, the specific features and acts described above are disclosed as example forms of implementing the claims appended hereto.