Patent Publication Number: US-7719666-B2

Title: Distributed optical fiber sensor

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a distributed optical fiber sensor capable of measuring strain and/or temperature in longitudinal direction with high accuracy and high spatial resolution using an optical fiber as a sensor. 
     2. Description of the Related Art 
     A method based on a Brillouin scattering phenomenon occurring in an optical fiber is known as a technology for measuring strains and temperatures. In this method, the optical fiber is used as a medium for detecting strain and/or temperature in an environment where the optical fiber is placed. 
     The Brillouin scattering phenomenon is such a phenomenon that, when two light rays having different frequencies pass each other in an optical fiber, power transfers via acoustic phonons in the optical fiber from the light having a higher frequency to the one having a lower frequency. If νd denotes a frequency difference between two light waves passing each other, the transferring power is proportional to a Brillouin gain spectrum BSg(νd) approximately defined by Equation 1:
 
 BSg (ν d )=1/(1+(2(ν d−νb )/Δν b ) 2 )  Equation 1
 
where νb is Brillouin frequency shift and Δνb is called Brillouin gain line width (full width at half maximum), these being parameters characterizing the Brillouin gain spectrum BSg(νd).
 
     The Brillouin frequency shift νb is given by Equation 2:
 
ν b= 2 nva/λ   Equation 2
 
where n is refractive index of the optical fiber, va is sound velocity in the optical fiber and λ wavelength of light incident on the optical fiber.
 
     Since the sound velocity va depends on the strain and temperature of the optical fiber, the strain and/or temperature can be measured by measuring the Brillouin frequency shift νb. 
     Accordingly, Brillouin gain spectra at the respective parts of the optical fiber may be measured in order to measure a strain distribution and/or a temperature distribution of the optical fiber in the longitudinal direction of the optical fiber. In order to attain a high spatial resolution, the lengths of these parts need to be shortened. 
       FIG. 24  are diagrams showing the construction of a distributed optical fiber sensor and a probe light according to a background art, wherein  FIG. 24A  is a block diagram showing the construction of the distributed optical fiber sensor according to the background art,  FIG. 24B  is a chart showing first probe light and  FIG. 24C  is a chart showing second probe light.  FIG. 25  is a chart showing Brillouin loss/gain spectra, wherein horizontal axis represents frequency and vertical axis represents loss/gain. 
     In  FIG. 24 , the distributed optical fiber sensor  500  according to the background art is provided with a probe light source  501 , an optical coupler  502 , a sensing optical fiber  503 , a pump light source  504  and a detector  505 . 
     The probe light source  501  generates an optical pulse shown in  FIG. 24B  and emits the generated optical pulse as probe light. The probe light emitted from the probe light source  501  is incident on one end of the sensing optical fiber  503  via the optical coupler  502 . The sensing optical fiber  503  is an optical fiber for detecting strain and/or temperature in an environment where this optical fiber is placed and used as a sensor. The pump light source  504  generates continuous light having a frequency lower than that of the probe light and emits the generated continuous light (CW light) as pump light. The pump light emitted from the pump light source  504  is incident on the other end of the sensing optical fiber  503 . In the sensing optical fiber  503 , the probe light and the pump light cause a Brillouin scattering phenomenon, and light attributed to this Brillouin scattering phenomenon is incident on the detector  505  via the optical coupler  502 . The distributed optical fiber sensor  500  measures the intensity of the light attributed to the Brillouin scattering phenomenon frequency by frequency in a time domain while successively changing the frequency of the pump light or the probe light, determines Brillouin gain spectra BSg(νd) in the respective parts along the longitudinal direction of the sensing optical fiber  503 , and determines a strain distribution and/or temperature distribution along the sensing optical fiber  503 . 
     Although the method for determining the strain and/or temperature from the Brillouin gain spectrum BSg(νd) is described above, strain and/or temperature can be similarly determined using a Brillouin loss spectrum BSl(νd) instead of the Brillouin gain spectrum BSg(νd) by setting the frequency of the pump light to be higher than that of the probe light. 
     The spatial resolution of this distributed optical fiber sensor  500  is restricted by the width of the optical pulse used for the measurement. Specifically, if v g [m/s] denotes velocity of the light in the optical fiber, spatial resolution Δz is v g Tp/2 [m] in a measurement using an optical pulse whose width is Tp[s]. More specifically, in normally used ordinary optical fibers in which the velocity of light slightly differs depending on the materials of the optical fibers, a Brillouin gain spectrum BSg(νd) or a Brillouin loss spectrum BLg(νd) (hereinafter, abbreviated as a “Brillouin loss/gain spectrum BSl/g(νd)) is represented by a Lorenz (Lorentzain) curve (curve “a” shown in  FIG. 25 ) up to the optical pulse width of 30 ns. If the optical pulse width is shortened to be below 30 ns, this results in a broadband curve (curve “b” shown in  FIG. 25 ) to lose a peak in the vicinity of a mean frequency, thereby taking a moderate shape. Thus, the spatial resolution Δz becomes about 2 to 3 m. Although the optical pulse having a short width is necessary to improve the spatial resolution, the spectral width of the optical pulse becomes wider in this case, with the result that strain measurement accuracy becomes poor. Therefore, it has been difficult to measure strain and/or temperature distributions with high spatial resolution (of, e.g. 1 m or less) with high accuracy (of, e.g. 200μ∈), and there has been a demand to measure strain and/or temperature distributions with high accuracy and high spatial resolution. 
     Accordingly, as disclosed, for example, in the following documents 1 to 3, the probe light source  501  causes an optical pulse having a specified light intensity As 2  to be incident on the sensing optical fiber  503  while causing continuous light (leakage light) having a weak light intensity Cs 2  to be incident thereon as shown in  FIG. 24C , whereby the Brillouin loss/gain spectra BSl/g(νd) substantially approximate to Lorenz curves having very steep peaks substantially at mean frequencies so that the mean frequencies can be clearly recognized as shown by the curve “a” in  FIG. 25 . In this way, it is known to measure strain and/or temperature with high accuracy and high spatial resolution. 
     Here, the Lorenz curve is generally expressed by Equation 3 that is a Lorenz function g(x):
 
 g ( x )=1 /πa /( a   2 +( x−a ) 2 )  Equation 3
 
Document 1
 
     X. Bao and A. Brown, M. DeMerchant, J. Smith, “Characterization of the Brillouin-loss spectrum of single mode fibers by use of very short (&lt;10-ns) pulses”, OPTICS LETTERS, Vol. 24, No. 8, Apr. 15, 1999 
     Document 2 
     V. Lecoeuche, D. J. Webb, C. N. Pannell and D. A. Jackson, “Transient response in High-resolution Brillouin-based distributed sensing using probe pulses shorter than the acoustic relaxation time”, OPTICS LETTERS, Vol. 25, No. 3, Feb. 1, 2000. 
     Document 3 
     Shahraam Afshar V., Graham A. Ferrier, Xiaoyi Bao, and Liang Chen, “Effect of the finite extinction ratio of an electro-optic modulator on the performance of distributed probe-pump Brillouin sensor systems”, OPTICS LETTERS, Vol. 28, No. 16, Aug. 15, 2003. 
     Since the setting of the light intensity Cs 2  of the leakage light depends on the length of the sensing optical fiber, it has been necessary to manually make a fine adjustment in conformity with the length of the sensing optical fiber every time a measurement is conducted. Thus, if the distributed optical fiber sensor is an industrial product, a user needs to make this difficult manual adjustment, which has hindered the productization of distributed optical fiber sensors as industrial products. 
     In document 2, authors confirmed the phenomenon disclosed in document 1 by means of simulation, but shed no light in terms of theoretical analysis. Document 2 shed no light on the cause of being able to measure strain and/or temperature with high accuracy and high spatial resolution by causing an optical pulse having a specified light intensity to be incident on the sensing optical fiber while causing continuous light (leakage light) having a weak light intensity to be incident thereon. Thus, it was not clear how physical quantities of the continuous light having the weak light intensity and the optical pulse having the specified light intensity should be adjusted in order to measure strain and/or temperature with high accuracy and high spatial resolution. 
     Further, with the distributed optical fiber sensor according to the background art, it has been difficult to sense minute strains equal to or below 200μ∈ if strains are equally distributed in a wide range (e.g. 25-fold or more than the set spatial resolution). 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a distributed optical fiber sensor necessitating no manual adjustment of the light intensity of leakage light in conformity with the length of a sensing optical fiber. It is another object thereof to provide a distributed optical fiber sensor capable of sensing minute strains equal to or below 200μ∈ if distributions are equally distributed in a wide range. 
     According to an aspect of the invention, a distributed optical fiber sensor is adapted for measuring strain and/or temperature utilizing a Brillouin scattering phenomenon. The distributed optical fiber sensor comprises a stepwise optical light source for generating an optical pulse having a stepwise distribution of light intensity to increase toward the center; a continuous light source for generating continuous light; a sensing optical fiber on which the optical pulse is incident as probe light and the continuous light is incident as pump light to thereby cause a Brillouin scattering phenomenon between the probe light and the pump light; and a Brillouin time domain detector for determining a Brillouin loss spectrum or a Brillouin gain spectrum from the light emerging from the sensing optical fiber and attributed to the Brillouin scattering phenomenon, and measuring strain caused in and/or temperature of the sensing optical fiber based on the determined Brillouin loss spectrum or the Brillouin gain spectrum. 
     These and other objects, features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments/examples with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  are charts showing the theoretical analysis of a Brillouin scattering phenomenon in connection with the invention. 
         FIG. 2  are graphs showing one example of simulation (No. 1) based on the theoretical analysis. 
         FIG. 3  are graphs showing one example of simulation (No. 2) based on the theoretical analysis. 
         FIG. 4  is a chart showing the waveform of an intensity-stepwise optical pulse. 
         FIG. 5  is a graph showing a simulation of H 2 /(H 1 +H 3 +H 4 ) in relation to ratio Prx based on the theoretical analysis. 
         FIG. 6  is a block diagram showing a construction of a distributed optical fiber sensor according to a first embodiment. 
         FIG. 7  is a block diagram showing a construction of a stepwise optical light source in the distributed optical fiber sensor. 
         FIG. 8  is a block diagram showing a construction of an automatic temperature controller in the distributed optical fiber sensor. 
         FIG. 9  are a block diagram showing a construction of an automatic frequency controller in the distributed optical fiber sensor and charts showing the principle of operation of the automatic frequency controller. 
         FIG. 10  are charts showing generation of the intensity-stepwise optical pulse. 
         FIG. 11  is a block diagram showing a construction of a light intensity/polarization regulator in the distributed optical fiber sensor. 
         FIG. 12  is a block diagram showing a construction of a CW light source in the distributed optical fiber sensor of the first embodiment. 
         FIG. 13  is a block diagram showing a construction of a light intensity regulator in the distributed optical fiber sensor. 
         FIG. 14  are charts showing distributions of Brillouin loss/gain spectra and Brillouin loss/gain spectra at distances L 1  and L 2 . 
         FIG. 15  is a block diagram showing a construction of a distributed optical fiber sensor according to a second embodiment. 
         FIG. 16  is a block diagram showing a construction of a CW light source in the distributed optical fiber sensor of the second embodiment. 
         FIG. 17  is a block diagram showing a construction of a distributed optical fiber sensor according to a third embodiment. 
         FIG. 18  are charts showing an intensity-stepwise optical pulse and a leakage optical pulse according to a fourth embodiment. 
         FIG. 19  is a flow chart showing an operation of a distributed optical fiber sensor according to the fourth embodiment. 
         FIG. 20  is a block diagram showing a construction of a distributed optical fiber sensor according to a fifth embodiment corresponding to the distributed optical fiber sensor according to the first embodiment. 
         FIG. 21  is a flow chart showing an operation of the distributed optical fiber sensor according to the fifth embodiment. 
         FIG. 22  is a graph showing a frequency error ratio-correction value characteristic curve representing correction value in relation to frequency error ratio. 
         FIG. 23  is a chart showing Brillouin frequency shift amount in the longitudinal direction of a sensing optical fiber. 
         FIG. 24  are a diagram showing a construction of a distributed optical fiber sensor according to a background art and charts showing probe lights. 
         FIG. 25  is a chart showing Brillouin loss/gain spectra. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First of all, main features of preferred embodiments according to the present invention will be described. A distributed optical fiber sensor is adapted for measuring strain and/or temperature utilizing a Brillouin scattering phenomenon, and comprises a stepwise optical light source for generating an optical pulse having a stepwise distribution of light intensity to increase toward the center; a continuous light source for generating continuous light; a sensing optical fiber on which the optical pulse is incident as probe light and the continuous light is incident as pump light to thereby cause a Brillouin scattering phenomenon between the probe light and the pump light; and a Brillouin time domain detector for determining a Brillouin loss spectrum or a Brillouin gain spectrum from the light emerging from the sensing optical fiber and attributed to the Brillouin scattering phenomenon, and measuring strain caused in and/or temperature of the sensing optical fiber based on the determined Brillouin loss spectrum or the Brillouin gain spectrum. 
     In the distributed optical fiber sensor, it may be preferable that the probe light is incident on one end of the sensing optical fiber; the pump light is incident on the other end of the sensing optical fiber; and the Brillouin time domain detector determines the Brillouin loss spectrum or the Brillouin gain spectrum from the light emerging from the one end of the sensing optical fiber and attributed to the Brillouin scattering phenomenon and measures strain caused in and/or temperature of the sensing optical fiber based on the determined Brillouin loss spectrum or Brillouin gain spectrum. 
     In the distributed optical fiber sensor, it may be preferable that the probe light is incident on one end of the sensing optical fiber; the pump light is incident on the other end of the sensing optical fiber; the sensing optical fiber reflects the pump light propagating therein at the other end thereof; and the Brillouin time domain detector determines the Brillouin loss spectrum or the Brillouin gain spectrum from the light emerging from the one end of the sensing optical fiber and attributed to the Brillouin scattering phenomenon and measures strain caused in and/or temperature of the sensing optical fiber based on the determined Brillouin loss spectrum or Brillouin gain spectrum. 
     In the above distributed optical fiber sensors, the Brillouin loss spectrum or the Brillouin gain spectrum in each longitudinal area section of equal to or shorter than 1 m of the sensing optical fiber from the light emerging from the sensing optical fiber and attributed to the Brillouin scattering phenomenon may be substantially represented by a Lorenz curve. 
     The above distributed optical fiber sensors may further comprise a light intensity/polarization regulator for regulating the light intensity of incident light and randomly changing a plane of polarization of the incident light; and a light intensity regulator for regulating the light intensity of the incident light. The probe light is incident on the sensing optical fiber via the light intensity-polarization regulator and the pump light is incident on the sensing optical fiber via the light intensity regulator. 
     In the above distributed optical fiber sensors, each of the stepwise optical light source and the continuous light source may include a light emitting element for continuously emitting light having a narrow line width, a specified frequency and a substantially constant light intensity; a temperature controller for keeping the temperature of the light emitting element substantially at a constant temperature; and a frequency controller for keeping the frequency of the light emitted from the light emitting element substantially at a constant frequency. 
     In the above distributed optical fiber sensors, the stepwise optical light source may include light emitting element for continuously emitting light having a narrow line width, a specified frequency and a substantially constant first light intensity; a first and a second light intensity modulators for modulating the light intensity of incident light; a first light-intensity-modulator driving portion for driving the first light intensity modulator to modulate the light intensity of the light continuously emitted from the light emitting element so that an optical pulse having the first light intensity exists in continuous light having a second light intensity lower than the first light intensity; and a second light-intensity-modulator driving portion for driving the second light intensity modulator to modulate the light intensity of the light incident from the first light intensity modulator so that the remaining light is eliminated while leaving continuous lights having the second light intensity over specified widths before and after the optical pulse. 
     In the above distributed optical fiber sensors, it may be preferable that the stepwise optical light source generates a first optical pulse having a stepwise distribution of light intensity to increase toward the center and a second optical pulse having the same time width as the first optical pulse and the same light intensity as the lowest light intensity of the first optical pulse; and the Brillouin time domain detector causes the stepwise optical light source to generate the second optical pulse and emit it as probe light to the sensing optical fiber and causes the continuous light source to generate the continuous light and emit it as pump light to the sensing optical fiber, thereby storing a first light intensity of the light emerging from the sensing optical fiber and attributed to the Brillouin scattering phenomenon, causes the stepwise optical light source to generate the first optical pulse and emit it as probe light to the sensing optical fiber and causes the continuous light source to generate the continuous light and emit it as pump light to the sensing optical fiber, thereby storing a second light intensity of the light emerging from the sensing optical fiber and attributed to the Brillouin scattering phenomenon, determines the Brillouin loss spectrum or the Brillouin gain spectrum based on the stored first and second light intensities, and measures strain caused in and/or temperature of the sensing optical fiber based on the determined Brillouin loss spectrum or Brillouin gain spectrum. 
     The above distributed optical fiber sensors may further comprise a reference optical fiber made of the same material as the sensing optical fiber, having a length corresponding to spatial resolution, and having a strain larger than the one expressed by an inverse of the time width of the optical pulse, wherein the Brillouin time domain detector causes the stepwise optical light source to generate the optical pulse and emit it as probe light to the reference optical fiber and causes the continuous light source to generate the continuous light and emit it as pump light to the reference optical fiber, determines the mean frequency of the Brillouin loss spectrum or the Brillouin gain spectrum from the light emerging from the reference optical fiber and attributed to the Brillouin scattering phenomenon, stores the determined mean frequency as a reference value, determines the mean frequency of the Brillouin loss spectrum or the Brillouin gain spectrum from the light emerging from the sensing optical fiber and attributed to the Brillouin scattering phenomenon as a detection value, calculates a correction value from the reference value, the detection value and the Brillouin loss spectrum or the Brillouin gain spectrum in accordance with a correction-value conversion equation stored beforehand, and measures strain caused in and/or temperature of the sensing optical fiber from the calculated correction value. 
     Further, in order to enable the detection of minute strains of 200μ∈ or smaller in the case where strains are equally distributed in a wide range, the sensing optical fiber may be an optical fiber in which a Brillouin frequency shift amount cyclically changes in the above distributed optical fiber sensors. 
     Further, in the above distributed optical fiber sensors, the sensing optical fiber may be fixed to a measurement object whose strain and/or temperature are to be measured. 
     In the distributed optical fiber sensor constructed as above, an optical pulse having a stepwise distribution of light intensity to increase toward the center (hereinafter, referred to as an “intensity-stepwise optical pulse”) is used as probe light. Thus, section(s) having a weak light intensity corresponding to the leakage light of the background art exist(s) only in a limited section before a section having the highest light intensity or only in both a limited section before the section having the highest light intensity and a limited section after the section having the highest light intensity, whereby there is no continuous leakage light as in the background art. As a result, the waveform of the intensity-stepwise optical pulse can be set beforehand, wherefore it is not necessary unlike the background art to manually adjust the light intensity Cs 2  of the leakage light of the probe light (optical pulse) in conformity with the length of the sensing optical fiber every time a measurement is conducted. Therefore, the distributed optical fiber sensor can be produced as an industrial product. 
     Hereinafter, a logic analysis of the Brillouin scattering phenomenon in the case where the intensity-stepwise optical pulse is used as probe light is described with reference to  FIGS. 1 to 3  and the waveform of the intensity-stepwise optical pulse is described with reference to  FIGS. 4 and 5 . 
       FIG. 1  are graphs showing the logic analysis of the Brillouin scattering phenomenon, wherein  FIG. 1A  shows a measurement system in the logic analysis of the Brillouin scattering phenomenon,  FIG. 1B  shows a pump light and  FIG. 1C  shows a probe light.  FIG. 2  are graphs showing one example of simulation based on the logic analysis (No. 1).  FIG. 3  are graphs showing one example of simulation based on the logic analysis (No. 2).  FIG. 4  is a graph showing the waveform of an intensity-stepwise optical pulse.  FIG. 5  is a graph showing a simulation of H 2 /(H 1 +H 3 +H 4 ) in relation to a ratio Prx based on the logic analysis. 
     First, the logic analysis is described. In  FIG. 1 , this logic analysis is for deriving a Brillouin loss spectrum in the case where a pump light in the form of a continuous light (CW) having a light intensity AL 2  is incident on one end of a sensing optical fiber SOF, an intensity-stepwise optical pulse OPs comprised of an optical pulse front light OPf having a time width Tf and a light intensity Cs 2  and an optical pulse OP having a pulse width D and a light intensity (As+Cs) 2  is incident as probe light on the other end of the sensing optical fiber SOF. The light intensity As 2  is a light intensity on the basis of the light intensity Cs 2  of the optical pulse front light OPf. 
     Here, in this specification, a continuous leakage light left to have a specified time width Tf before the optical pulse OP of the intensity-stepwise optical pulse OPs is referred to as the optical pulse front light OPf, whereas a continuous leakage light left to have a specified time width Tb after the optical pulse OP of the intensity-stepwise optical pulse OPs is referred to as the optical pulse back light OPb. 
     In this logic analysis, if it is assumed that L denotes the length of the sensing optical fiber SOF, z denotes a position coordinate in the longitudinal direction of the sensing optical fiber SOF (0≦z≦L: origin is one end of the sensing optical fiber SOF) and t denotes a time coordinate, Brillouin scattering equations in the case where the sensing optical fiber is strained are expressed by Equations 4 to 6. 
     
       
         
           
             
               
                 
                   
                     
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     Here, v g  denotes velocity of a group of lights in the sensing optical fiber SOF (v g =c/n where c: light velocity and n: refractive index of the sensing optical fiber SOF), E L  denotes electric field intensity of a pump light, E S  denotes electric field intensity of a Stokes light, E A  is Γ×ρ/Λ and * denotes conjugation, where Γ denotes Γ B /2, ρ denotes density of the sensing optical fiber, and Λ is (γ×q×q)/(16×π×Ω). Γ B =1/τ B  if τ B  denotes the life of an acoustic phonon. γ is called electrostrictive coupling constant and γ=ρ(δ∈/δρ) if ∈ denotes dielectric constant. If k L , k S  denote wavenumber of the pump light and that of the Stokes light, q=k L +k S . Ω denotes Brillouin angular frequency shift in the case where there is no strain, and Ω=ω L −ω S  if ω L , ω S  denote angular frequency of the pump light and that of the Stokes light, respectively. Ω B  denotes Brillouin angular frequency shift in the case where there is a certain strain, and Ω B =ω BL −ω BS  if ω BL , ω BS  denote angular frequency of the pump light and that of the Stokes light, respectively. i is complex unit and i×i=−1. β=κ×Λ/Γ, where κ=(γ×ω L )/(4×ρ 0 ×n×c)≈(γ×ω S )/(4×ρ 0 ×n×c). ρ 0  is an average value of the densities of the sensing optical fiber. If gSBS denotes gain coefficient of stimulated Brillouin scattering (SBS), gSBS=16×π×β/(n×c). gSBS=2.5×10 −11  m/W is disclosed, for example, in document 4. 
     Document 4 
     A. L. Gaeta and R. W. Boyd, “Stochastic dynamics of stimulated Brillouin scattering in an optical fiber”, Physical Review A, Vol. 44, no. 5, 1991, pp 3205-3209. 
     Equation 4 relates to the pump light, Equation 5 relates to the probe light and Equation 6 relates to the life of the acoustic phonon. Equations 7 to 11 can be obtained if Brillouin loss V(t, Ω) is obtained as an approximate solution by solving these Equations 4 to 6. 
     
       
         
           
             
               
                 
                   
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                   11 
                 
               
             
           
         
       
     
     Here, ξ, s, c.c denote position of the sensing optical fiber in the longitudinal direction, time, and a constant, respectively. If LL denotes the entire length of the sensing optical fiber, h(z,s)=Γ×e −(Γ+i(ΩB(z)−Ω))  at position z and time s and h c (ξ,s)=h(z,s)=h((LL−ξ),s). 
     H 1  given by Equation 8 denotes a Brillouin loss spectrum based on an acoustic phonon excited by the optical pulse OP and the pump light. H 2  given by Equation 9 denotes a Brillouin loss spectrum based on an acoustic phonon excited by the optical pulse front light OPf and the pump light and further by the optical pulse OP and the pump light. H 3  given by Equation 10 denotes a Brillouin spectrum based on an acoustic phonon excited by the optical pulse OP and the pump light and further by the optical pulse front light OPf and the pump light. H 4  given by Equation 11 denotes a Brillouin loss spectrum based on an acoustic phonon excited by the optical pulse front light OPf and the pump light. 
     As an example, assuming that the time width of the intensity-stepwise optical pulse OPs is 14 ns, the time width D of the optical pulse OP is 1 ns, 10×log((As+Cs) 2 /Cs 2 ) is 20 dB, the results of simulations to H 1 +H 3 , H 2  and H 4  at 3.05 m-0.2 m, 3.05 m, 3.05 m+0.2 m are shown in  FIGS. 2 and 3  for a case where a strain of 100μ∈ occurs at a position, 3.05 m, of a sensing optical fiber SOF (fiber taken from a bundle and having the entire length of 10 m and no strain) and there are no strains at positions, 3.05 m0.2 m.  FIG. 2A  shows the case of H 1 +H 3 ;  FIG. 2B  shows the case of H 2 ;  FIG. 3A  shows the case of H 4 ; and  FIG. 3B  shows the case of H (=H 1 +H 2 +H 3 +H 4 ). In each figure, horizontal axis represents frequency, solid line represents a case where there is a strain of 100μ∈, and broken line represents a case where there is no strain. Each figure shows only the right half of the Brillouin loss spectrum because the Brillouin loss spectrum is transversely symmetrical with respect to a mean frequency (frequency at the peak of the Brillouin loss spectrum) as a central axis. In other words, vertical axis represents the mean frequency and is the central axis in each figure. It can be understood from  FIG. 2  that only the component of H 2  notably shows a Brillouin frequency shift. 
     As can be understood from Equations 7 to 11, it is difficult to obtain information on local strain from H 3  and H 4  since energy is transferred from the probe light to the pump light over a wide range in H 3  and H 4 . On the other hand, it is easy to obtain information on local strain from H 1  and H 2  since energy is locally transferred from the probe light to the pump light. According to the simulation results, H 3  is smaller than other H 1 , H 2  and H 4  by one digit, and the full width at half minimum of H 1  reaches about 1 GHz as can be seen, for example, from  FIG. 2A . Therefore, as compared to H 2 , H 1  is a broader spectrum curve and it is difficult to find the mean frequency. 
     Accordingly, by such setting as to enable the detection of H 2 , strain and/or temperature can be detected with high accuracy and high spatial resolution using an optical pulse having a short pulse width. 
     Next, the waveform of the intensity-stepwise optical pulse OPs capable of detecting this H 2  is described. 
     Since the probe light does not require the setting of the waveform in advance and an adjustment in conformity with the length of the sensing optical fiber, it needs to be, for example, the intensity-stepwise optical pulse OPs shown in  FIG. 4 . 
     Here in this specification, a continuous leakage light left to have a specified time width Tf before the optical pulse OP of the intensity-stepwise optical pulse OPs is referred to as the optical pulse front light OPf of the intensity-stepwise optical pulse, and a continuous leakage light left to have a specified time width Tb behind the optical pulse OP of the intensity-stepwise optical pulse OPs is referred to as the optical pulse back light OPb. 
     In order to specify the waveform of this intensity-stepwise optical pulse OPs, it is necessary to specify the time width Tf of the optical pulse front light OPf, a pulse width (time width) Tp of the optical pulse OP, the time width Tb of the optical pulse back light OPb, a light intensity P 1  of the optical pulse OP and a light intensity P 2  of the optical pulse front light OPf (light intensity P 2  of the optical pulse back light OPb). 
     First, the time width Tf of the optical pulse front light OPf, the pulse width Tp of the optical pulse OP and the time width Tb of the optical pulse back light OPb are described. 
     The time width Tf of the optical pulse front light OPf may be Tp&lt;Tf≦( 1/35 MHz)=28.57 ns since the full width at half minimum of a Brillouin scattering spectrum is about 35 MHz if an acoustic phonon rises to 90% in accordance with the rise time thereof in the case where a single mode optical fiber for 1300 nm or a single mode optical fiber for 1550 nm generally used at present is, for example, used as the sensing optical fiber SOF. 
     If the time width Tf of the optical pulse front light OPf becomes longer (Tf&gt;28.57 ns), the shape of the Brillouin loss/gain spectrum BSl/g(νd) better approximates to a Lorenz curve, wherefore the value of the Brillouin frequency shift can be more accurately obtained to detect strain and/or temperature with higher accuracy. On the other hand, since the information of Brillouin frequency shifts around a specified position is mixed into a Brillouin frequency shift at the certain position of the sensing optical fiber SOF, the SNR (signal to noise ratio) of the Brillouin frequency shift at this certain position becomes poorer, with the result that strain and/or temperature at this certain position are detected with lower accuracy. In this way, the extension of the time width Tf of the optical pulse front light OPf acts both to improve and to lower the detection accuracy of strain and/or temperature. 
     The pulse width Tp of the optical pulse OP may be 10 ns≧Tp&gt;0 in order to obtain a high spatial resolution of 1 mm or less. The time width Tb of the optical pulse back light OPb is Tb&lt;Tf and the shorter, the better. The time width Tb may be 0. 
     The intensity-stepwise optical pulse OPs needs to be present in the waveform of  FIG. 4 , not in the one of  FIG. 24C  in the sensing optical fiber SOF. To this end, the time width Tf of the optical pulse front light OPf, the time width Tp of the optical pulse OP and the time width Tb of the optical pulse back light OPb are determined within the aforementioned ranges, and the minimum length of the sensing optical fiber SOF in which the intensity-stepwise optical pulse OPs having these determined time widths is present in the waveform of  FIG. 4  is specified. Accordingly, it is no longer necessary, unlike the background art, to manually adjust the probe light in conformity with the length of the sensing optical fiber SOF every time a measurement is conducted by using an optical fiber having a length equal to or longer than the specified minimum length. 
     Next, the light intensity P 1  of the optical pulse OP and the light intensity P 2  of the optical pulse front light OPf (light intensity P 2  of the optical pulse back light OPb) are described. 
     Here, a ratio Prx of the light intensity P 1  of the optical pulse OP to the light intensity P 2  of the optical pulse front light OPf is defined by Equation 12. 
     
       
         
           
             
               
                 
                   
                     
                       
                         Prx 
                         = 
                         
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                   12 
                 
               
             
           
         
       
     
     In order to check a condition to easily detect the aforementioned H 2 , H 2 /(H 1 +H 3 +H 4 ) in relation to the ratio Prx defined by Equation 12 was simulated. It should be noted that H 2 /(H 1 +H 3 +H 4 ) was calculated using peak values of H 1 , H 2 , H 3  and H 4 . As an example, a simulation result based on a logic analysis in the case of using the intensity-stepwise optical pulse OPs in which the time width Tf of the optical pulse front light OPf is 11 ns, the time width Tp of the optical pulse OP is 1 ns and the time width Tb of the optical pulse back light OPb is 0 ns is shown in  FIG. 5 . Horizontal axis of  FIG. 5  represents the ratio Prx shown in unit dB and vertical axis thereof represents H 2 /(H 1 +H 3 +H 4 ). It should be noted that the simulation was conducted, assuming that a strain of 100μ∈ occurred at a position, 3.05 m, of a sensing optical fiber SOF (fiber taken from a bundle and having the entire length of 10 m and no strain) and no strain was occurring at positions, 3.05±0.2 m. 
     As can be seen from  FIG. 5 , a curve representing H 2 /(H 1 +H 3 +H 4 ) in relation to the ratio Prx is a convex high-order curve having a peak at a specified value of the ratio Prx. Since H 2 /(H 1 +H 3 +H 4 ) may be 0.5 or larger for the detection of H 2 , a range of Prx (a≦ratio Prx≦b) where H 2 /(H 1 +H 3 +H 4 ) is 0.5 or larger can be determined by obtaining such a curve. For the detection of H 2  with highest accuracy, the ratio Prx may be set at a value (ratio Prx=c) where H 2 /(H 1 +H 3 +H 4 ) reaches a highest value, i.e. a peak. 
     Accordingly, in the case of using the intensity-stepwise optical pulse OPs having the respective time widths set as above, the ratio Prx of the intensity-stepwise optical pulse OPs may be set within the range where H 2 /(H 1 +H 3 +H 4 ) is about 0.5 or larger for the detection of strain and/or temperature with high accuracy and high spatial resolution and may be set at the value where H 2 /(H 1 +H 3 +H 4 ) reaches the peak for the detection of strain and/or temperature with highest accuracy and highest spatial resolution. 
     From the above description, it can be understood to determine the respective time widths Tf, Tp, Tb of the intensity-stepwise optical pulse OPs within the aforementioned ranges, to simulate H 2 /(H 1 +H 3 +H 4 ) in relation to the ratio Prx defined by Equation 12 in the intensity-stepwise optical pulse OPs having these determined time widths Tf, Tp, Tb in accordance with Equations 8 to 11, and to set the ratio Prx of the intensity-stepwise optical pulse at such a value where H 2 /(H 1 +H 3 +H 4 ) is 0.5 or larger, particularly at a value where H 2 /(H 1 +H 3 +H 4 ) reaches the peak in order to detect strain and/or temperature with high accuracy and high spatial resolution and to detect a change of a minute strain. Further, the usable minimum length of the sensing optical fiber SOF can be specified based on the intensity-stepwise optical pulse OPs having the determined time widths Tf, Tp, Tb. 
     Next, embodiments of the present invention are described with reference to the accompanying drawings in more details. In the respective drawings, the same constructions are identified by the same reference numerals and are not repeatedly described. 
     First Embodiment 
     A distributed optical fiber sensor according to a first embodiment of the present invention is for detecting a strain and/or temperature distribution based on a Brillouin frequency shift by causing an intensity-stepwise optical pulse as probe light to be incident on one end of a sensing optical fiber for detecting strain and/or temperature and causing a continuous light as pump light to be incident on the other end of this sensing optical fiber to receive light attributed to a Brillouin scattering phenomenon occurred in the sensing optical fiber, and by conducting a Brillouin gain spectrum time domain analysis (B Gain -OTDA) or a Brillouin loss spectrum time domain analysis (B Loss -OTDA). Hereinafter, the Brillouin gain spectrum time domain analysis and the Brillouin loss spectrum time domain analysis are abbreviated as Brillouin loss/gain spectrum time domain analyses. In this Brillouin loss/gain spectrum time domain analysis, the light attributed to the Brillouin scattering phenomenon is light subjected to Brillouin attenuation/amplification. 
       FIG. 6  is a block diagram showing the construction of the distributed optical fiber sensor according to the first embodiment;  FIG. 7  is a block diagram showing the construction of a stepwise optical light source in the distributed optical fiber sensor;  FIG. 8  is a block diagram showing the construction of an automatic temperature controller in the distributed optical fiber sensor;  FIG. 9  are a block diagram showing the construction of an automatic frequency controller in the distributed optical fiber sensor and charts showing the principle of operation of the automatic frequency controller;  FIG. 10  are charts showing the generation of an intensity-stepwise optical pulse;  FIG. 11  is a block diagram showing the construction of a light intensity/polarization regulator in the distributed optical fiber sensor;  FIG. 12  is a block diagram showing the construction of a CW light source in the distributed optical fiber sensor of the first embodiment; and  FIG. 13  is a block diagram showing the construction of a light intensity regulator in the distributed optical fiber sensor. 
     In  FIG. 6 , the distributed optical fiber sensor  1  of the first embodiment is provided with a stepwise optical light source  11 , an optical coupler  12 , a light intensity/polarization regulator  13 , an optical circulator  14 , an optical connector  15 , a control processor  16 , a Brillouin time domain detector  17 , a sensing optical fiber  18 , a CW light source  19 , an optical coupler  20 , a light intensity regulator  21  and an optical connector  22 . 
     The stepwise optical light source  11  is a light source device controlled by the control processor  16  and adapted to generate an optical pulse having a stepwise distribution of light intensity to increase toward the center. Such an optical pulse looks as if optical pulses having different light intensities were multiplexed. An output terminal (emitting terminal) of the stepwise optical light source  11  is optically connected with an input terminal (incident terminal) of the optical coupler  12 . 
     Such a stepwise optical light source  11  includes a substrate  101 , a temperature detector  102 , a light emitting element  103 , an optical coupler  104 , a Fabry-Perot etalon filter (hereinafter, abbreviated as “EF”)  105 , a first light receiving element  106 , a second light receiving element  107 , a temperature regulating element  108 , an automatic temperature controller (hereinafter, abbreviated as “ATC”)  109 , an automatic frequency controller (hereinafter, abbreviated as “AFC”)  110 , a first light intensity modulator  111 , a first light-intensity-modulator driving portion  112 , a second light intensity modulator  113 , and a second light-intensity-modulator driving portion  114 . 
     The substrate  101  is a mount on which the temperature detecting element  102 , the light emitting element  103 , the optical coupler  104 , the EF  105 , the first light receiving element  106  and the second light receiving element  107  are placed. 
     The temperature detecting element  102  is a component arranged in the vicinity of the light emitting element  103  for detecting the temperature of light emitting element  103  and, for example, a thermistor for detecting temperature through a change in its resistance value according to a temperature change. In order for the temperature detecting element  102  to more accurately detect the temperature of the light emitting element  103 , the substrate  101  is preferably made of one of metal materials (including alloys) having a high thermal conductivity such as aluminum or copper. Further, the substrate  101  preferably has a larger heat capacity so as to be able to reduce the temperature change. 
     The temperature regulating element  108  is a component for regulating the temperature of the substrate  101  through heat evolution and endotherm and, for example, a thermoelectric converting element such as a Peltier element or Seebeck element. In this embodiment, a Peltier element formed by soldering p-type and n-type thermoelectric semiconductors to a copper electrode is used, and this Peltier element is adhered to a surface of the substrate plate  101  opposite to the one where the light emitting element  103  and the like are arranged. 
     The ATC  109  is a circuit for automatically keeping the temperature of the substrate  101  substantially constant at a specified temperature by controlling the temperature regulating element  108  based on a detection output of the temperature detecting element  102 . 
     For example, as shown in  FIG. 8 , the ATC  109  includes an amplifier  201  for outputting a difference between a detection output of the temperature detecting element  102  and a reference voltage Vref 1  from the control processor  16  inputted thereto, an integrating circuit  202 , including, for example, a low-pass filter, to which the difference output of the amplifier  201  is inputted, a differentiating circuit  203 , including, for example, a high-pass filter, to which the difference output of the amplifier  201  is inputted, a proportioning circuit  204  for outputting a proportion of an integration output of the integrating circuit  202  and a differentiation output of the differentiating circuit  203  inputted thereto, amplifiers  205 ,  206  capable of obtaining outputs for driving a temperature-regulating-element driver  207  in accordance with a plus output and a minus output of the proportioning circuit  204 , and the temperature-regulating-element driver  207 , including a bridge circuit, for generating a drive current of the temperature regulating element  108  in accordance with the proportion output of the proportioning circuit  204 . In other words, the ATC  109  has such a construction as to PID control the temperature regulating element  108  based on the detection output of the temperature detecting element  102 . The reference voltage Vref 1  is set to be the same value as the detection output of the temperature detecting element  102  when the substrate  101  is at the specified temperature. 
     By such a construction, the ATC  109  drives the temperature regulating element  108  so that the temperature regulating element  108  absorbs heat if the temperature of the substrate  101  is higher than the specified temperature while driving the temperature regulating element  108  so that the temperature regulating element  108  evolves heat if the temperature of the substrate  101  is lower than the specified temperature. In this embodiment, a current of +1.4A is supplied to the Peltier element as the temperature regulating element  108  at the time of heat absorption, whereas a current of −0.6A is supplied thereto at the time of heat evolution. The temperature of the substrate  101  is automatically kept substantially constant at the specified temperature by the ATC  109  driving the temperature regulating element  108  in this way. As a result, the temperature of the light emitting element  103  is automatically kept substantially constant at a specified temperature. Thus, temperature dependency can be suppressed if the frequency of the light emitted from the light emitting element  103  is dependent on temperature. The specified temperature is a temperature at which the light emitting element  103  is supposed to oscillate at an oscillating frequency f 0 . Further, the proportion output of the proportioning circuit  204  is outputted to the control processor  16  after being analog-to-digital converted in order to monitor the temperature stability of the substrate  101 . 
     Referring back to  FIG. 7 , the light emitting element  103  is an element capable of emitting light having a narrow line width and a specified frequency and changing an oscillation wavelength (oscillating frequency) by changing the temperature of the element and a drive current and, for example, is a wavelength-variable semiconductor laser (frequency-variable semiconductor laser) such as a DFB laser having a multiple quantum well structure or a Bragg reflection-type laser having a variable wavelength distribution. The frequency of a laser beam emitted from a frequency-variable semiconductor laser is temperature-dependent, but the temperature dependence of the oscillating frequency is suppressed since temperature is automatically held substantially constant by the ATC  109  as described above, therefore the frequency-variable semiconductor laser can stably change the oscillating frequency by means of the drive current. 
     The optical coupler  104  is an optical component for projecting an incident light while dividing it into two light rays and, for example, is a beam splitter such as a half mirror. The EF  105  is a periodic filter having a period transmission frequency characteristic (periodic transmission wavelength characteristic) of periodically changing the intensity of a transmitted light according to a frequency (wavelength) change. An FSR (free spectral range) of the EF  105  is 100 GHz in this embodiment. The first and second light receiving elements  106 ,  107  are photoelectric conversion elements for generating a current corresponding to the light intensity of the received light and outputting the generated current after converting it into a voltage and includes, for example, a photodiode or a resistor. 
     Light rays (laser beams in this embodiment) emitted from the front and rear sides of the light emitting element  103  are incident on the first light intensity modulator  111  and the optical coupler  104 , respectively. The light incident on the optical coupler  104  from the rear side of the light emitting element  103  is divided into two light rays at a specified dividing ratio in the optical coupler  104 , wherein one divided light ray is incident on the second light receiving element  107  and the other one is incident on the first light receiving element  106  via the EF  105 . The first and second light receiving elements  106 ,  107  output voltages corresponding to the light intensities of the incident lights as received light outputs to the AFC  110 , respectively. 
     The AFC  110  is a circuit for controlling the light emitting element  103  based on received light outputs PDv 1 , PDv 2  from the first and second light receiving elements  106 ,  107  to automatically keep the frequency of the light emitted from the light emitting element  103  substantially constant at the specified frequency. 
     For example, as shown in  FIG. 9A , the AFC  110  includes an amplifier  211  for amplifying the received light output PDv 1  of the first light receiving element  106 , an amplifier  212  for amplifying the received light output PDv 2  of the second light receiving element  107 , a dividing circuit  213  for dividing the received light output PDv 1  of the first light receiving element  106  amplified by the amplifier  212  by the received light output PDv 2  of the second light receiving element  107  amplified by the amplifier  212 , an amplifier  216  for amplifying a division output PDv 1 /PDv 2  of the dividing circuit  213  and outputting it to the control processor  16  after applying an analog-to-digital conversion thereto, an amplifier  214  for outputting a difference between the division output PDv 1 /PDv 2  of the dividing circuit  213  and a reference voltage Vref 2  from the control processor  16  inputted thereto, an amplifier  217  for amplifying the difference output of the amplifier  214  and outputting it to the control processor  16  after applying an analog-to-digital conversion thereto, and an amplifier  215  for outputting a difference between the difference output of the amplifier  214  and a reference voltage Vref 3  from the control processor  16  inputted thereto. 
     There is described the operation of the AFC  110  for automatically keeping the frequency of the light emitted from the light emitting element  103  substantially constant at the specified frequency f 0  at which the light emitting element  103  is supposed to oscillate. 
     Since the aforementioned division output PDv 1 /PDv 2  is a value obtained by dividing the light intensity (received light output PDv 1 ) of the light received from the light emitting element  103  via the EF  105  having the periodic transmission frequency characteristic by the light intensity (received light output PDv 2 ) of the light directly received from the light emitting element  103 , it periodically changes according to a frequency change in conformity with the FSR of the EF  105  as seen in a curve “c” shown in  FIG. 9B . 
     It is assumed that a point on the curve “c” corresponding to the oscillating frequency f 0  at which the light emitting element  103  is supposed to oscillate (i.e. frequency f 0  of the light the light emitting element  103  is supposed to emit) is a lock point and the division output PDv 1 /PDv 2  at this point is a lock point value LP 0 . 
     Accordingly, the division output PDv 1 /PDv 2  becomes larger than the lock point value LP 0  if the frequency of the light emitted from the light emitting element  103  exceeds the oscillating frequency f 0  and, conversely, the division output PDv 1 /PDv 2  becomes smaller than the lock point value LP 0  if the frequency of the light emitted from the light emitting element  103  falls below the oscillating frequency f 0 . 
     Thus, the AFC  110  may drive the light emitting element  103  to reduce the frequency of the light emitted from the light emitting element  103  when the division output PDv 1 /PDv 2  is larger than the lock point value LP 0  since the frequency of the light emitted from the light emitting element  103  is higher than the oscillating frequency f 0 . On the other hand, the AFC  110  may drive the light emitting element  103  to increase the frequency of the light emitted from the light emitting element  103  when the division PDv 1 /PDv 2  is smaller than the lock point value LP 0  since the frequency of the light emitted from the light emitting element  103  is lower than the oscillating frequency f 0 . 
     Hence, the aforementioned reference voltage Vref 3  is set at the same value as the division output PDv 1 /PDv 2  if the frequency of the light emitted from the light emitting element  103  is driven at the oscillating frequency f 0 . The aforementioned reference voltage Vref 2  is a reference voltage for fine adjustment and set to precisely conform to the lock point value LP 0 . In this way, the reference voltages Vref 2 , Vref 3  are set to attain the lock point value LP 0  where the frequency of the light emitted form the light emitting element  103  is driven at the oscillating frequency f 0 . 
     By letting the AFC  110  operate with the reference voltages Vref 2  and Vref 3  set in this way, the AFC  110  can drive the light emitting element  103  to compensate for a deviation if the frequency of the light emitted from the light emitting element  103  deviates from the oscillating frequency f 0 . In this embodiment, the AFC  110  regulates an injection current in accordance with the deviation from the oscillating frequency f 0  because a frequency-variable semiconductor laser is used as the light emitting element  103 . Therefore, the frequency-variable semiconductor laser can emit a laser beam at the specified frequency f 0  automatically kept substantially constant. 
     To this end, the optical coupler  104 , the EF  105 , the first and second light receiving elements  106 ,  107  and the AFC  110  construct a so-called wavelength locker for substantially locking the wavelength (frequency) of the light emitted from the light emitting element  103 . 
     The first and second light intensity modulators  111 ,  113  are optical components for modulating the light intensity of the incident light and, for example, are Mach-Zehnder light modulators (hereinafter, abbreviated as “MZ light modulators”) or semiconductor light modulators of the electric field absorption type. 
     The MZ light modulator is such that a light guide, signal electrodes and a grounding electrode are formed on a substrate having an electro-optic effect and made, for example, of lithium niobate, lithium tantalate, lithium niobate and lithium tantalite solid solution. The light guide has intermediate portions thereof branched into two, i.e. first and second waveguide arms at two Y-branched waveguides, thereby constructing a Mach-Zehnder Interferometer. The signal electrodes are respectively formed on these two waveguide arms, and the grounding electrode is so formed on the substrate as to be parallel to the signal electrodes at specified intervals thereto. Light incident on the MZ light modulator propagates along the light guide and is branched into two, i.e. first and second lights, at the first Y-branched waveguide, and the branched lights further propagate along the respective waveguide arms, are joined again at the second Y-branched waveguide and emerge from the light guide. Here, if electrical signals, for example, high-frequency signals are applied to the respective signal electrodes, the speeds of the first and second lights propagating along the first and second waveguide arms change because the refractive indices of the respective waveguide arms change due to the electro-optic effect. Thus, the first and second lights are joined at different phases at the second Y-branched waveguide by setting a specified phase difference between the respective electrical signals, and the joined lights are in modes different from the one of the incident light, e.g. in high-order modes. These joined lights of different modes have the intensity thereof modulated since they cannot propagate in the light guide. The MZ light modulator modulates the light intensity of the incident light by the process of electrical signals→refractive index change→phase change→intensity change. Besides light modulators utilizing the electro-optic effect, there are also magnetooptic light modulators utilizing a magnetooptic effect, acoustooptic light modulators utilizing an acoustooptic effect, and electric-field absorption light modulators utilizing a Franz-Keldysh effect or a quantum-confined Stark effect. 
     The first and second light-intensity-modulator driving portions  112 ,  114  are driver circuits controlled by the control processor  16  for driving the first and second light intensity modulators  111 ,  113  and include, for example, pulse generating circuits for generating voltage pulses to be applied to the first and second light intensity modulators  111 ,  113 , and timing generating circuits for controlling the generation timings of the voltage pulses. The voltage pulses correspond to the aforementioned electrical signals if the first and second light intensity modulators  111 ,  113  are MZ light modulators. 
     An operation of generating an intensity-stepwise optical pulse by the stepwise optical light source  11  thus constructed is described.  FIG. 10A  is a chart showing an output light (position of arrow A shown in  FIG. 7 ) of the light emitting element  103 ,  FIG. 10B  is a chart showing an output light (position of arrow B in  FIG. 7 ) of the first light intensity modulator  111 , and  FIG. 10C  is a chart showing an output light of the second light intensity modulator  113 , i.e. an output light (position of arrow C in  FIG. 7 ) of the stepwise optical light source  11 . 
     As shown in  FIG. 10A , the light emitting element  103  is caused to continuously emit light CW 0 , which is light having a narrow line width, a specified frequency f 0  and a substantially constant light intensity P 1 , by the AFC  110  in accordance with a control of the control processor  16 . The continuous light CW 0  emitted from the light emitting element  103  is incident on the first light intensity modulator  111 . 
     In the background art, the light intensity modulator is normally off and turned on and off at specified timings to generate an optical pulse shown in  FIG. 24B . If the light intensity modulator is a Mach-Zehnder light modulator, voltages applies to the waveguides are regulated, whereby an OFF-state can be realized by setting a phase difference between the light propagating in the first waveguide arm and the one propagating in the second waveguide arm at 180°, and an ON-state can be realized by aligning the phases of the lights propagating in the first and second waveguide arms. 
     As shown in  FIG. 10B , the first light intensity modulator  111  of this embodiment is driven by the first light-intensity-modulator driving portion  112  in accordance with the control of the control processor  16  such that the light intensity of the light emitted from the first light intensity modulator  111  has a weak light intensity P 2  lower than the light intensity P 1  in a normal state, and is also so driven by the first light-intensity-modulator driving portion  112  in accordance with the control of the control processor  16  as to be turned on at timing T 1  and to return to the normal state at timing T 2 . By driving the first light intensity modulator  111  by the first light-intensity-modulator driving portion  112  in this way, the continuous light CW 0  incident on the first light intensity modulator  111  is modulated to have the light intensity P 2  up to timing T 1 , kept at the light intensity P 1  without being modulated between timing T 1  and timing T 2 , and modulated to have the light intensity P 2  again from timing T 2  on. Specifically, by driving the first light intensity modulator  111  by the first light-intensity-modulator driving portion  112  in this way, the first light intensity modulator  111  emits light in which an optical pulse OP having the light intensity P 1  is present in a continuous leakage light CWL having the light intensity P 2  as shown in  FIG. 10B . This light intensity P 1  corresponds to the light intensity P 1  (=(As +Cs) 2 ) in  FIGS. 1 and 4 , whereas the light intensity P 2  corresponds to the light intensity P 2  (=Cs 2 ) of the optical pulse front light OPf and the optical pulse back light OPb shown in  FIGS. 1 and 4 . If the light intensity modulator is a Mach-Zehnder light modulator, the first light-intensity-modulator driving portion  112  regulates a phase difference between the lights propagating in the first and second waveguide arms such that the light intensity is P 2  in a normal state and aligns the phases of the lights propagating in the first and second waveguide arms at timing T 1  and regulates the phase difference between the lights propagating in the first and second waveguide arms such that the light intensity in the normal state is P 2  at timing T 2  by regulating the voltages applied to the first and second waveguide arms of the Mach-Zehnder light modulator. 
     Then, the light comprised of the continuous leakage light CWL and the optical pulse OP and having a waveform shown in  FIG. 10B  emerges from the first light intensity modulator  111  to be incident on the second light intensity modulator  113 . As shown in  FIG. 10C , the second light intensity modulator  113  is so driven by the second light-intensity-modulator driving portion  114  in accordance with the control of the control processor  16  as to be off in a normal state, to be turned on at timing T 3  and to be turned off at timing T 4 . By driving the second light intensity modulator  113  by the second light-intensity-modulator driving portion  114  in this way, the light having the waveform shown in  FIG. 10B  and emerged from the first light intensity modulator  111  to be incident on the second light intensity modulator  113  is modulated to have a light intensity 0 (turned off) up to timing T 3 , kept as it is without being modulated between timing T 3  and T 4  and modulated to have the light intensity 0 (turned off) again from timing T 4  on. In other words, by being driven by the second light-intensity-modulator driving portion  114  in this way, the second light intensity modulator  113  removes the continuous leakage light while leaving only part thereof (optical pulse front light OPf) before the optical pulse OP. In this way, the second light intensity modulator  113  generates an intensity-stepwise optical pulse OPs in which the optical pulse having the light intensity P 1  higher than the light intensity P 2  is present in the optical pulse having the light intensity P 2 , and which has a narrow line width and a stepwise distribution of light intensity change in one step. It should be noted that the intensity-stepwise optical pulse OPs may include the optical pulse back light OPb by regulating the timing T 4 . 
     Here, a time width Tf of the optical pulse front light OPf, a time width Tp of the optical pulse OP and a ratio Prx defined by Equation 12 are set as described above. 
     In this embodiment, the time width Tf of the optical pulse front light OPf is set at, for example, 5 ns, 10 ns, 15 ns and 20 ns. As described above, the pulse width Tp of the optical pulse OP needs to be set equal to or below 10 ns in order to obtain a high spatial resolution of 1 m or higher. However, in order to suppress the inclusion of information on the Brillouin frequency shift around a certain position of the sensing optical fiber  18  into the Brillouin frequency shift at this certain position, the pulse width Tp is preferably adjusted to a sampling time interval in the case of measuring Brillouin loss/gain spectra BSl/g(νd) by means of a Brillouin time domain detector  17 . In this embodiment, the pulse width Tp of the optical pulse OP is set at 5 ns, 2 ns and 1 ns. As described above, the ratio Prx is set based on a simulation result of H 2 /(H 1 +H 3 +H 4 ) in relation to the ratio Prx in accordance with Equations 8 to 11. In this embodiment, the ratio Prx is set at a value to give a peak of H 2 /(H 1 +H 3 +H 4 ) in the simulation result because of being able to obtain a best Lorenz curve in detecting the peak of the Brillouin loss spectrum BSl/g(νd) and to detect strain and/or temperature with high spatial resolution and highest accuracy. Even if the ratio Prx is set at such a value that the value of H 2 /(H 1 +H 3 +H 4 ) in the simulation result is 0.5 or larger, strain and/or temperature can be detected with high accuracy and high spatial resolution. For example, if the time width Tf of the optical pulse front light OPf is 12 ns and the time width Tp of the optical pulse OP is 1 ns, the ratio Prx is set at a value between about 15 dB and about 27 dB and is set at about 21 dB to obtain a best Lorenz curve. 
     It should be noted that an optical amplifier  115  for amplifying the light may be arranged on a light path from the light emitting element  103  to the optical coupler  12  in order to compensate for losses in the first light intensity modulator  111  and the second light intensity modulator  113 . Particularly, in order to amplify while there is only a little amplifier spontaneous emission (ASE), which becomes noise, the optical amplifier  115  is preferably arranged on a light path between the first light intensity modulator  111  and the second light intensity modulator  113  as shown in broken line in  FIG. 7 . The optical amplifier  115  is, for example, an optical fiber amplifier or a semiconductor optical amplifier having a gain to the frequency of the light emitted from the light emitting element  103 . Optical fiber amplifiers include rare-earth added optical fiber amplifiers in which rare-earth elements such as erbium (Er), neodymium (Nd), praseodym (Pr) and thulium (Tm) are added to optical fibers and optical fiber amplifiers utilizing Raman amplification. 
     Referring back to  FIG. 6 , the optical couplers  12 ,  22  are optical components for projecting the incident light while dividing it into two lights and may, for example, employ optical splitter/couplers in the form of micro-optical elements such as half mirrors, those in the form of molten optical fibers and those in the form of light guides. One output terminal of the optical coupler  12  is optically connected with an input terminal of the light intensity/polarization regulator  13 , whereas the other output terminal thereof is optically connected with a first input terminal of the Brillouin time domain detector  17 . 
     The light intensity/polarization regulator  13  is a component controlled by the control processor  16  and adapted to regulate the light intensity of the incident light and project the incident light while randomly changing the plane of polarization of the incident light. An output terminal of the light intensity/polarization regulator  13  is optically connected with a first terminal of the optical circulator  14 . 
     The light intensity/polarization regulator  13  includes, for example, a variable optical attenuator  121  and a polarization controller  122  as shown in  FIG. 11 . The variable optical attenuator  121  is an optical component capable of projecting the incident light while attenuating the light intensity of the incident light, and changing an amount of attenuation. As the variable optical attenuator  121  can be employed, for example, a variable optical attenuator in which an attenuation disk is inserted between an incident light and an emergent light, a metal film whose thickness is continuously changed in a rotating direction is formed on the outer surface of the attenuation disk by deposition, and an amount of attenuation is changed by rotating this attenuation disk, or a variable optical attenuator in which a magnetooptical crystal is inserted between an incident light and an emergent light, a polarizer is inserted at an emergent side of this magnetooptical crystal, and a magnetic field is applied to the magnetooptical crystal to change the intensity of the magnetic field, thereby adjusting an amount of attenuation. The polarization controller  122  is an optical component for projecting the incident light while randomly changing the plane of polarization of the incident light. The light incident on the light intensity/polarization regulator  13  is incident on the polarization controller  122  after having the light intensity thereof regulated to a specified one in the variable optical attenuator  121  by the control of the control processor  16 , and is emitted after having the plane of polarization thereof randomly changed in the polarization controller  122 . 
     The optical circulator  14  is an irreversible optical component which has first to third terminals and in which an incident light and an emergent light change their positions in a circulating manner of the terminal numbers. Specifically, light incident on the first terminal emerges from the second terminal, but not from the third terminal; light incident on the second terminal emerges from the third terminal, but not from the first terminal; and light incident on the third terminal emerges from the first terminal, but not from the second terminal. The optical connectors  15 ,  22  are optical components for optically connecting optical fibers and/or optical components and optical fibers. The second terminal of the optical circulator  14  is optically connected with one end of the sensing optical fiber  18  via the optical connector  15 , and the third terminal of the optical circulator  14  is optically connected with a third input terminal of the Brillouin time domain detector  17 . 
     The CW light source  19  is a device controlled by the control processor  16  and adapted to emit a continuous light CWlump 1  having a substantially constant light intensity in a specified frequency range fr. As shown in  FIG. 12 , the CW light source  19  includes, for example, a substrate  131 , a temperature detecting element  132 , a light emitting element  133 , an optical coupler  134 , an EF  135 , a first light receiving element  136 , a second light receiving element  137 , a temperature regulating element  138 , an ATC  139  and an AFC  140 . Specifically, the CW light source  19  need not convert the continuous light emitted from the light emitting element  133  into an optical pulse having a stepwise distribution of light intensity as the stepwise optical light source  11  does because it is sufficient to emit the continuous light CWpump 1  having a substantially constant light intensity in the specified frequency range fr. Thus, the CW light source  19  does not include the first and second light intensity modulators  111 ,  113  and the first and second light-intensity-modulator driving portions  112 ,  114  of the stepwise optical light source  11 . Since the substrate  131 , the temperature detecting element  132 , the light emitting element  133 , the optical coupler  134 , the EF  135 , the first light receiving element  136 , the second light receiving element  137 , the temperature regulating element  138 , the ATC  139  and the AFC  140  of the CW light source  19  are respectively similar to the substrate  101 , the temperature detecting element  102 , the light emitting element  103 , the optical coupler  104 , the EF  105 , the first light receiving element  106 , the second light receiving element  107 , the temperature regulating element  108 , the ATC  109  and the AFC  110  of the stepwise optical light source  11  including optical connections and electrical connections except that the AFC  140  changes the frequency of the light emitted from the light emitting element  133  in accordance with the control of the control processor  16 , they are not described here. 
     The distributed optical fiber sensor  1  of this embodiment is for measuring the Brillouin loss/gain spectra BSl/(νd) and measuring the Brillouin frequency shift νb by locking the frequency f 0  of the intensity-stepwise optical pulse as probe light and scanning the frequency of the continuous light CWpump 1  as pump light within the specified frequency range fr. 
     To this end, a lock point value LP 0  of the AFC  140  is changed by changing reference voltages Vref 2  and Vref 3  in the AFC  140  in conformity with the frequency at which the control processor  16  scans, and the continuous light CWpump 1  kept at the scanning frequency is emitted. 
     In this embodiment, as shown in  FIG. 9C , the specified frequency range fr is 4 GHz, and the lock point value LP 0  changes within a range of LPL≦LP 0 ≦LPu if LPL, LPu denote lock point values respectively corresponding to a lower limit frequency fL and an upper limit frequency fu of this specified frequency range fr. 
     An output terminal of the CW light source  19  is optically connected with an input terminal of the optical coupler  20 . One output terminal of the optical coupler  20  is optically connected with an input terminal of the light intensity regulator  21 , whereas the other output terminal thereof is optically connected with a second input terminal of the Brillouin time domain detector  17 . 
     Referring back to  FIG. 6 , the light intensity regulator  21  is a component controlled by the control processor  16  and adapted to project the incident light after regulating the light intensity of the incident light. An output terminal of the light intensity regulator  21  is optically connected with the other end of the sensing optical fiber  18  via the optical connector  22 . 
     The light intensity regulator  21  includes, for example, a variable optical attenuator  151  and an optical isolator  152  as shown in  FIG. 13 . The variable optical attenuator  151  is an optical component for projecting the incident light after attenuating the light intensity of the incident light similar to the variable optical attenuator  121 . The optical isolator  152  is an optical component for transmitting light only in one direction from an input terminal to an output terminal, and is constructed, for example, by arranging a Faraday rotor between two polarizers displaced by 45°. The optical isolator  152  functions to prevent the propagation of reflected lights produced at connecting portions of the respective optical components in the distributed optical fiber sensor  1  and the propagation of the probe light to the CW light source  19 . The incident light incident on the light intensity regulator  21  is emitted via the optical isolator  152  after having the light intensity thereof adjusted to a specified one in the variable optical attenuator  151 . 
     It should be noted that the pump light having propagated in the sensing optical fiber  18  need not be incident on the light intensity/polarization regulator  13  and the stepwise optical light source  11  because it is incident on the Brillouin time domain detector  17  via the optical connector  15  and the optical circulator  14 . Here, if an optical coupler is used instead of the optical circulator  14 , an optical isolator or a light filter for blocking the pump light and permitting the transmission of the intensity-stepwise optical pulse OPs is preferably arranged at the output terminal of the light intensity/polarization regulator  13  in order to prevent the incidence of the pump light on the light intensity/polarization regulator  13  and the stepwise optical light source  11 . 
     The sensing optical fiber  18  is an optical fiber serving as a sensor for detecting strain and/or temperature. The probe light is incident on one end of the sensing optical fiber  18 , the pump light is incident on the other end thereof, and the probe light and the pump light subjected to the Brillouin scattering phenomenon emerge from the other and one ends thereof. Here, in the case of measuring strains caused in and/or temperatures of an object to be measured, which can be a structure such as a bridge, a tunnel, a dam or a building or the ground, such a measurement can be conducted by fixing the sensing optical fiber  18  to the object to be measured. 
     The control processor  16  is an electronic circuit for controlling the stepwise optical light source  11 , the light intensity/polarization regulator  13 , the CW light source  19  and the light intensity regulator  21  such that a strain and/or temperature distribution of the sensing optical fiber  18  in the longitudinal direction of the sensing optical fiber  18  can be measured with high accuracy and high spatial resolution by transmitting and receiving signals to and from the Brillouin time domain detector  17 . The control processor  16  includes, for example, a microprocessor, a working memory, and a memory for storing data such as the reference voltage Vref 1  for the ATC  109 , the reference voltages Vref 2 , Vref 3  for the AFC  110 , the reference voltage Vref 1  for the ATC  139 , the reference voltages Vref 2 , Vref 3  for the AFC  140 , the timings T 1 , T 2 , T 3 , T 4  and the ratio Prx. 
     The Brillouin time domain detector  17  controls the respective components of the distributed optical fiber sensor  1 ; determines the Brillouin loss/gain spectra BSl/g(νd) in the respective areas of the sensing optical fiber  18  in the longitudinal direction of the sensing optical fiber  18  by detecting the light attributed to the Brillouin scattering phenomenon and received at specified sampling intervals; determines the Brillouin frequency shifts νd of the respective areas based on the determined Brillouin loss/gain spectra BSl/g(νd) of the respective areas; and detects the strain distribution and/or the temperature distribution of the sensing optical fiber  18  based on the determined the Brillouin frequency shifts νd of the respective areas. The Brillouin time domain detector  17  has the aforementioned ratio Prx, from which a Lorenz curve is obtained, stored therein, detects the light intensity of the intensity-stepwise optical pulse OPs emitted from the stepwise optical light source  11 , and notifies to the control processor  16  so that a ratio P 1 /P 2  of the intensity-stepwise optical pulse becomes the ratio Prx. Further, the Brillouin time domain detector  17  has the frequencies of an optimal probe light and an optimal pump light to obtain a Lorenz curve stored therein, detects the light intensity of the intensity-stepwise optical pulse OPs emitted from the stepwise optical light source and the light intensity of the continuous light CWpump 1  emitted from the CW light source  19 , and notifies to the control processor  16  to regulate the light intensity/polarization regulator  13  and the light intensity regulator  21  in order to attain the light intensity of the optimal probe light and that of the optimal pump light. The Brillouin time domain detector  17  includes an optical switch, a spectrum analyzer and a computer. 
     Since the probe light is an intensity-stepwise optical pulse, the aforementioned ratio Prx from which the Lorenz curve can be obtained can be stored in the Brillouin time domain detector  17  beforehand. Further, since the aforementioned ratio Prx from which the Lorenz curve can be obtained can be stored in the Brillouin time domain detector  17  beforehand, it is not necessary, unlike the background art, to manually adjust the optical pulse in conformity with the length of the sensing optical fiber every time a measurement is conducted. 
     Next, the operation of the distributed optical fiber sensor according to the first embodiment is described. 
       FIG. 14  are charts showing a Brillouin loss/gain spectral distribution and Brillouin loss/gain spectra at distances L 1 , L 2 .  FIG. 14A  shows the Brillouin loss/gain spectral distribution, wherein x-axis represents distance from one end of the sensing optical fiber  18 , y-axis represents frequency, and z-axis represents light intensity.  FIG. 14B  shows Brillouin loss/gain spectra at the distances L 1 , L 2 , wherein x-axis represents frequency and y-axis represent light intensity. In order to facilitate the description, it is assumed that the sensing optical fiber  18  is not strained at the distance L 1  while being strained at the distance L 2 . 
     First, the Brillouin time domain detector  17  is prepared to measure the spectrum of light from the stepwise optical light source  11  via the optical coupler  12  and sends, to the control processor  16 , a signal to cause the stepwise optical light source  11  to emit light. 
     Upon receiving this signal, the control processor  16  applies the given reference voltage Vref 1  for the ATC  109  and the reference voltages Vref 2 , Vref 3  for the AFC  110  to the ATC  109  and the AFC  110 , respectively, thereby causing the light emitting element  103  to emit light and causing the stepwise optical light source  11  to emit light. 
     The light emitted from the stepwise optical light source  11  is incident via the optical coupler  12  on the Brillouin time domain detector  17 , which measures the spectrum of the incident light. The Brillouin time domain detector  17  confirms from this measurement result whether or not the stepwise optical light source  11  emits light of a specified oscillating frequency f 0 . Unless the specified oscillating frequency f 0  is confirmed, the Brillouin time domain detector  17  sends, to the control processor  16 , a signal to adjust the reference voltages Vref 2 , Vref 3  for the AFC  110  so as to set the specified oscillating frequency f 0 . 
     Upon receiving this signal, the control processor  16  adjusts the reference voltages Vref 2 , Vref 3  for the AFC  110  so as to set the specified oscillating frequency f 0 . If the oscillating frequency f of the light emitted from the stepwise optical light source  11  becomes the specified oscillating frequency f 0 , the Brillouin time domain detector  17  notifies the light intensity P 1  of the light to the control processor  16 . 
     The control processor  16  controls the first light intensity modulator  111  and the first light-intensity-modulator driving portion  112  based on the received light intensity P 1  and the stored ratio Prx so that a ratio of the light intensity P 1  of the optical pulse OP to the light intensity P 2  of the optical pulse front light OPf (of the optical pulse front light OPf and the optical pulse back light OPb if there is the optical pulse back light OPb) of the intensity-stepwise optical pulse OPs becomes the aforementioned specified ratio Prx, and controls the second light intensity modulator  113  and the second light-intensity-modulator driving portion  113  so that the intensity-stepwise optical pulse OPs is to be emitted. 
     The intensity-stepwise optical pulse OPs emitted from the stepwise optical light source  11  is incident via the optical coupler  12  on the Brillouin time domain detector  17 , which measures the spectrum of the intensity-stepwise optical pulse OPs. The Brillouin time domain detector  17  confirms from this measurement result whether or not the stepwise optical light source  11  emits the intensity-stepwise optical pulse OPs having the specified ratio Prx. Unless the specified ratio Prx is confirmed, the Brillouin time domain detector  17  sends, to the control processor  16 , a signal to adjust the first light intensity modulator  111  so as to set the specified ratio Prx. 
     Upon receiving this signal, the control processor  16  adjusts the first light intensity modulator  111  so as to set the specified ratio Prx. When the ratio of the intensity-stepwise optical pulse OPs emitted from the stepwise optical light source  11  becomes the specified ratio Prx by repeating such adjustments, the Brillouin time domain detector  17  sends, to the control processor  16 , a signal to adjust an amount of attenuation of the light intensity/polarization regulator  13  from the spectrum measurement result so that the intensity-stepwise optical pulse OPs emitted from the stepwise optical light source  11  is incident on the sensing optical fiber  18  at such an optimal light intensity as to give a Brillouin loss/gain spectrum BSl/g(νd) represented by a Lorenz curve. 
     Upon receiving this signal, the control processor  16  adjusts the amount of attenuation of the light intensity/polarization regulator  13  and sends, to the Brillouin time domain detector  17 , a signal indicating that the adjustment of the amount of attenuation was completed. 
     Upon receiving this signal, the Brillouin time domain detector  17  judges that the stepwise optical light source  11  is ready to emit the specified intensity-stepwise optical pulse OPs and causes the control processor  16  to control the stepwise optical light source  11  so as to prevent the emission of the specified intensity-stepwise optical pulse OPs. 
     Then, the Brillouin time domain detector  17  starts measuring the Brillouin loss/gain spectrum BSl/g(νd). 
     First, the Brillouin time domain detector  17  prepares to measure the spectrum of light from the CW light source  19  via the optical coupler  20 . Then, the Brillouin time domain detector  17  sends, to the control processor  16 , a signal to cause the CW light source  19  to emit a continuous light of a minimum frequency fL in the frequency range to be scanned. 
     Upon receiving this signal, the control processor  16  applies the given reference voltage Vref 1  for the ATC  139  and the reference voltages Vref 2 , Vref 3  for the AFC  140  corresponding to the minimum frequency fL to the ATC  139  and the AFC  140 , thereby causing the light emitting element  133  to emit light and the CW light source  19  to emit a continuous light. 
     The continuous light emitted from the CW light source  19  is incident via the optical coupler  12  on the Brillouin time domain detector  17 , which measures the spectrum of the incident light. The Brillouin time domain detector  17  confirms whether or not the CW light source  11  emits the continuous light of the minimum frequency fL. Unless the minimum frequency fL is confirmed, the Brillouin time domain detector  17  sends, to the control processor  16 , a signal to adjust the reference voltages Vref 2 , Vref 3  for the AFC  110  so as to set the minimum frequency fL. Further, from the spectrum measurement result, the Brillouin time domain detector  17  sends, to the control processor  16 , a signal to adjust an amount of attenuation of the light intensity regulator  21  so that the continuous light emitted from the CW light source  19  is incident on the sensing optical fiber  18  at such an optimal light intensity as to give a Brillouin loss/gain spectrum BSl/g(νd) represented by a Lorenz curve. 
     Upon receiving these signals, the control processor  16  adjusts the reference voltages Vref 2 , Vref 3  for the AFC  140  so as to set the minimum frequency fL. Further, the control processor  16  adjusts the amount of attenuation of the light intensity regulator  21  and sends, to the Brillouin time domain detector  17 , a signal indicating that the adjustment of the amount of attenuation was completed. 
     Upon receiving this signal, the Brillouin time domain detector  17  prepares to measure the spectrum of light attributed to the Brillouin scattering phenomenon and incident via the optical circulator  14  when the frequency of the light emitted from the CW light source  19  becomes the minimum frequency fL and sends, to the control processor  16 , a signal to cause the stepwise optical light source  11  to emit the intensity-stepwise optical pulse OPs. 
     Upon receiving this signal, the control processor  16  causes the stepwise optical light source  11  to emit the intensity-stepwise optical pulse OPs and sends, to the Brillouin time domain detector, a signal indicative of an emission timing. 
     The intensity-stepwise optical pulse OPs emitted from the stepwise optical light source  11  is incident on the light intensity/polarization regulator  13  via the optical coupler  12  and has the light intensity and the plane of polarization changed in the light intensity/polarization regulator  13 , and is incident on one end of the sensing optical fiber  18  as probe light via the optical circulator  14  and the optical connector  15 . The probe light (intensity-stepwise optical pulse OPs) incident on the one end of the sensing optical fiber  18  propagates from the one to the other end of the sensing optical fiber  18  while causing a Brillouin scattering phenomenon with the pump light (continuous light CWpump 1 ) incident on the other end of the sensing optical fiber  22  and propagating in the sensing optical fiber  18 . 
     The light attributed to the Brillouin scattering phenomenon is emitted from the one end of the sensing optical fiber  18  and incident on the Brillouin time domain detector  17  via the optical circulator  14 . The Brillouin time domain detector  17  applies a time domain analysis to the received light attributed to the Brillouin scattering phenomenon based on the emission timing of the intensity-stepwise optical pulse OPs notified from the control processor  16  and measures the distribution of the light intensity of the light attributed to the Brillouin scattering phenomenon in the longitudinal direction of the sensing optical fiber  18 . 
     In this way, the Brillouin time domain detector  17  causes the CW light source  19  to emit the continuous light of the minimum frequency fL as the pump light and causes the stepwise optical light source  11  to emit the intensity-stepwise optical pulse OPs as the probe light, thereby causing the Brillouin scattering phenomenon to occur in the sensing optical fiber  18  by these probe light and pump light; applies a time domain analysis to the light attributed to the Brillouin scattering phenomenon; and measures the distribution of the light intensity of the light attributed to the Brillouin scattering phenomenon in the longitudinal direction of the sensing optical fiber  18 . By conducting a measurement in this way, a curve m 1  shown in  FIG. 14A  which is the distribution of the light intensity of the light attributed to the Brillouin scattering phenomenon and corresponding to the pump light of the minimum frequency fL can be obtained. 
     Here, the degree of interaction between the probe light and the pump light attributed to the Brillouin scattering phenomenon depends on the relative relationship between the plane of polarization of the probe light and that of the pump light. Since the plane of polarization of the intensity-stepwise optical pulse OPs is randomly changed in the light intensity/polarization regulator  13  for each measurement in the distributed optical fiber sensor  1  according to this embodiment, this dependency can be substantially eliminated by adopting an average value obtained by repeating the measurement of the distribution of the light intensity of the light attributed to the Brillouin scattering phenomenon and corresponding to the pump light of the minimum frequency fL a plurality of times. Thus, the distribution m 1  of the light intensity of the light attributed to the Brillouin scattering phenomenon can be accurately obtained. In this embodiment, for example, the measurement is repeated 500 times or 1000 times. 
     Upon completing the measurement of the distribution m 1  of the light attributed to the Brillouin scattering phenomenon and corresponding to the pump light of the minimum frequency fL, the Brillouin time domain detector  17  first causes the CW light source  19  to emit a continuous light as pump light of a next frequency and causes the stepwise optical light source  11  to emit the intensity-stepwise optical pulse OPs as probe light by the same operation as above in order to measure the distribution of the light intensity of light attributed to the Brillouin scattering phenomenon and corresponding to the pump light of the next frequency. Then, the Brillouin time domain detector  17  applies a time domain analysis to the light attributed to the Brillouin scattering phenomenon, emerged from the one end of the sensing optical fiber  18  and incident on the Brillouin time domain detector  17  via the optical circulator  14  based on the emission timing of the intensity-stepwise optical pulse OPs, and measures a distribution m 2  of the light intensity of the light attributed to the Brillouin scattering phenomenon in the longitudinal direction of the sensing optical fiber  18  for the next frequency. 
     Upon completing the measurement of the distribution m 2  of the light attributed to the Brillouin scattering phenomenon and corresponding to the pump light of the next frequency, the Brillouin time domain detector  17  successively changes the frequency of the pump light up to a maximum fu and measures distributions m 3 , m 4 , . . . of the light intensity of the light attributed to the Brillouin scattering phenomenon and corresponding to the pump of the corresponding frequencies in the same manner as above. Here, a distribution mn represents the distribution m of the light intensity of the light attributed to the Brillouin scattering phenomenon and corresponding to the pump light of the n-th frequency in the scanning frequency range. 
     By measuring in this way, the distributions m 1 , m 2 , m 3 , . . . , mn of the light intensities of the lights attributed to the Brillouin scattering phenomenon in the longitudinal direction of the sensing optical fiber  18  at the respective frequencies of the scanning frequency range can be obtained with high accuracy and high spatial resolution as shown in  FIG. 14A , with the result that the Brillouin loss/gain spectra BSl/g(νd) in the respective longitudinal area sections of the sensing optical fiber  18  can be obtained with high accuracy and high spatial resolution. 
     Further, the Brillouin time domain detector  17  calculates the Brillouin frequency shift νb in each longitudinal area section of the sensing optical fiber  18  with high accuracy and high spatial resolution by calculating a difference between a frequency corresponding and a peak of the Brillouin loss/gain spectrum BSl/g(νd) in each longitudinal area section of the sensing optical fiber  18  to a frequency corresponding to a peak of the Brillouin loss/gain spectrum BSl/g(νd) in a section where the sensing optical fiber  18  is not strained. 
     If is, for example, assumed that the sensing optical fiber  18  is not strained in a section at the distance L 1  from the one end thereof while being strained in a section at the distance L 2  as shown in  FIGS. 14A and 14B . The Brillouin loss/gain spectrum BSl/g(νd) at the distance L 1  is represented by a solid-line curve “e” in  FIG. 14B , whereas the Brillouin loss/gain spectrum BSl/g(νd) at the distance L 2  is represented by a broken-line curve “f” in  FIG. 14B . In this case, the Brillouin frequency shift νb=νb 2 −νb 1  is calculated by subtracting a difference between a frequency νb 1  corresponding to the peak of the Brillouin loss/gain spectrum Brillouin loss/gain spectrum BSl/g(νd) at the distance L 1  from a frequency vb 2  corresponding to the peak of the Brillouin loss/gain spectrum BSl/g(νd) at the distance L 2 . 
     Then, the Brillouin time domain detector  17  determines strains and/or temperatures in the respective longitudinal area sections of the sensing optical fiber  18  with high accuracy and high spatial resolution from the Brillouin frequency shifts νb in the respective area sections. The distribution(s) of the determined strains and/or temperatures in the respective longitudinal area sections of the sensing optical fiber  18  are presented on an unillustrated output device such as a CRT display device, an XY plotter or a printer. 
     Since the intensity-stepwise optical pulse OPs is used as the probe light in the distributed optical fiber sensor  1  according to the first embodiment as described above, the aforementioned ratio Prx from which the Lorenz curve can be obtained can be stored in the Brillouin time domain detector  17  beforehand, wherefore it is not necessary, unlike the background art, to manually adjust the optical pulse in conformity with the length of the sensing optical fiber  18  every time a measurement is conducted. Accordingly, the distributed optical fiber sensor  1  can be produced as an industrial product. Further, since the optimal ratio Prx can be set as a result of a logic analysis, the Brillouin loss/gain spectrum BSl/g(νd) can be presented by a Lorenz curve, wherefore strain caused in and/or temperature of the sensing optical fiber  18  can be measured with high accuracy and high spatial resolution. 
     Second Embodiment 
     A distributed optical fiber sensor according to a second embodiment of the present invention is for detecting strain and/or temperature based on Brillouin frequency shifts by causing a probe light and a pump light to be incident on one end of a sensing optical fiber for detecting strains and/or temperatures, receiving the pump light subjected to the action of a Brillouin scattering phenomenon in the sensing optical fiber, and applying a Brillouin gain spectrum time domain reflection analysis (B Gain -OTDR, Brillouin gain optical time domain reflectometer) or a Brillouin loss spectrum time domain reflection analysis (B Loss -OTDR, Brillouin loss optical time domain reflectometer). Hereinafter, the Brillouin gain spectrum time domain reflection analysis or the Brillouin loss spectrum time domain reflection analysis is abbreviated as a Brillouin loss/gain spectrum time domain reflection analysis. In this Brillouin loss/gain spectrum time domain reflection analysis, light attributed to the Brillouin scattering phenomenon is a Brillouin scattering light. 
     First, the construction of the distributed optical fiber sensor according to the second embodiment is described.  FIG. 15  is a block diagram showing the construction of the distributed optical fiber sensor according to the second embodiment, and  FIG. 16  is a block diagram showing the construction of a CW light source of the distributed optical fiber sensor according to the second embodiment. 
     In  FIG. 15 , the distributed optical fiber sensor  2  according to the second embodiment is provided with a stepwise optical light source  11 , an optical coupler  12 , a light intensity/polarization regulator  13 , an optical circulator  14 , an optical coupler  33 , an optical connector  15 , a control processor  31 , a Brillouin time domain detector  17 , a sensing optical fiber  18 , a CW light source  32 , an optical coupler  20 , and a light intensity regulator  21 . 
     An output terminal of the stepwise optical light source  11  is optically connected with an input terminal of the optical coupler  12 . One output terminal of the optical coupler  12  is optically connected with an input terminal of the light intensity/polarization regulator  13 . An output terminal of the light intensity/polarization regulator  13  is optically connected with a first terminal of the optical circulator  14 . A second terminal of the optical circulator  14  is optically connected with one input terminal of the optical coupler  33 . An output terminal of the optical coupler  33  is optically connected with one end of the sensing optical fiber  18  via the optical connector  15 . 
     Further, an output terminal of the CW light source  32  is optically connected with an input terminal of the optical coupler  20 . One output terminal of the optical coupler  20  is optically connected with an input terminal of the light intensity modulator  21 . An output terminal of the light intensity modulator  21  is optically connected with the other input terminal of the optical coupler  33 . 
     The other output terminal of the optical coupler  12  is optically connected with a first input terminal of the Brillouin time domain detector  31 , the other output terminal of the optical coupler  20  is optically connected with a second input terminal of the Brillouin time domain detector  31 , and a third terminal of the optical circulator  14  is optically connected with a third input terminal of the Brillouin time domain detector  31 . 
     These stepwise optical light source  11 , optical coupler  12 , light intensity/polarization regulator  13 , optical circulator  14 , optical connector  15 , Brillouin time domain detector  17 , sensing optical fiber  18 , optical coupler  20  and light intensity regulator  21  are not described since they are the same as in the first embodiment. 
     The optical coupler  33  is an optical component for projecting an incident light while dividing it into two and is similar to the optical couplers  12 ,  22 . 
     The CW light source  32  is a device controlled by the control processor  16  and adapted to emit a substantially constant continuous light CWpump 2  in a specified frequency range fr. The continuous light CWpump 2  is emitted at a specified timing only for a specified period. As shown in  FIG. 16 , the CW light source  32  includes, for example, a substrate  131 , a temperature detecting element  132 , a light emitting element  133 , an optical coupler  134 , an EF  135 , a first light receiving element  136 , a second light receiving element  137 , a temperature regulating element  138 , an ATC  139 , an AFC  140 , a third light intensity modulator  161  and a third light-intensity-modulator driving portion  162 . Specifically, the third light intensity modulator  161  and the third light-intensity-modulator driving portion  162  are added to the CW light source  19  shown in  FIG. 12  since the CW light source  32  needs to emit the continuous light pump  2  at the specified timing only for the specified period. Since the substrate  131 , the temperature detecting element  132 , the light emitting element  133 , the optical coupler  134 , the EF  135 , the first light receiving element  136 , the second light receiving element  137 , the temperature regulating element  138 , the ATC  139  and the AFC  140  of the CW light source  32  are respectively similar to the substrate  101 , the temperature detecting element  102 , the light emitting element  103 , the optical coupler  104 , the EF  105 , the first light receiving element  106 , the second light receiving element  107 , the temperature regulating element  108 , the ATC  109  and the AFC  110  of the stepwise optical light source  11  including the optical connections and the electrical connections except that the AFC  140  changes the oscillating frequency of the light emitting element  133  in accordance with the control of the control processor  16 , no description is given thereon. 
     The third light intensity regulator  161  is an optical component for modulating the light intensity of an incident light similar to the first and second light intensity regulators  111 ,  113 . The third light-intensity-modulator driving portion  162  is a driver circuit controlled by the control processor  16  for driving the third light intensity regulator  161  similar to the first and second light-intensity-modulator driving portions  112 ,  114 . Light from the light emitting element  133  is incident on the third light intensity regulator  161 , which is turned on and off to emit and stop the emission of the continuous light CWpump 2  from the light emitting element  133  in accordance with the control of the third light-intensity-modulator driving portion  162 . The light emitted from the third light intensity regulator  161  when the third light intensity regulator  161  is on is incident on the optical coupler  20  as an output light of the CW light source  32 . The CW light source  32  emits the continuous light CWpump 2  at the specified timing only for the specified period by operating in this way. 
     It should be noted that an optical amplifier  163  for amplifying the light to compensate for a loss of the third light intensity regulator  161  may be arranged behind the third light intensity regulator  161  as shown in broken line in  FIG. 16 . 
     The control processor  31  is an electronic circuit for controlling the stepwise optical light source  11 , the light intensity/polarization regulator  13 , the CW light source  19  and the light intensity regulator  21  such that the pump light and the probe light are caused to propagate in the sensing optical fiber  18  in such a manner that the probe light is incident on one end of the sensing optical fiber  19  at a timing when the pump light having been incident on the one end of the sensing optical fiber  18  is reflected at the other end, and a strain and/or temperature distribution of the sensing optical fiber  18  in the longitudinal direction of the sensing optical fiber  18  can be measured with high spatial resolution by inputting and outputting signals to and from the Brillouin time domain detector  17 . The control processor  31  includes, for example, a microprocessor, a working memory, and a memory for storing data. 
     It should be noted that a mirror portion for reflecting the incident light may be further provided at the other end of the sensing optical fiber  18  to efficiently reflect the pump light having propagated from the one end to the other end of the sensing optical fiber  18  without losing its power. Alternatively, the other end of the sensing optical fiber  18  may be mirror-finished. 
     Next, the operation of the distributed optical fiber sensor according to the second embodiment is described. 
     The distributed optical fiber sensor  2  according to the second embodiment causes the stepwise optical light source  11  to emit an intensity-stepwise optical pulse OPs having a specified ratio Prx by an operation similar to that of the distributed optical fiber sensor  1  according to the first embodiment. The distributed optical fiber sensor  2  adjusts an amount of attenuation of the light intensity/polarization modulator  13  such that the intensity-stepwise optical pulse OPs is incident as probe light on the sensing optical fiber  18  at such an optimal light intensity as to give a Brillouin loss/gain spectrum BSl/g(νd) represented by a Lorenz curve by an operation similar to that of the distributed optical fiber sensor  1  according to the first embodiment. Upon completing the adjustment, the distributed optical fiber sensor  2  according to the second embodiment causes the control processor to control the stepwise optical light source  11  to stop the emission of the specified intensity-stepwise optical pulse OPs. 
     Then, the distributed optical fiber sensor  2  according to the second embodiment causes the CW light source  11  and the light intensity regulator  21  to adjust the light intensity of the pump light to such an optimal light intensity as to give a Brillouin loss/gain spectrum BSl/g(νd) represented by a Lorenz curve by an operation similar to that of the distributed optical fiber sensor  1  according to the first embodiment. Upon completing the adjustment, the distributed optical fiber sensor  2  according to the second embodiment causes the control processor  16  to control the CW light source  19  to stop the emission of the continuous light CWpump 2 . 
     Then, the distributed optical fiber sensor  2  according to the second embodiment starts measuring the Brillouin loss/gain spectra BSl/g(νd) to obtain distributions m 1 , m 2 , m 3 , m 4 , . . . , mn of the light intensities of lights attributed to the Brillouin scattering phenomenon in the longitudinal direction of the sensing optical fiber  18  at the respective frequencies of a scanning frequency range with high accuracy and high spatial resolution. 
     Here, since the distributed optical fiber sensor  2  according to the second embodiment applies the Brillouin loss/gain spectrum time domain reflection analysis, the pump light and the probe light are caused to propagate in the sensing optical fiber  18  such that the probe light is incident on the one end of the sensing optical fiber  18  at a timing when the pump light having been incident on the one end of the sensing optical fiber  18  is reflected at the other end at the respective frequencies of the scanning frequency range fr, and the lights attributed to the Brillouin scattering phenomenon are analyzed by the Brillouin scattering phenomenon detector  17 . 
     Based on the distributions m 1 , m 2 , m 3 , m 4 , . . . , mn of the light intensities of lights attributed to the Brillouin scattering phenomenon in the longitudinal direction of the sensing optical fiber  18  at the respective frequencies of a scanning frequency range, which were thus obtained with high accuracy and high spatial resolution, the distributed optical fiber sensor  2  according to the second embodiment determines strains and/or temperatures in the respective longitudinal area sections of the sensing optical fiber  18  with high accuracy and high spatial resolution and presents them on an unillustrated output device by an operation similar to that of the distributed optical fiber sensor  1  according to the first embodiment. 
     Since the distributed optical fiber sensor  2  according to the second embodiment uses the intensity-stepwise optical pulse as the probe light similar to the distributed optical fiber sensor  1  according to the first embodiment, the aforementioned ratio Prx from which a Lorenz curve can be obtained can be stored in the Brillouin time domain detector  17  beforehand, wherefore it is not necessary, unlike the background art, to manually adjust the optical pulse in conformity with the length of the sensing optical fiber  18  every time a measurement is conducted. Accordingly, the distributed optical fiber sensor  2  can be produced as an industrial product. Further, since the optimal ratio Prx can be set as a result of a logic analysis, the Brillouin loss/gain spectrum BSl/g(νd) can be presented by a Lorenz curve, wherefore strain caused in and/or temperature of the sensing optical fiber  18  can be measured with high accuracy and high spatial resolution. 
     Third Embodiment 
     A distributed optical fiber sensor according to a third embodiment of the present invention can singly apply a Brillouin gain spectrum time domain reflection analysis and a Brillouin loss spectrum time domain reflection analysis. 
     First, the distributed optical fiber sensor according to the third embodiment is described.  FIG. 17  is a block diagram showing the construction of the distributed optical fiber sensor according to the third embodiment. 
     In  FIG. 17 , the distributed optical fiber sensor  3  according to the third embodiment is provided with a stepwise optical light source  11 , an optical coupler  12 , a light intensity/polarization regulator  13 , an optical circulator  14 , an optical coupler  33 , an optical connector  15 , a control processor  41 , a Brillouin time domain detector  17 , a sensing optical fiber  18 , a CW light source  32 , an optical coupler  20 , a light intensity regulator  21 , an optical switch  42  and an optical connector  22 . 
     An output terminal of the stepwise optical light source  11  is optically connected with an input terminal of the optical coupler  12 . One output terminal of the optical coupler  12  is optically connected with an input terminal of the light intensity/polarization regulator  13 . An output terminal of the light intensity/polarization regulator  13  is optically connected with a first terminal of the optical circulator  14 . A second terminal of the optical circulator  14  is optically connected with one input terminal of the optical coupler  33 . An output terminal of the optical coupler  33  is optically connected with one end of the sensing optical fiber  18  via the optical connector  15 . 
     Further, an output terminal of the CW light source  32  is optically connected with an input terminal of the optical coupler  20 . One output terminal of the optical coupler  20  is optically connected with an input terminal of the light intensity modulator  21 . An output terminal of the light intensity modulator  21  is optically connected with an input terminal of the optical switch  42 . One output terminal of the optical switch  42  is optically connected with the other input terminal of the optical coupler  33 , and the other output terminal thereof is optically connected with the other end of the sensing optical fiber  18  via the connector  24 . 
     The other output terminal of the optical coupler  12  is optically connected with a first input terminal of the Brillouin time domain detector  17 , the other output terminal of the optical coupler  20  is optically connected with a second input terminal of the Brillouin time domain detector  17 , and a third terminal of the optical circulator  14  is optically connected with a third input terminal of the Brillouin time domain detector  17 . 
     An output of the Brillouin time domain detector  17  is inputted to the control processor  41 , which controls the stepwise optical light source  11 , the light intensity/polarization regulator  13 , the CW light source  32 , the light intensity regulator  21  and the optical switch  42 , respectively. 
     These stepwise optical light source  11 , optical coupler  12 , light intensity/polarization regulator  13 , optical circulator  14 , optical connector  15 , Brillouin time domain detector  17 , sensing optical fiber  18 , optical coupler  20 , light intensity regulator  21  and optical connector  22  are not described since they are the same as in the first embodiment. Further, the CW light source  32  and the optical coupler  33  are not described since they are the same as in the second embodiment. 
     The optical switch  42  is a 1×2 optical switch having one input and two outputs, wherein light incident on the input terminal is selectively emitted from either one of the two output terminals in accordance with the control of the control processor  41 . 
     A mechanical optical switch or a light guide switch can be, for example, used as the light switch  42 . The mechanical optical switch is a micro-optical element such as a prism, a rod lens or a mirror or an optical component for switching a light path by moving or turning the optical fiber itself or may be an optical switch of the opto micro electromechanical system for mechanically moving a refractive index matching fluid filled between light guides using a semiconductor fine processing technology or moving a mirror by means of an electrostatic actuator. The light guide switch is, for example, an optical component in which a Mach-Zehnder interference is constituted by a light guide, electric fields are applied to the respective light guide arms to change the refractive indices of the respective light guide arms, thereby switching a light path. There are also known semiconductor optical switches utilizing a change in refractive index by the carrier injection or semiconductor optical switches of the delivery and convergence type using an optical semiconductor amplifier as an on/off gate. 
     The control processor  41  is an electronic circuit for controlling the stepwise optical light source  11 , the light intensity/polarization regulator  13 , the CW light source  19 , the light intensity regulator  21  and the optical switch  42  such that the pump light and the probe light are caused to propagate in the sensing optical fiber  18  in such a manner that a strain and/or temperature distribution of the sensing optical fiber  18  in the longitudinal direction of the sensing optical fiber  18  can be measured at high spatial resolution through the Brillouin loss/gain spectrum time domain analysis and the Brillouin loss/gain spectrum time domain reflection analysis by inputting and outputting signals to and from the Brillouin time domain detector  17 . The control processor  31  includes, for example, a microprocessor, a working memory, and a memory for storing data. 
     Next, the operation of the distributed optical fiber sensor according to the third embodiment is described. 
     First, in the case of applying the Brillouin loss/gain spectrum time domain analysis using the distributed optical fiber sensor  3  according to the third embodiment, the control processor  41  controls the optical switch  42  such that light incident on the input terminal of the optical switch  42  emerges from the output terminal thereof optically connected with the other end of the sensing optical fiber  18  via the optical connector  22 . The operation of the distributed optical fiber sensor  3  according to the third embodiment is not described because it is similar to that of the distributed optical fiber sensor according to the first embodiment. 
     Next, in the case of applying the Brillouin loss/gain spectrum time domain reflection analysis using the distributed optical fiber sensor  3  according to the third embodiment, the control processor  41  controls the optical switch  42  such that light incident on the input terminal of the optical switch  42  emerges from the output terminal thereof optically connected with the one end of the sensing optical fiber  18  via the optical coupler  33  and the optical connector  15 . The operation of the distributed optical fiber sensor  3  according to the third embodiment is not described because it is similar to that of the distributed optical fiber sensor according to the second embodiment. 
     In this case, the pump light subjected to the interaction of the probe light and the Brillouin scattering phenomenon in the sensing optical fiber  18  and emerged from the one end of the sensing optical fiber  18  is divided into two by the optical coupler  33 , one of which is incident on the Brillouin time domain detector  17  via the optical circulator  14 . Specifically, the power of the pump light subjected to the interaction is lost in the optical coupler  33 . In order to compensate for this loss, an optical amplifier may be further provided on a light path from the optical circulator  14  to the Brillouin time domain detector  17 . Alternatively, the Brillouin time domain detector  17  may include an amplifier so as to amplify the pump light after being received by the light receiving element to be photoelectrically converted. 
     As described above, the distributed optical fiber sensor  3  according to the third embodiment uses the intensity-stepwise optical pulse as the probe light similar to the distributed optical fiber sensors  1 ,  2  according to the first and second embodiment. Thus, the aforementioned ratio Prx from which a Lorenz curve can be obtained can be stored in the Brillouin time domain detector  17  beforehand, wherefore it is not necessary, unlike the background art, to manually adjust the optical pulse in conformity with the length of the sensing optical fiber  18  every time a measurement is conducted. 
     Accordingly, the distributed optical fiber sensor  3  can be produced as an industrial product. Further, since the optimal ratio Prx can be set as a result of a logic analysis, the Brillouin loss/gain spectrum BSl/g(νd) can be presented by a Lorenz curve, wherefore strain caused in and/or temperature of the sensing optical fiber  18  can be measured with high accuracy and high spatial resolution. 
     Fourth Embodiment 
     The distributed optical fiber sensors  1 ,  2 ,  3  according to the first to third embodiments are for measuring strain caused in and/or temperature of the sensing optical fiber  18  with high accuracy and high spatial resolution without manually adjusting the optical pulse in accordance with the length of the sensing optical fiber  18 , by using the intensity-stepwise optical pulse OPs as the probe light. A distributed optical fiber sensor according to a fourth embodiment is such that distributions M 1 , M 2  of light intensities of lights attributed to the Brillouin scattering phenomenon in a pulsed light having the light intensity P 2  of the optical pulse front light OPf and the intensity-stepwise optical pulse OPs are respectively measured by the constructions of the distributed optical fiber sensors  1 ,  2 ,  3  according to the first to third embodiments, a difference M 2 −M 1  between the distributions M 1 , M 2  of the light intensities of these measured lights attributed to the Brillouin scattering phenomenon is calculated, and Brillouin loss/gain spectra BSl/g(νd) in the respective longitudinal area sections of the sensing optical fiber  18  are determined, thereby improving errors created in the detection results of the distributed optical fiber sensors  1 ,  2 ,  3  according to the first to third embodiments, particularly errors created in the vicinity of 1/(Tp+Tf). 
     Since the distributed optical fiber sensor according to the fourth embodiment is capable of improving errors by being thus constructed, the time width Tf of the optical pulse front light OPf of the intensity-stepwise optical pulse OPs can be set beyond the range Tp≦Tf≦28.57 ns, e.g. Tp=50 ns in the distributed optical fiber sensor according to the fourth embodiment although the time width Tf of the optical pulse front light Opf is set within the range Tp&lt;Tf≦( 1/35 MHz)=28.57 ns in the distributed optical fiber sensors  1 ,  2 ,  3  according to the first to third embodiments. Of course, it goes without saying that the time width Tf of the optical pulse front light OPf within the range Tp&lt;Tf&lt;28.57 nm can be used. Further, since being capable of improving errors by being thus constructed, the distributed optical fiber sensor according to the fourth embodiment is suitable in the case where minute strains of 200μ∈ or smaller are detected when strains are equally distributed in a wide range. 
     The construction of the distributed optical fiber sensor according to the fourth embodiment is not described since it is the same as in the first to third embodiments except that the Brillouin loss/gain spectra BSl/g(νd) in the respective longitudinal area sections of the sensing optical fiber  18  can be determined by an operation of the Brillouin time domain detector  17  as described below. 
       FIG. 18  are graphs showing an intensity-stepwise optical pulse OPs and a leakage optical pulse in the fourth embodiment, and  FIG. 19  is a flow chart showing the operation of the distributed optical fiber sensor according to the fourth embodiment. 
     In  FIG. 19 , in the fourth embodiment, the Brillouin time domain detector  17  causes a pump light in the form of a continuous light to be emitted and causes the stepwise optical light source  11  to emit a leakage optical pulse OPL shown in  FIG. 18B  as probe light (Step S 101 ), measures a distribution M 1  of the light intensity of light attributed to the Brillouin scattering phenomenon in the longitudinal direction of the sensing optical fiber  18  and saves this measurement result (Step S 102 ). 
     In the case of using an optical pulse OP having a time width Tp and a light intensity P 1  and the intensity-stepwise optical pulse OPs comprised of an optical pulse front light OPf having a time width Tf and a light intensity P 2  as an intensity-stepwise optical pulse, a leakage optical pulse OPL is an optical pulse having a time width (Tp+Tf) and the light intensity P 2  as shown in  FIG. 18B . In other words, the time width of the leakage optical pulse OPL is that of the intensity-stepwise optical pulse OPs and the light intensity thereof is that of the optical pulse front light OPf having a minimum light intensity in the intensity-stepwise optical pulse OPs. 
     Referring back to  FIG. 19 , the Brillouin time domain detector  17  subsequently causes a pump light in the form of a continuous light to be emitted and causes the stepwise optical light source  11  to emit the intensity-stepwise optical pulse OPs shown in  FIG. 18A  as probe light (Step S 103 ), and measures a distribution M 2  of the light intensity of light attributed to the Brillouin scattering phenomenon in the longitudinal direction of the sensing optical fiber  18  and saves this measurement result (Step S 104 ). 
     Subsequently, the Brillouin time domain detector  17  calculates a difference M 2 −M 1  between the distribution M 1  measured in Step S 102  and the distribution M 2  measured in Step S 104  and saves this difference M 2 −M 1  (Step S 105 ). This difference M 2 −M 1  corresponds to “m” in the first embodiment. 
     Such operations in Steps S 101  to S 105  are carried out at the respective frequencies of the scanning frequency range (Step S 106 ) to determine distributions m 1 , m 2 , m 3 , m 4 , . . . , mn of the light intensities of the lights attributed to the Brillouin scattering phenomenon in the longitudinal direction of the sensing optical fiber  18  and to determine the Brillouin loss/gain spectra BSl/g(νd) in the respective longitudinal area sections of the sensing optical fiber  18  from the determined distributions, and a strain distribution and/or a temperature distribution in the longitudinal direction of the sensing optical fiber  18  are determined based on the determined Brillouin loss/gain spectra BSl/g(νd) (Step S 107 ). 
     By operating in this way, the distributed optical fiber sensor according to the fourth embodiment is capable of improving errors better than the distributed optical fiber sensors  1 ,  2 ,  3  according to the first to third embodiments and measuring strain caused in and/or temperature of the distributed optical fiber sensor  18  with high accuracy and high spatial resolution even if the intensity-stepwise optical pulse OPs whose optical pulse front light OPf has the time width Tf beyond the range Tp&lt;Tf≦28.57 nm. The distributed optical fiber sensor according to the fourth embodiment is also capable of detecting minute strains of 200μ∈ or smaller in the case where strains are equally distributed in a wide range. Further, in the fourth embodiment as well, it goes without saying that it is not necessary, unlike the background art, to manually adjust the optical pulse in conformity with the length of the sensing optical fiber  18  every time a measurement is conducted. Accordingly, the distributed optical fiber sensor  3  can be produced as an industrial product. 
     Fifth Embodiment 
     Although the distributed optical fiber sensor according to the fourth embodiment is for improving errors created in the detection results of the distributed optical fiber sensors  1 ,  2 ,  3  according to the first to third embodiments by using the pulse light having the light intensity P 2  of the optical pulsed front light OPf and the intensity-stepwise optical pulse OPs as the probe light in the constructions of the distributed optical fiber sensors  1 ,  2 ,  3  according to the first to third embodiments, a distributed optical fiber sensor according to a fifth embodiment is for improving errors created in the detection results of the distributed optical fiber sensors  1 ,  2 ,  3  according to the first to third embodiments by connecting a reference optical fiber with one end of the sensing optical fiber  18  and correcting the mean frequencies of the Brillouin loss/gain spectra BSl/g(νd) as measurement results based on a reference value obtained from the reference optical fiber in the constructions of the distributed optical fiber sensors  1 ,  2 ,  3  according to the first to third embodiments. 
       FIG. 20  is a block diagram showing the construction of the distributed optical fiber sensor according to the fifth embodiment corresponding to the distributed optical fiber sensor according to the first embodiment. 
     In  FIG. 20 , the construction of the distributed optical fiber sensor  5  according to the fifth embodiment is not described since being the same as those of the first to third embodiments except that a reference optical fiber  51  is connected with one end of the sensing optical fiber  18 , i.e. an end where a pump light is incident in an example shown in  FIG. 20 , and a Brillouin time domain detector  52  in which a correction-value conversion equation to be described later is stored and which calculates a correction value based on a reference value obtained from the reference optical fiber  51  using the correction-value conversion equation by operating as described later and corrects the mean frequencies of the Brillouin loss/gain spectra BSl/g(νd) as measurement results using this correction value is used in place of the Brillouin time domain detector  17 . 
     The reference optical fiber  51  is an optical fiber made of the same material as the sensing optical fiber  18  and having a length corresponding to the spatial resolution of the distributed optical fiber sensor  5 . Strains sufficiently larger than a strain expressed by an inverse of a time width (Tp+Pf) of an intensity-stepwise optical pulse OPs used in the distributed optical fiber sensor  5 , for example, a strain expressed by 2/(Tp+Tf), the one expressed by expressed by 3/(Tp+Tf) and the one expressed by 4/(Tp+Tf) are given to the reference optical fiber  51 . 
       FIG. 21  is a flow chart showing the operation of the distributed optical fiber sensor according to the fifth embodiment, and  FIG. 22  is a graph showing a frequency error ratio-correction value characteristic curve representing a correction value in relation to a frequency error ratio. 
     In  FIG. 21 , the Brillouin time domain detector  52  first determines a Brillouin loss/gain spectrum BSl/g(νd) in the reference optical fiber  51 , finds out a frequency (mean frequency) at a peak of the determined Brillouin loss/gain spectrum BSl/g(νd) and sets it as a reference value f H0  (Step S 201 ). Specifically, the Brillouin time domain detector  52  measures the light intensity of the light attributed to the Brillouin scattering phenomenon in the reference optical fiber  51  at each frequency of a scanning frequency range. Subsequently, the Brillouin time domain detector  52  determines the Brillouin loss/gain spectrum BSl/g(νd) in the reference optical fiber  51  from the respective measured light intensities. Then, the Brillouin time domain detector  52  determines a mean frequency of this Brillouin loss/gain spectrum BSl/g(νd) and sets it as the reference value f H0 . 
     Subsequently, the Brillouin time domain detector  52  determines the Brillouin loss/gain spectra BSl/g(νd) in the respective longitudinal area sections of the sensing optical fiber  18  by the same operation as in the first to third embodiments, and finds out frequencies (means frequencies) at peaks of the respective Brillouin loss/gain spectra BSl/g(νd) in the respective area sections and sets them as detection values f Hn  (Step S 202 ). Specifically, the Brillouin time domain detector  52  measures distributions m 1 , m 2 , m 3 , m 4 , . . . , mn of the light intensities of the light attributed to the Brillouin scattering phenomenon in the longitudinal direction of the sensing optical fiber  18  at the respective frequencies of the scanning frequency range. Subsequently, the Brillouin time domain detector  52  determines the Brillouin loss/gain spectra BSl/g(νd) in the respective longitudinal area sections of the sensing optical fiber  18  from the measured distributions m 1 , m 2 , m 3 , m 4 , . . . , mn, finds out the means frequencies of the respective Brillouin loss/gain spectra BSl/g(νd) and sets them as detection values f Hn . Here, the detection value f Hn  is the detection value f H  in the n-th area section. 
     Subsequently, the Brillouin time domain detector  52  calculates a ratio frx (frequency error ratio) of the detection value f H  to the reference value f H0  defined by Equation 13 in each area section (Step S 203 ), and calculates a correction value f En  corresponding to the frequency error ratio frxn by substituting each calculated frequency error ratio frxn into the correction-value conversion equation (Step S 204 ). Here, the frequency error ratio frxn is a frequency error ratio Frx in the n-th area section, and the correction value f En  is a correction value f E  in the n-th area section. The correction-value conversion equation is an equation relating to an error H 4  gives to H 2 , a function expression obtained based on a difference between a value obtained by a simulation using the aforementioned Equations 7 to 11 and a specified strain assuming that the specified strain is given to the sensing optical fiber in accordance with the intensity-stepwise optical pulse OPs used as probe light. This conversion equation is, for example, a function expression of a frequency error ratio-correction value characteristic curve representing the correction value f E  in relation to the frequency error ratio Frx as shown in  FIG. 22 . The characteristic curve shown in  FIG. 22  is a result of a simulation carried out under conditions: Tp=1 ns, Tf=14 ns, ratio Prx=22 dB.
 
 frx =( f   H0   −f   H )/ f   H0   Equation 13
 
     Although the Brillouin time domain detector  52  converts the frequency error ratio frx into the correction value using the correction-value conversion equation stored in the form of a function expression in this embodiment, the frequency error ratio frx may be converted into the correction value using a look-up table relating the frequency error ratio frx and the correction value to each other in a table format. 
     Subsequently, the Brillouin time domain detector  52  calculates an area S Ln  at a low frequency side of the Brillouin loss/gain spectrum BSl/g(νd) in each area section and an area S Rn  at a high frequency side of the Brillouin loss/gain spectrum BSl/g(νd) in each area section from the corresponding detection value f Hn  (Step S 205 ). Here, the area S Ln  is an area S L  in the n-th area section, and the area S Rn  is an area S R  in the n-th area section. 
     Then, the Brillouin time domain detector  52  determines the sign of each correction value f En  calculated in Step S 204  based on the calculated corresponding areas S Ln  and S Rn  in the Brillouin loss/gain spectrum BSl/g(νd) in each area section (Step S 206 ). Specifically, the Brillouin time domain detector  52  determines a minus sign for the correction value f E  calculated in Step S 204  if S L &gt;S R  while determining a plus sign for the correction value f E  calculated in Step S 204  if S L &lt;S R . It should be noted that the correction value f E =0 if S L =S R . 
     Subsequently, the Brillouin time domain detector  52  adds the respective correction value f En  with the sign obtained in Step S 206  to the corresponding detection value f Hn  of the Brillouin loss/gain spectrum BSl/g(νd) in each area section (adds the correction value f E  to the detection value f H  in the case of the plus sign of the correction value f E  while subtracting the correction value f E  from the detection value f H  in the case of the minus sign of the correction value f E ), and calculates the detection values f Hn  (corrected detection values f HEn ) of the Brillouin loss/gain spectrum BSl/g(νd) after the correction (Step S 207 ). 
     Subsequently, the Brillouin time domain detector  52  calculates Brillouin frequency shifts νb in the respective longitudinal area sections of the sensing optical fiber  18  by calculating differences between the corrected detection values f HEn  of the Brillouin loss/gain spectrum BSl/g(νd) in the respective longitudinal area sections of the sensing optical fiber  18  and the mean frequency of the Brillouin loss/gain spectrum BSl/g(νd) in a section of the sensing optical fiber  18  where there is no strain, and determines a strain distribution and/or a temperature distribution in the longitudinal direction of the sensing optical fiber  18  based on the calculated Brillouin frequency shifts νb in the respective longitudinal sections of the sensing optical fiber  18  (Step S 208 ). 
     Although the distributed optical fiber sensor  5  of the fifth embodiment corresponding to the construction of the distributed optical fiber sensor  1  of the fifth embodiment is described above, distributed optical fiber sensors of the fifth embodiment corresponding to the constructions of the distributed optical fiber sensors  2 ,  3  of the second and third embodiments can be similarly constructed by carrying out the aforementioned Steps S 201  to S 208  in the constructions of the distributed optical fiber sensors  2 ,  3  of the second and third embodiments. 
     By operating in this way, the distributed optical fiber sensor according to the fifth embodiment is capable of improving errors better than the distributed optical fiber sensors  1 ,  2 ,  3  according to the first to third embodiments and measuring strain caused in and/or temperature of the distributed optical fiber sensor  18  with high accuracy and high spatial resolution. The distributed optical fiber sensor according to the fifth embodiment is also capable of detecting minute strains of 200μ∈ or smaller in the case where strains are equally distributed in a wide range. Further, in the fifth embodiment as well, it goes without saying that it is not necessary, unlike the background art, to manually adjust the optical pulse in conformity with the length of the sensing optical fiber  18  every time a measurement is conducted. Accordingly, the distributed optical fiber sensor  3  can be produced as an industrial product. 
     In order to enable the detection of minute strains of 200μ∈ or smaller out of those equally distributed in a wide range in the first to third embodiments, an optical fiber in which a Brillouin frequency shift amount cyclically changes may be used as the sensing optical fiber  18 . 
       FIG. 23  is a chart showing a Brillouin frequency shift in the longitudinal direction of a sensing optical fiber. 
     The sensing optical fiber  18  in which the Brillouin frequency shift amount cyclically changes in this way is, for example, such an optical fiber in which a section DM 11  having a first Brillouin frequency shift amount νb( 1 ) and a length l 1  and a section DM 12  having a length l 2  and a second Brillouin frequency shift amount νb( 2 ) different from the first Brillouin frequency shift amount νb( 1 ) alternately repeat as shown in  FIG. 23 . These first and second Brillouin frequency shift amounts νb( 1 ), νb( 2 ) can be realized by suitably straining the optical fiber. 
     In the first to fifth embodiments, the distributed optical fiber sensors  1 ,  2 ,  3 ,  5  are for measuring the Brillouin loss/gain spectra BSl/g(νd) and the Brillouin frequency shifts νb by locking the frequency f 0  of the intensity-stepwise optical pulse as the probe light and scanning the frequency of the continuous light CWpump 1  as the pump light within the specified frequency range fr. Thus, the light emitting element  103  of the stepwise optical light source  11  needs not always be a frequency-variable semiconductor laser and may be a semiconductor laser. In the aforementioned embodiments, the distributed optical fiber sensors  1 ,  2 ,  3 ,  5  may be constructed to measure the Brillouin loss/gain spectra BSl/g(νd) and the Brillouin frequency shifts νb by scanning the frequency f 0  of the intensity-stepwise optical pulse as the probe light within the specified frequency range fr and locking the frequency of the continuous light CWpump 1  as the pump light. 
     The present invention has been suitably and sufficiently described above by way of embodiments with reference to the accompanying drawings to be expressed. It should be appreciated that a person skilled in the art can easily modify and/or improve the aforementioned embodiments. Accordingly, modifications or improvements made by the person skilled in the art are interpreted to be embraced by the scope of the invention as claimed unless departing from the scope of the invention as claimed.