Patent Publication Number: US-2015069246-A1

Title: Information obtaining apparatus and information obtaining method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an information obtaining apparatus that obtains information of a sample by using a terahertz wave and an information obtaining method. 
     2. Description of the Related Art 
     A terahertz wave is an electromagnetic wave typically having a component at an arbitrary frequency band in a range between 0.03 THz and 30 THz. As an inspection apparatus using this terahertz wave, an information obtaining apparatus configured to obtain information of a sample by adapting a terahertz time domain spectroscopy (THz-TDS) technique has been proposed. 
     This THz-TDS technique is a method of obtaining a time waveform of the terahertz wave by relatively changing timings at which pulse light (probe light) for detecting the terahertz wave and a pulsed terahertz wave reach a terahertz wave detection unit. At this time, detection timings are changed by changing an optical path length in which the probe light reaches the terahertz wave detection unit from a light source or an optical path length in which pulse light (pump light) for generating a terahertz wave reaches a terahertz wave generation unit from the light source by using a retardation optical unit. 
     A time obtain the time waveform depends on a sweeping speed of the retardation optical unit. In view of the above, to shorten the time obtain the time waveform, a technique with which a fiber-type retardation optical unit is constituted by using fiber wound around a piezoelectric element is described in Proc. of SPIE, 7485, 748504 (2009), Handheld terahertz spectrometer for the detection of liquid explosives (hereinafter, referred to as Non-patent document 1). In the fiber-type retardation optical unit, the optical path length of the fiber is changed by changing a length of the fiber or applying an electric field to the fiber to change a refractive index. 
     The optical path length of the fiber may be changed in some cases due to factors including a change in a surrounding environment (such as a temperature or a moisture) of the information obtaining apparatus, a change in an elasticity modulus of the fiber caused by repetition of fiber extension and retraction, and the like. In a case where fiber extension and retraction are caused by using the piezoelectric element, the optical path length may be varied even when control signals are the same due to an influence from hysteresis of the piezoelectric element. These situations occur because of a change in a propagation speed or a propagation distance of light that propagates through the fiber. 
     For that reason, even when the retardation optical unit is controlled in the same manner, the optical path length of the fiber is varied, and the terahertz wave detection may not be performed at a constant interval in some cases. According to Non-patent document 1, interfering light of the pulse light that has propagated through the fiber and reference light is detected, and the interval for the terahertz wave detection is stabilized from an intensity change pattern of the interfering light. 
     The change in the propagation speed or the propagation distance of the light that propagates through the fiber also affects the timing for starting the terahertz wave detection. For that reason, even when the detection interval is stabilized by the method described in Non-patent document 1, the time waveform of the terahertz wave may start from a different timing. That is, in a case where a plurality of time waveforms are obtained, positions on time axes of the respective time waveforms constructed by the information obtaining apparatus may be varied. 
     To suppress the influence of this positional variation, the same sample is measured plural times, and those results are averaged to extract the time waveform having the highest appearance probability according to Non-patent document 1. For that reason, the number of measurements performed for one sample is high, and it may take long time to obtain the time waveform. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention, an information obtaining apparatus that irradiates a sample with a terahertz wave and obtains information of the sample includes a splitting unit that splits pulse light from a light source into pump light and probe light, a terahertz wave generation unit configured to generate a terahertz wave upon incidence of the pump light, a terahertz wave detection unit configured to detect the terahertz wave from the sample upon incidence of the probe light, a fiber through which the pump light or the probe light propagates, a changing unit configured to change an optical path length difference between an optical path length of the pump light and an optical path length of the probe light by changing an optical path length of the fiber, a waveform construction unit configured to construct a time waveform of the terahertz wave by using a detection result of the terahertz wave detection unit, and an obtaining unit configured to obtain information related to the optical path length of the fiber, wherein the obtaining unit includes a splitting unit that splits the pump light or the probe light before propagating through the fiber into first light and second light, a formation unit configured to form interfering light of the first light that has propagated through the fiber and the second light that has not propagated through the fiber, and an optical detection unit configured to detect an intensity of the interfering light as the information related to the optical path length of the fiber, and wherein the waveform construction unit constructs the time waveform of the terahertz wave by extracting the detection result of the terahertz wave detection unit based on a detection result of the optical detection unit. 
     Further aspects of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic configuration diagram of an information obtaining apparatus. 
         FIG. 2  is an explanatory diagram for describing a configuration of an information obtaining apparatus according to an example 1. 
         FIG. 3  is an explanatory diagram for describing a modified example of a retardation optical unit according to the example 1. 
         FIG. 4  is an explanatory diagram for describing a modified example of an interval monitoring unit according to the example 1. 
         FIG. 5  is an explanatory diagram for describing a configuration of an information obtaining apparatus according to an example 2. 
         FIG. 6A  illustrates an example configuration of an obtaining unit using a fiber measurement system. 
         FIG. 6B  illustrates an example configuration of an obtaining unit using a spatial measurement system. 
         FIG. 7A  is an explanatory diagram for describing a modified example of the retardation optical unit according to the example 1. 
         FIG. 7B  is a cross sectional view as seen from VIIB-VIIB of  FIG. 7A , illustrating the modified example of the retardation optical unit according to the example 1. 
         FIG. 8  is a flow chart illustrating a method of constructing a time waveform. 
         FIG. 9  is a flow chart illustrating another method of constructing the time waveform. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Exemplary Embodiment 
     An information obtaining apparatus according to the present embodiment will be described with reference to  FIG. 1 .  FIG. 1  is a configuration diagram of the information obtaining apparatus. The information obtaining apparatus according to the present embodiment is a THz-TDS apparatus using a fiber-type retardation optical unit. It is noted that a term “fiber” mentioned in the present specification is an optical waveguide for propagating light while the light is confined by using a refractive index distribution. 
     The information obtaining apparatus includes a light source  101 , a terahertz wave generation unit  102  (hereinafter, referred to as “generation unit  102 ”), a terahertz wave detection unit  103  (hereinafter, referred to as “detection unit  103 ”), a retardation optical unit  104 , an interval monitoring unit  105 , a waveform construction unit  106 , an obtaining unit  107 , and a splitting unit  108 . The information obtaining apparatus includes a computer provided with a CPU, a memory, a storage device, and the like, and the computer has a function of the waveform construction unit  106 . 
     The light source  101  outputs ultrashort pulse light. Herein, the ultrashort pulse light refers to pulse light having a pulse width in a femtosecond order. Typically, the light source  101  is a femtosecond laser having a pulse width that is larger than or equal to 10 femtoseconds and smaller than or equal to 100 femtoseconds and a repetitive frequency of 10 MHz. 
     The ultrashort pulse light (pulse light) output from the light source  101  is split into pump light L 1  for generating a terahertz wave  109  at the splitting unit  108  and probe light L 2  for detecting the terahertz wave  109 . The splitting unit  108  may be a unit that splits light such as a fiber coupler or a beam splitter. 
     The generation unit  102  is configured to generate the pulsed terahertz wave  109  upon incidence of the pump light L 1 . A pulse width of the generated terahertz wave  109  is typically several hundred femtoseconds to several picoseconds. 
     A photoconductive element in which an antenna is formed by a conductor on a semiconductor film can be applied as the generation unit  102 . In addition, a generation element or the like in a mode in which a semiconductor substrate or a surface of an organic crystal is irradiated with the pump light L 1  or a mode in which the pump light L 1  is guided to nonlinear crystal can be applied as the generation unit  102 . It is possible to apply a related art technology with which this purpose can be realized to the generation unit  102  so long as the pulsed terahertz wave  109  is generated by the input of the pump light L 1 . 
     The detection unit  103  is configured to detect the terahertz wave  109  by the incidence of the probe light L 2 . In detail, the detection unit  103  detects an instantaneous value of an electric field intensity of the terahertz wave  109  that reaches upon the incidence of the probe light L 2 . For example, a sample (not illustrated) is irradiated with the terahertz wave  109  generated in the generation unit  102 , and the terahertz wave  109  that has transmitted through the sample or has been reflected by the sample is detected by the detection unit  103 . A detail description related to a method of irradiating the sample (not illustrated) with the terahertz wave  109  will be omitted. 
     A technique of detecting an electric field by using the above-described photoconductive element or an electro-optical effect or a technique of detecting a magnetic field by using a magneto-optical effect can be applied to the detection unit  103 . It is sufficient if the detection unit  103  can detect the terahertz wave  109  by using the probe light L 2 , and it is possible to apply a related art technology with which this purpose can be realized to the detection unit  103 . 
     The retardation optical unit  104  is a fiber-type retardation optical unit configured to adjust an optical path length difference between the pump light L 1  that reaches the generation unit  102  and the probe light L 2  that reaches the detection unit  103 . The retardation optical unit  104  according to the present embodiment is constituted by a fiber  104   a  through which the probe light L 2  propagates and a changing unit  104   b  configured to change an optical path length of the fiber  104   a . In detail, the changing unit  104   b  changes a length of the fiber  104   a  or a physical property such as a refractive index to change the optical path length of the fiber  104   a.    
     It is noted that the “length of the fiber  104   a ” mentioned in the present specification refers to the physical length of the fiber  104   a  instead of an optical length (optical path length) of the fiber  104   a.    
     The interval monitoring unit  105  is configured to monitor an optical path length difference between an optical path length of the pump light L 1  and an optical path length of the probe light L 2  such that an interval at which the detection unit  103  detects the terahertz wave  109 , that is, a time difference between data pieces of instantaneous values of the adjacent terahertz waves  109  becomes constant. Since the optical path length of the fiber is changed due to a change in a surrounding environment, the elasticity modulus of the fiber itself, or the like, the interval at which the detection unit  103  detects the terahertz wave  109  may not be constant in some cases. In view of the above, the interval monitoring unit  105  monitors a propagation distance of the probe light L 2  such that the interval for detecting the terahertz wave  109  becomes constant by using the configuration described in Non-patent document 1 and outputs a detection trigger. 
     The interval monitoring unit  105  is not limited to the configuration in which a digital signal such as the detection trigger is output, and the interval monitoring unit  105  may output an analog signal such as a variation of the length of the fiber  104   a . The configuration of the interval monitoring unit  105  will be described in the following respective examples. 
     The waveform construction unit  106  is configured to construct a time waveform of the terahertz wave  109  by using a detection result of the detection unit  103 . Data used for the construction of the time waveform of the terahertz wave  109  includes electric field intensities of a plurality of terahertz waves  109  detected at different timings. 
     The obtaining unit  107  is configured to obtain information related to an optical path length of the fiber  104   a . The information related to the optical path length of the fiber  104   a  includes an intensity of interfering light of the light that has propagated through the fiber  104   a  and the light that has not propagated through the fiber  104   a , the variation of the changing unit  104   b , or the like. The information obtaining apparatus sets a measurement reference of the time axis constructed by the waveform construction unit  106  based on the information related to the optical path length of the fiber  104   a , and as a result, suppresses a variation of the position on the time axis of the time waveform of the terahertz wave  109 . 
     Specifically, the detection unit  103  starts the detection of the terahertz wave  109  based on the information related to the optical path length of the fiber  104   a . Alternatively, the waveform construction unit  106  extracts the detection result of the detection unit  103  and constructs the time waveform based on the information related to the optical path length of the fiber  104   a.    
     A method of suppressing the variation of the position on the time axis of the time waveform of the terahertz wave  109  by using the information related to the optical path length of the fiber  104   a  will be described in detail. The configuration of the obtaining unit  107  will be described in detail in the following respective examples. 
     First, a method for the detection unit  103  to start the detection of the terahertz wave  109  based on the information related to the optical path length of the fiber  104   a  will be described with reference to  FIG. 8 . According to this method, the information related to the optical path length of the fiber  104   a  is obtained while the optical path length is changed by the changing unit  104   b , and a reference (measurement reference) for starting the measurement is identified based on the information. Subsequently, the detection unit  103  starts the detection of the terahertz wave  109  from the measurement reference. 
     A specific example action flow will be described with reference to  FIG. 8 .  FIG. 8  is a flow chart representing an action of the information obtaining apparatus. 
     When the action of the information obtaining apparatus is started, the obtaining unit  107  starts to obtain the information related to the optical path length of the fiber  104   a  (S 801 ). Thereafter, the changing unit  104   b  starts the action and changes the optical path length difference by changing the optical path length of the fiber  104   a  (S 802 ). S 801  and S 802  may be performed at the same time, and also the order of S 801  and S 802  may be reversed. 
     The obtaining unit  107  obtains the information related to the optical path length of the fiber  104   a  each time the optical path length difference is changed while the changing unit  104   b  is operated (S 803 ). Thereafter, the waveform construction unit  106  determines whether or not the measurement reference is reached from the obtained information related to the optical path length of the fiber  104   a  (S 804 ). In a case where the measurement reference is not reached, the changing unit  104   b  is operated again to change the optical path length difference, and the action from S 802  to S 804  is repeatedly performed until the measurement reference is reached. The detection unit  103  starts the detection of the terahertz wave  109  from a time when the measurement reference is reached (S 805 ). 
     According to the present embodiment, the detection unit  103  starts the detection from the measurement reference. However, the present invention is not limited to the configuration. For example, the detection unit  103  may start the detection after a predetermined time elapses from the measurement reference. 
     When the terahertz wave  109  is detected and the measurement for the time waveform is completed, the waveform construction unit  106  constructs the time waveform of the terahertz wave  109  by using all or a part of the detection results (S 806 ). Even in a case where the time waveform is constructed by using a range where the measurement reference is not included among the detection results, since the position on the time axis can be identified based on the measurement reference, it is possible to suppress the variation of the position on the time axis of the time waveform. As a result, the process of extracting the time waveform having the high appearance probability by performing the measurement for the time waveform plural times can be shortened, and it is possible to reduce the time to obtain the time waveform. 
     In addition, according to this measurement method, the measurement for the time waveform of the terahertz wave  109  is started after the measurement reference is reached. For that reason, since the necessary minimum amount of data used for the construction of the time waveform is obtained, it is possible to reduce the amount of data to be dealt with. 
     Next, a method for the waveform construction unit  106  to extract the detection result of the detection unit  103  and construct the time waveform based on the information related to the optical path length of the fiber  104   a  will be described with reference to  FIG. 9 .  FIG. 9  is a flow chart representing the action of the information obtaining apparatus. According to this method, when the changing unit  104   b  is operated, the detection unit  103  also starts the detection of the terahertz wave  109 . Thereafter, the waveform construction unit  106  extracts the data used for the construction of the time waveform from the detection result of the detection unit  103 . 
     According to this method too, similarly as in the above-described method, the information related to the optical path length of the fiber  104   a  is obtained while the optical path length is changed by the changing unit  104   b , and a reference (measurement reference) for extracting the detection result is identified based on the information. Then, the detection unit  103  extracts data at and after the measurement reference. 
     When the action of the information obtaining apparatus is started, the obtaining unit  107  starts the action (S 901 ). Thereafter, the changing unit  104   b  starts the action to change and the optical path length of the fiber  104   a  (S 902 ). S 901  and S 902  may be performed at the same time, and also the order of S 901  and S 902  may be reversed. 
     The detection unit  103  detects the terahertz wave  109  along with the action of changing the optical path length of the fiber  104   a  by the changing unit  104   b  (S 903 ). Specifically, the detection unit  103  detects the terahertz wave  109  along with the action of the changing unit  104   b , and also the obtaining unit  107  obtains the information related to the optical path length of the fiber  104   a.    
     After the action of the changing unit  104   b  is confirmed, the detection unit  103  is configured to start the detection of the terahertz wave  109 . The configuration is not limited to this, and the detection unit  103  may take a configuration in which after the changing unit  104   b  starts the action, the detection is started when the changing unit  104   b  changes by a predetermined optical path length difference, a configuration in which the detection is started when the changing unit  104   b  passes through a sensor, or the like. The detection result of the detection unit  103  is recorded in a recording unit (not illustrated). The recording unit is a memory provided to a computer, an external storage apparatus such as a hard disc, or the like. 
     The measurement reference for identifying a range of the data used for the construction of the time waveform is identified from the obtained information related to the optical path length of the fiber  104   a  in parallel with the detection of the terahertz wave  109  (S 904 ). When the measurement reference is identified, the measurement reference is recorded in the recording unit (S 905 ). 
     In a case where the measurement reference is recorded, a column number of a data line in which the detection result is recorded may be recorded, or data converted into the optical path length of the fiber  104   a  at the measurement reference, the optical path length difference between the pump light L 1  and the probe light L 2 , or the like may be recorded. That is, any format of data to be stored may be employed so long as a started column of the data used for the construction of the time waveform can be found by referring to the measurement reference. 
     When the measurement of the terahertz wave  109  is completed, the waveform construction unit  106  extracts data including the detection result used for the construction of the time waveform of the terahertz wave  109  by using the measurement reference as a reference (S 906 ). According to the present embodiment, the data used for the construction of the time waveform is extracted while the detection result of the measurement reference is set as the starting column. It is noted that the present invention is not limited to the above-described configuration. For example, the data used for the construction of the time waveform may be extracted while a detection result at a time when a predetermined time elapses from the measurement reference is set as the starting column. 
     The waveform construction unit  106  constructs the time waveform of the terahertz wave  109  by using all the detection results included in the extracted data or a part of the detection results (S 907 ). Even in a case where the detection results corresponding to the part of the extracted data are used, while the detection result at the measurement reference is set as the reference, the variation of the position on the time axis of the time waveform of the terahertz wave  109  is suppressed, and it is possible to reduce the time to construct the time waveform. 
     According to this measurement method, after the terahertz wave  109  is measured so as to include the detection result used for the construction of the time waveform, the measurement reference is identified by referring to the information related to the optical path length of the fiber  104   a , and the data used for the construction of the time waveform is extracted. For that reason, it is possible to omit the process for checking whether or not the measurement reference is reached each time the optical path length difference is changed in the course in which the measurement of the terahertz wave  109  is performed, and the measurement at a higher speed can be carried out, so that it is possible to reduce the time to obtain the time waveform. 
     The time waveform of the terahertz wave  109  which is obtained by the above-described method includes the information of the sample irradiated with the terahertz wave  109 , and the information of the sample can be obtained by examining the time waveform. 
     For example, the time waveform of the terahertz wave  109  that has transmitted through the sample turns to have a time waveform that reflects the physical properties of the sample, and therefore optical characteristics of an object to be measured can be found by analyzing this time waveform. In a case where the terahertz wave  109  that has been reflected by the sample is measured, optical characteristics and the like of a refractive index interface provided to the object to be measured can be found from this time waveform. The “optical characteristics” mentioned in the present specification is defined to include a complex amplitude reflectivity, a complex refractive index, a complex permittivity, a reflectivity, a refractive index of, an absorption coefficient, a permittivity, an electrical conductivity, and the like of a test body. 
     When the configuration further includes a position changing unit configured to change a relative position between the sample and an irradiation spot of the terahertz wave  109 , this configuration can also be applied to an imaging apparatus that can obtain a transmission image, a reflection image, or the like that reflects the optical characteristics of the sample. At this time, from a position of an interface, a distribution of the optical characteristics, or the like, a shape of an object in the test body, a shape of an area having predetermined optical characteristics in the test body, or the like can be obtained as the information of the sample. 
     The outline of the information obtaining apparatus has been described above. Hereinafter, the information obtaining apparatus will also be described more specifically by way of examples. It is noted that descriptions on the same parts as those in the above explanations will be omitted. 
     EXAMPLE 1 
       FIG. 2  is an apparatus configuration diagram for describing an information obtaining apparatus according to an example 1. According to the present example, example configurations of the retardation optical unit  104 , the interval monitoring unit  105 , and the obtaining unit  107  will be described. 
     The fiber-type retardation optical unit  104  according to the present example is constituted by the fiber  104   a  through which the probe light L 2  split from the ultrashort pulse light (pulse light) which is output from the light source  101  propagates, and the changing unit  104   b . The changing unit  104   b  is provided with an expansion and contraction unit  2041  and a driver unit  2042 . 
     The expansion and contraction unit  2041  is a bobbin-type piezoelectric element. The fiber  104   a  is adhered and immobilized in a state of being wound around the expansion and contraction unit  2041 , and the fiber  104   a  is expanded and contracted since the expansion and contraction unit  2041  is deformed. The driver unit  2042  is a controller that applies the piezoelectric element with a voltage for deforming the piezoelectric element of the expansion and contraction unit  2041 . The length of the fiber  104   a  is changed along with the deformation of the expansion and contraction unit  2041 , and accordingly the optical path length of the fiber  104   a  is changed. Since the optical path length of the pump light L 1  that reaches the generation unit  102  is not changed, it is possible to adjust the optical path length difference between the pump light L 1  and the probe light L 2  by the retardation optical unit  104 . 
     For example, polarization retaining fiber is used as the fiber  104   a , and in a case where the fiber  104   a  having a length of 60 m is wound around the expansion and contraction unit  2041 , the fiber  104   a  expands and contracts at a rate of approximately 5.5 um/V with respect to the voltage of the driver unit  2042 . When the driver unit  2042  controls the expansion and contraction unit  2041  by the voltage of ±400 V at 300 Hz, it is possible to adjust the optical path length difference in a range of approximately 20 picoseconds. 
     It is noted that the range of the adjustment of the optical path length difference is changed depending on a type and a length of used fiber and an applied voltage of the driver unit  2042 . Similarly as in Non-patent document 1, it is also possible to adjust the optical path length difference exceeding 100 picoseconds by adjusting the type of the fiber and a condition of the applied voltage. 
     As another configuration of the fiber-type retardation optical unit  104 , a configuration illustrated in  FIG. 3  can also be used. In  FIG. 3 , the retardation optical unit  104  includes the changing unit  104   b  provided with an expansion and contraction unit  3041  and a driver unit  3042 , and the fiber  104   a.    
     The expansion and contraction unit  3041  is configured to change the length of the fiber  104   a . In detail, the fiber  104   a  is wound around the expansion and contraction unit  3041  in which DC motors are arranged in parallel, and the fiber  104   a  is reeled by rotations of the DC motors in the expansion and contraction unit  3041 , so that the optical path length of the fiber  104   a  is changed. The type of the motor is not limited to the DC motor, and a stepper motor or the like may be used. In addition, the number of motors is not limited to two. The driver unit  3042  is a controller configured to control the motors. 
     The method of adjusting the optical path length of the fiber  104   a  by using the changing unit  104   b  is not limited to the method of changing the length of the fiber  104   a . For example, the optical path length can be adjusted by changing a refractive index of the fiber  104   a . This example will be illustrated in  FIG. 7A  and  FIG. 7B .  FIG. 7A  is an explanatory diagram for describing a retardation optical unit  7040  according to a modified example of the retardation optical unit  104 , and  FIG. 7B  is a cross sectional view as seen from VIIB-VIIB of  FIG. 7A . 
     The retardation optical unit  7040  of  FIG. 7A  includes a first electrode  7045  and a second electrode  7046  provided to a bobbin-type member  7044 , the changing unit  104   b  including a driver unit  7042 , and the fiber  104   a  having an electro-optical effect. 
     As illustrated in  FIG. 7B , the fiber  104   a  is located between the first electrode  7045  and the second electrode  7046  and immobilized in a state of being wound around the member  7044  by an insulating part  7047 . The insulating part  7047  has a role of immobilizing and protecting the fiber  104   a.    
     The first electrode  7045  and the second electrode  7046  are electrodes that apply an electric field to the fiber  104   a  and formed in a concentric fashion to sandwich the fiber  104   a  on an outer side of the member  7044 . The driver unit  7042  is configured to apply an electric field to the first electrode  7045  and the second electrode  7046 . 
     According to this configuration, the adjustment of the optical path length of the fiber  104   a  in the retardation optical unit  104  can be performed by the external electric field. For that reason, the optical path length of the fiber  104   a  can be electrically adjusted, and an improvement in the measurement speed of the terahertz wave  109  can be expected as compared with a mechanical adjustment. 
     The interval monitoring unit  105  is constituted by an interference optical system. The fundamental configuration is the same as a technology of Mach-Zehnder interference system incorporated in a retardation optical unit in Non-patent document 1. In detail, the interval monitoring unit  105  includes a reference light source  2051 , an optical detector  2052 , splitting units  2053  and  2055 , and merging sections  2054  and  2056 . The merging section  2054  and the splitting unit  2055  use a wavelength division multiplexing coupler (hereinafter, referred to as WDM coupler). The merging section  2056  and the splitting unit  2053  are constituted by a coupler. 
     The interval monitoring unit  105  detects the interfering light of the light that has propagated through the fiber  104   a  and the light that has not propagated through the fiber  104   a  out of the light from the reference light source  2051  and as a result, detects a relative optical path length difference between the pump light L 1  and the probe light L 2 . 
     The reference light source  2051  outputs a continuous wave. Hereinafter, the continuous wave output by the reference light source  2051  will be referred to as reference light. The reference light is input to the splitting unit  2053  and split by two into reference light R 1  and reference light R 2 . The reference light R 2  is input to the merging section  2054 , and the reference light R 1  is input to the merging section  2056 . 
     In addition to the reference light R 2 , first light L 2   a  split from the probe light L 2  before reaching the retardation optical unit  104  is input to the merging section  2054 . An output of the merging section  2054  is input to the retardation optical unit  104 . A wavelength of the first light L 2   a  is preferably different from a wavelength of the reference light R 2  so as to avoid unwanted interference. The first light L 2   a  and the reference light R 2  are output from the merging section  2054  and propagate through the fiber  104   a  of the retardation optical unit  104 . 
     The first light L 2   a  and the reference light R 2  that have propagated through the fiber  104   a  are input to the splitting unit  2055  to be split by two. The reference light R 2  from the splitting unit  2055  is input to the merging section  2056 , and the first light L 2   a  is input to a splitting unit  2073  of the obtaining unit  107  which will be described below. 
     The reference light R 2  that has propagated through a route via the retardation optical unit  104  and the reference light R 1  that has not propagated through the route via the retardation optical unit  104  are input to the merging section  2056 . For that reason, a phase difference is caused between the reference light R 1  and the reference light R 2 , and interference occurs in the merging section  2056 . In detail, an intensity of the interfering light output by the merging section  2056  is changed in accordance with the optical path length of the fiber  104   a  which is adjusted by the retardation optical unit  104 . 
     The optical detector  2052  detects an intensity of the interfering light output from the merging section  2056  and outputs a signal (interfering signal). When strength and weakness patterns of this interfering signal are counted, it is possible to read the change in the optical path length difference between the pump light L 1  and the probe light L 2 . For example, laser having a wavelength of 1310 nm is used as the reference light source  2051 , and when a refractive index of fiber  2043  is assumed to be 1.5, strength and weakness of the interfering wave are repeated every approximately 3 femtoseconds. A change in the relative optical path length of the probe light L 2  with respect to the pump light L 1  is detected by counting these strength and weakness patterns. 
     The output of the optical detector  2052  is input to the waveform construction unit  106 . The waveform construction unit  106  refers to the output of the optical detector  2052  and decides on an interval for recording the detection result of the detection unit  103 . For example, a configuration in which the optical detector  2052  outputs the detection trigger every strength and weakness pattern, and when the waveform construction unit  106  obtains the detection trigger from the optical detector  2052 , an instantaneous value of the detection unit  103  is recoded may be used, but the present invention is not limited to this configuration. 
     In addition, the interval monitoring unit  105  may use a signal used for driving the retardation optical unit  104  without using the reference light. For example, according to the configuration of  FIG. 4 , the interval monitoring unit  105  includes a position conversion unit  4051  configured to refer to a control signal from the driver unit  2042  for adjusting a deformation manner of the expansion and contraction unit  2041  and convert this control signal to a position on the time axis of the time waveform. The position conversion unit  4051  stores a change in a propagation distance of the first light L 2   a  with respect to the control signal output by the driver unit  2042  or the like in advance and obtains the position on the time axis from the control signal by using the information to output the detection trigger. 
     Hereinafter, the configuration of the obtaining unit  107  will be described. The obtaining unit  107  according to the present example is constituted by an interference optical system. As illustrated in  FIG. 2 , the obtaining unit  107  includes splitting units  2072  and  2073 , a formation unit  2074 , and an optical detection unit  2071 . The splitting units  2072  and  2073  and the formation unit  2074  are constituted by a coupler. 
     A part of the pulse light output from the light source  101  is split by the splitting unit  108  into the pump light L 1  and the probe light L 2 . Thereafter, the probe light L 2  is input to the splitting unit  2072  and split into the first light L 2   a  and second light L 2   b . The second light L 2   b  is directly input to the formation unit  2074 . The first light L 2   a  is input to the merging section  2054  constituting the interval monitoring unit  105  described above. The first light L 2   a  input to the merging section  2054  is input to the splitting unit  2073  via the retardation optical unit  104  and the splitting unit  2055 . 
     The splitting unit  2073  further splits the first light L 2   a  that has propagated through the fiber  104   a  by two. One split light is input to the detection unit  103  as the probe light and used for the detection of the terahertz wave  109 . The other split light is input to the formation unit  2074 . 
     The light that has propagated through a route via the retardation optical unit  104  (part of the first light L 2   a ) and the light that has propagated through a route without the intermediation of the retardation optical unit  104  (the second light L 2   b ) are input to the formation unit  2074 , and interfering light thereof is formed. According to the present specification, the interfering light of the part of the first light L 2   a  and the second light L 2   b  is regarded as interfering light of the first light L 2   a  and the second light L 2   b.    
     The optical detection unit  2071  is configured to detect the intensity of the interfering light formed by the formation unit  2074 . The optical detection unit  2071  according to the present example detects the intensity of the interfering light as the information related to the optical path length of the fiber  104   a . A situation where the intensity of the interfering light becomes the highest is set as the measurement reference. 
     The intensity of the interfering light output from the formation unit  2074  is changed in accordance with the optical path length of the first light L 2   a  changed by the retardation optical unit  104 . Specifically, when the part of the first light L 2   a  and the second light L 2   b  are input to the formation unit  2074  at the same time, the intensity of the interfering light becomes the highest. According to the present example, the measurement reference is identified by using the change in the intensity of the interfering light. 
     When the measurement reference is identified, the optical detection unit  2071  outputs a digital signal such as a trigger indicating the measurement reference (hereinafter, referred to as measurement reference trigger) or continuously outputs a detection result of the optical detection unit  2071  as an analog signal. In a case where the analog signal is output, a value corresponding to the measurement reference (measurement reference value) is set in advance, and the waveform construction unit  106  includes a system configured to compare this measurement reference value with information related to the optical path length of the fiber. 
     Since the light input to the formation unit  2074  is the ultrashort pulse light in general, the change in the intensity of the interfering light is relatively smaller than the intensity of the input light. To detect this intensity change at a satisfactory sensitivity, the optical detection unit  2071  preferably takes a mode in which the intensity signals of the pulse light steadily detected from the interfering light are removed, and only the intensity change is detected. For example, to determine the position of the measurement reference at a satisfactory sensitivity, the difference of the intensity change is preferably increased by using a nonlinear effect. 
     An example method of improving the detection sensitivity for the optical detection unit  2071  by using the nonlinear effect will be described.  FIG. 6A  illustrates an example of the configuration of the obtaining unit  107  using a fiber measurement system. 
     In the example where the fiber measurement system illustrated in  FIG. 6A  is used, the interfering light output from the formation unit  2074  is input to the optical detection unit  2071  via nonlinear crystal fiber  6075  and a dividing unit  6076 . The formation unit  2074  is constituted by a counter. The dividing unit  6076  is constituted by a WDM coupler. 
     Herein, a wavelength of the interfering light from the formation unit  2074  is set as λ. The wavelength λ of the interfering light is a wavelength of the pulse light output by the light source  101 . When the interfering light propagates through the nonlinear crystal fiber  6075 , a fundamental wave λ and a higher harmonic wave λ/2 are generated by the nonlinear effect, and the fundamental wave λ and the higher harmonic wave λ/2 are divided from each other by the dividing unit  6076 . Thereafter, the optical detection unit  2071  detects the higher harmonic wave λ/2, so that the intensity of the interfering light is obtained. 
     In this manner, when the higher harmonic wave is detected by removing the fundamental wave of the interfering light by using the nonlinear crystal fiber  6075 , the sensitivity of the optical detection unit  2071  is increased, and it is possible to obtain the measurement reference at a satisfactory accuracy. In addition, since the obtaining unit  107  can be all constituted by the fiber, downsizing of the apparatus is facilitated. 
       FIG. 6B  illustrates an example of the configuration of the obtaining unit  107  for improving the detection sensitivity of the optical detection unit  2071  by using the nonlinear effect, corresponding to an example configuration using a spatial measurement system where fiber is not used. In a case where the spatial measurement system is used for the obtaining unit  107 , and the formation unit  2074  is formed by nonlinear crystal  6077 . In detail, the part of the first light L 2   a  and the second light L 2   b  are adjusted to be incident at angles where the nonlinear effect occurs with respect to the nonlinear crystal  6077 , and an interfering wave of the higher harmonic waves λ/2 of the respective pulse lights is detected as the interfering light by the optical detection unit  2071 . 
     When the spatial measurement system is used, since propagation paths of the higher harmonic wave λ/2 and the fundamental wave λ are spatially separated from each other, the signals of the higher harmonic waves λ/2 are easily taken out. In addition, the ultrashort pulse light propagates through the open space, and the pulse shape and the quality such power of the ultrashort pulse light are easily maintained, so that it is possible to obtain the measurement reference at a satisfactory accuracy. With the above-described configuration, the information obtaining apparatus according to the present example can shorten the time to obtain the time waveform than the time required in the related art. 
     It is noted that in a case where the retardation optical unit  104 , the interval monitoring unit  105 , and the obtaining unit  107  are constructed by a fiber system, characteristics of the fiber may be changed by an external environment such as a temperature or a moisture. To suppress the change in the characteristics of the fiber, a temperature adjusting mechanism is preferably provided at least to these units. Alternatively, a temperature adjusting mechanism that performs temperature adjustment on the entirety of the information obtaining apparatus may be provided. 
     EXAMPLE 2 
     An example 2 will be described with reference to  FIG. 5 . It is noted that descriptions of the same parts as those according to the above-described example 1 will be omitted.  FIG. 5  illustrates a configuration of an information obtaining apparatus according to the present example. 
     The information obtaining apparatus according to the present example is different from the example 1 the configuration of the obtaining unit  107 . Specifically, according to the example 1, the obtaining unit  107  uses the interference optical system to detect the intensity of the interfering light of the second light L 2   b  where the optical path length is constant and the first light L 2   a  that has propagated through the fiber  104   a  where the optical path length is changed and sets the intensity as the information related to the optical path length of the fiber. In contrast to this, according to the present example, a physical variation of the changing unit  104   b  is obtained as the information related to the optical path length of the fiber. The components other than the configuration of the obtaining unit  107  are the same as those according to illustrated in  FIG. 2 . 
     In  FIG. 5 , the obtaining unit  107  is constituted by an encoder  5072  functioning as a measurement unit configured to measure the physical variation of the expansion and contraction unit  2041  that changes the length of the fiber  2043  by expansion and contraction and a measurement reference output unit  5071 . In a case where the fiber is expanded and contracted by deforming the piezoelectric element constituting the expansion and contraction unit  2041  in a depth direction, the variation indicating how much the piezoelectric element is deformed in the depth direction is measured by the encoder  5072 . 
     The measurement reference output unit  5071  compares the measured variation with the measurement reference value to identify the measurement reference. With regard to the measurement reference value, the value indicated by the encoder  5072  in the measurement reference is measured in advance, and it is sufficient that the value may be recorded. When the measurement reference is identified, a signal indicating the measurement reference is output. 
     According to the present example, the obtaining unit  107  measures the variation of the expansion and contraction unit  2041  that changes the length of the fiber  104   a  and obtains the variation corresponding to the measurement result as the information related to the optical path length of the fiber  104   a . Since the obtaining unit  107  obtains the measurement reference by using the obtained information related to the optical path length of the fiber  104   a , the information obtaining apparatus according to the present example can further shorten the time to obtain the time waveform. 
     The variation of the expansion and contraction unit  2041  is measured by the encoder, and it is therefore possible to electrically treat the variation. For that reason, application of a signal processing technique in a related art is facilitated, and an SN ratio of the signal is improved, so that it is possible to identify the measurement reference at a satisfactory accuracy. In addition, since the variety of constitution parts is also abundant, wide choices for constitution elements constituting the obtaining unit  107  are available, and it is possible to provide a down-sized information processing assembly inexpensively. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     For example, according to the above-described exemplary embodiment, the retardation optical unit  104  is provided in the propagation path of the probe light corresponding to the pulse light for the detection but may be provided in the propagation path of the pump light corresponding to the pulse light for the generation. At this time, the interval monitoring unit  105  and the obtaining unit  107  are also provided on the pump light side. 
     Moreover, according to the above-described exemplary embodiment, the propagation path of the pulse light is constituted by the fiber, but the configuration is not limited to this. Fiber except for the fiber  104   a  constituting the retardation optical unit  104  can be replaced by another optical system. For example, according to the above-described exemplary embodiment, both the pump light and the probe light are set to propagate through the fiber, but the propagation path of the pump light without the intermediation of the retardation optical unit  104  may be constituted by a spatial optical system. 
     This application claims the benefit of Japanese Patent Application No. 2013-189445, filed Sep. 12, 2013, which is hereby incorporated by reference herein in its entirety.