JP2014222157A - Terahertz wave measuring apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent occurrence of inconveniences due to a positional displacement of a retroreflecting mirror.SOLUTION: A terahertz wave measuring apparatus (100) comprises: generation means (110) for generating a terahertz wave by being irradiated with a first laser beam; detection means (130) for detecting a terahertz wave with which a measurement object is irradiated, by being irradiated with a second laser beam; retroreflection means (121) for retroreflecting the second laser beam and guiding the second laser beam retroreflected to the detection means; adjustment means (220) for adjusting an optical path length difference between the first laser beam and the second laser beam by moving the retroreflection means; feature point detection means (420) for detecting a feature point of the detected terahertz wave; calculation means (430) for calculating an adjustment reference period which is a period from a reference time to the time when the feature point is detected ; and control means (150) for controlling the detection means to start detecting a terahertz wave at timing according to the adjustment reference period.

Description

  The present invention relates to a technical field of a terahertz wave measuring apparatus that analyzes characteristics of a measurement object using, for example, a terahertz wave.

  As a terahertz wave measuring apparatus, an apparatus using terahertz time-domain spectroscopy (Terahertz Time-Domain Spectroscopy) is known. The terahertz wave measuring apparatus analyzes the characteristics of the measurement object according to the following procedure. First, pump light (in other words, excitation light), which is one laser light obtained by branching ultrashort pulse laser light (for example, femtosecond pulse laser light), generates terahertz waves to which a bias voltage is applied. The element is irradiated. As a result, the terahertz wave generating element generates a terahertz wave. The terahertz wave generated by the terahertz wave generating element is irradiated to the measurement object. The terahertz wave irradiated to the measurement object is another laser light obtained by branching the ultrashort pulse laser light as reflected light or transmitted light from the measurement object, and is optically delayed with respect to the pump light. In other words, the terahertz wave detecting element irradiated with the probe light (in other words, excitation light) to which the (optical path length difference) is applied is irradiated. As a result, the terahertz wave detecting element detects a current signal corresponding to the intensity of the terahertz wave reflected or transmitted by the measurement object. The detected terahertz wave (that is, a terahertz wave in the time domain and a current signal) is Fourier-transformed to obtain a spectrum (that is, frequency response characteristics of amplitude and phase) of the terahertz wave. As a result, the characteristics of the measurement object are analyzed by analyzing the spectrum of the terahertz wave.

  Here, the optical delay imparted to the probe light (or pump light) is a retroreflective device (or such a retroreflective device including a retroreflector capable of retroreflecting incident light). In many cases, the probe light (or pump light) is incident on the optical delay device including the optical delay device. Here, “retroreflection” refers to a state in which incident light is reflected in a direction parallel to the incident direction of the incident light. The retroreflective device includes, for example, a plurality of rotatable retroreflecting mirrors (or prisms) (see, for example, Patent Documents 1 to 3).

JP 2008-515028 A JP 2001-228080 A Japanese Patent No. 3720335

  However, when an optical delay is generated by using a plurality of retroreflecting mirrors, an appropriate measurement result cannot be obtained due to misalignment of retroreflecting mirrors, variation in mounting accuracy, or the like. Technical problems arise.

Specifically, for example, in Patent Document 1, since a mechanism for adjusting the rotation operation and the detection timing is not provided, it is not possible to deal with a problem caused by a displacement or the like. Although Patent Document 2 describes a configuration for generating a timing pulse, it is necessary to separately arrange and adjust a photodiode, and the retroreflector position is always detected at the time of measurement. Resulting in. Further, when a retroreflecting mirror is configured by using two reflecting mirrors, only one of the reflecting mirrors can be detected by a photodiode. Furthermore, in Patent Document 3, since the timing pulse is generated using the reflected light from the light transmitting body, the optical axis needs to be arranged perpendicular to the light transmitting body, which increases the number of steps. In addition, it is necessary that the reflected light from the translucent body can be used as a timing pulse, the intensity of the terahertz wave applied to the object to be measured may decrease, and the SNR (Signal-Noise Ratio) may be deteriorated. Examples of the problem to be attempted include the above. An object of the present invention is to provide a terahertz wave measuring apparatus that can prevent inconvenience caused by a positional deviation of a retroreflecting mirror or the like with a relatively simple configuration.

  A terahertz wave measuring apparatus that solves the above-described problems is a generation unit that generates a terahertz wave by irradiating a first laser beam, and a measurement object from the generation unit by irradiating a second laser beam. Detecting means for detecting the terahertz wave irradiated to the second laser beam, retroreflecting means for retroreflecting the second laser light and guiding the retroreflected second laser light to the detecting means, and the retroreflecting means. An adjusting means for adjusting an optical path length difference between the first laser light and the second laser light by moving; a feature point detecting means for detecting a feature point of the terahertz wave detected by the detecting means; Calculation means for calculating an adjustment reference period that is a period from a reference time to detection of the feature point, and detection of the terahertz wave at a timing according to the adjustment reference period And control means for controlling the serial detection means.

It is a block diagram which shows the structure of the terahertz wave measuring device which concerns on an Example. It is a top view which shows the structure of the rotation type optical delay device based on an Example. It is a perspective view which shows each structure of a terahertz wave generation element and a terahertz wave detection element. It is a graph which shows the time waveform of a terahertz wave for every retroreflection mirror. It is a conceptual diagram which shows the amplitude of the time waveform of a terahertz wave with light and shade. It is a block diagram which shows the structure of an index signal generation part. It is a graph which shows the time waveform of a terahertz wave. It is a block diagram which shows the modification of an index signal generation part. It is a block diagram which shows the structure of a measurement start signal production | generation part. It is a flowchart which shows the process of the software which concerns on an Example.

  The terahertz wave measuring apparatus according to the present embodiment is configured to generate a terahertz wave by irradiating the first laser light and irradiate the second laser light to the measurement object from the generating means. Detecting means for detecting the terahertz wave irradiated to the second laser beam, retroreflecting means for retroreflecting the second laser light and guiding the retroreflected second laser light to the detecting means, and the retroreflecting means. An adjusting means for adjusting an optical path length difference between the first laser light and the second laser light by moving; a feature point detecting means for detecting a feature point of the terahertz wave detected by the detecting means; Calculating means for calculating an adjustment reference period, which is a period from a reference time to detecting the feature point, and a detection unit configured to start detecting the terahertz wave at a timing according to the adjustment reference period. And control means for controlling the detection means.

  According to the terahertz wave measuring apparatus of the present embodiment, the terahertz wave irradiated to the measurement object is detected using the terahertz time domain spectroscopy by the operation of the generating unit, the detecting unit, the retroreflecting unit, and the adjusting unit. The The detected terahertz wave is used for analyzing the characteristics of the measurement object. Note that an existing detection method may be used as the terahertz wave detection method itself using the terahertz time domain spectroscopy. An outline of a terahertz wave detection method using terahertz time domain spectroscopy will be briefly described below.

  Specifically, the generation unit generates a terahertz wave by irradiating the generation unit with the first laser light as excitation light (for example, pump light). The terahertz wave generated by the generating means is irradiated to the measurement object.

  The detection means detects the terahertz wave reflected by the measurement object or transmitted through the measurement object by irradiating the detection means with the second laser light as excitation light (for example, probe light).

  The retroreflective means retroreflects the second laser light and guides the retroreflected second laser light to the detection means. At this time, the optical path length difference between the optical path of the first laser beam and the optical path of the second laser beam is adjusted by moving the retroreflecting unit by the adjusting unit. Specifically, the optical path length of the second laser light emitted from the retroreflective means is changed by moving the retroreflective means in the optical axis direction of the second laser light. Therefore, according to the adjustment means, the optical path length difference between the optical path of the first laser beam and the optical path of the second laser beam can be set to a desired value. Note that such adjustment of the optical path length difference is performed in order to suitably detect the waveform of the terahertz wave that appears on the order of sub-picoseconds.

  In this embodiment, in particular, the feature point of the terahertz wave detected by the detection unit is detected by the feature point detection unit. Note that the “feature point” here is a point at which fluctuations in the time axis of the terahertz wave can be detected with high accuracy. For example, the maximum amplitude value, minimum amplitude value, maximum amplitude value, and minimum amplitude value of the time waveform Zero cross points that connect The detection of the feature points is typically performed in a state where the measurement object is not arranged (in other words, at the time of non-measurement).

  When the feature point is detected, the calculation unit calculates an adjustment reference period that is a period from the reference time until the feature point is detected. Here, the “reference time” is a reference time for calculating the adjustment reference period, and is determined by a predetermined reference signal or the like. It can be said that the adjustment reference period calculated in this way is a value indicating the time position of the feature point with respect to the reference time.

  When the adjustment reference period is calculated, the detection unit is controlled by the control unit so as to start detection of the terahertz wave at a timing according to the adjustment reference period. In other words, the detection start timing by the detection means is adjusted based on the adjustment reference period.

  If the above-described adjustment of the detection start timing is not executed, there is a possibility that the terahertz wave cannot be suitably detected due to, for example, the displacement of the retroreflective means or the variation in the mounting accuracy. Specifically, an unintended step or the like may occur in the tomographic image of the measurement object obtained by measurement.

  However, in the present embodiment, the detection start timing by the detection unit is adjusted based on the adjustment reference period as described above. That is, the detection start timing of the detection means is adjusted to a timing at which appropriate detection can be realized. For this reason, it is possible to prevent the occurrence of problems due to the displacement of the retroreflecting means and the variation in the mounting accuracy. Therefore, measurement using terahertz waves can be performed very suitably.

  In one aspect of the terahertz wave measuring apparatus according to this embodiment, a plurality of the retroreflective means are arranged on the circumference, and the adjustment means rotates the plurality of retroreflective means on the circumference. Rotating means for moving the

  According to this aspect, the optical path length difference between the first laser beam and the second laser beam is adjusted by rotating the retroreflective means on the circumference. Specifically, for example, when the rotating substrate on which the retroreflective means is arranged is rotated, the position of the retroreflective means in the optical axis direction with respect to the second laser light is changed, and the optical path length of the second laser light is changed. The

  In the above-described configuration, since a plurality of retroreflective means are used, a relatively high-speed rotation operation can be performed, and the measurement time can be shortened. However, since a plurality of arrangements are provided, problems due to the arrangement deviation of the retroreflective means and the variation in the mounting accuracy are likely to occur. However, in this aspect, the detection timing by the detection means is adjusted based on the adjustment reference period. Therefore, measurement using a terahertz wave can be suitably performed while preventing the occurrence of defects.

  In the aspect in which the plurality of retroreflective means described above are arranged, the feature point detecting means separately detects the feature points for each of the plurality of retroreflective means, and the calculating means is the plurality of retroreflective means. The adjustment reference period is calculated separately for each of the detection means, and the control means starts the detection of the terahertz wave at a timing corresponding to the adjustment reference time corresponding to each of the plurality of retroreflective means. You may comprise so that a means may be controlled.

  In this configuration, feature points are separately detected for each of the plurality of retroreflective means, and the adjustment reference period is calculated separately. Then, the adjustment of the detection start timing based on the adjustment reference period is also executed separately in each of the plurality of retroreflective means. For this reason, appropriate adjustment is performed for each retroreflective means, and it is possible to very effectively prevent problems caused by the displacement of the retroreflective means and variations in the mounting accuracy.

  In another aspect of the terahertz wave measuring apparatus according to the present embodiment, the feature point is at least one of a maximum value, a minimum value, and a zero cross point between the maximum value and the minimum value of the time waveform of the terahertz wave. One.

  According to this aspect, since the feature point of the terahertz wave can be detected easily and accurately, a more appropriate adjustment reference period can be calculated. Therefore, the detection start timing based on the adjustment reference period can be adjusted to a more appropriate timing.

  Note that the maximum value, the minimum value, and the zero cross point between the maximum value and the minimum value of the time waveform can be detected, and any one of them can be selectively detected. That is, the configuration may be such that more appropriate parameters are selected and detected according to the situation. Further, the adjustment reference period may be calculated using a maximum value, a minimum value, and a plurality of zero cross points between the maximum value and the minimum value of the terahertz wave. That is, the maximum value, minimum value, and zero cross point between the maximum value and the minimum value of the terahertz wave may be used in combination.

  In another aspect of the terahertz wave measuring apparatus according to the present embodiment, the feature point detection unit detects the feature point a plurality of times, the calculation unit calculates the adjustment reference period a plurality of times, and the control unit includes: The detection unit is controlled to start detection of the terahertz wave at a timing corresponding to the average value of the adjustment reference period calculated a plurality of times.

  According to this aspect, the feature point is detected a plurality of times, and the adjustment reference period is calculated a plurality of times based on each of the detected plurality of feature points. And when adjusting the detection start timing by a detection means, the average value of the adjustment reference period detected in multiple times is used.

  If the average value of the adjustment reference period is used, it is possible to prevent an adjustment based on an inappropriate adjustment reference period from being executed due to, for example, erroneous detection of a feature point. That is, the value of the adjustment reference period used for adjusting the detection start timing can be set to a more appropriate value. Note that the specific number of “multiple times” in the present embodiment is not particularly limited. However, the greater the number of times, the more remarkable the effect of the present embodiment described above. The smaller the number of times, the more simplified the process.

  In another aspect of the terahertz wave measuring apparatus according to the present embodiment, the feature point detecting unit includes an extracting unit that extracts a predetermined frequency component of the terahertz wave.

  According to this aspect, the predetermined frequency component of the terahertz wave is extracted (in other words, the frequency component other than the predetermined frequency component is cut) by the extraction unit configured as, for example, a low-pass filter. It is possible to more suitably detect the feature points.

  The operation and other gains of the terahertz wave measuring apparatus according to this embodiment will be described in more detail in the following examples.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  First, the terahertz wave measuring apparatus 100 according to the present embodiment will be described with reference to FIGS. 1 to 3.

<Overall configuration>
First, the overall configuration of the terahertz wave measuring apparatus 100 according to the present embodiment will be described with reference to FIG. FIG. 1 is a block diagram illustrating the configuration of the terahertz wave measuring apparatus 100 according to the embodiment.

  In FIG. 1, a terahertz wave measuring apparatus 100 irradiates a measurement target with a terahertz wave THz, and transmits a terahertz wave THz transmitted through the measurement target or reflected from the measurement target (that is, the terahertz wave irradiated on the measurement target). Wave THz).

The terahertz wave THz is an electromagnetic wave belonging to a frequency region (that is, a terahertz region) around 1 terahertz (1 THz = 10 ¹² Hz). The terahertz region is a frequency region that combines light straightness and electromagnetic wave transparency. The terahertz region is a frequency region in which various substances have unique absorption spectra. Therefore, the terahertz wave measuring apparatus 100 can analyze the characteristics of the measurement object by analyzing the frequency spectrum of the terahertz wave THz applied to the measurement object.

  In order to acquire the frequency spectrum of the terahertz wave THz irradiated on the measurement object, the terahertz wave measuring apparatus 100 employs terahertz time-domain spectroscopy (Terahertz Time-Domain Spectroscopy). The terahertz time domain spectroscopy irradiates the measurement target with the terahertz wave THz and performs Fourier transform on the time waveform of the terahertz wave THz that has passed through the measurement target or reflected from the measurement target. This is a method for acquiring a spectrum (that is, amplitude and phase for each frequency).

  Here, since the period of the terahertz wave THz is a period on the order of subpicoseconds, it is technically difficult to directly detect the time waveform of the terahertz wave THz. Therefore, the terahertz wave measuring apparatus 100 indirectly detects the time waveform of the terahertz wave THz by employing a pump-probe method based on time delay scanning.

  As shown in FIG. 1, a terahertz wave measuring apparatus 100 that employs such a terahertz time domain spectroscopy method and a pump-probe method includes a pulse laser device 101, a terahertz wave generating element 110, a beam splitter 161, and a reflecting mirror. 162, a reflecting mirror 163, a rotary optical delay unit 220, a terahertz wave detection element 130, a bias voltage generation unit 141, a measurement interval signal generation unit 142, a terahertz wave generation control unit 143, and an IV ( A current-voltage conversion unit 144, an arithmetic processing unit 150, a reference signal generation unit 310, an index signal generation unit 320, and a measurement start signal generation unit 330.

  The pulse laser device 101 generates pulse laser light LB in the sub-picosecond order or femtosecond order having light intensity corresponding to the drive current input to the pulse laser device 101. The pulse laser beam LB generated by the pulse laser device 101 is incident on the beam splitter 161 via a light guide (not shown) (for example, an optical fiber).

  The beam splitter 161 splits the pulsed laser light LB into pump light LB1 which is a specific example of “first laser light” and probe light LB2 which is a specific example of “second laser light”. The pump light LB1 is incident on the terahertz wave generating element 110 through a light guide path (not shown). On the other hand, the probe light LB2 enters the rotary optical delay unit 220 via a light guide path and a reflecting mirror 162 (not shown).

  The rotary optical delay unit 220 is a specific example of “adjustment means”, and adjusts a difference (that is, an optical path length difference) between the optical path length of the pump light LB1 and the optical path length of the probe light LB2. Specifically, the optical delay device 220 adjusts the optical path length between the optical path length of the pump light LB1 and the optical path length of the probe light LB2 by adjusting the optical path length of the probe light LB2. The timing at which the pump light LB1 enters the terahertz wave generation element 110 (or from the terahertz wave generation element 110) is adjusted by adjusting the optical path length difference between the optical path length of the pump light LB1 and the optical path length of the probe light LB2. The relative shift amount between the timing at which the outgoing terahertz wave THz enters the terahertz detection element 130) and the timing at which the probe light LB2 enters the terahertz wave detection element 130 can be adjusted. For example, when the optical path of the probe light LB2 is increased by 0.3 millimeters (however, the optical path length in the air) by the rotary optical delay unit 220, the timing at which the probe light LB2 enters the terahertz wave detection element 130 is 1 picosecond. Only slow down. In this case, the timing at which the terahertz wave detecting element 130 detects the terahertz wave THz is delayed by 1 picosecond. Considering that the terahertz wave THz having the same waveform repeatedly enters the terahertz wave detecting element 130 at intervals of about several tens of MHz, the timing at which the terahertz wave detecting element 130 detects the terahertz wave THz is gradually shifted. Thus, the terahertz detection element 130 can indirectly detect the time waveform of the terahertz wave THz.

  As shown in FIG. 2, the rotary optical delay unit 220 includes a rotary substrate 221 and a plurality of (four in FIG. 2) retroreflecting mirrors 121.

  The retroreflecting mirror 121 is a specific example of “retroreflecting means”, and retroreflects the probe light LB2 incident on the retroreflecting mirror 121. That is, the retroreflective mirror 121 reflects the probe light LB2 incident on the retroreflective mirror 121 in a direction parallel to the incident direction of the probe light LB2.

  The retroreflecting mirrors 121 are arranged at equal intervals on a circle C centering on the rotation axis of the rotating substrate 221 which is a specific example of “moving means”. Here, the rotating substrate 221 can be rotated by an operation of a motor or the like (not shown). Therefore, each of the plurality of retroreflecting mirrors 121 circulates on the circle C as the rotating substrate 221 rotates. By such a movement of the retroreflecting mirror 121, the optical path length of the probe light LB2 is adjusted.

  The retroreflecting mirror 121 is moved under the control of the arithmetic processing unit 150. That is, the arithmetic processing unit 150 controls the rotation operation of the rotating substrate 221 by outputting a control signal that specifies the driving amount of the motor that drives the rotating substrate 221.

  The probe light LB2 emitted from the rotary optical delay unit 220 is incident on the terahertz wave detecting element 130 via a light guide path and a reflecting mirror 163 (not shown).

  Here, the terahertz wave generating element 110 irradiated with the pump light LB1 and the terahertz detecting element 130 irradiated with the probe light LB2 will be described in more detail with reference to FIG. FIG. 3 is a perspective view showing the configuration of each of the terahertz wave generation element 110 and the terahertz wave detection element 130. The configurations of the terahertz wave generation element 110 and the terahertz wave detection element 130 illustrated in FIG. 3 are merely examples, and a photoconductive antenna or a photoconductive switch having a configuration different from the configuration illustrated in FIG. The terahertz wave detecting element 130 may be used.

  As shown in FIG. 3A, the terahertz wave generating element 110 includes a substrate 111, an antenna (in other words, a transmission line) 112, and an antenna (in other words, a transmission line) 113.

  The substrate 111 is a semiconductor substrate such as a GaAs (Gallium Arsenide) substrate. Each of the antenna 112 and the antenna 113 is a monopole antenna having a shape extending in the longitudinal direction. The antenna 112 and the antenna 113 are arranged on the substrate 111 so as to be parallel in the short direction. A gap (that is, a gap) 114 of about several micrometers is secured between the antenna 112 and the antenna 113. Therefore, the antenna 112 and the antenna 113 as a whole constitute a dipole antenna.

  A bias voltage output from the bias voltage generation unit 141 is applied to the gap 114 via the antenna 112, the antenna 113, and the terahertz wave generation control unit 143. When the pump beam LB1 is irradiated to the gap 114 in a state where an effective bias voltage (for example, a bias voltage other than 0 V) is applied to the gap 114, the terahertz wave generation element 110 receives carriers by photoexcitation by the pump beam LB1. Occur. As a result, the terahertz wave generating element 110 generates a pulse-shaped current signal in the order of subpicoseconds or in the order of femtoseconds corresponding to the generated carrier. As a result, the terahertz wave generation element 110 generates a terahertz wave THz resulting from the pulsed current signal.

  As shown in FIG. 3B, the terahertz wave detecting element 130 also has the same configuration as the terahertz wave generating element 110. That is, the terahertz wave detecting element 130 includes a substrate 131, an antenna (in other words, a transmission line) 132, and an antenna (in other words, a transmission line) 133. The substrate 131, the antenna 132, and the antenna 133 have the same configuration as the substrate 111, the antenna 112, and the antenna 113, respectively.

  When the probe light LB2 is irradiated to the gap 134, carriers are generated in the terahertz detection element 130 by light excitation by the probe light LB2. When the terahertz detection element 130 is irradiated with the terahertz wave THz while the probe beam LB2 is irradiated on the gap 134, a current signal having a signal intensity corresponding to the light intensity of the terahertz wave THz is generated in the gap 134. . The current signal is output to the IV conversion unit 144 via the antenna 132 and the antenna 133.

  In FIG. 1 again, the terahertz wave THz emitted from the terahertz wave generating element 110 is irradiated onto the measurement object via an optical system (not shown) (for example, a lens). The terahertz wave THz applied to the measurement object is incident on the terahertz wave detection element 130 through a not-shown optical system (for example, a lens) as reflected light or transmitted light from the measurement object. As a result, the terahertz wave detection element 130 outputs a current signal having a signal intensity corresponding to the light intensity of the terahertz wave THz.

  The current signal output from the terahertz wave detection element 130 is converted into a voltage signal by the IV conversion unit 144. Thereafter, the arithmetic processing unit 150 performs synchronous detection on the voltage signal using the bias voltage as a reference signal. As a result, the arithmetic processing unit 150 detects a sample value of the terahertz wave THz. Thereafter, the same operation is repeated while appropriately adjusting the optical path length difference between the optical path length of the pump light LB1 and the optical path length of the probe light LB2, so that the arithmetic processing unit 150 detects the time waveform of the terahertz waveform. be able to. In addition, the arithmetic processing unit 150 may acquire a frequency spectrum (that is, an amplitude and a phase for each frequency) of the terahertz wave by performing a Fourier transform on the detected time waveform of the terahertz waveform. Furthermore, the arithmetic processing unit 150 may analyze the characteristics of the measurement object by analyzing the frequency spectrum of the terahertz wave.

  In addition, the terahertz wave measuring apparatus 100 according to the present embodiment particularly includes a reference signal generation unit 310, an index signal generation unit 320, and a measurement start signal generation unit 330 as a configuration for adjusting the measurement start timing.

  The reference signal generator 310 generates a 1 pulse / 1 rotation reference signal synchronized with the rotation of the rotary optical delay unit 220. The reference signal is a signal for defining a reference time, and is generated using, for example, a drive control pulse from the rotary optical delay unit 220 or the arithmetic processing unit 150.

  The index signal generation unit 320 detects the feature point of the terahertz wave detected by the terahertz wave detection element 130 and calculates the position (time position) of the feature point with respect to the reference time defined by the reference signal. The detected time position of the feature point is output to the measurement start signal generation unit 330 as an index signal.

  The measurement start signal generation unit 330 adjusts the measurement start timing for each of the plurality of retroreflecting mirrors 121 in the rotary optical delay unit 220 according to the index signal input from the index signal generation unit 320. Is generated. The measurement start signal generated by the measurement start signal generation unit 330 is output to the arithmetic processing unit 150, and the arithmetic processing unit 150 controls the operation of each part so as to start measurement at a timing according to the measurement start signal.

A specific method for adjusting the measurement start timing will be described in detail later.
<Problems caused by misalignment>
Below, the malfunction which arises due to the position shift of the retroreflection mirror 121 is demonstrated in detail with reference to FIGS. FIG. 4 is a graph showing the time waveform of the terahertz wave for each retroreflector. FIG. 5 is a conceptual diagram showing the amplitude of the time waveform of the terahertz wave in shades.

  In FIG. 4, terahertz waves obtained from the plurality of retroreflecting mirrors 121 of the rotary optical delay unit 220 can be detected for each retroreflecting mirror 121 as time waveform signals as shown in the figure. Here, since the time waveform signal of the terahertz wave is used for displaying a tomographic image of the measurement object, for example, it is desirable that the peak positions of the time waveform signals of the retroreflecting mirrors 121 are aligned with each other. However, as shown in the figure, the peak positions of the time waveform signals may be shifted from each other. Such a waveform shift occurs due to, for example, a position shift of the retroreflector 121 or the like.

  In FIG. 5A, when terahertz time waveform signals as shown in FIG. 4 are arranged on the vertical axis and shades corresponding to the amplitude are given, a tomographic image is also displayed in accordance with the arrangement variation of the retroreflector 121. It can be seen that the X-Z tomographic image is shaken back and forth and has a step.

  In FIG. 5 (b), in order to obtain an XZ tomographic image without a step, the time waveform signals of terahertz waves may be in a state aligned with each other as shown in the figure. In the terahertz wave measuring apparatus 100 according to the present embodiment, the terahertz wave time waveform signal as shown in FIG. 5A is changed to the time waveform signal as shown in FIG. Processing for adjusting the measurement start timing for each retroreflector 121 is executed.

  Note that the terahertz wave measurement start timing adjustment processing is executed in a state where the measurement object is not arranged (that is, at the time of non-measurement) when transmission measurement is performed.

<Measurement start timing adjustment process>
Hereinafter, the specific configuration and operation of the index signal generation unit 320 will be described with reference to FIGS. FIG. 6 is a block diagram showing the configuration of the index signal generator. FIG. 7 is a graph showing a time waveform of the terahertz wave, and FIG. 8 is a block diagram showing a modification of the index signal generation unit.

  In FIG. 6, the index signal generation unit 320 includes a low-pass filter 410, a feature point detection unit 420, a time position extraction unit 430, and a selector 440.

  The low-pass filter 410 is a specific example of “extraction means”, and performs a filtering process on the detected time waveform of the terahertz wave to extract a predetermined frequency band component. According to the low-pass filter 410, for example, a time waveform signal as shown in FIG. 7A can be changed into a time waveform signal that smoothly transitions as shown in FIG. 7B.

  The feature point detection unit 420 is a specific example of “feature point detection means”, and detects a feature point from a time waveform signal of a terahertz wave. The maximum value detection unit 421 detects a point having the maximum amplitude as indicated by a point A in FIG. 7B as a feature point. The minimum value detection unit 423 detects a point having the minimum amplitude as indicated by a point C in FIG. 7B as a feature point. The zero cross detection unit 422 uses the maximum value detected by the maximum value detection unit 421 and the minimum value detected by the minimum value detection unit 423 to generate a zero cross point as indicated by a point B in FIG. Are detected as feature points.

  The time position extraction unit 430 is a specific example of “calculation means”, and calculates the time position of the detected feature point. The time position extraction unit 430 includes a first time position extraction unit 431 provided to correspond to the maximum value detection unit 421, a second time position extraction unit 432 provided to correspond to the zero cross detection unit 422, and a minimum A third time position extraction unit 433 provided to correspond to the value detection unit 423 is provided. The time position of the feature point is calculated as a period from the reference time indicated by the reference signal until the feature point is detected.

  The selector 440 selects one time position from the time positions calculated in each of the first time position extraction unit 431, the second time position extraction unit 432, and the third time position extraction unit 433, and selects the selected time. An index signal indicating the position is output.

  As described above, the index signal generation unit 320 detects a feature point from the input time waveform signal of the terahertz wave, and outputs an index signal indicating the time position of the feature point.

  In FIG. 8, a plurality of index signal generation units 320 may be provided so as to correspond to each of the plurality of retroreflecting mirrors 121. Specifically, an index signal generator 320a corresponding to the first retroreflector, an index signal generator 320b corresponding to the second retroreflector, ..., an index signal generator 320m corresponding to the mth retroreflector. May be provided.

  As described above, by providing the plurality of index signal generation units 320, it is possible to perform processing separately for each of the plurality of retroreflecting mirrors 121. Therefore, it is possible to prevent the processing executed by one index signal generation unit 320 from becoming complicated. Note that an average value calculation unit 340 that calculates an average value of the index signals may be provided after the index signal generation unit 320. In this way, it is possible to prevent the time position indicated by the index signal from becoming an inappropriate value due to erroneous measurement or the like.

  Next, a specific configuration and operation of the measurement start signal generator 330 will be described with reference to FIG. FIG. 9 is a block diagram showing the configuration of the measurement start signal generator.

  9A, the measurement start signal generation unit 330 includes a storage circuit 331a, an RW_CTL unit 332a, an offset unit 333a, and a measurement start unit 334a.

  The storage circuit 331a stores the index signal input from the index signal generation unit 320.

  The RW_CTL unit 332a performs write control and read control on the storage circuit 331a according to the mirror number assigned to each of the retroreflecting mirrors 121.

  The offset unit 333a subtracts a predetermined value from the storage index signal read from the storage circuit 331a. In other words, the adjustment amount of the measurement start timing is calculated based on the stored index signal.

  The measurement start unit 334a outputs a measurement start signal that defines the measurement start timing for the corresponding retroreflective means 121 using the reference signal. Note that, as a reference signal used by the measurement start unit 334, a multiplication clock of the rotation mechanism in the rotary optical delay unit 220 may be used.

  In FIG. 9B, the measurement start signal generation unit 330 may include a storage circuit 331b, an RW_CTL unit 332b, an offset unit 333b, a measurement start unit 334b, an addition unit 335b, and a division unit 336b.

  In this configuration, in particular, the addition unit 335b adds the past n stored index signals. Then, the past n stored storage index signals are divided by n in the division unit 336b. As a result, the average value of the past n stored index signals is calculated.

  By using the average value of the index signal, it is possible to prevent an inappropriate measurement start signal from being generated due to erroneous measurement or the like.

  In FIG. 9C, the measurement start signal generation unit 330 includes a storage circuit 331c, an RW_CTL unit 332c, an offset unit 333c, a measurement start unit 334c, and m (that is, the same number as the recursive reflectors 121) low-pass signals. A filter 337c may be provided.

  In this configuration, in particular, the m low-pass filters 337c perform filtering on the index signal, so that a measurement start signal can be generated while suppressing the influence of noise.

  By using the measurement start signal generated as described above, it is possible to suitably adjust the measurement start timing shift caused by the arrangement shift. That is, by using the feature points in the time waveform of the terahertz wave as a reference, the deviation between the retroreflecting mirrors 121 can be reduced. Therefore, the measurement start timing becomes appropriate, and the measurement using the terahertz wave can be executed more suitably.

<Software processing>
The hardware configuration for adjusting the measurement start timing described above can be realized by processing only with software. Hereinafter, measurement start timing adjustment processing by software will be described with reference to FIG. FIG. 10 is a flowchart illustrating the software processing according to the embodiment. Note that the software processing is generally the same as the processing executed by the hardware described above, and therefore, overlapping description will be omitted as appropriate. The software processing here is executed by a software processing unit (not shown).

  In FIG. 10, in the process by software, first, a predetermined pulse is sent from the software processing unit to the rotation mechanism of the rotary optical delay unit 220 (step S101). Thereby, the rotation mechanism is rotated at a constant speed. That is, the predetermined pulse is a pulse that defines the rotation speed of the rotation mechanism.

  Subsequently, the software processing unit determines whether or not the apparatus is being adjusted (that is, whether or not the measurement start timing is being adjusted) (step S102). Here, if it is determined that the apparatus is being adjusted (step S102: YES), an initial measurement start position is calculated for each of the plurality of retroreflecting mirrors 121. The initial measurement start position can be calculated using, for example, a stored time position.

  Subsequently, the software processing unit converts a current value corresponding to the terahertz wave detected by the terahertz wave detecting element 130 into a voltage value, and takes in a voltage value for a predetermined time from the measurement start value. Then, signal processing (for example, synchronous detection using a lock-in detection method using a modulated detection signal and a reference signal output from the terahertz wave generation element 110) is performed using the captured voltage value. A terahertz wave time waveform signal is generated (step S104).

  When the time waveform signal is generated, the feature point of the time waveform signal is detected, and the time position of the feature point is calculated (step S105).

  The calculated time position is stored for each corresponding retroreflector 121, and an appropriate measurement start position is calculated for each retroreflector from the stored time position (step S106). The measurement start position can be calculated by subtracting a certain offset time position. The calculated measurement start position is stored in a storage means such as a memory so that it can be used during measurement.

  With the above processing, a series of processing for calculating an appropriate measurement start position ends.

  On the other hand, when it is determined that the apparatus is not being adjusted (step S102: NO), when measurement of the measurement object is started (step S201: YES), the measurement start position calculated by the above-described process is output. (Step S202).

  Subsequently, the software processing unit converts a current value corresponding to the terahertz wave detected by the terahertz wave detecting element 130 into a voltage value, and takes in a voltage value for a predetermined time from the measurement start value. Then, signal processing is performed using the captured voltage value, and a time waveform signal of a terahertz wave is generated (step S203).

  In addition, the software processing unit sends a predetermined pulse for moving the rotation mechanism in order to obtain a tomographic image of the measurement object (step S204).

  The above-described processing from step S202 to step S204 is repeatedly executed when the movement of the rotation mechanism has not ended (step S205: NO). On the other hand, if it is determined that the movement of the rotation mechanism has ended, the series of processes ends.

  As described above, according to the terahertz wave measuring apparatus 100 according to the present embodiment, appropriate measurement can be realized based on the time position of the feature point detected for each retroreflecting mirror 121 at the measurement start timing. Adjusted to timing. For this reason, it is possible to prevent the occurrence of problems due to the displacement of the retroreflecting mirror 121 and the variation in the mounting accuracy.

  The adjustment process described above may be performed as an initial setting at the time of shipment from the factory unless a change is made to the optical path length arrangement inside the apparatus. In addition, when there is a change in the optical path length arrangement inside the apparatus, the adjustment process may be executed when the apparatus is started immediately after the change.

  The present invention is not limited to the above-described embodiment, and can be appropriately changed without departing from the spirit or idea of the invention that can be read from the claims and the entire specification, and terahertz wave measurement with such a change is possible. The apparatus is also included in the technical scope of the present invention.

DESCRIPTION OF SYMBOLS 100 Terahertz wave measurement apparatus 101 Pulse laser apparatus 110 Terahertz wave generation element 121 Retroreflector 130 Terahertz wave detection element 141 Bias voltage generation part 142 Measurement section signal generation part 143 Terahertz wave generation control part 144 IV conversion part 150 Operation processing part 161 Beam splitter 162, 163 Reflector 220 Optical delay device 221 Rotating substrate 310 Reference signal generator 320 Index signal generator 330 Measurement start signal generator 340 Average value calculator 410 Low pass filter 420 Feature point detector 430 Time position extraction Part 440 Selector LB Pulse laser beam LB1 Pump beam LB2 Probe beam THz Terahertz wave

Claims (6)

Generating means for generating a terahertz wave by being irradiated with the first laser beam;
Detecting means for detecting the terahertz wave irradiated to the measurement object from the generating means by being irradiated with the second laser beam;
Retroreflecting means for retroreflecting the second laser light and guiding the retroreflected second laser light to the detecting means;
Adjusting the optical path length difference between the first laser light and the second laser light by moving the retroreflective means;
Feature point detection means for detecting a feature point of the terahertz wave detected by the detection means;
A calculating means for calculating an adjustment reference period that is a period from a reference time to detecting the feature point;
A terahertz wave measuring device comprising: a control unit that controls the detection unit to start detection of the terahertz wave at a timing according to the adjustment reference period. A plurality of the retroreflective means are arranged on the circumference,
The terahertz wave measuring apparatus according to claim 1, wherein the adjusting unit includes a rotating unit that moves the plurality of retroreflecting units so as to rotate on the circumference. The feature point detecting means detects the feature point separately for each of the plurality of retroreflective means,
The calculating means calculates the adjustment reference period separately for each of the plurality of retroreflective means,
The control unit controls the detection unit to start detection of the terahertz wave at a timing corresponding to the adjustment reference time corresponding to each of the plurality of retroreflective units. The terahertz wave measuring device described.   2. The terahertz according to claim 1, wherein the feature point is at least one of a maximum value, a minimum value, and a zero-cross point between the maximum value and the minimum value of a time waveform of the terahertz wave. Wave measuring device. The feature point detection means detects the feature point a plurality of times,
The calculation means calculates the adjustment reference period a plurality of times,
2. The terahertz according to claim 1, wherein the control unit controls the detection unit to start detection of the terahertz wave at a timing corresponding to an average value of the adjustment reference period calculated a plurality of times. Wave measuring device.   The terahertz wave measuring apparatus according to claim 1, wherein the feature point detecting unit includes an extracting unit that extracts a predetermined frequency component of the terahertz wave.