JP2013057696A - Terahertz measuring method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a terahertz measuring device or the like realizing widening of a measuring frequency band and enhancement of frequency resolution.SOLUTION: A THz wave spectrum measuring device (1) comprises: a beam splitter (11) for splitting a femtosecond laser into pump light and probe light; a THz wave generating source (12) for generating a THz wave by receiving the pump light; a chirp generating optical device (22) for converting the probe light to a chirp pulse; a beam splitter (31) for splitting the chirp pulse into two; reference light detecting means for detecting one split reference light; an electrooptical crystal (43) for modulating the other split light by electrooptical effect of the THz wave: signal light detecting means for detecting signal light output from the electrooptical crystal (43); and an arithmetic circuit (50) for acquiring a time waveform of the THz wave on the basis of the reference light and the signal light detected at the same time.

Description

  The present invention relates to a terahertz measuring device using a terahertz wave (hereinafter referred to as a THz wave) having a frequency range of about 0.1 to several THz (wavelength is 30 μm to 3000 μm, hereinafter referred to as “THz region”), and transmission. The present invention relates to a terahertz measuring apparatus for a measurement object having a spectral component characterized by a THz wave in a THz region.

  Currently, THz time domain spectroscopy (hereinafter referred to as “A method”) is the mainstream for measuring THz waves using femtosecond lasers. In the method A, the laser pulse is branched into two. One laser pulse (referred to as a pump pulse) is applied to a THz wave generating element and used to generate a THz wave pulse. The other pulse (referred to as a probe pulse) is used to sample the time waveform of a THz wave pulse.

  In the method A, a probe pulse whose time width is sufficiently narrower than that of a THz wave pulse is moved using an optical time delay device, and the time waveform of the THz wave pulse is sampled point by point to obtain the entire time waveform of the THz wave pulse. Since the measurement is repeated at predetermined time intervals while mechanically moving the optical time delay device, it takes about several minutes to measure the entire time waveform of the THz wave pulse.

In the method A, the time waveform of the THz wave pulse is measured both when the measurement object is placed at the measurement position in the optical path of the THz wave pulse and when it is not placed, and each frequency spectrum (respectively, respectively) is measured by Fourier transform. Acquire Er and Es). By performing a predetermined calculation based on the two acquired frequency spectra, the absorption spectrum (log (| Er | ² / | Es | ² )) and complex amplitude transmittance spectrum (Er / Es) of the THz region of the measurement object are measured. ) And get. Hereinafter, the absorption spectrum in the THz region and the complex amplitude transmittance spectrum are referred to as a THz spectrum.

  As one method for shortening the measurement time in the A method, a multi-channel measurement method using chirp pulses (hereinafter referred to as B method) has been proposed by Jiang et al. In the B method, a chirp pulse is formed by extending the time width of the femtosecond laser pulse longer than the time width of the THz wave pulse, and the entire waveform information of the THz wave pulse is placed on the chirp pulse. Therefore, the measurement of the time waveform of the THz wave pulse is sufficient to detect one chirp pulse. Thereby, in the method B, it is possible to measure the time waveform of the THz wave pulse one time, and the measurement time of the time waveform of the THz wave pulse is the response time of the detection system that detects one chirp pulse. For example, a measurement time of about 1 millisecond or less is possible. In the method B, the measurement time can be significantly shortened compared to the method A.

  As another method for shortening the measurement time in the A method, there is a method in which a white light pulse having a wide spectrum width is generated by a femtosecond laser and then the white light pulse is chirped (Patent Document 1,). Hereinafter referred to as C method). In Non-Patent Document 1 that discloses the B method, the measurement frequency band and time resolution of terahertz spectroscopy are about 0.3 THz and 0.07 THz, respectively, whereas in the embodiment of the C method, the measurement frequency band and time resolution are respectively It is disclosed that it can be about 1 THz and about 0.1 THz.

  Although it is possible to expand the measurement frequency band from 0.3 THz of the B method to about 1 THz by using the C method, the time resolution of the C method of about 0.1 THz is the time resolution of the A method (several 0. It is several times worse than (01 THz).

Japanese Patent Laying-Open No. 2005-233683

Zhiping Jiang and X.-C. Zhang "Electro-optic measurement of THz field pulses with a chirped optical beam", Applied Physics Letters, Vol.72, No.16, 20 April 1998, pp.1945-1947.

  By the way, for THz spectroscopy, it is important that the measurement frequency band is wide and the frequency resolution is high.

  The chirp pulse carrying the waveform modulation information of the THz wave pulse is measured using a spectrometer with a one-dimensional multi-element detector. Since the linear chirp pulse has a linear relationship between frequency and time, the frequency axis of the spectrum of the chirp pulse measured by the spectroscope with a detector can be converted to the time axis. Therefore, the wavelength resolution of the spectroscope with a detector may be determined according to the time resolution required for THz pulse waveform measurement.

  In the B method and the C method, it is considered that the time resolution is determined by the following equation based on an approximation method called a stationary phase method.

  In order to follow this equation, the time resolution of the measuring device in the B method is several times wider than that in the A method. Since the time width of the time waveform of the THz wave pulse is inversely proportional to the measurement frequency band of terahertz spectroscopy, the measurement frequency band becomes narrow when the time resolution is wide. In the B method, the measurement frequency band is a fraction of that in the A method.

  On the other hand, to increase the frequency resolution, it is necessary to widen the observation time width and extend the time width of the chirp pulse. However, if the time width of the chirp pulse is increased in order to increase the frequency resolution, the time resolution is deteriorated from this equation, and the measurement frequency band of terahertz spectroscopy is narrowed.

  In Non-Patent Document 1 that discloses the B method, the measurement frequency band and time resolution of terahertz spectroscopy are about 0.3 THz and 0.07 THz, respectively. On the other hand, it is disclosed that the measurement frequency band and the time resolution can be about 1 THz and about 0.1 THz, respectively, in the embodiment of the method C.

  Although the measurement frequency band can be expanded from 0.3 THz in the B method to about 1 THz by using the C method, the time resolution of the C method of about 0.1 THz is the time resolution of the A method (several 0. It is several times worse than (01 THz).

  In order to obtain several tens of GHz, which is a typical frequency resolution of terahertz spectroscopy using the A method, by the B method and the C method, the time width of the chirp pulse needs to be about 20 picoseconds or more. However, according to the above equation, if the time width of the laser pulse of the femtosecond laser is 100 femtoseconds (= 0.1 picoseconds),

となるから、ＴＨｚ波の時間分解能は約１．４ピコ秒となる。時間分解能の逆数である測定周波数帯域は０．７１ＴＨｚであり、１ＴＨｚを超えない。

Therefore, the time resolution of the THz wave is about 1.4 picoseconds. The measurement frequency band that is the reciprocal of the time resolution is 0.71 THz and does not exceed 1 THz.

  By the way, in order to derive the time waveform of the THz wave pulse in the B method and the C method, the chirp pulse I (t) _on on which the waveform modulation information of the THz wave pulse is placed and the chirp pulse I (t on which the waveform modulation information is not placed. ) _off both need to be measured.

  However, neither the B method nor the C method measures both I (t) _on and I (t) _off at the same time, and derives the time waveform of the THz wave pulse using the results measured with separate chirp pulses. ing. Therefore, when the femtosecond laser pulse fluctuates for each pulse, it is difficult to improve the reliability of the measurement result.

  Furthermore, the formula based on the stationary phase approximation adopted in the B method and the C method cannot be used under all experimental conditions.

  The inventor formulated without using approximation and optimized the wavelength resolution of the spectrometer with a multi-element detector even if the time width of the chirp pulse is several tens of picoseconds or more, for example, 20 picoseconds. Invented a method that does not reduce the resolution.

The time waveform C (t) of the chirp pulse is expressed by the following equation, where T _c is the pulse width, 2α is the chirp plate, and ω _{0 is the} center frequency.

The THz electric field amplitude time waveform E _THz (t) is expressed by the following equation.

At this time, the THz waveform E _mes (ω) obtained by the measurement is expressed by the following equation.

である。 here,

It is.

  Further, the inventor can obtain the method B when the THz wave pulse and the chirp pulse meet in the electro-optic crystal at different timings, that is, in the time domain in the chirp pulse modulated by the THz wave pulse. It was discovered that the time waveform of THz wave pulses is different. This means that the device response of the measuring device changes with the timing of the two pulses.

  In general, when the measurement object has a refractive index different from 1 in the THz region, the time region in the chirp pulse modulated by the THz wave pulse varies depending on whether or not the measurement object is present. Therefore, there is a change due to the change in the device response between the measured time waveforms of the two THz wave pulses, and the THz spectrum of the measurement object is directly used by using the time waveforms of the two THz wave pulses. , The THz spectrum is distorted. The B method and the C method do not consider this effect.

  An object of the present invention is to provide a highly reliable terahertz measuring device and the like that realizes a wide measurement frequency band and a high frequency resolution.

  Hereinafter, means for solving the above-mentioned problems will be described. In order to facilitate understanding of the present invention, reference numerals in the accompanying drawings are added in parentheses, but the present invention is not limited to the illustrated embodiment.

  In order to solve the above problems, a terahertz measuring device (1) according to the present invention includes an optical pulse generator (10) that generates an ultrashort optical pulse and an ultrashort generated by the optical pulse generator (10). A first branching means (11) for branching the optical pulse into two parts, a pump light and a probe light, and a terahertz wave (on receiving the pump light of the ultrashort optical pulse branched by the first branching means (11)) A terahertz wave generating optical system (12) for generating THz waves) and a chirp generating optical system for converting the probe light of the ultrashort optical pulse branched by the first branching means (11) into a chirped pulse that is linearly chirped ( 22), a second branching means (31) for branching the chirp pulse converted by the chirp generation optical system (22) into two, and a chirp pulse branched by the second branching means (31) One side as reference light An electric field of a THz wave generated by the terahertz wave generating optical system (12) as the other of the chirp pulse branched by the reference light detecting means (32, 48) for detecting the spectrum and the second branching means (31) An electro-optic crystal (43) that modulates by an electro-optic effect induced by a signal, and a signal light detection means that receives light modulated and outputted by the electro-optic crystal (43) and detects a spectrum as signal light ( 45, 48) and arithmetic means for obtaining a time waveform of the THz wave based on the reference light by the reference light detection means (32, 48) and the signal light by the signal light detection means (45, 48) detected at the same time ( 50).

  Here, if the chirp generation optical system (22) is characterized by converting an ultrashort light pulse into a chirp pulse linearly chirped at 10 picoseconds or more, a frequency resolution of several tens of GHz equivalent to the method A is realized. it can.

  The optical system further includes an optical time delay (21) for delaying the probe light of the ultrashort optical pulse branched by the first branching means (11) by a predetermined time, and includes a terahertz wave generating optical system (12). The optical time delay (21) so that the THz wave generated from the other and the other of the chirped pulses branched by the second branching means (31) overlap in time in the electro-optic elementary crystal (43). ) Is characterized in that the probe light is delayed, the THz wave and the chirp pulse can be synchronized, and the modulation characteristic of the THz wave carried on the chirp pulse can be acquired.

  Further, the calculation means (50) includes a first time waveform of the THz wave acquired in a state where there is no measurement object at a predetermined measurement position and a second of the THz wave transmitted through the measurement object arranged at the measurement position. The time delay amount is calculated based on the time waveform, and the optical time delay device (21) delays the probe light by the time delay amount calculated by the calculation means (50). The THz wave modulates the same time domain in the chirp pulse with and without the measurement object. Therefore, it is possible to derive the time waveform of the THz wave excluding the dependency on the THz wave modulation region, and obtain a THz spectrum without distortion.

  Furthermore, the third branching means (101) for branching the pump light of the ultrashort light pulse branched by the first branching means (11) into two further, and the third branching means (101) And a shutter (103) that opens and closes one optical path of the branched ultrashort light pulse, and is branched by the third branching unit (101) with respect to the measurement object arranged at the measurement position. If the THz wave is irradiated from the terahertz wave generation optical system (12) receiving the other of the ultrashort light pulses while being irradiated with one of the ultrashort light pulses, the inside of the measurement object is generated by the light excitation. Functional analysis of chemical reactions and phase changes that occur in

In order to solve the above-mentioned problem, the time waveform acquisition method according to the present invention includes a first branching means (11) for branching an ultrashort light pulse into two parts, pump light and probe light, 11) A terahertz wave generating optical system (12) that receives pump light of the ultrashort light pulse branched in 11) to generate a terahertz wave (THz wave), and an ultrabranch branched by the first branching means (11). A chirp generation optical system (22) that converts the probe light of a short optical pulse into a chirp pulse that is linearly chirped, and a second branching means (31) that branches the chirp pulse output from the chirp generation optical system (22) into two. ) And the reference light detection means (32, 48) for detecting the spectrum using one of the chirp pulses branched by the second branch means (31) as the reference light, and the second branch means (31). Chirp Pal The electro-optic crystal (43) that modulates by the electro-optic effect induced by the electric field signal of the THz wave generated by the terahertz wave generation optical system (12), and the electro-optic crystal (43) Generated by the terahertz wave generating optical system (12) based on the signal light detecting means (45, 48) for detecting the spectrum using the modulated light as the signal light and the reference light and the signal light detected at the same time A time waveform acquisition method executed in a terahertz measurement device (1) having a calculation means (50) for calculating a time waveform of a THz wave, and a reference light R (t) _off and a signal light without generating a THz wave A first detection step of simultaneously detecting S (t) _off; a second detection step of simultaneously detecting the signal light S (t) _on and the reference light R (t) _on by generating a THz wave; Reference light R (t) _off and signal light S (t) _off detected in the first detection step, Beauty, using the signal light S (t) _on the reference light R (t) _on in the second detection step, and a calculation step of obtaining by calculating the THz wave time waveform E _THz (t) It is characterized by.

Here, the reference light R (t) _off and the signal light S (t) _off detected in the first detection step, and the reference light R (t) _off and the signal light S (t) _off are calculated. Storage step for storing any one of the branching ratios S (t) _off / R (t) _off. In the calculation step, the reference light R (t) _off and the signal light S ( t) _off and branching ratio S (t) _off / R (t) _off, and using the signal light S (t) _on and the reference light R (t) _on in the second detection step, If the time waveform E _THz (t) of the THz wave is calculated and acquired, the reference light R (t) _off and the signal light S (t) _off are detected simultaneously without generating the THz wave. If the first detection step is executed once, then only the second detection step of generating the THz wave and simultaneously detecting the signal light S (t) _on and the reference light R (t) _on is executed. The time waveform of the wave can be acquired.

Further, the branching ratio S (t) _off / R (t) _off is calculated based on the reference light R (t) _off and the signal light S (t) _off detected in the first detection step, and the second Using the reference light R (t) _on detected in the detection step and the calculated branching ratio S (t) _off / R (t) _off, a chirp pulse I (t) _off that is not modulated by the THz wave is obtained. Calculate the THz wave time waveform E _THz (t) by regarding the signal light S (t) _on detected in the second detection step as the chirp pulse I (t) _on modulated with the THz wave. It is possible to obtain the effective simultaneous measurement of chirp pulse I (t) _on modulated with THz wave and chirp pulse I (t) _off unmodulated with THz wave, and distortion caused by laser fluctuation. Can be removed.

  Further, a chirp pulse I (t) _off that is not modulated with a THz wave is expressed by the following equation (1).

により演算し、ＴＨｚ波の時間波形E_THz(t)を、式（２）

And calculate the time waveform E _THz (t) of the THz wave using equation (2)

により取得することを特徴とすれば、（２）式によりＴＨｚ波パルスの時間波形のレーザーの揺らぎに起因する歪みを除去できる。

(2), distortion caused by laser fluctuations in the time waveform of the THz wave pulse can be removed.

  In order to solve the above-described problem, a terahertz measurement method according to the present invention includes a first time waveform acquisition step of acquiring a first time waveform of a THz wave in a state where there is no measurement object at a predetermined measurement position, and a measurement position. A second time waveform acquisition step of acquiring a second time waveform of a THz wave that has passed through the measurement object disposed in the first time waveform, and a first time waveform and a second time waveform acquired in the first time waveform acquisition step A time delay amount calculation step of calculating a time delay amount based on the second time waveform acquired in the acquisition step, and delaying the probe light by the time delay amount calculated in the time delay amount calculation step, In the third time waveform acquisition step of acquiring the third time waveform of the THz wave that has passed through the measurement object, the first time waveform acquired in the first time waveform acquisition step, and the third time waveform acquisition step Based on the acquired third time waveform, And having a THz spectrum calculation step of calculating a spectrum of the THz wave transmitted through the constant object.

  Here, in the time delay amount calculation step, if the time delay amount is calculated based on the time difference between the main peaks of the first time waveform and the second time waveform, there is a case where there is a measurement object. When the THz wave is not modulated, the THz wave is modulated in the same time domain within the chirp pulse, and the time waveform of the THz wave can be derived without the dependence on the THz wave modulation domain.

  The first time waveform storing step for storing the first time waveform is further included. In the time delay amount calculating step, the first time waveform and the second time waveform stored in the first time waveform storing step are stored. If the time delay amount is calculated based on the above, the first time waveform acquisition step of acquiring the first time waveform of the THz wave in a state where there is no measurement object at the predetermined measurement position can be omitted. Measurement time can be shortened.

  Further, at least any one of the first to third time waveform acquisition steps is executed a plurality of times, and a THz wave time waveform is acquired based on the reference light and the signal light detected a plurality of times. The signal / noise ratio (S / N ratio) can be increased.

  In order to solve the above problems, the terahertz measurement method according to the present invention obtains a time waveform of a transmitted THz wave transmitted through a photoexcited measurement object by irradiating the measurement object with an ultrashort light pulse as excitation light. Acquired in the time waveform acquisition step in the excited state, the time waveform acquisition step in the ground state for acquiring the time waveform of the transmitted THz wave that has passed through the measurement object in the ground state, and the time waveform acquisition step in the excited state Of the measured THz wave in the excited state and the time waveform of the transmitted THz wave in the ground state acquired in the time waveform acquisition step in the ground state. And an analysis step for analyzing.

  In order to solve the above problems, an inspection apparatus (99) according to the present invention includes a transport unit (98) that transports a measurement object to a predetermined measurement position, and a measurement transported to the measurement position by the transport unit (98). And a terahertz measuring device (1) for measuring a transmitted THz wave that passes through an object.

  Here, the database (96) further stores a THz spectrum of various substances in the THz region, and is stored in the database (96) with the transmitted THz wave of the measurement object measured by the terahertz measuring device (1). If the measurement object is inspected based on the THz spectrum, the measurement object can be easily specified.

  According to the present invention, it is possible to provide a highly reliable terahertz measuring device and the like by realizing a wide measurement frequency band and a high frequency resolution.

It is a block diagram of the terahertz wave spectrum measuring apparatus concerning 1st Embodiment. It is a figure which shows the Fourier-transform spectrum of the time waveform of a THz wave pulse. It is a figure which shows the time waveform of the THz wave pulse detected by changing frequency resolution. It is a figure which shows the time waveform of the THz wave pulse obtained by changing the time timing of a THz wave pulse and a chirp pulse. It is a block diagram of the terahertz wave spectrum measuring apparatus concerning 2nd Embodiment. It is a general-view figure of the inspection apparatus using the terahertz wave spectrum measuring apparatus shown in FIG.

(First embodiment)
Hereinafter, embodiments for carrying out the present invention will be described in detail.

  FIG. 1 is a configuration diagram of a terahertz wave spectrum measuring apparatus 1 according to a first embodiment as an example of a terahertz measuring apparatus according to the present invention.

  As shown in FIG. 1, a terahertz wave spectrum measuring apparatus 1 is output from a laser generator 10 as an example of an optical pulse generator that generates a femtosecond laser as an example of an ultrashort optical pulse, and a laser generator 10. The beam splitter 11 as an example of the first branching means for branching the femtosecond laser into two of the pump light and the probe light, and the terahertz wave pulse (THz wave pulse) upon receiving the pump light branched by the beam splitter 11 And a THz wave generation source 12 as an example of a terahertz wave generation optical system.

  Further, the terahertz wave spectrum measuring apparatus 1 receives the probe light branched by the beam splitter 11 and delays it for a predetermined time and outputs it, and the optical time delay 21 delays the time. It has a chirp generation optical device 22 as an example of a chirp generation optical system that converts the output probe light into a linearly chirped chirp pulse and outputs it, and a polarizer 23 that transmits linearly polarized light having a predetermined vibration direction.

  Further, the terahertz wave spectrum measuring apparatus 1 includes a beam splitter 31 as an example of a second branching unit that splits the chirp pulse converted by the chirp generation optical device 22 into two of reference light and signal light, A reference light input unit 32 that takes in the reference light branched at 31 and outputs the reference light to a spectral detection unit 48 described later.

  Furthermore, the terahertz wave spectrum measurement apparatus 1 uses the electro-optic effect induced by the electric field signal of the THz wave pulse generated by the THz wave generation source 12 for the signal light of the chirp pulse branched by the beam splitter 31. An optical element array 40 to be modulated and a signal light input unit 45 that takes in the signal light modulated by the optical element array 40 and outputs the signal light to a spectral detection unit 48 described later.

  Furthermore, the terahertz wave spectrum measuring apparatus 1 acquires the reference light captured from the reference light input unit 32 and the signal light captured from the signal light input unit 45 via an optical fiber, and divides the reference light and the signal light. And a calculation circuit 50 as an example of a calculation means for acquiring a time waveform of a THz wave pulse based on the reference light and the signal light detected by the spectrum detection unit 48. Configured.

  Here, the laser generator 10 is constituted by, for example, a reproduction amplification system of a titanium / sapphire / laser. The laser generator 10 generates an ultrashort light pulse of a femtosecond laser having a spectral width of about 10 nm, a pulse width of 100 femtoseconds, a wavelength of 800 nm, a pulse energy of 1 mJ / pulse, and a pulse repetition of 1 KHz, for example. The ultrashort light pulse is not limited to this.

  The beam splitter 11 is composed of, for example, a half mirror type beam splitter, a polarization beam splitter, or the like. The beam splitter 11 branches the femtosecond laser output from the laser generation unit 10 into two, pump light supplied to the THz wave generation source 12 and probe light supplied to the chirp generation optical device 22.

  The THz wave generation source 12 includes, for example, a photoconductive antenna such as a semiconductor in which a metal antenna is formed on low-temperature grown gallium arsenide (LT-GaAs), a bulk semiconductor such as an indium arsenide (InAs) substrate, a semiconductor quantum well, and tellurium. It is composed of a nonlinear optical crystal such as zinc halide (ZnTe), a high-temperature superconductor, or the like. In the first embodiment, a semiconductor indium / arsenic substrate is used.

  The THz wave generation source 12 receives the pump light branched by the beam splitter 11 at a generation element (not shown), and generates a terahertz wave pulse (THz wave pulse). The THz wave generation source 12 is usually irradiated with pump light branched at a large branching ratio in order to increase generation efficiency and generation intensity.

  The optical time delay device 21 is configured by, for example, a combination of a plurality of folding mirrors (not shown). The optical time delay device 21 delays and outputs the probe light branched by the beam splitter 11 for a predetermined time based on a control signal from the arithmetic circuit 50 described later.

  The chirp generation optical device 22 is configured using, for example, a bulk diffraction grating pair, a prism, a holographic grating, a fiber grating, a collimator lens, and the like (all not shown). The chirp generation optical device 22 converts the probe light output after being time-delayed by the optical time delay device 21 into a chirp pulse having a linear chirp and outputs the chirp pulse. The chirp generation optical device 22 generates a chirp pulse having a time width of 10 picoseconds or more, for example, about 20 picoseconds.

  The beam splitter 31 is composed of, for example, a half mirror type beam splitter, a polarization beam splitter, or the like. The beam splitter 31 branches the chirp pulse converted by the chirp generation optical device 22 into two light sources, the reference light supplied to the reference light input unit 32 and the signal light supplied to the signal light input unit 45.

  The reference light input unit 32 is configured by, for example, an end portion of an optical fiber. The reference light input unit 32 receives the chirped pulse reference light branched by the beam splitter 31 and supplies the reference light to a spectral detection unit 48 described later.

  The optical element array 40 modulates the chirp pulse with a half mirror 41 that transmits a chirp pulse and reflects a THz wave pulse, an off-axis parabolic mirror 42 that collects and reflects parallel light, and an electric field signal of the THz wave pulse. The electro-optic crystal 43 and a polarizer 44 having a polarization direction orthogonal to the polarization direction of the polarizer 23 and transmitting a chirp pulse subjected to THz modulation are configured.

  Here, the electro-optic crystal 43 is composed of, for example, zinc telluride (ZnTe) crystal. When the electro-optic crystal 43 is irradiated with a THz wave pulse, the electro-optic crystal 43 modulates the time waveform of the signal light transmitted by the Pockels effect by the electric field signal.

  The signal light input unit 45 is constituted by, for example, an end portion of an optical fiber. The signal light input unit 45 receives the chirped pulse signal light branched by the beam splitter 31 and supplies the signal light to a spectral detection unit 48 described later.

  The spectroscopic detection unit 48 includes, for example, a spectroscope with a multi-element detector that integrally accommodates a spectroscope such as a diffraction grating and a detector such as a CCD (Charge Coupled Device). The spectral detection unit 48 of the first embodiment divides the light receiving region into two parts, and receives the reference light from the reference light input unit 32 and the signal light from the signal light detection unit 45. Here, the reference light input unit 32 and the spectral detection unit 48 constitute an example of the reference light detection unit of the present application, and the signal light input unit 45 and the spectral detection unit 48 are the signal light detection unit of the present application. An example is configured. The horizontal axes of the signal light and the reference light detected by the spectroscopic detection unit 48 are wavelengths or frequencies. Since the linear chirp pulse has a linear relationship between frequency and time, the horizontal axis of the detected signal light and reference light can be converted to the time axis using the value of the chirp plate. In order to actually obtain the value of the chirp plate, for example, 1) it is obtained by the frequency-resolved optical gate method, or 2) the timing at which the THz wave and the chirp pulse are overlapped by the electro-optic crystal 43 by the optical time delay device 21. It can be obtained by moving the THz time waveform on the horizontal axis of the wavelength or frequency and plotting the relationship between the wavelength (or frequency) change of the main peak of the waveform and the time at that time.

  The arithmetic circuit 50 is composed of, for example, a CPU (Central Processing Unit). The arithmetic circuit 50 outputs a control signal for instructing the time delay amount to the optical time delay device 21. In addition, the arithmetic circuit 50 acquires the electric field time waveform of the THz wave pulse from the spectroscopic detection unit 48, and Fourier transforms the electric field time waveform to calculate the frequency spectrum of the THz wave pulse. Here, as the amount of time delay indicated by the control signal, for example, a THz wave pulse output from the THz wave generation source 12 overlaps in time with a chirp pulse output from the chirp generation optical device 22 (in terms of time). There is a time delay amount for synchronizing). Further, for example, there is a time difference between main peaks of a time waveform of a THz wave pulse with and without a measurement object. This will be described later.

  In the terahertz wave spectrum measuring apparatus 1 according to the first embodiment described above, one spectroscopic detection unit 48 composed of a spectroscope with a multi-element detector or the like divides the light receiving region into reference light and signal light. However, it is also possible to adopt a configuration in which two spectroscopes with a multi-element detector share the detection of the reference light and the detection of the signal light (not shown).

(Overview of operation)
An outline of the operation of the terahertz wave spectrum measuring apparatus 1 having the above configuration will be described below.

  The femtosecond laser output from the laser generator 10 is split into two pump light and probe light by the beam splitter 11 (first branching step). The pump light is guided to the THz wave generation source 12 to generate a THz wave pulse (THZ wave generation process). After passing through the optical time delay device 21, the probe light is shaped into a chirp pulse having a time width of about 20 picoseconds or more using the chirp generation optical device 22 (chirp conversion step). From the time width of the chirp pulse, the frequency resolution Δω of the THz spectrometer is obtained according to the following equation.

  As a result, a frequency resolution of several tens of GHz, which is similar to the terahertz spectroscopy using the conventional method A, is obtained.

  The frequency resolution Δν of the spectroscopic detection unit 48 that detects the chirp pulse is determined based on the value of the time resolution necessary for resolving the time waveform of the THz wave pulse to be measured. If the required time resolution is Δτ and the size of the chirp plate is γ,

で得られる周波数分解能Δνが分光器の周波数分解能値の目安とされる。

The frequency resolution Δν obtained by the above is used as a standard for the frequency resolution value of the spectrometer.

Based on this guideline, it is possible to simultaneously realize a measurement frequency band and frequency resolution comparable to those of terahertz spectroscopy using the conventional method A. For example, if a laser with a pulse width of about 100 femtoseconds (800 nm center) is used to obtain a time resolution of about 200 femtoseconds with a chirp plate size of 0.2 THz ² , it is required for a spectrometer. The wavelength resolution can be calculated.

  The frequency resolution Δν is 0.04 THz from the following calculation formula.

  Since 800 nm = 375 THz, the wavelength resolution is

  Therefore, a spectrometer having a wavelength resolution of about 0.1 nm is used.

  The chirp pulse generated and output by the chirp generating optical device 22 is passed through the polarizer 23 and then branched into two by the beam splitter 31 (second branching step). One is guided to the reference light input unit 32 as it is. In the optical element array 40, the other is superimposed on the THz wave pulse output from the THz wave generation source 12 in the electro-optic crystal 43, and the chirp pulse is modulated by the THz wave electric field (modulation process). Thereafter, the light is guided to the signal light input unit 45 through the polarizer 44. The spectral detection unit 48 acquires reference light and signal light (reference light detection step, signal light detection step).

  As described above, the terahertz wave spectrum measuring apparatus 1 can acquire the reference light and the signal light.

(Time waveform acquisition method)
Since the operation outline has been described above, the time waveform acquisition method according to the first embodiment will be described.

(First detection step)
In the first detection step described below, the reference light R (t) _off and the signal light S (t) _off are detected simultaneously without generating a THz wave.

  The femtosecond laser generated by the laser generator 10 is split into two beams, pump light and probe light, by the beam splitter 11 (first branching step). The pump light is guided toward the THz wave generation source 12, but is blocked by a shutter (not shown). Thereby, the THz wave generation source 12 does not generate a THz wave pulse. A shutter (not shown) may be provided behind the THz wave generation source 12 to block the generated THz wave pulse. On the other hand, the probe light is shaped into a chirp pulse using the chirp generation optical device 22 after passing through the optical time delay device 21 (chirp conversion step).

  The chirp pulse is branched into two by the beam splitter 31 (second branching step). One is guided to the reference light input unit 32. The other is passed through the optical element array 40 and guided to the signal light input unit 45. Since no THz wave pulse is output from the THz wave generation source 12, no modulation is made in the optical element array 40. Thereafter, the light is guided to the signal light input unit 45. In the spectroscopic detection unit 48, the reference light R (t) _off and the signal light S (t) _off are simultaneously acquired.

  Based on the acquired reference light R (t) _off and signal light S (t) _off, the arithmetic circuit 50 calculates the branching ratio S (t) _off / R (t) _off of the beam splitter 31.

(Second detection step)
In the second detection step, a THz wave is generated and the reference light R (t) _on and the signal light S (t) _on are detected simultaneously.
The femtosecond laser generated by the laser generator 10 is split into two beams, pump light and probe light, by the beam splitter 11 (first branching step). The pump light is guided to the THz wave generation source 12, and the THz wave generation source 12 generates a THz wave pulse (THz wave generation step). On the other hand, the probe light is shaped into a chirp pulse by the chirp generation optical device 22 after passing through the optical time delay device 21 (chirp conversion step).

  The chirp pulse is branched into two by the beam splitter 31 (second branching step). One is guided to the reference light input unit 32. The other is superimposed on the THz wave pulse output from the THz wave generation source 12 in the electro-optic crystal 43 in the optical element array 40, and the chirp pulse is modulated by the THz wave electric field (modulation step). Thereafter, the light is guided to the signal light input unit 45. In the spectroscopic detection unit 48, the reference light R (t) _on and the signal light S (t) _on are acquired simultaneously.

(Calculation process)
Using the branching ratio S (t) _off / R (t) _off calculated in the first detection step and the reference light R (t) _on acquired in the second detection step, a THz wave A chirp pulse I (t) _off that is not modulated by a pulse is calculated by the following equation (1).

The signal light S (t) _on acquired simultaneously with the reference light R (t) _on is a chirp pulse I (t) _on modulated with a THz wave pulse (I (t) _on = S (t ) _on), the time waveform E _THz (t) of the THz wave pulse is calculated by the following equation (2).

  Thereby, effective simultaneous measurement of I (t) _on and I (t) _off is performed, and distortion caused by the fluctuation of the laser of the time waveform of the THz wave pulse derived from the equation (2) can be removed.

  Note that the branching ratio S (t) _off / R (t) _off calculated in the first detection step may be stored in a storage unit (not shown), and thereafter only the second detection step may be executed. .

(Transmission spectrum measurement method)
Since the time waveform acquisition method has been described above, the spectrum measurement method of the THz wave transmitted through the measurement object will be described next.

  In the transmission spectrum measurement method, the time waveform of the THz wave pulse when the measurement object is not disposed at the predetermined measurement position and when the measurement object is disposed is acquired based on the time waveform acquisition method described above (first). 1 time waveform acquisition step, second time waveform acquisition step). Based on the time difference between the main peaks of the two acquired time waveforms, the arithmetic circuit 50 calculates a delay amount of the second time waveform with respect to the first time waveform (a time delay amount of the transmitted THz wave) (time). Delay amount calculation step).

  The arithmetic circuit 50 outputs a control signal to the optical time delay device 21 so as to delay the time by the calculated time delay amount, delays the time, and acquires the time waveform of the transmitted THz wave of the measurement object again. (Third time waveform acquisition step). Then, based on the first time waveform and the third time waveform, the arithmetic circuit 50 calculates the spectrum of the THz wave that has passed through the measurement object (THz spectrum calculation step).

  As described above, since the transmitted THz wave with and without the measurement object at the measurement position modulates the same time region in the chirp pulse, the dependence of the device response of the terahertz wave spectrum measurement device 1 on the THz wave modulation region is excluded. The time waveform of the THz wave pulse can be acquired. And it is possible to measure a THz spectrum without distortion caused by the effect.

  In addition, acquisition of a time waveform of a THz wave pulse without a measurement object (first time waveform acquisition process), acquisition of a time waveform of a THz wave pulse with a measurement object (second time waveform acquisition process), Although the time waveform acquisition (third time waveform acquisition step) of the THz wave pulse in a state where there is a measurement object performed by correcting the time difference between the two THz time waveforms can be performed using a single laser pulse, respectively. In order to increase the signal / noise ratio (S / N ratio), data obtained by performing a plurality of measurements may be integrated to calculate a THz spectrum.

Example 1
In the terahertz wave spectrum measuring apparatus 1 having the configuration shown in FIG. 1, a femtosecond laser having a pulse width of about 150 femtoseconds, a pulse energy of about 0.5 mJ, a pulse repetition of about 1 kHz, and a center wavelength of about 800 nm is used. The output THz spectrum was measured. An indium arsenide (InAs) substrate having a thickness of 0.5 mm was used as the THz wave generating source 12, and a THz wave pulse was generated by irradiating the surface with a femtosecond laser. A chirp pulse of about 0.2 THz ² was generated using a diffraction grating pair of 1200 lines / mm as the chirp generation optical device 22. A zinc telluride (ZnTe) crystal was used as the electro-optic crystal 43. A spectroscope (30 cm, 1800 lines / mm) with a CCD camera (1340 × 400 pixels) was used for the spectroscopic detection unit 48. The resolution of the spectrometer is about 0.1 nm. The detection area was divided into two (1340 × 200 pixels), and the reference light and the signal light were simultaneously detected in the two areas.

  FIG. 2 shows a Fourier transform spectrum of a time waveform of a THz wave pulse derived by measurement. The horizontal axis indicates the frequency of the derived THz wave pulse, and the vertical axis indicates the electric field amplitude of the THz wave pulse.

  As shown in FIG. 2, since the measurement was performed in the atmosphere, there are some indentations due to water vapor absorption in the measured frequency domain.

  The time waveform of the THz wave pulse measured in Example 1 is that of the THz wave pulse acquired by using the A method from the low frequency region to the half value frequency (about 1.5 THz) of the maximum value of the THz wave electric field amplitude. It almost matches the time waveform. From this, it can be seen that Example 1 has almost the same measurement frequency band as the A method.

  Further, when the frequency resolution is evaluated by the full width at half maximum of the spectrum by paying attention to a sharp dent in the spectrum caused by absorption of water vapor in the vicinity of about 1.1 THz, the frequency resolution of Example 1 is about 0.03 THz, which is the same as that of the A method. Has reached. From this, it can be seen that Example 1 has substantially the same frequency resolution as the A method.

  As described above, the chirp pulse of about 20 picoseconds and the high wavelength resolution (about 0.1 nm) of the spectroscopic detection unit 48 enable the measurement frequency band and frequency resolution similar to those of the conventional method A to be achieved in the first embodiment. Achieved. Both values give results superior to those of the B and C methods.

  In the first embodiment, it is only necessary to chirp the femtosecond laser directly without using the method of generating a white pulse as in the C method.

  FIG. 3 shows the experimental results of the time waveform of the THz wave pulse detected by changing the frequency resolution of the spectroscopic detector 48. The horizontal axis represents time (picoseconds), and the vertical axis represents the electric field amplitude of the THz wave pulse.

  As shown in FIG. 3, when the frequency resolution of the spectroscopic detection unit 48 is deteriorated, the pulse width of the THz wave pulse is widened. That is, the frequency resolution of the spectroscopic detection unit 48 is an important parameter.

When the resolution of the spectroscope is Δω (THz) and the size of the chirp plate is γ (THz ² ), the time resolution Δτ of the measurement system can be roughly evaluated by Δω / γ.

In the case of Example 1, when using a 150 femtosecond laser (800 nm center), the size of the chirp plate is 0.2 THz ² , and the resolution of the spectrometer is 0.1 nm, the time resolution Δτ = Δω / γ = 0. 1 × 10 ⁻⁹ /(0.2×10 ¹² ) = about 200 femtoseconds, and has a time resolution Δτ that can sufficiently resolve the original waveform of the THz wave evaluated by the A method. In the case of this chirp plate, since the observation time width is several tens of picoseconds, a frequency resolution of several tens of GHz is obtained. This theoretical result corresponds well with Example 1 (FIG. 2).

  Derivation of the time waveform of the THz wave pulse is common to the B method, the C method, and the first embodiment, and is obtained by the equation (2).

  In the B method and the C method, only a signal corresponding to the signal light in the double beam method according to the first embodiment is measured, and the reference light is not measured simultaneously. Therefore, it is necessary to measure I (t) _on and I (t) _off with different probe pulses. In such a case, if the chirp pulse changes from pulse to pulse due to laser fluctuations, I (t) _on becomes a signal with changes due to laser fluctuations in addition to THz wave modulation on I (t) _off. The time waveform of the THz wave pulse derived from the equation (2) may be distorted.

  On the other hand, in the first embodiment, the reference light R (t) _off and the signal light S (t) _off are simultaneously measured without entering the THz wave pulse into the measurement system, and the branching ratio S (t) _off / R. (t) _off is acquired in advance. Then, a THz wave pulse is made incident on the measurement system, the reference light R (t) _on and the signal light S (t) _on are simultaneously measured, and I (t) _off used in equation (2) using the branching ratio value. Is calculated by equation (1). Thereby, the chirp pulse modulated by the THz wave pulse and the chirp pulse not modulated are measured substantially simultaneously.

  Therefore, in the first embodiment, the distortion of the time waveform of the THz wave pulse due to the fluctuation of the laser can be eliminated.

  FIG. 4 is a time waveform of the THz wave pulse measured when the time timing at which the THz wave pulse and the chirp pulse meet in the electro-optic crystal 43 is changed. The horizontal axis indicates the wavelength of the chirp pulse, and the vertical axis indicates the electric field amplitude of the THz wave pulse. The horizontal axis can be converted to a time axis using the value of the char plate. For example, when converting the frequency, it can be converted into time by dividing (measurement frequency step) by (char plate).

  As shown in FIG. 4, if the time timing at which the THz wave pulse and the chirp pulse meet in the electro-optic crystal 43 is changed, the time waveforms do not match. The apparatus response of the terahertz wave spectrum measuring apparatus 1 (see FIG. 1) is dependent on the temporal overlap of the THz wave pulse and the chirp pulse.

  When the THz wave pulse is transmitted through a measurement object having a refractive index different from 1 in the THz frequency region, the timing of encountering the chirp pulse in the electro-optic crystal 43 is changed as compared with the case where the THz wave pulse is not transmitted. Therefore, in order to remove the effect from the derivation of the THz spectrum of the measurement object, first, the time waveform of the THz wave pulse is measured when the measurement object is arranged at a predetermined measurement position and when it is not arranged And find the time difference between the main peaks. The optical time delayer 21 delays the time by the time difference, and the time waveform of the transmitted THz wave pulse of the measurement object is acquired again.

  Thereby, through the THz wave modulation in the same time region in the chirp pulse, the transmitted THz wave pulse can be measured when there is no object to be measured, and using the time waveform of both THz wave pulses, A THz spectrum of the measurement object is obtained. Therefore, it is possible to remove the distortion of the THz spectrum caused by the device response depending on the temporal overlap of the THz wave pulse and the chirp pulse.

  According to the terahertz wave spectrum measuring apparatus 1 according to the first embodiment, a time waveform of a THz wave is acquired based on the reference light and the signal light detected at the same time. Thereby, even when the laser pulse of the femtosecond laser fluctuates for each pulse, the reliability of the measurement result can be improved.

  The chirp generation optical device 22 converts a femtosecond laser having a spectral width of about 10 nm into a chirp pulse linearly chirped at several tens of picoseconds or more. Accordingly, it is possible to realize a frequency resolution of several tens of GHz, which is similar to the method A.

  Further, an optical time delay device (21) is used so that the THz wave generated from the THz wave generation source 12 and the chirp pulse branched by the beam splitter 31 overlap in time in the electro-optic crystal 43. Delays the probe light. Therefore, the THz wave and the chirp pulse can be synchronized, and the modulation characteristic of the THz wave carried on the chirp pulse can be acquired.

  Furthermore, the probe light is delayed by the optical time delay device 21 by the amount of time delay calculated based on the first time waveform and the second time waveform of the THz wave. Therefore, the THz wave that modulates the same time domain in the chirp pulse with and without the measurement object can remove the THz wave modulation domain dependence and can acquire a THz spectrum without distortion.

  Furthermore, the reference light and the signal light are detected by the spectroscopic detection unit 48 which is one (2 channels × spectrometer with multi-element detector). Therefore, it becomes easy to detect the signal light and the reference light at the same time.

  Furthermore, the reference light and the signal light are detected by two spectrometers with a multi-element detector. Therefore, a small spectroscope with a multi-element detector can be adopted, and the degree of freedom in arrangement can be increased.

According to the time waveform acquisition method according to the first embodiment, the reference light R (t) _off and the signal light S (t) _off detected in the first detection process, and the second detection process. Using the signal light S (t) _on and the reference light R (t) _on, the time waveform E _THz (t) of the THz wave is calculated and acquired. Thereby, effective simultaneous measurement of the chirp pulse I (t) _on modulated by the THz wave and the chirp pulse I (t) _off not modulated can be performed, and the distortion caused by the fluctuation of the laser can be removed.

Further, any one of the stored reference light R (t) _off, signal light S (t) _off, branching ratio S (t) _off / R (t) _off, and signal light in the second detection step A time waveform E _THz (t) of a THz wave is calculated and acquired using S (t) _on and the reference light R (t) _on. Therefore, after the first detection step is executed once, only the second detection step can be executed to acquire the time waveform of the THz wave.

Further, the chirp pulse I (t) _off that is not modulated with the THz wave is calculated using the branching ratio S (t) _off / R (t) _off, and the signal light S (t (t) detected in the second detection step is calculated. ) _on is used to calculate and acquire the _THz time waveform E _THz (t). Therefore, effective simultaneous measurement of the chirp pulse I (t) _on modulated by the THz wave and the chirp pulse I (t) _off not modulated can be performed, and distortion caused by the fluctuation of the laser can be removed.

  According to the terahertz measurement method according to the first embodiment, the THz wave transmitted through the measurement object by delaying the probe light by the time delay amount calculated based on the first time waveform and the second time waveform. The third time waveform is acquired and the spectrum of the THz wave is calculated. Thereby, the THz wave which modulates the same time domain in the chirp pulse can remove the THz wave modulation domain dependence, and can acquire a THz spectrum without distortion.

  Further, the time delay amount is calculated based on the time difference between the main peaks of the first time waveform and the second time waveform. Therefore, the THz wave that modulates the same time domain in the chirp pulse with and without the measurement object can derive a time waveform excluding THz wave modulation domain dependence.

  Further, a time delay amount is calculated based on the stored first time waveform and the acquired second time waveform. Therefore, the first time waveform acquisition step can be omitted, and the measurement time can be shortened.

  Furthermore, a time waveform of the THz wave is acquired based on the reference light and the signal light detected a plurality of times. Therefore, the signal / noise ratio (S / N ratio) can be increased.

(Second Embodiment)
FIG. 5 is a configuration diagram of a terahertz wave spectrum measuring apparatus 100 as a second embodiment of the terahertz measuring apparatus according to the present invention. The same components as those in the terahertz wave spectrum measuring apparatus 1 shown in FIG.

  As shown in FIG. 5, the terahertz wave spectrum measuring apparatus 100 further converts the femtosecond laser branched and output by the beam splitter 11 between the beam splitter 11 and the THz wave generating optical system 12 into excitation light and pump light. A beam splitter 101 as an example of a third branching unit that branches into the first and second optical time delay units that receive the pump light branched and output by the beam splitter 101 and delay the output for a predetermined time. It differs from the terahertz wave spectrum measuring apparatus 1 shown in FIG. 1 in that it has an optical time delay device 102 as an example and a shutter 103 that opens and closes the optical path of the excitation light branched by the beam splitter 101.

  Here, the beam splitter 101 is composed of, for example, a half mirror beam splitter, a polarization beam splitter, or the like. The beam splitter 101 splits the femtosecond laser branched by the beam splitter 11 into two, that is, excitation light irradiated to the measurement target and pump light supplied to the optical time delay unit 102.

  The optical time delay unit 102 is configured by a combination of a plurality of folding mirrors (not shown), for example. The optical time delay unit 102 delays and outputs the pump light branched by the beam splitter 101 based on the control signal from the arithmetic circuit 50.

  The shutter 103 is composed of, for example, a black metal thin plate and its driving mechanism (not shown). The shutter 103 blocks the femtosecond laser as excitation light for optically exciting the measurement object based on the control signal from the arithmetic circuit 50.

  In the terahertz wave spectrum measuring apparatus 100 having the above configuration, the femtosecond laser that has passed through the beam splitter 11 is further divided into two types of pump light and pump light by the beam splitter 101.

  Excitation light is applied to the measurement object via the shutter 103. The pump light is delayed by a predetermined time by the optical time delay unit 102 and guided to the THz wave generation source 12.

  A THz wave pulse is irradiated from the THz wave generation source 12 to the measurement object that has been excited and irradiated with the excitation light. The transmitted THz wave transmitted through the measurement object is superimposed on the chirp pulse output from the chirp generating optical device 22 in the electro-optic crystal 43. The signal light is detected by the signal light input unit 45 and the spectral detection unit 48, and the time waveform of the THz wave pulse is acquired (time waveform acquisition step in the excited state).

  Subsequently, the excitation light is blocked by the shutter 103, and the measurement object returns to the ground state. A THz wave pulse is irradiated from the THz wave generation source 12, and the transmitted THz wave is superimposed on the chirp pulse by the electro-optic crystal 43. Similarly, the signal light is detected by the signal light input unit 45 and the spectral detection unit 48, and the time waveform of the THz wave pulse is acquired (time waveform acquisition step in the ground state).

  The time waveform of the two acquired THz wave pulses is Fourier transformed by the arithmetic circuit 50, and each THz spectrum is acquired. From the change in the THz spectrum before and after photoexcitation, functional analysis such as chemical reaction and phase change occurring in the measurement object by photoexcitation is performed (analysis process).

  According to the terahertz wave spectrum measuring apparatus 100 according to the second embodiment, a measurement object placed at a predetermined measurement position is irradiated with a femtosecond laser to be photoexcited, and the THz wave generation source 12 generates THz. Irradiate waves. Thereby, functional analysis such as a chemical reaction or a phase change occurring in the measurement object by photoexcitation can be performed.

  According to the terahertz measurement method according to the second embodiment, a time waveform of a transmitted THz wave transmitted through a photoexcited measurement object and a time waveform of a transmitted THz wave transmitted through a ground state measurement object are acquired. Analyzes changes that occur in the measurement object due to photoexcitation. Thereby, functional analysis such as a chemical reaction or a phase change occurring in the measurement object by photoexcitation can be performed.

  A wavelength conversion optical element (not shown) that converts the excitation wavelength of the excitation light by a nonlinear optical effect may be disposed after the beam splitter 101. Excitation light can be irradiated after being converted into a wavelength suitable for the measurement object.

  Here, the wavelength conversion optical element is composed of, for example, a nonlinear optical crystal such as beta barium bolite (BBO). The wavelength conversion optical element generates an optical pulse as excitation light having an excitation wavelength suitable for the measurement object by a nonlinear optical effect using the excitation light dispersed by the beam splitter 101. The wavelength conversion optical element generates, for example, an 800 nm double wave or triple wave optical pulse or an optical pulse whose wavelength is continuously variable in the visible region.

(Inspection equipment)
FIG. 6 is a schematic diagram of an inspection apparatus 99 that uses the terahertz wave spectrum measuring apparatus 1 shown in FIG.

  As shown in FIG. 6, the inspection apparatus 99 includes a terahertz wave spectrum measuring apparatus 1, a belt conveyor 98 as an example of a conveying means that conveys a container 97 containing an object to be measured at a predetermined speed, and various substances. And a database 96 for storing the THz spectrum in the THz region.

  Here, the database 96 is composed of, for example, a personal computer with a built-in hard disk drive (not shown) as an example of storage means. The measurement result output from the terahertz wave spectrum measuring apparatus 1 is collated with the stored THz spectrum to make a predetermined determination.

  The terahertz wave spectrum measuring apparatus 1 irradiates a THz wave toward a container 97 moving on the belt conveyor 98. And the THz wave which permeate | transmits the measuring object contained in the container 97 is received. Here, the container 97 is made of, for example, paper or plastic that transmits a THz wave pulse.

  If the terahertz wave spectrum measuring apparatus 1 having the specification shown in the first embodiment is used, for example, measurement at a time interval of about 1 millisecond is possible. Since the measurement object conveyed by the belt conveyor 98 can be inspected at the measurement time interval of the terahertz wave spectrum measuring apparatus 1 at the fastest speed, high-speed inspection is realized.

  For example, it has recently become clear that prohibited substances such as stimulants and dangerous substances such as explosives have a characteristic fingerprint spectrum in the THz region. It is possible to determine whether or not such a drug is included in the measurement object by inputting such fingerprint spectrum data in advance into the database 96 and comparing it with the measurement result of the measurement object. As described above, since paper and plastic transmit a THz wave pulse, for example, even in a state of being packed in a mail or the like, it can be measured and determined.

  According to the inspection apparatus 99 according to the present embodiment, the transmitted THz wave that passes through the measurement object conveyed to the measurement position by the belt conveyor 98 is measured. Thereby, high-speed inspection is realized.

  The database 96 also stores THz spectra of various substances in the THz region. Therefore, by comparing the transmitted THz wave of the measurement object with the data stored in the database 96, the measurement object can be easily specified.

  The present invention relates to materials evaluation of semiconductors, ceramics, superconductors, polymers, etc., chemicals, food component analysis, biological molecule analysis, gene analysis, protein function analysis, cancer diagnosis, medical material inspection, illegal drugs It can be used in the field of environmental measurement, such as inspection of toxic substances, inspection of explosives, inspection of liquid in PET bottles, control of illegal gasoline, monitoring of exhaust gas and toxic gas.

DESCRIPTION OF SYMBOLS 1,100 ... Terahertz wave spectrum measuring device (terahertz measuring device), 10 ... Laser generating part (light pulse generating means), 11 ... Beam splitter (first branching means), 12 ... THz wave generating source (terahertz wave generating optics) System), 21 ... optical time delay device, 22 ... chirp generation optical device (chirp generation optical system), 31 ... beam splitter (second branch means), 32 ... reference light input section (reference light detection means), 43 DESCRIPTION OF SYMBOLS ... Electro-optic crystal, 45 ... Signal light input part (signal light detection means), 48 ... Spectral detection part (reference light detection means, signal light detection means, spectrometer with multi-element detector), 50 ... Arithmetic circuit (calculation means) ), 96 ... Database, 98 ... Belt conveyor (conveying means), 99 ... Inspection apparatus, 101 ... Beam splitter (third branching means), 103 ... Shutter

Claims (16)

An optical pulse generating means (10) for generating an ultrashort optical pulse;
A first branching means (11) for branching the ultrashort light pulse generated by the light pulse generating means (10) into a pump light and a probe light;
A terahertz wave generating optical system (12) that receives the pump light of the ultrashort light pulse branched by the first branching means (11) and generates a terahertz wave (THz wave);
A chirp generation optical system (22) for converting the probe light of the ultrashort light pulse branched by the first branching means (11) into a chirp pulse obtained by linear chirp;
Second branching means (31) for branching the chirp pulse converted by the chirp generation optical system (22) into two;
Reference light detection means (32, 48) for detecting a spectrum using one of the chirped pulses branched by the second branching means (31) as reference light;
The other one of the chirped pulses branched by the second branching means (31) is modulated by the electro-optic effect induced by the electric field signal of the THz wave generated by the terahertz wave generating optical system (12). An electro-optic crystal (43),
Signal light detection means (45, 48) for receiving light modulated and output by the electro-optic crystal (43) and detecting a spectrum as signal light;
Calculation means (50) for obtaining a time waveform of the THz wave based on the reference light detected by the reference light detection means (32, 48) and the signal light detected by the signal light detection means (45, 48) detected simultaneously. And a terahertz measuring device (1).   The terahertz measurement apparatus according to claim 1, wherein the chirp generation optical system (22) converts the ultrashort light pulse into the chirp pulse linearly chirped at 10 picoseconds or more. The terahertz measuring device according to claim 1 or 2,
An optical time delay device (21) for delaying the probe light of the ultrashort light pulse branched by the first branching means (11) by a predetermined time;
The THz wave generated from the terahertz wave generating optical system (12) and the other one of the chirp pulses branched by the second branching means (31) are temporally generated in the electro-optic elementary crystal (43). The terahertz measuring device, wherein the optical time delay device (21) delays the probe light so as to overlap. The terahertz measurement device according to claim 3,
The calculation means (50) includes a first time waveform of the THz wave acquired in a state where there is no measurement object at a predetermined measurement position, and the THz wave transmitted through the measurement object arranged at the measurement position. A time delay amount is calculated based on the second time waveform;
The terahertz measurement device, wherein the optical time delay device (21) delays the probe light by the time delay amount calculated by the computing means (50). In the terahertz measuring device according to claim 4,
Third branching means (101) for further branching the pump light of the ultrashort light pulse branched by the first branching means (11) into two;
A shutter (103) for opening and closing one optical path of the ultrashort light pulse branched by the third branching means (101),
The measurement object arranged at the measurement position is irradiated with one of the ultrashort light pulses branched by the third branching means (101) to be photoexcited, and the ultrashort light pulse The terahertz measuring device irradiates the THz wave from the terahertz wave generating optical system (12) receiving the other. A first branching means (11) for branching the ultrashort light pulse into two of pump light and probe light;
A terahertz wave generating optical system (12) that receives the pump light of the ultrashort light pulse branched by the first branching means (11) and generates a terahertz wave (THz wave);
A chirp generation optical system (22) for converting the probe light of the ultrashort light pulse branched by the first branching means (11) into a chirp pulse obtained by linear chirp;
Second branching means (31) for branching the chirp pulse output from the chirp generating optical system (22) into two;
Branched by reference light detection means (32, 48) for detecting a spectrum using one of the chirped pulses branched by the second branching means (31) as reference light and the second branching means (31) An electro-optic crystal (43) that receives the other of the chirped pulses and modulates the electro-optic effect induced by the electric field signal of the THz wave generated by the terahertz wave generating optical system (12);
Signal light detection means (45, 48) for detecting a spectrum using the light modulated by the electro-optic crystal (43) as signal light;
A terahertz measurement device comprising: a calculation means (50) for calculating a time waveform of the THz wave generated by the terahertz wave generation optical system (12) based on the reference light and the signal light detected simultaneously. A time waveform acquisition method executed in 1),
A first detection step of simultaneously detecting the reference light R (t) _off and the signal light S (t) _off without generating a THz wave;
A second detection step of generating a THz wave to simultaneously detect the signal light S (t) _on and the reference light R (t) _on;
The reference light R (t) _off and the signal light S (t) _off detected in the first detection step, and the signal light S (t) _on and the reference detected in the second detection step. And a calculation step of calculating and acquiring the time waveform E _THz (t) of the THz wave using light R (t) _on. In the time waveform acquisition method according to claim 6,
From the reference light R (t) _off and the signal light S (t) _off detected in the first detection step, and the reference light R (t) _off and the signal light S (t) _off A storage step of storing either of the calculated branching ratios S (t) _off / R (t) _off;
In the calculation step, one of the reference light R (t) _off and the signal light S (t) _off and the branching ratio S (t) _off / R (t) _off stored in the storage step, In addition, using the signal light S (t) _on and the reference light R (t) _on in the second detection step, the time waveform E _THz (t) of the THz wave is calculated and acquired. A time waveform acquisition method characterized by In the time waveform acquisition method according to claim 6 or 7,
A branching ratio S (t) _off / R (t) _off is calculated based on the reference light R (t) _off and the signal light S (t) _off detected in the first detection step;
The chirp not modulated with the THz wave using the reference light R (t) _on detected in the second detection step and the calculated branching ratio S (t) _off / R (t) _off Calculate pulse I (t) _off,
The signal light S (t) _on detected in the second detection step is regarded as the chirp pulse I (t) _on modulated with the THz wave, and the time waveform E _THz (t ) Is obtained by calculating the time waveform. In the time waveform acquisition method according to claim 8,
The chirp pulse I (t) _off that is not modulated with the THz wave is expressed by Equation (1).

Is calculated by
The time waveform E _THz (t) of the THz wave is expressed by Equation (2).

The time waveform acquisition method characterized by acquiring by this. A first time waveform acquisition step of acquiring a first time waveform of a THz wave in a state where there is no measurement object at a predetermined measurement position;
A second time waveform acquisition step of acquiring a second time waveform of the THz wave transmitted through the measurement object arranged at the measurement position;
A time delay amount for calculating a time delay amount based on the first time waveform acquired in the first time waveform acquisition step and the second time waveform acquired in the second time waveform acquisition step. A calculation process;
A third time waveform acquisition step of acquiring a third time waveform of the THz wave transmitted through the measurement object by delaying the probe light by the time delay amount calculated in the time delay amount calculation step;
Based on the first time waveform acquired in the first time waveform acquisition step and the third time waveform acquired in the third time waveform acquisition step, the measurement object is transmitted. A THz spectrum calculating step for calculating a spectrum of a THz wave.   The terahertz measurement method according to claim 10, wherein in the time delay amount calculating step, the time delay amount is calculated based on a time difference between main peaks of the first time waveform and the second time waveform. Characteristic terahertz measurement method. The terahertz measurement method according to claim 10 or 11,
A first time waveform storing step of storing the first time waveform;
In the time delay amount calculation step, the time delay amount is calculated based on the first time waveform and the second time waveform stored in the first time waveform storage step. Measurement method.   The terahertz measurement method according to claim 10, wherein the first and third time waveform acquisition steps are executed a plurality of times, and the time waveform of the THz wave is acquired based on the detected reference light and signal light. Characteristic terahertz measurement method. A time waveform acquisition step in an excited state in which a measurement object is irradiated with an ultrashort light pulse as excitation light, and a time waveform of a transmitted THz wave transmitted through the photo-excited measurement object is acquired;
A time waveform acquisition step in a ground state for acquiring a time waveform of the transmitted THz wave transmitted through the measurement object in the ground state;
The time waveform of the transmitted THz wave in the excited state acquired in the time waveform acquisition step in the excited state and the transmitted THz wave in the ground state acquired in the time waveform acquisition step in the ground state A terahertz measurement method, comprising: an analysis step of analyzing a change generated in the measurement object by optical excitation based on the time waveform. Transport means (98) for transporting the measurement object to a predetermined measurement position;
The terahertz measurement device (1) according to claim 1, which measures a transmitted THz wave transmitted through the measurement object conveyed to the measurement position by the conveyance means (98). 99). The inspection apparatus according to claim 15,
A database (96) for storing THz spectra of various substances in the THz region;
The measurement object is inspected based on the transmitted THz wave of the measurement object measured by the terahertz measurement device (1) and the THz spectrum stored in the database (96). Inspection device.

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