JP2010175332A - Terahertz spectrometer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a terahertz spectrometer capable of correcting a change in the light path length of terahertz light so as not to cause a measuring error. <P>SOLUTION: The terahertz spectrometer 1 is equipped with an emission element 10 for emitting terahertz light upon the reception of a laser beam, a light receiving element 20 receiving the terahertz light to detect a detection signal, the sample S arranged between the emission element 10 and the light receiving element 20, and an optical unit 40 for altering the light path length of the terahertz light in the light path up to the light receiving element 20 from the emission element 10 through the sample S. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a terahertz spectrometer using terahertz light.

In recent years, a terahertz time domain spectroscopy (THz-TDS: THz-TimeDomain) in which a sample is irradiated with light in a terahertz frequency range (0.1 THz to 10 THz) and transmitted light transmitted through the sample or reflected light reflected from the sample is detected. (Spectroscopy) is drawing attention.
This terahertz time-domain spectroscopy uses a terahertz pulse spectrometer to measure the electric field strength of terahertz light and performs Fourier transform on the time-series data to obtain frequency dependence such as the amplitude strength and phase of terahertz light. The complex refractive index and absorption spectrum of the sample can be known by comparing the electric field strength of the terahertz light when the sample is inserted and when nothing is inserted (see Patent Document 1).

The terahertz pulse spectrometer is configured such that terahertz light generated from a light emitting element is incident on a sample via a parabolic mirror, and terahertz light transmitted through the sample is incident on a light receiving element via a parabolic mirror. Measure.
At this time, the terahertz light generated from the terahertz light generating element is condensed in the vicinity of the sample by the first condensing optical unit, and the terahertz light that diverges after passing through the sample is incident on the light receiving element by the second condensing optical unit. There is a conventional example of collecting and receiving light (see Patent Document 2 and Patent Document 3).

WO00 / 079248 JP 2004-212110 A JP 2006-133178 A

However, in the conventional example shown in Patent Document 2, since the terahertz light focus is adjusted on the light receiving element in a state where the sample is not inserted, when the sample is inserted, the light receiving unit changes due to the optical path length change. There is a problem that the focal position is shifted, causing a measurement error.
In order to cope with this problem, in the conventional example shown in Patent Document 3, the condenser lens is moved to correct the optical path length. However, due to the wavelength dispersion of the optical material used for the lens, when the terahertz light is transmitted through the condenser lens, chromatic aberration with different focal positions depending on the wavelength occurs. For this reason, this chromatic aberration causes a measurement error, and there is a problem that an accurate measurement result cannot be obtained.

  An object of the present invention is to provide a terahertz spectrometer capable of correcting a change in optical path length of terahertz light so as not to cause a measurement error.

[Application Example 1]
The terahertz spectrometer according to this application example is
A terahertz spectrometer comprising: a light emitting element that emits terahertz light; a light receiving element that receives the terahertz light to detect a detection signal; and a sample disposed on an optical path between the light emitting element and the light receiving element. A first condensing optical part for condensing the terahertz light emitted from the light emitting element in the vicinity of the sample; a second condensing optical part for condensing the terahertz light diverging in the optical path to the light receiving element; and the first condensing part. An optical unit that varies the length of the optical path is provided between the optical unit and the second condensing optical unit, and the optical unit includes a position adjustment mechanism that moves in the optical axis direction of the optical path. Terahertz spectrometer.
In this application example of this configuration, when the terahertz light passes through the sample, the spectral absorption of the terahertz light caused by the sample passing through the sample and the sample storage part (for example, the sample cell, the measured object flow path, etc.) The optical path length changes. At this time, since the terahertz spectrometer includes an optical unit for changing the optical path length, the optical path length change can be easily corrected.
For this reason, it is possible to easily eliminate the optical path length change caused by the sample container that causes measurement errors, and it is possible to detect only the spectral absorption caused by the sample.
Therefore, by introducing an optical unit into the terahertz spectrometer, a terahertz spectrometer capable of realizing excellent measurement accuracy can be obtained.

[Application Example 2]
In the terahertz spectrometer according to this application example, the optical unit includes a plurality of plane mirrors.
In this application example having this configuration, since a mirror is used in the optical unit, there is no possibility of causing chromatic aberration as in the case where optical path length correction is performed by a lens.
Therefore, since only the optical path length change caused by the sample container that causes the measurement error is corrected, more excellent measurement accuracy can be realized. Further, since only a general-purpose plane mirror is used, the measurement accuracy of the terahertz spectrometer can be improved easily and at low cost.

[Application Example 3]
In the terahertz spectroscopic measurement apparatus according to this application example, the angles of the mirror surfaces of the plurality of plane mirrors intersect at right angles.
In this application example having this configuration, since the terahertz light incident on the optical unit and the emitted terahertz light are substantially parallel to each other, the optical path can be simply moved without changing the angles of the plurality of mirrors. The length can be changed.
Therefore, a mechanism for changing the angle of the mirror is unnecessary, and the optical path length changing device can be miniaturized.

[Application Example 4]
In the terahertz spectrometer according to this application example, the optical unit is arranged in a first optical path between the first condensing optical unit and the sample.
In this application example having this configuration, the same effects as described above can be achieved.

[Application Example 5]
In the terahertz spectrometer according to this application example, the optical unit is arranged in a second optical path between the sample and the second condensing optical unit.
In this application example having this configuration, the same effects as described above can be achieved.

[Application Example 6]
In the terahertz spectrometer according to this application example, the optical unit adjusts the optical axis position by holding and moving a sample between the plane mirrors.
In this application example having this configuration, the same effects as described above can be achieved.

Embodiments of the present invention will be described with reference to the drawings. Here, in each embodiment, the same components are denoted by the same reference numerals, and description thereof is omitted or simplified.
(First embodiment)
First, a terahertz spectrometer which is a first embodiment of the present invention will be described with reference to FIGS.
FIG. 1 shows a configuration diagram of a terahertz spectrometer in the first embodiment of the present invention.
The terahertz spectrometer 1 of the present embodiment includes a pulse light source 100 as a laser transmitter that generates pulses at a predetermined period, and a beam for separating a pulse wave generated from the pulse light source 100 into excitation light and probe light. A splitter 31, a pumping optical system 30 for guiding the pumping light, a light emitting element 10 for generating a terahertz optical path 32 based on the pumping light guided by the pumping optical system 30, and a terahertz generated by the light emitting element 10 A first condensing optical unit 13 that condenses light in the vicinity of the sample S as a sample, a second condensing optical unit 23 that condenses the terahertz light transmitted through the sample S and collected on the light receiving element 20, and the sample S An optical path length correcting device 40 as an optical unit that corrects a change in optical path length of the terahertz light caused by the sample cell, and the probe light is guided. Probe optical system 33, a light receiving element 20 that outputs a terahertz light detection signal by receiving probe light guided by the probe optical system 33 and terahertz light transmitted through the sample S, and detection from the light receiving element 20 It comprises a spectral processing unit 60 as an analysis means for processing signals.
Here, as the pulse light source 100, for example, a pulse laser device such as a femtosecond pulse laser can be used.

FIG. 2A shows a configuration diagram of the optical switch element (light emission) in the first embodiment of the present invention, and FIG. 2B shows the optical switch element (light reception) in the first embodiment of the present invention. The block diagram of is shown.
In this embodiment, an optical switch element 11 shown in FIG. 2A is used. The optical switch element 11 is formed of a semiconductor substrate 73 such as GaAs that responds at high speed, and a photoconductive thin film 74 such as low-temperature grown GaAs formed on the semiconductor substrate 73.
A parallel transmission line 72 including transmission lines 72a and 72b is formed on the photoconductive thin film 74, and a single optical switch unit 70 including a minute dipole antenna is provided at the center thereof.
In the center of the optical switch element 11, there is a minute gap 71 of about several μm, for example, and an appropriate bias voltage is applied to the gap 71 by a DC bias power supply 75.

  In such an optical switch element 11, when laser pulse light having energy higher than the semiconductor band gap is incident as an optical pulse between the gaps 71, free carriers are generated in the semiconductor, and a pulsed current flows. This pulsed current generates terahertz light in pulses.

In the present embodiment, the optical switch element 21 shown in FIG. 2B is used. The optical switch element 21 has the same configuration as the optical switch element 11.
However, the spectral processing unit 60 is connected to the gap 71 of the optical switch element 21 instead of the DC bias power supply 75.
In the optical switch element 21, the terahertz electromagnetic wave is focused on the optical switch unit 70 via the incident lens 22, and at the same time, the probe light pulse excites the gap 71. When carriers are generated by this excitation, a current proportional to the amplitude of the terahertz electromagnetic wave that reaches the optical switch element 21 at that moment flows. This current is supplied to the spectroscopic processing unit 60 as a current signal.

The traveling direction of the terahertz light generated by the optical switch element 11 is changed by the first condensing optical unit 13 that condenses near the sample (see FIG. 1).
In the terahertz optical path 32, a sample S having a sample cell or a measured object flow path in which a gas or liquid is accommodated or distributed as a measured object for spectroscopic measurement is disposed. The terahertz light is condensed and transmitted near the sample.

Further, when passing through the sample S, the optical path length of the terahertz light changes under the influence of the optical characteristics of the sample cell or the measured object flow path of the sample S. In the present embodiment, an optical path length correction device 40 that corrects this change in optical path length is provided.
FIG. 3 shows a block diagram of the optical path length correction apparatus in the first embodiment of the present invention.
As shown in FIG. 3, the optical path length correction device 40 includes a movable reflecting mirror 41 as an L-shaped mirror in which two flat mirrors are joined at a right angle, a support portion 43 that supports the mirror, and drive control thereof. And an optical path length correction control device 42.
The optical path length correction device 40 is used to drive and control the positions of the movable reflecting mirror 41 and the support portion 43 that supports the movable mirror 41 in parallel with the optical axis of the terahertz light by the optical path length correction control device 42. By driving and controlling the position of the movable reflecting mirror 41, the optical path length of the terahertz light is changed and controlled, and the change in the optical path length of the terahertz light transmitted through the sample S is corrected and eliminated.
Then, the corrected terahertz light is focused by the second condensing optical unit 23 and is incident on the optical switch element 21 by the incident lens 22 (see FIG. 1).
The drive control of the movable reflecting mirror 41 by the optical path length correction control device 42 is specifically performed by a support unit 43 placed on a rail (not shown) and a ball screw (not shown).

  Here, the exit lens 12 and the entrance lens 22 are formed of a translucent material that transmits terahertz light. For example, crystal, sapphire, polyethylene, polypropylene, silicon, germanium, fluororesin, diamond, translucency. It is made of a functional ceramic.

FIG. 4 shows a configuration diagram of the variable optical delay device in the first embodiment of the present invention.
The probe optical system 33 includes a variable optical delay device 50 as delay means (see FIG. 1). As shown in FIG. 4, the variable optical delay device 50 is for adjusting and setting the timing difference of the probe light with respect to the excitation light. The variable optical delay device 50 includes a movable reflecting mirror 51, a support portion 53 that supports the movable reflecting mirror 51, and an optical delay control device 52.
The optical delay control device 52 is for driving and controlling the position of the movable reflecting mirror 51. By driving and controlling the position of the movable reflecting mirror 51, the setting / change of the optical path length of the probe light is controlled, so that the irradiation timing difference between the excitation light and the probe light (terahertz light generation / detection timing difference) is set. Control changes.

As shown in FIG. 1, in the present embodiment, the spectral processing unit 60 includes a current-voltage conversion amplifier 61, a spectrum analysis device 62, and an analysis device 63.
The current-voltage conversion amplifier 61 is for converting the current signal supplied from the optical switch element 21 into a voltage signal.
The spectrum analyzer 62 is for performing frequency analysis of the voltage signal by obtaining an intensity distribution for each frequency of the voltage signal converted by the current-voltage conversion amplifier 61.
The spectrum analyzer 62 directly analyzes the frequency component of the input voltage signal, the horizontal axis is frequency, the vertical axis is level, and the relative magnitude of each frequency component of the input voltage signal, that is, the amplitude spectrum of the input voltage signal. Is displayed as a graph.

  The analysis device 63 is for obtaining the spectral characteristics of the sample S arranged in the terahertz optical system 32 based on the amplitude spectrum obtained by the spectrum analysis device 62. The analysis device 63 is composed of a personal computer or the like, performs an arithmetic process necessary for obtaining the terahertz light spectral characteristics of the sample S based on the amplitude spectrum data obtained by the spectrum analysis device 62, and performs the terahertz light spectroscopy of the sample S. Get properties.

The principle of spectroscopic measurement by the terahertz spectrometer 1 having the above configuration will be specifically described below.
When the optical path length of the probe light is changed by the variable optical delay device 50, the detection timing of the probe light changes with respect to the incident timing of the terahertz light that enters the optical switch element 21 as a detection target.
For example, the period of terahertz light having a frequency of 1 THz is 1 ps, which corresponds to 0.3 mm in terms of the optical path length. Therefore, for example, when considering the case where the movable reflecting mirror 51 is moved 1.5 mm away from the fixed reflecting mirror 54, the optical path length of the probe light increases by 3 mm in the reciprocating portion, and the probe light irradiation timing is delayed by 10 ps. Time is added.

Here, this delay time is given as step frequency fstep (THz) = (speed of light) / round-trip optical path length = 3 mm) = 1/10 ps = 0.1 (THz) in terms of frequency.
Therefore, as a comparative example, time domain measurement is performed in which the movable reflector 51 is moved only once in the movement range of 0 to 3 mm, and terahertz light components are sequentially measured at each movement position (each delay time). Think about it.
In this case, a time waveform of terahertz light having a full scale (horizontal axis) of the measurement time of 10 ps as shown in the time waveform diagram of terahertz light in FIG.

Assuming that data is obtained every 0.1 ps / step for this time waveform (that is, there are a total of 100 data points), and performing fast Fourier transform (FFT) calculation on such data, the full scale is obtained. A frequency spectrum of 10 THz corresponding to a data interval of 0.1 ps / step is obtained.
FIG. 5B shows a spectrum diagram of the frequency amplitude of the terahertz light. FIG. 5B is a frequency spectrum (horizontal axis: frequency, vertical axis: amplitude of each frequency component) in the range of 0 to 2 THz in the full scale of 10 THz. Here, the step interval (= step frequency fstep) of each FFT calculation point in this spectrum is 0.1 THz corresponding to the full scale 10 ps of the time waveform shown in FIG.

  On the other hand, in the terahertz spectrometer 1 according to the present embodiment, the variable optical delay device 50 performs measurement by reciprocatingly vibrating the optical path length of the probe light at a constant frequency and period. That is, the movable reflecting mirror 51 is driven and controlled by the optical delay control device 52 so as to reciprocate in parallel with the optical axis with respect to the fixed reflecting mirror 54 whose position is fixed in the variable optical delay device 50. As for the vibration of the movable reflecting mirror 51, as described above, the amplitude (maximum position change) of the position vibration may correspond to the full scale of the time waveform of the terahertz light to be measured. For example, the position amplitude of the vibration is set so as to correspond to the full scale of the measurement time axis sufficiently including the time waveform of the terahertz light pulse.

In the present embodiment having the above-described configuration, the following operational effects can be achieved.
(1) When the terahertz light passes through the sample S, the optical path length changes due to the optical characteristics of the sample cell or container of the sample S. On the other hand, in this embodiment, the optical path length correction device 40 performs optical path length correction so as to cancel out the change in the optical path length.
For this reason, when the terahertz light passes through the sample S, the terahertz light passes through the measurement object accommodated or distributed in the sample cell or the measurement object flow path. This causes spectral absorption of the terahertz light caused by the object to be measured and changes in the optical path length caused by the sample cell or the object flow path, but the optical path length is corrected by the optical path length correction device 40 so as to cancel out the change in the optical path length. Therefore, a change in the optical path length caused by the sample cell or the measured object flow path that causes measurement error can be easily eliminated, and only the spectral absorption caused by the measured object can be detected.
Therefore, by introducing the optical path length correction device 40 into the terahertz spectrometer 1, the terahertz spectrometer 1 that can realize excellent measurement accuracy can be obtained.

(2) In the present embodiment, since the movable reflecting mirror 41 is formed from a flat mirror, terahertz light is not affected by optical characteristics when reflected by the movable reflecting mirror 41, so only the optical path length is corrected. Is done. Specifically, there is no fear that chromatic aberration will occur as in the case where optical path length correction is performed by a condensing lens or the like.
Therefore, only the change in the optical path length caused by the sample cell or the measured object flow path causing the measurement error is corrected and eliminated, so that more excellent measurement accuracy can be realized. Further, since only a general-purpose flat mirror is used, the measurement accuracy of the terahertz spectrometer can be improved easily and at a low cost.

(3) In this embodiment, since the movable reflecting mirror 41 uses a combination of two flat mirrors in an L shape, the optical axis incident on the movable reflecting mirror 41 and the optical axis emitted are parallel, and It can be reflected away. For this reason, terahertz light can be folded back 180 °, so that the optical path length can be changed simply by moving the two mirrors together without changing the angle of the two mirrors, and a mechanism for changing the mirror angle is unnecessary. Yes, the optical path length changing device can be miniaturized.

(4) The drive control of the movable reflecting mirror 41 by the optical path length correction control device 42 is performed by a ball screw with the support portion 43 placed on the rail, so that it has a general-purpose and simple configuration. The movable reflecting mirror 41 and the support part 43 can be driven and controlled.

(5) In this embodiment, a variable optical delay device is provided, and terahertz light in a wide frequency range can be detected. For this reason, the spectral absorption of the terahertz light caused by the object to be measured can be measured in a wide frequency range.
In addition, since a spectral processing unit that performs analysis processing from the current signal detected by the optical switch element 21 is provided, it is possible to perform analysis and analysis of a current signal obtained for performing terahertz spectroscopy measurement.
Therefore, the terahertz spectrometer 1 can perform terahertz spectroscopy in a wide frequency range and analyze and analyze them at once.

Next, a second embodiment of the present invention will be described with reference to FIG.
The second embodiment differs from the first embodiment in the position of the optical path length correction device 40, and the other configurations are the same as those in the first embodiment.
FIG. 6 shows a block diagram of a terahertz spectrometer in the second embodiment of the present invention.
As shown in FIG. 6, in the second embodiment, the movable reflecting mirror 41 is disposed between the first condensing optical unit 13 and the sample S. In this case, the terahertz light is reflected by the movable reflecting mirror 41 and then passes through the sample S. That is, the change in the optical path length of the terahertz light due to the transmission of the sample S is corrected in advance. This can be corrected in advance by grasping the optical path length change of the terahertz light due to the optical characteristics of the sample cell or the like before the measurement. For this reason, the terahertz light transmitted through the sample is the same as the optical path length appropriately corrected.
Therefore, in 2nd Embodiment, there can exist an effect similar to the effect (1)-(5) of 1st Embodiment.

A third embodiment of the present invention will be described with reference to FIG.
The third embodiment is different from the first embodiment in the configuration of the optical path length correction device 40, and the other configurations are the same as those in the first embodiment.
FIG. 7 shows a configuration diagram of a terahertz spectrometer in the third embodiment of the present invention.
As shown in FIG. 7, in the third embodiment, the movable reflecting mirror 41 includes two reflecting mirrors 41a and 41b, which face each other in a vertical positional relationship. A sample S is disposed between the two reflecting mirrors 41a and 41b. The reflecting mirrors 41 a and 41 b and the sample S are moved in parallel along the optical axis by the optical path length correction control device 42. The sample S is interlocked with the reflecting mirrors 41a and 41b.

Therefore, in the third embodiment, the same effects as the effects (1) to (5) of the first embodiment can be achieved.
It should be noted that the present invention is not limited to the above-described embodiments, and modifications, improvements, and the like within the scope that can achieve the object of the present invention are included in the present invention.
For example, in the first embodiment and the second embodiment, the L-shaped movable reflecting mirror 41 is used. However, the present invention is not limited to this, and two reflecting mirrors as in the third embodiment may be used. And the angle which mutual mirror surfaces make, or the projection surfaces of the said mirror surface should just be a right angle.
Further, in the present embodiment, the embodiment using two reflecting mirrors has been described. However, three, four, or more reflecting mirrors can be used in combination.

  The present invention can be used for various optical measurement devices in addition to a terahertz spectrometer.

1 is a configuration diagram of a terahertz spectrometer in a first embodiment of the present invention. (A) The block diagram of the optical switch element (light emission) in 1st Embodiment of this invention. (B) The block diagram of the optical switch element (light reception) in 1st Embodiment of this invention. The block diagram of the optical path length correction | amendment apparatus in 1st Embodiment of this invention. The block diagram of the variable optical delay device in 1st Embodiment of this invention. (A) Time waveform diagram of terahertz light. (B) Frequency amplitude spectrum diagram of terahertz light. The block diagram of the terahertz spectrometry apparatus in 2nd Embodiment of this invention. The block diagram of the terahertz spectrometry apparatus in 3rd Embodiment of this invention.

  DESCRIPTION OF SYMBOLS 1 ... Terahertz spectrometry apparatus, 10 ... Light emitting element, 20 ... Light receiving element, 40 ... Optical path length correction apparatus (optical unit), 41 ... Movable reflector (mirror), 50 ... Variable optical delay device (delay means), 60 ... Spectral processing unit (analyzing means), 100 ... pulse light source (laser transmitter), S ... sample (specimen)

Claims (6)

A light emitting device emitting terahertz light;
A light receiving element that receives the terahertz light and detects a detection signal;
A sample disposed on an optical path between the light emitting element and the light receiving element;
In a terahertz spectrometer comprising:
A first condensing optical unit that condenses the terahertz light emitted from the light emitting element in the vicinity of the sample;
A second condensing optical unit for condensing the terahertz light diverging in the optical path onto the light receiving element;
An optical unit that varies the length of the optical path between the first condensing optical unit and the second condensing optical unit,
The terahertz spectrometer is characterized in that the optical unit includes a position adjusting mechanism that moves in the optical axis direction of the optical path. The terahertz spectrometer according to claim 1,
The terahertz spectrometer is characterized in that the optical unit has a plurality of plane mirrors. In the terahertz spectrometer according to claim 2,
In the terahertz spectroscopic measurement apparatus, the plane mirror has a right angle between the mirror surfaces. In the terahertz spectrometer according to any one of claims 1 to 3,
A terahertz spectrometer, wherein the optical unit is disposed in a first optical path between the first condensing optical unit and the sample. In the terahertz spectrometer according to any one of claims 1 to 3,
A terahertz spectrometer, wherein the optical unit is disposed in a second optical path between the sample and the second condensing optical unit. In the terahertz spectrometer according to claim 2 or claim 3,
The terahertz spectrometer is characterized in that the optical unit adjusts the optical axis position by holding and moving a sample between the plane mirrors.

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