JP2009036578A - Light absorption measuring instrument - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To detect many kinds of absorbing substances with high sensitivity by a compact device by detecting a plurality of absorbing lines while sweeping the wavelength of incident light without requiring a large-scaled multi-path cell. <P>SOLUTION: In this light absorption measuring instrument for measuring the optical absorbing characteristics of a substance to be measured by applying the beam, which is emitted from a laser beam emitting means, to the substance to be measured, a first tuning fork is arranged in the substance to be measured so as to cross the light path of the incident beam incident on the substance to be measured. Further, the light absorbing and measuring instrument includes modulation means for modulating the intensity of the incident beam by the signal in the vicinity of the resonance frequency of the first tuning fork, conversion means for converting the resonance vibration of the first tuning fork to a first electric signal, and measuring means that subject the first electric signal and the signal in the vicinity of the resonance frequency to lock-in detection to measure the optical absorbing characteristics of the substance to be measured. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a light absorption measurement device, and more particularly to a light absorption analyzer that performs optical trace detection of environmental gases, hazardous gases, and residual pesticides.

  From the viewpoint of environmental protection and safety and health, it is strongly desired to establish a trace analysis technique for environmental gases such as NOx, SOx, and ammonia, water absorption, or many organic gases or residual agricultural chemicals. As an ultra-trace analysis technique, a method in which a gas to be measured is adsorbed on a specific substance and a quantitative analysis by an electrochemical method or a characteristic optical absorption characteristic of the substance to be measured is generally used. Furthermore, studies on trace analysis using other optical methods have been made. The optical method has characteristics that real-time measurement is possible and a wide area through which measurement light passes can be observed.

  For the detection of extremely weak absorption typified by a rare gas, a multipath cell having a substantially long optical path length, an optical resonator, a reflecting plate or a scattering plate disposed far away are used. In these optical systems, the optical axis adjustment becomes more difficult as the optical path length becomes longer. In addition, the entire measurement system including these optical systems is inevitably large, and generality is poor.

  With reference to FIG. 1, the measurement principle of the conventional light absorption measurement will be described. FIG. 1A is a schematic diagram regarding amplification of a light absorption signal. Originally, since the light absorption measurement is a transmission measurement, the light absorption signal is superimposed on the background signal. Therefore, since the amplification of the light absorption signal is accompanied by the amplification of the background signal, an improvement in S / N cannot be expected, and the increase in the amplification factor is necessarily limited. Here, the limiting amplification factor is set to A.

  On the other hand, FIG. 1B is a schematic diagram relating to the null method. Photoacoustic spectroscopy detects an acoustic signal as a dense wave based on the presence or absence of gas expansion caused by light absorption. Therefore, a signal is generated only in a portion where absorption exists. Therefore, in principle, it is possible to amplify without causing a decrease in S / N. If the amplification factor in this case is G, G >> A is easy, and it can be seen that the null method is more advantageous than the optical absorption measurement.

As is often experienced in oscilloscope waveform observation, when observing changes in a DC signal, it is easier to see the change by observing only the AC component superimposed on the DC signal, and only the desired signal component can be amplified. . This is similar to the technique of performing phase-sensitive measurement and differential detection by using light that has been frequency-modulated by a sine wave. When using a phase reference signal having the same frequency as the modulation frequency f _{m of} the signal light, the first order differentiation, and when using a reference signal having a frequency 2f _{m that} is twice the modulation frequency of the signal light, a second order differentiation, a frequency 3f that is three times higher. _{When the m} reference signal is used, a third-order differential signal is obtained, and only the change rate can be observed. The magnitude of each signal also depends on the coefficient when the absorption line waveform is Taylor-expanded together with the differential coefficient.

  The photoacoustic spectroscopy used for the detection of the rare gas is the measurement by the null method shown in FIG. 1B, and the detection sensitivity is improved. However, it is still necessary to create a resonance gas cell unique to the device according to the object to be measured, and although it can be expected to be smaller than the measurement system for optical absorption measurement, it can basically be decompressed. A strong gas cell must be used. Therefore, there is a limit from the viewpoint of realizing generality.

With reference to FIG. 2, the measurement principle of the conventional phase sensitive measurement will be described. At the maximum absorption point f ₀ of the absorption line (minimum point of transmittance), even if waveform distortion is unavoidable, a signal having a harmonic wave of the modulation frequency of the signal light is generated. When the phase reference signal matches the modulation frequency of the signal light, or when the frequency is tripled, the sign of the output signal changes on the left and right with f ₀ as the center, so the frequency discrimination characteristics used for frequency stabilization etc. Is obtained. When the frequency of the phase reference signal is doubled, a second-order differential waveform of the absorption spectrum is obtained, and a spectrum narrower than the actual spectrum is obtained. As described above, differential detection to obtain these differential waveforms looks only at the rate of change as in the case of observing the alternating current component with an oscilloscope, so there are many direct current components in the background, such as the light transmission characteristics of a medium with weak absorption. This is a very effective observation technique for signal observation.

In the case of photoacoustic spectroscopy, the acoustic signal can be expected to become maximum at the maximum absorption point f ₀ of the absorption line. However, this is a case where intensity modulation is performed, and the situation is slightly different when frequency modulation is used. As shown in FIG. 2, when subjected to frequency modulation, in f ₀ is harmonic of the modulation frequency f _m, i.e. the frequency of the output waveform of the 2f _m primarily occur. Therefore, the amplitude of the 2f _m in f _0, the amplitude of the magnitude of f _m in f ₁ that is offset from f ₀ is the magnitude of the modulation amplitude Delta] f, depending on such rate of change with respect to frequency of the absorption line, unique to the not decided. For dilute gas, because inevitably absorption is reduced, in the case of the photoacoustic spectroscopy, the amplitude of the modulation frequency f _m at the center frequency f ₀ of the absorption sound signal is maximized, the peripheral portion of the f ₀ Even smaller. Thus, frequency modulation is not a good idea.

  Recently, photoacoustic spectroscopy using a quartz resonator has been proposed (see, for example, Non-Patent Document 1). However, the configuration of the measurement system is such that the gas to be measured is sealed in a decompression cell and an acoustic signal as a dense wave based on the presence or absence of gas expansion generated by light absorption is detected, so conventional photoacoustic spectroscopy Is the same. Instead of a piezoelectric element and a condenser microphone, a quartz resonator is merely used for detecting an acoustic signal in the photoacoustic cell. In addition, since a semiconductor laser (LD) is used directly, the wavelength sweep width cannot be secured, and the object to be measured is limited to one or two absorption lines of interest. In the example of Non-Patent Document 1, light emitted from an LD that oscillates in the near infrared near 1.6 μm where gas absorption is weak is used, and measurement of a rare gas in the atmosphere is impossible.

  In Non-Patent Document 1, direct frequency modulation that modulates the current of the LD is used for the light absorption measurement system. At this time, since the measurement is performed by the null method, the measurement of the differential waveform by frequency modulation using a sine wave is not necessarily a good measure as described above. On the other hand, the LD direct modulation method, which seems to be advantageous from the viewpoint of simplicity, superimposes intensity modulation as well as frequency modulation, and waveform distortion of the modulation signal is inevitable. In the example of Non-Patent Document 1, detection of a first-order differential waveform is employed, and the acoustic signal output maximum point is 0. By observing a double-wave signal waveform from which a second-order differential waveform is obtained, the acoustic signal output maximum point can be used effectively, but only a waveform close to the rectified waveform as shown in FIG. 2 can be obtained. Therefore, in the case of signal detection using a tuning fork including a crystal resonator, a technique using a mechanical chopper and intensity (amplitude) modulation by an external modulation method is more desirable. In the example of Non-Patent Document 1, half of the resonance frequency of the tuning fork is used for frequency modulation of the LD. This is effective for improving the monitor sensitivity, but is not necessarily a good measure for increasing the sensitivity of the absorption signal.

  Next, the measurement system laser light source will be described. The absorption peak of the substance to be measured is caused by the vibration rotation mode of the interatomic bond, and has a strong absorption band based on fundamental vibration mainly in the mid-infrared region of 2 μm to 20 μm. However, a laser that can stably obtain continuous oscillation at room temperature in the mid-infrared region with a wavelength of 2 μm or more is still difficult to put into practical use. Although quantum cascade lasers have been researched and developed, the lack of a laser light source that can be used in the optical method described above is a major obstacle.

  Since there is no practical laser light source in the mid-infrared region, an existing communication semiconductor laser (0.8 μm to 2 μm) is usually used. However, when performing microanalysis of various gases at this wavelength, the harmonics of the fundamental absorption wavelength of the original gas (= 1/2 of the fundamental absorption wavelength), the third harmonic (= 1/3 of the fundamental absorption wavelength), Alternatively, the absorption in the combined sound (= combination of a plurality of fundamental absorption wavelengths) is used. However, the required sensitivity may be obtained if it is about harmonics. However, when the measurement is performed at a high-order absorption peak corresponding to 3rd harmonic or more, detection is limited because the amount of absorption itself is small. Therefore, the sensitivity is reduced by about three orders of magnitude or more compared to the measurement at the original basic absorption peak.

As described above, the development of a mid-infrared laser light source is indispensable for obtaining high detection sensitivity when analyzing environmental gases, dangerous gases, and the like. Recently, it has been reported that mid-infrared light is generated at a wavelength of 3 μm or the like to confirm the gas sensor operation (for example, see Non-Patent Document 2). In this report, a mid-infrared light is generated by difference frequency generation using a wavelength conversion element made of lithium niobate (LiNbO ₃ ) having a periodic domain-inverted structure as a light source.

A.A.Kosterev et al., "Quartz-enhanced photoacoustic spectroscopy", Optics Letters, Vol.27, No.21, po.1902-1904 (2002) D. Richter, et al., "Development of an automated diode-laser-based multicomponent gas sensor", Applied Optics, Vol.39, No.24, pp.4444-4450 (2000) A. Miklos, et al., "Application of acoustic resonators in photoacoustic trace gas analysis and metrology", Review of scientific instruments, Vol.72, No.4, pp.1937-1955, 2001

  As described above, the conventional optical absorption measurement apparatus can detect only one or two absorption lines using a multipath cell having an optical path length of about 100 m. In contrast, (1) measurement is possible from the visible range to the infrared range (0.45 to 5.5 μm), particularly in the wide mid-infrared wavelength range of 1.9 to 5.5 μm, 2) The wavelength can be varied so that a plurality of absorption lines can be detected according to the environmental gas present in the atmosphere, and (3) the sensitivity is high so that the environmental gas in the atmosphere can be directly detected. And (4) A compact and portable light absorption measuring device is required.

  The object of the present invention is not to require a large-scale multi-pass cell, but to detect a plurality of absorption lines while sweeping the wavelength of incident light, and to absorb various kinds of absorbing substances with high sensitivity by a compact device. It is to provide a measuring device.

  In order to achieve such an object, the invention according to claim 1, the light emitted from the laser beam emitting means is incident on the substance to be measured, and the optical absorption characteristics of the substance to be measured are adjusted. In the light absorption measurement device to be measured, a first tuning fork disposed in the measured substance so as to intersect an optical path of incident light incident on the measured substance, and a vicinity of a resonance frequency of the first tuning fork Modulation means for modulating the intensity of the incident light with the signal, conversion means for converting the resonance vibration of the first tuning fork into a first electric signal, the first electric signal and a signal in the vicinity of the resonance frequency, And a measuring means for measuring the optical absorption characteristic of the measurement substance.

  According to this configuration, the intensity of incident light is modulated by a signal in the vicinity of the resonance frequency of the tuning fork, and the light enters the tuning fork disposed in the gas to be measured. The resonance vibration of the tuning fork is generated by the dense wave based on the expansion of the gas generated by light absorption. This resonance vibration is detected as an acoustic signal and converted into an electrical signal. The converted electrical signal and the signal in the vicinity of the resonance frequency are lock-in detected, and the optical absorption characteristic of the measurement substance can be measured.

  The modulating means may modulate the intensity of the incident light or may modulate the frequency of the incident light. The conversion means may convert an acoustic signal, which is a resonance vibration of the tuning fork, into an electric signal using a condenser microphone, or may convert a resonance vibration of the tuning fork into an electric signal using a strain sensor. It is also possible to use a crystal resonator as a tuning fork and include a preamplifier and an impedance converter in the conversion means.

  There may be further provided a resonator that interpolates a point where the optical path of the incident light and the first tuning fork intersect, and resonates near the resonance frequency of the tuning fork. A reflection mirror may be further provided that is disposed on the optical path of the incident light and is opposed to the point where the optical path of the incident light and the tuning fork cross each other.

  The invention according to claim 9 is the optical absorption measurement device according to any one of claims 1 to 8, wherein the branching means is arranged on the optical path of the incident light and branches a part of the incident light; The measurement measured by the measurement means by performing lock-in detection on the photoelectric conversion means for converting the branched incident light into a second electric signal, the second air signal and a signal in the vicinity of the resonance frequency. And a first correction unit that corrects the optical absorption characteristic of the substance.

  According to this configuration, since the intensity signal of the incident light and the detected acoustic signal are respectively detected, the wavelength dependence of the incident light intensity of the acoustic signal and the influence of intensity fluctuation can be offset.

  According to a tenth aspect of the present invention, in the light absorption measurement apparatus according to any one of the first to ninth aspects, the second tuning fork disposed in the measured substance so as not to intersect the optical path of the incident light, The second tuning fork resonance vibration is converted into a third electric signal, the third electric signal and a signal in the vicinity of the resonance frequency are lock-in detected, and measured by the measuring means. And a second correction unit that corrects the optical absorption characteristic of the measurement substance.

  According to this configuration, since the acoustic signal detected by the tuning fork that does not intersect the incident light and the detected acoustic signal are detected, the background acoustic noise can be removed.

An eleventh aspect of the present invention is the light absorption measurement apparatus according to any one of the first to tenth aspects, wherein the laser beam emitting means includes a _first laser that generates a laser beam having a wavelength λ1, and a wavelength λ. _{The second} laser that generates the laser beam _2, the laser beam having the wavelength λ ₁ , and the laser beam having the wavelength λ ₂ are input, and the relationship 1 / λ ₁ −1 / λ ₂ = 1 / λ ₃ is satisfied. And a non-linear optical crystal that outputs a difference frequency light having a certain wavelength λ ₃ .

More specifically, the first laser generates laser light with a wavelength λ ₁ = 0.9 to 1.1 μm, and the second laser has laser light with a wavelength λ ₂ = 1.25 to 1.75 μm. The nonlinear optical crystal outputs difference frequency light having a wavelength λ ₃ = 1.9 to 5.7 μm.

The nonlinear optical crystal is LiNbO ₃ (LN), LiTaO ₃ (LT) or a mixed crystal thereof, or LN, LT or a mixed crystal to which Zn or Mg is added, and a domain-inverted structure is periodically formed. Preferably it is. The nonlinear optical crystal preferably has a waveguide structure.

  As described above, according to the present invention, a large multipath cell is not required, a plurality of absorption lines are detected while sweeping the wavelength of incident light, and various types of absorbing materials are made highly sensitive with a compact device. It becomes possible to detect.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The present invention provides a novel photoacoustic spectroscopy that detects light absorption with a tuning fork. A major feature of the present invention is that the modulation frequency used, whether intensity modulation or frequency modulation, is almost the same as the resonance frequency of the tuning fork.

(Operating principle)
As long as there is no direct transition (radiative transition) in the light absorbing medium, heat H (r, t) is generated by light absorption when irradiated with light. H (r, t) is a function of position r and time t. The generation of heat causes the light absorbing medium to expand, and therefore, when an intensity-modulated light is applied, a density wave of light modulation frequency, that is, a sound wave is generated. Since the sound wave becomes the pressure change p as it is, an optical absorption analyzer can be realized by measuring this sound pressure.

  According to the generation theory of sound waves by the heat source H (r, t) (for example, see Non-Patent Document 3), the pressure change p is expressed by the following wave equation.

Here, c is the speed of sound, and γ is the specific heat ratio of gas (= C _p / C _v ). When the light source is modulated with a sine wave, Equation 1 can be solved by Fourier transforming the pressure change p using the normal acoustic mode p _j . The pressure change p can be expressed as the sum of all normal acoustic modes p _j and the following equation is obtained.

The normal acoustic mode p _j is a solution of the following wave equation.

When a sound wave resonator is used, the normal acoustic mode p _j is 0 on the wall surface, so that the following equation is satisfied for the cylindrical resonator.

Here, k, m, and n are axial, azimuthal and radial mode orders,

Is a Bessel function, P _j is a normalization constant, L is the length of the resonator, R is the radius of the resonator, α _{m, n} is a boundary condition dJ that is 0 on the wall of the resonator (r = R) _It is an n-th order root satisfying _m / dr = 0.

  In a normal situation where light incident on the absorption medium is incident along the central axis of the resonator and the absorption is weak, only the mode changing in the radial direction is excited as a higher-order mode. Therefore, the sound pressure excited by the incident light whose intensity is sinusoidally modulated at the angular frequency ω is given by the following equation on the wall surface when the diameter of the incident light is sufficiently smaller than the inner diameter of the resonator.

Here, Q _j is the Q value of the j-th mode, V _r is the internal volume of the resonator, and _Pabs is a value given by approximately ½ of the peak power absorbed in the resonator. The j-th resonance frequency is expressed by the following equation.

When ω = ω _j , Equation (5) becomes the following equation.

In addition,

When,

It is. When p (ω) reaches a measurable level in consideration of the S / N of the measurement system, it can be an optical absorption analyzer. Incidentally, in the present embodiment, when a crystal resonator used for a clock of a watch is used as a tuning fork, if the incident light intensity is sine-wave modulated at the resonance frequency of the crystal resonator, in principle, the sound pressure of Equation (7) is obtained. On the other hand, the Q value (around 10,000) of the detector is multiplied and amplified.

Here, the light source performs sum frequency generation, difference frequency generation using the nonlinear optical crystal and two excitation laser beams, or harmonic generation using the nonlinear optical crystal and one excitation laser beam. When the former is used for spectroscopic applications, the relationship between the wavelengths λ ₁ and λ _{2 of the} two laser beams and the wavelength λ ₃ of the generated output light is

Given in. Here, the magnitude relationship between the wavelengths λ ₁ and λ ₂ is not limited, but for the sake of convenience, λ ₃ > 0, so that λ ₁ <λ ₂ . In order to generate λ ₃ efficiently, it is necessary to satisfy the phase matching condition k ₃ = k ₁ ± k ₂ . k _i (i = 1, 2, 3) is a propagation constant of laser light propagating in the nonlinear crystal, and when the refractive index of the nonlinear optical crystal at λ _i is n _i ,

It becomes. However, it is generally difficult to satisfy the phase matching condition due to the dispersion characteristics of the crystal.

As a method for solving this, a quasi-phase matching method in which a nonlinear crystal is periodically poled is used. For the quasi phase matching method, a ferroelectric crystal such as LiNbO ₃ is advantageous, but the sign of these nonlinear optical constants corresponds to the polarity of spontaneous polarization. When this spontaneous polarization is modulated with a period Λ in the light propagation direction, the phase matching condition is

In the case where specific wavelengths λ ₁ and λ ₂ are used as excitation light, highly efficient output light λ ₃ can be generated.

However, when changing the wavelengths λ ₁ and λ ₂ to obtain output light having different wavelengths λ ₃ , if the wavelengths λ ₁ and λ ₂ vary, the equation (9) can be satisfied. However, the intensity of λ ₃ decreases. Here, the relationship between the wavelengths λ ₁ , λ ₂ , λ ₃ , the period Λ, and the output light generation efficiency η is as follows. First, the phase mismatch amount Δk is

It is defined as At this time, assuming that the sample length is l, the generation efficiency η of the difference frequency light depends on the product of the phase mismatch amount Δk and the sample length l,

It is expressed. In Equation (11), η _o is the difference frequency generation efficiency when k = 0. This η _o is determined by a nonlinear optical constant of crystal such as LiNbO ₃ , excitation light intensity, sample length, and the like. The generated intensity is proportional to the product of the two input light intensities.

The light source used in this embodiment is secured by the output light by the above method. Further, when the wavelength conversion element of LiNbO ₃ obtained by the quasi phase matching method has a waveguide structure, a light confinement effect can be obtained, and wavelength conversion can be performed more efficiently.

  Examples of the present invention will be described below, but the present invention is not limited to the following examples, and it goes without saying that various modifications can be made without departing from the scope of the invention.

  FIG. 3 shows a basic configuration of a measurement system according to Example 1 of the present invention. The light emitted from the light source 1, which is a laser beam emitting means, is intensity (amplitude) modulated at the resonance frequency of the tuning fork 32 by the external modulator 30 and is incident on the object to be measured 3. The signal output from the oscillator 31 has the resonance frequency of the tuning fork 32, is amplified by the amplifier 33A, and is input to the external modulator 30. The modulation waveform is basically a sine wave. However, as long as the fundamental wave component is the resonance frequency of the tuning fork 32, a square wave, a rectangular wave, a triangular wave, a sawtooth wave, or any other waveform may be used.

  It arrange | positions so that the light which injected into the to-be-measured object 3 which performs a spectroscopic measurement may pass the inner side of the U-shaped leg part of the tuning fork 32. FIG. That is, the tuning fork 32 is disposed so as to intersect the optical path of the light incident on the DUT 3. In FIG. 3, the tuning fork 32 may be arranged so that the tuning fork 32 is tilted 90 degrees to the left and the incident light strikes the crotch portion (U-shaped bottom) of the tuning fork 32.

  An acoustic signal detected by the tuning fork 32 is converted into an electric signal by a condenser microphone, or converted into an electric signal by a strain sensor which is a piezoelectric element, and is amplified by a preamplifier 33B as necessary. When a piezoelectric element such as a crystal resonator is used as the tuning fork 32, the preamplifier 33B has a role of an impedance converter. The signal amplified by the preamplifier 33B and the signal output from the oscillator 31 (used as a reference signal) are input to the lock-in amplifier 25.

  The lock-in amplifier 25 detects the frequency component of the oscillator 31, that is, the resonance frequency component of the tuning fork 32. If the frequency of the light emitted from the light source 1 is swept, the spectrum of the DUT 3 can be obtained. In this way, the light absorption measuring device can be embodied. The input light can be frequency-modulated or phase-modulated with a sine wave. However, since the resonance frequency of the tuning fork 32 is used, intensity modulation is performed. Further, when LD is used, direct modulation is possible, but external modulation is preferably performed. The output waveform obtained as the signal amplified by the preamplifier 33B is a differential waveform.

  FIG. 4 shows a basic configuration of a measurement system according to the second embodiment of the present invention. In the configuration of the second embodiment, in addition to the configuration of the first embodiment, a resonator 35 that resonates by resonance vibration in the vicinity of the resonance frequency of the tuning fork 32 is disposed. The resonator 35 is disposed so as to interpolate a point where the optical path of the light incident on the DUT 3 and the tuning fork 32 intersect. For example, in FIG. 4, two cylindrical resonators 35 having open ends are shown. The resonator 35 is disposed with the tuning fork 32 interposed therebetween with the optical axis of the light incident on the DUT 3 being the central axis of the cylinder. The resonator 35 may be a single cylinder, a slit may be provided at the center of the cylinder, and the tuning fork 32 may be inserted.

  For example, the diameter of the cylinder serving as the resonator 35 is set to 2.5 mmφ by performing aperture end correction to the half wavelength λ / 2 of the resonance frequency of the tuning fork 32 when a crystal resonator is used. At this time, the total length of the resonator 35 is about 4 mm. In addition, although the objection is advocated in the present theory regarding the opening end correction, and it is not strictly settled, the outline is approximately the same size. Further, some correction is necessary depending on the difference in gas type and gas pressure (for example, see Non-Patent Document 3). Further, the resonator 35 does not have to be open at both ends, and may be a closed tube with one end closed. In this case, when the diameter of the cylinder is 2.5 mmφ, the total length is about 1.5 mm. In the case of a tuning fork used for resonance of a musical instrument, a length of about 17 to 18 cm is appropriate for a closed tube resonator of about 6 cmφ.

  In order for Formula (4) to hold, it is necessary to cover the gas 3 to be measured with a wall such as a gas cell. However, if the sound pressure is sufficiently secured without using the resonance effect, it is not necessary to use a resonator. Of course, the resonator 35 may be disposed in the gas cell. Regardless of the shape of the resonator and the tube or box used as the resonator, whether it is a closed tube or an open tube, any resonator may be used. With the corrections used in normal acoustic vibration theory, the dimensions of the resonator can be determined approximately.

  FIG. 5 shows an application example in the first and second embodiments. In FIG. 5A, parallel plate reflectors 40 </ b> A and 40 </ b> B are arranged before and after the tuning fork 32 on the optical axis of the light incident on the DUT 3. The parallel plate reflecting mirrors 40A and 40B form a light resonator at a position opposite to each other across the point where the optical path of the incident light and the tuning fork 32 intersect. In FIG. 5B, concave mirrors 40 </ b> C and 40 </ b> D are arranged before and after the tuning fork 32. A concave reflection mirror 40C, 40D forms a multiple reflection optical path across the tuning fork 32. In any case, if a plurality of optical paths that pass through the tuning fork 32 can be secured, the absorption of the gas to be measured increases by the number of times of passage, so that the acoustic signal detected by the tuning fork 32 is enhanced.

  In FIG. 5C, parallel plate reflecting mirrors 40E and 40F are arranged before and after the resonator 35 in which the tuning fork 32 is inserted. In FIG. 5D, concave mirrors 40G and 40H are arranged before and after the resonator 35 in which the tuning fork 32 is inserted.

FIG. 6 shows a basic configuration of a measurement system according to Example 4 of the present invention. In this measurement system, the wavelength dependence of the incident light intensity of the acoustic signal detected by the tuning fork 32 and the influence of intensity fluctuation are canceled out. The intensity of the incident light directly affects the absorption measurement as can be inferred from the equation (7). The intensity of the mid-infrared light exhibits, for example, wavelength dependence close to a sinc function curve based on a phase matching curve of LiNbO ₃ crystal. In addition, optical fringes based on multiple reflections generated by optical components existing on the optical path of incident light also affect the absorption measurement as intensity changes. Therefore, the intensity of the acoustic signal is corrected to obtain a flat absorption spectrum having no wavelength dependency of the incident light intensity.

  In the fourth embodiment, incident light whose intensity is modulated before the object to be measured 3 is branched by a beam splitter 23 which is a branching means, and is input to a light receiver 24 which is a photoelectric conversion means. A signal (used as a reference signal) output from the oscillator 31 and an electric signal output from the light receiver 24 are input to the lock-in amplifier 25A. The acoustic signal detected by the tuning fork 32 is input to the lock-in amplifier 25B via the preamplifier 33B.

  The beam splitter 23 may branch incident light after passing through the DUT 3. However, if the thickness of the tuning fork 32 is such that the absorption of the object to be measured 3 is strong enough to be observed by light transmission, it is preferable to branch by the beam splitter 23 before entering the object to be measured 3. is there.

  The lock-in amplifiers 25A and 25B detect only the frequency component of the oscillator 31, that is, the resonance frequency component of the tuning fork 32, and divide each output to perform intensity correction. The acoustic signal due to light absorption is proportional to the incident light intensity. Since the intensity signal of the incident light and the detected acoustic signal are detected, the wavelength dependence of the incident light intensity of the acoustic signal and the influence of intensity fluctuations can be offset.

  FIG. 7 shows a basic configuration of a measurement system according to Example 5 of the present invention. In this measurement system, the background noise of the acoustic signal is canceled by using a plurality of tuning forks. The signal detected by the tuning fork 32 </ b> A includes not only the light absorption signal of the DUT 3 but also background acoustic noise. In order to remove the background acoustic noise, an acoustic signal detected by the tuning fork 32B that does not intersect the incident light is input to the lock-in amplifier 25. If the differential input type lock-in amplifier 25C is differentially amplified as it is, only the light absorption signal of the DUT 3 can be measured. In the case of a lock-in amplifier without a differential input terminal, the difference between the outputs may be calculated or divided using two lock-in amplifiers as in the fourth embodiment.

  FIG. 8 shows a basic configuration of a measurement system according to Example 6 of the present invention. In this measurement system, the wavelength dependence of the incident light intensity of the acoustic signal detected by the tuning fork 32 and the influence of intensity fluctuation are canceled, and the background noise of the acoustic signal is canceled by using a plurality of tuning forks. Since the fourth embodiment and the fifth embodiment are noise removal with respect to independent phenomena, both noises can be canceled by a simple combination.

  Incident light whose intensity is modulated in front of the DUT 3 is branched by the beam splitter 23 and input to the light receiver 24. A signal (used as a reference signal) output from the oscillator 31 and an electric signal output from the light receiver 24 are input to the lock-in amplifier 25A. The acoustic signal detected by the tuning fork 32A and the acoustic signal detected by the tuning fork 32B without light incidence are input to the differential input type lock-in amplifier 25C via the preamplifiers 33B and 33C, respectively. The background noise of the acoustic signal is canceled by the output of the lock-in amplifier 25C. If the outputs of the lock-in amplifiers 25A and 25C are divided, the influence of the wavelength dependency of the incident light intensity of the acoustic signal and the intensity fluctuation are canceled. The lock-in amplifier 25C may divide outputs from each other using two lock-in amplifiers.

FIG. 9 shows the configuration of a light absorption analyzer according to Example 7 of the present invention. The structure of a light absorption analyzer capable of changing the wavelength in the wide mid-infrared wavelength range of 1.9 to 5.5 μm and directly detecting environmental gas in the atmosphere is shown. The light source of the light absorption analyzer is a light emitted from the semiconductor laser 10 having a wavelength λ ₁ (λ ₁ = 0.9 to 1.1 μm band) and a wavelength λ ₂ (λ ₂ = 1.25 to 1.75 μm band). The light emitted from the semiconductor laser 11 (all or any wavelength region within this range) is combined by the multiplexer 18 and is incident on the LiNbO ₃ crystal 21 which is a nonlinear optical crystal. .

  The semiconductor laser 10 has a high reflection film with an end face 10A of 90% or more, and the opposite end face 10B has a low reflection film with a reflectance of 2% or less. An optical fiber 16 is connected to the output of the semiconductor laser 10, and a fiber Bragg grating 16A is provided to improve the wavelength stability. The light emitted from the optical fiber 16 is input to the multiplexer 36 via the optical isolator 14B and the external modulator 30.

  An optical fiber amplifier 15 is connected to the output of the semiconductor laser 11 and amplifies the emitted light as necessary. The light emitted from the optical fiber amplifier 15 is input to the multiplexer 36 via the optical isolator 14 </ b> A and the optical fiber 17. In the case where difference frequency generation is performed as in the present embodiment, the maximum mid-infrared light is generated when the polarization directions of the two excitation lights coincide with each other. When a wave holding fiber is used, simple and efficient generation of mid-infrared light can be obtained.

The outgoing light from the multiplexer 18 enters the LiNbO ₃ crystal 21 via the optical coupling lens 13. The LiNbO ₃ crystal 21 has a periodic domain-inverted structure, and an optical waveguide is formed. By controlling the temperature of the LiNbO ₃ crystal 21 and generating a difference frequency, mid-infrared light in the wavelength range of 1.9 μm <λ ₂ <5.5 μm can be obtained. When the wavelength and output of the semiconductor laser 11 are constant and the wavelength λ ₂ is changed, the change of the wavelength λ ₂ corresponds to the change of the wavelength λ ₃ of the mid-infrared light. The wavelength of the generated mid-infrared light can be confirmed by a spectroscope. In Example 8, a LiNbO ₃ crystal bulk having an element length of 10 mm was used, and the conversion efficiency was about 1% / W or less in all wavelength regions. When a waveguide element having an element length of 50 mm was used, it was 40% / W when the wavelength λ ₃ was 3.3 μm.

Although the generation efficiency of mid-infrared light is reduced, a bulk element of LiNbO ₃ crystal that does not have a waveguide structure may be used. Further, it may be a LiTaO ₃ crystal or a mixed crystal thereof, or LN, LT or a mixed crystal added with Zn or Mg.

The outgoing light from the LiNbO ₃ crystal 21 enters the device under test 3 via the optical coupling lens 12 and the filter 22. The optical coupling lenses 12 and 13 do not necessarily need to be lenses, and may be performed by an optical fiber butt joint. Filter 22 performs a mid-infrared light emitted from the LiNbO ₃ crystal 21, the separation of the incident light in the LiNbO ₃ crystal 21. In Example 7, a Ge filter is used. A dichroic mirror or the like may be used instead of the filter. However, at present, the Ge filter is superior in the extinction ratio for erasing input light.

  The external modulator 30 is used for the purpose of intensity (amplitude) modulation or frequency (phase) modulation of incident light on the DUT 3. In the eighth embodiment, an amplitude modulator is connected to the output side of the semiconductor laser 10. An amplitude modulator may be connected to the output side of the semiconductor laser 11. A phase modulator may be used as the external modulator 30. Further, although the electro-optic modulator (EO modulator) is used as the external modulator, an acousto-optic modulator (AO modulator) may be used.

The oscillator 31 matches the oscillation frequency with the resonance frequency of the tuning fork 32. The power required for driving input of the external modulator 30 is secured by using the amplifier 33A. When a crystal resonator is used for the tuning fork 32, for example, f _r = 33.7 kHz is the resonance frequency. In Example 7, a crystal resonator having an outer shape of 6 mm × 1.4 mm × 0.2 mm is used (described as a mechanical tuning fork 32 in FIG. 9).

  When a mechanical tuning fork is used, an acoustic signal is detected by directly connecting a pressure sensor and a strain sensor to the tuning fork 32. However, the resonance frequency is lowered and separation from background noise is not easy. There is also a method using a tuning fork and a condenser microphone, but it is necessary to overcome the problem of adhesion at the contact portion and to ensure total sound pressure-electrical conversion efficiency. The modulation waveform is basically a sine wave, but may be another waveform such as a square wave (can be handled as an overlap of sine waves of different frequencies) in amplitude modulation. If the fundamental frequency substantially matches the resonance frequency of the tuning fork 32, there is no problem.

Since the crystal resonator as the tuning fork 32 functions as a piezoelectric element, a pressure change is directly obtained as an electric signal. The preamplifier 33B efficiently amplifies the resonance frequency component of the tuning fork 32 in the obtained electrical signal. The amplifier 33B has a Q value of about 6000 and a gain of about 15 dB. The signal intensity of the absorption line is reproduced by performing lock-in detection by matching the oscillation frequency of the oscillator with the resonance frequency of the amplified electrical signal. An absorption line spectrum can be obtained by wavelength sweeping one of the incident light on the LiNbO ₃ crystal 21.

  In addition, as in the second embodiment shown in FIG. 4, a resonator may be used to efficiently extract the acoustic signal. In the eighth embodiment, the resonator 35 has a total length of 3.8 mm, an inner diameter of 2.5 mmφ, and a slit into which the tuning fork 32 is inserted at the center. In order to improve the detection sensitivity of the sound pressure, the electrical resonance phenomenon in the amplifier 33B described above is used, and the resonator 35 uses the resonance phenomenon of the sound wave, so that double sensitivity improvement is achieved.

  Although not necessary in the present embodiment, the light receiver 24 and the oscilloscope 34 are connected as a monitor of the modulated light incident on the DUT 3. For the light receiver 24, for example, a photoconductive detector for mid-infrared light such as PbSe is used.

  FIG. 10 shows a configuration of a measurement system for comparison with a light absorption analyzer using a conventional multipath cell. In a conventional optical absorption analyzer, a multipass cell 26 is filled with an object to be measured, the modulated light transmitted through the multipass cell 26 is detected by a light receiver 24B, and a lock-in amplifier as an electrical signal is passed through a switch 36. Output to 25A. The optical absorption analyzer according to the seventh embodiment has a tuning fork 32 and a resonator 35 (not shown) so that the modulated light incident on the DUT 3 (not shown) passes through the tuning fork 32. ) And arrange.

  Similarly to the fourth embodiment, the intensity-modulated incident light is branched by the beam splitter 23 and input to the light receiver 24A. The lock-in amplifier 25B receives a signal output from the oscillator 31 (used as a reference signal) and an electrical signal output from the light receiver 24A.

FIG. 11 shows a comparison result between the conventional optical absorption analyzer and the optical absorption analyzer according to Example 7. It is a measurement result of 1.78 ppm methane (CH ₄ ) contained in the atmosphere in the 3.3 μm band. FIG. 11A shows the result of measuring the transmitted light of the multipath cell 26 having the optical path length of 107 m with the changeover switch 36 connected to the A side. This is a result of dividing the outputs of the lock-in amplifiers 25A and 25B. FIG. 11B shows a photoacoustic signal based on light absorption detected by a tuning fork when the changeover switch 36 is connected to the B side.

It can be seen that the output in FIG. 11B changes corresponding to the absorption lines of water (H ₂ O) and methane (CH ₄ ) in FIG. That is, it can be seen that the absorption of methane in the atmosphere can be measured without passing through the 107 m multipass cell 26. The S / N can be further improved by the configuration of the signal detection unit of the tuning fork and the optimum design of the resonance tube. Considering that the thickness of the tuning fork is 0.2 mm, it can be seen that the sensitivity is high. If the signal output from the amplifier 33B and the electrical signal output from the light receiver 24B are input to the two lock-in amplifiers without passing through the changeover switch 36, the light transmitted through the multipath cell 26 is converted. And the photoacoustic signal detected by the tuning fork 32 can be directly compared.

  FIG. 12 shows the configuration of a light absorption analyzer according to Example 8 of the present invention. In this measurement system, in addition to the configuration of the seventh embodiment shown in FIG. 9, the wavelength dependence and the intensity fluctuation of the incident light intensity of the acoustic signal detected by the tuning fork 32, as in the sixth embodiment shown in FIG. The background noise of the acoustic signal is canceled using a plurality of tuning forks.

  In Example 8, incident light after passing through the DUT 3 is received by the light receiver 24, and an electric signal output from the light receiver 24 is input to the lock-in amplifier 25C. As shown in FIG. 8, the light branched by the beam splitter 23 may be referred to by the lock-in amplifier 25C. The lock-in amplifiers 25A and 25C detect only the frequency component of the oscillator 31, that is, the resonance frequency component of the tuning fork 32A, divide each output, and correct the wavelength dependence of the incident light intensity of the acoustic signal.

  Further, in order to remove background acoustic noise, an acoustic signal detected by the same tuning fork 32B without light incidence is input to the lock-in amplifier 25B. The lock-in amplifiers 25A and 25B detect only the frequency component of the oscillator 31 and divide each output. The lock-in amplifiers 25A and 25B can be replaced with a differential input type lock-in amplifier.

It is a figure for demonstrating the measurement principle of the conventional light absorption measurement. It is a figure for demonstrating the measurement principle of the conventional phase sensitive measurement. It is a figure which shows the basic composition of the measurement system concerning Example 1 of this invention. It is a figure which shows the basic composition of the measurement system concerning Example 2 of this invention. 6 is a diagram illustrating an application example in Examples 1 and 2. FIG. It is a figure which shows the basic composition of the measurement system concerning Example 4 of this invention. It is a figure which shows the basic composition of the measurement system concerning Example 5 of this invention. It is a figure which shows the basic composition of the measurement system concerning Example 6 of this invention. It is a figure which shows the structure of the light absorption analyzer concerning Example 7 of this invention. It is a figure which shows the structure of the measurement system for performing the comparison with the light absorption analyzer using the conventional multipass cell. It is a figure which shows the comparison result of the conventional light absorption analyzer and the light absorption analyzer concerning Example 7. FIG. It is a figure which shows the structure of the light absorption analyzer concerning Example 8 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Light source 3 Device to be measured 10, 11 Semiconductor laser 12, 13 Lens 14A, B Isolator 15 Optical fiber amplifier 16, 17 Optical fiber 16A Fiber Bragg grating 18 Multiplexer 21 LiNbO ₃ crystal 22 Wavelength filter 23 Beam splitter 24, 24A, B Light receivers 25, 25A to C Lock-in amplifier 26 Multipath cell 30 External modulator 31 Oscillator 32, 32A, B Tuning fork 33A to C Amplifier 34 Oscilloscope 35 Resonator 36 Changeover switch 40A, B, E, F Parallel plate reflector 40C, D, G, H Concave mirror

Claims (14)

In a light absorption measuring apparatus for measuring the optical absorption characteristics of the material to be measured by making the light emitted from the laser light emitting means incident on the material to be measured,
A first tuning fork disposed in the material to be measured so as to intersect an optical path of incident light incident on the material to be measured;
Modulation means for modulating the intensity of the incident light with a signal near the resonance frequency of the first tuning fork;
Conversion means for converting resonance vibration of the first tuning fork into a first electrical signal;
An optical absorption measurement apparatus comprising: a measuring unit that locks-in-detects the first electric signal and a signal in the vicinity of the resonance frequency to measure an optical absorption characteristic of the measurement substance.   The light absorption measurement apparatus according to claim 1, wherein the modulation unit modulates the intensity of the incident light.   The light absorption measuring apparatus according to claim 1, wherein the modulation unit modulates a frequency of the incident light.   4. The light absorption measuring apparatus according to claim 1, wherein the conversion means is a condenser microphone that converts an acoustic signal, which is resonance vibration of the first tuning fork, into a first electric signal. 5.   4. The light absorption measuring apparatus according to claim 1, wherein the converting means is a strain sensor that converts a resonance vibration of the first tuning fork into a first electric signal.   4. The light absorption measuring apparatus according to claim 1, wherein the tuning fork is a crystal resonator, and the conversion means includes a preamplifier and an impedance converter.   The resonator according to any one of claims 1 to 6, further comprising a resonator that interpolates a point where the optical path of the incident light and the first tuning fork intersect, and resonates by resonance vibration near the resonance frequency. The light absorption measuring apparatus as described.   8. The reflector according to claim 1, further comprising a reflecting mirror disposed on the optical path of the incident light and facing the point where the optical path of the incident light and the first tuning fork intersect. The light absorption measuring device according to 1. A branching unit arranged on the optical path of the incident light and branching a part of the incident light;
Photoelectric conversion means for converting the branched incident light into a second electric signal;
First correction means for correcting the optical absorption characteristics of the measurement substance measured by the measurement means by performing lock-in detection on the second electric signal and a signal in the vicinity of the resonance frequency. The light absorption measuring device according to claim 1. A second tuning fork disposed in the material to be measured so as not to intersect the optical path of the incident light;
Conversion means for converting resonance vibration of the second tuning fork into a third electrical signal;
And second correction means for correcting the optical absorption characteristics of the measurement substance measured by the measurement means by performing lock-in detection on the third electric signal and a signal in the vicinity of the resonance frequency. The light absorption measuring device according to any one of claims 1 to 9. The laser beam emitting means is
A _first laser that generates laser light of wavelength λ ₁ ;
A _second laser that generates laser light of wavelength λ ₂ ;
Nonlinear optics for inputting the laser light having the wavelength λ _{1 and} the laser light having the wavelength λ ₂ and outputting the difference frequency light having the wavelength λ _{3 in} the relationship of 1 / λ ₁ −1 / λ ₂ = 1 / λ _3. The light absorption measuring device according to claim 1, further comprising: a crystal. The first laser generates laser light having a wavelength λ ₁ = 0.9 to 1.1 μm,
The second laser generates laser light having a wavelength λ ₂ = 1.25 to 1.75 μm,
12. The optical absorption measurement apparatus according to claim 11, wherein the nonlinear optical crystal outputs difference frequency light having a wavelength [lambda] ₃ = 1.9 to 5.7 [mu] m. The nonlinear optical crystal is LiNbO ₃ (LN), LiTaO ₃ (LT) or a mixed crystal thereof, or LN, LT or a mixed crystal to which Zn or Mg is added, and a domain-inverted structure is periodically formed. The optical absorption measuring device according to claim 12, wherein   The optical absorption measurement apparatus according to claim 13, wherein the nonlinear optical crystal has a waveguide structure.

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