JPH10148573A - Method and device for spectral analysis - Google Patents

Abstract

PROBLEM TO BE SOLVED: To easily measure Raman scattering with no use of a spectroscope without affected by fluorescence. SOLUTION: By controlling a wavelength variable laser 40 with a control device 41, is the wavelength of monochrome light LB projected on a sample S is wavelength-swept, while alternately switching the wavelength between the first wavelength and the second wavelength which comprises a constant frequency difference Δνex to the first wavelength. The light emitted from the sample S is, through an interference filter 44 which allows an observation oscillation frequency νob to pass, detected with an optical detector 45. The output of the optical detector 45 is phase-synchronization-detected with a lock-in amplifier 46, and then processed with a signal processing device 47 to obtain Raman spectrum of the sample S.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for spectroscopically analyzing a sample, and more particularly, to a spectroscopic analysis method and an apparatus for measuring luminescence emitted from a sample.
In the present specification, fluorescence and Raman scattered light emitted from a sample are collectively referred to as light emission.

[0002]

2. Description of the Related Art A sample has a frequency ν ₀ (light speed c, wavelength λ).
When irradiating a laser beam of ν ₀ = c / λ),
Raman scattered light having a frequency component shifted from the frequency ν _{0 of the} incident light by ± Δν is obtained. Frequency ν of incident light
The difference ± Δν between ₀ and the frequency of the Raman scattered light is called Raman shift. Frequency low Raman line than vibration frequency [nu ₀ of the incident light in the Raman line (ν ₀ -Δν) is called the Stokes line, the frequency of high Raman lines from the vibration frequency [nu ₀ of the incident light (ν ₀ + Δν ) Is called the anti-Stokes line. Raman scattering reflects the state of molecular vibrations of the sample, similar to infrared absorption.However, infrared absorption can be observed for molecular vibrations with a change in dipole moment, whereas Raman scattering is polarized. Generated by molecular vibrations with changing rates, they give different information. In addition, while the infrared absorption analysis of an aqueous solution sample is very difficult, the Raman spectrum of water is weak. Therefore, the use of Raman scattering has an advantage that the analysis of a sample dissolved in water becomes easy.

Since Raman scattering is weak, a laser capable of obtaining high-intensity monochromatic light is used as a light source for measuring Raman scattering. As a spectroscope for Raman spectrum measurement, two diffraction gratings are used as a monochromator with sufficient resolution and low stray light to separate Raman scattered light from very strong Rayleigh scattered light. A double monochromator that uses three diffraction gratings or a triple monochromator that uses three diffraction gratings is used. As a detector, a type that uses a photomultiplier tube and scans a wavelength (wave number) by rotating a diffraction grating of a spectroscope,
There is a type in which a spectrum is measured at once using an optical multi-channel analyzer. Further, Fourierman spectroscopy using an interference type spectroscope as a spectroscope is also known.

[0004] By the way, as light detected at a frequency position shifted from the frequency of incident light, there is fluorescence other than Raman scattered light. Fluorescence may be generated from impurities mixed into the sample, but may also be generated from the sample itself. If the fluorescence is generated from the sample, the fluorescence cannot be reduced to zero even if the impurities are removed. In addition, the intensity of the fluorescent light is significantly higher than that of the Raman scattered light, which hinders the detection of the Raman scattered light.

As a method for measuring the Raman scattered light while suppressing the generation of fluorescence, there are two types of excitation light: infrared light having no fluorescence,
For example, it is conceivable to use 1.06 μm light from a YAG laser, but Raman scattered light is weakened by infrared excitation.

[0006] As another example of a method for measuring the Raman spectrum separated from fluorescence, Japanese Patent Application Laid-Open No. 49-59693 discloses a method of irradiating a sample with a laser beam having a modulated wavelength and irradiating the sample with the laser beam. There is described a method of detecting light after dispersing the light and obtaining only an AC component of the detected signal. In this method, when the wavelength of the incident laser light is shifted, the frequency of the Raman scattered light shifts with the wavelength shift of the incident laser light, whereas the wavelength of the fluorescent light shifts depending on the wavelength shift of the incident laser light. This is to take advantage of what is not done. The same technology is also described in JP-A-51-80282 and JP-A-53-39156.

Further, JP-A-49-60582 and JP-B-55-31893 disclose the use of the difference between the degree of depolarization of the Raman scattered light emitted from the sample and the degree of depolarization of the fluorescent light to obtain the fluorescence. A method of obtaining a Raman spectrum having no influence is described. Japanese Patent Publication No. 51-11511 discloses that Raman scattered light and fluorescence are separated by utilizing the difference between the lifetimes of Raman scattered light and fluorescence. ing.

[0008]

In the measurement of the Raman spectrum, in order to separate the weak Raman scattered light from the Rayleigh scattered light and to measure it at high resolution, as described above, it is necessary to use a spectrometer such as a double monochromator or a triple monochromator. Vessel is used. The brightness of the spectrometer is incompatible with the resolution,
When a high resolution is required, the brightness is sacrificed and the measurement takes a long time. In addition, the spectroscope occupies a large space, and its use involves complicated operations such as setting the slit width, scanning speed, time constant, and wavelength calibration.

In Raman spectroscopy, eliminating the influence of fluorescence is an important problem. However, any of the above-mentioned conventional methods for separating Raman scattered light from fluorescence uses a spectroscope as a spectral means. There is a similar problem. The present invention has been made in view of such problems of the related art, and provides a method and an apparatus that can easily measure Raman scattering without using a spectroscope and without being affected by fluorescence. With the goal.

[0010]

According to the present invention, as in the conventional method, if the wavelength of the excitation laser light is slightly changed, the wavelength of the Raman scattered light changes accordingly, but the fluorescence does not change. Utilize to separate Raman scattered light and fluorescence. However, as a means for dispersing the Raman scattered light, only one narrow band transmission filter such as an interference filter is used without using a spectroscope.

The principle of the present invention will be described with reference to FIGS. FIG. 1 is a schematic diagram of a Raman spectrum when no fluorescence is generated from a sample, and FIG. 2 is a schematic diagram of a spectrum in which fluorescence is superimposed on a Raman line.

In FIG. 1A, ν _ex is the frequency of the excitation light, and R ₁ , R ₂ , R ₃ and R ₄ are Raman lines. Although the anti-Stokes line appears on the frequency side higher than the frequency of the excitation light, the description will be made using the Stokes line appearing on the frequency side lower than the frequency of the excitation light. Raman line R
₁ appears at a frequency shifted from the excitation frequency ν _ex to a frequency lower by the frequency Δν ₁ . Similarly, the other Raman lines R ₂ , R ₃ , and R ₄ also appear at positions shifted by frequencies lower than the frequency ν _ex of the excitation light by frequencies Δν ₂ , Δν ₃ , and Δν ₄ , respectively. These Raman shifts Δν
₁ , Δν ₂ , Δν ₃ , and Δν ₄ are quantities specific to the substance.

As shown in FIG. 1B, when the frequency of the excitation light is shifted to a frequency v _ex ′ higher than v _ex, for example,
Raman lines R ₁ , R ₂ , R ₃ , R ₄ are frequency shift amounts Δν ₁ , Δν ₂ , Δ with respect to the frequency ν _ex ′ of the excitation light.
Similarly, it moves to the higher frequency side while keeping ν ₃ and Δν ₄ constant. Therefore, while sweeping the frequency of the excitation light to a higher frequency side, for example, by using a narrow band transmission filter such as an interference filter, the fixed observation frequency ν _ob shown in FIGS. 1A and 1B is used. Observing the scattered light from the sample, Raman lines R ₁ , R ₂ , R ₃ , and R ₄ are detected in order as Δν = ν _ex −ν _ob becomes ν ₁ , ν ₂ , ν ₃ , ν _4. Therefore, a Raman spectrum as shown in FIG. 1C is obtained.

However, when fluorescence is generated together with the Raman scattered light from the sample, the frequency of the excitation light is swept by the above method, and the scattered light from the sample is detected at a fixed observation frequency ν _ob . As shown by the solid line in FIG. 2A, a spectrum is obtained in which the Raman lines R ₁ , R ₂ , R ₃ , and R ₄ lie on the fluorescence excitation spectrum FL.

Therefore, in the present invention, the frequency of the excitation laser light is switched at high speed between two frequencies ν _ex1 and ν _ex2 . At this time, the Raman lines R ₁ , R ₂ , R ₃ , R
_4, in synchronism with the switching of the excitation frequency FIGS. 2 (a)
Is moved between the solid line position and the broken line position shown in FIG. On the other hand, the fluorescence hardly changes even when the excitation frequency is rapidly switched. Therefore, the frequency of the excitation laser beam is swept to, for example, the higher frequency side while switching the frequency of the excitation laser light between the two frequencies at high speed, and the signal detected at the fixed observation frequency ν _ob is determined by the switching signal of the excitation frequency. By performing synchronous detection, a Raman spectrum as shown in FIG. 2C can be obtained.

On the contrary, when the Raman spectrum shown in FIG. 2B is removed from the spectrum shown in FIG.
A fluorescence excitation spectrum FL as shown in (c) is obtained. That is, the present invention can be used for measuring a Raman spectrum of a sample, and can also be used for measuring a precise fluorescence excitation spectrum of the sample.

In order to detect a Raman spectrum by such a method, a frequency-variable laser capable of performing frequency scanning over a wide range while switching vibration (excitation wavelength) at high speed is indispensable. To achieve this, an electronically controlled wavelength tunable laser developed separately by the present inventor that enables high-speed wavelength sweeping by controlling the laser oscillation wavelength electrically without providing a mechanically movable part such as a rotating mechanism. [Hereafter, ETT (Electronically Tuned Tuna
ble) laser] can be used.

In the ETT laser, a laser medium capable of oscillating laser light in a predetermined wavelength region and a birefringent acousto-optical element are arranged in a laser resonator, and a light beam diffracted at a predetermined angle by the birefringent acousto-optical element. A wavelength tunable laser that constitutes a laser resonator only for the component and performs wavelength selection by selecting the frequency of the acoustic wave to be excited in the birefringent acousto-optic element.For example, when titanium sapphire is used as the laser medium , 680 nm to 1100 nm in a time within 1 second. In addition, since the wavelength is electrically selected using the birefringent acousto-optic element, the wavelength can be switched instantaneously, and, for example, switching between any two wavelengths can be performed stably in 1 ms or less. The present invention can be realized only for the first time by the development of the tunable laser.

That is, according to the present invention, in a spectroscopic analysis method of irradiating a sample with monochromatic light from a wavelength tunable laser and measuring light emission emitted from the sample, the wavelength tunable laser is provided within a laser resonator in a predetermined wavelength region. A laser medium capable of laser oscillation and a birefringent acousto-optical element are arranged, and a laser resonator is formed on a predetermined optical axis of a light component diffracted by the birefringent acousto-optical element. A tunable laser (ETT laser), which performs wavelength selection by selecting a frequency of an acoustic wave to be excited therein, is used to measure the emission intensity of a constant wavelength emitted from a sample.

The present invention also relates to a spectroscopic method for irradiating a sample with monochromatic light from a wavelength variable laser such as an ETT laser and observing the light emitted from the sample. Sweeping, observing light emitted from the sample at a predetermined wavelength, and obtaining an emission excitation spectrum at the observed wavelength.

Further, according to the present invention, in a spectroscopic analysis method of irradiating a sample with monochromatic light from a wavelength tunable laser and observing light emitted from the sample, the wavelength of the monochromatic light applied to the sample is changed to a first wavelength. And a second wavelength having a certain frequency difference with respect to the first wavelength, while alternately switching the wavelength, sweeping the wavelength, observing the light emitted from the sample at the third wavelength, and observing the third wavelength. Of the observation light at the wavelength of (i) is separated and observed as Raman scattered light.

At this time, the light emitted from the sample at the fourth wavelength different from the third wavelength is observed together with the third wavelength, and Raman scattering is determined from the correlation with the frequency difference between the third wavelength and the fourth wavelength. Light can also be separated precisely. The component that does not change over time due to wavelength switching is
It can be separately observed as a non-Raman component.

By irradiating the sample with monochromatic light from the wavelength tunable laser through an optical fiber and observing the light emitted from the sample through the optical fiber, it is possible to perform remote measurement of the sample. In addition, by scanning monochromatic light from the wavelength tunable laser relative to the sample, the distribution of light emission in a two-dimensional area of the sample can be measured.

A spectroscopic analyzer according to the present invention includes a wavelength tunable laser for irradiating a sample with monochromatic light, and an oscillation wavelength of the wavelength tunable laser, which is a first wavelength and a constant frequency difference with respect to the first wavelength. Wavelength control means for sweeping the wavelength while switching the wavelength between the second wavelength, a narrow band transmission filter transmitting the third wavelength, and a narrow band transmission filter emitted from the sample by irradiation of monochromatic light A photodetector for detecting light of a third wavelength transmitted through the light detector; and a phase-locked detection means for detecting a detection signal of the photodetector in phase with a switching signal of the first wavelength and the second wavelength. And a function of measuring a Raman spectrum of the sample.

Further, the spectroscopic analyzer according to the present invention provides a wavelength tunable laser for irradiating a sample with monochromatic light, an oscillation wavelength of the wavelength tunable laser at a first wavelength and a constant oscillation with respect to the first wavelength. Wavelength control means for performing wavelength sweeping while switching the wavelength between the second wavelength having a number difference, a first narrow-band transmission filter transmitting a third wavelength, and a fourth narrow-band transmission filter different from the third wavelength. A second narrow-band transmission filter that transmits light of a wavelength of, and a first that detects light emitted from the sample by irradiation of monochromatic light and transmitted through the first narrow-band transmission filter.
, A second photodetector for detecting light transmitted through the second narrow-band transmission filter, and a detection signal of the first and second photodetectors at a first wavelength and a second wavelength. A phase-locking detection means for detecting a phase-locked signal in synchronization with the switching signal, and a comparison means for comparing the two phase-locked detection signals of the phase-locking detection means with a function of measuring a Raman spectrum of the sample. And

Further, the spectroscopic analyzer according to the present invention provides a wavelength tunable laser for irradiating a sample with monochromatic light, an oscillation wavelength of the wavelength tunable laser at a first wavelength and a constant oscillation with respect to the first wavelength. Wavelength control means for sweeping the wavelength while switching the wavelength to a second wavelength having a number difference, an interferometer for performing interference spectroscopy on light emitted from the sample by irradiation of monochromatic light, and an output signal of the interferometer And a means for performing a Fourier transform on the output of the phase-locked detection means for detecting the phase-locked signal in phase synchronization with the wavelength switching signal of the wavelength control means, and having a function of measuring the Raman spectrum of the sample. Features.

The wavelength tunable laser used in the spectrometer according to the present invention comprises a laser medium having a laser medium capable of oscillating laser light in a predetermined wavelength region and a birefringent acousto-optic element in a laser resonator.
A laser that forms a laser resonator on a predetermined optical axis of a light component diffracted by a birefringent acousto-optic element and performs wavelength selection by selecting a frequency of an acoustic wave to be excited in the birefringent acousto-optic element That is, it is preferable to use an ETT laser.

By providing an optical fiber connecting between the tunable laser and the sample and / or between the sample and the photodetector, remote measurement of the sample can be performed. In addition, by providing a means for scanning monochromatic light from the wavelength tunable laser relative to the sample, light emission from the sample can be measured two-dimensionally.

According to the present invention, it is possible to detect Raman scattered light without being affected by fluorescence by using a narrow band transmission filter that is extremely easy to handle, such as an interference filter, without using a spectroscope.

[0030]

Embodiments of the present invention will be described below with reference to the drawings. Before describing the present invention, a wavelength tunable laser (ETT laser) capable of high-speed wavelength switching and used in the present invention will be described first. When an acoustic wave is excited in an acousto-optic crystal exhibiting birefringence, diffracted light of a specific wavelength according to the frequency of the acoustic wave in the light incident on the crystal becomes the acoustic wave, the incident light, and the diffracted light. Strongly diffracted in a direction that satisfies the phase matching condition between them. FIG.
FIG. 3 is a conceptual diagram showing the state of this diffraction.

Now, it is assumed that incident light 102 having an angular frequency ωi is incident on a birefringent acousto-optic element 100 in which a piezoelectric element 22 is attached to an acousto-optic crystal exhibiting birefringence such as a TeO ₂ crystal. Further, the piezoelectric element 22 causes the acoustic wave 104 having the angular frequency ωa into the birefringent acousto-optic element 100.
Is excited, the diffracted light 106 whose frequency is shifted to the following angular frequency ωo is obtained by the interaction between the incident light 102 and the acoustic wave 104. The incident light 1
02 is an extraordinary ray and diffracted light 106 is an ordinary ray. The plane of polarization of the diffracted light 106 is orthogonal to the plane of polarization of the incident light 102. Reference numeral 108 denotes undiffracted light.

[0032]

[Equation 1] ωo = ωi + ωa

Where ωa≪ωi, ωo, and ωi ≒
It can be considered as ωo. At this time, the wave number vector of the incident light 102 is ki, and the wave number vector of the acoustic wave 104 is k.
a, when the wave vector of the diffracted light 106 is ko, the vector equation represented by the following [Equation 2] is established from the phase matching condition.

[0034]

## EQU2 ## ko = ki + ka

FIG. 4 shows the relationship between the k-vector of an ordinary ray and the k-vector of an extraordinary ray propagating through the birefringent acousto-optic element 100. The magnitude of the k vector with respect to the ordinary ray is constant regardless of the traveling direction, and the trajectory of the end point of the k vector is a circle. On the other hand, the magnitude of the k vector for the extraordinary ray changes depending on the propagation angle of the birefringent acousto-optic element 100 with respect to the crystal axis, and the trajectory of the end point of the k vector becomes elliptical. The circle or ellipse formed by the trajectory of the k vector expands or contracts almost similarly when the wavelength is changed. FIGS. 4 (a) shows a state where the phase matching condition of expression (2) at the wavelength lambda ₁ is satisfied. In the figure, Va is the velocity of the acoustic wave 104 propagating through the crystal, and the magnitude of the wave number vector ka of the acoustic wave 104 is | ω
a / Va |.

[0036] Here, the frequency of the acoustic wave 104 is excited in the birefringent acousto 100 .omega.a, thus changing the magnitude of the wave vector ka, the phase matching condition of the wavelength lambda ₁ expression (2) is satisfied Disappears. At this time, the condition that the phase matching condition is satisfied is the wavelength λ ₂ as shown in FIG. Thus, the wavelength λ of light that satisfies the phase matching condition and the angular frequency ωa of the acoustic wave have a one-to-one correspondence.

As described above, the size of the circle or ellipse connecting the end points of the locus of the k vector changes depending on the wavelength.
Its shape changes little. Therefore, if the wavelength is λ ₁
To λ ₂ , and thus, even if the magnitudes of the vectors ki and ko of the incident light 102 and the diffracted light 106 change, the shapes become similar. Therefore, the vector (ko ₁ −ki ₁ ) and the vector (ko ₂ −ki ₂ ) The directions are parallel. As a result, ka
_An acoustic wave having a vector of ₁ = ko ₁ -ki ₁ and ka ₂ = ko ₂ -ki ₂ can be input simply by changing the acoustic frequency.

When the light of the wave vector ko emitted from the birefringent acousto-optical element 100 is reflected by the reflecting mirror 110 and is incident on the birefringent acousto-optical element 100 from the opposite direction, FIG. As shown, the returned light is also diffracted by the acoustic wave, and becomes -ki, which travels again in the opposite direction to the incident light ki, and traces the optical path of the incident light in reverse.

Therefore, when the total reflection mirror 110 and the exit side mirror 112 having a predetermined transmittance are arranged with the laser medium 14 and the birefringent acousto-optic element 100 interposed therebetween, for example, as shown in FIG. A laser resonator in which only light having a specific wavelength component only reciprocates between the two is constituted by 110 and the output side mirror 112. The wavelength λo of the diffracted light 106 becomes ka when the frequency ωa of the acoustic wave 104 generated in the birefringent acousto-optic element 100 is changed.
Is changed and ki is selected. As a result, the wavelength λi = 2π / |
ki | is determined. Therefore, the birefringent acousto-optic element 1
The laser oscillation wavelength λi can be controlled by driving the piezoelectric element 22 mounted at 00 with an RF signal of a predetermined frequency from the RF power supply 20.

Since the diffraction efficiency of the diffracted light 106 is determined by the intensity of the acoustic wave excited in the birefringent acousto-optic device 100, the amplitude of the RF signal output from the RF power source 20 is controlled. The intensity of the diffracted light 106, and thus the laser output, can be variably controlled.

Although it has been described above that the shape of the circle or ellipse connecting the end points of the locus of the k vector hardly changes depending on the wavelength, it actually changes slightly. Therefore, the diffraction angle also slightly changes depending on the wavelength, the condition of the resonator constituted by the total reflection mirror 110 and the partial transmission mirror 112 changes, and the direction of the emitted laser light changes slightly. This wavelength dependence of the diffraction angle can be compensated for by disposing a chromatic dispersion compensating element 28 such as a prism between the birefringent acousto-optic element 100 and the total reflection mirror 110. Direction can be made constant.

Laser medium 14 and birefringent acousto-optic device 1
The telescope 30 arranged between 00 is for adjusting the beam diameter, and the parallel light whose diameter has been enlarged by the telescope 30 passes through the birefringent acousto-optic device 100. According to this arrangement, the light reciprocating in the laser resonator passes through the laser medium 14 as a light beam having a high light intensity, so that the laser efficiency does not decrease. on the other hand,
At the position of the birefringent acousto-optical element 100, the light intensity irradiated per unit area decreases, so that optical damage to the birefringent acousto-optical element 100 can be suppressed.

As the laser medium, Ti: Al ₂ O ₃ , L
Any known tunable laser medium such as a laser crystal such as iSAF or LiCAF, or a dye solution can be used. The ETT laser can be a continuous wave laser by using a continuous wave laser (CW laser) as a pump laser source, or a pulsed laser by using a pulse laser as a pump laser source.
For example, when Ti: Al ₂ O ₃ is used as the laser medium, Nd: YAG laser, Nd: YLF laser, Nd:
YVO ₄ can be used a second harmonic and an argon ion laser of Nd solid state laser such as a laser, LiSAF laser crystal as the laser medium, in the case of using a LiCAF laser crystal be a semiconductor laser or a krypton ion laser it can.

The efficiency is increased by matching the pumping volume of the pumping laser in the laser medium with the optical mode volume in the laser resonator. Lasers and continuous wave lasers can also be used as pump lasers. For example, the laser resonator may be a Z-hold type laser resonator or an X-hold type laser resonator, and the pump laser light may be introduced along the optical path in the laser resonator, so that the pump light is efficiently used and the laser light is reduced. Laser oscillation can be caused by excitation light of energy.

FIG. 6 is a schematic diagram showing an example of an ETT laser using a Z-hold type laser resonator. The Z-hold type laser resonator includes an emission side mirror 112 having a predetermined transmittance and a total reflection mirror 110. Further, a first laser beam which enters the excitation laser beam A and reflects the light beam B which reciprocates between the exit side mirror 112 and the total reflection mirror 110.
An intermediate mirror 37 and a second intermediate mirror 38 for reflecting light B reciprocating between the emission side mirror 112 and the total reflection mirror 110 are provided. The optical path of the light B reciprocating in the laser resonator is represented by the alphabet Z. It is shaped like a letter.

The excitation laser light A generated by the excitation laser 32 is reflected by the total reflection mirror 34 on the total reflection mirror 36, collected by the total reflection mirror 36, and passed through the first intermediate mirror 37. The laser beam is incident so as to excite the laser medium 14 in the longitudinal direction. The frequency ωa of the RF power supply 20 is controlled by the personal computer 26 according to the wavelength (frequency ωi) of the emission laser light C to be emitted from the emission side mirror 112, and the piezoelectric element 22 is driven.

With this configuration, of the light in a wide wavelength range emitted from the laser medium 14 and incident on the birefringent acousto-optic device 100, light having a wavelength corresponding to the frequency of the RF power source 20 is Diffracted light D by the refracting acousto-optic element 100
(Frequency ωo). The diffracted light D is perpendicularly incident on the total reflection mirror 110 via the wavelength dispersion correction prism 28 of the diffraction angle, is reflected by the total reflection mirror 110, and reciprocates in the laser resonator along a Z-shaped optical path. (Angle frequency ωi at the position of the laser medium 14). Therefore, only the light having the wavelength corresponding to the frequency of the RF power supply 20 is amplified and laser oscillates, and the emitted laser light C (frequency ωi) having the wavelength is emitted from the laser resonator.

FIG. 7 shows a wavelength tunable characteristic of the ETT laser shown in FIG. T as the laser medium 14
i: An Al ₂ O ₃ crystal was used, and CW-
Using a Q switch pulse Nd: YLF laser, the second
The harmonic was used as the excitation laser light A. Excitation laser light A
Has a wavelength of 523 nm and a pulse repetition frequency of 1 kHz.
z, the output per pulse was 100 μJ. Also,
The diameter of the total reflection focusing mirror 36 is 200 mm, and the radius of the first intermediate mirror 37 and the second intermediate mirror 38 is 100 m.
m, and the output side mirror 112 has a reflectance of 97% (transmittance 3
%). The excitation area and the cavity mode diameter of the laser medium 14 are reduced to several tens of μm, and the excitation laser light A is condensed in this area by the total reflection condensing mirror 36, thereby improving the excitation efficiency. As is clear from FIG. 7, the wavelength variable range is from about 740 nm to about 870 nm. With the provision of the prism 28 for correcting the wavelength dispersion of the diffraction angle, the beam deflection observed at the time of tuning the wavelength of the laser was below the observation limit.

FIG. 8 is a schematic structural view of an example of the spectroscopic analyzer according to the present invention. The monochromatic laser beam LB generated from the tunable laser 40 is incident on the sample S. The scattered light generated from the sample S is collimated by the collimator lens 43, and then enters a narrow-band transmission filter 44 such as an interference filter that transmits only a narrow-band frequency centered on the frequency ν _ob. . The light beam transmitted through the narrow band transmission filter 44 is detected by a photodetector 45 such as a photomultiplier tube. The output of the photodetector 45 is a lock-in amplifier 46.
Supplied to The output of the lock-in amplifier 46 is supplied to a signal processing device 47, and the result of the signal processing is displayed on a display device 48 such as a CRT.

On the other hand, the tunable laser 40 is
Under the control of 1, the first frequency ν _ex1 and the second frequency ν _ex2 having a constant frequency difference Δν _ex with respect to the first frequency ν _ex1 alternately oscillate at two frequencies. Wavelength switching. The first frequency ν _ex1 and the second
_{Is swept toward the} high frequency side or the low frequency side while maintaining a constant frequency difference Δν _ex . The frequency difference Δν _ex between the first frequency ν _ex1 and the second frequency ν _ex2 may be any value, but is usually selected to be slightly wider than the Raman spectrum width. The control signal from the control device 41 is also input to the lock-in amplifier 46 as a synchronization signal.

FIG. 9A is a diagram schematically showing the output of the wavelength tunable laser 40. The horizontal axis is the time axis. As shown in the figure, the tunable laser 40 is controlled by the control device 41 to control the first frequency ν _ex1 and the second frequency ν _ex2.
(= Ν _ex1 + Δν _ex ). The tunable laser 40 used in the present invention can switch between two wavelengths at a wavelength switching frequency f (= 1 / t) of about 1 ms.

FIG. 9B is a diagram schematically showing the output signal of the detector 45. The light measured by the detector 45 is a scattered light component having a narrow band centered on the third frequency ν _ob (fixed frequency) transmitted through the narrow band transmission filter 44. The detection output I ₁ of the detector 45 is the first frequency ν
Corresponding to the scattered light detection signal generated from the sample S when irradiated with the laser light LB of _ex1 , the detection signal I ₂ is generated from the sample S when irradiated with the laser light LB of the second frequency ν _ex2 Corresponding to the detected scattered light signal.

When the frequency of the excitation light changes between the first frequency ν _ex1 and the second frequency ν _ex2 , the fluorescence intensity transmitted through the narrow band transmission filter 44 and received by the photodetector 45 since almost no change, the contribution of the fluorescence to the detection output I ₂ and the contribution of the fluorescence to the detection output I ₁ is equal. On the other hand, the frequency of the Raman scattered light changes according to the change in the frequency of the excitation light as shown in FIG. 2A. For example, when the laser beam LB having the first frequency ν _ex1 is excited, the Raman line Even if the frequency of the laser beam just coincides with the observation frequency ν _ob and is detected by the detector 55, when excited by the laser beam LB having the second frequency ν _ex2 , Δν _ex is larger than the Raman spectrum width. Large, Raman line frequency is observed frequency ν _ob
And is no longer detected by the detector 45. That is, although the Raman scattered light contributes to the detection output I ₁ ,
It does not contribute to the detection output I _2. Therefore, FIG.
As schematically shown in (b), the signal component of the frequency f / 2 is phase-locked detected by the lock-in amplifier 46, whereby the Raman component can be separated from the detection signal of the detector 45 and measured.

In order to measure the Raman spectrum, the frequency of the monochromatic laser beam LB generated from the frequency tunable laser 40 is calculated by _calculating the frequency difference Δν _ex between the first frequency ν _ex1 and the second frequency ν _ex2 ( = Ν _ex1 −ν _ex2 ) while keeping the frequency constant and alternately switching at two frequencies while continuously sweeping in the direction of higher frequency or in the direction of lower frequency.

FIG. 10 is a diagram schematically showing an output obtained from the lock-in amplifier 46 when such a frequency sweep is performed. Raman shift of a Stokes line is Δ
_Assuming that ν _R , when the Raman line of ν _ob is observed by the excitation of ν _ex1 or ν _ex2 , the following relationship is satisfied.

[0056]

Δν _R = ν _ex1 −ν _ob , Δν _R = ν _ex2 −ν _ob

Therefore, when the frequency of the wavelength tunable laser 40 is swept and the frequency ν _ex1 or ν _ex2 satisfies the above-described resonance relationship of _Equation 3, a large Raman signal ΔI is obtained. By setting the lock-in amplifier 46 for phase-locked detection, ΔI> 0 when the frequency ν _ex1 resonates, and ΔI <0 when the frequency ν _ex2 resonates, as shown in FIG. Peaks P ₂ , P ₄ , P ₆ , P
₈ is a peak based on the resonance of v _ex1 , and negative peaks P ₁ , P ₃ , P ₅ , and P ₇ are peaks based on the resonance of v _ex2 . Lock-in amplifier 46 for one Raman line
Gives a pair of positive and negative peaks appearing with a constant frequency difference Δν _ex (= ν _ex1 −ν _ex2 ), and similar peak pairs (P ₁ , P ₂ ), (P P ₃ ,
_{P 4), (P 5,} P 6), is obtained (P _7, P _8).

The signal processing device 47 includes the lock-in amplifier 4
6, the positive signal peak trains P ₂ , P ₄ , P ₆ ,
The Raman spectrum of the sample S is obtained by rearranging P ₈ with the difference Δν (= ν _ob −ν _ex1 ) between the observed frequency ν _ob and the excitation frequency ν _ex1 when each peak appears on the horizontal axis. The information is displayed on the display device 48. The same Raman spectrum can be obtained by performing the same processing on the negative signal peak strings P ₁ , P ₃ , P ₅ , and P ₇ instead of the positive signal peak strings. Further, in the signal processing unit 47, the positive signal peak train _{P 2, P 4, P 6} , waveform and signal peaks of the Raman spectrum obtained from P ₈ column _{P 1, P 3, P 5} , P
_It is also possible to compare the waveform of the Raman spectrum obtained from ₇ and output only a portion where both waveforms overlap with each other to the display device 48 as a Raman spectrum. In this case, 2
A portion where two spectral waveforms do not overlap can be regarded as noise.

FIG. 11 is a schematic configuration diagram showing an example of a spectroscopic analyzer in which the accuracy of Raman spectrum measurement is further improved by observing scattered light at two different frequencies. ETT
The wavelength variable laser 40 such as a laser oscillates alternately at a first frequency ν _ex1 and a second frequency ν _ex2 under the control of the control device 41. Second vibration frequency [nu _ex2 has a constant frequency difference .DELTA..nu _ex to the first vibration frequency [nu _ex1, first vibration frequency [nu _ex1 and second vibration frequency [nu _ex2 is constant vibration The frequency is swept toward the high frequency side or the low frequency side while maintaining the number difference Δν _ex .

The monochromatic laser beam LB generated from the wavelength tunable laser 40 is incident on the sample S. The scattered light generated from the sample S is converted into parallel light by the collimator lens 43a, and then transmitted to a narrow band transmission filter 44a such as an interference filter that transmits only light having a narrow band frequency centered on the frequency ν _ob1. Incident. Narrow band transmission filter 44
The light transmitted through a is a photodetector 45a such as a photomultiplier tube.
Is detected by Scattered light generated from the sample S at the same time, after being collimated by the collimator lens 43 b, transmits only light of the frequency of the narrow band around the second vibration frequency [nu _ob2 different from the vibration frequency [nu _ob1 The light passes through the narrow band transmission filter 44b and is detected by the photodetector 45b. Outputs of the photodetectors 45a and 45b are supplied to a lock-in amplifier 46.

The lock-in amplifier 46 uses the control signal from the control device 41 as a synchronization signal, and
The output signal b is subjected to phase-locked detection. Signal processing device 47
Calculates the Raman spectrum of the sample S from the output signal of the lock-in amplifier 46 obtained by phase-locking the output signal of the photodetector 45a as described above.
Lock-in amplifier 4 that performs phase-locked detection on the output signal of 5b
The Raman spectrum of the sample S is obtained from the output signal of No. 6.
Then, these two Raman spectra are compared, and the one having a strong correlation with the frequency difference is regarded as a true Raman spectrum, the one having a weak correlation is regarded as a noise, and the spectrum from which the noise has been removed is displayed on a display device 48 such as a CRT. I do. According to this spectroscopic analyzer, the Raman observation accuracy can be increased to obtain a precise Raman spectrum of the sample S, and the observation time can be shortened.

FIG. 12 is a schematic diagram showing an example of a spectroscopic analyzer of the present invention using an interference type spectroscope. The wavelength variable laser 40 such as an ETT laser oscillates alternately at a first frequency ν _ex1 and a second frequency ν _ex2 under the control of the control device 41. The second frequency ν _ex2 is _equal to the first frequency ν
_ex1 has a constant frequency difference Δν _ex , and the first frequency ν _ex1 and the second frequency ν _ex2 have a constant frequency difference _Δex.
high frequency side or keeping the [nu _ex is swept in a low frequency side.

The two-wavelength alternately oscillated laser beam LB of wavelengths λ ₁ and λ ₂ generated from the tunable laser 40 is incident on the sample S, and the scattered light generated from the sample S is collimated by the collimator lens L. Thereafter, the light enters the interference spectroscope including the fixed mirror M ₁ , the moving mirror M _2, and the half mirror HM. Scattered light from the sample S is divided into a component incident with component incident on the fixed mirror M ₁ by the half mirror HM to move the mirror M _2, reflected by the movable mirror M ₂ and the reflected components by the fixed mirror M ₁ The components thus combined are again combined by the half mirror HM and interfere on the photodetector D.

The control signal from the control device 41 is also input to the lock-in amplifier 46, which converts the output signal of the photodetector D into the switching frequency f of the wavelength alternate oscillation.
Is used for phase-locked detection. Δλ (= λ ₁ −λ ₂ ) is small, and the wavelength of the Raman scattered light shifts due to the wavelength switching, but the fluorescence does not shift. Therefore, the synchronously detected component becomes the Raman scattering output component. The output of the lock-in amplifier 46 is supplied to the signal processor 47, the spectrum obtained by Fourier transforming the detected while sweeping the movable mirror M ₂ output is displayed on a display device 48 such as a CRT. Thus, a Raman spectrum of the sample S is obtained.

FIG. 13 shows an example of a spectroscopic analyzer which enables remote measurement by connecting the wavelength tunable laser 40 such as an ETT laser and the sample S, and the sample S and the photodetector 55 by optical fibers 52a and 52b. FIG.

The light emitted from the wavelength tunable laser 40 such as ETT is transmitted from the optical coupler 51a to the optical fiber 52a for transmitting light.
To the sample S through the light transmitting optical fiber 52a and out of the irradiation optical coupler 51b at the other end.
The light scattered by the sample S enters the light-receiving optical fiber 52b from the light-receiving optical coupler 51c, and passes through the narrow-band transmission filter 54 into the optical coupler 51d provided at the other end of the light-receiving optical fiber 52b. It is detected by a photodetector 55 such as a photomultiplier tube. The output signal of the photodetector 55 is input to the lock-in amplifier 46.

As described above, the oscillation wavelength of the tunable laser 40 is controlled by the control device 41 to the first frequency ν _ex1 and the second frequency having a constant frequency difference Δν _ex with respect to the first frequency ν _ex1 . Wavelength switching so as to oscillate alternately at two frequencies of ν _ex2 , and at the same time, the first frequency ν _ex1
And the second frequency ν _ex2 is swept toward the high frequency side or the low frequency side while maintaining a constant frequency difference Δν _ex . At this time, the control signal of the control device 41 is transmitted to the lock-in amplifier 4.
The Raman spectrum of the sample S is obtained by the signal processing means 47 by performing phase-locked detection on the output signal of the photodetector 55 by inputting the signal to the signal detector 6 and displaying it on the display device 48.

As described above, the wavelength tunable laser 40 and the sample S,
And an optical fiber 52a between the sample S and the photodetector 55.
The method of tying the sample 52 with the wavelength tunable laser 4
This is effective when it is necessary to separately install the zero and other measuring devices 55, 46, 47, 48. Examples where such measurements are needed include isolated hospital rooms,
Measurement in a large area such as a high or low temperature area or an electromagnetic noise is included. According to this method, sample measurement at measurement positions set at a plurality of locations can be performed intensively in a measurement room several tens meters or several hundred meters away from the measurement positions.

FIG. 14 is a schematic explanatory view of a spectrometer for measuring the two-dimensional distribution of Raman scattered light from a sample. The sample S is held by the sample table 61 and can be moved in the X and Y directions by driving means such as a motor incorporated in the sample table 61. A tunable laser 40 such as an ETT laser is controlled by a control device 41 and has a first frequency ν _ex1 ,
Wavelength switching is performed so as to alternately oscillate at two frequencies of a second frequency ν _ex2 having a constant frequency difference Δν _ex with respect to the first frequency ν _ex1 , and at the same time, a constant frequency difference Δν _ex Is swept to the high frequency side or the low frequency side while maintaining

The monochromatic laser beam LB emitted from the wavelength tunable laser 40 is reflected by the reflecting mirror 62a, narrowed down by the focusing lens 63a, and irradiates a small area on the sample S with a spot from an oblique direction. The laser light scattered in the minute area of the sample S is condensed by the condenser lens 63b, reflected by the reflection mirror 62b, and enters the photodetector 65. The output signal of the photodetector 65 is input to the lock-in amplifier 46, detected in phase synchronization with the control signal of the control device 41, and the output signal is input to the signal processing means 47, and a Raman spectrum is obtained by signal processing. .

The sample stage controller 69 drives the sample stage 61 in the X and Y directions to thereby control the laser beam L on the sample S.
The irradiation position of B is changed, and the Raman spectrum at each position of the sample S is obtained. The signal processing means 47 is supplied with information on the laser beam irradiation position on the sample S from the sample stage controller 69, and displays a two-dimensional distribution of a specific Raman line intensity on a monitor 48 such as a CRT. As described above, by two-dimensionally scanning the laser beam irradiation position on the sample S, a Raman image of the sample surface can be obtained.

Here, as a method of separating the Raman light and the fluorescence by slightly changing the excitation wavelength, a method of detecting the Raman light and the fluorescence by the lock-in amplifier method using the effect of the frequency switching has been mainly described. However, this method
It is apparent that the present invention can be similarly applied to ordinary Raman spectroscopy of a type in which a spectrum is measured at a time by using an optical multi-channel analyzer by using spectrum difference analysis for λ _ex1 and λ _ex2 excitation light.

[0073]

According to the present invention, it is possible to detect Raman scattered light without being affected by fluorescence using a narrow band transmission filter such as an interference filter, which is extremely easy to handle, without using a spectroscope. it can.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a Raman spectrum when no fluorescence is generated from a sample.

FIG. 2 is a schematic diagram of a spectrum in which fluorescence overlaps a Raman line.

FIG. 3 is a conceptual diagram illustrating a wavelength selection action by a birefringent acousto-optic element.

FIG. 4 shows k of an ordinary ray propagating in a birefringent acousto-optic element.
The figure which displayed the vector and the k vector of the extraordinary ray.

FIG. 5 is an explanatory diagram of another example of the ETT laser.

FIG. 6 is an explanatory diagram of an ETT laser using a Z-hold type resonator.

FIG. 7 is a view showing a wavelength tunable characteristic of the ETT laser shown in FIG. 6;

FIG. 8 is a schematic configuration diagram of an example of a spectroscopic analyzer according to the present invention.

9A is a diagram schematically illustrating an output of a variable frequency laser, and FIG. 9B is a diagram schematically illustrating an output signal of a detector.

FIG. 10 is a diagram schematically showing an output of a lock-in amplifier.

FIG. 11 is a schematic configuration diagram showing an example of a spectroscopic analyzer with two types of observation frequencies.

FIG. 12 is a schematic configuration diagram showing an example of a spectroscopic analyzer using an interference spectroscope.

FIG. 13 is an explanatory view showing an example of a spectroscopic analyzer in which a wavelength tunable laser and a sample, and a sample and a photodetector are connected by an optical fiber.

FIG. 14 is a schematic explanatory view of a spectrometer for measuring a two-dimensional distribution of Raman scattered light from a sample.

[Explanation of symbols]

14 laser medium, 20 RF power supply, 22 piezoelectric element,
Reference numeral 24: Excitation laser beam, 26: Personal computer, 28: Prism, 30: Telescope, 32: Excitation laser, 40: Tunable laser, 41: Control device, 43
... Collimator lenses, 44, 44a, 44b ... Narrow band transmission filters, 45, 45a, 45b ... Photodetectors, 46
... lock-in amplifier, 47 ... signal processing device, 48 ... display device, 51a, 51b, 51c, 51d ... optical coupler, 5
4: narrow band transmission filter, 55: photodetector, 52a,
52b: optical fiber, 61: sample stage, 62a, 62b
... reflection mirror, 65 ... photodetector, 69 ... sample stage controller, 100 ... birefringent acousto-optic element, 104 ... acoustic wave,
106: diffracted light, 110: total reflection mirror, 112: output side mirror

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Kazuyuki Akakawa 19-1399, Koshiji, Nagamachi, Aoba-ku, Sendai, Miyagi Prefecture Photodynamics Research Center, RIKEN

Claims (14)

[Claims] In a spectroscopic analysis method of irradiating a sample with monochromatic light from a wavelength tunable laser and measuring light emission emitted from the sample, a laser oscillating in a predetermined wavelength region in a laser resonator as the wavelength tunable laser. Disposing a possible laser medium and a birefringent acousto-optical element, forming a laser resonator on a predetermined optical axis of a light component diffracted by the birefringent acousto-optical element, A spectroscopic analysis method comprising: measuring a light emission intensity of a constant wavelength emitted from the sample by using a tunable laser for selecting a wavelength by selecting a frequency of an acoustic wave to be excited therein. 2. A spectroscopic analysis method for irradiating a sample with monochromatic light from a tunable laser and observing light emission emitted from the sample, wherein the wavelength of the monochromatic light applied to the sample is swept at a high speed, and A spectroscopic analysis method comprising observing light emitted from the sample and obtaining an emission excitation spectrum at an observation wavelength. 3. A spectroscopic analysis method of irradiating a sample with monochromatic light from a wavelength tunable laser and observing light emitted from the sample, wherein the wavelength of the monochromatic light irradiated on the sample is a first wavelength and the second wavelength. Wavelength sweeping while alternately switching the wavelength between a second wavelength having a certain frequency difference with respect to one wavelength, observing light emitted from the sample at a third wavelength,
A spectroscopic analysis method characterized by separating and observing, as Raman scattered light, a component that changes in time in synchronization with the switching of the wavelength among the observation light at the third wavelength. 4. Observing light emitted from the sample at a fourth wavelength different from the third wavelength together with the third wavelength, and observing a correlation with a frequency difference between the third wavelength and the fourth wavelength. 4. The method according to claim 3, wherein the Raman scattered light is precisely separated. 5. The spectroscopic method according to claim 3, wherein a component that does not change with time due to the wavelength switching is separately observed as a non-Raman component. 6. A laser medium capable of oscillating laser light in a predetermined wavelength region and a birefringent acousto-optical element are arranged in a laser resonator as the wavelength-variable laser, and are diffracted by the birefringent acousto-optical element. A laser resonator is formed on a predetermined optical axis of a light beam component, and a wavelength tunable laser that performs wavelength selection by selecting a frequency of an acoustic wave to be excited in the birefringent acousto-optic element is used. Claim 2
6. The spectroscopic analysis method according to any one of 5. 7. The spectroscopic analysis according to claim 1, wherein the sample is irradiated with monochromatic light from the wavelength tunable laser through an optical fiber, and light emitted from the sample is observed through the optical fiber. Law. 8. The apparatus according to claim 1, wherein monochromatic light from said tunable laser is scanned relative to said sample, and a distribution of light emission in a two-dimensional area of said sample is measured.
The spectroscopic analysis method according to any one of claims 1 to 4. 9. A wavelength tunable laser for irradiating a sample with monochromatic light, and a second wavelength having an oscillation wavelength of the first wavelength and a constant frequency difference with respect to the first wavelength. Wavelength control means for sweeping the wavelength while switching the wavelength between the wavelengths, a narrow-band transmission filter transmitting the third wavelength, and the monochromatic light emitted from the sample and transmitted through the narrow-band transmission filter. A photodetector for detecting the light of the third wavelength, and a phase-locked detection means for detecting a detection signal of the photodetector in phase with a switching signal of the first wavelength and the second wavelength. And a function of measuring a Raman spectrum of the sample. 10. A wavelength tunable laser for irradiating a sample with monochromatic light, and a second wavelength having an oscillation wavelength of the first wavelength and a constant frequency difference with respect to the first wavelength.
Wavelength control means for sweeping the wavelength while switching the wavelength between the first and second wavelengths, a first narrow-band transmission filter transmitting the third wavelength, and transmitting a fourth wavelength different from the third wavelength. A second narrow-band transmission filter, a first photodetector that detects light emitted from the sample by the irradiation of the monochromatic light and transmitted through the first narrow-band transmission filter,
A second photodetector for detecting light transmitted through the second narrow-band transmission filter, and a detection signal of the first and second photodetectors for switching between the first wavelength and the second wavelength. A phase-locked detection means for detecting the phase-locked phase and a comparison means for comparing the two phase-locked detection signals of the phase-locked detection means with a function of measuring the Raman spectrum of the sample. Spectroscopic analyzer. 11. A wavelength tunable laser for irradiating a sample with monochromatic light, and a second wavelength having a constant frequency difference between the first wavelength and the first wavelength.
Wavelength control means for sweeping the wavelength while switching between the wavelengths, an interferometer for performing interference spectroscopy of light emitted from the sample by the irradiation of the monochromatic light, and controlling the output signal of the interferometer to the wavelength. A phase-locked detection means for detecting the phase-locked signal in phase synchronization with the wavelength switching signal, and a means for performing a Fourier transform on the output of the phase-locked detection means, and having a function of measuring the Raman spectrum of the sample. Spectroscopic analyzer. 12. The wavelength tunable laser includes a laser medium having a laser medium capable of oscillating laser in a predetermined wavelength region and a birefringent acousto-optic element, and is diffracted by the birefringent acousto-optic element. A laser resonator is formed on a predetermined optical axis of a light beam component, and wavelength selection is performed by selecting a frequency of an acoustic wave to be excited in the birefringent acousto-optic element. 12. The spectroscopic analyzer according to any one of 9 to 11. 13. The spectroscopic analysis according to claim 9, further comprising an optical fiber connecting between the tunable laser and the sample and / or between the sample and the photodetector. apparatus. 14. The spectroscopic analyzer according to claim 8, further comprising: means for scanning monochromatic light from the tunable laser relative to the sample.