JP3669632B2 - Spectroscopic measurement method and spectroscopic measurement apparatus - Google Patents

Description

  The present invention relates to a spectroscopic measurement method and a spectroscopic measurement device, and more particularly to a spectroscopic measurement method and a spectroscopic measurement device using a wavelength tunable laser as a spectroscopic light source.

  As a highly sensitive sample analysis method, the sample is irradiated with light in the infrared region, visible region, or ultraviolet region to measure light absorption by the sample, or reflected light from the sample, Rayleigh scattered light, Raman scattered light, fluorescence, etc. Spectroscopic methods are widely used. According to this spectroscopic measurement method, qualitative analysis such as sample identification and confirmation, quantitative analysis such as measurement of the concentration of characteristic components contained in the sample and component ratio of the mixture, analysis of the electronic state and three-dimensional structure of the molecule, etc. In addition, it is possible to perform a reaction process, a molecular structure analysis of a reaction intermediate, and the like by measuring an absorption spectrum by a sample after time resolution.

  In spectroscopic measurement, it is necessary to irradiate a sample with monochromatic light of a specific selected wavelength, or to irradiate light while continuously sweeping the wavelength of monochromatic light. Usually, a white light source and a monochromator are used. A spectrophotometer that extracts monochromatic light in combination is used. The wavelength sweep of the monochromatic light emitted from the exit slit of the monochromator is performed by rotating a wavelength dispersion element such as a diffraction grating incorporated in the monochromator.

In some spectroscopic measurements, a tunable laser is used as a light source. As the wavelength tunable laser, a solid-state laser using a crystal such as Ti: Al ₂ O ₃ (titanium sapphire) as a laser medium and a liquid laser using a dye solution or the like as a laser medium are known. As a wavelength selection method for causing such a wavelength tunable laser to oscillate at a desired wavelength, for example, a diffraction grating or a birefringent plate is disposed in a laser resonator containing a laser medium, and is mechanically rotated. Thus, a wavelength selection method is adopted in which only a specific wavelength can resonate in the laser resonator and a laser beam having a desired wavelength is extracted.

JP-A-8-13938 JP 9-172215 A

  As a technique for non-invasively quantifying oxygen concentration and the like in a living tissue, near infrared spectroscopy has attracted attention. However, in a measurement using a normal white light source and a spectroscope, a thick sample could not be measured due to scattering of living tissue. On the other hand, when a solid-state light source such as an LED is used, it is impossible to measure a continuous spectrum, and there is a problem in an object that can be quantified and its accuracy.

  In general, when measuring the absorption spectrum of a target component in a sample and performing a qualitative analysis or quantitative analysis, there is a disturbing component having a large absorption at a wavelength position overlapping the absorption spectrum of the target component in the sample. In this case, the absorption of the target component is hidden by the absorption of the interfering component, and high-accuracy measurement cannot be performed.

  FIG. 15 is a schematic diagram for explaining the problems of this conventional spectrum measurement. Here, a description will be given taking measurement using a normal white light source and a spectroscope as an example. FIG. 15A shows the spectrum of the light source, where the horizontal axis represents wavelength and the vertical axis represents energy. FIG. 15B shows the absorption spectrum of the interfering component in the sample. FIG. 15 (c) shows the absorbance measured when the absorption of the measured component overlaps the absorption spectrum of this interfering component. When the dynamic range of the detector is small, a spectrum with good S / N cannot be obtained. In normal spectroscopy, the lack of sensitivity of the detector may be compensated by electrically amplifying the output from the detector. However, with this method, noise that hinders measurement is also amplified simultaneously with the signal.

  In addition to the measurement of the absorption spectrum, for example, in normal Raman spectroscopy, a wide wavelength range is obtained by combining a plurality of lasers having a fixed oscillation wavelength, or a combination of lasers having an oscillation wavelength selection ability in a narrow range (<20 nm). The excitation wavelength dependence of the Raman spectrum at has been measured. However, even with the same laser, since the oscillation intensity changes when the oscillation wavelength is changed, the laser oscillation intensity is always monitored in the measurement of the excitation wavelength dependence, and the Raman spectrum intensity is measured for each excitation wavelength after the measurement. There was a need to calibrate. When a plurality of lasers are used in combination, it is very difficult to align the optical axes of the different lasers, and even if the laser oscillation intensity is monitored, a large error is found in the calibration of the Raman spectrum intensity. When measuring the wavelength dependence of the Raman spectrum, especially when a laser with a fixed oscillation wavelength is used as the light source, it is impossible to measure the spectrum for each desired wavelength, and it is difficult to interpret the measured spectrum. Therefore, it has been impossible to automatically measure the excitation wavelength dependence (Raman excitation profile) of the Raman spectrum using a conventional laser.

  The present invention has been made in view of such problems of the prior art, and enables high-accuracy spectrum measurement even for a sample containing a disturbing component, and facilitates measurement of a Raman excitation profile. It is an object of the present invention to provide a spectroscopic measurement method and a spectroscopic measurement apparatus that can be performed in the same manner.

  FIG. 1 is a schematic diagram for explaining the principle of an example of a spectroscopic measurement method according to the present invention, and shows a method for measuring a spectrum of an analyte having absorption at a position overlapping with an absorption spectrum of a disturbing component in a sample with high accuracy. . FIG. 1 (b) represents the absorption spectrum of the disturbing component. In the present invention, as shown in FIG. 1 (a), the emission spectrum of the light source is controlled so as to increase the light output at a wavelength where absorption is caused by an interference component and to decrease the light output in a wavelength region where there is no absorption. The emission spectrum is such that the absorption spectrum due to the component is canceled out. The spectrum detected at this time is a flat spectrum in which the influence of the disturbing component does not appear as shown in FIG. Accordingly, the absorption spectrum of the target component can be measured without being affected by the interference component within the dynamic range of the detector.

  Since the present invention compensates for the insufficient sensitivity of the detector by increasing the light source output, noise is not amplified. In addition, by combining the method of electrically amplifying the output from the detector and the method of increasing the light source output of the present invention, the intensity of the band having a very strong absorbance (the light transmittance is very low) is quantified. Can do. When a normal spectrometer was used, it was the limit to quantify the band intensity at an absorbance of 3.0 and a transmittance of 0.1%. According to the present invention, it is possible to measure the entire spectrum even when a portion where the absorbance is 0 (transmittance is 100%) and a band having very strong absorption coexist in the spectrum.

  The method of compensating the influence of absorption due to interference components in the sample by controlling the emission spectrum of the light source has been described with reference to FIG. However, according to the present invention, not only the influence of the absorption by the disturbing component but also the light attenuation in the sample based on the cause other than the absorption of the target component, including the attenuation due to light scattering in the sample, etc. It is possible to perform spectroscopic measurement with compensation by controlling the emission spectrum. Furthermore, even when the sensitivity of the detector is spectrally dependent, according to the present invention, the sensitivity variation of the entire measurement system including the sensitivity characteristics of the detector (sensitivity variation depending on the wavelength) is controlled by the emission spectrum control of the light source. It is possible to compensate.

  Further, the light source spectrum control method according to the present invention is not limited to a control method in which the absorption spectrum by the sample becomes flat when the target component is not contained in the sample. In general, the light source is controlled so that the emission intensity is relatively high in the wavelength region where the sample transmitted light exhibits a large attenuation, and the light emission intensity is relatively small in the wavelength region where the sample transmitted light does not exhibit a large attenuation. By controlling this, the dynamic range of the measurement system can be expanded.

  In the measurement of the Raman excitation profile, the intensity of the Raman scattered light may exceed the detection limit of the detector at an excitation wavelength with a strong resonance effect. In order to avoid this, in the present invention, in the wavelength region where the absorbance is large in the absorption spectrum of the sample and a strong resonance effect is expected, the detector is controlled by reducing the laser light output or increasing the wavelength sweep speed. Avoid saturation.

  In the measurement of the absorption spectrum and Raman scattering spectrum of a sample, by using a light source having different light intensity depending on the wavelength, the shape of the measured spectrum is influenced by the emission spectrum of the light source. However, if necessary, the original spectrum can be reconstructed from the measured spectrum.

  Control of the light source as shown in FIG. 1 is impossible in a spectroscopic measurement apparatus using a white light source because the intensity of the light source cannot be changed quickly and accurately in a time of 1 ms or less. In addition, even a spectroscopic measurement apparatus using a conventional wavelength tunable laser such as a dye laser as a light source is impossible because the wavelength tunable region is extremely narrow, 50 nm or less, and the intensity of the light source cannot be changed quickly and accurately. . In the present invention, an electronically controlled wavelength tunable laser (hereinafter referred to as an ETT (Electronically Tuned Tunable) laser) that has been previously developed by the present inventors and that enables high-speed wavelength sweeping by electrically controlling the lasing wavelength. By using JP-A-8-13938 and JP-A-9-172215 as a spectroscopic light source, a spectroscopic measurement method for performing such light source control is made possible.

  In an ETT laser, a laser medium capable of lasing in a predetermined wavelength region and a birefringent acoustooptic element are arranged in a laser resonator, and a light beam component diffracted at a predetermined angle by the birefringent acoustooptic element. Is a wavelength tunable laser that selects a wavelength by selecting a frequency of an acoustic wave excited in the birefringent acoustooptic device. For example, when titanium sapphire is used as a laser medium, Monochromatic light can be taken out in a wide near-infrared wavelength range of 1000 nm, and the wavelength range can be swept in a time within 1 second. When an SHG crystal is arranged in the resonator, a second harmonic of 350 to 500 nm can be extracted. Further, since the wavelength is electrically selected using the birefringent acoustooptic device, the wavelength can be switched instantaneously, for example, any two wavelengths can be switched stably in a time of 1 ms or less, A 1 kHz high repetition pulse laser can be generated. The light output can also be controlled electrically with high accuracy.

  Furthermore, when sweeping the wavelength of an ETT laser, the sweep speed is slow in the wavelength region at both ends of the laser oscillation region where the output intensity is less stable than in other wavelength regions, or in the wavelength region where the detection sensitivity of the detector is reduced. Alternatively, the wavelength sweep speed may be changed in accordance with the wavelength region by increasing the wavelength sweep rate in the center of the laser oscillation region where the output intensity is stable or in the wavelength region where the detection sensitivity of the detector is high. In this case, the overall spectrum measurement time can be shortened, and in the wavelength region where the noise is relatively high when the wavelength sweep is performed at a constant speed, the wavelength sweep speed is decreased to increase the number of detected pulses, and integration is performed. The S / N ratio can be improved by increasing the number of times.

  That is, the present invention provides a spectroscopic measurement method for measuring a spectrum of a sample by irradiating the sample with laser light obtained by sweeping the wavelength of the wavelength tunable laser so that the spectral sensitivity characteristic of the detector is compensated for. The oscillation light output is controlled.

  As the wavelength tunable laser, a laser medium capable of laser oscillation in a predetermined wavelength region and a birefringent acoustooptic element are arranged in a laser resonator, and a predetermined light component of a light beam diffracted by the birefringent acoustooptic element. It is particularly preferable to use a wavelength tunable laser that configures a laser resonator on the axis and performs wavelength selection by selecting the frequency of the acoustic wave excited in the birefringent acoustooptic device.

  For example, it is assumed that there is a detector having sensitivity at 700 to 1000 nm centered on 800 nm. When normal spectroscopic measurement is performed using a normal white light source, the S / N near 800 nm with high sensitivity is high, but the S / N near 700 nm and 1000 nm with low sensitivity is worse than 800 nm. On the other hand, when the output of the white light source is increased in order to increase the S / N in the vicinity of 700 nm or 1000 nm, the S / N in the vicinity of 700 nm and 1000 nm increases, but the detector saturates near 800 nm where the sensitivity is high from the beginning. The S / N may be lowered and the detector may be broken. When an ETT laser is used as a light source as in the present invention, in the wavelength region where the sensitivity of the detector is low (in this example, around 700 nm and around 1000 nm), the output of the ETT laser is increased to compensate for the sensitivity of the detector, and the sensitivity is high. In the wavelength region (in this example, around 800 nm), it is possible to prevent saturation and destruction of the detector by reducing the output of the ETT laser.

  The present invention also provides a spectroscopic measurement method for measuring a spectrum of a sample by irradiating the sample with a laser beam obtained by wavelength sweeping of a wavelength tunable laser, in which the laser beam in the sample is caused by causes other than absorption of the target component. The oscillation light output of the wavelength tunable laser is controlled so as to compensate the attenuation and the spectral sensitivity characteristics of the detector.

  When the spectrum of the reference sample, which is a sample that does not contain the target component, is known, the oscillation light output of the wavelength tunable laser is the RF intensity and laser oscillation light that are input to the birefringent photoacoustic optical element in the wavelength tunable laser. This can be done by investigating the output relationship at each laser oscillation wavelength and calculating the RF intensity that cancels the absorption of the reference sample. When a reference sample is present, the reference sample is irradiated with laser light, and the signal intensity from a detector that detects light transmitted through the reference sample (which may be scattered or reflected) is monitored ( Reference spectrum measurement of a reference sample), and an RF intensity that provides a laser output that constitutes a user's favorite spectrum, such as a substantially flat spectrum, is recorded at each laser oscillation wavelength. By controlling the wavelength tunable laser according to the relationship between the recorded RF intensity and the laser oscillation wavelength, a laser output that gives the user's favorite spectrum can be obtained from the next time.

  The present invention also provides a spectroscopic measurement method for measuring the absorption spectrum of a sample by irradiating the sample with a laser beam obtained by sweeping the wavelength of a wavelength tunable laser, and the laser beam output is increased in a wavelength region where the absorption by the sample is large. As described above, the oscillation light output of the wavelength tunable laser is controlled. The wavelength sweep speed can be varied according to the wavelength region.

The spectroscopic measurement method of the present invention can be applied to any absorption spectrum measurement of a sample that is measured using a technique such as a transmission method, a reflection method, or a diffuse reflection method.
The present invention also provides an absorption method for a target component in a spectroscopic measurement method in which a sample is irradiated with laser light obtained by wavelength sweeping of a wavelength tunable laser, and scattered light from the sample is measured to measure a Raman scattering spectrum of the sample. The oscillation light output of the wavelength tunable laser is controlled so as to compensate for the increase in the Raman scattered light due to.

The present invention also provides an absorption method for a target component in a spectroscopic measurement method in which a sample is irradiated with laser light obtained by wavelength sweeping of a wavelength tunable laser, and scattered light from the sample is measured to measure a Raman scattering spectrum of the sample. In a wavelength range with a small amount of light, the wavelength sweep speed is slowed in order to prevent a decrease in S / N due to a decrease in Raman scattered light.
By oscillating a wavelength tunable laser for each preset wavelength and sequentially measuring the Raman scattering spectrum using the laser light as excitation light, the Raman excitation profile can be easily measured.

  According to the present invention, a laser medium capable of laser oscillation in a predetermined wavelength region and a birefringent acoustooptic element are disposed in the laser resonator, and the predetermined light of the light component diffracted by the birefringent acoustooptic element is provided. A laser resonator is formed on the axis, and a wavelength tunable laser that selects a wavelength by selecting a frequency of an acoustic wave to be excited in the birefringent acoustooptic device, and an acoustic wave to be excited in the birefringent acoustooptic device In a spectroscopic measurement apparatus that includes a laser control device that controls frequency and intensity, and a photodetector, and that measures the spectrum of a sample by irradiating the sample with laser light obtained by sweeping the wavelength of a tunable laser, the laser control device Is characterized in that the oscillation intensity of the wavelength tunable laser is controlled so as to compensate for the spectral sensitivity characteristics of the photodetector.

  According to the present invention, a laser medium capable of laser oscillation in a predetermined wavelength region and a birefringent acoustooptic element are arranged in a laser resonator, and a predetermined light component of a light component diffracted by the birefringent acoustooptic element is arranged. A laser resonator is formed on the optical axis, and a wavelength tunable laser that performs wavelength selection by selecting a frequency of an acoustic wave to be excited in the birefringent acoustooptic device, and excitation to the birefringent acoustooptic device In a spectroscopic measurement apparatus comprising a laser control device for controlling the frequency and intensity of an acoustic wave, and a photodetector, and irradiating the sample with laser light obtained by sweeping the wavelength of the tunable laser to measure the spectrum of the sample The laser control device is characterized by controlling the oscillation intensity of the wavelength tunable laser so as to compensate for the attenuation of the laser beam in the sample due to causes other than the absorption of the target component and the spectral sensitivity characteristics of the photodetector. To.

The spectrometer can measure the absorption spectrum of the sample.
Further, according to the present invention, a laser medium capable of laser oscillation in a predetermined wavelength region and a birefringent acoustooptic element are arranged in a laser resonator, and a predetermined light component of a light beam diffracted by the birefringent acoustooptic element. A laser resonator is formed on the axis, and a wavelength tunable laser that selects a wavelength by selecting a frequency of an acoustic wave to be excited in the birefringent acoustooptic device, and an acoustic wave to be excited in the birefringent acoustooptic device Spectroscopic measurement that includes a laser control device that controls frequency and intensity, a spectroscope, and a photodetector, and that measures the Raman scattering spectrum of the sample by irradiating the sample with laser light obtained by sweeping the wavelength of the tunable laser. In the apparatus, the laser control device controls the oscillation light output of the wavelength tunable laser so as to compensate for an increase in Raman scattered light due to absorption of the target component.

  According to the present invention, high-accuracy spectrum measurement can be performed even on a sample containing an interference component, and the Raman excitation profile can be easily measured.

  First, the ETT laser will be briefly described. When an acoustic wave is excited in an acousto-optic crystal exhibiting birefringence, the diffracted light of a specific wavelength according to the frequency of the acoustic wave in the light incident on the crystal is the acoustic wave, incident light, and diffracted light. Strongly diffracted in the direction satisfying the phase matching condition. FIG. 2 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 acoustooptic device 100 in which a piezoelectric element 22 is attached to an acoustooptic crystal exhibiting birefringence such as a TeO ₂ crystal. Further, when the acoustic wave 104 having the angular frequency ωa is excited in the birefringent acoustooptic element 100 by the piezoelectric element 22, the angle represented by the following [Equation 1] is obtained by the interaction between the incident light 102 and the acoustic wave 104. The diffracted light 106 shifted in frequency to the frequency ωo is obtained. The incident light 102 is an extraordinary ray, and the diffracted light 106 is an ordinary ray. The polarization plane of the diffracted light 106 is orthogonal to the polarization plane of the incident light 102. Reference numeral 108 denotes non-diffracted light.

[Equation 1]
ωo = ωi + ωa
However, ωa << ωi, ωo, and it can be considered that ωi≈ωo. At this time, when the wave number vector of the incident light 102 is ki, the wave number vector of the acoustic wave 104 is ka, and the wave vector of the diffracted light 106 is ko, the following vector expression is established from the phase matching condition: To do.

[Equation 2]
ko = ki + ka
FIG. 3 shows the relationship between the k vector of ordinary rays propagating through the birefringent acoustooptic device 100 and the k vector of extraordinary rays. The magnitude of the k vector with respect to the ordinary ray is constant regardless of the traveling direction, and the locus of the end point of the k vector is a circle. On the other hand, the magnitude of the k vector with respect to the extraordinary ray changes depending on the propagation angle with respect to the crystal axis of the birefringent acoustooptic device 100, and the locus of the end point of the k vector becomes elliptic. The circle or ellipse formed by the locus of the k vector changes in a similar manner when the wavelength is changed. FIG. 3A shows a state where the phase matching condition of [Equation 2] is satisfied at the wavelength λ ₁ . In the figure, Va is the velocity of the acoustic wave 104 traveling through the crystal, and the magnitude of the wave number vector ka ₁ of the acoustic wave 104 is | ωa / Va |.

Here, if the frequency ωa of the acoustic wave 104 excited in the birefringent acoustooptic device 100, and hence the magnitude of the wave number vector ka, is changed, the phase matching condition of [Equation 2] is not satisfied at the wavelength λ ₁ . At this time, the phase matching condition is satisfied at 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 k-vector locus changes depending on the wavelength, but its shape hardly changes. Therefore, even if the wavelength is changed from λ ₁ to λ ₂ , and the magnitudes of the vectors ki and ko of the incident light 102 and the diffracted light 106 are thereby changed, a similar shape is obtained, so that the vector (ko ₁ −ki ₁ ) The direction of the vector (ko ₂ −ki ₂ ) is parallel. As a result, it is possible to input an acoustic wave having a vector of ka ₁ = ko ₁ -ki ₁ and ka ₂ = ko ₂ -ki ₂ only by changing the acoustic frequency.

  When the light having the wave number vector ko emitted from the birefringent acoustooptic device 100 is reflected by the reflecting mirror 110 and is incident on the birefringent acoustooptic device 100 from the opposite direction, as shown in FIG. The returned light is also diffracted by the acoustic wave and travels in the opposite direction to the incident light ki, and becomes -ki, and the optical path of the incident light is reversed.

  Therefore, when the total reflection mirror 110 and the emission side mirror 112 having a predetermined transmittance are arranged with the laser medium 14 and the birefringent acoustooptic device 100 interposed therebetween, as shown in FIG. The mirror 112 constitutes a laser resonator in which only light having only a specific wavelength component reciprocates between the two. The wavelength λo of the diffracted light 106 changes when the frequency ωa of the acoustic wave 104 generated in the birefringent acoustooptic device 100 is changed, and as a result of selecting ki, the wavelength λi = 2π / | ki | is determined. . Therefore, the laser oscillation wavelength λi can be controlled by driving the piezoelectric element 22 attached to the birefringent acoustooptic element 100 with an RF signal having a predetermined frequency from the RF power supply 20. Further, since the diffraction efficiency of the diffracted light 106 is determined by the intensity of the acoustic wave excited in the birefringent acoustooptic device 100, the diffracted light 106 is controlled by controlling the amplitude of the RF signal output from the RF power supply 20. The intensity of the laser, and thus the laser output can be variably controlled.

In the above, it has been stated that the shape of the circle or ellipse connecting the end points of the k-vector locus hardly changes depending on the wavelength, but actually changes slightly. Therefore, the diffraction angle also slightly changes depending on the wavelength, the conditions of the resonator constituted by the total reflection mirror 110 and the partial transmission mirror 112 change, and the direction of the emitted laser light changes slightly. This wavelength dependency of the diffraction angle can be compensated by arranging a wavelength dispersion correction element such as a prism between the birefringent acoustooptic element 100 and the total reflection mirror 110, and the wavelength of the emitted laser light can be reduced at all wavelengths. The direction can be made constant. As the laser medium, any known wavelength tunable laser medium such as a laser crystal such as Ti: Al ₂ O ₃ , LiSAF, LiCAF, or a dye solution can be used.

This 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, the second harmonic of an Nd solid state laser such as an Nd: YAG laser, an Nd: YLF laser, or an Nd: YVO ₄ laser and an argon ion laser may be used. In addition, when a LiSAF laser crystal, a LiCAF laser crystal, or the like is used as a laser medium, a semiconductor laser or a krypton ion laser can be used.

  By matching the pumping volume of the pumping laser in the laser medium with the optical mode volume in the laser resonator, the efficiency is increased and the pumping input is lowered, so that a high repetition rate pulsed pumping laser or a continuous laser whose output cannot be increased can be obtained. An oscillation laser can also be used as an excitation laser. For example, the laser resonator may be a Z-hold type laser resonator or an X-hold type laser resonator, and pump light is introduced along the optical path in the laser resonator to efficiently use the pump light. Laser oscillation can be generated by energy excitation light.

  FIG. 4 is a schematic view showing an example of an ETT laser using a birefringent acoustooptic device. In this example, a so-called Z-hold type laser resonator is used in which the optical path of light reciprocating in the laser resonator has an alphabetical Z shape. The Z-hold type laser resonator includes an output side mirror 112 and a total reflection mirror 110 having a predetermined transmittance. Further, between the first intermediate mirror 37 that makes the excitation laser light A incident and reflects the light B that travels back and forth between the emission side mirror 112 and the total reflection mirror 110, and between the emission side mirror 112 and the total reflection mirror 110. A second intermediate mirror 38 that reflects the reciprocating light B is provided, and the optical path of the light B reciprocating in the laser resonator is an alphabetic Z-shape.

  Between the first intermediate mirror 37 and the second intermediate mirror 38 on the optical path of the laser resonator, there is a laser medium 14 whose incident end face of incident light is Brewster cut as a wavelength tunable laser medium, and whose incident end face is incident light. Is arranged so that the Brewster angle becomes zero, and the laser oscillation is generated by the longitudinal coaxial excitation by the excitation laser light A. Between the second intermediate mirror 38 and the total reflection mirror 110 on the optical path of the laser resonator, a birefringent acousto-optic element 100 is disposed as a wavelength selection means.

  The birefringent acoustooptic device 100 is provided with a piezoelectric element 22 driven by an RF power source 20 whose frequency is controlled by a control computer 26 as acoustic wave input means. The RF power supply 20 and the control computer 26 constitute a laser control device 150, and the piezoelectric element 22 is driven by the RF power supply 20 set to an arbitrary frequency under the control of the control computer 26, and the sound corresponding to the frequency is obtained. By exciting the wave to the birefringent acoustooptic device 100, the birefringent acoustooptic device 100 diffracts the light D having the frequency ωo expressed by the above [Equation 1]. The piezoelectric element 22 is a diffraction in which the birefringent acoustooptic device 100 diffracts only light corresponding to the light B (frequency ωi≈ωo) having the wavelength of the outgoing laser light C desired to be emitted from the outgoing mirror 112 in a predetermined direction. The control computer 26 controls the frequency ωa of the acoustic wave that is input to the birefringent acoustooptic device 100 so that it can be emitted as light D and resonate.

  Between the birefringent acoustooptic device 100 and the total reflection mirror 110, a prism 28 is disposed as a wavelength dispersion correction device for correcting the dispersion of the diffracted light D. By using the wavelength dispersion correcting prism 28 of this diffraction angle, the direction of the emitted laser beam C can be made constant. A pulse laser or a continuous wave laser (CW laser) can be used as the excitation laser 32 for entering the excitation laser beam A into the laser resonator. The excitation laser light A generated by the excitation laser 32 is reflected by the total reflection collecting mirror 36 by the total reflection mirror 34, condensed by the total reflection collecting mirror 36, and the laser medium 14 through the first intermediate mirror 37. Are incident so as to be longitudinally coaxially excited.

  In order to obtain the emitted laser beam C, the laser medium 14 is excited using the excitation laser beam A incident by the excitation laser 32. Further, the frequency ωa of the RF power source 20 is controlled by the control computer 26 in accordance with the wavelength (frequency ωi) of the emitted laser beam C desired to be emitted from the emission side mirror 112, and the piezoelectric element 22 is driven. In this way, light having a wavelength corresponding to the frequency of the RF power source 20 among light in a wide wavelength band emitted from the laser medium 14 and incident on the birefringent acoustooptic device 100 is birefringent acoustic. It is diffracted as diffracted light D (frequency ωo) by the optical element 100. The diffracted light D enters the total reflection mirror 110 perpendicularly via the wavelength dispersion correction prism 28 having a diffraction angle, is reflected by the total reflection mirror 110, and travels back and forth in the laser resonator along the Z-shaped optical path. (Angular frequency ωi at the position of the laser medium 14). Accordingly, only light having a wavelength corresponding to the frequency of the RF power source 20 is amplified and laser-oscillated, and emitted laser light C (frequency ωi) having the wavelength is emitted from the laser resonator. As described above, the wavelength selection of the emitted laser beam C can be realized by selecting the frequency ωa of the RF power source 20 under the control of the control computer 26, so that the wavelength selection of the emitted laser beam C (frequency ωi) can be performed at high speed and at random. As a result, the wavelength variable speed of the emitted laser light can be increased.

FIG. 5 shows experimental results on the input / output characteristics of the ETT laser shown in FIG. Here, a Ti: Al ₂ O ₃ crystal was used as the laser medium 14, a CW-Q switch pulse Nd: YLF laser was used as the excitation laser 32, and the second harmonic was used as the excitation laser light A. The wavelength of the excitation laser light A is 523 nm, the pulse repetition frequency is 1 kHz, and the maximum output per pulse is 200 μJ. The diameter of the total reflection condensing mirror 36 was 200 mm, the radius of the first intermediate mirror 37 and the second intermediate mirror 38 was 100 mm, and the emission side mirror 112 had a reflectivity of 97% (transmittance of 3%). The pumping region and the resonator mode diameter are reduced to several tens of μm in the laser medium 14, and the pumping laser light A is condensed in this region by the total reflection condensing mirror 36, so that the pumping efficiency can be improved. FIG. 5 shows the input / output characteristics of the energy of the excitation laser beam A (input) and the energy of the output laser beam C (output) when the wavelength of the output laser beam C is fixed at 800 nm. As is apparent from FIG. 5, the laser oscillation threshold was reached when the energy of the excitation laser beam A was about 40 mJ per pulse.

FIG. 6 shows the wavelength tunable characteristics when the energy of the excitation laser light A is 100 μJ. As is apparent from FIG. 6, the wavelength tunable range is about 740 nm to about 870 nm. By providing the wavelength dispersion correcting prism 28 of the diffraction angle, the beam shake observed during the wavelength tuning of the laser is below the observation limit.
Next, the spectroscopic measurement method of the present invention using the above-described ETT laser will be described.

  FIG. 7 is a schematic diagram illustrating an example of a spectroscopic measurement apparatus according to the present invention. The aforementioned ETT laser 40 is used as a spectral light source. The ETT laser 40 can generate pulsed laser light or can generate continuous laser light. Here, an example in which pulsed laser light is generated will be described. A monochromatic laser beam 41 having a wavelength λ extracted from the ETT laser 40 is incident on the sample S, and a light beam transmitted through the sample S is detected by the photomultiplier tube 43.

  The oscillation wavelength and optical output of the ETT laser 40 are controlled by a laser control device 150 including the RF power source 20 and the control computer 26. The oscillation intensity of the ETT laser 40 is controlled according to information (control information) stored in the memory of the control computer 26. The control information is created manually or automatically by a computer program. For example, a table is created in the form of oscillation intensity with respect to the oscillation wavelength, and numerical values (control information) for control are stored on the computer memory according to the table.

  The signal processing circuit 160 receives the signal from the control computer 26 and the detection signal from the photomultiplier tube 43, and processes both in synchronization. For example, it is assumed that a pulse current having a time width of 200 ns is output from the photomultiplier tube 43 by detecting an incident laser pulse. If the laser pulse interval is 1 ms, only the noise component is detected from the photomultiplier tube for the remaining 999,800 ns. The signal processing circuit 160 uses the signal from the control computer 26 as a trigger, and integrates only the current within the time during which a significant signal is output from the detector using a gate type integration circuit. The integrated signal is transmitted as a voltage value to the analog / digital converter and stored in the memory of the display device 170. The measured absorption spectrum of the sample is displayed on the display device 170.

  A human palm (thickness of about 3 cm) was used as a sample, and the change in the degree of oxygenation of hemoglobin in the living body was measured from its absorption spectrum. Prior to the measurement, the light output of the ETT laser 40 was adjusted so as to compensate for scattering, absorption by water, and sensitivity characteristics of the photomultiplier tube 43. To adjust the output of the ETT laser 40, an interlipid dispersion in water was used as a reference scatterer. The electric signal from the detector was monitored through a signal processing circuit, and the laser was oscillated every 700 nm to 10 nm, and the RF intensity was set at each wavelength so that the output from the signal processing circuit was always a constant value. For wavelengths falling between every 10 nm, the RF intensity was defined by interpolation. FIG. 8 shows an oscillation spectrum of the ETT laser 40 whose output is adjusted so as to compensate for the water absorption and the sensitivity characteristic of the photomultiplier tube 43 in this way (the photomultiplier when the reference sample is placed at the position of the sample S). (Signal intensity obtained by integrating the detection signal of the double tube 43).

  FIG. 9 shows the absorption spectrum (solid line) of a human palm at rest, measured using an ETT laser whose output is adjusted so that the signal intensity transmitted through the reference sample becomes flat as shown in FIG. It is a measurement example of the absorption spectrum (long broken line) of the palm immediately after pushing up and the difference spectrum (broken line). It is known that hemoglobin has weak absorption in the wavelength region of 700 to 1000 nm, and its spectral change sensitively reflects the degree of oxidation of hemoglobin. The 760 nm and 940 nm bands observed in the difference spectrum can be attributed to deoxyhemoglobin, indicating that the relative amount has increased after the push-up.

  FIG. 10 shows an absorption spectrum of a human palm using an ETT laser whose light output characteristics are controlled so as to compensate only for the sensitivity characteristics of the detector without adjusting the output of the ETT laser using the scatterer reference sample. It is the example which measured. The ETT laser is designed so that when the laser light from the ETT laser that has passed through the air is attenuated by a neutral density filter and received by a photomultiplier tube, a detection output flat with respect to the wavelength can be obtained. The RF power source was controlled, and the control parameters of the RF power source for each wavelength were recorded in the memory. Thereafter, the neutral density filter was removed, and the RF power supply was controlled according to the control parameters recorded in the memory, and the absorption spectrum of the palm was measured. As is clear from comparison of FIG. 10 with FIG. 9, in this case, light absorption by blood flowing through the palm was buried in strong scattering and could not be observed.

  As a method for increasing the S / N of the measurement spectrum, integration of detection signals and slow wavelength sweep are used. The ETT laser has high output stability near the center of the oscillation wavelength region, and the output stability decreases at the end of the oscillation wavelength region. Since the wavelength sweep speed of the ETT laser can be freely controlled by a control computer, the same effect as the detection signal integration can be obtained by slowing the wavelength sweep speed at both ends of the oscillation wavelength region where the output stability is low. be able to.

When performing such processing, it is necessary to change the number of times of data integration on the computer for data processing in accordance with the wavelength sweep speed. For example, when the wavelength sweep speed at 990 nm or more is halved, at each wavelength of 990 nm or more, the number of data acquisition times twice the number of data acquisition at a wavelength less than 990 nm is obtained. By taking the average of these data, the S / N of the data in the wavelength region above 990 nm can be increased by 2 ^1/2 times.

  In the measurement of the absorption spectrum of the sample, by using a light source having a different light intensity depending on the wavelength, the shape of the measured spectrum is influenced by the shape of the emission spectrum of the light source. However, if necessary, the original spectrum can be reconstructed from the measured spectrum. If the sample (reference sample) without the target component can be destructively measured, the reference spectrum is measured with a cuvette cell with a short optical path length, and the target component is mathematically compared with the optical path length of the actual sample. The spectrum of the sample containing can be reconstructed.

  That is, when all the measured spectra are shown as absorbance, assuming that the measured spectrum is A and the spectrum of the reference sample is B, the spectrum C of the entire sample can be reconstructed by the following [Equation 3]. Where A = −log (a ′ / a ″), a ′ is the energy spectrum of the sample (wavelength versus voltage), and a ″ is the energy spectrum of the reference sample.

[Equation 3]
C = A + B
When it is impossible to measure the destruction of the reference sample, the laser output for each oscillation wavelength is obtained from the RF intensity for each oscillation wavelength used for controlling the ETT laser, and the sample is superimposed on the measured spectrum of the target component. All absorption spectra can be reconstructed. At this time, first, a characteristic curve of the voltage output with respect to the optical input to the detector is created while considering the sensitivity characteristic of the detector at each wavelength. Next, the laser oscillation intensity reconstructed from the RF intensity input to the birefringent acoustooptic device is applied to the above-mentioned characteristic curve, and output from the detector assuming that the dynamic range of the detector is infinite. A voltage output curve (virtual energy spectrum) d is obtained. The spectrum of the entire sample is obtained by the following [Equation 4].

[Equation 4]
C = −log (a ′ / d)
Further, according to the present invention, it is possible to construct a Raman spectroscopic measurement system in which the spectroscope and the laser are controlled in an integrated manner within the oscillation wavelength range of the ETT laser. The maximum oscillation wavelength region of the ETT laser ranges from 680 to 1050 nm. When an SHG crystal is used, an oscillation wavelength region of 345 to 500 nm can be covered. The oscillation wavelength and oscillation intensity of the ETT laser can be controlled by the control computer over the entire oscillation wavelength region. When the ETT laser is oscillated in this oscillation wavelength region, the optical axis does not change with the wavelength. Therefore, the spectroscope and the laser can be controlled centrally by the control computer.

  FIG. 11 is a schematic view of a spectrometer according to the present invention capable of measuring a Raman scattering spectrum. This apparatus includes an ETT laser 40, a laser controller 150 including an RF power source 20 for controlling the oscillation wavelength and oscillation intensity of the ETT laser 40, and a control computer 26, a spectrometer 115, and a multichannel detector 116 for spectrum measurement. A signal processing circuit 160 for processing the detection signal of the multi-channel detector, and a display device 170 such as a CRT for displaying the measurement result. The control computer 26 controls the ETT laser 40 and the spectroscope 115 in synchronization. The spectroscope 115 separates the Raman scattered light 42 generated from the sample S by the irradiation of the monochromatic laser light 41.

  When the Raman excitation profile is measured using the spectrometer shown in FIG. 11, the intensity of Raman scattering may exceed the detection limit at an excitation wavelength with a strong resonance effect, as shown in FIG. In order to prevent this, the light output of the ETT laser 40 and the wavelength sweep speed are changed in a wavelength region in which the absorption is large in the absorption spectrum where a strong resonance effect is expected.

An example is shown in FIG. In the case where the intensity of the Raman band a exceeds the detection limit in FIG. 12, for example, the excitation light intensity of the ETT laser 40 in the wavelength region λ _{2 to} λ ₃ where the resonance effect is large is represented by the excitation light intensity at other excitation wavelengths. Adjust to 1/2. When the measurement results for the excitation wavelength regions λ _{1 to} λ ₂ are obtained, the measurement is performed by reducing the output of the ETT laser 40 in the wavelength regions λ _{2 to} λ ₃ to ½. Further, in the wavelength region λ _{3 to} λ ₄ , the measurement is performed by returning to the original laser output. Thereafter, the data value in the wavelength region λ _{2 to} λ ₃ is doubled to reconstruct the original excitation profile as indicated by the broken line in FIG. Since the light output of the ETT laser 40 is changed according to the wavelength, the shape of the measured Raman scattering spectrum is influenced by the shape of the emission spectrum of the light source. The spectrum can be reconstructed.

Similarly, the same effect can be obtained by doubling the wavelength sweep speed of the ETT laser 40 in the wavelength region λ _{2 to} λ ₃ where the resonance effect is large. To double the wavelength sweep rate means to halve the integration time at each wavelength, which means to halve (slow down) the wavelength sweep rate in the wavelength region where absorption is weak. Is equivalent to

  If the oscillation intensity at each oscillation wavelength is defined in advance, it is not necessary to monitor the intensity of the excitation light 41 when measuring a Raman scattering spectrum with different excitation wavelengths, that is, measuring a Raman excitation profile. Since the ETT laser 40 can oscillate continuously over a wide oscillation wavelength range and no optical axis shift occurs, a Raman spectrum in which only the excitation wavelength is changed can be measured while other conditions are kept constant.

  Further, by measuring a Raman spectrum in which the excitation wavelength is changed at a wavelength interval set by the user, a series of Raman spectra having excitation wavelengths at equal intervals can be measured. FIG. 14 is a schematic diagram of equidistant Raman excitation profiles measured by the apparatus shown in FIG. A Raman excitation profile is a collection of Raman scattering spectra measured for various excitation wavelengths. In the Raman excitation profile, information such as the structural change in the excited state of the target molecule can be obtained by examining the intensity change and phase of each Raman band with respect to the excitation wavelength change. At this time, for example, by changing the laser oscillation wavelength every 5 nm in the wavelength range of 700 to 1000 nm and measuring the Raman spectrum at each wavelength to obtain equidistant Raman excitation profiles, the analysis results can be easily understood. .

  Here, the present invention has been described by taking the measurement of the absorption spectrum and the measurement of the Raman scattering spectrum as examples. However, the present invention can be similarly applied to measurement of emission spectra such as fluorescence and phosphorescence besides this.

The schematic diagram explaining the principle of the spectrometry method by this invention. The conceptual diagram explaining the wavelength selection effect | action by a birefringent acoustooptic device. The figure which displayed k vector of the ordinary ray which propagates in the birefringent acoustooptic device, and k vector of the extraordinary ray. Explanatory drawing of an example of an ETT laser. The figure which shows the input-output characteristic of the energy of an excitation laser beam, and the energy of an emitted laser beam when an ETT laser is excited with a pulse laser. The figure which shows the wavelength variable characteristic of the ETT laser shown in FIG. Schematic explaining an example of a spectrometer according to the present invention. The figure which shows the oscillation spectrum of the ETT laser which adjusted the output so that the absorption characteristic of the dispersion liquid in interlipid and the sensitivity characteristic of a photomultiplier tube may be compensated. The figure which shows the example of a palm absorption spectrum measurement. The figure which shows the example of a palm absorption spectrum measurement using the ETT laser which correct | amended only the wavelength dependence of detector sensitivity and gave the flat optical output characteristic as a light source. Schematic which shows the other example of the spectrometry apparatus by this invention. Explanatory drawing of the Raman excitation profile of equal intervals. Explanatory drawing of the method to reduce the ETT laser output of the excitation wavelength area | region with a strong resonance effect in the measurement of a Raman excitation profile. Schematic diagram of equally spaced Raman excitation profiles. The schematic diagram explaining the problem of the conventional spectrum measurement.

Explanation of symbols

DESCRIPTION OF SYMBOLS 14 ... Laser medium, 20 ... RF power supply, 22 ... Piezoelectric element, 24 ... Excitation laser beam, 26 ... Control computer, 28 ... Prism, 32 ... Excitation laser, 40 ... ETT laser, 41 ... Monochromatic laser beam, 42 ... Raman Scattered light, 43 ... photomultiplier tube, 100 ... birefringent acoustooptic device, 102 ... incident light, 104 ... acoustic wave, 106 ... diffracted light, 110 ... total reflection mirror, 112 ... emission side mirror, 115 ... spectrometer 116: Multi-channel detector, 150: Laser control device, 160: Signal processing circuit, 170: Display device

Claims (12)

In a spectroscopic measurement method for measuring a spectrum of a sample by irradiating the sample with laser light obtained by wavelength sweeping of a wavelength tunable laser,
A spectroscopic measurement method comprising controlling the oscillation light output of the tunable laser so as to compensate for attenuation of laser light in a sample due to causes other than absorption of a target component and spectral sensitivity characteristics of a detector.   2. The spectroscopic measurement method according to claim 1, wherein the reference spectrum of the reference sample is measured, and the oscillation light output of the wavelength tunable laser is controlled so that the measured spectrum becomes substantially flat. In a spectroscopic measurement method for measuring a spectrum of a sample by irradiating the sample with laser light obtained by wavelength sweeping of a wavelength tunable laser,
A spectroscopic measurement method comprising controlling the oscillation light output of the wavelength tunable laser so that the laser light output is increased in a wavelength region where absorption by a sample is large.   The spectroscopic measurement method according to any one of claims 1 to 3, wherein the wavelength sweep speed is varied according to the wavelength region.   The spectroscopic measurement method according to claim 1, wherein an absorption spectrum of the sample is measured. In a spectroscopic measurement method for measuring a Raman scattering spectrum of a sample by irradiating the sample with laser light obtained by sweeping the wavelength of a wavelength tunable laser, and dispersing the scattered light from the sample,
A spectroscopic measurement method comprising controlling an oscillation light output of the wavelength tunable laser so as to compensate for an increase in Raman scattered light due to absorption of a target component. In a spectroscopic measurement method for measuring a Raman scattering spectrum of a sample by irradiating the sample with laser light obtained by sweeping the wavelength of a wavelength tunable laser, and dispersing the scattered light from the sample,
A spectral measurement method characterized by slowing the wavelength sweep rate in order to prevent a decrease in S / N due to a decrease in Raman scattered light in a wavelength range where absorption of a target component is small.   8. The spectroscopic measurement method according to claim 6, wherein the wavelength tunable laser is oscillated for each preset wavelength, and a Raman scattering spectrum is sequentially measured using the laser light as excitation light.   A laser medium capable of lasing in a predetermined wavelength region and a birefringent acoustooptic element are disposed in the laser resonator as the wavelength tunable laser, and a predetermined beam component diffracted by the birefringent acoustooptic element is arranged. 9. A wavelength tunable laser comprising a laser resonator on an optical axis and performing wavelength selection by selecting a frequency of an acoustic wave excited in the birefringent acoustooptic device. The spectroscopic measurement method according to any one of the above. A laser medium capable of laser oscillation in a predetermined wavelength region and a birefringent acoustooptic element are arranged in a laser resonator, and laser resonance is performed on a predetermined optical axis of a light beam component diffracted by the birefringent acoustooptic element. A tunable laser that selects a wavelength by selecting a frequency of an acoustic wave to be excited in the birefringent acoustooptic device, and a frequency and intensity of the acoustic wave to be excited in the birefringent acoustooptic device In a spectroscopic measurement device that measures a spectrum of a sample by irradiating the sample with laser light obtained by wavelength sweeping the wavelength tunable laser, and a laser control device that controls
The laser control device controls the oscillation intensity of the wavelength tunable laser so as to compensate for attenuation of laser light in a sample due to causes other than absorption of a target component and spectral sensitivity characteristics of the photodetector. Spectral measurement device.   11. The spectrometer according to claim 10, wherein an absorption spectrum of the sample is measured. A laser medium capable of laser oscillation in a predetermined wavelength region and a birefringent acoustooptic element are arranged in a laser resonator, and laser resonance is performed on a predetermined optical axis of a light beam component diffracted by the birefringent acoustooptic element. A tunable laser that selects a wavelength by selecting a frequency of an acoustic wave to be excited in the birefringent acoustooptic device, and a frequency and intensity of the acoustic wave to be excited in the birefringent acoustooptic device In a spectroscopic measurement apparatus that measures a Raman scattering spectrum of a sample by irradiating the sample with laser light obtained by sweeping the wavelength of the wavelength tunable laser, a laser control device that controls the light, a spectroscope, and a photodetector ,
An apparatus for spectroscopic measurement, comprising controlling the oscillation light output of the wavelength tunable laser so as to compensate for an increase in Raman scattered light due to absorption of a target component.

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