JP4634956B2 - Light absorption measuring device - Google Patents

Description

  The present invention relates to a light absorption measurement device, and more particularly to a light absorption measurement device including a wavelength conversion device that generates coherent light in the 2 to 20 μm band in the mid-infrared region.

From the viewpoints of environmental protection and health and safety, greenhouse gases such as CH ₄ , CO ₂ , CO, and N ₂ O, environmental gases such as NOx, SOx, and ammonia, water absorption peaks, and many organic gases or residues There is a strong demand for the establishment of trace analysis technology for agricultural chemicals. As trace analysis techniques, a gas to be measured is adsorbed on a specific substance, a quantitative analysis by an electrochemical method, and an optical method for measuring a specific optical absorption characteristic of the substance to be measured are generally used. Among these, the optical method has a feature that real-time measurement is possible and observation of a wide area through which the measurement light passes is possible.

  The absorption peak of the substance to be measured is caused by the vibration mode of the interatomic bond, and is mainly in the mid-infrared region of 2 μm to 20 μm. In the mid-infrared region, there are strong absorption lines typified by gas reference vibration, and there is an increasing demand for small and inexpensive light sources that can be applied to concentration measuring instruments and analytical instruments based on optical absorption line observation. In recent years, the abundance of isotopomer species of these gases has attracted attention from the viewpoint of environmental measurement. However, since mass spectrometry is difficult for those having a small change rate of molecular weight, optical measurement is expected.

  On the other hand, there is an increasing demand for more compact and inexpensive optical absorption measurement devices. There is an increasing expectation for an optical absorption measurement apparatus that can reduce costs by a simple method that does not use phase-sensitive detection such as lock-in detection that has been conventionally used.

D. G. Lancaster et al., Opt. Lett., Vol. 24, 1744 (1999) N. Yoshida et al., Nature, Vol. 405, 330 (2000) A. Ubukata et al., Jpn. J. Appl. Phys., Vol. 40, 6406 (2001) G. Gagliardi et al., Appl. Phys., Vol. B 77, 119 (2003)

  Pb salt lasers and quantum cascade lasers are known as semiconductor lasers that oscillate on the long wavelength side from the 2 μm band. However, only oscillation at low temperature operation or pulse drive can be obtained, and it has not reached the practical range. Recently, a 2.7 μm band semiconductor laser made of an InP-based semiconductor and having a high strain active layer has been put into practical use. However, the maximum output is 3 mW, and since it is a DFB (Distributed Feedback) type laser (DFB-LD), the wavelength variable width with respect to the injection current is very narrow as 10 pm / mA or less, and the wavelength variable width of one laser is 1 nm. It is as follows.

  As a light source other than a semiconductor laser that can be used as a wavelength tunable light source in a 3 μm band to a 5 μm band, a wavelength converter using a lamp, a nonlinear optical crystal, or a quasi phase matching (QPM) type bulk element is known. . Since the spectral purity of the lamp is low and the output of the wavelength converter is low, linear absorption observation at the pressure width limit or Doppler width limit can be barely performed as a gas absorption observation method. Therefore, at present, nonlinear absorption observation such as saturated absorption observation that realizes high resolution spectroscopy and Doppler-free spectroscopy essential for natural width observation cannot be performed as a gas absorption observation method.

  In addition, in a light absorption measuring apparatus in the wavelength range of 2 μm to 20 μm, a light receiver that can be used at room temperature is a photoconductive element whose electrical conductivity changes due to light irradiation, or a thermopile that detects the presence or absence of light irradiation as a temperature change. Only thermal detection elements such as (pyrometers) are known. These light receivers cannot detect the presence or absence of light without a change in electrical conduction or a change in temperature due to a physical request for light detection under the use conditions at room temperature. Therefore, in general, a mechanical chopping operation is indispensable for the laser light used for observation. It is known that if this chopping operation is not performed, 1 / f noise is accumulated and the S / N of the measurement system is significantly reduced. Therefore, when the chopping operation is not performed, extreme deterioration of the light receiving sensitivity limit is observed, and it is indispensable to attach a cooling device.

  An object of the present invention is to provide a light absorption measuring device in a wavelength range from 2 μm to 20 μm that is highly efficient, highly stable, small, and low in cost.

In order to achieve the above object, according to the present invention, the invention described in claim 1 includes a laser beam emitting means, and a laser beam detector for detecting light emitted from the laser beam emitting means and passing through the substance to be measured. And an oscilloscope, wherein the laser light emitting means includes a _first laser that generates laser light having a wavelength λ ₁ , and a wavelength λ _2. there of a second laser for generating a laser beam, the type wavelength lambda ₁ between the laser beam and the wavelength lambda ₂ of the laser beam, the _{1 / λ 1 -1 / λ 2} = 1 / λ 3 relationships and a non-linear optical crystal to output a difference frequency light having a wavelength lambda _3, at least one of the first laser and the second laser is a semiconductor laser semiconductor laser of a current-driven DC drive for driving the semiconductor laser The current is ramped Is by intensity modulation with a modulation signal of the triangular waveform, and a drive circuit to continuously sweep the oscillation frequency of said semiconductor laser, said laser beam detecting means is a photoconductive element or thermal detection device, the An optical absorption characteristic is measured by inputting the output of the laser light detection means and the modulation signal to the oscilloscope.

The nonlinear optical crystal includes lithium niobate (LN), lithium tantalate (LT) or a mixed crystal thereof, LN, LT or a mixed crystal to which Zn or Mg is added, or AgGaS ₂ , GaAs, GaP, Te, CdGeAs ₂ , ZnGeP ₂ , GaSe, Tl ₃ AsSe ₃ , AgGaSe ₃ , CdSe, Ag ₃ SbS ₃ , Ag ₃ AsS _3, or a mixed crystal thereof, and a periodically poled structure is formed.

As described above, according to the present invention, the DC drive current for driving the semiconductor laser, by intensity modulating a ramp waveform or a triangular waveform of the modulation signal, because the oscillation frequency of the semiconductor laser can be continuously swept, It is possible to provide an optical absorption measurement device in the wavelength range of 2 μm to 20 μm that is highly efficient, highly stable, small, and inexpensive.

  In addition, according to the present invention, since a semiconductor laser is used, it is easy to increase the output by applying a laser used in the optical communication band, and includes observation of nonlinear absorption without using a conventional high-power laser. High resolution spectroscopy is possible.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. First, the principle of the light absorption measurement apparatus will be described with reference to FIG. The light output from the semiconductor laser 1 through the lens 5 is chopped by passing through a slit formed in the rotating disk 6 of the chopper, passes through the gas cell 11 and is incident on the light receiver 12. If reference light is required, a beam splitter 9 is inserted between the rotating disk 6 and the gas cell 11 and branched. The lock-in amplifier 13 receives a synchronization signal from a chopper control unit that controls the rotating disk 6, and locks in an electrical signal from the light receiver 12.

  This optical absorption measurement apparatus uses a semiconductor laser that directly oscillates in the wavelength band of 2 μm to 20 μm. In the spectroscopic measurement of light absorption, since a spectroscope using a spectroscopic element such as a prism or a grating is not used, a spectroscopic measurement with higher resolution can be performed. For the light receiver, a photoconductive element or a thermal detection element that can be easily obtained is used. For these light receivers, chopper operation is indispensable for the reasons described above, and therefore, it is assumed that lock-in detection that enables high-sensitivity observation is possible. The measurement object is not limited to the gas cell shown in the figure. The reference light branched by the beam splitter is used for the purpose of eliminating the influence of the fluctuation of the light source, the noise caused by the stability of the system, and the spectral shape of the light source. It goes without saying that the use of the reference light enables accurate spectrum observation.

  Next, with reference to FIG. 2, the principle of the optical absorption measurement apparatus using the laser beam obtained by the difference frequency generation in the nonlinear optical crystal will be described. The signal light output from the laser 21 and the excitation light output from the laser 22 are combined by the multiplexer 23 and are incident on the nonlinear optical crystal 24. Due to the difference frequency generation in the nonlinear optical crystal 24, the difference frequency light in the 2 μm band to the 20 μm band is emitted. From the light output from the nonlinear optical crystal 24 through the lens 25, only the difference frequency light is extracted by the two-color mirror 27 and the filter 28 that reflect the short wavelength side and transmit the mid-infrared light.

  The difference frequency light is chopped by passing through a slit formed in the rotating disk 26 of the chopper, passes through the gas cell 31, and enters the light receiver 32. If reference light is required, a beam splitter 29 is inserted between the filter 28 and the gas cell 31 and branched. The lock-in amplifier 33 receives a synchronization signal from a chopper controller that controls the rotating disk 26 and locks in an electric signal from the light receiver 32.

  In this way, it is possible to realize a light absorption measurement device using a light source of 2 μm band to 20 μm band. However, since these light absorption measurement devices are premised on lock-in detection, the system is still complicated, and it is difficult to reduce the size, portability, and cost.

(First embodiment)
FIG. 3 shows the configuration of the light absorption measurement apparatus according to the first embodiment of the present invention. Light output from the semiconductor laser 41 via the lens 45 passes through the gas cell 51 and enters the light receiver 52. If reference light is required, a beam splitter 49 is inserted between the lens 45 and the gas cell 51 and branched. The output of the light receiver 52 is input to the oscilloscope 57 via the amplifier 54, and the optical absorption characteristics of the substance to be measured are measured. A drive circuit 55 is connected to the semiconductor laser 41, and an oscillator 56 that outputs a signal having a ramp waveform or a triangular waveform is connected to the oscilloscope 57 and the drive circuit 55.

  A drive circuit 55 for driving the semiconductor laser 41 will be described. If the conventional chopping operation can create a binary state of the presence or absence of light, it can measure the sequential change of electrical conductivity and the sequential change of temperature, so just use the on / off function of light. Good. On the other hand, a spectroscopic function or a wavelength sweep function is indispensable for the light absorption measurement device, and it is necessary to sweep the wavelength of the light source. Therefore, in the drive circuit 55, the oscillation state of the semiconductor laser 41 is controlled by the injected current, and the intensity and frequency of the emitted light are simultaneously controlled. That is, the on / off function and the wavelength sweep function are realized at the same time.

  The mechanical chopping operation corresponds to rectangular wave modulation and can only take a binary state (on / off) of the light intensity. Therefore, even if the control by the drive circuit is performed, the frequency takes only a discrete binary value. I don't get it. Therefore, intensity modulation is performed with a waveform based on a triangular wave or a ramp wave. Since the transition between the extinction state and the light emission state changes smoothly, the frequency can be swept continuously as well. Further, since the frequency band of the light receiver in the long wavelength band is as low as several hundred kHz, even when the change rate of the light intensity is small, a low-noise amplifier can be used without causing a decrease in S / N. On-off state and frequency modulation including phase modulation can be detected simultaneously.

(Second Embodiment)
FIG. 4 shows the configuration of a light absorption measurement apparatus according to the second embodiment of the present invention. In the second embodiment, laser light obtained by difference frequency generation in a nonlinear optical crystal is used. The signal light output from the laser 61 and the excitation light output from the laser 62 are combined by the multiplexer 63 and are incident on the nonlinear optical crystal 64. The difference frequency light in the 2 μm band to the 20 μm band is emitted by the difference frequency generation in the nonlinear optical crystal 64. Only the difference frequency light is extracted from the light output from the nonlinear optical crystal 64 via the lens 65 by the two-color mirror 67 and the filter 68 that reflect the short wavelength side and transmits the mid-infrared light. The difference frequency light passes through the gas cell 71 and enters the light receiver 72. If reference light is required, a beam splitter 69 is inserted between the filter 68 and the gas cell 71 and branched.

  The output of the light receiver 72 is input to the oscilloscope 77 via the amplifier 74, and the optical absorption characteristics of the substance to be measured are measured. Drive circuits 75a and 75b are connected to the lasers 61 and 62, respectively, and an oscillator 76 for outputting a ramp waveform or a triangular waveform signal is connected to the oscilloscope 77 and the drive circuit 75a.

  In the second embodiment, since the energy of the difference frequency light is given by the energy difference between the signal light and the excitation light, the degree of freedom of the combination of the wavelengths of the difference frequency light is great. Therefore, if desired wavelength conversion efficiency is obtained, laser light having an arbitrary wavelength can be output. Since either the excitation light or the signal light may be modulated into the waveform based on the triangular wave or the ramp waveform, at least one of them uses a current-driven semiconductor laser. Variable wavelength light source including external cavity type semiconductor laser (ECLD), DFB (Distributed Feedback) type laser (DFB-LD), combination of semiconductor laser and fiber black grating (FBG), narrowing of line width, single A light source in a vertical mode can also be applied. Further, when a fiber amplifier or the like is used in combination with a laser that outputs signal light, the output can be increased, and observation of nonlinear absorption becomes easy. On the other hand, a high-power semiconductor laser that also uses FBG can be applied to a laser that outputs excitation light of 0.92 μm to 1.1 μm. Thus, by applying the laser used in the optical communication band, it is easy to increase the output, and high-resolution spectroscopy including observation of nonlinear absorption is possible without using a conventional high-power laser.

  According to the present embodiment, it is not necessary to use phase sensitive detection that performs chopping operation and lock-in detection. By applying intensity modulation using either a triangular wave or a ramp waveform to any of the excitation light, signal light, and difference frequency light, it is possible to suppress the accumulation of 1 / f noise and simultaneously perform frequency modulation. Therefore, it is possible to further configure a light absorption measuring device that realizes miniaturization, portability, and low cost.

  FIG. 5 shows a configuration of the light absorption measuring apparatus according to the first embodiment. The signal light output from the laser 81 and the excitation light output from the laser 82 are combined by the multiplexer 83 and are incident on the nonlinear optical crystal 84. The laser 81 uses a DFB-LD that oscillates at 1569 nm. As the laser 82, a Fabry-Perot laser (FP) equipped with an FBG having a center wavelength of 976.3 nm is used. For coupling of incident light to the nonlinear optical crystal 84, a coupling method using batting or lens coupling can be used. Due to the difference frequency generation in the nonlinear optical crystal 84, the difference frequency light in the 2 μm band to the 20 μm band is emitted.

In Example 1, the nonlinear optical crystal 84 uses a quasi-phase matching type waveguide made of LiNbO ₃ (LN), and generates a difference frequency.
1 / λ ₃ = 1 / λ ₁ −1 / λ ₂
The converted light with the wavelength λ ₃ = 2583 nm having the relationship of Since this wavelength matches the absorption line of water, the water vapor concentration in the atmosphere can be measured.

As the nonlinear optical crystal 84, in addition to LN, LiTaO ₃ (LT) or a mixed crystal thereof, or LN, LT to which Zn or Mg is added, or a mixed crystal thereof can be used. Alternatively, AgGaS ₂ , GaAs, GaP, Te, CdGeAs ₂ , ZnGeP ₂ , GaSe, Tl ₃ AsSe ₃ , AgGaSe ₃ , CdSe, Ag ₃ SbS ₃ , Ag ₃ AsS ₃ or a mixed crystal thereof can be used. The nonlinear optical crystal 84 is periodically formed with a domain-inverted structure. In addition to this, the wavelength conversion efficiency can be increased by having a waveguide structure.

From the light output from the nonlinear optical crystal 84 through the lens 85, only the difference frequency light is extracted by the two-color mirror 87 and the filter 88 that reflect the short wavelength side and transmit the mid-infrared light. The lens 85 uses anhydrous quartz, CaF ₂ , or KBr depending on the wavelength of the emitted light. Anhydrous quartz may be used in the 2 μm band, but CaF _{2 which} is easy to handle without absorption and deliquescence throughout the 2-7 μm band is preferable. In the 7 μm band or more, light is not transmitted unless KBr is used. The filter 88 is easy to use a wavelength filter using Ge, and can cut out the signal light and the excitation light and extract only the difference frequency light. If reference light is required, a beam splitter 89 is inserted between the filter 88 and the light receiver 92 and branched.

  The difference frequency light passes through the substance to be measured in the atmosphere and enters the light receiver 92. For the light receiver 92, a PbSe photoconductive element is used at room temperature. In addition to a photoconductive element such as PbS or HgCdTe, a thermal detection element such as a thermopile, a condenser microphone, or a pyroelectric element can also be used.

  The output of the light receiver 92 is input to the oscilloscope 97 via the amplifier 94, and the optical absorption characteristics of the substance to be measured are measured. The amplifier 94 uses a low noise DC amplifier. Although an AC amplifier can be used according to the frequency used, a DC amplifier is preferable because the sensitivity of the PbSe photoconductive element does not extend over a wide area and is about several kHz at most. Drive circuits 95a and 95b are connected to the lasers 81 and 82, respectively, and an oscillator 96 for outputting a ramp waveform or a triangular waveform signal is connected to the oscilloscope 97 and the drive circuit 95a.

  A repetitive ramp waveform of 100 Hz to 1 kHz is superimposed on the injection current from the drive circuit 95a to the laser 81 at about ± 50 mA. With the on / off function, an absorption spectrum can be observed over a wavelength width corresponding to 2 nm including adjacent absorption lines. Although a PbSe photoconductive element is used at room temperature as the light receiver 92, it operates satisfactorily without S / N degradation being observed. It was operated continuously for 8 hours a day and observed for 3 days, but there was no change in the results.

  Moreover, a transmission spectrum of 0 to 100% can be obtained by dividing the difference frequency light transmitted through the substance to be measured by the reference light. Here, air is used as the substance to be measured, but a transmission spectrum can be obtained in the same manner even if a sealed gas cell, a flow type gas cell, or a multi-pass cell is used. For use of the reference light, a reference cell is preferably used in the case of a gas cell, and in a multi-pass cell or the atmosphere, a light receiver may be disposed at a position immediately before light is incident on the substance to be measured.

  FIG. 6 shows a configuration of a light absorption measuring apparatus according to the second embodiment. Laser light emitted from the light source 101 is input to the multi-pass cell 111 via the reflection optical system including the concave mirror 103 and the mirrors 105 and 107 and the beam splitter 109. The light source 101 uses a quantum cascade laser (QCL) or a Pb salt laser (LSL). Of course, the light source is not limited to these, and a light source using difference frequency generation in a nonlinear optical crystal may be used.

In the second embodiment, a dilute gas that cannot be detected unless the object to be measured is observed using the multipass cell 111 is assumed. CO ₂ , CO, N ₂ O, CH ₄ , COS, NH _{3 and the} like in the atmosphere. The multi-pass cell 111 may be analyzed by performing a decompression operation in order to improve the resolution. In some cases, for example, when absorption of water vapor coexists, decompression is performed to separate the spectrum.

  The light output from the multipass cell 111 is incident on the light receiver 112a through a reflective optical system including the concave mirror 110 and the mirror 113. When the reference light is not used, for example, an absorption line is superimposed on the envelope of the ramp waveform and observed. When the reference light is used, the light branched by the beam splitter 109 enters the light receiver 112b. The light transmitted through the object to be measured and the reference light are each converted into an electric signal independently, amplified by the amplifiers 114a and 114b, the difference is extracted by the differential amplifier 119, and the optical absorption characteristic of the substance to be measured is obtained by the oscilloscope 117. taking measurement. Here, the intensity of the electrical signal of the reference light is adjusted by the attenuator 118. A driving circuit 115 is connected to the light source 101, and an oscillator 116 that outputs a signal having a ramp waveform or a triangular waveform is connected to the oscilloscope 117 and the driving circuit 115.

  When QCL that oscillates in the vicinity of 4868 nm is used as the light source 101, carbonyl sulfide (COS) existing in the atmosphere or exhaled breath can be observed. As the multipath cell 111, a cell having an optical path length of about several tens to 100 m is suitable. In order to separate the COS spectrum from the background absorption for easy observation, the multipass cell 111 is depressurized and used at about 100 Torr to 40 Torr, which is slightly lower than the atmospheric pressure (760 Torr). For example, the drive circuit 115 drives the QCL with a ramp waveform having a peak value of about 1 A at a repetition cycle of 100 Hz to 1 kHz.

  ADVANTAGE OF THE INVENTION According to this invention, the optical absorption measuring device which can be applied to measurement / analysis apparatus of gas, such as environmental gas and industrial process gas, can be provided. The light absorption measuring apparatus according to the present invention can directly observe light absorption, and can apply absorption emission observation to transitions showing a light emission process and photoacoustic spectroscopy to transitions showing a non-light emission process. In addition, saturated absorption observation known as a nonlinear phenomenon effective for natural width observation is also possible. Furthermore, it can be applied to gas absorption spectroscopy and lidar measurement in the field using scattered light and reflection from a soft target.

It is a figure for demonstrating the principle of a light absorption measuring device. It is a figure for demonstrating the principle of the optical absorption measuring apparatus using the laser beam obtained by the difference frequency generation | occurrence | production in a nonlinear optical crystal. It is a figure which shows the structure of the light absorption measuring apparatus concerning the 1st Embodiment of this invention. It is a figure which shows the structure of the light absorption measuring apparatus concerning the 2nd Embodiment of this invention. 1 is a diagram illustrating a configuration of a light absorption measurement apparatus according to Example 1. FIG. It is a figure which shows the structure of the light absorption measuring apparatus concerning Example 2. FIG.

Explanation of symbols

1, 41, 101 Semiconductor laser 21, 22, 61, 62, 81, 82 Laser 23, 63, 83 Multiplexer 24, 64, 84 Nonlinear optical crystal 5, 25, 45, 65, 85 Lens 6, 26 Rotating disk 27, 30, 67, 87, 105, 107, 113 Mirror 28, 68, 88 Filter 9, 29, 49, 69, 89, 109 Beam splitter 11, 31, 51, 71 Gas cell 12, 32, 52, 72, 92 , 112 Light receiver 13, 33 Lock-in amplifier 54, 74, 94, 114 Amplifier 55, 75, 95, 115 Drive circuit 56, 76, 96, 116 Oscillator 57, 77, 97, 117 Oscilloscope 103, 110 Concave mirror 111 Multipath Cell 118 attenuator 119 differential amplifier

Claims (3)

An optical absorption measuring device including laser light emitting means, laser light detecting means for detecting light emitted from the laser light emitting means and passing through the substance to be measured, and an oscilloscope, and measuring optical absorption characteristics of the substance to be measured In the laser beam emitting means,
A _first laser that generates laser light of wavelength λ ₁ ;
A _second laser that generates laser light of wavelength λ2,
Nonlinear optics for inputting the laser light having the wavelength λ _{1 and} the laser light having the wavelength λ ₂ and outputting the difference frequency light having the wavelength λ _{3 in} the relationship of 1 / λ ₁ −1 / λ ₂ = 1 / λ _3. Crystals,
At least one of the first laser and the second laser is a current-driven semiconductor laser, and the intensity of a direct-current drive current for driving the semiconductor laser is modulated with a ramp waveform or a triangular waveform modulation signal, A drive circuit for continuously sweeping the oscillation frequency of the semiconductor laser,
The laser light detection means is a photoconductive element or a thermal detection element,
An optical absorption measurement apparatus, wherein an optical absorption characteristic is measured by inputting an output of the laser light detection means and the modulation signal to the oscilloscope.   The nonlinear optical crystal is lithium niobate (LN), lithium tantalate (LT) or a mixed crystal thereof, or LN, LT or a mixed crystal to which Zn or Mg is added, and has a periodically poled structure. The light absorption measuring device according to claim 1, wherein the light absorption measuring device is formed. The nonlinear optical crystal _{is AgGaS 2, GaAs, GaP, Te} , CdGeAs 2, ZnGeP 2, GaSe, Tl 3 AsSe 3, AgGaSe 3, CdSe, Ag 3 SbS 3, Ag 3 AsS 3 or mixed crystals, periodic The optical absorption measuring device according to claim 1, wherein a polarization inversion structure is formed.

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