JP2014235103A - Light absorption measurement laser source and light absorption measurement apparatus using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-sensitivity light absorption laser source capable of eliminating an output variation of a laser beam and an intensity distribution due to interference generated by a resonator structure, easily identifying the position of an absorption line and of being downsized, and to provide a measurement apparatus using the light absorption laser source.SOLUTION: A light absorption measurement laser source 100 includes: a wavelength conversion element 105 composed of an optical waveguide having a periodically polarization-inverted structure; a DFB type semiconductor laser 101 outputting excitation light of 1.06 μm wavelength; a DFB type semiconductor laser 102 outputting signal light of 1.39 μm wavelength; a half mirror 106 splitting light output from the wavelength conversion element; and a small reference cell 107 encapsulating therein HO (water) of 5% concentration. The reference cell 107 does not contain NO gas which is a substance to be measured.

Description

  The present invention relates to a laser light source for light absorption measurement and a light absorption measurement apparatus using the same, and more specifically to a laser light source for light absorption measurement using a wavelength conversion element and a light absorption measurement apparatus using the same.

From the viewpoints of environmental protection and health and safety, extreme gases such as greenhouse gases such as CH ₄ , CO ₂ , CO, N ₂ O, environmental gases such as NO _x , SO _x , ammonia, many organic gases or residual agricultural chemicals The establishment of trace gas analysis technology is strongly desired. As a technique for detecting the gas concentration, a technique is known in which laser light is applied to a gas to be measured and its absorption characteristics are observed. Since each gas has its own absorption line, the gas concentration can be measured by sweeping the wavelength of a laser beam having a wavelength near the absorption line and observing the absorption spectrum.

  Most of the absorption lines of these substances to be measured exist in the mid-infrared region of a wavelength of 2 μm or more as fundamental vibrations or lower harmonics due to vibration modes of interatomic bonds. Accordingly, there is an increasing expectation for optical absorption measurement capable of measuring gas absorption lines with high sensitivity, particularly in the mid-infrared region having a wavelength of 2 μm or more.

  The reason why light absorption measurement in the mid-infrared region is difficult is that there are few appropriate light sources that can stably oscillate mid-infrared light having a wavelength of 2 μm or more at room temperature. 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, there are many light sources that can obtain stable oscillation only by low-temperature operation or pulse driving, and particularly, there are few light sources that stably operate at room temperature in the wavelength region of 3 μm band to 5 μm band.

  Recently, a 2.7 μm-band semiconductor DFB (Distributed Feedback) type laser made of an InP-based semiconductor and having a high strain active layer formed has been put into practical use. Hateful.

  As a light source that stably operates at room temperature in the wavelength region of 3 μm band to 5 μm band, a number of light sources that output laser light in the above-described wavelength region using the difference frequency generation by the quasi phase matching wavelength conversion element have been announced ( For example, refer nonpatent literature 1). Since this light source can use a technically stable semiconductor laser having a wavelength of 2 μm or less as excitation light and signal light input to the wavelength conversion element, it can be easily put to practical use.

  FIG. 5 shows a basic configuration diagram for stably measuring a gas concentration using a mid-infrared laser light source using a wavelength conversion element. The wavelength conversion laser light source 500 for generating light having a wavelength near the absorption line of the gas to be measured includes the semiconductor lasers 501 and 502 for inputting the excitation light and the signal light, and the combination of the excitation light and the signal light. The waver 503 includes a wavelength conversion element 504 made of a nonlinear optical material whose polarization is periodically inverted.

  In order to measure the absorption spectrum, it is necessary to sweep the wavelength of the laser beam to be output. Usually, this means that the DC drive current of either of the semiconductor lasers 501 and 502 that outputs the excitation light and signal light is ramped. Modulate into a waveform or triangular waveform. Since the wavelength of the laser has characteristics that can be approximated by a quadratic function of the drive current, the wavelength can be swept.

  When measuring minute absorption more precisely, the frequency f is modulated to the sweep wavelength by, for example, superimposing the sine wave of the frequency f on the ramp waveform or the triangular waveform, and responding to the modulation of the frequency f. There is a wavelength modulation spectroscopy (WMS) method in which a component or a 2f component of its harmonic wave is detected by a lock-in amplifier 509 or the like, and this method is often used for measurement of a trace gas.

  For modulation of a ramp waveform, a triangular waveform, a sine wave of frequency f, etc., a modulation signal from an arbitrary waveform generator such as a function generator 510 is mainly applied to one of the laser drivers (laser driving devices) of the semiconductor lasers 501 and 502. Do it by acting.

  The light emitted from the wavelength conversion laser light source 500 becomes parallel light by the action of the collimating lens 505 that passes after the emission, and then passes through the wavelength filter 506 in order to remove light other than the mid-infrared light. As the wavelength filter 506, a semiconductor such as Si, InP, GaAs, or Ge is often used.

  The mid-infrared light passes through the wavelength filter 506 and then is detected by the photodetector 508 through the multipass cell 507 in which the substance to be measured is enclosed. The detection result is sent to the signal processing device 511 after the specific frequency is extracted by the lock-in amplifier 509. In the case of wavelength modulation spectroscopy, a modulation signal from a pulse generator or the like is used as a reference signal, and a component in response to modulation is lock-in detected and displayed on an oscilloscope or the like, or data is sent to a recording device to obtain an absorption spectrum. In the gas concentration measurement, the concentration of the substance to be measured is calculated from the absorption spectrum thus obtained.

  The above is the basic configuration for stably measuring the gas concentration using the wavelength conversion laser light source 500.

DG Lancaster et al., “High-power continuous-wave mid-infrared radiation generated by difference frequency mixing of diode-laser-seeded fiber amplifiers and its application to dual-beam spectroscopy,” Opt. Lett., Vol. 24, 1744 (1999) "Near-infrared sensing technology for biological and environmental measurement", published by Science Forum, ISBN4-916164-24-5, pp.257-261, pp.269-276

  However, the following problems exist in gas concentration measurement using the conventional wavelength conversion laser light source 500. Since the output of the semiconductor lasers 501 and 502 that output the excitation light and the signal light may fluctuate due to the influence of the external environment or the like, the output of the mid-infrared light converted based on the semiconductor lasers 501 and 502 is also present. It will shake. When the output of the mid-infrared light fluctuates, the signal intensity due to light absorption also changes, which causes a problem that the fluctuation of the output is detected as a change in density.

  In addition, the output wavelengths of the semiconductor lasers 501 and 502 may shift due to the external environment or aging. For this reason, there existed a subject that the position of the absorption line on the acquired spectrum shifted with the passage of the external environment and time. As a result, when the absorption signal is strong, the position of the absorption line can be recognized by an absorption peak or the like. However, when the signal intensity is extremely weak, such as a trace gas measurement, it is difficult to accurately specify the position of the absorption line.

  Furthermore, when a wavelength conversion laser light source is used, a wavelength conversion element 504 made of a nonlinear optical crystal with periodically poled is used. The wavelength conversion element 504 receives excitation light and signal light output from the semiconductor lasers 501 and 502, and outputs mid-infrared light having a wavelength corresponding to the excitation light and signal light as converted light. Normally, an antireflection film is formed on the input end face and output end face to prevent interference due to internal reflection. However, the reflectivity is not completely 0%, and reflection of about 1 to 2% occurs. End up. Therefore, the element becomes a resonator, and the intensity distribution due to the interference effect is superimposed on the spectrum of the output light. Although this intensity distribution is small, when measuring minute absorption such as wavelength modulation spectroscopy, the peak of the absorption spectrum is as large as the intensity distribution due to this interference. There was a problem that accurate measurement of the position and intensity was hindered, and the sensitivity of the light absorption measurement was remarkably lowered.

  As a contrivance in measurement, as shown in FIG. 6, a configuration in which the influence is corrected using a reference substance may be employed (for example, see Non-Patent Document 2). However, when such a configuration is adopted, the mirror 512 for branching the laser beam, the gas cell 513 for the reference material, the photodetector 514, the lock-in amplifier 515, etc. must be arranged separately, and a considerable amount of measurement is required. Space is needed. For this reason, it becomes difficult to measure in a narrow space, and at the same time, it is necessary to align the optical axis in this reference optical path, which complicates the measurement work. In many cases, the same substance as the substance to be measured is selected as the reference substance. In this case, the output fluctuation of the laser beam and the position of the absorption line can be specified, but the intensity distribution of the spectrum derived from the interference effect can be removed. There was a problem that it was not possible.

  The present invention has been made in view of such a problem, and an object of the present invention is to determine the position of the absorption line by removing the intensity distribution due to the output fluctuation of the laser beam and the interference caused by the resonator structure. It is an object of the present invention to provide a light absorption laser light source and a light absorption measurement apparatus using the light absorption laser light source capable of realizing high-sensitivity and miniaturized light absorption measurement.

  In order to solve the above-described problems, the present invention is an optical absorption measurement device, which is a laser light source for optical absorption measurement, and includes two or more semiconductor lasers having different output frequencies, and output light from each of the semiconductor lasers A wavelength conversion element made of a periodically poled nonlinear optical material on which the output light output from the multiplexer enters, and the output light output from the wavelength conversion element is branched Branching means, a first output port for outputting the first branched light branched by the branching means, a reference cell containing a reference material into which the second branched light branched by the branching means is incident, and A second output port that outputs transmitted light that has passed through the reference cell, and the reference substance is not detected from the result of measuring the substance to be measured using the output light emitted from the first output port. Light absorption A laser light source for measurement; a multipass cell containing the substance to be measured; a first photodetector for detecting output light emitted from the first output port that has passed through the multipass cell; The second photodetector for detecting the output light emitted from the two output ports, the intensity of the measurement result of the first photodetector and the measurement result of the second photodetector are aligned, and the first detector And a signal processing device that removes interference fringes of the wavelength conversion element by subtracting the measurement result of the second photodetector from the measurement result of the first photodetector.

  According to a second aspect of the present invention, in the light absorption measurement apparatus according to the first aspect, the reference cell containing the reference substance can be exchanged according to the substance to be measured.

  According to a third aspect of the present invention, in the light absorption measurement apparatus according to the first or second aspect, the wavelength conversion element is added with lithium niobate (LN), lithium tantalate (LT), Zn or Mg. It has a waveguide structure composed of LN, LT, or a mixed crystal thereof.

  A fourth aspect of the present invention is the light absorption measurement apparatus according to any one of the first to third aspects, wherein the signal processing apparatus absorbs the reference substance from the measurement result of the second photodetector. The shift of the absorption line of the substance to be measured measured by the first photodetector is corrected based on the shift of the line.

According to a fifth aspect of the present invention, in the laser light source for light absorption measurement, two or more semiconductor lasers having different output frequencies, a multiplexer for multiplexing output light from each of the semiconductor lasers, and the multiplexer A wavelength conversion element made of a periodically poled nonlinear optical material, on which the output light output from the light enters, a branching means for branching the output light output from the wavelength conversion element, and branched by the branching means A first output port for outputting the first branched light; a reference cell containing a reference material into which the second branched light branched by the branching unit is incident;
A second output port that outputs transmitted light that has passed through the reference cell, and the reference substance is detected from a result of measuring the substance to be measured using the output light emitted from the first output port. It is a material that is not treated.

  The invention according to claim 6 is the laser light source for optical absorption measurement according to claim 5, wherein the wavelength conversion element is added with lithium niobate (LN), lithium tantalate (LT), Zn or Mg. It has a waveguide structure composed of LN, LT, or a mixed crystal thereof.

  According to the present invention, it is possible not only to correct the fluctuation of the laser beam output intensity and the shift of the output frequency of the laser beam, but also to correct by removing the influence of the intensity distribution derived from the resonator structure caused by the wavelength conversion element. Thus, the sensitivity of light absorption measurement can be increased. In addition, it is not necessary to secure a space for newly arranging the reference substance at the time of measurement, and complicated measurement work such as alignment of the optical axis of the reference light path is eliminated, and the measurement system can be reduced in size. As a result, it is possible to realize a light absorption measurement with higher sensitivity and a smaller size, and it is possible to detect a small amount of substance and measurement in a narrower space that could not be measured conventionally.

It is a figure which shows the measurement structure using the laser light source for light absorption measurement which concerns on one Embodiment of this invention. It is a figure which shows the simulation result of the absorption line of atmospheric component gas in 100 Torr in a measurement wavelength band. It is a diagram showing the raw waveform of the wavelength modulation spectroscopy spectrum measured by using N _{2 O} gas concentration 27Ppb. It is a figure which shows the difference spectrum which deducted the reference spectrum from the measurement spectrum. It is a figure which shows the structure of the light absorption measurement using the conventional wavelength conversion laser light source. It is a figure which shows the measurement structure using a reference cell in the light absorption measurement using the conventional wavelength conversion laser light source.

  Hereinafter, embodiments of the present invention will be described in detail.

FIG. 1 shows a laser light source for light absorption measurement and a measurement configuration according to an embodiment of the present invention. First, the light source 100 for measuring light absorption according to the present invention will be described. The laser light source 100 for light absorption measurement is assumed to output laser light having a wavelength of, for example, 2 μm to 20 μm. Here, a medium red of 4.6 μm band for measuring N ₂ O (nitrous oxide) is used. It was set as the structure which outputs external light. The laser light source 100 for light absorption measurement includes a wavelength conversion element 105 formed of an optical waveguide having a periodically poled structure, a DFB semiconductor laser 101 that outputs a wavelength of 1.06 μm as excitation light, and 1. A DFB semiconductor laser 102 that outputs a wavelength of 39 μm, a half mirror 106 that divides the light output from the wavelength conversion element, and a small reference cell 107 that contains H ₂ O (water) at a concentration of 5%. Yes.

Note that the reference cell 107 does not contain N ₂ O gas which is a substance to be measured. The term “not included” means that it contains only a detection limit or less that cannot affect the light absorption measurement. The reference cell 107 is mounted so that it can be easily replaced without the need for complicated operations such as optical axis adjustment, and the reference material can be easily replaced according to the substance to be measured.

  As shown in FIG. 1, an optical fiber amplifier 103 may be provided for amplifying the pumping light or the signal light. Here, in order to amplify the 1.06 μm pumping light that is the output light of the DFB type semiconductor laser 101. In addition, YDFA, which is a Yb-doped optical fiber amplifier, was used.

  The excitation light and the signal light are combined by a multiplexer 104 such as an optical coupler and introduced into the wavelength conversion element 105. The combined light is coupled to the waveguide element by a lens in the wavelength conversion element 105, and is converted into light of 4.6 μm band by the difference frequency generation. In this embodiment, a waveguide made of Zn-doped lithium niobate (LN) having a width of 20 μm, a height of 15 μm, a length of 50 mm, and a polarization inversion period of 26 μm is used as the wavelength conversion element 105. The waveguide was bonded to the base substrate by lithium tantalate (LT) by direct bonding using thermal diffusion, and a waveguide structure was produced by a dicing saw.

  In addition, the material, size, and manufacturing method of the wavelength conversion element 105 are examples, and are not limited to the above. The wavelength conversion element 105 may have a waveguide structure composed of LN, LT to which LN, LT, Zn, or Mg is added, or a mixed crystal thereof.

  Part of the light output from the wavelength conversion element 105 is output from the output port 109 as laser light for light absorption measurement, and the remaining light branched by the half mirror 106 that is a branching means is a compact in which a reference substance is enclosed. The light passes through the reference cell 107, the optical path is adjusted by the mirror 108, and is output from the output port 110 as reference light.

  When 500 mW was input as the amplified excitation light and 50 mW was input as the signal light, an output of 0.75 mW was obtained from the output port 109 and an output of 0.25 mW was obtained from the output port 2 in a wavelength region other than the water absorption line.

FIG. 2 shows a simulation result of atmospheric component gas absorption lines at 100 Torr using HITRAN, which is a gas absorption database for light. In this simulation, water having an absorption line in the vicinity of 2205.2 cm ⁻¹ was selected as a reference substance in order to measure an absorption line with a fraction of 220.69 cm ⁻¹ (wavelength 4533.7 μm). They can be freely selected and are not limited to the present embodiment.

When N ₂ O is measured, for example, CO ₂ (carbon dioxide), CO (carbon monoxide), or the like can be used in addition to the water described above. Similarly, the branching means for the light output from the wavelength conversion element 105 is not limited to this embodiment. If a small gas cell is employed as the reference cell 107, the size of the laser light source can be reduced.

  In addition, a collimating lens 113, a Ge filter 114 for removing near-infrared light, and a photodetector 118 can be installed immediately after the output port 110, so that the optical axis alignment for the reference light becomes very simple. As a result, the space required for the gas measurement system can be reduced by 30% or more compared to the case where the reference substance is installed outside the light source.

  Moreover, since it is not necessary to prepare the reference cell 107 and the troublesome adjustment of the optical axis associated therewith, the time required for the measurement can be shortened by about 20%. The light output from the output port 109 is introduced into the multipass cell 115 which is a measurement gas cell through the collimating lens 111 and the Ge filter 112, passes through the measurement gas, and is detected by the photodetector 116.

  In this embodiment, as the photodetectors 116 and 118 for detecting the measurement light and the reference light, an electronically cooled InSb is used, and a wavelength modulation measurement method is used for the measurement. A ramp wave for wavelength sweep and a sine wave for modulation were generated by the function generator 120, and wavelength sweep and modulation were performed via a laser driver for signal light. The period of the ramp wave was 0.1 Hz, and the modulation frequency was 15 kHz. Among the detection signals of the measurement light and the reference light, the 2f component that is a harmonic of modulation is extracted by the lock-in amplifiers 117 and 119, and is calculated by the signal processing device 121 and converted into a density. The type of the photodetector, the period of the lamp light, and the modulation frequency can be selected appropriately according to the measurement.

  In the laser light source for light absorption measurement according to the present invention, the semiconductor lasers 101 and 102 are processed by dividing the measurement signal by the reference signal in a state where the branching ratio between the measurement light and the reference light is measured in advance. The fluctuation of the output signal can be eliminated. Further, since the interval between the absorption line of the measurement gas and the absorption line of the reference material is constant and known, the shift in the output wavelength of the semiconductor lasers 101 and 102 is grasped from the deviation of the absorption line of the reference material, and the absorption line of the measurement gas The deviation can be corrected.

FIG. 3 shows a raw waveform of a wavelength modulation spectrum obtained by measuring N ₂ O gas having a concentration of 27 ppb. The presence of an intensity distribution (interference fringes) for interference originating from the resonator structure of the wavelength conversion element 105 can be confirmed around the absorption peak. At a concentration where the size of the absorption peak is reduced to the same level as the interference fringes, most of the components in the peak are occupied by the interference fringes, and this causes a measurement limit due to degradation in resolution. Therefore, after aligning the intensities, the interference fringes can be subtracted by subtracting the reference spectrum from the measured spectrum.

  FIG. 4 shows a difference spectrum obtained by subtracting the reference spectrum from the measured spectrum after aligning the intensities. In the present invention, since the measurement substance is substantially not included as the reference substance, only the interference fringes can be removed without affecting the absorption peak component to be measured when the difference spectrum is taken. As a result, the measuring ability is improved, and the limit concentration can be reduced to 1/3 of the conventional one. Reference light is required for such spectrum correction work, but by providing the reference cell 107 in the light source 100, correction can be performed very easily and detection sensitivity can be improved.

  As described above, a material that does not substantially contain the target substance to be measured is used as a reference material, and a reference cell is provided in the light source, resulting in a space-saving measurement configuration and a reference. Work such as optical axis adjustment, which is troublesome for cell preparation and replacement, is no longer necessary, and the time required for measurement can be shortened. In addition, because the reference light can be used easily, the influence of interference fringes derived from the fluctuations in the output of the semiconductor laser, the fluctuations in the output wavelength of the laser, and the resonator structure of the elements that existed in conventional light sources can be easily removed. Measurements can be made.

100, 500 Laser light source for optical absorption measurement 101, 102, 501, 502 DFB type semiconductor laser 103 Optical fiber amplifier 104, 503 multiplexer 105, 504 Wavelength conversion element 106, 512 Half mirror 107, 513 Reference cell 108 Mirror 109, 110 Output port 111, 113, 505 Collimating lens 112, 114, 506 Ge filter 115, 507 Multi-pass cell 116, 118, 508, 514 Photodetector 117, 119, 509, 515 Lock-in amplifier 120, 510 Function generator 121 511 Signal processing device

Claims (6)

A laser light source for measuring light absorption,
Two or more different semiconductor lasers with different output frequencies,
A multiplexer for multiplexing the output light from each of the semiconductor lasers;
A wavelength conversion element made of a nonlinear optical material periodically polarized in which the output light output from the multiplexer is incident;
Branching means for branching the output light output from the wavelength conversion element;
A first output port for outputting the first branched light branched by the branching means;
A reference cell containing a reference material into which the second branched light branched by the branching unit is incident, and a second output port that outputs transmitted light that has passed through the reference cell;
The reference substance is a substance that is not detected from the result of measuring the substance to be measured using the output light emitted from the first output port;
A multipass cell containing the substance to be measured;
A first photodetector for detecting output light emitted from the first output port that has passed through the multipath cell;
A second photodetector for detecting output light emitted from the second output port;
The intensity of the measurement result of the first photodetector and the measurement result of the second photodetector are aligned, and the measurement result of the second photodetector is subtracted from the measurement result of the first photodetector. A signal processing device for removing interference fringes of the wavelength conversion element;
A light absorption measuring device comprising:   The light absorption measuring apparatus according to claim 1, wherein a reference cell containing the reference substance can be exchanged according to the substance to be measured.   The wavelength conversion element has a waveguide structure composed of lithium niobate (LN), lithium tantalate (LT), LN to which Zn or Mg is added, or a mixed crystal thereof. The light absorption measuring device according to claim 1 or 2.   The signal processing device corrects the shift of the absorption line of the measured substance measured by the first photodetector based on the shift of the absorption line of the reference substance from the measurement result of the second photodetector. The light absorption measuring device according to any one of claims 1 to 3. Two or more semiconductor lasers having different output frequencies;
A multiplexer that multiplexes output light from each of the semiconductor lasers;
A wavelength conversion element made of a nonlinear optical material periodically polarized in which the output light output from the multiplexer is incident;
Branching means for branching the output light output from the wavelength conversion element;
A first output port for outputting the first branched light branched by the branching means;
A reference cell containing a reference material into which the second branched light branched by the branching unit is incident;
A second output port for outputting transmitted light that has passed through the reference cell;
And the reference substance is a substance that is not detected from the result of measuring the substance to be measured using the output light emitted from the first output port.   The wavelength conversion element has a waveguide structure composed of lithium niobate (LN), lithium tantalate (LT), LN to which Zn or Mg is added, or a mixed crystal thereof. The laser light source for light absorption measurement according to claim 5.

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