JP2018017989A - Middle infrared laser source and laser spectroscopy apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a light source causing less fluctuations in laser beam output and suitable for absorption spectrum.SOLUTION: A middle infrared laser source comprises: an excitation light source 100; a signal light source 101; a signal generator 102 modulating the wavelength of the signal light source 101; a wavelength conversion element 105 including an optical waveguide composed of a periodically polarization-inverted two-dimensional nonlinear optical material; a photodetector 115 detecting light passing through a reference cell 110 filled with reference substance; a lock-in amplifier 116 extracting a specific frequency component from the detection signal of the photodetector 115; and a PID control circuit 117 performing feedback control to the signal light source 101 based on the component extracted by the lock-in amplifier 116. The light source modulating the wavelength and the light source controlling the wavelength based on the optical signal passing through the reference cell 110 are unified into the signal light source 101 outputting the wavelength with lower end surface reflection rate of the wavelength conversion element 105.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a mid-infrared laser light source using a wavelength conversion element, and more particularly to a mid-infrared laser light source suitable for absorption spectroscopy with little fluctuation in laser light output, and a laser spectroscopic device using the mid-infrared laser light source. Is.

From the viewpoint of environmental protection and health and safety, 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. Therefore, in particular, in the mid-infrared region having a wavelength of 2 μm or more, there is an increasing expectation for optical absorption measurement capable of measuring gas absorption lines with high sensitivity.

  As a light source capable of stably outputting CW (Continuous Wave) light at room temperature in a wavelength region of 2 μm band to 5 μm band, a light source using difference frequency generation by a quasi-phase matching wavelength conversion element that periodically modulates a nonlinear optical constant is used. Many publications have been made (for example, see Non-Patent Document 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.

In order to form a periodic modulation structure with a nonlinear constant, a method of alternately inverting the sign of the nonlinear constant, or substantially alternately arranging a portion with a large nonlinear constant and a portion with a small nonlinear constant can be considered. In a ferroelectric crystal such as LiNbO ₃ , the sign of the nonlinear constant can be reversed by inverting the spontaneous polarization because the sign of the nonlinear constant corresponds to the polarity of the spontaneous polarization.

FIG. 12 shows a configuration of a wavelength conversion element based on difference frequency generation. A wavelength conversion element 10 shown in FIG. 12 includes a LiNbO ₃ substrate 11 on which an optical waveguide 12 is formed, an optical multiplexer 15, and two semiconductor lasers (not shown) that output signal light 13 and pump light 14, respectively. ). The signal light 13 and the excitation light 14 are multiplexed by the optical multiplexer 15 and are incident on the optical waveguide 12 formed on the LiNbO ₃ substrate 11 whose polarization is periodically inverted. In the optical waveguide 12, converted light (idler light) 16 that is a difference frequency light between the signal light 13 and the excitation light 14 is generated. Assuming that the wavelength of the excitation light is λa, the wavelength of the signal light is λb, and the wavelength of the converted light is λc, these three wavelengths satisfy the following formula (1).
1 / λc = 1 / λa−1 / λb (1)

For example, if the excitation light wavelength λa is 1.06 μm and the signal light wavelength λb is 1.56 μm, mid-infrared light with a converted light wavelength λc = 3.31 μm can be generated.
FIG. 13 shows a conventional difference frequency generation (wavelength conversion) light source using a wavelength conversion element. This light source is composed of the wavelength conversion element 10, the optical multiplexer 15, the excitation light source 17, and the signal light source 18 described with reference to FIG. When the pumping light wavelength λa is fixed and the signal light wavelength λb is changed, the converted light wavelength λc changes according to the equation (1). Alternatively, when the signal light wavelength λb is fixed and the pumping light wavelength λa is changed, the converted light wavelength λc is similarly changed. Utilizing this fact, the converted light wavelength λc is varied to observe the shape of the absorption line to be measured in the mid-infrared region.

  When observing a gas with a minute concentration, the absorption intensity may be 1% or less. If the absorption intensity falls below 1%, it becomes difficult to accurately grasp the absorption line intensity due to noise and fluctuations in the baseline. Therefore, wavelength modulation spectroscopy (WMS) for observing the second derivative component of the absorption line described in Non-Patent Document 2 and Non-Patent Document 3 is used. In this method, a minute sine wave modulation of the frequency f is applied to the wavelength of the laser beam used for observation, and a component that responds to the modulation at a frequency 2f that is twice the modulation frequency is observed by a lock-in amplifier. is there.

  In many cases, the modulation of laser light is performed by changing the injection current of a distributed feedback laser diode (DFB-LD). In order to observe the absorption line shape, it is necessary to observe the wavelength dependence of the absorption line. Therefore, in addition to a minute sine wave of frequency f, a sawtooth wave or a triangular wave having a large amount of change is superimposed and DFB- Modulates the current injection amount of the LD.

  14 and 15 show a general light source configuration when wavelength modulation spectroscopy is performed in a difference frequency light source. The light source includes a wavelength conversion element 10, an optical multiplexer 15, an excitation light source 17, a signal light source 18, a fiber amplifier 19, a signal generator 21, a photodetector 22, and a lock-in amplifier 23. , And a density calculation / display unit 24. The excitation light source 17 is provided with a fiber amplifier 19 for enhancing the light output.

  In the configuration of FIG. 14, the signal generator 21 modulates the injection current to the pumping light source 17 with a sine wave signal of the frequency f, so that the pumping light output from the pumping light source 17 has a minute sine of the frequency f. Apply wave modulation. The optical multiplexer 15 multiplexes the excitation light amplified by the fiber amplifier 19 and the signal light from the signal light source 18. When the combined light of the excitation light and the signal light is incident on the wavelength conversion element 10, converted light that is the difference frequency light between the excitation light and the signal light is generated as described above, and this converted light is applied to the measurement target 20 (gas). Is done.

  The photodetector 22 detects the light transmitted through the measuring object 20 and converts it into an electrical signal. The lock-in amplifier 23 extracts a frequency 2f component from the detection signal output from the photodetector 22 in accordance with the frequency 2f reference signal output from the signal generator 21. The concentration calculation / display unit 24 calculates the concentration of the measurement target 20 based on the component extracted by the lock-in amplifier 23 and displays the calculation result.

  On the other hand, in the example of FIG. 15, the signal generator 21 modulates the current injected into the signal light source 18 with a sine wave signal having the frequency f, so that the signal light output from the signal light source 18 has a minute frequency f. Apply sine wave modulation. Thus, when there are two light sources that can be modulated, either one may be modulated.

  Further, as described above, since it is necessary to measure the wavelength dependence of the absorption line of the measurement target 20, either the injection current to the excitation light source 17 or the injection current to the signal light source 18 is a sawtooth having a large amount of change. Either the excitation light or the signal light is swept in wavelength by modulating with a wave or a triangular wave. As described above, the sawtooth wave or the triangular wave for measuring the wavelength dependence of the absorption line of the measurement target 20 may be superimposed on the injection current of one of the two light sources, and the same light source as the sine wave modulation. It may be a different light source.

  As described above, there is a method of locking the wavelength of the light source as an example of concentration measurement different from the method of acquiring the target absorption spectrum by sweeping the wavelength of the light source and detecting the peak value of the absorption line. The spectral method described in FIG. 14 and FIG. 15 captures a change in the shape of the absorption line and identifies the peak position of the absorption line. On the other hand, the wavelength lock method checks the peak position of the absorption line in advance and determines the laser wavelength. Is locked (fixed) at the peak position and the peak value is continuously monitored.

  Compared with the spectrum method, this wavelength lock method is advantageous for data integration because only the peak value of the absorption line is monitored, and has an advantage that random noise can be easily reduced by statistical processing. In this case, it is important that the laser wavelength does not deviate from the peak position. As an example of countermeasures against this wavelength deviation, as disclosed in Patent Document 1, the wavelength of each component LD is set using an etalon. There is an etalon lock method to lock. Since the intensity of the light transmitted through the etalon changes at a frequency (wavelength) period determined by the refractive index of the etalon medium and the resonator length, a method of fixing the wavelength by performing feedback control to maintain a specific intensity It is. A light source configuration employing the etalon locking method is shown in FIG.

  In the configuration of FIG. 16, a part of the excitation light output from the excitation light source 17 is extracted by the optical demultiplexer 25, and this light is incident on the etalon 26. The PID / control driver 27 controls the injection current to the excitation light source 17 so that the wavelength of the excitation light is stabilized based on the intensity of the light transmitted through the etalon 26. Similarly, part of the signal light output from the signal light source 18 is taken out by the optical demultiplexer 28, and this light is incident on the etalon 29. The PID / control driver 30 controls the injection current to the signal light source 18 so that the wavelength of the signal light is stabilized based on the intensity of the light transmitted through the etalon 29. In the configuration of FIG. 16, the phase modulator 31 performs sine wave modulation of the frequency f on the signal light in accordance with the sine wave signal of the frequency f from the signal generator 21.

  However, in the configuration shown in FIG. 16, it takes time to adjust the wavelength lock points of the excitation light source 17 and the signal light source 18 so as to match the wavelength of the absorption line of the gas to be measured. In addition, since the wavelength lock point is determined by the etalons 26 and 29, even if the characteristics of the etalons 26 and 29 change and the wavelength lock point deviates from the target gas absorption line, It becomes impossible to detect and correct the deviation.

  In order to avoid such a problem, it is desirable to lock the wavelength of the light source in accordance with the absorption line of the gas. As a method for that purpose, as described in Non-Patent Document 4, the optical path is branched into two, one is an optical path that transmits the gas to be measured, and the other is an optical path that transmits a known reference gas, and then the reference gas A method is known in which feedback control is performed using the absorption line of the light to lock the wavelength of the light source.

JP, 2015-76467, A

DGLancaster 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”, Optics Letters, Vol. 24, No. 23 , P.1744-1746, 1999 O.Tadanaga, M.Asobe, Y.Nishida, H.Miyazawa, K.Yoshino, H.Suzuki, “763-nm Laser Light Source for Oxygen Monitoring Using Second Harmonic Generation in Direct-Bonded Quasi-Phase-Matched LiNbO3 Ridge Waveguide ”IEICE Trans. Electron, Vol. E89-C, No. 7, p. 1115-1117, 2006 I. Linnerud et al., “Gas monitoring in the process industry using diode laser spectroscopy”, Applied Physics B, Vol. 67, p.297-305, 1998. “Near-infrared light sensing technology for biological / environmental measurement”, published by Science Forum Inc., ISBN4-916164-24-5, pp. 269-276, 1999

  In the light source widely used in the near infrared region, there is one DFB-LD constituting the light source, and the configuration of the light source is easy to understand and easy. However, in the wavelength conversion light source using difference frequency generation, since there are a plurality of DFB-LDs constituting the light source, it is not possible to employ a simple wavelength locking method as in Non-Patent Document 4. The knowledge about how to use a plurality of LDs to fix the laser light wavelength at the peak position of the absorption line is desirable for absorption spectroscopy.

An object of the present invention is a light source that employs a method of locking the output wavelength to the wavelength of the absorption line to be measured, which is advantageous for data integration in view of the above-described problems of the prior art. It is an object of the present invention to provide a mid-infrared laser light source suitable for absorption spectroscopy with little fluctuation in laser light output.
It is another object of the present invention to provide a laser spectroscopic device capable of measuring density with higher sensitivity than conventional.

The mid-infrared laser light source of the present invention includes a wavelength conversion element including an optical waveguide made of a second-order nonlinear optical material whose polarization is periodically inverted, first and second semiconductor lasers having different output wavelengths, and the first A wavelength modulation unit that modulates the wavelength of one semiconductor laser; an optical multiplexer that multiplexes laser beams output from the first and second semiconductor lasers and enters the wavelength conversion element; and the wavelength conversion element A reference cell in which a reference material for locking the wavelength of the laser beam output from the laser beam is enclosed, and the laser beam output from the wavelength conversion element are branched and one of them is introduced into an optical path incident on the reference cell. Based on the optical branching means and the optical signal transmitted through the reference cell, a first wavelength for controlling the wavelength of the first semiconductor laser so that the wavelength of the laser light output from the wavelength conversion element becomes a desired wavelength. Feedback control means A semiconductor laser that modulates the wavelength and a semiconductor laser that controls the wavelength based on an optical signal output that has passed through the reference cell, and outputs the wavelength with the lower end face reflectance of the wavelength conversion element. It is characterized by unifying lasers.
In addition, one configuration example of the mid-infrared laser light source of the present invention is further branched by an optical demultiplexer for branching a part of the laser light output from the second semiconductor laser, and the optical demultiplexer. Second feedback control means for controlling the wavelength of the laser light output from the second semiconductor laser to be a desired wavelength based on the etalon having the input light as input and the optical signal transmitted through the etalon Are provided.

In addition, a mid-infrared laser light source of the present invention includes an optical waveguide made of a second-order nonlinear optical material whose polarization is periodically inverted, and a first wavelength conversion element for obtaining output light of the mid-infrared laser light source; A second wavelength conversion element having the same structure as the first wavelength conversion element, first and second semiconductor lasers having different output wavelengths, and wavelength modulation means for modulating the wavelength of the first semiconductor laser; First and second optical demultiplexers for branching the optical outputs of the first and second semiconductor lasers, respectively, and one light of the first semiconductor laser branched by the first optical demultiplexer A first optical multiplexer that causes the light combined with the output and one optical output of the second semiconductor laser branched by the second optical demultiplexer to be incident on the first wavelength conversion element; The wavelength of the laser beam output from the second wavelength conversion element is locked. A reference cell in which a reference material for sealing is encapsulated, wavelength shift means for changing a wavelength of the other optical output of the second semiconductor laser branched by the second optical demultiplexer by a predetermined amount, and the first A second wavelength conversion element that makes the second wavelength conversion element incident light that combines the other optical output of the first semiconductor laser branched by the first optical demultiplexer and the optical output of the wavelength shift means; Based on an optical multiplexer and an optical signal output from the second wavelength conversion element and transmitted through the reference cell, the wavelength of the laser light output from the second wavelength conversion element becomes a desired wavelength. First feedback control means for controlling the wavelength of the first semiconductor laser, and a semiconductor laser for controlling the wavelength based on an optical signal output transmitted through a reference cell and a semiconductor laser for modulating the wavelength. Low end face reflectance of wavelength conversion element Is characterized in that unifying said first semiconductor laser for outputting a square wave of.
Moreover, in one configuration example of the mid-infrared laser light source of the present invention, the amount of change in wavelength by the wavelength shift means is the wavelength of the laser light output from the first wavelength conversion element and the absorption line of the reference substance. The difference from the wavelength is set to a desired value.
In addition, one configuration example of the mid-infrared laser light source of the present invention further includes a third optical demultiplexer for branching a part of the laser light output from the second semiconductor laser, and the third light. Based on the etalon that receives the light branched by the branching filter and the optical signal transmitted through the etalon, control is performed so that the wavelength of the laser light output from the second semiconductor laser becomes a desired wavelength. And a second feedback control means.

In one configuration example of the mid-infrared laser light source of the present invention, the first feedback control means includes a photodetector that detects an optical signal transmitted through the reference cell, and a detection output from the photodetector. Of the signals, a lock-in amplifier that extracts a signal having the same or three times the modulation frequency of the first semiconductor laser, and an injection current of the first semiconductor laser based on the signal extracted by the lock-in amplifier. And a control circuit for controlling.
In one configuration example of the mid-infrared laser light source of the present invention, the optical waveguide includes lithium niobate, lithium tantalate, lithium niobate to which Zn or Mg is added, and lithium tantalate to which Zn or Mg is added. Or a mixed crystal of these materials.
Further, the laser spectroscopic device of the present invention comprises a mid-infrared laser light source and a concentration calculation means for calculating the concentration of the measurement object based on an optical signal output from the mid-infrared laser light source and transmitted through the measurement object. It is characterized by comprising.

  According to the present invention, a wavelength conversion element, a first and a second semiconductor laser, a wavelength modulation means, an optical multiplexer, a reference cell, an optical branching means, and a first feedback control means are provided, and the semiconductor laser modulates the wavelength. And integrating the measurement data by unifying the semiconductor laser that controls the wavelength based on the optical signal output transmitted through the reference cell into the first semiconductor laser that outputs the wavelength having the lower end face reflectance of the wavelength conversion element. In a mid-infrared laser light source that employs a method that locks the laser wavelength to the wavelength of the absorption line to be measured, it is possible to realize a configuration suitable for absorption spectroscopy with little fluctuation of the laser light output when the wavelength is locked. . As a result, by using the mid-infrared laser light source of the present invention, it is possible to realize a laser spectroscopic device that can measure the concentration with higher sensitivity than before, and detect a minute amount of material that could not be measured conventionally. Measurement in a narrower space is possible.

  Further, in the present invention, by providing the optical demultiplexer, the etalon, and the second feedback control means, it is possible to realize a laser spectroscopic device capable of measuring the concentration with higher accuracy.

  In the present invention, the first and second wavelength conversion elements, the first and second semiconductor lasers, the wavelength modulating means, the first and second optical demultiplexers, the first and second optical multiplexers, A reference cell, a wavelength shift unit, and a first feedback control unit are provided, and a semiconductor laser that modulates the wavelength and a semiconductor laser that controls the wavelength based on the optical signal output transmitted through the reference cell are connected to the second wavelength conversion element. By unifying the first semiconductor laser that outputs the wavelength with the lower end face reflectivity, even if the reference material in the reference cell is different from the measurement target, the wavelength of the mid-infrared laser light source is adjusted to the absorption line of the measurement target. It becomes possible to lock to the wavelength, and it is possible to realize a mid-infrared laser light source suitable for absorption spectroscopy with less fluctuation of the laser light output at the time of wavelength locking. As a result, by using the mid-infrared laser light source of the present invention, it is possible to realize a laser spectroscopic device that can measure the concentration with higher sensitivity than before, and detect a minute amount of material that could not be measured conventionally. Measurement in a narrower space is possible.

It is a block diagram which shows the structure of the laser spectrometer which concerns on the 1st Embodiment of this invention. It is a figure which shows the spectrum of the signal for the wavelength lock | rock of the laser light source in the 1st Embodiment of this invention. It is a figure which shows the spectrum of 2f signal when the wavelength of the excitation light by the side where feedback control is not performed in the 1st Embodiment of this invention has shifted | deviated. It is a figure which shows the time change of 2f signal at the time of performing wavelength lock in the 1st Embodiment of this invention. It is a figure which shows the spectrum of 2f signal when the wavelength of the excitation light by which the feedback control is not performed shifts in the case where the light source that performs wavelength modulation and the light source that performs feedback control are separated. It is a figure which shows the time change of 2f signal at the time of performing a wavelength lock in the case where the light source which performs wavelength modulation, and the light source which performs feedback control are isolate | separated. It is a figure which shows the analytical curve created by measuring various concentrations of gas using the laser spectrometer according to the first embodiment of the present invention. It is a figure which shows the analytical curve produced by measuring gas of various density | concentrations using the laser spectrometer which isolate | separated the light source which performs wavelength modulation, and the light source which performs feedback control. It is a block diagram which shows the structure of the laser spectrometer which concerns on the 2nd Embodiment of this invention. It is a figure which shows one example of the relationship between the wavelength of the light which injects into an etalon, and the output of the photodetector which detects the light after etalon transmission. It is a block diagram which shows the structure of the laser spectrometer which concerns on the 3rd Embodiment of this invention. It is a figure which shows the structure of the wavelength conversion element based on difference frequency generation | occurrence | production. It is a block diagram which shows the structure of the conventional difference frequency generation light source using a wavelength conversion element. It is a block diagram which shows the general light source structure in the case of performing wavelength modulation spectroscopy in a difference frequency light source. It is a block diagram which shows the other structure of the general light source in the case of performing wavelength modulation spectroscopy in a difference frequency light source. It is a block diagram which shows the structure of the conventional light source which employ | adopted the wavelength locking method using an etalon.

[First Embodiment]
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 shows the configuration of a laser spectroscopic apparatus according to the first embodiment of the present invention.
The laser spectroscopic device detects a light collected by the condensing lens 112, a condensing lens 112 that condenses light transmitted through the measurement target cell 109 in which a gas to be measured is sealed, and a mid-infrared laser light source. And a lock-in amplifier 118 for extracting a component of frequency 2f in accordance with a reference signal of frequency 2f output from a signal generator, which will be described later, among the detection signals output from the photodetector 114, and a lock A concentration calculation / display device 119 (concentration calculation means) is provided that calculates the concentration of the gas to be measured based on the component extracted by the in-amplifier 118 and displays the calculation result.

  The mid-infrared laser light source of this embodiment includes an excitation light source 100 made of a DFB type semiconductor laser, a signal light source 101 also made of a DFB type semiconductor laser, and a sine wave signal having a frequency f to the signal light source 101. The signal generator 102 (wavelength modulation means) to be superimposed on the injection current, the fiber amplifier 103 that amplifies the pumping light from the pumping light source 100, the optical multiplexer 104 including an optical coupler, and the like are periodically poled. A wavelength conversion element 105 having an optical waveguide made of a second-order nonlinear optical material, a collimator 106 that collimates the light output from the wavelength conversion element 105, an optical filter 107 for removing unnecessary light, and a wavelength A branch mirror 108 for branching the light output from the conversion element 105, a mirror 111 for reflecting one light branched by the branch mirror 108, A light collecting lens 113 that condenses the light transmitted through the reference cell 110 encapsulating the reference material, a light detector 115 that detects light collected by the light condensing lens 113, and the light output from the light detector 115. Of the detection signals, a lock-in amplifier 116 that extracts a frequency 3f component in accordance with a frequency 3f reference signal output from the signal generator 102, and feedback to the signal light source 101 based on the component extracted by the lock-in amplifier 116. And a PID control circuit 117 that performs (feedback) control.

  The branch mirror 108 constitutes an optical branching unit. The photodetector 115, the lock-in amplifier 116, and the PID control circuit 117 constitute a first feedback control means.

  Excitation light output from the excitation light source 100 is amplified by the fiber amplifier 103. The signal generator 102 modulates the injection current of the signal light source 101 with a sine wave signal having a frequency f via a signal light laser driver (not shown), thereby outputting the signal light output from the signal light source 101. Is subjected to minute sine wave modulation of frequency f. The optical multiplexer 104 multiplexes the excitation light amplified by the fiber amplifier 103 and the signal light from the signal light source 101. When the combined light of the excitation light and the signal light enters the wavelength conversion element 105, converted light that is a difference frequency light between the excitation light and the signal light is generated due to the nonlinear optical effect. The structure of the wavelength conversion element 105 is the same as that shown in FIG.

  The collimator 106 squeezes the output light of the wavelength conversion element 105 into parallel light. The optical filter 107 removes unnecessary signal light and excitation light from the output light of the wavelength conversion element 105. The branching mirror 108 branches the converted light that has passed through the optical filter 107 and makes one light incident on the measurement target cell 109. The mirror 111 reflects the other light branched by the branch mirror 108 and makes it incident on the reference cell 110.

The reference cell 110 is a cell for measuring an absorption line that performs wavelength locking, and is usually filled with the same gas as the measurement target as a reference substance, but the reference substance is absorbed at the same wavelength position as the measurement target. A gas having a line may be used, and a gas different from the measurement target may be used.
The condenser lens 112 collects the light transmitted through the measurement target cell 109 and causes the light to enter the photodetector 114. The condenser lens 113 collects the light transmitted through the reference cell 110 and causes the light to enter the photodetector 115. The photodetectors 114 and 115 each convert incident light into an electrical signal.

  In the example of FIG. 1, the fiber amplifier 103 for amplifying the pumping light is provided as the optical amplifier, but an optical amplifier for amplifying the signal light may be provided. In the example of FIG. 1, the collimator 106 and the optical filter 107 are provided after the wavelength conversion element 105, and the condensing lenses 112 and 113 are provided in front of the photodetectors 114 and 115. However, these are indispensable configurations. There is no need to provide these configurations.

  In the configuration of FIG. 1, the wavelength of the signal light source 101 is controlled to lock the wavelength of the mid-infrared laser light source, and the signal of the reference cell optical path is used for this control. The signal in the reference cell optical path is detected by the photodetector 115. The lock-in amplifier 116 extracts a frequency component corresponding to the reference signal output from the signal generator 102 from the detection signal output from the photodetector 115. The PID control circuit 117 performs feedback control for locking the wavelength of the signal light source 101 by controlling the injection current of the signal light source 101 based on the component extracted by the lock-in amplifier 116.

  A feedback control method that is usually used frequently will be described. In the measurement of minute absorption, 2f detection in which a frequency response component of 2f that is a second harmonic of the modulation frequency f is detected by a lock-in amplifier is widely used. The 2f signal has an upwardly convex peak structure at the wavelength position of the gas absorption line. On the other hand, for feedback control for wavelength locking of the mid-infrared laser light source, 1f detection for detecting at the modulation frequency f or 3f detection for detecting a frequency response component of 3f that is a third harmonic thereof is used. Although 1f detection is easy to detect because the signal is large, it is essential to adjust the offset, and 3f detection is also widely used because it requires a separate device.

  Hereinafter, the mechanism of wavelength locking will be described assuming that 2f detection is performed in the absorption measurement of the measurement target and 3f detection is performed in the feedback control. That is, when the reference signal of the frequency 2f is supplied from the signal generator 102 to the lock-in amplifier 118, the lock-in amplifier 118 extracts the component of the frequency 2f from the detection signal output from the photodetector 114. In addition, when the reference signal of frequency 3f is supplied from the signal generator 102 to the lock-in amplifier 116, the lock-in amplifier 116 extracts the component of frequency 3f from the detection signal output from the photodetector 115. When the 1f component is extracted, a reference signal having a frequency of 1f is supplied from the signal generator 102 to the lock-in amplifier 116, and the lock-in amplifier 116 has a frequency out of the detection signals output from the photodetector 115. What is necessary is just to take out the 1f component.

  In the present embodiment, it is assumed that the same gas type as that of the measurement object is sealed in the reference cell 110 in order to simplify the apparatus configuration. FIG. 2 shows the spectrum of the 2f signal extracted by the lock-in amplifier 118 and the 3f signal extracted by the lock-in amplifier 116. In FIG. 2, 200 is the spectrum of the 2f signal, and 201 is the spectrum of the 3f signal. In FIG. 2, the vertical axis represents signal intensity, and the horizontal axis represents wavelength.

  Reference numeral 202 denotes the wavelength of the absorption line to be measured. At the wavelength of this absorption line, the 2f signal peaks as described above, and the intensity of the 3f signal is zero. That is, if the wavelength of the mid-infrared laser light source is equal to the wavelength of the absorption line to be measured, the 3f signal is zero, and the wavelength of the mid-infrared laser light source is longer or shorter than the wavelength of the absorption line to be measured. When shifted to the side, the 3f signal takes a positive or negative value in proportion to the amount of shift. Therefore, if feedback control is performed based on the value of the 3f signal, control proportional to the wavelength shift of the mid-infrared laser light source can be realized, so that the mid-infrared laser is positioned at the wavelength position of the absorption line to be measured. The wavelength of the light source can be locked.

  As described above, when using difference frequency generation, there are two types of semiconductor lasers for excitation light and signal light. Which semiconductor laser is modulated and which is subjected to feedback control? Therefore, the configuration method of the mid-infrared laser light source is not uniquely determined.

  In the present embodiment, as a configuration of the mid-infrared laser light source, a semiconductor laser that performs wavelength modulation of frequency f and a semiconductor laser that performs feedback control are a semiconductor laser having a low end face reflectance of the wavelength conversion element 105. It has the characteristic of unifying. In other words, either the pumping light source side or the signal light source side may be used, but the semiconductor laser having the lower end face reflectivity of the wavelength conversion element 105 is subjected to wavelength modulation of the frequency f and simultaneously feedback control for locking the wavelength. Apply.

  A mechanism in which the present embodiment has excellent features will be described below. In the following description, modulation and feedback control are performed on the signal light source 101 as shown in FIG. 1, but the same description applies to the case of the excitation light source 100.

  Considering the influence that the wavelength of the mid-infrared laser light source is shifted with respect to the wavelength of the absorption line to be measured, the influence on the 2f signal is as follows. When the wavelength conversion element 105 using a nonlinear optical crystal is used, reflection occurs between both end faces of the element, thereby causing a variation in an optical signal having a wavelength dependency called fringe (interference fringe). Fringe is generated when the wavelength changes in all wavelength bands of pump light, signal light, and converted frequency (idler) light. Even if the influence of the fringe is small as the fluctuation of the optical signal itself, it has a large influence when it is detected as a differential signal after wavelength modulation. When modulation is applied to the signal light, the fringe that the differential signal (2f signal or 3f signal) is greatly affected is the fringe derived from the difference frequency light and the fringe generated by converting the fringe derived from the signal light into the difference frequency light. It is an ingredient.

  First, when the wavelength of the signal light on the side on which feedback control for locking the wavelength is deviated from a desired wavelength, the feedback control to the signal light source 101 is performed so as to correct the deviation. Does not occur. However, when the wavelength of a semiconductor laser different from the semiconductor laser for which feedback control is performed is shifted, the differential signal is characterized.

  FIG. 3A is a diagram illustrating a spectrum of the 2f signal when the wavelength of the excitation light on the side where feedback control is not performed in this embodiment is appropriate, and FIG. 3B is a diagram in which the wavelength of the excitation light is desired. FIG. 3C is a diagram showing the spectrum of the 2f signal when the wavelength of the excitation light is shifted to the short wave side with respect to the desired wavelength. It is. A straight line 300 parallel to the horizontal axis is a guide line indicating a peak position.

  The wavelength of the absorption line to be measured is unchanged if the temperature and pressure are constant. When the wavelength of the excitation light source 100 is deviated, the same difference frequency light (idler light) as the original is generated by changing the wavelength of the signal light source 101 on the side where feedback control is performed based on the relationship of the expression (1). Therefore, the wavelength is the same when the difference frequency light is considered as a reference, but the peak position of the 2f signal is different from the wavelength of the original signal light when the signal light is considered as a reference. Therefore, when the spectrum of the 2f signal is represented by the signal light wavelength, the signal light wavelength at which the peak occurs in the 2f signal is changed from the state of FIG. 3A to the state of FIG. The state or the state shown in FIG.

  Here, the effect of wavelength shift will be considered. The influence of the shift of the wavelength of the pumping light is that the optical signal fluctuates due to fringe, but since the pumping light is not subjected to wavelength modulation, the influence on the 2f signal or the 3f signal is small. Further, since the wavelength of the difference frequency light is not shifted by feedback control to the signal light source 101, it is not affected by the fringe derived from the difference frequency light.

  The signal light wavelength is shifted as shown in FIG. 3B or FIG. 3C by feedback control. At this time, since the wavelength modulation is performed, the fringe derived from the signal light (the fringe derived from the signal light is The 2f signal and the 3f signal output are affected by the fringe generated by the conversion to the difference frequency light. That is, the 2f signal or the 3f signal output fluctuates depending on how the signal peak position and the output fluctuation due to fringe overlap. The fluctuation of the 3f signal leads to the shift of the lock wavelength, that is, the shift of the wavelength of the mid-infrared laser light source with respect to the absorption line peak wavelength, and the fluctuation of the 2f signal as the detection signal leads to the fluctuation of the measured concentration. Therefore, each deviation causes a measurement error.

  However, in the present embodiment, modulation and feedback control are performed on the semiconductor laser (the signal light source 101 in the example of the present embodiment) on the side having the lower end face reflectance of the wavelength conversion element 105, so that the difference frequency light The effect of fringes converted to is minimized. FIG. 4 is a diagram showing a time change of the 2f signal output normalized by the incident light power of the measurement target optical path when wavelength locking is performed. According to FIG. 4, although the output fluctuation due to the fluctuation of the wavelength of the excitation light is seen, the intensity of the 2f signal is kept almost constant, and the mid-infrared laser light source is controlled by feedback control to the signal light source 101. The effect of locking the wavelength appears.

  Next, unlike the present embodiment, a case will be described in which a semiconductor laser that performs wavelength modulation at a frequency f and a semiconductor laser that performs feedback control of wavelength lock are separated. Here, the wavelength modulation is applied to the excitation light source and the feedback control is applied to the signal light source. Conversely, the same applies to the case where the wavelength modulation is applied to the signal light source and the feedback control is applied to the excitation light source. The explanation applies.

  When a semiconductor laser that performs wavelength modulation and a semiconductor laser that performs feedback control of wavelength lock are separated, a difference from the present embodiment occurs when the wavelength of an excitation light source that is not a light source for which feedback control is performed is shifted. FIG. 5A is a diagram showing the spectrum of the 2f signal when the wavelength of the excitation light on the side where feedback control is not performed is appropriate when the semiconductor laser that performs wavelength modulation and the semiconductor laser that performs feedback control are separated. FIG. 5B is a diagram showing the spectrum of the 2f signal when the wavelength of the excitation light is shifted to the long wave side with respect to the desired wavelength, and FIG. 5C is a short wave with respect to the desired wavelength of the excitation light. It is a figure which shows the spectrum of 2f signal at the time of shifting to the side. As in the case of FIGS. 3A to 3C, the straight line 300 parallel to the horizontal axis is a guide line indicating the peak position.

  The wavelength of the absorption line to be measured is unchanged if the temperature and pressure are constant. Similar to the present embodiment, when the wavelength of the excitation light source is shifted, based on the relationship of Expression (1), the wavelength of the signal light source on the side where feedback control is performed is changed to obtain the same difference frequency light as the original. (Idler light) is generated, so the wavelength is the same when considering the difference frequency light, but the peak position of the 2f signal is different from the wavelength of the original signal light when considering the signal light as a reference. ing. Therefore, when the spectrum of the 2f signal is represented by the signal light wavelength, the signal light wavelength at which the 2f signal has a peak is changed from the state of FIG. 5A to the state of FIG. 5B when the excitation light wavelength is shifted. The state or the state shown in FIG.

  Here, the influence of wavelength shift when a semiconductor laser that performs wavelength modulation and a semiconductor laser that performs feedback control of wavelength lock are separated will be considered. The influence of the shift of the wavelength of the excitation light is different from the present embodiment because the excitation light source is subjected to wavelength modulation. That is, in this case, since the excitation light source is subjected to wavelength modulation, the fringe component converted into the difference frequency light affects the 2f signal or the 3f signal. Since the wavelength modulation is performed around the fixed point of the injection current to the excitation light source, it is uncorrelated with the signal light wavelength, and the influence on the 2f signal or the 3f signal does not depend on the signal light wavelength.

  Also, the influence of the wavelength shift of the excitation light is superimposed on the 2f signal or 3f signal in the form of averaging the influence of the fringe derived from the excitation light that is converted into the difference frequency light that exists between the wavelengths that are fluctuating due to the modulation. Therefore, the influence on the spectrum in terms of the signal light wavelength appears in the offset of the spectrum intensity. When the wavelength of the excitation light is deviated, the fringe average is changed, and this wavelength deviation appears in the form of vertical fluctuation of the offset (translation of the spectral intensity). In the spectra of FIGS. 5A to 5C, the influence of the wavelength shift of the excitation light appears.

  Therefore, the offset of the 2f signal or 3f signal fluctuates, that is, fluctuates up and down by a constant value. However, the fluctuation of the 3f signal leads to the shift of the lock wavelength, and the fluctuation of the 2f signal as the detection signal fluctuates the measured concentration. As a result, the measurement error becomes a large value. On the other hand, since the wavelength of the difference frequency light is not shifted by feedback control to the signal light source, it is not affected by the fringe derived from the difference frequency light. Further, since the signal light is not subjected to wavelength modulation, the influence of the shift of the signal light wavelength that causes a peak in the 2f signal is slight.

  FIG. 6 shows the time change of the 2f signal output normalized by the incident light power of the optical path to be measured when wavelength locking is performed when the semiconductor laser that performs wavelength modulation and the semiconductor laser that performs wavelength lock feedback control are separated. FIG. Since the 2f signal and the 3f signal fluctuate due to the fluctuation of the wavelength of the pumping light, the influence of the shift wavelength shift and the 2f signal fluctuation overlaps, and a large intensity fluctuation is observed in the 2f signal. That is, it can be seen that the effect of locking the wavelength of the mid-infrared laser light source by feedback control to the signal light source is not so great.

  This is an effect of the wavelength shift on the output value of the signal. Usually, a larger output fluctuation occurs when wavelength modulation and feedback control are performed on different semiconductor lasers. This is due to the following reason. When a semiconductor laser that performs wavelength modulation and a semiconductor laser that performs feedback control are separated, the modulation amplitude of wavelength modulation is adjusted to a value that reduces the output fluctuation of fringes derived from mid-infrared light (difference frequency light). In terms of amplitude, fluctuations in fringe output in the wavelength band of excitation light or signal light, which is near infrared light, cannot be reduced. Further, in this adjustment, there is no method for knowing the influence of fringes derived from near-infrared light, and it is difficult to simultaneously adjust the output fluctuations of fringes derived from both near-infrared light and mid-infrared light to be small. For this reason, when the wavelength is shifted, the influence of the fringe is greatly detected, and the output fluctuation of the signal becomes large. On the other hand, when wavelength modulation and feedback control are applied to the same semiconductor laser, the modulation amplitude of wavelength modulation should be adjusted to a value that reduces the output fluctuation of both the fringe derived from mid-infrared light and the fringe derived from near-infrared light. Is relatively easy.

Next, a specific example of the present embodiment will be described. In this embodiment, it is assumed that the laser light source outputs laser light having a wavelength of, for example, 2 μm to 20 μm, but here, in order to measure the concentration of N ₂ O (nitrous oxide) as a measurement target. The 4.6 μm band mid-infrared light is output.

  As the excitation light source 100, a DFB semiconductor laser that outputs laser light having a wavelength near 1.06 μm is used. As the signal light source 101, a DFB semiconductor laser that outputs laser light having a wavelength near 1.39 μm is used. It was used. Since the end face reflectance of the wavelength conversion element 105 in the signal light wavelength band is lower than the end face reflectance in the excitation light wavelength band, feedback control for wavelength modulation and wavelength lock is performed on the signal light source 101. did.

  A YDFA (Ytterbium Doped Fiber Amplifier), which is a Yb-doped fiber amplifier, was used as the fiber amplifier 103 for amplifying 1.06 μm excitation light. The excitation light and the signal light are combined by an optical multiplexer 104 made of, for example, a fiber fusion type optical coupler, and introduced into the wavelength conversion element 105. The combined light is coupled to the optical waveguide by a lens in the wavelength conversion element 105, and is converted into converted light in the 4.6 μm band by the difference frequency generation.

The wavelength conversion element 105 includes an optical waveguide made of Zn-doped lithium niobate (LiNbO ₃ ) having a width of 20 μm, a height of 15 μm, a length of 50 mm, and a polarization inversion period of 26 μm. This optical waveguide was joined with lithium tantalate serving as a base substrate by direct joining using thermal diffusion, and a waveguide structure was produced by a dicing saw. The material, size, and manufacturing method of the element are examples, and are not limited to this example.

  The optical waveguide may be composed of any one of lithium niobate, lithium tantalate, lithium niobate to which Zn or Mg is added, lithium tantalate to which Zn or Mg is added, or a mixed crystal of these materials.

The branch mirror 108 is a half mirror such as a pellicle beam splitter. For example, a reference cell 110 made of quartz is filled with N ₂ O having a pressure of 100 Torr and a concentration of 5%. Similarly, N ₂ O is enclosed in the measurement target cell 109.

  From the converted light output from the wavelength conversion element 105, unnecessary near-infrared light is removed by the optical filter 107 (Ge filter in the example of the present embodiment), branched by the branch mirror 108, and one branched light is The light passing through the measurement target cell 109 is transmitted through the reference cell 110.

In this embodiment, an electronically cooled InSb detector is used as the photodetectors 114 and 115. As the collimator 106 and the condensing lenses 112 and 113, mid-infrared calcium fluoride (CaF ₂ ) lenses were used.
A sine wave signal for wavelength modulation was generated by a signal generator 102 (function generator), and wavelength modulation of the signal light was performed via a laser driver for signal light (not shown). The modulation frequency was 15 kHz.

  As described above, the lock-in amplifier 118 extracts the frequency 2f component from the detection signal output from the photodetector 114. The concentration calculation / display device 119 calculates the concentration of the gas to be measured based on the components extracted by the lock-in amplifier 118. The density calculation / display unit 119 is formed of, for example, a microcomputer, and performs density calculation processing according to a program stored in advance in a memory. Since the density calculation process is a well-known technique, a detailed description thereof is omitted.

  The lock-in amplifier 116 extracts a frequency 3f component from the detection signal output from the photodetector 115. The PID control circuit 117 controls the injection current of the signal light source 101 based on the component extracted by the lock-in amplifier 116. Specifically, the PID control circuit 117 performs feedback control so that the intensity of the 3f signal becomes zero.

When an amplifier is used as the output of the excitation light source 100, 500 mW of light is input to the wavelength conversion element as excitation light, and 50 mW of light as the signal light is input from the signal light source 101 to the wavelength conversion element. As an output from 105, 1 mW of converted light was obtained. This time, in order to measure an absorption line having a wave number of 2205.69 cm ⁻¹ (wavelength 4533.7 nm), the wavelengths of the signal light and the excitation light were adjusted. After adjusting the wavelength to the target absorption line, wavelength locking was performed, and the wavelength of the mid-infrared laser light source was fixed to the wavelength of the absorption line.

A measurement curve was created by encapsulating various concentrations of N ₂ O gas in the measurement target cell 109 as the measurement target. The concentration of the enclosed gas was confirmed by fully calibrated gas chromatography. The results are shown in FIG. The horizontal axis represents the concentration of the sealed gas confirmed by gas chromatography, and the vertical axis represents the concentration of N ₂ O gas measured by the laser spectrometer of the present embodiment. In this embodiment, since the effect of wavelength lock is large, there is little fluctuation of the measured value while averaging the data, and highly accurate concentration measurement is possible, and as a result, a calibration curve with little variation is obtained. I was able to.

  As a comparative example, FIG. 8 shows the result of performing the same calibration curve measurement with the configuration of the laser spectroscopic apparatus when the excitation light source is subjected to wavelength modulation and the signal light source is subjected to feedback control. In this configuration, since the fluctuation of the measured value during the averaging of the data is large, highly accurate data cannot be obtained, and as a result, the calibration curve has a large variation. The variation was about 25% larger than that of the laser spectrometer according to the present embodiment.

  As described above, according to the present embodiment, in the mid-infrared laser light source adopting the method of locking the laser wavelength to the wavelength of the absorption line to be measured, which is advantageous for integrating measurement data, the laser light output at the time of wavelength locking Therefore, it is possible to realize a configuration suitable for absorption spectroscopy. As a result, by using the mid-infrared laser light source of the present embodiment, it is possible to realize a laser spectroscopic device capable of measuring concentration with higher sensitivity than conventional.

[Second Embodiment]
Next, a second embodiment of the present invention will be described. FIG. 9 is a diagram showing a configuration of a laser spectrometer according to the second embodiment of the present invention, and the same components as those in FIG. 1 are denoted by the same reference numerals. Also in the present embodiment, the configuration is such that a 4.6 μm band mid-infrared light for measuring the concentration of N ₂ O (nitrous oxide) is output, and, similarly to the first embodiment, a wavelength conversion element Since the end face reflectance in the signal light wavelength band 105 was lower than the end face reflectance in the excitation light wavelength band, feedback control for wavelength modulation and wavelength lock was performed on the signal light source 101.

  The basic configuration of the laser spectroscopic device is the same as that of the first embodiment. However, as described in the first embodiment, when the wavelength of the excitation light source 100 where feedback control is not performed fluctuates, the signal light source Since the wavelength of 101 also changes accordingly, the laser light output is likely to fluctuate due to the influence of the fringe of the signal light.

  Therefore, in this embodiment, an optical demultiplexer 120, an etalon 121 with a photodetector, and a PID / control driver 122 are added to the mid-infrared laser light source of the first embodiment, and pump light The wavelength of the light source 100 is locked. The photodetector provided in the etalon 121 and the PID / control driver 122 constitute second feedback control means.

  For example, the optical demultiplexer 120 having a branching ratio of 1:10 branches the pumping light from the pumping light source 100 by a small amount and introduces it into the etalon 121. The etalon 121 has a resonator structure. The light transmitted through the etalon 121 exhibits an output fluctuation of a frequency (wavelength) period determined by the length of the etalon 121, the refractive index in the resonator, and the temperature of the etalon 121 due to the interference effect. The light transmitted through the etalon 121 is detected by a photodetector (not shown) fixed to the etalon 121. FIG. 10 shows an example of the relationship between the wavelength of light incident on the etalon 121 and the output of the photodetector that detects the light after transmission through the etalon.

If PID control is performed using the slope of a part of the output fluctuation of the photodetector (for example, 400 in FIG. 10) in the wavelength transmission characteristic as shown in FIG. 10, the wavelength of the excitation light is set to a desired value. Can be locked to wavelength. Specifically, the PID / control driver 122 may control the injection current of the excitation light source 100 so that the output of the photodetector becomes a desired value.
Other configurations are the same as those described in the first embodiment.

When a calibration curve was created by measuring the concentration of N ₂ O gas by the same method as in the first embodiment, it was possible to measure with higher accuracy than in the first embodiment. It was about 5% smaller than the laser spectroscopic device of the first embodiment.

[Third Embodiment]
Next, a third embodiment of the present invention will be described. FIG. 11 is a diagram showing a configuration of a laser spectroscopic device according to the third embodiment of the present invention. The same components as those in FIG. Also in the present embodiment, the configuration is such that a mid-infrared light of 4.6 μm band for measuring the concentration of N ₂ O (nitrous oxide) is output, and the end face reflectance of the wavelength conversion element 128 in the signal light wavelength band. Is lower than the end face reflectance in the excitation light wavelength band, and therefore, feedback control for wavelength modulation and wavelength lock is performed on the signal light source 101.

The basic configuration of the laser spectroscopic device is the same as that of the first embodiment, but in this embodiment, the gas type in the reference cell 110 is different from the measurement target. Here, CO ₂ having a pressure of 100 Torr and a concentration of 50% was sealed in the reference cell 110.

This time, the absorption line having a wave number of 21877.096 cm ⁻¹ (wavelength 4572.27 nm) is measured, and the wavelengths of the signal light and the excitation light are adjusted. However, the absorption line closest to the absorption line with a wave number of 21877.096 cm ^-1 among the absorption lines of CO _{2 is} the absorption line with a wave number of 2187.156 cm ^-1 (wavelength 4572.15 nm). If the wavelength of the mid-infrared laser light source is locked with the wavelength of the CO ₂ absorption line, it will be locked at a location deviated from the wavelength of the N ₂ O absorption line to be measured.

  Therefore, in the mid-infrared laser light source of the present embodiment, instead of branching the light output from the wavelength conversion element 105 into light that passes through the measurement target cell 109 and light that passes through the reference cell 110, A wavelength shifter 125 that changes the wavelength and a new wavelength conversion element 128 for the reference optical path are provided so that measurement is possible even when the wavelengths of absorption lines of the measurement target gas and the reference gas are different. The structure of the wavelength conversion element 128 is the same as that of the wavelength conversion element 105.

  For example, the optical demultiplexer 124 having a branching ratio of 1: 1 branches the signal light from the signal light source 101 into two. For example, the optical demultiplexer 123 having a branching ratio of 10: 1 branches the excitation light from the excitation light source 100 into two. The excitation light having the smaller branching ratio is amplified by the fiber amplifier 103. The optical multiplexer 104 combines the excitation light amplified by the fiber amplifier 103 and one signal light branched by the optical demultiplexer 124 and introduces the combined light to the wavelength conversion element 105.

On the other hand, the excitation light having the larger branching ratio is introduced into the wavelength shifter 125 that changes the wavelength by the wavelength difference between the N ₂ O absorption line and the CO ₂ absorption line. Thereby, the wavelength of the converted light (idler light) that has been wavelength-converted using the excitation light emitted from the wavelength shifter 125 becomes the same as the wavelength of the CO ₂ absorption line. The excitation light emitted from the wavelength shifter 125 is amplified by the fiber amplifier 126. The optical multiplexer 127 combines the excitation light amplified by the fiber amplifier 126 and the other signal light branched by the optical demultiplexer 124, and introduces the combined light to the wavelength conversion element 128. The reason why the excitation light having the larger branching ratio is introduced into the wavelength shifter 125 is that when the excitation light is introduced into the wavelength shifter 125, a loss occurs in the optical output.

In the present embodiment, the wavelength difference between the N ₂ O absorption line and the CO ₂ absorption line is 0.12 nm. If light having a wavelength of 1.38666935 μm is used as the signal light, the excitation light introduced into the wavelength conversion element 128 may be shifted to the short wavelength side by 0.0065 nm. Therefore, in this embodiment, the amount of wavelength change by the wavelength shifter 125 is set to −0.0065 nm.

By taking such a configuration, a wavelength difference of 0.12 nm is always generated between the difference frequency light output from the wavelength conversion element 105 and the difference frequency light output from the wavelength conversion element 128. If the wavelength of the difference frequency light output from the wavelength conversion element 128 is locked by the wavelength of the CO ₂ absorption line, the wavelength of the difference frequency light output from the wavelength conversion element 105 is the wavelength of the desired absorption line of N ₂ O. Will be locked.

The feedback control according to the present embodiment will be specifically described. The collimator 129 narrows the output light of the wavelength conversion element 128 into parallel light. The optical filter 130 removes unnecessary signal light and excitation light from the output light of the wavelength conversion element 128. The condenser lens 113 collects the light transmitted through the reference cell 110 and causes the light to enter the photodetector 115. The lock-in amplifier 116 extracts a frequency 3f component from the detection signal output from the photodetector 115. The PID control circuit 117 controls the injection current of the signal light source 101 via a signal light laser driver (not shown) so that the intensity of the 3f signal becomes zero.
Other configurations are the same as those described in the first embodiment.

A calibration curve was created by measuring the concentration of N ₂ O gas in the same manner as in the first embodiment. As in the first embodiment, the effect of wavelength locking is great, so the data is averaged. During measurement, the measurement value fluctuated little and accurate measurement was possible. As a result, a calibration curve with little variation was obtained.

  Note that the second embodiment may be applied by adding the optical demultiplexer 120, the etalon 121 with a photodetector, and the PID / control driver 122 shown in FIG. 9 to the present embodiment.

  The present invention can be applied to the technique of absorption spectroscopy.

  DESCRIPTION OF SYMBOLS 100 ... Excitation light source, 101 ... Signal light source, 102 ... Signal generator, 103, 126 ... Fiber amplifier, 104, 127 ... Optical multiplexer, 105, 128 ... Wavelength conversion element, 106, 129 ... Collimator, 107, 130 DESCRIPTION OF SYMBOLS ... Optical filter, 108 ... Branch mirror, 109 ... Measurement object cell, 110 ... Reference cell, 111 ... Mirror, 112, 113 ... Condensing lens, 114, 115 ... Photo detector, 116, 118 ... Lock-in amplifier, 117 ... PID control circuit, 119... Concentration calculator / display unit, 120, 123, 124, optical demultiplexer, 121, etalon, 122, PID / control driver, 125, wavelength shifter.

Claims (8)

A wavelength conversion element including an optical waveguide made of a second-order nonlinear optical material periodically poled;
First and second semiconductor lasers having different output wavelengths;
Wavelength modulating means for modulating the wavelength of the first semiconductor laser;
An optical multiplexer that multiplexes the laser beams output from the first and second semiconductor lasers and enters the wavelength conversion element;
A reference cell in which a reference material for locking the wavelength of the laser beam output from the wavelength conversion element is enclosed;
A light branching means for branching the laser light output from the wavelength conversion element and introducing one into an optical path incident on the reference cell;
First feedback control means for controlling the wavelength of the first semiconductor laser based on an optical signal transmitted through the reference cell so that the wavelength of the laser light output from the wavelength conversion element becomes a desired wavelength; With
The semiconductor laser that modulates the wavelength and the semiconductor laser that controls the wavelength based on the optical signal output transmitted through the reference cell are unified into the first semiconductor laser that outputs the wavelength having the lower end face reflectance of the wavelength conversion element. A mid-infrared laser light source. The mid-infrared laser light source according to claim 1,
An optical demultiplexer for branching a part of the laser light output from the second semiconductor laser;
An etalon with the light branched by this optical demultiplexer as input,
And second feedback control means for controlling the wavelength of the laser light output from the second semiconductor laser to a desired wavelength based on the optical signal transmitted through the etalon. Infrared laser light source. A first wavelength conversion element comprising an optical waveguide made of a second-order nonlinear optical material whose polarization is periodically inverted, and for obtaining output light of a mid-infrared laser light source;
A second wavelength conversion element having the same structure as the first wavelength conversion element;
First and second semiconductor lasers having different output wavelengths;
Wavelength modulating means for modulating the wavelength of the first semiconductor laser;
First and second optical demultiplexers for branching the optical outputs of the first and second semiconductor lasers, respectively;
One optical output of the first semiconductor laser branched by the first optical demultiplexer and one optical output of the second semiconductor laser branched by the second optical demultiplexer are combined. A first optical multiplexer for causing the waved light to enter the first wavelength conversion element;
A reference cell in which a reference material for locking the wavelength of the laser beam output from the second wavelength conversion element is enclosed;
Wavelength shifting means for changing a wavelength of the other optical output of the second semiconductor laser branched by the second optical demultiplexer by a predetermined amount;
A first light that combines the other optical output of the first semiconductor laser branched by the first optical demultiplexer and the optical output of the wavelength shift means is incident on the second wavelength conversion element. 2 optical multiplexers,
Based on the optical signal output from the second wavelength conversion element and transmitted through the reference cell, the first semiconductor has a desired wavelength so that the wavelength of the laser light output from the second wavelength conversion element becomes a desired wavelength. First feedback control means for controlling the wavelength of the laser,
The first semiconductor that outputs a wavelength having a lower end face reflectance of the second wavelength conversion element, a semiconductor laser that modulates the wavelength, and a semiconductor laser that controls the wavelength based on an optical signal output transmitted through the reference cell. A mid-infrared laser light source characterized by unifying lasers. The mid-infrared laser light source according to claim 3,
The amount of change in wavelength by the wavelength shift means is set so that the difference between the wavelength of the laser beam output from the first wavelength conversion element and the wavelength of the absorption line of the reference substance becomes a desired value. A mid-infrared laser light source. The mid-infrared laser light source according to claim 3 or 4,
A third optical demultiplexer for branching a part of the laser light output from the second semiconductor laser;
An etalon that receives light branched by the third optical demultiplexer;
And second feedback control means for controlling the wavelength of the laser light output from the second semiconductor laser to a desired wavelength based on the optical signal transmitted through the etalon. Infrared laser light source. The mid-infrared laser light source according to any one of claims 1 to 5,
The first feedback control means includes
A photodetector for detecting an optical signal transmitted through the reference cell;
A lock-in amplifier that extracts a signal having the same or three times the modulation frequency of the first semiconductor laser among the detection signals output from the photodetector;
A mid-infrared laser light source comprising a control circuit for controlling an injection current of the first semiconductor laser based on a signal taken out by the lock-in amplifier. The mid-infrared laser light source according to any one of claims 1 to 6,
The optical waveguide is composed of any one of lithium niobate, lithium tantalate, lithium niobate to which Zn or Mg is added, lithium tantalate to which Zn or Mg is added, or a mixed crystal of these materials. A mid-infrared laser light source. The mid-infrared laser light source according to any one of claims 1 to 7,
A laser spectroscopic apparatus comprising: a concentration calculation means for calculating a concentration of the measurement target based on an optical signal output from the mid-infrared laser light source and transmitted through the measurement target.