JP2011033941A - Intermediate-infrared light source, and infrared light absorption analyzer using the same - Google Patents

Abstract

A light source that emits light in the mid-infrared region of a wavelength band of 4 μm where absorption is strongest in many environmental gases is realized.
A first laser for generating first excitation light, a second laser for generating second excitation light, the first excitation light and the second excitation light are input, In a mid-infrared light source including a wavelength conversion element made of a nonlinear optical crystal that outputs converted light by generating a difference frequency, the first laser has a first wavelength of any wavelength between 0.97 μm and 1.04 μm. The second laser can vary the wavelength of the second excitation light in a wavelength range between 1.25 μm and 1.36 μm, and the wavelength conversion element Mid-infrared light having a wavelength of 3.5 μm to 5.8 μm is output as converted light.
[Selection] Figure 4

Description

  The present invention relates to an infrared light absorption analyzer capable of optically detecting an environmental gas, human breath, dangerous gas, residual agricultural chemicals, and the like, and a mid-infrared light source used in the infrared light absorption analyzer About.

  From the viewpoint of environmental protection and safety and health, establishment of trace analysis technology such as NOx, SOx, hydrocarbons in general, ammonia-based environmental gases, water absorption peaks, many organic gases, pesticide residues, etc. is strongly desired. It is rare. As trace analysis techniques, there are generally known a method in which a gas to be measured is adsorbed on a specific substance and quantitative analysis is performed by an electrochemical method, and a method in which the inherent optical absorption characteristics of the substance to be measured are measured. ing.

  The method of measuring the optical absorption characteristic has a feature that real-time measurement is possible and a wide area through which the measurement light passes can be observed. The absorption peak of the substance to be measured is mainly in the mid-infrared region of 2 μm to 20 μm due to the vibration rotation mode of the interatomic bond. At present, a laser capable of stable continuous oscillation at room temperature in the mid-infrared region having a wavelength of 2 μm or more has not been realized. Industrially, there is a high need for a laser light source that outputs mid-infrared light, but quantum cascade lasers have only been researched and developed, and the lack of usable laser light sources is a major obstacle.

  Since there is no practical light source in the mid-infrared region, the light source is configured using an existing communication semiconductor laser (0.8 μm to 2 μm). When performing trace analysis of various gases using this light source, overtones of the original fundamental absorption wavelength (= 1/2 of the fundamental absorption wavelength), 3rd overtone (= one third of the fundamental absorption wavelength), these Use the absorption in the combined sound.

D. Richter, et al., “Development of an automated diode-laser-based multicomponent gas sensor,” Applied Optics, Vol.39, No.24, p.4444-4450, 2000 Z. Cao, et al., “Broadband difference frequency generation around 4.2μm at overlapped phase-match conditions,” Opt. Commun. No.281, p.3878-3881, 2008

  However, although the required sensitivity may be obtained if harmonics are used, the amount of absorption itself is small when measured at higher-order absorption peaks of 3 or more harmonics, so that it is three orders of magnitude compared to the measurement at the original basic absorption peak. This will lead to a decrease in sensitivity. Therefore, development of a mid-infrared laser light source is indispensable for obtaining high detection sensitivity when analyzing environmental gases, dangerous gases, and the like.

Recently, it has been reported that mid-infrared light is generated at a wavelength of 3 μm and gas sensor operation has been confirmed (see, for example, Non-Patent Document 1). In these reports, mid-infrared light is generated by difference frequency generation using a lithium niobate (LiNbO ₃ : hereinafter referred to as LN) wavelength conversion element mainly having a periodic modulation structure.

Further, as an example of a wavelength tunable light source in the wavelength 4 μm band, a light source realized by difference frequency generation has been reported as described above. However, since the light source is limited to a light source using a Nd: YAG laser or a Ti: Al ₂ O ₃ laser, which is a solid-state laser (see, for example, Non-Patent Document 2), the disadvantage is that the system is large and not portable. is there.

  An object of the present invention is to provide an infrared light absorption analyzer that dramatically improves sensitivity by realizing a light source that emits light in the mid-infrared region of the wavelength 4 μm band where absorption is strongest in many environmental gases. There is to do. Specifically, using an LN wavelength conversion element, two energy-efficient lasers with high energy conversion efficiency, high wavelength control accuracy, and high power, and two available semiconductor lasers in the wavelength range, A light source that emits light in the mid-infrared wavelength range of 3.5 to 5.8 μm by generating a difference frequency is used. This provides an infrared light absorption analyzer that can detect various gases and the like easily and with high sensitivity.

  In order to achieve such an object, the present invention provides a first laser that generates first excitation light, a second laser that generates second excitation light, the first excitation light, In a mid-infrared light source including a wavelength conversion element made of a nonlinear optical crystal that inputs second excitation light and outputs converted light by generating a difference frequency, the first laser has a wavelength of 0.97 μm to 1. The first pumping light having an arbitrary wavelength between 04 μm is output, and the second laser can change the wavelength of the second pumping light in a wavelength range between 1.25 μm and 1.36 μm. The wavelength conversion element outputs mid-infrared light having a wavelength of 3.5 μm to 5.8 μm as converted light.

  The first laser is a laser that can vary the wavelength of the first excitation light in a wavelength range between 0.97 μm and 1.04 μm, and the second laser has a wavelength of 1.25 μm. By using a laser that outputs the second excitation light having an arbitrary wavelength between 1 and 1.36 μm, more preferably, mid-infrared light having a wavelength between 3.5 μm and 5.8 μm is output as converted light. can do.

The wavelength conversion element is made of LiNbO ₃ crystal and has an optical waveguide structure.

  This mid-infrared light source branches the output light output from the light source, measures the first light intensity using one output light as reference light, guides the other output light to the sample gas, An infrared light absorption analyzer that receives transmitted light, reflected light, scattered light, or fluorescence, measures a second light intensity, and measures the absorption of the sample gas from a ratio of the first and second light intensities. Can be applied.

  As described above, according to the present invention, since the laser used as the excitation light source is obtained from a product in the completed communication wavelength band, 3.5 to 5 is generated by the difference frequency generation using the LN wavelength conversion element. It becomes possible to obtain light in the mid-infrared wavelength range of .8 μm accurately and stably. Therefore, a highly accurate and highly sensitive optical absorption analyzer can be realized in the mid-infrared region of the wavelength 4 μm band where absorption is strongest in many environmental gases.

It is a figure which shows the wavelength dependence of the light absorption intensity of environmental gas. It is a figure which shows the structure of the mid-infrared light source concerning one Embodiment of this invention. It is a figure which shows the relationship between excitation light (pump light and signal light) and converted light (idler light). It is a figure which shows the structure of the infrared-light absorption analyzer using the gas cell concerning Example 1. FIG. It is a figure which shows the generation efficiency of the mid-infrared light source concerning Example 1. FIG. It is a figure which shows the light absorption measurement result by the infrared-light-absorption analyzer concerning Example 1. FIG. It is a figure which shows a prediction spectrum when the atmospheric CO of incomplete combustion warning concentration is detected. It is a figure which shows a prediction spectrum when CO is detected by the optical path of 100 m in air | atmosphere. It is a figure which shows the structure of the infrared-light absorption analyzer using the gas cell concerning Example 2. FIG. It is a figure which shows the wavelength conversion width | variety of the idler light in the case where the wavelength of signal light is made variable, and the case where the wavelength of pump light is made variable. It is a figure which shows the structure of the infrared-light absorption analyzer using the gas cell concerning Example 3. FIG.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
FIG. 1 shows the wavelength dependence of the light absorption intensity of a typical environmental gas. As described above, it can be seen that the absorption intensity of these environmental gases is highest in the mid-infrared region around the 4 μm band. This implies that the frequency of reference vibration (fundamental wave mode fundamental wave) of gas molecules is concentrated in this wavelength band. There is a need for a laser light source that can vary the wavelength around an arbitrary wavelength in the wavelength range of 3.5 to 5.8 μm.

  As described above, the coherent light source available in the above wavelength range is a light source including a quantum cascade laser or a wavelength conversion laser using an LN wavelength conversion element. Conventionally, quantum cascade lasers have not achieved stable room temperature operation at a practical level, and wavelength conversion lasers have been used for controllability as a light source, such as spectral width, wavelength variable operation, and optical amplification techniques. There are difficulties compared to light sources.

  FIG. 2 shows the configuration of a mid-infrared light source according to an embodiment of the present invention. A first laser 1 that outputs first excitation light (pump light), a second laser 2 that outputs second excitation light (signal light), and an LN wavelength conversion element 3 are provided. The output of the first laser 1 is connected to a multiplexer 10 via an optical isolator 6 by an optical fiber 8 on which a fiber Bragg grating (FBG) 5 is formed. The output of the second laser 2 is connected to the multiplexer 10 by the optical fiber 9 through the optical fiber amplifier 4 and the optical isolator 7. The excitation light combined by the multiplexer 10 is input to the LN wavelength conversion element 3 via the lens 11, and converted light (idler light) is output via the lens 12.

  The first laser 1 oscillates in a single longitudinal mode with any wavelength between 970 nm and 1.04 μm. The first laser 1 includes a high reflection film 1A having a reflectivity of 90% or more and a low reflection film 1B having a reflectivity of 2% or less, and the output wavelength is stabilized by a resonator composed of the high reflection film 1A and the FBG 5. Has improved. The second laser 2 can change the wavelength in the wavelength range of 1.25 μm to 1.36 μm. By the difference frequency generation (DFG) due to the nonlinear optical effect in the LN wavelength conversion element 3, coherent converted light (idler light) 13 having a wavelength of 3.5 to 5.8 μm can be obtained. In this way, a coherent mid-infrared light source can be realized by using two semiconductor lasers having a communication wavelength band excellent in controllability.

  FIG. 3 shows the relationship between excitation light (pump light and signal light) and converted light (idler light). When an optical waveguide is formed in the LN wavelength conversion element 3, it is necessary to consider waveguide dispersion. In this wavelength range, almost satisfactory element design is possible only by considering wavelength shift. . Therefore, the design of the polarization inversion period is facilitated by selecting the light of the three wavelengths according to FIG.

When the wavelength of the first pumping light (pump light) is λ ₁ , the wavelength of the second pumping light (signal light) is λ ₂ , and the wavelength of the output converted light (idler light) is λ ₃ , these relations Is

Given in. Here, in order to generate the converted light of wavelength λ ₃ efficiently, the phase matching condition

Need to be satisfied. In Equation (2), k _i (i = 1, 2, 3) is a propagation constant of laser light (excitation light, converted light) propagating in a nonlinear optical crystal such as LN, and the nonlinear optical crystal at λ _i Let n _{i be} the refractive index of

It becomes. However, it is generally difficult to satisfy the formula (2) due to the dispersion characteristics of the crystal.

  As a method for solving this, a quasi-phase matching method in which a nonlinear optical crystal is periodically poled is used. Ferroelectric crystals such as LN are advantageous for the quasi phase matching method. The sign of the nonlinear optical constant of the ferroelectric crystal corresponds to the polarity of spontaneous polarization. When this spontaneous polarization is modulated with a period Λ in the light propagation direction, the phase matching condition is

It is represented by When specific wavelengths λ ₁ and λ ₂ are used as excitation light, equations (1) and (4) are satisfied at the same time, and converted light with wavelength λ ₃ can be generated by high-efficiency difference frequency generation.

However, when changing the wavelengths λ ₁ and λ ₂ to obtain difference frequency light of different wavelengths λ ₃ , if the wavelengths λ ₁ and λ ₂ fluctuate, the equation (4) can be satisfied. In other words, the intensity of the difference frequency light λ ₃ decreases. Here, the relationship between the wavelengths λ ₁ , λ ₂ , λ ₃ , the period Λ, and the generation efficiency η of the difference frequency light is considered. First, the phase mismatch amount Δk is

It is defined as At this time, assuming that the length of the LN wavelength conversion element is l, the generation efficiency η of the difference frequency light depends on the product of Δk and l,

It is expressed. In Equation (6), η _o is the difference frequency generation efficiency when Δk = 0, and η _o is determined by the nonlinear optical constant of the ferroelectric crystal such as LN, the excitation light intensity, the length of the element, and the like. Therefore, η _o is proportional to the product of the light intensities input from the two lasers.

  EXAMPLES Hereinafter, although this invention is demonstrated in detail based on an Example, this invention is not limited to the following Example, It cannot be overemphasized that it can change variously in the range which does not deviate from the summary.

  FIG. 4 shows the configuration of an infrared light absorption analyzer using the gas cell according to the first embodiment. A first laser 21 that outputs first excitation light (pump light), a second laser 22 that outputs second excitation light (signal light), and an LN wavelength conversion element 23 are provided. The output of the first laser 21 is connected to a multiplexer 30 via an optical isolator 26 by an optical fiber 28 in which a fiber Bragg grating (FBG) 25 is formed. The output of the second laser 22 is connected to the multiplexer 30 by an optical fiber 29. The excitation light combined by the multiplexer 30 is input to the LN wavelength conversion element 23, and converted light (idler light) 33 is output via the lens 32.

The first laser 21 oscillates at an arbitrary wavelength between the wavelength λ ₁ = 0.97 and 1.04 μm. The first laser 21 includes a high-reflection film 21A having a reflectance of 90% or more and a low-reflection film 21B having a reflectance of 2% or less, and the output wavelength is stabilized by a resonator composed of the high-reflection film 21A and the FBG 25. Has improved. The second laser 22 can change the wavelength in the wavelength range of wavelength λ ₂ = 1.25 μm to 1.36 μm.

  An optical waveguide 23A is formed in the LN wavelength conversion element 23, and coherent converted light (idler light) 33 having a wavelength of 3.5 to 5.8 μm can be obtained by difference frequency generation (DFG) due to a nonlinear optical effect. . An optical fiber amplifier and an optical isolator may be inserted into the optical fiber 29 as necessary. Further, although the coupling between the output of the multiplexer 30 and the LN wavelength conversion element 23 is batting coupling, it may be lens coupling.

When the temperature of the LN wavelength conversion element 23 is adjusted, the wavelength λ ₁ of the first laser 21 is fixed, and the wavelength λ ₂ of the second laser 22 is varied, the wavelength of the converted light 33 is changed according to the change of the wavelength λ _2. It was confirmed that λ ₃ changed.

  The output of the LN wavelength conversion element 23 is collimated by the lens 32 and is input to the wavelength filter 34 via the chopper 40, and only the converted light 33 is extracted. The wavelength filter 34 is made of, for example, Ge or Si, and a dichroic mirror may be used. The converted light 33 is ON / OFF modulated by the chopper 40 in synchronization with the lock-in amplifiers 39A and 39B, and branched into two paths by the beam splitter 43. The chopper 40 and the lock-in amplifiers 39A and 39B are synchronized by a reference signal 41 which is an ON / OFF signal.

  The light intensity of one beam is measured as a reference beam 37 by a photodetector 38B. The other beam passes through the gas cell 35 in which the sample gas to be measured is sealed, and the light intensity is measured by the photodetector 38A as the transmitted light 36. The gas cell 35 has an optical path length of 20 cm, and the window material becomes an absorber in the mid-infrared region, but anhydrous quartz is used. The photodetectors 38A and 38B are made of, for example, InSb, HgCdTe, CdSe, or the like. The measurement results of the photodetectors 38A and 38B are input to the lock-in amplifiers 39A and 39B, and lock-in detection is performed at the modulation frequency of ON / OFF modulation.

FIG. 5 shows the generation efficiency of the mid-infrared light source according to Example 1. The element length of the LN wavelength conversion element 23 is 50 mm, has an inversion structure with an inversion period Λ = 25.2 μm, and an optical waveguide 23A is formed. The temperature of the LN wavelength conversion element 23 is adjusted to 33.3 degrees, and converted light is output using difference frequency generation. Pump light of wavelength lambda ₁ = 1.03 .mu.m of the first laser 21, power = fixed at 4 mW, the wavelength lambda ₂ = 1.33 band of the signal light of the second laser 22, the wavelength of the converted light (an idler light) 33 This is the wavelength conversion efficiency when a wavelength region in the vicinity of λ ₃ = 4.7 μm is obtained. The generation efficiency η was 4% / W under the temperature condition where the output was maximum near the wavelength λ ₃ = 4.7 μm.

  In the case of the LN wavelength conversion element 23 in which the optical waveguide 23A is formed with an element length of 50 mm, the conversion efficiency varies from 0.2% / W to 8% / W due to variations in element performance. On the other hand, when the LN wavelength conversion element was a bulk crystal element having an element length of 18 mm, the conversion efficiency was almost constant at around 0.01% / W.

Using the mid-infrared light source in the measurement system of FIG. 4, ¹³ CO and ¹² CO spectroscopic experiments known as isotopomers were conducted. Specifically, the absorption in the gas cell 35 in which 300 Torr ¹³ CO and 0.15 Torr ¹² CO were sealed was observed. Power of the transmitted light 36 and reference light 37 transmitted through the gas cell 35, a light detector 38A, and received by 38B, measuring the lock-in amplifier 39A, the reference beam intensity P _B and each of the transmitted light intensity P _A at 39B .

FIG. 6 shows the results of light absorption measurement by the infrared light absorption analyzer according to Example 1. 6 (a) is a detected spectrum P _A of the transmitted light 36 by the lock-in amplifier 39A, FIG. 6 (b) is a detected spectrum P _B of the reference light 37 by the lock-in amplifier 39B. 6 (c) is a transmittance characteristic of the gas cell obtained from P _A / P _B. A vibration rotation spectrum based on the reference vibration ν is obtained. In the transmittance curve of FIG. 6 (c), two absorption lines can be clearly confirmed. Regarding the assignment of each absorption line, ¹³ CO is R (24), and ¹² CO is R (8). Was confirmed from the analysis result of the HITRAN database. The concentration of ¹² CO used here corresponds to 190 ppm. Since the incomplete combustion alarm concentration = 550 ppm and the incomplete combustion warning concentration = 300 ppm, the measurement result is sufficient as the gas detection accuracy.

FIG. 7 shows a predicted spectrum when atmospheric CO having an incomplete combustion warning concentration is detected. FIG. 7A shows the result of analyzing the absorption in the optical path of 20 cm using the HITRAN database when 300 ppm of ¹² CO exists in the atmosphere. FIG. 7B is an enlarged view of the vicinity of R (8) of ¹² CO. Compared with FIG. 6 (c), it can be said that the result of CO observation in the gas cell is sufficiently durable.

FIG. 8 shows a predicted spectrum when CO is detected in an optical path of 100 m in the atmosphere. FIG. 8A shows absorption in the optical path 100m in the atmosphere, and FIG. 7B is an enlarged view of the vicinity of R (8) of ¹² CO. In the measurement in the atmosphere, there are many cases where desired observation cannot be performed due to the interference gas existing in the surrounding area and the pressure expansion of the absorption line. Even in the measurement of dilute concentrations of ¹² CO in the atmosphere of 100 ppb or less, since the absorption of interference gas does not affect R (8) of ¹² CO, it can be sufficiently detected with an optical path length of 20 cm or less. . Therefore, it became clear that a sensor with higher sensitivity can be realized as compared with the current alarm device that detects the incomplete combustion warning concentration.

In the first embodiment, the wavelength λ ₃ of the converted light 33 is a value obtained by measuring the wavelength λ ₁ of the first laser 21 that is the source of the excitation light with an optical spectrum analyzer, and the indicated wavelength of the second laser 22 having a variable wavelength. It can be estimated from the value of λ ₂ . Furthermore, the gas cell 35 can be used to accurately confirm the gas absorption line.

  In the first embodiment, the wavelength of the first laser 21 is fixed and the wavelength of the second laser 22 is variable. However, the wavelength of the second laser 22 is fixed and the wavelength of the first laser 21 is variable, and the light source is configured. There is no problem. In the first embodiment, the infrared light absorption analyzer is configured with the transmitted light as a measurement target. However, the system may be configured by using light emission such as reflected light, scattered light, or fluorescence. Example 1 is a result of using the LN wavelength conversion element 23 (waveguide element) in which the optical waveguide 23A is formed, but the waveguide is also used in the case of a bulk crystal element manufactured using a quasi-phase matching technique. Compared with an element, a decrease in mid-infrared light output is inevitable. However, the detection sensitivity of the measurement system in FIG.

  In Example 1, absorption measurement is performed using a gas cell having an optical path length of 20 cm. However, a multi-pass cell is convenient for detecting environmental gases typified by greenhouse gases contained in the atmosphere and detecting rare gases. is there. The gas cell 35 may be replaced with a multipass cell to constitute a measuring device. Moreover, there is no restriction | limiting in particular regarding this multipath cell, You may use a white cell, a hunt cell, an Elliott cell etc. according to a use. The use of these multi-pass cells enables measurement under reduced pressure depending on conditions when measurement under reduced pressure is desirable. Convenient.

  In the first embodiment, the converted light 33 is ON / OFF-modulated, but intensity modulation or frequency modulation may be performed on any of the excitation lights, and lock-in detection may be performed using the modulation signal as the reference signal 41. If direct modulation is used, the first laser 21 whose wavelength and output power are fixed is modulated, and if an external modulator is used, either the first laser 21 or the second laser 22 may be used. . However, in the case of using frequency modulation, although it depends on the frequency shift of the modulation, analytical detection is differential detection.

FIG. 9 shows a configuration of an infrared absorption analyzer using the gas cell according to the second embodiment. A different part from the structure of Example 1 shown in FIG. 4 is demonstrated. The first laser 121 that outputs the first excitation light (pump light) can vary the wavelength in the wavelength range of the wavelength λ ₁ = 0.97 to 1.04 μm. The output of the first laser 121 is connected to the multiplexer 30 via the optical isolator 26 by the optical fiber 28. The second laser 122 that outputs the second excitation light (signal light) oscillates at an arbitrary wavelength in the wavelength λ ₂ = 1.25 μm to 1.36 μm band. The output of the second laser 122 is connected to the multiplexer 30 by an optical fiber 29.

As shown in FIG. 3, in the case of obtaining converted light (idler light) of 3 μm to 6 μm, if the first pumping light (pump light) having the wavelength λ ₁ is not fixed, but the wavelength is variable, the second pumping is performed. It can be seen that the wavelength of the pump light can be swept over a wide range even if the light (signal light) has a constant wavelength. Since the wavelength sweep range of the pump light affects the wavelength variable range of the idler light, mid-infrared light in a wide wavelength band which is not conventionally obtained can be obtained. If an infrared light absorption analyzer is configured using this mid-infrared light, an infrared light absorption analyzer of a wide wavelength band can be obtained.

  The LN wavelength conversion element 123 is made of a bulk LN crystal having an element length of 18 mm and has a polarization inversion period Λ = 25 μm to 30 μm. As shown in FIG. 3, coherent converted light (idler light) 33 having a wavelength of 3 μm to 6 μm can be obtained by differential frequency generation (DFG) using a nonlinear optical effect.

FIG. 10 shows the wavelength conversion width of idler light when the signal light is tunable and when the pump light is tunable. Using the LN wavelength conversion element 123 having an element length of 18 mm and a polarization inversion period Λ = 27 μm, idler light in the 3.7 μm band is output. For comparison, FIG. 10 (a) shows that the λ ₁ of the first pumping light (pump light) is made constant and the wavelength λ ₂ of the second pumping light (signal light) is made variable as in the first embodiment. Shows the case. FIG. 10 (b), in Example 2, it shows the case where the second excitation light wavelength lambda ₂ (signal light) is kept constant, and the first excitation light lambda ₁ (pump light) variable.

  As can be seen at a glance, the conversion width of the converted light (idler light) is more than 10 times wider when the signal light is constant and the pump light is variable in wavelength. In FIG. 10A, the wavelength range of idler light is 20 nm in the 3.7 μm band (reference symbol Xa), whereas in FIG. 10B, the idler wavelength range is 220 nm (reference symbol Xb in the diagram). ). Even in other wavelength ranges, the conversion width can be expanded in the same way.

Similarly to Example 1, when an LN wavelength conversion element having an element length of 50 mm and an optical waveguide formed is used, the conversion width of converted light (idler light) exceeds 100 nm in the wavelength range of idler light. Compared with the case where the wavelength of the second excitation light (signal light) is variable (wavelength λ ₂ = 1.25 μm to 1.36 μm), this value provides a wide wavelength band.

  As a matter of course, when the set temperature of the LN wavelength conversion element 123 is changed, the phase matching wavelength changes. In Example 2, the result of setting the temperature of the element to 90 degrees was shown. Due to the nature of the material dispersion of the refractive index of LN, the phase matching curve exhibits both characteristics when the temperature is lowered. If this is allowed, each peak wavelength is about 0.5 nm / ° C. in terms of pump light wavelength (about 7 nm / ° C. in the region of 3.7 μm band idler light wavelength), and the wavelength range can be expanded. . Therefore, it can be seen that the wavelength range can be further expanded by simultaneously adjusting the temperature of the LN wavelength conversion element.

  FIG. 11 shows a configuration of an infrared absorption analyzer using the gas cell according to the third embodiment. A first laser 51 that outputs first excitation light (pump light), a second laser 52 that outputs second excitation light (signal light), and an LN wavelength conversion element 53 are provided. The output of the first laser 51 is connected to a multiplexer 60 through an optical isolator 56 by an optical fiber 58 in which a fiber Bragg grating (FBG) 55 is formed. The first laser 51 includes a high reflection film 51A having a reflectance of 90% or more and a low reflection film 51B having a reflectance of 2% or less, and the output wavelength is stabilized by a resonator composed of the high reflection film 51A and the FBG 55. Has improved. The output of the second laser 52 is connected to the multiplexer 60 by an optical fiber 59. The excitation light combined by the multiplexer 60 is input to the LN wavelength conversion element 53, and converted light (idler light) 63 is output via the lens 62.

In the LN wavelength conversion element 53, an optical waveguide 53A is formed, and coherent converted light (idler light) 63 can be obtained by difference frequency generation (DFG) due to a nonlinear optical effect. When the temperature of the LN wavelength conversion element 53 is adjusted, the wavelength λ ₁ of the first laser 51 is fixed, and the wavelength λ ₂ of the second laser 52 is varied, the wavelength of the converted light 63 is changed according to the change of the wavelength λ _2. It was confirmed that λ ₃ changed.

  The output of the LN wavelength conversion element 53 is collimated by the lens 62 and is input to the wavelength filter 64 via the chopper 70, and only the converted light 63 is extracted. The converted light 63 is ON / OFF-modulated by the chopper 70 in synchronization with the lock-in amplifiers 69A and 69B, and branched into two paths by the beam splitter 73. The chopper 70 and the lock-in amplifiers 69A and 69B are synchronized by a reference signal 71 which is an ON / OFF signal. The light intensity of one beam is measured as a reference beam 67 by a photodetector 68B. After the other beam is transmitted through the atmosphere, the light intensity is measured by the photodetector 68A as transmitted light 66. The measurement results of the photodetectors 68A and 68B are input to the lock-in amplifiers 69A and 69B, and lock-in detection is performed at the modulation frequency of ON / OFF modulation.

The difference from the first embodiment is that the infrared light absorption analyzer of the third embodiment is intended for gas measurement in the atmosphere related to detection of leaked gas. By changing the path of the reference beam 67 and the optical path length of the path of the transmitted light 66, in the same manner as in Example 1, it is possible to obtain the transmittance characteristics of the gas in the atmosphere from the P _A / P _B.

It has been reported that about 380 ppm of CO ₂ is contained in the Japanese atmosphere in 2005. The exhaled breath emitted from the human body usually contains 4% CO ₂ including carbon dioxide. By using the infrared light absorption analyzer of the third embodiment, if the optical path lengths of the reference light 67 path and the transmitted light 66 path are made equal, and exhalation is blown on one of the paths, exhalation analysis can be easily realized. Can do.

In the breath analysis, the absorption in the vicinity of 4.24 μm or 4.28 μm is the largest, and therefore analysis in a short optical path length is possible in this wavelength region. In these wavelength regions, λ ₁ of the first excitation light (pump light) of the first laser 51 is 980 nm, and the wavelength λ ₂ of the second excitation light (signal light) of the second laser 52 is 1.275 μm or 1 .271 μm, obtained by difference frequency generation. However, since CO ₂ in the atmosphere has a high content rate unlike other rare gases, absorption in this wavelength region is often too strong, and the spectrum waveform may be distorted. Care must be taken when it is necessary to quantify the absorption intensity.

The observation of N ₂ O absorption, which is attracting attention as a greenhouse gas in the same way as CO ₂ , is simple at around 4.531 μm or around 4.468 μm. These wavelength regions, the first excitation light lambda ₁ (pump light) and 980 nm, a second excitation light wavelength lambda ₂ (signal light) and 1.250μm or 1.255Myuemu, obtained by difference frequency generation . Observation is easy because 500 ppb of N ₂ O is present in the atmosphere. If an optical path length of several tens to 100 meters can be secured in the atmosphere, N ₂ O absorption observation of several to 10% can be performed regardless of whether or not a multipath cell is used.

It is also attracting attention in medical applications, such as finding patients with liver disease by breath analysis. COS is easy to detect in the vicinity of 4.86 μm. This wavelength region is obtained by generating a difference frequency by setting λ ₁ of the first excitation light (pump light) to 1.03 μm and the wavelength λ ₂ of the second excitation light (signal light) to 1.307 μm. If an optical path length of several tens to 100 m can be secured in the atmosphere, it is possible to observe absorption of COS in the atmosphere as with N ₂ O.

  The mid-infrared light source centered on the 4 μm band realized by the present embodiment is a wavelength tunable light source capable of operating at room temperature with high efficiency unprecedented by simply making the excitation light from the laser incident on the LN wavelength conversion element. It becomes. Since the laser used as the excitation light source is obtained from a product in the completed communication wavelength band, the light source specification is excellent, and the specification as a mid-infrared light source due to the difference frequency generation is also ensured. Specifically, performance that cannot be obtained from conventional mid-infrared light sources, such as center wavelength, frequency sweep accuracy, stable single longitudinal mode oscillation indispensable for spectroscopy, and ensuring spectral width, etc. are secured. Is done. It should also be noted that a compact light source can be constructed and resources are saved.

1, 21, 51, 121 First lasers 1A, 21A, 51A High reflection films 1B, 21B, 51B with a reflectance of 90% or more Low reflection films 2, 22, 52, 122 with a reflectance of 2% or less Second laser 3, 23, 53, 123 LN wavelength conversion element 4 Optical fiber amplifier 5, 25, 55 Fiber Bragg grating (FBG)
6, 7, 26, 56 Optical isolator 8, 9, 28, 29, 59 Optical fiber 10, 30, 60 Multiplexer 11, 12, 32, 62 Lens 13, 33, 63 Converted light (idler light)
23A, 53A Optical waveguide 34, 64 Wavelength filter 35 Gas cell 36, 66 Transmitted light 37, 67 Reference light 38A, 38B, 69A Detector 39A, 39B, 69B Lock-in amplifier 40, 70 Chopper 41, 71 Reference signal 43, 73 Beam splitter

Claims (4)

By inputting the first laser that generates the first excitation light, the second laser that generates the second excitation light, the first excitation light and the second excitation light, and by generating the difference frequency In a mid-infrared light source including a wavelength conversion element made of a nonlinear optical crystal that outputs converted light,
The first laser outputs the first excitation light having an arbitrary wavelength between 0.97 μm and 1.04 μm,
The second laser can vary the wavelength of the second excitation light in a wavelength range between 1.25 μm and 1.36 μm,
The wavelength conversion element outputs mid-infrared light having a wavelength of 3.5 μm to 5.8 μm as converted light. By inputting the first laser that generates the first excitation light, the second laser that generates the second excitation light, the first excitation light and the second excitation light, and by generating the difference frequency In a mid-infrared light source including a wavelength conversion element made of a nonlinear optical crystal that outputs converted light,
The first laser can vary the wavelength of the first excitation light in a wavelength range between 0.97 μm and 1.04 μm,
The second laser outputs the second excitation light having an arbitrary wavelength between 1.25 μm and 1.36 μm,
The wavelength conversion element outputs mid-infrared light having a wavelength of 3.5 μm to 5.8 μm as converted light. The mid-infrared light source according to claim 1, wherein the wavelength conversion element is made of LiNbO ₃ crystal and has an optical waveguide structure. The output light output from the light source is branched, the first light intensity is measured using one output light as the reference light, the other output light is guided to the sample gas, and the transmitted light, reflected light, and scattering from the sample gas In an infrared light absorption analyzer that receives light or fluorescence to measure the second light intensity, and measures the absorption of the sample gas from the ratio of the first and second light intensity,
The infrared light absorption analyzer according to claim 1, wherein the light source is the light source described in claim 1.

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