JP4568313B2 - Laser light source - Google Patents

Description

  The present invention relates to a laser light source, and more particularly, a laser light source that outputs a coherent light having a sodium D-line wavelength or a yellow region wavelength with high efficiency using a laser and a nonlinear optical crystal, and a mid-infrared region laser. The present invention relates to a laser light source that can vary light in a wavelength range of 2 to 3 μm and a laser light source that outputs laser light having a wavelength of 759 nm to 768 nm, which is an oxygen absorption line.

  Currently, gas lasers such as He—Ne laser and Ar laser, solid-state lasers such as Nd: YAG laser, dye lasers and semiconductor lasers are known as lasers in practical use. FIG. 1 shows the relationship between the laser wavelength region and the output. In recent years, small, lightweight, and inexpensive semiconductor lasers have become widespread, mainly in the wavelength band 102 in the visible and near infrared regions. Particularly in the field of optical communication, 1.3 μm band and 1.5 μm band semiconductor lasers for signal light sources and 0.98 μm band and 1.48 μm band semiconductor lasers for exciting fiber amplifiers are widespread. Semiconductor lasers are also used as CD lasers and red LDs, and semiconductor lasers are also used in the visible and ultraviolet wavelength bands 101 used for reading and writing on recording media such as DVDs and Blu-rays.

  However, semiconductor lasers have been put to practical use in the wavelength band 111 of green, yellow-green, and yellow regions with wavelengths of 0.5 to 0.6 μm and the wavelength band 112 of mid-infrared region of wavelengths of 2 to 5 μm. Gas lasers and solid-state lasers that are expensive and have high power consumption are used.

  Optical properties such as refractive index and absorption of optical media such as liquid and glass are important evaluation items in terms of prescribing the characteristics of optical equipment or quality control such as accuracy and purity of foods and pharmaceuticals. It has become. For the measurement of these optical characteristics, a light source that generates sodium D-rays having a wavelength of 589 to 590 nm in the yellow region included in the wavelength band 111 is used.

  For example, the relationship between the sugar content and the refractive index in the liquid is defined as a Brix value by ICUMSA (International Commission for Uniform Methods of Sugars Analysis), and a method for obtaining the sugar content from the measurement of the refractive index is defined. This method is applied to the measurement of sugar content of fruits and liquors and is widely used in industry.

  In the field of pharmaceuticals, as one of quality control of drugs, the refractive index of a solution in which a drug is dissolved is defined by the Japanese Pharmacopoeia. In drugs with a helical structure such as thalidomide, the “right-handed” system has medicinal effects, but the “left-handed” system can be a poison. It is impossible to physicochemically separate such substances having reverse helical structures. However, it is known to exhibit different optical rotations and can be easily identified optically. Thus, after a phytotoxicity accident such as thalidomide, the Japanese Pharmacopoeia stipulates the measurement of optical rotation using sodium D-line. In addition to thalidomide, there are many other drugs such as menthol, prostaglandins, β-lactam antibiotics, and quinolone antibacterial agents.

  At present, a laser light source that generates sodium D-line has not been realized, and a sodium lamp or a yellow LED is used as the light source. The light from the sodium lamp is divergent light radiated in all directions although it is excellent in monochromaticity. Therefore, it is difficult to make parallel light, and it is difficult to accurately measure optical characteristics. In addition, since the condensing energy does not increase, it is necessary to use a high output lamp.

  On the other hand, the yellow LED has a wide spectrum line width of about 20 nm. Thus, although the spectrum line width is narrowed by cutting out the spectrum near the sodium D line using an optical filter, there is a limit. Further, since there is no coherence, there is a limit to the improvement of measurement accuracy.

  Against this background, in many industrial fields such as food and pharmaceutical quality control, there is a need to improve the accuracy of the optical evaluation method defined by the sodium D-line wavelength. If a laser in the sodium D line can be realized, measurement using light interference becomes possible. By using light interference, the refractive index measurement accuracy of various liquids and optical media including food and pharmaceuticals can be improved by about two orders of magnitude compared to the current level, and further, low power consumption and downsizing can be achieved. .

The structure of sodium atoms and the characteristics of light generated from the energy transition will be described (for example, see Non-Patent Document 1). It is known that the wavelengths of light emitted from sodium atoms are 589.592 nm (D ₁ line) and 588.995 nm (D ₂ line). The D ₁ line and the D ₂ line are collectively referred to as a D line, and the wavelength of the D line is sometimes referred to as 589.3 nm taking an average of both. The energy level of the sodium atom is shown in FIG. The D line is generated with a transition from the 3P level that is the first excited state to the 3S level that is the ground state. 3P has a fine structure of 3P _1/2 and 3P _3/2 , the emission of the D ₁ line is due to a transition from 3P _1/2 to 3S _1/2 , and the emission of the D ₂ line is 3P _This is due to the transition from _3/2 to 3S _1/2 .

3S _1/2 , 3P _1/2 , 3P _3/2 have a very fine structure due to the interaction between the magnetic moment of electrons and the intrinsic magnetic moment of the nucleus, and 3S _1/2 has an energy difference of 7.3 μeV. Separating into two levels, 3P _1/2 separates into two levels with a width of 0.78 μe and 3P _3/2 separates into four levels with a width of 0.48 μeV.

In order to realize a laser with wavelengths of D ₁ line and D ₂ line, it is necessary to form an inversion distribution between energy levels corresponding to the respective wavelengths. In order to realize the population inversion, it is necessary to configure a three-level system or a four-level system. However, in the energy level shown in FIG. 2, the relaxation from 3P _3/2 to 3P _1/2 is a forbidden transition, and the relaxation time from 3P _1/2 to 3S _1/2 is 15.9 ns (for example, Non-Patent Document 2). For example, compared with the relaxation time of 3.2 μs of TiAl ₂ O ₃ laser, it is difficult to form an inversion distribution between 3S _1/2 and 3P _1/2 because it is shorter by two orders of magnitude or more. The laser oscillation has not been realized yet. Although laser oscillation using an ultrafine structure is also conceivable, the energy difference between 3S _1/2 , 3P _1/2 , and 3P _3/2 ultrafine structure in a sodium atom is 25.8 meV at room temperature (300 K). Is about 4 digits smaller than. Therefore, the excitation at room temperature is distributed almost evenly in both of the split ultrafine structures, and an inversion distribution cannot be formed. For these reasons, heretofore, sodium D ₁ line, it was not possible to realize a laser in D ₂ line.

  Conventionally, semiconductor lasers have been put into practical use only in a wavelength region of 500 nm or less or 620 nm or more. In the wavelength region of 500 nm to 620 nm, a solid-state laser of a specific wavelength is realized by the second harmonic generation method of the fiber laser and the Nd-YAG laser, but a solid-state laser of an arbitrary wavelength has not been realized yet. Absent.

On the other hand, a second harmonic generation method (SHG method) using a nonlinear crystal is known as a method for generating coherent light in the visible range. In order to generate D ₁ or D ₂ light by this method, a light source having a wavelength of 1179.2 nm or 1178.0 nm is required. Unfortunately, although these wavelengths can oscillate with a semiconductor laser, it is very difficult to obtain a laser that can provide the required output.

  Also, visible light can be obtained by generating a sum frequency of two excitation laser beams using a nonlinear crystal. In this method, the energy of the sum frequency light is given by the sum of the energy of the two excitation lights. In order to obtain the sum frequency of the desired wavelength, there is an advantage that the degree of freedom of the combination of the wavelengths of the two excitation lights is expanded. Therefore, it is the most practical method for realizing a laser having an arbitrary wavelength. However, in general, the nonlinear optical phenomenon has a problem of low efficiency. In order to solve this problem, it is important to select an existing laser apparatus that can improve the characteristics of the nonlinear optical crystal, obtain high excitation light intensity, and reduce the size and power consumption.

  Conventionally, a laser microscope that scans a sample with a confocal laser beam and obtains an optical tomographic image is known. Laser microscopes are used for tissue / subcellular distribution analysis of fluorescently labeled substances. In addition, a flow cytometer is known that irradiates a flow of cells arranged in a row with a laser beam and analyzes and sorts the cells according to the fluorescence intensity. A flow cytometer is a measuring device using a flow cytometry method that qualitatively identifies cell properties such as size and DNA content as optical parameters.

  In recent years, fluorescent dyes have been used as fluorescent labels. However, since fluorescent dyes are foreign substances for cells, there are problems such as affecting the properties of cells and killing cells. Therefore, a method of performing fluorescent labeling with green fluorescent protein extracted from jellyfish or the like is used. In addition, fluorescent proteins exhibiting yellow and red light emission are obtained by mutation and genetic manipulation of green fluorescent protein (see, for example, Non-Patent Document 3), and more detailed measurement and analysis using multicolor fluorescence is performed. ing.

  Since the red fluorescent protein has an absorption maximum at a wavelength of 560 to 590 nm (see, for example, Non-Patent Document 4), a laser light source having an oscillation wavelength in this wavelength band is desired. Since lasers having an oscillation wavelength in this wavelength band are only large lasers such as dye lasers, 532 nm solid-state lasers and 543 nm He—Ne lasers are used instead. However, these wavelengths are inconvenient for measurement and analysis using multicolor fluorescent proteins because the overlap between the fluorescence wavelength of green fluorescent protein and the absorption wavelength of yellow fluorescent protein is remarkable.

  Recently, a Kindling red fluorescent protein that stably emits red fluorescence over a long period of 72 hours or longer by irradiation with intense green laser light (wavelength 530 to 560 nm) has been reported (for example, see Non-Patent Document 5). ). If Kindling red fluorescent protein is used, it is expected that the state of cell division can be observed with fluorescence for a long time. However, in the conventional 532 nm solid laser and 543 nm He—Ne laser, the overlap between the fluorescence wavelength of the green fluorescent protein and the absorption wavelength of the yellow fluorescent protein is remarkable. Therefore, realization of a small solid laser having an oscillation wavelength as close to 560 nm as possible is desired.

  Metalloporphyrin is a molecule contained in a protein that plays an important role in animal and plant life activities such as photosynthesis and respiratory metabolism, and has an absorption maximum near a wavelength of 590 nm. Since the emission wavelength of these metal porphyrins shows a peak in the vicinity of 600 nm, when a laser having a wavelength of 589 nm is used, the overlap with the emission wavelength is large and measurement is difficult. Therefore, a yellow-orange laser having a wavelength of 585.0 nm is required.

  Furthermore, the wavelength 546.1 nm (yellowish green) corresponding to one of the emission lines (e-line) emitted by the mercury lamp is the wavelength with the highest human visibility, and is used as the standard refractive index wavelength of optical glass. ing. As shown in FIG. 1, a highly efficient and highly stable laser light source is required in the green, yellow-green, and yellow regions of 500 nm to 600 nm included in the wavelength band 111.

  However, as described above, the semiconductor laser has been put into practical use only in a wavelength region of 500 nm or less or 620 nm or more. In addition, a solid-state laser having an arbitrary wavelength has not been realized in the wavelength region of 500 nm to 620 nm. Furthermore, in order to generate light in the yellow region by the SHG method, a light source having a wavelength of 1092.2 nm, 1120.0 nm, or 1170.0 nm is required. However, although these wavelengths can be oscillated by a semiconductor laser, it is very difficult to obtain a laser capable of obtaining a necessary output.

  As described above, in applying the nonlinear optical phenomenon, it is important to select an existing laser apparatus that can improve the characteristics of the nonlinear optical crystal, obtain high excitation light intensity, and further reduce the size and power consumption.

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

  The absorption peak of the substance to be measured is mainly in the mid-infrared region of 2 μm to 20 μm due to the vibration mode of interatomic bonding. However, a laser capable of continuous oscillation at room temperature in the mid-infrared wavelength band 112 shown in FIG. 1 has not yet been put into practical use, and research and development of quantum cascade lasers have only been promoted. . Although the necessity of mid-infrared light is high in industry, the lack of a practical laser light source is a major obstacle.

  Since there is no practical light source in the mid-infrared region, when performing microanalysis of various gases using an existing communication semiconductor laser (0.8 to 2 μm), harmonics of the fundamental absorption wavelength (= The absorption at half the fundamental absorption wavelength and the third overtone (= one third of the fundamental absorption wavelength) is used. In the case of overtones, the necessary sensitivity may be obtained, but the measurement at the higher-order absorption peak of the third overtone or higher is limited in detection because the amount of absorption itself is small. Therefore, compared with the measurement at the original fundamental absorption wavelength, the sensitivity is reduced by about 3 digits.

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. In recent years, it has been reported that mid-infrared light is generated in the vicinity of a wavelength of 3 μm and its operation as a gas sensor has been confirmed (for example, see Non-Patent Document 6). The light source used for the gas sensor uses a lithium niobate (LiNbO ₃ ) wavelength conversion element having a periodic modulation structure to generate mid-infrared light by difference frequency generation.

  However, the wavelength conversion element having the periodic modulation structure only generates mid-infrared light having one fixed wavelength. Therefore, in order to make the wavelength variable so that many kinds of gases can be detected at one time, (1) various periods are provided in one wavelength conversion element (see, for example, Non-Patent Document 7). (2) The period is changed by a structure called Fanout Grating (see, for example, Non-Patent Document 6). (3) A technique is known in which excitation light is incident on an element obliquely to change an effective period (see, for example, Non-Patent Document 8).

  Although these methods can sweep the wavelength over a wide range, there is a problem that many working steps are required because elements having various periods must be bundled. In addition, the method of causing the excitation light to enter the element obliquely has a problem that it is difficult to make the element structure a waveguide structure in order to achieve high efficiency.

  In recent years, environmental problems have been greatly highlighted, and in particular, there is interest in the effects of dioxins on the human body. In an incinerator, which is one of the sources of dioxins, the generation of dioxins can be suppressed by controlling the combustion state of the furnace. In order to monitor the combustion state, a thermometer, a CO concentration meter, and an oxygen concentration meter are required.

  As a method for detecting the gas concentration, a method 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 detected by scanning a laser beam having a wavelength near the absorption line and observing the absorption spectrum. The points required for laser light at this time are monochromatic light, that is, single-mode laser light, output suitable for gas detection of several mW to several tens of mW, and stable wavelength scanning. And long life.

  The laser light used in the oximeter is in a wavelength region 113 including a plurality of oxygen absorption lines existing in a wavelength range of 759 nm to 768 nm, and a gallium arsenide semiconductor laser is used (see, for example, Patent Document 1). A gallium arsenide-based semiconductor laser is manufactured by growing a semiconductor crystal having a lattice constant substantially equal to that of gallium arsenide on a gallium arsenide substrate.

  Semiconductor lasers include an edge-emitting laser in which a waveguide is formed in parallel with a substrate and a surface-emitting laser that emits light perpendicular to the substrate. As a gallium arsenide-based edge emitting laser, a single mode laser having a relatively high output has been developed, but it does not have a structure for controlling an oscillation wavelength. Therefore, the oscillation wavelength of a gallium arsenide-based edge-emitting laser is determined by the point where the gain peak of the active layer and the resonance mode of the resonator coincide with each other. Difficult to do.

  As a structure for controlling the oscillation wavelength, a distributed feedback (DFB) type, a distributed Bragg reflection (DBR) type, and the like are well known. In these structures, it is necessary to produce semiconductor crystals having different refractive indexes, that is, different compositions periodically in a direction parallel to the substrate. In the manufacturing method, the surface of a semiconductor crystal is etched into a wave-like periodic structure, and semiconductor crystals having different compositions are grown thereon. In order to oscillate at a wavelength of 763 nm in order to detect the oxygen concentration, it is necessary to suppress absorption at that wavelength, and it is necessary to use a crystal with a high aluminum concentration. However, when the aluminum concentration is high, there is a problem that crystals are easily oxidized when a periodic structure is manufactured.

  The surface emitting laser is a kind of DBR type laser. Since the surface emitting laser has a light emitting direction perpendicular to the substrate, the surface emitting laser may have a DBR structure having a refractive index distribution in a direction perpendicular to the substrate. That is, it is only necessary to periodically produce semiconductor crystals having different compositions in parallel with the substrate, and the production is easy because only one semiconductor crystal growth is required. However, the surface emitting laser cannot obtain a large gain because light passes through the active layer in the vertical direction. In order to obtain a sufficient output, a method of increasing the light emission area is conceivable. However, when the light emission area is increased, oscillation having a plurality of transverse modes occurs and the mode is not single mode. If single mode oscillation is performed while suppressing the light emission area to obtain light emission intensity on the order of mW necessary for detecting the oxygen concentration, the current required for light emission is concentrated in a small area and the current density is increased. For this reason, there has been a problem that the lifetime of the surface-emitting laser becomes as short as several months.

JP-A-6-194343 US Pat. No. 5,036,220 (refer to Japanese Patent Publication No. 4-507299 for the corresponding Japanese application) Kenichi Kubo and Kenji Katori, “Spin and Polarization”, Baifukan, October 31, 1994, p. 21-24 Harold J. Metcalf and Peter van der Straten “Laser Cooling and Trapping”, Springer, 1999, pp.274 G. Patterson et al., J. Cell Sci. No. 114, pp.837-838 (2001) A.F.Fradkov et al., Biochem.J.No.368, pp.17-21 (2002) D.M.Chudakov et al., Nat.Biotechnol.No.21, pp.191-194 (2003) D. Richter, et al., Applied Optics, Vol. 39, 4444 (2000) I.B.Zotova et al., Optics Letters, Vol.28, 552 (2003) C.-W.Hsu et al., Optics Letters, Vol.26, 1412 (2001) A. Yariv, “Quantum Electronics”, 3rd Ed., Pp.392-398 (1988) http: //laserfocusworld.365media,comilaserfocusworld/search Result asp? cat = 48903 / & d = 453 & st = 1 R.M.Schotland, Proc. 3rd Symp. On Remote Sensing of Environment, 215 (1964) IEEE Photonics Technology Letters vol.11 (1999) pp.653-655 Proceedings of the 15th Annual Meeting of IEEE, Lasers and Electro-Optics Society, 2002 (LEOS2002), vol.1, pp.79-80 (2202)

  A first object of the present invention is to provide a laser light source that generates coherent light having a sodium D line wavelength, which has a narrow line width, excellent parallelism, and high energy efficiency.

  A second object of the present invention is to provide a laser light source that generates coherent light in a yellow region with a narrow line width, excellent parallelism, and high energy efficiency.

  A third object of the present invention is to provide a laser light source capable of changing the laser light in the mid-infrared region in the wavelength range of 2 to 3 μm.

  The fourth object of the present invention is to provide a laser light source having a high output and a long life at a wavelength of 759 nm to 768 nm which is an oxygen absorption line.

The present invention, in order to achieve the first object, a first laser for generating a laser beam having a wavelength lambda _1, a second laser for generating a laser beam having a wavelength lambda _2, the laser beam having a wavelength lambda ₁ And a laser beam having a wavelength λ ₂ and a non-linear optical crystal that outputs a coherent light having a wavelength λ ₃ of a sum frequency having a relationship of 1 / λ ₁ + 1 / λ ₂ = 1 / λ ₃ The wavelength λ ₃ of the sum frequency is a wavelength of 589.3 ± 2 nm corresponding to the sodium D line.

In order to achieve the second object, a _first laser that generates laser light having a wavelength λ _1, a _second laser that generates laser light having a wavelength λ _2, a laser beam having a wavelength λ ₁ , and a wavelength type and lambda ₂ of the laser beam, the laser light source and a nonlinear optical crystal for outputting coherent light having a _{1 / λ 1 + 1 / λ} 2 = 1 / λ wavelength lambda ₃ of the sum frequency in the _third relationship, The wavelength λ ₁ is 940 ± 10 nm, the wavelength λ ₂ is 1320 ± 20 nm, and the sum frequency wavelength λ ₃ is a wavelength 546.1 ± 5.0 nm corresponding to the yellow region.

If the wavelength λ ₁ is 980 ± 10 nm and the wavelength λ ₂ is 1320 ± 20 nm, the sum frequency wavelength λ ₃ becomes the wavelength 560.0 ± 5.0 nm corresponding to the yellow region. If the wavelength λ ₁ is 1064 ± 10 nm and the wavelength λ ₂ is 1320 ± 20 nm, the sum frequency wavelength λ ₃ becomes the wavelength 585.0 ± 5.0 nm corresponding to the yellow region. Further, if the wavelength λ ₁ is 940 ± 10 nm and the wavelength λ ₂ is 1550 ± 30 nm, the sum frequency wavelength λ ₃ becomes the wavelength 585.0 ± 5.0 nm corresponding to the yellow region.

Further, in order to achieve the third object, a _first laser that generates a laser beam having a wavelength λ _1, a _second laser that generates a laser beam having a wavelength λ _2, a laser beam having a wavelength λ ₁ , and a wavelength type and lambda ₂ of the laser beam, the laser light source and a nonlinear optical crystal for outputting coherent light having a _{1 / λ 1 -1 / λ 2} = 1 / λ wavelength lambda ₃ of frequency in the _third relationship The wavelength λ ₁ is 0.9 to 1.0 μm, and the nonlinear optical crystal has a polarization inversion structure with a period of 1. When the wavelength λ ₂ changes between 1.3 and 1.8 μm, the difference frequency The wavelength λ ₃ is configured to vary between wavelengths 3.1 to 2.0 μm.

Furthermore, in order to achieve the fourth object, the laser light source oscillates a laser beam having a wavelength twice the wavelength of one absorption line selected from oxygen absorption lines existing at wavelengths of 759 nm to 768 nm. A distributed feedback semiconductor laser, an optical waveguide having a second-order nonlinear optical effect, and a polarization maintaining fiber connecting the output of the distributed feedback semiconductor laser and one end of the optical waveguide, and the other end of the optical waveguide An optical fiber having a structure capable of guiding in a single mode with respect to the second harmonic light generated in the optical waveguide is connected, and the optical waveguide periodically inverts the polarization of the second-order nonlinear optical material. The wavelength of the absorption line is λ ₁ , the wavelength of the laser light having a wavelength twice the wavelength of the absorption line is λ _3, and the refractive index of the nonlinear optical material at wavelengths λ ₁ and λ ₃ , It When _{n 1,} n _3,
2πn ₃ / λ ₃ = 2πn ₁ / λ ₁ + 2πn ₁ / λ ₁ + 2π / Λ
It is characterized by a structure that is periodically inverted by a period Λ that satisfies

  The present invention provides a compact laser light source capable of freely designing the wavelength in a wavelength region not practically used in a semiconductor laser by combining a high-efficiency nonlinear optical crystal and a high-power semiconductor laser for optical communication. Can be provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In this embodiment, a high-efficiency nonlinear optical crystal and a high-power semiconductor laser for optical communication are combined. FIG. 3 shows a laser light source according to an embodiment of the present invention. The laser light source 120 includes two excitation lasers 121 and 122 for exciting the nonlinear optical crystal and a nonlinear optical crystal 123 for generating sum frequency light or difference frequency light. Depending on the wavelength, the output light from one excitation laser may be input to the nonlinear optical crystal and second harmonic generation may be used.

(First embodiment)
In sum frequency generation using a non-linear crystal, if the wavelength λ ₃ of the sum frequency light is two wavelengths λ ₁ and λ ₂ of the excitation light,
1 / λ ₃ = 1 / λ ₁ + 1 / λ ₂ (1)
Given in. In order to generate the sum frequency light corresponding to the sodium D ₁ line and the D ₂ line, λ ₁ and λ ₂ that satisfy λ ₃ = 5899.592 nm or 588.995 nm are selected in the equation (1), and two wavelengths of It is necessary to combine the excitation lasers 121 and 122 and the nonlinear optical crystal 123.

In order to increase the generation efficiency of the sum frequency light, the propagation constants k _i = 2πn _i / λ _i (i) of the two incident light (λ ₁ , λ ₂ ) and the sum frequency light (λ ₃ ) in the nonlinear crystal. = 1, 2, 3)
k ₃ = k ₁ + k ₂ (2)
Must hold. Here, n _i is the refractive index of the nonlinear crystal at λ _i . However, since dispersion characteristics exist in the optical medium, equation (2) is satisfied only under specific conditions. Specifically, there is a method of changing the polarization direction of one of incident light and sum frequency light and using an ordinary light refractive index and an extraordinary light refractive index (for example, see Non-Patent Document 9). In addition, a method is employed in which a periodic polarization inversion structure is formed in a nonlinear optical crystal and conversion efficiency is improved by quasi phase matching (see, for example, Patent Document 2).

  Since the generation intensity of the sum frequency light is proportional to the product of the two excitation laser intensities, the selection of the two excitation lasers uses a combination of wavelengths satisfying the formula (1) and a higher intensity laser. . Among the existing semiconductor lasers (for example, see Non-Patent Document 10), the wavelength bands where high output is realized are (1) 940 nm band, (2) 980 nm band, (3) 1060 nm band, and (5) 1480 nm. It is a belt. Also, (4) 100 mW class semiconductor lasers have been developed in the 1300 nm band and (6) 1550 nm band. In particular, in the regions (4), (5), and (6), DFB (Distributed FeedBack) lasers have been developed, and single longitudinal mode oscillation and wavelength stabilization are realized. High-power semiconductor lasers have also been developed in the 800 nm to 880 nm region. When a semiconductor laser in this region is used as the excitation laser 1, the wavelength of the excitation laser 2 is 1780 nm or more. Since it is difficult to realize a semiconductor laser with high output and high reliability in such a long wavelength region, it is excluded.

FIG. 4 shows the relationship between the wavelengths of the excitation laser 1 and the excitation laser 2 for obtaining the wavelength of the sodium D line by sum frequency generation. The relationship for obtaining the sum frequency light with the wavelength of the excitation laser 1 being λ ₁ and the wavelength of the excitation laser 2 being λ ₂ is shown by a curve 30. Further, the regions of the excitation laser 1 of (1) to (6) above are defined as 1- (1), 1- (2), 1- (3), 1- (4), 1- (5), 1- ( Hatching was applied as 6). In addition, the regions of the excitation laser 2 of (1) to (6) above are defined as 2- (1), 2- (2), 2- (3), 2- (4), 2- (5), 2- (6) shows hatching. From FIG. 4, the excitation laser 1 and the excitation laser 2 intersect on the curve 30 either one of 1- (1) to 1- (6) and one of 2- (1) to 2- (6). By using the combination, it is possible to increase the efficiency of sum frequency generation.

The areas (1) to (6) are
(1) 940 ± 10 nm
(2) 980 ± 10 nm
(3) 1060 ± 10 nm
(4) 1280 nm to 1350 nm
(5) 1480 ± 10 nm
(6) 1530 nm to 1600 nm
It was. Here, (5) is the O band in optical communication, and (6) is the C band. These two wavelength bands are the most widely used regions in wavelength division multiplexing (WDM) technology, and it is easy to obtain optical components such as high-power and high-reliability semiconductor lasers.

  The combination of any one of 1- (1) to 1- (6) and any one of 2- (1) to 2- (6) on the curve 30 reverses the wavelengths of the pump laser 1 and the pump laser 2 Nevertheless, take into account that the sum frequency is the same. As a result, the combinations of (1) and (6), (2) and (5), and (3) and (4) intersect on the curve 30, and using this combination, It can be seen that the wavelength can be generated efficiently.

  In general, there are single mode oscillation and multimode oscillation as laser forms, but the characteristics of the sum frequency generated light are determined by the characteristics of the two pumping semiconductor lasers. In order to perform single mode oscillation, it is necessary to oscillate two pumping semiconductor lasers in single mode. For this purpose, it is necessary to use a semiconductor laser having a DFB structure or a laser using a fiber Bragg grating as a resonator structure. In the case of multimode oscillation, a Fabry-Perot semiconductor laser, a fiber grating having a reflection spectrum with a full width at half maximum of about 0.1 nm to 0.5 nm, can be realized by using a semiconductor laser adapted to the resonator structure. it can.

The nonlinear optical crystal may be anything as long as it has a large nonlinear optical constant and is transparent at the two laser wavelengths used for excitation and the sodium D-line wavelength, but is lithium niobate (LiNbO ₃ , LN), lithium tantalate (LiTaO _3). , LT) can be given as specific examples. In addition, these nonlinear optical crystals desirably have a periodic domain-inverted structure and a waveguide structure in order to generate sum frequency with high efficiency.

The periodic polarization reversal structure is a grating structure in which the polarization direction is reversed by 180 degrees with a period Λ with respect to the traveling direction of light. This structure satisfies the quasi phase matching condition in which the phase mismatch amount is zero. If the refractive indices of the nonlinear optical crystals at wavelengths λ ₁ , λ ₂ , and λ ₃ are n ₁ , n ₂ , and n ₃ , respectively,
2πn ₃ / λ ₃ = 2πn ₁ / λ ₁ + 2πn ₂ / λ ₂ + 2π / Λ (3)
If the periodic domain-inverted structure with a period Λ satisfying the above is satisfied, the generation efficiency of the sum frequency light can be maximized.

  Further, if a waveguide is formed in the nonlinear optical crystal, incident light from the excitation laser can be effectively confined, so that sum frequency light can be generated with high efficiency. The periodic domain-inverted structure can be realized by an electric field application method, and the waveguide structure can be realized by a proton exchange method, a dry etching method, a machining method using a dicing saw, or the like. A method for manufacturing the waveguide will be described later as a fifth embodiment.

  In addition, in order to generate sum frequency light, it is necessary to combine two semiconductor laser beams and an LN waveguide. These technologies have been established as device technologies for optical communication, and are large in realization. It is also characterized by no hindrance.

For example, the line width of an existing semiconductor DFB laser is 1 MHz, and the line width of an external mirror resonator type semiconductor laser using a fiber Bragg grating is about 100 kHz. When these are used as the excitation laser, the line width of the sum frequency light is several MHz or less because it is given by the renormalization integral of the two excitation light beam widths. When measuring the refractive index of sodium D line (wavelength 589.3 nm, frequency of about 500 THz) by the interferometry, the measurement accuracy is given by the ratio of the frequency and the line width of the laser beam to be used, and the line width is 5 MHz. The measurement accuracy is 10 ⁻⁸ . Therefore, according to the present embodiment, it is possible to improve the accuracy of the refractive index measurement for the sodium D line by about two digits from the current level.

As described above, along with the improvement of the characteristics of the nonlinear optical crystal, by selecting an existing laser device, it is possible to generate sodium D ₁ line and D ₂ line wavelength coherent light with high efficiency and high stability. It becomes possible to reduce the size and improve the accuracy of refractive index measurement.

(Example 1-1)
FIG. 5 shows a sodium D-line laser light source according to Example 1-1 of the present invention. The laser light source includes two excitation lasers 140 and 141, LN 144 periodically poled, lenses 142a and 142b for collimating the laser beams of the excitation lasers 140 and 141, and multiplexing for combining the two laser beams. And a filter 145 that separates the laser light of the excitation lasers 140 and 141 transmitted through the LN 144 and the sum frequency light generated by the LN 144.

The wavelength lambda ₁ of the excitation laser 140, the wavelength lambda ₂ of the excitation laser 141,
1 / λ ₁ + 1 / λ ₂ = 1 / (589.3 ± 2.0)
Is a combination that satisfies Furthermore, λ ₁ and λ ₂ are
λ ₁ = 976 ± 10 nm, λ ₂ = 1485 ± 20 nm
λ ₁ = 1064 ± 10 nm, λ ₂ = 1320 ± 20 nm
λ ₁ = 940 ± 10 nm, λ ₂ = 1565 ± 35 nm
It is set as the range which satisfies either of these. The λ ₂ semiconductor laser may be a DFB laser.

When the wavelength λ ₁ of the excitation laser 140 is 1064 nm, the incident intensity to the LN 144 is 50 mW, the incident intensity of the excitation laser 141 to λ ₂ = 1320 nm and the LN 144 is 70 mW, the wavelength λ _{3 is} 589.1 nm and the output is 20 μW. Sum frequency light was obtained.

(Example 1-2)
FIG. 6 shows a laser light source with a sodium D-line wavelength according to Example 1-2 of the present invention. The difference from the laser light source of Example 1-1 is in the nonlinear optical crystal. As the nonlinear optical crystal, a periodically poled LN waveguide 151 in which an LN crystal is made into a waveguide is used. In addition, a lens 150 for efficiently coupling incident laser light to the periodically poled LN waveguide 151 and a lens 152 for collimating light emitted from the periodically poled LN waveguide 151 are provided.

When the wavelength λ ₁ = 1064 nm of the pump laser 140 and the incident intensity to the LN 144 are 50 mW, and the wavelength λ ₂ = 1320 nm of the pump laser 141 and the incident intensity to the LN 144 are 70 mW, the wavelength λ ₃ = 589.1 nm and the output 10 mW. Sum frequency light was obtained.

(Example 1-3)
In the configurations of Example 1-1 and Example 1-2 (FIGS. 4 and 5), the excitation laser 140 is a laser using Nd ions in the vicinity of a wavelength of 1064 nm (for example, an Nd: YAG laser), and the excitation laser 141 is used. Is a 1300 ± 10 nm semiconductor laser.

(Example 1-4)
FIG. 7 shows a laser light source having a sodium D-line wavelength according to Example 1-4 of the present invention. In the configuration of Example 1-2, in order to couple two laser beams to the periodically poled LN waveguide 151, polarization-maintaining fibers (or single mode fibers) 161 and 163 and a multiplexer 162 are used. It was. The light emitted from the polarization-maintaining fiber 163 directly enters the end face of the periodically poled LN waveguide 151 or is coupled by the lens 164.

(Example 1-5)
FIG. 8 shows a laser light source with a sodium D-line wavelength according to Example 1-5 of the present invention. This is a further application example of Example 1-4. In the excitation lasers 170 and 171, an AR coating having a reflectance of 2% or less is applied to the emission side end surfaces 170 a and 171 a, and an HR coating having a reflectance of 90% or more is applied to the opposite end surfaces 170 b and 171 b. The outputs of the excitation lasers 170 and 171 are coupled via lenses 172a and 172b to polarization plane maintaining fibers (or single mode fibers) 173 and 174 in which a fiber Bragg grating is formed on the end face or in the middle of the fiber. In this way, a resonator is configured between the HR coating on the end faces 170b and 171b and the fiber Bragg grating.

The oscillation wavelength of each laser is controlled by the reflection spectrum of the fiber Bragg grating. At this time, the center wavelength of the fiber Bragg grating reflection spectrum is
976 ± 10 nm, 1485 ± 20 nm
1064 ± 10nm, 1320 ± 20nm
940 ± 10nm, 1565 ± 35nm
The line width (full width at half maximum) is 0.3 nm or less.

(Second Embodiment)
The configuration of the laser light source in the yellow region according to the embodiment of the present invention is as shown in FIG. In order to generate the sum frequency light corresponding to the yellow region, λ ₁ and λ ₂ that satisfy λ ₃ = 546.1 nm, 560.0 nm, or 585.0 nm are selected in the equation (1), and excitation lasers of two wavelengths are used. 21 and 22 and the nonlinear optical crystal 23 need to be combined.

FIG. 9 shows the relationship between the wavelengths of the excitation laser 1 and the excitation laser 2 for obtaining the wavelength in the yellow region by generating the sum frequency. The relationship for obtaining the sum frequency light with the wavelength of the excitation laser 1 being λ ₁ and the wavelength of the excitation laser 2 being λ ₂ is shown by a curve 30. Further, the regions of the excitation laser 1 of (1) to (6) above are defined as 1- (1), 1- (2), 1- (3), 1- (4), 1- (5), 1- ( Hatching was applied as 6). In addition, the regions of the excitation laser 2 of (1) to (6) above are defined as 2- (1), 2- (2), 2- (3), 2- (4), 2- (5), 2- (6) shows hatching. The areas (1) to (6) are the same as those in FIG.

From FIG. 9, the excitation laser 1 and the excitation laser 2 are any one of 1- (1) to 1- (6) and any of 2- (1) to 2- (6), λ ₃ = 546.1 nm. become a combination of cross on the curve 21, by using a combination of cross on the curve 23 which is a combination or λ ₃ = 585.0nm, intersect on a curve 22 which is a λ ₃ = 560.0nm, sum frequency generation High efficiency can be achieved.

  The combination of any one of 1- (1) to 1- (6) and any one of 2- (1) to 2- (6) on the curves 21 to 23 is the wavelength of pump laser 1 and pump laser 2 Taking into account that the sum frequency wavelength is the same even if is reversed. As a result, when the combinations of (1) and (4), (2) and (4), (3) and (4), and (1) and (6) are used, the wavelength in the yellow region is efficiently converted. It can be seen that it can occur automatically.

  As explained above, along with improving the characteristics of the nonlinear optical crystal, by selecting an existing laser device, it is possible to generate coherent light in the yellow region with high efficiency and high stability. The accuracy can be improved.

(Example 2-1)
FIG. 10 shows a laser light source in a yellow region according to Example 2-1 of the present invention. The laser light source includes two pump lasers 240 and 241, LN 244 whose polarization is periodically inverted, lenses 242a and 242b that collimate the laser beams of the pump lasers 240 and 241, and multiplexing that combines the two laser beams. And a filter 245 that separates the laser light of the excitation lasers 240 and 241 transmitted through the LN 244 and the sum frequency light generated by the LN 244.

The wavelength λ ₁ of the excitation laser 240, and the wavelength λ ₂ of the excitation laser 241,
1 / λ ₁ + 1 / λ ₂ = 1 / (546.1 ± 5.0)
Is a combination that satisfies Further, λ ₁ and λ ₂ are combinations of the above (1) and (4),
λ ₁ = 940 ± 10 nm, λ ₂ = 1320 ± 20 nm
To a range that satisfies The λ ₂ semiconductor laser may be a DFB laser.

When the wavelength λ ₁ = 940 nm of the pump laser 240 and the incident intensity to the LN 244 are 40 mW, and λ ₂ = 1320 nm of the pump laser 241 and the incident intensity to the LN 244 are 70 mW, the wavelength λ ₃ = 546.1 nm and the output 20 μW. Sum frequency light was obtained.

(Example 2-2)
FIG. 11 shows a laser light source in the yellow region according to Example 2-2 of the present invention. The difference from the laser light source of Example 2-1 is in the nonlinear optical crystal. As the nonlinear optical crystal, a periodically poled LN waveguide 251 in which an LN crystal is made into a waveguide is used. In addition, a lens 250 for efficiently coupling incident laser light to the periodically poled LN waveguide 251 and a lens 252 for collimating light emitted from the periodically poled LN waveguide 251 are provided.

When the wavelength λ ₁ = 940 nm of the excitation laser 240 and the incident intensity to the LN 244 are 40 mW, and λ ₂ = 1320 nm of the excitation laser 241 and the incident intensity to the LN 244 are 70 mW, the wavelength λ ₃ = 546.1 nm and the output 10 mW. Sum frequency light was obtained.

(Example 2-3)
In the configurations of Example 2-1 and Example 2-2 (FIGS. 10 and 11), the excitation laser 240 is a laser using Nd ions in the vicinity of a wavelength of 1064 nm (for example, Nd: YAG laser), and the excitation laser 241 is used. Is a semiconductor laser of 1320 ± 20 nm. Therefore, the combination of (3) and (4) described above makes it possible to obtain a yellow frequency sum frequency light having a wavelength λ ₃ = 585.0 nm.

(Example 2-4)
FIG. 12 shows a laser light source in the yellow region according to Example 2-4 of the present invention. In the configuration of Example 2-2, in order to couple two laser beams to the periodically poled LN waveguide 251, polarization plane maintaining fibers (or single mode fibers) 261 and 263 and a multiplexer 262 are used. It was. The light emitted from the polarization-maintaining fiber 263 is directly incident on the end face of the periodically poled LN waveguide 251 or is coupled by the lens 264.

(Example 2-5)
FIG. 13 shows a laser light source in the yellow region according to Example 2-5 of the present invention. This is a further application example of Example 2-4. In the excitation lasers 270 and 271, the emission-side end surfaces 270 a and 271 a are subjected to AR coating with a reflectance of 2% or less, and the opposite end surfaces 270 b and 271 b are subjected to HR coating with a reflectance of 90% or more. Outputs of the pump lasers 270 and 271 are coupled to polarization plane holding fibers (or single mode fibers) 273 and 274 in which a fiber Bragg grating is formed in the end face or in the middle of the fiber via lenses 272a and 272b. In this way, a resonator is configured between the HR coating on the end faces 270b and 271b and the fiber Bragg grating.

The oscillation wavelength of each laser is controlled by the reflection spectrum of the fiber Bragg grating. At this time, the center wavelength of the fiber Bragg grating reflection spectrum is
940 ± 10nm, 1320 ± 20nm
980 ± 10nm, 1320 ± 20nm
1064 ± 10nm, 1320 ± 20nm
940 ± 10nm, 1550 ± 30nm
The line width (full width at half maximum) is 0.3 nm or less.

(Third embodiment)
In the method of generating mid-infrared light by difference frequency generation using a nonlinear optical crystal and two excitation laser beams, the wavelengths of the two excitation laser beams are λ ₁ and λ ₂ and the wavelength of the generated mid-infrared light With λ ₃ is

で与えられる。ここで、波長λ_１、λ_２の大小関係は問わないが、便宜上λ_３＞０とするため、λ_１＜λ_２とする。差周波光λ_３を効率よく発生させるために、

Given in. Here, the magnitude relationship between the wavelengths λ ₁ and λ ₂ does not matter, but for the sake of convenience, λ ₃ > 0, so that λ ₁ <λ ₂ . The difference frequency light lambda ₃ in order to generate efficiently,

となる位相整合条件を満足する必要がある。（４）式において、ｋ_ｉ（ｉ＝１，２，３）は、非線形結晶内を伝搬する各レーザ光の伝搬定数であり、ｋ_ｉにおける非線形光学結晶の屈折率をｎ_ｉとすると、

It is necessary to satisfy the phase matching condition. In equation (4), k _i (i = 1, 2, 3) is a propagation constant of each laser beam propagating in the nonlinear crystal, and the refractive index of the nonlinear optical crystal at k _i is n _i .

となる。しかし、結晶のもつ分散特性により、一般的には（４）式を満足することは難しい。

It becomes. However, it is generally difficult to satisfy the formula (4) due to the dispersion characteristics of the crystal.

As a method for solving this, a quasi-phase matching method in which a nonlinear crystal is periodically poled is used. For the quasi-phase matching method, a ferroelectric crystal such as LiNbO ₃ is advantageous, but the sign of these nonlinear optical constants 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

で表される。特定の波長λ_１、λ_２を励起光として用いた場合には、（３）、（６）式を同時に満足し、高効率に差周波光λ_３を発生することができる。

It is represented by When specific wavelengths λ ₁ and λ ₂ are used as excitation light, the equations (3) and (6) can be satisfied simultaneously, and the difference frequency light λ ₃ can be generated with high efficiency.

However, when changing the wavelengths λ ₁ and λ ₂ to obtain difference frequency light having different wavelengths λ ₃ , if the wavelengths λ ₁ and λ ₂ vary, the equation (6) may be satisfied. The intensity of the difference frequency light λ ₃ is reduced. Here, the relationship between the wavelengths λ ₁ , λ ₂ , λ ₃ and the period Λ and the generation efficiency η of the difference frequency light will be considered. First, the phase mismatch amount Δk is

と定義する。このとき、試料長をｌとすると、差周波光の発生効率ηは、Δｋとｌの積に依存し、

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

と表される。（８）式において、η_ｏは、Δｋ＝０の時の差周波光の発生効率であり、ＬｉＮｂＯ_３など結晶の非線形光学定数、励起光強度、試料長などで決まる。したがって、同一の試料においては、周期Λが固定されているため、波長λ_１またはλ_２の変化は、Δｋを増減させ、発生効率ηの低下をもたらす。与えられた周期Λに対して、η≧０．５η_ｏ、すなわち

It is expressed. In equation (8), η _o is the generation efficiency of the difference frequency light when Δk = 0, and is determined by the nonlinear optical constant of the crystal such as LiNbO ₃ , the excitation light intensity, and the sample length. Therefore, since the period Λ is fixed in the same sample, a change in the wavelength λ ₁ or λ ₂ increases or decreases Δk, resulting in a decrease in generation efficiency η. For a given period Λ, η ≧ 0.5η _o , ie

となる波長λ_１、λ_２の領域を周期Λにおける３ｄＢ領域という。この３ｄＢ領域を広く取ることができれば、発生効率ηを低下させることなく、差周波光λ_３の波長を可変にすることができる。

The region of the wavelengths λ ₁ and λ ₂ is called a 3 dB region in the period Λ. If this 3 dB region can be widened, the wavelength of the difference frequency light λ ₃ can be made variable without reducing the generation efficiency η.

In the following discussion, z-cut LiNbO ₃ is used, and the case where the polarization directions of the two excitation light and difference frequency light are both in the c-axis direction of the crystal will be dealt with. At this time, two excitation light propagation characteristics of the difference frequency light is determined by the extraordinary refractive index n _{e. ne} is from the Selmeier equation:

で与えられる。ここで、Ｔは温度（Ｋ）、波長λの単位はμｍである。

Given in. Here, T is temperature (K), and the unit of wavelength λ is μm.

FIG. 14 shows a 3 dB region obtained by assuming the period Λ and using the wavelength λ ₃ as an auxiliary variable. The 3 dB region for the wavelengths λ ₁ and λ ₂ is given by the equations (1), (5), and (7). At room temperature, the difference frequency light wavelength λ ₃ calculated from the formula (3) = 2.0 μm, 2.5 μm, 3.0 μm, 3.5 μm, 4.0 μm, 4.5 μm, 5.0 μm, 5.5 μm, The relationship between wavelengths λ ₁ and λ ₂ giving 6.0 μm is indicated by a dotted line. In addition, 3 dB regions with respect to the period Λ = 26 μm, 27 μm, 28 μm, 29 μm, and 30 μm were obtained from the equations (7) and (9), and each region was indicated by hatching. The element length was 10 mm.

[Eta] = [eta] _o that completely satisfies phase matching is present at substantially the center of the 3 dB region. That is, in the generation of difference frequency light in LiNbO ₃ having a periodically poled structure with a period Λ, a quasi phase matching element with a period Λ is used. When obtaining the desired difference frequency light λ ₃ , the wavelengths λ ₁ and λ _{2 at} which η = 0.5η _o are obtained from the equations (3), (7), and (9), and the 3 dB region of the period Λ and the desired It can be seen that the difference frequency light λ ₃ is given at the intersection with the curve of the equation (3).

As an example, consider a case where LiNbO ₃ having a periodically poled structure with a period Λ = 28 μm is used to generate difference frequency light with a wavelength λ ₃ = 3.0 μm. The regions of wavelengths λ ₁ and λ ₂ where the dotted line of wavelength λ ₃ = 3.0 μm and the 3 dB region of period Λ = 28 μm intersect (the portion surrounded by circles in A in the figure) are η = 0.5η _o Become.

Next, specific conditions are shown. The generation intensity in the difference frequency light generation is proportional to the product of the two excitation light intensities. Therefore, in the examples reported so far, an Nd: YAG laser (wavelength: 1.064 μm) in which high intensity is easily obtained is mainly used. Here, it is assumed that the wavelength λ ₁ = 1.064 μm and the wavelength λ ₂ is changed to realize the wavelength-variable difference frequency light λ ₃ . When a sample of LiNbO ₃ having a periodic domain-inverted structure with period Λ is used, the wavelength of the region where the 3 dB region with period Λ shown by hatching in FIG. 14 intersects with the straight line B with wavelength λ ₁ = 1.064 μm the η = 0.5η _o at λ _2.

FIG. 15 shows the normalized conversion efficiency η / η _o with respect to the wavelength λ ₂ when the period Λ = 27 μm and the wavelength λ ₁ = 1.064 μm. The width of the wavelength λ ₂ that satisfies η = 0.5η _o is only about 2 nm, and therefore the wavelength variable amount of the difference frequency light λ ₃ is limited to about 20 nm. Even when the period Λ is changed to 28 μm, 29 μm, and 30 μm, if the wavelength λ ₁ = 1.064 μm, in any case, the width of the wavelength λ ₂ that satisfies η = 0.5η _o is only about 2 nm, The wavelength variable amount of the difference frequency light λ ₃ is also limited in the same manner.

However, it can be seen from FIG. 14 that there is a region where the wavelength variable region of the difference frequency light λ ₃ can be greatly expanded by fixing the wavelength λ ₁ and changing the wavelength λ ₂ . That is, if the straight line in which the wavelength λ ₁ is constant and the 3 dB region having the period Λ intersect in a wider range, the wavelength variable region width of the difference frequency light λ ₃ increases dramatically. A 3 dB region with a period Λ = 25.5 μm to 29 μm has a wavelength λ ₁ = 0.9 μm to 1.0 μm and is substantially parallel to the vertical axis. In this wavelength region of 0.9 μm to 1.0 μm, the wavelength λ ₁ is It intersects with a straight line over a wide range. That is, even when the domain-inverted structure LiNbO ₃ having a single period Λ is used, the wavelength λ ₁ is fixed in the range of 0.9 μm to 1.0 μm, and the wavelength λ ₂ is set in the region of 1.3 μm to 1.8 μm. When changed, the difference frequency light λ ₃ satisfies the phase matching condition in almost the entire range of wavelengths 1.3 μm <λ ₂ <1.8 μm, and can make the wavelength variable with high efficiency.

For example, the period lambda = 27 [mu] m, when the wavelength lambda ₁ = 0.94 .mu.m, normalized conversion efficiency for the wavelength lambda _2, the wavelength lambda _2> in the region of 1.43μm η = 0.5η _o becomes substantially wavelength 2μm~ Difference frequency light can be generated in a wide wavelength range of 3 μm. In the vicinity of the wavelength λ ₃ = 3 μm, as will be described later, it can be generated with one period Λ by temperature adjustment.

  As described above, the first laser, the second laser, and the nonlinear optical crystal having a polarization inversion structure with one period are provided, and one of the wavelengths of the laser is between 1.3 and 1.8 μm. By changing it, the laser beam in the mid-infrared region can be varied in the wavelength range of 2 to 3 μm.

(Example 3-1)
FIG. 3 shows a laser light source that generates mid-infrared light according to an embodiment of the present invention. The laser light source includes a semiconductor laser having a wavelength λ ₁ (λ ₁ = 0.94 band) 310, and a semiconductor laser 311 having a wavelength λ ₂ (λ ₂ = 1.45 to 1.60 μm). , A multiplexer 318 that combines the output lights of the semiconductor lasers 310 and 311, and a LiNbO having a domain-inverted structure of one period that inputs the combined output light and generates difference frequency light, that is, mid-infrared light ₃ crystal bulk 321. The output of the semiconductor laser 310 is connected to the multiplexer 318 via the coupled lens systems 312 and 313 and the polarization plane holding fiber 316. The output of the semiconductor laser 311 is connected to the multiplexer 318 via the coupling lens systems 314 and 315 and the polarization plane holding fiber 317.

  In the semiconductor laser 310, a high reflection film of 90% or more is formed on the end face 310A, and a low reflection film having a reflectance of 2% or less is formed on the opposite end face 310B. The polarization maintaining fiber 316 is provided with a fiber Bragg grating 316A to improve the wavelength stability. Further, if necessary, a fiber amplifier can be coupled to the polarization plane holding fiber 317 in the middle thereof to increase the output light of the semiconductor laser 311.

The output of the multiplexer 318 is connected to the LiNbO ₃ crystal bulk 321 through the optical fiber 319 and the coupling lens system 320. The output of the LiNbO ₃ crystal bulk 321 is connected to the spectroscope 325 via the coupling lens systems 322 and 324 and the optical fiber 323 in order to measure the output light that is mid-infrared light.

As shown by the straight line C in FIG. 14, when the wavelength λ ₁ is 0.94 μm, the wavelength of the semiconductor laser 311 is 1.45 to 1.60 μm when the period Λ of the LiNbO ₃ crystal bulk 321 is 27 μm. Even if the range is changed, the above-described 3 dB region can be obtained in one period Λ. In other words, mid-infrared light can be obtained in a wide wavelength range by one period Λ. In the wavelength lambda ₁ = 0.94 .mu.m band, when changing the wavelength lambda ₂ in a range of 1.45～1.60Myuemu, the wavelength lambda ₃ of the mid-infrared light generated, a wide range of 2.3~2.7μm It can be seen that

FIG. 17 shows a 3 dB region in the first embodiment. The vertical axis represents the mid-infrared light intensity, and the horizontal axis represents the wavelength λ ₂ of the semiconductor laser 311. As expected from the calculation results of FIG. 14, mid-infrared light having a substantially constant intensity in a wide wavelength range of 1.45 μm <λ ₂ <1.60 μm by the LiNbO ₃ crystal bulk 321 having one period Λ. Can be obtained. The output of the semiconductor laser 311 is constant in all wavelength regions. A change of 1.45 μm <λ ₂ <1.60 μm corresponds to a change of 2.7 μm> λ ₃ > 2.3 μm of mid-infrared light. The wavelength of the generated mid-infrared light is confirmed by the spectroscope 325. In this example, a LiNbO ₃ crystal bulk 321 having an element length of 10 mm was used, but the conversion efficiency was 1% / W in all wavelength regions.

When performing the difference frequency generation experiment as in the present embodiment, the maximum mid-infrared light is generated when the polarization directions of the two excitation lights coincide. Here, if the polarization direction of the semiconductor laser 310 is fixed and the polarization direction of the semiconductor laser 311 is tilted by an angle θ, the light intensity I ₃ of the mid-infrared light is set to I ₁ . When the light intensity of the semiconductor laser 311 is I ₂ ,

となる。（１１）式は、中赤外光の発生を確認する手段となる。図１８に、実施例３−１において出力された中赤外光の偏波依存性を示す。実験結果は、計算によるものとほぼ一致することが確かめられた。

It becomes. Equation (11) is a means for confirming the generation of mid-infrared light. FIG. 18 shows the polarization dependence of the mid-infrared light output in Example 3-1. It was confirmed that the experimental results were almost the same as those calculated.

(Example 3-2)
In Example 3-1, the wavelength range of the output mid-infrared light was 2.3 to 2.7 μm. However, by changing the period Λ of the LiNbO ₃ crystal, the wavelength range can be further expanded. it can. In Example 3-2, the period Λ of the LiNbO ₃ crystal bulk 321 shown in FIG. 16 was set to 26 μm. The semiconductor laser 310 is a device that is tunable in a minute range in the wavelength range of 0.91 μm, and the semiconductor laser 311 is a device that is tunable in a wide range of wavelengths of 1.30 to 1.68 μm.

In the 3 dB region, mid-infrared light having substantially constant intensity can be obtained in a wide wavelength range of 1.30 μm <λ ₂ <1.68 μm by the LiNbO ₃ crystal bulk 321 having one period Λ. Since the wavelength λ ₂ was changed from 1.30 to 1.68 μm, the wavelength λ ₃ of the mid-infrared light was 3.1 to 2.0 μm. In this example, a LiNbO ₃ crystal bulk 321 having an element length of 10 mm was used, but the conversion efficiency was 1% / W in all wavelength regions.

As can be seen from the equation (10), the refractive index of the LiNbO ₃ crystal changes with temperature, so that the effective period Λ also changes accordingly. Therefore, if the temperature of the LiNbO ₃ crystal is finely adjusted, even if the difference frequency is generated with the LiNbO ₃ crystal having one period Λ, the effective one period Λ can be changed, so that high conversion efficiency can be achieved. Can keep. As shown in FIG. 14, a region where the conversion efficiency cannot be kept high if the wavelength of the semiconductor laser 310 is fixed (for example, a region where the characteristic curve is not completely parallel to the vertical axis, such as a period Λ = 28, 29 μm. ) Therefore, the temperature of the LiNbO ₃ crystal bulk 321 can be adjusted to always optimize the effective period Λ with respect to the wavelength of the semiconductor laser 310 and maintain high conversion efficiency.

In Example 3-2, under an appropriate temperature adjustment, the period Λ is changed between 25.5 to 29.3 μm at intervals of 0.1 μm, and the difference frequency is calculated using the LiNbO ₃ crystal bulk 321 having the period Λ. generate. As a result, when the wavelength λ ₁ is appropriately selected in the wavelength range of 0.9 to 1.0 μm for each period Λ, and the wavelength λ ₂ is changed in the range of 1.27 to 1.80 μm accordingly, The wavelength λ ₃ of infrared light can be continuously obtained in the range of 3.1 to 2.0 μm. However, since the period Λ exceeds 28.5 μm and the portion of the characteristic curve parallel to the vertical axis decreases as shown in FIG. 14, the temperature control necessary to obtain the difference frequency light with a constant intensity. The contribution of gradually increased. Temperature changes 100 degrees, and corresponds to a change amount of 0.005μm wavelength lambda _1.

(Example 3-3)
If the wavelength conversion element is changed from a bulk type LiNbO ₃ crystal to a waveguide type and has the same configuration as in Examples 3-1 and 3-2, mid-infrared light can be obtained with higher efficiency. In Example 3-3, an optical system in which the LiNbO ₃ crystal bulk 321 shown in FIG. 16 is replaced with a waveguide element is used. The element length of the LiNbO ₃ waveguide was 10 mm, the core cross-sectional size was 8 μm × 8 μm, and the period Λ was 26 μm. The semiconductor laser 310 is tunable over a very small range of 0.91 μm band, and the semiconductor laser 311 is tunable over a wide range of 1.3 to 1.65 μm band.

The 3 dB region in the waveguide element has a substantially constant intensity over a wide wavelength range of 1.3 μm <λ ₂ <1.65 μm with respect to the wavelength λ ₁ = 0.91 μm band under appropriate temperature adjustment. Infrared light λ ₃ is obtained in the wavelength range 3.1-2.0 μm. Conversion efficiency was improved in all wavelength regions, and an improvement of two orders of magnitude was seen compared to bulk devices.

Further, the period Λ is changed between 25.5 to 29.3 μm at intervals of 0.1 μm, and mid-infrared light is generated using a LiNbO ₃ waveguide having the period Λ under appropriate temperature adjustment. As a result, the wavelength lambda ₁ in each period Λ appropriately select a range of 0.9～1.0Myuemu, In accordance with this, when changing the wavelength lambda ₂ in 1.27～1.80Myuemu, mid-infrared light The wavelength λ ₃ can be continuously obtained in the range of 3.1 to 2.0 μm.

(Example 3-4)
As shown in FIG. 14, the phase matching curve has a region where the curve suddenly occurs. When this region is used, there is no significant advantage in terms of wavelength variability. However, when the difference frequency is generated, the tolerance in the wavelength stability of the two excitation lights is greatly improved, and in particular, the tolerance of the semiconductor laser on the short wavelength side is improved. For example, in FIG. 14, when the period Λ = 27 μm, in the region where λ _{2 of the} semiconductor laser 11 is 1.45 to 1.8 μm, even if the wavelength λ ₂ fluctuates, it does not deviate from the 3 dB region. The wavelength λ ₁ of 310 is caused to deviate from the 3 dB region by the slight fluctuation. However, in the curved portion where the wavelength λ ₂ is near 1.35 μm, the wavelength variation λ ₁ on the half-wavelength side also has the advantage that the allowable amount of wavelength variation with respect to the 3 dB region is doubled. The temperature adjustment amount of the LiNbO ₃ crystal bulk 321 also decreases. Here, although the permissible amount for the wavelength λ ₂ is reduced, it is still a sufficient width in view of the stability of an ordinary commercially available laser light source.

In Example 3-4, an optical system is used in which the reflection film on the end faces 310A and 310B of the semiconductor laser 310 and the fiber Bragg grating 316A of the polarization plane holding fiber 316 are removed. Fiber Bragg grating is a device that can obtain light having a wavelength that is designed to selectively, in Example 3-1, thereby had suppressed variation of wavelength lambda _1. Therefore, there is a case where it is difficult to obtain a stable 3 dB region except for the fiber Bragg grating 316A. However, in Example 3-4, a sufficiently stable operation can be performed without deviating from the 3 dB region without having a configuration for stabilizing such a wavelength. Here, the period Λ of the LiNbO ₃ crystal bulk 321 was 27 μm, the wavelength of the semiconductor laser 310 was 0.945 μm, and the wavelength of the semiconductor laser 311 was 1.35 μm.

(Example 3-5)
According to the laser light source that generates mid-infrared light according to the present invention, it is possible to accurately detect NOx of environmental gas. Since the basic absorption of NOx gas is 5 μm or more, considering the absorption characteristics of LiNbO ₃ (it is difficult to transmit light having a wavelength of 5.4 μm or more), it is convenient to use the following reaction formula.
4NO + 4NH ₃ + O ₂ → 4N ₂ + 6H ₂ O (12)
6NO ₂ + 8NH ₃ → 7N ₂ + 12H ₂ O (13)
That is, since NOx is decomposed by NH ₃ under the catalyst, the concentration of NO and NO ₂ can be indirectly calculated by examining consumed NH ₃ or newly generated H ₂ O. . It is also possible to detect by utilizing the fact that the harmonics of basic absorption of NO and NO ₂ are at a wavelength of 2 to 3 μm. Therefore, if there is a laser light source capable of changing the wavelength at a wavelength of 2 to 3 μm, the above-mentioned gas absorption can be examined collectively. The names of main fundamental absorption wavelengths, wave numbers, and absorptions at wavelengths of 2 to 3 μm for the respective gases are as follows.
H ₂ O 2.662 μm 3756 cm ⁻¹ reverse symmetric stretching vibration H ₂ O 2.734 μm 3657 cm ⁻¹ fully symmetric stretching vibration NH ₃ 2.904 μm 3444 cm ⁻¹ double degenerate vibration NH ₃ 2.997 μm 3337 cm ⁻¹ total symmetric vibration NO 5.330 μm 1876 cm ⁻¹ Inverse symmetric stretching vibration Overtone = 2.665 μm
NO ₂ 6.180 μm 1618 cm ⁻¹ Inverse symmetrical stretching vibration Overtone = 3.090 μm
FIG. 19 shows a light absorption analyzer according to an embodiment of the present invention. In particular, an optical system for detecting the NOx gas concentration is shown. The gas cell 344 in which the gas to be measured is sealed has a maximum optical path length of 18 m using the reflecting mirrors at both ends. The reaction gas is guided from the gas removal pipe 346 to the gas cell 344 by the pump 345 and discharged to the gas exhaust pipe 348 by the pump 347. The pressure in the gas cell can be changed by using the pump. The gas removal pipe 346 removes NOx by the reaction of the formula (12) or the formula (13). The detector 349 is a HgCdTe detector for mid-infrared light.

The laser light source includes a semiconductor laser λ ₁ having a wavelength λ ₁ (λ ₁ = 0.91 μm band fixed) 330, a semiconductor laser 331 having a wavelength λ ₂ (λ ₂ = 1.28 to 1.46 μm), and a semiconductor A multiplexer 338 that combines the output lights of the lasers 330 and 331, and a LiNbO ₃ crystal bulk 341 having a period Λ = 26 μm that receives the combined output light and generates mid-infrared light are provided. The output of the semiconductor laser 330 is combined via the coupling lens systems 332 and 333 and the polarization plane holding fiber 336, and the output of the semiconductor laser 331 is combined via the coupling lens systems 334 and 335 and the polarization plane holding fiber 337, respectively. Connected to the device 338.

The semiconductor laser 330 has a high reflection film of 90% or more formed on its end face 330A, and a low reflection film having a reflectance of 2% or less is formed on the opposite end face 330B. The polarization maintaining fiber 336 is provided with a fiber Bragg grating 336A to improve the wavelength stability. The output of the multiplexer 338 is connected to the LiNbO ₃ crystal bulk 341 via the optical fiber 339 and the coupling lens system 340. The output of the LiNbO ₃ crystal bulk 341 is connected to the gas cell 344 via the coupling lens system 342 and the optical fiber 343.

In Example 3-5, the measurement result accompanying the removal of NO ₂ gas is shown first. Measurement is performed in the following three stages.
(I) Introducing only the NO ₂ gas into the gas removal pipe without supplying any catalyst or NH ₃ gas
(Ii) NH ₃ gas is supplied without supplying a catalyst, and NO ₂ gas is introduced into the gas removal pipe.
(Iii) Provide catalyst and NH ₃ gas and introduce NO ₂ gas into gas removal pipe
In stage (i), the wavelength of the semiconductor laser 331 is adjusted to 1.290 μm, which corresponds to the absence of a chemical reaction, and the harmonic overtone absorption of NO ₂ antisymmetric stretching vibration at a wavelength of 3.090 μm. Can be detected. On the other hand, even if the wavelength of the semiconductor laser 331 is adjusted again to match the absorption wavelength of NH ₃ or H ₂ O, these two absorptions are not observed.

In stage (ii), even if NH ₃ is supplied, there is no catalyst, so that the chemical reaction does not proceed, and absorption of unreacted NO ₂ and NH ₃ is observed. However, in the stage (iii), since the catalyst is given, the chemical reaction proceeds, and NO ₂ is removed and NH ₃ is consumed. Therefore, the absorption of NO ₂ and NH ₃ begins to decrease, instead, Absorption of newly generated H ₂ O is observed. Further, when a large amount of NH ₃ is added, the absorption of NO ₂ completely disappears, and the absorption of excess NH ₃ and newly generated H ₂ O increases.

Here, using the equation (13), the concentration of NO ₂ can be quantitatively measured in the stage (iii). That is, when a large amount of NH ₃ is added, absorption of NO ₂ decreases, and absorption of NH ₃ added excessively and newly generated H ₂ O appears. NH ₃ added up to either the point at which NO ₂ absorption becomes zero, the point at which excessive NH ₃ absorption begins to take place, or the point at which the absorption intensity of H ₂ O begins to increase and then takes a constant value If the amount is measured, the concentration of NO ₂ contained in the gas removal pipe can be calculated by equation (13).

The NH ₃ concentration can be accurately measured because it is only necessary to measure the added amount. In Example 3-5, when the LiNbO ₃ crystal bulk 341 having a bulk length of 10 mm is used, the minimum detected concentration of NO ₂ is 1 ppm at 100 Torr. When a 10 mm long waveguide was used, the minimum detected concentration of NO ₂ could be reduced to the order of 10 ppb.

It is convenient to use the equation (12) for the detection of NO gas. NH ₃ and O ₂ are added to the gas removal pipe 346, the point at which NO absorption becomes zero, and excessive NH ₃ absorption begins to occur. The concentration of NO can be calculated by measuring the amount of NH ₃ at the point or the point at which the absorption intensity of H ₂ O increases and then starting to take a constant value (here, no absorption of O ₂ is seen). However, since the wavelength of absorption of harmonic overtone of NO and absorption of inversely symmetric stretching vibration of H ₂ O are very close, absorption of total symmetry stretching vibration of H ₂ O and absorption of NH ₃ are mainly utilized. The minimum detected concentration of NO gas was almost the same as NO ₂ .

In Example 3-5, since only one period Λ needs to be prepared for the LiNbO ₃ crystal bulk 341, the measurement is extremely simple and quick. Further, if it is only to check whether or not NO and NO ₂ gas are present, it is not necessary to check only the presence or absence of an absorption peak and to measure the amount of NH ₃ , so that the measurement is simpler and quicker. .

(Example 3-6)
If a gas meter such as NOx, CO ₂ , and CO is configured using a laser light source in the wavelength range of 2 to 3 μm and a variable wavelength mid-infrared region, various types of gas concentrations can be measured with one light source. Here, the simultaneous detection of four types of gases, NO, NO ₂ , CO, and CO ₂ will be described. The fundamental absorption wavelength, wave number, absorption name, and harmonic overtone absorption wavelength of each target gas are as follows.
CO ₂ 4.257 μm 2349 cm ⁻¹ inverse symmetrical stretching vibration overtone = 2.129 μm
CO 4.666 μm 2143 cm ⁻¹ stretching vibration overtone = 2.333 μm
NO 5.330 μm 1876 cm ⁻¹ Inverse symmetric stretching vibration Overtone = 2.665 μm
NO ₂ 6.180 μm 1618 cm ⁻¹ Inverse symmetrical stretching vibration Overtone = 3.090 μm
H ₂ O 2.662 μm 3756 cm ⁻¹ reverse symmetric stretching vibration H ₂ O 2.734 μm 3657 cm ⁻¹ fully symmetric stretching vibration NH ₃ 2.904 μm 3444 cm ⁻¹ double degenerate vibration NH ₃ 2.997 μm 3337 cm ⁻¹ total symmetric vibration In this example, each gas was sequentially removed through the following three stages, and the gas concentration was measured. The configuration is the same as that of Example 3-5 shown in FIG.
(A) Introduce NO, NO ₂ , CO ₂ , and CO into the gas removal pipe without giving a catalyst or a removal gas
(B) Remove NO and NO ₂ by supplying catalyst and NH ₃ and O ₂ gas
(C) After NO and NO ₂ are removed in (b) above, CO gas is burned by supplying O ₂ gas
In stage (a), since no chemical reaction proceeds in the gas removal pipe 346, overtone absorption of NO, NO ₂ , CO ₂ , and CO gas is observed at a wavelength of 2 to 3 μm.

In stage (b), in response to the removal of NO and NO ₂ and the consumption of NH ₃ , these absorptions begin to decrease, and instead the absorption of newly generated H ₂ O is observed. Become so. Furthermore, when excess NH ₃ and O ₂ are added, absorption of NO and NO ₂ disappears completely, and absorption of excess NH ₃ and newly generated H ₂ O increases (again, No absorption of O ₂ is seen). In the stage (c), the absorption of CO ₂ increases as the CO burns according to the following reaction formula (14).

2CO + O ₂ → 2CO ₂ (14)
In stage (b), the total concentration of NO and NO ₂ can be quantitatively measured. That is, when a large amount of NH ₃ and O ₂ are added, absorption of NO and NO ₂ decreases, and absorption of NH ₃ added excessively and newly generated H ₂ O appears. Added to the point at which absorption of NO and NO ₂ becomes zero, the point at which absorption of excess NH ₃ begins to appear, or the point at which the absorption intensity of H ₂ O increases and starts to take a constant value If the amount of NH ₃ is measured, the total concentration of NO and NO ₂ contained in the gas removal pipe can be calculated from the equations (12) and (13). To know the individual concentrations of NO and NO ₂ , it may be based on Example 3-5.

In stage (c), the concentration of CO can be measured. That is, when CO is burned in the presence of O ₂ , CO ₂ is generated. Therefore, when O ₂ is added, the absorption of CO disappears, or the CO ₂ absorption increases and reaches a peak after reaching a peak. If the amount of O ₂ added to any of the points to start taking is measured, the concentration of CO contained in the gas removal pipe can be calculated by the equation (12). Since it is only necessary to measure the amount of O ₂ added, it can be accurately measured. In Example 3-6, when the LiNbO ₃ crystal bulk 341 having a bulk length of 10 mm was used, the minimum detected concentration of NO ₂ was 1 ppm at 100 Torr. When a 10 mm long waveguide was used, the minimum detected concentration of NO ₂ could be reduced to the order of 10 ppb.

(Example 3-7)
If a laser light source that generates mid-infrared light according to the present invention is used, gases such as NOx, CO ₂ , and CO that absorb at a wavelength of 2 to 3 μm can be detected by remote control. In Example 3-7, environmental gas was detected by a two-wavelength differential absorption lidar (see, for example, Non-Patent Document 11). The two-wavelength differential absorption lidar uses the absorption wavelength and non-absorption wavelength of the gas to be measured. Since the absorption wavelength of the lidar signal is more attenuated than the non-absorption wavelength, the signal difference between the two wavelengths is used. The concentration of gas molecules can be measured.

In Example 3-7, four gases of NO, NO ₂ , CO, and CO ₂ are detected by a two-wavelength differential absorption lidar. The fundamental absorption wavelength, wave number, name of absorption, and overtone absorption wavelength of each gas are as follows.
CO ₂ 4.257 μm 2349 cm ⁻¹ inverse symmetrical stretching vibration overtone = 2.129 μm
CO 4.666 μm 2143 cm ⁻¹ stretching vibration overtone = 2.333 μm
NO 5.330 μm 1876 cm ⁻¹ Inverse symmetric stretching vibration Overtone = 2.665 μm
NO ₂ 6.180 μm 1618 cm ⁻¹ Inverse symmetrical stretching vibration Overtone = 3.090 μm
In the measurement, it is necessary to measure two wavelengths as close as possible in order to obtain accurate data. However, the laser light source according to the present invention can instantaneously emit the target two wavelengths, and LiNbO ₃ Since only one period Λ needs to be prepared for the crystal, the measurement of the four gases in the wavelength band of 2 to 3 μm can be performed very quickly.

FIG. 20 shows a measurement system for a two-wavelength differential absorption lidar. The two-wavelength differential absorption lidar 360 includes a laser beam emitting unit 360A and a laser beam detecting unit 360B. The laser light source included in the laser beam emitting portion 360A used a LiNbO ₃ crystal waveguide with an element length of 10 mm and had a period Λ = 26 μm. The wavelength of the semiconductor laser 330 was 0.91 μm, and the wavelength of the semiconductor laser 331 was variable between 1.28 and 1.46 μm. Under appropriate temperature adjustment, mid-infrared light having a wavelength of 2 to 3 μm is output from the laser emission port 361.

  The mid-infrared light 364 is emitted toward the detection gas 366, and scattered light (Rayleigh scattering, Mie scattering) 365 from the detection gas 366 is received by the reflecting mirror 362 inside the laser light detection unit 360B. The collected light is detected by a detector 363 that is a HgCdTe detector.

  In the measurement, the non-absorption wavelength is set to the 2-10 nm lower wavelength side from the harmonic absorption wavelength of the detection gas. Since the measurable distance increases as the intensity of the generated mid-infrared light increases, the intensity of the mid-infrared light is set to a high output of 10 mW. When the above four gases are diffused at a concentration of 1 ppm in a space 3 meters away (= spherical space having a diameter of 1 meter or more), absorption of all the gases can be observed. If the gas concentration is increased to 10 ppm, it can be detected even if the measurement space is 10 meters away.

(Example 3-8)
The laser light source that generates mid-infrared light according to the present invention is also useful for detecting agricultural chemicals remaining in agricultural products. CN groups and NO ₂ groups contained in pesticides are representative examples of particularly harmful functional groups, and if these can be detected, a guideline for residual pesticide concentration can be obtained. The CN group and NO ₂ group are contained in the pyrethroid pesticide fenpropatoline and the carbamate pesticide 1-naphthyl-N-methylcarbamate. The absorption wavelength is CN group = 4.44 μm (2250 cm ⁻¹ , stretching vibration), NO ₂ group = 6.15 μm (1625 cm ⁻¹ , stretching vibration).

FIG. 21 shows a measurement system of the residual pesticide measuring instrument. The residual pesticide measuring instrument 380 includes a laser beam emission unit 380A and a laser beam detection unit 380B. Light is emitted to the measurement object 383 made of agricultural products by the optical fibers 381 and 382 provided at the respective ends, and the scattered light is detected by the laser light detection unit 380B. As a detector provided in the laser beam detector 380B, an HgCdTe detector and a PbSe detector are used. The laser light source included in the laser beam emitting portion 380A uses a LiNbO ₃ crystal waveguide having an element length of 10 mm and a period Λ = 26 μm under moderate temperature adjustment. The wavelength of one semiconductor laser was 0.91 μm band, and the wavelength of the other semiconductor laser was variable in the 1.30 to 1.65 μm band.

Fenpropatoline and 1-naphthyl-N-methylcarbamate are applied to the skin of the apple to be measured (concentration: 1 ‰), and this is irradiated with mid-infrared light with an output of 10 mW. As a result, it is possible to sufficiently observe the overtone absorption of the CN group at a wavelength of 2.22 μm and the overtone absorption of the NO ₂ group at a wavelength of 3.08 μm. According to Example 3-8, the presence of a plurality of functional groups can be confirmed by the LiNbO ₃ crystal having one period Λ even in the detection of residual agricultural chemicals.

If the functional group to be detected is only the NO ₂ group, other advantages can be shown. That is, if the period Λ = 27 μm of the LiNbO ₃ crystal waveguide (the period Λ = 26 μm may be used, but will be discussed with the period Λ = 27 μm to show the magnitude of the effect), as described in Example 3-4 In addition, when the absorption wavelength of the detection body is in a region slightly exceeding 3.0 μm, the wavelength stability for both of the semiconductor lasers used is improved. Even using an optical system has been removed and a fiber Bragg grating of the reflection film and the optical fiber end face of the semiconductor laser, it is possible to observe the overtone absorption of sufficient NO ₂ group (Note that this effect is the aforementioned NO ₂ The same applies to gas detection).

(Fourth embodiment)
FIG. 22 shows a laser light source that generates a wavelength of an oxygen absorption line according to an embodiment of the present invention. A laser light source that generates a wavelength of an oxygen absorption line is a distribution that oscillates a laser beam having a wavelength twice as large as the wavelength of one absorption line selected from oxygen absorption lines existing at wavelengths of 759 nm to 768 nm. A feedback semiconductor laser module 401, an optical waveguide 403 having a second-order nonlinear optical effect, and a polarization maintaining fiber 402 that connects one end of the semiconductor laser module 401 and the optical waveguide 403 having a second-order nonlinear optical effect are provided. Yes.

  Unlike the prior art, since it oscillates at 1518 nm to 1536 nm, which is twice the wavelength of 759 nm to 768 nm, an indium phosphide-based material is used as the semiconductor laser. It is known that indium phosphide does not cause a so-called death phenomenon of the device as compared with gallium arsenide and has high reliability with respect to the device lifetime. Further, wavelengths from 1518 nm to 1536 nm belong to the S and C bands of the communication wavelength band, and due to recent developments in the optical communication field, a DFB type manufacturing technique is easy. Furthermore, a device with a high output of 40 mW can be manufactured.

  In an indium phosphide-based semiconductor laser, the wavelength can be changed by changing the temperature of the element and the injection current, and a stable wavelength scan without mode skipping can be performed by adopting a DFB type structure. it can. Laser light having a wavelength of 1518 nm to 1536 nm is output from light having a wavelength of 759 nm to 768 nm using second harmonic generation based on the second-order nonlinear optical effect.

Here, the second-order nonlinear optical effect will be described. The nonlinear optical effect is an effect that occurs because the electric polarization P in a substance has higher-order terms E ² and E ^{3 in} addition to the term proportional to the electric field E of light as described below.
P = χ ⁽¹⁾ E + χ ⁽²⁾ E ² + χ ⁽³⁾ E ³ + (15)
In particular, the second term is an effect that appears strongly in a material that is out of central contrast. When three lights ω ₁ , ω ₂ , and ω ₃ having different angular frequencies are in a relationship of ω ₁ + ω ₂ = ω ₃ ,
1) When ω ₁ and ω ₂ lights are input, ω ₃ light is generated (sum frequency generation)
2) When ω ₁ and ω ₂ have the same angular frequency during sum frequency generation, second harmonics are generated. 3) When ω ₁ and ω ₃ lights are input, ω ₂ (= ω ₃ −ω ₁ ) Generate light (difference frequency generation)
This produces the effect. That is, the wavelength of the input laser beam can be converted to another wavelength.

  A highly efficient wavelength converter is realized by periodically inverting the polarization of the secondary nonlinear optical material. This structure artificially matches the phases of the input light and the converted light by periodically inverting the influence of the refractive index dispersion caused by the material. As an example using this principle, for example, a wavelength converter is known in which the polarization of lithium niobate, which is a second-order nonlinear optical material, is periodically reversed and a waveguide is formed by proton exchange (for example, non-patented). Reference 12). It has been shown that in the lithium niobate optical waveguide having such a periodically poled structure, 90% or more of second harmonic generation is possible.

  An optical waveguide having such a second-order nonlinear optical effect has a problem relating to the lifetime, that is, the efficiency of second harmonic generation is reduced due to the photorefractive effect. Since such a problem does not occur with light having a wavelength of 1518 nm to 1536 nm, it is caused by the light intensity of wavelengths 759 nm to 768 nm which is the second harmonic. However, it is known that a decrease in efficiency can be avoided by raising the temperature of the optical waveguide having a second-order nonlinear optical effect from about 50 ° C. to about 100 ° C. or using a second-order nonlinear optical material to which zinc or magnesium is added. Therefore, it is easy to obtain a long-life optical waveguide (for example, see Non-Patent Document 13).

  An optical waveguide having such a second-order nonlinear optical effect produces a large effect on light polarized in a specific direction with respect to the crystal orientation. For example, in the case of lithium niobate, it is the z-axis direction. The semiconductor laser also oscillates with a certain polarization with respect to the substrate. Therefore, when the semiconductor laser module 401 and the optical waveguide 403 having the second-order nonlinear optical effect are connected by an optical fiber, the polarization maintaining fiber 402 is used in order to suppress fluctuations in the polarization direction of light incident on the optical waveguide. Is preferred. The second harmonic can be generated even if the polarization control element is inserted into the optical fiber by connecting with an optical fiber that is not a polarization maintaining type. However, since the polarization in the optical fiber fluctuates due to changes in the external environment such as temperature, it is difficult to stably generate the second harmonic in the long term.

  FIG. 23 shows a laser light source provided with a lens and a filter at the output. In addition to the laser light source of FIG. 22, at the other end of the optical waveguide 413 having a second-order nonlinear optical effect, a lens 414 that converts emitted light into parallel light, and light having a wavelength of 1518 nm to 1536 nm among the emitted light. And a filter 415 that transmits light having a wavelength of 759 nm to 768 nm. In this way, it is possible to extract light for performing stable wavelength scanning without mode skipping at wavelengths of 759 nm to 768 nm, which are oxygen absorption lines.

  FIG. 24 shows a laser light source provided with an optical fiber at the output. Instead of the embodiment of FIG. 23, an optical fiber 424 is connected to the other end of the optical waveguide 423 having a second-order nonlinear optical effect. If the optical fiber 424 has a structure capable of guiding in a single mode with respect to light with a wavelength of 759 nm to 768 nm, light having a wavelength of 759 nm to 768 nm, which is an oxygen absorption line, can be obtained by slightly bending the optical fiber 424. Can only be taken out. This is because light having a wavelength of 1518 nm to 1536 nm propagates as a wide mode in the optical fiber 424, and if there is a portion where bending is applied, the light is scattered and attenuated in the optical fiber 424. is there.

  As described above, the second harmonic generation based on the second-order nonlinear optical effect of the optical waveguide is used to output a laser beam having a wavelength of 759 nm to 768 nm, which is an oxygen absorption line, and a stable wavelength scan without mode skipping. Therefore, it is possible to provide a laser light source with high output and long life.

(Example 4-1)
FIG. 25 shows a laser light source according to Example 4-1. The laser light source according to Example 4-1 includes a distributed feedback semiconductor laser module 431 that oscillates laser light, an optical waveguide 433 having a second-order nonlinear optical effect, a semiconductor laser module 431, and an optical light having a second-order nonlinear optical effect. A polarization maintaining fiber 432 connecting one end 433a of the waveguide 433. The other end 433b of the optical waveguide 433 having a second-order nonlinear optical effect has a lens 435 that collimates the emitted light and does not transmit light near 1526 nm of the emitted light, and transmits light near 763 nm. A filter 436 for transmitting light is disposed.

  The semiconductor laser module 431 oscillates laser light in the vicinity of 1526.08 nm, which is twice the wavelength of 763.04 nm, which is one of the oxygen absorption lines output from the polarization maintaining fiber 432. The semiconductor laser module 431 has a built-in Peltier element (not shown) so that the temperature of the element can be changed. Further, the semiconductor laser module 431 includes an isolator (not shown) so that the reflected light on the end face of the optical waveguide 433 does not adversely affect the laser oscillation.

  The optical waveguide 433 having a second-order nonlinear optical effect has a periodically poled structure on a lithium niobate substrate, and a waveguide is formed using the method according to the fifth embodiment or the heat treatment proton exchange method. One end 433a of the optical waveguide 433 is provided with a coating that is non-reflective with respect to a wavelength of 1526 nm. Further, the other end 433b of the optical waveguide 433 is provided with a coating that does not reflect light with respect to a wavelength of 763 nm. Further, a Peltier element 434 for controlling the temperature of the optical waveguide 433 is disposed under the optical waveguide 433 so that the second harmonic generation efficiency at the incident light wavelength of 156.08 nm of the optical waveguide 433 is the best. The optical waveguide 433 is kept at a temperature of 90 ° C.

  When the semiconductor laser module 431 was set to 25 ° C. and operated at a wavelength of 1526.08 nm and an output of 30 mW, light having a wavelength of 763.04 nm and an output of 5 mW was observed as the output light 437. When the output light 437 was observed while the temperature of the semiconductor laser module 431 was continuously changed from 24 ° C. to 26 ° C., the wavelength was continuously changed from 762.99 nm to 762.09 nm, like a mode skip. The phenomenon was not seen. The light intensity of the output light 437 showed a stable operation from 4.7 mW to 5.0 mW. This operation was continuously performed for one year, but no decrease in output and wavelength jump were observed.

(Example 4-2)
FIG. 26 shows a laser light source according to Example 4-2. The laser light source according to Example 4-2 includes a distributed feedback semiconductor laser module 441 that oscillates laser light, an optical waveguide 445 having a second-order nonlinear optical effect, a semiconductor laser module 441, and an optical light having a second-order nonlinear optical effect. Polarization maintaining fibers 442 and 444 and an optical connector 443 that connect one end 445a of the waveguide 445 are provided. An optical fiber 447 is connected to the other end 445b of the optical waveguide 445 having a second-order nonlinear optical effect, and a lens 448 that converts emitted light into parallel light is disposed.

  The same semiconductor laser module 441 as that of the semiconductor laser module 431 of Example 4-1 was used. An optical waveguide 445 having a second-order nonlinear optical effect is obtained by forming a periodically poled structure on a Zn-doped lithium niobate substrate and forming the waveguide using the method according to the fifth embodiment or the heat treatment proton exchange method. Has been. One end 445 a of the optical waveguide 445 is coated with a coating that is non-reflective with respect to a wavelength of 1526 nm, and a polarization maintaining fiber 444 that is in a single mode with respect to light having a wavelength of about 1526 nm is connected. Further, the other end 445b of the optical waveguide 445 is provided with a coating that is non-reflective with respect to a wavelength of 763 nm, and an optical fiber 447 that is in a single mode with light near a wavelength of 763 nm is connected.

  A Peltier element 446 for temperature control is arranged under the optical waveguide 445, and the optical waveguide 445 is 25.0 so that the second harmonic generation efficiency at the incident light wavelength 1526.08 nm of the optical waveguide 445 is the best. Keep temperature at ℃. The optical fiber 442 and the optical fiber 444 are connected by the connector 443, and the light output of the optical fiber 447 is converted into parallel light by the lens 448.

  When the semiconductor laser module 441 was set to 25 ° C. and operated at a wavelength of 156.08 nm and an output of 30 mW, light having a wavelength of 763.04 nm and an output of 7 mW was observed as the output light 449. The output light 449 was observed while the temperature of the semiconductor laser module was continuously changed from 24 ° C. to 26 ° C., and the temperature of the optical waveguide 445 was continuously changed from 24 ° C. to 26 ° C. by the Peltier element 446. The wavelength continuously changed from 762.99 nm to 763.09 nm, and the light intensity of the output light 449 showed a very stable operation from 6.9 mW to 7.0 mW.

  At this time, in the output light 449, the light having a wavelength of 1526 nm that was transmitted without being converted into the second harmonic was below the observation limit. This is because light near 1526 nm propagates as a wide mode in the optical fiber 447, and if there is a portion where the optical fiber 447 is bent, the light is scattered and attenuated in the optical fiber 447. It is. For safety, a filter that removes the wavelength of 1526 nm may be attached after the lens 448. In Example 4-2, the polarization maintaining fiber is connected by the connector 443, but it goes without saying that it may be fused.

  In this example, a semiconductor laser was selected and focused on one of the oxygen absorption lines, 763.04 nm, but other absorption lines existing from 759 nm to 768 nm, for example, 760.4 nm, You may choose 1520.8nm which is a double wavelength.

  In the present embodiment, an optical waveguide having a second-order nonlinear optical effect having a periodically poled structure is used, but the same effect can be obtained by using another phase matching method. Also, the substrate used was lithium niobate or zinc doped with this, but a mixed crystal of lithium niobate and lithium tantalate, or a material with a small amount of elements added thereto, or other two Similar effects can be obtained by using a second-order nonlinear optical material. Further, the method according to the fifth embodiment or the heat treatment proton exchange method is used as a waveguide manufacturing method, but the same effect can be obtained by using a metal diffusion waveguide such as Ti diffusion, a ridge waveguide, or a buried waveguide. It goes without saying that it is obtained.

  A waveguide structure in the vicinity of both ends of an optical waveguide having a second-order nonlinear optical effect so that light can be easily coupled to an optical fiber connected to each end face, or to optimize the shape of light when spatially radiating Needless to say, it may be changed. In addition, an isolator is built in the semiconductor laser module, but the end face of the optical waveguide having the second-order nonlinear optical effect is attached to the end face, and the optical waveguide having the second-order nonlinear optical effect is cut out obliquely and an optical fiber and a lens are arranged. Or a combination of these may prevent reflected return light.

(Fifth embodiment)
Next, a method for forming a waveguide in the nonlinear optical crystal will be described. In the present embodiment, a ridge type waveguide using a wafer direct bonding substrate is used. In the wafer direct bonding method, an LiNbO ₃ substrate having a polarization inversion structure matched to an operating wavelength and a surface-treated substrate are directly bonded at room temperature without using an adhesive, and an annealing process is performed. For the waveguide, the domain-inverted structure of the bonded substrate is ground or thinned, and a ridge-type waveguide is formed using a dicing saw.

As a problem of the LiNbO ₃ substrate, there is an improvement in light damage resistance. Optical damage is the result of a change in refractive index (photorefractive effect) that is induced by the excitation of carriers from defects present in the crystal by light incident on the waveguide and subsequent trapping in the crystal. Is a phenomenon that shifts. Since the operating wavelength band of the waveguide is as narrow as 1 nm due to the LiNbO ₃ substrate, the power of the output light is greatly reduced or not output at all if there is optical damage. In order to realize sufficient optical damage resistance in a waveguide element produced on a non-doped LiNbO ₃ substrate using a proton exchange method, the operating temperature of the waveguide element needs to be 100 ° C. or higher. There was a problem that long-term stability could not be maintained due to proton re-diffusion. In the waveguide element prepared by using the proton exchange method on the LiNbO ₃ substrate doped with Mg or Zn instead of the non-doped LiNbO ₃ substrate, a certain improvement in optical damage resistance can be seen. It is necessary to heat more.

On the other hand, the ridge-type waveguide using a directly bonded substrate is a manufacturing method that does not degrade the original crystallinity of LiNbO ₃ and can suppress the generation of new defects, thus greatly improving optical damage resistance. Can be made.

Here, the wavelength conversion efficiency is the power Pa of the sum frequency light or difference frequency light.
^{_{P = ηL 2 P 1 P 2}} /100
The second harmonic power Pb is
^{_{Pb = ηL 2 P 3 2/}} 100
It becomes. η is the efficiency per unit length (% / W / cm ² ), L is the element length, and P ₁ P ₂ P ₃ is the output light power of the excitation laser.

In this embodiment, it can operate outside the wavelength band for optical communication, and a stable output of 10 mW or more can be obtained by combining with a high-power semiconductor laser of about 10 to 100 W. In this way, laser light having an arbitrary wavelength can be generated in the region of 450 nm to 5 μm where LiNbO ₃ is transparent.

(Example 5-1)
FIG. 27 shows a method for manufacturing a single-mode ridge waveguide. The first substrate 501 is a Z-cut Zn-added LiNbO ₃ substrate in which a periodic domain-inverted structure is prepared in advance, and the second substrate 502 is a Z-cut Mg-added LiNbO ₃ substrate. Each of the substrates 501 and 502 is a 3-inch wafer whose both surfaces are optically polished, and the thickness of the substrate is 300 μm. After the surfaces of the first substrate 501 and the second substrate 502 are made hydrophilic by normal acid cleaning or alkali cleaning, the substrates 501 and 502 are superposed in a clean atmosphere. The superposed substrates 501 and 502 are put into an electric furnace and subjected to diffusion bonding by heat treatment at 400 ° C. for 3 hours (first step). The bonded substrates 501 and 502 were void-free, and no cracks or the like occurred when returned to room temperature.

Next, using a polishing apparatus in which the flatness of the polishing surface plate is controlled, polishing is performed until the thickness of the first substrate 501 of the bonded substrates 501 and 502 becomes 5 to 10 μm. After the polishing process, a mirror-polished polishing surface is obtained by performing a polishing process (second step). When the parallelism of the substrate was measured using an optical parallelism measuring instrument, submicron parallelism was obtained almost entirely except for the periphery of a 3-inch wafer, and a thin film substrate suitable for making a waveguide was produced. can do. Note that an X-cut Zn-added LiNbO ₃ substrate may be used as the first substrate 501, and an X-cut Mg-added LiNbO ₃ substrate may be used as the second substrate 502.

A waveguide pattern is produced on the produced thin film substrate surface by a normal photolithography process, and then the substrate is set in a dry etching apparatus, and the substrate surface is etched using CF ₄ gas as an etching gas to have a width of 6 to 20 μm. A core was formed to produce a ridge-type waveguide (third step). A waveguide element of a nonlinear optical crystal having a length of 10 to 60 mm can be obtained by cutting out a ridge-type waveguide from a wafer and optically polishing the end face of the waveguide.

(Example 5-2)
The first substrate 501 is a Z-cut Zn-added LiNbO ₃ substrate in which a periodic domain-inverted structure is prepared in advance, and the second substrate 502 is a Z-cut LiTaO ₃ substrate. Each of the substrates 501 and 502 is a 3-inch wafer whose both surfaces are optically polished, and the thickness of the substrate is 300 μm. After the surfaces of the first substrate 501 and the second substrate 502 are made hydrophilic by normal acid cleaning or alkali cleaning, the substrates 501 and 502 are superposed in a clean atmosphere. The superposed substrates 501 and 502 are put into an electric furnace and subjected to diffusion bonding by heat treatment at 400 ° C. for 3 hours (first step). The bonded substrates 501 and 502 were void-free, and no cracks or the like occurred when returned to room temperature.

Next, using a polishing apparatus in which the flatness of the polishing surface plate is controlled, polishing is performed until the thickness of the first substrate 501 of the bonded substrates 501 and 502 becomes 6 to 10 μm. After the polishing process, a mirror-polished polishing surface is obtained by performing a polishing process (second step). When the parallelism of the substrate was measured using an optical parallelism measuring instrument, submicron parallelism was obtained almost entirely except for the periphery of a 3-inch wafer, and a thin film substrate suitable for making a waveguide was produced. can do. Note that an X-cut Zn-added LiNbO ₃ substrate may be used as the first substrate 501, and an X-cut LiTaO ₃ substrate may be used as the second substrate 502.

A waveguide pattern is produced on the produced thin film substrate surface by a normal photolithography process, and then the substrate is set in a dry etching apparatus, and the substrate surface is etched using CF ₄ gas as an etching gas to have a width of 6 to 20 μm. A core was formed to produce a ridge-type waveguide (third step). A waveguide element of a nonlinear optical crystal having a length of 10 to 60 mm can be obtained by cutting out a ridge-type waveguide from a wafer and optically polishing the end face of the waveguide.

(Example 5-3)
The first substrate 501 is a LiNbO ₃ substrate on which a periodic domain-inverted structure is prepared in advance, and the second substrate 502 is a quartz substrate. Thermal expansion coefficient of the plane perpendicular to the Z axis of the crystal is 13.6 × ^{10 -6} / K, close to the thermal expansion coefficient of the LiNbO _3, although the refractive index of the LiNbO ₃ is 2.1 On the other hand, since the refractive index of quartz is as small as 1.53, it is suitable for manufacturing a waveguide. A waveguide element of a nonlinear optical crystal can be obtained by the same manufacturing method as in Example 5-1.

In addition to the Zn-doped LiNbO ₃ substrate, the Mg-doped LiNbO ₃ substrate, the Sc-doped LiNbO ₃ substrate, the In-doped LiNbO ₃ substrate, the LiTaO ₃ substrate, the LiNb _x Ta _1-x O ₃ substrate, and the KNbO ₃ as the first substrate 501 A substrate, a KTiNbO ₃ substrate, or the like may be used.

(Example 5-4)
A waveguide is produced from the substrate produced up to the second step of Example 5-1 using a precision grinding technique using a dicing saw. The polished substrate is set on a dicing saw, and a ridge waveguide having a core having a width of 6 μm is produced by precision processing using a diamond blade having a particle diameter of 4 microns or less (third step). A waveguide element of a nonlinear optical crystal having a length of 10 to 60 mm can be obtained by cutting out a ridge-type waveguide from a wafer and optically polishing the end face of the waveguide. In addition, the board | substrate produced in Example 5-2 and Example 5-3 can also be used.

  According to the present embodiment, the refractive index measurement for the sodium D line can improve the accuracy by about two digits from the current level. Therefore, not only the quality control of foodstuffs or pharmaceuticals can be greatly improved, but also the safety can be greatly improved by improving the monitoring accuracy of contamination of foreign substances and poisons. In addition, with respect to a substance whose relationship between the refractive index and the density is known, it is possible to obtain the density from the measurement of the refractive index, and the accuracy improvement in the density measurement is also drastically improved.

  Further, according to the present embodiment, by adopting a laser light source with high energy efficiency, downsizing, and low power consumption, a small and economical laser microscope, flow cytometer, and the like can be realized.

  Furthermore, the laser light source that generates mid-infrared light according to the present embodiment can be applied to a measuring device that detects environmental gas with high accuracy and detects agricultural chemicals remaining on agricultural products.

  Furthermore, it is a light source used for an oximeter, and can be used as a laser light source that outputs laser light having a wavelength of 759 nm to 768 nm, which is an oxygen absorption line.

It is a figure which shows the relationship between the wavelength range of a laser, and an output. It is a figure which shows the energy level of a sodium atom. It is a block diagram of the laser light source concerning one Embodiment of this invention. It is a figure which shows the relationship of the wavelength of the excitation laser 1 and the excitation laser 2 for obtaining the wavelength of a sodium D line by sum frequency generation. It is a block diagram of the laser light source of the sodium D line wavelength concerning Example 1-1 of this invention. It is a block diagram of the laser light source of the sodium D line wavelength concerning Example 1-2 of this invention. It is a block diagram of the laser light source of the sodium D line wavelength concerning Example 1-4 of this invention. It is a block diagram of the laser light source of the sodium D line wavelength concerning Example 1-5 of this invention. It is a figure which shows the relationship of the wavelength of the excitation laser 1 and the excitation laser 2 for obtaining the wavelength of a yellow area | region by sum frequency generation. It is a block diagram of the laser light source of the yellow area | region concerning Example 2-1 of this invention. It is a block diagram of the laser light source of the yellow area | region concerning Example 2-2 of this invention. It is a block diagram of the laser light source of the yellow area | region concerning Example 2-4 of this invention. It is a block diagram of the laser light source of the yellow area | region concerning Example 2-5 of this invention. It is a figure which shows the 3dB area | region calculated | required by making wavelength (lambda) ₃ into an auxiliary variable supposing the period (lambda). It is a figure which shows normalized conversion efficiency (eta) / (eta) _o with respect to wavelength (lambda) ₂ when period (LAMBDA) = 27 micrometer and wavelength (lambda) ₁ = 1.064micrometer. It is a block diagram which shows the laser light source which generate | occur | produces the mid-infrared light concerning one Embodiment of this invention. It is a figure which shows 3 dB area | region in Example 3-1. It is a figure which shows the polarization dependence of the mid-infrared light output in Example 3-1. It is a block diagram which shows the light absorption analyzer concerning one Embodiment of this invention. It is a figure which shows the measurement system of the 2 wavelength difference absorption lidar concerning Example 3-7. It is a figure which shows the measurement system of the residual pesticide measuring device concerning Example 3-8. It is a block diagram which shows the laser light source which generate | occur | produces the wavelength of the oxygen absorption line concerning one Embodiment of this invention. It is a block diagram which shows the laser light source provided with the lens and the filter in the output. It is a block diagram which shows the laser light source provided with the optical fiber in the output. It is a block diagram which shows the laser light source concerning Example 4-1. It is a block diagram which shows the laser light source concerning Example 4-2. It is a figure which shows the preparation methods of a single mode ridge type | mold waveguide.

Explanation of symbols

401, 411, 421, 431, 441 Semiconductor laser modules 402, 412, 422, 432, 442, 444 Polarization-maintaining optical fibers 403, 413, 423, 433, 445 Optical waveguides 414, 435 having a second-order nonlinear optical effect , 448 Lens 415, 436 Filter 416, 437, 449 Optical output 424, 447 Optical fiber 434, 446 Temperature control element 443 Optical connector

Claims (4)

A distributed feedback semiconductor laser that oscillates a laser beam having a wavelength twice that of one absorption line selected from oxygen absorption lines existing at a wavelength of 759 nm to 768 nm;
An optical waveguide having a second-order nonlinear optical effect;
A polarization maintaining fiber connecting the output of the distributed feedback semiconductor laser and one end of the optical waveguide ;
An optical fiber having a structure capable of guiding in a single mode with respect to the second harmonic light generated in the optical waveguide is connected to the other end of the optical waveguide,
The optical waveguide has a structure in which the polarization of a second-order nonlinear optical material is periodically inverted, and the wavelength of the absorption line is λ ₁ , and the wavelength of a laser beam having a wavelength twice the wavelength of the absorption line is set. Assuming that λ ₃ and the refractive indexes of the nonlinear optical materials at wavelengths λ ₁ and λ ₃ are n ₁ and n ₃ , respectively ,
2πn ₃ / λ ₃ = 2πn ₁ / λ ₁ + 2πn ₁ / λ ₁ + 2π / Λ
A laser light source characterized by having a structure periodically inverted by a period Λ satisfying A temperature control element connected to the optical waveguide;
The laser light source according to claim 1, further comprising a temperature control circuit for controlling the temperature of the optical waveguide by controlling the temperature control element. 3. The laser light source according to claim 1, wherein the second-order nonlinear optical material is any one of lithium niobate, lithium tantalate, or a mixed crystal of lithium niobate and lithium tantalate. The laser light source according to claim 3 , wherein the secondary nonlinear optical material is added with either zinc or magnesium.

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