Optical device, method for installing optical resonator in optical device, and optical resonator

Abstract

This optical device comprises an optical resonator 13 that uses laser light, can reflect laser light, and is provided with a first mirror 15 and a second mirror 16 disposed at positions facing each other. There is no change in the laser light at a position t2 after emission from the optical resonator 13 between when the optical resonator 13 is arranged in the optical device and when the optical resonator 13 is removed from the optical device.

Description

Optical device, method for installing optical resonator in optical device, and optical resonator

[0001] The present invention relates to an optical device, in particular an optical device including an optical resonator, a method for installing an optical resonator in an optical device, and the optical resonator.

[0002] Conventionally, various optical devices using optical resonators have been known, such as laser oscillators for emitting laser beams and gas analyzers for performing gas analysis using optical resonators (see, for example, Patent Document 1). In such optical devices, the optical resonator has mirrors with high reflectivity, and repeatedly reflects incident laser light between the mirrors to amplify the light and increase the optical path length.

[0003] In order for laser light to enter an optical resonator, the direction of incidence of the laser light and the beam shape at the time of incidence must be aligned with the optical resonator. Furthermore, the higher the reflectivity of the mirror, the more difficult it becomes for light to exit the optical resonator. Therefore, unless the frequency of the laser light is aligned with the resonant frequency of the optical resonator with high precision, it becomes difficult to visually observe the light exiting the optical resonator. In such optical resonators, readjustment and restoration if the position shifts is extremely difficult. For these reasons, in the past, optical resonators in optical devices had to be firmly fixed in position beforehand.

[0004] U.S. Pat. No. 5,528,040

[0005] With regard to optical devices equipped with such optical resonators, there has long been a demand for a detachable optical resonator. For example, in gas analyzers using cavity ring-down spectroscopy or the like, there has been a demand for reducing the amount of material that needs to be brought to the sampling site where the sample gas to be analyzed is collected. If the optical resonator were detachable, it would be possible to carry only the optical resonator to the sampling site and directly collect the sample gas into the optical resonator, rather than carrying the optical device itself to the sampling site, thereby reducing the volume of components brought to the sampling site. Furthermore, with regard to optical devices equipped with optical resonators, there has been a demand for the ability to adjust the optical path in advance, i.e., adjust the position and angle of each component, without the optical resonator being installed. To this end, there has been a demand for a detachable optical resonator.

[0006] An object of the present invention is to provide an optical device including an optical resonator that can be easily attached to and detached from the optical device, a method for installing the optical resonator in the optical device, and the optical resonator.

[0007] The present invention solves the above problems by the following means.

[0008] (1) An optical device that uses laser light, comprising an optical resonator that is capable of reflecting the laser light and has a first mirror and a second mirror that are positioned opposite each other, wherein the beam diameter and beam divergence angle of the laser light at a position after it is emitted from the optical resonator do not change whether the optical resonator is placed in the optical device or removed from the optical device.

[0009] (2) The optical device according to (1), wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of the mirrors face each other, and are arranged so that normals at the centers of the reflective surfaces of the mirrors are aligned in the same straight line, and the radius of curvature of the reflective surfaces is R (mm), the refractive index of the base portion is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, and the refractive index of the gas outside the first mirror and the second mirror is n2, where La (mm) is a distance through which light travels between the first mirror and the second mirror and da (mm) is a thickness of the first mirror and the second mirror, where L0 (mm) is a distance and d0 (mm) are expressed by the following formulas 4 and 5: L0×0.8≦La≦L0×1.2 d0×0.9≦da≦d0×1.1.

[0010]

[0011]

[0012] (3) The optical device according to (1) above, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, have concavely curved reflective surfaces, face each other, and are arranged so that normals at the centers of the reflective surfaces of the mirrors are aligned in the same straight line, and a radius of curvature of the reflective surfaces is R (mm), a thickness of the first mirror and the second mirror is d (mm), a refractive index of the base portion is n, a refractive index of a gas filled between the first mirror and the second mirror is n1, and a refractive index of a gas outside the first mirror and the second mirror is n2, a distance La (mm) through which light travels between the first mirror and the second mirror satisfies L0×0.8≦La≦L0×1.2, where L0 (mm) is a distance shown in the following formula 1:

[0013]

[0014] (4) The optical device according to (1) above, wherein the first mirror and the second mirror have the same shape, have a base material and a reflective layer, have concavely curved reflective surfaces, face each other, and are arranged so that normals at the centers of the reflective surfaces of the mirrors are aligned in the same straight line, and a radius of curvature of the reflective surfaces is R (mm), a refractive index of the base material is n, a refractive index of a gas filled between the first mirror and the second mirror is n1, a refractive index of a gas outside the first mirror and the second mirror is n2, and a distance traveled by light between the first mirror and the second mirror is L (mm), where d0 (mm) is a thickness given by the following formula 3, the thickness da (mm) of the first mirror and the second mirror satisfies d0 × 0.9≦da≦d0 × 1.1.

[0015]

[0016] (5) An optical device according to any one of (1) to (4), wherein the position of the beam waist of the laser light in the region where the optical resonator is disposed remains unchanged between a state where the optical resonator is disposed in the optical device and a state where the optical resonator is removed from the optical device.

[0017] (6) The optical device according to any one of (1) to (5), wherein the optical resonator is detachably disposed in a sealed container that is transmissive to the laser light.

[0018] (7) A method for installing an optical resonator having a first mirror and a second mirror capable of reflecting laser light in an optical device that uses the laser light, wherein the distance through which light travels between the first mirror and the second mirror is set so that the beam diameter and beam divergence angle of the laser light at a position after emission from the optical resonator do not change when the optical resonator is placed in the optical device and when it is removed from the optical device.

[0019] (8) The method for installing an optical resonator in an optical device according to (7), wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concave curved reflective surfaces, the reflective surfaces of the mirrors face each other, and are arranged so that normals at the centers of the reflective surfaces of the mirrors are aligned in the same straight line, and the radius of curvature of the reflective surfaces is R (mm), the refractive index of the base portion is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, and the refractive index of the gas outside the first mirror and the second mirror is n2, where La (mm) is a distance through which light travels between the first mirror and the second mirror and da (mm) are the thicknesses da (mm) of the first mirror and the second mirror, where L0 (mm) and d0 (mm) are distances L0 (mm) and thicknesses d0 (mm) shown in the following equations 4 and 5, satisfy the following: L0×0.8≦La≦L0×1.2 d0×0.9≦da≦d0×1.1.

[0020]

[0021]

[0022] (9) The method for installing an optical resonator in an optical device according to (7), wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concave curved reflective surfaces, the reflective surfaces of the mirrors face each other, and are arranged so that normals at the centers of the reflective surfaces of the mirrors are aligned in the same straight line, and the radius of curvature of the reflective surfaces is R (mm), the thickness of the first mirror and the second mirror is d (mm), the refractive index of the base portion is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, and the refractive index of the gas outside the first mirror and the second mirror is n2, the distance La (mm) through which light travels between the first mirror and the second mirror satisfies L0 × 0.8 ≦ La ≦ L0 × 1.2, where L0 (mm) is a distance shown in the following formula 1:

[0023]

[0024] (10) The method for installing an optical resonator in an optical device described in (7) above, wherein the first mirror and the second mirror have the same shape, have a base material and a reflective layer, have concave curved reflective surfaces, face each other, and are arranged so that normals at the centers of the reflective surfaces of the mirrors are aligned in the same straight line, and the radius of curvature of the reflective surfaces is R (mm), the refractive index of the base material is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, the refractive index of the gas outside the first mirror and the second mirror is n2, and the distance traveled by light between the first mirror and the second mirror is L (mm), the thickness da (mm) of the first mirror and the second mirror satisfies d0 × 0.9 ≦ da ≦ d0 × 1.1 with respect to a thickness d0 (mm) shown in the following equation 3,

[0025]

[0026] (11) An optical resonator that can be attached to and detached from an optical device, the optical resonator comprising a first mirror and a second mirror that can reflect laser light and whose reflective surfaces are positioned opposite each other, wherein the beam diameter and beam divergence angle of the laser light at a position after it is emitted from the optical resonator in the optical device do not change when the optical resonator is attached and when the optical resonator is detached from the optical device.

[0027] (12) The optical resonator according to (11) above, wherein the first mirror and the second mirror have the same shape, have a base material and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of the mirrors face each other, and are arranged so that normals at the centers of the reflective surfaces of the mirrors are aligned in the same straight line, and when the radius of curvature of the reflective surfaces is R (mm), the refractive index of the base material is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, and the refractive index of the gas outside the first mirror and the second mirror is n2, the distance La (mm) through which light travels between the first mirror and the second mirror and the thickness da (mm) of the first mirror and the second mirror satisfy the following equations, where L0 (mm) and d0 (mm) are distances L0 (mm) and thicknesses d0 (mm) shown in the following equations 4 and 5: L0×0.8≦La≦L0×1.2 d0×0.9≦da≦d0×1.1

[0028]

[0029]

[0030] (13) The optical resonator according to (11) above, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, have concavely curved reflective surfaces, face each other, and are arranged so that normals at the centers of the reflective surfaces of the mirrors are aligned in the same straight line, and a radius of curvature of the reflective surfaces is R (mm), a thickness of the first mirror and the second mirror is d (mm), a refractive index of the base portion is n, a refractive index of a gas filled between the first mirror and the second mirror is n1, and a refractive index of a gas outside the first mirror and the second mirror is n2, a distance La (mm) through which light travels between the first mirror and the second mirror satisfies L0×0.8≦La≦L0×1.2, where L0 (mm) is a distance shown in the following formula 1:

[0031]

[0032] (14) The optical resonator according to (11) above, wherein the first mirror and the second mirror have the same shape, have a base material and a reflective layer, have concavely curved reflective surfaces, face each other, and are arranged so that normals at the centers of the reflective surfaces of the mirrors are aligned in the same straight line, and a radius of curvature of the reflective surfaces is R (mm), a refractive index of the base material is n, a refractive index of a gas filled between the first mirror and the second mirror is n1, a refractive index of a gas outside the first mirror and the second mirror is n2, and a distance traveled by light between the first mirror and the second mirror is L (mm), a thickness da (mm) of the first mirror and the second mirror satisfies d0 × 0.9 ≦ da ≦ d0 × 1.1 where d0 (mm) is a thickness given by the following formula 3:

[0033]

[0034] According to the present invention, it is possible to provide an optical device including an optical resonator that can be easily attached to and detached from the optical device, a method for installing an optical resonator in an optical device, and the optical resonator.

[0035] 1 is a diagram showing the configuration of the optical device 10 of the first embodiment. FIG. 2 is a diagram explaining the optical resonator 13 of the first embodiment. FIG. 3 is a diagram showing the change in beam diameter depending on whether or not the optical resonator 13 is present in the optical device 10 of the first embodiment. FIG. 4 is a diagram showing the change in beam diameter depending on whether or not the optical resonator 13B is present in the optical device 10B of the comparative example. FIG. 5 is a graph showing an example of the distribution of the distance L0 and the thickness d obtained by Equation 1. FIG. 6 is a diagram explaining a case where the laser light A does not pass through the center 15c of the first mirror 15 and the center 16c of the second mirror 16. FIG. 7 is a diagram explaining the optical resonator 23 of the second embodiment. FIG. 8 is a diagram explaining a method of installing the optical resonator 33 in the optical device of the third embodiment. FIG. 9 is a diagram explaining a method of installing the optical resonator 33 in the optical device of the third embodiment. FIG. 10 is a diagram explaining an optical resonator 43 of a modified form. FIG. 11 is a diagram explaining an optical resonator 43 of a modified form. FIG. 12 is a graph showing an example of the distribution of the thickness d0 and the distance L obtained by Equation 3.

[0036] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Note that the following figures, including FIG. 1, are schematic diagrams, and the size and shape of each part are appropriately exaggerated for ease of understanding. The numerical values, such as dimensions, and material names of each component described in this specification are examples of embodiments and are not limited thereto. They may be appropriately selected and used. In this specification, terms specifying shapes or geometric conditions, such as parallel and orthogonal, are intended to include not only their strict meanings but also states that exhibit similar optical functions and have an error that can be considered parallel or orthogonal.

[0037] First Embodiment FIG. 1 is a diagram showing the configuration of an optical device 10 according to a first embodiment. The configuration of the optical device 10 is simplified in FIG. 1 . The optical device 10 according to this embodiment is described as a gas analyzer that measures the concentration of a predetermined gas using cavity ring-down spectroscopy or the like. The optical device 10 includes a laser light source 11, a lens 12, an optical resonator 13, a lens 17, a detector 18, and the like, which are connected by optical fibers (not shown) as appropriate. The optical device 10 according to this embodiment irradiates laser light A into the optical resonator 13 when the optical resonator 13 is filled with a sample gas to be measured. The laser light A is repeatedly reflected within the optical resonator 13, and a portion of the laser light A is emitted from the optical resonator 13 and detected by the detector 18. The optical device 10 measures the concentration of the gas to be measured in the sample gas filled in the optical resonator 13 by observing the attenuation of the light intensity of the detected laser light A.

[0038] The laser light source 11 oscillates and emits laser light A at a predetermined output. The laser light A has a wavelength in a wavelength range in which the gas to be measured has an absorption band. For example, when the gas to be measured is water, carbon dioxide, methane, or the like, the wavelength of the laser light A is within the near-infrared and mid-infrared wavelength range of 700 to 10,000 nm. Suitable laser light sources 11 for outputting laser light A of such wavelengths include DFB lasers, external cavity lasers, tunable lasers, semiconductor lasers, and fiber lasers. The wavelength of the laser light A and the laser light source 11 can be changed as appropriate depending on the gas to be measured. The wavelength of the laser light A is not limited to the near-infrared and mid-infrared wavelength ranges, and may be in the ultraviolet wavelength range or the visible light wavelength range.

[0039] The lens 12 has a function of adjusting the beam diameter of the laser light A so that the laser light A enters the optical resonator 13 with a predetermined beam diameter.

[0040] FIG. 2 is a diagram illustrating the optical resonator 13 of the first embodiment. In this embodiment, the optical resonator 13 is used as a so-called sample cell. The optical resonator 13 is detachably held at a predetermined position from the optical device 10 by a support (not shown) or the like. The optical resonator 13 can be filled with a sample gas. The optical resonator 13 includes a cylindrical portion 14, a first mirror 15, and a second mirror 16. The cylindrical portion 14 is hollow. The first mirror 15 and the second mirror 16 are disposed at both ends of the cylindrical portion 14. Furthermore, although not shown, the cylindrical portion 14 is provided with an inlet through which the sample gas can flow in and an outlet through which the sample gas can flow out after measurement, and these openings can be opened and closed as appropriate.

[0041] In this embodiment, an example is given in which a sample gas is filled into the optical resonator 13 and measurement is performed in a sealed state, but this is not limiting. For example, during measurement, the sample gas may flow into the optical resonator 13 from the inlet at a predetermined flow rate and flow out from the outlet.

[0042] The first mirror 15 and the second mirror 16 are provided at both longitudinal ends of the cylindrical portion 14 and are arranged so that the reflective surfaces 15 a, 16 a face each other. In this embodiment, the first mirror 15 and the second mirror 16 have the same shape, and are arranged so that the reflective surfaces 15 a, 16 a of each mirror face each other and that normals at the centers 15 c, 16 c of the reflective surfaces 15 a, 16 a are aligned in the same line. In this embodiment, the reflective surfaces 15 a, 16 a are arranged so that the normals at the centers 15 c, 16 c of the reflective surfaces 15 a, 16 a face each other and are aligned in the same line. Furthermore, in this embodiment, the reflective surfaces 15 a, 16 a are arranged so that the normals at the centers 15 c, 16 c of the reflective surfaces 15 a, 16 a face each other and are aligned with the optical axis B, and the laser light A passes through the centers 15 c, 16 c of the reflective surfaces 15 a, 16 a of each mirror.

[0043] A large amount of light is reflected by the reflective layer 152 of the first mirror 15 and the reflective layer 162 of the second mirror 16. In this specification and claims, the reflective surfaces 15a, 16a refer to the surfaces of the reflective layers 152, 162 that form the boundary between the reflective layers 152, 162 of each mirror and the sample gas. The centers 15c, 16c are the most concave points of the reflective surfaces 15a, 16a, which are concave curved surfaces. In this embodiment, the first mirror 15 and the second mirror 16 are disposed symmetrically with respect to a plane (not shown) that passes through the center of the optical resonator 13 and is perpendicular to the longitudinal direction of the cylindrical portion 14.

[0044] The first mirror 15 and the second mirror 16 have a high reflectivity of 99% or more for the wavelength of the laser light A. In addition, an example will be described in which the first mirror 15 and the second mirror 16 have reflective surfaces 15a and 16a that are concavely curved and the other surfaces 15b and 16b that are flat.

[0045] The first mirror 15 and the second mirror 16 have substrate portions 151, 161 and reflective layers 152, 162. The substrate portions 151, 161 are formed of, for example, glass such as synthetic quartz glass, SiC (silicon carbide), Si (silicon), etc. The material of the substrate portions 151, 161 may be selected appropriately depending on the usage environment of the first mirror 15 and the second mirror 16, etc.

[0046] The reflective layers 152, 162 are film-like portions that reflect the laser light A. The reflective layers 152, 162 are formed of a dielectric multilayer film or a metal thin film. The material of the metal thin film that forms the reflective layers 152, 162 and the materials of the high-refractive-index layers and low-refractive-index layers of the dielectric multilayer film can be selected appropriately depending on the wavelength of the laser light A reflected by the first mirror 15 and the second mirror 16, etc.

[0047] Returning to Fig. 1, lens 17 is a lens that focuses laser light A emitted from optical resonator 13. When the frequency of laser light A matches the resonance frequency of optical resonator 13, laser light A is emitted from optical resonator 13. At this time, the intensity of laser light A emitted from optical resonator 13 increases. Detector 18 detects laser light A emitted from optical resonator 13. A photodetector (light detector) using a photodiode or the like is suitable for detector 18, but a power meter or the like may also be used.

[0048] As described above, the optical device 10 is a gas analyzer that measures the concentration of a predetermined gas using cavity ring-down spectroscopy or the like. The optical resonator 13 is filled with a sample gas to be analyzed, and laser light A output from a laser light source 11 is shaped into a predetermined beam diameter by a lens 12 and incident on the optical resonator 13. When the optical resonator 13 is placed at a predetermined position, the laser light A is repeatedly reflected by a first mirror 15 and a second mirror 16 within the optical resonator 13, and a portion of the laser light A is emitted from the optical resonator 13. The emitted laser light A is focused by a lens 17 and irradiated onto a detector 18, which detects it. Within the optical resonator 13, the laser light A is absorbed by the gas to be measured contained in the sample gas. Therefore, when the incidence of the laser light A is stopped, the intensity of the detected laser light A attenuates over time. The manner in which this light intensity attenuation varies depending on the presence or absence of the gas to be measured, the type and concentration of the gas, etc. By observing the attenuation of the intensity of the detected laser light A, the optical device 10 measures the concentration of the gas to be measured within the optical resonator 13, etc.

[0049] In order to make the optical resonator 13 of this embodiment detachable from the optical device 10, it is desirable that the distance between the two mirrors, i.e., the distance L (mm) through which light travels between the first mirror 15 and the second mirror 16, satisfies the following formula 1, i.e., L = L0. This distance L is the distance between the points on the reflecting surfaces 15a and 16a through which the laser light A passes, and in this embodiment, it is the distance between the centers 15c and 16c. The inventors of the present application have found that by configuring the optical resonator 13 to satisfy the following formula 1, the beam diameter and beam divergence angle of the laser light A at position t2 after emission from the optical resonator 13 do not change whether the optical resonator 13 is installed or removed from the optical device 10. Here, the beam divergence angle refers to the change (divergence) of the beam diameter at a certain point and corresponds to the slope of the tangent to the curve representing the beam diameter at a certain point in Figures 3 and 4, which will be described later.

[0050] In the following formula 1, L0 (mm) is the distance traveled by light between the first mirror 15 and the second mirror 16 (corresponding to the distance between the center 15c of the first mirror 15 and the center 16c of the second mirror 16 in FIG. 2 showing this embodiment), R (mm) is the radius of curvature of the reflecting surfaces 15a and 16a, d (mm) is the thickness of the first mirror 15 and the second mirror 16, n1 is the refractive index of the sample gas filled in the optical resonator 13, n2 is the refractive index of the gas outside the optical resonator 13, and n is the refractive index of the base material portions 151 and 161 of the first mirror 15 and the second mirror 16. It should be noted that the thickness d is the thickness at the center 15c of the first mirror 15 and the thickness at the center 16c of the second mirror 16.

[0051]

[0052] If the density of the medium filled between the first mirror 15 and the second mirror 16 and the medium outside the first mirror 15 and the second mirror 16 is low, both n1 and n2 can be considered to be approximately 1, and therefore Equation 1 can be transformed into the following Equation 2. This condition holds in many cases, such as when the medium is a gas or is an approximate vacuum (i.e., when a trace amount of material exists in the vacuum to the extent that it can be considered a vacuum).

[0053]

[0054] The optical device 10 of this embodiment satisfies the above-mentioned formula 1, i.e., L = L0, and therefore the beam diameter and beam divergence angle of the laser light A at position t2 after emission from the optical resonator 13 do not change when the optical resonator 13 is installed in the optical device 10 (i.e., when the optical resonator 13 is present) and when the optical resonator 13 is removed from the optical device 10 (i.e., when the optical resonator 13 is not present).

[0055] The divergence angle of the beam diameter also affects the thickness of the first mirror 15 and the second mirror 16, i.e., the thickness at the centers 15c, 16c of the reflecting surfaces 15a, 16a of the first mirror 15 and the second mirror 16. In order to ensure that the beam diameter and beam divergence angle of the laser light A at position t2 after emission from the optical resonator 13 do not change between when the optical resonator 13 is installed and when it is removed from the optical device 10 and to make the optical resonator 13 detachable from the optical device 10, it is desirable that the thickness d (mm) of the first mirror 15 and the second mirror 16 also satisfy the following formula 3, i.e., d = d0. The distance L shown in the following equation 3 is the distance traveled by light between the first mirror 15 and the second mirror 16, R (mm) is the radius of curvature of the reflecting surfaces 15a and 16a, n1 is the refractive index of the sample gas filled in the optical resonator 13, n2 is the refractive index of the gas outside the optical resonator 13, and n is the refractive index of the substrate portions 151 and 161 of the first mirror 15 and the second mirror 16.

[0056]

[0057] The optical device 10 of this embodiment satisfies the above-mentioned equation 3, i.e., d = d0, and therefore the beam diameter and beam divergence angle of the laser light A at position t2 after emission from the optical resonator 13 do not change when the optical resonator 13 is installed in the optical device 10 (i.e., when the optical resonator 13 is present) and when the optical resonator 13 is removed from the optical device 10 (i.e., when the optical resonator 13 is not present).

[0058] As described above, in this embodiment, the distance L and the thickness d satisfy L=L0 and d=d0. Therefore, from the above formulas 1 and 3, the optical device 10 of this embodiment satisfies the following formulas 4 and 5.

[0059]

[0060]

[0061] Fig. 3 is a diagram showing the change in beam diameter depending on whether or not the optical resonator 13 is present in the optical device 10 of the first embodiment. Fig. 4 is a diagram showing the change in beam diameter depending on whether or not the optical resonator 13B is present in the optical device 10B of the comparative example. In Figs. 3 and 4, for ease of understanding, only the first mirror 15 and the second mirror 16 are shown in a simplified manner. In Figs. 3 and 4, the beam diameter of the laser light A when the optical resonators 13 and 13B are installed in the optical device 10 is shown by a solid line, and the beam diameter when the optical resonators 13 and 13B are removed from the optical device 10 is shown by a dashed line.

[0062] The optical resonator 13B of the comparative example does not satisfy the above formula 1. Therefore, as shown in Fig. 4, in the conventional optical device 10B of the comparative example, even if the beam diameter at position t1 before entering the optical resonator 13B is the same, the beam diameter at position t2 after exiting from the optical resonator 13B differs between the state with and without the optical resonator 13B. However, in the optical device 10 of this embodiment, the optical resonator 13 satisfies formula 1, so as shown in Fig. 3, the beam diameter at position t2 is the same and does not change between the state with and without the optical resonator 13.

[0063] Therefore, in this embodiment, the beam diameter of the laser light A at position t2 after emission from the optical resonator 13 does not change in the optical device 10 whether the optical resonator 13 is present or not, so that the components of the optical device 10 can be arranged without the optical resonator 13, the laser light A can be oscillated, and the position of the detector 18 can be adjusted in a visible state without the need to tune the frequency of the laser light A to the resonance frequency. After the adjustment, when installing the optical resonator 13 in the optical device 10, high-precision readjustment of the laser is not required, so the optical resonator 13 can be easily installed in the optical device 10. Therefore, the optical resonator 13 can be easily attached to and detached from the optical device 10.

[0064] Next, the position of the beam waist will be considered. The beam waist is the portion where the beam diameter is smallest. In the optical device 10 of this embodiment, the above formula 1 is satisfied. The first mirror 15 and the second mirror 16 have the same shape, the reflecting surfaces 15 a and 16 a face each other, and the reflecting surfaces 15 a and 16 a are arranged so that the normals at the centers 15 c and 16 c of the reflecting surfaces 15 a and 16 a are aligned on the same line. Therefore, when laser light A, the frequency of which has been adjusted to cause resonance within the optical resonator 13, enters the optical resonator 13, the position of the beam waist of the laser light A coincides along the optical axis B of the laser light A, regardless of whether the optical resonator 13 is present. As shown in FIG. 3 , the position of the beam waist when the optical resonator 13 is present is position ta, and the position of the beam waist when the optical resonator 13 is absent is position tb, both of which coincide along the optical axis B. Furthermore, positions ta and tb are the midpoints of the distance L that the laser light A passes through between the first mirror 15 and the second mirror 16 (the points where the distance between the first mirror 15 and the second mirror 16 is L / 2).

[0065] On the other hand, in the optical device 10B of the comparative example, the above formula 1 is not satisfied, and therefore even if the first mirror 15 and the second mirror 16 have the same shape, the reflecting surfaces 15 a, 16 a face each other, and the normals at the centers 15 c, 16 c of the reflecting surfaces 15 a, 16 a are arranged to be collinear, when laser light A, the frequency of which has been adjusted to cause resonance within the optical resonator 13B, enters the optical resonator 13B, the position of the beam waist of the laser light A differs along the optical axis B depending on whether or not the optical resonator 13B is present. As shown in Fig. 4, the position of the beam waist when the optical resonator 13B of the comparative example is present is position ta, and the position of the beam waist when the optical resonator 13B is not present is position tb, which are different on the optical axis B.

[0066] Therefore, in this embodiment, when installing the optical resonator 13 in the optical device 10, the position of the optical resonator 13 in the direction of the optical axis B is adjusted so that the position tb of the beam waist of the laser light A when the optical resonator 13 is not present is the same as the position ta of the beam waist when the optical resonator 13 is present, thereby realizing a state in which the beam diameter and beam divergence angle at position t2 do not change regardless of the presence or absence of the optical resonator 13. Furthermore, when the optical resonator 13 is not present, the position tb of the beam waist can be visually confirmed without the need to tune the frequency of the laser light A to the resonance frequency. Therefore, it is easy to adjust the position of the laser light A in the direction of the optical axis B when installing the optical resonator 13 in the optical device 10.

[0067] In the present embodiment, the optical resonator 13 may have a distance L of light traveling between the first mirror 15 and the second mirror 16 that strictly satisfies the above-described formula 1 (i.e., L = L0), or it may have a distance L = La. This distance La satisfies L0 × 0.8 ≦ La ≦ L0 × 1.2, where L0 satisfies the distance L0 that satisfies formula 1. That is, the distance La is within a range of ±20% of the value of the distance L0 obtained by formula 1. If the distance L (in this embodiment, the distance between the center 15c of the first mirror 15 and the center 16c of the second mirror 16) in the optical resonator 13 satisfies the above range, the optical resonator 13 can be attached to and detached from the optical device 10, similar to when formula 1 is strictly satisfied, depending on the intended use of the optical device 10, etc. This distance La may be appropriately set within the above range depending on the usage environment of the optical device 10 in which the optical resonator 13 is used, the desired optical performance, etc.

[0068] FIG. 5 is a graph showing an example of the distribution of distance L0 and thickness d obtained by Equation 1. In the graph shown in FIG. 5, the vertical axis represents distance L (mm) and the horizontal axis represents thickness d (mm). The solid line represents the distribution of distance L0 and thickness d when n1 = n2 = 1, R = 100 mm, and n = 1.5 in Equation 1, while the dashed line represents the distribution of distance L0 and thickness d when n1 = n2 = 1, R = 100 ± 10 mm, and n = 1.5 ± 0.1 in Equation 1. In the graph shown in FIG. 5, for a given thickness d, the distance L0 indicated by the dashed line is within ±20% of the distance L0 indicated by the solid line and within the range of distance La calculated using the distance L0 indicated by the solid line. Therefore, the optical resonator 13 can be attached to and detached from the optical device 10 as long as the distance L is within the range of the two curves indicated by the dashed lines.

[0069] Furthermore, in this embodiment, the thickness d of the first mirror 15 and the second mirror 16 of the optical resonator 13 may be d = da. This thickness da satisfies d0 × 0.9 ≦ da ≦ d0 × 1.1, where d0 satisfies Equation 3. That is, the thickness da satisfies a range of ±10% of the value of the thickness d0 obtained by Equation 3. If the thickness d of the first mirror 15 and the second mirror 16 of the optical resonator 13 satisfies the above range, the optical resonator 13 can be attached to and detached from the optical device 10, similar to when Equation 3 is strictly satisfied, depending on the intended use of the optical device 10, etc. This thickness da may be appropriately set within the above range depending on the usage environment of the optical device 10 in which the optical resonator 13 is used, the desired optical performance, etc.

[0070] FIG. 10 is a graph showing an example of the distribution of thickness d0 and distance L obtained by Equation 3. In the graph shown in FIG. 10, the vertical axis represents thickness d (mm) and the horizontal axis represents distance L (mm). The solid line represents the distribution of thickness d0 and distance L when n1 = n2 = 1, R = 100 mm, and n = 1.5 in Equation 3, while the dashed line represents the distribution of thickness d0 and distance L when n1 = n2 = 1, R = 100 ± 10 mm, and n = 1.5 ± 0.1 in Equation 3. In the graph shown in FIG. 10, at a certain distance L, the thickness d0 indicated by the dashed line is within ±10% of the thickness d0 indicated by the solid line and is within the range of the thickness da calculated using the thickness d0 indicated by the solid line. Therefore, if the thickness d of each mirror of the optical resonator 13 is within the range of the two dashed curves, the optical resonator 13 is detachable from the optical device 10.

[0071] FIG. 6 is a diagram illustrating a case where laser light A does not pass through the center 15c of the first mirror 15 or the center 16c of the second mirror 16. For ease of understanding, FIG. 6 only shows the optical resonator 13, omitting the cylindrical portion 14. FIG. 6 also illustrates a case where laser light A does not strictly pass through the center 15c of the first mirror 15 or the center 16c of the second mirror 16, and the optical resonator 13 is detachable from the optical device 10. In this case, the actual distance traveled by laser light A between the first mirror 15 and the second mirror 16 is defined as L1, and the thickness of the first mirror 15 and the second mirror 16 is defined as d1. When calculating the distance L0 in Equation 1, it is necessary to substitute the thickness d1 for the thickness d. Furthermore, the calculated distance L0 and the distance L1 must be compared to determine whether the condition is met. Similarly, when calculating the thickness d0 in Equation 3, it is necessary to substitute the distance L1 for the distance L, and the calculated thickness d0 and the thickness d1 must be compared to determine whether the condition is met.

[0072] 7 is a diagram showing an optical resonator 23 of a second embodiment. The optical device of the second embodiment differs from the first embodiment in that the optical resonator 23 is disposed inside a sample cell 29 and is not sealed by a cylindrical portion 14, but otherwise has the same configuration. Therefore, parts that perform the same functions as those in the first embodiment are given the same reference numerals, and duplicated explanations will be omitted as appropriate.

[0073] The optical resonator 23 of the second embodiment is applicable to the optical device 10 shown in the first embodiment. The optical resonator 23 is not sealed by the cylindrical portion 14, but is disposed in a sample cell 29 as shown in Fig. 7. The sample cell 29 has a hollow cylindrical portion 291 and windows 292 that seal both ends of the cylindrical portion 291, and the internal space thereof can be sealed.

[0074] The cylindrical portion 291 is made of aluminum, stainless steel, invar, glass, or the like. The window 292 is provided at a location where the laser light A enters or exits, and is made of glass or the like that is transparent to the laser light A. The sample cell 29 is provided with an inlet and outlet (not shown) that can be opened and closed and that allow the inflow and discharge of the sample gas. The sample cell 29 also has an opening (not shown) that is large enough to allow the first mirror 15 and the second mirror 16 to be attached and detached, and that can be opened and closed.

[0075] The optical resonator 23 is detachably fixed inside the sample cell 29 along the optical axis B by a support (not shown). The optical resonator 23 is fixed by a support (not shown) at a position where the distance L between the centers 15c and 16c of the two mirrors satisfies the condition of the above-mentioned formula 1. That is, L = L0. The optical resonator 23 of this embodiment can be inserted into and removed from the sample cell 29 when the distance L satisfies the condition of formula 1 (i.e., when L = L0). The thickness d of the two mirrors also satisfies the condition of the above-mentioned formula 3, that is, d = d0. Therefore, the optical resonator 23 and the optical device of this embodiment satisfy the above-mentioned formulas 4 and 5. Note that this embodiment will be described taking an example in which the optical resonator 23 can be inserted into and removed from the sample cell 29 when the distance L satisfies L = L0. However, the present invention is not limited to this. The optical resonator 23 may also be inserted into and removed from the sample cell 29 when the distance L satisfies L = La. Similarly, the optical resonator 23 may have two mirrors each having a thickness d that satisfies d=da.

[0076] In this embodiment, a sample cell 29 is placed between positions t1 and t2. In the optical device of this embodiment, the beam diameter at position t2 does not change depending on whether or not the optical resonator 23 is present. Therefore, in this embodiment, a sample cell 29 without an optical resonator 23 is placed in the optical device, laser light A is actually irradiated, and the position of the detector 18 and other components is adjusted while the laser light A is visible. Thereafter, the optical resonator 23 is placed in the sample cell 29 and used. In other words, according to this embodiment, adjustments of each component of the optical device can be easily performed in advance using only the sample cell 29, making it possible to attach and detach the optical resonator 23. In this embodiment, a component that affects the optical path of the laser light A, such as a wedge window, can also be used as the window 292.

[0077] 8A and 8B are diagrams illustrating a method for installing an optical resonator 33 in an optical device according to a third embodiment. FIG. 8A illustrates a state in which the components of the optical device are adjusted using a temporary optical resonator 333, while FIG. 8B illustrates a state in which the temporary optical resonator 333 has been removed and replaced with the optical resonator 33. The optical device according to the third embodiment differs from the first embodiment in that the temporary optical resonator 333 is used to adjust the positions of the components of the optical device, but is otherwise similar to the first embodiment. Therefore, components that perform the same functions as those in the first embodiment are denoted by the same reference numerals, and redundant descriptions will be omitted as appropriate. The optical resonator 33 according to the third embodiment is applicable to the optical device 10 according to the first embodiment. The optical resonator 33 includes a first mirror 15 and a second mirror 16.

[0078] 8A , in the optical device of the third embodiment, first, each component of the optical device is adjusted using a temporary optical resonator 333. This temporary optical resonator 333 includes a temporary first mirror 315 and a temporary second mirror 316 having the same shapes as the first mirror 15 and the second mirror 16, and the distance between the center of the temporary first mirror 315 and the center of the temporary second mirror 316 (i.e., the distance traveled by light between the temporary first mirror 315 and the temporary second mirror 316) is equal to the distance between the center 15c of the first mirror 15 and the center 16c of the second mirror 16 (i.e., the distance traveled by light between the first mirror 15 and the second mirror 16) of the optical resonator 33. The temporary first mirror 315 and the temporary second mirror 316 correspond to the base portions 151 and 161 of the first mirror 15 and the second mirror 16 that do not have the reflective layers 152 and 162.

[0079] In this embodiment, a temporary optical resonator 333 including a temporary first mirror 315 and a temporary second mirror 316 is used to actually emit laser light A, and the positions of the detector 18 and the like are adjusted in a state in which the laser light A is visible. Next, as shown in FIG. 8B , the temporary optical resonator 333 is removed and replaced with the optical resonator 33.

[0080] In this embodiment, the beam diameter and beam divergence angle at position t2 are the same when the optical resonator 33 is placed and when the temporary optical resonator 333 is placed. As a result, according to this embodiment, the temporary optical resonator 333 is placed in the optical device 10, and the positions of each component can be adjusted while the laser light A is visible, and the optical resonator 33 can be easily attached and detached to and from the optical device. Furthermore, according to this embodiment, there are no conditions regarding the first mirror 15 and the second mirror 16 of the optical resonator 33, which increases the degree of freedom in selecting the optical resonator 33. It is also possible to select an optical resonator that does not satisfy the above-mentioned formula 1, such as an optical resonator in which the surfaces 15b, 16b opposite the reflecting surfaces 15a, 16a of the first mirror 15 and the second mirror 16 are not flat, or an optical resonator in which the surfaces 15b, 16b have an angle (wedge) with respect to the optical axis B.

[0081] (Modifications) The present invention is not limited to the above-described embodiment, and various modifications and changes are possible, and these are also within the scope of the present invention.

[0082] (1) In the first and second embodiments, an example was shown in which the optical axis of the laser light A between the first mirror 15 and the second mirror 16 was linear. However, this is not limited to this, and the optical axis may be bent in a predetermined direction at the position of the beam waist, etc. Figures 9A and 9B are diagrams illustrating a modified optical resonator 43. Figures 9A and 9B show an example in which the optical axis B of the laser light A forms an angle α with the original optical axis direction due to a mirror 434. The modified optical resonator 43 includes a mirror 434 at the position of the beam waist, and this mirror 434 reflects the laser light A in a predetermined direction. The mirror 434 has a flat reflective surface and is able to reflect and deflect the laser light A with high reflectivity.

[0083] 9A , the optical resonator 43 of a modified embodiment may be configured such that the optical axis B of the laser light A is deflected by a mirror 434 at a beam waist position in a direction that forms an angle α with respect to the original optical axis direction. In this case, the angle α is 0°<α<180°. Alternatively, as shown in FIG. 9B , the optical resonator 43 may be configured such that the angle α=0°, the laser light is reflected multiple times between the first mirror 15 and the mirror 434, and the laser light A is emitted from the first mirror 15. Alternatively, the optical resonator 43 may be configured such that the laser light A is emitted from the mirror 434, although this is not shown.

[0084] (2) In each embodiment, the first mirror 15 and the second mirror 16 have the same shape. However, this is not limiting. The radius of curvature of the concave surface and the thickness d of the base portions 151, 161 may be different. Furthermore, the refractive index n of the base portions 151, 161 may be different. If the first mirror 15 and the second mirror 16 have different shapes, the position of the beam waist may change depending on whether or not an optical resonator is present. However, if the beam diameter and beam divergence angle of the laser light A at position t2 after emission from the optical resonator do not change depending on whether or not an optical resonator is present, the components constituting the optical device 10 can be arranged without the optical resonator 13, and the positions of the components can be adjusted visually without the need to tune the frequency of the laser light A to the resonant frequency. After the adjustment, when installing the optical resonator 13 in the optical device 10, high-precision readjustment of the laser is not required, and therefore the optical resonator 13 can be easily installed in the optical device 10. Therefore, the optical resonator can be easily attached to and detached from the optical device, similar to the first embodiment and the like.

[0085] (3) In each embodiment, the optical device 10 is a gas analyzer using laser light A, the optical resonators 13 and 33 are used as sample cells, and the optical resonator 23 is disposed inside the sample cell 29. However, the present invention is not limited to this. For example, the optical device may be a laser oscillator that cuts unnecessary wavelength bands (side bands) from excited laser light and emits the laser light. The optical resonator may also be used to amplify the excited laser light.

[0086] The embodiments and modifications may be used in combination as appropriate, but detailed description thereof will be omitted. The present invention is not limited to the above-described embodiments.

[0087] REFERENCE SIGNS LIST 10 Optical device 11 Laser light source 12 Lens 13, 23, 33 Optical resonator 15 First mirror 16 Second mirror 17 Lens 18 Detector A Laser light B Optical axis

Claims

1. An optical device that uses laser light, comprising an optical resonator that is capable of reflecting the laser light and has a first mirror and a second mirror that are positioned opposite each other, wherein the beam diameter and beam divergence angle of the laser light at a position after it is emitted from the optical resonator do not change whether the optical resonator is placed in the optical device or removed from the optical device.

2. The optical device according to claim 1, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of each mirror face each other, and are arranged so that normals at the centers of the reflective surfaces of each mirror are collinear, and where the radius of curvature of the reflective surfaces is R (mm), the refractive index of the base portion is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, and the refractive index of the gas outside the first mirror and the second mirror is n2, the distance La (mm) over which light travels between the first mirror and the second mirror and the thickness da (mm) of the first mirror and the second mirror satisfy the following equations 4 and 5, where L0 (mm) and d0 (mm) are distances L0 (mm) and thickness d0 (mm) as shown in the following equations 4 and 5: L0 x 0.8≦La≦L0 x 1.2 d0 x 0.9≦da≦d0 x 1.1 3. The optical device according to claim 1, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of each mirror face each other, and are arranged so that normals at the centers of the reflective surfaces of each mirror are aligned on the same straight line, and the radius of curvature of the reflective surfaces is R (mm), the thickness of the first mirror and the second mirror is d (mm), the refractive index of the base portion is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, and the refractive index of the gas outside the first mirror and the second mirror is n2, where L0 (mm) is the distance L0 (mm) shown in the following formula 1, the distance La (mm) traveled by light between the first mirror and the second mirror satisfies L0 x 0.8 ≦ La ≦ L0 x 1.

2.

4. The optical device according to claim 1, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of each mirror face each other and are arranged so that normals at the centers of the reflective surfaces of each mirror are aligned on the same straight line, and the radius of curvature of the reflective surfaces is R (mm), the refractive index of the base portion is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, the refractive index of the gas outside the first mirror and the second mirror is n2, and the distance traveled by light between the first mirror and the second mirror is L (mm), where d0 (mm) is the thickness given by the following formula 3, the thickness da (mm) of the first mirror and the second mirror satisfies d0 x 0.9 ≦ da ≦ d0 x 1.

1.

5. An optical device according to any one of claims 1 to 4, wherein the position of the beam waist of the laser light in the region where the optical resonator is located does not change between a state where the optical resonator is located in the optical device and a state where the optical resonator is removed from the optical device.

6. The optical device according to claim 1, wherein the optical resonator is detachably disposed in a sealed container that is transmissive to the laser light.

7. A method for installing an optical resonator comprising a first mirror and a second mirror capable of reflecting laser light in an optical device that uses the laser light, wherein the distance through which light travels between the first mirror and the second mirror is set so that the beam diameter and beam divergence angle of the laser light at a position after emission from the optical resonator do not change when the optical resonator is placed in the optical device and when it is removed from the optical device.

8. The method of installing an optical resonator in an optical device according to claim 7, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of each mirror face each other, and are arranged so that normals at the centers of the reflective surfaces of each mirror are collinear, and the optical resonator is installed in the optical device in a state where the distance La (mm) over which light travels between the first mirror and the second mirror and the thickness da (mm) of the first mirror and the second mirror satisfy the following equations, where L0 (mm) and d0 (mm) are distances L0 (mm) and thicknesses d0 (mm) as shown in the following equations 4 and 5: L0 x 0.8≦La≦L0 x 1.2 d0 x 0.9≦da≦d0 x 1.1 9. The method of installing an optical resonator in an optical device according to claim 7, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of each mirror face each other, and are arranged so that normals at the centers of the reflective surfaces of each mirror are aligned in the same straight line, and the optical resonator is installed in the optical device in a state where the distance La (mm) traveled by light between the first mirror and the second mirror satisfies the following equation 1, where La (mm) is the distance L0 (mm) shown in the following equation 1: L0 x 0.8 ≦ La ≦ L0 x 1.2 10. The method of installing an optical resonator in an optical device according to claim 7, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of each mirror face each other, and are arranged so that normals at the centers of the reflective surfaces of each mirror are aligned in the same straight line, and the optical resonator is installed in the optical device in a state where the thickness da (mm) of the first mirror and the second mirror satisfies d0 × 0.9 ≦ da ≦ d0 × 1.1, where d0 (mm) is a thickness given by the following equation 3:

11. An optical resonator that can be attached to and detached from an optical device, comprising a first mirror and a second mirror that can reflect laser light and whose reflective surfaces are arranged in positions facing each other, wherein the beam diameter and beam divergence angle of the laser light at a position after it is emitted from the optical resonator in the optical device do not change when the optical resonator is attached and when the optical resonator is detached from the optical device.

12. The optical resonator according to claim 11, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of each mirror face each other, and are arranged so that normals at the centers of the reflective surfaces of each mirror are collinear, and wherein the radius of curvature of the reflective surfaces is R (mm), the refractive index of the base portion is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, and the refractive index of the gas outside the first mirror and the second mirror is n2, the distance La (mm) over which light travels between the first mirror and the second mirror and the thickness da (mm) of the first mirror and the second mirror satisfy the following equations, where L0 (mm) and d0 (mm) are distances L0 (mm) and thicknesses d0 (mm) as shown in the following equations 4 and 5: L0 x 0.8 ≦ La ≦ L0 x 1.2 d0 x 0.9 ≦ da ≦ d0 x 1.1 13. The optical resonator according to claim 11, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of each mirror face each other, and are arranged so that normals at the centers of the reflective surfaces of each mirror are aligned on the same straight line, and wherein the radius of curvature of the reflective surfaces is R (mm), the thickness of the first mirror and the second mirror is d (mm), the refractive index of the base portion is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, and the refractive index of the gas outside the first mirror and the second mirror is n2, where L0 (mm) is the distance L0 (mm) shown in the following equation 1, the distance La (mm) traveled by light between the first mirror and the second mirror satisfies L0 x 0.8 ≦ La ≦ L0 x 1.

2.

14. The optical resonator according to claim 11, wherein the first mirror and the second mirror have the same shape, have a base portion and a reflective layer, and have concavely curved reflective surfaces, the reflective surfaces of each mirror face each other and are arranged so that normals at the centers of the reflective surfaces of each mirror are aligned on the same straight line, and the radius of curvature of the reflective surfaces is R (mm), the refractive index of the base portion is n, the refractive index of the gas filled between the first mirror and the second mirror is n1, the refractive index of the gas outside the first mirror and the second mirror is n2, and the distance traveled by light between the first mirror and the second mirror is L (mm), where d0 (mm) is the thickness given by the following equation 3, the thickness da (mm) of the first mirror and the second mirror satisfies d0 × 0.9≦da≦d0 × 1.1.

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