Optical multi-pass cell

Abstract

The present disclosure relates to an infrared spectrometer, which includes: first and second relative reflectors, which are spaced apart by a spacing length; and a plurality of discrete concave reflective facets, which are facets of at least one reflector in the relative reflectors. An infrared laser source is arranged to form a laser beam. The relative reflector is arranged so that the laser beam is alternately reflected from each reflector in the relative reflector, including being reflected at least once by each reflective facet in the reflective facets. A detector is arranged to detect the spectral characteristics of the laser beam after being reflected from each reflective facet in the plurality of reflective facets, and an analyzer then determines the characteristics of a sample arranged between the first and second relative reflectors based on the detected spectral characteristics.

Description

Technical Field

[0001] The present invention relates to optical multi-pass cells for use, for example, in infrared spectroscopy techniques such as infrared absorption spectroscopy. Background Art

[0002] Optical multi-pass cells provide repeated reflections of a beam through space. They can be used for a variety of purposes, for example, optical delay lines and laser spectroscopy. For example, in the case of infrared spectroscopy, it may be desirable to increase the path length of the laser beam through the sample to increase the effect of the sample on the laser beam. Such an effect can include absorption of a specific laser beam wavelength in infrared absorption spectroscopy or other effects such as slight changes in the line width or wavelength of a spectral feature.

[0003] For some applications, it is desirable to provide an optical multi-pass cell that is more compact in length and / or width, and to provide a multi-pass cell that is more stable, easier to manufacture, etc. At the same time, it is generally desirable to minimize unnecessary optical losses of the laser beam within the multi-pass cell. For example, such losses may result from lower than ideal reflectivity of the reflective facets in the cell, as well as other effects.

[0004] The present invention solves the problems and limitations of the related prior art. Summary of the invention

[0005] The present invention seeks to provide a compact multi-pass cell suitable for applications such as laser spectroscopy (including infrared spectroscopy, for example, infrared absorption spectroscopy or dispersion spectroscopy), although the cell can be used for various other applications as described below. Typically, such a cell includes at least a first and a second relative reflector, each reflector having a plurality of reflective facets and / or planar reflective portions, as described in various ways below, and the cell can be considered, for example, as a combination of these reflectors and the spaces between the reflectors, which is used for multiple passes of the laser beam between these facets. If desired, the cell may also include walls arranged to retain a sample (usually a gas) at least in the space between the reflectors for appropriate optical analysis, although in an open cell arrangement, the sample gas may be present between the reflectors without forming a completely enclosed portion with a wall (if desired).

[0006] Aspects of the present invention relate to and are described by numerical modeling of multi-faceted optics to facilitate higher confinement of a laser beam between reflectors, for example by reducing incidence of the laser beam outside of defined reflective facet surfaces.

[0007] Specifically, according to one aspect, the present invention provides an infrared spectrometer, the infrared spectrometer comprising: first and second relative reflectors, the first and second relative reflectors are spaced apart by a spacing length, each reflector comprising a plurality of discrete concave reflective facets; an infrared laser source, arranged to form a laser beam; the reflective facets of the first and second reflectors are arranged so that the laser beam is reflected at least once from each of the plurality of reflective facets by alternately reflecting from the reflective facets of each of the first and second relative reflectors; a detector, arranged to detect the spectral characteristics of the laser beam after reflection from the plurality of reflective facets; and an analyzer, arranged to determine the characteristics of a sample disposed between the first and second relative reflectors based on the detected spectral characteristics. The sample can typically be a gas, liquid, or other fluid, although other sample types can be used.

[0008] According to another aspect, the present invention provides an infrared spectrometer, which includes: a first and a second relative reflector, which are separated by a spacing length; a plurality of discrete concave reflecting facets, which are facets of at least one of the relative reflectors; an infrared laser source, arranged to form a laser beam; the relative reflectors are arranged so that the laser beam is alternately reflected from each of the relative reflectors, including being reflected at least once by each of the reflecting facets; a detector, arranged to detect the spectral characteristics of the laser beam after being reflected from each of the plurality of reflecting facets; and an analyzer, arranged to determine the characteristics of a sample disposed between the first and second relative reflectors based on the detected spectral characteristics.

[0009] In either aspect, these discrete concave reflective facets can be considered discrete in the sense that each such facet is discontinuous with adjacent facets. This can be expressed, for example, by stating that convex curvature is required between adjacent concave facets, although the concave curvature can be provided in the form of convex edges and / or a more gently curved convex appearance.

[0010] The present invention may also be used for applications other than infrared spectroscopy, so more generally, the present invention may provide a multi-pass cell comprising: first and second opposing reflectors, the first and second opposing reflectors being spaced apart by a spacing length, each reflector comprising a plurality of discrete concave reflecting facets, the reflecting facets of the first and second reflectors being arranged so that a light beam (such as a laser beam, for example an infrared laser beam) is reflected at least once from each of the plurality of reflecting facets by alternating reflections from the reflecting facets of each of the first and second opposing reflectors. Thus, where appropriate, the embodiments described herein and the related more detailed aspects of the present invention should be considered to apply to such a multi-pass cell, and more specifically to an infrared spectrometer. Alternatively, this multi-pass cell may comprise: first and second opposing reflectors, the first and second opposing reflectors being spaced apart by a spacing length; a plurality of discrete concave reflecting facets, the reflecting facets being facets of at least one of the opposing reflectors, the reflecting facets being arranged so that a light beam (such as a laser beam, for example an infrared laser beam) is reflected alternately from each of the reflecting facets in the opposing reflectors, including being reflected at least once by each of the reflecting facets.

[0011] The above-mentioned spacing length may, for example, be considered as the spacing between opposing reflectors along the optical axis, and may be defined as the vertical spacing between planes or the like defining the average surface of the facets, if desired.

[0012] The number of reflective facets of the first reflector and the second reflector that reflect the laser beam can be selected according to the specific application and the desired design, but generally, the design can provide that the number of facets that will reflect the laser beam can be at least 3 or at least 6, and optionally not more than 100 or not more than 400, or more specifically 10 to 60. The pattern of reflective facets throughout the surface of each reflector can also be selected according to design requirements, such as distributed in a straight line or hexagonal grid.

[0013] In some embodiments, each reflection of the laser beam prior to a subsequent reflection from an opposing reflector is a reflection from a reflective facet, since reflective facets are provided on both reflectors.

[0014] In other embodiments, at least one reflection of the laser beam between the opposing reflectors may be a reflection of a plane reflection portion from one of the reflectors. Such a plane reflection portion may be provided on only one of the two reflectors or on both of the two reflectors.

[0015] In some embodiments, all reflective facets are disposed on only one of the reflectors, and the reflection of the laser beam at the opposite reflector comes from one or more planar reflective portions of the opposite reflector. For example, the opposite reflector may provide a single planar surface for all laser beam reflections. In this case, the opposite reflector may conveniently be provided by a plane mirror. In some embodiments, such a single planar surface or plane mirror may be replaced by a curved mirror with a suitably large radius of curvature or some other form of reflective surface.

[0016] Each reflective facet may have substantially spherical curvature, and each reflective facet may have the same radius of curvature or substantially the same radius of curvature as one another (e.g., all within about 1% of one another). Such a spherical arrangement may make the design and manufacture of an embodiment more straightforward. However, at least slightly aspherical surfaces may be used, in which case the radii of curvature referred to in this document may be understood with reference to, for example, the radius of curvature at the center point of the facet, the average radius of curvature, or other suitable means. In other embodiments, facets with different radii of curvature may be used.

[0017] Each reflective facet can have the same radius of curvature, and such radius of curvature can be a multiple of the spacing length, wherein the multiple is one of the following multiples: 2 / 3 times, 1 times, and 2 times. Such multiples do not have to be exact, for example, the radius of curvature can be within ±5% or ±-10% of such multiples. In other examples, the radius of curvature of the facets can be slightly different (e.g., within the above range) or can be greatly different.

[0018] These multiples are particularly advantageous for concave facets that reflect the laser beam to the concave reflective facets of the counter reflector. In the case where the counter reflector instead provides a planar reflective surface to reflect the laser beam back to the concave reflective facets of the first reflector, the radius of curvature of the first facet may alternatively be a multiple of 1, 4 / 3, 2, or 4 times the spacing length, or, for example, within + / - 5 or + / - 10% of this multiple.

[0019] The device can be arranged so that the laser beam forms multiple beam waists during reflection between the first reflector and the second reflector, for example, once for each reflection from the facet, or at least once every two reflections, or at least once every three reflections. Such a beam waist can be described as a focus that directs the laser beam to that point, and control of these multiple focal points is used in the design of the reflector and its facets to minimize the loss of the laser beam due to incidence outside the area of ​​each specific facet.

[0020] For example, the device including reflective facets can be configured so that the laser beam forms multiple beam waists in reflection between consecutive reflective facets in the beam path of the first and second relative reflectors, each beam waist being spaced apart from the two reflectors by at least 10%, more preferably at least 20%, of the spacing length.

[0021] If the next reflection will come from a planar reflection portion of the relative reflector, the reflecting facet can be configured so that the laser beam forms a beam waist at or near the next reflection and the planar reflection portion, wherein the adjacent relative reflector is optionally within 20% of the spacing length from the relative reflector, or optionally within 10% of the spacing length.

[0022] The infrared spectrometer may further include an adjustment optical device, which is arranged to form a waist before the laser beam is first reflected from any of the reflective facets. Specifically, such a waist may be located between the first reflector and the second reflector, and may be positioned to help minimize the loss of the laser beam due to incidence outside the surface area of ​​some or all of the reflective facets.

[0023] Each reflective facet may have a diameter of, for example, about 1 mm to 6 mm. The spacing length may be, for example, about 30 mm to 300 mm.

[0024] When a multi-pass cell is used for an infrared laser beam and for the described infrared spectrometer, the wavelength of the laser beam can be between 2000 nm and 20000 nm. Note that since laser beams can generally be described by Gaussian optics, the reflection and expansion of such beams depends on the wavelength, and the laser wavelength is highly dependent on the design of the device, for example, on the curvature of the reflective facets, the size, and the spacing between the reflective facets.

[0025] The reflectors may be designed to provide various combinations of single and / or multiple reflections of the laser beam from all subsets, or overlapping subsets, or discrete subsets of one or more configurations. However, in some embodiments, the reflective facets of the first reflector and the second reflector may be arranged so that the laser beam is reflected once and only once from each reflective facet in the plurality of reflective facets or each reflective facet in all reflective facets.

[0026] At least one of the reflective facets and / or planar reflective portions can be configured to be partially reflective and partially transmissive so that a first portion of the laser beam continues to reflect from a second subset of the reflective facets and is subsequently detected by a detector, and a second portion of the laser beam is transmitted through the partially transmissive facets or planar portions so as to be detected by the detector or another detector after reflecting from only the first subset of the reflective facets. For example, the ratio of reflection to transmission can be approximately 50:50, or some other ratio as desired and discussed below. In such an arrangement using at least one partially reflective facet or planar portion, the facets of the first subset and the second subset can have no intersection.

[0027] The infrared spectrometer may further include a spatial filter mask located between the opposing reflectors and arranged to reduce the proportion of the laser beam incident on a surface other than the intended reflection surface at each reflection, noting that the profile of the laser beam is typically Gaussian or similar. Specifically, the spatial filter mask may include a plurality of holes formed in a material that blocks portions of the laser beam that would otherwise be incident on a surface other than the intended next reflection surface.

[0028] For example, the spatial filter mask may include a plurality of holes, each hole being arranged to allow at least a portion of the laser beam directed to a corresponding next reflective facet in the path of the laser beam to reach the next reflective facet, and the spatial filter mask being arranged to block at least a portion of the laser beam that would otherwise be incident outside the next reflective facet.

[0029] The reflective facets and the spatial filter mask can be configured so that the laser beam forms a plurality of beam waists at or adjacent to the spatial filter mask during reflection between the first reflector and the second reflector. For example, if the radius of curvature of each reflective facet is approximately the same as the separation distance between the opposing reflectors, the infrared spectrometer can be arranged to form the beam waist at the center of the pool between the opposing reflectors, and the spatial filter mask can also be located at the center of the pool between the opposing reflectors.

[0030] If all reflective facets are arranged on a first reflector in the reflectors and only planar reflection is used at another second reflector, these reflective facets can be configured so that the laser beam forms multiple beam waists at or near the second reflector during reflection, and a spatial filter mask can be provided at or near the second reflector.

[0031] For each reflector, a plurality (e.g., greater than 80%) of the reflective facets or all of the reflective facets (and optionally any planar reflective portions) may be formed together or integrally on a single piece of material, such as by molding as a single piece, or formed from a single piece of material by diamond milling, laser ablation, or other cutting processes. Note, however, that coatings may also be applied to such a single piece of material. For example, at least some or all of the reflective facets may be coated with a reflective coating. In some embodiments, most of the reflective facets may be integrally formed on a single piece of material, but certain specific types of facets (e.g., partially reflective facets) may be provided in other ways, such as by using other materials bonded or otherwise mounted to the reflector.

[0032] Embodiments of the invention providing an infrared spectrometer may include a sample pool arranged to accommodate a sample in a space between first and second opposing reflectors so that a laser beam reflected between the reflectors passes through the sample multiple times. Such a sample pool may, for example, be provided by a pool wall extending between the opposing reflectors.

[0033] One or both of the opposing reflectors may include one or more ports, and the ports are arranged so that the sample flows into and / or flows out of the sample pool through the ports. For example, each of the opposing reflectors may include one or more of the ports, and the infrared spectrometer may then be arranged so that the sample flows into the sample gas pool through one or more ports of one of the reflectors and flows out of the pool through one or more ports of another of the reflectors.

[0034] Such a port may conveniently be located between two or more adjacent reflecting facets of the reflector.

[0035] The present invention also provides design, manufacturing, and operating methods corresponding to the described devices.

[0036] For example, in one aspect, the present invention provides a method of operating an apparatus (e.g., the above-mentioned infrared spectrometer), the method comprising: providing first and second opposing reflectors, each reflector comprising a plurality of discrete concave reflective facets; and directing a laser beam to pass between the reflectors multiple times by reflecting at least once from each of the plurality of reflective facets, the laser beam being alternately reflected from the reflective facets of each of the first and second opposing reflectors. A sample may be provided in the space between the opposing reflectors so that the laser beam passes through the sample multiple times, and the method may then include detecting spectral characteristics of the laser beam after the multiple reflections; and determining characteristics of the sample based on the detected spectral characteristics.

[0037] On the other hand, the present invention provides a method for operating an infrared spectrometer, the method comprising: providing a first and second relative reflector spaced apart by a spacing length; a plurality of concave reflecting facets, each reflecting facet being a facet of the reflector; providing a sample in the space between the relative reflectors; guiding the infrared laser beam to pass through the sample multiple times by reflecting the infrared laser beam at least once from each of the plurality of reflecting facets, the laser beam alternately reflecting from each reflector in the relative reflectors, including being reflected at least once by each of the reflecting facets; detecting the spectral characteristics of the laser beam after the multiple reflections; and determining the characteristics of the sample based on the detected spectral characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Embodiments of the present invention will now be described, by way of example only, with reference to the following drawings, in which:

[0039] Figure 1 An infrared spectrometer embodying the present invention is shown;

[0040] Figure 2 It is shown that in Figure 1 The arrangement uses partially reflective and partially transmissive facets;

[0041] Figure 3 It is shown that in Figure 1 or Figure 2 using a spatial filter mask located between reflective facets of an infrared spectrometer in an arrangement to reduce beam overlap to adjacent facets and / or other regions of the reflector;

[0042] Figure 4 Shows Figures 1 to 3 How a reflector of can be implemented to include one or more planar reflective facets;

[0043] Figures 5A to 5D Various forms of gap transition regions between reflective facets are shown in cross-section;

[0044] Figure 6 The perspective view shows a device including a Figures 1 to 5D A reflector having a reflective facet of an embodiment;

[0045] Figure 7 Shows Figure 6 The reflector is used to form the end wall of the closed sample cell;

[0046] Figure 8 is a graph showing the diameter at which 99% of the laser beam intensity at the last reflecting facet is obtained as a function of the wavelength of the laser and the radius of curvature of the reflecting facet; and

[0047] Fig. 9 The results for two specific wavelengths are shown. Figure 8 A cross section of a graph. DETAILED DESCRIPTION

[0048] Reference Figure 1 , schematically shows an infrared spectrometer 10 embodying the present invention. The first reflector 12 and the second reflector 14 opposite to each other are separated by a spacing length d. Each reflector 12, 14 includes a plurality of discrete concave reflective facets 16. Note that Figure 1 The shapes of the facets are not shown in FIG. 1 , but rather a dashed box is shown around the area of ​​each facet. The reflective facets are discrete in the sense that convex curvature is required between adjacent facets (e.g., by one or more edges or more slowly curved convex regions). Although the Figure 1 It is not obvious from the cross-sectional view of , but the reflective facets can be distributed across the opposing faces of each of the first reflector and the second reflector in various ways, such as in a straight line or hexagonal grid, in a single row of facets, or in other ways.

[0049] An infrared laser source 20 is used to form a laser beam 22, which is directed to be reflected at least once from each of the plurality of reflective facets 16 by alternately reflecting from the reflective facets of each of the first reflector and the second reflector, as shown. In some embodiments described below, some reflections may come from one or more planar portions of one or both of the first reflector and the second reflector, for example, all concave reflective facets are disposed on one of the reflectors, while reflections from the other reflector come from the planar portions of the reflector.

[0050] After reflection from the reflective facet 16, the laser beam 22 is incident on a detector 40, which is arranged to detect one or more spectral characteristics of the laser beam. If a sample 25 (such as a gaseous or liquid sample) is present in at least some of the paths of the laser beam 22 between the reflectors 12, 14, the sample may affect the spectral characteristics of the laser beam 22 (e.g., due to infrared absorption or dispersion), and the spectral characteristics detected at the detector 40 may therefore be used to determine characteristics of the sample. For example, an analyzer 42 may be used to receive a signal including the spectral characteristics from the detector 40 and determine characteristics of the sample based on the signal. The determined spectral characteristics and / or characteristics of the sample may be further stored, transmitted, displayed, and / or processed in various ways by a computer system 44 that receives relevant data from the analyzer 42, such as Figure 1 shown.

[0051] For example, a laser source can be arranged to form a laser beam having a wavelength consistent with or scanning across one or more absorption spectral features of one or more target substances to be detected in a sample. Then, the detection of the intensity of the laser beam at the wavelength of those absorption spectral features can be used to infer the concentration of those target substances in the sample. Typical applications of this technology include: detecting specific gaseous substances in human breath for clinical purposes; detecting specific gaseous substances in the atmosphere or in gases emitted from industrial processes for detecting pollutants, etc. Some techniques that can be used in this regard are discussed, for example, in WO2009 / 101374, WO2011 / 058330, and WO2018 / 078384, in each case these techniques can be implemented using the sample pool defined by the relative reflector described herein.

[0052] Since the type of substances to be detected in sample 25 are typically at low concentrations, they are difficult to detect with high accuracy or quickly enough using infrared absorption spectroscopy, and the use of multiple reflective facets provides a longer path length for the laser beam 22 through the sample, thereby providing a higher degree of infrared absorption or dispersion for detection of the target substance.

[0053] The laser source can form a laser beam that is in the infrared spectrum or more specifically in the mid-infrared spectrum, for example, having a wavelength between about 2000 nm and 20000 nm. The laser source can form a laser beam that is scanned over a certain wavelength range to cover more than one spectral feature (e.g., multiple absorption lines). Typically, for example, using various types of semiconductor-based lasers (e.g., in-band cascade lasers, or quantum cascade lasers, or optical parametric oscillation lasers, or external cavity lasers), a laser beam of about 1 cm can be used when it is necessary to detect a specific spectral feature. -1 Up to 3cm -1 The wavelength scanning range can be up to about 100cm when a wider spectral range is required. -1 wavelength scanning range.

[0054] The laser beam 22 may be directed through an entrance aperture 24 in the first reflector 12 and undergo a first reflection at a first reflecting facet or planar reflecting portion on the second reflector 14. The entrance aperture may be closed by a window (not shown in this figure) to prevent passage of gas or liquid. After reflection from the plurality of reflecting facets 16, the laser beam may then exit through an exit aperture 26 in the first reflector or the second reflector ( Figure 1 The exit hole is shown as exiting through the second reflector 14), and the exit hole can also be closed by a window to prevent gas or liquid from passing through.

[0055] The infrared laser beam 22 may generally be in the form of a Gaussian beam, in other words, dominated by a fundamental TEM00 transverse Gaussian mode, wherein the overall beam has a Gaussian intensity profile, although other beam components are generally present to some extent, such a mode may be denoted, for example, as an Hermite-Gaussian mode or a Laguerre-Gaussian mode. According to the general Gaussian beam principles and properties, the geometry of the laser beam may therefore be defined essentially by a combination of its wavelength and the position and radius of the beam waist. Figure 1 As shown, the conditioning optical device 28 can be located after the laser source 20 but before the first reflector 12 and the second reflector 14 to control the geometry of the light beam before the first reflection from the facet 16 by forming a beam waist of appropriate position and size before the first reflecting facet, as described in detail below. Such a beam waist can preferably be formed between the first reflector 12 and the second reflector 14.

[0056] The formation and relative arrangement of the facets, combined with control over the geometry of the beam upon reflection near the facets, controls the progression of the laser beam 22 as it reflects between the facets 16. Specifically, the facets may be formed and arranged to maintain the laser beam spot radius at each facet within a controlled range, to minimize the amount of laser beam incident on reflectors outside of each intended facet, and to maximize the amount of laser beam reaching the detector 40.

[0057] In this way, the sensitivity of the sensor can be increased by using a greater number of facets and therefore a longer laser beam path length through the sample without excessive loss of laser beam intensity due to stray reflections. This control of the laser spot size at each facet also allows the use of smaller facets, so a greater number of facets can be used for a given size reflector 12, 14. These effects can therefore be used to build a multi-pass cell that has a combination of the following properties: more compact due to having a smaller diameter and / or length, and greater sensitivity for applications such as spectroscopy due to having a longer optical path length and / or lower laser beam losses at the reflector.

[0058] Typically, the number of facets 16 on each reflector, or the number of facets 16 on two reflectors combined, and / or the number of facets for which the sensor is arranged to reflect the laser beam can be in the range of about 3 or about 6 to about 100 or about 400, or more specifically in the range of about 10 to about 60. Increasing the number of facets used for reflection results in an increase in the path length of the laser beam through the sample, but results in increased losses (due to the reflectivity at each facet being less than 100%, and the residual substantially Gaussian beam incident at each subsequent facet being less than 100%).

[0059] To form a compact spectrometer, each facet may have a diameter of between about 1 mm and 10 mm. Each reflector may typically be formed with an opposing face facing the other reflector having a surface area of ​​about 10 mm. 2 and 1000mm 2 The shape of each opposing facet can be conveniently circular, oval, square, or rectangular, but other shapes can also be used. The spacing length between reflectors can be selected taking into account the desired total laser beam path length, the number of facets available for reflection, and the available precision of the facets, but this spacing length can typically be between about 30 mm and 300 mm.

[0060] In some arrangements, the laser beam 22 is reflected only once from any one facet, although arrangements are contemplated in which one or more facets are used to reflect the laser beam 22 multiple times. In some embodiments, all reflective facets are used to reflect the laser beam 22 between the laser source 20 and the detector 40. However, this is not always necessary, and in some embodiments and some configurations of those embodiments, a subset of facets may be used for a particular configuration of the input laser beam 22. In some embodiments, the facets may be arranged so that more than one laser beam path is defined between the reflectors 12, 14. For example, two different reflection paths may be used, both between all reflectors or a subset of reflectors, optionally using different entry and / or exit apertures, and optionally using different laser beam sources and / or detectors, and optionally using the same subset, overlapping subsets, or non-overlapping subsets of reflectors.

[0061] In some arrangements, the spectrometer may be arranged to provide alternative longer and shorter total laser beam reflection paths between the first reflector and the second reflector, for example Figure 2 Schematically shown in FIG. Figure 2 Shows something like Figure 1 The first reflector 12 and the second reflector 14 are of the type shown in FIG. 1 , but in this case one of the reflective facets 16 of one of the reflectors 14 is a partially transmissive facet 18, followed by a further exit aperture 26′. The partially transmissive facet 18 is arranged so that part of the reflected light beam 22 continues to be further reflected from the facets of these reflectors as a longer path light beam 22′, and part of the transmitted light beam 22 is directed as a shorter path light beam 22″ via the further exit aperture 26′ in the reflector 14 to a detector 40, or alternatively to a further detector 40′, as shown in FIG. Figure 2 Such detector(s) and subsequent processing of the spectral signals have been described above. Figure 2A further exit aperture 26 ′ is shown in the same reflector 14 as the first described exit aperture 26 , but either aperture may be in the same reflector as the entrance aperture 24 .

[0062] Using this arrangement, the reflectors can be used to provide longer and shorter laser beam path lengths between the reflectors, and the beam intensity at the (one or more) detectors after each of the longer and shorter beam paths 22', 22" is lower. If in some cases the longer laser beam path length might provide a saturated signal at the detector or a signal in which the detector accuracy is affected at higher concentrations of sample material, a shorter laser beam path length can be used instead to provide a signal at the detector that is not affected in this way or at least is affected to a lesser extent. In other words, providing two or more paths of different lengths in this manner can provide the sensor with an enhanced dynamic range for different expected concentrations of a target substance in the sample.

[0063] To this end, if one partially transmissive facet is provided, it can conveniently be set between about 5% and 60% along the path of the laser beam 22 between the first reflector and the second reflector. If more than one partially transmissive facet is provided so as to provide more than two different lengths of beam paths, different beam paths can be selected accordingly.

[0064] Each partially transmissive facet may transmit between about 10% and 90% of the intensity of the beam incident thereon, thereby reflecting the remainder of the intensity of the beam. If only one partially reflective facet is used, it may transmit about 40% to 60% of the intensity of the beam.

[0065] The partially transmissive facets 18 can be formed, for example, by translucent crystal or glass materials (wherein the surface characteristics of the facets have been molded or processed). If desired, optical coatings can be used to adjust the reflection / transmission characteristics. Examples of suitable materials include lead chalcogenide glass or crystalline materials such as zinc selenide, calcium fluoride, and barium fluoride. For example, the partially transmissive facets can be glued in place or inserted using a mounting system.

[0066] In some arrangements, a spatial filter mask 15 may be disposed between the opposing reflectors 12, 14, for example Figure 3 As schematically shown in Figure 3 Shows something like Figure 1 or Figure 2The first reflector 12 and the second reflector 14. The spatial filter mask 15 is formed of a material that is substantially opaque to the laser beam 22 and can be constructed or coated to enhance absorption of the laser beam incident on the mask and thereby minimize scattering of stray laser beam light. The spatial filter mask 15 includes a plurality of holes that penetrate the mask material, each hole being arranged to allow a major portion or a central portion of the laser beam 22 to pass through the mask (particularly a portion of the laser beam that will be incident on the next reflective facet on the path of the beam) in each of some or all of the paths from one reflector to another reflector, while blocking some or all of the following portions of the laser beam 22: the portion will be incident on the reflector body outside the next reflective facet or outside a specific portion of the next reflective facet.

[0067] In this manner, the spatial filter mask 15 can be used to reduce or eliminate scattering of the laser beam 22 from portions of the reflectors 12, 14 other than the specific facets used for each reflection of the laser beam (e.g., other facets 16 or transition regions between facets). This is particularly useful for reducing unwanted scattering of laser beam light and thereby improving the performance of the spectrometer (e.g., improving the signal-to-noise ratio), but it should be noted that if the cross-section of the laser beam 22 is essentially Gaussian or even some other form, there are generally no clear boundaries of the laser beam.

[0068] If the beam waist of the essentially Gaussian laser beam 22 coincides with the spatial filter mask 15, then Figure 3 The spatial filter mask 15 shown may be particularly advantageous. For example, as described below, if the spacing length between the reflectors 12, 14 is approximately 1 times the radius of curvature of each reflective facet, the spectrometer can be arranged so that each beam waist is formed at the center of the space between the two opposing reflectors. Thus, the spatial filter mask 15 can be positioned to coincide with or be close to the beam waist in the center or central region of the space between the two reflectors to minimize the effect of the laser beam being incident on the peripheral region (and stray light) outside the intended reflective facet on the opposing reflector at each reflection.

[0069] As discussed above, some reflections between opposing reflectors may come from planar portions of those reflectors rather than from concave reflective facets. Figure 4 Similar to Figure 2, but an arrangement is shown in which all reflections from the first reflector 12 are from the concave reflecting facet 16 of the reflector, and all reflections from the second reflector are from the planar reflecting portion 116 of the second reflector 14. In this case, conveniently, all the planar reflecting portions can be provided by a single planar surface of the reflector, and in this way, one of the reflectors can be provided by a plane mirror, for example, thereby saving complexity of construction. Moreover, in some cases, such a reflector need not be strictly planar. For example, a single concave or convex surface with a sufficiently large radius of curvature can be used.

[0070] In an arrangement where all concave reflecting facets are provided on a single reflector, it may be convenient to provide a spatial mask 115 at or adjacent to another reflector, such as Figure 4 As shown. Thus, the beam waist of the laser beam 22, which is essentially Gaussian in form, can be consistent with the reflector providing the planar reflection portion, and therefore, if such a spatial filter mask 115 is provided, it can also be consistent with such a spatial filter mask 115. For example, if the spacing length between the reflectors 12, 14 is approximately half the radius of curvature of each reflective facet, the spectrometer can be arranged so that each beam waist is formed at the opposite reflector. Thus, the spatial filter mask 115 can be positioned to be consistent with or adjacent to the beam waist to maximize the impact of the laser beam on the peripheral area outside the intended reflective facet on the opposite reflector at each reflection.

[0071] In other embodiments, one or more planar reflective portions and one or more concave reflectors may be disposed on each of the first reflector and the second reflector to form a reflective element in the path of the laser beam 22 .

[0072] exist Figure 4 middle, Figure 2 One or more of the one or more partially reflective facets of the embodiment of the present invention are replaced by one or more partially reflective planar reflective portions 118 which may be constructed and function in substantially the same manner, although alternative embodiments may be used. Figure 1 and 3 An embodiment without a partially reflective portion is shown. Similarly, it is also possible to Figure 4 Such an arrangement (in which the laser beam path includes reflections from one or more planar reflective portions) provides for example Figure 3 The spatial filter mask 15 is shown.

[0073] FIG. 5A to FIG. 5D Some adjacent reflective facets 16 of a reflector 12 or 14 as discussed herein are shown in cross-section, illustrating how the boundaries or transitions between adjacent reflective facets 16 are formed or constructed. Figure 5AIn the embodiment of the present invention, the facets are not separated by any interstitial surfaces, and the reflector surface transitions from one facet to an adjacent facet without any significant gap. Therefore, the boundary between one facet and another facet generally takes the form of a step between the facets, although no step will occur in the case where the adjacent boundaries of two facets are located exactly at the same level.

[0074] exist Figure 5B In a convex interstitial surface, adjacent facets are separated by a raised interstitial surface, which thus typically takes the form of a wall structure between adjacent facets having steep or vertical sides and a flat, planar, and / or horizontal interstitial surface forming the top of the wall. Figure 5C In a PCB, adjacent facets are separated by a stepped clearance surface, which thus typically takes the form of a groove structure having steep vertical sides and a flat, planar, and / or horizontal clearance surface forming the groove bottom.

[0075] exist Figure 5D In , a gradual, smooth, or continuous transition between adjacent facets is used to provide a boundary region without any sharp edges or transitions. In the case of boundary 19', both sides of the transition (i.e., the transition to two adjacent facets) region are smooth, while in the case of boundary 19", one side of the transition region is smooth and the other side is provided by a sharp edge.

[0076] exist Figures 5A to 5D In all examples of , it should be noted that when considered in plan view, the two-dimensional distribution of the facets must be considered, e.g. Figure 6 For example, the facets can be arranged in a rectilinear or hexagonal grid. If each facet is substantially circular in plan view (e.g. Figure 6 shown), then Figures 5A to 5D The gap transitions can be viewed as those transitions observed in cross-sections through which the edges of the facets are at their closest approach, and where the edges of the facets are more widely separated, substantially the same or appropriately modified forms of wider transitions can be provided. Similarly, if the facets have a different shape in plan view, e.g., square, hexagonal, or some other form, then consideration can be given to FIG. 5A to FIG. 5D Transitions shown and modified appropriately.

[0077] By adopting certain forms of transition boundaries between facets, the extent to which stray scattered light affects the performance of the infrared spectrometer can be minimized, such as by using an appropriate modeling process. For example, certain forms of gap boundaries may result in much less laser beam light leaving the intended path of the beam, or much less laser beam light that does leave the intended path of the beam but is scattered back into the intended path of the beam (which would result in degraded spectrometer performance).

[0078] Figure 6 In perspective view is shown how the reflectors 12, 14 of the previous figures may be implemented in more detail. In the arrangement shown, each reflector comprises 15 discrete reflective facets 16 (one or more of which may be partially transmissive facets 18). The facets are arranged as a linear grid, although various other numerical and geometric arrangements may be used, such as hexagonal or triangular or somewhat unstructured or random grids. In order to provide a compact sensor, for example, the facets may be arranged so that for each reflector, the facets occupy at least 50% of the area of ​​the surface of the reflector that is arranged to face the other reflector.

[0079] Each facet is concave. Figure 6 The facets shown are substantially circular in plan view, although other shapes mentioned elsewhere herein may be used, for example, elliptical, square, hexagonal, etc. Figure 6 In FIG. 1 , the facets of the first reflector 12 are not visible, but in this particular example, the first reflector 12 may be substantially identical to the second reflector, but may be rotated so that the two reflectors face each other.

[0080] exist Figure 6 In the arrangement shown, with 15 facets in a 4×4 grid, the 16th grid position is used to provide an entry aperture 24 for the laser beam to enter the space between the opposing reflectors 12, 14 and an exit aperture 26 for the laser beam to exit therefrom. Figure 6 As shown, each of the entrance and exit apertures may be provided with a window 28 that is suitably transparent to the laser beam while preventing gas or liquid from moving through the aperture. Such a window may be formed, for example, of a molded or machined and polished crystal or glass material. If desired, optical coatings may be used to adjust the transmission properties. Example materials include lead chalcogenide glass or crystalline materials such as zinc selenide, calcium fluoride, and barium fluoride. The window may be, for example, glued in place or inserted using a mounting system.

[0081] exist Figure 6 In an arrangement of, the two reflectors 12, 14 may have the same or similar construction, each reflector including a plurality of concave reflective facets in the path of the laser beam alternately reflected between the two reflectors. Figure 4 As discussed, one of the reflectors 12, 14 may alternatively include a single planar reflector (or in some cases a smoothly curved planar portion) or a single mirror from which the laser beam is reflected multiple times, although typically from different points of the reflector or mirror. In other alternative arrangements, each reflector may include one or more concave reflective facets and one or more planar reflective portions, wherein each concave reflective facet or planar reflective portion provides one or more reflections of the laser beam 22.

[0082] Although not shown in the previous figures, Figure 6 It is shown that one or both of the reflectors 12, 14 may be provided with one or more ports 32 that extend through the particular reflector to allow sample gas or liquid to flow or be pumped into and / or out of the space between the reflectors. Such ports 32 may be positioned, for example, between two, three, or more reflecting facets, and may conveniently utilize areas between the facets that are not necessary for reflection of the laser beam, noting that the laser beam is typically expected to be Gaussian in distribution and thus to be approximately circular or elliptical in cross-section.

[0083] One, two, or more such ports 32 may be formed in one or both of the two reflectors and may be connected to suitable additional devices, such as conduits, pumps, and valves for controlling and / or directing sample flow into and through the space between the reflectors 12, 14. In this way, the flow of sample gas or liquid may be arranged to be primarily along the spacing direction between the first reflector and the second reflector, which may be desirable, for example, to improve the homogeneity of the sample between each beam path between the reflective facets. If a sample that flows across the spacing direction is used instead, rapid changes in the sample may cause certain beam paths to be affected earlier than other paths, resulting in poor sensing performance.

[0084] Figure 7 Shows such as Figure 6 The invention also relates to how the opposing reflectors 12, 14 of the sample cell form part or all of the end walls of the sample cell (the sample cell uses the cell wall 36 to surround the space between the opposing reflectors) and how such a sample cell is used to accommodate the sample. Then, the sample gas or liquid can be sent into and out of the sample cell through the ports 24, 26 and / or using other ports (e.g., through the cell wall 34).

[0085] Returning to the discussion of all the figures above, the discrete reflective facets 16 of the reflector can be generally formed as spherical concave surfaces, although other concave forms that are not completely spherical can be used, such as marginal aspherical surfaces. Where the radius of curvature of the reflective facets 16 is discussed herein, the term should be understood accordingly, for example, to define the average radius of curvature of the entire facet or the radius of curvature at the center of the facet where the shape of the facet is not spherical.

[0086] The geometry of the reflective facets 16 can be formed on or in the reflector in a variety of ways, including by milling (e.g., diamond milling), molding, ablation (such as, laser ablation), and / or replication (typically involving transferring the optical surface of a "negative" master to one or more "positive" copies or optical surfaces). Additive manufacturing (e.g., using 3D printing techniques) can also be used alone or in combination with other techniques. In another alternative, a piezoelectrically actuated surface can be used to provide the reflective facets. Essentially, the same techniques can be used to form one or more planar reflective portions on a reflector as described above. If one of the reflectors provides a single reflective surface without any concave facets, various known techniques for forming mirror surfaces can be used.

[0087] Each reflector can be formed of any of a variety of materials suitable for forming a facet, such as aluminum or other metals, plastics, glass, and ceramics. If the reflector (e.g., the material used to form the reflector itself) is not suitable for providing a sufficiently high reflectivity of the facet or some other change in the inherent optical properties of the underlying material is necessary, a suitable coating can be used on the facet surface (optionally on the entire facet surface or other portions).

[0088] For each reflector, all discrete reflective facets of the reflector and any planar reflective portion may be formed from a single piece of material or as a single integral unit. More specifically, each reflector may be integrally formed from a single piece of material. However, in any case, suitable coatings of one or more different materials may also be applied at least to the reflective facets themselves, to any planar reflective portion, and optionally to some or all portions of the opposing faces of the reflector.

[0089] For example, if the reflectors are formed by milling, all of the facets of the reflector may be milled on a single sheet of material. If each reflector is formed by molding, all of the facets of the reflector may be molded as part of a single sheet of material.

[0090] Alternatively, each reflector may be formed from a plurality of parts of a single piece of material, for example, each part providing one or more facets. In such an example, the plurality of parts may be formed adjacent to each other and may be held together, for example, by bonding or clamping.

[0091] In any of the above cases, one or more coatings may then be applied, for example to improve reflectivity and / or other optical properties, to improve longevity, for example by resisting contamination or chemical damage, and / or for other purposes.

[0092] The inventors have conducted numerical modeling experiments to determine the optimal geometrical parameters for the reflectors and the reflective facets so as to minimize the loss of laser beam intensity during reflection between reflectors and to minimize the overlap of laser beam intensity on adjacent reflectors, while aiming to provide a more compact form of the multi-pass cell and any device comprising such a cell by minimizing the size of the facets and reflectors. To this end, a numerical model was developed that uses the ABCD matrix formulation of ray optics combined with a Gaussian beam transformation, such as described in the university science book "Lasers" by Anthony E Siegman in 1986.

[0093] The numerical model accepts the beam quality factor M 2 and an assumed specific waist size of the laser beam before the first facet reflection as starting parameters, e.g. using Figure 1 The adjustment optical device 28 shown is used for adjustment. In the model, it is also required that the laser beam waist appears between the reflectors, for example, when the laser beam passes through Figure 1 The entrance aperture 24 is shown after entering the space between the reflectors, although the model subsequently allows for waist position variations when optimizing other calculated parameters. In this model, all reflections from the reflectors are from the concave reflective facets, rather than from any planar reflective portions included.

[0094] Figure 8 Shows M 2 =1.2 and the results of the model for an input laser beam with a waist size of 100 μm before the first facet reflection between the reflectors. The reflector spacing length is 179 mm, there are 6 facets at each reflector, each facet is a circle with a diameter of 5 mm, located in a rectilinear grid, and the spacing between the proximal edges of the facets is 0.25 mm. Every other reflection between the facets is between strictly opposing facets, so along the axis between the reflectors, and the beam angle of the alternating reflections between non-opposing (i.e., offset) reflecting facets is about 0.84 degrees.

[0095] Specifically, Figure 8 The diameter of 99% of the beam intensity on the last facet where the laser beam is reflected is shown as a function of the laser beam wavelength from 1000nm to 10000nm (abscissa) and the radius of curvature ROC of each facet (ordinate). The left scale shows the absolute value of the radius of curvature ranging from 0mm to 900mm, and the right scale shows the ratio of the radius of curvature to the spacing length between the two reflectors.

[0096] It can be seen that the beam diameter at the last facet shows strong minima for specific values ​​of facet curvature radius (corresponding to 1 / 2, 2 / 3, 1, and 2 times the spacing length. These minima extend over the entire wavelength range tested, but the minima become narrower and deeper at longer wavelengths, consistent with the Gaussian nature of the beam becoming more pronounced for a given optical scale at these longer wavelengths.

[0097] Fig. 9 Shows Figure 8 2, 1980, and 2000 nm. The 99% beam diameter at the last facet is clearly seen to show a strong minimum when the radius of curvature is equal to or close to multiples of 1 / 2, 2 / 3, 1, and 2 of the spacing length. Although beam diameter minima are seen at these radii of curvature, it can be observed that reasonable performance in terms of small beam diameters is seen within about + / - 10% of these multiples (more preferably within about + / - 5% of these multiples), for example, between radii of curvature of about 160 mm and 197 mm for 1 of the spacing length. Between the minima of beam width as a function of the radius of curvature of the facet, the increase in beam width is more pronounced for longer wavelengths, extending to about 6-7 mm for a 3300 nm laser beam and about 20 mm for a 9500 nm laser beam.

[0098] These results are consistent with various other laser beam parameters M 2 , beam waist width, reflector spacing, and other parameters (such as the number of reflective facets) are generally consistent, thereby enabling the design of an infrared absorption sensor embodying the present invention in which smaller diameter reflective facets can be used without significantly increasing beam intensity losses, thereby providing a more compact device. Reflector designs using a facet curvature radius of 2 / 3 times the spacing length between reflectors are relatively narrow width minimums, especially at longer wavelengths, which requires very high tolerances in the manufacture and positioning of the reflectors and adjusting optics. Reflector designs with a curvature radius of 1 / 2 times the spacing length require finer tolerances and alignment, and may therefore also be more difficult to use.

[0099] Reflector designs with a radius of curvature of approximately 1 or 2 times the spacing length are easier to implement because the minimum beam size at the facets is wider and therefore less affected by manufacturing tolerances. In the case of a radius of curvature of approximately 1 times the spacing length, re-imaging and therefore the beam waist occurs at the center of the cell. This also has the advantage of allowing the use of the beam size as described above and in Figure 3Advantages of the spatial filter mask shown in . In the case where the radius of curvature is approximately 2 times the spacing length, re-imaging and therefore the beam waist occurs at the center or facet of the cell for optimal placement.

[0100] Similar modeling by the inventors also suggests that it is advantageous to ensure that the reflective facets are designed and arranged so that the laser beam forms multiple beam waists (each beam waist is located at the center between or toward the center of the subsequent reflective facets) in terms of reducing facet size and reducing laser beam losses and overlap. For example, the sensor can be arranged so that some or all of the beam waists are spaced apart from the two reflectors by at least 10% of the spacing length, and more preferably at least 20% or at least 30% of the spacing length. As described above, the positioning of the beam waists between the reflectors allows the use of a spatial filtering mask that is close to or consistent with those beam waists, wherein the mask is arranged to reduce or eliminate the incidence of the beam on the reflector but outside the next expected reflective facet.

[0101] In the case where the laser beam reflects from a concave facet on a first reflector in the reflectors to a planar reflecting portion on an opposing reflector and then returns to a concave facet on the first reflector, the above modeling suggests that the curvature of the concave facet may be optimized to be approximately twice the above value, i.e., approximately multiples of 4 / 3, 2, and 4 of the spacing length, for example within approximately + / -10% of these multiples.

[0102] Similarly, where a planar reflective portion is used, it is advantageous to ensure that the reflective facets are designed and arranged so that the laser beam forms a beam waist located at or close to the planar reflective portion (e.g., within 10% or within 20% of the spacing length of the planar reflective portion) in terms of reducing facet size and reducing laser beam losses and overlap. If an arrangement is used in which all concave facets are disposed on a first reflector in the reflectors, the device can be arranged so that all beam waists occur at or near another second reflector. In this case, a spatial filter mask can be disposed at or near the second reflector so that it is close to or consistent with the beam waist at the second reflector, the mask being arranged to reduce or eliminate the beam from being incident on portions of the first reflector other than the next intended reflective facet.

[0103] Although specific embodiments of the invention have been described, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined by the claims. For example, the described arrangement of opposing reflectors, each comprising a plurality of concave facets, may be used for purposes other than the infrared absorption spectrometer discussed, such as providing an optical delay path for various purposes, including telecommunications, spectroscopy in various infrared and other spectral regions, and other fields.

Claims

1. An infrared spectrometer (10), include: first and second opposing reflectors (12, 14), the first and second opposing reflectors being spaced apart by a spacing length; a plurality of discrete concave reflective facets (16), said reflective facets being facets of said opposing reflector; an infrared laser source (20) arranged to form a laser beam (22); The opposing reflectors are arranged such that the laser beam is alternately reflected from the concave reflective facets of each of the opposing reflectors, including being reflected at least once by each of the plurality of concave reflective facets; a detector (40) arranged to detect a spectral characteristic of the laser beam after reflection from each of the plurality of concave reflective facets; as well as an analyzer (42) arranged to determine a characteristic of a sample (25) disposed between the first and second opposing reflectors based on the detected spectral characteristic, Wherein, each concave reflecting facet (16) among the plurality of reflecting facets (16) that reflects the laser beam to the opposite reflector has a curvature radius within ±10% of a multiple of the spacing length, wherein the multiple is the same for all the concave reflecting facets of the plurality of concave reflecting facets and the multiple is one of the following multiples: 2 / 3 times, 1 times, and 2 times.

2. An infrared spectrometer (10), include: first and second opposing reflectors (12, 14), the first and second opposing reflectors being spaced apart by a spacing length; a plurality of discrete concave reflective facets (16), the reflective facets being facets of only one of the opposing reflectors, the other of the opposing reflectors including one or more planar reflective portions (116); an infrared laser source (20) arranged to form a laser beam (22); The opposing reflectors are arranged such that the laser beam is alternately reflected from each of the opposing reflectors, including being reflected at least once by each of the concave reflective facets; a detector (40) arranged to detect a spectral characteristic of the laser beam after reflection from each of the plurality of reflective facets; as well as an analyzer (42) arranged to determine a characteristic of a sample (25) disposed between the first and second opposing reflectors based on the detected spectral characteristic, Wherein, each of the planar reflecting portions (116) of the plurality of concave reflecting facets (16) that reflects the laser beam to the opposite reflector for reflection back to another concave reflecting facet among the concave reflecting facets has a curvature radius within ±10% of a multiple of the spacing length, wherein the multiple is the same for all of the plurality of concave reflecting facets and the multiple is one of the following multiples: 4 / 3, 2, and 4.

3. An infrared spectrometer according to claim 1, arranged so that each reflection of the laser beam (22) before a subsequent reflection from an opposite reflector is a reflection from a reflective facet.

4. The infrared spectrometer according to claim 1, arranged so that at least one reflection of the laser beam (22) between the opposing reflectors is a reflection from a planar reflection portion (116) of one of the opposing reflectors.

5. The infrared spectrometer according to claim 2 is arranged so that all reflecting facets are arranged on only one of the opposing reflectors, and reflection of the laser beam at the opposing reflector comes from one or more planar reflecting portions (116) of the opposing reflector.

6. The infrared spectrometer according to claim 5, in, The one or more planar reflective portions (116) of the opposing reflector are provided by a single planar surface.

7. An infrared spectrometer according to any one of the preceding claims, in, The number of the reflective facets that reflect the laser beam at least once is at least 6.

8. The infrared spectrometer according to claim 7, in, The number of the reflective facets that reflect the laser beam at least once does not exceed 100.

9. The infrared spectrometer according to any one of claims 1 to 6, in, At least some of the reflective facets (16) are configured such that the laser beam (22) forms a beam waist between reflection from such reflective facet and the next reflection from a reflective facet of an opposite reflector.

10. The infrared spectrometer according to claim 9, in, Each such beam waist is spaced apart from two of the opposing reflectors (12, 14) by at least 20% of the spacing length.

11. The infrared spectrometer (10) according to any one of claims 1 to 6, in, At least some of the reflecting facets (16) are configured so that the laser beam forms a beam waist at or near a next reflection, wherein the next reflection comes from a planar reflecting portion (116) of an opposite reflector, wherein the vicinity of the opposite reflector is within 10% of the spacing length from the opposite reflector.

12. The infrared spectrometer according to any one of claims 1 to 6, further comprising conditioning optics (28) arranged to form a waist in the laser beam before a first reflection from any one of the opposing reflectors.

13. The infrared spectrometer according to any one of claims 1 to 6, in, The diameter of each concave reflective facet is 1 mm to 6 mm.

14. The infrared spectrometer according to any one of claims 1 to 6, in, The wavelength of the laser beam is between 2000nm and 20000nm.

15. The infrared spectrometer according to any one of claims 1 to 6, in, The reflective facets are arranged such that the laser beam is reflected once and only once from each of the plurality of reflective facets.

16. The infrared spectrometer according to any one of claims 1 to 6, in, Each reflective facet (16) has a substantially spherical curvature.

17. The infrared spectrometer according to any one of claims 1 to 6, in, The laser beam (22) is in the form of a Gaussian beam or is dominated by a fundamental TEM00 transverse Gaussian mode.

18. The infrared spectrometer according to claim 2, in, At least one of the reflective facets or at least one of the planar reflective portions is partially reflective such that after reflection from a first subset of the reflective facets: The first portion of the laser beam continues to reflect from the second subset of the reflective facets and is subsequently detected by the detector; and A second portion of the laser beam is transmitted through the partially reflective facets (18) or planar reflective portion to be detected by the detector or another detector after reflection from only the first subset of the reflective facets.

19. The infrared spectrometer according to claim 1, in, At least one of the reflective facets is partially reflective such that after reflection from a first subset of the reflective facets: The first portion of the laser beam continues to reflect from the second subset of the reflective facets and is subsequently detected by the detector; and A second portion of the laser beam is transmitted through the partially reflective facets to be detected by the detector or another detector after reflection from only the first subset of the reflective facets.

20. The infrared spectrometer according to claim 18 or 19, in, The facets of the first and second subsets have no intersection.

21. The infrared spectrometer according to any one of claims 1 to 6 further comprises a spatial filter mask (15) located between the relative reflectors, the spatial filter mask comprising a plurality of holes, each hole being arranged to allow at least a portion of the laser beam directed toward a corresponding next reflecting facet in a path of the laser beam to reach the next reflecting facet, and the spatial filter mask being arranged to block at least a portion of the laser beam that would otherwise be incident outside the next reflecting facet.

22. The infrared spectrometer according to claim 21, in, The reflective facets (16) and the spatial filter mask (15) are configured such that the laser beam forms a plurality of beam waists at or adjacent to the spatial filter mask during reflection of the laser beam between the first and second opposing reflectors.

23. The infrared spectrometer according to any one of claims 1 to 6, in, For each of one or both of the opposing reflectors (12, 14), all of the reflective facets are formed on a single piece of material.

24. The infrared spectrometer according to any one of claims 1 to 6, in, Some or all of the reflective facets are coated with a reflective coating.

25. The infrared spectrometer according to any one of claims 1 to 6, in, The reflective facets are formed by one of the following means: molding, diamond milling, laser ablation, and replication.

26. The infrared spectrometer according to any one of claims 1 to 6, comprising a sample cell arranged to accommodate the sample (25) between the first and second opposing reflectors.

27. The infrared spectrometer according to claim 26, in, One or both of the opposing reflectors may comprise one or more ports arranged for the sample to flow into and / or out of the sample cell.

28. The infrared spectrometer according to claim 27, in, Each of the opposing reflectors includes one or more of the ports, and the infrared spectrometer is arranged so that the sample flows into the sample gas cell through one or more ports in one of the reflectors and flows out of the sample gas cell through one or more ports in another of the reflectors.

29. A method of operating an infrared spectrometer (10), include: providing first and second opposing reflectors (12, 14), the first and second opposing reflectors being spaced apart by a spacing length; Providing a plurality of concave reflective facets (16), said concave reflective facets being facets of said opposing reflectors; providing a sample (25) in the space between the opposing reflectors; directing the infrared laser beam (22) through the sample multiple times by reflecting the infrared laser beam at least once from each of the plurality of concave reflective facets, the laser beam being alternately reflected from the concave reflective facets of each of the opposing reflectors; detecting, by a detector, the spectral characteristics of the laser beam after multiple reflections; as well as determining the characteristics of the sample based on the detected spectral characteristics, Wherein, each of the concave reflecting facets (16) among the plurality of concave reflecting facets (16) that reflects the laser beam to the opposite reflector has a curvature radius within ±10% of a multiple of the spacing length, wherein the multiple is the same for all of the plurality of concave reflecting facets and the multiple is one of the following multiples: 2 / 3, 1, and 2.

30. A method of operating an infrared spectrometer (10), include: providing first and second opposing reflectors (12, 14), the first and second opposing reflectors being spaced apart by a spacing length; providing a plurality of concave reflective facets (16), the concave reflective facets being facets of only one of the opposing reflectors, the other of the opposing reflectors including one or more planar reflective portions (116); providing a sample (25) in the space between the opposing reflectors; directing the infrared laser beam (22) through the sample a plurality of times by reflecting the infrared laser beam at least once from each of the plurality of concave reflective facets, the laser beam being alternately reflected from each of the opposing reflectors; detecting, by a detector, the spectral characteristics of the laser beam after multiple reflections; as well as determining the characteristics of the sample based on the detected spectral characteristics, Wherein, each of the planar reflecting portions (116) of the plurality of concave reflecting facets (16) that reflects the laser beam to the opposite reflector for reflection back to another concave reflecting facet among the concave reflecting facets has a curvature radius within ±10% of a multiple of the spacing length, wherein the multiple is the same for all of the plurality of concave reflecting facets and the multiple is one of the following multiples: 4 / 3, 2, and 4.

31. The method according to claim 29, in, Each reflection of the laser beam before reflecting from an opposing reflector is a reflection from the reflective facet.

32. The method according to claim 30, in, The reflective facets are all facets of a first one of the opposing reflectors, and the reflection of the laser beam from the other one of the opposing reflectors is a reflection from one or more planar reflective portions (116) of the other opposing reflector.

33. The method according to any one of claims 29 to 32, in, The number of reflective facets that reflect the laser beam at least once is at least 6.

34. The method according to claim 33, in, The number of reflective facets that reflect the laser beam at least once does not exceed 100.

35. The method according to any one of claims 29 to 32, in, The diameter of each reflective facet (16) is 1 mm to 6 mm.

36. The method according to claim 30, in, At least one of the reflective facets (16) or at least one of the planar reflective portions (116) is partially reflective, such that a first portion of the laser beam (22) continues to reflect from a second subset of the reflective facets and is subsequently detected by the detector, and a second portion of the laser beam is transmitted through the partially reflective facets or planar reflective portions to be detected by the detector or another detector after reflecting from only the first subset of the reflective facets.

37. The method according to claim 29, in, At least one of the reflective facets (16) is partially reflective, such that a first portion of the laser beam (22) continues to reflect from a second subset of the reflective facets and is subsequently detected by the detector, and a second portion of the laser beam is transmitted through the partially reflective facets to be detected by the detector or another detector after reflecting from only the first subset of the reflective facets.

38. A method according to any one of claims 29 to 32, in, Each reflective facet (16) has a substantially spherical curvature.

39. The method according to any one of claims 29 to 32, in, The laser beam (22) is in the form of a Gaussian beam or is dominated by a fundamental TEM00 transverse Gaussian mode.

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