Optical gas analyzer, optical gas analyzer, and method

Abstract

To provide an optical gas analyzer, system, and method that improves measurement accuracy and / or sensitivity while simultaneously being reasonably priced. [Solution] The optical gas analyzer (10) comprises a light source (30) that emits light into a gas cell (20) for the gas to be measured, and a photodetector (40) that receives the light after it has passed through the gas to be measured (21) in the gas cell. The light source (30) comprises an emitter (31), a first optical element (32), and a second optical element (33). The first optical element is a microlens mounted directly above the aperture of the emitter, which reduces the divergence angle of the light emitted by the emitter and provides pre-collimated light (61). The second optical element is configured to receive the pre-collimated light from the microlens and shape the light beam for emission into the gas cell, and the ratio f2 / f1 of the focal length f2 of the second optical element to the focal length f1 of the first optical element is at least 20.

Description

Technical Field

[0001] The present invention relates to an optical gas analyzer. The present invention further relates to a corresponding system and method for optical gas analysis.

Background Art

[0002] Optical gas analyzers can determine the concentration of a gas by utilizing the absorption of light by the gas. The gas to be measured may include a combination of one or more different gases. Gas analyzers can be used for several applications, such as in industrial fields, environmental monitoring, and / or medical applications.

[0003] Some gases absorb light at several narrow absorption lines or, in the case of larger molecules, absorption bands. The spectral position of an absorption line or absorption band is characteristic of each gas. Thus, it is often possible to identify a gas based on those absorption lines or absorption bands. The fraction of light absorbed by a gas depends on the gas itself, the gas concentration, and the optical path length over which the light can be absorbed. Beer-Lambert's law or Lambert-Beer's law is commonly applied to chemical analysis measurements to determine the concentration of a chemical species that absorbs light. Beer-Lambert's law thus provides the following simplified model.

Equation

[0004] Although the basic principles are known in the relevant field, there are numerous and varied practical methods for determining gas concentrations by optical measurement. Typical measurement principles of different gas analyzers are direct absorption spectroscopy (DAS) and wavelength-modulated spectroscopy (WMS).

[0005] Direct absorption spectroscopy (DAS) is a widely used technique for measuring gas concentrations by monitoring how much light a sample absorbs at a specific wavelength. In DAS, a laser or other light source is tuned to a specific wavelength corresponding to the characteristic absorption lines of the target gas. The light passes through the sample, and the decrease in intensity due to absorption by gas molecules is measured. This decrease in intensity, often described by the Beer-Lambert law shown above, is directly related to the concentration of the absorbing species. By accurately measuring the transmitted light and comparing it to the incident light, the gas concentration can be determined. Despite its simplicity and broad applicability, DAS is susceptible to problems such as baseline drift, electronic noise, and background interference, and is not very sensitive to detecting low-concentration species.

[0006] Wavelength modulation spectroscopy (WMS) is a more advanced technique that can improve the sensitivity and signal-to-noise ratio of conventional absorption measurements, and is particularly suitable for trace gas detection. In WMS, instead of measuring absorption at a fixed wavelength, the wavelength of the light source (usually a laser) is modulated at a high frequency around the central wavelength of the gas absorption line. This modulation shifts the measured signal to a higher frequency, moving it away from low-frequency noise and baseline fluctuations that commonly affect direct absorption spectroscopy (DAS). The detected signal is then demodulated at a specific harmonic of the modulation frequency, often the second harmonic, which provides a clearer representation of the absorption characteristics. Because this process not only enhances the ability to detect weak absorption signals even in the presence of noise, but also improves resolution and sensitivity, WMS is a preferred method in applications requiring high precision, such as atmospheric monitoring, industrial gas detection, or medical applications.

[0007] U.S. Patent No. 8,594,143 recognized that measurement performance may be impaired by changing interference patterns caused by temperature influences on or within the housing of a laser diode structure. U.S. Patent No. 8,594,143 proposes providing a laser diode structure integrating a temperature-controlled beam shaping element, and a gas detection method using the laser diode structure. In particular, this disclosure relates to a laser diode structure specifically for use in gas detection, having a sealed housing with an electrical connection section having a bottom and a window. A laser diode chip and a temperature control system for the laser diode chip are provided within the housing. A thermoelectric element, such as a Peltier element with a thermistor, forms the temperature control system, connected to the bottom of the housing via a lower flat surface and to the laser diode chip via an upper flat surface, with a temperature-controlled beam shaping element provided as a collimator between the laser diode chip and the window of the housing, which acts before the laser beam exiting the laser aperture of the laser diode chip passes through the window. The beam shaping element is in contact with the laser diode tip and is preferably connected to the laser aperture by face-to-face contact or adhesion via an interface, or is manufactured integrally with the laser aperture. Thus, U.S. Patent No. 8,594,143 provides fully temperature-controlled beam shaping. Therefore, the sensitivity of the gas sensor can be improved by avoiding temperature-dependent interference phenomena.

[0008] U.S. Patent No. 9,377,359 relates to an optical measuring system and a method for detecting a gas, the optical measuring system comprising an optical emitter and at least one photodetector arranged in at least one housing, the optical emitter emitting a modulated principal light beam having an average wavelength λ0 and a modulation span Δλ. At least one optomechanical component, for example, a housing window including an optically effective interface, is positioned between the optical emitter and the photodetector, causing a scattered light beam to interfere with the principal light beam, resulting in self-mixing and / or etalon formation. According to the disclosure, at least one optomechanical component is positioned relative to the optical emitter and / or photodetector at an optimized distance L, which is a function of the wavelength λ0 and modulation span Δλ of the principal light beam. This disclosure is based on the idea of ​​taking advantage of the specifics of wavelength-modulated spectroscopy (WMS), which adapts interference in the photodetector's measurement signal by adapting the distance within the measurement system according to the modulation span of the photoemitter in order to reduce interference in gas measurements, and as a result, prevents interference in the photodetector's measurement signal from being transferred to the demodulated signal, such as the 2f signal. By intelligently selecting distances within the measurement system, it is therefore possible to suppress, or at least reduce, all major interferences in the demodulated measurement signal. The relationship between distance and residual interference can be represented by an interference filter curve, as illustrated in Figure 4 of U.S. Patent No. 9,377,359, which shows the amplitude of the interference signal in the detection signal (output of the lock-in detector) with respect to the distance that determines the optical path length difference in the modulation of the principal light beam. The drawback of this approach is that interference mitigation is highly sensitive to the optical path length difference.

[0009] To achieve high sensitivity, especially in gas sensing applications targeting the ppb (parts per billion) range, a direct approach is to perform the measurement by probing the fundamental absorption line of the gas being measured, i.e., the spectral region where the strongest signal is expected. However, the drawback is that this often requires a rather expensive mid-infrared laser. [Overview of the Initiative] [Problems that the invention aims to solve]

[0010] Objectives of the present invention are to provide further improved apparatus, systems, and methods for optical gas analysis. In particular, it is desirable to further improve measurement accuracy and / or sensitivity while simultaneously enabling reasonable costs. [Means for solving the problem]

[0011] The subject matter of the claims is defined in the attached independent claims. Further improvements are provided in the dependent claims.

[0012] In a first aspect of the present invention, an optical gas analyzer is presented, comprising a gas cell for a gas to be measured, a light source adapted to emit light into the gas cell, and a photodetector adapted (and positioned) to receive light after it has passed through the gas to be measured in the gas cell, wherein the light source comprises an emitter, a first optical element, and a second optical element, wherein the first optical element is a microlens fitted directly to the aperture of the emitter and adapted to reduce the divergence angle of light emitted by the emitter to provide pre-collimated light, wherein the second optical element is adapted to receive the pre-collimated light from the microlens and shape the light beam for emission into the gas cell, wherein the ratio of the focal length f2 of the second optical element to the focal length f1 of the first optical element, f2 / f1, is at least 20.

[0013] In a further aspect of the present invention, an optical gas analysis system is presented, comprising the apparatus and analyzer described herein, wherein the analyzer is adapted to determine the gas concentration of at least one component of the gas to be measured based on light detected by a photodetector.

[0014] In yet another aspect of the present invention, an optical gas analysis method is presented, the method comprising the steps of: providing a gas cell containing a gas to be measured; emitting light into the gas cell from a light source; detecting the light after it has passed through the gas to be measured in the gas cell; and determining, based on the detected light, the gas concentration of at least one component of the gas to be measured, wherein the light source comprises an emitter, a first optical element, and a second optical element, wherein the first optical element is a microlens fitted directly into the aperture of the emitter and is adapted to reduce the divergence angle of the light emitted by the emitter and provide pre-collimated light, wherein the second optical element is adapted to receive the pre-collimated light from the microlens and shape the light beam for emission into the gas cell, wherein the ratio of the focal length f2 of the second optical element to the focal length f1 of the first optical element, f2 / f1, is at least 20.

[0015] Preferred embodiments of the present invention are defined in the dependent claims. The methods and systems described in the claims, as well as the optical gas analyzers described in the claims, should be understood to have similar and / or identical preferred embodiments, particularly as defined in the dependent claims and disclosed herein.

[0016] Aspects of the present invention are based on the idea of ​​providing an optical gas analyzer with a specifically modified light source configuration. At first glance, it may seem counterintuitive to those skilled in the art to use a more complex combination of the first and second optical elements defined in claim 1 in certain cases of an optical gas analyzer, because, at first glance, providing a single suitable collimating lens is sufficient and more cost-effective. Additional optical elements with further additional optical surfaces may lead to additional interference.

[0017] However, the inventors recognized that in order to provide sufficient optical power, such lenses must have a thickness that falls within the typical range of gas peaks to be analyzed at least at certain wavelengths and modulation spans in wavelength-modulated spectroscopy (WMS), and that the etalon effect could cause interference in trace gas measurements at the front and rear surfaces of the lens. Interference from both the front and rear can occur, and these cannot be canceled out in signal processing. Furthermore, although such lenses have a diameter, the difference in optical path length between the principal and peripheral rays from the light source to the lens also represents different optical path lengths to different parts of the lens, which surprisingly may not allow for the cancellation of interference in signal processing.

[0018] Therefore, it is proposed to provide an optical gas analyzer having an emitter equipped with a first optical element and a second optical element having significantly different focal lengths. The apparatus comprises a combination of a first optical element, which alone does not have sufficient optical power, and the proposed second optical element, where the focal length of the second optical element is much longer than that of the first optical element. The first optical element with a short focal length is a microlens directly attached to the aperture of the emitter. The adverse effects of residual light interference fringes from the first optical element are suppressed by a very short focal length, which ensures that the residual light noise exhibits a very long free spectral range in which it can be clearly distinguished from the absorbed signal. Furthermore, this particular configuration overcomes the problem that residual interference cannot be canceled out in signal processing because the principal and peripheral rays have substantially different optical path lengths. A second optical element with a much longer focal length is provided with pre-collimated light from a microlens, thereby reducing the optical path length difference between the principal and peripheral rays, making it possible to mitigate interference in signal processing and to match the relative positions of the emitter's (frequency) modulation depth and the second optical element. A particular combination of a second optical element and a microlens fitted directly into the emitter aperture to provide pre-collimated light, where the ratio f2 / f1 of the focal length of the second optical element to the focal length of the first optical element is at least 20, helps to further improve the performance of the optical gas analyzer.

[0019] A further advantage of the proposed solution is that it can further reduce residual noise in the optical gas analyzer while simultaneously enabling longer optical path lengths within the gas cell. The second optical element, having a much longer focal length, can provide better collimation compared to, for example, a single ball lens, and as a result further reduces the divergence angle of the light beam for radiation into the gas cell. The light beam with reduced divergence allows for longer optical path lengths through the gas being measured within the gas cell, for example, by using a gas cell with multipath. Longer optical path lengths improve sensitivity. It has been found that, despite the proposed solution using the second optical element adding another surface to the beam path that could introduce interference noise, the optical path length can be increased to such an extent that the sensitivity benefits from the increased optical path length more than compensate for the added interference noise. In particular, the sensitivity of measuring low gas concentrations can be improved to the extent that even harmonic absorption lines or side peaks of the gas being measured can be evaluated with a less expensive near-infrared light source at IR wavelengths, instead of using an expensive mid-infrared light source specialized for the main or fundamental absorption lines in the mid-infrared. Therefore, a reduction in overall system cost could be an advantage, as it may not be necessary to require expensive mid-infrared laser light sources to achieve high sensitivity.

[0020] The following provides a brief explanation and definition of some terms used throughout this application. The term "gas cell" may refer to a measuring volume for a measuring gas. Gas cells may have different shapes. For example, a cylindrical gas cell may have an inlet and an outlet for the measuring gas to be analyzed. The term "measuring gas" may refer to a measuring gas containing one or more gases to be analyzed, in particular a mixture that an optical gas analysis system may use to determine its components and / or concentrations.

[0021] In one embodiment, the light source does not include any additional beam shaping optical elements other than the first and second optical elements. In other words, the beam shaping optical elements of the light source are composed of the first and second optical elements. The advantage is a cost-effective implementation. Furthermore, the number of surfaces that can cause interference effects and other disturbances can be reduced. Nevertheless, additional surfaces such as the inlet and outlet windows / wedges for the gas cell, or optional alignment mirrors may still exist.

[0022] The emitter can be a NIR (near-infrared), IR (infrared), or MIR (mid-infrared) emitter, and in particular, can be a laser adapted to emit light in the near-infrared, infrared, or mid-infrared regions. As used herein, NIR can refer to a wavelength range from 750 nm to 2.5 μm, MIR can refer to a wavelength range from 2.5 μm to 25 μm, and the more general term IR can refer to a wavelength range from 750 nm to 1 mm. In one improvement, the emitter can be adapted to emit light in a wavelength range between 1.3 μm and 2.1 μm. The advantages of using a NIR laser are that it is a cost-effective means and there is good availability of light sources at different wavelengths and a wide range of optical powers. Furthermore, such light sources are very suitable for wavelength modulation spectroscopy (WMS).

[0023] As used herein, the microlens can refer to a small lens directly attached to the aperture of the emitter. In particular, the microlens may be directly attached to the semiconductor chip. The microlens may be structured on the chip surface. The microlens can be a beam shaping element as described in U.S. Patent No. 8,594,143 cited above.

[0024] The light beam radiated from the second optical element to the gas cell is also called the measurement beam or the measurement light beam. It can be a collimated beam or a focused beam. The second optical element can be adapted to shape the measurement light beam into a collimated beam having a divergence angle of 0.2° or less, particularly 0.15° or less, particularly 0.1° or less, particularly 0.05° or less. The divergence angle used herein refers to the half angle, particularly, for example, the half angle or the full width at half maximum (HWHM) of a Gaussian beam profile.

[0025] In a further improved form, the ratio f2 / f1 of the focal length f2 of the second optical element to the focal length f1 of the first optical element can be at least 35, particularly at least 50, particularly at least 100, particularly at least 150. The advantage of this embodiment is that substantial preliminary collimation is provided by the first optical element, and the size and / or thickness of the second optical element may result in only limited residual interference effects due to the different optical path lengths of the chief ray and the marginal ray, and / or due to reflections on the front and rear surfaces of the lens as the second optical element.

[0026] The microlens can be adapted to reduce the divergence angle of the light emitted by the emitter by at least 70%, particularly at least 80%, particularly at least 90%. Therefore, it is proposed to provide a certain degree of preliminary collimation, and as a result, the size and / or thickness of the second optical element can be reduced to reduce the interference effect.

[0027] The first focal length f1 of the microlens may be less than 0.8 mm, particularly less than 0.6 mm, particularly less than 0.4 mm, particularly less than 0.3 mm, and particularly less than 0.2 mm. Furthermore or alternatively, the second focal length of the second optical element may be at least 10 mm, particularly at least 20 mm, particularly at least 30 mm, and particularly at least 50 mm. The advantage is that a compact design of the light source can be implemented while providing a measurement light beam sufficient for sensing low concentrations of gas. The focal lengths used herein should be understood to refer to the focal lengths measured with respect to refractive index n=1. Nevertheless, the microlens may be attached to the emitter in a refractive index matching manner, for example, using refractive index matching adhesive.

[0028] A second optical element in an optical gas analyzer may be an optical thin lens. In optics, a thin lens can refer to a lens whose thickness (the distance along the optical axis between the two surfaces of the lens) is negligible compared to the radius of curvature of the lens surface. More precisely, as used herein, a thin lens may refer to a lens having a thickness such that both the front and rear surfaces of the lens are at a distance near the interference filter curve, for example, the interference minimum of a 2f signal. Further details regarding the interference minimum of a 2f signal and the interference filter curve are described in U.S. Patent No. 9,377,359, which is incorporated herein by reference in its entirety. The distance from the front and / or rear surfaces of the lens to the minimum of the interference filter curve may be 30% or less, particularly 20% or less, particularly 10% or less, of the separation between adjacent interference minimums in the interference filter curve. In the proposed solution, for example, both surfaces of the second optical element can be positioned near the interference minimum distance, so the interference effect can be significantly reduced. Therefore, it is proposed to provide a thin lens, which does not possess sufficient optical power on its own but has a thickness that does not cause interference due to the separation of its front and rear surfaces in optical trace gas measurement, in combination with the proposed first optical element.

[0029] The second optical element in an optical gas analyzer may be a mirror, particularly an off-axis parabolic (OAP) mirror. The advantage is that residual interference can be further reduced because there is only one optical surface, rather than the front and rear surfaces of lenses that can cause interference in trace gas measurements. Furthermore, combined with pre-collimation provided by microlenses, the difference in optical path length between peripheral rays is substantially reduced, resulting in reduced residual interference. The advantages of this embodiment may include one or more of the following: low divergence angle, low spherical aberration, and reduced stray light due to the off-axis configuration. While conventional OAP mirrors can be prohibitively expensive, the proposed combination with pre-collimation by microlenses allows for the use of much smaller and therefore less expensive OAP mirrors.

[0030] In a further improved configuration, the mirror can be an adjustable mirror for adjusting the direction of the light beam for radiation into the gas cell. Optionally, a separate mirror can be provided for adjusting the direction of the light beam for radiation into the gas cell. An advantage of the separate mirror is that it can also be used in conjunction with a lens as a second optical element. However, a further synergistic effect can be obtained if the second optical element is configured not only to provide a ratio of their respective focal lengths, but also to further adjust the direction of the measured light beam. For example, the number of paths in a gas cell with multiple paths can be changed by adjusting the direction in which the light beam enters the gas cell. The position and / or tilt angle of the mirror can be adjustable.

[0031] Microlenses can be attached to the emitter, for example, using a refractive index matching adhesive, so that their refractive indices are matched. In an improved embodiment, the microlens can be a ball lens directly attached to the aperture of the emitter using a refractive index matching adhesive, where the refractive index of the adhesive matches the refractive index of the microlens. The advantage of this embodiment is that there is one less surface that could cause interference.

[0032] In an advantageous improved configuration, the microlens can be mounted directly to the emitter aperture so as to match the refractive index, and the second optical element can be a mirror. Advantageously, the emitter's optical system providing the light beam for radiation into the gas cell is thus defined by only two beam-shaping surfaces. When the microlens is mounted so as to match the refractive index, for example, when the microlens is bonded to the emitter aperture with refractive index matching adhesive, only the rear or outer surface of the microlens performs beam shaping. When the second beam-shaping element is implemented as a mirror, there is only one surface compared to a lens having two surfaces. Thus, potential sources of error leading to interference effects can be further reduced.

[0033] The optical emitter and the first optical element are housed within a common optical emitter housing. The optical emitter housing may include an exit window for pre-collimated light. The exit window may be positioned in the optical path between the first and second optical elements. The exit window may be tilted with respect to the optical axis so that back reflections are deflected away from the emitter. The exit window may also be positioned at a distance corresponding to the minimum value of the interference filter curve.

[0034] Similar to tilting the emission window, it is also possible to deflect back-reflected light away from the emitter by tilting the second optical element relative to the optical axis. This can reduce interference artifacts.

[0035] The position of the second optical element relative to the optical emitter is adapted to reduce interference caused on the optical emitter by the optical surface of the second optical element. In particular, the second beam shaping element can be positioned to yield the minimum residual interference. Furthermore, or alternatively, the position of the second optical element relative to the photodetector is adapted to reduce (in particular minimize) (residual) interference caused on the photodetector by the optical surface of the second optical element.

[0036] The optical emitter can be configured to emit light with a central wavelength λ0 and a modulation span (sometimes called modulation amplitude) Δλ. The distance between the second optical element and the optical emitter, and / or the distance between the second optical element and the photodetector, and the modulation span Δλ are matched to each other so as to minimize interference caused to the measurement signal demodulated by the optical surface of the second optical element.

[0037] The gas cell can be a gas cell having a multipath, and in particular can be a gas cell having a multipath adapted to provide an optical path length of at least 5 m, at least 10 m, and at least 20 m. The optical path length of the measurement beam passing through the measurement gas can therefore be improved. An advantage of this embodiment is improved measurement sensitivity. Certain combinations of the first and second optical elements described herein have been found to enable longer optical path lengths than prior art solutions. [Brief explanation of the drawing]

[0038] These and other embodiments of the present invention will become clear and clarified by referring to the embodiments described below. [Figure 1] This is a schematic diagram of a system for optical gas analysis equipped with an apparatus according to one aspect of the present disclosure. [Figure 2] This is a schematic diagram of a first embodiment of a light source for an apparatus for optical gas analysis according to one aspect of the present disclosure. [Figure 3] This is a schematic diagram of a further light source. [Figure 4] This is a schematic diagram of a second embodiment of a light source for an apparatus for optical gas analysis according to one aspect of the present disclosure. [Figure 5] This figure shows an interference filter curve illustrating the noise contribution due to direct feedback. [Figure 6] This figure shows interference filter curves illustrating the noise contribution due to lens thickness. [Figure 7] This is a flowchart of a method according to one aspect of the present disclosure. [Modes for carrying out the invention]

[0039] Figure 1 schematically illustrates an embodiment of a system for optical gas analysis according to one aspect of the present disclosure. The system as a whole is denoted by reference numeral 1. System 1 comprises an optical gas analyzer 10 and an analyzer 50. The analyzer 10 comprises a gas cell 20 for a gas to be measured 21, a light source 30 adapted to emit light into the gas cell, and a photodetector 40 adapted to receive light after it has passed through the gas to be measured in the gas cell. The analyzer 50 is adapted to determine the gas concentration of at least one component of the gas to be measured 21 based on the light detected by the photodetector 40. As shown in Figure 1, the analyzer 50 is coupled to the photodetector 40 and may also be coupled to the light source 30. The analyzer 50 can be adapted to control the light source 30 for a desired measurement task. For example, System 1 can be a TDLAS, i.e., a tunable diode laser absorption spectroscopy system, and in particular, a system adapted to perform wavelength-modulated spectroscopy (WMS).

[0040] The gas cell 20, also called an absorption cell, may have one or more gas connections to allow gas exchange within the gas cell. In the example shown, an inlet 22 and an outlet 23 are provided for the measurement gas. The measurement gas 21 can therefore be introduced into the gas cell 20 via the inlet 22 and exit the gas cell via the outlet 23. The gas cell 20 may be filled with the measurement gas to be analyzed. However, it is also possible to provide a gas flow from the inlet 22 to the outlet 23, thereby allowing continuous analysis of the gas flow. The inlet 22 and outlet 23 may also be located in different positions, for example, on opposite side walls of the gas cell 20 so that the measurement gas 21 passes through the gas cell 20.

[0041] As illustrated in Figure 1, the gas cell 20 can be a multipath gas cell such as a Pfand cell, a White cell, or a Heliot cell. The gas cell 20 may be equipped with a first mirror 24 and a second mirror 25 on opposite sides so that the light beam 60 emitted into the gas cell 20 is reflected multiple times and the gas to be measured 21 passes through multiple times before reaching the photodetector 40. The gas cell may be equipped with an inlet opening or inlet wedge or window 26 for receiving light from the light source 30 and an outlet opening or outlet window 27 for supplying light to the photodetector 40. The gas cell 20 can be adapted to provide an optical path length of at least 5 m, in particular at least 10 m, and in particular at least 20 m. The gas cell 20 can therefore provide a long optical path length through which the gas to be measured 21 passes, thereby providing highly sensitive trace gas measurement. However, longer optical path lengths require a collimated or focused laser beam.

[0042] Figure 2 shows an exemplary embodiment of a light source 30 of an optical gas analyzer 10. The light source 30 of Figure 2 can be used in a system such as the one shown in Figure 1. The light source 30 comprises an emitter 31, a first optical element 32, and a second optical element 33. The first optical element 32 is a microlens fitted directly to the aperture 35 of the emitter 31 and is configured to reduce the divergence angle of the light emitted by the emitter and provide pre-collimated light 61. The second optical element 33 is configured to receive the pre-collimated light 61 from the microlens and shape the light beam 60 emitted into the gas cell, where the ratio f2 / f1 of the focal length f2 of the second optical element 33 to the focal length f1 of the first optical element 32 is at least 20. In the shown embodiment, the light source 30 does not include any further beam-shaping optical elements other than the first and second optical elements 32, 33. In other words, the beam-shaping optical element of the light source consists of a first and a second optical element as two beam-shaping elements.

[0043] In the exemplary embodiment shown in Figure 2, the emitter 31, adapted to emit light through the aperture 35, can be mounted on a temperature control element 36, such as a Peltier element. A thermistor 37 may be provided to control the temperature. The first optical element 32 is also preferably in thermal contact with the temperature control element 36, either directly or indirectly via the emitter 31. This allows for the "freezing" of interference fringes. In the example shown in Figure 2, the second optical element 33 is not temperature-controlled.

[0044] The optical emitter 31 and the first optical element 32 can be housed in a common optical emitter housing 71. The optical emitter housing 71 may include an exit window 72 for pre-collimated light 61. The exit window 72 can be located in the optical path between the first optical element 32 and the second optical element 33. The exit window 72 can be tilted, for example, with respect to the optical axis of the pre-collimated light so that the back-reflected light is deflected away from the emitter 31. The exit window 72 is sometimes also called a laser cap window. The exit window 72 can also be located at a distance corresponding to the minimum interference value of the interference filter curve, as will be further described below.

[0045] The microlens can be applied to the emitter 31 in a refractive index matching manner, for example, using a refractive index matching adhesive 38. In an improved embodiment, the microlens can be a ball lens directly attached to the aperture 35 of the emitter 31 using a refractive index matching adhesive, where the refractive index of the refractive index matching adhesive matches the refractive index of the microlens. Thus, there is one less surface that could cause interference.

[0046] In a further improvement of the exemplary embodiment shown in Figure 2, the first beam shaping or optical element 32 is a ball lens (e.g., with a diameter of less than 300 μm) bonded to the emitter 31 with refractive index matching adhesive. The short focal length f1 of the ball lens allows for a very short working distance (approximately 100 μm). Only the outer side of the ball lens (the side away from the laser) functions as the beam shaping surface.

[0047] The first optical element 32, mounted as a microlens, here as a ball lens, directly above the aperture 35 of the emitter 31, advantageously provides very low interference noise. The residual interference between the laser and the lens surface is very short, resulting in a long free spectral range.

number

[0048] The relatively small aperture 35 of the emitter 31 yields a diffracted beam. In the case of a VCSEL, the divergence angle is given by equation 3.

number

number

[0049] For comparison, Figure 3 shows a light source 80 comprising an emitter 81 and a single optical element 82 for beam shaping. Thus, in comparison with Figure 2, the light source 80 shown in Figure 3 does not include a combination of a first optical element mounted directly above the aperture of the emitter and functioning as a microlens, and a second optical element adapted to receive pre-collimated light from the microlens and supply pre-collimated light to a second optical element that shapes the light beam for emission into the gas cell.

[0050] As can be seen from Figure 3, the principal ray 83 and the respective peripheral rays 84 and 85 from the emitter 81 to the single optical element 82 have substantially different optical path lengths. Therefore, the difference in optical path lengths between the principal ray 83 and the peripheral rays 84 and 85 from the emitter 81 to the single optical element 82 may not allow for interference cancellation in signal processing, as described, for example, in U.S. Patent No. 9,377,359, when considering the different optical path lengths to different parts of the lens. In Figure 3, the optical element 82 is illustrated as a beam-shaping mirror. However, the problem of different optical path lengths also applies to lenses with expanded diameters.

[0051] Referring again to Figure 2, this particular configuration overcomes the problem that the principal and peripheral rays have substantially different optical path lengths, resulting in residual interference that cannot be canceled in signal processing. A second optical element with a much longer focal length is supplied with pre-collimated light from the microlens, thereby reducing the optical path length difference between the principal and peripheral rays, making it possible to mitigate interference in signal processing and to match the relative positions of the emitter's (frequency) modulation depth and the second optical element. In a particular combination of the second optical element and the microlens 32, which is mounted directly above the aperture 35 of the emitter 31 and adapted to supply pre-collimated light, the ratio f2 / f1 of the focal length f2 of the second optical element to the focal length f1 of the first optical element is at least 20. Thus, the proposed solution helps to further improve the performance of the optical gas analyzer.

[0052] The first optical element 32, in this case a ball lens, can further reduce beam divergence to approximately 0.4° (HWHM, half width at half maximum), and significantly reduce the illumination area on the second optical element, in this case an off-axis parabolic (OAP) mirror. As a result, the distance between the regions of the first optical element 32 and the second optical element 33 is now clearly defined and does not deviate significantly from the principal and peripheral rays. This allows the distance and / or modulation span to be adjusted to the minimum value of the interference filter curve, thereby reducing interference effects.

[0053] Furthermore, the size or diameter of the second optical element 33, in this case the OAP mirror, is significantly reduced, as shown by the dashed line in Figure 2. Therefore, a much smaller OAP mirror 33' can be used. This affects the manufacturing cost of such mirrors. Since OAP mirrors as high-precision optical elements are quite expensive, the cost savings from using smaller OAP mirrors can even offset the additional effort required to provide additional microlenses. The mirror as the second optical element 33 may be an adjustable mirror for adjusting the direction of the light beam 60 for radiation into the gas cell 20. Optionally, a separate mirror can be provided for adjusting the direction of the light beam for radiation into the gas cell. The advantage of a separate mirror is that it can also be used together with a lens as the second optical element. However, further synergistic effects can be achieved if the second optical element is configured not only to provide the respective ratios of focal lengths, but also to adjust the direction of the measured light beam. For example, the number of paths in a gas cell with multipaths can be changed by adjusting the direction in which the light beam enters the gas cell. The position and / or tilt angle of the mirror may be adjustable.

[0054] The working distance of the second beam shaping optical element relative to the emitter is understood to be adjustable according to the respective system requirements to achieve appropriate beam parameters, such as a desired beam divergence angle, so that the beam becomes collimated or focused for radiation into the gas cell 20.

[0055] Figure 4 shows a second exemplary embodiment of the light source 30 of the optical gas analyzer 10. Similar elements are indicated by the same reference numerals. Compared to the embodiment shown in Figure 2, the second optical element 33 is implemented as a lens instead of an OAP mirror. In particular, the second optical element may be implemented as an optical thin lens as described above.

[0056] Figure 5 shows an interference filter curve that appears to illustrate the noise contribution due to direct feedback. Direct feedback noise can be caused by light reflected back from the optical surface onto or into the emitter. This can lead to (self-mixing) interference noise. The horizontal axis represents the distance (in arbitrary units, au) between the emitter 31 and the second optical element 33. The vertical axis represents the noise contribution (in arbitrary units, au). As can be seen from Figure 5, the curve 91 features several distances at which the noise contribution is minimized. With respect to the minimum, it is understood that the modulation span is scanned over integer multiples of the interference fringes. The annihilation of the interference effect occurs at a specific distance by the lock-in method. The location of the minimum is understood to depend on the frequency modulation span to which the emitter is modulated. The optical emitter can be configured to emit light with a center wavelength λ0 and a modulation span Δλ. The distance between the second optical element and the optical emitter and / or photodetector, and the modulation span Δλ, can be adapted to each other so as to minimize interference caused to the measurement signal demodulated by the optical surface of the second optical element. Thus, the optical interference fringes arising from the second optical element 33 can be suppressed by a clearly defined optical path length of the pre-collimated light received from the microlens, where the distance is selected to efficiently suppress the interference fringes in the filter function shown in Figure 5. For example, the exit window 72 may be positioned at the minimum value indicated by reference numeral 92, and the second optical element may be positioned at the minimum value indicated by reference numeral 93 or 94.

[0057] Residual light interference fringes from microlenses are suppressed by a very short focal length, which ensures that the residual light noise exhibits a very long free spectral range that is well distinguishable from the absorption signal, for example, when refractive index matching adhesive is used only from the outside of a ball lens.

[0058] Conveniently, referring again to the example in Figure 2, there are only two optically active beam shaping surfaces involved. The first optical beam shaping surface is the outside of the first beam shaping optical element, which is bonded to the laser aperture with refractive index matching adhesive. The second beam shaping surface belongs to a second optical element that can be implemented as a mirror with a reflective surface. Furthermore, when using an OAP mirror, the off-axis properties of the OAP mirror ensure that back-reflected light to the laser is minimized.

[0059] Figure 6 shows an interference filter curve illustrating the noise contribution due to lens thickness. The horizontal axis represents the lens thickness or width (in arbitrary units, au) between the front and back of the lens. The vertical axis represents the noise contribution (in arbitrary units, au). As can be seen from curve 98 in Figure 6, using an optically thin lens can substantially reduce the noise contribution. Considering a specific combination of a first optical element, which is a microlens mounted directly above the aperture of the emitter and adapted to reduce the divergence angle of the light emitted by the emitter and supply pre-collimated light, and a second optical element adapted to receive the pre-collimated light from the microlens and shape the light beam for emission into the gas cell, where the ratio f2 / f1 of the focal length of the second optical element to the focal length of the first optical element is at least 20, the lens thickness of the second optical element can be further reduced when implemented as a lens instead of a mirror. Thus, the noise contribution can be advantageously reduced, thereby improving sensitivity to low gas concentrations.

[0060] Figure 7 shows a flowchart of an optical gas analysis method 100 according to one aspect of the present disclosure. In a first step S101, a gas cell containing a gas to be measured is provided. In a second step S102, light from a light source is emitted into the gas cell. In a third step S103, the light is detected after passing through the gas to be measured in the gas cell. In step S104, based on the detected light, the concentration of at least one gas of at least one component of the gas to be measured is determined. The light source comprises an emitter, a first optical element, and a second optical element. The first optical element is a microlens mounted directly above the aperture of the emitter and is adapted to reduce the divergence angle of the light emitted by the emitter and to supply pre-collimated light. The second optical element receives the pre-collimated light from the microlens and is adapted to shape the light beam for emission into the gas cell, where the ratio f2 / f1 of the focal length f2 of the second optical element to the focal length f1 of the first optical element is at least 20.

[0061] In conclusion, further improved apparatus, systems, and methods for optical gas analysis are provided that help to further improve measurement accuracy and / or sensitivity while simultaneously enabling reasonable costs. In contrast to conventional approaches, the proposed solutions may result in designs with significantly reduced optical interference. In one embodiment, this is made possible by a first lens having a very short focal length (e.g., f1 < 0.4 mm, < 0.6 mm, < 0.8 mm), which allows for a very short working distance to the laser. The short working distance results in optical noise characteristics that hardly interfere with the measurement signal. The drawbacks arising from the short focal length lens are compensated by adding another beam shaping element exhibiting a much larger focal length than the first optical element, where the ratio of the focal length f2 of the second optical element to the focal length f1 of the first optical element, f2 / f1, is at least 20 (e.g., f2 > 10 mm > 20 mm > 30 mm > 50 mm). This makes it possible to increase the beam diameter and, consequently, achieve a small divergence angle. Since the beam parameter product (BPP) is constant, increasing the beam size allows for smaller divergence. Reducing beam divergence with the help of the first lens makes it possible to achieve beam path lengths defined for different rays to the second beamforming element, which enables efficient noise suppression by filtering. In the advantageous improved configuration, only two optically active surfaces are involved.

[0062] Although the present invention is illustrated and described in detail in the drawings and the foregoing description, such illustrations and descriptions should be considered illustrative or exemplary and not limiting, and the present invention is not limited to the disclosed embodiments. Other variations of the disclosed embodiments can be understood and implemented by those skilled in the art in carrying out the claimed invention, based on a consideration of the drawings, disclosure and the appended claims.

[0063] In a claim, the word “equipment” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude plurals. A single element or other unit may perform the functions of multiple items described in a claim. The mere fact that certain means are described in different dependent claims does not imply that combinations of these means cannot be used advantageously.

[0064] No reference numeral in a claim should be construed as limiting the range.

Claims

1. An optical gas analyzer (10), A gas cell (20) for the measuring gas (21), A light source (30) configured to emit light into a gas cell (20), The system includes a photodetector (40) configured to receive light after it has passed through the measurement gas in the gas cell, The light source (30) comprises an emitter (31), a first optical element (32), and a second optical element (33). The first optical element (32) is a microlens directly mounted on the aperture (35) of the emitter (31), and is configured to reduce the divergence angle of the light emitted by the emitter to provide pre-collimated light (61). An optical gas analyzer comprising a second optical element (33) configured to receive pre-collimated light (61) from a microlens and to shape a light beam (60) for radiation into a gas cell (20), wherein the ratio f2 / f1 of the focal length f2 of the second optical element (33) to the focal length f1 of the first optical element (32) is at least 20.

2. The optical gas analyzer according to claim 1, wherein the ratio f2 / f1 of the focal length f2 of the second optical element (33) to the focal length f1 of the first optical element (32) is at least 35, in particular at least 50, in particular at least 100, and in particular at least 150.

3. The microlens is configured to reduce the divergence angle of light emitted by the emitter (31) by at least 70%, in particular at least 80%, and in particular at least 90%. The optical gas analyzer according to claim 1 or 2.

4. The optical gas analyzer according to any one of claims 1 to 3, wherein the first focal length f1 of the microlens is less than 0.8 mm, more particularly less than 0.6 mm, more particularly less than 0.4 mm, more particularly less than 0.3 mm, and more particularly less than 0.2 mm.

5. The optical gas analyzer according to any one of claims 1 to 4, wherein the second focal length f2 of the second optical element (33) is at least 10 mm, particularly at least 20 mm, particularly at least 30 mm, and particularly at least 50 mm.

6. The optical gas analyzer according to any one of claims 1 to 5, wherein the second optical element (33) in the optical gas analyzer is an optical thin lens.

7. The optical gas analyzer according to any one of claims 1 to 5, wherein the second optical element (33) in the optical gas analyzer is an off-axis parabolic mirror.

8. The optical gas analyzer according to any one of claims 1 to 7, wherein the microlens is a ball lens directly attached to the aperture (35) of the emitter (31) with a refractive index matching adhesive (38), and the refractive index of the refractive index matching adhesive matches the refractive index of the microlens.

9. The optical gas analyzer according to any one of claims 1 to 7, wherein the microlens is directly mounted on the aperture (35) of the emitter (31) so as to match the refractive index, and the second optical element (33) is a mirror.

10. The optical gas analyzer according to any one of claims 1 to 9, wherein the emitter (31) and the first optical element (32) are arranged in a common optical emitter housing (71), the optical emitter housing comprises an exit window (72) for pre-collimated light (61), and the exit window is located in the optical path between the first optical element (32) and the second optical element (33).

11. The optical gas analyzer according to any one of claims 1 to 10, wherein the position of the second optical element (33) relative to the emitter (31) is adapted to reduce interference caused to the emitter (31) by the optical surface of the second optical element (33).

12. The optical gas analyzer according to any one of claims 1 to 10, wherein the emitter (31) is configured to emit light having a central wavelength λ0 and a modulation span Δλ, and the distance between the second optical element (33) and the emitter (31) and / or the photodetector (40) and the modulation span Δλ are adapted to minimize interference caused to the measurement signal demodulated by the optical surface of the second optical element (33).

13. The optical gas analyzer according to any one of claims 1 to 12, wherein the gas cell (20) is a gas cell having a multipath, and in particular a gas cell having a multipath configured to provide an optical path length of at least 5 m, at least 10 m, and at least 20 m.

14. An optical gas analysis system (1), The apparatus (10) according to any one of claims 1 to 13, Equipped with an analyzer (50), The analyzer (50) is configured to determine the concentration of at least one gas of at least one component of the measurement gas (21) based on the light detected by the photodetector (40), in an optical gas analysis system.

15. A method for optical gas analysis (100), The steps include providing a gas cell containing a measurement gas (S101), Step (S102) involves emitting light from a light source into the gas cell, The steps include detecting the light after it has passed through the measurement gas in the gas cell (S103), The process includes the step (S104) of determining the gas concentration of at least one component of the measurement gas based on the detected light, The light source comprises an emitter, a first optical element, and a second optical element. The first optical element is a microlens mounted directly above the aperture of the emitter, and is configured to reduce the divergence angle of the light emitted by the emitter, thereby providing pre-collimated light. An optical gas analysis method characterized in that the second optical element is configured to receive pre-collimated light from a microlens and to shape the light beam for radiation into a gas cell, wherein the ratio f2 / f1 of the focal length f2 of the second optical element to the focal length f1 of the first optical element is at least 20.

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