Absorption spectroscopy apparatus

Abstract

The present invention relates in one aspect to an absorption spectroscopy apparatus for measuring one or more gases in an atmosphere, wherein the apparatus comprises: a tunable laser source configured to emit laser radiation; an optical system for transmitting the emitted laser radiation as a probe laser beam, and for collecting return light, in particular emitted laser radiation reflected by a remote surface, the optical system defining one or more apertures; a photodetector for detecting the collected return light, wherein the apparatus defines a transmission beam path between the tunable laser source and a transmission aperture of said at least one apertures, and the apparatus defining a receiving beam path between a receiving aperture of said at least one aperture and the photodetector; wherein the optical system is configured to have a polarization-insensitive transmittance.

Description

[0001] Absorption spectroscopy apparatus

[0002] The present disclosure relates in one aspect to an absorption spectroscopy apparatus.

[0003] BACKGROUND

[0004] In recent years there has been an increased interest in monitoring the concentration of greenhouse gases and trace gases in the atmosphere, i.a. for assessing their impact on the environment and for making informed decisions to mitigate their adverse effects.

[0005] In particular, precise computation of emissions from localized facilities is of pivotal importance for comprehending and quantifying emission trends from these facilities, which have the potential to substantially contribute to the rise in global levels of greenhouse gases.

[0006] Therefore, ground-based, long-distance open-path measurement stations offer a compelling solution to oversee such local emissions, capitalizing on the higher likelihood of sampling emissions as opposed to spatially localized (point) measurements.

[0007] Measurements based on laser absorption spectroscopy have, in the last few decades, emerged as a mature technique for quantitative assessment of gas concentration with applications ranging from atmospheric pollution monitoring, industrial process control, monitoring of agricultural activities, combustion diagnostics, medical breath analysis, etc.

[0008] One interesting aspect of this development have been the advancements in reliable and versatile laser light sources, in particular inexpensive tunable narrow linewidth diode lasers. The latter are available in the telecommunication band, which are easily adaptable and accessible at various wavelengths. With numerous environmentally relevant gas species exhibiting characteristic absorption signatures in this telecommunication band, tunable diode laser absorption spectroscopy (TDLAS) has emerged as a viable technique for selective and precise gas identification and quantification in the near-infrared region (approximately 0.8 - 2 pm). In contrast to traditional methods like chemical and chromatographic measurements, which are often time-consuming and intrinsically destructive, TDLAS techniques have become a popular approach for high-precision and non-intrusive measurement of gas concentrations in the atmosphere. TDLAS systems based on robust distributed feedback (DFB) lasers, widely accessible in the telecommunication band, offer a key advantage owing to the narrow band nature of the light source, leading to enhanced sensitivity and spectral resolution. Furthermore, the open-path TDLAS technique stands out as an attractive means for achieving flexible, cost-effective sensing of atmospheric gases with high selectivity, enabling seamless monitoring in harsh environments where target gases coexist with a complex atmospheric environment.

[0009] A substantial body of literature is dedicated to atmospheric monitoring of greenhouse gases in general. Among the greenhouse gases, carbon dioxide (CO2) and methane (CH4) have been of particular interest due to their substantial contributions to global warming. Various attempts have been made to employ TDLAS for openpath CO2 and CH4 monitoring.

[0010] For instance, the scientific paper Bailey et al. “An open-path tunable diode laser absorption spectrometer for detection of carbon dioxide at the Bonanza Creek Long-Term Ecological Research Site near Fairbanks, Alaska” Appl. Phys. B (2017) 123:245, DOI 10.1007 / s00340-017-6814-8 discloses an open-path TDLAS CO2 sensor centred around a laser wavelength of 1.572 pm, with a typical path length of 100 meters. Bailey et al. reported a sensitivity of 5.4 ppm (based on Allan deviation minimum) with 100 seconds of integration time.

[0011] J. Xia et al., “Probing greenhouse gases in turbulent atmosphere by long-range open-path wavelength modulation spectroscopy”, Optics and Lasers in Engineering 117 (2019) 21-28, discloses a 2.6 km open-path TDLAS-based CO2 and CH4 sensor with sensitivities of approximately 20 ppm and 20 ppb, respectively, using integration times of 20 and 60 seconds. W. Liang et al., “A novel wavelength modulation spectroscopy in TDLAS”, Infrared Physics & Technology 114 (2021) 103661 , discloses a 4.5 m open-path CH4 sensor operating at a laser centre wavelength of 1.65 pm. The authors of this prior art document reported a sensitivity of 27 ppb with 50 seconds of integration time against a background of 77 ppm, albeit involving intricate laser modulation techniques.

[0012] J. Li et al., “Simultaneous standoff sensing for methane and hydrogen sulfide using wavelength-modulated laser absorption spectroscopy with non-cooperative target”, Sensors & Actuators: B. Chemical 374 (2023) 132825 discloses a CH4 sensitivity of approximately 10 ppb with 9 seconds of integration time against a background of 100 ppm at a path length of 20 meters. However, this prior art system employed complex laser modulation and data processing.

[0013] The diverse sensor configurations discussed above fall under the broader umbrella of TDLAS. Prominent variants of TDLAS include direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS), and cavity ring-down spectroscopy (CRDS). CRDS, due to its methodological nature, is primarily suited for spatially localized (point) measurements. On the other hand, DAS and WMS are more suitable for long-distance open-path concentration measurements. While WMS has traditionally been considered superior to DAS, recent advancements in electronics, including high-speed and high-bit-depth analog-to-digital converters (ADCs), have bridged the performance gap between DAS and WMS under comparable conditions, both theoretically and experimentally. Moreover, limitations in achievable sensing speed and complexities associated with implementing the WMS technique have rendered DAS more appealing for gas spectroscopy.

[0014] The article Siozos, P.; Psyllakis, G.; Samartzis, P.C.; Velegrakis, M.: “Autonomous Differential Absorption Laser Device for Remote Sensing of Atmospheric Greenhouse Gases”, Remote Sens. 2022, 14, 460, https: / / doi.org / 10.3390 / rs14030460 discloses A ground-based, integrated path, differential absorption light detection device capable of measuring multiple greenhouse gas species in the atmosphere.

[0015] As remote sensing devices are often placed in outdoor or other environments that are difficult to control, and since it is often desirable to perform the gas sensing over extended periods of time, it is desirable that the absorption spectroscopy apparatus is robust and provides stable measurements despite changes in the environmental conditions, such as mechanical vibrations etc.

[0016] It is further generally desirable to provide an apparatus that is robust and insensitive to noise.

[0017] It is further generally desirable to provide an apparatus that is compact and has low complexity.

[0018] It is further generally desirable to provide an apparatus that is easy to configure and install.

[0019] It is further generally desirable to provide an apparatus that is flexible in use.

[0020] On this background, despite previous efforts, it remains desirable to provide an apparatus that solves one or more of the above problems and / or other problems, and / or that has other benefits, or that at least provides an alternative to existing solutions.

[0021] SUMMARY

[0022] According to one aspect, the present disclosure relates to an absorption spectroscopy apparatus, in particular a direct absorption spectroscopy apparatus, for measuring one or more gases in an atmosphere, wherein the apparatus comprises:

[0023] - a tunable laser source configured to emit laser radiation;

[0024] - an optical system for transmitting the emitted laser radiation as a probe laser beam, and for collecting return light, in particular a reflected portion of the emitted laser radiation, reflected by a remote surface, the optical system defining one or more apertures;

[0025] - a photodetector for detecting the collected return light, wherein the apparatus defines a transmission beam path between the tunable laser source and a transmission aperture of said at least one apertures, and the apparatus defining a receiving beam path between a receiving aperture of said at least one aperture and the photodetector; wherein the optical system is configured to have a polarization-insensitive transmittance.

[0026] In particular, the inventors have realized that the use of an optical system that has a polarization-insensitive transmittance provides a considerable increase in the longterm stability of the measurement results.

[0027] The transmittance of the optical system may be defined as the fraction of incident electromagnetic power that is transmitted by the optical system, i.e. the fraction of the received electromagnetic power that is output by the optical system. In particular the optical system may define a transmission transmittance and a receiving transmission, and the optical system may be configured to have a polarization-insensitive transmission transmittance and / or a polarization-insensitive receiving transmission. The transmission transmittance may be defined as the fraction of the electromagnetic power of the laser radiation emitted by the tunable laser source that is transmitted by the optical system through the transmission aperture. The receiving transmittance may be defined as the fraction of the incident electromagnetic power of the return light received at the receiving aperture that is forwarded by the optical system to the photodetector. Preferably, the transmission transmittance and the receiving transmittance are polarization-insensitive. It will be appreciated that an optical system may include transmissive and / or reflective optical components, i.e. the transmittance of the optical system may depend on the transmittances and / or reflectances of the individual components of the optical system.

[0028] The transmittance of the optical system is thus polarization-insensitive when the fraction of the incident electromagnetic power of the incident electromagnetic radiation transmitted by the optical system is independent of the polarization of the incident electromagnetic radiation, i.e. the optical system transmits substantially the same fraction of the incident electromagnetic power regardless of the polarization of the incident electromagnetic radiation. Preferably, the difference of transmittance of the optical system for incident electromagnetic radiation of respective polarization is less than 5%, preferably less than 2%, more preferably less than 1 % of the transmittance for incident electromagnetic radiation of at least one polarization, e.g. of the polarization for which the optical system has a maximum transmittance. In this respect, the transmittance of the optical system is preferably polarization insensitive at all wavelengths detected by the photodetector and used for measuring the one or more gases in the atmosphere, in particular for measuring the concentration(s) of said one or more gases in the atmosphere.

[0029] Generally, the term atmosphere as used herein is intended to refer to the gaseous medium at the location of the measurement performed by the apparatus, in particular the gaseous medium in the space between the absorption spectroscopy apparatus and the remote surface. The atmosphere may be the terrestrial atmosphere of the earth, or it may be the atmosphere of another astronomical body. Other examples of atmospheres include gas mixtures inside a confined space, such as a cave, a mine, a building or other structure, or a gas mixture that results from deployment of one or more gases at an outdoor or indoor location. Embodiments of the apparatus disclosed herein may be used for detecting various gas components of different types of atmospheres, including atmospheres at different temperatures, pressure and / or atmospheres having different gaseous components at different relative amounts.

[0030] Various embodiments of the apparatus disclosed herein are suitable for accurate, real-time open-path measurements over long distances. While use of a dedicated retro-reflector for providing the remote surface is possible, various embodiments of the apparatus disclosed herein allow for long-range open path measurements even without the need for a dedicated retro-reflector.

[0031] Various embodiments of the apparatus are configured to measure the concentration of one or more gases in an atmosphere, i.e. in a mixture of two or more gases, i.e. to measure the concentration of molecules in gaseous state. The one or more gases whose concentrations are measured by the apparatus will also be referred to as one or more target gases.

[0032] The optical system may include one or more optical components. One or more of the optical components may be transmitting optical components, i.e. optical components where the beam path passes through the optical component. One or more of the optical components may be reflecting optical components, i.e. optical components where the beam path is reflected by the optical component. In particular, all transmitting optical components of the optical system that form part of the transmission beam path and / or of the receiving beam path, may have a polarization-insensitive transmittance. Similarly, all reflecting optical components of the optical system that form part of the transmission beam path and / or of the receiving beam path, may have a polarization-insensitive reflectance. More generally, each optical component of the optical system that forms part of the transmission beam path and / or of the receiving beam path may pass a fraction of the electromagnetic power incident onto the optical component on along the transmitting beam path or the receiving beam path, respectively, such that said fraction is independent of the polarization of the electromagnetic radiation.

[0033] In some embodiments, the optical system is a monostatic optical system, i.e. a system comprising a combined transmission and receiving aperture. The monostatic optical system is configured to transmit the probe laser beam through said combined transmission and receiving aperture and to collect the return light through said combined transmission and receiving aperture. Accordingly, the apparatus is easy to align and includes only relatively few optical components.

[0034] In some embodiments, the optical system comprises a telescope for transmitting the probe laser beam and for collecting the return light, thus allowing for efficient remote sensing. As the telescope is used for transmitting the probe beam and for collecting the return light, the system is easy to align and insensitive to mechanical disturbance that might otherwise result in misalignment of the transmission and receiving paths.

[0035] In some embodiments, the optical system includes a polarization-insensitive beam splitter positioned between the one or more tunable laser source and the one or more apertures and configured to direct a portion of the emitted laser radiation towards the transmission aperture and to direct a portion of the collected return light towards the photodetector. Accordingly, in some embodiments the transmission and receiving beam paths form a combined transmission and receiving beam path between the polarization-insensitive beam splitter and the combined transmission and receiving aperture.

[0036] In some embodiments, said portion of the emitted laser radiation and / or said portion of the collected return light is within a percentage interval around a predetermined percentage, in particular around 50%, independently of a polarization of the emitted laser radiation, wherein the percentage interval has a width of 10 percentage points or less, such as of 5 percentage points or less, such as of 4 percentage points or less. For example, the inventors have found that the long-term drift of the measurement results can be considerably decreased when the polarization-insensitive beam splitter directs 50% + / - 2% of the emitted laser radiation towards the one or more apertures, in particular to the combined transmission and receiving aperture, regardless of the polarisation of the emitted laser radiation. Similarly, it is preferred that the polarization-insensitive beam splitter directs 50% + / - 2% of the collected return light towards the photodetector, regardless of the polarisation of the collected return light. Accordingly, polarisation fluctuations of the emitted laser radiation and / or the collected return light do not significantly affect the effective output power and the corresponding measured light intensity. The inventors have surprisingly found that polarization fluctuations otherwise would be a major cause of noise and measurement drift.

[0037] In some embodiments, the apparatus further comprises a beam dump, wherein the polarization-insensitive beam splitter is configured to direct a remaining portion, in particular 50%, of the emitted laser radiation into the beam dump. The inventors have realized that the beneficial effects of employing a polarization-insensitive beam splitter, in particular in a monostatic system, more than compensate for the sacrifice of 50% of the emitted laser radiation.

[0038] Instead of directing the remaining portion into a beam dump, the remaining portion of the emitted light may be utilized. In some embodiments, the apparatus further comprises one or more additional optical components, and the polarization-insensitive beam splitter may be configured to direct a remaining portion, in particular 50%, of the emitted laser radiation towards the one or more additional components. Examples of the one or more additional optical components include: an additional photodetector, a reference gas cell and an additional photodetector, an additional optical system for emitting an additional probe beam, and optionally including additional receiving optics and an additional photodetector for receiving and detecting return light resulting from reflection of the additional probe beam by a remote surface.

[0039] The additional photodetector may be operable for monitoring a potential degradation of the reference gas cell (e.g. a possible leak of gas(es) in the cell over time), and / or for monitoring potential degradation of the laser, e.g. changes to power and / or wavelength versus laser drive current behavior over time. Accordingly, the provision of such an additional photodetector may serve to improve the accuracy and precision of the sensor and diagnose the “health” of the sensor, e.g. for maintenance or servicing such as replacement of gas cell or laser.

[0040] The additional optical system may be configured to output an additional probe beam and may include receiving optics and an additional photodetector for receiving and detecting return light resulting from reflection of the additional probe beam from a remote surface. This can be beneficial for example for applications that require two probe beams for measuring gas concentrations in their respective beam paths simultaneously. Especially when a beam scanner is used, the number of scan angles may thus be reduced by a factor of 2 by simultaneously scanning the two probe beams. Generally, some embodiments of the apparatus may comprise one or more beam scanners for scanning the probe beam across different emission directions. In embodiments that emit more than one probe beam, all of the probe beams may be scanned, e.g. across respective emission directions.

[0041] Generally, the optical system emitting the probe beam and for collecting the return light may be implemented in a variety of ways.

[0042] When the optical system is a monostatic system comprising a telescope and a beam splitter, the system may be implemented using relatively few optical components, and embodiments of the apparatus may be implemented using readily available, relatively inexpensive components.

[0043] In alternative embodiments, the optical system comprises a receiving mirror, in particular a parabolic or otherwise curved mirror for collecting the return light. The receiving mirror may comprise a through hole or an otherwise transparent / transmis- sive portion that allows the emitted laser radiation to pass. The apparatus is thus operable to transmit the probe beam through the through hole or transparent / transmis- sive portion of the receiving mirror. The through hole or transparent / transmissive portion of the receiving mirror may be a central through hole or a central transparent / transmissive portion of the receiving mirror that is positioned at or proximal to the center of the reflective surface. The through hole or transparent / transmissive portion of the receiving mirror may define the transmission aperture. When employing such an embodiment, the need for a beam splitter is avoided and the receiving aperture, which may be defined by the diameter of the receiving mirror may be made relatively large, thereby allowing receipt of a relatively large amount of return light. Moreover, a mirror-based optical system is suitable for a wide range of wavelength, including long wavelengths in the long-wave IR region. The receiving mirror has preferably a polarization-insensitive reflectance. To this end, the receiving mirror is preferably be coated with a coating that provides a polarisation-insensitive reflectance, at least in the desired wavelength range, e.g. an unprotected high-reflectivity metal coating, in particular an unprotected gold coating.

[0044] In some embodiments, the apparatus comprises two or more tunable laser sources, tunable at respective target wavelengths, and one or more optical switches configured to selectively direct emitted laser radiation from one of the two or more tunable laser sources towards the optical system, in particular towards the polarization-in- sensitive beam splitter or towards the through hole or transparent / transmissive portion of the receiving mirror. Accordingly, the apparatus allows for a seamless and efficient time-multiplexing between different target wavelengths, thus allowing the apparatus to detect a variety of different target gases. It will be appreciated that the apparatus may include three or even more tunable laser sources and / or more than one optical switches, e.g. in a cascaded arrangement so as to allow for a seamless multiplexing between three or more different target wavelengths. In some embodiments, all optical elements of the optical system, in particular all optical elements of the apparatus, positioned in the transmission beam path and / or in the receiving beam path are polarization-insensitive, in particular have a polarization-insensitive transmittance and / or reflectance. Accordingly, the long-term stability of the measurements is further improved. The inventors have found that polarization fluctuations otherwise would be a major cause of noise and measurement drift. In some embodiments, the variation of transmittance and / or reflectance, as measured in percent, of the optical elements for different polarizations is below 5 percentage points, such as below 2 percentage points, such as below 1 percentage point.

[0045] In some embodiments, the tunable laser source has a wavelength tuning range including an absorption wavelength of a target gas to be detected and preferably including at least one other spectral component displaced from said absorption wavelength, thereby allowing accurate determination of the gas concentration based on a measurement of two or more spectral components at or around an absorption wavelength of the target gas.

[0046] In some embodiments, the target gas is an atmospheric gas, in particular a greenhouse gas and / or a trace gas. In some embodiments the target gas is an atmospheric gas selected from: N2O, NH3, CO2, CH4, CO, C2H2, H2O. Various embodiments of the apparatus may be used to measure multiple gases, in particular concurrently. Here the term concurrently is intended to include measurements of multiple gases made in sufficiently fast succession of each other that the atmospheric conditions do not significantly change between the measurements.

[0047] In some embodiments, the target gas may be an irritant gas (e.g. tear gas), a hazardous production material (HPM), a toxic industrial chemical (TIC), a chemical warfare agent or CWA (e.g. VX, Sarin, other nerve agent), and / or their simulants (e.g. dimethyl methylphosphonate or DMMP). Each of these gases may be probed using a laser operating at a wavelength at which the target gas of interest has a sufficiently strong and distinguishable absorption feature. Alternatively or additionally to a laser source operating in the 1.4 pm to 3 pm region (SWIR or short-wave IR), a laser source operating in the 3 pm to 8 pm region (MWIR or mid-wave IR) or in the 8 m to 15 pm region (LWIR or long-wave IR) can be employed. The use of longer wavelength lasers has the advantage of probing the strong fundamental rotational and vibrational absorption lines of most gas molecules, resulting in more sensitive gas sensors.

[0048] VX and Sarin gas, for example, have prominent absorption lines around 1040 cmA-1 (9.6 pm) and 1015 cmA-1 (9.85 pm), respectively. The Sarin-simulant gas DMMP, which is used for the test and calibration of Sarin sensors and other organophosphorus detectors, has a dominant absorption line around 1060 cmA-1 (9.4 pm).

[0049] In some embodiments, the apparatus is a differential lidar absorption apparatus. In some embodiments, the apparatus further comprises a control unit configured to control the tunable laser source to alternatingly emit laser radiation at the absorption wavelength and at a reference wavelength displaced from said absorption wavelength, in particular a reference wavelength that close to, but spaced apart from the absorption peak at the absorption wavelength. The actual distance between the absorption wavelength and the reference wavelength may depend on the width of the absorption peak. For example, the distance may be selected to be between 2 or 3 times the FWHM of the absorption peak. In some embodiments, the distance may be 0.05 nm and 1 nm, such as between 0.05 nm and 0.3 nm. Preferably, the control unit may cause the tunable laser source to toggle between the absorption wavelength and the reference wavelength sufficiently fast, e.g. at a rate of at least 8 kHz, such as at least 10 kHz, to make the concentration measurement insensitive against variations of other factors, e.g. atmospheric fluctuations, that might change the intensity of the return light.

[0050] In other embodiments, the apparatus further comprises a control unit configured to control the tunable laser source to scan the wavelength of the emitted laser radiation across at least a portion of said wavelength tuning range, said portion including the absorption wavelength and, preferably, multiple other spectral components, wherein the apparatus further comprises a signal processing system configured to receive detector signals from the photodetector, the detector signals being indicative of a plurality of received return light intensities at respective times of said wavelength scan, and to compute an absorbance at said absorption wavelength from said received detector signals. Accordingly, the apparatus provides measurements at a plurality of wavelengths across the tuning range, thereby allowing for an accurate determination of the concentration of the absorbing gas. The apparatus may perform a continuous wavelength scan or a step-wise wavelength scan across at least the portion of the tuning range. The absorption wavelength may be a wavelength at which the target gas has an absorption peak or another distinguishable absorption signature.

[0051] In some embodiments, the apparatus further comprises a reference gas cell, partly or completely filled with a reference gas, the reference gas including said target gas, preferably at a known concentration. The inventors have found that a particularly accurate concentration measurement may be performed even at low concentrations and / or small measurement ranges, when the reference gas cell is positioned in the receiving beam path and / or in the transmission beam path, in particular between the tunable laser source and the polarization-insensitive beam splitter or between the tunable laser source and the through hole or transparent / transmissive portion of the receiving mirror. The inventors have found that the insertion of the reference gas cell in the transmission path improves the limit of detection of the target gases and aids the signal processing for computing the gas concentration from the measured detector signals. The reference gas cell may also be placed in the receiving beam path instead of in the transmission beam path, e.g. between detector and non-polarizing beam splitter or between the detector and the receiving mirror. The reference gas cell may also be placed in a path common to the transmission beam path and the receiving beam path, e.g. between the non-polarizing beam splitter and the telescope of a monostatic system.

[0052] In some embodiments, the apparatus comprises only a single photodetector, i.e. avoids the need for a separate reference detector, thereby reducing the complexity of the apparatus and avoiding noise and / or other inaccuracies resulting from differences in the detector properties of multiple detectors.

[0053] In some embodiments, the apparatus comprises: - an optical head housing accommodating at least the optical system and the one or more apertures,

[0054] - a laser housing accommodating at least the tunable laser source,

[0055] - a detachable fibre-optic coupling between the optical head housing and the laser housing. Accordingly, a modular system is provided that aids system maintenance and allows for an easy reconfiguration of the apparatus for measuring different types of target gases.

[0056] The present disclosure relates to different aspects including the apparatus described above and in the following, corresponding apparatus, systems, methods, and / or products, each yielding one or more of the benefits and advantages described in connection with one or more of the other aspects, and each having one or more embodiments corresponding to the embodiments described in connection with one or more of the other aspects and / or disclosed in the appended claims.

[0057] In particular, another aspect disclosed herein relates to an absorption spectroscopy apparatus, in particular a direct absorption spectroscopy apparatus, for measuring one or more gases in an atmosphere, wherein the apparatus comprises:

[0058] - a tunable laser source configured to emit laser radiation, wherein the tunable laser source has a wavelength tuning range including an absorption wavelength of a target gas to be detected;

[0059] - an optical system for transmitting the emitted laser radiation as a probe laser beam, and for collecting return light, in particular emitted laser radiation reflected by a remote surface, the optical system defining one or more apertures;

[0060] - a photodetector for detecting the collected return light, wherein the apparatus defines a transmission beam path between the tunable laser source and a transmission aperture of said at least one apertures, and the apparatus defining a receiving beam path between a receiving aperture of said at least one aperture and the photodetector;

[0061] - a reference gas cell, filled with a reference gas, the reference gas including said target gas, wherein the reference gas cell is positioned in the transmission beam path and / or in the receiving beam path. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Embodiments of the various aspects disclosed herein will be described in more detail in connection with the appended drawings, in which

[0063] FIG. 1 schematically shows an embodiment of an absorption spectroscopy apparatus;

[0064] FIG. 2 schematically illustrates another embodiment of an absorption spectroscopy apparatus;

[0065] FIGs. 3A-B depict comparative power measurements at the output of different types of beam splitters when used in the apparatus of FIG. 2;

[0066] FIGs. 4A-D illustrate the fitting of experimental spectra for all three lasers of the embodiment of the apparatus of FIG. 2;

[0067] FIGs. 5A-D illustrate the long-term performance of an embodiment of the absorption spectroscopy apparatus disclosed herein;

[0068] FIG. 6 shows plotted data from CH4 absorbance measurements.

[0069] FIGs. 7A-B illustrate the long-term performance of an embodiment of the absorption spectroscopy apparatus disclosed herein as compared to a reference method;

[0070] FIG. 8 schematically shows yet another embodiment of an absorption spectroscopy apparatus.

[0071] DETAILED DESCRIPTION

[0072] In the following, aspects of an absorption spectroscopy apparatus, in particular an open-path, multi-gas sensor apparatus, will be described.

[0073] Various embodiments of the apparatus disclosed herein provide a sensitive, accurate, reliable, low-power consumption, and cost-effective environmental gas monitoring solution. Various embodiments of the apparatus are operable to perform openpath multi-gas sensing. Various embodiments of the apparatus disclosed herein are capable of remote, continuous quantification of gas emissions. Various embodiments of the apparatus can be used for long-term field deployment with minimal power consumption and labour. Some embodiments may even be suitable for deployment by an UAV. Other applications include the fenced-line monitoring of critical facilities, extracting spatially averaged concentrations of emitted gas species. Further applications include the continuous monitoring of emissions from industrial facilities, facilitating emissions quantification, and thereby enabling regulatory compliance enforcement.

[0074] Various embodiments of the apparatus disclosed herein employ tunable diode laser absorption spectroscopy (TDLAS), in particular direct tunable diode laser absorption spectroscopy (occasionally referred to as dTDLAS in the literature).

[0075] Various embodiments of the apparatus are multi-gas sensors designed to measure various species of gases, in particular greenhouse gases such as carbon dioxide (CO2) and methane (CH4), as well as tracer gases such as acetylene (C2H2).

[0076] In some embodiments, the tunable laser source comprises a tunable diode laser, in particular tunable diode laser operable to emit laser radiation in a telecommunication wavelength band, such as a wavelength range around 1.5 pm.

[0077] In some embodiments the tunable laser source comprises more than one tunable diode laser, in particular more than one fibre-coupled laser diode. Fibre-coupled laser diodes facilitate seamless and readily deployable multiplexing of lasers for multi-gas sensing.

[0078] Some embodiments of the apparatus facilitate multiplexing of multiple fibre-coupled diode lasers via optical switches based on micro-electro-mechanical systems (MEMS), thereby leveraging the capabilities of fibre-coupled diode lasers. This facilitates the easy integration of new lasers for detecting additional or alternative target gases based on application requirements. For example, in one embodiment, in addition to CO2 and CH4, the apparatus can measure the concentration of a tracer gas like acetylene (C2H2), which plays a vital role in understanding atmospheric dynamics and source attribution.

[0079] In some embodiments, the apparatus comprises an optical head housing accommodating the optical system and the photodetector. The optical head housing may be made of a suitable material, such as from composite plastic or other lightweight material. In some embodiments the optical head housing may be manufactured by additive manufacturing methods such as 3D printing.

[0080] In some embodiments, the apparatus comprises a signal processing system for processing the detector signals and compute resulting gas concentrations, thereby obtaining accurate gas concentrations from the absorption measurements, e.g. in real time. In some embodiments the signal processing system is programmed to use an analytical model, which may be based on a multi-parameter (e.g. five-parameter) fitting to extract real-time ambient path-averaged concentration. The analytical model may comprise a fitting of the acquired detector signal to a model involving simultaneous multi-parameter fitting to account for gas absorption features, dynamic baseline correction, and / or other instrumental factors.

[0081] Some embodiments of the apparatus provide a sensitivity of 0.07% for CO2 and 0.1% for CH4 ambient level for 1 -second averaging and for a path length of 51 m.

[0082] In some embodiments, the apparatus comprises a reference gas cell in the transmission beam path and / or in the receiving beam path, thereby providing elevated limits of detection as well as enhanced overall stability of computation of the gas concentration from the detector signals, e.g. based on a fitting algorithm. The reference gas cell is filled with a reference gas that includes the target gas, in particular at a known concentration. The reference gas cell is preferably sealed so as to avoid gas leaking into and out of the reference gas cell. The provision of the reference gas cell in the transmission path causes the probe beam to be pre-modulated with the absorption feature of the reference gas, in particular with the target gas. This improves the performance of the signal processing of the resulting detector signal, by providing a known starting point far from the noise levels, which is particularly valuable for measurements involving weak absorption features where the signal processing could fail due to the low signal-to-noise ratio (SNR) of the absorption feature. In one embodiment, the apparatus achieves a limit of detection of 2% of the ambient level for CO2, and 2.5% of the ambient level for CH4, both for a 51 m open path length. An extended limit of detection allows for shorter path lengths and improves the dynamic range of the sensor, augmenting the efficacy of the sensor across diverse application scenarios.

[0083] Embodiments of the apparatus enable real-time gas concentration quantification with high precision and reliability, offering an extensive dynamic range for concentration measurements. Various embodiments of the apparatus can produce real-time concentration values at a high update rate and at sub-percent sensitivity of the ambient background concentration in a matter of a few seconds of averaging. The update rate thus defines the available measurement rate at which measurements may be obtained. In one embodiment the update rate is 100 Hz or more, such as 130 Hz or more. In other embodiments, the update rate is even faster, such as 500 Hz or more, 800 Hz or more, or 1 kHz or more, or 1.5 kHz or more, or even 2 kHz or more. Providing fast measurement rates is beneficial for ensuring that the system captures a "frozen" scenario during data acquisition, thereby minimizing the effects of atmospheric variations such as turbulence, pressure changes, temperature fluctuations, etc. At slower measurement rates, these atmospheric parameters can change during the measurement process, introducing uncertainties in the gas concentration values. By operating at kHz-level rates, e.g. at update rates of 500 Hz or more, 800 Hz or more, or at 1 kHz or more, or 1 .5 kHz or more, or 2 kHz or more, these influences can significantly be reduced, thereby improving both the accuracy and reliability of the concentration measurements.

[0084] Some embodiments of the apparatus employ a monostatic optical configuration, where at least a portion of the signal transmitting and receiving paths share the same optical components, thereby simplifying system design and contributing to compactness. This choice is contrasted with a more complex bistatic approach, which entails alignment complexities and limits easily adapting to longer distances.

[0085] Some embodiments of the apparatus disclosed herein include a polarization-insensitive beam splitter, i.e. a beam splitter whose optical properties, in particular the beam-splitting ratio between reflected and transmitted portions, does not depend on the polarization of the incoming beam. Various embodiments of the apparatus utilize the polarization-insensitive beam splitter for the separation of the transmitted signal and the received open-path return signal at the photodetector, which is useful in particular in the monostatic configuration. The polarization-insensitive beam splitter considerably reduces high-intensity fluctuations.

[0086] FIG. 1 schematically shows an embodiment of an absorption spectroscopy apparatus, generally designated by reference numeral 1.

[0087] The apparatus comprises a tunable laser source 12 that is configured to emit laser radiation. The apparatus further comprises an optical system 113 for transmitting the emitted laser radiation as a probe laser beam 3, and for collecting return light, in particular emitted laser radiation reflected by a remote surface 2.

[0088] The optical system 113 defines an aperture 1138. In this example, the aperture 1138 is defined by an output lens 11342 of the optical system. In other embodiments, the aperture may be defined by another optical element, e.g. by a dedicated aperture element, which may be integrated with an optical head housing of the apparatus.

[0089] The apparatus further comprises a photodetector 114 for detecting the collected return light.

[0090] The apparatus defines a transmission beam path and a receiving beam path. The transmission beam path is schematically illustrated by arrow 116, while the receiving beam path is schematically illustrated by arrow 117. In the example of FIG. 1 , the optical system is a monostatic optical system where the aperture 11383 is a combined transmission and receiving aperture, i.e. it is both a transmission aperture of the transmission beam path 116 and a receiving aperture of the receiving beam path 117.

[0091] In the present example, the optical system 113 comprises a collimator 1131 , a beam splitter 1132, a telescope 1134 and a focussing lens 1133. It will be appreciated, however, that other embodiments of the optical system may include alternative or additional optical components. Generally, all of the components of the optical system within the transmission and receiving beam paths are preferably polarization- insensitive components, i.e. their transmittance and / or reflectance and / or other optical properties are substantially insensitive to the polarization of the light propagating along the transmission and / or receiving beam paths.

[0092] The tunable laser source 12 comprises three tunable diode lasers 121A-C, respectively. Each of the diode lasers is tunable within a tuning range chosen to correspond to selected absorption properties of respective target gases. In particular, each tuning range comprises an absorption wavelength of the respective target gas, i.e. a wavelength at which the target gas absorbs light and / or exhibits another detectable absorption signature. In the present example, laser diode 121A is tunable around an absorption wavelength of 1.52 .m, which corresponds to an absorption wavelength of C2H2. Laser diode 121 B is tunable around an absorption wavelength of 1.645 .m, which is an absorption wavelength of CH4. Laser diode 121C is tunable around an absorption wavelength of 1572 nm, which is an absorption wavelength of CO2. It will be appreciated that other embodiments of the apparatus may include alternative or additional diode lasers, e.g. fewer than three or more than three diode lasers, and / or diode lasers having different tuning ranges that match absorption wavelengths of other target gases.

[0093] The tunable laser source further comprises a MEMS optical switch unit 122, which may comprise one or more MEMS optical switches and which is operable to selectively transmit the laser radiation from one of the tunable diode lasers and to selectively block the laser radiation from the corresponding other two diode lasers. Instead og MEMS optical switches, alternative embodiments may use other types of optical switches, such as opto-mechanical switches or optical waveguide optical switches etc. The tunable diode lasers may be tunable by adjusting the drive current to the respective diode lasers as is known as such in the art. Accordingly, the diode lasers may be operated to scan across the tuning range or to selectively toggle between two or more discrete wavelengths within the tuning range.

[0094] The diode lasers 121A-C are optically connected to the MEMS optical switch unit 122 via suitable fibre-optics. Similarly, the MEMS switch unit 122 is optically connected to the optical system 113 via a fibre-optic connection 123. Preferably, the apparatus comprises at least two separate housings, in particular a laser housing accommodating the laser source 12 and a separate optical head housing 11 accommodating the optical system 113 and the photodetector 114. To this end, the fibre-optic connection 123 may comprise a detachable fibre-optic connector so as to allow the laser source and the optical system to selectively be optically connected and disconnected from each other, e.g. for maintenance purposes or in order to replace one laser source, configured for the detection of one set of one or more target gases, by another laser source, configured for the detection of a different set of one or more target gases.

[0095] The optical system 113 receives the emitted laser radiation from the laser source 12 via the fibre-optic connection 123. In the present example, the collimator 1131 is a collimating lens that receives a diverging beam from the optical fibre and forwards a collimated laser radiation to the polarization-insensitive beam splitter 1132. In other embodiments, the collimator 1131 may be implemented as a fibre-collimator or otherwise. The polarization-insensitive beam splitter 1132 forwards a portion of the laser radiation towards the telescope 1134 and directs a remaining portion of the laser radiation into a beam dump 115. Preferably, the polarization-insensitive beam splitter is a 50 / 50 beam splitter that forwards 50% of the laser radiation towards the telescope 1134 and that directs 50% of the laser radiation into the beam dump. In particular, the polarization-insensitive beam splitter is operable to split the laser radiation irrespective of the polarisation of the laser radiation, preferably polarization-in- sensitive at least such that 50% + / - 2% of the incoming laser radiation is forwarded to the telescope regardless of the polarization of the incoming laser radiation. Other embodiments may include other types of polarization insensitive beam splitters, e.g. a polarization-insensitive beam splitter having a different split ratio, e.g. a polarization insensitive 70 / 30 beam splitter.

[0096] The telescope 1134 receives the forwarded laser radiation from the non-polarization beam splitter 1132 and transmits the laser radiation as a probe beam 3 towards a remote surface 2. To this end, the telescope may include one or more lenses 11341 and 11342, respectively. The remote surface 2 may be a dedicated retroreflector or another suitable surface. For example, when the apparatus is deployed airborne, e.g. on a UAV, the apparatus may be configured to direct the probe downward towards the ground, which thus may function as the remote surface 2. In any event, the remote surface 2 is separate from and preferably not physically attached to the apparatus 1, i.e. the probe beam traverses an open path outside of the apparatus. A portion of the probe beam is reflected from the remote surface, by specular reflection, back-scattering or otherwise. A portion of the reflected probe beam returns to the apparatus 1 and is detectable by the apparatus.

[0097] To this end, the telescope 1134 is further operable to receive return light reflected back from the remote surface 2 and to direct the collected return light back towards the polarization-insensitive beam splitter 1132.

[0098] The polarization-insensitive beam splitter 1132 directs a portion of the collected return light via focussing lens 1133 towards the photodetector 114. In particular, the polarization-insensitive beam splitter is preferably operable to direct 50% + / - 2% of the collected return light forwarded to the photodetector regardless of the polarization of the collected return light.

[0099] The photodetector 114 detects the collected return light and outputs a detector signal indicative of the detected return light.

[0100] It will be appreciated that the transmission beam path between the polarization beam splitter and the aperture 1138 coincides with the receiving beam path between the aperture 1138 and the polarization-insensitive beam splitter. The polarization-in- sensitive beam splitter separates the transmission and receiving beam paths such that only the collected return light is directed towards the photodetector 114.

[0101] It will be appreciated that other embodiments may include different optical systems, e.g. employing alternative or additional optical components, or even fewer optical components. For example, in some embodiments, the polarization-insensitive beam splitter may direct the remaining portion of the emitted laser radiation into a refer- ence arm or into an additional optical system rather than into a beam dump. The reference arm may include a reference gas cell and / or a reference photodetector. However, in a preferred embodiment, the apparatus only includes a single photodetector. To this end, as will be described in greater detail with reference to FIG. 2 below, the apparatus may include a reference gas cell in the receiving beam path and / or in the transmission beam path 116.

[0102] The apparatus further comprises a control unit 13 and a signal processing unit 14. The control unit 13 and the signal processing unit 14 may be provided as separate modules or integrated into a single module, e.g. into a single processing unit. Moreover, one or both of the control unit 13 and the signal processing unit 14 may be distributed across multiple sub-modules. For example, the control unit 13 may include one or more separate or combined control circuits for controlling the diode lasers and the MEMS optical switch. The control unit 13 and / or the signal processing unit 14 may completely or partly be accommodated within the laser housing of the laser source 12, within the optical head housing 11 of the optical system and photodetector and / or within a separate housing. Yet further, it will be appreciated that at least some of the signal processing may be performed by a remote data processing system.

[0103] FIG. 2 schematically illustrates another embodiment of an absorption spectroscopy apparatus. The apparatus 1 of FIG. 2 is similar to the apparatus of FIG. 1 in that it comprises a tunable laser source 12, an optical head housing 11 , a monostatic optical system 113, a photodetector 114, a control unit 13 and a signal processing unit 14, all as described in connection with FIG. 1.

[0104] The control unit 13 and the signal processing unit 14 may be implemented by a single unit or by multiple units, also as described in connection with FIG. 1.

[0105] The tunable laser source 12, includes three tunable diode lasers 121A-C as described in connection with FIG. 1. The selection of laser diode wavelengths depends on the target gases to be detected. The selection may be guided by spectral windows with minimal interference from other gases in the vicinity of the target gas absorption feature around the 1.55 pm telecommunication band. Nonetheless, water vapour emerges as the primary source of interference due to its proximity to some of the target gases of interest. In the present embodiment, laser centre wavelengths of 1.521 pm, 1.572 pm, and 1.645 pm have been chosen for C2H2, CO2, and CH4 gases, respectively. However, other embodiments may employ alternative or additional wavelengths. Each of the diode lasers may be any suitable type of tunable diode laser. In the present example, the diode lasers are supplied by Eblana Photonics Ltd., and they have a narrow line width of 100 kHz and output power of 5 mW. However, other diode lasers may be used instead.

[0106] The laser diodes are fibre-coupled, and the light is emitted through a polarizationmaintaining (PM) fibre. In this example, the fibre is 1m long. However, other embodiments may employ different fibre configurations.

[0107] The tunable laser source 12 further comprises laser drive circuits 124A-C for driving diode lasers 121A-C, respectively. To reduce the form factor of the laser source 12, the present embodiment utilizes a compact, lightweight current and temperature driver, e.g. the DX1 laser diode controller module by Eblana Photonics Ltd. It will be appreciated, however, that other embodiments may use one or more other laser drive circuits.

[0108] The tunable laser source 12 further comprises a MEMS optical switch unit 122 as described in connection with FIG. 1, thereby facilitating quick and easy switching between the lasers. The use of fibre-coupled MEMS switches also simplifies the integration of new lasers due to the fibre coupling nature. In the present embodiment, the MEMS switch unit 122 includes a cascade of two individual switches and an associated control circuit. For example, two 2x1 PM fibre-fibre switches (e.g. using 0.8 m long fibres or fibres of another suitable length) may be used in tandem to facilitate the switching between the three lasers, controlled by electronic signals from the associated control circuit. It will be appreciated that other embodiments may employ other types of switch units, e.g. comprising other types of optical switches. In the present embodiment, MEMS optical switches from Agiltron, Inc. were employed. However, alternative embodiments may use other types of optical switches, e.g. other types of MEMS optical switches, opto-mechanical switches, or optical waveguide optical switches. The MEMS switch unit 122 thus forwards the laser radiation from a selected one of the three diode lasers 121 A-C at a time to the optical system 113. All the remaining optics of the optical system 113 are thus time-shared between the three lasers.

[0109] The control unit 13 controls operation of the laser drive circuits 124A-C and of the MEMS switch unit 122. In the present embodiment, the control unit 13 is implemented by a CompactRIO (cRIO) system from National Instruments, optionally combined with a computer. However, it will be appreciated that other embodiments of the control unit may implemented by other suitable control circuitry or devices, e.g. implemented by a suitable microcontroller, a FPGA, an ASIC and / or the like. The control unit 13 generates control signals for the temperature and current of the laser diode, as well as for the MEMS switch, e.g. via suitable digital-to-analogue converters (DACs). In the present embodiment, the control unit 13 also receives the detector signal from the photodetector 114, e.g. using a suitable analogue-to-digital converter (ADC) or otherwise. The control unit 13 may then forward the detector signal to the signal processing unit 14, which may be implemented by a suitably programmed microprocessor, a computer, or otherwise. The signal processing may maybe conducted collaboratively by the control unit 13 and the signal processing unit 14 or otherwise.

[0110] The output fibre of the MEMS optical switch unit 122 is linked via fibre connection 123 to the common optical system 113, which is accommodated in the optical head housing 11 and which is shared by all three diode lasers. In some embodiments the MEMS switch unit 122 provides switching times ranging from 50 ms to 200 ms, thereby efficiently alternating between the three lasers and allowing concurrent multi-gas measurements.

[0111] The optical system 113 comprises a fibre collimator 1331 , a polarisation-insensitive beam splitter 1132, a telescope 1134, and a focussing lens 1333, as described in connection with FIG. 1. The telescope includes lenses 11341 and 11342, and which is operable to expand the incoming beam from the fibre to the open path probe beam and output the expanded beam via aperture 1138. The focussing lens 1133 is configured to focus the retro-reflected return light toward the photodetector 114. In the present example, the optical system further comprises mirrors 1136 and 1137 that serve to fold the beam path.

[0112] As described in connection with FIG. 1 , the optical system employs a monostatic design. It is accommodated with in a suitable optical head housing 11 , which, in one embodiment may be 3D printed, e.g. from a polycarbonate blend infused with carbon fibre filament, or made using another suitable manufacturing technique and / or another suitable material. The optical system 113 and the photodetector 114 are accommodated in a single optical head housing 11 to ensure structural rigidity and enhanced mechanical stability. The fibre collimator 1131 is employed to collimate the output from the MEMS optical switch unit 122, which then traverses through a reference gas cell 1135, containing known concentrations of the target gas(es) to be measured, in this example known concentrations of CO2, CH4, and C2H2. The reference gas cell 1135 may e.g. be a gas cell from Wavelength References, Inc. In this example, the reference gas cell has end facets that are wedged and treated with antireflection (AR) coating to avoid stray light. In the present embodiment, the reference gas cell is 5 cm long and has 12 mm diameter open aperture. It holds a gas mixture comprising 90% CO2, 3% CH4, and 1% C2H2, balanced with N2 to reach a total pressure of 740 Torr (roughly 1 atm). However, other embodiments may include another type of reference gas cell or no reference gas cell at all. The reference gas cell may be filled with other reference gases, and / or at other concentrations. The purpose of incorporating the reference gas cell will be elaborated later.

[0113] The reference gas cell 1135 is succeeded by a polarization-insensitive beam splitter 1132, which is operable to distinguishing the transmitted laser radiation and the received return light (i.e. the reflected signal from the remote surface 2).

[0114] Generally, as in the previous embodiment, all of the components of the optical system of the present embodiment that are located within the transmission and receiving beam paths are preferably polarization-insensitive components, i.e. their transmittance and / or reflectance and / or other optical properties are substantially insensitive to the polarization of the light propagating along the transmission and / or receiving beam paths. The inventors have performed measurements with the polarization-insensitive beam splitter as well as comparative measurements with a cube polarizing beam splitter (PBS) instead of a polarization-insensitive beam splitter. The comparative measurements exposed considerable instability and drift in measuring concentration when using the PBS. Subsequent tests uncovered significant power and intensity modulation at the output of the PBS. The inventors have realised that the underlying cause related to random polarization fluctuations in the fibre, resulting in power fluctuations after the beam splitter.

[0115] To illustrate this, the collimated laser light from one of the fibres (in this case the laser emitting at a CO2 absorption wavelength) was directed to pass directly through the PBS and, in a separate measurement, through the polarization-insensitive beam splitter (without involving the reference gas cell), and the resulting transmitted output power was measured for both scenarios. FIGs. 3A-B depict the power measurements at the output of the beam splitters obtained during the respective experiments. In FIG. 3A, the power was measured under undisturbed mechanical conditions, whereas FIG. 3B demonstrates the impact of merely touching the fibre. Even in the mechanically stable scenario of FIG. 3A, the power after the PBS exhibited significant fluctuations compared to the polarisation-insensitive beam splitter. It is important to note that the power directly at the output of the fibre was also measured for reference. FIG. 3A confirms that the utilization of the polarization-insensitive beam splitter does not compromise system performance - similar thickness and drift were observed in the time series for both laser power and power after the polarization-insensitive beam splitter. Additionally, subjecting the laser fibre to equivalent mechanical vibrations highlighted the superiority of the polarization-insensitive beam splitter over the PBS configuration, with differences in the peak-to-peak drift of the curves reaching a factor of at least 40.

[0116] Specifically, FIG. 3A illustrates measured laser power after the polarization-insensitive beam splitter in transmittance mode (line 301), compared to a comparative example of a PBS in transmittance mode (line 302). FIG. 3A further shows the laser power measured directly after the fibre (line 303). The graphs show that the fluctua- tion in power, due to dynamically changing polarization in the laser PM fibre, is completely reduced when using the polarization-insensitive beam splitter. FIG. 3B shows the corresponding laser power measured while the fibre is mechanically perturbed by an external force. The change in polarization state due to the induced mechanical vibration to the fibre results in causing a significant change in power after the PBS (line 312), whereas it is not even felt by the polarization-insensitive system consisting of the polarization-insensitive beam splitter (line 311).

[0117] Accordingly, the present embodiment employs a polarization-insensitive beam splitter, in particular a polarization-insensitive beam splitter with a 50:50 split ratio, e.g. a beam splitter available from MKS Instruments, Inc. (10B20NP.31). However, it will be appreciated that other embodiments may utilize other types of polarisation-insensitive beam splitters.

[0118] It will be appreciated that employing a 50:50 polarization-insensitive beam splitter may result in a loss of 50% of the emitted laser radiation in comparison to e.g. a PBS when considering transmitted light (as ca e.g. be seen form the legend of FIG. 3A for voltages). Additionally, another 50% is lost in the case of return light, resulting in an overall power efficiency of only 25% at the photodetector. However, the power instability caused by polarization shifts in the PBS scenario far outweighs any potential advantages of enhanced power efficiency. Balanced detection, which involves utilizing a reference detector to counteract common mode power fluctuations, could be employed to mitigate the polarization-induced power variation in the PBS scenario. However, the signal from the reference detector is still unable to compensate for dynamic polarization changes in the return signal, which might encompass polarization shifts due to the dynamic nature of the atmospheric environment. Additionally, any mismatch between the detector performance ends up as an imperfect correction of the common mode signal and hence the noise.

[0119] Generally, it is preferred that the polarization-insensitive beam splitter is arranged such that the incident beam is incident at a large angle, e.g. between 30° and 60°, such as between 40° and 50°, e.g. about 45°. It is further preferred that the beam splitter is provided on a thick substrate, e.g. a substrate having a thickness of at least 4mm, such as at least 5m, such as at least 6 mm, e.g. between 4 mm and 10 mm. Accordingly, undesired interferences between reflections at the front and rear surface of the beam splitter are reduced.

[0120] Still referring to FIG. 2, the remaining 50% reflected by the polarization-insensitive beam splitter is not utilized and is instead terminated using a beam dump 115 to prevent stray reflection towards the photodetector. In the present embodiment, the polarization-insensitive beam splitter is wedged and AR coated to mitigate stray and internal reflections. However, other types of polarization-insensitive beam splitters may be used in other embodiments.

[0121] The polarization-insensitive beam splitter 1132 is followed by two metallic mirrors 1136 and 1137, e.g. mirrors available from Thorlabs, Inc. These mirrors serve two purposes: (i) folding the beam and increasing the distance from the photodetector 114 to the front surface of lens 11341 to achieve a 1 / r2reduction in surface reflections from lens 11341 at the photodetector and (ii) facilitating better manoeuvrability of the beam. The lenses themselves are also AR coated. Lenses 11341 and 11342 constitute the telescope stage designed to expand the beam for long-distance operation. Bistatic systems commonly rely on a parabolic reflector with a central pinhole to collect the return signal. However, this approach employs a continuously diverging beam and thus is not readily adaptable for operation at varying and large distances. In the present embodiment, lens 11341 has a focal length of 7.5 mm, while lens 11342 has a focal length of 75 mm, resulting in a magnification factor of 10. Consequently, a 1 mm diameter collimated beam at the output of the fibre collimator expands to 10 mm after passing through the telescope 1134. This configuration yields a comfortable Rayleigh length of 200 m, allowing the apparatus to operate with a 25 m open path distance between the aperture 1138 and the remote surface. It will be appreciated, however, that other embodiments may employ a different telescope design, e.g. optimized for different open path distances. Yet further, alternative embodiments may include fewer or more mirrors, e.g. no mirrors.

[0122] Since the optical head housing 11 is not hermetically sealed and does not enclose a vacuum, the folded beam path length between mirrors 1136 and 1137 should also be factored into the total open-path length for the purpose of the signal analysis. In the present example, the total open path length thus amounts to 25.5 m one-way and 51 m round-trip. However, other embodiments may have different open path lengths.

[0123] In some embodiments, the apparatus 1 may be used in combination with a retroreflector that provides the remote surface 2. For example, a hollow retroreflector from Thorlabs, Inc., with a diameter of 50 mm and metal coating on all facets may be used to retro-reflect the beam. Using a retroreflector offers several advantages, including simplified sensor deployment due to alignment ease and strong reflected signals compared to the diffuse reflection from random targets. In other embodiments, the apparatus may be used in combination with another type of retroreflector or without any designated retroreflector. In the latter case, an existing surface, such as a building, landscape feature, and / or the like may serve as remote surface for reflecting a portion of the probe beam.

[0124] The apparatus 1 uses the same telescope 1134 to collect the retro- reflected signal, i.e. the signal retro- reflected by a dedicated retroreflector or by another remote surface. Accordingly, the return light, reciprocally, retraces its path back to the polarization-insensitive beam splitter 1132 via telescope 1134 and via mirrors 1137 and 1136. At the polarization-insensitive beam splitter, the return light undergoes reflection towards the photodetector 114. A lens 1133 may be utilized to concentrate the light onto the photodetector 114. In the present example, the lens 1133 has a focal length of 18.4 mm. However, it will be appreciated that other lenses may be used.

[0125] The photodetector 114 may be positioned slightly beyond the focal point and at a minor angle (of e.g. 5° or the like) to the incoming beam to prevent interference from stray light sources. This arrangement also serves to mitigate the potential for multiple reflections within the Ge window located in front of the detector. To eliminate undesired background radiation from being detected, a long-pass 1.5 pm filter, together with the decreasing sensitivity of an InGaAs photodetector beyond 1.7 pm, may be employed. The photodetector 114 may e.g. be an InGaAs detector, e.g. model FGA21 from Thorlabs, Inc., or another suitable photodetector. The photodetector may be integrated with a suitable detector electronics, e.g. an electronic circuit with a suitable trans-impedance gain and noise performance. The signal de- tected by the detector may be subjected to digitization, e.g. via a 16-bit ADC operating at a data sampling rate of 250 kS / s within the control unit 13, or otherwise. Subsequently, this digitized signal may promptly be transmitted to a signal processing unit 14, which may be implemented by a computer or otherwise. The signal processing may be performed in real-time for analysis and processing aimed at extracting the concentration of the target gas.

[0126] Various embodiments of the apparatus disclosed herein employ DAS for measuring gas concentration. To this end, in some embodiments, the laser source 12 is controlled by the control unit 13 to conduct a wavelength sweep across a suitable wavelength range that includes an absorption characteristic of the target gas. As the target molecules along the open path between the optical head 11 and the remote surface 2 absorb energy from the probe beam 3, the transmitted light intensity undergoes modulation, causing a corresponding change in power of the return light detected by the photodetector. To achieve the wavelength sweep, the laser current may be subjected to modulation using a ramp signal or otherwise. This modulation causes both the laser wavelength and intensity to vary linearly with the current. Consequently, the laser wavelength is scanned ramp-wise and the resulting detected transmittance directly encodes information regarding the concentration of the target gas.

[0127] The ramp speed may e.g. be selected high enough to (i) have a frozen scene where the atmospheric conditions stay the same during the scan, (ii) to partly benefit from improvement in the 1 / f noise, and (iii) to have more measurements per time to improve from signal averaging. In one example, the ramp speed is about 2 kHz but other ramp speeds may be selected.

[0128] Other embodiments may include other measurement schemes, e.g. the so-called WMS scheme. WMS operates on the basis of lock-in detection, where the current scans used in WMS are further modulated by a high-frequency signal. The necessity for this high-frequency signal to possess a frequency several orders of magnitude higher than the ramp signal places an upper limit on the speed of concentration measurement while maintaining manageable complexity in the electronics. The challenge with increasing ramp speeds, which is beneficial for further enhanced 1 / f noise performance and improved averaging, is that it leads to a reduction in the number of data points obtained per scan, thus, reducing the wavelength resolution. Consequently, the accuracy of concentration extraction may be compromised. This may be mitigated by utilizing an ADC with a higher sampling rate, such as 1 MS / s. The present embodiment uses a sampling rate of 250 kS / s and obtains 128 data points per ramp cycle.

[0129] The drive current for all three lasers is adjusted over a suitable range, e.g. a range of 100 mA, which approximately corresponds to a wavelength span of 300 pm, or over another suitable range.

[0130] In one embodiment, the concentration may be evaluated by employing a fitting process, wherein the detected signal is fitted to an analytical model. In one embodiment, the model is parameterized by five independent parameters. Other embodiments may utilize a different number of independent parameters. This fitting procedure may be carried out using the nonlinear least squares method or another suitable method. When the wavelength varies linearly with time during the scan, the fitting of the scan may be conducted while retaining the temporal x-axis, without converting it to wavelength. Otherwise, a suitable conversion may be performed. Mathematically, an example of the analytical model may be expressed as follows:

[0131] The model can be conceptually divided into two segments. The first part, the term inside the first curly brackets, consists of a second-order polynomial in which of denotes the DC offset voltage detected, and sf signifies the slope connecting the alteration in current at the laser diode with the corresponding voltage change at the detector. This slope term encompasses factors such as the current-to-power (l-to-P) characteristics of the laser diode, the current-to-wavelength (l-to-A) characteristics of the laser diode as well as other optical elements, and the power-to-voltage (P-to-V) characteristics of the detector. The parameter sl2represents the coefficient of the second-order term in equation (1). This term is introduced to account for potential nonlinearities within the system. Although higher-order nonlinear terms may also be included, the present example is limited to the second-order term for the sake of simplicity. The term tcoffers an adjustment of the temporal location at which the second-order term is centred. In one example, this term may be centred at the midpoint of the temporal frame employed for fitting. The first part addresses what is typically known in the literature as the baseline correction of the detected spectra. Unlike other approaches, the present example involves real-time baseline correction. This approach compensates for variations in the baseline that might be attributed to factors such as laser aging, power fluctuations due to beam wander and mechanical vibrations, and dynamic nonlinearities encountered during the measurement process. However, other approaches may be employed as well.

[0132] In and around the 1.5 pm spectral region, the interference stemming from adjacent gas lines is typically predominantly due to the presence of a closely positioned water vapour line. This neighbouring line exerts an impact manifested, primarily, as a baseline shift accompanied by a certain degree of nonlinearity. The inventors believe that the baseline correction using the first part of equation (1) possesses the capability to sufficiently mitigate this interference effect to a large extent by effectively minimizing its influence at the peak absorbance wavelength of the target gas. Furthermore, the real-time dynamic nature of the baseline fitting process addresses any real-time alterations in the concentration of water vapour. However, other embodiments may utilise alternative mechanisms for mitigating interference from adjacent gas lines, such as water vapour and / or others.

[0133] The second part of the model accounts for the wavelength-dependent absorbance of the light caused by the presence of gas in the path, i.e., the absorption line of the target gas. The parameter abs represents the value of absorbance at the peak of the absorption line and is directly proportional to the concentration that can be easily calculated. The parameter tdP0Sis a temporal offset to centre the absorption line. This parameter may, in some embodiments, be kept fixed. However, in a presently preferred embodiment, the parameter tdP0Sis maintained as a fitting parameter to effectively account for even the slightest wavelength shifts in the laser diode. Variations like changes in temperature or shifts in current offset can induce wavelength changes that subsequently alter the position of the spectra. Consequently, fixing tdpos might yield less accurate fitting outcomes.

[0134] The term w denotes the characteristic width of the absorption line and it may be maintained at a predetermined value, e.g. a value sourced from the HITRAN database. Nevertheless, some embodiments may retain the flexibility to adjust it slightly. This adjustment may e.g. be guided by observing the real-time residual of the fit and minimizing it.

[0135] Various models can be employed to fit the absorption line, e.g. using the Lorentzian or the Voigt line shapes. The Voigt line shape lacks an analytical form and necessitates approximations, while the Lorentzian shape boasts a straightforward analytical expression. For practical measurements conducted in open environments and those close to the ground, the Lorentzian shape may in some situations be a preferred choice. Additionally, the computational demands of the Lorentzian shape are significantly lower compared to the Voigt line shape, rendering it advantageous for realtime concentration measurements. Thus, the present example uses the Lorentzian shape to fit the absorption line. The second part of the model signifies the gas transmittance, with the Lorentzian shape manifesting in the exponent of equation (1). The model of equation (1) may be adapted to specific target gases. For example, the CH4absorption line comprises two absorption peaks. Consequently, equation (1) may be adjusted so as to include an additional Lorentzian term. The present example limits the fitting to five free parameters, relying on the characteristic peak displacement and amplitude difference drawn from the HITRAN database. These parameters can be further fine-tuned by minimizing the fit residual. Moreover, other fit procedures, including fit procedures using a different number of free parameters and / or different absorption models may be employed.

[0136] One interesting aspect of the fitting process involves determining initial values that serve as starting points for the fitting parameters, which the algorithm uses to initiate the fitting procedure. The accuracy of these initial starting points directly influences the speed of convergence towards a solution and the efficiency of processing. To significantly expedite this process, some embodiments of the fitting process may employ a strategy of reusing the solution obtained from a previous scan as the initial starting point for a subsequent scan. While this approach might result in slower convergence for the initial scan, all subsequent scans converge much more rapidly. This adaptability of the initial starting point to changing environmental conditions facilitates dynamic and swift convergence. In the present example, the rate of switching between gases is ultimately constrained by the recycling of fitting parameters. The switching speed between gases post-fitting ranges from the slowest duration of 1.4 seconds to the fastest duration of 100 milliseconds. Other embodiments may provide other switching speeds. In any event, the ability to switch between three gases (or between another number of gases) within seconds underscores the capability of semi-simultaneous multi-gas detection.

[0137] The real-time five-parameter fitting process is computationally relatively intensive. The maximum rate at which concentration measurements has been performed in initial experiments is 200 Hz on a computer equipped with a processor clock speed of 2.6 GHz and four cores. For the purposes of the examples discussed in the following, a computer with a clock speed of 1 .9 GHz and four cores was employed, resulting in an update rate of 130 Hz. The difference in update rate suggests the potential for achieving even higher rates, e.g. when using a more powerful processor. While aiming to sustain a high scan speed, e.g. a scan speed of 2 kHz, to enable rapid measurements and achieve a temporally frozen measurement environment, adjustments may be made to attain a lower fitting rate and hence update rate. For example, this may involve selecting one spectrum out of / V adjacent spectra or averaging / V adjacent spectra prior to conducting the fitting.

[0138] Higher update rates can be achieved by performing various optimizations of the signal processing. For example, the fitting process described above involves five parameters - three baseline (background) parameters and two gas absorption parameters. The complexity of the fitting process is directly tied to the number of parameters being used. Since the baseline parameters are not expected to fluctuate significantly, particularly due to the small wavelength scan involved, one way of streamlining the signal processing is to fit the baseline parameters initially at a slower rate (e.g. at, 1 Hz or at another suitable update rate), and then apply a common scaling factor to adjust them at faster update rate. Alternatively, the baseline parameters can be fitted at a lower frequency, while maintaining rapid fitting of the gas absorption parameters. Both of these approaches reduce the number of parameters that are fitted at a fast rate. Hence, both approaches reduce the computational load and allow for faster concentration measurement rates without sacrificing accuracy.

[0139] Initial tests by the inventors have shown that an efficient fitting process may be implemented in e.g. Python or another efficient software environment where update rates of 1.5 kHz or more may be achieved. By incorporating the above and / or other suitable optimizations, kHz-level measurement rates may be achieved for real-time gas concentration measurement, significantly enhancing measurement accuracy and reducing the influence of atmospheric parameters on the data.

[0140] In order to mitigate the impact of vibrations, various embodiments of the apparatus may be implemented mechanically robust. The optical head housing 11 including the optical system and photodetector may e.g. be provided with vibration dampeners.

[0141] An embodiment of the apparatus has been tested in an outdoor environment using a retroreflector positioned on a stable tripod. The raw detector voltage was monitored at 2 kHz (as fitting was not yet involved), demonstrated good stability, confirming the insensitivity of the sensor to vibrations. Therefore, for the purpose of testing, the inventors have adopted the approach of averaging 15 adjacent spectra to transition from a 2 kHz scan rate to a 130 Hz concentration update rate (resulting from 1953 / 15=130). This yielded an improvement of > / (15) in signal-to-noise ratio (SNR) due to averaging instead of picking one out of 15 adjacent spectra.

[0142] It will be appreciated that alternative embodiments may provide different scan speeds and / or different concentration update rates.

[0143] FIGs. 4A-D illustrate the fitting of experimental spectra for all three lasers using the embodiment of the apparatus described in connection with FIG. 2 above. Due to the ramp nature of wavelength scan, which for the present experiments occurred at 2 kHz with an individual scan duration of 500 ps, there is an abrupt wavelength (and power) transition at the start of each cycle. This generated consistent distortions (ringing) at the start of the cycle, which persists even after averaging 15 adjacent spectra. Consequently, out of the available 128 data points per scan, the initial and final transition portions of each cycle were excluded for fitting purposes. This leaves 100 data points per fitting. FIGs. 4A-C show the raw voltage at the detector due to individual laser current scans as well as the corresponding five parameter fitted results applied to raw spectra. The vertical dotted line signifies the temporal window employed for fitting data. These figures showcase a notable degree of fit. In FIG. 4D, the baseline-corrected transmittance for the three gases is depicted. A strong degree of fit is evident, with the residual at the dip amounting to approximately 4 x 10-4.

[0144] The SNR of the fit can be computed as the ratio of the absorbance peak to the fit noise. For all three lasers, the fit noise, defined as the standard deviation of the fit residual, fell within the range of 10'4. Calculating for the weakest signal, which is CO2 (see FIG. 4C), the SNR equated to 0.0114 over 1.6 x 10-4, resulting in a value of 70. Similarly, for the strongest signal, CH4, the SNR was 110. As a result, the real-time SNR of this sensor system ranges between 70 and 110. This level of SNR is good from an instrumental perspective and is in line with previous works where SNR was reported. The transmittance dip levels observed in FIG. 4D differ from those anticipated for ambient concentrations in a 51 m open path. This discrepancy arises from the presence of a gas cell that introduces a known dip, which adds to the absorbance dip caused by the ambient gas. To ascertain ambient levels, the absorbance of the reference gas cell is subtracted from the fitted absorbance result. FIG. 4D also shows that the fitting procedure for CH4successfully captures the double peak feature.

[0145] FIGs. 5A-D illustrate the long-term performance of the system. FIG. 5A shows the 10-hour time series of the fitted absorbance value (where the gas cell absorbance is subtracted) for all three gases for an indoor measurement. Curve 511 corresponds to the measured absorbance values for C2H2, curve 512 corresponds to the measured absorbance values for CO2, and curve 513 corresponds to the measured absorbance values for CH4. The value for C2H2 is very close to zero due to its sparse presence in the atmosphere. The real-time peak-to-peak thickness of the time series over the 10-hour period is between 2 x 1c4and 3 x 10'4at a 130 Hz update rate. This shows that the uncertainty of the measurements in real-time absorbance is quite small. The thickness can further be improved by a factor of - (130) for a 1-sec- ond update rate, which is the typical update rate of a commercial point sensor. The peak-to-peak drift over the entire time series was 7 x 10-4for C2H2, 4 x 10-4for CO2, and 2.5 x 10-4for CH4. These numbers demonstrate that the apparatus exhibits comparable behaviour for all three gases, with the highest long-term drift amounting to only 2 to 3 times the real-time time series thickness. This further indicates the capacity of the sensor for sustained long-term operation.

[0146] Another interesting aspect associated with the performance of the apparatus is its sensitivity, here defined as the minimum change in concentration that the instrument can accurately measure. Traditionally, the Allan variance has served as a tool for this purpose, assessing the impact of noise on system stability. FIGs. 5B-D present Allan deviation (the square root of Allan variance) plots of the concentrations measured for C2H2, CO2, and CH4gases, respectively, using the time series shown in FIG. 5A. The initial sensitivity increase with extended averaging time highlights the enhancement possible against white noise; however, other system drift makes the curve go up with averaging. The sensitivity is quantified as the minimum point of the Allan deviation curve. In this example, for C2H2, CO2, and CH4, maximum sensitivities of 0.6 ppb at 4 seconds, 0.3 ppm at 4 seconds, and 0.96 ppb at 16 seconds could be achieved, respectively. These values represent roughly 0.05% of the ambient level for CO2, which is 560 ppm, and CH4, which is 1720 ppb. This underscores the excellent sensitivity of the sensor. Even at a 1 -second update rate, sensitivity remains robust: 0.07% for CO2 and 0.1 % for CH4. The increased ambient level for CO2 and decreased ambient level for CH4in this experiment, compared to the global averages, could be attributed to operating the apparatus within an enclosed space. For instance, CO2 might accumulate rapidly within the room during system preparation. The sensitivity achieved in this example surpasses previously reported results for DAS, and is either superior or comparable to outcomes obtained through the WMS approach. In FIGs. 5B-D, the solid curves 501 represent the Allan deviation plots of the respective gas concentrations as measured with the embodiment of FIG. 2.

[0147] As mentioned above, some embodiments of the apparatus include a reference gas cell in the transmission beam path and / or in the receiving beam path. The present example illustrates the benefits of such a reference gas cell, in particular when measurements trace gases, such as C2H2.

[0148] In particular, when measuring gasses that are present in small amounts, the measured absorbance values may be so small that they are comparable to the fit noisemaking it difficult for the signal processing, in particular a fitting algorithm, to accurately fit the absorbance and converge towards a solution. The introduction of a reference gas cell into the transmission path adds a DC absorbance on top of the ambient level. The presence of a DC absorbance significantly helps to stabilize the signal processing, in particular the fitting algorithm, as the convergence is made possible for small concentrations of target gases, e.g. C2H2. Hence it becomes possible for the fitting algorithm to produce the correct absorbance value for even smaller absorbance values. With the gas cell integrated, some embodiments of the apparatus disclosed herein can comfortably measure absorbance values as low as about 10'4.

[0149] Insertion of the reference gas cell into the transmission path and / or the receiving beam path also allows reliable measurements to be made with shorter measurement paths. Generally, insertion of the reference gas cell in the transmission path and / or the receiving beam path improves the limit of detection.

[0150] This is illustrated in FIG. 6, which shows plotted data from CH4 absorbance measurements. The square dots represent comparative ambient CH4measurements taken without a reference gas cell in the transmission path, while the diamonds correspond to corresponding measurements obtained with the reference gas cell in the transmission path of the apparatus, as in the embodiment of FIG. 2. Both sets of measurements were taken at varying open-path lengths. The circles represent theoretical values calculated using SpectralCalc under comparable conditions. The curves for cases with and without the gas cell closely mirror the theoretical predictions. However, for the square dots, fitting ceases to be effective at absorbance levels below 1 x io-3. In essence, the reference gas cell had little or no impact on CH4absorbance measurement above 1 x 10'3. Below this threshold, the absence of a gas cell results in inaccurate absorbance readings due to inadequate SNR. As a result, the detection limit of the embodiment of the apparatus used for this particular measurement without the gas cell was determined to be 1 x 10-3. However, the introduction of a reference gas cell in the transmission path can extend the detection limit even further, approaching levels close to the fit noise of 1 x 10-4. With the reference gas cell in place, the detection limit is improved by an order of magnitude, resulting in 10 ppm for CO2 (2% of ambient level) and 42.5 ppb for CH4(2.5% of ambient level), both for a path length of 51 m. Thus, inserting the reference gas cell in the transmission path of the apparatus enables measuring at a large dynamic range of concentration, even significantly below the ambient levels, and can operate even at shorter distances providing versatility of the apparatus to adapt to varying situational demands.

[0151] The slight mismatch between the diamonds and the circles at 2 m open-path length is due to the unavoidable fringing noise present in the scan, present only at extremely short distances, producing slightly inaccurate fitted results.

[0152] FIGs. 7A-B illustrate the long-term performance of an embodiment of the absorption spectroscopy apparatus disclosed herein as compared to a reference method. FIG. 7A shows measurements of C2H2 concentration in an environment where the C2H2 concentration gradually decreased over a period of 15 h. The measurements were performed with the embodiment of FIG. 2 and compared to reference measurements performed by a Picarro sensor. As can be seen from FG. 7A, the measurements are virtual indistinguishable. This is further illustrated in quantified by the correlation plot of FIG. 7B showing the correlation between the measurement results of the embodiment of the sensor disclosed herein and the reference sensor. The correlation could be fitted with a linear fit Y=0.9977X-3ppb with an R2=0.9999.

[0153] FIG. 8 schematically shows yet another embodiment of an absorption spectroscopy apparatus. The apparatus 1 of FIG. 8 is similar to the apparatus of FIGs. 1 and 2 in that it comprises a tunable laser source 12, a photodetector 114, a control unit 13 and a signal processing unit 14, all as described in connection with FIG. 1 and / or FIG. 2. The embodiment of FIG. 8 also comprises an optical system, but the optical system of FIG. 8 differs from the optical system of FIGs. 1 and 2 in that the optical system of FIG. 8 is based on reflective components and defines a transmission aperture different from the receiving aperture. The laser source 12 may e.g. comprise a fiber-coupled or free-space IR laser, e.g. a quantum cascade laser (QCL) or an interband cascade laser (ICL) with any operating wavelength in the range of 3 pm to 12 pm, and / or another suitable laser source, e.g. a telecommunication diode laser. As in the previous embodiments, the choice of laser source may depend on the target gas to be detected so as to match the wavelength of the transmitted light with prominent absorption features of the target gas. In some embodiments, the laser source may include more than one laser and one or more optical switches, e.g. as described in connection with the previous embodiments or otherwise.

[0154] The optical system of the present embodiment is mainly based on reflective components. In particular, the optical system comprises a parabolic mirror collimator 1131 configured to collimate the output from the laser source 12. The optical system further comprises a parabolic receiving mirror 1150 with a hole 1151 , The parabolic mirror collimator 1131 directs the output from the laser source 12 as a collimated beam through the hole 1151 , which thus functions as a transmission aperture of the optical system. The collimated beam then travels as an open path probe beam 3 and is reflected by a remote surface 2. The parabolic receiving mirror 1150 receives reflected return light 4 from the return surface 2 and focusses and directs the received return light towards the detector 114. Accordingly the reflective surface 1152 of the receiving mirror 1150 defines the receiving aperture of the apparatus.

[0155] Optionally, as described in connection with the embodiment of FIG. 2 above, the apparatus further comprises a reference gas cell 1135, which may comprise one or more target gas(es) to be detected, e.g. at known concentration(s). The reference gas cell 1135 may be positioned in the transmission path of the optical system, in particular such that the collimated output from the laser source passes through the reference gas cell before exiting the apparatus through the hole 1151 in the receiving mirror 1150.

[0156] The embodiment of FIG. 8 is particularly, though not exclusively, suitable for longer (MWIR and LWIR) wavelength operation. As in the previous embodiments, it is pre- ferred that each optical component of the optical transmitter / receiver system is polarization insensitive, i.e. has a polarization-insensitive transmittance. In particular, the transmittance of the optical system has preferably the same magnitude for the p-polarized component of the transmitted laser beam as for the s-polarized component. To this end, each of the optical components of the optical system preferably has a polarization-insensitive reflectance (of the mirrors) or transmittance (of the gas cell).

[0157] Generally, for reflective elements such as parabolic mirrors, optimal polarization insensitivity can be achieved by using unprotected metallic coating (e.g. unprotected gold coating layer). For the optional reference gas cell, polarization insensitivity can be achieved by using anti-reflection (AR) coated optical windows made of materials with equal or well-balanced p-polarized and s-polarized transmittances. As in the previous embodiments, the distant reflector 2 can either be a specular reflector (e.g. a plane mirror or a retroreflector) or a diffusely reflecting surface.

[0158] The reflective surface of the parabolic mirror 1131 and / or the reflective surface 1152 of the parabolic receiving mirror 1150 is preferably coated by a suitable coating providing a polarization-insensitive reflectance. To this end, the inventors have found unprotected (i.e. without protective dielectric overcoat) gold high-reflective (HR) coatings, e.g. as available from Thorlabs Inc., to be particularly suitable for wavelengths from MWIR to LWIR and also suitable for SWIR wavelengths, as they provide substantially the same reflectance for p-polarized and s-polarized light at different incident angles. In this respect it is noteworthy that the use of curved or plane mirrors involves various angles of incidence from normal incidence to as large as 45°.

[0159] Generally, the provision of a polarization-insensitive optical system has been found to be an interesting aspect of various embodiments of the apparatus disclosed herein.

[0160] The term, “optical fringing” is often used as term for interference of unwanted stray light, coming from various components inside the optical instrument (e.g. from nonperfect AR coatings), with the wanted beam. The wanted beam could be a reference beam or the signal beam itself. The consequence of optical fringing is a sinusoidal intensity modulation of the wanted beam (on top of a DC component), whose amplitude now depends on amplitudes of the interfering beams as well as their relative phase difference. Optical fringing and changes in the gas concentration can usually not be distinguished and is thus deteriorating the sensitivity of the instrument. Optical fringing constitutes a significant technical challenge, that e.g. has led to more complicated bistatic designs where signal and detection paths has been separated.

[0161] The present inventors have observed a second kind of fringing, which may be referred to as polarization fringing. Polarization fringing is similar to optical fringing, in that it will typically also give rise to a sinusoidal variation in the wanted beam. Consequently, polarization fringing will also deteriorate the quality of the gas measurement. Polarization fringing can occur when the polarization state of a beam changes followed by interaction with a polarization sensitive component, e.g. when a simple beam splitter or reflecting surface, is used to split or combine a beam or beams. In that case the ratio between the two beams will vary depending on the exact polarization state.

[0162] Changes in the polarization state of a (laser) beam can occur in various ways, e.g. if the laser is launched via a polarization maintaining (PM) fibre. If the laser beam unintentionally is coupled to both eigenstates of the PM fibre the exiting beam from the fibre is in an elliptical polarization state. Such an unintentional coupling described above can result from the coupling of a linearly polarized laser, whose polarization direction is slightly misaligned (due to mechanical precision) with respect to the physical direction of the PM eigenstate. The precise polarization state, described by a Jones vector is very sensitive to both small temperature changes of the laser fibre as well as to bending of the fibre.

[0163] In practice, it is difficult to keep the polarization state constant throughout the system, particularly when using fibre coupled lasers. Further slight misalignment of optical components may induce polarization changes. In particular the atmosphere may induce polarization changes depending on weather conditions. Accordingly, the in- ventors have found that it is beneficial to make the optical system polarization insensitive, In this respect, the term optical system refers to the optical components that can affect the actual concentration measurement.

[0164] When the optical system is made polarization insensitive, much larger acceptance in polarization changes in the system can be accepted. Mathematically the polarization properties of an optical system can be described using Jones matrix analysis. Using such a methodology, the requirement of “polarization insensitive” can in general be formulated as having the Jones matrix to be expressed as a complex constant times a unitary matrix. Unitarity will ensure the insensitiveness to polarization changes. Note that, when all optical components are unitary, then the total Jones matrix will also be unitary. In practice each system implementation may require a separate Jones matrix analysis to be performed as indicated above. For example, some of the embodiments disclosed herein employ a non-polarizing 50 / 50 beamsplitter, which ensures that all components in the set-up are then unitary. The inventors have observed that this may give rise to more than an order of magnitude improvement (appr. 40 times) in long term stability.

[0165] The above and other embodiments of the apparatus disclosed herein thus provide a cost-effective, and energy-efficient open-path multi-gas sensor. Various embodiments of the apparatus employ TDLAS for the quantification of greenhouse gases such as carbon dioxide and methane, as well as tracer gases, such as acetylene. Some embodiments of the apparatus may alternatively or additionally be used to detect other types of gasses such as irritant gases, toxic gases, pollutants, chemical warfare gases, and / or the like. Various embodiments of the apparatus leverage the availability of inexpensive fibre-coupled diode lasers in the telecommunication band, allowing for seamless multiplexing of lasers via MEMS-based optical switches. This approach enhances the adaptability of the sensor to easily add and measure additional gases as per application requirements. Various embodiments of the apparatus provide a high degree of application versatility. Various embodiments of the apparatus are compact and lightweight, thereby facilitating various deployment scenarios, including drone-mounted applications. Various embodiments of the apparatus provide good sensitivity and precision for multi-gas concentration measurements, and they provide accurate and real-time quantification of gas concentrations.

[0166] Another general aspect disclosed herein relates to the integration of a sealed reference gas cell in the transmission path and / or in the receiving beam path of some embodiments of the apparatus, thereby enhancing the overall performance of the signal processing by providing a reliable starting point, thus contributing to the high precision and stability of the sensor. Additionally, a notable benefit of the insertion of the gas cell is the improved limit of detection. For an open-path length of 51 m, the limit of detection was found to be 10 ppm for CO2 (2%) and 42.5 ppb for CH4 (2.5%). The extended limits of detection, which allows for shorter path length measurements and contributes to an improved dynamic range and efficacy of the sensor across diverse application scenarios. With a demonstrated sensitivity of 0.07% for CO2 and 0.1% for CH4 for a 51 m open path length in a matter of a few seconds, embodiments of the apparatus disclosed herein exhibit promising capabilities for emissions monitoring from high-emission facilities. The real-time capabilities of the apparatus, which in some embodiments offer concentration updates at a 130 Hz update rate or even at a kHz-level update rate, underline its potential for continuous live monitoring needs.

[0167] Various embodiments of the apparatus provide a robust and versatile solution for monitoring greenhouse gas emissions and / or emissions of other types of gases. While the above examples have mainly been discussed in the context of some specific examples of target gases, it will be appreciated that embodiments of the apparatus disclosed herein may be used for the measurement of gas concentrations of a variety of other gases, including other greenhouse and / or environmental gases. Examples of such gases include, but are not limited to N2O, NH3, CO, CO2, CH4, C2H2, H2O. Other examples of target gases include irritant gases, toxic gases, pollutants, chemical warfare gases, and / or the like.

[0168] Various embodiments of the apparatus may be used as a valuable tool for regulatory compliance enforcement, environmental research, emissions quantification, detection of hazardous materials, and / or other applications. Some embodiments of the aspects disclosed herein may be summarized as follows:

[0169] Embodiment 1 : An absorption spectroscopy apparatus for measuring one or more gases in an atmosphere, wherein the apparatus comprises:

[0170] - a tunable laser source configured to emit laser radiation;

[0171] - an optical system for transmitting the emitted laser radiation as a probe laser beam, and for collecting return light, in particular emitted laser radiation reflected by a remote surface, the optical system defining one or more apertures;

[0172] - a photodetector for detecting the collected return light, wherein the apparatus defines a transmission beam path between the tunable laser source and a transmission aperture of said at least one apertures, and the apparatus defining a receiving beam path between a receiving aperture of said at least one aperture and the photodetector; wherein the optical system is configured to have a polarization-insensitive transmittance.

[0173] Embodiment 2: The apparatus according to any one of the preceding embodiments, wherein the optical system includes a polarization-insensitive beam splitter positioned between the one or more tunable laser source and the one or more apertures and configured to direct a portion of the emitted laser radiation towards the transmission aperture and to direct a portion of the collected return light towards the photodetector.

[0174] Embodiment 3: The apparatus according to embodiment 2, wherein said portion of the emitted laser radiation and / or said portion of the collected return light is within percentage interval around a predetermined percentage, in particular around 50%, independently of a polarization of the emitted laser radiation, wherein the percentage interval has a width of 10 percentage points or less, such as of 5 percentage points or less, such as of 4 percentage points or less.

[0175] Embodiment 4: The apparatus according to any one of embodiments 2 through 3, further comprising a beam dump, wherein the polarization-insensitive beam splitter is configured to direct a remaining portion, in particular 50%, of the emitted laser radiation into the beam dump. Embodiment 5: The apparatus according to any one of the preceding embodiments, wherein all optical elements of the optical system positioned in the transmission beam path and / or in the receiving beam path are polarization-insensitive, in particular have a polarization-insensitive transmittance and / or a polarization-insensitive reflectance.

[0176] Embodiment 6: The apparatus according to any one of the preceding embodiments, wherein the tunable laser source has a wavelength tuning range including an absorption wavelength of a target gas to be detected.

[0177] Embodiment 7: The apparatus according to embodiment 6, further comprising a reference gas cell, filled with a reference gas, the reference gas including said target gas, wherein the reference gas cell is positioned in the receiving beam path and / or in the transmission beam path, in particular between the tunable laser source and the polarization-insensitive beam splitter.

[0178] Embodiment 8: A spectroscopy apparatus for measuring one or more gases in an atmosphere, wherein the apparatus comprises:

[0179] - a tunable laser source configured to emit laser radiation, wherein the tunable laser source has a wavelength tuning range including an absorption wavelength of a target gas to be detected;

[0180] - an optical system for transmitting the emitted laser radiation as a probe laser beam, and for collecting return light, in particular emitted laser radiation reflected by a remote surface, the optical system defining one or more apertures;

[0181] - a photodetector for detecting the collected return light, wherein the apparatus defines a transmission beam path between the tunable laser source and a transmission aperture of said at least one apertures, and the apparatus defining a receiving beam path between a receiving aperture of said at least one aperture and the photodetector;

[0182] - a reference gas cell, filled with a reference gas, the reference gas including said target gas, wherein the reference gas cell is positioned in the transmission beam path and / or the receiving beam path. Embodiment 9: The apparatus according to any one of embodiments 6 through 8, wherein the target gas is an atmospheric gas, in particular a greenhouse gas or a tracer gas.

[0183] Embodiment 10: The apparatus according to embodiment 9, wherein the target gas is an atmospheric gas selected from: N2O, NH3, CO2, CO, CH4, C2H2, H2O.

[0184] Embodiment 11 : The apparatus according to any one of embodiments 6 through 10, further comprising a control unit configured to control the tunable laser source to al- ternatingly emit light at the absorption wavelength and at a reference wavelength displaced from the absorption wavelength.

[0185] Embodiment 12: The apparatus according to any one of embodiments 6 through 10, further comprising a control unit configured to control the tunable laser source to scan the wavelength of the emitted laser radiation across at least a portion of said wavelength tuning range, said portion including the absorption wavelength and, preferably, a plurality of other spectral components, wherein the apparatus further comprises a signal processing system configured to receive detector signals from the photodetector, the detector signals being indicative of a plurality of received return light intensities at respective times of said wavelength scan, and to compute an absorbance at said absorption wavelength from said received detector signals.

[0186] Embodiment 13: The apparatus according to any one of the preceding embodiments, wherein the optical system is a monostatic optical system comprising a combined transmission and receiving aperture and wherein the optical system is configured to transmit the probe laser beam through said combined transmission and receiving aperture and to collect the return light through said combined transmission and receiving aperture.

[0187] Embodiment 14: The apparatus according to embodiment 13, wherein the optical system comprises a telescope for transmitting the probe laser beam and for collecting the return light. Embodiment 15: The apparatus according to any one of the preceding embodiments, comprising two or more tunable laser sources, tunable at respective target wavelengths, and one or more optical switches, such as one or more MEMS optical switches, configured to selectively direct emitted laser radiation from one of the two or more tunable laser sources towards the optical system.

[0188] Embodiment 16: The apparatus according to any one of the preceding embodiments, comprising a single photodetector. Embodiment 17: The apparatus according to any one of the preceding embodiments, comprising:

[0189] - an optical head housing accommodating at least the optical system and the one or more apertures,

[0190] - a laser housing accommodating at least the tunable laser source, - a detachable fibre-optic coupling between the optical head housing and the laser housing.

Claims

CLAIMS1. An absorption spectroscopy apparatus for measuring one or more gases in an atmosphere, wherein the apparatus comprises:- a tunable laser source configured to emit laser radiation;- an optical system for transmitting the emitted laser radiation as a probe laser beam, and for collecting return light, in particular emitted laser radiation reflected by a remote surface, the optical system defining one or more apertures;- a photodetector for detecting the collected return light, wherein the apparatus defines a transmission beam path between the tunable laser source and a transmission aperture of said at least one apertures, and the apparatus defining a receiving beam path between a receiving aperture of said at least one aperture and the photodetector; wherein the optical system is configured to have a polarization-insensitive transmittance.

2. The apparatus according to claim 1 , wherein the optical system is a monostatic optical system comprising a combined transmission and receiving aperture and wherein the optical system is configured to transmit the probe laser beam through said combined transmission and receiving aperture and to collect the return light through said combined transmission and receiving aperture.

3. The apparatus according to claim 2, wherein the optical system comprises a telescope for transmitting the probe laser beam and for collecting the return light.

4. The apparatus according to claim 1 , wherein the optical system comprises a receiving mirror for collecting the return light, the receiving mirror comprising a hole or transparent / transmissive portion for transmitting the probe beam through the hole or transparent / transmissive portion of the receiving mirror.

5. The apparatus according to claim 4, wherein the receiving mirror is coated with a coating that provides a polarisation-insensitive reflectance.

6. The apparatus according to any one of the preceding claims, wherein the optical system includes a polarization-insensitive beam splitter positioned between the oneor more tunable laser source and the one or more apertures and configured to direct a portion of the emitted laser radiation towards the transmission aperture and to direct a portion of the collected return light towards the photodetector.

7. The apparatus according to claim 6, wherein said portion of the emitted laser radiation and / or said portion of the collected return light is within percentage interval around a predetermined percentage, in particular around 50%, independently of a polarization of the emitted laser radiation, wherein the percentage interval has a width of 10 percentage points or less, such as of 5 percentage points or less, such as of 4 percentage points or less.

8. The apparatus according to any one of claims 6 through 7, further comprising a beam dump, wherein the polarization-insensitive beam splitter is configured to direct a remaining portion, in particular 50%, of the emitted laser radiation into the beam dump.

9. The apparatus according to any one of claims 6 through 7, further comprising an additional photodetector and / or a reference gas cell and / or an additional optical system, the additional optical system being configured to emit an additional probe beam, wherein the polarization-insensitive beam splitter is configured to direct a remaining portion, in particular 50%, of the emitted laser radiation towards the additional photodetector and / or towards the reference gas cell and / or towards the additional optical system.

10. The apparatus according to any one of the preceding claims, comprising two or more tunable laser sources, tunable at respective target wavelengths, and one or more optical switches configured to selectively direct emitted laser radiation from one of the two or more tunable laser sources towards the optical system.

11. The apparatus according to any one of the preceding claims, wherein all optical elements of the optical system positioned in the transmission beam path and / or in the receiving beam path are polarization-insensitive, in particular have a polarization-insensitive transmittance and / or a polarization-insensitive reflectance.

12. The apparatus according to any one of the preceding claims, wherein the tunable laser source has a wavelength tuning range including an absorption wavelength of a target gas to be detected.

13. The apparatus according to claim 12, wherein the target gas is an atmospheric gas, in particular a greenhouse gas or a tracer gas, in particular an atmospheric gas selected from: N2O, NH3, CO2, CH4, CO, C2H2, H2O.

14. The apparatus according to claim 12, wherein the target gas is selected from the group consisting of: an irritant gas, a hazardous production material, a toxic industrial chemical, a chemical warfare agent, and / or a simulants of a chemical warfare agent.

15. The apparatus according to any one of claims 12 through 14, further comprising a control unit configured to control the tunable laser source to alternatingly emit light at the absorption wavelength and at a reference wavelength displaced from the absorption wavelength.

16. The apparatus according to any one of claims 12 through 14, further comprising a control unit configured to control the tunable laser source to scan the wavelength of the emitted laser radiation across at least a portion of said wavelength tuning range, said portion including the absorption wavelength, wherein the apparatus further comprises a signal processing system configured to receive detector signals from the photodetector, the detector signals being indicative of a plurality of received return light intensities at respective times of said wavelength scan, and to compute an absorbance at said absorption wavelength from said received detector signals.

17. The apparatus according to any one of claims 12 through 16, further comprising a reference gas cell, filled with a reference gas, the reference gas including said target gas, wherein the reference gas cell is positioned in the receiving beam path and / or in the transmission beam path,18. the apparatus according to claim 17 when dependent on any one of claims 6 through 8, wherein the reference gas cell is positioned between the tunable laser source and the polarization-insensitive beam splitter.

19. The apparatus according to any one of the preceding claims, comprising a single photodetector.

20. The apparatus according to any one of the preceding claims, comprising:- an optical head housing accommodating at least the optical system and the one or more apertures,- a laser housing accommodating at least the tunable laser source,- a detachable fibre-optic coupling between the optical head housing and the laser housing.

21. The apparatus according to any one of the preceding claims, comprising two or more tunable laser sources, tunable at respective target wavelengths, and one or more optical switches, such as one or more MEMS optical switches, configured to selectively direct emitted laser radiation from one of the two or more tunable laser sources towards the optical system.

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