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 wavelength-tunable laser source, a control circuit configured to provide an electrical drive current to the wavelength-tunable laser source; wherein the control circuit is configured to create the electrical drive current as a modulated drive current, wherein the modulated drive current alternates between at least a first and a second drive current, the first drive current configured to cause the wavelength-tunable laser source to emit laser radiation at a first wavelength and the second drive current configured to cause the wavelength-tunable laser source to emit laser radiation at a second wavelength, different from the first wavelength, and wherein the modulated drive current defines a sequence of pulses, each pulse having a non-rectangular pulse shape.

Description

[0001]Absorption spectroscopy apparatus The present disclosure relates in one aspect to an absorption spectroscopy appa- ratus. BACKGROUNDIn recent years there has been an increased interest in monitoring the concentrationof greenhouse gases and trace gases in the atmosphere, e.g. for assessing their im-pact on the environment and for making informed decisions to mitigate their adverse effects.In particular, precise computation of emissions from localized facilities is of pivotalimportance 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. Therefore, ground-based, long-distance open-path measurement stations offer a compelling solution to oversee such local emissions, capitalizing on the higher likeli- hood of sampling emissions as opposed to spatially localized (point) measurements. Measurements based on laser absorption spectroscopy have, in the last few dec- ades, emerged as a mature technique for quantitative assessment of gas concentra- tion with applications ranging from atmospheric pollution monitoring, industrial pro- cess control, monitoring of agricultural activities, combustion diagnostics, medical breath analysis, etc.One interesting aspect of this development have been the advancements in reliableand versatile laser light sources, in particular inexpensive tunable narrow linewidthdiode lasers. The latter are available in the telecommunication band, which are eas-ily adaptable and accessible at various wavelengths. With numerous environmen- tally relevant gas species exhibiting characteristic absorption signatures in this tele- communication band, tunable diode laser absorption spectroscopy (TDLAS) has emerged as a viable technique for selective and precise gas identification and quan-tification in the near-infrared region (approximately 0.8 – 2 µm) and the mid-infraredregion (approximately 2.5 – 15 µm). In contrast to traditional methods like chemicaland chromatographic measurements, which are often time-consuming and intrinsi-cally destructive, TDLAS techniques have become a popular approach for high-pre-cision and non-intrusive measurement of gas concentrations in the atmosphere. TDLAS systems based on robust distributed feedback (DFB) lasers, widely accessi- ble 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 forachieving 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. A substantial body of literature is dedicated to atmospheric monitoring of green- house gases in general. Among the greenhouse gases, carbon dioxide (CO2) andmethane (CH4) have been of particular interest due to their substantial contributionsto global warming. Various attempts have been made to employ TDLAS for open- path CO2and CH4monitoring.For instance, the scientific paper Bailey et al. “An open-path tunable diode laser ab-sorption spectrometer for detection of carbon dioxide at the Bonanza CreekLong-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 sen-sor centred around a laser wavelength of 1.572 µm, with a typical path length of 100meters. Bailey et al. reported a sensitivity of 5.4 ppm (based on Allan deviation mini-mum) with 100 seconds of integration time. J. Xia et al., “Probing greenhouse gases in turbulent atmosphere by long-range open-path wavelength modulation spectroscopy”, Optics and Lasers in Engineering117 (2019) 21-28, discloses a 2.6 km open-path TDLAS-based CO2 and CH4 sensorwith sensitivities of approximately 20 ppm and 20 ppb, respectively, using integra- tion times of 20 and 60 seconds. W. Liang et al., “A novel wavelength modulation spectroscopy in TDLAS”, InfraredPhysics & Technology 114 (2021) 103661, discloses a 4.5 m open-path CH4 sensoroperating at a laser centre wavelength of 1.65 µm. The authors of this prior art docu-ment reported a sensitivity of 27 ppb with 50 seconds of integration time against abackground of 77 ppm, albeit involving intricate laser modulation techniques. J. Li et al., “Simultaneous standoff sensing for methane and hydrogen sulfide usingwavelength-modulated laser absorption spectroscopy with non-cooperative target”,Sensors & Actuators: B. Chemical 374 (2023) 132825 discloses a CH4 sensitivity ofapproximately 10 ppb with 9 seconds of integration time against a background of100 ppm at a path length of 20 meters. However, this prior art system employedcomplex laser modulation and data processing. 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 spectros- copy (CRDS). CRDS, due to its methodological nature, is primarily suited for spa- tially 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 ren- dered DAS more appealing for gas spectroscopy. The article Siozos, P.; Psyllakis, G.; Samartzis, P.C.; Velegrakis, M.: “Autonomous Differential Absorption Laser Device for Remote Sensing of Atmospheric Green- house Gases”, Remote Sens.2022, 14, 460, https: / / doi.org / 10.3390 / rs14030460discloses A ground-based, integrated path, differential absorption light detection de-vice capable of measuring multiple greenhouse gas species in the atmosphere. 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. It is further generally desirable to provide an apparatus that is robust and insensitive to noise. It is further generally desirable to provide an apparatus that is capable of acquiring measurements at a high rate. It is further generally desirable to provide an apparatus that is compact and has low complexity. It is further generally desirable to provide an apparatus that is easy to configured and install. It is further generally desirable to provide an apparatus that is flexible in use.On this background, despite previous efforts, it remains desirable to provide an ap-paratus 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 so- lutions. SUMMARYAccording to one aspect, the present disclosure relates to an absorption spectros-copy apparatus, in particular a direct absorption spectroscopy apparatus, for meas-uring one or more gases in an atmosphere, wherein the apparatus comprises:- a wavelength-tunable laser source, in particular a tunable diode laser,- a control circuit configured to provide an electrical drive current to the wavelength-tunable laser source;- wherein the control circuit is configured to create the electrical drive current as amodulated, in particular a pulsed, drive current, wherein the modulated drive currentalternates between at least a first and a second drive current, the first drive current configured to cause the wavelength-tunable laser source to emit laser radiation at a first wavelength and the second drive current configured to cause the wavelength- tunable laser source to emit laser radiation at a second wavelength, different fromthe first wavelength, and wherein the modulated drive current defines a sequence ofpulses, each pulse having a non-rectangular pulse shape. In particular, the inventors have realized that the use of non-rectangular pulses fordriving the wavelength-tunable laser source, in particular a tunable diode laser, pro-vides a fast switching between the first and the second wavelength, thereby obtain- ing a high sampling rate of absorption measurements. This in turn provides accurate measurements despite changes in the environmental conditions.In some embodiments, the modulated drive current defines transitions between acurrent one of the first and second drive currents and a subsequent one of the firstand second drive currents, wherein each transition defines an initial transition be-tween the current one of the first and second drive currents and an overshoot drive current, the overshoot drive current being further displaced from the current drive current than the subsequent drive current is displaced from the current drive current, and wherein each transition further defines at least one relaxation portion from the overshoot drive current towards the subsequent one of the first and second drivecurrents. In particular, in some embodiments, the modulated drive current definesupward transitions between a lower one of the first and second drive currents and a higher one of the first and second drive currents, wherein each upward transition de- fines an initial rising edge between the lower one of the first and second drive cur- rents and an upper overshoot drive current, the upper overshoot drive current being higher than the higher one of the first and second drive currents, and wherein eachupward transition further defines at least one decay portion from the upper over-shoot drive current and towards higher one of the first and second drive currents. Al-ternatively or additionally, in some embodiments, the modulated drive current de-fines downward transitions between the higher one of the first and second drive cur-rents and the lower one of the first and second drive currents, wherein each down-ward transition defines an initial trailing edge between the higher one of the first andsecond drive currents and a lower overshoot drive current, the lower overshoot drivecurrent being lower than the lower one of the first and second drive currents, andwherein each downward transition further defines at least one rising portion from thelower overshoot drive current towards the lower one of the first and second drivecurrents. Accordingly, by providing an initial overshoot, the wavelength-tunable lasersource more quickly reaches an operational state where the emitted wavelength cor-responds to the higher or lower drive current, respectively. In particular, in some embodiments, the non-rectangular pulse shape, in particularthe overshoot drive current and the relaxation portion, is / are selected to compensatefor a delayed temperature change of the wavelength-tunable laser source in re-sponse to a changing drive current, thereby providing a faster temperature transitionand, hence, a faster toggling between the first and second wavelengths. In someembodiments, the control circuit is configured to alternate the drive current at apulse frequency of at least 8 kHz, such as at least 10 kHz. The selection of the non-rectangular shape may be selected based on the propertiesof the wavelength-tunable laser source. To this end, in some embodiments, thewavelength-tunable laser source defines a transfer function between the time-de- pendent electrical drive current driving the wavelength-tunable laser source and a time-dependent intensity of the laser radiation emitted by the wavelength-tunable la-ser source when driven by the time-dependent electrical drive current. In particular,the transfer function may be defined between the time-dependent electrical drivecurrent and the time-dependent intensity of the emitted laser radiation when the la-ser radiation is transmitted through a wavelength-selective reference optical filter having a first optical density at the first wavelength and a second optical density, dif- ferent from the first optical density, at the second wavelength, and wherein the non- rectangular pulse shape is selected to be defined by an inverse of said transfer func-tion applied to a rectangular pulse shape. The inventors have found that a surpris-ingly accurate wavelength toggling with little noise can be achieved at high rates.In some embodiments, the control circuit is configured to operate the wavelength-tunable laser source mode-hop free, thereby avoiding undesired perturbations of thelaser behaviour. It will be appreciated that a mode-hop free operation can beachieved by suitable choice of diode laser and / or suitable choice of a drive current interval. In some embodiments, the apparatus is a direct absorption spectroscopy apparatus,in particular a differential absorption lidar apparatus, for measuring one or moregases in an atmosphere. The apparatus may further comprise:- an optical system for transmitting the emitted laser radiation as a probe laser beamand for collecting return light,- a measurement photodetector for detecting the collected return light.The apparatus may further comprise signal processing circuit, e.g. a signal pro-cessing unit or system, configured to receive detector signals from the measurementphotodetector and to and compute one or more resulting gas concentrations fromthe received detector signal, thereby obtaining accurate gas concentrations from theabsorption measurements, e.g. in real time. The signal processing circuit may partly or completely be integrated with the control circuit or be provided as a separate cir- cuit. The probe laser beam may be transmitted into an atmosphere of which concentra- tions of one or more target gases are to be measured. The return light may thus be a portion of the probe laser beam that has passed through the atmosphere. The at- mosphere may be accommodated in a gas cell through which the probe beam is transmitted. In other embodiments, the apparatus may be an open-path apparatus where the return light is a portion of the probe laser beam having been reflected back towards the optical system of the apparatus by a remote surface. Accordingly, the laser radiation may interact with, in particular be absorbed by, a target gas pre-sent in the atmosphere, e.g. in the atmosphere in a gas cell or the atmosphere be-tween the optical system and the remote surface, thereby allowing detection of suchtarget gas in the atmosphere. To this end, the first wavelength may be an absorptionwavelength of a target gas to be detected and the second wavelength may be a ref-erence wavelength displaced from said absorption wavelength. The target gas maybe an atmospheric gas, in particular a greenhouse gas, in particular an atmosphericgas selected from: N2O, NH3, CO2, CH4, C2H2. By toggling between emitting laserradiation at the absorption wavelength and emitting laser radiation at a reference wavelength, the amount of attenuation of the return light attributable to absorption by the target gas can be determined. When the toggling is fast, many measurements can be taken over a short period of time. Moreover, rapid toggling reduces potential measurement errors caused by atmospheric changes or other variations occurring between a measurement at the absorption wavelength and at the reference wave- length. 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 compo- nents where the beam path passes through the optical component. One or more of the optical components may be reflecting optical components, i.e. optical compo- nents where the beam path is reflected by the optical component. In some embodi- ments, some or 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, some or all reflecting optical compo- nents 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 gen- erally, each optical component of the optical system that forms part of the transmis- sion beam path and / or of the receiving beam path may pass a fraction of the electro- magnetic power incident onto the optical component on along the transmitting beam path or the receiving beam path, respectively. In some embodiments, said fraction is independent of the polarization of the electromagnetic radiation. In some embodiments, the optical system is a monostatic optical system, where atleast a portion of the signal transmitting and receiving paths share the same opticalcomponents, thereby simplifying system design and contributing to compactness.This choice is contrasted with a more complex bistatic approach, which entails align-ment complexities and limits easily adapting to longer distances. The monostatic op-tical system is configured to transmit the probe laser beam through said combinedtransmission and receiving aperture and to collect the return light through said com-bined transmission and receiving aperture. Accordingly, the apparatus is easy toalign and includes only relatively few optical components. In some embodiments, the optical system comprises a telescope for transmitting theprobe laser beam and for collecting the return light, thus allowing for efficient remotesensing. 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 disturb- ance that might otherwise result in misalignment of the transmission and receiving paths.In some embodiments, the optical system includes a beam splitter, preferably a po-larization-insensitive beam splitter, i.e. a beam splitter whose optical properties, inparticular the beam-splitting ratio between reflected and transmitted portions, does not depend on the polarization of the incoming beam. The polarization-insensitive beam splitter considerably reduces high-intensity fluctuations. The beam splitter may be positioned between the wavelength-tunable laser source and the transmis-sion / receiving aperture of the optical system. The beam splitter may be configuredto direct a portion of the emitted laser radiation towards the transmission apertureand to direct a portion of the collected return light towards the measurement photo-detector. Accordingly, in some embodiments the transmission and receiving beampaths form a combined transmission and receiving beam path between the polariza-tion-insensitive beam splitter and the combined transmission and receiving aperture.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 orless. For example, the inventors have found that the long-term drift of the measure-ment results can be considerably decreased when the polarization-insensitive beamsplitter directs 50% + / - 2% of the emitted laser radiation towards the one or moreapertures, in particular to the combined transmission and receiving aperture, regard- less of the polarisation of the emitted laser radiation. Similarly, it is preferred that thepolarization-insensitive beam splitter directs 50% + / - 2% of the collected return lighttowards the measurement photodetector, regardless of the polarisation of the col- lected 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 may cause of noise and measurementdrift. Accordingly, the long-term stability of the measurements is further improved. In some embodiments, the apparatus is configured to, e.g. intermittently or continu-ously, determine a current output power of the wavelength-tunable laser source. Tothis end, in some embodiments, the apparatus comprises a laser reference photode-tector configured to receive a reference portion of the emitted laser radiation, the ref-erence portion being indicative of a current output power of the wavelength-tunablelaser source. Provision of such a laser reference photodetector allows the apparatusto determine a degree of attenuation of the laser probe beam on its round trip be-tween the optical system and the remote surface, thereby allowing accurate meas- urements despite a varying output power of the wavelength-tunable laser source. Insome embodiments, the laser reference photodetector is different from the measure-ment photodetector, thereby avoiding the need for alternatively directing the return light or the reference portion onto a single detector. In other embodiments, the laser reference photodetector and the measurement photodetector may be embodied as a single laser reference and measurement photodetector, thereby reducing the num- ber of required photodetectors. In such an embodiment, the apparatus may include a mechanism for alternatingly directing either the return light of the reference portion onto the combined laser reference and measurement detector. For example, such a mechanism may include a mirror that is operable be moved in and out of the beamor otherwise. Accordingly, at selected times, the mechanism may direct the refer-ence portion onto the detector so as to obtain a laser reference measurement.In some embodiments, the apparatus further comprises a wavelength referencephotodetector configured to receive a modified reference portion of the emitted laser radiation, in particular such that the modified reference portion is modified in a wave- length-dependent manner. To this end, the apparatus may comprise a wavelength- dependent element configured to receive a reference portion of the emitted laser ra- diation and to modify, e.g. attenuate, the received reference portion, where the mod- ification, e.g. the degree of attenuation, is wavelength-dependent. Provision of awavelength reference photodetector for detecting a modified reference portion of theemitted laser radiation where the modification of the reference portion is wavelength dependent, allows the apparatus to monitor a desired spectral property of the emit- ted laser radiation, in particular a relative alignment of the emitted laser radiation with a predetermined spectral property of the wavelength-dependent element. Forexample, the wavelength-dependent element may comprise a reference gas cell po-sitioned in the beam path of the reference portion. Accordingly, the apparatus may monitor alignment of the wavelength of the emitted laser radiation with the absorp- tion wavelength (or the wavelength of another absorption feature) of a known refer- ence gas accommodated within the reference gas cell. In the context of gas analy- sis, such monitoring may be used to control the drive current driving the wavelength- tunable laser source so as to lock the drive current to the wavelength of a predeter- mined absorption feature of the target gas whose concentration is to be measured. To this end, the reference gas cell may include the target gas. In particular, the con- trol of the drive current may be performed accurately even when the target gas in the atmosphere to be measured is only present at low concentrations or not present at all.In some embodiments, the control circuit is further configured to:- receive detector signals from the measurement photodetector and / or from thewavelength reference photodetector, the detector signals being indicative of a time-dependent intensity of the collected return light and / or of a time-dependent intensity of a modified reference portion of the emitted laser radiation, when the tunable diode laser is alternatingly driven at the first and second drive currents, and- adjust the first and / or second drive current responsive to the time-dependent inten-sity, in particular responsive to a pulse shape of the time dependent intensity. The inventors have realised that the time-dependent intensity, in particular the pulse shape of the time dependent intensity, carries information about whether the firstwavelength matches the absorption wavelength of the target gas. Accordingly, thecontrol circuit may use information extracted from the time-dependent intensity toadjust the first wavelength such that the first wavelength matches the absorptionwavelength of the target gas. To this end, the control circuit may be configured toextract one or more indicator features from the time-dependent intensity. A suitable example of a useful indicator feature is a derivative, with respect to time, of the time- dependent intensity, in particular a derivative at a time immediately preceding the subsequent transition to the second drive current. In particular, the sign of the deriv- ative may indicate whether the first wavelength is larger than the absorption wave- length or smaller than the absorption wavelength. In some embodiments, the wavelength-tunable laser source comprises a tunable di- ode laser, in particular a tunable diode laser operable to emit laser radiation in a tel-ecommunication wavelength band, such as a wavelength range around 1.5 µm, e.g.between 0.8 µm and 2 µm. Other examples of tunable diode lasers include Quan-tum Cascade Lasers and Interband Cascade Lasers, which may operate in the mid-infrared region, in particular in the region approximately 2.5 – 15 µm.In some embodiments, the apparatus comprises two or more wavelength-tunable la-ser sources, in particular two or more tunable diode lasers, tunable at respective tar- get wavelengths. The two-or more wavelength-tunable laser sources may be two ormore fibre-coupled laser diodes. The apparatus may further comprise one or moreoptical switches configured to selectively direct emitted laser radiation from one of the two or more wavelength-tunable laser sources towards the optical system, inparticular towards the polarization-insensitive beam splitter. Accordingly, the appa-ratus 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 morewavelength-tunable laser sources and / or more than one optical switches, e.g. in acascaded arrangement so as to allow for a seamless multiplexing between three ormore different target wavelengths. Some embodiments of the apparatus facilitatemultiplexing of multiple fibre-coupled diode lasers via optical switches based on mi- cro-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 exam- ple, in one embodiment, in addition to CO2and CH4, the apparatus can measure the concentration of a tracer gas like acetylene (C2H2), which plays a vital role in under- standing atmospheric dynamics and source attribution. In some embodiments, the wavelength-tunable laser source has a wavelength tun- ing range including an absorption wavelength of a target gas to be detected andpreferably including at least one other spectral component displaced from said ab-sorption wavelength, thereby allowing accurate determination of the gas concentra- tion based on a measurement of two or more spectral components at or around anabsorption wavelength of the target gas. The absorption wavelength may be a wavelength where the target gas provides strong absorption or another detectable absorption feature. In some embodiments, the target gas is an atmospheric gas, in particular a green-house gas and / or a trace gas. In some embodiments the target gas is an atmos-pheric gas selected from: N2O, NH3, CO2, CH4, CO, C2H2, H2O. Various embodi-ments of the apparatus may be used to measure multiple gases, in particular con-currently. Here the term concurrently is intended to include measurements of multi-ple gases made in sufficiently fast succession of each other that the atmospheric conditions do not significantly change between the measurements.In some embodiments, the apparatus is a differential absorption lidar apparatus. Insome embodiments, the apparatus further comprises a control circuit configured tocontrol the wavelength-tunable laser source to alternatingly emit laser radiation atthe absorption wavelength and at a reference wavelength displaced from said ab-sorption wavelength, in particular a reference wavelength that is close to, butspaced apart from, an absorption peak / dip at the absorption wavelength. The actualdistance between the absorption wavelength and the reference wavelength may de- pend on the width of the absorption peak / dip. For example, the distance may be se- lected to be between 2 or 3 times the FWHM of the absorption peak / dip. In some embodiments, the distance may be between 0.05 nm and 1 nm, such as between 0.05 nm and 0.3 nm, such as between 0.05 nm and 0.2 nm. Preferably, the controlcircuit may cause the wavelength-tunable laser source to toggle between the ab-sorption 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 in- sensitive against variations of other factors, e.g. atmospheric fluctuations, that might change the intensity of the return light.In some embodiments, the apparatus further comprises a reference gas cell, partlyor completely filled with a reference gas. The reference gas may include said targetgas, preferably 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 refer- ence gas cell may be positioned in the transmission and / or receiving optical path of the emitted laser radiation and / or the return light, i.e. such that the radiation de- tected by the measurement photodetector has pass through the reference gas cell,thereby providing elevated limits of detection and / or enhanced overall stability ofcomputation of the gas concentration from the detector signals. 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 beampath, in particular between the wavelength-tunable laser source and the polariza-tion-insensitive beam splitter. The inventors have found that the insertion of the ref- erence 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 themeasured detector signals. The reference gas cell may also be placed in the receiv-ing beam path instead of in the transmission beam path, e.g. between the beam-splitter and the measurement detector. The reference gas cell may also be placed ina path common to the transmission beam path and the receiving beam path, e.g.between the beam splitter and the telescope of a monostatic system. Alternatively or additionally, the reference gas cell may be positioned in a reference arm of the optical system, i.e. on the optical path of a reference portion of the emit- ted laser radiation that is not transmitted through the atmosphere to be measured and that is not received by the measurement detector during the concentration measurement. Instead the reference portion of the emitted laser radiation may be received by a reference photodetector after having passed through the referencegas cell. If the same detector is used as laser reference detector and wavelengthreference detector, the concentration measurements and wavelength referencemeasurements may be made in a time-multiplexed manner. Similarly, if the samedetector is used as measurement detector and wavelength reference detector, the respective measurements may be made in a time-multiplexed manner. The provi- sion of the reference gas cell in a reference arm provides a wavelength-sensitive modification of a wavelength reference portion of the emitted laser radiation. As will be described herein, a detection of the wavelength-sensitive modification may be used to accurately tune the drive currents. In some embodiments, the apparatus comprises:- an optical head housing accommodating at least the optical system,- a laser housing accommodating at least the wavelength-tunable laser source,- a detachable fibre-optic coupling between the optical head housing and the laserhousing. Accordingly, a modular system is provided that aids system maintenance and allows for an easy reconfiguration of the apparatus for measuring different typesof target gases. In other embodiments, the optical system and the wavelength-tuna-ble laser source may be accommodated within a single housing.The present disclosure relates to different aspects including the apparatus describedabove 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 em- bodiments corresponding to the embodiments described in connection with one ormore of the other aspects and / or disclosed in the appended claims.In particular, another aspect disclosed herein relates to a laser apparatus, whereinthe laser apparatus comprises:- a wavelength-tunable laser source, in particular a tunable diode laser,- a control circuit configured to provide an electrical drive current to the wavelength-tunable laser source; wherein the control circuit is configured to create the electricaldrive current as a modulated, in particular a pulsed, drive current, wherein the mod-ulated drive current alternates between at least a first and a second drive current,the first drive current configured to cause the wavelength-tunable laser source to emit laser radiation at a first wavelength and the second drive current configured to cause the wavelength-tunable laser source to emit laser radiation at a second wave- length, different from the first wavelength,- an optical system for transmitting at least an output portion of the emitted laser ra-diation as an output laser beam and for providing at least a wavelength referenceportion of the emitted laser radiation,- a wavelength-dependent radiation modifying element configured to receive thewavelength reference portion and to cause a wavelength-dependent modification ofthe wavelength reference portion,- a wavelength reference photodetector for detecting the modified reference wave-length portion.In some embodiments, the control circuit is further configured to:- receive detector signals from the wavelength reference photodetector, and- adjust the first drive current and / or the second drive current responsive to the re-ceived detector signals.In some embodiments, the detector signals are indicative of a time-dependent inten-sity of the modified wavelength reference portion when the wavelength-tunable lasersource is alternatingly driven at the first and second drive currents.In some embodiments, the control circuit is configured to derive a derivative with re-spect to time of the time-dependent intensity of the modified wavelength referenceportion when the wavelength-tunable laser source is driven at the first or seconddrive current the received detector signals, and to adjust the first drive current and / orthe second drive current responsive to the derived derivative.BRIEF DESCRIPTION OF THE DRAWINGSEmbodiments of the various aspects disclosed herein will be described in more de-tail in connection with the appended drawings, in whichFIG.1 schematically shows an embodiment of an absorption spectroscopy appa-ratus;FIG.2 schematically illustrates another embodiment of an absorption spectroscopyapparatus;FIGs.3A-B illustrate that the wavelength response of the diode laser of the embodi-ment of FIG.2;FIGs.4A-D illustrate an example of the ON / OFF wavelength toggling in accordancewith an embodiment;FIGs.5A-B show the ON transmittance data for different mean drive currents of theembodiment of FIG.2;FIG.6 shows the slope of the ON pulses versus mean drive current for the embodi-ment of FIG.2;FIGs.7A-C illustrate results of a long-term gas sensing test with feed-back loop us-ing the embodiment of FIG.2;FIG. 8 schematically illustrates another embodiment of an absorption spectroscopyapparatus;FIG. 9 schematically illustrates another embodiment of an absorption spectroscopyapparatus;FIG. 10 schematically illustrates another embodiment of an absorption spectroscopyapparatus; FIG.11 schematically illustrates an embodiment of a laser apparatus. DETAILED DESCRIPTIONIn the following, aspects of a laser apparatus will be described. In particular, in thefollowing, embodiments of an absorption spectroscopy apparatus will be described.Various embodiments of the apparatus disclosed herein provide a sensitive, accu-rate, reliable, low-power consumption, and cost-effective environmental gas monitor- ing solution. Various embodiments of the apparatus are operable to perform open-path sensing for sensing one or more gases. Various embodiments of the apparatusdisclosed 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 suit- able for deployment by an UAV. Other applications include the fenced-line monitor- ing of critical facilities, extracting spatially averaged concentrations of emitted gasspecies. Further applications include the continuous monitoring of emissions fromindustrial facilities, facilitating emissions quantification, and thereby enabling regula- tory compliance enforcement. Various embodiments of the laser apparatus disclosed herein may be employed for other applications, e.g. for spectroscopy of samples accommodated in a sample container such as a gas cell, and / or for other applications where a rapid and accu- rate and stable switching between two or more wavelengths is desirable.Various embodiments of the apparatus disclosed herein employ tunable diode laserabsorption spectroscopy (TDLAS), in particular direct tunable diode laser absorptionspectroscopy (occasionally referred to as dTDLAS in the literature). Various embodiments described herein use measurements at two wavelengths, in particular measurements at only two wavelengths, to deduce the gas concentration. Using Lamberts absorption law, a transmission measurement ON / OFF the gas ab- sorption line is sufficient to calculate the integrated gas concentration, calibrationfree. Here and in the following, the terms ON and OFF refer to entities related to theabsorption wavelength and to a reference wavelength, respectively. For example, reference to a measurement ON the gas absorption line refers to a measurement based on a probe laser beam having a wavelength that corresponds to the absorp- tion wavelength of an absorption line of the target gas. Similarly, reference to a measurement OFF the gas absorption line refers to a measurement based on a probe laser beam having a wavelength that is displaced from the absorption wave- length of an absorption line of the target gas. Various embodiments described herein provide relatively simple data processing for computing the gas concentration from the measurements, thereby facilitating fast update rates and providing robust- ness in data processing. High update rates are beneficial for measurements in out- door settings or fast monitoring in industrial applications. In prior art systems, the inherent thermal time constants of typical diode lasers, ranging to the ms scale, limits the tuning rate of the center wavelength to typically 10-100 Hz update rate. When high wavelength precision is needed as for DLAS, this is an important limitation, which is addressed by various embodiments disclosedherein. In various embodiments described herein, when the electrical drive current ismodulated such that it comprises non-rectangular pulses, the toggling rate betweenthe two wavelengths can be increased considerably.Generally, the current modulation induces a temperature change of the wave guide, which in turn changes the refractive index and the length of the diode laser cavityand, thus, the center wavelength of the laser. Hereby, wavelength tuning can be ob-tained. In practice the wavelength response as function of modulation current is as- sociated with several time constants reflecting the heat dissipating characteristics ofthe chip itself (on a 10-100 ns scale), of the sub-mount (on a 10 µs scale) and of theheat sink (on a 100 µs scale), respectively. In experiments by the inventors, a laserdriver itself had a 3 dB limit of 150 kHz. In an experimental setup, which will be de-scribed in more detail below with reference to FIG.2, 27 ms was the delay experi-mentally observed for the diode laser to come within 1 pm of its final wavelength.Without employing the pulse modulation described herein, this delay severely limits the toggling frequency between two fixed wavelengths given a predetermined preci- sion. Generally, in some embodiments, the modulated drive current may be a pulsed drive current comprising pulses having a pulse shape that comprises a step function and an exponential function, in particular a step function superposed with an expo- nential function. The exponential function may include one or more, such as two, ex- ponential terms having respective decay rates. By employing two or more exponen-tial terms with respective decay rates, the process is capable of compensating forthermal processes associated with different time constants, e.g. the time constants discussed above, thereby providing an even faster wavelength toggling between thetwo desired wavelengths. In some embodiments, the exponential function has amaximum amplitude of between 10% and 50% of the amplitude of the step function, such as a maximum amplitude between 20% and 40%, such as between 25% and 35% of the amplitude of the step function. In some embodiments, the exponential function has a decay rate defined by a time constant between 10% and 50%, such as between 20% and 40%, such as between 25% and 35% of the pulse width of the rectangular pulse, The above choices of the amplitude and decay rate of the expo- nential function have been found to allow for the wavelength to be toggled between two spaced-apart wavelengths at a fast transition time.The following describes an example of an adapted current pulse shape that com-prises a step current function and an exponential function. The inventors have found that by suitable amplitudes and time constant, the toggling transition time can be re- duced by offsetting the slow thermal decay constant. In particular, a current pulse may be generated using the sum of a step function and an exponential decay func-tion, that, by a suitable amplitude ratio p (i.e. ratio of the amplitude of the exponen-tial term to the peak-to-peak amplitude of the step function) and time constant, mini-mizes the toggling transition time by compensating thermal time constants of thesystem. In one embodiment, the pulse shape is based on the measured step response func-tion, ^^^^(^). The pulse shape may further be based on the system’s linearity andtime-invariance properties. Now, where ^^^^^(^) is the unit step current function, h(t) is the system impulse response,* is the convolution operator, ^^^^(^) is the step response, which may be approxi-mated with a double exponential function, where a, b and c are derived from experi-ments, y(^) is the wavelength change. ^^ and^^ are measured time constants.The Laplace transfer function, H(s), can then be expressed as,^(^) = ^^^^^(^)^For a combination of a step input and an exponential input, ^^^^^(^) + ^^^^^, theoutput, ^(^) is given by, After insertion of ^^^^(^) and explicit calculation, The inverse Laplace transform yields: Setting a= −^ − ^ eliminates any bias current as can be seen from ^^^^(0). In sum-mary, a,b, c, ^^ and ^^ may be found from the step response, while p and ^^ may be determined by optimizing for a fast toggling response. For the purpose of thepresent description, when it is desirable to reach a fast steady state wavelength fol-lowed by a relatively long constant period before toggling back, a single step func-tion and not a full toggling cycle may be considered for the purpose of the followinganalysis. This simplifies calculations.Gas sensing based on Beer-Lamberts’s absorption law may be based on the follow-ing equation: ^^ = ^^^^^^^^^(^)^,where ^^ and ^^ is the transmitted and incident power respectively, ^^^^(^) is thewavelength-dependent absorption coefficient of the target gas in the path, ^ is thelength of the beam path through the atmosphere including the target gas, and ^ ac-counts for all coupling losses as well as the detector response. Since current is usedto change the wavelength, power will change when toggling between the two wave-lengths. However, a laser reference detector can be used to normalize the power.Assuming that the measurement detector and the laser reference detector are lin-ear, i.e. of the form ^(^) = ^^ + ^, where U is the detector voltage, P is incidentpower on the detector, ^ is the responsivity and ^ any (typically unwanted) offset ofthe detector. Using the ratio of an ON transmission measurement (^^^) and an OFF measurement (^^^^), the gas absorption can be derived as: where ^^^^(^)^ is the wavelength dependent absorbance of the gas. From ^^^^^ ,the actual gas concentration, C can then be derived. It is assumed above that theOFF measurement is substantially free of the absorption line of the measured gas, i.e. that the OFF wavelength is sufficiently far removed from the ON wavelength. It is worth mentioning that as long as the two detector response curves are linear, their detector response curves may be mutually different, without changing the ratio above. This guarantees a high detector insensitivity, in part the explanation for the high gas sensitivity that may be achieved by embodiments of the apparatus dis- closed herein.Embodiments of the apparatus enable real-time gas concentration quantificationwith high precision and reliability, offering an extensive dynamic range for concen- tration measurements. Various embodiments of the apparatus can produce real-timeconcentration values at a high update rate and at sub-percent sensitivity of the am-bient background concentration in a matter of a few seconds of averaging. In oneembodiment the update rate is 100 Hz or more, such as 130 Hz or more.FIG.1 schematically shows an embodiment of an absorption spectroscopy appa-ratus, generally designated by reference numeral 1. The apparatus comprises a tun-able diode laser 12 and a control circuit 13.The tunable diode laser 12 is configured to emit laser radiation. The control circuit13 is configured to provide an electrical drive current to the tunable diode laser 12so as to cause the tunable diode laser to emit the laser radiation. The wavelength ofthe emitted laser radiation depends on the drive current, and the control circuit 13 isconfigured to create the electrical drive current as a modulated drive current,wherein the modulated drive current alternates between at least a first and a seconddrive current, the first drive current configured to cause the tunable diode laser to emit laser radiation at a first wavelength and the second drive current configured to cause the tunable diode laser to emit laser radiation at a second wavelength, differ-ent from the first wavelength. Accordingly, the emitted laser radiation toggles be-tween the first and the second wavelengths. In the present example, the apparatus further comprises an optical system 113 fortransmitting the emitted laser radiation as a probe laser beam 3, and for collectingreturn light, in particular emitted laser radiation reflected by a remote surface 2.The optical system 113 defines an aperture through which the probe beam is emit-ted from the optical system and through which the return light is received at the opti- cal system. In this example, the aperture is defined by an output lens 11342 of the optical system. In other embodiments, the aperture may be defined by another opti- cal element, e.g. by a dedicated aperture element, which may be integrated with anoptical head housing of the apparatus. The apparatus further comprises a measurement photodetector 114, in the followingalso referred to as the target gas detector, for detecting the collected return light.The apparatus defines a transmission beam path and a receiving beam path. In theexample of FIG.1, the optical system is a monostatic optical system where the aper- ture is a combined transmission and receiving aperture, i.e. it is both a transmission aperture of the transmission beam path and a receiving aperture of the receiving beam path. In the present example, the optical system 113 comprises a collimator 1131, a beamsplitter 1132, a telescope 1134, a pair of mirrors 1136 and 1137 and focussinglenses 1133 and 1139. It will be appreciated, however, that other embodiments ofthe optical system may include alternative or additional optical components. In someembodiments, all of the components of the optical system within the transmissionand receiving beam paths are polarization-insensitive components, i.e. their trans- mittance and / or reflectance and / or other optical properties are substantially insensi- tive to the polarization of the light propagating along the transmission and / or receiv- ing beam paths. The tunable diode laser 12 is tunable within a tuning range chosen to correspond toselected absorption properties of a target gas whose concentration is to be meas-ured. In particular, the tuning range comprises an absorption wavelength of the tar-get gas, i.e. a wavelength at which the target gas absorbs light and / or exhibits an-other detectable absorption signature. For example, the laser diode 12 may be tuna-ble around an absorption wavelength of 1.52 ^m, which corresponds to an absorp-tion wavelength of C2H2. Alternatively, the laser diode 12 may be tunable around anabsorption wavelength of 1.645 ^m, which is an absorption wavelength of CH4. Yetalternatively, the laser diode 12 may be tunable around an absorption wavelength of1572 nm, which is an absorption wavelength of CO2. It will be appreciated that otherembodiments of the apparatus may include alternative or additional diode lasers, e.g. one or more diode lasers having different tuning ranges that match absorption wavelengths of other target gases. For example, in some embodiments, the appa- ratus may include more than one, e.g. two, three or more laser diodes which may each be tunable within a respective tuning range, e.g. corresponding to absorption wavelengths of respective target gases. The multiple laser diodes may be operable, e.g. via a suitable fiber-optic system including one or more MEMS switches, opto- mechanical switches, optical waveguide optical switches or otherwise, to selectively emit their laser radiation into the transmission path of the optical system 113.The choice of laser diode wavelength(s) may depend on the target gases to be de-tected. The choice may be guided by spectral windows with minimal interferencefrom other gases in the vicinity of the target gas absorption feature around the 1.55 µm telecommunication band. Nonetheless, water vapour emerges as the primarysource of interference due to its proximity to some of the target gases of interest. Inthe present embodiment, laser centre wavelengths of 1.521 µm, 1.572 µm, and1.645 µm have been chosen for C2H2, CO2, and CH4 gases, respectively. However,other embodiments may employ alternative or additional wavelengths. The diode la- sers may be any suitable type of tunable diode laser. In one example, the diode la- ser may have a narrow line width of 100 kHz and output power of 5 mW. However, other diode lasers may be used instead.Accordingly, the diode laser 12 may be operated to selectively toggle between twoor more wavelengths within the tuning range. The diode laser 12 is optically con-nected to the optical system 113 via a fibre-optic connection 123. The optical system 113 receives the emitted laser radiation from the diode laser 12via the fibre-optic connection 123. In the present example, the collimator 1131 is acollimating lens that receives a diverging beam from the optical fibre and forwards acollimated laser radiation to the beam splitter 1132. In other embodiments, the colli- mator 1131 may be implemented as a fibre-collimator or otherwise. The beam split-ter 1132 may be a polarization-insensitive beam splitter. The beam splitter 1132 for-wards a portion of the laser radiation towards the telescope 1134 and directs a re- maining portion of the laser radiation via focussing lens 1139 onto a laser reference photodetector 116. The focussing lens 1139 and the laser reference photodetector 116 thus form a reference arm of the apparatus. The telescope 1134 receives the forwarded laser radiation from the 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, re- spectively. The remote surface 2 may be a dedicated retroreflector or another suitable surface.For example, a hollow retroreflector may be used to retro-reflect the beam. Using aretroreflector 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 combina- tion 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. For ex- ample, 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 mayfunction as the remote surface 2. In any event, the remote surface 2 is separatefrom 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 re- flected 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. 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 towardsthe beam splitter 1132. The apparatus 1 thus uses the same telescope 1134 to col-lect the retro-reflected signal, i.e. the signal retro-reflected by a dedicated retrore- flector or by another remote surface. Accordingly, the return light, reciprocally, re- traces its path back to the beam splitter 1132 via telescope 1134 and via mirrors1137 and 1136. The beam splitter 1132 directs a portion of the collected return lightvia focussing lens 1133 towards the measurement photodetector 114. It will be ap-preciated that the transmission beam path between the beam splitter and the aper-ture coincides with the receiving beam path between the aperture and the beamsplitter. The beam splitter 1132 separates the transmission and receiving beampaths such that only the collected return light is directed towards the photodetector 114. The measurement photodetector 114 detects the collected return light and outputs a detector signal indicative of the detected return light. 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 reference arm may include a reference gas cell and / or more than one reference photodetector.In the present example, the control circuit 13 further includes a signal processingunit for analysing the detector signals from the measurement photodetector 114 andfrom the laser reference detector 116. It will be appreciated that the control circuit and the signal processing unit may be provided as separate modules or integrated into a single module, e.g. into a single processing unit. Moreover, the control circuit 13 and, optionally the signal processing unit, may be distributed across multiple sub-modules. For example, the control circuit 13 may include one or more separate orcombined control circuits for controlling the tunable laser source 12 and other com- ponents of the apparatus, e.g. one or more MEMS optical switch and / or the like. Thecontrol circuit 13 and / or a signal processing unit may completely or partly be accom-modated within the laser housing of the laser source module 12, within an optical head housing of apparatus, e.g. together with 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 sys- tem. The control circuit 13 may further comprise a laser drive circuit for driving diode laser 12. To reduce the form factor of the apparatus, some embodiments may utilize a compact, lightweight current and temperature driver, e.g. the DX1 laser diode con- troller module by Eblana Photonics Ltd. It will be appreciated, however, that other embodiments may use one or more other laser drive circuits.It will generally be appreciated that the control circuit of various embodiments maybe implemented by a variety of suitable control circuitry or devices, e.g. imple- mented by a suitable microcontroller, a FPGA, an ASIC and / or the like.The control circuit 13 generates control signals for the current and, optionally thetemperature, of the laser diode, e.g. via a suitable digital-to-analogue converter(DACs). In the present embodiment, the control circuit 13 also receives the detectorsignals from the measurement photodetector 114 and from the laser reference pho-todetector 116, e.g. using suitable analogue-to-digital converters (ADC) or other-wise. The control circuit 13 may then provide the detector signal to a signal pro-cessing unit, which may integrated into the control circuit 13 or provided as a sepa- rate component.Generally, the signal processing unit of various embodiments may be implementedby a suitably programmed microprocessor, a computer, or otherwise. The signalprocessing may maybe conducted collaboratively by the control circuit and the sig-nal processing unit or otherwise. In the present example, the optical system further comprises mirrors 1136 and 1137that serve to fold the beam path. These mirrors serve two purposes: (i) folding thebeam and increasing the distance from the measurement photodetector 114 to the front surface of lens 11341 to achieve a 1 / r² reduction in surface reflections fromlens 11341 at the photodetector and (ii) facilitating better manoeuvrability of thebeam. The lenses themselves may also be AR coated. Lenses 11341 and 11342 constitute the telescope stage designed to expand the beam for long-distance oper- ation. Alternative embodiments may include fewer or more mirrors, e.g. no mirrors.The optical system may be accommodated within a suitable optical head housing.Generally, in some embodiments, all of the components of the optical system thatare located within the transmission and receiving beam paths may be polarization-insensitive components, i.e. their transmittance and / or reflectance and / or other opti- cal properties are substantially insensitive to the polarization of the light propagating along the transmission and / or receiving beam paths.The measurement photodetector 114 may be positioned slightly beyond the focalpoint and at a minor angle (of e.g. 5° or the like) to the incoming beam to prevent in-terference from stray light sources. This arrangement also serves to mitigate the po-tential for multiple reflections within a window located in front of the detector. Toeliminate undesired background radiation from being detected, a long-pass 1.5 µmfilter, together with the decreasing sensitivity of an InGaAs photodetector beyond 1.7µm, may be employed. The measurement photodetector 114 may e.g. be an In-GaAs detector, e.g. model FGA21 from Thorlabs, Inc., or another suitable photode- tector. The measurement photodetector may be integrated with a suitable detector electronics, e.g. an electronic circuit with a suitable trans-impedance gain and noiseperformance. The signal detected by the measurement photodetector may be sub-jected to digitization, e.g. via a 16-bit ADC operating at a data sampling rate of 250kS / s within the control circuit 13, or otherwise. The subsequent signal processingmay be performed in real-time for analysis and processing aimed at extracting theconcentration of the target gas. The laser reference photodetector 116 may be any suitable photodetector, e.g. as described in connection with the measurement photodetector, or otherwise. The la- ser reference photodetector may be of the same type as the measurement photode- tector 114 or it may be a different type of photodetector. Various embodiments of the apparatus disclosed herein employ DAS for measuringgas concentration of a target gas. To this end, in some embodiments, the diode la-ser 12 is controlled by the control circuit 13 to toggle between an absorption wave-length of the target gas and a reference wavelength displaced from the absorption wavelength. As the target molecules along the open path between the optical sys-tem 113 and the remote surface 2 absorb energy from the probe beam 3, the trans-mitted light intensity undergoes modulation, causing a corresponding change in power of the return light detected by the photodetector. The modulation at the ab- sorption wavelength is different from the modulation at the reference wavelength and the difference can be used to compute the concentration of the target gas. Toachieve the wavelength toggling, the laser drive current may be provided as a mod-ulated drive current as described herein.In particular, when the pulse shape of the modulated drive current is non-rectangu-lar, the toggling rate can be increased while maintaining accurate toggling betweenpredetermined wavelengths. The laser reference detector 116 detects the light reflected by the beamsplitter 1332, i.e. the light detected by the laser reference detector 116 is indicative of thelaser power. The measurement photodetector 114 detects the retroreflected lightfrom the target gas detection path i.e. light indicative of a combination of the laser power and of the gas transmittance in the beam path. By dividing the signal from the measurement photodetector with the laser reference detector signal the transmit-tance at the two (ON and OFF) wavelengths in the target gas detection path can bedetermined.FIG.2 schematically shows another embodiment of an absorption spectroscopy ap-paratus, generally designated by reference numeral 1. The apparatus of FIG. 2 issimilar to the apparatus of FIG.1 and comprises a tunable diode laser 12, a controlcircuit 13, a measurement photodetector 114, and a laser reference detector 116, all as described in connection with FIG.1. The apparatus of FIG.2 represents an ex- perimental setup of a closed-path spectroscopy apparatus. Accordingly, the optical system 213 of this embodiment includes a probe gas cell 2135. The probe gas cellaccommodates an atmosphere that includes the target gas to be measured. In theexample of FIG.2, the optical system is a fiber-optic system including a linear polar-izer 2131, a 2x2 fiber coupler 2132 with 50:50 split ratio and the probe gas cell2135, which is a fiber-coupled gas cell. In this example, the laser diode 12 is a tuna-ble 1.55 ^m laser diode.The set-up according to this embodiment is based on polarization maintaining fibers.The 1.55 mm laser diode 12 (from Eblana Photonics) is coupled to a polarizationmaintaining (PM) fiber. The linewidth is 100 kHz and provides approximately 5 mWof stable output power. The PM fiber is connected to the linear polarizer 2131 toeliminate any polarization cross talk in the first section. The 50:50 polarization main-taining 2x2 fiber coupler 2132 divides the beam into a reference beam used to measure the actual output power of the laser diode at all times, and a measurementbeam to probe the wavelength-selective element, i.e. the probe gas cell. The fiber-coupled probe gas cell 2135 contains HCN gas as target gas to demonstrate close-path gas sensing based on Beer-Lamberts’s absorption law as described herein. Inthis example, α was assumed Lorentzian with a FWHM of 19.1 pm as relevant for HCN. In order to determine the wavelength step response of the system, the laser wave- length was tuned to be at approximately the HWHM point of the gas line, so that a linear wavelength response of the gas cell could be assumed for small current per-turbations. In an experimental set-up using the embodiment of FIG.2, the fiber-cou-pled gas cell had an optical path length of 16.5 cm and contained HCN in nominally100% volumetric concentration. In the simulation performed by the inventors,^^^^(^) has been assumed to follow a Lorentzian profile with a full-width-at-half-maximum (FWHM) of 19.1 pm for a chosen HCN absorption line (λON = 1549.73nm). In converting the measured absorbance to concentration, a constant gas cell temperature (25^C) and pressure (1 atm) were assumed in order to assume a fixed Lorentzian profile of the absorption line. Since the diode laser current is varied to change the wavelength, optical power also changes accordingly when toggling be-tween the two wavelengths, hence the laser reference detector of various embodi-ments disclosed herein can be used to normalize the power at all times.FIGs.3A-B illustrate that the wavelength response of the diode laser 12 of the em-bodiment of FIG.2, i.e. the transmittance T, is proportional to the wavelengthchange. A total wavelength change of 3.2 pm is produced. FIG.3A shows the wave-length response in the few tens of ms range.27 ms is measured for the transmit-tance to reach within approximately 1E-3 of its final value – corresponding to a pre-cision in wavelength of 1 pm if the target is a total wavelength change of 90 pm (in-stead of 3.2 pm). FIG.3B shows time constants relevant for the 10 µs range.As indicated by the dot 301 in FIG. 3A, the wavelength settling time to reach 1 pm offinal value (chosen for a 90 pm difference between initial and final wavelengths typi-cally required for differential laser absorption spectroscopy) is 27 ms. This will effec-tively limit the update rate of the gas sensor to the 10-100 Hz range. Therefore, asimple, periodic square current pulse approach was found ill-suited for use in noisyenvironments where fast external perturbations may interfere with the sensor sig- nals, effectively ruin the quality of measurement. Such applications will require a full gas concentration measurement acquired so fast that the ambient parameters canbe considered constant. FIG. 3B shows the wavelength response in the 10 µs range. From this graph, which corresponds to a suitable target time scale, parame- ters for a non-rectangular pulse shape can be deduced. In particular for the example pulse shape discussed above, the parameters a,b, c, ^b and ^c can be deduced. Inthe present example, they can be deduced as follows:a b c ^b ^c0.07 -0.054 -0.016 3 µs 22 µsWith the objective of toggling fast and precise between two wavelengths, in this ex-ample 90 pm spaced wavelengths, corresponding to an ON and OFF wavelength of the absorption dip of the HCN gas line, the diode laser current pulse was modifiedby adding an exponentially decaying current pulse as described above. This choicewas motivated by 1) current tuning of the wavelength being much faster than from the passive dissipation of heat to or from a heat sink (responsible for the long set- tling time), 2) the exponential shape is heuristically chosen from the observed dou- ble exponential decay of the step response function. Experimentally it was foundthat an exponential time constant, ^^, of 18.75 µs and a relative amplitude (to thesquare pulse amplitude), r, of 0.3 was optimal for fastest stabilization of the wave-length during toggling. This was confirmed by a theoretical model using the exactsame parameters as listed. However, the theoretical model predicts that this condi- tion does not occur at precisely the dip position (minimum transmission value), butrather shifted 0.4 pm. This tiny and constant difference is included in the model tomimic experimental data correctly. For practical purposes, this difference may beeliminated when calibrating the system.FIGs.4A-D illustrate an example of the ON / OFF toggling at 8 kHz in the embodi- ment of FIG.2 using the example pulse shape discussed above. FIG.4A shows theapplied current pulse 401 compared to a rectangular pulse 402. The current stepsize is approximately 43 mA and the exponential part has an amplitude of 13 mA,with a time constant, ^m=18.75 µs. FIG.4B shows the HCN absorption dip illustrat-ing the toggling range. FIG.4C shows the transmittance of a full cycle, comparingthe transmittance for a square pulse 412 with the transmittance for the modified pulse 411. FIG.4C further shows the corresponding transmittances 421 and 422 de-rived from a theoretical model. FIG.4D shows a close-up of the transition from OFFto ON demonstrating a 14 µs settling time.In particular, FIG. 4A shows a square pulse and an added exponential current. Theamplitude of the exponential in this example is 0.3 of the pulse amplitude. FIG.4Bshows the transmittance of the gas cell of the embodiment of FIG. 2 including cou-pling losses. The FWHM of the gas line is seen to be 19.1 pm. FIGs.4C-D show thetransmittance with and without the modified current pulse. As can be clearly seen, using a periodic square pulse alone, the wavelength of the diode laser never reaches the minimum transmittance, i.e. the desired ON wavelength in the 64 µs ON period. However, when applying the modified current pulse, the wavelength stabi- lizes within 14 µs as seen in FIG.4B. The model shows close agreement for the modified pulse case. However, a somewhat faster decay is theoretically predicted for the case without the exponential component, see FIG.4D. It may be speculatedthat the difference may be due to not accounting for the 150 kHz cut off frequency ofthe current driver, the limitation of using only a double exponential model or a possi- ble nonlinearity in scaling the amplitude of the wavelength response function, see FIG.3B. The inventors have found an improved update rate of the DLAS system by more than 3 orders of magnitude over a similar system without a modified pulse. The ex-ponential current term compensates for the thermal time constants.In a different field of application, where power stability rather than wavelength stabil-ity is of primary concern, the authors of a scientific paper (A. K. Hansen et al., "Effi-cient generation of 1.9  W yellow light by cascaded frequency doubling of a distrib-uted Bragg reflector tapered diode," by A. K. Hansen et al., Applied Optics 55,9270–9274 (2016)) have reported that by modifying the square-wave current pulseshape, the slow, thermally limited wavelength response of a diode laser could be overcome to increase the toggling rate between two wavelengths. In the above arti- cle, the authors were concerned with toggling between one wavelength close to and another wavelength far from the phase-matched fundamental wavelength of a sec- ond-harmonic (SH) generation crystal, thus providing a means to generate SH pulses with power modulation depths above 90%. The authors of the above articleused a tailored current waveform to compensate for the wavelength chirp throughoutan individual pulse. However, they only reported slow toggling rates of up to 50 Hzfor small wavelength steps of 37 pm. Some of the same authors then used a differ-ent mechanism, namely exploiting an abrupt diode laser mode hop, to obtain highertoggling rates and wavelength steps (see M. Christensen et al., "Deep modulation ofsecond-harmonic light by wavelength detuning of a laser diode," Applied Optics 56(8), 2250–2254 (2017)). The present inventors have surprisingly found that non- square current pulses can be employed to provide sufficiently accurate wavelength toggling at a rate sufficiently high and with a wavelength separation sufficiently highto provide accurate gas sensing and to allow sensing a spectral feature at a highrate so as to overcome unwanted perturbations from environmental changes. Theinventors have further realized that laser power changes during wavelength togglingare acceptable, as a laser reference detector may be used to compensate measure-ment results for such power changes if needed.Various embodiments of the apparatus provide a robust and versatile solution formonitoring greenhouse gas emissions. While the above examples have mainly been discussed in the context of some specific examples of target gases, it will be appre- ciated 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. Various embodiments of the apparatus may be used as a valuable tool for regula- tory compliance enforcement, environmental research, emissions quantification and / or other applications. Another interesting aspect relates to the control of the drive current so as to lock the ON wavelength onto the absorption dip of the target gas. In order to keep the ON wavelength locked at the absorption dip for extended meas- urement periods, e.g. hours, an active current control system may be implemented. The control loop may be based on the time-dependence of the measured transmit-tance during the ON pulse. To this end, the control may be based on a suitable fea-ture, e.g. an average slope, of the individual ON pulse section. The locking criterion may then be defined as a predetermined value of the selected feature, e.g. as a zero slope. A deviation of the extracted feature of the measured transmittance may then be used as an error signal for controlling the ON drive current. When the laser wavelength drifts to either side of the dip position, the slope willchange and depart from the predetermined value. When the laser wavelength is toolong, as may be simulated by adding a small bias current, the slopes at the end ofthe ON period are positive. The positive slope in the end of a cycle, can be explained as the laser wavelength being scanned past the minimum of the gas dip, thus ending at a higher transmit- tance level. Conversely, slight wavelength drift to the shorter side, results in a nega- tive slope, corresponding to not reaching the minimum value.The control scheme may be illustrated by the example shown in FIGs. 5A-B, whichshow the ON transmittance data for different mean drive currents of the embodimentof FIG.2. FIG.5A shows experimentally obtained transmittance curves for differentvalues of the drive current. In this example, I=224.6 mA (curve 511), I=224.8 mA(curve 512), I=225.0 mA (curve 513), I=225.2 mA (curve 514), and I=225.4 mA(curve 515). FIG.5B illustrates corresponding theoretical curves. The time period il-lustrated by band 501 indicates the last part of the pulse where the slopes (and signof the slopes) have been computed and can be used as error signal for the currentcontrol. In this example, the zero slope is obtained at I=225.0 mA. Smaller drive cur-rents result in a negative slope while larger driver currents result in a positive slope. The error signal based on the slope is extraordinarily sensitive to drift. A 0.1 mAchange corresponds to a mere 300 fm shift of the center wavelength, easily de-tected by the proposed method. FIG.6 illustrates the slope of the ON pulses versusmean current, Imean. In the example of FIG.2, at 225.0 mA, the slope is nominallyzero. In the example of FIG.6, the system was operating for 20 hours with the cur-rent control loop. In an actual long-term measurement campaign, the inventors used12 out of the 16 data points (from 20 µs-64 µs) to calculate the average slope. FIG.6 shows the experimentally measured slope for five current settings used in FIGs5A-B. The horizontal lines represent the standard deviation error bar for the slope ateach current – showing the high sensitivity of slope to change in current.FIGs.7A-C illustrate results of a long-term gas sensing test with feed-back loop us-ing the embodiment of FIG.2. The reference gas concentration was 100%, so any deviation represents drift (noise) in the set-up. FIG.7A shows experimentally meas-ured concentrations at respective laboratory temperatures. A linear correlation existsas indicated by the linear fit 701. FIG.7B illustrates measured concentrations 702 versus time at 20 Hz update rate over 20 hours. Removal of the linear correlation with temperature is shown as temperature compensated curve 703. As can be ob-served, the system is highly stable over long periods of time. FIG.7C shows the Al-lan Deviation, showing the rms value (^) in measured absorbance as function ofmeasurement time. At 25 ms integration time the rms value is a mere 5E-6 and donot exceed 1.5E-4 at any integration time.Since the setup was not temperature stabilized, the correlation of the gas concentra-tion and ambient temperature was also measured, see FIG. 7A. Using a linear fit(curve 701), the temperature dependency can be eliminated from measured data.Removing the linear temperature dependency, using the linear fit from FIG.4A, thecurve 703 in FIG.7B is obtained. At 20 Hz update rate, the variation in concentrationover the 20 hours was <0.2% peak-to peak over the full campaign. As can be ob-served from FIG. 7C, at 1 s update rate, the sensitivity is 2.7E-5 (or 0.0045 % of themean absorbance) at 1 s and reaches a minimum of 5E-6 (or 0.001 % of the meanabsorbance) at 0.1 s at mean absorbance of 0.6021.FIG.8 schematically illustrates another embodiment of an absorption spectroscopyapparatus. The apparatus of FIG.8 is similar to the apparatus of FIG.1 and com-prises a tunable diode laser 12, a control circuit 13, a measurement photodetector114, a laser reference detector 116 and an optical system 113, all as described in connection with FIG.1. The embodiment of FIG.8 differs from the embodiment of FIG.1 in that the optical system 113 further includes a reference gas cell 1135 posi-tioned in the target gas detection path such that the return light passes through thereference gas cell 1135 after the reflection of the return light by the beam splitter1132 towards the measurement photodetector 114, resulting in a single pass of thelight through the reference gas cell 1135. The insertion of the reference gas cell pro-vides an increased signal strength and results in stable small signal detection.The reference gas cell 1135 may contain known concentrations of the target gas(es)to be measured. The reference gas cell 1135 may e.g. be a gas cell from Wave-length References, Inc., or another suitable reference gas cell. In this example, thereference gas cell has end facets that are wedged and treated with antireflection (AR) coating to avoid stray light. In one example, the reference gas cell 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 in- clude another type of reference gas cell or no reference gas cell at all. The refer- ence gas cell may be filled with other reference gases, and / or at other concentra- tions. The provision of a reference gas cell in the transmission and / or receiving beam path causes the probe beam to be pre-modulated with the absorption feature of the refer- ence gas, in particular with the target gas. This improves the performance of the sig- nal processing of the resulting detector signal, 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. Theresulting extended limit of detection allows for shorter path lengths and improves thedynamic range of the sensor, augmenting the efficacy of the sensor across diverse application scenarios.FIG. 9 schematically illustrates yet another embodiment of an absorption spectros-copy apparatus. The apparatus of FIG.9 is similar to the apparatus of FIG.8 andcomprises a tunable diode laser 12, a control circuit 13, a measurement photodetec-tor 114, a laser reference detector 116 and an optical system 113, all as described in connection with FIG.1. The embodiment of FIG.9 further includes a reference gas cell 1135, also as described in connection with FIG.8, except that the referencegas cell 1135 of the embodiment of FIG.9 is positioned in the combined transmis-sion and receiving path between the beam splitter 1132 and the telescope 1134, re- sulting in a double pass of the light through the reference gas cell. The insertion ofthe gas cell provides an increased signal strength for stable small signal detection,as was described above.FIG. 10 schematically illustrates yet another embodiment of an absorption spectros-copy apparatus. The apparatus of FIG.10 is similar to the apparatus of FIG.1 andcomprises a tunable diode laser 12, a control circuit 13, a measurement photodetec-tor 114, a laser reference detector 116 and an optical system 113, all as described in connection with FIG.1. The embodiment of FIG.10 differs from the embodiment of FIG.1 in that the optical system further includes a reference gas cell 1135 posi-tioned in a reference arm of the apparatus. To this end, the optical system 113 maycomprise another beam splitter 1138 for directing a portion of the reference beam, which is reflected off the beam splitter 1132, via the reference gas cell 1135 to an- other photodetector 117, which will also be referred to as a wavelength reference detector.The reference gas cell 1135 may contain known concentrations of the target gas(es)to be measured and be of the type described in connection with FIG.8, or other-wise.Accordingly, the embodiment of FIG. 10 is an example of a gas sensing apparatusbased on the wavelength toggling technique described herein and involving a dedi-cated reference arm for gas detection. The reflected beam from the beam splitter1132 is again split into two parts by the additional beam splitter 1138. The laser ref-erence detector 116 detects the transmitted light from the additional beam splitter1138, which contains information about the laser power. The wavelength referencedetector 117 detects the reflected light from the additional beam splitter 1138, whichcontains information about the laser power and the wavelength-dependent intensity of a wavelength-sensitive element, in particular the wavelength-dependent transmit-tance of the reference gas cell 1135. The ratio of these signals effectively provides awavelength-to-intensity measure and enables monitoring of the ON and OFF wave-length toggling response for the target gas. The signals from detectors 116 and 117 may thus be used to control the drive current of the diode laser 12 so as to lock in on the absorption wavelength of the target gas, e.g. as described with reference to FIGs.5A-B above, or otherwise.In the present embodiment, the reference gas cell 1135 is operable as a wave-length-dependent radiation modifying element configured to receive the reflectedlight from the additional beam splitter 1138, and to cause a wavelength-dependentmodification of the received light. It will be appreciated that, in an alternative embod- iment, the reference gas cell may be replaced by another type of wavelength-de- pendent radiation modifying element that is configured to receive a wavelength ref- erence light portion and to cause a wavelength-dependent modification of the wave- length reference light portion, e.g. as described below with reference to FIG.11 inthe context of a more general embodiments of a laser apparatus. Generally, exam-ples of a wavelength-dependent radiation modifying element include an optical ”eta-lon” designed with a transmission peak tuned to the desired wavelength, or an opti-cal band pass filter designed for the relevant center wavelength. These band pass filters are typically based on a (glass) substrate with a multilayer dielectric coating. It will be appreciated that various modifications may be made to the various embodi-ments. For example, additional temperature and / or pressure sensors may be de-ployed to improve the gas sensing precision by accounting for the small temperature and pressure related changes in the atmospheric gas spectra. The fast wavelength toggling and accurate control of the laser drive current to match a target wavelength may be applied to other types of laser systems than DAS spec-troscopy. An example of a more general setup is illustrated in FIG. 11, which sche-matically illustrates an embodiment of a laser apparatus.The apparatus of FIG.11 is similar to the apparatus of FIG.10 in that it comprises awavelength-tunable diode laser 12, a control circuit 13, a wavelength reference de-tector 117 and a laser reference detector 116, all as described in connection withFIG.10. The embodiment of FIG.11 differs from the embodiment of FIG.10 in that the apparatus of FIG.11 does not necessarily provide the emitted laser radiation asa probe beam for gas spectroscopy, but provides an output beam 3 to a down-stream beam recipient 4. To this end, the optical system 113 of this embodimentmay comprise two beam splitters 1132 and 1138 as described in connection withFIG.10, a collimator 1131 and focussing lenses 1133 and 1139. It will be appreci- ated that other embodiments may include other types of optical systems, which may depend on the type of beam recipient 4. In this embodiment, the reflected beam from the first beam splitter 1132 is again split into two parts by a second beam split-ter 1138. The laser reference detector 116 detects the transmitted light from the sec-ond beam splitter 1138, which contains information about the laser power of the di-ode laser 12. The wavelength reference detector 117 detects the reflected light fromthe second beam splitter 1138 after it has interacted with a wavelength-sensitive el-ement 1135. The wavelength-sensitive element 1135 is configured to affect the in- tensity of the light in a wavelength-dependent manner. Accordingly, the light de- tected by the wavelength reference detector 117 includes information about the la- ser power and about the wavelength-dependent intensity modification by the wave-length-sensitive element 1135. The ratio of these signals effectively provides awavelength-to-intensity measure and enables monitoring of the wavelength-togglingresponse as described above. The transmitted light from the first beam splitter 1132represents the laser wavelength toggling signal and can be applied to applications 4 requiring wavelength toggling at high precision. Examples of wavelength-sensitive elements include a reference gas cell, an optical ”etalon” designed with a transmis-sion peak tuned to the desired wavelength, or an optical band pass filter designedfor the relevant center wavelength. Examples of applications of embodiments of the laser apparatus disclosed herein in-clude open- or closed-path DLAS, injection seeding of OPOs and pulsed lasers,wavelength-modulation based beam steering or as a stable reference for stand-offdetection (open path). Accordingly, it will be appreciated that the apparatus of FIG.11 may include additional components and / or the apparatus may otherwise be con-figured depending on the intended application. For example, when used as part ofan absorption spectroscopy apparatus, such as a TDLAS apparatus, the apparatusmay include a measurement photodetector. Alternatively or additionally, the controlcircuit may be configured to create a modulated drive current. For example, themodulated drive current may vary, e.g. alternate, between at least a first and a sec-ond drive current, the first drive current configured to cause the wavelength-tunable laser source to emit laser radiation at a first wavelength and the second drive cur- rent configured to cause the wavelength-tunable laser source to emit laser radiationat a second wavelength, different from the first wavelength. The modulated drivecurrent may define a sequence of pulses, each pulse having a non-rectangularpulse shape. The first wavelength may be an absorption wavelength of a target gasto be detected, and the second wavelength may a reference wavelength displacedfrom said absorption wavelength, all as described in connection with the previous embodiments, or otherwise.

Claims

CLAIMS1. An absorption spectroscopy apparatus for measuring one or more gases in an at-mosphere, wherein the apparatus comprises:- a wavelength-tunable laser source,- a control circuit configured to provide an electrical drive current to the wavelength-tunable laser source;- wherein the control circuit is configured to create the electrical drive current as amodulated drive current, wherein the modulated drive current alternates between atleast a first and a second drive current, the first drive current configured to cause the wavelength-tunable laser source to emit laser radiation at a first wavelength and the second drive current configured to cause the wavelength-tunable laser source to emit laser radiation at a second wavelength, different from the first wavelength, andwherein the modulated drive current defines a sequence of pulses, each pulse hav-ing a non-rectangular pulse shape.

2. The apparatus according to claim 1, wherein the modulated drive current definestransitions between a current one of the first and second drive currents and a subse-quent one of the first and second drive currents, wherein each transition defines aninitial transition between the current one of the first and second drive currents andan overshoot drive current, the overshoot drive current being further displaced from the current drive current than the subsequent drive current is displaced from the cur-rent drive current, and wherein each transition further defines at least one relaxationportion from the overshoot drive current towards the subsequent one of the first and second drive currents.

3. The apparatus according to claim 1 or 2, wherein the non-rectangular pulseshape is selected to compensate for a delayed temperature change of the wave- length-tunable laser source responsive to a changing drive current.

4. The apparatus according to any one of claims 1 through 3, wherein the wave-length-tunable laser source defines a transfer function between the time-dependentelectrical current driving the wavelength-tunable laser source and a time-dependent intensity of the laser radiation emitted by the wavelength-tunable laser source whendriven by the time-dependent electrical current, and wherein the non-rectangularpulse shape is selected to be defined by an inverse of said transfer function applied to a rectangular pulse shape.

5. The apparatus according to any one of the preceding claims, wherein the controlcircuit is configured to alternate the drive current at a pulse frequency of at least 8kHz, such as at least 10 kHz.

6. The apparatus according to any one of the preceding claims, wherein the appa-ratus is a direct absorption spectroscopy apparatus for measuring one or more gases in an atmosphere, wherein the apparatus further comprises:- an optical system for transmitting the emitted laser radiation as a probe laser beamand for collecting return light, in particular emitted laser radiation reflected by a re- mote surface,- a measurement photodetector for detecting the collected return light.

7. The apparatus according to claim 6, wherein the direct absorption spectroscopyapparatus is a differential absorption lidar apparatus,8. The apparatus according to claim 6 or 7, wherein the control circuit is further con-figured to:- receive detector signals from the measurement photodetector, the detector signalsbeing indicative of a time-dependent intensity of the collected return light when the tunable diode laser is alternatingly driven at the first and second drive currents, and- adjust the first drive current and / or the second drive current responsive to the time-dependent intensity, in particular responsive to a pulse shape of the time dependent intensity.

9. The apparatus according to claim 8, wherein the control circuit is configured toadjust the first drive current and / or the second drive current responsive to a pulse shape of the time dependent intensity.

10. The apparatus according to any one of claims 6 through 9, wherein the opticalsystem comprises a telescope for transmitting the emitted laser radiation as a probelaser beam and for collecting return light, in particular emitted laser radiation re- flected by a remote surface.

11. The apparatus according to claim 10, wherein the return light includes emittedlaser radiation reflected by a remote surface.

12. The apparatus according to any one of claims 6 through 11, comprising a refer-ence photodetector configured to receive a reference portion of the emitted laser ra- diation.

13. The apparatus according to claim 12, comprising a reference gas cell positionedin the beam path between the wavelength-tunable laser source and the reference photodetector.

14. The apparatus according to any one of claims 6 through 13, comprising two ormore wavelength-tunable laser sources, tunable at respective target wavelengths,and one or more optical switches configured to selectively select emitted laser radia-tion of one of the two or more wavelength-tunable laser sources.

15. The apparatus according to any one of claims 6 through 14, wherein all opticalcomponents of the apparatus in the beam path of the emitted laser radiation and of the return light are polarisation-insensitive.

16. The apparatus according to any one of the preceding claims, wherein the firstwavelength is an absorption wavelength of a target gas to be detected and wherein the second wavelength is a reference wavelength displaced from said absorption wavelength.

17. The apparatus according to claim 16, wherein the target gas is an atmosphericgas.

18. The apparatus according to claim 16 or 17, wherein the target gas is a green-house gas.

19. The apparatus according to any one of claims 16 through 18, wherein the targetgas is an atmospheric gas selected from the group of gases consisting of: N2O, NH3,CO2, CH4, and C2H2.

20. The apparatus according to any one of the preceding claims, comprising:- an optical system for providing at least a wavelength reference portion of the emit-ted laser radiation,- a wavelength-dependent radiation modifying element configured to receive thewavelength reference portion and to cause a wavelength-dependent modification of the wavelength reference portion,- a wavelength reference photodetector for detecting the modified reference wave-length portion.

21. The apparatus according to any one of the preceding claims, comprising ameasurement photodetector for detecting light after the light has interacted with theone or more gases in an atmosphere.

22. A laser apparatus, comprising:- a wavelength-tunable laser source,- a control circuit configured to provide an electrical drive current to the wavelength-tunable laser source, wherein the control circuit is configured to create the electricaldrive current as a modulated drive current, wherein the modulated drive current al-ternates between at least a first and a second drive current, the first drive current configured to cause the wavelength-tunable laser source to emit laser radiation at a first wavelength and the second drive current configured to cause the wavelength- tunable laser source to emit laser radiation at a second wavelength, different from the first wavelength,- an optical system for transmitting at least an output portion of the emitted laser ra-diation as an output laser beam and for providing at least a wavelength referenceportion of the emitted laser radiation,- a wavelength-dependent radiation modifying element configured to receive thewavelength reference portion and to cause a wavelength-dependent modification of the wavelength reference portion,- a wavelength reference photodetector for detecting the modified reference wave-length portion.

23. The laser apparatus according to claim 22, wherein the wavelength-tunable la-ser source is a wavelength-tunable diode laser.

24. The laser apparatus according to claim 22 or 23, wherein the control circuit isconfigured to create the electrical drive current as a pulsed drive current.

25. The apparatus according to any one of claims 21 through 24, wherein the controlcircuit is further configured to:- receive detector signals from the wavelength reference photodetector, and- adjust the first drive current and / or the second drive current responsive to the re-ceived detector signals.

26. The apparatus according to claim 25, wherein the detector signals are indicativeof a time-dependent intensity of the modified wavelength reference portion when thewavelength-tunable laser source is alternatingly driven at the first and second drive currents.

27. The apparatus according to claim 26, wherein the control circuit is configured toderive a derivative with respect to time of the time-dependent intensity of the modi-fied wavelength reference portion when the wavelength-tunable laser source isdriven at the first or second drive current the received detector signals, and to adjustthe first drive current and / or the second drive current responsive to the derived de-rivative.

Images