Patent Description:
Measuring purity of a gas is essential for a manufacturer of gas. A purity measurement allows gas to be sold at a specified quality. Any gas produced that does not meet the quality requirements set by a manufacturer is likely to be wasted, for example flared. Therefore, to avoid waste, it is important that the purity measurement is reliable.

Continuous emission monitoring instruments are increasingly needed to monitor industrial pollution output in various industrial sites, for example at power plants, process industry factories and commercial shipping facilities. The need arises from efficiency improvements, health and safety considerations and legislative requirements. Legislative requirements often cover measurements on a range of multiple emitted compounds, for example: sulphur dioxide, nitrogen oxides, carbon monoxide, carbon dioxide, methane, water and oxygen.

Known gas analysis systems are sensitive to single compounds or a small number of compounds. To cover multiple compounds using known systems it may be necessary to install several different continuous emission monitoring instruments, which can be inefficient, complicated and take up significant amount of space.

Known gas analysis systems also include one or more optical cells, for example one or more Herriott cells, for containing a gas sample to be analysed and through which a laser beam passes in order to interact with the gas sample. Arrangement of optical cells and other optical components affects the geometry and size of a gas analysis system, and in order to make a system compact, for example to be able to contain the system within a compact, transportable housing, it can be important to provide a suitable arrangement of the optical cell and other optical components.

Known gas analysis systems may be at risk to potential cross-interference effects arising from atmospheric air. This is a problem when typical impurities being measured include compounds present in air, particularly when attempting to measure a quantity of these impurities below levels found in the atmosphere. Impurities that can be found in the atmosphere include, but are not limited to methane, water and carbon dioxide. Purge systems using dry nitrogen may be used to alter the atmosphere in the volume surrounding the gas detector. Alternatively, chemical scrubbing can also be used to address this problem, for example a carbon dioxide and/or oxygen scrubber. Both can be difficult to engineer and rely on the presence of utilities such as dry nitrogen or instrument air.

Other approaches also exist. These include using analysis software that takes into account cross-interference effects. However, backgrounds can be variable thus limiting this approach. In addition, if light from a laser experiences significant interference it may be degraded before reaching an optical cell and produce a weak and unreliable measurement.

<CIT> describes a gas sensor using the principle of infra-red (IR) absorption. <CIT> describes a quarternary tunable Mid-IR laser to measure both particles and gas at the same time. The measurement is done within an area of which the gas of interest will absorb the Mid-IR radiation. By widely tuning the emission wavelength of the laser, several wavelengths can be measured in order to accurately find both gas composition and particle density with one laser based sensor.

<NPL>) describes conducting tunable diode laser absorption spectroscopy (TDLAS) of methane at <NUM> using a vertical-cavity surface-emitting laser (VCSEL). <CIT> describes a device for combining two or more separate components of an optical analysis system, to use common entrance and exit apertures for optical measurements across a measurement space such as a stack, combustion chamber, duct or pipeline, in such way that the optical paths from the respective light sources to detectors are substantially the same, enabling multiple optical measurements over a single optical path or closely aligned optical paths with equivalent ambient conditions such as temperature and pressure distribution and background substance concentrations. The device and a set of interconnectable devices forming a modular system are useful, for example, in absorption spectroscopy, such as for measuring the amount fraction of the chemical constituents of a fluid in a measurement volume.

<CIT> describes a gas spectroscopy device comprising a laser diode/lens assembly, wherein a planar reference surface of a base portion of the laser diode/lens assembly is pressure fitted into a cylindrical mounting opening first end of a laser mounting member until the laser diode housing compresses a rubber o-ring and a ledge reference surface of base portion is pressed against and therefore spatially referenced to an inside ledge surface of the laser mounting opening. A second end of the laser mounting member is disposed in a first end of a joining tube member, and a first end of a sample cell tube member is disposed in a second end of the joining tube member so that an axial hole in the sample cell tube member is aligned with an axial hole in the laser mounting member.

<CIT> describes a gas analyser including a first housing with a laser and a detector, wherein by means of a flange the first housing is captively attached to a counterflange of a gas probe.

The present invention is defined in claim <NUM> and provides a laser detection system.

The sample chamber may comprise an optical cell. The optical cell may be configured to perform the receiving and containing of the sample gas. The material(s) may comprise any suitable compound(s), for example any suitable gaseous compounds.

The close-coupling arrangement may be such that in operation the laser beams are substantially not absorbed by gas present between the laser housing and the at least one window. For example, the intensity of the laser beams may be reduced by less than <NUM>% optionally less than <NUM>% by passage between the laser housing and the at least one window. The separation between the laser housing and the at least one window may be less than <NUM>%, optionally less than <NUM>% of a path length of each laser beam within the housing.

The system may further comprise a second optical interface having at least one window to the sample chamber that is at least partially transparent to light output from the sample chamber, wherein the detector apparatus may be arranged to be in a close-coupling arrangement with the second interface such that, in use, the laser beams may be substantially unmodified by passage from an output of the sample chamber to the at least one window of the second optical interface.

The sample chamber may be sealed and/or the system may comprise means for controlling the pressure and/or gaseous content of the sample chamber.

Each laser housing may be sealed and/or may be under vacuum or may contain a selected gas and/or be at a selected pressure.

The detector apparatus may comprise a housing and at least one of: the detector apparatus housing may be sealed and/or may be under vacuum or may contain a selected gas and/or is at a selected pressure.

The close coupling arrangement(s) may be such that at least one of: there is a separation of less than <NUM>, optionally less than <NUM>, optionally less than <NUM> between at least one output aperture of the laser housing and the at least one window of the first optical interface; there is a separation of less than <NUM>, optionally less than <NUM>, optionally less than <NUM> between the at least one window of the second optical interface and the detector apparatus.

The at least one aperture of the at least one laser housing may comprise at least one window through which the laser beams pass in operation, and the at least one window of the at least one laser housing is in direct contact with the at least one window of the first optical interface. The at least one window of the second optical interface may be in direct contact with the detector apparatus.

The at least one laser housing may comprise an exit aperture through which the respective laser beam exits the laser, and in the close coupling arrangement, each of the laser exit apertures may be in direct contact with, or separated by less than <NUM> from, the window or a respective one of the windows of the first optical interface.

The detector apparatus may comprise a housing having an entry aperture for passage of the light output from the sample chamber and in the close coupling arrangement, the detector entry aperture may be in direct contact with, or separated by less than <NUM> from, the window or a respective one of the windows of the second optical interface.

The system may further comprise at least one coupling means configured to hold the at least one laser housing in the close coupling arrangement.

The at least one window of the first optical interface may comprise one or more flat or wedged optical windows, wherein each window may be associated with a respective at least one of the lasers.

The system may comprise at least one collimating lens associated with the window or at least one of the windows of the first optical interface.

The system may further comprise at least one directing device inside the sample chamber configured to direct laser beams passing through the one or more windows to an optical cell in the sample chamber, wherein the at least one directing device may be configured to direct the laser beams along a common optical path to the optical cell.

The at least one directing device inside the sample chamber may comprise a plurality of optical components arranged such that, for each laser beam a respective at least one of the optical components is arranged to direct said laser beam along the common optical path.

The plurality of optical components may be arranged substantially in a straight line. At least one of the optical components may comprise a flat or non-wedged optical component.

Each of the lasers and associated windows may be arranged such that in operation each of the lasers transmits its laser beam to its corresponding at least one of the optical components in a direction substantially orthogonal to said straight line.

The plurality of optical components may comprise at least one partially reflective mirror and/or at least one dichroic mirror.

The optical components may be arranged in series and may be configured such that in operation each optical component directs a laser beam from its associated laser to join said common optical path, and directs or allows passage of laser beam(s) from preceding optical components in the series along said common optical path.

Each of the optical components may be at least partially reflective and at least partially transmissive.

The at least one directing device may comprise steering optics between the last of said plurality of optical components and the optical cell and configured to direct the laser beams into the optical cell.

The detector apparatus may one or more detectors, each detector being configured to detect radiation of a respective wavelength or range of wavelengths.

The at least one window of the second optical interface may comprise one or more flat or non-wedged optical windows, wherein each window is associated with a respective one of the detectors.

The system may comprise further steering optics inside the sample chamber between the optical cell and the second optical interface and configured to direct light from the optical cell to the second optical interface.

Each of the windows and/or plurality of optical components may have a thickness in a range <NUM> to <NUM>.

The system may further comprise a controller configured to control operation of the one or more lasers such that the laser beams are pulsed laser beams interleaved in time.

The controller may be configured to synchronise operation of the detection apparatus and the lasers, thereby to obtain a series of detection signals, each detection signal being associated with a respective one of the lasers.

The controller may be configured to control operation of the lasers such that each laser beams is pulsed at a rate in a range <NUM> to <NUM>, optionally in a range <NUM> to <NUM>, and/or the controller may be configured to control the lasers such that each laser beams is pulsed with pulse lengths in a range <NUM> ns to <NUM>,<NUM> ns.

The sample gas may comprise at least one of ethylene, H<NUM>, N<NUM>, or natural gas.

The plurality of materials may comprise at least one of: CO<NUM>, CO, H<NUM>O, CH<NUM> and NH<NUM>.

The plurality of materials may comprise at least one of: H<NUM>O, MeOH, NH<NUM>, C<NUM>H<NUM>, O<NUM>, HF, HCl, H<NUM>S, CO and CO<NUM>.

Each of the plurality of lasers may be configured to produce infrared laser radiation.

Each of the lasers may be configured to produce a laser beam of a respective different wavelength or range of wavelengths and/or the or each detector apparatus is configured to detect radiation of a respective different wavelength or range of wavelengths.

At least one of the wavelengths or ranges of wavelengths may be selected from the following ranges: <NUM> to <NUM>; <NUM> to <NUM> or <NUM> to <NUM>; <NUM> to <NUM>; <NUM> to <NUM>; and <NUM> to <NUM>. The sample gas may comprise H<NUM> or N<NUM>.

At least one of the wavelengths or ranges of wavelengths may be selected from the following ranges: <NUM> to <NUM>; <NUM> to <NUM>; <NUM> to <NUM>; <NUM> to <NUM>; <NUM> to <NUM>; and <NUM> to <NUM>. The sample gas may comprise ethylene.

At least one of the wavelengths or ranges of wavelengths may be selected from the following ranges: <NUM> to <NUM>; <NUM> to <NUM>; and <NUM> to <NUM>. The sample gas may comprise natural gas or a gas from a flue.

At least one of the wavelengths or ranges of wavelengths may be selected from the following range: <NUM> to <NUM>. The sample gas may comprise one of H<NUM> or N<NUM>, ethylene, natural gas, or gas from a flue. The at least one material may comprise O<NUM>. The detector apparatus may be arranged on the opposite side of the optical cell to the one or more lasers and the at least one directing device.

The system may further comprise a gas supply arrangement configured to supply a sample gas to the sample chamber.

The optical cell may comprise a Herriott cell.

The optical cell may comprise an astigmatic Herriott cell.

The system may comprise a continuous emission monitoring system (CEMs) or an H<NUM> purity measurement system or an N<NUM> purity measurement or a natural gas purity measurement system or an ethylene purity measurement system. The CEMs may be configured to measure gas from a flue, for example a flue of a power plant, a process industry plant, or a shipping facility.

In a further aspect, there is provided a method of detecting the presence, absence or amount of at least one material, optionally a plurality of materials, in a sample gas according to claim <NUM>.

The method may comprise using radiation of wavelengths or wavelength ranges as claimed or described herein.

Various aspects of the invention will now be described by way of example only, and with reference to the accompanying drawings, of which:.

<FIG> is a schematic representation of a laser spectroscopy system for analysing gas collected in an optical cell <NUM>. The system comprises a laser apparatus <NUM>, a controller <NUM>, a sample apparatus <NUM> and a detector apparatus <NUM>. The controller <NUM> is electronically, electrically or otherwise connected to the laser apparatus <NUM> and the sample apparatus <NUM>. The laser apparatus <NUM> is optically coupled to the sample apparatus <NUM> and the sample apparatus <NUM> is optically coupled to the detector apparatus <NUM>. The laser apparatus <NUM> comprises one or more lasers <NUM>. Each of the one or more lasers <NUM> may be comprised in a laser module having a respective housing and aperture that may include a window through which the laser beam(s) pass in operation.

The sample apparatus <NUM> includes the optical cell <NUM>. The sample apparatus <NUM> also includes a first optical interface <NUM>, a second optical interface <NUM> and a sample chamber <NUM>. The sample apparatus <NUM> also includes at least one directing device in the form of a plurality of optical components <NUM> arranged to direct laser beams from the one or more lasers <NUM> along a common optical path into the optical cell <NUM>, as described in more detail below in relation to <FIG>.

The optical cell <NUM> is contained inside the sample chamber <NUM>. The at least one directing device and the steering components <NUM> are also contained inside the sample chamber <NUM>. The sample chamber <NUM> is optically coupled to the laser apparatus <NUM> by the first optical interface <NUM>. The sample chamber <NUM> is optically coupled to the detector apparatus <NUM> by the second optical interface <NUM>.

The detector apparatus <NUM> comprises a plurality of detectors. The detectors are configured to detect light from the optical cell <NUM>. The light may be infra-red or visible light or light of any other suitable wavelength or from any suitable part of the electromagnetic spectrum. The controller <NUM> comprises a control module <NUM> and a signal processor <NUM>. The control module <NUM> is configured to control operation of the lasers <NUM> and the signal processor <NUM> is configured to process signals obtained from the detector apparatus <NUM>. The controller <NUM> may be, for example, in the form of a suitably programmed PC or other computer, or may comprise dedicated circuitry or other hardware, for example one or more ASICs or FPGAs or any suitable mixture of hardware and software. The control module <NUM> and processing module may be provided as separate, distinct components in some embodiments, for example separate processing resources, rather than being provided within the same controller component as shown in <FIG>.

The optical cell <NUM> has an optical entrance aperture and an optical exit aperture. The optical cell <NUM> may, for example, be a Herriott cell or any other suitable type of optical cell. The optical cell <NUM> is set inside the sample chamber <NUM>. The sample chamber <NUM> of <FIG> defines a first enclosed volume into which a sample of gas can be introduced and collected. The optical cell <NUM> defines a second open volume smaller than the first enclosed volume and inside the first enclosed volume of the sample chamber <NUM>. A sample of gas introduced and collected into the first enclosed volume is therefore introduced and collected into the second open volume defined by the optical cell <NUM>.

The gas can comprise one or more different compounds or other materials of interest. An indication of the presence of these compounds in the gas collected in the optical cell <NUM> can be determined by passing light from the lasers <NUM> through the optical cell <NUM>. If the light is in a wavelength range that corresponds to the absorption spectrum or absorption lines of the compound of interest, then any absorption of light as it passes through the cell may be due to the presence of the compound of interest in the sample. The level of absorption, once determined, can be used to determine a physical property of the compound of interest in the sample, for example, concentration. As different compounds have absorption spectra at different wavelength, different wavelengths of light are provided to the optical cell <NUM>.

The one or more lasers <NUM> are directly coupled to the first optical interface <NUM> in a close-coupling arrangement. Likewise, the detectors of the detector apparatus <NUM> are directly coupled to the second optical interface <NUM> in a close-coupling arrangement. In some embodiments, the close-coupling arrangement may be such that the optical components are separated by a distance less than <NUM>. In some embodiments, in the close-coupled arrangement the optical components may be in contact, touching and/or butt-coupled.

Direct coupling of the lasers and detectors to the optical interface of the sample chamber offers the advantage that the optical path between laser and detector, traversed by a laser beam, is contained substantially, within the sample chamber. The path length of laser light outside the optical cell <NUM> is less than <NUM> preferably less than <NUM>. This is to be compared to typical prior art arrangements, where the path length outside the cell can be <NUM> to <NUM>. This has the effect that the laser beam is not exposed to atmospheric air outside the sample chamber and the compounds contained therein. In other words, the laser beam may be exposed to only the sample gas contained in the sample chamber.

<FIG> is a more detailed schematic view of the sample apparatus <NUM>. <FIG> shows a representative laser <NUM> of the laser apparatus <NUM> having a housing and a representative detector <NUM> of the detector apparatus <NUM> having a housing. The housing of the laser <NUM> includes an aperture that has a window through which a laser beam from the laser passes in operation. <FIG> also shows the sample chamber <NUM>. As discussed with reference to <FIG>, the sample chamber <NUM> is coupled to the laser apparatus <NUM> via the first optical interface <NUM> and to the detector apparatus <NUM> via the second optical interface <NUM>. The first optical interface <NUM> is represented by a first window <NUM> and a lens <NUM> in <FIG>. The second optical interface <NUM> is represented by a second window <NUM>. The first window <NUM> and the second window <NUM> are flat or non-wedge shaped windows. The lens <NUM> is a collimating lens and is positioned inside the sample chamber <NUM> at the first window <NUM>.

In <FIG>, the first window <NUM> and second window <NUM> are both positioned on the same face of the sample chamber <NUM>. However, it is noted that <FIG> is a schematic diagram only and the positions of the first and second window may be different. In particular, if more than one laser or more than one detector are coupled to the sample chamber, or if a different optical cell type is used, then the layout of the sample apparatus <NUM> may be different.

<FIG> also shows the optical cell <NUM> inside the sample chamber <NUM> comprising a first reflecting element <NUM> and a second reflecting element <NUM>. In particular, the optical cell <NUM> may be a Herriott cell comprising a first reflecting element <NUM> and a second reflecting element <NUM>. The first reflecting element <NUM> is positioned closest to the first window <NUM>. The first and second reflecting elements have an entrance aperture <NUM> and an exit aperture <NUM>. In <FIG>, the entrance and exit apertures are both positioned in the first reflecting element <NUM>. Alternatively, the entrance aperture <NUM> may be positioned in the first reflecting element <NUM> and the exit aperture <NUM> may be positioned in the second reflecting element <NUM>. Advantageously, this leads to a more compact system design.

<FIG> also shows a first set of optical steering components, inside the sample chamber <NUM>, for directing light introduced into the sample chamber <NUM> via the window <NUM> and lens <NUM> to the entrance aperture <NUM> of the optical cell <NUM>. The first set of optical steering components <NUM> includes a first and second steering mirror. The first and second steering mirrors are configured to act together to redirect, position and adjust the incident angle of a laser beam. <FIG> also shows a second set of optical steering components <NUM> for directing light from the exit aperture <NUM> to the second window <NUM>. The second set of optical steering components <NUM> includes a third and fourth steering mirror. The third and fourth steering mirrors are configured to act together to redirect, position and adjust the incident angle of a laser beam.

<FIG> is a more detailed schematic view of a part of the laser apparatus <NUM> of the laser spectroscopy system shown in <FIG>. <FIG> shows how a plurality of lasers is incorporated into the laser spectroscopy system. <FIG> show the lasers <NUM>, the sample chamber <NUM>, the first optical interface <NUM> and the optical components <NUM> of <FIG> in more detail.

The optical components <NUM> comprise a set of partially reflective mirrors <NUM> and a dichroic mirror <NUM>. The dichroic mirror is included as this embodiment may be used in relation to the measurement of O<NUM>. In other embodiments no dichroic mirror is used, as discussed further below. The partially reflective mirrors <NUM> comprise a first mirror <NUM>, a second mirror <NUM>, a third mirror <NUM>, a fourth mirror <NUM> and a fifth mirror <NUM>. The lasers <NUM> comprise a first laser <NUM>, a second laser <NUM>, a third laser <NUM>, a fourth laser <NUM>, a fifth laser <NUM> and a sixth laser <NUM>. The first optical interface <NUM> comprises a first window <NUM>, a second window <NUM>, a third window <NUM>, a fourth window <NUM>, a fifth window <NUM> and a sixth window <NUM>. The first optical interface <NUM> also comprise a first lens <NUM> associated with the first window <NUM>, a second lens <NUM> associated with the second window <NUM>, a third lens <NUM> associated with the third window <NUM>, a fourth lens <NUM> associated with the fourth window <NUM>, a fifth lens <NUM> associated with the fifth window <NUM> and a sixth lens <NUM> associated with the sixth window <NUM>. Each of the lasers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> has a corresponding window <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>. Each of the lasers <NUM> is directly coupled to their corresponding window. Light from each of the lasers is input into the sample chamber <NUM> via their corresponding window and lens.

The partially reflective mirrors <NUM> and the dichroic mirror <NUM> are configured to direct laser beams from the lasers <NUM> along a common optical path to point <NUM>. The additional steering optical components to steer a combined laser beam <NUM> from point <NUM> along the common optical path to the optical cell <NUM> are included in the system but not shown in <FIG>. The additional steering optical components are shown schematically in <FIG>. Each of the lasers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> has a corresponding mirror <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>. The partially reflective mirrors <NUM> and the dichroic mirror <NUM> are arranged in a straight line. Each mirror is tilted with respect to this straight line at a <NUM> degree angle. The straight line defines a direction of propagation from the first mirror <NUM> to the dichroic mirror <NUM> and then to point <NUM>. The combined laser beam <NUM> propagates along the direction of propagation.

Any suitable partially reflective mirrors may be used. In the embodiment of <FIG>, each of the partially reflective mirrors comprise coated infrared BaF<NUM> or CaF<NUM> windows that have an optical coating applied to control broadband reflection of the front surface. Any other suitable materials can be used in alternative embodiments. In the embodiment of <FIG>, two coatings are used, an <NUM>:<NUM> (<NUM>% transmission, <NUM>% reflection) and a <NUM>:<NUM> (<NUM>% transmission, <NUM>% reflection). This can allow the variety of laser powers to be adjusted to harmonise the output power to a consistent value for each laser (within practical limits). More or fewer coatings can be used in alternative embodiments. The coatings of the partially reflective mirrors of <FIG> are designed to be broadband, such that any variation in their response to a change in wavelength, particularly around wavelengths of interest, is reduced or minimised.

Any suitable dichroic mirror may be used. In the embodiment of <FIG>, the dichroic mirrors comprises a coated infrared BaF<NUM> window that has an optical coating applied to cause light lower than a specified wavelength to be reflected and light higher than said specified wavelength to be transmitted. Any other suitable materials can be used in alternative embodiments. In the embodiment of <FIG>, the coating is such as to reflect light less than <NUM> in wavelength and to transmit light greater than <NUM> in wavelength.

Any suitable windows may be used. In the embodiment of <FIG>, each of the windows comprise coated infrared BaF<NUM> or CaF<NUM> windows.

In other embodiments, other suitable types of mirror or optical devices may be used in place of the partially reflective mirrors and the dichroic mirror. For example, in some embodiments a mirror other than a dichroic mirror or partially reflective mirror may be used at the position of the dichroic mirror <NUM>, e.g. at the last mirror position before point <NUM>. Such a mirror may be used at the last position to introduce more power into the cell. This can be possible as or if the last position does not have any additional lasers behind it such that no lasers need to pass through the last position. In alternative embodiments, any suitable number and combination of partially reflective mirrors and dichroic mirrors may be used.

Each of the partially reflective mirrors <NUM> is configured to partially reflect and partially transmit light incident on it. The reflection and transmission properties of the mirror are chosen to direct laser beams from the lasers <NUM> along the common optical path. In the embodiment of <FIG>, each of the partially reflective mirrors <NUM> reflects <NUM>% of the incident light and transmits <NUM>% of the incident light from the corresponding one of the lasers <NUM>. The partially reflective mirrors <NUM> may have different reflection and transmission properties in alternative embodiments. The dichroic mirror <NUM> is defined by a reflection wavelength range and is configured to reflect light that has a wavelength in the reflection wavelength range and transmit light with a wavelength outside the reflection wavelength range. The reflection wavelength range of the dichroic mirror <NUM> is chosen to correspond to a wavelength range of the sixth laser <NUM>, such that light from the sixth laser <NUM> is reflected and light from the first to fifth lasers is transmitted. The mirrors are flat or non-wedged optical components. Advantageously this allows the system to operate in an orthogonal fashion. For example, the system has a geometrical arrangement such that the direction of propagation from the first mirror <NUM> to the dichroic mirror <NUM> is substantially orthogonal to the laser beams output from the lasers <NUM>.

Another advantage of using flat or non-wedged optical components in embodiments is that the directing of the laser beams to the common optical path may be substantially independent of wavelength, for example such that any distortion effects or other artefacts caused by the optical components may be substantially independent of wavelength. However, the use of partially reflective mirrors may cause the resulting optical signal to be subject to fringe interference effects. These effects can be reduced by selecting the dimensions, in particular the thickness, of the optical components to control the Free Spectral Range of the system. The Free Spectral Range is a measure of the wavelength difference between two successive maxima or minima. The Free Spectral Range may be represented by FSR = <NUM>/(<NUM> x n x L) where L is thickness of the glass and n is the refractive index. Typically, a suitable thickness of the optical components is less than <NUM>. This choice presents, for certain choices of materials such as BaF<NUM> for example, at worse a Free Spectral Range of <NUM>-<NUM> or greater. By controlling the Free Spectral Range, the frequency at which fringing effects occur can be shifted to not coincide and/or interfere with the measurement of the compounds in the optical cell <NUM>.

The Free Spectral Range of this magnitude provides a spectral window that is similar in width to the spectral window covered by an entire laser scan. An expected effect is a curvature on the background of the laser pulse. This background can be easily removed using spectral fitting algorithms as part of the processing the signal. Additional fringing effects are avoided in the steering optical components <NUM> in the sample apparatus <NUM> and optics used to steer light to the optical cell <NUM> through the use of non-flat or wedged optical components.

Each laser in <FIG> has a corresponding mirror belonging to the set of five partially reflective mirrors <NUM> and a dichroic mirror <NUM>. In operation a laser beam from the first laser <NUM> passes through the first window <NUM> and first lens <NUM> into the sample chamber <NUM>. The laser beam continues to the first mirror <NUM> and then from the first mirror <NUM> to the point <NUM>. The first mirror <NUM> is tilted such that the laser beam from the first laser <NUM> is reflected at a right angle by the first mirror <NUM>. Likewise, each of the second to fifth lasers has a corresponding optical path defined by the second to fifth windows, lenses and mirrors. A sixth optical path is defined in the same way from the sixth laser <NUM> to the dichroic mirror <NUM> and to the point <NUM>. All of the mirrors are arranged at the same tilted angle as the first mirror <NUM> such that each of the optical paths bends at a right angle at its point of intersection with its corresponding mirror.

The mirrors are arranged such that laser beams from the lasers <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> pass along a common optical path to the cell <NUM> via point <NUM> after passing through their corresponding window <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>, being focussed by their corresponding lens <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM> and being reflected by their corresponding optical components <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and <NUM>. The common optical path may, for example, have one end at the first mirror <NUM> and the other end at the entrance aperture to the optical cell <NUM> and may extend through point <NUM> and when directed to pass along the common optical path, the optical paths of each respective laser joins the common optical path. Hence, the optical paths of each laser may substantially overlap.

The laser modules can be swapped for other laser modules. Therefore, <FIG> and the corresponding description above is an illustrative example of one choice of configuration of laser modules. In some embodiments the laser spectroscopy system can host up to six laser modules. The configuration of <FIG> includes the sixth laser <NUM> with a sub-range that is suitable to detect O<NUM>. The dichroic mirror <NUM> corresponding to the sixth laser <NUM> is also included in the system. As discussed later, if the system is configured to detect a set of components that does not include O<NUM> the sixth laser <NUM> is replaced by a laser in a suitable wavelength range and the dichroic mirror <NUM> is replaced by a sixth partially reflective mirror.

<FIG> is a more detailed schematic view of the detection part of the laser spectroscopy system shown in <FIG>. <FIG> shows how a plurality of detectors is incorporated into the laser spectroscopy system. <FIG> show the optical cell <NUM>, the optical steering components <NUM>, the sample chamber <NUM>, the second optical interface <NUM> and the detector apparatus <NUM>, which includes an outer housing, of <FIG> in more detail.

<FIG> shows the second interface <NUM> comprising a first output window <NUM> and a second output window <NUM>. The detector apparatus <NUM> has a first detector <NUM> and a second detector <NUM>. The steering optical components <NUM> in the sample chamber <NUM> are configured to steer light from the optical cell <NUM> to a first detector <NUM> of the detector apparatus <NUM> or to a second detector <NUM> of the detector apparatus <NUM>. The first detector <NUM> is directly coupled to the first output window <NUM>. The second detector <NUM> is directly coupled to the second output window <NUM>. The first detector <NUM> is sensitive to light from one or more lasers in a first subset of lasers of the laser apparatus <NUM>. The second detector <NUM> is sensitive to light from one or more lasers in a second subset of lasers of the laser apparatus <NUM>. For the configuration described with respect to <FIG>, with the dichroic mirror, the first detector is sensitive to light from the first to fifth lasers and the second detector is sensitive to light from the sixth laser.

Table <NUM> provides, in further detail, possible configurations of laser modules in the spectroscopy system and corresponding detectors that can be implemented in the system. Careful selection of wavelength ranges of the lasers allows multiple measurements per laser wavelength. The first column of Table <NUM> shows the compound that is to be detected. The second and third column shows one or more wavelength ranges suitable for detection of the compound. The third column of Table <NUM> shows the detector type. In the final column, a typical but non-limiting application of the choice of wavelength range is shown.

As can be seen from Table <NUM>, three detectors A, B and C are listed. Each of the detectors A, B and C is sensitive to light from a different wavelength or wavelength ranges. Due to compatibility of physical housing of the system and the detectors, as described with reference to <FIG>, certain combinations of detectors can be included in the system. Options include: (i) only detector A; (ii) only detector B; (iii) only detector C; (iv) detector A and detector B; and (v) detector A and detector C. As described elsewhere, if detector C is included (options (iii) and (v)) for detecting O<NUM>, the dichroic mirror <NUM> must be included. For options (i), (ii) and (iv), no dichroic mirror is included and all mirrors are partially reflective mirrors. In alternative embodiments, any suitable combination of detectors may be provided in order to detect any of the listed materials of interest using one or more of the listed wavelengths or wavelength ranges.

The operation of the system is now described with reference to <FIG>, <FIG> and <FIG>. In operation, the lasers <NUM> are controlled by the control module <NUM>, or other control component in other embodiments, to sequentially produce pulses. The sequence may be as follows. The first laser <NUM> produces a first pulse that is directed to point <NUM> by the optical components and passes onward to the optical cell <NUM>. Subsequently, the second laser <NUM> produces a second pulse that is directed to point <NUM> by the optical components and passes onward to the optical cell <NUM>. This is followed, in turn, by a third pulse produced by the third laser <NUM> that is directed to point <NUM> by the optical components and passes onward to the optical cell <NUM>, a fourth pulse produced by the fourth laser <NUM> that is directed to point <NUM> by the optical components and passes onward to the optical cell <NUM>, a fifth pulse produced by the fifth laser <NUM> that is directed to point <NUM> by the optical components and passes onward to the optical cell <NUM>, and a sixth pulse produced by the sixth laser <NUM> that is directed to point <NUM> by the optical components and passes onward to the optical cell <NUM>. Following the sixth pulse, this sequence is repeated. The pulsed beams from each of the lasers are interleaved and/or non-overlapping in time and propagate along the common path to the optical cell <NUM>.

Following the above sequence, the first pulse is incident on, and passes through, the first window <NUM> and the first lens <NUM> and is then incident on, and reflected by, the first mirror <NUM> and is then transmitted by the second, third, fourth, fifth mirrors and the dichroic mirror to point <NUM> and continues to the optical cell <NUM> and the detector apparatus <NUM>. Subsequently, the second pulse is incident on, and passes through, the second window <NUM> and the second lens <NUM> and is then incident on and reflected by, the second mirror <NUM> and is then transmitted by the third, fourth and fifth mirrors and the dichroic mirror to point <NUM> and onward to the optical cell <NUM> and detector apparatus <NUM>. Subsequently, the third pulse is incident on, and passes through, the third window <NUM> and the third lens <NUM> and is then incident on, and reflected by, the third mirror <NUM> and then transmitted by the fourth and fifth mirrors and the dichroic mirror to point <NUM> and onward to the optical cell <NUM> and detector apparatus <NUM>. Subsequently, the fourth pulse is incident on, and passes through, the fourth window <NUM> and the fourth lens <NUM> and is then incident on, and reflected by, the fourth mirror and is then transmitted by the fifth mirror and the dichroic mirror <NUM> to point <NUM> and onward to the optical cell <NUM> and detector apparatus <NUM>. Subsequently, the fifth pulse is incident on, and passes through, the fifth window <NUM> and the fifth lens <NUM> and is then incident on, and reflected by, the fifth mirror <NUM> and is then transmitted by the dichroic mirror <NUM> to point <NUM> and onward to the optical cell <NUM> and detector apparatus <NUM>. The last pulse in the sequence is the sixth pulse and this pulse is incident on, and passes through, the sixth window <NUM> and the sixth lens <NUM> and is then incident on, and reflected by, the dichroic mirror <NUM> to point <NUM> and onward to the optical cell <NUM> and detector apparatus <NUM>. The pulse sequence is then repeated.

The pulses propagate through the optical cell <NUM> towards the second optical interface <NUM>. The pulses pass through the second optical interface <NUM> to the detector apparatus <NUM>. The steering optical components <NUM> in the sample apparatus <NUM> steer light (originating from the first to fifth lasers) from the optical cell <NUM> to the first detector <NUM> via the first output window <NUM>. The first detector is sensitive to light from the first to fifth lasers. Thus, in this embodiment one of the detectors is sensitive to light from more than one of the lasers. The steering optical components <NUM> in the sample chamber <NUM> steer light (originating from the sixth laser) from the cell to the second detector <NUM> via the second output window <NUM>. The second detector is sensitive to light from the sixth laser <NUM>. The steering optical components <NUM> include a second dichroic mirror to direct light of the sixth laser <NUM> towards the second detector <NUM> and to direct light of the first to fifth lasers to the first detector <NUM>. The optical properties of the second dichroic mirror may match the properties of the dichroic mirror <NUM>. The steering optical components <NUM> include two separate off-axis parabolic mirrors to focus the two different branches of light onto the two detectors. The control module <NUM> synchronises operation of the lasers and the first and second detectors, such that each of the detection signals corresponds to light received from a respective one of the lasers.

The lasers <NUM> of <FIG> are semiconductor diode lasers that are operable to produce light over a sub-range of wavelengths. The lasers may be quantum cascade lasers, for example pulsed, chirped quantum cascade lasers, although any other suitable types of laser may be used in alternative embodiments. For example, interband cascade lasers and tuneable diode lasers may be used. The lasers may, for example, produce beams of <NUM> to <NUM> in diameter, or of any other suitable size.

The sub-ranges of wavelengths may be in the infra-red spectrum. The wavelength ranges are chosen to correspond to the measurement of one or more compounds. Together the instrument may provide multiple wavelength ranges of light and combines, for example, visible, near infrared and/or mid infrared light to take advantage of the most suitable wavelengths for each compound. The laser spectroscopy system is configured to measure impurities found in atmospheric air. Impurities that can be found in normal air include, but are not limited to methane, water and carbon dioxide.

Table <NUM> shows a first example implementation of wavelength ranges for lasers <NUM>, the corresponding wavenumber range and the corresponding compound detected by light in this wavelength range. The configuration of lasers of <FIG> is suitable for measuring ethylene purity. For the first example, only one type of detector is required (corresponding to detector type A of Table <NUM>) and no dichroic mirror is required.

Table <NUM> shows an alternative example implementation of wavelength ranges for lasers <NUM>, the corresponding wavenumber range and the corresponding compound detected by light in this wavelength range. The laser wavelengths shown in Table <NUM> are suitable for measuring H<NUM>/N<NUM> purity.

To implement Table <NUM> requires modification to the embodiment shown in <FIG>. In particular, there is no sixth laser <NUM> and corresponding dichroic mirror <NUM>, or sixth window <NUM> and sixth lens <NUM>.

The control module <NUM> is configured to send one or more electronic control signals to the lasers <NUM>. In response to the electronic control signals, the lasers <NUM> produce the combined laser beam <NUM>. The control signal acts to pulse the lasers <NUM> sequentially. In other words, the control signal acts to drive each of the lasers <NUM> in a sequence, such that over a sample time interval only light from one laser is provided to the optical components <NUM>. Thus, although reference is made to a combined laser beam, at any one point in time and position along the laser beam path, the combined laser beam at that position and point of time may consist of light from only one of the lasers. The combined laser beam may consist of interleaved, non-overlapping pulses from the different lasers. The optical components <NUM> are configured to direct the light from each laser along the optical path of the laser to follow the common path to the optical cell <NUM>. In this way, the control module <NUM> controls the laser apparatus <NUM> to produce the combined laser beam <NUM> and provide the combined laser beam <NUM> to the optical cell <NUM>. The combined laser beam <NUM> comprises pulsed beams from each of the lasers interleaved and/or non-overlapping in time.

The switching frequency between the lasers is selected to ensure a reliable measurement in the detector apparatus <NUM>. In particular, the time taken for a pulse of light to traverse its optical cell optical path is dependent on the physical properties of the pulse and the dimensions of the optical cell <NUM>. Pulse lengths and frequency of subsequent pulses are controlled and selected to prevent physical overlapping of pulses. Overlapping of pulses leads to interference and unreliable measurements. Two or more pulses can be present in a multi-pass cell at the same time provided that the two or more pulses are not physically overlapping. Suitable pulse durations for pulses from the lasers <NUM> may be between <NUM> nanoseconds and <NUM>,<NUM> nanoseconds. The frequency of sequential pulsing may be up to <NUM> in some embodiments.

The signal processor <NUM> processes the detection signals from the detectors to determine the concentrations and/or relative amounts of the different compounds under investigation, or to determine any other desired properties. The signal processor <NUM> uses any suitable known processing techniques to determine the concentrations, relative amounts or other properties.

Optionally, an alignment mechanism may be provided. An example alignment mechanism comprises a camera and a mirror adjustment mechanism. The camera or is positioned at or near the point <NUM> to intersect a desired direction of propagation of the combined laser beam <NUM>. The desired direction of propagation is such that the combined laser beam <NUM> will, in normal operation, enter the optical cell <NUM> via the common optical path. During an alignment step, sample beams are produced by the lasers <NUM> and the sample beams are directed by the optical components <NUM> to the camera. The camera detects the position of the sample beams incident on it relative to the desired direction of propagation. The mirror adjustment mechanism adjusts the position, in particular the tilt relative to the direction of propagation, of the partially reflective mirrors <NUM> and dichroic mirror <NUM> to substantially align the optical paths of the lasers <NUM> with the desired direction of propagation and substantially align the optical paths with each other. For example, the optical paths are substantially aligned within a <NUM>° tolerance. The alignment step is repeated for each of the lasers <NUM>.

<FIG> is a perspective view of internal casing for the laser spectroscopy system. The casing has an upper optical plate <NUM> and a lower base section <NUM>. The upper optical plate <NUM> is sized and dimensioned to receive the laser apparatus, here shown including six laser modules <NUM>, including a laser module <NUM>. The upper optical plate <NUM> also has a first detector <NUM>, a second detector <NUM>, a pressure gauge <NUM> and a temperature gauge <NUM>. Also located on the upper optical plate <NUM> is a calibration mechanism <NUM>. The calibration mechanism <NUM> can comprise an etalon for laser calibration. In addition, a removable mirror <NUM> may be added for optional attenuation of the laser beam. The lower base section <NUM> contains the sample chamber <NUM>. The optical cell <NUM> is attached to a lower surface of the optical plate <NUM>. The lower base section <NUM> is configured to be coupled with a gas supply arrangement via a gas inlet <NUM> and a gas outlet <NUM>. Aside from the gas inlet <NUM> and gas outlet <NUM>, the lower base section <NUM> is sealed.

<FIG> is a cross-sectional perspective view of an example optical interface between a laser and a sample chamber. <FIG> shows the laser module <NUM> fitted to an interface plate <NUM> of the laser spectroscopy system. <FIG> also shows the upper optical plate <NUM> and the lower base section <NUM> in which the sample chamber is located. An optical interface <NUM> between the laser module <NUM> and the sample chamber of the lower base section <NUM> is indicated.

<FIG> is a zoomed view of <FIG> that shows a closer view of the optical interface <NUM>. A laser package <NUM> of the laser module <NUM> is shown. The laser package <NUM> has a lower surface <NUM>. The lower surface <NUM> is flat. At a central point of the lower surface <NUM> there is an aperture <NUM> for laser light to exit the laser module <NUM>. The aperture <NUM> houses a window that serves to hermetically seal the laser package.

The interface plate <NUM> is configured to accommodate the laser module <NUM> to the spectroscopy system. In the embodiment of <FIG>, the interface plate <NUM> has a coupling member <NUM> for the corresponding laser module <NUM>. The coupling member <NUM> is a hollowed cylindrical protrusion extended from the upper surface of the interface plate <NUM>. At the upper end of the coupling member <NUM> is a rimmed circular seating <NUM>. <FIG> show a circular window <NUM> fixed in the rimmed circular seating <NUM>. The circular window <NUM> and rimmed circular seating <NUM> are dimensioned such that, with the window <NUM> positioned in the rimmed circular seating <NUM>, the upper surface of the coupling member <NUM> presents a flat and horizontal surface to the laser module <NUM>.

The laser module <NUM> has a cavity <NUM> dimensioned to complement and conform to the shape of the coupling member <NUM> of the interface plate <NUM>. The cavity is positioned directly below the lower surface <NUM> of the laser package <NUM>. To fit the laser module <NUM> to the interface plate <NUM>, the cavity <NUM> of the laser module <NUM> is placed over the coupling member <NUM> and thereby mounted on the coupling member <NUM>. The size of the cavity <NUM> is such that, when mounted, the lower surface <NUM> of the laser package <NUM> is flush with the upper surface of coupling member <NUM>. In this way, the laser module <NUM> is in contact and/or butt coupled to the window <NUM> of the coupling member <NUM>. At least partially located in the hollow of the coupling member is a collimating lens <NUM>. The collimating lens <NUM> is held in position by a lens holder <NUM> connected to and hanging from a lower inner surface of the interface plate <NUM>. The collimating lens <NUM> acts to collimate the laser beam incident on it.

<FIG> is a perspective view of housing for the laser spectroscopy system in a closed configuration. The housing has a lift-off cover <NUM> that is secured in a closed position to a base section <NUM> by collection of release catches <NUM> located about the periphery of the housing.

A gas supply arrangement in the form of sample supply tube provides gas to the sample chamber, for example, via a gas inlet. A sample return tube provides an outlet for gas from the sample chamber, for example, via a gas outlet. Ventilation is provided to the optical cell via a vent. The lift off cover <NUM> has a local operator display <NUM> and user input pad <NUM>. In the embodiment of <FIG> the user display and user input pad is for interaction with the analyser and visual communication of measurements and status. Some maintenance functionality is provided by the user input display in this embodiment, however its purpose is mostly communication of measurement values and status.

The housing is manufactured to be capable of containing an ignition event. The housing includes flame arrestors. The housing is tested to ensure that it can withstand a sudden high pressure event, for example, an explosion. This housing may obviate the need for additional purge apparatus.

Also connected to the housing are three output conduits (not shown). The conduits provide electrical breakthroughs that allow power and control signals to be sent to the system and to allow data to be transmitted from the system. The data transmitted may, for example, be in the form of digital signals, digital health signals, analogue signals for example <NUM>-20mA signals indicating measured values of gases, more sophisticated protocols such as Modbus, or in any other suitable format. The arrangement described above provides a compact system. In some embodiments, the housing may have dimensions of <NUM> by <NUM> by <NUM>.

The sample supply tube and the sample return tube provides a fluid to the communication path through the sample chamber and the optical cell. The sample gas can be collected from a remote location and can be delivered via the sample supply tube to the sample chamber to be sampled. The sample gas can then be exhausted from the sample chamber via the sample return tube. Together, the sample supply tube and the sample return tube allow for the instrument to operate remotely, in contrast to in-situ emission sensing. Any other suitable gas supply arrangement may be used in alternative embodiments.

A sample handling system (SHS) unit (not shown) may be provided to control pressure of the gas in the optical cell <NUM>. Any suitable SHS unit or other pressure control device may be used, which may or may not comprise or be driven by a pump and may or may not comprise other pressure control components such as an arrangement of valves. In the embodiment of <FIG>, the SHS unit includes an aspirator rather than a pump, although a pump or other pressure control device or components may be used in other embodiments.

Any suitable optical cell may be used as optical cell <NUM>. For example, a Herriott cell is used as the optical cell. Any suitable Herriott cell may be used, or any suitable multipass spectroscopic absorption cell, or for example any other cell which is configured to provide interaction between the laser beam(s) and the sample gas, for instance by way of reflection of the laser beam between surfaces of a chamber containing the gas. For example, the optical cell may be an astigmatic Herriott cell.

In various embodiments, the sample chamber may be sealed and/or the system may comprise means for controlling the pressure and/or gaseous content of the sample chamber. Similarly, each laser may comprise a housing and at least one of: each laser housing may be sealed and/or is under vacuum or contains a selected gas and/or is at a selected pressure. In various embodiments, the detector apparatus comprises a housing and at least one of: the detector apparatus housing is sealed and/or is under vacuum or contains a selected gas and/or is at a selected pressure.

Claim 1:
A laser detection system comprising:
a sample chamber (<NUM>) comprising an enclosed volume configured to receive and contain a volume of sample gas;
one or more laser modules, wherein each laser module comprises
a laser package(<NUM>) within a laser housing, wherein each laser package of the one or more laser modules is configured to produce a respective laser beam for excitation of one or more different materials in the sample gas and the one or more laser modules are outside the sample chamber (<NUM>),
wherein the laser package comprises a lower surface and an output aperture on the lower surface for laser light to exit the laser module, wherein the aperture houses a window and wherein the laser module further comprises a cavity positioned below the lower surface of the laser package;
a detector apparatus (<NUM>) for detecting light output from the sample chamber (<NUM>);
wherein the system comprises an interface plate configured to accommodate the laser module, wherein the interface plate provides a first optical interface (<NUM>) between the laser module and the sample chamber, the first optical interface having at least one window that is at least partially transparent to the laser beams from the one or more laser modules (<NUM>), wherein the system further comprises coupling means configured to position and hold the at least one laser housing in a close-coupling arrangement relative to the at least one window of the first optical interface (<NUM>) such that, in use, the laser beams are substantially unmodified by passage between the laser housing and the at least one window,
wherein the interface plate comprises a coupling member for a corresponding laser module, wherein the coupling member comprises a protrusion extended from the upper surface of the interface plate and having a rimmed seating at the upper end thereof, wherein the at least one optical window of the first optical interface is fixed in the rimmed seating,
wherein the coupling means comprises the coupling member and the cavity of the corresponding laser module , wherein the cavity is dimensioned to complement and conform to the shape of the protrusion
and wherein the cavity is configured to be mounted on the protrusion.