The present invention relates to a system and method for the detection of gaseous chemicals, based on Quantum Cascade Laser (QCL) technology.

A basic gas-sensing module is formed when laser radiation from a wavelength tuneable source is passed through a multipass gas cell for signal amplification, onto a detector that generates an electrical signal. The signal is subsequently processed by the electronics and analysed by software. As the wavelength of the laser is swept through the absorption spectra of the gas, the intensity of the radiation detected is found to change, and in particular, correlate with a concentration of the gas which is present in the cell.

Quantum cascade lasers (QCLs) are semiconductor lasers that emit light in the mid-infrared wavelength range of the electromagnetic spectrum, namely 3-20pm, and more recently QCLs have been used to generate THz frequencies (corresponding to a wavelength lOOpm - lmm). QCLs operating in the single lateral mode have an emitting area that typically measures 5-15pm by 2-3pm and the laser cavity is several millimetres in length. Lasing devices are soldered on to a heat carrying substrate and electrically connected. QCLs may be supplied in this soldered format as a chip on sub-mount, or there are further industry standard packaging options available such as chip on C-mount, or the QCL chip may be sealed and collimated using a high heat load package.

QCLs differ from semiconductor diode lasers in that they do not use a p-n junction for light emission. Multiple active regions are "cascaded" so that each injected electron can emit multiple photons and therefore enhance laser gain. Each active region is composed of a multi-layered semiconductor material structure designed to have an electronic band structure, which gives the desired emission wavelength, and is manufactured with extremely precise nanometre-level thickness control. There are several reasons why QCLs are being increasingly utilised across a wide range of applications but the most important of all relate to gas-sensing and analysis. This is because most molecules, ranging from the simple to the moderately complex (including CH ₄ , HCL, CH ₂ 0, CO, C0 ₂ , NO, H ₂ 0, etc.), have characteristic absorption bands in the spectral region between 3 -14pm. The key features of QCLs combined with their wavelength emission agility make the development of QCL technology crucial for applications in spectroscopy, chemical and bio molecular sensing, security and non-invasive medical diagnostics. Infrared spectroscopy has been used in these applications for many years, however, highly sensitive industrial systems are very expensive and not sufficiently compact to make them easily accessible; on the other hand, the sensitivity of widely available and affordable compact gas detection modules is relatively poor, such that these systems are unsuitable for use in high demanding application areas such as breath analysis, environmental trace gas detection and the like.

In an exemplary use, the system and method of the present invention can be used to diagnose and detect Colorectal Cancer (CRC), also commonly referred to as bowel cancer and other pathological conditions, by identifying volatile organic compounds (VOCs) present in flatus and faeces. Colorectal cancer is the third most common cancer in the UK after breast and lung cancer, with approximately 40,000 new cases registered each year. The occurrence of colorectal cancer is strongly related to age, with almost three-quarters of cases occurring in people aged 65 or over. Colorectal cancer (CRC) is the second most common cause of cancer death in the UK (NICE, 2011).

In an effort to reduce the number of deaths linked to colorectal cancer the UK government introduced the National Bowel Cancer Screening Program in 2009. Many countries have instituted a variety of screening programmes to reduce mortality and morbidity of this disease.

The UK screening program involves the use of a home faecal occult blood test (FOBt), which takes samples from three bowel motions. The test requires patients to obtain a sample by using a small scraper to scrape faeces off toilet tissue. The national screening program for the detection of colorectal cancer only has a 50% uptake (Weller, 2006 & Hoi, 2010), which is thought to be due to the perceived issues from the general population in terms of discomfort and social acceptability in using the home testing kit provided by the scheme. Alternative methods of diagnosis include colonoscopy or flexible sigmoidoscopy, during which a colonoscope is inserted through a patients anus and used to inspect the lining of the bowel. The patient is required to take medication prior to the tests to empty the bowel and the test itself can be uncomfortable, requiring the use of sedatives and pain relief for many patients. In addition, there are patients in whom it is not possible to complete the procedure due to inadequate bowel cleansing or patients not being able to tolerate the procedure. Moreover, such procedures carry the risk of puncturing the bowel (perforation) during the procedure, which may result in the patient requiring hospital admission or even surgery. Another option is virtual colonoscopy, which involves the insertion of a dye through the bowel with air insufflated through the large bowel, and then a computerised tomography (CT) scan performed to visualise the inside of the bowel wall to look for abnormalities. Patients are still required to take medications to empty the bowel prior to the investigation and the technique cannot be tolerated by certain patients, in particular the frail and elderly.

The current screening options used are not without problems, mainly relating to patient acceptance and uptake, cost and effectiveness. As a result, there has been an expansion in the research into new forms of CRC screening. There are non-invasive techniques described for the detection of colorectal cancer by products using DNA and micro RNA within faeces. Another approach is the measurement and analysis of by-products of cellular metabolism, namely volatile organic compounds.

Volatile organic compounds (VOCs) are a diverse group of carbon-based chemicals that are volatile at ambient temperature. Metabolomics is the most recently developed research area, investigating the end points of cellular metabolism. It has been determined that the VOCs excreted vary from healthy to diseased tissue and different diseases are associated with a varying mixture of VOCs. VOCs may be emitted from bodily fluids and as a result may be odorous. Currently these differences have been measured through the use of nuclear magnetic resonance spectroscopy, high performance liquid chromatography and gas chromatography linked to mass spectrometry. VOCs are increasingly being thought of as potential biomarkers generated in the development of cancer, infections and chronic inflammatory conditions. Uses of VOCs could include: screening and monitoring tools, studying pathogens responsible for disease, and, treatment efficacy (effectiveness).

The use of faeces and flatus (gas generated in or expelled from the digestive tract) for the detection of VOCs in infectious conditions of the bowel (e.g. Campylobacter and Clostridium difficile) and inflammatory conditions (e.g. ulcerative colitis and Crohns disease) has been described (Garner et a I, 2007, Volatile organic compounds from faeces and their potential for diagnosis of gastrointestinal disease).

Some other exemplary uses of the system and method are: in industrial process control, such as in pressurised canister monitoring; oil & gas exploration and extraction; surveillance; combustion diagnostics; chemical agent detection; continuous emissions monitoring; air and water quality; blood and breath analysis etc.

There are two main benefits for the current invention with respect to the example of CRC detection; it can be more accurate than current screening methods and is very likely to be considered more socially acceptable. If the device is at least as reliable as current screening tools to detect colorectal cancer and is considered by the screening population focus groups to be a more socially acceptable method, then the current uptake of 50% for the national cancer screening program in the UK could be substantially increased, with a subsequent impact on the morbidity and mortality of a common and lethal disease.

The invention can produce similar benefits in the other exemplary uses with the device being more compact than current devices. Other benefits include more stable laser intensity, a suppressed jitter effect (quasi-continuous wave regime of an active mode locked laser), substantially higher repetition frequency (less noise) and narrow emission spectra. Effectively, the invention has high sensitivity for trace gas detection and is cheaper, simpler to manufacture as the driving electronics can be simpler than for a short pulsed distributed feedback (DFB) laser as there is no need to use a more expensive DFB laser; the wavelength selection approach involving a narrow bandpass filter or diffraction grating, and a high finesse etalon allows the use of any laser and reduces the operating costs. The invention removes the requirement for an expensive external multipass gas cell but offers laser intracavity gas absorption geometry which provide very long (up to several hundred meters) paths for the light interacting with a gas.

STATEMENTS OF INVENTION

In accordance with the present invention there is provided a system for detecting gaseous chemicals, the system comprising:

a volume configured to receive gaseous chemicals;

an external cavity quantum cascade laser arranged to transmit lasing radiation in a mid-infrared wavelength range through the volume, the volume being defined by a cell disposed within the external cavity of the laser;

a detector for detecting the intensity of the laser radiation transmitted through the gaseous chemicals; and,

an analyser for processing the detected radiation to identify the one or more gaseous chemicals;

wherein the gaseous chemicals are organic compounds having a boiling point less than or equal to 250°C measured at a standard atmospheric pressure of substantially 101.3kPa.

In an embodiment, the cell may be arranged to receive the gaseous chemicals extracted by a clinician directly from a patient's colon through a patient's anus, or alternatively obtained from a toilet, bathroom, sanitation fixture or capture vessel. The system can very easily be adapted to fit multiple environments, making it suitable for clinical and home use.

In an embodiment, the external cavity quantum cascade laser is actively mode locked. Additionally, the laser operates in the wavelength range of substantially 3-20pm. These features of the laser are particularly suitable for the detection and analysis of VOC's.

Additionally, the gaseous chemicals may be produced from a gas, a liquid, a solid, a plasma, or their combinations. Having, the gas cell inside the laser external cavity makes for a simple, inexpensive, compact laser with a sensitivity that is comparable with an external multipass cell. Alternatively, the gaseous chemicals may occupy all the space of the external cavity.

In an embodiment, the external cavity quantum cascade laser may comprise a linear cavity geometry. In this geometry a laser emission from an antireflection coated laser facet may pass through the gas cell, reflect from an external cavity mirror, propagate through the cell again, prior to passing back to the same laser facet. In this embodiment the components of the QCL may be relatively easily aligned.

In an embodiment, the external cavity quantum cascade laser comprises a ring cavity geometry. In this geometry there may be several external cavity mirrors and a laser emission from an antireflection coated laser facet is arranged to pass through the gas cell at least twice, prior to passing back to an opposing antireflection coated laser facet. In this embodiment, the laser alignment is more challenging but has improved intensity/emission wavelength stability due to unidirectional operation.

In an embodiment, the external cavity quantum cascade laser may be designed for a single roundtrip through the volume with a path length of less than 10cm or may be designed for multiple passes through the volume with a path length over 100m. The choice between path lengths is dependent on the trade-off between cost and simplicity, and accuracy. Alternatively, the compact external cavity quantum cascade laser may be designed for two passes through the volume (toward the cavity mirror(s) and back) with an external laser cavity length of less than approximately 10cm, or may be designed for multiple passes through such a volume, by introducing additional mirror reflections in the external cavity system, similar to a multipass cell geometry.

In an embodiment, the laser, detector and analyser may form a single device. This single compact device may be placed within a clinical setting (such as a GP surgery or hospital), a toilet, bathroom, or a home environment, thereby providing a simple, effective instrument to monitor and detect possible colorectal cancer. In accordance with the present invention there is provided a method for detecting gaseous chemicals, the method comprising the steps of:

receiving gaseous chemicals in a volume defined by a cell;

transmitting laser radiation from an external cavity quantum cascade laser, operating across a mid-infrared wavelength range, through the volume comprising the gaseous chemicals, the volume being disposed within the external cavity of the laser;

detecting the intensity of the laser radiation transmitted through the gaseous chemicals; and

analysing the detected radiation to identify the one or more gaseous chemicals; wherein the gaseous chemicals are organic compounds having a boiling point less than or equal to 250°C measured at a standard atmospheric pressure of substantially 101.3kPa.

The external cavity quantum cascade laser may be operated in a continuous wave regime or be driven with a modulating electrical current including amplitude modulation at a frequency corresponding to the round-trip time of the lasing radiation through the external cavity. The laser current modulation frequency may be a sinusoidal signal and is inversely proportional to the length of the external cavity, within the range of 100MHz to 3GHz. These features allow pulsed (<50% duty cycle) driving conditions which result in a smaller thermal impact, and enables the use of a simple sinusoidal generator with very relaxed demand on output signal shape, or a more sophisticated generator with very short (<100 ps) pulse width and a repetition rate corresponding to the round-trip time of the lasing radiation through the external cavity may also be used.

Wavelength tuning of the external cavity quantum cascade laser within its optical gain range may be arranged in the following ways. Setting a wavelength may include adjusting the angle of incidence of a narrow bandpass interference filter inserted in the external cavity. The transmission range of the narrow bandpass filter includes many longitudinal modes of the external cavity, which is found to affect laser performance and generate wavelength tuning instabilities. For example, these may include unwanted mode hopping. Accordingly, the operating wavelength range may be coarsely set by the narrow band pass filter by fixing it in an appropriate position. The wavelength may subsequently be finely set by adjusting the optical thickness of a high finesse etalon also disposed within the external cavity. The etalon may comprise a much narrower transmission peak than the filter and may comprise a highly parallel plate substantially transparent to mid-infra red radiation. The optical thickness of the etalon defines the free spectral range which in turn, defines the fine wavelength tuning range, which may be up to 20cm-l. The etalon may be attached to a piezo/Galvo actuator providing angular/rotational movement to the etalon and change to the angle of incidence. Alternatively, the etalon may be attached to a mini thermoelectric cooler to effect a change to the refractive index of the etalon material and, as a result, a corresponding change to the optical thickness of the etalon. This may also be achieved when the etalon is manufactured from a material with piezoelectric properties providing precise control on the geometrical thickness of the etalon.

The operating wavelength may be alternatively set using a diffraction grating. For example, the wavelength may be set using a diffraction grating by adjusting an incidence angle of the lasing radiation upon the diffraction grating. In an embodiment, the diffraction grating may replace one or more of the mirrors. The diffraction grating may be positioned in a Littrow configuration and be coupled with a piezo/Galvo actuator to provide angular/rotational movement. Also, similar to the case of the narrow band pass filter, an additional high finesse etalon might be used to increase selectivity of the longitudinal modes and improve stability of the laser operation.

Alternatively, the operating wavelength may be finely set by adjusting the refractive index of the laser chip, which may be conducted while other wavelength selecting elements such as the filter or the diffraction grating or the etalon are kept in their predefined and fixed positions. The variation of the laser refractive index may be achieved by varying the offset bias applied to the laser or by varying the temperature of a substrate of the lasing device.

I n an embodiment, the detector can be positioned randomly around the laser system to catch the light which is scattered or reflected from internal optical elements or transmitted through partially transparent cavity mirrors. Measuring the results may include reading a signal from a mid-infrared detector. Measuring can include demodulating the detector signal at a current modulation frequency corresponding to the round trip of the lasing radiation through the laser external cavity. Measuring may include sweeping the operating wavelength across a selected wavelength range substantially close to an absorption feature of a volatile organic compound to obtain a laser intensity spectrum.

Sweeping may include supplying a sweep (ramp) signal to the laser (sweeping of the current on top of the mode locking modulation), or to a sub-mount thereof, or directly to a wavelength setting element, to control its oscillating movement (for a filter or for a diffraction grating) or its optical thickness (for an etalon). Alternatively, sweeping can include simultaneous modulating the operating wavelength range at a selected frequency, waveform, and amplitude. The wavelength sweeping frequency may be at least 1000 times smaller than the modulation frequency of the laser corresponding to the light round trip along the laser external cavity.

Measuring may include further demodulating the detector signal at the sweeping frequency, the waveform, and the amplitude to obtain a demodulated laser intensity spectrum.

Measuring can further include demodulating the detector signal at a current dither modulation frequency. The sinusoidal dither modulation is applied to the laser together with the laser sweeping modulation and its frequency may be at least 100 times smaller than the modulation frequency of the laser current corresponding to the lasing radiation round trip time through the laser external cavity. The amplitude of the dither modulation is less than the amplitude of the sweeping modulation. The frequency of the sweeping modulation may be at least 100 times smaller than the dither modulation frequency.

Measuring may include further demodulating the detector signal at the dither frequency, to obtain a first or higher order derivative of the laser intensity spectrum. In an embodiment analysing can include comparing the laser intensity to a known value thereof measured in the absence of the one or more volatile organic compounds to determine the presence of at least one volatile organic compound.

In an embodiment, analysing can include comparing and adjusting the laser emission wavelength to a known absorption wavelength of the one or more volatile organic compounds. For that purpose one or more reference absorption cells filled with appropriate gas or gases may be introduced to the system.

Identifying the at least one volatile organic compounds can include identifying one or more features of the intensity spectrum corresponding to absorption wavelengths of the one or more volatile organic compounds. Identifying the at least one volatile organic compounds can include determining a concentration of the at least one volatile organic compounds.

Identifying the at least one volatile organic compounds may include comparing the one or more features of the intensity spectrum with absorption wavelengths appearing in at least one known spectrum from a spectral database of known volatile organic compounds associated with colorectal cancer.

The invention may be performed in various ways and embodiments thereof will now be described, by way of example only, reference being made to the accompanying drawings, in which:

Figure 1 is a representation of a system according to an embodiment of the present invention;

Figure 2 is a flow diagram outlining the steps associated with a method according to an embodiment of the present invention;

Figure 3 is an embodiment of a gas sensor which may be used with the system of figure 1; and, Figure 4a illustrates potential longitudinal modes of a QCL of figure 3, defined by the gain spectrum of the laser;

Figure 4b illustrates the transmission bandwidth of a narrow bandpass interference filter in the QCL of figure 3;

Figure 4c illustrates longitudinal modes of the QCL, selected by the narrow bandpass interference filter;

Figure 4d illustrates the transmission spectrum of an etalon in the QCL of figure 3;

Figure 4e illustrates a single longitudinal mode selected by the narrow bandpass interference filter and the etalon in combination; and,

Figure 5 is an alternative embodiment of a gas sensor which may be used with the system of figure 1.

Referring to Figure 1 of the drawings, there is illustrated a system 10 according to an embodiment of the present invention, for detecting colorectal cancer from flatus. The system 10 is arranged to receive a sample of flatus 14 from a patient 12. The system comprises a cell 16 defining a volume 16a which is configured to receive and contain a sample of the flatus. The cell is disposed or disposable within the cavity of an external cavity quantum cascade laser (ECQCL) 18. A detector 18A is provided to detect the intensity of the laser radiation following the transmission of the radiation through the sample of flatus 14. An analyser 20 is arranged to receive the detected results from the detector 18A and process the results to identify one or more volatile organic compounds 15 within the flatus which may be associated with colorectal cancer, for example. The system may be realised as a single integrated arrangement of components or the separate components may be optically and electrically coupled together.

In an embodiment, the system 10 further comprises a capture vessel 17, such as a cassette or thermal desorption tube, for capturing/extracting a sample of flatus from the patient 12. The method of collecting a sample may include inserting a capture vessel 17 into the patient via the rectum for example. The collected sample may subsequently be transferred to the cell 16 for insertion within the cavity of the external cavity QCL 18.

The system 10 may additionally include features to separate individual compounds and molecules prior to analysis by the ECQCL 18. For example; where a sample has been collected using a capture vessel 17, the capture vessel may be heated through a gas chromatography (GC) process. Different molecules/compounds will release from vessel at different temperatures and times and will separate out in a GC column, thereby producing a train of different gaseous species over time.

Referring to Figure 2 of the drawings, there is illustrated a method 200 for detecting colorectal cancer from flatus according to an embodiment of the present invention. The method includes the steps of: receiving in a volume at step 202, flatus from a patient; operating an ECQCL 18 across a mid-infrared wavelength range corresponding to an absorption feature of one or more volatile organic compounds at step 204; detecting the intensity of the laser radiation affected by the flatus at step 206; and analysing the results at step 208 to identify the one or more volatile organic compounds associated with colorectal cancer.

Referring to Figure 3 of the drawings, there is illustrated an embodiment of an intra-cavity gas sensor for the detection of gaseous chemicals. The intra-cavity gas sensor includes a QCL 300 which comprises a gain medium 300a, having a front facet 301 and a rear facet 302. The sensor further comprises a lens 304, an external mirror 312, a wavelength-selecting element or device such as an etalon 308, and a narrow band pass interference filter 310 and/or a diffraction grating (not shown) arranged in along a substantially linear optical path. The optical laser cavity (OLC) is defined as the region between the front facet 301 of the gain medium 300a and the mirror 312, whereas the external cavity (EC) is defined as the region between the rear facet 302 of the gain medium 300a and the mirror 312.

The gain medium 300a comprises a broad gain spectrum, such as a quantum cascade gain medium or an interband cascade gain medium. The gain medium has a front facet 301 and a rear facet 302. The gain medium emits a laser beam from the rear facet 302 which is collimated by lens 304. Lens 304 also couples the laser beam from the external cavity back into the gain medium.

Reflections from the rear facet 302 give rise to the possibility that the QCL 300 will operate on longitudinal modes (figure 4a) of one of three separate cavities: the external cavity, the gain medium cavity between facets 301 and 302, or a combined cavity incorporating the external cavity and the gain medium cavity. Operating on modes of either the gain medium cavity or the external cavity is undesirable, as the gain medium cavity and external cavity will tend to compete for dominance causing instability and mode hopping. It is preferred for the QCL 300 to operate on modes of the combined cavity instead. To encourage the QCL 300 to operate on the preferred combined cavity, the rear facet 302 is anti-reflection coated to eliminate reflections from the rear facet 302 as much as possible, while the front facet comprises a high-reflective coating. Minimising reflections from the rear facet 302 also improves feedback from the external cavity back into the gain medium 300a. Preferably, reflections from the rear facet 302 are 0.1% or less, more preferably 0.01% or less.

The narrow bandpass interference filter 310 is positioned between the lens 304 and the cavity mirror 312. Relying on the interference filter 310 alone to select a single longitudinal mode has problems, since narrow transmission bandwidth interference filters suitable to select a single mode tend to have low transmission efficiency (typically around 50% or less). Such low transmission efficiency will reduce feedback from the external cavity back into the gain medium 300a, reducing the efficiency of the wavelength selectivity and resulting either in unstable single mode operation or more likely multi-mode operation.

The efficiency of the wavelength selectivity improves as the transmission efficiency of the interference filter 310 is increased, and mid-infrared interference filters with a much higher transmission efficiency (85% or more) can be easily produced. However, high transmission efficiency interference filters in the mid-infrared tend to have broader transmission bandwidths (of around 0.5 - 1 % of the peak transmission wavelength of the interference filter 310) which is not sufficient to select a single longitudinal mode. So, instead, the interference filter 310 has a transmission bandwidth (Figure 4b) which selects several emitting longitudinal modes (Figure 4c), supressing laser emission at the wavelengths that are rejected by the interference filter 310.

As the transmission bandwidth interference filter 310 used in the cascade laser 300 cannot select a single mode by itself, the interference filter 310 is combined with the etalon 308 which selects a single mode from the plurality of longitudinal modes selected by the interference filter 310. Additionally or alternatively, the mode selection may be achieved using a diffraction grating, such as a volume Bragg grating.

The etalon 308 may be produced from a highly polished and parallel plate, for example, with a tilt error of less than around 2 arcsec. The plate for the etalon 308 is made from a material transparent in the mid-infrared range, for example, fused silica, sapphire, calcium fluoride, silicon, germanium or chalcogenide glasses. The plate is then cut into pieces with the size required for a compact laser package. The required finesse of the etalon 308 is achieved by partly reflection coating (providing more than around 70% reflection) on both sides of the etalon 308. Higher finesse values are required for better single mode selection. The optical thickness of the etalon 308, taking into account the refractive index of the material, is chosen to provide the required free spectral range. For single mode operation, the free spectral range of the high finesse etalon 308, that is, the energy separation between its Fabry-Perot fringes Av _et (Figure 4d), needs to be larger than the emission bandwidth Av _L (Figure 4c) of the cascade laser 300 operating with the interference filter 310 of bandwidth Av _f (Figure 4b).

The incidence angle upon the interference filter 310 is adjusted to select the longitudinal modes from the plurality of potential longitudinal modes of the cascade laser. Ideally, the desired emission wavelength of the cascade laser 100 is located at or close to a point of maximum transmission efficiency of the interference filter 310 to enhance the efficiency of the cascade laser 300.

An angle of the etalon 308 is then adjusted to the single mode, from the modes selected by the interference filter 310. The etalon 308 will be used to select a single longitudinal mode (Figure 2e) which is as close as possible to a wavelength required for a particular application, for example, in gas sensing the longitudinal mode will be selected which corresponds most closely to a particular gas absorption line for trace gas spectroscopy, detection and/or monitoring.

The etalon 308 and/or filter 310 may be mounted on an actuator for fine manipulation and control. The sensor may also include a signal-generating device (not shown) that supplies the actuator with a scanning signal for moving the etalon and/or filter through a predefined range of movement.

The sensor may further include a current controller (not shown), which supplies the QCL device with electrical current. A compliance voltage appearing across the QCL (and thus simultaneously across the current controller) may be measured with a detector (not shown) which may include amplifiers, mixers, filters, and computers etc.

The presence of one or more gaseous chemicals in an external cavity of the QCL 300 can be passively introduced by way of a natural presence of the gases in ambient air pervading the external cavity, or more particular, the gases may by deliberately introduced into the external cavity by locating a gas cell 306 in the optical path of the laser radiation.

Gaseous chemicals present in the gas cell 306 will affect the intensity of the laser radiation transmitted therethrough, which in turn results in a change in the compliance voltage detected across the QCL 300 which is measured by a detector. Gaseous chemicals are thus detected by measuring and analysing changes in the compliance voltage.

In use, the gas cell 306 is disposed within the external cavity and laser radiation is arranged to pass several times through the gas cell 306 via reflections between the front facet 301 and mirror 312. The laser radiation is subsequently directed onto a detector (not shown in figure 3) which generates an electrical signal characteristic of the intensity of the laser radiation. The electrical signal is processed by the electronics and analysed by software. As the wavelength of the laser is varied, it is found that the intensity of the laser radiation will also vary depending on the particular wavelength of the laser radiation propagating through the gas cell 300. The change in intensity is found to correlate with a concentration of the gas present in the cell.

Referring to Figure 5 of the drawings, there is illustrated an alternative embodiment of an gas sensor for the detection of gaseous chemicals. The gas sensor is similar to the gas sensor illustrated in figure 3, however, the laser radiation is arranged to follow a ring configuration. The gas sensor of figure 5 includes a QCL 400 that includes a gain medium 400a having opposing facets 401, 402. The sensor further comprises lenses 404, cavity mirrors 412, an etalon 408, a narrow band pass interference filter 410 and a gas cell 406 for holding a volume of gas which is the subject of the analysis. However, the skilled reader will recognise that one or more of the mirrors 412 may be replaced by diffraction gratings (not shown).

The external cavity (EC) is defined as the region outside of the gain medium 400a, via which the lasing radiation propagates whereas the optical laser cavity (OLC) is defined as the region including the EC and the region directly between the facets 401, 402 of the gain medium 400a.

The more complicated ring-cavity geometry alignment gives this embodiment improved intensity stability due to uni-directional operation around the ring.

The QCL device 400 comprises a dielectric anti-reflection coating 401, 402 disposed on the facets thereof. These coatings 401, 402 are arranged to suppress longitudinal modes of the laser chip since the existence of multiple modes may result in external cavity laser instabilities. To further reduce the remaining reflectivity, the gain medium 400a is fabricated so that both of the facets 401, 402 are at an angle to an optical axis of the gain medium to reduce retro-reflections that could be coupled back into the laser cavity. The facets 401, 402 for example, may be angled by forming a laser ridge for example, which is curved towards one end before cleaving or by etching the facet 401, 402 at an angle between 2 degrees and 16 degrees. The combination of an angled facet 401, 402 with an anti-reflection coating with low residual reflectivity reduces reflections from the facet 401, 402 that are coupled back into the laser cavity to 0.01 % or less, improving the reliability with which the QCL 400 operates. In the case of the linear external cavity, the laser ridge may be curved towards one facet 301 and then cleaved so one laser facet facing the external cavity is at some angle to the laser ridge while another facet is still perpendicular to the laser ridge and which may be high-reflectivity coated to increase reflection on this non-angled facet. Combining the angled facet geometry with the dielectric anti-reflection coating may decrease the remaining reflectivity to less than 0.01%.

To improve collection efficiency from an angled laser facet and to make the external cavity alignment easier, an additional antireflection coated, miniature hyper semi-spherical, semi- spherical or elliptical lens (acting as solid immersion lenses) may be attached/glued (for example, with an epoxy) directly onto this facet. Such a lens may be manufactured from any mid-infrared transparent material with a high refractive index, for example, silicon or germanium. If such a material is also transparent in the visual range, for example, zirconium oxide, sapphire or calcium fluoride, this may dramatically simplify the alignment of this lens on top of the laser ridge.

To improve the feedback from the external cavity and provide the coarse wavelength selectivity, the narrow band pass filter may be manufactured from a material transparent to laser radiation having a wavelength in the mid-infrared spectral region, for example, silicon, sapphire or germanium and may have the peak transmission of more than 80% with the transmission bandwidth of less than ~2%, relative to the central wavelength of the transmission.

To improve the feedback from the external laser cavity, the cavity mirror(s) 412 may be high reflectivity coated (>99%).

To provide a fine and dynamic adjustment to the system alignment (to keep the laser intensity maximised during the operation or calibration) one or more of the cavity mirrors may be attached to, for example, piezo actuators to facilitate fine positional control.

To provide the long term alignment stability one or more system components may be mounted onto one or more thermoelectric coolers. All the sub-mounts for the optical elements and for the laser chip have to be manufactured from materials with high thermal conductivity and low thermal expansion, for example, ceramic, AIN or CuW alloys.

Select embodiments of the invention only have been described and illustrated, and it will be readily apparent that other embodiments, modifications, additions and omissions are possible within the scope of the invention.

The system and method of the invention may be varied according to requirements, having as its ultimate objective detecting the presence of certain gaseous chemicals in an improved manner.