Optoacoustic Fluid Sensing Apparatus

Abstract

An apparatus for photo-acoustic measurement of a measurement target in a fluid flow comprises:an ellipsoidal measurement chamber (3) having a first focal point and a second focal point; a duct (6, 7, 8) configured to guide a fluid flow through the measurement chamber (3) along a first axis (X) through the first focal point; light source means for generating an excitation light beam of modulated intensity; means configured to pass the excitation light beam through the measurement chamber (3) along a second axis (Y), which is different from the first axis (X), such that the excitation light beam crosses the fluid flow at the first focal point and that the crossing of the fluid flow and the excitation light beam defines an excitation volume (4) within which the fluid flow is excited by the excitation light beam to generate acoustic waves; and detecting means (5) arranged at the second focal point and configured to detect said acoustic waves, wherein the detecting means has no direct contact with the fluid flow, and wherein the ellipsoidal measurement chamber has inner walls that are configured to focus the acoustic waves generated by the excitation light beam within the excitation volume (4) onto the detecting means (5).

Claims

1.-35. (canceled)

36. An apparatus for measuring a measurement target in a fluid flow (contaminant in an aqueous flow), the apparatus comprising: an ellipsoidal measurement chamber (3) having a first focal point (37) and a second focal point (38); a duct (6, 7, 8) configured to guide a fluid flow through the measurement chamber (3) along a first axis (X) through the first focal point (37); light source means (21, 22) for generating an excitation light beam (19) of modulated intensity; means (12, 15, 16) configured to pass the excitation light beam (19) through the measurement chamber (3) along a second axis (Y), which is different from the first axis (X), such that the excitation light beam crosses the fluid flow at the first focal point (37) and that the crossing of the fluid flow and the excitation light beam defines an excitation volume (4) within the fluid flow is excited by the excitation light beam to generate acoustic waves; and detecting means (5) arranged at the second focal point (38) and configured to detect said acoustic waves, wherein the detecting means has no direct contact with the fluid flow, and wherein the ellipsoidal measurement chamber has inner walls that are configured to focus the acoustic waves generated by the excitation light beam (19) within the excitation volume (4) onto the detecting means (5).

37. Apparatus for measuring a measurement target in a fluid flow (contaminant in an aqueous flow) according to claim 36, comprising light source means (21, 22), measuring means (44) with a chamber (3) and detecting means (5), wherein said measuring means constituting a measuring cell consist of an optical absorption sensor (44), the chamber (3) whereof comprises ducting means (70) to duct the flow of a fluid containing a measurement target (MT) and concentrates the energy produced in response to the excitation for detection, wherein said excitation is produced within an excitation volume (4), which is formed at the crossing of a beam (19) generated by said light source means (21), wherein the acoustic chamber (3) has a curved shape that is such that the excitation volume (4) is formed at a first focal point (F.sub.1) thereof and the detection area (5) at its second focal point (F.sub.2) located at a distance (d) from said first point (F.sub.1), characterized in that said light source means (21, 22) is configured to generate an excitation beam (19) of modulated intensity and in that the said detector means (5) is configured to detect an acoustic signal, whereby a signal is produced within the chamber (3) by excitation light, with the flow of said measurement target (MT) in their intersection with said beam (19), wherein the flow path has a first axis (X), which is defined by a sample inlet (6) and a sample outlet (8) for the sample flow, and the light beam (19) has a second axis (Y), which is defined by a light inlet (6) and inlet and exit windows (12) for the light passage, by which the species flow and the incident light beam (19) follow a mutually different optical path according to said X-, resp. Y-direction, within said chamber (3), further wherein the excitation produces energy comprising a thermal and an acoustic component, either of which is sensed by said detecting means (5), wherein said acoustic chamber (3) focuses said sample flow to a remote detection means (5) over said distance (d) from said first point (F.sub.1) of said excitation volume (4) thereby avoiding a direct contact between said measurement target (MT) and the detector (5), by virtue whereof the sensitivity in detecting the energy produced on a measurement target (MT) in response to its excitation, is increased using light of modulated intensity.

38. Apparatus for measuring a measurement target (MT) according to claim 37, wherein said chamber (3) has an ellipsoidal shape with a first focal point (F.sub.1) and a second focal point (F.sub.2), wherein said guiding means (70) guide a gas flow across the first focal point (F.sub.1) along said X axis; said light beam (19) and said flow define an intersection volume that allows excitation of said measurement target (MT), wherein said intersection volume forms said excitation volume (4) of said measurement target (MT) flow by the light beam (19) at their intersection and wherein the ellipsoid acoustic chamber (3) has said excitation volume (4) located at its first ellipsis focal point corresponding to said first point (F.sub.1) and the detection area (5) located at its second ellipsis focal point corresponding to said second point (F.sub.2), in that said ellipsoid chamber (3) concentrates acoustic energy, which is generated at the said excitation volume (4) in response to the light of modulated intensity, and focuses said acoustic energy to a remote sound detection area (5) corresponding to said second point (F.sub.2) located at a distance (d) from said first point (F.sub.1) of said excitation volume (4) along a third axis (Z), wherein said two axes (X, Y) are mutually positioned with a certain angle to each other thus forming a plane (), which said third axis (Z) does not belong to, wherein said second point (F.sub.2) defines a sound detection area, wherein said detector (5) is located to detect the energy generated in response to the light of modulated intensity, wherein said acoustic detector (5) is located away from the incoming measurement target (MT) pollutant flow, thus being remote at said distance (d) therefrom, particularly wherein said axes (X, Y, Z) are perpendicular to each other having a mutual angle of 90, wherein the species flow (MT) and the incident light beam (19) follow a mutually different optical path within said chamber (3) according to said X-, resp. Y-direction, that is mutually orthogonal, wherein the optical path of the said light beam (19) according to said Y-direction is perpendicular to said measurement target (MT) pollutant flow according to said X-, direction thereby avoiding the optics coming close to pollutant contaminants in that said acoustic detector (5) is thus kept remote from the species flow (MT) containing said pollutant contaminants.

39. Apparatus according to claim 38, comprising a light source (21, 22) for generating an excitation light beam (19) and a detector (5) for detecting acoustic waves; an ellipsoidal chamber (3) having a first and a second focal point (F.sub.1, resp. F.sub.2); guiding means (70) to guide a gas flow across the first focal point (F.sub.1) along an X axis; means for introducing the excitation light beam (19) along a Y direction passing through the first focus (F.sub.1), thereby forming an excitation volume (4); wherein said ellipsoidal chamber (3) comprises inner walls (63) configured to reflect acoustic waves generated in the excitation volume (4) towards the acoustic detector (5) located at the second focal point (F.sub.2), wherein sound is refocused by the ellipsoidal chamber (3) for an optoacoustic detection, whereas the sample configuration consists of said flow along one single axis (X) without any circulation, wherein said guiding means (70) consist of a straight section located at the reduced focal end section (37) of said ellipsoidal chamber (3) thereby passing the flow remaining a minimum of time in said ellipsoidal chamber (3), further wherein optoacoustic detection is applied without trapping medium, wherein the deposition of contaminants is minimized, wherein the concentration of contaminants in said MT is measured and determined, whereas the sample flows inside the chamber (3) and the optics, the acoustic detector (5) and the chamber (3) are then protected from contamination through an optical path (Y) perpendicular to incoming pollutant flow (X); wherein the said detector (5) is located away from the flow path being remote over said distance (d).

40. Apparatus according to claim 37, comprising multiple optical detectors that are positioned at different angles over said plane () to evaluate light scattering, in particular wherein in addition to said light of modulated intensity (21), said sensor (44) comprises additional light sources (22) and corresponding sensing means associated thereto, thereby providing complementary reading means for said measurement target (MT) via optical detection, more particularly wherein that said additional light sources (22) are at multiple wavelengths (.sub.i), wherein the light beam (19) is formed by said plurality of modulated light sources (21, 22) at different wavelengths (.sub.i), in particular laser diodes (LD) or Light Emitting Diodes (LED), respectively as low cost and compact light sources, wherein said laser diodes and LEDs are driven with very high repetition rates (duty cycles), allowing for an improved signal to noise ratio (SNR) through averaging without increasing the acquisition time, more particularly low-cost modulated by means of pulses, notably nanosecond modulation, esp. sinusoidal.

41. Apparatus according to claim 37, wherein the axis ratio a/b of said ellipsoid chamber (3) is comprised in a range between 1.5 and 4, where (a) is its major axis and (b) is the small one, particularly wherein the eccentricity of said ellipsoidal chamber (3) or the scaling factor is fine-tuned, by virtue whereof sensitivity is additionally increased; and/or wherein said chamber (3) is provided with high density solid walls (63) with high reflection power, in particular thin high density plastic walls (63) and/or metallic, preferably with metal plating of said plastic; and/or wherein said acoustic detector (5) is separated from said measurement target (MT) flow by means of a separating means made of an acoustically transparent but particle non-permeable material, thereby protecting it from contamination; and/or wherein said chamber (3) comprises two casing halves (1, 2), each having a recess in the shape of half said ellipsoid being aligned mutually according to said third axis (Z) one of which (1) shelters said acoustic detector (5), whereas the other half (2) shelters said light source (21, 22), the transmission of the light beam (19) and the flow of the measurement target (MT).

42. Apparatus according to claim 41, wherein fiber power combiners (27) are incorporated into said apparatus by means whereof the output signals of the plurality of said light sources (21, 22) is combined into one single fiber (72).

43. Apparatus according to claim 37, wherein said detector (5) is a quartz tuning fork (QTF) that is responsive only on narrow bands of acoustic frequencies, main frequency and its harmonics, thus delivering a high Q-factor, wherein said QTF delivers a high signal to noise ratio (SNR), thus increasing the sensitivity of said sensor (44), even with low power light sources (21, 22).

44. System comprising an array of sensing apparatus as defined in claim 37, wherein said array comprises at least two sensors (44, 45) which are arranged mutually in parallel, wherein a first sensor (44) is connected in normal operation, whereas the second sensor (45) is incorporated with its said measurement target (MT) flow blocked by an absorbing species (87) at said measurement target (MT) for which said first sensor (44) produces a signal, particularly wherein said second sensor (45) is equipped with a device, notably a filter, removing black carbon (BC) before it reaches said excitation volume (4), more particularly wherein said array of sensors is arranged as a control circuit of sensors wherein a feedback is incorporated for control of the signal of said second sensor (45) that is used to improve the measured signal from said first sensor (44), notably by means of a signal correction means, particularly wherein both said sensors (44, 45) are identical; especially wherein said sensor (44, 45) is portable.

45. Method for operating a high sensitivity optical absorption sensing apparatus as defined in claim 36, wherein said measurement target (MT) flow is introduced in said chamber (3) by entering said measurement target (MT) through the chamber's inlet (6), which is further passed through said ducting means (70) which are straight in parallel with said small axis (b) providing a shortened path to said measurement target flow (MT) thereby involving a way of reduced resistance to said measurement target flow (MT) and which measurement target flow (MT) is further exited at the chamber's outlet (8), wherein the inlet pipe (6) contains a reduced section (7) involving an acceleration for said measurement target (MT) flow and a smooth rim (76) upstream the chamber (3), having an end section (77) with a diameter corresponding to the diameter of said light beam (19) just before it enters therein, under the action whereof (76) said measurement target (MT) flow is accelerated and then focused in said first ellipsis focal point (F.sub.1).

46. Method according to claim 45, wherein said chamber (3) shelters said measurement target (MT) flow, wherein said measurement target (MT) is excited by the modulated incident light, and wherein the energy derived from the excitation of said measurement target (MT) is concentrated by its ellipsoidal configuration, thereby yielding an effective measurement with high sensitivity, wherein the energy produced by said incident light excitation on said measurement target (MT) has a thermal component with slight increase of local temperature, and an acoustic component with the generation of an ultrasound wave being detected along said third axis (Z) in said chamber (3), wherein both thermal and acoustic energies relate to the amount of light energy incident to said excitation volume (4) and the quantity of absorbing species (87) present in said measurement target; particularly wherein sound is refocused at low frequencies in the range 10-200 kHz yielding a large acoustic focal area of the order of the mm, by virtue whereof sensitivity is made independent from the exact positioning of the acoustic detector or external vibrations.

47. Method according to claim 45, for operating an apparatus as defined in claim 5, for environmental application, wherein the said thermal component of the energy produced at said measurement target flow (MT) is measured by optical detection of the temperature gradient (T) along the third axis (Z), wherein the energy dissipated by the absorbing species (87) in the said measurement target flow (MT), following their excitation by a modulated light incident beam (18), produces a temperature gradient (T) along said third detection axis (Z), wherein this local temperature increase (T) is measured by reading the heat-induced index of refraction changes in response to excitations, wherein a beam of light of wavelengths (.sub.2) different from the one (.sub.1) of said modulated incident light (19) is targeted along said third axis (Z) in the vicinity of the excitation volume (4), wherein a deflection of the beam is caused resulting from the local temperature difference (T) and the corresponding change of the refraction index at the said measurement target flow (MT) vicinity that generates a decrease of the light which is sensed by the photosensing detector (5) located on an opposite wall (63) of the chamber (3) along the beam axis (Y), wherein said decrease in light intensity is then linked to the quantity of said light absorbing species (87) in the said measurement target flow (MT).

48. Method according to claim 46, wherein the excitation volume (4) for a maximum signal to noise ratio (SNR), the high sensitivity and low detection limit of the sensor (44) are optimized, wherein the excitation volume (4) is adjusted by modifying the cross-section of said measurement target (MT) flow, the flowrate thereof, the cross-section of the modulated light beam (19) and the angle () formed between said two axes (X, Y), wherein said measurement target (MT) flow cross-section is increased by sizing the inlet (6) and outlet (8) of the sensor (44) for said MT-flow, further wherein said two openingsinlet (6) and outlet (8)are enlarged, which increases the flowrate (MT) by virtue whereof more absorbing species (87) per unit of time is brought in said excitation volume (4), by virtue whereof the sensor's sensitivity is increased; particularly wherein the cross-section of said sample flow (MT) and the light beam (19) are monitored for having the same diameters where they cross each other at their intersection.

49. Method according to claim 45, wherein said measurement target (MT) is submitted to optical monitoring, wherein particles or gases are illuminated, after which light gets both absorbed and scattered, and black carbon (BC) is identified in that optoacoustics is responsive only to light absorption and said BC is identified, whereas light detection in 180 is sensitive to both absorption and scattering while detection in other angles, such as 45 or 90, is only sensitive to scattering, wherein the sensor (44) with its ellipsoidal geometry combines both optoacoustic detection and detection of scattered light in various angles between 0 and 180 stereoscopic; in particular wherein for particles, information for particle size and potentially non-carbonaceous composition in addition to said BC mass is obtained therewith; more particularly wherein fibers guide the light scattered in different angles in sensitive photodetectors, whereas the scattering angle distribution of the light depends on the size distribution of the particles that the light illuminates, by virtue whereof a particle size distribution is derived accordingly, thereby producing the required identification data, wherein the characteristics of the pollutants being measured are deduced from the scattering versus absorption measurements thus enabling to distinguish between light absorption and scattering.

50. Method according to claim 46, wherein a moderate positive thermal gradient (T) is maintained between the pollutant path and the sensitive elements, esp. sensing elements, thereby further protecting by thermo-repulsion, thereby further avoiding contaminants deposition by buoyancy and natural convection; in particular wherein said plurality of laser diodes (21, 22) excites various substances, notably gases and particles, and wherein the total signal is spectrally unmixed to measure different pollutants.

51. Method for the detection of acoustic signals, scattered light and absorption signals at different angles according to claim 45, wherein a plurality of sample's characteristics is evaluated by means of the sensor's multiple signals, wherein the optoacoustic signal provides the mass concentration of certain gas and particulate species (87) to be identified, wherein light scattering is additionally monitored in different angles and wherein the size distribution of the particles is then calculated; further wherein the gas sample component notably including NO.sub.2, BC resp. other carbonaceous particles, CO.sub.2, SO.sub.2, dust, ashes is distinguished, by means whereof light absorption and scattering are mutually distinguished from one another.

52. Method according to claim 45, wherein different absorbers and pollutant species (87) are separated by spectral unmixing, wherein the optoacoustic signal (S.sub.) is proportional to the excitation energy of the laser source (21) the absorption value of the species (87) in said measurement target (MT) and the concentration of these species (87) as $S_{}=I_{}\underset{i}{\overset{n}{.Math.}}{}_{}^i{C}_i$ where (S.sub.) is the optoacoustic signal of the laser source with wavelength , I.sub. is the optical energy of the laser source with wavelength , .sub..sup.i the absorption of the gas or particulate i at wavelength and C.sub.i the concentration of the i.sup.th gas or particulate in said measurement target (MT), wherein a system of n equations of n unknowns is formed that is solved analytically thereby yielding the concentration of the n pollutant gases or particulates in said measurement target (MT), which are thus determined with n wavelengths.

53. Method according to claim 45, wherein electronics circuitry (20) is integrated on the sensor (44) by means whereof the laser diodes (21, 22) are driven, wherein the detected optoacoustic signal is amplified, the optical as well as the optoacoustic signals are digitized and acquired, processed and transmitted to a collection and data storage point.

54. Method according to claim 45, for operating a photoacoustic device as defined in claim 2, wherein sound is refocused by the ellipsoidal chamber (3) yielding an optoacoustic detection, whereas the sample configuration consists of said flow (MT) along one single axis (X) without any circulation, and wherein the flow (MT) remains a minimum time in the ellipsoidal chamber (3), further wherein an optoacoustic detection is applied without trapping medium, still further wherein the deposition of the contaminants to be measured is minimized, yet further wherein the concentration of contaminants in a flow is measured and determined, whereas the sample (87) flows inside the chamber (3) and the optics, the acoustic detector (5) and the chamber (3) are then protected from contamination through an optical path (Y) perpendicular to incoming pollutant flow (MT), wherein the detector (5) is located away from the flow path (Y); and wherein through thermo-repulsion, a mild positive thermal gradient is maintained between the pollutant path (X) and the sensitive elements (5).

55. A method of measuring gaseous and particulate species (87) using a photoacoustic apparatus as defined in claim 1 to measure in the exhaust of different combustion systems including cars, vessels, aircraft, stationary engines, comprising the steps of detecting and monitoring gaseous or particulate pollutants from combustion, including engines, boilers, burners and other combustion setups, are detected and monitored, wherein said sensor (44, 45) provides real-time evaluation of pollutants' concentration at said exhaust of said combustion systems, stationary engines and combustion devices, wherein said sensor in engine exhaust serves as on-board detection (OBD) sensor or on-board monitoring (OBM) sensor, wherein said sensor is configured as a particle number (PN) sensor; or wherein air quality is detected and measured for atmospheric pollution concentrations and/or wherein light at different wavelengths is used and/or wherein the optoacoustic sensor is used as a multicomponent sensor, wherein the CO.sub.2 is measured to monitor the actual emissions of CO.sub.2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0065] FIG. 1 is a prospective view of an embodiment of the optoacoustic apparatus with sensor of the present invention;

[0066] FIG. 2 is a sectional view of an enlarged representation of the apparatus with sensor of FIG. 1 taken along the line A-A;

[0067] FIG. 3 is a sectional view of the apparatus with sensor of FIG. 1 taken along the plane B-B;

[0068] FIG. 4 is a detail of an implementation of the light beam, in an apparatus with sensor;

[0069] FIG. 5 is a sectional view of an embodiment of the optoacoustic apparatus with sensor of the present invention where the sensor is used for the measurement of pollutants in an exhaust line;

[0070] FIG. 6 is a sectional view in 90 degrees angle difference to the embodiment according to the invention of FIG. 5;

[0071] FIG. 7 is a schematic plan view of an embodiment according to the invention using two sensors in parallel to increase sensitivity.

DESCRIPTION

[0072] The invention relates to an optoacoustic apparatus with high sensitivity optical absorption sensor and its use for environmental applications, a first embodiment whereof is shown in FIG. 1, wherein the sensitivity in detecting the energy produced on a measurement target in response to its excitation is increased, using light of modulated intensity. The sensitivity increase is based on amplification techniques, as explained in the following.

[0073] Said apparatus comprises a sensor 44 provided with a chamber 3 having a remarkable design, which encompasses three axes X, Y, Z, allowing in a first axis X the flow of said measurement target, in a second axis Y showing the direction of incident light of modulated intensity, and in a third axis Z the detection of the corresponding energy produced. Said measurement target flow central axis X and said central axis Y of the incident light of modulated intensity are directed so as to form a plane . Said axes X, Y are at different angles to each other, also allowing for the inclusion of additional axes Z, as required for measuring optical properties of the measurement target. The chamber geometry provides protection of the optics and detectors 5 which are sheltered therein, by decomposing the axes X of the measurement target flow path MT and Y of light beam(s) 19. Measurement amplification enables the use of cheap light sources, such as laser diodes 21, 22.

[0074] Measurement target MT means herein the fluid flow containing a quantity of absorbing species present in the MT passing through the chamber 3 that is excited by the incident light beam(s) 19. Said MT could be flue or exhaust gas from any emission source, ambient air or a solute of molecules, including biomolecules. The fluid may contain different pollutants, such as black carbon or other particles, nitrogen oxides, carbon dioxide, sulfur oxides, and others that need to be detected by said sensor to determine air quality. Said MT may also be a portion of the entirety of the exhaust of an engine or of flue gas of other combustion activity, such as exhaust and flue gases produced by transport devices, like vehicles, vessels, trains, airplanes, etc. or industrial activities like combustors, incinerators, boilers, etc.

[0075] The sensor 44 has the chamber 3 where the MT enters. The excitation volume 4 in the chamber 3 is formed by the crossing of the MT flow axis X and the different axis Y of incident light of modulated frequency beam 19, at their intersection, wherein they form an angle. Preferably these axes X, Y are perpendicular to each other. However, other relative angles can be foreseen as well. The purpose of the chamber 3 is to enable the MT flow and the excitation of the MT by the modulated incident light, as well as to provide a configuration whereby the energy derived from the excitation of the MT is contained or concentrated in order to achieve an effective measurement with high sensitivity. The energy produced by incident light excitation on the MT contains a thermal component representing a slight increase of local temperature, and an acoustic component corresponding to the generation of an ultrasound wave, that is detected along the third axis Z in the chamber 3. Both thermal and acoustic energies relate to the amount of light energy incident to the excitation volume 4 and the quantity of absorbing species present in the MT. This relation is described in published literature by the thermoacoustic equationoptoacoustic, photoacoustic, or as the photothermal equationphotothermal heat generation.

[0076] The chamber 3 concentrates the acoustic component of energy generated sound and refocuses this sound generated at the excitation volume 4, to a remote sound detection area 5 along the third axis Z, where the acoustic detector 5 is located. It is thus expected to increase the reliability of signal detection, due to avoidance of pollutant contamination of the optics and the acoustic detector 5, since they are both located away from the pollutant flow. Moreover, refocusing sound at low frequencies, 10-200 kHz, results in a relatively large acoustic focal area, of the order of mm, which means that sensitivity is not dependent on the exact positioning of the acoustic detector 5 or to external vibrations, thus alleviating the operating requirements. Said concentration of sound, said avoidance of contamination and the relaxed requirements in terms of acoustic sensor positioning set out above entail that cheap light sources, such as laser diodes, are good enough to generate light while maintaining the needed sensitivity. With the use of a chamber 3 instead of a resonator, the speed of sound and hence, the temperature of the sample, does not have an important effect on the output signal, in contrast with the commonly used resonator. Moreover, a chamber may provide several degrees of freedom for combining various optoacoustic with optical detection methods to better characterize MT properties.

[0077] An ellipsoid chamber 3 is selected as the preferred geometry to achieve passive concentration and refocusing of sound. In addition, such a geometry is characterised by two mutually remote focal points F.sub.1, F.sub.2, which allows for an adequate distance in the mm to cm range between the excitation volume 4 located at the first focal point F.sub.1, and the detection volume 5 located at the second focal point F.sub.2. In an ellipsoid, the acoustic energy generated at the excitation volume 4 travels over the same distance by reflection in all directions on the ellipsoid walls 63 to reach the detection volume 5; sound is thus concentrated and refocused away from the excitation volume 4. The ellipsoid also provides enough space and wall area to evaluate optical scattering from the sample along additional axes in different angles.

[0078] In another embodiment, the measurement of the thermal component of the energy produced in the excitation volume 4 is conducted along the third axis Z. This can be achieved by a thermal detector by inferring the temperature variation in the vicinity of the MT due to the modulated energy of the incident light, as further set out below. By introducing optical detection of temperature variation along this third axis Z, relevant optics again remain at a distance from the MT and along an axis Y forming an angle with the MT flow axis X. This again serves as protection of the optics from contamination. Moreover, the measurement of the thermal component of the produced energy at the vicinity of the MT is expected to lead to an increased sensitivity over optoacoustic sensors, as the acoustic energy is only a fraction of the thermal energy produced.

[0079] The sensing apparatus preferentially utilizes low cost and compact light sources such as laser diodes LD 21, 22 or Light Emitting Diodes LED respectively. LDs and LEDs, although small in size and low cost, usually suffer from low output peak power. However, LDs can also be overdriven with up to 40-fold higher current than their continuous wave referred to as CW absolute maximum value, for only a few nanoseconds, providing up to 30-fold higher peak power than their CW absolute maximum rating, without getting damaged. In this way, the delivered power can be increased, and the SNR can be improved, therefore the sensor's sensitivity as well [7]. Additionally, the LDs and LEDs can be driven with very high repetition ratesduty cycles, that allows for improved SNR through averaging without increasing the acquisition time.

[0080] In order to keep the size of the sensing apparatus small and not to use multiple acoustic chambers 3 for different wavelengths, fiber power combiners 27 are incorporated so as to combine the output of different light sources 21, 22 into a single fiber. Fiber combiners 27 present high coupling efficiency up to >98%, therefore not limiting the sensor's sensitivity.

[0081] LDs or LEDs, as current-driven devices, can be modulated using different waveforms, such as sine waves, square pulses with different duty cycles and repetition rates, a frequency comb, triangular pulse.

[0082] Clever modulation and coding techniques, such as the Golay Codes, can be applied in order to simultaneously excite the sample with all the selected wavelengths and later disentangle the respective signals. In this way the sensor signal acquisition rate can be increased.

[0083] As to acoustic detector, sound detection along the third axis Z of the chamber 3 can be based in any sensitive kHz implementation including a so-called quartz tuning fork referred to hereafter as QTF, a microphone, MEMS or other piezoelectric detector, or optical detection of sound as elaborated in [8].

[0084] In a first embodiment of the sensing apparatus, the detector 5 is a QTF, which is responsive only on narrow bands of acoustic frequenciesmain frequency and its harmonics, thus delivering a high Q-factor. The QTF is expected to deliver a high signal to noise ratio because of its inherent characteristics, thus increasing the sensitivity even using low power light sources, i.e. without current overdriving.

[0085] In another embodiment of the sensing apparatus, a sensitive microphone, which can detect all frequencies that derive from an overdriven light source, is used as acoustic detector 5. High sensitivity is then achieved by the relatively high acoustic energy produced by the overdriven light source 21, 22. The microphone selected should be sensitive to the frequency of modulated light and its harmonics produced at the MT, and insensitive to sound frequencies of the environment that the sensor 44 operates at.

[0086] FIG. 1 shows a perspective schematic of an embodiment of the optoacoustic sensor 44. It comprises two casing halves 1, 2 having both the same recess so as to enclose a chamber 3 formed inside it, with an ellipsoid profile. One of the halves 1 provides for the positioning of the acoustic detector 5. The other half 2 provides for the positioning of the light source 21, 22, the transmission of the light beam 19 and the flow of the measurement target MT. The axes Y, X of the light beam 19 and the measurement target MT respectively are preferably perpendicular to each other and form a plane . The intersection of the two axes X, Y forms the excitation volume 4 which, together with the point of acoustic detector location F.sub.2 form third axis Z of chamber 3.

[0087] A cross-section of the said ellipsoid chamber 3 is shown in FIG. 2 along the A-A plane of FIG. 1. The chamber 3 is formed by proper hollow shaping of the two casing halves 1 and 2, preferably as two identical half ellipsoid recesses. These are made of plastic for low temperature applications or metal, or even another material, e.g. ceramic, for higher temperature applications. Proper sealing of said two halves 1, 2 is achieved using an O-ring 9 of elastomer, copper, or any other suitable material.

[0088] The measurement target MT is introduced in the chamber 3 by entering through inlet 6 and exiting through outlet 8. The inlet pipe 6 contains a proper accelerating section 7 getting narrower downstream, e.g. linearly for the MT flow in progress according to arrow F and a smoothened rim 76 just before this enters the chamber, i.e. upstream therefrom. This allows to accelerate and focus the measurement target MT flow to reduce particle loss by diffusion on the walls 63 thereof and to give it the same diameter with the light beam 19 where they cross each other, at their intersection in F.sub.1. This results in that the sample flow remains in the sensor's chamber 3 for a short time only, which minimizes particle deposition thereon. The measurement target MT crosses the light beam 19, which is perpendicular to the measurement target flow, at the first focal point F.sub.1 of the ellipsoid. The intersection of the two axes X, Y forms the excitation volume 4 which, together with the point of acoustic detector location F.sub.2, form the third axis Z of the chamber 3. The acoustic energy produced by the optoacoustic phenomenon at the excitation volume 4 is refocused by the ellipsoid chamber 3 to the second focal point F.sub.2 of the ellipsoid where a QTF is located as acoustic detector 5. Said QTF is held at position, using a conical section 10 that allows easier assembling. The second focal point F.sub.2 location is determined within several hundreds of micrometers, resulting in that precisely locating the QTF is not a requirement. An electronic circuit 11, which is used to capture the QTF's signal, is located on top of the sensor 47. An O-ring 9 is also used there for tightly sealing both casing halves 1, 2 in perfect alignment of the corresponding recessed shells of said sensor 44.

[0089] FIG. 3 shows a sensor cross-section along plane B-B of FIG. 1, which is at an angle of 90 degrees compared to the cross-section of FIG. 2 showing the path of the light beam 19. An optical fiber 18 transfers the light from the light source 21, 22 to the optical path of the sensor 44. Transparent windows 12 made of glass are used to allow the light to travel in and out of the sensor 44. A set of lenses 15, 16 are used to focus the light beam 19 to the first focal point F.sub.1 of the ellipsoid. The light beam diameter starts increasing when it exits the optical fiber 18 until it meets the first lens 16. The first lens 16 collimates the light beam 19 before it reaches the second lens 15, i.e. the second lens focuses the light beam 19 so that it has the proper diameter when it reaches the excitation volume 4. The diameter of the cross section of the focused beam at the location of the excitation volume 4 depends on the modulation frequency of the light, in order to achieve maximum sensitivity. Moreover, focusing of the beam 19 by lens 15 helps in reducing the amount of light that is lost at the walls 63 of the sensor 44 causing background acoustic noise. The light beam 19 may comprise more than one light source 21, 22, i.e. wave lengths, carried by the same optical fiber 18 to allow detection of more than one species of the measurement target.

[0090] A hollow cap 13 is used to hold the glass windows 12 in place on the side where the light exits the chamber 3 while O-rings 9 are sealing these glass windows. A second cap 14 holds the glass at the inlet of the light beam 19. That cap 14 also accommodates the two lenses 15, 16, which provide a proper space modulation of the light beam 19. A fiber connector adapter 17, which allows the optical fiber to be integrated with SM-threaded components, is also screwed on an SM-thread of the second cap 14, in a very small distance from the second lens 16. An optical fiber 18, which carries the light beam 19, is finally connected to the fiber connector adapter 17.

[0091] The use of an optical fiber 18 allows flexibility wherein the light source 21, 22 is located in relation to the sensor body. This may be needed for example where the sensor 44 operates in a high temperature environment or where the light source 21, 22 should be otherwise protected. There is an additional reason for using an optical fiber, which consists of the possibility to combine more than one light source 21, 22, for example at different wavelengths .sub.i, for detection of different species. FIG. 4 shows how the light from two different sources 21, 22 enters the fiber 18 for being transferred to the sensor 44. Two Laser Diodes 21, 22 at different wavelengths .sub.1, .sub.2 are used as light sources in that exemplary embodiment. The Laser Diodes are connected to electronic circuitry 20 which modulates their output. A twin set of two lenses 23/24, 25/26 couples the light beams 28, 29 to a 12 fiber optic coupler 27. The fiber optic coupler 27 produces a mixed light beam 30, which contains both wavelengths .sub.1, .sub.2.

[0092] FIG. 5 illustrates a second exemplary embodiment of the sensor, wherein it is configured for installation in a vehicle's exhaust line or in an industrial stack for flue gases. The sensing part of the sensor is identical to the one presented in FIGS. 1 to 3. However, the measurement target MT flow path upstream of the sensor is changed to allow resistance of the sensor to high temperatures and a self-induced flow for the measurement target (exhaust or flue gases). The sensor is held on said exhaust line or stack by means of a retaining screw 31, e.g. of an M20 size, similar to the one used for automotive exhaust sensors today. The desired flowrate through the sensor is created by a tip 32 which employs the Bernoulli principle. Based on this, the forced motion of the exhaust or flue gas in said exhaust line or stack, respectively, creates an underpressure at tip outlet 33. This creates a flowrate of the exhaust gas through an inlet 34, which travels in a sleeve 35 and enters the sensor chamber 3 through an inlet 36. An ellipsoid chamber as in FIGS. 1, 2 and 3 is used as well in this case. Thanks to this remarkable configuration, the sound waves created in the first focal point 37 are refocused therein where the light beam is focused to the QTF placed in the second focal point 38. Again, the latter is located away from the pollutant source to avoid contamination of QTF as sensitive material. An electronic circuit 39 is used again to capture the signal. An O-ring 40 is used for sealing the sensor.

[0093] FIG. 6 shows the same embodiment in a cross-section perpendicular to the plane of FIG. 5. In this section, the path of the light beam is illustrated. An optical fiber 43 transfers the light from the light source to the sensor inlet 34. The harness 42 for the optical fiber 43 may also carry the wiring for the sensor signals 41. In this way, the light sources and the electronics box are at a distance from the sensor to avoid impacts of vibration and temperature, as well as connection flexibility to the signal's bus of the vehicle or the industrial plant where the sensor is installed.

[0094] Finally, FIG. 7 shows a third exemplary embodiment where two identical sensors 44, 45 are used in a differential way. This configuration assists in improving the sensitivity, in particular for black carbon. First sensor 44 and second sensor 45 receive the measurement target sample from a common inlet line 46. A high efficiency particulate air filter 47 is used upstream of second sensor 45 in order to filter out all particulate matter, including black carbon, before such species enters the sensor chamber. Thus, the signal from second sensor 45 is a weak signal due to any interference from gas phase species and noise due to light diffusion on the sensor walls 63. By subtracting the signal of sensor 45 from first sensor 44, one creates a differential which is proportional to the concentration of black carbon. This configuration allows increase of the sensitivity for ambient measurement of black carbon, in this way also correcting the impact of environmental conditions such as humidity, temperature, etc. on the sensor signal. When more than one light source is used in each sensor 44, 45, e.g. one for black carbon and one for a different gaseous species, such as CO.sub.2 or NO.sub.2, the signal of second sensor 45 in this gaseous species, which is not influenced by contamination from particulate matter, can be used as a reference to correct the signal of first sensor 44, when comparing the response of the sensors for the same gas species.

[0095] The operation of said optoacoustic device is set out hereafter.

[0096] Thermal Detector

[0097] Optical detection of the temperature gradient along the third axis Z is the preferred method to measure the thermal component of the energy produced at the MT. The energy dissipated by the absorbing species at the MT, following their excitation by the modulated light incident beam 19, produces a temperature gradient along the third detection axis Z. This local temperature increase can be measured by means of reading the heat-induced index of refraction changes in response to excitations, as also captured in [8], [9].

[0098] In such an embodiment, an optical beam 19 of light of different wavelength of the modulated incident light is targeted along the third axis Z in the vicinity of the excitation volume 4 and a photosensing element is located on the opposite wall of the chamber 3 along the beam axis 19. The deflection of the beam 19 as a result of the local temperature difference and the corresponding change of the refraction index at the MT vicinity results in a decrease of the light sensed by the photosensing element. The decrease in light intensity sensed is then linked to the quantity of light absorbing species at the MT.

[0099] Sensor Sensitivity

[0100] By adjusting the pulse width, the repetition rate of the pulse train, the parameters of the frequency comb, or other parameters of the modulation function, the sensor's sensitivity can be further improved. The excitation volume 4 can also be optimized for maximum SNR, high sensitivity and low detection limit of the sensor 44. The excitation volume 4 is adjustable by modifying the cross-section of the MT flow, the flowrate of the MT flow, the cross-section of the modulated light beam and the angle of the two axes X, Y formed. The MT flow cross-section can be increased by properly sizing the inlet 6 and outlet 8 of the sensor 44 for the MT flow. Increasing the two openings, inlet and outlet, may also be used to increase the flowrate which results in more absorbing species per unit of time brought in the excitation volume 4.

[0101] Ideally, the cross-section of the MT flow and the light beam 19 should have the same diameters where they cross each other. For a higher cross-section of the MT flow, a wider beam for the incident light is required. With the appropriate choice of collimation and focusing optics, the shape of the laser beam 19 on the excitation volume 4 can vary from a small, tightly focused, point to a large collimated beam. The spatial shape and time modulation waveforms can be chosen so that the sensor's sensitivity is maximum and the detection limit low.

[0102] The chamber wall 63 material is selected for optimal sound reflection and minimum transmission and absorption of incident waves to improve the sensitivity. Thin, high-density solid walls 63 offer such characteristics. High-density plastic offers a good compromise between low material cost and high sound reflection. Metals, such as steel, and aluminum and bronze X offer advanced sound reflection but may entail higher manufacturing costs. Metal plating of the plastic wall entails decreased costs and enhanced sound reflection properties. Care is taken to avoid material corrosion in specific sensor applications that are exposed to corrosive fluids.

[0103] Minimum light absorption of the wall 63 material is also required to avoid thermal and acoustic energy generation by any diffuse light in the chamber 3. Such energy generation would result in increasing the background of the measurement thus reducing sensitivity. Techniques to increase the light reflectance by means of surface polishing, metal plating, or painting with light color of the surface walls are expected to improve the signal to noise ratio.

[0104] By attaching the amplifier circuit on the sensor 44, optimal amplification and transmission of the signal to the acquisition unit is achieved for minimum losses and maximum SNR. Increased sensitivity is achieved by optimizing the chamber's 3 geometry, and with its said ellipsoid shape by fine-tuning the eccentricity or the scaling factor in order to increase sensitivity without any compromise on contamination, notably by selecting an a/b ratio possibly in the range 1.5 to 4.

[0105] Sensitivity is also increased by improving the signal to noise ratio SNR. For promoting this, two sensors 44, 45 are arranged in parallel, with the first sensor 44 in normal operation and the second one 45 with its MT fluid flow blocked to the species for which the first sensor 44 produces a signal. The second sensor 45 can be equipped with a device to remove black carbon before this reaches the excitation volume 4. The signal of the second sensor 45 can be used to improve the measured signal from the first sensor 44, using known signal correction techniques, including interference bias notably from environmental pollutants or cross-interference, offset and linearity corrections.

[0106] An optical monitoring of measurement target is performed as follows. When particles or gases are illuminated, light will be both absorbed and scattered. Optoacoustic is responsive only to light absorption which makes them ideal for BC identification. Light detection in 180 is sensitive to both absorption and scattering while detection in other angles, e.g. 45 or 90, is only sensitive to scattering. The sensor 44 may combine both optoacoustic detection and detection of scattered light in various angles between 0 and 180 stereoscopic. In particular for particles, this will allow obtaining information for particle size and potentially non-carbonaceous composition in addition to BC mass. For this purpose, the sensor 44 may utilize fibers to guide the light scattered in different angles in sensitive photodetectors. The scattering angle distribution of the light depends on the size distribution of the particles that the light illuminates. According to the scattering theories of Mie, Rayleigh or Rayleigh-Debye-Gans, a particle size distribution can be derived.

[0107] Regarding decrease of contamination, retaining clean optics and a clean sound detector is essential to retain long-term sensor durability. Accumulation of particles on the optical path decreases light sensitivity, while accumulation on the sound detector changes its natural frequency. Long term operation is naturally achieved by locating the sensitive components away from the flow path. The acoustic detector 5 is by design located away from the incoming pollutant flow. The optical path Y is also by design perpendicular to the pollutant flow X to avoid the optics coming close to pollutant contaminants. To further avoid contaminants deposition also by buoyancy and natural convection, a mild positive thermal gradient can be retained between the pollutant path and the sensitive elements to enable further protection by thermo-repulsion. An acoustically transparent but particle non-permeable material can be used to separate the acoustic detector from the MT flow to protect it from contamination.

[0108] Measurement of the target flowrate is carried out with the chamber 3 introducing minimum resistance to the MT flow thanks to its simplified design as shown in FIG. 2. Therefore, the MT flow can be generated with multiple means. A small flow generator such as a pump, can be connected to the outlet duct of the chamber 3 and create a flow by underpressure: This guarantees a steady flowrate in the order of some liters per minute and can be used for both ambient and emission source measurements.

[0109] For environmental applications, a pump can be avoided by proper shaping of the inlet and outlet ducts of the sensor. Several methods can be used to establish a small flowrate through the sensor. Directing a weak flow stream created by a fan at a properly angled outlet duct creates an MT flow due to the Bernoulli effect. The fan creates an air stream at a direction with an angle to the outlet duct axis. If the angle is above 90 degrees, the accelerating flow around the duct exit creates underpressure and generates an MT flow in the chamber 3. A second method involves the formation of a small temperature gradient in the chamber along the MT flow axis X. This can be created by a small heat source, such as an electrical resistance positioned at the outlet duct walls, which generates an MT flow by natural convection.

[0110] In conditions where the species of interest are in forced motion before entering the sensor 44, such as in exhaust lines of vehicles or vessels, or flue gases in stacks, the line transporting the species can serve as the sensor chamber 3. In this case, the MT flow is the actual flow of the transported fluid in said exhaust line or stack. In another configuration, the forced motion of the measurement species can be employed to create a flowrate through the sensor 44, due to Bernoulli principle. In such an embodiment, a Bernoulli-based sensor tip is placed in said exhaust line/stack creating a pressure difference between the inlet 6 and outlet 8 of the sensor 44 and consequently a flowrate through the measurement chamber 3.

[0111] By multi-wavelength illumination and spectral separation, the different absorbers and pollutant species can later be separated using spectral un-mixing methods. The Optoacoustic signal S.sub. is proportional to the excitation energy of the laser source, the absorption value of the species in MT and the concentration of these species:

[00002]$S_{}=I_{}\underset{i}{\overset{n}{.Math.}}{}_{}^i{C}_i$

where S.sub. is the optoacoustic signal of the laser source with wavelength , I.sub. is the optical energy of the laser source with wavelength , .sub..sup.i the absorption of the gas or particulate i at wavelength and C.sub.i the concentration of the i-th gas or particulate in the MT. Therefore, a system of n equations of n unknowns is formed that can be easily solved analytically to determine the concentration of the n pollutant gases or particulates in the MT, as long as there are n wavelengths.

[0112] Laser diodes and LEDs are available in many wavelengths, covering the range from the UV, the visible and the NIR (300 nm up to 1500 nm). The different wavelengths will excite different gases such as NO.sub.2 (350 nm-600 nm), black and brown carbon particles, mainly in the visible and NIR spectrum, CO.sub.2 (1400 nm), SO.sub.2 (300-320 nm). The different absorbers, pollutant gases and particulates, can later be separated using the abovementioned spectral unmixing methods. Using this method, the sensor 44 is able to detect and monitor in real time multiple gases and light-absorbing particulate species simultaneously.

[0113] When only a limited number of wavelengths is available, filters can alternatively be used to remove certain gases or particulates from the MT and extract information about the different pollutants by simple subtraction methods. Such an implementation is presented in FIG. 5.

[0114] Proper electronics circuitry is integrated on the sensor 44 to drive the laser diodes 21, 22, to amplify the detected optoacoustic signal, to digitize and acquire the optical as well as the optoacoustic signals, process them and transmit them to a collection and data storage point. In order to achieve this, microprocessors, e.g. Arduino, field programmable gate arrays (FPGA), analog to digital converters (ADC), operational and trans-impedance amplifiers, Bluetooth or other transmitting technologies can be used.

[0115] A wide variety of applications of the present sensing system notably includes the sensor detecting and monitoring gaseous or particulate pollutants from combustion, including engines, boilers, burners and other combustion setups. In such applications, it can provide real-time evaluation of pollutants' concentration at the exhaust of cars, vessels, airplanes and stationary engines and combustion devices. One particular application of such a sensor in engine exhaust would be to server as on-board detection sensor or on-board measurement sensor referred to hereafter as OBD and OBM respectively. In particular for such applications, knowledge of the concentration and size distribution of particles in the MT mean the sensor can be configured as a particle number sensor referred to hereafter as PN.

[0116] In addition, use of light at different wavelengths entails the sensor's use as a multicomponent sensor. In particular, the possibility to measure CO.sub.2 is important because this can be used to monitor the actual emissions of CO.sub.2, a measure which is not possible today.

[0117] In applications in ships, the measurement of SO.sub.2 is important because of the regulation of sulfur in fuels. This is achieved again by using the suitable wavelength for the light source.

[0118] The low energy consumption, packed size, low cost and increased sensitivity means the sensor can also be used for ambient studies. Primarily, it can be used for pollutants concentration monitoring of singular locations. In such a case, the sensor is positioned in the location where measurements on the concentration of pollutants are required. Such a location can be either in the open environment (atmosphere) or in specific location close to an emissions source (field) or an enclosed location for the monitoring of occupational or general indoor air quality.

[0119] A distributed network of such sensors for environmental sampling provides information about the air quality of the environment. The sensor can be combined with the signal transmission functionality that will allow storage of information to the cloud. That network also provides an excellent input for aerosol modeling by climate models, which traditionally have to assume particle mass concentration or particle absorption cross section.

[0120] Artificial Intelligence, Meta-data is further involved in that a great quantity of data will be made available by utilizing a network array on a regional or even global level. These data can then be collected and stored. By means of artificial intelligence algorithms, data can be processed in a very efficient way and complex patterns can be identified. For example, the source of pollution for remote areas can be identified and countermeasures can be designed with increased accuracy. Patterns regarding pollutants' aging can also be evaluated.

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