Photoacoustic Chemical Detector

Abstract

A laser vibrometer for measurement of ambient chemical species includes a laser that produces a beam that is split into a reference readout beam and a signal readout beam. A probe laser beam is tuned to an absorption feature of a molecular transition, and generates acoustic signals when incident on a gaseous species via the photo acoustic effect. The scattered acoustic signals are incident on a thin membrane that vibrates. The readout laser beam reflected from the vibrating membrane is mixed with the reference beam at the surface of a photo-EMF detector. Interferrometric fringes are generated at the surface of the photo-EMF detector. Electric current is generated in the photo-EMF detector when the fringes are in motion due to undulations in the signal readout beam imparted by the vibrating membrane. A highly sensitive photo-EMF detector is capable of detecting picoJoules or less laser energy generated by vibrating processes.

Claims

1-11. (canceled)

12. A laser vibrometer capable of detecting chemical species, comprising: a light source configured to produce a beams of monochromatic light including: an external probe beam, having a wavelength corresponding to an absorption feature of the chemical species to be detected; a reference beam; and a sensing beam, a pressure-sensing diaphragm which when impacted by the pressure waves resulting from the external probe beam interacting with a chemical species that is located away from the sensing beam responsively vibrates; a photo-electromotive force (photo-EMF) sensor; wherein the sensing beam is directed against the second side of the pressure sensing diaphragm; and wherein the sensing beam is directed to the photo-EMF sensor from the pressure-sensing diaphragm which photo-EMF sensor outputs a signal corresponding to the displacement of the diaphragm caused by the incident pressure wave.

13. The laser vibrometer of claim 12, wherein the light source includes: a laser configured to produce a beam of monochromatic light having a wavelength corresponding to an absorption feature of the chemical species to be detected; a first beam splitter configured to split the beam of monochromatic light into the external probe beam and an internal beam; and a second beam splitter configured to split the internal beam into the reference beam and the sensing beam, the reference beam being directed to a photosensor;

14. The laser vibrometer of claim 13, including: an external mirror configured to reflect the external probe beam.

15. The laser vibrometer of claim 14, including: a lens configured to direct the external probe beam after the external probe beam reflects from the external mirror.

16. A laser vibrometer for detecting predefined substances disposed in a gaseous medium, the laser vibrometer comprising: at least one laser configured to generate a beam of light having a wavelength that corresponds to an absorption feature of a chemical species that is to be detected that generates an acoustic signature comprising pressure waves having identifying characteristics via a photo-acoustic effect when the beam of light interacts with a chemical species to be detected; a beam splitter configured to split the beam of light into a reference beam and a signal beam; a diaphragm having a mirror surface that moves in response to pressure waves resulting from the photo-acoustic effect, wherein the mirror surface reflects the signal beam and causes the signal beam to carry intensity modulations due to movement of the mirror surface; a photo-electromotive force (photo-EMF) detector having a detecting surface arranged to receive the reference beam and the signal beam after the signal beam has been reflected from the diaphragm, and wherein moving fringes are generated on the detecting surface of the photo-EMF detector whereby the photo-EMF detector is capable of detecting motion of the mirror surface of the diaphragm on the order of 10.sup.15 meters or smaller can be detected.

17. The laser vibrometer of claim 16, wherein: the photo-EMF detector comprises CdSe detector material into which multiple transition elements are doped to enhance at least one of spectral sensitivity and spectral range.

18. The laser vibrometer of claim 16, wherein: the photo-EMF detector comprises nanoparticles of CdSe arranged in various sizes to provide bandgap tuning to enhance at least one of spectral sensitivity and spectral range.

19. The laser vibrometer of claim 16, wherein: the diaphragm comprises ZnO that is nanolayered on silicon.

20. (canceled)

21. The laser vibrometer of claim 12, wherein; the laser produces a beam of light having a wavelength of about 2.3 microns to detect carbon monoxide.

22. The laser vibrometer of claim 12, wherein: the laser produces a beam of light having a wavelength of 1.6 or 3.3 microns to detect methane.

23. The laser vibrometer of claim 12, wherein: the laser comprises a nonlinear device configured to generate tunable laser wavelengths.

24. The laser vibrometer of claim 12, including: a housing defining an interior space, and wherein the light source, the pressure-sensing diaphragm, and the photo-EMF sensor are is-disposed in the interior space.

25. The laser vibrometer of claim 24, wherein: the external probe beam travels outside of the housing.

26. The laser vibrometer of claim 12, wherein: the pressure-sensing diaphragm comprises ZnO that is nanolayered onto a silicon-based layer of material.

27. The laser vibrometer of claim 26, wherein: the silicon-based layer of material comprises a silicon carbide.

28. The laser vibrometer of claim 12, wherein: the photo-EMF sensor comprises detector material defining a bandgap that is tuned based on the absorption features of a chemical species that is to be detected.

29. The laser vibrometer of claim 28, wherein: the detector material comprises CdSe having multiple doping of transition elements into the CdSe.

30. The laser vibrometer of claim 28, wherein: the photo-EMF sensor comprises a nanotechnology based bandgap tuned device.

31. The laser vibrometer of claim 28, wherein: Wherein the reference beam is shifted to a different frequency than that of the sensing beam.

32. A chemical species detector, comprising: a light source configured to produce beams of monochromatic light including: a first beam, having a wavelength corresponding to an absorption feature of the chemical species to be detected; a second beam; and a third beam; a pressure-sensing diaphragm which when impacted by the pressure waves resulting from the first beam interacting with a chemical species that is located away from the third beam responsively vibrates; a photo-electromotive force (photo-EMF) sensor, configured and arranged to detect displacements of the pressure sensing diaphragm as little as 10 femtometers; a housing defining an interior space, and wherein the light source the pressure-sensing diaphragm, the photo-EMF sensor, the second beam and the third beam are contained within the interior space; wherein the chemical species is located outside of the interior space and the first beam is directed outside of the interior space; wherein the third beam is directed against the second side of the pressure sensing diaphragm; and wherein the third beam is directed to the photo-EMF sensor from the pressure-sensing diaphragm which photo-EMF sensor outputs a signal corresponding to the displacement of the diaphragm caused by the incident pressure wave; and wherein the second beam is frequency shifted from that of the third beam and the sensor is configured to perform phase measurements by heterodyning a frequency shifted second beam and the third beam.

Description

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

[0018] FIG. 1 depicts a laser vibrometer according to one aspect of the present invention;

[0019] FIG. 2 depicts a laser vibrometer according to another aspect of the present invention;

[0020] FIG. 3 depicts a laser vibrometer according to another aspect of the present invention;

[0021] FIG. 4 depicts a laser vibrometer according to another aspect of the present invention;

[0022] FIG. 5 depicts a laser vibrometer configured for wavelength calibration; and

[0023] FIG. 6 depicts a laser vibrometer configured for wavelength calibration according to another aspect of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] For purposes of description herein, the terms upper, lower, right, left, rear, front, vertical, horizontal, and derivatives thereof shall relate to the invention as oriented in FIG. 1. However, it is to be understood that the invention may assume various alternative orientations and step sequences, except where expressly specified to the contrary. It is also to be understood that the specific devices and processes illustrated in the attached drawings, and described in the following specification, are simply exemplary embodiments of the inventive concepts defined in the appended claims. Hence, specific dimensions and other physical characteristics relating to the embodiments disclosed herein are not to be considered as limiting, unless the claims expressly state otherwise.

[0025] With reference to FIG. 1, a laser vibrometer 1 according to one aspect of the present invention includes a light source such as a laser 10. Laser 10 can be either a continuous or a pulsed laser, preferably a pulsed laser, which may be either a conventional modestly average-powered, Q-switched and mode-locked laser, such as, a neodymium-doped yttrium aluminum garnet laser (Nd:YAG), which emits a light beam 12. The emitted light beam 12 is then split into two branches by a standard beam splitting element 14. The reference light beam 16, newly split from beam 12, is directed via a mirror 20 to a photo-EMF sensor 25. As discussed in more detail below, photo-EMF sensor 25 is preferably configured to provide enhanced absorption sensitivity and spectral range.

[0026] The sensing light beam 32 is directed onto a diaphragm 34, whose motion is being affected and controlled by the incident pressure waves 36, the acoustic signature of interest produced as a result of interaction of sensing light beam 32 and chemical species 8. The pressure-sensing diaphragm 34 has a mirror-like surface finish on at least the face 38 where the sensing beam 32 impinges thereon, to minimize any reflection and scattering optical losses that might be suffered by the reflected light beam 40. The initial reflected sensing light beam 40 may be re-directed by an optional reflective mirror assembly 42 back onto the pressure-sensing diaphragm 34 a set number of times by appropriately sizing and curving the reflective mirror assembly to enhance the measurement of the acoustic signature of pressure waves 36. While only two bounces are made by the probe light beam onto the pressure-sensing diaphragm in the embodiment shown in FIG. 1, it is to be understood that the total number of bounces can be more or less than 2, with the upper bound number being determined by the loss characteristics of the system/components involved, the distance between the diaphragm element 34 and the reflective mirror assembly 42, the power level of laser 10, and in the case of the shown pulsed light source, the laser pulse width.

[0027] The final sensing beam 44, upon completion of the desired number of bounces, exits the diaphragm mirror assembly. The final sensing beam 44 is directed onto the photo-EMF sensor 25, which heterodynes this final sensing beam 44 with the reference beam 16 to output an analog signal whose phase modulation is proportional to the displacement of the diaphragm 34 caused by the incident pressure wave 36. This analog signal is the photocurrent signal 46 that can be converted into voltage signal using a transimpedance amplifier 48, which voltage signal is displayed or sent to a digital computer 50 for analysis and reporting via a display screen 52.

[0028] The photocurrent signal 46 generated by the photo-EMF sensor 25 can be expressed approximately as:

j.sup.(t)=(t)P.sub.probe(t)(1)

where P.sub.probe(t) is the back-scattered sensing light beam power density impinging onto the photo-EMF sensor 25 and is a constant determined by the geometric arrangement of the light beams, sensor material characteristics, photon energy, as well as the reference light beam intensity. Herein (t) represents the total amount of phase modulation imposed onto the sensing light beam 32 by the pressure-sensing diaphragm 34. Equation (1) shows that stronger signal photocurrents are generated when the amount of phase modulation is increased or a higher optical power density of the sensor light beam is available. The output signal strength and its detection sensitivity may be increased by using a multi-bounce arrangement as described in U.S. Pat. No. 8,072,609 as well as the deployment of a pulsed light source, including but not limited to, Q-switched and mode-locked lasers where the laser energy is concentrated within short time periods to produce transiently very high peak optical power density levels while maintaining modest optical power density level averaged over time. However, it will be understood that the present invention is not limited to a multi-bounce arrangement, and the reflective mirror assembly 42 is therefore optional.

[0029] Considering the presence of a temporal pressure wave of sinusoidal nature and that, upon its interaction with the pressure-sensing diaphragm 34, causes the pressure-sensing diaphragm 34 to conform and exhibit surface vibrations that can be described mathematically as:

d sin(t)(2)

where is the angular frequency of the vibration as well as that of the impinging pressure wave 36 and d is the maximal displacement of the pressure-sensing diaphragm 34 under the effects of the impinging pressure wave 36. A conversion relationship exists between the strength of the impinging pressure wave 36 and the resultant surface displacement by the sensing diaphragm 34. This relationship is determined by the design, dimensions, and the characteristics of the materials forming the pressure-sensing diaphragm 34. The amount of phase modulation imposed onto the sensing light beam 32 upon its one bounce from the pressure-sensing diaphragm 34 is given by:

4d sin(t)/(3)

where is the wavelength of the light beam. By repeatedly bouncing the sensing light beam 32 onto the diaphragm 34, as shown schematically in FIG. 1, it can be shown that the total amount of phase modulation suffered by the sensing light beam 32 upon its final exit from the pressure-sensing diaphragm 34, as the final sensing beam 44, and reflective mirror assembly 42 is given by:

(t)=.sub.nd sin [t+(n1).sub.0]4/(4)

where n=1, 2 . . . N, with N representing the total number of bounces the sensing beam strikes the pressure-sensing diaphragm. The static phase .sub.0=2 L/c, where L is the separation between the pressure-sensing diaphragm 34 and the reflective mirror assembly 42 and c is the speed of light, is the additional phase delay experienced by the sensing light beam 32 upon its round-trip passage between the pressure-sensing diaphragm 34 and the reflective mirror assembly 42. It can be seen readily from Equation (4) that if the additional phase shift N.sub.0 is negligible due to, for example, the limited number of bounces or minimal separation between the pressure-sensing diaphragm 34 and the reflective mirror assembly 42 (i.e., N2 L<<the spatial extent of the laser pulses), the total phase modulation suffered by the probe light beam can then be approximated by:

(t)Nd sin(t)4/(5)

which is greater than the phase modulation imposed by the single-bounce embodiment, Equation (3), by a factor of N. Thus, under these conditions, the vibration amplitude of sensing diaphragm caused by the incident pressure waves can be effectively amplified by multi-bounce arrangements which proportionally enhance the resultant output signal strength, as clearly indicated by Equation (1). Indeed, the enhancement in the detected output signal strength expressed in power spectral density is given by N.sup.2. Thus by increasing the number of total bounces, N, the detected output signal strength caused by the impinging pressure waves can be increased.

[0030] The laser vibrometer 1 may optionally include a second laser 10A that produces a second beam of light 12A that is incident on chemical species 8. The wave length of light 12A is selected on the basis of a desired molecular transition or an absorption feature of a molecule of interest to generate acoustic signatures 36 via the photo-acoustic effect. If a second laser 10A is utilized, the beam of light 12A comprises a probe beam. The lasers 10 and 10A may be mounted in a housing 4 having one or more openings (not shown) that permit entry of gas and other substances comprising chemical species 8 into the interior space 6 of the housing 4 to permit use of sensing light beam 32 as a probe. The housing 4 may comprise a compact, hand-held module that may be utilized for in-situ and short distance measurements. The laser vibrometer 1 may also be extended for use at longer ranges by energy scaling the probe laser 10A and by efficiently collecting the acoustic signals generated from desired chemical species. Alternatively, the probe beam 12A from laser 10A may be directed to a remote location outside of the housing 4 to produce pressure waves 36 that are then transmitted into the interior space 6 of housing 4 through openings (not shown) in housing 4.

[0031] The lasers 10 and/or 10A may comprise solid-state lasers, semi-conductor lasers, or quantum cascade lasers. Furthermore, the interferometric setup can be either direct or coherent type. As discussed above, the coherent technique allows sensitive phase measurements via heterodyning that is achieved by frequency shifting of the reference beam 16 to improve concentration resolution.

[0032] The probe laser wavelength is preferably a single longitudinal mode. It is preferably pulsed or intensity modulated to obtain substantial photoacoustic signatures. The probe laser beam may be focused using a lens to increase the magnitude and range. The probe laser wavelengths can be derived from any nonlinear processes including from a tunable solid state laser, a semiconductor laser, an optical parametric oscillator providing tunable laser wavelengths, a quantum cascade semiconductor laser, oan optical parametric oscillator providing tunable laser wavelengths, or a quantum cascade laser.

[0033] Laser vibrometers according to the present invention permit sub-ppb and below measurements to be achieved. The laser virbometers can be used to sense and measure any of the atmospheric trace gasses and traces of chemical species including toxic agents such as those present in TEDs, nerve gas, etc.

[0034] According to one example, laser vibrometer 1 may be configured to sense ambient CO.sub.2 utilizing a probe laser operating at 1.571 micron spectral band. If a single laser 10 is utilized, the sensing light beam 32 comprises a probe beam. Alternatively, if a second laser 10A is utilized, the beam 12A comprises a probe beam. The laser radiation is tuned to the center of a strong transition in this spectral band and therefore generates acoustic signals 36 if CO.sub.2 is present in the gaseous medium through which the probe beam passes. As discussed above, the readout laser beam 32 reflects from the membrane 34 and carries intensity undulations due to vibrations of the diaphragm 34 and therefore generates moving fringes on surface 26 of photo-EMF detector 25. The magnitude of the photo-EMF current corresponding to the minute acoustic vibrations of the membrane 34 may be calibrated for the laser specifications, range, and probe volume of chemical species to thereby provide concentration information of the gaseous species of interest.

[0035] Diaphragm 34 may comprise a silicon carbide membrane that is several microns thick. To response to tiny vibrations generated by impinging photoacoustic signatures, the diaphragm 34 has to be highly sensitive. Diaphragm 34 is preferably capable of detecting displacements in the order of Picometer and Femtometer levels or less. This significantly benefits ambient chemical detection. Diaphragm 34 may comprise a membrane including ZnO that is nanolayered on Silicon (Si) or Silicon Carbide (SiC). Diaphragms fabricated using ZnO nanostructures on Si or SiC membranes provide enhanced response for impinging photoacoustic vibrations. ZnO has a relatively large direct bandgap energy and exhibits long lifetimes of optical phonons that facilitates vibration sensing.

[0036] Absorption sensitivity and spectral range can be enhanced by tuning the bandgap of the photo-EMF detector material of photo-EMF detector 25 to the laser vibrometer transmitter wavelengths which are selected based on the absorption feature of specific chemicals to be detected. In order to enhance the spectral sensitivity and spectral range, two techniques may be used. In the first case, multiple doping of transition elements into CdSe or similar photo-EMF detector material host matrix is utilized. In the second case, a nanotechnology based bandgap tuned photo-EMF detector 25 is utilized. Using nanoprocessing techniques, nanoparticles of photo-EMF detector material such as CdSe, arranged in various ranges of sizes enables bandgap tuning. Bandgap tuning expands the detector's spectral range. These two techniques can be used independently or simultaneously for the fabrication of advanced photo-EMF detectors 25. However, simultaneous use of both techniques may provide better performance. The field-of-view, and hence the detection sensitivity, may be enhanced by encapsulating the photo-EMF detector surface 26 in a spatially matched and integrated lens (not shown) of appropriate refractive index that provides maximum transmission to reference wavelength.

[0037] With reference to FIG. 2, a laser vibrometer 1A according to another aspect of the present invention includes a reference laser 10 and a probe laser 10A. The vibrometer 1A includes a second beam splitter 14A that combines the reference light beam 16 and final sensing beam 44 and directs the resulting beam onto surface 26 of photo-EMF sensor 25. Vibrometer 1A does not include a reflective mirror assembly 42 as shown in the laser vibrometer 1 of FIG. 1. The laser vibrometer 1A may include a support structure such as housing 4A having openings or the like (not specifically shown) that permit the light beams 12A to propagate outside of the housing 4A to detect chemical species 8 disposed outside of laser vibrometer 1A. If vibrometer 1A includes a housing, openings (not shown) in the housing permit sound waves 36 to travel from the chemical species 8 to the diaphragm 34. The wavelength of light beam 12A produced by laser 10A is selected to correspond to an absorption feature of a specific chemical species to be detected. Beam 12 produced by laser 10 may have virtually any suitable wavelength of light that is compatible with the photo-EMF detector 25 and related components.

[0038] With further reference to FIG. 3, a laser vibrometer 1B according to another aspect of the present invention includes a beam splitter 14B positioned between laser 10 and beam splitter 14. The beam splitter 14B produces an external sensing or probe beam 54 that is reflected by a mirror 20A. The external beam 54 is then focused/directed by a lens 18 to form second beam of light 12A that is directed towards the chemical species 8. Laser 10 is selected to provide a wavelength of light corresponding to an absorption feature of a specific chemical species to be detected.

[0039] With further reference to FIG. 4, a laser vibrometer 1C according to another aspect of the present invention includes a probe/reference laser 10 that produces a light beam 12 that is split into a reference light beam 16 and a sensing light beam 32. The laser vibrometer 1C includes a housing 4C having openings (not specifically shown) whereby the chemical species 8 to be detected can enter the housing 4C. The sensing light beam 32 passes through the chemical species 8 and produces pressure waves 36 that are reflected by diaphragm 34. A second beam splitter 14A combines the reference light beam 16 and sensing light beam 32 and directs the resulting beam onto surface 26 of photo-EMF sensor 25. The photo-EMF sensor 25 is preferably configured to detect extremely small changes in sensing light beam 32 due to movement of diaphragm 34. The wavelength of light beam 12 is preferably selected to correspond to an absorption feature of a chemical species 8 that is to be detected.

[0040] It will be understood that the beam combiner 14A is optional in the configurations of FIGS. 2-4 because moving fringes can be formed on the surface 26 of the photo-EMF detector 25 itself.

[0041] With further reference to FIG. 5, a laser vibrometer 1D according to another aspect of the present invention is configured for wavelength calibration. The laser vibrometer 1D does not include a dedicated laser, but rather utilizes a laser beam 120 that is generated by a chemical detection LIDAR system 150. The chemical detection LIDAR system 150 may comprise a known LIDAR system. The laser vibrometer 1D includes a housing 4D or other suitable structure that is hermitically sealed with a known quantity of a given chemical species 8. When the test beam is incident on this device, the generated signal strength may be utilized to determine the extent of the test laser beam 120 that is coincident with the absorption feature of the chemical species 8. Based on the vibration signal strength, the laser wavelength of the LIDAR system 150 can be tuned to optimize its performance.

[0042] With further reference to FIG. 6, a laser vibrometer 1E for wavelength calibration according to another aspect of the present invention includes a first beam splitter 140 (typically 50:50), a total reflector such as a mirror 142, and a beam splitter/combiner 143. A diaphragm 34 is attached to a prism 110 with AR coating at laser wavelength. A gas compartment or cell 108 is filled with a known gas of a specific concentration.

[0043] In use, a laser beam 120 from a LIDAR system 150 is initially split into a sensing light beam 122 that passes through the chemical species in the gas cell 108, and a reference light beam 126 that is reflected by the mirror 142 to form beam 128 prior to being recombined by the beam combiner 143 whereby the combined beam 130 is incident on surface 26 of photo-EMF detector 25. The configurations shown for wavelength calibration (FIGS. 5-6) may be used in a feedback loop to automatically tune the test wavelength to the maximum absorption feature of a given chemical species.

[0044] The probe laser may have one of the following wavelengths corresponding to a the listed chemical species:

[0045] Carbon Di Oxide=1.571, 2.06, and 1.6 microns

[0046] Methane=1.65 and 3.3 microns

[0047] Oxygen=0.765 and 1.26 microns

[0048] Carbon Monoxide=2.34 microns

[0049] Ozone=2.91 and 3.08 microns

[0050] Nerve gas(Sarin)=9.35 microns

[0051] RDX=7.6 microns

[0052] Sulphur Di Oxide=300 nm

[0053] Nitrous Oxide=448 nm

[0054] It will be understood that the wavelength of the probe laser of the present invention is not limited to these specific examples.

[0055] It will also be understood that the sensor devices of the present invention may be used for photoacoustic imaging for profiling inhomogeneities in test samples. The same devices may also be used for estimating/monitoring stored energies in a photochemical reaction including photosynthesis processes in various environments.