Photoacoustic chemical detector
Inventors
Assignees
National Aeronautics and Space Administration NASA
Publication Number
US-9995674-B2
Publication Date
2018-06-12
Expiration Date
2034-12-29
Interested in licensing this patent?
MTEC can help explore whether this patent might be available for licensing for your application.
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.
Core Innovation
The invention is a laser vibrometer designed for the detection of ambient chemical species based on the photoacoustic effect. It utilizes a laser to produce a monochromatic light beam that is split into a reference beam and a sensing beam. A probe laser beam, tuned to an absorption feature corresponding to a molecular transition of the chemical species, excites the species to generate acoustic signals. These acoustic signals cause a thin membrane or diaphragm to vibrate, which is then detected interferometrically by mixing the sensing beam reflected from the vibrating diaphragm with the reference beam at the surface of a photo-electromotive force (photo-EMF) detector. The photo-EMF detector converts the motion of the interference fringes into an electric current proportional to the diaphragm's vibration, enabling highly sensitive detection of chemical species.
The problem being addressed arises from limitations of conventional microphones and optical microphones in detecting very weak acoustic signals such as those generated by trace chemical species. Traditional microphones feature mechanical components that add weight and limit low-frequency response and sensitivity. Optical interferometer-based detection is constrained by the wavelength of light and typically cannot detect diaphragm movements below nanometer or picometer scales. This limits their applicability for detecting extremely weak acoustic signals remotely, such as trace gases or chemical agents at distances of tens of meters, where detection sensitivity at femtometer displacement scales is needed.
This invention overcomes these limitations by employing a laser vibrometer configuration coupled with a photo-EMF sensor to detect diaphragm displacements at the femtometer or sub-femtometer scale. The vibrometer may use multiple bounces of the sensing beam on the diaphragm to amplify the phase modulation and enhance detection sensitivity. The use of a probe laser with wavelength matched to absorption features of specific chemical species efficiently generates photoacoustic signals for detection. The photo-EMF detector is optimized by tuning its bandgap via doping or nanotechnology to enhance sensitivity and spectral range consistent with the laser wavelengths used. The sensitive diaphragm may be fabricated with ZnO nanolayers on silicon carbide or silicon substrates to provide the necessary mechanical properties for femtometer-level vibration detection.
Claims Coverage
The patent includes two independent claims that define a laser vibrometer system and a chemical species detector with key inventive features related to light sources, pressure-sensing diaphragms, photo-EMF sensors, and methods of beam handling and signal detection.
Laser vibrometer with external probe beam tuned to chemical absorption
A laser vibrometer comprising a light source producing monochromatic light beams including an external probe beam having a wavelength corresponding to the absorption feature of a target chemical species, a reference beam, and a sensing beam. It uses a pressure-sensing diaphragm that vibrates responsively when impacted by pressure waves from the interaction of the external probe beam with the chemical species. The sensing beam is directed to the diaphragm and subsequently to a photo-EMF sensor which outputs a signal corresponding to diaphragm displacement caused by incident pressure waves.
Multi-level beam splitting for generation and handling of beams
The laser includes a beam splitter arrangement: a first beam splitter divides the monochromatic light into an external probe beam and an internal beam; a second beam splitter splits the internal beam into the reference beam and sensing beam, with the reference beam directed to a photosensor. An external mirror and lens may be employed to direct the external probe beam.
Spectral tuning of photo-EMF sensor material
The photo-EMF sensor material bandgap is tuned based on absorption features of targeted chemical species through techniques such as multiple doping of transition elements into CdSe or nanotechnology-based bandgap tuning. This improves the sensor's absorption sensitivity and spectral range.
Highly sensitive diaphragm structure
The pressure-sensing diaphragm includes ZnO nanolayered onto silicon-based substrates such as silicon carbide to achieve detection sensitivities down to displacements as small as 10 femtometers or less.
Frequency shifting and heterodyning for phase measurement
The reference beam is frequency shifted relative to the sensing beam, enabling the sensor to perform heterodyne phase measurements that enhance the detection resolution of diaphragm displacements.
Integrated housing and beam arrangement for chemical species detection
The light source, diaphragm, and photo-EMF sensor are housed within an interior space, while the chemical species to be detected is external. The external probe beam is directed outside the housing, and the sensing beam interacts with the diaphragm inside for detection.
These inventive features collectively enable detection of chemical species by photoacoustic excitation and sensitive interferometric measurement of pressure-sensing diaphragm vibrations with enhanced spectral and displacement sensitivity, employing tailored laser wavelengths, beam configurations, and advanced sensor materials.
Stated Advantages
Enables detection of diaphragm displacements at the femtometer scale or less, significantly improving sensitivity to weak acoustic signals from chemical species.
Enhanced absorption sensitivity and spectral range of the photo-EMF sensor due to bandgap tuning by doping and nanotechnology.
Multi-bounce arrangements amplify phase modulation, resulting in signal strength enhancement proportional to the square of the number of bounces.
Sensitive diaphragm materials such as ZnO nanolayers on silicon carbide provide improved response to photoacoustic vibrations.
Capability for in-situ, short-range, and extended range chemical detection with high concentration resolution.
Documented Applications
Ambient chemical species detection including toxic agents such as explosives, nerve gas, and atmospheric trace gases.
Wavelength calibration for laser-based chemical detection LIDAR systems.
Photoacoustic imaging for profiling inhomogeneities in test samples.
Monitoring stored energies in photochemical reactions including photosynthesis processes in various environments.
Interested in licensing this patent?