Photoacoustic chemical detector

Patent metadata

Field	Value
Publication number	US-9995674-B2
Application number	US-201414584004-A
Country	US
Kind code	B2
Filing date	Dec 29, 2014
Priority date	Dec 26, 2013
Publication date	Jun 12, 2018
Grant date	Jun 12, 2018

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.

First claim

What is claimed is:

1. A laser vibrometer capable of detecting chemical species, comprising:
a light source configured to produce 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.

2. The laser vibrometer of claim 1 , 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.

3. The laser vibrometer of claim 2 , including:
an external mirror configured to reflect the external probe beam.

4. The laser vibrometer of claim 3 , including:
a lens configured to direct the external probe beam after the external probe beam reflects from the external mirror.

5. The laser vibrometer of claim 1 , wherein
the laser produces a beam of light having a wavelength of about 2.3 microns to detect carbon monoxide.

6. The laser vibrometer of claim 1 , wherein:
the laser produces a beam of light having a wavelength of 1.6 or 3.3 microns to detect methane.

7. The laser vibrometer of claim 1 , wherein:
the laser comprises a nonlinear device configured to generate tunable laser wavelengths.

8. The laser vibrometer of claim 1 , including:
a housing defining an interior space, and wherein the light source, the pressure-sensing diaphragm, and the photo-EMF sensor are disposed in the interior space.

9. The laser vibrometer of claim 8 , wherein:
the external probe beam travels outside of the housing.

10. The laser vibrometer of claim 1 , wherein:
the pressure-sensing diaphragm comprises ZnO that is nanolayered onto a silicon-based layer of material.

11. The laser vibrometer of claim 10 , wherein:
the silicon-based layer of material comprises a silicon carbide.

12. The laser vibrometer of claim 1 , 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.

13. The laser vibrometer of claim 12 , wherein:
the detector material comprises CdSe having multiple doping of transition elements into the CdSe.

14. The laser vibrometer of claim 12 , wherein:
the photo-EMF sensor comprises a nanotechnology based bandgap tuned device.

15. The laser vibrometer of claim 12 , wherein:
Wherein the reference beam is shifted to a different frequency than that of the sensing beam.

16. 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.

Assignees

• Nasa
• Nasa

Inventors

• Prasad Narasimha S

Classifications

• G01N2021/1704

  in gases · CPC title

• G01N29/46

  by spectral analysis, e.g. Fourier analysis {or wavelet analysis (spectral signal processing per se G06F17/14)} · CPC title

• G01N29/2418

  using optoacoustic interaction with the material, e.g. laser radiation, photoacoustics (photoacoustic cells G01N21/1702; measuring characteristics of vibrations by using radiation-sensitive means G01H9/00; acousto-optical conversion techniques for short-range imaging G01S15/8965; sound-producing devices using laser bundle G10K15/046) · CPC title

• G01H9/006

  the vibrations causing a variation in the relative position of the end of a fibre and another element · CPC title

• G01N21/1702 Primary

  with opto-acoustic detection, e.g. for gases or analysing solids · CPC title