Broad band intra-cavity total reflection chemical sensor

• US 5,835,231 A
• Filed: 10/31/1997
• Issued: 11/10/1998
• Est. Priority Date: 10/31/1997
• Status: Expired due to Fees

• Current Assignee: The United States of America As Represented By The Secretary of Commerce
• Original Assignee: THE GOVERNMENT OF THE UNITED STATES OF AMERICA, AS REPRESENTED BY THE SECRETARY OF COMMERCE
• Inventors: Pipino, Andrew C. R.
• Primary Examiner(s): McGraw, Vincent P.
• Application Number:
  US08/962,171
• Time in Patent Office: 375 Days
• Field of Search: 356/436, 356/437, 356/440
• US Class Current: 356/440
• CPC Class Codes: G01J 3/42 Absorption spectrometry; Double beam spectrometry; Flicker spectrometry; Reflection spectrometry beam switching arrangements G01J3/08
• CPC Class Codes: G01N 21/031 Multipass arrangements
• CPC Class Codes: G01N 21/31 Investigating relative effect of material at wavelengths characteristic of specific elements or molecules, e.g. atomic absorption spectrometry G01N21/72 takes precedence
• CPC Class Codes: G01N 21/552 Attenuated total reflection
• CPC Class Codes: G01N 30/74 Optical detectors measurement of intensity, velocity, spectral content, polarisation, or phase of infrared, visible or ultraviolet light G01J

Abstract

A broadband, ultrahigh-sensitivity chemical sensor is provided that allowsetection through utilization of a small, extremely low-loss, monolithic optical cavity. The cavity is fabricated from highly transparent optical material in the shape of a regular polygon with one or more convex facets to form a stable resonator for ray trajectories sustained by total internal reflection. Optical radiation enters and exits the monolithic cavity by photon tunneling in which two totally reflecting surfaces are brought into close proximity. In the presence of absorbing material, the loss per pass is increased since the evanescent waves that exist exterior to the cavity at points where the circulating pulse is totally reflected, are absorbed. The decay rate of an injected pulse is determined by coupling out an infinitesimal fraction of the pulse to produce an intensity-versus-time decay curve. Since the change in the decay rate resulting from absorption is inversely proportional to the magnitude of absorption, a quantitative sensor of concentration or absorption cross-section with 1 part-per-million/pass or better sensitivity is obtained. The broadband nature of total internal reflection permits a single device to be used over a broad wavelength range. The absorption spectrum of the surrounding medium can thereby be obtained as a measurement of inverse decay time as a function of wavelength.

Claims

1. An intra-cavity total reflection apparatus for high sensitivity measurement of the optical absorption of a test material, said apparatus comprising:

injecting means for producing light for a predetermined length of time;

a closed stable optical cavity, comprising an internal total reflecting surface, for receiving said light within said cavity and for providing total internal reflection of said light at said internal surface so as to generate a plurality of evanescent waves which escape from said cavity at spaced points and which decay within a length outside of said cavity beyond said surface, said test material being disposed outside of said cavity within the decay length of one of said evanescent waves; and

measuring means for monitoring one of said evanescent waves to determine the decay time of said light.

2. An apparatus as claimed in claim 1, wherein said closed stable optical cavity has one of a polygonal, spherical, cylindrical or toroidal shape.

3. An apparatus as claimed in claim 2, wherein said surface further comprises an input convex interfacing facet for receiving said light and an output convex interfacing facet through which one of said evanescent waves is transmitted to said measuring means.

4. An apparatus as claimed in claim 3, wherein said input and output convex interfacing facets have a radius of curvature that is dependent on the size of said cavity.

5. An apparatus as claimed in claim 3, wherein said injecting means includes at least one lens for transverse mode matching said light.

6. An apparatus as claimed in claim 1 wherein said cavity is comprised of fused silica.

7. An intra-cavity total reflection apparatus for high sensitivity measurement of the optical absorption of a test material, said apparatus comprising:

injecting means for producing light for a predetermined length of time;

input coupling means for receiving said light and transmitting a portion of said light;

a polygonal shaped stable optical cavity, comprising a total reflecting internal surface including an input convex interfacing facet, for receiving at said input convex interfacing facet said light portion by way of photon tunnelling and for providing total internal reflection of said light portion at said surface within said cavity while generating a plurality of evanescent waves which escape from said cavity at spaced points and which decay within a length outside of said cavity, said test material being disposed outside of said cavity within the decay length of one of said evanescent waves; and

measuring means for monitoring one of said evanescent waves to determine the decay time of said light portion.

8. An apparatus as claimed in claim 7, wherein said surface further comprises an output convex interfacing facet through which one of said evanescent waves is transmitted to said measuring means.

9. An apparatus according to claim 8, further comprising an output coupling means disposed between said output convex interfacing facet and said measuring means.

10. An apparatus as claimed in claim 9, wherein said input and output convex interfacing facets have a radius of curvature that is dependent on the size of said cavity.

11. An apparatus as claimed in claim 9, said input coupling means includes a lens for transverse mode matching said light.

12. An apparatus as claimed in claim 9, wherein each of said input and output coupling means each comprises one of a piezoelectric translator, a fiber optic coupling, a prism and a wave guide.

13. An apparatus as claimed in claim 7 wherein said cavity is comprised of fused silica.

14. A method of high sensitivity measuring the optical absorption of a test material, said method comprising the steps of:

injecting light having a predetermined duration by way of photon tunnelling through a surface of a closed polygonal stable shaped optical cavity having multiple reflecting facets forming said surface so that said light circulates within said optical cavity off of said facets and generates a plurality of evanescent waves which decay within a decay length outside of said optical cavity beyond said surface;

disposing said test material outside of said cavity within said decay length of one of said evanescent waves so as to affect the decay time taken by said light to decay within said cavity; and

measuring one of said evanescent waves to determine the decay time of said light.