Dual interferometer spectroscopic imaging system

• US 5,285,261 A
• Filed: 07/06/1992
• Issued: 02/08/1994
• Est. Priority Date: 07/06/1992
• Status: Expired due to Term

• Current Assignee: Sabic Innovative Plastics IP BV (Government of Saudi Arabia)
• Original Assignee: General Electric Company
• Inventors: Dumoulin, Charles L.
• Primary Examiner(s): Warden, Robert J.
• Assistant Examiner(s): JASTRZAB, KRISANNE MARIE
• Application Number:
  US07/909,277
• Time in Patent Office: 582 Days
• Field of Search: 356/432, 356/345, 356/355, 356/357-358, 356/360
• US Class Current: 356/432
• CPC Class Codes: G01B 9/02004 using frequency scans
• CPC Class Codes: G01B 9/02014 by using pulsed light
• CPC Class Codes: G01N 2021/1725 Modulation of properties by light, e.g. photoreflectance
• CPC Class Codes: G01N 2021/1731 Temperature modulation
• CPC Class Codes: G01N 2021/1776 Colour camera
• CPC Class Codes: G01N 21/1702 with opto-acoustic detection, e.g. for gases or analysing solids
• CPC Class Codes: G01N 21/171 with calorimetric detection, e.g. with thermal lens detection

Abstract

To obtain spectra from the surface of a sample, a first of a pair of interferometers includes a broadband radiation source and modulates the radiation at a frequency which is inversely proportional to wavelength. The modulated radiation impinges on the surface of interest where it is absorbed. The absorption of radiation causes the surface of the sample to expand. This change in dimension is then detected by a second interferometer which employs a monochromatic radiation source to measure the instantaneous distance between the sample surface and the second interferometer. The detection system of the second interferometer can be an imaging device such as a video camera to obtain the spatial distribution of chemical composition of the sample surface.

Claims

1. A system for creating images representing chemical composition of a sample surface based upon the electromagnetic absorption spectra thereof comprising:

a) a first interferometer means for irradiating a portion of a sample surface with a time modulated sample excitation beam having a plurality of frequencies, each frequency having an amplitude varying with time inversely according to its frequency;

b) a second interferometer means for detecting changes in distance over time between said sample surface and the second interferometer responsive to absorption by the sample of the broadband radiation beam generated by the first interferometer; and

c) image formation means for calculation of absorption spectra of the portion of said sample surface from the detected changes in distance versus time between said sample surface and the second interferometer, for creating absorption spectra for portions of said sample surface from the detected changes in distance indicating chemical composition of said sample surface, and creating an image indicating chemical composition over the portions of said sample surface.

2. The system of claim 1 further comprising positioning means for moving said sample relative to the first and second interferometers.

3. The system of claim 1, wherein the first interferometer means comprises:

a1) a beam splitter for creating a partially transmitted and partially reflected beam from an incident beam;

a2) a broadband radiation source for creating a broadband radiation beam incident upon the beam splitter such that the beam splitter creates a first transmitted beam and a first reflected beam;

a3) a fixed mirror for reflecting the first transmitted beam back toward the beam splitter, so as to be split by the beam splitter into a second transmitted and a second reflected beam; and

a4) a movable mirror being movable in a substantially uniform manner, for reflecting the first reflected beam back to the beam splitter so as to be split into a third transmitted beam and a third reflected beam such that the third transmitted beam constructively and destructively interferes with the second reflected beam to create a time modulated sample excitation beam having a plurality of frequencies, each frequency having an amplitude varying with time inversely according to its frequency.

4. The system of claim 1, wherein the second interferometer means comprises:

b1) a beam splitter for creating a partially transmitted and partially reflected beam from an incident beam;

b2) a monochromatic radiation source for creating a radiation beam having a frequency which is reflected from said sample surface, the radiation beam being incident upon the beam splitter causing the beam splitter to create a first transmitted beam and a first reflected beam, the first transmitted beam being reflected off of said source surface back toward the beam splitter which is split by the beam splitter into a second transmitted and a second reflected beam;

b3) a movable mirror, the movable mirror reflecting the first reflected beam back to the beam splitter which is split into a third transmitted beam and a third reflected beam such that the third transmitted beam constructively and destructively interferes with the second reflected beam creating a detection beam; and

b4) a detection means for sensing the detection beam and determining changes in distance versus time between the second interferometer means and said sample surface.

5. The system of claim 3 further comprising mirror control means for controlling motion of the movable mirror of the first interferometer and providing information regarding the motion of the movable mirror to the image formation means.

6. The system of claim 4, wherein said detection means simultaneously determines changes in distance between the interferometer means and a plurality of points on said sample surface versus time from the detection beam, thereby obtaining spatially resolved information from the irradiated portion of said sample surface.

7. The system of claim 6 including means for generating images of said sample surface from the detected changes in distances versus time obtained by said detection means in response to broadband sample excitation beam generated by the first interferometer means.

8. The system of claim 3, wherein the broadband radiation source comprises means for generating radiation at a wavelength in one of the group of wavelengths consisting of:

infra-red wavelengths, visible light wavelengths and ultra-violet wavelengths.

9. The system of claim 4, wherein the monochromatic radiation source of the second interferometer is a laser.

10. The system of claim 1, wherein the image formation means includes Fourier transformation means for calculating Fourier transform values of the detected changes in distance versus time for at least one point of the irradiated portion of said sample surface to generate an absorption spectrum for the at least one point.

11. The system of claim 10 further including an output device for creating an image indicating distribution of a desired chemical type from the absorption spectrum of sample at the at least one point.

12. A method of creating images representing chemical composition of a sample surface based upon the electromagnetic absorption spectra thereof, comprising the steps of:

a) irradiating a portion of said sample surface with a time modulated output beam having a plurality of frequencies, each frequency having an amplitude frequency of the output beam which varies with time inversely according to its frequency, causing the sample surface to absorb a portion of the output beam and expand periodically changing distance between the sample surface and a measurement point;

b) detecting changes in distance between the sample surface and the measurement point over time using an interferometer;

c) calculating Fourier transform values of the detected changes in distance versus time for at least one point of the irradiated portion of said sample surface; and

d) generating an absorption spectrum of said sample surface from the Fourier transform values, indicating chemical composition of said sample surface.