Optics: measuring and testing – By dispersed light spectroscopy – With raman type light scattering
Reexamination Certificate
2000-03-20
2001-10-23
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
With raman type light scattering
Reexamination Certificate
active
06307626
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for collecting and analyzing Raman scattered light. The apparatus includes an atomic vapor cell configured to spectrally disperse the light by resonant dispersion while simultaneously suppressing the Rayleigh Scattering and other background scattering through resonant absorption.
2. Related Art
Raman spectroscopy has long been a primary tool for molecular diagnostics. The Raman mechanism is based on light scattering from molecules in a sample volume and is usually done using a narrow linewidth laser as a source. When a sample is illuminated, the light that scatters from it echoes the character of the molecules in the sample volume. If the source has a narrow frequency band, then one can distinguish several molecular scattering mechanisms by the spectrum of the light that is collected. Assuming that the illumination frequency is far away from a resonant absorption feature associated with any of the molecules in the sample volume, then there are three primary components of the scattered light. The strongest component is the elastic scattering from the molecules. This light is usually termed Rayleigh Scattering, and it is only very slightly shifted in frequency from the source frequency due to the Doppler effect associated with the translational motion of the molecules. A second component arises from scattering off of the acoustic waves in the medium and is frequency shifted by the Doppler shift associated with the speed of sound. The second component is often call Brillouin Scattering, and it appears most notable in high pressure or low temperature gases where the mean free path of the molecules is small compared to the relevant acoustic wavelength. The third component, Raman scattering, is due to inelastic scattering from the molecules themselves and is associated with the internal modes of molecular energy. Since the internal modes of energy are unique to each molecular species, this type of scattering is exceptionally useful. The presence of specific spectral features indicates the presence of particular molecules in the sample volume, and the strength of the features can be interpreted to give the concentration of those molecules and their temperatures. This means that Raman spectroscopy can be used to identify what molecular species are present within a sample volume and the temperature of those species. In certain cases, the molecules may not be in thermal equilibrium. Under these circumstances, the spectral features and their relative strengths can give the nonequilibrium state of the molecule, yielding such information as the translational, rotational and vibrational temperatures. Thus, Raman spectroscopy may also be used to produce images of complex environments such as combusting gases, mixing gas streams, etc.
The Raman Spectrum is separated into two regions, one associated with scattering from purely rotational transitions, called Rotational Raman Scattering, and one associated with scattering from vibrational-rotational transitions, called Vibrational Raman Scattering. These two regions have different features. The, Vibrational Raman scattering has features associated with each molecular species clustered together at various locations in the spectrum, whereas the rotational Raman spectrum has all the Raman active species interleaved within it. In additional, since the rotational modes of a molecule have significantly lower energy than the vibrational modes, the Raman shift associated with rotational motion is rather small, on the order of a few wavenumbers. The vibrational shifts, on the other hand, are hundreds to thousands of wavenumbers. This means that the strong Rayleigh line is very close to the rotational spectrum and in most cases, obscures it. Thus, even though rotational Raman scattering is usually on the order of a factor often stronger than vibrational Raman, it is not often used for spectral analysis.
Much prior work has been done to develop various Raman devices that can effectively collect and analyze the Raman spectrum. This has been difficult because Raman scattering is exceptionally weak and occurs in the presence of strong Rayleigh scattering plus background from the sample cell walls and, in some cases, fluorescence. The typical Raman set-up consists of a light collection system which images the scattering onto a narrow slit, through which the light passes into a spectrometer, is dispersed by a grating and then passes out of a second slit onto a detector. In some cases the second slit is removed, and the detector is replaced by a camera. The scattering spectrum is spread out in space by diffraction from the grating. The grating is turned so the Raman line of interest passes through the slit onto the detector or the Raman spectrum is imaged by the camera, and the strong spectral background features are not seen by the camera or the detector. Often this arrangement does not give enough extinction to the other light components, and a second spectrometer stage must be added.
Various approaches to improving this set-up have been proposed.
Barrett, U.S. Pat. No. 3,909,132, proposed using an interference filter to transmit the rotational lines in order to measure temperature. This concept matches the approximately equal spacing of the rotational lines from one another to the regularly spaced transmission frequencies of an interferometric filter and recognizes that the progressive mismatch at higher energy may be used to determine the temperature.
Other attempts in the past include:
Stamm, U.S. Pat. No. 2,483,244, recognizes the need to provide a device of high dispersion to separate light of closely adjacent wavelengths to the maximum degree, such as found in Raman spectra. To accomplish this, the spectrometer system taught uses a Wernicke prism that includes in combination the use of a central liquid prism sandwiched between two end prisms of crown glass. The glass and prisms are symmetrically arranged and have substantially identical optical properties, with the central liquid section prism being filled with a clear ester or an acid. Various acids are disclosed in this reference.
Cary, U.S. Pat. No. 2,940,355, discloses two monochromator sections which receive monochomatic light from a source for permitting illumination of a sample over a greatly extended range relative to the use of a single monochromator, for reducing the intensity of background radiation relative to the intensity of the Raman light scattering or spectra-lines to be measured. The scattered light or Raman spectrum from the double monochromator is amplified and recorded.
Tochigi, et al., U.S. Pat. No. 4,586,819, discloses a Raman-scattered light is separated from the reflector laser beam by a filter, and the extracted Raman-scattered light is then passed through a single-monochromator for analyzing a sample. In the example given, the sample can be foreign matter of one micron or less in diameter on an integrated circuit wafer. The filters utilized, are indicated as being commercially available filters such as those that consist of dielectric-coated glass filters, that are selected for transmitting a predetermined wavelength band of light. The filters are used for separating the wavelength bands of the laser beam into spectra in shorter and longer wavelength regions. By comparing the spectra in these wavelength regions, the temperature of the sample under analysis can be exactly detected.
Webster, U.S. Pat. No. 4,684,258 teaches the reduction of interference fringes in a light beam passed through a passive cavity by including a Brewster-plate spoiler in the cavity, and oscillating it back-and-forth over an angle for causing the cavity resonances to tune in a frequency over a range that is a multiple of the period of the interference fringes. A diode laser is used to direct a beam of coherent light through a lens into a prism-like cell, from which cell the beam is directed to a Brewster-plate spoiler.
Wada, U.S. Pat. No. 5,217,306, discloses the passage of light through an optical fil
Finkelstein Noah
Lempert Walter R.
Miles Richard B.
Evans F. L.
Plasma Tec, Inc.
Wolff & Samson
LandOfFree
Dispersive atomic vapor raman filter does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dispersive atomic vapor raman filter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dispersive atomic vapor raman filter will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2608415