Optics: measuring and testing – By dispersed light spectroscopy – With raman type light scattering
Reexamination Certificate
2003-06-13
2004-06-08
Evans, F. L. (Department: 2877)
Optics: measuring and testing
By dispersed light spectroscopy
With raman type light scattering
Reexamination Certificate
active
06747735
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a multiplex coherent Raman spectroscopic detector and method for generating and detecting coherent Raman radiation from a sample. More specifically, the present invention relates to a multiplex coherent Raman radiation detector and method for generating and detecting coherent Raman radiation scattered by the components of an unknown sample and using the coherent Raman radiation to determine the identity of the sample's constituents.
The present invention also relates to an apparatus and method for illuminating a gaseous sample with broadband light with a continuous range of more than 3000 wavenumbers and with a narrowband light having a bandwidth of less than 1 wavenumber, and preferably about 0.003 wavenumbers to produce the entire gas phase vibrational Raman spectrum of the sample, thereby permitting accurate identification of the sample.
In addition, the present invention relates to an apparatus and method for increasing the intensity of the backward-propagating, phase-conjugate, coherent Raman radiation produced by a Raman cell. Moreover, the present invention also relates to an apparatus and method for using this enhanced, backward-propagating, phase-conjugate, coherent Raman radiation to drive a device capable of producing broadband light of more than 3000 wavenumbers.
2. Description of Related Art
In many fields, such as scientific research, industrial research, and forensics, it is often necessary to identify the chemical composition of an unknown sample. This task is often performed by first isolating the different compounds in the sample, and then applying an identification technique to each isolated compound. One standard method for isolating unknown compounds is called gas chromatography, where the unknown sample is transformed into a gas, if not already in the gaseous state, and the various compounds in the gas are separated due to their differing gaseous properties, such as polarity. Once the compounds are isolated, they may be identified. The simplest way to identify the compounds is by noting the time it takes for each compound to pass through the gas chromatograph, since different compounds take different amounts of time to do so. But this method is limited to samples where much is known about the components. A more powerful method for identifying isolated compounds examines the intensity of different wavelengths of light emitted, transmitted, reflected, or scattered by the compound. This technique, called spectroscopy, works if each compound emits, transmits, reflects, or scatters light differently and if the spectroscopic instrument has sufficient spectral resolution to detect these differences. More specifically, different chemical compounds emit, transmit, reflect, or scatter different wavelengths of light with differing intensities. A graph or picture of such data is called the spectrum of that compound. Different types of spectroscopy reproduce the spectrum of a compound over different wavelengths and/or under different conditions. If the type of spectroscopy used provides a unique spectrum for each chemical compound, an unknown compound can be identified by producing its spectrum (for example, by illuminating the compound and measuring the light reflected, scattered, or emitted therefrom) and comparing its spectrum with the spectra of known compounds. As a result, gas chromatographs, which isolate compounds from a sample, are often used with spectrometers, which identify the compounds once they are isolated.
One popular type of spectroscopy detector used with gas chromatographs requires the gas isolated by the gas chromatograph to be embedded in or condensed onto a substrate before spectroscopic examination. Such detectors provide advantages, such as low detection limits, but are complicated because they require the isolated gas to be condensed, trapped, or adsorbed onto a substrate. In addition, such detectors suffer from unwanted effects such as nearest-neighbor effects, sample decomposition, and a slow detection speed. As a result, detectors that operate “on the fly” with little or no sample modification are often faster and freer from unwanted effects.
One type of frequently-used “on the fly” spectroscopy is infrared spectroscopy. But infrared spectroscopy is sometimes unable to accurately determine the identity of an unknown sample because certain characteristics of some samples (i.e., those with spectra that are highly state-(phase) dependent and those that produce strong rotational side bands in the infrared light absorbed by the sample that cause a loss of spectral resolution) reduce its accuracy. Furthermore, certain molecules, such as homonuclear diatomics, have no infrared spectrum, and optical components designed to direct and process the infrared light used in an infrared spectrometer are often inferior to the optical components designed for use in the visible spectrum.
A type of spectroscopy that is less susceptible to these problems is called Raman spectroscopy. In this type of spectroscopy, light in the visible wavelength region or the near-visible wavelength region is projected onto a sample and a small fraction of this light is scattered in all directions by the sample and is measured. The light is scattered because the molecules of the sample inelastically scatter the light due to the vibrational or rotational motions in the molecules of the sample. Such scattered light is of two types: light whose wavelength is not shifted, which is called Rayleigh scattering, and light whose wavelength is shifted, which is called Raman scattering. The Raman scattered light is much less intense than the Rayleigh scattered light. Since the Raman scattered light is scattered and shifted in wavelength because of the vibration of molecules of the sample, a graph of the Raman scattered light from a sample is called the vibrational Raman spectrum of the sample and provides information about the internal vibrational motion of the molecules of the sample. Moreover, the entire vibrational Raman spectrum of each compound (which is approximately 3000 wavenumbers wide) is unique to that compound. As a result, unknown compounds can be identified by their vibrational Raman spectrum. But, the intensity of the Raman spectrum must be sufficiently strong to be detected by currently-developed detectors with a high signal-to-noise ratio, and the entire Raman vibrational spectrum, covering a range of at least 3000 wavenumbers (indicating a large number of wavelengths of light are measured) must be produced. If only a partial Raman vibrational spectrum is produced, the identity of the compound may not be determined with high accuracy, since many compounds can share the same partial Raman vibrational spectrum. When Raman spectroscopy is used to detect gases, such as those isolated by a gas chromatograph, it is called gas phase Raman spectroscopy.
Gas phase Raman spectroscopy provides several advantages over gas phase infrared spectroscopy. First, Raman spectroscopy is less susceptible to phase transitions in the sample and to unwanted broadening of scattered or absorbed light due to rotational sidebands, so species identification may be more accurate using Raman spectroscopy. Second, Raman spectroscopy can be used to identify more types of molecules than infrared spectroscopy, since certain molecules do not appear in infrared spectroscopy, while all molecules will appear in Raman spectroscopy. Third, several advanced techniques are available with Raman spectroscopy that improve its accuracy and generate additional, valuable data not available in infrared spectroscopy, including resonance Raman spectroscopy, surface enhanced Raman spectroscopy, and coherent Raman spectroscopy. Finally, the optical components commercially for use in the visible region are often superior to those available for use in the infrared region. For example, extremely sensitive and rapid multichannel detectors are available in the visible region but not in the infrared region.
But gas phase Raman spectroscopy
Chen Peter
Guyer Dean R.
Joyner Candace C.
Patrick Sheena T.
Evans F. L.
Fitzpatrick ,Cella, Harper & Scinto
Spelman College
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