A class of miniature probes has been proposed to supplant the fiber-optic probes used heretofore in some Raman and fluorescence spectroscopic systems. A probe according to the proposal would include a capillary tube coated with metal on its inside to make it reflective. A microlens would be hermetically sealed onto one end of the tube. A spectroscopic probe head would contain a single such probe, which would both deliver laser light to a sample and collect Raman or fluorescent light emitted by the sample.
The capabilities of most prior Raman and fluorescence fiber-optic probes are limited because of spurious emission of Raman and fluorescent light by the cores of the optical fibers themselves. To prevent the spurious emissions from overwhelming the desired Raman and/or fluorescence signals, it is necessary to incorporate spectral filters. In a given application, such a filter undesirably limits the probe to a single laser wavelength. Filtration is usually performed by means of free-space optics in the probe head. These optics are vulnerable to misalignment. Alternatively, thin-film filters can be deposited directly on optical fibers or in proximity to the fibers, but the performances of such filters are inferior to those of free-space optical filters.
A typical probe according to the proposal would have an outside diameter of <1 mm. Relative to a typical prior Raman probe, this probe would be much smaller and lighter and could be manufactured at lower cost. The interior of the probe could be evacuated or filled with a gas (e.g., argon) that does not emit any Raman or fluorescent light in the spectral region of interest. Hence, there would be no need for filters, ancillary filter optics, and associated mounting hardware vulnerable to misalignment, and so the probe would be better able to withstand such adverse environmental conditions as higher pressures, temperatures, and mechanical shocks. The elimination of filters would make it possible to operate the probe at more than one wavelength. A probe according to the proposal could perform well in the deep ultraviolet spectral region, which is important for spectral detection of organisms. Unlike fiber-optic probes containing filters, these probes could be used to measure Raman wave-number shifts smaller than 50 cm-1.
In the development of these probes, it should be possible to take advantage of the knowledge base already accumulated in the use of metallized capillary tubes to deliver laser beams for surgery and cutting metal. Provided that the reflectivity of the interior metal coat of a capillary is sufficiently high, the numerical aperture of the capillary sufficiently low, and the diameter of the capillary sufficiently large, it should be possible to achieve efficient transmission of light along the capillary.
The small size, low cost, and multiwavelength capability of these probes make them attractive for many Raman and fluorescence applications. Matrices of samples generated by combinatorial chemistry could be analyzed by an array of these probes interfaced to a single fiber-optic probe head or directly to a single Raman spectrograph. Access ports to chemical reactors could be kept small, reducing the potential hazards of a seal failure. Probes inserted into large chemical reactors could be much smaller in diameter due to the greatly relaxed stiffness requirements compared to traditional Raman probes. Also, because of their smallness, they would be capable of withstanding high pressures and would be well suited for use in undersea exploration.
This work was done by Michael Pelletier of Caltech for NASA's Jet Propulsion Laboratory.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
Intellectual Assets Office
Mail Stop 202-233
4800 Oak Grove Drive
Pasadena, CA 91109-8099
Refer to NPO-30711.