Novel Miniature Spectrometer for Remote Chemical Detection Page: 3 of 4
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and a 12 bit digitizer, an additional improvement of 10-100 over this sensitivity should
be obtained. The evanescent optical field at the cavity facets, where chemical detection
occurs, has a well-defined, easily calculated direction and magnitude for the in-plane and
out-of-plane polarization directions, which allows molecular orientation to be probed.
Since the orientation of an adsorbed molecule can significantly affect an optical
absorption measurement, this effect was addressed. Adsorbed iodine, which has an
optical transition moment that is parallel to the molecular axis, showed a much stronger
out-of-plane polarized (S-state) absorption than in-plane absorption (P-state), indicating
that the adsorbed form of iodine is oriented parallel to the cavity surface.
Similar results for detection of adsorbed iodine were obtained using a new non-
ring resonator design, which combines high-reflectivity coatings with TIR to form a
stable optical cavity (disclosed last year in the proprietary information section). Although
the non-ring design has a narrow spectral bandwidth due to the use of high-reflectivity
coatings, this single element cavity can be excited directly by an incident laser. Direct
excitation of the cavity through a coated surface facilitates interfacing the cavity to
experiments and will speed application testing. Experiments on reactions of NO,
compounds are currently being explored. The TIR-ring cavity design is superior in
sensitivity and spectral bandwidth in comparison to the non-ring design, but is excited by
a photon tunneling mechanism which requires the use of coupling prisms. These small
coupling prisms must be mounted on the cavity with a precise and stable separation
between the cavity and the coupling optic. Strategies to achieve this goal have been
devised that will be carried out in collaboration with IDI through a CRADA. The goal of
the CRADA is to engineer prototype miniature spectrometers that will have mounted
coupling optics, mode-matching optics (for single-mode excitation which improves
measurement precision), polarization-selective optics (for orientational information), and
fiber-optic connectors (for remote detection and general convenience of use).
Finally, a hexagonal TIR-ring cavity has been designed and successfully
fabricated from sapphire. The high index of refraction of sapphire permits dense liquids
to be probed by immersion of the hexagonal cavity design in the medium of interest. A
miniature spectrometer based on the hexagonal sapphire cavity should be field
deployable in the cone penetrometer, for example, since sapphire is a durable material.
Simple cleaning procedures, which can be easily field implemented, have been developed
to recover the baseline response after chemical exposure.
PLANNED ACTIVITIES:
Fused-silica, square and sapphire, hexagonal TIR-ring cavities will be
incorporated in prototype miniature spectrometers during the next year. The square and
hexagonal designs are appropriate for vapor and liquid sensing applications, respectively,
in the visible and near-IR. Diode lasers will be employed in addition to a dye laser.
Vapor and liquid sensing applications will be explored when miniature spectrometers
have been successfully engineered. For the mid-IR, cavities may be fabricated from un-
doped YAG, CaF, or fluoride glass, depending on laser source availability (an OPO may
be available but will not be purchased; mid-IR quantum cascade lasers (QCL) may also
be available).
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Pipino, Andrew C.R. Novel Miniature Spectrometer for Remote Chemical Detection, report, June 1, 1999; United States. (https://digital.library.unt.edu/ark:/67531/metadc781781/m1/3/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.