Optical apparatus for forming correlation spectrometers and optical processors. The optical apparatus comprises one or more diffractive optical elements formed on a substrate for receiving light from a source and processing the incident light. The optical apparatus includes an addressing element for alternately addressing each diffractive optical element thereof to produce for one unit of time a first correlation with the incident light, and to produce for a different unit of time a second correlation with the incident light that is different from the first correlation. In preferred embodiments of the invention, the optical apparatus is in the form of a correlation spectrometer; and in other embodiments, the apparatus is in the form of an optical processor. In some embodiments, the optical apparatus comprises a plurality of diffractive optical elements on a common substrate for forming first and second gratings that alternately intercept the incident light for different units of time. In other embodiments, the optical apparatus includes an electrically-programmable diffraction grating that may be alternately switched between a plurality of grating states thereof for processing the incident light. The optical apparatus may be formed, at least in part, by a micromachining process.

Optical apparatus for forming correlation spectrometers and optical processors

Three methods of remote air pollution detection-Raman backscattering, resonance backscattering, and resonance absorption-are discussed and compared. Theoretical expressions are derived for the minimum detectable pollutant concentration, and in each case the depth resolution and the problems of interference, pump depletion, and background noise are discussed. A brief discussion of possible laser sources is included, numerical examples of the detectabilities based on present technology are given. The atmospheric transparency limits the useful range to a few kilometers for the Raman and resonance backscattering schemes. For the resonance absorption technique the useful range can be as great as 50 kilometers.

Comparison of laser methods for the remote detection of atmospheric pollutants

The applications of optical correlation techniques to the remote sensing of SO2 plumes using sky light

Independent measurements of nitrogen oxides (NOx = NO + NO2) were made in proximity to lightning using both a chemiluminescent NOx analyzer and an absorption spectrometer (COSPEC) tuned to the NO2 absorption at Langmuir Laboratory (elevation 3300 m), near Socorro, New Mexico. COSPEC data obtained from three separate lightning events, two from distant storms and one from direct observation of the plume from a single flash a few hundred meters away, yielded 4 to 10 × 1026 molecules NO2/flash after normalizing to flashes at sea level. Measurements from the NOx analyzer during two different thunderstorms indicated that the ratio of NO to NOx can remain high (0.7–0.8±0.1) for several minutes after a flash. Combining the data from the COSPEC with the NOx analyzer yields a production rate of approximately 3×1027 molecules N0x/flash. If 100 flashes/second is an assumed global lightning flash frequency, this production rate becomes 100 TgN/yr (TgN/yr = 1012 g nitrogen/yr) as an order of magnitude estimate.

Nitrogen oxides produced from lightning

Determination of sulfur dioxide by ultraviolet absorption spectrometry

The method of spectrometric determination of gas concentration by use of a mask is examined and described. Calibration curves for four gases, NH(3), C(6)H(5)CH(3), NO(2), and SO(2), are reported in detail, and results are included for C(6)H(6), NO, and SiF(4). The calibration curves are shown to be linear in a log-log plot over ranges of practical interest. The slopes are reproducible. The technique used in making masks is described. The proposed manner of making measurements involves the use of a small reference cell containing a sample of the gas being measured. When such a method of measurement is used, several gases of practical importance are concluded to be determined as to concentration to a few parts per billion with a precision of 10% or less, if path lengths of the order of several hundred meters are used. The results appear to be independent of the presence of unknown absorbing gases under many circumstances.

Byron L. Kolitz

Papers

Molecular correlation spectrometry.