A Combined Optical Emission and Absorption Spectrometer Using Micromachined Optical Components, 15-9160

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Principal Investigators
Gregory P. Miller
Martin P. Wüest

Inclusive Dates: 09/15/99 - Current

Background - Atmospheric gas composition measurements can contribute significantly to an understanding of the origin and evolution of planets, cometary material, and Earth's terrestrial environment. Atmospheric composition has primarily been measured with mass spectrometer systems. These systems provide a high degree of specificity, but they are highly complex, expensive, and difficult to miniaturize because of the vacuum system that must be integrated into their design. Optical spectrometers, on the other hand, can be used to identify an unknown compound with a high degree of specificity by the compound's unique absorbance or emission line spectra. Micromachining technology has been applied successfully to the construction of highly miniaturized gratings and to the detector design to reduce the size of the instruments to palm-size devices housing all electronics. Furthermore, the use of MEMS components is ideal for applications in the space sciences where severe constraints with regard to mass and power must be observed.

This focused internal research and development effort on MEMS devices uses micromachined optical and mechanical devices for developing a breadboard optical microspectrometer and gas sampling system. The microspectrometer system is being designed to make gas-phase emission and absorbance measurements in the ultraviolet and visible region between 200 to 800 nanometers. A series of MEMS microvalves and a chemical getter micropump are being used for the gas introduction system.

Approach - Emission and absorbance measurements make use of a gas introduction system, a light source, monochromater, and a detector. MEMS devices are incorporated for the construction of the optical spectrometer system using microminiaturized ion-etched corrected holographic grating for dispersing the light onto a photodiode array. Micromachined optical components are interfaced using fiber optic connections to simplify the alignment and layout to the gas emission source and absorbance cell. Gas is introduced by the use of several MEMS vacuum valves that have been specifically designed and built for this application. Atmospheric gas is transferred into the absorbance and emission cell by first evacuating the entire system using a miniaturized vacuum pump working on the principle of chemical gettering. Emission spectra are obtained using a high-voltage discharge. Absorbance measurements are made in a long-path gas cell.

Accomplishments - A microminiaturized ion-etched corrected holographic grating and photodiode array and fiber optic system has been obtained. Design, construction, and delivery of microvalves have recently been made. A microvalve electronic driver board has been constructed, and control software for microvalve sequencing has been written. A micropump and heater unit has been constructed for the vacuum system.           

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