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  • IR Beamsplitter Coatings Reduce Gas Analyzer Size, Cost

Photonics Spectra
Nov 1997
Ruth A. Mendonsa

A manufacturer of gas-analysis equipment had developed a product that could quickly and accurately measure up to 250 hydrocarbon gases, but it was not without its problems. It was a large, expensive instrument. The company determined that a beamsplitter would shorten optical paths, reducing the optics, electronics and sensor requirements. But the technical constraints of the device's two IR channels presented some unique problems, including ghost images, unacceptable changes in the transmission and operating temperature ranges, channels that could not be balanced and polarization difficulties. The company contacted OFC Corp.'s Optical Coating Div. to develop a multilayer thin-film design that would address these challenges.

The OFC custom coating chamber has a 3-m double-pass spectrometer to monitor the coating process.

A formidable challenge

Because few thin-film materials are suitable for the IR, typical production methods were inadequate. The instrument's two bandpass filters created a very long path length, and the temperature requirements, center wavelength, unusual angles of incidence at two wavelengths, available materials, and steep operating angle of optical radiation and variable angle of incidence presented a challenge.

A very tight match between design and the real-time manufacturing evaporation in the coating chamber was required. Coatings usually tolerate slight variations between computed values and actual deposition results, but conventional OFC control tolerances of 0.1 percent between theoretical benchmarks and actual film thickness would render the beamsplitter unusable. Usual methods of monitoring layer thickness also were inadequate.

Software solution

The solution involved OFC-developed computer thin-film modeling and production software to narrow the tolerance between design and production films. New in situ monitoring techniques controlled layer thicknesses, and a 1.5-m double-pass spectrometer -- effectively a 3-m device -- helped monitor the process. Increased resolution enabled precise control to match individual layer thickness requirements. Co-evaporation varied layer characteristics and reduced inherent stresses.

Because of the magnitude of the process variables, these results would not have been easily reproducible in production quantities without OFC's customized automatic computer controls. Specialized test equipment included an environmental chamber within the cavity of a spectrometer reflectance attachment to permit testing over varied environmental conditions.

The new beamsplitter allowed the instrument manufacturer to improve the temperature stability of the device by eliminating one temperature sensor and reducing path length. Size and weight were reduced to one-fourth of the original product, increasing its utility. The company says sales of the new instrument have increased by a factor of 10.

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