Southwest Sciences Inc

Etalon Reduction Patent

US Patent #4,934,816

This patent describes a novel method for reducing the effects of etalon fringes on laser spectra. Etalon fringes consist of periodic ripple in the transmitted laser power. This ripple is usually the factor that limits the sensitivity of a diode laser measurement of a trace gas concentration.

Etalon fringes arise when light follows two paths from the laser to the detector. For instance, in addition to the main beam, some of the laser light may be reflected off a window, back to the laser, and from there on to the detector. This stray between follows a longer optical path. Therefore, the phase of its electric field differs from that of the main beam. The phase difference depends on the extra path length and the wavelength of the light.

The detector measures the total power, which is proportional to the square of sum of the electric fields from both the main beam and the stray beam. Because the electric field is a vector, the phase must be considered when forming the sum. As a result, when the laser wavelength is tuned to measure a molecular absorption feature, the stray beam electric field goes in and out of phase, resulting in a rippling baseline. Is this important? If a millionth of the laser power follows a stray path, the ripple is +/- one thousandth of the laser power. This doesn't sound like much, but it is 10,000 times greater than the noise of the laser. Measuring a weak molecular absorption feature against this rippling background is like measuring the height of a buoy bobbing in mile high waves!

Our invention involves mechanically dithering the distance traveled by the stray beam, for instance by vibrating the window or the laser. This causes the optical phase to vary, thus making the etalon ripples to move. By averaging over the positive and negative parts of the etalons, their effect is reduced or eliminated. Measuring the molecular absorption feature becomes much easier.

Contact Information

Southwest Sciences, Inc.
1570 Pacheco St., Suite E-11, Santa Fe, NM 87505
tel. (505) 984-1322/ fax (505) 988-9230



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