Gas insensitive interferometric apparatus and methods

Interferometric apparatus for measuring distances occupied by a gas whose optical properties may vary over the measured distance, said system comprising:

• interferometer means comprising first and second measurement legs, said first and second measurement legs having optical paths structured and arranged such that at least one of them has a variable physical length and at least one of them is at least in part occupied by the gas and one of them may at least in part be occupied by a predetermined medium, the optical path length difference between said first and second measurement legs varying in accordance with the difference between the respective physical lengths of their optical paths and the properties of said gas and said predetermined medium;
• means for generating at least two light beams having different wavelengths;
• means for introducing first and second predetermined portions of each of said light beams into said first and second measurement legs, respectively, of said interferometer means so that each of said first and second predetermined portions of said light beams travels through said first and second measurement legs along predetermined optical paths, said predetermined first and second portions of said light beams emerging from said interferometer means as exit beams containing information about the respective optical path lengths through said first and second measurement legs at said wavelengths;
• means for combining said exit beams to produce mixed optical signals containing information corresponding to the phase differences between each of said exit beams from corresponding ones of said predetermined optical paths of said first and second measurement legs at said wavelengths;
• means for detecting said mixed optical signals and generating electrical interference signals containing information corresponding to the effects of the index of refraction of the gas at said different beam wavelengths and the physical path length of said first and second measurement legs occupied by said gas;
• means for measuring intrinsic optical properties of the gas and generating corrective information for compensating for the variable optical properties of the gas in said first and second measurement legs;
• and
• electronic means for determining difference in physical length between said first and second reference legs based on the information contained in said interference signals and said corrective information.

A gas insensitive interferometric apparatus for measuring physical length, said interferometric apparatus comprising:

• an interferometer having a measurement leg of variable physical length that is to be measured while occupied by a gas, said interferometer being adapted to generate at least one signal indicative of the optical path length of said measurement leg and at least one other signal indicative of the non-intrinsic optical properties of the gas in said measurement leg;
• a monitor for measuring the intrinsic optical properties of the gas and generating a corrective signal indicative of said intrinsic optical properties;
• and
• means for receiving said at least one signal and said at least one other signal and determining the actual physical length of said measurement leg by substantially compensating for the presence of the gas in said measurement leg.

A gas insensitive interferometric apparatus for measuring variable physical length in a gas, said interferometer comprising:

• first interferometer means comprising a reference leg and a measurement leg whose physical length is variable and is occupied by the gas;
• second interferometer means for use in the compensation of the presence of the gas in said first interferometer means, said second interferometer means comprising a reference leg and a measurement leg each of which has a predetermined physical path length, said reference leg being configured and arranged to be occupied by a predetermined medium and said measurement leg being configured and arranged to be occupied by the gas;
• means for generating at least two light beams having different wavelengths;
• means for introducing at least a portion of each of predetermined ones of said light beams into preselected ones of said reference and measurement legs of said first and second interferometers to generate optical signals that contain information corresponding to:
• (a) the optical path length in the measurement path of said first interferometer means at a first one of said wavelengths,
• (b) the optical path length in the measurement leg of said first interferometer means at at least one other one of said wavelengths, and
• (c) intrinsic optical properties of the gas in said measurement leg of said second interferometer means at first and said at least one other one of said wavelengths;
• means for converting said optical signals to electrical signals;
• and
• electronic means for processing said electrical signals to compensate for the presence of the gas in the measurement leg of said first interferometer and determine the physical path length of said measurement leg of said first interferometer means by substantially correcting for the presence of the gas in said measurement leg of said first interferometer means.

A gas insensitive interferometric apparatus comprising:

• an interferometer having reference and measurement legs where said measurement leg is configured so that the physical path length, L, thereof can vary and is occupied by the gas;
• an interferometric Γ-monitor;
• means for generating at least two light beams having different wavelengths;
• means for introducing at least a portion of predetermined ones of each of said light beams into said interferometer and said interferometric Γ-monitor and generating:
• (a) a first signal for said physical path length, L, where L=L1−Γ(L2−L1), L1 is the optical path length of said measurement leg divided by p1k1 where k1 is the wavenumber and p1 the number of passes through said measurement leg at λ1, L2 is the optical path length of said measurement leg divided by p2k2 where k2 is the wavenumber and p2 the number of passes through said measurement leg at wavelength λ2, and Γ=(n1−1)/(n2−n1) where n1 and n2 are, respectively, the indices of refraction of the gas in said measurement leg at λ1 and λ2, and
• (b) a second signal containing information for calculating Γ to correct said first signal for errors in said first signal related to the presence of the gas in said measurement leg at λ1;
• and
• signal processing means for receiving said first and second signals and calculating Γ and then said actual physical length, L, of said measurement leg.

A gas insensitive interferometric method for measuring physical displacement, said interferometric method comprising the steps of:

• generating at least two light beams having different wavelengths;
• providing an interferometer having at least one measurement leg arranged to have a variable physical displacement that is to be measured while occupied by a gas and adapted to receive said light beams and generate non-intrinsic measurement information that varies in accordance with the optical path length and the instantaneous column density of the gas in said measurement leg;
• determining the intrinsic optical properties of the gas in said measurement leg and generating a monitor signal indicative of said intrinsic optical properties;
• and
• receiving said non-intrinsic measurement information and said monitor signal and determining the actual physical displacement in said measurement leg by substantially compensating for the presence of the gas in said measurement leg in accordance with both said non-intrinsic information and said intrinsic optical properties of the gas.