Patent ID: 8179534

Claim:
A fixed wavelength absolute distance interferometer including: a first interferometer comprising: a first light source comprising at least a first point source that transmits at least a first light beam having a wavelength W along a first-interferometer measurement optical path toward a measurement target, wherein the first-interferometer measurement optical path includes a variable optical path length portion having a length that varies with a position of the measurement target, and the at least a first light beam has a wavefront radius that varies with distance along the first-interferometer measurement optical path and; a wavefront radius detector configured to provide a first measurement responsive to the wavefront radius at the wavefront radius detector, wherein the first measurement is indicative of the first-interferometer measurement optical path length; and a first path length calculating portion configured to calculate a coarse resolution absolute path length measurement R of the first-interferometer measurement optical path length on the basis of the first measurement; and a second interferometer comprising: a beam transmitting device transmitting a second-interferometer light beam that provides a synthetic measurement wavelength Λ; a beam splitting/combining device separating the second-interferometer light beam into a reference beam traversing a fixed optical path and a measurement beam traversing a second-interferometer measurement optical path that includes the variable optical path length portion having the length that varies with a position of the measurement target, and providing a combined beam by combining the reference beam returning from the fixed optical path with the measurement beam reflected from the measurement target and returning along the second-interferometer measurement optical path; a second-interferometer detector configured to receive the combined beam and provide signals indicative of a phase φ associated with the synthetic measurement wavelength Λ in the combined beam; and a second path length calculating portion configured to determine a medium resolution absolute path length measurement Z M of the second-interferometer measurement optical path length on the basis of a number of wavelengths N Λ , the wavelength Λ, and the phase φ of the combined beam indicated by the signals of the second-interferometer detector, wherein: the number of wavelengths N Λ is a calculated integer number of complete wavelengths Λ included in the second-interferometer measurement optical path length; and the number of wavelengths N Λ is calculated based on the coarse resolution absolute path length measurement R and an offset constant that is related to a constant difference C 12 between the first-interferometer measurement optical path length and the second-interferometer measurement optical path length.