The invention is a method for obtaining a measure of the light propagation time difference for two light-propagating-media paths. The first step consists of generating two substantially-identical frequency-modulated light waves whereby the frequency of the light waves is offset from a reference frequency by a different frequency increment for each basic time interval in each of a plurality of groups of three or more basic time intervals. Each frequency increment is the sum of a specified increment and a frequency-modulation-error increment. The frequency-modulation-error increments associated with the specified increments are independent of each other and unknown. The second step consists of feeding the two light waves into the entry points of two light-propagating-media paths having a light propagation time difference and obtaining a combination light wave by summing the light waves emerging from the exit points of the two light-propagating-media paths. The third step consists of calculating an estimated corrected or estimated uncorrected phase measure of the light propagation time difference for the two paths for a plurality of groups using only measured properties of the combination light wave.
An optical phase transient is added to the optical signal input to an interferometer sensor array in addition to a CW phase generated carrier used for acquisition of the interferometer phase shift. Detection of an error signal associated with the transient is used to adjust the modulation depth of the CW phase generated carrier to a null the error signal so that there is a known ratio between the coefficients of the sine and cosine terms in the interferometer output signal to allow for normalization.
6646723 - High precision laser range sensor - Owned by The United States of America as represented by the Administrator of the National Aeronautics and Space Administration (Washington, DC)
The present invention is an improved distance measuring interferometer that includes high speed phase modulators and additional phase meters to generate and analyze multiple heterodyne signal pairs with distinct frequencies. Modulation sidebands with large frequency separation are generated by the high speed electro-optic phase modulators, requiring only a single frequency stable laser source and eliminating the need for a first laser to be tuned or stabilized relative to a second laser. The combination of signals produced by the modulated sidebands is separated and processed to give the target distance. The resulting metrology apparatus enables a sensor with submicron accuracy or better over a multi-kilometer ambiguity range.
In an interferometric alignment system provides an alignment signal for reproducibly registering a reticle 10 with a wafer 12. The radiation from a laser 50, which is the illumination source for the interferometer, is modulated by a phase modulator 52 to eliminate spurious noise from the alignment signal.
A direct detection method and apparatus for a fiber optic acoustic sensor array systems using an in-line Michelson sensor TDM array and an interferometric section having two acousto-optic modulators that produce optical pulses that are frequency shifted with respect to each other. Direct detection is accomplished according to the equation: I(t)=A+B cos [.phi..sub.1-.phi..sub.2+2.pi.(f.sub.1-f.sub.2)t], with the phase shift difference .phi..sub.1-.phi..sub.2 between two paths containing the acoustic phase information and the frequency f.sub.1-f.sub.2 being the difference between the RF frequencies for the two acousto-optic modulators.
