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US 6,646,723 B1Granted
High precision laser range sensor
AssigneeThe United States of America as represented by the Administrator of the National Aeronautics and Space Administration
InventorsSerge Dubovitsky, Oliver P. Lay
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Abstract
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.
Claims17
A high precision laser range sensor comprising:
- a single laser for generating a carrier frequency;
- an optical splitter to split the carrier frequency into local and target arms;
- a frequency shifting unit to introduce a frequency shift offset between the target and local arms;
- a pair of high speed modulators that apply modulation to the carrier frequency in the local and target arms to produce a series of modulation sidebands, the high speed modulators having modulation frequencies of FL and FT;
- and
- a comparing unit for comparing a phase difference between the local and target arms with modulation sidebands where each modulation sideband forms a low frequency heterodyne pair.
3 more elements
A method for measuring a distance between two points comprising the steps of:
- providing a single frequency stable source of electromagnetic signal;
- dividing the signal into a target signal and a local signal;
- applying a sinusoidal phase modulation to the target signal and the local signal;
- applying a frequency shifting operation to the modulated target signal and the modulated local signal to generate at least two heterodyne pairs;
- directing a portion of the target signal and a portion of the local signal to a reference photodetector, while transmitting a portion of the target signal and the local signal;
- passing the transmitted local signal to a signal photodetector, while deflecting the transmitted target signal to a pair of retro-reflectors before deflecting the target signal to the signal photodetector;
- measuring a phase difference of a first heterodyne pair between the reference photodetector and the signal photodetector;
- measuring a phase difference of a second heterodyne pair between the reference photodetector and the signal photodetector;
- and
- comparing the phase differences between the first and second heterodyne pairs.
7 more elements
A high precision laser range sensor comprising:
- a single laser for generating a carrier signal;
- a first modulator for applying a modulation of a first modulation frequency to a portion of the carrier signal, the first modulator outputting a first modulated signal with a first series of sidebands spaced at plus and minus the first modulation frequency comprising a first series of upper and lower sidebands;
- a second modulator for applying a modulation of a second modulation frequency to a portion of the carrier signal, the second modulator outputting a second modulated signal with a second series of sidebands spaced at plus and minus the second modulation frequency comprising a second series of upper and lower sidebands;
- a first frequency shifter for applying a first frequency shift to the first modulated signal, the first frequency shifter outputting a target signal;
- a second frequency shifter for applying a second frequency shift to the second modulated signal, the second frequency shifter outputting a local signal,
- wherein the sidebands of the target and local signals generate at least two heterodyne pairs;
- a reference photodetector for detecting a portion of the each of the target and local signals;
- a first partial deflector for deflecting a portion of each of the target and local signals to the reference photodetector, the first partial deflector passing a portion of each of the target and local signals;
- a signal photodetector for detecting a portion of the each of the target and local signals;
- a second partial deflector for deflecting a portion of the target signal to a first retro-reflector, the second partial deflector passing a portion of the local signal, the first retro-reflector being placed at a first distance from the second partial deflector, the first retro-reflector reflecting a portion of the target signal from the second partial deflector to a second retro-reflector, the second retro-reflector being placed at a second distance from the first retro-reflector and a third distance from the second partial deflector, the second retro-reflector reflecting a portion of the target signal from the first retro-reflector to the second partial deflector, the second partial deflector deflecting a portion of the target signal from the second retro-reflector to the signal photodetector, the second distance being equal to the sum of the first and third distances,
- a first phase meter for measuring a first phase difference between the reference photodetector and the signal photodetector of the first heterodyne pair, the first phase difference corresponding to a first set of distance measurements that repeat on a first ambiguity range;
- a second phase meter for measuring a second phase difference between the reference photodetector and the signal photodetector of the second heterodyne pair, the second phase difference corresponding to a second set of distance measurements that repeat on the period of a second ambiguity range;
- and
- a comparing unit for comparing the first phase difference and the second phase difference, the comparing unit determining a measured distance corresponding to the solution of the first and second distance measurements, the solution indicating the sum of the first, second, and third distances comprising twice the distance between the first and second retro-reflectors.
11 more elements
Dependencies are inferred from the claim text.