Optics: measuring and testing – By light interference – Having light beams of different frequencies
Reexamination Certificate
1999-04-01
2001-06-05
Font, Frank G. (Department: 2877)
Optics: measuring and testing
By light interference
Having light beams of different frequencies
C356S484000
Reexamination Certificate
active
06243168
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally related to micrometers, and more particularly to a dynamic optical micrometer that linearly detects movement over a distance spanning several optical wavelengths.
2. Description of the Prior Art
Present small motion detectors are limited in the distance over which they can be used and the upper dynamic response that they can follow. Physical micrometers, although not limited in distance, typically do not have a fast dynamic response. Small fast motions typically are measured using conventional optical interferometry. In such conventional optical interferometry a beam of light is transmitted upon a moving object and its reflected light is optically interfered with a portion of the original transmitted light. The resulting interference pattern creates a change in optical intensity that varies sinusoidally as a function of the relative state of phase of the light in the two interfering beams. The magnitude of the varying intensity is correlated to a particular position of the object under test with an accuracy that is within a fraction of the wavelength of the light transmitted. However, this conventional approach only allows for the detection of motion over a maximum distance associated with the variation of a single wavelength of the transmitted light. Further motion repeats the described intensity variation and creates ambiguous results.
What is needed, therefore, is an apparatus and a method for dynamically etecting the motion of an object spanning several wavelengths of light.
In addition, it is desirable to produce an indication that is linearly related to the movement of the object.
SUMMARY OF THE INVENTION
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides generally a dynamic optical micrometer.
Briefly, the optical micrometer detects the motion of an object that moves several microns, spanning several wavelengths of light, with a fast dynamic response. The approach utilizes optical interference in combination with RF modulation and simple digital processing to simultaneously achieve a detection range spanning many wavelengths, an output voltage that is linearly related to position, and a rapid response to changes in position. The invention employs an optical frequency shifter, such as an acousto-optic modulator, that up shifts or down shifts a portion of a beam of transmitted light to a new optical frequency by an amount corresponding to an RF modulation frequency. The transmitted light beam and the shifted light beam are then directed through an interferometer where one of the two beams is reflected off a moving object that is being tested. The two beams are then interferometrically combined and sent to a detector that delivers an electrical signal at the optical beat frequency that corresponds to the frequency of the RF modulation. The phase of the detected RF beat frequency relative to the original RF modulation signal contains the position information of the object under test. The detection of the relative phase of the RF signals is performed by a digital dividing technique that provides a high resolution determination. More particularly, each RF frequency is converted to a digital waveform that is digitally divided by an equal number of powers of two so that the resulting outputs are square waves. The divided signals are applied to an exclusive OR gate that provides a pulse waveform having a duty cycle that reflects the relative phase state of the two RF frequencies. The pulse waveform is filtered to create an average voltage that linearly corresponds to a particular position of the object and is able to follow rapid motions of the object. It does not repeat over a position distance that equals one-half the wavelength of the beam of transmitted light multiplied by the divisor used in the digital division process. Hence, the position distance can be many wavelengths in length.
REFERENCES:
patent: 5493395 (1996-02-01), Otsuka
patent: 5537209 (1996-07-01), Lis
Dr. Robert E. Brooks, “Surface Acoustic Wave Signal Processing”, Mar. 31, 1984, pp. 3-437-3-442.
Heflinger Donald G.
Heflinger Lee O.
Font Frank G.
Keller Robert W.
Natividad Philip
TRW Inc.
Yatsko Michael S.
LandOfFree
Dynamic optical micrometer does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Dynamic optical micrometer, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Dynamic optical micrometer will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2545445