초록 ▼

A method and apparatus for detecting small shifts in optical wavelength is provided. Light having a known wavelength is directed towards a target volume in the atmosphere. Light reflected from the target volume is received and mixed with differentially delayed copies of itself to produce three inter

A method and apparatus for detecting small shifts in optical wavelength is provided. Light having a known wavelength is directed towards a target volume in the atmosphere. Light reflected from the target volume is received and mixed with differentially delayed copies of itself to produce three interference signals. The interference signals are taken at delays spanning one-half the wavelength of the transmitted signal. The intensity of the signal at the three different delays is measured simultaneously, from which the wavelength of the reflected signal can be determined.

대표청구항 ▼

What is claimed is: 1. A method for determining a shift in optical wavelength, comprising: generating light having a first wavelength at a first location; directing said generated light towards an area of interest; receiving light reflected from said area of interest at about said first location, w

What is claimed is: 1. A method for determining a shift in optical wavelength, comprising: generating light having a first wavelength at a first location; directing said generated light towards an area of interest; receiving light reflected from said area of interest at about said first location, wherein at least some of said reflected light comprises at least a portion of said generated light; directing a first portion of said received reflected light along a first optical path to form a first beam; directing a second portion of said received reflected light along a second optical path to form a second beam: delaying a first portion of said received light directed along said first optical path by a first delay interval at time t, wherein said delaying a first portion of said received light includes reflecting said first portion of said received light off a first portion of a first mirror assembly; delaying a second portion of said received light directed along said first optical path by a second delay interval at time t, wherein said delaying a second portion of said received light includes reflecting said second portion of said received light off a second portion of said first mirror assembly; delaying a third portion of said received light directed along said first optical path by a third delay interval at time t, wherein said delaying a third portion of said received light includes reflecting said third portion of said received light off a third portion of said first mirror assembly, wherein at time t a position of said first mirror assembly is fixed, and wherein time t is about equal to a travel time of light to and from said area of interest; combining at least some of said first beam with said delayed first portion of said second beam to obtain a first interference signal; combining at least some of said first beam with said delayed second portion of said second beam to obtain a second interference signal; combining at least some of said first beam with said delayed third portion of said second beam to obtain a third interference signal; determining at a common instant in time an intensity of said received interference signals at each of said three different delay intervals, wherein said delays are spaced at known intervals of said first wavelength from one another, wherein time t comprises one instant in time, and wherein said determining an intensity of said received reflected light at three different delay intervals at time t includes measuring an intensity of each of said first, second and third interference signals at time t; and determining from a ratio of said first, second and third interference signal intensities a change in wavelength between said generated light and said reflected light. 2. The method of claim 1, further comprising: reflecting said first beam from a common mirror; reflecting said second beam from said first mirror assembly, wherein said first mirror assembly includes a segmented mirror, wherein said first portion of said second beam is reflected by a first segment of said segmented mirror, wherein said second portion of said second beam is reflected by a second segment of said segmented mirror, and wherein said third portion of said second beam is reflected by a third segment of said segmented mirror. 3. The method of claim 2, wherein said first, second and third segments of said segmented mirror are equal in area. 4. The method of claim 2, wherein said first, second and third segments of said segmented mirror are all parallel to a common plane and wherein a plane of a reflective surface of said first segment is separated from a plane of a reflective surface of said second segment by a distance equal to one-eighth of said first wavelength, and wherein said plane of said reflective surface of said second segment is separated from a plane of a reflective surface of said third segment by a distance equal to one-eighth of said first wavelength. 5. The method of claim 1, further comprising separating said first, second and third portions of said second beam from one another by aperture division. 6. The method of claim 1, wherein said delaying a second portion comprises passing said second portion of said received reflected light of said second beam through a phase retarder. 7. The method of claim 1, wherein said delaying first, second and third portions of said received light directed along said first optical path further includes passing said first, second and third portions of said received reflected light of said second beam through at least one of different portions of a phase retarder and different phase retarders. 8. The method of claim 1, further comprising: directing said first interference signal to a first detector; directing said second interference signal to a second detector; directing said third interference signal to a third detector. 9. The method of claim 1, wherein said directing a first portion of said received reflected light along a first optical path and said directing a second portion of said received reflected light along a second optical path comprises splitting said received reflected light into said first and second portions using a beam splitter. 10. The method of claim 1, further comprising: calibrating said first and second delays relative to one another, wherein said calibrating includes adjusting at least one of said first delay and said second delay such that light wherein received reflected light at said first wavelength produces first and third intensities that are equal to one another. 11. The method of claim 1, further comprising selecting a first average delay for greater resolution and a second average delay for greater measurement range, wherein said first average delay is greater than said second average delay. 12. The method of claim 1, further comprising: calibrating said three delay intervals, wherein said calibrating includes adjusting said first and third delay intervals so that said first and third interference signals each are of equal intensity when light of said first wavelength is subjected to said first and third delays, and wherein said determining from a ratio of said interference signal intensities a change in wavelength between said generated light and said reflected light includes: fitting a sine curve to said measured interference signal intensities; calculating a change in phase in said sine curve as compared to a calibrated phase; calculating a change in wavelength from said calculated change in phase; calculating a velocity of a reflector included in said area of interest relative to said first location from said calculated change in wavelength. 13. A method for determining a shift in optical wavelength, comprising: generating light having a first wavelength at a first location; directing said generated light towards an area of interest; receiving light reflected from said area of interest at about said first location, wherein at least some of said reflected light comprises at least a portion of said generated light; determining an intensity of said received reflected light at each of three different delay intervals at time t, where t is equal to a travel time to and from said area of interest, and wherein said delays are spaced at intervals of �� said first wavelength from one another; and determining from a ratio of said intensities a change in wavelength between said generated light and said reflected light; directing said received light to a polarizing beam splitter as light linearly polarized at 45�� to a plane of incidence of said polarizing beam splitter, wherein said polarizing beam splitter forms a first beam having a first polarization and a second beam having a second polarization that is orthogonal to said first polarization; passing said first beam through a first quarter wave plate; reflecting said first beam back through said first quarter wave plate; passing said second beam through a second quarter wave plate; reflecting said second beam back through said second quarter wave plate; recombining said first and second beams in said polarizing beam splitter, wherein said first and second beams are directed along a third optical path; directing a first portion of said first and second beams from said third optical path to a first polarization scrambler and to a first detector; passing a second portion and a third portion of said first and second beams through a third quarter wave plate; after said passing said second portion and third portions of said first and second beams through a third quarter wave plate, directing said second portion of said first and second beams to a second polarization scrambler and to a second detector; passing said third portion of said first and second beams through a fourth quarter wave plate; after said passing said third portion of said first and second beams through a fourth quarter wave plate, passing said third portion of said first and second beams through a third polarization scrambler and to a third detector; and wherein said determining an intensity of said received reflected light at each of three different delay intervals at time t includes determining an intensity of a signal received at each of said first, second and third detectors at time t. 14. A device for measuring small shifts in optical wavelengths, comprising: a light source, wherein said light source provides light at a first wavelength; a first beam splitter, wherein light from said light source and reflected back to said device is divided into first and second beams that traverse first and second optical paths having different path lengths; a first mirror, wherein light directed along said first optical path is reflected by said first mirror; a second mirror, wherein light directed along said second optical path is reflected by said second mirror; at least one of a phase retarder and a stepped surface included in said second mirror, wherein said second beam is sub-divided into at least first, second and third delay paths each having different magnitudes of delay, wherein said delays span at least one-half said first wavelength, and wherein at a time during which said second beam is subdivided into said first, second and third delay paths a position of said second mirror is fixed; a detector assembly, wherein an intensity of each signal resulting from interference between light that has been delayed by said at least three different delays and light that has been directed along said first path is measured; and a trihedral prism, wherein a first division of said second beam comprising light delayed by a first amount combined with a first portion of said first optical path light is directed to a first area of said detector assembly, wherein a second division of said second beam comprising light delayed by a second amount combined with a second portion of said first optical path light is directed to a second area of said detector assembly, and wherein a third division of said second beam comprising light delayed by a third amount combined with a third portion of said first optical path light is directed to a third area of said detector assembly. 15. The device of claim 14, wherein said second mirror of said device has a stepped surface. 16. The device of claim 14, wherein said second mirror has a stepped surface that includes three parallel reflective segments, wherein a first of said reflective segments is separated from a second of said reflective segments by a distance equal to one-eighth said first wavelength, wherein said second reflective segment is separated from a third of said reflective segments by a distance equal to one-eighth said first wavelength, and wherein said first reflective segment is separated from said third reflective segment by a distance equal to one-fourth said first wavelength. 17. The device of claim 14, wherein said device includes a phase retarder. 18. The device of claim 14, wherein said device includes a phase retarder comprising a number of quarter wave plates. 19. The device of claim 14, wherein said beam splitter comprises a polarizing beam splitter, and wherein said device further comprises: a linear polarizer, wherein said light reflected back to said device is provided by said linear polarizer to said polarizing beam splitter, and wherein said polarizing beam splitter has an axis of polarization that is 45 degrees to a plane of incidence of said polarizing beam splitter. 20. The device of claim 19, further comprising: a first quarter wave plate in a path of said first beam and adjacent to said first mirror; and a second quarter wave plate in a plate in a path of said second beam and adjacent said second mirror. 21. The device of claim 14, wherein said detector assembly comprises a charge coupled device. 22. The device of claim 14, wherein said detector assembly includes: a first detector element coincident with said first area of said detector assembly and associated with said first delay path; a second detector element coincident with said second area of said detector assembly and associated with said second delay path; a third detector element coincident with said third area of said detector assembly and associated with said third delay path. 23. The device of claim 14, wherein said light source comprises a pulsed laser light source. 24. The device of claim 23, wherein said pulsed laser light source has a coherence length of at least 0.5 meters. 25. The device of claim 14, wherein said first mirror is integral to said beam splitter. 26. The device of claim 14, further comprising: a processor, wherein an intensity of an interference signal from each of said first, second and third delay paths is provided to said processor by said detector device, and wherein a ratio of said intensities is taken in connection with determining a velocity of a reflector within said target area. 27. The device of claim 14, further comprising an actuator, wherein a length of said first, second and third delay paths can be altered simultaneously in response to movement of said actuator. 28. An apparatus for determining a relative velocity of a target, comprising: means for generating light, wherein said light includes a first wavelength; means for receiving a reflected signal, wherein said reflected signal includes a reflected portion of said light from said means for generating light; means for directing said received reflected signal, wherein at least a first portion of said received selected signal is directed along a first path and wherein at least a second portion of said received selected signal is directed along a second path; means for differentially delaying three segments of said second portion of said received reflected signal relative to one another; means for mixing said light directed along said first paths with said light directed along said second of said paths, wherein three interference signals are obtained; means for simultaneously detecting an intensity of each of said three interference signals; means for determining from a ratio of said intensities a change in wavelength between said generated light and said reflected light. 29. The apparatus of claim 28, wherein said means for differentially delaying three segments of said second portion of said received reflected signal relative to one another includes a stepped mirror, said apparatus further comprising: means for separating said three interference signals into three separate paths for delivery to said means for simultaneously detecting, wherein said means for separating includes a trihedral prism. 30. The apparatus of claim 28, wherein said means for differentially delaying three segments of said second portion of said received reflected signal relative to one another includes a phase retarder. 31. The apparatus of claim 28, wherein said means for differentially delaying three segments of said second portion of said received reflected signal relative to one another includes a quarter wave plate. 32. The apparatus of claim 28, further comprising: means for determining a ratio of said intensities of said interference signals. 33. The apparatus of claim 32, further comprising: means for determining a velocity of said target from said ratio of said intensities. 34. The apparatus of claim 28, wherein said second portion of said received reflected signal comprises a beam, and wherein said three segments of said second portion of said received reflected signal each comprise an equal area of a transverse section of said beam.