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Various systems and methods for performing optical analysis downhole with an interferogram (a light beam having frequency components with a time variation that identifies those frequency components. The interferogram is produced by introducing an interferometer into the light path, with the two arms

Various systems and methods for performing optical analysis downhole with an interferogram (a light beam having frequency components with a time variation that identifies those frequency components. The interferogram is produced by introducing an interferometer into the light path, with the two arms of the interferometer having a propagation time difference that varies as a function of time. Before or after the interferometer, the light encounters a material to be analyzed, such as a fluid sample from the formation, a borehole fluid sample, a core sample, or a portion of the borehole wall. The spectral characteristics of the material are imprinted on the light beam and can be readily analyzed by processing electronics that perform a Fourier Transform to obtain the spectrum or that enable a comparison with one or more templates. An interferometer designed to perform well in the hostile environments downhole is expected to enable laboratory-quality measurements.

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1. A downhole tool that comprises: a downhole light detector;a downhole dual-beam interferometer on an optical path between two light sources and the light detector, wherein the interferometer produces, from light received along the optical path, a spectralized beam having a spectral composition wit

1. A downhole tool that comprises: a downhole light detector;a downhole dual-beam interferometer on an optical path between two light sources and the light detector, wherein the interferometer produces, from light received along the optical path, a spectralized beam having a spectral composition with multiple frequency components measurable from time variations of the spectralized beam's intensity;a window on the optical path that enables light received along the optical path to illuminate a material to be analyzed, wherein the material is a borehole fluid or part of a borehole wall; andprocessing electronics coupled to the light detector to detect an electrical signal representing time variations of the spectralized beam's intensity and to determine therefrom the multiple frequency components and a parameter of the material. 2. The tool of claim 1, wherein the material is a fluid drawn from a port seated against a borehole wall. 3. The tool of claim 2, wherein the window is on a sample cell that stores the fluid for transport to the surface, and the optical path passes through the fluid. 4. The tool of claim 2, wherein the parameter is a contamination level for the fluid. 5. The tool of claim 2, wherein the parameter includes a quantity of at least one fluid type. 6. The tool of claim 2, wherein the parameter includes a concentration of at least one substance. 7. The tool of claim 2, wherein the parameter is a size distribution of particles in the fluid. 8. The tool of claim 1, wherein the parameter is a fluid density or a quantity of at least one fluid type. 9. The tool of claim 1, wherein the light detector senses light reflected from the material. 10. The tool of claim 1, wherein the light detector senses light transmitted through the material. 11. The tool of claim 1, wherein the interferometer includes an integrated light path component comprised of a solid block of a transparent material. 12. The tool of claim 1, wherein one of the two light sources is a reference light source providing light that traverses at least that portion of the light path that includes the interferometer and the light detector, wherein the processing electronics determine a motion compensation based on measurements of light from the reference light source. 13. The tool of claim 1, wherein the tool is adapted to be part of at least one of: a wireline logging assembly, a drilling assembly, and a tubing-conveyed logging assembly. 14. The tool of claim 1, wherein the processing electronics correlate the signal with one or more templates to determine a relative concentration of at least one substance. 15. The downhole tool of claim 1, wherein the interferometer comprises a fixed mirror and a movable mirror, wherein the movable mirror is movable as a function of time to spectralize the light received along the optical path to produce the spectralized beam as a function of time. 16. The downhole tool of claim 1, wherein the interferometer produces the spectralized beam using a reciprocating or rotating mirror, and wherein the processing electronics determine the multiple frequency components using mirror position as a function of time. 17. A downhole tool that comprises: a downhole light detector;a downhole interferometer on an optical path between a broadband light source and the light detector, wherein the interferometer produces, from light received along the optical path, a spectralized beam having a spectral composition with multiple frequency components measurable from time variations of the spectralized beam's intensity;a window on the optical path that enables light received along the optical path to illuminate a material to be analyzed, wherein the material is a borehole fluid or part of a borehole wall; andprocessing electronics coupled to the light detector to detect an electrical signal representing time variations of the spectralized beam's intensity and to determine therefrom the multiple frequency components and a parameter of the material,wherein the interferometer includes a spinning retroreflector having a position encoder coupled to the processing electronics. 18. A downhole analysis method that comprises: directing light from two downhole light sources along an optical path that includes a downhole interferometer and a window for downhole sample illumination, wherein the downhole sample is a borehole fluid or part of a borehole wall;modulating one arm length of the interferometer as a function of time to produce a spectralized beam before the light reaches a downhole detector, the spectralized beam having a spectral composition with multiple frequency components measurable from time variations of the spectralized beam's intensity;measuring time variations of the spectralized beam's intensity with the downhole detector; andusing the measured time variations of the spectralized beam's intensity to determine the multiple frequency components and a property of the downhole sample. 19. The method of claim 18, wherein the downhole sample is a fluid drawn from a port seated against a borehole wall. 20. The method of claim 18, wherein the property is at least one of; a contamination level, a quantity of at least one fluid type, a concentration of at least one substance, and a size distribution of particles. 21. The method of claim 18, wherein modulating one arm length of the interferometer as a function of time comprises reciprocating or rotating a mirror, and wherein mirror position as a function of time is used to determine the multiple frequency components. 22. The method of claim 18, wherein one of the two light sources is a reference light source providing light that traverses at least the interferometer and a light detector, wherein using the measured time variations of the spectralized beam's intensity to determine the multiple frequency components and a property of the downhole sample is based on measurements of light from the reference light source.