2π ambiguity-free optical distance measurement with subnanometer precision with a novel phase-crossing low-coherence interferometer
We report a highly accurate phase-based technique for measuring arbitrarily long optical distance with sub-nanometer precision. The method employs a Michelson interferometer with a pair of harmonically related light sources, one cw and the other low coherence. By slightly detuning (∼2 nm) the center wavelength of the low-coherence source between scans of the target sample, we can use the phase relationship between the heterodyne signals of the cw and the low-coherence light to measure the separation between reflecting interfaces with subnanometer precision. As this technique is completely free of 2π ambiguity, an issue that plagues most phase-based techniques, it can be used to measure arbitrarily long optical distances without loss of precision. We demonstrate one application of this technique, the high-precision determination of the differential refractive index. © 2002 Optical Society of America.
Yang, C; Wax, A; Dasari, RR; Feld, MS
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