Although traditional interferometry can be ideal for displacement metrology, it is often ill-suited for dimensional metrology such as length or thickness measurement due to fringe ambiguities. A number of related technologies compromise the laser's monochromatic nature, and therefore typically the measurement precision, in exchange for the ability to measure over larger windows without ambiguity. These systems rely on large information bandwidths to synthesize full, unambiguous range profiles. For instance, white light or low-coherence interferometry offer very large bandwidths (tens or hundreds of THz) and can be used to profile surface features down to the nanometer level. However, the short coherence length typically limits the measurement windows to tens of microns. A reference delay line in these systems can be physically scanned to enable larger effective windows, but this is typically slow, requires continual and precise calibration and monitoring of the reference delay, and is prone to failure due to the physical movement.
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