Most light enabled sensing and imaging techniques have depth resolutions that are larger than a few micrometers. An exception to this is the single wavelength laser coherent interferometry, known for its sensitivity and resolution lambda/1000. However, due to 2pi periodicity of the cosine function, the unambiguous range of single wavelength interferometry systems is limited to lambda/2.
Researchers in the past have performed interferometry with multiple wavelengths and detected phase on a single photodetector. The beat signal of the multiple wavelengths has been used to extend the range, but this technique worsens the resolution by the same factor.
To alleviate this range-resolution tradeoff, our system uses multiple wavelengths for the interference, but wavelength demultiplexes the interferograms before detecting them on separate photodetectors. The resolution is preserved to that of a single wavelength interferometer, while the range is extended to the lowest common multiple of the multiple wavelengths.
In our experiment, we use two wavelengths, 1525 nm and 1550 nm, and employ techniques of simultaneous phase interferometry to extract and unwrap the phase. We are able to measure discrete positions of a piezo electric stage up to an unambiguous distance of 94.55µm, with nanometer precision. Furthermore, by incorporating 4F lens systems with our technique, we demonstrate the capabilities of measuring samples with high precision. Our system is instantaneous, computationally cheap, utilizes inexpensive components, and has the highest dynamic range of 1e5.