# Coherence or low-coherence?

I don't understand what means coherent interferometry and low-coherence interferometry under subtopic "types of interferometry". http://en.wikipedia.org/wiki/Interferometer

Low-coherent interferometry uses white light so that its coherent length is very short, able to measure very small path difference.

Coherent interferometry uses laser so that its coherent length is very long, is that means its resolution is very low? But why it says the interference is capable of very accurate (nanometer) measurement?

What means distance can be measured by phase difference? Two coherent sources differ by 2pi can be said in-phase (phase diff zero) right? Phase difference 0.5pi same with phase difference 1.5pi right? Then how to measure distance?

Why coherent interferometry suffers from a 2π ambiguity problem but low-coherent interferometry not?

Why OCT resolution is given by (2ln2/pi )*coherence length?

Any references for the applications of both interferometry?

Relevance

Answers to a few (but by no means all) of your interesting questions.

"Coherent length is long" means the laser beam is coherent over a good distance. But the resolution is high (order of fraction of a wavelength) if the ambiguity of "how many whole wavelengths do I add to the fractional-wavelength difference?" is resolved.

Yes 2pi difference of a single-frequency source is in-phase; but 0.5pi is out of phase with 1.5pi. So you can measure any fractional wavelength difference from 0 to 2pi.

Low-coherent interferometry doesn't have a 2pi ambiguity since 2pi is a meaningless length when you are dealing with many different wavelengths simultaneously.