How do laser interferometers measure distances greater than 1wavelengt

[blin1994]

Hi,

My question is how do do laser interferometers measure distances of more than one wavelength?
I know they rely on phase difference to calculate distances but phases only differ at 180 degrees at max so how do interferometers factor in going over 180 degrees in phase difference to calculate larger distances?


Thanks

 

[UltrafastPED]

There are multiple interference fringes - all across the wavefronts of the interfering beams.

See http://cmi.epfl.ch/metrology/files/Wyko/Interferometery_Basics.pdf

 

[Andy Resnick]

Hi,

My question is how do do laser interferometers measure distances of more than one wavelength?
I know they rely on phase difference to calculate distances but phases only differ at 180 degrees at max so how do interferometers factor in going over 180 degrees in phase difference to calculate larger distances?


Thanks

Google "Phase unwrapping", for example:

http://earth.esa.int/landtraining07/D1LB4-Rocca.pdf
http://www.acsu.buffalo.edu/~jlv27/index_files/Publications_files/PhaseUnwrapEncyclopedia.pdf