A method and an apparatus for a precision optical distance measuring device which can be used to determine both external and internal target geometries. The beam from a frequency modulated laser diode is split into reference and target beams. A closed loop control system modulates the ...http://www.google.com/patents/US5781297?utm_source=gb-gplus-sharePatent US5781297 - Mixed frequency and amplitude modulated fiber optic heterodyne interferometer for distance measurement 
