1. Cause a required amount of the difference frequency: This frequency difference between the measured amount may be converted to amplitude phase measurement. Specifically, after a slight difference by the method, to obtain a difference frequency sweep signal frequency, and when there are other things affect the phase (mechanical vibration corresponding frequency plasma, etc.) was added into the beam path, the optical path will influence the phase the reflected. To two difference frequency wave sweep brings a slight amount, no static interference pattern, but according to this difference frequency swept.
reference:
1. Khan Academy, Young's double-slit: https://tr.khanacademy.org/science/physics/light-waves/interference-of-light-waves/v/youngs-double-split-part-1
The wavelength of the laser by two closely adjacent its aperture, diffracted, interference between the diffracted waves form an interference pattern (interference pattern). Briefly, the whole difference between the two wave wavelength was strengthened point, an integral multiple of a half wavelength It was weakening point. It means a stable pattern is to enhance / decrease the fixed position of a point, rather than static (the human eye to distinguish between light and dark fringes, a is the average amount of power).
