Principles of Interferometers
When you split a light wave having regular periods such as a laser beam into two waves and then recombine them, you can observe an interference phenomenon of light waves.When superimposing the two waves, the resultant wave has a part with amplified intensity and a part with diminished intensity alternately, making fringes of light and dark.
These interference fringes indicate the phase difference between the two optical path lengths, and one fringe is equivalent to the phase difference of the length of light source wavelength (half of the wavelength in the case of round-trip optical path).
Since it is impossible to identify components of phases different by an integral multiple of waves of interference fringes,what you can actually observe is limited to either variation of phases different by lower than an integral multiple of waves or continuous phase variation.
The wavelength of the light source is 632.8nm in the case of a He-Ne laser, therefore the one phase between fringes of an interferometer of round-trip optical path becomes very short, 0.3µm. This is why an interferometer is capable of measuring minute displacement or variation.
High sensitivity provides precise measured values however, at the same time detects influences of disturbance such as vibration or air turbulence. To prevent this, experiments need to be performed on a vibration isolator bench or the entire experiment system needs to be shut off from the outside with black-out curtains or the like.
To measure surface accuracy with an interferometer, install a sample in one optical path, and superimpose the wavefront reflected by the measuring surface with the plane wave reflected by the reference surface. Interference fringes generated at this time are folded, reflecting the shape of the measuring surface.
The shape of the measuring surface can be found from the amount of fold of these fringes.