Construction of real interferometer
Displacement measurements with the use of a laser interferometer allow obtaining the accuracy of a displacement measurement of 0.4 ppm in air and 0.02 ppm in vacuum.
The interferometer was first built by A.A. Michelson in 1881. The simplified schematic of the interferometer is shown in the figure below. Coherent light beam falls on a semi-transparent mirror. This mirror splits the light into two beams. The first goes to the reference arm and reflects from the reflector Z1; the second goes to the measurement arm and reflects form the reflector Z2. The reflected beams meet again on the detector. Because these beams come from the same, coherent, source, they will interfere. When the moving reflector is being displaced, the frequency of the reflected beam in the measurement arm changes. The detector counts the frequency difference between reflected beams – fD. The measured value of the displacement is calculated according to: