Laser Interferometer – Introduction, Construction, Working

Laser interferometer uses A.C laser as the light source and thus it enables the measurements to be made over longer distance. Laser represents a source of intensively monochromatic optical energy, which can be collimated into a directional beam. The laser beam wavelength is exact and pure for highly accurate measurements. The laser interferometers utilize the principles of both optical techniques and digital electronics.

Laser Interferometer (AC)


The AC laser interferometer has the following components.

  1. Two frequency zeeman laser.
  2. Beam splitters.
  3. Fixed internal cube corners.
  4. External cube corners.
  5. Photo detectors.
  6. Amplifiers.
  7. Pulse converter.

Two frequency zeeman laser

It is generally He Ne type that generates stable coherent light beams of two frequencies, one is polarized vertically and the other one is polarized horizontally relative to the plane of the mounting.

Beam splitters

It divides the laser beam into separate beams along different axis. It is possible to adjust the splitted laser’s output, intensity by having a choice of beam splitter reflectivities.

Cube corners

The cube corners can be plane mirrors roof prisms. Each AC laser interferometers are required at least two retro frequencies. One is fixed external cube corner and another one is external cube corner. Both the cube corners are used to reflect the laser beam.

Photo detectors

Photo detectors receives the signal from the beam splitter and changes into electrical signal.


There are two amplifiers used in AC laser interferometer. It is used to separate the frequency difference.

Pulse converter

The pulse converter is used to extracts change in frequency Δƒ.


The two frequencies zeeman laser generates light of two slightly different frequencies with opposite circular polarizations. The beams are splitted by the beam splitter B1. In this, one part travels towards B2 and from B2 to external cube corners, where the displacement is measured.

Beams splitter B2 optically separates the frequencies ƒ1 which is sent to a fixed reflector and then rejoins ƒ1 at the beam splitter B2 to produce alternate light and dark interference. Now, the external cube corner moves which it will produce a change in ƒ1 (i.e., Δƒ1) in the returning beam frequency. So, the light beams move towards photo detectors p2 having frequencies ƒ2 (i.e., ƒ1 to Δti) and p2 will be changed into electrical signal.

The photo detector p1 receives signal from beam splitter B1 and changes the references beam ƒ1 and ƒ2 into electrical signal. The two amplifiers A1 and A2 separate frequency difference signals ƒ21 and f2 – (ƒ1 + Δƒ). The pulse converter extracts Δƒ and displays in the form of pulses in analog or digital form in the output.

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Santhakumar Raja

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