Single frequency DC interferometer system

Single frequency DC interferometer system

Single frequency DC interferometer system

   

          It uses a single frequency circular polarised laser beam.

   

     On reaching the polarising beam splitter, the beam splits into two components, the reflected beam being vertically polarised light and the transmitted beam being horizontally polarised light.

    These two beams referred to as reference arm and measurement arm respectively travel to their retroreflectors and are then reflected back towards the beam splitter.

    The recombined beam at beam splitter consists of two superimposed beams of different polarisation; one component vertically polarised having travelled around reference arm and other component horizontally polarised having travelled around the measurement arm.

    These two beams being differently polarised do not interfere.

    The recombined beam then passes through a quarter wave plate which causes the two beams to interfere with one another to produce a beam of plane polarised light.

    The angular orientation of the plane of this polarised light depends on the phase difference between the light in the two returned beams.

    The direction of plane of polarisation spin is dependent on the direction of movement of the moving retroreflector.

    The beam after quarter wave plate is split into three polarisation sensitive detectors.

    As the plane of polarised light spins, each detector produces a sinusoidal output waveform.

    The polarisation sensitivity of the detectors can be set so that their outputs have relative phases of 0°, 90°, and 180°.

    The outputs of three detectors can be used to distinguish the direction of movement and also the distance moved by the moving retroreflector attached to the surface whose displacement is to be measured.

    For linear measurements (positional accuracy or velocity), the retroreflector is attached to the body moving along the linear axis. For angular measurement (For pitch and yaw), the angular beam splitter is placed in the path between the laser head and the angular reflector.

    In this way it is possible to measure flatness, straightness, rotatory axis calibration.

    Arrangements also need to be made for environmental compensation because the refractive index of the air varies with temperature, pressure and humidity.

    Interferometry is now an established and well developed technique for high accuracy and high resolution measurement.