AC laser interferometer

AC laser interferometer

AC laser interferometer

    In case of AC laser interferometer (ACLI) position information is carried as phase deviation rather than as a signal amplitude deviation, thus giving a much improved signal to noise ratio over amplitude modulation, because the noise sources that affect signal amplitude have little effect on phase.

    In this way, ACLI is much more tolerant of environmental factors that attenuate the intensity of a laser beam, such as dust, smoke, air turbulence etc.

    It requires no warm-up time or standby power. Thus ACLI has the following advantages: high repeatability and resolution of displacement measurement (0.1 um), high accuracy, long-range optical path (60 m), easy installation, and no change in performance due to ageing or wear and tear.

    A single laser source can be used for as many as six simultaneous measurements in different axes.

    However, it is very much expensive; since the basic instrument measures physical displacement in terms of wavelength instead of traditional units, conversion instrumentation is required for conventional read out.

    Highest possible accuracy is obtainable only by compensating changes in air pressure and temperature which affect wavelength of the laser beam.

    It uses two frequency laser system, thus overcoming the shortcoming of d.c. laser interferometer. Whereas the d.c. system mixes out of phase light beams of the same frequency, the a.c. system mixes beams of two different frequencies thus permitting the distance information to be carried on a.c. waveform.

    Use is made of the fact that the AC amplifiers are insensitive to d.c. variation of a.c. inputs.

    Two frequency Zeeman laser generates light of two slightly different frequencies with opposite circular polarisations.

    These beams get split up by beam splitter B1: one part travels towards B2 and from there to external cube corner where the displacement is to be measured.

   It may be noted that mirror is not employed here like Michelson Interferometer, because mirror alignment is a critical procedure.

    This interferometer, instead, uses cube-corner reflectors (retroreflectors) which reflect light parallel to its angle of incidence regardless of retroreflector alignment accuracy.

    Beam splitter B2 optically separates the frequency f1 which alone is sent to the movable cube-corner reflector.

    The second frequency f2 (optically separated) from B2 is sent to a fixed reflector which then re-joins f1 at the beam splitter B2 to produce alternate light and dark interference flicker at about 2 Mega cycles per second.

    Now if the movable reflector (external cube corner) moves, then the returning beam frequency will be Doppler-shifted slightly up or down by Δf1.

    Thus the light beams moving towards photo-detector P2 have frequencies f1 and (f1 ± Δ f1) and P2 changes these frequencies into electrical signal. (Photocells convert light-intensity variations into voltage pulses which can be processed by electronic instruments to give the amount and direction of position change).

    Photo detector P1 receives signal from beam splitter B1 and changes the reference beam frequencies f1 and f2, into electrical signal.

    An A.C. amplifier A1 separates frequency difference signal f2 – f1 and A2 separates frequency difference signal [(f2 - (f1 ± Δ f1)].

    The pulse converter extracts Δ f, one cycle per half wavelength of motion.

    The up-down pulses from the pulse converter are counted electronically and displayed in analog or digital form on the indicator.

    It may be noted that output in case of ACLI is in the form of pulses, whereas in d.c. systems, the output is in the form of a sinusoidal wave, the amplitude (intensity) of which depends upon laser aging, air turbulence or air pollutant and thus the change of amplitude leads to improper triggering and counting errors.