Heterodyne Interferometry Technique (Metrology)

6.9.
Simple d.c. fringe counting techniques suffer from problems of intensity level changes
in source and also on account of motion of source or object. Fringe contrast changes and d.c.
level shifts result in miscounting of the fringes. Heterodyne interferometer is an a.c. device
and the problems of d.c. fringe counting techniques are overcome. In this type of interferometer,
a Zeeman laser source emits two closely spaced orthogonal polarisation frequencies separated
by around 1 MHz. Abeam splitter placed in front of laser source separates off part of the signal
Working principle of Heterodyne Interferometer
Fig. 6.38. Working principle of Heterodyne Interferometer.
from both polarisations which are mixed on detector Dx to provide a reference beat ft – ft. The
transmitted component travels upto polarising beam splitter where it is splitted. Part of it
travels to reference fixed arm and other to measurement arm connected with target movement.
The two signals are recombined at the polarising beam splitter and detected by detector D2- If
target is stationary, the detected beam is ft – ft. When it moves, then detected beat is ft – ft
± A/. The reference and Doppler-shifted beats are counted by two independent counters and
subtracted to give A/-. Integration of the count over time t measures 2d/X.
6.9.1.


Dual-frequency Laser Interferometer.

This instrument is used to measure
displacement, high-precision measurement of lengths, angles, speeds and refractive indices as
well as derived static and dynamic quantities. It operates on heterodyne principle. The two
resonator modes (frequencies ft and ft) are generated in a laser tube such that ft – ft = 640
MHz. These are controlled so that their maxima are symmetrical to the atomic transition. This
permits a long reliable stability. The frequency stability of He-Ne laser is responsible for
outstanding performance of the interferometer.
An amplitude beam splitter branches off part of the laser output create a reference beam,
which an optical fibre cable relays to a photodetector 1. This detects the beat signal of the 640
MHz frequency difference produced by the heterodyning of the two modes. The other portion
of the light serves as measuring beam. Via an interferometer arrangement it is directed to a
movable measuring mirror and a stationary reference mirror, which reflects it on to a
photo-detector 2. The two frequencies in the measuring beam are separated by a polarisation-
sensitive beam splitter so that the measuring mirror receives light of frequency/i only, whereas
the light that strikes the reference consists exclusively of frequency f2. With the measuring
mirror at rest, detector 2 also senses the laser differential frequency of/i – f2 = 640 MHz. If
the measuring mirror is being displaced at a speed v, the partial beam of frequency/i reflected
by it is subjected to a Doppler shift dfx; where dfx = (2v)/Xx-
Accordingly, detector 2 now receives a measuring frequency of fx-f2± dfx (+ dfx or – dfx)
depending on the direction of movement of the measuring mirror. The reference frequency
f\—fi ana the measuring frequency f\—f%± dfx are compared with each other by an electronic
counting chain. The result is the frequency shift ± dfx due to the Doppler effect, a measure of
the wanted displacement of the measuring mirror. In a fast, non-hysteric comparator, the
 Schematic of dual frequency laser interferometer
Fig. 6.39. Schematic of dual frequency laser interferometer.
Pl = Photo detector for reference signal
P2, P3 and P4 = photo detectors for measuring
Ix = Basic Instrument signals with HF signal processing and interpolation facilities.
Doppler frequency df\ is digitised and then fed to a counter, which registers the number of zero
passages per unit time.
The forward and return movements of the measuring mirror can be distinguished by
outcoupling the measuring signal /i – f2 + dfx at ‘n’ phase angles, via a delay line and feeding
to ‘re’ mixers. The mixers are connected with the reference signal f\ —f2 (common feeding point
for all mixers). Thus n Doppler frequencies get shifted in phase by njn at the mixer outputs.
They are symmetrical relative to zero. After comparison they are made available to low-fre-
quency counting logic as TTL signals. The n phase angles and their tolerances are implemented
by the geometry of the delay line.
This system can be used for both incremental displacement and angle measurements.
Due to large counting range it is possible to attain a resolution of 2 nm in 10 m measuring
range. Means are also provided to compensate for the influence of ambient temperature,
material temperature, atmospheric pressure and atmospheric humidity fluctuations.

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