Page 189 - Instrumentation Reference Book 3E

Interferometric sensing approach 173
Sensor cross-sensitivity relates to the fact that laser sources can either be separate devices having
sensors are usually required to measure one narrow and stable spectral lines, or they can be
specific variable parameter in the presence of the same source beam (Figure 12.4) that has been
other changing conditions such as pressure in amplitude divided and with one beam then fre-
the presence of temperature and vibration, etc.; quency shifted, for example, by an acousto-optic
minimizing such undesirable effects is important Bragg cell modulator (see, e.g., Tsubokawa et 21.
in any sensor design exercise. 1988; Meggitt etal. 1991). For source wave-
lengths of A1 and A', with a wavelength separ-
ation of AA, the associated beat frequency AfH,,
12.3 Interferometric sensing between the two optical frequencies is then given
approach by:
The use of interferometric techniques in optical (12.1)
fiber sensing has become a well established and
one of the most widely used methods, since they
can be applied to a large range of measurement and where c is the velocity of light. In the case
parameters providing both a high resolution and where the two wavelengths are closely spaced
large dynamic range capability. Initially, single such as in the use of a Bragg cell element, the
mode fiber interferometric systems were demon- associated heterodyned wavelength (or synthetic
strated (e-g., Jackson etal. 1980: Giallorenzi et al. wavelength) of the beat frequency is given by:
1982), but more recently multimode methods
A'
have also shown themselves to be equally viable ASyn = - (12.2)
(eg, Boheim 1985) under the right operating AA
conditions There are three basic modulation For the case mentioned above where a Bragg cell
methods suitable for interferometric sensor sys-
tems that can produce a carrier signal, the phase element introduces a relative frequency shift of.
say, 80MHz between the two beams (typically
of which is then used to monitor for optical path
length changes in the associated sensing inter- between 40 MHz and 1 GHz), then the associated
synthetic wavelength caused by the beating
ferometer. These include heterodiwe techniques between the beams is 3.75m in air. It is also
which are useful with gas laser sources, yseudo- possible to down-convert the r.f. output signal
heterodyne rechrziqiies for use with injection- (MHz) to a lower and more manageable inter-
current modulated single-mode laser diode devices, mediate frequency (kHz) by mixing the output
and "white-light " interferometric teclzniqtres for use signal in a double balanced mixer with a stable
with low coherence. broadband sources such as r.f. oscillator having a slightly differing fre-
LED and multimode laser diode devices. quency. Mixing the 80MHz carrier with a
80.002 MHz local oscillator, for example, will
produce a carrier signal at 2kHz. When using
12.3.1 Heterodyne interferometry
APD photodetectors it is also possible to carry
In this technique two (or more) laser sources are out this down-conversion within the detector
used such that interferometric mixing occurs to device itself by applying the mixing frequency
produce a heterodyned output carrier signal. The directly onto one terminal of the detector while

Gas Laser

su rtace
(J Photo-Detector
4-J
Output Frequency at fBragg
(Phase Shiner by Target Movement)
Figure 12.4 Illustration of a typical heterodyne processing scheme for generating an intermediate carrier signal in a
displacement measurement device.