Page 191 - Instrumentation Reference Book 3E

Interferometric sensing approach 175

parameters, notably for acoustic detection in in the use of single mode laser diodes. These
hydrophone developments, its use in the monitor- include a greatly reduced degree of wavelength
ing of low frequency quasi-static measurands stabilization of the source and the elimination of
such as temperature, pressure, and strain fields feedback problems in the lasing cavity since the
ha.s seen less progress. white-light system can operate with multimode
This lack of progress results from a serious laser diodes or LED devices. White-light inter-
limitation of the current modulation method ferometry is dependent on the relatively short
which requires a high degree of wavelength stabil- coherence lengths of this type of source and oper-
ization of the laser diode source. For example, ates by connecting the source, sensing interfero-
even withlatit the necessary wavelength modula- meters, and processing system via an optical fiber
tion, there is a stringent requirement on the network to establish a remote sensing device.
degree of wavelength stabilization needed. In
order to provide a SN resolution in 12.3.3.2 Ternyorally scariried method'
the interferometric fringe, the required wave-
length stability SA of the source wavelength is In order to conveniently measure optical phase
related to the cavity length L by: changes introduced in the sensing interferometer;
a carrier signal is produced by modulation of the
A2
SA = -6N (12.4) path imbalance in a second processing interfero-
2nL meter, as will be described below. In the con-
For a modest 1/100 of a fringe resolution and a ventional white-light interferometric method this
1 cm arm imbalance, the required stabilization in is carried out by the periodic displacement of one
the central wavelength needed is therefore about interferometer mirror in a linear ramp fashion by
2 x 10-'3rn. Considering that the variation in a piezoelectric modulator, therefore producing
source wavelength is about 0.02 ndmA and a sinusoidal fringe output signal. This type of
about 0.3nd°C, this implies a control in the system is classified as the tenzporalfiinge method
drive current of < 10 pA and at a temperature of (e.g., Boheim and Fritsch 1987). A major advan-
< 1/1000 "C. These represent demanding control tage of the white-light interferometric technique
conditions on the laser diode source, especially is its relative insensitivity to wavelength fluctu-
when it is realized that the drive current of the ations of the source.
laser diode needs to be linearly modulated over a Basically, the white-light technique has a sens-
2 x lo-* nm range at the same time as its central ing interferometer and a reference or processing
wavelength is stabilized to the indicated levels. interferometer along with a broad spectral band-
Attempts have been made to stabilize the central width source such as a LED, a multimode laser
wavelength of the source to atomic absorption diode or a super-radiant source (see, e.g., Ning
lines (see, e.g., Villeneuve and Tetu 1987), and et al. 1989). The broadband source is launched
also to a linearly scanned Fabry-Perot reference into the core of the optical fiber and transmitted
interferometer (Change and Shay 1988). How- to the sensor head interferometer, as shown in
ever, the stringent control requirements needed Figure 12.6(a). The path imbalance of the sensing
for this technique have led to the rise in an alter- interferometer, L1, is made a distance greater
native interferometric method-that based on the than the coherence length, I,, of the source radi-
"white-light" technique that makes use of a broad ation such that light reflected back from the two
spectral bandwidth source. This method is con- cavity mirrors does not interfere but is passed
sidered in the following sections. down the connecting fiber leads and via a direc-
tional coupler into the processing or reference
interferometer cavity. The imbalance, L?, of this
12.3.3 White-light interferometry interferometer is made comparable to that of the
sensing interferometer and necessarily within the
12.3.3.1 Introduction
coherence length of the source, &I&?. Here, part
-'White-light" optical fiber interferometry has of the radiation is brought back into temporal
established itself as a powerful sensing technique coherence by the interaction of the two inter-
in the development of a wide range of sensing ferometers, and the resulting interference signal
systems. The method was initially confined to is detected on the output photodetector, Modula-
the use of single mode optical fiber components tion of either the sensor or processing interfero-
(see Al-Chalabi etal. 1983). More recently this meter path imbalance will lead to a modulation in
"white-light" or low-coherence interferometric the phase of the output interference signal. A
method has attracted a broad interest (e.g., carrier signal is then introduced on the output
Bosselmann and Ulrich 1984; Mariller and signal by a serrodyne displacement modulation
Lequine 1957; Valleut etal. 1987) due to its on one mirror of the reference interferometer.
ability to overcome some of the major limitations Using a piezoelectric transducer the mirror is