Fully Distributed Systems

The fully distributed optical-fiber sensing (FDOFS) system possesses the singular advantage of allowing a measurement of the external field to be made at any point along the length of the fiber, within the limitation of the spatial resolution interval. Thus, it may be possible to make a distributed measurement of a field over 2 km with a resolution interval of 1 m, giving 1000 measurement points and, as a result, effectively linearly multiplexing 1000 transducers.

Most FDOFS systems use time-domain techniques, rather than (the equivalent) frequency domain techniques, owing to their considerably reduced complexity and increased system bandwidth. Hence, this review will concentrate on these. FDOFS systems fall into three primary subclasses:

1. Linear backscatter: In this class the propagating optical pulse lies within the linear regime, and light backscattered from the pulse is time-resolved and analyzed to provide the spatial distribution of the measurand field (Figure 5.12(a)).

2. Nonlinear backscatter: The difference here is that the optical pulse has sufficient peak power to enter the nonlinear regime, and the (linear) backscattered power has to be analyzed differently (Figure 5.12(b)). The advantages of entering the nonlinear regime are that there is a diverse range of nonlinear optical effects offering specific responses to external measurands and ready discrim-ination at the detector. The main disadvantage is that the magnitude of the effect is strongly dependent upon optical power and, therefore, can vary significantly along the fiber as a result of attenuation.

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3. Nonlinear forward-scatter: Another advantage of the nonlinear regime is that it allows independent optical signals to interact. Thus, it is possible for counter-propagating (CP) radiations (e.g., a pulse and a CP continuous wave or two CP pulses) to interact (see Figure 5.12(c)). When the interaction is influenced by the external field, the field can be mapped by a forward-scattered (as opposed to a backscattered) light propagation. However, the same disadvantage of strong power dependence also applies, of course, to this mode of operation.

Linear systems are less complex; in particular, they are less demanding with respect to source require-ments and fiber properties. Nonlinear backscatter systems require high-power pulse sources and fibers appropriate for the nonlinear effect in question, but they do provide a broader range of measurand interactions and a ready discrimination at the detector. Nonlinear forward-scatter systems possess the same advantages and disadvantages as nonlinear backscatter systems but have the added advantage of a much higher signal level, which means a larger signal-to-noise ratio, and the added disadvantage of requiring two high-performance optical sources and, in most cases, access to both ends of the fiber.

An example of a linear, fully distributed optical-fiber measurement system will now be described.

5.4.1 Polarization-Optical Time-Domain Reflectometry (POTDR)

Polarization-optical time-domain reflectometry (POTDR) was, in fact, the first fully distributed optical-fiber measurement method to be studied in the laboratory [Rogers, 1980, 1981]. It is a polarimetric extension of OTDR. Whereas in OTDR the power level of the Rayleigh-backscattered radiation, from a propagating optical pulse, is time-resolved to provide the distribution of attenuation along the length of the fiber, in POTDR it is the polarization state of the backscattered light that is time-resolved; this provides the spatial distribution of the fiber’s polarization properties. Only monomode fibers can be FIGURE 5.12 Schemes for fully distributed sensing: (a) linear backscattering; (b) nonlinear backscattering;

(c) nonlinear forward-scattering.

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involved in POTDR, because only with monomode propagation can there be a single, determinable polarization state of the light at any point in space or time within the fiber; each mode in multimode propagation will possess its own independent polarization state.

With the determination of the spatial distribution of the polarization properties of the fiber comes a capability for measurement of the distribution of any external field that modifies those properties. These include strain, pressure, temperature, electric field, magnetic field, etc.

Figure 5.13(a) shows the basic arrangement for POTDR, and Figure 5.13(b) shows the result obtained for the distribution of strain induced in a fiber when wound on a drum [Ross, 1981]. The measurement gave an accuracy of 1% for measurement of 3-m strain over 0.1 m of spatial resolution. The drum diameter was 185 mm. But the technique does possess several disadvantages. First, it cannot discriminate among the various effects (e.g., simultaneous temperature and strain), all of which are capable of modifying the polarization properties. Second, polarization information is lost in backscatter. This is most clearly appreciated by considering the propagation of light in an optically active (i.e., circularly birefringent) crystal. Any rotation of the polarization state that occurs on the forward passage of light through the crystal is canceled on back reflection through the crystal. As a result, all knowledge of a pure rotation is lost in backscatter. Consequently, the loss of information prevents full knowledge of the FIGURE 5.13 Polarization-optical time-domain reflectometry (POTDR): (a) basic arrangement; (b) a trace for bend birefringence on a drum-wound fiber.

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distribution of fiber polarization properties. There are two possible approaches to the solution of this problem: either some prior knowledge of the fiber’s polarization properties must be available (as is the case in a hi-bi fiber, for example) or more sophisticated interrogation and processing techniques must be used. This problem now needs a convenient and practical solution in two arenas as interest in POTDR as a diagnostic tool has developed recently as a result of the polarization mode dispersion (PMD) problem in optical communications [Ono et al., 1994; Poole et al., 1986]. This problem derives from the fact that asymmetries in communications-grade fiber lead to small, generally elliptical, local birefringences in the fiber, which vary randomly or quasi-randomly with axial position. The result is that an optical pulse (in a digital communications system) will suffer from accumulated differential group delay (DGD) and thus become broadened by the experience, leading to a reduction in bandwidth. POTDR provides the means by which the local value of DGD can be mapped along the length of a fiber so that sections of fiber with large values can be identified and replaced [Gisin et al., 1991]. It also provides the means for comparing—and so improving—fiber fabrication processes with respect to their effectiveness in mini-mizing PMD in manufactured fibers [Ellison and Siddiqui, 1998].

As a result of these requirements various improvements in the POTDR technique have been proposed [Gisin et al., 1994; Zhou et al., 1997], and these have clear implications for improved DOFS systems.

In general, however, the lack of discrimination among the many effects that can modify the polarization properties of a monomode fiber has led to investigations of techniques that are more measurand specific.

This leads naturally to chemical and to nonlinear methods.