Basics of DOFS

The basic principles of distributed optical fiber sensing have been covered in earlier reviews [Rogers, 1986; Dakin, 1987; Kersey and Dandridge, 1988], but for convenience they will be summarized here.

DOFS systems offer a unique measurement capability—that of making a spatially distributed measure-ment of a measurand field with a spatial resolution of 0.1 to 1 m, and a measuremeasure-ment accuracy of ∼1%, over widely varying distances, according to the measurement method and application, from ∼10 m to

∼100 km.

The ability to determine the spatial and temporal features of a measurand field with a medium that is nonintrusive, dielectric, passive, flexible, and easy to install—even retrospectively—offers a new dimension in the monitoring, diagnosis, and control of large extended structures of all kinds. No conventional measurement techniques can compete effectively.

The singular problem, then, to be solved for DOFS systems, when compared with almost all other types of measurement systems, is to determine the value of a measurand continuously as a function of FIGURE 5.5 Types of high-birefringence fiber: (a) with an elliptical core and (b) with a strained core.

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position along the length of an optical fiber with some definable spatial resolution and sensitivity. This implies that each spatially resolved measurement that is made must be identified in some way with a particular fiber section whose position is known. Clearly, in doing this it is not possible to identify the position with some kind of active, coded transmitter, if the important advantages of the fiber as a passive, dielectric medium are to be retained. Hence, the identification should be made from one or the other of the fiber ends; in some cases both ends are used. There are several ways in which this might be done.

As mentioned above, the subject of DOFS was stimulated by the OTDR technique, which uses the temporal resolution of light continuously Rayleigh-backscattered from an optical pulse propagating in the optical fiber [Barnoski and Jensen, 1976]. Clearly, if the delay between the launch of the pulse and the time at which the backscattered light is received is τ, then the fiber section from which the backscatter occurred is identified as that which lies at distance s from the launch end of the fiber, where:

(5.1) and c is the velocity of light in the fiber. Such temporal resolution can be used in both quasi-distributed (Figure 5.6(a)) and fully distributed (Figure 5.6(b)) arrangements.

FIGURE 5.6 Schemes of distributed sensing: (a) quasi-distributed and (b) fully distributed.

s c

= τ2

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However, all measurement technology embraces the art of compromise and trade-off. In particular circumstances, this temporal resolution technique may not be optimal; for example, it may not provide the sensitivity required as backscatter power levels are very low (∼10⁻⁶ of the pulse power per meter of fiber).

As a result, other methods also are used. For example, the individual sensors in a quasi-distributed system may be wavelength selective and can thus be interrogated with a broadband continuous wave (CW) source. The identification in this case is made in the frequency domain, via a detection grating, prism, or tunable filter.

A rather more subtle method for positional coding is illustrated in Figure 5.7. This arrangement involves two optical paths with differing effective light velocities. (These might, for example, comprise the two polarization modes of a high birefringence fiber.) The effect of the measurand field is to couple light from one path to the other. Light of low coherence is launched into one of the paths. When the measurand field causes coupling into the other path at a particular point, the two components then travel at different velocities to the exit end and experience a relative delay that renders them mutually incoherent.

Optical interference between them occurs at the exit end only if a delay is inserted between them of just the right amount to correspond to their travel delay, which identifies the position at which the coupling occurs. Hence a variable delay at the exit will allow the fiber couplings—and the measurand field—to be scanned along the fiber length.

So far, we have considered only the possibilities offered by linear optical systems; but there is also a class of distributed sensors that use nonlinear effects, and these effects provide another option. This option is that of forward-scatter pulse–wave or pulse–pulse interaction. Consider the arrangement shown in Figure 5.8. In this case a pulse of light with high peak power is launched into a fiber, and it generates a local nonlinear effect as it propagates. A counter-propagating CW will experience the nonlinearity as the pulse passes through it, and it will be modulated in a way that depends upon the nature of the nonlinearity. Upon emergence, the CW’s temporal variation will map the passage of the pulse through it, FIGURE 5.7 Distributed coherence sensing.

FIGURE 5.8 Basics of forward-scattering DOFS.

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so that, if the nonlinear interaction is influenced by an external field, this field will be correspondingly mapped along the fiber. Such systems possess the considerable advantage of temporal resolution without the low sensitivity inherent in backscatter methods. Their disadvantage is that they require a high-power, pulsed laser source in order to enter the nonlinear regime.