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Optical fibers play an important role in many applications beyond communications, including sensing, control, and instrumentation. In these areas, optical fibers have made a significant impact and continue to be the subject of substantial research. In general, for these applications, fibers are made more sensitive and susceptible to the very external mechanisms that would render telecommunications fibers ineffective. In its simplest form, an optical fiber sensor is composed of a light source, an optical fiber, a sensing element, and a detector (see Fig. 2.1).
The principle of operation of a fiber sensor is that the sensing element modulates some parameter of the optical system (intensity, wavelength, polarization, phase, etc.), which gives rise to a change in the characteristics of the optical signal received at the detector. Figure 2.2 depicts the various changes that can be introduced to the light traveling along the fiber.
A fiber optic sensor (FOS) can affect one or more of the light-guiding characteristics of the transmitting fiber and correlate the particular parameter of interest (e.g., temperature, strain, pressure, chemical species, etc.) to these particular light changes. Hence, the fundamental principle is to measure an external physical parameter by inducing changes in one or more of the optical properties of a light beam traveling inside and along an optical fiber. The fiber is thus both the sensing element and transmission medium.
Fiber optic sensors represent a technology base that can be applied to a multitude of sensing applications. The following are some characteristic advantages of fiber optics that make their use especially attractive for sensors:
Nonelectrical
Explosion proof
Often do not require contact
Remotable
Small size and lightweight
Allow access into normally inaccessible areas
Potentially easy to install
Immune to radio frequency interference (RFI) and electromagnetic interference (EMI)
Solid-state reliability
High accuracy
Can be interfaced with data communication systems
Secure data transmission
Resistant to ionizing radiation
Can facilitate distributed sensing
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