Shared medium, but not entirely. In fiber optic sensors, the information about the parameter to be measured is determined by a change in the phase, polarization, frequency, or intensity of the optical signal (as well as any combination thereof). Sensors that detect changes in signal intensity are very simple. In contrast, those that work with phase, polarization, or frequency modulation are more complex, since the signal must be pre-processed because the photoreceptor only detects optical power. In these cases, interferometric structures of the Mach-Zehnder, Michelson, Fabry-Perot, or Sagnac type are used, which in turn provide high sensitivity. More recently, Bragg gratings are also being used as sensors for spectral measurements or wavelength discrimination.

Fiber optic sensors can be broadly classified into two categories. Those in which the optical fiber is used solely as a means to guide light from the emitter to the sensing element, and from the latter to the photodetector (called extrinsic sensors). Those in which the fiber itself serves as both the sensing and reference element (called intrinsic sensors). The various configurations proposed to date are numerous. Below, we will discuss some of the most representative ones.

Among interferometric structures, the most typical is the Mach-Zehnder interferometer. Its application as a sensor is shown in Figure 1. A coherent optical source generates a signal that is split into two paths by means of a 3 dB directional coupler. One path is the reference path, while the other contains a transducer that converts a specific physical measurement parameter into a phase shift of the optical signal. Subsequently, both signals are recombined in an output coupler, resulting in two signals (sum and difference) that are received by separate photodetectors. In this way, the phase shifts of the optical signal in one of the interferometer's branches can be measured from the optical powers in both detectors. The equations are given by:

where L1 and L2 are the lengths of the interferometer arms, k is the propagation constant, and Df is the phase shift produced in the transducer.
On the other hand, with regard to intensity sensors, there are also numerous implementation possibilities. Among the best-known techniques are: light interruption by displacement of a foil (Figure 2), relative displacement of two fibers, modulation of core or cladding losses due to bending, or coupling of evanescent modes with another fiber, etc. As an example, Figure 2 shows the fundamentals of the technique based on a foil that obstructs light coupling between two fibers. A mechanism sensitive to a specific physical parameter (e.g., pressure or vibration) displaces the foil to a greater or lesser degree, thus modulating the optical intensity detected at the system output.

Products and Applications
As previously mentioned, fiber optic sensors have applications in a wide range of fields. Below are some examples, along with the companies that market them.

High energy prices and the need to replenish reserves are incentivizing oil companies to invest in heavy oil fields. Heavy, viscous oils present challenges in fluid analysis and obstacles to recovery, which are being overcome with new technology and modifications of methods developed for conventional oils. Steam-assisted gravity drainage (SAGD) is a method used for extra-heavy oils. A pair of parallel horizontal wells are drilled, with one well positioned 5 to 7 meters above the other. Steam injected into the upper well heats the heavy oil, reducing its viscosity. Gravity causes the mobilized oil to flow downwards towards the lower horizontal producer. Initial communication between the injector and the producer is established through steam injection, cyclic steam injection, or solvent injection. The estimated recovery factor for this method ranges from 50 to 70%. However, formation stratification can significantly impact SAGD recovery. The SAGD method is used in many fields in Canada, including the Christina Lake and MacKay River fields. Figure 3 shows a schematic diagram of the SAGD method for heavy oil extraction. Pressure and temperature control are clearly crucial in this process, requiring sensors for measurement. The Canadian company Opsens Inc., a leader in fiber optic sensor manufacturing, has developed products for this specific application. For example, the OPP-W pressure and temperature sensor, based on a Fabry-Perot interferometer. Its specifications are shown in Table I. In combination with the WFC fiber optic cable (8-fiber rigid cable for harsh environments) and the WellSens measuring equipment (white light polarization interferometry), heavy oil extraction processes can be effectively monitored (Figure 3), as well as any other similar application. Pressure sensors also find applications in the medical field. One example is the fiber optic sensors from FISO Technologies Inc. (Figure 4). Their applications range from measuring fluid pressure in the human body during critical hospital procedures to animal testing in environments with high levels of electromagnetic interference. The FOP-MIV and FOP-M260 models are manufactured using silicon microfabrication technologies, making them smaller, more accurate, more reliable, and easier to use than traditional catheters. Additionally, the FOP-F125 sensor, with a diameter of only 125 microns, is the smallest commercially available sensor for these applications. It is positioned directly at the end of the fiber, without the need for any adhesive, allowing for integration into minimally invasive medical and surgical devices.