**1. Introduction**

Telemetry is a technology that allows remote measurement and monitoring of data. It normally refers to one-way direction of information, that is, from the sensor to the interrogation system or data logger system. Telemetry could be defined as a sub-class of telecom, a more complex way of exchanging information such as Internet, telephone calls or video transmission.

Telecommand, the counterpart of telemetry, occurs when the remote systems require remote instructions and data to operate, which means that the information goes on the other direction.

Telemetry finds applications in aerospace, automotive, consumer, engineering, industrial manufacturing, medical, military, electric power industry etc.

Although the term telemetry commonly refers to wireless data transfer mechanisms (e.g. using radio or infrared systems), it also encompasses data transferred over other media, such as a telephone or computer network, optical link or other wired communications.

In the applications mentioned above and particularly in the electric power industry, we find normally protocols that can be either bidirectional or mono directional, such as Fieldbus, RS-485, Ethernet, 4-20 mA, 0-10 V, all working in a twisted-pair basis. These protocols, although being among us for many decades, have disadvantages, particularly when applied to the electric power industry. One of these disadvantages is that data transmitted through electric wires normally need electric energy at the sensor end, or in other words, the transducer needs to be powered in order to measure and transmit data. However, it occurs that sometimes providing electric energy at the sensor location is difficult for it could be far away from any appropriated power supply. This happens in long high voltage transmission lines or along pipe-lines or in deep ocean, for instance. The other problem with these protocols is that they electrically connect the sensor location with the interrogation location. The main consequence of this is that short circuits due to malfunctioning or atmospheric discharges can easily be transferred to the operation room and furthermore putting the substation personnel and equipment at risk.

With the invention of the practical optical fiber in the 70´s the world watched a boom in the telecommunication technology. In the 80´s, with the popularity of optical fiber technology, scientists started to develop a new class of sensors and transducers: the optical fiber sensors. They came offering many advantages over the other technologies and soon started to be applied in telemetry with very good return in costs, maintenance and efficiency.

Optical Fiber Sensors 5

The high immunity to electromagnetic interference (EMI) is a strong requirement for sensing in electromagnetic contaminated environments, e.g. RF-field and high electric and magnetic

The insulation is another special requirement, because as these sensors are inherently electrically insulated (dielectric) and do not require external power, this means that there is no electric path from the power line to ground, which means high personnel security. Therefore the optical fiber sensors can work at high electrical potentials and in potentially

Optical fibers can be used as sensors by modifying a fiber so that the measurand interferes on the guided light and modulate light parameters such as intensity, phase, polarization, wavelength, or transit time of light over the fiber. Sensors that vary the intensity of light are

We can divide OFS in three basic categories: intrinsic, extrinsic and evanescent field based. Extrinsic fiber optic sensors use an optical fiber, normally multi-mode, to transmit modulated light from either a non-fiber optical sensor or an electronic sensor connected to an optical transmitter. In this case the optical fiber is used only to transmit light to and from the sensor. This kind of sensor sometimes is called hybrid sensor for it enclosures different

In intrinsic sensors the light does not leave the fiber and the light modulation takes place inside the fiber. This kind of sensor presents the major benefit to have the ability to reach otherwise inaccessible places and without the need of electrical energy at the sensing location. The third category is the evanescent field based sensor. Due to the total internal reflection phenomenon that occurs in the core-cladding interface of the fiber, the light propagating in the fiber has two components - an oscillatory field in the core and an exponentially decaying field in the cladding. The latter field, referred to as the evanescent field, is the key to sensing and is based on the modulation of the light amplitude in the core of the fiber by the optical

When developing an OFS we can use the fiber for: a) conducting light; b) to be the sensor itself; and c) for both applications, that is, sensing and conducting light to and from the

An optical fiber is a thin, flexible, transparent glassy filament that acts as a waveguide, or "light pipe", to transmit light from the light source to the photodetector located at the two ends of the fiber. They are mainly used for telecom and sensing but find many uses in the

By the 70's all telephone cables and microwave links in the planet were already saturated. The solution came when Charles Kao and George Hockham of the British company Standard Telephones and Cables (STC) promoted the idea that the attenuation in the existing optical fibers could be reduced below 20 decibels per kilometer (dB/km), making fibers a practical communication medium. They proposed that the attenuation in fibers available at the time was caused by impurities that could be removed by chemical processes. They correctly and systematically theorized the light-loss properties for optical fiber, and pointed out the right material to use for such fibers — silica glass with high purity. This

Easy maintenance

Work over long distances

fields present in power lines.

explosive environments.

Chemically inert even against corrosion

technologies such as optics and electronics.

properties of the surrounding medium.

industry, research sciences, medicine, entertainment etc.

discovery earned Kao the Nobel Prize in Physics in 2009.

sensing area.

Several sensors can be multiplexed on the same fiber

the simplest, since only a simple source and detector are required.

In conclusion, when it comes to telemetry, optical fibers perform telemetric measurements at distances much longer than conventional telemetry protocols and media. Additionally, due to its virtually infinite capacity to multiplex, one can mix different kinds of signals in one single fiber therefore saving many kilometers of copper wires, which is also welcome by the maintenance personnel.

In this article we will concentrate on applications of telemetry over optical fiber and on optical fiber sensors which encompass telemetry and sensor in one single media.
