1. Introduction

Nowadays, it is impossible to imagine modern industry without automated control systems. Complex structures and mechanisms require constant control of various parameters and processes that happen in them. Monitoring of complex engineering structures and industrial systems is an integral part of their day-to-day operation. To perform such control, sensors and control systems of various physical quantities such as temperature, mechanical deformation, pressure, etc. are required.

At present, the market of measuring systems and sensors was dominated by electronic measuring technologies, which involve the transformation of the measuring parameter into an electrical signal and its subsequent processing [1]. An alternative to such an approach is the use of fiber-optic measurement systems, where the measured parameter is converted into an optical signal transmitted over an optic fiber. The market of fiber-optic sensors has been growing from the moment they appeared, 2.5 billion dollars in 2004 with an annual increase of 11% in all

industries, according to Frost and Sullivan's marketing agency. Fiber-optic sensors have some advantages over more traditional electronic technologies: (a) explosion safety; (b) zero sensitivity to electromagnetic interference (noise immunity); and (c) high separation ability. Despite all mentioned above, the relative share of fiberoptic sensors in the overall market of measuring systems remains small. In essence, fiber-optic sensors occupy only a very small position where traditional measuring instruments cannot be used or their use is very expensive.

However, it is worth noting two trends that are observed in our time. Firstly, the rapid development of related technologies, fiber-optic transmission of information, reception and processing of images using digital photo and video equipment, microprocessor technology, contributes to the development of fiber-optic measuring technology and in cheapening their production. Secondly, industry and regulators put increasingly stringent requirements on the operating conditions of such devices, namely, noise immunity requirements, measurement safety, accuracy, etc. These are the criteria that can satisfy modern fiber-optic sensors. It can be seen that these two trends can lead to a situation where the fiber-optic measuring systems will become more competitive in than traditional systems [2]. This is due, not only, to the performance characteristics but also to the cost of individual measuring channel.
