The work presents an overview and classification of combined fiber-optic resonant structures, which are essentially a combination of a Fabry-Perot interferometer (FPI) and a fiber Bragg structure. The latter can be represented either as a conventional fiber Bragg grating (FBG), or an addressed fiber Bragg structure (AFBS) of various types. It is shown that such combined resonant structures can serve as sensing elements of various fiber-optic measurement systems, detecting such physical impacts as temperature, pressure, acoustic waves, etc. One of the proposed structures is a fiber optic FabryPerot interferometer based on an open macro cavity at the end face of an optical fiber. Immersing the end face of the optical fiber with macro cavity in liquid leads to the formation of an air bubble where interfacial surfaces act as FabryPerot mirrors, which allows to use the structure for detecting pressure or acoustic waves in liquids. Another considered type of an FPI is composed of layer of borosilicate glass at the end of an optical fiber, acting as a temperature sensor. It is demonstrated that the combination of FPI and fiber Bragg structure can be used as a unified basic element for the various optical fiber sensors, utilizing the Fabry-Perot cavity at the end face of the fiber and the Bragg structure as fundamental sensing elements.
It is well known that the speed limitations inherent in conventional electronics can be overcome using photonic circuits, one of the main building blocks of which is a time differentiator. This device provides the time derivative of the complex envelope of an arbitrary input optical signal. Possible applications include optical pulse shaping, optical computing, information processing systems and ultra-high-speed coding, among other applications. The paper discusses the principles of constructing fractional order differentiators. Using the example of an asymmetric fiber Bragg grating with a π-phase shift operating in reflection, based on addressed fiber Bragg structure of Moiré type, the possibility of implementing a photonic fractional differentiator is shown. We present modeling of the characteristics of the spectral structures of a differentiator, which uses gratings of the same length, but with different modulation depths on both sides of the localization of the π-phase shift, using a new method of homogeneous layers.
Based on the analysis of methods and devices development for all-optical differentiation of signals and addressable fiber Bragg structures as sensitive elements of sensors, the possibility of combining the advantages of the latest versions of their implementation based on symmetric Moiré Bragg gratings has been found. For differentiators, which, as a rule, are considered as separate devices, the issues of their network multiplexing for a single module of photonic processing and computing of multiple signals, as well as the stability of differentiation characteristics with deviations in the temperature regimes of the laser and differentiator with a corresponding mismatch of their central wavelengths, are not touched upon. The advantages of microwave photonic processing of information used for addressable fiber Bragg structures will make it possible to solve the above problems of photonic differentiators when they are combined into a network. This conclusion is proved by the procedure presented in the report for the synthesis of addressable fiber Bragg structures of the Moiré type and the unity of their structure with the structure of photon differentiators.
Seven years ago, we proposed the concept of addressed fiber Bragg structures (AFBS), which simultaneously perform the functions of: a complexed sensitive element based on two FBGs (2λ-AFBS) with different Bragg frequencies or FBG with two π-phase shifts (2π-AFBS), the difference frequency of which is the AFBS address and the value of it is invariant to measured physical fields; a two-frequency laser radiation source, which can operate as in reflection, so as transmission mode respectively to structure above, a self-multiplexed set of sensors, if the difference frequency will be unique for each AFBS, enabling their address multiplexing. In this article, we consider the ontology of AFBS, including the parent structures with 2λ- or 2π-components, successor AFBS with three spectral components and various combinations of difference frequencies: symmetrical and asymmetric, performing the functions of the addressing and converting information signals to the low-frequency region at the same time, along with the functions of rejecting collisions caused by the relative movement of structures relative to each other during measurements. The subjects of interrogation of these structures and their calibration are discussed as well as prospects of AFBS further development based on common tasks born by ontology formalization and decisions of applicability tasks.
This work presents results of test series, performed for earlier on designed and successfully fabricated twisted silica fewmode microstructured optical fibers (MOF) with six GeO2-doped cores. While Part I introduces results of differential mode delay map measurements, Part II is focused on researches of spectral responses, measured for fiber Bragg gratings, recorded in these multi-core MOFs with core graded refractive index profiles and induced twisting 100 revolutions per meter. Specially setup for spectral response measurement for described complicated fiber optic element was developed, that provides selected alignment of matching singlemode optical fiber with particular single core of MOF via free space and reducing of reflection by precision 8 angle cleaving. Comparing analysis of measured spectral responses confirmed written FBGs in 2 of 6 cores, and demonstrated potentiality of fabricated complicated structure, containing multi-core MOF with FBG, for applications in multichannel fiber optic sensors with spatial division multiplexing technique.
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