L.I. Yusupova – Student, 

Department «Radio Engineering, Opto- and Nanoelectronics», Ulyanovsk State Technical University E-mail: lesanusupova@gmail.com

O.V. Ivanov – Dr.Sc.(Phys.-Math.),

Department «Radio Engineering, Opto- and Nanoelectronics», Ulyanovsk State Technical University Ulyanovsk Branch of Kotel’nikov Institute of Radio Engineering and Electronics of RAS E-mail: olegivvit@yandex.ru

Fiber structures with various insertions of nonstandard fibers, gratings, and tapers that are used to excite cladding modes have attracted considerable interest in recent years due to their compactness, simplicity of fabrication, and good sensitivity to parameters of external medium. These structures find application in fiber sensors of humidity, temperature, refractive index, and liquid level. Insertions of multimode, thin-core, coreless, depressed cladding, and microstructured fibers are spliced with the standard fiber to provide coupling from the core mode to cladding modes.

In order to enhance sensitivity of fiber structures employing cladding modes, interferometers are created by combining two elements that couple different fiber modes. For example, it can be two sections of multimode fiber, pair of tapers or taper and grating, etc. For sensors working in reflection, an interferometer can be made using just one element that is passed by the modes twice. A modal interferometer is formed by a section of thin-core fiber spliced between standard fibers. In such a structure, the cladding modes are excited at the junction of two fibers due to strong dissimilarity of their refractive index profiles.

In this paper, a new type of fiber interferometer based on two short thin-core fiber insertions SM600 and SM450 with a section of standard fiber in between is proposed. The thin-core fiber insertions excite cladding modes and provide energy exchange between different fiber modes. The standard fiber section is used to accumulate phase differences between core and cladding modes propagating through the structure. Transmission spectra are measured for various lengths of the interferometer. The optimum length of thin-core fiber insertions for obtaining maximum amplitude of spectral oscillations is 0.3−0.8 mm for SM600 fiber and 0,1−0,4 mm for SM450 fiber.

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