Novel multifunctional structures based on redistribution of optical power as basis for fiber optic sensors

Abstract

Sensors built with the help of optical fibers can measure almost all magnitudes in these days [3]. In our research we design new novel multifunctional structures that afford concurrently utilizing of optical fiber for telecommunications and measurements. These fibers are designed to work on two wavelengths. On telecommunication wavelength of 1550 nm these fibers are operating in single mode regime and on measurement wavelength of 850 nm they are working in quasisingle mode regime. Complicated profiles of refractive indexes provide four LP modes on 850 nm that are supported by fibers and that transmit a significant amount of power. First samples of these hybrid fibers have already been made thanks to grant cooperation with Academy of Science of Czech Republic. These refractive index profiles have to be designed in such way that all supported modes should carry approximate the same amount of optical power. The usage of both wavelength means that the light of communication wavelength must not be affected by the fiber activities at the wavelength of 850nm. The consequence is that only redistribution of optical power among supported modes can be applied. The Fourier and wavelet analysis is used to find out the significant points in the progression of the optical power. There are changes in the Fourier spectra and changes in wavelet coefficients. From the Fourier analysis we can predict the progression; wavelet analysis [2] enables us to find out singularities. It is expected, that every change on the fiber has its own fingerprint in the redistribution of the optical power. The main instrument is the coupled mode equations [1] following directly from the wave equations for individual modes. They contain a detailed description of the phase and amplitude of all the modes at any point z along the waveguide. But usually we are not interested in the phases and amplitudes of the individual modes. For the most of practical intentions, it is sufficient to know the average of the amount of power carried by each mode. Coupled power equations for the average mode power are derived for weak coupling between modes. It means that coupling requires a distance of 1000 wavelengths for a complete exchange of power between two modes. It is a set of finite number of first-order differential equations with symmetric constant coefficients. Coupling to forward traveling guided modes occurs for distortion function f(z) whose Fourier spectra are limited to spatial frequencies. In the contribution refractive index profiles will be presented together with first camera photos analysis of such fiber temperature sensor. Temperature crosstalk to communication wavelength of 1550nm will be also specified.