An optical fibre yeast concentration sensor based on inter-fibre distributed coupling

lb); second, an X-coupler with a reference channel (Fig. IC); third, a pair of fibers whose ends approach each other to a maximum extent, one of the fibers being input, and the other one receiving the useful autocollimation signal (Fig. Id). The later version is applied for more sensitive scheme of measurement due to the presence of an initial displacement of the receiving fiber and the absence of a Fresnel reflection as in the cases of couplers. The use of single-mode fibers ensures the formation of a Gaussian distribution of the modal field, which allows for the precise collimation and focusing of the input and backscattered emission, as well as the application of an analytical method for loss de~cription.~,~ Depending on whether investigations are carried out at a single wavelength or within a broad spectrum range, laser or wide-spectrum light sources are used, respectively. The suggested scheme allows for taking an independent measurement both of the autocollimation angle a at a given wavelength and of the prism apex angle 8. The refractive index of the optical material is determined by the measured values of these angles by means of the dependence: n = sin(ci)/sin(O). The accuracy determining the n depends on the accuracy of tuning the rotation goniometric table. The refractive index of modal prism is determined with accuracy 4,8 . by means of the setup shown in Fig. l a (with a pair of single-mode fibers) using a precise micrometer screw providing a linear displacement accuracy 0.254 pm, which corresponds to a minimum rotation angle 0.762 arcsec. The accuracy of measurement the angle and determining the refractive index is confirmed by the experimentally registered dependence of the photocurrent variation during scanning of the autocollimation angle, which is illustrated in Fig. 2. The proposed scheme of a fiber-optic autocollimation refractometer features specific advantages as compared with the conventional autocollimation schemes, that can be formulated in the following basic directions: first, the used singlemode fibers can be treated as a point light source, which, in the absence of diffraction effects, provides near-to-the-ideal conditions both for formation of a probing emission and for tuning and receiving of the autocollimation signal; second, the optical fibers are forming a Gaussian distribution of the field (even from nonlaser sources), which allows for precise collimation and focusing of the emission; third, high accuracies of measuring angles and determining the refractive index of solid-state and liquid (by hollow reservoir prism) optical materials; fourth, it possesses a simplified and compact principal scheme.