This paper presents a solution to one of the major problems of plasmonic fiber Bragg grating sensors concerning their high sensitivity to changes in the polarization state of light propagating through optical fiber. For the first time these kind of sensors have been produced using polarization maintaining fibers, thereby stabilization has been achieved using mechanical action and bending the supplied fiber. Comparative experiments have demonstrated that the sensor readings stability is at least an order of magnitude higher relative to other sensors, which record in a standard fiber with an isotropic structure.
An optical scheme for polarization encoding BB84 protocol is described. Fiber electro-optical LiNbO<sub>3</sub> phase modulators based on Pockels cells provide both polarization states generation for Alice and basis choosing for Bob at high frequency, requiring low operation voltages (V<sub>π</sub> < 5V). Proposed scheme uses only one laser source, which guarantee the indistinguishability of the pulses and active basis choosing allows the system to have two single photon detectors in contrast to the four in the standard polarization encoding schemes. Two phase modulators compensate each others’ polarization mode dispersion, caused by the natural birefringence of the lithium niobate crystal. The system consists of standard components and has simple configuration. The principle of operation is experimentally demonstrated.
We have experimentally investigated fundamental mode propagation in few-meter-long adiabatic step-index tapers with high numerical aperture, core diameter up to 117μm (V=38), and tapering ratio up to 18. We confirmed single fundamental mode guiding in tapers with uniform core index profile by several experiments. We observed an annular near field distribution and degraded beam quality for large output core diameters, found to occur due to intrinsic mechanical stress in the fibers. We expect that eliminating the stress would prevent the mode deformation and allow constructing single-mode, diffraction-limited tapered large-mode-area amplifiers with a good beam shape.
We present a comparative theoretical and experimental study of different schemes of fiber lasers with tapered doubleclad
fiber (T-DCF) as an active medium. We have developed a theoretical model for obtaining the distribution of power
and power density inside the fiber for the considered laser schemes with different longitudinal shapes of T-DCFs. The
slope efficiencies, spectral characteristics, and beam quality deterioration associated with the mode conversion in the TDCF
were also studied experimentally for different laser schemes. In addition, a spontaneous transition to a self-pulsing
regime under certain conditions, and the associated problems have been investigated. Characteristics of T-DCF lasers
constructed in co- and counter-propagating schemes are discussed and compared to those of regular fiber lasers,
equivalent in active volume or in brightness.