Fibre Optic Gyroscope With Single-Mode Fibre And Loop-Back Phase Shift Compensation

An all-fibre optical sensor of an angular velocity (fibre-optic gyroscope) based on the Sagnac interferometer and using a loop-back phase shift compensation is presented. The sensing loop consists of 760 metres of an ordinary single-mode fibre, which makes this setup cost-effective. To ensure principles of beams reciprocity, the randomly induced changes of polarization in the fibre are reduced by using unpolarized light. This is achieved by a fibre Lyot depolarizer and a superfluorescent fibre source consisting of an erbium-doped fibre pumped by a laser diode. Unlike common approaches to the unpolarized fibre-optic gyroscope with a single-mode fibre, whose output is naturally nonlinear, we use a loop-back compensation of a rotation-induced phase shift to achieve a linear response. In most cases, this technique requires fast electro-optical modulator, which is compatible only with an expensive polarization-maintaining fibre. We use a novel loop-back modulation scheme utilizing only harmonic signals and thus compatible with a piezoelectric fibre stretcher, which can be used with any kind of optical fibre. As a result of the closed-loop operation, the range of the gyroscope's linearity is greatly increased and a sensitivity to source power changes is suppressed. We describe the gyroscope setup with the proposed modulation method and provide a comparison to the common open-loop setup.