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In traditional fiber-optic gyroscopes (FOG), the polarization state of counter
propagating waves is critically controlled, and only the mode polarized along one particular
direction survives. This is important for a traditional single mode fiber gyroscope as the
requirement of reciprocity. However, there are some fatal defects such as low accuracy and poor
bias stability in traditional structures. In this paper, based on the idea of polarization multiplexing,
a double-polarization structure is put forward and experimentally studied. In highly birefringent
fibers or standard single mode fibers with induced anisotropy, two orthogonal polarization modes
can be used at the same time. Therefore, in polarization maintaining fibers (PMF), each pair of
counter propagating beams preserve reciprocity within their own polarization state. Two series of
sensing results are gotten in the fast and slow axes in PMF. The two sensing results have their own
systematic drifts and the correlation of random noise in them is approximately zero. So, beams in
fast and slow axes work as two independent and orthogonal gyroscopes. In this way, amount of
information is doubled, providing opportunity to eliminate noise and improve sensitivity.
Theoretically, this double-polarization structure can achieve a sensitivity of 10-18 deg/h. Computer
simulation demonstrates that random noise and systematic drifts are largely reduced in this novel
structure. In experiment, a forty-hour stability test targeting the earth's rotation velocity is carried
out. Experiment result shows that the orthogonal fiber-optic structure has two big advantages
compared with traditional ones. Firstly, the structure gets true value without any bias correction in
any axis and even time-varying bias does not affect the acquisition of true value. The unbiasedness
makes the structure very attractive when sudden disturbances or temperature drifts existing in
working environment. Secondly, the structure lowers bias for more than two orders and enhances
bias stability for an order higher (compared with single axis result), achieving a bias stability of
0.01 deg/h. The evidences from all aspects convincingly show that the orthogonal fiber-optic
structure is robust against environmental disturbance and material defects, achieving high stability
and sensitivity.
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