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Polarization-maintaining fiber (PMF) has attracted great attentions not only due to its ability to realize coherent optic
communication with ultrahigh-capacity, but also because it can be widely employed in high-precise fiber-optic sensors
and optic information processors, e.g., fiber-optic gyroscope and fiber-optic amperemeter. Distinct with common
monomode optic fibers, additional birefringence is introduced in the PMF, minimizing the effect of mode coupling
between two orthogonal polarization modes in optic fibers. Thus, light polarization can be maintained after long
distance transmission in the PMF. However, beat-length, which reflects the characteristic of induced birefringence and
evaluates the ability of polarization maintaining, is one of the most important parameters for PMF. Accurate
measurement of PMF beat length becomes more and more important along with the expanding of its application field.
The uniqueness of the measurement results determines the identity of PMF fabrication and the resolution of fiber-optic
sensors. The existing measurement methods for PMF beat length are introduced first. Most testing setups are established
by several individual components, which lead to complicated setup, inconvenient testing, and unsteady measurement.
In this paper, a novel all fiber beat length measurement system is presented, where optic source, integrated optic
modulator, stress apply structure, polarization detector, and polarization indicator of output light are included. The
configuration and working principle of the whole system, and the designing and realization of each components, are
addressed in the paper. Furthermore, the influence on measurement results by the relative direction between applied
stress and principal axis of the optic fiber is analyzed. Moreover, optimized designs, including polarized light incidence,
pressure apply, and precise movement systems, are performed, which ensure the identical pressure apply direction in the
moving process. Finally, measurement of PMF samples is carried out on experimental setup. In comparison with the
other existing system, due to its all fiber light incidence structure, the novel beat length measurement system represents
great stability and high reliability.
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