Its principle is that the acceleration is converted to polarization beat frequency of optic fiber laser and the acceleration is obtained by monitoring beat signal.

There is inevitable unbalance in the dual-rotor system of the aero-engine, the beat vibration will occur to engine when two rotors are operating at approximately the same speed.

Consequently, the vibration was investigated and found to be beat vibration.

Under the periodical loads, the shaft must have the torsional vibration, named "beat", because the shafting has not only inertia but also elasticity.

The paper mainly studies the influence to sensitivity by the polarized style and analyzed style in the measurement system of beat length that relies on magnetooptic modulation techniques in which the magnetic gap and magnetic intensity have been given.

The restriction to the cut length of fiber is relaxed and the measurement range of beat length is extended, so that the cutback method can be suitable for the short beat-length measurement of hi-bi holey optical fiber.

According to the dependence of the phase difference between the two orthogonally polarized modes HEx11 and HEy11 on the wavelength of the injected laser,the wavelength-sweeping modulation method is adopted to measure the beat length of the fiber.