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Measurement of Optical Fiber Polarization Mode Dispersion and Beat Length
The optical interference technique is an tool with high temporal and spatial resolution in the fields of measurement and metering. Combined with laser which has unique temporal and spatial coherence, the interference method has large measurement area (It belongs to a different catalogue from resolution, and not described here). Interference technique is widely used in fiber optic test field. For example, the interference test techniques of PMD and beat length are the typical ones, which base on the same principle.
 
The PMD test system, PMD-6000B, in Information Transmission Lines Quality Inspect Test Center of M.I.I(the Inspection and Testing Center of No 23 Research Institute, CETC) has a temporal resolution of 0.6 fs, and uses Michelson interferometer. The tested sample is placed on one of the two optical circuit arms. The beams of polarized light, from the two optical circuit arms of the interferometer, are injected into the sample in the directions of two orthogonal characteristic optical axes. By adjusting the optical path of the movable optical circuit arms, the polarized light beams from the two characteristic directions will engender interference at the output end, and we can get the time delay difference between two PDMs  obtained from the sample by the adjustment, that is PMDτ=2d/c. Here d is the quantity of optical path adjustment, and c is optical speed.
  
According to the definition of beat length, we can get the formula below: 
                      
                                                                       Lp·Δβ=2πLp·Δn =λ
    
When the propagation phase difference of the two orthogonal linearly polarized
modes achieves 2π the spatial distance, Lp, they pass through is called beat length.
                      
                                                                           Lp=2π/Δβ=λn
    
Where Δβ is propagation constant deviationΔβn2π/λ; Δn is modal birefringence; λ is the testing optical wavelength; Δn=d/L, L is the length of sample.
 
  
As can be seen from the above formula, the adjustment of the optical path, d, is the base of measuring PMD and Lp in the interference technique. In addition, we can get the conclusion from analysis above,  the interference method may be more reliable theoretically compared with such technique as common used external modulation method.
 
 
 
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