4. What is the beat length?
If the polarization direction of the incident linearly polarized light is consistent with the fast axis or slow axis of the fiber, the polarization state of the light remains unchanged during transmission.
If the polarization direction of the incident light forms an angle with the fast axis or the slow axis, two orthogonal polarization modes with different propagation constants will be excited simultaneously, and the periodic exchange of power between the two polarization components is called the beat length.
The beat length is a very objective parameter of the birefringence degree of the fiber, which has nothing to do with the length of the fiber, the polarization and alignment of the input light, etc., and can well reflect the periodic change of the polarization state of the fiber.
Periodicity refers to the process of the polarization state along the length of the fiber from linear deviation → elliptical deviation → circular deviation → elliptical deviation → linear deviation → elliptical deviation → circular deviation → elliptical deviation → linear deviation, and returns to the original linear deviation after an integer number of beat lengths state.
The formula for the beat length is Lp=λ/B, where λ is the wavelength, and B is the birefringence. The smaller the beat length, the larger the difference in light speed between the fast and slow axes, and the stronger the birefringence. Typical B value: ordinary fiber B=10 (-7), polarization maintaining fiber B=10 (-4)
5. What is the extinction ratio?
How does the polarization-maintaining fiber realize the polarization-maintaining principle? When the polarization direction of polarized light is aligned with one of the axes, the polarization component assigned to the other axis will be very small, thereby maintaining the polarization state of the transmitted light. At this time, the extinction ratio (ER) parameter is introduced to reflect the degree to which the optical fiber maintains the polarization state.
When the polarization direction of polarized light is aligned with one of the fast and slow axes, two orthogonal polarization modes are generated by the components. The ratio of the polarization component along the original direction axis to the polarization component in the vertical direction is the extinction ratio. The extinction ratio is an important parameter to measure the quality of the polarizer, the larger the extinction ratio, the higher the quality of the polarizer.
ER = 10 log(Pmax/Pmin)
