What is polarization maintaining fiber
Polarization Maintaining Optical Fiber (PMF, Polarization Maintaining Optical Fiber) is a special type of optical fiber, which overcomes the influence of environmental factors on the polarization state of the optical fiber during transmission by increasing the inherent birefringence of the optical fiber, and keeps the polarization state of the light wave transmitted in the optical fiber unchanged. In the field of optical fiber communication, the stability and communication capacity of the optical transmission system can be improved, and in the optical fiber sensing system, the error caused by polarization coupling in the optical path can be significantly reduced.
Classification of Polarization Maintaining Fiber
Polarization-maintaining fibers generally include geometrical polarization-maintaining fibers and stress-type polarization-maintaining fibers. The structure of geometrical polarization-maintaining fibers is mainly elliptical core polarization-maintaining fibers, which are used in some special applications.
Stressed PM fibers mainly include Panda, Ellipse, Butterfly and Bow Tie PM fibers.
Panda-type polarization-maintaining fiber has natural advantages in terms of fiber performance, whether it is birefringence, geometric characteristics, structural symmetry or longitudinal uniformity of the fiber, and because the manufacturing process of panda-type polarization-maintaining fiber does not need to be completed in one step , independent manufacturing of each component can effectively control the structure and composition of the optical fiber, which can ensure that each preform can be drawn into a uniform polarization-maintaining optical fiber product of tens of kilometers or even more than one hundred kilometers, which is suitable for mass production, so the panda type is commonly used in China Polarization maintaining fiber.
What are the parameters of polarization maintaining fiber?
01 Fast axis and slow axis
Two stress zones different from the expansion coefficient of the fiber cladding are made on both sides of the single-mode fiber core. During the drawing process of the fiber preform, the fiber is rapidly dropped from high temperature to a lower temperature. During the cooling process, 2 Each stress zone contracts, the amount of which is hindered by the surrounding quartz. The core of the optical fiber is pulled outward along the direction of the line connecting the two stress zones (x-axis), and there is a compressive stress in the direction perpendicular to it (y-axis), thereby generating stress birefringence in the core and forming 2 axes that are orthogonal to each other.
The refractive index of the two main axis directions is different. In the direction of stress application, the effective refractive index of the fiber core is higher, and the transmission speed of light is slower, which becomes the slow axis, which is the main axis of the polarization-maintaining fiber. In the other direction, light travels faster, known as the fast axis.
02 beat length
The beat length of the polarization-maintaining fiber is characterized by the length of the fiber transmitted by the complex polarization state of the transmitted light in the polarization-maintaining fiber jumper to complete a periodic change.
Physically, the beat length reflects the fiber length Lp corresponding to the phase difference between the two linearly polarized modes of the fundamental mode changing from 0 to 2Π.
The shorter the beat length, the more flexible the fiber is to the irregular effect of polarization, and the stronger the polarization-maintaining ability of the fiber is to linearly polarized light.
03 Extinction ratio
Extinction ratio is an index to measure the stability of polarization maintaining fiber performance.
When a beam of linearly polarized light is accurately incident along a certain optical axis of the polarization maintaining device, the polarization mode will be excited on the orthogonal axis, and the power ratio of the two orthogonal axes is the extinction ratio.
Pmin and Pmax are the measured minimum and maximum optical power
The larger the extinction ratio, the higher quality the polarizer.
✳ The beat length describes the change of state as the light polarization propagates
✳ Extinction ratio describes the polarization state of light after transmission
What are the applications of polarization maintaining fiber?
Optical devices prepared by polarization-maintaining fibers are widely used in polarization multiplexing communications, lasers, polarization optical devices, polarization sensing, fiber optic gyroscopes, fiber optic hydrophones and other polarization mode compensation.