When using laser as a carrier wave for communication or tools for processing, medical treatment, sensing, and detection, it is usually necessary to manage the polarization state of the laser. If the system needs to maintain a certain special polarization state of the laser, in the case of non-free space, the polarization-maintaining fiber or the circular-preserving fiber will be a practical solution to maintain the laser polarization state in a closed channel mode.
For polarization-maintaining fibers, the most common type of special fiber is a type of special fiber that increases a stress zone near the core of a traditional single-mode fiber. It can actually transmit two orthogonal linearly polarized light, in this sense, it is not "single-mode". During use, linearly polarized light input and accurate alignment (regardless of fast axis or slow axis) is required. Otherwise, elliptically polarized light with random polarization states will be obtained because the components on the fast axis and the slow axis are comparable and the transmission constants are different. The shaft includes a series of methods, tools and test equipment, and practitioners also need to have a sufficient understanding of polarization-maintaining fiber.
If stress areas or voids that are obviously close to the core are added on both sides of the traditional single-mode fiber core, the propagation constants of the polarization components in the two orthogonal directions will be significantly different, and one of the polarization components will be absorbed, scattered or escaped. If it produces significant attenuation, it will be made into a single-polarization fiber-from the point of view of fault finding, it is a true single-mode fiber. It can polarize the input light of any polarization state, but its attenuation is related to the input polarization state and its alignment with the main axis of the single-polarization fiber. Introducing "defects" in the working axis direction of the polarization-maintaining fiber, such as grinding to a certain depth and applying light absorption or dissipation treatment, can also make the conventional polarization-maintaining fiber have a polarization function. In this grinding processing range, it is also a special Form of single-polarization fiber.
The production method of using photonic crystal fiber can make the photonic crystal polarization maintaining fiber easily and flexibly according to the needs of the designer. Because its numerical aperture is easier to adjust and control, the fiber core can be pure fused silica, and its application in high-power laser systems has considerable technical advantages.
Although the polarization-maintaining fiber can maintain linear polarization under normal conditions and is insensitive to general environmental changes (such as temperature, vibration, humidity, etc.), when the external stress is large enough to affect the inherent internal stress of the polarization-maintaining fiber, the polarization-maintaining fiber The fiber's maintenance of linear polarization will be degraded accordingly. Once degraded, the original linear polarization will have a certain component coupled into the orthogonal direction. This situation is not easy to compensate. What's more serious is that only one point in the optical fiber link will be degraded, and the subsequent parts will be affected accordingly. Therefore, the protection of the polarization-maintaining fiber is very important in the process.
The stress caused by the coiled fiber and the twisting force generated by the fiber wiring process will inevitably cause the degradation of the performance of the polarization-maintaining fiber, and degrade the linearly polarized light transmitted therein. Some test processes, and even some polarization devices, instead obtain desired parameters or characteristics based on the effects of these stress processes, such as the need to generate polarized light with a specific polarization state.
In addition to maintaining linear polarization, there are rotating fibers that maintain a specific polarization state. This kind of fiber can be produced based on almost all existing single-mode fibers and polarization-maintaining fibers, and even special stress regions and refractive index distributions can be designed to form highly similar or different propagation constants for polarized light of different rotation directions. To achieve the purpose of maintaining the specific polarization state and even filtering out the specific polarization.