Polarization maintaining coupler is the key device to realize coupling, splitting and multiplexing of linearly polarized light. Its characteristic is that it can stably transmit two orthogonal linearly polarized light and keep their polarization state unchanged, thus becoming the key optical device needed by various military interferometric sensors, coherent optical communication, fiber optic gyroscopes and fiber optic hydrophones
Optical fiber coupler is the core component of these optical fiber sensors, and its performance has a great influence on the overall performance of optical fiber sensors.
Laser interference method is to hit the He-Ne laser on the polarization maintaining fiber from the side, rotate the two fibers respectively, and determine the polarization axis direction of the fiber through the change of its interference fringes during the rotation. In this method, the optical fiber is placed between two orthogonal polarizers, and the polarization axis of the optical fiber can be detected according to the birefringence generated by the stress application part.
After finding the polarization axes of the two polarization maintaining fibers, the two fibers should be drawn in parallel and then fixed. However, in this process, the optical fiber will rotate, which will make the polarization maintaining axis deviate. At the same time, the optical fiber itself may also slightly rotate and generate stress. These factors will also affect the performance of the device. Therefore, overcoming these difficulties is also the difficulty in manufacturing polarization-maintaining fiber couplers.
The main performance indexes and influencing factors of polarization maintaining fiber coupler are the same as those of single-mode fiber coupler for communication. To measure the performance of polarization maintaining fiber coupler, additional loss and coupling ratio are two important indexes.
In which, it is the sum of the optical powers of the main and branch main polarization axes of the optical fiber coupler, which are the optical powers injected into the pm coupler along the main polarization axis. The diameter of the bipyramid of polarization maintaining coupler is an important factor affecting the additional loss. The coupling ratio can be controlled by the flame temperature to obtain different values.
Different from single-mode fiber coupler, pm fiber coupler is made of polarization maintaining fiber, so it has the index extinction ratio to evaluate its polarization maintaining performance.
High extinction ratio is the ratio of the optical power transmitted along the main axis X and the orthogonal polarization axis Y in an output port of polarization maintaining fiber coupler, which reflects the degree of polarization maintaining of linearly polarized light by the pm fiber coupler. Therefore, polarization maintaining fiber couplers are mainly used in optical fiber sensors, such as fiber optic gyroscopes, fiber optic hydrophones, fiber optic current sensors and so on. It is one of the basic components of high-precision optical fiber sensors.
Polarization maintaining coupler is mainly made of single-mode fiber, which is simple in manufacturing craft and low in cost. However, because it does not have the function of polarization maintaining, the birefringence caused by external disturbance will cause the zero drift and signal fading of optical fiber sensor, which will lead to the unstable performance of the couplers.
Polarization maintaining fiber coupler made of polarization maintaining fiber is a special type of polarization maintaining fiber coupler, which not only has the function of combining light and splitting light from ordinary coupler but also has the property of keeping the polarization state of linearly polarized light unchanged. Therefore, it is of great significance to analyze and study the polarization maintaining fiber coupler.