Polarization-maintaining Fiber Coupler is One of the Basic Components of High Precision Fiber Sensor

The polarization-maintaining fiber coupler is a key component to realize the coupling, splitting and multiplexing of linearly polarized light. Its characteristic is that it can stably transmit two orthogonal linearly polarized lights and keep their respective polarization states unchanged, thus becoming key optic devices for various military interference sensors, coherent optical communications, fiber optic gyroscopes, and fiber optic hydrophones etc.

1. Difficulties in the manufacture of polarization-maintaining fiber couplers

The optical fiber coupler is the core component of these optical fiber sensors, and its performance has a great influence on the overall performance of the optical fiber sensor.

The laser interferometry is to hit the helium-neon laser from the side onto the polarization-maintaining fiber, rotate the two fibers separately, and determine the polarization axis direction of the fiber through the change of the interference fringes during the rotation. In this method, the fiber is placed between two polarizers placed orthogonally, and the polarization axis of the fiber can be detected according to the birefringence generated by the stress application part.

After finding the polarization axis of the two polarization-maintaining fibers, the two fibers must be brought close together in parallel and then fixed before drawing. However, during this process, the optical fiber will rotate to deviate the polarization-maintaining axis. At the same time, the optical fiber itself may also rotate slightly, causing stress. These factors will also affect the performance of the device. Therefore, overcoming these difficulties is also a difficulty in the manufacture of polarization-maintaining fiber couplers.

2. The performance of the polarization-maintaining fiber coupler

The main performance indicators and influencing factors of polarization-maintaining fiber couplers are the same as those of single-mode fiber couplers for communication. Additional loss and coupling ratio are two important indicators to measure the performance of polarization-maintaining fiber couplers. The diameter of the double cone of the polarization-maintaining coupler is an important factor affecting the additional loss. The coupling ratio can be controlled by the flame temperature to control the stretched length and obtain different values.

Different from the single-mode fiber coupler, since the polarization-maintaining fiber coupler is made of polarization-maintaining fiber, it has an index extinction ratio to evaluate its polarization-maintaining performance.

The high extinction ratio is the ratio of the optical power transmitted along the main axis X and the polarization axis Y at the output port of the polarization-maintaining fiber coupler. It reflects the degree of polarization-maintaining of the linearly polarized light by the coupler. Therefore, polarization-maintaining fiber couplers are mainly used in fiber optic sensors, such as fiber optic gyroscopes, fiber optic hydrophones, and fiber optic current sensors. It is one of the basic components that constitute a high-precision fiber sensor.

3. The significance of polarization-maintaining fiber couplers

The polarization maintaining fiber coupler is mainly made of single-mode fiber. The manufacturing process of this type of coupler is simple and the cost is low. However, because it does not have the polarization maintaining function, the birefringence caused by the external disturbance will cause zero drift of the fiber sensor and signal fading, resulting in unstable performance of the coupler.

The polarization-maintaining fiber coupler made of polarization-maintaining fiber is a special polarization-maintaining fiber coupler. In addition to the function of ordinary coupler to combine and split light, it 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.

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