Introduction to polarization

As light passes through a point in space, the direction and amplitude of the oscillating electric field travels along a path over time. An electromagnetic field vector at right angles to each other in a transverse section (a plane perpendicular to the direction of advance) represents a polarized light wave signal. The polarization is defined using the electric field vector as a function of time, in accordance with the pattern traced across the cross-section. Polarization can be divided into linear, elliptic or circular polarization, of which linear polarization is the simplest. Polarization of any kind is a problem in fiber optic transmission.

Any radio communication and fiber-optic measurement system is a device capable of analyzing interference between two types of light waves. We cannot use the information given by the interference unless the amplitudes of the combinations remain stable over time, that is, the light waves are in the same polarization state. In this case it is necessary to use optical fibers capable of transmitting stable polarization states. So in order to solve this problem, optical fibers that can maintain polarization were developed.

What is PM fiber?
on the wavelength) and depends on any bending of the fiber as well as the temperature

The diffusion of the polarization of the light in the fiber becomes uncontrolled (depending state. Special optical fibers are needed to achieve the desired optical properties, which are affected by the polarization of light as it passes through the fiber. Many systems, such as fiber interferometers and sensors, fiber lasers, and electro-optical modulators, also have polarization-dependent losses that affect system performance. This problem can be solved by using special optical fibers called PM fibers.

The Principle of PM Fiber

If the polarization of the light emitted into the fiber is coaxial with a birefringence axis, it will remain so even if the fiber is bent. According to the principle of uniform mode coupling, the physical principle behind this phenomenon can be understood. Because of the strong birefringence phenomenon, the propagation constants of the two polarization modes are different, so the relative meeting of the modes involved tends to drift rapidly. Therefore, as long as any interference along the light has an effective spatial Fourier component (and a wave number corresponding to the difference between the propagation constants of the two modes), it can be effectively matched to both modes. If the difference is large enough, the general disturbance in the light will change gradually and slowly to achieve effective mode coupling. So the principle of PM fiber makes enough difference.

Among the most common applications of optical fiber long-distance communication, PM fiber is used to introduce light from one place to another into the state of linear polarization. In order to achieve this result, several conditions must be met. The input fiber must be highly polarized to avoid transmitting slow axis and fast axis modes, in which the output polarization state is unpredictable.

For the same reason, the electric field in the optical fiber must be aligned precisely and accurately with the main axis of an optical fiber (which is usually the slow axis in industrial practice). If the PM fiber path cable is composed of segmented fibers connected by fiber connectors or splicing joints, matching fiber rotation and positioning is a very critical problem. In addition, the connector must be installed on the PM fiber, and during the installation of the connector, the internal stress generated will not cause the electric field to be projected onto the optical axis not used on the fiber.

Applications of PM fiber

PM fibers are used in areas where polarization drift is not allowed, such as temperature changes. Examples of this are fiber interferometers and some fiber lasers. The disadvantage of using such fibers is that they usually require precise orientation of the polarization, which can cause more trouble. At the same time, propagation loss is higher than that of standard optical fibers, and it is difficult to keep all types of optical fibers in polarization retaining form.

PM fibers are used in specific applications such as fiber sensing applications, interferometry, and quantum key distribution. It is also commonly used in long-distance communications between laser generators and modulators, which require polarized light as input. It is rarely used for long-distance transmission because PM fiber is very expensive and has a higher attenuation than single-mode fiber.

Requirements for the use of PM fiber

Terminal: When the terminal of a PM fiber is an optical connector, it is important to connect the stress rod to the connector, usually by means of a key.

Splicing: Splicing PM fibers should also be done very carefully. When the fiber is fused, the X, Y, and Z axes should be well positioned and the rotation positioning must be well positioned so that the stress bar can be precisely positioned.

Another requirement is that the incident condition at the end of the fiber must be consistent with the direction of the transverse principal axis of the fiber cross-section.