Fiber Core

- Nov 18, 2020-

The fiber core is a double-layer concentric cylinder with a small cross-sectional area made of quartz glass. It is brittle, easy to break, and needs a protective layer. It can be divided into microstructure optical fiber and polarization maintaining optical fiber, which mainly involve military, national defense, aerospace, energy and environmental protection, industrial control, medical and health, measurement and testing, food safety, household appliances and many other fields.

In 1966, Mr. Gao Kun first proposed the use of dielectric optical fiber to transmit information with optical carrier in an article, thus laying the theoretical foundation for optical fiber as a medium to transmit light. After several years of research, Corning in the United States produced the first optical fiber with a loss of 20dB/Km in 1970, which greatly reduced the transmission loss of the optical fiber and made the development of optical fiber communication technology possible. In recent years, researchers have discovered that optical fiber sensing technology has become one of the active branches in the field of optoelectronic technology due to its high sensitivity, strong anti-electromagnetic interference ability, small size, and easy integration.

Optical fiber sensing technology covers a wide range of fields, including military, national defense, aerospace, energy and environmental protection, industrial control, medical and health, measurement and testing, food safety, household appliances and many other fields. The main sensors involved mainly include: fiber optic gyroscopes, fiber optic hydrophones, fiber grating temperature sensors, fiber optic current transformers and other optical fiber sensing technologies. Micro-structured fibers and polarization-maintaining fibers have become the backbone of the field of optical fiber sensing due to their flexible structure and unique characteristics.

Microstructure fiber (Microstructure dfiber, MOF) can be divided into the following two categories according to its structure and transmission mechanism: one is the refractive index guided microstructure fiber; the other is the band gap type photon with periodic air hole arrangement Crystal fiber. The index-guided microstructure fiber mainly includes capillary fiber, parallel array core fiber and multi-core fiber according to its structure. Capillary fiber was first proposed by Hidaka et al. in 1981. As the name suggests, capillary fiber is a hollow structure inside its core, which leads to many special properties. In the field of sensing, capillary fiber has its unique advantages in measuring liquids and gases. In 1997, the ITO.H research group used hollow-core optical fibers to control the movement of hot rubidium atoms to achieve a more in-depth understanding of the atomic field. The Intelligent Materials and Structure Aerospace Science and Technology Laboratory of Nanjing University of Aeronautics and Astronautics realizes the diagnosis and repair of composite materials by injecting glue on the hollow fiber, thereby realizing the application of the special structure of the capillary fiber. Parallel array core fiber refers to a fiber in which multiple cores are arranged according to a certain rule and share the same cladding. This will produce mutual coupling between the cores, and thus produce many strange characteristics. Harbin Engineering University Optical Fiber Sensing Laboratory has produced a series of index-guided multi-core microstructure optical fibers. Multi-core optical fiber was proposed in the late 1970s. Its main purpose is to integrate the fiber core into a single optical fiber, so that the manufacturing cost of optical fiber and cable can be greatly reduced, and the integration of optical fiber can be improved. In 1994, France Telecom first produced a four-core single-mode fiber. In 2010, American OFS company B. Zhu and others designed and produced a seven-core multi-core optical fiber, and the seven cores were arranged in a regular hexagon. In 2012, R.Ryf and S.Randel et al. used few-mode fibers to produce three-core microstructure fibers, which reduced the core crosstalk of multi-core fibers. Although these waveguide-type microstructure optical fibers have problems such as optical coupling between cores and crosstalk in long-distance optical fiber communication, this undoubtedly provides a new idea for the field of optical fiber sensing.

There are two orthogonal> polarization states in a single-mode fiber. In the ideal case where the fiber structure is strictly symmetrical, the propagation of these two modes is equal. However, in actual production and application, because single-mode fiber is affected by external environment such as temperature and stress, and the stress generated during manufacturing, there is always a certain degree of ellipticity, refractive index distribution, and stress asymmetry. There is a difference in the propagation constant, so an additional phase difference occurs during propagation, which is called birefringence in optics. This kind of birefringence will inevitably lead to polarization mode dispersion. In the fields of optical fiber sensing and optical fiber metrology, it is required that the light propagating in the optical fiber should have a stable polarization state. In many integrated optical devices, the polarization state of the input light is also selective. Due to this polarization-mode dispersion phenomenon, ordinary single-mode optical fibers limit the development of optical fiber sensing and other fields, and polarization-maintaining optical fibers are produced.

At present, there are two main methods to solve the problem of instability of the polarization state in a single-mode fiber. The first is: try to reduce the asymmetric characteristics of single-mode fiber, try to solve the influence of the ellipticity and internal residual stress of the fiber, so that the birefringence effect of this single-mode fiber is minimized to two The two modes can be mutually degenerate. When the normalized birefringence propagation constant B is less than 10^-6, this kind of fiber is usually called Low Birefringent Fiber (LBF). The second method is to increase the asymmetry of the single-mode fiber, increase its birefringence characteristics, and make the light between the two modes difficult to couple with each other. We call this kind of polarization-maintaining fiber high birefringence polarization-maintaining fiber (High Birefringence Fiber, referred to as HBF), and its normalized birefringence propagation constant B is greater than 10^-5. High birefringence polarization-maintaining fibers can be divided into dual-polarization fibers and single-polarization fibers according to their propagation characteristics. The dual polarization fiber separates the two polarization modes so that the polarization mode remains basically unchanged during the transmission process; while the single polarization fiber can only transmit one mode of the two orthogonal polarization modes, and the other mode is suppressed and cannot propagate. We call this fiber a single-polarization fiber or an absolute single-mode fiber.

According to the different ways of birefringence in optical fibers, polarization maintaining fibers can be divided into geometric shape effect fibers and stress-induced fibers. As shown in the figure, there are several common polarization-maintaining fiber end-face structures. Among them, bow-tie, panda, inner elliptical cladding, and rectangular stress-clad polarization-maintaining fibers are stress-sensitive fibers; elliptical core, side slot, Polarization maintaining fibers such as side tunnel type are geometric shape effect type fibers. At present, most polarization-maintaining fibers are manufactured by methods that generate residual stress in the fiber.

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