Background
In 1966, Mr. Gao Kun first proposed the use of dielectric optical fiber to transmit information by 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 capability, small size, and ease of 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 fiber (MOF) can be divided into the following two categories according to its structure and transmission mechanism: One is the refractive index guided microstructure Optical fiber; the other is a band-gap photonic crystal fiber with periodic air hole arrangement. 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 implies, 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 humans in the field of atoms. 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, so that mutual coupling and other effects between the cores will be produced, which will 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, and 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, with the seven cores arranged in a regular hexagon. In 2012, R.Ryf and S.Randel, etc. used few-mode fibers to produce three-core microstructure fibers, which reduced the core crosstalk of multi-core fibers. Although these waveguide-type micro-structured optical fibers have problems such as optical fiber-core coupling and crosstalk in long-distance optical fiber communication, this undoubtedly provides a new idea for the field of optical fiber sensing.
Polarization-maintaining fiber
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 the single-mode fiber is affected by the 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.
Solving the problem of polarization instability in single-mode fiber is mainly divided into two methods. 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 birefringence polarization maintaining fiber (Low Birefringent Fiber, referred to as LBF). The second method is to increase the asymmetry of the single-mode fiber and increase its birefringence characteristics, so that the light between the two modes is not easily coupled to each other. We call this kind of polarization-maintaining fiber as High Birefringence Fiber (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 the fiber, polarization-maintaining fibers can be divided into geometric shape effect fibers and stress-induced fibers. Figure 1 shows the end face structure diagrams of several common polarization-maintaining fibers. Among them, bow-tie, panda, inner elliptical cladding, and rectangular stress-clad polarization-maintaining fibers are stress-sensitive fibers; elliptical core and side-groove types Polarization-maintaining fibers such as, side tunnel type, etc. are geometric shape effect type fibers. Most polarization-maintaining fibers are made by methods that generate residual stress in the fiber.