Rare Earth Doped Fibers and Raman Fibers and Eccentric Fibers
Rare Earth Doped Fiber
In the core of the optical fiber, the optical fiber is doped with rare earth elements such as erbium (Er), chin (Nd), and praseodymium (Pr). In 1985, Payne and others from Southampton University in the United Kingdom first discovered that the Rare Earth DoPed Fiber (Rare Earth DoPed Fiber) has the phenomenon of laser oscillation and optical amplification. Therefore, the veil of optical amplification such as miserable bait has been lifted since then. The already practical 1.55pm EDFA uses a single-mode fiber doped with bait, and uses a 1.47pm laser for excitation to obtain 1.55pm optical signal amplification. In addition, praseodymium-doped fluoride fiber amplifiers (PDFAs) are under development.
Raman fiber
The Raman effect means that when a monochromatic light of frequency f is injected into a material, scattered light with frequencies other than f±fR and f±2fR will appear in the scattered light. This phenomenon is called the Raman effect. . Because it is produced by the energy exchange between the molecular motion of matter and the lattice motion. When matter absorbs energy, the vibrational number of light becomes smaller, and the scattered light is called Stokes line. Conversely, the scattered light whose vibrational number becomes larger is called anti-Stokes line. Therefore, the deviation FR of the vibration number reflects the energy level and can show the value inherent in the substance. Optical fibers made of this nonlinear medium are called Raman fibers (RF: Raman Fiber). In order to encapsulate light in a small core and propagate it over a long distance, the interaction effect between light and matter will occur, so that the signal waveform is not distorted and long-distance transmission is achieved. When the input light is enhanced, coherent induced scattered light is obtained. The Raman fiber laser is used as a device for inductive Raman scattered light, which can be used as a power source for spectroscopic measurement and a power source for fiber dispersion test. In addition, the application of induced Raman scattering as an optical amplifier is being studied for long-distance communication of optical fibers.
Eccentric fiber
The core of a standard optical fiber is arranged in the center of the cladding, and the cross-sectional shapes of the core and the cladding are concentric circles. However, due to different uses, the position of the core, the shape of the core, and the shape of the cladding are also made into different states, or the cladding is perforated to form a special-shaped structure. Compared with standard fibers, these fibers are called special-shaped fibers. Excentric Core Fiber is a kind of special-shaped fiber. Its core is set off-center and close to the eccentric position of the outer line of the cladding. Due to the proximity of the core to the surface, part of the light field will propagate beyond the cladding (called Evanescent Wave). Using this phenomenon, the presence or absence of adhering substances and the change in refractive index can be detected. Eccentric fiber (ECF) is mainly used as a fiber optic sensor for detecting substances. Combined with the optical time domain reflectometer (OTDR) test method, it can also be used as a distributed sensor.