Drop Termination Closures
The carrier of optical fiber transmission is light, although the bandwidth is very wide, but not fully utilized, this is due to optical transmission in the optical fiber dispersion (mold colors bulk, materials and waveguide dispersion). They affect fiber bandwidth to varying degrees.
Colors dispersion is due to different modes of light in the core-package interface of the full reflection angle, twists and turns forward the length of the distance. Therefore, after a beam of optical pulse incident fiber, it contains the mode of transmission at a certain distance to the end of the time will have successively, resulting in pulse broadening. It allows a narrow pulse to widen to about 20 nanoseconds/km, and the corresponding bandwidth of the fiber is about 20 MHz km.
The dispersion of materials is a kind of mode dispersion, and the light transmitted by optical fibers, even the laser, also contains the light components of different wavelengths with a certain spectral width. For example, the GaAlAs semiconductor laser emits a laser spectrum of about 2 nm. The transmission velocity of light in medium is related to refractive index n, and the refractive index of quartz medium changes with the wavelength, so when a beam of light pulse incident fiber, even the same mode, the transmission group velocity will vary with the light wavelength, resulting in the pulse broadening after reaching the end, which is the material dispersion. In the vicinity of 1.3 microns, the refractive index varies very little with the wavelength, so the material dispersion is very small (for example, 3 picosecond/km/NM). Eliminating the colors powder can greatly increase the fiber bandwidth. Pure Quartz has 0 dispersion characteristics at 1.27 micron wavelength.
The waveguide dispersion is also a mode dispersion, which is caused by the difference of the group velocity caused by the variation of the mode propagation constant with the wavelength. Waveguide dispersion is smaller. In the vicinity of the 1.3 micron wavelength, the dispersion of materials is significantly reduced so that they are roughly the same and may counteract each other.