Intermodal dispersion
Intermodal dispersion is a signal distortion mechanism that occurs in multimode fibers and other waveguides. In a multimode fiber, the light rays entering the fiber at different angles of incidence are all defined with a path or a mode. Because the transmission path of each mode is different, its transmission speed (ie group speed) is also different, so the signal transmission between modes has a time difference to reach the fiber terminal. Generally speaking, some light rays will pass directly through the core (axial mode), while other light rays will be reflected back and forth between the cladding/core boundary and propagate forward along the zigzag waveguide, which is the step refraction shown in the figure below. The rate of multimode fiber is shown. The fact is that once the light is refracted, intermodal dispersion/modal dispersion occurs immediately.
Among them, the inter-mode dispersion is positively correlated with the transmission path, that is, the inter-mode dispersion caused by the high-order mode (the light enters at a larger angle is longer) than the low-order mode (the light enters the shorter angle at a shorter distance)
Multimode fiber can accommodate up to 17 light propagation modes at the same time, and its inter-mode dispersion is much higher than single-mode fiber. This is because the single-mode fiber has a single propagation mode, that is, the light propagates along the core (axial mode) without being reflected to the cladding boundary, so no inter-mode dispersion occurs.
However, if you use graded index multimode fiber, the situation is different. Although the light also propagates in different modes, due to the uneven refractive index of the core, the path of the light is no longer a straight line but a curve, and the propagation speed of the light also changes. Therefore, the inter-mode can be greatly reduced by choosing an appropriate refractive index distribution Dispersion.
Chromatic dispersion
Chromatic dispersion refers to the light pulse broadening phenomenon caused by the different group velocities of different wavelength components in the optical fiber, including material dispersion and waveguide dispersion.
Material dispersion is caused by the dependence of the refractive index on the wavelength of the core material, while waveguide dispersion is the dependence of the modal propagation constant on fiber parameters (core radius, core and cladding refractive index difference) and signal wavelength. Caused by sex. At certain frequencies, material dispersion and waveguide dispersion can cancel each other out, resulting in a wavelength close to zero chromatic dispersion.
