What is fiber dispersion?How to compensate dispersion?

Fiber dispersion shows the propagation state of the input signal in the fiber. It refers to the signal distortion caused by the propagation of different frequency components or different mode components of the optical signal at different speeds.It mainly includes three cases: intermode dispersion, chrominance dispersion and polarization mode dispersion.

Intermode dispersion is a signal distortion mechanism that occurs in multimode fibers and other waveguides.In a multimode fiber, light rays entering the fiber at different incidence angles are defined as a path or a pattern.As the transmission path of each mode is different, its transmission speed (i.e., group speed) is also different, so there is a time difference between the signal transmission modes to reach the optical fiber terminal.In general, some light passes directly through the core (in axial mode), while others bounce back and forth between cladding/core boundaries and travel zigzag along the waveguide, as shown in the step-index multimode fiber below.The fact is that as soon as light is refracted, intermode/mode dispersion occurs.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 ray enters at a larger Angle for a longer distance) is higher than that caused by the low-order mode (the ray enters at a smaller Angle for a shorter distance).

Multimode fiber can accommodate up to 17 ray propagation modes at the same time, and its intermode dispersion is much higher than that of single mode fiber.This is because single-mode fibers have a single propagation mode, i.e. light travels along the core (axial mode) without reflecting off the cladding boundary, so no intermode dispersion occurs.However, if a graded index multimode fiber is used, the situation is different.Although light also propagates in different modes, due to the uneven refractive index of the fiber core, the path of light rays is not a straight line but a curve, and the propagation speed of light rays also changes. Therefore, the inter-mode dispersion can be greatly reduced by selecting the appropriate refractive index distribution.

The material dispersion is caused by the dependence of refractive index on the wavelength of the core material, while the waveguide dispersion is caused by the dependence of the mode propagation constant on the fiber parameters (core radius, the difference in refractive index between the core and the cladding) and the signal wavelength.At certain frequencies, material dispersion and waveguide dispersion can cancel each other out, so that a wavelength approximating 0 chroma dispersion is obtained.In fact, chrominance dispersion is not always detrimental.Light travels at different speeds in different wavelengths or materials, causing light pulses to be broadened or compressed in the fiber, which makes it possible to customize refractive index profiles to produce fibers for different purposes.The G.652 optical fiber is one example.

Polarization mode dispersion (PMD) reflects the polarization dependence of the propagation characteristics of light waves in optical fibers.In actual optical fibers, there are two polarization modes that are perpendicular to each other. Ideally, the two polarization modes should have the same wave propagation characteristics, but in general, there are subtle differences between different polarization modes.This is due to the change or disturbance of temperature, pressure and other factors in the propagation process, resulting in different transmission speeds of the two polarization modes, resulting in time delay and polarization mode dispersion.