Dispersion is a physical phenomenon that causes distortion of the transmission signal due to the different group speeds of different frequency components or different mode components of the signal transmitted by the optical fiber. In an optical module, the dispersion of the optical module will limit the transmission distance. The following 4 parts help you to know more.
1. Why have dispersion?
Because electromagnetic waves of different wavelengths propagate at different speeds in the same medium, different wavelength components of the optical signal arrive at the receiving end at different times due to the accumulation of the transmission distance, resulting in pulse broadening, and the signal value cannot be resolved.
2. The Classification of dispersion
Optical fiber dispersion is mainly composed of modal dispersion, material dispersion, and waveguide dispersion. Among them, material dispersion and waveguide dispersion are related to wavelength, so they are collectively called wavelength dispersion.
(1) Mode dispersion
In multimode fiber, there are many transmission modes. Different modes have different transmission paths, different distances, and different time to reach the end point, which causes the broadening of the pulse. This is the mode dispersion.
(2) Material dispersion
Material dispersion is caused by the characteristics of the fiber material itself, and each material has different values for different transmission wavelengths.
(3) Waveguide dispersion
After the light pulses from a light source with a certain pop width are taken into the optical fiber, the optical transmission paths of different wavelengths are not exactly the same, so the time to reach the end point is also different, which results in pulse broadening. Waveguide-induced, so called waveguide dispersion.
3. The effect of dispersion
System performance impairments related to fiber dispersion can be caused by a variety of reasons, two of which are more important: inter-symbol interference, mode allocation noise, and so on.
(1) Inter-symbol interference
Fiber dispersion results in broadening of the transmitted light pulses. The actual received waveform is composed of many line spectra of the laser in the optical module. Even if the receiver can ideally equalize the waveform formed by a single line spectrum, it is staggered because of the different dispersion experienced by the same waveform generated by each line spectrum. , Making the combined waveform different from a single line spectrum waveform, will still cause non-ideal equalization.
(2) Modular distribution noise
This is due to the system damage caused by the fiber dispersion and the spectral characteristics of the laser in the optical module. Although the total power of the laser spectral lines is constant, the power of each spectral line fluctuates randomly. After the various spectral lines of the laser pass through the optical fiber, due to the inherent dispersion of the optical fiber, the spectral lines of different wavelengths have different delays, resulting in different receiving waveforms for different bits and forming a widening of the received pulse.
4. The dispersion tolerance of optical modules
10G XFP optical module: 1600ps / nm; GE SFP optical module: 246ps / nm; GE color optical module: 0.11ps / nm; 100M / 155M SFP optical module: 96ps / nm
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