For an optical transceiver, dispersion also is the factor not to be ignored. Maybe many people are unfamiliar with this term and have little knowledge of what impacts it has on the optical transceiver module. Then Gigalight will have an introduction to it in this article from these comprehensive perspectives.
What is the Dispersion?
Dispersion is a kind of physical phenomenon. The reason why it forms is that the different group velocity under different frequency/mode of the transmission signal of optic fiber causes the distortion of transmission signals. In optical transceivers, the dispersion of optical modules will limit the transmission distance. Let’s learn from the following aspects.
The Reasons Why Dispersion Appears
On account that the speed of electromagnetic wave with a different wavelength is different in the same media, the optical signal with different wavelength reaches the receiving end in a different time due to the accumulation of transmission distance. After that, the pulse to be widened and then the signal values cannot be distinguished.
Types of Dispersion
The dispersion of optic fiber is mainly composed of modal dispersion, material dispersion, and waveguide dispersion. Thereinto, the material dispersion and waveguide dispersion are related to wavelength, so they are collectively referred to as wavelength dispersion.
1. Modal Dispersion: among the multimode optic fibers, there are various modes of transmission. For different modes, the path of transmission is different so that the time of reaching at the end is also different, which causes the broadening of the pulses. This is the modal dispersion.
2. Material Dispersion: material dispersion is caused by the characteristics of optic fiber materials, and each material has different values for different transmission wavelengths.
3. Waveguide Dispersion: after taking the optical pulse from the light source with certain spectrum into the optical fiber, the time of arrival at the end is also different because the optical transmission paths of different wavelengths are not same. Then the pulse is broadened. This kind of dispersion is caused by the optical waveguide of the optical fiber. Therefore it is also called the waveguide dispersion.
The Impacts of Dispersion on Optical Transceiver
The damage of system performance related to fiber dispersion can be caused by many factors, which mainly include intersymbol interference, mode partition noise and so on.
1. Intersymbol Interference:
the dispersion of optical fibers will lead to the broadening of the transmitted light pulse. The actually received waveform is combined with many line spectrum of the laser in the optical module. Even if the receiver can perfectly balance the waveform of the single line spectrum, but it will be disconnected because the same waveform generated by the line spectrum is with different dispersion. After that, the combined waveform is different from that of the single line spectrum, and it will still cause the non-ideal balance.
2. Mode Partition Noise:
It is one kind of system damage caused by the combination of the dispersion effect with the spectral characteristics of the laser in the optical module. Although the sum of the power of each spectral line in the laser is definite, and the power of each spectral line is fluctuant. When the spectral lines of the laser pass through the optic fiber, the receiving waveforms with different bits are different on account that the original dispersion of optic fiber makes the delay of spectral lines different, forming the broadening of the receiving pulse.
Above all is about the introduction to dispersion. Hoping it can be beneficial to those who get involved in it for the first time and want to have a knowledge of it.
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