Hi there, Community friends!
This post is about fiber dispersion. Please find more details on the topic as you read further down.
INDICATION OF FIBER DISPERSION
In a fiber, signals of different speeds travel over the same distance with different delays generated. As a result, delay inequality occurs. The severity of dispersion increases with the delay inequality. Hence, the delay inequality is used to indicate the dispersion degree. The delay inequality, however, does not equal the dispersion. If the delay of each signal is the same, dispersion does not occur and the signals are not distorted during transmission.
The delay inequality can be caused by different transmission rates of frequencies or modes of the signals.
DISPERSION COEFFICIENT OF THE SINGLE-MODE FIBER
In a single-mode fiber, there is only basic-mode transmission. The total dispersion of this fiber is a sum of the material dispersion and waveguide dispersion in the fiber. All these dispersion values are relevant to the wavelengths transmitted in the fiber. Hence, the total dispersion of a single-mode fiber is also called wavelength dispersion.
Dispersion coefficient refers to the difference between the arriving time values of two 1 nm spaced wavelengths that travel over 1 km. It is expressed in ps/nm·km.
RELATIONSHIP BETWEEN DISPERSION AND TRANSMISSION RATE
The impact of the group velocity dispersion on the bit rate should comply with the criterion that adjacent pulses do not overlap, that is, B∆T < 1.
Where, "B" denotes the bit rate and "∆T" denotes the broadened pulse width caused by group velocity dispersion.
The higher the transmission rate is, to ensure the correct transmission of signals, the weaker the impact of the dispersion on the on the bit rate must be.
∆T = DLδλ
Where, "L" denotes the transmission distance; "D" denotes the dispersion coefficient; "δλ" denotes the root mean square spectral width of the light source. In the following formula, "δλ-20" denotes –20 dB spectral width.
δλ = δλ-20/6.07
SYSTEM PERFORMANCE IMPAIRMENT CAUSED BY FIBER DISPERSION
Various factors can impair the system performance due to fiber dispersion. The major two factors are intersymbol interference and mode partition noise.
INTERSYMBOL INTERFERENCE
Fiber dispersion broadens transmitted pulses. The waveform of actually received signals is generated by multiple spectral lines from the laser on the transmitter. Even if the receiver can thoroughly equalize the waveform generated by a single spectral line and thus each spectral line generates the same waveform, such waveforms stagger in tandem after enduring different dispersion during transmission. Hence, the waveform combined by such waveforms differs from the waveform generated by a single spectral line. As a result, the receiver cannot thoroughly equalize the combined waveform.
MODE PARTITION NOISE
Mode partition noise impairs the system performance because of the combined effect of fiber dispersion and laser spectral characteristics. Though the total optical power of the spectral lines generated by a laser is certain, the optical power of each spectral line randomly fluctuates. When each spectral line of this laser travels over a fiber, the spectral lines of different wavelengths are subject to different delays because of the inherent dispersion of the fiber. As a result, the waveform of received spectral lines varies with the bit rate and the received pulses are broadened.
That would be all on fiber dispersion. Thanks for reading this post!
