Hi there, everybody!
This post is about dispersion reduction techniques. Please see more details below.
Background information
At the 1550 nm window, the typical dispersion of G.652 fiber is 17 ps/nm·km. When the attenuation problem of the fiber is solved, the dispersion limit becomes the key factor that determines the transmission distance of a WDM system. The dispersion accommodation (DA) technology adopts some techniques to reduce or cancel the impact of dispersion and extend the transmission distance. DA adopts the following techniques:
Narrowing the spectral width of the light source
The major impact of dispersion on optical pulse transmission is optical pulse broadening. The magnitude of optical pulse broadening within a certain transmission distance depends on the dispersion coefficient of the transmission fiber and the spectral width of the light source. The magnitude of optical pulse broadening caused by fiber dispersion increases with the spectral width (frequency chirp coefficient) of the light source. Using a laser with a small frequency chirp coefficient can reduce the impact of dispersion in the transmission line.
Frequency chirp is a type of system performance impairment that only the single-longitudinal mode (SLM) laser has. When the SLM laser operates in the direct modulation mode, any change of the input current of the laser can change the carrier density and further change the refractive index of the active area. As a result, the optical path length of the optical cavity of the laser changes and the wavelengths deviate with time. This is the frequency chirp phenomenon in which the wavelength stability is low and the spectrum is broad. After optical pulses travel over the fiber, the waveforms of the optical pulses affected by frequency chirp are broadened because of fiber dispersion.
The methods of reducing the light source chirp coefficient are as follows:
1. Use an externally modulated laser (namely, an indirectly modulated light source) that consists of a constant light source and an optical modulator. The constant light source prevents the exciting current from changing and thus reduces the deviation of optical wavelengths that the light source generates. In this way, the frequency chirp coefficient is reduced.
2. Use an SLM DFB laser with very narrow spectral width and set the laser with negative pre-chirp.
When you select an optical module, note its launched optical power, central wavelength, –20 dB spectral width and maximum dispersion (dispersion tolerance). Consider the SS32L1605 as an example: its operating wavelength is 1550.12 nm; its launched optical power ranges from –2 dBm to +3 dBm; its transmission distance specification is 170 km (with amplifiers), and its maximum dispersion is 6500 ps/nm.
Selecting a streamlined fiber
Some customers' newly constructed laser is G.655 fiber (non-zero dispersion shift fiber). At the 1550 nm window, the chromatic dispersion coefficient of the fiber is about 4 ps/(nm·km).
Using dispersion compensation technologies
The technology of using the dispersion compensation fiber (DCF) to compensate for the dispersion of the transmission line is mature.
The DCF is a special fiber. Its negative chromatic dispersion cancels the positive chromatic dispersion of G.652 fiber. The typical chromatic dispersion of the DCF is –90 ps/(nm km). The dispersion of the entire link is close to zero when the length of the DCF is only one-fifth of that of G.652 fiber in the entire link.
The attenuation of the DCF, however, is large (about 0.5 dB/km) and needs to be compensated by using the EDFA. The EDFA, however, causes severe nonlinear effects on intensive light. The nonlinear effect needs to be avoided. The dispersion compensation by using the DCF is a passive compensation method that is easy to use.
This would be all on dispersion reduction techniques.
Thanks for reading this post!
