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Dispersion Details in Optical Fiber

Dispersion Details in Optical Fiber: What is the meaning of the dispersion in the words of the telecommunications network, today we will try to understand. Broadening of an optical signal that result from the many wavelength components travelling at different rates.We can also say that dispersion is that different wavelength is propagating at a different velocity in fiber that causes the pulse to broaden.
There are two kinds of dispersion, the most common is called chromatic dispersion and is routinely compensated for by DWDM systems for proper operation. The effects of polarization mode dispersion (PMD) are much more insidious and difficult to make compensation for in the real networks.
DCM/DCU Dispersion compensation module/unit used in DWDM
Image of DCM/DCU used in DWDM

Chromatic Dispersion
Chromatic dispersion is a measure of fiber delay for different wavelengths. Different wavelengths travel at different velocities through fiber. The difference in velocity is called “delay” or chromatic dispersion of the signal. The faster the rate of transmission, the greater the dispersion effects.

Chromatic Dispersion Tolerance
Standard SMF fiber has an average of 17 ps/nm/km of dispersion.

A 10-Gb/s receiver can tolerate about 800 ps/nm of dispersion.
A 500-km system generates 17 ps/nm/km x 500 km = 8500 ps/nm of dispersion.

The DWDM system must compensate for dispersion to support 10-Gb/s transmission using methods like those shown in below image.
function of Compensation Modules in DWDM
X Axis is Time and Y Axis is The Change In Dispersion Between Modules

2.5-Gb/s transmission is 16 times less sensitive than 10 Gb/s.
2.5-Gb/s signals tolerate up to 12,200 ps/nm without problems.
40-Gb/s transmission is 16 times more sensitive than 10 Gb/s; only tolerates 50 ps/nm and requires special attention to dispersion compensation.

Dispersion Compensators
A compensator is a device that has the opposite chromatic dispersion effect as the transmission fiber. Various technologies are available that can compensate for all wavelengths in a band or for each wavelength. Compensating for all wavelengths greatly reduces the cost of compensation. Per-band compensation is used in some DWDM products. The various methods include:
Dispersion compensation module (DCM)

Fiber Bragg gratings
High order mode devices
Virtual image phase array (VIPA) is a free space dispersion device.

Dispersion Slope and Limits
Because dispersion varies with wavelength, it is difficult to compensate all wavelengths simultaneously. This becomes more difficult over longer distances. New compensators are providing better “slope” compensation. (Slope is variance of dispersion with wavelength.) below image shows both slope and limits.

Dispersion slope and limits details for DWDM
Dispersion Details in DWDM
The dispersion limit of the system depends on many factors:
Receiver tolerance
Depends on pulse type; NRZ is better than RZ pulses.

Depends on data rate.
Depends on transmitter chirp.
Variability of fiber dispersion
Dispersion of fiber has a range of values.
Some temperature dependence occurs
Variability of DCM values.
Includes slope of dispersion and manufacturing tolerances.
Variability of components in network elements (NEs).
Receiver dispersion tolerance must be less than sum of dispersion

What is Chirp?
Chirp is an abrupt change of the center wavelength of a laser, caused by laser instability. When the modulator pulses the laser there is a difference in the refractive index of the laser output that can cause chirp in a DWDM system.

Chirp is the phenomenon of the rising edge of a pulse having a slightly different frequency than the falling edge. It is a common effect in devices that generate optical pulses (optical modulators), and interacts with fiber dispersion and thereby may provide more or less dispersion tolerance.

Chirp usually occurs with a value of +1 GHz to –1 GHz. Each laser transmits coherent light at a different center frequency for each λ.Chirp can be provisioned to match the system input requirements on many DWDM systems. On systems that allow changes the technician may adjust the Chirp value to support the network requirement, commonly the technician can only report the presence and degree of chirp.

Chromatic dispersion near the tolerance limit for DWDM receivers may be worse due to the chirp effect, and may require dispersion compensation or closer spacing of ILA systems.

Polarization Mode Dispersion
Light has polarization. Comparable to polarized sunglasses, some light is vertical and
some is horizontal.Different polarizations travel at different velocities, because fiber is not perfectly round. Different velocities cause dispersion.

As light is refracted within the fiber, slight changes in the polarization of the light may occur. Light which takes different paths within the fiber, will have polarization differences resulting in “dispersion”.

Polarization Mode Dispersion Effect
Although known, polarization mode dispersion (PMD) was not considered in early fiber manufacturing because of the limited impairments that polarization mode dispersion represented at the lower data rates prevalent at that time.

Later, as faster data transmission rates became practical, various manufacturers began to provide solutions that helped manage the 
polarization mode dispersion effects.
The outside vapor deposition method produced low PMD fibers (Corning).
The inside vapor deposition method produced high PMD fibers (Lucent).

Polarization Mode Dispersion details in optical fiber
Polarization mode dispersion became an issue in the early 1980s. Manufacturing methods were improved and now fibers can be manufactured that have low PMD. The PMD standard, Standard Reference Materials (SRM) 2518, published by the National Institute of Standards and Technology (NIST), states 0.5 ps of PMD per the square root of the fiber length in kilometers as the proven PMD management interface. 0.5ps/Гkm. The new fiber types have less than 0.5 PMD.

Polarization Mode Dispersion Compensation
A PMD compensator (PMDC) compensates for polarization mode dispersion. The PMDC device has tunable PMD and is new to optical networking. The compensator applies the opposite amount of PMD as that produced by the physical attributes of the fiber network itself. Current technology compensates for PMD at the receiver. This requires a PMDC for each wavelength.
PMD compensation in optical fiber
PMD Compensation in optical fiber
Effective Polarization Mode Dispersion Compensation
Polarization Mode Dispersion Compensators (PMDC) must consider the following
conditions in their dynamic PMDC operation.

Signal rate
Noise accumulation limit of amplifiers
Chromatic dispersion compensation limit
PMD compensation limit

Friends I have tried to cover the full details about dispersion in "Dispersion details in Optical fiber".If you have more ideas about dispersion you can update me. Dispersion can be decreases in DWDM by the using of dispersion compensation unit or dispersion compensation module. It is also a coil of optical fiber.

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  1. Such an amazing blog you have written. I am a tuition teacher and yesterday I taught my students, Dispersion details in Optical Fiber. But after reading your blog, I think I have missed a few things to teach like Chromatic Dispersion Tolerance, Polarization Mode Dispersion Compensation and Effective Polarization Mode Dispersion Compensation. I do not know how I forget to teach them these important topics. Anyways, thanks to you!