Fiber Optic Dispersion testing was not necessary for most 2.5 Gig communication systems. However, as bit rates increase to 10 Gig and beyond (40 Gig & 100 Gig), fiber characterization, particularly dispersion testing, have become increasingly important. The acceptable tolerances for dispersion greatly decrease as the transmission speeds increase.

10 Gig Workaround?

Rather than install new fiber optic cable, which can be extremely costly, some service providers are upgrading their existing networks by implementing Dense Wave Division Multiplexing (DWDM) to increase their bandwidth and capacity to handle higher data rates. In a DWDM system, multiple wavelengths are transmitted down a single fiber. These faster data rates can result in higher bit error rates due to a phenomenon called “dispersion.”

What is Dispersion?

Dispersion is the broadening or spreading of a light pulse as it propagates through an optical fiber. This spreading causes bits to overlap making it difficult for the receiver to interpret the signal. Dispersion increases with distance and bit rate and it limits a network’s ability to transmit at higher speeds, degrades optical signal quality, and increases bit error rates.

Receiver Tolerance

Based on 1dB Power Penalty
2.5 Gb/s 16,000ps/nm
10 Gb/s 1,000ps/nm
40 Gb/s 60ps/nm

Compensating for Dispersion

Optical Fiber Dispersion testing can tell you if a legacy network can handle higher data rates before you invest a lot of money in equipment upgrades. Test results can also be used to create and implement a dispersion compensation plan. Such a plan may involve using DCF (dispersion compensating fiber) and RDF (reverse dispersion fiber) fibers, Bragg Gratings, decreasing impulse spectral width and implementing other remedies. Once the remedy is in place, the span must be re-verified.

Test equipment is available that can accurately measure dispersion. They typically use PC software for documentation and certification reporting.

Types of Dispersion

Chromatic Dispersion (CD)

A laser pulse contains several wavelengths of light, which travel down the fiber at different speeds. This difference in speed causes the laser pulse to spread out and bits to overlap making it difficult for the receiver to interpret. Chromatic Dispersion increases with fiber link distance and bit rate.

Polarization Mode Dispersion (PMD)

Light consists of two perpendicular waves. When light is sent down a fiber these waves travel two different paths (Polarization Modes).

Due to imperfections in the fiber from manufacturing and stresses applied to the fiber during installation, the light in each path travels at different speeds. Polarization Mode Dispersion creates a slow and a fast axis, which causes the pulse to distort or broaden. This broadening of the laser light pulse causes interference at the receiver which limits the network’s signal quality, its ability to transmit at higher speed, and causes signal blending making it difficult for the receiver to interpret. PMD increases with fiber link distance and bit rate.

PMD is not constant and varies over time due to stress and environmental changes. For example, a fiber installed along a railroad track will exhibit a different PMD value when the train is running (due to vibration) than on those days when there is no train. Therefore, PMD should be tested periodically to re-verify the PMD value of an installed fiber link.

Bit Error Rates (BER)

Since dispersion results in higher Bit Error Rates (BER), you should include BER in your testing routine. BER is the percentage of bits that have an error compared to the total number of bits received. The BER represents how often a data packet has to be re-transmitted due to errors.

A high Bit Error Rate increases the number of re-transmissions required. This in turn increases the amount of time needed to send data through the system, slowing transmission speeds. Bit Error Rate testing involves sending simulated data through a communication system and comparing the input data to the output data.