Chromatic Dispersion (CD) and Polarization Mode Dispersion (PMD) in Single-Mode Fibers
As the needs of transmission range and rate grow rapidly in fiber-optic communication networks, it's very important now to test and analyze some additional factors than we were used to.
Chromatic Dispersion (CD)
- Is a major limiting factor in high-speed transmission.
- Chromatic Dispersion is a variation in the velocity of light according to wavelengths.
- It causes the pulses of modulated laser source to broaden so that bit error rate increases.
- CD is determined by the cable manufacturing process and it's not affected by installation or weather effects.
Chromatic Dispersion Measurements:
Why to test?:
- The CD is a major limiting factor in high speed transmission systems. Therefore, to ensure adequate quality of service, it's very important to test CD properly and compensate it.
- The widespread use of DWDM systems covering the C and L bands (1530 nm to 1625 nm) creates a need for accurate wideband determination of CD.
Polarization Mode Dispersion (PMD)
- adversely affects signal transmission rates of fiber-optic networks.
PMD is the broadening of a pulse due the difference in speed of the polarization modes of a light wave travelling in an optical fiber. This difference in speed results from birefrigence which is caused by small defects in the manufacturing process, bends, mechanical stresses that may affect the circular fiber geometry.
Why to test?:
- PMD is now an important consideration in high-speed digital and analog networks (10Gbit/s and higher).
- Older fibers that have never been tested for PMD may have extremely high PMD delays.
- While CD can be corrected and fairly well controlled, it is difficult to compensate for a stochastic phenomen such as PMD. PMD should be measured not only after cable manufacturing but also after cabling and installation, since mechanical bends and pressure points can also change fiber characteristics.
Measuring set for PMD analyzis:
- Measuring set FTB 400 + PMD Analyser FTB 5500
- This set uses the interferometric method.
|Polarized LED source
|wavelength l = 1550 nm
||Range: 0,06 - 115 ps