The broadening of light pulses, called dispersion, is a critical factor limiting the quality of signal transmission over optical links. Dispersion is a consequence of the physical properties of the transmission medium. Dispersion limits the transmission distance in optical communications. In simple terms, the broadening of the pulse as it travels along the length of an optical fiber is called dispersion. In fiber-optic measurements, dispersion is often called Chromatic dispersion or short CD. Chromatic Dispersion is variation in the speed of propagation of a light signal with wavelength. Currently installed fiber-optic network spans are limited to around 80 to 100 kilometers that are penalized by dispersion.
Chromatic dispersion is a single-mode fiber characteristic. Chromatic dispersion is not a multimode fiber characteristic. Group delay happens as the signals travel long distances. Multimode fibers are used for short-distance applications. Single-mode fibers, used in high-speed optical networks, are subject to Chromatic Dispersion (CD) that causes pulse broadening depending on wavelength, and to Polarization Mode Dispersion (PMD) that causes pulse broadening depending on polarization.
Chromatic Dispersion is expressed in ps/nm.km (picoseconds/nanometer.kilometer). The delays are in picoseconds and hence the development of an accurate measurement system for chromatic dispersion has many challenges. The optical source in a high-speed communication system is typically a single line diode laser with nonzero spectral width. Pulse modulation increases the spectral width. Each wavelength component of the signal travels at a slightly different speed, resulting in the pulse broadening.
Excessive spreading will cause bits to “overflow” their intended time slots and overlap adjacent bits. The receiver may then have difficulty discerning and properly interpreting adjacent bits, increasing the Bit Error Rate. To preserve the transmission quality, the maximum amount of time dispersion must be limited to a small proportion of the signal bit rate, typically 10% of the bit time.
Chromatic dispersion results from the interplay of two underlying effects in single-mode fibers. Chromatic dispersion occurs from the nonlinear dependence of wavelength with the refractive index. This nonlinear dependency leads to group delay. The higher the refractive index, the lesser will be the speed of light signals. Waveguide dispersion is rooted in the wavelength-dependent relationships of the group velocity to the core diameter and the difference in index between the core and the cladding.
Control of total chromatic dispersion of optical fibers is critical to the design and construction of long haul, high-speed telecommunications systems. The first objective is to reduce the total dispersion to the point where its contribution to the error rate of the system is acceptable. The dispersion of a single channel system can be controlled by concatenating fibers of differing dispersion such that the total dispersion is near zero.
The precision and accuracy of chromatic dispersion measurements depend on test equipment design. The impact of phase instability and phase measurement resolution depends upon the modulation frequency. A higher modulation frequency will produce a phase change that is larger in comparison with a given phase measurement uncertainty. Wavelength accuracy is important because the actual phase shift is proportional to the wavelength step. Depending upon measurement objectives, the inherent wavelength accuracy of a tunable laser or filtered broadband optical source may be sufficient. Thermal transients in the measurement setup and the fiber under test can contribute significant measurement error.
Laboratory Chromatic dispersion test set up is provided by many leading fiber optic test equipment supplies such as Photon Kinetics, EXFO, JDSU, etc. Photon Kinetic’s S-18 is a model in which I have experience. It can measure Chromatic dispersion, Polarization mode dispersion, and fiber strain.
Chromatic dispersion of a conventional single-mode fiber as per ITU-T G.652 has zero dispersion at around 1310nm. International standards specify the wavelength range of 1300 nm to 1324 nm for zero-dispersion wavelength. The dispersion graph of a conventional single-mode fiber used in telecommunication cables is shown below.