OFC ASSIGN02

7/29/2019 OFC ASSIGN02

1/4

Dispersion in Fiber Optics

Introduction

Dispersion is the spreading out of a light pulse in time as it propagates down the fiber. The pulseat the output of the fiber is wider than the input since as it travels along the fiber it becomeswider. Dispersion is measured in units of time, ususally nanoseconds or picoseconds. The totaldispersion of a fiber depends on its length. A longer fiber causes more pulse broadening and haslarger dispersion. There are basically three types of dispersion, Model Dispersion, MaterialDispersion, and Waveguide Dispersion

Chromatic Dispersion (CD)CD Definition and OriginLight within a medium travels at a slower speed than in vacuum. The speed at which light

travels isdetermined by the mediums refractive index. In an ideal situation, the refractive indexwould not depend onthe wavelength of the light. Since this is not the case, different wavelengths travel atdifferent speeds withinan optical fiber.

Figure 1: CD in single-mode fiber

Laser sources are spectrally thin, but not monochromatic. This means that the input pulsecontains severalwavelength components, traveling at different speeds, causing the pulse to spread. Thedetrimental effectsof chromatic dispersion result in the slower wavelengths of one pulse intermixing withthe fasterwavelengths of an adjacent pulse, causing intersymbol interference.The Chromatic Dispersion of a fiber is expressed in ps/(nm*km), representing thedifferential delay, or timespreading (in ps), for a source with a spectral width of 1 nm traveling on 1 km of thefiber. It depends on the

fiber type, and it limits the bit rate or the transmission distance for a good quality of service.


2/4

Polarization Mode Dispersion (PMD)PMD Definition and Origin

Testing for Polarization Mode Dispersion (PMD) is becoming essential before upgrading

a network to a higher bit rate because PMD can highly degrade the quality oftransmission. It is a difficult parameter to measure, however, because it varies with timeand depends on environmental conditions.PMD is known to stem from the difference inthe propagation constants of a fiber due to geometrical imperfections in the fiber. Theterm PMD denotes both the physical phenomenon and the associated temporal delay.PMD also causes a system penalty because of the associated pulse spreading in ahighspeed digital transmission system. The physical origin of PMD is essentially linearbirefringence due to core eccentricity and ovalization. These appear during themanufacturing process or result from external stresses on the fiber, such as bends andtwists, and can be considered constant over a length called the coupling length. Thetypical value of the coupling length is several hundred meters and depends on fiber

manufacturing parameters. This means that for distances that are practical fortransmission applications, the actual length of the fiber is much greater than the couplinglength. The PMD phenomenon is characterized by Differential Group Delay (DGD).DGD is the difference in propagation time between the two polarization eigenstates,which are the states of polarization with minimum and maximum propagation time foreach wavelength.

Figure 9: DGD of PM and random coupling fibers

Phase-shift method CD-OTDR method

application long-haul link metro & access linkmeasurement constraint two-ended measurement one-ended measurementaccuracy

good accuracy (depending on

number of acquired points)


3/4

fiber lengths), the optical fiber acts like many short birefringent elements stackedtogether and the alignmentof fast- and slow-axes is random from element to element. Consequently, we speak aboutrandom (orstrong) mode coupling. In that case, the DGD varies as a function of wavelength and the

PMD, expressed inps, is the average value of the DGD spectral distribution.The average DGD scales as the square root of the length of the fiber. So the PMDcoefficient, expressed inps/km, is often calculated. In addition, the second-order PMD coefficient, in ps/(nm.km), expresses thePMD dependency with the wavelength.PMD needs to be tested on the C&L bands. But, depending on the wavelengthtransmission window of thenetwork, there is a need to also test PMD at 1310 nm as PMD values could be differentfrom 1310 nm to

1550 nm.The Statistical Nature of PMDFor a practical transmission system, DGD determines the system penalty and depends toa large extent onthe wavelength of operation within the operating wavelength band. But DGD alsochanges withenvironmental conditions over time. The next two traces show DGD as a function ofwavelength, for thesame fiber at different times.

Model Dispersion in Multimode Fibers

This is related to the fact that a pulse of light transmitted through a fiber optic cable is composedof several rays of light instead of only one single beam, therefore it is called modal dispersion.They have a much larger core size. Each mode enters the fiber at a different angle and thentravels at different paths in the fiber. Since the rays of the light pulse are not perfectly focusedtogether into one beam, each mode of light travels a different path, some short and some long.Therefore, the modes will not be received at the same time and the signal will be distorted orpossibly even lost over long distances. The light pulse spreads out and eventually can causesignal overlapping so bad that you can't distinguish them anymore. It is not a problem in singlemode fibers since there is only one mode that can travel in the fiber.

Material Dispersion

This is also known as chromatic dispersion. Since the refractive index of the fiber mediumvaries. And since the light source nor the fiber optic cable is 100 percent pure, the pulse beingsent becomes less and less precise as the light's wavelengths are separated over long distances.This same effect happens when a glass prism disperses light into a spectrum.


4/4

Waveguide Dispersion

This is very similar to material dispersion in that they both cause signals of different wavelengthsand frequencies to separate from the light pulse. However, with wave-guide dispersion itdepends on the shape, design and chemical composition of the fiber core. Only 80 percent of the

power from a light source is confined to the core in a standard single-mode fiber, the other 20percent actually propagates through the inner layer of the cladding. This 20 percent travels fasterbecause the refractive index of the cladding is lower than that of the core. Therefore, signals ofdiffering frequencies and wavelengths are dispersed and the pulse becomes indistinguishable. Anincrease in the wave-guide dispersion in an optical fiber can be used to counterbalance materialdispersion and change the wavelength of zero chromatic dispersion to 1550 nanometers.

Documents

OFC ASSIGN02