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1、光纤通信与高压中应用光纤通信与高压中应用Information Capacity?The usual rate for new systems is 2.4 Gbps or even 10 Gbps.?By sending many (“wavelength division multiplexed”) channels on a single fibre?capacity to carry about 30 million uncompressed telephone callsNo Electrical Connection?protected from high voltages bec
2、ause of the danger to people touching the wire.No Electromagnetic Interference?optical communication has so few errors.?Cables may be located near water or power lines without risk to people or equipment.Distances between Regenerators?the repeater spacing is typically 40 kilometres,Some recently ins
3、talled systems have spacings of up to 120?kilometres?This compares with 12 km for the previous coaxial cable electrical technology.some limitations:?Joining Cables“fusion splicing”.?Bending Cables?Optics for Transmission Only:no available optical amplifier.?Gamma Radiation:cause glass to discolor?El
4、ectrical Fields:Very high-voltage electrical fields also affect some glasses tends to emit light and discolor. fibre communication cables ?wrapped around high-voltage electrical cables on transmission towers, only of 30 000 volts or belowLight as an Electromagnetic WaveNaming Modes ?Transverse Elect
5、ric (TE) Modes?Transverse Magnetic (TM) Modes?Transverse ElectroMagnetic (TEM) Modes:both the electric and magnetic fields are perpendicular to the z- direction. the only mode of a single- mode fibre.?Helical (Skew) Modes (HE and EH)Hard Polymer (plastic) Clad (silica) Fibre (HPCF)Transmitting Light
6、 on a FibreFibre Refractive Index ProfilesTransmission through a Sheet of GlassSnells law.critical anglecritical angle?At the critical angle we know that equals 90 and sin 90 = 1Numerical Aperture (NA)what kind of light?Fibre Transmission Windows (Bands)?Short Wavelength Band (First Window): the ban
7、d around 800-900 nm. This was the first band used for optical fibre communication in the 1970s and early 1980s.?Medium Wavelength Band (Second Window): the band around 1310 nm which came into use in the mid 1980s. fibre attenuation is only about 0.4 dB/km?Long Wavelength Band (Third Window):?The ban
8、d between about 1510 nm and 1600 nm has the lowest attenuation available on current optical fibre (about 0.26 dB/km). it is difficult (expensive) to make optical sources and detectors that operate herelPPpinout/log10=Transmission Capacity?1 nm wide waveband at 1500 nm has?a bandwidth of about 133 GH
9、z. 1 nm wide waveband at 1300 nm has abandwidth of 177 GHz. In total, this gives a usable range of about 30 Tera Hertz(3 1013Hz). one bit per Hz of analog bandwidth is available, then we can expect a digital bandwidth of 3 1013 bits per second.Operational PrinciplesDispersionMaterial dispersion (chr
10、omatic dispersion)?Both lasers and LEDs produce a range of optical wavelengths (a band of light) rather than a single narrow wavelength. The fibre has different refractive index characteristics at different wavelengths and therefore each wavelength will travel at a different speed in the fibre. Thus
11、, some wavelengths arrive before others and a signal pulse disperses (or?smears out).Modal dispersion?When using multimode fibre, the light is able to take many different paths or “modes” as it travels within the fibre. The distance traveled by light in each mode is different from the distance trave
12、lled in other modes. When a pulse is sent, parts of that pulse (rays or quanta) take many different modes (usually all available modes). Therefore, some components of the pulse will arrive before?others. The difference between the arrival time of light taking the fastest mode versus the slowest obvi
13、ously gets greater as the distance gets greater.Waveguide dispersion?Waveguide dispersion is a very complex effect and is caused by the shape and index profile of the fibre core.?由于光纤的几何结构、形状等方面的不 完善，使得光波部分在包层中传播，由 于纤芯和包层的折射率不同，造成脉冲 展宽Characteristics of Glasses?Ordinary window glass is made from a m
14、ixture of sodium carbonate,calcium carbonate and silicon dioxide. ?The predominant material in optical fibre is pure fused silica (silicon dioxide, SiO,?sand). In optical fibre we usually mix a proportion of germanium dioxide (4% to 10%) with the silica to increase the RI when required. RI can be de
15、creased by adding borontrioxide (BO). Other common dopants?used to increase the RI are phosphorus pentoxide (PO5), titanium dioxide (TiO)?and aluminium oxide (AlO).Optical Sources?There are two kinds of devices that are used as light sources: Lasers and LEDs?(Light Emitting Diodes).Light Production?
16、Spontaneous emission?Stimulated emissionLight Emitting Diodes (Light Emitting Diodes (LEDsLEDs) )?Almost all light sources used in communications today are made from?semiconductors.?The Semiconductor Junction DiodeBandgap Energy and Possible Wavelength Ranges in Various MaterialsHeterojunctions (Practical LEDs)?The heterojunction allows us to have a small active region where the light is?produced. In addition, the m
