Optical fiber-a blessing to the communications industry

 Courtesy: 1. Anis Ahmed, Lecturer, Department of EEE, Green University of Bangladesh

                  2. Biswajit Dey, Lecturer, Department of EEE, Green University of Bangladesh

Brief history of Optical Communication Technology:

Before 1792, fire beacons or smoke were used to send information. In 1792, Claude Chappe invented optical telegraphy. He succeeded in transmitting information between Paris and Lille. By 1830, the network had extended to Europe. In 1950, it was realized that bandwidth (BW) product could be further increased if optical waves were used as the carrier but the absence of a coherent optical source as well as a suitable transmission medium. In 1960, the first LASER was developed (coherent light source). The first optical fiber was made by Kao and Hockham in 1966, but line loss was 1000 dB/km. The simultaneous availability of compact light sources and low-loss fiber led to a worldwide effort to develop FO communication systems.

Generation of Optical Fiber:

Parameter	1st generation	2nd generation	3rd generation	4st generation	5st generation
Invention	1974	1978	1982	1994	2000
Technology	0.8 μm	1.3 µm	1.55 µm	1.55 µm	1.65 µm
Repeater spacing	10 km	50 km	60~70 km	21000 km	24000 km – 35000 km
Attenuation	3 dB/km	0.5 dB/km	0.2 dB/km	0.2 dB/km	0.2 dB/km
Bit rate	45 Mb/s	1.7 Gb/s	2.5 Gb/s	2.5 Gb/s	160 Gb/s

BW product in several generations of light wave systems

Optical fiber structures:

A typical bare fiber consists of a core, a cladding and a polymer jacket (buffer coating). The polymer coating is the first line of mechanical protection. The coating also reduces the internal reflection of the cladding, so light is only guided by the core.

A typical optical fiber structure

Both the core and the cladding are made from a type of glass known as silica (SiO2), which is almost transparent in the visible and near-IR. In this case, the refractive index changes in a "step" between the core and the cladding. This fiber structure is known as step-index fiber. The higher core refractive index (0.3% higher) is typically obtained by doping the silica core with germanium dioxide (GeO2).

Transmission characteristics of optical fibers:

The transmission characteristics of an optical fiber are mainly dependent on attenuation (loss) and bandwidth. Silica-based glass fibers have losses of about 0.2 dB/km (i.e.95%) of launched power remains after 1 km of fiber transmission). This is essentially the fundamental lower limit for attenuation in silica-based glass fibers. Fiber bandwidth is limited by the signal dispersion within the fiber. Bandwidth determines the number of bits of information transmitted in a given time period. Now, fiber bandwidth has reached many 10’s Gbit/s over many km’s per wavelength channel.

Minimum loss of 0.2 dB/km @ 1.55 µm Low loss short wavelength (close to fundamental limit of 0.15 dB/km of silica fibres) whereas 0.5 dB/km @ 1.3 µm (this is also the wavelength of minimum dispersion) and approximately 2 dB/km @ 0.85 pm.

 Fiber attenuation and dispersion characteristics

Advantages of optical fiber communication:

·         Greater bandwidth

·         Low attenuation

·         Electrical immunity (no RFI, EMI)

·         Greater security

·         Flexibility

·         Falling cost

·         Long repeater spacing

·         Smaller size and weight than copper cables