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DATA COMMUNICATION TECHNOLOGY
Introduction and Historical Perspective
Alexander Graham Bell was a very curious and inventive man. In 1880, four year after he invented a telephone, he patented an “apparatus for signaling and communicating, called photophone.” This device shown in bellow figure, transmitted a voice signal over a distance of 2000 meters using a beam of sunlight a the carrier. As he spoke into the Photophone, the speaking trumpet vibrate the mirror, varying the light energy that reflect onto the photovoltaic cell. The electric current produce by the cell varied in conjunction with varying light energy.
The Photophone demonstrate basic principal of optical transmission
The two requirements for commercial success, however, where almost a hundred year away. These requirement were a powerful and reliable light source and reliable and low-cost medium for transmission.
In 1960, the laser was recognized as the long sought light source, and system were tried using both the atmosphere and beam waveguides as the transmission medium. The proposals for optical communication via electric waveguides or optical fibers fabricated fro glass to avoid degradation of the optical signal by the atmosphere were made almost simultaneously in 1966 by Kao and Hockham and Werts. Such system were viewed as a replacement for coaxial cable or carrier transmission system.
The Fundamental of Fiber-Optic System
Optical fiber guide light ray within the fiber material. They can do this because light rays bend or change direction when they pass from one medium to another. They bend because the speed of propagation of light in each medium is different. This phenomenon is called refraction. One common example of refraction occurs when you stand at the edge at a pool and look at an object at the bottom of the pool. Unless you are directly over the object, it appears to be farther away than it really is. This effect occurs because the speed of the light rays from the object increases as the light ray pass from the water to the air.
Ray Theory Transmission
Total Internal Reflection
The reflective index of the medium is define as the ratio of the velocity of the light in a vacuum to the velocity of the light in the medium. A ray of light travels more slowly in an optically dense medium than in one that is less than dense, and the reflective index gives a measure of this effect. When a ray incident on the interface between two dielectrics of different refractive indices (e.g. glass and air). Is may be observed that the ray approaching the interface is propagating in a dielectric of refractive index n1 and is at angle Ø1 to the normal at the surface of the interface. If the dielectric on the other side of the interface have the refractive index n2 which is less than n1, than the refractive is such that the ray path in this lower index medium is at an angle Ø2 to the normal, where Ø2 is grater than Ø1. The angle of incidence Ø1 and refraction Ø2 are related to each other and to the refractive indices of the dialectics by Snell’s low of refraction, which is stated that:
n1 sin Ø1 = n2 sin Ø2
OR
sin Ø1 / sin Ø2 = n2 / n1
As n1 is greater than n2, the angle of the refraction is always greater than the angle of incidence. Thus when the angle of the refraction is 90o and the refracted ray emerged parallel to the interface between the dielectrics the angle of incidence must be less than 90 o . This is the limiting case of refraction and the angle of incidence is now known as the critical angle Øc ,the value of the critical angle is given by :
Øc = n2/n1
At angle of incidence greater than the critical angle the light is reflected back into the originating dialectical medium with high efficiency. Total internal refraction occurs as the interface between two dielectrics of different refractive indices when light is incident on the dielectrics of lower index from the dielectrics of higher index, and the angle of incidence of the ray exceeds the critical value.
Acceptance Angle
The geometry concern with launching a light ray into an optical fiber is shown in bellow figure, which illustrate a meridional ray A at the critical angle Өc within the fiber at the core-cladding interface. It may be observed that this ray enters the fiber core at an angle Өa to the fiber axes and is refracted at the air-core interface before transmission to the core-cladding interface at the critical angle. Hence, any rays which are incident into the fiber core at an angle greater than Өa will be transmitted to the core-cladding interface at an angle less than Өc, and will not be totally internally refracted. Thus for rays to be transmitted by total internal reflection within the fiber core they must be incident on the fiber core within an acceptance cone defined by the conical half angle Өa. Hence Өa is the maximum angle to the axes at which light may enter the fiber in order to be propagated, and is often referred to as the acceptance angle for the fiber.
Numerical Aperture
The numerical aperture of the optical fiber is a measure of its light-gathering capability. The numerical aperture is defined as the maximum angle of incidence of a ray that is totally reflected at the core/classing interface. Mathematically aperture, NA, is expresses in this way :
NA=n2 core – n2 cladding
Skew Ray
Another category of ray exists which is transmitted without passing through the fiber axes. These ray, which greatly outnumber the meridional rays, follow a helical path through the fiber and are called skew rays. Hence, unlike meridional rays, the point of emergence of skew rays from the fiber in air will depend upon the number of reflection they undergo rather than the input condition to the fiber. When the light input to the fiber is nonuniform, skew rays will therefore tend to have a smoothing effect on the distribution of the light as it is transmitted, giving a more uniform output. The amount of smoothing is dependent on the number of reflections encountered by the skew rays.
Attenuation
The loss in signal power as the light travels down the fiber is called attenuation. Attenuation in the fiber is controlled mainly by four factor: radiation of the propagated light, called scattering; conversion of the light energy to heat, called absorption; connection losses at splices and joints in the fiber; and losses at bend in the fiber.
Scattering Losses
Scattering occurs due to microscopic imperfection in the fiber, such as the inclusion of water in the glass. The effect of impurities in the transmission medium is evident when we look up at sky and see a blue color. In fact, deep space has no color, but due to the scattering of sunlight by the dust in the atmosphere, the sky appears bright blue.
There is a limit below which scattering cannot be reduced, no matter how perfectly the glass fiber is made, because of irregularities in the molecular structure of glass. This limit, called the Rayleight scattering limit, has a strong wavelength dependence (1/14). Thus, as the wavelength of the light source increases, the effect of Rayleight scattering on optical loss is reduced. For light with a wavelength of 0.8 μm it is about 2.9 dB/km. At a wavelength of 1.3 μm, the value is about 0.3 dB/km, and at 1.55 μm wavelength, the limit is about 0.15 dB/km.
Absorption Losses
Absorption refers to the conversion of the power to the light beam to heat in some material or imperfection that is partially or completely opaque. This property is useful, as in the jacket of the fiber, to keep the light from escaping the cable, but is a problem when it occurs as inclusion or imperfections in the fiber.
a) Intrinsic Absorption
An absolutely pure silicate glass has little intrinsic absorption due to its basic material structure in the near infrared region.
b) Extrinsic Absorption
In practical optical fibers prepared by the conventional melting techniques, a major source of signal attenuation is extrinsic absorption from transition metal element impurities. It may be noted that certain of this impurities, namely chromium and copper, in their worst valence state can cause attenuation in excess of 1 dB/km in the near-infrared region.
Connection Losses
Connection losses are inevitable. They represent a large source of loss in commercial fiber-optic system. In addition to this installation connections, repair connection will be required. The alignment of optical fiber required at each connection is considerable mechanical feat. Production techniques have been developed to splice single-mode fiber whose total diameter is less than 10 μm by using a mounting fixture and small electric heater.