Optical Fiber Communication Lecture 1 Optical Fiber Communication

Optical Fiber Communication Lecture 1: Optical Fiber Communication: An Overview Dr. Ghusoon Mohsin Ali M. Sc. in Electronics & Communication Department of Electrical Engineering College of Engineering Al-Mustansiriya University 1

INTRODUCTION ØFiber-Optic Communication is the most modern and advanced mode of data communication (not more than 60 years ago). ØFiber optics has revolutionized the way we receive information and communicate with one another, and it has played a major role in leading in the Information Age. Since the communication scientists were in an incessant search of a wideband lowloss medium of data communication which could be used at high data rates with the least amount of lost possible.

HISTORY ØThe first revolution in the communication came when Sir Alexander Graham Bell successfully converted voice signals into electrical signals which were transmitted on electrical wires and then converted back to voice signals. ØRight from this time there has been a continuously increasing need of bandwidth for communication due to continuously increasing number of users.

ELECTROMAGNETIC SPECTRUM Fig. 1. 1 ELECTROMAGNETIC SPECTRUM

Fig. 1. 2

H. W

HISTORY Ø Initial communications started at lower operating frequencies (f 0) of about 30 MHz (low bandwidths (BW)). Since then the f 0 have drastically increased due to large requirements in BW. Ø The medium of transmission that were used for f 0 up to about 1 GHz were coaxial cables. These cables had a loss figure of about 20 d. B/km. Ø Waveguides basically hollow structures which guide the electromagnetic energy from one point to another through them. But the f 0 further increased to few hundreds of gigahertz. Ø Hence this led to a strong need of a search for other alternatives The idea was that, optical frequencies

HISTORY This new communication should answer two questions; • Whether or not there are transmitters and receivers available for this new communication technology? • Whether or not there exists such a wideband loss-less medium for carrying optical signals & conversely, photons control the flow of electrons.

History of attenuation The figure shows the loss profiles of manufactured glass in the early 1970 s, 80 s and 90 s. ØIn the 1970 s, wavelength of 800 nm, Ga. As LASERs also were invented which usintrinsically is capable of emitting light of wavelength 800 nm. it is called as the “First Window” of optical communication. ØOptical communications were now shifted to these regions and were called as the “Second Window” and “Third Window” of optical communication. today, most of the optical transmission take place in the 1550 nm Fig. 1. 4: History of Attenuation graph in the manufacture of glass

CHARACTERISTICS OF LIGHT In this course, we will treat light in the following three models: ØRay Model Ø Wave Model Ø Quantum Model

CHARACTERIZATION OF A LIGHT SOURCE A source of light can be characterized by the following factors: ØIntensity of the light: is defined as the power per unit solid angle. ØWavelength of Light (λ): If we look into figure 1. 4, the choice of wavelength of transmission has a direct relation to the SNR of the transmission. ØSpectral Width of Source (Δλ): It is the range of wavelengths emitted by the source. That is, if we have a source with a wide spectral width, say for example if it emits all the wavelengths ranging from blue to red, we get a light from the source which will look like white light. If we reduce the spectral width to near red, we would get a sharp red color light. Smaller the value of Δλ more will be the purity of the source and also higher would be the data rate of the source (higher bandwidth).

The electromagnetic wave If the dimensions of the medium of propagation are very large compared to the wavelength of the light, light can be considered as a transverse electromagnetic (TEM) wave. This means that the direction of electric field, direction of magnetic field and the direction of propagation of light are mutually perpendicular to one another according to the right hand thumb rule as shown in figure 1. 8. Fig. 1. 8 : TEM nature of Light

The electric and magnetic fields of light are hence related to each other through the medium parameter η which is called the intrinsic impedance of the medium. That is, Where, η= intrinsic impedance of medium |E 0 x| = magnitude of electric field E. |H 0 y| =magnitude of magnetic field H. μ = Permeability of the medium. [H/m] ε = Permittivity of the medium. [F/m] So, if the electric field is known completely, the magnitude of magnetic field can be determined using the above relation and its direction would be perpendicular to the direction of electric field. The behavior of the electric field as a function of time is called the polarization of light.

Polarization is one of the very important parameters of any electromagnetic wave. It is a quantity which illustrates the vector nature of light unlike other quantities like intensity, wavelength and spectral width which show scalar nature of light. It shows that light is made up of varying electric and magnetic fields which are vector quantities. If we look at the locus of the tip of the electric field vector with respect to time, this locus gives the polarization of the wave. There may be different shapes that the tip of the electric field vector can trace with respect to time. Based on these shapes there are different types of polarization which are called as the states of polarization; Ø Linear Polarization Ø Elliptical Polarization ØCircular Polarization Ø Random Polarization Linear and circular polarizations are special cases of elliptical polarization

Light as a Wave • Light is a transverse electromagnetic wave, composed of an electric field and a magnetic field. • As a vector description Ø Electric field points in the x direction. Ø Wave is propagating in the z direction. ω is angular frequency and found by: Øk is propagation constant and found by: Fig. 1. 9: Field distributions in plane E&M waves

Polarization Has an Orientation From the figure we can clearly see that the electric field vector E traces out line with respect to time and so the light is said to be linear polarized.

General form of linearly polarized plane waves Any two orthogonal plane waves Can be combined into a linearly Polarized wave. Conversely, any arbitrary linearly polarized wave can be resolved into two independent Orthogonal plane waves that are in phase. (2. 2) (2. 3) • Consider a slightly more general Ey wave: where δ is relative wave difference with respect to the Ex wave. Fig. 1 -10: Linearly polarized waves

General Linear Polarization When δ is 0 or a multiple of π, then this is always linear • Has an angle with respect to the X-axis under this condition given by: • This is called the orientation of polarization • The magnitude of this wave is:

Circular Polarization When Amplitudes are equal and δ is π/2 then we call this circularly polarized • Traces a circle in the XY plane • Has NO orientation angle Fig. 1 -11: Circular polarized waves

General Elliptical Polarization When Amplitudes and δ are arbitrary values, then we call this most general case elliptically polarized • Traces an ellipse in the XY plane • Still has an orientation angle (now called α) • But now also has property called ellipticity (2. 8) Fig. 1. 12: Elliptically polarized light

Final Comments on Polarization • Light from many sources is unpolarized (incandescent lamp, sun, LEDs). Incoherent lights in general do not have any kind of polarization and are said to be randomly polarized. This means random E-field. • Polarized light has the E-field pointing in same direction for all photons. • Note that light can be partially polarized (in between the above extremes) • Polarization describes two things: – (a) the direction of the electric field (orientation) – (b) the rotation of the electric field (ellipticity) • Any polarization state can be broken down into two ORTHOGONAL polarizations

1. “Optical Fiber Communication”- G. Keiser, Mc. Graw Hill, 4 rd edition 2009. H. W

WAVE-MODEL OF LIGHT The wave function is a generalized function of space (x) and time (t). (2. 2) Where, A= Amplitude of the wave. ω = Angular Frequency of the wave (radian/second) β = Phase Constant (Radian/metre) The term (ωt-βx) is the phase function of ψ(x, t). Thus the phase of the wave is a function of space and time.

H. W

RAY-MODEL OF LIGHT Ø e. g. a point source (like a stone dropped in water) Ø Light is emitted in all directions as a spherical wave – series of crests and troughs forming spherical wavefronts Ø Distance between adjacent crests is the wavelength λ Fig. 2. 2: Spherical and plane wave fronts

Plane waves and their associated rays ØLight-rays are imaginary lines that determine the direction of propagation of light energy. ØSpherical waves – wavefronts are spherical (rays are perpendicular to the wavefronts) ØPlane waves – wavefronts are planes (rays are perpendicular to plane wavefronts in the direction of propagation) ØSpherical waves from a point source at a far distance from an observer appears as a plane wave

Refractive index • Refractive index of a medium is defined as: Relative magnetic permeability Relative electric permittivity • For non-magnetic media : e. g. For air and gases, v ~ c, so that n ~ 1. At optic frequencies, the refractive index of water is 1. 33. e. g. Glass has many compositions, each with a slightly different n. An approximate refractive index of 1. 5 is representative for the silica glasses used in fibers; more precise values for these glasses lie between ~1. 45 and ~1. 48.

Optical fiber Ø Constructional, an optical fiber is a solid cylindrical glass rod called the core, through which light in the form of optical signals propagates. This rod is surrounded by another coaxial cylindrical shell made of glass of lower refractive index called the cladding Ø The diameter of the cladding is of the order of 125 μm and the diameter of the core is even smaller than that. Ø The light propagates inside the core-cladding arrangement and throughout the length of the fiber by a phenomenon called the Total Internal Reflection of light. This phenomenon occurs only when the refractive index of core is greater than the refractive index of cladding. . Fig. 2. 3: Constructional Details of an Optical Fiber

Launching of light into an optical fiber ØIf a ray of light is incident on the core of an optical fiber from the side, the ray of light simply refracts out from the fiber on the other side. Any light that enters the fiber from the side does not propagate along the fiber. ØThe only option thus available with us is to launch the light through the tip of the fiber. . Figure 2. 4: Launching of light into an optical fiber.

ØQ: If a partial reflection at the core-cladding interface suffices the propagation of light along the fiber over long distances? ØNO, the reason is that, at each reflection a part of the optical energy launched into the optical fiber would be lost and after a certain distance along the length of the fiber the optical power would be negligibly low to be of any use. Ø Thus Total Internal Reflection is an absolute necessity at each reflection for a sustained propagation of optical energy over long distance along the optical fiber. This is the sole reason of launching light into the fiber at particular angles so that light energy propagates along the fiber by multiple total internal reflections at the core-cladding interface.

1. “FIBER OPTICS”- Prof. R. K. Shevgaonkar Indian Institute of Technology, Handout. 2. “Optical Fiber Communication”- G. Keiser, Mc. Graw Hill, 4 rd edition 2009. 3. “Optical Fiber Communications”- J. M. Senior, Prentice Hall, Englewood Cliffs, NJ, 2 nd edition 1992.

Thank you