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Fiber-Optic Communications James N. Downing

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Chapter 2 Principles of Optics

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Chapter 2 2.1 Geometrical Optics –A model by which the nature of light is used to explain refraction, reflection, and propagation of light Refraction : –The bending of light as it passes through a medium –Index of refraction: The ratio of the speed of light in a vacuum to the speed of light in the medium –Phase velocity: The speed of light in a medium –Optical path length: apparent length of an optical element

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Chapter 2 2.1 Geometrical Optics Snell’s Law –Mathematical determination of the index of refraction at the interface of two media –Critical angle is the angle at which the refracted ray is at 90 0 to the normal

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Chapter 2 2.1 Geometrical Optics Reflection –Bouncing off of rays from a material interface –Depends on the smoothness of the surface and the refractive indices of the media Fresnel reflection law –Determines the fraction of light reflected as a function of the incident ray as well as the amount of light refracted or transmitted into the medium

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Chapter 2 2.2 Wave Optics Electromagnetic Waves –Result of the dual properties of electricity and magnetism and their relationship –Derived from Maxwell’s equations –Electric waves and magnetic equations are perpendicular to each other –Function of both space and time –Electromagnetic spectrum consists of all forms of electromagnetic energy

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Chapter 2 2.2 Wave Optics Polarization –Describes the direction of the electric field oscillations –Induced by preferential reflection, transmission, scattering, or passing light through a birefringent material –May be either perpendicular, horizontal, z-axis, circular, or elliptical

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Chapter 2 2.2 Wave Optics Coherence –Phase difference is the shift between two waves along their axis of propagation –Coherent light—no phase shift –Incoherent light—phase is continually shifting –Temporal coherence —waves are equal –Spatial coherence—waves are in phase at a point in space

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Chapter 2 2.2 Wave Optics Interference –Due to the linear superposition of electromagnetic waves such that the amplitude at any point is equal to the sum of the individual amplitudes at that point Constructive interference –Phase shift is zero Destructive interference –Phase shift is 180 0

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Chapter 2 2.2 Wave Optics Diffraction –Diffraction describes how light can spread out after going through a small aperture. –Diffraction grating is the separation of the diffracted light into different bands of different colors.

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Chapter 2 2.2 Wave Optics Scattering –Scattering is the spreading apart of light caused by interaction with matter. –Rayleigh scattering, or molecular scattering, is caused by small particles of matter (less than or equal to 1/10 wavelength) interacting with light. –Mie scattering is due to interaction with matter larger than 1/10 wavelength of light.

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Chapter 2 2.3 Quantum Optics Bohr Model –Consists of nucleus and orbitals –Nucleus contains the protons and neutrons –The orbital contains the electrons

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Chapter 2 2.3 Quantum Optics Absorption –Ground state is the minimum level of energy needed to keep an electron associated with its orbit. –Excited state is that in which the electron has absorbed some energy. –Absorption is the process in which light energy is converted into electrical energy. –Beer’s Law describes the absorption transfer function.

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Chapter 2 2.3 Quantum Optics Emission –Emission is the process by which electrical energy is converted to light. –Spontaneous emission occurs naturally. –Stimulated emission occurs when an external photon causes a photon to lose energy. –Linewidth is the length of a wavelength of light (defined at the 50% power level).

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Chapter 2 2.3 Quantum Optics Planck’s Law –This law describes the energy released when an electron moves from one energy level to another.

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Chapter 2 2.4 Nonlinear Optics Four-Wave Mixing –Four-wave mixing results in a fourth frequency when three frequency signals are combined. –Can be used to generate a fourth frequency, if needed. –Problems arise when the fourth frequency is already in use.

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Chapter 2 2.4 Nonlinear Optics Phase Modulation –The result of a change in the refractive index with a change in light intensity –Self-phase results in a broadening of the linewidth of a particular signal –Cross-phase occurs when self-phase modulation causes phase changes in another signal. which results in a linewidth broadening at another wavelength.

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Chapter 2 2.4 Nonlinear Optics Brillouin Scattering –Occurs at optical powers high enough to generate small acoustic waves in the material –Alters the refractive index, and shifts the frequency –Scattering increases as power increases

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Chapter 2 2.4 Nonlinear Optics Raman Scattering –Light is absorbed and some energy is lost or gained from molecular vibrations. –Can be used to transfer energy from one wavelength to another resulting in signal amplification. –Cross-talk may be enhanced if more than one wavelength is used.

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Chapter 2 2.5 Optical Power Radiometric and Photometric Quantities –Photometric quantities describe the visual brightness of a light and exist only between 400nm and 700nm with a peak at 550nm. –Radiometric quantities are consistent throughout the spectrum and are proportional to the square of the energy.

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Chapter 2 2.5 Optical Power Power –The ratio of energy per unit time (measured in watts or dBm) –Transfer function: T dB = P out-dBm – P in-dBm

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