may be regarded as a form of electromagnetic radiation, consisting of interdependent, mutually perpendicular transverse oscillations of an electric and magnetic field. It forms a narrow section of the the wavelength range being approximately 390nm (violet) to 740nm (red). According to the quantum theory, light is absorbed in packets of light quanta, or photons. Source: Dictionary of Physics

Oscillations and Waves Oscillation – a periodic variation of any physical quantity Wave – oscillation of an extended medium which transmits a disturbance Some definitions: Amplitude - the difference between the maximum displacement and minimum displacement of the wave. Cycle (Period), T - one complete oscillation of a periodic wave, after which the wave is returned to its original form. (measured in sec) Frequency, f - the number of cycles that a periodic wave undergoes per second. ( measured in Hz = 1/sec) Wavelength, - the distance from one peak to the next of a periodic wave. (measured in m)

Electromagnetic (EM) Waves These are produced by vibrating charges, either positive (protons) or negative (electrons). EM waves are described as all other waves – Amplitude – magnitude of the electric (or magnetic) field Intensity – proportional to (Amplitude) 2 Frequency – color Wavelength Definition: Spectrum – a range of frequencies EM travel in empty space at the speed of light – c = 299,792,457 m/sec  3×10 8 m/sec

Source:

The wave on the left has vertical polarization and the wave on the right has horizontal polarization. Polarization

Light in transparent media Glass and other transparent media transmit light, which travels at different speeds inside of various materials (media). The speed is given in terms of a parameter called the refractive index, denoted by n, of the medium. The wavelength of a light wave inside a medium also depends on the refractive index. The refractive index, n:. In air n  1 medium, n = 2air, n  1 c  3×10 8 m/sec n  1

Snell’s Law    n2n2 n1n1 n1n1 Light rays bend when traversing boundaries between media with different refractive index: in out See

Light refraction When a wave moves from one medium into another in which the light’s speed is different, the direction of the wave’s travel bends. The wavefronts remain continuous across the boundary between the two media. n1n1 n 2 > n 1 n1n1 wavefront n 2 > n 1

MEDIUMn(visible) vacuum1 air water1.3 glass1.5 diamond2.4 gallium arsenide 3.5 Some values for the refractive index of common optical materials

Total internal reflection If light traveling inside a medium with a higher refractive index than the surrounding medium, and it hits the inner surface of the medium at a steep enough angle, then the light is reflected completely. This angle is known as the “critical angle”. This is the basis of optical fiber, which is used to transmit light over long distances. Angle smaller than the critical angle Angle equal to the critical angle Angle greater than the critical angle: Total Internal Reflection n > n’ See

Optical Waveguides and Fibers n > n’ always Light is guided by total internal reflection Slab waveguide n n’ Confines light by total internal reflection only along one direction in space in out

Optical fiber Cladding Core n n’ n n Optical fibers are cylindrical waveguides, providing light confinement by total internal reflection along all directions which are perpendicular to the propagation direction. These are essentially bendable “light pipes”. Cross-section 1 – 10 μm ~ 100 μm size n > n’ always

Fibers are made of ultrapure SiO 2 glass (silica). Different dopants are added both to the core and cladding, such that the refractive index of the core is slightly larger than that of the cladding. Optical loss in fiber-quality fused silica. (circa 1995) Optical loss in fiber-quality fused silica. (circa 2001) To optimize fibers for telecommunications applications it was necessary to purify them to a very high degree and remove all traces of water. This eliminated the high absorption losses in the “communications window”. Communications window

Fiber-Optic Communications Systems Example of fiber-optical communication link. Electrical current pulses representing digital data drive a semiconductor laser. The emitted light pulses pass through a fiber and are detected by a photo-detector at the far end. Laser Input electric pulses ~10Gb/sec Light pulses travel in fiber (short or long) Output electric pulses

Amplifying optical signals How far can an optical signal (light) travel in fiber before absorption causes significant losses and signal deterioration? Communications window Fibers can typically transmit information over a distance of 80km, after which signals require amplification and/or regeneration. Fibers also have a very large bandwidth – the communications window where absorption losses in the fiber are small is broad. This allows transmitting many wavelengths (frequencies) simultaneously.

Amplifiers can be integrated into the fiber, by doping fibers with Erbium atoms. EDFA – Erbium Doped Fiber Amplifier In the amplifier, Erbium atoms are pumped by a separate pump semiconductor laser (PSCL). Once in the excited state, these atoms will undergo stimulated emission when the signal pulses arrive at the EDFA. In this way, energy from the EDFA is added to the signal pulses, leading to their amplification. Laser Pump laser

Connecting fibers – optical communications systems MUX = Multiplexing DEMUX = Demultiplexing SCL = semiconductor laser Mod = modulator Det = detector Different frequency for each channel

Techniques for multiplexing and demultiplexing. Prisms or diffraction gratings deflect light beams into different angles depending on their frequencies. Some useful applets: Multiplexing and Demultiplexing optical signals prisms diffraction gratings

For tutorials about light refraction and total internal reflection see To visualize injection of light into optical fibers and fiber networks see