Fast Optical Communication Components

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1 Fast Optical Communication Components

2 Fiber optics In optical communications, fiber replaces copper coaxial cables used in wired networks

3 Fiber optics

4 Fiber optic telephone communication system

5 Optical Fiber An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding.

6 Optical Fiber Numerical Aperture The Numerical Aperture of the fiber is the sine of the maximum angle of an incident beam that can be guided in the core 2 2 core clad NA= n n ; For example, taking n core =1.62 and n clad =1.52, we find the NA to be.56. The corresponding angle, Θ = arcsin(.56) = 34 deg The acceptance angle = 2Θ = 68 deg

7 Dispersion in optical fibers

8 Optical Fiber Attenuation For long distance communications optical sources and detectors operating at 1300 nm and 1600 nm are needed.

9 Dispersion in optical fibers

10 Dense Wavelength Division Multiplexing Nortel has demonstrated 6 Tb/s 1000 km DWDM using 160 different wavelengths 40 Gb/s each

11 Optoelectronic modulation The role of modulator is to impress the information onto the optical signal from laser or from LED. Modern optical communication systems require ultra-fast ( Gb/s) modulation

12 Direct laser current modulation R B Current E Time Simplest way to impress the information onto the optical beam direct laser current modulation Optical power Ideal laser response to the current modulation Time

13 Direct laser current modulation issues: relaxation oscillations frequency Laser high-frequency equivalent circuit Electron Photon interaction in the cavity Electron concentration Laser response to a step-like bias Threshold concentration Optical power

14 Direct laser current modulation issues: Dispersion and chirp Chirp is the shift of the laser s center wavelength during single pulse durations

15 Chirp in directly modulated laser

16 Effective pulse broadening due to chirp and fiber dispersion different levels of fiber dispersion

17 External optical modulation Direct modulation scheme Signal (information) Laser (optical carrier) Modulator Channel (Fiber) External modulation scheme

18 Franz-Keldysh Effect In strong electric field the band edges get tilted There is a probability for the electron, which absorbed the photon with the energy less than the bandgap to transfer into the conduction band

19 Quantum confined Stark Effect: the analogous of Franz-Keldysh effect in QWs Electron wave function compression in the quantum well structure

20 E abs Electro-absorption in QW structure in electric field

21 MQW Electro-absorption modulator

22 Electro-Optical Modulators The principle of operation is based on the linear electro-optic effect, or the Pockels effect: a change of optical refractive index in the waveguide due to application of an electric field. Pockels effect: r ij is the electrooptic coefficient

23 Mach Zehnder modulator l V The light velocity in the waveguide: v = c/n eff ; The time to travel the distance l (the length of the interferometer): t = l/v = l n eff /c; When the electric field is applied, the refractive index changes by n; The travel time change: t = l n /c The electric field causes the beam to delay by π if t=t/2 (where T is the wave period); For the light beam, T = λ/c; T/2 = λ/(2c) = l n /c; The required index change n π = λ/(2l)

24 Mach Zehnder modulator (continued) l V The required index change n π = λ/(2l) For the Pockels effect, the difference between top and bottom indices: 3 r0 ij n= n r E If the thickness is d, then E = (V/d)/2 - the applied voltage is divided between the two arms. From this, we find the voltage V π needed to delay the beam by π (i.e. by λ/2) 3 0 ( 2 ) ( nπ = nr rijvπ / d = 3 ) λ /( 2l) Vπ = dλ lr ij n r0

25 Mach Zehnder interferometer: electro-optical RF modulator

26 Example: Mach Zehnder modulator V p calculation l V ( 3 ) ij r0 V = dλ lr n π Interferometer waveguide is made of LiNbO 3 n = 2.3; r ij = 10.8 *10-12 m/v; The waveguide thickness d = 1 µm The modulator length, l = 1mm; The optical beam wavelength, λ = 1.3 µm V = 9.9 V