OPTICAL LIGHT SOURCES Light Emitting Diode LED Light

ΟΠΤΙΚΕΣ ΠΗΓΕΣ OPTICAL ή LIGHT SOURCES (Light Emitting Diode – LED) (Light Amplification by Stimulated Emission of Radiation – LASER)

ΕΚΠΟΜΠΗ ΦΩΤΟΣ LIGHT EMISSION ΚΒΑΝΤΟΜΗΧΑΝΙΚΗ (QUANTUM MECHANICS)

Light Emitting Diode LED

LED: ΟΠΤΙΚΗ ΣΥΧΝΟΤΗΤΑ «λ» & ΑΠΌΣΤΑΣΗ ΕΝΕΡΓΕΙΑΚΩΝ ΖΩΝΩΝ (Eg) Eg= h c / λ = 1240 e. V-nm/λ (c = λ ν) h = Plank's Constant = 4. 13 x 10 -15 e. V • s c = speed of light = 2. 998 x 108 m/s λ = wavelength in nm

LED: ΥΛΙΚΑ ΚΑΙ ΕΚΠΕΜΠΟΜΕΝΟ «λ» Material Formula Energy Gap (Eg) Wavelength (λ) Gallium Phosphide Ga. P 2. 24 e. V 550 nm Aluminum Arsenide AIAs 2. 09 e. V 590 nm Gallium Arsenide Ga. As 1. 42 e. V 870 nm Indium Phosphide In. P 1. 33 e. V 930 nm Aluminum. Gallium Arsenide AIGa. As 1. 42 -1. 61 e. V 770 -870 nm Indium-Gallium. Arsenide. Phosphide In. Ga. As. P 0. 74 -1. 13 e. V 1100 -1670 nm

ΕΞΩΤΕΡΙΚΗ ΑΠΟΔΟΤΙΚΟΤΗΤΑ (External Quantum Efficiency – next) n 1 n 2 c p n substrate next = Αριθμός φωτονίων που εξέρχονται της LED Μηχανισμοί απωλειών που επηρεάζουν το next : (1) Απορρόφηση μέσα στην LED (2) Απώλειες Fresnel : part of the light gets reflected back, reflection coefficient: R={(n 2 -n 1)/(n 2+n 1)} (3) Απώλειες Critical angle: all light gets reflected back if > C with C=sin-1(n 1/n 2) critical angle [e. g. C=17° for Ga. P/air interface with n 2=3. 45, n 1=1]

ΓΩΝΙΑ ΚΑΙ ΛΟΒΟΣ ΕΚΠΟΜΠΗΣ ΤΗΣ LED SLED Πηγή εκπομπής τύπου Lambertian P = P 0 cosθ θ ΗETEROSTRUCTURE θ ELED HOMOSTRUCTURE

ΤΕΧΝΙΚΑ ΧΑΡΑΚΤΗΡΙΣΤΙΚΑ ΤΗΣ LED Active Material Type Radiating wavelengt h (nm) Spectral width (nm) Output power into fiber (µW) Forward current (m. A) Rise/fall time (ns) AIGa. As SLED 660 20 190– 1350 20(min) 13/10 ELED 850 35– 65 10– 80 60– 100 2/2– 6. 5/6. 5 SLED 850 40 80– 140 100 — ELED 850 35 10– 32 100 6. 5/6. 5 SLED 1300 110 10– 50 100 3/3 ELED 1300 25 10– 150 30– 100 1. 5/2. 5 ELED 1550 40– 70 1000– 7500 200– 500 0. 4/0. 4– 12/12 Ga. As In. Ga. As. P ΓΙΑ ΜΙΑ LED TO ΓΙΝΟΜΕΝΟ “BW * PO ” = ΣΤΑΘΕΡΟ

ΣΥΓΚΡΙΣΗ LED ΜΕ LASER Comparison of LEDs and Lasers Characteristics LEDs Lasers Output Power Linearly proportional to drive current Proportional to current above threshold Current Drive Current: 50 to 100 m. A Peak Threshold Current: 5 to 40 m. A Coupled Power Moderate High Speed Slower Faster Output Pattern Higher Lower Bandwidth Moderate High Wavelengths Available 0. 66 to 1. 65 µm 0. 78 to 1. 65 µm Spectral Width Wider (40 -190 nm FWHM) Narrower (0. 00001 nm to 10 nm FWHM) Fiber Type Multimode Only SM, MM Ease of Use Easier Harder Lifetime Longer Long Cost Low ($5 -$300) High ($100 -$10, 000)