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exam2sol - ECE 414 Fiber Optics and Optoelectronics...

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Unformatted text preview: ECE 414 Fiber Optics and Optoelectronics Mid—Term Exam #2 3.24.2011 ‘ a n ' \ Name:3¥2l lifié Signature: :HlUl (GI/l3 You are permitted to use one 8.5" x 11” paper and a graphing calculator to complete the exam. Remember to show all work to receive credit on each problem. Possibly useful constants Boltzmann's constant: k = 1.381 x 10'23 ]/K0 = 8.617 X 10'5 eV/KO .1 Electronic charge: e = 1.6021 x 10 '19 C Electron volt: 1 eV = 1.6021 x 10 '12 erg = 1.6021 x 10 '19 ] ' Planck's constant: h = 6.6256 x 10-34 I-s = 4.1356 x 10-15 eV—s Vacuum permittivity: so = 8.854 X 10'12 F/m 1 [25 Points) Short Answer. Answer the following questions with a short response [and / or diagrams explaining your argument). a] “Laser” is a word that has become used in every-day language, while it is actually an acronym. What does each letter in L.A.S.E.R. stand for? £5th Empliflmrm lag §l€mulalad fimfism" 0G Katalidtfen b) While silicon is ubiquitously used as a semiconductor in the microelectronics industry, why does silicon not work well as a semiconductor in diode lasers? Slr'ldei’l (3 0W\ baMgaP Mule/mod (c) What is the key difference between direct bandgap and indirect bandgap semiconductors? Give examples of semiconductors of both types. What is the difference in photon emission process in direct bandgap and indirect bandgap semiconductors? W } w GMAS ~ ‘ GaSb ‘ “ H \ l . band 609% CW aligned /' Band Mlng (Me My aligned d) There are three processes that describe the relationship between the change in number of photons in a material, and electrons changing energy levels within L a material. Draw a diagram of each process, and give the name for each process. 390“ lanai-05 emr‘fisslo“ Slimulaled gmbsion Absar p‘r‘rmn « Beam Mater 8e10an l Aflw ‘ BOW may i .96... t... s... .. l Av W l :t: w l \ 3 2 [25 Points] Consider the three-level system shown below, where E2-E1=2eV and E3- E1=1.7eV. Relaxation from level 2 to level three happens very fast. The spontaneous emission lifetime from level 3 is 2.4 microseconds. You may assume a density of 1015 atoms / cm3. Wm E 2 ' _ (very fast} E3 a) What is the wavelength of a photon emitted in the level 3 to level 1 transition? What is the wavelength of a photon absorbed in the level 1 to level 2 transition? m fl 1 .. u—f . : Q \ \ ,, - .- Egl’hv/m“)? 51:» h’U "‘7 ’U- WHO "'37 I\ “pg 1' é.v9\5\kl0?m b) The system is pumped so that in steady state with no stimulated emission, 95% of the atoms are in level 3. How much power [in watts/cm3) is absorbed? W E in flaw/g 5%, «Afil: 0/46 9?; fiPWmM mrsslbm,fiw muff W . Mi‘al‘ed W mf‘f' WW :(63'59 r E (W) {W 10"“ 376$) = l0l W/mj c; aw, é, abggwcélox toa ' . )‘l "'4 . c) A phoén’ IS feleased when an electron falls from a higher energy stag lower energy state. This condition is satisfied within materials that have two 2 l energy levels. Why can’t we use a two-level system for lasers (or can we)? A +wo~lml 534m“ CamoJr (Ichtth P®©Uk&+‘fo’r\ MWSEOA (see male/s) which {5 0\ @f‘f‘ldel properlj 04 (age/r1 3 (25 Points) A four-level system is shown in the diagram below. Lasing occurs between levels 3 and 4. All spontaneous emission coefficients (rm, k = 1, 2, 3) are known, as well as pump and lasing probabilities Ap and A1,. TAbsorpt‘ian lSfimuiated Emission : :Spantanenus Emission v .__.. _ ~ ALWHWB)- + M: .t 19.; (13am [(9) '1 Ewae m it Lama Mgefims AW Creme 04 electrsz (Tm/{swim in [aw/l9 33H ‘1be (llch ft) in [ml H Rambilitg (19,019 heme/n (cud-1K c) What does ‘steady state’ condition mean in the description of such many-level ; systems? M:O qm-Q/‘aminfl “MK HUI/14W 01L awry/‘5 "A 6W 0” flak (‘5 OWL“me with respect in fine 4 (15 Points) A simple cavity is shown in the image below for a laser operating at 532nm. The material in the center of the cavity is a gain medium. Both mirrors are 98% reflective. D = 2m is the total separation of the two mirrors. The gain material inside the cavity has a gain coefficient g, and length L. The dashed line shows the photon’s path through the resonator. What is change in density of photon states [Ad)5f) after each complete pass through the cavity? Give a brief description of each term in this equation. cu .1 rote \ . f + @9216 [Qinscpmwl—(Er (Ll (@5663 due +0 Male/rial MSFOLZ msmisr f \‘n h Fer NHw‘m lnPU‘l phOl‘Zr/‘x did/W311 \ P 0 n Coefiftdmfg damn-’3 densitfi 39 the m Micros 5 [10 Points) A two-level system at room temperature is in equilibrium with energy levels E2 = ZeV, and E1 = 1.2eV. What is the fraction of electrons is in higher energy state E2? Qmm +€mpercuLUwQ T: <QOGC, 7‘ QQSOK ...
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This note was uploaded on 02/12/2012 for the course ECE 414 taught by Professor Alenxendra during the Spring '11 term at Purdue.

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exam2sol - ECE 414 Fiber Optics and Optoelectronics...

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