12-4-08

12-4-08 - ’ Types of Semiconductor Lasers Homojunction...

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Unformatted text preview: ’ Types of Semiconductor Lasers Homojunction laser region * fight sififiin Double heterostructure >optical waveguide >no reabsorption £443? : Evaluating threshold current In steady state: . (electrons in per sec) = (electrons recombining per see) In; {‘33: gfiplT/Ts) Require minimum 11 for transparency (gain>0) GaAs at room 31% 1 M {gmwg temperature: Stripe laser diode (buried heteIOStructure) A (3349.5 DH laser has an active region of fihicknesa 0.1 gm: mid width gm. Determine (akhe thresheid current density, and (Mike threahald currenti c: l’ K“ Salution: (a) The threshoid current‘density can be wtimated fmm Eq.( 13324 ) as i ‘7 V 3 I _ ' , Jm m (1.6 x 15*-19e)(.1¢1 x W19 x 3.0318 (swig): (1 x 10”“5 em) m 1.1. K 193 “5:5 £3111 This caimlated value is in geod agreement with experimental current thmaholds fer DH lasers, which are w 1133 A/cmi’. (b) The arses sectianal area {at currem flaw is A m 132;; m (10"1cm) (3 x lflmécm) : 8 x. 10431212 30 the thrwhald sweat is m m (L1 x magi-:3) (a x 19*5cm2) : (3118818. :2 88:91.4. Quantum Well Lasers If (1 is very small, need “optical confinement layers” to form optical waveguide for mode confinement Advantages: 1. Can adjust wavelength by changing d 2. Lower threshold due to small d 3. Less temperature dependence These are now commonly used even for inexpensive diode lasers. Single Frequency Lasers DF B laser (distributed feedback laser) A {3333: laser 11% a mvity iefigth 9f 8.8; mm, and eperates at SE8 11m. The refrains» We index ef GaAs i3 313. Detmnine the medufiatienparioé it): E: Bragg reflectar £23; this laser: and the maximam allnwable Width sf the grating refiectian spectrum” Swizz-tiara: Far first. {31'er diffracgien, m = 11, $0 86011131 “A” 2&6) W“ 119nm T313 patterning (if a semiconduater surfam an this length male is feasihle: 33:11; it represenfis a manuiacmring ehaflenge. The Width fif the refleefien symtmm mast be Smaller film: (86%} 17am)"; Am” 2(3,fi){8 x 185 um) n 0128111311 DBG (distributed Bragg grating) laser Similar to the DF B laser, except that the reflective region is spatially separated from the gain region. The Bragg grating regions act like mirrors, with reflection occuring not a one point, but over a region. VCSEL (vertical cavity surface emitting laser) > Emission is perpendicular to actiVe layer > 7 Reflectivityof Bragg grating layers is high (R > 0.99) w .. Symmetrical output beam (7-10O cone 1/2 angle) High speed (small area, low capacitance) Low threshold (small d). Large mode spacing, so single mode operation Make large arrays on one substrate Individually test lasers during production VVVVVVV Can vary cavity length to change wavelength Quantum Cascade Lasers > No pn junCtion (acts like resistor in circuit) > Lasing transition in between QW levels in CB > Wavelength does not depend on bandgap > Many photons emitted for one electron traversing semiconductor > Laser levels not thermally populated, so easier to lase in infrared than other lasers > Applications in remote sensing, trace detection of contaminants ...
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12-4-08 - ’ Types of Semiconductor Lasers Homojunction...

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