Lecture 3-7 Semiconductor Lasers.

Transcription

1 Laser LED Stimulated emissio Spotaeous emissio Laser I th I Typical output optical power vs. diode curret (I) characteristics ad the correspodig output spectrum of a laser diode.?1999 S.O. Kasap, Optoelectroics (Pretice Hall) At I > Ith, why may photos are comig out? - What is most likely to happe iside of active regio is a stimulated emissio. As soo as curret(carrier) is ijected, may photos are comig out by stimulated emissio. -Threshold curret poit is a poit whe coheret radiatio gai i the active layer by stimulated emissio just balaces all the cavity losses (represeted by τ p ) plus losses by spotaeous emissio which is radom. The, how may photos (or much power) are emitted through the facet? Thik about the Slope!! d ph dt I = Ith ph qv τ p ( I ) ==> = η ( ) = η Ith ph i th i τ p qv 1 ==> τ p = v g ( αm + αit ) ; at ateady-state d/dt ; ηi iteral quatum efficiecy. -That meas coversio from oe carrier to oe photo iside of the cavity ; τ p photo lifetime. - That represets cavity losses, mirror loss ad iteral loss

2 From the above relatio, Pout = v g α m ph V hw photo iside of cavity photo eergy iside of cavity photo eergy that comes out of the laser light power that comes out of the laser α P m out = αm + αit hw ηi ( I I th ) q - The relatioship betwee light comig out loss ad ijected curret. dp out αm hw = ηi di αm + αit q α η m ex = ηi αm + αit η ex : exteral quatum efficiecy Emissio spectrum - More tha oe lasig peaks exist i the emissio spectrum. But why? X gai spectrum (~1m) > (mode spacig) = at 2 2L = 1.5um, =3.5, L=5um =.6um more tha oe mode several peaks i emissio spectrum Source has more tha oe mode : Differet modes accordig to each differet wavelegth results i sigificat problem, i.e. modal dispersio, i optical commuicatio. The,. how to make sigle mode laser diode?

3 Sigle mode (frequecy) Semicoductor LD - If decreases, the L extremely small L is too difficult. Thus it is ot useful. Rather tha makig LD havig Febry-Perot mirror, someoe adds somethig, which acts like mirror, ito the laser i order to replace the mirrors. The somethig is Gratig. DFB (Distrubuted Feedback) LD ad DBR (Distributed Bragg Reflector) LD - Review of gratig properties d (si θm siθi) = θ i θ m For LD that light comes from horizetal directio, θi = 9 act as mirrors. d( 1 1) = m d = ( m = 1) 2 Thus ; Gratig acts as mirrors! At particular wavelegth, feedback ca be provided. - Picture of DBR ad DFB Laser Corrugated gratig Guidig layer Active layer (a) Λ (a) Distributed feedback (DFB) laser structure. (b) Ideal lasig emissio Typical output spectrum from a DFB laser. <DFB LD> Ideal lasig emissio Optical power (b)?1999 S O K O l i(p ti H ll) B (c).1 m (m) θm should be -9 to P 2 P 2 I 1 I 1 N 2 N 2 at 1.5 um, eff =3.5. The=.2um 3 Distributed Bragg reflector A B Λ q( B /2) = Λ (a) Active layer Corrugated dielectric structure (b) (a) Distributed Bragg reflectio (DBR) laser priciple. (b) Partially reflect at the corrugatios ca oly costitute a reflected wave whe the wavele satisfies the Bragg coditio. Reflected A waves ad B iterfere costructive whe q( B /2) = Λ.?1999 S.O. Kasap, Optoelectroics (Pretice Hall) <DBR LD>

4 - DBR is a mirror that has bee desiged like a reflectio type diffractio gratig ; it has a periodic corrugated structure. Ituitively, partial reflectios of waves from the corrugatios iterfere costructively to give a reflected wave oly whe the wavelegth correspods to twice the corrugatio periodicity. - I DFB, the crystal faces provide the ecessary optical feedback ito the cavity to build the photo cocetratio. I DFB LD, there is a corrugated layer, called the guidig layer, ext to the active layer. These corrugatios i the refractive idex act as optical feedback over the legth of the cavity by producig partial reflectios. Thus optical feedback is distributed over the cavity legth. VCSEL(Vertical Cavity Surface Emittig Laser) - If L, frequecy separatio betwee modes is icreased. This esures better modal purity tha a covetioal cleaved cavity laser. - I priciple, VCSEL requires a vertical cavity with top ad bottom mirrors which are quarter-wave Bragg reflectors realized by stackig may dielectric materials. - if L, the R is required to maitai α m. R (High reflectivity) meas that we eed HR coatig. High HR coatig is solved by multiple dielectric layers. (quarter wavelegth reflector) ( ) m 4 i = ==> R = 3 ; as m, R 4 ( 2 ) m 4 3 -HR coatig is accomplished by stackig layers. -VCSEL provides sigle mode without cleavig, easy to make(just stackig),cheap -However, so far, it is t possible to make 1.55um optical source but possible for.85 um optical source. cf) why 1.55 um laser is so importat?

5 More o cocept of lifetime ph e αz ph e αvt z t ph e αvt τ p = dt 1 ==> τ p = αv τ p t Threshold curret ad threshold carrier P o th Threshold populatio iversio I th P o = Lasig output power N ph I Simplified ad idealized descriptio of a semicoductor laser diode based o rate equatios. Ijected electro cocetratio ad coheret radiatio output power P o vs. diode curret I. - After reachig threshold curret, all excess carrier ijected is coverted ito photo. Thus carrier is clamped, ad optical power (light) is liearly icreases as icrease of curret.