realizarea fizică a dispozitivelor...
TRANSCRIPT
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Curs 7
2016/2017
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2C/1L Optoelectronică, structuri, tehnologii, circuite, OSTC
Minim 7 prezente curs + laborator
Curs - sl. Radu Damian◦ Joi 15-18, P5◦ E – 70% din nota 20% test la curs, saptamana 4-5?
◦ probleme + (?1 subiect teorie) + (2p prez. Curs) 2prez=0.5p
◦ toate materialele permise
Laborator – sl. Daniel Matasaru◦ Joi 8-14 par
◦ L – 15% din nota◦ C – 15% din nota
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Curs 6
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Bilantul puterilor
i
itot AA dBdB
dBdBmdBdBm MSAP rtote
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efecte succesive se adună liniar
dar pe fiecare fibra exista efecte simultane(pentru dispersie) care se adună pătratic
21 tot
2
1mod,
2
1,1 cr
1 2
1,1mod, , cr 2,2mod, , cr
2
2mod,
2
2,2 cr
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Atenuare in fibra Atenuare datorata conectorilor Atenuare datorata splice-urilor Atenuare datorata diferentelor de apertura
numerica◦ apare numai la trecerea de la un dispozitiv cu NA
mai mare la un dispozitiv cu NA mai mic◦ neglijabil intre 2 fibre monomod sudate
Atenuare datorata diferentelor de diametru◦ apare numai la trecerea de la un dispozitiv cu
diametru mai mare la un dispozitiv cu diametru mai mic
◦ bidirectional la fibre monomod sudate
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0 dBm = 1 mW
3 dBm = 2 mW5 dBm = 3 mW10 dBm = 10 mW20 dBm = 100 mW
-3 dBm = 0.5 mW-10 dBm = 100 W-30 dBm = 1 W-60 dBm = 1 nW
0 dB = 1
+ 0.1 dB = 1.023 (+2.3%)+ 3 dB = 2+ 5 dB = 3+ 10 dB = 10
-3 dB = 0.5-10 dB = 0.1-20 dB = 0.01-30 dB = 0.001
dB = 10 • log10 (P2 / P1) dBm = 10 • log10 (P / 1 mW)
[dBm] + [dB] = [dBm]
[dBm/Hz] + [dB] = [dBm/Hz]
[x] + [dB] = [x]
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in
out
P
PPierderi
in
out
P
P10log10dBPierderi
]lungime[km
B]Pierderi[ddB/kmAtenuare
dBmdBmdBPierderi inout PP
√
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Dioda electroluminescenta
Capitolul 7
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Caracteristica putere optica emisa functie de curentul direct prin LED este liniara la nivele mici ale curentului.
Nu exista curent de prag
La nivele foarte mari puterea optica se satureaza
Responzivitatea
Tipic r=50μW/mA
A
W
I
Pr o
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Cea mai simpla schema de control: un rezistor in serie cu LED◦ Atentie! Tensiunea directa poate varia
semnificativ (>>0.7V) si trebuiepreluata din catalog
mai ales la intensitate luminoasa mare
datorita materialelor diferite de realizarea LED-urilor
dependenta de lungimea de unda
mai mica la lungimi de unda mai mari
cd/mcd]mA[Fv IfI
R
VVI Fcc
F
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Variatii mici ale tensiunii (mai ales in jurultensiunii de deschidere) pot duce la variatiimari ale curentului
Se prefera de multe ori controlul in curent al LED-ului
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Control in tensiune◦ Schema electrică a emiţătorului în impuls
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Control in curent◦ Schema electrică a emiţătorului optic analogic
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Capitolul 8
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Avantaje◦ Putere optica ridicata (50mW functionare continua, 4W
functionare in impulsuri)◦ Precizie ridicata a controlului (impulsuri cu latimea de
ordinul fs - femptosecunde) – viteza mare de lucru◦ Spectru ingust, teoretic LASER ofera o singura linie
spectrala◦ Lumina coerenta si directiva (~80% poate fi cuplata in fibra)
Dezavantaje◦ Cost (dispozitiv si circuit de comanda: controlul puterii si al
temperaturii)◦ Durata de viata◦ Senzitivitate crescuta cu temperatura◦ Modulatie analogica dificila (de obicei cu dispozitive
externe)◦ Lungime de unda fixa
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LASER = Light Amplification by the Stimulated Emission of Radiation = Amplificarea Luminiiprin Emisie Stimulata
Un foton incident poate cauza prin absorbtietranzitia unui electron pe un nivel energetic superior
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Emisia spontana – electronul trece in stareaenergetica de echilibru emitand un foton
Trecerea se realizeaza prin recombinarea uneiperechi electron-gol
Directia si faza radiatiei emise sunt aleatoare
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Emisia stimulata – un foton incident cu energie corespunzatoare poate stimula emisiaunui al doilea foton fara a fi absorbit
Noul foton are aceeasi directie si faza cu fotonul incident, Lumina rezultata e coerenta
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Recombinarea unei perechi electron-gol necesitaconservarea impulsului
In Si si Ge aceasta conditie presupune aparitiaunui foton intermediar (tranzitie indirecta) a caruienergie se transforma in caldura
Se utilizeaza aliaje de Ga Al As sau In Ga As P
Spatierea atomilor in diferitele straturi trebuie safie egala (toleranta 0.1%) pentru a nu se introduce defecte mecanice la jonctiune◦ limitare a aliajelor utilizabile◦ aparitia defectelor creste ineficienta (recombinari neradiative)
scade durata de viata a dispozitivului
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eV
240.1μm;;
ggg
EE
hchE
h constanta lui Plank6.6261·10-34 Ws2
c viteza luminii in vid2.998·108 m/s
e sarcina electronului1.6·10-19 C
benzi energetice: λ0, Δλ
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Lungimi de unda mici (spectru vizibil –1000nm)◦ GaP (665nm), GaAsyP1-y
◦ GaAs (900nm), Ga1-xAlxAs (AlAs – 550nm)
Lungimi de unda mari (1000÷1700nm)◦ InxGa1-xAsyP1-y
◦ x,y concentratii relative in aliaj a materialelorcorespunzatoare
◦ x,y alese din considerente privind
lungimea de unda
spatierea atomilor
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Lungimi de unda mici◦ Ga1-xAlxAs
◦ substrat GaAs
◦ limitare pentru tranzitiedirecta, x<0.45
◦ Eg (in eV)
45.0,247.1424.1 xxEg
45.0,143.0125.09.1 2 xxxEg
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Lungimi de unda mari◦ InxGa1-xAsyP1-y
◦ Tipic substratul este InP
Spatierea atomilor (lattice spacing) corespunzatoare InP
◦ Eg (in eV)
◦ Exemplu: 1300nm se obtinecu y=0.611 si x=0.282,
In0.282Ga0.718As0.611P0.389
y
yx
031.01
4526.0
212.072.035.1 yyEg
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Inversiune de populatie◦ necesara deoarece electronii au capabilitatea de a
absorbi energie la aceeasi frecventa la care are loc emisia stimulata
◦ se defineste probabilistic: probabilitatea de emisiestimulata sa fie mai mare decat probabilitatea de absorbtie
Materialele capabile sa genereze inversiunede populatie au starea excitata metastabila
avec pnpn
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La un material cu 4 nivele energetice tranzitiaradianta a electronului (3) se termina intr-o stare instabila, starea de echilibru obtinandu-se prin emisia unui fonon
Inversiunea de populatie se obtine mult maiusor datoritaelectronilor din stareaintermediara
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Pentru ca emisia stimulata sa apara, fotoniiemisi trebuie sa ramana in contact cu materialul o perioada mai mare de timp – 2 oglinzi necesare
Pentru a permite extragerea radiatiei e necesar ca una din oglinzi sa fie partial reflectanta
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Pentru diodele laser utilizate in comunicatiireflectivitatea oglinzilor nu trebuie sa fie foarte mare
Interfata semiconductor aer ofera un coeficient de reflexie de ~6% dar poateajunge la 36% pentru lungimea de unda de operare (vezi lamela dielectrica)
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Pentru a realiza◦ coerenta radiatiei
◦ interferenta constructiva intre radiatiile incidente sireflectate de oglinzi,
distanta intre oglinzi trebuie sa fie un multiplu a jumatate din lungimea de unda
Pentru eficientizarea pomparii de energie din
exterior L=100÷200μm, k 400
nkL 0
2
1
fn
ckL
2
0
Ln
ckf
2
0
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Definirea directiilor in dioda LASER
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Ln
ckfk
2
0
Ln
cf
2
0
Ln
2
2
0
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Castigul diodei laser (eficacitatea aparitieiemisiei stimulate) depinde◦ de caracteristicile energetice ale materialului din
care e realizata dioda
◦ de energia pompata din exterior (curentul prindioda)
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Filtre spatiale in regiunea activa
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Pentru operarea in impulsuri, un salt de λ/4 ingusteaza suplimentar spectrul diodei laser
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Se utilizeaza suprafete reflective selective pentru filtrare optica
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Amorsarea emisiei stimulate necesitapomparea unei anumite cantitati de energie –curent de prag
A
W
I
Pr o
thII
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Curentul de prag variaza cu temperatura si cu timpul
Variatia tipica 1-2%/°C
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Dependenta de temperatura a curentului de prag este exponentiala
I0 e o constanta determinata la temperaturade referinta
0/0
TTth eII
Material Lungime de unda T0
InGaAsP 1300 nm 60÷70 K
InGaAsP 1500 nm 50÷70 K
GaAlAs 850 nm 110÷140 K
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Gain guided – 8÷20 linii spectrale (5÷8 nm)
Index guided – 1÷5 linii spectrale (1÷3 nm)
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Sursa lambertiana
◦ Eficienta cuplarii in fibra
Aproximatie Lambertiana pentru surse cu directivitate crescuta
cos)( 0 PP
mPP cos)( 0
2
2
ss
f
r
aNA
P
P
2
2
1NA
m
P
P
s
f
2
2
2
g
g
r
aNA
P
P
ss
f
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La alimentarea cu curent a diodei laser emisiaeste initial spontana, devenind stimulata dupaamorsarea acesteia
emisia spontanaeste un fenomenintrinsec aleator
Intarzierea estevariabila - jitter
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Frecventa de oscilatie depinde de indicele de refractie al materialului
Indicele de refractie depinde de concentratiade purtatori
Cand curentul este modulat in impuls apare o modulatie a frecventei luminii cu efectulcresterii latimii spectrale a diodei (un ordin de magnitudine)
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oscilatii de relaxare – x GHz
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Generate de schimbul de energie intre electronisi fotoni
Amorsarea emisiei stimulate duce la descrestereanumarului de electroni in starea excitata, ceea ceduce la micsorarea emisiei de fotoni
Acumularea din nou a electronilor in stareaexcitata duce din nou la cresterea puterii
f1 = 1÷4 GHz
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Cresterea vitezei si minimizarea erorilor date de oscilatiile de relaxare si variatiile timpuluide amorsare dioda este partial stinsa in timpul transmisiei unui nivel 0 logic
Raport de stingere
Raportul semnalzgomot scade cu (1-α)
Tipic ER = 10÷15dB
1
H
H
P
PER
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Pentru viteze mari se prefera utilizarea emisieicontinue si modulareaoptica a radiatiei
In LiNbO3 viteza luminiidepinde de campulelectric, ceea ce permiteintroducerea unui defazajegal π
Creste complexitateacircuitului de control
Tensiuni de 4÷6 Vnecesare
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Jonctiunea intre doua materiale conductoarediferite poate genera sau absorbi caldura in functie de sensul curentului
Tipic se utilizeaza doua regiunisemiconductoare puternic dopate (tipic teluritde bismut) conectate electric in serie iar termicin paralel
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Poate produce o diferenta maxima de temperatura de 70°C
Lucreaza la nivele mici de caldura disipata Devine cu atat mai ineficient cu cat fluxul
termic disipat e mai mare De 4 ori mai putin eficiente decat sistemele cu
compresie de vapori
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Control putere optica
Control temperatura
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Mode hopping – salt de mod (hole burning)
RIN – Relative Intensity Noise (generat de emisiaspontana)
Zgomot de faza (idem) – necesitatea modulatiei in amplitudine
Zgomot intercavitati (reflexiile din exterior in zonaactiva)
Drift – variatia parametrilor cu varsta si temperatura(in special distanta intre oglinzi)
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Heterojunctiuneingropata
Heterojunctiunemuchie (ridge)
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Concentrare verticala a purtatorilor◦ Electronii sunt atrasi din zona n in zona activa◦ O bariera energetica existenta intre zona activa si
zona n concentreaza electronii in zona activa◦ Situatie similara corespunzatoare golurilor◦ Purtatorii sunt concentrati in zona activa, crescand
eficienta
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Cand lumina e pastrata in cavitati mai micidecat lungimea de unda nu mai poate fimodelata prin unda, modelul devine cuantic
Daca inaltimea zonei active scade la 5-20 nm comportarea diodei laser se schimba◦ energia necesara pentru inversarea de populatie se
reduce, deci curentul de prag scade
◦ dimensiunea redusa a zonei active duce la scadereaputerii maxime
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multiple straturi subtiri suprapuse – Multiple Quantum Well
Avantaje◦ curent de prag redus
◦ stabilitate crescuta a frecventei la functionarea in impuls
◦ latime mica a liniilor spectrale
◦ zgomot redus
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1565 nm
RL +0.00 dBm5.0 dB/DIV
1545 nm
Emisie spontanăAmplificată (ASE)
Canale: 16Spaţiere: 0.8 nm
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Necesitate◦ In sistemele WDM exista necesitatea (in propuneri
pentru arhitecturi viitoare de retele) pentru reglajfoarte rapid al lungimii de unda pe un anume canal - zeci de ns
◦ In aceleasi sisteme intervine necesitatea rutarii prinlungime de unda - timp de reglaj necesar de ordinul secundelor)
◦ realizarea cererilor de date - timp de reglaj de ordinul sute de μs
◦ reglarea emitatorilor individuali in sistemele WDM lipsa necesitatii controlului strict la productia diodelor
degradarea lungimii de unda in timp
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Curentul trece prin zona activa ducand la amplificarea luminii
curentul ce parcurge zona corespunzatoarereflectorului Bragg modifica indicele de refractieal acestei zone deci lungimea de unda
zona centrala suplimentara permite reglaj fin suplimentar in jurul valorii impuse de reflectorulBragg
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Dezavantajul metodelor anterioare e dat de limita redusa a reglajului (~10nm)
Reflectorul Bragg esantionat (periodic) produce spectru de filtrare discret
Regland unul din reflectori se obtinerezonanta la suprapunerea celor douaspectre
Dezavantaj : reglajul e discret
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Oglinzile pot fi realizate din straturisuccesive din semiconductori cu indici de refractie diferiti – reflector Bragg
Prelucrarea laterala se rezuma la taierea materialului
Caracteristici puteri de ordinul 1mW lungimi de unda 850 si 980 nm radiatie de iesire circulara cu divergenta redusa Curenti de prag foarte mici (5mA) si putere
disipata redusa circuite de control speciale nu sunt necesare Banda de modulatie mare (2.4GHz) Stabilitate mare cu temperatura si durata de viata
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Caracteristici◦ VCSEL produce mai multe moduri transversale
insensibila la pierderile selective la mod din fibrele multimod (principala limitare in utilizarea diodelor laser in fibrele multimod)
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Dependenta de temperatura a curentului de prag este exponentiala
I0 e o constanta determinata la temperaturade referinta
0/0
TTth eII
Material Lungime de unda T0
InGaAsP 1300 nm 60÷70 K
InGaAsP 1500 nm 50÷70 K
GaAlAs 850 nm 110÷140 K
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Curentul de prag variaza cu temperatura si cu timpul
Variatia tipica 1-2%/°C
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Puterea scade in timp exponential
τm – timpul de viata
Diodele laser sunt supuse la conditii extreme de lucru◦ densitati de curent in zona activa 2000÷5000A/cm2
◦ densitati de putere optica: 105÷106 W/cm3
Diverse definitii ale timpului de viata faccomparatiile dificile
mtePtP
/0
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Cresterea curentului duce la scaderea duratei de viata
◦ n = 1.5÷2 (empiric)◦ dublarea curentului duce la scaderea de 3-4 ori a duratei de viata
Cresterea temperaturii duce la scaderea duratei de viata
◦ E = 0.3÷0.95eV (valoarea tipica in teste 0.7eV)◦ cresterea temperaturii cu 10 grade injumatateste durata de viata
kTE
m e/
~
n
m J
~
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Coerenta radiatiei emise◦ LED: tc ≈ 0.5ps, Lc ≈ 15μm
◦ LASER : tc ≈ 0.5ns, Lc ≈ 15cm
Stabilitatea frecventei◦ detectie necoerenta (modulatie in amplitudine)
◦ mai ales in sistemele multicanal
Timpul de raspuns
Viteza, interval de reglaj
20
cc tcL
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Amorsarea emisiei stimulate necesita pompareaunei anumite cantitati de energie – curent de prag
A
W
I
Pr o
thII
Apare saturare la nivelemari de curent
thII regim LED
regim LASER
ineficient!,
tho IIrP
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eficienta de conversie electro-optic (randament)
tipic, randamente sub 10% sunt intalnite
eficienta cuantica◦ interna
◦ externa
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