Óptica Moderna Fundamentos e aplicações - Fotonica.ifsc.usp.br ...
Óptica Moderna Fundamentos e aplicações - Fotonica.ifsc.usp.br ...
Óptica Moderna Fundamentos e aplicações - Fotonica.ifsc.usp.br ...
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Coerência 153<<strong>br</strong> />
τ0<<strong>br</strong> />
⎡τ<<strong>br</strong> />
0 ⎤<<strong>br</strong> />
g ( ω) = sinc⎢<<strong>br</strong> />
( ω − ω0<<strong>br</strong> />
)<<strong>br</strong> />
2π<<strong>br</strong> />
⎥⎦<<strong>br</strong> />
(7.20)<<strong>br</strong> />
⎣ 2<<strong>br</strong> />
2 1<<strong>br</strong> />
∫ ∞ +<<strong>br</strong> />
π<<strong>br</strong> />
−∞<<strong>br</strong> />
2<<strong>br</strong> />
A intensidade do feixe é I α E = E(<<strong>br</strong> />
t)<<strong>br</strong> />
dt.<<strong>br</strong> />
Entretanto, através do<<strong>br</strong> />
teorema de Parceval podemos relacionar<<strong>br</strong> />
2<<strong>br</strong> />
1<<strong>br</strong> />
2π<<strong>br</strong> />
∫<<strong>br</strong> />
∞<<strong>br</strong> />
−∞<<strong>br</strong> />
E(<<strong>br</strong> />
t)<<strong>br</strong> />
dt =<<strong>br</strong> />
E(<<strong>br</strong> />
t)<<strong>br</strong> />
S. C. Zilio <strong>Óptica</strong> <strong>Moderna</strong> – <strong>Fundamentos</strong> e Aplicações<<strong>br</strong> />
2<<strong>br</strong> />
∫<<strong>br</strong> />
∞<<strong>br</strong> />
−∞<<strong>br</strong> />
2<<strong>br</strong> />
2<<strong>br</strong> />
2<<strong>br</strong> />
e como:<<strong>br</strong> />
g(<<strong>br</strong> />
ω)<<strong>br</strong> />
g( ω)<<strong>br</strong> />
2<<strong>br</strong> />
dω<<strong>br</strong> />
(7.21)<<strong>br</strong> />
Vamos chamar g( ω)<<strong>br</strong> />
de G(ω), que é a função de distribuição espectral,<<strong>br</strong> />
ou seja, a energia do trem compreendida entre ω e ω +d ω . As duas<<strong>br</strong> />
funções g(ω) e G(ω) estão esboçadas na Fig. 7.6. G(ω ) é dado por:<<strong>br</strong> />
2 τ0<<strong>br</strong> />
G ( ω)<<strong>br</strong> />
= sinc2φ<<strong>br</strong> />
, onde<<strong>br</strong> />
τ0<<strong>br</strong> />
φ = ( ω − ω ) .<<strong>br</strong> />
4π2<<strong>br</strong> />
0<<strong>br</strong> />
2<<strong>br</strong> />
-2π<<strong>br</strong> />
π<<strong>br</strong> />
g(ω) ~ sincφ<<strong>br</strong> />
G(ω) ~ sinc 2 φ<<strong>br</strong> />
π 2π<<strong>br</strong> />
Fig. 7.6 - Composição espectral do campo elétrico, g(ω) e função de distribuição<<strong>br</strong> />
espectral, G(ω).<<strong>br</strong> />
2 τ<<strong>br</strong> />
Notando que 0<<strong>br</strong> />
G(<<strong>br</strong> />
ω0<<strong>br</strong> />
) = , podemos encontrar as freqüências que dão a<<strong>br</strong> />
4π2<<strong>br</strong> />
meia largura do pico central através de:<<strong>br</strong> />
G<<strong>br</strong> />
1<<strong>br</strong> />
2<<strong>br</strong> />
( ω ) = G(<<strong>br</strong> />
ω ) = G(<<strong>br</strong> />
ω )<<strong>br</strong> />
±<<strong>br</strong> />
0<<strong>br</strong> />
0<<strong>br</strong> />
sen<<strong>br</strong> />
⎡<<strong>br</strong> />
⎢<<strong>br</strong> />
⎣<<strong>br</strong> />
2<<strong>br</strong> />
τ0<<strong>br</strong> />
2<<strong>br</strong> />
⎡<<strong>br</strong> />
⎢<<strong>br</strong> />
⎣<<strong>br</strong> />
τ0<<strong>br</strong> />
2<<strong>br</strong> />
φ<<strong>br</strong> />
( ω − ω )<<strong>br</strong> />
( ) 2<<strong>br</strong> />
⎤<<strong>br</strong> />
ω − ω<<strong>br</strong> />
±<<strong>br</strong> />
±<<strong>br</strong> />
0<<strong>br</strong> />
⎥<<strong>br</strong> />
⎦<<strong>br</strong> />
0<<strong>br</strong> />
⎤<<strong>br</strong> />
⎥<<strong>br</strong> />
⎦<<strong>br</strong> />
(7.22)