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 151<<strong>br</strong> />
A segunda somatória é nula pois as variações de fase são<<strong>br</strong> />
aleatórias e quando somamos exp{i Δ n+1}, os vários termos se cancelam.<<strong>br</strong> />
Assim sendo, substituímos a eq. (7.14) em (7.13) e obtemos:<<strong>br</strong> />
γ<<strong>br</strong> />
12<<strong>br</strong> />
Caso b)<<strong>br</strong> />
⎡ 1 ⎤ ⎛ τ ⎞<<strong>br</strong> />
exp ⎢<<strong>br</strong> />
0 ⎥ ⎜<<strong>br</strong> />
(7.15)<<strong>br</strong> />
n→∞<<strong>br</strong> />
⎣nτ<<strong>br</strong> />
0 ⎦ ⎝ τ0<<strong>br</strong> />
⎠<<strong>br</strong> />
() { } { } ⎟ τ = iωτ<<strong>br</strong> />
lim n(<<strong>br</strong> />
τ − τ)<<strong>br</strong> />
= exp iωτ<<strong>br</strong> />
⎜1−<<strong>br</strong> />
τ> τ0<<strong>br</strong> />
Agora, Δφ será sempre diferente de zero, pois em t e t +τ0 as fases<<strong>br</strong> />
são diferentes. Assim, temos um termo exp iΔ<<strong>br</strong> />
= e não teremos<<strong>br</strong> />
o termo não nulo em que Δφ = 0. Logo, para<<strong>br</strong> />
γ () τ = 0 .<<strong>br</strong> />
12<<strong>br</strong> />
[ ] 0<<strong>br</strong> />
S. C. Zilio <strong>Óptica</strong> <strong>Moderna</strong> – <strong>Fundamentos</strong> e Aplicações<<strong>br</strong> />
∑<<strong>br</strong> />
n 0<<strong>br</strong> />
∞<<strong>br</strong> />
=<<strong>br</strong> />
n + 1<<strong>br</strong> />
τ > τ teremos sempre<<strong>br</strong> />
Para utilizarmos a eq. (7.4), devemos tomar a parte real de γ12(τ),<<strong>br</strong> />
dada por:<<strong>br</strong> />
⎧ ⎛ τ ⎞<<strong>br</strong> />
⎪cos<<strong>br</strong> />
ωτ<<strong>br</strong> />
τ τ<<strong>br</strong> />
() ⎨ ⎜<<strong>br</strong> />
⎜1<<strong>br</strong> />
−<<strong>br</strong> />
⎟ para < 0<<strong>br</strong> />
Re γ12<<strong>br</strong> />
τ =<<strong>br</strong> />
⎝ τ<<strong>br</strong> />
(7.16)<<strong>br</strong> />
0 ⎠<<strong>br</strong> />
⎪⎩ 0<<strong>br</strong> />
para τ > τ<<strong>br</strong> />
Com este resultado, podemos fazer o gráfico de I(τ), mostrado na Fig. 7.4.<<strong>br</strong> />
= I = I<<strong>br</strong> />
2I<<strong>br</strong> />
1+<<strong>br</strong> />
cos ωτ 1−<<strong>br</strong> />
τ / τ<<strong>br</strong> />
Se I [ ( )( ) ]<<strong>br</strong> />
1 2 0, temos I(τ) = 0<<strong>br</strong> />
0 para τ < τ0 e 2I0<<strong>br</strong> />
para τ > τ .<<strong>br</strong> />
0<<strong>br</strong> />
( ) 2<<strong>br</strong> />
I + I<<strong>br</strong> />
1<<strong>br</strong> />
I + I<<strong>br</strong> />
2<<strong>br</strong> />
( ) 2<<strong>br</strong> />
I − I<<strong>br</strong> />
1<<strong>br</strong> />
1<<strong>br</strong> />
2<<strong>br</strong> />
2<<strong>br</strong> />
I(τ)<<strong>br</strong> />
Fig. 7.4 - Interferência entre dois feixes parcialmente coerentes.<<strong>br</strong> />
τ0<<strong>br</strong> />
0<<strong>br</strong> />
τ<<strong>br</strong> />
0