Biomedical Engineering Reference
In-Depth Information
3.80
3.75
3.70
3.65
3.60
3.55
3.50
3.45
0.75
0.78
0.81
0.84
0.87
0.90
3.40
3.35
3.30
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
Aluminum concentration
X
FIGURE 5.17
Al
x
Ga
1 −
x
As
n
(
X
) versus
X
and vacuum wavelength.
=
−α
3
n r E
41
n
for (011)propagation
(5.20)
2
where
r
41
is the electro-optic coefficient (for GaAs
r
41
= 1.1 × 10
−12
m V
−1
)
α is an overlap figure with a value between 0 and 1 depending on the over-
lap of the electric and optical fields
One way to get a larger field overlap is to grow highly conducting
n
+
layer
on top of the SI GaAs substrate before growing the buffer layer. This causes
the electric field to be perfectly vertical. It is apparent from the equations that
because of the small size of
r
41
a device structure which can sustain a high
electric field is a necessity if unduly long devices are to be avoided.
5.8 CouplingConsiderations
5.8.1 Fiber to Waveguide Coupling
The problem of coupling an optical fiber to a waveguide (or a waveguide
to a fiber) is critically important to the overall performance of integrated/
fiber-optic systems. The most straightforward way of coupling a fiber and a
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