Biomedical Engineering Reference
In-Depth Information
+
O/P
O/P
(TE/TM)
-
Electrode
Waveguide
FIGURE 2.23
Polarization TE-TM modulator/converter.
2.3.4 Polarization Controllers
Another important device is the polarization controller (Figure 2.23) [47]. In
these devices, two interdigitized electrode fingers are formed over a single
waveguide. The electrodes affect the two components of the general ellipti-
cal polarization state (TE and TM) modes by modifying their relative ampli-
tudes and phases. The voltage in the interdigitized section of the electrode
changes the relative polarization amplitudes of the modes, while the volt-
age in the uniform portion acts to change the relative phases of the modes.
Effective coupling between these orthogonal modes may be achieved with
conversion efficiencies better than 99% [48], with an applied voltage of 13 V
and an electrode period of 7 μm. The phase matching condition is [49]
2
Λ
π
λ
π
(2.6)
2
(
N
N
)
=
TE
TM
where
Λ is the electrode period
N TE and N TM are the effective waveguide indices
2.3.5 Integrated Systems
Several other device types exist such as Bragg effect modulators (Figure 2.24),
frequency translators [50], and tunable wavelength selectors. These all oper-
ate in similar fashion to those devices already described. Although any of
these devices may be separately fabricated and then assembled into packages
via single-mode splices or connectors, this approach is simply not practical.
This is because the insertion losses and the system costs would be exces-
sive, especially for systems requiring a large number of components. These
systems can be made practical via the integration of several components
onto a single substrate. Examples of such integrated devices that have been
fabricated include: a coherent optical receiver (Figure 2.25) [51] and direc-
tional couplers (Figure 2.26) [52]. The only limit to integration arises from the
 
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