iXBlue MBC-AN-LAB, Руководство по эксплуатации

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MBC-AN-LAB 18 Version 2.0
Figure 7: typical electrode and waveguide architecture of a Mach-Zehnder modulator
B. Drift effects
DC drift
is the optical output power variation under the constant DC bias
voltage application. Figure 8 shows the output power of the modulator as a function
of the applied voltage. The broken line shows the output power as a function of
applied voltage when only AC voltage is applied (and no DC is applied, at t = 0) and
the solid line shows the same, after t = t1, when DC voltage is also applied in
addition to the previous AC signal voltage. The shift between these two curves,

V,
is the measure of the DC drift.
Figure 8: Output power of the modulator as the function of the driving voltage, with/without the DC
applied voltage.
The DC drift physical mechanism and its causes are not yet well understood. But
this can be attributed to the ions movement, including OH- ions, inside the Lithium
Niobate substrate and the buffer layer. It is influenced by the balance of the RC time
constants, in both horizontal and vertical directions in the equivalent circuit model,
as is displayed in Figure 9. It was also found that the DC drift is more affected by
the buffer layer. In the circuit model of Figure 8, all layers, including the LiNbO3
substrate, the Ti:LiNbO3 optical waveguide and the buffer layer, are represented in
terms of resistances R and capacitances C, in both vertical and horizontal directions
of the crystal.