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ModBox VNA
–
User’s Manual
Ref. Ed. B
–
12/05/2020
33
Figure 18: Typical electrode and waveguide architecture of an MZM
B
D
RIFT
E
FFECTS
DC drift is the optical output power variation under the constant DC bias voltage
application. Fig.15 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 19: MZM output power vs driving voltage w and w/o DC voltage
The DC drift physical mechanism and its root causes are not yet very 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 fig.16. It was also reported that the
DC drift is more affected by the buffer layer. In the circuit model of fig.16, all layers,
including the LIN
b
O
3
substrate, the Ti: LiN
b
O
3
optical waveguide and the buffer
layer are represented in terms of resistances R and capacitances C, in both the
vertical and horizontal directions of the crystal.
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