The crystal cut affects both modulator efficiency, denoted as driving voltage and modulator chirp.
In the case of the Z-cut structure, as a hot eletrode is placed on top of the waveguide, RF field flux is more concentrated, and this results in the improvement of overlap between RF and optical field. However overlap between RF in ground electrode and waveguide is reduced in the Z-cut structure so that overall improvement of driving voltage for Z-cut structure compared to X-cut is approximately 20%.
The different overlap between the two waveguides for the Z-cut structure results in a chirp parameter of 0.7 whereas X-cut has almost zero-chirp due to its symmetric structure.
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