Jamal,
The predistortion component applies a function to input signal. The allowed types are arcsin and polynomial. Polynomial is the general predistorion since using a taylor series most functions can be easily approximated. The arcsin is used because MZMs modulate an optical signal according to the sin of the voltage on both branches.If you predistort your electrical signal with an arcsin then the MZMs sin distortion will cancel it out and give a nice linear response.
This effect is hard to see with NRZ modulations but becomes more apparent if a Triangle pulse generator is used. This MZM distortion is also what causes the levels of different QAM symbols to have different separations in amplitude. The constellation diagram will not appear square for large input voltages.
Check the screenshot I attached.
Damian
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