The transfer function of the phase modulator is as follows:

3. Intensity modulator
The intensity modulator is also called Mach Zehnder modulator (MZM), and its basic structure is shown in the figure below

The transfer function is as follows:

4. IQ modulator

The IQ modulator can be seen as the combination of phase modulator and intensity modulator, widely used in QAM and other high-order modulation formats. The basic structure is shown in the following figure:

The transfer function is as follows: