The phase modulator must have an input electrical signal（even a electrical null） to operate，Then when using the feedback electrical signal as the input electrical signal, the whole system does not work.As shown in Figure 1，the homodyne receiving system can work（the low pass rectangle filter and the electrical constellation visualizer 2 have images）,however, when the electrical signal from the low pass rectangle filter is input to the phase modulator 1 as in Figure 2, the electrical constellation visualizer 2 has notion，because the phase modulator 1 does not output light.How can I solve the problem? Reply me via email zxb291037695@163.com,Thank you.
Sincerely,
Xiaobing Zhang
