In optical communications, amplitude modulation is widely used to transmit data and the most common equipment for that is Mach-Zhender modulators. Despite being widely used, has certain obstacles. The transfer function is an sinusoidal curve that relates input voltage to output optical power. Therefore, when using it is necessary to connect a bias voltage that makes it operate in the most linear regime possible. However, as the time passes by, the bias voltage required for linear operation shifts. This project will analyze a method of autonomously correcting the bias error. The method will be performed by modulating a low frequency tone so as not to disturb communication and serve as an indicator of drift. Basically, the system monitors the modulator operation from the modulated signal, comparing between the magnitude of the low frequency tone and it s second harmonic. Therefore, spectral analysis algorithms were studied to detect the influence of the frequencies of interest and concluding performing a PID control with the ratio of their magnitudes.