Insufficient Frequency Response and IQ Skew Compensations for Coherent Optical Transmitters Using a Low Bandwidth Photodetector

Driven by the fast-growing demand for higher bit rates, higher speed (400Gb/s or 800Gb/s) long-haul coherent optical transmissions with higher symbol rate and higher order modulation formats are highly desirable. However, insufficient frequency response and IQ skew of optical transmitters will seriously affect the performance of Polarization Multiplexing- Multiple Quadrature Amplitude Modulation (PM-MQAM) Nyquist coherent optical communication systems with high symbol rate and high-order modulation. In this paper, an algorithm scheme is proposed to quasi-dynamically compensate insufficient frequency response and IQ skew of coherent optical transmitters by detecting the beat frequency signal of time-alternating IQ two-tone training sequences using a low bandwidth PD built in Mach-Zehnder Modulator.