Pre-compensation of nonlinear distortion of a silicon microring modulator in short reach systems
Silicon photonics has attracted significant attention due to its compatibility with CMOS technology and the potential for both discrete and integrated devices. As a critical component in silicon-based communication systems, silicon optical modulators have been widely investigated, including solitary Mach-Zehnder modulators (MZMs), IQ MZMs, and micro-ring modulators (MRMs). Compared with Si-MZM, Si-MRM has smaller chip size, lower power consumption, and enhanced modulation efficiency, and is potentially preferable for short reach transmission systems. This research focuses on the implementation of an MRM in a high-speed short reach communication system. One issue with Si-MRMs is the nonlinear electrical-to-optical modulation dynamics. This work investigates pre-compensation of this nonlinear distortion by back-calculation. An MRM model is firstly set up using Matlab. Through back-calculation of the model, an electrical drive signal is calculated based on a desired modulator output optical intensity. The back-calculated drive signal has a broad frequency spectrum which would be difficult to generate using a practical digital-to-analog converter (DAC). Truncation of the drive signal bandwidth shows that a DAC sampling rate working at twice the symbol rate produces relatively good results. The system bit-to-error ratio (BER) performance is investigated with and without fiber transmission for a 28 Gbaud PAM-4 signal. The systematic simulation includes drive signal generation, electrical to optical conversion by the MRM model, fiber transmission, optical to electrical conversion and receiver side digital signal processing. Since the pre-compensation technique only deals with the distortion in modulation, fiber dispersion during transmission becomes the main factor affecting the system BER performance. Thus a linear decision-directed least mean square (DD-LMS) equalizer and a Volterra nonlinear equalizer are implemented at the receiver to equalize the signal distortion induced by fiber dispersion. In order to verify the simulation results, experiment has been performed using an un-packaged MRM chip. The chip exhibits bit error free transmission for 28 Gb/s on-off keying (OOK) modulation, but signal distortion is induced during the modulation, which produces poor results for PAM-4 signal modulation. Thus a direct comparison between simulation and experimental results was not possible.