The paper reviewed the recent progress in transmission of 400Gb/s wavelength division multiplexed (WDM) channels for optical networks based on flexible grid arrangement. High speed optical technology has become essential for realizing greater network capacity and enabling network operators to meet the increasing bandwidth challenges from new generation broadband, video and cloud related services and applications. Research efforts on coherent transponders software reconfigurable are now focusing on reducing the implementation penalties through improved equalization and exploring higher level modulation formats and multiple sub-carriers/channels (m-QAM, OFDM), using super-channels to achieve Terabit transmission. By joint optimization of modulation spectral efficiency and FEC code rate, and by using adaptive optical networking technologies and optimized digital pulse shaping with advanced DSP, we demonstrated the best way to improve network efficiency and to increase the total transmission capacity.