Contents 1. Introduction
Within the last years, digital-to-analog converters (DAC) have turned out to be the main bottleneck for scaling the symbol rate of optical communication systems beyond 100GBd. While single DAC modules provide bandwidths up to 55GHz [1], higher bandwidths have been achieved by time interleaving of parallel DAC [2], by the use of electrical bandwidth doublers [3], or by digital bandwidth interleaving [4]. However, all of these fully-electronic concepts require broadband analog multiplexers or mixer circuits that are challenging to design. On the other hand, optical IQ modulators allow to generate optical waveforms that cover twice the bandwidth of the underlying DAC [5]. Balanced heterodyne downconversion of these signals in high-bandwidth photodetectors [6] with a local oscillator (LO) tone that is tuned to the edge of the optical spectrum [7], may open a door towards broadband photonic-electronic arbitrary waveform generation (AWG). However, the generated electrical signal sensitively depends on the phase relation between the LO tone and the optical signal at the input of the receiver. This phase is subject to random drift in the underlying optical setup, thereby rendering targeted waveform generation impossible. As a consequence, the demonstration in [7] was restricted to subcarrier transmission of quadrature amplitude modulation (QAM), for which LO phase drifts are automatically compensated by the digital phase tracking of the coherent receiver.
