Fast digital-to-analog converters (DAC) are indispensable components for modern signal processing systems. Bandwidth and effective number of bits (ENOB) are important metrics for the performance of DACs. At the same time, those parameters constitute a trade-off in the design of a DAC: The more broadband the DAC, the less typically the ENOB. This is due to clock signal jitter limitations as well as linearity limitations of utilized transistors . These fundamental, physical limitations motivate the search for new DAC concepts. To our opinion the most promising approach is presented by electronic-photonic DAC concepts and its integration by means of silicon photonics.
The goal of PONyDAC project is the investigation of electronic-photonic DACs based on optical time-interleaving and broadband optical pulse synthesis which can be implemented in modern silicon photonics technology through monolithic co-integration of photonic and electronic components on the same substrate. This novel approach has the potential to multiply today`s DAC bandwidths.
The functional principle is shown in figure 1. A Mach-Zehnder modulator (MZM) is fed optically by a continuous wave laser (CW) and driven electronically by a low noise radio frequency generator (RFG). By tuning both the amplitude and frequency of the drive signal as well as the MZM`s bias voltage one can generate precise, periodic Nyquist pulses with adjustable repetition rate and FWHM. In a following optical power splitter the Nyquist pulse train will be distributed into N arms and delayed in phase in respect to each other. MZMs located in those arms are driven by electronic DACs and modulate the light signals in the respective arms according to the digital input [ ]. The optical pulses are then combined by an interferometric structure with matching phase relation.
The concept of optical time-interleaving allows for a very high output signal bandwidth, which is a multiple of the bandwidth of state-of-the-art DACs. In the project an electronic-photonic DAC will be realized in modern silicon photonic technology, which targets for a DAC bandwidth of more than 100 GHz.
The PONyDAC project is funded by the Deutsche Forschungsgemeinschaft in context of the priority program „Electronic Photonic Integrated System for Ultrafast Signal Processing (SPP2111) “. Our project partner is the Institut für Hochfrequenztechnik TU Braunschweig under the direction of Prof. Dr. Thomas Schneider.
 M. Khafaji, J. C. Scheytt, et. al., "SFDR considerations for current steering high-speed digital to analog converters," 2012 IEEE Bipolar/BiCMOS Circuits and Technology Meeting (BCTM), Portland, OR, 2012
 M. A. Soto et al., “Optical sinc-shaped Nyquist pulses of exceptional quality,” Nat. Commun., vol. 4, no. May, pp. 1–11, 2013.
 L. Zimmermann et al., “BiCMOS Silicon Photonics Platform,” Opt. Fiber Communication Conference (OFC), San Diego, p. Th4E.5, 2015.
Research contact: Prof. Dr.-Ing. Christoph Scheytt