InP DHBT Very High Speed Power-DACs for Spectrally Efficient Optical...

This paper reports on very high speed large swing digital-to-analog converter (DAC) suiting the generation of spectrally efficient optical transmission signals. 2-bit and 3-bit DACS with up to 4 Vpp swing, operating up to 50 GBaud, have been fabricated using our InP DHBT technology (FT ~280 GHz, FMAX ~270 GHz, BVCE0 ~5 V). These chips have been packaged and used to generate 16- and 64-QAM signals in optical transmission experiments. Introduction Optical fiber networks are under pressure to satisfy the continuously increasing demand for data transmission bandwidth. Thanks to coherent detection scheme, modulation formats offering higher spectral efficiency [1] than on-off keying are presently assessed, targeting beyond 100 Gbps per channel data rates. However for the implementation of these advanced modulation formats, such as M-ary Quadrature Amplitude Modulation (QAM), complex transmitters are needed. These are often based on 6bit DACs ([2]-[4]), followed by linear amplifier to get a signal large enough to drive the electro-optical modulator. Fastest reported DAC ([4]) operates at 34 GBaud. Using a power-DAC, which essentially combines both DAC and large swing output functions, allows getting better signal definition at very high symbol rates, up to 50 GBaud. Circuits Design: The proposed circuit architecture realizes the combination of two or three single-ended input data to create multilevel signal (i.e. DAC function) with a large output signal. The input data are optionally retimed, using DFFs, and an active combiner is used to build up the multi-level signal, achieving the weighted summation function, for the 3-bit power-DAC [5].