An Ultrahigh-Speed Low-Power DAC Using InP HBTs for Multi-Level Optical Transmission Systems

This paper presents an ultrahigh-speed low-power digital-to-analog converter (DAC) for multi-level optical transmission systems. To achieve both high-speed and low-power operation, we used a simple R-2R ladder-based current-steering architecture and devised a timing alignment technique. The 6-bit DAC test chip was fabricated with our InP HBT technology, which yields a peak f_t of 175 GHz and a peak f_max of 260 GHz. The measured differential and integral non-linearity (DNL and INL) are within +0.68/-0.16 LSB and +0.17/-0.68 LSB, respectively. The measured spurious-free dynamic range (SFDR) remains above 36 dB up to the Nyquist frequency at a sampling rate of 13.5 GS/s, which was the limit of our measurement setup. The expected linear ramp-wave outputs at a sampling rate of 27 GS/s are also obtained. The DAC chip was assembled into a metal package for use in an optical transmission experiment, where it provides a clear 4-level signal for 16-QAM optical transmission at a sampling rate of up to 28 GS/s. The DAC consumes only 0.95 W and has a better figure of merit (FOM) (0.53 pJ) than any other previously reported DAC with a sampling rate above 20 GS/s. Our DAC module can also be applied to beyond-100-Gb/s/ch multi-level optical transmission systems.

I. Introduction

Coherent optical data transmission schemes with multilevel modulation formats are now attracting a great deal of attention and are being investigated as to their suitability for the future cost-effective optical transport networks (OTNs). High-order multi-level formats, such as 16-level quadrature amplitude modulation (16-QAM) and 64-QAM, are especially promising techniques for constructing beyond-100-Gb/s/ch optical transmission systems [1]–[3]. Figure 1 illustrates an example of a transmitter for such systems. In the transmitter, high-speed digital-to-analog converters (DACs) are key components for generating optical QAM signals. The performance requirements for the DAC depend on the modulation format and include a sampling rate of several tens of GS/s, medium resolution, and low-power operation. For example, a sampling rate of around 28 GS/s is needed for beyond-100-Gb/s/ch optical transmission systems. Recently, high-speed DACs for such optical transmission systems have been reported [4]–[7]. We previously reported a 32-GS/s 6-bit DAC [8].