Abstract
High-speed, low-power optical interconnects, such as intensity modulation direct detection (IMDD) optical links, are increasingly deployed in data centers to keep pace with the growing bandwidth requirements. High-sensitivity low-power optical receivers (RXs) are the key components that enable energy-efficient IMDD optical interconnects. This article presents a low-power nonreturn-to-zero (NRZ) optical RX using a combination of a limited-bandwidth trans-impedance amplifier (TIA) and duobinary sampling to improve RX sensitivity at high data rates. Duobinary sampling leverages the well-controlled TIA inter-symbol interference (ISI) to recover the transmitted data, making it much more hardware efficient than canceling the ISI using a decision feedback equalizer (DFE). The proposed optical RX employs a CMOS-based analog front-end (AFE) to achieve high linearity and excellent power efficiency. Fabricated in 65-nm CMOS process, the prototype RX achieves optical modulation amplitude (OMA) sensitivity of -11.6 dBm at 16 Gb/s with 0.7-pJ/bit efficiency.
Highlights
L OW-POWER, high-density, low-cost, and high-speed short-reach interconnects are needed in modern hyperscale data centers
It consists of a multi-stage SF-trans-impedance amplifier (TIA) (MS-Shunt feedback TIA (SF-TIA)) [5], a single-to-differential (S2D)-ended conversion stage, and a differential variable-gain amplifier (VGA), which form the analog front-end (AFE) followed by a quarter-rate duobinary sampling back end
It is important to note that the dc offset-compensation (DCOC) loop does not impact the AFE noise performance because it corrects the AFE offset at the VGA-TIAs inputs where the received signal is already amplified by the front-end TIA and the VGA-GM
Summary
L OW-POWER, high-density, low-cost, and high-speed short-reach interconnects are needed in modern hyperscale data centers. This tradeoff can be alleviated by using an equalizer at the output of a low-bandwidth TIA [4]–[7], in which noise is reduced by increasing RF , and an equalizer suppresses the ISI resulting from reduced TIA bandwidth Following this line of argument, Ahmed et al [5] achieved 3-dB sensitivity improvement by using TIA with a bandwidth of only 25% the data rate and a decision feedback equalizer (DFE) that incurs minimal noise penalty compared to a continuous linear equalizer (CTLE) [8]. DFE is susceptible to error propagation, especially when the first post-cursor ISI is large, which is the case with a low-bandwidth TIA [4] Given these drawbacks, this article presents a high-sensitivity low-power optical RX using duobinary sampling in conjunction with a low-bandwidth TIA.
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