A Carrier Phase Recovery Loop for a 3.2-pJ/b, 24-Gb/s QPSK Coherent Optical Receiver

The increasing demand for data throughput in modern data centers has intensified the need for high-speed, energy-efficient optical interconnects. Although traditional intensity-modulation direct detection schemes have served short-reach links well, their scalability to higher data rates is limited. Coherent detection, which leverages the amplitude, phase, and polarization of optical signals, offers significantly higher spectral efficiency but at the cost of increased power consumption due to the reliance on complex digital signal processing (DSP). This article proposes a low-complexity, energy-efficient coherent optical receiver (RX) architecture based on analog signal processing (ASP) techniques, specifically tailored for short-reach data center interconnects (DCIs). A key innovation is the integration of a wide-bandwidth (BW), monolithic analog carrier phase recovery (CPR) loop designed for quadrature phase shift keying (QPSK) modulation. This on-chip CPR loop eliminates the need for external optical phase recovery feedback loops, enhances phase-tracking capability, and simplifies system integration. Fabricated in a 28-nm CMOS process, the QPSK RX operates error-free at 24 Gb/s, with a CPR loop BW of 10–100 MHz and a frequency tracking range of 600 MHz, while maintaining an energy efficiency of 3.2 pJ/bit. These results demonstrate the potential of analog-intensive coherent RX architectures for energy-constrained, short-reach optical links.