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

The increasing intra-datacenter traffic is pushing the demand for ultra-high-speed optical interconnect that maximizes both power efficiency and data rate per wavelength. Intensity modulation-direct detection (IM-DD) links are used in these short-reach applications because of their simplicity and low power consumption; however, increasing their data rates is becoming exceedingly difficult due to technology- and packaging-imposed constraints. Coherent links, traditionally used in long-reach applications, are gaining traction as an alternative to short-reach lM-DD links. Compared to lM-DD, coherent links can deliver $4\times$ spectral efficiency by utilizing three degrees of freedom of the optical signal (i.e., intensity, phase, and polarization states). Still, it comes at the expense of the receiver complexity needed to perform polarization demultiplexing, chromatic dispersion (CD) compensation, and carrier phase recovery (CPR). Such complex functions are usually implemented on dedicated DSP chips separate from the analog front-end, resulting in very high power consumption. Recently, analog-based implementations of polarization demultiplexing, CD compensation and CPR have been successfully demonstrated [1–4]. But the CPR in [1] suffers from limited phase tracking bandwidth (~100kHz) and requires high-quality tunable lasers with very narrow linewidth to avoid adding much phase noise, degrading phase recovery capabilities. While a wide CPR loop bandwidth (~1.1GHz) was achieved in [4] at the expense of high power consumption (75pJ/b). Moreover, the feedback signals are routed off-chip with external loop filters, making the sensitive control signal susceptible to external noise.