Cover story: Li XF, Li JC, Ni X et al. Direct detection with an optimal transfer function: toward the electrical spectral efficiency of coherent homodyne detection. Opto-Electron Sci 4, 240020 (2025).

Complex-valued double-sideband (CV-DSB) direct detection (DD) receiver can reconstruct the optical field as a homodyne coherent receiver but without requiring a costly local oscillator laser. Recently, various CV-DSB DD schemes have been proposed. However, a fundamental issue to explore is: what is the optimal DD receiver structure to approach the electrical spectral efficiency of the coherent homodyne detection with the simplest design?

To answer the question, the authors start from the receiver structure of conventional phase-diversity coherent homodyne detection and derive an optimal DD receiver architecture comprising a coupler, two single-ended photodiodes, two analog-to-digital converters, and a frequency-selective phase shifter with an optimal transfer function. By considering the impact of both noise and signal-to-signal beat interference (SSBI) on system performance, the optimal transfer function exhibits an all-pass amplitude response accompanied by a phase response that introduces an adjustable phase shift on either the carrier or the information-bearing signal. In practice, the operators need to judiciously select an appropriate phase shift according to the operating conditions. At the receiver-side digital signal processing, the authors use a convolutional neural network to seamlessly achieve signal reconstruction and SSBI mitigation in a unified step, thus enabling optimal system performance.

The proposed DD receiver with an optimal transfer function may pave the way for new applications in large-capacity and cost-effective data center interconnections, metro networks, and mobile backhauls.