This review explores recent advances in the convergence of optical and millimeter-wave (mmWave) technolo-gies to simplify BS designs in UDNs. State-of-the-art developments in mmWave front ends, including passive lens-based arrays and additive manufacturing, and innovations in optical front-haul. The following introduces a low-latency optical pass-gate circuit, opto-electronic converters, and an optical nonlinear device as three key components deemed essential to developing an opto-electronic integrated accelerator (Fig. This has become a hot alternative for the chip industry.