The Cisco 400G QSFP-DD Ultra Long-Haul Coherent Optics Module enables 400G traffic anywhere over dense wavelength division multiplexing amplified networks, and is available in both C-band and L-band. Cisco has expanded the range of 400G digital coherent QSFP-DD transceivers with the 400G QSFP-DD. Traditional fiber optic modules can hardly meet the demand of large bandwidth (over 100Gbps) and long distance (over 80km) transmission at the same time, but coherent fiber optic modules can perfectly fill this gap. Traditional solutions face limited bandwidth and restricted transmission distances. When 400G was introduced, the question was – how can we get it to 80km, taking into account the dispersion compensation and optical power. Developed by the Optical Internetworking Forum (OIF) and released in March 2020, 400ZR is profile-optimized for high-density acce s and point-to-point DCI applications. It can deliver 400 Gb/s up to 40 km over a single dark fib r span without external. Unlike conventional short-reach 400G Ethernet transceivers designed only for intra-data-center links, 400G DWDM coherent optics are engineered to transmit a full 400 Gigabit signal over tunable Dense Wavelength Division Multiplexing (DWDM) channels, allowing operators to carry massive bandwidth. Optech's 400 G QSFP‑DD digital coherent modules provide exactly that: a compact, high‑density form factor supporting up to 400 Gbps per wavelength, enabling both dense data‑center interconnect (DCI) links and regional long‑haul connectivity. Coherent Optics Module Overview — QDD‑400G‑ZR, ZR+.