Small coherent optical transceivers have been demanded to provide a high data rate density in next-generation metro area networks and data-center interconnects. Therefore, the mechanical sizes of optical modules and sub-assemblies have been shrinking, and accordingly, highly integrated optical modules have been demanded. We present two types of highly integrated optical modules for ≥400-Gb/s coherent optical links. Our wavelength-tunable laser module consists of an ultra-compact DBR/Ring laser chip and a PLC-based wavelength locker. The laser module covers the wavelength range of full C-band with a narrow linewidth of >100 kHz and a fiber output power of 17 dBm. Furthermore, we demonstrate a tiny transmitter-receiver optical sub-assembly that integrates all required optical components into a single gold-box of 26 × 14 × 4 mm in compliant with the Optical Internetworking Forum (OIF) specifications. The electrical-optical and optical-electrical bandwidths measure >40 GHz, which is wide enough to realize ≥400-Gb/s per lambda operations.