This chapter presents the development and experimental validation of an optic-electronic-optic (OEO) interferometer supporting flexible coherent add-drop operations. The system leverages coherent detection, real-time digital signal processing, and in-phase/quadrature (I/Q) electro-optic remodulation within one interferometer arm to enable programmable manipulation of the optical field. We demonstrate successful add-drop operation of a quadrature phase-shift keying (QPSK) signal and quantify system performance under various conditions. In addition to single-channel demonstrations, we present new experimental results using intradyne detection in a dense wavelength-division multiplexing (WDM) scenario, showing scalability to multi-carrier systems. Long-term operational stability is ensured by continuous phase tracking implemented in the digital signal processing (DSP) chain. These results highlight the potential of the OEO architecture for future coherent optical networks requiring dynamic channel control, integrated processing, and signal reconfigurability. Two generations of indium phosphide (InP)-based integrated OEO interferometer PICs are introduced to improve phase and polarization stability. As a preliminary step, a partially integrated OEO interferometer PIC is developed and assembled into a module. Finally, a fully integrated OEO interferometer PIC is presented, incorporating all optical components required for the OEO interferometer setup.