Systems and techniques of incorporating an optical resonator (121) in an optical part of a feedback loop (120) in opto-electronic oscillators (100). This optical resonator (121) provides a sufficiently long energy storage time and hence to produce an oscillation of a narrow linewidth and low phase noise. Certain mode matching conditions are required. For example, the mode spacing of the optical resonator (121) is equal to one mode spacing, or a multiplicity of the mode spacing, of an opto-electronic feedback loop that receives a modulated optical signal and to produce an electrical oscillating signal.