Improving the frequency stability and phase noise of opto-electronic oscillators by harmonic feedback

Optical technology offers a new method for generating microwave signals with extremely low noise. For that purpose a long fiber is used in the feedback loop of the oscillator. However, by the application of a long fiber, a lot of resonances appear in the output spectrum impairing the stability of oscillation. This paper introduces two new approaches for improving the frequency stability and reducing the phase noise of opto-electronic oscillators. That goal is achieved in two different ways: one by the insertion of an additional third-harmonic optical loop and another by nonlinear electronic feedback techniques. This way the conversion between the fundamental and harmonic components in the nonlinear active element of the oscillator can be controlled and utilized both for improving the frequency stability and reducing the noise. The combination of these methods results in an optimum adjustment.