A novel measurement technique to estimate the RF beat-linewidth of free-running heterodyning system using a photonic discriminator

We propose a novel technique to our knowledge to estimate the beat spectrum linewidth of a free-running heterodyning scheme using an optical discriminator. Utilizing a dense wavelength division multiplexing (DWDM) filter as an optical discriminator, the phase modulation (PM) to intensity modulation (IM) conversion is achieved. Thus converted phase noise of the laser into RIN (relative intensity noise). We establish a relationship between the converted RIN and the beat spectrum linewidth. The proposed concept on beat spectrum linewidth estimation is demonstrated and compared with direct beat spectrum measurement. We show that our proposed technique estimates the beat linewidth more accurately for a free-running heterodyning scheme.