Optoelectronic Oscillator Based Microwave Frequency Downconverter with Low Phase Noise and High Conversion Efficiency

This paper presents a microwave photonic structure that uses a parity time (PT) symmetric optoelectronic oscillator (OEO) to generate a high spectral purity local oscillator (LO) for radio frequency (RF) signal frequency down conversion. It is based on a dual-polarization Mach Zehnder modulator (DPol-MZM) in series with a 45° polarization beam splitter (PBS). The OEO is operated under the PT symmetric condition to enhance the gain difference between the oscillation mode and the adjacent modes so that the OEO has a large sidemode suppression ratio. The components used to construct the OEO loop are optimized to ensure the LO generated by the OEO loop has a low phase noise. The optical carrier is suppressed at the 45° PBS output enabling a high-power intermediate frequency (IF) signal to be generated at the system output. This overcomes the low conversion efficiency problem in the reported OEO based microwave frequency down converters. Experimental results are presented that show an LO with a low phase noise of -133.2 dBc/Hz at a 10 kHz frequency offset and a large sidemode suppression ratio of over 65 dB. This leads to a high-quality IF signal generated at the frequency downconverter output. Experimental results also demonstrate a high -3 dB conversion efficiency when down converting an RF signal with a frequency of 6 to 18 GHz into a 100 MHz IF signal.