Microwave Downconversion by a Tunable Optoelectronic Oscillator Based on PS-FBG and Polarization-Multiplexed Dual loop

A novel photonic method for microwave downconversion by using a tunable optoelectronic oscillator (OEO) based on a phase-shifted fiber-Bragg grating (PS-FBG) and the polarization-multiplexed dual loop is proposed and investigated. By the virtue of the narrow notch bandwidth of the PS-FBG and the side-mode suppression of the optimized dual loop, the frequency tunable local oscillator (LO) signal with high spectral purity and low phase noise is generated by changing the frequency of the optical carrier from the laser source. The microwave downconversion is conducted directly by inputting the radio frequency (RF) signal into the OEO without any additional components. The experiment of the LO signal generation and the microwave downconversion is carried out. The frequency tunable LO signals from 2.5 to 7 GHz with the side-mode suppression ratio as high as 56 dB and the phase noise of -109 dBc/Hz at 10 kHz are generated successfully. By tuning the LO frequency generated by the proposed OEO, the RF signals with different frequencies from 3.5 to 8 GHz are converted to the same intermediate frequency band of 1 GHz. The spurious-free dynamic range of the downconversion system is measured to be 102.2 dB·Hz
^2/3
. The downconversion performance for RF carrying 16 quadrature amplitude modulation signal is also analyzed.