Wideband Microwave Phase Noise Analyzer Based on an All-Optical Microwave I/Q Mixer

Digital phase demodulation can greatly simplify the calibration of microwave phase noise analyzers (PNAs) based on frequency discriminators, and avoid the use of feedback loops, but the required microwave in-phase and quadrature (I/Q) mixer always has a small operational bandwidth, leading to a limited phase noise measurement frequency range. In this paper, a novel photonic microwave PNA based on digital phase demodulation is proposed and investigated, using an all-optical microwave I/Q mixer that consists of a Mach-Zehnder modulator, a polarization modulator and two polarizers. Different from conventional electrical I/Q mixers, which rely on electrical hybrids to introduce a 90-degree phase difference between the I and Q channels, the all-optical microwave I/Q mixer achieves the 90-degree phase difference by properly setting the polarizers, ensuring a broad operational bandwidth. The major problems associated with the use of the optical mixer in the PNA, i.e., DC interference and I/Q mismatch, are solved by tuning a variable delay-line together with post signal processing. In addition, to improve the phase noise measurement sensitivity, the time delay required in the frequency discriminator is implemented by a span of low-loss fiber. Experimental results show that the I/Q mixer can maintain a good performance within a frequency range from 5 to 40 GHz, and the proposed PNA has a high sensitivity (<;-100 dBc/Hz@1 kHz and <;-130 dBc/Hz@10 kHz).