A Simple and All-Optical Microwave Doppler Frequency Shift and Phase Measurement System Based on Sagnac Loop and I/Q Detection

Microwave measurement system, which acquires the microwave parameters including frequency, phase, and amplitude information, is one of the backbone parts in modern radio frequency (RF) systems. In this article, a simple and novel optical approach is proposed to perform broadband in-phase/quadrature (I/Q) downconverter and is applied in Doppler frequency shift (DFS) and phase measurement. In the proposed approach, a Mach-Zehnder modulator (MZM) is used to modulate both RF and local oscillator (LO) signals. A Sagnac loop structure and optical coupler are used to suppress the optical carrier. A dense wavelength division multiplexer (DWDM) and two photodiodes (PDs) are used to build I/Q channels. The proposed approach has an all-optical structure and no frequency-dependent electronic devices are needed. In the experimental demonstration, the I/Q downconverter achieves low phase imbalance (<0.9 degrees), low I/Q amplitude imbalance (<0.5 dB), and large dynamic range (106.5 dB . Hz(2/3)) over an ultrawide operating frequency. When DFS measurement is implemented using the I/Q downconverter, the microwave DFS is measured with a maximum measurement error of 8 Hz, and a clear direction can be obtained. When the system is applied for phase detection, the phase shift is successfully estimated with a measurement error of less than 7 degrees.