IEEE Photon. J. http://doi.org/cfxs (2017)

As carrier frequencies in microwave communication systems increase in order to enable higher rates of data transmission, detection and digitization of high-frequency signals with higher resolution and lower noise are required. Now, Yunxin Wang and co-workers from China have developed an all-optical microwave photonic frequency down-converter that offers tunable, continuous phase control between 0° and 360°. The system consists of a laser source, a dual-parallel Mach–Zehnder modulator (DPMZM), an optical bandpass filter (OBPF) and a photodetector (PD). The laser provides the optical carrier, and the radio-frequency (RF) and local oscillator (LO) signals are fed to the top and bottom sub-Mach–Zehnder modulators (MZMs), respectively. The two sub-MZMs are both biased at the minimum transmission point to implement the carrier-suppressed double-sideband modulation. The RF and LO sidebands are obtained by the OBPF and sent to a PD to produce an intermediate frequency (IF) signal. In this way, an RF signal can be down-converted to a phase-tunable IF signal. The phase of the IF signal can be linearly shifted from 0° to 360° by adjusting the bias voltage of the parent MZM in the DPMZM. The measured spurious-free dynamic range can reach values as high as 100.2 dB Hz2/3. The phase deviation and power ripple of the IF signal are less than 2° and 0.26 dB, respectively.