Microwave photonic frequency up-converter with LO doubling based on carrier suppression single-sideband modulation

Abstract

Frequency up-converter as an essential component of the transmitter, which is used to implement the frequency up-conversion by mixing a low-frequency intermediate frequency (IF) signal with a local oscillator (LO) signal. However, only the 1st-order sideband of the LO signal and the IF signal are used in the tradtioanal microwave photonic up-converser, thus the frequency of the up-conversion signal is <i>ω</i><sub>LO</sub> + <i>ω</i><sub>IF</sub>. In this case, an LO with a higher frequency is needed for generating a high-frequency up-converted signal. In order to reduce the frequency requirement of the LO signal, the high-order LO singals or secondary modulation can be used to achieve high-frequency up-conversion. A microwave photonic up-converter with LO doubling based on carrier suppressing single-sideband modulation is proposed based on the cascaded structure of a Mach-Zehnder modulator (MZM) and a dual-parallel Mach-Zehnder modulator (DPMZM). The MZM is driven by an LO signal biased at the minimum transmission point for carrier suppressing double-sideband (CS-DSB) modulation. A fiber Bragg grating (FBG) is used to separate the +1st-order from －1st-order of the LO signal. The －1st-order of LO signal is then sent to a DPMZM for the secondary modulation, and the carrier suppressing single-sideband (CS-SSB) modulation is realized in order to generate the －1st-order of the IF signal by using an electrical 90° hybrid coupler. The modulated IF signal is then combined with the +1st-order LO signal reflected by the FBG and sent into the photodetector (PD) to implement the photoelectric detection. The upconverted signal with a frequency of 2<i>ω</i><sub>LO</sub> + <i>ω</i><sub>IF</sub> can be detected by a PD. The experimental results show that the spur suppression ratio of the optical spectrum and the up-converter signal reach 22.5 dB and 23.6 dB, respectively. The spurious-free dynamic range of the system is 96.1 dB·Hz<sup>2/3</sup>. The proposed system can effectively reduce the frequency requirement of LO signal, and the purity of the electrical spectrum is largely improved which benefits from the CS-SSB modulation. The proposed microwave photonic up-converter provides an effective way for high-frequency emissions in systems such as radio-over-fiber and optically controlled phased array radar.