Sagnac Interferometer-Assisted Microwave Photonic Link With Improved Dynamic Range

Abstract

In this paper, we propose a simple method to improve the spurious-free dynamic range (SFDR) of a microwave photonic link (MPL). The proposed method is based on a Sagnac interferometer structure; signal output of the Sagnac interferometer consists of two kinds of optical components: the clockwise (CW) modulated optical signal and the counter clockwise (CCW) unmodulated optical carrier. An appropriate situation is then created to satisfy the phase and power ratio condition of the two counter propagating light waves. Eventually, in photodiode, part of the third-order intermodulation distortion (IMD3) component is generated by the CW light wave and another part is generated by the CW sidebands beating with the CCW optical carrier. Under appropriate circumstances, these two kinds of IMD3s have same amplitude but opposing phase; hence, the IMD3 can be cancelled, and an enhanced SFDR is achieved. An experiment is conducted, and results show that, as compared with a conventional MPL, the SFDR of the proposed link is improved by 10.3 dB, and for a 16-quadrature amplitude modulation (QAM) 10-MSym/s signal centered at 15 GHz, the error vector magnitude is reduced by 5.9%.