Wenwu Zhu

Sagnac Interferometer-Assisted Microwave Photonic Link With Improved Dynamic Range

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%.

Dynamic Range Improvement of a Microwave Photonic Link Based on Brillouin Processing

A novel method to improve the spurious-free dynamic range (SFDR) of a microwave photonic link based on stimulated Brillouin scattering (SBS) processing is proposed and experimentally demonstrated. In the proposed system, an optical single-sideband with suppressed carrier (SSB-SC) signal is generated using a dual-parallel Mach–Zehnder modulator (MZM) by biasing its two sub-MZMs at minimum transmission point and the parent MZM at quadrature transmission point. In order to generate SBS, the SSB-SC signal is amplified and injected into a length of dispersion-shifted fiber where the MPL’s double-sideband optical signals are transmitted. Due to the SBS effect, a phase shift is induced to MPL’s optical carrier band, and hence, the third-order intermodulation distortion (IMD3) components originated from two different sources can cancel each other. Experimental results show that the SFDR of the link is improved by 10.3 dB, and the IMD3 is reduced by more than 23 dB.

Dual-loop optoelectronic oscillator based on a compact balanced detection scheme

Abstract. A compact dual-loop optoelectronic oscillator (OEO) employing a dual-output Mach–Zehnder intensity modulator (DOMZM) and a balanced photodetector (BPD) is theoretically analyzed and experimentally demonstrated. The fundamental idea of the scheme is based on double loops formed by two complementary output ports of DOMZM and two input ports of BPD, which could be naturally combined without any additional optical coupler or polarization beam splitter devices. This simple structure makes it possible to enhance side-mode suppression ratio (SMSR) and reduce phase noise of the OEO. Compared with the traditional dual-loop OEO (SO-OEO) based on single-output Mach–Zehnder modulator (MZM), optical coupler, and BPD, the advantages of our proposed dual-loop OEO (DO-OEO) with DOMZM and BPD are presented. Experimental results show that a 16-GHz single-mode OEO is obtained with measured SMSR of 72 dB and phase noise of −133.2  dBc/Hz at 10-kHz frequency offset.

Tunable optoelectronic oscillator based on coupled double loops and stimulated brillouin scattering

We propose and demonstrate a tunable optoelectronic oscillator (TOEO) based on coupled double loops (CDLs) and stimulated Brillouin scattering (SBS). By the incorporation of CDLs and SBS, we not only improve the side-mode suppression ration (SMSR) of the SBS-based OEO, but also enhance the stability of the generated signals. Microwave signals with a tunable range from 2 GHz to 18 GHz are generated. The phase noise is lower than −90 dBc/Hz at 10 kHz frequency offset and the SMSR is superior to 60 dB when the oscillation frequency tuned from 2 GHz to 18 GHz. Furthermore, the power excursion is below 0.2 dB and the frequency excursion is less than 0.3 ppm at 10 GHz in lab condition within 30 minutes, which proved the stability of the generated signals.

Recent progress of research on microwave photonic signal transmission and processing

In this paper, we would like to review and present our latest progress on microwave photonic signal transmission and processing, including linearization of analog photonic link, generation of ultrapure radiofrequency signal and elimination of self-interference.

Performance improvement for an SBS-based optoelectronic oscillator

We propose and demonstrate a tunable optoelectronic oscillator (TOEO) based on Stimulated Brillouin scattering (SBS). By the joint of coupled double loops (CDLs), we not only increase the side-mode suppression ratio (SMSR) of the SBS-based TOEO, but also enhance the stability of the system.

Linearization of microwave photonic link based on nonlinearity of distributed feedback laser

Abstract. A microwave photonic link (MPL) with spurious-free dynamic range (SFDR) improvement utilizing the nonlinearity of a distributed feedback (DFB) laser is proposed and demonstrated. First, the relationship between the bias current and nonlinearity of a semiconductor DFB laser is experimentally studied. On this basis, the proposed linear optimization of MPL is realized by the combination of the external intensity Mach–Zehnder modulator (MZM) modulation MPL and the direct modulation MPL with the nonlinear operation of the DFB laser. In the external modulation MPL, the MZM is biased at the linear point to achieve the radio frequency (RF) signal transmission. In the direct modulation MPL, the third-order intermodulation (IMD3) components are generated for enhancing the SFDR of the external modulation MPL. When the center frequency of the input RF signal is 5 GHz and the two-tone signal interval is 10 kHz, the experimental results show that IMD3 of the system is effectively suppressed by 29.3 dB and the SFDR is increased by 7.7 dB.

Optical upconversion of single sideband signal using low-cost radio-over-fiber system

A novel optical single-sideband (OSSB) signal generation with simultaneous IF signal up conversion technique is proposed to overcome the fiber dispersion problem. With this up-conversion technique, a high frequency OSSB signal is generated by using two low bandwidth intensity modulators in combination with fiber gratings. The low frequency local oscillator (LO) signal is modulated by employing frequency doubling technique or frequency quadrupling technique respectively. The OSSB radio frequency (RF) signal generated by mixing the intermediate frequency (IF) signal and low frequency local oscillator (LO) signal, is transmitted over standard single-mode fiber successfully. The received signal error vector magnitude (EVM) is 5.8% rms and 13% rms.