All-Opticla Frequency Down-Coversion For Radio-Over-System

In recent years, the ROF (Radio-over-Fiber) technology is considered to be the effective ways to provide broadband wireless access. Frequency down-conversion technology as a main method to solve the high performance requirements for the receive module of ROF system lead to the high cost, and it has become one of the hot spot. While how to improve the system spectrum efficiency has become a concerned problem in a multiple channel ROF system. In this paper, realizing frequency down-conversion of the ROF system uplink links using DP-MZM is proposed. While for the spectrum efficiency problem of wavelength-division-multiplexing ROF (WDM-ROF) system, a photonic frequency down-conversion approach for high-spectral-efficiency WDM-ROF uplinks is proposed by using an optical frequency comb and single-sideband (SSB) modulation. The research contents in detail are as follows:(1) We have analyzed the MZM external modulation principle and the different modulation mode influenced performance by fiber dispersion. The results show that the double sideband modulation (DSB) signal is affected by fiber dispersion most seriously, and the receiving signal periodicity disappeared after the photoelectric detection. While the optical carrier suppression (OCS) modulation signal is not influenced by fiber dispersion theoretically.(2) The scheme of the frequency down-conversion using DP-MZM is proposed, and the simulation for frequency down-conversion based on different modulation is established. A RF signal at40GHz is successfully down-converted to an IF signal at10GHz, and the bandwidth is reduced. By analyzing the signal eye diagram of different transmission distance of optical fiber length, the OCS modulation signal performance is best. The two arms of the modulator for DSB modulation and signal sideband (SSB) modulation signal are improved in the conclusion of OCS modulation. By adjusting the third bias of DP-MZM modulator can realize OCS modulation on the output port, its performance is superior to the above two kinds of modulation of frequency down-conversion.(3) A high-spectral-efficiency photonic frequency down-conversion approach for WDM-ROF uplinks is proposed. A two-channel experiment is established to verify the proposed approach of frequency down-conversion using optical frequency comb and SSB modulation. A15-GHz RF signal carrying lOMbit/s quadrature phase shift keying (QPSK) data is successfully down-converted to an IF signal at2-GHz. The eye diagrams and constellation diagrams are measured for uplinks with different transmission distances. In order to prove that this scheme can be applied to the higher transmission rate and frequency of the WDM-ROF system, a40-GHz RF signal carrying a2.5-Gbit/s data is successfully down-converted to a10-GHz IF signal using the software. And the simulation for the transmission the BER curve after150km is established. The simulated results show that this scheme can apply in high data rata and frequency in the condition of communication.