Tunable Microwave Photonics Filter Based On A Broadband Optical Source

In recent decades, microwave photonics signal processing using photonic technologies to transmit and process radio frequency (RF) signals has been in rapid development with unique advantages as compared to traditional microwave technologies. Microwave filter has the same filtering to ordinary microwave filter within an RF system or link, in addition, photonics technology has the advantages of low loss, high bandwidth, immunity to electromagnetic interference(EMI) etc, and also provides features such as fast tunability and reconfigurability.Firstly, in the approach of a broadband optical source (BOS) sliced by Mach-Zehnder interferometer (MZI), the Optisystem simulation and numerical analysis are applied and compared using the intensity and phase modulation:the performance of transmission response, tunability and filtering with intensity and phase modulation are theoretical analyzed and system simulated with some variables such as bandwidth of the source, length and dispersion slope of the fiber, the optical path difference of interferometer respectively. Secondly, given the above analysis, microwave photonic filter based on variable optical carrier time shift structure is demonstrated to improve tunability, the frequency response and tunability with intensity modulation and carrier suppression modulation are discussed, as well the simulation and experiment verification are applied using carrier suppression modulation and the limited frequency of MZI structure are loaded to compare the filtering effect. Finally, in order to specify the performance of transmission signal of the filter, the filter is applied as subcarrier multiplex (SCM) demultiplexer which modulated by base signal. The two obtained subcarriers signal, eye diagram and BER are simulation analyzed.The results show that the tunability of the filter is controlled through the time difference, in respect of frequency response, both MZI structure and variable optical carrier time shift structure can achieve single bandpass response with baseband resonance, as well owing to fiber dispersion, the MZI structure with intensity modulation has notch points at high frequency, but using phase modulation at both low and high frequency without baseband resonance, however the response of variable optical carrier time shift structure has an advantage of not affected by dispersion due to the response of frequency and dispersion shifting simultaneously when tuning. In respect of tunability, owing to frequency notch, the MZI structure with intensity modulation has a continuous tunable frequency window within1～12.2GHz in25km fiber transmission, but using phase modulation within3-17.2GHz, however the variable optical carrier time shift structure with smooth tunable within20GHz range which confirmed well by the experiment results. In respect of bandwidth and Q factor, the influence of third order fiber dispersion leading to broadening of the filter bandwidth with a BOS is serious, in order to achieve totally smooth tunability, the bandwidth of BOS in both structures should be control within critical value which is the major reason of limited frequency window and Q value. Thus the variable optical carrier time shift structure filter in terms of frequency response and tunability are improvement with smooth over a wider frequency window compared to the MZI structure.