Filters are fundamental components of signal processing systems. Unlike traditional amplitude manipulating filters，the all-pass filter (APF) changes signal phase information through its phase-frequency response characteristics. Except for a fixed insertion loss, the amplitude spectrum of signal cannot be changed by the APF. This unique characteristic can be potentially applied to many systems, such as microwave photonic (MWP) phase shifters, optical variable delay lines, and Hilbert transformers i.e. In the past, to the best of our knowledge, the exploration of APF is focused on theoretical research and no actual all-pass filter have been realized.
In August 2020, ACS Photonics published online the latest research progress of Associate professor Yu Yuan and Professor Zhang Xinliang from the Wuhan National Research Center for optoelectronic of Huazhong University of Science and Technology called "The Optical All-pass Filter in silicon-on-insulator." Researchers proposed a novel method of analyzing the all-pass filter response, and pointed out that the end of the vector forms a closed curve with the wavelength varies. This method can be potentially used to analyze the structures based on all-pass microring resonator intuitively and simpliy. On this basis, the researchs implemented the all-pass filter on a silicon-on-insulator consisting of Mach-Zehnder interferometer (MZI) and microring resonator (MRR) structures. The experimental results show that the amplitude variation of the all-pass filter are approximately 0.8 dB from 1545nm to 1555nm. The APF-based broadband MWP phase shifter exhibit that the microwave phase can be continuously adjusted from 0 to 1.84π, the operating bandwidth is 4-40 GHzandthe maximal RF power variation of is less than 1 dB.
Fig. 1. Schematic diagram of the proposed APF.
Figure2. Principle of the APF analyzed in the complex plane.
Figure 3. Experimental results.
Weijun Jiangis thefirst author of the paper, and associate professor Yuan Yu and professor Xinliang Zhang are the corresponding authors. The research was supported by the National Key Research and Development Program and the National Natural Science Foundation of China.
The article links：https://pubs.acs.org/doi/10.1021/acsphotonics.0c00848
