Filters are key devices in signal processing systems, and are usually used to select desired signals and suppress interference and noise based on their amplitude-frequency response. The all-pass filter (APF) is a special filter that has a constant amplitude response but a varied phase response in target frequency band. It has been used in phase shifter, dispersion compensation, time delay, ultra-fast all-optical clock recovery, and Hilbert transform. However, most of the APFs are obtained by ignoring the transmission loss. In fact, the waveguide loss always exists in passive waveguides and the amplitude response of the obtained filter is not constant.
To solve the problem of amplitude response deterioration caused by loss, Professor Zhang Xinliang and Associate Professor Yu Yuan from Wuhan National Laboratory for Optoelectronics (WNLO), Huazhong University of Science and Technology, came up with a new method called self-compensation of loss, which separated part of input signal to compensate the loss of signal to achieve all-pass filtering. Researchers compared the microring resonator (MRR) with self-compensation of loss and ideal all-pass MRR to calculate the accurate loss compensation condition. The input signal and the compensation signal satisfying the all-pass condition were input from the input and the add ports of a silicon-based add-drop MRR respectively. The two beams of light interfered with each other, and were finally output from the through port, achieving an optical APF. The experimental results show that the amplitude variation of the APF is approximately 0.3 dB from 1545 nm to 1555 nm. Based on the optical APF, researchers also realized a microwave photonic phase shifter adjusted from 0 to 1.8 π in the range of 0-40 GHz, a microwave photonic notch filter with a rejection ratio over 50 dB,and an adjustable time delay with a maximum time delay of approximately 200 ps.
Fig. 1. Schematic of the proposed APF
Fig. 2 MRR-based APF
Fig. 3 Experimental results
The work was published online in ACS Photonics under the title Optical APF Realized by Self-Compensation of Loss. Professor Zhang Xinliang and Associate Professor Yu Yuan are the corresponding authors.
Paper link: https://pubs.acs.org/doi/10.1021/acsphotonics.1c00745
Written by: Yu Yuan
Reviewed by: Dong Jianji, Gou Bingbing
