As the revolution of new-generation information technology continues to advance around the world, the data traffic of the Internet is growing exponentially, and the demand for network bandwidth in various fields is increasing. The optical communication system, the core of networks, should continuously improve the frequency spectrum efficiency and the transmission capacity to meet the demands of future development. Hence, it faces great challenges. Silicon-based photonic integration technology has been developing rapidly due to its advantages of high integration, low costs, large bandwidth, and compatibility with CMOS technology. It is widely applied in optical fiber communication, optical sensing, optical modulation, quantum communication, optical neural networks, and other fields. The tunable silicon photonic filter is one of the most important components and plays a vital role in the wavelength division multiplexing (WDM) system of optical fiber communication. Optical filters include the Mach-Zenhnder interferometer (MZI), microring resonator (MRR), Bragg-gratings, arrayed waveguide grating (AWG), echelle diffraction grating (EDG), and MRR-assisted MZI (MRR-MZI). However, these filters have limitations in the size of devices, power consumption, flexibility, and adjustable range. Therefore, a tunable filter based on SOI material is proposed. The bandwidth and center wavelength of the filter can be tuned by the cascading of two filter units of the double-ring-assisted MZI, which guarantees that the filter output has a good shape factor and a flat passband.

The tunable filter of the cascaded double-ring-assisted MZI based on SOI material proposed in this paper is composed of two cascaded MRR-MZI filters. The spectral response of the tunable filter is the intersection of the transmission spectrum response of two double-ring-assisted MZI filters, and its bandwidth is determined by the overlapping region of the transmission spectrum response of the two units. Therefore, on the premise that the bandwidth and center wavelength of the filter can be adjusted, the output with a good shape factor and a flat passband is guaranteed. In design, the transmission function of the filter is calculated by the transmission matrix method, and the mathematical model of the filter structure is built by MATLAB. The appropriate range of the self-coupling coefficient between microring and MZI arm can be found when the stopband extinction ratios (SER), passband loss (PL), shape factor, and bandwidth tuning range of the filter output are used as the performance criteria. At the same time, MATLAB is employed to analyze the phase of the microring and obtain the appropriate phase to ensure that the output performance of the filter meets the design requirements. Considering the influence of loss and dispersion on the performance of the filter, we leverage the optical simulation software RSoft and the finite-difference time-domain (FDTD) method to simulate the performance of the component. The structural parameters of the microring are determined according to the self-coupling coefficient obtained by mathematical analysis. The effective refractive index of the microring is changed to simulate the phase control of the microring and achieve the bandwidth and center wavelength tuning of the filter, which is realized by changes in the temperature of the microring.

The bandwidth of the cascaded double-ring-assisted MZI filter proposed in this paper can be tuned by simultaneous changes in the phase of the microring in the second unit filter (Fig. 11). As the effective refractive index of the microring increases, the 3 dB bandwidth of the filter changes from 5 nm to 1.5 nm, and the passband loss is between 0.73 dB and 0.88 dB, which meets the needs of optical fiber communication. The center wavelength of the filter can be tuned from 1548.05 nm to 1559.6 nm (Fig. 12) when the phase of the four microrings of the cascaded double-ring-assisted MZI filter is simultaneously changed, and the phase change in the big ring keeps twice as much as that in the small ring. During the center wavelength tuning, the shape factor changes from 0.78 to 0.60, and the 3 dB bandwidth changes from 5.0 nm to 4.6 nm, both of which remain unchanged.

We propose a tunable filter with a large bandwidth tuning range of the cascaded double-ring-assisted MZI filter. The system transmission function of the filter is derived by the transmission matrix method, and the theoretical analysis and performance simulation optimization are carried out. The simulation results show that the SER of the filter is greater than 20 dB, and the PL is less than 1 dB; the size of the device is 60 μm×87 μm. When the thermal-optical effect changes the refractive index of the microring resonator, the center wavelength can move in the whole free spectral range, and the output bandwidth can be tuned between 1.5 nm and 5 nm, which meets the requirements of different wavelength signal screening. The filter has the advantages of small size, wide bandwidth adjustment range, and low loss and can be widely applied in optical switching and optical signal processing.