With the continuous development of scientific and technological information, people's demand for communication system capacity and speed is increasing, and wavelength division multiplexing technology plays an important role in improving system capacity. As a key component of wavelength division multiplexing systems, the demultiplexer has become a research hotspot in this field. At present, a large-bandwidth filter with a high rectangular degree is mainly used to realize the function of demultiplexing. High-bandwidth and ultra-high-bandwidth filter schemes are frequently proposed and implemented, involving photonic crystals, Mach-Zehnder interferometers, and waveguide Bragg gratings. Although traditional schemes based on photonic crystals and waveguide Bragg gratings can achieve large bandwidths, additional magneto-optical devices such as optical circulators need to be added to separate the reflected signals due to their working in the reflection mode, which undoubtedly increases the complexity of the system. In addition, the difficulty in the large-scale integration of magneto-optical materials with silicon-based devices also affects the application range of the above schemes. The Mach-Zehnder interferometer scheme is also difficult to integrate on a large scale due to its large structure size. Therefore, to realize a compact ultra-high-bandwidth filter, this paper proposes a filtering scheme based on a grating-assisted contra-directional coupler structure. The filter has the advantages of a high rectangular degree, an ultra-high bandwidth, and a low loss and can meet the needs of demultiplexing in coarse wavelength division multiplexing systems.

To reduce the use of magneto-optical devices such as optical circulators and improve integration, this paper uses a grating-assisted contra-directional coupler structure to realize the filter function. According to the coupled mode theory, the bandwidth of the contra-directional coupler is inversely proportional to the sum of the effective refractive indices of the two waveguides and proportional to the coupling coefficient. Therefore, it is necessary to reduce the effective refractive indices of the two waveguides and increase the coupling coefficient for enhancing the bandwidth of the filter. This paper first compares the effective refractive indices of sub-wavelength grating and conventional waveguide grating and finds that the former has a lower refractive index. Considering that a smaller waveguide width corresponds to a lower effective refractive index, the paper proposes to gradually change the input waveguide into a narrow waveguide through a taper so that the effective refractive index can be further reduced. When the width of the waveguide is reduced to a certain extent, the effective refractive index is lower than that of the current common scheme. Moreover, the narrow waveguide has a more dispersed electric field, which is beneficial to improve the coupling coefficient and achieve an ultra-high bandwidth. Therefore, in this paper, the narrow waveguide structure and the grating-assisted contra-directional coupler structure are used to address the low bandwidth, and the problem of a large structure size is also solved by continuing to optimize parameters.

The width of the proposed narrow waveguide is 180 nm; the widths of the conventional grating and the sub-wavelength grating are both 500 nm; the duty cycle is 0.5. The effective refractive index of the narrow waveguide in the wavelength range of 1530-1570 nm is 1.441-1.447, which is smaller than those of the conventional grating and the sub-wavelength grating (Fig. 2). To further verify that the filter can achieve higher bandwidth, the transmission matrix method is used to calculate its spectral characteristics. At this time, the 3 dB bandwidth at the drop end is 70 nm (Fig. 3), which is in line with the above theoretical derivation. The grating comb and the waveguide are apodized at the same time to further improve the side-lobe suppression ratio (Fig. 5). In such a case, the 3 dB bandwidth of the filter is 92.9 nm; the side-lobe suppression ratio is greater than 11.1 dB; the shape factor is 0.99; the insertion loss is less than 0.38 dB; the in-band ripple is less than 0.28 dB (Fig. 6). This paper also designs a filtering scheme with a more compact structure and a higher side-lobe suppression ratio. After re-optimization, the 3 dB bandwidth is 87 nm; the side-lobe suppression ratio is 25.6 dB; the device length is 96 μm; the insertion loss is 0.16 dB; the in-band ripple is 0.08 dB (Fig. 7).

This paper proposes an ultra-high-bandwidth filter based on a grating-assisted contra-directional coupler. Firstly, in light of the coupled mode theory, the expression of the filter bandwidth is deduced, and the factors influencing the bandwidth are analyzed. Secondly, the paper develops a method of changing one waveguide in the contra-directional coupler to a narrow waveguide. After design and optimization, an ultra-high-bandwidth silicon-based photonic filter is finally realized with a 3 dB bandwidth of 92.9 nm, a side-lobe suppression ratio of 11.1 dB, a shape factor of 0.99, an insertion loss of 0.38 dB, and an in-band ripple of 0.28 dB. In addition, this paper also designs a filtering scheme with a more compact structure and a higher side-lobe suppression ratio. After re-optimization, the 3 dB bandwidth is 87 nm; the side-lobe suppression ratio is 25.6 dB; the device length is 96 μm; the insertion loss is 0.16 dB; the in-band ripple is 0.08 dB. The two filter schemes proposed in this paper adopt the contra-directional coupling method. The filter bandwidth in scheme 1 is larger than the largest bandwidth of similar structures, and the scheme greatly reduces the size of the filter. Scheme 2 improves the side-lobe suppression ratio and reduces the length of the device, which is suitable for application scenarios that are more sensitive to side-lobe suppression ratio and size. In addition, this paper also focuses on analyzing the filter performance changes when there are process errors in the apodization coefficient and comb width of the grating. A method of increasing the design value of the gap G between the narrow waveguide and the grating and fine-tuning the grating period is proposed to reduce the influence of process errors on filter performance.