With the rapid development of applications such as artificial intelligence, Internet of Things, cloud computing and data centers, the current optical fiber transmission network needs to adopt more advanced technologies to cope with the increasing transmission rate and bandwidth requirements [[1], [2], [3], [4], [5], [6]]. Various devices applied to optical network nodes, such as wavelength division multiplexer [7,8] and modulator [9,10], have also become the focus of research in optical communication systems. Since the traditional single-function optical devices have been difficult to meet the needs of transmission system performance, multifunctional integrated devices have become an inevitable trend in the future development of optical communication. In optical networks, many independent devices are needed to cascade and work together in order to realize a complex function. The integrated devices consisting of a modulator and a wavelength division multiplexer have received widespread attention from scholars from various countries due to their compact size and large capacity in the highly integrated optical communication systems. The traditional integration method of modulator and wavelength division multiplexer is to cascade each functional device through optical waveguide. For example, Zhang et al. designed an 8-channel integrated device through simulation [11], the wavelength division multiplexing (WDM) module of the device consists of two 1×4 Multiple Mode interferometers (MMI) and one 1×2 Mach Zehnder Interferometer (MZI). The modulation bandwidth of a modulator unit is 35.83 GHz, the modulation rate of the 8-channel integrated device can reach 800 Gb/s, and the channel crosstalk is −23dB. The integrated device can realize the modulation and WDM function of 8 channels, with large transmission capacity and high rate. However, the structure requires 11 waveguides to complete the cascading of the modulation module and WDM module. As a result, the insertion loss is as high as 2.26 dB, and the extinction ratio is only 7.54 dB. Such integrated devices that rely on waveguides to cascade each functional device are generally large in size, which not only reduces the reliability of the system, but also increases the cost. A more common method in recent years is to introduce a photonic crystal (PhCs) based devices into large-scale integrated optical communication systems [[12], [13], [14]]. Debnath et al. [15] designed a cascade structure of multiple modulators applied to WDM. Two-dimensional air-hole PhCs structure adopted, and photonic crystal resonator (PCR) used as the modulator. The size of single channel structure and the channel spacing are 100μm2 and 0.44 nm, respectively, but the insertion loss of the whole device was as high as 6 dB and the extinction ratio was only 3.5 dB. The integrated device for EO modulation and WDM based on PCR proposed by Liu et al. [16] is small in size and easy to integrate, but the channel interval is 20 nm. Pan et al. [17] proposed a PhCs integrated devices, which are composed of PCR-based modulation module and PCR-based wavelength splitters. Using one-dimensional PCR to achieve the function of modulation and WDM at operation wavelength 1570 nm and 1573.2 nm. The proposed device has the size of 53×10μm2 and the extinction ratio of 25.8 dB. The crosstalk and modulation bandwidth are 23.3 dB and 20 GHz, respectively, but the channel spacing is too wide (3.2 nm). Therefore, there are three main problems in the current designed integrated devices of modulation and WDM. First, the insertion loss is large. It is mainly caused by the coupling loss between the electro-optical (EO) modulator and the WDM. Second, the channel spacing is wide. Considering the coarse wavelength division multiplexing (CWDM) technology is widely used and it is difficult to carry the demand of large communication capacity in the limited operating frequency band. Third, the structure is complex. Using waveguide to cascade each functional unit not only increases the excess coupling loss, but also makes the integrated device take up a lot of space on chip.

We propose an eight-channel integrated device for EO modulation and DWDM based on PhCs. The designed integrated device is capable of simultaneously executing the modulation functions for eight channels and the dense wavelength division multiplexing (DWDM) with a channel spacing of 0.8 nm. The device is composed of eight PhCss Aubry-André-Harper (AAH) cavities forming the modulation module and eight reflective cavity-type filters constituting the dense wavelength division multiplexing (DWDM) module. Simulation results demonstrate that the overall dimensions of this eight-channel integrated device are approximately (114.81×16×0.22)μm3, indicating a compact structure. It can support eight-channel transmission with a channel spacing of 0.8 nm. The lowest insertion loss and maximum channel crosstalk of the modulator in the “on” state are 0.24 dB and −17.8 dB, respectively. In the “off” state, the extinction ratio is greater than or equal to 19.3 dB, and the modulation voltage is less than 0.98 V. This device exhibits characteristics such as low loss, small size, high capacity, and ease of integration, making it of significant practical value in data centers and high-capacity optical communication systems.