For optical communication, the dimensions that can carry information include: wavelength, polarization, amplitude, phase, etc. Therefore, wavelength division multiplexing (WDM) [1] and polarization multiplexing (PDM) [2] have been used to increase the transmission capacity successfully. However, the communication capacity of single-mode waveguides is limited by the Shannon limit, which cannot be infinitely improved. Recently, it has been getting closer and closer to its limit value [3]. Under the situation that the wavelength and polarization of light have been fully utilized, it is necessary to develop new dimensions to further improve the communication capacity. The mode division multiplexing (MDM) technology has attracted a lot of attentions. In mode division multiplexing system, wider optical waveguides which can support more modes are adopted. These modes are orthogonal to each other, and thus can transmit information as independent channels, thereby can increase the communication capacity. As the key component of MDM technology, the mode division multiplexer has attracted more and more attention, recently. Typically, silicon-based mode division multiplexers can be constructed by using multimode interference (MMI) Coupler [4], adiabatic coupler (AC) [5], Y-branch [6] and asymmetric directional coupler (ADC) [7]. Among them, the MMI based and AC based mode division multiplexers can achieve large operating bandwidth and relatively large fabrication tolerances. However, relative long coupling lengths are required for mode evolutions in these two schemes, thus leading to relatively large device size. Mode division multiplexers based on the Y-branch can achieve wideband, but high precision fabrications are needed. The ADC based mode division multiplexers can have compact size and easy to design, but it is highly wavelength dependent and can be greatly affected by the inevitable fabrication errors. Compared with the mode division multiplexer based on the two-waveguide directional coupler, the advantage of the mode division multiplexer based on the three-waveguide directional coupler is that with the aid of the central waveguide, the coupling strength among the waveguides can be enhanced and the coupling length can be shortened [8], therefore, the device can be made more compact.

The TM mode multiplexer is one of the most important components of the short-reach optical links [9]. Therefore, it is necessary to research and improve the TM mode multiplexer. Recently, some ADC-based TM mode division multiplexers have been reported [10], [11], [12]. A TM mode division multiplexer based on a 3D triple-waveguide directional coupler [10] and a TM mode division multiplexer using a triple plasmonic dielectric waveguide based directional coupler were proposed by Weifeng Jiang et al. in 2018 [11]. Then a TM mode division multiplexer based on a gold nano-cube array assisted directional coupler was proposed by Zarifkar et al. in 2020 [12]. The advantages of above three schemes are the compact coupling lengths of 5.5 um, 7.5 um and 1 um respectively. However, due to the dispersions, relative narrow working bandwidths of 40 nm, 50 nm and 110 nm were achieved, respectively. On the other hand, subwavelength grating (SWG) with period in the sub-wavelength range, whose dispersion characteristics can be tailored can be introduced to enlarge the working bandwidth.

As known, when the period of the grating reaches the sub-wavelength range, the diffraction effect of the grating is suppressed and the SWG can be equivalent to an anisotropic material with an equivalent refractive index. The equivalent refractive index of the SWG can be changed by varying the grating period and the duty cycle. Compared to the conventional silicon waveguides, SWG has a stronger evanescent field, resulting in higher coupling efficiency at the same gap size and coupling length. Until now, some SWG based mode-division multiplexers have been reported [13], [14], [15]. In 2018, González-Andrade et al. proposed a MMI-SWG based mode division multiplexer [13], whose insertion loss is 0.84 dB, crosstalk is less than −20 dB, and working bandwidth of 300 nm is obtained. However, relative long device length of up to 36 um is needed. In 2019, Luhua et al. proposed an AC-SWG based mode division multiplexer [14], the device has an insertion loss of 0.32 dB, crosstalk less than −18.5 dB, and broad working bandwidth of 740 nm. But the device length is up to 55 um. In 2017, Jafari et al. proposed an ADC-SWG based mode division multiplexer [15], which has a smaller device size of 25 um compared to the previous two schemes. Moreover, relative wide working bandwidth of 120 nm can be achieved when the crosstalk is less than −10 dB. Besides, another advantage of this ADC-SWG based scheme is its simple structure and small size, but there is also a problem of the large crosstalk.

In this paper, we propose a TM mode division multiplexer based on an asymmetric directional coupler constructed by three SWG waveguides to achieve mode multiplexing of the fundamental TM mode (TM0) and first order TM mode (TM1). Furthermore, a mode blocker is also added in the proposed structure to reduce the crosstalk. By using the three-dimensional finite difference time domain method (3D-FDTD), the proposed TM mode division multiplexer was designed and optimized. The simulation results show that low insertion loss of 0.7 dB and low crosstalk of −35.9 dB at 1550 nm can be obtained. Moreover, large working bandwidth of 270 nm can be achieved when the insertion loss and the crosstalk less are less than 3 dB and −15 dB.