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A compact two-mode (de)multiplexer [(DE)MUX] based on symmetric Y-junction and multimode interference (MMI) waveguides was designed by 3D beam propagation method (BPM). The phase evolution in the structure was discussed in detail. Simulations show that the optical bandwidth is as large as 100 nm (1500 nm ~1600 nm). The two-mode (DE)MUX is very compact compared with the other kind of mode (DE)MUX. The length of the structure is only 48.8 μm. Simulation also shows the fabrication tolerance is as large as ± 75 nm.
© 2014 Optical Society of America
1. Introduction
Due to its high capacity and compatibility with CMOS technology, silicon photonics has been proposed as a promising platform to bring revolution to the on-chip communication [1,2]. There are several ways to explore the capacity of photonics for multiplexing schemes, such as wavelength-division multiplexing (WDM), and polarization division multiplexing (PDM). Complementarily, mode-division multiplexing (MDM) is introduced to improve the transmission capacity of optical on-chip networks further [3]. A mode (DE)MUX meeting the requirement of low cross talk, low insertion loss, broad bandwidth, small footprint and large fabrication tolerance is a key component for realizing the on-chip MDM. Previously, several mode multi/demultiplexers, such as asymmetrical directional couplers (ADCs), adiabatic couplers (ACs), asymmetrical Y-splitter, cascaded asymmetric Y-junctions (CAYJs) and multimode interference (MMI), have been proposed, but they are all not meet the requirement. For example, although the mode (DE)MUX based on ADCs is broadband, its fabrication tolerance is very tight and it requires accurate controlling of the coupling length and waveguide width [3–5]. The mode (DE)MUX based on ACs [6], asymmetrical Y splitter [7–10] and CAYJs [11] have large footprints, whereas it is harmful to the high density integration. The mode (DE)MUX based on MMI is proved to be compact and has very large operation optical bandwidth [12], however, the fabrication tolerance limited by the cascaded phase shifter is very tight. A compact mode (DE)MUX with large fabrication tolerance is required to realize more dense integration and practical application.
It is well known that both symmetric Y-junctions and MMI waveguides are commonly used in various photonics integrated circuits with the advantages such as compact size and high fabrication tolerance [13–16]. In this paper, we propose and design a compact two-mode (DE)MUX by combining a symmetric Y-junction and MMI waveguide for TE mode operation. The similar structure was also experimently demonstrated for polarization splitter and rotator with large fabrication tolerance [17]. The beam propagation method (BPM) is used to analyze the behavior of two-mode (DE)MUX. Compared with the conventional mode (DE)MUX, the designed device is compact and broadband. Especially, there is at least ± 75 nm fabrication tolerance. Numerical simulation shows the two-mode (DE)MUX has a demultiplexer crosstalk lower than −22 dB from 1500 nm to 1600 nm, and the crosstalk is reduced to less than −31dB over the whole C band (1530 nm-1560 nm).
2. Operation principle and structure design
The proposed two-mode (DE)MUX design composed of a symmetric Y-junction, 2 × 2 MMI waveguide and a tapered π/2 phase shifter between them, is schematically shown in Fig. 1.The narrow arms of the symmetric Y-junction (W2) support just the fundamental mode (FM) and the stem supports two modes. The blue parts denote the first mode and the red ones stand for the second-order mode. Y-junction is a 3dB splitter. For the second-order mode, the two outputs have a π phase difference. After the Y-junction, a 2 × 2 MMI waveguide based on paired interference mechanism is cascaded. The input and output waveguides are placed at ± Wmmi/6 (Wmmi is the width of the MMI region). The length of the MMI waveguide is equal to Lπ/2 (Lπ is the beat length of the MMI waveguide). Therefore, based on the paired interference, the output ports would be different when the two inputs have the equal amplitude but with π/2 (-π/2) and 3π/2 (π/2) phase difference. To achieve the required phase difference, a tapered phase shift is placed at one of the arms between the Y junction and MMI region. For this structural configuration, the FM launched into the stem of the Y-junction is coupled into the down output of the MMI region with a phase difference evolving from 0 (after the Y-junction) to π/2 (propagating through the phase shift). On the other hand, the second-order mode in the stem is coupled into upper output of the MMI region with phase difference evolution from π/2 (after the Y-junction) to 3π/2 (propagating through the phase shift).
Fig. 1 Schematic representation and operation principle of the two-mode (DE)MUX based on symmetric Y-junction and MMI waveguide
In this design, a SOI wafer with a silicon thickness of h = 220 nm is considered. The refractive indexs of SiO2 and Si are 1.444 and 3.467 respectively. The structure is covered by air with the refractive index of 1. TE mode at the operating wavelength of 1550 nm is choosed to examine the behavior of the two-mode MUX. The stem width of the Y-junction (W1) is set as 800nm, which supports two guided modes. The output port of the Y-junction is composed of two mirrored S-bends single mode waveguides. The width of S-bends (W2) is selected to be 500nm. The radius of the first half curve and the second half curve of the S-bend are 500μm and 200 μm respectively. Both the bending angles are 2.3°. One arm connecting the Y-junction and the MMI region is a straight waveguide with the width (W2) of 500 nm. To expand the mode size for reducing the coupling loss, tapered waveguides with a length (Ltaper) of 10μm are introduced at the end of the arm. The width W3 at the end of the mode-size expander is set as 1.2 μm. The other arm is the tapered phased shift. The widths at start and end are selected to be W2 = 500 nm and W3 = 1200 nm. Figure 2 shows the calculated phase difference between the two arms cascading with mode-size expander when changing the length of the phase shift. In the following simulation, only TE modes are considered. It can be seen that, the length of the phase shift (Lps) should be set to 12.3 μm to achieve the phase difference π/2. We choose Wmmi = 4.28 μm to prevent crosstalk between the input and output waveguides. Based on the simulations, the beat length of the MMI region is equal to 45μm. Therefore the length of the paired interference MMI coupler is set to be 22.5 μm. Figure 3(a) and Fig. 3(b) show the simulated field distribution of the two-mode MUX at an operating wavelength of 1550nm when the input is the fundamental mode and the second-order mode respectively. When the fundamental mode is injected, the optical power is routed into the fundamental mode of the left output. On the other hand, when the input is the second-order mode, it is split and converted into the fundamental mode by the symmetric Y-junction and then the optical power is routed into the fundamental mode of the right output. Therefore, the demultiplexer function is achieved when the input is the stem of the Y-junction.
Fig. 3 BPM simulated field distribution of the two-mode MUX at the operating wavelength 1550nm when the (a) fundamental mode and (b) second-order mode is launched.
Optical bandwidth is a very important parameter for mode (DE)MUX. Figure 4 show the wavelength dependence of the transmission spectra from the right port of the MMI region in Fig. 1 when the input is the fundamental mode and the second-order transverse electric (TE) mode of the stem of the Y-junction. It can be seen, within a bandwidth from 1500 nm to 1600 nm, the lowest demultiplexed crosstalk of the two modes is up to −48dB, while in the worst case it is −22dB. In the whole C band (1530 nm~1565 nm), the crosstalk is lower than −31 dB. Simulation shows that the insertion loss of the designed two-mode MUX is smaller than 0.3 dB within the bandwidth from 1500 nm to 1600 nm.
Fig. 4 Wavelength dependence of the designed two-mode MUX using as a “mode demultiplexer” when the input is the fundamental modes and the second-order mode respectively
Based on the above discussion, the core theory of the two-mode MUX is the different phase evolution of the two modes. The π/2 phase shift, based on the changes of the waveguide widths, plays a key role on the phase evolution. In this design, we set the width of the narrowest point of the phase shift to W2 = 500 nm. The length of the phase shift is chosen to be 12.3 μm. Figure 5 shows the output power of the devices as a function of W3 at the operating wavelength of 1550 nm. It can be seen that the lowest crosstalk is achieved at the width W3 of 1.2 μm. Moreover, when the width variation is as large as ± 100nm, the cross talk is lower than −27 dB. The fabrication tolerance of the length of the phase shift is also examined (as shown in Fig. 6). The crosstalk is lower than −30 dB when the length of the phase shift is changed within ± 75 nm.
Figure 7(a)- Fig. 7(c) show the simulated light propagation during the process of two modes multiplex and demultiplex in our designed two-mode (DE)MUX. It can be seen that light is efficiently coupled into the corresponding output and almost no power is coupled into the neighbor’s output when the input is the fundamental mode in one of the two arms of the MMI region. When we launch the fundamental modes into the two arms simultaneously, the two modes will be multiplexed in the stem of the symmetric junction and then be demultiplexed into the desired arms of the MMI region. The theoretical excess loss of the designed two-mode MUX in the whole process of the multiplexing and demultiplexing is as low as 0.3dB at the wavelength of 1550nm. It can be seen in the Fig. 5 and Fig. 6 that the power loss mainly comes from the fundamental mode of the stem of the Y-junction which can be further reduced by increasing the curve radius.
Fig. 7 the simulated field distribution of the designed two-mode (DE)MUX when the fundamental mode is launched into the access arms at the wavelength of 1550nm. (a) right input arm (b) left arm (c) both the input arms
3. Conclusions
A novel design for high-performance two-mode (de) multiplexer with low loss, large bandwidth, compact size and high fabrication tolerance is proposed. According to the phase evolution of the symmetric Y-junction and the MMI waveguide based on paired interference theory, the fundamental modes in the two arms excite the fundamental mode and the second-order mode in the stem respectively in the multiplexing case, and are adiabatically coupled out into the fundamental mode of the output arms in the demultiplexing case. The simulation shows that, within the common optical bandwidth from 1500 nm to 1600 nm, the crosstalk is lower than −22dB. The designed length of the two-mode MUX is as small as 48.8 μm (do not include the input and output arms) and at least ± 75 nm fabrication tolerance is achieved. The large optical bandwidth, compact size and high fabrication tolerance make the proposed two-mode (DE)MUX a promising candidate in the MDM system.
Acknowledgments
This work was supported by National Natural Science Foundation of China (Grant No. 61036003, 61176013, and 61177038), the National High Technology Research and Development Program of China (Grant No. 2011AA010302).
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