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Mode-Division Multiplexing for Silicon Photonic Network-on-Chip
Tool Used: OptoDesigner
Introduction
Since the clock frequency of the microprocessors are limited to the range of 4 - 5 GHz, modern microprocessors have migrated towards parallel computing with many-core processors (MCPs) and systems-on-chip (SoC). Optical interconnect is a potential solution to attain the large bandwidth on-chip communications needed in high performance computers. Wavelength-division multiplexing (WDM) is one of the most mature multiplexing technologies used in telecommunications and has been widely explored for on-chip optical interconnects. However, WDM has limitations in bandwidth density scalability and requires multiple precise wavelength sources, which are not currently available within the cost scenarios of a single-user computer. Mode-division multiplexing (MDM) is an emerging technology that scales the capacity of a single wavelength carrier by the number of modes in a multimode waveguide, and is attractive as a cost-effective means. In this application note we demonstrate an intra-chip MDM communications link employing advanced modulation formats with two waveguide modes based on IMEC ISIPP25G platform [1].
The Challenge
For this project IMEC ISIPP25G MPW [2] run was selected, since high speed moderators was a key building block for the circuit. The high-speed modulation for microring modulator was achieved by controlling the depletion width of the PN junction in the microring waveguide, and thus change the waveguide’s efractive index and resonant wavelength. To make the design compliant to the foundry design rule and get the first time right design in a time effective way, an professional PIC design tool is necessary.
The Solution
With a flexible and integrated environment focused on manufacturable designs, OptoDesigner [3] enables efficient, high-quality PIC device development. The Key capabilities include: Photonic chip and mask layout, advanced connectors and autorouting, simulation modules and Photonic design verification. OptoDesigner has the most comprehensive photonic foundry support in the industry, with more than 30 Process Design Kits (PDKs) available from foundries around the world for photonic processes such as silicon, silicon nitride, indium phosphide, polymers, and silica on glass. OptoDesigner has enabled more than 500 tapeouts (including commercial designs) over the last three years. The IMEC PDK is a plug-in library for OptoDesigner and supports the Multi Project Wafer runs provided by imec. In addition to the photonic elements from the standard OptoDesigner library, the PDK contains technology specific information like mask layer names, design rules, validated building blocks, die sizes and GDS file settings.
The Result
The schematic of the demonstration network reported in this paper using two microring modulators, two germanium photodetectors (Ge PDs) and an MDM link is shown in Fig. 1(a). In this optical link, a single continuous-wave (CW) laser acts as the light source. The laser is coupled to the waveguide via a shallow-etched focusing grating coupler. Two separate silicon microring modulators encode the electrical orthogonal frequency-division multiplexing (OFDM) signal onto different optical modes of the waveguide. The modulated signals in the single-mode access waveguides are converted to different high order modes in the multimode bus waveguide using cascaded asymmetric directional couplers (ADCs). In the ADC, the signal encoded on the fundamental mode of the access waveguide can be evanescently coupled to the higher order mode of the multimode bus waveguide.
Fig. 1. (a) MDM optical link, including integrated microring modulators, integrated germanium photodetectors and mode add-drop (de)multiplexers. (b) Microscope images of the 2-channel MDM on-chip communication link
The mask design in OptoDesigner is shown in Figure 2 (a) and the corresponding gds is shown in Figure 2(b). As we can see, the mask includes not only the circuit mentioned above but also individual structures for test purpose and circuits for other topics. We easily fitted all the structures into IMEC die and made the necessary modification. Since OptoDesigner is script driven, for-loops with parameter changes were used to create the identical test structures.
Fig.2. (a) Design shown in OptoDesigner with logical view. (b) Design in GDSII with the gds layers
The MDM monolithically integrated circuits were fabricated by IMEC. Microscope images of the 2-channel MDM on-chip communication link is shown on figure 1(b). There is good agreement between the circuit and fabricated chip.
Figure. 3(a) and (b) show the measured BERs at different data rate for the two mode channels, respectively. Using 20% coding overhead for hard-decision forward error correction (HD-FEC) [36], which has a BER threshold of 1.5 × 10−2, the TE0 channel can achieve a data rate of ∼128 Gb/s with >105 Gb/s net information rate and TE1 channel can achieve ∼120 Gb/s with ∼100 Gb/s net information rate. At 7% overhead HD-FEC, which has a BER threshold of 3.8 × 10−3, 100 Gb/s aggregate data rate could be achieved for both channels with 84 Gb/s net information rate.
Fig. 3. (a) (b) BER performance of OFDM signal modulation at different data rate for TE0 channel and TE1 channel, respectively. Insets are the constellation diagrams of the received 100 Gb/s line rate signal for each channel
Summary
With OptoDesigner plus IMEC PDK an intra-chip optical communication link by using a monolithically integrated MDM photonic integrated circuit was demonstrated. The proposed on-chip MDM system is promising for high density optical interconnects and cost effective optical networks on chip.
References:
[1] Xinru Wu et al, “Mode-Division Multiplexing for Silicon Photonic Network-on-Chip”, J. Lightw. Technol., vol. 35, no. 15, pp. 304–309, Aug. 2017.
[2] http://europractice-ic.com/mpw-prototyping/siphotonics/imec/access-contacts/
[3] https://www.synopsys.com/photonic-solutions/pic-design-suite.html