Abstract
In this work, a bidirectional grating coupler for perfectly vertical coupling is proposed. The coupling efficiency is enhanced using a silicon nitride (Si3N4) layer above a uniform grating. In the presence of Si3N4 layer, the back-reflected optical power into the fiber is diminished and coupling into the waveguide is increased. Genetic algorithm (GA) is used to optimize the grating and Si3N4 layer simultaneously. The optimal design obtained from GA shows that the average in-plane coupling efficiency is enhanced from about 57.5% (−2.5 dB) to 68.5% (−1.65 dB), meanwhile the average back-reflection in the C band is reduced from 17.6% (−7.5 dB) to 7.4% (−11.3 dB). With the help of a backside metal mirror, the average coupling efficiency and peak coupling efficiency are further increased to 87% (−0.6 dB) and 89.4% (−0.49 dB). The minimum feature size of the designed device is 266 nm, which makes our design easy to fabricate through 193 nm deep-UV lithography and lowers the fabrication cost. In addition, the coupler proposed here shows a wide-band character with a 1-dB bandwidth of 64 nm and 3-dB bandwidth of 96 nm. Such a grating coupler design can provide an efficient and cost-effective solution for vertical fiber-to-chip optical coupling of a Wavelength Division Multiplexing (WDM) application.
Highlights
Silicon photonics has enabled a wide range of applications ranging from light-based communication to interconnect to low-cost lab-on-a-chip systems [1]
We propose a new design of a bidirectional grating coupler with enhanced efficiency
As can be seen clearly, the device we propose is based on a uniform grating with two opposite in-plane transmission ports
Summary
Silicon photonics has enabled a wide range of applications ranging from light-based communication to interconnect to low-cost lab-on-a-chip systems [1]. Benefitting from the mature complementary metal-oxide-semiconductor (CMOS) technology, various photonic devices have been demonstrated base on low loss silicon-on-insulator (SOI) waveguide, showing great promise for electronic-photonic integrated circuits, high-density photonic integrated circuits (PICs), and three-dimensional (3D) photonic integration [2,3,4,5,6]. Since there is no mature silicon-based light source, a fiber coupler is still needed to couple off-chip light from a single mode fiber (SMF). Inverse tapers and grating couplers are the two most commonly used interfaces between a silicon PIC and SMF. High coupling efficiency (CE) and wide bandwidth can be achieved simultaneously using inverse tapers [8]. Very precise optical alignment is needed, which drastically increases the Materials 2020, 13, 2681; doi:10.3390/ma13122681 www.mdpi.com/journal/materials
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