INFORMATION TECHNOLOGY

Sub-wavelength gratings successfully achieve zero crosstalk between waveguides


Guide

Recently, the team of Professor Sangsik Kim, Department of Electrical Engineering and Korea Advanced Institute of Science and Technology, Texas Institute of Technology, successfully achieved zero crosstalk between waveguides by using anisotropic sub-wavelength gratings. This is of great significance for the large-scale integration of on-chip micro-nano optics.

Research background

With the development of artificial intelligence, quantum information and micro-nanophotonics, large-scale and integrable on-chip optical systems have attracted widespread attention because of their powerful computing power and multifunctional optical information processing. However, when waveguides become more integrated, the leakage patterns between waveguides create strong coupling, resulting in crosstalk. This makes the transmission of information severely distorted, which limits the integration of the optical system. The traditional method suppresses the skin depth of the evanescent wave by adding a sub-wavelength grating, thereby greatly suppressing crosstalk between waveguide modes. However, this method has its limitations, one is that it does not completely eliminate crosstalk, and the other is that it is only effective for TE mode in waveguides. In some specific applications, such as bioimaging, gas sensing, etc., TM mode waveguides play a very important role. Therefore, eliminating crosstalk between TM modes is also an urgent problem to be solved.

Innovative research

In this paper, sub-wavelength grating (SWG) metamaterials are innovatively used to achieve zero crosstalk. By introducing non-uniform perturbations in the optical waveguide, a leaky wave pattern can be created, which combines a standing wave field and a radiated wave, resulting in a weak electromagnetic coupling. Figure 1 shows four typical waveguide modes. Figure 2 numerically simulates the electric field distribution of TM0 modes for different types of waveguides. The local nature of blend mode is demonstrated.

Then, by engineering the medium perturbation in different directions, the coupling coefficients in each direction can cancel each other out, thus achieving complete zero crosstalk. As shown in Figure 3, the authors calculated the effective refractive index and effective coupling length of the subwavelength grating waveguide and found that by modulating the dissipation coefficients in all directions, crosstalk can be greatly suppressed in a specific wavelength range.

The author’s team also etched the waveguide model for experimental verification. The results show (see Figure 4) that crosstalk is suppressed by about 40 dB compared to conventional strip waveguides between the same leaky SWG waveguides that are closely arranged, corresponding to a coupling length of about 100 times.

The method presented in this paper overcomes the limitations of strong electromagnetic coupling in traditional methods and enables higher integration densities in photonic circuits. In addition, the sub-wavelength grating metamaterials used in this technology can be applied to other research fields, such as microwave engineering or antenna design.

Figure 1: Schematic diagram of a waveguide with a sub-wavelength grating. (Blue: Si, gray: SiO2). (a) A typical strip waveguide supporting a guide mode with an exponentially decaying eviction field. (b) Placing an infinite flat plate near the guide mode will cause leakage losses in the radiation pattern. (c) Infinitely large subwavelength grating arrays can replace flat plates, support leakage mode, but provide anisotropic field oscillations. (d) By truncating the subwavelength grating, the mode will be guided without any radiation loss. At the same time, oscillations of leakage mode are preserved in the anisotropic grating cladding. When coupled to other waveguides, this leakage can exhibit non-traditional anisotropic oscillations and can result in zero crosstalk.

Figure 2: Mode characteristics of banded, infinite sub-wavelength gratings, and finite sub-wavelength grating waveguides. Figures (a)-(c) are cross-sectional views of these three waveguides, respectively. (d)-(f) Mode profile for TM0 mode in each waveguide scheme. Re is plotted from top to bottom in each figure[Ey], Re[Ex]and Im[Ez]。 The profile realizes (d) for guidance mode, (e) for leakage mode, and (f) for mixed mode with oscillation mode.

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Figure 3: Zero crosstalk in TM0 mode coupled with subwavelength grating waveguides.

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Figure 4: Experimental demonstration of a coupled SWG waveguide with near-zero crosstalk in TM0 mode.

The article was recently published in Light: Science & Applications in the top international academic journal “Anisotropic leaky-like perturbation with subwavelength gratings enables zero crosstalk”. Md Faiyaz Kabir is the first author and Sangsik Kim is the corresponding author. (Source: LightScience Applications WeChat public account)

Related paper information:https://www.nature.com/articles/s41377‍-023-0‍1184-5

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