CUHK announces breakthrough in photonic integration published in Nature Communications

Researchers in Department of Electronic Engineering announced a breakthrough on how to use light instead of electrons to convey large rates of data in advanced optical chips. In their paper titled “High-dimensional communication on etchless lithium niobate platform with photonic bound states in the continuum” published in Nature Communications on 25 May 2020, they explained how they made use of an old concept, the so-called bound states in the continuum (BICs), first proposed by John von Neumann and Eugene Wigner in 1929 during the early historical development of quantum theory, to confine light without necessarily having the high-refractive-index channel that is needed for the conventional total-internal-reflection-based waveguiding in optical fibers and photonic integrated circuits.

Applying BICs in photonic integrated circuits enables low-loss light guidance and routing in easy-to-fabricate, low-refractive-index channels on high-refractive-index substrates. Professor Xiankai Sun explained that the BIC concept makes it unnecessary to invent new high-refractive-index polymers to form waveguide channels on the high-refractive-index substrate nor etch the substrate in order to guide light in channels in the substrate. Optical interconnections with ultrahigh data capacity are needed in high-performance computers and data centers. To further enhance the data transmission capacity, optical multiplexing technologies are used to transmit multiple channels of data in parallel. By making use of carefully engineered high-order BICs on a planar lithium niobate substrate, the researchers demonstrated the viability of the BIC concept for use in high-capacity optical communication links by using four different spatial modes for mode-division multiplexing.

Figure 1a shows an optical microscope image of the fabricated mode (de)multiplexer. Figures 1b–1e show the measured normalized spectra of light transmission from each of the four input ports to each of the four output ports. All the channels can operate in the wavelength range of 1.51–1.58 μm, and can thus be combined with conventional wavelength-division multiplexing. Figure 1f shows the experimental setup for measuring high-dimensional data transmission through the fabricated mode (de)multiplexer. Figure 1g shows the eye diagrams of data transmission through the four channels at 40 Gb/s per channel, indicating an aggregate data rate of 160 Gb/s per wavelength for the mode (de)multiplexer.

Figure 2a shows an optical microscope image of the fabricated mode (de)multiplexer integrated with on-chip electro-optic modulators. Light sent into the four input channels was first modulated by a microcavity electro-optic modulator in each channel, then passed through the mode (de)multiplexer before being directed to the corresponding output channels. Figure 2b shows the experimental setup for measuring the devices with electro-optic modulation and mode (de)multiplexing on the same chip. Figure 2c shows the measured signals in the four channels, demonstrating that the fabricated devices are capable of both electro-optic modulation and mode (de)multiplexing on a single chip.

Images 

Fig. 1. Experimental demonstration of mode (de)multiplexing and high-dimensional data transmission with BICs.

Fig. 2. Experimental demonstration of on-chip electro-optic modulation and mode (de)multiplexing.

Relevant information

1. Zejie Yu, Yeyu Tong, Hon Ki Tsang, and Xiankai Sun, “High-dimensional communication on etchless lithium niobate platform with photonic bound states in the continuum,” Nature Communications 11, 2602 (2020). https://doi.org/10.1038/s41467-020-15358-x
2. Nature Communications (https://www.nature.com/ncomms/) is an open access journal that publishes high-quality research from all areas of the natural sciences. Papers published by the journal represent important advances of significance to specialists within each field.
3. Prof. Xiankai Sun’s Photonic and Optomechanical Nanodevice Laboratory. http://www.ee.cuhk.edu.hk/~xksun/