INFORMATION TECHNOLOGY

Multi-wavelength multiplexing: ultra-compact thin-film lithium niobate emitting chip

With the rapid growth of data in artificial intelligence, 5G systems, cloud computing, and the Internet of Things, data communication urgently needs to have large-capacity transmitters, the key of which is ultrafast optical modulation and multi-channel multiplexing.

In recent years, thin-film lithium Niobate electro-optic modulators have attracted more and more attention with their advantages of large modulation bandwidth, high linearity and low transmission loss, and the use of multi-dimensional multiplexing technology (space division multiplexing, wavelength division multiplexing, modulo division multiplexing, etc.) to build multi-channel transmitters (Transmitters) can further significantly improve the system capacity. Therefore, the challenges of monolithically integrating high-speed modulators with high-performance multiplexed devices to achieve large-capacity optical reflection chips on thin-film lithium niobate platforms are very attractive, and the challenges include two main aspects. On the one hand, due to the characteristics of various anisotropic materials and inclined sidewall structures, the implementation of thin-film lithium niobate wavelength division multiplexing devices is not easy, and it is difficult to meet the needs of high integration and high performance. On the other hand, the overall size of thin-film lithium niobate electro-optical modulators based on Mach-Zeng interferometers or ring resonators is still too large, which is difficult to meet the requirements of high integration.

In response to this situation, Professor Daozin Daozin’s research group of Zhejiang University published a report entitled “Ultra-compact lithium niobate photonic chip for high-capacity and energy-efficient wavelength-division-multiplexing” in Light: Advanced Manufacturing transmitters”.

Using a novel 2×2 FP cavity electro-optical modulator array and a quad-channel multimode waveguide grating wavelength division multiplexer, this work realizes a monolithically integrated thin-film lithium niobate photoemitting chip (Figure 1a) with a functional area size of only 0.3×2.8 mm², demonstrating the high-capacity transmission of 320 Gbps (4×80 Gbps) OOK signals and 400 Gbps (4×100 Gbps) PAM4 signals.

Figure 1: Schematic diagram of a four-channel wavelength division multiplexed thin-film lithium niobate light emitter chip

In this work, the four-channel WDM adopts a cascaded antisymmetric multimode waveguide grating filter (Figure 1b), the principle of which is: TE mode incident is converted into reflective TE mode by multimode waveguide grating, and then converted by the mode demultiplexer into download segment TE mode, so as to avoid returning to the incident port. Here, through the amplitude toe cut design, a single-channel band-pass filter with high side-mode rejection ratio and large free spectral range is realized, and then a multi-channel wavelength division multiplexer can be realized by cascading. At the same time, a new 2×2 FP cavity structure is constructed based on the multimode waveguide grating structure, and a small-size, high-efficiency electro-optical modulator with an equivalent modulation zone length of only 50 μm is realized instead of the traditional ring resonant cavity structure (Figure 1c). In this design, the antisymmetric multimode waveguide grating structure adopted can solve the dilemma that traditional Bragg grating devices/FP cavity devices need to introduce external rings or isolators, and there is no need to introduce curved waveguides in the functional area, avoiding a series of problems caused by mode hybridization in the traditional design due to curved waveguides.

Figure 2: FP cavity modulator unit

Based on the team’s independent etching process, a thin-film lithium niobate photoemitting chip with a monolithic integration of a 2×2 FP cavity electro-optical modulator array and a four-channel multimode waveguide grating wavelength division multiplexer was successfully developed, and each channel has excellent uniformity (additional loss of only ~0.8 dB, extinction ratio >20 dB), and can obtain 80 Gbps OOK and 100 Gbps PAM4 high-speed signal eye diagram (Figure 3), demonstrating its superior performance. In general, optical filters based on multi-mode waveguide gratings and 2×2 FP cavity optical modulators have outstanding advantages such as compact structure, simple design and flexible expansion, and have important potential in the fields of wavelength division multiplexing optical interconnection and optical computing. (Source: Advanced Manufacturing WeChat public account)

Figure 3: Eye diagram of four lanes of 40 Gbps/80 Gbps OOK and 100 Gbps PAM4 signals

Related paper information:https://doi.org/10.37188/lam.2023.013

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