A new generation of high-speed optical module photonic chips for data centers based on silicon-based hybrid integration technology - Xunshi optical communication network (iccsz.com)

Abstract:Based on the characteristics of high linearity and high bandwidth of thin-film lithium niobate modulator, Yilan Micro has designed and fabricated a silicon-based hybrid integrated optical chip for single-wave 200Gbps LPO optical module, which is currently being tested in cooperation with some customers.

With the vigorous development of the global AIGC industry, the construction of data centers of major IT companies at home and abroad is in full swing, and the layout depth of 5G and 6G networks in the field of communications is also increasing rapidly. In this era, the market demand and update and iteration speed of the new generation of data center optical modules will be greatly improved, which puts forward higher requirements for the core device inside the optical module - high-speed photonic integrated chips, which will face changes and challenges in terms of bandwidth, cost, rate, and power consumption.

Traditional silicon photonics and EML solutions

Silicon-based optoelectronics technology is a technology platform for a large number of applications in the field of data centers and communications. The traditional pure silicon photonics solution (silicon photonics V1.0) is based on its mature tape-out process, which is easy to mass produce and has certain advantages in terms of integration. However, due to the fact that silicon materials themselves have no electro-optical response, and the silicon-based modulation principle is based on the plasma dispersion effect, and the modulation bandwidth is limited (the upper limit of 30GHz at 2-3V driving voltage corresponds to PAM4 single-wavelength 100Gbps), so various hybrid integration (silicon photonics V2.0) platforms based on silicon and silicon-based substrates and multi-material systems have become hot technologies.

Recently, mainstream academic journals in China have pointed out that there are many materials that can be used as photonic integration platforms, such as silicon (Si), thin-film lithium niobate (TFLN), silicon nitride (Si3N4), indium phosphide (InP) and gallium arsenide (GaAs), which are popular materials for integrated photonics research, and great breakthroughs and progress have been made on these material platforms. It should be pointed out that there is currently no perfect platform material that can achieve the requirements of efficient on-chip light generation, transmission, manipulation and detection, which is also one of the general consensus of the scientific research community. Therefore, hybrid integration and heterogeneous integration are considered to be the only way to solve the problem of all-optical integrated devices or chips.

As a new integrated optoelectronic material, thin-film lithium niobate has attracted extensive attention in recent years, and its excellent characteristics such as high electro-optical coefficient, large bandwidth, high power, high stability, low driving voltage, and low loss can achieve ultra-fast electro-optical response and highly integrated optical waveguide. With the breakthrough of etching process, the thin-film lithium niobate modulator prepared based on CMOS process can give full play to its high-speed modulation characteristics, and can meet the rate requirements of PAM4 single-wavelength 200Gbps or even single-wavelength 400Gbps.

Due to the relatively high level of technology maturity and more mature supply chains, it is based on III-V The EML solution of the family of semiconductors is still the mainstream solution of 400G optical modules. However, under the demand of 800G or even higher speed in data centers, although the traditional EML solution can meet the requirements of a single-wavelength 200 Gbps rate in terms of modulation rate, its cost is high, the process is complex, and the product yield is not optimistic, which limits the application of EML solutions in high-speed optical modules in data centers. At the same time, all EML suppliers with single-wavelength 100Gbps are overseas vendors (mainly in the United States and Japan). Compared with the traditional silicon photonics platform (silicon photonics V1.0), the wafer-level etching process of thin-film lithium niobate is still immature, and the device size does not have advantages. In the face of the current market demand and technical bottlenecks, Yilan Micro has developed a high-speed hybrid integrated photonic chip based on the silicon-based hybrid integration technology (silicon photonics V2.0) platform. Compared with pure silicon photonics (silicon photonics V1.0) or pure thin film lithium niobate wafer platform, the easy-cable micro-hybrid integration technology (silicon photonics V2.0) platform gives full play to the advantages of silicon photonics passive and lithium niobate active modulation, and perfectly combines silicon and lithium niobate materials.

Easy cable micro-silicon-based hybrid integration technology platform

Based on the silicon-based hybrid integration technology (silicon photonics V2.0) platform with independent intellectual property rights, Yilan Micro has successfully designed and prepared high-speed optical module optical engine chips for data centers. In the chip modulation region, the input light source is coupled from the silicon-based waveguide layer to the thin-film lithium niobate waveguide layer, which is then coupled back to the silicon-based waveguide layer to the optical output after the optical modulation is realized by the thin-film lithium niobate MZ modulator.

Easy cable micro-silicon-based hybrid integration technology platform

In this technical solution, the efficient coupling of silicon-based waveguide and lithium niobate waveguide ensures low insertion loss of the chip. After design optimization, the measured single insertion loss of the vertically coupled silicon-based/lithium niobate waveguide is less than 0.2dB. The thin-film lithium niobate MZ modulator is used to achieve high-speed optical signal modulation, and through optical and traveling wave electrode design, the modulator can achieve an electro-optical modulation bandwidth greater than 67GHz, and can support single-wave 200Gbps high-speed signal modulation.

Silicon-based/lithium niobate hybrid integrated electro-optical response test

Schematic diagram of LN/SiN heterogeneous integrated MZM chip structure

R&D design

In addition to realizing the performance requirements of high bandwidth of chips, Yilan Micro has also made great progress in the research of low driving voltage and small size of chips. The conventional thin-film lithium niobate modulator has a longer size, and its half-wave voltage length product (Vπ· L) usually at 2~3 Around V·cm, which corresponds to a half-wave voltage of 2.4V, the length of the modulator is usually about 10 mm, which results in an oversized chip size (more than 10 mm on the long side). The micro-design of Easy Cable can effectively shorten the size and length of the modulator by bending and folding the modulator, which can shorten the length of the modulator to less than 4mm, which can fully reach the length of the traditional silicon photonics modulator chip, and can match the existing optical module specifications to complete the package.

The modulation efficiency of an electro-optical modulator is usually expressed in terms of Vπ· L, the lower this value is, the higher the modulation efficiency. Vπ· corresponding to a specific value L, the length of the modulation zone and the half-wave voltage will theoretically constrain each other, that is, it is difficult to achieve low driving voltage and small size at the same time. In order to achieve low displacement voltage and small size of the thin-film lithium niobate electro-optical modulator at the same time, Yilanwei increased the buffer layer between the electrode and the lithium niobate plate to reduce the absorption loss of the metal electrode, and reduced the Vπ· L. After this design optimization, the Vπ· L can be made up of 2 Above V·cm, it is reduced to 1.2 V·cm, which greatly eases the limitation between low driving pressure and small size.

With completely independent intellectual property rights and technical advantages on the silicon-based hybrid integration technology (silicon photonics V2.0) platform, Yicable Micro can design and prepare customized hybrid integrated photonic chips according to customer needs. In the face of the market demand in the optical communication industry, Yilan Micro will continue to release hybrid integrated photonic chip products with multiple specifications and rates, including 400G/800G/1.6T DR-4(8) photonic chip, 400G/800G/1.6T FR-4(8) photonic chips and other products. At the same time, in order to meet the higher bandwidth rate requirements of the optical communication market in the future, Yicable Micro will continue to invest in the research and development of higher rate photonic chip products, in the newly designed silicon-based hybrid integrated optical chip, its electro-optical modulation bandwidth has been simulated to exceed 130GHz, this solution can support the future single-wave 400Gbps optical module 3.2T product requirements.

Easy cable micro single wave 400G optical chip electro-optical response

Silicon-based hybrid integrated optical chips adapted to LPO optical modules

With the increasing demand for 400G/800G optical modules for AI computing power, the cost advantage is outstanding and can meet the needs of LPO (Linear-drive) with short distance, large bandwidth, and low latency Pluggable Optics) optical transceiver solutions have gradually become a popular choice in the market. LPO technology uses a linear direct drive method, which is 200Gbps for a single wavelength required by the current market LPO optical transceiver has high requirements for the linearity of all components in the module link, especially the electro-optical modulator. In the work that has been reported, the SFDR of the intermodulated signal is often used to evaluate the linearity of the modulator. The SFDR of a conventional pure silicon modulator is 97dB· Hz2/3, while the thin-film lithium niobate modulator can reach 120dB· Hz2/3。 The linearity of the EML scheme is lower than that of pure silicon and thin-film lithium niobate modulators. Based on the characteristics of high linearity and high bandwidth of thin-film lithium niobate modulator, Yilan Micro has designed and fabricated a silicon-based hybrid integrated optical chip for single-wave 200Gbps LPO optical module, and this product is currently being tested in cooperation with some customers.