Data Center High-Speed Optical Interconnect Technology

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Rapid growth of optical interconnect rates in data

Due to the explosive growth of network traffic, existing data centers cannot meet the requirements in terms of transmission bandwidth, transmission rate, delay, and scalability. Therefore, future data centers will move from 10/40 Gbit/s to 25G/100G/400 Gbit/s architecture upgrade, correspondingly, data center optical module products are also facing the upgrade from 10G, 40G optical modules to 100G, 400G optical modules.

In this case, the traditional electrical interconnection architecture is faced with challenges such as insufficient transmission bandwidth, limited communication distance, excessive network complexity, and excessive energy consumption, making it difficult to meet the needs of future data centers. There is a huge opportunity. Since Kao discovered the potential of optical fiber for communication in 1966, optical fiber communication technology has developed for more than 50 years.

The first commercial optical fiber communication system was developed by AT&T in 1977, with a capacity of 45 Mbit/s. Today, the capacity of single-mode fiber can reach 100 Tbit/s, and the transmission distance spans more than 10,000 kilometers, covering Most information transmission scenarios that have become the cornerstone of today’s information society.

40G/100G/400G optical interconnection technology and modules

At present, most data centers have reached 40 Gbit/s transmission rate, 100 Gbit/s architecture is being deployed, and the next-generation architecture will skip 200 Gbit/s and directly upgrade to 400 Gbit/s rate. Parallel optical transmission is an important method of internal communication in data centers.

This transmission method can not only greatly improve the communication rate, but also can be combined with the parallel data channel structure in the large-scale network architecture, which greatly improves the speed of data processing.

In the 40G architecture, a multimode transmission scheme is usually used. The 40G SR4 optical module uses a vertical cavity surface laser (VCSEL) as the emission source and a multimode fiber (MMF) as the transmission medium. This scheme not only has low cost, The advantage of low power consumption, and it is easy to achieve rate matching between electrical and optical signals;

For architectures upgraded to 100 G and higher rates, due to the limitation of multi-mode fiber transmission distance and the influence of modal dispersion, it is difficult for VCSEL-MMF-based solutions to break through the rate bottleneck, so single-mode fiber (SMF) is mainly used, and a wide range of using wavelength division multiplexing (WDM) technology.

The current 100G transmission technology mainly includes 3 types: parallel single-mode 4-channel (100G PSM4), coarse wavelength division multiplexing system (100G CWDM4), and short-distance optical module (100G SR4), these types are 4 25 Gbit/s per channel, the first 2 are based on distributed feedback (DFB) laser and single-mode fiber technology, SR4 still uses VCSEL and multi-mode fiber.

For the next-generation rate standard of 400 Gbit/s, higher requirements for the bandwidth of optoelectronic devices are required, and new technologies are required to be applied in optical interconnection. These new technologies include advanced signal modulation technology, dispersion compensation, etc., as well as the evolution of parallel multi-channel technology.

In terms of implementation, the capacity of the existing network can be increased by increasing the channel rate, increasing the number of parallel fibers, and increasing the number of wavelength channels to reach the 400 Gbit/s standard, but no matter which method is adopted, 400 Gbit Neither the cost per bit per s nor the power consumption should be higher than 100 Gbit/s.

The Institute of Electrical and Electronics Engineers (IEEE) completed the standardization of 400 Gigabit Ethernet in December 2017, but the industry has already conducted a series of technical research and development before that. 400G upgrade currently has two main challenges, one is how to achieve the 4x rate increase from 100G to 400G, and the other is the signal encoding method from non-return-to-zero (NRZ) to 4-level Signal integrity issues with Pulse Amplitude Modulation (PAM4).

At present, the transmission technology supporting 400 G in the data center is multi-mode SR4.2 and a single-mode long-distance optical module (DR4). Among them, SR4.2 adopts 4 pairs of multi-mode optical fibers, which is more suitable for transmission within 100 meters, while DR4 is more suitable for transmission within 100 meters. Can reach 500 m using 8×50 Gbit/s PAM4.

Read also: avplemedia

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