800 Gbit/s Optical Module Technology

Source: bt-pon.com

Market Forecast and Application Scenarios

The emergence of new services such as 4K virtual reality (VR), Internet of Things, and cloud computing has put forward higher requirements for network bandwidth, concurrency, and real-time performance. According to Omdia’s prediction, with the continuous increase in bandwidth demand in the next few years, although 100, 200, and 400 Gbit/s optical modules will still retain the largest market share, 800 Gbit/s optical modules will be commercially available in 2023. Large-scale deployment will be achieved in 2025.

According to the 800 GE network structure, the connection distance from the top-of-rack switch (TOR) to the leaf switch may be tens of meters short or hundreds of meters long. For this part of the connection, large Internet companies generally adopt 100 Gbit/s connection technology, and will gradually upgrade to 200 Gbit/s or 400 Gbit/s speed technology starting in 2021. Some leading companies will start trials of 800 Gbit/s technology in 2023. The connection between the leaf and the spine switch, or the connection between the spine switch and the core router, may solve the interconnection problem within a campus or between adjacent campuses.

This connection distance can reach 2 km, or even 10 km. The interface rate will also be gradually replaced from 100 Gbit/s to 200 Gbit/s or 400 Gbit/s from 2021. Some companies will begin trials of 800 Gbit/s technology in 2023. Data center interconnection (DCI) generally refers to the connection between several adjacent data centers for load balancing or disaster recovery backup, and the connection distance may be as long as tens of kilometers. For such a long distance, due to the preciousness of fiber resources, people mainly use dense wavelength division multiplexing plus coherent communication to reuse fiber resources as much as possible. We divide the application scenarios of 800 Gbit/s optical modules into SR (100 m scenario), DR/FR/LR (500 m/2 km/10 km scenario), and ER/ZR (40 km/80 km scenario).

Technical solutions

Source: carritech.com

The evolution of 800 Gbit/s technical solution includes 3 generations. The first generation is 8 optical 8 electrical: optical interface 8 × 100 Gbit/s, electrical interface 8 × 100 Gbit/s, commercial time is 2021; the second generation is 4 optical 8 electrical: optical interface 4 × 200 Gbit/s, electrical interface The interface is 8×100 Gbit/s, and the commercial time is expected to be 2024; the third generation is 4 optical 4 electrical: optical interface 4×200 Gbit/s, electrical interface 8×100 Gbit/s, and the commercial time is expected to be 2026. In the long run (within 5 years), optical/electrical single-channel 200 Gbit/s technology will be popularized; in the short term (within 3 years), since single-channel 200 Gbit/s optical chip devices and equalization technology are not yet available Mature, the industry still needs time to break through related technical bottlenecks.

Electrical Interface and Packaging

Source: fiberopticshare.com

From the perspective of the development of 100 Gbit/s direct adjustment and direct detection optical modules, when the single-channel rate of the electrical interface is the same as the single-channel rate of the optical interface, the architecture of the optical module will reach the best state, and it will have the advantages of low power consumption and low cost. . The single-channel 100 Gbit/s electrical interface will be an ideal electrical interface for 8×100 Gbit/s optical modules, and the single-channel 200 Gbit/s electrical interface will be an ideal electrical interface for 4×200 Gbit/s optical modules. In terms of packaging, 800 Gbit/s optical modules may have different forms such as double-density four-channel small form-factor pluggable (QSFPDD800) and eight-channel small form-factor pluggable (OSFP). Pluggable optical modules based on 200 Gbit/s electrical interfaces still face many challenges due to factors such as wiring within the module and loss of connectors.

Optical Interface

Source: blog.router-switch.com

(1) 8×100 Gbit/s 4-level pulse amplitude modulation (PAM4) optical module: PAM4 transceiver operates at 53 Gbd, using 8 pairs of digital-to-analog converters (DACs) and analog-to-digital converters (ADCs), 8 Lasers, 8 pairs of optical transceivers, and 1 pair of 8-channel coarse wavelength division multiplexer (CWDM) or Ethernet channel-based wavelength division multiplexing (LAN-WDM) (depending on fiber dispersion loss) multiplexer and demultiplexer device (not required for SR/DR application scenarios).

(2) 4×200 Gbit/s PAM4 optical module: PAM4 transceiver operates at 106 Gbd, using 4 pairs of DAC and ADC, 4 pairs of optical transceivers (including 4 lasers), and 1 pair of 4-channel CWDM or LAN-WDM (depending on fiber dispersion loss) Multiplexer and demultiplexer (not required for SR/DR application scenarios).

(3) 800 Gbit/s coherent optical module: operating at 128 Gbd under dual-polarization sixteen-quadrature amplitude modulation (16QAM). It uses 4 pairs of DACs and ADCs, 1 laser and 1 pair of optical transceivers, and can use fixed-wavelength lasers in data center coherent optical modules to reduce cost and power consumption.

The 8×100 Gbit/s direct adjustment and direct detection scheme can use the existing technical framework, the related technologies and standards are relatively mature, and the supply chain is relatively complete. In the SR scenario, the vertical cavity surface emitting laser (VCSEL) 100 Gbit/s technology faces challenges. Improving the performance of multimode solutions and reducing the cost of multimode fiber will be the key factors for the continuous evolution of this technology.

Single-mode technologies represented by silicon photonics (SiPh) and directly modulated lasers (DML) are developing rapidly. Among them, SiPh technology is developing more rapidly, and it is expected to compete with multi-mode solutions in application scenarios with transmission distances of 100 m and below in the future. In the DR/FR scenario, there are three schemes: electroabsorption modulated laser (EML), DML and SiPh. In the LR scenario, there are 800 Gbit/s LR8 schemes based on coarse wavelength division multiplexing (CWDM), fine wavelength division multiplexing (LWDM) and narrowband fine wavelength division multiplexing (nLWDM), and these schemes are still in the research stage.

In terms of wavelength selection, LWDM8 is superior to CWDM8 in terms of dispersion penalty due to the large dispersion of the edge wavelength of the O-band. At present, the direct adjustment and direct detection scheme with a distance of 10 km and above mainly faces the challenge of “worst case” dispersion and narrow dispersion tolerance matching.

Constructing a new wavelength system and compressing the multi-channel wavelength range can narrow the worst-case dispersion accordingly, thereby simplifying the design of digital signal processing (DSP) and reducing theoretical power consumption. For example, when the 8×100 Gbit/s PAM4 direct modulation and direct detection scheme adopts the LWDM scheme with 800 GHz spacing, the dispersion-limited distance is about 10 km, and when the nLWDM scheme with 400 GHz spacing is used, the dispersion-limited distance can be extended to 20 km.

The dispersion-limited distance can be further extended to 40 km with nLWDM at 200 GHz spacing. At the same time, compressing the zero-dispersion point distribution or drift range and reducing the corresponding dispersion range is also one of the solutions. However, since the distribution of zero dispersion points of optical fiber products from different manufacturers is not uniform, large-scale compression is still difficult.

For the 4×200 Gbit/s direct modulation and direct detection scheme, the single-channel 200 Gbit/s continues to use the PAM4 modulation code type, which can use the relatively mature PAM4 industry basic conditions (but the possibility of new modulation code types is not ruled out). In 4×200 Gbit/s DR and FR application scenarios, there are currently two technical solutions: 4-way single-mode parallel (PSM4) and CWDM4. These two schemes still face many challenges and need further research.

For LR application scenarios, there are 800 Gbit/s LR4 solutions based on CWDM, LWDM, and nLWDM. These solutions are still in the research and discussion stage, and require high-bandwidth optoelectronic chip devices, stronger equalization techniques, and forward error correction (FEC) to ensure post-correction bit error rate (BER).

The device bandwidth of the 800 Gbit/s coherent optical module needs to be greatly improved, and it is difficult to double the bandwidth in one step in device design. 800 Gbit/s coherent optical modules based on 96 GBd devices must use higher order modulation patterns. This method has disadvantages such as low optical signal-to-noise ratio (OSNR), limited transmission distance and application scenarios. The dual polarization (DP)-16QAM coherent optical module based on 128 GBd has better OSNR and transmission capability, and will become the mainstream implementation of 800 Gbit/s coherent.

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