400G Optical Transceivers in Long-Distance & High-Capacity Transmission Networks

NADDOD Claire Optical Module Engineer Aug 11, 2023

As new businesses continue to emerge, the bandwidth requirements for long-distance transmission in modern communication networks are steadily increasing. In response to this challenge, transmission technology is constantly evolving to improve spectral efficiency and approach the Shannon limit of information transmission. In this fast-paced era of communication, multiple forms of 400G transceivers have emerged as solutions to meet the transmission demands of different scenarios.


Scenario Description

With the emergence of new businesses, the pressure on long-distance bandwidth remains high. The continuous growth of network traffic drives the increase in transmission network port bandwidth. WDM-based coherent transmission technology always remains the optimal solution for meeting distance and bandwidth requirements.


Illustration of Long-Distance Transmission Scenario

Illustration of Long-Distance Transmission Scenario


With the maturity of 400G coherent solutions, the adoption of 400G coherent ports is expected to rapidly increase after 2020. This growth is driven in part by the growing demand for network bandwidth.


Additionally, the gradual increase in customer-side 400GE ports will also promote the adoption of coherent 400G ports, as using a single 400G wavelength to carry 400GE traffic is the most cost-effective approach.


NADDOD's predictive report highlights that 400G coherent ports will be the fastest-growing ports in the next 5 years, with increasing adoption across networks.


Simultaneously, as network traffic and scale continue to surge, the number and scale of wavelengths in networks are growing, leading telecommunication network operators to demand greater flexibility in network management and scheduling.


This has driven the widespread commercial use of Reconfigurable Optical Add-Drop Multiplexers (ROADM) and Optical Cross-connects (OXC) in networks.


Wavelength-based optical switching technology allows operators to dynamically configure wavelength paths based on business requirements, enabling direct wavelength routing and reducing latency and power consumption.


Currently, an increasing number of operators are embracing this network architecture approach. For instance, in 2017, a certain operator constructed a ROADM network with a staggering 364 wavelengths.


Another key technology driving the evolution of wavelength division multiplexing networks towards greater flexibility and intelligence is flexible rate modulation and flexible grid technology. Traditional DWDM systems use fixed 50/100 GHz grids, with predetermined center frequencies and channel widths.


With the introduction of flexible modulation and flexible grid technologies, the most suitable modulation format and channel width can be selected for each port's capacity and transmission distance, thereby enhancing spectral efficiency and transmission capacity.


Flex-rate & Flex-grid Enable Flexible Network Configurations

Flex-rate & Flex-grid Enable Flexible Network Configurations


These changes at the network architecture level have raised demands for more flexible and stronger non-linear effect-resistant capabilities in high-speed line-side ports.


Technology Trends

Continuously improving spectral efficiency, approaching the Shannon Limit to drive advancements in transmission technology. In general, coherent optical transceivers evolve in the following three directions:


Spectral Efficiency

Continuously improving spectral efficiency through advancements in oDSP (optical digital signal processing) algorithm technology to increase the capacity per fiber and meet the growing demands of traffic.


Symbol Rate

Increasing the data rate per wavelength to accommodate the requirements of high-bandwidth single-port applications, while also helping to reduce bit costs and power consumption.


Smaller Size/Lower Power Consumption

Leveraging integrated optoelectronic devices, advanced semiconductor process technology, and oDSP algorithms specifically designed for low-power scenarios to continuously reduce transceiver power consumption and size.


Due to the limitations of Shannon's Law, the transmission performance of 400G wavelength at a symbol rate of 64Gbaud is insufficient for long-haul optical transmission applications.


To meet the transmission requirements of regional and even long-haul backbone networks, higher symbol rates (above 90Gbaud) and more sophisticated and powerful oDSP algorithms are necessary.


To support long-haul links exceeding 1000km, the symbol rate of 400G wavelengths needs to be increased to above 90Gbaud, which requires bandwidth enhancements in optoelectronic devices and synchronous upgrades in the ADC/DAC components of oDSP.


Simultaneously, with the increase in symbol rates, the cost and difficulty of compensating for impairments in fiber transmission also rise. Stronger compensation algorithms are needed to mitigate these channel impairments. One common application of compensation algorithms is for compensating filtering impairments.


Due to the widespread use of ROADM, end-to-end wavelength links often need to pass through multiple ROADM nodes, and the filtering effects of wavelength selective switches (WSS) in these ROADM nodes narrow the effective bandwidth of the entire link. This poses higher requirements for compensation algorithms in oDSP.


Impact of Multi-stage ROADM on Optical Channel Bandwidth

Impact of Multi-stage ROADM on Optical Channel Bandwidth


Additionally, many network operators desire the flexibility to choose different modulation formats and symbol rates based on their specific port speeds and transmission distances. For example, for long-distance transmission requirements at 400G, they can opt for 400G 16QAM modulation to achieve extended reach.


Conversely, for scenarios like metropolitan core interconnects spanning a few tens of kilometers, they can choose 800G 64QAM modulation to enhance spectral efficiency and reduce cost per bit. This flexible modulation technology, combined with the flexibility of the optical layer's grid (Flex-grid), maximizes fiber capacity and saves on expensive fiber optic cable investments.


Solution: Multiple 400G Transceivers Meet Different Transmission Needs

NADDOD offers solutions for long-distance and high-capacity 400G coherent optical transceivers to cater to various customer requirements. Each transceiver supports Flex Rate of 100G/200G/400G and comes in different form factors. In addition to the commonly used 40nm C-Band spectrum width, NADDOD also supports a 48nm Super C-Band with a maximum of 120 channels to meet customers' high-capacity demands. Depending on different application scenarios, NADDOD utilizes small-sized silicon photonics devices or high-performance, high-bandwidth InP devices, supporting various packaging options to match specific needs.


For 400G coherent optical transceivers, the underlying principles are the same across different packaging options. The transmitter of a 400G coherent optical transceiver consists of oDSP, data drivers, wavelength-tunable light sources, and PDM-I/Q modulators. The incoming data from the motherboard is mapped and encoded, undergoes spectrum shaping and data link bandwidth compensation by the TX-oDSP, and is then amplified by the data drivers before being input to the modulator to generate the modulated optical signal. On the receiver side, the optical signal is input to the coherent receiver ICR, where it interferes with the local oscillator wavelength-tunable laser to achieve optical-to-electrical conversion. The electrical signal is then subjected to high-speed ADC sampling and undergoes compensation processing for CD, SOP, and PMD, ultimately recovering the data signal.


Block Diagram of Coherent Optical Transceiver

Block Diagram of Coherent Optical Transceiver


In Conclusion

With the continuous development of communication networks, the pressure on bandwidth for long-distance transmission will persist. However, by continuously improving spectral efficiency and driving advancements in transmission technology, we can meet this challenge.


Multiple forms of 400G transceivers, as solutions, offer the flexibility to address the transmission demands of different scenarios, injecting new vitality into the telecommunications industry and providing users with more efficient and reliable transmission experiences.


As technology continues to evolve, we can expect to see more innovative solutions in the future, bringing greater breakthroughs and advancements in long-distance transmission.