Application of Space Division Multiplexing (SDM) in Submarine Optical Cable

In the age of big data, data transmission has become a critical link for social and economic development. With the growing demand for data transmission, optical fiber communication technology is constantly improving and developing. The emergence of space division multiplexing technology provides a new development direction for the capacity improvement of optical cable transmission systems. In recent years, people are constantly researching and applying it to submarine optical cables to expand the transmission capacity.

What Is Space Division Multiplexing?

Space division multiplexing technology, or SDM for short, refers to a way to realize optical fiber multiplexing by using space division. The main principle is to combine multiple fiber cables into a bundle to realize space division multiplexing or realize space division multiplexing in the same fiber cable. Expert research shows that long-distance submarine optical cable transmission is one of the most promising application scenarios of space division multiplexing technology.

The reason why space division multiplexing technology plays an important role in submarine optical cable transmission is that the optical signal gets limited from the power supply system at both ends and cannot increase the transmission capacity by increasing the power and number of amplifiers in the submarine system. The submarine cable system using SDM technology can achieve the lowest power consumption per bit.

Three Transmission Systems of SDM Technology

Based on the research of SDM technology in the application of submarine optical cable, space division multiplexing technology can be divided into three transmission systems: single-core fiber C-band transmission, single-core fiber C+L-band transmission, multicore fiber C-band transmission. Here is a system light path diagram of the three systems.

In submarine optical cable transmission, single-core fiber C-band transmission system is only equipped with EDFA equipment to enhance the signal. The single-core fiber C+L-band transmission system needs two EDFAs to enhance the two band signals respectively. The multicore fiber C-band transmission system is more complicated, and needs to fan out each fiber core and input it to the signal amplifier, and then fan the signal of the amplifier into the multi-core fiber cable.

Signal-to-noise Ratio Analysis of Three SDM Transmission Systems

When the signal-to-noise ratio(SNR) of the three-channel transmission system is 9.5dB, the single-core fiber C+L-band transmission system requires 37 pairs of optical fibers to achieve the highest optical cable capacity transmission. Multicore fiber C-band transmission system requires 19-20 pairs of fibers (usually 7 or 4 pairs of multi-core fiber cables) to achieve maximum transmission capacity. Single-core fiber C+L-band transmission system only needs 13 pairs of fiber cables to transmit the maximum capacity, but its maximum capacity is only 70% of the single-core C-band fiber transmission. As shown in the figure below, the number of optical fiber cores that the submarine optical cables can support under different transmission distances.

Maximum Submarine Optical Cable Capacity at Different SNR Values

Here, each submarine optical cable distance is set to 60km to measure the voltages required by the three transmission systems in space division multiplexing technology. Single-core C-band and C+L-band require lower voltages, with a maximum voltage of 15 kV. Compared with multi-line fiber optic transmission systems, their voltages are lower. The main reason is that multi-core fiber transmission systems require additional amplifiers to complete the transmission.

To sum up, we can know that in three transmission systems in space division multiplexing, the transmission capacity of single-core fiber C+L-band and multi-core C-band is smaller than that of single-core fiber C-band transmission. Single-core fiber C-band and C+L-wave systems can use lower voltages and power consumption than multi-core systems if the same capacity is achievable with multi-core.

Summary

space division multiplexing technology conforms to the development trend of optical fiber communication. It is a major innovation and developed direction of optical fiber communication technology. Comparing the three transmission capacity configurations, the single-core C-band system has significant capacity advantages and lower power consumption per bit compared with single-core C+L and multi-core C-band in submarine optical cable applications.