World’s First Standard Cladding Diameter 19-core Optical Fiber with Record Transmission Capacity

In the experimental system shown in Figure 6, the transmission capacity (data-rate) of the system was estimated by directly applying error coding on the received bits. The blue dots in Figure 7 show the total data rate of all cores after applying error corrections with flexible overheads to maximize the throughput of each wavelength channel. Data rates of approximately 5 terabits per second in the C-band and 2.5 to 5 terabits per second in the L-band were obtained. The total transmission capacity of all wavelength channels was 1.7 petabits per second.

International standards specify that the outer diameter of the glass (cladding) of the optical fiber is 0.125 ± 0.0007 mm, and the outer diameter of the coating layer is 0.235 - 0.265 mm. The optical fiber widely used in current optical communication systems is a single-core single-mode fiber with a cladding diameter of 0.125 mm, and the transmission capacity is limited to approximately 250 terabits per second.

Standard single-core/single-mode optical fiber (see Figure 3(a)) has a capacity limit of approximately 250 terabits per second. To address this problem, multi-core fibers with increased number of cores (light paths) (see Figure 3(b)) and multi-mode fibers have been investigated. If the cores of a multi-core fiber are close to each other, the signal in one core leaks into the other cores, causing signal quality degradation due to interference (see Figure 3(c)). To suppress signal interference, core spacing of an uncoupled multi-core fiber is appropriately increased.

Randomly coupled multi-core fiber (see Figure 3(d)) is a kind of multi-core fiber in which cores are densely arranged on the premise of eliminating signal interference between cores through MIMO digital signal processing. Compared to multi-mode fiber, randomly coupled multi-core fiber has uniform signal propagation characteristics in each core, making it suitable for long-distance transmission. However, to ensure the randomness of coupling between cores required for long-distance transmission, the core spacing must be strictly controlled.

In the transmission using multi-mode fiber or randomly coupled multi-core fiber, MIMO processing for mode/core separation is almost inevitably required at the receiver side. MIMO is a signal processing technology for eliminating multipath interference in wireless communications. In optical communications, it is used to eliminate interference between various optical signals propagating in the same optical fiber.

The power consumption of MIMO processing depends mainly on the number of multipliers and is proportional to the length of the filter function to revert the effects of signal propagation in the fiber. In multi-mode fiber transmission, the length of the filter function depends on the propagation time difference of each mode, which generally increases in proportion to the fiber length. Therefore, the power consumption of digital signal processing required for long-distance transmission tends to be large. In randomly coupled multi-core fiber transmission, the propagation characteristics of each spatial channel (core) are uniform, and the length of the filter function is proportional to the square root of the transmission distance. Transoceanic-class (10,000 km) transmission systems using randomly coupled 19-core fibers will be able to reduce the number of multipliers per channel in MIMO processing by up to thousands compared to the case of 55-mode fibers (see Figure 4).

A multi-mode fiber has a large core diameter, and multiple modes exist within the core (see Figure 3(e)). Inter-modal signal interference occurs during multi-mode fiber propagation, at the fiber connections, and also at the input/output. Therefore, it is important to eliminate the interference by using MIMO digital signal processing. Multi-mode fiber transmission of 55 modes at maximum was realized.

Multi-mode optical fiber generally has mode-dependent propagation loss and time delay. When the mode dependent loss is large, the quality of the signal deteriorates. When the propagation time delay difference is large, the power consumption required for MIMO digital signal processing becomes large.

The wavelength bands used for telecommunication applications are the C band (wavelength 1,530 to 1,565 nm) and L band (1,565 to 1,625 nm); other bands include the O band (1,260 to 1,360 nm), E band (1,360 to 1,460 nm), S band (1,460 to 1,530 nm), and U band (1,625 to 1,675 nm). In this study, C-band and L-band were used.