World’s Highest Density Optical Fiber for Space Division Multiplexing with Deployable Reliability
Space division multiplexing (SDM) technology has been intensively investigated in order to substantially increase the network capacity of optical fiber telecommunication. Multi-core or multi-mode fiber is a promising candidate for next-generation optical fiber. In this article, we describe our optical fiber for SDM transmission that can realize 100 times larger capacity than that of standard single-mode fiber while maintaining deployable mechanical reliability.
Keywords: optical fiber, space division multiplexing, multi-mode multi-core
Existing telecommunication networks mainly utilize single-mode fiber (SMF), whose low loss and broadband characteristics have resulted in increased network transmission capacity. The progress in the capacity per optical fiber of the core network in Japan is shown in Fig. 1. The transmission capacity has steadily increased year by year, and the rate of increase grew by 1000 times in a 20-year period (from 1.6 Gbit/s in 1987 to 1.6 Tbit/s in 2007). This rapid increase in capacity is due to the development of transmission technologies such as time division multiplexing and wavelength division multiplexing, as well as the development of optical amplifiers such as erbium-doped fiber amplifiers.
A transmission system using digital coherent technology was recently introduced, and 8-Tbit/s capacity has been achieved by using a multi-level modulation format with high spectral efficiency. However, it is expected that the capacity limit of the standard SMF is around 100 Tbit/s . Therefore, it is necessary to develop a new transmission medium to achieve 1000 times larger capacity in the next 20 years. Space division multiplexing (SDM) technology using multi-core or multi-mode fiber has been intensively investigated as a promising candidate for a next-generation transmission system. In the following section, we report our latest research results on the development of optical fiber for an SDM system taking the spatial density and mechanical reliability into account.
2. Optical fiber for SDM system
Multi-core and multi-mode fibers are depicted in Figs. 2(a) and (b). Multi-core fiber has multiple cores within a cladding, and multiple signals can be transmitted in parallel by using multiple cores. Multi-mode fiber typically has a larger core than that of SMF, and multiple modes can propagate within a core. Each mode is orthogonal to each other and is treated as an independent transmission path. Thus, mode division multiplexing (MDM) is realized, where multiple signals can be transmitted through multiple modes.
Multi-mode multi-core fiber has been studied recently in order to achieve ultra-high capacity transmission. This fiber combines multi-core and multi-mode technologies. The structure of the multi-mode multi-core fiber is shown in Fig. 2(c). There are multiple cores, and multiple modes can propagate within each core. This enables us to employ MDM transmission using each core. As a result, m×n transmission channels are obtained with n-mode m-core fiber.
The transmission channels per fiber as a function of fiber diameter of recently reported multi-core fiber are shown in Fig. 3. The results for multi-core fiber with single-mode cores and multi-mode cores are respectively plotted as open and solid circles. It can be seen that the multi-mode multi-core fiber makes it possible to achieve substantially more transmission channels compared to single-mode multi-core fiber. In fact, more than 100 transmission channels can be obtained with multi-mode multi-core fiber. However, more transmission channels results in a larger fiber diameter. Next, we describe some important parameters when designing the SDM fiber.
2.1 Mechanical reliability
Failure probability, which means the probability that a fiber will be broken, increases as the fiber diameter increases when fiber is bent owing to the inherent nature of the glass. A failure in a transmission line causes a disconnection of the network service and should be avoided so as not to degrade the network reliability. Thus, the fiber diameter cannot be increased in an unlimited fashion in order to deploy a larger number of cores, and a fiber diameter design that maintains flexibility and reliability is required.
2.2 Spatial density
It is important for the telecom operator to design a multi-core fiber with the cores packed as closely as possible because we need to efficiently utilize the limited space of the telecom infrastructure such as the cable duct space underground. The spatial density, namely the number of cores per unit area, is one of the parameters used to evaluate the density.
2.3 Transmission characteristics
The transmission channels in the SDM fiber need to have better performance than those of the SMF. This means that the SDM fiber should have low transmission loss characteristics comparable to the SMF, and the inter-core crosstalk should be suppressed by properly designing the distance between neighboring cores. The differential mode delay (DMD) is also an important parameter for multi-mode multi-core fiber. DMD is the group delay difference between the propagation modes. The propagation modes typically have different group velocities. Reducing the DMD in an MDM system is strongly required because it becomes more difficult to recover the transmitted signals at the receivers when the DMD is large.
3. Fabricated fiber with world’s highest spatial density
A cross section of the fabricated fiber is shown in Fig. 4. It has a hexagonally arranged 19-core structure with 6-mode cores, and there are 114 transmission channels per fiber in total. This fiber has the following advantageous features.
3.1 Deployable mechanical reliability
Our fiber was designed to have a suitable fiber diameter for maintaining deployable mechanical reliability. The relationship between the fiber failure probability and fiber diameter is shown in Fig. 5. It is clear that an increase in the fiber diameter causes an increase in failure probability, which reduces the reliability of the fiber. Fiber reliability depends on the bending radius and proof testing as well as the fiber diameter. Proof testing is a process to improve the reliability of the fiber by applying longitudinal stress to the fiber during the fabrication process. A 1–2% proof level is commonly used in the current manufacturing process, and a bending radius of less than 15 mm is assumed for recent high density optical fiber cable design, so we have targeted a fiber diameter of less than 250 μm to obtain the same mechanical reliability as that of SMF. The fabricated fiber has a fiber diameter of 246 μm. Thus, deployable mechanical reliability for a telecommunication network is obtained.
3.2 Highest spatial density
Our fiber has the world’s highest spatial density among reported SDM fibers. To achieve this, we investigated the optimum core structure to incorporate the transmission channels efficiently within a fiber diameter of less than 250 μm. The relationship between the spatial density and the fiber diameter of various multi-mode multi-core structures is shown in Fig. 6. The number of spatial channels is noted in parentheses. Here, the spatial density is the number of transmission channels divided by unit area of the cross section, and it is normalized by that of SMF. We assumed 12–21 core structures with 3-mode cores (red symbols) or 6-mode cores (blue symbols). We found that 6-mode multi-core fiber can achieve larger spatial density than that of 3-mode multi-core fiber, and the 6-mode 19-core structure can be obtained within a fiber diameter of less than 250 μm. We fabricated the 6-mode 19-core fiber as shown in Fig. 4 and achieved the world’s highest spatial density of more than 60.
3.3 Optical properties suitable for long-haul transmission
Our fiber has suitable optical properties for long-haul transmission owing to the well-controlled fabrication process. The refractive index profile of the core is shown in Fig. 7(a). It has a graded index core with a low index trench. The graded index core profile enables us to reduce the DMD, and the low index trench can reduce the inter-core crosstalk. The transmission loss as a function of the DMD value of reported multi-mode multi-core fibers is plotted in Fig. 7(b). As shown in the graph, our fiber had the lowest loss (less than 0.24 dB/km) and DMD (less than 0.33 ns/km) among various fibers. In addition, the inter-core crosstalk was suppressed below –30 dB/100 km, which corresponded to having the potential to transmit quadrature phase shift keying (QPSK) signals over 1000 km with negligible power penalty induced by the crosstalk. We experimentally confirmed that QPSK signals through 114 transmission channels were successfully transmitted over an 8.85-km-long fiber . This indicates that our fiber has suitable transmission characteristics for long-haul SDM networks.