Figure 3: Image of optical fiber communication using a single core, single mode fiber
Space Division Multiplexing (SDM)
Single-core, single-mode fibers (SMF; see Figure 3), which are widely used commercially, have a transmission capacity limit of several hundred Tb/s. To overcome this problem, optical fibers with an increased number of optical paths (spatial channels) using multiple cores or modes have been studied (see Figure 4). Fiber-optic communication technology that uses such optical fibers is collectively called space-division multiplexing (SDM). This study used a 38-core 3-mode optical fiber (Sumitomo Electric Industries, Ltd.).
Figure 4: Image of SDM using a multicore multimode fiber, multiband WDM, and multilevel modulation
Wavelength Division Multiplexing (WDM)
WDM is used to transmit optical signals of different wavelengths within a single optical path. The optical signals are assigned to carrier frequency slots within the wavelength band. The total frequency bandwidth is then determined by the spacing and number of wavelength channels. Hence, the transmission capacity can be increased by increasing the number of wavelength channels.
However, the wavelength band suitable for telecommunication applications is limited. The C-band (wavelength of 1,530-1,565 nm) is mainly used in current optical communication systems, and the number of wavelength channels for the 100 GHz frequency grid is ~50. L-band (1,565-1,625 nm) has recently been used in the commercial systems for additional capacity. By contrast, the T-band (1,000-1,260 nm), O-band (1,260-1,360 nm), E-band (1,360-1,460 nm), S-band (1,460-1,530 nm), and U-band (1,625-1,675 nm) have not yet been commercialized. WDM that includes these bands is commonly called multi-band WDM. The widest frequency bandwidth used in previous multi-band WDM experiments was 20 THz in the S-, C-, and L-bands. Recently, we also demonstrated multi-band WDM transmission with 27.4 THz total bandwidth using the E-, S-, C-, and L-bands, to be presented in another press release.
Multicore fiber (MCF)
An MCF has many cores (physical optical paths) in a common cladding region, and its total transmission capacity can be increased by transmitting different data through each core. Two types of MCFs are commonly investigated: uncoupled and randomly coupled. Uncoupled MCFs that confine each signal to a corresponding core are suitable for early adaptation. Randomly coupled MCFs with numerous cores are under research as candidates for next-generation transmission media.
A multimode fiber has a large core diameter that can support multiple modes within the same core. Intermodal signal interference occurs at the fiber connections, inputs/outputs, and during multimode fiber propagation. Therefore, MIMO receivers that undo the interference through MIMO digital signal processing are required to recover transmitted signals. Multimode fiber transmission with a maximum of 55 modes has been realized. This study used a multicore multimode optical fiber containing 38 cores, each supporting 3-mode propagation and an addition core supporting single-mode propagation unused (see Figure 4).
MIMO is a signal processing technique used to eliminate multipath interference in wireless communications. It is also used in SDM optical communication systems to eliminate the signal interference between different cores (in the case of randomly coupled MCFs) and different modes (in the case of multimode fibers).
Polarization-multiplexed 256 QAM
Multi-level modulation is a technique that encodes multiple bits on a lightwave by precisely controlling its amplitude and/or phase. Multi-level modulation, in which the amplitude and phase are used simultaneously, is called Quadrature Amplitude Modulation (QAM). Because 256 QAM uses 256 different points in the complex phase space, it can encode eight bits of information (28 = 256) in each transmitted symbol. Thus, the spectral density of 256 QAM is eight times higher than that of a simple modulation format, such as on-off keying. The data-rate can be further doubled by polarization multiplexing, in which different data signals are transmitted in two orthogonal polarization states.