One terabit is one trillion (1012) bits. One gigabit is one billion (109) bits.
Figure 2 Profile of standard single-mode optical fiber
Standard optical fiber
According to international standards, the outer diameter of the glass (cladding) of optical fibers is 0.125 ± 0.0007 mm, and the outer diameter of the coating layer is 0.235 to 0.265 mm. The optical fiber widely used in optical communication systems is a single-core single-mode fiber with an outer diameter of 0.125 mm, and the capacity limit is considered to be about 100 terabits per second in the conventional C and L-bands.
Optical fibers have a very small transmission loss compared to coaxial and other electrical cables, but since data is often transmitted over long distances, it is necessary to compensate for attenuation periodically, typically after several tens of kilometers. This is usually done in an optical amplifier which may amplify many wavelength (WDM) channels simultaneously. A common practical amplification method uses rare-earth doped fibers. By adding a small amount of rare earth ions such as erbium/thulium ions or in the case of our new amplifier, bismuth and germanium to the base material of an optical fiber, amplification can be achieved by exciting these ions with lower wavelength pump lasers and then amplifying signal photons through stimulated emission. Such amplifiers have significantly increased the transmission range of optical fiber communication and allowed amplification of many wavelength channels simultaneously. For recent wide-band transmission systems other amplification schemes such as Raman amplification and semiconductor optical amplifiers, have been also employed.
Optical gain equalizer
Equipment that adjusts the relative intensity of light signals at different wavelengths. Among various technologies one approach for gain equalization uses an optical diffraction grating and a spatial light modulator. In this study, we developed an optical intensity adjuster that adjusts a large number of wavelengths in the E band within a single unit.
Wavelength bands (Optical fiber transmission windows)
Various wavelength bands for optical fiber transmission are defined, distinguished by regions with different transmission characteristics arising from physical properties of the fiber, as summarized in Figure 3. The C band (Conventional band, wavelength 1,530 - 1,565 nm) and L band (Long wavelength band, 1,565 - 1,625 nm) are most commonly used for longer commercial transmission, with O band (Original band, 1,260 - 1,360 nm), currently used only for short-range or inter data-centre communications. Although the T band (Thousand band, 1,000 - 1,260 nm）, U band (Ultralong wavelength band, 1,625 - 1,675 nm) is rarely used due to lack of suitable amplification, new amplifier technologies have recently enabled research into the use of S band (Short wavelength band, 1,460 - 1,530 nm). In this experiment we utilize the E band (Extended band,1,360 - 1,460 nm), for the first time, combining with S, C and L-band for dense WDM transmission.
Figure 3 Optical communication wavelength band
Multi-band wavelength division multiplexing (WDM) technology
Wavelength division multiplexing (WDM) is a method of transmitting optical signals of different wavelengths within a single optical fiber. WDM is a widely used technology to increase the transmission capacity in proportion to the number of wavelengths.
In the current optical fiber transmission system, typically only C-band and occasionally L-band wavelength are used. Wavelength bands such as T-band, E-band, S-band, and U-band have not yet been commercialized but currently under research in labs around the world. Large WDM systems using many bands are often called Multi-band WDM systems.
Raman amplification is based on stimulated Raman scattering, when signal photons induce the inelastic scattering of a lower wavelength 'pump' photon in a non-linear optical medium. When this occurs, additional signal photons are produced, with the surplus energy resonantly passed to the vibrational states of molecules in the fiber core. This process, as with other stimulated emission processes, allows all-optical amplification in optical fibers with the gain depending on material of the fiber core.
Quadrature-amplitude modulation (QAM) - Dual polarization (DP)
QAM is a technique for modulating information data on optical signals using multiple levels of both phase and amplitude of the optical wave, that can enable very high spectral information density. 256 QAM uses 256 different signal symbols and can therefore encode 8 bits of information (28 = 256 bits) in each symbol. The spectral density of 256 QAM is therefore 8 times higher than for simple modulation formats such as on-off keying. 64QAM symbols can encode 6 bits in 64 levels while 16QAM symbols code 4 bits in 16 symbol sets. QAM symbols may also be transmitted in both polarizations simultaneously, increasing the number of bits transmitted in each dual polarization (DP) symbol to 16, 12 or 8 for DP-256QAM, DP-64QAM and DP-16QAM respectively.
Generalized mutual information (GMI)
Generalized mutual information is a measure amount of shared information between the transmitted and received signals and provides the number bits per symbol that can be successfully transmitted after decoding, assuming the presence of an optimal error correction code. The data-rate estimated from the GMI is and upper bound and typically higher than the data-rate obtained with the available error correcting codes implemented in real systems.
Figure 4 summarized previous wideband (>100nm), high data-rate (>100 Tb/s) transmission experiments in single-mode fibers. Previous contributions from NICT are also highlighted in red. The previous record was a 2022 journal paper submission using 157 nm over S, C and L-bands.
‘Benjamin J. Puttnam, et.al., “S-, C- and L-band transmission over a 157 nm bandwidth using doped fiber and distributed Raman amplification”, Opt. Express, vol. 30, no. 6, p. 10011, Mar. 2022.’
Figure 4 Recent wideband experiments in single mode fiber
New type of optical fiber
An alternative method of increasing the transmission capacity of optical fibers is by utilizing the spatial domain to support multiple communications channels in the same fiber. This can be in multi-core fibers where many cores are supported in the same cladding or multi-mode fibers where an enlarged core supports many modes. Such fiber requires large changes to optical communications system design, particularly if the diameter of the fiber increases. Hence, even when designing systems with such fibers, it is advantageous to maximize the capacity of each spatial channel to increase data-rates whilst maintaining the same outer-diameter as standard optical fibers that are widely used for optical communications.