World Record 402 Tb/s Transmission in a Standard Commercially Available Optical Fiber

The growth of internet and data-services has driven demand for optical transmission bandwidth. To meet this demand, multi-band wavelength division multiplexing (WDM) technology, where new spectral windows are used to increase optical fiber transmission bandwidth, has become a popular research topic. Utilizing new transmission windows in deployed fibers also offers a potentially significant benefit in the near-term as a method of extending the life of existing fiber systems to provide additional transmission capacity without the large capital expenditure associated with new fiber deployment. However, moving away from the lowest loss regions of standard silica fibers requires new amplification schemes beyond the standard erbium (E-) doped fiber amplifier (DFA) that is a staple of C-band or C+L-band systems. Previously, S/C/L-band transmission has been explored with various amplifier solutions. In addition to thulium (T-) DFAs, semiconductor optical amplifiers (SOAs), distributed and discrete Raman amplification has been used, with maximum estimated data-rates of 256 Tb/s utilizing almost 20 THz bandwidth. Even wider transmission demonstrations have used bismuth (B-DFAs) for O-band and lumped Raman amplifiers for U-band channels for 119 Tb/s with a cumulative bandwidth of 25 THz. E-band BDFAs were also used with distributed Raman amplification for E/S/C/L-band transmission over 27.8 THz with <320 Tb/s GMI estimated data-rate, as shown in Table 1.

In this demonstration, we expand dense wavelength division multiplexed (DWDM) transmission to cover all the major transmission bands in the low-loss window of standard optical fibers to enable more than 1,500 parallel transmission channels within an aggregate 37.6 THz (275 nm) optical bandwidth.

It is expected that the data-rate of optical transmission systems required to enable “Beyond 5G” information services will increase enormously. New wavelength regions enable deployed optical fiber networks to perform higher data-rate transmission and extend the useful life of existing network systems. It is also anticipated that new bands can address the increasing demand of next generation communications services by combining with new types of optical fibers.

The paper containing these results was presented at the Optical Fiber Communication (OFC) Conference 2024, the largest international optical communications conference, having been selected as a post-deadline paper. The post-deadline session is a special session to showcase the latest important research achievements and was held on Thursday March 28, 2024 at the San Diego Convention Center, California, USA.

NICT will continue to promote research and development into new amplifier technologies, components and fibers to support new transmission windows for both near and long-term applications. NICT will also aim to extend the transmission range of such wideband, ultra-high-capacity systems and their compatibility for field deployed fibers.

The research team participating in this study are as follows.
・NICT Photonic Network Laboratory: Design and development of transmission system
・Aston University (UK): Development of Raman amplifiers, supported by EPSRC Grants EP/V000969/1, EP/R035342/1 and Royal Society grant IES/R3/223001.
・Nokia Bell Labs (USA): Development of optical gain equalizers
・Amonics (Hong Kong): Development of optical fiber amplifiers and Raman amplifiers
・University of Padova (Italy): Participated in transmission experiments
・University of Stuttgart (Germany): Participated in transmission experiments

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.

Various wavelength bands for optical fiber transmission, as summarized in Figure 3, are distinguished by regions with different transmission characteristics arising from physical properties of the fiber and amplifier technology. The C-band (wavelength 1,530 - 1,565 nm) and L-band (1,565 - 1,625 nm) are most commonly used for long-haul commercial transmission, with O-band (1,260 - 1,360 nm), used for short-range or inter data-center links. Recently, S-band (1,460 - 1,530 nm) transmission experiments have been enabled by development of Thulium (T-) doped fiber amplifiers (DFAs) and Bismuth (B-) and Praseodymium (P-) DFAs have been developed for O-band and E-band (1,360 - 1,460 nm). Although T-band (Thousand band, 1,000-1,260 nm） is limited by high fiber loss and limited amplifier options, discrete Raman amplifiers have recently been proposed for U-band (1,625 - 1,675 nm) amplification. In this experiment we use P/B/TDFAs for O/E/S-band, Erbium (E-)DFAs for C/L-bands and new discrete Raman amplifiers for U-band.

Optical fibers have a very small transmission loss compared to coaxial and other electrical cables, but it is necessary to compensate for attenuation periodically (10s of km) to transmit over long distances. 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 or bismuth ions to the base material of an optical fiber, amplification can be achieved by exciting these ions with shorter wavelength pump lasers to amplify signal photons through stimulated emission. Such amplifiers have significantly increased the fiber transmission range and allow amplification of many wavelength channels simultaneously. Recent wide-band transmission systems have also employed alternative amplifiers such as Raman amplification and semiconductor optical amplifiers.

Raman amplification is based on stimulated Raman scattering, where signal photons induce the inelastic scattering of a shorter 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 allows all-optical amplification in optical fibers with the gain depending on the material of the fiber core. Discrete Raman amplifiers typically use dedicated non-linear fibers and pump lasers to provide Raman gain at specific wavelengths. Distributed Raman amplification injects pumps directly into the transmission fiber to boost signal channel power during transmission.

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 used custom designed optical gain equalizers to control hundreds of WDM channels across O and E-band signals.

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 current optical fiber transmission systems, typically only C-band and occasionally L-band wavelengths are used. Wavelength bands such as T-band, O-band, E-band, S-band, and U-band have not yet been commercialized but are currently under research in labs around the world. Large WDM systems using many bands are often called Multi-band WDM systems.

Generalized mutual information is a measure of the amount of shared information between the transmitted and received signals and provides the number of 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 an upper bound and typically higher than the data-rate obtained with the available error correcting codes implemented in real systems.

Figure 4 shows wideband (>15 THz), high data-rate (>100 Tb/s) transmission experiments in single-mode fibers. Previous NICT contributions are highlighted in red. The previous record was a 2023 European Conference on Optical Communications (ECOC) paper ‘301 Tb/s E, S, C+L-Band Transmission over 212 nm bandwidth with E-band Bismuth-Doped Fiber Amplifier and Gain Equalizer’.

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.