402Tbit/s down commercial optical fibre

402Tbit/s has been sent down 50km of commercial-grade optical fibre by an international team led by the Japanese National Institute of Information and Communications Technology (NICT).

1,505 WDM (wavelength division multiplexed) channels were used simultaneously, spread across six optical bands spanning 37.6THz (275nm) of spectrum.

Signals were boosted with six doped-fiber amplifiers, with lumped and distributed Raman-amplification, “to cover all of the low-loss transmission bands of silica fibers”, according to NICT.

Band	Wavelength (nm)	Channel count	Modulation	Amplifier technology
O	1,260 – 1,360	305	16QAM	bismuth + praseodymium
E	1,360 – 1,460	314	64QAM	bismuth
S	1,460 – 1,530	318	256QAM	thulium
C	1,530 – 1,565	195	256QAM	erbium
L	1,565 – 1,625	253	256QAM	erbium
U	1,625 – 1,675	123	64QAM	Raman

Transmission along optical fibres is normally at the lowest-loss wavelengths: C-band, with L-band added occasionally for more capacity – both are amenable to erbium doped fiber amplification.

Adding more bands can increase the capacity of installed links without the cost of adding more fibres, but this means coping with higher loss and finding suitable amplifiers.

To achieve the record, six doped fibre amplifiers provided gain for O, E, S, C and L-bands, with novel discrete Raman amplification in U-band, and distributed Raman amplification along with custom equalisers for optical gain profile shaping in O and E-bands.

Praseodymium, bismuth and thulium-doped amplifiers were used for the O, E and S-bands, and erbium for C and L-bands.

The fibre was water-absorption-peak-suppressed single-core single-mode with 0.125mm diameter cladding, and a coating taking this to between 0.235 to 0.265mm.

In commercial C-band plus L-band systems, this type of fibre is limited to around 100Tbit/s, increasing to 150Tbit/s in research demonstrations, Ben Puttnam of NICT told Electronics Weekly.

Signals in the six-band NICT demonstration were added using dual-polarisation QAM (quadrature-amplitude modulation) with 16, 64 or 256 symbols per constellation.

NICT collaborated with: Aston University (UK, Raman amplifiers), Nokia Bell Labs (USA, optical gain equalisers), Amonics (Hong Kong, optical fiber amplifiers and Raman amplifiers), University of Padova (Italy, transmission experiments) and the University of Stuttgart (Germany, transmission experiments).

This work was first revealed at OFC 2024 in San Diego earlier this year in the paper ‘402 Tb/s GMI data-rate OESCLU-band transmission’.