Mitch Leslie

Senior Technology Writer

In August of 2020,engineers at University College London(UCL)in the United Kingdom reported a new record for transmitting data through a single optical f iber,hitting 178 terabits per second(Tbps)and beating the old mark by 20%[1].And in June 2020,tw o US telecommunications companies,Sunnyvale,CA-based Inf inera and Little Rock,AK-based Windstream,reached another milestone.For the f irst time,the collaborators maintained an 800 gigabits per second(Gbps)transmission rate in one f iber over a long distance,using a line running more than 730 km from San Diego,CA,to Phoenix,AZ,USA[2].

These achievements demonstrate tw o of the novel approaches that may help meet the insatiable demand for new capacity in optical f iber netw orks.They are all the more signif icant because they did not require new types of cable,said Alan Willner,a professor of electrical engineering at the University of Southern California in Los Angeles,CA,USA.‘‘Even w ith an existing deployed f iber,one can dramatically upgrade the capacity of the system by innovating at the terminals.”

An explosion in internet traff ic is driving the demand for more optical f iber bandw idth.Netw orking giant Cisco(San Jose,CA,USA)has estimated that internet traff ic w ill be about three times higher in 2021 than it w as in 2016[3].How ever,that estimate did not account for effects of the coronavirus disease 2019(COVID-19)pandemic,w hich has forced more people to w ork from home and increased their reliance on the internet for entertainment,shopping,and other activities they once did off line[4].In addition,the rollout of f ifth-generation(5G)cellular netw orks in the United States and other countries w ill further boost the need for f iber optic capacity[5].‘‘People used to think you didn’t need terabit transmission rates—but w e are there now,”said Dan Blumenthal,a professor of electrical and computer engineering at the University of California,Santa Barbara.

Increasing the capacity of optical f ibers could bring another benef it,said Blumenthal.Fiber optic netw orks consume a large amount of pow er,so‘‘energy eff iciency is paramount,”he said.‘‘The more bandw idth you get,the better.”

New types of optical f iber may be able to take some of the load.In the last 40 years,various improvements have increased the capacity of optical f ibers by more than ten million times[6],and engineers are pushing for more.The glass core of a f iber is w hat transmits light,and one route to boosting capacity is so-called multicore f iber that is essentially several f ibers in one[6].

Multicore designs have set transmission records in the laboratory.In 2020,researchers at the National Institute for Communications Technology in Japan delivered data at a rate of 172 Tbps over a f iber w ith three cores[7].(The UCLteam’s record w as for single-core f iber.)The f irst f ield tests of a multicore f iber started in Italy in 2019[8].Researchers are pursuing other promising avenues[9],including hollow f ibers through w hich light travels 30 times faster[10].

But new f ibers face a huge hurdle—the cost of installation(Fig.1).Laying f iber optic lines can cost up to 500 000 USD·km-1[1],spurring a search for cheaper alternatives.‘‘There has to be installation of new f iber,”said John Ballato,a professor of materials science and engineering at Clemson University in Clemson,SC,USA.‘‘But there is already a ton of f iber in the ground.The question is,can w e squeeze more data through existing netw orks?”

Much of the installed optical f iber w as designed to transmit at 100 Gbps.To expand its data-carrying ability,‘‘w e are now playing around w ith the attributes of the light and using it to pack more information into the f iber w ith minimal distortion,”said Ballato.One strategy,know n as w avelength-division multiplexing,involves increasing the number of w avelengths of light traveling through the f iber,thus adding information-carrying channels[6].

Fig.1.Workers bury new f iber optic cable in Wagga Wagga,NSW,Australia.The high cost of installing such cable has helped drive research into expanding the amount of information existing optical f iber netw orks can carry.Credit:Bidgee,Wikimedia Commons(CCBY-SA 3.0 AU).

The UCL group took this strategy a step further by exploiting a block of spare w avelengths.Fiber optic lines for long-distance transmission typically carry only light in the so-called C band,w hich spans 1530-1565 nm,although some cables make use of portions of the L band betw een 1568 and 1605 nm[1].The UCL team added w avelengths in the little-used Sband betw een 1484 and 1520 nm.Deploying three lasers and a combination of amplif iers,they achieved a spectral range of 16.8 THz,the most for a f iber w ith one core and about tw ice the value for the best commercial f iber optic netw orks.With the added bandw idth,the researchers w ere able to send data at 178 Tbps for 40 km over a standard f iber,w hich had a nominal capacity of 100 Gbps[11].‘‘This a beautiful demonstration of using a broader w avelength range,”said Willner.

How ever,transmitting more w avelengths typically means adding lasers and other equipment,w hich is expensive and consumes large amounts of pow er.In 2020 another team demonstrated that they could pack much of the electronics into one microchip[12].Bill Corcoran,a lecturer in electrical and computer systems engineering at Monash University in Melbourne,VIC,Australia,and colleagues developed a microcomb,a device that pairs a laser w ith an optical resonator that splits the laser’s light into 80 w avelengths.The researchers reported a transmission rate of 40 Tbps for nearly 77 km through an installed f iber linking Monash University to Royal Melbourne Institute of Technology University,another university in the Melbourne area[13].‘‘We’ve provided a w ay to reduce the amount of equipment needed,”said Corcoran.

The Inf inera-Windstream collaboration took on a different transmission problem.Telecommunications companies are debuting netw orks w ith capacities of 400 Gbps and even 800 Gbps[14].But because performance declines w ith distance,a 400 Gbps line typically can sustain that transmission rate for only about 100 km[15].The Inf inera-Windstream team w as able to maintain the 800 Gbps rate over 700 km in part because it used so-called Nyquist subcarriers,w hich are generated by carving the beam from each laser into multiple data streams[2].One benef it to the approach is that it reduces chromatic dispersion as the light travels through the f iber[16].

Ballato said that demonstrations like the one by the UCLgroup are important because they show w hat is possible,but the Inf inera-Windstream test illustrates w hat performance is attainable now.‘‘They are show ing 800 Gbps over a real netw ork,w hich is impressive.”

The upshot,the experts agree,is that large gains in capacity are on the w ay.‘‘The result from the UCL group is a sign of things to come.It may take a few years,but the commercial demand isthere,”said Ballato.But innovation w ill have to continue,said Willner.‘‘Even w hen you double capacity,it w ill only last for a few years.”