Hollow-core fiber loss below 1 dB/km could lead to applications

Postdeadline reports of record low loss and record high-speed data transmission in hollow-core fibers at the European Conference on Optical Communications 2019 (ECOC 2019; held in Dublin, Ireland) show the growing maturity of a technology that has long been in the laboratory stage. Thomas Bradley of the University of Southampton's Optoelectronic Research Centre (Southampton, England) and colleagues measured an attenuation of 0.65 dB/km across the full C and L telecommunication bands.¹ Antonio Nespola of the LINKS Foundation (Torino, Italy) and colleagues from Southampton and elsewhere transmitted digital signals through a total of 341 km of hollow-core fiber in a loop experiment, a record for hollow-core fibers.

Both are important steps for hollow-core dielectric waveguides, a concept proposed by Bell Labs in the 1960s. Largely shelved after the development of low-loss, solid-core glass fibers in the early 1970s, the concept later was revived for transmission of carbon-dioxide laser beams. Development of photonic-bandgap fibers revived interest in hollow-core fibers around the turn of the century, with developers hoping they could reduce nonlinear effects at the high power densities in single-mode fibers and transmit light significantly faster through air than is possible in glass. A series of new designs and demonstrations followed, but losses remained in the tens of decibels per kilometers until a few years ago.

Antiresonant design reduces light loss

That changed a few years ago after Walter Belardi and Jonathan Knight of the University of Bath (Bath, England) proposed an “antiresonant” design for thin glass membranes running the length of the fiber. Hollow-core fibers have resonances that make the glass membranes transparent at certain wavelengths depending on their geometry. However, that leaves a broad range of wavelengths between resonances where very little of the light can enter the membranes, so virtually all of the light remains trapped in the hollow core (see figure), says Francesco Poletti of the Southampton group.

At last year's ECOC, Southampton reported an antiresonant hollow-core fiber with a then-record-low attenuation of only 1.3 dB/km. This year's postdeadline paper reported half that level. “It has [taken] many years of research to reach this point,” says Knight, who was impressed by the rapid improvements. “Further loss reductions might be expected, and that makes it a very exciting result,” Knight adds, holding out hope of reducing losses below those of conventional fibers, which “would be a genuinely major achievement.”

Two key design refinements in the structure of the thin cylindrical glass membranes helped Southampton reduce attenuation in its new antiresonant fibers, says Poletti. One was the elimination of nodes where the tubes touch each other. That avoids thick regions of glass at the nodes, which can produce resonances and affect fiber performance. The second was nesting smaller tubes inside the bigger ones, which reduces light leakage from the core by up to a factor of a thousand. Thickness of the tubes also was reduced by more than half, to 0.5 µm. In addition to reducing the loss, the changes also increased the range of the flat band of low loss by about a factor of three to 120 nm between 1520 and 1650 nm, compared to 1420 to 1460 nm in the fiber they described in 2018.

Another improvement was in the length of fiber drawn, from 0.5 km in the previous record-setter to 1.23 km in the new fiber. Southampton has been scaling up its draw operations to work with larger preforms, says Poletti. An important benefit of their circular design is that the fluid dynamics of fiber drawing pulls internal structures into circular shapes, so the preform can be a scaled-up version of the desired fiber design, and the process should scale to longer lengths.

By comparing measured results with finite element simulations, the Southampton group broke down the total loss of 0.65 dB/km into 0.31 dB/km arising from light leakage, 0.24 dB/km due to microbending, and 0.1 dB/km due to surface scattering. They consider surface scattering as “the only truly fundamental loss mechanism of the fiber,” so they hope for further reduction in total attenuation.

Low nonlinearity and high power

The most immediate attractions of hollow-core fibers come from their much lower nonlinearity and potentially much higher damage threshold than solid-core silica fibers. "Laser beam delivery is clearly a very attractive area," says Poletti. "Some of these fibers are already available for delivery of ultrashort laser pulses," where peak powers can be extremely high.

Another potential application is transmission of wavelengths strongly absorbed by conventional glasses. With only about 50 parts per million of the light in the best hollow-core fibers overlapping the internal glass structures, Poletti says it's possible to make beam-delivery fibers even for wavelengths where silica is opaque. That could open up the mid-infrared, which has required exotic fluoride glasses, and the ultraviolet, which darkens silica glasses.

Hollow-core fibers also can contain gases to give long interaction lengths with laser light passing through the core for sensing, selective excitation, or other applications.

Telecommunication transmission experiments

The transmission experiments were preliminary efforts to assess signal transmission in hollow-core fibers. The Southampton group used the longest continuous length of antiresonant hollow-core fiber ever drawn in a single pull, but that was only 3.4 km, to which they added a 1.4 km length to give 4.8 km of fiber with average loss of 1.18 dB/km in the erbium-amplifier C band. They added 51 km of standard single-mode fiber to provide the delay needed for looped transmission with a fiber amplifier. That let them demonstrate transmission of PM-16QAM signals 21 times through the loop, spanning 125 km of the hollow-core fiber, with signals on 61 optical channels. They also sent more-robust PM-QPSK signals through 71 loops, including 341 km of hollow-core fiber, but did not have the time needed to test wavelength-division-multiplexed signals.

“The main message is that we did not find any problem in going [what was] a very short distance for standard fiber, but a very long distance for hollow-core fiber,” Poletti says. The group also presented simulations suggesting that hollow fibers with attenuation reduced well below that of conventional silica fibers might have transmission capacities two to five times higher. He explains that enhanced capacity would come from the much lower nonlinear effects in hollow fibers because nonlinearities now limit capacity of standard single-mode fiber.

However, Peter Winzer, director of optical transmission subsystems research at Nokia Bell Labs, sees little near-term hope for practical use of hollow-core fibers other than reducing latency for high-frequency security traders. They are willing to pay a steep premium to save milliseconds in placing trades, but they remain a niche application. Other time-sensitive applications such as cloud computing also could benefit from low latency, but Winzer says “those applications are very cost-sensitive and would first require the cost and performance of low-latency fiber to be comparable to standard fiber.”

REFERENCE

1. T. D. Bradley et al., “Antiresonant hollow core fibre with 0.65 dB/km attenuation across the C and L telecommunication bands,” postdeadline paper PD.3.1, presented at ECOC 2019, Dublin, Ireland.