Conventional optical fibers are fabulously successful, but they have profound limitations. They contain a glass core at the center of the fiber through which light is transmitted. However, not only does this glass center limit the speed of the light as it passes through, but it also adversely affects other aspects of its propagation, thereby limiting the performance of the fiber and the associated optical system. These include a finite spectral transparency, susceptibility to optical damage, dispersion — which restricts the ability to deliver short and ultrashort pulses — and nonlinear optical response.
Hollow-core optical fibers replace this conventional glass core with air, or a vacuum, do away with the cladding and replacing it with photonic crystals. The light shoots down the hollow core, and when it strikes the edge, the photonic crystals bounce the photons.
By doing away with the plastic/glass, these hollow-core fibers have lower signal loss (allowing for longer distances between repeaters), and the increased speed of light (about 30% faster than plastic/glass) reduces latency. As more than 98% of the mode is confined in air, the fibers are also very radiation insensitive making them suitable for radiation hard environments like space. Other advantages with these fibers are that they are almost entirely bend-insensitive – they may be bent down <1 cm bend diameter without any change of optical transmission. The intensity of light that propagates through glass optical fiber is fundamentally limited by the glass itself, therefore they can handle large powers.
The increased bandwidth with lower latency, greatly improved power handling, and improved overall quality of light transmission, allow for networks that are faster, have more bandwidth, and traverse greater distances as well as open up many new applications. These include delivery of powerful picosecond/subpicosecond pulses throughout the visible and near-IR, damage-free delivery of laser light in the UV, and low-loss transmission into the mid-IR — far beyond the usable spectral window of conventional fibers. They also have huge potential for data centers and the internet backbone.
High-frequency traders have long sought ways to trim the time it takes to complete a transaction to gain an edge on fellow traders. Shaving off microseconds can mean gains in the millions of dollars. Traders have cut the time taken to execute trading algorithms by using specialist hardware such as field-programmable gate arrays (FPGAs), and by writing the algorithms in assembly language code. They run faster on the processor, but require specialist skills to code. High-frequency traders have also employed microwave radio links to streamline the network connection to a trading exchange’s computers. Microwave links are simpler to deploy than laying optical fiber across a city and also have a lower latency. Radio links span up to 100 km and can deliver a gigabit of data—sufficient capacity for the trading data.
In Oct 2020, it was reported that one remaining part of the network where high-frequency traders can trim latency is the last-mile connection between the microwave tower and the datacenter hosting the trades and intra-datacenter connections. It is here that low-latency hollow-core fiber trounces glass fiber. Hollow-core fiber is ideal here because these links are typically shorter than a few kilometers, the data rates are 1 to 10 Gbit/s, and the signals are transmitted via intensity modulation and received via direction detection. The traders implemented the link with OFS (Somerset, NJ) photonic bandgap fiber which provides many unique features. The OFS fiber can replace microwave transmission in the “last-mile” connection between the microwave tower and the datacenter; its speed advantage over glass fiber shaves valuable milliseconds from the time between trading transactions, increasing traders’ profits.
High-power laser light can be easily ducted to its point of application via optical fiber. However, existing solid-core chalcogenide glass fibers used in mid-infrared (mid-IR) applications such as surgery and some types of materials processing absorb enough light that they can overheat, even resulting in damage. A mid-IR version of antiresonant hollow-core fiber developed at the Optoelectronics Research Centre, University of Southampton (England) and the Center of Materials and Nanotechnologies, University of Pardubice (Czech Republic) solves this problem; the outer surface of the fiber is coated with fluorinated ethylene propylene (FEP) polymer to increase durability and protect the fiber from moisture. The tellurite-glass fiber material has high thermal stability and can be synthesized in an ambient air environment. The fiber is close to single-mode in operation.
Gyroscopes for rotation sensing are based on several design platforms, each with their advantages and disadvantages, depending on the application. Advancements in optical fibers have enabled the fiber optic gyroscope (FOG) to become an attractive choice for demanding applications. The fiber optic gyroscope offers high performance, reduced size and weight, high reliability (no moving parts and a high resistance to shock and vibration), and low power consumption. Fiber optic gyroscopes are gaining in addressing many of the markets traditionally dominated by mechanical and ring laser gyroscopes, in particular those used in navigation and guidance of aircraft and spacecraft where performance requirements are demanding. DARPA has utilized these fibers to make incredibly accurate gyro that can be used for navigation where GPS is being actively or passively denied (i.e. in a warzone or indoors). The technology was actually developed as part of DARPA’s Compact Ultra-Stable Gyro for Absolute Reference (COUGAR) program.
Advancements in hollow core photonic bandgap fibers offer an attractive alternative to conventional silica core fibers. The “controlled free-space” guiding properties of the hollow core fibers can significantly reduce many of the performance-limiting characteristics found in conventional fibers, enabling gyroscopes with high performance, improved stability (less drift), and improved packaging. Compared to conventional fibers, the hollow core fibers are much less sensitive to radiation and temperature making the fibers suitable for FOGs for extreme environments.
Inner-surface-coated fibers have long been known to be effective for certain applications, but they are hard to manufacture in long lengths and typically transmit light through multiple spatial modes. Fibers based on the use of a photonic bandgap cladding were developed around the turn of the century and represented a step change improvement in performance characteristics.
Since around 2011, a far simpler family of structures has been shown to represent a dramatic advancement, while being much easier to produce. Like photonic bandgap fibers, these new designs rely on the presence of air holes running down the fiber length to control the flow of light. They trap light in an optical mode of the hollow core by controlling the microstructure and hence the other optical modes in the immediate vicinity of the core. The fibers work by surrounding the central hole with a wall of glass that is antiresonant, enhancing the reflection of light back into the core. These fibers are often referred to as antiresonant hollow-core fibers.
Other elements of the design are also important, such as the shape of the core wall and the number and size of the surrounding air holes. In contrast to photonic bandgap fibers, antiresonant fibers do not require a 2D periodic array of wavelength-scale air holes in the cladding, making them significantly easier and quicker to produce. They also have relatively large core diameters (typically around 30× the wavelength) compared to either conventional fibers or photonic bandgap fibers, which provides several advantages in performance characteristics.
In manufacturing, in biomedicine, and in other fields, ultrashort-pulse fiber lasers have become inexpensive and reliable tools. However, conventional single-mode optical fibers are not able to deliver the ultrashort pulses these lasers produce, often restricting the deployment of these lasers and making them uncompetitive. Conventional fibers literally “fail to deliver” due to their optical nonlinear response and their dispersion. Taken together, these two effects rapidly lengthen ultrashort optical pulses as they travel through even very short lengths of fiber, reducing the peak intensity and making the pulses ineffective for most applications.
Antiresonant hollow-core fibers address both of these unwanted effects. The nonlinear response of the fibers is reduced by many orders of magnitude — first, because the intrinsic Kerr nonlinearity of air is around 1000× less than that of silica glass, and second, because the larger core size reduces the intensity of light in the core.
Conventional optical fibers can suffer from various forms of optically induced damage, but one of the hardest to overcome is caused by the absorption of short-wavelength (UV) photons. The higher photon energy at these short wavelengths causes disruption of the glass matrix at a microscopic scale, leading to the glass suffering from increased absorption. This effect makes it hard to find optical fibers that can stably transmit UV light, although some special fibers with improved characteristics for this spectral window are available. But even these special fibers have limitations and will rapidly degrade when used at short wavelengths and high powers.
Antiresonant hollow-core fibers reduce these problems by several orders of magnitude and offer virtually damage-free transmission of UV light, even for short wavelengths and high optical powers. The light travels mostly through the hollow core, which greatly reduces the rate of damage to the glass. And even when the glass is affected, it makes almost no difference to the light trapped in the core, resulting in a minimal reduction of transmission.
Silica-based fibers are fantastically transparent, but only over a finite spectral range. For wavelengths longer than around 2500 nm, in the MIR, silica becomes absorbing, and optical fibers usually need to be made of different glasses that can transmit these longer wavelengths. Such fibers are available, but they do have drawbacks when compared to the silica fibers that are used for shorter wavelengths. Nonsilica or soft glass fibers are not as strong or as easy to work with, nor are they as transparent. They also tend to have high nonlinearity and dispersion, severely limiting their use for short-pulse delivery.
Despite being made of silica, which becomes absorbing at MIR wavelengths, hollow-core antiresonant fibers can transmit MIR light with low loss because the light travels mainly in the hollow core (Figure 4). Indeed, the optical attenuation of the fibers can be reduced by as much as 20,000× compared to the attenuation of the bulk silica. As a result, in the spectral range between 3 and 5 µm, hollow-core fibers formed from silica have the attractive strength of silica fibers, very low nonlinearity and dispersion, and optical attenuation that is comparable to or better than that of conventional optical fibers formed from IR glass. The combination of these characteristics makes them a potentially valuable technology in this rapidly developing spectral band.
Current research involves using the fibers to develop a new generation of fiber-based optical sources at MIR wavelengths — with high efficiencies and output powers. With clear market advantages, the identification of solutions to the most pressing problems, and a growing number of businesses with the capacity to serve as fiber providers, there is every reason to believe the time has arrived for hollow-core fibers to make a commercial impact.
Similar to ring laser gyroscopes, the FOG is based on the Sagnac effect. In the FOG, a length of optical fiber is set up as a ring interferometer. A signal is launched into the fiber in both directions of the loop, i.e., clockwise (CW) and counterclockwise (CCW), where the optical path length would nominally be the same; however, the Sagnac effect results in a difference in the optical path lengths when the system undergoes rotation. By detecting the two signals on the receive end and combining them, the interference and corresponding phase shift can be related to an angular rotation.
Within the area of fiber optic gyroscopes, the choice of fiber type can play a key role in determining the gyroscope capabilities. For fiber gyroscopes, hollow core fibers offer a number of advantages over solid core silica fibers. This technology provides for
– Low nonlinearities. As more than 98% of the mode is confined in air, not silica, the fibers are less sensitive to nonlinearities such as the Kerr effect.
-Pure silica material, with no co-dopants that can add to fiber environmental sensitivities.
– No Fresnel reflections at open fiber end when free-space coupling in air.
– Polarization maintaining design using form birefringence for low temperature sensitivity.
– Low bend sensitivity, fibers may be bent to very small diameters of less than 1 inch without added losses, thereby enabling smaller form factor designs.
The University of Southampton’s Optoelectronics Research Centre received (6.1 million pound) nearly 8.2 million dollar Engineering and Physical Sciences Research Council (EPSRC)-funded program in 2018 to develop the next generation of fiber optics and position the UK as a world-leader in this technology. The new program will investigate and explore the performance limits of hollow-core fiber technology. It will also develop innovative means of manufacturing and reliable methods of interconnecting these new optical fibers to other more conventional fiber types as well as other optical components such as lasers.
DARPA creates hollow-core optical fiber for faster networks, more accurate sensors
DARPA succeeded in creating hollow-core photonic-bandgap optical fiber in 2013, which allowed light to travel along its length at around 99.7% the speed of light, or a 30% improvement over conventional (silica glass) optic fibers. In almost every fiber-optic network, light travels through plastic or glass fibers — in DARPA’s fiber, light travels through an air gap, allowing for networks that are faster, have more bandwidth, and traverse greater distances.
The cross-section of a submarine fiber optic cable. #6 and #7 are the cladding that would usually destroy a hollow-core signal
According to DARPA, the fact that each fiber is physically separated (single-spatial-mode) allows for higher bandwidth, and any polarization of the light is kept intact (important for sensing, secure communications, and other interesting applications. Specialists from Honeywell International have changed empty central element, significantly enhancing execution and making it the principal empty main element to show single-spatial-mode, low-misfortune, and polarization control.
According to DARPA, the fact that each fiber is physically separated (single-spatial-mode) allows for higher bandwidth, and any polarization of the light is kept in tact (important for sensing, secure communications, and other interesting applications).
The power of light that proliferates through glass optical fiber is on a very basic level restricted by the glass itself. A novel fiber configuration utilizing an empty, air-filled center evacuates this impediment and significantly enhances execution by compelling light to go through channels of air, rather than the glass around it.
“Past instantiations of an empty central element have demonstrated these high proliferation speeds, yet they couldn’t do as such in a blend with the properties that make it valuable for military applications,” said Josh Conway, DARPA program director. “The genuine leap forward with COUGAR fiber is that it can accomplish a solitary spatial-mode, keep up polarization and give low misfortune, all while keeping more than 99 percent of the optical shaft noticeable all around.”
DARPA isn’t the first to create hollow-core fiber, but this is the first time that it has been produced in the US, and with specifications suitable for military use. The technology was actually developed as part of DARPA’s Compact Ultra-Stable Gyro for Absolute Reference (COUGAR) program. COUGAR aims to create an incredibly accurate gyro that can be used for navigation where GPS is being actively or passively denied (i.e. in a warzone or indoors). The new hollow-core photonic-bandgap fiber should allow for the creation of a very accurate optical gyro.
OFS’s photonic bandgap fiber
OFS (Somerset, NJ) introduced a photonic bandgap fiber with unique features . The fiber uses a relatively large air core encapsulated by an array of smaller hexagonal cells that run along the length of the fiber. The cladding also has six large holes—a unique feature—that surround the core. These are “shunts” and their role is explained below.
The hollow-core fiber operates as follows: The cladding confines the light to the air-core along the length of the fiber, as with a solid-core fiber. The periodic air-glass cladding prohibits light from penetrating into the cladding due to the photonic bandgap effect. At some discrete wavelengths, though, such light that is confined to the fiber can be guided not only within the core, but also in the many glass features around the core and in the cladding. Due to the inevitable microscopic roughness of the air-glass interfaces in the fiber, a small fraction of this light keeps being reflected at random angles while it propagates along the fiber, creating narrow spectral regions of high optical loss. So, the transmission spectrum of the hollow-core fiber is characterized by windows of low loss separated by high loss at certain wavelengths. This contrasts with traditional optical fibers that enable transmission over a wide continuous range of wavelengths.
The photonic bandgap fiber has a relatively large core—25 μm in diameter —that provides freedom in how light is distributed so that multiple modes can guide. As mentioned above, the dominant loss mechanism in a hollow-core fiber is scattering at the air-glass interfaces. The fiber loss can be reduced by increasing the core diameter, which has the effect of limiting the amount of light at these interfaces. The disadvantage of multimode transmission is that each mode of light travels at a different speed. The digital information carried by the light doesn’t arrive all at once, but is spread over time, making data recovery harder at the optical receiver.
Higher-order modes have another undesirable effect in that small imperfections in the fiber and disturbances—such as pressure on the fiber due to compression or bending—can cause the modes to couple and exchange energy with each other. This coupling creates an echo of the original signal. Such interactions are known as multipath interference and complicate the data recovery, as the echoes are effectively noise added to the signal. More significantly, multipath interference hinders the working of the fiber cable, as explained below.
OFS has solved the issue of multipath interference by embedding the shunts in the cladding. The shunts deliberately perturb the cladding’s periodic structure to increase significantly the loss experienced by the higher-order modes. The shunts act as an optical filter, stripping out the light in higher-order modes so that the fiber is effectively single-mode.
This fiber exhibits an approximately 30% lower latency when compared to a conventional solid-core fiber, but for a high-frequency trader to realize this benefit, an optical cable package is required to deploy the fiber. And, cabling this fiber has proven to be as complicated as designing it because cabling changes the fiber’s behavior to a degree such that the fiber may not even work. In essence, hollow-core fiber is highly sensitive to disturbances, making practical cabling a challenge. The ideal approach is to avoid modal interference altogether. But this isn’t possible in practice, so the next best thing is to increase the loss experienced by the higher-order modes to stop coupling from arising. This is what the shunts visible in Figure provide—a pathway for the higher-order modes to leak out of the fiber even when protected in a cable. The fiber attenuates the higher-order modes such that multiple-path interference is mitigated.
The hollow-core optical fiber is protected by a cable designed for indoor/outdoor operation; it has been successfully deployed and is carrying live traffic in several high-frequency trading networks. To prepare the fiber for transmission, standard LC or SC connectors are fusion-spliced to the hollow-core fiber. These connectors have a small length of solid-core fiber and are designed to be spliced to the hollow-core fiber while achieving a splice loss better than 2.5 dB.
Attenuation is the most important parameter in closing these transmission links. A 1 km cable has an assumed loss of 10 dB (5 dB/km of the fiber and 5 dB for the connectors), while a 2 km link has a 15 dB loss (10 dB for 2 km of fiber and 5 dB for the connectors). A transceiver with an output power of 0 dBm and a power budget of 20 dB means that the receiver’s sensitivity should accommodate 15 dB of cable and connector loss, while providing an ample safety margin of 7 dB for changes the fiber may experience. Note that there is sufficient margin to include wavelength-division multiplexing (WDM); this technology has been implemented.
These advances, matching low-latency transmission demand of high-frequency traders with the technological status of hollow-core fiber, are significant. However, these are the first steps to realizing the full potential of hollow-core fiber transmission. Solid-core fiber has set the standard for low loss—less than 0.2 dB/km, as well as a large transmission band, although only about 40 nm is routinely used. Clearly, the next steps are to continue to industrialize hollow-core fiber technology to expand its applicability to opportunities for longer lengths by including technologies like forward-error correction and the use of low-latency amplifiers.
Tellurite hollow-core anti resonant fiber for mid-IR is rugged and flexible enough for practical use
High-power mid-infrared (mid-IR) lasers are used for laser surgery, as well as in some types of materials processing—both of which require that the optical power be easily, precisely, and safely moved to the right spot, wherever that may be. Delivering light via optical fiber is a leading approach, with one large drawback of the conventional solid-core chalcogenide glass fibers being absorption of the mid-IR light and the resulting fiber heating and damage. Hollow-core microstructured fibers greatly reduce light absorption due to the air core. Numerous designs, including those of hollow-core antiresonant fibers (HC-ARFs), are being developed; these fibers simultaneously have a large core size and effectively single-mode guidance.
However, HC-ARFs, which usually consist of delicate, air-filled structures within a solid cladding, are difficult to fabricate—in addition, the chalcogenide glass they are often made from can be somewhat toxic. A group of researchers from the Optoelectronics Research Centre, University of Southampton (England) and the Center of Materials and Nanotechnologies, University of Pardubice (Czech Republic) is experimenting with tellurite glass as a material from mid-IR HC-ARFs, and has designed and fabricated tellurite HC-ARFs that have optical losses of only 8.2 ±0.6 dB/m, 4.8 ±0.4 dB/m, and 6.4 ±0.4 dB/m at mid-IR wavelengths of 5, 5.6, and 5.8 μm, respectively. Light was transmitted through a 13-cm-long test sample with near-single-mode quality (M2 factor of 1.2). The fiber preforms are coated with fluorinated ethylene propylene (FEP) polymer such that, when drawn, the finished fibers have an FEP coating that increases durability and protects the fibers from moisture.
The researchers chose a tellurite glass formulation (70TeO2-13ZnO-10BaO-7K2O) with a cutoff wavelength of 6.1 µm; they note that fluoride glasses, another mid-IR material candidate, have much lower thermal stability than tellurite glasses. And unlike chalcogenide and fluorite glasses, tellurite glasses can be synthesized in an ambient air environment (in contrast to needing a protective atmosphere). Both these factors, along with nontoxicity, made it straightforward for the researchers to synthesize the glass in-house, which they did by melting the precursors in a gold crucible at 800°C for 2 hours.
Attenuation measured via FTIR and OPO
Two experimental specimens were made—fibers A and B, both 9 m long, fabricated with 6 and 5.2 mbar of pressure into the fiber’s capillaries during fiber drawing, respectively. The transmission loss of a 36 cm length of fiber A was measured using a Fourier-transform IR (FTIR) spectrometer, with light from a Thorlabs SLS202L stabilized tungsten lamp as the source; minimum measured loss was at a 5.1 μm wavelength (see figure). The results were checked by measuring transmission using an optical parametric oscillator (OPO) at 4.9 and 5 μm as the source; results of the two different approaches matched well. Fiber B was measured using the OPO as a source. In addition, the experimental losses were compared to simulations using COMSOL multiphysics, in which the experimental fiber cross-sections were measured and entered into the simulation software (using the real part of the tellurite glass refractive index).
In another test, Fiber B was subjected to bending at various curvatures from 60 to about 7 cm, with loss doubling at a 9 cm radius. However, the fiber can be bent to a radius of only 4 cm without breaking. The researchers expect that the measured mode quality of M2 = 1.2 for a 13 cm fiber will improve (get closer to 1) as longer lengths of fiber are used, due to the preferential absorption of higher modes. They also note that simulations show that, with improved fabrication techniques, the fiber loss could be lowered to less than 0.1 dB/m in the mid-IR.
Spectral attenuation of a 36-cm length of tellurite glass hollow-core antiresonant fiber (HC-ARF) was measured using both an FTIR spectrometer and an OPO (a); the fiber attenuation was also simulated in software. A section of fiber was measured for bending loss (b). The air-core fiber itself has six-fold symmetry (inset).
Using air to amplify light
In a promising breakthrough for the future of communications, EPFL researchers have developed a technology that can amplify light in the latest hollow-core optical fibers. Today’s optical fibers usually have a solid glass core, so there’s no air inside. Light can travel along the fibers but loses half of its intensity after 15 kilometers. It keeps weakening until it can hardly be detected at 300 kilometers. So to keep the light moving, it has to be amplified at regular intervals.
Luc Thévenaz, the head of the Fiber Optics Group in EPFL’s School of Engineering, has developed a technology to amplify light inside the latest hollow-core optical fibers. Thévenaz’s approach is based on new hollow-core optical fibers that are filled with either air or gas. “The air means there’s less attenuation, so the light can travel over a longer distance. That’s a real advantage,” says the professor. But in a thin substance like air, the light is harder to amplify. “That’s the crux of the problem: light travels faster when there’s less resistance, but at the same time it’s harder to act on. Luckily, our discovery has squared that circle.”
So what did the researchers do? “We just added pressure to the air in the fiber to give us some controlled resistance,” explains Fan Yang, postdoctoral student. “It works in a similar way to optical tweezers – the air molecules are compressed and form into regularly spaced clusters. This creates a sound wave that increases in amplitude and effectively diffracts the light from a powerful source towards the weakened beam so that it is amplified up to 100,000 times.” Their technique therefore makes the light considerably more powerful. “Our technology can be applied to any type of light, from infrared to ultraviolet, and to any gas,” he explains. Their findings have been published in Nature Photonics in August 2020.
Going forward, the technology could serve other purposes in addition to light amplification. Hollow-core or compressed-gas optical fibers could, for instance, be used to make extremely accurate thermometers. “We’ll be able to measure temperature distribution at any point along the fiber. So if a fire starts along a tunnel, we’ll know exactly where it began based on the increased temperature at a given point,” says Flavien Gyger, PhD student. The technology could also be used to create a temporary optical memory by stopping the light in the fiber for a microsecond – that’s ten times longer than is currently possible.
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