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.
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