For over 50 years, optical fibres have revolutionized global communications and connectivity. Today, hundreds of thousands of miles of optical fibre carry vast amounts of data and information across continents and oceans. However, all commercial fibres guiding light through a solid glass core are approaching fundamental limits imposed by the materials. To meet ever-growing demands for bandwidth, capacity and speed, new innovations in fibre technology are required. Hollow core fibres might be the answer.
One route to overcome the limitations of conventional fibres is to guide light through air rather than solid glass. While the concept of air-guiding fibres has existed for decades, practical implementations with suitably low loss have only emerged in the last 20 years. Ongoing research on hollow core fibres aims to unlock their full potential and displace standard fibres for diverse applications.
In a paper published in Optics Express, researchers at the University of Southampton propose a novel hollow core fibre design that combines the advantages of existing air-guiding fibres. Through simulations and tests, they show this new fibre, called a Hollow Core Nested Anti-Resonant Nodeless Fiber (HC-NANF), can potentially achieve lower losses than any hollow or conventional fibre developed to date.
The Two Routes for Optical Guidance in Air
Hollow core fibres guide light using a principle called total internal reflection. Rays propagating along the hollow core hit the surrounding lower-index cladding at angles sharper than a critical value, leading to essentially 100% reflection. This enables low-loss optical guidance in the air through a self-contained system.
To date, two main types of hollow core fibres have been demonstrated. Each uses a distinct cladding design to achieve tight light confinement in the core:
Photonic Bandgap Fibers (PBGFs) – These fibres have a microstructured cladding formed by a periodic array of air holes that creates a photonic bandgap. The light within this bandgap is forbidden from propagating in the cladding and is thus trapped in the core. PBGFs can achieve losses of around 1 dB/km over a narrow bandwidth.
Anti-Resonant Fibers (ARFs) – These fibres guide light by a combination of inhibited coupling and anti-resonance. Thin glass membranes in the cladding act as Fabry-Perot resonators to reflect certain wavelengths. ARFs provide extremely broad bandwidths spanning over an octave but with higher loss.
Each approach has advantages and tradeoffs. Generally, PBGFs offer lower loss while ARFs provide broader bandwidth. Ongoing research aims to improve both designs and expand their applications.
A Combined Approach – Hollow Core Nested Anti-Resonant Nodeless Fibers
The team from Southampton proposes merging the two existing fibre types into one design that leverages the strengths of both. Their proposed HC-NANF consists of nested tubular elements arranged in the cladding without touching nodes. This nodeless structure comes from ARFs and enables ultra-wide transmission bandwidths.
Meanwhile, adding an extra nested tube inside each element provides enhanced antiresonance for strong light confinement, an idea adapted from PBGFs. This additional tube acts as an extra reflecting boundary to create tighter modal confinement.
First simulations indicate this hybrid approach could achieve a remarkably low loss under 0.2 dB/km across a broad spectral range. For context, the loss of modern solid-core single-mode fibres has plateaued around 0.2 dB/km despite intensive development over 40+ years. The designs represent an exciting step to exceed the performance of conventional optical fibres.
Two other important advantages emerge from the simulations:
Rapid Scaling of Loss with Core Size – Due to the nested anti-resonant structure, the loss follows a λ7/R8 dependence on wavelength and core radius. This indicates losses could be rapidly reduced by simply increasing the core size. The team shows losses below 0.1 dB/km may be possible, outperforming any existing hollow-core or solid-core fibre.
Effective Single-Mode Guidance – The dimensions of the nested tubes can be tuned to couple and eliminate higher-order core modes. This resonantly filters out unwanted modes, enabling effectively single-mode behavior which is ideal for many applications like telecommunications.
The Evolution of Record-Breaking Hollow Core Fibres
In 2018, the first low-loss NANF was experimentally demonstrated, achieving 1.3 dB/km loss and effective single-mode behaviour. While record-breaking for a hollow core fibre, losses were still above conventional fibres.
By 2019, improvements in fabrication enabled a NANF to operate in the first anti-resonant window with 6.6 dB/km loss. This provided the broadband low-loss transmission required for high-speed optical data links across the full telecom spectrum.
Just a year later, losses dropped below 1 dB/km for the first time in any hollow core fibre. Advances in draw control and preform assembly produced a record 0.65 dB/km NANF. Simulations reveal room for further improvement, with losses below 0.2 dB/km possible.
In 7 years, careful optimization of the NANF design reduced losses by over 2X per year on average. This rapid evolution showcases the potential of emerging hollow core fibres to soon outperform conventional technologies. NANFs highlight how combining physical understanding with advanced modelling can drive breakthroughs in fibre innovation.
Ongoing research aims to build on these results for real-world deployment. Future work must transfer NANF developments from lab demos into robust manufacturing processes. With sustained progress, hollow core NANFs promise unprecedented capabilities for communications and sensing.
Remarkable Properties to Unlock New Applications
The proposed HC-NANF combines the strengths of PBGFs and ARFs to push beyond the limits of both existing hollow core fibre platforms. Some of the performance advantages HC-NANFs could provide include:
- Losses below conventional solid-core fibres
- Broad transmission bandwidth exceeding an octave
- Effectively single-mode guidance
- Low latency and non-linearity due to air core
- Wide operating wavelength range
- High laser power handling and damage thresholds
According to the researchers, this exceptional combination of properties could make HC-NANFs key enablers for numerous applications, including:
- High-capacity long haul optical data transmission
- Fiber delivery of ultrafast laser pulses
- Non-linear optics and high peak power handling
- Mid-infrared molecular sensing and spectroscopy
- Low noise interferometry and metrology
- Navigation, gyroscopes, and current sensing
HC-NANFs highlight the immense possibilities opened up by hollow-core fibres. Ongoing advances promise to unlock performance far beyond conventional optical fibres and open doors for novel applications and technologies.
Experimental realization of these or related hollow-core designs could provide a pathway to replace existing fibre infrastructure across a range of fields. The proposed fibre concept shows that substantial room for innovation remains to address growing demands for higher capacity, lower latency, and advanced capabilities.
Future Outlook of Hollow Core Fibres
Hollow-core fibre research is a thriving area of study in academia and industry. These intriguing structures continue demonstrating immense possibilities beyond standard glass fibers. Ongoing modelling and optimization are essential to lay the groundwork for next-generation designs and gauge their potential.
However, expanded efforts are critically needed to translate simulations into fabricated fibres for validation and practical use. This requires dedicated and sustained funding for specialized facilities and technical expertise. Bridging modelling and experiments remains the largest hurdle before hollow-core fibres can unlock their full disruptive potential.
While risks exist, the enormous performance enhancements promised by hollow fibres demand focused development efforts. With continued research driving innovations like HC-NANFs, hollow-core fibres are poised to open up new horizons for optics and photonics worldwide. Guided by science and engineering creativity, these smart fibre designs will help address tomorrow’s communication, sensing and light source needs.