Newswise — Significant advancements have recently been achieved in enhancing the efficiency of optical fibers by developing cables capable of transmitting data faster and over wider bandwidths. Among these advancements, hollow-core fibers, despite being prone to leakage, have demonstrated exceptional potential for numerous applications.
In a groundbreaking achievement, a team of scientists, including Dr. Leah Murphy, a recent PhD graduate, and Emeritus Professor David Bird from the Centre for Photonics and Photonic Materials at the University of Bath, has finally unraveled the mystery behind the varying efficiencies of air-filled fiber designs.
Through their theoretical and computational analysis, the researchers have provided a compelling explanation for a phenomenon observed by other physicists: the incorporation of glass filaments in the design of hollow-centered optical fibers results in an unprecedentedly low loss of light during its transmission from the source to the destination.
Dr. Murphy expressed her excitement about the work, emphasizing its ability to shed new light on the 20-year-long discussion surrounding the guidance of light in antiresonant, hollow-core fibers. She expressed optimism that this breakthrough will inspire researchers to explore innovative designs of hollow-core fibers, ensuring ultra-low light loss.
The communication revolution
In recent years, optical fibers have revolutionized communication systems, playing a crucial role in facilitating the remarkable growth of high-speed data transmission. Moreover, specially engineered fibers have emerged as essential components in imaging, lasers, and sensing applications, enabling the development of pressure and temperature sensors for challenging environments.
The most exceptional fibers possess remarkable properties. For instance, a pulse of light can traverse over 50km along a standard silica glass fiber while retaining more than 10% of its original intensity—a comparable feat to seeing through 50km of water.
However, the reliance on solid materials for guiding light in current fibers introduces certain limitations. Silica glass, the primary material used, becomes opaque when transmitting light in the mid-infrared and ultraviolet regions of the electromagnetic spectrum. Consequently, applications requiring these wavelengths, such as spectrometry and astrophysics instruments, cannot employ standard fibers.
Moreover, standard fibers distort high-intensity light pulses, sometimes leading to the destruction of the fiber itself. To overcome these challenges, researchers have been diligently seeking solutions by developing optical fibers that guide light through air instead of glass.
Nonetheless, this approach presents its own set of obstacles. Light inherently resists confinement within low-density regions like air. As a result, optical fibers utilizing air as the guiding medium inherently suffer from leakage, akin to how water would seep through the sides of a hosepipe.
The confinement loss, also known as leakage loss, quantifies the reduction in light intensity as it propagates through these fibers. Consequently, a primary objective of ongoing research is to enhance the fiber's structural design to minimize this loss, ultimately improving its performance.
Hollow cores
The most promising designs for these hollow-core fibers incorporate a central void surrounded and confined by a specially engineered cladding. Within the cladding, ultra-thin-walled glass capillaries are slotted, interconnected with an outer glass jacket.
Remarkably, this configuration enables the hollow-core fibers to achieve loss performance comparable to that of conventional fibers.
Interestingly, despite the significant experimental advancements, the theoretical understanding of how and why these fibers efficiently guide light has lagged behind.
For approximately two decades, scientists have possessed a solid physical understanding of how the thin walls of the glass capillaries facing the hollow core (depicted in green in the diagram) act as reflectors, redirecting light back into the core and preventing leakage. However, a theoretical model solely based on this mechanism significantly overestimates the confinement loss. Consequently, the question of why real fibers exhibit superior light guidance compared to what the simplistic theoretical model predicts has remained unanswered—until now.
In a recently published paper in the esteemed journal Optica, Dr. Murphy and Professor Bird present their model, shedding light on this matter. Their theoretical and computational analysis focuses on the role played by sections of the glass capillary walls (shown in red in the diagram) that face neither the inner core nor the outer wall of the fiber structure.
In addition to their role in supporting the core-facing elements of the cladding, the researchers from the University of Bath have discovered that these elements play a crucial part in guiding light by imposing a structured pattern on the propagating light's wave fields (depicted as grey curved lines in the diagram). They have coined this phenomenon "azimuthal confinement."
Although the underlying principle behind azimuthal confinement is simple, its impact is remarkably influential in elucidating the relationship between the cladding structure's geometry and the fiber's confinement loss.
Dr. Murphy, the paper's lead author, expressed, "We anticipate that the concept of azimuthal confinement will prove significant to other researchers studying light leakage in hollow-core fibers, as well as those involved in the development and fabrication of new designs."
Professor Bird, who spearheaded the project, added, "This has been an immensely fulfilling endeavor that required dedicated time and a fresh perspective to explore every detail. We initiated this investigation during the initial lockdown, and it has kept me engaged throughout my first year of retirement. The paper introduces a novel framework for researchers to contemplate light leakage in hollow-core fibers, and I am confident that it will inspire the exploration of innovative designs."
Dr Murphy was funded by the UK Engineering and Physical Sciences Research Council.
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