MSGID: 1:317/3 64a3a081   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Limiting loss in leaky fibers    
    A theoretical understanding of what makes some hollow-core optical fibers   
   more efficient than others will inspire the design of new low-loss fibers    
      
     Date:   
         July 3, 2023   
     Source:   
         University of Bath   
     Summary:   
         Scientists have developed a mathematical model to explain how   
         antiresonant hollow-core fibers guide light in a way that keeps data   
         loss ultra-low. Until now, scientists had no complete explanation   
         for this well-observed phenomenon.   
      
      
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   A theoretical understanding of the relationship between the geometrical   
   structure of hollow-core optical fibres and their leakage loss will   
   inspire the design of novel low-loss fibres.   
      
   Immense progress has been made in recent years to increase the efficiency   
   of optical fibres through the design of cables that allow data to   
   be transmitted both faster and at broader bandwidths. The greatest   
   improvements have been made in the area of hollow-core fibres -- a   
   type of fibre that is notoriously 'leaky' yet also essential for many   
   applications.   
      
   Now, for the first time, scientists have figured out why some air-filled   
   fibre designs work so much more efficiently than others.   
      
   The puzzle has been solved by recent PhD graduate Dr Leah Murphy and   
   Emeritus Professor David Bird from the Centre for Photonics and Photonic   
   Materials at the University of Bath.   
      
   The researchers' theoretical and computational analysis gives a clear   
   explanation for a phenomenon that other physicists have observed in   
   practice: that a hollow-centred optical fibre incorporating glass   
   filaments into its design causes ultra-low loss of light as it travels   
   from source to destination.   
      
   Dr Murphy said: "The work is exciting because it adds a new perspective   
   to a 20-year-long conversation about how antiresonant, hollow-core fibres   
   guide light. I'm really optimistic that this will encourage researchers to   
   try out interesting new hollow-core fibre designs where light loss is kept   
   ultra-low."  The communication revolution Optical fibres have transformed   
   communications in recent years, playing a vital role in enabling the   
   enormous growth of fast data transmission. Specially designed fibres have   
   also become key in the fields of imaging, lasers and sensing (as seen, for   
   instance, in pressure and temperature sensors used in harsh environments).   
      
   The best fibres have some astounding properties -- for example, a pulse   
   of light can travel over 50km along a standard silica glass fibre and   
   still retain more than 10% of its original intensity (an equivalent   
   would be the ability to see through 50km of water).   
      
   But the fact that light is guided through a solid material means current   
   fibres have some drawbacks. Silica glass becomes opaque when the light it   
   is attempting to transmit falls within the mid-infrared and ultraviolet   
   ends of the electromagnetic spectrum. This means applications that need   
   light at these wavelengths (such as spectrometry and instruments used   
   by astrophysicists) cannot use standard fibres.   
      
   In addition, high-intensity light pulses are distorted in standard fibres   
   and they can even destroy the fibre itself.   
      
   Researchers have been working hard to find solutions to these drawbacks,   
   putting their efforts into developing optical fibres that guide light   
   through air rather than glass.   
      
   This, however, brings its own set of problems: a fundamental property   
   of light is that it doesn't like to be confined in a low-density region   
   like air.   
      
   Optical fibres that use air rather than glass are intrinsically leaky   
   (the way a hosepipe would be if water could seep through the sides).   
      
   The confinement loss (or leakage loss) is a measure of how much light   
   intensity is lost as it moves through the fibres, and a key research goal   
   is to improve the design of the fibre's structure to minimise this loss.   
      
   Hollow cores The most promising designs involve a central hollow core   
   surrounded and confined by a specially designed cladding. Slotted within   
   the cladding are hollow, ultra-thin-walled glass capillaries attached   
   to an outer glass jacket.   
      
   Using this set-up, the loss performance of the hollow-core fibre is   
   close to that of a conventional fibre.   
      
   An intriguing feature of these hollow-core fibres is that a theoretical   
   understanding of how and why they guide light so well has not kept up   
   with experimental progress.   
      
   For around two decades, scientists have had a good physical understanding   
   of how the thin glass capillary walls that face the hollow core (green   
   in the diagram) act to reflect light back into the core and thus prevent   
   leakage. But a theoretical model that includes only this mechanism greatly   
   overestimates the confinement loss, and the question of why real fibres   
   guide light far more effectively than the simple theoretical model would   
   predict has, until now, remained unanswered.   
      
   Dr Murphy and Professor Bird describe their model in a paper published   
   this week in the leading journal Optica.   
      
   The theoretical and computational analysis focuses on the role played   
   by sections of the glass capillary walls (red in the diagram) that face   
   neither the inner core nor the outer wall of the fibre structure.   
      
   As well as supporting the core-facing elements of the cladding, the Bath   
   researchers show that these elements play a crucial role in guiding   
   light, by imposing a structure on the wave fields of the propagating   
   light (grey curved lines in the diagram). The authors have named the   
   effect of these structures 'azimuthal confinement'.   
      
   Although the basic idea of how azimuthal confinement works is simple, the   
   concept is shown to be remarkably powerful in explaining the relationship   
   between the geometry of the cladding structure and the confinement loss   
   of the fibre.   
      
   Dr Murphy, first author of the paper, said: "We expect the concept   
   of azimuthal confinement to be important to other researchers who are   
   studying the effect of light leakage from hollow-core fibres, as well   
   as those who are involved in developing and fabricating new designs."   
   Professor Bird, who led the project, added: "This was a really rewarding   
   project that needed the time and space to think about things in a   
   different way and then work through all the details.   
      
   "We started working on the problem in the first lockdown and it has now   
   been keeping me busy through the first year of my retirement. The paper   
   provides a new way for researchers to think about leakage of light in   
   hollow-core fibres, and I'm confident it will lead to new designs being   
   tried out."  Dr Murphy was funded by the UK Engineering and Physical   
   Sciences Research Council.   
      
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   Materials provided by University_of_Bath. Note: Content may be edited   
   for style and length.   
      
      
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   Journal Reference:   
      1. Leah R. Murphy, David Bird. Azimuthal confinement: the missing   
      ingredient   
         in understanding confinement loss in antiresonant, hollow-core   
         fibers.   
      
         Optica, 2023; 10 (7): 854 DOI: 10.1364/OPTICA.492058   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230703133108.htm   
      
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