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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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
==========================================================================

Link to news story:
https://www.sciencedaily.com/releases/2023/07/230703133108.htm

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