1/11/22, 9:40 AM
Hollow-Core Optical Fibers Offer Advantages at Any Wavelength | Features | Apr 2019 | Photonics Spectra
Hollow-Core Optical Fibers Offer
Advantages at Any Wavelength
As demands on optical fiber performance increase, researchers show that hollow-core fibers
may prove useful in the MIR and UV, and for delivering ultrashort pulses in the visible and
near-IR.
JONATHAN KNIGHT, UNIVERSITY OF BATH; DUNCAN HAND, HERIOT-WATT
UNIVERSITY; AND FEI YU, CHINESE ACADEMY OF SCIENCES
Conventional optical fibers are fabulously successful, but they have profound limitations.
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.
Such limitations are fundamental to conventional optical fibers because the light travels
through a solid or a liquid material. In most conventional fibers, fused silica is the material
that forms the glassy core of the fiber. Researchers are now demonstrating that an
alternative optical fiber technology, based on the use of silica fibers but with hollow cores
and using different optical physics, can substantially outperform standard fiber designs for
numerous applications.
https://www.photonics.com/Articles/Hollow-Core_Optical_Fibers_Offer_Advantages_at/a64402
1/8
1/11/22, 9:40 AM
Hollow-Core Optical Fibers Offer Advantages at Any Wavelength | Features | Apr 2019 | Photonics Spectra
Figure 1. Antiresonant hollow-core fiber is delivering 300-fs ultrafast laser pulses with
average power up to 1 W. The glowing of fiber comes from the supercontinuum generated
by laser pump in the air of the hollow core. Courtesy of Heriot-Watt University.
In some applications — for example, for data transmission in terrestrial optical
telecommunications systems, or as gain fibers for short-pulse fiber lasers — conventional
fibers will remain preeminent. But numerous emerging applications are being enabled by
alternative fiber designs. 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. With these and other applications, this alternative hollow-core
technology is earning a place in the toolkit of photonic components for the future (Figure 1).
Trapping light in a hollow core
The lure of hollow-core optical fibers is not new. Inner-surface-coated fibers have long been
known to be effective for certain applications1, but they are hard to manufacture in long
lengths and typically transmit light through multiple spatial modes. Fibers based on the use
of a photonic bandgap cladding were developed around the turn of the century and
https://www.photonics.com/Articles/Hollow-Core_Optical_Fibers_Offer_Advantages_at/a64402
2/8
1/11/22, 9:40 AM
Hollow-Core Optical Fibers Offer Advantages at Any Wavelength | Features | Apr 2019 | Photonics Spectra
represented a step change improvement in performance characteristics.
Figure 2. Cross sections of functional designs of antiresonant hollow-core fibers developed
at the University of Bath. Dark regions are air and lighter regions are fused silica. Courtesy
of University of Bath.
Since around 2011, a far simpler family of structures has been shown to represent a
dramatic advancement2, while being much easier to produce. Like photonic bandgap fibers,
these new designs rely on the presence of air holes running down the fiber length to control
the flow of light. They trap light in an optical mode of the hollow core by controlling the
microstructure and hence the other optical modes in the immediate vicinity of the core. The
fibers work by surrounding the central hole with a wall of glass that is antiresonant,
enhancing the reflection of light back into the core. These fibers are often referred to as
antiresonant hollow-core fibers.
Other elements of the design are also important, such as the shape of the core wall and the
number and size of the surrounding air holes. In contrast to photonic bandgap fibers,
antiresonant fibers do not require a 2D periodic array of wavelength-scale air holes in the
https://www.photonics.com/Articles/Hollow-Core_Optical_Fibers_Offer_Advantages_at/a64402
3/8
1/11/22, 9:40 AM
Hollow-Core Optical Fibers Offer Advantages at Any Wavelength | Features | Apr 2019 | Photonics Spectra
cladding, making them significantly easier and quicker to produce. They also have relatively
large core diameters (typically around 30× the wavelength) compared to either conventional
fibers or photonic bandgap fibers, which provides several advantages in performance
characteristics (Figure 2).
Figure 3. Characteristics of Ti:sapphire femtosecond pulse delivered through a 4-m
antiresonant hollow-core fiber. FWHM: full width half maximum (a). Hollow-core fiber
delivered picosecond laser pulses can be used to machine-fuse silica (the same material as
the fiber). Scale bar is 200 µm (b). Multiphoton autofluorescence images of two planes of
the epidermis of an ex vivo mouse skin sample excited by fiber-delivered ultrashort pulses at
different wavelengths (left to right: 740, 780, 820, and 860 nm). Scale bar is 50 µm. (c).
Figures a and c reproduced from reference 4 under Creative Commons. Figure b courtesy
of Heriot-Watt University.
Ultrashort-pulse delivery
https://www.photonics.com/Articles/Hollow-Core_Optical_Fibers_Offer_Advantages_at/a64402
4/8
1/11/22, 9:40 AM
Hollow-Core Optical Fibers Offer Advantages at Any Wavelength | Features | Apr 2019 | Photonics Spectra
In manufacturing, in biomedicine, and in other fields, ultrashort-pulse fiber lasers have
become inexpensive and reliable tools. However, conventional single-mode optical fibers
are not able to deliver the ultrashort pulses these lasers produce, often restricting the
deployment of these lasers and making them uncompetitive. Conventional fibers literally “fail
to deliver” due to their optical nonlinear response and their dispersion. Taken together, these
two effects rapidly lengthen ultrashort optical pulses as they travel through even very short
lengths of fiber, reducing the peak intensity and making the pulses ineffective for most
applications.
Antiresonant hollow-core fibers address both of these unwanted effects3,4. The nonlinear
response of the fibers is reduced by many orders of magnitude — first, because the intrinsic
Kerr nonlinearity of air is around 1000× less than that of silica glass, and second, because
the larger core size reduces the intensity of light in the core. This same combination of large
core size and reduced material contribution simultaneously reduces the fiber dispersion to
far below that found in standard single-mode fibers. As a result, the fibers can transmit even
powerful ultrashort pulses, with almost no pulse distortion (Figure 3a)4.
Unlike conventional fibers, where the fiber dispersion is strongly dependent on the operating
wavelength, antiresonant fibers exhibit low dispersion across a broad spectral range and
have been used successfully for ultrashort-pulse delivery at a number of important
wavelengths, including 1550, 1060, and 532 nm. This capability will enable numerous
applications — for example, delivery of pulsed light to a scanning head for machining glass
(Figure 3b), or simplifying coupling light from a laser source into a multiphoton microscope
(Figure 3c).
Silica-based fibers for the MIR
Silica-based fibers are fantastically transparent, but only over a finite spectral range. For
wavelengths longer than around 2500 nm, in the MIR, silica becomes absorbing, and optical
fibers usually need to be made of different glasses that can transmit these longer
wavelengths. Such fibers are available, but they do have drawbacks when compared to the
silica fibers that are used for shorter wavelengths5. Nonsilica or soft glass fibers are not as
strong or as easy to work with, nor are they as transparent. They also tend to have high
nonlinearity and dispersion, severely limiting their use for short-pulse delivery.
https://www.photonics.com/Articles/Hollow-Core_Optical_Fibers_Offer_Advantages_at/a64402
5/8
1/11/22, 9:40 AM
Hollow-Core Optical Fibers Offer Advantages at Any Wavelength | Features | Apr 2019 | Photonics Spectra
Figure 4. Comparison between low-loss, single-mode antiresonant hollow-core fiber
(ARHCF) and commercial multimode solid-core InF3 fiber (see reference 5) in the MIR. The
diameters of the hollow-core and InF3 fibers are 106 and 100 μm, respectively.
The attenuation of the hollow-core fiber was measured by cutback from 226 to 46 m. The
sharp cutoff of transmission around 4.2 μm is due to a strong atmospheric CO2 absorption
line. The red dashed line is the measured material absorption of F300 Heraeus fused-silica
glass (right axis). The absorption peaks around 3.5 μm are from gaseous HCl (hydrogen
chloride) molecules present in the fiber core, which can be removed by purging. OH:
hydroxyl ion. Courtesy of University of Bath.
Despite being made of silica, which becomes absorbing at MIR wavelengths, hollow-core
antiresonant fibers can transmit MIR light with low loss because the light travels mainly in
the hollow core (Figure 4). Indeed, the optical attenuation of the fibers can be reduced by as
much as 20,000× compared to the attenuation of the bulk silica. As a result, in the spectral
range between 3 and 5 µm, hollow-core fibers formed from silica have the attractive strength
of silica fibers, very low nonlinearity and dispersion, and optical attenuation that is
comparable to or better than that of conventional optical fibers formed from IR glass. The
combination of these characteristics makes them a potentially valuable technology in this
rapidly developing spectral band.
Damage-free fibers for the UV
Conventional optical fibers can suffer from various forms of optically induced damage, but
https://www.photonics.com/Articles/Hollow-Core_Optical_Fibers_Offer_Advantages_at/a64402
6/8
1/11/22, 9:40 AM
Hollow-Core Optical Fibers Offer Advantages at Any Wavelength | Features | Apr 2019 | Photonics Spectra
one of the hardest to overcome is caused by the absorption of short-wavelength (UV)
photons. The higher photon energy at these short wavelengths causes disruption of the
glass matrix at a microscopic scale, leading to the glass suffering from increased absorption.
This effect makes it hard to find optical fibers that can stably transmit UV light, although
some special fibers with improved characteristics for this spectral window are available. But
even these special fibers have limitations and will rapidly degrade when used at short
wavelengths and high powers.
Antiresonant hollow-core fibers reduce these problems by several orders of magnitude and
offer virtually damage-free transmission6 of UV light, even for short wavelengths and high
optical powers. The light travels mostly through the hollow core, which greatly reduces the
rate of damage to the glass. And even when the glass is affected, it makes almost no
difference to the light trapped in the core, resulting in a minimal reduction of transmission.
Practical challenges and solutions
The obvious questions that arise with such fibers, including bending7 (which ensures that
the fibers deliver light single-mode) and fabrication, are largely resolved. Other questions —
including optimizing fiber designs, demonstrating the lowest possible attenuation, controlling
the polarization state, and ensuring that the fiber end face is protected against ingress of
contaminants — remain topics of active investigation for specific applications.
Current research involves using the fibers to develop a new generation of fiber-based optical
sources at MIR wavelengths — with high efficiencies and output powers. With clear market
advantages, the identification of solutions to the most pressing problems, and a growing
number of businesses with the capacity to serve as fiber providers, there is every reason to
believe the time has arrived for hollow-core fibers to make a commercial impact.
Meet the authors
Jonathan Knight is a researcher in fiber optics at the University of Bath. He works on the
design and fabrication of new fibers and also on their properties and applications;
email: pysjck@bath.ac.uk.
Duncan Hand leads research at Heriot-Watt University, where he focuses on applications of
high-power lasers. These include use of novel fiber optics for delivery and manipulation of
ultrashort-pulsed laser light; email: d.p.hand @hw.ac.uk.
Fei Yu was formerly with the Department of Physics at the University of Bath, and is now an
associate professor at the Shanghai Institute of Optics and Fine Mechanics, Chinese
Academy of Sciences; email: yufei@siom.ac.cn.
References
https://www.photonics.com/Articles/Hollow-Core_Optical_Fibers_Offer_Advantages_at/a64402
7/8
1/11/22, 9:40 AM
Hollow-Core Optical Fibers Offer Advantages at Any Wavelength | Features | Apr 2019 | Photonics Spectra
1. J.A. Harrington (2000). A review of IR transmitting, hollow waveguides. Fiber Integr Opt,
Vol. 19, pp. 211-227.
2. A.D. Pryamikov et al. (2011). Demonstration of a waveguide regime for a silica hollowcore microstructured optical fiber with a negative curvature of the core boundary in the
spectral region >3.5 μm. Opt Express, Vol. 19, pp. 1441-1448.
3. B. Wedel and M. Funck (2016). Industrial fiber beam delivery system for ultrafast
lasers. Laser Tech J, Vol. 13, pp. 42-44.
4. B. Sherlock et al. (2016). Tunable fibre-coupled multiphoton microscopy with a negative
curvature fibre. J Biophotonics, Vol. 9, pp. 715-720.
5. Thorlabs Inc. (2014). Mid-infrared fluoride optical fiber,
www.thorlabs.de/newgrouppage9.cfm?objectgroup_id=7062.
6. F. Yu et al. (2018). Single-mode solarization-free hollow-core fiber for ultraviolet pulse
delivery. Opt Express, Vol. 26, p. 10879.
7. R.M. Carter et al. (2017). Measurement of resonant bend loss in anti-resonant hollow
core optical fiber. Opt Express, Vol. 25, p. 20612.
Photonics Spectra
Apr 2019
https://www.photonics.com/Articles/Hollow-Core_Optical_Fibers_Offer_Advantages_at/a64402
8/8