Holey Fibers!

An introduction to the weird and wonderful world of holey fibers * What they are * Why they are special * How they can be used

November 21, 2002

12 Min Read
Holey Fibers!

Back in the 1920s, John Logie Baird had the idea of using hollow tubes to transmit images in an early incarnation of television. Some 80 years later, Logie Baird's television is the ubiquitous source of home entertainment, so-called [ed. note: and, of course, a cartoon series about his exploits in Jellystone park has rocked the world]. But what about his hollow waveguide brainchild?Well, the idea of transmitting light through hollow waveguides has become cutting-edge technology once again in the form of "Holey Fibers." Such fibers have a regular arrangement of air holes running along their length to act as the cladding, whereas the core is formed by a glitch in the center of the regular pattern – either a solid region or an additional air hole.It is all really rather clever, with such fiber structures leading to a range of new and interesting properties with a variety of applications. These are now being exploited by a handful of startups, including: BlazePhotonics Ltd., Crystal Fibre A/S, INO, and OmniGuide Communications Inc.; as well as the usual big fiber players like Corning Inc. (NYSE: GLW).The following overview of the field of holey fibers is authored by Tanya Monro who leads a team at the University of Southampton's Optoelectronics Research Centre that is working on the design and development of holey fibers.Here's a hyperlinked summary of what follows:

  • The Basics
    Some background on holey fibers – what they are, how they work, and the history of their development.

  • Fiber Fabrication & Photo Gallery
    An outline of how a holey fiber is fabricated, and a range of mesmerizing holey fiber photos.

  • Novel Properties
    A look at the peculiar optical properties of holey fibers, focusing on endless singlemode guidance, large mode-areas, nonlinearity tailoring,
    and dispersion.

  • Applications
    Some of the telecom applications of holey fibers are discussed, including optical switching and Raman amplification.

Next Page: The BasicsIntroduction by Craig Williamson, Associate Editor, Light Reading
www.lightreading.comWhat is a holey fiber?Regular optical fibers have a cylindrical core surrounded by an envelope of a different glass, which is called the cladding. They guide light by total internal reflection, which requires that the refractive index of the cladding be lower than that of the core (see the Light Reading Beginner's Guide: Optical Fiber).

The term "holey fiber" refers to a new structure of optical fiber. Instead of applying concentric layers of glass around a solid core, the fiber is built up by stacking hollow glass rods. The result is a fiber with multiple holes running along its entire length – which could extend to hundreds of kilometers.

The core of a holey fiber is defined by a "mistake" in the otherwise regular pattern of holes, as show by these images below. The core can take the form of an extra hole or a missing hole in the pattern, or by a hole that's in the right place but is a different size from the others in the pattern.

Slide1.gif The air holes have a diameter denoted by d, and a spacing from the center of one hole to the next (the period) of &#923, which is typically in the range of 1 to 10 microns. The material used for the fiber is usually silica, although dopants can be added to make active devices, just as with conventional fiber.Guidance mechanismsThere are two methods of guiding light within a holey fiber, depending on its structure:

  • Effective index guiding
    This guiding mechanism relies on the fact that the holes in the fiber are smaller than the wavelength of the light being guided. As a result, light experiences an average or "effective index" in the cladding. If the cladding, which is full of holes, has a lower average refractive index than the core, then light is guided by total internal reflection – again, just as with ordinary fibers.

  • Photonic bandgap guiding
    A holey fiber can guide light even when the refractive index of the core is lower than that of the cladding – if, for example, the core of the fiber comprises an air hole. Total internal reflection doesn't work under these circumstances. A new mechanism – photonic bandgap guidance – is responsible, which relies on the regular arrangement of the holes. The cladding acts like a mirror, with reflections at multiple air/silica interfaces adding up to produce strong reflection overall. This works in much the same way as thin-film filters, or multilayer mirrors, the difference being that the TFFs are periodic in one dimension, while fibers are two-dimensional.

Here's a table summarizing some of the nomenclature encountered, specifically looking at the types of fiber that are formed from the different guidance methods. Table 1: Guidance Mechanisms and Holey Fiber Types

Guidance Method

Fiber Configuration

Fiber Type

Effective Index Guidance

Bandgap Guidance



Some historyA brief look at recent developments in holey fiber research:

  • 1995
    Birks et al. from the University of Southampton propose a method for making holey fibers (HFs) using a combination of capillary stacking and fiber fabrication technology

  • 1996
    First holey fiber fabricated

  • 1998
    First bandgap guiding fiber fabricated

  • 1999
    First air-guiding photonic band gap fiber produced, first predictions and realizations of novel optical properties in index-guiding HFs

  • 2000 – present
    Practical advances in index-guiding HFs (improved fiber robustness, lower loss, longer lengths), device demonstrations, new materials

Next Page: Fiber Fabrication & Photo GalleryHoley fibers are fabricated via the process illustrated in the diagram below:

  1. Capillaries are stacked around a rod to form a cm-scale macroscopic replica of the desired transverse HF cross-section.

  2. This stack can be drawn down directly into fiber on a fiber drawing tower. If small core dimensions are required, a mm-scale cane is drawn.

  3. The cane can be inserted into a solid jacket tube and drawn to fiber.

The process is flexible in that a range of fibers can be drawn from one preform as illustrated below for an 80oC temperature range. The key element is to strike a balance between surface tension and viscosity. Photo GalleryBelow are some of the fascinating structures that are created in the field of holey fibers (images courtesy of the ORC, University of Southampton). Several of the designs will be discussed in further detail in the following sections. Slide6.gif Slide7.gif Slide8.gif Next Page: Novel PropertiesHoley fiber can exhibit some unusual optical properties not encountered in ordinary fiber. First let's look at those effects, and then (next page) see how they can be exploited.

Reasons for novel optical properties A wide range of novel optical properties are possible in holey fibers because of the following features:

  • The cladding features can be on the scale of the wavelength

  • The core/cladding index contrast can be large

  • The cladding index is a strong function of wavelength

  • Cladding design is flexible, with many different possible hole arrangements

Endless singlemode guidanceStandard optical fibers become multimode as the size of the core increases relative to the wavelength. Some holey fibers can guide a single mode regardless of the wavelength [Birks, Optics Letters, 1997]. Light at shorter wavelengths is more tightly confined to the core, so the core/cladding refractive index difference is reduced with decreasing wavelength. This strong wavelength-dependence of the cladding index can prevent the fiber from supporting more than one mode at short wavelengths, and so a single fiber can exhibit singlemode operation from the UV region to beyond 2 microns.Large mode-area holey fibersIn conventional fibers, large core diameters and low numerical aperture can be combined to produce large mode-area (LMA) fibers. Similarly, the guided mode size can be increased in holey fibers by having a larger hole spacing (thus resulting in a larger core diameter) and using smaller diameter holes (creating a lower numerical aperture).Endless singlemode guidance is essentially scale-independent, and large-mode HFs provide a new route to singlemode operation (especially in the UV and visible range). They give a potentially simpler fabrication process than conventional fibers for extremely low numerical aperture; and the largest practical mode size is determined by bend losses that are comparable to those of similarly sized conventional fibers. Practical singlemode holey fibers with mode areas as large as 680 µm2 have been demonstrated [Baggett et al., Optics Letters, 2001].This type of fiber has a variety of potential applications including laser/amplifier development, transmission fiber, and low nonlinearity telecommunications.Nonlinearity tailoringThe nonlinearity of an optical fiber is proportional to the intrinsic nonlinearity of the material used to make the fiber and is also inversely proportional to the mode area of light guided by the fiber. Therefore low fiber nonlinearity can be achieved by increasing the mode area or by using a glass host with a low material nonlinearity, such as silica. In contrast, high nonlinearity can be achieved by combining a small mode area with a high nonlinearity glass (typically found in high-index glasses such as chalcogenides, etc.). Slide9.gif Holey fibers can be made with extremely small cores, offering opportunities for making fibers with unusually high nonlinearity. These can be exploited in nonlinear devices operating at low powers using short fiber lengths, such as soliton generation, optical switching, polarization-maintaining fiber, and Raman devices. Slide10.gif DispersionDispersion can be broken into two components: material dispersion and waveguide dispersion (see Chromatic Dispersion and Polarization Mode Dispersion (PMD) Beginner's Guide). In terms of holey fibers, waveguide dispersion is due to the wavelength-dependent change in the refractive index induced by the air holes. This can be unusually large in holey fibers if either the air holes are large or the core is small.Illustrated below are some of the various dispersion regimes possible in holey fibers, dependent upon the hole diameter/spacing ratio. With the zero dispersion wavelength shifted to shorter wavelengths, such fibers could have applications in soliton transmission at 1300 nm. Dispersion flattened profiles could be used for Wavelength Division Multiplexing (WDM) and nonlinear devices, and areas of large normal dispersion could be used for dispersion compensation (not shown here). Practical considerationsThe current state of the art in making holey fibers is km-lengths of polymer-coated fiber with losses as low as 0.5 dB/km at 1550 nm for index-guiding fiber [BlazePhotonics Ltd., ECOC 2002] and 13 dB/km for air-guiding photonic band gap fiber [Corning Inc., ECOC 2002] and the ability to be spliced to conventional fibers. Producing low-loss holey fibers requires careful thought. In single-material holey fibers, light can always leak out to the cladding, as there are no truly bound modes, only leaky ones. Increasing the number of rings of holes is one way to reduce the confinement loss.There are also other issues of importance. With small-core fibers, birefringence, coupling, confinement loss, and dispersion need to be considered. In large mode area fibers, bend loss and the requirements for singlemode guidance should be considered. There are always tradeoffs to be had among dispersion, birefringence, high nonlinearity, etc.Next Page: ApplicationsOptical SwitchA nonlinear holey fiber can cause self-phase modulation of a signal being transmitted through it. This can broaden out the wavelength/frequency distribution of the signal, and then an offset narrowband filter can be used to only pass the nonlinearly generated spectral components. Noisy 0 bits are suppressed whereas noisy 1 bits are equalized, giving the application of an optical switch as illustrated below [Petropoulos et al., Optics Letters, 2001]. Slide12.gif Raman amplifierFiber Raman amplifiers offer a wide bandwidth and a gain band tuneable through control of the pump wavelength (see Raman Amplification Beginner's Guide). In conventional fibers, a long fiber is required to give the desired results, but using highly nonlinear holey fiber, a much shorter fiber length can be used.Soliton generationThe combination of dispersion and nonlinearity leads to the possibility of generating solitons at shorter wavelengths than in conventional fibers. As already mentioned in the dispersion section of this report, zero dispersion wavelengths can be made as low as 500 nm – which makes soliton generation at lower wavelengths a possibility.GratingsThe core rod within a holey fiber can chosen to be photosensitive, and hence Bragg gratings and long-period gratings (LPGs) can be produced using conventional phase mask techniques (see Fiber Bragg Gratings (FBGs) Beginner's Guide). Using large holes around the outside of the fiber profile can provide good environmental isolation, for example with an LPG spacing of 155 µm in a holey fiber, low temp sensitivity of 2nm/100oC can be achieved, together with low polarization splitting of 0.1nm. Alternatively, the outer holes can be filled with a temperature sensitive polymer to give highly temperature-sensitive LPGs for broadband gain flattening and tunable filter applications. With an LPG spacing of 220 µm a temperature sensitivity of 74nm/100oC is possible [Eggelton, Optics Letters, 1999].Other applicationsA host of other present and future applications for holey fibers exist, including: atom pipes, bend sensing, broadband devices, continuum generation, controlled light/matter interactions, dispersion compensation, dispersion-controlled devices, gas sensing, efficient continuum sources, frequency conversion, high power fibers, low loss propagation, new source wavelengths, new fiber compositions, nonlinear devices, metrology, mode-shape tailoring, optical switching, parametric processes, pulse compression, quantum optics, second harmonic generation, soliton lasers, UV transmission, wavelength standards, WDM devices, and
X-ray generation.AcknowledgementsWith thanks to: David Richardson, Kentaro Furusawa, Walter Belardi, Ju Han Lee, Periklis Petropoulos, Joanne Baggett, Bill Brockelsby, Chris Hillman and The Royal Society (for a Royal Society University Research Fellowship).

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