We present a compact-cavity, picosecond, mid-infrared optical parametric oscillator (OPO) employing a length of hollow-core-fiber (HCF) inside the cavity and operating at 1-MHz repetition rate for high pulse energy. Pumped by an ytterbium-doped fiber laser, the periodically-poled-lithium-niobate-based OPO generates output beam with tunable wavelengths ranging from 1.3 µm to 4.8 µm. The OPO provides 137-ps pulses with maximum energies of 10 µJ for signal output at 1.6 µm and 5 µJ for idler output at 3 µm, respectively. Output power performance with respect to the wavelength tunability and optimization of beam quality for the OPO are numerically and experimentally investigated.
Hollow-core fibers (HCFs) have been under intense research interest thanks to their many advantages including low latency, low nonlinearity, and temperature insensitivity. The most recent progress on the double nested antiresonant nodeless fiber (DNANF) demonstrated fiber losses of only 0.174 dB/km. Transmission of ultra-short, high-peak-power pulses can greatly benefit from low nonlinearity of HCFs. However, the waveguide dispersion in HCFs such as DNANF is typically 2-3 ps/(nm·km) in the low-loss transmission region, still causing unwanted pulses broadening. Here, we demonstrate a low-loss interconnection between HCF and a dispersion-compensating fiber (DCF), enabling to obtain HCF+DCF link with zero-net dispersion. To adapt the relatively small mode-filed diameter (MFD) of DCF (4.9 μm) to the MFD of the HCF, we first splice a short segment of graded-index (GRIN) multi-mode fiber on the DCF. The GRIN fiber is then polished to a specific length to obtain an optimal MFD adaptation to our HCF, which was a nested antiresonant nodeless fiber (NANF) with 26.3 μm MFD at 1550 nm. We obtained a loss of only 0.55 dB for the whole DCF-GRIN-NANF component. By depositing an anti-reflective coating on the mode-field adapter end-face, the interconnection loss can be further reduced to 0.39 dB.
The performance of quantum key distribution (QKD) is heavily dependent on the physical properties of the channel over which it is executed. Propagation losses and perturbations in the encoded photons’ degrees of freedom, such as polarisation or phase, limit both the QKD range and key rate. The maintenance of phase coherence over optical fibres has lately received considerable attention as it enables QKD over long distances, e.g., through phase-based protocols like Twin-Field (TF) QKD. While optical single mode fibres (SMFs) are the current standard type of fibre, recent hollow core fibres (HCFs) could become a superior alternative in the future. Whereas the co-existence of quantum and classical signals in HCF has already been demonstrated, the phase noise resilience required for phase-based QKD protocols is yet to be established. This work explores the behaviour of HCF with respect to phase noise for the purpose of TF-QKD-like protocols. To achieve this, two experiments are performed. The first, is a set of concurrent measurements on 2 km of HCF and SMF in a double asymmetric Mach-Zehnder interferometer configuration. The second, uses a TF-QKD interferometer consisting of HCF and SMF channels. These initial results indicate that HCF is suitable for use in TF-QKD and other phase-based QKD protocols.
This conference presentation was prepared for the Quantum Technology: Driving Commercialisation of an Enabling Science III conference at SPIE Photonex, 2022.
Austin Taranta, Francesco Poletti, Hesham Sakr, Greg Jasion, John Hayes, Seyed Resz Sandoghchi, Lucy Hooper, S. Mohammad Mousavi, Eric Numkam Fokoua, Arsalan Saljoghei, Hans Christian Mulvad, Marcelo Alonso, Thomas Bradley, Ian Davidson, Yong Chen, David Richardson
In recent years Hollow Core fibres (HCF) technology has improved its performance indicators by orders of magnitude in many directions, making it a contender for the next generation of numerous fibre based optical devices, as well as an enabler for novel applications currently unthinkable with standard glass-guiding fibres.
Loss wise, air guiding fibres with lower loss than fundamentally achievable in any other glass are now possible at wavelengths spanning from the visible to the VCSEL and laser delivery wavelengths of 850 and 1060 nm, respectively. At telecommunication wavelengths, the loss of HCFs is now down to 0.22dB/km, with a rate of progress that seems to indicate that further improvements are possible. And in the mid-infrared, HCFs made of silica of soft glasses with broad bandwidth and sub dB/m or lower are becoming available.
Besides, the latest generation of HCFs is now capable of producing better polarization purity, transmitting higher CW powers over longer distances without incurring in nonlinear spectral degradation, and of transmitting high-capacity data signals over thousands of kilometers.
We will review some of these recent highlights, with a particular emphasis on the results achieved in our group at the University of Southampton.
Austin Taranta, Francesco Poletti, Hesham Sakr, Greg Jasion, John Hayes, Seyed Resz Sandoghchi, Lucy Hooper, S. Mohammad Mousavi, Eric Numkam Fokoua, Arsalan Saljoghei, Hans Christian Mulvad, Marcelo Alonso, Thomas Bradley, Ian Davidson, Yong Chen, David Richardson
In recent years Hollow Core fibres (HCF) technology has improved its performance indicators by orders of magnitude in many directions, making it a contender for the next generation of numerous fibre based optical devices, as well as an enabler for novel applications currently unthinkable with standard glass-guiding fibres.
Loss wise, air guiding fibres with lower loss than fundamentally achievable in any other glass are now possible at wavelengths spanning from the visible to the VCSEL and laser delivery wavelengths of 850 and 1060 nm, respectively. At telecommunication wavelengths, the loss of HCFs is now down to 0.22dB/km, with a rate of progress that seems to indicate that further improvements are possible. And in the mid-infrared, HCFs made of silica of soft glasses with broad bandwidth and sub dB/m or lower are becoming available.
Besides, the latest generation of HCFs is now capable of producing better polarization purity, transmitting higher CW powers over longer distances without incurring in nonlinear spectral degradation, and of transmitting high-capacity data signals over thousands of kilometers.
We will review some of these recent highlights, with a particular emphasis on the results achieved in our group at the University of Southampton.
The attenuation of hollow-core fibers (HCFs) is predicted to surpass the minimum intrinsic attenuation of standard single-mode fibers (SMFs) in the near future. Recent advances in HCF performance and drawing technology have motivated their application not only in telecommunications but also in sensing and high-power delivery. Among HCFs, nested antiresonant nodeless fibers (NANFs) have shown the lowest attenuation values with 0.28 dB/km at 1550 nm and 0.22 dB/km at 1625 nm. Furthermore, the latest generation of NANFs effectively mitigates higher-order modes, which in some applications introduces a significantly limiting factor. As HCFs are becoming more available, their incorporation into standard SMF-based systems needs to be efficiently addressed.
Various solutions to the HCF-SMF interconnection have already been proposed, such as the commonly employed fusion splicing with bridge fibers, using tapers to match the mode-fields, employing micro-optics, or using the fiber-array approach. Based on the fiber-array approach we have recently demonstrated losses of only 0.16 dB per interconnection and back reflection below -60 dB.
But what if the interconnection itself can provide some additional functionality beyond low loss and low back reflection?
Such an approach was already proposed in the micro-optics interconnection providing a function as an optical isolator or a wavelength-division multiplexer. Still, the relatively high complexity of such a device might limit its wider application.
In this talk, I will overview current trends in HCF-SMF interconnection techniques which are enabling their incorporation into current SMF-based fiber-optic systems. I will present a future outlook of providing additional functionality to the HCF-SMF interconnection. I will focus on an interconnection technique we developed, based on the fiber-array approach. I will show how components such as an optical filter, a gas cell, or a Fabry-Perot cavity can be easily formed by simple tailoring of the HCF-SMF interconnection.
We demonstrate transient changes in the optical properties, specifically the loss, of antiresonant hollow core fibres (HCFs) due to a combination of the sub-atmospheric gas pressure inside the fibre holes post-fabrication and the subsequent gas induced differential refractive index (GDRI) between the core and cladding elements of the fibre; this is temporarily created while the gas pressures inside the core and cladding elements are evolving after the HCF ends are opened up to surrounding atmospheric pressure. Here we show experimental evidence of this effect in two different HCF designs; for both fibres, the transmitted power initially increases, reaches a maximum, and then reduces to its initial level. We show via gas flow simulations that the timeline of this behaviour is consistent with the gas flow rates into the core and cladding elements of the tubular HCF studied and the subsequent transient differential gas pressure. The experimental results also show (in line with GDRI expectations) that this transmission (loss) change is higher at shorter wavelengths. Our results imply that this transient change in the fibre’s optical properties must be considered for accurate fibre characterisation; this is particularly true for long fibre lengths where the equalisation of the fibre’s internal gas pressure with atmospheric pressure could take many weeks.
Realizing compact picosecond Optical Parametric Oscillators (OPOs) capable of generating high-energy mid-IR pulses at MHz repetition rates is a challenge due to the correspondingly long cavity length requirements. Intracavity fiber delay lines can be used to increase the cavity length but the achievable peak powers are then severely constrained by fiber nonlinearity.
Here we report a compact, ytterbium-fiber-laser pumped, periodically poled lithium niobate based OPO that incorporates a 298 m length of hollow-core-fiber as an ultralow nonlinearity intracavity delay line. The OPO is capable of generating 1-MHz, 100-ps mid-IR pulses with an energy of 1.64-μJ and 12.8-kW peak power.
Silica glass optical fibers have revolutionized data transmission, sensing and laser development over the past 50 years. Moreover, dielectric waveguides with a hollow core offer exciting development possibilities beyond traditional technology. Hollow Core Optical Fibers (HCFs) have been fabricated over the past 20 years with various geometries and refinements, yet their attenuation has remained significantly higher than can be routinely achieved in standard silica single mode fibers. Here we present recent developments in Nested Anti-resonant Nodeless Fiber (NANF) design over the last few years and show how this rapidly developing technology has been refined to produce state of the art HCFs at wavelengths between 850 – 1625 nm.
Quantum Key Distribution (QKD) technology has been considered as the ultimate physical layer security due to its dependencies on the physical laws of physics to generate quantum keys. However, for QKD to become functional for practical scenarios, it must be integrated with the classical optical networking infrastructure. Coping with optical nonlinearity from the classical represents a major challenge for QKD systems. In this paper, we take the advantage of the ultra-low nonlinearity of Hollow Core Nested Antiresonant Nodeless Fibre (HC-NANF) to demonstrate the coexistence of discrete-variable quantum key distribution channel with carrier-grade classical optical channels over a 2 km HC-NANF.
Hollow core optical fibres have many unique properties, especially compared to traditional glass-core optical fibres [1]. Firstly, the light path is accessible and light can thus interact with the gas inside over long lengths, making them interesting for applications in gas sensing or for nonlinear processes in gasses. Hollow core fibres can also operate at wavelengths, where silica glass has poor transmission and their chromatic dispersion is not compromised by the chromatic dispersion of bulk glass. Yet another unique feature is weak interaction of light with the guiding medium (typically air), significantly increasing the damage threshold and thus making them a good candidate for high-power (average or peak power) light delivery. Another group of unique features is related to how their properties (little) change with temperature.
In the presentation, we will firstly show where the common fibre optics wisdom (gained from work with standard optical fibres) tends to fail. In the second part, we will discuss how differently hollow core fibre change with temperature as compared to standard optical fibres and how it can be used for various applications, including fibre interferometry and time-stable signal transmission.
In this paper, we present results of long-term stability tests of a low-loss (<0.55 dB) hollow core fiber (HCF) to standard optical fiber interconnection prepared by modified gluing-based fiber-array technology. We measured insertion loss of three interconnected HCF samples over a period of 100 days at room temperature, observing a variation in insertion loss of less than 0.02 dB. Subsequently, we placed the HCF samples in a climatic chamber and heated to +85°C in four cycles. Maximum insertion loss variation of 0.10 dB was observed for HCF samples with angled 8° interconnections and only 0.02 dB for a HCF sample with a flat interconnection.
Silica glass optical fibers have revolutionized data transmission, sensing and laser development over the past 50 years, however, dielectric waveguides with a hollow core offer exciting development possibilities beyond traditional technology. Hollow Core Optical Fibers (HCFs) have been fabricated over the past 20 years with various geometries and refinements reported over this time. Despite numerous design developments and predictions from theoretical studies, one of the key performance indicators of optical fibers – attenuation - has remained significantly higher than can be routinely achieved in standard silica single mode fibers. Here we present recent developments in Nested Anti-resonant Nodeless Fiber (NANF) design over the last few years and show how this rapidly developing technology has been refined to produce state of the art HCFs with attenuation = 0.28 dB/km at 1550 nm.
Propagation time through standard optical fibres changes with temperature at a rate of 40 ps/km/K. This can pose significant challenges in many diverse application areas of optical fibres in physics and engineering. Primary examples lie in applications in which very precise timing signals need to be disseminated for synchronization purposes in large experimental infrastructures such as synchrotrons, linear particle accelerators, large telescope arrays, and in phase arrayed antennae. A value of 40 ps/km/K equates to a phase temperature sensitivity of about 48 rad/m/K. This can adversely affect many applications relying on fibre interferometers (e.g. fibre optic sensors, quantum-optics, interferometric measurement techniques, and so on), in which maintaining stable interference would require temperature stabilization below mK level. Similarly, a few key optical metrology applications require the dissemination of optical signals at a precise frequency, for example to compare distant ultra-precise clocks (e.g., national standard clocks) with a precision (fractional stability) at/below the 10-18 level. Such a level of precision is easily compromised by thermally-induced changes in optical path length (temperature drift) with time that unavoidably result in a Doppler frequency shift.
Here, we review our recent results in which we show why and how Hollow-Core Fibres (HCF) are significantly better than solid-core fibres in terms of their sensitivity of propagation time and accumulated phase change to temperature and thus are a better alternative to standard fibres in the above-mentioned fields.
We have recently developed a novel electro-optic modulator via external electrical gating of 2D MoS2 bilayers deposited within the inner regions of a silica hollow core anti-resonant fiber. The MoS2 film acts as the electro-optically active material, responding with increased absorption of waveguided modes when in the presence of an externally applied electric field. The bilayer is formed via a liquid phase deposition method, in which the single source precursor ammonium tetrathiomolybdate is thermally decomposed into MoS2. The device has to date demonstrated modulation depths of >3.5dB, at an operating DC voltage of 1500 V with an optical insertion loss of 7.5dB. We have thus developed a novel, active, composite material anti-resonant fiber (CM-ARF) technology platform, which despite high voltage requirements, show excellent potential for all-fiber electro-optical design and operation.
We propose and demonstrate the fabrication of wire array metamaterial fibers and hyperlenses based on Ag72Cu28 wires embedded in borosilicate glass for imaging applications at mid-infrared frequencies. Initial numerical modeling indicates smaller optical losses for such a system in comparison to the equivalent Sn/soda-lime structures reported recently in the literature. Modeling of the proposed and fabricated magnifying hyperlenses shows promising overall optical losses between 11 dB to 16 dB (depending on the structural parameters) when operating at 5 μm wavelength. Their experimental characterization and the proof-of-concept far-field sub diffraction imaging experiment are in progress.
Flexible dielectric optical fibers guiding light in a hollow core were conceptually imagined at the end of the 19th century, but first demonstrated in practice about 2 decades ago. Since then, many geometric variants have been described and implemented, and theoretical models developed and finessed. Despite this, for a fairly long time the key metric by which their performance was judged – attenuation – has remained quite considerably higher than standard all-glass fibers. In this paper, we describe the recent breakthroughs in hollow core fiber technology. We trace the story of this breakthrough from the theoretical exploration of a new design of hollow core fiber, through early implementations, up to the staggering results achieved over the last 18 months. The progress reported concerns not only a reduction in the fiber attenuation level, but also a considerable improvement in modal quality of the fibers, which have led to excellent data transmission performance. These fabricated fibers tell a story of improvements in all aspects of the technology, including preform preparation, performance modelling, fiber draw dynamics and coatings.
In this paper we present an exploration of the stability and repeatability of a hollow core microstructured fibre (HCMOF) Raman gas sensor. Raman gas detection using HC-MOFs is an exciting technique as it enables high sensitivity, multi-species detection using a small gas volume and within a small physical space. Several previous works have demonstrated the utility of HC-MOF fibres as Raman gas cells for the detection of a wide range of gas species such as methane and hydrogen. Here we take a first look at the Raman signal stability (in a single fibre) and signal reproducibility (from fibre-to-fibre). We show that a HC-MOF Raman system can achieve low within-day variability of 0.3 %CV and fibre-to-fibre variability of 7.6 %CV. Understanding the error within systems such as the one presented is critical in the development of HC-MOF-based gas sensors for practical applications.
Hollow Core Anti-resonant fibers allow for guidance of mid-infrared light at low attenuation and can be used for a variety of applications, such as high power laser transmission and gas sensing. Recent work has seen the integration of silicon into such fibers with linear losses potentially as low as 0.1dB/m. Due to the change in refractive index difference of silicon via for example the free carrier plasma dispersion effect, the prospect of an all optical modulator using such a fiber has been proposed. Here, further work has been undertaken on the integration of functional materials inside hollow core fibers via the deposition of the TMD semiconductor material MoS2, in its few-layered form. Through the use of a liquid precursor, a high quality MoS2 film can be deposited over 30cm length of fiber, as confirmed via Raman spectroscopy. The transmission spectra of these novel composite material hollow core fibers has also been analysed, showing additional loss of around 5dB/m, despite being only around 2nm in thickness. This implies that the refractive index of the integrated material is potentially able to modify the guidance properties of the fiber sample. We will present a comparison of the composite material hollow core fibers we have fabricated to date and discuss the prospects for using these novel waveguides in the active manipulation of light, including optical switching, sensing and frequency generation.
We have demonstrated Raman frequency conversion and supercontinuum light generation in a hollow core Kagomé fiber filled with air at atmospheric pressure, and developed a numerical model able to explain the results with good accuracy. A solid-state disk laser was used to launch short pulses (~6ps) at 1030nm into an in-house fabricated hollow core Kagomé fiber with negative core curvature and both ends were open to the atmosphere. The fiber had a 150 THz wide transmission window and a record low loss of ~12 dB/km at the pump wavelength. By gradually increasing the pulse energy up to 250 μJ, we observed the onset of different Kerr and Raman based optical nonlinear processes, resulting in a supercontinuum spanning from 850 to 1600 nm at maximum input power. In order to study the pulse propagation dynamics of the experiment, we used a generalized nonlinear Schrödinger equation (GNLSE). Our simulations showed that the use of a conventional damping oscillator model for the time-dependent response of the rotational Raman component of air was not accurate enough at such high intensities and large pulse widths. Therefore, we adopted a semiquantum Raman model for air, which included the full rotational and vibrational response, and their temperature-induced broadening. With this, our GNLSE results matched well the experimental data, which allowed us to clearly identify the nonlinear phenomena involved in the process. Aside from the technological interest in the high spectral density of the supercontinuum demonstrated, the validated numerical model can provide a valuable optimization tool for gas based nonlinear processes in air-filled fibers.
We study in detail the macrobending effects in a wide transmission bandwidth (~200nm) 19 cell hollow-core photonic bandgap fiber operating at 1550nm. Our results indicate low bend sensitivity over a ~130nm wide interval within the transmission window, with negligible loss (<0.1dB) for bending radii down to 5mm. The “red shift” and “blue shift” of the bandgap edge have been observed at the short and long wavelength edges, respectively. The cutoff wavelengths where air-guiding modes stop guiding can be extracted from the bending loss spectra, which matches well with the simulated effective refractive index map of such fiber.
While hollow core-photonic crystal fibres are now a well-established fibre technology, the majority of work on these speciality fibres has been on designs with a single core for optical guidance. In this paper we present the first dual hollow-core anti-resonant fibres (DHC-ARFs). The fibres have high structural uniformity and low loss (minimum loss of 0.5 dB/m in the low loss guidance window) and demonstrate regimes of both inter-core coupling and zero coupling, dependent on the wavelength of operation, input polarisation, core separation and bend radius. In a DHC-ARF with a core separation of 4.3 μm, we find that with an optimised input polarisation up to 65% of the light guided in the launch core can be coupled into the second core, opening up applications in power delivery, gas sensing and quantum optics.
We use an inverse-scattering (IS) approach to design single-mode waveguides with controlled linear and higher-order
dispersion. The technique is based on a numerical solution to the Gelfand-Levitan-Marchenko integral equation, for the
inversion of rational reflection coefficients with arbitrarily large number of leaky poles. We show that common features
of dispersion-engineered waveguides such as trenches, rings and oscillations in the refractive index profile come
naturally from the IS algorithm without any a priori assumptions. Increasing the leaky-pole number increases the
dispersion map granularity and allows design of waveguides with identical low order and differing higher order
dispersion coefficients.
We review our recent progress in the design and fabrication of lead silicate glass fibers with high nonlinearity and tailored near-zero dispersion at telecommunication wavelengths. We have explored a range of different fiber structures, including suspended core fibers, holey fibers, all-solid microstructured and conventional W-type profiled fibers. The optical properties of the fabricated fibers are assessed both experimentally and through accurate numerical simulations. The relative merits of each fiber design are discussed and the significant potential of lead silicate highly nonlinear fibers for all-optical signal processing at telecommunication wavelengths is shown through reference to a number of key experimental demonstrators.
We report recent advances in the development of fibers for the delivery and generation of both single-mode and heavily
multimode laser beams as well as recent progress in fibers for supercontinuum generation in spectral regimes spanning
the visible to mid-IR.
Air/silica Microstructured Optical Fibers (MOFs) offer new prospects for fiber based sensor devices. In this paper, two
topics of particular significance for gas sensing using air guiding Photonic Bandgap Fibers (PBGFs) are discussed. First,
we address the issue of controlling the modal properties of PBGFs and demonstrate a single mode, polarization
maintaining air guiding PBGF. Secondly, we present recent improvements of a femtosecond laser machining technique
for fabricating fluidic channels in PBGFs, which allowed us to achieve cells with multiple side access channels and low
additional loss.
We report the generation of white light comprising red, green, and blue spectral bands from a frequency-doubled
fiber laser in submicron-sized cores of microstructured holey fibers. Picosecond pulses of green light are launched
into a single suspended core of a silica holey fiber where energy is transferred by an efficient four-wave mixing
process into a red and blue sideband whose wavelengths are fixed by birefringent phase matching due to a slight
asymmetry of the structure arising during the fiber fabrication. Numerical models of the fiber structure and
of the nonlinear processes confirm our interpretation. Finally, we discuss power scaling and limitations of this
white light source.
A simple fabrication technique for a silica suspended-core holey fiber design is presented that features a higher air-filling fraction than most holey fibers, making it ideal for evanescent-field-sensing applications. The holes in the fiber are defined through mechanical drilling of the preform, which is a significantly quicker and more straightforward approach to the customary stacking method. During the draw, the shape of the holes are manipulated so that the final fiber design approximates that of an air-suspended rod with three fine struts supporting the core. Modeling reveals that the modal overlap is greater than 29% at 1550 nm for a core diameter of 0.8 µm, which is significantly higher than any previously reported index-guiding structure used for sensing. A basic gas sensor is demonstrated using acetylene as the sensing medium and the results are reported.
We have performed numerical simulations to investigate the optimization of compound glass microstructured optical
fibers for mid IR supercontinuum generation beyond the low loss transmission window of silica, using pump
wavelengths in the range 1.55-2.25 mm. Large mode area fibers for high powers, and small core fiber designs for low
powers, are proposed for a variety of glasses. Modeling results showed that for Bismuth and lead oxide glasses, which
have nonlinearities ~10 x that of silica, matching the dispersion profile to the pump wavelength is essential. For
chalcogenide glasses, which have much higher nonlinearities, the dispersion profile is less important. The pump pulses
have duration of <1 ps, and energy <30 nJ. The fiber lengths required for generating continuum were <40 mm, so the
losses of the fibers were not a limiting factor. Compared to planar rib-waveguides or fiber-tapers, microstructured fiber
technology has the advantages of greater flexibility for tailoring the dispersion profile over a broad wavelength span, and
a much wider possible range of device lengths.
Microstructured fibers (MOFs) are among the most innovative developments in optical fiber technology in recent years. These fibers contain arrays of tiny air holes that run along their length and define the waveguiding properties. Optical confinement and guidance in MOFs can be obtained either through modified total internal reflection, or photonic bandgap effects; correspondingly, they are classified into index-guiding Holey Fibers (HFs) and Photonic Bandgap Fibers (PBGFs). MOFs offer great flexibility in terms of fiber design and, by virtue of the large refractive index contrast between glass/air and the possibility to make wavelength-scale features, offer a range of unique properties. In this paper we review the current status of air/silica MOF design and fabrication and discuss the attractions of this technology within the field of sensors, including prospects for further development. We focus on two primary areas, which we believe to be of particular significance. Firstly, we discuss the use of fibers offering large evanescent fields, or, alternatively, guidance in an air core, to provide long interaction lengths for detection of trace chemicals in gas or liquid samples; an improved fibre design is presented and prospects for practical implementation in sensor systems are also analysed. Secondly, we discuss the application of photonic bandgap fibre technology for obtaining fibres operating beyond silica's transparency window, and in particular in the 3μm wavelength region.
Fiber delivery of intense laser radiation is important for a broad range of application sectors, from medicine through to industrial laser processing of materials, and offers many practical system design and usage benefits relative to free space solutions. Optical fibers for high power transmission applications need to offer low optical nonlinearity and high damage thresholds. Single-mode guidance is also often a fundamental requirement for the many applications in which good beam quality is critical. In recent years, microstructured fiber technology has revolutionized the dynamic field of optical fibers, bringing with them a wide range of novel optical properties. These fibers, in which the cladding region is peppered with many small air holes, are separated into two distinct categories, defined by the way in which they guide light: (1) index-guiding holey fibers (HFs), in which the core is solid and light is guided by a modified form of total internal reflection, and (2) photonic band-gap fibers (PBGFs) in which guidance in a hollow core can be achieved via photonic band-gap effects. Both of these microstructured fiber types offer attractive qualities for beam delivery applications. For example, using HF technology, large-mode-area, pure silica fibers with robust single-mode guidance over broad wavelength ranges can be routinely fabricated. In addition, the ability to guide light in an air-core within PBGFs presents obvious power handling advantages. In this paper we review the fundamentals and current status of high power, high brightness, beam delivery in HFs and PBGFs, and speculate as to future prospects.
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