Air-clad Fiber Research Articles

High-spatial density snapshot imaging spectrometer enabled by 2-photon fabricated custom fiber bundles.

We report on a proof-of-concept snapshot imaging spectrometer developed using an array of optical fibers fabricated with 2-photon polymerization (2PP). The dense input array maps to an output array with engineered void spaces for spectral information. Previously, the development and fabrication of custom fiber arrays for imaging spectrometers have been a complex, time-consuming, and costly process, requiring a semi-manual assembly of commercial components. This work applies an automatic development process based on 2PP additive manufacturing with the Nanoscribe GmbH Quantum X system. The technique allows printing of arbitrary optical quality structures with submicron resolution with less than 5 nm roughness, enabling small core fibers/integrated arrays. Specifically, we developed an array prototype of 40 × 80 with 6-micron pitch at the input and 80-micron pitch at the output. The air-clad fibers had a core diameter of 5 µm. Fabricated optical fiber arrays were incorporated into a prism-based imaging spectrometer system with 48 spectral channels to demonstrate multi-spectral imaging. Imaging of a USAF target and color printed letter C as well as spectral comparisons to a commercial spectrometer were used to validate the performance of the system. These results clearly demonstrate the functionality and potential applications of the 3D-printed fiber-based snapshot imaging spectrometer.

Investigation of mode coupling in step-index air-clad silica optical fibers

This paper analyzes the state of mode coupling in a high numerical aperture (NA) step-index (SI) air-clad silica optical fiber using the power flow equation. The coupling coefficient of this equation was fine-tuned at first to allow it to correctly reconstruct previously reported experimental intensity distributions of output far-field (FF) patterns. At a length of 2100 m, the equilibrium mode distribution in the investigated fiber was obtained to be established. In our best knowledge for the first time, this paper reports on the strength of mode coupling and output beam quality of SI air-clad silica optical fiber. The results obtained can be exploited to employ SI air-clad fibers in broadband high-NA astro-photonics systems.

Thermal modeling of gain competition in Yb-doped large-mode-area photonic-crystal fiber amplifier.

We propose a new model for gain competition effects in high-power fiber amplifiers, which accounts for the thermal effects of heat load on the doped core overlap of the propagating light field. The full-vectorial nature of the fiber modes is modeled by an embedded finite-element method modal solver, and the temperature profile is calculated by a simple and efficient radial heat propagation solver. The model is applied to a Yb³⁺-doped LPF45 air-clad photonic-crystal fiber amplifier for coand counter-propagating pumping setups, showing gain competition in conditions of severe heat load.

1.9 μm lasing with Tm3+ /Yb3+ co-doped air-clad fiber and 931 nm pumping

A 1.9 μm single mode laser is demonstrated by employing a newly developed large mode area Tm3+/Yb3+ co-doped air-clad photonic crystal fiber (PCF) in conjunction with 931 nm pumping for the first time. The PCF has an Yb3+ and Tm3+ ion concentrations of about 16 × 10−19 and 4 × 10−19 ions/cc, respectively. The laser produces a maximum output power of 4.6 mW with an efficiency slope of 0.70% at a multimode pump power of 2.3 W with a 2.7 m long PCF in a linear cavity with two fiber Bragg grating. The threshold of the input pump power is obtained at around 1.3 W. © 2013 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:1124–1126, 2013; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.27523

Multimode interference in outer cladding large-core, air-clad photonic crystal fiber

It is described a large-core air-clad photonic crystal fiber-based sensing structure using the outer cladding as a light guide, which is highly sensitive to refractive index. The sensing head is based on multimodal interference, and relies on a single mode/large-core air-clad photonic crystal fiber/single mode fiber configuration. Using this configuration and controlling the light to travel in a segment of the outer cladding multimode fiber, it was possible to implement a sensing head and the results were obtained independently from variations of temperature, strain and refractive index. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 54:1009–1011, 2012; View this article online at wileyonlinelibrary.com. DOI 10.1002/mop.26726

Airclad fiber laser technology

High-power fiber lasers and amplifiers have gained tremendous momentum in the last 5 years. Many of the traditional manufacturers of gas and solid-state lasers are now pursuing the fiber-based systems, which are displacing the conventional technology in many areas. High-power fiber laser systems require reliable fibers with large cores, stable mode quality, and good power handling capabilities-requirements that are all met by the airclad fiber technology. In the present paper we go through many of the building blocks needed to build high-power systems and we show an example of a complete airclad laser system. We present the latest advancements within airclad fiber technology including a new 100 μm single-mode polarization-maintaining rod-type fiber capable of amplifying to megawatt power levels. Furthermore, we describe the novel airclad-based pump combiners and their use in a completely monolithic 350 W cw fiber laser system with an M2 of less than 1.1.

Fabrication of air-clad fibers for near-IR laser application

We report the fabrication of ytterbium (Yb)-sensitized thulium (Tm)-doped air-clad fibers for near-IR laser applications. The effect of change in the concentration ratio of Yb and Tm was studied using three such fibers, which were pumped with a 980 nm laser diode and subsequently characterized to test their lasing properties. Lasing at around the 2 μm wavelength was obtained using a simple cavity design, and improved performance was seen for increased Yb∶Tm ratio.

Photonic bandgap fibre oscillators and amplifiers

In the search for ever higher output power or energy from fibre oscillators or amplifiers a nowadays mature technology relies on enlarging the fibre mode area. Broadening of the core diameter, all other things being equal, inevitably yields a multimode fibre, thereby dramatically limiting the device usefulness. Various strategies have been deployed to design and manufacture single transverse mode fibre oscillators and amplifiers, among which making use of the so-called photonic bandgap effect to restrict the modal population seems promising. Helped by efficient and reliable numerical tools the design of large mode area singlemode photonic bandgap fibres is presented. Two fibres with 20-μm and 40-μm core diameter, both of them heavily doped with Yb 3+ ions, have been fabricated by the widespread modified chemical vapour deposition process and are shown to behave properly when used as the core element of either continuous wave oscillators or femtosecond amplifiers. Good output beam quality (M 2 parameter spanning from 1.12 to 1.5 for the set of fibres studied) and high slope efficiency of 80% in cw oscillation regime are demonstrated. Furthermore the 40-μm core diameter fibre is shown to be resilient to tight bending down to 7.5-cm radius. The stack-and-draw process makes it easy to tailor the outer cladding so that a large numerical aperture can be reached. Subsequently, from this air-clad fibre, 500 fs 47 W pulses at 35 MHz are obtained from a two-stage chirped pulse amplification system.

Full-vectorial modal analysis for circular optical waveguides based on the multidomain Chebyshev pseudospectral method

What we believe to be a new one-dimensional full-vectorial mode solver for circular waveguides in step-index optical fibers is introduced based on the multidomain Chebyshev pseudospectral method and the perfectly matched layer. The formulations are derived with two transverse components on the radial axis in the cylindrical coordinate system. The electromagnetic fields are expanded inside each homogeneous layer by the Chebyshev basis functions, and then matched rigorously at the radial discontinuities by the interface conditions. The numerical analysis on the guided and leaky modes of the two layer air-clad fiber, hybrid surface plasmon polariton, and Bragg fibers demonstrates the validity and good performance of the present method, which has remarkably high accuracy and fast convergence.

Controlling the chromatic dispersion of soft glass highly nonlinear fiber through complex microstructure

Soft glass highly nonlinear fibers have high nonlinearity and a broad transparency range, but their chromatic dispersion is far from being freely tailored until now due to the immaturity in fabrication technology. In this research, the chromatic dispersion of soft glass highly nonlinear fibers was controlled by using the complex microstructure in the cladding. A tellurite glass fiber which had a 1.8 μm core surrounded by four ring holes was fabricated. The preform was fabricated by the method of cast rod in tube and stack. The chalcogenide–tellurite glass composite fibers which had a 1.5 μm core surrounded by tellurite microstructure cladding were demonstrated. Their preform was fabricated by the method of stack and draw. In the fiber-drawing process of both types of fibers an inflation pressure of nitrogen gas was pumped into the holes of the preform to overcome the surface tension and to reshape the microstructure. The tellurite complex microstructured fiber has a chromatic dispersion much more flattened than that of step-index air-clad fiber. The chalcogenide–tellurite glass composite fibers have the zero dispersion wavelength (ZDW) in the near-infrared range. Having the ZDW in the near-infrared has not been realized before for the fibers made from chalcogenide glass. Meanwhile, the composite microstructured fiber with large holes in the cladding has the highest nonlinearity of all highly nonlinear fibers if the tapered fibers are excluded. Supercontinuum spectra covering over one octave, free of fine structures, were demonstrated by the fabricated fibers.

A highly non-linear tellurite microstructure fiber with multi-ring holes for supercontinuum generation

We have fabricated a highly nonlinear complex microstructure tellurite fiber with a 1.8 micron core surrounded by four rings of holes. The cane for the fiber was prepared by combining the methods of cast rod in tube and stacking. In the process of fiber-drawing a positive pressure was pumped into the holes of cane to overcome the collapse of holes and reshape the microstructure. The correlations among pump pressure, hole size, surface tension and temperature gradient were investigated. The temperature gradient at the bottom of the preform's neck region was evaluated quantitatively by an indirect method. The chromatic dispersion of this fiber was compared with that of a step-index air-clad fiber. It was found that this fiber has a much more flattened chromatic dispersion. To the best of our knowledge this is the first report about a soft glass microstructure fiber which has such a small core together with four rings of holes for the dispersion engineering. The SC generation from this fiber was investigated under the pump of a 1557 nm femtosecond fiber laser. Infrared supercontinuum generation, free of fine structure, together with visible third harmonic generation was obtained under the pump of a femtosecond fiber laser with a pulse energy of several hundred pJ.

Increasing the Numerical Aperture of Large-Core Microstructured Polymer Optical Fibers Using a ‘Y’-Bridge Cladding

There has been considerable recent success in fabricating large-core highly multimoded polymer air-clad fibers suspended by thin bridges. These thin bridges are constructed by surrounding a solid core preform by many capillary tubes, the adjacent walls of the capillary tubes fuse together and form bridges when the preform is drawn to fiber. Recently a different cladding technique has been demonstrated experimentally where the interstitial holes between capillary walls do not completely collapse and leave a gap at the core or cladding side of the bridge forming a 'Y'-shape. This paper investigates this 'Y'-bridge microstructured cladding and the potential for increased numerical aperture of different geometries of the bridge.

Insights into tunnelling rays: outperforming guided rays in fiber-optic sensing device

We report that, for an evanescent-wave (EW) based fiber-optic sensor, the less skewed tunnelling rays and a small number of very steep guided rays experience total internal reflection (TIR) at one end-face of the air-clad fiber segment. As a result, they demonstrate nine times greater efficiency in collecting EW-power than rays that do not exhibit end-face- TIR. This tunneling ray group originates from a launching ring that is seven times wider than that of end-face-TIR capable guided rays, enabling it to dominate the overall detected EW-power level. Strongly supporting this conclusion is the counter-intuitive experimental observation that the detected power at one end of the fiber abruptly drops 91% when the back reflection at the opposite end is eliminated, while an isotropic light source is placed at the core-cladding interface between these two ends. The mechanism of non-propagating ray excitation accounts for this high efficiency. We also indicate that many highly skewed tunnelling rays are naturally isolated from the external world because, outside the fiber core, they are accessible only via their EW fields even at the fiber end-faces. The findings reported here have significant implications for the design of future high-performance EW-based fiber sensing devices for the analysis of surface-event.

Air-clad fibres for astronomical instrumentation: focal-ratio degradation

Focal-ratio degradation (FRD) of light launched into high-numerical aperture (NA) single-annulus all-silica undoped air-clad fibres at an NA of 0.54 is reported. The measured annular light distribution remained Gaussian after 30 m of propagation, but the angular FWHM of the output annulus doubled from 4° after 1 m propagation to 8.5° after 30 m, which is significantly larger than that reported of standard doped-silica fibres (NA < 0.22). No significant diffractive effects were observed. The design of air-clad fibres for broad-band, high-NA astrophotonics applications is discussed.

BPM-Numerical Study of Microstructured Fiber With High Difference Index Profile

A scalar beam propagation method is used to study confinement losses of air-clad fibers versus their geometrical parameters. The method is also applied to the computation of multimode guidance and pump absorption in two different microstructured fibers with high refractive index difference.

Air-clad fibers: pump absorption assisted by chaotic wave dynamics?

Wave chaos is a concept which has already proved its practical usefulness in design of double-clad fibers for cladding-pumped fiber lasers and fiber amplifiers. In general, classically chaotic geometries will favor strong pump absorption and we address the extent of chaotic wave dynamics in typical air-clad geometries. While air-clad structures supporting sup-wavelength convex air-glass interfaces (viewed from the high-index side) will promote chaotic dynamics we find guidance of regular whispering-gallery modes in air-clad structures resembling an overall cylindrical symmetry. Highly symmetric air-clad structures may thus suppress the pump-absorption efficiency eta below the ergodic scaling law etainfinity Ac/Acl, where Ac and Acl are the areas of the rare-earth doped core and the cladding, respectively.

Heat transport in solid and air-clad fibers for high-power fiber lasers

An analytical approach for the thermal design for high-power fiber lasers and fiber components is presented. The approach is based on defining a thermal resistance for each fiber layer. Thus the importance of each layer for the heat transport is made transparent and the influence of the parameters can be studied for each layer separately. Furthermore the analysis and analytic optimization of interacting effects of groups of layers is possible. The approach is applied to air-clad-fiber with results differing up to 40 % from previous works. Furthermore the heat transport from splices is analyzed and recommendations for the thermal packaging of splices and fiber integrated components are given.

Air-clad optical fibre filament for generating broadband radiation

A simple broadband optical source is demonstrated by using a high numerical aperture air-clad optical fibre to collect broadband emission of blackbody radiation from the fibre during exposure to high intensity electric arcs.

The influence of skew rays on angular losses in air-clad fibres

Detailed experimental transmission properties of high numerical aperture air-clad undoped fibres are presented. Measurements of the angular transmissivity indicate a 15% lower numerical aperture (NA = 0.75) compared to theoretical predictions for the bridge thickness and wavelength ratio of δ/ λ = 0.226. The discrepancy is attributed to skew rays that are deliberately launched and which are subject to larger loss rates through diffractive tunnelling in frustrated total internal reflection than meridional rays. Measurements of the angular propagation losses show that light launched at angles less than the critical angle for total internal reflection experiences low initial losses as well as low propagation losses. Light launched at steeper angles experiences a significant initial loss attributed to loss of skew rays within 1.5 m of propagation as well as larger general propagation losses.

Square Core Jacketed Air-Clad Fiber

We report the fabrication of a highly multi-mode square core jacketed air-clad fiber with a top-hat near-field intensity profile. Using this fiber to deliver Q-switched pulses at 1 mum we have successfully produced square shaped ablation marks in indium tin oxide (ITO) without the use of complex beam shaping optics.