Karl W. Koch

Low-loss hollow-core silica/air photonic bandgap fibre

Photonic bandgap structures use the principle of interference to reflect radiation. Reflection from photonic bandgap structures has been demonstrated in one, two and three dimensions and various applications have been proposed. Early work in hollow-core photonic bandgap fibre technology used a hexagonal structure surrounding the air core; this fibre was the first demonstration of light guided inside an air core of a photonic bandgap fibre. The potential benefits of guiding light in air derive from lower Rayleigh scattering, lower nonlinearity and lower transmission loss compared to conventional waveguides. In addition, these fibres offer a new platform for studying nonlinear optics in gases. Owing largely to challenges in fabrication, the early air-core fibres were only available in short lengths, and so systematic studies of loss were not possible. More recently, longer lengths of fibre have become available with reported losses of 1,000 dB km-1. We report here the fabrication and characterization of long lengths of low attenuation photonic bandgap fibre. Attenuation of less than 30 dB km-1 over a wide transmission window is observed with minimum loss of 13 dB km-1 at 1,500 nm, measured on 100 m of fibre. Coupling between surface and core modes of the structure is identified as an important contributor to transmission loss in hollow-core photonic bandgap fibres.

Surface modes in air-core photonic band-gap fibers

We present a detailed description of the role of surface modes in photonic band-gap fibers (PBGFs). A model is developed that connects the experimental observations of high losses in the middle of the transmission spectrum to the presence of surface modes supported at the core-cladding interface. Furthermore, a new PBGF design is proposed that avoids these surface modes and produces single-mode operation.

Soliton pulse compression in photonic band-gap fibers.

We report on pulse compression using a hollow-core photonic band-gap fiber filled with Xe. Output pulses with megawatt peak powers and durations of 50 fs have been generated from 120-fs input pulses. The large third-order dispersion inherent in these fibers degrades the optimal compression ratio and prevents generation of even shorter pulses. Nevertheless, for picosecond input pulses, compression to less than 100 fs is predicted.

Observation of transverse Anderson localization in an optical fiber

We utilize transverse Anderson localization as the waveguiding mechanism in optical fibers with random transverse refractive index profiles. Using experiments and numerical simulations, we show that the transverse localization results in an effective propagating beam diameter that is comparable to that of a typical index-guiding optical fiber.

Greater than 100% photon-conversion efficiency from an optical parametric oscillator with intracavity difference-frequency mixing

Using 100-ps Nd:YAG pump pulses, we synchronously pump an optical parametric oscillator with intracavity difference-frequency mixing (DFM) between the signal and the idler. The cavity is singly resonant at the signal frequency. The signal, idler, and difference wavelengths are near 1.5, 3.5, and 2.8 µm, respectively. Periodically poled lithium niobate is used for both interactions. Results show an 80% enhancement in the idler power-conversion efficiency and an idler photon-conversion efficiency of 110% when the DFM interaction is phase matched. Backconversion of the pump is suppressed when the DFM interaction is phase matched, and pump depletion increases from 65% to 79% at full pump power.

Image transport through a disordered optical fibre mediated by transverse Anderson localization

Transverse Anderson localization of light allows localized optical-beam-transport through a transversely disordered and longitudinally invariant medium. Its successful implementation in disordered optical fibres recently resulted in the propagation of localized beams of radii comparable to that of conventional optical fibres. Here we demonstrate optical image transport using transverse Anderson localization of light. The image transport quality obtained in the polymer disordered optical fibre is comparable to or better than some of the best commercially available multicore image fibres with less pixelation and higher contrast. It is argued that considerable improvement in image transport quality can be obtained in a disordered fibre made from a glass matrix with near wavelength-size randomly distributed air-holes with an air-hole fill-fraction of 50%. Our results open the way to device-level implementation of the transverse Anderson localization of light with potential applications in biological and medical imaging.

Durable, superhydrophobic, antireflection, and low haze glass surfaces using scalable metal dewetting nanostructuring

AbstractIn this paper we report a multifunctional nanostructured surface on glass that, for the first time, combines a wide range of optical, wetting and durability properties, including low omnidirectional reflectivity, low haze, high transmission, superhydrophobicity, oleophobicity, and high mechanical resistance. Nanostructures have been fabricated on a glass surface by reactive ion etching through a nanomask, which is formed by dewetting ultrathin metal films (< 10 nm thickness) subjected to rapid thermal annealing (RTA). The nanostructures strongly reduce the initial surface reflectivity (∼4%), to less than 0.4% in the 390–800 nm wavelength range while keeping the haze at low values (< 0.9%). The corresponding water contact angle (θc) is ∼24.5°, while that on a flat surface is ∼43.5°. The hydrophilic wetting nanostructure can be changed into a superhydrophobic and oleophobic surface by applying a fluorosilane coating, which achieves contact angles for water and oil of ∼156.3° and ∼116.2°, respectively. The multicomponent composition of the substrate (Corning® glass) enables ion exchange through the surface, so that the nanopillars’ mechanical robustness increases, as is demonstrated by the negligible changes in surface morphology and optical performance after 5,000-run wipe test. The geometry of the nanoparticles forming the nanomask depends on the metal material, initial metal thickness and RTA parameters. In particular we show that by simply changing the initial thickness of continuous Cu films we can tailor the metal nanoparticles’ surface density and size. The developed surface nanostructuring does not require expensive lithography, thus it can be controlled and implemented on an industrial scale, which is crucial for applications.

Detailed investigation of the impact of the fiber design parameters on the transverse Anderson localization of light in disordered optical fibers

We recently reported the observation of transverse Anderson localization as the waveguiding mechanism in optical fibers with random transverse refractive index profiles [Opt. Lett. 37, 2304 (2012)]. Here, we explore the impact of the design parameters of the disordered fiber on the beam radius of the propagating transverse localized beam. We show that the optimum value of the fill-fraction of the disorder is 50% and a lower value results in a larger beam radius. We also explore the impact of the average size of the individual random features on the value of the localized beam radius and show how the boundary of the fiber can impact the observed localization radius. A larger refractive index contrast between the host medium and the disorder sites results in smaller value of the beam radius.

Transverse Anderson localization in a disordered glass optical fiber

We report the first observation of transverse Anderson localization in a glass optical fiber. The strong localization happens near the outer boundary of the fiber and no trace of localization is observed in the central regions. However, these observations complement previous reports that the boundary of a disordered medium has a de-localizing effect. Our observations can be explained by considering the non-uniform distribution of disorder in the fiber, where the substantially larger disorder near the outer boundary of the fiber offsets the de-localizing effect of the boundary.

Highly birefringent hollow-core photonic bandgap fiber

A highly birefringent hollow-core photonic bandgap fiber is fabricated and characterized. The fiber group birefringence is found to be 0.025 at 1550 nm through wavelength scanning method and direct measurement of differential group delay.