Katja Lyytikainen

Temperature independent highly birefringent photonic crystal fibre

A highly birefringent photonic crystal fibre has been characterised as a function of temperature. The modal birefringence has been found to be independent of temperature from -25 to 800 degrees C.

Water-core Fresnel fiber

A water core photonic crystal Fresnel fiber exploiting a hole distribution on zone plates of a cylindrical waveguide was developed and characterized. This fiber has similar guiding properties as the pristine air-hole guiding fiber although a large loss edge ~900nm is observed indicating that the bandgap associated with Fresnel guidance has shifted to longer wavelengths. The absorption bands of the water in the region of the NIR were observed. The application to biosensing is discussed.

Strain and temperature characterization of photonic crystal fiber Bragg gratings.

A Bragg grating in a photonic crystal fiber was written and its dependence with temperature and strain analyzed. The two observed Bragg wavelengths correspond to a fundamental and a higher-order mode in the optical fiber. The temperature and strain calibration curves for both modes are measured and found to be distinct. The general properties of gratings in these fibers, and their implications, are enunciated.

Intermodal interference in a photonic crystal fibre

Intermodal interference in photonic crystal fibres, single mode over long lengths, is measured over a short length. Akin to conventional fibres, this poses a potential problem for practical device utilisation of photonic crystal fibres. We note that given the existing widespread fabrication capability of this fibre and indications that some commercial use in devices will come to fruition, the need for standardising measurement techniques, analogous to ITU standards for conventional fibre, specific to photonic crystal fibres will be required.

Dopant diffusion during optical fibre drawing

Diffusion of Ge and F was studied during drawing of silica optical fibres. Preforms were drawn using various draw conditions and fibres analysed using the etching and Atomic Force Microscope (AFM) technique. The results were confirmed by comparison with fibre Refractive Index Profiles (RIP). Both Ge and F were found to diffuse at high temperature, 2100 degrees C, and low draw speed, 10m/min. Diffusion simulations showed that most diffusion occurred in the neck-down region. The draw temperature and preform feed rate had a comparable effect on diffusion, whereas preform diameter did not significantly affect the diffusion.

All-fibre photonic crystal distributed Bragg reflector (PC-DBR) fibre laser

We describe an Er3+-doped aluminosilicate core photonic crystal fibre laser incorporating distributed Bragg reflectors written by two-photon 193nm irradiation through an optical phase mask as the feedback elements. The laser is diode pumped at 980nm and evidence of dual linewidth laser operation close to threshold is observed. However, at higher pumping levels gain competition preferentially selects one laser line.

Distributed feedback photonic crystal fibre (DFB-PCF) laser

A distributed feedback laser is fabricated in Er3+-doped photonic crystal fibre. Preferential single-mode lasing is obtained with no special consideration of polarisation issues. The results demonstrate practical implementation of a multi-photon writing process for complex structures in these optical fibres. No hydrogen loading and no germanium are involved.

Multiple source generation using air-structured optical waveguides for optical field shaping and transformation within and beyond the waveguide

In this paper we review recent results describing the generation of optical modes within waveguides based on coherent scattering from artificially structured interfaces. The generation of optical waveguide propagation similar to free space propagation enables possible solutions to controlling and shaping optical field generation in free space using coherent scattering of multiple sources. It is shown that the controlled fabrication of such sources can be done simply with air-material structured waveguides such as air-silica structured fibres. Further, the technique of coherent superposition is well known in Fresnel optics, exploiting zone plates to correct the necessary phase adjustments for a desired lens performance. Similarly, in waveguide form this allows fine control of the interference process resulting in the desired mode field and its properties within the waveguide, at the end of the waveguide in the near field regime and well beyond the waveguide into the far field. A factor that can contribute significantly to the coherent scattering within the Fresnel waveguide is resonant-like scattering inside the low index regions since the critical angle of propagation can be very small, increasing Fresnel reflections between interfaces. The results presented here open up a range of hitherto unexplored possibilities in controlling and shaping at first glance disparate phenomena, including free space diffraction.

Retaining and characterising nano-structure within tapered air-silica structured optical fibers.

Air-silica fiber 125m in diameter has been tapered down to ~15m. At this diameter, it is commonly assumed that the nanostructured fiber holes have collapsed. Using an Atomic Force Microscope, we show this assumption to be in error, and demonstrate for the first time that structures several hundred nanometers in diameter are present, and that hole array structures are maintained. The use of Atomic Force Microscopy is shown to be an efficient way of characterising these structures.

Complex mode coupling within air-silica structured optical fibres and applications

Abstract Novel twin-core coupling at shorter wavelengths (∼633 nm) but not at longer wavelengths (∼1.5 μm) is achieved within an air–silica structured optical fibre. This is the opposite of a conventional twin-core fibre coupler. The implications for a new class of all-optical devices are discussed.