V Shahraam Afshar

A full vectorial model for pulse propagation in emerging waveguides with subwavelength structures part I: Kerr nonlinearity

The propagation of pulses through waveguides with sub-wavelength features, inhomogeneous transverse structure, and high index contrast cannot be described accurately using existing models in the presence of nonlinear effects. Here we report the development of a generalised full vectorial model of nonlinear pulse propagation and demonstrate that, unlike the standard pulse propagation formulation, the z-component of guided modes plays a key role for these new structures, and results in generalised definitions of the nonlinear coefficient gamma, Aeff , and mode orthognality. While new definitions reduce to standard definitions in some limits, significant differences are predicted, including a factor of approximately 2 higher value for gamma, for emerging waveguides and microstructured fibers.

Porous fibers: a novel approach to low loss THz waveguides.

We propose a novel class of optical fiber with a porous transverse cross-section that is created by arranging sub-wavelength air-holes within the core of the fiber. These fibers can offer a combination of low transmission loss and high mode confinement in the THz regime by exploiting the enhancement of the guided mode field that occurs within these sub-wavelength holes. We evaluate the properties of these porous fibers and quantitatively compare their performance relative to that of a solid core air cladded fiber (microwire). For similar loss values, porous fibers enable improved light confinement and reduced distortion of a broadband pulse compared to microwires.

THz porous fibers: design, fabrication and experimental characterization

Porous fibers have been identified as a means of achieving low losses, low dispersion and high birefringence among THz polymer fibers. By exploiting optical fiber fabrication techniques, two types of THz polymer porous fibers--spider-web and rectangular porous fibers--with 57% and 65% porosity have been fabricated. The effective refractive index measured by terahertz time domain spectroscopy shows a good agreement between the theoretical and experimental results indicating a lower dispersion for THz porous fiber compared to THz microwires. A birefringence of 0.012 at 0.65 THz is also reported for rectangular porous fiber.

Terahertz dielectric waveguides

Several classes of non-planar metallic and dielectric waveguides have been proposed in the literature for guidance of terahertz (THz) or T-ray radiation. In this review, we focus on the development of dielectric waveguides, in the THz regime, with reduced loss and dispersion. First, we examine different THz spectroscopy configurations and fundamental equations employed for characterization of THz waveguides. Then we divide THz dielectric waveguides into three classes: solid-core, hollow-core, and porous-core waveguides. The guiding mechanism, fabrication steps, measured loss, and dispersion are presented for the waveguides in each class in chronological order. The goal of this review is to compare and contrast the current solutions for guiding THz radiation.

Enhancement of fluorescence-based sensing using microstructured optical fibres

We develop a generic model of excitation and fluorescence recapturing within filled microstructured optical fibres (MOFs) with arbitrary structure and demonstrate that the light-matter overlap alone does not determine the optimal fibre choice. Fibre designs with sub-wavelength features and high-index glasses exhibit localised regions of high intensity, and we show that these regions can lead to approximately two orders of magnitude enhancement of fluorescence recapturing. Here we show how this regime can be exploited for sensing and demonstrate experimentally in-fibre excitation and fluorescence recapturing within a filled, solid-core MOF.

Small core optical waveguides are more nonlinear than expected: experimental confirmation

For the first time, to our knowledge, we demonstrate the experimental confirmation of a new vectorially based expression of the effective nonlinear coefficient gamma in bismuth suspended core fibers with core diameters of around 500 nm. The new expression predicts a significantly higher value of gamma than what is expected based on the standard expression. We confirm that there is a distinguishable difference between the standard and our new vectorially based gammas, owing to the high index glass and subwavelength dimension of this fiber, and we show that the experimental result of gamma matches the new expression within the experimental error.

Low loss, low dispersion and highly birefringent terahertz porous fibers

We demonstrate that porous fibers in addition to low loss and high confinement, have near zero dispersion for 0.5–1 THz resulting in reduced terahertz signal degradation compared to microwires. We also show for the first time that these new fibers can be designed, introducing asymmetrical sub-wavelength air-holes within the core, to achieve high birefringence �0.026. This opens up the potential for realization of novel polarization preserving fibers in the terahertz regime.

Enhanced fluorescence sensing using microstructured optical fibers: a comparison of forward and backward collection modes

A general model of excitation and fluorescence recapturing by the forward and backward modes of filled microstructured optical fibers (MOFs) is presented. We also present experimental results for both backward and forward fluorescence recapturing within a MOF as a function of fiber length and demonstrate a good qualitative agreement between the numerical model and experimental results. We demonstrate higher efficiency of fluorescence recapturing into backward modes in comparison with that of forward modes.

Direct probing of evanescent field for characterization of porous terahertz fibers

We develop a technique based on a micromachined photoconductive probe-tip to characterize a terahertz (THz) porous fiber. Losses less than 0.08 cm−1 are measured in the frequency range from 0.2 to 0.35 THz, with the minimum of 0.003 cm−1 at 0.24 THz. Normalized group velocity greater than 0.8, which corresponds to dispersion values in between −1.3 and −0.5 ps/m/μm for 0.2<f<0.35 THz are obtained. Moreover, we directly measure the evanescent electric field as a function of frequency. Good agreement between the measured curves and expected theoretical values indicates the low invasiveness of the applied probe-tip.

A genetic algorithm based approach to fiber design for high coherence and large bandwidth supercontinuum generation

We present a new approach to the design of optical microstructured fibers that have group velocity dispersion (GVD) and effective nonlinear coefficient (gamma ) tailored for supercontinuum (SC) generation. This hybrid approach combines a genetic algorithm (GA) with pulse propagation modeling, but without include it into the GA loop, to allow the efficient design of fibers that are capable of generating highly coherent and large bandwidth SC in the mid-infrared (Mid-IR) spectrum. To the best of our knowledge, this is the first use of a GA to design fiber for SC generation. We investigate the robustness of these fiber designs to variation in the fibers structural parameters. The optimized fiber structure based on a type of tellurite glass (70TeO(2) - 10 Na(2)O - 20 ZnF(2)) is predicted to have near-zero group velocity dispersion (< +/-2 ps/nm/km) from 2 to 3 microm, and a effective nonlinear coefficient of gamma approximately 174 W(-1)km(-1) at 2 microm. The SC output of this fiber shows a significant bandwidth and coherence increase compare to a fiber with a single zero group velocity dispersion wavelength at 2 microm.