Low loss, low dispersion and highly birefringent terahertz porous fibers

We demonstrate that porous fibers have low effective material loss over an extended frequency range, 4.5 times larger bandwidth than that can be achieved in sub-wavelength solid core fibers. We also show that these new fibers can be designed to have near zero dispersion for 0.5-1 THz resulting to overall less terahertz signal degradation. In addition, it is demonstrated that the use of asymmetrical sub-wavelength air-holes within the core leads to high birefringence ~0.026. This opens up the potential for realization of novel polarization preserving fibers in the terahertz regime.Comment: The following article has been submitted to Applied Physics Letters. If it is published,it will be found online at http://apl.aip.or

Microwire fibers for low-loss THz transmission

This paper was presented at Smart Structures, Devices, and Systems, which was a sub-conference contained within Smart Materials, Nano- and Micro-Smart Systems Symposium (10-13 December 2006 : Adelaide, South Australia). This paper was published in Smart Structures, Devices, and Systems III, edited by Said F. Al-Sarawi, Proc. of SPIE Vol. 6414, 64140I and is made available as an electronic reprint with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited.In this paper, we will investigate microwire fibers for low-loss terahertz transmission. Microwires, air-clad wire waveguides with diameter smaller than the operating wavelength (a few μm), have an enhanced evanescent field and tight wave confinement resulting in a low loss waveguide structure for the terahertz (T-ray) frequency regime. Based on our experimental data for the bulk material absorption of four glasses (F2, SF6, SF57 and Bismuth) and a polymer (PMMA), we calculate the normalized field distribution, power fraction outside the wire and effective loss. It will be shown that regardless of material, the effective loss of all microwires converges to the same order < 0.01 cm -1.Shaghik Atakaramians, Shahraam Afshar Vahid, Bernd M. Fischer, Heike Ebendorff-Heidepriem, Tanya Monro and Derek Abbotthttp://spie.org/Documents/ConferencesExhibitions/au06-final.pd

A prism based magnifying hyperlens with broad-band imaging

Magnification in metamaterial hyperlenses has been demonstrated using curved geometries or tapered devices, at frequencies ranging from the microwave to the ultraviolet spectrum. One of the main issues of such hyperlenses is the difficulty in manufacturing. In this letter, we numerically and experimentally study a wire medium prism as an imaging device at THz frequencies. We characterize the transmission of the image of two sub-wavelength apertures, observing that our device is capable of resolving the apertures and producing a two-fold magnified image at the output. The hyperlens shows strong frequency dependent artefacts, a priori limiting the use of the device for broad-band imaging. We identify the main source of image aberration as the reflections supported by the wire medium and also show that even the weaker reflections severely affect the imaging quality. In order to correct for the reflections, we devise a filtering technique equivalent to spatially variable time gating so that ultra-broad band imaging is achieved

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. © 2011 American Institute of Physics.Shaghik Atakaramians, Shahraam Afshar V., Michael Nagel, Henrik K. Rasmussen, Ole Bang, Tanya M. Monro, and Derek Abbot

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.Shaghik Atakaramians, Shahraam Afshar V., Heike Ebendorff-Heidepriem, Michael Nagel, Bernd M. Fischer, Derek Abbott and Tanya M. Monr

Suspended core subwavelength fibers: practical designs for the low-loss terahertz guidance

In this work we report two designs of subwavelength fibers packaged for practical terahertz wave guiding. We describe fabrication, modeling and characterization of microstructured polymer fibers featuring a subwavelength-size core suspended in the middle of a large porous outer cladding. This design allows convenient handling of the subwavelength fibers without distorting their modal profile. Additionally, the air-tight porous cladding serves as a natural enclosure for the fiber core, thus avoiding the need for a bulky external enclosure for humidity-purged atmosphere. Fibers of 5 mm and 3 mm in outer diameters with a 150 \mu m suspended solid core and a 900 \mu m suspended porous core respectively, were obtained by utilizing a combination of drilling and stacking techniques. Characterization of the fiber optical properties and the near-field imaging of the guided modes were performed using a terahertz near-field microscopy setup. Near-field imaging of the modal profiles at the fiber output confirmed the effectively single-mode behavior of such waveguides. The suspended core fibers exhibit broadband transmission from 0.10 THz to 0.27 THz (larger core), and from 0.25 THz to 0.51 THz (smaller core). Due to the large fraction of power that is guided in the holey cladding, fiber propagation losses as low as 0.02 cm-1 are demonstrated. Low-loss guidance combined with the core isolated from environmental perturbations make these all-dielectric fibers suitable for practical terahertz imaging and sensing applications.Comment: 13 pages, 7 figure

Bendable, low-loss Topas fibers for the terahertz frequency range

We report on a new class of polymer photonic crystal fibers for low-loss guidance of THz radiation. The use of the cyclic olefin copolymer Topas, in combination with advanced fabrication technology, results in bendable THz fibers with unprecedented low loss and low material dispersion in the THz regime.We demonstrate experimentally how the dispersion may be engineered by fabricating both high- and low-dispersion fibers with zero-dispersion frequency in the regime 0.5-0.6 THz. Near-field, frequencyresolved characterization with high spatial resolution of the amplitude and phase of the modal structure proves that the fiber is single-moded over a wide frequency range, and we see the onset of higher-order modes at high frequencies as well as indication of microporous guiding at low frequencies and high porosity of the fiber. Transmission spectroscopy demonstrates low-loss propagation (< 0.1 dB/cm loss at 0.6 THz) over a wide frequency range.Imaging Science and TechnologyApplied Science

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.Shaghik Atakaramians, Shahraam Afshar V., Bernd M. Fischer, Derek Abbott, and Tanya M. Monr