Carlos M. Bledt

Low-Loss Hollow Waveguide Fibers for Mid-Infrared Quantum Cascade Laser Sensing Applications

We report on single mode optical transmission of hollow core glass waveguides (HWG) coupled with an external cavity mid-IR quantum cascade lasers (QCLs). The QCL mode results perfectly matched to the hybrid HE11 waveguide mode and the higher losses TE-like modes have efficiently suppressed by the deposited inner dielectric coating. Optical losses down to 0.44 dB/m and output beam divergence of ∼5 mrad were measured. Using a HGW fiber with internal core size of 300 μm we obtained single mode laser transmission at 10.54 μm and successful employed it in a quartz enhanced photoacoustic gas sensor setup.

Terahertz wave transmission in flexible polystyrene-lined hollow metallic waveguides for the 2.5-5 THz band

A low-loss and low-dispersive optical-fiber-like hybrid HE₁₁ mode is developed within a wide band in metallic hollow waveguides if their inner walls are coated with a thin dielectric layer. We investigate terahertz (THz) transmission losses from 0.5 to 5.5 THz and bending losses at 2.85 THz in a polystyrene-lined silver waveguides with core diameters small enough (1 mm) to minimize the number of undesired modes and to make the waveguide flexible, while keeping the transmission loss of the HE₁₁ mode low. The experimentally measured loss is below 10 dB/m for 2 < ν < 2.85 THz (~4-4.5 dB/m at 2.85 THz) and it is estimated to be below 3 dB/m for 3 < ν < 5 THz according to the numerical calculations. At ~1.25 THz, the waveguide shows an absorption peak of ~75 dB/m related to the transition between the TM₁₁-like mode and the HE₁₁ mode. Numerical modeling reproduces the measured absorption spectrum but underestimates the losses at the absorption peak, suggesting imperfections in the waveguide walls and that the losses can be reduced further.

Loss and modal properties of Ag/AgI hollow glass waveguides

Hollow glass waveguides, composed of Ag/AgI coatings, have been studied at 10.6 µm. The losses for different bore sizes equal the theoretical loss, which for the 700 µm bore guide was about 0.15  dB/m. The losses for the guides increase upon bending, varying linearly with increasing curvature. These hollow guides propagate a single mode when the bore size of the guide is approximately 30λ. In addition, the best single-mode transmission is obtained when the thickness of the glass wall is large. These smaller bores, thick wall hollow guides, can also be used to filter higher order modes from poor quality input laser beams.

Modes in silver-iodide-lined hollow metallic waveguides mapped by terahertz near-field time-domain microscopy

Thin dielectric layers inside hollow metallic waveguides are used to improve the waveguide transmission characteristics as the dominant waveguide mode changes into the hybrid HE11 mode. We investigate the effect of 1 μm thick silver iodide (AgI) coatings on the fundamental modes in cylindrical waveguides at terahertz (THz) frequencies, in the regime of the dielectric layer being thinner than the optimal thickness hopt� 2 THz� ≈ 20 μm. In the region of 1–3.2 THz, the lowest-order modes are similar in profile to the TE11 and TM11 modes, as determined by the timeresolved near-field measurements and verified numerically. Higher-order modes are detected experimentally as mode mixtures due to the multimode propagation. Numerical electromagnetic modeling is applied to resolve the mode structure ambiguity, allowing us to correlate experimentally detected patterns with a superposition of the TM11 and the higher-order mode, TE12. Mode profiles determined here indicate that in the regime of ultrathin dielectric (h ≪ 0.1λeff), the dielectric layer does not transform the dominant mode into the low-loss HE11 mode. Experimental mode patterns similar to the HE11 and the TE01 modes nevertheless can be formed due to mode beating. The results indicate that the Ag/AgI waveguides can be used for guiding THz waves in the TE01 mode or the TE12 mode with high discrimination against other modes.

Coherent hollow-core waveguide bundles for thermal imaging

There has been very little work done in the past to extend the wavelength range of fiber image bundles to the IR range. This is due, in part, to the lack of IR transmissive fibers with optical and mechanical properties analogous to the oxide glass fibers currently employed in the visible fiber bundles. Our research is aimed at developing high-resolution hollow-core coherent IR fiber bundles for transendoscopic infrared imaging. We employ the hollow glass waveguide (HGW) technology that was used successfully to make single-HGWs with Ag/AgI thin film coatings to form coherent bundles for IR imaging. We examine the possibility of developing endoscopic systems to capture thermal images using hollow waveguide fiber bundles adjusted to the 8-10?mum spectral range and investigate the applicability of such systems. We carried out a series of measurements in order to characterize the optical properties of the fiber bundles. These included the attenuation, resolution, and temperature response. We developed theoretical models and simulation tools that calculate the light propagation through HGW bundles, and which can be used to calculate the optical properties of the fiber bundles. Finally, the HGW fiber bundles were used to transmit thermal images of various heated objects; the results were compared with simulation results. The experimental results are encouraging, show an improvement in the resolution and thermal response of the HGW fiber bundles, and are consistent with the theoretical results. Nonetheless, additional improvements in the attenuation of the bundles are required in order to be able to use this technology for medical applications.

Theory and practical considerations of multilayer dielectric thin-film stacks in Ag-coated hollow waveguides.

This analysis explores the theory and design of dielectric multilayer reflection-enhancing thin film stacks based on high and low refractive index alternating layers of cadmium sulfide (CdS) and lead sulfide (PbS) on silver (Ag)-coated hollow glass waveguides (HGWs) for low loss transmission at midinfrared wavelengths. The fundamentals for determining propagation losses in such multilayer thin-film-coated Ag hollow waveguides is thoroughly discussed, and forms the basis for further theoretical analysis presented in this study. The effects on propagation loss resulting from several key parameters of these multilayer thin film stacks is further explored in order to bridge the gap between results predicted through calculation under ideal conditions and deviations from such ideal models that often arise in practice. In particular, the effects on loss due to the number of dielectric thin film layers deposited, deviation from ideal individual layer thicknesses, and surface roughness related scattering losses are presented and thoroughly investigated. Through such extensive theoretical analysis the level of understanding of the underlying loss mechanisms of multilayer thin-film Ag-coated HGWs is greatly advanced, considerably increasing the potential practical development of next-generation ultralow-loss mid-IR Ag/multilayer dielectric-coated HGWs.

Investigation of tapered silver / silver halide coated hollow glass waveguides for the transmission of CO2 laser radiation

The present study focuses on the theoretical and practical infrared radiation propagation properties of tapered silver / silver iodide coated Hollow Glass Waveguides (HGWs). Tapered HGWs with inner diameters ranging from 300 μm to 650 μm with a linear taper increasing at an approximate rate of 1.5 μm/cm were fabricated and optimized for low-loss transmission of CO2 laser radiation at of 10.6 μm. The theoretical losses in these tapered silver / silver iodide coated HGWs are calculated for light transmitted from the big to the small and vice versa. Theoretical calculations used in this study are based on ray-optics. Experimental loss measurements are likewise presented, along with the calculated and measured output beam divergence. The experimental bending losses of the tapered HGWs are studied and compared with those measured and for those for non-tapered, straight bore sizes from 300 to 700 μm. Experimental losses for tapered Ag/AgI HGWs ranged from 0.732 - 1.340 dB/m depending on configuration and bending radius.

Efficient coupling of double-metal terahertz quantum cascade lasers to flexible dielectric-lined hollow metallic waveguides

The growth in terahertz frequency applications utilising the quantum cascade laser is hampered by a lack of targeted power delivery solutions over large distances (>100 mm). Here we demonstrate the efficient coupling of double-metal quantum cascade lasers into flexible polystyrene lined hollow metallic waveguides via the use of a hollow copper waveguide integrated into the laser mounting block. Our approach exhibits low divergence, Gaussian-like emission, which is robust to misalignment error, at distances > 550 mm, with a coupling efficiency from the hollow copper waveguide into the flexible waveguide > 90%. We also demonstrate the ability to nitrogen purge the flexible waveguide, increasing the power transmission by up to 20% at 2.85 THz, which paves the way for future fibre based terahertz sensing and spectroscopy applications.

Investigation of metal sulfide optical thin film growth in low-loss IR hollow glass waveguides

In this study, the film growth kinetics for near and mid-IR reflection enhancing CdS and PbS dielectric thin films in HGWs is experimentally established. Crucial fabrication parameters including solution concentrations, pH, and fluid velocity are optimized. The film thickness of these films in HGWs is studied as a function of deposition time and temperature. Through IR spectral response analysis, the dielectric thin film thicknesses were determined and found to have a strong linear time dependence. Accurate metal sulfide film growth models in HGWs were developed, allowing for direct determination of necessary deposition times to yield metal sulfide HGW thin film coatings having a desired response.

Theoretical and experimental investigation of infrared properties of tapered silver/silver halide-coated hollow waveguides

Silver/silver halide-coated hollow-glass waveguides (HGWs) are capable of low-loss, broadband transmission at infrared wavelengths with the advantage of optical response tunability through alteration of a number of key design parameters. Generally, the design of circular HGWs has primarily involved optimization of the waveguide bore size and deposited film structure in order to obtain the desired optical response, with the waveguide bore size being held constant as a function of length. In this study, the effects of HGW structures consisting of linearly tapered inner diameters on the optical response at infrared wavelengths are theoretically and experimentally investigated. Theoretical analysis involving numerical ray optics methods accounting for the dynamic nature of bore size, and consequently light propagation, along the waveguide length is presented and compared to experimental results in order to gain a deeper understanding of these atypical HGW structures.