As the ambition behind THz quantum cascade laser based applications continues to grow, abandoning free-space optics in favor of waveguided systems promises major improvements in targeted, easy to align, and robust radiation delivery. This is especially true in cryogenic environments, where illumination is traditionally challenging. Although the field of THz waveguides is rapidly developing, most designs have limitations in terms of mechanical stability at low temperatures, and are costly and complicated to fabricate to lengths > 1 m. In this work, we investigate readily available cylindrical metal waveguides which are suitable for effective power delivery in cryogenic environments, and explore the optimal dimensions and materials available. The materials chosen were extruded un-annealed and annealed copper, as well as stainless steel, with bore diameters of 1.75, 2.5, and 4.6 mm. Measurements were performed at three different frequencies, 2.0, 2.85 and 3.2 THz, with optimal transmission losses <3 dB/m demonstrated at 2.0 THz. Additionally, novel optical couplers are also presented and characterised, with the ability to change the beam path by 90° with a coupling loss of just 2.2 dB whilst maintaining mode quality, or thermally isolate sections of waveguide with a coupling loss as low as 0.5 dB. The work presented here builds on previous work1, and forms a comprehensive investigation of cryogenically compatible THz waveguides and optical couplers, paving the way for a new generation of systems to utilize THz QCLs for a host of low-temperature investigations.
Metal coated Hollow Glass Waveguides (HGWs) incorporating single dielectric thin films have been widely used for the low-loss transmission of infrared radiation in applications ranging from surgery to spectroscopy. While the incorporation of single dielectric film designs have traditionally been used in metal/dielectric coated HGWs, recent research has focused on the development of alternating low/high refractive index multilayer dielectric thin film stacks for further transmission loss reduction. Continuing advances in the deposition of optically functional cadmium sulfide and lead sulfide thin films in HGWs have allowed for the simultaneous increase in film quality and greater film thickness control necessary for the implication of such multilayer stack designs for enhanced reflectivity at infrared wavelengths. This study focuses on the theoretical and practical considerations in the development of such multilayer stack coated waveguides and presents novel results including film growth kinetics of multilayer stack thin film materials, IR spectroscopic analysis, and IR laser attenuation measurements. The effects of incorporating progressive alternating cadmium sulfide and lead sulfide dielectric thin films on the optical properties of next generation dielectric thin film stack coated HGWs in the near and mid infrared regions are thoroughly presented. The implications of incorporating such dielectric multilayer stack coatings based on metal sulfide thin films on the future of IR transmitting hollow waveguides for use in applications ranging from spectroscopy, to high laser power delivery are briefly discussed.
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.
This study involves the optimization of thin silver coated Hollow Glass Waveguides (HGWs) for low-loss delivery of
laser radiation at visible wavelengths and discusses preliminary work in the development of polystyrene dielectric thin
films in silver coated HGWs for low-loss radiation delivery at near and far infrared wavelengths. The optimization of the
silver thin film deposition procedure in HGWs for reduced transmission losses at λ = 500 - 1050 nm is presented along
with experimental results. Such low-loss hollow waveguides are capable of delivering high power / high energy laser
light with no functional damage. The benefits and use of novel polystyrene thin films in HGWs is likewise presented and
preliminary experimental results are discussed along with potential applications of said polystyrene coated waveguides.
Polystyrene is an attractive material for use as a dielectric thin film in HGWs due to its relatively low refractive index
nearing the optimal refractive index of n = 1.414 for use as a single dielectric thin film in HGWs. Furthermore, its nontoxicity,
low cost, and chemical inertness add to its beneficial use as a transparent thin film at visible and infrared
wavelengths ranging from λ = 500 - 3,000 nm and λ > 50 μm. Its broadband transparency additionally allows for its
simultaneous use as a dielectric film in HGWs at infrared and visible wavelengths. Preliminary results in the
development of polystyrene coated HGWs optimized for transmission at short and long wavelengths are presented,
primarily through FTIR spectroscopic methods. The design for the optimization of deposited polystyrene thin films in
HGWs based on desired transmission wavelength range is discussed.
The objective of this study is to validate a method for the measurement of tissue oxygen saturation level via a
thermal imaging bundle in-vitro and in-vivo. The method consists of a thermal imaging system and an algorithm
which estimates the compound concentration according to the temperature rise of the tissue. A temperature rise
is obtained by illuminating the tissue in the NIR range and is measured using a thermal camera and a coherent
thermal imaging bundle for non-invasive transendoscopic use. The system was validated using agar phantoms of
varying concentrations of Methylene Blue and ICG as well as blood samples. The algorithm estimated the
Methylene Blue relative amount and the results were compared to the real relative amount. The calculated RMS
of the error was 5.12%, a satisfying value for this stage. In the blood samples, for oxygenation levels higher than
50% the RMS of the error was 5.79%. Once the system was verified a portable system was built for clinical use,
this system was also evaluated on agar phantoms and the RMS of the error was 10.64%. As a result of the
encouraging experiments in-vivo, animal trials were performed. The oxygenation levels of mice were decreased
and were estimated respectively using our system. The system determined a small decrease in the tissue oxygen
saturation of the mice. These results verify the algorithm's and bundle's suitability for the use in a non-invasive
system. They provide motivation for performing more complex in-vitro experiments and moving on to clinical
trials.
Hollow Glass Waveguides (HGWs) incorporating single dielectric thin film designs deposited on silver coated silica
hollow waveguides have been used for low-loss transmission of infrared radiation in the 2 - 14 micrometer region. Silver
iodide has traditionally been the material of choice as a dielectric thin film in HGWs, with other dielectric thin film
materials such as cadmium sulfide and lead sulfide being used as well. The incorporation of multilayer stacks of
alternating low and high refractive index dielectric thin films in HGWs has been theoretically shown to further reduce
the optical attenuation. Theoretically, lower losses are achieved when the refractive index contrast of the two thin film
materials used is high and the number of films incorporated in the HGW film structure increases. Theoretically, such
multilayer dielectric stack designs can give rise to the appearance of 1-D photonic bandgap structures with
omnidirectional reflection properties as long as critical design parameters are met and scattering contributions due to
surface roughness and similar defects are sufficiently low. This study involves the practical design of multilayer
dielectric stacks in HGWs, with lead sulfide as a high refractive index material and cadmium sulfide as low refractive
index material. The design, optimization, and processing methodology for achieving low-loss multilayer dielectric stacks
in HGWs at desired infrared wavelengths is discussed. Characterization of multilayer dielectric coated HGWs includes
FTIR spectroscopy for determining the optical response and infrared laser measurements for determining the optical
attenuation properties of said multilayer dielectric stack coated HGWs. The experimental loss dependency of dielectric
coated HGWs incorporating such metal chalcogenide materials on the particular thin film materials used and number of
dielectric layers incorporated is presented and challenges in the current fabrication methodology are discussed.
We describe the development and testing of hollow core glass waveguides (i.e., fiber optics) for use in Mid-Wave
Infrared (MWIR) and Long-Wave Infrared (LWIR) spectroscopy systems. Spectroscopy measurements in these
wavelength regions (i.e., from 3 to 14 μm) are useful for detecting trace chemical compounds for a variety of security
and defense related applications, and fiber optics are a key enabling technology needed to improve the utility and
effectiveness of detection and calibration systems. Hollow glass fibers have the advantage over solid-core fibers (e.g.,
chalcogenide) in that they are less fragile, do not produce cladding modes, do not require angle cleaving or antireflection
coatings to minimize laser feedback effects, and effectively transmit deeper into the infrared. This paper
focuses on recent developments in hollow fiber technology geared specifically for infrared spectroscopy, including
single mode beam delivery with relatively low bending loss. Results are presented from tests conducted using both
Quantum Cascade Lasers (QCL) and CO2 lasers operating in the LWIR wavelength regime. Single-mode waveguides
are shown to effectively deliver beams with relatively low loss (~ 1 dB/m) and relatively high beam quality. The fibers
are also shown to effectively mode-filter the "raw" multi-mode output from a QCL, in effect damping out the higher
order modes to produce a circularly symmetric Gaussian-like beam profile.
Spectroscopy in the long-wave infrared (LWIR) wavelength region (8 to 12 μm) is useful for detecting trace
chemical compounds, such as those indicative of weapons of mass destruction (WMD). To enable the development
of field portable systems for anti-proliferation efforts, current spectroscopy systems need to be made more robust,
convenient, and practical (e.g., miniaturized). Hollow glass waveguides have been used with a Quantum Cascade
Laser source for the delivery of single-mode laser radiation from 9 to 10 μm. The lowest loss measured for a
straight, 484 μm-bore guide was 0.44 dB/m at 10 μm. The smallest 300 μm-bore waveguide transmitted singlemode
radiation even while bent to radii less than 30 cm.
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