Hollow fibers for compact infrared gas sensors

2007 IEEE Sensors, 2007

In the present study, bending losses in conventional hollow waveguides (internally Ag/AgI coated) and in photonic bandgap (PBG) hollow waveguides (HWG) are compared based on studies via FT-IR spectroscopy and quantum cascade lasers (QCL). To date, literature on bending losses in hollow waveguides focuses on conventional HWG structures (e.g., silica structural tube with internal Ag/AgI coating), whereas the results discussed here compare relative bending losses in novel photonic bandgap waveguides, a new type of HWG progressively more integrated in gas sensors, versus conventional HWGs for the first time. Photonic bandgap waveguides are expected to exhibit lower polarizationdependent relative bending losses due to radiation propagation via omnidirectional reflection, in contrast to conventional HWGs. Accordingly, photonic bandgap waveguides offer superior flexibility and robustness against bending losses in coiled configurations rendering them promising structures for next-generation miniaturized QCL-based HWG gas sensors.

Applied optics, 2016

Hollow silica waveguides (HSWs) are used to produce long path length, low-volume gas cells, and are demonstrated with quantum cascade laser spectroscopy. Absorption measurements are made using the intrapulse technique, which allows measurements to be made across a single laser pulse. Simultaneous laser light and gas coupling is achieved through the modification of commercially available gas fittings with low dead volume. Three HSW gas cell configurations with different path lengths and internal diameters are analyzed and compared with a 30 m path length astigmatic Herriott cell. Limit of detection measurements are made for the gas cells using methane at a wavelength 7.82 μm. The lowest limit of detection was provided by HSW with a bore diameter of 1000 μm and a path length of 5 m and was measured to be 0.26 ppm, with a noise equivalent absorbance of 4.1 × 10 −4. The long-term stability of the HSW and Herriott cells is compared through analysis of the Allan-Werle variance of data collected over a 24 h period. The response times of the HSW and Herriott cells are measured to be 0.8 s and 36 s, respectively.

Quantum Sensing and Nanophotonic Devices VI, 2009

Stand-off and extractive explosive detection methods for short distances are investigated using mid-infrared laser spectroscopy. A quantum cascade laser (QCL) system for TATP-detection by open path absorption spectroscopy in the gas phase was developed. In laboratory measurements a detection limit of 5 ppm*m was achieved. For explosives with lower vapor pressure an extractive hollow fiber based measurement system was investigated. By thermal desorption gaseous TATP or TNT is introduced into a heated fiber. The small sample volume and a fast gas exchange rate enable fast detection. TNT and TATP detection levels below 100 ng are feasible even in samples with a realistic contaminant background.

Journal of Applied Physics, 2011

We present an optical gas sensor based on the classical nondispersive infrared technique using ultracompact photonic crystal gas cells. The ultracompact device is conceptually based on low group velocities inside a photonic crystal gas cell and low-reflectivity antireflection layers coupling light into the device. Experimentally, an enhancement of the CO2 infrared absorption by a factor of 2.6 to 3.5 as compared to an empty cell, due to slow light inside a 2D silicon photonic crystal gas cell, was observed; this is in excellent agreement with numerical simulations. We show that, theoretically, for an optimal design enhancement factors of up to 60 are possible in the region of slow light. However, the overall transmission of bulk photonic crystals, and thus the performance of the device, is limited by fluctuations of the pore diameter. Numerical estimates suggest that the positional variations and pore diameter fluctuations have to be well below 0.5% to allow for a reasonable transmi...

Scientific Reports, 2018

Mid-infrared (mid-IR) optical fibers have long attracted great interest due to their wide range of applications in security, biology and chemical sensing. Traditionally, research was directed towards materials with low absorption in the mid-IR region, such as chalcogenides, which are difficult to manipulate and often contain highly toxic elements. In this paper, we demonstrate a Polyethylene Terephthalate Glycol (PETG) hollow-core fiber (HCF) with guiding properties in the mid-IR. Guiding is provided by the fiber geometry, as PETG exhibits a material attenuation 2 orders of magnitude larger than the HCF propagation loss. The structured plastic fiber preforms were fabricated using commercial 3D printing technology and then drawn using a conventional fiber drawing tower. The final PETG fiber outer diameter was 466 µm with a hollow-core diameter of 225 µm. Thermal imaging at the fiber facet performed within the wavelength range 3.5–5 µm clearly indicates air guidance in the fiber hollow-core. Since the invention of laser sources in the mid-IR range spectral region (2.5 to 25 μm), there has been a growing interest in the development of optical fibers transparent at these wavelengths for applications in chemical, biological and atmospheric sensing 1–4 , where the unique molecular absorption associated to the excitation of specific fundamental vibrational and rotational modes allows for an accurate chemical fingerprinting 5. Although over the last 3 decades silica optical fibers have become the backbone of optical communications and distributed optical sensing, they cannot guide light in the mid-IR as the silica network has strong overtone absorptions above 2 μm, resulting in an overall attenuation that increases for increasing wavelengths. Thus, the search for optical fiber operating in the mid-IR has focused on materials that have high transmittance in the wavelength range λ = 2–20 μm such as chalcogenide glasses, heavy metal fluorides, polycrystalline silver halides (AgX), single-crystal sapphire, and tellurium halides (TeX) 6–8 , which still requires extraordinary purification processes and are extremely hard to handle. An alternative approach relies on the use of hollow-core fibers (HCFs) such as photonic bandgap fibers and anti-resonant fibers, where light is confined within the hollow-core, greatly decreasing the influence of the material optical properties. Such fibers have been already exploited for numerous applications in infrared spectroscopy such as laser surgery, gas sensing, label-free biological sensing, thermal imaging and infrared countermeasures 9–13. In initial attempts, mid-IR HCFs have been fabricated by coating the inner surface of a capillary tube with a metallic film and dielectric material. However, the coating procedure is relatively cumbersome and used toxic materials. Lately, pure silica HCFs have been investigated 14. As the mode propagates in the microstructured fiber hollow-core, the effect of the silica's high material absorption is minimized owing to the low overlap (<1%) of the guided mode with the glass. Silica HCFs were manufactured using the stack and draw technique, where capillaries are manually assembled into a hexagonal structure prior to draw. Despite this being the most popular method, it is labor intensive, mainly limited to the hexagonal packed periodic structure and difficult to implement for complex designs. Because of the continuous improvement in additive manufacturing technologies and the related cost reduction , the fabrication of prototypes using 3D printing has gained interest in many applications such as medical , art, engineering and science. The most common manufacturing techniques to print 3D models include

2007 IEEE Sensors, 2007

A hollow waveguide mid-infrared gas sensor operating from 1000 cm -1 to 4000 cm -1 has been developed, optimized, and its performance characterized by combining a FT-IR spectrometer with Ag/Ag-halide hollow core optical fibers. The hollow core waveguide simultaneously serves as a light guide and miniature gas cell. CH 4 was used as test analyte during exponential dilution experiments for accurate determination of the achievable limit of detection (LOD). It is shown that the optimized integration of an optical gas sensor module with FT-IR spectroscopy provides trace sensitivity at the few hundreds of parts-per-billion concentration range (ppb, v/v) for CH 4 .

Proceedings of SPIE, 2012

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.

The detection and measurement of gas concentrations using the characteristic optical absorption of the gas species is important for both understanding and monitoring a variety of phenomena from industrial processes to environmental change. This study reviews the field, covering several individual gas detection techniques including non-dispersive infrared, spectrophotometry, tunable diode laser spectroscopy and photoacoustic spectroscopy. We present the basis for each technique, recent developments in methods and performance limitations. The technology available to support this field, in terms of key components such as light sources and gas cells, has advanced rapidly in recent years and we discuss these new developments. Finally, we present a performance comparison of different techniques, taking data reported over the preceding decade, and draw conclusions from this benchmarking.

SPIE Proceedings, 2011

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 CO 2 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.

Sensors, 2013

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 HE 11 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.