Hollow Glass Waveguides Research Articles

Delivery of the diode pumped Er:YLF laser radiation by special hollow glass waveguides

Delivery of the diode pumped Er:YLF laser radiation by special hollow glass waveguides

Hollow waveguide CO2 laser sensing system for rapid detection of trace sulfur hexafluoride (SF6) gas

Sulfur hexafluoride (SF6) has been widely applied in electrical facilities due to its excellent electrical insulation and high-voltage arc extinguishing properties, although it is a potent greenhouse gas. Rapid detection of trace SF6 gas is significant for monitoring the operation safety of high-voltage power grid facilities and greenhouse gas emissions. In this work, a SF6 gas sensing system was proposed and built using a CO2 laser as the light source and a 700 μm bore Ag/AgI hollow glass waveguide (HGW) as the gas chamber and laser transmission channel. Sensing performances of the SF6 gas HGW sensors with different lengths were investigated. The gas sensing sensitivity and standard deviation (SD) of blank increase as the waveguide length increases. The limit of detection (LOD) decreases with waveguide length increase but the decline becomes less pronounced after the length exceeds 120 cm. The HGW length exhibits minor effect on the response and recovery time of the sensor system. The sensor sensitivity undergoes a slight reduction as the HGW is bent. Optimization of the sensing performance was achieved based on the straight 120 cm-length HGW SF6 gas sensor system. Its sensitivity reaches 0.384 dB/ppm and the LOD is 148 ppb with good cyclic response characteristics (response and recovery time 2–3 s). The present work provides a feasible idea for building up a trace SF6 gas sensing system with simple structure, easy operation and high performances.

Mid-infrared saturated absorption spectroscopy inside a hollow glass waveguide

We report the saturation spectroscopy of carbon dioxide inside a hollow glass waveguide (GW) using a tunable mid-infrared difference frequency generation (DFG) source. This DFG source was generated in a periodically poled lithium niobate crystal with a tuning range from 3.97 to 4.71μm. Saturated spectroscopy was performed on the R(60) transition of CO20001 ← 0000 band at 4.2μm inside a hollow GW of diameter 300μm and length 1 m. The saturation dip signal was found with a pump power as low as 0.18 mW inside the hollow GW. The linewidth of the dip was determined to be 4.40 MHz.

Observation of saturated absorption spectrum of 12C2H2 at 1536 nm in a 300 μm hollow glass waveguide

A system for saturated absorption spectroscopy is firstly built (to our knowledge) based on a gas-filled hollow glass waveguide with a core diameter of 300 μm. The performance of the system is demonstrated by observing the saturated absorption of the P(17) transition of the (ν1 +ν3) combination band of acetylene around 1536 nm using an erbium-doped fiber amplifier boosted external cavity diode laser. The saturated absorption spectrum is observed and the observed linewidth of the saturation dip varies from 10 to 20 MHz depending on the gas pressure and the optical launching condition into the GW. Its derivative spectrum is obtained using wavelength modulation spectroscopy and the third derivative signal is used as the error signal to lock the external cavity diode laser to the transition center. A frequency stability of ~60 kHz at 1 s integration time was achieved. Our study shows it is possible to build a mid-IR frequency reference using a large core gas-filled hollow GW.

Radiotherapy fiber dosimeter probes based on silver-only coated hollow glass waveguides.

Manifestation of Čerenkov radiation as a contaminating signal is a significant issue in radiation therapy dose measurement by fiber-coupled scintillator dosimeters. To enhance the scintillation signal transmission while minimizing Čerenkov radiation contamination, we designed a fiber probe using a silver-only coated hollow waveguide (HWG). The HWG with scintillator inserted in its tip, embedded in tissue-mimicking phantoms, was irradiated with clinical electron and photon beams generated by a medical linear accelerator. Optical spectra of the irradiated tip were taken using a fiber spectrometer, and the signal was deconvolved with a linear fitting algorithm. The resultant decomposed spectra of the scintillator with and without Čerenkov correction were in agreement with measurements performed by a standard electron diode and ion chamber for electron and photon beam dosimetry, respectively, indicating the minimal effect of Čerenkov contamination in the HWG-based dosimeter. Furthermore, compared with a silver/dielectric-coated HWG fiber dosimeter design, we observed higher signal transmission in the design based on the use of silver-only HWG.

Optic nerve sheath fenestration using a Raman-shifted alexandrite laser.

Optic nerve sheath fenestration is an established procedure for relief of potentially damaging overpressure on the optic nerve resulting from idiopathic intracranial hypertension. Prior work showed that a mid-IR free-electron laser could be delivered endoscopically and used to produce an effective fenestration. This study evaluates the efficacy of fenestration using a table-top mid-IR source based on a Raman-shifted alexandrite (RSA) laser. Porcine optic nerves were ablated using light from an RSA laser at wavelengths of 6.09, 6.27, and 6.43 μm and pulse energies up to 3 mJ using both free-space and endoscopic beam delivery through 250-μm I.D. hollow-glass waveguides. Waveguide transmission was characterized, ablation thresholds and etch rates were measured, and the efficacy of endoscopic fenestration was evaluated for ex vivo exposures using both optical coherence tomography and histological analysis. Using endoscopic delivery, the RSA laser can effectively fenestrate porcine optic nerves. Performance was optimized at a wavelength of 6.09 μm and delivered pulse energies of 0.5-0.8 mJ (requiring 1.5-2.5 mJ to be incident on the waveguide). Under these conditions, the ablation threshold fluence was 0.8 ± 0.2 J/cm(2) , the ablation rate was 1-4 μm/pulse, and the margins of ablation craters showed little evidence of thermal or mechanical damage. Nonetheless, nominally identical exposures yielded highly variable ablation rates. This led to fenestrations that ranged from too deep to too shallow-either damaging the underlying optic nerve or requiring additional exposure to cut fully through the sheath. Of 48 excised nerves subjected to fenestration at 6.09 μm, 16 ex vivo fenestrations were judged as good, 23 as too deep, and 9 as too shallow. Mid-IR pulses from the RSA laser, propagated through a flexible hollow waveguide, are capable of cutting through porcine optic nerve sheaths in surgically relevant times with reasonable accuracy and low collateral damage. This can be accomplished at wavelengths of 6.09 or 6.27 μm, with 6.09 μm slightly preferred. The depth of ex vivo fenestrations was difficult to control, but excised nerves lack a sufficient layer of cerebrospinal fluid that would provide an additional margin of safety in actual patients.

Silver/cyclic olefin copolymer hollow glass waveguides for infrared laser delivery.

Metal/dielectric-coated hollow glass waveguides (HGWs) have been studied extensively for the efficient transmission of radiation over a broad spectral range. In this study, a low-absorption optical polymer, cyclic olefin copolymer (COC), is investigated as a dielectric material for HGWs designed for the delivery of various IR lasers. Using established silver (Ag) plating techniques and a newly optimized polymer-coating procedure, Ag/COC HGWs with low attenuation coefficients are fabricated for operation at the following three wavelengths: 808 nm, 1.064 μm, and 2.94 μm. The spectral responses of the HGW designs are used to develop a film thickness dependency of the COC layer formation based on the concentration of the solution used in the polymer deposition procedure. Further, the attenuation coefficients of the HGWs are measured using the cutback method at the three wavelengths as a function of the curvature of the waveguide. In order of increasing operation, the attenuation coefficients are measured to be 0.549, 0.095, and 0.298 dB/m for the HGWs in the straight configuration. These experimental values for the straight attenuation coefficients are compared to theoretical values calculated using a ray transfer matrix approach and are found to be in good agreement.

Single mode operation with mid-IR hollow fibers in the range 5.1-10.5 µm.

Single mode beam delivery in the mid-infrared spectral range 5.1-10.5 μm employing flexible hollow glass waveguides of 15 cm and 50 cm lengths, with metallic/dielectric internal layers and a bore diameter of 200 μm were demonstrated. Three quantum cascade lasers were coupled with the hollow core fibers. For a fiber length of 15 cm, we measured losses down to 1.55 dB at 5.4 μm and 0.9 dB at 10.5 μm. The influence of the launch conditions in the fiber on the propagation losses and on the beam profile at the waveguide exit was analyzed. At 10.5 µm laser wavelength we found near perfect agreement between measured and theoretical losses, while at ~5 µm and ~6 µm wavelengths the losses were higher than expected. This discrepancy can be explained considering an additional scattering loss effect, which scales as 1/λ(2) and is due to surface roughness of the metallic layer used to form the high-reflective internal layer structure of the hollow core waveguide.

Delivery of 3–5 μm laser radiation by a hollow waveguide

Radiation delivery is required for various applications—mainly in medicine or industry. Due to the necessity of delivering IR, UV or powerful radiation, its transfer by hollow waveguides was investigated because standard glass fibers cause serious losses. Our special waveguide was based on a fused silica glass capillary tube with an inner silver layer and dielectric film (cyclic olefin polymer (COP)). Three lasers were designed and constructed as mid-infrared sources, namely Er:YAG (2.94 μm), Dy: PbGa2S4 (4.3 μm) and Fe:ZnSe (4.45 μm). The delivered spatial beam profile and transfer capability of 3−5 μm radiation by the COP/Ag hollow glass waveguide were characterized.

Miniature forward‐imaging B‐scan optical coherence tomography probe to guide real‐time laser ablation

Investigations have shown that pulsed lasers tuned to 6.1 µm in wavelength are capable of ablating ocular and neural tissue with minimal collateral damage. This study investigated whether a miniature B-scan forward-imaging optical coherence tomography (OCT) probe can be combined with the laser to provide real-time visual feedback during laser incisions. A miniature 25-gauge B-scan forward-imaging OCT probe was developed and combined with a 250 µm hollow-glass waveguide to permit delivery of 6.1 µm laser energy. A gelatin mixture and both porcine corneal and retinal tissues were simultaneously imaged and lased (6.1 µm, 10 Hz, 0.4-0.7 mJ) through air. The ablation studies were observed and recorded in real time. The crater dimensions were measured using OCT imaging software (Bioptigen, Durham, NC). Histological analysis was performed on the ocular tissues. The combined miniature forward-imaging OCT and mid-infrared laser-delivery probe successfully imaged real-time tissue ablation in gelatin, corneal tissue, and retinal tissue. Application of a constant number of 60 pulses at 0.5 mJ/pulse to the gelatin resulted in a mean crater depth of 123 ± 15 µm. For the corneal tissue, there was a significant correlation between the number of pulses used and depth of the lased hole (Pearson correlation coefficient = 0.82; P = 0.0002). Histological analysis of the cornea and retina tissues showed discrete holes with minimal thermal damage. A combined miniature OCT and laser-delivery probe can monitor real-time tissue laser ablation. With additional testing and improvements, this novel instrument has the future possibility of effectively guiding surgeries by simultaneously imaging and ablating tissue.

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.

Fabrication and characterization of improved Ag/PS hollow-glass waveguides for THz transmission

This study involves the fabrication and characterization of improved quality silver (Ag)/polystyrene (PS) thin-film-coated hollow-glass waveguides for the low-loss transmission of terahertz radiation via modified dynamic liquid phase deposition techniques. High-quality PS thin films were deposited from aqueous PS solutions, and the spectral response of fabricated samples was measured from λ=1-100 μm. Fabricated samples exhibited highly defined spectral responses throughout this entire range indicative of PS films of excellent quality. The spectra of experimental samples were compared to the theoretical and bulk PS spectra in the near-IR and far-IR regions. The thickness of deposited PS thin films was found to depend on total sample length and to vary from approximately 10-16 μm for sample lengths ranging from 115 to 140 cm. Such PS film thicknesses are adequate for low-loss delivery from approximately 2-4 THz. Furthermore, film thickness was found to vary minimally along the waveguide length regardless of total sample length.

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.

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.

Resolution limits of laser spectroscopic absorption measurements with hollow glass waveguides

In this paper, resolution limits of laser spectroscopy absorption measurements with hollow capillary fibers are investigated. Furthermore, a concept of sensitive near-infrared sensing utilizing hollow fiber directly coupled with vertical-cavity surface-emitting lasers is developed. By performing wavelength modulation spectroscopy, the smallest absorbance that can be detected by the fiber sensor was determined to be 10(-4), limited by a random modulation of the fiber transmission function (modal noise). By mechanically vibrating the fiber, a sensor resolution of 10(-5) in absorbance is achieved. Because the random modulation on the fiber transmission function limits the detection sensitivity, its physical reasons are analyzed. One contribution is found to be the partial integration of the far field, and the amplitude of the spectral features is inversely proportional to the square root of the integrated speckle points number. Therefore, careful design of the fiber-detector outcoupling is necessary. It turned out that incoupling alignment is not of much influence with respect to the spectral background. The residual spectral background is caused by mode-dependent effects and can be lowered by vibrating the fiber mechanically.

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.

Application of mid-infrared laser radiation for lithotripsy

The perforation effects of Er:YAG (2940 nm) and Ho:YAG (2100 nm) lasers radiation on human urinary stones model made from compressed plaster and real human samples were compared in vitro. For mid-infrared laser radiation delivery the special COP/Ag hollow glass waveguides were used. From the interaction experiments the perforation rates were derived and compared for both lasers. From the results it can be evaluated that Er:YAG laser radiation is favourable in comparison with Ho:YAG laser in case of artificial samples perforation efficiency.

Efficient excitation of the TE_01 hollow metal waveguide mode for atom guiding

We demonstrate excitation of the azimuthally-polarized TE(01) cylindrical waveguide mode in hollow glass and metal waveguides with 780 nm light. Experimentally, we demonstrate formation of the vectorial vortex beams, and measure attenuation lengths of the TE(01) mode in hollow optical fibers with diameters of 50-100 microns. By silver-coating the inner walls of the dielectric fibers, we demonstrate a approximately 200% increase in the attenuation length.

Theoretical and experimental investigations of metal sulfide dielectric coatings for hollow waveguides

Continuous efforts to develop low-loss flexible waveguides, to transmit mid-IR laser energy for minimally invasive surgical and diagnostic procedures, have been carried out by us and other groups. We have introduced sulfide dielectric films coated over an Ag reflecting layer as another potential solution. The metal sulfides used have high transparency in the IR spectrum, and their thickness can be tailored to minimize the attenuation over a selected wavelength range. The high refractive index contrast of the two metal sulfide materials enables us to produce multilayer hollow waveguides. These waveguide will have low attenuation in both straight and bent conditions, low sensitivity to coupling and to surface roughness, and a broad wavelength range. The lowest loss for a straight guide, measured at 1.55 µm for a 1,000-µm-bore Ag/CdS/PbS/CdS hollow glass waveguide (HGW), was 0.06 dB/m. This loss is three times less than that measured for a single-layer Ag/CdS-coated HGW at 1.55 µm. A theoretical simulation applying the same conditions showed the same pattern with a good potential for improvement.