Characterization of a mid-infrared hollow waveguide gas cell for the analysis of carbon monoxide and nitric oxide.

Infrared spectroscopy is commonly applied to the analysis of small gas-phase molecules. One of the limitations of using Fourier transform infrared (FT-IR) spectroscopy for these applications is the time response of long path length gas cells. Hollow waveguides (HW) that transmit in the mid-infrared spectral range have higher optical efficiencies compared to long path length cells due to smaller cell volumes. This study characterizes a silver coated, 2 mm inner diameter HW for the analysis of carbon monoxide (CO) and nitric oxide (NO) and compares the performance to a 3 m gas cell and traditional gas analyzers. The HW was found to have a CO response time less than the NDIR analyzer and approximately one-tenth of the response time on the FT-IR system equipped with a 3 m gas cell. The utility of the increased response time was demonstrated by measuring CO concentrations in sidestream cigarette smoke at the same temporal resolution as an NDIR analyzer. A 10 to 60% increase in sensitivity using various frequencies for both CO and NO was observed using the HW compared to the 3 m multipass gas cell. However, cost savings for gas-sensing applications can be achieved on a per analyte basis by using FT-IR spectroscopy, especially in combination with a HW gas-sensing module, which is significantly less expensive than a multipass gas cell.

Broad band and low loss mid-IR flexible hollow waveguides.

This paper introduces a deposition method to create a multilayered waveguide with alternating layers of high index of refraction contrast. A very thin Ag layer, practically transparent in the mid-IR radiation wavelengths of CO(2) and Er-YAG lasers, was created. This enabled a good contrast of the indices of refraction of silver/silver iodide. Theoretical calculations as well as experiments have shown that transmission was higher at these wavelengths for two pair layers, in comparison to one pair of silver/silver iodide. Windows of transmittance and small sensitivity to bending were demonstrated for those two pair layer waveguides. This method could be extended to an increased number of pairs to configure a true photonic band gap waveguide.

Flexible waveguides for Er-YAG laser radiation delivery.

Flexible plastic waveguides (FPW) were devised for the delivery of Er-YAG laser radiation. The FPW characteristics were studied under various conditions. In vitro studies were carried out to explore the drilling procedure on extracted teeth and the FPW-tissue mutual effects. The results which were obtained proved that the FPW as a delivery device might be a substitute hand applicator for the pneumatic turbine for drilling in teeth.

Modal analysis and cut-off conditions of multichannel surface-acoustic-waveguide structures.

Multichannel guides for surface acoustic waves can improve the efficiency of SAW (surface acoustic-wave) devices significantly. Focusing, steering, and modulating the propagating acoustical modes can be achieved similarly to optical waveguided devices. A general formulation is presented for the analysis of the lateral waveguiding properties of Rayleigh modes in surfaces loaded with deposited strips of different materials. General expressions are obtained for the number of modes and cutoff conditions in these structures. As examples of applications, a simple directional coupler and an electrically controlled coupler are proposed.

Experimental evaluation of a hollow glass fiber.

Very high interest in making a low-loss fiber for the infrared has been stimulated by important applications in optical communication, surgery, cutting, welding, and heat treatment. The leaky waveguide is one of the most promising types of future fiber in the infrared region where low-loss materials are not available or not suitable for making fibers (i.e., CO2 laser light lambda = 10.6 microm). In this paper a comparative model of a He-Ne laser beam and an oxide glass leaky hollow fiber for a CO2 laser light beam and a chalcogenide glass leaky hollow fiber are studied. Measurements of attenuation, dependence of output power on diameter and angle, and the angular dependence of output angle vs input angle were made. The experimental data were compared with theoretical calculations, and the critical value of the wall thickness for minimum attenuation is given.

Studies on the relation between the diffusion process and optical properties in Ti-diffused planar optical waveguides.

The kinetics of Ti diffusion into Y-cut LiNbO(3) was investigated. Scanning electron microscopy and Auger electron spectroscopy techniques were used to study the diffusion process, while end-fire and prism coupling of light at different wavelengths was used for optical characterization. A Gaussian profile was chosen as a solution to the diffusion equation, and the diffusion constant and the activation energy for this profile were found. By comparison of the result of optical measurements with numerical theoretical calculations, we obtained an empirical relation between the initial Ti thickness and the extraordinary refractive-index change.

Characterization of hollow fibers for the transmission of infrared radiation.

Plastic hollow fibers were made from plastic tubes covered on the internal wall with a metal layer (a-type) or a metal layer and dielectric layer on top of it (b-type). The CO(2) laser energy transmission through the hollow fiber was measured as a function of the radius of curvature and the coupling lens (focal length at a constant fiber length). The yield of the transmission decreased in subtle curvatures (radius of curvature up to 100 cm) and remained almost constant as the curvature became sharper (down to radius of curvature of 13 cm). For the a-type fibers, the characteristics of attenuation depended on the focal length of the coupling lenses. The energy distribution at the output was measured and mapped. The experimental results showed that the maximum of the energy distribution is asymetrically positioned relative to the center and closer to the internal wall at a smaller bending radius. This was predicted in our previous theoretical calculation. The value of transmitted power attenuation was up to 1.4 dB/m. Maximum power at the output was 30 W, for a fiber of 50-cm length and a cross-sectional diameter of 1.9 mm. These types of hollow fiber have already been used in surgical experiments on dogs.

Nondestructive method for attenuation measurements in optical hollow waveguides.

A new method for characterizing hollow waveguides has been developed in which the laser radiation is coupled into the waveguide hollow bore through an optical fiber. By moving the distal end of the fiber along the waveguide we achieved scanning of the incident radiation in the waveguide at various points on the internal walls. This method can be employed for measuring attenuation without cutback or for detecting point defects on the waveguide's guiding layers.

Broadband flexible waveguides for free-electron laser radiation.

We refined flexible waveguides previously developed for CO(2) and Er:YAG laser radiation to transmit free-electron-laser (FEL) radiation. One can tune this laser over several segments of the radiation spectrum. This laser has a high peak power of as much as 10 MW with pulse energy of as much as 100 mJ. We made the waveguides of either Teflon or fused-silica tubes internally coated with metal and dielectric layers. We optimized the internal coatings specifications for transmission of various radiation wavelengths in the mid-IR range and enabled transmission of high-peak radiation. We performed experiments in three major FEL sites in the United States over a more than 1-year period when we measured and examined various characteristics of transmission. We used the analysis of these experiments as feedback to further improve these waveguides. The good preliminary results encourage us to invest more effort to further develop these waveguides until a suitable waveguide is obtained for this type of laser and make possible its introduction to the medical field where its characteristics can be exploited in surgical applications.

Hollow waveguide for mid and thermal infrared radiation.

OBJECTIVE: A novel type of flexible hollow waveguide that facilitates delivery of infrared radiation from source to treatment site, and made of polyimide tubes, was developed. SUMMARY BACKGROUND DATA: Polyimide tubes with flat internal wall and optimal conditions of deposition of guiding metal and dielectric layers were used to obtain low losses of radiation and high quality of beam shape. METHODS: Etching of the internal wall of polyimide tubes, high reflective metal (silver) layers and suitable dielectric layers (index of refraction and thickness) were deposited as guiding elements to obtain a waveguide suitable for infrared transmission. RESULTS: Polyimide waveguides of very low attenuation (transmission of approximately 95% through 1 meter length) beam profile of variable shape as a function of bending radius and type of tbe, large delivered radiation power (more than 25 Watts), constant delivered power as a function of time (during more than 250 seconds), and very low divergence of the delivered beam (angle of divergence less than 1 degree), were obtained. The polyimide waveguides also are very flexible and may be bent to a radius of bending less than 5 cm. The polyimide plastic hollow waveguide is suitable for medical applications (chemical and physical stable and nontoxic). CONCLUSIONS: The optical and mechanical parameters correspond with those requested for applications in medicine for infrared laser treatments or radiometric measurements (non-contact thermometry). Waveguides for applications in surgery are under preparation.

Flexible waveguides for IR laser radiation and surgery applications.

Flexible plastic waveguides were developed to deliver IR radiation, especially at 10.6 microns, which is the CO2 laser radiation wavelength. The waveguide is made from teflon tube with the inner wall coated with a metal layer and a dielectric overlayer. The internal diameter (ID) is 1.0 mm, length 1.0-1.2 m, and the distal tip decreases moderately to ID approximately 0.6 mm. The distal part on the last 10 centimeters is coated externally with a metal layer. Maximum power that can be delivered at the outlet is approximately 30 W and 10.6 x 10(3) W/cm2. This type of waveguide was used in several medical operations to evaluate its cutting characteristics and the resistances to heat reflection from the tissue while operating in orifices containing liquid substances.

Experimental surgery on dog's stomach and liver using CO 2 laser plastic hollow fibers: technical method.

Plastic hollow fibers for the transmission of CO 2 laser energy in curved paths were produced by plating the inner surface of plastic tubes with a metal film and dielectric film upon it. These fibers can transmit high power up to 40 W at the outlet, with high transmission yield even through several bendings. A three-dimensional simulator was built to simulate paths in the dog's body and the outlet power was measured. From the achieved data the expected transmitted power during real surgery was appreciated. The fibers were checked for their influence on live tissues of dogs. Incisions were made in the liver and ulcers in the dogs' stomachs were treated. The fibers were inserted into the stomach through the dogs' esophagus. Complete healing was observed after four weeks.

Preliminary experiments of possible uses in medicine of novel plastic hollow fibers for transmission of CO2 radiation.

Plastic hollow fibers for the transmission of CO2 laser energy in curved paths were produced by plating the inner surface of plastic tubes with a metal film and dielectric film upon that. These fibers can transmit high power, up to 40 W at the outlet, with high transmission yield even through several bendings. To demonstrate a possible use of the fibers, they were checked for their influence on live tissues of dogs. Incisions were made in the liver, and ulcers in the dogs' stomachs were treated. The fibers were inserted into the stomach through the esophagus. Complete healing was observed after 4 weeks.

Ray model for transmission of metallic-dielectric hollow bent cylindrical waveguides.

The problem of transmitting CO(2) laser radiation through metallic or metallic with inner dielectric coating (metallic-dielectric) bent hollow cylindrical waveguides is investigated using a ray model. Computer calculations of transmission as a function of the geometrical dimensions of the waveguide are performed. The coupling of laser radiation at the entrance of the waveguide is also taken into account. The theoretical calculated transmission is compared with previously published experimental data and good agreement is obtained for a large range of curvatures. The devised ray model contributes to a better understanding of the role of the dielectric layer in the metallic-dielectric waveguide, increasing the transmission of the radiation. The calculation of the transmission as a function of the radius of the cross section of the waveguide shows that, for a best metallic-dielectric waveguide, an optimal cross-sectional diameter appears where the transmitted energy is maximum. The method presented will be of value as a tool in the design of hollow cylindrical waveguides.

Plastic hollow waveguides: properties and possibilities as a flexible radiation delivery system for CO2-laser radiation.

BACKGROUND AND OBJECTIVES: One significant inconvenience of the CO2 laser is the lack of flexible fibers essential for endoscopic applications. The goal of this study is to test the feasibility of hollow waveguides in view of a practical use in medicine. STUDY DESIGN/MATERIALS AND METHODS: Various types of plastic hollow waveguides for flexible delivery of CO2 laser radiation were examined. The transmission losses, divergence angle, damage threshold, and input and output beam profiles were determined. The interaction process between radiation transmitted through these guides with soft as well as hard tissues was studied. RESULTS: Plastic hollow waveguides can transmit high power (up to 50 W) with low losses (straight guide 1 dB/m) even through bendings. The divergence angle is < 13 degrees. Cutting quality and extent of thermal damage are comparable to incisions performed with a free laser beam. CONCLUSION: The results of this study show good cutting quality and durability of these flexible plastic hollow waveguides, which render possible to deliver CO2 radiation in the power range needed for most surgical applications with affordable transmission losses. Plastic hollow waveguides are, therefore, a real alternative to replace the mirror arms.

Spectroscopy in the gas phase with GaAs/AlGaAs quantum-cascade lasers.

We demonstrate what we believe is the first application of the recently developed electrically pumped GaAs/AlGaAs quantum-cascade lasers in a spectroscopic gas-sensing system by use of hollow waveguides. Laser light with an emission maximum at 10.009 microm is used to investigate the mid-infrared absorption of ethene at atmospheric pressure. We used a 434-mm-long silver-coated silica hollow waveguide as a sensing element, which served as a gas absorption cell. Different mixtures of helium and ethene with known concentrations are flushed through the waveguide while the laser radiation that passes through the waveguide is analyzed with a Fourier-transform infrared spectrometer. The experimentally obtained discrete ethene spectrum agrees well with the calculated spectrum. A detection threshold of 250 parts per million is achieved with the current setup.