Daniel J. Gibson

Processing and characterization of silver films used to fabricate hollow glass waveguides

Hollow glass waveguides are an increasingly popular fiber for the delivery of high-power IR laser radiation. At CO(2) laser wavelengths the measured and theoretical losses agree, but at the 3-microm Er:YAG laser wavelength the losses remain higher than expected. The reason for this is the surface roughness of the silver film used to form the first layer of the Ag/AgI thin-film structure. We found that the roughness of the silver film increases fivefold as silvering times increase from 5 to 80 min. This increased surface roughness produces a concomitant linear increase in the attenuation coefficient for the silver-only guides for wavelengths shorter than approximately 5 microm.

Extrusion of hollow waveguide preforms with a one-dimensional photonic bandgap structure

An extrusion technique is used to make an all-dielectric, hollow waveguide preform. The structure consists of radially alternating dielectric layers of high/low refractive index pairs. By requiring that the two dielectric materials have a high index contrast, it is possible to make a preform that will have a photonic bandgap structure when drawn into a fiber optic. The preform is made by an extrusion process in which a stack-of-plates, composed of alternating disks of chalcogenide glass and a polymer, is extruded through a die into both solid and hollow-core structures. Laminar flow during extrusion forces the periodicity from an axial to a radial orientation in the final extruded preform. For these experiments the high index material was arsenic selenide glass (As2Se3,n=2.6) and the low index material was polysulfone (PSU,n=1.55), which gives an index contrast of 1.68.

Recent advances in the fabrication of hollow glass waveguides

Hollow glass waveguides are an attractive fibers delivery system for a broad range of IR wavelengths, including the 3 μm Er:YAG and 10.6 μm CO2 lasers. The losses for these waveguides are as low as 0.2 dB/m at the 10.6 wavelength for waveguides with a 700 μm bore. At the shorter wavelengths, like that of the Er:YAG laser, losses are higher than those predicted theoretically. This is shown to be a result of the increasing effect of surface roughness of the inner coatings. Variation of attenuation from waveguide to waveguide is examined, and the variation in the dielectric layer is evaluated.

Tapered and noncircular hollow glass waveguides

Linearly tapered hollow-glass waveguides (HGW) were fabricated using tapered silica glass tubing and wet chemistry techniques. Attenuation constants for these tapered HGWs were found to be higher than for similarly sized non-tapered HGWs, but the tapered guides showed reduced loss on bending. HGWs with rectangular and square cross-sections were also fabricated from non-circular bore silica glass tubing using wet chemistry techniques. These guides were able to maintain linear polarization of CO2 laser light better than circular bore HGWs fabricated by the same methods, with as high as 97% of the input polarization preserved for a 227 μm X 1253 μm bore guide. The non-circular bore HGWs had higher attenuation constants than similarly sized circular bore HGWs and sacrificed some spatial purity of the output beam.

Polarization-maintaining hollow glass waveguides with noncircular bore

Hollow waveguides are made from square and rectangular glass tubing for the transmission of polarized CO2 laser radiation. These hollow noncircular cross section glass waveguides have Ag and AgI thin film coatings deposited on the inside surface using liquid-phase chemis- try techniques. The losses for straight square (5003500 mm) and rect- angular (22531250 mm) hollow glass waveguides at 10.6 mm were 0.8 and 2.2 dB/m, respectively. The degree of polarization is more than 90% for the rectangular waveguides, and this degree of polarization is pre- served when rectangular guides are bent or twisted to rotate the polar- ization.

Coiled hollow waveguides for gas sensing

Coiled hollow waveguide gas absorption cells were designed and fabricated using polymer and glass tubing. These coiled waveguides were found to be acceptable for the detection of gases in the IR region of the spectrum. Through the in-depth investigation of straight and bent losses in hollow waveguides, an equation was developed to calculate the loss of the coiled system. By setting the straight loss to zero, it was found that additional loss on bending was approximately 1 dB per turn, independent of the bore size or bending radius.

Gradually tapered hollow glass waveguides for the transmission of CO~2 laser radiation

Hollow glass waveguides with bores tapered from 1000 to 500 microm and from 700 to 500 microm over a length of 2 m were coated with silver and silver iodide inner films. These waveguides were designed for low attenuation at the 10.6-microm CO2 laser wavelength. The straight losses, which were measured to be 0.8 and 1.6 dB/m, respectively, decreased when the guides were bent. A simple ray-trajectory model is presented to explain this unexpected behavior.

Producing an intense collimated beam of sound via a nonlinear ultrasonic array

We have designed and built an ultrasonic parametric array with an emphasis on creating an intense, collimated beam of low frequency sound. With this device, we can insonify a small area of ground or a small target at range and induce vibrations. These vibrations can be synchronously detected with any stand-off device such as a laser-Doppler vibrometer or the millimeter wave vibrometer we describe in Smith et al. [J. Appl. Physics 108, 024902 (2010)]. Despite nonlinear conversion losses, the array produces sound pressure levels in excess of 90u2009dB at 1u2009kHz, 1.5u2009m in front of the array using 25 low-cost 40 KHz transducers.

Extrusion method for fabrication of hollow waveguides with 1D photonic bandgap structure

An all-dielectric hollow waveguide structure consisting of radially alternating dielectric layers and exhibiting a photonic bandgap is proposed. Fabrication involves the extrusion of a stack-of-plates into a hollow structure composed of alternating high/low index pairs. Specifically, a billet consisting of alternating glass layers is forced through a die such that laminar flow forces the periodicity from an axial to a radial orientation. The concept is demonstrated using lead-borosilicate and As2Se3 glasses.

Transmission properties of hollow glass waveguides

IR transmitting hollow waveguides are an attractive alterative to solid-core IR fibers. Hollow waveguides are made from silica glass or plastic tubing which has highly reflective coatings deposited on the inside surface. These guides have losses as low as 0.1 dB/m at 10.6 micrometer and may be bent to radii less than 5 cm. For laser-power delivery applications the hollow glass guides have been shown to be capable of transmitting up to 1 kW of CO2 laser power. In some power delivery applications it is necessary to have a distal tip configured to bend and/or concentrate the light for more efficient ablation. Curved tips (2 cm in length) are shown to increase the loss of a 1-m long straight guide from 22 to 34%. New research is described on the fabrication of coherent hollow glass bundles for IR imaging. The number of guides in the bundle is currently less than 20 but the results indicate that the hollow bundle can be coated to achieve an identical spectral response for each individual guide.