Werner Fuss

Liquid-core light guides for near-infrared applications

The strong absorption of tissue water is responsible for the low ablation threshold for biological tissues at the Er:YAG and Er:YSGG laser wavelengths. These lasers are therefore considered to be promising tools for medical treatments. As the existing transmission systems are still unsatisfactory, three types of liquid-filled light guides are investigated here as alternatives to conventional near-IR fibers. In addition to mechanical advantages, the minimum attenuation is below 3 dB/m, and losses at bending radii down to 20 mm are negligible. The maximum output energy densities of 14.2 J/cm(2) (free-running Er:YAG) or power densities of 7 MW/cm(2) (Q-switched Er:YAG) are sufficient for soft-tissue ablation. When the liquid was circulated, much higher energy densities, exceeding the hard-tissue ablation threshold, were achieved. These properties make liquid-core light guides promising delivery systems for many near-IR applications, including medical ones.

Liquid-core fused silica capillary lightguides for applications in the UV/VIS and NIR spectral range

Fused silica capillaries proved to be a suitable cladding material for liquid core lightguides. They have small diameters, a good mechanical stability and a high flexibility. Due to these properties such lightguides can be used for applications in minimally invasive surgery for endoscopical treatments. Filled with carbontetrachloride (CCl4), the optical properties were investigated spectrometrically and with various lasers. The transmission measurements of straight and 180 degree bent fibers were performed with a free-running Er:YAG, Er:YSGG, Ho:YAG, and Nd:YAG laser. Due to an overlap of the refractive indices of CCl4 and fused silica between 500 nm and 1 micrometer, laser wavelengths in this spectral range, e.g., of the Nd:YAG and the HeNe laser, cannot be transmitted in this lightguide. If the core liquid is a mixture of CCl4 and tetrachloroethylene (C2Cl4) the lightguide is transparent from the near UV (380 nm) up to the NIR (3 micrometer) and consequently, it is suitable also for the Nd:YAG laser. With the CCl4-filled fused silica capillaries distal energy densities up to 30 J/cm2 were achieved and thus the ablation threshold of soft tissue is exceeded.

High power laser waveguide with a circulating liquid core for IR applications

Due to the high absorption coefficient of water in biological tissues at 3 micrometer, the Er:YAG (2.94 micrometer) and the Er:YSGG laser (2.79 micrometer) are very effective tools for medical applications. A widespread use of these lasers is prevented by the lack of a lightguide which satisfies the demands of medical practice, e.g. flexibility, low bending radius and unbreakability. If teflon (FEP) tubes filled with carbontetrachloride (CCl4) are used for high laser energies, absorbing and scattering particles in the liquid core are responsible for the decreasing transmission and damaging of the lightguide. A system was developed therefore to purify the liquid continuously even during laser irradiation. Two concentric FEP tubes are used to realize a circulation system. Different inner diameters of cladding tubes (1.6 mm and 0.5 mm) were tested with an Er:YAG and a Ho:YAG laser. A theoretical model of the different intrinsic attenuation losses in those lightguides was developed, which is in good agreement with the experimental results. Distal energy densities of 88 J/cm2 (Er:YAG) up to 220 J/cm2 (Ho:YAG) could be achieved with the small cladding tubes. These values were only limited by the output power of our laser devices. This waveguide setup has a better flexibility than those with a solid core. Due to the high numerical aperture (NA equals 0.54) the bending losses are negligible down to a bending radius of 15 mm. The high flexibility and transmittable power of the presented circulation system is particularly attractive for medical laser treatments.