Hollow waveguide delivery of ultrashort pulses for tissue ablation

Abstract

Tissue ablation with pulses in the femtosecond regime is generally more efficient and causes less collateral and thermal damage to the surrounding tissue compared to ablation with longer pulsewidths. A compact, flexible fiber delivery system that could transmit these pulses would be advantageous over free-space beam delivery, since it would allow ultrashort pulse tissue ablation in vivo. However, the extremely high intensities associated with ultrashort pulses have deleterious effects in conventional silica fibers such as nonlinearities and fiber damage. Hollow silica waveguides with a silver inner coating essentially guide the pulses in air, thereby avoiding many of these problems. The transmission characteristics of four hollow waveguides with bore diameters of 300, 500, 750 and 1000µm and lengths up to 1m were tested using pulses from a femtosecond regime Ti:Sapphire laser operating with input pulses <150fs duration and energy up to 700 microjoules at a repetition rate of 1kHz. Coupling was primarily to the HE 11 mode and straight and bending losses were measured. Beam profiles were also taken at the output of straight and bent waveguides. Autocorrelation measurements show minimal pulse broadening for straight waveguides and increasing pulsewidth with waveguide bend. Diffractive micro-optics were used to focus the output and ablation of fresh cadaveric porcine liver and heart tissues was accomplished using an x-y-z translational stage moving at 1 mm/second. Targeted tissues were then processed for light and electron microscopic examination. Light and scanning electron microscopy demonstrated near a-thermal ablation with depth correlating to energy application.

Pulse dispersion in hollow optical waveguides

“…In order to further validate the described theoretical model, these theoretical calculations were compared with experimental results obtained by Matsuura et al 19 and Nguyen et al 11 Table 1 shows the theoretical pulse dispersion ͑with and without roughness͒ compared with Matsuura et al's measurements ͑f = 300 mm, l = 1 m, ID= 1 mm, pulse width 196 fs, and = 775 nm͒. Table 2 shows the comparison of theoretical calculations ͑with and without roughness͒ with measurements by Nguyen et al ͑f = 100 mm, l = 1 m, ID= 0.5, 0.75, and 1 mm, pulse width 150 fs, and = 800 nm͒.…”

“…There are a few basic methods for measuring a short laser pulse width. One is to use an autocorrelator, 11 where the input beam is split before entering the waveguide and then compared with the output beam using an autocorrelator ͑Positive Light, Los Gatos, CA͒. This method was used by Nguyen et al 11 for studying femtosecond dispersion in waveguides.…”

“…One is to use an autocorrelator, 11 where the input beam is split before entering the waveguide and then compared with the output beam using an autocorrelator ͑Positive Light, Los Gatos, CA͒. This method was used by Nguyen et al 11 for studying femtosecond dispersion in waveguides. Another method was suggested by Amirmadhi et al 12 and used by Pratisto et al to measure pulse dispersion in waveguides transmitting picosecond pulses from a free-electron laser ͑FEL͒.…”

Ex vivo evaluation of femtosecond pulse laser incision of urinary tract tissue in a liquid environment: Implications for endoscopic treatment of benign ureteral strictures

“…In recent years, special hollow optic fibers have been developed to successfully deliver high‐power FS lasers. These fibers include hollow silica waveguides with a thin metallic inner coating 18–20 and hollow‐core photonic crystal fibers 21, 22. These hollow fibers are capable of delivering pulses with less than 150 fs duration and at energies up to 700 mJ at a repetition rate of 1 kHz 20.…”

“…These fibers include hollow silica waveguides with a thin metallic inner coating 18–20 and hollow‐core photonic crystal fibers 21, 22. These hollow fibers are capable of delivering pulses with less than 150 fs duration and at energies up to 700 mJ at a repetition rate of 1 kHz 20. Although, to date, there has been no report of application of these hollow fibers in endourology, recent advances in optic fiber technology enables the FS laser to be transmitted into the urinary tract and used in endourological surgery.…”