Paul S. Golding

Investigation of a 791-nm pulsed-pumped 2.7-µm Er-doped ZBLAN fibre laser

A pulsed Er3+-doped ZBLAN fibre laser with 1.9 mJ of output energy at 2.7 µm in near transverse-single-mode operation is reported. A maximum slope efficiency of 13.5% with respect to launched pump energy at 791 nm was determined. The laser line shifts to a longer wavelength at the end of the output pulse because of reabsorption losses from the lower laser level at shorter wavelengths. The temporal behaviour of the laser output cannot be explained by the common rate-equation models. Possible quenching processes of the pump level are discussed.

PHOTOSENSITIZATION OF STAPHYLOCOCCUS AUREUS WITH MALACHITE GREEN ISOTHIOCYANATE : INACTIVATION EFFICIENCY AND SPECTROSCOPIC ANALYSIS

The potential of malachite green isothiocyanate as a photosensitizer for the inactivation of bacteria has been evaluated. Samples of Staphylococcus aureus are treated with the dye and exposed to continuous-wave red light from a filtered xenon lamp. Reduction in cell viability is seen to increase with radiation dose, whilst non-photosensitized samples are largely unaffected with exposure. The mechanism of photosensitization and the subsequent inactivation is addressed. UV-Vis and Fourier-transform infrared absorption spectrometry have been applied to this biological system, revealing the rapid hydrolysis of the isothiocyanate group of the dye and the transition to the colourless carbinol base when in solution. On binding to Staphylococcus aureus via a complexation mechanism, the dye is seen to be stabilized in its cationic form. Involvement of the excited triplet state of the photosensitizer is suggested and identification of reduced dye photoproducts is made following irradiation.

Efficient high power operation of a Tm-doped silica fiber laser pumped at 1.319 μm

Efficient cw and Q-switched operation of a Tm-doped silica fiber laser, pumped with the 1.319-μm output from a Nd:YAG laser, is described. An unsaturated continuous wave maximum output power of 310 mW and a slope efficiency of 23% was obtained for a near optimised cavity. A relatively low measured absorption cross section in conjunction with excited state absorption (ESA) of the pump light results in a minimum threshold pump power of 0.9 W. Q-switching by use of a mechanical chopper resulted in 16 W/280 ns pulses at a repetition rate of 20 kHz.

Development and application of fiber lasers for medical applications

Recent progress in fiber laser technology has provided lasers operating at mid-infrared wavelengths, at power levels and temporal regimes previously inaccessible to bulk crystal lasers. This paper reports on the development of new continuous wave fiber lasers; a Tm-silica fiber laser operating at 1.98 micrometers , an Er,Pr:ZBLAN fiber laser at 2.78 micrometers and a Yb:Er-silica fiber laser at 1.5 micrometers , and pulsed fiber laser sources; gain-switched and Q-switched Tm- silica systems, specifically targeting applications in medicine. The first studies on the interaction of high- power, continuous wave mid-infrared fiber laser light and soft biological tissues are presented, demonstrating the ability of 2 and 3 micrometers fiber lasers to remove soft tissue cleanly, and with minimum collateral damage.

Eradication of bacterial species via photosensitization

Photosensitization and inactivation efficacy of three bacterial species: Prevotella nigrescens, Staphylococcus aureus and Escherichia coli have been investigated. Samples of Staphylococcus aureus and Escherichia coli were treated with the triphenylmethane dye malachite green isothiocyanate and exposed to light from a variety of continuous and pulsed light sauces at a wavelength of approximately 630 nm. Inactivation of the Gram-positive species Staphylococcus aureus was found to increase with radiation dose, whilst Gram-negative Escherichia coli was resistant to such treatment. Samples of the pigmented species Prevotella nigrescens were found to be inactivated by exposure to light alone. The mechanism of photosensitization and inactivation of Staphylococcus aureus with malachite green isothiocyanate is addressed. The possible roles of the excited triplet state of the photosensitizer, the involvement of molecular oxygen, and the bacterial cell wall are discussed. Photosensitization may provide a way of eliminating naturally pigmented species responsible for a variety of infections, including oral diseases such as gingivitis and periodontitis.

Energy recycling versus lifetime quenching in erbium-doped 3-/spl mu/m fiber lasers

Summary form only given. In recent years, there have been enormous research efforts to improve the performance of lasers emitting at 3 /spl mu/m mainly because of their potential applications in laser surgery. Due to the high absorption of 3-/spl mu/m radiation in water, high-quality cutting or ablation is demonstrated in biological tissue using erbium-doped solid-state lasers. We have recently demonstrated a CW output power of 17 W at 2.7 /spl mu/m with a slope efficiency of 17% from an Er/sup 3+/,Pr/sup 3+/ codoped ZBLAN fiber. In this paper, we investigate whether energy recycling by energy-transfer upconversion (ETU) from the lower laser level with its inherent enhancement of the slope efficiency by a factor of 2 can operate this fiber laser more efficiently than lifetime quenching of the lower laser level by the Pr/sup 3+/ codopant.

Laser-tissue interaction with high-power fiber lasers operating in the mid-infrared wavelength region

We describe the results of preliminary studies on the interaction of continuous wave (cw), mid-infrared fiber laser light and soft biological tissues. An Er, Pr:ZBLAN fiber laser operating at a wavelength of 2.71 micrometer was used at 800 mW output power, a Tm-doped silica fiber laser at 1.98 micrometer provided up to 5 W output power, and a Yb:Er-doped silica fiber laser at 1.5 micrometer was used at 800 mW output power. Surface changes in tissue samples are described qualitatively and quantitatively, ablation velocity in tissue is measured, where observed, at the 800 mW power level, and sample sections are described with reference to histologically recognizable markers of thermal damage. The basic science described prepares the way toward ultimate clinical evaluations and applications of these new laser sources.

Determination of energy transfer parameters in Er3+-doped and Er3+,Pr3+-codoped ZBLAN glasses

A detailed characterization of energy level lifetimes and energy-transfer processes in Er3+-doped and Er3+, Pr3+-codoped ZBLAN bulk glasses is presented. Energy transfer upconversion parameters from the Er3+ 4I13/2 and 4I11/2 levels have been measured and are compared to energy transfer from Er3+ to a Pr3+ codopant. To assess the practicality of energy transfer a high power, diode pumped, 3 μm, Er3+, Pr3+- codoped ZBLAN fiber laser is presented.

Potential of malachite green isothlocyanate for photosensitization and inactivation of bacteria

Summary form only given. Malachite green isothiocyanate has been used to photosensitize and inactivate the bacteria Staphylococcus aureus upon exposure to light. Spectroscopic analysis following laser and non-laser illumination reveals details of dynamic and photochemical processes.

Laser inactivation of periodontal bacteria using photosensitizing dyes

We demonstrate the killing of the oral bacteria Prevotella nigrescens using a photosensitizer and light from a 10 Hz, frequency doubled, Q-switched Nd:YAG pumped dye laser, with modified oscillator to increase output power. This system produced light at wavelengths close to 620 nm, the absorption maximum of the photosensitizing agent, malachite green isothiocyanate, a wavelength that is not significantly absorbed by tissue. A bacterial reduction of 97.5 percent was achieved at an energy density of 0.67 J/cm2 and exposure times of 300 seconds.