[Importance of wavelength for ultrashort laser pulses in healthy and pathological corneas]. / Bedeutung der Wellenlänge für ultrakurze Laserpulse in der normalen und pathologischen Hornhaut.

BACKGROUND: A study on the role of laser wavelength in keratoplasty assisted by ultrashort pulse lasers is presented. MATERIAL AND METHODS: This article gives a summary of the principal physical mechanisms contributing to the transparency of healthy corneas and presents transparency measurements as well as laboratory experiments on tissue with lasers at different wavelengths. RESULTS: The transparency of a healthy cornea is strongly related to its regular structure at micrometer and nanometer length scales. Many indications for keratoplasty are associated with a perturbation of this structure and therefore with a sometimes strongly reduced tissue transparency. This explains the often unsatisfactory results obtained when using ultrashort pulse lasers for the procedure. Theoretical considerations and laboratory experiments show that the light scattering processes responsible for the loss in laser beam quality depend strongly on wavelength and the use of wavelengths longer than those presently used allows these processes to be almost completely eliminated. The use of a spectral transparency window close to 1.65 µm is suggested. CONCLUSION: The use of laser wavelengths close to 1.65 µm represents an interesting alternative for the improvement of keratoplasty assisted by ultrashort pulse lasers.

All-solid-state neodymium-based single-frequency master-oscillator fiber power-amplifier system emitting 5.5 W of radiation at 1064 nm.

We demonstrate a master-oscillator fiber power-amplifier system consisting of a diode-pumped monolithic nonplanar ring oscillator as the master oscillator and a Nd:glass double-clad fiber as the power amplifier. The system emits up to 5.5 W of single-frequency radiation at a wavelength of 1064 nm with an M(2) value of ~1.1 . The optical emission spectrum is investigated with respect to the background of residual amplified spontaneous emission. Spectrally resolved amplitude-noise behavior is examined. Further power-scaling possibilities are discussed.

In vivo evaluation of the penetration of topically applied drugs into human skin by spectroscopic methods.

Spectroscopic techniques are reported on which allow to study in vivo the penetration behaviour of topically applied light-absorbing drugs into human skin. Remittance spectroscopy, a purely optical method, provides a good tool in both, skin adaptation by use of a remote viewing head coupled to the spectrometer via optical fibres, and adequate sensitivity for the detection of small amounts of the applied drugs. The measuring depth in the skin is determined by the wavelength-dependent optical penetration depth, which itself depends on light absorption and light scattering. In the UV-spectral region the optical penetration depth is of the order of the thickness of the stratum corneum (UV-A) or of only a superficial part of it (UV-B, UV-C). Fluorescence spectroscopy, another optical method, offers two kinds of drug detection, a direct one in case of self-fluorescent drugs or an indirect one being based on the light absorption of the drug, which may give rise to a screening of the self-fluorescence of the skin itself or of an applied marker. The measuring depth is comparable to that achieved with remittance spectroscopy. A third method is photothermal spectroscopy which is determined by thermal properties of the skin in addition to optical properties. Photothermal spectroscopy is unique in that it allows depth profiles of drug concentration to be measured non-invasively, as the photothermal measuring depth can be changed by varying the modulation frequency of the intensity-modulated incident light. Results of measurements demonstrating the potentials of these spectroscopic methods are presented.

Measurement of the thermal diffusivity of human epidermis by studying thermal wave propagation.

The thermal diffusivity of dry human epidermis was determined in vitro by studying thermal wave propagation in thin epidermal layers at frequencies between 10 and 200 Hz. Transmission measurements were performed on samples applied to a plane copper support at the underside of which thermal waves were generated by means of a square voltage controlled power transistor. Additionally, measurements were performed on epidermal layers with metal and air backing, in which thermal waves were generated by the absorption of intensity modulated light in a thin, superficially applied graphite layer (short and open circuit measurements). Thermal waves were detected by means of the laser beam deflection technique which allows the contactless measurement of the oscillatory surface temperature of a sample with respect to amplitude and phase. A critical discussion of methods shows that the thermal diffusivity is most reliably determined by transmission experiments. From experimental data obtained by this method a mean value alpha = (2.8 +/- 0.9) x 10(-4) cm2 s-1 was evaluated for the thermal diffusivity of dry epidermis.