Numerical and ex vivo studies of a bioprobe developed for laser-induced thermotherapy (LITT) in contact with liver tissue.

This work is based on the production of a bioprobe that is compatible with magnetic resonance imaging (MRI) for laser-induced thermotherapy (LITT) in liver cancer laser therapy. This probe is made of an alumina tube (3-mm diameter) in which an optical fibre is centred and fixed. A shooting window (20mm) is created using a mechanical rectifier. The device is then consolidated by the injection of a transparent and heat-resistant resin. Through numerical modelling, the thermal power damping of the laser source is evaluated as well as the propagation of the heat in the ex vivo liver tissue according to different heating scenarios. These analyses allow for an estimation of the irradiated volume. Ex vivo tests were performed on bovine liver to confirm the adequacy of the bioprobe for LITT and of the irradiated volumes predicted by the numerical model. There was a difference of 8% between the simulations and ex vivo experiments. The pulsed mode heating scenario was the most effective under the experimental conditions.

Aggregation in colloidal suspensions: evaluation of the role of hydrodynamic interactions by means of numerical simulations.

Numerical simulations constitute a precious tool for understanding the role of key parameters influencing the colloidal arrangement in suspensions, which is crucial for many applications. The present paper investigates numerically the role of hydrodynamic interactions on the aggregation processes in colloidal suspensions. Three simulation techniques are used: Brownian dynamics without hydrodynamic interactions, Brownian dynamics including some of the hydrodynamic interactions, using the Yamakawa-Rotne-Prager tensor, and stochastic rotation dynamics coupled with molecular dynamics. A system of monodisperse colloids strongly interacting through a generalized Lennard-Jones potential is studied for a colloid volume fraction ranging from 2.5 to 20%. Interestingly, effects of the hydrodynamic interactions are shown in the details of the aggregation processes. It is observed that the hydrodynamic interactions slow down the aggregation kinetics in the initial nucleation stage, while they speed up the next cluster coalescence stage. It is shown that the latter is due to an enhanced cluster diffusion in the simulations including hydrodynamic interactions. The higher the colloid volume fraction, the more pronounced the effects on the aggregation kinetics. It is also observed that hydrodynamic interactions slow down the reorganization kinetics. It turns out that the Brownian dynamics technique using the Yamakawa-Rotne-Prager tensor tends to overestimate the effects on cluster diffusion and cluster reorganization, even if it can be a method of choice for very dilute suspensions.

Tracer diffusion in colloidal suspensions under dilute and crowded conditions with hydrodynamic interactions.

We consider tracer diffusion in colloidal suspensions under solid loading conditions, where hydrodynamic interactions play an important role. To this end, we carry out computer simulations based on the hybrid stochastic rotation dynamics-molecular dynamics (SRD-MD) technique. Many details of the simulation method are discussed in detail. In particular, our choices for the SRD-MD parameters and for the different scales are adapted to simulating colloidal suspensions under realistic conditions. Our simulation data are compared with published theoretical, experimental and numerical results and compared to Brownian dynamics simulation data. We demonstrate that our SRD-MD simulations reproduce many features of the hydrodynamics in colloidal fluids under finite loading. In particular, finite-size effects and the diffusive behavior of colloids for a range of volume fractions of the suspension show that hydrodynamic interactions are correctly included within the SRD-MD technique.

Low loss microstructured chalcogenide fibers for large non linear effects at 1995 nm.

Microstructured optical fibers (MOFs) are traditionally prepared using the stack and draw technique. In order to avoid the interfaces problems observed in chalcogenide glasses, we have developed a new casting method to prepare the chalcogenide preform. This method allows to reach optical losses around 0.4 dB/m at 1.55 µm and less than 0.05 dB/m in the mid IR. Various As(38)Se(62) chalcogenide microstructured fibers have been prepared in order to combine large non linear index of these glasses with the mode control offered by MOF structures. Small core fibers have been drawn to enhance the non linearities. In one of these, three Stokes order have been generated by Raman scattering in a suspended core MOF pumped at 1995 nm.

Measurement of the elliptical birefringence of single-mode optical fibers.

The polarization properties of single-mode optical fibers are theoretically modeled with use of the Jones formalism. The fiber is described as an elliptical birefringent plate. The properties predicted by this model are discussed and lead to the development of a simple experimental method to extract the parameters that describe a real fiber. A magneto-optic method that measures the beat length of the fiber is also presented and gives a more complete description of the fiber. The validity of the model is then clearly established. Finally, the wavelength dependence of the parameters characteristic of the fiber is experimentally investigated.

All-fiber tunable ytterbium-doped double-clad fiber ring laser.

We have designed and realized a cw all-fiber tunable Yb-doped double-clad fiber laser in a unidirectional ring cavity. The side-pumping V-groove technique is used, yielding very good launching efficiency of the high-power pumping laser diode, together with a compact configuration. The fiber laser delivers as much as 800 mW of power and is tunable in the spectral range 1.04-1.10 microm . The laser linewidth is ~0.1 nm .

Comparison of the 585 nm pulse dye laser and the short pulsed CO2 laser in the treatment of striae distensae in skin types IV and VI.

BACKGROUND: Striae distensae, or stretch marks, are a very common cosmetic problem. The successful management of stretch marks has long been a source of frustration and curiosity for both the clinician and the researcher. Recent studies suggest lasers may have a role in their management. As yet, no study has reported on the effects of either of these lasers in the treatment of stretch marks on persons with skin types 4 to 6. OBJECTIVE: To assess the efficacy of short pulsed CO2 laser and pulsed dye laser for the treatment of stretch marks in skin types 4, 5, and 6. METHODS: Four patients were initially enrolled in the study. All were female with abdominal stretch marks, present for a range of 8 to 19 years. The patients had skin types ranging from 4 to 6. The test area consisted of a stretch mark long enough to be divided into three contiguous 2 cm sections, labeled A, B, and C. Section A served as the short pulsed CO2 test site, section B served as a control, while section C served as the 585 nm pulsed dye site. Patients were seen for evaluation after 1 week, then every 4 weeks for a total of 20 weeks. Patients were evaluated subjectively by the investigators, and the patients' own self-evaluation was reported as well. RESULTS: Following the 585 nm pulse dye laser, at 20 week follow-up patients with type 4 skin showed no improvement, while type 6 skin showed hyperpigmentation. The short pulsed CO2 test site showed persistent erythema in type 4 skin and marked hyperpigmentation in type 6 skin. CONCLUSIONS: For patients with types 4, 5, and 6 skin, laser treatment of striae should be avoided or used with great caution.

Combination treatment of melasma with pulsed CO2 laser followed by Q-switched alexandrite laser: a pilot study.

BACKGROUND: Melasma is very difficult to treat and often refractory to treatment with topical creams and pigmented-lesion lasers. OBJECTIVE: Pulsed CO2 laser alone is compared with the combination of pulsed CO2 laser followed by Q-switched alexandrite laser in the treatment of dermal-type melasma. This combination is proposed to be effective by first destroying the abnormal melanocytes with the pulsed CO2 laser and then selectively eliminating the dermal melanin with the alexandrite laser. METHODS: Four patients were randomly chosen for each treatment arm. There were multiple follow-up visits for examination by an objective blinded investigator. RESULTS: All patients in the combination laser group showed complete resolution, and two patients in the CO2 laser only group had peripheral hyperpigmentation in the long-term follow-up evaluation. CONCLUSION: These laser therapies are safe, as there was no scarring and no infection. The combination laser therapy was highly effective in removing the hyperpigmentation and all patients in this group showed complete resolution without any peripheral hyperpigmentation.

A high SPF sunscreen's effects on UVB-induced immunosuppression of DNCB contact hypersensitivity.

Several studies in mice of the protection afforded by sunscreens from UVB-induced suppression of contact hypersensitivity have yielded conflicting reports ranging from complete protection to no protection. Firstly, we sought to determine the effects of sunscreen and UVB on Langerhans cells; secondly we sought to determine whether the effect of preapplication of sunscreen with a sun protection factor of 30 could prevent local UVB-induced suppression of contact hypersensitivity to dinitrochlorobenzene in humans. In the first part of the study we compared a control group with a sunscreen plus UVB group and enumerated the number of Langerhans cells in each group. In the second part of the study we had four groups: a control group, a UVB group, a sunscreen group, and a sunscreen plus UVB group. Our results show that application of a sunscreen prior to UVB can prevent the decrease in number of Langerhans cells in an irradiated site. In the second part of the study, our results indicate that sunscreen, in itself, does not interfere with contact hypersensitivity, and that a high SPF sunscreen applied prior to UVB irradiation partially prevents suppression of contact hypersensitivity.

Optical feedback effects in Nd-doped fiber lasers with broadband spectra.

A detailed experimental and theoretical investigation of optical feedback effects in Nd-doped fiber lasers is presented. Of particular interest is an output-intensity reduction attributed to the broadband nature of the laser spectrum combined with a mode-selection mechanism of the external Fabry-Perot cavity. The theoretical approach, based on a simple multimode rate-equation model, provides an excellent description of the experimental results.

Output-coupling optimization of Nd-doped fiber lasers.

A simple theoretical modeling of the static properties of a fiber laser that includes distributed losses and inhomogeneous pumping is presented. Closed-form expressions for both the output and the backward (at the input mirror) intensities are obtained. The model is based on an extended formulation of the Rigrod's theory. It is shown that the laser responds differently depending on the length of the fiber. In particular, we show that for long (short) lasers optimal output power is achieved with low (high) output-coupler reflectivities. Experimental evidence of these results is obtained with Nd-doped fiber lasers with various lengths.

Channeled spectrum of a fiber laser.

We describe the polarization properties of a long Nd(3+)-doped optical fiber laser. An intracavity prism acts as a dichroic element and fixes the polarization of the laser beam essentially in its plane of incidence. The slight birefringence of the fiber makes a wavelength-dependent phase plate that modifies the state of the polarization. In the laser output spectrum, the azimuth for the elliptically polarized field can thus rotate by several times pi/2 if the fiber is long enough. This gives a channeled spectrum when observed through a polarizer.