Abstract:We developed a high power supercontinuum source at a center wavelength of 1.7 μm to demonstrate highly penetrative ultrahighresolution optical coherence tomography (UHR-OCT). A single-wall carbon nanotube dispersed in polyimide film was used as a transparent saturable absorber in the cavity configuration and a high-repetition-rate ultrashort-pulse fiber laser was realized. The developed SC source had an output power of 60 mW, a bandwidth of 242 nm full-width at half maximum, and a repetition rate of 110 MHz. The average power and repetition rate were approximately twice as large as those of our previous SC source [20]. Using the developed SC source, UHR-OCT imaging was demonstrated. A sensitivity of 105 dB and an axial resolution of 3.2 μm in biological tissue were achieved. We compared the UHR-OCT images of some biological tissue samples measured with the developed SC source, the previous one, and one operating in the 1.3 μm wavelength region. We confirmed that the developed SC source had improved sensitivity and penetration depth for low-water-absorption samples. tomographic imaging of human tissue at 1.55 μm and 1.81 μm using Er-and Tm-doped fiber sources," J.

The power-off phase of pulsed low-pressure plasmas (the so-called afterglow) in noble gases is a rich field for both fundamental and application oriented research. The physics of these plasmas is complex and involves various processes: Initially, electrons cool rapidly to temperatures close to the gas temperature by evaporative cooling. At sufficiently high plasma densities the low kinetic electron energy strongly enhances three-body recombination into Rydberg states. Finally, subsequent collisional-radiative decay leads to emission of radiation and populates the metastable states of the atoms. The various steps are investigated experimentally and are compared to analytical models. This allows us to follow all steps throughout in a single experiment involving diagnostics of electron density, metastable density, and emission. Excellent agreement with the models is achieved. The mechanisms included are: (i) for electrons, balance between evaporative cooling and Coulomb collisions with ions leading to thermalization; (ii) consistent combination of re-ionization and microfield reduction of the ionization energy in the recombination rate; (iii) adiabatic balance of recombination and collisional and radiative de-excitation; and (iv) radiative population and diffusional and pooling collisional loss of metastable levels. Although the experiment is carried out in argon, the underlying physics is generally applicable for the afterglow of high-density low-pressure discharges in atomic gases.

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The flow structure of ions in a diverging magnetic field has been experimentally studied in an electron cyclotron resonance plasma. The flow velocity field of ions has been measured with directional Langmuir probes calibrated with the laser induced fluorescence spectroscopy. For low ion-temperature plasmas, it is concluded that the ion acceleration due to the axial electric field is important compared with that of gas dynamic effect. It has also been found that the detachment of ion stream line from the magnetic field line takes place when the parameter ͉f ci L B / V i ͉ becomes order unity, where f ci , L B , and V i are the ion cyclotron frequency, the characteristic scale length of magnetic field inhomogeneity, and the ion flow velocity, respectively. In the detachment region, a radial electric field is generated in the plasma and the ions move straight with the E ϫ B rotation driven by the radial electric field.

We have developed a high-density H2 plasma source excited by helicon-wave discharge. By optimizing the antenna shape, the diameter of the discharge tube, and the magnetic field strength, a high electron density close to 1×1013 cm-3 was achieved at an rf power of 3 kW and a H2 gas pressure of 30 mTorr. The high-density H2 plasma source can be used as a compact divertor simulator in nuclear fusion research.

A novel method to determine electron densities in a low pressure (1 Pa) pulsed ICP discharge via absorption spectroscopy on argon metastables is presented. By use of an external cavity diode laser tuned at a vacuum wavelength of 696.73 nm the time behaviour of the absorption from metastable argon atoms in the Ar1s 5 state in the afterglow is recorded. An analytical model motivates the assumption of a homogeneous metastable density distribution under our experimental conditions. Further, a detailed model for the metastable density decay is developed, including the spatial electron density distribution and its temporal decay. This allows determination of the electron density from the measured data. Comparison between densities obtained by this technique and by Langmuir probe measurements shows excellent agreement. Furthermore, the electron density decay time, the ambipolar diffusion constant and the effective electron temperature in the afterglow are determined comparing data and model. The scaling of the electron decay time with density indicates recombination. The obtained low electron temperature reduces diffusion very much and can be expected to cause an increased recombination rate.

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The gas temperatures in high-density H2 plasmas excited by helicon-wave discharges were measured by absorption spectroscopy using a diode laser as the light source. The gas temperature was evaluated from the Doppler broadening of the absorption line profile at Hα. The gas temperature increased with rf power from 0.05 to 0.18 eV at a gas pressure of 50 mTorr. The temporal variations of the gas temperature after the initiation of discharge and the termination of the rf power were investigated. The power consumed by heating the gas was evaluated using the temperature and the time constant of the temporal variation.