In recent years, photonic crystal fibres (PCF) have attracted much interest because of their unique possibilities in mode selection. They allow high index as well as low index core geometries and light guidance is provided either by modified total internal reflection or by the photonic bandgap. An example for the high index guiding type of PCF is the fibre with air-cladding, the subject of our investigation. We report on experiments to produce this type of fibre. The silica preforms consist of a central rod surrounded by tubes, all packed into a larger tube. Fibres have been drawn at various furnace temperatures, drawing forces, and pressures. A suitable set of parameters has been determined that allows drawing airclad fibres. The parameters are a furnace temperature of 1750°C, a drawing force of 0.25 N, and a pressure of 10 kPa. An air-clad fibre with only one hexagonal ring of holes produced with this set of parameters already shows guiding of light. Light propagation in this fibre is simulated with BeamPROP 6.0 computer code. The experimental results are compared to the simulations.
The ideal adaptive optical mirror combines large aperture with high spatial and temporal resolution and a phase shift of at least 2π. Further, a simple low-cost solution is preferred. No adaptive system can perfectly fulfill all these requirements. We present a system that has the potential to reach this goal with the exception of high temporal resolution. But even with a moderate temporal resolution of one second such a system can find practical applications. For example as a laser resonator mirror that allows to modify the intensity distribution of the emission, or to correct slowly varying aberrations of optical systems. Two possible mechanisms can be used to change the optical path length of the adaptive mirror: thermal expansion of the mirror substrate or the thermally induced change of the refractive index (thermal dispersion) of a medium in front of the mirror. Both mechanisms have been shown to lead to promising results. In both cases heating was performed by irradiation of light in the active medium. The thermal dispersion based adaptive mirror is built with a thin layer of a liquid in front of a mirror. To allow a modification of the refractive index by irradiation with a diode laser at 808 nm, a suitable absorber is dissolved in the water. With chopped irradiation a resolution of 3.8 Hz at 30 % contrast is measured. This mirror has been used in a laser resonator to modify the output distribution of the laser. The thermal expansion based adaptive mirror is built with a thin layer of a silicon elastomer with a gold coated front side. We present a preparation method to produce thin films of Sylgard on sapphire. With an irradiated intensity of only 370 mW/cm<sup>2</sup> surface modulations of up to 350 nm are obtained. With a test pattern a resolution of 1.6 line-pairs per millimeter at 30 % contrast is measured. The temporal resolution is better than one second.
We report on the possibility to use lasers as a demining tool to dispose mines from a safe distance. Most anti personnel (AP) mines consist of 10 g to 500 g of an explosive, a fuse and a plastic case which makes them very difficult to detect. In 90% of all AP mines trinitrotoluene (TNT) or a combination of TNT and other explosives is used. The interaction of laser radiation with TNT and possible mine wrapping materials is investigated based on spectroscopy and practical considerations. With a CW Nd:YAG laser the desired burning of the explosive is achieved. The interaction is rather based on the absorption of the mine case than on the weak absorption of the explosive. A portable CW Nd:YAG laser is described and experiments with real AP mines are performed. We have investigated the behavior of four different representative blast AP mines under laser irradiation at Bofors test centre in Sweden. Disposal of all available mines from a safe distance up to 50 meters is achieved. Laser incident power was in the range from 20 W to 60 W. Due to partial burning of the explosive charge the resulting detonation of mines is considerably reduced.
The optical excitation of the I6 level in Yb:Ho:YLF is studied. Pumping with a wavelength of 953 urn populates the 2F572 upper level of yi3+fromwhere the energy is efficiently transferred to the '6upper laser level ofHo3. Pumping is performed with a Ti:sapphire laser. Based on the measured rise times and decay times of the fluorescence from the y3 2F level, the Ho3 5S2 and '6 levels the cross relaxation coefficients w yi3+ 2F512 + Ho3 '8 —3 y132F712 + Ho3 '6), w (Ho3I6+ Ho3 '6 Ho3 5S2 +Ho3 '8) and w 2F512 + Ho3I8 y3 2F712 + Ho3 5S2) as well as the ESA cross-section o6 (Ho3 '6 — H35S2) are estimated with the aid of a numerical analysis
In contrast to flashlamps the emission of single stripe laser diodes is highly directional and can be focused rather easily to small spots, which gives access to very high pump intensities. Numerical arrangements are possible for transferring the pump radiation to the solid state laser media. In this paper the most important concepts of diode laser systems for pumping solid state lasers are summarized and described. Thereby the aim is to find the most efficient and powerful method for endpumping a Yb<sup>3+</sup>-double clad fiber.
We report on the experimental study of the excitation mechanism of fluorescence in a 10000-ppm wt. Tm<sup>+3</sup>-doped ZBLAN fiber. UV (at 365 nm), visible (at 453, 480 and 650 nm) and near infrared (~800 nm) radiation was observed under excitation at 1.064 μm. The responsible up-conversion mechanisms are investigated based on experimental data. The results show that the <sup>3</sup>H<sub>4</sub> and the <sup>1</sup>D<sub>2</sub> levels are predominantly excited by ion-ion cross relaxation processes.
The effects of laser-induced structural modifications of the diamond surface are considered in relation to i) laser activation of diamond for direct electroless metallization ii) enhanced electron field emission from laser-ablated diamond films. It is shown that both the catalytic activity for electroless metal plating and reduced emission fields of the laser-ablated diamond surface correlate with the modifications in the valence-band structure, a specific feature of which is the appearance of the occupied electronic states in the gap. The laser ablation technique is applied for fabricating periodical microstructures at the diamond film surface, and results of the field emission testing of the produced 1D microstructures are presented.
The results of laser polishing of 350 micrometers thick free- standing diamond films are reported. The polishing was performed with a grazing beam of a copper vapor laser. It is shown that the laser polishing conditions and the resulting surface roughness are controlled by varying an angle of incidence of a scanning laser beam during polishing. The surface roughness of the as-grown films was reduced by an order of magnitude and a minimum roughness of R<SUB>a</SUB> equals 0.38 micrometers was achieved as a result of the two-step polishing. Optical transmission in the UV-visible spectral range of the diamond films polished under the optimized conditions was found to be close to the optical transmission of the mechanically polished diamond film. Properties of the laser-graphitized layer at the diamond surface were studied with optical spectroscopy techniques in the process of oxidative removal of the layer with increasing temperature of the oxidation in ambient air. The optical properties and oxidation stability of the laser-modified surface layer were found to change throughout its thickness from the surface to the diamond interface, depending on the laser polishing regime.
Ultra-fast processes of recombination of free charge carriers optically generated in natural and chemical vapor deposited (CVD) diamond materials; GaAs and Si, were investigated with a 1 nanosecond time resolution by applying a developed microwave-radiation-based technique. Time-dependent responses of reflection and transmission of 2 mm wavelength electromagnetic CW-radiation were recorded when the tested specimens were irradiated by single laser pulses of IR, visible and UV spectral range. A waveguide configuration was used to measure the exponential fall of the reflection/transmission signals corresponding to the recombination times of free carriers. Depending on the material, impurity contamination and laser wavelength, the measured signals were varied from a few to several hundred nanoseconds. The measured recombination times in the bulk of natural and CVD diamond specimens were found to be of 2-4 ns. The distinguished difference between the surface and bulk recombination times was clearly demonstrate in the case of GaAs and Si. Applicability and relevance of the applied technique to non-equilibrium carrier lifetimes measurements and the validity of the result obtained are discussed and confirmed on the base of the analytical analysis of the time- dependent microwave radiation reflection/transmission in a solid excited with high intense laser pulses.
An 'electrode-free' transient photoconductivity technique was applied to investigate excitation, drift and recombination of non-equilibrium free charge carriers in high quality synthetic polycrystalline diamond films, natural diamond crystals and low-conductive GaAs with a time resolution better than 200 ps. Picosecond laser pulses of UV, visible and Ir spectral range were applied for single- photon excitation of free charge carriers with initial concentrations of (10<SUP>12</SUP>-10<SUP>19</SUP>) cm<SUP>-3</SUP>. Dependences of amplitude and duration of photocurrent on laser intensity/carrier density were measured. Lifetimes, drift mobilities and carrier photoexcitation cross sections as a function of electron concentration were estimated. Computer calculations of conduction and displacement currents, induced space charge and electric field spatial distribution have been performed for the real experimental conditions. Based on the obtained results, high voltage diamond-based switches triggered by ultra-short laser pulses have been designed. Special attention was paid to metal- dielectric interface investigation and ohmic contacts formation. The developed diamond-base module permitted to switch electric fields as high as 100 kV/cm within a time interval less than 200 ps. The amplitude of photocurrent reached 100 A and the electrical resistance reduce by a factor of 10<SUP>10</SUP>.
In a first part we report on a Ti:sapphire pumped Er<SUP>3+</SUP>:YLF laser. The dopant concentration and the polarization of the pump beam is optimized. The highest slope efficiency is obtained with the polarization of the pump beam parallel to the c-axis and a dopant concentration of 20 at. %Erbium. Slope efficiencies up to 50% are achieved. This value clearly exceeds the Stokes limit of 35%. Further, we report on an Er<SUP>3+</SUP>:BYF laser pumped with two polarization-coupled diode lasers. The dependence of slope efficiency on the reflectance of the resonator mirrors, on the focus of the pump light, and on the resonator length is investigated. The best slope efficiency of (tau) equals 24% is obtained for a nearly hemiconcentric resonator with a reflectance of R<SUB>in</SUB>(DOT)R<SUB>out</SUB> equals 97.6% and a pump beam focused with a lens of f equals 20 mm. To obtain a higher pump power, we investigate the coupling of sixteen diode lasers, each one operated to emit a maximum power of 800 mw. The diode laser emission is collimated and then focused using aspherical lenses. The point spread function of this system is investigated. Laser action in a longitudinally pumped Er<SUP>3+</SUP>:YLF is already achieved using only one single diode laser.
In this paper we show that 2 micrometers Tm<SUP>3+</SUP>:Ho<SUP>3+</SUP> silica fiber lasers have promising properties for applications in laser medicine and in lidar applications. 2 micrometers lasers are interesting for measurements in the atmosphere since in this range there are several absorption lines of water vapor. 2 micrometers lasers can be used for lidar applications if their wavelength can be controlled. In contrast to crystal lasers glass fiber lasers can be tuned continuously over a range of about 80 nm. The excitation can be performed in a wide range of pump wavelengths and with different geometrical arrangements. With Ti:sapphire pumping a laser output of 71 mW with a slope efficiency of 17.5 percent has been reached. The possibility of side-pumping together with the development of fibers with lower losses will allow to reach the power levels that are necessary for the mentioned applications.
Bright light can temporarily reduce the visual capacity of the human eye. Especially in traffic or in dealing with lasers disability glare can considerably reduce the visual faculty and lead to accidents. Even after the glare light has disappeared out of the field of vision, the visual capacity can still remain reduced for seconds or even minutes. It is therefore indispensable to investigate the recovery time of the human eye after glaring and to determine limiting exposures in order to avoid accidents or possible eye damage. We report on measurements of the recovery time in the range from 475 nm to 650 nm performed with thermal sources. Experiments are reported on the influence of varying intensities of test and glare light applied in the line of sight or extrafoveally. These studies allow us to give an empirical formula for the recovery time. In the present contribution we also report on measurements as a function of the duration of glaring. The range from 20 microsecond(s) to 1 s has been covered with an Argon laser at 488 nm as glare source. The resulting recovery time in different regions of this very large time scale will be discussed.
One of the most promising applications of the laser is its use as an optical knife for surgery, especially for microsurgery. Efficient cutting with a minimum of undesired damage can be achieved if the laser energy is absorbed within a very thin layer and if a pulse duration is chosen that prevents diffusion of the heat by thermal conduction. Absorption of the laser energy can occur either in specific absorbers such as blood or melanin or unspecifically in water. According to the absorption properties of water, a good laser for cutting should emit radiation with about 3 micrometer wavelength.
The limit of the slope efficiency with respect to absorbed pump power is investigated in erbium-doped 3-micrometer crystal lasers. It depends on the major population mechanisms of the system. Fluoride hosts are favorable due to the long lifetime of the upper laser level. The calculated slope efficiency in Er:YLF approaches 56% when pumping at 970 nm. This value clearly exceeds the Stokes limit of 35% because of energy recycling via interionic upconversion. A laser slope efficiency of 40% in Er(15%):YLF is experimentally obtained under Ti:sapphire pumping.
The saturation of the 2.71 micrometer laser in a 5000 ppm erbium doped ZBLAN single-mode fiber pumped at 791 nm was spectroscopically analyzed. The bleaching of the ground state, the absorption coefficient at the pump wavelength and the fluorescence intensities over a wide wavelength range have been measured simultaneously during laser emission. The saturation of the 2.71 micrometer emission is explained by co-lasing at 850 nm.
The erbium 2.7-micrometer fluorozirconate fiber laser is investigated in computer simulations and experiments. An output power of 158 mW at 2.7 micrometer is achieved from an erbium- doped single-mode fiber cascade laser. The output power is limited only by the pump power available from the Ti:sapphire laser. The slope efficiency of 23.3% in the cascade-lasing regime is close to the calculated limit of 27%. It is shown that this cascade regime represents the most efficient system for the operation of an erbium 2.7-micrometer fluorozirconate fiber laser.
The output power of cw monomode fiber lasers, commonly operated far below 1 W, can be enhanced using a double-clad fiber structure. Double-clad fibers can be excited with powerful multimode laserdiode arrays. An output power of several watts has already been demonstrated in a Nd<sup>3+</sup>-doped double-clad fiber. A further enhancement of the laser power can be achieved by side pumping the fiber. An upper limit of the order of 100 W given by the damage threshold of the fiber can be estimated for a multi-side-pumped system. As for the case of crystal lasers, even more output power in the fundamental mode can be obtained by coherent combination of several lasers. This procedure can be performed by active phase adjustment between identical lasers: The adjustment of the phase can be realized rather elegantly in fibers by squeezing or stretching the fibers with a piezoelectric element. Even if we are still far from the practical realization of a 100-W monomode fiber laser, all the different steps leading to high output power have been demonstrated to work.
In meteorological and climatological fields, the scientific community will increasingly need global measurements of key atmospheric parameters with high spatial resolution (horizontal as well as vertical): the spaceborne lidars are the most suitable instruments for those missions. While backscatter lidar (ATLID, currently studied as ESA) is presently first candidate for space deployment, the next generation of lidars will be DIAL and Doppler wind lidars, presenting a higher level of complexity, mainly due to the large power and complex signal processing required. The present considered wind lidars are based on CO<SUB>2</SUB> lasers, whose space compliance still needs confirmation, while alexandrite lasers are considered for water vapor and temperature measurements, but they need flashlamp pumping which poses a lot of several thermal constraints and lifetime problems: on the other side, the recent developments achieved in solid-state technology allow to envisage diode pumping as most promising possibility for both previous applications.