Search

A procedure of inspection of femtosecond laser induced mass transfer processes has been suggested that is based on pump-probe Doppler tomography. In the experiments we used mode-locked Ti:sapphire laser with output power up to 150 mW.. In our experiments focused laser radiation induced on the surface of the target mass transfer processes (melting and convection). The Doppler shifted backscattered radiation was registered by heterodyne scheme based on Michelson interferometer. We have obtained information about depth and distribution of convection flows within melted paraffin bath with high spatial resolution (Dz~15mm). We propose optical heterodyning technique based on OCT for obtaining temporal distribution of backscattered photons from media irradiated by Cr:forsterite femtosecond laser. The temporal resolution is about 60 fs. The technique was used for sheet paper porosity diagnostics.© (2004) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Papermaking process consists in a succession of unit operations for having main objective the expression of water out of the wet paper pad. The three main stages are successively, the forming section, the press section and finally the drying section. Pressing is not on the scope of this paper but the influence of formation and drying on anisotrophy of paper properties is analyzed. More specifically, optical measurements are described in order the anisotropic structure and physical properties of paper to be analyzed.

On-line measurement of material properties is a goal of many manufactures for production improvement and quality control. In the paper industry, the elastic stiffness of the paper web needs to be accurately measured and controlled online. Currently, contact ultrasonic methods are employed to measure the elastic constants of paper offline. Piezoelectric transducers are placed in contact with the paper surface to generate and detect plate wave modes, known as Lamb wave modes, in the paper in both the machine and cross directions. Laser ultrasonics offers a new approach to making elastic wave measurements that does not require contacting the paper web. A pulsed laser generates ultrasound in the paper by delivering a high intensity short impulse to the paper through thermoelastic expansion. An optical interferometer is used to demodulate the small phase modulation that the elastic wave produces on a continuous probe laser beam. Difficulty in applying this approach to online measurements of rapidly moving pape...

Stiffness properties of paper materials can readily be characterized in the laboratory using conventional ultrasonic techniques. For on-line inspection on a paper machine, due to the high translation velocity and the somewhat fragile nature of the moving paper web, contact ultrasonic techniques using piezoelectric transducers are of limited use. To overcome this limitation, non-contact laser- based ultrasonic techniques can be used. Due to the rough surface of the paper, the reflected light is composed of many speckles. For efficient detection, the receiver must be able to process as many speckles as possible. Adaptive receivers using the photorefractive or photo-emf effects are characterized by a large etendue, and thus, are well suited for detection on paper and paperboard. Moreover, the translation velocity of the moving web implies that the detection system must adapt extremely quickly to the changing speckle pattern. In this work, a photo-emf receiver was used to detect Lamb waves excited using a pulsed Nd:YAG laser in moving paper. Experiments were performed using a variable-speed web simulator at speeds much higher than 1 m.s -1 . Results corresponding to various translation speeds are shown, demonstrating the feasibility of laser- based ultrasound for on-machine inspection of paper and paperboard during production.

Laser-Based Ultrasound (LBU) systems are now entering their maturity years by penetrating the factory in both the areas of non-destructive testing and process control. A LBU system can be used for the on-line characterization of a paper web in a paper mill. Compared to contact techniques, LBU is able to generate and detect on a paper web both symmetric and antisymmetric waves with a non-contact tool which is the spot of the generation laser. This provides all at once a rich amount of data about the stiffness properties of the sheet. To demonstrate this concept we made some experiments on static paper first, our ultimate goal being to monitor the stiffness properties of a paper web, on-line, at industrial speeds. A photorefractive interferometer using the two-wave mixing method with a continuous electric field applied to a BSO crystal has been built for ultrasound detection. Results are presented on different paper grades, using a Nd:YAG laser for generation. Both A0 (anti- symmetric) and S0 (symmetric) modes of Lamb waves have been detected with acceptable signal to noise ratio in single laser shot. The dispersive nature of A0 wave is clearly visible as well as the higher frequency content of S0 wave.© (1999) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The physical principles underlying a three-dimensional (3D) laser microstructuring technique are outlined, its applications for the fabrication of 3D (nano)micro-structured materials are presented. The direct laser writing and holographic recording in SU-8 photoresist are described. The limits of the lateral and axial light localization of a Gaussian pulse/beam at the focus are derived for the multi-photon absorption taking into account the threshold of photomodification. Prospective holographic patterns formed by interference of circularly and linearly polarized beams are discussed.

Numerous equally-spaced emission lines can be generated by stimulated Raman scattering and subsequent four-wave Raman mixing. The spectral domain extends from the near-infrared to the deep-ultraviolet, and then an ultimately-short optical pulse can be generated by phase locking in the process of four-wave Raman mixing. In fact, an intense 17-fs optical pulse has been generated using this technique. A comb of 1.6-fs optical pulses, i.e., the shortest optical pulses, has also been generated by other researchers. On the other hand, it is possible to generate highly-repetitive pulses using a continuous wave (CW) laser as a pump source and molecular hydrogen as a Raman medium in a high-finesse cavity. Coherent superposition of the two-color beam comprising of the fundamental beam and the Stokes beam provides a sinusoidal wave modulated at 17 THz. It is also possible to generate more than three emission lines simultaneously, which should generate an impulsive wave. It is noted that such Raman emission can be generated in a hollow-core fiber, making the device extremely simple. Such a device may have a potential for use in basic science and technology, e.g., the generation of a three-primary-color laser for display.

Limiting factors for short-wavelength QCL designs are discussed, and a model is described to predict the short-wavelength limit for strain-balanced QCL structures. High performance is predicted at wavelengths as short as 3.0mm based on a conduction band offset of 0.9 eV in the GaInAs/AlInAs materials. Recent work is presented on the growth of strained materials using gas-source molecular beam epitaxy to investigate the model predictions. Advanced material characterization, including HR-STEM, high-resolution x-ray diffraction, photoluminescence, atomic force microscopy, and wafer-scale uniformity and repeatability are demonstrated for strain-balanced QCL structures. Laser testing results are presented for QCLs operating at ~4.8mm, and lastly, predictions for further performance improvement at short wavelengths are discussed.

We analyze the impact of slow intraband relaxation and strong carrier localization on the characteristics of quantum dot (QD) lasers. Relatively long intraband relaxation times and population filling of the QD ground state lead to carrier pile-up on excited states, reducing the laser efficiency and maximum output power. Strong carrier localization in the QDs and consequently large thermal hopping time within the QD ensemble results in the absence of quasi-thermal equilibrium under lasing conditions, as evidenced by stimulated and spontaneous emission spectra. The impact of these specific physical characteristics of QD active regions on the laser high-frequency modulation properties is analyzed, particularly with regards to the differential gain, the gain compression and the linewidth enhancement factors.

We have performed theoretical and experimental researches of spatio-temporal structure of femtosecond laser pulses, scattered in suspension of polysterol beads in water. The spatio-temporal structure of scattered pulses was measured by means of nonlinear optical gating at noncollinear second harmonic generation with subpicosecond temporal resolutions. Testing of phototoxic action of high-intensity femtosecond pulses on biological tissues has been performed with serum of rat's blood. Samples were irradiated with trains of 50-fs Ti:sapphire laser pulses with intensity up to a threshold of optical breakdown. The phototoxic stress of blood serum was defined with a several techniques including biochemoluminescence, sonoluminescence, and morphology analysis of dehydrated droplet, total concentration of proteins, viscosity and proteins oxidation measurements. We have determined regimes of blood serum irradiation with JR femtosecond laser pulses, corresponding to nonivasive influence, phototoxic stress and optical damage.

The anisotropic nature of the mechanical loss factor, tan u, ofex vivo porcine skin was evaluated using a novel acousto-optical method as well as by more tranditional dynamic mechanical testing. The acousto-optical method called acousto-optical elastography (AOE) relies upon launching low frequency (0.5Hz) acoustic stress waves into the skin with an acoustic speaker and quantifying the displacement ofthe skin surface by tracking a backscattered laser speckle pattern with a linear array CCD camera. Two different optical wavelengths were used to monitor the surface acoustic waves, 543 nm and 633 nm. The 633 nm light has a penetration depth on the order of 3 times that of the 543 nm light. The value of tan δ was found to be dependent upon the orientation in which the skin samples were tested. For both the AOE employing 543nm light and the dynamic mechanical testing approaches, tan δ was greatest in the dorso-ventral direction and smallest in the cranial caudal direction. The actual values of tan δ were significantly greater when they were determined using traditional mechanical testing than with the AOE method. This was attributed to the greater strain range over which tan δ was calculated when using dynamic mechanical testing. The AOE method using the 633 nm light provided data not only on the skin, but the underlying tissue as well, masking the anisotropic behavior of the porcine skin. The results of this study indicate that AOE may be a suitable testing method for quantifying aspects of mechanical behavior of tissues in vivo.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The backscattered intensity from low-intensity laser illumination of the skin in the area of vascular plexus is investigated in vivo. The exposure of blood to low power laser light in the absorption range of haemoglobin leads to increasing intensity of the backscattered light. Theoretical evaluation using an existing optical model of erythrocyte aggregation has suggests that the fragmentation of erythrocyte aggregates is the most probable mechanism leading to the enhanced backscattering.

Novel types of thin-film microoptical components have been found very advantageous for beam shaping of high-power and ultrashort-pulse lasers. Measuring techniques, nonlinear optics, materials processing, and further advanced photonic applications, will benefit from specific advantages. Compared to conventional microoptics, low dispersion and absorption as well as added degrees of freedom in structure and functionality are accessible. Single or multilayer designs, spherical and non-spherical profiles, extremely small angles, and flexible substrates enable key components for the tailoring of lasers in spatial, temporal, and spectral domain at extreme parameters. By vacuum deposition and selective etching transfer, a cost-effective fabrication of single or array-shaped refractive, reflective, or hybrid components is possible. During the last decade significant progress in this field could be achieved. Including very recent applications for spatio-temporal shaping and characterization of complex and non-stationary laser fields, the state of the art is presented here. Particular emphasis is put on the generation of localized few-cycle wavepackets from Ti:sapphire laser beams by the aid of broadband microaxicons. Special microoptics are capable of transforming vacuum ultraviolet radiation. Wavefronts of strongly divergent sources can be analyzed by advanced Shack-Hartmann sensors based on microaxicon-arrays. Single-maximum nondiffractive beams are generated by different approaches for self-apodizing systems. Prospects for future developments, like robust multichannel information processing with arrays of self-reconstructing X-pulses, are discussed.

We present work on design of monolithic mode-locked semiconductor lasers with focus on the gain medium. The use of highly inverted quantum wells in a low-loss waveguide enables both low quantum noise, low-chirped pulses and a large stability region. Broadband noise measurements are performed and used to confirm the design principles.

Device characteristics of photonic crystal lasers formed in InGaAsP membranes bonded to a sapphire substrate are discussed. Also discussed are waveguide loss mechanisms in type-A and type-B photonic crystal waveguides and the transmission properties of photonic crystal waveguide bends.

The ideal source of radiation for extreme ultraviolet lithography will produce intense light in a 2% bandwidth centred at 13.5 nm, while the debris and out-of-band radiation produced will be limited to prevent adverse effects to the multilayer optics in the lithography system. In this study ways to optimise plasma sources containing tin are presented. The optimum power density for a tin slab target, with a fixed spotsize, is determined, while the effects of power density on ceramic targets, where tin is present only as a few percent in a target of mainly low Z elements, is also investigated. It has been found that the in-band radiation is increased when the concentration is 5-6%, while the out-of-band radiation is dramatically reduced, due the the low average Z of the target constituents, with conversion effciencies of over 2.5% recorded. The power density needed to optimise the emission from ceramic targets was found to be greater than that required for the pure tin case. In addition, if the target is first irradiated with a pre-pulse, the conversion effciency is seen to increase.

The wavelength spectra of ridge waveguide Fabry Perot lasers can be modified by perturbing the effective refractive index of the guided mode along very small sections of the laser cavity. One way of locally perturbing the effective index of the lasing mode is by etching features into the ridge waveguide such that each feature has a small overlap with the transverse field profile of the unperturbed mode, consequently most of the light in the laser cavity is unaffected by these perturbations. A proportion of the propagating light is however reflected at the boundaries between the perturbed and the unperturbed sections. Suitable positioning of these interfaces allows the mirror loss spectrum of a Fabry Perot laser to be manipulated. In order to achieve single longitudinal mode emission, the mirror loss of a specified mode must be reduced below that of the other cavity modes. Here we briefly review one procedure for calculating the mirror loss spectra of devices containing such features. We then go on to describe a method for synthesising onedimensional slot patterns. This technique allows the lasers emission wavelength to be specified to a high degree of accuracy.

In silicon, direct electronic transitions leading to light emission have a low probability of occurrence due to the momentum mismatch between upper and lower electronic levels. Until recently, this had prevented the realization of the long waited silicon optical amplifier and laser. Raman scattering, which describes the interactions of light with vibrational levels, can be used as a way to bypass the indirect band structure of silicon and to obtain amplification and lasing. The Raman approach is very appealing because device can be made in pure silicon with a spectrum that is widely tuneable though the pump laser wavelength. While a new research topic, amplifiers with pulsed gain of 20dB and CW gain of 3 dB have already been demonstrated. Using parametric Raman coupling, wavelength conversion from 1550nm to 1300nm has been achieved. A distinguishing feature of silicon Raman devices, compared to fiber devices, is the electronic modulation capability. By integrating a p-n junction with the silicon gain medium, electrically switched lasers and amplifiers have already been demonstrated. These have many exciting applications. For example, the laser can be directly modulated to transmit data, and can be part of a silicon optoelectronic integrated circuit. At the same time, electrically switched amplifiers represent loss-less optical modulators.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Recent progress toward implementing high-density, optical-digital building blocks necessary to accomplish efficient, end-to-end optical interconnect architecture on low cost FR-4 boards has been demonstrated. The optical interconnect system consists of fabricating an optical buffer layer separating board metallurgy from the optical lightwave circuit layer, and implementing optical links between embedded lasers and detectors. We will show an example of 1310 nm light from an edge emitting distributed-feedback or Fabry-Perot laser operating at 10 Gb/s being guided to the photodetector by a polymer waveguide. Both lasers and detector are embedded in the waveguide and all construction is built on a low-cost FR-4 board with 3 levels of metallurgy.

An approach that modifies an analog fiber optic link with a recirculating optical loop as a means to realize a high-speed, high-resolution Analog-to-Digital Converted (ADC) is presented. The loops stores a time-limited microwave signal so that it may be digitized by using a slower, conventional electronic ADC. Detailed analytical analysis of the dynamic range and noise figure shows that under appropriate conditions the microwave signal degradation is sufficiently small so as to allow the digitization of a multi-gigahertz signal with a resolution greater than 10 effective bits. Experimental data is presented which shows that a periodic extension of the input signal can be sustained for well over one hundred periods that in turn suggests an electronic ADC speed-up factor of over 100. The data also shows that polarization effects must be carefully managed to inhibit the loops tendency to lase even though the loop itself contains no frequency-selective elements.

A direct comparison between continuous and discrete forms of analysis of control and stability is investigated theoretically and numerically. We demonstrate that the continuous method provides a more energy-efficient means of controlling the switching of a periodically-driven class-B laser between its stable and unstable pulsing regimes. We provide insight into this result using the close correspondence that exists between the problems of energy-optimal control and the stability of a steady state.

Detecting quantum fluctuations of a mechanical resonator is a long-standing goal of experimental physics. Recent progress has been focussed on high frequency (MHz to GHz) resonators inserted in a milli-Kelvin environment, with motion detection performed by single electron transistor means. Here we propose a novel experimental approach based on high-sensitivity optical monitoring of the displacement of the resonator and feedback cooling. The experimental setup is based on a micro-mechanical resonator inserted in a high-finesse optical cavity and monitored by a highly-stabilized laser system. Available optical technologies provide an unequalled sensitivity, in the 10-20m/√Hz range. The displacement signal is used in real-time to perform a feedback cooling in order to set the resonator's fundamental mode of vibration in its quantum ground state. With the resonator at cryogenic temperature, the feedback cooling mechanism should allow to reach an effective temperature in the micro-Kelvin range.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We model a new THz laser device structure based on a semiconductor quantum dot (QD) gain medium, where the lasing occurs through discrete conduction states. An ensemble of QDs is selectively placed in a high quality cavity, called a microdisk, which is resonant with an intersublevel QD transition. We simulate the rate equations governing lasing and discuss a variety of processes affecting lasing including nonradiative recombination and the ground state decay rate.

We demonstrate temperature-insensitive eye-opening under 10-Gb/s direct modulation of 1.3-μm p-doped quantum-dot lasers without current adjustments, which show 6.5-dB extinction ratio between 20 and 70°C. The active region consisting of ten quantum-dot layers with p-type doping enabled this highly temperature-stable dynamic performance, much superior to conventional 1.3-μm quantum-well lasers. This result opens a way to uncooled 1.3-μm quantum-dot lasers without current adjustments.

There is an increasing need for single-spatial-mode, edge-emitting semiconductor lasers with reliable cw output power of around 1 W for applications such as pumps for rare-earth-doped fiber amplifiers and free-space communications. The design of respective devices is still a challenging task for experimenters, and software can assist very much in doing analyses of potentially perspective designs. We have developed a 3D numerical code supplied with a user-friendly interface for active diode-laser structures, taking into account light diffraction and carrier diffusion. The Watt-Ampere characteristics are calculated by changing the drive current density in the equation for the carrier-number density. To evaluate a single-mode stability limit, a procedure is developed to calculate 3-5 higher order optical modes on a 'frozen background': gain, carrier-induced index variation, as produced by the operated mode at a fixed drive level. Modal gains of these modes are compared to the calculated threshold gains for each mode. Because of non-uniform gain saturation by the operated mode, modal gains for higher-order modes increase with drive current due to beneficial overlap of their fields with the gain. When one of the higher-order modes approaches its threshold, this puts an upper limit for stable single-mode operation. A graphical interface allows for viewing near- and far-field patterns of any mode in the form of 3D surfaces or contour-plots. Scanning of profiles of mode intensity in an arbitrary cross section in the output plane and in far-field zone is available, too. Results of analyses of a number of published designs are reported.

The Tilted Cavity (TC) concept has been proposed to combine advantages of edge- and surface-emitting lasers (detectors, amplifiers, switches, etc.). Tilted Cavity Lasers (TCL) enable wavelength-stabilized high-power edge and surface emitters (TCSEL) in low-cost single-epitaxial step design. The concept covers numerous applications including mode-locked TCL for light speed control, dispersion and linewidth engineering, GaN-based light-emitters, electrooptic wavelength tunable devices, and other applications. Presently, wavelength stabilized TC operation is realized between -200°C and 70°C in broad TCL diodes with cleaved facets based on quantum dots (QDs). The spectral width is below 0.6 nm in broad area 100 μm-wide-stipe devices. The far fields are: 4° (lateral) and 42° (vertical). Wavelength-stabilized 1.16 μm and 1.27 μm edge-emitting QD TCL lasers are demonstrated. Quantum well TCL demonstrate high-temperature operation up to 240°C with a low threshold, high temperature stability and improved wavelength stability. The tilted cavity approach can also be applied in wavelength-optimized photodetectors, switches, semiconductor optical amplifiers, including multi-channel devices, in optical fibers, in photodetectors, in light-emitting diodes and in many other applications. Moreover, microelectronic devices based on similar tilted angle resonance phenomena in quantum wells and superlattices can be realized in electron- or hole-wavefunction-engineered structures, thus, merging the fields of nanophotonics and nanoelectronics. The tilted cavity concept can be further complimented by lateral patterning and (or) processing of three-dimensional photonic crystal structures further extending horizons of modern optoelectronics.

By introducing triangular holes into oxide confined 850nm monolithic vertical-cavity surface-emitting lasers (VCSELs), single-transverse-mode operation has been obtained for large oxide apertures of 14-15 microns. The two-dimensional triangular holes etched on the device surface were aligned circumferentially along the aperture perimeter, with their tips surrounding the device center. When the holes had a relatively large lateral penetration into the oxide aperture, the holey VCSEL lased with a single spot near field pattern with a high side-mode suppression ratio (SMSR) of 45-50dB, and an output power of 2mW. In this case, it is assumed that the triangular holes are acting as a highly mode selective loss mechanism. On the other hand, when the penetration of the holes was relatively small, an SMSR of 40dB was obtained from a large area "floral" near field pattern, with a record high single-mode output power of 7mW. The lasing spectrum and far field intensity profile of this "floral" type emission indicates that it is a somewhat deformed fundamental mode that is extending over the whole device, and oscillating in-phase. The ability of triangular holes to suppress high order modes in large area oxide confined VCSELs should be effective for systems with wavelengths other than 850nm as well.

Rare earth doped ternary lead salts are being studied for use as mid-IR laser materials. We summarize progress at the Naval Research Labs on the production and evaluation of this important class of solid-state laser.

For years the international laser community has been discussing what the "best" concept for high power solid state lasers might be. With the advent of high power diode lasers of lifetimes in excess of 10,000 hours at long term operational power levels around 50W per diode bar, diode pumped concepts moved into their rightful role as next generation sources. The main reason for this trend is the combination of laser efficiencies in the range of 15 to 25% at powers of several kW with the well known and increasingly applied advantage of fiber coupling. A particularly promising perspective similar to electrical networks is provided by concepts of laser power networks distributing the optical energy on demand in the production environment, especially for large scale production.

We have demonstrated efficient operation of the eyesafe laser transition ( 4 I 13/2 -> 4 I 15/2 ) in Er:YAG by resonantly pumping with 1470nm diodes. Quasi-cw powers in excess of 30W have been achieved at 10% duty cycle with 47% slope efficiency, 26% conversion efficiency, and beam quality of M 2 =1.4 x 2.2. In energy storage mode, we have generated near-diffraction-limited 41mJ / 58ns pulses, more than 700kW of peak power, at 10Hz. Storage lifetimes in the range of 5 to 7msec have been measured, and pulses as short as 25ns have been obtained at reduced energy. We believe this to be the first-ever demonstration of a resonantly diode pumped (bulk) erbium laser.

Laser-pumped cesium magnetometers allow highly sensitive small field magnetometry at room temperature. We report on applications of that technique in biomagnetic diagnostics and in a new experiment searching for a neutron electric dipole moment (nEDM) experiment. In the biomagnetic application the magnetiv field from the beating human heart is detected using a gradiometer which reaches an intrinsic sensitvity of 80 fT/Hz1/2 with a spatial resolution of 28 mm. The device can record time-resolved magnetic field maps above the human body surface and its performance is comparable to devices based on Hi-Tc-SQUID (superconducting quantum interference device) technology. In the nEDM experiment laser-pumped cesium magnetometers will be used to measure and stabilize a DC magnetic field of 2μT and to generate the oscillating filed for the neutron Ramsey resonance. Those devices reach an intrinsic sensitivity of about 14 fT/Hz1/2 with a measurement bandwidth of 1kHz. The general principle of operation and specific results are presented.© (2005) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Advances in high power fibre lasers and amplifiers and in novel non-linear fibres that can be readily integrated with such pumps have led to a family of high power super-continuum sources that extend throughout the complete window of transparency of silica based fibres. The systems have been operated femtosecond, picosecond and nanosecond as well as cw. Average powers of 10’s Watts can be easily achieved, giving flat spectral power densities in excess of 10’s mW per nm from 400 nm to beyond 2.2 μm.

Optical tweezers use the gradient force created by tightly focused single laser beam to trap dielectric microparticles. While this technique has been used for over 20 years to manipulate particles without mechanical contact, it is only recently htat accurate and quantitative photonic forces measurements have been considered. Moreover of great relevance has resulted the monitoring of Brownian motion of particles confined in optical traps since that provides precious information on local visco-elastic properties of the surrounding fluid. In this work we consider the still open question concerning the calibration of an optical tweezers which represents the key point for any absolute measurements. In particular, we discuss a novel method to calibrate a quadrant photodiode used as sensor position in the forward scattering scheme. The voltage signals provided by the qudrant photodiode are converted in length units by comparing them with the absolute bead positions measured by means of a calibrated CCD camera. Finally we briefly discuss how calibrated optical tweezers, combined with thermal analysis of the Brownian motion, are potentially of great relevance for microrheological studies of complex fluids.

We demonstrated pulse compression at 1 μm based on the use of microstructured optical fibers. Two different approaches have been investigated. On the first hand, we used fibers with negative group velocity dispersion at the laser emission to obtain self-compression based on solitonic effect. On the second hand, we used fibers with zero-dispersion at the laser emission to generate very broad spectra (by self-phase modulation) recompressed afterward using prism-based compressor. We studied experimentally and theoretically these two techniques of compression with a special care regarding their limitations. For the experiments, the compressions have been obtained using a home-made diode-pumped Yb:SYS laser producing 110-fs pulses directly injected in the microstructured optical fibers. We demonstrated compressed pulses as short as 20-fs centered at 1070 nm which is to our best knowledge the shortest pulses obtained with a diode-pumped system in this wavelength range.

A simple set-up for observation of CPT effect at single hyperfine ground state excitation has been examined for application in measurements of non-vanishing magnetic fields. The CPT is prepared using low-frequency modulated (in the kHz range) diode laser. Magnetic field measurements down to 500 pT sensitivity and reproducibility were obtained for integration time of 30 ms. One of the important characteristics of the proposed methodology is a proper Cs excitation in order to overcome effectively the huperfine optical pumping effefct. This is possible working in narrow ranges of laser beam intensity, buffer gas pressures and Cs vapor densities. The best conditions have been experimentally determined and discussed.

The increasing use of lasers in biomedical research and clinical praxis leads to the development and application of new, non-invasive, therapeutic, surgical and diagnostic techniques. In laser surgery, the theory of ablation dictates that pulsed mid-infrared laser beams exhibit strong absorption by soft and hard tissues, restricting residual thermal damage to a minimum zone. Therefore, the development of high quality 3 μm lasers is considered to be an alternative for precise laser ablation of tissue. Among them are the high quality oscillator-two stages amplifier lasers developed, which will be described in this article. The beam quality delivered by these lasers to the biological tissue is of great importance in cutting and ablating operations. As the precision of the ablation is increased, the cutting laser interventions could well move to the microsurgery field. Recently, the combination of a laser scalpel with an optical trapping device, under microscopy control, is becoming increasingly important. Optical manipulation of microscopic particles by focused laser beams, is now widely used as a powerful tool for 'non-contact' micromanipulation of cells and organelles. Several laser sources are employed for trapping and varying laser powers are used in a broad range of applications of optical tweezers. For most of the lasers used, the focal spot of the trapping beam is of the order of a micron. As the trapped objects can vary in size from hundreds of nanometres to hundreds of microns, the technique has recently invaded in to the nanocosomos of genes and molecules. However, the use of optical trapping for quantitative research into biophysical processes requires accurate calculation of the optical forces and torques acting within the trap. The research and development efforts towards a mid-IR microbeam laser system, the design and realization efforts towards a visible laser trapping system and the first results obtained using a relatively new calibration method to calculate the forces experienced in the optical trap are discussed in detail in the following.

We discuss the basic physical principles of laser optical pumping double-resonance spectroscopy, which form the basis of state-of-the-art vapour-cell atomic frequency standards using laser optical pumping of Rb atoms. The main effects limiting the frequency stability of Rb vapour-cell atomic clocks are identified, and their impact on the development of high-performance frequency standars and their transfer from research laboratories to industry and space is discussed. As examples, the impact of the AC stark effect and the realated issue of laser frequency stabilisation are dealt with in more detail. The main features of the present state-of-the-art Rb atomic frequency standards will be illustrated using the example of the development of atomic clocks for satellite navigation and positioning systems (GPS, GLONAS, GALILEO, etc.) as well as some directions for further improvements that could overcome present day limitations. Such compact Rb clocks find their applications in, for example, telecommunications, local timekeeping and synchronisation, and space applications like satellite navigation and science missions. An overview of other, alternative clock schemes is given and critical issues for future developments towards further performance improvement or device miniaturisation in the field of vapour-cell atomic clocks are discussed.

Information about atmospheric variables, such as presence and characteristics of supended particulates (aerosols), concentration of chemical species, humidity, temperature, etc., can be obtained remotely in an energetic-efficient manner from radiation produced by optical sources and backscattered by the atmosphere, drawing on the relatively strong interaction between electromagnetic radiation at optical wavelengths and particulates and molecules in the atmosphere. Atmospheric probing systems based on laser sources can provide information as a function of range and, being their operating principle essentially the same as that of radar (except for the fact that in atmospheric probing the target is by definiti8on the atmosphere zone illuminated by the laser, instead of a hard target), they are usually called laser radars or lidars (from Light Detection And Ranging). In addition to their ability to perform remote measurements- a characteristic that they share with other systems-lidars feature the availability of quasi-real-time data, high spatial resolution and a relative convenience to perform three-dimensional scans of the atmospheric volume under study. Basic lidar principles and capabilities are discussed in this work.

The production of thin films by pulsed laser deposition (PLD) has become a standard method, even though many of the basic processes are not known in detail. The key quantitites are the ablation rate (yield) and the angular distribution of the ablated particles. The starting point for this study is the comparatively simple case of one-component metals rather than metal oxides which in the past have been comprehensively studied. The ablation rate depends primarily on the thermal properties of the metal, in such a way that a low cohesive energy leads to a high ablation rate. The angular distribution of the ablated atoms is important for the uniformity of the film thickness as well as the deposition rate on a substrate. However, if the ablation takes place in a background gas, the angular distribution of collected ablated atoms becomes comparatively broad. Combined diagnostic measurements of dposition rates and ion time-of-flight (TOF) signals have been used to study the dynamics of a laser albation plume in backgound gases. The angluar distribution and the TOF signals exhibit three separate regimes with increasing pressure, a vacuum-like regime, a transition regime with increasing plume broadening and splitting of the ion signal, and at the highest pressures a diffusion-like regime with a broad angular distribution.

Fiber lasers are pumped by fibercoupled, multimode single chip devices at 915nm. That’s what everybody assumes when asked for the type of fiber laser pumps and it was like this for many years. Coming up as an amplifier for telecom applications, the amount of pump power needed was in the range of several watts. Highest pump powers for a limited market entered the ten watts range. This is a range of power that can be covered by highly reliable multimode chips, that have to survive up to 25 years, e.g. in submarine applications. With fiber lasers entering the power range and the application fields of rod and thin disc lasers, the amount of pump power needed raised into the area of several hundred watts. In this area of pump power, usually bar based pumps are used. This is due to the much higher cost pressure of the industrial customers compared to telecom customers. We expect more then 70% of all industrial systems to be pumped by diode laser bars. Predictions that bar based pumps survive for just a thousand hours in cw-operation and fractions of this if pulsed are wrong. Bar based pumps have to perform on full power for 10.000h on Micro channel heat sinks and 20.000h on passive heatsinks in industrial applications, and they do. We will show a variety of data, “real” long time tests and statistics from the JENOPTIK Laserdiode as well as data of thousands of bars in the field, showing that bar based pumps are not just well suitable for industrial applications on high power levels, but even showing benefits compared to chip based pumps. And it’s reasonable, that the same objectives of cost effectiveness, power and lifetime apply as well to thin disc, rod and slab lasers as to fiber lasers. Due to the pumping of fiber lasers, examples will be shown, how to utilize bars for high brightness fiber coupling. In this area, the automation is on its way to reduce the costs on the fibercoupling, similar to what had been done in the single chip business. All these efforts are part of the JENOPTIK Laserdiode’s LongLifeTechnologie.

A tapered fiber bundle is one of the leading approaches to coupling pump light into cladding-pumped fibers. This article compares it to other pump coupling schemes, and describes the tapered fiber bundle in detail. In addition tapered fiber bundles which maintain the polarization state of the signal and transform the size of the mode are described.

In a context where ultrafast lasers have become ideal tools for material probing and processing we present various concepts for process control and optimization. Temporal tailoring of ultrashort laser pulses enables synergies between radiation and material and, therefore, new opportunities for optimal processing of materials. The concept of optimizing laser interactions is based on the possibility to adjust energy delivery so that control of laser-induced processes can be achieved and particular states of matter can be accessed. We present recent results related to the implementation of adaptive feedback loops based on temporal shaping of ultrafast laser pulses to control laser-induced phenomena for practical applications. The chosen example indicates the possibility to manipulate the kinetic properties of ions emitted from ultrafast laser irradiated semiconducting samples, using excitation sequences synchronized with the phase-transformation characteristic times. Versatile sub-keV ion beams are obtained exploiting transitions to supercritical fluid states with minimal energetic expenses, while achieving very efficient energy coupling and thermodynamic paths towards highly volatile states. Temporally selective irradiation can thus open up efficient thermodynamic paths towards critical points, delivering at the same time an extended degree of control in material processing.

Both ultrafast thermal and photocurable nanoimprint lithography (NIL) are studied and high fidelity transfers of nanopatterns from molds to resists have been achieved. In ultrafast thermal NIL, we use a single excimer laser pulse to melt a NIL resist polymer and imprint it using a fused silica mold. The entire imprint process, from melting the polymer to completion of the imprint, takes less than 200 ns. This technique, termed laser assisted nanoimprint lithography (LAN), has patterned nanostructures in various polymer films with high fidelity over the entire mold area. In LAN, the short laser pulse is absorbed primarily by the resist and the laser energy is minute, hence substrate heating and distortion are negligible. In ultrafast photocurable NIL, a flash lamp (pulse width 94 μs) is used to crosslink photo curable resists over a 4 in. wafer with high uniformity by a single pulse. The significant reduction of the heating of the substrate and mold will greatly benefit overlay alignment.

Picosecond acoustic phonon pulses are generated with ultrashort laser pulses in a sample containing three GaAs-Al 0.3 Ga 0.7 As quantum wells of different thickness. The pump photon energy is tuned through the hh 1- e 1 transitions of each well (1.44 - 1.64 eV) and the probe photon energy is chosen to allow detection of the phonon pulses at the sample surface (3.06 eV). Transient optical reflectance and phase changes are recorded as a function of the delay time between the pump and probe light pulses using an interferometric technique. The transition between the valence and conduction sublevels of the wells is observed to strongly influence the pump-photon-energy dependence of the acoustic phonon pulse generation. The data are analyzed with a model that relates the carrier wavefunctions in the quantum wells to the acoustic strain through the deformation potential, and the acoustic strain to the transient optical reflectance and phase.