Mode-locked Er-doped fiber laser systems built on single-mode fiber technology continue to see a remarkable improvement in their performance characteristics. In this contribution, we present an extremely compact and powerful version of such a laser source, delivering elevated peak powers well in excess of 10 kW in combination with ultrashort pulse durations below 100 fs. Eliminating the need for costly pump sources, external cooling as well as daily re-alignment routines, this laser system opens possibilities for an entirely new class of experiments and applications to a much larger group of users than only dedicated laser institutes. The accessible wavelength range is greatly enhanced by generation of a supercontinuum inside an integrated highly nonlinear fiber. We report output spectra with a bandwidth exceeding one full octave which we utilize for phase stabilization of the laser source. As a first proof of principle, a precise frequency measurement is carried out on a cavity-stabilized diode laser over a time interval of 88 hours without interruption. With regard to the time domain pulse structure, the user can select to re-compress defined parts of the continuum to achieve pulse durations below 30 fs. At the same time, the central wavelength of these pulses is easily shifted over a wavelength interval from 1130 nm to 1400 nm. Based on these findings, we demonstrate the generation of widely tunable light pulses in the visible spectral range by efficient frequency doubling. Potential applications for this novel light source are discussed.

Dynamics of electronic excitation, heating and charge-carrier transport in different materials (metals, semiconductors, and dielectrics) under femtosecond pulsed laser irradiation is studied based on a unified continuum model. A simplified drift-diffusion approach is used to model the energy flow into the sample in the first hundreds of femtoseconds of the interaction. The laser-induced charging of the targets is investigated at laser intensities slightly above the material removal threshold. It is demonstrated that, under near-infrared femtosecond irradiation regimes, charging of dielectric surfaces causes a sub-picosecond electrostatic rupture of the superficial layers, alternatively called Coulomb explosion (CE), while this effect is strongly inhibited for metals and semiconductors as a consequence of superior carrier transport properties. Various related aspects concerning the possibility of CE for different irradiation parameters (fluence, wavelength and pulse duration) as well as the limitations of the model are discussed. These include the temporal and spatial dynamics of charge-carrier generation in non-metallic targets and evolution of the optical (reflection and absorption) characteristics. A controversial topic concerning CE probability in laser irradiated semiconductor targets is also a subject of this work.

We describe experiments using ultrashort pulses of laser light to ablatively remove opaque and partially transmitting materials from transparent substrates. Pulses of 100 femtosecond duration at a wavelength of 266 nm were used to repair defects on photomasks used in lithographic printing of integrated circuits, with better than 100 nm spatial resolution. Details of the development and implementation of a photomask repair tool, presently operating in manufacturing, which exploits the advantages of ablation with femtosecond pulses, are presented. We further describe experiments where pulses of 400 nm light were used to photolytically deposit Cr metal with better than 200 nm resolution. Finally we describe a gas phase 35 femtosecond laser source used to extend this approach to ablative mask repair at 193 nm.

The reduced thermal nature of ultrafast laser processing attracts many to develop the laser technology and to explore their applications in semiconductor manufacturing. We report here for the first time the systematic study and the results of using pico-second lasers for semiconductor memory repair. We found that the thermal effect is dramatically reduced by the reduction in laser pulse duration from the conventional nanoseconds to pico-seconds. The neighboring damage caused by such thermal effect is therefore reduced, allowing further reduction in spacing (pitch) between links into the required dimensions for the next generation of memory devices like DRAM. We have also found that due to the ultrashort pulses, the link blowing process is less chaotic compared to the conventional laser process, which results in much better control on substrate damage.

Alumina ceramic, Al₂O₃, presents a challenge to laser micro-structuring due to its neglible linear absorption coefficient in the optical region coupled with its physical properties such as extremely high melting point and high thermal conductivity. In this work, we demonstrate clean micro-structuring of alumina using NIR (λ=775 nm) ultrafast optical pulses with 180 fs duration at 1kHz repetition rate. Sub-picosecond pulses can minimise thermal effects along with collateral damage when processing conditions are optimised, consequently, observed edge quality is excellent in this regime. We present results of changing micro-structure and morphology during ultrafast processing along with measured ablation rates and characteristics of developing surface relief. Initial crystalline phase (alpha Al₂O₃) is unaltered by femtosecond processing. Multi-pulse ablation threshold fluence F_th ~ 1.1 Jcm^-2 and at low fluence ~ 3 Jcm^-2, independent of machined depth, there appears to remain a ~ 2μm thick rapidly re-melted layer. On the other hand, micro-structuring at high fluence F ~ 21 Jcm^-2 shows no evidence of melting and the machined surface is covered with a fine layer of debris, loosely attached. The nature of debris produced by femtosecond ablation has been investigated and consists mainly of alumina nanoparticles with diameters from 20 nm to 1 micron with average diameter ~ 300 nm. Electron diffraction shows these particles to be essentially single crystal in nature. By developing a holographic technique, we have demonstrated periodic micrometer level structuring on polished samples of this extremely hard material.

A principal challenge facing nanotechnology is consistently producing well-defined features much smaller than the wavelength of visible light. We find that the remarkably sharp threshold for femtosecond laser-induced material damage enables nanomachining with unprecedented precision and versatility, allowing highly reproducible machining of structures with nanoscale features. Using this methodology, we demonstrate, in glass, surface trenches that are only tens of nanometers in width but micron in depth, sub-surface channels that are hundreds nanometers in diameter, tens of microns deep, and hundreds microns in length, and 3D microstructures such as cantilevers. Furthermore, we demonstrate reproducible nanometer scale features in mixed and amorphous materials that differ significantly from glass, such as gold and onion cells. This technique is versatile, not material specific, and has potentially broad applications for MEMS construction and design, high density microelectronics, nanofluidics, material science, and optical memory.

As applications demanding microJoule level pulses at "real-time" rates of delivery increase, and the expectations in terms of long-term, reliable, high quality performance become greater, fiber lasers are becoming increasingly attractive sources. When a combination of excellent beam quality, flexibility in design for repetition rate over 100 kHz - 5 MHz, and robust design for operation in a variety of environments, in a plug-and-play, non-water cooled package are necessary, IMRA’s fiber chirped-pulse amplifier (FCPA) system delivers in a compact, single-box solution. This type of laser has particular promise in precision material processing applications, enabling the use of technology that was previously considered too unstable or difficult to use. The basis for this advanced technology is a novel Yb:fiber oscillator/amplifier combination. The modular design architecture ensures a very robust construction that is well-suited to integration into commercial systems. To show the utility of such a laser in commercial applications, results of ablation thresholds and processing tests of various materials including metals and dielectrics are presented using IMRA’s FCPA μJewel femtosecond fiber laser.

Based on a femtosecond Cr:forsterite laser, harmonics optical microscopy (HOM) provides a truly “noninvasive” tool for in vivo and long-term study of vertebrate embryonic development. Based on optical nonlinearity, HOM provides sub-micrometer 3D spatial resolution and high 3D optical-sectioning power without using invasive and toxic fluorophores. Since only virtual-level-transition is involved, HOM is known to leave no energy deposition and no photodamage. Combined with second harmonic generation, which is sensitive to specific structure such as nerve and muscle fibers, HOM can perform functional studies of early developmental dynamics of many vertebrate physiological systems. Recently, zebrafish has become a standard model for many biological and medical studies of vertebrates, due to the similarity between embryonic development of zebrafish and human being. Here we demonstrate in vivo HOM studies of developmental dynamics of several important embryonic physiological systems in live zebrafish embryos, with focuses on the developments of brains, eyes, ears, and hearts. Based on a femtosecond Cr:forsterite laser, which provides the deepest penetration (~1.5mm) and least photodamage in the zebrafish embryo, complete developing processes of different physiological systems within a period of time longer than 20 hours can be non-invasively observed inside the same embryo.

The general utility of two compact femtosecond laser sources for Third Harmonic Generation (THG) and Multiphoton absorption microscopy is demonstrated. The effect of aberrations due to a dielectric interface on THG microscopy is investigated both from a theoretical and experimental point of view. The significance of these aberration issues in third order nonlinear optical susceptibility (χ⁽³⁾) measurements using THG microscopy is discussed.

Second harmonic generation (SHG) imaging microscopy is an important emerging technique for biological research, with many advantages over existing one- or two-photon fluorescence techniques. SHG is of growing interest to those in the biomedical community studying structural proteins such as collagen and to those in neuroscience using voltage-sensitive dyes. An important consideration in the application of non-linear phenomena such as SHG to routine microscopy is the complexity of the laser source used for excitation. Almost all applications in ultrafast microscopy currently employ mode-locked Ti:sapphire lasers, and though these systems have improved considerably in recent years, they are still expensive, large and complicated for those with skills outside of ultrafast optics. Here we report on SHG microscopy using a high power femtosecond fiber laser. The Femtopower1060 from Fianium Ltd. is an ultrafast fiber laser operating at 1064nm. With a passively mode-locked master source, a high power fiber amplifier and a built-in pulse compressor, the laser produces high quality pulses shorter than 200fs with a repetition rate of 100MHz and an average power of 1W. The unit is turn-key, air-cooled and maintenance free with a small footprint and proves to be an excellent source for SHG and two-photon microscopy at this wavelength outside the range of most Ti:sapphire systems and without those systems' complexity.

The two-photon excitation fluorescence (TPEF) process of an enhanced green fluorescent protein (EGFP) for fluorescence signals was adaptively controlled by the phase-modulation of femtosecond pulses. After the iteration of pulse shaping, a twofold increase in the ratio of the fluorescence signal to the laser peak power was achieved. Compared with conventional pulses optimized for peak power, phase-optimized laser pulses reduced the bleaching rate of EGFP by a factor of 4 while maintaining the same intensity of the fluorescence signal. The ratio of two-photon (2P) fluorescence from EGFP and three-photon (3P) fluorescence from the essential amino acid L-Tryptophan was also enhanced by using the adaptive pulse shaping technique. To achieve a trade-off between the 2P/3P fluorescence ratio and the 2P fluorescence intensity, we then engineered the cost function in the self-learning algorithm. These methods have much potential for immediate application to various important biological and medical studies.

Optical coherence tomography (OCT), so far mainly used in the biomedical field, has a high potential as non-destructive and contactless technique for material characterization and analysis. For these applications, OCT systems with ultra-high resolution in the micrometer range and capable of high imaging speeds are required. In this work, we combine ultra-high resolution imaging using a femtosecond Ti:sapphire laser as light source with the concepts of transversal OCT. Based on acquisition by heterodyne detection via acousto-optic modulators (AOMs), and by using an xy-galvano scanner unit we are able to obtain en-face scans with sizes as large as 3 x 3 mm² within a few seconds. The ultra-high resolution of our OCT system of 2.95 μm axially and 4 μm laterally, both in air, is shown to be essential for imaging of different compounds and fibre materials. We demonstrate the benefits of en-face scanning OCT for various applications in material investigation where in-plane information is of interest which can hardly be obtained by cross-sectional OCT.

Two-photon excitation (TPE) via a microscope objective lens produces a spatially confined excitation volume where UV-excited caged molecules may be broken (uncaged) to release active products. We describe an optical system that creates a stationary parfocal TPE uncaging spot on the stage of a conventional confocal microscope. With this system, we have examined the ability of two dyes to track microscopic calcium changes produced by TPE photolysis of DM-nitrophen. We find that, even when EGTA is used with a low affinity indicator, the dye signals are complicated by diffusion of both indicator-Ca complex and CaEGTA to produce a signal that does not simply report the spatial dimensions of the calcium release site. In addition, the time course of calcium release is poorly reported. This suggests that considerable caution must be applied to the interpretation of spatially resolved calcium signals inside cells. We have also used TPE of CMND-caged fluorescein to measure the rate of fluorescein production in test solution (2500 s^-1) as well as the diffusion of fluorescein in drops of solution and within and between between eye lens fiber cells. While diffusion of uncaged fluorescein was about an order of magnitude slower inside fiber cells than in aequeous solution, slower diffusion between cells could also be detected and could be explained by the gap junctions joining the cells behaving as a barrier to diffusion. By using a computer model, parameter fits to experimental data gave estimates for both intracellular and intercellular diffusion coefficients. From this analysis, the gap junctions in eye lens fiber cells permit exchange of low molecular weight compounds between cells at about 0.4% of the rate of free diffusion.

In the focal region of tightly focused ultrashort laser pulses, sufficient high intensities to initialize nonlinear ionization processes are easily achieved. Due to these nonlinear ionization processes, mainly multiphoton ionization and cascade ionization, free electrons are generated in the focus resulting in optical breakdown. A model including both nonlinear pulse propagation and plasma generation is used to calculate numerically the interaction of ultrashort pulses with their self-induced plasma in the vicinity of the focus. The model is based on a (3+1)-dimensional nonlinear Schroedinger equation describing the pulse propagation coupled to a system of rate equations covering the generation of free electrons. It is applicable to any transparent Kerr medium, whose linear and nonlinear optical parameters are known. Numerical calculations based on this model are used to understand nonlinear side effects, such as streak formation, occurring in addition to optical breakdown during short pulse refractive eye surgeries like fs-LASIK. Since the optical parameters of water are a good first-order approximation to those of corneal tissue, water is used as model substance. The free electron density distribution induced by focused ultrashort pulses as well as the pulses spatio-temporal behavior are studied in the low-power regime around the critical power for self-focusing.

We show that GRENOUILLE, the experimentally simple version of frequency-resolved optical gating (FROG) can measure the two spatio-temporal distortions, namely spatial chirp and pulse-front tilt, in addition to the pulse intensity and phase. This is done without a single alteration in the experimental setup. Specifically, pulse-front tilt yields a displacement of the otherwise centered trace along the delay axis while spatial chirp causes a shear to the otherwise symmetrical GRENOUILLE trace. We develop a more general FROG pulse-retrieval algorithm based on the Levenberg-Marquardt algorithm, which can retrieve not only the pulse intensity and phase but also both the spatial chirp and pulse-front tilt from GRENOUILLE traces. Lastly, we also show that, by employing the exotic nonlinear crystal Proustite, GRENOUILLE can be extended to measure fiber-laser pulses with wavelengths near 1.5μm. The high nonlinearity of Proustite compensates for the lower output power of fiber lasers. Also Proustite has so high dispersion that it can spectrally resolve these relatively narrowband pulses. We experimentally test all of these innovations and obtain perfect agreement with the expected results.

Optical waveguides provide rich environment for various nonlinear processes thanks to the long interaction lengths, sustained high intensities and diverse dispersion regimes. Nonlinear and dispersion properties of fibers and waveguides can be widely controlled through microstructuring resulting in a broad family of photonic crystal and bandgap waveguides. This flexibility can be used to realize previously impossible nonlinear interaction regimes for solitons and quasi-continuous waves. The dynamics of femtosecond optical pulses in such dispersive and nonlinear materials provide a truly challenging measurement task, but reward us with most spectacular images of nonlinear wave interactions. We visualized the dynamics of solitons and continua in several such structures using cross- correlation frequency-resolved optical gating, the technique which provides experimentally the most complete information about an optical pulse. These detailed time and frequency-resolved measurements infinitely surpass the simple spectral measurements or even the time axis-symmetric FROG spectrograms. Soliton dynamics in the vicinity of the second zero-dispersion point of a silica PC fiber, Cherenkov continuum generation, stabilization against the Raman self-frequency shift and other resonant interactions as well as the supercontinuum generation in soft-glass fibers were characterized. Recent theoretical studies were brought about to develop a fundamental understanding of these resonant interactions and excellent agreement was found.

We discuss two techniques for measuring space-time coupling in ultrashort pulses using spectral phase interferometry for direct electric-field reconstruction (SPIDER). The first technique, Spatially Encoded Arrangement for SPIDER (SEA SPIDER), requires reduced spectral resolution as compared to conventional SPIDER techniques and is therefore ideally suited for very large bandwidth pulses. In addition, this method results in a spatially resolved reconstruction of the temporal field and allows for the characterization of some types of space-time coupling. The second technique, Space Time SPIDER (ST SPIDER), couples spatial shearing and spectral shearing interferometry to fully characterize any arbitrary space-time field without assumptions about the ultrashort pulse or the type of coupling present. Experimental demonstrations of both techniques are presented.

Results of diode-pumped cw and fs laser operation of an Yb³⁺:CaF₂ single crystal are reported. With a 5-at.-% Yb³⁺-doped sample we obtained 5.8 W output power at 1053 nm, for 15 W of incident pump power at 980 nm. In passively mode-locked diode-pumped regime, using a Brewster-cut, 5-at.-% Yb³⁺-doped sample and prisms for dispersion compensation, the oscillator provided pulses as short as 150 fs, with 880 mW of average power and up to 1.4 W average output power, with pulse duration of 220 fs, centred at 1049 nm. The laser wavelength could be tuned from 1018 nm to 1072 nm in cw regime and from 1040 nm to 1053 nm in mode-locked regime. Using chirped mirrors for dispersion compensation, we obtained up to 1.74 W of average power, with pulse duration of 230 fs. For all these reasons, Yb:CaF₂ crystal is showing the great potential as active medium for high average power femtosecond oscillators and as amplifier medium for femtosecond pulses.

Ultra-high bandwidth continuum generation has been attracting enormous interest for applications in optical frequency metrology, low-coherence tomography, laser spectroscopy, dispersion measurements, sensor techniques and others. The acceptance of this new technology would greatly benefit from the availability of compact and user-friendly sources. High index planar devices provide a versatile and unique approach to continuum generation. The dispersion can be carefully engineered by choosing the material and the geometry of the waveguides. Optical integration can also be provided on the same platform. Hundreds of different waveguides having different and calibrated dispersions can be integrated in few tens of millimeters. Input and output of the 2D guides can be tailored to provide mode matching to fibers and pump lasers by means of single element bulk optics. In this paper for the first time we demonstrate a low-noise, ultra-high bandwidth continuum at 1.55 μm. A bandwidth in excess of 390 nm is obtained by launching energy as low as 50 pJ in a 12 mm short tapered planar waveguides. The pump wavelength was in the normal dispersion regime and was provided by a compact, fiber-based sub-100 femtosecond source.

Ultra-high bandwidth continua generated by ultrashort fs pulses have been attracting enormous interest for applications such as general spectroscopy, Optical Coherence Tomography and metrology. Dispersion engineering is one of the key aspects of optimised continuum generation in optical waveguides. However in addition, the dispersion of the pump pulse can be continuously adapted to control bandwidth and spectral characteristics of the generated continua. In this work we report on a systematic investigation of how 2^nd, and 3^rd order dispersion affects the continuum generated in strongly nonlinear planar waveguides. A ~30 fs Ti:Sapphire tuned to 800 nm was used as a pump source delivering ~3 nJ pulses. The chirp of the pulses was controlled completely-arbitrarily by an acousto-optic programmable dispersive filter (Dazzler). The power launched into the structures was kept constant to compare the generated continua as the pulse dispersion is varied. High refractive index tantalum pentoxide (Ta₂O₅) waveguides grown by standard silicon processing techniques were used. The devices investigated were specially designed tapered ridges with ~5 mm² input modal volume and zero group velocity dispersion at ~l - 3.7 mm. Self-phase modulation, which is responsible for the spectral broadening of the continua, is tracked by finely tuning the both 2^nd and 3^rd order dispersions. The nonlinear propagation is dramatically influenced by the simultaneous presence of these dispersive effects resulting in a change of bandwidth and spectral shape. Pulse widths of up to Dl > 100 nm for launched powers as low as 300 pJ. Spectral peak intensity can also be systematically modulated by simply scanning the 2^nd and 3^rd order dispersion around their relative zeros. Specific combinations of high order dispersion contribution are currently targeted as a route to control and optimise the continua bandwidths and to control dispersion lengths in specifically engineered waveguides.

Recently, an electron-based ultrashort hard-x-ray source has been developed at the Laser Zentrum Hannover e.V. In this source x-ray pulses are produced by combining femtosecond laser technology with a specially designed x-ray diode. At first, ultrashort electron pulses are generated by photoemission from a photocathode. Then, these electron pulses are accelerated over a short distance towards a high-Z anode. Hard-x-rays are produced via Bremsstrahlung and characteristic line emission. Now detailed measurements of the hard-x-ray pulse duration have been performed using an advanced streak camera. The streak camera has a sub-picosecond time resolution in the keV range. With this camera hard-x-ray pulse durations of less than 5 ps were observed for electron pulse charges of the order of several pC. In this contribution we present our results on the x-ray pulse duration measurements and their dependence on different experimental parameters. A comparison with theoretical simulations is given.

Time-integrated spatially resolved emission spectra of soft x-rays were obtained in the case of 100-fs-laser irradiation of tantalum and aluminum targets by controlling the time intervals between the main pulse and artificial pre-pulse. The intensities of the main pulse and pre-pulse were fixed to 1.3x10¹⁶ and 2.0x10¹⁴ W/cm². X-ray intensities were enhanced more than several ten fold at the optimal pulse-separation time of 2 ns. We confirmed that the main emission area for each target was less than 50-μm long from the target surface. In contrast to the small size of main emission area, we observed that line emissions from the aluminum target expanded far from the target.

X-ray pulses from a fs-laser plasma were focused by an X-ray capillary lens, generating a spot smaller than 100 μm. Fe Kα radiation (λ = 0.194 nm) is produced by focusing 200 mJ/130 fs pulses from the ATLAS titanium-sapphire laser at 10 Hz onto a moving iron tape. The capillary lens enhanced the intensity by a factor of about 1600. Diffraction from samples of small size is demonstrated by producing diffractograms from a Si (111) crystal in only about 10 seconds. The model of a novel ultrafast streak camera which takes advantage of the different path lengths of rays propagating through the lens is demonstrated. Preliminary experiments using a semi-lens for collimating X-rays are also reported.

The fabrication of telecom active devices, such as waveguide amplifiers and lasers, with femtosecond laser pulses is of great industrial interest due to the simplicity, low cost and 3D capabilities of this technology with respect to the standard ones. In this work we will present the various improvements that brought us to demonstrate net gain and the first waveguide laser fabricated with femtosecond laser pulses on an erbium-ytterbium-doped phosphate glass. The first results have been obtained with an amplified, low repetition rate (1 kHz), Ti:Sapphire system. The target of matching the mode field of the fabricated waveguides to that of standard telecom fibers pushed us to develop a novel astigmatic focusing of the writing beam to overcome the asymmetry of the waveguide transverse profile intrinsic in the transversal writing geometry. Despite the circularization of the transverse profile, the high coupling losses allowed only for internal gain in an all-fiber coupling configuration. The best results have been obtained with a very compact, unamplified, diode-pumped Yb:glass laser, with a higher repetition rate (166/505 kHz) and lower energy. In this case, the waveguides exhibited almost perfect mode matching with a telecom fiber allowing coupling losses as low as 0.18 dB and propagation losses of 0.5 dB/cm. Such figures enabled net gain when pumping with 980-nm laser diodes and laser action by terminating the waveguide with two fiber Bragg gratings. These results pave the way to a transfer of femtosecond waveguide writing into the industrial arena for the realization of practical telecom components.

Waveguides were written in soda lime silicate glasses with a composition of xNa₂O xCaO (1-2x)SiO₂, where x = 15 and 20, using an amplified femtosecond laser. The waveguides formed around, not inside the exposed regions. This is similar to the waveguide behavior our group first observed in a phosphate glass, Schott IOG-1, and is distinctly different from fused silica in which the waveguides are inside the exposed regions. This data supports the rapid quenching theory, i.e. that the exposed regions cool rapidly, locking in a glass structure with a high fictive temperature, with the dependence of the refractive index on the glass cooling rate determining the qualitative behavior of the waveguides.

Integrated optics in silicon is interesting for various optoelectronic devices, since photonics and electronics could be realized together. However, to be compatible with the standard CMOS technology, optical waveguides which rely on structuring the silicon surface are inappropriate. An alternative solution is the direct structuring inside the bulk medium by using ultrashort laser pulses. In recent years true three-dimensional photonic structures have been fabricated inside crystals and various optical glasses with this technique. Here, we demonstrate the use of femtosecond laser pulses to directly inscribe optical waveguides into the bulk of crystalline silicon. Due to the bandgap of 1.1 eV of silicon, the 800 nm pulses of the typically used Ti:Sapphire lasers cannot penetrate into the silicon. Therefore, the wavelength was converted to 2.6 μm using an optical parametric amplifier and the pulses were then focused into the bulk silicon by a Schwarzschild reflective objective. This way the laser energy was deposited in the focal region by three-photon absorption. Waveguides have been produced by translating the sample at a constant velocity of 2 mm/min. The waveguides are single-mode at the telecommunication wavelengths of 1550 nm and 1300 nm. Propagation losses were found to be less than 1 dB/cm. This technique is inherently capable of generating three-dimensional structures below the surface of silicon and therefore offers the potential to have a common platform for photonics and electronics.

Direct waveguide writing with femtosecond lasers can be divided into two general categories based upon the type of lasers used: amplified systems that emit high pulse energy (>2 μJ) at low repetition rates (<250 kHz), and oscillators that produce low energy (<200 nJ) at high repetition rates (>1 MHz). In this presentation, we report on waveguide writing with a novel commercial femtosecond fiber laser system (IMRA, FCPA μJewel) that bridges the gap between these two regimes, providing sub-400 fs pulses with pulse energies of >2.5 μJ at 100 kHz and >150 nJ at 5 MHz. The laser repetition rate can be varied from 100 kHz to 5 MHz in 1 kHz increments through a computer controlled user interface. The ability to quickly and easily vary the repetition rate of this laser was critical in identifying and optimizing laser processing windows for different target glasses. An overview of laser processing windows and waveguide characteristics are presented for borosilicate and fused silica glasses exposed to fundamental (1045 nm) and second harmonic (522 nm) laser light.

In recent years, ultrashort laser pulses have drawn increasing interest for the direct writing of photonic structures into different materials. Several optical devices have already been demonstrated, e.g. optical waveguides, waveguide amplifiers and lasers, beam splitters, couplers, stacked waveguides and three-dimensional waveguide arrays and gratings. The investigations were mainly focused on glasses where the laser irradiation causes a rise in refractive index. However, for different applications the realization of waveguides in crystalline media is interesting. Here, we present investigations on femtosecond laser induced modifications in crystalline quartz. We show that the irradiation leads to a refractive index decrease which is due to amorphization of the focal volume. A detailed analysis of the structures is performed with transmission electron microscopy and X-ray diffraction and topography. Our investigations show that the irradiated amorphous core creates a stress field in the surrounding material that possesses a positive index change and therefore supports the guiding of light. The results of the X-ray experiments allow a quantitative characterization of the stress field. We are able to simulate the stress distribution by a simple model based on the density difference between the amorphous and crystalline material. From this the refractive index profile can be calculated and compared to experimental results. The light guiding properties of the compressed regions and the fact that only one polarization is guided can be verified by the simulation results.

Femtosecond laser ablation of materials with high thermal conductivity is of paramount importance, because the chemical composition and properties of the area ablated with femtosecond laser are kept unchanged. The material processing by femtosecond laser can well control the heat-affected zone, compared to nanosecond laser ablation. We report on the heat-affected zone of crystalline copper (Cu) by use of femtosecond laser experimentally and theoretically. Laser ablation of Cu is investigated theoretically by two temperature model and molecular dynamics (MD) simulation. The MD simulation takes into account of electron temperature and thermal diffusion length calculated by two temperature model. The dependence of lattice temperature on time and depth is calculated by the MD simulation and two temperature model. The heat-affected zone estimated from the temperature is mainly studied and calculated to be 3 nm at 0.02 J/cm² which is below the threshold fluence of 0.137 J/cm². In addition, the thickness of heat-affected zone of copper crystal ablated with femtosecond Ti:sapphire laser is experimentally studied. As a result of X-ray diffraction (XRD) of the ablated surface, the surface crystallinity is partially changed into disordered structure from crystal form. The residual energy left in the metal, which is not used for ablation, will induce liquid phase, leading to the amorphous phase of the metal during resolidification. The thickness of heat-affected zone depends on laser fluence and is experimentally measured to be less than 1 μm at higher laser fluences than the ablation threshold.

We have demonstrated the ablation processing of dry-collagen with double pulsed femtosecond lasers through a bent hollow fiber. Using a 320 μm core-diameter hollow fiber, we have delivered an ultrashort pulse with a pulse width of 200 fs (straight condition), 320 fs (bent condition), and a near Gaussian beam profile. Furthermore, a precise biomedical material processing is obtainable with a transmitted femtosecond laser beam through a bent hollow fiber. The material processing speed is increased using femtosecond double pulses and by optimizing the delay time. The optimal delay time between the double pulses is from 1 ps to 3 ps, because the ablation rate decreased due to a plasma shielding effect beyond a 5 ps delay time. Our results will open up new medical endoscopic applications.

Micromachining applications require high pulse energy (>1μJ) short pulse (<1ps) laser systems at high repetition rates. Rare-earth doped fibers are attractive to generate these target values by the amplification of ultrafast femtosecond seed sources. Two favored techniques have been used: the chirped pulse amplification (CPA) scheme where the pulses are stretched in the time domain to reduce nonlinearity in the amplifier stage and the parabolic pulse amplification scheme where the combined effect of nonlinearity, normal dispersion and gain in the fiber generate linearly chirped parabolic shaped pulses. Both approaches can be scaled to higher power by reducing the nonlinearity in the amplifiers. To achieve this, we discuss novel photonic crystal fiber designs which allow for larger single-mode core diameter and reduced absorption length and therefore reduced nonlinearity. The so generated high average power of >100 W at repetition rate up to several tens of MHz cannot be compressed by gold gratings to femtosecond pulse duration due to thermal heating. We focus on the development of dielectric gratings in fused silica which can handle this power levels due to their high damage threshold. Two kinds of gratings are discussed. Firstly, the transmission gratings with a period of 800 nm were designed to possess 96% diffraction efficiency over a spectral range from 1.03μm to 1.09μm. The fabrication of the rectangular groove profile was done using electron beam lithography and reactive ion beam etching into the fused silica substrate. The measured diffraction efficiency was 96.5% @ 1060nm. Secondly, dielectric reflection gratings, which consist of a dielectric grating on top of a high-reflective layerstack, can theoretically exhibit a diffraction efficiency of even higher than 99%. To achieve this we chose a period of 1060nm. The fabrication was done similar to the transmission gratings, though a HR-coated substrate had to be used instead of the simple fused substrate. The fabricated gratings show a diffraction efficiency of 99.6%. Both are applied to the discussed high power fiber amplifier stages to generate linearly polarized femtosecond pulses at ~100 W average power with a repetition rate of 80 MHz.

We demonstrate sub-micron scale surgery with femtosecond lasers in a tiny living organism. By just cutting few nano-scale nerve connections inside the nematode C. elegans, we succeeded to stop the whole animal from moving backwards. This delicate axotomy keeps the surrounding of the severed axons un-damaged so that the axons can regrow back, and the worms recover and can move backwards again. These results demonstrate, for the first time, nerve regeneration in such a tiny organism, in its evolutionarily simplest form. The ability to perform precise sub-micron scale axotomy on such organisms provides tremendous research potential for rapid screening of drugs and discovery of new biomolecules affecting regeneration and development.