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We show that the classic Raman threshold formula is unsuitable to accurately predict the onset of Raman scattering in high-power CW fiber amplifiers. We propose a new analytical formula which accuracy is tested with simulations.

We report on an ytterbium-doped fiber CPA system delivering millijoule energy 800fs pulses at high repetition rates and average powers exceeding 100 W. A micromachining application and average power scaling potential are also presented.

Phase-compensation in ultra-compact nonlinear CPA systems employing chirped volume Bragg gratings as stretcher and compressor is demonstrated. It is based on compensation of nonlinear phase with phases due to the dispersion of the amplifiers.

An optical parametric amplifier pumped by a fiber amplifier producing ultrashort pulses with durations of 15.6 fs at 2MHz repetition rate is presented together with scaling considerations to tens of μJ pulse energy.

In this contribution it is reported that circularly polarized light is advantageous if the Kerr-effect has to be minimized during laser-amplification. The experimental demonstration is based on a fiber CPA-system. The different polarization states result in different B-integrals, which are measured using phase-only pulse-shaping. The theoretical value of 2/3 for the ratio of the B-integrals of circularly and linearly polarized light is experimentally verified. In laser-amplifiers circularly polarized light reduces the detrimental impact of the Kerr-nonlinearity, and thus, increases the peak-power and the self-focussing threshold.

We analyze the impact of small amplitude and phase modulations of the input pulse on the pulse contrast at the output of nonlinear CPA-systems. The Kerr-effect precludes nonlinear amplification if the initial pulse-quality is low.

We derive an expression describing pre-compensation of pulse-distortion due to saturation effects in short pulse laser-amplifiers. The analytical solution determines the optimum input pulse-shape required to obtain any arbitrary target pulse-shape at the output of the saturated laser-amplifier. The relation is experimentally verified using an all-fiber amplifier chain that is seeded by a directly modulated laser-diode. The method will prove useful in applications of high power, high energy laser-amplifier systems that need particular pulse-shapes to be efficient, e.g. micromachining and scientific laser-matter-interactions.

We report on what we believe to be the first successful inscription of continuously chirped fiber Bragg gratings (CCFBG) into a nonphotosensitive single-mode fiber (SMF) with near-IR femtosecond laser pulses. The continuous chirp was achieved by inscribing into a bent fiber using a phase mask scanning technique. We fabricated a 20 mm long CCFBG with a bandwidth of 6 nm and a maximal reflectivity of 50%. The second-order dispersion of the CCFBG of up to 28 ps(2) was measured with spectral interferometry.

The optimum seed pulse-profile for a desired output pulse-profile in saturated ns-pulsed fiber-amplifiers is determined with an analytical solution for the amplification process. It can be regarded as an inverse Frantz-Nodvik-equation for the seed photon-density.

We numerically and experimentally analyze gain limitations due to pump light bleaching in large core short length fiber amplifiers and discuss consequences such as the efficiency and accumulated nonlinear phase.

A detrimental pulse distortion mechanism inherent to nonlinear chirped-pulse amplification systems is revealed and analyzed. When seeding the nonlinear amplification stage with pulses possessing weak side-pulses, the Kerr-nonlinearity causes a transfer of energy from the main pulse to side pulses. The resulting decrease in pulse contrast is determined by the accumulated nonlinear phase-shift (i.e., the B-integral) and the initial pulse-contrast. The energy transfer can be described by Bessel-functions. Thus, applications relying on a high pulse-contrast demand a low B-integral of the amplification system and a master-oscillator that exhibits an excellent pulse-contrast. In particular, nonlinear fiber CPA-systems operated at B-integrals far beyond pi have to be revised in this context.

We report on a noncollinear optical parametric amplifier, which is pumped by a fiber-amplifier, producing ultra-short pulses with pulse-durations down to 15.6 fs and pulse-energies up to 500 nJ at 2 MHz repetition rate.

Noncollinearly phase-matched optical parametric amplifiers (NOPAs) - pumped with the green light of a frequency doubled Yb-doped fiber-amplifier system 1, 2 - permit convenient generation of ultrashort pulses in the visible (VIS) and near infrared (NIR) 3 . The broad bandwidth of the parametric gain via the noncollinear pump configuration allows amplification of few-cycle optical pulses when seeded with a spectrally flat, re-compressible signal. The short pulses tunable over a wide region in the visible permit transcend of frontiers in physics and lifescience. For instance, the resulting high temporal resolution is of significance for many spectroscopic techniques. Furthermore, the high magnitudes of the peak-powers of the produced pulses allow research in high-field physics. To understand the demands of noncollinear optical parametric amplification using a fiber pump source, it is important to investigate this configuration in detail 4 . An analysis provides not only insight into the parametric process but also determines an optimal choice of experimental parameters for the objective. Here, the intention is to design a configuration which yields the shortest possible temporal pulse. As a consequence of this analysis, the experimental setup could be optimized. A number of aspects of optical parametric amplifier performance have been treated analytically and computationally 5 , but these do not fully cover the situation under consideration here.

We report on an Yb-doped photonic crystal fiber based CPA system delivering 90.4 W average power of 500 fs pulses at a repetition rate of 0.9 MHz corresponding to a pulse energy of 100 mJ.

We report on the experimental demonstration of the control of the influence of nonlinearity in fiber-based chirped-pulse amplification (CPA) using active spectral amplitude shaping. By applying a liquid crystal spatial light modulator, the influence of the spectral profile on the recompressed pulse quality is experimentally revealed. The parabolic spectrum is experimentally determined to be very suitable for CPA-systems in which nonlinearity is present. The corresponding nonlinear phase contribution can be efficiently compensated by a conventional grating compressor. In a proof-of-principle experiment using an Yb-doped fiber- CPA-system, control at a B-integral as high as 16 rad is demonstrated. The method allows significant performance improvement of fiber-based chirpedpulse amplification.

We report on a fiber CPA-system operating beyond the B-integral limit. Pulse shaping using a spatial-light-modulator allows for the amplification of stretched parabolic pulses. Very clean recompressed pulses are obtained at a B-integral of 16.

We report on an ytterbium-doped fiber chirped-pulse amplification (CPA) system delivering millijoule level pulse energy at repetition rates above 100 kHz corresponding to an average power of more than 100 W. The compressed pulses are as short as 800 fs. As the main amplifier, an 80 microm core diameter short length photonic crystal fiber is employed, which allows the generation of pulse energies up to 1.45 mJ with a B-integral as low as 7 at a stretched pulse duration of 2 ns. A stretcher-compressor unit consisting of dielectric diffraction gratings is capable of handling the average power without beam and pulse quality distortions. To our knowledge, we present the highest pulse energy ever extracted from fiber based femtosecond laser systems, and a nearly 2 orders of magnitude higher repetition rate than in previously published millijoule-level fiber CPA systems.

Optics letters 40(24), 5762 -(2015). doi:10.1364/OL.40.005762, We present an efficiency scaling study of optical rectification in cryogenically cooled periodically poled lithium niobate for the generation of narrowband terahertz radiation using ultrashort pulses. The results show an efficiency and brilliance increase compared to previous schemes of up to 2 orders of magnitude by exploring the optimal pump pulse format at around 800 nm, and reveal saturation mechanisms limiting the conversion efficiency. We achieve >10 −3 energy conversion efficiencies, μJ-level energies, and bandwidths, Published by Soc., Washington, DC

We investigate supercontinuum generation with a femtosecond dual-pumping scheme. A 10 MHz oscillator delivering 300 femtosecond pulses at 1028 nm is frequency doubled and both the fundamental and second harmonic are coupled into a micro structured fiber. When the two pulses are temporally overlapped in the fiber a broad supercontinuum appears. By tuning the temporal delay between the two pulses, different regions of the spectrum can be enhanced which allows either improved flatness of the spectrum or selective amplification of regions of interest. The interesting result is shown to arise from cross phase modulation imposed on the visible pulse by fundamental solitons. Results from similar experiments with picosecond and nanosecond dual-wavelength pumping show the same qualitative behaviour and demonstrate that the governing mechanism in all cases is soliton fission and subsequent cross phase modulation of the co-propagating visible pulse.

We demonstrate a continuously electronically tunable (435-1150 nm) ultrafast source for fluorescence imaging applications that is derived from a visible supercontinuum generated by injecting infrared femtosecond pulses in to a microstructured fibre. We demonstrate this source applied to confocal and wide-field microscopy, as well as multiwell-plate imaging. We also report first application of a tunable supercontinuum source (TSS) to FLIM and to in situ measurements of relative excitation spectra. We are currently applying this source to hyperspectral imaging and working towards a system for acquiring the full fluorescence excitation-emission matrix. Fluorescence lifetime imaging (FLIM) can provide further information about fluorescent samples and requires a rapidly modulated or pulsed light source, in general, (tunable) ultrafast light sources, particularly in the visible, are complex and expensive and usually require a significant degree of adjustment to achieve tunability

We demonstrate an optically sectioned fluorescence lifetime imaging microscope with a wide-field detector, using a convenient, continuously tunable (435-1150 nm) ultrafast source for fluorescence imaging applications that is derived from a visible supercontinuum generated in a microstructured fiber.

Multi-beam confocal sectioned fluorescence lifetime imaging microscopy is demonstrated using a Yokogawa spinning disk. The single-photon excitation source is a supercontinuum generated from a Ti:sapphire seeded photonic crystalline fibre.

A model for THz generation by optical rectification using tilted-pulse-fronts is developed. It simultaneously accounts for (i) the spatio-temporal distortions of the optical pump pulse, (ii) the nonlinear coupled interaction of THz and optical radiation in two spatial dimensions (2-D), (iii) self-phase modulation and (iv) stimulated Raman scattering. The model is validated by quantitative agreement with experiments and analytic calculations. We show that the optical pump beam is significantly broadened in the transverse-momentum (kx) domain as a consequence of the spectral broadening caused by THz generation. In the presence of this large frequency and transverse-momentum (or angular) spread, group velocity dispersion causes a spatio-temporal break-up of the optical pump pulse which inhibits further THz generation. The implications of these effects on energy scaling and optimization of optical-to-THz conversion efficiency are discussed. This suggests the use of optical pump pulses with elliptical beam profiles for large optical pump energies. It is seen that optimization of the setup is highly dependent on optical pump conditions. Trade-offs of optimizing the optical-to-THz conversion efficiency on the spatial and spectral properties of THz radiation is discussed to guide the development of such sources., 21 pages, 8 figures

We both theoretically and experimentally investigate the optimization of femtosecond Yb-doped fiber amplifiers (YDFAs) to achieve high-quality, high-power, compressed pulses. Ultrashort pulses amplified inside YDFAs are modeled by the generalized nonlinear Schrödinger equation coupled to the steady-state propagation-rate equations. We use this model to study the dependence of compressed-pulse quality on the YDFA parameters, such as the gain fiber's doping concentration and length, and input pulse pre-chirp, duration, and power. The modeling results confirmed by experiments show that an optimum negative pre-chirp for the input pulse exists to achieve the best compression.

We introduce Bessel-Gauss beam enhancement cavities that may circumvent the major obstacles to more efficient cavity-enhanced high-field physics such as high-harmonic generation. The basic properties of Bessel-Gauss beams are reviewed and their transformation properties through simple optical systems (consisting of spherical and conical elements) are presented. A general Bessel-Gauss cavity design strategy is outlined, and a particular geometry, the confocal Bessel-Gauss cavity, is analyzed in detail. We numerically simulate the confocal Bessel-Gauss cavity and present an example cavity with 300 MHz repetition rate supporting an effective waist of 33 μm at the focus and an intensity ratio from the focus to the cavity mirror surfaces of 1.5 × 10(4).

We show that the peak powers of ytterbium-doped fiber chirped pulse amplification (CPA) can be scaled by at least 1 order of magnitude (in the transform limit) as compared to current systems by using a different spectral region of operation. A simple and fast model for saturated broadband fiber CPA systems is developed and applied to study the impact of the interplay between the spectrally dependent small signal gain and the saturation on the output bandwidth. The influence of self-phase modulation on the recompression of the pulse is discussed. It can be shown that the novel operation regime exhibits superior performance even if nonlinear effects are considered. The numerical results are significant for the design of the next generation of ultrafast high power fiber lasers.

Using phase shaping, the impact of the Kerr effect in a fiber-based chirped-pulse amplification (CPA) system is experimentally controlled. The technique is based on an analytical model describing the spectral phase owing to self-phase modulation in CPA systems. The method relies neither on complex phase measurements nor on time-consuming optimization routines. Nearly transform-limited pulses with energies as high as 1 mJ are produced, and a B integral being as high as 8 rad is accumulated in the main amplifier. The value of the B integral is determined by the method itself.

An efficient and simple approach for converting pulsed near-IR laser radiation into visible and mid-IR light by exploiting degenerate four-wave-mixing in an endlessly single-mode, large-mode-area photonic-crystal fiber is presented. Coupling a 1 MHz, 200 ps, 8 W average power pulsed source emitting at 1064 nm into this fiber results in average powers of 3 W at 673 nm signal wavelength and of 450 mW at 2539 nm idler wavelength, respectively. The excellent pulse energy conversion efficiencies of 35% for the signal and 6% for the idler wavelength are due to the unique combination of characteristics of this type of fiber. (C) 2009 Optical Society of America

We report on a significant improvement of the total bandwidth amplified in an optical parametric process. By pumping a parametric amplifier with a broadband pump, we demonstrate amplification of a supercontinuum whose spectrum expands over nearly an octave ranging from less than 600 nm up to 1200 nm. Our amplifier stage is set to provide amplification at degeneracy in the quasi-collinear configuration with a temporally as well as angularly dispersed pump.

We experimentally demonstrate compensation of the impact of Kerr-nonlinearity with positive dispersion in chirped-pulse systems. The condition for the phase-compensation is derived and design guidelines are presented. The technique is shown with a fiber-based system employing conventional diffraction gratings as well in a system that is based on chirped volume Bragg-gratings. Practical requirements on the stretching unit are discussed.

It is analytically shown that weak initial spectral phase modulations cause a pulse-contrast degradation at the output of nonlinear chirped-pulse amplification systems. The Kerr-nonlinearity causes an energy-transfer from the main pulse to side-pulses during nonlinear amplification. The relative intensities of these side-pulses can be described in terms of Bessel-functions. It is shown that the intensities of the pulses are dependent on the magnitude of the accumulated nonlinear phase-shift (i.e., the B-integral), the depth and period of the initial spectral phase-modulation and the slope of the linear stretching chirp. The results are applicable to any type of laser amplifier that is based on the technique of chirped-pulse amplification. The analytical results presented in this paper are of particular importance for high peak-power laser applications requiring high pulse-contrasts, e.g. high field physics.

The choice of optimum phase-matching conditions for nobcollinear optical parametric amplifiers is usually made on the basis of the linear spectral dispersion characteristics of the anisotropic nonlinear crystal. However, for high-peak-power operation, where pump depletion is involved, it is shown that the tolerance of the parametric gain with regard to k-vector mismatch is to change the optimum phase-matching parameters. Our calculations show that, with the revised parameters, an enhancement in peak power approaching 50% could be achieved.

We report on an ytterbium-doped fiber based chirped-pulse amplification system delivering 100 microJ pulse energy at a repetition rate of 900 kHz, corresponding to an average power of 90 W. The emitted pulses are as short as 500 fs. To the best of our knowledge, this is the highest average power ever reported for high-energy femtosecond solid-state laser systems.

Our investigation is focused on the influence of the temporal pulse shape in chirped pulse fiber amplifier systems in which the recompression is limited due to a nonlinear chirp caused by self-phase modulation (SPM). We show that specific pulse shapes can overcome the usual limit but have to be maintained during amplification, which is not fulfilled if gain shaping occurs. We analyze numerically the possibility of spectral preshaping in order to prevent gain shaping and nonlinear distortion due to SPM.

We report on an ytterbium-doped fiber CPA system delivering millijoule level pulse energy at repetition rates above 100 kHz corresponding to an average power of more than 100 W. The compressed pulses have a pulse duration of 800 fs. To our knowledge, this constitutes the highest pulse energy ever extracted from a fiber based femtosecond laser system and nearly two orders of magnitude higher repetition rate than in previously published mJ-level fiber CPA systems.

An extended bandwidth of the amplified pulse is a key option to increase the peak-power from Yb-doped fiber CPA-systems. We analyze the gain characteristics of state-of-the-art Yb-doped fiber amplifiers with regard to the broadest gain-bandwidth.

We present two scaling concepts for fiber based system. Ultrashort pulses with high peak power are generated while maintaining the advantages of fiber laser systems such as high repetition rate and good beam quality.

The unique properties of a 10µm endlessly-single-mode LMA PCF allowed for the efficient conversion of a 8W @ 1MHz fiber amplified 200ps, 1064nm microchip laser to 3W @ 673nm and 450mW @ 2.539µm by degenerated FWM. Scaling potential as well as tuneability options are supported by numerical simulations and will be discussed in detail.

In the past, generation of high power, pulsed, red light was performed using dye lasers, OPOs or second harmonic generation of the 1.3µm transitions from neodymium based solid state lasers [1]. Using FWM in photonic-crystal fibers (PCF) in combination with pulsed 1µm laser sources has so far only accessed the spectral region around 730nm [2]. We present a simple approach to generate picosecond or nanosecond pulses in the visible by efficient FWM in an endlessly single-mode large mode area (LMA) PCF fiber. At a moderate pump peak power of 40kW the conversion efficiency can reach 35%. When pumping the fiber at 1030/1064nm, the resulting wavelength in the visible is 626/673nm, making it interesting for many applications like photodynamic therapy and fluorescence spectroscopy [3]. The experimental setup used to demonstrate the effect consists of the laser source emitting 50ps pulses at 1064nm at a repetition rate of 100kHz with variable peak power. This light is coupled into an endlessly single mode LMA PCF fiber with a length of ∼1m and a core diameter of 10µm. The spectrum at the output of the fiber shows two narrow peaks, the pump wavelength at 1064nm and the FWM generated idler wavelength at 673nm. No supercontinuum trace or Raman scattering can be observed as can be seen in figure 1. At an input peak power of 30kW, the conversion efficiency starts to saturate at ∼35% as shown in figure 2. This high conversion efficiency can be explained by the excellent modal overlap provided by the endlessly single mode nature of the fiber at all wavelengths. Our simulations show that the contribution of the waveguide dispersion for phase matching is almost negligible when employing fused silica PCF fibers with core diameters ≥10µm. This means that with bigger cores even higher peak powers could be handled while still fulfilling the phase matching conditions. In our institute, a master oscillator fiber amplifier system employing a LMA PCF fiber amplifier with a core diameter of 40µm delivered up to 700mW at 632nm wavelength, 100ps pulse duration and a repetition rate of 100kHz. The conversion efficiency from 1064nm to 632nm was ∼18%. The phase match diagram for the LMA PCF fiber with a 10µm core used in our experiment is presented in figure 3. It indicates a broad tuning range from ∼600 to ∼750nm when using a tuneable pump source covering the spectrum between 1020 and 1100nm, e.g. a tuneable pulsed fiber laser.