We present a femtosecond laser system at 920 nm delivering ultrashort pulses via a hollow-core photonic bandgap fiber (HC-PBGF). The laser system is designed to simplify two-photon microscopy applications and can be used for miniaturized two-photon microscopes. While previously presented solutions have been tailored to a specific length and dispersion coefficient of the HC-PBGF, we now show a compact and flexible scheme for dispersion compensation which is compatible with a wide range of fiber types and lengths.
In addition, this new approach fully maintains the capability of software-controlled dispersion compensation in the range from 0 to -40,000 fs2 after the pulse delivery fiber. Hence, the dispersion of common two-photon microscopes can be pre-compensated in order to obtain compressed pulses at the sample plane. Our newly developed system displays excellent long-term fiber coupling stability under varying environmental conditions. It is capable of polarization-preserving femtosecond pulse delivery at 920 nm and reaches Watt-level power after the delivery fiber, making it suitable for in-vivo brain imaging of GCaMP in mouse models.
Traveling-Wave Thomson-Scattering (TWTS) allows for the realization of optical free-electron lasers (OFELs) from the interaction of short, high-power laser pulses with brilliant relativistic electron bunches. The laser field provides the optical undulator which is traversed by the electrons. In order to achieve coherent amplification of radiation through electron microbunching the interaction between electrons and laser must be maintained over hundreds to thousands of undulator periods. Traveling-Wave Thomson-Scattering is the only scattering geometry so far allowing for the realization of optical undulators of this length which is at the same time scalable from extreme ultraviolet to X-ray photon energies. TWTS is also applicable for the realization of incoherent high peak brightness hard X-ray to gamma-ray sources which can provide orders of magnitude higher photon output than classic head-on Thomson sources.
In contrast to head-on Thomson sources TWTS employs a side-scattering geometry where laser and electron propagation direction of motion enclose an angle. Tilting the laser pulse front with respect to the wave front by half of this interaction angle optimizes electron and laser pulse overlap. In the side-scattering geometry the tilt of the pulse-front compensates the spatial offset between electrons and laser pulse-front which would be present otherwise for an electron bunch far from the interaction point where it overlaps with the laser pulse center. Thus the laser pulse-front tilt ensures continuous overlap between laser pulse and electrons while these traverse the laser pulse cross-sectional area. This allows to control the interaction distance in TWTS by the laser pulse width rather than laser pulse duration as is the case for head-on Thomson scattering. Utilizing petawatt class laser pulses with millimeter to centimeter scale width allows for the realization of compact optical undulators with thousands of periods.
When laser pulses for TWTS are prepared, care has to be taken of laser dispersion. Especially for scenarios featuring interaction angles of several ten to over one hundred degree the angular dispersion originating from laser pulse-front tilt can significantly prolong the pulse duration during the interaction which leads to a decrease in optical undulator amplitude and eventually terminates the interaction long before the target interaction distance is reached. In the talk it is shown how a pair of two gratings can be used to first generate the pulse-front tilt and second control and compensate dispersion during the interaction by utilizing the plane of optimum compression. Furthermore an experimental setup strategy is presented allowing for an interaction outside the laser pulse focus. This is a necessity for TWTS OFELs requiring focusing to reach optical undulator strengths on the order of unity since the centimeter scale laser pulse width at the interaction point result in turn in Rayleigh lengths on the order of one hundred meter and thus in laser focusing distances of several hundred meter. The talk shows how an out-of-focus interaction geometry utilizing strong focusing of the incident laser pulse needs to be designed in order to regain compactness by reducing the focusing distance by one to two orders of magnitude.
We introduce the directly diode-pumped PEnELOPE laser-system which is designed for a pulse energy of 150 J, a repetition rate of 1Hz and a pulse duration of 120 fs. The principle setup of amplifier and stretcher-compressor system as well as the pumping, energy extraction and cooling scheme of the power amplifiers will be reported. In this paper we focus on numerical modeling as well as design studies.
To further scale the peak-power of state-of-the-art fiber CPA-systems, a careful optimization of the spectral as well as
temporal dynamics is required. The wavelength dependence of the small-signal gain, as well as the saturation of the
amplifier, strongly affect the signal bandwidth. For unsaturated amplifiers only a spectral optimization is required. It
can be shown that both the spectral center and the width of the input spectrum strongly affect the output bandwidth. An
optimization regarding these two parameters will be given. Design guidelines are presented. We develop a simple yet
efficient model to simulate the impact of saturation in broadband Ytterbium-doped fiber CPA-systems. Using this
model, we reveal that significant peak-power scaling up to 10 GW of current fiber CPA-systems is possible.
We present an experimental study of the drilling of metal targets with ultrashort laser pulses with pulse durations from
800 fs to 19 ps at repetition rates up to 1 MHz, average powers up to 70 Watts, using an Ytterbium-doped fiber CPA
system. Particle shielding and heat accumulation have been found to influence the drilling efficiency at high repetition
rates. Particle shielding causes an increase in the number of pulses for breakthrough. It occurs at a few hundred kHz,
depending on the pulse energy and duration. The heat accumulation effect is noticed at higher repetition rates. Although
it overbalances the particle shielding thus making the drilling process faster, heat accumulation is responsible for the
formation of a large amount of molten material that limits the hole quality. The variations of the pulse duration reveal
that heat accumulation starts at higher repetition rates for shorter pulse lengths. This is in agreement with the observed
higher ablation efficiency with shorter pulse duration. Thus, the shorter pulses might be advantageous if highest
precision and processing speed is required.
We present a degenerated-parametric amplifier with gigawatt peak power operating at 1030 nm and 30 kHz repetition
rate. Pulses of a fiber chirped pulse amplification (FCPA) system with 650 fs pulse duration and 1 mJ pulse energy are
frequency doubled and used as pump source for a two stage optical parametric amplifier. Both the FCPA and the optical
parametric amplifier (OPA) are seeded by the same YB:KGW oscillator. Spectral broadening of the OPA seed signal in a
short-polarization-maintaining-step-index fiber creates enough bandwidth for sub 30 fs pulse generation, while temporal
synchronization of pump and signal is realized by means of a multipass cell in the OPA signal beam path. Parametric
amplification of the broadband signal takes place in two 1 mm BBO crystals. Pulse compression via chirped mirrors
yields 81 μJ pulses as short as 29 fs. The corresponding pulse peak power is estimated to be as large as 2 GW. Together
with the good beam quality (measured M2<1.8) this device enables high intensity experiments at high repetition rates.
We report on an ytterbium-doped fiber CPA system delivering 325 W of average power at 40 MHz repetition rate
corresponding to 8.2 μJ pulse energy. The pulse duration is as short as 375 fs resulting in 22 MW of peak power.
We report on a high repetition rate noncollinear optical parametric amplifier system (NOPA) seeded by a cavity
dumped Ti:Sapphire oscillator. The pump pulses for parametric amplification are generated via soliton generation in a
highly nonlinear photonic crystal fiber with a subsequent fiber-based amplification stage and are therefore
synchronized. The system is capable of producing high energy ultra-short pulses at repetition rates up to 2 MHz.
We show spectral combination of pulsed fiber laser systems for the first time to our knowledge. In this proof of principle
experiment, two directly modulated wavelength-stabilized tunable external cavity diode lasers (ECDL) serve as
independent seed sources. Each signal is amplified in a two stage ytterbium-doped fiber amplifier. The spatial overlap is
created using a transmission grating with a combining efficiency as high as 92 %. No beam quality degradation has been
observed for the combined beam compared to a single emission. An electronic delay is used to adjust the temporal
overlap of the pulses from the spatially separated amplifier setups. The presented approach offers an enormous scaling
potential of pulsed fiber laser systems, which are generally limited by nonlinear effects or fiber damage. We show that
the huge gain bandwidth of Yb-doped fiber amplifiers and the high diffraction efficiency of dielectric reflection gratings
in this wavelength range yield potential for a combination of up to 50 channels. For state-of-the-art ns-amplifier systems
> 100 MW of peak power, > 100 mJ of pulse energy and average powers of > 10 kW seem feasible.
In this contribution we report a high repetition rate optical parametric amplifier (OPA) pumped by a chirped pulse fiber amplifier system. Fiber CPA systems have demonstrated power scaling and open the route to OPAs at repetition rates in the 100 kHz-10MHz range. The OPA stage is seeded by a continuum generated in a Sapphire plate and more than 50 nm bandwidth is efficiently amplified, resulting in 3 &mgr;J, 29 fs pulses.
We report on an optical parametric amplification system which is pumped and seeded by fiber generated laser radiation.
Due to its low broadening threshold, high spatial beam quality and high stability, the fiber based broad bandwidth signal
generation is a promising alternative to white light generation in bulky glass or sapphire plates. As pump source we
propose the use of a high repetition rate ytterbium-doped fiber chirped pulse amplification system.
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 &mgr;J.
The different concepts of combining fiber lasers for power-scaling are discussed. We report on three combined fibers with an output power of 100 W. Several proposals are made for further power scaling and the capacitance of a grating is tested in a simulation-experiment.
We report on the generation of 50 fs pulses with an average output power of more than 50W. This is done by combining a high average power fiber CPA system with a microstructured large-mode-area fiber for nonlinear compression. The fiber CPA system delivers 300 fs pulses with a repetition rate of 73MHz in a linearly polarized beam with diffraction-limited quality. The average output power can exceed 100W. Nonlinear compression of these pulses is done by launching the light into a very short piece of a microstructured fiber and then removing the phase with a pair of chirped mirrors.
The performance of high average power and high energy femtosecond fiber laser systems is discussed. Remarkable evolutions in fiber technology made it possible to overcome restrictions due to nonlinear pulse distortions in the amplification fiber and revealed the full potential of rare-earth-doped fibers as a power-scalable solid-state laser concept in the short pulse regime. State-of-the-art femtosecond fiber lasers in our labs deliver average powers well above 100 W and pulse energies of several 100 μJ in the 1 μm wavelength region. This performance, in particular the significantly higher repetition rate compared to conventional femtosecond lasers, allows for unique approaches in several application fields. Beside the fiber designs, the setup, performance and limitations of these systems we will discuss selected applications.
Photonic crystal fibers consisting of a solid fused silica core surrounded by a regular array of sub-micron air-holes have been shown to operate with single mode core well above 30 μm in active laser geometries as well as passive beam delivery fibers. Novel designs based on the combination of stress applying elements that are index matched to the holey cladding have recently been emerged. In this report we summarize the properties of these polarization maintaining photonic crystal fibers. Beside the characterization of the polarizing window and birefringence, high power laser and amplifier configurations using these fibers are demonstrated and first experiments concerning the temperature sensitivity of the polarizing properties are presented.
The generation of high energy femtosecond pulses in Optical Parametric Amplifier (OPA) pumped by fiber laser at a repetition rate of 1MHz is reported. Highly nonlinear fibers are used to create an intrinsically synchronized signal for the parametric amplifier. Seeding the OPA by a supercontinuum generated in a photonic crystal fiber, large tunability extending from 700 nm to 1500 nm of femtosecond pulses is demonstrated, with pulse energies as high as 1.2 μJ. Generating the seed using only SPM in a standard fiber, broadband amplification over more than 85 nm and subsequent compression down to 46 fs in a prism sequence are achieved. Pulse peak powers pulses above 10 MW together with 0.5 W of average power is achieved. This system appears to be very interesting due to scalability of pulse energy and average power of both involved concepts: fiber laser and parametric amplifier.
In the last years a dramatic increase of the output power of rare-earth-doped fiber lasers and amplifiers with diffraction limited beam quality has been observed. These demonstrates impressively that fiber lasers and amplifiers are an attractive and power scalable solid-state laser concept. The main limiting factors for the laser output power are the damage of the fiber ends, heating of the fiber due to the quantum defect and nonlinear effects. To overcome these problems, an increasing of the core diameter and keeping the fiber single mode, by using solid core step-index large-mode-area fibers, allow the power scaling beyond 1 kW at diffraction limited beam quality. A further scaling is possible by using novel highly doped air-clad photonic crystal fibers with increased mode field diameters of the active core. This type of fibers has several new preferable features. In our contribution we will discuss the advantages of microstructured fibers to reduce nonlinear effects inside the fiber and the possibility to scale the output power of fiber lasers and amplifiers with excellent beam quality. We also show experiments with pulsed fiber amplifier systems using these microstructured large mode area fibers.
We report on the high power fiber based amplification of parabolic pulses. The output is compressed using transmission gratings to 300 fs and an average power of 38 W at 75 MHz repetition rate.
The fiber based generation of nearly transform-limited 10-ps pulses with 200 kW peak power (97 W average power) based on SPM-induced spectral compression is reported. Efficient second harmonic generation applying this source is also discussed.
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.
Experimental results based on rare-earth-doped fibers have impressively shown that fiber lasers and amplifiers are an attractive and power scalable solid-state laser concept. Based on ytterbium-doped large-mode-area double-clad fibers, in the continuous regime, output powers approaching the kW-range with diffraction limited beam quality have been shown. Average output powers in the order of 100 W have been demonstrated in the pulsed regime even for femtosecond fiber lasers. Further power scaling is limited by the end facets damage, thermo-optical problems or nonlinear effects. To overcome these restrictions microstructured fibers with several new preferable features can be used. In our contribution we will discuss power scaling of fiber lasers and amplifiers in the multi kW-range with excellent beam quality based on rare-earth-doped photonic crystal fibers.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.