Thermal effects in high average power optical parametric amplifiers

High-efficiency, single-stage tunable optical parametric amplifier for visible photocathode applications

We present a single-stage optical parametric amplifier (OPA) with an average conversion efficiency up to 38%, tunable between 1.01 and 1.18 µm. The OPA seed is produced by a gain-managed nonlinear fiber amplifier. Numerical modeling of the seed pulse generation shows a linear chirp, a smoothly broadened redshifted spectrum, and a high spectral energy density. When up-converted to the visible through second-harmonic generation, the signal pulses are suitable for visible photocathode excitation.

Adding a second AgGaS2 stage to Ti:sapphire/BBO/AgGaS2 setups increases mid-infrared power twofold

We present a method for increasing the power of mid-infrared laser pulses generated by a conventional beta-barium borate (BBO) optical parametric amplifier (OPA) and AgGaS2 difference frequency generation (DFG) pumped by a Ti:sapphire amplifier. The method involves an additional stage of parametric amplification with a second AgGaS2 crystal pumped by selected outputs of the conventional DFG stage. This method does not require additional pump power from the Ti:sapphire laser source and improves the overall photon conversion efficiency for generating mid-infrared light. It merely requires an additional AgGaS2 crystal and dichroic mirrors. Following difference frequency generation, the method reuses near-infrared light (∼1.9 µm), typically discarded, to pump the additional AgGaS2 stage and amplifies the mid-infrared light twofold. We demonstrate and characterize the power, spectrum, duration, and noise of the mid-IR pulses before and after the second AgGaS2 stage. We observe small changes in center frequencies, bandwidth, and pulse duration for ∼150-fs pulses between 4 and 5 µm.

Polarization-based idler elimination: enhancing the efficiency of optical parametric amplification

This study presents a novel way to increase the energy conversion efficiency of optical parametric amplification by eliminating the idler wave from the interaction using consecutive type-I and type-II amplification processes. By using the aforementioned straightforward approach the wavelength tunable narrow-bandwidth amplification with exceptionally high 40% peak pump-to-signal conversion efficiency and 68% peak pump depletion was achieved in the short-pulse regime, while preserving the beam quality factor of less than 1.4. The same optical layout can also serve as an enhanced idler amplification scheme.

Mid-infrared quasi-parametric chirped-pulse amplification based on Sm:LGN crystals

We numerically demonstrate highly efficient mid-infrared quasi-parametric chirped-pulse amplification (QPCPA) based on a recently developed Sm³⁺-doped La₃Ga_5.5Nb_0.5O₁₄ (Sm:LGN) crystal. At pump wavelength around 1 µm, the broadband absorption of Sm³⁺ on idler pulses can enable QPCPA for femtosecond signal pulses centered at 3.5 or 5 µm, with a conversion efficiency approaching the quantum limit. Due to suppression of back conversion, such mid-infrared QPCPA exhibits robustness against phase-mismatch and pump-intensity variation. The Sm:LGN-based QPCPA will provide an efficient approach for converting currently well-developed intense laser pulses at 1 µm to mid-infrared ultrashort pulses.

Simultaneously Wavelength- and Temperature-Insensitive Mid-Infrared Optical Parametric Amplification with LiGaS2 Crystal

Ultrafast mid-infrared (mid-IR) lasers with a high pulse repetition rate are in great demand in various fields, including attosecond science and strong-field physics. Due to the lack of suitable mid-IR laser gain medium, optical parametric amplifiers (OPAs) are used to generate an ultrafast mid-IR laser. However, the efficiency of OPA is sensitive to phase mismatches induced by wavelength and temperature deviations from the preset points, which thus limits the pulse duration and the average power of the mid-IR laser. Here, we exploited a noncollinear phase-matching configuration to achieve simultaneously wavelength- and temperature-insensitive mid-IR OPA with a LiGaS2 crystal. The noncollinearity can cancel the first-order dependence of phase matching on both wavelength and temperature. Benefitting from the thermal property of the LiGaS2 crystal, some collinear phase-matching solutions derived from the first-order and even third-order wavelength insensitivity have comparatively large temperature bandwidths and can be regarded as approximate solutions with simultaneous wavelength and temperature insensitivity. These simultaneously wavelength- and temperature-insensitive phase-matching designs are verified through numerical simulations in order to generate few-cycle, high-power mid-IR pulses.

Nonlinear absorption in lithium triborate frequency converters for high-power ultrafast lasers

We report on an analysis of the nonlinear absorption in lithium triborate (LBO) used for second and third harmonic generation of ultrashort laser pulses at average powers in the order of kW and with sub-picosecond pulse duration. Thermographic imaging of the LBO crystals together with a simple analytical model revealed the presence of nonlinear absorption in both harmonic generation processes. Subsequent processing with a numerical model considering the nonlinear mixing, the absorption, and the heat conduction was used to estimate the absorption coefficients. Average powers exceeding 100 W in the ultraviolet and 400 W in the visible spectral range were obtained while maintaining a good beam quality by avoiding excessive nonlinear absorption.

Wavefront Restoration Technology of Dynamic Non-Uniform Intensity Distribution Based on Extreme Learning Machine

Non-uniform intensity distribution of laser near-field beam results in the irregular shape of the spot in the wavefront sensor. The intensity of some sub-aperture spots may be too weak to be detected, and the accuracy of wavefront restoration is seriously affected. Therefore, an extreme learning machine method is proposed to realize high precision wavefront restoration under dynamic non-uniform intensity distribution. The simulation results show that this method has better accuracy of wavefront restoration than the classical modal algorithm under dynamic non-uniform intensity distribution. The root mean square error of the residual wavefront for the proposed method is only 2.9% of the initial value.

Analysis of angular dispersion induced by wavefront rotation in nanosecond optical parametric chirped pulse amplification

Angular dispersion observed in a nanosecond optical parametric chirped-pulse amplification (ns-OPCPA) amplifier adopted in the frontend of a multi-PW laser was analyzed. The theory on the angular dispersion, extended by including the wavefront rotation and the pulse front tilt of a strongly chirped laser pulse, revealed that the wavefront rotation is a major contributor to the angular dispersion, as compared to the pulse front tilt, in a ns-OPCPA amplifier. It was also shown that the wavefront rotation could be introduced by the phase mismatch and the noncollinear propagation angle in the noncollinear ns-OPCPA amplifier. The theoretical prediction was experimentally verified by measuring the angular dispersion of the ns-OPCPA frontend installed in the 20-fs, 4-PW Ti:Sapphire laser. We emphasize the importance of the proper characterization and control of the angular dispersion in the ns-OPCPA amplifier since the focus intensity of an ultrahigh power laser could be significantly reduced due to the spatiotemporal effect even for small induced angular dispersion.

Temperature-insensitive broadband optical parametric chirped pulse amplification based on a tilted noncollinear QPM design

Ultrafast pulsed laser of high intensity and high repetition rate is the combined requisite for advancing strong-field physics experiments and calls for the development of thermal-stable ultrafast laser systems. Noncollinear phasing matching (PM) is an effective solution of optimizing the properties of optical parametric chirped pulse amplification (OPCPA) to achieve broadband amplification or to be temperature-insensitive. But as a cost, distinct noncollinear geometries have to be respectively satisfied. In this paper, a noncollinear quasi-phase-matching (QPM) scheme of both temperature- and wavelength-insensitive is presented. With the assistance of the design freedom of grating wave vector, the independent noncollinear-angle requirements can be simultaneously realized in a tilted QPM crystal, and the temperature-insensitive broadband amplification is achieved. Full-dimensional spatial-temporal simulations for a typical 1064 nm pumped mid-IR OPCPA at 3.4 µm are presented in detail. Compared with a mono-functional temperature-insensitive or broadband QPM scheme, the presented QPM configuration shows a common characteristic that simultaneously optimizes the thermal stability and the gain spectrum. Broadband parametric amplification of a ∼40 fs (FWHM) pulsed laser is achieved with no signs of gain-narrowing. Both of the beam profiles and the amplified spectra stay constant while the temperature is elevated by ∼100°C. Finally, influence of the QPM grating errors on the gain spectrum is discussed.

Power-scaling of nonlinear-mirror modelocked thin-disk lasers

We present a first power-scaled nonlinear-mirror (NLM) modelocked thin-disk laser based on an Yb-doped gain material. The laser oscillator delivers average output powers up to 87 W and peak powers up to 14.7 MW with sub-600-femtosecond pulses at ≈9-MHz repetition rate. We demonstrate a threefold improvement in average output power and sixfold improvement in pulse energy compared to previous NLM-modelocking results. We obtain peak powers in excess of 10 MW for the first time from an NLM-modelocked laser oscillator. In our laser, the NLM is assisted by a semiconductor saturable absorber mirror (SESAM) to reliably initiate pulsed operation. We validate the high-power suitability of the NLM modelocking technique using low-absorption χ⁽²⁾ crystals and optimized dichroic-mirror coating designs. Furthermore, we discuss stability against Q-switching and study how the tuning of the nonlinear mirror affects the laser performance.

Measurements of heat flow from surface defects in lithium triborate

We present what is, to our knowledge, the first measurement of temperature distributions in a nonlinear optic resulting from absorption in a localized surface defect. These measurements were performed on principal cut samples of lithium triborate with damage spots centered on their front surfaces, pumped by a kW-scale continuous-wave laser. The changes in optical-path length associated with this heating were measured with a Mach-Zehnder interferometer, from which the temperature distribution could be inferred. These distributions have sharper features with larger magnitudes than would be expected with bulk-absorption heating. Comparison with both numerical and analytical models is used to qualify the measurements and to estimate the total power absorbed at a given site using this bulk material response. While sensitivity is dependent on the properties of the material of study, we demonstrate measurements of absorption levels of one part in 10⁵.

High average power 88 W OPCPA system for high-repetition-rate experiments at the LCLS x-ray free-electron laser

We present a 100 kHz, sub-20 fs optical parametric chirped-pulse amplifier (OPCPA) system delivering 88.6 W average power at a center wavelength of 800 nm. The seed pulses are derived from the pump laser via white-light continuum generation and are amplified in three non-collinear OPCPA stages. The final two high-power stages are pumped with a 661 W Yb:YAG InnoSlab amplifier. A simple and robust design is used for the OPCPA system to avoid thermal effects and enhance long-term stability, resulting in excellent beam quality and high conversion efficiency. To the best of our knowledge, this is the highest average power OPCPA system reported to date.

Comprehensive analyses of the thermal insensitive noncollinear phase-matching for power scalable parametric amplifications

Thermal-induced phase-mismatch distortion, which will dramatically deteriorate the efficient energy transfer, has become a critical obstacle to power scaling of optical parametric amplifiers. To ease this efficiency deterioration, the noncollinear optical parametric amplification (OPA) configuration widely employed to achieve broadband phase-matching (PM) may also serve as a promising approach to optimize the temperature acceptance. In this paper, starting from the noncollinear wave-vector equations, a required thermo- and angle-relationship analogous to that of noncollinear broadband PM is firstly inferred. Based on the presented mathematical criterion, we have explored the potential spectral boundaries of this ingenious temperature insensitive OPA scheme. Full-dimensional simulations of two types of typical OPA processes were quantitatively discussed. Compared with traditional collinear PM designs, the presented noncollinear PM configurations show significant common characteristics on improving the temperature acceptance and subsequently the overall amplification efficiency. For a typical high power parametric process of the 532 nm pumped near-IR OPA at 800 nm especially, incredible temperature bandwidth exceeding 8000 °C was obtained while a YCOB (xz plane) crystal is adopted. What is more, it can also be applied to ameliorate the gain-spectrum thermo-instability of OPA.

Temperature-insensitive frequency conversion by phase mismatch self-compensation in the same type of crystals

Aiming at high-power laser frequency conversion, we present a new scheme that can self-compensate for the thermally induced phase mismatch. The basic design of the scheme is that three crystals with the same type are cascaded, of which the crystals at both ends are used for frequency conversion and the middle crystal is used for compensating phase mismatch. By configuring the polarization states of the interacting waves in the middle crystal, the sign of the first temperature derivative of the phase mismatch is opposite to that of the frequency conversion crystals. The thermally induced phase mismatch in the first crystal can thus be self-compensated in the middle crystal. To verify the utility of the proposed scheme, we experimentally demonstrated temperature-insensitive second and third harmonic generation using KH₂PO₄ crystals. The results show that the temperature acceptance bandwidth is about two times larger than that of using a single crystal. Since the crystals used are of the same type, this scheme has excellent universal applicability and is almost completely free from the limitations of the laser wavelength, crystal and phase-matching type. Therefore, the scheme can be widely applied to various frequency conversion processes and is scarcely any limitations.

Ultra-short pulse fiber optical parametric oscillator

This work explores the frequency conversion and generation of short pulses with an optical parametric oscillator based on micro-structured fibers. Depending on the operation regime, the optical cavity can either behave as a normal-dispersion cavity delivering linearly chirped pulses, which were externally compressed down to only 26 fs, or as a dispersion-managed oscillator, which directly delivered compressed pulses with a pulse duration of only 39 fs.

Thermal effects in an ultrafast BiB3O6 optical parametric oscillator at high average powers

An ultrafast, high-average-power, extended-cavity, femtosecond BiB₃O₆ optical parametric oscillator was constructed as a test bed for investigating the scalability of infrared parametric devices. Despite the high pulse energies achieved by this system, the reduction in slope efficiency near the maximum-available pump power prompted the investigation of thermal effects in the crystal during operation. The local heating effects in the crystal were used to determine the impact on both phase matching and thermal lensing to understand limitations that must be overcome to achieve microjoule-level pulse energies at high repetition rates.

Highly stable, 54mJ Yb-InnoSlab laser platform at 0.5kW average power

We report on the leap of Yb-InnoSlab laser technology towards high pulse energies of 54mJ combined with high average power exceeding half a kW. The system features pulse durations of 1.5 picoseconds (ps) at 10kHz repetition rate with excellent beam properties (M² of 1.1) combined with superb power and pointing stability in the sub-% range. It provides different output ports to facilitate optical synchronization for pumping parametric amplifiers. Tunable, femtosecond seed pulses are derived directly from the ps Yb pump pulses. We investigate the long term stability of this ps driven white light continuum and demonstrate 100-fold pulse compression down to 10fs duration. Ultra-broadband IR spectra centred at 2µm wavelength are subsequently generated via difference frequency generation of selected white light components.

Temperature- and wavelength-insensitive parametric amplification enabled by noncollinear achromatic phase-matching

Optical parametric chirped-pulse amplification (OPCPA) has been demonstrated to be a promising approach for pushing femtosecond pulses towards ultra-high peak powers. However, the future success of OPCPA strongly relies on the ability to manipulate its phase-matching (PM) configuration. When a high average power pump laser is involved, the thermal effects in nonlinear crystals induce phase-mismatch distortions that pose an inherent limitation on the conversion efficiency. Here, we demonstrate that the noncollinear configuration previously adopted for wavelength-insensitive PM can be employed for temperature-insensitive PM when the noncollinear angle is properly reset. Simultaneous temperature- and wavelength-insensitive PM is realized for the first time by imposing such a temperature-insensitive noncollinear configuration with an angularly dispersed seed signal. Based on the lithium triborate crystal, the proposed noncollinear achromatic PM has a thermal acceptance 6 times larger than that of the conventional wavelength-insensitive noncollinear PM and has a sufficient spectral acceptance to support pulse durations of ~20 fs at 800 nm. These achievements open new possibilities for generating ultra-high peak power lasers with high average power.

Energetic sub-2-cycle laser with 216 W average power

Few-cycle lasers are essential for many research areas such as attosecond physics that promise to address fundamental questions in science and technology. Therefore, further advancements are connected to significant progress in the underlying laser technology. Here, two-stage nonlinear compression of a 660 W femtosecond fiber laser system is utilized to achieve unprecedented average power levels of energetic ultrashort or even few-cycle laser pulses. In a first compression step, 408 W, 320 μJ, 30 fs pulses are achieved, which can be further compressed to 216 W, 170 μJ, 6.3 fs pulses in a second compression stage. To the best of our knowledge, this is the highest average power few-cycle laser system presented so far. It is expected to significantly advance the fields of high harmonic generation and attosecond science.

100 W average power femtosecond laser at 343 nm

We present a femtosecond laser system delivering up to 100 W of average power at 343 nm. The laser system employs a Yb-based femtosecond fiber laser and subsequent second- and third-harmonic generation in beta barium borate (BBO) crystals. Thermal gradients within these BBO crystals are mitigated by sapphire heat spreaders directly bonded to the front and back surface of the crystals. Thus, a nearly diffraction-limited beam quality (M2 < 1.4) is achieved, despite the high thermal load to the nonlinear crystals. This laser source is expected to push many industrial and scientific applications in the future.

Thin disk amplifier-based 40 mJ, 1 kHz, picosecond laser at 515 nm

We report on a frequency-doubled picosecond Yb:YAG thin disk regenerative amplifier, developed as a pump laser for a kilohertz repetition rate OPCPA. At a repetition rate of 1 kHz, the compressed output of the regenerative amplifier has a pulse duration of 1.2 ps and pulse energy of 90 mJ with energy stability of σ < 0.8% and M² < 1.2. The pulses are frequency doubled in an LBO crystal yielding 42 mJ at 515 nm.

Scalability of components for kW-level average power few-cycle lasers

In this paper, the average power scalability of components that can be used for intense few-cycle lasers based on nonlinear compression of modern femtosecond solid-state lasers is investigated. The key components of such a setup, namely, the gas-filled waveguides, laser windows, chirped mirrors for pulse compression and low dispersion mirrors for beam collimation, focusing, and beam steering are tested under high-average-power operation using a kilowatt cw laser. We demonstrate the long-term stable transmission of kW-level average power through a hollow capillary and a Kagome-type photonic crystal fiber. In addition, we show that sapphire substrates significantly improve the average power capability of metal-coated mirrors. Ultimately, ultrabroadband dielectric mirrors show negligible heating up to 1 kW of average power. In summary, a technology for scaling of few-cycle lasers up to 1 kW of average power and beyond is presented.

250 W average power, 100 kHz repetition rate cryogenic Yb:YAG amplifier for OPCPA pumping

A cryogenically cooled, bulk Yb:YAG, four-pass amplifier delivering up to 250 W average power at 100 kHz repetition rate is reported. The 2.5 mJ amplified optical pulses show a sub-20 ps duration before temporal compression and a spectrum supporting a transform-limited duration of 3.6 ps. The power instabilities were measured to be <0.5% rms over 30 min at full power, and the spatial intensity profile showed a flat-top distribution and near diffraction-limited beam quality. This compact amplifier is an ideal source for pumping either near-IR or mid-IR optical parametric chirped pulse amplifiers.

High-average-power, 50-fs parametric amplifier front-end at 1.55 μm

An average-power-scalable, two-stage optical parametric chirped pulse amplifier is presented providing 90-μJ signal pulses at 1.55 μm and 45-μJ idler pulses at 3.1 μm at a repetition rate of 100 kHz. The signal pulses were recompressible to within a few percent of their ~50-fs Fourier limit in anti-reflection coated fused silica at negligible losses. The overall energy conversion efficiency from the 1030-nm pump to the recompressed signal reached 19%, significantly reducing the cost per watt of pump power compared to similar systems. The two-stage source will serve as the front-end of a three-stage system permitting the development of novel experimental strategies towards laser-based imaging of molecular structures and chemical reactivity.

High-average-power 2 μm few-cycle optical parametric chirped pulse amplifier at 100 kHz repetition rate

Sources of long wavelengths few-cycle high repetition rate pulses are becoming increasingly important for a plethora of applications, e.g., in high-field physics. Here, we report on the realization of a tunable optical parametric chirped pulse amplifier at 100 kHz repetition rate. At a central wavelength of 2 μm, the system delivered 33 fs pulses and a 6 W average power corresponding to 60 μJ pulse energy with gigawatt-level peak powers. Idler absorption and its crystal heating is experimentally investigated for a BBO. Strategies for further power scaling to several tens of watts of average power are discussed.

High-power, high-repetition-rate performance characteristics of β-BaB₂O₄ for single-pass picosecond ultraviolet generation at 266 nm

We report a systematic study on the performance characteristics of a high-power, high-repetition-rate, picosecond ultraviolet (UV) source at 266 nm based on β-BaB2O4 (BBO). The source, based on single-pass fourth harmonic generation (FHG) of a compact Yb-fiber laser in a two-crystal spatial walk-off compensation scheme, generates up to 2.9 W of average power at 266 nm at a pulse repetition rate of ~80 MHz with a single-pass FHG efficiency of 35% from the green to UV. Detrimental issues such as thermal effects have been studied and confirmed by performing relevant measurements. Angular and temperature acceptance bandwidths in BBO for FHG to 266 nm are experimentally determined, indicating that the effective interaction length is limited by spatial walk-off and thermal gradients under high-power operation. The origin of dynamic color center formation due to two-photon absorption in BBO is investigated by measurements of intensity-dependent transmission at 266 nm. Using a suitable theoretical model, two-photon absorption coefficients as well as the color center densities have been estimated at different temperatures. The measurements show that the two-photon absorption coefficient in BBO at 266 nm is ~3.5 times lower at 200°C compared to that at room temperature. The long-term power stability as well as beam pointing stability is analyzed at different output power levels and focusing conditions. Using cylindrical optics, we have circularized the generated elliptic UV beam to a circularity of >90%. To our knowledge, this is the first time such high average powers and temperature-dependent two-photon absorption measurements at 266 nm are reported at repetition rates as high as ~80 MHz.

167.75-nm vacuum-ultraviolet ps laser by eighth-harmonic generation of a 1342-nm Nd:YVO4 amplifier in KBBF

We demonstrate a ps 167.75-nm vacuum-ultraviolet (VUV) laser by cascaded second-harmonic generation (SHG). The VUV laser is produced by eighth-harmonic generation (EHG) of a mode-locked ps 1342-nm Nd:YVO4 amplifier through three stages cascaded SHG with two LiB3O5 crystals and one KBe2BO3F2 crystal, successively. The 167.75-nm laser provides up to 65-μW output power, and the corresponding photon flux and photon flux density are 5.5×10(13)  s(-1) and 1.6×10(18)  s(-1)·cm(-2), respectively.

CEP-stable, sub-6 fs, 300-kHz OPCPA system with more than 15 W of average power

We report on a CEP-stable OPCPA system reaching multi-GW peak powers at 300 kHz repetition rate. It delivers 15 W of average power, over 50 µJ of compressed pulse energy and a pulse duration below 6 fs. By implementing an additional pump-seed-synchronization, the output parameters are stabilized over hours with power fluctuations of less than 1.5%.

Tm-based fiber-laser system with more than 200 MW peak power

Tm-based fiber-laser systems are an attractive concept for the development of high-performance laser sources in the spectral region around 2 μm wavelength. Here we present a system delivering a pulse-peak power higher than 200 MW in combination with 24 W average power and 120 μJ pulse energy. Key components enabling this performance level are a Tm-doped large-pitch fiber with a mode-field diameter of 65 μm, highly efficient dielectric gratings, and a Tm-based fiber oscillator operating in the stretched-pulse regime.

Performance of MgO:PPLN, KTA, and KNbO₃ for mid-wave infrared broadband parametric amplification at high average power

The performance of potassium niobate (KNbO₃), MgO-doped periodically poled lithium niobate (MgO:PPLN), and potassium titanyl arsenate (KTA) were experimentally compared for broadband mid-wave infrared parametric amplification at a high repetition rate. The seed pulses, with an energy of 6.5 μJ, were amplified using 410 μJ pump energy at 1064 nm to a maximum pulse energy of 28.9 μJ at 3 μm wavelength and at a 160 kHz repetition rate in MgO:PPLN while supporting a transform limited duration of 73 fs. The high average powers of the interacting beams used in this study revealed average power-induced processes that limit the scaling of optical parametric amplification in MgO:PPLN; the pump peak intensity was limited to 3.8  GW/cm² due to nonpermanent beam reshaping, whereas in KNbO₃ an absorption-induced temperature gradient in the crystal led to permanent internal distortions in the crystal structure when operated above a pump peak intensity of 14.4  GW/cm².

53 W average power few-cycle fiber laser system generating soft x rays up to the water window

We report on a few-cycle laser system delivering sub-8-fs pulses with 353 μJ pulse energy and 25 GW of peak power at up to 150 kHz repetition rate. The corresponding average output power is as high as 53 W, which represents the highest average power obtained from any few-cycle laser architecture so far. The combination of both high average and high peak power provides unique opportunities for applications. We demonstrate high harmonic generation up to the water window and record-high photon flux in the soft x-ray spectral region. This tabletop source of high-photon flux soft x rays will, for example, enable coherent diffractive imaging with sub-10-nm resolution in the near future.

Temperature nonuniformity occurring during the cooling process of a KDP crystal and its effects on second-harmonic generation

The temperature nonuniformity occurring during the cooling process of a KDP crystal is studied, along with its effects on the second-harmonic generation (SHG) of a high-average-power laser. A comprehensive model is proposed incorporating principles of thermodynamics, mechanics, and optics, and it is applied to investigate the temperature nonuniformity and its effects. The temperature rise caused by linear absorption is calculated, while the temperature nonuniformity occurring during the cooling process is analyzed using the finite-element method (FEM). The stress induced by the nonuniformity is then studied using the FEM, and the trend of its change is determined. Moreover, the changes in refractive index caused by the stress are calculated, the results of which are used to determine the variations in the induced phase mismatch. The SHG efficiency considering the phase mismatch is eventually obtained by solving the coupling wave equations. The results demonstrate that the temperature nonuniformity has negative effects on the SHG efficiency.

Thermal properties of borate crystals for high power optical parametric chirped-pulse amplification

The potential of borate crystals, BBO, LBO and BiBO, for high average power scaling of optical parametric chirped-pulse amplifiers is investigated. Up-to-date measurements of the absorption coefficients at 515 nm and the thermal conductivities are presented. The measured absorption coefficients are a factor of 10-100 lower than reported by the literature for BBO and LBO. For BBO, a large variation of the absorption coefficients was found between crystals from different manufacturers. The linear and nonlinear absorption coefficients at 515 nm as well as thermal conductivities were determined for the first time for BiBO. Further, different crystal cooling methods are presented. In addition, the limits to power scaling of OPCPAs are discussed.

Two MHz tunable non collinear optical parametric amplifiers with pulse durations down to 6 fs

We report on the development of a 2 MHz non collinear optical parametric amplifier (NOPA) for high repetition rate time resolved X-ray or optical spectroscopy. Our modular and very flexible device is pumped by the second and third harmonics of a commercial femtosecond Ytterbium-doped fiber laser. The amplified pulses are tunable from 520 nm to 1000 nm with pulse durations between 15 and 30 fs over the full tuning range. The same setup is also suitable for broadband amplification and we demonstrate the generation of 6 fs pulses at a central wavelength of 850 nm as well as the generation of a broadband spectrum supporting 4.2 fs transform limited pulse duration at a central wavelength of 570 nm. Very high stability and compactness is achieved thanks to an optimized mechanical design.

Frequency domain optical parametric amplification

Today's ultrafast lasers operate at the physical limits of optical materials to reach extreme performances. Amplification of single-cycle laser pulses with their corresponding octave-spanning spectra still remains a formidable challenge since the universal dilemma of gain narrowing sets limits for both real level pumped amplifiers as well as parametric amplifiers. We demonstrate that employing parametric amplification in the frequency domain rather than in time domain opens up new design opportunities for ultrafast laser science, with the potential to generate single-cycle multi-terawatt pulses. Fundamental restrictions arising from phase mismatch and damage threshold of nonlinear laser crystals are not only circumvented but also exploited to produce a synergy between increased seed spectrum and increased pump energy. This concept was successfully demonstrated by generating carrier envelope phase stable, 1.43 mJ two-cycle pulses at 1.8 μm wavelength.

Sub-20 fs μJ-energy pulses tunable down to the near-UV from a 1 MHz Yb-fiber laser system

Optical parametric amplifiers render widely tunable ultrashort pulses, but for full spectral coverage, complex mixing schemes are needed. In particular, the blue and near-UV part of the spectrum is not directly reached with the 800 nm pump from Ti:sapphire systems or the 1030 nm pump of Yb-based lasers. We combine third harmonic pumping at 343 nm with seeding by a second harmonic (SH) pumped continuum to tune a noncollinear optical parametric amplifier down to 395 nm. Together with a SH pumped branch, the full range from 395 to 970 nm is covered with 20 fs pulse length or less. Pulse energies up to the μJ-level with an average power of up to 200 mW at 200 kHz and 480 mW at 1 MHz are achieved. With additional frequency doubling, the full range down to 210 nm is reached without gap. Two-photon absorption in the amplifier crystal is discussed as the critical issue in UV-pumped systems.

Power scaling of supercontinuum seeded megahertz-repetition rate optical parametric chirped pulse amplifiers

Optical parametric chirped-pulse amplifiers with high average power are possible with novel high-power Yb:YAG amplifiers with kW-level output powers. We demonstrate a compact wavelength-tunable sub-30-fs amplifier with 11.4 W average power with 20.7% pump-to-signal conversion efficiency. For parametric amplification, a beta-barium borate crystal is pumped by a 140 W, 1 ps Yb:YAG InnoSlab amplifier at 3.25 MHz repetition rate. The broadband seed is generated via supercontinuum generation in a YAG crystal.

Design considerations for a high power, ultrabroadband optical parametric chirped-pulse amplifier

A conceptual design of a high power, ultrabroadband optical parametric chirped-pulse amplifier (OPCPA) was carried out comparing nonlinear crystals (LBO and BBO) for 810 nm centered, sub-7.0 fs pulses with energies above 1 mJ. These amplifiers are only possible with a parallel development of kilowatt-level OPCPA-pump amplifiers. It is therefore important to know good strategies to use the available OPCPA-pump energy efficiently. Numerical simulations, including self- and cross-phase modulation, were used to investigate the critical parameters to achieve sufficient spectral and spatial quality. At high output powers, thermal absorption in the nonlinear crystals starts to degrade the output beam quality. Strategies to minimize thermal effects and limits to the maximum average power are discussed.

Temporal and spatial effects inside a compact and CEP stabilized, few-cycle OPCPA system at high repetition rates

We present a compact and ultra-stable few-cycle OPCPA system. In two non-collinear parametric amplification stages pulse energies up to 17 µJ at 200 kHz repetition rate are obtained. Recompression of the broadband pulses down to 6.3 fs is performed with chirped mirrors leading to peak powers above 800 MW. The parametric amplification processes were studied in detail employing (2 + 1) dimensional numerical simulations and compared to experimental observations in terms of spectral shapes, pulse energy, spatial effects as well as delay dependent nonlinear mixing products. This gives new insights into the parametric process and design guidelines for high repetition rate OPCPA systems.