Megawatt-scale average-power ultrashort pulses in an enhancement cavity

Harmonically mode-locked laser pulse accumulation in a self-resonating optical cavity

Optical enhancement cavities enabling laser pulses to be coherently stacked in free space are used in several applications to enhance accessible optical power. In this study, we develop an optical cavity that accumulates harmonically mode-locked laser pulses with a self-resonating mechanism for X-ray sources based on laser-Compton scattering. In particular, a Fabry-Perot cavity composed of 99% reflectance mirrors maintained the optical resonance in a feedback-free fashion for more than half an hour and automatically resumed the accumulation even if the laser oscillation was suspended. In contrast to conventional optical enhancement cavity systems with a dedicated feedback controller, this characteristic is highly beneficial in practical applications, such as for laser-Compton scattering X-ray sources. Lastly, upscaling and adoption of the proposed system might improve the operability and equipment use of laser Compton-scattering X-ray sources.

Prior-damage dynamics in a high-finesse optical enhancement cavity

An observation of prior-damage behavior inside a high-finesse optical resonator is reported. Intra-cavity average power drops appeared with magnitude and time scale depending on the power level. Increasing further the incident laser beam power led to irreversible damage of the cavity coupling mirror surface. The origin of this phenomenon is investigated with post mortem mirror surface imaging and analysis of the signals reflected and transmitted by the enhancement cavity. Scattering losses induced by surface deformation due to a hot-spot surface contaminant is found to be most likely the dominant physics process behind this phenomenon.

100 MHz frequency comb for low-intensity multi-photon studies: intra-cavity velocity-map imaging of xenon

We raise the power from a commercial 10 W frequency comb inside an enhancement cavity and perform multi-photon ionization of gas-phase atoms at 100 MHz for the first time, to the best of our knowledge. An intra-cavity velocity-map-imaging setup collects electron-energy spectra of xenon at rates several orders of magnitude higher than those of conventional laser systems. Consequently, we can use much lower intensities ${\sim}{{10}^{12}} \;{\rm W}/{{\rm cm}^2} $∼10¹²W/cm² without increasing acquisition times above just a few seconds. The high rate and coherence of the stabilized femtosecond pulses are known to be transferred to the actively stabilized cavity and will allow studying purely perturbative multi-photon effects, paving the road towards a new field of precision tests in nonlinear physics.

Modal instability suppression in a high-average-power and high-finesse Fabry-Perot cavity

An experimental method to remove modal instabilities induced by thermoelastic deformation in optical high-finesse resonators is presented and experimentally investigated in this paper. The method is found suitable for multi-mirror folded monolithic and compact cavities, such as those used in the particle accelerator environment. It is also suitable for very high stacked average power. Here we demonstrate stable operation at the 200 kW intracavity average power.

Coherently combined 16-channel multicore fiber laser system

We present a coherently combined laser amplifier with 16 channels from a multicore fiber in a proof-of-principle demonstration. Filled-aperture beam splitting and combination, together with temporal phasing, is realized in a compact and low-component-count setup. Combined average power of up to 70 W with 40 ps pulses is achieved with combination efficiencies around 80%.

Near-concentric Fabry-Pérot cavity for continuous-wave laser control of electron waves

Manipulating free-space electron wave functions with laser fields can bring about new electron-optical elements for transmission electron microscopy (TEM). In particular, a Zernike phase plate would enable high-contrast TEM imaging of soft matter, leading to new opportunities in structural biology and materials science. A Zernike phase plate can be implemented using a tight, intense continuous laser focus that shifts the phase of the electron wave by the ponderomotive potential. Here, we use a near-concentric cavity to focus 7.5 kW of continuous-wave circulating laser power at 1064 nm into a 7 µm mode waist, achieving a record continuous laser intensity of 40 GW/cm². Such parameters are sufficient to impart a phase shift of 1 rad to a 10 keV electron beam, or 0.16 rad to a 300 keV beam. Our numerical simulations confirm that the standing-wave phase shift profile imprinted on the electron wave by the intra-cavity field can serve as a nearly ideal Zernike phase plate.

Ultrafast optomechanical pulse picking

State-of-the-art optical switches for coupling pulses into and/or out of resonators are based on either the electro-optic or the acousto-optic effect in transmissive elements. In high-power applications, the damage threshold and other nonlinear and thermal effects in these elements impede further improvements in pulse energy, duration, and average power. We propose a new optomechanical switching concept which is based solely on reflective elements and is suitable for switching times down to the ten-nanosecond range. To this end, an isolated section of a beam path is moved in a system comprising mirrors rotating at a high angular velocity and stationary imaging mirrors, without affecting the propagation of the beam thereafter. We discuss three variants of the concept and exemplify practical parameters for its application in regenerative amplifiers and stack-and-dump enhancement cavities. We find that optomechanical pulse picking has the potential to achieve switching rates of up to a few tens of kilohertz while supporting pulse energies of up to several joules.

Extraction of enhanced, ultrashort laser pulses from a passive 10-MHz stack-and-dump cavity

Periodic dumping of ultrashort laser pulses from a passive multi-MHz repetition-rate enhancement cavity is a promising route towards multi-kHz repetition-rate pulses with Joule-level energies at an unparalleled average power. Here, we demonstrate this so-called stack-and-dump scheme with a 30-m-long cavity. Using an acousto-optic modulator, we extract pulses of 0.16 mJ at 30-kHz repetition rate, corresponding to 65 stacked input pulses, representing an improvement in three orders of magnitude over previously extracted pulse energies. The ten times longer cavity affords three essential benefits over former approaches. First, the time between subsequent pulses is increased to 100 ns, relaxing the requirements on the switch. Second, it allows for the stacking of strongly stretched pulses (here from 800 fs to 1.5 ns), thus mitigating nonlinear effects in the cavity optics. Third, the choice of a long cavity offers increased design flexibility with regard to thermal robustness, which will be crucial for future power scaling. The herein presented results constitute a necessary step towards stack-and-dump systems providing access to unprecedented laser parameter regimes.

High flux circularly polarized gamma beam factory: coupling a Fabry-Perot optical cavity with an electron storage ring

We report and discuss high-flux generation of circularly polarized γ-rays by means of Compton scattering. The γ-ray beam results from the collision of an external-cavity-enhanced infrared laser beam and a low emittance relativistic electron beam. By operating a non-planar bow-tie high-finesse optical Fabry-Perot cavity coupled to a storage ring, we have recorded a flux of up to (3.5 ± 0.3) × 10⁸ photons per second with a mean measured energy of 24 MeV. The γ-ray flux has been sustained for several hours. In particular, we were able to measure a record value of up to 400 γ-rays per collision in a full bandwidth. Moreover, the impact of Compton scattering on the electron beam dynamics could be observed resulting in a reduction of the electron beam lifetime correlated to the laser power stored in the Fabry-Perot cavity. We demonstrate that the electron beam lifetime provides an independent and consistent determination of the γ-ray flux. Furthermore, a reduction of the γ-ray flux due to intrabeam scattering has clearly been identified. These results, obtained on an accelerator test facility, warrant potential scaling and revealed both expected and yet unobserved effects. They set the baseline for further scaling of the future Compton sources under development around the world.

High-power ultrafast Yb:fiber laser frequency combs using commercially available components and basic fiber tools

We present a detailed description of the design, construction, and performance of high-power ultrafast Yb:fiber laser frequency combs in operation in our laboratory. We discuss two such laser systems: an 87 MHz, 9 W, 85 fs laser operating at 1060 nm and an 87 MHz, 80 W, 155 fs laser operating at 1035 nm. Both are constructed using low-cost, commercially available components, and can be assembled using only basic tools for cleaving and splicing single-mode fibers. We describe practical methods for achieving and characterizing low-noise single-pulse operation and long-term stability from Yb:fiber oscillators based on nonlinear polarization evolution. Stabilization of the combs using a variety of transducers, including a new method for tuning the carrier-envelope offset frequency, is discussed. High average power is achieved through chirped-pulse amplification in simple fiber amplifiers based on double-clad photonic crystal fibers. We describe the use of these combs in several applications, including ultrasensitive femtosecond time-resolved spectroscopy and cavity-enhanced high-order harmonic generation.

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.

Power enhancement of burst-mode ultraviolet pulses using a doubly resonant optical cavity

We report a doubly resonant enhancement cavity (DREC) that can realize a simultaneous enhancement of two incoming laser beams at different wavelengths and different temporal structures. The double-resonance condition is theoretically analyzed, and different DREC locking methods are experimentally investigated. Simultaneous locking of a Fabry-Perot cavity to both an infrared (1064 nm) and its frequency-tripled ultraviolet (355 nm) pulses has been demonstrated by controlling the frequency difference between the two beams with a fiber-optic frequency shifter. The DREC technique enables novel applications of optical cavities to power enhancement of burst-mode lasers with arbitrary macropulse width and repetition rate.

High-intracavity-power thin-disk laser for the alignment of molecules

We propose a novel approach for strong alignment of gas-phase molecules for experiments at arbitrary repetition rates. A high-intracavity-power continuous-wave laser will provide the necessary ac electric field of 10(10)-10(11) W/cm(2). We demonstrate thin-disk lasers based on Yb:YAG and Yb:Lu(2)O(3) in a linear high-finesse resonator providing intracavity power levels in excess of 100 kW at pump power levels on the order of 50 W. The multi-longitudinal-mode operation of this laser avoids spatial-hole burning even in a linear standing-wave resonator. The system will be scaled up as in-vacuum system to allow for the generation of fields of 10(11) W/cm(2). This system will be directly applicable for experiments at modern X-ray light sources, such as synchrotrons or free-electron lasers, which operate at various very high repetition rates. This would allow to record molecular movies through temporally resolved diffractive imaging of fixed-in-space molecules, as well as the spectroscopic investigation of combined X-ray-NIR strong-field effects of atomic and molecular systems.

Femtosecond Enhancement Cavities in the Nonlinear Regime

We combine high-finesse optical resonators and spatial-spectral interferometry to a highly phase-sensitive investigation technique for nonlinear light-matter interactions. We experimentally validate an ab initio model for the nonlinear response of a resonator housing a gas target, permitting the global optimization of intracavity conversion processes like high-order harmonic generation. We predict the feasibility of driving intracavity high-order harmonic generation far beyond intensity limitations observed in state-of-the-art systems by exploiting the intracavity nonlinearity to compress the pulses in time.

High average power coherent vuv generation at 10 MHz repetition frequency by intracavity high harmonic generation

Intracavity high harmonic generation was utilized to generate high average-power coherent radiation at vacuum ultraviolet (vuv) wavelengths. A ytterbium-doped fiber-laser based master-oscillator power-amplifier (MOPA) system with a 10 MHz repetition frequency was developed and used as a driving laser for an external cavity. A series of odd-order harmonic radiations was generated extending down to ∼ 30 nm (41 eV in photon energy). The 7th harmonic radiation generated was centered at 149 nm and had an average output power of up to 0.5 mW. In this way, we developed a sub-mW coherent vuv-laser with a 10 MHz repetition frequency, which, if used as an excitation laser source for photo-electron spectroscopy, could improve the signal count-rate without deterioration of the spectral-resolution caused by space-charge effects.

Enhancement cavities for zero-offset-frequency pulse trains

The optimal enhancement of broadband optical pulses in a passive resonator requires a seeding pulse train with a specific carrier-envelope-offset frequency. Here, we control the phase of the cavity mirrors to tune the offset frequency for which a given comb is optimally enhanced. This enables the enhancement of a zero-offset-frequency train of sub-30-fs pulses to multi-kW average powers. The combination of pulse duration, power, and zero phase slip constitutes a crucial step toward the generation of attosecond pulses at multi-10-MHz repetition rates. In addition, this control affords the enhancement of pulses generated by difference-frequency mixing, e.g., for mid-infrared spectroscopy.

Balancing of thermal lenses in enhancement cavities with transmissive elements

Thermal lensing poses a serious challenge for the power scaling of enhancement cavities, in particular when these contain transmissive elements. We demonstrate the compensation of the lensing induced by thermal deformations of the cavity mirrors with the thermal lensing in a thin Brewster plate. Using forced convection to fine-tune the lensing in the plate, we achieve average powers of up to 160 kW for 250-MHz-repetition-rate picosecond pulses with a power-independent mode size. Furthermore, we show that the susceptibility of the cavity mode size to thermal lensing allows highly sensitive absorption measurements.

Energy scaling of Kerr-lens mode-locked thin-disk oscillators

Geometric scaling of a Kerr-lens mode-locked Yb:YAG thin-disk oscillator yields femtosecond pulses with an average output power of 270 W. The scaled system delivers femtosecond (210-330 fs) pulses with a peak power of 38 MW. These values of average and peak power surpass the performance of any previously reported femtosecond laser oscillator operated in atmospheric air.

Carrier-envelope phase drift measurement of picosecond pulses by an all-linear-optical means

Carrier-envelope phase (CEP) drift of a pulse train of 2 ps pulses has been measured by a multiple beam interferometer. The round trip time of the interferometer is slightly mistuned from the pulse sequence, leading to spectral interference fringes. We extract the pulse-to-pulse CEP drift from the position of the spectral interference pattern. The length of the interferometer has been actively stabilized to ±10  nm, which sets the ultimate limit on the accuracy of the measurement to 78 mrad, while the CEP-drift (rms) noise of the measurement was 127 mrad (at 800 nm).