We present a high peak and average power optical parametric chirped pulse amplification system driven by diode-pumped Yb:KGW and Nd:YAG lasers running at 1 kHz repetition rate. The advanced architecture of the system allows us to achieve >53 W average power combined with 5.5 TW peak power, along with sub-220 mrad CEP stability and sub-9 fs pulse duration at a center wavelength around 880 nm. Broadband, background-free, passively CEP stabilized seed pulses are produced in a series of cascaded optical parametric amplifiers pumped by the Yb:KGW laser, while a diode-pumped Nd:YAG laser system provides multi-mJ pump pulses for power amplification stages. Excellent stability of output parameters over 16 hours of continuous operation is demonstrated.
© 2017 Optical Society of America
Optical parametric chirped pulse amplification (OPCPA), first demonstrated in 1992 , is becoming the standard technique for producing few cycle, high intensity pulses for attosecond science and high field experiments . So far, the dominant technology for generation of intense pulses with durations down to the near-single-cycle regime has been nonlinear pulse compression in gas-filled capillaries [3, 4]. Although scaling of this approach to 216 W of average power has been demonstrated , the output energy from hollow fiber compressors is limited to a few mJ [6, 7] due to limited aperture of the capillaries. On the other hand, multimilijoule, CEP stabilized pulses with TW-level peak powers can be produced by advanced Ti:sapphire systems, but the pulse duration is on the order of 25 fs . OPCPA schemes based on BBO crystals pumped by the second harmonic of lasers operating at 1 μm have been demonstrated to be capable of reaching TW peak power in low repetition rate flashlamp pumped systems [9–11], and produce few cycle pulses with much lower pulse energies, but at average power up to 22 W, when pumped by fiber-based lasers . The recent massive improvements in the quality and availability of diode-pumped, high average power solid-state lasers [2,13–19], providing multimillijoule pulses for OPCPA pumping at ≥1 kHz repetition rates allows boosting the average powers of few cycle TW-scale OPCPA system to the range of tens of watts.
Alongside peak and average power, pulse temporal contrast is an extremely important parameter that must be controlled in strong field experiments. In terms of output pulse contrast, OPCPA is advantageous to traditional chirped pulse amplification systems because of the instantaneous nature of the process, which limits amplification to the temporal window defined by the pump pulse. However, it has been noted in the literature that if the OPCPA seed pulse is contaminated by amplified spontaneous emission (ASE), the ASE background is amplified along with the signal within the temporal window of the pump pulse . Furthermore, the contrast of OPCPA output is always degraded by amplified parametric fluorescence (APF). Although APF is inevitable in a parametric amplifier, its level can be reduced by careful design of high-gain amplification stages , optimizing seed and pump pulse durations , using higher energy seed pulses, and employing nonlinear pulse cleaning techniques [11,22]. Carrier envelope phase (CEP) control is another essential prerequisite for a number of the few cycle laser applications in strong-field research, in particular for experiments on generation of isolated atosecond pulses. Nowadays, both active and passive CEP stabilization are well established techniques providing CEP control in different types of laser systems. However, only a few CEP stable few cycle OPCPA systems producing CEP-stable pulses with energies exceeding 1 mJ have been reported [23,24].
In this paper we present an OPCPA system consisting of femtosecond and picosecond noncollinear parametric amplifiers seeded by passively stabilized broadband pulses from a continuum generator. The system delivers sub-3-cycle, 53.8 mJ pulses, centered at 880 nm. Using a diode-pumped Nd:YAG laser system as the pump source allows us to produce these pulses at 1 kHz repetition rate, corresponding to >53 W average power. We show how several improvements to the well-known OPCPA scheme based on BBO crystals pumped at 532 nm allow us to combine the few-cycle pulse duration and multi-TW peak power with passive, sub-220 mrad CEP noise, high temporal contrast, and excellent long-term stability. To the best of our knowledge, the system described here delivers the highest average power currently achieved among CEP-stabilized, few-cycle, TW-class laser systems.
2. OPCPA setup
The layout of the OPCPA setup is presented in Fig. 1. The system consists of a front-end based on a commercial industrial-grade femtosecond Yb:KGW laser (Pharos, Light Conversion Ltd.), a specially designed diode-pumped Nd:YAG picosecond pump laser (Ekspla UAB), and 4 picosecond OPCPA stages, preceded by a seed pulse stretcher (grism pair + acoustooptic programmable dispersive filter) and followed by a compressor (bulk glasses and positive GDD chirped mirrors).
The design of the femtosecond front-end is based on our previous setup described in detail in . Passively CEP stabilized pulses  at 1.5 μm are produced by generating the difference frequency between frequency doubled Yb:KGW laser pulses and the long-wave wing of a continuum produced by the second harmonic of Yb:KGW laser pulses. The difference frequency (DF) pulses are focused into a sapphire plate to generate a CEP stabilized continuum, whose short-wave wing is amplified in two noncollinear optical parametric amplification (NOPA) stages pumped by the major part of the Yb:KGW regenerative amplifier (RA) output energy. All three-wave mixing stages are realized in BBO crystals. Although a broader gain bandwidth can be achieved with BBO crystals pumped at 515 nm, the fs NOPA is optimized to maximize energy transfer to the 700 nm – 1100 nm range, compatible with the picosecond amplification stages pumped at 532 nm. The front-end output energy is 80 μJ. Short-term CEP noise of the front-end was measured to be below 100 mrad.
Figure 2 shows a simplified schematic drawing of the picosecond pump laser (PPL), which is based on Nd:YAG active material. The PPL is seeded by the Yb:KGW master oscillator, which enables straightforward pump-seed synchronization for the OPCPA. The oscillator is optimized to provide ≈ 50 pJ energy within the amplification band of Nd:YAG. This is sufficient to reliably seed the first Nd:YAG-based regenerative amplifier (RA1), in which the oscillator pulses are amplified to ≈ 100 μJ and stretched to 95 ps. Output of RA1 is split in equal parts to seed three more regenerative amplifiers (RA2-RA4), each producing 1.8 mJ pulses.
RA2 output pulses are directed to a pulse envelope shaper based on cascaded second harmonic generation . The shaped pulses are amplified to 10 mJ in a diode pumped power amplifier, and frequency doubled in a 12 mm type I LBO crystal (SH1), providing 6 mJ at 532 nm. The SH1 pulse features an intensity plateau, extending over 100 ps. The flattened temporal pulse profile, measured with a streak camera (C5680, Hamamatsu Photonics) is shown in Fig. 3(a). Meanwhile, the beam profile remains nearly Gaussian, as shown in Fig. 3(b). The pulse shaper is operated at ≈ 50% pump depletion, as our research has indicated that around this value the pulse shape is the most robust against minor variations of input parameters. All nonlinear crystals in the pump laser are held in temperature-controlled ovens to maintain stable phase matching and prevent crystal degradation due to moisture condensation.
Gaussian output beams from the other two regenerative power amplifiers are converted to super-Gaussian beams using spatially variable beam shapers , and directed to two identical power amplifier chains. The amplifier chains are comprised of five quasi-continuous-wave diode side-pumped laser modules (Northrop Grumman Ltd) with 76 mm long Nd:YAG laser rods. The PPL amplifier chains are described in detail in . Outputs from first amplifier chain are collinearly combined in an LBO crystal for type II second harmonic generation, providing Beam 3 with 120mJ pulse energy (see Fig. 2). The length of the SH crystal was chosen to maintain the temporal shape of the residual fundamental beam similar to that of Beam 1. This residual beam is frequency doubled once more, providing Beam 2 with an energy of 60 mJ and a flattened temporal shape. Meanwhile, outputs from the second amplifier chain are combined in a type II LBO crystal for non-collinear second harmonic generation, providing Beam 4 with 150 mJ of energy. The noncollinear configuration simplifies the separation of the second harmonic from the fundamental, avoiding the use of dichroic optics in the highest energy beam. Beams 2, 3 and 4 possess super-Gaussian (roughly 12th order) spatial profiles (see Fig. 3(c)). The combined power of the 532 nm outputs exceeds 300 W, while energy noise amounts to ≈ 0.5% RMS in all channels. The configuration of the picosecond pump laser with four different output channels is well suited for OPCPA pumping, because the temporally flattened pulses in the first two output channels help to maintain a broad signal spectrum in the high-gain OPCPA stages, while the super-Gaussian spatial profiles of beams 2–4 ensure efficient energy extraction in the low gain, power amplifier-type OPCPA stages.
The seed pulses are stretched to ≈ 65 ps in a negative dispersion grism stretcher . An acousto-optical programmable dispersive filter (AOPDF; Dazzler, Fastlite) is used for fine control of the spectral phase and compensation of slow CEP drift. After the stretcher and AOPDF, slightly more than 1 μJ of energy remains for seeding the first picosecond OPCPA stage. Type I phase matching and internal noncollinearity angle ≈ 2.1° are chosen for the four BBO-based parametric amplification stages pumped by the picosecond Nd:YAG laser. Due to the long duration of the Nd:YAG laser pulses and the correspondingly high seed pulse stretching factor, the OPCPA system is not very sensitive to pump-seed timing jitter, and the synchronization provided by seeding the seed and pump arms of the system from one master oscillator is sufficient to avoid timing jitter-related effects. However, a slow feedback loop, operating on a motorized delay line, had to be implemented to compensate for slow thermal drifts of oscillator and regenerative amplifier cavity lengths, which introduce a pump-seed delay drift on the scale of a few picoseconds per hour. At the output of the final amplification stage, signal energies up to 67 mJ are measured, and bandwidth sufficient for < 9 fs pulses is maintained. The pump beams for all picosecond stages are relay imaged from the pump laser, ensuring excellent long-term stability and uniform beam profiles. The parameters of the picosecond amplification stages are summarized in Table 1.
The energy conversion efficiency in the first two stages is low due to the long flat-top pump pulse duration. Furthermore, the first stage is deliberately kept unsaturated to minimize parametric fluorescence. The power amplifier stages are strongly saturated and show pump-to-signal conversion ηp−s ≈ 25%. 2.5 mm and 4 mm crystals were tested for the final amplification stage. Due to the high seed energy, gain saturation is easily reached even with the modest pump intensity Ip ≈ 3 GW/cm2 and relatively thin 2.5 mm crystal. With the 4 mm crystal, the efficiency is substantially degraded due to strong back-conversion. However, this improves the energy stability σ(Esignal), which may be a desirable trade-off in some cases. The stability values given in the table are the standard deviation (STD) of pulse energies sampled over 2 seconds (2000 shots). It is notable that even though there is strong back-conversion of energy from the signal to the pump with the longer crystal, the beam profiles obtained in the two cases were very similar. We attribute this to the highly uniform top-hat profiles of both signal and pump beams.
The amplified signal beam is expanded to 70 mm diameter (1/e2 level), reflected from a deformable mirror (ILAO Star, Imagine Optic SA) and sent to a bulk glass compressor, where the pulse is shortened to ≈ 500 fs. The compressor consists of, in total, 350 mm of SF-57 and 100 mm of fused silica, all with a clear aperture 100 mm. Final compression to transform-limited <9 fs duration is performed with 8 positively chirped mirrors (Ultrafast Innovations) placed in a vacuum chamber. When paired with the AOPDF, a single chirped mirror design produced sufficiently good results, eliminating the need to use matched chirped mirror pairs. The total throughput of the components after the final amplifier is ≈ 80%, which yields an output of 53.8 mJ, corresponding to 5.5 TW peak power. Due to limited availability, we had to use several FS windows to reach the required total length. This limits the efficiency of the compressor quite severely; we estimate that replacing the numerous windows with a single FS block could improve the transmission to 88%.
A reflection from an uncoated FS window before the vacuum chamber is sent to an array of diagnostics, including measurements of pulse energy, duration, beam profile, CEP, pointing stability, and wavefront. The diagnostics path includes an identical set of chirped mirrors to equalize GDD oscillations in the two beams. The energy available for diagnostics is ≈ 1.5 mJ, which allows us to generate the continuum required for CEP measurement simply in ambient air.
3. OPCPA output parameters and long-term performance
3.1. Long-term stability of the femtosecond front-end
The reliability of the front-end is a crucial factor because most of the optimization work cannot be performed during downtime of the front-end. Although passive CEP stabilization eliminates the typically problematic step of CEP-stabilizing the laser oscillator, the passive CEP stabilization module is still quite sensitive to input beam drift because of the tight focusing required for producing and amplifying the long-wave wing of the 515 nm-pumped continuum. Therefore, care was taken to minimize the physical dimensions of the CEP stabilization module to make it mountable directly on the Yb:KGW laser assembly. As can be seen in Fig. 4, this approach enables us to run the CEP stabilization module for more than a week without any adjustments. We note that the signal beam of the DFG stage was used for this measurement; however, it is well known that the energy of the DFG signal beam is directly related to the energy of the idler beam. A two-point beam stabilization system after the fs NOPA ensures a constant seed beam path through the stretcher, minimizing change of dispersion through days of operation, even though the front-end and the stretcher are on different optical tables.
3.2. OPCPA output characteristics
An overview of the output parameters of our system is given in Fig. 5. Although most of the measurements discussed here were performed in the diagnostics beam, care was taken to check that they correspond well to the parameters of the main beam.
Figure 5(a) shows a temporal pulse profile measured with a self-referenced spectral interferometry (SRSI) device (Wizzler USP, Fastlite), and the corresponding pulse spectrum. The envelope is virtually indistinguishable from the transform limited one. Roughly 3-cycle pulse durations were also comfirmed with other pulse characterization methods (chirpscan  and autocorrelation). We note that achieving this <8 fs pulse duration typically requires careful optimization of the all parameters of the picosecond parametric amplification stages. As shown later, the typical pulse duration achieved without special effort is <9 fs. The spectrum produced by the femtosecond frontend is also shown for comparison in Fig. 5(b).
An exceptional feature of our system is the high output pulse contrast. A contrast measurement, performed with a high dynamic range third order autocorrelator (Tundra, Ultrafast Innovations), is given in Fig. 5(c). The measured pre-pulse contrast in the <−20 ps delay range is higher than 1011, limited by the dynamic range of the autocorrelator. Several factors allow for such contrast. First, the seed pulse in our system is derived from a white light continuum (WLC) pumped at 1.5 μm. We use strong spectral filtering of the 1.5 μm pulse to remove any parasitic radiation before WLC generation. Although this does not attenuate parametric fluorescence at the DF wavelength, any noise at this wavelength is well outside the transmission band of the stretcher. Therefore, the continuum pulse is background-free. Although some parametric fluorescence is generated in the femtosecond preamplifier, it is contained within a ≈ 250 fs temporal window defined by the Yb:KGW laser pulse duration, and therefore does not degrade the temporal contrast on the picosecond timescale. Meanwhile, the seed energy reaching the first picosecond stage is >1 μJ, which is orders of magnitude higher than the energy available from Ti:sapphire oscillators, commonly used to seed OPCPA systems. Also favorable is the tendency of Nd:YAG amplifiers to form a steep leading pulse edge, enabling us to delay the pump pulses in a way that maximizes the pre-pulse contrast without losing output energy nor bandwidth. We emphasize that this contrast is achieved without nonlinear pulse cleaning after the amplification stages. We also note that, although the autocorrelator is specially adapted for broadband pulses, the acceptance bandwidth of the nonlinear crystals used in the autocorrelator is still too narrow for our system. Therefore, the intensity of the main peak is underestimated, and the actual contrast values could be still higher by a factor of ≈ 4.
An output beam profile, measured after the bulk compressor, is given in Fig. 5(d). As in the pump beam, intensity modulation in the flat-top area of the beam is on the order of several percent. The focusability of the OPCPA beam is evaluated by computing the far-field beam profile from the beam intensity distribution and wavefront data measured with a Shack-Hartmann sensor (HASO3-128, Imagine Optic SA). Although a Strehl ratio ≈ 0.9 was measured directly after the final ps amplification stage (before beam expansion and pulse compression), a deformable mirror was required to maintain good output focusability due to imperfections of the magnifying telescope and compressor glasses. With the deformable mirror, residual RMS wavefront error of < λ/17 and Strehl ratio S = 0.89 are achieved (Figs. 5(e) and 5(f)). The faint ring around the main peak in Fig. 5(f) is a consequence of the top-hat beam profile. The beam was also investigated for spatial chirp and angular dispersion, but no significant amounts of either were found. Detailed measurements of the complete space-and-time dependent electric field using the TERMITES method are planned in the near future.
3.3. Long-term OPCPA stability
To demonstrate the long-term stability of our system, we log the key output parameters during a 16-hour run. The measurements are shown in Fig. 6. The displayed parameters are: pulse energy E and its stability σ(E(2000)) (STD of 2000 shots), pulse duration τFWHM, f − 2f interferogram, and CEP stability evaluated from the interferogram. The CEP stability graph in Fig. 6(d) shows the STD of CEP values measured over a 10 minute temporal window. CEP is calculated from spectra measured at 1 kHz. No averaging was performed. The mean values and standard deviations of the experimental parameters are also indicated in the figure.
Overall, 53.8 mJ pulse energy, sub-9 fs pulse duration, and sub-220 mrad CEP jitter was maintained throughout the test. The stability values are among the best demonstrated for laser systems of comparable peak and average power [8,19]. We note that automated feedback loops had to be set up to correct slow drifts of CEP, output beam direction and OPCPA seed-pump delay.
4. Conclusion and outlook
We have demonstrated an OPCPA system delivering sub-9fs, 53.8 mJ near-IR pulses at a repetition rate of 1 kHz, corresponding to >53 W average and 5.5 TW peak power. The advanced design of the system enables us to also achieve excellent temporal contrast and maintain <220 mrad CEP stability through multiple hours of operation. To the best of our knowledge, our system produces the highest average power among CEP-stabilized, multi-TW, few-cycle OPCPA systems reported in the literature.
In the near future, an upgrade is planned, which is expected to enable generation of 6 fs pulses by adding LBO-based OPCPA stages to extend the spectrum to the long-wave side. The upgraded laser system will be installed at the ELI-ALPS laser facility in Szeged, Hungary.
Lietuvos Mokslo Taryba (MIP-055/2014); Seventh Framework Programme (284464).
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