1. Introduction

The variety of Optical Parametric Chirped Pulse Amplification (OPCPA) systems [1] is increasing quickly due to the huge potential and variability of this concept to reach ultrashort laser pulses with high energies at high repetition rates ranging over a wide range of wavelength regimes from the visible to the mid-infrared [2]. In all these systems, the output specifications and the complexity are mainly determined by the implemented pump laser concept. Traditional approaches rely on frequency-doubled Ti:sapphire amplifiers operating with several mJ of energy at low kHz repetition rates. Higher energies and repetition rates can be achieved by Yb-doped gain materials. Here, highest energies up to 500 mJ with >2 ps pulse duration [3] can be realized by regenerative amplifiers using Yb:YAG thin-disks. Fiber-based pump lasers are best suited for high repetition rates of more than 100 kHz due to the efficient heat removal in ytterbium doped large mode area fibers [4]. New record values for OPCPAs are published on a regular basis [5, 6]. One common feature of the above mentioned concepts is the need for a stretcher and compressor in the pump source, which adds additional losses, complexity, and potential jitter to the system. In fact, these grating sequences contribute substantially to the size of the full OPCPA, so that state-of-the-art systems easily fill a complete optical table.

In this work we present a simple high repetition rate picosecond pump concept delivering 190 µJ of energy and nearly Fourier limited pulse durations with optical synchronization to an few-cycle seed oscillator pumping a multipass fs OPCPA system. Our ps power amplifier approach consists of a multi-crystal scheme with Nd:YVO₄ as high gain material. In contrast to fiber-based pump lasers this approach exhibits a negligible B-integral, allowing for a setup without stretcher and compressor. Therefore losses from the compressor which are typically in the order of 20% can be avoided and efficient second harmonic generation can be achieved due to the narrow bandwidth. This allows for a very compact and easy setup which can open up new fields of application of OPCPAs e.g. in life science and biophysics. The narrow gain bandwidth of Nd:YVO₄ results in rather long pump pulses for the OPA stages, which causes problems in achieving a sufficient temporal overlap between signal and pump pulses, while still being able to recompress the few-cycle pulses with a pure chirped mirror compressor. Partial use of a pump pulse in the parametric amplification stages with several subsequent crystals was investigated theoretical [7] and realized experimentally with a rather complicated setup at 10 Hz repetition rate with 8 ns pump using the so called time-shear power amplification based on three nonlinear crystals [8]. This system includes a bulk grating stretcher to reach stretched pulse durations in the ns-range as well as a grating compressor for recompression. Alternatively, multipass parametric amplifier concepts for nanosecond pump pulses were reported from several groups, where the seed signal (stretched to 300 ps) was folded back several times with slightly different angles to the nonlinear crystal pumped by a ~20 times longer pump pulse [9–12]. For pump pulses with picosecond duration, the seed pulse was typically chosen to fit. Re-using of the pump was previously reported by focusing the amplified signal and the depleted pump into an additional nonlinear crystal [13–15].

Here, we present for the first time to our knowledge a simple sequential pump depletion approach for 10 ps pumped OPA stages at high repetition rate (>100 kHz) which avoids excessive chirping of the signal pulses by partial pump depletion in a simple double-pass configuration. For this configuration no additional nonlinear crystal and only two curved mirrors are necessary. The double-pass alignment is very easy, as pump and signal are back reflected with only a slight vertical offset for pickup. This configuration allows for using a significantly longer pump pulse with several ps pulse duration, while keeping the signal pulses only moderately stretched. This greatly facilitates the recompression of the signal pulses which is especially important in case of few-cycle pulses as a broadband compression is required up to at least fourth order dispersion. In this work, we demonstrate the clean recompression with a pure chirped mirror compressor. Several experiments as coincidence measurments with a reaction microscope [13], photoemission electron microscopy [16, 17] or photoelectron spectroscopy [18] can benefit from the increased statistics at repetition rates of 100 kHz or higher.

2. Setup

The presented compact few-cycle amplifier system with high repetition rate is based on a simple direct diode-pumped solid-state amplifier design. This concept enables a pumping scheme without the need of stretcher and compressor leading to a very compact (75 cm x 210 cm) OPCPA system with reduced complexity (see Fig. 1).

 

Fig. 1 Setup of the OPCPA System: neoVAN-2P: Nd:YVO₄ amplifier; SHG 1-2: second harmonic generation; NOPA 1-4: non-collinear optical parametric amplifiers; DCM: double chirped mirrors.

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The OPCPA system is based on a commercial octave spanning 80 MHz Ti:sapphire oscillator (venteon^dual, Laser Quantum), which delivers an octave spanning spectrum from 600 nm – 1200 nm without any external spectral broadening.

By use of a dichroic mirror this spectrum is divided into a broadband spectrum spanning from 600 nm to 1020 nm, supporting sub 6 fs pulse duration, while the infrared part in the second arm provides sufficient power at 1064 nm for seeding a fiber-based pre-amplifier (venteon pre-amp3). The polarisation maintaining two stage Yb-doped pre-amplifier includes an integrated fiber-coupled pulse picker (AOM) which divides the fundamental repetition rate by a factor of 800 down to 100 kHz between the two stages. At the output it delivers up to 70 nJ of pulse energy at 100 kHz, (in a 10 nm bandwidth) which can be directly used for further amplification in two subsequent direct diode pumped solid-state amplifier modules (neoVAN-2P, neoLASE GmbH) based on Nd:YVO₄. The dispersion of about 3 ps/nm from the pre-amplifier can be neglected for the subsequent narrowband (< 0.5 nm) high-power amplification stages.

Each module is equipped with two crystals pumped with 30 W each at 808 nm. The first amplifier module delivers in single pass configuration amplification from 70 nJ to 5 µJ of pulse energy; in double-pass configuration, up to 50 µJ are achieved. Further amplification in a second single-pass amplifier module leads to up to 190 µJ of pulse energy with a reproducible day-to-day performance.

For the amplifier output an M² value of 1.30 x 1.42 was measured. For the amplification process no stretching/compressing was required, leading to a nearly Fourier limited output pulse duration of 10 ps. The amplifier output was frequency-doubled with 58% efficiency in a 2 mm long LBO crystal with about 100 GW/cm² intensity. After the doubling crystal up to 113 µJ (at 532 nm) and 58 µJ (at 1064 nm) were observed with a Gaussian beam profile (see Fig. 2(d)). After the first doubling stage the pulse duration at 1064 nm shows an increase of 25% due to pump depletion together with a decrease of the observed beam profile as shown in Fig. 2(e). Nevertheless, this beam can be converted once more with about 50% efficiency in a second 3 mm LBO (intensity ~55 GW/cm²), giving additional 35 µJ at 532 nm of pulse energy for pumping the first OPA stage as shown in Fig. 2(b). The beam profile of this recycling SHG scheme are depicted in Fig. 2(f). With 11.3 W average output power for the second OPA stage and 3.5 W for the first OPA stage, a total pump power of 14.8 W at 532 nm could be extracted from the pump amplifier. This corresponds to a record conversion efficiency of more than 77.9% from generated IR pump power to useable pump power for the OPCPA.

 

Fig. 2 a) Output power at 532 nm (red circle) plotted together with the remaining power at 1064 nm (blue squares) and the conversion efficiency (black dots, right axis). b) Results of the second SHG stage pumped with the remaining 1064 nm light from the first SHG stage. The output power (red circle / left axis) is plotted together with the conversion efficiency (black squares / right axis). c) Autocorrelation trace before (red) and behind (black) the first SHG crystal. d) Beam profile behind the first SHG stage at full power. e) Profile of the remaining 1064 nm light behind the first SHG stage at maximum conversion. f) Beam profile behind the second SHG stage.

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The two SHG outputs are used to pump a BBO crystal each, non-collinearly phase matched for broadband amplification of the fundamental Ti:sapphire oscillator seed spectrum. In each case, the amplification was realised in a double pass configuration to increase the efficiency which will be described in more details in the following. After amplification, pulse compression of the broadband spectrum was realised by the use of double-chirped-mirror (DCM) pairs [19].

3. Parametric amplification results

The non-collinear optical parametric amplification stages (NOPA) were based on 5 mm long BBO crystals each cutted with a theta angle of 24.3°. By choosing an internal non-collinear angle of ~2.4° between pump and signal beam, broadband phase matching in the range from 650 nm −1100 nm is supported, which was quantitatively confirmed by (2 + 1)D numerical simulations. For both parametric amplification stages the Poynting-Vector Walk-off configuration was chosen for high output beam quality with long crystals and small pump spots [20].

In order to achieve sufficient overlap with the 10 ps pump pulses in a traditional single-pass approach, strong chirping of the signal pulses and recompression with more than 60 reflections on chirped mirror pairs would be required. At this excessive number, imperfections in the dispersion compensation would cause a significant deterioration of the temporal pulse profile. Instead, a multipass scheme has been applied within this work which was reported up to date only for low repetition rate systems (<< 100 kHz). Due to a temporal shifting of the second pass, efficient energy conversion could be reached with significantly shorter signal pulse duration. The seed pulse was stretched in front of the first NOPA stage by 70 mm fused silica which results in a signal pulse duration of approx. 3.3 ps which was a compromise between the number of reflections on the chirped mirror compressor for optimum pulse compression and efficiency during the amplification.

In the first NOPA stage with 35 µJ of pump energy and the slightly longer pulses, a quite low single-pass amplification from 1.6 nJ to 30 nJ was reached at a pump intensity of 20 GW/cm². However, by simple refocusing pump and signal with two curved mirrors back into the NOPA stage, further amplification up to 170 nJ was obtained. The temporal delay between pump and signal pulses have been optimized for maximum output power. The beam pickup after the double-pass case was realised by implementing a slight vertical offset, so that the non-collinear angle can be maintained in both passes.

For the second amplification stage up to 100 µJ of pump energy with an intensity of ~50 GW/cm² have been applied. In single-pass operation up to 15 µJ (see Fig. 3(a); blue square) was measured. In double-pass, signal energies up to 26 µJ (see Fig. 3(a); blue circles) have been obtained before compression while maintaining a broadband spectrum (see Fig. 5(a)). When the pump for the first double-pass NOPA stage was blocked, 1 nJ of seed energy was measured in front of the second NOPA and 7 µJ (see Fig. 3(a); red square) and 17 µJ (see Fig. 3(a); red circle) behind in single- and double-pass amplifier configuration. This confirms the advantage of the IR recycling to pump another NOPA stage.

 

Fig. 3 a) Pulse energies behind the third and fourth NOPA stages: Amplification with running first double pass NOPA stage in single-pass (blue squares) and double-pass (blue circles) compared to the case that the pump of the first double pass NOPA was blocked with 1 nJ seed energy (single-pass - red squares / double-pass - red circle). b) Pulse profile from a SPIDER measurement at maximum power with a pulse duration of 8.7 fs.

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70 mm Fused Silica stretcher, several meters of air, and 4x5 mm BBO were compensated by 32 reflections on specially designed broadband chirped mirror pairs (DCM11, Laser Quantum). Compressor efficiency of 69% leads to compressed pulse energies of 18 µJ. Pulse characterization after dispersion fine tuning via a BaF₂ wedge pair was performed with a SPIDER device (Laser Quantum), leading to a pulse FWHM of 8.7 fs (see Fig. 3(b)) with 69.4% of the energy in the central peak and a reasonable beam profile (see Fig. 4(b)). For the output power stability an rms value of 0.4% was measured over 20 minutes (see Fig. 4(a)) without active stabilisation of the temporal delay or the temperature of the SHG crystals. On longer timescales power drifts were observed which could be corrected by minor adjustment of the phase matching angle of the first SHG crystal.

 

Fig. 4 a) Power stability with an rms of 0.4% measured at the output of the system. b) Beam profile obtained after the NOPA stages and the mirror compressor.

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For the case that the seed in front of the first NOPA stage is blocked, the multipass concept generates quite significant Optical Parametric Generation (OPG) background with average power up to 1.4 W (see Fig. 5(b)). As shown in Fig. 5(a) the spectral shape of the seeded and the un-seeded cases looks quite different. Naturally, the amount of superfluorescence is much lower in the seeded than in the un-seeded case [21]. To measure the OPG underground in the seeded case we used an optical cross correlation setup. A surface reflection on an uncoated fused silica plate in the seed beam was used to separate a fraction from the short-pulse seed oscillator in front of the bulk stretcher. This fraction was non-collinearly overlapped with about 20% of the amplified and compressed output in a 20 µm thick Type II phase matched BBO crystal. The sum-frequency signal was separated from the two SHG signals with a pinhole and measured with a photomultiplier tube and a lock-in amplifier (Stanford Research SR830 DSP). Tuning of the relative temporal delay was realized by a manual linear stage. The results of the measurement for the seeded and un-seeded case are shown in Fig. 5(c). The OPG forms a background pulse with durations close to the pump pulse. For the seeded case, the main pulse is accompanied by a weak OPG background. 87.5% of the energy are located in the main pulse inside a time window of 5% of the peak.

 

Fig. 5 a) Measured seed spectrum (black), behind the two double-pass NOPA stages (red), superfluorescence (blue). b) Comparison of the slope efficiency of the second double-pass NOPA stage with seed (red) and for the superfluorescence output (blue). c) Optical cross correlation on a logarithmic scale; the signal was measured at the output of the system for the compressed NOPA output (red) and the superfluorescence (blue).

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The presented measurement is well suited to estimate the OPG underground in OPCPA systems and a simple alternative to the spectral-hole technique [22]. Considering the OPG background, the pulse still reveals a peak power of 1.26 GW. Therefore, this simple setup is well suited for non-linear experiments like high-order harmonic generation at high repetition rates [23–25]. For future experiments the CEO-phase of the octave-spanning oscillator can be easily stabilized. Slow thermal phase drifts during the amplification process can be straightforward counteracted by means of a second f-to-2f interferometer (see e.g [26].).

4. Conclusion

The concept presented here is suited to realize a multi-µJ, few-cycle light source with high repetition rate which comes with a significant reduction of complexity in comparison with other state-of-the-art OPCPA systems. It is based on an ultra-broadband Ti:sapphire seed oscillator with all-optical synchronization to an efficient CPA free, directly diode pumped solid-state pump source. The nearly Fourier limited amplifier output could be easily frequency doubled in a two-crystal setup with high efficiency. Parametric amplification in two double-pass non-collinear amplifier stages leads to 18 µJ of pulse energy at 8.7 fs pulse duration with 1.26 GW of peak power after mirror compressor with well characterized fluorescence background. The multipass concept shows a simple and cost-efficient way to increase the output energy of parametric amplifiers with 10 ps pump pulses. For the best of our knowledge, this is the first OPCPA system which combines a CPA-free ps amplifier with two-stage SHG and a multipass NOPA concept.