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%.
© 2015 Optical Society of America
Since the last decade, high power and ultrafast optical parametric chirped pulse amplification (OPCPA) concepts have gained increasing recognition in the field of ultrafast science . Meanwhile, these systems can be used for high harmonic generation (HHG) and the generation of isolated attosecond pulses [2,3]. High repetition rate OPCPAs offer great potential for applications such as photoemission electron microscopy (PEEM)  and cold target recoil ion momentum spectroscopy (COLTRIMS) . These experiments especially benefit from the high repetition rates of more than 100 kHz allowing reduced integration times, improved signal-to-noise ratios and only weak space charge effects affecting the measurements .
Along with pulse peak powers in the gigawatt range and durations in the few-cycle regime, a constant carrier-envelope-phase (CEP) is essential in combination with a stable and constant output. Most commonly, those systems are based on a broadband Ti:Sapphire oscillator synchronously seeding the infrared pump amplifier and the parametric amplifier itself , hence the involved pump and seed pulses are optically synchronized. The CEP of these Ti:Sapphire oscillators is now routinely stabilized using f-to-2f interferometry [8,9]. Owing to the phase preserving nature of the parametric amplification process itself, CEP-stable output pulses are directly achievable [10–12].
Output powers reaching 22 W at 1 MHz (i.e. 22 µJ of pulse energy) and pulse energies up to 29 µJ at 150 kHz repetition rate were presented so far , demonstrating the power and energy scalability of the OPCPA approach at high repetition rates. Later, heat deposition inside the nonlinear crystals (i.e. BBO) caused by unwanted idler absorption was reported to be one major challenge towards the further power scaling of these systems . Nonetheless, output powers of up to 100 W were reported to be feasible even with BBO when slightly tuning the output spectrum and using specially designed crystals containing a sapphire sandwich structure [15,16].
In this publication we report on a CEP-stabilized, highly-efficient, thin-disk regenerative amplifier pumped, high repetition rate OPCPA system. The average power before compression reached 20 W at 300 kHz repetition rate. After temporal compression to less than 6 fs and CEP stabilization, the average power was reduced to 15 W leading to a pulse energy of 50 µJ. With respect to the reconstructed temporal pulse shape, the calculated pulse peak power exceeds 4 GW. The overall optical efficiency from the infrared pump amplifier output to the compressed few-cycle output is beyond 10%. The carrier-envelope-phase of the amplified pulses was measured at the system output and further optimized by applying a slow-loop feedback to the Ti:Sapphire stabilization unit resulting in a remaining phase noise below 240 mrad averaged over 200 pulses and recorded over a 20-min. time frame. The complete system runs stable with long-term power fluctuations of less than 1.5% recorded over 2 hours and the energy fluctuations were measured to be less than 3% over 30000 consecutive pulses.
2. System setup
The setup is schematically shown in Fig. 1. It is based on a broadband CEP-stabilized Ti:Sapphire oscillator (VENTEON Pulse:One OPCPA) simultaneously seeding the infrared pump amplifier and the parametric amplification stages themselves. The oscillator provides a pulse energy of more than 2 nJ at 75 MHz repetition rate. The spectrum spans over more than one optical octave (600-1200 nm) and supports a pulse duration of less than 5 fs. A 10-nm broad fraction centered at 1030 nm is extracted from the output spectrum and amplified from ~30 pJ to 500 pJ by means of an integrated fiber amplifier (VENTEON PreAmp2). In addition, the repetition rate of the pulse train is reduced by a factor of 4 to enable the separation and amplification of one single pulse in the regenerative amplifier cavity. Prior to amplification, the pulses are temporally stretched along a fiber Bragg grating (FBG) introducing a dispersion of −64 ps/nm and supporting a bandwidth of 5 nm to prevent nonlinear distortions and damages from high peak powers. The higher order dispersion terms of the stretcher up to fourth order are matched to those of the grating compressor after the amplification to attain pulse recompression close to the Fourier-limit. The grating compressor consists of two dielectric reflection gratings (1760 l/mm) and is built in a near-Littrow configuration. The pulses are amplified by a modified commercial Yb-based thin-disk regenerative amplifier (TruMicro 5070) .
Once recompressed, the infrared 1030 nm-pulses are frequency converted to the green inside a 1.5-mm-long LBO crystal (Type I; ϕ = 13.8°, θ = 0°). The crystal is mounted inside a copper inlay whose temperature is stabilized to 40° C ensuring stable long-term operation. The infrared and green parts are subsequently separated using a dicroic beam splitter. The 515-nm component is again splitted by a combination of waveplates and thin film polarizers to pump the two OPA stages and feed an additional Pump-Seed-Synchronization (PSS) unit based on frequency resolved optical cross correlation . This PSS-unit is required to maintain the temporal overlap between the pump and seed pulses inside the OPA crystals. Reducing the timing fluctuations below 5 fs, its implementation is essential for the long-term stability of the system.
The parametric amplification process itself takes place in two BBO crystals (θ = 24.5°), which are 2-mm and 5-mm long for the 1st and 2nd stage, respectively. The OPA crystals are temperature-stabilized inside a copper mount, similarly to that of the SHG unit. For broadband amplification, a noncollinear geometry is used with an internal pump-seed angle of ~2.4° while both stages are set-up in the Poynting-vector-walk-off configuration. Despite its slightly reduced efficiency associated with parasitic phase-matching effects, we experimentally verified an improved beam profile and pulse compressibility. Prior to amplification, the seed pulses are temporally stretched inside a 5-mm long fused silica substrate. Together with the dispersion of the ambient air, the seed pulse is stretched to ~200 fs (at −10 dB) directly in front of the first OPA stage. Here, the pulse is amplified but also further stretched in time due to the highly dispersive BBO. Before entering the second OPA stage, the seed pulse duration was calculated to be ~380 fs (at −10 dB) ensuring an optimized temporal overlap with the ps-pump pulses and hence a high conversion efficiency. The compressor consists of a pair of double chirped mirrors (VENTEON: DCM 11) of which the dispersion is matched to that of the BBO crystals. Additionally, a CaF2 wedge pair is used for fine tuning the pulse duration. In both stages, the pump beam diameter is adjusted to reach pump intensities of up to 120 GW/cm2. The diameter of the seed beam is approximately 5% bigger compared to that of the pump beam.
3. Experimental results
3.1 Infrared pump amplifier & second harmonic generation (SHG)
Due to gain narrowing effects in the amplification process, the bandwidth of the amplified output spectrum is reduced from 5 nm to 1.4 nm corresponding to transform-limited pulses of ~1.1-ps duration. The repetition rate of the infrared pump amplifier is tunable between 100 kHz and 300 kHz delivering over 200 W of average power and pulse energies of more than 2 mJ. In the OPA setup, the repetition rate was set to 300 kHz. Using highly efficient dielectric reflection gratings, the compression efficiency was measured to be higher than 95% and resulted in pulse durations of 1.25 ps (Fig. 2(a)) close to the supported Fourier-Limit.
Although some small side wings are visible in the measured autocorrelation trace, the excellent temporal and spatial quality of the amplified pulses is confirmed by the high SHG conversion efficiency with up to 70% (Fig. 2(b)). The side wings originate from uncompressible high order dispersion terms of the FBG. Due to the nonlinearity of the SHG process, they are significantly reduced resulting in a nearly Gaussian temporal beam profile. According to the theory, the green pulse duration is slightly shortened from 1.25 ps to 1.15 ps. The excellent long term stability (< 0.5% rms during 2.5 hours), pulse-to-pulse energy stability (< 2% over 30000 consecutive pulses), beam quality (M2x,y < 1.2) and pointing-stability (< 55 µrad over 3000 consecutive pulses) make this source well suited for pumping OPCPA.
3.2 OPCPA output
For the following parametric amplification, the infrared pump amplifier was set for generating ~95 W (i.e. ~317 µJ at 300 kHz repetition rate) at 515 nm. This output was split into three parts using a set of half-wave plates and thin-film polarizers. Only ~3 W were required to operate the PSS leaving ~15W and ~75 W for the first and second parametric amplification stage, respectively. Using a broadband beam splitter for the seed, 20% of the Ti:Sapphire output (~0.4 nJ) was reserved for the PSS, while the remaining 80% (~1.6 nJ) were seeding the OPA stages. In the first stage, the nJ-seed pulses were amplified by a factor of ~103 to approximately 1.5 µJ (~500 mW). In the second amplification stage this energy was further boosted to more than 65 µJ (~20 W). Finally, with ~75 W of pump power applied in the second stage, we obtained optical-to-optical efficiencies of nearly 27%. Despite the losses introduced by the multiple bounces on the DCM-pair and a glass wedge for feeding the slow-loop CEP stabilization withdrawing additionally ~5%, the compressed output power still exceeded 15 W (50 µJ). The duration of the output pulses was measured using a SPIDER apparatus. The amplified spectrum and the retrieved phase information led to a reconstructed temporal pulse profile with more than 4.4 GW of peak power and less than 6 fs duration (see Fig. 3) with ~60% of energy located inside the main pulse. The spectral phase was well defined over the complete supported range from 650 to 1000 nm. The spectral and phase modulations around 900 nm however indicate the presence of back and forth conversion, parasitic effects and the imprinted parametric phase was manifested by a small temporal pulse background located within −100 and 100 fs from the main pulse.
Using a 3-mm-long BBO crystal in the second stage instead, we were still able to reach more than 10 W of compressed output power (i.e. more than 14 W before DCM-compressor and pick-off mirror). Although back and forth conversion effects were still present, the phase of the compressed pulse was slightly smoother and the pulse duration was measured to be only 5.4 fs with a peak power of 3.1 GW. However, since the pulse peak power is higher using the 5-mm-long crystal, it was finally used in the system.
The stability of the OPCPA output was verified by tracking the output power over time. The long-term power stability error was measured to be less than 1.4% when running at full power over two hours (see Fig. 4). On even longer time scales, only slight adjustment of the spatial overlap in the second OPA stage was necessary to reestablish the original performances. The energy stability was recorded using a GHZ-oscilloscope and a fast photodiode. Measured over 30000 subsequent pulses, the rms-error is below 2.7%, additionally attesting the excellent performance of the system. The signal beam exhibits a near-Gaussian spatial profile, even at the highest output power (see inset Fig. 3(b)).
To estimate the pulse contrast, we additionally investigated the generated superfluorescence background. When blocking the seed beam in each individual stage, it was observed to be negligible. Nonetheless, when blocking the seed beam in front of the first OPA stage, the fluorescence level increased to ~150 mW measured after an iris aperture with a diameter of ~5 mm placed approximately 0.75 m behind the second stage, which can be attributed to a weak fluorescence generated in the first stage being further amplified in the second OPA stage. The final ratio of fluorescence background compared to the amplified signal was estimated to be less than 10−6 taking additionally the effect of gain quenching (i.e. the presence of a seed) and the pulse compression of the amplified pulses into account .
To measure the CEP-stability of the amplified pulses, a modified common path f-to-2f interferometer (Menlo APS 800) was used. For this purpose, approximately 5% of the output energy was split up using an uncoated fused silica substrate. In a first step, the spectrum was broadened to more than one optical octave by white light generation in a thin sapphire substrate. Subsequently the infrared part of the spectrum was frequency doubled in a thin BBO crystal and the interference pattern between fundamental and frequency doubled components was analyzed using a spectrometer.
When switching off the oscillator CEP-stabilization, the interference fringes immediately vanished. To quantify the CEP-stability of the output, the phase drifts were analyzed by means of a FFT of the observed interference signal. For the measurements, the integration time of the spectrometer was set to 630 µs. Thus every spectrum was averaged over 200 shots. Figure 5 shows the measured phase noise with the slow-loop feedback activated. A residual phase noise of less than 240 mrad over more than 20 minutes was measured. Due to the averaging over 200 pulses, the information about the pulse-to-pulse CEP-fluctuations is lost which might slightly deteriorate this value. In the free running system, the CEP-phase oscillates with amplitudes of more than π/2 with a ~5-s periodicity (see Fig. 5(a)). This behavior might be attributed to cooling cycles of the pump laser and the breadboard temperature control.
We demonstrated a high power, CEP-stabilized, few-cycle laser system with more than 15 W of compressed output power at 300 kHz repetition rate delivering 50 µJ of pulse energy. The OPCPA system is ideally suited for applications such as high repetition rate HHG and ultrafast spectroscopy. With an uncompressed output power of 20 W, the optical-to-optical efficiency of the second amplification stage is above 27%. The conversion from the initial infrared pump into the compressed amplified few-cycle pulses is still more than 10% underlining the high efficiency of this concept.
The author thanks Thomas Binhammer from VENTEON Technologies for the support with the Ti:Sapphire oscillator and the fruitful discussions as well as Martin Wolferstetter from MENLO Systems for the modification and implementation of the slow loop phase stabilization unit.
This research has been supported in part by Global Research Laboratory Program [Grant No 2009-00439], by Leading Foreign Research Institute Recruitment Program [Grant No 2010-00471] and by Max Planck POSTECH/KOREA Research Initiative Program [Grant No 2011-0031558] through the National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT & Future Planning.
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