We present a compact TW-class OPCPA system operating at 800 nm. Broadband seed pulses are generated and pre-amplified to 25 μJ in a white light continuum seeded femtosecond NOPA. Amplification of the seed pulses to 35 mJ at a repetition rate of 10 Hz and compression to 9 fs is demonstrated.
© 2014 Optical Society of America
Optical parametric chirped pulse amplification (OPCPA)  is a well established method to produce high-energy, sub-10 fs pulses. Generation of multimilijoule sub-10 fs pulses around 800 nm has already been demonstrated in OPCPA systems pumped by picosecond Nd:YAG [2, 3] and Ti:sapphire lasers . In the majority of works devoted to the development of few-cycle OPCPA systems, the main source of the broadband seed for the parametric amplification in vicinity of 800 nm was the output of a broadband mode-locked Ti:sapphire oscillator . In this case the output of picosecond parametric amplifiers often exhibits a substantial uncompressible amplified parametric fluorescence (APF) background due to small seed energy. The scaling of the seed energy by employment of the Ti:sapphire amplifier in combination with the noble gas-filled hollow core fiber  has led to a significant improvement in the output pulse contrast reducing APF caused background level down to 10−10 . However, this technique does not eliminate temporal pedestal consisting of amplified spontaneous emission (ASE) coming from the Ti:sapphire frontend that is amplified within temporal window of OPCPA pump. A source of broadband and ASE-free seed pulses around 800 nm is a white light continuum (WLC) generated in a bulk materials by femtosecond pulses of OPAs operated at longer wavelengths  or ytterbium-doped laser systems, which automatically provides the possibility to perform seed generation and amplification in a femtosecond non-collinear optical parametric amplifier (NOPA) [8–10]. Employment of femtosecond pump pulses in the initial OPCPA stages is advantageous, since thinner crystals and narrower pump beams can be used, implying an increased amplification bandwidth  and a reduced level of APF which is proportional to the pump beam area . It is also important to point out that the recompressed APF resides within the time window defined by the duration of the femtosecond pump pulse, since the contribution of APF arising in the subsequent picosecond amplifications stages is practically negligible.
Nd:YAG lasers are front-rank sources of picosecond pump pulses for high energy OPCPA systems. However, if they are used in the systems based on Ti:sapphire front-ends, the synchronization between the seed and pump pulses becomes cumbersome and is realized by generation of an optical soliton at 1064 nm in a photonic crystal fiber  or implementation of an additional phase-locked oscillator . When a femtosecond Yb laser is used for broadband seed generation, all-optical synchronization is straightforward because of a partial overlap of the Yb and Nd:YAG spectral lines .
Here we present a sub-10 fs TW-class OPCPA system operating at 10 Hz repetition rate and consisting of multiple parametric amplification stages driven by femtosecond ytterbium and picosecond neodymium pump sources. WLC seeded femtosecond NOPA provides 25 μJ energy in a broadband seed pulses. Amplification of these pulses up to 35 mJ in Nd:YAG laser pumped OPCPA and pulse compression down to 9 fs is demonstrated.
2. The OPCPA setup
The setup of our OPCPA system is outlined schematically in Fig. 1. The system can be conceptually divided into a non-collinear optical parametric pre-amplifier of white light continuum, pumped by femtosecond pulses (fs NOPA), and a high energy picosecond parametric amplifier.
The front end is based on a solid-state Kerr lens mode-locked Yb:KGW oscillator, which delivers 7 nJ, 80 fs pulses at 1030 nm. Most of the oscillator output is sent through a transmission grating stretcher and then used to seed an Yb:KGW regenerative amplifier (Light Conversion Ltd.), delivering up to 1 mJ pulses at 1 kHz. Also, using a polarizing spectrum splitter, a fraction of the oscillator pulse energy (12 pJ within 0.6 nm bandwidth at 1064 nm) is delivered to a Nd:YAG regenerative amplifier. Thus, by seeding the two amplifiers from one master oscillator, we realize an all-optical synchronization of the seed and pump pulses.
The fs pre-amplifier used in our current setup differs somewhat from the previously reported by our group . The compressor of the Yb:KGW regenerative amplifier is detuned slightly to deliver down-chirped pulses of ∼450 fs at full width half-maximum (FWHM). A few percent of the amplifier output are compressed in a 4 cm ZnSe rod and focused into a 4 mm sapphire plate to generate a smooth white light continuum (see Fig. 2(a)). The main portion of the amplifier pulse was frequency doubled in a 0.7 mm thick BBO crystal and then used to pump the two stages of the fs NOPA. Using chirped pump pulses in the fs NOPA allows us to match the durations of the seed and the pump without any dispersion management of the white light continuum, thus minimizing the losses and making the system simpler and more compact. Parametric amplification is carried out in Type-I BBO crystals at a non-collinearity angle α ≈ 2.5° and phase matching angle θ ≈ 24.6°. In order to minimize the parametric superfluorescence, the first femtosecond NOPA stage is operated in a low-gain regime, keeping the pump intensity ∼3 times below the saturation level. The continuum pulses are amplified from ∼10 nJ to 0.6 μJ by using 15 μJ pump pulses. The seed and pump beams were both focused to a spot size FWHM of ∼140 μm onto 2.5 mm BBO crystal. The second stage (1.8 mm BBO long, beam diametrer 1 mm) are pumped by pulses of 300 μJ and increases the signal energy up to 25 μJ, while the spectrum (see Fig. 2(a)) corresponds to a Fourier-limited pulse duration of ∼6.7 fs. The amplified seed pulses are sent to a compact stretcher (discussed in more detail in Section 3) and then to the picosecond amplification stages.
Pump pulses for ps parametric amplifiers was produced by Nd:YAG amplification system (EKSPLA Ltd.) comprising of regenerative and linear amplification stages (for details see ). A two-stage cascaded second harmonic (SH) generation scheme was used for conversion of 380 mJ of the fundamental Nd:YAG harmonic (FH) pulse into two SH pulses with different temporal shapes. After the first SHG crystal (DKDP type I, 10 mm long), SH pulses with a nearly Gaussian envelope (70 ps FWHM) were generated with 50% efficiency. In the second SHG stage (DKDP type I, 20 mm long) the remainder of the FH pulse was used for generation of a flat-top SH pulse . In this paper we present for the first time the application of this technique in the OPCPA system providing an efficient use of the pump and a favorable conditions for mitigation of the spectral gain narrowing in the high-gain stages of the OPCPA. The shapes of picosecond pump pulses measured by the cross-correlation technique using femtosecond pulses from the Yb:KGW laser as a probe are presented in Fig. 2(b,c).
Both picosecond OPCPA stages are based on 5 mm long BBO crystals. The first stage, pumped with 15 mJ, 100 ps flat-top pulses focused to a spot of 0.8 mm diameter (FWHM), amplifies the seed pulses to 0.5 mJ. Next, the signal beam is expanded to a diameter of 8 mm (FWHM) to match main pump beam and amplified to 35 mJ in the second OPCPA stage. This stage is operated at low gain and strong saturation to avoid narrowing the signal spectrum. A typical OPCPA output spectrum is shown in Fig. 2(a) (red curve). Wavelengths above 970 nm have undesirable spectral phase modulation, caused by a filter inserted after the WLC generator to block the 1030 nm pump pulses. Therefore, these wavelengths are intentionally filtered out in the grisms. The asymmetry of the spectrum results from the slight asymmetry of the pump pulse in the last OPCPA stage and from the dispersion of the stretcher, since in our case, the shorter wavelengths are more dispersed in time as compared to longer ones, and thus interact with more pump energy per unit spectral interval.
In oder to inspect the level of amplified superfluorescence, we have measured the OPCPA output energy while the white light continuum was blocked. We found that this energy is mostly determined by the pump intensity in the first amplification stage of the fs NOPA and was lower than 10 μJ (<0.03% of the amplified signal energy) under normal operating conditions.
Although the initial profile of the signal beam is nearly Gaussian, the signal adopts the top-hat shape of the pump beam due to the saturation in the last OPCPA stage. The profiles of the pump and amplified signal beams in the last OPCPA stage are shown in Fig. 3(a, b). The beam profiles at various parts of the signal spectrum were determined by inserting narrow-band filters into the beam path (see Fig. 3(c)). Although some variations in the beam profiles are visible, no evident spatial chirp was detected. The focusability of the output beam was evaluated by measuring its intensity distribution in the focal plane of a f =1 m concave mirror. The focused beam is close to Gaussian and does not exhibit any significant ring structures (see Fig. 3(d)). The spot size is only ∼1.2 times larger than that calculated for an ideal 3rd-order super-Gaussian beam under equivalent conditions.
3. Pulse stretching and compression
In our system, we employ the down-chirped pulse amplification technique. The stretching module consists of an acoustooptic programmable dispersive filter (AOPDF) with a 45 mm-long TeO2 crystal (Dazzler, Fastlite), followed by a home-built grism stretcher for flexible dispersion control . The stretcher is composed of SF-10 prisms with an apex angle of 19 degrees and reflective gratings with a groove density of 300 grooves/mm. At the exit of the stretcher, the seed pulses have a duration of ∼50 ps. The achievable duration of the stretched seed pulses was limited by the aperture of our grisms. However, it would be desirable to stretch the seed pulses more in order to match the ∼100 ps flat-top pump pulses better.
The compressor consists of several rods of H-ZF52A glass (SF-57 equivalent), adding up to a total length of 420 mm, and a 100 mm of fused silica (FS). The final stage of the compression is performed by 6 bounces from the chirped mirrors (Optida Ltd.) with a group delay dispersion (GDD) of approximately +50 fs2/bounce. Due to the small aperture of the H-ZF57A glass rods available in our laboratory, the OPCPA output was attenuated to 50 μJ before being sent to the compressor, thus avoiding nonlinear propagation effects.
The compressed pulses were characterized simultaneously by chirpscan , utilizing the AOPDF in the stretcher, and Frequency Resolved Optical Gating (FROG) . The apparatus we have used allowed us to perform SHG FROG and chirpscan measurements in the same optical setup, thus it is meaningful to compare the results obtained by these two methods. The chirpscan trace of the compressed pulse is shown in Fig. 4(a). The trace exhibits good left-to-right symmetry, which is a strong indication of a nearly transform-limited pulse, thus even using the stationary phase approximation one should get a reasonable estimate of the spectral phase, shown in Fig. 4(b) (curve no. 3). Applying this phase to the measured OPCPA output spectrum yields a pulse that is virtually indistinguishable from the transform-limited one. The FROG trace was measured without altering dispersion settings of the system and the corresponding FROG inversion results are shown in Fig. 4(b)–4(d). The FROG retrieval error was 1.8% on a 128×128 grid. Although the FROG measurement shows a certain amount of residual chirp, the measured pulse duration still differs by less than 9% from the transform limit and ∼60% of the pulse energy is delivered within a ±5 fs temporal window.
In conclusion, we have developed a table-top OPCPA system pumped by fs Yb:KGW and ps Nd:YAG lasers. Employing a femtosecond Yb:KGW laser driven WLC generator and NOPA stages, a compact grism and an AOPDF based pulse stretcher and a flat-top picosecond pump pulses, we have obtained high spatio-temporal quality output pulses with the energy of up to 35 mJ. The fraction of the APF energy was measured to be smaller than 3×10−4. Attenuated output pulses were compressed down to 8.9 fs. Equipped with compressor optics of appropriate aperture, the system would be capable of producing pulses with peak powers exceeding 3 TW. The footprint of the whole setup is 4×1.5 m. Together with recent results regarding stabilization of carrier-envelope phase of the Yb:KGW laser systems [20,21] this continuum-seeded OPCPA system appears to be a promising source for high-field science applications.
This work was partially funded by Lithuanian Agency for Science, Innovation and Technology (Grant No. 31V-29), the European Community’s social foundation (grant agreement No. VP1-3.1-ŠMM-08-K-01-004/KS-120000-1756) and EU Seventh Framework Programme (grant agreement No. 284464)
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