1. Introduction

Today, ultra-short laser pulses have found widespread applications. The process of high order harmonic generation (HHG) in noble gases is one impressive example where intense ultra-short laser pulses are required [1]. The emitted radiation provides laser like brilliance and coherence in the XUV wavelength range. Furthermore, attosecond pulse generation became feasible by using driving laser pulses as short as a few optical cycles [2]. However, the photon flux in the XUV range is low, since the conversion efficiency of the HHG process is typically below 10⁻⁶. Hence, a high laser output power and repetition rate is desirable for many applications, improving signal to noise ratio and minimizing necessary detector integration times.

Another interesting application for XUV radiation, generated via HHG of a high repetition rate laser source, is seeding of a free electron laser (FEL) [3]. By seeding the FEL with high order harmonics of an external infrared laser it is possible to achieve higher shot-to-shot stability and shorter pulse duration and improved coherence. Additionally, the output of the FEL is precisely synchronized with the external seed laser, thus enabling pump-probe experiments with femtosecond resolution.

The infrared laser pulses for HHG are typically delivered by Ti:Sapphire (Ti:Sa) lasers, which nowadays deliver up to 100 Gigawatt peak power pulses at repetition rates up to several kHz [4]. Additional post- compression, based on hollow core fibers, enabled the generation of pulses as short as 3.8 fs [5]. However, due to thermo-optical effects, e.g. thermal lensing, the output power of these laser systems is limited to the Watt level. Cryogenically cooling of the Ti:Sapphire crystal helped to increase the average output powers up to 40 W [6], but the cooling technique is complex and expensive. Additionally, further power scaling is limited by the required high power pump lasers with excellent beam quality.

An alternative approach to generate intense few cycle pulses is optical parametric amplification (OPA). In noncollinear geometry, which offers enormous gain bandwidth, pulses as short as 3.9 fs have been generated [7] and the generation of sub-10 fs pulses at up to 1 MHz repetition rate has been reported [8]. By combination of noncollinear optical parametric amplifier (NOPA) and the chirped pulse amplification principle (CPA) extremely powerful optical parametric chirped pulse amplification (OPCPA) few cycle laser sources have been developed [9,10]. Due to low absorption and lack of energy storage in the nonlinear crystals, thermal lensing is negligible in these amplifiers.

Today, high power pump lasers for OPCPA are based on rare earth ions such as Nd³⁺ or Yb³⁺, which offer a very low quantum defect compared to Ti:Sa and can be pumped directly by high power laser diodes. Especially the fiber geometry, which offers a large surface to active volume ratio, provides excellent thermal properties. State of the art femtosecond fiber amplifiers are based on chirped pulse amplification and deliver pulse energies up to 1 mJ [11] and average powers up to 325 W [12]. They have been used successfully as pump for OPA and pulses as short as 29 fs [13], with a pulse peak power as high as 2 GW have been generated. Additionally an average output power as high as 4 W [14] has been achieved for 50 fs pulses at 100 kHz repetition rate. However, these systems are rather complex, and in particular, to achieve the high pulse energies of 1 mJ [11], a large stretcher and compressor is needed in the pump laser to keep the accumulated nonlinear phase (measure: B-Integral) low, and therefore, to prevent pulses from nonlinear distortion. In addition, to match the broadband signal pulse duration to those of the pump pulses, a second stretcher and compressor is needed for the signal pulses.

An alternative approach is direct (unchirped) amplification of picosecond pulses without the need of large and complex CPA in the pump laser. Recently, this concept has been implemented in an OPCPA system delivering 50 fs, 16 µJ pulses, with 180 MW peak power at 80 kHz repetition rate [15].

In this contribution we present a significantly improved, table-top OPCPA system. A schematic of the experimental setup is shown in Fig. 1 . Both the OPA and the fiber amplifier chain are seeded by a cavity dumped Ti:Sa oscillator. A two stage birefringent pulse shaper generates 460 ps long flat-top pulses, which allows us to extract 1 mJ pulses from the power amplifier. Additionally, the efficiency of second harmonic generation (SHG) and optical parametric amplification is improved, due to the uniform temporal profile. The broadband signal pulses are stretched by a Martinez type grating stretcher and parametrically amplified up to 72 µJ pulse energy in two BBO crystals. The pulse compressor consists of two dielectric transmission gratings, which provide high diffraction efficiency and high power capability. We measured a compressor throughput as high as 74% resulting in 53 µJ energy for the compressed pulses. The compressed pulse duration is 35 fs.

 

Fig. 1 Schematic overview of the experimental setup including the Ti:Sapphire oscillator, the pre- amplifier and spectral filter, the pulse shaper, the power amplifier, second harmonic generation (SHG), the optical parametric amplifier (OPA) and the grating stretcher and compressor.

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Overall, the improved system provides a pulse peak power of 1.1 GW, which is a factor of 6 higher, than the previously reported [15]. The pulse duration is significantly reduced and the pulse energy is increased more than 3 times.

2. The pump laser

For parametric amplification, due to lack of energy storage in the nonlinear medium, synchronized pump pulses are required. They are generated by soliton formation and self frequency shift in a small-core photonic crystal fiber with enhanced nonlinearity and tailored dispersion [16, 17]. The frequency shifted pulses are directly amplified in two step-index single mode fiber pre-amplifiers, which are both operated in double pass configuration. The spectral bandwidth is reduced by fiber Bragg gratings to 15 pm resulting in transform limited 100 ps Gaussian-like pulses. The pre-amplifiers offer turn-key, stable and alignment-free operation, since all fiber components are polarization-maintaining and spliced together [15].

To lower gain narrowing effects in the OPA, which occur for non-uniform pump pulse profiles [18], and to increase the efficiency of SHG and OPA, we implemented a two stage birefringent pulse shaper [19], to generate a flat-top temporal pulse profile. The schematic setup of the pulse shaper is shown in Fig. 2 .

 

Fig. 2 Schematic setup of the two stage birefringent pulse shaper

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The device is based on birefringent Yttrium Orthovanadate (YVO4) crystals. In each stage, the polarisation of the optical pulses is set 45 degrees with respect to the optical axis of the crystal. Therefore, the same part of optical power is polarized ordinary and extraordinary. At the end of each crystal the two pulse replicas experience a temporal delay and interfere at a polarizer, which is orientated 45 degrees to the optical axis and is located at the crystals output. The two stages provide 133 ps / 266 ps delay with a crystal length of 180 mm / 360 mm. Temperature control of the crystals allows for fine tuning of the relative phases of the interfering sub-pulses. The relative amplitudes of the sub-pulses are controlled by half-wave-plates, which are placed in front of each crystal set. The generated pulses consist of four sub-pulses, have a flat-top like pulse shape and 460 ps pulse duration.

These pulses are further amplified in a 1.5 m long double-clad Yb-doped photonic crystal fiber which has a mode field diameter of 33 µm. The amplifier is used in a double pass configuration while a Tellurium Dioxide (TeO₂) acousto-optical modulator is used to reduce the repetition rate to 40 kHz after the first pass of the amplifier fiber. The AOM provides a modulation contrast better than 10⁻⁵, when operated in double pass. However due to the rise- and fall-time of the modulator the neighbouring pulses are suppressed by 10⁻². In the second amplification pass the pulse energy is increased to 5 µJ which corresponds to an average power of 200 mW.

The power amplifier consists of a 1.2 m long Yb-doped rod type photonic crystal fiber with a mode field diameter of 71 µm, delivering up to 41.5 W of average power. It is pumped by a 976 nm fiber coupled laser diode, imaged into the 200 µm diameter pump core of the rod type fiber. The amount of amplified spontaneous emission (ASE) is measured to be below 2% and unwanted pre- and post-pulses amount to 2%, hence the pulse energy of the amplified pulses is as large as 1 mJ. It is worth mentioning that even at the highest output power we could not observe significant degradation of the polarization contrast and that the amplifier is operated at a B-Integral as high as 25 rad. It has to be stressed that such a high accumulated nonlinear phase leads to severe degradation of the pulse quality in fiber CPA systems [20].

In contrast, self-phase modulation only results in moderate spectral broadening in the herein presented approach. Most important, the amplified picosecond pulses are not disturbed by the nonlinear phase in the temporal domain. The measured spectral bandwidth of the amplified pulses is below 0.2 nm (FWHM), which is acceptable for efficient SHG. When focusing into a 6 mm long LBO crystal, efficient second harmonic generation is observed, resulting in up to 660 µJ pulses at 515 nm wavelength. The corresponding conversion efficiency is as high as 66%, owing to the uniform temporal pulse profile and the excellent beam quality of the fiber amplifier. The output characteristics of the fiber amplifier and the generated second harmonic are shown in Fig. 3 (a). The temporal pulse profile at 1 mJ pulse energy, measured with a fast photodiode (rise time: 18.5 ps) and a 50 GHz sampling scope, is shown in Fig. 3(b).

 

Fig. 3 a) Infrared output power of the fiber amplifier (red, squares), generated second harmonic power (green, triangles) and second harmonic conversion efficiency (black, circles) versus pump power of the fiber amplifier. b) Temporal pulse profile of the amplified pulses at 1mJ pulse energy.

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3. Optical parametric chirped pulse amplifier

3.1 Grating stretcher

The signal pulses, delivered by the cavity dumped output of the Ti:Sa oscillator (95 nm FWHM), are stretched to match the pump pulse duration. For this purpose a Martinez-type grating stretcher is applied which is based on a dielectric transmission grating with 700 nm period used under Littrow angle. It is worth mentioning, that the material dispersion of the 5 mm thick fused silica substrate can be compensated easily at least in the 2nd and 3rd order by the compressor, and is therefore negligible even for 15 fs pulses. The stretcher provides low aberrations, e.g. no measurable spatial chirp, and a stretched pulse duration of 400 ps for 90 nm transmitted bandwidth (FWHM).

3.2 Parametric amplifiers

Suitable candidates for the nonlinear material are several crystals belonging to the borate group (BBO, LBO, BIBO). They offer a high effective nonlinear coefficient, large transparency range and high damage thresholds as well as proper phase matching conditions for broadband amplification around 800 nm central wavelength.

In [15] the properties of these borate crystals are discussed in detail. Especially for relatively low peak power (~MW) pump lasers BIBO is superior due to its higher nonlinear coefficient and angular acceptance. However, when pumped at 515 nm, BIBO shows two photon absorption and photo-induced damages which make these crystals unsuitable for high power operation [21]. For this reason we choose BBO as nonlinear material for the presented OPCPA system, even though, the single pass gain is low due to the relatively low angular acceptance of BBO. Experimentally, a single pass gain of 50 is measured for a pump pulse peak power of 1.4 MW. Hence, the first parametric amplifier (20 mm long BBO) is used in double pass configuration to achieve sufficient gain. The experimental setup of the two stage parametric amplifier is shown in Fig. 4 . When a pump intensity of 1.3 GW/cm² is applied, the 4.4 nJ stretched signal pulses are amplified to 11 µJ in the first stage, corresponding to a gain factor of 2500.

 

Fig. 4 Schematic of the optical parametric amplifier

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The second amplifier is an 8 mm long BBO crystal operated at 2.1 GW/cm² pump intensity (pump beam diameter 275 µm). A single amplification pass boosts the pulse energy to 72 µJ, which corresponds to an overall pump to signal conversion efficiency of 11%. Note that the signal beam diameters in the nonlinear crystals have been chosen to be slightly larger than the pump diameters, in order to compensate for spatial gain narrowing effects during each amplification pass.

The angle between the wave vectors of signal and pump is chosen to be 2.6 ° inside the crystal to provide broadband phase matching over a large spectral range around 800 nm. The calculated gain curve for a 20 mm long BBO used in this configuration is shown in Fig. 5 a ) (grey dashed) together with the measured seed spectrum (blue dashed) and the amplified spectra for only the first OPA pumped (red) and both OPAs pumped (black). The amplified spectral bandwidth is 63 nm (FWHM), which corresponds to a Fourier limited pulse duration of 22 fs.

 

Fig. 5 a) Measured amplified spectra with one (red) and both (black) OPAs pumped. Blue dashed: Seed spectrum, grey dashed: calculated OPA gain curve for 20 mm BBO crystal. b) Measured Autocorrelation trace (black), and 4th order Dispersion corrected Fourier transform of the measured spectrum (grey).

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3.3. Grating compressor

For recompression, the amplified pulses are sent through a grating compressor, which consists of the same type of dielectric transmission gratings used in the stretcher. These gratings provide more than 94% diffraction efficiency for a wavelength range of 760 nm to 820 nm. Experimentally a compressor throughput as large as 74% is measured, including reflection losses at a silver coated roof mirror, resulting in 53 µJ energy of the compressed pulses. The grating angles and their distance are optimized for compensation of 2nd and 3rd order dispersion to achieve the minimum compressed pulse duration. The measured autocorrelation trace is shown in Fig. 5(b) (black). For comparison a Fourier transform of the measured spectrum is shown (grey). It is corrected by 4th order dispersion to the same autocorrelation width. Both curves agree well, despite a small pre- or post- pulse which is present in the measured trace and caused by higher order phase distortions in stretcher and compressor. The assumption of 90% peak power of the corresponding numerical result (grey), leads to a pulse peak power as high as 1.1 GW for the recompressed pulses.

4. Conclusion and outlook

In this paper, we presented an OPCPA system delivering 35 fs pulses with 53 µJ pulse energy and 1.1 GW pulse peak power. The system is pumped by a fiber amplifier, operated at 40 kHz repetition rate and seeded by a cavity dumped broadband Ti:Sa oscillator.

The implementation of a two stage birefringent pulse shaper, with accurate temperature control, enables the generation of 460 ps long flat-top pulses. These pulses allow for extraction of 1 mJ pulse energy out of a rod type amplifier fiber. Additionally, the uniform temporal profile results in improved SHG and OPA efficiency.

Emphasis is also placed on average power scalability of the setup. Highly efficient dielectric transmission gratings are used in the compressor and provide significantly improved high average power capability and efficiency. The use of BBO crystals as nonlinear material avoids thermo optical distortion as well as crystal damage. The presented system currently delivers 2.1 W average power, which is among the highest for high repetition rate OPCPA systems [13, 14]. In future, an increase of the average output power by up to one order of magnitude seems feasible by incorporating a several 100 W fiber amplifier [12] as pump laser and optimization of the OPCPA efficiency.

Due to a revised stretcher, which provides very low aberrations, the compressed pulse duration is reduced to 35 fs and the pulse quality is significantly improved compared to the previously reported results [15].

Overall, the presented system provides a pulse peak power as high as 1.1 GW, which is a factor of 6 higher, than the previously reported result [15]. The pulse peak power and the pulse duration are comparable to the current record of 2 GW and 29 fs [13], which has been achieved by a fiber CPA system pumped OPCPA.

Please note that the herein presented system is less complex and easier to operate, since no stretcher and compressor is needed for the pump laser. Furthermore short free space beam paths and looser timing requirements, due to the longer pump pulse duration, lead to superior stability. In the future we plan to implement a fiber based pump pulse shaper [22] as well as a fiber coupled AOM to have a fully monolithic and alignment-free fiber preamplifier and pump pulse shaper.

The pulse energy of the presented system can be further increased by incorporating longer pump pulses, which allow for extraction of higher pump pulse energies from the fiber amplifier. The limiting nonlinear effect is self focusing, which is observed at ~4.3 MW pulse peak power for linear polarized light [23]. For example a 1 ns pulse could therefore extract more than 4 mJ pulse energy from the amplifier fiber. Additionally, the simplicity of the pump laser allows for the use of multiple amplifier fibers as pump for the OPA, which could increase the pulse energy of the amplified ultrashort pulses to several hundred microjoule. Additionally, the use of an active phase shaper for the broadband signal could reduce the compressed pulse duration of the system below 20 fs.