1. Introduction

In a variety of topical applications in physics, chemistry, biology, and medicine high intensities are required for research, imaging, or micro-machining. MHz repetition rates ensure high signal-to-noise ratios in sensing applications and reduced processing times. Additionally, investigation of ultrafast dynamics occurring for example in molecules requires pulses with durations shorter than 20 fs. Nowadays, few-cycle pulse generation almost completely relies on Ti:sapphire laser systems for low and for high pulse repetition rates. This technique is well established (see e.g. [1, 2, 3]), but so far systems with high repetition rates provide only low pulse energies or the other way around high energies but low repetition rates.

Recently, different groups reported non-collinear parametric amplifier (NOPA) systems at MHz repetition rates pumped by Yb-doped fiber-amplifiers. In one case the NOPA is pumped by a commercial fiber MOPA system delivering pulses with durations down to 20 fs with 825 nJ pulse energy [4]. In the second case, the NOPA is seeded by a Ti:sapphire oscillator with cavity-dumping whereby sub-20-fs pulses and energies up to 500 nJ from the NOPA are achieved [5].

In a previous publication, we demonstrated a MHz-NOPA with sub-10-fs pulses and energies of 45 nJ directly pumped by 400 fs pulses from a cavity-dumped Yb:KYW-oscillator. The white-light seed was generated in sapphire, requiring fiber-based nonlinear pulse compression to durations in the region of 50 fs [6].

In this paper we report on few-cycle pulses with durations of sub-10-fs with substantially higher pulse energies beyond 400 nJ from a NOPA seeded by a white-light filament directly generated with 420 fs pulses in YAG. Using YAG instead of sapphire enables to omit the complex fiber-based nonlinear pulse compression resulting in a less complex and more stable NOPA setup. With the resulting peak power of 29 MW at 1 MHz repetition rate we aspire high harmonic generation in noble gases.

2. Experimental setup

An overview of our experimental setup is given in Fig. 1. It can be subdivided into different modules: the femtosecond seed laser, an optical isolator, the fiber amplifier, the white-light generation stage, the NOPA itself, and finally the pulse recompression stage.

The pump source is a femtosecond fiber-based chirped pulse amplifier (CPA) system [7]. It consists of a grating stretcher, a rod-type fiber amplifier, and a fused-silica transmission grating compressor. As seed oscillator a diode-pumped passively mode-locked Yb:KYW laser oscillator with electro-optical cavity-dumping is used [8], which operates in the positive-dispersion regime and emits chirped pulses with a duration of a few picoseconds and a pulse energy of 1.5 μJ at 1 MHz repetition rate. Due to the well adapted intracavity dispersion the output spectrum of the chirped-pulse oscillator supports a Fourier limit of 360 fs. This seed source is ideally suited for our application since chirped pulses are required for the amplification anyway. After an optical isolator the 2-3ps pulses are further stretched to about 50 ps with an aberration-free all-reflective Offner triplet [9], which consists of two spherical concentric mirrors and one single grating with 1250 lines/mm. Afterwards, the pulses are amplified in a 50 cm long Yb-doped rod-type fiber, which is pumped from the opposite side with a fiber-coupled diode laser capable of delivering up to 75 W at 976 nm. The amplified pulses are dechirped with a compressor consisting of two fused-silica transmission gratings with 1250 lines/mm (see e.g. [10 ]). This quite compact system delivers 9 μJ pulses at a center wavelength of 1030 nm with a pulse duration of 420 fs [7].

 

Fig. 1. Experimental setup of the MHz-NOPA system. DCM: Double chirped mirrors, ROC: Radius of curvature.

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The left hand side of Fig. 1 illustrates the experimental setup of the parametric amplification stage in detail. Approximately 70 % of the incident power is used to generate the pump light for the NOPA process by frequency doubling in a 1.2 mm long LBO crystal cut for type I phase matching (Θ = 90°, ϕ = 13.1°) with a conversion efficiency of approximately 50 %. The remaining 30 % of the infrared light are separated to generate the ultrabroadband signal seed. For white-light generation we focus the beam with an achromatic lens (f = 50 mm) into a 2 mm long YAG plate. With a substantially lower threshold for continuum generation in YAG [11], we achieve a stable white-light filament even with 420 fs long input pulses- this is a drastic improvement against the white-light generation in sapphire, where pulse compression down to 50fs was unavoidable [6]. With a short pass filter the fundamental radiation at 1030 nm is blocked and only the white-light continuum in the range from 600 nm to 1000 nm is seeded as the signal beam into the parametric amplifier. By using double-chirped mirrors (DCM) [12] behind the YAG crystal the seed is stretched for a good temporal overlap of signal and pump pulses. Pump and seed beam are overlapped in a non-collinear geometry in a 5 mm thick BBO crystal cut for non-collinear type I phase matching at an internal angle of 2.4° (Θ = 23°, ϕ= 0°). The pump spot was calculated to be slightly elliptical with 20 × 30 μm radii. The divergent beam from the NOPA stage is collimated with a spherical silver mirror (ROC = 200 mm). For recompression, we employ 18 bounces on a broadband double-chirped mirror pair in combination with CaF₂ wedges for fine tuning.

3. Results and discussion

The measured average power behind the NOPA was 420mW at 1MHz repetition rate, leading to an output pulse energy of 420 nJ. Both the white-light generation and the NOPA process reveal an excellent long-term stability and can be operated for many hours without realignment. A trace of the NOPA output power for one hour is shown in Fig. 2. The prominent power oscillation with a period of approximate 8 minutes stems from residual temperature fluctuations from the chiller of the seed laser. In the frequency range below 10 Hz we determined a pulse energy rms-noise of less than 2 % for the NOPA signal in spite of the mentioned interference with the chiller, which is an excellent number compared to other NOPAs, keeping the complexity of the whole system in mind. The spectral and temporal characteristics of the pulses were recorded with a spectrometer (OceanOptics USB 4000) and a home-built interferometric autocorrelator optimized for few-cycle pulse measurement. The results are presented in Fig. 3, which shows the power spectrum of the NOPA pulses located around 850 nm. The Fourier-limited pulse duration calculated from the spectrum is 5.9 fs. The associated measured interferometric autocorrelation (IAC) of the pulses is given on the right-hand side of Fig. 3. The blue dots illustrate the measured IAC, the red solid line represents the fitted curve. The pulse reconstruction by optimizing the spectral phase to get an agreement between the calculated spectrum and the measured IAC (see e.g. [13] and references therein) delivers a pulse duration as short as 9.7 fs with nearly 70 % of the power inside the main pulse. The resulting GD curve is shown in Fig. 4 on the right hand side in green color. Due to imperfect compression of third- and higher order dispersion approximately 30 % of the power is located outside the central part of the pulse. The resulting peak power with respect to the pulse shape calculated from the spectrum is 29 MW.

 

Fig. 2. Output power of the NOPA versus time. The oscillation is from remaining temperature oscillations in the oscillator cooling system.

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To understand the discrepancy between the calculated Fourier-limit and the measured pulse duration we did a comprehensive dispersion analysis of the DCMs used for compression with a white-light interferometry setup. We recorded directly the GD (group delay) values of the DCMs to get more information about the influence of the DCMs to the phase (see the blue dashed line in the left hand side of Fig. 4). On the opposite side, we calculated the GD from all the components of the NOPA. This includes 2mm YAG, 5mm BBO, an achromatic lens, a short pass filter, air and three bounces on additional dispersive mirrors (black solid line in the left hand side of Fig. 4). The sum of the solid black and the dashed blue curve is depicted in red dots and reveals the final pulse phase (see the more detailed view in the right hand side of Fig. 4 which is in good agreement with the retrieved GD curve from the autocorrelation). With our 18 bounces DCM-compression scheme some distinct dispersion ripples are obvious. Especially in the wavelength range from 700 nm to 900 nm some strong oscillations appear. In the group delay dispersion curve a deep minimum with an absolute value of more than 2000 fs² occurs around 800 nm. Furthermore, some remaining curvature indicates non compensated dispersion of third and higher order. Nevertheless, our DCM-compression scheme allows for the dechirping of the pulse duration from approximately 1 ps to 10 fs (see Fig. 3), but leaves much room for improvement to approach the Fourier limit of 5.9 fs. In a future perspective, compression close to the transform-limit could be exploited with chirped mirrors specially designed and produced to meet the requirements of this application. Another possibility is the use of adaptive pulse shaping (see e.g. [14]) to manipulate the phase resulting in transform-limiting pulse durations.

 

Fig. 3. Left: Optical output spectrum of the MHz-NOPA supporting a Fourier-limited pulse duration of 5.9 fs. Right: Interferometric autocorrelation of the compressed pulses. The measurement (blue dots) indicates a pulse duration of 9.7 fs calculated from the best-fit curve (red solid line).

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Fig. 4. Left: Comparison between the summation of the estimated GD due to the material inside the NOPA (black solid line) [YAG: 2mm, BBO: 5mm, achromatic lens, short pass filter, air and dispersive mirrors] and the GD compensation of the compressor-mirrors (blue dashed line) resulting in a mean-GD value (red dots) after compression. Right: The resulting mean-dispersion curve in detail (red dots), and the reconstructed dispersion curve from the IAC measurement (green solid line).

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4. Conclusion and outlook

In conclusion, we demonstrated the generation of sub-10-fs pulses from a MHz NOPA with 420mW average output power resulting in a peak power close to 30MW. The NOPA was seeded with a chirped-pulse amplified cavity-dumped Yb:KYW laser oscillator. In YAG we were able to create a very stable white-light continuum even with pulses as long as 420 fs. We observe further potential for an increase of the peak power by a factor of two by improved pulse compression, and the opportunity of an additional increase by using a double-stage NOPA layout.

Generally the MHz-NOPA becomes an interesting alternative to Ti:sapphire based systems, and will be contributing in many applications such as nonlinear bio-imaging, precision laser spectroscopy, or high harmonic generation (HHG). Focusing down to a beam diameter in the range of 5 μm will result in intensities between 10¹³ and 10¹⁴ W/cm² which will allow for HHG in Xenon with MHz repetition rates with this system.