1 Introduction

The transfer from new laser technology into the application laboratories is well established. Many different applications in physics, chemistry, biology, and medicine profit from ultrafast lasers with high pulse energies, from lasers with short pulses, and from lasers with high pulse repetition rates. The literature is full of reports on lasers which can fulfill one or two of the desired requirements. But some applications such as nonlinear bioimaging or spectroscopy need them simultaneously, and here we propose a laser system which is capable of meeting all three goals simultaneously: a noncollinear optical parametric amplifier pumped by a MHz/μJ-oscillator with cavity-dumping (MHz-NOPA). It can produce few-cycle pulses with MHz repetition rates and pulse energies / peak powers well beyond the typical oscillator level. Furthermore, it can be easily scaled with the anticipated progress in pump laser technology.

With just a few exceptions all reported OPA systems have been pumped by Ti:sapphire laser amplifiers at low repetition rate (typical up to 100 kHz) [1, 2]. Due to the short Ti:sapphire pulses (typically < 100 fs) and the high pulse energies both the white-light generation in a filament in sapphire as well as the parametric amplification process are very efficient, and NOPA pulses with durations below 6 fs and pulse energies up to 5 μJ have been reported at 1 kHz repetition rate [3, 4].

Figure 1 shows the normalized parametric gain sinc²(∆kL/2) in BBO with the wave number mismatch ∆k and the crystal length L = 2 mm for collinear and noncollinear geometries and for the two pump wavelengths 400 nm and 520 nm. It becomes very clear, that the concept of the ‘magic angle’ can be well transfered from the Ti:sapphire harmonic to a pump wavelength around 520 nm. In [5] a 1 MHz repetition rate OPA driven by a chirped pulse fiber amplification system is reported. It delivered pulses with energies of 1.2 μJ, but due to the fact that the super-continuum seed was generated in a photonic crystal fiber (mainly through soliton fission and four wave mixing) the pulses could not be compressed below several hundred femtoseconds. Only at degeneracy pulse durations in the 40 fs range could be demonstrated.

 

Fig. 1. Calculated parametric gain in BBO for different geometries and pump wavelengths. α is the angle between signal and pump beam.

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In a previous letter, we demonstrated a collinear OPA directly pumped with a MHz repetition rate femtosecond oscillator [6]. We were able to generate broadly wavelength tunable pulses with durations below 20 fs. The tuning range was extended up to 2.5 μm due to the collinear idler emission. Shifting now the angle between pump and seed beam to 2.4 °, according to Fig. 1 the amplification bandwidth is increased to more than 120 THz. Furthermore, the spatial walk-off is reduced allowing for longer interaction lengths [7] and therefore for higher conversion efficiencies. Employing this scheme, it is possible to generate ultrabroadband few-optical-cycle pulses, and in this contribution we report on the generation of sub-10-fs-pulses at an energy of 45 nJ with a repetition rate of 1 MHz.

2 Experimental set-up

The experimental setup of the MHz-NOPA is given in Fig. 2. It consists of four stages: A femtosecond pump laser with μJ pulse energy and MHz repetition rate, the white-light generation stage, the NOPA itself, and finally the pulse recompression.

 

Fig. 2. Setup of the MHz-NOPA experiment. The abbreviations are explained in the text.

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The pump laser is a directly diode-pumped Yb:KYW femtosecond oscillator with electro-optical cavity-dumping [8]. The laser is operated at a center wavelength of 1040 nm, and bandwidth limited pulses with 340 fs duration and 1.2 μJ pulse energy are emitted at a repetition frequency of 1 MHz. The pulses are frequency doubled in a 1.2 mm long LBO crystal cut for type I phase matching with a conversion efficiency of 50 %. After frequency doubling the infrared is separated from the green light at a dichroic beam splitter (BS). The green pulses are directed to the amplifier for pumping, whereas the infrared pulses are used to generate the white-light seed.

The leftover infrared pulse energy of about 600 nJ at a duration of 390 fs is not sufficient for direct white-light generation in sapphire, so that the peak power has to be enhanced by a pulse compression scheme: The optical spectrum is broadened by self-phase-modulation in a 3.4 cm long large-mode-area microstructured fiber (LMA). Afterwards dispersive mirrors (DM) are used with a group delay dispersion of approximately -300 fs² realized on a bandwidth of about 50 nm to recompress the pulses down to a duration of roughly 50 fs, leading to a tripled peak power of 4 MW. Focusing the light into a 3 mm long sapphire plate (SP) with an aspherical lens, a stable white-light filament is achieved. In contrast to the radiation generated in the photonic crystal fiber from [5] this continuum has proven to be compressable down to few-cycle-pulses.

The white-light seed is pre-chirped with double-chirped mirrors (DCM, see below) to optimize the temporal overlap with the pump pulses. For this purpose we introduce a sum negative dispersion of -300 fs². Pump and seed beam are overlapped in the noncollinear geometry in a 2 mm thick BBO crystal cut for type I phase matching at an internal angle of 2.4° (pump: e, signal: o, idler: o). The focal diameter of the signal seed is about 50 μm, the pump spot slightly larger. The temporal overlap is adjusted by a variable delay line (DL). After the NOPA stage the amplified pulses are collimated with a spherical silver mirror.

For recompression, we employ a prism sequence made of two CaF₂-prisms at an apex distance of 55 cm in combination with six bounces on broadband double-chirped mirror pairs (DCM), which have a group delay dispersion of approximately -60 fs² per bounce covering one whole octave of bandwidth [9].

3 Results and discussion

The MHz-NOPA shows an excellent long-term stability and can be operated for days without any realignment. The average power behind the NOPA was measured to be 55 mW, the power behind the prism compressor was 45 mW, leading at 1 MHz repetition rate to an output pulse energy of 45 nJ. The pulses were characterized with a spectrometer and an interferometric autocorrelator optimized for few-cycle pulses. Figure 3(a) shows the power spectrum of the amplified pulses located around 850 nm. The measured interferometric autocorrelation of the pulses is shown in Fig. 3(b). The measurement indicates a pulse duration of 9.7 fs. The dots are the calculated IAC for a pulse with a flat spectral phase. The resulting peak power is 4 MW, well beyond the level of the input pump pulses and well beyond the peak powers of typical few-cycle laser oscillators. Furthermore, we measured in the frequency range below 10 Hz a pulse energy rms noise of only 1 %, which is an excellent number compared to other NOPA systems.

 

Fig. 3. a) Optical spectrum of the MHz-NOPA pulses. b) Interferometric autocorrelation.

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From the calculations for Fig. 1 as well as from former experiments it is well known that much shorter pulses are supported by the NOPA process itself. We expect to reduce the pulse duration substantially by optimizing the white-light generation process. This might be easily achieved by increasing the input pulse energy, which is expected for the near future.

4 Conclusion

In conclusion we have demonstrated a few-cycle NOPA directly pumped by a femtosecond oscillator with a MHz repetition rate. A pulse duration of 9.7 fs and a pulse energy of 45 nJ has been obtained, resulting in a peak power of 4 MW.

The MHz-NOPA becomes an interesting alternative to Ti:sapphire based systems, and many applications such as nonlinear bioimaging or precision laser spectroscopy might profit from this technology.