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We present a compact and stable femtosecond wavelength-tunable optical parametric chirped pulse amplification (OPCPA) system. A novel OPCPA front-end was constructed using a multi-channel picosecond all-in-fiber source for seeding DPSS pump laser and white light supercontinuum generation. Broadband chirped pulses were parametrically amplified up to 1 mJ energy and compressed to less than 40 fs duration. Pulse wavelength tunability in the range from 680 nm to 930 nm was experimentally demonstrated.
© 2016 Optical Society of America
1. Introduction
Femtosecond wavelength-tunable optical parametric amplification (OPA) systems are indispensable for ultrafast science applications such as time-resolved pump-probe experiments (transient absorption, fluorescence up-conversion, time-resolved photoelectron spectroscopy, time-resolved mass spectrometry), femtosecond digital holography and CARS (Coherent Anti-Stokes Raman Spectroscopy) [1–8]. Incorporation of optical parametric chirped pulse amplification (OPCPA) technique [9,10] leads to extremely high peak intensity of light which is required by modern laser-matter interaction applications, e.g. HHG (High-Order Harmonic Generation) and attosecond science [11]. A seed source for such system is commonly a Ti:Sapphire femtosecond oscillator [12,13]. Alternatively, white light supercontinuum (WLC) generated by the femtosecond pulses from the regenerative amplifier (Ti:Sapphire, Yb:KGW) can be used as a seed [14,15]. In both cases seed pulses have energy in nJ range and need to be preamplified in femtosecond OPA stage before the amplification in OPCPA stage, where pulses are stretched, parametrically amplified using long picosecond pulses as a pump and finally compressed to femtosecond duration. Picosecond Nd3+ doped DPSS (Diode Pumped Solid State) lasers are usually used as a pump.
Conventional approach requires to use a complex femtosecond CPA laser system and optical or electronic synchronization of the femtosecond and picosecond laser systems, which typically operate at different wavelengths [16]. Furthermore, in many strong field applications it is important to have very high temporal contrast in picosecond to nanosecond time scales of the generated femtosecond pulses [10,17]. If pulses from regenerative amplifiers are used in all parametric amplification stages, achievable pulse contrast is limited or requires sophisticated techniques for contrast improvement [17]. During the past few years novel schemes were proposed to overcome the mentioned shortcomings. A passive seed and pump pulse synchronization was experimentally demonstrated by R. Riedel et al. [18] performing the generation of WLC pulses with a small fraction of light from a pump laser based on Yb:glass fiber amplifier. A few-cycle seed formation using a femtosecond Kerr-lens mode-locked thin-disk Yb:YAG oscillator and implementing a two-stage external pulse broadening and compression technique was realized by O. Pronin et al. [19].
In this work, we developed a compact femtosecond wavelength-tunable OPCPA system with a novel front-end, which uses a spectrally broadened picosecond fiber oscillator for seeding picosecond DPSS regenerative amplifier and WLC generation. This approach eliminates the need of seed and pump pulse synchronization therefore greatly simplifying the system and does not require generation of femtosecond pulses for the regenerative preamplifier potentially increasing temporal contrast of final pulses.
2. Experimental setup and results
The principle scheme of our femtosecond wavelength-tunable OPCPA system is presented in Fig. 1. The experimental setup consisted of OPCPA front-end based on white light supercontinuum generation and parametric amplification by femtosecond pulses and picosecond OPCPA amplifier. Picosecond DPSS Nd:YVO4 laser was used as a pump.
Fig. 1 Principal experimental scheme of the femtosecond wavelength-tunable OPCPA system. Multiple channel all-in-fiber picosecond laser was used to seed DPSS pump laser and a non-collinear optical parametric amplifier (ps NOPA) in order to form pulses for white light supercontinuum (WLC) generation. WLC pulses were preamplified in femtosecond NOPA (fs NOPA) performing wavelength tuning, then stretched to picosecond duration, amplified in one stage OPCPA amplifier and recompressed.
The seed source of the OPCPA front-end and the pump laser was all-in-fiber picosecond laser (Fig. 2) which consisted of three main parts: the picosecond oscillator, the seed formation chain for the regenerative amplifier (RA) and the seed formation chain for non-collinear parametric amplifier (ps NOPA).
Fig. 2 Principle scheme of picosecond all-in-fiber laser. Passively mode-locked fiber oscillator generated 2 ps pulses which were divided into two branches. In one branch narrowband and chirped pulses were formed in order to seed the DPSS regenerative amplifier. The other branch was used for broadband (~14 nm) pulses formation which were later amplified parametrically, compressed to femtosecond duration and then used to generate WLC.
A passively mode-locked fiber oscillator generated 2 ps transform-limited pulses at 1064.15 nm central wavelength. Pulse duration and the central wavelength of the oscillator were determined by reflectivity spectral profile of chirped fiber Bragg grating (CFBG1). Stable single-pulse mode-locking regime at 35 MHz repetition rate was achieved using semiconductor saturable absorber mirror (SESAM) as the end mirror of the resonator.
An Yb3+ doped polarization maintaining single-mode fiber was used as a gain medium which was pumped with a 976 nm laser diode (LD) through the CFBG1. Fiber oscillator had 2 output ports realized by the 70/30 beamsplitter (BS) which was fusion spliced inside the resonator. Photodetector connected to one port of the beamsplitter was used to synchronize fiber laser with a regenerative amplifier. Average output power from the fiber oscillator was 2 mW.
Ultrafast pulses from the oscillator were amplified in Yb3+ doped fiber amplifier to 25 mW and then divided into two branches by 50/50 splitter. In the first branch pulses were stretched to ~200 ps duration by a narrowband CFBG2 while back-reflected to 50/50 splitter to RA seed output port (Fig. 3(a)). A resistive heater element was used to tune the central wavelength of CFBG2 in order to match the seed spectrum to the gain maximum of the RA (Fig. 3(a)). Stretched pulses then were amplified in Nd:YVO4 regenerative amplifier and a single pass Nd:YVO4 booster operated at 1 kHz repetition rate to 11.5 mJ energy. The spectral bandwidth and the duration of the amplified chirped pulse was approximately half that of the input pulse due to gain narrowing effect in RA. The resulting duration of the amplified OPCPA pump pulses was 98 ps as spectral narrowing of chirped pulses results in pulse shortening (Fig. 3(b)).
Fig. 3 (a) Spectrum of the regenerative Nd:YVO4 amplifier seed pulse: red line corresponds to measured RA output spectrum; (b) Autocorrelation trace of the regenerative amplifier output pulses. The retrieved pulse duration was 98 ps at FWHM when fitted with Gaussian function.
The second branch of the fiber laser was used as a seed for generation of WLC. At the beginning, pulses were amplified to 2 nJ energy in Yb3+ doped fiber amplifier (Fig. 2). After the amplification pulse spectrum due to self-phase modulation (SPM) was broadened to ~14 nm in a single-mode polarization-maintaining fiber (Fig. 4(a)). Then, pulses were stretched to 125 ps by a broadband CFBG3. The CFBG3 was designed in such a way that cumulative second and third order dispersion of the CFBG3 and of the nonlinear fiber was compensated by the grating compressor to achieve shorter pulse duration and better contrast. After CFBG3 stretcher pulses were amplified in another fiber amplifier to 350 mW (10 nJ pulse energy) and directed to a non-collinear optical parametric amplifier (NOPA).
Fig. 4 (a) Spectrum of the NOPA seed pulses; (b) Autocorrelation trace of the compressed pulses used to generate WLC. The retrieved compressed pulse duration was 325 fs at FWHM when fitted with Gaussian function.
Pulses from DPSS laser were frequency doubled with 74% conversion efficiency (8.5 mJ@532 nm) in 6 mm length LBO crystal. Then the second harmonic pulses were split to two parts – the first was used as NOPA pump. 7 mm type I BBO picosecond NOPA crystal (phase matching angle θ ≈23°) was pumped by 2.5 mJ at 532 nm wavelength and produced 190 µJ amplified signal pulse energy. The parametric amplification factor was 1.9·104 with 7.6% pump energy conversion to signal wave. The pulse-to-pulse energy stability of the amplified signal measuring every pulse during 15 s period was ~0.6% rms which was comparable to the pump pulse stability which demonstrates that the parametric amplifier was working close to saturation regime. The ratio of the signal and pump pulse duration was set to ~1.25 aiming to achieve high amplification efficiency and to apodize the oscillatory spectrum structure (Fig. 4(a)) caused by SPM in fiber and improve compressed pulse contrast. After first NOPA stage, pulses were compressed down to 325 fs duration (Fig. 4(b)) in a diffraction grating compressor. 1600 grooves/mm transmission diffraction gratings working in Littrow configuration were used. The efficiency of the compressor was 63%.
A small part (~1 µJ) of the parametrically amplified and compressed pulses was focused on a 5 mm sapphire plate to generate WLC (Fig. 5, gray shaded area). Focusing conditions and pulse energy were chosen to produce smooth and stable WLC. The remaining energy (115 µJ) was frequency doubled in a 2.5 mm thick BBO crystal with a ~57% efficiency (65 μJ@532 nm) and later used as a pump for femtosecond NOPA. Parametric amplification was realized in a 1.2 mm thick BBO crystal (type I) at a phase matching angle θ = 24° and a non-collinearity angle α ≈2.4° in order to reach the broadest amplification bandwidth [9,20]. The spot size of the pump beam in the crystal was adjusted to achieve the highest intensity but avoiding significant generation of the parametric fluorescence.
Fig. 5 White light continuum generated in 5 mm Sapphire plate (grey curve), femtosecond NOPA output in a few cycle regime (red curve) and wavelength-tunable regime (blue curve). In order to remove 1064 nm radiation longer wavelengths (>960nm) were filtered using short-pass filter.
The broadband supercontinuum spanning from 700 to 1050 nm (Fig. 6(d)) was amplified up to 2.5 µJ. In order to achieve shortest possible pulse duration from the femtosecond NOPA, we had to compensate the material dispersion of the system. Although only reflective optics were used after the WLC generation, pulses experienced temporal broadening passing through the sapphire plate and BBO crystal. We used a fused silica prism compressor and a pair of N-BK7 glass wedges for the fine tuning of the dispersion in the system. The chirpscan method [21] was used to characterize output pulses. Chirpscan diagrams and retrieved pulse envelope (red curve) are shown in Fig. 6. Obtained pulse duration at FWHM was 8.5 fs compared to 7.9 fs transform-limited pulse duration corresponding to the measured spectrum (Fig. 6(d)).
Fig. 6 The results of pulse characterization after a non-collinear parametric amplification of a broadband supercontinuum seed and dispersion compensation with a prism compressor. The measurement was realized using chirpscan method: (a) experimentally measured chirpscan trace; (b) numerically retrieved chirpscan trace; (c) retrieved pulse envelope compared with transform-limited pulse; (d) measured and retrieved pulse spectra and retrieved spectral phase (dashed green trace).
The next aim of this experimental work was to realize a wavelength-tunable OPCPA. In order to achieve this, WLC pulses were stretched to ~1.2 ps in 10 mm thickness SF10 glass block. Wavelength tuning was performed by varying the delay between the pump and the chirped signal pulses in the femtosecond NOPA stage. Close to 300 fs duration pump pulse was slicing stretched WLC pulses during amplification in NOPA crystal and limiting their bandwidth. Pre-chirped WLC was amplified up to 8 μJ energy and spectrum of the pulses corresponded to ~30 fs transform-limited pulse duration (Fig. 5, blue curve). The tuning range of NOPA output pulses was 670-1000 nm, and was limited by the amplification bandwidth of BBO crystal at phase matching conditions described before.
For further amplification in the OPCPA stage, pulses were stretched to the duration of 40 ps by diffraction grating stretcher in Treacy configuration [22]. We used 1500 grooves/mm density diffraction gratings. The efficiency of the stretcher was ~85%. Stretched pulse duration was chosen in order to optimize efficiency and bandwidth of the amplifier [23]. BBO crystal (type I) with the length of 7 mm was used for the OPCPA amplifier. Pulses were amplified up to 1 mJ energy by using 5 mJ 100 ps frequency doubled pump pulses from DPSS laser. Amplified pulse-to-pulse energy stability was 0.5% rms measuring every pulse during 15 s period. Offner-type compressor [24] with ~85% efficiency was used to recompress pulses after the amplification. The compressor consisted of one 1500 grooves/mm diffraction grating and two spherical concentric silver coated mirrors. The first mirror was concave (ROC = 500 mm) and the second was convex (ROC = −250 mm). When beam incidence angle to the diffraction grating of Offner compressor is equal to the one of Treacy stretcher, this stretcher-compressor tandem is self-compensating for all orders of dispersion [24,25].
The output spectra from the OPCPA at different central wavelengths are shown in Fig. 7(a). For each central wavelength the distance between the diffraction grating and the concave mirror in Offner-type compressor was optimized to achieve the shortest pulse duration. The pulses were characterized using multiple-shot SHG FROG autocorrelator. Mechanical construction of our experimental setup limited the pulse duration and contrast achieved throughout the full range of wavelength tuning (Fig. 7(b)).
Fig. 7 (a) Pulse spectra at the OPCPA system output when performing wavelength tuning; (b) experimentally measured autocorrelation traces of the compressed pulses at different central wavelengths. The pulse duration was calculated at FWHM assuming Gaussian deconvolution parameter (~1.41). The legend shows the measured pulse duration (tM) compared with transform limit calculated from the pulse spectrum.
There was no possibility to properly align and equalize the critical geometric parameters of pulse stretcher and compressor performing the wavelength tuning at the same time. Such misalignment caused pulse phase distortions due to the third order residual dispersion which can be clearly seen from the autocorrelation trace at 860 nm (Fig. 7(b)) where the measured pulse duration was nearly twice longer than the transform limit and had characteristic pedestal indicating uncompensated third order dispersion. The stretcher-compressor scheme will be optimized in near future and should lead to compressed pulse duration of less than 40 fs in the whole tuning range.
In this experiment the alignment of pulse stretcher and compressor was optimized to achieve shortest pulse duration in the central wavelength range of 705-800 nm. The measured pulse durations in this range were less than 40 fs and close to the transform limit calculated from the pulse spectra (28-35 fs). We performed retrieval calculations of the pulse envelope using FROG algorithm [26] for the measurement at 750 nm central wavelength (Fig. 8). The FROG retrieval error was 0.38% on a 256 × 256 grid. The pulse duration calculated from the autocorrelation trace and FROG retrieval were identical and equal to 36 fs. As it can be seen from the Fig. 8, small amount of residual phase was still present after the optimization of the compressor which resulted in an asymmetrical tail at the trailing edge of the pulse.
Fig. 8 Envelope of the compressed pulse retrieved from SHG FROG measurement at λ0 = 750 nm compared with transform-limited pulse calculated from the measured spectrum. Inset - measured pulse spectrum compared with retrieved spectrum and retrieved spectral phase (dashed green trace).
In order to characterize the beam quality at the output of the system, we measured the beam radius versus the distance from the beam waist. At low amplified pulse energy (<0.3 mJ) the beam quality parameter M2 was 1.21 (Fig. 9(a)). At the highest achievable pulse energy the beam quality got worse (M2 = 1.52) (Fig. 9(b)) likely due to parametric back-conversion at the peak of Gaussian beam in the BBO crystal.
Fig. 9 4σ beam radius at the system output versus distance from the waist location measured (a) at low output pulse energy (<0.3 mJ) and (b) at the highest achievable pulse energy at the system output (0.85 mJ); (c) beam profiles at the plane of OPCPA crystal at 0.3 mJ and 0.85 mJ amplified pulse energies.
This effect was verified by measuring amplified signal beam profile at the plane of BBO crystal perpendicular to propagation axis using 4-f imaging lens system (Fig. 9(c)). At the maximum pump intensity the peak of the beam profile shifted from the beam center due to parametric back-conversion and walk-off.
3. Conclusions
We have developed a compact femtosecond tunable OPCPA system with a picosecond all-in-fiber seed laser and a picosecond DPSS pump laser. Pulses from fiber laser were spectrally broadened in optical fiber, parametrically amplified, compressed to femtosecond duration and then used to generate white light supercontinuum signal which was amplified in a femtosecond broadband non-collinear optical parametric amplifier and compressed down to 8.5 fs. This corresponded to 3 optical cycles and nearly transform-limited pulse duration.
After OPCPA amplifier and compressor we obtained high spatio-temporal quality pulses with energy up to 0.85 mJ and pulse duration down to 40 fs. In this case the wavelength tunability in the spectral range of 680-930 nm was experimentally demonstrated.
This concept opens a path for the development of compact femtosecond high energy tunable hybrid laser systems, incorporating advantages of fiber and solid-state laser technologies, which may be adopted in a variety of ultrafast laser applications.
Funding
Research Council of Lithuania (LAT-10/2016).
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