Visible pulse compression to 4 fs by optical parametric amplification and programmable dispersion control

Summary form only given. The coherent synthesis of custom-tailored, intense, sub-cycle optical waveforms is promising for attosecond science and strong-field physics, e.g., for precision control of strong-field interactions in atoms, molecules and solids, for the generation of intense isolated attosecond pulses, and for attosecond pump-probe spectroscopy.Recently, coherent pulse synthesis based on supercontinuum generation in a hollow-core fiber compressor allowed the generation of sub-cycle -300-μJ optical pulses. However, in this case ionization losses in the gas medium prevent further scaling to the mJ level. In contrast, parametric synthesizers do not face an energy scaling limit, and allow for spectral extension into the particularly appealing MIR region. In previous works we demonstrated coherent pulse synthesis of two optical parametric chirped-pulse amplifiers and of two optical parametric amplifiers (OPAs) on the few-μJ level [3]. Here we present the ongoing development of a novel 3-channel parametric synthesizer for generating a 2-octave-wide spectrum, which can easily be upscaled to the mJ-level. We start from a cryogenically cooled Ti:sapphire chirped-pulse amplifier (150 fs, 22 mJ, 0.8 μm, 1 kHz) and generate a CEP-stable continuum (0.5-2.3 μm) [5], by white-light generation in a YAG crystal pumped by the second harmonic (1.06 μm) of the CEP-stable idler of a NIR OPA. The continuum is split with custom-designed dichroic beam splitters (which will also be used for the final beam recombination) and seeds three OPAs, a VIS noncollinear OPA (NOPA), a NIR and an IR degenerate OPA (DOPA), each composed of 2 (later 3) amplification stages, and pumped by the pulses at 0.8 μm (IR DOPA), and by its second harmonic at 0.4 μm (VIS NOPA, NIR DOPA). To synthesize a coherent ultrashort pulse from these three OPAs, the relative timing of the pulses will be tightly locked using feedback loops with balanced optical cross-correlators, that can achieve sub-cycle synchronization with s0-as RMS timing jitter [2,3]. Figure 1(a) shows the measured output spectra and the energies from the second amplification stages operating in parallel. The transform-limited (TL) pulse duration from the synthesis of these spectra is 1.9 fs FWHM, corresponding to 0.7 optical cycles at 785 nm center wavelength. Our current work aims to scale up the energies (by a third amplification stage) to -0.5 mJ for the VIS NOPA/NIR DOPA and -2 mJ for the IR DOPA using the 18.5 mJ of pump remaining after the first two amplification stages. State-of-the-art double-chirped mirror pairs (at present in fabrication) will allow for the final pulse compression with ultralow residual ripple in the resulting total group-delay dispersion over the full bandwidth (0.52-2.3μm). Temporal characterization of the synthesized two-octave-spanning optical waveforms will be performed by two-dimensional spectral shearing interferometry (2DSI). We foresee that our 3-channel synthesizer, when further scaled in energy to the mJ level, will become a versatile tool for controlling strong-field interactions in atoms, molecules and solids and for attosecond pumpprobe spectroscopy employing ultrashort pulses in the VIS/IR and XUV/soft-X-ray regions.

Tilting the pump pulse front has been proposed for efficient phase-matched THz generation by optical rectification of femtosecond laser pulses in LiNbO3 (Hebling et al., 2002). By using amplified Ti:sapphire laser systems for pumping, this technique has recently resulted in generation of near-single-cycle THz pulses with energies on the 10-μJ scale (Yeh et al., 2007, Stepanov et al., 2008). Such high-energy THz pulses have opened up the field of subpicosecond THz nonlinear optics and spectroscopy (Gaal et al., 2006, Hebling et al., 2008a). The method of tilted-pulse-front pumping (TPFP) was introduced as a synchronization technique between the optical pump pulse and the generated THz radiation. Synchronization was accomplished by matching the group velocity of the optical pump pulse to the phase velocity of the THz wave in a noncollinear propagation geometry. Originally, TPFP was introduced for synchronization of amplified and excitation pulses in so called traveling-wave laser amplifiers (Bor et al., 1983). By using such traveling-wave excitation (TWE) of laser materials, especially dye solutions, extremely high gain (109) and reduced amplified spontaneous emission could be obtained (Hebling et al., 1991). Contrary to the case of TWE, when TPFP is used for THz generation by optical rectification, a wave-vector (momentum) conservation condition or, equivalently, a phase-matching condition has to be fulfilled. It was shown (Hebling et al., 2002), that such condition is automatically fulfilled if the synchronization (velocity matching) is accomplished. The reason is that in any tilted pulse front there is present an angular dispersion of the spectral components of the ultrashort light pulse and there is a unique connection between the tilt angle of the pulse front and the angular dispersion (Bor & Racz, 1985, Martinez 1986, Hebling 1996). Angular dispersion was introduced into the excitation beam of so called achromatic frequency doubler (Szabo & Bor, 1990, Martinez, 1989) and sum-frequency mixing (Hofmann et al., 1992) setups in order to achieve broadband frequency conversion and keeping the ultrashort pulse duration. It was pointed out that in non-collinear phasematched optical parametric generators (OPG) and optical parametric amplifiers (OPA) tilted pulse fronts are expected (Di Trapani et al., 1995). TPFP was used in the non-collinear OPA (NOPA) producing sub-5-fs pulses (Kobayashi & Shirakawa, 2000). The different aspect of tilted pulse front and angular dispersion is usually not mentioned in these papers dealing with broadband frequency conversion. It is well known that the bandwidth of parametric processes is connected to the relative group velocities of the interacting pulses (Harris, 1969). Phase matching to first order in frequency

Femtosecond pulses are primarily generated at 800 or 1030 nm in Tiisapphire or Yb based systems. To generate fully tunable pulses over the full visible and NIR range, optical parametric amplifiers and in particular white light seeded noncollinear optical parametric amplifiers (NOPA) are now widely used. For highest energy pulses this scheme is expanded to OPCPAs. Energy and beam pointing stability is a crucial topic in the application of all these sources. We investigated — to the best of our knowledge for the first time — how the beam pointing instability of the pump laser source translates to the NOPA output.

We have developed a tunable, high-repetition-rate, broadband noncollinear optical parametric amplifier (NOPA) system pumped by a 1030 nm Yb:KGW laser. The system employs a prism-based dispersion compensation scheme that effectively corrects the wavefront tilt between the pump and seed pulses. Experimental results show that the NOPA provides single-shot output covering a spectral range of approximately 680–980 nm and enables tunable signal generation within 620–980 nm. The proposed design demonstrates excellent robustness and broad spectral coverage, offering a practical and efficient solution for realizing high-repetition-rate broadband light sources with great potential for applications in ultrafast spectroscopy and imaging.

The spontaneous fluorescence background in optical parametric amplifiers is generally attributed to the zero-point fluctuations of the electromagnetic field. These are amplified in parallel to the seed light and lead to an uncompressible superfluorescence background that deteriorates the contrast in optical parametric chirped pulse amplifiers (OPCPA). The absolute level of the underlying parametric fluorescence has not been reported so far. Comparing the fluorescence to low level cw seed light and quantitatively monitoring the output of a noncollinear optical parametric amplifier for both sources, the level is now determined. In a situation of 50 nm visible output bandwidth and low Gaussian spatial modes about 58 photons are found in the signal direction within the femtosecond time window of the amplifier. The superfluorescence level is observed to be proportional to the pump area for constant signal amplification. The implications for the background in high power OPCPA are discussed.

We report a framing imaging based on noncollinear optical parametric amplification (NCOPA), named FINCOPA, which applies NCOPA for the first time to single-shot ultrafast optical imaging. In an experiment targeting a laser-induced air plasma grating, FINCOPA achieved 50 fs-resolved optical imaging with a spatial resolution of ∼83 lp∕mm and an effective frame rate of 10 trillion frames per
\nsecond (Tfps). It has also successfully visualized an ultrafast rotating optical field with an effective frame rate of 15 Tfps. FINCOPA has simultaneously a femtosecond-level temporal resolution and frame interval and a micrometer-level spatial resolution. Combining outstanding spatial and temporal resolutions with an ultrahigh frame rate, FINCOPA will contribute to high-spatiotemporal resolution observations of ultrafast transient events, such as atomic or molecular dynamics in photonic materials, plasma physics, and laser
\ninertial-confinement fusion

We have investigated the generation of new undesirable frequencies in a two-pump optical parametric amplifier, which are generated by the four-wave mixing of a signal or idler with one of the strong pump waves. The powers of the new frequencies impose upper bounds on the device gain and output power if cross talk is to be avoided. In the parametric amplifier we investigated, however, we found that, for 7.5 dB of gain and −3.5 dBm of output power, the ratio of power in the new frequencies and the signal is less than −30 dB. This demonstrates that cross talk is acceptable for low gain and output power applications of these devices such as frequency conversion. We present simple scaling relationships for the power in the new undesirable frequencies that can be used to help design optical parametric amplifiers.

Noncollinear optical parametric amplification (NOPA) is now established as a unique technology to generate ultrashort tunable pulses with high flexibility for spectroscopic applications. With the availability of commercial ultrafast fiber lasers delivering energies of several tens of microjoules, such devices can be designed to operate at unprecedented repetition rates of several MHz, enabling shorter acquisition time as well as better signal-to-noise ratio. Fiber laser-pumped tunable sources are also valuable tools for X-ray time resolved studies, to match the MHz repetition rate of synchrotrons or free electrons laser facilities. We report here on the development of a new tunable ultrashort NOPA pumped by a fiber laser at 500 kHz repetition rate with improved performances over previously published similar designs.

We have investigated the possibility of using optical parametric amplifiers (OPA) as optical parametric chirped pulse amplifiers (OPCPA), in which energy from a narrow bandwidth pump source is efficiently transferred to a chirped signal pulse prior to its recompression (1).

The generation and characterization of femtosecond pulses at three independently tunable visible wavelengths is reported. Selected spectral portions of a common continuum generated in sapphire are amplified in noncollinear optical parametric amplifiers. The phase relation of the pulse trains is analyzed with a nonlinear interferometer based on coherent anti-Stokes Raman scattering and is found to be locked to better than 250 mrad rms. Small spectral shifts of the pulses lead to interference behavior that is consistent with 1 kHz frequency combs.

Dual-pumped non-collinear parametric amplification of a chirped laser pulse is numerically studied by employing the pump beams of different wavelengths. Our results reveal that the output bandwidth and the efficiency-bandwidth product are improved, when non-collinear angles and phase matching conditions are separately optimized for each individual pump beam. Moreover, by studying the dependence of the bandwidth of the amplified signal on the pump intensity, gain stability and its fluctuations are compared in small signal gain and saturation regimes. It is verified that the effective interaction length in this dual-pumped configuration is reduced as a consequent of increased bandwidth, compared to that in a single pump scheme.

At signal critical wavelength degeneracy (CWD), super-broad parametric gain bandwidths (SBGBs) can be obtained in nonlinear crystals. A 280 nm SBGB, for ∼60 small signal peak gain, has been calculated considering a collinear optical parametric chirped pulse amplification (OPCPA) at CWD in a 40 mm long, 58.3% partially deuterated KDP (P-DKDP) crystal, pumped by a frequency doubled nanosecond Nd:glass laser at 1 GW/cm2 pump intensity. This gain bandwidth in the 1 µm spectral range is significantly broader compared to the 150 nm gain bandwidth, calculated under similar conditions, for the broad-band non-collinear (NC) OPCPA in DKDP crystals in the 900 nm spectral range. It can support the amplification of significantly shorter femtosecond pulses, giving rise to a higher peak power at the same amplified pulse energy. The spectral profile of the gain bandwidth can be shaped by slightly adjusting the deuteration ratio (DR) of the P-DKDP crystals. A small-angle non-collinear OPCPA near the CWD can be used for the spatial separation of the signal and idler beams with the same polarization, keeping at the same time an ultra-broad gain bandwidth. A SBGB of 255 nm FWHM has been calculated for a non-collinear OPCPA stage with a 40 mm long P-DKDP crystal, 10.3 mrad signal-pump wavevectors internal angle, 57.8% DR, pumped by a few-ns 527 nm wavelength laser at 1 GW/cm2 pump intensity. The half-meter size P-DKDP crystals, pumped by frequency doubled few-ns multi-kJ Nd:glass lasers, are able to amplify super broad-band nanosecond stretched pulses up to the kJ energy level. They might be appropriate amplifying media for the development of single-beam 100-PW class femtosecond laser systems based on OPCPA at CWD.

A noncollinear optical parametric amplifier in which blue pump pulse generation as well as parametric amplification takes place in the same nonlinear crystal is presented. Broadband spectra tunable throughout over 100 nm in the visible were generated in a simple setup. The green output pulses were compressed to 65 fs, and efficiency (IR-to-visible) of 1.8% was achieved.

The development of attosecond pump–probe experiments at high repetition rate requires the development of novel attosecond sources maintaining a sufficient number of photons per pulse. We use 7 fs, 800 nm pulses from a non-collinear optical parametric chirped pulse amplification laser system to generate few-pulse attosecond pulse trains (APTs) with a flux of >106 photons per shot in the extreme ultraviolet at a repetition rate of 100 kHz. The pulse trains have been fully characterised by recording frequency-resolved optical gating for complete reconstruction of attosecond bursts (FROG-CRAB) traces with a velocity map imaging spectrometer. For the pulse retrieval from the FROG-CRAB trace a new ensemble retrieval algorithm has been employed that enables the reconstruction of the shape of the APTs in the presence of carrier envelope phase fluctuations of the few-cycle laser system.

High-energy infrared (IR) femtosecond laser sources are important for applications in ultrafast and strong-field laser science; thus, considerable effort has recently been made to develop such laser systems. In this paper, we review our recent work on developing TW-class mid-infrared (MIR) femtosecond laser pulses in the 1-4 $\mu$m region using a dual-chirped optical parametric amplification (DC-OPA) method. By employing a Ti:sapphire laser with sub-joule energy to pump the DC-OPA system, MIR femtosecond pulses with 100-mJ-class energy are demonstrated. Efficient energy scalability and flexible wavelength tunability in DC-OPA are confirmed experimentally. Different features of DC-OPA from those of conventional OPA and narrow-band pumped optical parametric chirped pulse amplification (OPCPA) are observed. By precisely optimizing the chirps of the seed and pump pulses in the DC-OPA system, bandwidth narrowing of amplified pulses can be minimized in an energy-scaling strategy. Moreover, DC-OPA can be universally employed for the energy scaling of near-IR, MIR, and far-IR pulses, regardless of the type of nonlinear crystal, and is helpful for efficiently generating few-cycle carrier-envelope phase stable IR pulses with TW-class peak power.