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Abstract
We demonstrate a dual-beam infrared optical parametric source featuring a noncollinear KTA booster amplifier and straightforward angular dispersion compensation of the idler beam. Through careful beam and pulse characterization, and high-harmonic generation in a crystalline solid, we show that the corrected idler beam is diffraction-limited, astigmatism-free, and compressible to its transform-limited, 5-cycle pulse duration. Pumped by only 40-µJ pulses at 1.03 µm, the parametric source delivers 7.8-µJ, 38-fs, 1.53-µm and 2.3-µJ, 53-fs, CEP-stable, 3.1-µm pulses at a repetition rate of 100 kHz. The scheme provides a promising route to scale the pulse energy and average power beyond PPLN- or KTA-based collinear OPA architectures.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The generation and amplification of high-repetition-rate, few-cycle optical pulses at the short wavelength edge of the mid-infrared, MIR (i.e., ~2.7-4 µm or 3700-2500 cm−1) benefit a wide range of applications from ultrafast vibrational spectroscopy [1] to strong-field physics [2] and high-harmonic generation [3]. Optical parametric amplifiers (OPAs) pumped by sub-ps and ps, diode-pumped Yb and Nd lasers can directly satisfy the requirements for large spectral bandwidths and power-scalability in this spectral region.
Although occasionally bulk, angle-tuned crystals are also used [4,5], most often, periodically poled lithium niobate (PPLN) with high effective nonlinearity is employed as the nonlinear medium. However, photorefractive and photochromic effects and the limited aperture size restrict the use of PPLN to low-to-medium power systems [6–11]. At high average and peak powers, one needs to resort to bulk, angle-tuned crystals, which, in the collinear geometry, typically suffer from narrow parametric gain bandwidths. Thinner crystals pumped at higher intensities are employed in this case, but reaching broad enough gain bandwidths at a certain amplification factor is often prevented by laser-induced damage. The noncollinear amplifier arrangement can frequently increase the gain bandwidth and can also provide high gain at lower pump intensities thanks to the larger crystal thickness that can be employed. However, in a given nonlinear crystal, broadband amplification in the conventional scheme (i.e., the signal beam is free from angular dispersion) is restricted to spectral regions where matching the projection of the idler group velocity onto the signal group velocity is possible at a suitable noncollinearity angle. The idler beam generated in such a scheme exhibits angular dispersion and is typically dumped. As a solution, one can either compensate the angular dispersion of the idler output beam [12,13] or use a non-conventional scheme, where the seed signal beam is angularly dispersed [14,15]. Alternatively, in certain applications, one can tolerate the tilted pulse front and employ simultaneous spatial and temporal focusing to minimize the temporal duration of the pulses in the focal plane [16].
For MIR-seeded, conventional noncollinear OPAs, there are only a few materials (e.g., LiNbO3, LiIO3, and KNbO3) that are widely available, with KNbO3 being the most promising nonlinear crystal for amplification with pump wavelengths near 1 µm at both high intensities and average powers [6]. Even though KNbO3 has been successfully employed in booster amplifier stages with 21-W MIR average output power [11], its susceptibility to thermal shock, photodarkening [6] and its strong OH absorption near 2.8 µm [17] limit its scalability to simultaneously high average and peak powers. Compared to LiNbO3 and KNbO3, KTiOAsO4 (KTA) is a superior material with respect to resistance to laser-induced damage, photodarkening, and photorefractive effects, and has a significantly reduced OH absorption [7,18–20]. However, broadband, noncollinear amplification is prohibited in KTA for MIR seeding, and therefore, it had only been considered for use in collinear geometry in the MIR [6,7,21].
Here, we demonstrate that using KTA in the noncollinear arrangement for the parametric generation of broadband, high-quality MIR pulses is viable through a straightforward angular dispersion compensation scheme based on a single spherical mirror and a reflection grating. To this aim, we started from a white-light-seeded, infrared OPA based on two collinear PPLN amplifiers pumped at 1.03 µm [22] and added a noncollinear KTA test stage module that can serve as the second OPA stage. Removable mirrors were used to redirect the seed and pump beams for the second stage, leaving the previously implemented PPLN booster stage intact and available for comparison. The two systems share the PPLN-based first stage. This way, a reasonably direct comparison could be made between the performances of the collinear PPLN versus the noncollinear KTA booster stage. We demonstrate that the performance of the noncollinear, KTA-based booster stage for dual-beam (i.e. signal + idler) operation at a given broad gain bandwidth is superior to that of the collinear, PPLN-based booster stage in terms of conversion efficiency, beam quality, and carrier-envelope phase (CEP) noise. We also show that with the application of the angular dispersion compensation scheme, the corrected idler beam from the noncollinear KTA stage exhibits diffraction-limited quality and, in a benchmark experiment, was successfully used for generating high-harmonics in solids up to the 9th order. The high corrected idler beam quality with the current setup is vastly superior to that obtained with our previous compensation scheme based on a silicon prism, which was tested on a large-scale, non-CEP-stable OPCPA system [10]. The presented setup based on noncollinear amplification in KTA and consecutive angular dispersion correction of the idler beam is scalable to high average powers and pulse energies presumably beyond what is offered by KNbO3 and LiNbO3, and simultaneously provides high-quality signal pulses in the near infrared.
2. Design of angular dispersion compensation unit
The angular dispersion compensation unit relies on an optical system that images the plane of the nonlinear crystal onto an angularly dispersive element, which then cancels the angular dispersion of the laser beam. Due to the imaging arrangement, the spatial chirp of the corrected laser beam is ultimately limited by the spatial chirp at the position of the nonlinear crystal, which is negligible in a properly designed noncollinear OPA stage.
The design starts by estimating the idler angular dispersion that needs to be compensated. Our phase-matching scheme is shown in Fig. 1(a). Type II interaction in the XZ plane of KTA is used with an extraordinary signal polarization and a pump-signal wave vector angle of 3.58°. The internal signal angle relative to the Z-axis is 49.3°. For this type of interaction, KTA behaves like a positive uniaxial crystal and the direction of signal energy flow (Poynting vector) deviates from the signal wave vector towards the crystal Z axis. To partly compensate this spatial walk-off effect, the pump propagates between the Z-axis and the signal. When pumped at 1.028 µm and seeded by angular-dispersion-free, broadband pulses at a center wavelength of 1.53 µm, the broadband, 3.13-µm idler pulses exhibit an angular dispersion of ~65.6 µrad/nm inside the crystal and ~115 µrad/nm in air. The wavelength-dependent angular dispersion inside the crystal varies only slightly across the idler spectral range. Assuming a monochromatic pump and plane signal and pump waves with fixed k-vectors, the absolute value of the idler angular dispersion varies in the narrow range of 64.8-66.6 µrad/nm within the whole 2.8-3.5-µm region. Furthermore, the total deviation of the idler angle from a linear fit remains below 320 µrad (including both positive and negative deviations) over this wavelength range. As a comparison, the half-angle divergence of a diffraction-limited Gaussian beam with our idler beam parameters is >2.0 mrad inside the crystal. Under the above approximations, the angular spread of the idler beam generated inside the crystal is 2.65°. The small angle of incidence [i.e., 3.9 ± 1.3°, cf. Figure 1(a)] on the exit face of the crystal introduces a negligible angular dispersion of only ~0.6 µrad/nm.
Fig. 1 (a) Noncollinear phase-matching in the walk-off compensated geometry using a KTA crystal cut at θ = 42°. in the XZ plane. The two optic axes located at ± 14° from Z are not shown. kp, ks, ki are the pump, signal, and idler wave vector, respectively. (b) Angular dispersion introduced by a 40.96-line/mm reflection grating in the unit of µrad/nm as a function of angle of incidence and wavelength. The dashed line corresponds to our experimental, wavelength-dependent angle of incidence on the grating.
In the next step, the angular dispersion of the compensation unit and the magnification of the imaging system have to be chosen for the given power, intensity, and angular dispersion of the laser beam. Depending on the laser power and spectral bandwidth, a prism or a diffraction grating can both be used as the angularly dispersive element. Various grating-based schemes have been demonstrated for the compensation of angular chirp of idler pulses in the near infrared below 1.6 µm [12,13]. In [10], we extended the technique to the MIR and used a silicon prism for correcting the 3.1-µm beam generated in a noncollinear KTA stage. Compared to prisms, reflection gratings are a better choice, as i) the scaling of the grating angular dispersion with wavelength can be made arbitrarily small, ii) astigmatism is avoided, and iii) gratings do not introduce a varying group delay dispersion (GDD) across the laser beam. In this work, we use a reflection grating with a groove density of 40.96 lines/mm and a blaze wavelength of 3.2 µm. Figure 1(b) shows the angular dispersion introduced by the grating. In contrast to a prism, the angular dispersion of a low-groove-density grating close to the Littrow arrangement is practically independent of the angle of incidence [cf. Fig. 3 in [10] versus Fig. 1(b) in this paper]. The imaging system consists of a single spherical reflector with a focal distance of f = 100 mm placed at a distance of ~135 mm from the crystal. Using the Gaussian thin lens equation from paraxial geometric optics, the corresponding magnification factor for the imaged spot size is 2.8 and the demagnification factor for the angular dispersion is 1/2.8, which can then be cancelled by the angular dispersion added by the grating (i.e., 115 µrad/nm / 2.8 = 41 µrad/nm).
3. Experimental results
The schematic layout of our two-stage OPA system is shown in Fig. 2. Compared to our previous arrangement [22], which was based on two PPLN-based amplifier stages, the second stage was replaced by a noncollinear KTA stage in the present work. As was explained in the Introduction, removable mirrors were used to redirect the seed and pump beams, leaving the previously implemented PPLN booster stage intact and available for comparison. Other than the pump and seed spot size in the second stage, the scheme is essentially unchanged. Therefore, only a brief overview is given here. The OPA system is pumped by a commercial Yb:KGd(WO4)2 laser oscillator-amplifier system (Pharos-SP, Light Conversion Ltd.) operated at a center wavelength of 1.028 µm at a variable repetition rate up to 100 kHz and a pulse duration of 180 fs. While the total pulse energy of the pump laser is 60 µJ, only an energy of 40 µJ is employed for seeding and pumping the two-stage OPA. The remaining 20-µJ pulse energy is applied for generating narrowband visible pulses for vibrational sum frequency generation spectroscopic applications [22,23]. From the 40-µJ pump pulses, a small part (110 mW i.e., 1.1 µJ) is split off to generate white-light-continuum seed pulses in an uncoated 6-mm-thick YAG plate. The 1.4-1.7-µm, down-chirped part of the continuum is amplified to a pulse energy of 0.4 µJ in a collinear, AR-coated, 1-mm-thick MgO-doped PPLN based first stage (cf. OPA1 in Fig. 2) and is then sent to the second amplifier stage. The residual pump pulses and the generated MIR idler pulses are filtered out by a dichroic mirror after the first stage.
Fig. 2 Schematic of the dual-beam setup for the generation of broadband, angular dispersion compensated MIR pulses. BSp: beam splitter, BS: beam sampler, WLCG: white-light continuum generator, WP: half-wave plate, PB: Brewster-type thin film polarizing beam splitter, Delay: delay stage, FS: fused silica window, CHM: chirped mirror, Si: silicon window, LPF: long-pass filter, BD: beam dump. DM: dichroic mirror, CM1 and CM2: spherical concave mirrors, G: grating. OPA1 and OPA2: optical parametric amplifier no. 1 and 2 based on PPLN and KTA, respectively. All lenses, wave plates, crystals, and filters are AR-coated.
In this work, the second stage is based on an AR-coated, 2-mm-thick KTA crystal used in noncollinear geometry (cf. OPA2 in Fig. 2) and pumped by the remaining pump pulse energy of 27 µJ. The reflectivity of the AR coating on the KTA crystal is < 0.5% in the 1.45-1.65-µm signal range and 4-5% in the 2.8-3.5-µm idler range. Due to the lower deff coefficient of KTA compared to PPLN, the pump beam waist at the second stage was reduced from 500 to 300 µm. The seed signal pulses are focused onto the crystal with an AR-coated, achromatic lens to a beam waist radius of 290 µm. The external noncollinearity angle between the pump and seed pulses was initially set to ~6.4° and was experimentally optimized to obtain broadband amplified signal pulses at minimum angular dispersion. The gain reached a factor of 20 resulting in 7.8-µJ output signal pulses (i.e., 780-mW average power) directly after the KTA crystal corresponding to a pump-to-signal energy conversion efficiency in excess of 27%. Despite the high conversion efficiency, the amplified parametric superfluorescence background in the signal, measured by blocking the seed, was below the resolution of our power meter head (< 1 mW). As a comparison, the pump-to-signal energy conversion efficiency of our previous, PPLN-based second stage was only 15%, which was limited by parasitic frequency conversion processes and photorefractive effects [22].
The pulse energy of the generated, angularly dispersed 3.1-μm idler beam directly after the KTA stage is 3.3 µJ corresponding to a pump-to-idler energy conversion efficiency above 12%. As a comparison, the pump-to-idler energy conversion efficiency of our previous, PPLN-based second stage was ~7%. The internal quantum conversion efficiency (or pump depletion) calculated either from the signal or the idler output energy, neglecting the Fresnel reflection of the pump beam at the input KTA surface, is 39-40%, which is significantly higher not only than that of the PPLN-based booster in [22], but also that in [10], where the booster stage was based on noncollinear KTA. We note that the compressed 1.5/3.1-µm pulse energy to pump pulse energy ratios (overall conversion efficiency including losses) was 8.5% of our system, which exceed those of similar MIR systems [8,9,21].
The idler pulses generated in OPA2 are imaged by a spherical mirror (cf. CM1 in Fig. 2) onto the ruled grating (40.96 lines/mm) at a transverse magnification of 2.8 and then up-collimated by another spherical mirror (cf. CM2 in Fig. 2). The measured diffraction efficiency of the grating for our MIR pulses is 77%, significantly higher than the transmission of the AR-coated prism in [10]. We note that the higher throughput and beam quality offered by reflection gratings compared to prisms can be preserved at higher average powers provided the gratings are manufactured on substrates with high thermal conductivity. The distances between the KTA crystal, the imaging spherical mirror, and the grating were optimized by minimizing the angular dispersion. For this purpose, the up-collimated idler beam was sent through a 200-µm-thick type I AgGaS2 (AGS) crystal and the spatial chirp of the second-harmonic in the far field was monitored by scanning the input fiber of a high-resolution InGaAs spectrometer.
3.1 Chirp compensation and pulse characterization
For chirp compensation, the signal and idler beams are transmitted through AR-coated windows. In order to find optimum compression, the pulses were characterized using a home-built, multi-shot second-harmonic generation frequency resolved optical gating apparatus (SHG-FROG). For the idler/signal beam, a 0.2-mm-thick AGS/0.5-mm-thick β-BaB2O4 (BBO), both type I, crystal was used, respectively.
The signal pulses were chirp-compensated by transmission through 24 mm of AR-coated fused silica (i.e., GDD = −610 fs2) and reflection off a chirped mirror (cf. CHM in Fig. 2, GDD = −300 fs2) at negligible losses. Figure 3 shows the measured and retrieved FROG traces [cf. Fig. 3(a) and Fig. 3(b), respectively], and the reconstructed temporal and spectral profiles [cf. Fig. 3(c) and Fig. 3(d), respectively] obtained for the amplified signal pulses at the highest power. The spectral intensity profile measured using an InGaAs spectrometer was in good agreement with the retrieved spectrum [cf. dashed line in Fig. 3(d)]. The pulse duration at FWHM intensity is 38 fs corresponding to 7.5 optical cycles at 1.53 μm.
Fig. 3 SHG-FROG data obtained for the compressed, 38-fs, 7.8-μJ signal pulses. Measured (a) and retrieved (b) SHG-FROG spectrogram, and reconstructed temporal (c) and spectral (d) intensity and phase. The measured spectrum is shown by the symbols in (d) and the symbols in (c) correspond to its Fourier transform. The FROG-error of the reconstruction for a grid size of 64 × 64 points was 0.006.
The angular-chirp-corrected, collimated idler pulses are transmitted through 4.5 mm of silicon (i.e., GDD = + 2200 fs2). The idler pulse energy after temporal compression is 2.3 µJ. Figure 4 shows the corresponding measured and retrieved FROG traces [cf. Fig. 4(a) and Fig. 4(b), respectively] and the reconstructed temporal and spectral profiles [cf. Fig. 4(c) and Fig. 4(d), respectively] of the MIR idler pulses at maximum power. The idler spectrum was measured using a Fourier-transform optical spectrum analyzer and also shows good agreement with the retrieved spectrum [cf. dashed line in Fig. 4(d)].
Fig. 4 SHG-FROG data obtained for the compressed, 53-fs, 2.3-μJ idler pulses. Measured (a) and retrieved (b) SHG-FROG spectrogram, and reconstructed temporal (c) and spectral (d) intensity and phase. The measured spectrum is shown by the symbols in (d) and the symbols in (c) correspond to its Fourier transform. The FROG-error of the reconstruction for a grid size of 64 × 64 points was 0.005.
3.2 Beam quality of the signal and the angular-chirp-corrected idler pulses
High conversion efficiencies typically come at the cost of a deteriorated beam quality, which can lower the Strehl-ratio, increase the M2 values, and can even affect the near-field beam profile. In addition, chromatic aberrations such as angular dispersion can drastically reduce the focal intensity of a laser pulse. In order to assess the beam quality of our signal and idler pulses, we performed near-field beam profile and M2 measurements.
Figure 5 presents the near-field profile of the 1.53-µm, 780-mW signal beam, measured before passing through the chirp compensation module without any collimation optics in the beam. An InGaAs camera with a pixel size of 30 µm was used to record the distributions. The 1/e2-diameter is 4.8 mm in the horizontal and 4.2 mm in the vertical plane, with an ellipticity dictated by the pump beam profile at the KTA crystal (not shown). Figure 6 displays the near-field profile of the angular-dispersion-compensated, 3.13-µm, 231-mW idler beam taken along a 3.5-m path length after the collimating spherical mirror, measured using a pyroelectric camera with an 80-µm pixel size. At the shortest propagation distances, the idler beam shows an ellipticity similar to that of the signal beam. The signal and idler near-field beam profiles follow Gaussian spatial distributions and the idler beam propagation exhibits very small astigmatism. The expanding beam diameter of the idler beam is due to diffraction-limited beam divergence starting from the relatively small up-collimated beam waist (w1/e2 ~1.0 mm) after the collimating mirror, CM2 (cf. Fig. 2). Compared to our previous arrangement employing a PPLN booster stage, the current, KTA-based booster amplifier provides improved near-field signal and idler beam profiles at a higher conversion efficiency (cf. Figure 4 in [22] with Fig. 5 and Fig. 6 in this paper). Relative to the angular dispersion scheme based on a Si prism, the grating-based setup shows dramatically improved idler beam quality and smaller losses (cf. Figure 4 in [10] with Fig. 6 in this paper).
Fig. 5 Near-field spatial beam profile of the 780-mW signal pulses measured at a distance of 0.8 m behind the KTA crystal with w1/e2(horizontal) = 2.4 mm and w1/e2(vertical) = 2.1 mm.
Fig. 6 (a)-(d) Near-field spatial beam profiles of the angular-dispersion-corrected, 231-mW idler pulses measured at different distances from the up-collimating spherical mirror. The 1/e2 Gaussian radii in the horizontal and vertical planes are 1.1 and 0.9 mm (a), 1.6 and 1.4 mm (b), 2.0 and 1.9 mm (c), and 2.8 and 2.7 mm (d), respectively. The Rayleigh length is ~1.0 m, which agrees with that of a diffraction-limited TEM00 beam.
The beam quality factor, M2, was also measured for both the signal and the idler beam. While the large pixel size of our pyroelectric camera prevents a straightforward M2 measurement in the available space on the optical table, the second harmonic of the idler beam could be characterized using the InGaAs camera allowing better spatial resolution. The so-obtained M2 values represent an upper limit of the actual values. Therefore, characterization of the beam quality of both the signal and the idler pulses were carried out using the InGaAs camera. In order to generate the second-harmonic of the idler beam, the up-collimated MIR pulses were propagated through the 200-µm-thick AGS crystal. The results are summarized in Fig. 7. Both beams show essentially diffraction-limited quality with negligible astigmatism despite the very high conversion efficiency in the KTA booster stage. The M2 values obtained from the caustic data for the horizontal and vertical plane are 1.28 and 1.37 for the signal beam and 1.33 and 1.21 for the idler beam, respectively. The excellent beam quality of the corrected idler beam, which is on an equal level to that of the signal beam, shows no contributions from uncompensated, residual angular dispersion.
Fig. 7 Beam caustic data measured for the 780-mW, 38-fs signal (a) and the 231-mW, 53-fs, angular-dispersion-compensated idler (b) beam. The extracted M2 values in the horizontal and vertical plane are 1.28 and 1.37 for the signal pulses and 1.33 and 1.21 for the idler pulses, respectively.
3.3 Open-loop CEP stability of the angular-chirp-corrected idler beam
The generated MIR idler pulses are expected to be CEP-stable, since the pump and seed pulses in the booster amplifier (OPA2) are originating from the same pump laser [24]. However, beam pointing fluctuations on a diffraction grating can deteriorate CEP stability [25]. In order to investigate the CEP jitter and compare it to that of our previous OPA system, we performed f-2f interferometry following the same procedure we used in [22]. Briefly, a multi-octave, single-filament super-continuum was generated by focusing the idler pulses into a 2-mm-thick YAG plate. The blue edge of the generated continuum reached 450 nm, which is a limit dictated by the material. The fact that generation of such a multi-octave continuum was possible confirms that residual angular dispersion in the idler beam was negligible. The continuum was collimated and focused into a 1-mm-thick BBO crystal (type I), which was angle-tuned to frequency-double the wavelength range around 1.37 µm. The fundamental and the generated second-harmonic waves at 685 nm were relay-imaged onto the slit of a CCD spectrometer. A wire grid polarizer was used to maximize the visibility of the spectral fringes. Figure 8 shows the resulting spectral fringes as a function of time. In order to prove that the fringes were caused by f-2f beating, data were taken with a 2-mm thick, uncoated UVFS window temporarily in the collimated continuum during the measurement [cf. Fig. 8(a)]. The extracted fringe period with and without the UVFS plate in place [cf. at 7.7 and 9.0 s in Fig. 8(a), respectively, and Fig. 8(b)] was 3.36 and 2.95 nm respectively. The change in fringe spacing is in excellent agreement with the expected change in the group delay between the 1.37-µm and 685-nm components predicted by the Sellmeier equation of fused silica.
Fig. 8 Spectral f-2f interferometric data taken for the angular-dispersion-compensated 3.1-µm idler beam. No active CEP stabilization was employed. (a) f-2f fringes as a function of time with and without a 2-mm thick UVFS plate in the supercontinuum prior to SHG. The plate was removed at 8.25 s. (b) f-2f spectra extracted from panel (a) at the dashed lines at 7.7 and 9.0 s corresponding to data recorded with and without the UVFS plate, respectively. (c) f-2f fringes recorded over a 1-min time period at a sampling rate and integration time of 10 Hz and 100 ms, respectively. (d) The time-dependent CEP jitter extracted from the data in panel (c) corresponding to an RMS value of 54 mrad.
Figure 8(c) shows the f-2f fringe stability over a 60-s time period using an integration time of 100 ms per interferogram (i.e., 104 shots). The corresponding RMS CEP jitter is 54 mrad [cf. Fig. 8(d)], which is significantly lower than what we measured using the PPLN-based booster stage (i.e., 180 mrad in [22]). The improved CEP noise performance obtained with KTA compared to PPLN is in agreement with our previous observation that photorefractive effects in PPLN increase CEP jitter [22].
Despite the long, 100-ms integration time used for taking one f-2f interferogram, our unaveraged fringe visibility [i.e., (Imax-Imin)/(Imax + Imin)] presented in Fig. 8(c) is ~0.40, which compares well to that obtained for a 100-kHz, 1.8-µm, passively CEP stabilized source under identical acquisition time (cf. right panel of Fig. 4 in [26]). We note that the quoted 54 mrad value must not be understood as the RMS shot-to-shot CEP jitter, as it was extracted from 10000-shot averages. Our shot-to-shot CEP noise can be estimated by simulating the resulting f-2f fringe visibility and was found to be < 700 mrad, which matches the high passive CEP noise performance of the Ti:sapphire laser-driven 1-kHz infrared OPA source in [27].
3.4 High-harmonic generation using the angular-dispersion-corrected idler beam
Efficient HHG in solids has occasionally been used as a criterion to evaluate beam quality for small scale systems [21]. While it is difficult to infer quantitative information on beam quality from HHG in a solid, such a benchmark experiment can provide qualitative confirmation that our corrected MIR beamline constitutes a practically useful source on par with traditional ultrafast sources. As the only goal here was to test the corrected idler beam, no attempt was made to explain the features of the observed harmonics.
For this experiment, the MIR idler pulses were transmitted through a variable attenuator consisting of an AR-coated zero-order half-wave plate and an AR-coated wire-grid polarizer, and were then chirp compensated by an AR-coated, 5-mm-thick silicon plate resulting in a transmitted pulse energy of 1.7 µJ. The pulses were focused into an undoped, 200-µm-thick YAG window using an f = 25-mm parabolic reflector. The generated harmonics were collimated with an f = 50-mm uncoated, MgF2 singlet lens and were then focused onto the slit of a 320-mm spectrograph equipped with a Peltier-cooled, back-illuminated, deep-depletion CCD. Odd harmonics up to the 9th-order were obtained [cf. Fig. 9(a)]. The width of the 9th-harmonic indicates significant spectral broadening, possibly due to self-phase modulation similarly to the data in [21]. In order to confirm that the observed spectrum is indeed a result of HHG, we measured the intensity scaling of the 5th and 7th-harmonics by varying the incident idler power by rotating the half-wave plate in the variable attenuator. Figure 9(b) shows the so-obtained data together with the trend lines according to the corresponding perturbative power laws. The scaling suggests that these, below-gap harmonics were generated in the perturbative regime similarly to the scaling of the 5th and 7th-harmonics generated by high-power 3.8-µm pulses in ZnO (cf. Figure 2 in [28]).
Fig. 9 High-harmonics generated by angular-dispersion-compensated, 3.1-µm pulses in a 200-µm-thick YAG window. (a) 9th-harmonic recorded using an integration time of 1 s. A UG5 color glass filter was used to suppress possible stray light effects. (b) Intensity dependence of the 5th and 7th-order harmonic yield. The lines show intensity scaling following a 5th and 7th-order power law (green and red line respectively).
4. Conclusions and outlook
We have presented a white-light-seeded double-stage OPA based on a collinear, PPLN pre-amplifier and a noncollinear KTA booster amplifier providing two optically synchronized beams at a repetition rate of 100 kHz: signal pulses at 1.53 µm and CEP-stable idler pulses 3.13 µm. The pump-to-signal and pump-to-idler energy conversion efficiency in the KTA stage exceed an unprecedented level of 27% and 12% compared to existing MIR systems, respectively, without deteriorating the beam quality or the CEP stability. Dual-beam operation was made possible by angular dispersion correction of the idler beam, which was accomplished by a straightforward scheme leading to a diffraction-limited idler beam performance. The dual-beam source delivers 7.8-µJ, 38-fs, 1.53-µm signal pulses and 2.3-µJ, 53-fs, 3.1-µm idler pulses with an open-loop CEP jitter on par with traditional supercontinuum-seeded, passively CEP-stabilized OPA’s. The measured M2 values of both beams are below 1.4. We also demonstrated that the high beam quality requirements dictated by efficient HHG in solids can be met by the corrected idler beam. Using a noncollinear KTA as the booster stage, we obtained higher beam quality and better CEP-stability at much higher conversion efficiencies than with a PPLN booster amplifier. Thanks to the high conversion efficiencies, our compact, small-scale OPA system constitutes a promising source not only for high-repetition-rate broadband sum-frequency generation spectroscopy [22,23], but also for high-harmonic spectroscopy of solids. Finally, a noncollinear KTA amplifier combined with the presented angular dispersion compensation scheme relying on a reflection grating manufactured on a substrate with high thermal conductivity promises great potential for scaling the average power beyond the current state-of-the-art.
Funding
Deutsche Forschungsgemeinschaft (DFG) (GSC 1013 SALSA); European Union’s Horizon 2020 Research and Innovation Programme (654148).
Acknowledgments
The authors thank Janina Kneipp for her support and helpful suggestions. Z.H. acknowledges funding by a Julia Lermontova Fellowship from DFG, No. GSC 1013 SALSA.
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