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Abstract
We demonstrate pulse compressibility from 840 fs to 38 fs of 10 mJ pulses from a thin-disk amplifier at a repetition rate of 3 kHz after nonlinear broadening in a multipass cell. In addition, the temporal-intensity contrast is enhanced via nonlinear ellipse rotation of more than a factor 50 with an optical efficiency of 56%. We believe this is the first published experimental combination of multipass cell-based nonlinear compression and nonlinear ellipse rotation-based contrast enhancement preserving both pulse compressibility and beam quality.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Modern ultrafast laser sources allowed the development of novel applications in science [1–4] and industry due to their ability to reach extremely high peak powers. Many of these applications not only benefit from ultrashort pulses but also from high repetition rates and high energies, implying high average powers. Ti:Sapphire-based systems deliver pulse durations well below 50 fs, thanks to the broadband emission cross-section of the gain media. However, this class of lasers is limited in average power to approximately 10–20 W [5–7]. In contrast, Yb-based amplifiers can achieve much higher average powers, albeit typically longer pulse durations [8,9]. Among these systems, thin-disk amplifiers distinguish themselves for achieving ultra-high pulse energies at these high repetition rates, resulting in average powers of nearly 2 kW [10] and pulse energies of >500 mJ [11] but with pulse durations typically of >500 fs, due to the narrowband emission cross-section of their gain medium.
External nonlinear pulse compression can extend the bandwidth of Yb-based systems and simultaneously preserve the high repetition rate and high pulse energies. Such approach has the potential to take the lead in a novel class of ultrafast high energy sources. Well established schemes for nonlinear compression are often using waveguides or quasi-waveguides, such as fibers, capillaries, or multi-plates [12–16]. These waveguides are subject to several limitations, making them poorly suited for high pulse energies or high optical transmission efficiencies [17–20]. Another largely diffused arrangement is based on optical parametric amplification [21,22]. However, their limited efficiency restricts the applicability of this broadening method at higher average powers. In contrast, a gas-filled Herriott-type multipass cell (MPC) combines both scalability to high energies and high optical efficiencies [23–27].
In addition to requiring short pulse durations, high peak intensities and high temporal-intensity contrasts are essential in numerous applications such as the acceleration of protons and electrons [28,29]. Cross-polarized wave (XPW) generation and nonlinear ellipse rotation (NER) are commonly implemented to improve pulse contrast [30–34]. Similarly to self-phase modulation (SPM), both techniques are χ(3) processes, therefore spectral broadening and contrast enhancement can be merged in a single experimental setup. As opposed to NER relying on noble gas as nonlinear medium, XPW requires a crystal. Typically, it will be driven above the critical power for self-focusing, which may result in a degradation of the output beam profile [32]. Hence, NER in gas is preferably chosen for contrast improvement at high peak and average powers. First simulations of multipass cell-based nonlinear compression in combination with NER were performed by Pajer et al. [33] and show the NER suitability for high peak power applications.
In this work, we report on a Herriott cell-based nonlinear compression setup with a compression factor of 22 seeded by 840 fs long pulses with a pulse energy of 10 mJ at a repetition rate of 3 kHz. Furthermore, we experimentally demonstrate pulse contrast enhancement by more than a factor of 50 using NER without degradation of the temporal compressibility and spatial beam quality.
2. Nonlinear compression
The outline of the experimental setup for nonlinear pulse compression is sketched in Fig. 1. The laser source used in this experiment is a Yb-doped thin-disk regenerative amplifier (DIRA 200-5) [35] that provides an average power of 200 W and pulse energies of up to 40 mJ. Before the experiment, the pulse energy of the laser was set to 10 mJ at a repetition rate of 3 kHz, namely the design value of the following nonlinear compression setup. At the laser output a pulse duration of 840 fs and a beam quality of M2x,y = 1.28 ${\times} $ 1.33 was measured.
Fig. 1. Schematic layout of the nonlinear pulse compression setup. The pulses generated by a Yb-doped thin-disk chirped-pulse amplifier (CPA) are coupled into an argon-filled Herriott-type multipass cell (MPC) for spectral broadening. Chirped mirrors are used for subsequent temporal re-compression at low pulse energy. M1: mode matching mirror, M2: in- and outcoupling mirror, M3: recollimation mirror, PM: power meter, CMs: chirped mirrors. QWP1: broadband achromatic λ/4 wave plate, QWP2: broadband achromatic λ/4 wave plate, GTP: Glan-Taylor calcite polarizer. The light grey components will be added to the experimental setup only in the second part of this work, which deals with nonlinear ellipse rotation.
The nonlinear spectral broadening is performed inside a Herriott-type multipass cell. The cell is placed in a 2.7 m long and 0.4 m wide low-pressure chamber with 3.5 mm thick fused silica windows for incoupling and outcoupling. The cell consists of two concave 4” dielectric mirrors with a radius of curvature (ROC) of 1.2 m spaced apart by ∼2.4 m. These mirrors are designed to meet a high reflectivity of >99.99% and a $|{\textrm{GDD}} |$< 50 fs2 between 970 nm and 1090 nm. The Gaussian beam diameters (1/e2) are calculated to be 0.62 mm at the focal plane and 3.0 mm on the mirror surface. The incident beam is matched to the cell’s eigenmode using a 10 m ROC concave mirror (M1 in Fig. 1). A plane rectangular mirror (M2 in Fig. 1) steers the beam into the MPC. The chamber is filled with argon at a pressure of 650 mbar. For the design and layout, a nonlinear refractive index for argon of n2 ${\approx} $ 1 ${\times} $ 10−23 m2/W at atmospheric pressure was assumed [36]. The B-integral accumulated over a single pass is ∼ 2.1 rad. After 37 focal passes resulting in a total B-integral of 77.7 rad, the beam is guided out of the chamber by a second rectangular mirror and recollimated by another concave mirror (M3 in Fig. 1). The energy transmitted through the MPC is virtually unchanged with 9.7 mJ at its output. To avoid both nonlinear interactions with air and damage on the following chirped mirrors, the energy sent towards the compressor is attenuated to ∼ 3.5%. In future implementations, a possibly necessary vacuum atmosphere and larger beam diameters on the chirped mirrors will be implemented to achieve the temporal re-compression of the full energy.
The positive group delay dispersion (GDD) acquired during propagation through the gas-filled MPC and the resulting SPM is compensated by reflecting the beam 18 times over chirped mirrors with a negative GDD of $- $250 fs2 per bounce. The measured optical efficiency of the chirped mirror compressor is 98.5%.
The compressed pulses after the chirped mirror compressor are analyzed by a second-harmonic frequency resolved optical gating (SH-FROG, VideoFROG by Mesa Photonics Inc.). Figures 2(a)–2(b) displays the measured and retrieved SH-FROG trace on a 512 ${\times} $ 512 grid. The SH-FROG error is computed to be 0.32%. In Fig. 2(d), the retrieved and measured spectra are plotted along with the retrieved spectral phase. The measured spectrum exhibits a distinctive central peak around 1030 nm. This peak coincides with the amplifier spectrum with a -20 dB bandwidth of roughly 5 nm and can be attributed to the typical pre- and post-pulses of a regenerative amplifier as well as amplified spontaneous emission (ASE). The spectral bandwidth at -20 dB is calculated to be 73 nm. The temporal profile (Fig. 2(c)) shows a compression of the pulse to 38.4 fs (FWHM), close to the Fourier-transform limit (FTL) of 37.7 fs, with 80.0% of the energy contained in the main peak. The pedestals around the main peak originate from the SPM-induced spectral shape. In summary, the pulse compression from 840 fs to 38.4 fs leads to a compression factor of ∼22.
Fig. 2. Temporal characterization of the output pulse from the nonlinear pulse compression setup after the temporal re-compression. (a) Measured and (b) retrieved SH-FROG (Mesa Photonics Inc.) spectrogram after the chirped mirror compressor (grid size, 512 ${\times} $ 512 points; G-error = 0.32%). (c) Retrieved temporal profile of the compressed pulse. (d) Measured and retrieved spectra of the compressed pulses. The green dotted line reports the spectral phase.
For preserving M2 (Fig. 3), a reasonable spatio-spectral homogeneity is required [24,25,37]. The homogeneity as a figure of merit is defined as Homogeneity${\; = }\left[ {\smallint {\textrm{I}_\mathrm{\lambda}}\mathrm{(\lambda )}\cdot {\textrm{I}_{\mathrm{\lambda 0}}}\mathrm{(\lambda )d\lambda }} \right]^2/[ \smallint {\textrm{I}_\mathrm{\lambda }}(\mathrm{\lambda } )\mathrm{^2d\lambda }\cdot \\ \smallint {\textrm{I}_{\mathrm{\lambda 0}}}(\mathrm{\lambda } )\mathrm{^2d\lambda } ] \times $100, where each spectrum Iλ(λ) overlaps with the spectrum on the beam axis Iλ0(λ) [37]. To characterize this spatio-spectral coupling, we probed the spectrum across two orthogonal axes of the transverse beam profile of the collimated and compressed beam. The beam diameter is determined to 2wx ${\times} $ 2wy = 3.4 mm ${\times} $ 3.6 mm (Fig. 4). The spectra are scanned in steps of 0.25 mm by moving a multimode fiber (200 µm core diameter) mounted behind a 100 µm diameter precision pinhole through the collimated beam. The spectra are recorded with a HR4000 spectrometer from Ocean Insight Inc. and analyzed in terms of homogeneity. The spatio-spectral homogeneity values within the 1/e2 diameter are determined to be to >80% (Fig. 4(c-d)). The Fourier transform limits across the two orthogonal axes of the transverse beam profile are depicted in Fig. 4(b).
The excellent beam propagation factor is measured to M2x,y = 1.17 ${\times} $ 1.17 (Fig. 3) after the pulse compression.
Fig. 3. Measurement of beam propagation factor after pulse compression along transverse x (red) and y (blue) axes.
Fig. 4. Spatio-spectral characterization after temporal re-compression. (a) Transverse beam profile of the collimated and compressed beam. (b) Calculated Fourier transform limits across two orthogonal axes. The normalized intensity for x-axis (c) and y-axis (d) of (a) is represented by the light filled curve. The grey areas mark the positions outside the 1/e2 diameter. The dark blue (c) and red (d) curves show the corresponding spatio-spectral homogeneity values.
3. Contrast enhanced nonlinear compression by nonlinear ellipse rotation
During the nonlinear compression experiments, the composition of the prominent central peak around the center wavelength of the broadened spectrum (Fig. 2(d)) was investigated in more detail. This peak is not retrieved by the SH-FROG and therefore can be explained as unbroadened low intensity components in the pulse. The central peak contains about 19% of the spectral energy. By using a photodiode (DET10A/M, Thorlabs Inc.) to investigate the temporal distribution, 8% of the energy content can be attributed to the typical pre-and post-pulses of a regenerative amplifier (Fig. 5(a)). The pre-pulses (Fig. 5(a-b), blue) originate from the leakage of the Pockels cell inside the regenerative amplifier at each round-trip. The spacing between two consecutive pre-pulses corresponds to the round-trip time of the amplifier cavity. Simulations (Fig. 6(a)) show that 7% of the energy of the broadened spectrum would be contained within the bandwidth of the amplifier spectrum anyway. The broadening was simulated by solving the extended nonlinear Schrödinger equation with a split-step approach as described in [38]. The remaining 4% of the energy are most likely attributed to ASE in the fiber-laser based pre-amplifier chain of the laser source. To reduce the weighting of the pre-pulses and improve the pulse contrast, we implemented the method of nonlinear ellipse rotation [30,39]. It should be noted that the pre- and post-pulses of the regenerative amplifier could also be suppressed very efficiently by a factor of 500-1000 using a pulse picker.
Fig. 5. Characterization of contrast enhancement. (a) Full scale photodiode signal (red) and zoomed-in representation (blue) without NER (a) and with NER (b). The maximum main and pre-pulse intensities without NER (a) and with NER (b) are noted. Main pulse starts at time 0. (c) Efficiency as a function of θNER at a constant pressure of 650 mbar.
Fig. 6. Temporal characterization of the NER setup. (a) Simulated (black) and measured broadened spectrum without (red) and with (blue) NER. (b) Retrieved temporal profile of the compressed pulse and the calculated Fourier-transform limited profile after NER.
In this work, the pre-pulses are conveniently used to determine the contrast improvement by NER. We are aware that contrast-sensitive processes such as laser wakefield acceleration of electrons or laser plasma generated X-rays can benefit from the contrast improvement of the main pulse. Therefore, it would have made more sense to investigate the contrast improvement of the main pulse. At the time of the experiments, no measuring device such as a third-order autocorrelator was available to examine the contrast of the main pulse more closely. Therefore, measurement of the pre-pulses was used, as only a photodiode and an oscilloscope are sufficient. In contrast to pre-pulses, at a certain level of contrast improvement, post-pulses disappear in the trailing edge of the photodiode signal from the main pulse. Due to the comparative nature of the measurements, inaccuracies related to, for example, the sampling rate of the oscilloscope and the load of the photodiode can be neglected.
The initial state for NER is described by an elliptical polarization. Due to the different intensity-dependent refractive indices of the left- and right circular polarization components, the orientation of the polarization ellipse is rotated during the propagation of a high intense laser pulse [34,40]. This results in different polarization ellipses between the high intense main pulse and the less intense parts such as pre- and post-pulses. After differentiation of the low and high intense part, the temporal intensity contrast of the high intense part is improved.
The input parameters and the main setup for spectral broadening are taken from the above experiments (Fig. 1). The polarization rotation from linear to elliptical polarization is performed by a broadband achromatic λ/4 wave plate (QWP1), which is positioned in the collimated beam before M1 (Fig. 1). A second broadband achromatic λ/4 wave plate (QWP2) is placed after mirror M3 in the recollimated beam (Fig. 1) to compensate the introduced ellipticity. The orthogonal polarizations are split by a Glan-Taylor calcite polarizer after the attenuating uncoated 0° wedge (Fig. 1). The simplified sketch of the NER setup is depicted in Fig. 7.
Fig. 7. Simplified sketch of the NER setup. QWP1: broadband achromatic λ/4 wave plate, MPC: Herriott-type multipass cell, QWP2: broadband achromatic λ/4 wave plate, GTP: Glan-Taylor calcite polarizer, PM: power meter, CMs: chirped mirrors.
In general, at low intensities, QWP1 and QWP2 are adjusted to achieve a maximum in the polarizer reflection for s-polarized light. Therefore, the transmitted p-polarization is at its minimum. The polarizer transmission increases at increasing intensities due to elliptical polarization rotation of the intense part of the beam [30,41].
To achieve the same compressed pulse duration after NER as above and to get the highest possible efficiency of the whole experimental setup, we used a slightly modified approach. First, at low intensity, the polarizer reflection (s-pol) was maximized. Afterwards, the angle of QWP1 (θNER) was rotated in one-degree increments. θNER is defined relative to the input polarization. QWP2 was readjusted at high intensity to minimize the dominant central peak in the spectrum while maintaining the broadened spectral bandwidth and the spectral intensity at the outer edges of the spectrum.
With this procedure in the spectral domain, the dominant peak around the central wavelength for θNER = 5° is drastically reduced and the bandwidth of the broadened pulse is preserved (Fig. 6(a)). The main and pre-pulses are measured by a photodiode (DET10A/M, Thorlabs Inc.) before compression and recorded using an oscilloscope (WaveRunner 640Zi, Teledyne LeCroy Inc.). Dividing the maximum pre-pulse photodiode (PD) signal by the maximum main pulse PD signal results in the corresponding pulse contrast without (Fig. 5(a)) and with (Fig. 5(b)) NER. In our experiments, the temporal contrast with NER is improved by more than a factor of 50 in comparison to that without NER. As described in [30,34], the contrast can be enhanced up to three orders of magnitude by using the general procedure described above and by reducing the gas pressure inside the chamber [33]. The average power of the experimental setup after attenuation in Fig. 1 is 1.01 W. After inserting the two waveplates and the polarizer in the beam path, the power is reduced to 0.57 W at the optimized setpoint. Hence, the overall efficiency of the NER setup before compression corresponds to approximately 56% (Fig. 5(c)). The θNER dependent efficiency values at 650 mbar argon (Fig. 5(c)) are in a good agreement to the simulations performed by Pajer et al. [33]. The small deviations could have been caused by inaccurate adjustment of the QWP1 due to the used rotation mount (RSP2/M, Thorlabs Inc.).
Compared to the first experiments described above without NER, the required negative GDD to compensate the second order phase term has slightly increased due to a different nonlinear phase shift and propagation through the 22 mm thick Glan-Taylor calcite polarizer. The 20 bounces off the chirped mirrors introduce a total GDD of -5000 fs2 and compress the output to 37.8 fs (FWHM). In Fig. 6(b), the temporal profile of the compressed pulses is depicted alongside the calculated FTL of 37.2 fs. The overall compression factor of ${\sim} $22 is not affected through the NER.
The spatio-spectral homogeneity values within the 1/e2 diameter exceed 80% after compression. The M2 parameters for both axis were recorded to M2x,y = 1.17 ${\times} $ 1.19.
4. Conclusions and outlook
In conclusion, we demonstrated an efficient nonlinear spectral broadening in an argon-filled multipass cell with 10 mJ input energy at a repetition rate of 3 kHz. The initial pulse duration of 840 fs was compressed to 38 fs resulting in a compression factor of 22 with 80% of the energy in the main temporal peak. The cell and the chirped mirror compressor allow a transmission of >95% with excellent M2 values of 1.17 ${\times} $ 1.17 and a spatio-spectral homogeneity of >80% within the 1/e2 diameter.
In a second step, to the best of our knowledge, we presented for the first time experimentally a combination of multipass cell-based nonlinear compression and nonlinear ellipse rotation-based contrast enhancement. The temporal characterization revealed a significant contrast improvement of more than a factor of 50 with an optical efficiency of 56%. Both the re-compressed pulse duration and the beam quality were preserved by the NER.
In further steps, the multipass cell-based nonlinear compression scheme will be scaled towards 200 mJ pulse energy and an average power of 1 kW. In addition, improved studies of the NER with a third-order autocorrelator are planned to determine if a contrast enhancement at the main pulse of three orders of magnitude can be achieved.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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