High-power ultrafast laser sources are highly demanded tools in various ultrafast applications^[1^–3^]. The high peak power enables these applications to be driven, while the high average power satisfies their need for high photon flux and high signal-to-noise ratio. Thanks to the high quantum and Stokes efficiency, as well as the excellent thermal management of state-of-the-art fiber, thin-disk and Innoslab laser architectures, Yb-based lasers are capable of reaching several hundred watts, and even more than 1 kW^[4^–7^].

Solid-state amplifiers shine in terms of high pulse energy owing to the negligible nonlinear effect and high damage threshold of key devices, especially the Innoslab amplifier endowed with a compact configuration and high amplification factor. The fiber laser can work as an outstanding frontend owing to its robustness, excellent beam quality and the integration of chirped fiber Bragg gratings (CFBGs) as the stretcher without misaligning like the grating stretcher. Hence, the Innoslab amplifier seeded by a fiber frontend provides an attractive choice for achieving a superior high-power laser systems^[8^–10^]. However, there are still some drawbacks that cannot be ignored. Firstly, the narrow gain bandwidth limits the generation of sub-100 fs ultrashort pulses, which is a common drawback of Yb-based lasers. Secondly, the compressed pulses always show obvious pedestals, which can be attributed to the inability of the grating compressor in offsetting the higher-order dispersion, mainly from fiber frontend. Finally, the output beam of the Innoslab amplifier is always elliptical and accompanied with spatial distortion. The implementation of high-quality beam generally requires state-of-the-art reshaping and spatial filtering.

Post-compression technology is an effective route for overcoming the narrow bandwidth limitation, which mainly relies on spectral broadening by self-phase modulation (SPM). Due to the long-distance nonlinear effect with gas medium and the waveguide structure, many pulse compression results with large compression factor and high spatial quality are demonstrated by the gas-filled hollow core fiber technology^[11^–14^]. However, the low fiber coupling efficiency, alignment sensitivity and increased susceptibility to damage at high power are the inescapable drawbacks, which restrict its application to high power lasers. The multi-pass cell approach is also a widely applied technique^[15^–21^]. In contrast, high transmission efficiency pulse compression can be achieved because of the absence of fiber coupling loss. Nonetheless, the large number of reflections on the cavity mirrors requires a state-of-the-art coating with high reflectivity and low (or negative) group-delay dispersion, which is costly and technically challenging. Impressive performances are also shown in multi-thin-solid-plates (MTSP) technique which is simple, robust and economical^[22^–26^]. But the MTSP technique usually needs an elaborate strategy in placing the thin plates to avoid the catastrophic spatial Kerr effect which can cause energy loss, beam deterioration and irreversible material damage^[27^–30^]. Vlasov et al.^[27^] proposed a periodical self-focusing system where a repeating (stationary) propagation is achieved by balancing the spatial nonlinear effect and beam divergence, which allows a high-efficiency spectral broadening without catastrophic self-focusing effect even though the peak power is far beyond the self-focusing critical power. Zhang et al.^[28^] introduced the Fresnel–Kirchhoff diffraction (FKD) integral to identify a more exact solution and demonstrated an eight-fold pulse compression with more than 85% efficiency. In our previous work, we proposed a simple Gaussian beam optics model to find the solution from a more practical perspective and demonstrated a 17-fold pulse compression with 94% efficiency^[30^].

In this work, we demonstrate dual-stage periodically placed thin fused silica plate post-compression used for the Innoslab laser system seeded by a fiber frontend, eventually implementing an efficient, compact and robust high-power ultrafast laser source with 64 fs pulse duration and 96 W average power at 175 kHz repetition rate. The high average power laser is provided by the compact Innoslab laser system, but the spatial quality degraded from the amplification process and temporal quality is undesirable because of the mismatched spectral phase between the stretcher and grating compressor. Not only is more than eight-fold pulse compression achieved with 94% transmission, but also the spatial mode and pulse quality are improved in the dual-stage all-solid-state post-compression. In the time domain, the final pulses are almost pedestal-free and quite close to the Fourier-transform-limited (FTL) pulse duration, because of the compensation for high-order dispersion during the nonlinear process. In the space domain, the increasingly circular and clean beam profile can be attributed to the spatial mode self-cleaning effect. A similar effect was reported in Ref. [28], which was considered as the effect of spatial self-organization of the laser beam and was previously observed in the nonlinear process of filamentation^[31^], self-focusing collapse^[32^] and multimode fibers^[33^]. This spatial improvement resulted in the increase of spatial energy concentration, which strengthens the application of the Innoslab laser system. These results demonstrate that periodically placed thin-solid-plate post-compression is competent to overcome the narrow bandwidth limitation of Yb-based lasers with exceptional efficiency and enable a high-quality laser output by the spatiotemporal self-cleaning effect. This work indicates that periodically placed thin-solid-plate post-compression is an efficient and economical complement for the Innoslab laser system and this robust and compact combination may be a promising scheme for generating higher-power few-cycle lasers with good spatial and temporal quality.

As illustrated in Figure 1, efficient all-solid-state post-compression based on dual-stage periodically placed fused silica plates is used for a high-power ytterbium-doped yttrium aluminum garnet (Yb:YAG) Innoslab laser system, which is seeded by a commercial all-fiber frontend, eventually yielding a high-power ultrafast laser output.

The high-power amplifier system is described in detail in Ref. [34], which employs chirped pulse amplification, a spatial filter and a grating compressor. It delivers 102 W average power at 175 kHz repetition rate, corresponding to a pulse energy of approximately 0.6 mJ. The amplifier output spectrum is shown in Figure 3(a) detailed later, which is centered at 1030 nm and corresponds to an FTL pulse duration of 250 fs. The autocorrelation trace is shown in Figure 2(a), indicating 532 fs pulses with pedestals assuming a Lorentz pulse shape (APE GmbH, pulseCheck-NX50), which is larger than the FTL pulse duration (factor ~ 2.1). This can be attributed to the nonlinear effect of the fiber frontend and the mismatch in higher-order dispersion between the grating compressor and the CFBGs. The amplifier output beam quality is characterized as M² = 1.29 × 1.14 (Ophir BeamSquared), as shown in Figure 2(b). Although it is nearly diffraction-limited after spatial reshaping and filtering, a few high-order spatial modes and spatial distortion are difficult to completely eliminate, as evidenced by the images measured at different locations.

The dual-stage all-solid-state post-compression consists of periodically placed thin-solid plates for nonlinear spectral broadening, focusing mirrors for mode-matching and chirped mirrors for dispersion compensation. The first-stage mode-matching is achieved by a curved mirror with 2500 mm radius of curvature (ROC), focusing the beam to a waist with 0.5 mm diameter. In the first-stage spectral broadening system, twelve 0.5-mm-thick fused silica plates are periodically arranged with a distance of 40 mm, that is, the period of the system. In order to maximize the transmission efficiency, these uncoated plates are all placed at the Brewster angle and the laser is linearly p-polarized. By placing the first plate 20 mm behind the waist, the stationary mode will propagate in the system at the balance of the self-focusing effect and diffraction when the full power is applied, which enables high-efficiency spectral broadening with high spatial quality. The second-stage mode-matching comprises ten 0.5-mm-thick fused silica plates at a period of 75 mm. The mode-matching condition is placing the first plate of the second-stage spectral broadening system 38 mm behind a 0.7-mm-diameter waist formed by a 4000-mm ROC curved mirror, corresponding to an estimated peak intensity of about 1 TW/cm² on the plates which is similar to that of the first stage. The above-mentioned mode-matching condition and spectral broadening system design are all determined by the Gaussian beam optics model detailed in Ref. [30]. It identifies the stationary mode propagation by approximating the spatial Kerr effect of thin-solid plates as a nonlinear lens group. The focal length can be expressed by the following^[35^]:

$$\begin{align}f\approx \frac{hw^2}{4{n}_2{I}_0{l}_{\mathrm{eff}}},\\[-18pt]\nonumber\end{align}$$  (1)

The spectrally broadened pulses are compressed by chirped mirrors where the first-stage compressor provides a total GDD of –17,500 fs² and that of the second-stage compressor is –2000 fs². The reason why numerous chirped mirrors are employed in the first-stage compressor is the massive residual GDD after the grating compressor. If the GDD is completely offset in the grating compressor, an obvious pedestal and many sidelobe pulses with considerable intensity will appear in addition to a narrower center-peak pulse, which are mainly caused by the residual third-order dispersion (TOD) and higher-order dispersion. During the nonlinear process, the nonlinearity induced dispersion can offset the residual high-order dispersion to some extent. According to the theory in Ref. [37], the residual TOD after the nonlinear process decreases to a negligible level of 10⁻⁶–10⁻⁷ ps³ with the accumulation of nonlinear effect, eventually resulting in the pulse self-cleaning even though chirped mirrors only offset GDD. As a result, the dispersion to be compensated in the second-stage compressor is substantially reduced thanks to the pulse-cleaning in the first stage.

The spectral evolution of the high-power ultrafast system is illustrated in Figure 3(a). An obvious gain narrowing is introduced as the power enhancement in the Yb:YAG Innoslab amplifier and the spectral bandwidth is narrowed to 2.9 nm (full width at half maximum, FWHM) from 8.3 nm (FWHM) of the seed. After the dual-stage periodically placed fused silica plates, the spectrum is significantly broadened due to the SPM effect. The first-stage spectral broadening basically offsets the impact of gain narrowing, extending the spectral range close to that of the seed. The second stage exhibits a stronger ability for spectral broadening, eventually broadening the spectral range to 990–1065 nm at the –20 dB level. Although the accumulated B-integral is similar, the spectral broadening effectiveness shown in the dual stage is quite different. This can be attributed to the massive residual chirp in the first-stage initial pulses, which weaken the capability of spectral broadening^[38^]. The asymmetry of spectral broadening is caused by the high-order dispersion of the initial pulses and the self-steepening effect^[37^,38^]. Figure 3(b) shows the good spatial homogeneity of spectral broadening, which is characterized by the spectra measured across the transverse beam profile of the final output beam at the location with a beam radius of about 6 mm, indicating that the spatial chirp is well controlled by the strategically placed thin plates.

The measured and Lorentz fitted autocorrelation traces characterized at the first-stage and second-stage post-compression are shown in Figure 4. After the first-stage post-compression, the pulses are actually compressed to 239 fs, which is larger than the FTL pulse duration (factor ~ 1.3) and corresponds to a compression factor of 2. Although the first-stage spectral broadening is slight, much shorter pulses can be achieved due to the nonlinear dispersion compensation. Compared with the initial pulse, the pulse width is closer to the FTL pulse duration and the pedestals are cleaned to some degree. As shown in Figure 4(b), the pedestal-free pulses of 64 fs are measured after the second-stage post-compression, corresponding to a four-fold pulse compression. The pulse width is very close to the FTL pulse duration (factor ~ 1.07), and the pedestals of the amplifier output pulses are almost completely cleaned. These results indicate that the dual-stage all-solid-state post-compression achieves not only a large-factor pulse compression but also a pulse-cleaning effect for the driving pulse quality stemming from the fiber frontend. The pulse-cleaning effect is introduced by the high-order dispersion compensation in the nonlinear process^[37^], providing the ultrafast pulses with a pulse width closer to the FTL pulse duration and cleaner pedestals in the case in which only the negative GDD can be provided in the chirped-mirror compressor.

The beam quality and beam profile at different stages are illustrated in Figure 5. The first-stage output beam quality is measured to be M² = 1.42 × 1.28 and that of the second-stage output is M² = 1.53 × 1.38. The beam profiles are measured at the near field, focal spot and after focus, respectively. The M² factor increases slightly with the nonlinear propagation in the system, while the spatial mode is significantly improved. The beam profiles are no longer elliptical and the sidelobes and spatial distortion have been almost eliminated. A similar spatial mode self-cleaning effect in periodically placed thin-solid plates was also reported in Ref. [28] and previously observed in other nonlinear processes^[31^–33^]. According to the investigation of Ref. [28], the spatial mode self-cleaning effect only occurs during stationary mode propagation in the periodically placed thin-solid plates and can be attributed to the spatial self-organization effect in this waveguide-like propagation.

To further investigate this nonlinear spatial mode self-cleaning effect, we measured the evolution of the beam profile at different locations and the final beam quality with the increase of power, as shown in Figure 6. The spatial mode self-cleaning effect can also be observed in this power-related evolution, which becomes more apparent as the power approaches the design conditions. The beam profile becomes increasingly circular and clean regardless whether at the near field, focal spot or after focus, which indicates an improvement of the spatial mode. Although the increase of the M² factor cannot be completely avoided due to the intensity-dependent spatial nonlinear effect, only a slight change would be introduced by controlling the accumulated B-integral per plate, whose impact can be ignored in applications^[15^]. Following the dual-stage post-compression, the final output average power is 96 W, demonstrating an exceptional efficiency of 94%. This high-efficiency spatial mode self-cleaning effect in the post-compression is quite suitable for the Innoslab laser system, which overcomes the limitations of narrow gain width and improves the spatial mode simultaneously. These results suggest that only this nonlinear effect used for the spatial filtering without additional spatial filtering modules such as a slit may be feasible, which would significantly enhance the overall efficiency and unleash the power-boost capability of this high-power laser system.

In summary, we demonstrate efficient all-solid-state post-compression based on dual-stage periodically placed thin-solid plates. It is driven by a compact and robust Yb:YAG Innoslab laser system, which is seeded by a fiber frontend and yields a 100 W level average power with mJ level pulse energy. This economical all-solid-state post-compression not only achieves a more than eight-fold pulse compression with exceptional efficiency of 94%, but also improves the spatial mode and pulse quality of this high-power laser system, which are helpful in many applications. Eventually, an efficient, robust and compact high-power ultrafast laser source is implemented with 64 fs pulse duration, 96 W average power at 175 kHz repetition rate and good spatial mode and pulse quality. This work proves the potential of periodically placed thin-solid-plate post-compression combined with the Innoslab laser seeded on a fiber frontend in enabling the generation of an efficient, robust and compact laser source with higher-power, few-cycle pulses, and good spatial and temporal quality.