1 Introduction

Femtosecond Ti:sapphire lasers have been developed with merits of high peak power and short pulse duration. In particular, 100-kHz-level GW Ti:sapphire lasers with energy above a few µJ have been utilized actively for scientific and industrial applications [1–4]. In high harmonic generation (HHG) producing coherent femtosecond and attosecond pulses in extreme ultraviolet and soft x-ray regions, a high-repetition rate high-power Ti:sapphire laser is vital for the investigation of photoelectron dynamics in atoms and molecules requiring high efficiency data acquisition [5, 6]. In addition, 100-kHz high-spatial-quality femtosecond lasers are advantageous for high-speed machining of µm-scale patterns in various materials [7]. Since experimental controllability is better with shorter laser pulses for such applications as HHG, there have been efforts to reduce laser pulse duration of high-repetition rate femtosecond lasers as much as possible.

So far there has been a limitation in generating sub-30-fs pulses from 100-kHz-level high-power Ti:sapphire lasers. Generally, in order to obtain shorter pulses, a spectral width has to be broadened while generating a flat spectral phase to reach Fourier-transform-limited pulse duration. Though the flattening of spectral phases could be made in 100-kHz-level Ti:sapphire lasers, the spectral width could not be made broad enough to support a pulse duration below 30 fs due to the severe gain narrowing effect in a high-gain amplifier [2–4]. As a consequence, some kind of a spectral shaping method needs to be adopted to broaden the spectral width.

Several spectral shaping methods, such as a thin etalon, a birefringent filter, and a programmable spectral shaper [8, 9], have been used to make the spectral width broad enough to support 20-fs-level duration. In particular, an intracavity programmable spectral control filter (PSCF) is one of the most powerful spectral shaping devices for the case of a regenerative amplifier. The PSCF with collinear interaction geometry and simple optical alignment can make the bandwidth as wide as possible while allowing the adjustment of the central wavelength. When the PSCF was applied to regenerative amplifiers with 10–100 Hz repetition rates, the spectral widths were limited only by finite bandwidths of other optical components, resulting in 20-fs-level pulse durations [10, 11]. We, thus, adopted a PSCF to the 100-kHz regenerative amplifier to obtain an amplified spectrum broad enough to support 20-fs pulses from a 100-kHz high-power Ti:sapphire laser.

In this paper, we describe the output performance of a high-power Ti:sapphire laser with a 100-kHz repetition rate. The bandwidth was fully broadened through the application of a PSCF to the 100-kHz regenerative amplifier, and the spectral phase was optimized with a grism compressor. For stable operation and high spatial quality of the regenerative amplifier, the suppression of thermal effects in the Ti:sapphire crystal and in the PSCF crystal was found to be critical. Lastly, the beam quality factor, M 2, was characterized as a spatial beam parameter for applications requiring high-spatial-quality lasers.

2 System setup

A 100-kHz Ti:sapphire regenerative amplification laser has been developed based on the chirped-pulse amplification technique. The laser consisted of a Ti:sapphire oscillator, a material stretcher, a regenerative amplifier, and a compressor, as shown in Fig. 1. The home-made Ti:sapphire oscillator, pumped by a frequency-doubled Nd:YVO4 laser (Verdi, Coherent), generated sub-15-fs laser pulses with 5-nJ energy at 75-MHz repetition rate. The femtosecond laser pulses were stretched temporarily after passing through a SF57 glass block. Here the glass-block stretcher has higher throughput with an 84 % efficiency, smaller size and easier alignment than a grating based stretcher. The stretched laser pulses were amplified in the regenerative amplifier. The amplified laser pulses were recompressed by the grism compressor consisting of prism and grating pairs.

Fig. 1
figure 1

Schematic of the 100-kHz high-power femtosecond Ti:sapphire laser. M1, M4, end mirror; M2, M3, curved mirror with 0.5-m radius curvature; TFP thin-film polarizer, PC BBO Pockels cell, VC vacuum chamber, QWP quarter wave plate, FR Faraday rotator, PBS polarizing beam splitter, PSCF programmable spectral control filter

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The regenerative amplifier was configured to produce broadband amplified pulses at 100-kHz repetition rate. The regenerative amplifier consisted of two curved mirrors with 500-mm radius curvature, a 6-mm-long Brewster-cut Ti:sapphire crystal with an absorption coefficient of 4.78 cm−1, a thin-film polarizer, a quarter waveplate, a BBO Pockels cell with a 3.5-mm-diameter clear aperture, and a PSCF (Mazzler, Fastlite), as shown in Fig. 1. A seed beam was focused onto the Ti:sapphire crystal by the first curved mirror (M2) and collimated by the second curved mirror (M3) with a beam size of about 2 mm (1/e 2 diameter) on the end mirror (M1 and M2), while a pump beam was focused onto the Ti:sapphire crystal with a beam size of 175 µm (1/e 2 diameter) after passing through the curved mirror (M2). An astigmatism induced by an oblique angle of incidence on the Ti:sapphire crystal and two Brewster windows attached on the vacuum chamber (VC) was compensated for by an off-axis angle of incidence on the two curved mirrors (M2 and M3). Here, the incidence angle on the curved mirrors was 5.2°. Using the thin-film polarizer, quarter waveplate and BBO Pockels cell, laser pulses were injected into the amplifier at 100 kHz and extracted from the amplifier after reaching the maximum achievable energy.

In regenerative amplifiers with 100-kHz-level repetition rates, strong thermal loads are commonly induced by high-average-power pump lasers focused tightly, resulting in distortion of spatial beam profiles [3]. In our 100-kHz regenerative amplifier, thermal loads were induced in two optical components, a Ti:sapphire crystal and a TeO2 crystal of the PSCF. When a 37-W green pump laser is focused to the Ti:sapphire crystal, the induced strong thermal lensing distorts the spatial beam profile and so optical damage can be induced in the crystal and other optical components. To suppress the thermal lensing effect, the Ti:sapphire crystal was cooled down to 70 K in a vacuum chamber with a cryogenic cooling system (RS80 Refrigerator, Ulvac Cryogenics), resulting in a thermal focal length longer than 1.5 m. The 25-mm-long TeO2 crystal of the PSCF was heated up when a RF power around 2.5 W was launched into it. Its thermal load induced small thermal lensing and thermal beam deviation, resulting in unstable amplifier operation. To alleviate the thermal load, top and bottom surfaces of the TeO2 crystal were cooled with water and thus the calculated thermal focal length of the crystal became longer than 5 m. As a result, the thermal loads in the regenerative amplifier could be managed so well that the high-average-power pump laser and the high-power RF signal of the PSCF did not disturb stable amplifier operation and high-spatial-quality beam generation.

The amplified pulses were recompressed after double-passing through the grism compressor (Grism, Fastlite) consisting of two anti-parallel SF57 prisms, two parallel transmission gratings and a retro-reflecting roof mirror [12, 13]. The input and output faces of prisms were anti-reflection coated. To compensate for dispersions of all transparent materials in the stretcher and in the amplifier, an incidence angle on the first grating, the separation distances of the prism pair and of the grating pair, and the prism insertion were adjusted precisely. After the grism compressor with a throughput efficiency of 85 %, amplified pulses were recompressed to reach near Fourier-transform-limited pulse duration.

3 Spectral control and laser characterization

The spectral control was managed using the PSCF to maximize the output spectral width in the regenerative amplifier. The spectral width typically becomes narrowed due to the gain narrowing effect during high-gain amplification. To compensate for the gain narrowing, the high-gain part of the laser spectrum around 800 nm was suppressed using the PSCF, as shown in Fig. 2. The dotted line in Fig. 3 shows that the amplified laser pulse without the PSCF has a narrow bandwidth of 24 nm at full width at half maximum (FWHM). When the TeO2 crystal of the PSCF was not cooled, the RF power level was limited because the amplifier operation became unstable at the high RF power required for sufficient spectral shaping. As a consequence, only 15 % of the full RF power could be launched into the TeO2 crystal in a stable amplifier operation state and the gain narrowing was only partially compensated for, as shown in the dashed line of Fig. 3. On the other hand, when the TeO2 crystal was cooled by water, the spectrum was fully broadened with 88 % of the full RF power. Here, the maximum spectral width was limited just by the bandwidths of intracavity components such as the BBO Pockels cell, the thin-film polarizer and the reflection mirrors. As a result, the bandwidth was extended greatly from 24 to 78 nm (FWHM) without instability in amplifier operation, as shown in the solid line of Fig. 3.

Fig. 2
figure 2

Input spectrum of the regenerative amplifier (solid line) and single-pass transmission of the PSCF (dotted line)

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Fig. 3
figure 3

Spectrum of the amplified laser pulse before applying the PSCF (dotted line) and spectra after applying the PSCF without (dashed line) and with (solid line) water cooling

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In the regenerative amplifier, the amplified laser output power was optimized by controlling the roundtrip number of a laser pulse passing through the Ti:sapphire crystal. The regenerative amplifier was pumped with 37-W green laser pulses from a 100-kHz Q-switched frequency-doubled Yb:YAG laser (Cyber-Laser) [14]. When no RF power was launched into the PSCF, the maximum output power of the amplifier was 4.8 W with an optimized roundtrip of 16. Here, the maximum output power was limited mainly by the thin-film polarizer with a low reflectivity of 75 %. On the other hand, when a high RF power was launched into the PSCF for the widest bandwidth, the maximum output power decreased to 3.5 W, while the roundtrip number increased to 22. The single-pass transmittance of the PSCF with a high RF power was 95 %, while that of the PSCF with no RF power was 99 %. This implies that the energy loss at the PSCF could be partially compensated for by increasing the roundtrip number.

The laser pulse duration was optimized by minimizing a spectral phase error through the careful adjustment of the grism compressor. The temporal profile was measured with spectral phase interferometry for direct electric-field reconstruction. The spectral dispersions of all transparent materials including the PSCF for one round-trip in the amplifier, calculated up to the fourth-order term at 800 nm, corresponded to a group delay dispersion (GDD) of 3.3 × 104 fs2, a third-order dispersion (TOD) of 2.2 × 104 fs3 and a fourth-order dispersion (FOD) of 1.1 × 104 fs4. Before applying the PSCF to the amplifier, the final pulse duration was as long as 39 fs (FWHM). After applying the PSCF, the pulse duration was shortened to 22 fs (FWHM), which is close to Fourier-transform-limited pulse duration, as shown in Fig. 4. Here the total GDD, TOD and FOD values compensated for by the grism were 1.1 × 106 fs2, 7.0 × 105 fs3 and 3.0 × 105 fs4, respectively. Consequently, although the compressor output power decreased from 4.1 to 3.0 W after applying the PSCF, the final laser peak power increased from 1.0 to 1.4 GW.

Fig. 4
figure 4

Measured temporal profiles of the compressor output pulses before (dashed line) and after (solid line) applying the PSCF, and a calculated temporal profile of the Fourier-transform-limited pulse (dotted line)

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The beam quality factor, M 2, of the final laser pulse was obtained by measuring spatial profiles at different positions near the focal plane with a CCD camera. The beam quality factors were 1.17 and 1.07 in the horizontal and the vertical directions, respectively, as shown in Fig. 5. The beam quality factors less than 1.2 and the focal spot image in Fig. 5 show that the thermal loads in the regenerative amplifier were alleviated sufficiently by cooling the Ti:sapphire crystal and the TeO2 crystal. This 100-kHz femtosecond laser with a high spatial quality and a sub-30-fs duration can be well utilized for scientific and industrial applications.

Fig. 5
figure 5

M 2 factor and a focal spot image of the 100-kHz high-power femtosecond Ti:sapphire laser

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4 Conclusion

The 100-kHz 22-fs 1.4-GW Ti:sapphire laser with a high spatial quality has been developed. The short pulse duration of 22 fs was achieved by broadening the spectral width to 78 nm using the PSCF and compensating for the spectral phase error with the grism compressor. The PSCF has been applied successfully for the first time at a 100-kHz repetition rate. The beam quality factor less than 1.2 was achieved by cooling the Ti:sapphire crystal and the PSCF crystal efficiently in the regenerative amplifier. This 100-kHz high-power femtosecond Ti:sapphire laser can be applied valuably to scientific and industrial fields requiring very high-repetition rate intense laser pulses, thanks to its high temporal and spatial qualities.