An Yb:KYW femtosecond regenerative amplifier with combination of two gain spectra from a single thin disk is presented. The amplifier generated an average power of 10 W after compression at a repetition rate of 20 kHz resulting in a pulse energy of 500 µJ. A bandwidth of 18.6 nm at −20 dB allowed a compression to nearly Fourier-limited pulses with a duration of 185 fs resulting in a peak power of 2.7 GW.
©2009 Optical Society of America
Ultrafast, high-energy laser systems based on regenerative amplification are of great importance in a variety of applications such as high precision micro and nano material processing, microsurgery, photo-polymerization, high energy physics, OPA pumping, time-resolved spectroscopy and pump/probe analysis. Prominent representatives of these systems are based on Ti:Sapphire. They are commercially available with repetition rates up to 10 kHz, pulse energies from µJ to mJ with pulse durations of some 10 fs to 200 fs. However, drawbacks are their cost intensive and bulky structure in combination with high maintenance and operation costs. Promising alternatives are based on Ytterbium doped tungstates. Ultra-short pulse laser oscillators with pulse durations of 101 fs and 71 fs [1,2] have been demonstrated. A strong absorption at 981 nm, a very low quantum defect  and a moderate thermal conductivity makes them to the most interesting materials for diode-pumped laser systems.
Unfortunately these up-to-date laser systems suffer from the accessible gain-bandwidth. In combination with the gain-narrowing effect  the typical high-energy pulse duration is above 300 fs. To overcome this limited pulse duration several spectral broadening techniques like spatial dispersive amplification [5,6], regeneratively spectral shaping using intracavity elements , nonlinear pulse compression [8,9] and seed-pulse shaping in phase and amplitude [10,11] can be used. Optical spectra exceeding the gain bandwidth can only be achieved with residual distortion of the spectral shape and the phase characteristics.
A promising method to reduce gain narrowing and to enhance the effective gain bandwidth is to combine gain media with separated gain maxima and overlapping broadband gain, which has been realized in oscillators .
This approach can become even simpler if the spectra to be combined originate from a single birefringent gain medium, where the optical axes are perpendicular to each other. Each optical axis can successively amplify the laser pulse just by alternate addressing the axes by a rotation of the linear polarization. The reduction of gain narrowing is optimal, if the combined gain has separated maxima of equal amplitude. A proper material for this purpose is Ytterbium doped potassium yttrium monoclinic double tungstate oxide, Yb:KY(WO4)2 or short Yb:KYW. Due to the different maximum emission cross sections of the optical axes Nm and Np , the maxima of the combined gain have to be equalized by using an appropriate ratio between the passes amplified by the optical axes Nm and Np. We already demonstrated this concept at lower pulse energies and compared the performance with a similar setup with single Nm-gain [14,15]. In this paper we demonstrate a high energy ultrafast Yb:KYW regenerative amplifier with combined gain spectra from a single thin disk. This is to the best of our knowledge the first setup of this type, generating pulses with a duration of sub 200 fs with an energy of 500 µJ.
The setup was realized as a chirped-pulse amplification system (CPA)  consisting of an Ytterbium doped fiber oscillator with a fiber pre-amplifier, Faraday isolators, the regenerative amplifier cavity and a transmission compressor.
The output pulses of our ultra-short pulse fiber oscillator had a pulse duration of 4 ps. We used a 35 m fiber stretcher to increase the pulse duration to 31 ps. With a fiber based preamplifier the pulse energy was boosted to 9 nJ. The resulting seed pulses had a central wavelength of 1034 nm, a spectral full width at half maximum (FWHM) of 14.4 nm and an autocorrelation function with a FWHM of 206 fs. This corresponds to a Fourier-limited pulse duration of 152 fs assuming a deconvolution factor of 1.36 which was calculated by using the ratio of the FWHM of the Fourier-limited autocorrelation and the FWHM of the Fourier transform of the power spectrum assuming a zero phase. Within a measurement uncertainty of 4% we were able to compress these pulses to the Fourier-limit. A good overlap of the seed beam and the cavity mode of the regenerative amplifier were ensured by mode matching resulting in a high seeding efficiency. No pulse picker was used to reduce the pulse background. Because of an expected overall amplification of > 70 dB we protected the seed oscillator against feedback by the use of three Faraday isolators resulting in a total isolation of > 90 dB. The Faraday isolator nearest to the regenerative amplifier was used to separate the amplified pulses from the seed pulses.
We used the thin disk concept [17,18] and designed a regenerative amplifier cavity for TEM00 operation and 12 passes per roundtrip through an Yb:KYW thin disk (Fig. 1 ).A cavity length of about 3 m corresponded to a roundtrip time of 20 ns. A Pockels cell in the amplifier consists of two β-barium borate crystals (BBO) with an aperture of 6 mm and a length of 2 x 20 mm. A relative low quarter-wave voltage of 3.8 kV allows repetition rates of > 200 kHz. Polarization switching was initially established with this Pockels cell in combination with a thin film polarizer (TFP) and an optically contacted zero order quarter-wave plate. Due to strong etalon effects at high pulse energies we replaced the wave plate by a static quarter-wave phase shift realized by proper alignment of the Pockels cell. To avoid etalon effects from the humidity protecting windows, we replaced them by two wedged quartz windows with a center thickness of 1 mm.
An Ytterbium concentration of 10 at% in KY(WO4)2 was used for our thin disk with Ng-cut orientation. The crystal had a diameter of 7 mm with a center thickness of 102 µm and a wedge of 0.33 degree. The thin disk was pumped at a wavelength of 981 nm with a fiber coupled laser diode emitting a maximum output power of 78 W. The pump light was guided 24 times through the thin disk resulting in an absorption efficiency of > 98% and an integrated pump intensity of 19 kW/cm2.
The polarization direction of the first four passes through the thin disk was parallel to the Np-axis. A low order half-wave plate (176 µm quartz) rotates the polarization by 90 degrees, in order to be parallel to the Nm-axis when the thin disk was passed again in the direction of the end mirror. Astigmatism of the cavity resulting from the relative low astigmatic convex curvature of 7 m by 24 m of the thin disk was compensated by different folding mirrors.
The dispersion per roundtrip caused by the thin disk and the Pockels cell was calculated to be 2921 fs2 for second order dispersion (SOD) and 5061 fs3 for third order dispersion (TOD) resulting in a TOD/SOD ratio of 1.733 fs. Dechirping of the pulses was performed by a compressor arrangement consisting of a transmission grating combined with a prism (GRISM ). This configuration has the advantage of a constant TOD/SOD ratio over a wide range of dispersion compensation.
3. Experimental Results
In order to investigate the spectral characteristics of our regenerative amplifier we applied different pump powers from 35 W to 75 W and adapted the number of roundtrips to achieve constant output pulse energy of 200 µJ at a repetition rate of 10 kHz. The obtained output spectra are shown in Fig. 2 .At a relative low pump power of 35 W the amplification was dominated at the longer wavelength part (corresponding to the Np-gain) because of the higher reabsorption at shorter wavelengths. At medium pump powers between 49 W and 63 W the optical spectrum was nearly symmetrical and at a pump power of 75 W the Nm-gain dominated with a center wavelength at 1030 nm.
The output pulse energy of our regenerative amplifier could easily be adjusted by changing the pump power and/or the number of roundtrips and was limited by the onset of bifurcation . Figure 3 (a) shows the pulse energy after compression as a function of the number of roundtrips at a pump power of 75 W and a repetition rate of 20 kHz. A pulse energy of 500 µJ was achieved at a roundtrip number of 53. Further increase of the number of roundtrips resulted in the onset of bifurcation.The optical spectrum at a pulse energy of 500 µJ is shown in Fig. 3 (b), having a peak wavelength at 1028 nm and a bandwidth of 4.5 nm at −3 dB and 18.6 nm at −20 dB. The spectral modulation was a consequence of an etalon occurring between the planar surfaces of the intracavity half-wave plate. The modulation frequency was 0.58 THz resulting in a satellite pulse at 1.8 ps as confirmed in the autocorrelation trace, shown in Fig. 4 (a) . The satellite pulse contains less than 3% of the total energy. The calculated Fourier-limited pulse duration was 185 fs at FWHM corresponding to an autocorrelation function with a FWHM of 268 fs. Within a measurement uncertainty of 4% the measured intensity autocorrelation indicates a pulse compression to the calculated Fourier-limit. The residual wings with about 10% of the total energy are due to incompletely compensated higher order dispersion. Figure 4 (b) shows the calculated Fourier-limited pulse duration after compression as a function of the number of roundtrips at a pump power of 75 W and a repetition rate of 20 kHz. For different pulse energies between 100 µJ and 500 µJ the corresponding pulse duration varies only by 5%. This indicates an efficient reduction of gain narrowing for this setup.
In this paper we presented an ultrafast Yb:KYW regenerative amplifier with combined gain spectra of the optical axes Nm and Np from a single thin disk. This was to the best of our knowledge the first setup of this type, operating at an average power of 10 W with pulses with a duration of sub 200 fs. This regenerative amplifier provided pulses with an energy of 500 µJ after compression at a repetition rate of 20 kHz and a pump power of 75 W. By using a transmission GRISM compressor nearly Fourier-limited pulses with a duration of 185 fs could be achieved.
We are grateful to Guido Palmer from the Institut für Quantenoptik at the Leibniz Universität Hannover for helpful discussions. This work was partly funded by the German Federal Ministry of Education and Research under contract no. 13N8722.
References and links
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