We report on substantial pulse energy increase in Yb:KYW femtosecond laser oscillators by utilizing multiple laser crystals for an enhanced net-gain at higher pump power. The two-crystal oscillator generates pulse energies of 7 µJ at 1 MHz repetition rate which is, to our knowledge the highest energy ever reported from an Yb-doped tungstate fs-laser oscillator. The external pulse compression yields a pulse duration of 416 fs with a peak power of 12 MW being enough for stable white light generation in YAG.
©2010 Optical Society of America
Compact and stable femtosecond laser oscillators with high pulse energies and MHz-repetition rates are essential laser sources for a wide variety of applications in science and industry, ranging from fields such as micro-machining over ophthalmologic surgery to quantum metrology [1–3].
In recent years a lot of effort has been addressed to energy and power scalability of laser oscillators in order to substitute complex and high-cost amplifier systems with low repetition rates. As an example, extraordinary high average output power has been obtained from Yb:Lu2O3 mode-locked thin disk lasers with up to 140 W . Beside these results pulse energies exceeding 25 µJ could be achieved using an Yb:YAG thin-disk oscillator with an active multi-pass cell . The shortest pulse duration of these systems is in the range of 800 fs. However, just the combination of energies beyond 2 µJ and pulse durations considerably shorter than 800 fs would be beneficial for various applications. The cavity-dumping technique is a promising method to obtain such pulse specifications. In previous research activities it has been shown that solitary thin disk oscillators are not well suited for pulse energy scaling with cavity-dumping, owing to the low dumping efficiencies that originate from the low small signal gain [6,7]. Moreover, with the high peak power from solitary mode-locking both the electro-optical modulator and the air inside the resonator give rise to a high B-integral that makes dispersion management particular demanding [5,6,8].
On the other hand, it could be demonstrated that bulk crystals with their higher small signal gain are ideally suited for efficient cavity-dumping with high output coupling ratios [9,10]. In order to avoid high peak power within the resonator and to provide broad band femtosecond pulse spectra at the same time it is of advantage to operate the laser in the positive net-dispersion regime with chirped-pulses. However, pulses need to be externally compressed to reach the femtosecond durations [9,11,12].
The challenging aspect of both power and energy scaling in bulk crystals is to overcome the limitation of applicable pump power which can initiate strong thermal lensing and crystal damage. An ideal approach is to split up the higher pump power among multiple crystals as previously reported in [13,14] which in turn delivers a higher net gain that is especially beneficial for an increase of output coupling ratio and dumping depth, respectively.
Beside the fact that a wide selection of Yb-doped hosts has been investigated [15,16] towards directly-diode pumped short pulse generation, the use of Yb:KYW as active medium still holds the advantage of commercial availability in reasonable good and reproducible quality. General aspects of choosing Yb:KYW imply the low quantum defect, the comparable broad emission spectrum and the high absorption- and emission cross sections.
In addition, the crystal exhibits different spectral distributions of the emission cross-section in respect to its three optical axes. This property reveals the opportunity for a simultaneous mode-locking of spectra with different maxima (i.e. Np: 1040 nm and Nm: 1030 nm ) in order to broaden the complete pulse spectrum and to generate even shorter pulses accordingly. This can be accomplished by a consecutive propagation through media with different spectra. In the case of a regenerative thin-disk amplifier this has already been demonstrated  whereas it is not explored for oscillators, to our knowledge. Beside pump power and energy scalability oscillators utilizing multiple gain crystals are perfectly qualified for a combination of differing spectra since not only one type of medium but several different types of laser crystals can be applied at the same time.
In the following article we present a chirped-pulse femtosecond oscillator system with cavity-dumping utilizing two gain crystals with significantly enhanced pulse energies; furthermore, we report on first results of mode-locking under combined Yb:KYW spectra.
3. Experimental set-up
Figure 1 shows a schematic of the resonator. Two Yb:KYW crystals are used as active media. The 5 at % doped crystals are wedged and AR-coated for both laser and pump wavelength. The crystals are cut parallel to the Nm- and Np-optical axis while laser propagation occurs parallel to the Ng-optical axis (according crystal size: 4 x 1.5 x 2 mm3).
Each crystal is pumped by a 30 W fiber coupled laser diode (Jenoptik, core: 200 µm, NA: 0.22, λpump: 980 nm). The pump system consists of a collimation (ƒ1 = 25 mm) and a focusing lens (ƒ2 = 30 mm) and gives rise to a mode radius of about 160 µm in the crystals. The actual laser mode radius at the same position is in the range of 130 µm. With respect to the integrated Herriott-type cell which was employed to make the set-up more compact and stable the total resonator length equals 8.2 m yielding a repetition rate of 18.2 MHz. An actively cooled SESAM with a modulation depth of ~2% induces passive mode-locking. The combination of a β-barium-borate (BBO) based Pockels-cell and a thin film polarizer (TFP) enables cavity-dumping at 1 MHz repetition rate. Therefore each 17th pulse is coupled out by the Pockels-cell / TFP. The dumping efficiency and thus the extracted pulse energy are directly controlled by the high-voltage applied to the Pockels-cell crystals.
In order to optimize the intra cavity group delay dispersion (GDD), standard resonator mirrors were replaced by negative dispersive Gires-Tournois-interferometer mirrors (GTI).
The entire net-material dispersion of one roundtrip without GTI-mirrors was calculated to be around + 4700 fs2.
The oscillator was embedded in a sealed box to reduce external disturbances and thus to reduce the noise. The entire set-up features a compact footprint with a dimension of 1 x 0.5 x 0.3 m3.
The external compressor consists of one dielectric transmission grating (1250 lines/mm), a delay line as well as a retro-reflector. The grating is operated under Littrow-angle (θLitt: 40.5°) and s-polarization. Separation of the incoming beam from the output is accomplished by a slight vertical beam displacement. Before the beam entered the compressor a collimating telescope altered the beam diameter to approximately 1 mm (not shown in Fig. 1).
4. Results and discussion
Without cavity-dumping and with laser polarization parallel to both Np-axes stable mode-locking occurred within a net GDD range between + 1500 fs2 and + 4700 fs2. Larger GDD values have not been investigated due to the lack of positive dispersive mirrors. Tuning the GDD to a value of + 3500 fs2 offered the widest range for stable mode-locking in terms of pulse energy. From an internal average power of 27 W to 49 W (equivalent average energy: 1.5 µJ to 2.7 µJ) Q-switching occurred. Exceeding this threshold led to stable single pulse mode-locking. Beyond 10.5 µJ double pulsing could be observed.
With cavity-dumping mode-locking also became unstable when internal pulse energies reached either the upper or the lower limit. The maximum dumping depth of more than 67% was reached at a GDD of + 3500 fs2 (limited only by electronics). At this point the laser generated 7 µJ of pulse energy and an average output power of 7 W at 20 W of absorbed pump power. At higher pump power double pulsing was initiated. The maximum dumping depth for less GDD was substantially lower (i.e. GDD: + 2700 fs2, max. dumping depth: 54%). Increasing the maximum pump power for single pulsing by means of a higher net GDD (> + 3500 fs2) also narrowed the spectral width. In contrast the pulse energy did not increase to the same extend which made the stronger chirp less desirable with regard to the achievable peak power after compression. Figure 2 displays the according power spectrum revealing a Fourier-limit of 264 fs assuming a flat zero phase. The calculated pulse shape was close to a Gaussian shape. The measured autocorrelation (AC, Fig. 2b) of the uncompressed pulses confirms the Gaussian shape yielding a pulse duration of τP = 13.8 ps.
With the compressor set-up from Fig. 1 pulses were externally compressed. The compressor losses are lower than 13% providing an output pulse energy of 6.1 µJ. The measured autocorrelation is depicted in Fig. 3 . At the best obtainable compressor alignment we measured a FWHM-value of τA = 604 fs indicating a pulse duration of τP = 416 fs (shape factor: 0.69) and a peak power of 12 MW when taking the shape into account. The AC-trace clearly exhibits pre- and post-wings which we attribute to residual higher order dispersion that prevents compression down to the Fourier-limit of 264 fs.
By adding second, third and fourth order dispersion to the calculated autocorrelation (at Fourier-limit) we could reproduce the measured AC-trace. However the residual TOD contribution was so high that it could not originate from the grating (TODgrating ≈ + 0.007 ps3). By means of a GDD-measurement we found that the high third order value mainly derives from one of the internal GTI-mirrors where the TOD is accumulated (entire Resonator TOD ≈ + 0.017 ps3) over the number of round-trips. Replacing this GTI by one with a flat-GDD at the given bandwidth would allow for significantly shorter pulse durations. With the given Fourier-limit and similar compressor losses a total peak power of more than 18 MW should be feasible in the future.
The oscillator shows good long term stability with hardly any necessary manual adjustment within period of weeks. The observed rms-noise is below 1% and beam quality is excellent with a measured M2-value of 1.1.
As a direct proof of peak power we investigated white light generation. In previous experiments we could generate white light in crystals like sapphire or YAG only by means of fiber based spectral broadening or further amplification techniques [19,20]. Here we verify white light generation directly from the oscillator in a 2 mm long YAG-plate without any further components. The threshold energy for the white light process was around 2.3 µJ. Two lenses, each with a focal length of 30 mm were used to widen and focus the beam into the crystal. The most stable and broad spectrum did not occur directly in the focus. The beam diameter could not be measured with high precision at that position but it is in the range of 10 to 40 µm. The white light spectrum is shown in Fig. 4 . It spreads from 450 nm to 750 nm.
In line with a first approach we investigated mode-locked operation with combined Yb:KYW spectra. The aim was to generate broader pulse spectra and consequently both shorter Fourier-limits and pulse durations. Therefore simultaneous operation under the Nm- and the Np-optical axis was accomplished by using a λ/2-retardation plate (not shown in Fig. 1) to alter the polarization between both laser crystals.
In addition the power of each pump diode was adjusted with respect to the different emission cross-sections in both crystal orientations. At net roundtrip GDD-values between + 1000 fs2 and + 3500 fs2 we could observe the most stable mode-locking behavior. Despite combined spectra we observed a center wavelength of the power spectrum at 1048 nm. Moreover the bandwidth of the spectrum was rather poor with a FWHM-bandwidth below 3 nm. Thus the Nm-spectrum (crystal I) amplified the 1048 nm of the Np-axis instead of delivering spectral components at its own maximum around 1025 nm. Apart from this, Q-switching instabilities destabilized the operation and the average output power was lower than it was when operating both crystals under the same polarization.
We address the spectral effects to the strong reabsorption of 1025 nm from the Np-axis. However we could not increase the Nm-pumping to overcome the losses since Q-switching at high average power (>150 W) would have destroyed the laser crystals. In order to understand the oscillators’ dynamics in more detail and to overcome the difficulties a deeper experimental and numerical study of the underlying processes will be carried out in the future.
5. Conclusion and outlook
In summary we report on a passively mode-locked chirped-pulse Yb:KYW laser oscillator with cavity-dumping that generates 7 µJ of pulse energy at a 1 MHz repetition rate. In comparison to previous systems two laser crystals were utilized to enhance the total pump and average output power at the same thermal load. To the best of our knowledge the oscillators’ energy is the highest ever reported for any fs-Yb-tungstate laser oscillator as well as for any fs-oscillator with cavity-dumping (more than a factor of 2 ,). With an external compressor pulses could be compressed from almost 14 ps down to 416 fs. Taking the compressor losses and the temporal shape into account a total peak power of more than 12 MW was reached which was enough for stable white light generation in YAG. Emission cross-section combing has been realized to generate broader spectra but the unstable operation and the low bandwidth demand further investigations and parameter optimization.
The concept of using multiple laser crystals is promising concerning further pulse energy scaling. Since no limitations (i.e. thermal load, SESAM, pump power etc.) could be figured out, a higher number of crystals will soon be studied. With better internal GTI-mirrors pulse durations below 300 fs are in reach with peak powers in excess of 18 MW. In the near future, the oscillator will be used without the compressor as a seed source for a single stage fiber CPA system to increase the pulse energies up to a factor of 10.
The authors thank Martin Siegel from High-Q-Laser and Bergmann Messgeräte Entwicklung KG for support, discussions and cooperation. This work was partly funded by the German Federal Ministry for Education and Research (BMBF) under support codes 13N8723, 01QE0904C and the Eurostars-project “E! 4589 NEOWARP”, respectively.
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