Ultrafast Yb:CALGO laser oscillator based on cross-polarization pumping with a cost-efficient multi-mode diode

. We demonstrate a multimode diode-pumped Yb:CALGO laser oscillator based on bandwidth-optimized cross-polarization pumping targeting sub-30-fs operation. In our first proof of principle experiment we achieved mode-locked operation at 83 MHz repetition rate with 0.4 W of average power and a 33-nm-bandwidth optical spectrum supporting sub-40-fs pulses. This concept offers a simple and cost-efficient alternative to green-pumped Ti:sapphire lasers.


Introduction
Numerous applications rely on transform-limited fewcycle laser pulses and are typically based on greenpumped Ti:sapphire laser oscillators [1,2].Yb-doped bulk laser oscillators are a promising alternative, thanks to their excellent thermal and spectral properties [3], combined with the availability of cost-efficient multi-watt pump diodes.Among various Yb-doped gain materials, Yb:CALGO is an excellent candidate for generating ultrashort pulses at high average power directly from a laser oscillator.More than 10 W of average power in sub-100-fs pulses have been achieved [4,5].However, efficient operation in the sub-30-fs regime remains challenging.The low quantum defect of the Yb-doped gain media restricts the spectral bandwidth toward shorter wavelengths.Moreover, bulk oscillators are typically collinearly pumped using a dichroic mirror as depicted in Fig. 1a).Here, the transition of the reflectivity leads to a strongly varying group delay dispersion (GDD) as shown in Fig. 1c) by the red dashed line.This limits the expansion of the optical spectrum towards shorter wavelengths and impedes the generation of shorter pulses.
In 2022, we have shown that this limitation can be overcome by using a cross-polarized pumping scheme [6] as depicted in Fig. 1b).For orthogonal polarizations of the pump and the laser under a large angle of incidence, the pump transmitting mirror can be designed with much broader reflectivity and flatter dispersion for the laser wavelength as shown by the blue dashed curve in Fig. 1c).This element enabled an unprecedented performance of our Yb:CALGO laser oscillator delivering 0.7 W of average power at 22 fs pulse duration and 25% of opticalto-optical efficiency.Nevertheless, the result was achieved using a diffraction-limited 976-nm fiber laser for pumping, which strongly increased the price and the size of the setup.
In this contribution, we apply the cross-polarization pumping scheme to a multimode diode-pumped oscillator, providing a much more compact and costefficient solution.In our initial proof-of-principle experiments with a not yet fully optimized setup, we achieve 0.4 W of average power with a spectrum supporting sub-40-fs pulses.These initial results confirm that cross-polarization pumping is also suitable for multimode pumping, in which the pump propagation in the gain medium is subject to additional constraints.We are currently optimizing the components and we expect pulse durations in the sub-30-fs regime at several Watt of average power at the time of the conference.Such oscillators will be very attractive as simpler and cheaper alternative for many applications currently relying on Ti:sapphire lasers.A schematic of the hard-aperture Kerr-lens mode-locked Yb:CALGO bulk laser oscillator is depicted in Fig. 2. The 4-mm-long gain crystal is mounted in a water-cooled copper mount and is end-pumped using a fiber-coupled multimode 20-W laser diode (VBG stabilized at 980 nm, 105-µm-core diameter, 0.15 NA).The pump beam is refocused into the crystal by a pair of 60-mm focal length lenses.Only the p-polarized component is selected (higher absorption in the crystal and transmitted by the cross-polarized pump mirror) by a polarized beam splitter.This results in maximum of 12.8 W available for pumping the crystal.Compared to our previous result using a single-mode pump, the 60° angle of incidence on the dichroic mirror introduces astigmatism for the multimode focused pump beam.Thus, an antireflection-coated compensation plate is placed in front of the laser diode in the orthogonal plane.With the compensation plate, the pump features a circular spot of 60-µm radius inside the crystal.The cavity dispersion is set by three dispersive mirrors over-compensating the dispersion of the gain material, leading to a total GDD per cavity roundtrip of -200 fs 2 .For Kerr lens mode-locking, a hard aperture is placed directly in front of the output coupler introducing higher losses for the continuous wave beam than for the mode-locked beam.Mode-locking is initiated by shaking the output coupler, which is mounted on a linear translation stage.
The mode-locked parameters are shown in Fig. 3.The laser operated at 83 MHz with a 33-nm full width at half maximum spectral bandwidth, supporting sub-40-fs transform-limited pulses.The small peak on the right side of the optical spectrum is a Kelly side band commonly present in these oscillators.The autocorrelation trace shows a longer 57-fs pulse duration indicating an uncompensated chirp of the pulse.The radio frequency spectrum shows no side peaks down to -60 dB indicating clean mode-locking.The measurement was performed very recently, and due to the lack of time, no compensation for dispersion after the output coupler was implemented.At the conference, we will present a detailed analysis of our system and the steps towards full optimization.

Conclusion
We have demonstrated the combination of the crosspolarized pumping scheme for ultrafast Yb-based bulk oscillators with cost-effective multimode diode pumping.Our first results using a Yb:CALGO show operation with a 33-nm bandwidth optical spectrum supporting sub-40-fs transform-limited pulses at 0.4 W of average power and 80 MHz repetition rate.We expect that further optimization will lead to operation in the sub-30-fs regime at several-watt-level power.This will offer a simple and cost-efficient alternative to Ti:sapphire lasers.

Fig. 1
Fig. 1 a) Standard collinear pumping scheme.b) Crosspolarization pumping scheme with a 60° pump mirror.c) Typical reflectivity (solid line) and GDD (dashed line) of a standard dichroic (red) in comparison to cross-polarized dichroic mirror (blue), pump wavelength at 976 nm (purple line).

Fig. 3 .
Fig. 3. Experimental results: a) Intensity autocorrelation with a sech 2 -fit for soliton pulses.b) Optical spectrum of the laser output.c) Radio-frequency spectrum of the fundamental repetition rate of 83 MHz measured with a resolution bandwidth (RBW) of 10 kHz.