We present a femtosecond regenerative Yb:YGAG (Y3Ga2Al3O12) ceramic slab amplifier delivering 405 fs pulses at a wavelength of 1030 nm with a bandwidth limit of 306 fs, 1.1 W of average power, 8 μJ of pulse energy, and a repetition rate of 100 kHz. The amplifier is seeded by 9 pJ pulses generated by a Yb-doped fiber ring oscillator with extra-cavity spectral shaping to minimize gain narrowing. The net-gain of the pulses is 60 dB, the spectral bandwidth is 4.1 nm (FWHM), and the M2 beam quality factor is < 1.2. Due to similar optical and thermo-mechanical properties to Yb:YAG, the Yb:YGAG gain medium is a promising alternative for upgrading the existing Yb:YAG picosecond disk amplifiers to the femtosecond regime.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Over the past decades, ytterbium-doped gain media has become one of the most promising materials for high power and ultrashort pulse lasers. Especially Yb:YAG material is very well established in picosecond laser systems with kW level output power and diffraction-limited beam quality due to its excellent optical and mechanical properties. However, the limiting factor for amplification of high energy sub-picosecond and femtosecond pulses in Yb:YAG power amplifiers is the spectral gain narrowing, i.e. spectral narrowing during amplification due to the bandwidth limited spectral gain profile. Majority of existing Yb:YAG systems cannot generate pulses shorter than approx. 0.9 ps [1–6]. One approach how to overcome the spectral gain narrowing is to introduce a nonlinear effect such as self-phase modulation (SPM) to increase the spectral bandwidth . The other, more straightforward approach is to use gain medium with broader spectral bandwidth. One of the best candidates to exchange the Yb:YAG is the Yb:YGAG (Y3Ga2Al3O12) material.
Yb:YGAG material belongs to the mixed garnet group, where a small amount of aluminium ions were substituted by gallium ions. These modifications of the material composition result in broadening of the absorption and emission spectra of the material . Spectroscopic and lasing properties of Yb-doped YGAG ceramics were investigated at room and cryogenic temperatures both for conventional pumping (940 nm) and zero-phonon line-pumping (969 nm) [9,10]. The emission bandwidth of Yb:YGAG is 7.14 nm FWHM at 100 K (5 times wider than Yb:YAG) and the gain cross-section profile is broader (9-nm FWHM) at 300 K and in the case of Yb:YAG 6-nm FWHM . Yb-doped YGAG material thus offer favourable conditions for femtosecond pulse generation at room and cryogenic temperatures. The main aim of the presented study is the investigation of a diode pumped regenerative amplifier based on Yb:YGAG ceramics in femtosecond pulse amplification.
2. Thermo-mechanical properties of Yb:YGAG
To fully exploit the potential of Yb:YGAG as a gain medium for high power amplifier, it is necessary to know thermo-mechanical properties, especially the thermal conductivity of the Yb-doped YGAG ceramics. The thermal conductivity of uncoated ceramic samples with 5 and 10 at. % doping concentration produced by Konoshima Chemical Co. Ltd. was measured using a TCi thermal conductivity analyzer. The TCi developed by C-Therm Technologies Ltd. is a device for measuring the thermal conductivity of a small sample, by using the Modified Transient Plane Source (MTPS) method . The results are listed in Table 1 together with other parameters measured on the same sample. The thermal conductivity of Yb:YGAG ceramic samples is about 15% lower compared to comparable Yb:YAG , however, it is about 30-50% higher than thermal conductivity of e.g. Yb:CaF2 with the same doping concentration level [14–16], hence still acceptable for high power amplification. In addition, mechanical properties of Yb:YGAG are better than properties of brittle CaF2 crystals.
3. Design of regenerative amplifier
To design a stable laser cavity, measured values from Table 1 were used as input parameters for finite-element method (FEM) simulation of ceramic slab using Comsol Multiphysics software. The heat sources and thermo-optical characteristics were calculated for different pump power considering measured 56% absorption of 969-nm pump light in 1.5 mm thick slab, flat-top pump profile, pump spot diameter 500 μm, 6% quantum defect, 18°C cooling water temperature, heat sink geometry identical to experimental setup, and thermal conductivity and density from Table 1. Other unknown mechanical and thermo-optical parameters for simulation were estimated from YAG material . From the temperature distribution and deformation of the slab, optical path difference (OPD) was calculated and focal length of the thermal lens was obtained. With respect to the calculated data, the cavity of regenerative amplifier was designed to compensate the thermal lens for maximum pump power of 30 W and thus realize stable single mode operation. The calculated focal length of the thermal lens was about 17 cm and this value was experimentally verified by the method of changing the cavity length with one mirror while monitoring the output power and the regions of stability. The maximum temperature measured by thermal imaging camera FLIR A65 in the center of the pump spot was about 66 °C which is in good agreement with our calculation (Fig. 1).
4. Experimental results
Experimental setup of the regenerative amplifier is shown in Fig. 2. The regenerative amplifier consists of a 10 at. % Yb-doped YGAG bulk ceramic with dimensions of 3.6 × 3.6 × 1.5 mm3 with AR coating for 940, 969 nm pump and 1030 nm laser wavelengths. Gain medium was wrapped with indium foil, mounted in copper water-cooled holder and pumped by a fiber-coupled 30W continuous-wave volume-Bragg-grating-stabilized (VBG) laser diode module with 969 nm central wavelength, fiber core diameter of 200 μm, and NA of 0.22. The pump fiber output was imaged by achromatic doublet lenses with magnification 1:2.5 on the slab surface to a pump spot with diameter of 500 μm.
The repetition rate of the regenerative amplifier was controlled by a Pockels cell with a 25 mm long β-BaB2O4 (BBO) crystal with 5x5 mm aperture. The repetition rate was limited to 100 kHz due to insufficient HV power supply driver but, in principle, up to 250 kHz operation is possible. The system was seeded by an all-normal-dispersion (ANDi) type Yb-doped fiber ring oscillator which provides 50 mW maximum output power at 63 MHz repetition rate, 9 pJ of pulse energy, 4 ps stretched pulses with spectral bandwidth (FWHM) of 20 nm. The seed laser beam was mode-matched to the 400 μm mode diameter in the cavity of the regenerative amplifier by means of a lens telescope. A train of incoming pulses from the oscillator enters the cavity of the regenerative amplifier with horizontal polarization after passing through an isolator consisted of a polarizing beam splitter, Faraday rotator and half-wave plate. Quarter-wave voltage of 5.2 kV was applied to the Pockels cell to pick up the single seed pulse and lock the pulse inside the cavity and thus to allow amplification in vertical polarization state.
Our experimental goal was to achieve μJ-level output pulse energy with a gain of at least 106, while preserving the output pulse spectrum as broad as possible. Therefore, we employed a spectral shaping technique based on combination of a birefringent filter and a polarizer. We placed a 1 mm thick quartz plate outside the cavity of the regenerative amplifier to modify the input seed spectrum and to compensate spectral gain narrowing. By rotating and tilting of the quartz plate we were able to shift and change the spectral modulation depth of the injected seed spectrum and broaden the output pulse spectrum of the regenerative amplifier accordingly.
As shown in Fig. 3, for the pulse regime of the regenerative amplifier with a gain of 60 dB in pulse energy, a gain-narrowed pulse spectrum bandwidth of 3 nm FWHM was obtained, whereas with inserted birefringent filter we broadened the output pulse spectrum to 4.1 nm FWHM with the same gain. In both cases the seed pulse energy was identical Eseed = 9 pJ. Unlike direct amplification in Yb:YAG gain medium, Yb:YGAG material provides pulses with almost twice as broad spectral bandwidth compared to Yb:YAG power amplifiers and so supports generation of femtosecond pulses, as obvious from the measured data.
Figure 4 shows the output power characteristics of the regenerative amplifier. An average output power of 1.3 W was achieved in CW regime for the maximum pump power of 30 W. In pulsed operation the maximal output power was 1.1 W at a repetition rate of 100 kHz. The total amplification time was set to 2.05 μs, which corresponds to 171 round-trip passes in the cavity. The optical-to-optical efficiency was only about 6% mainly due to unoptimized AR coating of BBO and HR coating of some mirrors. The B-integral of the system was kept below 1 and the pulse operation of the regenerative amplifier did not show any signs of drift or instability.
Figure 5 shows the M2 beam quality parameter which was measured according ISO 11146 standard with M2x = 1.21 and M2y = 1.16. Amplified output pulses with pulse energy of 10 μJ and spectral bandwidth FWHM of 4.1 nm were compressed in a Treacy-type compressor  by means of two reflective diffraction gratings (600 l/mm groove density) with 52 cm grating separation. Since we used simple diffraction gratings with aluminium reflective coating, the throughput efficiency was only 45%. The output pulse duration was measured with a pulseCheck autocorrelator from APE GmbH. Figure 6 shows second harmonic autocorrelation traces after compression with pulse duration of 405 fs and 505 fs (FWHM) assuming sech2 pulse shape for with and without birefringent filter, respectively. The corresponding Fourier-transform limit calculated from the measured optical spectra is 306 fs and 406 fs. The difference between the measured and FTL pulse duration is attributed to uncompensated high order dispersion of the seed pulse from ANDi oscillator.
In conclusion, we have demonstrated an Yb:YGAG regenerative amplifier operating with high gain of 106 in pulse energy and providing broader bandwidth than comparable Yb:YAG (4.1 nm pulse bandwidth from Yb:YGAG). Unlike direct amplification in Yb:YAG it supports generation of ultrashort pulses with duration of 405 fs. The measurements performed in this study clearly shows the potential of Yb:YGAG ceramic material as an excellent substitute for existing Yb:YAG high power laser systems in terms of generation of femtosecond pulses. The next step will be further broadening amplified spectral bandwidth by means of programmable optical filter which enable arbitrary user-generated modulation shapes of the seed spectrum. In addition, employing SPM during process of amplification within the regenerative amplifier, another broadening of pulse spectral bandwidth is anticipated. Furthermore, for generation of high output power with diffraction limited-beam, an Yb:YGAG thin-disk regenerative amplifier will be built. If we take into consideration lower thermal conductivity of Yb:YGAG ceramic material compared to Yb:YAG and our experiences with the thin-disk regenerative amplifiers at the HiLASE center , we believe, we can get up to 50 W level output at 100 kHz repetition rate.
European Regional Development Fund and the state budget of the Czech Republic project HiLASE CoE: Grant (No. CZ.02.1.01/0.0/0.0/15_006/0000674); European Union's Horizon 2020 research and innovation programme (No. 739573); Ministry of Education, Youth and Sports of the Czech Republic Programmes NPU I Project (No. LO1602); Large Research Infrastructure Project (No. LM2015086); Joint Research Projects of the Japan Society for the Promotion of Science; Czech Academy of Sciences (project JSPS 16-13).
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