Powerful diode-pumped ultrafast solid-state laser oscillators based on bulk Yb:KGd(WO4)2 crystals
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Yb-ion doped gain media have become the material of choice for reliable generation of ultrashort pulses at wavelength around 1 μm. At present, however, operation at high average power (>1 W) with sub-100 fs pulses still remains challenging. The efforts of developing an Yb-ion oscillator towards this goal, therefore, are the main focus of this thesis. In this work, the Yb:KGd(WO4)2 (Yb:KGW) crystals were chosen to serve as the gain media. To achieve high power operation, two fundamental issues have been carefully considered: 1) a new pumping scheme was proposed to alleviate the thermal issues in the Yb:KGW crystals; 2) a new method was introduced to characterize intracavity losses in the broadband Yb-ion oscillators. As a side effect observed during the optimization of the CW operation, simultaneous two-wavelength emission was also discussed. With the knowledge and experimental understanding of the fundamental issues in laser oscillators operated in the continuous-wave regime, the next step of this work demonstrated their operation in a pulsed regime. The dual action of the Kerr-lens and saturable absorber (KLAS) mode locking was proposed in this work and resulted in greatly enhanced laser performance. The laser delivered pulses with 67 fs duration at a repetition rate of 77 MHz. The average output power reached 3 W, which, to the best of our knowledge, is the highest average output power produced to date from the Yb-ion based bulk lasers with such a short pulse duration. The scalability of pulse energy and peak power was also demonstrated by reducing the repetition rate to either 36 MHz or 18 MHz. The cavity with the latter repetition rate produced 85 fs pulses with the pulse energy up to 83 nJ, which corresponds to a peak power as high as 1 MW. As required by many biomedical applications, the wavelength of the generated pulses (~1 μm) can be tuned in the near-infrared region by coupling them into an optical parametric oscillator (OPO). The feasibility of this approach was demonstrated in the last part of this thesis, through a thorough theoretical analysis of two OPO materials suitable for excitation at 1.04 μm.
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