In order to reduce the influence of thermal lens effect on beam quality of the multi-pass laser amplifier, a new method of spherical-aberration self-compensation based on ZEMAX physical optical simulation is proposed. Firstly, the change of quality of gaussian beam after passing the thermal lens is simulated according to the principle of geometric optics. And the simulation results show that if two identical thermal lenses are placed symmetrically near the focal point of the laser beam, the degradation of the beam quality caused by the first thermal lens can be compensated by the second thermal lens. Secondly, a four-pass laser amplifier based on the spherical-aberration self-compensation theory is designed by the sequence mode of ZEMAX software. Finally, according to the theoretical model, we design a picosecond fiber-solid hybrid laser amplifier which is seeded by an all polarization-maintaining (PM) fiber laser. The output power of the all PM fiber laser is 2 W. After the solid-state amplifier, the final output laser power reaches 8.5 W with M2 factor of 1.2. The beam quality is well preserved by the four-pass amplification structure which is favorable to the spherical-aberration compensation. This system, which combines the advantages of the all PM fiber amplifier and the solid-state laser amplifier, enables high repetition rate and good beam quality with high gain picosecond pulses. It makes significant contributions to many applications such as material micro-processing, laser ranging and laser detection.
We propose a fiber-solid hybrid amplifier system which consists of a semiconductor saturable absorber mirror (SESAM) mode-locked fiber seed with pulse width of 10.6 ps and repetition rate of 17.6 MHz, a two-stage fiber pre-amplifier and a Nd: YAG regenerative amplifier. This structure not only endures high peak power, but also reduces the influence of fiber nonlinear effects. In the regenerative amplifier, a Pockels cell made up of BBO crystal is used as the electro-optical Q switch. It can effectively amplify the fiber seed source while keeping the beam quality constant. Besides, the pulse frequency, round trip time and thermal lens effect of the regenerative amplifier are considered and the stability of the high repetition rate regenerative cavity is further improved. The system achieves average powers of 1.5, 2.5, 3.4 W at the repetition rates of 1, 3, 5 kHz, corresponding to single pulse energies of 1.5, 0.83, 0.68 mJ respectively. And the beam quality factor M2 reaches 1.5 at the output power of 3.4 W. This system will make significant contributions to many fields such as material micro-processing, laser ranging, and laser detection. To make it more systematizing and engineering, we design an engineering prototype of the fiber-solid hybrid amplifier system.
In this paper, we report a simpler and efficient hybrid laser amplification system, which delivers a single pulse energy of 20 mJ at 500 Hz with a pulse width of 278 ps. This hybrid amplification system mainly consists of a hundred-picosecond all-fiber structure seeder, a regenerative amplifier and a single-pass amplifier. The hundred-picosecond laser pulse from the fiber seed is first amplified to 4 mJ by a regeneration amplifier with a normally doped Nd:YAG rod. For the improvement of the laser energy, the QCW side-pumped Nd:YAG single-pass amplifier has been adopted at the second stage of solid-state amplifiers. To reduce the serious thermal effect, the bonded composite YAG-Nd:YAG-YAG rod crystal is used as the gain medium, which possesses 6 mm in diameter and 85 mm in effective pumped length, and a doping concentration of 0.8%. Finally, 20 mJ output energy and 278 ps pulse duration are obtained with a peak power of 72 MW, and its beam quality factor is 1.7. In addition, this laser amplifier is confirmed by latter theoretical simulations of laser-induced plasma in silicon-based photodiode. The high-energy amplification of the hundred-picosecond pulse is realized in this hybrid amplifier system, which will be used as laser irradiation source in laser-induced damage application.