A laser-diode (LD) pumped Nd:YLF laser with 1.5 ps duration was constructed as a front-end of a chirped pulse amplification (CPA) Nd:glass laser and its operational characteristics was investigated. The Nd:YLF laser was mode locked using the additive pulse mode locking (APM) technique. The external cavity of the APM Nd:YLF laser includes a single mode fiber as a nonlinear medium. The output pulse was stretched to 170 ps using a four-pass grating stretcher, and regeneratively amplified.
The TEM00 mode of the diode-pumped Nd:YLF laser was analysed and the CW operating characteristics were investigated. When the reflectivity of the output mirror was 94 % and the diode pumping power was 2.8 W, the CW output power of 1.1 W was obtained without an extra cooling system. The total power conversion efficiency was 39 % and the slope efficiency was 41 %.
For additive pulse mode locking, a 1.2 m single mode fiber was installed in the external cavity as nonlinear medium, and two types of APM lasers were configured. In a Fabry-Perot type APM laser, the shortest pulsewidth was 1.8 ps and average output power was 215 mW with 3 W pumping power (conversion efficiency: 7.2 %). The peak power of a mode locked pulses was 1.2 kW. A Michelson type APM laser including a SF-19 Brewster prism pair was set up to compensate for the group velocity dispersion induced from the fiber. When the distance between the prism pair was 30 cm, the shortest pulse of 1.5 ps was generated, and the average output power of 150 mW with 1.6 W pumping power was obtained ( conversion efficiency: 9.4 %, slope efficiency: 14.6 %). The energy fluctuation of the APM laser was about 1 %. A cylindrical PZT wound with the fiber was utilized for stabilizing the external cavity and the mode locking was operated stably over several hours.
The birefringence effect of the single mode fiber on the self-starting threshold of the APM laser was investigated. By controlling the polarization state of pulses in the external cavity, the mode locking could be self-started with CW power coupled to the fiber as low as 10 mW. The lowest diode pumping power for operating the APM Nd:YLF laser in self-starting mode was 0.9 W.
The effect of the cavity length mismatch between the main and external cavity was also investigated. The APM operation could be maintained up to the mismatch range of 290 um. The pulsewidth varied from 1.5 ps to 2.9 ps and the pulse shape also changed to asymmetric sech2 with asymmetric parameter ranged from a = 1(symmetric case) to a = 7.
A regenerative amplifier with a 7-mm-diameter Nd:phosphate glass was designed and constructed. The mode locked pulse was stretched to 170 ps by utilizing a four-pass grating stretcher for regenerative amplification. After twenty-four round trips in the regenerative amplifier, the stretched pulse with 46 pJ/pulse was amplified to 0.44 mJ (amplification ratio was $9.6×10^6$ ). This regenerative amplifier, with modification to increase the output energy, will be utilized as a preamplifier to Tera-Watt Nd:glass laser system.
