Investigation and development of mode-locking techniques of diode pumped solid-state lasers
Development of diode laser pumped mode-locked sources for use as short pulse oscillators for chirped pulse amplification (CPA) experiments using the VULCAN Nd:glass laser has been investigated. To achieve subpicosecond pulses we studied the performance of a recently reported laser crystal, NdxLa1-xMgAl11O19 (Nd:LMA). Using an active mode-locking technique (frequency modulation, or FM mode-locking) bandwidth-limited pulses of 16 W peak power and 14 psec duration were obtained. The pump source was a 0.5 W single stripe diode-laser. In order to obtain sub-picosecond pulses it was necessary to pump the Nd:LMA crystal with higher pump powers and use the more powerful mode-locking technique of additive-pulse mode-locking (APM). A detailed discussion is given of the performance of the Nd:LMA laser pumped with high power diode-lasers and also the pumping and cavity configurations which lead to successful laser operation. In free running operation up to 560 mW and 1.1 W output power was achieved in single and multi mode operation respectively when the crystal was pumped with a 3 W and two 3 W diode-lasers. A brief study is also made of the aspects of the thermal effects, such as thermal lensing and thermally induced birefringence, which may affect the laser performance. Both the Fabry-Perot and Michelson additive-pulse mode-locking configurations were used, and are discussed in detail. With the Fabry-Perot set up, transform-limited pulses of 420 fsec duration were obtained. The average output power was 18 mW, corresponding to an energy per pulse of 0.19 nJ. In the Michelson arrangement the average output power was 120 mW with nearly the same pulse duration which corresponds to a pulse energy of 1.6 nJ. The use of the additive pulse mode-locked laser-diode-pumped Nd:LMA laser as a seed source for the VULCAN Nd:glass amplifier system at the Rutherford Appleton Laboratory is discussed. Use of this seed source allowed the achievement of extremely high peak power (30 TW) sub-picosecond (800 fsec) pulses from this amplifier system. We also present detailed measurements of the main characteristics of this laser.