Long-cavity variable-repetition-rate passively mode-locked Nd:YVO4 laser for ultra-fast pulsed laser deposition

Full text: Ultra-fast ablation with high-repetition-rate lasers is a promising new technology for deposition of thin films with extremely high homogeneity and down to atomic level surface quality. Optimised process conditions require short ps or fs pulses with repetition rate 1-40 MHz, depending on the target material. Mode-locked lasers capable of producing short pulses, usually operate at repetition rates around 70-100 MHz determined by the cavity length. We report on development of a diode-end-pumped Nd:YVO4 laser with a long extendable cavity. An ion implanted InGaAs quantum well semiconductor saturable absorber mirror (SESAM) was used for passive cw mode-locking, resulting in stable output train of 13-ps pulses. The cavity was extended by a zero-q-transformation multi-pass cell (MPC) incorporated into a waist-scaling system. The latter is essential for keeping fluence on the SESAM in the operational limits given the different intracavity pulse energy at different repetition rates. During laser operation, we were able to switch easily between three cavity configurations with repetition rates ∼70 MHz, 5.7 MHz and 3.1 MHz. The output power decreased from 3.5 W at ∼70 MHz down to 1.1 W at 3.1 MHz mainly due to the additional loss introduced by the mirrors (R∼99.85%) of the MPC. Low-loss mirrors (R>99.997%) will allow operation at repetition rates down to 1.7 MHz with insignificant output power reduction. After further amplification and frequency conversion, the laser will be used for deposition of nonlinear optical films and formation of nano clusters for applications in optical science and technology