High energy, high repetition rate picosecond pulses from a Quasi-CW diode pumped Nd:YAG system

Abstract

Summary form only given. High-harmonic generation offers the possibility to generate table-top coherent radiation in the soft X-ray regime [1]. This enables high-resolution microscopy of biological and solid state samples. The high intensity required for HHG can be generated by amplifying pulses from a Ti:sapphire oscillator in an optical parametric chirped-pulse amplification (OPCPA) scheme, enabling the production of few-cycle pulses with terawatt peak powers [2]. To maximize the achievable flux of soft X-ray HHG, a high repetition rate pump system for OPCPA is required.Here we report on a new pump laser system based on quasi-CW diode pumped Nd:YAG modules. This system produces 60 ps pulses, with an energy of 80 mJ at 532 nm wavelength, at a repetition rate of 300 Hz. The transition from CW to quasi-CW diode pumping reduces the thermal load on the crystal rods and allows much more efficient pumping and high gain. This enables us to achieve an order of magnitude higher repetition rate compared to flashlamp pumped systems, while maintaining similar pulse energies [2]. One of the issues with side-pumped Nd:YAG rods is the occurrence of thermal birefringence, causing a radially dependent polarization [3]. After a single pass through a Nd:YAG rod, this can result in up to 30% depolarization, and therefore a similar intensity loss at polarizing optics. A double-pass scheme with a Faraday rotator almost completely compensates for this effect [4]. To limit the peak intensity while keeping the pulse energy as high as possible, a top-hat beam profile is used. Therefore relay-imaging is implemented between every critical surface.A schematic of the pump laser is shown in Fig. 1. A Nd:YV04 oscillator produces 10 ps pulses, which are amplified to 1 mJ in a regenerative amplifier. The pulses are stretched in this amplifier to 60 ps by applying an intracavity etalon. The output beam is enlarged by a telescope and then passed through an aperture to create a top-hat beam profile, reducing the pulse energy to 0.1 mJ. These pulses are then used as a seed for the postamplifiers. The aperture is relay-imaged in every pass through the Nd:YAG rods (taking into account thermal lensing), and then relay-imaged on a BBO crystal for efficient second-harmonic generation. The maximum pulse energy (80 mJ at 532 nm) is currently limited by the damage threshold of the second Nd:YAG rod. This pump laser system forms an ideal starting point for the development of an OPCPA system that produces terawatt pulses at high repetition rate. As such, it is an important step towards the production of high-flux soft-Xray radiation with a table-top source.