Chirped-pulse two-crystal Yb:KYW oscillator with cavity-dumping

Introduction

Although thin-disk oscillators have proofed to be excellent sources for the generation of high average power and high energies the combination of energies beyond 2 µJ and pulse durations considerably shorter than 800 fs could not be provided to this stage. We have established a bulk-crystal oscillator which allows for pulse energies up to 7 µJ and pulse durations of 416 fs (6.1 µJ). With a repetition rate of 1 MHz this system is of particular interest for apllications such as micromachining, non-linear spectroscopy or ophthalmology.

Refering to the research on thin-disk oscillators the cavity-dumping concept is restricted to bulk-crystals since thin-disk configurations have not been able to allow for efficient output coupling owing to the comparably low small signal gain.

Generally with bulk-crystals scaling the average power and the pulse energy by increasing the pump power is limited due to thermal lensing and opto-thermal damage of the crystals. In order to apply higher pump power we split up the total power onto two gain crystals which in turn drastically reduced the thermal load on each crystal. Fig.1 depicts the experimental set-up of the laser. In that way we could apply a total pump power of more than 40 W without observing any thermally induced distortion of the laser beam. At that pump power we could reach dumping efficiencies of 67%. The power values account for an optical-to-optical efficieny of almost 20% which is outstanding for a mode-locked oscillator with cavity-dumping.

Since the necessary dispersion management would be demanding for solitary mode-locking with group-delay-dispersion (GDD) values of < -75 000 fs² it is much more beneficial to operate the system in the normal dispersion regime with way lower GDD values (here: 3500 fs²) and externally compress the pulses as shown in Fig.1.

Beside the good power noise (rms<1 %) and beam properties (M²: 1.1) the high peak power is enough to generate white light in YAG-plates. In contrast to previous systems white light was therefore directly generated by the oscillator without any further spectral broadening and amplification schemes.