Optimizing the power of pulses in the picosecond regime is important for advances in micromachining, direct pumping of parametric devices, and high-field physics. For these applications, high-power thin-disk lasers offer better thermal management and smaller optical nonlinearity than other ultrafast solid-state laser geometries. Now, physicists at Trumpf-Laser (Schramberg, Germany) and the Center for Applied Photonics at the University of Konstanz (Konstanz, Germany) have achieved record-energy ultrafast pulses directly from a passively modelocked thin-disk laser oscillator.
Thomas Dekorsy, professor of physics at the Center for Applied Photonics, University of Konstanz, and colleagues have reported stable single-pulse operation with an average output power exceeding 50 W, excluding a CW background of 8%, at a repetition rate of 3.8 MHz.1 Maximum pulse energy of 13.4 µJ occurred at a pulse duration of 1.36 ps with a time-bandwidth product of 0.34. At an average output power of 55 W, these pulses had a spectral bandwidth of 0.88 nm centered at 1030.3 nm (see figure).
The setup used a passively modelocked ytterbium-doped YAG (Yb:YAG) thin-disk laser with a thickness of only 60 µm, a diameter of 10 mm, and a wedge angle of 6′. To compensate for the low single-pass gain of a thin disk, the pumping chamber provided 20 successive passes through the gain medium, undergoing absorption loss of less than 60% for the 940 nm pump power.
The group achieved passive soliton modelocking of the laser through use of a semiconductor saturable absorber mirror (SESAM) in an active multipass geometry based on angular multiplexing of the gain element, a novel setup never before used in an oscillator, according to the researchers. The active multipass cell consisted of a pair of Gires-Tournois spherical mirrors forming a telescopic image that reproduced itself after 11 passes through the cell (corresponding to 44 passes through the gain medium round trip). The increased roundtrip gain enabled output coupling efficiency well above 50%—the highest of any thin-disk laser ever reported.
The total cavity length of 39.93 m corresponded to a repetition rate of 3.79 MHz. In addition to increasing gain and output coupling efficiency, the multipass geometry enabled easy alignment of components, and sufficient group dispersion delay to compensate for self-phase modulation due to air. While no degradation of the SESAM occurred during several hours of operation, the team will conduct a thorough evaluation of reliability as operation continues.
Single-pulse operation
An autocorrelation trace revealed single-pulse operation across a 100 ps time window. A fast photodiode also measured the output pulse train rise time of 300 ps, confirming single-pulse behavior. The group then took advantage of the thin-disk oscillator’s high peak powers to generate pulses at 515 nm in a critically phase-matched, 3-mm-long, uncooled beta barium borate (BBO) crystal with a beam diameter of 0.8 mm inside the crystal. With 48 W of power incident on the BBO crystal (some loss was due to imperfect beam delivery), the conversion efficiency of this second-harmonic generation was measured to be 60%. This further confirms single-pulse operation of the oscillator.
“We think that the achieved high pulse energy from an oscillator without the need for an oscillator-amplifier system is significant progress in the field,” said Dekorsy. “Our system is the first to operate at these pulse energies at room-temperature conditions, in contrast to recent developments in helium atmosphere at ETH Zurich.2 Hence our system is relevant for real applications and will challenge systems based on fiber lasers.”
While initial published results stated maximum pulse energies of 13.4 µJ at pulse durations of 1.36 ps (which exceeded previously reported values by a factor of seven), subsequent eleventh-hour increases to the pump spot size achieved energies up to 16 µJ at 54 W average power without any extraneous CW background. At average output powers below 8 W, Q-switching occurred. The group plans to announce the latest improved results of their research, which is supported by the by the German Federal Ministry of Education and Research, at the Conference on Lasers and Electro-Optics (CLEO) in San Jose, May 4-9 (see www.cleoconference.org).
REFERENCES
1. J. Neuhaus et al., Optics Lett. 33 (April 1, 2008).
2. S. Marchese et al., Proc. CLEO-Europe 18, paper CF3-2, Optical Society of America, Washington D.C. (2007).
