# Advances in High-Power, Ultrashort Pulse DPSSL Technologies at HiLASE
## Mid-IR Picosecond Pulse Generation  
Sources at wavelengths in the mid-IR range (between 2 and 8 \(\upmu\)m) are under development due to their important applications [11; 39], such as minimally-invasive neurosurgery and plastic and polymer processing. Scientific applications, for instance high harmonic generation [40], dielectric laser acceleration [41], or vibrational spectroscopy [42], also benefit from the use of a mid-IR source.  
A high average power wavelength tunable picosecond mid-IR source based on optical parametric generation (OPG) and amplification (OPA) in a nonlinear optical crystal is being developed. In this parametric down-conversion process, a powerful pump beam of the shortest wavelength generates signal and idler beams of longer wavelengths. In the wavelength conversion setup (Figure 4), the Yb:YAG thin-disk laser delivers the pump beam of 100 W average power at a repetition rate of 77 kHz, wavelength of 1030 nm, and pulse width of about 2 ps. Part of this fundamental beam pumps an OPG in a 10-mm long, periodically poled lithium niobate crystal (PPLN). The generated wavelength is determined by the PPLN's poling period and temperature. Tunability of the signal wavelength between 1.46 \(\upmu\)m and 1.95 \(\upmu\)m was achieved. The corresponding idler wavelengths were in the range of 2.18 \(\upmu\)m to 3.50 \(\upmu\)m. The signal beam up to 80 mW was generated at 2 W of pump power, when the double pass of the beams through the PPLN crystal was employed.  
The main part of the pump beam is delivered into an OPA stage, in which the signal beam from the OPG stage is amplified. The stage consists of two 10-mm long potassium titanyl phosphate (KTP) crystals arranged in the walk-off compensating arrangement. Signal and idler tunability between 1.70-1.95 \(\upmu\)m and 2.18-2.62 \(\upmu\)m, respectively, was demonstrated by changing the phase-matching angle in KTP crystals. The signal beam was amplified up to 8.5 W at 42 W pump. The power of the generated idler was up to 5 W.  
Improvements of the system for the achievement of higher output powers and broader tuning ranges were proposed. These are based on higher pump power, another OPA stage, and the use of Potassium Titanyle Arsenate (KTA) crystals, which have a broader transparency range in comparison to KTP crystals. Thermal effects caused by residual absorption [43] of the beams in nonlinear optical crystals at high average power will be studied as well.  
Besides the OPG/OPA-based mid-IR source of radiation, a six-year project on the development of a Ho-doped picosecond thin-disk laser system emitting near 2.1 \(\upmu\)m is starting at HiLASE to support OPA in the 4-6 um spectral region (Perla D in Figure 1). In the first phase, a 100-W regenerative  
Figure 4: Tunable picosecond optical parametric generator (OPG) followed by optical parametric amplifier stage (OPA): (**a**) Optical scheme of the frequency conversion system (PPLN—periodically poled lithium niobate, BPF—bandpass filter, BD—beam dump, PUMP—Perla C thin-disk laser, KTP—potassium titanyl phosphate, At—delay line); (**b**) Spectral tunability of idler (**top**) and signal (**bottom**) beams.  
amplifier with a 10-kHz repetition rate and a fully Ho-doped fiber-based front-end will be constructed. In the second phase, we are planning to boost pulse energy in order to generate 1-J picosecond pulses and 1 kW of average power. All of the Ho-based systems will be pumped by Tm-doped CW fiber lasers, which are under construction now. Detailed technical concept is still under consideration. Direct applications for this kind of laser system besides OPA pumping are modifications of semiconductors, processing of plastics, laser-induced damage threshold measurement, etc.