The usage of coherent radiation in the mid-infrared (mid-IR) wavelength range (2 – 8 μm) covers a wide spectrum of applications in many different fields. To satisfy the demand for a high-power, picosecond mid-IR source, we are developing an optical parametric system tunable between 1.45 and 3.5 μm in wavelength. The pumping of the system is provided by an in-house built Yb:YAG thin-disk laser, delivering 80 W of average power at 93 kHz pulse repetition rate, 1030 nm wavelength and ~1.3 ps pulse duration. The optical parametric system consists of a double-pass optical parametric generator (OPG) based on a periodically poled lithium niobate. By utilizing four periods of poling and temperature tuning, wavelength tunability range is from 1.45 to 3.5 μm. Maximum signal output power was around 85 mW at 1850 nm wavelength at 2 W pump power. Subsequent amplification of the signal generated in the OPG stage takes place in an optical parametric amplifier (OPA), which consists of a pair of walk-off compensating KTA crystals, pumped by up to 50 W average power. Maximum output signal and idler sum power after the OPA stage was 8.4 W. The wavelength tunability of the amplifier spans from 1.5 to 3.2 μm. Further increase in the tunability range as well as gapfree tuning is currently crystal mount-limited.
We report on the generation of picosecond pulses at 515 nm and deep ultraviolet pulses at 257 nm and 206 nm. They are generated as second, fourth and fifth harmonic frequencies of the high power diode-pumped Yb:YAG thin-disk laser at the fundamental wavelength of 1030 nm. The laser at the fundamental wavelength is based on a chirped-pulse amplification of oscillator pulses in two-stage regenerative amplifier based on thin-disks as active media. The diode pumping at the zero phonon line is used. The pulses are produced at ~100 kHz repetition frequency, 2-10 ps pulse duration and ≤2 mJ pulse energy. The fundamental beam is doubled in an LBO crystal at an efficiency of 42%. Subsequently the fourth harmonic frequency (257 nm) is produced by frequency doubling of the second harmonic frequency in a CLBO crystal. The unconverted part of the fundamental beam after SHG is used with the fourth harmonic beam as 1ω+4ω frequency sum in a further CLBO crystal for the fifth harmonic frequency (206 nm) generation. Two issues are important in the efficient optical conversion: nonlinear absorption given mainly by two-photon absorption (TPA) and proper timing of interacting pulses. In our simulations we take into account TPA and study the consequences of different timing of 1ω- and 4ω-pulses on the CLBO when generating the 5th harmonic. It was found that 1.4 ps delay of the 1ω-pulse after the 4ω-pulse is necessary to get the highest 5ω-output. Also the 5ω-pulse duration is affected by the timing.
Mid-IR wavelength range (between 2 and 8 μm) offers perspective applications, such as minimally-invasive neurosurgery, gas sensing, or plastic and polymer processing. Maturity of high average power near-IR lasers is beneficial for powerful mid-IR generation by optical parametric conversion. We utilize in-house developed Yb:YAG thin-disk laser of 100 W average power at 77 kHz repetition rate, wavelength of 1030 nm, and about 2 ps pulse width for pumping of a ten-watt level picosecond mid-IR source. Seed beam is obtained by optical parametric generation in a double-pass 10 mm long PPLN crystal pumped by a part of the fundamental near-IR beam. Tunability of the signal wavelength between 1.46 μm and 1.95 μm was achieved with power of several tens of miliwatts. Main part of the fundamental beam pumps an optical parametric amplification stage, which includes a walk-off compensating pair of 10 mm long KTP crystals. We already demonstrated the OPA output signal and idler beam tunability between 1.70-1.95 μm and 2.18-2.62 μm, respectively. The signal and idler beams were amplified up to 8.5 W and 5 W, respectively, at 42 W pump without evidence of strong saturation. Thus, increase in signal and idler output power is expected for pump power increase.