Broadly-tunable near- and mid-IR source by direct pumping of an OPA with a 42 MHz femtosecond multi-Watt Yb:KGW oscillator

Laser sources delivering high repetition rate broadband mid-IR radiation enable a number of key applications [1], for example in near-field and FTIR spectroscopy [2]. Here, we present an OPA that is directly pumped by a diode-pumped solid-state oscillator without the need for any amplifier or cavity dumping, generating tunable radiation in the near- and mid-IR spectral region.We employ a passively mode-locked dual-crystal Yb:KGW oscillator delivering up to 7.4 W average power with 425 fs pulse duration and 41.7 MHz repetition rate at 1040 nm as pump source [3]. A fraction of the light is used to generate the supercontinuum seed in an 8 cm long tapered fiber with 4 μm waist diameter, whereas the remaining light is used to pump the parametric amplifier. Pump and signal beam are combined with a dichroic mirror and focused separately into the crystal with a collinear interaction geometry. Using a 5 mm long PPLN crystal for frequency conversion, the OPA signal wavelength is continuously tunable between 1380 and 1830 nm, which corresponds to idler wavelengths from 2.41 to 4.22 μm, by changing the poling period of the crystal (28.0 to 31.0 μm in 0.5 μm steps) and by changing the crystal temperature between 20 and 200 °C. The signal and idler spectra measured at room temperature are shown in Fig. 1(a) and 2(b), respectively. We exceed 325 mW over the whole signal range with a maximum power of 540 mW at 1400 nm and we exceed 140 mW over the whole idler range with a maximum power of 220 mW at 3.06 μm. The pump power was set to 2 W in order to remain well below the damage threshold of the PPLN crystal. Using the maximum available pump power of 4 W, signal powers up to 800 mW were achieved. To generate mid-JR radiation at longer wavelengths we used a 2 mm long GaSe crystal instead of the PPLN crystal. We performed type J phase matching (e - o → o). The pump power was set to 4 W. Broadband mid-JR pulses tunable from 4.85 t- 9.33 μm were generated with up to 830 μW average power at 6.46 μm, see Fig. 1(c).Power and spectra are temporally stable with power fluctuation of 0.9 % rms in the NJR and 3.3 % rms in the MJR region. We numerically model the parametric process using a χ(2) Nonlinear Envelope Equation [4] and show good agreement between simulations and experiments. Our source is compact and cost efficient and has the potential to replace more complex Ti:sapphire/OPO systems for ultrafast spectroscopy and mid-JR applications.