Power-scalable ultrafast laser sources in the midwave-infrared (MWIR) are a key element for basic research and applications in material processing and medicine. Optical amplifiers based on chirped pulse amplification (CPA) are used to generate high intensity pulses, a technique awarded with the Nobel Prize in physics in 2018. In the CPA scheme, a weak temporally stretched seed pulse is amplified to high energy in a laser amplifier and finally re-compressed resulting in an ultrashort pulse of very high intensity. Applying this concept a new system was developed at MBI delivering few-ps pulses at 2 μm wavelength with peak power beyond 10 GW (10 billion watt) at a 1 kHz repetition rate. The emitted pulses are characterized by excellent stability and brilliant beam quality. The results are reported in the latest issue of Optics Letters .
The main amplifiers of the 2-μm CPA system are based on Ho:YLF crystals and consist of a highly stable regenerative amplifier and two booster amplifiers. All of them are operated at room temperature and pumped by continuous-wave Tm:fiber lasers with a total power of 270 W. Starting from a 2-μm supercontinuum source the seed pulses are stretched and pre-amplified and subsequently fed into the Ho:YLF amplifier chain. The re-compressed pulse energy of the Ho:YLF CPA amounts to 52.5 mJ and reveals an excellent pulse-to-pulse stability of <0.23% rms. The long-term pulse stability together with the beam quality, measured to be better than a M2 of 1.2, is presented in Fig. 1. The emitted spectrum centered at 2050 nm with a bandwidth of 3.5 nm (FWHM) supports a ~1.7 ps Fourier-transform limited pulse duration. After amplification the pulses are re-compressed in a Treacy-type grating arrangement with an efficiency >93%. The recorded autocorrelation trace exhibits a FWHM of 4.1 ps. This corresponds to a duration of the main pulse of 2.4 ps (FWHM) with an estimated energy content of 85%, translating into 17 GW peak power. The latter and the pulse energy of >50 mJ represent the highest values ever achieved for few-ps pulses at 2 μm wavelength yet.
This source is currently being applied as pump in a system for the generation of few-cycle pulses around 5-μm with multi-millijoule energies. Applications in nonlinear optics, spectroscopy and materials processing are underway.
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Optics Letters