Development of a Kerr-lens mode-locked Cr:ZnS laser and a nonlinear frequency conversion stage for broadband mid-infrared spectroscopy

The detection of diseases by a quick measurement of a sample of breath or blood would provide a minimal invasive and valuable method for medical diagnostics. Broadband and coherent mid-infrared sources could enable the sensitive detection of biomarkers in the human breath that are related to diseases or metabolic changes. Coherent infrared radiation could help to increase the sensitivity by employing multipass cells with large propagation distances in small probe volumes. Furthermore, broadband coherent ultrafast lasers can be employed to realize dual-comb spectrometers, which promise Fourier-transform spectroscopy with a fine temporal resolution due to fast data acquisition and a fine spectral resolution. Moreover, these coherent and ultrafast infrared sources are needed for near-field optical microscopy to acquire spectroscopic information with a spatial resolution on the nanometer scale, and these sources are generally needed for ultrafast pump-probe spectroscopy. In this thesis, a laser system for the generation of broadband and coherent infrared radiation was developed. This radiation was generated by nonlinear frequency conversion of the radiation emitted by a Kerr-lens mode-locked Cr:ZnS oscillator lasing at 2.3 µm central wavelength. Broad phase-matching bandwidths at this wavelength lead to the efficient generation of broad infrared spectra, spanning from 10 to 39 THz (7.7 – 30 µm, 334 – 1300 cm⁻¹). Therefore, a Cr:ZnS laser oscillator was developed, which provides similar to Ti:sapphire lasers extreme emission bandwidths of about 14 THz (FWHM) and ultrashort pulses with a duration of 36 fs, but around a center wavelength of 2.3 µm instead of 0.8 µm. The output power of the Cr:ZnS oscillator was boosted by a compact laser amplifier to 2 – 3 W, which was accompanied by beneficial spectral broadening of the pulsed laser radiation. Furthermore, a novel laser amplifier design based on quasi-waveguide propagation in bulk laser gain media was investigated theoretically and experimentally on the example of Ti:sapphire. A distribution of the pump absorption over a larger volume inside the gain medium leads to a reduction of the absolute temperature. The novel amplifier geometry is promising for pumping a Cr:ZnS gain medium. Besides that, the direct peak power scaling of Kerr-lens mode-locked bulk laser oscillators was also investigated. However, this was done on the example of a Yb:YAG gain medium to simplify the alignment, mode-locking, and diagnostics. The broadband radiation of the Cr:ZnS laser oscillator and amplifier was used to generate infrared radiation via intra-pulse difference frequency generation in the nonlinear crystals ZGP und GaSe. With GaSe, a major part of the molecular fingerprint region was covered and wavelengths up to almost 30 µm (10 THz) were generated. In addition, infrared radiation was also generated by sending temporally compressed pulses with a duration of 7 fs from an ytterbium-based laser system with an emission wavelength of 1 µm into GaSe and LGS crystals. Difference frequencies of up 97 THz (3.1 µm, 3200 cm⁻¹) were reached due to the broad spectrum of the short pulses. For sensitive spectroscopic measurements, a multipass cell with a path length of 94 m was developed and the noise of the Cr:ZnS laser in a FTIR spectrometer was evaluated. Additionally, an attempt was made to systematically compare spectroscopic approaches based on thermal infrared emitters and coherent laser sources in combination with FTIR and dual-comb spectroscopy.