Mid-infrared supercontinuum generation for spectroscopic applications

Spectroscopy in the mid-infrared spectral region has several applications ranging from gas sensing to optical coherence tomography and medical applications because of unique molecular absorption signatures. Current light sources used for applications in spectroscopy are not suitable for all cases because of numerous parameters such as narrow bandwidth or low power levels. Using supercontinuum generation in nonlinear optical fibers is a field of interest as the laser sources can be made compact and cost effective but still have broad bandwidth towards the mid-infrared with high coherence properties and high power. Creating an all-fiber supercontinuum into the mid-infrared requires the use of soft-glass fibers that have transmission window extending to the desired region. Silica fibers are highly studied due to their good availability and transmission to the near-infrared region, but reaching the mid-infrared spectral region is not possible due to material absorption above 2500 nm. Soft-glass fibers with lower damage threshold but higher nonlinearity and transmission are cascaded with the silica in order to create supercontinuum extending to the mid-infrared, as demonstrated in this thesis. This thesis discusses current supercontinuum-based commercial light sources in the mid-infrared. They are based on using single-mode optical fibers with a lower damage threshold and this limits the achieved power spectral density crucial for applications such as remote sensing or molecular spectroscopy. One must seek for solutions allowing for the increase of the supercontinuum source power output. This work investigates the possibility of using multimode fibers with larger core size, and therefore, a higher damage threshold, for supercontinuum generation light sources in the mid-infrared. The physical phenomena behind supercontinuum generation are introduced in this thesis. Additionally, basic fiber properties are shown focusing on different refractive index profiles. The experimental part demonstrates supercontinuum generation in a cascade of step-index multimode silica and soft glass fibers with a nanosecond fiber laser. Different combinations of silica and soft glass fibers were used to optimize broadening, power levels and clean transverse intensity distribution. With a multimode step-index silica and indium(III)fluoride, the generated supercontinuum spans from 730 to 3000 nm with coupling efficiency of 70 %, extending in the mid-infrared region