Mid-infrared beam shaping concepts are presented, which rely on coherent emission from QCLs. Grating coupled
surface emitting quantum cascade ring lasers allow for far-field tuning, ranging from highly symmetric spot- to ringshaped
beam patterns, depending on the grating period. In single-mode operation, the devices exhibit low beam
divergence, represented by a full width at half maximum of ~3°. Moreover, a tree shaped resonator is investigated, which
enables coherent parallel coupling of six laser elements into a single waveguide by means of several Y-junctions. The
lasers were investigated in terms of optical power, near and far field characterization. Phase-locking was observed and
leads to in-phase emission on both sides of the devices. Both concepts demonstrate the feasibility of high-brightness midinfrared
quantum cascade lasers with prospective applications in spectroscopy and high power laser arrays.
A monolithic coupling scheme in which two active waveguides merge into a single waveguide to form a Y-shaped
resonator is demonstrated for mid-infrared quantum cascade lasers. Lasers with emission wavelengths of 10.5 μm and
4.2 μm were processed from lattice-matched GaAs/AlGaAs and strain-compensated InP/InGaAs/AlAs/AlInAs
structures. Phase-locking is observed in the laser cavities, resulting in coherent interference of the emitted radiation. Far
fields were recorded on both sides of the devices and analyzed in respect to their radiative origin. By matching the
recorded far field intensity profiles to corresponding near field distributions, the lateral mode distribution within the
resonator is derived. Depending on the length of the coupling section, even or odd cavity modes evolve. Moreover, a
comparison between the fabricated devices shows the emission wavelength's impact on the coupling performance of the
Y-junction. The results demonstrate the feasibility of coherent laser resonators with prospective applications in
interferometric sensing and high power laser arrays.
We present design and comprehensive characterization of a versatile, small-scale photoacoustic sensor stick. Due to its optimized forward-looking directional characteristic, it is a valuable tool for spatially resolved PA depth scanning and 3D imaging. The pencil-formed, optical fiber-coupled sensor has a diameter of only 6 mm, with a length of 15 cm. For characterization of its fundamental parameters, we applied a pulsed frequency-doubled Nd:YAG laser (532 nm) with a pulse repetition rate of 10 Hz. Different designs of the sensor tip are compared. We present a full characterization of the qualities of the system as imaging tool, i.e. lateral and depth resolution in dependence on light absorption and scattering properties of the samples as well as of the surrounding matrix. Specially tailored phantoms are introduced for these experiments. The phantoms in combination with a xy-scanning stage are applied to produce 2D and 3D images with the sensor. The imaging properties of the endoscope are explored by several methods of characterization. We test the sensitivity to absorbing structures of different size and absorptivity, which can be summarized as contrast. Finally, we present first tomographic images of tissue phantoms resembling the optical properties of human tissue.