Parametric Processes for Generation and Low Noise Detection of Infrared Light: Experimental and Numerical Investigation of Fundamental Parameters for the Design of PPLN-based Light Sources and Detector Systems

This thesis describes an experimentally based, and application oriented investigation of sum- and difference frequency generation for photon conversion,from one spectral domain to another. The applications in focus are coherent gas spectroscopy in the near- and mid-infrared regimes. The investigations analyse and measure the influence of the different parameters that defines the physical properties of the applied nonlinear optical processes. The emphasis is on how to use the parametric processes as an engineering tool for designing versatile lightsources and low noise detection systems. The first chapter of the thesis introduces and motivates the work with frequency conversion, sketching the potential of the noise properties for upconversion based detection systems and the increased wavelength availability for parametric light sources. A selection of prior work is presented to give an overview of the focus in the field and to place the thesis in a general context. The second chapter introduces the basic concepts of nonlinear parametric interaction in the context of this work, where phasematching is a key factor in the work on both light sources and detection systems. Third chapter presents the work on infrared light sources. An optical parametric generator was constructed, and worked as an optical parametric amplifier for both a near- and a mid-infrared seed. The setups are analyzed spectrally and temporally, and discussed with respect to spectroscopic applications. It is concluded that the light source experiments have provided a solid understanding and experience with parametric processes and key aspects of the choice of light sources. It is furthermore concluded that with the resources and scope of the project it was not feasible to continue with application testing using light sources produced in-house. Chapter four changes the focus to upconversion based detection systems, where the prior work is sparser. A compact intracavity upconversion module was constructed and described in the context of an efficient and low noise detection system. A special emphasis is on the noise sources that originate from the upconversion process and how these relate to the poling errors in the nonlinear crystal. The poling errors are measured, and a model is constructed to explain the measured noise features and dependencies of the generated noise. In the fifth and last main chapter, two different upconversion modules are tested as part of a detection system in two different applications for spectroscopic gas measurements. The first test was done at Lund University together with the Combustion Physics Group, where the detection level of acetylene gas in a four-wave mixing setup was improved a factor of 500 compared to previous measurements. The second test was performed at the Atmospheric Physics Department at the German Aerospace Center in Oberpfaffenhofen. Here the upconversion detector was tested as an alternative to an InGaAs detector in a differential absorption lidar setup for long range measurements of atmosphericCO2. The test demonstrated great potential for the upconversion system in combination with photomultiplier tubes for the visible region, and revealed special challenges regarding the etendue of long range detection systems based on an upconversion process. Finally the thesis is concluded with a summary of the major findings together with an outlook sketching the future potential for the application of the upconversion technology within infrared spectroscopic.