Mid-Infrared upconversion imaging and spectroscopy using short pulse light source

Research output: Book/ReportPh.D. thesis

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Abstract

As Frank J. Low once said, ‘‘Every object in the universe with a temperature above absolute zero radiates in the infrared, so this part of the spectrum contains a great deal of information’’. The infrared region of the electromagnetic spectrum is rich in molecular absorption features, but in general, it is challenged by the lack of coherent light sources and detectors. Parametric upconversion is a viable technology that can address these challenges. Though frequency conversion techniques were demonstrated as early as the 1970s, advancements in the nonlinear crystal and laser technology have recently improved the overall efficiency of frequency conversion processes. This thesis work primarily focuses on generating and detecting infrared light in the 1.5 – 4 µm range. We first develop a cheap compact tunable infrared light source based on spontaneous parametric down-conversion (SPDC) using a high intensity passively Q-switched laser, pumping a periodically poled lithium niobate crystal. The Q-switched laser delivers pump
pulses at 1030 nm with 3 nanosecond duration and maximum energy of 180 µJ. The extremely high gain for the parametric process provides a conversion efficiency of ~ 55%. A theoretical description of the high gain regime is presented here for the first time. The
model allows us to accurately predict the generated SPDC power and the spectral properties in the high gain regime. We then quantify the pulse-to-pulse energy and the spectral intensity stability of the SPDC light source for the first time to the best of our
knowledge. The spectral stability is critical when using the light source for infrared sensing applications, for example, spectroscopy. Furthermore, we demonstrate the fast continuous tuning capability of the SPDC light source using a fan-out crystal covering the 2 to 4 µm range with a tuning rate of 100 nm/sec. We test the light source for spectroscopy of a polystyrene sample using a simple thermal power meter, thus eliminating the need for a conventional spectrometer. In the last part of the thesis, we describe an upconversion system to perform infrared imaging in the femtosecond pulse regime. A mode-locked Ti:Sapphire laser at 804 nm
with a pulse duration of 100 femtosecond pumps an optical parametric oscillator generating tunable infrared light in the 2.7 – 4 µm range with a mid-IR pulse duration of ~ 200 femtosecond. Synchronous mixing of the infrared light with a portion of the pump
inside an unpoled lithium niobate crystal placed in the Fourier plane of a 4f imaging setup facilitates efficient upconversion to the vis/near-infrared range. This enables easy imaging in the femtosecond regime based on conventional silicon detectors. For the first time, a theoretical model is developed to describe the broad angular and spectral acceptance bandwidths of a short-pulsed upconversion system. We also identify a blurring effect that deteriorates the imaging quality of short-pulse upconversion
imaging.

Original languageEnglish
PublisherTechnical University of Denmark
Number of pages181
Publication statusPublished - 2021

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