Yttrium Aluminum Garnet (YAG, Y_3 〖Al〗_5 O_12) crystals doped with Erbium have posed an interesting position in the field of rare earth solid state lasers as they possess the property of self-saturation, in which the upper energy level in a laser has a much shorter lifetime than that of the lower level. This is the case of the 2.94μm transition in Er:YAG. That property at first seems to make this material unfit for use as a laser gain medium, however further research into these classes of rare earth materials revealed interesting non-liner energy transfer mechanisms that allow it to be a useful mid wave infra-red (MWIR) coherent source in the continuous wave (CW) and pulsed regimes. This came to fruition due to decades of modeling and spectroscopic research investigating the non-linear energy transfer mechanisms of excited state absorption (ESA), energy transfer up-conversion (ETU), and cross relaxation (XR). These effects show up in the rise and fall times of the levels observed through fluorescence. Resulting in the non-exponential rise and fall characteristics and having a squared or even cubed relation to the population. The population evolution for each of the lasing levels is now affected by ions recycling energy and in turn cause the lifetimes of the levels to “effectively” change to a point where simple linear models do not adequately describe the system and its performance.

The non-linear energy transfer dynamics of Er:YAG are modeled under high resonant pump conditions. By pumping with selective resonant pumps, the interaction dynamics of the (_ ^4)I_(11/2) and the (_ ^4)I_(13/2) levels in the Erbium ion reveal the contributions from generally ignored non-linear energy transfer mechanisms. Specifically, the multi-photon effect known as excited state absorption (ESA) is modeled by measuring and characterizing the cross section in single crystal Er:YAG samples utilizing a pump and probe technique coupled with transient fluorescence measurements. The measured ESA cross section is then included in the rate equation modeling.

Fluorescence lifetime imaging is an established imaging technique that has been extensively explored over the past few decades but has yet to break through as a diagnostic tool in clinical applications. This work will characterize and analyze the quasi-fluorescence lifetime technique with improvements in contrast by a novel approach to manipulate LEDs photonic characteristics to push forward a real time video rate relative lifetime imaging technique. This thesis establishes the framework of using quasi-fluorescence lifetime imaging in the specific long gate time acquisition mode and instating the parameters for any future research of multi-spectral lifetime imaging. Results and analysis of changes in spectral lifetime signatures of collagen and elastin in dry, hydrated, and in acetone chemical environments are presented for the first time using this technique.

Failed wounds in the extremities are characterized with abnormal collagen production and inappropriate bone growth in soft tissue. This condition can occur after trauma to a limb and can cause a disruption to the healing process, and is known as Heterotopic Ossification (HO). In HO, regions in the tissue start to mineralize and form microscopic bone-like structures. These structures continue to calcify and develop into large, non-functional bony masses that cause pain, limit limb movement, and expose the tissue to reoccurring infections; in the case of open wounds this can lead to amputation as a result of a failed wound. Both Magnetic Resonance Imaging (MRI) and X-ray imaging have poor sensitivity and specificity for the detection of HO, thus delaying therapy and leading to poor patient outcomes. This work presents a bi-modality approach using real-time fluorescence lifetime imaging and Raman imaging for tissue differentiation. The fluorescence system is based on relative lifetime contrast generation to characterize surface tissue morphology and detect margins of abnormal tissue, specifically connective tissue biomarkers. The Raman system utilizes a biocompatible, fast, and a large field of view (1 cm2) acquisition to differentiate bone tissue from soft tissue without using a spectrometer, this is in contrast to conventional Raman microscopy systems. This capability may allow for the development of instrumentation which permits bedside complementary diagnosis of HO.

This thesis reports the experimental study of a new, promising class of solid- state laser gain media, namely erbium doped ceramic yttria. Optical quality ceramics have numerous advantages over single-crystal hosts and glass hosts, such as general mechanical robustness in extreme conditions, the ability to engineer doping profiles and tailor the laser gain media, and depending on the material system chosen, higher thermal conductivity these materials demand greater attention of researchers in the field. Ceramic gain media can also be made in large volumes much like glass, leveraging cheap flash-pumping methods to generate very high powers for military and scientific purposes.

The work reported here spans many areas that when taken together present a full evaluation of a future laser host. We have experimentally ascertained thus far the refractive indices of varying doping levels of active ion erbium in polycrystalline yttria and the materials stress-optic behavior. Spectroscopic data such as absorption and emission spectra in the near and mid infra-red regions, and the calculated cross-sections are also included. Multiphonon studies were also completed, the data provides vital information on the dominant phonon frequency and the radiative lifetimes. Energy transfer amongst the erbium ions and upconversion behavior is also analyzed. The theoretical work provided included an up-to-date Judd-Ofelt analysis of the ion-host system.

The multiphonon analysis above was expanded to include some other oxide host materials and differing forms of the same material. Some new upconversion pathways were also discovered in a novel two-pump experiment. Comparisons have been drawn to the current standard Er:YAG system where necessary.

Lithium Niobate (LiNbO3) is an excellent photo sensor that can be used in medical imaging, weapons guidance systems, radiation detection, and many other light sensitive applications. It can operate at its natural state without requiring extensive heating or cooling modifications. However, when LiNbO3 is used as a photo detector, its functionality becomes limited because it produces many signal oscillations. These oscillations make the real signal and the background noise indistinguishable. This phenomenon is known as the "Ringing Effect." To address this issue, Lithium Niobate is cut at a crystal orientation that reduces signal oscillations. In this paper I present: the experimental set up, device performance, along with detailed derivations of the circuit model, temperature, and charge models that are specific to LiNbO3.

Ringing suppressions were tested under three sets of variables: 1) various sample thicknesses; 2) different irradiation angles; 3) the addition of a black absorbing layer on the illuminated surface. The resulting signal response time under the previously mentioned circumstances were measured and compared.

Our conclusion is that ringing effect can be reduced by a correct crystal cut. The attained signal response time under ringing suppression was in the sub-nanosecond range. The added black coating layer on the illuminated surface did not always give faster performance. It was observed that a decrease in the sample thickness could reduce response time.

Erbium-doped in YAG (Y3Al5O12) and in yttira (Y2O3) are the most often used laser

gain media for generation of 1.6 μm and 3 μm laser radiation. Of all the laser gain media

studied, Er-based materials are the most complicated due to the presence of rich non-linear energy transfer kinetics processes. Energy transfer upconversion (ETU) and excited state absorption (ESA) processes in Er-based laser gain materials has been investigated by many groups over the last four decades using the fluorescence decay time as an indirect measure of the ETU and ESA rates. The values reported for ETU and ESA vary two-to-three orders of magnitudes for the same materials. When attempting to model and prototype high power Er-based lasers, such a large uncertainty in the values for ETU and ESA coefficients leads to large (and sometimes unrealistic) predictions in terms of laser performance. To allow the laser community to better model and prototype high power Er-based lasers, a more accurate knowledge of ETU and ESA coefficients is needed. The objective of the present effort was to develop a faster and more accurate technique for quantifying, in absolute terms, the values for ETU and ESA coefficients in Er-based materials. The technique reported here emphasized measuring the rise and decay times of the fluorescence signal from the energy states that directly impact ETU and ESA as a function of pump wavelength and pump intensity. ETU and ESA was investigated in 0.5% and 50% single crystal and polycrystalline (ceramic) Er:YAG and 1%, 15%, and 25% polycrystalline Er:Yttria with the fluorescence signal quantified for the 4S3/2, 4F9/2, 4I9/2, 4I11/2, and 4I13/2 states. Results for the rise and fall time of the fluorescence signal in the case of 0.5% Er:YAG do not show any appreciable ETU component. This result is consistent with results already published in literature and

thus validating that the technique developed in the present study. In the case of the 50%

Er:YAG, measurements were made, which allowed the direct observation of the effects of

ESA and ETU of the 4I11/2, and 4I13/2 energy states (laser states). This technique appears to be applicable to many other rare earth ion systems.

Room temperature, multi wavelength Q-switched operations in 2% Er:YAlO under flashlamp excitation was demonstrated. Lasing occurred predominantly at 1658 nm and 1665 nm with the emission of the wavelengths orthogonally polarized. The use and orientation of a intracavity polarizer dictated the lasing wavelength. Spectral and temporal high resolution analysis of the emission from the Er:YAlO laser revealed dynamic cascaded lasing between the 4S3/2 4I9/2 ( = 1658 nm) and 4I9/2 4I13/2 ( = 1665 nm). In Q-switch mode, the output ranged from 3 – 10 mJ/pulse and pulse duration of 90-110 ns (FWHM).