This dissertation is mainly concerned with increasing the pump power absorption in optical refrigeration of solids and photo-acoustic spectroscopy of trace gases using optical cavities. Enhancing the absorption is key to reaching lower temperatures in optical refrigeration and achieving better sensitivity in photo-acoustic spectroscopy.;We have used intra-cavity and coupled-cavity absorption enhancement techniques to increase the absorption in Ytterbium doped Yttrium Lithium Fluoride (Yb 3+:YLF) crystals. For this purpose, we have developed tunable high-power narrow-linewidth InGaAs/GaAs vertical external-cavity surface-emitting lasers (VECSELs) operating at 1020 nm, the optimal cooling wavelength for Yb:YLF. By inserting a 7% Yb:YLF sample inside the resonator of the VECSEL, we have cooled it to 130+/-1 K. It has been shown that due to high intra-cavity power, saturation of pump absorption reduces the absorbed power in intra-cavity cooling. We have also utilized a coupled-cavity geometry to enhance the absorption. In this method, the cooling sample is placed inside a Fabry-Perot cavity which is used as an effective output coupler for the VECSEL. With this technique we have been able to cool a 10% Yb:YLF crystal to 145+/-1 K. Advantages and challenges, including cavity design, wavelength stabilization techniques, and cooling sample choice for optimal cooling are discussed in both cases.;We have also utilized critical coupling (or impedance) matching condition in two coherently coupled Fabry-Perot cavities to enhance the absorption in photo-acoustic detection of trace gases. In this novel technique, by adjusting the reflectivity of the first Fabry-Perot cavity, the impedance matching can be achieved for a wide range of absorption coefficients for the second cavity, where the acoustic detection is performed. Normalized noise-equivalent absorption coefficient of [Special characters omitted] is measured.
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