The increasing demand for high power laser sources with excellent beam quality operating in the visible and near-infrared electromagnetic radiation spectrum has prompted significant research effort into developing robust, reliable and efficient diode-pumped solid-state lasers (DPSSLs). The scope of this thesis is to study the planar waveguide laser (PWL) architecture, with semiconductor diode lasers (SDLs) pumping, along with the spectroscopic investigation of promising materials with embedded trivalent rare earth (RE) ions that may enable upconversion (UC) lasers of the future. The PWL architecture has distinct advantages over their bulk counterparts in terms of exceptional thermal management, compatibility with SDLs, typically high figures of merit regarding the intensity-interaction length product, leading to the possibility of enabling weak optical transitions. Several experiments are detailed in this thesis in order to point out unique properties of PWLs. The first, a demonstration of a Tm:Y2O3 PWL with a maximum output power of 35mW at 1.95 µm and slope efficiency of 9 %, with respect to the incident pump power. A 12 µm thick active film was grown by the pulsed laser deposition (PLD)technique on top of undoped Y3Al5O12 (YAG) crystal. That was the first demonstration at its kind,i.e. in terms of a sesquioxides PWL, one operating with Tm3+ as the optically active ion and in the 2 micron wavelength regime. Despite waveguide propagation losses on the order of 2 dB/cm, the efficiency was respectable, demonstrating the potential of these PLD films. The second PWL experiment was conducted with a state of the art double-clad Nd:YAG structure fabricated by Adhesive-Free Bonding technique, operating on a weak optical transition of this material, this is, at a wavelength of 1.83 µm, with an output laser power in excess of 1W in a near diffraction-limited optical beam. The main goal of this thesis, however, is to investigate the suitability of several different solid-state gain materials for generation of laser radiation in UV and purple-blue optical spectrum by means of the upconversion process via a sequential step pumping scheme. Nd- and Tm-doped crystals have been investigated for their spectroscopic properties associated with the pump absorption bands, emission strengths, and lifetimes of the main intermediate energy level of Tm3+, which would be likely a reservoir for the first excitation step. These comprehensive spectroscopic studies have been carried out in order to develop rate equations and gain analysis for prospective UC lasers from different host media. The prior literature results of upconversion lasers have been gathered for comparison with herein data and some general guidelines have been pointed out towards the future work. High resolution, absolute excited-state absorption (ESA) spectra, at room temperature,for the long-lived thulium and neodymium metastable levels (the 3F4 and 4F3/2 manifolds, respectively) were measured using a bespoke purpose built spectrophotometer based upon diodes and a dual lock-in amplifier technique. The aim of that investigation was to determine the strength of ESA channels at wavelengths addressable by commercially available SDLs operating around 630-680 nm and 440-470 nm wavelength. From those measurements, for Tm3+, the effective stimulated emission cross-sections were derived and used in the modelling of the potential gain for the UC transitions (wavelength below 500 nm) in a variety of hosts, including Y3Al5O12, YAlO3, LiYF4 and KY(WO4)2. Waveguides ordered for this project were not delivered. Notwithstanding, preparation of SDL pump sources for the double-excitation method, and the necessary optical components, was undertaken. With the gain studies, and initial hardware preparations, the ground has been laid that will enable the demonstration of new class of UV or purple-blue UC laser architecture in the near future.