Temperature dependent measurement of spectroscopic properties in cr:Colquiriites and development of diode pumped Cr:LiCAF, Yb:YLF and Yb:LLF laser systems

The general framework of this thesis includes the characterization of the temperature dependent spectroscopic properties and the demonstration of continuous-wave, passively Q-switched and mode-locked regimes with various transition metal and rare earth element ion doped solid-state gain media under laser diode pumping. In various experimental studies under this scope, transition metal ion (Cr+3) doped Cr:LiCAF, Cr:LiSAF, Cr:LiSGaF and rare earth element ion (Yb+3) doped Yb:YLF and Yb:LLF gain media were used. In order to design successful laser/amplifier systems, to ensure their practical installation, efficient operation and to predict the durability of the gain media under the expected operating conditions, it is necessary to know in detail the optical, mechanical and thermal properties of the gain media, as well as the parametric (e.g. temperature, dopant ion content) dependence of spectroscopic properties such as cross-sections (absorption and emission), small signal gain and fluorescence lifetime. For a given gain medium, such properties are a combination of the active ion properties added to the structure and the intrinsic properties of the host material. In this context, the first aim of this thesis is the experimental investigation of temperature dependence of the fluorescence lifetime and emission cross section of three different Cr:Colquiriite (named after the Colquiri tin mine in Bolivia where it was discovered) gain media (Cr:LiCAF, Cr:LiSAF, Cr:LiSGaF). In the realization of this objective, temperature dependent variation of fluorescence lifetime (at 1 K intervals) and emission cross section (at 5 K intervals) of the given Cr:Colquiriite gain media are measured at cryogenic (down to 77 K with liquid nitrogen cooling) and above room temperatures (up to 618 K) and are reported in detail. Analytical descriptions of the fluorescence lifetime, radiative lifetime, quantum efficiency and the variation of the intensity, peak wavelength, and width of the emission in the E//c and E//a axes over the relevant temperature range are given with the relevant equations and fit coefficients. The results reported in this context provide a solid basis for the modeling of Cr:Colquiriite laser systems operating in different regimes (such as amplification, power scaling, passive Q-switching, and mode-locking with low timing-jitter) and for their practical implementation, especially for their potential at cryogenic temperatures. In order to demonstrate laser operation in different regimes with diode-pumped Cr:Colquiriites, a Cr:LiCAF laser passively Q-switched by an AlGaAs-based saturable absorber mirror (SESAM) and pumped by low-cost multimode diodes has been realized. The system is experimentally characterized using four different output couplers and two SBRs with different modulation depths, where the system achieves average output powers up to 125 mW at repetition frequencies between 18-40 kHz, with pulse widths in the range of 1.5-4 µs. The experimental results are compared with the basic Q-switching theory and further performance improvements are emphasized in terms of pulse length reduction and peak power scaling. In the last part of the thesis, studies on (i) power scaling in continuous wave operation, (ii) wavelength tuning, (iii) passively mode-locked femtosecond pulse generation, (iv) scaling of mode-locking frequency to GHz were carried out with Yb:YLF and Yb:LLF laser systems pumped with single-mode and multi-mode laser diodes. In this context, femtosecond pulses at GHz repetition frequency have been achieved for the first time with Yb:YLF by SESAM mode-locking in a compact, single-mode diode-pumped cavity. 210 fs long pulses (with a pulse energy of 36.04 pJ) with a repetition frequency of 1.1 GHz and an average power of 40 mW were obtained with the TEM00 profile at laser output. As for the continuous-wave Yb:LLF laser system pumped by a multimode laser diode (960 nm, 10W), experiments using nine different output couplers in the 0.5%-60% range have demonstrated (i) average output power up to 3.2W and slope efficiency of 42.3%, (ii) wavelength-tunable continuous-wave laser output in the 996-1073 nm range using a quartz birefringent filter (BRF). This result is the widest wavelength tunability demonstrated with Yb:LLF compared to the 1029-1069 nm range previously reported in the literature. Mode-locking experiments with SESAM in multimode diode-pumped Yb:LLF systems demonstrated 337 fs pulses at a repetition frequency of 200 MHz with an average output power of 2.5 W. In the following experiments, by adjusting the intra-cavity dispersion and cavity setups of different lengths, (i) 181 fs pulses at 257 MHz with 3 W output power, (ii) 110 fs pulses at 214 MHz with 780 mW output power, (iii) 110 fs pulses at 355 MHz with 220 mW output power (similar results were repeated with Yb:YLF) were demonstrated. Finally, pumping of Yb:LLF with with single-mode laser diodes (960 nm, 260 mW) are demonstrated where continuous-wave output powers of up to 80 mW (for single diode pumping) with 52% slope efficiency are reported.