Alexandrite ve Cr:LiSAF kazanç ortamı tabanlı yakın-kızılaltı femtosaniye lazer kaynakları

Alexandrite (Cr3+:BeAl2O4) ve Cr:LiSAF (Cr3+:LiSrAlF6 ) are promising near-infrared gain media. They have tuning ranges extending from 700 nm to 820 nm and from 780 nm to 1110 nm, broad enough to support sub-10 fs pulse generation. They can be directly pumped with low-cost red diodes. Furthermore, since the emision cross section-fluorescence lifetime products of these media are greater than that of Ti:sapphire (Ti3+:Al2O3), they have low laser thresholds. Due to their high quantum efficiency and high figure of merit, these gain media can be utilised as low-cost, robust, compact and efficient alternatives to Ti:sapphire lasers near 800 nm. In particular, Alexandrite has superior thermo-mechanical properties which make it suitable for the development of high power laser applications. Cr:LiSAF has low passive losses, making it advantageous in the development of ultralow-jitter sources. However, the main drawback of these media is their low Kerr-nonlinearity. In this thesis, we present experiments aimed at the investigation of various emerging mode locking techniques in order to generate ultrashort pulses from Cr:LiSAF and Alexandrite lasers. The first part of the thesis focuses on the use of a gain matched output coupler (GMOC) for robust and efficient Kerr-lens mode locking (KLM) of Cr:LiSAF lasers. As a pump source, a tapered diode (TD) laser with a power of 1W was utilized at 685 nm. In the experiments, it was shown that GMOC provided a significant improvement in the efficiency and robustness of the KLM operation by reducing the gain-filtering effect. The tapered diode pumped Cr:LiSAF oscillator generated pulses as short as 14.5 fs with 106 mW of average output power and 60kW of peak power at an incident pump power of 940 mW. A femtosecond tuning range extending from 807 nm to 919 nm was also achieved with sub-50-fs long pulses. In the second part of this thesis, we present the femtosecond pulse generation experiments from a multipass-cavity Alexandrite laser. First, to the best of our knowledge, we demonstrated the generation of the shortest femtosecond pulses reported to date from a Kerr-lens mode-locked Alexandrite laser operating near 750 nm. A continuous-wave, 532-nm laser was used as a pump source and the performance of both the short and extended resonators was investigated. We increased the optical path length of the short-cavity Alexandrite resonator with a multipass-cavity extension to scale up the pulse energy and to eliminate the multi-wavelength spectral instabilities observed during continuous-wave operation. The resulting increase in the intracavity pulse energy provided enhanced Kerr-nonlinearity and eliminated the Q-switching instabilities during mode-locked operation. The Kerr-lens mode-locked MPC Alexandrite laser produced nearly transform-limited, 70-fs pulses at a pulse repetition rate of 5.6 MHz with only 1W of pump power. The time-bandwidth product was further measured to be 0.331. We then present the graphene mode-locked operation of a femtosecond Alexandrite laser at 750 nm. We adopted a resonator configuration similar to what was previously used for Kerr-lens mode locking experiments. We used a monolayer graphene saturable absorber to obtain mode locking with a pump power of 5W at 532 nm. The resonator generated nearly transform-limited, 65-fs pulses with a time-bandwidth product of 0.319 at a pulse repetition rate of 5.56 MHz. During mode-locked operation of the cavity, the output power was around 8 mW, corresponding to a pulse energy and peak power of 1.4 nJ and 22 kW. These experiments further show that graphene can be used to initiate mode locking at wavelengths as low as 750 nm. We believe that the results described in this thesis will pave the way for the development of low cost and efficient alternative femtosecond laser sources with a wide tunability around 800 nm, a wavelength range which is important for various scientific and technological applications.