Development of sub-100 fs Yb:CALGO lasers with high peak power

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Reza, Mohammad Anisur Rahman
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High power laser sources of ultrashort pulses (~fs, 10-15 s, femtosecond) have numerous applications in science, medicine, and technology such as spectroscopy, optical coherence tomography, microscopy, laser micromachining and processing of materials (e.g., all smartphone glass screens are cut with ultrashort laser pulses). The generation of ultrashort laser pulses requires a laser gain medium to have a broad emission spectrum. Generation of ultrashort pulses with high average power, on the other hand, also requires the gain medium to have good thermal conductivity. These two requirements (i.e., emission spectrum vs thermal conductivity), unfortunately, are contradictory to each other. Broad emission spectra (>30 nm) are generally observed from materials with disordered structure, i.e., glasses, which have poor thermal conductivity (~1W/m/K). Crystals, on the other hand, have good thermal conductivity (~5-10 W/m/K), but narrow emission spectra (<10 nm). At the same time propagation of intense laser pulses induces strong nonlinear optical effects in laser components (laser crystals, lenses, and even in air, etc.) which all should be minimized to avoid pulse spreading in time. Therefore, generation of ultrashort laser pulses with high average power is not a trivial task not only from the material properties point of view, but also from the way that these pulses are generated and stabilized. A method of generation of ultrashort pulses is called mode locking. Fortunately, a new laser crystal, Yb:CALGO, recently became available which has very good spectroscopic and thermal properties, i.e. 50 nm of bandwidth and 6.9 W/m/K of thermal conductivity. The first stage of the work focused on the continuous-wave (CW) operation of the laser. To enhance the lasing efficiency, the intracavity losses were estimated and made sure that this loss was low. After that, the tuning performance of the laser was evaluated using a 12 mm and 24 mm optimized off-surface axis birefringent filter. Also, Conical refraction (CR) output from the laser was obtained by using a separate conerefringent element (CRE). The rest of the work was focused on the development of ultrashort pulse generation by mode locking method. Applications like nonlinear microscopy and fluorescence imaging require low repetition (1-20 MHz) rate oscillators. But typically, the pulse repetition rate of the conventional laser sources is around 70-100 MHz. The repetition rate can be reduced by a pulse picker or cavity dumper which are complex in design and costly. As the repetition rate is inversely proportional to the cavity length, the alternative simple way is to increase the cavity length which is known as the extended-cavity method. Two experiments were performed in the mode-locked regime by using a 1:1 imaging telescope and a White cell configuration to lower the repetition rate.
Solid-state laser, Yb:CALGO, Mode locking, Ultrafast laser