Ultrafast Dual-gain-media Neodymium:glass Laser
This dissertation presents an experimental demonstration of an ultrafast dual-gain-media Nd:glass laser. This laser contains both the Nd:fluorophosphate glass (Schott glass LG-810) and the Nd:silicate glass (Schott glass LG-680) in a single cavity. Record-short pulses of 34 fs were generated by Kerr-lens mode-locking from the dual-gain-media Nd:glass laser as a result of the overall gain broadening. Compared with the shortest pulses generated from a single-medium Nd:glass laser, the pulse was shortened by nearly a factor of 2. Neodymium-doped glass is characterized by inhomogeneous broadening and multiple Stark-split sub-transitions. This dissertation presents a comprehensive study of passive mode-locking of inhomogeneously broadened lasers. For a broadband inhomogeneously broadened laser, mode-locking becomes unstable when the lasing spectrum exceeds the maximum lockable spectrum. This mode-locking instability due to the insufficient gain filtering does not appear in homogeneously broadened lasers. The mode-locking by the pure self-amplitude modulation is essentially a phase-locking process. The soliton mode-locking resists best the impact of insufficient gain filtering, while the mode-locking under a strong self-phase modulation and positive group-delay dispersion is the weakest and a certain amount of gain filtering (narrowing) is necessary. A minimum unsaturated absorber loss is required for stable mode-locking in most situations. When the multiple Stark-split sub-transitions are involved in the laser gain, it generally broadens the overall gain linewidth and narrows the free-running spectrum. Those Stark-split sub-transitions located near the gain peak primarily affect the gain curvature and therefore how strong the gain filtering effect is. The free-running bandwidth is primarily determined by those Stark-split sub-transitions located within the inhomogeneous linewidth. In passive mode-locking, a larger GDD region of stable mode-locking exists as a result of the involvement of multiple Stark-split sub-transitions compared to the single-transition gain. When mode-locking strength is strong or GDD is small, the gain including the multiple Stark-split sub-transition can be approximated by a single-transition gain of which the homogeneous linewidth is equal to the effective homogeneous linewidth of the Stark-splitting gain.
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2 PASSIVE MODELOCKING THEORY FOR BROADBAND
3 EXPERIMENTAL STUDY OF PASSIVELY MODELOCKED
abs,ave broadband CW lasing bandwidth CW lasing spectrum degree of inhomogeneity dispersion compensation dual-gain-media Nd:glass laser effective homogeneous cross-section emission cross-section F2 prism fast saturable absorber femtosecond free-running gain medium gain model gain profile GDD region homogeneous linewidth inhomogeneous gain inhomogeneously broadened laser insufficient gain filtering Kerr-lens mode-locking locking master equation mode-locked dual-gain-media Nd:glass mode-locked pulse mode-locking characteristics mode-locking instability mode-locking mechanism mode-locking regime multiple Stark-split sub-transitions narrower Nd:fluorophosphate glass laser Nd:silicate glass laser negative GDD Output coupler passive mode-locking peak positive GDD prism pair pulse bandwidth pulse shaping pulse spectrum pulse width pump power pure SAM mode-locking saturable absorber Schott Glass self-phase modulation SF10 prisms shorter pulses shown in Figure single peak-sub-transition single-medium Nd:glass laser solid-state lasers soliton mode-locking soliton pulse soliton-like pulse SPM and positive stability zone stable mode-locking Stark-splitting gain sub-levels tuning range ultrashort pulses unsaturated absorber loss