Study On Passively Q-switched Fiber Lasers Based On Ho-Doped Zblan &Tm-Doped Silica Fiber

In recent years, mid-infrared pulsed laser sources operating around 2 μm and 3 μm have attracted large amount of attention because of their wide applications in defence, medical, scientific research etc. On the other hand, compared to conventional bulk lasers, the optical fiber based lasers have exhibited a series of merits e.g., compactness, robustness, high flexibility, excellent heat dissipation, great beam quality and convenient intergration. In this dissertation, our study mainly fouced on Q-switched fiber lasers pulsing around 2 μm and 3 μm as a result of the combination of above two research highlights and can be divided into following four sections:(1) A comprehensive numerical model mainly involving rate equations, power evolution equations and heat conduction equations capable of describing the cascaded transitions of Ho3+-doped ZBLAN fiber laser was constructed. A set of significant parameters including gain fiber length, dopant concentration, output coupling, pump direction and pump wavelength were optimized in terms of the slope efficiencies, thresholds and thermal dissipation. Furthermore, the effects of ETU and ESA on the performance of cascaded stystem were also investigated.(2) Based on the diluently Ho3+-doped ZBLAN fiber, the ASE operation under both co- and counter-propagating pumped arrangements were demonstrated. In contrast to the co-propagating pumping, counter-propagating pumped scheme can operate at a higher slope efficiency and broadest spectrum coverage of ~130 nm and maximum ASE output power of 219.76 mW were achieved. Moreover, the cascasde laser output performances of Ho3+-doped ZBLAN fiber laser were also investigated.(3) Employing a reverse design mid-IR SESAM as the saturable absorber, the Q-switching performance at 3 μm range was investigated. Firstly, the passively Q-swithced fiber laser with longest wavelength of 2971 nm was demonstrated. Moreover, the passively switched dual wavelength pulsed fiber laser operating at mid-infared region was also realized, the achieved maximum energy of ~3 μm Q-switched pulses and ~2.1 μm gain switched pulses were 7.47μJ and 3.44 μJ, respectively which was the highest level from passively Q-switched ~3 μm fiber laser.(4) Using the Fe2+:ZnSe and Cr2+:ZnSe crystal as the saturable absorber, the influences of crystal initial transmission on the Q-switching performances at ~3 μm and ~2 μm ranges respectively were investigated. By optimizing the initial transmission, passively Q-switched Ho3+-doped ZBLAN fiber laser centerred at ~2958 nm yleiding pulse duration and energy of 1.36 μs and 5.12 μJ, and Tm3+-doped silica fiber laser centered at ~1985 nm delivering pulse duration and average power of 160 ns and 1.32 W also the highest level from ~2 μm Cr2+:ZnSe-based Q-switched fiber laser were demonstrated.