High power all-fiber Q-switched and Mode-locked fiber lasers for industrial and medical applications

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High power pulsed fiber lasers have recently become a topic of intensive research due to their high energy and high peak power, which are essential in many applications such as material processing, laser range finding, and second harmonic generation. Among many technologies for pulsed lasers, passively Q-switched and mode-locked fiber lasers are especially interesting because of their simple structure, easy maintenance, and low cost. Picosecond and femtosecond lasers with a pulse energy of several tenths of nJ and peak power at the kW level have been demonstrated. However, despite the progress made so far, many of the reported lasers may not be suitable for industrial and medical applications due to the structure and components used in their designs. For example, some Q-switched lasers use lenses and dichroic mirrors to couple light into and out of the fiber and saturable absorbers; which diminish the advantages of the fiber laser. Some mode-locked lasers use polarization controllers to adjust polarization, which is clearly not suitable for field applications. Many pulsed lasers employ semiconductor saturable absorbers (SESAM) or carbon nanotube absorbers, which are susceptible to high energy pulses and whose long-term stability remains an issue. These issues need to be resolved in order to develop high energy and high peak power pulsed fiber lasers, particularly those of stable, compact, and low-cost modules for industrial and medical applications. Because of this, I propose to develop high power all-fiber Q-switched and mode-locked lasers for industrial and medical applications over the next five years. I will concentrate my research on all-fiber designs so that all components will be spliced together with no optical alignment needed; this will make the laser compact, stable, and low-cost. My research will have two directions: 1. Q-switched laser: the goal is to achieve all-fiber Q-switched oscillators with over 200-µJ pulse energy and over kW-level peak power at variable repetition rates. The oscillator can be used to cut metal sheets by itself, or used as a seed laser to achieve 20- to 30-kW peak power with a single-stage amplifier. Fibers with different core diameters will be used for the gain medium and saturable absorber in the laser cavity. The optimization of the laser cavity structure, fiber core diameters of gain and saturable absorber, their lengths, and other design parameters will be studied. The dynamics of Q-switching will be simulated. 2. Mode-locked laser: the goal is to achieve stable output pulses of 10- to 30-ps pulse width and over 5-nJ pulse energy in linearly-polarized state. When this laser is used as a seeder, a one-stage amplifier will bring its peak power to multiple kW. One approach is to use a SESAM for mode-locking, in which an all-fiber coupling device will be designed to focus the beam onto the SESAM with a proper power density for stable mode-locking. Another approach is to use nonlinear polarization rotation as an equivalent saturable absorber so the power density will not be limited by the lower damage threshold of the saturable absorbers. The Q-switched laser with sub-mJ pulse energy and kW-level peak power can be used for precision cutting and drilling. The Q-switched laser with long pulses can also be used for welding. The mode-locked ps laser, due to its fast energy deposition, will significantly reduce the heat-affected zone in material processing, which will make it an ideal tool for precise micromachining of metals and semiconductors. It will also have many interesting applications in medicine.