High-power Short-pulsed Hybrid Laser Systems

Ultrafast fiber lasers have found wide application in medicine, biology, and ranging. Their high-power counterparts are vital for high-precision material processing, light frequency conversion, and optical parametric oscillator (OPO) pumping. This thesis concerns the research and development of a high-power ultrafast laser system in a master oscillator power amplifier (MOPA) configuration. It is based on a gain-switched laser diode (GSLD) acting as a seed laser and an active tapered double-clad fiber (T-DCF) operating as the main amplifier.

The constraints for the pulse compression in laser systems seeded by GSLDs were demonstrated both theoretically and experimentally. Guidelines for effective pulse reshaping via the Mamyshev regenerator scheme were developed to bypass the limitations imposed by the use of GSLD with a pulse duration of several tens of picoseconds. 50- and 20-fold compression of 47 ps pulses from GSLD was demonstrated experimentally in the single Mamyshev regenerator scheme based on non-polarization-maintaining (non-PM) fiber and in the double Mamyshev regenerator scheme with PM fiber, respectively.

The active T-DCF operating as an amplifier was applied for power scaling in MOPA system. Two new types of active tapered double-clad fibers were manufactured. The main accent was made on the amplification of polarized radiation. The first type was PM T-DCF and utilized the traditional approach of creating high birefringence in the cladding by adding borosilicate rods. The second type (spun T-DCF) was made to emphasize minimizing the magnitude of birefringence by fast rotation a preform during fiber drawing since it was demonstrated that stress-induced birefringence in PM T-DCF significantly affects the state of polarization under intense pumping. A comparative study of the state of polarization drift in various types of T-DCF has been carried out. Spun T-DCF was found to be less sensitive to the pump-induced heating and better preserves the state of polarization at high power than PM T-DCF. The influence of the geometry of the spun T-DCF on the amplification properties and the quality of the output beam has been shown. Excessive twisting of spun T-DCF leads to mode coupling in the core and deterioration of the mode composition, as well as to pump vignetting and degradation of the amplifying properties. Insufficient twisting, in turn, does not provide effective mixing of cladding modes and also impairs the amplifying properties. It was demonstrated that for a spun T-DCF with a certain geometry the optimal pitch length can be found at which its amplification properties are comparable to those of PM T-DCF while polarization stability is better.