An applied research project that was focused on the research of the picosecond high power hybrid laser was conducted by the Laboratory for photonics and laser systems at Faculty of Mechanical Engineering, University of Ljubljana in cooperation with photonics companies (Fotona d.o.o., Optacore d.o.o and LPKF d.o.o.) and foreign partners.
In the framework of the project, research on highly doped fibers was carried out to reduce the length of fiber amplifiers and increase the diameter of the doped core to alleviate the effect of nonlinear effects, such as self-phase modulation. The production of test fibers was based on the technology developed by Optacore (Improved MCVD method), supported by fiber characterization system developed in collaboration with the UL-FS partner. The developed system allowed for the characterization of active optical fibers with core diameter up to 30 μm. With its help, an evaluation of the thermal effects of photo-darkening on highly doped core pumped amplifiers was made, representing one important project outcome
The research continued on the development of an all fiber pump laser which converts multimode light at 915 nm into single mode at 976 nm. The result of more than 5 W single mode laser output with a measured beam quality of less than M2 <1.3 was achieved. This represents very high brightness and allows for pumping small single-core active fibers without major losses.
For the realization of the project goals, the research was carried out on the optimization and realization of a multi-stage fiber amplifier. It was necessary to investigate the ways of controlling the amplifying chain from the seed diodes, the pulse picker based on the acoustic optical modulator and the synchronization. The results of the research have been reflected in three SCI articles publishing and innovative approach of controlling the pulse train amplitude in the fiber amplifiers, controlling the gain fluctuations in the two stage amplifier when producing pulses on demand, and controlling the amplified spontaneous emission at low repetitions together with the advanced control of the seed diode. Based on these results, a compact picosecond laser with an average power of more than 17 W, with adjustable pulses between 400 fs and 15 ps, and a repetition frequency adjustable between 50 kHz and 1 MHz was developed. The achieved pulses reached energies up to 20 μJ and peak powers up to 10 MW. The so-produced picosecond laser pulses were then coupled in an output amplification stage based on a single crystal in a free-space configuration. The last stage was pumped with pump system developed within the project with optical power of 200 W operating at wavelength of approx. 970 nm, which reduced the thermal load of the crystal and avoided thermal lensing. The maximum average output power from the laser was over 60 W and maximum peak power approx. 100MW. It was limited by currently available pumping power and therefore could be scaled further.
