50W, 30ns all fiber ytterbium laser Yigit Ozan Aydin 1 , Koray Eken 1 , F.Omer Ilday 2 1 FiberLAST Inc., METU Technopark, 06800 Ankara, Turkey 2 Middle East Technical University, Micro and Nanotechnology, 06800 Ankara Turkey 2 Physics Department, Bilkent Üni versitesi, 06800 Ankara, Turkey *aydin@fiberlast.com.tr Fiber lasers have the advantage of high beam quality, high efficiency, small size, and air cooling therefore much interest in the development of high power fiber laser systems have arisen recently in the world [1]. However, apart from research laboratories, using these lasers for the industrial and scientific applications, all-fiber structures which are misalignment free (requiring no optomechanic adjustment) are to be provided. Almost every commercially developed fiber laser with nanosecond pulse duration, that are being used for material processing, are Q-switched systems. Vital parameters in the material processing such as repetition rate, pulse energy and pulse duration are correlated with each other therefore cannot be adjusted independently in the Q- switch mechanism. Pulse energy should be 0.5-1 mJ (for high beam quality) and as the repetition rate determines the processing speed, aiming for high average power leads to best results [2]. In nanosecond pulse duration all-fiber lasers, average powers up to 150 W have been reported [3], however it is the peak power (<1 kW) and pulse energy which determines the interaction with the material, and these parameters are not satisfactory in these high power delivering systems. All- fiber lasers with high pulse energy (1mJ) and high peak power (0,5 MW) were also developed [4], however in these systems which are working under 10-20 Hz repetition frequency, considering the excited level lifetime of Yb is approximately 1 ms, probably most of the energy is delivered between pulses as amplified spontaneous emission(ASE). Additionally, energies of these systems are at mW ranges which are not suitable to be used for material processing. Therefore, all-fiber laser systems with high energy, high repetition rate (at high average power), whose parameters are to be adjusted independently and having diffraction limited beam quality are still needed. In this study, Ytterbium doped all-fiber laser with 50 W average power and more than 12 kW peak power at 1 µm wavelength was developed. This master-oscillator power-amplifier (MOPA) architectured system is composed of pulses, produced by electronically pumped diode, and amplified by a series of fiber amplifiers. Apart from Q-switch lasers, MOPA architecture allows us to adjust pulse duration, repetition rate and power independently. Beam quality is diffraction limited, and typically the value is M 2 < 1,2. As far as we know, this is the first commercial all-fiber laser produced under MOPA architecture. Laser Parameters Average Power Beam Quality Repetition Rate Pulse Width Peak Power Up to 50W <1.2 100-500 kHz 40-200ns >12 kW Table 1 : Laser parameters at the output of the system The architecture is summarized in graph 1. Produced by 1064 nm wavelength laser diode, the pulses with the approximate duration of 200 ns or longer are amplified to 1,5W average power by a middle amplifier and then to 50W average power by an amplifier stage. System is all-fiber and the amplified laser output is delivered from the isolated collimator following the 2 m long beam delivery fiber. Due to multi-stage architecture and special precautions, the system works without