High performance pulsed fiber laser systems for scientific & industrial applications

This thesis reports an investigation of the power scaling of pulsed fiber laser systems towards the hundreds of Watts regime whilst keeping the impact of fiber nonlinearities such as Stimulated Raman Scattering (SRS) at a manageable level. Two regimes of pulsed operation are investigated: the nanosecond pulse regime and the picosecond pulse regime. Some of the work reported in this thesis was carried out in collaboration with SPI Lasers and Institute for Manufacturing, University of Cambridge under the TSB funded SMART LASER programme. In the nanosecond regime, two kinds of MOPA configurations are investigated. In the first instance a high accuracy active pulse shaping technique is implemented. Using the combination of a fast electrical Arbitrary Waveform Generator (AWG) and an Electro-Optic Modulator (EOM), optical pulses can be shaped into various custom defined pulse shapes with high temporal resolution feature definition, allowing faster pulse rise and fall times than previously possible. This MOPA has the capability to generate a maximum average output power of ~70 W, pulse energy close to 1 mJ, all within a diffraction limited output beam. The second instance a fully-fiberized system capable of producing up to 45 W of average output power with a pulse energy of ~1 mJ was developed in collaboration with SPI Lasers Ltd. Unlike the first system, which uses an EOM for optical pulse shaping, an Acousto-Optic Modulator (AOM) is instead used to pre-shape the leading edge of the optical seed pulse in order to reduce the impact of nonlinear effects caused by the high peak powers otherwise associated with gain-saturation assisted reshaping of long nanosecond square input pulses, providing a cost-effective solution for the SMART Laser system. The successful development of the SMART Laser system allowed SPI Lasers Ltd to introduce a new product line, namely the G4 pulsed fiber laser system. Both types of fiber laser system were used in material processing experiments to investigate their performance and capabilities. Using a nanosecond fiber MOPA as a pump source, a synchronously pumped, tuneable, Raman fiber laser is demonstrated both in the near infrared (NIR) and visible regions. A continuous tuning range of 28nm in the NIR and 2.8nm in the visible region are achieved with efficiencies in the range of 12% to 18% respectively. The conversion efficiency can be increased further with the use of a feedback signal. Furthermore, with the presence of a feedback signal, the linewidth of the Raman Stokes lines in both visible and NIR regions shows a significant narrowing effect. This technique will allow the generation of wavelengths which are not easily generated with rare-earth doped fiber lasers and will be useful in the fields of spectroscopy, archaeology, biomedical and many more. Next, optical pulses in the picosecond regime are investigated. A gain-switching technique is used to generate a stable train of picosecond optical pulses from a semiconductor laser diode (SLD). Gain switching of different types of commercially available SLDs shows different temporal and spectral characteristics which are primarily influenced by the design of the specific chip used. The shortest pulse durations achieved through direct gain switching resulted in ~50 ps pulses; however these were far from transform-limited. However, an external FBG seeded gain switched SLDs was shown to be capable of producing transform-limited optical pulses. I show that a mode-locking mechanism is responsible for the short, transform limited optical pulses observed. This is the first demonstration of a mode locked SLD at the 1.06 μm waveband. With this technique, 18 ps optical pulses with pulse energy of 7.2 pJ and peak power of 400 mW were obtained. The single polarization, stable, picosecond optical pulses were fed into a chain of polarization maintaining fiber amplifiers to investigate the power scaling capability of this system. A maximum average output power of 513 W is demonstrated in a diffraction-limited output beam. The system operated at a repetition frequency of 215 MHz, corresponding to an estimated pulse energy of 2.4 μJ and a peak power of ~ 69 kW. At the maximum operating output power, the OSNR is measured to be well above 26 dB with a polarization extinction ratio (PER) of 17 dB. A pulse energy of 3.23 μJ is achieved from a similar system at a reduced operating frequency of 53 MHz and an average optical power of 200 W, corresponding to a pulse peak power of 107 kW. In both cases, further power scaling is limited by the SRS. These results represent the highest optical power demonstrated from a fiber MOPA producing tens of picosecond optical pulses.