Compact femtosecond chirped pulse amplification system based on thulium doped fibre

Abstract

High power and high energy ultra-short-pulse fibre lasers at 2 µm are an attractive option for use in many applications including free space optical communications, sensing, material processing and mid-IR generation. High energy applications in the 2 µm range in particular, such as for high harmonic generation and laser-driven electrons acceleration, would benefit from repetition-rate scaling. In this thesis, I focus on developing systems with conventional, flexible, fibre technology as this is an important research target that will have immediate practical application, including in material processing, precision surgery and mid-IR light generation. This thesis describes a systematic study of the development of 2 µm thulium-doped fibre laser systems based on chirped pulse amplification (CPA). It includes the development of the seed source, and the exploration of two types of amplification chains for germanate and silica thulium-doped-fibre, as these greatly increase their output power, compactness and potential for practical applications.
A carbon nanotube (CNT) based dissipative soliton mode-locked fibre laser which emitted a central wavelength of around 1960 nm with a bandwidth of 18 nm was created. Unfortunately, after a prolonged operation, the CNT deteriorated to the point that it could no longer produce a stable mode-lock pulse. To replace this damaged carbon nanotube, an artificial saturable absorber based on a nonlinear polarisation rotation mechanism was used. A conventional soliton which pulsed at 1942 nm and had a spectrum that consisted of strong Kelly sidebands was also achieved, but it was deemed unsuitable as a seed for the intended CPA system. In addition, it was difficult for the dispersion fibre managed cavity to achieve mode-locking through nonlinear polarisation rotation. A commercial SESAM was then used but the oscillator generated constant noise-like pulses lasting a nanosecond each. These were not suitable to seed a CPA system and I were therefore forced to explore alternative solutions. Finally, using a new SESAM, a dissipative soliton mode-locked fibre laser at 1925 nm was investigated. The oscillator could generate self-started mode-locked pulses at a pulse energy of 2.5 nJ with a bandwidth of 40 nm at a 15.6 MHz repetition rate. Furthermore, the chirped pulses with a duration of 25 ps were able to be compressed to 240 fs. In summary, this mode-locked fibre laser was an ideal seed source and qualified for amplification in a future CPA system. In the CPA part, both commercial silica and in-house fabricated germanate thulium doped fibre (TDF) were investigated for both the cladding and the core pumping scheme. For the TDF silica version, cladding pumping a 1.5 m TDF produced 297 fs compressed pulses with an average power of 21.5 W and a peak power of 4.2 MW. Core pumping a 29 cm TDF meanwhile, produced 285 fs pulses with an average power of 2.8 W and a peak power of 20 MW at a repetition rate of 0.39 MHz. Shortening the fibre length and increasing the mode area are two common methods to minimise the nonlinearity accumulation in the gain fibre. However, for simple fibres, the TDF silica used in the amplifier had the largest mode area and a higher doping concentration than commercial silica. Therefore, there was not enough room to scale pulse peak power for these types of fibres.
To overcome this limitation, two versions of the high doping TDF were developed, both of which were able to shorten the fibre length. The first investigation was the 1st generation of the thulium doped germanate fibre (TDGF1) with a lower doping concentration (3×1020 ions/cm3). Cladding pumping of a 65 cm length of TDGF1 produced pulses with an average power of 14.1 W and a peak power of 2.55 MW. Core pumping a 19 cm TDGF1 meanwhile, produced pulses with an average power of 2.3 W and a peak power of 17 MW at the output of the CPA system. In terms of peak/average power, the TDGF1 version had comparable results with the silica fibre version. To achieve higher pulse energy/peak power therefore, the 2nd generation thulium doped germanate fibre (TDGF2) with a higher doping (8.5×1020 ions/cm3) was developed to shorten the amplifier length. Core pumping of a 9.5 cm length of TDGF produced pulses with an average power of 1.9 W and a peak power of 42 MW, which, in terms of peak power, beat the silica fibre’s results. This confirms that the highly doped Tm3+ germanate glass single mode fibre can work as an alternative to silica-based fibre for the generation of 2 μm high energy/peak power pulses. This thesis is not just limited to the generation of 2 μm high energy/peak power pulses through thulium doped fibre master oscillator power amplifier (MOPA) systems. In the final part, it also covers an exploration for an interesting oscillator with ultra-high energy/peak power output pulses in the germanate fibre oscillators.

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Identifiers

ORCID for Zhengqi Ren: orcid.org/0000-0003-1877-893X

ORCID for David Richardson: orcid.org/0000-0002-7751-1058

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