Mode-locked fibre lasers are stable, compact, and are capable of producing ultra-short pulses. Because of these desirable salient characteristics, the range of fibre laser
applications are on the rise, and so too is the need for improved performance . We present here a new design for mode-locked fibre laser operating in the all-normal
dispersion regime. Previously we have presented a mode-locked fibre laser design that contains two segments; the main loop cavity that is uni-directional and a
bi-directional nonlinear amplifying loop mirror (NALM) that acts as the mode-locking mechanism of the laser. Here we demonstrate that incorporating a third
Yb-doped fibre third gain medium inside the main cavity significantly improved both the output power and the ease of mode-locking. Remarkable both the
additional gain segment and the NALM can be pumped by the same laser diode making this design more efficient than the previous one.
Previous endeavours have shown that with the two gain media architecture, the laser successfully mode-locks at repetition rates between 500 KHz and 10MHz,
however, this architecture suffered from lack of pulse energy, hence requiring additional amplifiers before they could be used in practical applications. The incorporation
of a third gain medium has proved useful in improving the output power and bandwidth, and because the new design utilises the same number of components (which
are all polarizing-maintaining), we also preserve costs.
Operating at 1030nm and with a repetition rate of 5.6MHz, we characterised and compared the laser before and after the insertion of the third gain media. We saw an
improvement of average output power from 4mW to 9mW of power corresponding to 1.6 nJ of pulse energy. The spectral bandwidth is about 17 nm and upon taking a
FROG trace, we find a pulse width of 16ps with a linear chirp. The linear chirp of the pulse allows us to compress the pulse using a single transmission grating (1600 lines/mm). The prism-grating compressor has an efficiency of 70%, and produced a compressed pulse duration of 291fs with an output pulse energy of 1.1nJ.
The autocorrelation shows that the bulk of the energy is contained in the centre of the peak and that the side-lobes, although noticeable, hold little energy.
It has been observed that the laser mode-locks and single pulses more easily with a third gain medium. We attribute this to the bi-directional pumping of the gain medium in
the NALM which produces a more uniform inversion profile. In conclusion the incorporation of a third gain medium has shown favourable results; increased output pulse energy, reliability in self-starting, ease of mode-locking and output pulses exhibiting linear chirp which allows for compression. Said features are largely attributed to the interactions of the third gain medium with the NALM to create uniform gain. All the while, utilising the same number of components and preserving costs. Further work will be done to test and optimise the laser architecture which will be tailored towards the purposes of micro-machining.
 M. E. Fermann and I. Hartl, “Ultrafast fiber laser technology,” IEEE Journal of Selected Topics in Quantum Electronics, vol. 15, no. 1, pp. 191–206, 2009.
 C. Aguergaray, N. G. R. Broderick, M. Erkintalo, J. S. Y. Chen, and V. Kruglov, “Mode-locked femtosecond all-normal all-pm yb-doped fiber laser using a nonlinear amplifying loop mirror,” Opt. Express, vol. 20, pp. 10545– 10551, May 2012.