Bandwidth-controllable tunable Q-switched thulium fibre laser

Sources that offer both high peak power and wavelength flexibility within the two-micron wavelength band are of interest for a number of applications, including spectroscopic characterisation, laser processing of transparent polymers and laser surgery. They also provide a platform for nonlinear frequency conversion to the mid-infrared wavelength band. In this paper, we present a high-peak-power pulsed single-mode fibre source that offers flexibility in operating wavelength as well as spectral bandwidth. The source employs an acousto-optic modulator in combination with a diffraction grating in a novel external feedback cavity arrangement to achieve `Q-switching´ and the desired spectral control. In this configuration the role of the acousto-optic modulator is two-fold: Firstly, it provides the means to time gate the round-trip loss for the fed back first-order diffracted beam to generate Q-switched pulses. Secondly, it has the effect of supressing longitudinal mode build-up by introducing a frequency shift upon on the fed back beam by twice the acousto-optic drive frequency (per round trip). The frequency shift in combination with the relatively small number of round trips needed for efficient energy extraction causes the Q-switched fibre source to behave like a time-gated multi-pass amplifier spontaneous emission (ASE) source. The resulting spectral profile is broadband with bandwidth determined by the laser gain bandwidth and the condition for Bragg diffraction in the AOM, with no evidence of the discontinuous spectral structure typical of conventional fibre laser sources. By introducing a further dispersive element such as diffraction grating it is possible for wavelength selection to be achieved as well as spectral shaping of the emission spectrum through the use slits or apertures.