Q-switching in fibre-lasers

One of the most attractive features of fibre lasers is their ability to produce significant pulse powers when Q-switched. Potential applications for such lasers include non-linear switching, OTDR measurements, materials processing and range-finding. We have, already demonstrated 100W:15ns pulses from a diode pumped Nd3+ fibre-laser and have obtained 15W:70ns at 1.56µm with a diode pumped Er3+/Yb3+ system. Pulses of 8W:155ns have been obtained in the visible from a Sm3+ fibre-laser (although this required an Ar+ pump laser). Considerably higher powers than these are predicted for diode-pumped lasers. In particular, theoretical analysis shows that the Er3+ laser is capable of producing pulses in excess of 1kW. To date the performance of the Q-switched fibre laser has been limited by the modulator used to effect the Q-switching. problems involved in Q-switching will be discussed and we will look at ways of overcoming them. Theoretical modelling allows us to predict many of the properties required of a modulator and the results of such calculations will be presented. In addition, Q-switched pulses are seen experimentally to be modulated at the round trip frequency - for some applications this effect may be very important and its origin will be discussed