Scaling of the pulse energy generated from a fiber laser, however, requires the use of fibers with unusually large mode areas. It is not hard to appreciate that even when the numerical aperture of these fibers is kept extremely low, higher order mode content can become a limiting factor at such high power levels. Microstructured holey fibers (or photonic crystal fibers as they are commonly termed) have been used to successfully tackle this problem in the past. In this work, a special all-solid fiber (a chirally-coupled core fiber) has been chosen for easier splicing to a fused pump-signal combiner, which in turn ensures robust operation and a compact footprint. The fiber design acts as a mode filter and gives rise to effectively single-mode operation in the laser. Eventually, pulses with energy of 61 nJ are generated, which could be compressed to yield a staggering 0.5 MW of peak power.
It is exciting to consider the remarkable rise in power that has been achieved at the output of fiber lasers over the past few years. But it is not just the power arguments that make fiber lasers attractive. First of all, the fiber geometry itself makes these sources less sensitive to heat generation than the more traditional bulk lasers and gives rise to extremely high quality beam profiles. Furthermore, fiber gain media provide high values of gain and broad gain linewidths, which are necessary for ultrashort pulse generation. Additionally, pump-to-signal conversion efficiencies in fiber lasers are extremely high. All of these factors, fuelled by significant advances in critical fiber technologies, have led to a rapid penetration of fiber laser systems to applications that were previously dominated by other laser types. Results such as those highlighted here demonstrate that the field is still undergoing rapid development and show that there is certainly plenty of scope for further exciting work to be carried out.
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