Increasing the effective area Aeff past 200-300 um2 while still maintaining single mode output and low bend loss is not trivial – large core fibers are multimoded unless the NA is very small, and low NA fibers are susceptible to bend-induced mode distortion and loss. For the largest mode areas demonstrated to date, the latter problem is removed by using short, rigid rod-type fibers. Another strategy is to embrace multimode structures and selectively excite a stable, large Aeff high order mode. Ideally, one would like to have an LMA design which provides a Gaussian-like output and retains all the mechanical advantages of fiber in that it can be easily handled, cleaved, spliced, and coiled, and so a common approach is to use few moded designs for which there is significant differential loss or differential gain between the fundamental and higher order modes. All -solid photonic bandgap fiber (AS-PGBF), which guides light based on anti-resonant reflections from discrete high index regions in the cladding, is one such class of fiber which provides large Aeff in the 500-1000 um2 range while still maintaining excellent mode purity. The anti-resonant nature of AS-PBGFs provide some additional benefits: since these fibers transmit only discrete frequency bands of light, they can be used as distributed filters to shape the gain spectrum and, for example, suppress the Yb gain peak at 1030 nm, enabing high power sources at 1150-1200 nm. Kramers-Kronig demands that AS-PBGFs also have strong waveguide dispersion near the transmission band edges, so they can be used to control cavity dispersion or stretch/compress pulses. Several research groups have demonstrated LMA AS-PGBFs in both passive and active configurations, but typically the minimum bend radius Rmin is 20-50 cm, and may require coiling the fiber along a specific orientation.
Kashiwagi and co-authors have now demonstrated a passive LMA AS-PBGF with bend loss < 0.1 dB/m at bend radii below 10 cm, a necessary step towards making this design amenable to compact packaging and thus commercially attractive. The AS-PBGF is based on an array of high index rods with a 7-cell defect core. They draw a few fiber diameters and demonstrate Aeff = 712 um2 with Rmin = 10 cm for their largest fiber and Aeff = 520 um2 with Rmin = 5 cm for their smallest, with an M2 factor of 1.05, all at 1064 nm. The unbent fiber is weakly multimoded, so near diffraction-limited output is achieved only when the fiber is coiled within a relatively small range of diameters. Within this range, however, both simulations and experiment indicate only a minor reduction in Aeff (values quoted above are for the largest coiling radius) and no significant mode distortion. Going to an active design entails a number of additional challenges, specifically with regards to maximizing the gain per length. One would want to dope only the core with Yb while preserving the index profile, and even with fluorine co-doping, this may limit Yb concentration. Furthermore, in a cladding-pumped configuration, the high index rods will trap some of the pump light and reduce the net absorption in the core. More importantly, the authors do not specify what jacketing they use, so it may be that the excellent mode stripping properties of this fiber are not maintained a double clad embodiment. Nonetheless, this work shows AS-PBGFs to be very promising for compact LMA fiber devices.
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