Long-working-distance expanded-beam fibre microlenses

Single-mode-fibre output couplers for diode lasers have been fabricated to have long working distances. The fibres have (i) tapered cores expanded at one end to 25 mu m diameter by thermal diffusion of the core dopant and (ii) outside diameters etched down to 30-60 mu m. Hemispherical lenses were made by melting the ends. The resulting working distances are 35-40 mu m, and coupling losses, as low as 3 dB, are mostly between 3.5 and 5.5 dB.