Tolerant low-loss three-lens coupling system for 1.48-μm unstable-cavity lasers

A 1.48-μm unstable-cavity laser is coupled into single-mode fiber using three microlenses. Reproducible coupling of very high power is demonstrated with different types of lenses (plano-convex or biconvex, with different apertures). Over 550 mW in single-mode fiber (SMF) were reproducibly reached; to our knowledge, it is the highest power coupled into an SMF from a single semiconductor laser at this wavelength. Tolerance measurements on all of the coupling elements of a three-lens system are reported for the first time; an unexpected very large tolerance an the axial displacement of the second lens was measured. Results and interpretation with the aid of Gaussian and aberration simulations are also presented.