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It is prohibitively expensive to deposit customized dielectric coatings onindividual optics. One solution is to batch-coat many optics with extradielectric layers, then remove layers from individual optics as needed. Here wepresent a low-cost, single-step, monitored wet etch technique for reliablyremoving (or partially removing) individual SiO$_2$ and Ta$_2$O$_5$ dielectriclayers, in this case from a high-reflectivity fiber mirror. By immersing inacid and monitoring off-band reflected light, we show it is straightforward toiteratively (or continuously) remove six bilayers. At each stage, wecharacterize the coating performance with a Fabry-P\'{e}rot cavity, observingthe expected stepwise decrease in finesse from 92,000$\pm$3,000 to3,950$\pm$50, finding no evidence of added optical losses. The etch alsoremoves the fiber's sidewall coating after a single bilayer, and, after sixbilayers, confines the remaining coating to a $\sim$50-$\mu$m-diameter pedestalat the center of the fiber tip. Vapor etching above the solution produces atapered "pool cue" cladding profile, reducing the fiber diameter (nominally 125$\mu$m) to $\sim$100 $\mu$m at an angle of $\sim$0.3$^\circ$ near the tip.Finally, we note that the data generated by this technique provides a sensitiveestimate of the layers' optical depths. This technique could be readily adaptedto free-space optics and other coatings.

Monitored wet-etch removal of individual dielectric layers from high-finesse Bragg mirrors