An apparatus may include an upper transparent plate to hold a wafer of bottom-emitting or bottom-detecting optical devices, wherein the upper transparent plate comprises a set of holes in an area of the upper transparent plate for holding the wafer. The apparatus may include a lower transparent plate and a structure supporting the upper transparent plate and the lower transparent plate to form a cavity bounded by the upper transparent plate, the lower transparent plate, and the structure, wherein the structure comprises an opening in fluid communication with the cavity, wherein applying suction through the opening, via the cavity and the set of holes, holds the wafer flat on the upper transparent plate, and wherein an optical path, between a bottom-emitting or bottom-detecting optical device of the bottom-emitting or bottom-detecting optical devices of the wafer and a testing device, passes through the upper transparent plate, the cavity, and the lower transparent plate.