Abstract: A micro-fluidic reactor may comprise a photosensitive glass substrate with a plurality of features produced by etching. The features may include micro-channels, micro-lenses, and slots for receiving optical fibers. During operation of the micro-fluidic reactor, the optical fibers may transmit optical signals for measuring characteristics of fluid reagents and reactions taking place. The micro-lenses may focus optical signals from the optical fibers to create an approximately collimated optical path for the optical signals, reducing optical spread and enhancing fiber-to-fiber optical power coupling.