A method of manufacturing a cylindrical glass optical waveguide preform having a low water content centerline region, for use in the manufacture of optical waveguide fiber, is disclosed. Two laser-optical sensors are arranged opposite one another above and below the cut package units. This design may provide advantages over the existing technology by miniaturizing the sensing element to provide potentially faster sensor light off times and thereby reduce undesired exhaust gas emissions. The tissue may be irradiated in vivo through the skin or via an incision, for example.