An injector provides optical access into a process chamber along an axial path from a diagnostic end point outside the process chamber through an optical access window. A hollow housing body receives first and second process gases, and surrounds the axial path. A sleeve in the body is urged against the body to minimize particle generation, and defines a first gas bore injecting the first process gas into the process chamber. A second gas bore of the sleeve surrounds the axial path for injecting the second process gas into the process chamber, allowing an optical signal to have a desired signal-to-noise ratio (SNR) at the end point. Methods provide a septum in the second bore dividing the second bore into apertures configured to reduce etching of and deposition on the optical access window and to maintain the desired SNR at the diagnostic end point.