An apparatus for processing a substrate is provided. A processing chamber is provided. A substrate support is within the processing chamber. A gas inlet provides a process gas into the processing chamber. A gas source provides the process gas to the gas inlet. An exhaust pump pumps gas from the processing chamber. A parameter measurement system comprises a cavity ring down device in fluid communication with the processing chamber, comprising a first cavity ring down mirror on a first side of the cavity ring down device and a second cavity ring down mirror on a second side of the cavity ring down device spaced apart from the first cavity ring down mirror. At least one laser light source is optically coupled to the first cavity ring down mirror. A light detector is optically coupled to either the first cavity ring down mirror or the second cavity ring down mirror.