a spectrometer configured to receive an optical signal through at least one optical channel connected to the plasma processing chamber, and to perform spectral analysis on the optical signal in response to a synchronization signal corresponding to each state of a plurality of states of a multi-level pulse applied to the plasma processing chamber;

an internal sensor configured to detect an envelope of the optical signal; and

a synchronizer configured to generate the synchronization signal corresponding to each state of the plurality of states of the multi-level pulse,

wherein during each state a different process is conducted in the plasma processing chamber,

wherein the synchronizer is configured to generate the synchronization signal according to the detection of the envelope of the internal sensor, and

wherein the internal sensor includes a fast time photo meter.