Very high resolution spectrometer for monitoring of semiconductor processes

An optical instrument for spectroscopically processing an optical signal, comprising:

• an optical interface of a spectrometer that receives an optical fiber;
• a narrow band pass filter that filters a portion of an optical signal corresponding to a predetermined portion of an operational wavelength range of the spectrometer received via the optical fiber;
• a wide band pass filter fixed in a portion of a collimated light path within the spectrometer that filters the optical signal received via the optical fiber corresponding to the operational wavelength range of the spectrometer, wherein the collimated light path includes a collimating mirror, a grating, and a focusing mirror, and the wide pass band filter is fixed between the grating and either the collimating mirror or the focusing mirror to contribute to a desired resolution of the spectrometer by reducing optical background signals, wherein at least one of the narrow band pass filter and the wide band pass filter provide a spectral out-of-band rejection of optical density 6 or greater for the suppression of undesired optical signals;
• optical components that are selectively combined to spectroscopically process at least a portion of the optical signal according to the desired resolution, wherein the optical components include a sensor that receives the optical signal;
• and
• one or more processors that process electrical signals from the sensor.