Spectrometry is the deconstruction of a light wave into its constituent colors. This class of tools range from simple prisms, to space telescopes. This particular spectrometer shines light through a grating and detects an interference pattern from path difference. This will separate the light based on wavelength.
Lens-Grating-Lens is a spectrometer configuration that collimates light from a slit or point source, sends that column of light through a holographic grating, then focuses the output on the grating onto a the detector. By illuminating as many lines as possible, and setting the second focal length very far, this spectrometer can detect the separation between laser harmonics.
Although lasers are accepted as monochromatic sources, the light they emit is spread among several harmonics across a band of visible light. Inside the semiconductor cavity of a laser diode, standing electromagnetic waves reflect internally until they emit out of the laser as in-phase light.