A longitudinal mode spectrum shows the results of laser light separated into components by an optical spectrum analyzer, etc., and expressed in terms of the wavelength vs. light intensity distribution.
Spectrum where many laser emission modes are excited are called multi-longitudinal mode (multi mode) emission, whereas spectrum where only a single laser emission mode is excited are called single-longitudinal mode (single mode) emission.
The wavelength of a laser diode shifts toward the longer wavelength side as the ambient temperature rises. Therefore, for high accuracy spectrum measurement, the temperature of the laser diode must be controlled by a Peltier element, etc., to maintain its constant level.