The gain bandwidth of the He-Ne laser is dominated by Doppler broadening rather than pressure broadening due to the low gas pressure and is thus quite narrow: only about 1.5 GHz full width for the 633 nm transition. With cavities having typical lengths of 15 to 50 cm, this allows about 2 to 8 longitudinal modes to oscillate simultaneously (however, single-longitudinal-mode units are available for special applications). The visible output of the red He-Ne laser, long coherence length, and its excellent spatial quality, makes this laser a useful source for holography and as a wavelength reference for spectroscopy. A stabilized He-Ne laser is also one of the benchmark systems for the definition of the meter.