A laser can only oscillate at frequencies where the round trip is a whole number of wavelengths, giving a comb of modes spaced by the free spectral range. Any mode that falls under the gain curve and above threshold can lase, so most lasers run on many longitudinal modes at once unless an etalon or short cavity forces one. Locking their phases together is what mode locking does.
Scanning Fabry-Perot shows three longitudinal modes beating, so it's not single frequency no matter what the datasheet says.
Also heard as
- axial mode
Do not mix it up with
- TEM00: Transverse modes such as TEM00 describe the beam's cross-section; longitudinal modes describe its frequency comb.
Related terms
Free spectral range
The frequency spacing between neighboring resonances of a laser cavity, etalon or interferometer.
Mode locking
Fixing the phases of many longitudinal modes so they add up into a train of ultrashort pulses.
Mode hop
A sudden jump of a laser's output frequency from one cavity mode to a neighboring one.
Linewidth
The spectral width of a single-frequency laser's output, usually given in Hz, kHz or MHz at half maximum.
Etalon
A pair of parallel reflecting surfaces that transmits a comb of sharp frequency peaks, used to select or measure wavelengths.