The full width at half maximum of the optical spectrum of a laser running on one longitudinal mode, set by phase noise from spontaneous emission plus technical noise such as vibration and current noise. Free-running DFB diodes sit around 1 MHz, external-cavity diodes near 100 kHz, DFB fiber lasers a few kHz, and lasers locked to high-finesse cavities reach the Hz level. Linewidth sets coherence length and limits sensing, spectroscopy and coherent communication.
Delayed self-heterodyne says 3 kHz linewidth, good enough for the 50 km sensing run without a reference cavity.
Also heard as
- laser linewidth
- spectral linewidth
Do not mix it up with
- Transform-limited: For ultrafast pulses the relevant width is the broad spectral bandwidth of the pulse, not the linewidth of a single-frequency laser.
Related terms
Coherence length
The path difference over which a light source keeps a fixed phase relationship and can still form interference fringes.
Longitudinal mode
One of the discrete frequencies at which a standing wave fits a whole number of half-wavelengths into the resonator.
Mode hop
A sudden jump of a laser's output frequency from one cavity mode to a neighboring one.
SBS
Stimulated Brillouin Scattering: narrow-linewidth light scattering backward off sound waves it creates in fiber.
Finesse
The sharpness of an optical cavity's resonances: free spectral range divided by the resonance linewidth.