Glossary entry

Linewidth

The spectral width of an optical source, reported as the FWHM of the intensity spectrum. For lasers, typically Hz or MHz.

Laser linewidth quantifies spectral purity — the frequency span over which the optical power spectrum drops to half maximum.

The Schawlow–Townes limit gives the theoretical minimum linewidth for an idealized laser:

\Delta\nu_\text{ST} \;=\; \frac{\pi \, h \nu \, (\Delta\nu_\text{cavity})^2}{P_\text{out}},

where $\Delta\nu_\text{cavity}$ is the cold-cavity linewidth. Real semiconductor lasers exceed this by the linewidth enhancement factor:

\Delta\nu \;=\; (1 + \alpha^2) \, \Delta\nu_\text{ST}.

Typical $\alpha = 2$ – $5$ for InGaAsP/InP MQW devices broadens the Schawlow–Townes value by a factor of 5–26.

Below a technical noise floor (thermal drift, mechanical vibration, current noise, carrier fluctuation), linewidth is dominated by environmental effects rather than quantum noise. Sub-kHz stabilized lasers achieve this only with active feedback to a reference (atomic transition, ultra-stable cavity).

Typical values:

Source	Linewidth (FWHM)
Multimode Fabry–Pérot laser diode	100 GHz – 1 THz
Single-mode DFB telecom laser	1 – 10 MHz
External cavity diode laser (ECDL)	100 kHz – 1 MHz
Narrow-linewidth fiber laser	1 – 10 kHz
Stabilized HeNe	1 kHz – 1 MHz
Cavity-stabilized optical clock laser	$<$ 1 Hz

OSA-based linewidth measurement is limited by the resolution bandwidth: typical RBW of 0.01–0.1 nm corresponds to 1–10 GHz at 1550 nm. Sub-RBW linewidth measurement requires beat-note techniques (with a second reference laser) or delayed self-heterodyne with a fiber delay of length $\geq c / (2 \pi \Delta\nu)$ for self-coherence breakdown.

Linewidth matters for: coherent optical communication (carrier phase recovery), interferometric sensing, atomic spectroscopy, and frequency metrology.