Glossary entry

Linewidth

The spectral width of a light source, usually the full width at half maximum of its optical power spectrum. For single-frequency lasers it ranges from megahertz for DFB lasers to hertz and below for lasers locked to reference cavities.

Lasers & gainUpdated October 2026

The linewidth of a laser is the width of its optical spectrum, usually given as the full width at half maximum (FWHM) of the power spectrum of one lasing mode, in hertz. A width in frequency converts to one in wavelength as $Δ λ = λ^{2} Δ ν / c$ , so 1 MHz at 1550 nm is 8.0 fm. Telecom DFB lasers have linewidths of 1 to 10 MHz, external cavity lasers of kilohertz to hundreds of kilohertz depending on how long the measurement lasts, and lasers locked to ultrastable reference cavities have reached below 40 mHz (Kessler and colleagues). The linewidth sets the laser's coherence length, $c / (π Δ ν)$ for a Lorentzian line: 95 m for 1 MHz and 95 km for 1 kHz.

Where the width comes from

Even with every technical noise source removed, spontaneous emission into the lasing mode keeps adding photons of random phase, so the phase of the laser field diffuses and the spectrum becomes a Lorentzian line. Its width is the Schawlow–Townes linewidth,

Δ ν_{ST} = \frac{π h ν (Δ ν_{c})^{2} n_{sp}}{P_{out}},

where $Δ ν_{c}$ is the linewidth of the passive (cold) cavity, set by its mirror and internal losses, $n_{sp}$ the population inversion factor and $P_{out}$ the output power. In a semiconductor laser the carrier density links gain and refractive index, and each spontaneous event also moves the frequency; Henry showed that this multiplies the width by $1 + α^{2}$ , where $α$ is the linewidth enhancement factor, 2 to 5 in InGaAsP quantum-well lasers, a factor of 5 to 26. For a 300 µm cleaved diode laser emitting 10 mW at 1550 nm, with the formula multiplied by 0.80, the fraction of the cavity loss that leaves as output, the result is 0.22 MHz, or 3.7 MHz with $α = 4$ . The formula names the ways to a narrower line: a longer or lower-loss cavity, which lowers $Δ ν_{c}$ , and more power.

On top of that intrinsic width, current noise, temperature drift, vibration and acoustic pickup make the frequency wander, mostly at low Fourier frequencies. That wander widens the line further, into a shape closer to a Gaussian, by an amount that grows with the time over which the spectrum is recorded.

Intrinsic and effective linewidth

A linewidth is therefore quoted in one of two ways. The intrinsic, or Lorentzian, linewidth is the width that white frequency noise alone would produce, $π S_{0}$ for a one-sided white level $S_{0}$ in the frequency-noise spectrum; it is read from the flat floor of that spectrum at high Fourier frequencies. The effective, or integrated, linewidth is the width of the line recorded over a stated observation time, which the β-separation line estimates from the same spectrum. The two can differ widely: the laser used as the example in the β-separation entry, with a 5.9 kHz intrinsic linewidth and flicker frequency noise, has an effective linewidth of about 150 kHz over 1 ms and 250 kHz over 1 s. A datasheet value without the observation time, or without saying which of the two it is, cannot be compared with another.

Typical values

Laser	Linewidth	Source
Cleaved Fabry-Perot diode, 300 µm, 10 mW, one mode	3.7 MHz intrinsic	computed, Schawlow–Townes entry
Telecom DFB laser	1 to 10 MHz	DFB laser entry
External cavity laser, 1 mm chip and 20 mm of air, 10 mW	5.9 kHz intrinsic	computed, external cavity laser entry
Hybrid InP–silicon nitride laser	40 Hz intrinsic	Fan et al., 2020
Diode laser self-injection locked to a silicon nitride resonator	hertz-level intrinsic	Jin et al., 2021
Laser locked to a silicon reference cavity	below 40 mHz	Kessler et al., 2012

A multimode Fabry-Perot laser also has a spectral width, that of the envelope of its several modes, which is a different quantity from the linewidth of any one mode and is specified separately, as an RMS or −20 dB width in nanometers.

Measuring it

A grating optical spectrum analyzer resolves 0.02 nm at best, 2.5 GHz at 1550 nm, far wider than a single-frequency laser's line, so linewidths are measured with interferometric methods instead. The delayed self-heterodyne method, introduced by Okoshi and colleagues, beats the laser against a copy of itself delayed by more than its coherence time and shifted in frequency; for a Lorentzian line the beat is a Lorentzian twice as wide. The delay has to exceed the coherence length in fiber, $c / (n π Δ ν)$ , about 65 m for 1 MHz and 6.5 km for 10 kHz, so narrower lasers need a heterodyne beat with a second, narrower laser, or a frequency discriminator that records the frequency-noise spectrum, from which both linewidths follow. How to measure laser linewidth compares the methods.

Emitters and ensembles

The linewidth of an emitter or an ensemble of emitters, as opposed to that of a laser field, splits into a homogeneous part, set by the coherence time $T_{2}$ of each emitter, and an inhomogeneous part, set by the spread of transition frequencies among them; the time-domain counterpart of the inhomogeneous width is the dephasing time T2*. Saturating such a line with a narrowband laser burns a spectral hole whose width reveals the homogeneous part.

The Photonics Unit Converter converts a linewidth between frequency, wavelength and wavenumber at any center wavelength and gives the corresponding Q and coherence length. The Coherence Length Calculator gives the coherence time and length for Lorentzian, Gaussian and flat-top spectra and the fringe visibility at a chosen path difference.

Common questions

What is laser linewidth?

The width of a laser's optical spectrum, usually the full width at half maximum of the power spectrum of one lasing mode, in hertz. It measures how far the laser's frequency spreads, and it sets the coherence length.

What is a typical laser linewidth?

1 to 10 MHz for a telecom DFB laser, kilohertz to hundreds of kilohertz for an external cavity laser depending on the observation time, and below a hertz for lasers locked to reference cavities.

For a Lorentzian line the coherence length is $c / (π Δ ν)$ in vacuum and $c / (n π Δ ν)$ in a medium of index $n$ : 95 m in vacuum for a 1 MHz line. Conventions differ by factors of order one, so the definition used should be stated.

What is the difference between intrinsic and effective linewidth?

The intrinsic linewidth is the Lorentzian width from white frequency noise, set by spontaneous emission. The effective linewidth also includes the slower wander of the frequency and depends on the observation time, and it is never the smaller of the two.

References: A. L. Schawlow and C. H. Townes, "Infrared and optical masers," Physical Review 112, 1940 (1958). C. H. Henry, "Theory of the linewidth of semiconductor lasers," IEEE Journal of Quantum Electronics 18, 259 (1982). T. Okoshi, K. Kikuchi, and A. Nakayama, "Novel method for high resolution measurement of laser output spectrum," Electronics Letters 16, 630 (1980). G. Di Domenico, S. Schilt, and P. Thomann, "Simple approach to the relation between laser frequency noise and laser line shape," Applied Optics 49, 4801 (2010). Y. Fan et al., "Hybrid integrated InP-Si3N4 diode laser with a 40-Hz intrinsic linewidth," Optics Express 28, 21713 (2020). W. Jin et al., "Hertz-linewidth semiconductor lasers using CMOS-ready ultra-high-Q microresonators," Nature Photonics 15, 346 (2021). T. Kessler et al., "A sub-40-mHz-linewidth laser based on a silicon single-crystal optical cavity," Nature Photonics 6, 687 (2012).