Laser Linewidth and Bandwidth Calculator

Your details

Choose whether you know the bandwidth in nm (e.g. from a spectrometer) or in GHz (e.g. from a beatnote measurement).
The central emission wavelength of your laser. Common values: 405 nm (violet diode), 532 nm (green DPSS), 633 nm (He-Ne), 808 nm (NIR diode), 1064 nm (Nd:YAG), 1550 nm (telecom C-band).
nm
Full-width at half-maximum (FWHM) spectral width in nanometres. For a single-mode telecom laser this is typically 0.001-0.01 nm; for a multimode diode it can be several nm.
nm
Refractive index n of the propagation medium. Use 1.000 for vacuum or air, 1.5 for borosilicate glass, 1.44-1.47 for silica fibre at 1550 nm. Affects coherence length only.
Frequency bandwidthBroadband source
12.4784GHz

Spectral width expressed in GHz (exact endpoint method)

Coherence time80.139ps
Coherence length24.025mm
Quality factor Q15,500
Approx. bandwidth (small-Δλ)12.4784GHz
Approximation error-0%
15,500
Broadband<1000000Moderate1000000-1000000000Narrow1000000000-1000000000000Ultra-narrow1000000000000+

Frequency bandwidth: 12.4784 GHz at 1550 nm

  • The coherence length is 24.0 mm. Interference is visible over path differences up to that distance.
  • The coherence time is 80.1 ps, the window over which the phase of the output field stays predictable.
  • The quality factor Q = 1.55e+4 indicates how spectrally pure the source is relative to its center frequency.
  • This bandwidth is typical of free-running single-mode lasers or narrow multimode diode lasers.

Next stepFor interferometry, check that your target path-length difference is well below the coherence length. For OCT imaging, the axial resolution is approximately lambda0^2 / (2 * delta_lambda), so a broader bandwidth gives finer depth resolution.

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