Center Wavelength
Center wavelength (CWL, also written centre wavelength) is the representative wavelength that locates a finite-bandwidth spectrum—of a laser, LED/SLD, optical filter, Fiber Bragg Grating (FBG), or similar photonic component—rather than a single monochromatic line.
It is distinct from peak wavelength (the wavelength of maximum spectral power density). For a symmetric spectrum they coincide; for an asymmetric spectrum (common in laser diodes, LEDs, and some filters) they differ.
Common definitions:
Two definitions appear most often:
Power-weighted mean (centroid / first moment / “center of gravity”):
λc = ∫p(λ) λ dλ
∫p(λ) dλ
where p(λ) is the power spectral density. This is the most rigorous definition for broadband or multi-mode sources.
Midpoint of the half-power (FWHM) points:
λc = λ1+λ2
2
where λ1 and λ2 are the wavelengths at 50 % of peak intensity. This is widely used on laser and bandpass-filter datasheets. For multilayer interference filters the arithmetic mean is sometimes taken in wavenumber (1/λ) because the response is more linear in frequency.
ISO 13695 also defines a weighted-average wavelength of discrete modes for multi-line lasers.
Wavelengths are normally quoted as vacuum wavelengths (or “wavelength in air” with a small correction of order 0.3 nm near 1 µm). Units are usually nanometers in the visible–telecom range.
Technical notes:
A real laser is never perfectly monochromatic. The emission spectrum has a finite width set by the gain bandwidth, cavity modes, and (for diodes) temperature and drive current. The CWL is the number used to specify that spectrum.
Typical temperature coefficients: laser diodes ~0.2–0.4 nm/°C; uncompensated FBGs ~10 pm/°C; athermal FBGs and many telecom lasers are much more stable.
Related specs that almost always accompany CWL: FWHM (or RMS) bandwidth, side-mode suppression ratio (SMSR), and wavelength tolerance/accuracy.
In WDM the ITU-T grids define channel center frequencies (hence wavelengths); e.g., the C-band 100 GHz grid is anchored at 193.1 THz ≈ 1552.52 nm.
Applications:
Laser specification and pumping. Datasheets quote CWL ± tolerance so the source can be matched to an absorption line (808 nm for Nd:YAG, 980 nm for EDFAs, 1480 nm for Raman pumps, etc.).
Optical bandpass / laser-line filters. CWL and FWHM are the two primary filter parameters used in fluorescence microscopy, Raman spectroscopy, and laser safety.
Fiber Bragg gratings. The Bragg wavelength is the CWL of the reflection peak. FBGs stabilize pump-laser wavelength, form fiber-laser cavities, serve as DWDM add/drop filters, and act as strain/temperature sensors (the peak shifts with the measurand).
WDM / CWDM / DWDM systems. Channel plans and multiplexers/demultiplexers are designed around defined center wavelengths.
Tunable lasers and spectrometers. The operating or design wavelength is given as a CWL plus a tuning or free-spectral range.
LEDs, SLDs, and broadband sources. Both peak and centroid/center wavelengths are often listed because the spectra are asymmetric.
Center wavelength is the practical “address” of a spectral feature when the source or component is not a single infinitely narrow line.