Optical Bandwidth
Optical bandwidth is the width of a range of optical frequencies (or the corresponding wavelength interval) associated with a light source or a photonic device. In lasers and photonics it most often means either the spectral width of emitted light or the frequency range over which a component can usefully operate.
It is commonly specified as full width at half-maximum (FWHM) of the optical power spectrum, though other measures (1/e width, 10 dB width, etc.) are also used.
Source spectrum vs. device passband:
For a laser or other source, optical bandwidth (sometimes called linewidth when the spectrum is a single narrow line) is the width of the output spectrum. Stabilized single-frequency lasers can have bandwidths below 1 Hz; few-femtosecond pulses can span tens of terahertz.
For components, it is the frequency (or wavelength) range over which the device performs its function: gain bandwidth of an amplifier or laser medium, reflection bandwidth of a mirror, transmission window of a fiber, or phase-matching bandwidth of a nonlinear crystal.
Wavelength and frequency intervals are related by
Δν ≈ (c/λ2)Δλ
so a given Δλ corresponds to a larger Δν at shorter wavelengths.
Optical bandwidth is distinct from modulation bandwidth (how fast optical power can be varied or detected, typically given in MHz–GHz). The two are sometimes confused in detector and transceiver specs.
Relation to coherence:
Narrower optical bandwidth implies longer coherence time τcoh (and coherence length Lcoh=cτcoh). For a Lorentzian lineshape the FWHM linewidth and coherence time are related by Δν=1/(πτcoh). High temporal coherence is useful for interferometry and holography; low coherence (broad bandwidth) is useful when path-length differences or speckle must be suppressed.
Typical magnitudes:
HeNe laser gain bandwidth: ~1.5 GHz (~0.002 nm at 633 nm).
Ti:sapphire: tens to >100 THz (hundreds of nm around 800 nm).
Telecom silica fiber low-loss windows: on the order of 10 THz or more across the C+L bands and beyond.
Applications:
Narrow bandwidth: high-resolution spectroscopy, optical frequency standards and clocks, coherent optical communications, long-coherence LIDAR, precision interferometry, and holography.
Broad bandwidth: generation of ultrashort pulses (time–bandwidth product), optical coherence tomography (axial resolution scales with bandwidth), wavelength-division multiplexing (many independent channels in one fiber), supercontinuum sources, and broadband sensing.
Device bandwidths: erbium-doped fiber amplifiers (C-band gain window), dielectric mirrors and filters, photodetectors (sometimes specified by a –3 dB optical-power bandwidth), and nonlinear frequency-conversion crystals (phase-matching acceptance bandwidth).
In fiber communications the enormous optical bandwidth of silica fiber, combined with WDM, is what enables terabit-scale links; the term is sometimes loosely used for data rate itself, but that usage is imprecise.