Spectral linewidth
Encyclopedia
The spectral linewidth characterizes the width of a spectral line
, such as in the electromagnetic emission spectrum
of an atom, or the frequency spectrum
of an acoustic or electronic system. For example, the emission of an atom usually has a very thin spectral linewidth, as only transitions between discrete energy levels are allowed, leading to emission of photons with a certain energy.
Several definitions are used to quantify the spectral linewidth, e.g. the full width at half maximum
(FWHM).
While the spectral width of a resonator in electronics depends on the parameters of the components, and therefore can be easily adjusted over a wide range, linewidths are typically more difficult to adjust in physics. For example, even a resting atom which does not interact with its environment has a non-zero linewidth, called the natural linewidth (also called the decay width), which is a consequence of the Fourier transform limit (classical description) and the Heisenberg uncertainty principle (quantum mechanical description).
According to the uncertainty principle the uncertainty in energy, ΔE, of a transition is inversely proportional to the lifetime, Δt of the excited state:
In practice lines are further broadened by effects such as Doppler broadening
.
Spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
, such as in the electromagnetic emission spectrum
Emission spectrum
The emission spectrum of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted by the element's atoms or the compound's molecules when they are returned to a lower energy state....
of an atom, or the frequency spectrum
Frequency spectrum
The frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...
of an acoustic or electronic system. For example, the emission of an atom usually has a very thin spectral linewidth, as only transitions between discrete energy levels are allowed, leading to emission of photons with a certain energy.
Several definitions are used to quantify the spectral linewidth, e.g. the full width at half maximum
Full width at half maximum
Full width at half maximum is an expression of the extent of a function, given by the difference between the two extreme values of the independent variable at which the dependent variable is equal to half of its maximum value....
(FWHM).
While the spectral width of a resonator in electronics depends on the parameters of the components, and therefore can be easily adjusted over a wide range, linewidths are typically more difficult to adjust in physics. For example, even a resting atom which does not interact with its environment has a non-zero linewidth, called the natural linewidth (also called the decay width), which is a consequence of the Fourier transform limit (classical description) and the Heisenberg uncertainty principle (quantum mechanical description).
According to the uncertainty principle the uncertainty in energy, ΔE, of a transition is inversely proportional to the lifetime, Δt of the excited state:
In practice lines are further broadened by effects such as Doppler broadening
Doppler broadening
In atomic physics, Doppler broadening is the broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting particles result in different shifts, the cumulative effect of which is the line broadening.The...
.
See also
- Bandwidth (signal processing), a more generic term for a range of electromagnetic frequencies
- Q factorQ factorIn physics and engineering the quality factor or Q factor is a dimensionless parameter that describes how under-damped an oscillator or resonator is, or equivalently, characterizes a resonator's bandwidth relative to its center frequency....
and linewidths in electronics, acoustics and optics - Spectral lineSpectral lineA spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from a deficiency or excess of photons in a narrow frequency range, compared with the nearby frequencies.- Types of line spectra :...
s in optics. (This article also includes a list of sources of linewidth broadening.) - Spectral widthSpectral widthIn telecommunications, spectral width is the wavelength interval over which the magnitude of all spectral components is equal to or greater than a specified fraction of the magnitude of the component having the maximum value....
in telecommunications - Oscillator linewidthOscillator linewidthThe concept of a linewidth is borrowed from laser spectroscopy. The linewidth of a laser is a measure of its phase noise. The spectrogram of a laser is produced by passing its light through a prism. The spectrogram of the output of a pure noise-free laser will consist of a single infinitely thin...
- Laser linewidthLaser linewidth←Laser linewidth is the spectral linewidth of a laser beam.Two of the most distinctive characteristics of laser emission are spatial coherence and spectral coherence. While spatial coherence is related to the beam divergence of the laser, spectral coherence is evaluated by measuring the laser...