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Definition: the width of some frequency or wavelength range
In photonics, the term bandwidth occurs with a variety of meanings:
- the bandwidth (or linewidth = width of the intensity spectrum) of the output of some light source, e.g. a superluminescent source or a laser; the bandwidth of ultrashort pulses can be particularly large
- the width of the frequency range which can be transmitted by some element, e.g. an optical fiber
- the gain bandwidth of an optical amplifier
- the width of the range of some other phenomenon, such as a reflection, the phase matching of a nonlinear process (→ phase-matching bandwidth), or some resonance
- the maximum modulation frequency (or range of modulation frequencies) of an optical modulator
Optical bandwidth values may be specified in terms of frequency or wavelength. Due to the inverse relationship of frequency and wavelength, the conversion factor between gigahertz and nanometers depends on the center wavelength or frequency. For converting a (small) wavelength interval into a frequency interval, the equation
can be used. This shows that 1 nm is worth more gigahertz if the center wavelength is shorter.
The term bandwidth is also often used for the data rate (e.g. in Gbit/s) achieved in an optical communication system. Precisely, the data rate is limited by the optical bandwidth, but is not really itself a bandwidth.
The optical bandwidth of a light source is strongly related to the temporal coherence, characterized with the coherence time.
Both for passive resonators (e.g. optical cavities) and for the output of oscillators (e.g. lasers), the Q factor is the oscillation frequency divided by the bandwidth.
See also: time–bandwidth product, gain bandwidth, coherence time, transform limit, Q factor, phase-matching bandwidth
