1.  Laser bandwidth | | A bandwidth is the width of some frequency range. In photonics, the term bandwidth occurs in a variety of meanings:
-the bandwidth (or linewidth = width of the intensity spectrum) of the output of some light source, e.g. an ASE 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 (e.g. 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
-the range of frequencies in which some measurement apparatus (e.g. a powermeter) can operate
-the data rate (e.g. in Gbit/s) achieved in an optical communication system
The latter use of the term is somewhat sloppy; precisely speaking, the data rate is limited by the optical bandwidth, but is not really itself a bandwidth.
The bandwidth of a light sources 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 term Q factor is sometimes used; it is the oscillation frequency divided by the bandwidth.
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. To convert a (small) wavelength interval into a frequency interval, one can use the formula
Δυ=c*Δλ/(λ^2)
This shows that one nanometer is worth more gigahertz if the center wavelength is shorter.
from:http://www.rp-photonics.com/bandwidth.html
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