In the common case of DC-coupled photodetectors, the bandwidth is equated to the maximum detectable modulation frequency according to some criterion. Typically, that frequency range would start from zero frequency, but in some cases (AC-coupled photodetectors) that is not the case. Bandwidth of PhotodetectorsĪ photodetector has a limited bandwidth, here meaning the frequency range in which modulations of the optical power can be detected. Although a large data transmission rate is not possible without a large optical bandwidth, different communications devices can differ substantially in terms of spectral efficiency, i.e., concerning what data rate is achievable per megahertz of optical bandwidth. Note that the data transmission capacity has only a limited relation to the optical bandwidth. A more appropriate term would be data rate or data transmission capacity, avoiding any confusion with optical bandwidth. in units of Gbit/s) achieved in an optical communication system. In the area of optical fiber communications, the term bandwidth is also often inaccurately used for the data rate (e.g. Bandwidth of ModulationsĪ bandwidth can also indicate the maximum frequency with which a light source can be modulated, or at which modulated light can be detected with a photodetector. lasers), the Q factor is the oscillation frequency divided by the bandwidth. optical cavities) and for the output of oscillators (e.g. The optical bandwidth of a light source is strongly related to the temporal coherence, characterized with the coherence time.īoth for passive resonators (e.g. If you choose a longer center wavelength, you will see that one nm is worth fewer GHz!Īttention: The buttons do not work, as Javascript is turned off in your browser! Conversion between Frequency and Wavelength Bandwidth Center wavelength:Įnter input values with units, where appropriate.Īfter you have modified some values, click a “calc” button to recalculate the field left of it. ) This shows that 1 nm is worth more gigahertz if the center wavelength is shorter. (It can be obtained by considering the derivative of with respect to. $$\Delta \nu = \frac\Delta \lambda $$Ĭan be used. For converting a (small) wavelength interval into a frequency interval, the equation Due to the inverse relationship of frequency and wavelength, the conversion factor between gigahertz and nanometers depends on the center wavelength or frequency. Optical bandwidth values may be specified in terms of frequency or wavelength. Its full width at half maximum bandwidth is 8.9 nm, corresponding to 3.9 THz. Figure 1: The optical spectrum of an unchirped 80-fs ultrashort light pulse. For example some authors use the half width at half maximum (HWHM), which is just half the FWHM. For example, it can be the reflection bandwidth of a mirror, the optical transmission bandwidth of an optical fiber, the gain bandwidth of an optical amplifier, or the phase-matching bandwidth of a nonlinear optical device.Ī common definition of spectral width is the full width at half maximum (FWHM), but other definitions are also used. An optical bandwidth can be the width of a frequency range which can somehow be handled by an optical element or photonic device.On the other hand, ultrashort pulses with few-femtosecond pulse durations can have very large bandwidth – easily tens of terahertz. For narrow-linewidth lasers, the bandwidth can be extremely small – in extreme cases below 1 Hz, which is many orders of magnitude less than the mean optical frequency. A light source can have some optical bandwidth (or linewidth), meaning the width of the optical spectrum of the output. In the following cases, bandwidth means the width of a range of optical frequencies: The following sections discuss some important cases. In photonics, the term bandwidth occurs in many different cases. How to cite the article suggest additional literature More specific terms: gain bandwidth, resonator bandwidth, modal bandwidth, phase-matching bandwidthĬategories: light detection and characterization, physical foundations Encyclopedia > letter B > Bandwidth Bandwidthĭefinition: the width of some frequency or wavelength range
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