An important concept in many system fields including, for example, data, voice, and video transmission; information theory; radio communications; and spectroscopy. In a communications channel, bandwidth is the maximum range of frequencies (for an analog line) or bit rate (for a digital link) the channel will transmit without exceeding some specified loss. It is measured in hertz. The term also refers to data rates when communicating over certain media or devices. According to the Shannon-Hartley theorem, the data rate of reliable communication is directly proportional to the frequency range of the signal used for the communication. In this context, the word bandwidth can refer to either the data rate or the frequency range of the communication system (or both). Bandwidth also refers to the range of frequencies in which the signal remains strong and beyond which the signal drops by at least some specified value, often 3 dB below (i.e., less than half of) the maximum value. For analog signals bandwidth is the width, measured in hertz, of a frequency range in which the signal’s Fourier transform is nonzero. The word bandwidth could also apply to systems. Saying that a system has a certain bandwidth is essentially the same as saying that the transfer function of the system has a certain bandwidth. In photonics, the term bandwidth has a variety of meanings: it can be applied to optical pulse duration, the frequency range of some optical element such as an optical amplifier, modulation frequency, or frequency range of specific instruments.
