RP Photonics logo
RP Photonics
Encyclopedia
Technical consulting services on lasers, nonlinear optics, fiber optics etc.
Profit from the knowledge and experience of a top expert!
Powerful simulation and design software.
Make computer models in order to get a comprehensive understanding of your devices!
Success comes from understanding – be it in science or in industrial development.
The famous Encyclopedia of Laser Physics and Technology – available online for free!
The ideal place for finding suppliers for many photonics products.
Advertisers: Make sure to have your products displayed here!
… combined with a great Buyer's Guide!
VLib part of the
Virtual
Library

Gain Bandwidth

<<<  |  >>>

Definition: the width of the optical frequency range in which significant gain is available from an amplifier

German: Verstärkungsbandbreite

Categories: optical amplifiers, lasers

Formula symbol: Δνg, Δλg

Units: Hz, nm

How to cite the article; suggest additional literature

Optical amplification (quantified as a gain value) can only occur for a finite range of optical frequencies. The gain bandwidth is essentially the width of this frequency range.

Definitions

Unfortunately, there is a lot of confusion about the exact meaning of the term gain bandwidth (also called amplification bandwidth), because the gain of, e.g., a laser amplifier decreases fairly smoothly, and also there are different ways of quantifying gain, so that there is not a single straightforward definition. Possible definitions are:

Because of these manifold definitions, which can lead to different values, the specification of a gain bandwidth is meaningful only when the definition used is indicated. Additional difficulties arise when the shape of the gain spectrum is complicated, e.g. containing multiple peaks.

Note also that in some cases (e.g. the generation of ultrashort pulses with mode locking) the relevant quantity is not the total width of the gain region, but rather the “curvature” of the gain curve near its maximum. For a given spectral shape, that curvature is directly related to the gain bandwidth, and some effective gain bandwidth can be defined to quantify that mentioned curvature. Such a quantity can be used, e.g., in the Haus Master equation.

Importance of the Gain Bandwidth

The gain bandwidth of the gain medium of a laser, or of an optical parametric amplifier, can be important e.g. in the following cases:

In the case of an optical amplifier, e.g. in an optical fiber communications system, the gain bandwidth can limit the usable transmission bandwidth and thus the achievable data rate.

Particularly in the context of mode-locked lasers and short-pulse amplifiers, the phenomenon that a limited gain bandwidth leads to spectral compression is called gain narrowing.

Physical Aspects Behind the Gain Bandwidth

Even the simplest laser transition, occurring between two different energy levels in absolutely equal atoms or ion without external influences, would exhibit some (small) finite gain bandwidth. The reason is the finite upper-state lifetime (and, if applicable, also the lower-state lifetime), which leads to lifetime broadening.

A much larger gain bandwidth can be obtained in gain media with strong homogeneous or inhomogeneous broadening. Some examples are:

Parametric amplifiers also have a finite gain bandwidth, which in that case depends on chromatic dispersion, the phase-matching configuration, and the crystal length. Frequently used equations involve the group velocities of pump, signal and idler waves for some idealized cases (e.g. zero pump bandwidth). In some special phase-matching configurations, the gain bandwidth can be very large.

Measurement of the Gain Spectrum

The measurement of an optical gain spectrum of a laser medium may be based on a direct gain measurement, where a tunable laser provides a signal input, the amplification factor of which is measured at different wavelengths. This, however, requires relatively expensive equipment.

In the case of four-level gain media with negligible excited-state absorption, it may be easier to measure the width of the fluorescence spectrum. The logarithmic gain is then proportional to the power spectral density of the fluorescence. This method, however, is not valid for three-level gain media, as reabsorption can reduce the effective gain without substantially influencing the fluorescence spectrum. The same holds for media with excited-state absorption.

See also: gain, bandwidth, gain narrowing, laser crystals, laser crystals versus glasses, parametric amplification, wavelength tuning, mode locking, titanium–sapphire lasers, vibronic lasers, Spotlight article 2007-10-25
and other articles in the categories optical amplifiers, lasers

If you like this article, share it with your friends and colleagues, e.g. via social media:

arrow