Sech2-shaped Pulses | previous | next | feedback |
Definition: pulses with a temporal intensity profile which has the shape of a sech2 function
Ultrashort pulses from mode-locked lasers often have a temporal shape which can be described with a squared hyperbolic secant (sech) function:
The full width at half-maximum pulse duration is approximately 1.76 times the parameter 
.
(That parameter itself is sometimes called the pulse duration.)
In many practical cases (e.g. soliton mode locking), sech2 pulses have hardly any chirp, i.e., they are close to transform-limited. The time–bandwidth product is then ∼ 0.315.
Compared with a Gaussian function with the same half-width, the sech2 function has stronger wings, as shown in Figure 1.
Figure 1: Temporal shapes of sech2 and Gaussian pulses.
The peak power of a sech2 pulse is ∼ 0.88 times the pulse energy divided by the FWHM pulse duration.
The sech2 shape is typical of fundamental soliton pulses (in the absence of higher-order dispersion and self-steepening). Therefore, this pulse shape also occurs in soliton mode-locked lasers. However, it is also found in other situations; e.g. passive mode locking with a slow absorber usually leads to a pulse shape which is relatively close to the sech2 shape.
See also: pulses, Gaussian pulses, solitons, soliton mode locking, transform limit
Since October 2008, the Encyclopedia of Laser Physics and Technology is also available in the form of a two-volume book. Maybe you would enjoy reading it also in that form! The print version has a carefully designed layout and can be considered a must-have for any institute library, laser research group, or laser company.
You may order the print version via Wiley-VCH.
