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:
Here, the parameter 
is the full width half maximum pulse duration divided by the numerical factor 1.76.
In many practical cases (e.g. soliton mode locking), sech2 pulses have hardly any chirp, i.e., 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 somewhat stronger wings:
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 for 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
