Filling factor, , is a quantity measuring the efficiency of absorption of pump in the core of a double-clad fiber

Double-clad fiber

Double-clad fiber is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding...

.
The efficiency of absorption of pumping energy in the fiber is an important parameter of a double-clad fiber

Double-clad fiber

Double-clad fiber is a class of optical fiber with a structure consisting of three layers of optical material instead of the usual two. The inner-most layer is called the core. It is surrounded by the inner cladding, which is surrounded by the outer cladding...

 laser. In many cases this efficiency can be approximated with
where is the cross-sectional area of the cladding is the radius of the core (which is taken to be circular) is the absorption coefficient of pump light in the core is the length of the double-clad fiber, and is a dimensionless adjusting parameter, which is sometimes called the "filling factor"; .

The filling factor may depend on the initial distribution of the pump light, the shape of the cladding, and the position of the core within it.
The large (close to unity) filling factor is important in double-clad amplifiers; it allows them to reduce the requirements for the brightness

Brightness

Brightness is an attribute of visual perception in which a source appears to be radiating or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target...

 of the pump and to reduce the length of the fiber laser.