When energy is given to laser medium (solid, liquid or gas), then only a small fraction of laser medium shows lasing action. This part of laser medium is called active medium or active centre. Thus, due to this reason, the laser medium is also called the heart of a laser. For example, in case of ruby laser, Al2O3 is doped with Cr2O3.
The laser is due to doped chromium ions. Thus Cr3+ ions are active centres. In He-Ne laser, laser is produced due to Ne atoms, therefore, Ne atoms are active centres.
Note: Dear students, next time, we will learn about optical pumping and its types.